introduction
Programmable Logic Controllers (PLCs) are widely used in modern industrial control due to their advantages such as simple and convenient programming, stable and reliable control, and powerful functions. Touchscreens, as human-machine interfaces, reduce the use of external I/O points for PLCs and simplify the wiring complexity of external buttons and switches, while also improving ease of operation and maintenance. With the increasing demands for miniaturization, ease of operation, and intelligence in industrial settings, the application prospects of AC variable frequency speed control systems based on PLCs and touchscreens are very broad. This paper designs an AC variable frequency speed control experimental system for two three-phase asynchronous motors using a Mitsubishi PLC (Fx2N-64MR), a Hitech touchscreen (PWS6AOOT), a Lenze frequency converter, and external buttons. Actual operation results show that the system operates stably and reliably with good control performance.
1. Control System Requirements
This system is required to control two three-phase asynchronous motors in the following states: forward rotation; reverse rotation; stop; jogging; acceleration; and deceleration. These functions should be implemented via a touchscreen or external buttons, with both digital input methods redundantly configured to improve the reliability of the control system. Additionally, indicator lights should display the various digital input states and any hardware malfunctions.
2 Control System Hardware Design
The hardware structure of the AC variable frequency speed control system is shown in Figure 1.
The hardware structure of the control system mainly includes: a programmable control module, a control command input module, a D/A conversion module, and a frequency converter regulation module.
2.1 Module Functions
2.1.1 Programmable Control Module
This module is the core processor of the entire control system, serving as the execution center for touchscreen and push-button switch commands, and the triggering element for inverter commands.
2.1.2 Control Command Input Module
This module loads control instructions onto the PLC to achieve corresponding output operations. Instruction input can be achieved via touchscreen buttons or external switches, with both methods serving as backups for each other. To avoid complex wiring due to numerous instructions being implemented via buttons and switches, redundant backups can be implemented for critical switching quantities, while less critical switching quantities are implemented solely via touchscreen buttons.
2.1.3D/A Module
A D/A converter converts the digital output from a PLC into an analog voltage signal to achieve variable frequency speed control. This system uses the FX2N-2DA module, which has two analog outputs to control two frequency converters.
2.1.4 Inverter Regulation Module
The frequency converter can perform corresponding adjustments according to the instructions loaded on its input control terminals by the PLC, and can execute various working states of a three-phase asynchronous motor.
2.2 Module Communication
The PC communicates with the PLC and touchscreen modules via dedicated cables. Once the program is written and debugged, it can be directly downloaded to the program memory of both the touchscreen and the PLC. The PLC module and the touchscreen are connected by a dedicated cable, and touchscreen button commands can be loaded into the control program via the communication cable to execute corresponding operations. Additionally, external button commands are directly loaded onto the PLC input terminals to achieve corresponding instruction operations. The PLC and D/A module are connected by an extension cable, converting the PLC's digital output into two corresponding voltage signals, which are then loaded onto the inverter's input terminals to achieve speed control.
3 Control System Software Design
3.1 PLC Programming
3.1.1 Input and Output Address Allocation
Based on the requirements of the control system, determine the number of digital inputs and outputs and assign addresses to the PLCs accordingly.
X00～X04 and X10～X14 are respectively assigned to the switch input terminals (including forward rotation, reverse rotation, jogging, and stop) of motors 1 and 2 and the inverter fault input terminals.
Y00～Y04 and Y10～Y14 are respectively assigned to the switch output indicators (including forward rotation, reverse rotation, jogging, and stop) and touch screen fault indicators of motors 1 and 2.
Y20～Y22 and Y30～Y32 are connected to the E1, E2 and 28 control terminals of the two frequency converters, respectively. Among them, the function of the E1 terminal is to activate the fixed given speed when the level is high; the E2 terminal controls the rotation direction; and the 28 terminal controls the motor start and stop.
M00～M05 and M10～X15 are respectively assigned to the PLC write addresses of the 1 and 2 key instructions of the touch screen (including forward rotation, reverse rotation, jogging, acceleration, deceleration, and stop).
In addition, registers D1 and D2 store the D/A conversion values of the two motors, respectively, and register M8000 monitors the operating status of the PLC.
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, which facilitates application expansion. The corresponding program flowchart is shown in Figure 2.
3.2 Touchscreen Programming
The development platform for the touchscreen human-machine interface of this system is HitechADP programming software for Hitech touchscreens. This software is similar to configuration software, using a graphical programming method. Users simply drag relevant components onto a predefined screen, set the necessary parameters, and configure the PLC write addresses to complete the operation.
HitechADP programming software has low requirements for the programming PC. It utilizes the touchscreen's built-in RS232 serial port or USB interface for communication, allowing the designed human-machine interface to be downloaded to the touchscreen. For the two motors, control keys for forward rotation, reverse rotation, jogging, acceleration, deceleration, stop, and speed display were designed, enabling control and monitoring of the motor's operating status. After the touchscreen is powered on, it automatically enters the designed screen, allowing operators to directly control the lower-level PLC via human-machine interaction as needed. The human-machine interface is shown in Figure 3.
4 Conclusion
This AC variable frequency speed control system for a three-phase asynchronous motor, based on a programmable logic controller (PLC) and a touchscreen, fully utilizes the PLC's powerful logic processing capabilities and the user-friendly interface, avoiding the complex wiring of traditional relay-contactor control circuits and reducing the technical requirements for operators. Simultaneously, it implements redundant backup modes for important switch inputs via touchscreen buttons and external buttons, improving system reliability and facilitating real-time monitoring and maintenance by on-site operators. Furthermore, as a subsystem of a professional laboratory integrated experimental system, this system combines theory with practice, providing excellent guidance and practical significance for students to master new technologies and concepts and improve their hands-on skills.
