The semiconductor refrigeration device TES 1- 12739 selected in this system is produced by Tianjin Lantian Hi-Tech Power Supply Co., Ltd., with a maximum temperature difference voltage of 14.7V, a maximum temperature difference current of 3.9A and a maximum cooling power of 33.7W
1.5 Other parts
The system uses vacuum fluorescent digital display VFD produced by Samsung to display the current temperature in real time to observe the control effect. Keyboard and serial communication interface are used to set control temperature and adjust PID parameters. The schematic diagram of the system circuit is shown in Figure 3.
2 system software design
When the system starts to work, the microcontroller control software sends out a temperature reading instruction, and the digital temperature sensor DS 18B20 samples the current temperature value T 1 of the controlled object and sends it to the display screen for real-time display. Then, the measured temperature value is compared with the set value t, and the difference is sent to the PID controller. After processing, the PID controller outputs a certain value of control quantity, which is converted from DA to analog voltage quantity. The voltage signal is loaded on the semiconductor refrigeration device after passing through the high current driving circuit, so as to improve the current driving ability, thereby heating or cooling the temperature-controlled object. Heating or cooling depends on the positive or negative voltage applied to the refrigerator. If the difference between the current temperature measured value of the temperature-controlled object and the set value is positive, a negative voltage signal is output, and when the refrigerator is loaded with negative voltage, the temperature of the temperature-controlled object decreases. On the contrary, when DC voltage is applied to the refrigerator, the temperature of the controlled object will increase. Repeat the above process: temperature sampling-temperature difference calculation-PID adjustment-signal amplification output, and finally control the temperature of the temperature-controlled object to fluctuate up and down near the set value. With the increase of the number of cycles, the fluctuation amplitude will gradually decrease to a small level until the control requirements are met. In order to speed up the control speed, the temperature difference judgment program is added before entering PID control. When the temperature difference is greater than the set threshold Δ t, the system performs full-power heating or cooling until the temperature difference is less than Δ t. Figure 4 is the software flow chart of the main program of the system.
3 Conclusion
The precision temperature control system based on digital PID control of single chip microcomputer designed in this paper has achieved good control effect in practical application, and the temperature control accuracy reaches 0.65438 0℃. After 48 hours of continuous operation, the system works stably, which effectively reduces the temperature coefficient of the radiance standard detector, so that the radiance standard detector can still maintain high accuracy in the environment with large temperature changes, laying a foundation for the wide application of high-precision radiation calibration based on this detector.
The author's innovation: Based on the original PID temperature control system based on PC, a precise temperature control system is designed, which consists of a single chip microcomputer, a digital temperature sensor DS 18B20 and a semiconductor refrigerator. The application of the temperature control system provides favorable conditions for miniaturization and intelligence of high-precision optical radiation measuring instrument-radiance standard detector.