How does the PLC control the servo motor? How to design a servo system?

Before answering this question, we must first understand the purpose of the servo motor. Compared with the ordinary motor, the servo motor is mainly used for precise positioning. Therefore, what we usually call controlling the servo is actually the position control of the servo motor. In fact, the servo motor also uses two other working modes, that is, speed control and torque control, but the application is relatively small.

How does the PLC control the servo motor?

Before answering this question, we must first understand the purpose of the servo motor. Compared with the ordinary motor, the servo motor is mainly used for precise positioning. Therefore, what we usually call controlling the servo is actually the position control of the servo motor. In fact, the servo motor also uses two other working modes, that is, speed control and torque control, but the application is relatively small.

Speed ​​control is generally realized by frequency converter, and servo motor is used for speed control, which is generally used for fast acceleration and deceleration or precise speed control, because compared with frequency converter, servo motor can reach thousands of revolutions within a few millimeters. Since the servos are all closed loop, the speed is very stable. Torque control mainly controls the output torque of the servo motor, also because the response of the servo motor is fast. Applying the above two controls, the servo drive can be regarded as a frequency converter, which is generally controlled by analog quantity.

The main application of servo motor is positioning control. Position control has two physical quantities that need to be controlled, that is, speed and position. To be precise, it is to control how fast the servo motor reaches and stops accurately.

The servo drive controls the distance and speed of the servo motor by the frequency and number of pulses it receives. For example, we stipulate that the servo motor makes one revolution every 10,000 pulses. If the PLC sends 10,000 pulses in one minute, then the servo motor completes one circle at a speed of 1r/min, and if it sends 10,000 pulses in one second, then the servo motor completes one circle at a speed of 60r/min lock up.

Therefore, the PLC controls the servo motor by controlling the sent pulses. Sending the pulses in a physical way, that is, using the transistor output of the PLC is the most commonly used method. Generally, this method is used by low-end PLCs. The mid-to-high-end PLC transmits the number and frequency of pulses to the servo drive by means of communication. for example:
Profibus-DP CANopen, MECHATROLINK-II, EtherCAT, etc.

The only difference between these two methods is the implementation channels, but the essence is the same, and it is the same for our programming. This is what I want to tell you, to learn principles, to learn by analogy, and not to learn for the sake of learning.

For programming, the difference is very big. Japanese PLCs use instructions, while European PLCs use function blocks. But the essence is the same. For example, to control the servo to perform absolute positioning, we need to control the output channel of the PLC, the number of pulses, the pulse frequency, the acceleration and deceleration time, and we need to know when the servo drive is positioned and whether it encounters the limit, etc. Wait. No matter what kind of 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.

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