CNC control using DROs?

[JAZZCNC]

I use the analogy or driving an old banger along a winding hilly track with half a turn of slop in the steering wheel. You (the closed loop controller) have to work bloody hard to keep on track.

Good Analogy and reminds me of an old Transit van I used to have with a steering box, used to have to move the steering wheel +/-20 degrees either way from the center just to keep it in a straight line...

Often better machines have both encoders on the servos/drives and linear scales on the axis that feedback to the controller which closes the loop.
Many moons ago I nearly bought a very high-end 5 axis Mill made by a company called "JOBS" that was built for McLaren F1 team which cost £250,000 and that was 25yrs ago.
This had Mitsubishi servos with absolute encoders and Glass linear scales on the Axis that both went back to the controller, the thing was unbelievably accurate but the complexity and setup was a nightmare apparently.

To me, if you are going to this level of complexity then the WHOLE machine needs to match, because like most things in life can only be as good as the weakest link.

[alanambrose]

Yeah, I wan't thinking about tarting up a dodgy machine tool so much as a way, using absolute encoders, to avoid homing and get 'limit switches' which I don't have now. Also I like the idea of controlling for the 'actual' position. I acknowledge that nothing's perfect and that the encoders have measurement errors and the mill will have some bendiness / alignment errors. I like the concept better though than relying on the stepper/servo's idea of where the position is plus a fixed value per axis of 'backlash'.

My mill has no missed steps and about 40 microns backlash on X and 80 on Y and Z - although I plan to measure those more accurately soon.

Alan