The development of new-generation information technology has made unmanned, IoT-enabled, and digitally connected factories possible. The key to unmanned, digital, and intelligent manufacturing lies in the electromechanical automation and digitization of various industrial machine tools. In my country, servo motors are widely used, with the largest application area being machine tools (especially CNC machine tools), accounting for 20.4%. This is followed by electronic manufacturing equipment, packaging machinery, textile machinery, industrial robots, and plastics machinery, with proportions of 16.5%, 12.6%, 12.1%, 8.7%, and 8.2%, respectively.
Servo motor system
Data indicates that in the future, with the rapid development of related industries, the growth rate of the next generation of electromechanical equipment will slow down over the next five years, but will still grow slowly. The market size of the servo motor industry is expected to reach 22.5 billion yuan by 2026.
Market size of servo motors
So, what are the fundamental differences between stepper motors and servo motors? And how are synchronous and asynchronous motors, both belonging to the servo motor family, distinguished?
Stepper motors and servo motors
Stepper motor
It is a synchronous motor with "high-precision positioning operation". Its driver controls the motor's stepping synchronization according to the pulse signal output from the controller, and operates at the designed step angle (resolution). Its feature is that it is a motor that can perform simple, high-precision positioning operation using only a controller, driver, and motor in an open-loop control mode.
The most distinctive feature of a stepper motor in terms of its structure is that it has "small teeth" on the inside of the stator where the coils are assembled and on the outside of the rotor, and permanent magnets are assembled inside the rotor.
Servo motor
The servo motor itself is equipped with a rotation detector (encoder) that feeds back the rotational position/speed of the motor shaft to the driver. The driver calculates the error between the pulse signal (position command/speed command) sent from the controller and the feedback signal (current position/speed), and controls this error to zero to control the motor's rotation. By using the motor, driver, and encoder to form a closed-loop control system, a high-precision positioning operation can be achieved for the motor.
In terms of the main structure, the servo motor has a rotary detector (encoder, etc.) mounted at the tail of the motor to detect position and speed.
In addition, the following are also major reasons for prioritizing servo motors:
There are also significant differences in the selection of power output. Stepper motors and AC servo motors differ in their construction and application, requiring different power specifications. When high output power is necessary, such as exceeding 100W, AC servo motors are usually the preferred choice.
The speed-torque characteristics and positioning time differ. For short positioning distances, stepper motors rotate synchronously based on received pulse signals, resulting in high torque at low speeds (superior responsiveness during start-up/deceleration). Servo motors, on the other hand, experience a delay in response to the pulse signal (affected by accumulated pulses), and thus have lower torque than stepper motors at low speeds. For long positioning distances, stepper motors have lower torque than servo motors at high speeds, while servo motors have higher torque at high speeds (superior high-speed performance).
The difference in stopping accuracy is significant. The stationary angle error of a stepper motor is affected by the machining accuracy of the small teeth; when the small tooth pitch is 7.2°, the error tends to decrease (because the excitation phase is exactly the same, it is not affected by the error of the stator small teeth). Therefore, when positioning in 7.2° units, the stationary angle error also decreases. The stopping position accuracy of a servo motor is affected by the encoder resolution and the assembly accuracy of the encoder and motor. Therefore, simply adjusting the encoder to a high resolution will not improve the positioning accuracy.
Regarding synchronization: Stepper motors, being open-loop controlled, operate synchronously with the input pulses. As long as the pulse commands are input synchronously, multiple motors can perform almost identical operations. Servo motors, however, are closed-loop controlled, resulting in a delay relative to the pulse commands. When controlling multiple motors, the varying delay times can lead to mutual interference and unstable operation.
So, what are the differences between servo synchronous motors (permanent magnet synchronous motors) and asynchronous motors, both of which are servo motors?
Servo motor classification
If we only observe the operation of the motor, we can easily draw the following simple and easy-to-understand conclusions.
asynchronous servo motor
Three-phase AC asynchronous motor
Structure of synchronous servo motor
1. Different control speeds
Synchronous servo motors have a fast control speed, taking only a few milliseconds from start to additional speed, while asynchronous motors require several seconds under the same conditions.
2. Different starting torque
Synchronous servo motors have a larger starting torque than asynchronous servo motors, enabling them to move objects with large inertia.
3. Different power densities
For the same power rating, synchronous servo motors can be made smaller and lighter.
4. Different operating efficiencies
Synchronous servo motors have high operating efficiency and can support long-term low-speed operation, while asynchronous servo motors support long-term high-speed operation.
5. Different situations during power outages
Synchronous servo motors do not rotate when power is off and can be quickly controlled to stop the action, while asynchronous servo motors do rotate when power is off.
A diagram illustrating the differences between synchronous and asynchronous servos.
In summary, servo motors offer highly accurate speed and position control, converting voltage signals into torque and speed to drive the controlled object. The rotor speed of a servo motor is controlled by the input signal and responds quickly. In automated control systems, when used as an actuator, it possesses characteristics such as a small electromechanical time constant, high linearity, and low starting voltage, converting received electrical signals into angular displacement or angular velocity output on the motor shaft. Therefore, the advantages of servo motor systems in terms of accuracy, torque-frequency response, and overload performance give them a wider range of applications than stepper motor systems. They are widely used in machine tools, textile machinery, printing and packaging machinery, as well as in next-generation robots, new energy vehicles, and clean energy equipment such as photovoltaic and wind power.
