ANGLE SENSOR
The invention relates to an angle sensor comprising at least one flat magnetoresistive sensor element, each of which has:
• at least one first and at least one second supply terminal for being acted upon by at least one supply voltage for generating a drop in the supply voltage in a predefined direction along the flat dimension of the sensor element and/or at least one supply current for generating a flow of the supply current in the predefined direction,
• at least two tapping electrodes for tapping off a differential voltage which is produced by the drop in the supply voltage or by the flow of the supply current under the effect of the field strength and/or the direction of a magnetic field acting on the angle sensor between the tapping electrodes.
In European patent application No. 04 102 117.1 of 14.05.2004, a description is given of a magnetoresistive sensor element which can be acted upon by at least one supply voltage or by at least one supply current by means of at least one supply terminal, in particular by means of at least one first supply terminal, and by means of at least one reference terminal, in particular by means of at least one second supply terminal, for example at ground potential, wherein the differential voltage which is produced as a function of the angle between the direction of the drop in the supply voltage or of the flow of the supply current and the direction of the resulting internal magnetic field can be tapped off between at least one first, in particular positive, tapping electrode and at least one second, in particular negative, tapping electrode, and wherein the direction of the resulting internal magnetic field is given by superposing the intrinsic magnetization and at least one external magnetic field, wherein furthermore the magnetoresistive sensor element is flat and in particular the drop in the supply voltage is continuous. In said application, a description is furthermore given of an angle sensor which comprises a combination of a first sensor element assigned to the first supply voltage or to the first supply current and a second sensor element assigned to the second supply voltage or the second supply current. In particular, the direction of the drop in the first supply voltage or of the flow of the first supply current is rotated or offset by a
defined angle, for example by about 45 degrees, with respect to the direction of the drop in the second supply voltage or of the flow of the second supply current.
Sensor elements of the described type, which are designed not with meandering conductor strips but rather with closed, flat conductor structures which comprise e.g. rectangular or circular layers of the material Permalloy applied to a silicon substrate, are characterized by a very low ohmic resistance. Typical resistance values lie in the region of a few Ohms. It has been found that these resistance values thus come to lie in a value range which lies close to the value range of the resistance values of the leads to the flat conductor structures for supplying the supply voltages or supply currents, that is to say close to the value range of the lead resistances of the bond connections to the actual sensor structure. The resistance values of the leads to the flat conductor structures thus have an appreciable influence on the supply voltages acting on the flat conductor structures or on the flow of the supply currents. The voltage losses which occur along these lead resistances during operation considerably falsify the measurement result and thus have a significant influence on the quality of the measurement signal that is produced, said signal being intended to be an indication of the angle of the field lines of a magnetic field acting on the angle sensor relative to the angle sensor.
The object of the invention is to avoid the above-described defects in an angle sensor of the type mentioned above.
According to the invention, in an angle sensor of the generic type, this object is achieved in that at least the first supply terminal or one of the first supply terminals and at least the second supply terminal or one of the second supply terminals of at least one of the sensor elements is coupled to a power source for supplying an impressed supply voltage to the at least one sensor element.
As a result, the situation is achieved in a simple manner whereby the sensor element is acted on by a defined supply current or the sensor elements are acted on by a common defined supply current and thus a defined supply voltage is present across said sensor elements, said supply voltage depending only on the ohmic resistance of the sensor element itself but not on the lead resistances. The manufacturing variation thereof thus no longer has any effect on the measurement result of the angle sensor. With the design of the power supply to the angle sensor according to the invention, there is no need for any complicated compensation devices.
Preferably, the angle sensor according to the invention is designed with two flat magnetoresistive sensor elements, each of which has a first and a second supply terminal and also two tapping electrodes, and the magnetoresistive sensor elements are arranged in a series connection with one another and with their common power source via their supply terminals. This ensures that the two sensor elements are fed the same supply current, so that the ratio of the amplitudes of the measurement signals output by the sensor elements no longer depends on variations in the current intensity of this supply current. The influence of the lead resistances on the tapping electrodes, which also exists in principle, is reduced to a negligible level by virtue of a measurement circuit with high-ohmic inputs. In an angle sensor designed according to the invention with two sensor elements, said angle sensor preferably being designed for angle measurement over a large angle range, the magnetoresistive sensor elements are advantageously of at least substantially the same geometric shape and size. In particular, to this end, the magnetoresistive sensor elements, with respect to the plane of their flat dimension, are furthermore arranged at least substantially parallel and in this plane with respect to one another and in a manner rotated by a predefined angle with respect to one another. Preferably, the predefined angle of the rotated arrangement of the magnetoresistive sensor elements with respect to one another corresponds to at least almost 45°. With such a configuration of the angle sensor according to the invention, advantageously two signals which have the same amplitude and are almost sinusoidal and are offset by 45° with respect to one another are measured at the tapping electrodes, and these signals can be used with particular advantage to determine the measurement signal which is intended to be a indication of the angle of the field lines of a magnetic field acting on the angle sensor relative to the angle sensor.
In an angle sensor of the type described above, the sensor element is or the sensor elements are advantageously designed as sensors which are based on the principle of the anisotropic magnetoresistive effect, so-called A[nisotropic]M[agneto]R[esistive] sensors.
The invention will be further described with reference to an example of embodiment shown in the drawing to which, however, the invention is not restricted.
Fig. 1 shows an example of a schematic circuit diagram comprising two magnetoresistive sensor elements which according to the invention are connected to one another and to their common power source via their supply terminals.
Fig. 2 shows an example of a schematic circuit diagram of an angle sensor in the configuration according to the invention, with an evaluation device for angle measurement.
Fig. 1 schematically shows a first flat magnetoresistive sensor element 1 and a second flat magnetoresistive sensor element 2 of an angle sensor. Their magnetoresistive, current-conducting layers, which are preferably made of Permalloy, have a circular contour. A design with rectangular contours, as shown in European patent application No. 04 102 117.1 of 14.05.2004, is likewise possible. The magnetoresistive sensor elements 1, 2 are preferably designed as AMR sensor elements.
The first sensor element 1 is provided with a first 10 and a second supply terminal 11. Via the supply terminals 10, 11, the first sensor element can be acted on by a supply voltage or a supply current. As a result, during operation of the angle sensor, a drop in the supply voltage in a direction along the flat dimension of the first sensor element 1, said direction being shown schematically and being denoted by the reference UVl, is produced in the first sensor element 1; this direction UVl is also the direction of the flow of the supply current in the first sensor element 1. Accordingly, the second sensor element 2 is provided with a first 20 and a second supply terminal 21 via which it can be acted on by a supply voltage and a supply current. As a result, during operation of the angle sensor, a drop in the supply voltage in a direction along the flat dimension of the second sensor element 2, said direction being denoted by the reference UV2, is produced in the second sensor element 2; this direction UV2 is also the direction of the flow of the supply current in the second sensor element 2. The first sensor element 1 is furthermore provided with a first 12 and a second tapping electrode 13, via which a first measurement voltage UMl is tapped off from the first sensor element 1 during operation. Accordingly, the second sensor element 2 is provided with a first 22 and a second tapping electrode 23, via which a second measurement voltage UM2 is tapped off from the second sensor element 2 during operation. The measurement voltages UMl, UM2 are shown schematically as arrows in Fig. 1.
Via their supply terminals 10, 11 and 20, 21, the sensor elements 1 and 2 are connected in a series connection with one another and with a common power source 30, said series connection being arranged in the illustrated example between a voltage source terminal 31 and ground 32. In this series connection, a first terminal of the power source 30 is
connected to the voltage source terminal 31 and a second terminal of the power source 30 is connected to the first supply terminal 10 of the first magnetoresistive sensor element 1. The second supply terminal 11 of the first magnetoresistive sensor element 1 is connected to the first supply terminal 20 of the second magnetoresistive sensor element 2 via a connecting line 33, and the second supply terminal 21 of the second magnetoresistive sensor element 2 is connected to ground 32. In this way, a common current which is denoted IS in Fig. 1 is passed through both sensor elements 1 and 2 during operation.
Fig. 2 schematically shows an example of embodiment of the use of an angle sensor with two sensor elements 1, 2 for angle measurement. Elements of the drawing which correspond to Fig. 1 are again provided with the same references.
In the block diagram shown in Fig. 2, a first and a second measurement voltage input 41 and 42 of an evaluation device 40 are connected to the first and second tapping electrode 12 and 13 of the first magnetoresistive sensor element 1 in order to supply the first measurement voltage UMl to the evaluation device 40. A third and a fourth measurement voltage input 43 and 44 of the evaluation device 40 are connected to the first and second tapping electrode 22 and 23 of the second magnetoresistive sensor element 2 in order to supply the second measurement voltage UM2 to the evaluation device 40. The measurement voltage inputs 41, 42, 43, 44 are designed to be high-ohmic, so that the sensor elements are not appreciably loaded at the tapping electrodes and thus no signal falsifications occur.
In the evaluation device 40, which is preferably designed in a manner shown in European patent application No. 04 102 117.1 of 14.05.2004, the measurement voltages UMl, UM2 are processed in the manner described in said application to form an output signal which is output during operation at an output 45, said output signal forming an output signal of an angle measurement system comprising the angle sensors 1, 2 and the evaluation device 40. This output signal is an information item regarding the angle, with respect to the angle sensor, of the component of the field lines of a magnetic field acting on the angle sensor which runs in the plane of the flat dimension of the sensor elements 1, 2. This component of the field lines is shown schematically in Fig. 2 by an arrow bearing the reference H.
In a computer simulation and in a measurement structure designed accordingly, a resulting angle error of approx. 0.1° could be achieved in a homogeneous external magnetic field with an angle sensor of the described type.
LIST OF REFERENCES
1 first magnetoresistive sensor element
2 second magnetoresistive sensor element
10 first supply terminal of the first magnetoresistive sensor element
11 second supply terminal of the first magnetoresistive sensor element
12 first tapping electrode of the first magnetoresistive sensor element
13 second tapping electrode of the first magnetoresistive sensor element
20 first supply terminal of the second magnetoresistive sensor element
21 second supply terminal of the second magnetoresistive sensor element
22 first tapping electrode of the second magnetoresistive sensor element
23 second tapping electrode of the second magnetoresistive sensor element
30 power source
31 voltage source terminal
32 ground
33 connecting line
40 evaluation device
41 first measurement voltage input of the evaluation device, connected to the first tapping electrode of the first magnetoresistive sensor element
42 second measurement voltage input of the evaluation device, connected to the second tapping electrode of the first magnetoresistive sensor element
43 third measurement voltage input of the evaluation device, connected to the first tapping electrode of the second magnetoresistive sensor element
44 fourth measurement voltage input of the evaluation device, connected to the second tapping electrode of the second magnetoresistive sensor element 45 output of the evaluation device
H component of the field lines of a magnetic field acting on the angle sensor which runs in the plane of the flat dimension of the sensor elements IS common current through the sensor elements
UMl first measurement voltage, tapped off via the first and second tapping electrode of the first sensor element UM2 second measurement voltage, tapped off via the first and second tapping electrode of the second sensor element UVl direction of the drop in the supply voltage or of the flow of the supply current along the flat dimension of the first sensor element UV2 direction of the drop in the supply voltage or of the flow of the supply current along the flat dimension of the second sensor element