Re: stepper vs. servo

> Actually the DRIVER adjusts the current levels into 2 multi-
> stepped "square" waves, 90 electrical degrees apart; which causes

the

> motor to move in microsteps. As the number of current steps

increase,

> finer microsteps are possible. As we increase the number of current
> levels available (from the drive), we can more closely approximate
> the sinewave "ideal". The values used for the current levels are
> taken from the sin/cos table. They may be "adjusted" from these
> theoretical values due to motor construction/response. (FWIW, the
> Allegro A3977 spec sheet--type a3977 in a yahoo search-- does a
> pretty good job of explaining this. And allegro uses the sin/cos
> values exactly)

Interestingly, Allegro's paper notes that at the 1st and 16th step,
the current is so close to full that the value (98.089%) is smaller
than the motor error (3%) so they ignore the very end of the ranges.
I'd quote the pages, but Allegros site is not responding. If anyone
wants the reference, please let me know.

But that opens the question of how others can get 256 microsteps or
more. And THAT really is a more general motion control questions as
even though we tend to think our interests are 98% of the universe,
CNC machining applications of motion control are probably more in the
2% range. I'll bet Hewlett Packard (one supplier) sells more Laser
Printers in one year using motion control than all the home brew CNC
stuff ever built.

Dave