sounds like an optical angle
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(and therefore velocity) encoder:
http://abrobotics.tripod.com/Ebot/using_encoder.htm
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How smashed up was it? If the disc is still in one piece, you should be ok
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- they are normally enclosed in automotive applications to stop junk and crap cloging the slots in the disc
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(hence screwing up the angle measurement).
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Once you're happy you've got the code wheel
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(circular bit with slots) and the encoder
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(small black box with wires going in!) sorted
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(if you can't see the encoder and it works
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- leave it!) then if you can, a piece of gaffer tape to hold the casing together and to keep the crap out.
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HTH,
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Anna