Bearing sensor having magneto resistive elements

Details Bearing sensor having magneto resistive elements Bearing sensor having magneto resistive elements

2002-01-29

2003-04-29

Lefkowitz, Edward (Department: 2862)

Electricity: measuring and testing

Magnetic

Magnetometers

C324S260000, C033S35500D

Reexamination Certificate

active

06556007

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bearing sensor having magneto resistive elements and a process for bearings, in particular, to a bearing sensor and a process for bearings in which bearings are measured by applying a biasing magnetic field to the magneto resistive elements.
2. Description of the Related Art
When a current is applied to a magneto resistive element in a direction of an easy axis of magnetization, and at the same time, a magnetic field is applied in a direction perpendicular thereto, a resistance in the current direction has a magneto-resistance effect, that is, it is reduced depending on a magnetic field strength. A relationship between the resistance and the applied magnetic field strength can substantially be approximated by a quadric as shown in FIG.
24
and as follows:
R=R
0
(1−&agr;(
H/Hk
)
2
),
where reference symbol R
0
denotes a resistance when no magnetic field is applied, reference symbol &agr; denotes a resistivity variation ratio, and reference symbol Hk denotes a saturation magnetic field.
When a biasing magnetic field on the order of 1/2-Hk is applied to the magneto resistive element, there is a substantially linear relationship between an external magnetic field and the resistance. Since the maximum horizontal component of the earth magnetism is 0.4 Oe, bearings can be measured by applying an appropriate bias.
There is used a bearing sensor comprising a full bridge constituted by four magneto resistive elements
91
,
92
,
93
, and
94
that are orthogonal to each other as shown in
FIG. 25
, and two bias coils
101
and
102
that are wound around a holder mounted outside of the magneto resistive elements so that two orthogonal biasing magnetic fields can be applied both at an angle of 45 degrees with respect to the current directions of the magneto resistive elements.
FIG. 26
is a schematic cross-sectional view thereof, and
FIG. 27
is a perspective view thereof
In measurement of bearings, a +x-direction bias is applied by one bias coil
101
(referred to as an x-direction coil) to the four magneto resistive elements
91
,
92
,
93
, and
94
constituting the full bridge to measure an intermediate potential difference among the magneto resistive elements, and then, a −x-direction bias is applied by the same bias coil
101
to the magneto resistive elements to measure the intermediate potential difference among the magneto resistive elements. A difference between the intermediate potential differences measured when the +x-direction bias is applied and when the −x-direction bias is applied is proportional to sin &thgr;, the angle &thgr; being an angle between the horizontal component of the earth magnetism and the x-axis.
Next, a +y-direction bias is applied by the other bias coil
102
(referred to as a y-direction coil) to the four magneto resistive elements
91
,
92
,
93
, and
94
constituting the full bridge to measure an intermediate potential difference among the magneto resistive elements, and then, a −y-direction bias is applied by the same bias coil
102
to the magneto resistive elements to measure the intermediate potential difference among the magneto resistive elements. A difference between the intermediate potential differences measured when the +y-direction bias is applied and when the −y-direction bias is applied is proportional to sin(&pgr;/2-&thgr;), that is, cos &thgr;.
From the y-directional output Vy and the x-directional output Vx, the bearings can be measured as the direction &thgr; of the horizontal component of the earth magnetism as follows:
&thgr;=
a
tan(
Vx/Vy
).
While the above description is true in terms of theory, the relationship between the magnetic field applied to the magneto resistive element and the resistance involves a hysteresis as shown in
FIG. 28
, rather than FIG.
24
. It is said that when the applied magnetic field strength is increased, it reaches a level of saturation via the upper curve in
FIG. 28
, and when it is decreased from the level, it traces the lower curve.
Therefore, when measuring bearings, the saturation magnetic field is applied before the application of the biasing magnetic field in consideration of the hysteresis.
For example, as disclosed in Japanese Patent Laid-Open No. 5-157565, when measuring bearings using the bearing sensor composed of the magneto resistive elements and two orthogonal bias coils as described above, the saturation magnetic field Hk is applied in +x direction, and then the intermediate potential difference between the magneto resistive element pairs is measured while applying the +x-direction biasing magnetic field Hb. Then, the saturation magnetic field −Hk is applied in −x direction by the same bias coil, and then the intermediate potential difference between the magneto resistive element pairs is measured while applying the −x-direction biasing magnetic field −Hb. The difference between the intermediate potential differences at the time of applications of the +x-direction bias and the −x-direction bias thus obtained is defined as an x-direction output Vx.
Then, the saturation magnetic field is applied in the +y direction by the other bias coil, and then the intermediate potential difference between the magneto resistive element pairs is measured while applying the +y-direction biasing magnetic field. Then, the saturation magnetic field is applied in the −y direction by the same bias coil, and then the intermediate potential difference between the magneto resistive element pairs is measured while applying the −y-direction biasing magnetic field. The difference between the intermediate potential differences at the time of applications of the +y-direction bias and the −y-direction bias thus obtained is defined as an y-direction output Vy. Based on the Vx and Vy, bearings are measured in the manner as described above.
As the orthogonal four magneto resistive elements assembled into the full bridge described above, zigzag magneto resistive thin plates formed by etching a Ni-based alloy film deposited on one ceramic substrate may be used. Thus, the magneto resistive elements can be quite small and thin. However, since the two bias coil wound around them in x direction and y direction are provided outside the magneto resistive element bridge, the bearing sensor has, at the smallest, a thickness of the order of 3 mm and an area of the order of 10 mm×10 mm. Due to such a thickness, a wristwatch incorporating the bearing sensor has a large size.
In the procedure of measuring bearings has been explained in the above description, it is required to carry out measuring four times because the bias is applied in +x direction and −x direction by the x-direction coil, the bias is applied in +y direction and −y direction by the y-direction coil, and then calculation is carried out.
Furthermore, in order to eliminate the effect of the hysteresis of the magneto resistive element, before the biasing magnetic field is applied, the saturation magnetic field of the same direction as that of the biasing magnetic field is applied. Application of the biasing magnetic field after the application of the saturation magnetic field make the gradient of the curve for the resistance of the magneto resistive element and the magnetic field (see
FIG. 28
) be decreased, so that the output to be measured becomes low.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a bearing sensor of a significantly reduced thickness and area.
Furthermore, another object of the present invention is to provide a bearing sensor in which the number of applications of a current to a coil and the number of measurements are less than before.
Furthermore, another object of the present invention is to provide a process for bearings in which the number of applications of a current to a coil and the number of measurements are less than before.
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