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How does PLC control servo motor?

2020-02-01 11:47

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 is commonly referred to as controlling servo is actually controlling the position of the servo motor. In fact, the servo motor also uses two other working modes, that is, speed control and torque control, but there are relatively few applications.

Speed control is generally implemented by a frequency converter. Servo motors are used for speed control. They are generally used for fast acceleration and deceleration or precise speed control. Because compared to frequency converters, servo motors can reach thousands of revolutions within a few millimeters. Since the servos are all closed loop, the speed is very stable. Torque control is mainly to control the output torque of the servo motor, also because the servo motor responds quickly. Applying the above two types of control, the servo driver can be regarded as an inverter, and generally analog control is used.

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

The servo driver controls the distance and speed of the servo motor by receiving the frequency and number of pulses. For example, we have agreed that the servo motor rotates once every 10,000 pulses. If the PLC sends 10,000 pulses in one minute, the servo motor completes one revolution at a speed of 1r / min. If it sends 10,000 pulses in one second, the servo motor completes one at a speed of 60r / min ring.

Therefore, the PLC controls the servo motor by controlling the pulses sent. The pulses are sent physically, that is, the transistor output of the PLC is the most commonly used method. Generally, this method is used by low-end PLCs. The middle and high-end PLCs transmit the pulse number and frequency to the servo driver through communication, such as Profibus-DP CANopen, MECHATROLINK-II, EtherCAT and so on. These two methods are just different implementation channels, the essence is the same, and for our programming, it is the same. This is what I want to tell you, to learn the principle, to bypass the class, not to learn for learning.

For programming, this is very different. Japanese PLCs use instructions, while European PLCs use function blocks. But the essence is the same. For example, to control the servo to perform an absolute positioning, we need to control the PLC's output channel, pulse number, pulse frequency, acceleration and deceleration time, and need to know when the servo driver has completed positioning, and whether it hits the limit. Wait. No matter what kind of PLC, it is nothing more than the control of these several physical quantities and the reading of motion parameters, but different PLC implementation methods are different.

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