RE: [CAD_CAM_EDM_DRO] Microstepping high detent steppers

I believe that the positional errors introduced by detent torque are
repeatable. This suggests mapping them out at the controller level, but the
computational costs of that are high and additional feedback is necessary.
The feedback is necessary since the controller only sends out steps, it does
not know where the motor/drive combination is within the sine wave of a
microstep table. The rewards of doing this are questionable, since the
error introduced is pretty small compared to the cumulative effect of
ballscrew manufacturing error, shaft windup, thermal effects, and all the
other things that make a system less than perfect.

If you really want to cancel detent torque, the way to do it is through the
lookup table for the sine wave. The sine wave is, in reality, only an
approximation. Most systems have a digital counter which goes to a lookup
table for points on the "sine" wave. This is the source of the analog
command for current levels in each microstep. The way to remove detent
torque offsets is through that lookup table. The microstep wave form can be
altered to include offset currents, thereby nulling the detent torque. I
believe this is described in greater detail in Unitrode application note
U-112, using Unitrode UC3637 chip. They claim a "servo-like" torque when
this is done.

Never the less, this is a lot of screwing around for limited gains. A
custom table would need to be created for each brand/model of stepper motor.
When I read about this it seemed more like an interesting theoretical
curiosity rather than a practical approach.


G. Jackson

-----Original Message-----
From: echnidna [mailto:echnidna@...]
Sent: Sunday, December 01, 2002 3:27 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Microstepping high detent steppers

Hi Group,
I seem to recall a very brief comment about microstepping accuracy with high
detent steppers but cannot locate anything in the archives.

I assume the positional accuracy of microsteps is innaccurate with high
detent motors. But are those positional inaccuracies repeatable for the
precisely same coordinates of the tool cutting path?

List wisdom seems to generally be that reducing stepper current below the
maximum rated motor current has certain advantages.

Am I correct in believing that if one were microstepping a high detent motor
then reduced motor current would enable the motors inherent detent force to
become more influential on a particular microstep position than it would be
if the motor had the maximum rated current flowing through its coils?

Regards
Bob Thomas



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