A motion sensor having non-volatile direction memory

A simple, noncontacting device for determining the sense of an ongoing, or last occurring, motion, as well as the speed, of a smooth ferromagnetic target is described. The device consists of a small permanent magnet and a magnetic field sensor displaced from each other along the line of motion with each at a small fixed distance from the target. As the initially uniformly magnetized target passes the magnet it undergoes a polarity reversal within a small region downstream of the magnet. The nonzero div M within this localized transition zone is the source of an additional field having unequal intensities on the two sides of the magnet axis. This asymmetry allows the direction of motion to be discerned by the field sensor. In conductive targets a velocity-dependent, asymmetrical field component arises from the reversal in the circulation direction of eddy currents at the magnet axis. The use of two field sensors on either side of the magnet provides symmetrical directional response