Re: [CAD_CAM_EDM_DRO] Re: EMC and lead screw compensation

Paul Corner wrote:

> With the current low cost of laser modules, and a cheap source of suitable
> gratings - It would at least be do-able. Would need to be meticulous in
> keeping the optics clean and free from vibration. You may be able to achieve
> sub micron accuracies. <bg>

I'm not sure where you'd use the gratings. Low cost (diode) lasers have
a VERY short coherence length, generally less than one cm, and change
wavelength quite a bit over a small temperature change. This would play
hob with trying to make a measuring system with it by interferometry.

You could also use a diode laser with a master ruling and an analyzer,
but that is essentially making an equivalent to a glass scale DRO
encoder, and you could use a light bulb with that.

To make an interferometer, the method I've seen that allows measurements
that give both distance and direction information requires circularly
polarized light, which a diode laser definitely does not give.
You can turn a Helium-Neon gas laser's output into circularly polarized
coherent light with a spatial filter and a retardation plate, both fairly
expensive parts. I tried to build one of these a long time ago, and
never got any fringes at all. You bounce the light off a corner
reflector on the movable arm, and then mix it with a sample of the
original beam with a beamsplitter. You are supposed to get
waves of light and dark that sweep across the spot after the
beamsplitter. Placing photodetectors at the right locations
should give quadrature type signals.

Jon