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

> Hi, this is Dave:

Just to be dumb and innocent....how often is the gui getting updated? Since the system is really in program mode....updating the
gui at 10-20 Hz might look klutzy but certainly not affect the control: which is more important?

Dave

> Hi again,
>
> Remember me, the guy with the old rusty Gorton on an island? I'm back
> home now and the mill's still on that island, to stay as far as I know.
> But that's beside the point.
>
> I feel much more in my element with this topic, though it's a lot like
> coming in late to someone else's design review meeting. I'll make a few
> quick points and get out of the way:
>
> 1. I must agree that trying to use a PC printer port to generate a nice
> even pulse train while running a GUI at the same time is a silly idea.
>
> 2. Trying to do it with a PIC class micro is sort of like trying to use
> a wrench for a hammer. It's a waste of a good wrench and it makes a
> really bad hammer.
>
> 3. I think Jon is on the right track here with the CPLD approach.
>
> Jon, are you doing this as a product design to sell, or is it an "open
> source" type project? Also, I read about you doing battle with the EPP
> fast transfer protocol. How many different parallel port chips have you
> tested with. Some don't play nice at all. And, in your spare time, try
> one of those USB to printer thingies. Big fun.
>
> Jeff
>
> On Tue, 29 Aug 2000 18:05:31 -0500, Jon Elson wrote:
>
> >
> >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
> >
> >
> >
>
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