Hi Craig
Yes, it's very robust. And I have it. :-) Plus some spares by EEG I think.
It is fitted with an encoder - 256 line or 512 line, A+B+I. Old HP (or subsequent) unit, same as on axis servos. Reliable.
Either analog or digital control. If I went for a phase-lock loop, it would of course be 'analog'. I have built those before for some very strange things, and they work very well. Some delicate design needed to get the start-up right, to avoid massive hunting at the start. Most of my designs in the past have had a known centre frequency: this would be different.
If I go digital with an up/down counter, it would be an s-plane servo control using a microP.
In either case I would expect the ESS to be outputting the Step pulses at the required rate of spin*encoder. In this case I would expect overall spin accuracy of well under 0.01% at most speeds. That is good enough for some very fine threading. Stability - not so sure, but with a 512 line encoder I would be expecting fairly fast and effective response to any transients. The spindle drive PS is a KBWT-26 MOSFET switcher. Very gutsy with huge overload capacity, albeit also with incredible RF noise possibilities. I had to deal with that to keep the CNC positions stable.
To my mind, the difficulties in either case would be handling the transients at start-up and what happens when you accidentally stick a threading tool into the stock a bit too far. So I am wondering whether anyone else has already done this and is willing to share info. Much easier if someone else has banged their knuckles first.
S-plane vs z-plane: well, we tend to use the phrase s-plane even though we are working in digital space. But yes. As for wind-up on the integrator - one can do all sorts of good tricks there in the digital space, like limiting the size of any term in the equation. One can also do some very nice things like introducing 'anti-dither' at the digital level, to prevent servo grumbling. I WISH the Gecko servos had this.
Cheers
Roger