Before answering this question, it's essential to understand the purpose of servo motors. Compared to ordinary motors, servo motors are primarily used for precise positioning. Therefore, what people usually refer to as controlling a servo motor is actually position control. Servo motors also operate in two other modes: speed control and torque control, but these are less commonly used.
Speed control is typically achieved using frequency converters. Using servo motors for speed control is generally for applications requiring rapid acceleration/deceleration or precise speed control. This is because, compared to frequency converters, servo motors can reach thousands of revolutions per minute within milliseconds, and since servo motors operate in closed-loop systems, their speed is extremely stable. Torque control primarily controls the output torque of the servo motor, again due to its fast response. By applying these two control methods, the servo drive can be used as a frequency converter, typically employing analog signal control.
The primary application of servo motors is positioning control. Position control involves controlling two physical quantities: speed and position. More precisely, it involves controlling how fast the servo motor reaches a certain location and stops accurately .
A servo driver controls the distance and speed of a servo motor by receiving the frequency and number of pulses. For example, we can define a servo motor as rotating once every 10,000 pulses. If the PLC sends 10,000 pulses per minute, the servo motor will complete one revolution at a speed of 1 r/min. If it sends 10,000 pulses per second, the servo motor will complete one revolution at a speed of 60 r/min.
Therefore, PLCs control servo motors by sending pulses . Sending pulses physically, specifically using the PLC's transistor outputs, is the most common method, typically found in low-end PLCs. Mid-range and high-end PLCs, on the other hand, transmit the number and frequency of pulses to the servo driver via communication, such as PROFIBUS-DP, CANopen, MECHATROLINK-II, and EtherCAT. These two methods differ only in the implementation method; they are essentially the same.
Japanese PLCs use instructions, while European PLCs use function blocks. However, the essence is the same. For example, to control a servo motor to perform an absolute positioning, you need to control the PLC's output channels, pulse count, pulse frequency, acceleration/deceleration time, and know when the servo drive completes positioning and whether it has hit a limit switch, etc. Regardless of the type of PLC, it essentially involves controlling these physical quantities and reading motion parameters; the only difference lies in the implementation methods.
