Stepper motor
and AC servo motor are two most
common types driving motor on CNC router machine.
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.
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’.
Stepper 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.
8) Noise
AC servo has lower
noise than stepping. If you have requirement on noise, AC servo is a better
choice.
9) Maintenance
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.
10) Cost
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.
No comments:
Post a Comment