Abstract: A stepper motor is an electrical component based on pulse control. This paper introduces the application of Siemens S7-200 in stepper motor control. Furthermore, this control system also features human-machine dialogue functionality and a high performance-price ratio. Keywords: PLC , Pulse control, Human-machine dialogue, Stepper motor 0. Introduction A stepper motor is a control component that converts electrical pulse signals into angular or linear displacement. Under non-overload conditions , the motor's speed and displacement depend only on the frequency and number of pulses, and are unaffected by load changes. That is, applying a pulse signal to the motor causes it to rotate one step angle. This linearity, coupled with the stepper motor's characteristic of having only periodic errors and no accumulated errors, contributes to its performance. This allows stepper motors to precisely control rotation angle and rotation speed. Stepper motors are ideal for use in speed, position and other control fields. With the development of digital technology and the improvement of stepper motor technology, stepper motors will be used in more fields. This article will introduce a stepper motor control system based on PLC pulse. 1. Block diagram of the control system The block diagram of the stepper motor control system is shown in Figure 1. It mainly consists of human-machine interface, PLC controller, stepper motor driver and stepper motor. [align=center] Figure 1[/align] Due to the structural characteristics of the stepper motor itself, it must have an acceleration process to achieve high-speed operation. If a high-frequency pulse is suddenly applied during startup, the motor will produce a whistling sound, lose steps, or even fail to start. The same is true during the stopping phase. When the high frequency suddenly drops to zero, the motor will also produce a whistling sound and vibration. Therefore, there must be an acceleration and deceleration phase during acceleration and stopping [1]. In this article, specifically: first, control the stepper motor to start steadily, then move at high speed (the speed when the motor is working normally), reach the designated position, decelerate and run at low speed for a period of time, and then stop. The backward process is the same as the forward process. The motor working process is shown in Figure 2: [align=center] Figure 2[/align] 2. System software design According to the requirements of the control system, the system software flowchart is shown in Figure 3: It mainly includes the main program and subroutines. The main functions of the main program are: at the beginning of power-on, the output port is initialized to zero, the direction of the motor is set, and the subroutines are called to complete the forward and backward control of the stepper motor, and the start and stop control of the motor. The subroutines mainly complete the parameter and network table settings of the high-speed pulse train output. [align=center] Figure 3[/align] The stepper motor adopts an automatic lifting method, that is, the stepper motor starts at a low frequency and then gradually rises to the operating frequency. When the motor stops, the frequency of the pulse signal is reduced to below the starting frequency before the output pulse is stopped. The stepper motor can stop without losing steps. After the stepper motor starts normally at a frequency lower than the limit starting frequency, the control pulse can be slowly increased to run normally[3]. The motor pulse characteristics are shown in Figure 4. [align=center]Figure 4[/align] 3. Stepper Motor and Driver The stepper motor is the actuator of this system, and its accuracy affects the control accuracy of the entire system. The selected motor should meet the functional requirements of the system. Simultaneously, the pulse output frequency of the selected PLC should be calculated based on the motor parameters to ensure it meets the requirements of the stepper motor. In the control system, the PLC generates control pulses; a certain number of square wave pulses are output through PLC programming to control the rotation angle of the stepper motor and thus the feed amount of the servo mechanism; simultaneously, the pulse frequency—that is, the feed speed of the servo mechanism—is controlled through programming; the ring pulse distributor distributes the control pulses output by the programmable controller to the corresponding windings according to the stepper motor's energizing sequence. Using a hardware ring distributor, although the hardware structure is slightly more complex, can save on the number of I/O ports occupied by the PLC. Currently, there are various dedicated chips available on the market. The stepper motor power driver amplifies the control pulses output by the PLC to tens to hundreds of volts and a driving capability of several amps to tens of amps. The output interface of a general PLC has a certain driving capability, while the load capacity of a typical transistor DC output interface is only tens to hundreds of volts and tens to hundreds of milliamps. However, power stepper motors require a driving capability of tens to hundreds of volts and several amps to tens of amps, so a driver should be used to amplify the output pulses. Feed direction control is the direction control of the stepper motor. The direction of the stepper motor can be changed by changing the energizing sequence of each winding of the stepper motor; for example, when the energizing sequence of a three-phase stepper motor is A-AB-B-BC-C-CA-A…, the stepper motor rotates forward; when the windings are energized in the order of A-AC-C-CB-B-BA-A…, the stepper motor rotates in reverse. Therefore, the direction control signal output by the PLC can be used to change the output sequence of the hardware ring distributor, or the energizing sequence of the stepper motor windings can be changed by programming [4]. According to the working principle and characteristics of the stepper motor, the total rotation angle of the stepper motor is proportional to the number of input control pulses; therefore, the number of pulses output by the PLC can be determined according to the displacement of the servo mechanism: (1) Where D[sub]l[/sub]——displacement of the servo mechanism (mm); d——pulse equivalent of the servo mechanism (mm/pulse) The feed speed of the servo mechanism depends on the speed of the stepper motor, and the speed of the stepper motor depends on the input pulse frequency; therefore, the pulse frequency output by the PLC can be determined according to the feed speed required by the process: (2) Where V[sub]f[/sub]——feed speed of the servo mechanism (mm/min) 4. Human-machine interface Using the PPI communication protocol of S7-200, the system can be monitored in real time [2]. The PC issues control commands to start, stop, and reverse the motor. When the system is running, the software program converts the pulse frequency into speed and the number of pulses into distance and sends them to the PC for display, so as to monitor the speed and displacement. 5. Conclusion The PLC-controlled open-loop servo mechanism was used in the CNC slide of a large production line. Its pulse equivalent was 0.01-0.05 mm and the feed speed was V[sub]f[/sub]=3-15 m/min, which fully met the process requirements and machining accuracy requirements[4]. It is shown that the application of PLC pulse control stepper motor technology to medium and small power traction equipment has the characteristics of simple control, stability and low cost, and is a practical electrical control scheme. If protection circuits and anti-interference measures are added to the system, the stability of the system can be improved. References: [1] Dai Jincan. Application of Siemens S7-200 series PLC in development [M]. China Water Resources and Hydropower Press. 2007. [2] Deng Xingzhong. Electromechanical transmission control (fourth edition) [M]. Huazhong University of Science and Technology Press. 2007. [3] E20001-H5400-C400-V2-5D00.S7-200 Chinese system manual. 2002. H31—H36 [4] Guan Lina, Shao Qiang. PLC direct control of stepper motor [J]. Journal of Dalian Nationalities University. 2004, No. 1. 45-47