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. 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. 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, using the PLC's transistor outputs, is the most common method, typically found in low-end PLCs. Mid-range and high-end PLCs, such as Profibus-DPCANopen, Mechatronics-II, and EtherCAT, transmit the number and frequency of pulses to the servo driver via communication. 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.
