Re: stepper vs servo

Hi,

Here's my 2 cents worth on the stepper vs. servo debate.

Steppers are not prone to losing steps any more than a servo is prone
to losing position. Both are the result of a motor being asked to do
more than it can. The major difference is the servo lets you know it
has given up while a stepper does not.

Think of a stepper as a carosel with 50 horses and only one rider.
The carosel is the rotating magnetic field and the rider is the
motor's rotor sitting in one of the 50 possible stable locations. As
the field rotates, it drags the rotor along. Overload the motor and
the "rider" will fall back onto the next "horse" behind it. If the
field is rotating fast enough, the rider will not be able to do that
and will come to a stop while the field continues to rotate on.

At low speeds, an overloaded stepper will fall back 4 full steps and
make a heck of a racket as it does so. At high speeds the motor will
simply stall and stop. Either way you will notice.

The idea a stepper can "miss" steps dates back to full step drives
where the motor would resonate so badly it would actually knock
itself onto the adjoining "horses". Furthermore the motor made so
much noise and vibration that missed steps sounded just like normal
ones. Microstepping has taken care of that.

You can put an encoder on a stepper to notify you it has stalled or
missed steps. A servomotor requires it in order to be stable, so
the "fault" feedback you get is a freebe. However if you go to all
the trouble and expense of adapting an encoder, why not finish the
job and replace the motor with a servo and get better performance?

The virtue of a stepper is it can operate open-loop very reliably.
Equipping it with an encoder cancels that virtue, and in my opinion,
is like dressing a sparrow up in peacock's plumage. As long as you
are getting the feathers, get the bigger bird as well.

Or, on the otherhand, don't overload the stepper and it will be just
as reliable and accurate.

Mariss