GB2315333A

GB2315333A - Magnetic sensor

Info

Publication number: GB2315333A
Application number: GB9714035A
Authority: GB
Inventors: Werner Fischer; Juergen Moessinger; Matthias Schnell
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1996-07-16
Filing date: 1997-07-02
Publication date: 1998-01-28
Also published as: JPH10115502A; GB9714035D0; FR2751408A1; DE19628566A1

Abstract

The invention relates to a sensor for detecting changes in a magnetic field, the said sensor having a field plate (1) which has resistors (3) made of magnetoresistive material which are mounted on a substrate (2) in a meander-shaped manner and the resistance value of which depends upon the strength and direction of the magnetic field passing through them. The magnetic field can be influenced by an external metal object (5). The metal object (5) is preferably a wheel which is provided with regularly arranged depressions (6) which can be guided successively past the field plate (1), under which circumstances the lines of force of a magnet (4) arranged behind the sensor are intersected.

Description

2315333

Sensg-r which is sensitive to a maggetic field

Prior art

The invention relates to a sensor of the type in the main claim, which is sensitive to a magnetic field.

A device is already known from EP 0 427 882 B 1, in which a permanent magnet is moved, as the moving body, III the region of a sensor which is sensitive to a magnetic field. In this known device, the movement of the body extends parallel to the plane of the sensor at a distance which is predetermined by the design. In this instance, there is present as a stationary sensor, a so-called "magnetoresistive sensor" which detects, in the plane of its sensitive layer and using the anisotropic magnetoresistive effect (AMR), changes in the field strength or flux as a result of movement of the magnet.

The sensor of the known device consists of a number of resistors sensitive to a magnetic field, which are connected to form a bridge in each case and the resistance values of which change, when there is a passing-through of lines of magnetic force in their sensitive plane, and thus lead to detuning of the bridge. The bridge output signal can thus be used as a detection signal.

Advantages of the invenAon The sensor of the type initially described is advantageous, in the finther development according to the invention having the characterising features of claim 1, because of the fact that the path of the lines of force can be periodically varied or modulated in a simple marmer, in the course of a passing movement, by the depressions on the surface of the object to be subjected to detection. In the event of the sensor being used as a sensor for angles of rotation, it is preferably possible to determine the rotational speed and also the simple change in the angle of rotation of a Wheel, if the depressions are located on the outer surface of the wheel and are moved past the field plate of the sensor.

A path of the lines of force which can readily be detected emerges if the thickness of the layer of substrate of the field plate is less than 0.2 mm in the case of ferrite, or is about 0.05 mm in the case of silicon. The thin substrate permits a small distance between the magnets on the field plate and the surface to be subjected to detection, a fact which results in a higher magnetic induction in this region and thus an improved resolution in the modulation of the lines of force.

The use of mildly magnetic materials as the substrate, for example pure iron, steel, or SiFe, CoFe or NiFe alloys, is also particularly advantageous. Located between this substrate and the magnetoresistive resistor elements is an electrically insulating layer. In the case of this substrate, with a high saturation flux density, the share of the total flux accounted for by the modulated magnetic flux rises; this advantageously enlarges the percentage modulation of the field at the location of the magnetoresistive resistors, and the output signal of the detector arrangement.

According to another advantageous form of embodiment, the magnetoresistive resistors are mounted directly on the magnets via a thin insulating layer. As a result of this, a further reduction in distance is achieved and assembly is also made easier.

Drawings 3 Exemplified embodiments of a sensor according to the invention will be explained with reference to the drawings, in which:

Figure I shows a basic representation of a sensor with a wheel as the modulator or pulse-generator; Figure 2 shows an exemplified embodiment of a sensor with a mildly magnetic substrate, and Figure 3 shows an exemplified embodiment with resistors belonging to the sensor which are attached directly to the magnet.

Descripon of the exeMlified embodiments Figure I shows a field plate I which consists of magnetoresistive resistors 3 mounted on a substrate 2. Located on the field plate I is a magnet 4 the lines of force of which intersect the field plate I and a metal object 5 lying opposite the said field plate 1. In the exemplified embodiment represented, the metal object 5 is a wheel which is provided, on its outer surface, with depressions 6 which are arranged periodically behind one another. When the wheel 5 is guided, together with its depressions 6, past the field plate I as a result of rotation, the magnetic flux of the magnet 4 is also modulated in the region of the field plate I during the running-through of the depressions 6, since the lines of force or the magnetic flux adopt a changed course at this point.

The evaluation of this change in flux takes place in the known manner described in EP 0 427 882 B I via the magnetoresistive resistors 3 which are located on the field plate I III a meander-shaped manner and the resistance value of which likewise changes when there is a change in flux. In order to increase the resolution of the signal to be evaluated, ferrites having a thickness of less than 0.2 min or silicon having a thickness of about 0.05 min are particularly suitable as

4 particularly thin materials for the substrate.

An expanded form of embodiment of the field plate I is represented in figure 2. Here, the substrate 2 of the field plate I is made of a mildly magnetic material such as, for example, pure iron, steel, or SiFe, CoFe or NiFe alloys. With the high saturation flux density of this material, there is a distinct rise in the share of the total flux accounted for by the modulated flux. In this exemplified embodiment, an electrically insulating layer 9 is arranged between the substrate 2 and the resistors 3.

According to figure 3, the magnetoresistive resistors 3 are attached directly to the magnets 4 via the insulating layer 9.

Claims

Patent claims

1. Sensor for detecting changes in a magnetic field, having - resistors (3) which are made of magnetoresistive material and the resistance value of which depends upon the strength and direction of the magnetic field passing through them, which magnetic filed can be influenced by an external metal object and having - a detector arrangement, the output signal of which depends upon the size of the change in resistance, characterised in that - the metal object (5) is provided with regularly arranged depressions (6) which can be guided successively past the field plate (1) and which intersect the lines of force of a magnet (4) arranged behind the sensor.

2. Sensor according to claim 1, characterised in that - the metal ob ect is a wheel (5) and the depressions (6) are arranged on the outer j periphery of the said wheel (5).

3. Sensor according to claim 1 or 2, characterised in that - the resistors (3) are mounted, in a meander-shaped manner, on a substrate (2) on a field plate (1), the said substrate (2) being ferrite having a thickness of less than 0.2 mm.

4. Sensor according to claim 1 or 2, charsderised in that - the substrate (2) is silicon having a thickness of about 0.05 nun.

5. Sensor according to claim 1 or 2, characterised in that - the substrate (2) is a mildly magnetic material having a high saturation flux 6 density, and that - an electrically insulating layer (9) is arranged between the substrate (2) and the resistors (3).

6. Sensor according to claim 1 or 2, characterised in that the meander-shaped resistors (3) are mounted directly on the magnets (4) via an insulating layer (9).

7. A sensor for detecting changes in a magnetic field substantially as herein described with reference to Figures 1 and 2, or Figure 3 of the accompanying drawing.