Re: [CAD_CAM_EDM_DRO] Re: stepper vs. servo

Mariss Freimanis wrote:

>A step motor's ultimate accuracy is 2,000 locations (steps) per
>revolution. This is based on the "+/-5% non-accumulative tolerance"
>spec.
>
>Any microstep setting past 10 gives empty resolution.
>
>

Yes, we are trying to come at the same point from different directions.
But, the accuracy when saddled with external loads and friction make the
case even worse than this.

>I can think of only two reasons for using a higher resolution than 10:
>
>1) Need smooth rotation at speeds <<1 rev per second.
>2) The motor is used closed loop with a high resolution encoder.
>
>

Although I haven't made an attempt to measure this with instruments, my
feeling from playing with stepper motors is that their open-loop stiffness
is WAY lower than a servo system with high DC gain. Given a system
with a good deal of external friction (for instance a machine with an
Acme leadscrew) and a closed-loop control system, I might expect a
serious case of hunting. Ie. error is sensed, an offset is applied to
the motor
position to attempt to correct the position error, and at some point it
jumps past the desired position. Due to the motor's inductance, and the
possibility of some delays between the detection of the position error
and the change in winding currents, it might be impossible to make this
work at all with standard drive components. For instance, I would expect
that a Gecko 210 would be totally unsuitable for such an arrangement,
but I don't know if a 201 would do a whole lot better, with typical motors.
Have you tried doing closed-loop control with your stepper drivers?
(Not that I really think this is a great idea, just wondering. If you have
to have encoders on your motors, then going with DC brush servos starts
to look like a much better idea, and the cost difference gets much closer.)

Jon