Patent ID: 7573259

Claim:
In a system for sensing the relative displacement between first and second members, a magnetic structure and at least two detector devices for sensing variations in magnetic properties of the magnetic structure, the magnetic structure comprising: a plurality of encoding tracks comprising at least a first track and a second track mounted for rotation about a central rotational axis, the first and second tracks being located at respective predetermined radial locations relative to the central rotational axis and extending generally parallel to each other; the first track having a first predetermined number of sectors and the second track having a second predetermined number of sectors, each sector defining a magnetic pole facing toward a respective one of the detector devices; each of the first and second tracks passing within a triggering range of a respective one of the detector devices for causing the respective one of the detector devices to produce a signal upon sensing a transition between adjacent sectors in a respective track; and a blocker track located between the first and second tracks, wherein the blocker track includes a predetermined number of sectors corresponding to the number of sectors on the first track, each sector of the blocker track being aligned with and comprising an opposite magnetic pole to an adjacent sector of the first track.