The Differences Between Stepper Motors And Servo Motors

Servo motor and stepper motor are the two most common motor drives in industrial CNC, what is the difference between them, we will make an ultimate guide to stepper motor vs servo motor.

Servo motors are used as actuators in control systems, converting received electrical signals into angular displacement or angular velocity output on the motor shaft. The rotor inside the servo motor is a permanent magnet. The U/V/W three-phase electricity controlled by the driver forms an electromagnetic field. The rotor rotates under the action of this magnetic field. At the same time, the encoder of the motor feeds back the signal to the drive, and the drive compares the target value with the feedback value. Compare the values and adjust the rotation angle of the rotor.

The accuracy of the servo motor is determined by the accuracy of the encoder, that is to say, the servo motor itself has the function of sending pulses. It sends out a corresponding number of pulses every time it rotates an angle, so that the pulses of the servo drive and the servo motor encoder form a response. So it is closed loop control.

A stepper motor is a stepper motor device with an open-loop control element that converts electrical pulse signals into angular displacement or linear displacement. Under non-overload conditions, the motor speed and stop position only depend on the frequency and number of pulses of the pulse signal, and are not affected by load changes. When the stepper driver receives the pulse signal, it drives the stepper motor to rotate a fixed angle in the set direction, which is called "step angle".

Stepping can control the angular displacement by controlling the number of pulses, so as to achieve the purpose of precise positioning. The speed and acceleration of motor rotation can also be controlled by controlling the pulse frequency of the driver, so as to achieve the purpose of high speed. Therefore, stepper motors are open-loop controlled.

The difference between stepper motor and servo motor controller A stepper motor controller is an electronic product that can send out uniform pulse signals. After the signal it sends enters the stepper motor driver, the driver converts it into a strong current signal required by the stepper motor to drive the stepper motor to run.

The stepper motor controller can precisely control the stepper motor to rotate through various angles. The driver receives a pulse signal. Each time a pulse is received, the driver will give the motor a pulse to turn the motor by a fixed angle. Due to this feature, stepper motors are widely used in all walks of life today.

A servo motor controller is a controller used to control servo motors. Its function is similar to that of a frequency converter acting on an ordinary AC motor. It is part of the servo system and is mainly used in high-precision positioning systems. Generally, the servo motor is controlled by three methods of position, speed and torque to achieve high-precision positioning of the transmission system. It is a high-end product of current transmission technology.

Control Accuracy The more phases and beats a stepper motor has, the more accurate it will be. Servo motors are based on built-in encoders. The larger the encoder scale, the higher the accuracy. One of the control methods is open-loop control and the other is closed-loop control. Low-frequency characteristics Stepper motors are prone to low-frequency vibration at low speeds. Damping or subdivision techniques are generally used to overcome low frequency vibrations, while servo motors do not vibrate at low speeds. The AC servo system has the function of resonance suppression, which can make up for the lack of rigidity of the machine. The system has a frequency analysis function (FFT), which can detect the resonance point of the machine and facilitate system adjustment. Torque frequency characteristic The torque output of stepper motors will decrease with the increase of rotational speed, while the torque output of AC servo motors is constant. Overload capability Stepper motors do not have overload capability, while AC motors have stronger overload capability. Running performance The stepper motor adopts open loop control.If the starting frequency is too high or the load is too large, it is easy to lose steps or stall. When the speed is too high, it is easy to overshoot. The servo system is closed loop control. The drive can directly sample the feedback signal of the motor encoder. A position loop and a velocity loop are formed inside. Generally, the stepper motor will not appear out of step or overshoot, and the control performance is more reliable. Speed response performance It takes hundreds of milliseconds for a stepper motor to accelerate from a standstill to a working speed, while the acceleration performance of an AC servo system is better, generally only a few milliseconds, which can be used to start and stop in applications requiring fast control.

AC servos outperform stepper motors in many performance areas. However, in some less demanding occasions, stepper motors are often used as actuators. Therefore, in the design process of the control system, various factors such as control requirements and costs should be comprehensively considered, and the appropriate control motor should be selected.