Comparison Between Stepper Motor and AC Servo
Stepper motor and AC servo motor are two most common types driving motor on wood engraving cnc router. When buy a CNC, many users are very concerned about what’s the difference between them? Which one is more proper for my production requirement? Let us make a detailed comparison between stepper motor and AC servo here.
1) Working principle
Please refer to article “Work Principle Stepper VS Servo” for comparison of their working principle.
2) Control precise
Stepping angle of stepper motor is 1.8 degree or 3.6 degree.
The precise of AC servo motor is guaranteed by the encoder which is near the motor shaft at back-end.
For a servo motor with a 17 bit encoder, the motor is taken one turn when the driver received 131027 pulse single from the control system. The pulse equivalent of servo motor is only 1/655 of stepper motors’.
3) Low frequency performance
Step motor will have low frequency vibration when it runs under a low speed. Low frequency vibration which is caused by working principle of stepper motor is not good for CNC router’s performance. But, it is usually solved by using a driver with subdivision function.
AC servo motor run smoothly, vibration does not occur even at low speed. The AC servo system has resonance suppression function which can compensate mechanical parts rigidity. With frequency analysis functions (FFT), mechanical resonance can be detected and easy to adjust by the servo system.
4) RPM and output torque
The output torque of the stepping motor decreases as RPM increases, and it will decrease sharply at high RPM, so the maximum operating speed is generally 300~600RPM.
AC servo motor is exporting constant torque. And it can output rated torque all the time when the RPM is no over its rated value which is normally 2,000RPM or 3,000RPM.
5) Overload capacity
Stepping motors do not have capacity of overload. AC servo motor has a strong overload capacity that can be used to overcome the inertia moment of mass load in the starting off action. Because there is no such overload capacity, we need to select a larger torque stepper motor when we make the selection. The machine needs not such a big torque under normal condition, so there is a waste torque on stepper.
6) Run performance
Stepping motor is open loop system. It may lose step when start frequency too high or too much load; and it is easily to occur overshoot if RPM value too high in stop moment. But please do not worry; there are usually no problems of loosing steps, when you have good parameters for speed and acceleration.
AC servo is close loop system. Servo pack takes sample the feedback signal from encoder and forms the position loop and speed loop. So there is no loosing steps or overshoot on AC servo system. The control performance is more reliable and stable.
7) Speed and acceleration
It takes 200~400 milliseconds for stepper motor from zero RPM to operating RPM.
For AC servo, it takes only several milliseconds in this case. So it is ideal for quick start and stop control. AC servo is superior to stepper in capacity of acceleration and deceleration.
AC servo has lower noise than stepping. If you have requirement on noise, AC servo is a better choice.
Stepper motors do not require routine maintenance.
AC servo is a more precise close loop system and there is feedback from encoder, alarm code will appear on servo pack sometimes when there is any error been detected even it is a tiny error. Meantime servo system instructs machine to stop working as error detected. Under this case, we have to find out the reason and solve it, then error code will disappear and machine works again.
AC servo is more costly than stepping system.
Although AC servo is superior to step in many aspects, we can often see stepping system on basic cnc router machines. So my advice is that we should take all factors like production requirement, control performance, maintenance and cost into our consideration and select the appropriate motor for our machines.
Please feel free to ask if any question.
Original article from Jinan EagleTec CNC Machinery Co.,Ltd.
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