Re: Stepper resonance - not a problem

On 3 Mar, CAD_CAM_EDM_DRO@onelist.com wrote:

> Message: 1
> Date: Thu, 2 Mar 2000 11:39:34 -0500
> From: "Joel Jacobs" <jj@...>
> Subject: Stepper resonance - not a problem
>
> I had been reluctant to use half-steps because every other step only one
> field energizes and I thought it wouldn't have as much torque - after
> thinking about it a bit, that's wrong. It still gets full power every other
> step and even if it couldn't move at all on the half step it would surely
> move on the next full step.

I think you're a bit mistaken on a couple of things here.

If the motor stopped on a particular half-position step as a result of
insufficient torque, it may not move on the next "full" step either.
That's the whole point programmers are trying to make with SMOOTH
accelerations tied to hardware timers. Resonance is a problem that can
cause missed steps, but "unsmooth" accelerations will also cause
missed steps.

The constant torque "secret" is to increase the current to the winding,
when only one winding is on. Appropriately scaled, this will result in
constant motor torque for each position.

The theory is clear and well understood. For any given angle, the
current relationship should be sin(a) cos(b). For simple half-step, this
translates to increasing the current by 40% when only a single winding
is on.

Note that at no time does the current through the windings exceed the
current in the both windings on position, so no motor overheating
should result. ( Your 4 amp steppers, for example, will have 8 amps
total in "both windings on" position and 5.6 amps though one winding
only in the "half" step position ) If the motor is stopped in the
one-winding on position, though, the motor current must be reduced, or
the winding will overheat.


I've been working with DanM on this for a bit, and Dan has found that
going to quarter step solves almost all resonance problems, and that
going finer than that, to so-called microstepping is really not
necessary.

Be aware that absolute positioning is not as accurate in any of the
intermediate positions, and this position error is increased as the
steps get finer and finer. ( Yet another reason to avoid uStepping )
Moral: Don't change your pulley ratios to keep the speed up.

Alan


--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
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