10.2 Magnetic Encoding

Recent semiconduction development has made Hall-effect sensors very cost effective. There are several kinds of hall-effect sensors. The most common type is effectively a semiconductor ``reed-relay''. It is a transistor that is turned on if and only if the magnetic field perpendicular to it exceeds a threshold.

With hall-effect sensors, we can encode motion by placing small magnets on a wheel or a gear. The encoding resolution is somewhat limited with this approach because magnets are fairly big (compared to a slot on an optical encoder disk). Furthermore, because the magnet is on the rotating part, it can fall off when the rotational speed is high.

An alternative is to place a ``bias'' magnet near the sensor, then use a metalic (with iron component) gear or slotted disc for encoding. The teeth of the gear serves as magnetic field concentrators. As the gear turns, the magnetic field experienced by the sensor changes, and these transitions indicate motion.

See http://rocky.digikey.com/WebLib/Melexis Web Data/MLX90217.pdf for more information.

Magnetic encoding is not affected by most dirt, dust and oil. It is suitable for robots that need to operate in dirty or outdoor environments.