Abstract This article introduces the structure of a PID control variable frequency speed regulation constant pressure water supply system, including tables, PID algorithm formulas, and the functions of the S7-200 PID loop instruction programming wizard. It also describes the structure of a variable frequency speed regulation constant pressure water supply system with built-in PID control in a frequency converter, including built-in PID parameter settings and functions. This information is provided for future reference. Keywords : PID; PLC; frequency converter; constant pressure water supply 1 Introduction The rapid development of frequency conversion technology has led to the widespread application of variable frequency speed regulation in various fields. Variable frequency speed regulation constant pressure water supply systems offer advantages such as energy saving, safety, and high-quality water supply, greatly facilitating people's lives and bringing significant economic benefits to enterprises. The advent of AC frequency converters and the integrated application of PLC and PID control have facilitated the smooth and continuous constant adjustment of water pump speed. The PID control constant pressure water supply speed regulation system achieves stepless speed regulation of the water pump motor, automatically adjusting the system's operating parameters based on changes in set values ​​and water consumption. It maintains constant water pressure to meet water requirements when water consumption changes, making it a currently advanced and reasonable energy-saving water supply system. In our daily work practice, the PID-controlled variable frequency speed regulation constant pressure water supply system mainly adopts two methods: ① Using the PLC's own PID control instructions to complete the variable frequency speed regulation constant pressure water supply control. ② Using the built-in PID function of the frequency converter in conjunction with the PLC to complete the variable frequency speed regulation constant pressure water supply control. The following is a description of these two PID control methods. 2 PLC PID-controlled variable frequency speed regulation constant pressure water supply control system 2.1 System Composition The PLC PID-controlled variable frequency speed regulation constant pressure water supply control system consists of a PLC, frequency converter, pressure sensor, control switching circuit, and motor pump group, etc. The system block diagram is shown in Figure 1. The system uses one frequency converter to drive four water pump motors, operating in a cyclical manner. The pressure sensor is responsible for sampling the pipeline pressure, and the frequency converter controls the speed of the water pump motors. The water pump motors are the output stage. The pressure sensor installed on the water supply pipe converts the outlet pressure signal into a standard 4-20mA signal and feeds it back to the PLC's PID module for PID regulation. After calculation and comparison with the given pressure parameters, an adjustment parameter is obtained and a control signal is issued to control the switching and operation of the frequency converter and water pump motor. The frequency converter controls the speed of the water pump to adjust the water supply of the system, so that the pressure in the water supply system network is maintained at the given pressure. When the water consumption changes, the PLC automatically controls the increase or decrease of the number of working pumps and the speed adjustment of the water pump by the frequency converter according to the amount of water consumption, so as to achieve constant pressure water supply. [align=center] Figure 1 Block diagram of PLC PID control frequency conversion speed regulation constant pressure water supply system[/align] 2.2 S7-200 built-in PID function PLC selection should be a model with built-in integrated IPD adjustment calculation instruction function. We will take the Siemens 57-200 series CPU224 as an example for introduction. The S7-200 CPU224 provides 8-loop PID function (8 PID instruction function blocks) to realize PID automatic adjustment control of signals such as temperature, pressure, and flow. The PID function input generally needs to be analog input to reflect the actual value of the controlled physical quantity as feedback. The user-set adjustment target value is given. The task of P-D calculation is to calculate the result based on the relative difference between the feedback and the given value, according to the PID calculation law, and output it to the frequency converter to drive the water pump motor to operate at variable speed. In S7-200, the PID function is implemented through the PID instruction function block. The PID function block is executed at regular intervals (according to the sampling time). According to the PID calculation law, the control quantity is calculated based on the given value, feedback, and proportional-integral-derivative data at that time. The HD function block exchanges data through a PID loop table, which is allocated in the V data storage area and has a length of 36 bytes. Therefore, each PID function block needs to specify two elements when called: the PID control loop number and the starting address of the control loop table (represented by VB). 2.2.1 PID Loop Table The core of the PID function in S7-200 is the PID instruction. The PID loop table provides data entry points such as the given value, feedback, and PID parameters, and the PID calculation result is also output in the loop table. The loop table contains 9 parameters used to control and monitor the PID calculation. Table 1 PID Command Receiving Loop Table 2.2.2 PID Algorithm Formula In the formula, M(t) — the output of the IPD loop, a function of time; Kc — the gain of the IPD loop; E — the deviation value of the PID loop; Minital — the initial value of the PID loop output; Tl, TD — the integral and derivative time constants. To enable the computer to process this control formula, the formula must be discretized into a periodically sampled deviation formula to calculate the output value. The computer-processed formula is as follows: Where Mn is the IPD loop output value at the nth sampling time; En is the deviation value at the nth sampling time; en-1 is the deviation value at the (n-1)th sampling time; Ts is the sampling period; Mx is the initial value of the integral term; Further simplification of the above formula yields the control formula for IPD operations by the PLC: Where PVn is the output of the controlled object; SPn is the setpoint of the controlled object; 2.2.3 PID Instruction Programming Wizard The S7-200 series PLC is equipped with STEP7/MICRO1NVIN32 programming software, which can run on a computer, providing a good programming environment for users to develop, edit, and monitor their own applications. STEP7/MICRO1NVIN32 provides a PID instruction wizard, which can help users easily generate a PID algorithm for a closed-loop control process. This wizard can automatically program most PID calculations. Users only need to call the subroutine generated by the PID wizard in the main program to complete the PID control task. Select Tools>instructionwizard in the command menu, and then select the PID instruction in the instruction wizard window for programming. 3. PID Variable Frequency Speed ​​Control Constant Pressure Water Supply System Built into the Inverter In recent years, many foreign manufacturers have launched a series of new inverter products. Examples include ABB's ACS60O and ACS55O series, and Fuji's GllslPllS series. These products integrate the functions of a PID controller and a simple programmable controller into the inverter, forming a new type of inverter with various application macros. The emergence of the new inverter makes PID control very convenient. We will take the ABB ACS550 series inverter as an example for a brief introduction. 3.1 System Overview This water supply system consists of a new inverter with built-in IPD function, a PLC, pressure sensors, control switching circuits, and motor pump sets. The system block diagram is shown in Figure 2. The water supply equipment uses four water pumps, which are switched by the programmable controller. If the water consumption is large, the inverter can also send signals to the programmable controller through the programmable interface, allowing the programmable controller to control the four pumps to operate at mains frequency or variable frequency. In Figure 2, the water pressure signal fed back by the sensor is directly sent to the input ports AIZ and AGND of the PID controller built into the frequency converter. The pressure setting can be set digitally using the frequency converter's keypad or analogly using a potentiometer fed into AIZ and AGND. Multiple pressure settings can be configured daily to adapt to the water supply pressure requirements. Specific daily water supply pressure control can also be set. The panel can directly display the pressure feedback value (MPa). Thus, by selecting appropriate PID parameters in the frequency converter's PID options through the control panel and performing on-site debugging and calibration, the requirements for variable frequency speed regulation and constant pressure water supply in the pipeline network can be met. 3.2 Built-in PID Parameter Settings and Functions of ACS550 The ACS550 frequency converter provides two sets of PID parameters, Group 40 and Group 41. The two sets of parameter tables are identical and can be selected via the PID signal. [align=center]Figure 2 Block Diagram of PID Constant Pressure Water Supply System Built into Frequency Converter[/align] [align=center]Table 2 Main Parameter Settings Table of Group 4oPlo[/align] The frequency converter provides an application macro: HD Control. This application macro is used for various closed-loop control systems, such as pressure control, flow control, etc. To call it, the value of parameter 9902 must be set to 6 (PIDCTRL). Frequency converter terminal AI1 is the external setpoint, analog input terminal PID: 0...10V=>0...100%PID setpoint. AI2 is the actual feedback signal analog input terminal (PID): 0...20mA. Frequency converter output frequency: 0...20mA<=>0...50Hz. Since the PID calculation is performed internally by the frequency converter, this eliminates the requirement for programmable controller storage capacity and PID algorithm programming, and online debugging of PID parameters is very easy. This not only reduces production costs but also greatly improves production efficiency. Because the built-in PID regulator of the frequency converter uses an optimized algorithm, the water pressure regulation is very smooth and stable. To ensure the accuracy and integrity of the water pressure feedback signal, a filtering time constant can be set for the signal, and the feedback signal can also be converted. 4. Conclusion In our daily work, both of the above-mentioned HD-mode variable frequency speed control constant pressure water supply systems are widely used in practice. The specific choice can be made based on system process requirements and complexity. Through repeated use, we have found that variable frequency speed control constant pressure water supply systems using the built-in PID function of the frequency converter effectively reduce equipment costs, save installation and commissioning time, and the online debugging of PID parameters is very easy. Therefore, this system should be given priority when process requirements are met. References [1] S7-200 System Manual [M]. Siemens (China) Co., Ltd. 2002 [2] ACS550 Inverter User Manual [M]. Beijing ABB Electric Drive Co., Ltd. 2004 [3] Wang Xianfang, Du Zhiyong. Application of PLC PID instruction in variable frequency speed regulation constant pressure water supply system [J]. Water Supply and Drainage, 2005, (3); Author Introduction Zuo Pengjun (1973-) Male. Electrical engineer, research direction: industrial control