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

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.