Siemens PLC control servo motor speed - Solutions - Huaqiang Electronic Network

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1. Introduction

The servo motor is a key actuator in automatic control systems, converting received control signals into angular displacement or angular velocity output of the shaft. There are three common control methods: communication mode, analog quantity control, and differential signal control. In the communication mode, the system communicates with the host computer via RS232 or RS485 to achieve control. In the analog quantity control mode, the size and polarity of the analog signal determine the motor speed and direction. The differential signal control mode uses the frequency of the differential signal to regulate the motor speed. Achieving precise and simple control of the servo motor's speed is an important goal in industrial automation. This paper focuses on using the analog output from a PLC to achieve more accurate control of the servo motor speed.

2. Control System Circuit

The control device selected for this system is the Siemens S7-200 series PLC, specifically the CPU224XPCN model. This PLC includes input and output points, as well as one analog input and one analog output point. It also supports an analog module that meets the requirements for controlling the servo motor. The human-machine interface (HMI) used is the Siemens TP177B touch screen. The specific control scheme is illustrated in Figure 1. The touch screen serves as the user interface, where initial commands are entered. These commands are then sent to the PLC through a communication port. After processing, the PLC outputs an analog signal connected to the servo controller’s analog input. The controller performs internal calculations and drives the motor to reach the desired speed. A feedback mechanism from the motor ensures stable operation by adjusting the speed in real-time.

Figure 1: Control Scheme

The designed working speed range of the servo motor is 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 enter a four-digit value. This value is mapped to a corresponding variable in the PLC (e.g., VW310). The goal is for the motor to reach the same speed as the entered value. The PLC’s analog output ranges from 0 to 10V, corresponding to a shaping value of 0 to 32000. The servo motor’s analog input also operates within 0 to 10V, which corresponds to a speed range of 0 to 6500 RPM. However, due to theoretical limitations, the final goal is for the input value to correspond directly to the actual speed. Therefore, the analog signal acts as an intermediate reference. Key considerations include the input value, shaping value, and actual speed. After direct measurement, the test results are shown in Table 1.

From Table 1, it is clear that the input value does not match the actual speed. To resolve this, the input value must be converted into an integer that accurately reflects the actual speed. First, we need to determine the values corresponding to the highest and lowest speeds. Through experiments, the relationship is shown in Table 2.

Both the PLC’s analog output and the servo motor’s speed output follow a linear relationship. Using the data from Table 2, we can derive the linear equation:

2711 = 500 × a + b

30854 = 6000 × a + b

Solving these equations gives: a = 5117, b = 152

Let x represent the actual speed and y the shaping value. The relationship is:

y = 5117 × x + 152

This formula is implemented in the PLC using digital operations. As shown in Figure 2, after performing the calculation, the result is transmitted to the analog output port. Since the output port only accepts word data (e.g., VB2232), double-word data is not supported. As shown in Figure 3, the analog output sends the signal to the servo controller, which drives the motor to match the input speed. After testing, the input value, shaping value, and actual speed are recorded in Table 3.

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 straightforward, easy to implement, and effectively meets the requirement of ±3 RPM speed accuracy. This solution offers a reliable and efficient way to achieve precise motor control in industrial applications.