Research on the application of Beckhoff in stepper motor control

This system is a host electronic control system, which completes parameter diagnosis, data acquisition, closed-loop control and other real-time control functions in the process of beam alignment, as well as communication with the main control system. Stepper motors are known for their high cost-effectiveness, good real-time effect, and no need for expensive feedback systems. More than one thousand stepper motors are used in this system to achieve beam alignment. The Shenguang prototype uses a control card to control a large number of stepper motors. The control card controls the stepper motor without a feedback signal of the load position, the real-time effect is poor, and fault location is very difficult; in the experiment, Siemens PLC (Programmable Logic Controller) is used to control these motors with good reliability, but it is relatively simple to control a small number of stepper motors. When controlling a large number of stepper motors, the timing is complex, the wiring is complex, the cost is high, and debugging is difficult. The final solution uses automation products from Beckhoff of Germany, such as embedded controllers, field bus terminals, stepper motor bus terminals and other equipment to control motors. This solution has the advantages of low cost, simple operation, and small space occupation, and also improves the performance and reliability of the system. It has now passed the test and review and entered the debugging stage.

1 Overall design concept of stepper motor motion control system

The overall design concept of the electric control system is: the control system design should meet the requirements of on-site independent control and remote centralized control respectively. Each subsystem has the function of real-time monitoring of the status of various equipment in the system, including fault location detection and alarm; and has the functions of parameter setting, parameter acquisition, storage, and processing for various equipment in the system. The main control computer is located in the main control room and communicates with the sub-control computer through a 1 000 Mb/s optical fiber network. The sub-control computer communicates with the control equipment through an industrial Ethernet through an optical fiber switch. The network topology is shown in Figure 1.

2 Stepper Motor Motion Control System Design

2.1 Beckhoff stepper motor overall control architecture design

The modular motion controller based on embedded PC is adopted, model CX1020. The motion controller itself integrates dual Ethernet interfaces, one of which is connected to the on-site industrial Ethernet to receive the call instructions of motion control, and the other can be used as a network interface for local debugging. The motion controller, motion control network interface components, and stepper motor driver form a motion control network system through hard real-time ultra-high-speed Ethernet EtherCAT (Ethernet for Control and Automation Technology). Using EtherCAT to replace the previous field bus transmission technology can significantly reduce the load of the control system CPU, and no intelligent master card is required, which is suitable for centralized and distributed control systems.

The end terminal is connected to the bottom EtherCAT bus coupler terminal BK1120. The bus coupler in this rack is connected to the motor drive bus terminals KL2531 and KL2541 through the K-Bus bus interface. The stepper motor is directly connected to these two terminals. If the KL2541 terminal is used, the digital input terminal KL1002 needs to be installed on the rack to receive the stepper motor limit signal. Finally, the BK9010 bus terminal terminal is placed at the end of the rack.

The other network port of the bus coupler BK1120 on each rack is cascaded downward to the bus coupler on another rack, and finally a bus-type control network topology is formed from the controller to the network nodes of each rack. KL2531 and KL2541 are stepper motor bus terminals, providing a 20 kHz pulse, each of which can drive a stepper motor. The overall control architecture of the stepper motor is shown in Figure 2.

The specific configuration of controllers and communication modules is shown in Table 1.

2.2 Hardware Composition

The motor control system is mainly composed of a control server, motion controller, power module, Ethernet data exchange communication module and bus terminal.

2.3 Overview of Important Components

The motion controller is a modular motion controller based on embedded PC, model CX1020, with Intel Celeron MULV1 GHz processor, 256 MB/512 MB/1 024 MB memory, and Microsoft Windows XP Embedded or Microsoft Windows CE operating system. This motion controller is used in medium-sized control systems and has two industrial Ethernet EtherCAT interfaces, one of which is connected to the field industrial Ethernet to receive motion control call instructions, and the other can be used as a local debugging network interface. The software development platform TwinCAT PLC (The Windows Control and Automation Technology PLC) is programmed in a programming language that complies with IEC61131-3. The motion control function library complies with the PLCopen standard and has a universal, open software interface.

EtherCAT is a communication system that matches PC control technology. It is a real-time industrial Ethernet solution developed by Beckhoff, which is characterized by superior performance and easy operation. This technology uses standard Ethernet frames and supports Internet technology. Ethernet can directly reach the terminal, and the Ethernet process interface can be expanded from 2 bits to 64 bytes; 350μs to process 12,000 digital I/Os; 100μs to process 100 motor axes, etc. EtherCAT is suitable for a variety of network topologies and can be used for equipment control, robots, embedded systems, building automation, transportation systems, etc.

Stepper motors have a long history, good explosive power, easy control and high torque, and they occupy a very important position in the field of control. Stepper motors are actuators that convert electrical pulses into angular displacement. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle (called "step angle") in the set direction. Its rotation runs step by step at a fixed angle. The angular displacement can be controlled by controlling the number of pulses, so as to achieve the purpose of accurate positioning. At the same time, the speed and acceleration of the motor rotation can be controlled by controlling the pulse frequency, so as to achieve the purpose of speed regulation. We use a two-phase hybrid stepper motor from Beijing Sitong, model 28BYG250C, step angle of 0.9/1.8, static phase current of 0-7 A, holding torque of 0.065 N/m, positioning torque of 0.005 N/m, and moment of inertia of 12 gcm2.

KL2531 and KL2541 are integrated stepper motor controllers, which are cost-effective products for traditional drives in Beckhoff products. Both KL2531 and KL2541 can provide performance similar to that of small servo drives, providing a maximum of 64 maximum intermediate steps and providing a 20 kHz pulse, one pulse to drive a stepper motor. The KL2531 bus terminal is used for lower performance occasions. The terminal is designed for simple integration into a 24 V DC voltage control system with a peak current of 1.5 A per phase. KL2541 can provide a maximum current of up to 5 A and can drive stepper motors with a torque of up to 5 Nm. The output stage of the terminal has overload protection in the form of overtemperature alarm and shutdown, as well as short-circuit protection. Diagnostic data can be obtained through the process image area of ​​the controller. It has two high-speed input channels to quickly and accurately record the current position or realize the emergency stop function. The output pulse can be started through the "EnableBit" and the motor current can be set through the parameter value. The configuration of the motor can be done through software, so that the bus terminals can be easily replaced, and the parameters can be stored or transferred to the next project without having to set them through potentiometers or dip switches.

3 Software Configuration

Beckhoff's PC-based real-time core technology can transform ordinary PCs with Microsoft's WindowsNT/2000/XP/CE.NET/XP embedded operating system and Beckhoff's embedded control system CX1000 into PC controllers with real-time capabilities. Beckhoff's PC-based real-time core technology is combined with the TwinCAT programming environment provided by 3S and the system manager's drivers for Beckhoff's various I/O devices, making TwinCAT CP, I/O, PLC, PTP and NCI form a complete set of PC-based real-time controller solutions.

TwinCAT software has four independently running PLC real-time cores, each of which can be assigned 4 tasks, and the minimum task cycle time can be set to 50μs (depending on the CPU processing power used). TwinCAT PLC control is a complete development environment for PLC. The editor and debugging functions used are based on the high-level programming language development environment. In TwinCAT PLC, five different languages ​​of IEC61131-3 can be used to create PLC programs. The load rate of TwinCAT real-time task operation (the ratio of TwinCAT real-time task running time to operating system running time) can be adjusted from 10% to 90%. On this basis, PTP, NCI and HMI technologies in servo control are expanded to the system manager and TwinCAT programming environment.

4 Conclusion

The design has passed the experimental stage and entered the formal debugging stage. In this process, it can be found that Beckhoff products have many advantages and occupy little space. The main features of the fieldbus terminal are high cost performance, reliable quality, compact structure, ruggedness and durability, and very convenient installation and wiring. EtherCAT technology is highly versatile, and the system based on EtherCAT can greatly shorten the development speed.