Re:Re: PWM stepper driver with external bridge

On 8 Jan, CAD_CAM_EDM_DRO@egroups.com wrote:

> Message: 1
> Date: Mon, 08 Jan 2001 14:34:24 -0000
> From: "Roman Black" <fastvid@...>
> Subject: Re: PWM stepper driver with external bridge
>

> Hi, it's my first post here, hope I don't offend
> anyone but I want to dispute a couple of points.

Speak up .. we tend to fight in a "friendly" manner in this group.

> having designed power control hardware for many
> years I can tell you 5A to 10A drivers are as
> different as 2A to 5A drivers are...

I've run that design at 8 amps with no problems and have no reason to
believe that 10 amps won't work as well .. although I do admit that 10
amps IS NEAR, if not ON, on the edge.

> Wrong! Yes microstepping CAN be "at all" accurate,
> the question is "how accurate"?

Forgive me the lack of precision in my previous post. I was assuming
1/16 or 1/32 step as "microstep" .. to me, 1/4 step is some kind of
"middle ground". Also, I was assuming the current ratios to be
uncompensated.

Moreover, as far as static friction goes, some quick testing has shown
that the behaviour of a motor on a bench and connected to a lead screw
moving a table are quite different.

> The a3955 will do as Lawrence requires, it will
> drive ONE PHASE perfectly as it was designed.
> Obviously you need two for both phases. It will
> drive a h-bridge fine with a few resistors.
> Remember a stepper motor requires TWO h-bridges
> for bipolar operation.

I think he wants the chip as a driver for a higher current output stage
AND to maintain the various current decay modes. Not sure how this can
be accomplished with just resistors.

>
> This sounds somewhat similar to the commercial
> project I am currently working on. :o)

My goal was not commercial, just an exercise in "improving" what I
already had. Just about the time I completed the design, I talked
myself into a servo system.

> I am curious how you generate vref with the PIC,
> by hardwired resistor ladder? Or pwm??

First step was PWM. Some quick math convinced me that I couldn't change
the PWM widths quickly enough for the step rates I had in mind. ( I
could be wrong about this, but it was too close for comfort )

Next step was an R2R ladder ( right out of the Microchip App note ).
Great idea, just ran out of I/O pins with the processor I had in mind.

Last step was to use discrete resistors of JUST the right value,
because, as you rightly point out, there's not that many values
involved.

> There is not a lot of complexity in a h-bridge
> if it is designed well. Gate capacitance should
> not be huge problem.

I agree, except that relatively small value resistors are required
mostly to evacuate the current nice and quick, meaning slightly more
power sourced or sunk by the driver transistors, generally ruling
out TO-92 devices.

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