Christian wrote:
> reading heads from discdrives should be working!?
A magentic disc is not a static system. You will need a significant 
speed of the disc to induct enough voltage into the heads coil. So 
stopping and turning the direction will cause tricky problems...

Christian wrote:
> If the magnetic system does not work cd's could be used (is optical)!
Indeed, but you will have to implement a highly sopisticated controller. 
and due to the usually very bad disc quality (most of the job in DVD and 
CD is done by error correction) the whole system will not be 
reproduceable and each encoder will be a prototype...

IIRC people used stepper motors for such purposes in the past... of 
course, this will not yield absolute position.

For an optical encoder, a 600 dpi laser printer will likely be capable 
of printing an encoder wheel on plastic film which will yield a rather 
high resolution when assuming the encoder wheel can be multiple inches 
in diameter.

The theoretical angular resolutions of floppy media (>10000bpi) seem 
favourable, but there are probably good reasons (mechanical stability I 
think) that only at most a few hundred tracks per inch were realized in 
floppy disk systems.
A hard drive(!) platter and bearing would probably give you the 
mechanical stability required. However, the way hard drive heads work 
will turn into a huge problem with varying rotational speeds and 
directions. IF what you want to do is get the exact current position of 
a constantly spinning shaft this might work though - in the past, hard 
drives used to do exactly that "for their own use" with a one side of a 
platter dedicated to this task.