Magnetic encoders！

Magnetic encoders have many useful features. They have lower power requirements than optical encoders, a simple and robust structure, good performance characteristics, excellent resistance to humid and dirty environments, and they are well suited to large-volume manufacturing techniques. Some of the disadvantages are sensitivity to temperature effects, and generally lower resolution capabilities.
Principles of Operation. Magnetic encoders utilize a magnetoresistive (MR) sensing element and a magnetic code wheel (Fig. 10.11). The MR sensor changes resistance in the presence of a magnetic field.  sensors are slightly more sensitive than Hall devices. MR elements can sense fields of approximately 0.005 T (50 G). This is at the low end for Hall devices. This is necessary because flux density is proportional to the pole width, so as line count goes up, density goes down. High-resolution magnetic encoders result in very modest magnetic flux densities.

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