Design of Industrial Temperature Transmitter

蓝色天使

Design of Industrial Temperature Transmitter [Copy link]

 

1. Introduction (100-200 words)

Industrial automation systems often process signals such as temperature, flow, and pressure. The transmitter system usually collects the above signals and converts them into 4-20mA current for long-distance transmission to the PLC system to achieve automatic control. Based on ADICUP360, high-precision 24-bit AD and programmable constant current source output can provide an excitation current source for three-wire and four-wire thermal resistors, which can eliminate the problem of inaccurate measurement due to long leads of two-wire thermal resistors. ADICUP360 has a built-in 16-bit PWM, which can achieve high-precision DA conversion in an integral manner, or through a dedicated 4-20mA IC through the IIC interface. This example is implemented through the IIC interface. ADICUP resources used include programmable excitation current source, ADC acquisition, serial port print output, GPIO status indication, and IIC interface resources.

2. System Block Diagram (Combined Graphics and Text)

Industrial scenarios often require wide-range and high-precision temperature acquisition. Ordinary digital semiconductor sensors often have the shortcomings of narrow measurement range and insufficient accuracy. Traditional three-wire and four-wire thermal resistor RTD measurements have the characteristics of wide measurement temperature range and accurate temperature measurement, but there are also design difficulties. The design of high-precision excitation source and 4-20mA circuit design, ADICUP360 solves the above difficulties at the same time. The programmable excitation current source can effectively solve the problems of accuracy and consistency of external excitation current source. At the same time, the high-precision 24-bit AD effectively solves the problem of expensive external reference voltage source. Due to limited time, the 4-20mA output part can achieve high-precision output through PWM integration. This time, IIC interface IC is used for implementation.

3. Functional description of each part (combined with pictures and text)

In the excitation current part, the AIN6 pin is configured to have an excitation output current of 200uA. AIN1 and AIN0 are used as ADC acquisition input terminals to acquire the absolute resistance of the RTD. This ensures that the acquisition accuracy is not affected in the case of long leads. Then the AD acquisition value is converted into the corresponding resistance value, and then the resistance value is converted into the corresponding temperature value.

This part converts the collected RTD temperature value into 4-20mA output through the IIC interface. R17 is the sampling end of the feedback current, and U7 is the differential amplifier circuit that converts the current sampled by R17 into a differential voltage to the feedback end. The maximum output current I= 2.5/R17 = 25mA.

4. Source Code

[New reminder] Temperature transmitter RTD acquisition code - 2022 Digi-Key Innovation Design Competition - Electronic Engineering World Forum (eeworld.com.cn)

The following procedure mainly completes four steps.

1. Measure the precise excitation current from AIN1 to AGND.

2. Measure the voltage from AIN0 to AGND.

3. Measure the voltage of the RTD between AIN1 and AIN0.

4. Calculate the RTD resistance and temperature values and print them out. The P1.3 output LED indicates that the program is running.

5. Convert the RTD temperature value to 4-20mA output. (The simulator has some problems and is not adjusted properly)

5. Demonstration video of the work’s functions

The simulator is not currently connected, so the physical printout cannot be displayed.

6. Project Summary

ADICUP360 is a fully functional, high-precision analog signal acquisition MCU that can perform programmable amplification and acquisition of weak electrical signals. The ADC accuracy is up to 24 bits, which saves analog signal acquisition engineers from expensive ADCs and reference sources. At the same time, the programmable output excitation current source can save complex constant current source design and batch consistency temperature drift lamp complex problems. High-precision PWM output can easily achieve high-precision DA. It is an MCU with very good integration designed for high-precision analog signal acquisition.

火辣西米秀

The program uses IIC interface IC to achieve the effect. It mainly completes four steps of operation. How is the effect accuracy?