Design of AC frequency conversion speed regulation system based on PLC and touch screen

0 Introduction
Programmable logic controller (PLC) is widely used as a controller in the field of modern industrial control due to its advantages of simple and convenient programming, stable and reliable control, and powerful functions. As a human-computer interaction interface, the touch screen reduces the use of external I/O points of the PLC and reduces the complexity of wiring of the system's external button switches to a certain extent, while also improving the convenience of operation and maintenance. With the continuous improvement of the requirements for miniaturization, easy operation, and intelligence of control equipment in industrial sites, the application prospects of AC variable frequency speed regulation systems based on PLC and touch screens will be very broad. This paper uses Mitsubishi PLC (Fx2N-64MR), Hitech touch screen (PWS6AOOT), Lenze inverter and external buttons to realize the design of AC variable frequency speed regulation experimental system for two three-phase asynchronous motors. The actual operation results show that the system operates stably and reliably with good control performance.

1 Control system requirements
This system is required to be able to control the following states of two three-phase asynchronous motors: forward; reverse; stop; jog; acceleration; deceleration. It is required that the above functions can be realized by touch screen or external buttons, and the two switch input methods are redundant to improve the reliability of the control system. In addition, indicator lights are required to display various switch states and abnormal hardware states.

2 Control system hardware design
The hardware structure of the AC variable frequency speed regulation system is shown in Figure 1.

The hardware structure of the control system mainly includes: programmable control module, control instruction input module, D/A conversion module and inverter adjustment module.

2.1 Module Function
2.1.1 Programmable Control Module
This module is the core processor of the entire control system, the execution center of touch screen instructions and button switch instructions, and the inverter instruction trigger element.
2.1.2 Control Instruction Input Module
This module is to load control instructions to the PLC to realize the corresponding output operation. Here, the instruction input can be realized by the touch screen button or by the external switch button, and the two instruction input methods are mutually redundant. In order to avoid the complicated wiring caused by the implementation of many instructions by the button switch instructions, the important switch quantity can be redundantly reserved, and the non-important switch quantity is only realized by the touch screen button.
2.1.3 D/A Module
D/A converts the digital quantity output by the PLC into an analog voltage quantity to achieve the purpose of variable frequency speed regulation. This system uses the FX2N-2DA module, which has two analog outputs to realize the control of two inverters.
2.1.4 Inverter Adjustment Module
The inverter can perform corresponding adjustments according to the instructions loaded by the PLC on its input control terminal, and can execute various working states of the three-phase asynchronous motor.
2.2 Module Communication
The PC communicates with the PLC module and the touch screen module through a dedicated cable. After the program is compiled and debugged, it can be directly downloaded to the touch screen and PLC program memory. The PLC module and the touch screen are connected by a dedicated cable. The touch screen key commands can be loaded into the control program by the communication cable to perform the corresponding operations. In addition, the external button commands are directly loaded into the PLC input terminal to realize the corresponding command operation. The PLC and the D/A module are connected by an extension cable, and the PLC output digital quantity is converted into the corresponding two-way voltage signal and loaded into the inverter input terminal to realize speed control.

3 Control system software design
3.1 PLC program design
3.1.1 Input and output address allocation
According to the control system requirements, determine the number of switch inputs and outputs and allocate addresses to the PLC.
X00~X04, X10~X14 are respectively allocated to the switch input terminals (including forward, reverse, jog, stop) of motors 1 and 2 and the inverter fault input terminals.
Y00~Y04, Y10~Y14 are respectively allocated to the switch output indications (including forward, reverse, jog, stop) of motors 1 and 2 and the touch screen fault indication.
Y20~Y22, Y30~Y32 are respectively connected to the E1, E2 and 28 control terminals of the two inverters. Among them, the E1 terminal function activates the fixed given speed when the high level; the E2 terminal controls the rotation direction; the 28 terminal controls the start and stop of the motor.
M00~M05, M10~X15 are respectively allocated to the PLC write addresses of the 1 and 2 key commands of the touch screen (including forward, reverse, jog, acceleration, deceleration, stop).
In addition, registers D1 and D2 store the D/A values ​​to be converted of the two motors respectively, and register M8000 monitors the PLC operation status.
3.1.2 Program flow design
The PLC program is implemented using Mitsubishi FXGP-WIN-E programming software. The program adopts a modular and functional structure to facilitate expansion and application. The corresponding program flow chart is shown in Figure 2.

3.2 Touch screen programming
The development platform of the touch screen human-machine interaction interface of this system is implemented using Hitech ADP programming software of Hitech touch screen. This software is similar to configuration software and adopts a graphical programming method. You only need to drag the relevant components to the pre-defined screen, set the relevant parameters as needed, and reasonably configure the PLC write address to complete the operation.
Hitech ADP programming software has low requirements for the programming PC. It uses the RS 232 serial port or USB interface communication of the touch screen to download the designed human-machine interaction interface to the touch screen. For the two motors, forward, reverse, jog, acceleration, deceleration, stop and speed display control keys are designed respectively, and the corresponding control keys can be used to control and monitor the motor operation status. After the touch screen is powered on, it automatically enters the designed screen. The operator can directly control the lower computer PLC through human-machine interaction as needed. The human-machine interface is shown in Figure 3.

4 Conclusion
The AC variable frequency speed regulation system of three-phase asynchronous motor based on programmable controller and touch screen makes full use of the powerful logic processing function of PLC and the good interactivity of human-machine interface, avoiding the complex wiring of traditional relay-contactor control circuit and reducing the technical requirements for operators; at the same time, it realizes the redundant standby mode of touch screen buttons and external buttons for important switch input, improves the reliability of the system, and brings convenience to the real-time monitoring and maintenance of the operation process for on-site operators. At the same time, as one of the subsystems of the comprehensive test system of professional laboratories, this system combines theory with practice, which has good guiding significance and practical significance for students to master new technologies and new concepts and improve their hands-on ability.