Distributed multi-motor synchronous control system based on CAN bus

The coordinated control of multiple asynchronous motors plays an important role in industrial production. At present, there are two main frequency conversion control methods: speed sensorless type and speed sensor type. The speed sensorless type has a low price, but poor accuracy. The speed feedback type has high accuracy, but the setting is complicated and the price is high. Based on the Lenze9326 vector servo inverter (with speed feedback) group control system, the CAN bus network structure is adopted. The field control unit concentrates the functional advantages of the field bus and regulator. The control function is powerful and has extremely high reliability. The line speed error of each section of the production line is small, and the control target of dynamic synchronization can be achieved during the acceleration and deceleration process. This scheme has a high reference value for the design and research of chemical production line control systems.

System speed control scheme

This solution is used in places with high process requirements, small conveyor belt speed error, strong production line process continuity, high degree of automation, and relatively high requirements for operation reliability, speed coordination and stability. In the process of acceleration and deceleration, good dynamic synchronization performance is required. When running at a constant speed, the conveyor belt conveys continuous sheet objects with approximately zero tension. It is particularly suitable for the continuous production of textiles and chemical raw materials. The control goal is to minimize the tension caused by speed mismatch and prevent the continuous production of sheet materials from being broken due to rapid changes in tension.

The scheme is shown in Figure 1. A PC is used as the basic speed setting. Through real-time data acquisition and processing, the operating status of each station is monitored online, and the given linear speed value is transmitted from the monitoring station to the system controller (PLC) and the main inverter 11 through the field bus. The auxiliary speed of the main inverter 11 is given by the rotary transformer connected to the main command motor. The main speed setting of the inverter 12 is sent by the inverter 11 through the CAN bus, and the auxiliary speed is given by the laser displacement sensor. The main speed setting of the inverter 13 is sent by the inverter 12 through the CAN bus, and the auxiliary speed is given by the laser displacement sensor. The main speed setting of the inverter 14 is sent by the inverter 13 through the CAN bus, and the auxiliary speed is given by the laser displacement sensor. Through the sequential transmission of speed, the speed of each section of the conveyor belt can be smoothly followed. Thereby, the real-time adjustment and status monitoring of the running speed of the production line are realized. The running speed of the conveyor belt is from 35 to 120m/s, and stepless speed regulation is achieved within this range.

Control system hardware design

The system is based on Lenze's 93 series inverter 9326 and PCI card based on SJA1000. It mainly includes the monitoring station PC monitoring machine, four 9326 inverters, four 45kW frequency conversion dedicated motors produced by Lenze, CAN bus interface card, laser displacement sensor, and rotary transformer. The model of the distributed multi-motor synchronous control system based on CAN bus designed in this paper is shown in Figure 1.

Figure 1 System hardware connection diagram

Figure 2 Conveyor belt connection diagram

As can be seen from hardware diagram 2, the sheet material being transported between the two conveyor belts has no equipment to support it, and will bend in an arc shape due to the effect of gravity. When the speed of conveyor belt 1 is faster than that of conveyor belt 2, the tension will reduce the curvature of the curved part of the sheet material being transported between conveyor belts 1 and 2, and the displacement measured by the laser displacement sensor will become smaller. The laser displacement sensor will send a signal to the inverter to correct the speed of the drive motor of conveyor belt 2, so that the speed of the drive motor of conveyor belt 2 increases, so that the speed of conveyor belt 1 matches that of conveyor belt 2, and the tension on the curved part of the sheet material being transported between conveyor belt 1 and conveyor belt 2 is reduced. When the speed of conveyor belt 1 is slower than that of conveyor belt 2, the feedback signal of the laser displacement sensor will reduce the speed of the motor driving conveyor belt 2, so that the speed of conveyor belt 1 matches that of conveyor belt 2, and the tension on the curved part is reduced. The speed correction of conveyor belts 3 and 4 is the same as that of conveyor belt 2.

Features and function selection of Lenze inverter

Technical indicators and hardware features
According to the literature [1], Lenze inverters are divided into two series according to different control methods: 82 series and 93 series. The 82 series is a common v/f frequency control type. The 93 series is a vector control type. This design uses the 93 series inverter numbered 9326. The inverter has the characteristics of high power factor, small output ripple, reliable performance, and good system stability. The inverter panel is equipped with a rich set of digital and analog I/O interfaces, resolver interfaces, digital frequency input, digital frequency output, and CAN bus interfaces. The controller also has an expansion slot for additional communication boards. PROFIBUS-DP, RS-485 or fiber optic interfaces can be realized through additional plug-in boards.

Choice of communication network
Lenze9326 inverter has strong communication capability. It has multiple ways to communicate with the outside. For high-level automation systems, communication through PROFIBUS-DP network can be achieved by installing additional boards. CAN follows the standard model of ISO, which uses the data link layer and the physical layer. Products from different manufacturers can work together as long as they follow the ISO standard. Therefore, Lenze9326 inverter can be connected to any device with CAN interface. Using CAN bus, Lenze9326 inverter has a fixed CAN interface, and you only need to buy a CAN communication card for PC. If you use PROFIBUS fieldbus, you need to buy a PROFIBUS communication card for PC, and you also need to buy a PROFIBUS bus module dedicated to Lenze inverter. The dedicated module is expensive. The transmission rate of PROFIBUS bus is ten times that of CAN bus, but the communication capacity of CAN bus can meet the requirements of real-time information transmission of the system. Therefore, the fieldbus used for networking is CAN bus.

9326 Inverter Control Mode Selection
Lenze inverter's vector control mode can accurately measure and control the torque component and excitation component of the motor current. Its control performance is comparable to that of DC drive. It has the functions of automatic adjustment parameters, automatic fault display and alarm. It has the functions of flexible setting and changing process data channels. Speed ​​control, torque control or phase control mode can be selected by setting the code. This system is used for line speed control of the production line, so the speed control mode is selected.

CAN communication card in PC
In this project, Advantech PCM3680 is used. According to the literature [3], this is an embedded dual-port CAN bus communication card. The CAN controller uses Philips' independent CAN controller SJA1000 chip, and the CAN transceiver uses Philips' P82C250, providing a transmission speed of up to 1Mb/s.

Integrated laser displacement sensor
Use German Miiri OptoNCDT1400 integrated laser displacement sensor. It uses the principle of light triangulation reflection to measure displacement non-contactly. The light beam output by the sensor is focused and becomes a very fine light spot projected onto the surface of the measuring body. And it is refracted to the extremely sensitive optical linear detector CCD through imaging. The signal processing uses an integrated digital processor. The measurement range is 0 to 20cm. The output current is 4 to 20mA according to the measurement results.

Control system software design

For CAN bus nodes to complete tasks effectively and in real time, software design is the key and also the difficulty. Software design mainly includes two parts: signal flow design of the inverter internal module and design of CAN communication and control software in PC.

Figure 3 PC control system software block diagram

Design of CAN communication and control software in PC
The software in PC mainly completes the initialization of CAN card, the setting of main motor speed, and the adjustment of main motor speed to meet the process requirements of producing different products. At the same time, it should have fault alarm function. The program flowchart is shown in Figure 3.

Design of the control signal flow chart of the master motor inverter
Lenze9326 is a vector control inverter. A total of 996 parameters are provided inside the inverter to set the working status of 70 software modules inside the inverter. However, at most 50 function modules can be used each time. The inverter provides the running time of each module. The total running time of the selected modules cannot exceed the specified time limit.

The signal flow chart of the inverter 11 for controlling the main motor is given as shown in Figure 4. It is made to work in the speed control mode with maximum torque limitation. The function blocks in Figure 4 are all provided by the inverter and can be used after setting. X5/E1~E5 in Figure 4 are digital input terminals, which are connected to the digital output points of the PLC and are used to control the emergency stop and interlock of the motor. DIGIN is a digital input unit (setting the digital input high or low level to be effective), which controls the R/L/Q input and the start of the motor jog (JOG). R/L/Q is used to control the forward/reverse and emergency stop of the motor. Here, the two functions of the motor being always forward and accepting the external emergency stop signal input are set. The QSP output of R/L/Q is connected to the QSP input of the MCTRL block, and the motor emergency stop is finally realized through the MCTRL block.

AIN1 is an analog input unit, which completes the amplification function of the given speed signal and sends this signal to the inverter 12 and NSET block (speed setting adjustment block) through the CAN bus. The NSET block selects to output N (given speed) or JOG1 (fixed speed) to MCTRL (motor control block) according to the input of the NSET block LOAD point and JOG1 point. The LOAD point input in the NSET block is determined by the output of the MCTRL block QSP-OUT point (emergency stop). The MCTRL block implements the phase control, speed control and motor torque control of the motor. The RESOLVER of the MCTRL block connects the rotary transformer to achieve speed feedback. The LO-M-LIM and HI-M-LIM of the MCTRL block set the lower and upper maximum torque limit values ​​of the motor. The FLD-WEAK point of the MCTRL block sets the motor excitation according to the variable frequency motor description used. The PHI-LIM of the MCTRL block sets the correction value of the phase control function. The MMAX and IMAX outputs of the MCTRL block show whether the motor is running in the torque limit state and the current limit state. The NSET output point of the MCTRL block (the actual linear speed of the motor) is connected to the input of the CMP block (comparison block) to detect whether the motor speed is greater than 50r/min, and send the result to the STAT block (this block information is automatically sent to the CAN bus for PLC to receive, and determines whether the production line is in manual control state/automatic control state).

The DCTRL block controls the inverter in different working states (trip, trip reset, quick stop, normal), and outputs the status information through the DIGOUT block (digital output block) and the STAT block, which are received by the PLC and PC monitoring machines respectively. In the signal flow chart of the control inverter of the follower motor, the main speed setting comes from the CAN bus, and the auxiliary speed setting for correction comes from the integrated laser displacement sensor.

Figure 4 Inverter 11 signal flow chart

CAN bus parameter setting
Since the inverter is about 50m away from the controlled motor. According to the reference [1] Lenze inverter 9326 setting manual, the transmission rate should be set to 1Mbits/s to ensure normal communication. The transmission rate of the CAN card in the PC and the CAN interface of the programmable controller must also be set to 1Mbits/s. In order to achieve automatic following of the running speed of each section of the production line, the CAN address of inverter 11 is set to 1, the sending number is set to 258, the CAN address of inverter 12 is set to 2, the CAN-IN2 receiving number is set to 258, and the CAN-OUT2 sending number is set to 259. The CAN address of inverter 13 is set to 3, the CAN-IN2 receiving number is set to 259, and the CAN-OUT2 sending number is set to 260. The CAN address of inverter 14 is set to 4, and the CAN-IN2 receiving number is set to 260.

in conclusion

The advantages of CAN bus have been recognized. It has been favored by more and more R&D personnel due to its outstanding advantages such as high performance and simple implementation. The control strategy in this article has been applied in the distributed control system of a certain factory production line. The production operation shows that the whole system has the characteristics of high control accuracy, stable operation, simple operation and convenient maintenance. It can meet the requirements of producing high-quality and high-precision products.