Inaccuracies of Anisotropic Magneto- Resistance Angle Sensors Due to Assembly Tolerances

A large class of angle sensors uses a small permanent magnet attached to the rotor. The magnet is polarized perpendicularly to the axis of rotation, and a magnetic field sensor is placed ahead on the axis. The sensor circuit consists of two full bridges at 45◦, each having four anisotropic magneto-resistive (AMR) elements. Even though the electronic system may be calibrated to have nearly no errors like offset, nonlinearity, and mismatch, still significant angle errors may result from assembly tolerances of the magnet and the sensor. This work gives an analytical description of the angle error caused by tilts and eccentricities of magnet and sensor elements against the axis of rotation. Particular emphasis is given to worst case combinations of all tolerances. One part of the angle error can be cancelled by an optimized layout of the AMR-resistors. The remaining part is identical to the case of giant magneto-resistive (GMR) angle sensors. Errors of both AMR and GMR angle sensors are effectively reduced by identical optimization of the shape of magnets. One such optimized shape is disclosed.