Thermistor three-wire dual constant current source measurement circuit and temperature and time drift error correction method

Abstract

The application relates to a thermistor three-wire double constant current source measurement circuit and a temperature and time drift error correction method. The circuit comprises a thermistor, the thermistor and a line resistance are connected in series to form a first resistance branch, the line resistance is a second resistance branch, the line resistance is a third resistance branch, A1 and B1 are connected to a positive branch and a negative branch through a double switch respectively, and the positive branch and the negative branch are connected to the positive and negative poles of an analog / digital converter; one end of a resistor and the other end of the resistor are connected to the positive branch and the negative branch through a double switch; a constant current source is connected to the positive branch, and a constant current source is connected to the negative branch. Compared with the prior art, the application has the advantages of eliminating constant current source error, improving temperature measurement accuracy and the like.

Description

Technical Field

[0001] This invention relates to the field of temperature measurement technology in industrial production process control, and in particular to a three-wire dual constant current source measurement circuit for resistance temperature detectors (RTDs) and a method for correcting temperature and time drift errors. Background Technology

[0002] In industrial process control, resistance temperature detectors (RTDs) are the most common temperature sensors in the medium and low temperature range. Due to their high accuracy and stable performance, especially platinum resistance temperature detectors (Pt100), they are widely used in industrial temperature measurement. RTDs measure temperature based on the thermal effect of resistance, converting temperature changes into changes in resistance. Therefore, by measuring the resistance change of the RTD, the corresponding temperature can be measured. Generally, RTD measurement involves applying a known excitation current (constant current source) to the RTD and measuring the voltage across its terminals to obtain the resistance value, which is then converted into a temperature value. Because the constant current source method has strong anti-interference capabilities in industrial environments, it is widely used. To automatically compensate for errors caused by wire resistance and improve measurement accuracy, while also considering the cost of signal cable laying, RTD measurement typically employs a three-wire dual constant current source method.

[0003] Figure 2 The diagram shows a typical three-wire dual constant current source measurement circuit for resistance temperature detectors (RTDs). Its working principle is as follows: Constant current source I1 flows in from terminal A, through the line resistor R... LA Resistance temperature detector (RTD) R t Line resistance R Lc The current flows out from terminal C, then through resistor R0 back to reference ground GND; the constant current source I2 flows in from terminal B, through line resistor R LB Line resistance R Lc The current flows out from terminal C, then through resistor R0 back to reference ground GND; according to:

[0004] U AB =U A -U B =[R LA *I1+R t *I1+(R LC +R0)*(I1+I2)]-[R LB *I2+(R LC +R0)*(I1+I2)],

[0005] Since the two constant current sources are equal, i.e., I1 = I2, the connecting wires use the same multi-core cable, i.e., R. LA =R LB =R LC Therefore, U AB =R t*I1. Therefore, the three-wire dual constant current source measurement method of RTD can automatically compensate for the measurement error caused by the resistance of the measurement circuit wires. However, the dual constant current sources may drift with changes in operating temperature and time, and the temperature and time drift of the constant current sources will also introduce measurement errors, resulting in inaccurate measurement results. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects of the prior art by providing a three-wire dual constant current source measurement circuit for resistance temperature detectors (RTDs) and a method for correcting temperature and time drift errors. The measured RTD resistance value of the circuit is independent of the constant current source, thereby eliminating the measurement error caused by the temperature and time drift of the constant current source and improving the measurement accuracy.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] This is a three-wire dual constant current source measurement circuit for resistance temperature detectors (RTDs). The circuit includes an RTD, a line resistor connected in series to form a first resistance branch, a second resistance branch, and a third resistance branch. One end of each of these branches is connected. The other end of the first, second, and third branches is designated as terminal A1, terminal B1, and terminal C, respectively. Terminal C is connected to one end of a resistor, and the other end is connected to ground. Terminals A1 and B1 are connected to the positive and negative branches via a dual-switch, respectively, and are then connected to the positive and negative terminals of an analog-to-digital converter (ADC). One end of the resistor is connected to another resistor, and the other end is connected to the positive and negative branches via the same dual-switch. A constant current source is connected to both the positive and negative branches. The ADC is sequentially connected to an optical isolator and an MCU (Microcontroller Unit).

[0009] The dual-channel switch and its state are set to two states: dual-channel switch closed, dual-channel switch open, and dual-channel switch open and dual-channel switch closed. In the first state, the MCU microprocessor measures the voltage across the thermal resistor, and in the second state, the MCU microprocessor measures the voltage across the resistor. The resistance value of the thermal resistor is determined based on the voltage across the thermal resistor and the voltage across the resistor.

[0010] Furthermore, the resistance is a 300Ω manganese copper wire-wound resistor with a temperature coefficient of 5ppm.

[0011] Furthermore, the expression for the resistance value of the thermal resistor is: R t =300*V Rt / V RG , where R t V is the resistance value of the thermal resistor. Rt V is the voltage across the resistance temperature detector (RTD). RG This is the voltage across the resistor.

[0012] Furthermore, the resistance is greater than or equal to 100Ω.

[0013] Furthermore, the analog-to-digital converter includes a differential operational amplifier and an AD converter. The positive and negative branches are connected to the positive and negative input terminals of the differential operational amplifier, respectively. The output terminal of the differential operational amplifier is connected to the input terminal of the AD converter, and the output terminal of the AD converter is connected to optical isolation.

[0014] The present invention also provides a method for correcting temperature drift error, using the above-mentioned circuit, and the method includes the following steps:

[0015] S1. Perform circuit self-calibration by connecting calibration resistors to terminals A1, B1, and C, and controlling the dual-channel switch to close and open to obtain the voltage V across the calibration resistors. Ex The system controls the opening and closing of the dual-channel switch and samples the voltage V across the resistor. In The two voltages obtained are then stored in the MCU microprocessor.

[0016] S2. During circuit operation, the dual-way switch is controlled to close and open, and the MCU microprocessor samples the voltage V across the thermal resistor. Rt The control switches open and close, and the voltage V across the resistor is sampled. RG ;

[0017] S3, the voltage V across the self-calibrating resistor of the MCU microprocessor. In and the voltage V across the resistor during operation RG Calculate the temperature drift coefficient;

[0018] S4, the MCU microprocessor calculates the temperature drift coefficient and the voltage V across the thermal resistor. Rt and the voltage V across the calibrated terminals Ex Calculate the resistance of the thermal resistor.

[0019] Furthermore, the calibration resistors connected to terminals A1, B1, and C have a resistance of 300Ω.

[0020] Furthermore, the expression for the temperature drift coefficient is: β=(V RG -V In ) / V In , where V In V is the voltage across the resistor during circuit self-calibration. RG This is the voltage across the resistor when the circuit is running.

[0021] Furthermore, when the calibration resistor has a resistance of 300Ω, the expression for the resistance of the thermal resistor obtained based on the temperature drift coefficient is: R t =300*V Rt / [(1+β)*V Ex ], where β is the temperature drift coefficient, VRt The thermal resistance (R) during circuit operation t Voltage across the terminals, V Ex This is the voltage across the calibration resistor connected during circuit self-calibration.

[0022] Furthermore, the calibration resistor is disconnected from terminals A1, B1, and C after the circuit self-calibration is completed.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) Using the circuit of the present invention, when the dual-channel switch SW1 is closed and SW2 is open, the thermal resistance R is measured. t Voltage value V at both ends Rt =G*R t *I1, When the dual-way switch SW1 is open and SW2 is closed, measure the resistance R. G Voltage value V at both ends RG =G*R G *I1=G*300*I1, where I1 is the current of the constant current source, G is the differential operational amplifier gain of the analog-to-digital converter U1, and the measured value R of the thermal resistor is calculated at this time. t =300*V Rt / V RG The measured resistance value is independent of the constant current source, thus eliminating the measurement error caused by the temperature drift of the constant current source and improving the measurement accuracy.

[0025] (2) The temperature drift error correction method of the present invention introduces a resistor R. G The temperature drift coefficient is determined using the same circuit, i.e., resistance R. G The sampling value processing data of the circuit uses only the relative change coefficient of one circuit to correct the error caused by the change of the other circuit. Because they are two similar measurement circuits, the absolute measurement value changes are generally different, but the relative change value coefficients are the same. Compared with the existing methods that use two sampling values ​​from different circuits to process data, the error correction method of this invention is more in line with the high precision requirements of real-time measurement of thermal resistance in industry and improves the accuracy of temperature measurement. Attached Figure Description

[0026] Figure 1 This is a circuit diagram of the present invention;

[0027] Figure 2 For existing three-wire dual constant current source measurement circuits for resistance temperature detectors (RTDs);

[0028] Figure 3 This is a flowchart of the error elimination method of the present invention;

[0029] Among them, the thermal resistance R t Line resistance R LA Line resistance RLB Line resistance R Lc Dual-channel switch SW1, resistor R G Dual-channel switch SW2, constant current source I1, constant current source I2, analog-to-digital converter U1, opto-isolation U2, MCU microprocessor U3, resistor R0. Detailed Implementation

[0030] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0031] This invention proposes a three-wire dual constant current source measurement circuit for resistance temperature detectors (RTDs), the circuit diagram of which is shown below. Figure 1 As shown. The circuit includes a thermal resistor R. t thermal resistance R t and line resistance R LA The first resistive branch is formed by connecting them in series, and the line resistance is R. LB This is the second resistor branch, with a line resistance R. Lc The third resistor branch connects one end of the first, second, and third resistor branches. The other end of the first resistor branch is designated as terminal A1, the other end of the second resistor branch as terminal B1, and the other end of the third resistor branch as terminal C. Terminal C is connected to one end of resistor R0, and the other end of resistor R0 is connected to reference ground GND. Terminals A1 and B1 are connected to the positive and negative branches respectively via a two-way switch SW1. The positive and negative branches are connected to the positive and negative terminals of the analog-to-digital converter U1 respectively. One end of resistor R0 is connected to resistor R... G One end, resistor R G One end and resistor R G The other end is connected to the positive branch and the negative branch respectively via a dual-channel switch SW2; a constant current source I1 is connected to the positive branch and a constant current source I2 is connected to the negative branch; the analog-to-digital converter U1 is connected to the opto-isolation U2 and the MCU microprocessor U3 in sequence.

[0032] In some embodiments, the resistor R G Set to a high-precision, 300Ω manganese copper wire-wound resistor with a temperature coefficient of 5ppm. The resistance R0 is greater than or equal to 100Ω.

[0033] The circuit of this invention is used to measure the resistance temperature R. t The process of determining the resistance value is as follows:

[0034] With the dual-circuit switch SW1 closed and SW2 open, the constant current source I1 flows in from terminal A1, passing through the line resistor R. LA Resistance temperature detector (RTD) R t Line resistance R Lc It flows out from terminal C, then through resistor R0 back to reference ground GND.

[0035] The constant current source I2 flows in from terminal B1, through the line resistor R. LB Line resistance R Lc It flows out from terminal C, then through resistor R0 back to reference ground GND.

[0036] According to the formula:

[0037] U A1B1 =U A1 -U B1 =[R LA *I1+R t *I1+(R LC +R0)*(I1+I2)]-[R LB *I1+(R LC +R0)*(I1+I2)],

[0038] Since the two constant current sources are equal, i.e., I1 = I2, the connecting wires use the same multi-core cable, i.e., R. LA =R LB =R LC Therefore, U A1B1 =R t *I1.

[0039] The differential voltage U across A1 and B1 A1B1 The analog-to-digital converter U1 converts the signal into a digital value, which is then transmitted via optical isolation U2 to the MCU microprocessor U3, resulting in a signal equivalent to a thermal resistor R. t Voltage value V at both ends Rt V Rt =G*R t *I1 (G is the differential operational amplifier gain of analog-to-digital converter U1).

[0040] When the dual-switch SW1 is open and SW2 is closed, the circuit becomes a resistor measuring circuit R. G (A high-precision 300Ω manganese copper wire-wound resistor with a temperature coefficient of 5ppm (1 / ℃) is selected for the circuit.) A high-precision 300Ω manganese copper wire-wound resistor R with a temperature coefficient of 5ppm (1 / ℃) is used. G The purpose is to reduce measurement errors caused by changes in the resistance of the reference resistor. Generally, temperature measuring devices operate in environments between -30℃ and 85℃, using 25℃ as the reference temperature. The maximum temperature variation is approximately 60℃. At this temperature, the maximum deviation affecting a 300Ω resistor is 5.0*10⁻⁶. -6 *60 = 0.03%. Similarly, we can obtain:

[0041] V RG =G*R G *I1=G*300*I1.

[0042] From formula V Rt =G*R t *I1 and V RG =G*300*I1, to obtain the measured value R of the thermal resistance. t =300*V Rt / V RG .

[0043] Thus, we obtain the measurement result R, which is independent of the constant current source. t =300*V Rt / V RG This eliminates the measurement error caused by the temperature drift of the constant current source.

[0044] In this invention, the function of resistor R0 is to increase the common-mode bias voltage of the differential input of analog-to-digital converter U1. Generally, the value of resistor R0 is greater than or equal to 100Ω.

[0045] By using the above-mentioned three-wire dual constant current source measurement circuit for resistance temperature detectors (RTDs), the measurement error caused by the temperature-time drift of the constant current source can be eliminated.

[0046] Based on the above circuit, this invention also proposes a method for correcting temperature drift error, the flowchart of which is shown below. Figure 3 As shown. The method includes the following steps:

[0047] S1. Perform circuit self-calibration by connecting a high-precision calibration resistor to terminals A1, B1, and C, and controlling the dual-channel switch SW1 to close and SW2 to open, obtaining the voltage V across the calibration resistor. Ex The dual-channel switch SW1 is opened and SW2 is closed, and the resistance R is sampled. G Voltage V across the terminals In The voltages across the two resistors are then stored in the MCU microprocessor U3.

[0048] S2. During circuit operation, the dual-channel switch SW1 is closed and SW2 is open, and the MCU microprocessor U3 samples the thermal resistance R. t Voltage V at both ends Rt The dual-channel switch SW1 is opened and SW2 is closed, and the resistance R is sampled. G Voltage V at both ends RG ;

[0049] S3, MCU microprocessor U3 is based on self-calibrating time resistor R G Voltage V across the terminals In and operating resistance R G Voltage V at both ends RG Calculate the temperature drift coefficient;

[0050] S4, MCU microprocessor U3, based on temperature drift coefficient and thermal resistance R t Voltage V at both ends Rtand the voltage V across the resistor Ex Calculate the thermal resistance R t The resistance value.

[0051] In S1, during the self-calibration of the measurement circuit, high-precision 300Ω resistors (generally precision resistance boxes) can be connected to terminals A1, B1, and C to sample and obtain the voltage V across them. ExR300 And sampled to obtain the internal resistance R G The voltage V across (300Ω) InR300 The two measured values ​​are saved to the EEPROM memory of the MCU microprocessor U3. Then, S2 is executed. When the measurement circuit is running, the resistance R is sampled and obtained respectively. t and R G Voltage V at both ends Rt and V RG Then execute S3 to process R. G The coefficient of variation β of the measurement circuit due to temperature drift is β=(V RG -V InR300 ) / V InR300 Finally, execute S4 to process the thermal resistor R. t Measured value: R t =300*V Rt / [(1+β)*V ExR300 ].

[0052] The calibration sampling code for the external and internal 300Ω resistance of the measuring device is used as CV. Ex_R300 =16310, CV In_R300 =16300 and R t Taking a resistance of 150Ω as an example (in actual measurement, the sampling code of analog-to-digital converter U1 is generally used; the sampling codes for external and internal 300Ω resistors are different due to differences between the two measurement circuits), the error correction method is analyzed as follows using the traditional method:

[0053] a. When the measuring circuit first starts working, the thermal resistance R... t The sampling code CV obtained by analog-to-digital converter U1 Rt The value is 8155, and the internal reference resistor R of the device is... G The sampling code CV obtained by analog-to-digital converter U1 RG The value is 16300. The result obtained using the traditional method is R. t =300*CV Rt / CV RG =300*8155 / 16300≈150.09,

[0054] The error is: |150.09-150| / 150≈0.06%;

[0055] b. After the measuring circuit has been running for a long time, the thermal resistance R... t The sampling code CV obtained by analog-to-digital converter U1 Rt The value is 8255, and the internal reference resistor R in the circuit is... G The sampling code CV obtained by analog-to-digital converter U1 RG The value is 16500. The result obtained using the traditional method is R. t =300*CV Rt / CV RG =300*8255 / 16500≈150.09,

[0056] The error is: |150.09-150| / 150≈0.06%.

[0057] If the thermal resistance R t If the type is Pt100, then 150Ω corresponds to a temperature of 130.5℃, and 150.09Ω corresponds to a temperature of 130.7℃. The temperature error is |130.7-130.5| / 130.5≈0.15%.

[0058] After the measurement circuit has been running for a long time, the measurement error remains basically unchanged, which is why the traditional processing method has been adopted.

[0059] The analysis process of the temperature drift error correction method proposed in this invention is as follows:

[0060] a. When the measuring circuit first starts working

[0061] β=(CV RG -CV InR300 ) / CV InR300 = (16300-16300) / 16300 = 0;

[0062] R t =300*CV Rt / [(1+β)*CV ExR300 ]=300*8155 / [(1+0)*16310]=150,

[0063] b. After the measuring device has been running for a long time

[0064] Processing the reference resistor R G The coefficient of variation due to temperature drift

[0065] β=(CV RG -CV InR300 ) / CV InR300 = (16500-16300) / 16300≈0.01227;

[0066] Processing the thermal resistor Rt Measured values

[0067] R t =300*CV Rt / [(1+β)*CV ExR300 = 300 * 8255 / [(1 + 0.01227) * 16310] ≈ 149.999

[0068] The error is: |149.999-150| / 150≈0.0007%<<0.06%.

[0069] Clearly, the measurement error obtained by the correction method of this application for the same sampled code data is much smaller than that of the traditional processing method. This is because the traditional method uses two sampled values ​​from different loops to process the data, while the present invention uses sampled values ​​from the same loop to process the data. It only uses the relative change coefficient of one loop to correct the error caused by the change in the other loop, because the absolute changes in the measured values ​​of two similar measurement loops are generally different, but the relative change coefficients are the same.

[0070] Therefore, the method of this invention can achieve the high-precision requirements of real-time measurement of resistance temperature detectors (RTDs) in industry, while also improving the accuracy of the RTD. t The measurement error caused by temperature drift in the measurement circuit has been corrected.

[0071] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for correcting temperature-time drift error based on a three-wire dual constant current source measurement circuit for resistance temperature detectors (RTDs), wherein the three-wire dual constant current source measurement circuit is used to measure the resistance temperature detector (RTD) R. t The resistance value is characterized by, The circuit includes a thermal resistor R t thermal resistance R t and line resistance R LA The first resistive branch is formed by connecting them in series, and the line resistance is R. LB This is the second resistor branch, with a line resistance R. Lc The third resistor branch connects one end of the first, second, and third resistor branches. The other end of the first resistor branch is designated as terminal A1, the other end of the second resistor branch as terminal B1, and the other end of the third resistor branch as terminal C. Terminal C is connected to one end of resistor R0, and the other end of resistor R0 is connected to reference ground. Terminals A1 and B1 are connected to the positive and negative branches respectively via a two-way switch SW1. The positive and negative branches are connected to the positive and negative terminals of the analog-to-digital converter U1 respectively. One end of resistor R0 is connected to resistor R... G One end, resistor R G One end and resistor R G The other end is connected to the positive branch and the negative branch respectively via a dual-channel switch SW2; a constant current source I1 is connected to the positive branch and a constant current source I2 is connected to the negative branch; the analog-to-digital converter U1 is connected to the optical isolation U2 and the MCU microprocessor U3 in sequence; The states of dual-channel switches SW1 and SW2 are set to two states: dual-channel switch SW1 closed and dual-channel switch SW2 open, and dual-channel switch SW1 open and dual-channel switch SW2 closed. In the first state, the MCU microprocessor U3 measures the thermal resistance R. t The voltage across the two ends; in the second state, the MCU microprocessor U3 measures the resistance R. G The voltage across the two ends, the thermal resistor R t The resistance value is based on the thermal resistor R. t Voltage and resistance R at both ends G The voltage at both ends is determined; The method includes the following steps: S1. Perform circuit self-calibration by connecting a calibration resistor to terminals A1, B1, and C, and controlling the dual-channel switch SW1 to close and SW2 to open, obtaining the voltage V across the calibration resistor. Ex The dual-channel switch SW1 is opened and SW2 is closed, and the resistance R is sampled. G The voltage V across the terminals In The two voltages obtained are stored in the MCU microprocessor U3; S2. During circuit operation, the dual-channel switch SW1 is closed and SW2 is open, and the MCU microprocessor U3 samples the thermal resistance R. t Voltage V at both ends Rt The dual-channel switch SW1 is opened and SW2 is closed, and the resistance R is sampled. G Voltage V at both ends RG ; S3, MCU microprocessor U3 is based on self-calibrating time resistor R G The voltage V across the terminals In and operating resistance R G Voltage V at both ends RG Calculate the temperature drift coefficient; S4, MCU microprocessor U3, based on temperature drift coefficient and thermal resistance R t Voltage V at both ends Rt and the voltage V across the calibrated terminals Ex Calculate the thermal resistance R t The resistance value.

2. The method according to claim 1, characterized in that, resistor R G It is a 300Ω manganese copper wire-wound resistor with a temperature coefficient of 5ppm.

3. The method according to claim 2, characterized in that, The thermal resistor R t The expression for the resistance value is: R t =300*V Rt / V RG , where R t V is the resistance value of the thermal resistor. Rt For thermal resistance R t Voltage across terminals, V RG For resistor R G Voltage at both ends.

4. The method according to claim 1, characterized in that, The resistance R0 is greater than or equal to 100Ω.

5. The method according to claim 1, characterized in that, The analog-to-digital converter U1 includes a differential operational amplifier and an AD converter. The positive and negative branches are connected to the positive and negative input terminals of the differential operational amplifier, respectively. The output terminal of the differential operational amplifier is connected to the input terminal of the AD converter, and the output terminal of the AD converter is connected to the optical isolation U2.

6. The method according to claim 1, characterized in that, The calibration resistor connected to terminals A1, B1, and C has a resistance of 300Ω.

7. The method according to claim 1, characterized in that, The expression for the temperature drift coefficient is: β = (V RG - V In ) / V In , where V In The resistor R during circuit self-calibration G The voltage across the two ends, V RG The resistance R during circuit operation G Voltage at both ends.

8. The method according to claim 7, characterized in that, When the resistance of the calibration resistor is 300Ω, the thermal resistance R obtained based on the temperature drift coefficient is... t The expression for the resistance value is: R t =300* V Rt / [(1+β)*V Ex ], where β is the temperature drift coefficient, V Rt The thermal resistance R during circuit operation t Voltage across terminals, V Ex This is the voltage across the calibration resistor connected during circuit self-calibration.

9. The method according to claim 1, characterized in that, After the circuit self-calibration is completed, disconnect the calibration resistor from terminals A1, B1, and C.

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