Patent ID: 7619407
Filing Date: 2009-11-17
Classification: G01B,G01D,G01P

Abstract:
1. A method to measure rotational speed of a toothed wheel whose teeth include ferromagnetic material, comprising: providing at least two permanent magnets; disposing said permanent magnets so as to form a zero field region; placing in said zero field region an even number of MR (magneto-resistive) devices divided into equal-sized first and second groups; arranging for said first group to be immersed in a different magnetic environment from that in which said second group is immersed, whereby said permanent magnets and said MR sensors, including said magnetic environments, together form a rotational speed detector; and then placing said rotational speed detector near said toothed wheel whereby, when said toothed wheel rotates, a differential signal taken between said first and second groups of MR detectors may be used to determine at what rate teeth from said toothed wheel pass by said rotational speed detector.