Re: [CAD_CAM_EDM_DRO] Re: Sherline doing circles

> Congradulations!
> Seeing it inspired me to go work on my x-y table. Been meaning to
> learn FPGAs, so your implementation is interesting to me. Mind
> giving us a few words why you decided to do it that way?
>
> - Steven Ciciora

I was originally going to use a PIC microcontroller because I've used it in
other projects. However, I quickly found out that it had no chance of
keeping up. My second choice was the upcoming DSPic series, but it has been
upcoming for a long time :(. When I started looking at FPGAs I realized that
they have enough hardware to move beyond linear interpolation. I'm now using
a $54 chip that does 4-axis quadratic interpolation at 50MHz. That's a
maximum step rate of 25MHz! Of course I'm running way below the Gecko's
capability of 200KHz on my mill, but the 20ns spacing creates jitter way
below anything that would be noticable.

$54 is a lot for a single chip; but it's reconfigurable and is also doing
the job of a microcontroller. I put 256kB external RAM on it that it uses
for an instruction queue. After I have it cutting parts I'll probably go
back and see what other types of interpolation I can add.

Todd Fleming
flemingcnc.com