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

Wally K wrote:

> So for the time being you would have to code the numbers for the dead
> zone into the driver for the rate generator. Does this mean what you
> program in would be dependent on the encoder mounting position.
> Either on the motor or ballscrew, encoder resolution, steps per rev
> and belt ratio. And we would have to provide this information to you.

Well, no, you could change the value and recompile. It only takes a couple
of minutes to do that on the 333 MHz machine, and about 10 on the
100 MHz. I think trying to align the encoder to the motor steps would
be a tricky business, as there is a fair bit of error in motor step position,
so it would vary depending where the motor is sitting at the moment.
So, I think it would be best to compute the worst case 'jitter zone'
and set the dead zone to about that many counts. If you had a stepper with
200 steps/rev, but are half-stepping to get 400 steps/rev, and you had
a 1000 cycle encoder giving 4000 counts/rev, then you would have
10 encoder counts per half-step. So, the dead zone would have to be
set to at least 10 counts, or it would start bouncing around trying to get
to a position the motor can't sit still at. Something I should warn people
about is trying to take the above example and get a x10 microstepping
drive, and expect that you can microstep stably to each encoder count.
The micro-stepping drive has much lower gain than the servo amp, and
will just build up error until the motor, leadscrew and machine (maybe
depending on backlash) all jump to a new position. This will lead to
HORRIBLE hunting behavior, and you will be forced to turn up the
dead zone so as to totally negate the microstepping, at least at standstill.
It could work well while moving, however.

>
> Ok i forgot the DAC came in a single 4 channel package. And i did
> not understand if you can run an axis in open loop if you want to for
> rough spindle control.

Yes, you definitely can. The software needs to know, of course, that
there will be no feedback, and a 16-bit DAC is overkill, since it will
probably be re-digitized by an 8-bit ADC in the VFD, anyway!

On the other hand, if you only had 3 servo axes, and the 4th axis was
going to waste, you CAN use it for spindle speed. And, there is a
provision in EMC that tells what DAC channel has spindle speed on it.
I haven't tested this part, though.

Jon