Probe type intelligent temperature controller

1 Introduction
At present, the working temperature of industrial operations is very wide. If you want to accurately measure the temperature, you need to use an intelligent temperature controller. Therefore, the intelligent temperature controller uses a thermistor plus a constant current source as a temperature sensor, collects the temperature signal in the form of a voltage signal, converts it through an amplified A/D converter, transmits it to the microcontroller for processing, and then obtains the temperature value and displays it on the digital tube. The instrument has fast processing speed, high accuracy, and is widely used in fields such as high-temperature furnaces, industrial metallurgy, and water temperature measurement.

2 System hardware design
Figure 1 is the structural block diagram of the system hardware design, which is mainly composed of sensors, displays, A/D conversion sampling, single-chip control unit, serial port communication, human-computer interaction and other modules.

2.1 Sensor module
The sensor module uses a PT-100 thermistor, whose resistance increases with temperature. The PT-100 plus a constant current source has a measurement temperature range affected by the temperature resistance of the PT-100. The range is -50℃～620℃. PT-100 is a thermistor with a metal front end and a wire at the back end. If it is wrapped with anti-corrosion and high-temperature resistant materials, it can be used as a probe to probe into a high-temperature furnace or a low-temperature ice cellar to achieve temperature measurement. The PT-100 thermistor comes with two wires. Changing its two wires to a four-wire measurement can effectively eliminate the influence of lead resistance (i.e., the connecting wire or cable between the sensor signal and the remote secondary instrument), making the measurement result more accurate. Figure 2 is the sensor front-end interface circuit. The regulator TL431 provides a reference voltage V-ref of 2.491 V (the reference voltage range is 2.440～2.550 V) to the constant current source in the circuit. Here, a voltage of 2.491 V is selected to facilitate subsequent digital quantity calculations. Connect the reference power supply to amplifier 0P07 to provide a constant current source for PT-100.

Since the PT-100 used in this system design is a passive sensor, a sensor interface circuit needs to be designed. This interface circuit can provide a constant current source to convert the resistance value into a voltage value. Its constant current value is 2.491 V, 2.5 kΩ≈1 mA. R2 in Figure 2 is a precision resistor that can provide a stable constant current value without temperature drift. Software filtering is added to the algorithm to minimize external interference. [page]

2.2 A/D converter sampling module
The sampling module of the probe-type intelligent temperature controller is mainly composed of the reference device A/D converter AD623 and AD7705. Since the collected temperature sensor voltage is in the millivolt level, it needs to be completed through the AD623 with amplification function. Its voltage amplification factor is determined by RC, and the amplification factor G=(100 kΩ/RG)+1. Since the sensor output voltage is 80~320 mV and the reference voltage is 2.491 V, RG is selected as 20 kΩ, and the amplification factor G≈6. After amplification by the operational amplifier, its output voltage signal is 0.48~1.92V, which fully meets the requirement that the input voltage is not higher than the reference voltage. Figure 3 is the signal amplification circuit.

2.3 Display module
As shown in Figure 4, a serial input/parallel output latch 74HC595 is connected between the single-chip microcomputer and the LED display to drive the LED light. The single-chip microcomputer I/O interface controls the bit selection of the four LED digital tubes, using the saturation and cutoff characteristics of the transistor. The digital tube is connected to the transistor through four bit selection lines. The single-chip microcomputer I/0 interface shifts 8-bit binary numbers into the shift register bit by bit. When the rising edge signal is received, the memory shifts in 1-bit binary numbers. After the memory is full, the single-chip microcomputer controls the memory to output 8-bit binary numbers and light up the corresponding digital tubes, which can reduce the consumption of single-chip microcomputer resources.

3 System software design
3.1 Temperature algorithm
Figure 5 is a signal conversion block diagram. The digital quantity can be obtained from the resistance value. On the contrary, the single-chip microcomputer processes the digital quantity sampled by the A/D converter. The input analog voltage value can be calculated by the formula 2.5×(digital quantity/65.536), and the resistance value can be inversely deduced. Since the resistance value and temperature of the thermistor PT-100 have a piecewise nearly linear characteristic, and referring to the comparison table of PT-100 resistance value and temperature, the temperature value is divided into a group of 10℃ and the corresponding resistance value is calculated to find its slope, and the corresponding temperature value can be calculated according to the digital quantity sampled by the single-chip microcomputer.

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3.2 Program flow design
Figure 6 shows the main program flow of the system software design, and its subroutines include digital-to-analog conversion, key interruption, resistance value linear conversion, etc. The intelligent temperature controller uses VB to write the host computer program. The mainboard and the host computer are connected by serial port. The experimental data is observed through the host computer and is not limited by the storage range. The VB program is connected to the serial port of the lower computer through the MSComm control, and the collected data is transmitted to the host computer, and then stored in the hard disk.

4 Experimental results
The values ​​collected by the host computer are stored in the computer, and the results listed in Table 1 are obtained after sorting. The intelligent temperature controller uses software filtering, delay de-jittering and other methods to display the program, so that the temperature value is displayed stably. The data shows that the measurement error does not exceed 1%, which can be used in those application fields with high requirements on environment and accuracy.

5 Conclusion
In harsh environments, the probe-type intelligent temperature instrument can accurately measure the temperature. Under the condition of ensuring the normal operation of the single-chip microcomputer and its components, the probe-type temperature sensor can measure the temperature in harsh environments such as furnaces and ice caves. This temperature sensor, which does not have high requirements on the outside world, can be used separately from the central processing unit, without considering whether the working environment of the temperature controller affects the operation of the mainboard.