Re: What is a sense coil? Not all 8 wire steppers are unipolar!

Dave,

I just did an archive search and it ws YOU who posted the findchips
url I saw first (couple hunerd msgs' ago) so my THANKS go to you for
letting me know about it; and Doug for reminding me I didn't thank
anyone properly.

Ballendo

--- In CAD_CAM_EDM_DRO@y..., davemucha@j... wrote:
> To find a supplier, try www.findchips.com
>
>
> > I am looking at the l2998 from allegro Microsystems,
> > which includes internal diodes, but can't find a supplier/price.
>
>
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., yes@g... wrote:
> > I am working with some 8 wire stepper motors that have a very
> strange
> > characteristic. I had done lots of research before purchasing,
and
> > everywhere I read that an eight-wire stepper motor would always
be
> > universal. I found that two coils have a high resistance while
the
> > other two have a low resistance. This was really confusing until
I
> > got the manufactures data sheet. It turns out that they are
> bipolar
> > motors, with two sense coils. From what I gather, you can use
the
> > sense coils to determine if the stepper has actually moved to
where
> > you wanted it to go. Has anyone ever heard of this before? Is
> this
> > correct? Seems like an expensive way to implement an encoder!
> What
> > type of product would use such a feature?
> >
> > I have successfully driven them in bipolar mode (297/298
chopper),
> > ignoring the sense coils, and that works as expected. I have
many
> of
> > these motors and I am trying to design the cheapest/best driver
for
> > them. I would like to include the sense coil feature if it is
> useful.
> >
> > The bipolar drive coils are 1.25 ohm rated at 2.1 volts 1.65
amps.
> > My applications for them always include driving at least three
> > motors, so I think I will do the logic in a $5 PIC versus $24 for
3
> > 297s. I am having trouble selecting the drive circuit. The
> > complication comes in when I try to get pricing information. I
am
> > looking at the l2998 from allegro Microsystems, which includes
> > internal diodes, but can't find a supplier/price. I am
interested
> to
> > hear others opinions on the easiest/ cheapest drive circuitry. I
> am
> > also interested in references to learn about h bridge design if
> that
> > would be cheaper for this current level.
> >
> > It seems that drivers are limited by the amount of current they
> can
> > provide, which makes me wonder why the coil wouldn't be higher
> volts,
> > lower amps to accommodate. The 5804 is such an easy solution,
wish
> > they were unipolar
> >
> > Thanks for any input.