"Many capacitive sensors can very reliably resolve a dozen microns or so. They work great for plastics. They are not overly expensive. The resolution required determines the cost of the drive equipment."
"Capacitive sensors give very high performance. Unfortunately, they require a driver system and generally some tuning. Optical reflective sensors can require a bit of dong to make work well for a gear, but they are very cheap, come in a number of…"
"An often overlooked encoder for some low to moderate speed applications is the optical mouse chip. It can work with performance similar to a 1024 cpi linear encoder, can provide feedback for multiple axes (the output is a vector rather than a simple…"