piclist 1999\08\15\150151a >
Thread: Driving a Stepper
www.piclist.com/techref/io/steppers.htm?key=stepper
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face BY : Mark Willis email (remove spam text)



Paul B. Webster VK2BZC wrote:
>
> Mark Willis wrote:
>
> > <G>) - You know me, though, I want to know MORE, MORE, MORE <G>  I
> > have a LOT of old steppers around here & will be using them for other
> > uses (some of which will have to be PIC driven, and are no-where near
> > as simple as his needs.
>
>   I just feel that someone's done virtually *all* the design work to
> make those things and I doubt they would have failed to optimize the
> design, at least to any significant extent.  IOW, you should have a 12V
> motor with the appropriate driver (chip) driven in the optimal manner.
>
>   Now, you might begrudge the current drawn by the other parts of the
> disk drive electronics but of course you will have split off the capstan
> motor anyway, and any components obviously unnecessary can be removed
> (or disabled) and I doubt you can make any other efficiencies.

 Oh, sure, all true ...  But, it's no FUN if I cannot breadboard it
myself! <G>

> >  I want to automate feeding the cats, some day, for one thing.
>
>   Hmmm.  That'll be interesting.

 The good thing is the cats'll tell me if the design goes awry (They're
good at leading me to their food dish <G>)

> >  He wants an absolute minimal cost solution,
>
>   Using the drive electronics is certainly that.

 Definitely.  Just a 555 & power source...

> > I also was already talking to him about half-stepping the motor, which
> > I'd probably do here - but I doubt he'll do, for finer smoother
> > rotation of the motor.
>
>   Is it *really* necessary?

 For some things I'll be doing, later, it'd be fun, for his job,
probably not <G>  May have to full step using twin windings activated at
the same time, or may have to spin at 24Hz step rate with a 4:1 gear
down, to get him enough torque to do what he wants to do.  We'll see.

> >  (I've wondered - Does half-stepping give you more effective torque
> > out of the same stepper?  Or no effect there?)
>
>   Mmm, various "scraps" on similar topics on this list.  It seems to me
> you *may* get marginally more torque by half-stepping (having adjacent
> coils simultaneously energized for alternate steps) but really, the half
> step from two-coils-driven to one-coil-driven must by definition be cor-
> respondingly weaker than its converse.

 Makes sense - you can choose to always drive pairs of coils, though,
for higher torque.

>   Assuming each step is met with limiting friction, if the one-to-two
> coil transition is only just sufficient to overcome the friction, the
> next half-step would be too weak to overcome that friction and the rotor
> would tend to jam.  A subsequent two-coil activation involves the
> opposite drive on one of these coils and would have some net torque away
> from the previous position, but you'd have lost resolution.  I don't see
> that the analysis would be much better in continuous motion with
> "flywheel" effect.

 He'll probably gear down and increase the step rate, if he has a
torque problem.

> > I might scavenge driver parts off floppy boards, but want to be able
> > to be independent (on some designs) of a PC power supply <G>
>
>   Well...  The 5¹" drives use 12V motors, so you need a 12V supply for
> that.  If you use logic (PICs) to drive them, you need 5V.  The (tiny)
> steppers in many 3¸" drives use 5V of course.  You can be independent of
> a PC power supply, but you'll end up supplying the same voltages.  And
> I'm sure you have *plenty* of spare plugs on the end of RD-BK-BK-YL
> leads!

 "Never enough", despite the dozen spare power supplies in the garage
<G>

>   Cheers,
>         Paul B.

<37B70E1E.7F5B75CA@nwlink.com>

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