How does PLC control servo motor? The main process of servo transmission system design

How does PLC control servo motors?

Before answering this question, we must first understand the purpose of servo motors. Compared with ordinary motors, servo motors are mainly used for precise positioning. Therefore, what we usually call servo control is actually the position control of servo motors. In fact, servo motors also have two other working modes, namely speed control and torque control, but they are less used.

Speed ​​control is generally achieved with a frequency converter. Using a servo motor for speed control is generally used for fast acceleration and deceleration or precise speed control, because compared with a frequency converter, a servo motor can reach thousands of revolutions within a few millimeters. Since the servo is closed-loop, the speed is very stable. Torque control mainly controls the output torque of the servo motor, also because the servo motor has a fast response. Applying the above two types of control, the servo drive can be regarded as a frequency converter, which is generally controlled by analog quantity. The most important application of servo motors is positioning control. Position control has two physical quantities to control, namely speed and position. To be precise, it is to control how fast the servo motor reaches where and stop accurately. The servo drive controls the speed and distance of the servo motor by the frequency and number of pulses received. For example, we agree that the servo motor rotates once every 10,000 pulses.

If the PLC sends 10,000 pulses in one minute, the servo motor will complete one circle at a speed of 1r/min. If it sends 10,000 pulses in one second, the servo motor will complete one circle at a speed of 60r/min. Therefore, the PLC controls the servo motor by controlling the pulses sent. Sending pulses in a physical way, that is, using the transistor output of the PLC is the most common way, and low-end PLCs generally use this method. The mid-to-high-end PLC transmits the number and frequency of pulses to the servo drive through communication. For example: Profibus-DP CANopen, MECHATROLINK-II, EtherCAT, etc. These two methods are just different in terms of the channels of implementation, but the essence is the same, and it is the same for our programming. This is what I want to tell you, to learn the principles and apply them to other cases, rather than learning for the sake of learning.

There is a big difference in programming. Japanese PLC uses instructions, while European PLC uses function blocks. But the essence is the same. For example, to control the servo to an absolute positioning, we need to control the PLC output channel, pulse number, pulse frequency, acceleration and deceleration time, and know when the servo driver completes positioning, whether it hits the limit, etc. No matter which PLC, it is nothing more than the control of these physical quantities and the reading of motion parameters, but the implementation methods of different PLCs are different.

How to design a servo system?