What is the difference between a servo motor and a stepper motor?

Stepper motors and servo motors are both common precision motion control actuators widely used in industrial automation, robotics, and CNC machinery. While both can achieve accurate position and speed control, their operating principles, performance characteristics, and applications differ significantly.

Operating Principles

Stepper Motor

A stepper motor operates on an open-loop control system. Its rotor consists of permanent magnets, and the stator windings are designed in multiple phases. By systematically energizing and de-energizing these windings, the motor rotates in fixed, discrete increments, known as "step angles." Each electrical pulse causes the motor to move one step. Therefore, to make the motor rotate a specific angle, you only need to send the corresponding number of pulses to its controller.

Servo Motor

A servo motor, on the other hand, uses a closed-loop control system. It consists of three main parts: the motor, an encoder, and a drive. The encoder provides real-time feedback on the motor's position and speed. The drive then compares this feedback with the set target value and adjusts the motor's current and voltage to ensure it accurately reaches the desired position and speed. This closed-loop control allows the servo motor to continuously correct any position errors, resulting in higher precision and dynamic performance.

Performance Characteristics

Stepper Motor

• Precision: A stepper motor's precision depends on its step angle, typically ranging from 0.9° to 1.8°. Micro-stepping technology can further improve resolution, but this may reduce accuracy and torque.

• Torque: Stepper motors have high torque at low speeds, but their torque rapidly decreases as the speed increases. They provide a strong holding torque when at a standstill, eliminating the need for an external brake.

• Speed: The maximum speed of a stepper motor is generally low, usually a few hundred to a thousand revolutions per minute (RPM).

• Overload Capability: Stepper motors do not have overload protection. If the load is too high, they can "lose steps," failing to follow the control pulses. This leads to position errors that the system cannot automatically correct.

Servo Motor

• Precision: A servo motor has very high precision, which is primarily determined by the encoder's resolution. It can achieve sub-micron positioning accuracy and maintain this precision even at high speeds.

• Torque: A servo motor provides consistent high torque across its entire speed range. Its torque decline at high speeds is far less than that of a stepper motor. It also has a strong overload capability and can withstand short-term overloads of several times its rated torque.

• Speed: The maximum speed of a servo motor is much higher than that of a stepper motor, reaching several thousand or even tens of thousands of RPM.

• Overload Capability: A servo motor system has strong overload capability and dynamic responsiveness. When the load suddenly changes, it can quickly adjust to maintain the set position and speed, preventing lost steps.

Applications

Stepper Motor

Due to their simple structure, lower cost, and high torque and holding force at low speeds, stepper motors are often used in applications where speed is not critical, the load is relatively constant, and real-time feedback is not required. Examples include:

• 3D printers

• Laser engravers

• Small-scale CNC machines

• Vending machines

• Textile machinery

Servo Motor

Because of its high precision, speed, torque, and strong dynamic response, the servo motor is widely used in applications with extremely high performance requirements. Examples include:

• Industrial robot arms

• High-precision CNC machine tools

• Automated production lines

• Printing and packaging equipment

• Medical devices

Summary