Performance comparison between servo motor and stepper motor

Stepper motor, as an open loop control system, has an essential connection with modern digital control technology. In the current digital control system, the application of stepper motor is very extensive. With the advent of full digital AC servo system, that is also more and more used in digital control system. In order to adapt to the development trend of digital control, the stepper motor or digital AC servo motor is used as the executive motor in the motion control system. Although the two methods are similar in control mode (pulse train and directional signal), there are great differences in performance and application.

1, the control accuracy is different

The step angle of two phase hybrid stepping motor is generally 1.8 degree and 0.9 degree, and the step angle of five phase hybrid stepping motor is generally 0.72 degree and 0.36 degree. There are some high performance stepper motor subdivision by smaller step angle. such as Sanyo Corporation (SANYO DENKI) two phase hybrid step motor production through the step angle dial switch is set to 1.8 degrees, 0.9 degrees, 0.72 degrees, 0.36 degrees, 0.18 degrees, 0.09 degrees, 0.072 degrees, 0.036 degrees, compatible with the step angle two-phase and five phase hybrid stepping motor.

The control precision of AC servo motor is guaranteed by the rotary encoder at the back of the motor shaft. Taking SANYO full digital AC servo motor as an example, for the motor with standard 2000 line encoder, the pulse equivalent is 360 degrees /8000=0.045 degrees due to the adoption of four octave technology in the driver. With 17 bit encoder motor, drive each receive 131072 pulse motor of a turn, the pulse equivalent to 360 degrees /131072=0.0027466 degrees, 1.8 degrees is the step angle of stepping motor pulse equivalent of 1/655.

2, low frequency characteristics are different

Low frequency vibration is easy to occur in stepper motor at low speed. The vibration frequency is related to the load and the performance of the driver. It is generally believed that the vibration frequency is half of the no-load take-off frequency of the motor. The low frequency vibration, which is determined by the working principle of the stepping motor, is very unfavorable to the normal operation of the machine. When stepping motor works at low speed, damping technology should be used to overcome the low frequency vibration phenomenon, such as adding dampers on the motor, or using subdivision technology on the driver.

AC servo motor runs very smoothly, even at low speed, there will be no vibration phenomenon. AC servo system has resonance suppression function, can cover mechanical rigidity shortage, and the system has frequency analysis function (FFT), can detect the resonance point of the machine, easy to adjust the system.

3, the torque frequency characteristics are different

The output torque of the stepper motor decreases with the increase of the speed, and it will decrease sharply at high speed, so the maximum working speed is generally 300 ~ 600RPM. AC servo motor is constant torque output, that is, at its rated speed (generally 2000RPM or 3000RPM), can output rated torque, above rated speed is constant power output.

4, overload capacity is different

Stepper motors generally do not have overload capacity. AC servo motor has strong overload capability. Taking SANYO AC servo system as an example, it has the ability of speed overload and torque overload. Its maximum torque is two to three times of the rated torque, which can be used to overcome the inertia moment of the inertia load at the start moment. Stepper motor because without the overload ability, in the selection in order to overcome this inertia moment, often need to select large torque motor, and the machine during normal operation and does not need so large torque, the torque will appear the phenomenon of waste.

5, different operation performance

Step motor control for open loop control, the start frequency is too high or too much load there is lost or blocking the phenomenon of step turn off high speed, prone to overshoot phenomenon, so as to ensure the control accuracy, should handle the ascending and descending velocity. AC servo drive system for closed-loop control, drive directly to the motor encoder feedback signal sampling, the internal structure of position loop and speed loop, is generally not a stepping motor lose step or overshoot phenomenon, the control performance is more reliable.

6, the speed response performance is different

Stepping motor from standstill to working speed (usually hundreds of revolutions per minute) takes 200~400 milliseconds. The acceleration performance of AC servo system is better. Taking the 400W AC servo motor as an example, it is only a few milliseconds from static acceleration to its rated speed 3000RPM, which can be used in the control occasions requiring quick start and stop.

To sum up, AC servo system is superior to stepping motor in many performance aspects. But in some occasions where the requirements are not high, stepper motors are often used as executive motors. Therefore, in the design process of the control system, the control requirements, cost and other factors should be taken into account comprehensively, and the appropriate control motor should be selected.