Before answering this question, it is first important to understand the purpose of a servo motor. Unlike ordinary motors, servo motors are primarily used for precise positioning. Therefore, what people generally refer to as servo control is actually the position control of the servo motor. In fact, servo motors have two other operating modes: speed control and torque control, though these are less commonly applied.

Speed control is typically achieved by a frequency converter. Using a servo motor for speed control is usually reserved for scenarios requiring rapid acceleration/deceleration or highly precise speed regulation. Compared with frequency converters, a servo motor can reach several thousand revolutions in a few milliseconds, and its speed is extremely stable due to the closed-loop control inherent to all servo systems. Torque control is focused on regulating the output torque of the servo motor, and it also leverages the fast response of servo motors. For these two control modes, the servo drive can be treated like a frequency converter, with analog control being the standard method of operation.

The primary application of servo motors remains positioning control, which involves regulating two physical quantities: speed and position. Specifically, it means controlling how fast the servo motor moves to a specific position and ensuring it stops accurately there.

A servo drive controls the travel distance and speed of a servo motor based on the frequency and number of received pulses. For example, we can set a specification where the servo motor rotates one full revolution for every 10,000 pulses received. If a PLC sends 10,000 pulses in one minute, the servo motor will complete one revolution at a speed of 1 r/min; if 10,000 pulses are sent in one second, the servo motor will complete one revolution at 60 r/min.

Thus, a PLC controls a servo motor by regulating the pulses it transmits. The most common method is physical pulse transmission via the PLC’s transistor outputs, a solution typically adopted by low-end PLCs. Mid-to-high-end PLCs, by contrast, transmit pulse count and frequency to the servo drive through communication protocols such as Profibus-DP, CANopen, MECHATROLINK-II, and EtherCAT. These two methods differ only in implementation, not in their fundamental principle.

 

Japanese-brand PLCs implement this control through dedicated instructions, while European-brand PLCs use function blocks—yet the core principle is identical. For instance, to perform an absolute positioning move with a servo motor, it is necessary to control the PLC’s output channel, pulse count, pulse frequency, acceleration/deceleration time, as well as to detect signals from the servo drive such as positioning completion and limit switch activation. Regardless of the PLC brand, the operation essentially boils down to controlling these physical quantities and reading motion parameters, with only the implementation methods varying across different PLC models.