Displacement Sensor Using GMR Elements, Angle Sensor Using GMR Elements, And Semiconductor Device Used For Them

• US 20080116886A1
• Filed: 10/29/2007
• Published: 05/22/2008
• Est. Priority Date: 10/30/2006
• Status: Active Grant

• Current Assignee: Hitachi, Ltd.
• Original Assignee: Hitachi, Ltd.
• Inventors: Yamada, Masamichi, Kanekawa, Nobuyasu, Nakabayashi, Kenji
• Granted Patent: US 7,852,070 B2
• Time in Patent Office: Days
• US Class Current: 324/207.21
• CPC Class Codes: G01D 5/145 influenced by the relative movement between the Hall device and magnetic fields see G01R33/06

Abstract

A highly accurate displacement sensor using GMR elements for detecting a displacement of a physical quantity such as angle is to be provided wherein a waveform distortion of output voltage is diminished.

There are installed at least two Wheatstone bridge circuits having a predetermined angular offset and each comprising a plurality of GMR elements, the GMR elements each having a fixed magnetic layer set to a predetermined magnetization direction. An AC power supply is use as a power supply of the Wheatstone bridge circuits and a displacement of a physical quantity such as a rotational angle is detected on the basis of AC-modulated outputs from the Wheatstone bridge circuits.

An anisotropic self-bias effect of a free magnetic layer in each GMR element can be diminished and hence it is possible to remedy a waveform distortion of an output signal based on the anisotropic self-bias effect of the free magnetic layer.

Claims

1. A displacement sensor comprising:

a sensor element having a first magnetic layer whose magnetization direction is set in a predetermined direction without being influenced by an external magnetic field and a second magnetic layer whose magnetization direction varies against the external magnetic field;

magnetic field generating means for supplying the external magnetic field; and

power supply means for supplying drive voltage to said sensor element;

wherein a change in resistance value of said sensor element proportional to the difference in a magnetized state between a magnetized state of said second magnetic layer and a magnetized stage of said first magnetic layer, which occurs depending on a relative positional relation between said magnetic field generating means and said sensor element, is detected to detect a relative positional relation between said magnetic field generating element and said sensor element,

said displacement sensor further comprising suppressing means for suppressing an anisotropic magnetoresistive effect which occurs in said second magnetic layer and which is attributable to an electric current flowing in said sensor element on the basis of said drive voltage.

2. A displacement sensor according to claim 1, wherein said suppressing means is current control means which inverts a flowing direction of said electric current at a specific timing.

3. A displacement sensor according to claim 1, wherein said suppressing means is AC power supply means which applies an alternating voltage as said drive voltage to said sensor element.

4. A displacement sensor according to claim 1, wherein the relative positional relation between said magnetic field generating means and said sensor element, which relation is to be detected, is a rotational angle displacement between the two.

5. A displacement sensor according to claim 1, wherein the relative positional relation between said magnetic field generating means and said sensor element, which relation is to be detected, is a slant displacement between the two.

6. A displacement sensor according to claim 1, wherein the relative positional relation between said magnetic field generating means and said sensor element, which relation is to be detected, is an axial displacement between the two.

7. An angle sensor comprising:

a stator section having a plurality of giant magnetoresistive elements of a multi-layer structure, said giant magnetoresistive elements each having at least a fixed magnetic layer having a fixed magnetization direction, a non-magnetic conductive layer and a free magnetic layer; and

a rotor section adapted to form a magnetic field and rotate in opposition to said stator section, a rotational angle of said rotor section being detected from a change in resistance value caused by the magnetic field provided from said rotor section to said GMR elements,

characterized in that at least two Wheatstone bridge circuits having a predetermined angular offset are installed in said stator section, said Wheatstone bridge circuits each comprising a plurality of GMR elements each having a fixed magnetic layer set in a predetermined magnetization direction, an AC power supply is used as a power supply for said Wheatstone bridge circuits, an alternating current is made flow in said plural GMR elements and a rotational angle is detected on the basis of AC-modulated outputs provided from said Wheatstone bridge circuits.

8. An angle sensor according to claim 7, wherein said stator section comprises:

a first Wheatstone bridge circuit for comprising four GMR elements, said GMR elements each having a fixed magnetic layer set in a predetermined magnetization direction parallel or anti-parallel with respect to a reference direction in which a rotor section rotational angle θ

is zero; and

a second Wheatstone bridge circuit for comprising four GMR elements, said GMR elements each having a fixed magnetic layer set in a predetermined ±

90°

direction with respect to a reference direction in which the rotor section rotational angle is zero, and

a signal processor for detecting the rotational angle on the basis of a differential-amplified AC-modulated output signal sin θ

sin(ω

t) provided from the first Wheatstone bridge circuit and a differential-amplified AC-modulated output signal cos θ

sin(ω

t) provided from the second Wheatstone bridge circuit, when an AC voltage of said AC power supply is set to sin(ω

t) (ω

;

angular frequency, t;

time)

9. An angle sensor according to claim 7, further comprising a signal processor for calculating the rotational angle (θ

) from output signals sin θ and

cos θ and

in accordance with the relationship θ

=arctan(sin θ

/cos θ

), the output signals sin θ and

cos θ

being obtained from a differential-amplified AC-modulated output signal sin θ

sin(ω

t) provided from the first Wheatstone bridge circuit and a differential-amplified AC-modulated output signal cos θ

sin(ω

t) provided from the second Wheatstone bridge circuit, through a synchronous detector circuit, said synchronous detector circuit synchronously detecting an AC voltage sin(ω

t) of said AC power supply.

10. An angle sensor according to claim 9, wherein said synchronous detector circuit has at least a multiplier and an integrator (circuit).

11. An angle sensor according to claim 7, further comprising a signal processor for detecting the rotational angle on the basis of output phase information on a composite output signal sin(ω

t−

θ

), said composite output signal being obtained by combining an output signal sin θ

cos(ω

t) resulting from a 90°

phase shift of a differential-amplified AC-modulated output signal sin θ

sin(ω

t) provided from the first Wheatstone bridge circuit with a differential-amplified AC-modulated output signal cos θ

sin(ω

t) provided from the second Wheatstone bridge circuit.

12. An angle sensor according to claim 7, wherein said stator section comprises:

a first Wheatstone bridge circuit for comprising four GMR elements, said GMR elements each having a fixed magnetic layer set in a predetermined magnetization direction parallel or anti-parallel with respect to a reference direction in which a rotor section rotational angle θ

is zero; and

a second Wheatstone bridge circuit for comprising four GMR elements, said GMR elements each having a fixed magnetic layer set in a predetermined ±

90°

direction with respect to a reference direction in which the rotor section rotational angle is zero,

AC voltages of said AC power supply are supplied as sin(ω

t) (ω

;

angular frequency, t;

time) and cos(ω

t) to said first and second Wheatstone bridge circuits, respectively, and

a signal processor for detecting the rotational angle on the basis of output phase information on a composite output signal sin(ω

t−

θ

), said composite output signal sin(ω

t−

θ

) being obtained by combining differential-amplified AC-modulated output signals sin θ

cos(ω

t) and cos θ

sin(ω

t) provided from said first and second Wheatstone bridge circuits.

13. An angle sensor according to claim 7, further comprising a signal processor which causes a phase variable φ

to be generated from a sequential phase generator circuit, causes cos φ and

sin φ

to be generated from a sin/cos generator circuit on the basis of the phase variable φ

, multiplies, with use of a multiplier circuit, a differential-amplified AC-modulated output signal sin θ

sin(ω

t) provided from the first Wheatstone bridge circuit by cos φ

, multiplies, with use of a multiplier circuit, a differential-amplified AC-modulated output signal cos θ

sin(ω

t) provided from the second Wheatstone bridge circuit by sin φ

, and detects the rotational angle from the phase variable φ

(=0) in which the term of sin(φ

−

θ

) in a composite signal sin(φ

−

θ

)sin(ω

t) resulting from combining both said output signals by a subtractor circuit becomes zero.

14. An angle sensor according to claim 7, wherein a signal processor for detecting the rotational angle provides a phase detector circuit which comprises at least a reference signal generator circuit, a multiplier and an integrator (circuit), said multiplier being configured to multiply an input signal by a reference signal, an output of said multiplier being inputted to said integrator (circuit), and said integrator (circuit) being configured to integrate the output of said multiplier, and said reference signal generator being configured to produce the reference signal.

15. An angle sensor according to claim 14, further comprising a phase detector circuit having a comparator circuit, an output of an integrator (circuit) being inputted to said comparator circuit, said comparator circuit comparing whether an output value of said integrator (circuit) is within a predetermined range or not, and wherein when an input value of said comparator circuit deviates from the predetermined range, it is detected as an error by said phase detector circuit.

16. An angle sensor according to claim 14, further comprising a phase detector circuit having a comparator circuit, and wherein when an input value of said comparator circuit deviates from a predetermined range for a period of time longer than a predetermined duration time, it is detected as an error by said phase detector circuit.

17. An angle sensor according to claim 15, wherein there are plural said predetermined ranges, said predetermined duration time is defined for each of said plural predetermined ranges, and said phase detector circuit detects a deviation from any of said predetermined ranges as an error.

18. An angle sensor according to claim 7, further comprising means for biasing AC power-side terminals and earth-side terminals of said Wheatstone bridge circuits to a predetermined DC reference potential, and voltage obtained by the addition of a DC reference potential and a DC voltage is applied to the AC power-side terminals.

19. An angle sensor according to claim 7, further comprising a signal processor which stores in memory means a deviation angle α

between a reference direction in which the rotor section rotational angle θ

is zero and the magnetization direction of the fixed magnetic layers of the GMR elements constituting said Wheatstone bridge circuits, and then corrects and outputs the deviation angle α

in accordance with the rotational angle (θ

+α

) obtained on the basis of the AC-modulated outputs of said Wheatstone circuits.

20. A semiconductor device for a displacement sensor, wherein at least four GMR elements are connected together using conductors to constitute a Wheatstone bridge, the GMR elements being each constituted by a four-layer laminate comprising an antiferromagnetic layer, a fixed magnetic layer (pin magnetic layer), a non-magnetic conductive layer and a free magnetic layer all formed on a semiconductor substrate, power supply terminals of two of the GMR elements that constitute said Wheatstone bridge being connected electrically to an AC power supply, power supply terminals of the remaining two GMR elements being connected electrically to an earth potential section, so that an alternating current is supplied as a bias current to the four GMR elements.