Re: [CAD_CAM_EDM_DRO] Re: Lost Steps => time for microprocessors!

Ron Ginger wrote:

> Ive got to bang my drum again- a dedicated microprocessor to generate
> the pulses. I think FlashCut has the right idea, if only they were
> into
> Open Standards.

And, I'm going to jump right in and BUST your drum, again!

A microprocessor can just about do the job, but it is too slow, unless
you
REALLY want to spend some money on a bunch of DSP machines, like
the AD 29060 super-SHARC.

I can EASILY do this for 4 channels in a single $18 FPGA, and have pulse

timing resolution down to 100 nS by running the chip's counters at 10
MHz.
Your micro can't execute more than one instruction in that time.

If I clock it at 40 MHz, it can get resolution down to 25 nS! The chip
should
handle this with no sweat at all, now it is clearly out of the range of
any
micro! The CPU just outputs how fast the pulses should come out, and
the
timing chip counts them out with extremely fine resolution, so at a
constant
commanded velocity, the pulses will come out with +/- 1 clock cycle
of timing resolution. As long as the pulse rate is very low compared
to the clock rate, this appears pretty close to being an inverse linear
relationship, and everybody should be happy. Depending on how many
bits of resolution the timer has, it gets a little jumpy at the bottom
end,
but that's all relative, since the stepper is TOTALLY jumpy (ie.
discrete)
at these speeds, anyway. A 24-bit counter running at 10 MHz can count
out steps up to one every 1.6 seconds. When moving slower than this,
the counter needs to be turned off. If you don't turn it off, a step
comes out
~ every 1.6 seconds, and the CPU would notice the count change in the
position counter, and then order a very slow move in the other
direction.
This would cause a 1 step hunting behavior at 1/3.2 Hz, which actually
doesn't sound like much of a problem.

I just don't think you can come even CLOSE to this with a micro,
especially
a PIC-class one. The only parts I need are a 10 MHz crystal oscillator
and a Xilinx Spartan XCS10-series chip! That's IT! So, $20 in parts,
plus the circuit board. (This assumes another unit has the position
counters, also on one of these Spartan chips. That allows you to
have the position counters counting simulated encoder phase signals
coming from the timing chip, or getting real encoder signals from a
shaft encoder on the motor. Note that the shaft encoder does not need
to match the resolution of the motor. But, if the motor has LOWER
resolution that the encoder (a common practice) then a deadband
will have to be set on the CNC or it will hunt, possibly an annoyance.)

Jon