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The difference between stepping motor and servo motor

August 19,2022

Stepping motors are mainly classified according to the number of phases, among which two-phase and five-phase stepping motors are widely used in the market at present. Two-phase stepping motor can be divided into 400 equal parts per revolution, and five-phase stepping motor can be divided into 1000 equal parts. Therefore, the five-phase stepping motor has better characteristics, shorter acceleration and deceleration time and lower dynamic inertia.

With the appearance of all-digital AC servo system, AC servo motor is more and more used in digital control system. In order to adapt to the development trend of digital control, stepping motor or all-digital AC servo motor is mostly used as executive motor in motion control system. Although they are similar in control mode (pulse train and direction signal), they are quite different in performance and application.

First, the control accuracy is different

The step angle of two-phase hybrid stepping motor is generally 3.6 degrees and 1.8 degrees, and that of five-phase hybrid stepping motor is generally 0.72 degrees and 0.36 degrees. There are also some high-performance stepping motors with smaller step angles. For example, a stepping motor for wire cut machine tools produced by Sitong Company has a step angle of 0.09 degrees; The step angle of three-phase hybrid stepping motor produced by bergerlahr can be set to 1.8, 0.9, 0.72, 0.36, 0.18, 0.09, 0.072 and 0.036 by dip switch, which is compatible with the step angle of two-phase and five-phase hybrid stepping motors.

The control accuracy of AC servo motor is guaranteed by the rotary encoder at the rear end of the motor shaft. Taking Panasonic all-digital AC servo motor as an example, for the motor with standard 2500-line encoder, the pulse equivalent of the motor is 360 degrees /10000=0.036 degrees because of the quadruple frequency technology in the driver. For the motor with 17-bit encoder, every time the driver receives 217=131072 pulses, the motor makes one revolution, that is, its pulse equivalent is 360 degrees /131072=9.89 seconds. It is 1/655 of the pulse equivalent of a stepping motor with a step angle of 1.8 degrees.

Second, low frequency characteristics are different.

Stepping motor is prone to low-frequency vibration at low speed. The vibration frequency is related to the load condition and the performance of the driver, and it is generally considered that the vibration frequency is half of the no-load take-off frequency of the motor. This low-frequency vibration phenomenon, which is determined by the working principle of stepping motor, is very unfavorable to the normal operation of the machine. When stepping motor works at low speed, damping technology should generally be adopted to overcome the phenomenon of low-frequency vibration, such as adding dampers to the motor or adopting subdivision technology on the driver.

The AC servo motor runs very smoothly, and even at low speed, there will be no vibration. The AC servo system has resonance suppression function, which can cover the lack of rigidity of machinery, and the frequency analysis function (fft) inside the system can detect the resonance point of machinery, which is convenient for system adjustment.

Third, the moment-frequency characteristics are different

The output torque of stepping motor decreases with the increase of rotating speed, and it will drop sharply at higher rotating speed, so its maximum working speed is generally 300 ~ 600 rpm.

AC servo motor is a constant torque output, that is, it can output rated torque within its rated speed (generally 2000rpm or 3000rpm), and it is a constant power output above the rated speed.

Four: different overload capacity

Stepping motors generally do not have overload capability. AC servo motor has strong overload capability. Take Panasonic AC servo system as an example, it has the capability of speed overload and torque overload. Its maximum torque is three times of the rated torque, which can be used to overcome the inertia moment of the inertia load at the moment of starting. Stepping motors do not have this overload capability, so in order to overcome this inertia moment, it is often necessary to select a motor with larger torque. However, when the machine works normally, it does not need such a large torque, which leads to the phenomenon of torque waste.

Five: different running performance

The control of stepping motor is open-loop control, so it is easy to lose step or stall when the starting frequency is too high or the load is too large, and it is easy to overshoot when the speed is too high when it stops. Therefore, in order to ensure its control accuracy, the problems of speed up and speed down should be handled properly. AC servo drive system is a closed-loop control. The driver can directly sample the feedback signal of the motor encoder, and the position loop and the speed loop are formed inside. Generally, the stepping motor will not lose step or overshoot, so the control performance is more reliable.

Six: Speed affects performance differently.

It takes 200 ~ 400 milliseconds for stepping motor to accelerate from standstill to working speed (usually several hundred revolutions per minute). The AC servo system has a good acceleration performance. Taking Panasonic msma400w AC servo motor as an example, it takes only a few milliseconds to accelerate from static to its rated speed of 3000rpm, which can be used in control occasions requiring quick start and stop.