Re: [CAD_CAM_EDM_DRO] Re: PC based CNC system architecture

Wally K wrote:

> > For steppers, I think having an encoder with more resolution than
> the
> > stepper could cause problems. But, we could put a dead zone in the
> > control loop to prevent fast hunting. (The planned design will
> cause steppers
> > to hunt at a rate of one step every .6 seconds, as that is as slow
> as the
> > rate generator can go.)
> >
>
> Wally K.
>
> For this dead zone in the control loop for steppers. Is this a
> number in an EMC file. Or something on your control.

I don't think EMC has a dead zone, but it could be put into the
code that gets these numbers from the .ini file and passes
them through to the real-time section. For now, it would have to
be coded into the driver for the rate generator.

> Also i do not understand your hunt rate. "to hunt at a rate of one
> step every .6 seconds, as that is as slow as the rate generator can
> go." maybe i am thinking of the wrong term. Is hunt rate different
> than correcting for missing steps or encoder counts.

Yes. I originally had the idea to be able to turn the rate generator
off. Then, I realized that with 24 bits on the counter, I don't really
have to turn it off. Just set it to the lowest possible rate, and the
pulse counter feedback will cause EMC to flip the direction bit, and
then it will go back the other way. Maybe I will put in the control bit to
stop each axis individually, by freezing the counter. Simple to do.
But, without that, the steppers would get a pulse every .596 second
(10 MHz / 2^24). So, each stepper axis would bump one step
back and forth, completing a cycle in about 1.2 seconds. This might
drive people crazy, the machine sitting there going bump-bump-bump
all the time. So, I guess I will put in the stop logic.

> And, If i have a stepper system and want to add 1 spindle control
> axis for my bridgeport to use for a VFD. Can your 4 axis servo card
> be populated for 1 axis and maybe even a cheap DAC put on board. (i
> think you said before the Burr Brown DACS are a little pricey, 4 of
> these would probably add right up).

There is no servo card. There is an encoder counter card and a DAC
card. The DAC chip IS pretty expensive, close to $40 in small quantity.
But, at least, it does 4 axes with one chip.

Now, for open-loop spindle speed control only, a much lower resolution
DAC would do just fine. Unfortunately, we don't really have plans for
one like that right now. There are probably other alternatives to this,
cheap ISA plug-in ADC and DAC boards. But, maybe you don't want
to get into reprogramming the system, and I don't blame you there!

EMC is well constructed, but it IS complicated as can be!

Jon