Re: [CAD_CAM_EDM_DRO] more RE black boxes

Very nice explanation. Thanks. I happen to teach a senior design course
that can include electrical, mechanical, and computer engineers. I'm
always on the lookout for good projects. This might make an excellent
project - probably continuing over several semesters.

-Denny

ccs@... wrote:

>
> > So your saying a single board computer can do the job? Then in
> > principle, you can send G-code to the SBC, which can interpret the code
> > and issue control signals to the servos or steppers. So where does the
> > proprietary equipment come into play? Thanks.
>
> A single board computer can indeed to this job - in fact that is more
> or less how cnc machines traditionally worked - a little (as in
> processing power and memory, not necessarily physical size) computer
> read g-code off of paper tape or had it drip fed via a serial port.
> Then it did the necessary calculations (probably using fixed-point
> math) and controlled servo drives. I believe Mariss is looking into
> making a modern version of this, only using a much more powerful
> single board computer.
>
> Some of the windows controls do most of the work on the pc, but have a
> little black box that generates step pulses at a more consistent
> spacing and finely variable rate (speed) than is easily accomplished
> under windows. I believe the computer simply sends them some sort of
> velocity and/or distance command. The g-code interpretation and fancy
> math is still in the computer.
>
> FPGAs and [embedded] Microprocessors/Microcontrollors have different
> but somewhat overlapping capabilities. (In fact, you can 'build' a
> moderate powered microprocessor inside of a large FPGA).
> Microprocessors are good for thinkinig things out - how do I
> accelerate and decelerate, take corners, do circular interpoloation,
> etc. FPGA's are better at doing lots of simple things at once -
> counting a bunch of quadrature inputs from multiple encoders, counting
> step commands from a computer, running fast servo loops, generating
> pulse width modulation or chopping, etc. Many of these things are
> much more difficult to do in microcontrollor software where only one
> thing can happen at any given time - ie, it is harder to watch
> multiple pulse sources without the risk of missing one of them if they
> come too quickly. So some of the microcontrollors have built in
> peripherals like pulse counters and generators - the problem is that
> these are not always well suited to cnc applications - we need things
> like counters that can count up AND down, in order to track encoder
> motion, wheras the usual ones supplied can only count in one
> direction. You can buy dedicated chips like the LS7266, but that 2
> channel encoder counter costs nearly as much as a small fpga - and the
> more chips on the board the more expensive and special purpose it is.
>
> A box that runs g-code or the like itself is probably best built with
> a microprocessor. A device which helps improve the performance of a
> pc-based cnc controllor may do better with an fpga, as may some of the
> stepper and servo driver modules themselves. My personal preference
> would be to have both a processor and an FPGA working together in a
> box, for maximum flexibility.
>
> But I'd also like to see the software-only controls continue to offer
> as much as possible, since there is quite a lot to be said for a
> controller that one can download off the web and wire up through the
> parallel port.
>
> Chris Stratton
>
> -------------------------------------------------
> Christopher C. Stratton
> Engineer, Instrument Maker, and Horn Player
> ccs@... 617 628 1062
> http://web.mit.edu/~stratton/www/brassbuild.html
>
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