How to Choose Between Stepper and Servo Motor

In the last post, we performed a detailed analysis of the stepper motor and servo motor from the perspective of the working principle of the two. Here, we will look at the differences between the two from other perspectives.

Acceleration and deceleration performance

● It takes 200 to 400 milliseconds for the step motor to accelerate from standstill to the working speed.

● The AC servo motor accelerates from standstill to its rated speed in just a few milliseconds, so it is much better for quick start-stop control applications when compared with stepper.

Noise

Servo motor has lower noise than NEMA stepper motor during working. If you have a higher requirement on noise, CNC servo motor is a better choice.

Control and positioning accuracy

● Step angle of geared stepper motor is generally 1.8 or 3.6 degree.

● The control accuracy of the AC linear servo motor is determined by the encoder at the end of the motor. For a servo with 17-bit encoder, it makes one turn after its driver receives 31072 pulse. The pulse equivalent is only 1/655 of the stepper motor with a step angle of 1.8 degree.

RPM and output torque

● The output torque of the bipolar stepper motor decreases as the RPM increases, and it drops sharply at higher RPM, so the maximum operating speed for it is generally 300~600RPM.

● The AC servo motor torque is constant, that is, it can output the rated torque within the rated RPM (generally 2000RPM or 3000RPM).

Overload capability

● Stepmotor generally do not have overload capability. Based on this point, a high torque stepper motor is required in the selection, but the cnc machine does not need such a big torque during normal operation, and this caused a waste of torque..

● The electric servo motor has a strong overload capability and can be used to overcome the moment of inertia generated by the inertia load at the moment of starting.

Low frequency vibration

● Linear stepper motors are prone to low frequency vibration at low speeds. This low-frequency vibration phenomenon has a bad influence on the normal run of the cnc machine, which is determined by the working principle of stepper. This problem is usually solved by using a stepper driver with subdivision.

● The AC servo runs very smoothly, and no vibration even at low speeds. Furthermore, it has a resonance suppression function, which compensates for the lack of rigidity of the mechanical part. The system also has a frequency analysis function (FFT) inside, which can detect the resonance point of the machine to make compensation there.

Daily Maintenance

● DC stepper motors do not require routine maintenance. As long as the motor does not break, the machine can always run.

● Since the AC servo motors have a self-test function, sometimes when the detected feedback value exceeds a reasonable interval, the fault alarm code will appear and the machine will be automatically stopped. In this case, we need to eliminate the alarm code before we can continue working on the machine.

Cost of both

The step motor price is low, and the servo motor price is usually several times that of the stepper motor cost.

In summary, AC servo motors are superior to stepper motors in many performance aspects. While, stepmotors are less troublesome during use. Therefore, bipolar linear stepper motors are often used on some basic CNC. EagleTec suggest, in the process of selecting the machine motor, it is necessary to comprehensively consider various factors such as control precision requirements, speed performance, daily maintenance and cost, and select the appropriate control motor for your CNC.

Original Post from EagleTec

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