Temperature sensor high-precision measuring circuit for heat meter

Abstract

The utility model provides a temperature sensor high-precision measuring circuit for a heat meter, which belongs to the technical field of temperature sensor high-precision measurement, and comprises R1 and R2 which are contact resistance and line resistance of a thermocouple on a line, r3 and R4 are line resistors from the voltage collector to the two ends of the thermocouple, R5 is an internal resistor of the voltage collector, the output end of the DAC is a controlled voltage output end, the value of Vi determines the size of a controlled constant current source at the rear end, R5 / R6 is a protection series resistor, R7 is a sampling resistor, and Q1 is a switch NMOS tube; the variable constant current source is a constant current output power supply controlled by the MCU, the constant current source flows out to the load circuit through the R1 and the R2, the current is shunted at the a1 point according to the Thevenin current quantification, the resistance value of the thermocouple can be accurately tested through the circuit design, the interference of the line resistance and the contact resistance is reduced, and the test precision is greatly improved.

Description

Technical Field

[0001] The utility model relates to the technical field of high-precision measurement of temperature sensors, in particular to a high-precision measurement circuit of a temperature sensor used in a heat meter. Background Art

[0002] In heat meter applications, the key parameters are flow data and temperature data. There are usually two solutions for obtaining temperature data. One is a digital thermometer, that is, the MCU directly obtains the sensor temperature value through the communication interface. The other solution is a thermocouple solution, that is, according to the resistance change characteristics of the thermocouple sensor at different temperatures, the resistance value at both ends of the thermocouple is measured, and then the relevant temperature data is obtained by looking up the table. Therefore, the accuracy of the thermocouple test resistance value directly determines the accuracy of the back-end temperature data.

[0003] However, the thermocouple R0 is the item to be measured

[0004] R is the actual series voltage divider resistor, which is a known term

[0005] R1 and R2 are the online resistance or contact resistance

[0006] R3 and R4 are online resistors

[0007] R5 is the internal resistance of the voltage collection terminal

[0008] V is the voltage of the constant voltage source, V1 is the terminal voltage of the thermocouple. The theoretical ideal calculation formula for thermocouple resistance is: V*R0 / (R0+R)=V1. The actual calculation formula for thermocouple resistance is: It can be seen from the calculation formula that since R1 / R2 / R3 / R4 / R5 are interference items introduced by the outside world, they are mainly restricted by the welding process and the manufacturing process of wires and PCBs, and are not within the design theory range. Therefore, the accuracy of the test using this solution is not high.

[0009] Therefore, it is necessary to provide a high-precision measurement circuit of a temperature sensor used in a heat meter to solve the above technical problems. Utility Model Content

[0010] In order to solve the above technical problems, the utility model provides a high-precision measurement circuit of a temperature sensor used in a heat meter.

[0011] The utility model provides a high-precision measurement circuit of a temperature sensor used in a heat meter, comprising:

[0012] R1, R2, are the contact resistance and line resistance of the thermocouple on the line;

[0013] R3, R4 are the line resistances from the voltage collector to both ends of the thermocouple;

[0014] R5 is the internal resistance of the voltage collector;

[0015] The DAC output is a controlled voltage output, and the value of Vi determines the size of the back-end controlled constant current source;

[0016] R5 / R6 are protective series resistors;

[0017] R7 is the sampling resistor;

[0018] Q1 is a switch NMOS tube;

[0019] The variable constant current source is a constant current output power source controlled by the MCU, and the constant current source flows out to the load circuit through R1 and R2.

[0020] Preferably, according to the quantification of the Thevenin current, it can be known that the current is split at point a1, a part of which flows to a2 through the thermocouple R0, and the other part flows to a2 through R3, b1, R5, b2, and R4. The thermocouple used in the heat meter is a PT500 or PT1000 thermocouple, and depending on the usage scenario, the temperature range is between 4-100°C. By looking up the table, it can be known that the resistance range of the thermocouple in the working state is between 1KΩ and 1.5KΩ. R5 is the internal resistance of the voltage collector, and the internal resistance value is >1M ohms.

[0021] Preferably, based on the comparison between the resistance value of the thermocouple R0 and the internal resistance value of R5, and equivalent to R5 being open-circuited, the circuit is regarded as current flowing back to the constant current source input terminal through a1, R0, and a2, and the terminal voltage at both ends of R0 calculated by the equivalent circuit is equal to the constant current source current I*thermocouple R0, that is, the terminal voltage V between a1 and a2 is equal to I*R0.

[0022] Preferably, according to Kirchhoff's voltage law, the voltage V0 at the voltage collection end is V(R3)+V(R0)+V(R4), and since the loop current flowing through R3 / R0 / R4 is close to 0, V(R3) and V(R4) are close to 0, and the terminal voltage at the voltage collection end is equal to the terminal voltage V of a1 and a2, through the above calculation, it can be calculated that the thermocouple resistance R0=terminal voltage V÷constant current source current I, and the temperature value corresponding to the thermocouple can be obtained by looking up the table.

[0023] Preferably, according to the virtual-off principle of the amplifier, the voltage at point a is equal to Vi, and according to the virtual-off principle of the amplifier, the voltage at point a is equal to the voltage at point b, that is, the voltage at point b is equal to Vi, the current flowing through R7 is Vi / R7, and according to the loop characteristics of the amplifier.

[0024] Preferably, the output voltage at point d of the output end of the amplifier is V=Vi+Vg, and the switch tube Q1 is in the amplification area or the saturation area. According to the working characteristics of the switch tube and the amplifier, the current at point b=amplifier (Vout-Vd) / R6+I(switch tube), Vout≈Vd, and the current at point b=I(switch tube), the current flowing through the switch tube Q1 is equal to the output current of the controlled constant current source, the DAC output end, i.e., Vi, is fixed, and the current flowing through R7 is constant.

[0025] Compared with the related art, the high-precision measurement circuit of the temperature sensor used in the heat meter provided by the utility model has the following beneficial effects:

[0026] 1. A high-precision measurement circuit for a temperature sensor used in a heat meter. Through circuit design, the resistance value of the thermocouple is accurately tested, the interference of line resistance and contact resistance is reduced, and the test accuracy is greatly improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the series resistance voltage division test method of the utility model;

[0028] Figure 2 It is a schematic diagram of the circuit topology of the utility model;

[0029] Figure 3 This is a schematic diagram of the variable constant current source process of the utility model;

[0030] Figure 4 The topological diagram of the circuit design of the constant current source of the utility model is shown. DETAILED DESCRIPTION

[0031] The following will be combined with the drawings in the embodiments of the utility model to clearly and completely describe the technical solutions in the embodiments of the utility model. Obviously, the described embodiments are only part of the embodiments of the utility model, not all of the embodiments. Based on the embodiments in the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.

[0032] See also Figures 1 to 4 The embodiment of the utility model provides a high-precision temperature sensor measurement circuit for a heat meter, comprising:

[0033] R1, R2, are the contact resistance and line resistance of the thermocouple on the line;

[0034] R3, R4 are the line resistances from the voltage collector to both ends of the thermocouple;

[0035] R5 is the internal resistance of the voltage collector;

[0036] The DAC output is a controlled voltage output, and the value of Vi determines the size of the back-end controlled constant current source;

[0037] R5 / R6 are protective series resistors;

[0038] R7 is the sampling resistor;

[0039] Q1 is a switch NMOS tube;

[0040] The variable constant current source is a constant current output power source controlled by the MCU, and the constant current source flows out to the load circuit through R1 and R2.

[0041] According to the quantification of Thevenin current, the current is divided at point a1, part of which flows to a2 through thermocouple R0, and the other part flows to a2 through R3, b1, R5, b2, R4. The thermocouple used in the heat meter is a PT500 or PT1000 thermocouple, and the temperature range is between 4-100°C depending on the usage scenario. By looking up the table, it can be seen that the resistance range of the thermocouple in the working state is between 1KΩ-1.5KΩ. R5 is the internal resistance of the voltage collector, and the internal resistance value is >1M ohm.

[0042] According to the comparison between the resistance value of thermocouple R0 and the internal resistance value of R5, which is equivalent to R5 being open-circuited, the circuit is regarded as the current flowing back to the constant current source input terminal through a1, R0, and a2. The terminal voltage across R0 calculated by the equivalent circuit is equal to the constant current source current I*thermocouple R0, that is, the terminal voltage V between a1 and a2 is equal to I*R0.

[0043] According to Kirchhoff's voltage law, the voltage V0 at the voltage acquisition end is V(R3)+V(R0)+V(R4), and because the loop current flowing through R3 / R0 / R4 is close to 0, V(R3) and V(R4) are close to 0, and the terminal voltage at the voltage acquisition end is equal to the terminal voltage V of a1 and a2. Through the above calculation, it can be calculated that the thermocouple resistance R0 = terminal voltage V ÷ constant current source current I, and the temperature value corresponding to the thermocouple can be obtained by looking up the table;

[0044] Kirchhoff's voltage law: The law states that in any closed loop, the algebraic sum of the voltage drops on each component is equal to the algebraic sum of the electromotive force, that is, when starting from a point and going around the loop and returning to the point, the algebraic sum of the voltages of each segment is always equal to zero, that is, ∑U=0. Kirchhoff's voltage law shows that for any closed loop of the circuit, the algebraic sum of the voltage drops across each component (which can be a resistor, capacitor, inductor, etc.) is equal to zero. In other words, the sum of the potential increases must be equal to the sum of the potential decreases when going around the closed loop. This law is not only applicable to closed loops, but can also be extended to part of the loop. Kirchhoff's voltage law is a law that determines the relationship between the voltages in any loop in the circuit, so it is also called the loop voltage law. It is one of the basic laws in circuit analysis, and together with Kirchhoff's current law (KCL), it constitutes the basis of circuit analysis. These two laws are the embodiment of the law of conservation of charge in circuits. They are used together to solve problems in complex circuits. In addition, Kirchhoff's voltage law can also be used to calculate the current value of each branch in the circuit. When Kirchhoff's first (Kirchhoff's current law) and second (Kirchhoff's voltage law) equations are used together, the current values ​​of each branch in the circuit can be calculated accurately and quickly. The application scope of Kirchhoff's law can also be extended to AC circuits. It can not only be used for the analysis of DC circuits and quasi-stable circuits, but also for the analysis of nonlinear circuits containing electronic components.

[0045] According to the virtual break principle of the amplifier, the voltage at point a is equal to Vi, and according to the virtual short principle of the amplifier, the voltage at point a is equal to the voltage at point b, that is, the voltage at point b is equal to Vi, the current flowing through R7 is Vi / R7, and according to the loop characteristics of the amplifier;

[0046] The virtual-break principle of the amplifier: refers to the fact that when analyzing the linear state of the operational amplifier, the two input terminals can be regarded as equivalent to open circuits. This feature is called a false open circuit, or virtual break for short. Since the differential input resistance of the operational amplifier is very large, the input resistance of general-purpose operational amplifiers is generally above 1MΩ. Therefore, the current flowing into the input terminal of the operational amplifier is often less than 1uA, which is much smaller than the current of the circuit outside the input terminal. Therefore, the two input terminals of the operational amplifier can usually be regarded as open circuits, and the larger the input resistance, the closer the two input terminals are to open circuits. The virtual-break principle is one of the important characteristics of the amplifier. It allows the input terminals to be regarded as open circuits when analyzing the amplifier circuit, thereby simplifying the circuit analysis. At the same time, the virtual-break principle is also one of the factors that need to be considered in the design of the amplifier circuit. It is necessary to ensure that the input resistance is large enough to meet the virtual-break condition, so as to obtain better circuit performance. It should be noted that the virtual-break principle is only applicable to the case where the amplifier is in a linear state. When the amplifier is in a nonlinear state, the virtual-break principle is no longer applicable, and other methods are needed to analyze the circuit. In addition, the virtual-break principle is also not applicable to the case where there is a large signal at the input terminal of the amplifier, because the large signal will cause the input resistance to decrease, so that the virtual-break condition no longer holds.

[0047] The output voltage at the output end of the amplifier is V = Vi + Vg, and the switch tube Q1 is in the amplification area or saturation area. According to the working characteristics of the switch tube and the amplifier, the current at point b = amplifier (Vout-Vd) / R6 + I (switch tube), Vout ≈ Vd, and the current at point b = I (switch tube). The current flowing through the Q1 switch tube is equal to the output current of the controlled constant current source. The DAC output end, i.e., Vi, is fixed, and the current flowing through R7 is constant.

[0048] Switching diodes and amplifiers: Switching diodes and amplifiers are two important components in electronic circuits. They each have unique working characteristics. Switching diodes are a special type of diode. Their main characteristics are fast switching speed, small size, long life and high reliability. The time from the switching diode from cutoff (high resistance state) to conduction (low resistance state) is called the turn-on time, and the time from conduction to cutoff is called the reverse recovery time. The sum of these two times is called the switching time. Generally speaking, the reverse recovery time is greater than the turn-on time, so only the reverse recovery time is usually given in the use parameters of the switching diode. The switching speed of the switching diode is very fast. For example, the reverse recovery time of the silicon switching diode is only a few nanoseconds, and even the reverse recovery time of the germanium switching diode is only a few hundred nanoseconds. Therefore, switching diodes are widely used in switching circuits, detection circuits, high-frequency and pulse rectification circuits, and automatic control circuits of electronic equipment. The amplifier is an electronic component that can amplify the input signal, and its core component is the output transistor. The output transistor has a high current amplification capability. When the input signal is applied to the amplifier circuit, the output transistor is responsible for amplifying the signal. Amplifiers can be divided into two categories: linear amplifiers and digital amplifiers. Linear amplifiers can achieve linear amplification of signals and are divided into Class A, Class B, and Class AB according to different working areas. Digital amplifiers use switching technology, which is more efficient and smaller than linear amplifiers, and are suitable for compact audio power amplifiers. Amplifiers have many important characteristics, such as high power, low distortion, high efficiency, wide application range, and easy control.

[0049] When working, the working test principle of the circuit is:

[0050] The constant current source flows out to the load circuit through R1 and R2. According to the quantification of the Thevenin current, the current is divided at point a1, part of which flows to a2 through the thermocouple R0, and the other part flows to a2 through R3, b1, R5, b2, and R4. The thermocouple used in the heat meter is a PT500 or PT1000 thermocouple. Depending on the usage scenario, the temperature range is between 4 and 100 °C. By looking up the table, it can be seen that the resistance range of the thermocouple in the working state is between 1KΩ and 1.5KΩ. R5 is the internal resistance of the voltage collector, and the internal resistance value is greater than 1M ohms. According to the comparison between the resistance value of the thermocouple R0 and the internal resistance value of R5, it is equivalent to R5 being open. Therefore, the circuit can be regarded as the current flowing back to the constant current source input through a1, R0, and a2, such as Figure 2, the terminal voltage at both ends of R0 calculated by the equivalent circuit is equal to the constant current source current I*thermocouple R0, that is, the terminal voltage V of a1 and a2 is equal to I*R0. According to Kirchhoff's voltage law, the voltage V0 at the voltage acquisition end is equal to V(R3)+V(R0)+V(R4). Since the loop current flowing through R3 / R0 / R4 is close to 0, V(R3) and V(R4) are close to 0. The terminal voltage at the voltage acquisition end is equal to the terminal voltage V of a1 and a2. Through the above calculation, it can be calculated that the thermocouple resistance R0 is equal to the terminal voltage V÷constant current source current I, and then the temperature value corresponding to the thermocouple is obtained by looking up the table;

[0051] Working principle of constant current source:

[0052] According to the virtual open principle of the amplifier, the voltage at point a is equal to Vi. According to the virtual short principle of the amplifier, the voltage at point a is equal to the voltage at point b, that is, the voltage at point b is equal to Vi. Therefore, the current flowing through R7 is Vi / R7. According to the loop characteristics of the amplifier, the output voltage at the output end of the amplifier is V=Vi+Vg, and the switch tube Q1 is in the amplification area or saturation area. According to the working characteristics of the switch tube and the amplifier, the current at point b is equal to the amplifier (Vout-Vd) / R6+I(switch tube), Vout≈Vd, so the current at point b is equal to I(switch tube). Therefore, the current flowing through the Q1 switch tube is equal to the output current of the controlled constant current source, because the DAC output end, that is, Vi, is fixed, that is, the current flowing through R7 is constant.

[0053] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0054] Although the embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A high-precision temperature sensor measurement circuit for a heat meter, characterized in that: include: R1, R2, are the contact resistance and line resistance of the thermocouple on the line; R3, R4 are the line resistances from the voltage collector to both ends of the thermocouple; R5 is the internal resistance of the voltage collector; The DAC output is a controlled voltage output, and the value of Vi determines the size of the back-end controlled constant current source; R5 / R6 are protective series resistors; R7 is the sampling resistor; Q1 is a switch NMOS tube; The variable constant current source is a constant current output power source controlled by the MCU, and the constant current source flows out to the load circuit through R1 and R2.

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