A temperature measurement system and method based on a Wheatstone bridge controlled by a directional excitation

Abstract

The application relates to a temperature measurement system and method based on directional excitation control of a Wheatstone bridge, and belongs to the technical field of precise measurement of sensor signals. The application solves the problem of temperature measurement deviation caused by the output nonlinearity of a traditional temperature measurement circuit based on a Wheatstone bridge, non-synchronous sampling error and output offset voltage of a differential amplifier circuit. The application relates to a bridge arm resistance value optimization strategy for suppressing output nonlinearity, reducing the complexity of a temperature measurement value calculation model and improving the reliability of precise temperature measurement. Based on synchronous sampling of an analog-to-digital converter and a directional excitation control method, non-synchronous sampling error caused by different sampling times is eliminated, measurement deviation caused by the inherent output offset voltage of a differential amplifier is offset, and through optimization design of the parameters of the precise temperature measurement method, the output nonlinearity of the temperature measurement circuit can be well suppressed, the non-synchronous sampling error and the measurement deviation are eliminated. The method can be applied to the field of temperature measurement based on a Wheatstone bridge.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sensor signal precision measurement, and particularly relates to a temperature measurement system and method based on directional excitation control of a Wheatstone bridge. BACKGROUND

[0002] Temperature is a physical parameter representing the hot and cold state of an object, and its level directly affects the characteristics of the object, such as water, air, metal, etc. In various fields of national production, temperature plays an important role. In chemical production, distillation methods rely on different temperatures to separate complex mixtures and obtain production materials of different purity, different composition and different types; in smart agriculture, unmanned aerial vehicles use infrared cameras to monitor the surface temperature of the planting area in real time, and reasonably irrigate according to the planting needs of different types of crops; in industrial production, the intelligent control system of the storage tank monitors the temperature of the industrial storage tank in time to indicate potential safety hazards and avoid dangerous accidents. Temperature plays a key role in production practice, and even a decisive role in some special application occasions, therefore, accurate temperature measurement has been an important research direction in the field of instruments and meters.

[0003] The platinum resistance PT1000 has the advantages of high precision, large range and good repeatability, which lays a good foundation for the detection of weak changes in target temperature, and the weak signal detection method based on platinum resistance is widely used in precision temperature measurement methods. Therefore, the key to the accuracy and reliability of temperature measurement lies in the measurement method and the elimination of inherent errors. The Wheatstone bridge is good at detecting weak resistance changes, but the nonlinearity of the temperature measurement circuit output will increase the complexity of the temperature measurement value calculation model and reduce the temperature measurement reliability; the constant voltage source output voltage for exciting the Wheatstone bridge has fluctuations, which will cause the output sampling time and the excitation time of the Wheatstone bridge to be out of sync and introduce non-synchronous sampling errors; the output of the temperature measurement bridge needs to be further conditioned by the differential amplification circuit, and the output offset voltage will lose part of the input signal and cause measurement deviation. Therefore, suppressing the nonlinearity of the temperature measurement circuit output, eliminating non-synchronous sampling errors and measurement deviation are important ways to improve the accuracy and reliability of precision temperature measurement.

[0004] Therefore, a high-precision temperature measurement system based on directional excitation control of a Wheatstone bridge is proposed, which specifically eliminates the influence of key factors that reduce the accuracy of weak signal sampling, thereby improving the accuracy and reliability of precision temperature measurement, and further simplifying the complexity of the temperature measurement circuit. This is of great significance for expanding the application of precision temperature measurement methods in the field of precision temperature measurement. SUMMARY

[0005] The application aims to solve the problem of temperature measurement deviation caused by the output nonlinearity of the traditional Wheatstone bridge-based temperature measurement circuit, the non-synchronous sampling error and the output offset voltage of the differential amplifier circuit, and proposes a temperature measurement system and method based on the directional excitation control of the Wheatstone bridge to improve the accuracy and reliability of temperature measurement.

[0006] The technical solution adopted by the application to solve the above technical problems is:

[0007] Based on one aspect of the application, a temperature measurement system based on the directional excitation control of the Wheatstone bridge, the system comprises a Wheatstone bridge, a directional excitation control module, a differential amplifier, an analog-to-digital converter, a resistance calculation module, a temperature calculation module and a Butterworth digital filter; wherein:

[0008] The Wheatstone bridge comprises a constant voltage source, a first measurement bridge arm and a second measurement bridge arm, the first measurement bridge arm comprises resistors R1 and R3, and the second measurement bridge arm comprises resistors R2 and R4;

[0009] Among them, the resistors R1, R2 and R3 are fixed value resistors, and the resistor R4 is a platinum resistor PT1000;

[0010] The resistor R4 in the Wheatstone bridge is used to sense the target temperature;

[0011] The directional excitation control module is used to perform forward excitation and reverse excitation on the Wheatstone bridge;

[0012] The voltage at the reference point of the first measurement bridge arm and the voltage at the reference point of the second measurement bridge arm in the forward excitation mode are input to the differential amplifier, and the output voltage of the differential amplifier in the forward excitation mode is obtained;

[0013] The voltage at the reference point of the first measurement bridge arm and the voltage at the reference point of the second measurement bridge arm in the reverse excitation mode are input to the differential amplifier, and the output voltage of the differential amplifier in the reverse excitation mode is obtained;

[0014] The analog-to-digital converter is used to synchronously sample the constant voltage source voltage and the output voltage of the differential amplifier in the forward excitation mode, and is also used to synchronously sample the constant voltage source voltage and the output voltage of the differential amplifier in the reverse excitation mode;

[0015] The resistance calculation module is used to calculate the value of the resistor R4 according to the sampling results of the analog-to-digital converter;

[0016] The temperature calculation module is used to calculate the initial value T' of the target temperature according to the resistor R4;

[0017] The Butterworth digital filter is used to process the initial value T' of the target temperature calculated by the temperature calculation module to obtain the target temperature detection result.

[0018] Based on another aspect of the present application, a temperature measurement method for controlling a Wheatstone bridge based on directional excitation, the method specifically comprises the following steps:

[0019] Step one, using a platinum resistance PT1000 in the Wheatstone bridge as a temperature sensor to sense the target temperature;

[0020] Step two, using a constant voltage source DC to forward excite the Wheatstone bridge, taking the voltage of the reference point on the first measurement bridge arm and the voltage of the reference point on the second measurement bridge arm in the forward excitation mode as the input of a differential amplifier A F , and using an analog-to-digital converter to synchronously sample the constant voltage source voltage and the output voltage of the differential amplifier in the forward excitation mode, recording the sampled output of the differential amplifier A F as U Z (t), and recording the sampled constant voltage source voltage as U Z ;

[0021] Step three, using a constant voltage source DC to reverse excite the Wheatstone bridge, taking the voltage of the reference point on the first measurement bridge arm and the voltage of the reference point on the second measurement bridge arm in the reverse excitation mode as the input of a differential amplifier A F , and using an analog-to-digital converter to synchronously sample the constant voltage source voltage and the output voltage of the differential amplifier in the reverse excitation mode, recording the sampled output of the differential amplifier A F as U F (t), and recording the sampled constant voltage source voltage as U F ;

[0022] Step four, using the sampled U Z (t), U Z , U F (t) and U F to calculate the resistance R4 of the platinum resistance PT1000;

[0023] Step five, calculating the target temperature initial value T' according to the resistance R4 calculated in step four, and taking the calculated target temperature initial value as the target temperature initial value obtained by one sampling;

[0024] Step six, processing the target temperature initial value T' using a Butterworth digital filter to obtain a processed target temperature filtered value T'', i.e., taking the processed target temperature filtered value T'' as the target temperature filtered value obtained by one sampling;

[0025] Step seven, using the method of steps two to six to obtain the target temperature filtered value corresponding to each sampling, until the target temperature filtered value converges, and stopping sampling, and taking the target temperature filtered value obtained by the last sampling as the temperature detection result.

[0026] The beneficial effects of the present application are:

[0027] The present application is based on the precise measurement of the resistance change of platinum resistance PT1000 by Wheatstone bridge to detect the weak temperature change, and a bridge arm resistance value optimization strategy is constructed to suppress the output nonlinearity, reduce the temperature measurement value calculation model complexity and improve the reliability of precise temperature measurement; based on the synchronous sampling and directional excitation control method of analog-to-digital converter, the non-synchronous sampling error caused by different sampling of signals is eliminated, the measurement deviation caused by the inherent output offset voltage of the differential amplifier is offset, and the temperature measurement accuracy is further improved; the precise temperature measurement method parameters are optimized and designed, the output nonlinearity of the temperature measurement circuit is well suppressed, the non-synchronous sampling error and measurement deviation are eliminated, and the temperature measurement accuracy is better than ±0.001℃, which simplifies the complexity of the temperature measurement circuit and reduces the implementation difficulty, and has important significance for expanding the application of precise temperature measurement method in the field of precise temperature measurement. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is a framework diagram of a temperature measurement system based on a directional excitation control Wheatstone bridge of the present application;

[0029] Figure 2 is a schematic diagram of a traditional precise temperature measurement system based on a Wheatstone bridge;

[0030] Figure 3 is a value distribution diagram of f(R2) when R2∈[0,10kΩ];

[0031] Figure 4 is a value distribution diagram of g(R2) when R2∈[0,10kΩ];

[0032] Figure 5 is a simulation result of the temperature measurement circuit output;

[0033] Figure 6 is the increment of the temperature measurement circuit output simulation result;

[0034] Figure 7 is a non-synchronous measurement error diagram under the excitation of a constant voltage source output with ±1% precision error;

[0035] Figure 8 is a schematic diagram of a high-precision temperature measurement system based on a directional excitation control Wheatstone bridge of the present application;

[0036] Figure 9 is a 10.014℃ temperature measurement curve;

[0037] Figure 10 is a 52.244℃ temperature measurement curve;

[0038] Figure 11 is a 87.603℃ temperature measurement curve. DETAILED DESCRIPTION

[0039] Specific implementation one: combination Figure 1 This embodiment describes a temperature measurement system based on a steering excitation control Wheatstone bridge. The system includes a Wheatstone bridge, a steering excitation control module, a differential amplifier, an analog-to-digital converter, a resistance solving module, a temperature solving module, and a Butterworth digital filter. Wherein:

[0040] The Wheatstone bridge includes a constant voltage source, a first measurement bridge arm, and a second measurement bridge arm. The first measurement bridge arm includes resistors R1 and R3, and the second measurement bridge arm includes resistors R2 and R4.

[0041] Wherein, resistors R1, R2, and R3 are fixed value resistors, and resistor R4 is a platinum resistor PT1000.

[0042] Resistor R4 in the Wheatstone bridge is used to sense the target temperature.

[0043] The steering excitation control module is used to perform forward excitation and reverse excitation on the Wheatstone bridge.

[0044] The voltage at the reference point of the first measurement bridge arm and the voltage at the reference point of the second measurement bridge arm in the forward excitation mode are input to the differential amplifier, and the output voltage of the differential amplifier in the forward excitation mode is obtained.

[0045] The voltage at the reference point of the first measurement bridge arm and the voltage at the reference point of the second measurement bridge arm in the reverse excitation mode are input to the differential amplifier, and the output voltage of the differential amplifier in the reverse excitation mode is obtained.

[0046] The analog-to-digital converter is used to synchronously sample the constant voltage source voltage and the output voltage of the differential amplifier in the forward excitation mode, and is also used to synchronously sample the constant voltage source voltage and the output voltage of the differential amplifier in the reverse excitation mode.

[0047] The resistance solving module is used to calculate the value of resistor R4 according to the sampling results of the analog-to-digital converter.

[0048] The temperature solving module is used to solve the target temperature initial value T' according to resistor R4.

[0049] The Butterworth digital filter is used to process the target temperature initial value T' solved by the temperature solving module to obtain the target temperature detection result.

[0050] Specific implementation two: The difference between this embodiment and specific implementation one is that the resistance value of the platinum resistor PT1000 is: R4 = R0(1+AT+BT 2), where A and B are coefficients, T is the target temperature, and R0 = 1000 Ω.

[0051] The steps and parameters are the same as in the first embodiment.

[0052] In this embodiment, A = 3.8623139728 x 10 -3 , and B = -6.534932626 x 10 -7 .

[0053] In this embodiment, the resistance R1 and R2 satisfy R1 = R2 >> R0, which is different from the first or second embodiment.

[0054] The other steps and parameters are the same as in the first or second embodiment.

[0055] The preferred method of the resistance R1 and R2 is:

[0056] The first and second measurement bridge arms are driven by a constant voltage source DC, based on which the precision temperature measurement circuit output U(t) can be represented as:

[0057]

[0058] where U is the voltage of the constant voltage source;

[0059] As can be seen from equation (1), the factors affecting the accuracy and reliability of U(t) mainly include the temperature measurement circuit output nonlinearity caused by the target temperature T, the non-synchronous measurement error caused by the constant voltage source output fluctuation, and the measurement deviation caused by the inherent output offset voltage of the differential amplifier circuit. Eliminating the above errors is the most effective way to improve temperature measurement accuracy.

[0060] When the target temperature T changes linearly, U(t) changes nonlinearly, so a temperature measurement value solving model with the ability to accurately describe the nonlinear relationship is needed to solve T based on U(t) in real time. The simpler the nonlinear relationship, the simpler the temperature measurement value solving model, the smaller the solving error, and the higher the temperature measurement accuracy. Therefore, suppressing the temperature measurement circuit output nonlinearity is an important prerequisite for improving temperature measurement accuracy.

[0061] Since the PT1000 is generally used as a temperature sensor in the precision temperature measurement method, its resistance R t is as follows:

[0062] R t = R0(1 + AT + BT 2 ) (2)

[0063] In equation (2), A = 3.8623139728 x 10 -3 , and B = -6.534932626 x 10 -7, R0 = 1000 Ω. Let R4 = R t , formula (1) can be further represented as:

[0064]

[0065] In the formula, C = R2 / R0. Considering [A 2 -4B(R2 / R0+1)] > 0, [T 2 + AT / B + (C + 1 / B)] exists real roots r1 and r2, therefore, formula (3) can be further derived as:

[0066]

[0067] Since the resistance of PT1000 is related to the quadratic term of the target temperature T, after the second-order Taylor expansion of formula (4), the following formula can be obtained:

[0068]

[0069] In the formula, f(R2) is the coefficient of the target temperature T, and g(R2) is the coefficient of the square of the target temperature T 2 . When R2 increases relative to R0, the non-linear relationship becomes smaller. Therefore, R2 is appropriately selected to reduce the influence of f(R2)T and g(R2)T 2 on U(t). Based on this, further derivation of f(R2) can be obtained as follows:

[0070]

[0071] As shown in formula (6), Figure 3 the value distribution diagram of f(R2) when R2 ∈ [0, 10kΩ] is given. When R2 = R0, f(R2) takes the maximum value, and at this time, f(R2)T takes the maximum value. Therefore, R2 should take a value greater than R0 to reduce the influence of f(R2)T on U(t), that is, R2 >> R0.

[0072] Further, after derivation of g(R2), the following formula can be obtained:

[0073]

[0074] Based on this, the extreme point of formula (7) is:

[0075]

[0076] As shown in formula (8), Figure 4 the value distribution diagram of g(R2) when R2 ∈ [0, 10kΩ] is given. When R2 = 0.516R0, g(R2) takes the maximum value and g(R2)T 2Take the maximum value. When R2>0.516R0 and increases, g(R2)T 2 The smaller the value, the smaller the nonlinearity of U(t), thereby suppressing the nonlinearity of the output temperature measurement circuit and reducing the complexity of the temperature measurement value solution model, that is, R2>>0.516R0. Based on this, the optimal strategy for the bridge arm resistance value is as follows:

[0077] R1=R2>>R0 (9)

[0078] Specific implementation method four: Combination Figure 1 This embodiment describes a temperature measurement method based on a Wheatstone bridge controlled by a directional excitation, the method specifically comprising the following steps:

[0079] Step 1: Use the platinum resistor PT1000 in the Wheatstone bridge as a temperature sensor to sense the target temperature. The resistance of the platinum resistor PT1000 can sensitively reflect the slight change of the target temperature.

[0080] Step 2: Use a constant voltage source DC to forward excite the Wheatstone bridge, and use the voltage of the reference point on the first measurement bridge arm and the voltage of the reference point on the second measurement bridge arm in the forward excitation mode as the differential amplifier A. F The analog-to-digital converter is used to synchronously sample the constant voltage source voltage and the output voltage of the differential amplifier in the forward excitation mode, and the sampled differential amplifier A F The output is denoted as U Z (t), the sampled constant voltage source voltage is recorded as U Z ;

[0081] Step 3: Use a constant voltage source DC to reverse excite the Wheatstone bridge, and use the voltage of the reference point on the first measurement bridge arm and the voltage of the reference point on the second measurement bridge arm in the reverse excitation mode as the differential amplifier A. F The analog-to-digital converter is used to synchronously sample the constant voltage source voltage and the output voltage of the differential amplifier in the reverse excitation mode, and the sampled differential amplifier A F The output is denoted as U F (t), the sampled constant voltage source voltage is recorded as U F ;

[0082] Step 4: Using the sampled U Z (t), U Z 、U F (t) and U F Calculate the resistance R4 of the platinum resistor PT1000;

[0083] Step 5: Calculate the target temperature initial value T' based on the resistance R4 calculated in step 4, and use the calculated target temperature initial value as the target temperature initial value obtained by a single sampling;

[0084] Step six, processing the target temperature initial value T' by using a Butterworth digital filter to obtain a processed target temperature filtered value T", i.e. taking the processed target temperature filtered value T" as the target temperature filtered value obtained by one sampling;

[0085] Step seven, obtaining the target temperature filtered value corresponding to each sampling by using the method of step two to step six, stopping sampling until the target temperature filtered value converges, and taking the target temperature filtered value obtained by the last sampling as the temperature detection result.

[0086] Specific implementation five: the difference between this implementation and the specific implementation four is that the output U F (t) of the differential amplifier A Z in the forward excitation mode is:

[0087]

[0088] In the formula, V OO is the differential amplifier offset voltage, α is the differential amplifier amplification factor, U Z is the forward excitation instantaneous value of the constant voltage source, and U(t) is the differential voltage of the reference point voltage on the first measurement bridge arm and the reference point voltage on the second measurement bridge arm.

[0089] The steps and parameters are the same as those in the specific implementation four.

[0090] Specific implementation six: the difference between this implementation and the specific implementation four or five is that the output U F (t) of the differential amplifier A F in the reverse excitation mode is:

[0091]

[0092] In the formula, U F is the reverse excitation instantaneous value of the constant voltage source.

[0093] The other steps and parameters are the same as those in the specific implementation four or five.

[0094] Specific implementation seven: the difference between this implementation and one of the specific implementations four to six is that the calculation method of the differential amplifier amplification factor α is:

[0095] The analog-to-digital converter is an 18-bit synchronous sampling analog-to-digital converter AD7608 with 8 inputs, ADR421 is selected as the reference source of AD7608, and the resolution U res of the analog-to-digital converter AD7608 is:

[0096]

[0097] wherein U0 is the reference voltage converted by the A / D converter from the voltage outputted by the reference source;

[0098] Taking the target temperature 25℃ as the reference temperature, when the target temperature changes by 0.001℃, the resistance change amount AR4 of the platinum resistance PT1000 is calculated; according to the resistance change amount, the differential voltage change amount AU(t) of the reference point voltage on the first measuring bridge arm and the reference point voltage on the second measuring bridge arm outputted by the Wheatstone bridge when the target temperature changes by 0.001℃ is calculated;

[0099] The amplification factor a of the differential amplifier is:

[0100]

[0101] The other steps and parameters are the same as one of the fourth to sixth embodiments.

[0102] The eighth embodiment is different from one of the fourth to seventh embodiments in that the sampled U Z (t), U Z , U F (t) and U F The resistance R4 of the platinum resistance PT1000 is calculated, specifically as follows:

[0103]

[0104] The other steps and parameters are the same as one of the fourth to seventh embodiments.

[0105] The ninth embodiment is different from one of the fourth to eighth embodiments in that the specific process of the step five is as follows:

[0106]

[0107] The other steps and parameters are the same as one of the fourth to eighth embodiments.

[0108] The tenth embodiment is different from one of the fourth to ninth embodiments in that the working process of the Butterworth digital filter is as follows:

[0109]

[0110] wherein T' is the initial value of the target temperature of the current sampling before being processed by the Butterworth digital filter, T" is the filtered value of the target temperature of the current sampling after being processed by the Butterworth digital filter, z -1 is the initial value of the target temperature obtained relative to the previous sampling of the current sampling, z -2 is the initial value of the target temperature obtained relative to the second previous sampling of the current sampling, z"-1 T'(n) is the target temperature initial value corresponding to the current sampling time, T'(n-1) is the target temperature initial value corresponding to the previous sampling time, and T'(n-2) is the target temperature initial value corresponding to the second previous sampling time. -2 T'(n) is the target temperature initial value corresponding to the current sampling time, T'(n-1) is the target temperature initial value corresponding to the previous sampling time, and T'(n-2) is the target temperature initial value corresponding to the second previous sampling time.

[0111] In order to realize the digital filter in the program, formula (16) can be further expressed as:

[0112] T'(n) is the target temperature initial value corresponding to the current sampling time, T'(n-1) is the target temperature initial value corresponding to the previous sampling time, and T'(n-2) is the target temperature initial value corresponding to the second previous sampling time. -6 T'(n) is the target temperature initial value corresponding to the current sampling time, T'(n-1) is the target temperature initial value corresponding to the previous sampling time, and T'(n-2) is the target temperature initial value corresponding to the second previous sampling time. -5 T'(n) is the target temperature initial value corresponding to the current sampling time, T'(n-1) is the target temperature initial value corresponding to the previous sampling time, and T'(n-2) is the target temperature initial value corresponding to the second previous sampling time. -6 T'(n) is the target temperature initial value corresponding to the current sampling time, T'(n-1) is the target temperature initial value corresponding to the previous sampling time, and T'(n-2) is the target temperature initial value corresponding to the second previous sampling time.

[0113] In formula (16), T"(n) is the target temperature filter value corresponding to the current sampling time of the Butterworth digital filter output, T"(n-1) is the target temperature filter value corresponding to the previous sampling time of the Butterworth digital filter output, T"(n-2) is the target temperature filter value corresponding to the second previous sampling time of the Butterworth digital filter output, T'(n) is the target temperature initial value corresponding to the current sampling time, T'(n-1) is the target temperature initial value corresponding to the previous sampling time, and T'(n-2) is the target temperature initial value corresponding to the second previous sampling time.

[0114] The other steps and parameters are the same as one of the fourth to ninth embodiments.

[0115] The temperature measurement method of the present application needs to be repeatedly sampled at the same target temperature. The sampling frequency can be set according to the actual situation. Each sampling can use a target temperature initial value obtained by the method of steps 2 to 5, and then the target temperature initial value obtained by each sampling is filtered to obtain the target temperature filter value obtained by each sampling. It should be noted that when the target temperature initial values obtained at the first and second sampling times are filtered, the related parameters of the previous two sampling times involved in the filtering formula are set to 0. Until the target temperature filter value obtained by sampling converges, the target temperature detection result is obtained.

[0116] The present application will be further described in detail below with reference to the accompanying drawings. By measuring the voltage difference of the bridge arm reference point, the Wheatstone bridge accurately measures the slight change of the bridge arm resistance value. Based on this, the Wheatstone bridge is widely used in the field of precise measurement of slight resistance value change, especially in the field of precise temperature measurement. Figure 2 The principle diagram of the precise temperature measurement method based on the Wheatstone bridge is given.

[0117] In Figure 2In the formula, R1, R2 and R3 are fixed resistances on the bridge arm, R4 is a platinum resistance PT1000, and its resistance value can be expressed as R4=f(T), A F The precision temperature measurement circuit is a differential amplifier, DC is a constant voltage source, and its output voltage is U. R1 and R3 constitute a first measurement bridge arm, R2 and R4 constitute a second measurement bridge arm, and the first measurement bridge arm and the second measurement bridge arm are driven by the constant voltage source DC. Based on this, the output U(t) of the precision temperature measurement circuit can be expressed as:

[0118]

[0119] As can be seen from formula (17), the factors affecting the accuracy and reliability of U(t) mainly include the temperature measurement circuit output nonlinearity caused by the target temperature T, the non-synchronous measurement error caused by the constant voltage source output fluctuation, and the measurement deviation caused by the inherent output offset voltage of the differential amplifier circuit. Eliminating the above errors is the most effective way to improve the temperature measurement accuracy.

[0120] 1. Temperature measurement circuit output nonlinearity suppression

[0121] As can be seen from formula (17), when the target temperature T changes linearly, U(t) changes nonlinearly, so a temperature measurement value solving model with the ability to accurately describe the nonlinear relationship is needed to solve T based on U(t) in real time. The simpler the nonlinear relationship, the simpler the temperature measurement value solving model, the smaller the solving error, and the higher the temperature measurement accuracy. Therefore, suppressing the temperature measurement circuit output nonlinearity is an important prerequisite for improving the temperature measurement accuracy. Since the PT1000 is generally used as a temperature sensor in the precision temperature measurement method, its resistance R t As follows:

[0122] R t = R0(1+AT+BT 2 ) (18)

[0123] In the formula, A=3.8623139728x10 -3 , B=-6.534932626x10 -7 , and R0=1000Ω. Let R4=R t , and formula (17) can be further expressed as:

[0124]

[0125] In the formula, C=R2 / R0. Considering that [A 2 -4B(R2 / R0+1)]>0, [T 2 +AT / B+(C+1 / B)] has real roots r1 and r2, therefore, formula (19) can be further derived as shown below:

[0126]

[0127] Since the PT1000 resistance is related to the quadratic term of the target temperature T, the second-order Taylor expansion of equation (20) can be obtained as follows:

[0128]

[0129] where f(R2) is the coefficient of the target temperature T, and g(R2) is the coefficient of the target temperature square T 2 . Considering that the nonlinear relationship between the PT1000 resistance and the target temperature T is an inherent property, the nonlinear relationship between U(t) and the target temperature T cannot be eliminated. However, when R2 increases relative to R0, its nonlinear relationship becomes smaller. Therefore, R2 is appropriately selected to reduce the impact of f(R2)T and g(R2)T 2 on U(t). This is an important way to suppress the nonlinearity of the output of the temperature measurement circuit. Based on this, further derivation of f(R2) can be obtained as follows:

[0130]

[0131] As shown in equation (22), Figure 3 a value distribution diagram of f(R2) when R2 ∈ [0, 10 kΩ] is given.

[0132] As shown in equation (22), Figure 3 when R2 = R0, f(R2) takes the maximum value, and f(R2)T takes the maximum value at this time. Therefore, R2 should take a value greater than R0 to reduce the impact of f(R2)T on U(t), that is, R2 >> R0. Further, the derivative of g(R2) can be obtained as follows:

[0133]

[0134] Based on this, the extreme point of equation (23) is:

[0135]

[0136] As shown in equation (24), Figure 4 the value of g(R2) when R2 ∈ [0, 10 kΩ] is given. When R2 = 0.516R0, g(R2) takes the maximum value and g(R2)T 2 takes the maximum value. When R2 > 0.516R0 and increases, g(R2)T 2 takes a smaller value, the nonlinearity of U(t) is smaller, thereby realizing the suppression of the nonlinearity of the output of the temperature measurement circuit and reducing the complexity of the temperature measurement value solving model, that is, R2 >> 0.516R0. Based on this, the bridge arm resistance value optimization strategy is as follows:

[0137] R1 = R2 >> R0 (25)

[0138] To illustrate the effect of the bridge arm resistance value optimization strategy on simplifying the temperature measurement value calculation model, a typical temperature measurement circuit based on the Wheatstone bridge is taken as an example, and U(t) is simulated under different R1 and R2 conditions and different target temperatures in the temperature measurement range of 0-100°C. Let R1 = R2 ∈ [0, 10 kΩ], R3 = 1 K, R4 be PT1000, and the constant voltage source be DC 5 V. The simulation conditions are shown in Table 1.

[0139] Table 1. Simulation conditions

[0140]

[0141] Figure 5 The simulation results of U(t) under different target temperatures in the range of 0-100°C under the simulation conditions are shown in the following table, Figure 6 The increments dU(t) are shown in the following table. Figure 5 Figure 6 As shown in the above tables, when R1 and R2 increase, the non-linear relationship between U(t) and R4 weakens, which reduces the complexity of the temperature measurement value calculation model based on U(t) inversion calculation R4, and further reduces the calculation error and improves the temperature measurement accuracy and reliability. Based on this, the bridge arm resistance value optimization strategy plays an important role in reducing the complexity of the temperature measurement value calculation model, and can provide an effective reference for temperature measurement circuit parameter selection.

[0142] 2. Non-synchronous measurement error elimination

[0143] As shown in equation (17), U(t) is also closely related to the constant voltage source excitation voltage U. If U fluctuates, U(t) will also fluctuate, and the greater the fluctuation of U, the greater the fluctuation of U(t), and the more significant the error caused by excitation on the temperature measurement accuracy. When the constant voltage source output time t DC is not synchronized with the target temperature sampling time t s , the constant voltage source output voltage instantaneous value U DC is not synchronized with the target temperature sampling value U S , both of which contain non-synchronous measurement errors caused by the fluctuation of the constant voltage source output voltage. When the target temperature is 20°C, the instantaneous resistance of PT1000 can be obtained from equation (18) as follows:

[0144] R4 = R0(1 + AT + BT 2 ) = 1076.985 Ω (26)

[0145] Under the excitation of the constant voltage source DC 5 V, U(t) can be represented as:

[0146]

[0147] ​If the constant voltage source output is accompanied by a ±1% accuracy error, and the precision temperature measurement circuit output is U″(t), then the temperature measurement circuit has an asynchronous measurement error ΔU(t) = |U′(t)-U″(t)|. Figure 7 The asynchronous measurement error ΔU(t) is given when the constant voltage source output is excited with ±1% accuracy error.

[0148] Depend on Figure 7 As shown in the figure, under the constant voltage source excitation with ±1% accuracy error, U(t) is a fluctuating output with a maximum error of approximately ±5×10 -4 In precision temperature measurement, the asynchronous measurement error ΔU(t) can be significant enough to affect the accuracy of precision temperature measurement. To effectively eliminate this asynchronous measurement error, ΔU(t), the measurement interval Δt between the precision temperature measurement circuit output U(t) and the constant voltage source U should be shortened as much as possible. The smaller Δt, the smaller the asynchronous measurement error ΔU(t), and the smaller its impact on temperature measurement accuracy. Ideally, Δt should be set to 0, meaning that U(t) and U are measured synchronously. This requires the use of an analog-to-digital converter with synchronous sampling and holding capabilities for signal sampling.

[0149] 3. Measurement deviation offset

[0150] Depend on Figure 2 As shown in equation (17), U(t) needs to be conditioned by a differential amplifier and then processed by an analog-to-digital converter to obtain a digital value for precise temperature measurement. The actual target temperature value can then be obtained through temperature measurement and calculation. An important performance indicator of a differential amplifier, "input offset voltage," refers to the difference in DC voltages applied to the two input terminals in order to obtain a constant zero-voltage output at the output of the differential amplifier. This performance parameter characterizes the matching level of the differential amplifier. The input offset voltage of the differential amplifier is superimposed on the input voltage difference, allowing the measurement signal to be precisely measured even with errors. At this point, if the resolution of the input offset voltage and the target signal are similar, the input offset voltage will "distort" the target signal, making it impossible to accurately complete the measurement. For example, using Equation (26), INA118 is used as a differential amplifier. It is known that the input offset voltage of INA118 is 10 μV. When the target temperature changes by 0.001°C, the input differential voltage of INA118 is 2.1 μV. At this time, its input offset voltage will introduce an error to the input differential voltage and cause measurement deviation. Therefore, the introduction of the steering excitation control excitation Wheatstone bridge to eliminate the input offset voltage. Figure 8 The schematic diagram of a high-precision temperature measurement system based on directional excitation controlled Wheatstone bridge is given.

[0151] The direction of excitation control is controlled by the direction switch K to adjust the excitation direction of the constant voltage source to the precision temperature measurement circuit based on the Wheatstone bridge. The excitation direction includes forward excitation mode and reverse excitation mode. Figure 8As shown, S1 is the forward excitation mode switch K point, S2 is the reverse excitation mode switch K point. When the precision temperature measurement circuit works in the forward excitation mode, the precision temperature measurement circuit output U Z (t) can be expressed as:

[0152]

[0153] In the formula, V OO is the differential amplifier offset voltage, a is the differential amplifier amplification, U Z is the forward excitation instantaneous value of the constant voltage source. When the precision temperature measurement circuit works in the reverse excitation mode, the precision temperature measurement circuit output U F (t) can be expressed as:

[0154]

[0155] In the formula, U F is the reverse excitation instantaneous value of the constant voltage source. After adding equation (28) and equation (29), we can get:

[0156]

[0157]

[0158] After the forward excitation mode processing, the input offset voltage of the differential amplifier is completely offset, and the sampling value at this time is the more accurate measurement precision temperature measurement circuit output U(t). In addition, the forward excitation mode can further eliminate the accidental error of the Wheatstone bridge measurement, and further improve the accuracy and reliability of the signal measurement.

[0159] 4. Parameter design

[0160] As shown in Figure 5 , under normal circumstances, the temperature measurement range of the precision temperature measurement method is 0℃-100℃, considering that the target temperature is 0℃, the Wheatstone bridge should have no output, so R3=R4=1kΩ. In order to minimize the non-linearity of the output of the Wheatstone bridge, R1=R2>>R0. In addition, considering that the bridge arm resistance needs to be composed of low-temperature drift and high-precision resistance, a fixed resistance with an accuracy of 1‰ and a resistance of 10kΩ is selected to form R1=R2=5kΩ in parallel. When the target temperature changes by 0.001℃, the resistance change of the platinum resistance PT1000 is:

[0161] ΔR4=f(25.001)-f(25)=3.8296×10 -3 Ω (32)

[0162] In order to achieve the requirements of non-synchronous measurement error elimination, 18-bit synchronous sampling analog-digital converter AD7608 with 8-way input is selected, which has synchronous sampling and holding function, can synchronously monitor the output of Wheatstone bridge and the output of constant voltage source exciting Wheatstone bridge. At the same time, ADR444B with output voltage U DC = 4.096V is selected as constant voltage source to reliably excite Wheatstone bridge; ADR421 is selected as reference source of AD7608, whose output voltage 2.5V is converted to 4.5V reference voltage by internal circuit of AD7608. Therefore, the resolution U res of AD7608 is as follows:

[0163]

[0164] When the target temperature changes 0.001℃, the transformation amount ΔU(t) of Wheatstone bridge output can be expressed as shown in formula (17):

[0165] ΔU(t) = 2.1105 × 10 -6 V (34)

[0166] In order to ensure that AD7608 can accurately distinguish 0.001℃ temperature change, the output of Wheatstone bridge needs to be amplified by the following multiple α:

[0167]

[0168] Based on this, the INA118-based differential amplifier is designed to collect and amplify the output of Wheatstone bridge, and input AD7608 for analog-digital conversion. The output of Wheatstone bridge after INA118 conditioning is as follows:

[0169]

[0170] In the formula, 50K represents the built-in high-precision resistor of INA118, and R5 represents the external bypass high-precision resistor. Therefore, R5 can be expressed as:

[0171]

[0172] Three resistors with resistance of 10KΩ and accuracy of 1‰ are selected to be connected in parallel as R5≈3.333KΩ, and the amplification multiple α of INA118 is approximately 16 at this time. When the output of Wheatstone bridge after INA118 conditioning is the full-scale value of AD7608 input, the target temperature at this time is the upper limit value of temperature measurement range. Given that the reference voltage of AD7608 is 4.5V, formula (30) can be obtained:

[0173]

[0174] In the formula, U Z = UF = U DC = 4.096 V. Thus, the upper limit of the temperature measurement range is 105.9578 °C after inversion calculation. The direction control of excitation is realized based on the Omron relay G6S-2. The common terminal of G6S-2 is connected to ADR444B, the normally closed contact is connected to the Wheatstone bridge in the forward excitation mode, and the normally open contact is connected to the Wheatstone bridge in the reverse excitation mode. Then, the STM32F103 drives the transistor S9012 to drive the coil of G6S-2, thereby realizing the excitation of the Wheatstone bridge in the forward excitation mode and the reverse excitation mode in turn.

[0175] In view of the nonlinearity of the platinum resistance PT1000, a temperature measurement value solving model is designed according to formula (18) to solve the target temperature in real time, and its expression is as follows:

[0176]

[0177] In the formula, A = 3.8623139728 × 10 -3 , B = -6.534932626 × 10 -7 , R0 = 1000 Ω, R t = R4. A second-order Butterworth digital filter is designed to filter high-frequency measurement noise in the temperature measurement data. Considering the characteristics of temperature inertia and lag, the digital expression of the second-order Butterworth low-pass filter is as follows:

[0178]

[0179] Therefore, the target temperature can be further expressed as:

[0180]

[0181] Where T' is the initial value of the target temperature of the current sampling before the Butterworth digital filter processing, T" is the filtered value of the target temperature of the current sampling after the Butterworth digital filter processing, z -1 is the initial value of the target temperature obtained relative to the previous sampling of the current sampling, z -2 is the initial value of the target temperature obtained relative to the second previous sampling of the current sampling, z" -1 is the filtered value of the target temperature obtained relative to the previous sampling of the current sampling, and z -2 is the filtered value of the target temperature obtained relative to the second previous sampling of the current sampling.

[0182] In order to realize the digital filter in the program, formula (41) can be further expressed as:

[0183] T" (n) = 7.9625 × 10 -6T'(n-1) + 1.5925 x 10 -5 T'(n-2) + 1.992 x T"(n-1) - 0.992 x T"(n-2) -6 T'(n-2) + 1.992 x T"(n-1) - 0.992 x T"(n-2)

[0184] In the formula, T"(n) is a target temperature filtering value corresponding to a current sampling time of a Butterworth digital filter output, T"(n-1) is a target temperature filtering value corresponding to a previous sampling time of the Butterworth digital filter output, T"(n-2) is a target temperature filtering value corresponding to a second previous sampling time of the Butterworth digital filter output, T'(n) is a target temperature initial value corresponding to the current sampling time, T'(n-1) is a target temperature initial value corresponding to the previous sampling time, and T'(n-2) is a target temperature initial value corresponding to the second previous sampling time.

[0185] Based on this, a high-precision temperature measuring system based on the Wheatstone bridge controlled by the directional excitation can realize the precision temperature measurement with the temperature measuring precision of ±0.001 ℃ and the temperature measuring range of about 0 ℃ to 100 ℃.

[0186] 5. Experimental verification

[0187] In order to verify the precision and reliability of the temperature measuring system, three target temperature reference values are selected randomly in the temperature measuring range of 0 ℃ to 100 ℃ to carry out the temperature measuring precision test. Therefore, 10KΩ and 1KΩ resistors with low temperature drift, high precision and precision of 1‰ are selected to construct corresponding resistance values of 10.014 ℃, 52.244 ℃ and 87.603 ℃ in a series-parallel manner, and three continuous tests of 10 minutes are carried out. Figure 9 The 10.014 ℃ temperature measuring curve is given, Figure 10 The 52.244 ℃ temperature measuring curve is given, Figure 11 The 87.603 ℃ temperature measuring curve is given.

[0188] It can be seen from Figure 9 , Figure 10 , Figure 11 that the high-frequency noise in the measurement signal is effectively suppressed, the temperature measuring value fluctuates around the target temperature reference value, and the temperature measuring precision is better than ±0.001 ℃. Moreover, with the increase of the running time of the temperature measuring method, the envelope effect of the temperature measuring result around the target temperature reference value is more and more significant. This shows that the high-precision temperature measuring system based on the Wheatstone bridge controlled by the directional excitation can effectively improve the precision and reliability of the precision temperature measurement.

[0189] The above calculation examples of the present application are only used to illustrate the calculation model and calculation process of the present application, and are not used to limit the embodiments of the present application. Based on the above description, other different forms of changes or variations can be made by those skilled in the art, and all the embodiments cannot be exhausted here. Any obvious changes or variations derived from the technical solutions of the present application are still within the protection scope of the present application.

Claims

1. A temperature measurement system based on steering excitation control of a Wheatstone bridge, characterized by, The system comprises a Wheatstone bridge, a directional excitation control module, a differential amplifier, an analog-to-digital converter, a resistance solution module, a temperature solution module and a Butterworth digital filter; wherein: The Wheatstone bridge comprises a constant voltage source, a first measuring bridge arm and a second measuring bridge arm, the first measuring bridge arm comprising a resistor and , the second measuring bridge arm comprising a resistor and ; Wherein, the resistance , the resistance and the resistance are constant value resistances, and the resistance is a platinum resistance PT1000; resistors in the wheatstone bridge for sensing a target temperature; The directional excitation control module is used for forward excitation and reverse excitation of the Wheatstone bridge; After the voltage of the first measurement bridge arm reference point and the voltage of the second measurement bridge arm reference point in the forward excitation mode are input to the differential amplifier, the output voltage of the differential amplifier in the forward excitation mode is obtained; Differential amplifier in positive excitation mode The output is: （10） wherein is the differential amplifier offset voltage, is the differential amplifier amplification factor, is the forward excitation transient value of the constant voltage source, is the differential voltage of the reference point voltage on the first measurement bridge arm and the reference point voltage on the second measurement bridge arm; After the voltage of the first measurement bridge arm reference point and the voltage of the second measurement bridge arm reference point in the reverse excitation mode are input to the differential amplifier, the output voltage of the differential amplifier in the reverse excitation mode is obtained; Differential amplifier in reverse excitation mode The output Is: （11） Where, is the instantaneous value of reverse excitation of the constant voltage source; The analog-to-digital converter is used for synchronous sampling of the constant voltage source voltage and the output voltage of the differential amplifier in the forward excitation mode, and is also used for synchronous sampling of the constant voltage source voltage and the output voltage of the differential amplifier in the reverse excitation mode; The resistance calculation module is configured to calculate the value of the resistance according to the sampling result of the analog-to-digital converter. the value of the resistance. Utilizing the sampling , , and calculating the resistance of the platinum resistance PT1000 , in particular: （14） The temperature solving module is configured to solve the target temperature initial value according to the resistance solves the target temperature initial value ; The specific process is as follows: （15） wherein and are coefficients, ; The Butterworth digital filter is used to process the target temperature initial value calculated by the temperature calculation module The target temperature detection result is obtained by processing.

2. A temperature measurement system based on a steered excitation controlled Wheatstone bridge according to claim 1, characterized in that The resistance value of the platinum resistance PT1000 is: wherein, is the target temperature.

3. A temperature measurement system based on the control of a Wheatstone bridge with directive excitation according to claim 2, characterized in that, The resistance And Satisfies .

4. A temperature measurement method based on steering excitation control of a Wheatstone bridge, characterized by, The method specifically comprises the following steps: Step one, using the platinum resistance PT1000 in the Wheatstone bridge as a temperature sensor to perceive the target temperature; Step two, using a constant voltage source DC to forward excite the Wheatstone bridge, taking the voltage of the reference point on the first measurement bridge arm and the voltage of the reference point on the second measurement bridge arm in the forward excitation mode as the input of the differential amplifier , and using an analog-to-digital converter to synchronously sample the constant voltage source voltage in the forward excitation mode and the output voltage of the differential amplifier, recording the sampled output of the differential amplifier as , and recording the sampled constant voltage source voltage as ; The output of the differential amplifier in the positive excitation mode is: is: （10） wherein is the differential amplifier offset voltage, is the differential amplifier amplification factor, is the forward excitation transient value of the constant voltage source, is the differential voltage of the reference point voltage on the first measurement bridge arm and the reference point voltage on the second measurement bridge arm; Step three, using the constant voltage source DC to reverse excite the Wheatstone bridge, taking the voltage of the reference point on the first measurement bridge arm and the voltage of the reference point on the second measurement bridge arm in the reverse excitation mode as the input of the differential amplifier , and using the analog-to-digital converter to synchronously sample the constant voltage source voltage in the reverse excitation mode and the output voltage of the differential amplifier, recording the sampled output of the differential amplifier as , and recording the sampled constant voltage source voltage as ; The output of the differential amplifier in the reverse excitation mode is: ​ （11） In the formula, is the instantaneous value of the reverse excitation of the constant voltage source; Step four, using the samples , , and calculating the resistance of the platinum resistor PT1000 ; （14） Step five, calculating the resistance value according to the resistance value calculated in step four solving the initial value of the target temperature using the solved initial value of the target temperature as the initial value of the target temperature obtained by the first sampling The specific process of step five is as follows: （15） wherein and are coefficients, ; Step six, using a Butterworth digital filter to process the target temperature initial value to obtain a processed target temperature filtered value , i.e. the processed target temperature filtered value is the target temperature filtered value obtained by one sampling; Step seven, the target temperature filtering values corresponding to each sampling are obtained respectively by using the method of steps two to six, until the target temperature filtering value converges, and the target temperature filtering value obtained by the last sampling is taken as the temperature detection result.

5. The temperature measurement method based on directional excitation control of Wheatstone bridge according to claim 4, characterized in that: The differential amplifier amplification factor The calculation method is: The analog-digital converter is an 18-bit synchronous sampling analog-digital converter AD7608 with 8 inputs, and the resolution of the analog-digital converter AD7608 is: : （12） wherein, is a reference voltage converted from the voltage outputted by the reference source by an analog-to-digital converter; The resistance change amount of the platinum resistance PT1000 is calculated when the target temperature changes 0.001 ℃ with the target temperature 25 ℃ as the reference temperature The differential voltage transformation amount of the reference point voltage on the first measurement bridge arm and the reference point voltage on the second measurement bridge arm output by the Wheatstone bridge is calculated according to the resistance change amount when the target temperature changes 0.001 ℃ ​ The amplification factor of the differential amplifier is then is: （13）。 6. A temperature measurement method based on the control of a Wheatstone bridge with directive excitation according to claim 5, characterized in that, The working process of the Butterworth digital filter is as follows: wherein, is a target temperature filtered value corresponding to a current sampling time of a Butterworth digital filter output, is a target temperature filtered value corresponding to a previous sampling time of a Butterworth digital filter output, is a target temperature filtered value corresponding to a second previous sampling time of a Butterworth digital filter output, is a target temperature initial value corresponding to a current sampling time, is a target temperature initial value corresponding to a previous sampling time, is a target temperature initial value corresponding to a second previous sampling time.

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