A mad idea: place a sample of that cloth on a table, can you reliably use an optical mouse over it?. So it could double as speed sensor in your process as well. In every burst of bytes towards the computer or microcontroller, the mouse is telling how much it moved. An appropiate software would adjust motor speed to have an approximately constant value. 12''/minute... 1'' every 5 seconds... I am simulating it with my mouse... yes, it IS slow indeed, so your application should take decisions after having integrated several communications from the mouse.