Magnetostrictive torque sensor and torque detection method

1. A magnetostrictive torque sensor that detects a torque of a rotating shaft and/or a stationary shaft by using a magnetostriction inverse effect that occurs on a shaft surface, the magnetostrictive torque sensor comprising:

a first detection coil that is disposed so as to detect a magnetic permeability change in a first direction on the shaft surface, the first detection coil detecting the magnetic permeability change in the first direction as an inductance change of the first detection coil;

a second detection coil that is disposed so as to detect a magnetic permeability change in a second direction on the shaft surface, the second detection coil detecting the magnetic permeability change in the second direction as an inductance change of the second detection coil;

a first oscillation circuit that oscillates autonomously at a predetermined reference frequency and causes phase shifts in oscillating waves according to the inductance change of the first detection coil;

a second oscillation circuit that oscillates autonomously at a predetermined reference frequency and causes phase shifts in oscillating waves according to the inductance change of the second detection coil;

a first direction magnetic permeability detector that performs an oscillating wave counting processing and detects the magnetic permeability change in the first direction based on a time required for the oscillating wave counting processing that counts a plurality of oscillating waves that are output from the first oscillation circuit and determines whether a counted number reaches a predetermined number N;

a second direction magnetic permeability detector that performs an oscillating wave counting processing and detects the magnetic permeability change in the second direction based on a time required for the oscillating wave counting processing that counts a plurality of oscillating waves that are output from the second oscillation circuit and determines whether a counted number reaches a predetermined number N; and

a torque detector that detects the torque of the rotating shaft and/or the stationary shaft based on a difference between the magnetic permeability of the first and second directions.

2. The magnetostrictive torque sensor according to claim 1 , wherein the first and second detection coils form closed magnetic paths between the first and second detection coils and the shaft surface in order to limit detection regions and/or detection directions on the shaft surface.

3. The magnetostrictive torque sensor according to claim 2 , wherein the first and second detection coils are formed using a high magnetic-permeability material, the first and second detection coils including:

a core, the closed magnetic paths being between the core and the shaft surface; and

a coil that is wound around the core.

4. The magnetostrictive torque sensor according to claim 1 , wherein:

the first oscillation circuit includes a plurality of first detection coils that are connected in series or parallel; and

the second oscillation circuit includes a plurality of second detection coils that are connected in series or parallel.

5. The magnetostrictive torque sensor according to claim 4 , wherein the plurality of first and second detection coils are lined up on a same circumference of the rotating shaft and/or the stationary shaft and disposed such that a detection region of the first detection coil alternates with a detection region of the second detection coil.

6. The magnetostrictive torque sensor according to claim 4 , wherein the plurality of first and second detection coils are lined up on a same circumference of the rotating shaft and/or the stationary shaft and disposed such that a detection region of the first detection coil overlaps with a detection region of the second detection coil.

7. The magnetostrictive torque sensor according to claim 1 , wherein the first and second oscillation circuits are driven alternately so as to avoid a mutual interference.

8. The magnetostrictive torque sensor according to claim 1 , wherein a counting number changer is provided for changing the predetermined number N of the oscillating waves in one oscillating wave counting processing.

9. The magnetostrictive torque sensor according to claim 1 , wherein the shaft surface includes a magnetostrictive film that is formed by plating.

10. A torque detection method for a magnetostrictive torque sensor that detects a torque of a rotating shaft and/or a stationary shaft using a magnetostriction inverse effect that occurs on a shaft surface, the magnetostrictive torque sensor including:

a first detection coil that is disposed so as to detect a magnetic permeability change in a first direction on the shaft surface, the first detection coil detecting the magnetic permeability change in the first direction as an inductance change of the first detection coil;

a second detection coil that is disposed so as to detect a magnetic permeability change in a second direction on the shaft surface, the second detection coil detecting the magnetic permeability change in the second direction as an inductance change of the second detection coil;

a first oscillation circuit that oscillates autonomously at a predetermined reference frequency and causes phase shifts in oscillating waves according to the inductance change of the first detection coil; and

a second oscillation circuit that oscillates autonomously at a predetermined reference frequency and causes phase shifts in oscillating waves according to the inductance change of the second detection coil,

wherein the method includes the steps of:

performing an oscillating wave counting processing and detecting the magnetic permeability change in the first direction based on a time required for the oscillating wave counting processing that counts a plurality of oscillating waves that are output from the first oscillation circuit and determines whether a counted number reaches a predetermined number N;

performing an oscillating wave counting processing and detecting the magnetic permeability change in the second direction based on a time required for the oscillating wave counting processing that counts a plurality of oscillating waves that are output from the second oscillation circuit and determines whether a counted number reaches a predetermined number N; and

detecting the torque of the rotating shaft and/or the stationary shaft based on a difference between the magnetic permeability of the first and second directions.

11. The torque detection method according to claim 10 , wherein the first and second detection coils form closed magnetic paths between the first and second detection coils and the shaft surface in order to limit detection regions and/or detection directions on the shaft surface.

12. The torque detection method according to claim 11 , wherein the first and second detection coils are formed using a high magnetic-permeability material, the first and second detection coils including:

a core, the closed magnetic paths being between the core and the shaft surface; and

a coil that is wound around the core.

13. The torque detection method according to claim 10 , wherein:

the first oscillation circuit includes a plurality of first detection coils that are connected in series or parallel; and

the second oscillation circuit includes a plurality of second detection coils that are connected in series or parallel.

14. The torque detection method according to claim 13 , wherein the plurality of first and second detection coils are lined up on a same circumference of the rotating shaft and/or the stationary shaft and disposed such that a detection region of the first detection coil alternates with a detection region of the second detection coil.

15. The torque detection method according to claim 13 , wherein the plurality of first and second detection coils are lined up on a same circumference of the rotating shaft and/or the stationary shaft and disposed such that a detection region of the first detection coil overlaps with a detection region of the second detection coil.

16. The torque detection method according to claim 10 , wherein the first and second oscillation circuits are driven alternately so as to avoid a mutual interference.

17. The torque detection method according to claim 10 , wherein the predetermined number N of the oscillating waves in one oscillating wave counting processing is changed.

18. The torque detection method according to claim 10 , wherein the shaft surface includes a magnetostrictive film that is formed by plating.

19. A magnetostrictive torque sensor that detects a torque of a rotating shaft and/or a stationary shaft by using a magnetostriction inverse effect that occurs on a shaft surface, the magnetostrictive torque sensor comprising:

a first detection coil that is disposed so as to detect a magnetic permeability change in a first direction on the shaft surface, the first detection coil detecting the magnetic permeability change in the first direction as an inductance change of the first detection coil;

a second detection coil that is disposed so as to detect a magnetic permeability change in a second direction on the shaft surface, the second detection coil detecting the magnetic permeability change in the second direction as an inductance change of the second detection coil;

a first oscillation circuit that oscillates autonomously at a predetermined reference frequency and causes phase shifts in oscillating waves according to the inductance change of the first detection coil;

a second oscillation circuit that oscillates autonomously at a predetermined reference frequency and causes phase shifts in oscillating waves according to the inductance change of the second detection coil; and

a controller that:

performs an oscillating wave counting processing and detects the magnetic permeability change in the first direction based on a time required for the oscillating wave counting processing that counts a plurality of oscillating waves that are output from the first oscillation circuit and determines whether a counted number reaches a predetermined number N;

performs an oscillating wave counting processing and detects the magnetic permeability change in the second direction based on a time required for the oscillating wave counting processing that counts a plurality of oscillating waves that are output from the second oscillation circuit and determines whether a counted number reaches a predetermined number N; and

detects the torque of the rotating shaft and/or the stationary shaft based on a difference between the magnetic permeability of the first and second directions.