Measures to improve the measurement accuracy of thermal resistors and thermocouples!

2.2 Measures to improve the measurement accuracy of thermocouples
Similar to the three-wire balanced bridge method, the output voltages V1 and V2 of the circuit shown in Figure 2 are relatively small, and a voltage amplifier should be added before A/D conversion. The reference voltage VR is generally provided by a precision constant voltage source to provide a stable voltage signal. In addition, when the microcontroller software selects an appropriate algorithm and word length in mathematical calculations, the calculation error can also be ignored. However, the amplification factor β and RV of the amplifier circuit will vary from component to component, especially in mass production, when the accuracy of components is difficult to ensure uniformity. Therefore, for a specific input circuit, the errors caused by β and RV must also be considered.
In order to eliminate the errors caused by β and RV, the calibration method can be used to automatically calibrate and calculate the actual circuit β and RV values ​​during instrument production, and then these two parameters are recorded in the non-volatile memory of the instrument. When the instrument measures temperature, the parameters are read and calculated according to formula (1) to obtain accurate measured temperature.
If the long wire in Figure 2 is replaced by the shortest wire possible (i.e. r=0), and the thermal resistor Rt is replaced by a precision resistor R, VAD is the reference voltage of the A/D converter, β is the voltage magnification factor, and the rest remain unchanged, then:

In formula (4), R is a precision resistor with a known resistance value; D is the result of the A/D conversion, which can be easily read from the instrument display device; VR and VAD are reference voltages, which are constants; β is the total magnification factor of the circuit; K is the proportional factor of the A/D conversion, such as K=214 for a 14-bit A/D converter. Then there are only two unknowns RV and β in formula (2). For a specific input circuit, if two precision resistors R1 and R2 with known resistance values ​​are connected to the circuit shown in Figure 2 for calibration (when calibrating, try to make r=0), a set of two-variable linear equations can be obtained. Thus, for a specific input circuit, β and RV can be solved from the equations, and the results are as follows:

The above calibration method can be summarized as follows: two precision standard resistors R1 and R2 with known resistance values ​​are connected to the input terminals of the instrument respectively, and the resistance of the connecting wire is minimized as much as possible. At this time, the instrument readings D1 and D2 are recorded, and the unknown parameters β and RV of the calibrated instrument can be calculated by substituting them into formula (5). In use, it is recommended to use the same reference source for VR and VAD, so that the calculation of β in formula (5) is independent of the accuracy of the reference voltage. This method reduces the differences between different reference sources, especially the influence of time drift and temperature drift of different references.
2.3 Measurement circuit
Figure 3 is the front input circuit of the high-precision Pt100 temperature measurement system, in which the Pt100 reference voltage and the reference voltage of the A/D converter ICL7135 are the same voltage reference source. The two measurement input signals V1 and V2 of Pt100 are amplified by the same operational amplifier (1+R3/R4) times and then enter the A/D converter. The micro relay K1 is used for channel selection. This method shares the operational amplifier, A/D converter, and reference voltage source, reducing the impact of differences between different devices on the measurement results. The A/D conversion result of ICL7135 is connected to the microcontroller in serial mode, which can greatly save the IO port of the microcontroller. When calibrating the circuit, standard resistors 100Ω and 300Ω are used for calibration, and the calibration results β and RV are stored in the EEPROM of the microcontroller system. In actual measurement, the microcontroller system takes out β and RV as known values, and calculates the resistance Rt value by formula (3).

2.4 Analysis of measurement circuit test

Compared with the three-wire balanced bridge method, the detection results of this circuit have been greatly improved. Table 1 is a comparison of the standard resistance values ​​measured by two different methods. Among them, r is the line resistance.

It can be seen from Table 1 that the theoretical measurement results of the three-wire balanced bridge method have large errors, and the error caused by the increase of the line resistance r is larger. As the resistance value of the thermal resistor to be measured increases, the absolute error also increases. In Table 1, the absolute error is 2.57% when the measured resistance Rt=300 Ω and the line resistance r=20 Ω. The actual measurement results of the improved three-wire method used in this paper have an absolute error of only 0.3 Ω and a relative error of ±0.1% within the measured data range. The A/D converter used in the circuit is only equivalent to 14-bit A/D conversion accuracy. If a higher-precision A/D converter is used, higher measurement accuracy can be achieved. In the actual thermal resistor sensor temperature measuring instrument, it is also necessary to add a related program that converts the measured resistance into the corresponding temperature. That is, after Rt is measured, the actual temperature value can be accurately solved by formula (1).

3 Conclusion

The three-wire balanced bridge method is widely used in thermal resistor measurement, but there is a problem that the measurement error caused by the sensor lead resistance cannot be eliminated. This paper analyzes the problems existing in the balanced bridge method for measuring thermal resistors, proposes a constant voltage divider three-wire measurement method, analyzes the causes and influencing factors of the measurement circuit error, derives and establishes the influencing parameters and formulas of the resistance to be measured, and designs a complete measurement circuit, including a signal amplifier and an A/D converter as well as an interface circuit with a single-chip microcomputer. Finally, the test accuracy of the designed circuit is tested and determined. The test shows that when the three-wire balanced bridge method measures the standard resistance value of 100~300Ω and the line resistance is 0~20Ω, the **** measurement error reaches 2.57%, while the balanced three-wire measurement error is only ±0.1%. Thus, a high-precision three-wire thermal resistor measurement circuit is obtained.