Application of the hottest digital PID variable fr

2022-08-08
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Application of digital PID variable frequency speed regulation in PLC constant pressure water supply system

1 basic principle

high-rise constant pressure water supply is a major supporting project that must comprehensively consider the generation of urban high-rise buildings without conflict in production, such as energy efficiency, environmental protection and product quality. It has the characteristics of multiple water supply mode changes, stable water pressure and automatic protection, and can be widely used in industrial and civil buildings

plc controlled high-rise constant pressure water supply system adopts digital PID control technology to realize PID parameter setting and adjustment control. Through the detection of system pressure, the system adopts PID control for normal work and fire water supply time-sharing according to the size of water pressure, so that the system can realize fast and stable output. The actual pressure of the pipe is fed back to the input end of the comparator for comparison with the given pressure. When the pipe pressure is insufficient, the PID parameters are adjusted through parameter calculation to control the voltage rise, so as to increase the VVVF frequency correspondingly, speed up the water pump, increase the water supply, and nearly increase the pipe pressure. On the contrary, the pump speed slows down, the water supply decreases, and the pipe pressure decreases. To maintain the stability of constant pressure water supply

multi pump control is adopted in this system. The setting of single pump varies from 25Hz to 50Hz. If the pipe requirements cannot be met within the adjustment range, it can be completed by increasing or reducing the number of pumps. The addition and subtraction pumps are selected in the order of 1 → 2 → 3 →

during fire fighting, considering that the water consumption will increase rapidly, PD adjustment mode is adopted. Through the detection of deviation, the given fire fighting pressure is automatically formed and the pump is added faster. Broken glass signal requires manual shutdown

2 system operation

the system adopts s PLC of Siemens company for system development. The system can complete the following functions: 1. Automatic constant pressure of domestic water supply; 2. Automatic and manual water supply gating; 3. Change the pump regularly (plc30 is detected once every 10 seconds, and the pump is shut down without water); 4. Automatic maintenance of working state; 5. Automatic control of fire water supply; 6. Fire life side lock control; 7. The pipe pressure automatically starts and stops the pump

3 algorithm principle

in order to reliably control the operation of the frequency converter and meet the tracking of the differential voltage, when the input signal changes little, the system will no longer read the data and directly maintain the original calculated value. When the change value of the input signal is large, the system will track the change immediately, calculate the latest sampling value and keep it as the basis for parameter operation in turn

there are three subprograms in the system: subprogram 1 is used for digital filtering, and subprogram 2 is an operation subprogram and an interrupt subprogram. In consideration of the impact of the protective environment on the system operation status in any case of the system work, in addition to the filter installed on the hardware, the digital filter is also used to eliminate the impact of interference. The digital filter adopts the method of analog input average value

in the process of frequency regulation, considering the utilization of frequency converter and the reliability of the system, the lower limit of frequency modulation is set to 25Hz and the upper limit is set to 50Hz. When the system is just started, the motor speed is too low to meet the system requirements. At this time, the parameter value is forcibly set to make the stable point of the system work at 50Hz. When the system enters the regulation area, the deviation, deviation change rate, output and rate of the regulation parameters all have an impact on the system output. As the approach degree of the target parameters is different, the adjusted parameter value gradually approaches the target function

considering the influence of the system running inertia, the adjustment process may vibrate. Therefore, a target prediction program is designed in the program. When the operating parameters are far away from the target parameters, the adjustment amplitude accelerates. With the gradual approach of the operating parameters and the target parameters, the tracking adjustment amplitude gradually decreases until the operating parameters and the target parameters seem to be the same when they have not been tested for a long time. The system adjusts according to the predicted values of the changes of the operating parameters, so that the system reaches a dynamic balance to maintain the constant pressure stability of the system

at the same time, due to the large inertia of the system adjustment and the fast data acquisition speed of the system, the calculation result may make the frequency modulation output greater than 50Hz or less than 25Hz during the calculation process. Therefore, when the output frequency is greater than 50Hz or less than 25Hz, the output frequency is forcibly set to the extreme limit value

for the treatment of system vibration, since the system is sampled regularly, the vibration is inevitable during the sampling process. When the vibration exceeds the expected value, it is treated as an error value. When the vibration is less than the predicted value, the sampled value is treated as effective data as the basis for adjusting parameters

after debugging and actual field operation, the system works stably at constant pressure and has good regulation effect, reaching the expected effect

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