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The scheme design and application scope analysis of the probe type intelligent temperature controller

At present, the working temperature of industrialized operation 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 and a constant current source as the temperature sensor, collects the temperature signal in the form of a voltage signal, converts it by an amplified A/D converter, and transmits it to the single-chip microcomputer for processing. Show on the tube. The instrument has a fast processing speed and a wide range of applications, such as high-temperature furnaces, industrial metallurgy, water temperature measurement and other fields.

introduction

At present, the working temperature of industrialized operation 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 and a constant current source as the temperature sensor, collects the temperature signal in the form of a voltage signal, converts it by an amplified A/D converter, and transmits it to the single-chip microcomputer for processing. Show on the tube. The instrument has high processing speed, high processing speed and wide application, such as high temperature furnace, industrial metallurgy, water temperature measurement and other fields.

System hardware design

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

The scheme design and application scope analysis of the probe type intelligent temperature control instrument

Sensor module

The sensor module adopts PT-100 thermistor whose resistance value increases with the increase of temperature. PT-100 adds a constant current source, and its measuring temperature range is affected by the temperature resistance of 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 into high-temperature furnaces and low-temperature ice cellars to achieve temperature measurement. PT-100 thermistor comes with two wires. Changing its two wires to four-wire measurement can effectively eliminate the influence of lead resistance (that is, the connecting wire or cable between the sensor’s signal to the remote secondary instrument). The measurement result is more accurate.

Figure 2 is the sensor front-end interface circuit, through the regulator TL43l to provide the constant current source in the circuit with a reference voltage V-ref of 2.49l V (reference voltage range of 2.440 ~ 2.550 V). Choose 2.491 V voltage here to facilitate subsequent digital calculations. Connect the reference power supply to the amplifier 0P07 to provide a constant current source for the PT-100.

Because the PT-100 used in the system design is a passive sensor, the sensor interface circuit needs to be designed. The 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.

The scheme design and application scope analysis of the probe type intelligent temperature control instrument

R2 in Figure 2 is a precision resistor, which can provide a stable constant current value without temperature drift.And add software filtering to the algorithm, which can reduce external interference to

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 by the AD623 with amplification function, and its voltage amplification factor Determined by RC, the magnification G=(100 kΩ/RG)+1. Since the output voltage of the sensor is 80-320 mV and the reference voltage is 2.491 V, RG is selected as 20 kΩ, and the amplification factor G≈6. Amplified by the operational amplifier, the output voltage signal is 0.48 to 1.92V, which fully meets the requirement that the input voltage is not higher than the reference voltage. Figure 3 shows the signal amplifier circuit.

The scheme design and application scope analysis of the probe type intelligent temperature controller

Display module

As shown in Figure 4, a serial-in/parallel-out latch 74HC595 is connected between the microcontroller and the LED display to drive the LED light. The I/O interface of the one-chip computer controls the position selection of 4 LED nixie tubes, using the saturation and cut-off characteristics of the triode. The nixie tube is connected to the triode through the 4-bit selection line. The I/0 interface of the one-chip computer shifts into the 8-bit binary number to the shift register one by one. When the signal is on the rising edge, the memory moves into a 1-bit binary number. After the storage is full, the single-chip microcomputer controls the memory to output an 8-bit binary number and lights up the corresponding digital tube, which can reduce the resource consumption of the single-chip microcomputer.

The scheme design and application scope analysis of the probe type intelligent temperature control instrument

System software design

Temperature algorithm

Figure 5 is a block diagram of signal conversion. The digital value can be obtained from the resistance value. On the contrary, the single-chip microcomputer processes the digital value sampled by the A/D converter. The input analog voltage can be calculated by the formula 2.5×(digital quantity/65.536) Value, inversely deduce the resistance value. Since the resistance of the thermistor PT-100 and the temperature have the characteristics of linearity in segments, and referring to the comparison table of resistance and temperature of PT-100, the temperature value is divided into a group and the corresponding resistance value to find its slope. The corresponding temperature value can be calculated according to the digital quantity sampled by the single-chip microcomputer.

The scheme design and application scope analysis of the probe type intelligent temperature controller

Program flow design

Figure 6 is the main program flow of the system software design, and its subroutines include digital-to-analog conversion, key interruption, resistance value linear conversion and so on. The intelligent temperature controller uses VB to write the upper computer program, the main board and the upper computer serial port are connected, and the experimental data is observed through the upper computer, and it 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 upper computer, and then stored in the hard disk.

Experimental result

The value collected by the host computer is stored in the computer, and the results listed in Table 1 are obtained after sorting. The intelligent temperature controller adopts software filtering, delay debounce and other methods to display the program, so that the temperature value is displayed stably, and the data shows the measurement error If it does not exceed 1%, it can be used in applications where the environment and requirements are higher.


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