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CNC position feedback has been exclusively encoder or glass scales in every instance that I've encountered or heard of. However there are other technologies which I've encountered outside the world of CNC which seem also potentially suitable for the job (LVDT, inductance to digital sensing, et. al.), and I'm curious if there are any examples of alternative feedback being used in a CNC application and how well it worked out.
Hi,
LVDTs of high resolution have very limited stroke.
For instance an LVDT with 1um resolution might have a stroke of plus/minus 1mm, whereas a 50mm stroke unit will have much lower resolution, say 10um.
They are good for accurate positioning of optical parts for example, which need to be precisely placed but within a narrow range, but less useful for CNC.
Laser interferometry is another technique capable of truly remarkable precision, down to nm and yet can be used with displacements of metres.
Interferometry requires extremely coherent lasers, coherence lengths of tens of metres are called for. Note most people interpret a laser with a very
narrow beam as coherent, but that is mistaken. The term for a tightly focused beam is collimated. You will no doubt have seen well collimated lasers
on building sites for leveling and so on.
Such lasers, red diode lasers commonly, have coherence lengths of only millimetres. They may be useful for leveling a floor say but cannot
be used for interferometry.
A coherent laser on the other hand may even have a diverging beam BUT be absolutely 'single mode'. Such distinction comes at a cost.
One very clever technique used by HP (back in the day) is a short dual mode locked Helium-Neon laser tube with the two modes locked by Zeeman splitting.
The two modes are orthogonally polarised and this allows a very clever use of differential interferometric measurement. A good Zeeman Split HP laser
can still be had for around $2000. Just as a side note these lasers typically are only 200uW output and have beams that diverge by 0.5 degree or more.
Most people would consider them hopeless as lasers but they have a coherence length of 15m or so....and that stirs my loins!!!
A HP laser setup can measure repeatably to 30nm.
Genuine single mode Helium-Neon lasers tend to be more expensive, upwards of $5000 with coherence lengths of 20m or more.
Iodine cell stabilised mode locked Helium-Neon lasers with coherence lengths of hundreds of metres and more start at $20k second hand.
For CNC purposes you would require additional beam splitters/combiners, retro mirrors, optical squares and interferometer cells, all costing big dollars.
Further they need to be in harms way......so not practical at all.
Interferometry is used extensively by the largest CNC machine manufacturers to measure and qualify their machines. A good metrology setup for this
kind of operation is north of $100,000.
Where interferometry is used is the semiconductor industry where measurements of sub nano metre are called for. Presumably in the ultra-clean, often vacuum
environments in semiconductor manufacturing cells worth tens of millions maybe interferometric components are not seen as expensive!!!
Craig
Your laser talk way way over my head but I think I got the gist of it: lasers would work but not practical for CNC.
At the place I used to work we built all-electric subsea tooling and needed a way to accurately measure distance traveled by axes on the tools. I had the idea to use an inductance measuring circuit on a long coil of wire wrapped around a tube, with a steel rod that passes through the tube. I built a prototype and it worked but I didn't get a chance to pursue it as the firm we had working on the mechanical design incorporated encoders in the leadscrews and that was deemed "good enough" despite the fact that position was lost each time power was lost. I've had the itch to rekindle that idea and see if it could be applied to CNC. I don't know if the accuracy and repeatability is good enough.