Variable speed temperature control system based on 8051

In some production processes, the temperature needs to be raised or lowered quickly or slowly or kept constant according to the process requirements. If manual control is used, it is not only time-consuming and laborious, but also the control accuracy is related to the operator's experience, physical condition, mental state, etc., and large errors are prone to occur. Therefore, it is necessary to design a high-performance temperature control system.
With the MCS-51 series single-chip microcomputer 8051 as the core component and the thermocouple as the temperature measuring element, a fully automatic variable speed temperature control system is designed. The heating time of the heating furnace can be automatically adjusted according to the process temperature curve, thereby realizing intelligent temperature control.

1 Working principle
The temperature preset values ​​of each time period are stored in the memory. The system queries every 20 seconds whether the current preset value is within the constant temperature time through the program. If it is not within the constant temperature time, the next set of preset values ​​is taken out, otherwise the current set value is maintained and the constant temperature time is accumulated.
After the temperature value measured by the thermocouple is converted into an electric potential, it is converted into a digital quantity by the analog-to-digital conversion circuit ADC0809 and sent to the single-chip microcomputer through the P1 port. The measured temperature is sampled every 10 seconds by the time interrupt control, and the collected temperature value is compared with the set value. If the actual temperature exceeds the preset control amount, the system executes the cutoff or full power output command to control the conduction angle of the thyristor and control the rise and fall of the temperature. If the actual temperature does not exceed the preset control range, the system controls the number of pulses of the thyristor per second according to the preset value to maintain the preset temperature curve.
The interference to the control system is that the fluctuation of the grid voltage causes the electric heating furnace to lose control; the second is the change of the condition of the electric heating furnace itself. In view of these two situations, the system samples the grid voltage and the working condition of the electric heating furnace, and transmits them to the control system through the feedback network to change the on-off time ratio of the thyristor, thereby offsetting the influence of the above interference.

2 Hardware Design
8051 is widely used, powerful and low-priced. It has 4 kBROM program memory, 128 kBRAM data memory, 32 I/O lines, 2 16 b timers/counters, and 5 interrupt sources. The hardware circuit is shown in Figure 1.

Since 8051 has 4 kBROM integrated inside, the memory can be expanded according to the program needs. An EPROM 2716 is used as the external program memory of 8051. The low 8 bits of the address of 2716 transmitted from the P0 port of 8051 are latched by 74LS273 and connected to the low 8 bits of 2716. The high 3 bits A8~A10 are directly transmitted from the low 3 bits P2.0~P2.2 of the P2 port. The falling edge of ALE is used to latch the external address, and PSEN is used as the selection signal of the external EPROM. The set value of the temperature curve and the number of thyristor conduction pulses per second are stored in the internal program memory of 8051.
When the internal memory is selected and the next set of temperature values ​​is taken out, the next set of preset pulses per second are output at the same time, and sent to the preset counter composed of two CD40192s in cascade through the latch 74LS273 for preset counting. When the PE end of CD40192 is "0", the preset number latched in 74LS273 is placed in CD40192, and CD40192 starts counting. When the preset number is counted, the counter is cleared and a control signal is sent to the zero-crossing trigger to turn off the thyristor.
The thyristor zero-crossing trigger only conducts the preset number of pulses per second. When it is required to conduct, a control signal is sent before the power supply crosses the zero point, so that the high-power bidirectional thyristor conducts at the power supply zero point.
The temperature analog signal collected by the thermocouple is sent to 8051 through the P1 port by ADC0809 for data processing. The deviation value after comparing the measured temperature with the set temperature is sent to the display through the interface chip 8255 for temperature display.

3 Program Design
The system software mainly consists of the main program module, A/D conversion module, data processing module, and display module.
The program flow charts of the main program module and A/D conversion module are shown in Figure 2 and Figure 3.

During the program running, the timing clock count of 8051 runs through the whole process. Under the control of the clock, the number of cycles required for a delay of 10 s can be calculated by software frequency division. Each cycle adds 1 to the time accumulation unit. During the delay period of the time cycle process, other subroutines can be called to complete other operations. When the timing time of 10 s or 20 s is reached, the sampling time interval is identified by setting a mark.

4 Summary
The design of this system is simple and flexible. Its characteristics are that the preset temperature curve value can be stored in the memory. The system adjusts the temperature according to this preset value, changes the preset temperature curve and the corresponding software, so as to adapt to different process requirements and has universality.

［1］Wang Furui. Design of Single-Chip Microcomputer Measurement and Control System［M］．Beijing: Beijing University of Aeronautics and Astronautics Press, 1999．
［2］Chen Guangdong. Principles and Interface Technology of Single-Chip Microcomputer［M］．Wuhan: Huazhong University of Science and Technology Press, 1998．
［3］Lin Youde. Sensor and Application Technology［M］．Shanghai: Shanghai Science and Technology Literature Press, 1992．