The servo motor is controlled by a servo system, which includes a controller, a servo drive and a servo motor, as well as feedback devices (such as encoders).
The working principle of the servo motor can be simply summarized as: input control signal → servo controller → servo motor → output motion.
Specifically, the control method and principle of the servo motor are as follows:
Control method
There are three main control methods for servo motor systems:
1. Torque control (current loop):
- The magnitude of the motor shaft's external output torque is set by external analog input or direct address assignment.
- Mainly used in situations where torque needs to be strictly controlled.
2. Speed control (current loop, speed loop):
- The speed of rotation is controlled by analog input or pulse frequency.
- The speed loop performs negative feedback PID adjustment by detecting the signal of the servo motor encoder, and the PID output in the loop is directly the setting of the current loop.
3. Position control (current loop, speed loop, position loop):
- The most common control mode in servo.
- Generally, the magnitude of the rotation speed is determined by the frequency of the external input pulse, and the angle of rotation is determined by the number of pulses.
- Mainly used in positioning devices.
Control principle
The control principle of servo motor involves three main links, namely current loop, speed loop and position loop, which together constitute the three-loop control structure of servo system:
1. Current loop:
- It is completely carried out inside the servo drive.
- The output current of each phase of the motor from the drive is detected by the Hall device, and negative feedback is given to the current setting for PID adjustment to make the output current as close to the set current as possible.
- The current loop is the key link to control the motor torque.
2. Speed loop:
- Negative feedback PID adjustment is performed by detecting the signal of the servo motor encoder.
- The PID output in the speed loop is directly the setting of the current loop, so the speed control includes the speed loop and the current loop.
3. Position loop:
- It is the outermost loop, which can be built between the drive and the servo motor encoder, or between the external controller and the motor encoder or the final load.
- The internal output of the position loop is the setting of the speed loop.
- In the position control mode, the system performs operations in all three loops. At this time, the system has the largest amount of calculation and the slowest dynamic response.
Summary
Servo motors are precisely controlled through servo systems, including torque control, speed control, and position control.
These control methods rely on the coordinated work of the current loop, speed loop, and position loop to ensure that the motor can run accurately according to the input control signal.
The fast response, high precision, and stability of servo motors make them widely used in industrial automation, robotics, CNC machine tools, and other fields.