Two dimensional magnetoresistive position sensor

Details Two dimensional magnetoresistive position sensor Two dimensional magnetoresistive position sensor

1999-03-15

2001-12-04

Snow, Walter E. (Department: 2862)

Electricity: measuring and testing

Magnetic

Displacement

C324S207210, C338S03200R

Reexamination Certificate

active

06326782

ABSTRACT:

TECHNICAL FIELD
The present invention relates to position sensors and more particularly to a two dimensional positional sensor utilizing a magnetoresistive (MR) die characterized by two orthogonally oriented magnetoresistive pairs which are electrically independent and equally magnetically biased.
BACKGROUND OF THE INVENTION
The principle of using a pair of matched MRs in a differential arrangement for the purpose of measuring small linear displacements along one dimension is well known in the art.
FIG. 1
depicts one such example. In
FIG. 1
, MR die
10
is composed of two matched MRs, MR
1
and MR
2
, a first terminal
16
, a second terminal
18
, and a third terminal
20
. A small moving target
22
, in the form of a permanent magnet if the die is not biased by an external magnetic field, or, if the MR die
10
is biased by an external magnetic field, then the target would, instead, be a ferromagnetic material. The target
22
is suspended, usually a fraction of a millimeter, above the MR die
10
. A two dimensional Cartesian (X-Y) coordinate system
30
consisting of an X axis and a Y axis is superimposed on the MR die
10
as shown in
FIG. 1
, whereby the target
22
is movable along the X axis.
It is well known in the art that the resistance, R
MR
, of an MR can be modulated by a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The portions of MR
1
and MR
2
under the target
22
are exposed to a considerably higher magnetic field than the portions of MR
1
and MR
2
not under the target. Thus, the more area of MR
1
or MR
2
covered by the target
22
, the greater the resistance of MR
1
or MR
2
, respectively. When the center line
24
of the target
22
coincides with the Y axis, which is aligned midway between MR
1
and MR
2
, the areas of MR
1
and MR
2
covered by the target are equal and, thus, the resistance R
MR1
of MR
1
is the same as the resistance R
MR2
of MR
2
, since MR
1
is matched with MR
2
. Once the target
22
is moved in, for example, the positive X direction to the point where the center line
24
of the target is at the position
36
on the X axis, the area of MR
1
covered by the target is less than the area of MR
2
covered by the target, thereby causing the resistance of MR
2
to increase while the resistance of MR
1
decreases. A properly designed electrical circuit, as will be discussed shortly, can incorporate this change in resistance and produce an output voltage which is a linear function of the position of the target
22
relative to MR
1
and MR
2
.
Such a circuit is depicted in FIG.
2
. The first terminal
16
of MR
1
is connected to the positive terminal
38
of a constant voltage source V
IN
whereas the third terminal
20
of MR
2
is connected to ground
40
. Resistors R
1
and R
2
have, preferably but not necessarily, the same value. V
OUT
is measured with respect to a pair of output terminals
42
,
44
.
With resistors R
1
and R
2
having the same value, V
OUT
can be expressed in terms of the current I
MR
passing through MR
1
and MR
2
and the resistances, R
MR1
and R
MR2
, of MR
1
and MR
2
, respectively, as:
V
OUT
=(
I
MR
/2)(
R
MR2
−R
MR1
) where
I
MR
=V
IN
(
R
MR2
+R
MR1
).
The movement of the target
22
of
FIG. 1
in the X direction increases the resistance of one MR and decreases the resistance of the other MR. However, since the MRs are matched, the magnitude of the increase of the resistance of one MR is the same as the magnitude of the decrease in resistance of the other MR, thereby causing the total resistance (R
MR2
+R
MR1
) to remain constant whereby the current MR also remains constant.
Thus, the output voltage, V
OUT
, is directly proportional to the difference in the respective resistance of MR
2
and MR
1
and, hence, is a linear function of (R
MR2
−R
MR1
). Since the resistance of each MR is proportional to the area covered by the target
22
and the area covered is proportional to the position of the target along the X axis (wherein, the position of the target along the Y axis remains constant), the output voltage, V
OUT
, is directly proportional to the position of the target along the X axis, as well.
However, there are many applications requiring two dimensional position sensors. Probably, the most famous application of this kind is the ubiquitous computer mouse employing a rubber covered ball, two rollers, and two position encoders. A two dimensional position sensor utilizing MR elements could be envisioned as an MR die consisting of an overlay of two orthogonal MR layers, with each layer composed of two MR elements. Unfortunately, two layer MRs are not practical and the die pattern would be, in effect, reduced to one layer consisting of four independent MR elements which would require the use of active components, such as operational amplifiers, to realize a two dimension position sensor.
Accordingly, what remains needed in the art is a solution to the problem of providing an MR die for a two dimensional position sensor which does not, necessarily, require the use of active components.
SUMMARY OF THE INVENTION
The present invention is an MR die, and actualizing circuit therefor, by which a plurality of individual MR elements are arranged and configured so as to produce a two dimensional position sensor which does not, necessarily, require the use of active components.
Structurally, the present invention is composed of an MR die consisting of a number of interdigitated MR elements, wherein each MR element is composed of a number of serially connected straight segments, each characterized by a magnetosensitive material on which a multiplicity of conductive shorting bars are deposited in regularly spaced intervals therealong. In the preferred form of the present invention, the segments are composed of indium antimonide (InSb) epitaxial film mesas, and the conductive shorting bars are composed of gold bars deposited thereupon. The ends of the segments of each MR element are serially connected by conductive bridges, preferably of gold. The techniques to fabricate the MR elements is elaborated in U.S. Pat. No. 5,153,557, issued Oct. 6, 1992 and U.S. Pat. No. 5,184,106, issued Feb. 2, 1993, both patents being owned by the assignee hereof and are hereby incorporated by reference herein.
In the MR die according to the preferred embodiment of the two dimensional position sensor of the present invention, four MR elements are provided, wherein each MR element has an orthogonally serpentine configuration. A first MR sensor is formed of two MR elements that are diametrically opposed along a first axis, and a second MR sensor is formed of the remaining two MR elements which are diametrically opposed along a second, orthogonal axis.
Each MR element is interdigitated with both of the MR elements of the other MR sensor, such that each MR sensor is electrically independent and orthogonally oriented with respect to the other MR sensor. Accordingly, one MR sensor consisting of two MR elements senses position along a first axis, and the other MR sensor consisting of the two other MR elements senses position along a second axis that is orthogonal to the first axis, whereupon an electronic circuit consisting of passive components (i.e. operational amplifiers are not required) is employed, as is analogously done for the one dimensional sensor of FIG.
2
.
The interdigitation of the MR elements may be accomplished with geometries other than orthogonally serpentine. It is preferable that the MR elements be matched to each other and that the geometry of the interdigitation of the MR elements is such that the magnitude of the increase of the resistance of one MR element is the same as the magnitude of the decrease in resistance of the other MR element, but this is not essential. Proper circuit design with appropriate weighting factors determined empirically or theoretically can be applied by those of ordinary skill in the art to accommodate MR element mismatch and interdigitation geometries, based upon the principles elaborated in the

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