Re: Axis Stall Problems

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Paul Kelly" <tech@c...> wrote:

>
> I run my 297+298 driver (circuit pretty much straight from
> the datasheet) at 18kHz.
>
> Thinking about your setup (I've built drives very similar to
> this and had similar problems) I reckon 20kHz sample rate is
> about a factor of 10 too slow.
>
> A typical PWM drive starts with a square wave at the PWM
> frequency. Turns on the drive transistor for a minimum period
> (ie ignores the current spike from switching transients),
> then uses a comparator to turn off the drive when the
> current reaches a threshold. All this is happening VERY
> fast. I can remember having to change an op amp I was using
> to amplify a current sense voltage because it wasn't fast
> enough.. If you are switching at say 10KHz and sampling at
> 20kHz then you effectively only have two current settings.
> (it's a bit more complex I know..)

I understand what you are saying, but the measured current
does indeed average around 1 Amp.

> I'm not trying to rubbish your (obviously) hard work BTW.

Ultimately, the only thing that matters is whether it works
or not. Right now, it barely works.

> It all looks fairly well thought out. My advice is still
> to buy a 297, build a drive and test..

If I can't figure out what is going wrong, I will do that.

> Slightly off topic. I've just build some three axis Allegro
> 2977 based boards. These little microstep chips work really
> well. Only good for 35V and 2-2.5A though.... If you end up
> deciding to rebuild your drives, get in touch and I'll send
> you some schematics for them.

I've looked at the A2977 and there are three problems (for me):

1) 35V is on the hairy edge for 24VAC transformer.
Finding a smaller transformer is a pain.

2) I can not find any Allegro spec. sheet that
explains how to heat sink their product. At
least with the L297, I can just buy a heat sink
that clips right on.

3) Surface mount. I do not really have adequate
equipment to mount the chips.

I'm sure they are all workable, but in the end, all I
really want is something that is easy to build and just
works. Obviously, the L298/L298 pairs meet that criteria,
but I was hoping I could get a little cheaper and a little
more flexiblilty.

-Wayne

[snippage]