How does a servo motor work?

The term 'servo' is derived from the Greek word 'σερβ', which originally means 'slave'. It was first used to describe a system capable of automatically executing instructions, akin to slaves following their masters' commands. A servo motor can thus be understood as a motor that strictly adheres to control signals.

How does a servo motor work? When a machine receives a command, the command is relayed to the servo system, the system employs a closed-loop feedback control loop to adjust the rotation speed and torque of the corresponding servo motor with remarkable speed and precision. This ensures that the related joint moves smoothly and accurately. Next, this article will reveal how the servo motor operates under the control of the servo system.

How does a servo motor work?

The servo system usually consists of a controller, a driver, an actuator and a feedback device. Its working principle is illustrated in the figure below.

Servo system operation flow chart 1. After receiving the target command, the controller will first transmit the control signal to the driver; 2. The driver then converts the control signal into a drive signal and transmits it to the actuator; 3. After receiving the drive signal, the actuator initiates mechanical movement and sends the movement information to the feedback device. Concurrently, the feedback device starts relaying real-time movement information from the actuator back to the driver; 4. After comparing the feedback signal with the control signal, the driver continues to output the adjusted drive signal to the actuator, which promptly modifies its movements. This cycle continues until the actuator outputs the mechanical movement required by the target command. At this point, the servo system has achieved the purpose of controlling motion according to the target command. The servo system is a closed-loop feedback control system that can accurately control the position, speed, acceleration or other parameters of the actuator. The servo motor, a specialized actuator, is specifically designed to accurately control mechanical motion. The core components of the servo motor include the stator, rotor, and encoder, as shown in the figure below.

Servo motor and encoder structure After receiving the drive signal through the cable, the servo motor's rotor initiates the shaft rotation, thereby completing the torque output. Simultaneously, the encoder provides real-time feedback to the driver on the servo motor's rotation angle, number of turns, speed, direction, and other motion-related data. A key characteristic of the servo motor is its ability to respond swiftly and accurately to changes in the drive signal, maintaining high-precision position control during movement. From a motion control perspective, it is the servo system that endows the servo motor with the capability to control movement. The precision of this motion control depends on the motor's structural design, the encoder's resolution, and the overall performance of the servo system in which it operates. The most significant structural difference between servo motors and other motors is the built-in encoder, which forms the basis for a closed-loop feedback control system. Servo motors typically employ three closed-loop controls: current (torque), speed, and position, to enhance the precision of their motion control. In addition, thanks to the application of digital signal processing technology, high-resolution encoders, and advanced control algorithms, the precision control capabilities of servo motors are significantly superior to those of other motors.

The difference between servo motors and other motors.

Specifically, traction motors are noted for their ability to generate high torque and power under heavy loads, yet their motion control capabilities significantly lag behind those of servo motors. Variable frequency motors boast a board power range and robust speed variability, but they also fall short in motion control precision compared to servo motors. Stepper motors achieve high position accuracy through pulse signals, are vulnerable to load variations that can cause missed steps, and they lack a closed-loop feedback mechanism for self-correction. Although stepper motors are also frequently used in motion control applications, servo motors equipped with 17-bit encoders offer control accuracy approximately 600 times greater than that of stepper motors. The superior control accuracy of servo motors vastly exceeds that of stepper motors. Details of the comparison between servo motors and other motor types are presented in the table below.

Comprehensive comparison of servo motors and other motors.

Motor Type	Main Features	Control Method	Control Accuracy	Application Scenarios
Traction Motor	Provides stable high torque and high speed for a long time under heavy load	Open-loop control is usually used to adjust the speed and output power through magnetic field weakening technology		Used to drive transportation vehicles such as trains, subways and trams
Variable Frequency Motor	Wide power range, strong speed control ability, energy saving	Open-loop control is usually used to control the motor speed by adjusting the power supply frequency through the inverter		Applicable to occasions that require frequent speed changes or energy saving, such as compressors, fans, water pumps, air conditioners and elevators
Stepper Motor	Simple control and high precision, but easy to lose steps when overloaded	Open-loop control is usually used to rotate by receiving pulse signals, and each pulse causes the motor shaft to rotate a fixed angle	The control accuracy of the two-phase stepper motor is 0.18°	ATM machines, copiers, 3D printers, image scanners, camera lenses, computer embroidery machines, etc.
Servo Motor	Fast response, high precision, strong overload capacity	Closed-loop control is strictly used, with three control modes: position, speed, and torque	The control accuracy is 360°/number of lines after encoder frequency doubling, and the positioning accuracy can reach 0.001mm	Used for high-precision automation equipment, robots, CNC machine tools, etc.

It is important to note that while other motors can achieve closed-loop control with external sensors and controllers, but the "innate" closed-loop control structure of servo motors allows them to surpass other types such as traction, variable frequency, and stepper motors in terms of control accuracy. Furthermore, the control objectives differ among these motors. Traction motors are focused on power management, efficiency optimization and overload protection; variable frequency motors aim to adjust the speed; stepper motors are designed to address the issue of lost steps; whereas servo motors are specifically aimed at precisely controlling the position, speed, and torque of the output motion. MOONS' offers several servo motors. Servo Motors from MOONS'

For more comparison information, please refer to these technical articles: Servo Motor Working Principle What is the type of DC servo motor? What is the control mode of the DC servo motor? What are the advantages of the step-servo motor instead of the stepper motor?

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