The working principle of a servo motor is based on its “perception-decision-making-execution” ability through closed-loop control. It converts electrical energy into precisely controllable mechanical motion, serving as the core power source for modern high-precision equipment. Whether it’s the millimeter-level positioning of a robotic arm or the complex trajectory processing of a numerically controlled machine tool, it relies on the servo motor’s precise control over position, speed, or torque. This article will deeply analyze how a servo motor achieves such precise control, covering its structure, control principle, and application scenarios.
Core Components
Motor Body
Types: DC servo motors (less common) or AC servo motors (mainstream, such as permanent magnet synchronous motors).
Structure: The stator (coil winding) and the rotor (permanent magnet or electromagnet) generate torque through electromagnetic induction.
Encoder (Feedback Device)
Function: It monitors the position, speed, or torque of the rotor in real-time and converts the mechanical motion into electrical signals to be fed back to the controller.
Types: Photoelectric encoders (high precision) or magnetic encoders (strong anti-interference ability).
Driver (Servo Amplifier)
It amplifies the low-power signal from the controller into a high-power current to drive the motor and adjusts the output waveform (such as three-phase alternating current).
Controller (such as PLC, motion control card)
It receives instructions (such as the target position), calculates the deviation between the actual value and the target value, and generates correction signals through algorithms like PID.
Closed-loop Control Process
Instruction Input
The user sets the target parameters (such as rotating 90° or maintaining a constant speed of 1000 rpm, etc.).
Start and Feedback
The motor starts to rotate, and the encoder collects position/speed data in real-time and sends it back to the controller.
Deviation Calculation
The controller compares the target value with the feedback value and calculates the error (such as the angular difference Δθ).
Output Adjustment
PID Control: It generates correction signals through proportional (P), integral (I), and derivative (D) algorithms to optimize the response speed and stability.
Driver Execution: It adjusts the phase, frequency, or amplitude of the current in the motor windings to change the torque and speed.
Dynamic Correction
Through continuous feedback-calculation-adjustment, the error approaches zero to achieve precise control.
Key Technical Features
Multi-loop Control Structure
Current Loop (Inner Loop): Controls the motor torque and has the fastest response.
Speed Loop (Middle Loop): Regulates the stability of the rotational speed.
Position Loop (Outer Loop): Ensures the final positioning accuracy.
Pulse Control Mode (Common in Position Control)
The controller sends pulse signals (such as the number of pulses = target displacement), and the driver interprets the pulses to drive the motor. The encoder verifies the actual position.
Anti-disturbance Ability
When there is a sudden change in the load, the encoder detects the deviation, and the controller immediately compensates to maintain stable operation.
Application Scenarios
High-precision Positioning: Joints of industrial robots, nozzles of 3D printers.
Speed Control: Spindles of numerically controlled machine tools, speed adjustment of conveyor belts.
Force Control Scenarios: Tactile operation of collaborative robots, precision assembly.
Servo vs. Stepper Motor
Advantages of Servo: Closed-loop control without step loss, faster response, and strong overload capacity.
Disadvantages of Stepper: Open-loop control is prone to step loss, poor high-speed performance, but low cost.
From factory assembly lines to medical robots, from the servos of spacecraft to smart homes, servo motors, with their precise, fast, and stable characteristics, are quietly promoting the development of various industries towards efficiency and intelligence. With the continuous breakthroughs in material science, algorithm optimization, and communication technology, the performance boundaries of servo motors are constantly expanding. In the future, they will play the role of “precision commanders” in more complex scenarios. Understanding its working principle is not only the key to mastering automation technology but also the key to unlocking the next generation of intelligent manufacturing.