In the industrial automation control system, the most common is the combined application of PLC and inverter, and a variety of methods of PLC control inverter have been produced. Among them, the use of RS-485 communication to implement control is widely used: because It has strong anti-interference ability, high transmission rate, long transmission distance and low cost. However, RS-485 communication must solve a series of technical problems such as data encoding, checksum calculation, framing, sending data, receiving data parity, overtime processing and error retransmission, and a simple inverter operation instruction , Sometimes it takes dozens of PLC ladder diagram instructions to be realized. The programming workload is large and cumbersome, which makes designers daunting.
This article introduces a very simple way to control the inverter in Mitsubishi FX series PLC communication mode: it only needs to install an RS-485 communication board or hook an RS-485 communication module on the PLC host; embed one under the PLC panel The "function expansion storage box" with a cost of only a few hundred yuan, and writing 4 extremely simple PLC ladder diagram instructions, can realize the reading and writing of parameters of 8 inverters, monitoring and control of various operations, and the communication distance can be Up to 50m or 500m. This method is very simple, convenient and easy to master. This article takes Mitsubishi products as an example, and briefly introduces this simple method of "using extended memory communication to control the inverter".
2.1 System hardware composition
FX2N series PLC (product version V 3.00 or higher) 1 set (software adopts FX-PCS/WIN-C V 3.00 version);
1 FX2N-485-BD communication template (the longest communication distance is 50m);
Or 1 FX0N-485ADP communication module + 1 FX2N-CNV-BD board (the longest communication distance is 500m);
1 FX2N-ROM-E1 function expansion storage box (installed in the PLC body);
8 Mitsubishi inverters with RS485 communication port (S500 series, E500 series, F500 series, F700 series, A500 series, V500 series, etc., can be mixed with each other, the total number does not exceed 8; the communication parameter numbers of all Mitsubishi inverters , Command code and data code are the same.);
RJ45 cable (5 core with shield);
Terminal impedance device (terminal resistance) 100Ω;
Option: 1 set of man-machine interface (small touch screen such as F930GOT).
2.2 Hardware installation method
(1) Use the special crimping pliers for the network cable to crimp one end of the cable with the RJ45 crystal plug; connect the other end to the FX2N-485-BD communication module as shown in Figure 1 to Figure 3. The two unused P5S ends are not Pick up.
(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 inverter, and connect a 100Ω terminal resistor between the receiving signal terminals RDA and RDB of the inverter at the end of the network to eliminate the possibility of reflection due to signal transmission speed and transmission distance. Communication barriers caused by the impact of
2.3 Inverter communication parameter setting
In order to establish communication correctly, communication-related parameters such as "station number", "communication rate", "stop bit length/word length", "parity check", etc. must be set in the inverter. The Pr.117~Pr.124 parameters in the inverter are used to set the communication parameters. The parameter setting adopts the operation panel or the inverter setting software FR-SW1-SETUP-WE to carry out on the PU port.
2.4 Examples of inverter setting items and instruction codes
2.5 Example of frequency converter data code table
2.6 PLC programming methods and examples
(1) Communication method
The communication between PLC and frequency converter adopts master-slave mode, PLC is the master and the frequency converter is the slave. There is only one master in a network, and the master distinguishes different slaves by station number. They use half-duplex two-way communication, and the slave only sends data after receiving a read and write command from the host.
(2) PLC instruction specifications for inverter control
(3) PLC statement list program example and comments for inverter operation monitoring
LD M8000 operation monitoring;
EXTR K10 K0 H6F D0 EXTR K10: run monitoring command; K0: station number 0; H6F: frequency code (see Table 1); D0: PLC read address (data register).
Instruction explanation: PLC always monitors the speed (frequency) of the inverter with station number 0.
(4) PLC statement list program example and notes for inverter operation control
LD X0 running command is input by X0;
SET M0 set M0 auxiliary relay;
LD M0 EXTR K11 K0 HFA H02 EXTR K11: run control command; K0: station number 0; HFA: run command H02: forward rotation command.
AND M8029 instruction execution ends;
RST M0 Reset M0 auxiliary relay.
Instruction explanation: PLC sends forward rotation instruction to the inverter with station number 0.
(5) PLC statement list program example and comments for inverter parameter reading
LD X3 parameter read command is input by X3;
SET M2 set M2 auxiliary relay;
LD M2 EXTR K12 K3 K2 D2 EXTR K10: inverter parameter reading instruction; K3: station number 3; K2: parameter 2-lower limit frequency; D2: PLC reading address (data register).
OR RST M2 Reset M2 auxiliary relay.
Instruction explanation: PLC always reads the No. 2 parameter-lower limit frequency of the inverter with station No. 3.
(6) PLC statement list program example and comments for inverter parameter writing
LD X1 parameter change command is input by X3;
SET M1 Set M1 auxiliary relay
LD M1 EXTR K13 K3 K7 K10 EXTR K13: inverter parameter write command; K3: station number 3; K7: parameter 7-acceleration time; K10: written value.
EXTR K13 K3 K8 K10 EXTR K13: inverter parameter write command; K3: station number 3; K8: parameter 8-deceleration time; K10: written value.
AND M8029 instruction execution ends;
RST M1 Reset M1 auxiliary relay.
Instruction explanation: PLC changes the No. 7 parameter-acceleration time and the No. 8 parameter-deceleration time of the inverter with station number 3 to 10.
3. Comprehensive review and comparison of various methods of Mitsubishi PLC control inverter3.1 PLC's switch signal control frequency converter
The output point and COM point of PLC (type MR or MT) are directly connected to the STF (forward start), RH (high speed), RM (medium speed), RL (low speed), input terminal SG and other ports of the inverter. The PLC can control the start, stop, and reset of the inverter through programs; it can also control the different combinations of the inverter's high-speed, medium-speed, and low-speed terminals to achieve multi-speed operation. However, because it uses a switch to implement control, its speed control curve is not a continuous and smooth curve, and it cannot achieve fine speed adjustment. This kind of switch quantity control method, its speed regulation accuracy cannot be compared with adopting the expansion memory communication control.
3.2 PLC's analog signal control frequency converter
Hardware: FX1N, FX2N PLC host, equipped with a simple FX1N-1DA-BD extended analog output board; or analog input and output mixed module FX0N-3A; or two output FX2N-2DA; or four channels Output FX2N-4DA module, etc.
Advantages: PLC programming is simple and convenient, the speed control curve is smooth and continuous, and the work is stable.
Disadvantages: In a large-scale production line, the control cable is long, especially when the DA module uses voltage signal output, the line has a large voltage drop, which affects the stability and reliability of the system. In addition, from an economic point of view, for example, to control 8 inverters, 2 FX2N-4DA modules are required, and the cost is 5 to 7 times that of the expansion memory communication control.
3.3 PLC uses RS-485 protocol-free communication method to control the inverter
This is the most commonly used method. PLC uses RS serial communication instructions to program.
Advantages: simple hardware, lowest cost, and can control 32 inverters.
Disadvantages: The programming workload is relatively large. From the second chapter of this article, it can be seen that the programming using extended memory communication control is extremely simple. As long as the technicians who have engaged in PLC programming know how to look up the meter, they can master it in just a few hours, and the additional hardware costs are also very low. The ease of programming in this method is incomparable to the method of controlling the inverter using RS-485 protocol-free communication.
3.4 PLC uses RS-485 Modbus-RTU communication method to control the inverter
Mitsubishi's new F700 series inverter uses RS-485 terminals to communicate with PLC using Modbus-RTU protocol.
Advantages: PLC programming in Modbus communication mode is simpler and more convenient than RS-485 protocol-free mode.
Disadvantages: PLC programming workload is still relatively large.
3.5 PLC uses fieldbus to control the frequency converter
Mitsubishi inverters can be built-in various types of communication options, such as FR-A5NC option for CC-Link fieldbus; FR-A5AP(A) option for Profibus DP fieldbus; For DeviceNet fieldbus FR-A5ND option and so on. Mitsubishi FX series PLC has corresponding communication interface module to connect with it.
Advantages: fast speed, long distance, high efficiency, stable work, simple programming, and a large number of inverters that can be connected.
Disadvantages: The cost is relatively high, which is much higher than the cost of using extended memory communication control.
To sum up, the method of PLC using extended memory communication to control the inverter has the advantages of low cost, easy to learn and use, and reliable performance; if the man-machine interface is configured, the inverter parameter setting and monitoring will become more convenient.
The AC variable frequency drive system composed of 1 PLC and no more than 8 inverters is a common small industrial automation system, which is widely used in small paper production lines, single-faced corrugated board machinery, plastic film production lines, printing, dyeing, cooking and bleaching machinery, loopers Metal wire drawing machine and other industrial fields. The use of simple control methods can make the project plan have many advantages of communication control, and can save many complicated calculations in RS-485 data communication, so that the project quality and work efficiency are greatly improved. However, this simple method also has its shortcomings: it can only control the inverter but not other devices; in addition, the number of control inverters is also limited.
4 ConclusionThis article introduces in more detail the simple method of PLC using extended memory communication to control inverter, and comprehensively reviews various methods of Mitsubishi PLC to control inverter. A thorough understanding of these methods will help improve the scientific, advanced and economical design of the AC variable frequency drive control system. Readers can choose the appropriate program according to the specific situation of the system. Despite its shortcomings, the simple method highlighted in this article is still a good method with popularization value.
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