some older link to someone using the mouse as a scanner (he however relies on the mouse telling him positions): http://hackaday.com/2006/01/07/optical-mouse-based-scanner/
(link to original is broken, but project is still here: http://spritesmods.com/?art=mouseeye )

@Sheldon
Has fingered a potential problem.

See, you don’t actually know where the stage is because this isn’t actually an “encoder”. It’s more like a rotopulser. Anyone with experience of a CP-80 type printer (or maybe FDD’s) will have seen a jammed carrage vibrating in place while the driving processor is blissfully unaware that the carrage is nowhere near where it thinks (until maybe it returns to zero prematurely and causes an error).

@mungewell
said the magic word “Gray code”.

One way to know exactly where the carrage is at all times is to fully encode the traverse with as many bits of Grey code as required.

@Jim
just reinvented slide-wire feedback, used in early plotters.

@simpleplan
now THAT I really like. Hat tip.

Which reminds me of a metrology setup that used a fine inter-digital ladder and a matching short reader strip. In the macro this would count off the phase reversals for each ladder step (~1mm) but for the fine did a phase resolution between the ladder steps to better than 1 thou overall (retrofit gear for old mills, lathes, &c; Nixie readouts, all TTL btw).

I’d suggest the world of PCB photography would be a good starting point for large, very fine and regular structures.

@urlax
Slidewires worked, honestly. And they were far from the most troublesome part of the recorder. Wound slides were also used, but for home construction the evenness of the winding would be critical for positioning, and the wire thickness would limit fine resolution.

@reboots
Yer. But say we combine mouse opticals with a fine printed Grey code strip, then read the mouse video directly, and in software interpolate based on the position of the code in the scan. Give all that processing power something useful to do for once. :)

@Jim
“Twang-wire” rebalance? That has to be either brilliant or crazy. You might consider what happens if you measure the twang frequency on *both* sides of the head.

http://home.roadrunner.com/~maccody/robotics/mouse_hack/mouse_hack.html

http://home.roadrunner.com/~maccody/robotics/croms-1/croms-1.html

At 120-150 lines per inch, much lower resolution than even the cheapest optical mouse. But, assuming clean optics, they yield very precise measurement. Optical mice have relatively poor accuracy: losing track of their position, skipping pixels, etc. Great for rough motion tracking, terrible for quantifying it.

you can better use resistive wire isolated with enamel and wind it on a wooden rod. or a pvc one. that way, your resistance will be much higher, reducing the error. (the wiper will oxydate, introducing uncertainty.)

for instance, if you want to measure an axis of Oddlers device, you’d use 2 meters of wire. (1mm² nichrome would have an resistance of 2,1 Ohm.)

if you wound the wire around an bar with an circumference of 1cm, you would need 2000 turns, using 20 meters of wire. the resistance would become 21 Ohms, which requires less current, and gives a better resolution.

it would look like this, but longer:
http://www.zsa-one.com/Upfiles/Prod_X/2007122474655.jpg

“I thought the best way might be to use some resistance wire and a wiper on the carriage… It would be very cheap and absolute though”

This is very similar to what was done back in the 60s and 70s for electronic console organs (and probably portable ones too). There was a long bus bar (with some voltage across it) that ran the length of the console, and each key was connected to a stiff wire that touched the bus bar, providing a voltage for a VCO. Tuning was accomplished by bending the wires.