Design and implementation of temperature transmitter based on Ptl00 platinum thermal resistor

Temperature is a physical quantity that characterizes the degree of hotness or coldness of an object. It is a very important parameter in industrial production, life applications and scientific research. In the process of industrial control, it is necessary to monitor the temperature of the controlled object to prevent the controlled object from being damaged due to excessive temperature. Therefore, real-time monitoring of temperature becomes even more important. Real-time monitoring of temperature is conducive to timely inspection and protection of the controlled object, and timely adjustment of the temperature. According to different design requirements of the control system, the design of the temperature monitoring system has also changed. Some use integrated chips, and some use constant current source devices and constant voltage source devices. Because platinum thermal resistors have the advantages of large measurement range, good stability, high indication reproducibility and oxidation resistance, the system uses Pt100 platinum thermal resistors as temperature sensing elements to design and implement temperature sensors. In the design, the voltage signal is converted into a standard 4-20 mA current signal, which not only saves expensive compensation wires, but also improves the anti-interference ability of the signal during long-distance transmission.

1 Overview of Ptl00 platinum thermal resistor

The degree of change of resistance value with temperature is called temperature drift coefficient. The temperature drift coefficient of most metal materials is positive, and the temperature drift coefficient of many pure metal materials remains constant within a certain temperature range. Which metal material (platinum, copper, nickel, etc.) is selected in a specific application depends on the range of the measured temperature. The temperature response characteristics of metal platinum (Pt) resistor are good, the cost is low, and it can measure higher temperatures; its rated resistance value at 0 ℃ is 100 Ω, and it is a standardized device. Operating temperature range: -200～+850 ℃. Considering the actual industrial application, the measurement range of this system is designed to be 0～120℃. Because the resistance value of thermistor is proportional to temperature, you only need to know the current flowing through the resistor to get the output voltage proportional to the temperature. According to the known resistance-temperature relationship, the measured temperature value can be calculated. Ptl00 temperature sensor is a resistance temperature detector made of platinum (Pt). The relationship between its resistance and temperature change is:

Where: R0 is the resistance value at 0 ℃, R0=10O Ω; T is Celsius temperature.
Therefore, the resistance temperature detector made of platinum, also known as the Ptl00 temperature sensor, is:

Obviously, the relationship between resistance and temperature is nonlinear, but when the measurement accuracy requirement is low, the functional relationship between resistance value and temperature can be simplified as:

In practical applications, the connection method of Ptl00 can be two-wire, three-wire or four-wire. This system can meet the requirements with a three-wire connection. When connected in a two-wire system, the lead resistance is connected in series with Ptl00, which increases the resistance and affects the measurement; when connected in a three-wire system, an additional third wire is added to Ptl00. Since the lead resistance has the same characteristics, the line resistance can be compensated; when connected in a four-wire system, Kelvin detection can be realized, eliminating the voltage difference between the two wires.

2 System structure and working principle

The hardware design of Pt100 temperature sensor is carried out. The structure diagram of the whole data acquisition system is shown in Figure 1.

In Figure 1, the temperature is detected by using the characteristics of platinum thermal resistance, and the temperature is converted into a voltage signal; then the voltage signal is converted into a standard current signal of 4-20 mA through a V/l conversion unit, which not only saves expensive compensation wires, but also improves the anti-interference ability of the signal during long-distance transmission; on the single-chip microcomputer system, the current-voltage conversion chip RCV420 converts 4-20 mA into a 0-5 V voltage signal. After A/D conversion into a digital signal, the single-chip microcomputer system identifies and processes the read digital signal and converts it into a digital signal corresponding to the temperature, and finally displays the output temperature value on the LCD. The CPU mainly completes the processing of the data collected by A/D, including the filtering of the A/D value and the conversion of the A/D value to the actual temperature, and finally sends it to the display for display.

3 Hardware Design

The hardware mainly consists of four parts: stable power supply circuit, operational amplifier circuit, voltage-to-current conversion circuit, and current-to-voltage conversion circuit.

3.1 Stable power supply circuit

The stable power supply circuit is shown in Figure 2. LP2951 is a low-power voltage regulator launched by S1PEX, which is very suitable for some battery-powered systems, such as cordless phones, wireless control systems and portable computers. It has low quiescent current, low voltage drop and other characteristics (the voltage drop is 50 mV when lightly loaded; the voltage drop is 380 mV when 100 mA load). LP2951 has a very small initial tolerance (generally 0.5%), very good load and line regulation characteristics (generally 0.05%), and a very low temperature coefficient (20 ppm/℃), so it is very suitable for use as a low-power voltage source. LP-2951 can obtain a 5 V voltage through pin connection, and use the internal voltage divider to obtain the output, sense, feedback, and 5 V terminal voltages through pin 1 (output), pin 2 (Sense) and pin 7 (feedback). Similarly, other output voltages can be obtained through a 1.235 V reference source, with a maximum of 30 V.

3.2 Main circuit analysis

R7, R8, R4, RP1 and Ptl00 form a sensor measurement bridge. In order to ensure the stability of the bridge output voltage signal, the input voltage of the bridge is provided by the LP2951 power supply chip. The differential voltage signal obtained from the bridge is amplified by a two-stage operational amplifier, and then converted into a standard current signal output of 4-20 mA through a voltage/current conversion circuit. Because A/D can only collect voltage signals, before the signal is collected by A/D, the 4-20 mA is converted into a 0-5 V voltage signal through the chip RCV420. One arm of the bridge uses an adjustable resistor RP1. By adjusting RP1, the size of the differential voltage signal input to the operational amplifier can be adjusted, which is usually used to adjust the zero point. The amplifier circuit uses the LM258 integrated operational amplifier. In order to prevent the nonlinear error caused by the excessively high single-stage amplification factor, the amplifier circuit uses a two-stage amplification, in which the adjustable resistor RP2 is used to adjust the gain of the amplifier circuit. The temperature changes from 0 to +120 ℃. When the temperature rises, the resistance of Ptl00 increases, the differential signal of the input amplifier circuit increases, the output voltage of the amplifier circuit increases accordingly, and the output current also increases accordingly. The actual selected resistance of R8 is much higher than that of PtlOO, so the measured current changes little due to the change of the resistance of Ptl00, and the linear output of the approximate constant current method is obtained.

3.3 Current-voltage conversion circuit

RCV420 is a precision current-to-voltage converter produced by BB Company in the United States. It can convert 4-20 mA loop current into 0-5 V voltage output. As a monolithic integrated circuit, it has reliable performance and very low cost. In addition to precision operational amplifiers and resistor networks, it also integrates a 10 V reference voltage source. Without the need for external adjustment, it can obtain a common-mode rejection ratio of 86 dB and a common-mode voltage input of 40 V. Within the full range, the input impedance has a voltage drop of only 1.5 V, which has a good conversion capability for loop current.

4 Experimental Tests and Conclusions

Since there are errors in measuring tools such as multimeters and temperature testers, it is obvious that from the above experimental data analysis, the experimental slope K1 is approximately equal to the theoretical slope, that is, within the allowable range of industrial error (-0.5 ~ +0.5 ℃), the experimental value is equal to the theoretical value. This design meets the actual requirements, as shown in Figure 4. Figure 5 shows the actual experimental picture.

5 Conclusion

In the temperature measurement system, the Ptl00 platinum thermal resistor is sealed in a metal rod, so that the temperature measurement system can not only measure the indoor gas temperature, but also measure the oil temperature in the oil cylinder, the temperature in the soil, liquid, seeds, etc., which greatly improves the application range of the temperature measurement system. Using the Ptl00 platinum thermal resistor as a temperature detection element can effectively reduce the development cost and expand the application. The circuit design of the transmitter is simple and convenient, with good practicality, stable circuit operation and high reliability. The product has been actually used in the temperature measurement system of the inverter dedicated to the air compressor, and it works stably and reliably.