“The industrial site environment is complex, and the sensor is often far away from the controller. For the temperature measurement sensor PT100, the sensor resistance changes by 0.385Ω/℃, so the line impedance of the too long wire cannot be ignored. Eliminating the measurement error caused by the wire is to improve the PT100 measurement. The problem of accuracy must be solved.
Abstract: The industrial field environment is complex, and the sensor is often far away from the controller. For the temperature measurement sensor PT100, the sensor resistance changes by 0.385Ω/℃, so the line impedance of the too long wire cannot be ignored. Eliminating the measurement error caused by the wire is to improve PT100 measurement accuracy must be solved.
The commonly used platinum thermal resistance PT100 is a temperature sensor with a positive temperature coefficient, that is, as the temperature increases, the resistance of the sensor changes in a positive correlation, as shown in Figure 1.1, the resistance of the PT100 is for every 1°C increase in the temperature of the measured object Increase by 0.385In the range of -200℃~850℃, the platinum thermal resistance PT100 has good linearity, which provides developers with a guarantee of high-precision measurement. This is one of the reasons why PT100 is widely used.
Figure 1.1 PT100 curve
PT100 can be used as a high-precision temperature sensor with excellent functions, but in the case of a complex industrial environment, the sensor’s lengthy wires will bring line impedance, which is 0.385, If you want to ensure the accuracy of 0.1 degree, you need to ensure that the error caused by the line impedance must be less than 38.5, And in order not to introduce the error caused by the thermal effect of the sensor PT100, the constant current source current of the PT100 is generally set at 0.1mA~1mA. If the constant current source is set at 1mA, it can be calculated by formula 1.1. The error voltage must be less than 38.5uV. This is an estimate excluding the influence of noise.
Therefore, in order to obtain the high-precision temperature measurement effect, the influence of the error caused by the sensor line impedance must be eliminated. There are generally three ways of wiring in the industry, as shown in Table 1.1.
Table 1.1 PT100 general connection method
It can be seen from Table 1.1 that the impedance error of the two-wire connection is the largest and cannot be ruled out; the four-wire connection is also called Kelvin’s connection, which is the most accurate and simple, but requires more interface resources; the three-wire connection can Eliminating line errors through calculations saves interface resources, but three lines with equal impedance are required, which is relatively easy to implement.
In response to the complex industrial environment and customers’ needs for high-precision temperature measurement products, our company has launched a dual-channel high-precision thermal resistance PT100 isolation temperature measurement module TPS02R, which can achieve a measurement accuracy of 0.02 in the range of -200℃~850℃ %0.1℃, 10ppm low temperature drift index.
Figure 1.2 Application of three-wire system connection.
As shown in Figure 1.2, the three-wire connection method collects the voltage signal, and the back-end PGA provides gain amplification, after ADC analog-to-digital conversion, the output digital signal is sent to the MCU for processing, because the three wires A, B, and C all have line impedance,,.Here you can install a standard three-wire PT100 to ensure
The constant current source flows through the common-mode resistance R to provide a certain common-mode voltage for the ADC. The ADC differential input channels measure the voltages across AB and AC respectively..A known
Combining formula 1.2, after calculation
Through formula 1.4, the PT100 measurement value that completely eliminates the line impedance error can be obtained, and high-precision temperature measurement can be achieved. The three-wire connection method can achieve the same measurement accuracy as the four-wire system while saving a terminal. This has great advantages for users to develop miniaturized products. The three-wire connection method only needs to ensure the impedance of A, B, C, etc. It can be ensured that the introduced error of the line impedance is basically negligible.
Our TPS02R optimizes the constant current source on the basis of the three-wire connection method, uses the op amp negative feedback principle to improve the stability of the constant current source, and provides a certain common-mode voltage for the ADC input, which guarantees the ADC differential measurement. , And give the monitoring alarm function. The specific measurement data is shown in Table 1.2
Table 1.2 Verification data of our temperature measurement program