Re: stall detection without encoder?

Hi Mariss,

Yes, I'm starting to remember a little of what you told me before! Too
bad it's not an easy thing to do (I mean the stall detection, not
necessarily the remembering!). I think the "missing steps" flag would
be useful, IMO, 'tho.

Alan KM6VV


mariss92705@... wrote:

>
> Alan,
>
> Stall detect on a step motor consists of seperating the "back-EMF"
> current component from the motor winding current.
>
> Without this component, the winding current is entirely inductive,
> meaning it is a triangle wave at medium and higher speeds.
>
> The problem is the amplitude of the inductive component increases
> while the the "back EMF" component decreases with decreasing speed.
>
> The stall detect method I investigated was to use two identically
> driven bridges; one driving an operating motor while the other one
> drove the same type motor except its shaft was super-glued so it
> couldnt turn.
>
> A dual-channel 'scope probed both bridge's current sense resistors
> differentially, (same V/div both channels, channel "B" inverted,
> scope set to "ADD"). This cancelled the inductive component, leaving
> only the "back EMF" if the operating motor was turning.
>
> The phase angle of the "back EMF" component relative to the step
> pulse gave an accurate "pecentage of stall torque" being delivered by
> the motor. This would have been very useful as feedback to the step
> pulse source to slow down if the motor load were to approach stall.
> Not only would you have stall detect but stall prevention as well!
>
> A practical circuit was designed where the cancelling signal was
> derived from a current source driven capacitor ( 1/C*di/dt = L*dv/dt).
>
> I ultimately abandoned this enquiry because below a cetain speed the
> signal simply became too small and was buried in the noise. Put
> another way, "back EMF" goes to zero at zero speed, so motor load
> cannot be detected for a stopped motor.
>
> I suppose missed steps could be detected due to the generated pulse
> of "back EMF" but I didn't feel this was enough to justify the
> additional circuit complexity.
>
> Mariss
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., Alan Marconett KM6VV <KM6VV@a...> wrote:
> > Hi Peter,
> >
> > Interesting idea. I don't think it's new. I believe Mariss and I
> have
> > discussed monitoring V/A's of the stepper motors, for this or
> similar
> > purposes. The Software would have to read A/D's, and look for the
> > signature of a stall in the data. I believe Mariss' hardware might
> > sense? Not sure. If we knew what to look for... "missed" step
> > detection would be valuable!
> >
> > Alan KM6VV