1. Introduction With the continuous development of 3C technologies (computer, communication, and control) and frequency conversion technologies, water supply systems in residential communities, industrial water supply, various waterworks, large factories and mines, and fire-fighting water supply systems have traditionally relied on equipment such as water towers and elevated water tanks. These systems not only occupy large areas and require significant investment, but are also increasingly unable to meet the demands of modern water supply. Furthermore, traditional methods struggle to guarantee real-time water supply, and because water pumps are designed for maximum capacity, peak water usage periods are short, easily leading to energy waste and pipeline damage due to excessive pressure. This paper takes the control system of the Yongcheng Coal and Power Group water plant as an example to introduce a constant pressure water supply system based on PLC and touchscreen control. 2 System Introduction The Yongcheng Coal and Power Group's water plant variable frequency constant pressure water supply system consists of a PLC controller, two variable frequency speed controllers, a soft starter, a touch screen display, AC contactors, thermal relays, pressure transmitters, water level transmitters, flow transmitters, and other electrical control equipment, as well as four 75kW water pumps and one 45kW small water pump, as shown in Figure 1. Figure 1: Constant Pressure Water Supply System Diagram. A water level transmitter is installed in the water storage tank of the water supply system, and a pressure transmitter and flow transmitter are installed on the main outlet pipe. These detect the water level, pressure, and flow rate, converting them into 4-20mA signals, which are input to the PLC's analog input module. The detected pressure signal and the pressure set via the touch screen are processed by PID calculation, and the output frequency of the frequency converter is controlled to adjust the speed of the water pump motors to maintain constant water pressure. Simultaneously, water level detection determines the number of water source wells opened and the water production capacity of the reverse osmosis system is adjusted based on the water level. Meanwhile, the touchscreen display shows the current, frequency, water level, water pressure, operating time of both mains frequency and variable frequency drives for each motor, as well as the operating status of each pump. System information can also be published to the network via the company's intranet for remote system diagnostics and control. 3. Controlled Objects Based on the real-time situation of the Yongcheng Coal and Power Group's water plant water supply system, the system controls the opening, closing, stopping, and operation of on-site equipment (pumps, valves, etc.); the opening and closing of solenoid valves; the opening and stopping of each water source well; and the starting/stopping of the frequency converter, to achieve automatic control of the water plant's constant pressure water supply. 4. System Working Principle The system has two operating modes: manual and automatic. 4.1 Manual Operation Mode: In manual operation mode, each pump can be controlled by pressing the start and stop buttons as needed. This mode is only used when a system malfunction occurs. 4.2 Automatic Operation Mode (1) Start-up Program During automatic operation, the system first checks if any of the five pumps are faulty. If pump #1 is faulty, the system modifies the pump's status online via the touchscreen and sets pump #1 to a faulty state. When the system starts, pump #1 will not start automatically. Next, the system checks the water level in the reservoir. If the water level meets the requirements and pump #1 is also fault-free, the system starts the vacuum pump to draw a vacuum. If the requirements are met, the AC contactor of pump #1 engages, connecting the motor and the frequency converter. At the same time, the solenoid valve of pump #1 is opened. By detecting the pressure, the PLC performs PID calculations. At this time, the output frequency of the frequency converter starts to rise from 0 Hz. If the pressure is insufficient, it rises to 50 Hz. After a delay, the system switches pump #1 to the power frequency via the soft starter and then starts pump #2. This process continues until the outlet pressure reaches the set pressure. (2) Pump Switching Program Based on the flow rate detected by the flow sensor, if the outlet flow decreases and the outlet pressure is too high, the PLC controls the output frequency of the frequency converter to reduce the outlet flow and stabilize the outlet pressure. If the output frequency of the inverter is 20Hz, the PLC will start timing. If the water pressure drops, the timing will be stopped. If the water pressure is always higher than the set pressure, after a certain time, according to the principle of first-in, first-out, the PLC will first close the solenoid valve of the pump that has been running for the longest time, and then close the pump until the water pressure reaches the set value. (3) Starting the small power pump For the domestic water supply of Yongcheng Coal and Power Company, it is a system with a strong time period of water use. Therefore, among the 5 water pumps, pumps 1#-4# are 75kW and pump 5# is 45kW. Therefore, when the water use is low, a small power pump of 45kW is enough to maintain the water supply pressure. At this time, the small power pump 5# can be operated by frequency conversion to make it work in a low frequency state, so as to maintain the water pressure stability of the water supply system. (4) Remote control and fault diagnosis The control system sends information to the host computer located in the logistics department of the bureau through the communication module CP340 of the PLC. The communication between the two adopts RS-485 mode. When a fault is found on the host computer, such as the water level in the reservoir being too low, the water source well and reverse osmosis device can be opened to raise the water level in the reservoir and prevent the water pump from running dry due to the low water level. Since the host computer of the system can publish to the network, when a fault occurs and the on-site personnel cannot solve it, the manufacturer can help to handle it through the remote diagnosis function. 5 Control system hardware configuration and software programming 5.1 Hardware configuration (1) PLC configuration The programmable controller adopts Siemens S7-300 series CPU-314 host, 1 power module, 2 32-bit I/O modules, 2 16-bit output modules, 2 12-bit AD modules, 1 12-bit DA module and 1 CP340 module. The CP340 communicates with the host computer through RS-485. (2) Touch screen The touch screen adopts Siemens TP270 series, which has its own rich operating system. It can communicate with the programmer through serial communication and can also communicate with the PLC through the MPI network. The touch screen can display motor current, pipeline pressure, water level, flow rate, working status of each water source well, and various fault information in real time; it can also control the PLC online. (3) Selection of frequency converter and control mode In this system, according to the principle of redundancy design, two Mitsubishi dedicated water supply frequency converters are used to provide mutual backup in case of failure. The start/stop of the frequency converter is controlled by the PLC switch output. 5.2 Software programming (1) PLC programming The PLC has system program and user program in software configuration. The system program is installed on the CPU module and comes with the hardware product. The user program is the program written by the programmer and input into the storage module of the programmable controller. The program adopts a block structure and has five forms: organization block (0b), program block (pb), function block (fb), data block (db), and sequence block (sb) (not used in this system). Therefore, in the PLC software, the program modules of various functions are organically combined through the main program. Therefore, the PLC program mainly solves the starting program, switching program, starting of small power pumps, analog quantity processing, and communication data processing functions of the host computer and touch screen. (2) Host computer program The host computer uses VB to write communication programs and establish databases. The command frame format issued by the communication program must fully comply with the RS-485 communication protocol of Siemens PLC; the host computer must disassemble and identify the response frames sent back by the PLC in order to correctly separate the exchange data and useful status information. (3) Touch screen program The touch screen program mainly consists of the main screen, parameter settings, power supply circuit, reverse osmosis, real-time curve, report statistics, fault maintenance and other screens. Its structure is shown in Figure 2. Figure 2 Touch screen program structure 5 Conclusion In the water supply system, the variable frequency constant pressure water supply system based on PLC and touch screen control can automatically adjust the speed or deceleration of the water pump according to the actual water pressure change, realize constant pressure water supply, reduce energy consumption; and extend the service life of the main pump motor. Thanks to the use of dual frequency converters, the system can automatically switch to the other frequency converter when one fails, maintaining a virtually uninterrupted constant pressure water supply, demonstrating its advanced technology. Currently, after more than a year of operation at the Yongcheng Coal and Power Group water plant, the system is performing well, meeting the expected design requirements, exhibiting significant energy-saving effects, and receiving high praise from users, showing promising application prospects.