Design of boiler pressure controller based on fuzzy PID of single chip microcomputer

0 Introduction

The boiler automatic control system is a typical process control system with large inertia, large lag and multiple variables. It involves the detection and control of multiple physical parameters such as pressure, temperature and water level. It needs to control the fan, water supply pump and heating device at the same time, automatically troubleshoot faults, etc. Due to the large number of analog inputs, many hardware circuits are required, and the control is not simple. At present, many manufacturers use PLC to control boilers. Its automation and reliability are high, but the cost is also high, and program modification and parameter setting are difficult. Intelligent control systems with single-chip microcomputer as the control core are favored by more companies due to their low cost, good reliability and high safety [1]. Based on the existing various single-chip microcomputer temperature control systems, this paper designs a mid-range single-chip microcomputer boiler pressure controller system. The system uses STC89C51 single-chip microcomputer as the core, and the specific control adopts fuzzy PID method.

1 System hardware circuit design

The control system of the boiler pressure controller based on fuzzy PID mainly includes a single-chip microcomputer chip, a keyboard, an LED display, a buzzer alarm circuit, a switch circuit and an input signal acquisition circuit. The hardware schematic diagram is shown in Figure 1.

Figure 1 Hardware system block diagram

The design of this solution is relatively simple. The STC89C51 used in the hardware circuit is a new generation of super anti-interference/high-speed/low-power microcontroller launched by Taiwan Macrochip Technology. The instruction code is fully compatible with the traditional 8051 microcontroller, and 12 clocks/machine cycles and 6 clocks/machine cycles can be selected at will. The latest D version integrates the MAX810 dedicated reset circuit.

The input signal acquisition circuit mainly collects water level signals and pressure controller signals. The water level signals include extremely low water level, low water level, normal water level, high water level and extremely high water level; the pressure controller includes low pressure, high pressure and ultra-high pressure; the switch circuit is mainly responsible for controlling the first-stage burner, the second-stage burner, the water supply pump and the blower; the LED display is mainly responsible for the display of various water levels, various pressures and various faults; the buzzer alarm circuit is mainly responsible for the buzzer alarm of water shortage level, ultra-high water level, ultra-high pressure state and various faults.

The functions of this system are mainly to control whether the water pump replenishes water according to the water level and pressure of the boiler, control the heating of the first and second stage fire switches, and control the fan to cool down according to the specific situation, and can issue a fault alarm at any time. And the corresponding status can be displayed through the LED on the control panel. Figure 2 is a first stage fire switch control circuit.

Burner control process: During operation, when the pressure status displays "low", the first and second stages of fire are started at the same time; when the pressure status displays "normal", the second stage of fire is extinguished and the first stage of fire is still on; when the pressure status displays "high", the first and second stages of fire are completely extinguished, and the burner fan continues to be controlled to purge for 30 seconds. When the pressure drops and the status displays "low" again, the first and second stages of fire are started again at the same time, and the cycle continues until the operation stops. In short: low pressure starts, normal pressure is maintained, high pressure stops, and ultra-high pressure protection is used. If you press the "Run" button to start the operation when the pressure status displays "normal", neither the first nor the second stage of fire will start, and they will start only when the pressure drops to "low".

Figure 2 First stage ignition switch control circuit [page]

2 Selection of fuzzy PID regulation law

The system adopts the commonly used PID regulation law:

Among them: is the input signal of the single-chip microcomputer, is the given value. However, since the single-chip microcomputer can only process digital signals, it is represented by digital PID:

In the above formula, is the deviation value at the sampling time and the output of PID; and is the proportional, integral and differential coefficients.

In real systems, the objects being measured and controlled often have characteristics such as large lag, strong coupling, and time variation. Using only PID control often results in long adjustment time, large overshoot, and difficulty in determining PID parameters. Here, fuzzy thinking is introduced to adjust PID parameters in real time in order to obtain better control effects.

The specific adjustment method is shown in the following formula, where a, b and c are the adjustment coefficients of the proportional coefficient, integral coefficient and differential coefficient respectively; and – the proportional coefficient, integral coefficient and differential coefficient used to obtain the PID output after adjustment; and – the initial proportional coefficient, integral coefficient and differential coefficient.

3. System software design

The software in this system is mainly designed using C language according to system requirements. The software can reliably implement various functions of the system. The system software design adopts modularization. According to the function, the system software is divided into initialization system, data acquisition and processing of the acquired data. The main flow chart of the system software and the flow chart of the interrupt handling program are shown in Figure 3.

Figure 3 System software main flow chart

The automatic control process of the water pump of this system is as follows: when the boiler water level drops below the low level, pump 1 starts to automatically replenish water; when the boiler water level rises above the high level, pump 1 stops; when the boiler water level drops below the low level again, pump 1 starts to automatically replenish water again. This cycle continues until it stops. If you press the "Run" button to start running when the water level status displays "Normal", pump 1 will not start, and will only start when the water level drops to the "low level". During the pressure control operation, when there are water shortages, excessive pressure, excessive water levels, burner failures, excessive smoke temperature, etc., sound and light alarms will be issued for shutdown protection.

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The following is the burner fault procedure:

else if(status_ERR==1)

{

LED_RUN=LED_ON;

LED_WAIT=LED_OFF;

LED_BURNFAULT=LED_ON;

MotorAlm=0; //Alarm on

MotorAir=1; //Forced to turn off the fan

SecondBurn=1; //Turn off the diode fire

FirstBurn=1;

status_Start=0;

}

else if(status_Watered==0&&WaterLowest==0)

{

MotorWater=0; //Turn on the water pump

MotorAir=1; //Turn off the fan

SecondBurn=1; //Turn off the diode fire

FirstBurn=1; //Turn off the first level fire

if(WaterHigh==0) //Add water to high water level and stop

status_Watered=1; //Set the watered flag

LED_RUN=LED_ON;

LED_WAIT=LED_OFF;

status_Start=1; //Set the running flag

}

……

else if(PrESSureHighest==1&&Pressurelower==1&&PressureHigh==1) //No pressure

{

status_PressureHighest=0;

status_pressuresilence=0;

LED_PRESSURE=LED_OFF;

}

4 Conclusion

This paper designs a mid-range single-chip boiler pressure controller system with universal practical significance. The system uses STC89C51 as the core, and the specific control algorithm uses a fuzzy PID method that can be adjusted in real time. The system can overcome some shortcomings of conventional single-chip PID control systems and achieve a more ideal control effect. The system has been used by manufacturers and the system runs stably.