Re:Re:Re: PWM stepper driver with external bridge

On 9 Jan, CAD_CAM_EDM_DRO@egroups.com wrote:

> Sure, I see this attitude a lot lately, seems there
> is a very definite "microstepping bad - gearing
> good" attitude with CNC guys in particular. In
> many other areas microstepping is quite accepted.
> I suppose current compensation stuff is quite new
> and not as widely understood. :o)

Maybe not quite so simple as that.

Remember that machining is somewhat unique among other computer
controlled applications in that the load being moved is frequently
unknown or unpredictable in terms of size or "stiffness".

In most other CNC applications that I can think of ( that use motors of
more or less this size ), the forces required are predictable in
advance and hence genuine "engineering" can be done.

A machinist, on the other hand, is likely to try anything and see what
happens.

That makes the torque multiplication of pulleys is a big deal for many
of us, and given the ( usually ) added cost for microstepping
controllers, the "gearing good" attitude is somewhat understandable.

> Good point, the different current decay modes
> make it more compliacted, but from what I know
> about the 3955 it turns on both the bottom
> bridge semis for fast decay, this will still
> work when it is driving a h-bridge I think,
> but you could be right. :o)

Yeah, I'll have to stare at the logic for a while to figure out exactly
what they're doing .. and then hope the diagram actually matches the
chip !

> Yeah, it gets hard to get resistors just right,
> you either have to go to 1%'ers in non standard
> values or stat using a number of R in combination
> to get the right values.

For a 1/4 step controller, I was able to pick standard value resistors
and stay within 1% of the theoretical ideal. Just took about two hours
with Excel to do it. For the eighth and sixteenth step prototype, I
didn't even try .. I chickened out and just used a whack of ten turn
trimmers.

Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta