Anisotropic Magneto Resistive or AMR sensors offer robust non-contact measurement of changes in the magnetic field as seen by the sensor. This effect allows for the creation of sensors that can detect disturbances ranging from extremely weak fields, as found in traffic detection sensors, to strong field sensors that are used in precision encoders.
Also available: MR_Basics, MR_Angle_WhitePaper, MR_Rotational Measurement with H-Magnet