Composition, classification, selection, control and use of servo motors
1、 Composition of servo motor
1. Motor body
The core part of a servo motor is usually a DC brushless motor or an AC synchronous motor. Its main function is to provide rotational power.

brushless dc motor

AC synchronous motor

2. Position/velocity sensor
Used for real-time detection of the rotor position and speed of the motor, and feedback to the control system. Common sensors include:

Encoder: divided into incremental and absolute types, used for measuring angular displacement and velocity.

Rotary transformer: Strong anti-interference ability, commonly used in industrial environments.

Hall sensor: measuring rotor position in low-cost applications.

3. Driver
Also known as a servo controller, it is the control center of a motor used to adjust the torque, speed, and position of the motor based on control signals.

High efficiency, easy to control, commonly used in small and medium power applications.

Suitable for high-power and high-precision applications.

photoelectric sensor

Accept external command signals (such as pulse signals sent by PLC or controller).

Internally perform closed-loop control (speed loop, position loop, torque loop).

Operation mode

1. Position control operation mode

The position control mode generally determines the displacement of movement by the number of pulses, and the external input pulse frequency determines the magnitude of the rotational speed.

How to control the stroke accuracy of servo motors?

Adjust the working status of the motor based on the feedback signal.

Encoder resolution (r): The number of pulses per revolution of the motor is determined by the encoder (ppr): The number of pulses that the higher-level controller needs to emit to make the motor rotate one revolution

Due to its strict control over position and speed, the position mode is generally applied to positioning devices. It is the most widely used control mode for servo applications, mainly used in robotic arms, surface mount machines, engraving and milling, CNC machine tools, etc.

2. Speed control operation mode

The concept of electronic gears: It is analogous to the gear reduction ratio in machinery. For example, if the original speed of a motor is 10000 revolutions per minute and it is reduced to 1000 revolutions per minute by gears, then the reduction ratio is 10:1.

There may be more than one mechanism that affects the speed of the entire system, and there may be multi-stage deceleration mechanisms. The relationship between the final stroke and the control pulse can be calculated by configuring the deceleration ratio on the mechanism to tell the driver.

If the encoder resolution is 10000

Without electronic gear ratio: If the motor rotates once and the workpiece moves 6mm, the corresponding length of the unit pulse is 6mm/10000=0.6um; Moving the workpiece by 10mm requires 16666.66 pulses of 10mm/0.6um. However, when sending the pulses, decimals will be rounded off, resulting in errors!

With electronic gear ratio: After using electronic gear ratio, the motor rotates once and the workpiece moves 6mm. The encoder resolution is 10000, and the configurable electronic gear numerator is 10000 and denominator is 6000. Therefore, if the motor rotates once and requires 6000 pulses, the corresponding length of the unit pulse is 6mm/6000=1um; Moving the workpiece 10mm requires 10mm/1um=10000 pulses, and there will be no decimals or errors caused by rounding when actually sending the pulses!

Another factor that requires an electronic gear ratio is the maximum PWM frequency that can be achieved by the upper computer and the highest PWM frequency that can be recognized by the motor driver. For example, if the output frequency of the pulse generator is 200KHz and the resolution of the servo encoder is 8388608, the servo motor needs to send 8388608 pulses to make one revolution. So to make the motor turn once, the fastest time required is 8388608/200000=41.943 seconds. In order to balance speed control, some control accuracy is sacrificed through electronic gear ratios.