Temperature measurement system and method based on direction-adjusting excitation control Wheatstone bridge

Abstract

The invention discloses a temperature measurement system and method based on a direction-adjusting excitation control Wheatstone bridge, and belongs to the technical field of sensor signal precision measurement. According to the invention, the problem of temperature measurement deviation caused by non-linear and asynchronous sampling error output by a traditional temperature measurement circuit based on a Wheatstone bridge and offset voltage output by a differential amplification circuit is solved. According to the invention, a bridge arm resistance value optimization strategy is used for suppressing output nonlinearity, the complexity of a temperature measurement value calculation model is reduced, and the precision temperature measurement reliability is improved; based on an analog-to-digital converter synchronous sampling and direction-adjusting excitation control method, asynchronous sampling errors caused by asynchronous signal sampling are eliminated, and measurement deviation caused by inherent output offset voltage of a differential amplifier is counteracted. The output nonlinearity of the temperature measurement circuit can be well inhibited, and asynchronous sampling errors and measurement deviations are eliminated. The method can be applied to the field of temperature measurement based on the Wheatstone bridge.

Description

Technical Field

[0001] The invention belongs to the technical field of precise measurement of sensor signals, and in particular relates to a temperature measurement system and method based on directional excitation control of a Wheatstone bridge. Background Art

[0002] Temperature is a physical parameter that characterizes the hot or cold state of an object. Its level directly affects the properties of an object, such as water, air, metal, etc. Temperature plays an important role in all areas of national production. In chemical production, the distillation method relies on different temperatures to separate complex mixtures and obtain production materials of different purities, different components, and different types; in smart agriculture, drones use infrared cameras to monitor the surface temperature of the planting area in real time, and irrigate reasonably according to the planting needs of different types of crops; in industrial production, the intelligent control system of storage tanks monitors the temperature of industrial storage tanks in time to indicate safety hazards and avoid dangerous accidents. Temperature plays a key role in production practice, and even plays a decisive role in some special applications. Therefore, accurate temperature measurement has always been an important research direction in the field of instrumentation.

[0003] The platinum resistor PT1000 has the advantages of high accuracy, large range, and good repeatability, which lays a good foundation for the detection of weak changes in target temperature. The weak signal detection method based on platinum resistors is widely used in precision temperature measurement methods. Therefore, the key to affecting the accuracy and reliability of temperature measurement lies in the measurement method and its inherent error elimination. 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 solution model and reduce the reliability of temperature measurement; the output voltage of the constant voltage source that excites the Wheatstone bridge fluctuates, which will cause the output sampling time of the Wheatstone bridge to be out of sync with the excitation time and introduce asynchronous sampling errors; the output of the temperature measurement bridge needs to be further conditioned by the differential amplifier circuit, and its output offset voltage will lose part of the input signal and cause measurement deviations. Therefore, suppressing the nonlinearity of the temperature measurement circuit output and eliminating asynchronous sampling errors and measurement deviations are important ways to improve the accuracy and reliability of precision temperature measurement.

[0004] Based on this, a high-precision temperature measurement system based on directional excitation control of 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 simplifies 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 of the invention

[0005] The purpose of the present invention is to solve the problem of temperature measurement deviation caused by output nonlinearity, asynchronous sampling error and output offset voltage of the traditional Wheatstone bridge-based temperature measurement circuit, and to propose a temperature measurement system and method based on directional excitation control of the Wheatstone bridge to improve the accuracy and reliability of temperature measurement.

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

[0007] According to one aspect of the present invention, a temperature measurement system based on a directional excitation control Wheatstone bridge 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 measuring bridge arm and a second measuring bridge arm, the first measuring bridge arm comprises resistors R1 and R3, and the second measuring bridge arm comprises resistors R2 and R4;

[0009] Among them, the resistors R1, R2 and R3 are all 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 direction adjustment excitation control module is used to perform forward excitation and reverse excitation on the Wheatstone bridge;

[0012] After the voltage at the first measurement bridge arm reference point and the voltage at 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;

[0013] 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;

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

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

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

[0017] The Butterworth digital filter is used to process the target temperature initial value T′ calculated by the temperature calculation module to obtain the target temperature detection result.

[0018] According to another aspect of the present invention, a temperature measurement method based on directional excitation control of a Wheatstone bridge is provided, the method specifically comprising the following steps:

[0019] Step 1: Use the platinum resistor PT1000 in the Wheatstone bridge as a temperature sensor to sense the target temperature;

[0020] Step 2: Use a constant voltage source DC to positively 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 positive 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 ;

[0021] Step 3: Use a constant voltage source DC to reversely 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 ;

[0022] Step 4: Using the sampled U Z (t), U Z , U F (t) and U F Calculate the resistance value R4 of the platinum resistor PT1000;

[0023] Step 5, calculating the target temperature initial value T' according to the resistance value R4 calculated in step 4, and using the calculated target temperature initial value as the target temperature initial value obtained by a single sampling;

[0024] Step 6: Use a Butterworth digital filter to process the target temperature initial value T' to obtain a processed target temperature filtered value T", that is, the processed target temperature filtered value T" is used as the target temperature filtered value obtained by a single sampling;

[0025] Step 7: Use the methods of Step 2 to Step 6 to obtain the target temperature filter value corresponding to each sampling, stop sampling until the target temperature filter value converges, and use the target temperature filter value obtained by the last sampling as the temperature detection result.

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

[0027] The present invention accurately measures the resistance change of the platinum resistor PT1000 based on the Wheatstone bridge to detect slight temperature changes, and constructs a bridge arm resistance value optimization strategy to suppress output nonlinearity, reduce the complexity of the temperature measurement value solution model and improve the reliability of precise temperature measurement; based on the synchronous sampling and directional excitation control method of the analog-to-digital converter, the asynchronous sampling error caused by asynchronous signal sampling is eliminated, and 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 parameters of the precise temperature measurement method are optimized and designed, which better suppresses the output nonlinearity of the temperature measurement circuit, eliminates the asynchronous sampling error and measurement deviation, and the temperature measurement accuracy is better than ±0.001°C, simplifies the complexity of the temperature measurement circuit and reduces the difficulty of implementation, which is of great significance for expanding the application of the precise temperature measurement method in the field of precise temperature measurement. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0029] Figure 2 This is the schematic diagram of the traditional Wheatstone bridge-based precision temperature measurement system;

[0030] Figure 3 When R2∈[0,10kΩ], the distribution diagram of f(R2) values;

[0031] Figure 4 When R2∈[0,10kΩ], the distribution diagram of g(R2) values;

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

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

[0034] Figure 7 This is the error diagram of the asynchronous measurement under the constant voltage source output accompanied by ±1% accuracy error excitation;

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

[0036] Fig. 9 It is the temperature measurement curve of 10.014℃;

[0037] Fig.10 It is the temperature measurement curve of 52.244℃;

[0038] Fig.11 It is the temperature measurement curve of 87.603℃. DETAILED DESCRIPTION

[0039] Specific implementation method 1: Combination Figure 1 This embodiment is described. The temperature measurement system based on the directional excitation control Wheatstone bridge described in this embodiment includes 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:

[0040] The Wheatstone bridge comprises a constant voltage source, a first measuring bridge arm and a second measuring bridge arm, the first measuring bridge arm comprises resistors R1 and R3, and the second measuring bridge arm comprises resistors R2 and R4;

[0041] Among them, the resistors R1, R2 and R3 are all fixed value resistors, and the resistor R4 is a platinum resistor PT1000;

[0042] The resistor R4 in the Wheatstone bridge is used to sense the target temperature;

[0043] The direction adjustment excitation control module is used to perform forward excitation and reverse excitation on the Wheatstone bridge;

[0044] After the voltage at the first measurement bridge arm reference point and the voltage at 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;

[0045] 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;

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

[0047] The resistance calculation module is used to calculate the value of the resistor R4 according to the sampling result of the analog-to-digital converter;

[0048] The temperature calculation module is used to calculate the target temperature initial value T' according to the resistor R4;

[0049] The Butterworth digital filter is used to process the target temperature initial value T′ calculated by the temperature calculation module to obtain the target temperature detection result.

[0050] Specific implementation method 2: The difference between this implementation method and the specific implementation method 1 is that the resistance 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 = 1000Ω.

[0051] Its steps and parameters are the same as those of the first specific implementation method.

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

[0053] Specific implementation method three: This implementation method is different from specific implementation method one or two in that the resistors R1 and R2 satisfy R1=R2>>R0.

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

[0055] The preferred method for resistors R1 and R2 is:

[0056] The first measurement bridge arm and the second measurement bridge arm are driven by a constant voltage source DC. Based on this, the output U(t) of the precision temperature measurement circuit can be expressed as:

[0057]

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

[0059] It can be seen from formula (1) that the factors affecting the accuracy and reliability of U(t) mainly include the nonlinearity of the temperature measurement circuit output caused by the target temperature T, the asynchronous measurement error caused by the output fluctuation of the constant voltage source, 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.

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

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

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

[0063] Where A = 3.8623139728 × 10 -3 , B = -6.534932626 × 10 -7, R0=1000Ω. Let R4=R t , formula (1) can be further expressed as:

[0064]

[0065] In the formula, C = R2 / R0. Considering [A 2 -4B(R2 / R0+1)]>0,[T 2 +AT / B+(C+1 / B)] has real roots r1 and r2, so 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, the second-order Taylor expansion of equation (4) yields:

[0068]

[0069] Where f(R2) is the coefficient of the target temperature T, g(R2) is the square of the target temperature T 2 When R2 increases relative to R0, the nonlinear relationship becomes smaller. Therefore, R2 is appropriately selected to reduce f(R2)T and g(R2)T 2 The impact on U(t) is an important way to suppress the nonlinearity of the temperature measurement circuit output. Based on this, further deduction of f(R2) yields:

[0070]

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

[0072] Furthermore, by taking the derivative of g(R2), we can obtain:

[0073]

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

[0075]

[0076] As shown in formula (8), Figure 4 The value 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 calculation 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 The present embodiment describes a temperature measurement method based on directional excitation control of a Wheatstone bridge, 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 positively 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 positive 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 reversely 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 value R4 of the platinum resistor PT1000;

[0083] Step 5, calculating the target temperature initial value T' according to the resistance value R4 calculated in step 4, and using the calculated target temperature initial value as the target temperature initial value obtained by a single sampling;

[0084] Step 6: Use a Butterworth digital filter to process the target temperature initial value T' to obtain a processed target temperature filtered value T", that is, the processed target temperature filtered value T" is used as the target temperature filtered value obtained by a single sampling;

[0085] Step 7: Use the methods of Step 2 to Step 6 to obtain the target temperature filter value corresponding to each sampling, stop sampling until the target temperature filter value converges, and use the target temperature filter value obtained by the last sampling as the temperature detection result.

[0086] Specific implementation mode 5: This implementation mode is different from the specific implementation mode 4 in that the differential amplifier A in the forward excitation mode F The output U Z (t) is:

[0087]

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

[0089] Its steps and parameters are the same as those of the fourth specific implementation method.

[0090] Specific implementation method 6: This implementation method is different from specific implementation methods 4 or 5 in that the differential amplifier A in the reverse excitation mode F Output U F (t) is:

[0091]

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

[0093] The other steps and parameters are the same as those in the fourth or fifth embodiment.

[0094] Specific implementation method 7: This implementation method is different from any one of specific implementation methods 4 to 6 in that the calculation method of the differential amplifier gain 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. The resolution of the analog-to-digital converter AD7608 is U res for:

[0096]

[0097] Among them, U0 is the reference voltage after the voltage output by the reference source is converted by the analog-to-digital converter;

[0098] Taking the target temperature of 25°C as the reference temperature, when the target temperature changes by 0.001°C, calculate the resistance change ΔR4 of the platinum resistor PT1000; calculate the differential voltage change ΔU(t) between 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 when the target temperature changes by 0.001°C based on the resistance change;

[0099] Then the gain α of the differential amplifier is:

[0100]

[0101] The other steps and parameters are the same as those in Specific Implementation Methods 4 to 6.

[0102] Specific implementation eight: This implementation differs from any one of specific implementations four to seven in that the U using sampling Z (t), U Z , U F (t) and U F Calculate the resistance value R4 of the platinum resistor PT1000, specifically:

[0103]

[0104] The other steps and parameters are the same as those in Specific Implementation Methods 4 to 7.

[0105] Specific implementation method 9: This implementation method is different from any one of specific implementation methods 4 to 8 in that the specific process of step 5 is as follows:

[0106]

[0107] The other steps and parameters are the same as those in Specific Implementation Methods 4 to 8.

[0108] Specific implementation method 10: This implementation method is different from any one of specific implementation methods 4 to 9 in that the working process of the Butterworth digital filter is as follows:

[0109]

[0110] Where T′ is the initial value of the current sampling target temperature before being processed by the Butterworth digital filter, T″ is the filtered value of the current sampling target temperature after being processed by the Butterworth digital filter, and z -1 is the initial value of the target temperature obtained in the previous sampling relative to this sampling, z -2 is the initial value of the target temperature obtained from the previous two samplings relative to this sampling, z″-1 is the target temperature filter value obtained from the previous sampling relative to this sampling, z -2 It is the target temperature filtered value obtained by the previous two samplings relative to this sampling.

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

[0112] T″(n)=7.9625×10 -6 T′+1.5925×10 -5 T′(n-1)+7.9625×10 -6 T′(n-2)+1.992×T″(n-1)-0.992×T″(n-2)

[0113] Wherein, T″(n) is the target temperature filter value corresponding to the current sampling moment output by the Butterworth digital filter, T″(n-1) is the target temperature filter value corresponding to the previous sampling moment output by the Butterworth digital filter, T″(n-2) is the target temperature filter value corresponding to the previous two sampling moments output by the Butterworth digital filter, T′(n) is the target temperature initial value corresponding to the current sampling moment, T′(n-1) is the target temperature initial value corresponding to the previous sampling moment, and T′(n-2) is the target temperature initial value corresponding to the previous two sampling moments.

[0114] The other steps and parameters are the same as those in Specific Implementation Methods 4 to 9.

[0115] The temperature measurement method of the present invention needs to perform repeated sampling at the same target temperature. The sampling frequency can be set according to the actual situation. Each sampling can use the method of step 2 to step 5 to obtain an initial value of the target temperature, and then filter the initial value of the target temperature obtained by each sampling to obtain the target temperature filter value obtained by each sampling. It should be noted that when filtering the initial value of the target temperature obtained at the first and second sampling moments, the relevant parameters of the first two sampling moments involved in the filtering formula are set to 0. When the target temperature filter value obtained by sampling converges, the target temperature detection result is obtained.

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

[0117] exist Figure 2In the figure, R1, R2 and R3 are fixed resistors on the bridge arm, R4 is a platinum resistor PT1000, and its resistance can be expressed as R4 = f(T), A F is a differential amplifier, DC is a constant voltage source and its output voltage is U. R1 and R3 form the first measurement bridge arm, R2 and R4 form the 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] It can be seen from formula (17) that the factors affecting the accuracy and reliability of U(t) mainly include the nonlinearity of the temperature measurement circuit output caused by the target temperature T, the asynchronous 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 nonlinear suppression

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

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

[0123] Where A = 3.8623139728 × 10 -3 , B = -6.534932626 × 10 -7 , R0=1000Ω. Let R4=R t , formula (17) can be further expressed as:

[0124]

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

[0126]

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

[0128]

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

[0130]

[0131] As shown in formula (22), Figure 3 The distribution diagram of f(R2) values ​​when R2∈[0,10kΩ] is given.

[0132] Depend on Figure 3 As shown, when R2=R0, f(R2) reaches the maximum value, and at this time f(R2)T reaches the maximum value. Therefore, R2 should be greater than R0 to reduce the influence of f(R2)T on U(t), that is, R2>>R0. Further, after taking the derivative of g(R2), we can get:

[0133]

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

[0135]

[0136] As shown in formula (24), Figure 4 The value of g(R2) when R2∈[0,10kΩ] is given. When R2=0.516R0, g(R2) reaches the maximum value and g(R2)T 2 Take 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 calculation model, that is, R2>>0.516R0. Based on this, the optimal strategy for the bridge arm resistance value is as follows:

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

[0138] In order to illustrate the effect of the bridge arm resistance value optimization strategy in simplifying the temperature measurement value solution model, a typical temperature measurement circuit based on Huygens bridge is taken as an example to simulate U(t) at different target temperatures under different R1 and R2 conditions within the temperature measurement range of 0℃~100℃. Let R1=R2∈[0,10kΩ], R3=1K, R4 is PT1000, and the constant voltage source is DC5V. The simulation conditions are shown in Table 1:

[0139] Table 1 Simulation conditions

[0140]

[0141] Figure 5 The simulation results of U(t) under simulation conditions are given for different target temperatures between 0℃ and 100℃. Figure 6 Its increment dU(t) is given by Figure 5 and Figure 6 As shown in the figure, when R1 and R2 increase, the nonlinear relationship between U(t) and R4 weakens, which reduces the complexity of the temperature measurement value solution model based on U(t) inversion solution of R4, thereby reducing the solution error and improving 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 solution model and can provide an effective reference for the selection of temperature measurement circuit parameters.

[0142] 2. Elimination of asynchronous measurement errors

[0143] As shown in formula (17), U(t) is also closely related to the constant voltage source excitation voltage U. If U fluctuates, U(t) will produce measurement fluctuations. The greater the fluctuation of U, the greater the fluctuation of U(t), and the more significant the influence of the excitation error on the temperature measurement accuracy. DC and the target temperature sampling time t s When asynchronous, the instantaneous value of the constant voltage source output voltage U DC and the target temperature sampling value U S is an asynchronous sampling value, both of which contain the asynchronous measurement error caused by the fluctuation of the constant voltage source output voltage. When the target temperature is 20℃, the instantaneous resistance of PT1000 can be obtained from formula (18) as follows:

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

[0145] Under the excitation of constant voltage source DC5V, U(t) can be expressed 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) under the excitation of a constant voltage source with ±1% accuracy error is given.

[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 and the maximum error value is about ±5×10 -4 V. In precision temperature measurement, the asynchronous measurement error ΔU(t) is sufficient to affect the accuracy of precision temperature measurement. In order to effectively eliminate the asynchronous measurement error ΔU(t), the measurement interval Δt of the precision temperature measurement circuit output U(t) and the constant voltage source U should be shortened as much as possible. The smaller Δt is, the smaller the asynchronous measurement error ΔU(t), and the smaller its impact on the temperature measurement accuracy. Ideally, Δt=0 is required, that is, U(t) and U are measured synchronously, which requires the use of an analog-to-digital converter with synchronous sampling and holding functions for signal sampling.

[0149] 3. Measurement deviation compensation

[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. Then, the actual measured value of the target temperature can be obtained through temperature measurement and solution. 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 terminal of the differential amplifier. This performance parameter characterizes the matching degree of the differential amplifier at this stage. The input offset voltage of the differential amplifier will be superimposed on the input voltage difference, and the measurement signal will be precisely measured in the presence of errors. At this time, if the resolution of the input offset voltage and the target signal are similar, the input offset voltage will cause the target signal to be "distorted", which will result in the inability to accurately complete the measurement. For example, taking equation (26) as an example, INA118 is used as a differential amplifier. It is known that the input offset voltage of INA118 is 10uV. When the target temperature changes by 0.001°C, the input differential voltage of INA118 is 2.1uV. 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 is used 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 excitation control is controlled by the direction switch K to adjust the excitation direction of the constant voltage source to the precise temperature measurement circuit based on the Wheatstone bridge. The excitation direction includes the forward excitation mode and the reverse excitation mode. Figure 8As shown, S1 is the switch K position in the forward excitation mode, and S2 is the switch K position in the reverse excitation mode. When the precision temperature measurement circuit works in the forward excitation mode, the precision temperature measurement circuit outputs U after conditioning by the differential amplifier. Z (t) can be expressed as:

[0152]

[0153] Where V OO is the differential amplifier offset voltage, α is the differential amplifier gain, U Z is the instantaneous value of the forward excitation of the constant voltage source. When the precision temperature measurement circuit works in the reverse excitation mode, the precision temperature measurement circuit outputs U after conditioning by the differential amplifier. F (t) can be expressed as:

[0154]

[0155] Where U F is the instantaneous value of the reverse excitation of the constant voltage source. Adding equation (28) and equation (29) together, we get:

[0156]

[0157]

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

[0159] 4. Parameter design

[0160] Depend on Figure 5 As shown, under normal circumstances, the temperature measurement range of the precision temperature measurement method is 0℃~100℃. Considering that the Wheatstone bridge should have no output when the target temperature is 0℃, R3=R4=1kΩ. In order to minimize the output nonlinearity of the Wheatstone bridge, R1=R2>>R0 is required. In addition, considering that the bridge arm resistors need to be composed of low temperature drift and high precision resistors, fixed value resistors with an accuracy of 1‰ and a resistance of 10kΩ are selected in parallel to form R1=R2=5kΩ. Taking the target temperature of 25℃ as a reference, when the target temperature changes by 0.001℃, the resistance change of the platinum resistor PT1000 is:

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

[0162] In order to meet the needs of eliminating asynchronous measurement errors, the 18-bit synchronous sampling analog-to-digital converter AD7608 with 8 inputs is selected. It has a synchronous sampling and holding function and can synchronously monitor the output of the Wheatstone bridge and the constant voltage source output that excites the Wheatstone bridge. At the same time, the output voltage U DC =4.096V ADR444B is a constant voltage source to stably and reliably excite the Wheatstone bridge; ADR421 is selected as the reference source of AD7608, and its output voltage of 2.5V is converted into a 4.5V reference voltage by the internal circuit of AD7608. Therefore, the resolution of AD7608 is U res As shown below:

[0163]

[0164] When the target temperature changes by 0.001°C, as shown in equation (17), the output change of the Wheatstone bridge ΔU(t) can be expressed as:

[0165] ΔU(t)=2.1105×10 -6 V (34)

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

[0167]

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

[0169]

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

[0171]

[0172] Select three resistors with a resistance of 10KΩ and an accuracy of 1‰ and connect them in parallel to form R5≈3.333KΩ. At this time, the amplification factor of INA118 is α≈16. When the output of the Wheatstone bridge after conditioning by INA118 is the full-scale value of the AD7608 input, the target temperature at this time is the upper limit of the temperature measurement range. It is known that the reference voltage of AD7608 is 4.5V, and it can be obtained from formula (30):

[0173]

[0174] Where U Z =UF =U DC =4.096V. Therefore, after inverse calculation, the upper limit of the temperature measurement range is 105.9578℃. Directional excitation control is realized based on Omron relay G6S-2. The common end 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, STM32F103 uses its GPIO to drive the transistor S9012 to drive the G6S-2 coil, thereby realizing the forward excitation mode and the reverse excitation mode to excite the Wheatstone bridge in turn.

[0175] In view of the nonlinearity of platinum resistor PT1000, the temperature measurement value calculation model is designed according to formula (18) to calculate the target temperature in real time. Its expression is as follows:

[0176]

[0177] Where A = 3.8623139728 × 10 -3 , B = -6.534932626 × 10 -7 , R0=1000Ω, R t =R4. Design a second-order low-pass Butterworth digital filter to filter out the high-frequency measurement noise in the temperature measurement data. Considering the characteristics of temperature as an inertial link and a lag link, the digital expression for designing a second-order Butterworth low-pass filter is as follows:

[0178]

[0179] Based on this, the target temperature can be further expressed as:

[0180]

[0181] Where T′ is the initial value of the current sampling target temperature before being processed by the Butterworth digital filter, T″ is the filtered value of the current sampling target temperature after being processed by the Butterworth digital filter, and z -1 is the initial value of the target temperature obtained in the previous sampling relative to this sampling, z -2 is the initial value of the target temperature obtained from the previous two samplings relative to this sampling, z″ -1 is the target temperature filter value obtained from the previous sampling relative to this sampling, z -2 It is the target temperature filtered value obtained by the previous two samplings relative to this sampling.

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

[0183] T″(n)=7.9625×10 -6T′+1.5925×10 -5 T′(n-1)+7.9625×10 -6 T′(n-2)+1.992×T″(n-1)-0.992×T″(n-2)

[0184] Wherein, T″(n) is the target temperature filter value corresponding to the current sampling moment output by the Butterworth digital filter, T″(n-1) is the target temperature filter value corresponding to the previous sampling moment output by the Butterworth digital filter, T″(n-2) is the target temperature filter value corresponding to the previous two sampling moments output by the Butterworth digital filter, T′(n) is the target temperature initial value corresponding to the current sampling moment, T′(n-1) is the target temperature initial value corresponding to the previous sampling moment, and T′(n-2) is the target temperature initial value corresponding to the previous two sampling moments.

[0185] Based on this, a high-precision temperature measurement system based on directional excitation controlled Wheatstone bridge can achieve precise temperature measurement with an accuracy of ±0.001℃ and a temperature measurement range of approximately 0℃ to 100℃.

[0186] 5. Experimental Verification

[0187] In order to verify the accuracy and reliability of the temperature measurement system of the present invention, three target temperature reference values ​​are randomly selected within the temperature measurement range of 0℃~100℃ to carry out temperature measurement accuracy test. Therefore, 10KΩ and 1KΩ resistors with low temperature drift, high precision and accuracy of 1‰ are selected to construct the corresponding resistance values ​​of 10.014℃, 52.244℃ and 87.603℃ in series-parallel mode, and three 10-minute continuous tests are carried out. Fig. 9 The temperature measurement curve of 10.014℃ is given. Fig.10 The temperature measurement curve of 52.244℃ is given. Fig.11 The temperature measurement curve of 87.603℃ is given.

[0188] Depend on Fig. 9 , Fig.10 , Fig.11 It can be seen that the high-frequency noise in the measurement signal is effectively suppressed, the temperature measurement value fluctuates around the target temperature reference value, and the temperature measurement accuracy is better than ±0.001°C. Moreover, as the running time of the temperature measurement method increases, the envelope effect of the temperature measurement result around the target temperature reference value becomes more significant. This shows that the high-precision temperature measurement system based on the directional excitation control Wheatstone bridge of the present invention can effectively improve the accuracy and reliability of precision temperature measurement.

[0189] The above calculation examples of the present invention are only used to explain the calculation model and calculation process of the present invention in detail, and are not intended to limit the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the scope of protection of the present invention.

Claims

1. A temperature measurement system based on directional excitation control of Wheatstone bridge, characterized in that: The system includes a Wheatstone bridge, a steering 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: The Wheatstone bridge comprises a constant voltage source, a first measuring bridge arm and a second measuring bridge arm, the first measuring bridge arm comprises resistors R1 and R3, and the second measuring bridge arm comprises resistors R2 and R4; Among them, the resistors R1, R2 and R3 are all fixed value resistors, and the resistor R4 is a platinum resistor PT1000; The resistor R4 in the Wheatstone bridge is used to sense the target temperature; The direction adjustment excitation control module is used to perform forward excitation and reverse excitation on the Wheatstone bridge; After the voltage at the first measurement bridge arm reference point and the voltage at 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; 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; The analog-to-digital converter is used for synchronously sampling the constant voltage source voltage and the output voltage of the differential amplifier in a forward excitation mode, and is also used for synchronously sampling the constant voltage source voltage and the output voltage of the differential amplifier in a reverse excitation mode; The resistance calculation module is used to calculate the value of the resistor R4 according to the sampling result of the analog-to-digital converter; The temperature calculation module is used to calculate the target temperature initial value T' according to the resistor R4; The Butterworth digital filter is used to process the target temperature initial value T′ calculated by the temperature calculation module to obtain the target temperature detection result.

2. A temperature measurement system based on directional excitation control of a Wheatstone bridge according to claim 1, characterized in that: The resistance 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 = 1000Ω.

3. The temperature measurement system based on directional excitation control of Wheatstone bridge according to claim 2, characterized in that: The resistors R1 and R2 satisfy R1=R2>>R0.

4. A temperature measurement method based on directional excitation control of Wheatstone bridge, characterized in that: The method specifically comprises the following steps: Step 1: Use the platinum resistor PT1000 in the Wheatstone bridge as a temperature sensor to sense the target temperature; Step 2: Use a constant voltage source DC to positively 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 positive 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 ; Step 3: Use a constant voltage source DC to reversely 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 ; Step 4: Using the sampled U Z (t), U Z , U F (t) and U F Calculate the resistance value R4 of the platinum resistor PT1000; Step 5, calculating the target temperature initial value T' according to the resistance value R4 calculated in step 4, and using the calculated target temperature initial value as the target temperature initial value obtained by a single sampling; Step 6: Use a Butterworth digital filter to process the target temperature initial value T' to obtain a processed target temperature filtered value T", that is, the processed target temperature filtered value T" is used as the target temperature filtered value obtained by a single sampling; Step 7: Use the methods of Step 2 to Step 6 to obtain the target temperature filter value corresponding to each sampling, stop sampling until the target temperature filter value converges, and use the target temperature filter value obtained by the last sampling as the temperature detection result.

5. The temperature measurement method based on directional excitation control of Wheatstone bridge according to claim 4 is characterized in that: In the forward excitation mode, the differential amplifier A F The output U Z (t) is: Where V OO is the differential amplifier offset voltage, α is the differential amplifier gain, U Z is the instantaneous value of the forward excitation of the constant voltage source, and U(t) is the differential voltage between the reference point voltage on the first measurement bridge arm and the reference point voltage on the second measurement bridge arm.

6. The temperature measurement method based on directional excitation control of Wheatstone bridge according to claim 5, characterized in that: In the reverse excitation mode, the differential amplifier A F The output U F (t) is: Where U F is the instantaneous value of reverse excitation of the constant voltage source.

7. The temperature measurement method based on directional excitation control of Wheatstone bridge according to claim 6, characterized in that: The calculation method of the differential amplifier gain α is: The analog-to-digital converter is an 18-bit synchronous sampling analog-to-digital converter AD7608 with 8 inputs. The resolution of the analog-to-digital converter AD7608 is U res for: Among them, U0 is the reference voltage after the voltage output by the reference source is converted by the analog-to-digital converter; Taking the target temperature of 25°C as the reference temperature, when the target temperature changes by 0.001°C, calculate the resistance change ΔR4 of the platinum resistor PT1000; calculate the differential voltage change ΔU(t) between 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 when the target temperature changes by 0.001°C based on the resistance change; Then the gain α of the differential amplifier is:

8. The temperature measurement method based on directional excitation control of Wheatstone bridge according to claim 7, characterized in that: The use of sampling U Z (t), U Z , U F (t) and U F Calculate the resistance value R4 of the platinum resistor PT1000, specifically:

9. The temperature measurement method based on directional excitation control of Wheatstone bridge according to claim 8, characterized in that: The specific process of step five is as follows:

10. The temperature measurement method based on directional excitation control of Wheatstone bridge according to claim 9, characterized in that: The working process of the Butterworth digital filter is: T″(n)=7.9625×10 -6 T′+1.5925×10 -5 T′(n-1)+7.9625×10 -6 T′(n-2)+1.992×T″(n-1)-0.992×T″(n-2) In the formula, T″(n) is the target temperature filter value corresponding to the current sampling moment output by the Butterworth digital filter, T″(n-1) is the target temperature filter value corresponding to the previous sampling moment output by the Butterworth digital filter, T″(n-2) is the target temperature filter value corresponding to the previous two sampling moments output by the Butterworth digital filter, T′(n) is the target temperature initial value corresponding to the current sampling moment, T′(n-1) is the target temperature initial value corresponding to the previous sampling moment, and T′(n-2) is the target temperature initial value corresponding to the previous two sampling moments.

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