I. Introduction
In industrial automation control systems, the combined use of PLCs (Programmable Logic Controllers) and variable frequency drives has become a key method for achieving efficient and precise control. Mitsubishi PLCs have emerged as leaders in the field of industrial automation due to their outstanding performance and wide range of applications. This article will provide a detailed overview of various methods for controlling AC drives with Mitsubishi PLCs, including digital signal control, analog signal control, RS-485 protocol-free communication control, Modbus-RTU communication control, and fieldbus control, supported by relevant data and information.
II. Digital Signal Control
Digital signal control is a fundamental method for controlling variable frequency drives (VFDs) using Mitsubishi PLCs. This method connects the PLC's output and COM terminals to the VFD's STF (Forward Start), RH (High Speed), RM (Medium Speed), and RL (Low Speed) ports to control the VFD's start, stop, reset, and multi-speed operation. However, because this method relies on digital signal control, the speed control curve is not a continuous, smooth curve. Consequently, it cannot achieve fine-grained speed regulation, and speed control accuracy is relatively low.
In terms of hardware connections, the output points and COM points of the Mitsubishi PLC (MR or MT series) are directly connected to the corresponding ports on the VFD. In terms of software programming, the PLC controls different combinations of VFD terminals via programming to achieve multi-speed operation. For example, when PLC output point Y0 is activated, the inverter's STF terminal receives the signal, and the motor begins to rotate forward; when Y1 is activated, the inverter's RH terminal receives the signal, and the motor runs at high speed; and so on, thereby achieving multi-speed control.
III. Analog Signal Control
Analog signal control is another commonly used method for controlling inverters with Mitsubishi PLCs. This method involves configuring the PLC's analog output modules (such as FX1N-1DA-BD, FX0N-3A, FX2N-2DA, FX2N-4DA, etc.) to convert the PLC's digital outputs into analog signals (such as voltage or current), which are then input to the inverter's analog input terminals, thereby enabling continuous and smooth adjustment of the inverter's speed.
In terms of hardware connections, the PLC must be equipped with the appropriate analog output modules, and the module's output terminals must be connected to the inverter's analog input terminals. In terms of software programming, the PLC controls the output values of the analog output modules via a program, thereby achieving continuous speed regulation of the VFD. For example, when the motor needs to run at a specific speed, the PLC can calculate the corresponding analog output value and send it to the VFD via the analog output module, causing the motor to operate at the specified speed.
It should be noted that in large-scale production lines, due to the length of control cables, analog signal control may be affected by voltage drops along the line, thereby impacting system stability and reliability. Additionally, from an economic perspective, analog signal control is relatively costly.
IV. RS-485 Protocol-Free Communication Control
RS-485 protocol-free communication control is a widely used method for controlling inverters with Mitsubishi PLCs. This method connects the PLC's RS-485 communication interface to the inverter's RS-485 communication interface to facilitate data exchange and command transmission between the PLC and the inverter. This method offers the advantages of simple hardware and low cost, and can control up to 32 inverters.
In terms of hardware connection, simply connect the PLC's RS-485 communication interface to the inverter's RS-485 communication interface. For software programming, the PLC uses RS serial communication instructions to program the system, enabling data exchange and command transmission with the inverter. For example, the PLC can send commands such as start, stop, and speed setting to the inverter. Upon receiving these commands, the inverter executes the corresponding operations and feeds back its operating status to the PLC.
V. Modbus-RTU Communication Control
Modbus-RTU communication control is a new method for controlling inverters using a Mitsubishi PLC. This method connects the PLC's RS-485 communication interface to the inverter's RS-485 communication interface and uses the Modbus-RTU protocol for communication. While this method offers the advantage of simple and convenient programming, the PLC programming workload remains substantial.
In terms of hardware connection, the setup is identical to that of protocol-free RS-485 communication control; simply connect the PLC's RS-485 communication interface to the inverter's RS-485 communication interface. Regarding software programming, the PLC is programmed using the Modbus-RTU protocol to facilitate data exchange and command transmission with the inverter. For example, the PLC can send commands to the inverter to read its operating status or set parameters. Upon receiving these commands, the inverter executes the corresponding operations and returns the response data to the PLC.
VI. Fieldbus Control
Fieldbus control is an advanced method for controlling inverters using Mitsubishi PLCs. This method utilizes fieldbus technology to facilitate data exchange and command transmission between the PLC and the inverter. Mitsubishi inverters can be equipped with various types of communication options, such as the FR-A5NC option for CC-Link fieldbus and the FR-A5AP(A) option for Profibus DP fieldbus. This method offers advantages such as high communication speed, large data transmission capacity, and strong interference resistance.
In terms of hardware connections, the PLC and the inverter must be equipped with the corresponding fieldbus communication interfaces and communication options. Regarding software programming, the PLC is programmed using the fieldbus protocol to facilitate data exchange with the inverter.