1. Introduction
Servo motors are widely used as actuators in automatic control systems, where they convert incoming control signals into angular displacement or angular velocity output. There are three common control methods:
- Communication mode: Using RS232 or RS485 to communicate with a host computer for control.
- Analog control mode: Controlling motor speed and direction based on the magnitude and polarity of analog signals.
- Differential signal control mode: Using differential signal frequency to regulate motor speed.
Achieving precise and simple control of servo motor speed is a key goal in industrial automation. This paper explores how to use the analog output from a PLC to achieve more accurate control over the motor's speed. 2. Control System Circuit
The control system uses a Siemens S7-200 series PLC model CPU224XPCN, which includes input and output points, along with one analog input and one analog output point. This type of PLC is suitable for controlling a servo motor, especially when paired with an analog module. A Siemens TP177B touch screen is used as the human-machine interface (HMI), allowing users to input commands and monitor system status. The control scheme is illustrated in Figure 1. The servo motor in this setup operates within a range of 500 to 6000 RPM, with an accuracy requirement of ±3 RPM. The control process involves setting up a dialog box on the touch screen where users can enter a four-digit value. This value is then mapped to a variable in the PLC (e.g., VW310). The PLC outputs an analog signal ranging from 0 to 10V, corresponding to shaping data from 0 to 32000. The servo controller receives this signal and adjusts the motor speed accordingly. A feedback loop ensures stability by comparing the actual speed with the desired value. 3. Control Process
The analog output from the PLC is 0–10V, matching the input range of the servo controller. However, direct mapping between the input value and the actual motor speed does not yield accurate results. Through testing, it was found that the relationship between the input value and the actual speed is non-linear. By performing experiments, the correct shaping values were determined for specific speeds. For example, at 500 RPM, the shaping value was 2711, and at 6000 RPM, it was 30854. Using these values, linear equations were derived to calculate the appropriate shaping value for any given input speed. The equation obtained was:
y = 5117 × x + 152
where y is the shaping value and x is the desired speed. This equation was implemented in the PLC using digital operation instructions. After processing, the result was sent to the analog output port, allowing the motor to reach the intended speed accurately. Table 3 shows the measured values after implementing the control algorithm: 4. Conclusion
This paper presents a practical method for controlling a servo motor using the analog output module of a Siemens S7-200 PLC. The approach is straightforward, easy to implement, and meets the required speed accuracy of ±3 RPM. It demonstrates how PLCs can be effectively used in precision control applications, making them a valuable tool in modern industrial automation systems. Three Phase Vfd,Ac Single Phase Gasoline Generator,220V Vfd,Ac Inverter WuXi Spread Electrical Co.,LTD , https://www.vfdspread.com
Input Value Shaping Value Actual Speed 500 2711 500 1000 5269 999 2000 10386 1998 3000 15503 3000 4000 20620 4002 5000 25737 5001 6000 30854 6000
