Research on PLC Asynchronous Motor Control System

Abstract: This paper designs a PLC-based asynchronous motor speed control and positioning integrated control system, which uses the fuzzy-PI composite control algorithm to realize the speed control of the asynchronous motor, and uses the proportional factor self-adjusting fuzzy control algorithm to realize the position control of the asynchronous motor. The system integrates the asynchronous motor speed control and position control, achieving a certain control accuracy.

1 Introduction

In the spindle servo system of processing equipment and machine tools, the spindle should have both speed and position control functions; in the constant pressure water supply system of residential areas and high-rise buildings, the motor is required to have a high speed regulation performance; in the accurate positioning of steelmaking converters and the position control system of stacking machinery, the motor is required to have a precise positioning function. In the above application occasions, asynchronous motors have a place with their unique advantages of high power and high cost performance, but at the same time, their speed regulation performance and positioning performance are not perfect and need to be improved.

2 Hardware Design

The hardware of the asynchronous motor integrated control system is shown in Figure 1. In Figure 1, the host computer and PLC control the speed and position of the asynchronous motor through the frequency converter. The speed and position information of the asynchronous motor can be obtained through the pulse count value of the rotary encoder. The pulse count is completed by the PLC, and it continuously communicates with the host computer and transmits the count value to the host computer. The host computer obtains the speed and position information based on the pulse count value transmitted by the PLC, and obtains the output control quantity - the speed given value according to different control strategies, and then transmits it to the PLC. After the A/D conversion module of the PLC, the analog value of the speed given value is sent to the analog control end of the frequency converter for control, forming a closed-loop control.

In the system experiment, the asynchronous motor used is YZA5*S three-phase 180W asynchronous motor, with a rated voltage of 380V, a rated frequency of 50Hz, a rated speed of 1400r/min, and a rated current of 0.66A; the rotary encoder used is E6B2-CWZ6C three-phase rotary encoder, with a resolution of 1000P/R (up to 4000P/R using up and down counting method), a rated voltage of 5VDC-5%~24VDC+5%, and open collector output; the frequency converter is Siemens' MICRO MASTER440; the PLC uses OMRON CQM1H.

Under open-loop control conditions, the MICRO MASTER440 inverter can receive start, stop, forward and reverse commands through the BOP board or digital input port to control the motor operation mode, and receive speed adjustment commands through the BOP board, analog input port and digital input port. This system implements closed-loop control, so the PLC's switch output port issues the motor operation mode control command to the inverter's digital input port, and the PLC A/D conversion module's output port issues the speed adjustment command to the inverter's analog input port.

CQM1H PLC provides an RS232C communication port. It can be connected to the host computer serial port with a cable to form a simple monitoring network. The host computer realizes the monitoring function, including the display of the human-computer interaction interface, the input of control instructions and parameters, the implementation of complex control algorithms, the display of experimental result curves, the storage and printing of data, etc. The communication between the host computer and PLC adopts the fixed format communication protocol required by CQM1H.

3 Software Implementation

3.1 Implementation of communication and monitoring functions

In order to realize the communication function between PLC and host computer, PLC and host computer must be set separately. In CQM1H, it is necessary to set the relevant bytes of the system storage area to determine the baud rate, start bit, data bit, stop bit and other communication frame formats of serial communication. For example, set DM*6 to: baud rate 9600K, even parity, start bit 1 bit, bit length 7 bits, stop bit 2 bits, etc. In the host computer, high-level language is used to write program code to realize communication and monitoring functions. Visual Basic language is used here. The software is easy to use. It has both the excellent performance and graphical working environment unique to Windows and the simplicity of programming. In Visual Basic, there is a special serial communication control-MSComm communication control, through which the communication frame format can be set to realize the communication function. The specific setting steps are as follows:

MSComm1.CommPort = 1

MSComm1.Settings = "9600,e,7,2" ; Set the communication frame format

MSComm1.PortOpen = True ; Open the communication port

MSComm1.InBufferCount = 0 ; Clear the receive buffer

MSComm1.OutBufferCount = 0 ; Clear the send buffer

MSComm1.InputMode=ComInputMode Text; data is retrieved in text format

There are two ways for the host computer to monitor CQM1H. One is that the host computer issues an initial command to the PLC, requiring the PLC to return relevant operating data, and then displays the results and performs control operations based on the data, and issues control instructions to the PLC to directly change the digital or analog output of the PLC to achieve speed and position control; the other is that the PLC actively sends an initial command to the host computer, provides relevant operating data, and the host computer receives the data and performs result display and control operations based on the data. The former is adopted here.

To match the communication data format of CQM1H, the format of the command sent by the host computer is as follows:

The command to be sent must start with @, the node number indicates the PLC communicating with the host computer, the header code is a two-character command code, the body sets the command data, FCS is the group check sequence, and the terminator is fixedly set to * and ↙.

The PLC response format is as follows:

The tail code returns the completion status (whether an error occurred), and the body is returned only when there is data. When the data is longer than 131 characters, it can be transmitted in groups. The data returned by the PLC is in ASCII format. Since @, node number, FCS, terminator, etc. are automatically added, it is necessary to write a related processing program in the host computer to intercept the required data content when the communication is normal.

3.2 Implementation of control algorithm

The host computer has powerful and fast computing capabilities, so various control strategies such as PID algorithm, fuzzy control algorithm, artificial neural network control algorithm, genetic algorithm, etc. can be easily implemented. The fuzzy control algorithm is mainly used here.

3.2.1 Speed ​​Control Algorithm

The three-phase asynchronous motor model is a high-order nonlinear, strongly coupled, multivariable model [1]. Many factors need to be considered when controlling it. For example, the rotor voltage, frequency, magnetic flux, rotor speed and position and other parameters are coupled to each other, the winding has electromagnetic inertia, the rotor has electromechanical inertia, the rotor resistance value changes with temperature, and the load torque and moment of inertia may have various random disturbances. The influence of these factors brings difficulties to control. It is difficult to achieve high accuracy requirements by using only classical control methods. The fuzzy control algorithm does not require an accurate mathematical model of the controlled object, is insensitive to parameter changes of the controlled object, and has strong robustness. Therefore, the inverter and the asynchronous motor are regarded as a whole, and the fuzzy control algorithm is used to achieve speed control.

At the same time, considering that the fuzzy controller is an error controller, the steady-state stage after reaching the desired speed has a relatively large fluctuation. When the error is small, PI control has a higher steady-state accuracy [2,3]. Therefore, the incremental PI control strategy is introduced into the fuzzy controller to form a fuzzy-PI composite control to improve the steady-state performance of the fuzzy controller. The asynchronous motor speed control system based on fuzzy-PI composite control is shown in Figure 2(a), and the experimental results are shown in Figure 2(b) (given speed 100 r/min, sampling period T=150ms, speed error ≤1.0 r/min).

The fuzzy control system based on self-adjustment of the proportional factor is shown in Figure 3(a), and the experimental results are shown in Figure 3(b) (the motor is positioned at 0° when running at rated speed, the sampling period T = 150ms, and the position error ≤ 0.09°, that is, the position interval of 1 counting pulse).

4 Conclusion

Based on the PLC hardware platform, this paper makes full use of PLC's own resources to design a speed and position integrated control system for asynchronous motors. Experiments show that the system has achieved a certain control accuracy and has a certain practical reference value.