Sensor chip with magnetoresistive wheatstone bridges for determining magnetic field directions

An arrangement for a magnetoresistive sensor chip has two Wheatstone brid to determine the sine and cosine of the angle formed between a chip edge and the direction of the magnetic field. All resistances of the bridges consist of a plurality of magnetoresistive laminated elements (2) with current connections made of highly conductive thin films with parallel edges. When the resistances of a bridge are directly electrically interconnected, these edges form angles (5) of 90.degree. each. The parallel edges of the corresponding resistances of the sine and cosine bridges are mutually offset by 45.degree.. The magnetoresistive laminated elements (2) are distributed on the chip surface to reduce angle measurement errors to a minimum. Also disclosed are arrangements that allow the sensor chips to be used for measuring angles and positions.

The subject matter of the present invention is a sensor chip having 
resistive layers being dependent on a magnetic field to be used for 
contactless measuring angles and positions. 
The advantages of employing sensor chips, the thin-film resistances of 
which exhibit the anisotropic magnetoresistance effect are already set out 
for such measurements for example in a paper by A. Petersen and T. 
Rinschede "Beruhrungslose Winkelmessung mit magnetoresistiven Sensoren", 
in Elektronik 6/1994, pages 91-93. A disadvantage of using a single sensor 
bridge is the highly limited measuring range and the high temperature 
dependence of the output signal of the bridge, Therefore, sensor chips 
having two bridges for determining the sine and cosine values of the angle 
of the magnetic field with respect to an edge of the sensor chip are 
already disclosed in the Patent Specification DE 43 17 512 and in a paper 
by A. Petersen "Beruhrungslose Winkelmessung" in Design & Elektronik 
Sensortechnik, May 1995, pages 64-66. The angle is obtained therein from 
the ratio of the output signals of both bridges. As both sensor chips are 
arranged on the same chip the difference of their temperatures is merely 
minor, and, hence, the ratio of both output signals becomes essentially 
independent of the temperature. However, a disadvantage of both sensor 
chips is that the magnetoresistive strip conductors which form the bridge 
resistances have a form-anisotropy which substantially exceeds the 
inherent anisotropy of a layer extending to infinity. The higher the total 
anisotropy of the respective layer strip, the higher is also the deviation 
of the direction of the external magnetic field from the direction of 
magnetization within the interior of the layer material. The output signal 
of the bridge, however, always corresponds to the direction of the 
internal magnetization. Therefore, deviations occur being observed as 
corresponding errors of the angle measurement, During evaluation it is 
extremely complicated to correct these errors because magnetoresistive 
layer strips with different longitudinal directions and different lengths 
are used. Therefore, the deviations of each magnetoresistive layer strip 
are of different magnitudes. Therefore, in the disclosed arrangements, the 
only possibility for eliminating the errors is to employ magnets having 
very high field strengths. This means that magnets having considerable 
volumes of magnetic materials which provide high field strengths, have to 
be employed and that only relatively small distances between the magnet 
and the sensor chip are allowed. The first necessity mentioned results in 
high cost, the latter in narrow assembly tolerances. 
A further drawback of employing long magnetic layer strips of different 
longitudinal directions on the chip surface is solely caused by geometry. 
The total chip surface cannot be used to accomodate the layer providing 
the magnetic field dependent resistance due to the angles of about 
45.degree. between the strips. Hence, a larger chip surface is absolutely 
necessary which results in higher chip cost. 
The influence of the anisotropy of the layers is substantially eliminated 
in an arrangement which utilizes the planar Hall-effect in 
magnetoresistive layers, disclosed in the Patent Specification EP 0 217 
478 B1. The disadvantage connected therewith is, however, that only very 
small resistances of the elements can be implemented. This directly leads 
to small output voltages of the measuring elements as these voltages are 
proportional to the applied operating voltages. Additionally, when 
employing circular thin-film areas having nearly dot-like current feeding 
points at the circular circumference, the directions of the current lines 
are not parallel. Hence, a distribution of the current directions always 
exists with respect to the direction of the magnetization, and the maximum 
resistance variation of the magnetic layer achievable with parallel 
current lines is hardly reached. In correspondence with such a reduced 
resistance variation, the output signal is also reduced. A distribution of 
the current directions also exists when using resistance strips in form of 
circular arcs instead of said circular ones. This results in a reduction 
of the amplitude of the output voltage with respect to the maximum 
possible voltage by a factor of .pi./2. When using narrow conductor lines 
as used for the laminated conductors in form of circular arcs the 
form-anisotropy, more than that, has already again a considerable value so 
that greater deviations between the field direction to be measured and the 
magnetization of the layer occur. In case of these narrow laminated 
conductors in the proposed rotationally symmetric arrangement, 
furthermore, the utilization of the chip surface as sensible sensor 
surface is only possible to a small extent. 
Therefore, it is an object of the invention to provide the arrangement and 
use of a sensor chip for determining the sine and cosine values of the 
angle between a magnetic field and a line on the sensor chip, as used for 
measuring angles and positions, such that only small field strengths are 
required, that the chip surface can be minimized, that the sensor chip 
supplies large output signal amplitudes and that large distances between 
the magnet and sensor chip are allowable. 
This object is solved by this present invention. The bridge resistances are 
composed of a plurality of magnetoresistive laminated elements. These 
laminated elements comprise highly conductive thin-film surfaces each 
having two opposing edges by means of which the current direction is 
determined in the laminated elements. As the current directions within the 
resistances of the same bridge are mutually rotated by 90.degree. the 
oppositely varying resistance values necessary for the bridges are 
obtained in each bridge arm. The angle of 45.degree. between the edges of 
the laminated elements of the sine and cosine bridges constitutes the 
necessary requirement for the phase shift of 45.degree. which exists 
between the two bridge output signals, The laminated elements are designed 
such that they have only negligible form anisotropies. The only reason 
therefore is the fact that the length of the elements is not essentially 
different from the width thereof. Therefore, a plurality of them is 
connected in series. This results in a high resistance to which a high 
operating voltage can be applied without a high generation of heat on the 
chip surface. Hence, high output voltages of the bridges are possible as 
they are proportional to the applied operating voltage. The small 
form-anisotropy which is possible in this arrangement and a small inherent 
anisotropy of the magnetoresistive layer result in a small deviation of 
the direction of the internal magnetization of the laminated elements from 
the external field direction so that the field direction can be determined 
from the output signals of both sensor bridges with high precision. 
As the same geometric figure and the same dimensions are used for each 
magneoresistive laminated element the total anisotropies thereof are 
identical at any location, and systematic angle dependent variations do 
not arise when angularly rotating the sensor chip with respect to the 
magnetic field. 
Providing the magnetoresistive laminated elements in form of squares leads 
to the advantage that, when adjusting the structure of the highly 
conductive thin-film surfaces with respect to the magnetoresistive squares 
in the four different mutually rotated positions, a deviation of 
adjustment does not cause a resistance variation of the magnetoresistive 
laminated elements, and that thereby the manner of forming the layers 
already provides bridges having very small inherent offset voltages. 
Providing all contact surfaces for connecting the operating voltage and the 
output voltages along an edge of the sensor chip leads to the advantage 
that the magnetic field sensitive surface of the sensor is situated 
adjacent the opposite edge and thus can be positioned in close distance 
with regard to parts producing magnetic fields such as coils or permanent 
magnets. Here, far higher magnetic field strengths exist than at larger 
distances so that measuring the sine and cosine values of the angles with 
respect to the magnetic field exhibits smaller errors. 
Providing all magnetoresistive laminated elements, the connecting lines and 
the contact surfaces in one plane eliminates crossovers of lines and 
therewith the necessity of providing insulating layers. 
The discussed sensor chip arrangement is suitable to measure an angle which 
is defined between an edge of the sensor chip and the direction of a 
magnet. In a particular case the sensor is provided above a bar magnet 
such that the mid-vertical of the sensitive surface of the sensor chip is 
aligned with the mid-vertical of the bar magnet, and the magnetizing 
direction of the bar magnet is orthogonal to this mid-vertical. As in 
known magnetoresistive sensors a full sine and cosine period is measured 
at both bridge outputs when the bar magnet rotates by 180.degree.. The 
tangent of the angle to be measured is generated by forming the ratio from 
the sine and cosine signals. Forming the ratio leads to a measurement 
result which is independant of both the temperature of the sensor chip and 
the temperature of the magnet. 
In order to achieve a high angle resolution the sensor chip is arranged 
near the circumference of a magnet wheel that, at least on its surface, is 
alternatingly magnetized in opposite directions. As the magnetic field 
orthogonally emanates from the north poles and orthogonally enters into 
the south poles a. rotation of the magnetic field direction by 180.degree. 
exists between both poles. Thereby, a full period of the sine and cosine 
signals, respectively, appears at the outputs of the sensor chips. The 
number of poles passing the sensor is determined by means of known 
incremental counting techniques. The exact angle position between the 
poles is again obtained by forming the ratio of both output signals. 
Advantageously, the angle determining sensor chip is suitable for the 
measurement at different pole distances of the magnet wheels. 
For measuring a position, a magnetic field generating arrangement, which, 
among other solutions, may be a coil or a bar magnet, is moved in a 
direction in respect to the sensor chip. In case the sensor chip is, for 
example, arranged such that it moves along a parallel line of the axis of 
the bar magnet, which axis is also the direction of magnetization of the 
bar magnet, the position can be uniquely determined from the angle of the 
magnetic field with respect to the parallel line, which is measured by the 
sensor chip. Here again the advantage is that the temperatures of the 
magnet as well as of the sensor chip do not influence the result of 
measurement. In addition, it is possible to linearly relate the measured 
angle to the positon with small error. The distance between the magnet and 
sensor chip can have the dimension of half the magnet length. Distance 
variations do not cause a positional error when limiting the path length 
to be measured to about half the magnet length. 
If, instead of a bar magnet, a simple magnet is used which is magnetized in 
alternating directions in areas which have a certain length in the 
direction of movement, an incremental length measurement with high 
resolution is possible in an analogous manner to the above outlined angle 
measurement using the magnet wheel. Again, the inventive sensor chip 
provides the advantage that it is suitable for all period lengths of the 
magnet which is magnetized in alternating directions.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
FIG. 1 shows a circuit diagram of two parallely connected Wheatstone 
bridges. The first bridge consists of the resistances 1.1; 1.2; 1.3 and 
1.4 and is designated as sine bridge. The second bridge consists of the 
resistances 1.5; 1.6; 1.7 and 1.8 and is the cosine bridge. This circuit 
can be implemented both in the prior art sensor chip and the sensor chip 
according to the invention. 
FIG. 2 shows a diagram of a special embodiment of the sensor chip 12 
according to the invention. Here, all bridge resistances 1.1 to 1.8 are 
formed from identical magnetoresistive laminated elements 2. Such a 
laminated element 2 is shown in a larger scale in FIG. 3. The 
magnetoresistive element 2 comprises, besides a square surface of a layer 
exhibiting an anisotropic magnetoresistance effect, highly conductive 
thin-film surface elements 3 for establishing an electric contact, the 
thin-film surface element edges 4 facing the center of the squares are in 
parallel to each other. The magnetoresistive laminated elements 2 are each 
arranged in rows as linear resistance areas 7, only two of which are 
circumscribed by broken lines and indicated in the figure. The two 
resistances 1.1 and 1.2, 1.3 and 1.4, 1.5 and 1.6 as well 1.7 and 1.8 
belonging to each bridge arm are each arranged on the sensor chip 12 in 
symmetry to the center line 10 of the chip surface. The edges 4 of the 
magnetoresistive laminated elements 2 which are each connected in series 
to form a bridge arm in the electric circuit of the two individual bridges 
form an angle 5 of 90.degree. with each other. The edges 4 of the 
magnetoresistive laminated elements 2 of the resistances which correspond 
to each other in the sine and cosine bridges 1.1 and 1.5; 1.2 and 1.6; 1.3 
and 1.7; 1.4 and 1.8 form angles 6 of 45.degree. with each other. Both 
bridges are connected to the operating. voltage contacts 8. The linear 
resistance areas 7 which are connected to the same operating voltage 
contact 8 alteratingly belong to the sine and to the cosine bridge. Each 
resistance 1.1 to 1.8 consists of two linear resistance areas 7 which are 
connected in series and which are arranged in a meander-shaped manner such 
that, in each meander branch 9, the adjacent linear resistance areas 7 
each alternatingly belong to the sine and to the cosine bridge. In actual 
embodiments of the sensor chip, contrary to FIG. 2 not only three 
magnetoresistive laminated elements 2, but a plurality of them are 
arranged on the sensor chip 12, and not only two meander branches but 
substantially more thereof. The dimensions of the sensor chips 12 are in 
the range of several millimeters, those of the magnetoresistive laminated 
elements 2 in a range of about 10 micrometers. The contact surfaces 13 for 
the output signals of both bridges and the operating voltage contacts 8 
are all adjacent to a chip edge 14. Here, the balancing surfaces 11 for 
zero voltage adjustment are also provided for both bridges. Hence, the 
sensitive surface 17 (FIGS. 4 and 6) of the sensor chip 12, in which 
surface the magnetic field dependent magnetoresistive laminated elements 2 
are arranged, is displaced towards the rim lying opposite to this edge 14. 
FIG. 4 shows an arrangement for angle measurement. A permanent magnet 15 
with poles N and S and with the mid-vertical 18 of its surface facing the 
sensor chip 12 is rotatably provided above the sensor chip 12, the 
sensitive surface 17 of which and the mid-vertical 16 of this surface 17 
are indicated in the drawing. Both mid-verticals 16 and 18 have to be 
aligned in order to measure the angle between the edge 14 of the sensor 
chip 12 and a longitudinal edge of the magnet 15 with an error as small as 
possible. 
FIG. 5 shows the arrangement for an angle measurement with high resolution 
using the sensor chip 12. The plane of the sensor chip 12 is also the 
plane of the magnet wheel 19 which, at least along its circumference 20, 
is alternatingly magnetized in opposite directions. Therein, edge 21 being 
opposite to edge 14 near which the contact surfaces 13 are located, faces 
the. magnet wheel. When rotating the magnet wheel by an angle 
corresponding to the distance of a south pole S from a north pole N, the 
field direction rotates by 180.degree. at the location of the sensor chip 
12, and the output signals of the sine and cosine bridges cycle through a 
full period. Thereby, a sensitivity of the angle measurement is achieved, 
which is higher by the number of the poles of the magnet wheel 19 than in 
case of using a bar magnet 15 of FIG. 4. The count of the poles already 
having passed the sensor chip 12 can be determined in accordance with 
known incremental measuring techniques so that the total angle variation 
is always known with respect to an initial value. 
FIG. 6 shows the arrangement of the sensor chip 12 for measuring a linear 
position variation with respect to a permanent magnet 22 which can perform 
a reciprocating movement in the direction of an indicated arrow. The 
magnet comprises in the direction of movement a plurality of areas 23 
which are alternatingly magnetized in opposite directions. The edge 21 of 
sensor chip 12 is aligned in parallel to the direction of movement of the 
permanent magnet. The field direction varies by a full period length at 
the location of the sensor chip 12 when the magnet 22 performs a passing 
movement by the distance of a north pole N from a south pole S. Thereby, 
both sensor output signals cycle through a full period. Advantageously, 
the relation of the field direction angle determined therewith is 
substantially linear to the position. The count of magnet poles having 
already passed the sensor chip 12 starting from an initial position can be 
again determined by known incremental measuring techniques. 
FIG. 7 shows the substantial linearity of the relationsship between the 
field direction angle determined by the sensor chip and the position 
variation. The results presented here have been obtained under the 
condition of using a magnet 22 consisting of only a single area 23. This 
area has a length of 20 mm and a cross-section of 10.times.10 mm.sup.2. In 
this graphical representation, the error resulting by linearity relating 
the measured angle to the position in direction of movement is plotted. 
Graph 24 applies to a distance of 10 mm between the sensor chip 12 and the 
magnet 22, graph 25 to a distance of 10.7 mm and graph 26 to a distance of 
12 mm. For a total measuring length of 20 mm, the error remains under 0.1 
mm for the optimum distance of 10.7 mm, If only a measuring distance of 10 
mm is utilized with a magnet having a length of 20 mm, the measuring error 
is still smaller than 0.1 mm when the distance between the sensor chip 12 
and the magnet varies by 2 mm. It is to be appreciated that these results 
of measurement are neither dependent on the temperature of the sensor chip 
12 nor on the temperature of the magnet 22.