Re: [CAD_CAM_EDM_DRO] servo vs stepper

cdoughtynz wrote:

> Hi all,
>
> I am sure I have read something about this before but can't find
> anything now.
> Are servo's better then stepper motors? what are the pro's and con's?

If you want to have manual as well as CNC control, and use the CNC as
a DRO during manual operation, then a servo system with encoders is
better. (steppers with encoders will also do this.)

I feel that servos with tachometers can provide smoother motion, as the tach
effectively interpolates between encoder counts during VERY slow movement
in one axis. A good microstepping stepper system can be a LOT better than
a bad servo, however.

If the thought of ever losing your position reference and not knowing about
it immediately worries you, then a system with encoder feedback is better than
not having encoders. An open-loop stepper can lose a step, and you
won't know it until you discover the part doesn't fit.

Another major difference is that steppers lose torque very early as the speeds
rise. This can be extended by using a higher DC power supply voltage, up
to a point. Servo motors develop full torque right up to the point that they
are running at nearly the full power supply voltage. This means you can
develop more acceleration to much higher speeds with a small servo motor.
Stepper motors run hot, as they require a good deal of power to just hold
position, whether they are resisting a load or not. Servos draw no power
unless producing torque, so they draw power depending on the loads they
must move or resist. Servos will attempt to keep the error between actual
and commanded position as small as possible, and when slightly overloaded,
will just keep trying until the error is reduced. The CNC control can watch
the error, and decide whether to call an emergency stop when the error exceeds
some threshold. The threshold could be different depending on whether it is
a cutting move (G01) or a positioning move (G00). Under similar conditions, a
stepper will lose position (skip steps).

> Do servo's come as 3 phase?

Most modern servo systems use 3-phase motors of either permanent magnet
construction (called, somewhat erroneously, "brushless, DC") or AC
induction type.

Older systems used familiar DC brush motors. For light use, both hobby
and small shop, these are completely fine, and may last many years before
the brushes need attention. The drive electronics for these motors is simpler
than for the other types.

Jon