Re: [CAD_CAM_EDM_DRO] Geckodrive

Jack Hudler wrote:

>Unless I'm missing something in your post (you said "step motor").
>
>A stepper motor by its very nature is stallable via load, inertia, and
>resonance. Unless you're suppressing mid range instability caused by parametric
>oscillations then you're always in danger of stalling regardless of loading.
>These suppression or damping features are usually found on high end
>micro-stepper drivers.
>You can fix this some what by having a stepper with extra torque than needed but
>that's a wasteful and expensive route and all you've really done is changed the
>rotor inertia.
>
>Once a stepper stalls it enters a very chaotic state and at this point the
>pulses from the incremental encoder are all invalid. You always have to re-index
>after a stall to remove the chaotic error. Still the end effect is; you're part
>is screwed.
>
>At any rate an incremental encoder is not going to give you advance warning of a
>stall in order to avoid it. You might try putting a tachometer on it and
>programming a DSP to sense the instability but that's not a cheap route either.
>
>
>

Why would an encoder fail to provide accurate position feedback (within
its basic
resolution) when the motor stalls? Other than electrical interference,
(which has to be
combatted in all situations), a quadrature encoder should report
position reliably, no
matter whether the motion is chaotic or smooth. They handle vibrations
in machines
all the time.

And, yes, given a suitable encoder resolution relative to the motor step
resolution, it SHOULD
give advance warning of a stall, as the difference between electrical
commanded position
and actual measured position will increase. You have to reduce the step
input rate
very sharply before this angle exceeds 180 electrical degrees, or the
stall has ocurred.
That is not 180 degrees on the motor shaft, it is 180 degrees of the
electrical step
waveform.

Jon