Angle sensing using differential magnetic measurement and a back bias magnet

1. A magnetic field sensor, comprising:

a back bias magnet configured to generate a DC magnetic field;

first and second magnetic field sensing elements disposed along a sensing element line and disposed proximate to at least one ferromagnetic surface of a ferromagnetic target object, the first and second magnetic field sensing elements configured to generate first and second electronic signals, respectively, in response to first and second sensed magnetic fields corresponding to the DC magnetic field but influenced by the at least one ferromagnetic surface;

wherein the magnetic field sensor is operable to generate a difference signal that is a difference of amplitudes of the first and second electronic signals, and the difference signal is indicative of a rotation measurement of an absolute relative rotation of the ferromagnetic target object and the magnetic field sensor about a rotation axis, wherein the sensing element line is substantially parallel to the rotation axis, wherein the at least one ferromagnetic surface of the ferromagnetic target object comprises a first surface having a first radius and a second surface having a second different radius, the first and second surfaces disposed on the ferromagnetic target object, the second surface displaced from the first surface in a direction along the rotation axis.

2. The magnetic field sensor of claim 1 , wherein the first and second surfaces are configured to provide the difference signal that is substantially linear with respect to the absolute relative rotation of the ferromagnetic target object and the magnetic field sensor about the rotation axis.

3. The magnetic field sensor of claim 1 , wherein the first magnetic field sensing element is aligned with the first surface and the second magnetic field sensing element is aligned with the second surface, wherein the magnetic field sensor comprises a differential amplifier coupled to receive the first and second electronic signals and configured to generate the difference signal as the difference between the amplitudes of the first and second electronic signals, wherein an amplitude of the difference signal is related to the absolute relative rotation of the ferromagnetic target object and the magnetic field sensor about the rotation axis.

4. The magnetic field sensor of claim 1 , wherein the first radius and the second radius have respective decreasing spirals, wherein a first distance between the first surface and the first magnetic field sensing element decreases while a second distance between the second surface and the second magnetic field sensing element increases in response to changes of the absolute relative rotation of the ferromagnetic target object and the magnetic field sensor.

5. The magnetic field sensor of claim 1 , wherein the first radius and the second radius are selected to provide the difference signal that is substantially linear with respect to the rotation of the ferromagnetic target object about the rotation axis.

6. A method of sensing an absolute relative rotation of a ferromagnetic target object and a magnetic field sensor about a rotation axis, the method comprising:

generating a DC magnetic field by a back bias magnet;

generating first and second electronic signals, respectively, by first and second magnetic field sensing elements disposed along a sensing element line and disposed proximate to at least one ferromagnetic surface of a ferromagnetic target object, the first and second electronic signals in response to first and second sensed magnetic fields corresponding to the DC magnetic field but influenced by the at least one ferromagnetic surface;

generating, by the magnetic field sensor, a difference signal that is a difference of amplitudes of the first and second electronic signals, and the difference signal is indicative of a rotation measurement of an absolute relative rotation of the ferromagnetic target object and the magnetic field sensor about a rotation axis, wherein the sensing element line is substantially parallel to the rotation axis, wherein the at least one ferromagnetic surface of the ferromagnetic target object comprises a first surface having a first radius and a second surface having a second different radius, the first and second surfaces disposed on the ferromagnetic target object, the second surface displaced from the first surface in a direction along the rotation axis.

7. The method of claim 6 , comprising:

providing the difference signal that is substantially linear with respect to the absolute relative rotation of the ferromagnetic target object and the magnetic field sensor about the rotation axis based upon the first and second surfaces.

8. The method of claim 6 , comprising:

aligning the first magnetic field sensing element with the first surface and aligning the second magnetic field sensing element with the second surface;

receiving, by a differential amplifier of the magnetic field sensor, the first and second electronic signals; and

generating the difference signal as the difference between the amplitudes of the first and second electronic signals, wherein an amplitude of the difference signal is related to the absolute relative rotation of the ferromagnetic target object and the magnetic field sensor about the rotation axis.

9. The method of claim 6 , wherein the first radius and the second radius have respective decreasing spirals, wherein a first distance between the first surface and the first magnetic field sensing element decreases while a second distance between the second surface and the second magnetic field sensing element increases in response to changes of the absolute relative rotation of the ferromagnetic target object and the magnetic field sensor.

10. The method of claim 6 , comprising:

providing the difference signal that is substantially linear with respect to the rotation of the ferromagnetic target object about the rotation axis based upon the first radius and the second radius.