Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2003-01-22
2003-12-09
Barrera, Ramon M. (Department: 2832)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S207210
Reexamination Certificate
active
06661225
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a revolution detecting device using a tunneling magnetoresistance sensor (TMR sensor).
2. Description of the Related Art
As a revolution detecting device used in wheel speed sensors for vehicles or the like, an MR element (magnetoresistance element) or the like are used. The MR element has low magnetoresistance (MR) ratio (magnetoresistance change) of approximately 3% so that the detection signals of the MR element are weak.
Therefore, it is necessary for an amplifier to amplify the detection signals. A large gain of the amplifier is required for amplifying the weaken detection signals of the MR element. Due to the large gain of the amplifier, error component included in the detection signals are large so that an error correction circuit with large-scale must be required for correcting the error component included in the detection signals of the MR element.
In order to solve the above problems, revolution detecting devices each having a TMR sensor with a high MR ratio, such as 18% and over at room temperature, compared with the MR ratio of the MR element are disclosed in many Japanese Patent Publications of No. 8-70148, No. 11108689, No. 2000-304568, 2000-123328 and so on.
The TMR sensor comprises an element which is formed with a pinned layer composed of ferromagnetic material formed on a substrate, a tunneling film (tunneling layer) composed of insulating film and a free layer composed of ferromagnetic material. When the TMR sensor is located in a magnetic field, tunneling current between the pinned layer and the free layer via the tunneling film varies according to the change of magnetic field so that the change of the tunneling current allows change of magnetoresistance of the element to be detected.
Concretely, when discotic magnetic rotor with a circumferential side surface which is magnetized so that S poles and N poles are alternately arranged in the circumferential direction thereof is revolved, a number of changes of the magnetic field caused by the revolution of the rotor is detected by the TMR sensor, making it possible to obtain a number of revolutions of the rotor. In this case, the detected number of revolutions includes not only a plurality of revolutions but also a revolution less than one revolution, such as a half revolution.
However, the revolution detecting device using the TMR sensor is still developing so as to leave room for many improvements.
SUMMARY OF THE INVENTION
The invention is made on the background of the need of the related arts.
Accordingly, it is an object of the invention to provide a revolution detecting device having a TMR sensor and capable of efficiently detecting a change of sensitivity of the TMR sensor.
In order to achieve the object, inventors performed considerations as follows.
That is, a magnetic field applied in a region in which an element of a tunneling magnetoresistance sensor (TMR sensor) is located allows a magnetization direction of a free layer of an element of the TMR sensor to be changed with respect to the magnetization direction of a pinned layer of the element, causing a change of a magnetoresistance of the element.
Then, when Cartesian coordinate system is determined such that a center of the free layer of the element is taken as an origin, a magnetization direction of the pinned layer passing through the origin is taken as an X axis, a direction passing through the origin and orthogonal to the X axis and a layer plane of the pinned layer is taken as a Z axis and a direction passing through the origin and orthogonal to an X-Z plane including the X axis and the Z axis is taken as the Y axis.
In Cartesian coordinate system, changes of the magnetoresistance, that is, changes of MR ratio of the element with respect to revolutions of the magnetic field in planes of X-Y plane including the X axis and the Y axis, X-Z plane including the X axis and the Z axis and Y-Z plane including the Y axis and the Z axis were examined, wherein the magnetic field is applied to the region in which the element is located.
FIGS. 1
to
3
represent the results of the changes of magnetoresistance.
FIG. 1
is a view showing a relationship between an angle &thgr; of a direction of the magnetic field with respect to the X axis and the change of magnetoresistance of the element when the magnetic field revolves around the Z axis in the X-Y plane,
FIG. 2
is a view showing a relationship between an angle &thgr; of a direction of the magnetic field with respect to the X axis and the change of magnetoresistance of the element when the magnetic field revolves around the Y axis in the X-Z plane and
FIG. 3
is a view showing a relationship between an angle &thgr; of a direction of the magnetic field with respect to the Y axis and the change of magnetoresistance of the element when the magnetic field revolves around the X axis in the Y-Z plane.
In each of
FIGS. 1
to
3
, each ratio between each of the maximum values of each magnetoresistance and each of the minimum values thereof substantially corresponds to an MR ratio.
In
FIG. 1
, using a range of region A in which the magnetoresistance rapidly changes allows grate change of the magnetoresistance to be detected. That is, it is possible to detect the change of magnetoresistance at high sensitivity when the magnetic field pivotally changes with respect to the Y axis in parallel to the X-Y plane.
In
FIG. 2
, using a range of region B in which the magnetoresistance rapidly changes allows grate change of the magnetoresistance to be detected. That is, it is possible to detect the change of magnetoresistance at high sensitivity when the magnetic field pivotally changes with respect to the Z axis in parallel to the X-Z plane.
In other words, each of the regions A and B is a range such that the MR ratio is mostly changed according to the change of the magnetic field. Incidentally, in
FIG. 3
, no change of the magnetization direction of the free layer occurs so that, in
FIG. 3
, the magnetoresistance of the element is constant irrespective of the change of the magnetic field.
That is, using characteristics of the elements of the TMR sensor shown in
FIGS. 1 and 2
to contrive a positional relationship between the revolution member and the element allows the change of sensitivity of the element to be efficiently detected.
According to one aspect of the present invention, there is provided a revolution detecting device comprising: a tunneling magnetoresistance sensor having an element located in a region, the element comprising: a substrate; a pinned layer composed of ferromagnetism material and located to one side of the substrate; a tunneling layer composed of insulating film and located to one side of the pinned layer; and a free layer composed of ferromagnetism film and located to one side of the tunneling layer, the element being configured to detect a change of magnetoresistance of the element according to a magnetic field applied in the region in which the element is located, the change of the magnetoresistance of the element being based on a change of current flowing through the tunneling layer between the pinned layer and the free layer; when determining Cartesian coordinate system such that a center of the free layer is taken as an origin, a magnetization direction of the pinned layer passing through the origin is taken as an X axis, a direction passing through the origin and orthogonal to the X axis and a layer plane of the pinned layer is taken as a Z axis and a direction passing through the origin and orthogonal to an X-Z plane including the X axis and the Z axis is taken as the Y axis, a revolution member disposed in a vicinity of the element in the Y axis from a viewpoint of the element, the revolution member having a surface portion opposite to the element, the surface portion being formed with S poles and N poles which are alternately arranged; and a magnet disposed in a vicinity of the element and generating the magnetic field, a direction of the magnetic field being substantially parallel to the Y ax
Murata Yuichiro
Suzuki Yasutoshi
Toyoda Inao
Uenoyama Hirofumi
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