DRO stuff

The HEDS-9XXX encoder optics are set up for a specific grating pitch, so
using them with a higher pitch won't work. They contain multiple very
narrow detectors connected to each amplifier. With a different pitch they
won't all be in phase, hence lower output. There is some chance that a 360
x 3 = 1080 lines/inch there might be some output, maybe not enough to work
the electronics.

It is really unfortunate that HP didn't bring the analog outputs out
somewhere. It would save a huge amount of time in making a high res
interpolated encoder head.

There are some practical limits to how much interpolation can be achieved
with analog levels.
For one thing, the output values can be a triangle wave, a sine wave, or
inbetween, with harmonic strengths varying with spacing from object to
readout grating. An interpolation routine needs to know what the functional
relationship is. With enough detectors the output to position function can
be determined on the fly.

As an extreme case, I took a 100 lpi grating and set up grating optics to
measure its position. A HP laser interferometer was used to measure grating
position, and a 5 digit voltmeter used to measure output of the detector
amplifier. Adjusting grating spacings, I was able to generate a sine wave
output with harmonic distortion at the 2 microinch level. It would have
been possible in principle to have a 2 microinch readout with a 100 lpi
grating.

The spacing between the object and readout grating was very sensitive. A
third harmonic term was introduced quite easily which spoiled the purity of
the sine wave.

This was a 5000x interpolation. A 100x interpolation could well be
practical, giving 0.000027" with a 360 lpi grating.

There are design advantages besides low cost to making the optics in a small
area. Sensitivity to tilt is reduced considerably. There are some small
quadrant detectors made, used for CD players, that could be used, with a
couple of quad op amps.

How to do the counting electronics?

Elliot Burke