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What is the difference between servo motor and stepper motor?

Superwave Laser Technology Co.,Ltd | Updated: Apr 01, 2018

Six major differences between servo motor and stepper motor:

1. Different control accuracy.

The step Angle of the two-phase hybrid stepping motor is generally 3.6 and 1.8, and the step Angle of the five-phase hybrid stepping motor is 0.72 and 0.36. There are also some high performance stepping motors with smaller step angles. Such as cross company produces a stepper motor for wire cutting machine tool, the interval Angle is 0.09 °. Berger lahr company in Germany (BERGERLAHR) production of three-phase hybrid stepping motor where the step Angle can dial the code switch is set to 1.8 ° and 0.9 ° and 0.72 ° and 0.36 ° and 0.18 ° and 0.09 ° and 0.072 ° and 0.036 °, combine the two and five phase hybrid stepper motor step Angle.
The control precision of ac servo motor is guaranteed by the rotary encoder at the back end of the motor shaft. Panasonic digital ac servo motor, for example, in line with standard 2500 encoder of the motor, to drive internal adopted four times the frequency conversion technology, the pulse equivalent to 360 ° / 10000 = 0.036 °. For with 17 encoder of the motor, drive each receive 217 = 131072 pulse motor turns a circle, namely the pulse equivalent to 360 ° / 131072 = 9.89 seconds. Interval Angle is 1.8 ° is 1/655 of the pulse equivalent of stepper motor.

The low frequency characteristics are different.
The low frequency vibration of stepping motor is easy to occur at low speed. The vibration frequency is related to the load condition and drive performance, and the vibration frequency is generally considered to be half the frequency of the motor no-load. The low frequency vibration caused by the principle of stepping motor is very bad for the normal operation of the machine. When the stepper motor works at low speed, the damping technique should be used to overcome the low frequency vibration phenomenon, such as adding a damper on the motor, or using subdivision technology on the drive. Ac servo motor runs very smoothly, even at low speed, there is no vibration phenomenon. Ac servo system has a resonance suppression function, can cover the mechanical rigidity, and internal system has the function of frequency resolution (FFT), can detect the mechanical resonance point, is advantageous for the system adjustment.

Three, the moment frequency characteristic is different.
The output torque of stepping motor decreases with increasing speed, and decreases sharply at higher speed, so its maximum working speed is generally 300 ~ 600RPM. Ac servo motor is the constant torque output, namely, within its rated speed (generally 2000RPM or 3000RPM), the rated torque can be output, and the constant power output is above the rated speed.

4. The overload capacity is different.
Stepper motor is generally not overloaded. Ac servo motor has strong overload capacity. Take panasonic ac servo system as an example, it has the capacity of speed overload and torque overload. The maximum torque is three times the rated torque, which can be used to overcome the inertia moment of inertia load. Stepper motor because there is no this kind of overload capacity, in order to overcome the inertia moment when selection, often need to select a larger torque of the motor, however the machine during the normal work and don't need that big torque, there is a moment of waste phenomenon.

5. Different operation performance.
Stepper motor control for the open loop control, start frequency is too high or too much load step easy appear lost or blocked, stops the phenomenon of high speed prone to overshoot, so to ensure the control accuracy, should handle up, slow down. Ac servo drive system for closed-loop control, the actuator can be directly to the motor encoder feedback signal sampling, internal position loop and speed loop, general won't appear step motor lose step or overshoot phenomenon, more reliable control performance.

6. Different speed response performance.
It takes between 200 and 400 milliseconds for stepping motor to accelerate from rest to working speed (usually a few hundred revolutions per minute). The ac servo system has a good acceleration performance, taking panasonic MSMA400W ac servo motor as an example, from static acceleration to its rated speed of 3000RPM for only a few milliseconds, which can be used to request the quick start-stop control situation. To sum up, ac servo system is superior to stepper motor in many aspects of performance. However, it is often used in some cases where the motor is used to perform the motor. Therefore, in the design process of the control system, the control requirements, costs and other factors should be taken into consideration, and appropriate control motors should be selected.