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IntroductionThermal relay is a protective device.  are protective devices. It is used in conjunction with a contactor to protect electric motors. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying over current of the system, it bends and makes normally open contacts. Getting to know more about the thermal basics just check the following note. CatalogIntroductionⅠ The Working Principle and Structure of Thermal Relay  1.1 The Role and Classification of Thermal Relay  1.2 Protection Characteristics and Working Principle of Thermal RelayⅡ Selection and Setting Principle of Thermal Relay  2.1 Thermal Relay Selection Overview  2.2 Type Selection of Thermal Relay  2.3 Selection of Rated Current of Thermal Relay  2.4 Selection of Thermal Element Setting Current  2.5 Reliable and Reasonable Protection Characteristics of The Thermal Relay  2.6 Other ConsiderationsⅢ Other Matters Needing Attention  3.1 Installation Direction  3.2 Selection of Connecting Wires  3.3 Use Environment  3.4 Adjustment of Thermal RelayⅣ Frequently Asked Questions about Thermal RelayⅠ The Working Principle and Structure of Thermal Relay1.1 The Role and Classification of Thermal RelayIn the electric drag control system, when the three-phase AC motor runs under abnormal conditions such as long-term under-load and under-voltage operation, long-term overload operation, and long-term single-phase operation, it will cause the motor winding to overheat and even burn out. In order to give full play to the overload capacity of the motor, to ensure the normal start and operation of the motor, and once the motor is overloaded for a long time, it can automatically cut off the circuit, so that there are electrical appliances that can change the operating time with the degree of overload and that is thermal relay.  Obviously, the thermal relay is used for overload protection of the three-phase AC motor in the circuit. It must be pointed out that, due to the thermal inertia of the heating elements in the thermal relay, instantaneous overload protection cannot be done in the circuit, and short circuit protection cannot be done either. Therefore, it is different from overcurrent relays and fuses. According to the number of phases, there are three types of thermal relays: single-phase, two-phase, and three-phase. Each type has different specifications and model numbers according to the rated current of the heating element. Three-phase thermal relays are often used in three-phase AC motors for overload protection. Divided by function, there are two types of three-phase thermal relay. One is without phase failure protection and the other one is with phase failure protection.1.2 Protection Characteristics and Working Principle of Thermal Relay1) Protection Characteristics of Thermal Relay Because the contact action time of the thermal relay is related to the overload of the motor being protected, before analyzing the working principle of the thermal relay, the relationship between the motor's overload current and the motor's energizing time must be clarified under the condition that the motor does not exceed the allowable temperature rise. This relationship is called the overload characteristic of the motor. When an overload current occurs during the motor operation, it will inevitably cause the winding to heat up. According to the thermal equilibrium relationship, it is not difficult to draw the conclusion that under the condition of allowable temperature rise, the motor energizing time is inversely proportional to the square of its overload current. According to this conclusion, it can be concluded that the motor's overload characteristics have inverse time characteristics, as shown by curve 1 in Figure 1. Figure 1. Overload Characteristics of the Motor and Protection Characteristics of the Thermal Relay and Their Coordination In order to adapt to the overload characteristic of the motor and play the role of overload protection, it is required that the thermal relay should also have the inverse time characteristic as the motor overload characteristic. For this reason, the thermal relay must have a resistance heating element. The thermal effect generated by the overload heating current through the resistance heating element causes the sensing element to act, thereby driving the contact to complete the protection function.  The relationship between the overload current passed in the thermal relay and the action time of the thermal relay contact is called the protection characteristic of the thermal relay, as shown by curve 2 in figure 1. Considering the effects of various errors, the overload characteristics of the motor and the protection characteristics of the relay are not a curve, but a belt. Obviously, the larger the error, the wider the belt; the smaller the error, the narrower the belt. It can be known from the curve 1 in the figure that when the motor is overloaded, it is safe to work below the curve 1. Therefore, the thermal relay's protection characteristics should be close to the motor's overload characteristics. In this way, if an overload occurs, the thermal relay will operate before the motor reaches its allowable overload limit, cutting off the power to the motor to prevent damage. 2) Working Principle of Thermal Relay The heat-generating heating element in the thermal relay should be connected in series with the motor circuit. In this way, the thermal relay can directly reflect the overload current of the motor. The sensing element of a thermal relay generally uses a bimetal. The so-called bimetallic sheet is to mechanically roll two metal sheets with different linear expansion coefficients into one body. The larger the expansion coefficient is called the active layer, the smaller the expansion coefficient is called the passive layer. The bimetallic sheet undergoes linear expansion when heated. Because the linear expansion coefficients of the two layers of metal are different and the two layers of metal are closely attached together, the bimetallic sheet is bent to the passive layer side, and the mechanical force generated by the bending of the bimetallic piece drives the contact action. There are four types of bimetal heating methods, namely direct heating, indirect heating, composite heating, and current transformer heating. The direct heating type uses the bimetal as a heating element and allows current to pass through it directly; the heating element of the indirect heating type is made of resistance wire or tape, is wound around the bimetal and is insulated from the bimetal; the composite heating type is between the above two methods; the heating element of the current transformer heating type is not directly connected to the motor circuit, but is connected to the secondary side of the current transformer. This method is mostly used in situations where the motor current is relatively large to reduce the current passes through the heating element. Figure 2. Structural Schematic of the Thermal Relay The thermal element 3 is connected in series to the motor stator winding, and the motor winding current is the current flowing through the thermal element. When the motor is running normally, although the heat generated by the thermal element can bend the bimetal 2, it is not enough to make the relay operate; when the motor is overloaded, the heat generated by the thermal element increases, causing the bending displacement of the bimetal to increase.  After a certain period of time, the bimetal is bent to push the guide plate 4, and the contacts 9 and 6 are separated by the compensating bimetal 5 and the push rod 14, the contacts 9 and 6 are normally-closed contacts in which the thermal relay is connected to the contactor coil circuit, and the contactor is de-energized after being disconnected. The normally-open contacts of the contactor disconnect the power supply of the motor to protect the motor. The adjusting knob 11 is an eccentric wheel, which constitutes a lever with the support 12, and 13 is a compression spring. Turning the eccentric wheel and changing its radius can change the contact distance between the compensating bimetal 5 and the guide plate 4 so that the purpose of adjusting the setting action current is achieved. In addition, the position of the normally-open contact 7 is changed by adjusting the reset screw 8 so that the thermal relay can work in two working states: manual reset and automatic reset. When debugging the manual reset, after the fault is excluded, button 10 must be pressed to restore the movable contact to the contact position of the static contact 6. 3) Thermal Relay with Open Phase Protection One of the main reasons for a three-phase asynchronous motor to burn out is that wiring of a three-phase motor is loosened or a phase fuse is blown. If the motor protected by the thermal relay is Y connection method when one phase power failure occurs in the line, the current of the other two phases will increase a lot. Since the line current is equal to the phase current, the current flowing through the motor windings and the current flowing through the thermal relay are increased by the same proportion, so ordinary two-phase or three-phase thermal relays can protect this.  If the motor is △ connection method, the phase current and line current of the motor will not be the same when the phase failure occurs, the current flowing through the motor windings and the current flowing through the thermal relay will increase in different proportions, and the thermal element is connected in series with the power supply line of the motor, and it is set according to the rated current of the motor, that is, the line current, and the setting value is relatively large. When the fault line current reaches the rated current, in the motor winding, the fault current of the phase winding with the larger current will exceed the rated phase current, and there is a danger of overheating and burning. Therefore, the △ connection method must use a thermal relay with phase failure protection. The thermal relay with phase failure protection is a differential mechanism added to the ordinary thermal relay to compare the three currents. The structural principle of the differential phase-open protection device is shown in figure 3. The guide plate of the thermal relay is changed to a differential mechanism, which is composed of an upper guide plate 1, a lower guide plate 2 and a lever 5. They are connected by a rotating shaft. Figure 3a shows the positions of the components of the mechanism before power is applied. Figure 3b shows the position during normal energization. At this time, the three-phase bimetals are bent to the left by heating, but the bending deflection is not enough. Therefore, the lower guide plate is moved to the left for a short distance, and the relay does not operate. Figure 3c shows the situation when the three phases are overloaded simultaneously. The three-phase bimetal is bent to the left at the same time, and the lower guide plate 2 is pushed to move to the left. The normally-closed contact is immediately measured by lever 5. Figure 3 shows the disconnection of phase C.  At this time, the phase C bimetal gradually cools down, the end moves to the right and pushes the upper guide plate 1 to the right. While the temperature of the other two-phase bimetals rises, the ends are bent to the left, pushing the lower guide plate 2 to continue to move to the left. Because the upper and lower guide plates move left and right, a differential function occurs, and the normally-closed contacts are opened by the amplification of the lever. Due to the differential function, the thermal relay is accelerated to protect the motor when the phase failure occurs. Figure 3. Schematic Diagram of Differential Relay Phase Failure Protection Mechanism of Thermal Relay Ⅱ Selection and Setting Principle of Thermal Relay2.1 Thermal Relay Selection OverviewThe thermal relay is mainly used to protect the motor from overload. In order to ensure that the motor can obtain both necessary and sufficient overload protection, it is necessary to fully understand the performance of the motor, and assign it with a suitable thermal relay to perform the necessary settings. Generally, conditions related to the motor are the working environment, starting current, load nature, working system, allowable overload capacity and so on. In principle, the ampere-second characteristic of the thermal relay should be as close as possible or even overlap the motor's overload characteristic, or under the motor's overload characteristic, and at the same time, the thermal relay should not be affected (not actuated) at the moment when the motor is temporarily overloaded and started. The correct selection of the thermal relay is closely related to the working system of the motor. When the thermal relay is used to protect the motor for long-term or intermittent long-term operation, it is generally selected according to the rated current of the motor. For example, the setting value of the thermal relay may be equal to 0.95-1.05 times of the rated current of the motor, or the median value of the setting current of the thermal relay is equal to the rated current of the motor, and then adjust. When the thermal relay is used to protect a motor that is repeatedly operated for a short time, the thermal relay has only a certain range of adaptability. If there are many operations per hour, a thermal relay with a speed saturation current transformer must be selected. For special working motors with frequent forward and reverse phase on and off, it is not appropriate to use thermal relays as overload protection devices. Instead, use temperature relays or thermistors embedded in the motor windings to protect them.2.2 Type Selection of Thermal RelayThe thermal relay can be divided into the two-pole types and three-pole types from the structural type. The three-pole type is divided into phase-open protection and no phase-open protection, which should be selected according to the stator wiring of the protected motor. When the motor stator winding is in delta connection, a three-pole thermal relay with phase failure protection must be used; for a motor using the star connection method, a thermal relay without phase failure protection is generally used. Because the general motor does not have a neutral wire when using the star connection method, the two-pole or three-pole type of the thermal relay can be used. However, if the motor is set to use a star connection method with a neutral wire, the thermal relay must use a three-pole type. In addition, generally a two-phase structure thermal relay should be selected for light-load starting, long-term working motors or intermittent long-term working motors; when the current and voltage balance of the motor is poor, the working environment is poor, or there are fewer people to look after, three-phase thermal relay can be used.2.3 Selection of Rated Current of Thermal Relay1) Ensure the normal operation and starting of the motorIn the case of normal starting current and starting time and infrequent starting, it must be ensured that the starting of the motor does not cause the thermal relay to malfunction. When the starting current of the motor is 6 times the rated current, the starting time does not exceed 6s, and rarely starts continuously, the thermal relay can generally be selected according to the rated current of the motor. (In practice, the rated current of the thermal relay can be slightly larger than the rated current of the motor) 2) Consider the object of protection-the characteristics of the motorModels, specifications, and characteristics of motors. The insulation materials of motors are classified into A, E, and B grades. Their allowable temperature rises are different, so their ability to withstand overload is also different, which should be paid attention to when selecting a thermal relay. In addition, the open-type motor is easier to dissipate heat, while the closed-type motor is much more difficult to dissipate heat. With a slight overload, its temperature rise may exceed the limit. Although the selection of the thermal relay is based on the rated current of the motor in principle, the rated current of the thermal relay (or thermal element) that it is equipped with should be appropriately small for the motor with poor overload capacity. In this case, the rated current of the thermal relay (or thermal element) can also be taken as 60% -80% of the rated current of the motor. 3) Consider load factorsIf the nature of the load is not allowed to stop, even if the overload will shorten the life of the motor, the motor should not be allowed to trip unexpectedly, so as not to suffer a huge loss many times higher than the price of the motor. At this time, the rated current of the relay can be selected to a larger value (of course, the selection of the motor under this working condition generally also has a strong overload capacity). In this case, it is best to use the protective measures of the organic combination of thermal relays and other protective appliances, and only consider tripping when a very dangerous overload occurs. In short, this is not a dogmatic formula and should be considered comprehensively. 2.4 Selection of Thermal Element Setting CurrentAccording to model number of the thermal relay and the rated current of the thermal element, the adjustment range of the setting current of the thermal element can be found out. Generally, the setting current of the thermal relay is adjusted to the rated current of the motor; for motors with poor overload capacity, the setting value of the thermal element can be adjusted to 0.6-0.8 times of the rated current of the motor; when the motor starts for a long time, drags the impact load or is not allowed to stop, the setting current of the thermal element can be adjusted to 1.1-1.15 times of the rated current of the motor. 2.5 Reliable and Reasonable Protection Characteristics of The Thermal RelaySpecifically, it should have an inverse time characteristic similar to the allowable overload characteristic of the motor, and it should be below the allowable overload characteristic of the motor, and it should have high accuracy to ensure the reliability of the protective action. 2.6 Other Considerations1) Operating frequency: When the operating frequency of the motor exceeds the operating frequency of the thermal relay, such as the motor's reverse braking, reversible operation, and dense on-off, the thermal relay cannot provide protection. At this time, you can consider using a semiconductor temperature relay for protection. 2) It is not necessary to set overload protection for motors with short working hours and long intervals (such as rocker lifting motors for rocker drilling machines, etc.), and motors that have little possibility of overload despite long-term work(such as exhaust fans, etc.). 3) Thermal relays are generally not suitable for motors with a jog, heavy load starting, continuous forward and reverse rotation, and reverse braking. 4) It should have a certain temperature compensation: due to the change in the temperature of the surrounding medium, under the same overload current, the operation of the thermal relay will cause an error. To eliminate this error, temperature compensation measurements should be set up. 5) In general, the principle that the protected motor should not restart automatically even after the thermal relay is automatically reset after the thermal relay protection action, otherwise, the thermal relay should be set to the manual reset state. This is to prevent the motor from being repeatedly restarted several times to damage the equipment before the fault is eliminated.  For example: Generally, for the control circuit using button control to manually start and stop, the thermal relay can be set to the automatic reset form; for the automatic start circuit using automatic component control, the thermal relay should be set to the manual reset form; Any thermal relay that can be automatically reset should be able to be automatically reset reliably within 5 minutes after operation, while the manual reset one should be reset reliably when the manual reset button is pressed by hand within 2 minutes after the operation. Most products generally have both manual and automatic reset methods and can be adjusted to any method with screws to meet the needs of different occasions. 6) The operating current value should be adjustable to meet the needs in production and use, and reduce the specification-grade, so the thermal relay of a certain specification should be able to be realized by adjusting the cam. 7) Since it takes time for the thermal element to deform due to heat, the thermal relay can only be used as overload protection for the motor, not as short circuit protection. Therefore, when using a thermal relay, a fuse should be installed as short circuit protection. For heavy load, frequent-starting large-capacity important motors, overcurrent relays (time-delay action type) can be used for its overload and short circuit protection. Ⅲ Other Matters Needing Attention3.1 Installation DirectionThe installation direction of the thermal relay is easily overlooked. In the thermal relay, there is a current that generates heat through the heating element, which promotes the action of the bimetal. There are three ways of heat transfer: convection, radiation and conduction. Convection is directional, and heat is transferred from the bottom up. During the placement, if the heating element is under the bimetal, the bimetal will heat up quickly and the action time will be short; if the heating element is next to the bimetal, the bimetal will heat slowly and the action time of the thermal relay will be long.  When the thermal relay is installed with other electrical appliances, it should be installed below the electrical appliances and away from other electrical appliances by more than 50 mm to avoid the influence of other electrical appliances. The installation direction of the thermal relay should be in accordance with the specifications of the product manual to ensure that the thermal relay's operating performance is consistent during use.3.2 Selection of Connecting WiresThe connecting wires at the output end should be selected according to the rated current of the thermal relay. Too thick or too thin will also affect the normal operation of the thermal relay. If the connecting wire is too thin, the heat generated by the connecting wire will be transferred to the bimetallic sheet, and the heat-generating component will dissipate less heat along the wire, which shortens the trip time of the thermal relay.  On the contrary, if the connecting wire is too thick, this will extend the trip time of the thermal relay. For thermal relays with a rated current of 10 A, the cross-sectional area of the connecting wire at the output end is preferably 2.5 mm2 (single-strand copper-core plastic wire), the one of 20 A is preferably 4 mm2 (single-strand copper-core plastic wire), 16 mm2 is suitable for the one of 60 A(multi-strand copper-core rubber flexible wire), and the one of150 A is preferably 35 mm2 (multi-strand copper-core rubber flexible wire).  Because the material and thickness of the wire will affect the heat conduction from the termination of the thermal element to the external heat, if the wire is too thin, the axial thermal conductivity is poor, and the thermal relay may act in advance; if the wire is too thick, the axial heat conduction is fast, and the thermal relay may lag behind. The connecting wire at the output end of the thermal relay is generally a copper core wire. If an aluminum core wire is used, the cross-sectional area of the wire should be increased by 1.8 times, and the end of the wire should be tinned. Reference table for selection of cross-section of connecting wire:Setting current of thermal relay I / ACross-sectional area of connecting wire MM20 < IN ≤81.08 < IN ≤121.512 < IN ≤202.520 < IN ≤254.025 < IN ≤326.032 < IN ≤5010.050 < IN ≤6516.065 < IN ≤8525.085 < IN ≤11535.0115 < IN ≤15050.0150 < IN ≤16070.03.3 Use EnvironmentThis mainly refers to the ambient temperature, which has a greater impact on the speed of the thermal relay. The temperature of the medium surrounding the thermal relay should be the same as the temperature of the medium surrounding the motor, otherwise, the adjusted fit will be destroyed. For example: when the motor is installed in an environment of high temperature and the thermal relay is installed in an environment of lower temperature, the action of the thermal relay will lag (or the action current is large); otherwise, its action will be advanced (or the action current is small). For thermal relays without temperature compensation, they should be used at the place where there is little difference in an ambient temperature between the thermal relay and the motor. For the thermal relay with temperature compensation, it can be used in the place where the environmental temperature of the thermal relay and the motor is different, but the influence caused by the environmental temperature change should be minimized as much as possible. The ambient temperature of the thermal relay and the protected motor should be considered. When the ambient temperature of the thermal relay is lower than the ambient temperature of the protected motor by 15℃, a thermal relay with a larger rated current rating should be used; when the ambient temperature of the thermal relay is lower than the ambient temperature of the protected motor by 15℃, a thermal relay with a smaller rated current rating should be used. In addition, the load of the motor and the adjustment range that the thermal relay may require should also be considered.3.4 Adjustment of Thermal RelayBefore putting it into use, the setting current of the thermal relay must be adjusted to ensure that the set current of the thermal relay matches the rated current of the protected motor. Before the thermal relay is used in the circuit, the specific current of the thermal relay must be adjusted according to the rated current of the motor to meet the requirements of corresponding occasions. For example, for a 10kW, 380V motor with a rated current of 19.9A, a XX20-25 thermal relay can be used. The setting current of the thermal element is 17 ～ 21 ～ 25A. First, set it at 21A according to the general situation. If it is found that it often moves in advance and the temperature rise of the motor is not high, you can change the setting current to 25A and continue to observe; if the motor temperature rises at 21A, and the thermal relay lags, you can change the setting current to 17A and observe to get the best fit. It is used to adjust the rated current when overload protection of the motor is repeatedly operated for a short time. Multiple tests and adjustments in the field can get more reliable protection. The method is: first adjust the rated current of the thermal relay to be slightly smaller than the rated current of the motor. If it is found that it often moves during operation, then gradually increase the rated value of the thermal relay until it meets the operating requirements. There should be motor protection during the special operations. Motors with forward, reverse, and frequent on-off operations should not be protected by thermal relays. The ideal method is to protect it with a temperature relay or thermistor embedded in the winding.  Ⅳ Frequently Asked Questions about Thermal Relay1. What is a thermal relay?A relay that opens or closes contacts with a bending mechanism as a result of the difference in the expansion coefficients of a bimetal, which is heated by the current. ... The thermal relay is combined with a magnetic contactor because it cannot switch the main circuit by itself. The operating point can be changed. 2. How does a thermal relay work?A thermal relay works depending upon the above-mentioned property of metals. The basic working principle of thermal relay is that, when a bimetallic strip is heated up by a heating coil carrying overcurrent of the system, it bends and makes normally open contacts. 3. What is the purpose of thermal overload relay?Thermal overload relays are economic electromechanical protection devices for the main circuit. They offer reliable protection for motors in the event of overload or phase failure. The thermal overload relay can make up a compact starting solution together with contactors. 4. What are the two types of thermal overload relays?Thermal Overload RelaysThermal overloads can be divided into two types: solder melting type, or solder pot, and bimetal strip type. Because thermal overload relays operate on the principle of heat, they are sensitive to ambient (surrounding air) temperature. 5. How do you use a thermal overload relay?Overload relays protect a motor by sensing the current going to the motor. Many of these use small heaters, often bi-metallic elements that bend when warmed by the current to the motor. When the current is too high for too long, heaters open the relay contacts carrying current to the coil of the contactor. 6. How do you test a thermal overload relay?CEP7 Overload Relay test procedures:Measure the normal motor running current (i motor).Turn off the motor and let it cool for about 10 minutes.Calculate the following ratio: i (motor) / i (overload min FLA).Set the overload to its minimum FLA and turn on the motor.Wait for the overload to trip. 7. What is thermal overload relay how it functions?The function of a thermal overload relay, used in motor starter circuits is to prevent the motor from drawing excessive current which is harmful to motor insulation. It is connected either directly to motor lines or indirectly through current transformers. 8. What is the purpose of a thermal overload?Thermal overload relays are protective devices. They are designed to cut power if the motor draws too much current for an extended period of time. To accomplish this, thermal overload relays contain a normally closed (NC) relay. 9. What does a thermal overload relay consist of?Bimetallic thermal overload relays (sometimes referred to as heater elements) are made of two metals, with different coefficients of thermal expansion, that is fastened or bonded together. A winding, wrapped around or placed near the bimetallic strip, carries current. 10. How do I know if my overload relay is bad?Unplug the start relay from the compressor and give it a shake. If you can hear rattling on the inside of the start relay, then the part is bad and will have to be replaced. If it's not rattling and appears to be in good condition, you may have a problem with the actual compressor. 

Ⅰ IntroductionA differential transformer is an electromagnetic inductive displacement sensor that converts mechanical displacement into an electrical signal. It mainly relies on the displacement of the movable iron core in the cylindrical coil and establishes a mutual induction relationship between the input coil and the output coil of the cylindrical coil, and the displacement of the movable core can be obtained by measuring the induced voltage of the output coil proportional to it.  CatalogⅠ IntroductionⅡ The Working Principle and Structure of the Differential TransformerⅢ The Type of Differential TransformerⅣ Linearity and SensitivityⅤ The Cause of the ErrorⅥ The Measurement Circuit  6.1 Differential DC output circuit  6.2 DC differential transformer circuitⅦ The Application of Differential TransformerⅧ Application Circuit Examples of Differential Transformer  8.1 MZK-4R Grinding Machine Automatic Control Device  8.2 ZD41B Short Cylindrical Roller Sorting Machine  8.3 Discussion of Differential Transformer ApplicationⅨ FAQ Characteristics of Differential Transformer(1) There are many types of linear ranges, and it is easy to select according to the use. Usually, there are about 10 types between ±2 mm and ±200 mm.(2) The structure is simple, so the vibration resistance and impact resistance are strong.(3) It does not wear, does not deteriorate, and has excellent durability.(4) The output voltage has a precise ratio to the displacement of the core, that is, the linearity is good. Generally, the full stroke deviation of this sensor is less than 1%, and it can be guaranteed to be ±0.2% to ±0.3% in high-grade products.(5) Because of the high sensitivity, a large output voltage can be obtained, and a small displacement can be detected without requiring an advanced circuit.(6) Since the output changes smoothly, high-resolution detection is possible.(7) The zero point is stable, and its use as a reference point for measurement is good for maintaining accuracy.(8) A high response speed from 500 Hz to 100 Hz can be obtained. Ⅱ The Working Principle and Structure of the Differential TransformerThe structure of the differential transformer is divided into two types: variable-gap type and solenoid type. Since the variable-gap type differential transformer has a small stroke and a complicated structure, it is rarely used at present, and the solenoid type is usually adopted. The basic components of the solenoid type differential transformer include an armature, a primary coil, a secondary coil, and a coil frame. The primary coil acts as excitation and corresponds to the primary side of the transformer. The secondary coil is formed by inverting two coils of the same structural size and parameters to form the secondary side of the transformer. There are two-section, three-section and multi-section according to the initial and secondary arrangement. The zero potential of the three-section is small, the two-section is more sensitive than the three-section, and the linear range is large. The four-section and five-section are all efforts to improve the linearity of the sensor. The working principle of the differential transformer can be explained by the principle of the transformer. The difference is: the general transformer is the closed magnetic circuit, and the differential transformer is the open magnetic circuit; the mutual inductance of the original transformer and the secondary side is constant, and the mutual inductance between the primary and secondary sides of the differential transformer changes as the armature moves. The operation of the differential transformer is based on the change of mutual inductance. The construction principle of the differential transformer is as shown in figure 1, and is composed of a cylindrical coil and a core that is completely separated from it. A typical differential transformer has three cylindrical coils, each of which is one-third of the total length, with a primary coil in the middle and a secondary coil on each side. The iron core added to the cylindrical coil is used to link the magnetic lines of force in the coil to form a magnetic circuit. Figure 1. The Construction Principle of the Differential Transformer When an alternating voltage is applied to the primary coil in the middle (ie, excitation), an electromotive force is generated due to the mutual inductance with the coils at both ends (this is the same as that of a normal transformer). Since the secondary coils are connected in series with each other in opposite polarity, the induced electromotive forces in the two secondary coils are opposite in phase, and as a result of the addition, a potential difference between the two is generated at the output end. At the center of the coil length direction, the induced voltages of the two secondary coils are equal in opposite directions, and thus the output is zero. This position is called the mechanical zero point of the differential transformer (or simply zero points). When the iron core changes position from zero points to a certain direction, the voltage of the secondary coil in the displacement direction increases, and the voltage of the other secondary coil decreases. The product design guarantees that the potential difference is proportional to the displacement of the core. When the iron core moves from zero points to the opposite direction, a proportional voltage is generated, but the phase is 180° different from the previous one. The relationship between the secondary coil voltage and the output voltage difference with respect to the core displacement is shown in figure 2. The range in which the voltage difference is proportional to the core displacement is called the linear range, and its proportionality is called linearity, which is the most important indicator of the differential transformer. Figure 2. The Core Displacement — Output Relationship of Differential Transformer Ⅲ The Type of Differential TransformerThe standard differential transformer consists of a cylindrical coil and a rod-shaped iron core. In actual use, there is also a structure with a guide and a spring. The basis for the classification of differential transformers is as follows: • According to the voltage input to the primary coil(excitation type)Commercial power supply type is suitable for practical measuring instruments of 50-60Hz, 6.3V power supply excitation;Oscillation power supply type is an excitation circuit of 1~5KHz, it is suitable for application measuring instruments requiring certain accuracy and response characteristics;DC power supply type, the semiconductor device is installed in the coil part of the differential transformer to form the excitation oscillation circuit and the secondary output detection circuit inside the coil. It is a differential transformer whose input and output are both DC, called DC-DT. • According to the displacement range of the iron core (displacement type)Small displacement type considers how to measure the small displacement below 0.5mm from the structure;General displacement type is designed for measuring the displacement about 100mm or less;The long-stroke type is designed for long stroke measurement of 120 to 400 mm.  • According to the use environment (environment type)Standard type is used in a normal environment with a temperature of -30℃ to +90℃ and a humidity of about 80%;Environmentally friendly type is the sensor for high temperature, high humidity, waterproof and radioactive environments. Features and SpecificationsWhen using a differential transformer as a position sensor, the selected specifications are as follows:◆ Excitation power supply (frequency, voltage, waveform, etc.);◆ Structure (whether guides and springs are required);◆ Linear range (it is usually ±1%, and that of high-grade products is ±0.5%～±0.2%);◆ Sensitivity (corresponding to the output of the core displacement of 1mm);◆ Impedance (input, output impedance);◆ Connection conditions (cables, sockets, input circuits, etc.);◆ Assembly method (connection method with the object to be tested, etc.);◆Environmental conditions (temperature, humidity, dust, water resistance, rust-proof conditions, etc.). Ⅳ Linearity and Sensitivity• Linearity. The linear range of the differential transformer is affected by the non-uniform magnetic field of the solenoid coil. A reasonable design guarantees the required linear range and linearity.• Sensitivity. The sensitivity of the differential transformer refers to the change of the output potential generated by the armature unit displacement. It can be expressed by mV/mm. In practice, considering the influence of the excitation voltage, it is also commonly expressed by mV/mm/V, that is, the potential change generated by the armature unit displacement divided by the excitation voltage value. The sensitivity of the differential transformer is related to the primary voltage, the number of secondary winding turns, and the frequency of the excitation voltage:• Relationship with secondary turnsThe number of secondary turns increases and the sensitivity increases, which is linear. However, the number of secondary turns cannot be increased indefinitely because the residual voltage at the zero points of the differential transformer also increases.• Primary voltageThe sensitivity is proportional to the primary voltage, but the primary voltage should not be too large. When the voltage is too large, the differential transformer coil will heat up and cause the output signal to drift. Generally, 3~8V is used.• Excitation power frequencyWhen the frequency is very low, the sensitivity increases with increasing frequency; when the frequency increases, the inductance of the coil is much higher than its resistance, the sensitivity is independent of the frequency; when the frequency exceeds a certain value (the value varies depending on the armature material), the effective resistance of the wire increases due to the skin effect of the wire at a high frequency, and the eddy current loss and hysteresis loss of the armature increase, and the output decreases. Figure 3 is the relationship between the input frequency and sensitivity of a certain magnetically permeable material, which can be used as a reference for selecting the excitation frequency. Figure 3. Relationship Between Excitation Frequency and Sensitivity of Differential Transformer Ⅴ The Cause of the ErrorThe error refers to the deviation between the actual and ideal characteristics of the sensor. Here, the system error inherent in the sensor itself and random error is mainly analyzed, and the error in the measurement method is not involved.• Influence of amplitude and frequency of excitation power supplyFluctuations in the magnitude of the excitation supply voltage cause changes in the strength of the excitation field of the coil to directly affect the output potential. The frequency fluctuations have little effect.• The effect of temperature changesChanges in ambient temperature cause changes in the magnetic permeability of the coil and the magnet, causing a change in the magnetic field of the coil to cause temperature drift. This effect is more severe when the coil quality factor is low. The use of constant current source excitation is more advantageous than the constant voltage source. Properly increasing the quality factor of the coil and using a differential bridge can reduce the effects of temperature.• Zero residual voltageWhen the armature of the differential transformer is in the neutral position, the ideal output voltage should be zero. But in fact, when using a bridge circuit, there is always a small voltage value (from a few millivolts to tens of millivolts) at zero point, which is called the zero residual voltage. Figure 4 is an enlarged output characteristic of the zero residual voltage. The dotted line is the ideal characteristic and the solid line indicates the actual characteristics. The presence of a zero residual voltage causes an insensitive zone near the zero point. Figure 4. Zero Residual Voltage of the Differential TransformerThe waveform of the zero residual voltage is very complicated and irregular. It is analyzed to include the fundamental wave in-phase component, the fundamental wave orthogonal component, and the second and third harmonics as well as the electromagnetic interference waves with small amplitude. The reasons why the zero residual voltage is generated are as follows:• Fundamental wave component: Since the winding of the two secondary windings of the differential transformer can not be completely identical in process, its equivalent circuit parameters (mutual inductance, self-inductance and loss resistance, etc.) cannot be completely equal, thus two induced potential values are not equal. The copper loss resistance of the primary coil, the iron loss and material non-uniformity of the magnetically permeable material and the presence of the inter-turn coil capacitance cause the excitation current to be out of phase with the generated magnetic flux.The above factors cause the induced potentials in the two secondary coils to be not only unequal in value but also in phase. The zero residual voltage generated by the difference in phase cannot be eliminated by adjusting the armature displacement. • High-order harmonics: The high-order harmonics are mainly caused by the nonlinearity of the magnetization curve of the magnetically permeable material. Due to the effects of hysteresis loss and magnetic saturation, the excitation current is inconsistent with the magnetic flux waveform, resulting in a non-sinusoidal wave (mainly the third harmonic flux), thereby inducing a non-sinusoidal potential in the secondary winding. The general method for eliminating zero residual voltage:— From the design and process, try to ensure the symmetry of the coil and the magnetic circuit. The structure can adopt the magnetic circuit adjustment mechanism; when selecting the working point of the magnetic circuit, it should be ensured that the magnetic field does not work in the saturation region of the magnetization curve.— Use the appropriate measurement line. The phase-sensitive detection circuit can not only identify the moving direction of the armature but also eliminate the high-order harmonic zero residual voltage of the armature in the middle position. As shown in figure 5, after using the phase-sensitive detection, the characteristic curve of the armature reverse stroke changes from 1 to 2, thereby eliminating the zero residual voltage. Figure 5. Output Characteristics After Phase-sensitive Detection— Use compensation lines. In applications of a differential transformer, there are many circuit types used to eliminate the zero residual voltage, which can be summarized as follows:▲Add series resistors to eliminate the in-phase component of the fundamental wave; generally the resistance of the series resistor is very small such as 0.5~5Ω, and is wound with constant wire.▲Add parallel resistors to eliminate the fundamental wave orthogonal component, but it has an effect on the in-phase component of the fundamental wave; the resistance of the shunt resistor is from tens to hundreds of kiloohms.▲Shunt capacitor, change phase shift, and compensate for high-order harmonics; parallel capacitor value is in the range of 100 ~ 500pf.▲Add feedback winding and feedback capacitor to compensate for fundamental wave and high-order harmonics.In fact, these values are determined experimentally; based on the working principle of the differential transformer and the cause of the zero residual voltage, the above methods can be modified and combined, and it is also possible to design a new compensation circuit. Figure 6 shows some line schematics for compensating for zero residual voltage for reference. Figure 6. Zero Residual Voltage Compensation Circuit of Differential Transformer Ⅵ The Measurement Circuit6.1 Differential DC output circuitThe output voltage of the differential transformer is an AC signal whose amplitude is proportional to the armature displacement. If the output value is measured with an AC voltmeter, it can only reflect the magnitude of the armature displacement and cannot reflect the direction of the displacement. Secondly, there is a certain zero residual voltage in the AC voltage output. Even with various compensation methods, it can only be reduced and cannot be completely eliminated. Therefore, the DC output circuit is commonly used in engineering practice, which can reflect the displacement direction of the armature and compensate for the zero residual voltage. The DC output circuit has two forms: one is a differential phase-sensitive detector circuit, and the other is a differential rectifier circuit.The differential rectifier circuit is shown in Figure 7. This circuit is relatively simple. It does not need to compare the voltage windings. It does not need to consider the influences if the phase adjustment and the zero residual voltage. The influence on the sensing and distributed capacitance can also be ignored. In addition, since the rectifying portion is on the differential output side, the two DC conveying lines are convenient to connect, and can be transported at a long distance, and are widely used. Figure 7. Differential Rectifier Circuita) full-wave current output         b) half-wave current outputc) full-wave voltage output         d) half-wave voltage outputDifferential phase sensitive detector circuits come in many forms. Figure 8 shows two examples, one is a full wave circuit and the other is a half-wave circuit. The phase sensitive detector circuit requires that the comparison voltage and the secondary transformer output voltage of the differential transformer have the same frequency and the same phase or opposite phase.  To ensure this, a phase-shifting circuit is usually connected to the circuit. In addition, it is required that the comparison voltage amplitude should be as large as possible (because the comparison voltage acts as a switch in the detector circuit, and if it is less than the signal voltage, the switch cannot be turned on), generally it should be 3 to 5 times the signal voltage. In the figure, Rw is the bridge zero potentiometer. For the case of measuring small displacements, since the output signal is small, the input amplifier is also connected to the circuit. Figure 8. Differential Phase-sensitive Detection Circuita) full-wave detection         b) half-wave detection6.2 DC differential transformer circuitThe working principle of the DC differential transformer is exactly the same as that of the ordinary differential transformer described above. The only difference is that the power supply used in the instrument is a DC power supply (dry battery, battery, etc.). The schematic diagram of the DC differential transformer is shown in figure 9. It consists of a DC power supply, a multivibrator, a differential rectifier circuit, a filter and so on. Figure 9. Schematic Diagram of DC Differential Transformer CircuitThe multivibrator provides a high frequency excitation power supply for the differential transformer, which can be a square wave, a triangular wave or a sine wave. DC differential transformers are commonly used in the following applications:◆ The measuring point is far from the control room (more than 100m);◆ Simultaneous use of multiple differential transformers and requires no interference with each other and with other equipment;◆ Where explosion protection is required;◆ Requires easy to carry, such as working in the field. Ⅶ The Application of Differential TransformerDisplacement measurement is the most important use of differential transformers. Any physical quantity that can be transformed into a displacement can be measured with a differential transformer. It is noted that the differential transformer measurement is generally contact type. In some cases, it will affect the state of the measured object (such as vibration), which is the so-called “load effect”. In this case, other types of sensors must be used such as eddy current sensors, etc.◆ It can be used as the main component of many precision measuring instruments, such as making high-precision inductance comparator with corresponding measuring devices, which can perform various precise measurements on parts: length, inner diameter, outer diameter, non-parallelism, non-flatness, non-perpendicularity, vibration, eccentricity, and ellipticity.◆ As the main measuring part of the bearing rolling element automatic sorting machine, it can sort large and small steel balls, large and small cylinders, large and small round vertebrae, needle roller and so on.◆ It is used to measure the expansion, elongation, strain, movement, etc. of various parts. With a variety of sensors, its displacement measurement range can be from ±3μm to over 1000mm.◆ Vibration and acceleration measurements. An accelerometer for measuring vibration can be constructed by using a differential transformer and a cantilever beam elastic support.◆ Pressure measurement. The differential transformer and the elastic sensitive component (diaphragm, bellows, spring tube, etc.) can be combined to form a pressure sensor of the open-loop system and a force-balanced pressure gauge of the closed-loop system. Due to the excellent characteristic of differential transformer as the displacement sensor, it has been applied in almost all industrial fields and several specific examples are described below.• Steel industry: blast furnace top-level detection, continuous casting roll gap, sand type vibration, convexity detection, position detection of sliding water nozzles such as ladle and tundish.• Heavy motor industry: the main valve of the steam turbine, the valve lift detection of the bypass valve, and the posture monitoring of the elevator.• Construction machinery industry: Measuring head for numerical control machine tool simulation test.• Ceramic industry: thermal expansion testing of refractory materials, shape detection of templating glass.• Ship and vehicle industry: fuel classification position detection of diesel engine, dynamic characteristic detection of fuel injection valve of an automobile engine, and eccentricity detection of tire and wheel.• Weighing machine industry: a device that automatically measures the weight of the bag, and a weighting machine for the asphalt carrying device.• Measuring instrument, testing machine industry: used for traction test, creep test of metal materials and plastics, signal conversion part of flow meter and liquid level meter, a mechanical test of civil building components.• General industry: spacer separators for assembling bearings, motion deviation detection during stamping, and measurement of workpiece size and shape deviation. Ⅷ Application Circuit Examples of Differential Transformer8.1 MZK-4R Grinding Machine Automatic Control DeviceThis device is used on automatic or semi-automatic grinding machines. During the grinding process of the workpiece, the control device can accurately output 4 signals according to the amount of the pre-regulation to control the introduction of the grinding head, rough grinding, fine grinding, light grinding and exit, etc., thus realizing the automatic measurement and control of the grinding process. • Working process of the grinding machineWhen the workpiece is loaded, the measuring device first enters the workpiece for measurement. If the workpiece size meets the pre-adjusted result, the control device sends a “starting” signal, the grinding head enters the workpiece and moves forward quickly to the machining direction, and coarse grinding starts. Taking the internal grinding as an example, as the workpiece size of the grinding wheel becomes smaller, the output signal of the measuring head also becomes smaller.  When the preset position is reached, the trigger sequentially sends three signals, that is, the “rough grinding end” signal, indicating that the rough grinding is finished, so that the moving speed of the grinding wheel is reduced, and the fine grinding starts; when the fine grinding is finished, the “finishing end” signal is issued, so that the grinding wheel stops moving and the light grinding starts; when the preset size is reached, the “light grinding end” signal is issued to make the grinding wheel and the detection device exit quickly. • Working principle of measuring head (sensor)The measuring head adopts a differential transformer type displacement sensor, and its structure is as shown in figure 10(a). The iron core moves to the right, so that the induced potential of the winding A decreases, and the induced potential of the winding B increases (and vice versa). The two windings and the resistors R1 and R2 in the measuring device form a bridge to realize differential output, as shown in figure 10(b). Figure 10. Schematic Diagram of the Differential TransformerThe primary coil is excited by a square wave generator with a square wave frequency of 3 kHz and an effective voltage value of 3.5V. Along with the change of the displacement of the core, a corresponding voltage variable is generated between the boom of the potentiometer Rw and the secondary common tap (ground) of the measuring head. The displacement-voltage characteristic curve in figure 11 is obtained after the voltage variable is amplified and phase-sensitive rectified. Figure 11. Output Characteristic Curve of Differential TransformerIn the figure, the S-T segment is the full linear range, wherein the H-E segment (high precision) is the ×1 gear indication range, and the K-C segment (low precision) is the ×10 gear indication range. The“start”signal 0 is sent in the D-A segment, the“rough grinding end” signal 1 is sent in the G-B segment, and the“fine grinding end” signal is 2 sent in the 0-F segment. The“light grinding end”signal 3 is sent at point 0. • Principle of the circuit①The circuit block diagram shown in figure 12. Figure 12. Circuit Block Diagram of Control Device②Explanation of the circuit principleThe device consists of six parts: ▲Input bridge, the two arms are composed of two secondary windings of the measuring head, the other two arms are composed of R84 and R85, the potentiometer VR1 is used for the electrical zero points coarse adjustment, the VR2 is used for the zero points fine adjustment, and the R86 is used to limit the zero point adjustment range.In order to obtain the reference voltage for amplifier calibration, a voltage is obtained by the square wave generator, and another bridge is formed by transformers TR4, R88, and R89, and VR4 is used to adjust the reference voltage. ▲The amplifier amplifies the weak signal obtained in the input circuit to have sufficient amplitude to complete the measurement and control. T15, T17, and T18 form a voltage amplifier with gains of about 10, 20, and 20 dB, respectively. T16 is a buffer stage. T19 and T20 form a push-pull power amplifier stage, and the voltage gain of the amplifier is about 60 to 70 dB. In order to achieve higher stability and linearity, deeper negative feedback is added to each stage. The negative feedback of first stage is adjustable, and the total gain of the amplifier is adjusted by VR3. ▲Phase-sensitive rectification and indicating circuit are used to complete the rectification and identify the phase of the input signal. The half-wave rectification circuit is composed of D15 and D16, and the blocking voltage is 13V, which is provided by the square wave generator.The rectified DC ramp signal is used as an input to the trigger on the one hand and as a panel indicator on the other. The μ meter is a microampere meter with a full-scale of 150μA, and full-scale indications of 50μ and 500μ are obtained with shunt resistors R90 and R91. D33 is used as a voltage clamp to protect the meter head. ▲Square wave generator, which is used to generate the excitation voltage required by the measuring head and the blocking voltage required for phase-sensitive rectification. The high rectangular coefficient multivibrator circuit is composed of T21 and T22, which is easy to start, high in frequency and amplitude stability, and its oscillation frequency is 3 to 3.5 kHz. ▲Trigger, according to the comparison of the output voltage of the phase-sensitive rectification and the pre-adjustment voltage, four different control signal outputs are sequentially generated. The circuit adopts a trigger with an emitter coupled by a Zener tube, which has a small temperature drift and convenient backlash adjustment. Among them, VR5, VR6, VR7, and VR8 are used as the adjustment potentiometers for the four signals of “0”, “1”, “2”, and “3” on the panel. ▲Power:-24V, used for power relay after rectification and filtering;-15V, generated by the series regulator circuit and is used as the collector voltage of each transistor and the trigger pre-call;+6V, generated by the shunt regulator circuit and is supplied for the bias and pre-call of the trigger. • Main technical indicators✿ Instrument indexing and error:High precision (G) 1μ/division; full scale -10～+50μ; error ≤1.5μLow precision (D) 20μ/division; full scale -100～+500μ; error ≤30μ✿ Adjustable range of control signal :Signal "0", 350～500μ;Signal "1", 30～100μ;Signal "2", 0 ~ 30μ;Signal "3", -10 ~ +10μ✿ Electrical zero adjustable range:Not less than 100μ, and ±5μ fine adjustment✿ Repeat error:No more than 1μ✿ (Grid) voltage adjustment error:No more than 3μ✿ Instability:Time drift is no more than 10μ/4 hours; temperature drift is no more than 10μ/ 10℃ 8.2 ZD41B Short Cylindrical Roller Sorting MachineThis machine is composed of high-precision micrometer (differential transformer), combined with transistor circuit to form measurement and logic control device to complete the task of automatically sorting short cylindrical bearing rollers. • Main technical indicators◆ Measurement range:length is no more than 15mm5 to 15 mm in diameter◆ Accuracy:1μ, 2μ, 3μIf the magnification and radial grouping potentiometer are re-tuned, any grouping in the range of 0.5 to 5μ can be obtained.◆ Number of groups:10 groups.◆ Speed:28/min to 65/min,can be adjusted arbitrarily • Working principleThe measurement and classification of the radial dimensions of the roller are automated. The roller to be tested is manually placed in a disc-shaped hopper, passed through the vibrating roller to the feeding position along the pipe, and then pushed into the measuring portion by the reciprocating push rod for radial measurement. When different sizes of rollers enter the measurement site for measurement, the differential transformer guide core is displaced in the coil, so that the differential transformer outputs an alternating current signal proportional to the change in the size of the roller, and tiny electrical signal is amplified, rectified, and then amplified by the DC amplifier, so that the corresponding trigger drives the relay and the electromagnet to open the storage valve of the sorting group, so that the measured rollers of different diameters are placed in different sorting bins for automated measurement and sorting. Here we mainly introduce the radial dimension measurement part, namely the differential transformer and its secondary circuit. The measuring part of the roller consists of a differential transformer, a 4KHz oscillator, an attenuator, a low-frequency AC amplifier, a phase-sensitive rectification, a DC amplifier, a regulated power supply, etc. ① Micrometer (differential transformer): The differential transformer is used to convert the diameter of the roller into a change in the amount of electricity. The primary coil is excited by a rectangular wave with a frequency of 4 kHz and an amplitude of 2 to 3 volts. Thus, the voltages of u2 and u3 are induced in the secondary coil. The different names of the secondary coils are connected as a common point ground, and the other ends serve as a differential output and form a bridge balance loop with the resistors R1, R2 and the potentiometer VR.  When the iron core is at the center position of the two secondary coils, since the magnetic resistance of the two coils is equal, the bridge is in a balanced state, and the differential transformer output E2=0 (u2=u3). In a static state, due to the self-weight of the iron core and the guide rod, the iron core is located at the lowermost end of the secondary coil, thus outputting a negative polarity voltage; when the roller is measured, the guide rod is displaced upward, and the iron core is also displaced upward in the differential coil. The output voltage varies with the displacement. When the displacement exceeds the center position, the differential transformer outputs a positive voltage. ② Oscillator: A high-frequency triode is used as a capacitive voltage divider oscillator with an oscillation frequency of 4KHz. This circuit feature avoids the difficulty of inductive oscillator winding. The intermediate transformer is used to couple the output, and then through the first-stage voltage amplification, the two pairs of Zener diodes are used to limit the clipping to form a rectangular wave with constant amplitude (2~3V), one way is for the primary excitation of the differential transformer and the other is for the phase-sensitive rectification comparison voltage. ③ AC amplifier: three-stage amplification circuit and transformer-coupled output. In order to keep the amplifier gain stable, 20dB negative feedback is introduced between the first and second stage, and the total gain is 75~80dB. ④ Phase-sensitive rectifier circuit: diode half-wave phase-sensitive rectification is used, the comparison voltage amplitude is high, and both diodes are turned on in the positive half cycle. The signal voltage is small, the positive voltage is output in the same phase with the comparison voltage, and the negative voltage is output in the opposite phase with the comparison voltage. ⑤ DC differential amplifier: The DC voltage output from the phase-sensitive rectifier circuit is further amplified and the polarity is converted. When inputting ±50mV, the differential output is 4~12V. 8.3 Discussion of Differential Transformer Application(1) The above example uses the two directions of the differential transformer and is determined for the special purpose of roller sorting. When measuring with a roller of nominal size, the differential transformer core is just adjusted to the center position, the positive tolerance roller produces a positive displacement, and the positive voltage is output; the negative tolerance roller produces a negative displacement and outputs a negative voltage. This makes full use of the linear range of the differential transformer. For different applications, especially for small-range, high-precision measurements, there is no need to distinguish the direction of the displacement. It is also possible to use only the displacement of the differential transformer in one direction, and the corresponding circuit can be simplified. (2) This product was a product of the 1970s, so a transistor discrete component circuit was used. Today's electronic technology and the component levels are no longer the same. AC amplifiers and DC amplifiers can be used with operational amplifiers, and performance is much better than discrete component circuits. The basic principles of the circuit and the various functional parts are still applicable and can be designed accordingly. (3) Nowadays, the application of single-chip microcomputers can completely replace the various logic circuits in the past. In the case of a single-chip microcomputer, the entire circuit design may vary greatly. For example, the oscillation source can be digitized (crystal oscillator frequency division may be directly generated by a single-chip microcomputer), and the measurement result is digitized (via A/D conversion), and a large number of analog comparators, triggers can be replaced by program judgment methods.  Furthermore, with the precise timing and synchronization function of the single-chip microcomputer, A/D conversion can be directly performed on the AC signal sampling, and the phase-sensitive rectifier circuit can be omitted. After the measurement results are digitized, data transmission can be used instead of analog transmission, thus precision will not be lost, interference will not exist and transmission distance will be long. Ⅸ FAQ1. Why does LVDT use high voltage?It is a type of electrical transformer used for measuring linear displacement.The linear variable differential transformer has three solenoidal coils placed end-to-end around a tube. The center coil is the primary, and the two outer coils are the top and bottom secondaries. A cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides along the axis of the tube. An AC current drives the primary and causes a voltage to be induced in each secondary proportional to the length of the core linking to the secondary.Cutaway view of an LVDT. Current is driven through the primary coil at A, causing an induction current to be generated through the secondary coils at B. When the core is displaced toward the top, the voltage in the top secondary coil increases as the voltage in the bottom decreases. The resulting output voltage increases from zero. This voltage is in phase with the primary voltage. When the core moves in the other direction, the output voltage also increases from zero, but its phase is opposite to that of the primary. The phase of the output voltage determines the direction of the displacement (up or down) and amplitude indicates the amount of displacement.  2. What are the advantages of using an LVDT?• Very reliable: Long sensor lifespan due to near frictionless operation of most models.• Very high resolution: Because of the near-frictionless movement they provide virtually infinite resolution. Even the smallest changes can be detected.• Damage resistant: In some models, both ends of the tube are open, preventing sensor damage if the test article pushes the rod farther than expected (except for collision with the tube itself).• Null point stability: The zero or null point of the sensor is extremely repeatable due to the construction of the sensor itself.• Wide range of operating temperatures: There are LVDT models available that can withstand cryogenic temperatures (-200℃/ -328℉) as well as high temperatures (650℃/ 1200℉)• Low hysteresis/ high positional accuracy and repeatability• Absolute reading output device: As opposed to an incremental output device, the reading from an LVDT will be the same before and after its power is cycled (assuming that the object under test did not move).  3.What are the disadvantages of using an LVDT?• Limited measurement distance: Even the largest LVDTs are limited to less than 1m (~27'') measurement ranges.• Can be affected by magnetic fields (models with shielding are common as a result).• AC models require a precise AC excitation from an LVDT signal conditioner.• DC LVDT models have an inferior shock, vibration and temperature specifications compared to AC LVDT models. 4. How do I interface an LVDT output with PLC?The output is voltage so you will need an analog input card which can take in a voltage input and then inside the PLC you will scale what that voltage corresponds to. For eg 10 V could mean 10 mm or 10 degrees etc. If the output is current them you would need an analog IP card which accepts current. Most common current used is 4–20 mA. 5. What are the applications of the Bourdon tube and the LVDT method for pressure measurement?A bourdon tube is a curved, hollow, closed end tube which can be pressurized. The pressure will attempt to straighten out the tube as it is increased. The amount of movement is typically very small but can be mechanically amplified. The translation of the end of the tube can be a linear indication of the pressure applied to the other end.Pressure gauges have used this technique for over a century and a half to indicate pressure manually on a dial gauge with a linkage that moves a dial pointer.To make electronic readouts of pressure to remote dials or to computers, a linear movement to electrical voltage is needed. An LVDT satisfies this need. An LVDT is a variable transformer consisting of a movable magnetic core sliding inside a tube with a primary and secondary winding. As the core is displaced the coupling between windings is varied linearly. If a small AC voltage is applied to the primary then the amplitude of the secondary output can be measuered in amplitude to indicate proportional to the pressure.So this makes a hybrid sensor or transducer, pressure to displacement connected to a displacement to variable voltage resulting in a pressure to variable voltage device.Technically this is an older way of converting pressure to volts… and is subject to hysteresis or mechanical backlash. Most modern P-V transducers use strain gauge bridge followed by an instrumentation amplifier to have fewer moving parts and less hysteresis. 6. Discuss various applications where LVDT’s can be used?They can be used in any application where a highly accurate measurement of linear displacement or position is needed. This includes precision gaging systems for measurement and metrology, feedback transducers for precision servomechanisms, torque, force and moment transducers, materials testing equipment (tensile testers, rheometers, fatigue testers, etc.). 7.What is the accuracy of a LVDT and an inductance transducer in a displacement measurement?Accuracy for both devices depends on the way they are designed, made and used, and the materials from which they are made. As well as calibration.However, neither on their own give “readings”. They need conditioning and interface circuitry. (I do recognise that the “inductance transducer” is a two-word item, and that the second word - transducer - implys that at least some form of signal conditioning exists therein.)That cicuitry is at least as important as the device itself.To give typical values for accuracy, is difficult without more specifics, though it is usual to be able to achieve several significant figures of accuracy out of each. 8. What is LVDT in measurement?A Linear Variable Differential Transducer is a sensor based on the idea of transformers. As its name shows it's a Linear sensor used in measuring displacements.  It has an iron core that moves up and down in the gap separating the primary and secondary coils. So the coils are not physically connected. The secondary coils are connected in opposition such that the output voltage is the difference between the voltages induced in the first and second secondary coils. The components whose Displacement is required to be measured should be connected to the core, so the input to the sensor is the displacement، the output would be the differential voltage output and after some manipulation using the sensitivity and sensor resolution, the displacement can be obtained. 9. How does a DC LVDT work?An oscillator/demodulator circuit built into the displacement transducer supplies the excitation and converts the return signal to a dc voltage. ... As the transducer contains internal signal conditioning electronics, there is no need for external signal conditioning. 10. Is LVDT an active transducer?The active transducer is also called a self-generating type transducer. ... Example of an active transducer is the bourdon tube. An example of a passive transducer is LVDT (linear variable differential transformer). It generates electric current or voltage directly in response to environmental stimulation.

CatalogⅠ IntroductionⅡ What is an infrared sensor?Ⅲ How does the infrared sensor work?Ⅳ The basic law of infrared radiationⅤ The working principle of the infrared sensorⅥ Types of infrared sensors  6.1 Thermal sensor  6.2 Photon sensorⅦ Application and Prospect of the infrared sensorⅧ SummaryⅨ FAQⅠ IntroductionAny object in the universe can produce infrared radiation as long as its temperature exceeds zero. In fact, like visible light, its radiation can be refracted and reflected, which leads to infrared technology. Infrared detector is widely used in military and civil fields because of its unique advantages. In the military, infrared detection is used for guidance, fire control tracking, alert, target detection, weapon thermal sight, ship navigation, etc.; in the civil field, it is widely used in industrial equipment monitoring, safety monitoring, disaster relief, remote sensing, traffic management, medical diagnosis technology, etc.With the development of science and technology, the proportion of automatic control and automatic detection in people's daily life and industrial control is more and more heavy, which makes people's life more and more comfortable and the efficiency of industrial production higher and higher. The sensor is an important component of the automatic control, and an important component of the information acquisition system.  Through the sensor, the feeling or response is measured and converted into a signal suitable for transmission or detection (generally electrical signal), and then the computer or circuit equipment is used to process the signal from the sensor to achieve the function of automatic control. Because the response time of the sensor is generally short, the real-time control of industrial production can be carried out through the computer system. The infrared sensor is a common type of sensor.  Because an infrared sensor is a kind of sensor to detect infrared radiation, and any object in nature will radiate infrared energy as long as its stability is higher than absolute zero, so infrared sensor is called a very practical type of sensor. Many practical sensor modules can be designed by using infrared sensors, such as infrared temperature measurement Instruments, infrared imagers, infrared human detection alarms, automatic door control systems, etc.Ⅱ What is an infrared sensor?Infrared sensor is a sensor that uses the physical properties of infrared to measure. Infrared light, also known as infrared light, has the properties of reflection, refraction, scattering, interference, absorption, etc. It is a kind of invisible light, its spectrum is located outside red in visible light, so it is called infrared. In engineering, the position (band) of infrared rays in the electromagnetic spectrum is divided into four bands: near infrared band, mid infrared band, far infrared  band and extremely far infrared band. Any substance can radiate infrared ray, as long as it has a certain temperature (higher than absolute zero).Ⅲ How does the infrared sensor work?First of all, let's learn about infrared light. Infrared light is a part of the solar spectrum. The biggest characteristic of infrared light is its photothermal effect and radiant heat. It is the largest photothermal effect area in the spectrum. An invisible light, like all electromagnetic waves, having the properties of reflection, refraction, scattering, interference, absorption, etc. The propagation speed of infrared light in a vacuum is 300000 km / s. The transmission of infrared light in the medium will produce attenuation, and the transmission attenuation in the metal is very large, but the infrared radiation can pass through most semiconductors and some plastics, and most liquids absorb the infrared radiation very much.Different gases have different absorption levels, and the atmosphere has different absorption bands for different wavelengths of infrared light. The results show that the infrared light with the wavelength of 1-5 μ m and 8-14 μ m has a relatively large "transparency". That is to say, these wavelengths of infrared light can penetrate the atmosphere well. Any object in nature, as long as its temperature is above absolute zero, can produce infrared radiation. The photothermal effect of infrared light is different for different objects, and the intensity of heat energy is also different.  For example, blackbody (an object that can fully absorb the infrared radiation projected on its surface), mirror body(an object that can fully reflect the infrared radiation), transparent body(an object that can fully penetrate the infrared radiation) and gray body (an object that can partially reflect or absorb the infrared radiation) will produce different photothermal effects. Strictly speaking, there are no blackbody, mirror body and transparent body in nature, and most of the objects belong to gray body. These characteristics are the important theoretical basis for the application of infrared radiation technology in military and scientific research projects such as satellite remote sensing and infrared tracking. The physical essence of infrared radiation is thermal radiation. The higher the temperature of an object, the more infrared it radiates, the stronger the energy of the infrared radiation. It is found that the thermal effect of various monochromatic light in the solar spectrum increases gradually from violet light to red light, and the largest thermal effect occurs in the frequency range of infrared radiation, so people call infrared radiation as thermal radiation or thermal ray.Ⅳ The basic law of infrared radiation① Kirchhoff's Law: at a certain temperature, the ratio of the radiation flux W per unit area of the ground object to the absorption rate is a constant for any object, and is equal to the radiation flux w of a blackbody of the same area at that temperature. At a given temperature, the emissivity of the object = the absorptivity (the same band); the higher the absorptivity, the higher the emissivity. The thermal radiation intensity of the ground object is directly proportional to the fourth power of the temperature, so the small temperature difference of the ground object will cause the obvious change of the infrared radiation energy. This feature constitutes the theoretical basis of infrared remote sensing.② Boltzmann's Law: that is, the total radiation flux of blackbody increases rapidly with the increase of temperature, which is proportional to the fourth power of temperature. Therefore, a small change in temperature will cause a great change in radiation flux density. It is the theoretical basis of measuring temperature with an infrared device. ③ Wien displacement law: with the increase of temperature, the peak wavelength corresponding to the maximum radiation value moves to the short wave direction.Ⅴ The working principle of the infrared sensorThe working principle of the infrared sensor is not complicated. The entities of each part of a typical sensor system are as follows:• Target to be tested: the infrared system can be set according to the infrared radiation characteristics of the target to be tested.• Atmospheric attenuation: when the infrared radiation of the target to be measured passes through the earth's atmosphere, the infrared radiation from the infrared source will be attenuated due to the scattering and absorption of gas molecules, various gases and various colloidal particles.• Optical receiver: it receives part of the infrared radiation of the target and transmits it to the infrared sensor. Equivalent to radar antenna, usually objective lens.• Radiation modulator: it can modulate the changed radiation light from the target to be tested, provide the target orientation information, and filter out large-area interference signals. Also known as modulation disk and chopper, it has a variety of structures.• Infrared detector: This is the core of the infrared system. It is a sensor that uses the physical effect of the interaction between infrared radiation and matter to detect infrared radiation. In most cases, it uses the electrical effect of the interaction. These detectors can be divided into two types: photon detector and heat-sensitive detector.• Detector Cooler: because some detectors must work at low temperatures, the corresponding system must have refrigeration equipment. After refrigeration, the equipment can shorten the response time and improve the detection sensitivity.• Signal processing system: amplify and filter the detected signals, and extract information from these signals. Then, this kind of information is transformed into the required format, and finally transmitted to the control equipment or display.• Display device: This is the terminal device of infrared device. Commonly used displays include oscilloscopes, picture tubes, infrared sensitive materials, indicating instruments and recorders.According to the above process, the infrared system can complete the corresponding physical quantity measurement. The core of infrared system is infrared detector. According to the different detection mechanism, it can be divided into two categories: thermal detector and photon detector. The thermal detector absorbs all the radiant energy of all kinds of incident wavelengths. It is an infrared sensor with no choice for infrared light wave. The common photon effects of photon detectors are external photoelectric effect, internal photoelectric effect (photovoltaic effect, photoconductive effect) and photoelectromagnetic effect. The thermal detector uses the radiation heat effect to make the temperature rise after the detector receives the radiation energy, and then the temperature-dependent performance of the detector changes.  Radiation can be detected by detecting a change in one of the properties. In most cases, radiation is detected by thermoelectric changes. When the element receives radiation and causes the physical change of non electric quantity, the corresponding electric quantity change can be measured after appropriate transformation. The response time of thermal detector to infrared radiation is much longer than that of photodetector. The response time of the former is generally more than MS, while that of the latter is only ns. Thermal detectors do not need to be cooled, most photon detectors need to be cooled.Ⅵ Types of infrared sensorsCommon infrared sensors can be divided into thermal sensors and photon sensors.6.1 Thermal sensorThe thermal sensor uses the incident infrared radiation to change the temperature of the sensor, and then make the relevant physical parameters change accordingly. The infrared radiation absorbed by the infrared sensor is determined by measuring the changes of the relevant physical parameters. The main advantage of the thermal detector is that it has a wide band, can work at room temperature and is easy to use. However, the thermal sensor has a long response time and low sensitivity, which is generally used in low frequency modulation.The main types of thermal sensors are thermal sensor type, thermocouple type, gaolai pneumatic type and heat release electric type. ① Thermistor sensorThe thermistor is made of manganese, nickel and cobalt oxides. The thermistor is usually made into thin sheet. When the infrared radiation irradiates the thermistor, its temperature increases and the resistance decreases. By measuring the change of the thermistor value, we can know the intensity of the incident infrared radiation, thus we can judge the temperature of the object generating the infrared radiation.② Thermocouple sensorThermocouples are made of two materials with a great difference in thermal power. When infrared radiation reaches the contact of the closed circuit composed of these two metal materials, the contact temperature increases. The other contact which is not irradiated by infrared radiation is at a lower temperature, at this time, the temperature difference current will be generated in the closed circuit. At the same time, thermoelectric potential is generated in the loop, and the magnitude of thermoelectric potential reflects the strength of infrared radiation absorbed by the contact. The infrared sensor made of thermoelectric potential is called thermocouple infrared sensor. Because of its large time constant, long corresponding time and poor dynamic characteristics, the modulation frequency should be limited below 10Hz.③ Lai pneumatic sensorAfter absorbing the infrared radiation, the temperature and volume of the gas are increased to reflect the intensity of the infrared radiation. It has an air chamber connected to a flexible sheet by a small pipe.  One side of the back pipe of the sheet is a reflector. The front of the gas chamber is attached with an absorption mode, which is a thin film with low heat capacity. The infrared radiation is incident on the absorption mode through the window, and the absorption mode transmits the absorbed heat energy to the gas, which makes the gas temperature and pressure increase, so that the flexible mirror moves.  On the other side of the chamber, a beam of visible light is focused on the flexible mirror through the grating light bar, and the grating image reflected by the flexible mirror is projected onto the photoelectric cell through the grating light bar. When the flexible mirror moves due to the change of pressure, the relative displacement between the grating image and the grating light bar will change the amount of light falling on the photocell, and the output signal of the photocell will also change, which reflects the intensity of the in-out infrared radiation.  This sensor is characterized by high sensitivity and stable performance. But the response time is long, the structure is complex and the intensity is poor, so it is only suitable for laboratory use. ④ Pyroelectric sensorPyroelectric sensor is a kind of thermal crystal or ferroelectric with polarization phenomenon. The polarization intensity (charge per unit area) of ferroelectrics is related to temperature. When infrared radiation irradiates the surface of the polarized ferroelectric sheet, the temperature of the sheet increases, the polarization intensity decreases, and the surface charge decreases, which is equivalent to releasing part of the charge, so it is called pyroelectric sensor.  If the load resistor is connected to a ferroelectric sheet, an electrical signal output is generated on the load resistor. The size of the output signal depends on the speed of the temperature change of the chip, which reflects the intensity of the incident infrared radiation. It can be seen that the voltage response rate of the pyroelectric infrared sensor is directly proportional to the change rate of incident radiation. When the constant infrared radiation irradiates on the pyroelectric sensor, the sensor has no electrical signal output.  Only when the temperature of ferroelectrics is in the process of change can the electrical signal be output. Therefore, it is necessary to modulate the infrared radiation (or chopping light) so that the constant radiation becomes the alternating radiation, which constantly causes the temperature change of the sensor, so as to generate pyroelectric and output the alternating signal.6.2 Photon sensorThe photon sensor uses some semiconductor materials to produce photon effect under the illumination of incident light, which changes the electrical properties of materials. By measuring the change of electrical properties, we can know the intensity of infrared radiation. The infrared sensors made by photon effect are called photon sensors. The main characteristics of photon sensor are high sensitivity, fast response speed and high response frequency, but generally it must work at low temperature and the detection band is narrow. According to the working principle of photon sensor, it can be divided into internal photoelectric sensor and external photoelectric sensor. The latter is divided into photoconductive sensor, photovoltaic sensor and magnetoelectric sensor. ① External photoelectric sensorWhen the light radiates on the surface of some materials, if the photon energy of the incident light is large enough, the electrons of the materials can escape from the surface. This phenomenon is called external photoelectric effect or photoelectron emission effect. Photodiode, photomultiplier tube and so on belong to this type of electronic sensor. Its response speed is relatively fast, generally only a few nanoseconds. However, electron escape requires a large amount of photon energy, which is only suitable for near-infrared radiation or visible light. ② Photoconductive sensorWhen infrared radiation irradiates on the surface of some semiconductor materials, some electrons and holes in the semiconductor materials can change from the original non-conductive bound state to the conductive free state, which increases the conductivity of the semiconductor. This phenomenon is called the photoconductivity phenomenon. The sensors made of photoconductive phenomena are called photoconductive sensors.  For example, lead sulfide, lead selenide, indium antimonide, mercury telluride and other materials can be used to make photoconductive sensors. When using photoconductive sensor, we need to cool and add a certain bias voltage, otherwise, the response rate will be reduced, the noise will be large, the response band will be narrow, and the infrared sensor will be damaged.③ Photovoltaic sensorWhen the infrared radiation irradiates on the PN junction of some semiconductor materials, the free electrons move to the N-region under the action of the electric field in the junction. If the PN junction is open, an additional potential will be generated at both ends of the PN junction, which is called the photogenerated electromotive force.  The sensors or PN junction sensors based on this effect are usually made of materials such as indium arsenide, indium antimonide, mercury telluride, lead-tin telluride, etc. ④ Magnetoelectric sensorWhen the infrared radiation irradiates on the surface of some semiconductor materials, some electrons and holes in the semiconductor materials will diffuse to the interior. If the diffusion is affected by a strong magnetic field, the electrons and holes will each deviate to one side, resulting in an open circuit voltage. This phenomenon is called the optical magnetoelectric effect. The infrared sensor made of this effect is called magnetoelectric sensor. The response band is about 7 μ m, the time constant is small, the response speed is fast, there is no bias, the internal resistance is very low, the noise is small, and it has good stability and reliability. However, its sensitivity is low and it is difficult to make low noise preamplifier, which affects its use.Ⅶ Application and Prospect of the infrared sensor (1) The application of infrared sensor is mainly reflected in the following aspects:    1. Infrared radiometer: used for radiation and spectral radiation measurement.    2. Search and tracking system: used to search and track the infrared target, determine its spatial position and track its motion.    3. Thermal imaging system: it can form the infrared radiation distribution image of the whole target.    4. Infrared ranging system: to measure the distance between objects. (it uses the non-proliferation principle of infrared propagation, because the refractive index of infrared is very small when it passes through other substances, so infrared will be considered in long-distance distance distance rangefinders.)    5. Communication system: infrared communication as a way of wireless communication.    6. Hybrid system: refers to two or more combinations of the above systems.Infrared sensor applications can be used for non-contact temperature measurement, gas composition analysis, nondestructive testing, thermal image detection, infrared remote sensing and military target reconnaissance, search, tracking and communication. With the development of modern science and technology, the application prospect of infrared sensor will be more broad. In the future, the performance and sensitivity of infrared sensor will be improved greatly.  (2) Development trend    1. Intellectualization: at present, the infrared sensor is mainly used in combination with peripheral equipment. The built-in microprocessor of the intelligent sensor can realize the two-way communication between the sensor and the control unit. It has the advantages of miniaturization, digital communication, simple maintenance, etc., and it can work independently as a module.     2. Miniaturization: an inevitable trend of sensor miniaturization. Now in application, because of the volume problem of infrared sensor, its use degree is far worse than that of thermoelectric corner. Therefore, whether the infrared sensor is miniaturized and portable or not can't be ignored.     3. High sensitivity and high performance: in medicine, the infrared sensor has been widely used for the measurement of human body temperature, but it can not replace the existing temperature measurement method due to its low accuracy. Therefore, the high sensitivity and high performance of infrared sensor is the inevitable trend of its future development.Ⅷ SummaryAlthough there are many deficiencies in the current infrared sensor, the infrared sensor has played a huge role in modern production practice. With the improvement of detection equipment and other parts of the technology, the infrared sensor can have more performance and better sensitivity and will have a broader application range. Ⅸ FAQ1. What is the working principle of the IR sensor?Active infrared sensors both emit and detect infrared radiation. Active IR sensors have two parts: a light-emitting diode (LED) and a receiver. When an object comes close to the sensor, the infrared light from the LED reflects off of the object and is detected by the receiver. 2. Why is an infrared sensor important?An infrared sensor is an electronic instrument that is used to sense certain characteristics of its surroundings. It does this by either emitting or detecting infrared radiation. Infrared sensors are also capable of measuring the heat being emitted by an object and detecting motion. 3. What is an IR sensor for kids?Light waves longer than red light waves are called infrared light (IR). We cannot see either UV and IR light without special equipment or photography. In the case of infrared sensors, an infrared light source, which is typically an IR LED, is used to transmit light to a receiving infrared sensor. 4. Can IR sensors detect humans?The Passive Infrared (PIR) sensor is used to detect the presence of humans. But this detects the human only if they are in motion. Every human radiates the infrared energy of a specific wavelength range. The absorbed incident radiation changes the temperature of a material. 5. Where are IR sensors used?A passive infrared sensor (PIR sensor) is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. They are most often used in PIR-based motion detectors. PIR sensors are commonly used in security alarms and automatic lighting applications. 6. How do you bypass an infrared sensor?Most motion detectors, even newer ones, use infrared to detect significant changes in the surrounding room's temperature, Porter said. Normally, walking around in a room would set off these sensors, but using something as simple as a piece of styrofoam to shield your body can trick them, he said. 7. What is the difference between an IR sensor and an ultrasonic sensor?The biggest difference between IR sensors vs. ultrasonic sensors is the way in which the sensor works. Ultrasonic sensors use sound waves (echolocation) to measure how far away you are from an object. On the other hand, IR sensors use Infrared light to determine whether or not an object is present. 8. What is the range of the IR sensor?An infrared sensor (IR sensor) is a radiation-sensitive optoelectronic component with spectral sensitivity in the infrared wavelength range 780 nm ...50 µm. IR sensors are now widely used in motion detectors, which are used in building services to switch on lamps or in alarm systems to detect unwelcome guests. 9. Can an IR sensor detect temperature?Infrared temperature sensors sense electromagnetic waves in the 700 nm to 14,000 nm range. ... Because the emitted infrared energy of any object is proportional to its temperature, the electrical signal provides an accurate reading of the temperature of the object that it is pointed at. 10. How do IR sensors detect obstacles?An infrared sensor emits and/or detects infrared radiation to sense its surroundings. ... The basic concept of an Infrared Sensor which is used as an Obstacle detector is to transmit an infrared signal, this infrared signal bounces from the surface of an object and the signal are received at the infrared receiver. 

IntoductionAn operational amplifier, or op-amp for short is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op-amp produces an output potential that is typically hundreds of thousands of times larger than the potential difference between its input terminals, and is a voltage amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. They are used extensively in signal conditioning, filtering or to perform mathematical operations such as add, subtract, integration and differentiation.In this video, the basic introduction of the Operational Amplifier (Op-Amp) has been given and different characteristics of ideal and real Op-amp (General Purpose 741 Op-Amp) has been discussed. CatalogIntroductionⅠ Operational Amplifier Basics1.1 Amplification Principle1.2 Balance Resistor1.3 Feedback Resistor in Parallel with a Capacitor 1.4 Pulling Down Resistor and Pulling Up the Capacitor1.5 Parasitical Resistor as an Integrator1.6 Parasitical Resistors and Capacitors1.7 Balance Resistor Failure1.8 Magnification, Input Impedence, Voltage1.9 Open Loop Gain1.10 Virtual ShortⅡ Op Amp ApplicationⅢ Op Amp SamplingⅣ Op Amp Reference VoltageⅤ Importance of Op AmpⅠ Operational Amplifier BasicsWhen using op amp, there will be more problems confusing us, what are they? Listing all of them is impossible, but we can seek the core of these problems, which are the following lists.1.1 Amplification PrincipleThere are many types of op amps with many functions, and their circuits are inconsistent, but the internal block diagrams are basically the same. It consists of three parts: input stage, intermediate stage, and output stage. The input stage consists of a differential amplifier circuit that uses circuit symmetry to improve overall circuit, and the main function of the intermediate voltage amplifier stage is to increase the voltage gain. It can be composed of one or more stages of amplifying circuits; the output stage has a voltage gain of 1, but can provide a certain amount of power, and the circuit consists of two power supplies V+ and V-. The entire amplifier circuit is designed with two inputs P and N, and one output O. The voltages of the three terminals are represented by Vp, Vn, and Vo, respectively. The two ends of P and N are respectively called the non-inverting input terminal and the inverting input terminal, which means that when the P terminal is added with the voltage signal Vp (Vn = 0), it is obtained at the output end. The voltage Vo is in-phase with Vp, when the voltage signal Vn (Vp = 0) is applied to the N terminal, the output voltage Vo obtained at the output is inverted from Vp.The operational amplifier is actually a differential amplifier. Look at its structure, two transistors are connected back to back to share the crossing current source. One of the transistors is the positive input of the op amp and the other is the inverting input. The positive input is amplified and sent to a power amplifier circuit to amplify the output. Thus, if the voltage at the forward input rises, the output naturally becomes larger. If the voltage at the inverting input rises, the reverse current is large, and the forward current is small, because the inverting tertiary tube and the forward tube share a same current source.1.2 Balance ResistorGenerally, there is a balance resistor in the inverting / non-inverting amplifier circuit. What is the role of this balance resistor?(1) Provide a suitable static bias for the transistors inside the chip.The internal circuit of the chip is usually directly coupled, and it can automatically adjust the static operating point, but if an input pin is directly connected to the power supply or the ground, its automatic adjustment function can not work normally. Because the voltage of the ground cannot be raised by the inside transistors, and the voltage of the power supply cannot be reduced, which causes the chip to fail to meet the conditions of virtual short and virtual open.(2) Eliminate the influence of the static base current on the output voltage, and the value should be balanced with the equivalent resistance value of the external DC of the two input terminals.(3) In non-inverting op amp circuit, if it not connecting a balance resistor, the op amp will be burned, because the resistor acts as a voltage divider. 1.3 Feedback Resistor in Parallel with a Capacitor What is the role of the feedback resistor in parallel with a capacitor when using non-inverting op amp?(1) The feedback resistor and capacitor form a high-pass filter, so that local high-frequency amplification is particularly noticeable.(2) Prevent self-excitation. 1.4 Pulling Down Resistor and Pulling Up the CapacitorWhat role does the role of pulling down resistor and pulling up the capacitor at the input of the op amp play?To get positive feedback and negative feedback, depending on the specific circuit connection. For example, if the input voltage signal and the output voltage signal are taken to the input, the partial output signal passes through the balance resistor to obtain a new voltage value, that is, shunting the input voltage to make the input voltage smaller, and this is a negative feedback. Since the signal output from the signal source is always constant, the output signal can be corrected by negative feedback. 1.5 Parasitical Resistor as an IntegratorWhat is the function of the resistor RF connected to the op amp as an integrator at the two ends of the integrating capacitor?Adjust resistance to prevent the output voltage from running out of control. 1.6 Parasitical Resistors and CapacitorsWhy are resistors and capacitors connected in series at the input of the op amp?Regardless of the type of op amp, it consists of transistors or MOS transistors. In the absence of an external components, the op amp is a comparator actually. When the voltage of the non-inverting terminal is high, it will output a level similar to the positive voltage, and vice versa, but this op amp does not seem to have much use. Only when the external circuit is formed to generate the feedback will make the real op amp function. 1.7 Balance Resistor FailureWhat is the consequence of the balance resistor doesn't work well in non-inverting amplifier circuit?(1) The non-inverting end is unbalanced. For example, there will be an output although the input is 0. When the input signal is output, the output value is always larger (or smaller) than the theoretical output value by a fixed number.(2) The error caused by the input bias current cannot be eliminated. 1.8 Magnification, Input Impedence, VoltageWhat is the amplification factor and input impedence of an ideal integrated operational amplifier? What is the voltage between the non-inverting input and the inverting input?The magnification is infinite, the input impedance is infinitesimal, and the voltage is almost the same (the voltage is not 0V, for example, the non-inverting end is 10V and the inverting end is 9.99V). 1.9 Open Loop GainWhy is the open loop gain of an ideal op amp infinite?1) The actual open loop gain of the op amp is very large, so imagine it as infinity and derive the virtual ground from it.2) Deriving virtual ground is not only an inverting amplifier for the negative feedback connection, because there is no virtual ground for positive feedback.The open-loop gain of the op amp is infinite, when design the circuit, the closed-loop gain can be independent of the open-loop gain, and only depends on the external components. It is to use the large open loop gain in exchange for the stability of the closed loop gain.3) Assuming that the gain is small, the difference between the voltages applied across the op amp is relatively large for an output voltage. If it is connected to a negative feedback state, the voltage across the op amp will be different, causing amplification.We all know that the op amp’s output voltage Vo is equal to the difference Vid between the non-inverting input voltage and the inverting input voltage, multiplied by the op amp’s open-loop gain A, that is, Vo = Vid * A = (VI + - VI-) * A ( 1 ). Since the output voltage of the op amp does not exceed the supply voltage in practice, it is a finite value. In this case, if A is large, (VI+ - VI-) is necessarily small; if (VI+ - VI-) is small enough, then we can actually treat it as 0, at this time,  there will be VI+ = VI-, that is, the voltage at the non-inverting input of the op amp is equal to the voltage at the inverting input. This is what we call “virtual short”. Note that they are not really connected together, and there is resistance between them.In the above discussion, how did we get the result of “virtual short”? Our starting point is the formula (1), which is based on the characteristics of the op amp. Then, we made two important assumptions, one is that the output voltage of the op amp is limited, and it not exceed the power supply voltage; the second is that the open loop gain A of the op amp is large. The A of a normal op amp usually reaches 106 or 107 or even larger, but the actual open loop gain of the op amp is also related to its working state. For example, if the op amp is not working in the linear area, the value A may be small, so second assumption is conditional.Therefore, we know that when the open loop gain A of the op amp is large, the op amp can have a virtual short. But it is one of the possibilities, and it is not suitable for every op amp in any case to say their inputs are virtual short, in other words, virtual short can only be achieved in circuits under certain conditions.The conditions of virtual short:a. The open-loop gain of operational amplifier should be large enough.b. There should be a negative feedback circuit. From the above we know when we need to analyze the virtual short in the circuit. In reality, condition (1) is true for most op amps, and the important point is to look at the work area. If it is a circuit drawing, judge by calculation; if it is an actual circuit, it is reasonable to use the instrument to measure amplifier output voltage.There is also a situation related to virtual short called “virtual ground”, that is, there is a virtual short when the input is grounded. Some books say that virtual short will be exist under deep negative feedback conditions, but in reality, the op amp is more likely to work in the linear region under this situation. But this is not absolute, when the input signal is too large, the op amp with deep negative feedback will still be saturated. Therefore, it should be judged to be the most reliable with the output voltage value. 1.10 Virtual ShortAdd the input signal directly to the non-inverting input, and the inverting input is grounded through the resistor. Why is U-= U+ = Ui≠0? Is it not a virtual short? What are the conditions that the virtual ground exists?(1) In the non-inverting amplifier circuit, the output affects by the feedback, so that U(+) automatically tracks U(-), so they will be close to zero. It seems that the two ends are short circuit, so it is called virtual short.(2) Due to the virtual short phenomenon and the high input resistance of the op amp, the current flowing through the two input terminals is small, approaching 0. This phenomenon is called virtual open, which is derived from virtual short.(3) The virtual ground is in the inverting op amp circuit, the (+) terminal is grounded, and the (-) is connected to the input and feedback network. Due to the virtual short, U(-) and U(+) are very close, which is said to be virtual ground.(4) About the conditions: the virtual short is an important feature of the closed-loop (negative feedback) operating state of the non-inverting amplifier circuit; the virtual ground is an important feature of the inverting amplifier circuit in the closed-loop operating state. Ⅱ Op Amp ApplicationWhen a operational amplifier is connected as a non-inverting amplifier, the potentials of the two inputs are the same. If the waveform of the input is measured, it will be the same. This is like a common-mode signal. In fact, there are still small differential mode signal on the two inputs, but the differential mode signal can not be measured by the general instrument. As a result, the virtual short artificially increases the common-mode signal at the two inputs, which poses a challenge to the performance of the operational amplifier. Why is an op amp used like this?(1) The common mode signal of the non-inverting amplifier is much larger than the inverting amplifier, and strict to the common mode rejection ratio.(2) For single-ended input, the equivalent common-mode value is half of the input value, whether non-inverting or inverting input. However, since the input impedance of the non-inverting amplifier is usually larger than the inverting amplification, the anti-interference ability is a little poor.As mentioned above, when the inverting input is performed, the voltage at the inverting terminal is almost zero, so the differential influence on the tube collector voltage that has only one tube change. When the input is in phase, the voltage at the inverting terminal is equal to the non-inverting terminal voltage, so the common mode voltage and the input voltage are equivalent. That is to say, the collector voltage of the differential tube has variable quantity that changes in the same direction when the two tubes have portions that change in different directions at the same time, which is the common mode output voltage. It is added in phase with the voltage of one of the tubes. Therefore, it is easy to cause the tube to become saturated (or cut off), fortunately, the amplification of the common mode voltage is only tens of thousands of parts of the differential mode amplification.However, this does not mean that the common mode rejection suppression ratio of the differential mode input and the common mode input of the amplifier is different. It should be that the non-inverting input is added with a common mode signal equivalent to the input volume, so it should be careful to use non-inverting amplification mode when the input signal is large. Ⅲ Op Amp SamplingWhy is the amplifier circuit composed of operational amplifiers generally sampling the inverting input mode?(1) The significant difference between the inverting input and the non-inverting input mode is:When inverting input, because there is a balanced resistor connected to the ground at the same phase, and there is no current on this resistor (because the input resistance of the op amp is extremely large), this non-inverting terminal is approximately equal to the ground potential, and  the potential at the non-inverting terminal is extremely close to the inverting terminal, so there is a virtual ground at the inverting end. The advantage of having a virtual ground is that there is no common mode input signal, even if the common mode rejection ratio is not high, there is no common mode output. The non-inverting input mode has no virtual ground. When a single-ended input signal is used, a common-mode input signal is generated. Even if an operational amplifier with a high common-mode rejection ratio is used, there is still a common-mode output. Therefore, it is best to use the inverting input method.(2) The positive phase is the oscillator, and the inverting can stabilize the amplifier and access the negative feedback.(3) From the principle point of view, it is possible to connect to the same analog circuit. However, the signal (differential mode signal) that is amplified during the actual application tends to be small, thus it is necessary to pay attention to suppressing noise (usually expressed as a common mode signal). In the same way, the amplification circuit has a poor ability to suppress the common mode signal, and the signal that needs to be amplified is submerged in the noise, which is not conducive to post processing. Therefore, an inverting proportional amplification circuit with better suppression capability is good.Ⅳ Op Amp Reference VoltageSome op amps will have an output even if no voltage is input after power-on, and the output is not small, so VCC/2 is often used as the reference voltage.The output is output signal without any input, this is called the input offset voltage Vos, which is caused by the asymmetry of the design structure of the op amp. It is a very important performance indicator of the op amp. The op amp commonly used VCC/2 as the reference voltage is because the op amp is in a single power supply state. At this time, the real reference of the op amp is VCC/2, so a DC offset of VCC/2 is often provided at the positive terminal of the op amp. When having positive and negative dual power supply, it is often referenced to the ground.The selection of op amps requires attention to many things. Under less stringent conditions, it is often necessary to consider the operating voltage, output current, power consumption, gain bandwidth product, and price of the op amp. Of course, when using it under special conditions, different factors must be considered in practice. Ⅴ Importance of Op Amp(1) If the voltage on both inputs of the op amp is 0V, the output voltage should also be equal to 0V. But in fact, there is always some voltage at the output, that is, the offset voltage Vos. If the offset voltage at the output is divided by the noise gain of the circuit, the calculated result is called the input offset voltage or the input reference offset voltage. The Vos is considered to be a voltage source in series with the inverting input of the op amp. A differential voltage must be applied to both inputs of the amplifier to produce a 0V output.(2) The input impedance of an ideal op amp is infinite, so no current flows into the input. However, a real op amp using a bipolar junction transistor (BJT) in the input stage requires some operating current, which is called bias current (IB). There are usually two bias currents: IB+ and IB-, which flow into the two inputs, respectively. The range of IB values is large, with bias currents of lower at 60fA for special op amps and up to tens of mA for some high-speed op amps.(3) The power supply voltage range required for the first single-chip op amp to operate normally is ±15V. Today, op amps are moving toward low voltages due to increased circuit speeds and power supplies from low-power sources such as batteries. Although the op amp’s voltage specifications are usually specified as symmetrical two-pole voltages ±15V, these voltages do not necessarily require a symmetrical voltage or a two-pole voltage. For an op amp, as long as the input is biased in the active region (within the common-mode voltage range), the ±15V supply is equivalent to a +30V/0V supply, or a +20V/-10V supply. The op amp does not have a ground pin unless the negative voltage rail is grounded in a single-supply application.The input voltage swing of high speed circuits is smaller than that of low speed devices. The higher the speed of the device, the smaller its geometry, which means the lower the breakdown voltage. Due to the low breakdown voltage, the device must operate at a lower supply voltage. Today, op amps typically have a breakdown voltage of around ±7V, so high-speed op amps can work at a supply voltage of ±5V, and they can also operate at a single supply voltage of +5V.For general-purpose op amps, the supply voltage can be as low as +1~8V. These op amps are powered by a single power supply, but this does not mean that a low supply voltage must be used. Because the terms single supply voltage and low voltage are two related and independent concepts. Frequently Asked Questions about Operational Amplifiers Problems1. How can you tell if an op amp is blown?Re: how to tell whether an op amp is burned out? measure the DC voltage at the +input. then measure the DC voltage at the output. if the results are significantly different, the opamp is most likely shot. 2. How do I know if my op amp is broken?measure the DC voltage at the +input. then measure the DC voltage at the output. if the results are significantly different, the opamp is most likely shot. if they are the same, the opamp is most likely ok and the problem is something else. 3. What errors you have to consider with real operation amplifiers?These errors include input bias current, input offset current, input offset voltage, CMRR, PSRR, and finite input impedance. In reality, all these errors will occur at the same time. 4. How do op amps fail?The common failures I have seen including with comparators involve either the output being shorted or open to one supply or the input differential pair or input protection circuits being damaged causing excessive input bias current and/or input offset voltage which usually ends up pinning the undamaged output. 5. Why do op amps fail?The common failures I have seen including with comparators involve either the output being shorted or open to one supply or the input differential pair or input protection circuits being damaged causing excessive input bias current and/or input offset voltage which usually ends up pinning the undamaged output.

Ⅰ AbstractIn portable electronic devices such as mobile phones, notebook computers, and small video cameras, lithium-ion batteries have developed rapidly due to their sound performance, such as high working voltage, large specific energy, long cycle life, low self-discharge rate, no memory effect and so on, which are compared with traditional NiCd batteries and NiMH batteries.Figure 1. Lithium-ion Movement in Li-ion BatteryCatalogⅠ AbstractⅡ Charging Characteristics of Lithium BatteriesⅢ Performance Description of Several Different Charging States3.1 On Standby3.2 Precharging3.3 Constant Current3.4 Constant VoltageⅣ Charging Process Analysis4.1 High Voltage Constant Current Mode4.2 Low Voltage High Current Mode4.3 High Voltage High Current ModeⅤ Li-ion Battery Charging Security5.1 Common Sense in the Daily Use of Batteries5.2 Charging RulesⅥ One Question Related to Lithium-ion Battery and Going Further6.1 Question6.2 AnswerThe charge and discharge of lithium-ion batteries do not transfer electrons through traditional methods, but energy changes occur through the entry and exit of lithium ions in the crystals of layered materials. Under normal charge and discharge conditions, the in and out of lithium ions cause changes in the interlayer spacing, but will not cause damage to the crystal structure, so lithium-ion batteries can be regarded as an ideal reversible battery. During charging and discharging, lithium ions come and go between the positive and negative electrodes of the battery, and they shake between the positive and negative electrodes like a rocking chair.Lithium-ion Battery Charging BasicCharging batteries is common for people's daily life, as we all know, Li-ion batteries play a very important role in our social life with their excellent performance, in order to get longest service life, proper charging of Li-ion batteries is essential. Li-ion battery charging mode is voltage limit and constant current, which is controlled by IC chip. The typical charging method is: detect the voltage of the battery to be charged firstly, if its voltage is lower than 3V, pre-charge is required necessarily, and the the charging current is 1 ≤ 10 of the set current. After the voltage rises to 3V, then transferring into the standard charging process. The standard charging process is: having constant current charging with set current. When the battery voltage rises to 4.20V, it is changed to constant voltage charging mode, and the charging voltage is kept at 4.20V. At this time, the charging current gradually decreases till the current drops to 1/10 of the set charging current, the charging ends.The charging process of a Li-ion battery can be divided into three processes: trickle charging (low voltage precharging), constant current charge, and constant voltage charge. Ⅱ Charging Characteristics of Lithium BatteriesFigure 2. Typical Charge ProfileAs can be seen from the above figure, the charging current and voltage of the lithium battery are dynamically changed, which is determined by the chemical content of the lithium battery itself. Therefore, it is necessary to configure the performance of the charging IC according to the charging characteristics of the lithium battery itself to achieve a correct, safe and efficient use of the lithium battery. The "lithium-ion battery charging current" in the daily expression is for the charging current of fast charging. As a dynamic process, the optimal charging current of the lithium battery is actually divided into three stages. Ⅲ Performance Description of Several Different Charging StatesFigure 3. Li-ion Battery Process3.1 On StandbyThe standby state is handled in the following cases:1) The input voltage is lower than the minimum operating voltage of the circuit.2) After the battery voltage is approach to the limit.3) Using external switch to turn offmanagement IC to stop charge.Voltage and current characteristics in standby mode: The charging IC has no charging voltage output, and the IC input current is in the uA level, which can reduce power loss. 3.2 PrechargingAs shown in above figure. Optimal current during precharging: that is, when the initial/no-load voltage of the lithium battery is lower than the prechargeing threshold, it needs a pre-charging stage. For a single lithium-ion battery, this threshold is generally 3.0V, in the phase, the precharge current is about 10% of the current in the constant current charging phase. 3.3 Constant CurrentAs shown in the figure above, when the battery voltage is greater than the preset voltage threshold and less than the maximum voltage of 4.2V, the IC will charge the battery with the maximum charging current set by the external resistor. When the battery voltage is equal to the maximum charging voltage (near 4.2V), the charge stop.The best current for constant current charging: when stay in constant current stage, the voltage gradually rises, then enter the fast charging phase. Most of the constant current charging current is set between 0.5 and 1.0C, and the best set is 0.8C, because the battery can be full charged about two hours without consider other factors. The case is a good balance between charging time and charging safety.Several problems that should be paid attention to when batteries at constant current charging:1) In this state, the IC is in the state of maximum charging current, and the loss at this time is also the largest. The linear voltage drop loss calculation is L = (Vin-Vout) × Iout, it is necessary to pay attention to the maximum operating temperature of the IC.2) The increasing temperature due to the highest charging current, the IC will automatically reduce the maximum charge current, and this is why the charging current drops during overheating. 3.4 Constant VoltageThe maximum charging voltage portion shown in the above figure, when it is detected that the battery voltage is equal to or close to the battery charging voltage, at this time, the charging mode will be stepped down with a constant charging voltage of 4.2V. When it is detected that the charging current is less than 1/10 of the maximum set current, charging will stop. Charging current during constant voltage charging: In the case of a single-cell lithium-ion battery, as the battery voltage rises to 4.2 V, the constant current charging ends and the constant voltage charging stage begins. In order to achieve the best performance, the voltage stabilizer tolerance should be better than +1%.At this stage, the voltage is keeping constant and the current is reduced, and this current reduction is a sequential decrement process. Most lithium battery protection selects 0.1C as the termination current, which means that the charging process enters the end state. Once charging is finished, the charging current drops to zero. The problem to be noted in this state is that the battery can be automatically turned off when the battery is charged to the highest setting voltage. At the same time, when the overvoltage protection of the IC is in the abnormal battery state, it can be automatically locked. Unlike nickel batteries, continuous trickle charging is not recommended. Because it will cause plate plating effect to the lithium metal, making batteries failure.The core of the best charging current of lithium battery is the current design of constant current charging. It should be emphasized that most portable lithium batteries should be designed to charge 0.5C~0.8C. For example 1400mAh capacity of iPhone battery(capacity mAh= current mA × time /h), choosing 0.7C, that is, Apple’s charging current is about 1A, so that most of the batteries between 0.5C~0.8C you can choose.When charging, the voltage of the battery should be detected first. If the voltage is lower than 3V, pre-charging should be performed first. When the charging current is 1/10 of the set current, 0.05C is selected generally. After the voltage rises to 3V, it enters the standard charging process. The standard charging process is constant current charging with set current. Till the battery voltage rises to 4.20V, it is changed to constant voltage charging, and the charging voltage is kept at 4.20V. At this time, the charging current gradually decreases, and when the current drops to 1/10 of the set charging current, the charging ends.Generally, the charging current of the lithium battery is set between 0.2C and 1C. The larger the current, the faster the charging, and the greater the heat of the battery. Moreover, when lies in excessive current charging, the capacity is not full, because the electro-chemical reaction inside the battery takes time. Ⅳ Charging Process Analysis Figure 4. Charging Characteristics of Lithium-ion Battery 4.1 High Voltage Constant Current ModeIn general, the charging process of the mobile phone is to first reduce the 220V charging voltage to the 5V charger voltage, and the 5V charger voltage reduce to the 4.2V battery voltage. During the entire charging process, if the voltage is increased, heat is generated, therefore, the charger will heat up and the phone will heat up. Moreover, the greater the power consumption, the greater the damage to the battery. 4.2 Low Voltage High Current ModeWhen the voltage is constant, the current can be increased by using a parallel circuit. Under this situation, the smaller the volume shared by each circuit after parallel shunting, each circuit has the smaller load damage, so as to the phones charging process. 4.3 High Voltage High Current ModeThis method increases the current and voltage at the same time, so that from the previous formula P=UI, we can know that this method is the best way to increase the power, but it will generate more heat when the voltage is increased. In this way, the more energy is consumed, but the voltage and current are not freely increased without limitation.The maximum charging current of a lithium battery is strictly determined by the structure of the battery. Therefore, the specifications of the lithium battery manufacturers are not consistent, some are set to 0.6C, and the highest current specification for portable lithium batteries is 1C.  Of course, the current design of pre-charging and constant voltage charging cannot be ignored. In the two processes, if the initial voltage is not lower than the pre-charging threshold of 3.0V, there is no pre-charging process. In general, there is a process to check batteries charging voltage that is beneficial to keep the long-term use of lithium batteriess.Ⅴ Li-ion Battery Charging Security5.1 Common Sense in the Daily Use of BatteriesMisunderstanding: “Battery activation”, charging for more than 12 hours in the first three times.For the “activation” problem of lithium batteries, many sayings are: charging time must be more than 12 hours, and repeat three times in order to activate the battery. This statement that “the first three charges have to be charged for more than 12 hours” is obviously a continuation of nickel batteries (such as nickel cadmium and nickel hydride), in other words, this kind of statement can be said to be misinformation of the other batteries. After a sample survey, it conformed that a considerable number of people have confused the charging methods of the two batteries. Lithium-ion battery activation does not require a special method, they will be activated naturally in the normal use.The charge and discharge characteristics of lithium and nickel batteries are very different. All the professional technical data reviewed emphasize that overcharge and overdischarge can cause huge damage to lithium batteries, especially liquid Li-ion batteries. Therefore, charging is preferably performed in accordance with standard methods, especially for ultra-long charging of more than 12 hours. For example, the charging method described in the mobile phone manual is a standard charging method suitable for the mobile phone. It is not suitable to charge for a long time, also the battery is completely dischargedand thenThe lithium battery phone or charger will automatically stop charging when the battery is fully charged. There is no so-called “turbulent” charging over 10 hours for nickel battery chargers. If the lithium battery is fully charged, it will not be charged anymore continuously.Over-time charging and power off completely will cause over-charging and over-discharging, which will cause permanent damage to the positive and negative electrodes of lithium-ion batteries. At the molecular level, over-discharge will cause the anode carbon to release lithium ions excessively causing the layer structure collapses, and overcharging will hardly plug too much lithium ions into the negative carbon structure, and some of the lithium ions will no longer be released. Regular deep charge and discharge for battery calibrationLi-ion batteries generally have a management IC and a charge control IC. The management IC has a series of registers, which contain values such as capacity, temperature, ID, state of charge, and discharge times. These values will gradually change during use, so the main function of the “The batteries should be fully charged and discharged when used once a month or so” is to correct the improper values in these registers. 5.2 Charging RulesThe following rules should be noted when charging and discharging lithium ion batteries:Figure 5. Typical Li-ion Battery Discharging DiagramCharge currentItmust limited for li-ion batteries. Typically the maximum value is 0.8C, but lower values are more usually set to give some margin. Charge temperature  Itshould be monitored. The cell or battery must not be charged when the temperature is lower than 0°C or greater than 45°C. Short circuit protectionItis required to prevent damage or explosion as a result of short circuits. Over-voltage protectionItis required to prevent a voltage that is too high being applied across the battery terminals. Over-charge protectionItis required to stop the Li-ion charging process when voltage per cell rises above 4.30 volts. Reverse polarity protectionItis needed to make sure the battery is not charged in the wrong direction as this could lead to serious damage or even explosion. Over-discharge protectionItis required to prevent the battery voltage falling below about 2.3V dependent upon the manufacturer, when battery voltage less than 2.3V will make battery damage irreversibly. Over temperature protectionIt is necessary to prevent the battery operating in a high temperature, because heating will age batteries and reduce their service life. if the temperature rises too high. Temperatures above 100°C can cause irreparable damage.Ⅵ Questions Related to Lithium-ion Batteries1. How many years does a lithium ion battery last?three yearsThe typical estimated life of a Lithium-Ion battery is about two to three years or 300 to 500 charge cycles, whichever occurs first. One charge cycle is a period of use from fully charged, to fully discharged, and fully recharged again. 2. What is the difference between a lithium battery and a lithium ion battery?Lithium batteries feature primary cell construction. This means that they are single-use—or non-rechargeable. Ion batteries, on the other hand, feature secondary cell construction. This means that they can be recharged and used over and over again. 3. Why are lithium ion batteries bad for the environment?Recycling Lithium-IonUnwanted MP3 players and laptops often end up in landfills, where metals from the electrodes and ionic fluids from the electrolyte can leak into the environment. Because lithium cathodes degrade over time, they cannot be placed into new batteries. 4. Is there a better battery than lithium ion?Zinc-air batteries can be considered superior to lithium-ion, because they don't catch fire. The only problem is they rely on expensive components to work. 5. What is the best way to charge a lithium ion battery?Simple Guidelines for Charging Lithium-based BatteriesTurn off the device or disconnect the load on charge to allow the current to drop unhindered during saturation.Charge at a moderate temperature.Lithium-ion does not need to be fully charged; a partial charge is better.

CatalogⅠ What is Biosensor?Ⅱ Principle of Biosensors Ⅲ Characteristics of Biosensors Ⅳ Types of Biosensors  4.1 Acoustic Biosensor  4.2 Optical Biosensor  4.3 Magnetic Biosensor  4.4 Electrochemical Biosensor  4.5 Optical Fiber Nano BiosensorⅤ FAQⅠ What is Biosensor?Biosensor is an instrument that is sensitive to biological substances and converts its concentration into an electrical signal for detection. Biosensor has the function of receiver and converter. Because enzyme membrane, mitochondrial electron transport system particle membrane, microbial membrane, antigen membrane and antibody membrane have the selective recognition function to the molecular structure of biomaterials and only have the catalytic activation function to specific reactions, so biosensors have very high selectivity. The disadvantage is that the biofilm is not stable.Biosensors are mainly used in clinical diagnosis, treatment monitoring, fermentation industry, food industry, environment and robotics. Biosensor is an interdisciplinary subject combining bioactive materials(Enzyme, protein, DNA, antibody, antigen, biofilm, etc)with physical and chemical transducers. It is an advanced detection method and monitoring method necessary for the development of biotechnology, and it is also a rapid and microanalysis method at the molecular level. In the 21st century, in the development of the knowledge economy, biosensor technology will be a new growth point between information and biotechnology. It will have a wide application prospect in clinical diagnosis, industrial control, food and drug analysis(including biopharmaceutical research and development), environmental protection, biotechnology, biochip and other research in the national economy. All kinds of biosensors have the following common structures: including one or several related bioactive materials(Biofilm) and physical or chemical transducers(sensors) that can convert the signals expressed by bioactivity into electrical signals. The two are combined to reprocess the biological signals with modern microelectronics and automatic instrument technology to form a variety of usable biosensor analysis devices, instruments and systems.Ⅱ Principle of Biosensors The substance to be measured enters into the bioactive material through diffusion, and after molecular recognition, biological reaction occurs. The information generated is then transformed into a quantitative and treatable electrical signal by the corresponding physical or chemical transducer, and then amplified and output by the secondary instrument, the concentration of the substance to be measured can be known.Ⅲ Characteristics of Biosensors     （1）The biosensor uses the immobilized bioactive substance as the catalyst, and the expensive reagent can be reused many times, which overcomes the shortcomings of the high cost of enzyme analysis reagent and complicated chemical analysis in the past.    （2）Strong specificity only reacts to a specific substrate, and not affected by color and turbidity.     （3）The analysis speed is fast, and the results can be obtained in one minute.    （4）High accuracy; general relative error can reach 1%.    （5）The operating system is simple and easy to realize automatic analysis.    （6）Low cost; only a few cents per measurement in continuous use.     （7）Some biosensors can reliably indicate the oxygen supply and by-products in the microbial culture system. In the process of production control, much complex information can be obtained only by the comprehensive action of physical and chemical sensors. At the same time, they also pointed out the direction of increasing the yield of products.Ⅳ Types of BiosensorsAccording to the classification of life substances used in biosensors, biosensors can be divided into microbial sensors, immune sensors, tissue sensors, cell sensors, enzyme sensors, DNA sensors, etc. According to the principle of sensor detection, it can be divided into a thermosensitive biosensor, FET biosensor, piezoelectric biosensor, optical biosensor, acoustic channel biosensor, enzyme electrode biosensor, mediator biosensor, etc.According to the type of interaction between sensitive substances, it can be divided into two types: affinity type and metabolism type. 4.1 Acoustic BiosensorAcoustic biosensor is a kind of sensor to detect the change of acoustic frequency caused by the substance to be detected. Among them, quartz crystal microbalance（QCM） biosensor has been studied most. In the piezoelectric crystal of quartz crystal microbalance biosensor, AT mode is often used to form two parallel metal((Au，Ag，Pt，Ni，Pd etc.)) membrane electrodes on both sides of the crystal by ion beam deposition. (At cutting means that the cutting surface is 25.15 ° to the main optical axis of quartz crystal.  At this moment, the temperature coefficient of crystal resonance is close to zero at room temperature)The recognition molecules are fixed on the surface of the membrane electrode. Because of their specificity, the recognition molecules combine with the molecules to be detected, causing the quality change of the electrode surface, thus changing the oscillation frequency of the quartz crystal. If the nanoparticles are modified on the molecules to be detected, the quality of the molecules to be detected will be significantly improved, and the detection signal will also be enhanced. Ward et al. Labeled the antibody with nano colloidal particles, and combined it to the surface of quartz crystal by antibody antigen immunoassay.  Because the modified colloidal particles(The diameter of sol particles is 5 "100nm) improved the quality of the labeled molecules, according to Sauerbrey equation, the oscillation frequency of quartz crystal was correspondingly increased, so the detection signal was amplified, the detection sensitivity was improved, and the detection lower limit was also reduced.4.2 Optical BiosensorNano metal particles can be used for optical resonance detection. Bauer et al. Fixed nano-metal particles on the surface of conductive materials by antigen-antibody or protein receptor binding methods. Due to the interaction of reflection dipoles of nanoparticles, the resonance of reflected light is enhanced. The materials to be detected can be detected by detecting resonance signals. Nanoparticles can also be used to locate tumors. Fluorescein labeled recognition factors bind to tumor receptors, and then the size and location of tumors can be displayed in vitro. Nano metal particles can also be used as a general fluorescent annihilation group. Maxwell and other scientists labeled gold nanoparticles and fluorescence excitation groups at both ends of oligonucleotide probe molecules respectively. The probe formed a "hairpin" structure due to complementary bases, and the proximity of fluorescence excitation group and gold nanoparticles resulted in excitation fluorescence annihilation. When the probe combined with specific target DNA, its conformation changed, and the gold nanoparticles and fluorescence excitation groups were separated, so as to excite Fluorescence. The principle can be used for real-time fluorescence detection of nucleic acids and single base mutation polymorphism detection.4.3 Magnetic BiosensorMagnetic nanoparticles have important application value in biological detection and drug analysis. By using magnetic materials to label biomolecules and molecular recognition technology, complex operations such as sample mixing, separation and detection can be realized. Scientists label molecules with magnetic materials, and realize the separation and detection of samples under the magnetic field gradient. Richardson et al. Used magnetic counter to detect magnetic labeled molecules by magnetic immunoassay.  In addition, the distribution and position of magnetic particles in vivo can be measured in vitro after the identification factor is labeled with nanomagnetic particles and combined with the target recognition device on the tumor surface, so as to locate the tumor. Chemla and other scientists used paramagnetic nanoparticles and a microscope based on high-temperature transient DC superconducting quantum interface device (SQUID) to propose a novel rapid detection technology for biological samples. Firstly, the magnetic particles of the fixed antibody are suspended in the solution, and then the magnetized nanoparticles are generated under the instantaneous magnetic field pulse.  When the magnetic field disappears, the particles tend to be free distribution, because the particles without the antibody are Brownian motion, so there is no detection signal; while the nanoparticles with the target molecule move in the way of Neel relaxation, resulting in a slowly attenuated magnetic signal, The substance to be detected can be analyzed by the signal collected by the squid. This technology can directly detect the labeled molecules without separating the nanoparticles which are not combined with the molecules to be detected, which shortens the detection time and improves the detection efficiency.4.4 Electrochemical BiosensorColloidal gold is the most common metal nanoparticles, which can be used to mark biomolecules, thus realizing signal detection and amplification; in addition, it can also be widely used in TEM, SEM characterization and paper strip color. Many literatures also reported the signal amplification of colloidal gold in various biosensors. Gonzalez Garcia and other scientists used colloidal gold labeling and electrochemical methods to study the interaction between biotin and avidin.  By modifying biotinylated albumin on the electrode surface and then reacting with avidin labeled by colloidal gold with a diameter of 10 nm, scientists found that the current response caused by colloidal gold was linearly related to the concentration of avidin(2.5×10-9mol/L "2.5×10–5mol/L). Nanoparticles have an excellent specific surface area, which can be used to immobilize biomolecules, increase the number of fixed molecules, and achieve signal amplification. Singh and other scientists used the sol-gel method to synthesize silicon nanoparticles with a diameter of 20 nm or 200 nm. Acetylcholinesterase immobilized on the surface of nanoparticles can be used to make organophosphorus pesticide biosensor.  Because of its high specific surface activity, combined with the detection of ion-sensitive field effect tube, the metal nanoparticles with rapid response can be used as the carrier of catalyst, which can greatly improve the performance of catalyst. Enzyme colloidal gold is fixed on the surface of the electrode and can be used for the electrochemical detection of H2O2, glucose, xanthine and hypoxanthine. Xu et al. Modified the surface of the screen-printed carbon electrode with colloidal gold, combined with immunity and horseradish peroxidase (HRP) to make H2O2 biosensor. The results showed that the electrocatalytic performance and current response of HRP were significantly improved, the linear range of signal was greatly improved(0.8μM"1.0mM), and the detection limit was also reduced to 0.4 μ M.4.5 Optical Fiber Nano BiosensorCompared with other types of biosensors, fiber-optic nano biosensors are not only small in size and high insensitivity, but also free from electromagnetic interference and do not need reference devices. It can enter the interior of cells and measure the changes of structure and cytoplasm in vivo. （1）Optical fiber nano fluorescence biosensorKopelman was the first to use a fluorescent fiber-optic nanosensor to detect the pH value in the microenvironment. Its working principle is to fix the fluorescent agent at the head of the optical fiber. When the fluorescent agent reacts reversibly with the proton, the optical property of the liquid changes.  According to the change of the fluorescence intensity, the pH value can be determined. The optical fiber processing method is as follows: the optical fiber is drawn into a fiber probe with a head diameter of 100nm "1000nm by a fiber drawing instrument, and aluminum is plated on the surface of the optical fiber by a vacuum evaporator to prevent light from leaking during transmission. Then, the exposed optical fiber head is silanized, and the surface is modified into an active surface containing hydroxyl or amino group, and the antigen or antibody of the molecule to be detected is fixed and identified.  Finally, the light The fiber head is combined with a pH selective fluorescent dye polymer. The response time of the nano sensor is 250ms, and it can detect the ion concentration of μ M. These characteristics are suitable for the detection of single cell and subcellular structure, such as the detection of pH value of mouse embryonic cell fluid. （2）Optical fiber nano immune biosensorOptical fiber nano immunosensor is a kind of sensor which applies optics and photonics technology to immunoassay. It can convert the amount of antigen or antibody to optical signal by using the characteristic that antigen and antibody can combine specifically. This kind of sensor combines the advantages of traditional immunoassay, optics and biosensor technology, and has high specificity, sensitivity and stability.  At the same time, the fiber-optic nano immune sensor only uses nano products on sensitive components, so it not only retains many advantages of the original but also makes it suitable for the measurement of single cells. Dinh et al. Have successfully developed an optical fiber nano immunosensor for the detection of BPT (benzopyrene tetrol, a biomarker of DNA damage related to exposure to carcinogenic benzo [α] pyrene). They first made quartz fiber with a diameter of 10nm "100nm with a fiber drawing instrument, then silanized the fiber head, modified the fiber head with BPT antibody, and then plated the whole length of the fiber (except the modified fiber head) with silver to prevent light from leaking out.  Finally, cell puncture and detection experiments were carried out on a single cell operated micromanipulator/microinjector, they used photomultiplier PMT to record the fluorescence produced by the binding of BPT and antibody, and detected the content of BPT in cells by measuring the change of fluorescence intensity. The minimum detection limit of the sensor can reach 10 – 21mol. Ⅴ FAQ1. What is the principle of piezoelectric biosensors?Piezoelectric Biosensors are also known as Acoustic Biosensors as they are based on the principle of sound vibrations i.e. acoustics. When a mechanical force is applied to a piezoelectric biosensor, they produce an electrical signal. The biological elements are attached to the surface of the piezoelectric biosensor. 2. What are the different types of biosensors?• Electrochemical Biosensors.• Magnetic Biosensors.• Thermometric Biosensors.• Acoustic Biosensors.• Optical Biosensors. 3. How does a basic biosensor work?The term ‘biosensor’ is short for ‘biological sensor.’ The device is made up of a transducer and a biological element that may be an enzyme, an antibody or a nucleic acid. The bio element interacts with the analyte being tested and the biological response is converted into an electrical signal by the transducer. 4. What are the main components of biosensors?A biosensor typically consists of a bio-receptor (enzyme/antibody/cell/nucleic acid/aptamer), transducer component (semi-conducting material/nanomaterial), and electronic system which includes a signal amplifier, processor & display. Transducers and electronics can be combined, e.g., in CMOS-based microsensor systems. 5. How do you classify biosensors?Biosensors can be classified according to the transduction methods they utilize (Fig. 4). Most forms of transduction can be categorized in one of five main classes: electrochemical, electrical, optical, piezoelectric (mass detection methods) and thermal detection. 6. What are wearable biosensors?Wearable systems are devices that allow physicians to overcome the limitations of technology and provide a response to the need for monitoring individuals over weeks or months. Wearable Biosensors typically rely on wireless sensors enclosed in bandages or patches or in items that can be worn. 7. What is an amperometric biosensor?Amperometric biosensors are self-contained integrated devices based on the measurement of the current resulting from the oxidation or reduction of an electroactive biological element providing specific quantitative analytical information. 8. What is voltammetric biosensor?Cyclic Voltammetry (CV) Voltammetry belongs to a category of electro-analytical methods, through which information about an analyte is obtained by varying potential and then measuring the resulting current. It is, therefore, an amperometric technique. 9. What is the electrochemical biosensor?An electrochemical biosensor is a self-contained integrated device, which is capable of providing specific quantitative or semi-quantitative analytical information using a biological recognition element (biochemical receptor) which is retained in direct spatial contact with an electrochemical transduction element. 10. What are Piezoelectric Biosensors?Piezoelectric biosensors are a group of analytical devices working on the principle of affinity interaction recording. A piezoelectric platform or piezoelectric crystal is a sensor part working on the principle of oscillations change due to a mass bound on the piezoelectric crystal surface.