Re: [CAD_CAM_EDM_DRO] Re: stepper vs servo

Hi,

This is something I've been pondering for a while and wonder if someone can
answer one point at least. While it is obvious that stepper motors do just
that and have discrete and easily seen steps, do servo motors not also have
irregular or somewhat stepping motion? As the normal DC servos are basically
a DC motor with a set number of field coils and a certain number of segments
on an armature, surely there will be points in the revolution which have
greater torque than others. I know that in high precision servos, DC motors
are not used and they use a solid rotor which is driven by induced currents
which does give a very smooth but not very powerful motion but surely, in
servos using DC motor technology as are usually found on machine tools,
there is some 'cogging' action relating to the number of segments in the
armature.

I also wondered about the starting performance of servos. In a stepper, the
initial current produces as powerful a torque as when it is running at speed
but, in a DC servo, presumably the current must build up the torque until
the motor can overcome the friction - a more gradual effect if still only
taking microseconds. Does this have any actual effect on a machine's
capabilities or does it mean you need more powerful servos than you would
steppers?

Ian
--
Ian W. Wright
Sheffield UK
www.iw63.freeserve.co.uk

----- Original Message -----
From: "Jon Elson" <elson@...>
>
> Well, it depends on what you are doing, but this is NOT splitting hairs.
> For extremely
> fine machining, which I sometimes do, the added precision and smoothness
of
> a
> servo system CAN NOT BE BEAT! This is why ALL high end machine tools
> use some form of servo, and NOT steppers.
>