PLC control of servo motor speed - News - Global IC Trade Starts Here.

1. Introduction Servo motors are widely used as actuators in automatic control systems, converting received control signals into angular displacement or angular velocity output of the motor shaft. There are three common control methods: 1) Communication mode: Using RS232 or RS485 to communicate with a host computer for control. 2) Analog control mode: Using the size and polarity of an analog signal to control motor speed and direction. 3) Differential signal control mode: Using the frequency of a differential signal to regulate motor speed. Achieving precise control of servo motor speed is a key goal in industrial automation. This paper focuses on using the analog output from a PLC to achieve more accurate speed control of the servo motor. 2. Control System Circuit The control device selected is the Siemens S7-200 series PLC, model CPU224XPCN. It has both digital input/output points and one analog input/output point, making it suitable for controlling the servo motor. The touch screen used is the Siemens TP177B, serving as the human-machine interface. The control scheme is illustrated in Figure 1. The touch screen is where initial commands are entered, which are then sent to the PLC via the communication port. The PLC outputs an analog signal connected to the servo controller’s analog input. The controller processes the signal and drives the motor to reach the desired speed. A feedback mechanism ensures stable performance by comparing actual speed with the target value. Figure 1: Control Scheme

The servo motor in this setup operates within a speed range of 500 to 6000 RPM, with a precision requirement of ±3 RPM. 3. Control Process A dialog box is set up on the touch screen to allow users to input a four-digit value. This value is mapped to a variable in the PLC (e.g., VW310). The goal is to ensure the motor reaches the same speed as the input value. The PLC's analog output ranges from 0 to 10V, corresponding to a shaping value of 0 to 32000. The servo motor accepts an input of 0 to 10V, corresponding to 0 to 6500 RPM. However, theoretical values may not match real-world results, so direct measurements were taken. Table 1: Direct Measured Value Table | Input Value | Shaping Value | Actual Speed | |-------------|----------------|---------------| | 500 | 500 | 70 | | 2000 | 2000 | 360 | | 4000 | 4000 | 750 | | 6000 | 6000 | 1145 | From the table, it's clear that the input value doesn't directly correlate with the actual speed. Therefore, a conversion formula was developed based on experimental data. Table 2: Measured Corresponding Value Table | Shaping Value | Actual Speed | |----------------|---------------| | 2711 | 500 | | 30854 | 6000 | Both the PLC's analog output and the motor's speed response are linear. Using these values, a linear equation was derived: y = 5117x + 152 Where y is the shaping value and x is the actual speed. This equation allows the PLC to calculate the correct output value for the desired speed. As shown in Figure 2, the digital operation instruction in the PLC performs the calculation, and the result is sent to the analog output port. Since the output port only accepts word data (e.g., VB2232), the conversion is completed accordingly. Figure 3 shows the analog output process. When the user inputs a speed value through the dialog box, the PLC calculates the shaping value, sends the analog signal to the servo controller, and the motor runs at the desired speed. Table 3: Measured Value Table After Operation | Input Value | Calculated Value | Actual Speed | |-------------|-------------------|---------------| | 500 | 2711 | 500 | | 1000 | 5269 | 999 | | 2000 | 10386 | 1998 | | 3000 | 15503 | 3000 | | 4000 | 20620 | 4002 | | 5000 | 25737 | 5001 | | 6000 | 30854 | 6000 | 4. Conclusion This paper presents a method for controlling a servo motor using the analog output module of the Siemens S7-200 series PLC. The approach is simple, easy to implement, and meets the required speed accuracy of ±3 RPM.