A Theory of Magnetic Angle Sensors with Hall Plates and without Fluxguides

Magnetic angle sensors detect the angular position of a permanent magnet attached to a rotating shaft. The magnet is polarized diametrically to the rotation axis. No soft magnetic flux guides are present. The semiconductor die is placed on and orthogonal to the rotation axis. There are two kinds of systems: (i) perpendicular systems detect the field components perpendicular to the rotation axis, and (ii) axial systems detect the component parallel to the rotation axis. The former use magneto-resistive sensors or vertical Hall effect devices; the latter use Hall plates. This paper focuses on axial systems, derives their conceptual limitations, and compares them with perpendicular systems. An optimized system and optimum shapes of magnets are reported. Angle errors due to assembly tolerances of magnet and sensor versus shaft are explained. It is proven that assembly tolerances of optimized axial systems give three times larger errors than perpendicular systems.