In industrial automation control systems, the most common application is the combination of PLC and frequency converter , resulting in various methods for PLC control of frequency converters. Among these, the RS-485 communication method is widely used because of its strong anti-interference capability, high transmission rate, long transmission distance, and low cost. However, RS-485 communication requires solving a series of technical problems, such as data encoding, checksum calculation, framing, data transmission, parity checking of received data, timeout handling, and error retransmission. A simple frequency converter operation instruction sometimes requires writing dozens of PLC ladder diagram instructions, making the programming workload large and tedious, which is daunting for designers.
This article introduces a very simple method for controlling frequency converters using Mitsubishi FX series PLCs via communication: It only requires installing an RS-485 communication board or connecting an RS-485 communication module to the PLC host; embedding a "function expansion memory box" costing only a few hundred yuan under the PLC panel; and writing four extremely simple PLC ladder diagram instructions to achieve the reading and writing of parameters for eight frequency converters, as well as the monitoring and control of various operations, with a communication distance of up to 50m or 500m. This method is very simple, convenient, and easy to master. This article uses Mitsubishi products as an example to briefly introduce this simple method of "using extended memory communication to control frequency converters."
2. Mitsubishi PLC system configuration using extended memory communication to control frequency converters
2.1 System Hardware Composition
One FX2N series PLC (product version V3.00 or above) (software using FX-PCS/WIN- CV3.00 version);
One FX2N-485-BD communication template (maximum communication distance 50m);
Alternatively, one FX0N-485ADP communication module + one FX2N-CNV-BD board (maximum communication distance 500m);
One FX2N-ROM-E1 function expansion storage box (installed inside the PLC body);
Eight Mitsubishi inverters with RS485 communication ports (S500 series, E500 series, F500 series, F700 series, A500 series, V500 series, etc., can be used interchangeably, with a total quantity not exceeding eight units; the communication parameter numbers, command codes, and data codes are the same for all Mitsubishi inverter series).
RJ45 cable (5 cores with shield);
Terminating impedance (terminating resistor) 100Ω;
Optional: One human-machine interface (such as a small touchscreen like the F930GOT).
2.2 Hardware Installation Method
(1) Use a network cable crimping pliers to crimp one end of the cable to the RJ45 connector; connect the other end to the FX2N-485-BD communication module as shown in Figures 1 to 3. Do not connect the two unused P5S terminals.
(2) Uncover the panel cover on the left side of the PLC host , install the FX2N-485-BD communication module and FX2N-ROM-E1 function expansion memory, and then cover the panel.
(3) Connect the RJ45 cables to the PU port of the frequency converter respectively. Connect a 100Ω terminating resistor between the receiving signal terminals RDA and RDB of the frequency converter at the network end to eliminate communication obstacles that may be caused by reflection due to signal transmission speed, transmission distance and other reasons.
2.3 Inverter Communication Parameter Settings
To establish communication correctly, communication-related parameters such as "station number," "communication rate," "stop bit length/word length," and "parity check" must be set on the frequency converter. Parameters Pr . 117 to Pr . 124 within the frequency converter are used to set these communication parameters. Parameter settings are performed via the operation panel or the frequency converter setting software FR-SW1-SETUP-WE through the PU port.
2.4 Examples of Inverter Setting Items and Command Codes
2.5 Example of a frequency converter data code table
2.6 PLC Programming Methods and Examples
(1) Communication method
The PLC and frequency converter communicate in a master-slave manner, with the PLC acting as the master and the frequency converter as the slave. There is only one master in a network, and the master distinguishes different slaves by their station numbers. They use half-duplex bidirectional communication; the slave only sends data after receiving read/write commands from the master.
(2) PLC instruction specifications for frequency converter control
(3) Example and comments of PLC statement list program for inverter operation monitoring
LDM8000 operation monitoring;
EXTRK10K0H6FD0EXTRK10: Run monitoring instruction; K0: Station number 0; H6F: Frequency code (see Table 1); D0: PLC read address (data register).
Instruction Explanation: The PLC continuously monitors the speed (frequency) of the inverter with station number 0.
(4) Example and comments of PLC statement list program for inverter operation control
The LDX0 execution command is input via X0;
SETM0 sets the M0 auxiliary relay;
LDM0EXTRK11K0HFAH02EXTRK11: Run control command; K0: Station number 0; HFA: Run command; H02: Forward rotation command.
The execution of the ANDM8029 instruction has ended;
RSTM0 resets the M0 auxiliary relay.
Instruction Explanation: The PLC sends a forward rotation command to the frequency converter with station number 0.
(5) Example and comments of PLC statement list program for reading inverter parameters
LDX3 parameter reading commands are input via X3;
SETM2 sets the M2 auxiliary relay;
LDM2EXTRK12K3K2D2EXTRK10: Inverter parameter read instruction; K3: Station number 3; K2: Parameter 2 - lower limit frequency; D2: PLC read address (data register).
ORRSTM2 resets the M2 auxiliary relay.
Instruction Explanation: The PLC continuously reads parameter 2 - the lower limit frequency of the frequency converter at station number 3.
(6) Example and comments of PLC statement table for writing inverter parameters
LDX1 parameter change commands are input via X3;
SETM1 sets the M1 auxiliary relay;
LDM1EXTRK13K3K7K10EXTRK13: Inverter parameter write command; K3: Station number 3; K7: Parameter 7 - Acceleration time; K10: The value to be written.
EXTRK13K3K8K10EXTRK13: Inverter parameter write command; K3: Station number 3; K8: Parameter 8 - deceleration time; K10: The value to be written.
The execution of the ANDM8029 instruction has ended;
RSTM1 resets the M1 auxiliary relay.
Instruction Explanation: The PLC changes parameter 7 (acceleration time) and parameter 8 (deceleration time) of the frequency converter at station number 3 to 10.
3. A comprehensive review and comparison of various methods for controlling frequency converters using Mitsubishi PLCs.
3.1 PLC-based digital signal control of the frequency converter
The output points and COM points of the PLC (MR or MT type) are directly connected to the inverter's STF (forward start), RH (high speed), RM (medium speed), RL (low speed), and input SG ports, respectively. The PLC can control the inverter's start, stop, and reset via a program; it can also control different combinations of the inverter's high-speed, medium-speed, and low-speed terminals to achieve multi-speed operation. However, because it uses switching signals for control, its speed regulation curve is not a continuous and smooth curve, and it cannot achieve precise speed adjustment. The speed regulation accuracy of this switching control method cannot be compared with that using extended memory communication control.
3.2 PLC analog signal control of frequency converter
Hardware: FX1N or FX2N PLC main unit, configured with a simple FX1N-1DA-BD extended analog output board; or an analog input/output mixed module FX0N-3A; or a two-output FX2N-2DA; or a four-output FX2N-4DA module, etc.
Advantages: PLC programming is simple and convenient, the speed regulation curve is smooth and continuous, and the operation is stable.
Disadvantages: In large-scale production lines, control cables are long, especially when the DA module uses voltage signal output, resulting in significant voltage drop in the lines, affecting system stability and reliability. Furthermore, from an economic perspective, controlling eight frequency converters requires two FX2N-4DA modules, which costs 5 to 7 times more than using extended memory communication control.
3. The PLC uses RS-485 protocol-free communication to control the frequency converter.
This is the most common method, where the PLC is programmed using RS serial communication instructions.
Advantages: Simple hardware, lowest cost, can control 32 frequency converters.
Disadvantages: The programming workload is relatively large. As discussed in Chapter 2, programming using extended memory communication control is extremely simple. Technicians with PLC programming experience can master it in just a few hours if they know how to look up tables, and the additional hardware costs are very low. The ease of programming with this method is incomparable to the method of controlling frequency converters using RS-485 protocol-less communication.
3.4 The PLC uses RS-485 Modbus-RTU communication to control the frequency converter.
Mitsubishi's new F700 series inverters use RS-485 terminals to communicate with PLCs via the Modbus-RTU protocol.
Advantages: PLC programming using Modbus communication is simpler and more convenient than RS-485 without protocol.
Disadvantages: PLC programming still requires a significant amount of work.
3.5 The PLC uses a fieldbus method to control the frequency converter.
Mitsubishi inverters can be equipped with various communication options, such as the FR-A5NC option for CC-Link fieldbus; the FR-A5AP(A) option for ProfibusDP fieldbus; and the FR-A5ND option for DeviceNet fieldbus, etc. Mitsubishi FX series PLCs have corresponding communication interface modules for interface with them.
Advantages: High speed, long distance, high efficiency, stable operation, simple programming, and can connect to a large number of frequency converters.
Disadvantages: The cost is high, far exceeding the cost of using extended memory for communication control.
In conclusion, the method of using extended memory to communicate and control frequency converters with PLCs does have the advantages of low cost, ease of learning and use, and reliable performance; if a human-machine interface is configured, the setting and monitoring of frequency converter parameters will become even more convenient.
An AC variable frequency drive system consisting of one PLC and no more than eight frequency converters is a common small-scale industrial automation system, widely used in various industrial fields such as small paper production lines, single-face corrugated cardboard machinery, plastic film production lines, printing and dyeing bleaching machinery, and looper metal drawing machines. Employing a simplified control method allows the engineering solution to possess the many advantages of communication control while eliminating the complex calculations required for RS-485 data communication, greatly improving project quality and work efficiency. However, this simplified method also has its drawbacks: it can only control the frequency converters and not other devices; furthermore, the number of frequency converters that can be controlled is also limited.
