In the field of industrial field control, programmable logic controllers (PLCs) have always played an important role. As the country’s investment in the water supply industry has increased, the automation level of the water plant has been continuously improved, and the use of PLC in the water supply industry has gradually increased. Touch screen and PLC supporting the use of PLC applications makes it more flexible, while you can set parameters, display data, animation and other situations depict the automation process, making the application of PLC visualization.
The frequency conversion constant pressure water supply has become a mainstream in the water supply industry and is an important means to ensure that the water supply pipe network is under constant pressure. The perfect network communication engineering of modern frequency converters, Wei Motor's synchronous operation, long-distance centralized control and online monitoring provide necessary support. Through the touch screen connected with the PLC, the control can be made more intuitive and the operation is more simple and convenient.
Combine the application of PLC, touch screen and frequency converter, use the communication method to control the frequency converter to achieve constant pressure water supply.
2, system structure
The principle of frequency conversion constant pressure water supply system is shown in Fig. 1; it is mainly composed of PLC, frequency converter, touch screen, pressure transmitter, power and control circuit and pump group. The user can control the operation of the system through the touch screen, and can also understand and control the operation of the system through the indicator lights and buttons on the control panel and the switch. Through a pressure transmitter installed on the outlet pipe network, the outlet pressure signal is changed to 4 to 20 mA or 0 to 10 V. The standard signal is sent to the built-in PID controller of the PLC. The PID operation is compared with the given pressure parameter to output the operating frequency. To the frequency converter. In the control system, the speed of the water pump is controlled by the inverter to adjust the water supply. According to the difference in water consumption, the PLC frequency output gives the frequency converter's operating frequency, so as to adjust the pump speed and achieve constant pressure water supply. The internal program set by the PLC drives the output of the I/O port switch quantity to realize switching of the AC contactor group, so as to coordinate the number of the pump motor that is put into operation, and complete the switching of the motor start and stop, frequency conversion and power frequency. By adjusting the number of motors that are put into operation and controlling the frequency conversion speed of a motor in a motor group, the working pressure of the system pipe network is always stable, and the purpose of constant pressure water supply is achieved.
3, working principle
The system has two operating modes, manual and automatic. In the manual mode, the pump can be controlled through the start and stop buttons on the control cabinet. The pump can be started and stopped as required. This mode is mainly used for equipment debugging, automatic fault and maintenance. In automatic operation, firstly, the frequency conversion of 1# pump runs, and the output frequency of the inverter rises from 0HZ. At the same time, the PID regulator compares the received signal with the given pressure and sends it to the inverter for control. If the pressure is not enough, then the frequency rises to 50HZ, the program set by the PLC to drive the output of the I/O port switch amount to achieve the switch AC contactor group, so that the 1# pump frequency conversion quickly switches to the power frequency, 2# pump frequency conversion start, If the pressure still fails to reach the set pressure, the 2# pump is switched from the variable frequency to the industrial frequency, and the 3# pump is started with the frequency conversion; if the water consumption is reduced, the PLC control starts with the pump that started, and the system is stabilized according to the PID adjustment parameters. Run, always maintain the pressure of the pipe network.
If there is a momentary outage of the power supply, the system is shut down. After the power is restored to normal, the system is manually started and the system automatically returns to the initial state and starts operation. The frequency conversion automatic function is the most basic function of this system, the system finishes all operation processes of starting, stopping, circulating frequency conversion of many pumps automatically.
4, the device parameter settings
The communication parameters of the PLC, the touch screen and the frequency converter must be set correctly before the communication. This system is defined as Modbus protocol with a baud rate of 9600, a data bit of 8, no parity, and a stop bit of 1. In addition to setting the communication parameters, the inverter must also enable "freewheel stop" to protect the motor.
PLC communication parameter settings: TWDLCAA24DRF - hardware - port - port settings, port parameter settings in the port settings; touch screen communication parameter settings: IO Manager - ModbusRTU01 [COM1] - Modbus Equipment, double-click "Modbus Equipment" You can set communication parameters.
5, PLC control system
The system uses Schneider's TWDLCAA24DRF, I/O points 24 points, relay output, PLC programming using Schneider PLC programming software Twidosoft, the software provides a complete programming environment, offline programming, online connection and debugging. In order to increase the cost-effectiveness of the entire system, the system uses the programmable controller's digital input and output to control the start and stop of the motor, automatic input, periodic switching, frequency conversion of the water supply pump and fault alarm, etc., and is given by the built-in PID of the PLC. Motor speed, set pressure, frequency, current, voltage and other analog signal.
Schneider PLC's programming instructions are easy to understand and the program design is flexible. The step counter function module (%SCi) provides a series of steps that can be assigned to actions. Moving from one step to another depends on external or internal events. Through the analog input and output module TWAMM3HT and the built-in PID calculator, the sequential pumping as shown in Fig. 2 is realized.