Linear encoder and rotary motor servo

This has been discussed before: a linear encoder combined with a rotary
motor and some sort of rotary motion/linear motion conversion. The
consensus has been that this is a bad idea because play in the rotary/linear
motion conversion will cause the servo to go wild, hunting and vibrating.

I was convinced that this was so until I took apart a inkjet printer
yesterday and found that it had a rotary motor and a linear encoder. The
rotary to linear motion bit is a cogged belt.

Since millions are of these were made, it clearly works. Is the common
problem with servos of this sort caused by backlash between a screw and nut?


The encoder grating pitch is 0.0055 inch, so by detecting all edges they
could get the precision they need.

The motor cogs less than some DC motors its size, but still is not exactly
smooth when turned by hand.

Does anyone know if this 0.0055"/4 is about the limit of what can be done
with a cogged belt? It would be a really neat way to make a hexapod.

regards-
Elliot

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