How does a PLC control a servo motor?
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 servo control is actually the position control of a servo motor. Servo motors also have two other operating modes: speed control and torque control, but these are less commonly used.
Speed control is generally achieved using frequency converters. Using servo motors for speed control is typically used for rapid acceleration/deceleration or precise speed control. Compared to frequency converters, servo motors can reach thousands of revolutions per second within millimeters, and because 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. Using these two control methods, the servo drive can be treated as a frequency converter, typically employing analog signal control. The most important application of servo motors is positioning control. Position control requires controlling two physical quantities: speed and position. More precisely, it controls the speed at which the servo motor reaches a certain location and stops accurately. The servo drive controls the distance and speed of the servo motor by receiving pulse frequency and quantity. For example, if we define a servo motor as rotating once every 10,000 pulses, and the PLC sends 10,000 pulses per minute, the servo motor will complete one revolution at 1 r/min. If it sends 10,000 pulses per second, the servo motor will complete one revolution at 60 r/min. Therefore, PLCs control servo motors by sending pulses. Sending pulses physically, using the PLC's transistor outputs, is the most common method, typically found in low-end PLCs. Mid-range and high-end PLCs, however, transmit the number and frequency of pulses to the servo driver via communication. Examples include Profibus-DP, CANopen, Mechatronics-II, and EtherCAT. These two methods differ only in their implementation channels; the essence is the same, and the programming principles are also the same. This is what I want to emphasize: learn the principles to understand broader concepts, rather than learning for the sake of learning.
The differences in programming are significant. 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, we 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; only the implementation methods differ.
How to design a servo system?
