WO1993007505A1

WO1993007505A1 - Device for determining the strength of a magnetic field

Info

Publication number: WO1993007505A1
Application number: PCT/EP1992/002288
Authority: WO
Inventors: Norbert Normann; Gunter Schulze; Ralf Kessler
Original assignee: Doduco Gmbh + Co. Dr. Eugen Dürrwächter
Priority date: 1991-10-04
Filing date: 1992-10-03
Publication date: 1993-04-15
Also published as: DE4200156A1; EP0606313A1

Abstract

In order to determine the strength of a magnetic field of a predetermined direction there are a magnetic field sensor (1) and an electric assessment circuit (2) connected thereto. The magnetic field sensor (1) is a fixed arrangement of an electromagnet (3) and an electric resistor (4) in series therewith having a magnetic field-dependent conductivity (magnetoresistor). The assessment circuit (2) has a controllable current source (7) powering the electromagnet (3). The resistance of the magnetic field sensor (1) is measured and kept at a constant value by a control circuit (10). A tapping circuit (14) taps a signal proportional to the current I and takes it to the assessment circuit (2, 9) for further processing or for display.

Description

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Device for determining the strength of a magnetic field

The invention relates to a device for determining the strength of a magnetic field from a predetermined direction with a magnetic field sensor and with an associated electrical evaluation circuit. 5

It is known to use a Hall sensor as a magnetic field sensor. For example, EP-B-0 263 775 shows a tire pressure monitor in which a bellows is slidably mounted in the rim ring of the wheel and is acted upon from the inside by the tire's air pressure, parallel to the tire

Extends the wheel axle and carries a permanent magnet at its outer end. Since the expansion of the bellows from the air pressure

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depends, the position of the permanent magnet depends on the air pressure. The permanent magnet rotates past a Hall sensor attached to the wheel suspension, which emits a signal which is dependent on the magnetic flux and thus on the position of the permanent magnet and which is fed to an electrical evaluation circuit.

The disadvantage here is that the Hall sensor is an active component and therefore requires an energy supply. That is why at least three lines emanate from the Hall sensor. Today, however, vehicles already contain so many electrical and electronic components that it is a major problem for the engineers to accommodate the cable harnesses required for this. There is therefore a need to keep the number of electrical lines as small as possible. Another disadvantage of the Hall sensors is that they are sensitive to temperature. Their working range is limited to temperatures from -40 to approximately + 120 ° C-150 ° C. In applications on vehicle wheels, however, higher temperatures can occur because the nearby brake discs temporarily radiate considerable heat.

The present invention is therefore based on the object of providing a device of the type mentioned at the outset, which does not require more than two supply lines and withstands higher temperatures than a Hall sensor.

This object is achieved by a device with the features specified in claim 1. Advantageous developments of the invention are the subject of the dependent claims.

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According to the invention, a fixed arrangement of an electromagnet and an electrical resistor connected in series therewith with a magnetic field-dependent conductivity (magnetoresistor) is used as the magnetic field sensor. 5 Such a magnetic field sensor manages with two supply lines and is suitable for use in temperature ranges from -40 ° C to at least + 200 ° C. It is also advantageous that magnetoresistors have a very low price compared to Hall sensors; the disadvantage with them, however, is that

10 the dependence of their electrical resistance on the magnetic flux follows a quadratic characteristic and is therefore strongly non-linear. Their resistance increases from their basic resistance R _Q , which is present in the absence of a magnetic field, with increasing magnetic flux with the square

15 of the amount of magnetic flux. Another disadvantage is that magnetoresistors of a series differ greatly in their basic resistance R_: scattering of the basic resistance by 20% is normal. Therefore, magnetoresistors cannot easily be used for accurate magnetic field measurements,

Rather, they are typically used as differential probes for measuring changes in the magnetic field, but not as probes for determining the absolute value of magnetic fluxes.

According to the invention, the magnetoresistor is now operated in a manner which nevertheless enables a very precise magnetic field measurement. For this purpose, the magnetoresistor is first coupled to an electromagnet and connected in series with it. The field of the electromagnet

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sets the magnetoresistor and shifts its resistance from the basic resistance R_ along the resistance-magnetic flux characteristic to a higher resistance R, which is also referred to below as the operating point of the magnetic field sensor. The working point is on an increasing branch of the parabolic curve. Due to the increase in steepness of the characteristic curve, the sensitivity of the magnetic field sensor also increases. An external magnetic field to be measured strengthens or weakens the self-generated field of the magnetic field sensor, depending on whether it is the same as or opposite to the field of the electromagnet. The resistance of the magnetoresistor thus shifts out of the operating point R ₌ along the characteristic curve. However, due to the parabolic shape of the characteristic, the measurement is strongly non-linear.

In order to compensate for the non-linearity, it is provided that the evaluation circuit contains a control circuit which regulates the resistance of the magnetoresistor constantly to the value R by controlling the voltage u or the current I of the current source feeding the magnetic field sensor. For this purpose, current and voltage are continuously measured at the magnetic field sensor and the resistance R is determined therefrom by means of a dividing device. From the deviation of the determined resistance from the working point

a manipulated variable is derived which controls the current flow through the magnetic field sensor so that its self-generated magnetic field compensates for the influence of the external magnetic field on the magnetoresistor and thereby its resistance to the operating point Rα. leads back.

REPLACEMENT LEAF In a control circuit functioning in this way, the current I is linearly dependent on the magnitude of the magnetic flux of the external field at the location of the magnetoresistor. If you tap an electrical signal proportional to I, you get a measurand for this magnetic field.

The circuit which picks up this signal can either bring it to further processing in the evaluation circuit or display it.

In a development of the invention, a temperature drift of the magnetic field sensor can be compensated for by providing a reference magnet which generates a magnetic field of known strength and orientation at the location of the magnetic field sensor. From time to time, the magnetic field sensor is exposed to the known magnetic field of the reference magnet instead of the magnetic field to be determined. If it is established that the signal supplied by the tapping circuit and proportional to the manipulated variable, which signal originates from the influence of the reference magnet, is of a normal value, e.g. determined at a temperature of 20 ° C, then the next time the unknown magnetic field is determined, the signal proportional to the manipulated variable is compensated by the drift previously determined by means of the reference magnet and the temperature influence is thereby compensated. Other influences falsifying the measurement result, e.g. an aging can be compensated in this way.

Basically, an air coil can be used as an electromagnet. However, in order to get by with smaller currents, it is preferred

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to arrange the coil on a core with higher magnetic susceptibility μ. It is best to use a cylindrical coil with a rod core, on one end of which the magneto resistor is attached. In the case of an air coil, the magnetoresistor could also be connected to a non-magnetic, e.g. Attach ceramic core that is sufficiently heat-resistant and can also serve as a support for the coil. A field plate magnetic sensor is preferably used as the magnetoresistor, in particular one whose magnetic field sensitive material contains a semiconductor based on indium antimonide / nickel anti onide.

The device according to the invention is preferably used in a tire pressure monitor as described in EP-B-0 263 775: the magnetic field sensor is fixed in place on part of the wheel suspension and is oriented parallel to the wheel axle. On the rim ring of the wheel there is a permanent magnet at the end of a bellows which is acted upon by the tire pressure on the inside. The bellows and the permanent magnet are oriented parallel to the wheel axis, so that the distance of the permanent magnet from the magnetic field sensor, if it rotates past it, depends on the tire pressure. Two lines lead from the magnetic field sensor into the interior of the vehicle to the evaluation circuit, which is preferably located in the area of the dashboard. A reference magnet provided for the temperature compensation is fixed in place on the rim ring, namely at such a distance from the magnet on the bellows that its fields practically do not influence one another; in front-

the reference magnet is preferably located at a position of the rim ring diametrically opposite the bellows.

In a further development of the invention, a magnetic field sensor can also be used

Nuclear coil can also be used to detect soft magnetic materials. Such a sensor itself generates a magnetic field and has a magnetic circuit, the permeability U of which determines the proportionality factor between the coil current I and the generated magnetic field. Since this μ. is increased by the presence of soft magnetic material near the sensor, the control loop reacts to this material by lowering the current I.

There is a preferred application of this development in a motor vehicle anti-lock braking system as a sensor for detecting the teeth of a rotating ring gear on the anti-lock wheel.

An embodiment of the invention is illustrated in the accompanying drawings and is described below.

Figure 1 is a block diagram of an apparatus for

Determining the strength of a magnetic field with a magnetic field sensor and with an evaluation circuit,

FIG. 2 shows the structure of the magnetic field sensor in detail,

Figure 3 shows a typical example of the course of the characteristic of the magnetic field sensor, which shows the change in resistance depending on the magnetic

Power flow indicates

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Figure 4 shows the arrangement of the magnetic field sensor on a wheel suspension, and

FIG. 5 shows in detail the interaction of the magnetic field sensor with a pressure sensor on the vehicle wheel.

The device shown in FIG. 1 consists of a magnetic field sensor 1 and an evaluation circuit 2 connected to it. The magnetic field sensor consists of an electromagnet 3 and a magnetoresistor 4, in particular a field plate magnetic sensor, the resistance R of which is square of the amount of the magnetic flux B (FIG. 3) changes.

The electromagnet 3 consists of a cylindrical coil 6 on a rod-shaped core 5, at one end of which the magneto-resistor 4 is attached.

The evaluation circuit 2 contains a controllable current source 7, which feeds a current I into the magnetic field sensor 1, through which a magnetic force flow B is generated at the location of the magnetoresistor 4, so that the resistance R of the magnetoresistor via the basic resistance R _Q , that exists in the absence of a magnetic field increases. The voltage U of the current source 7 is controlled so that the resistance R assumes a predetermined value R 3, which is referred to here as the operating point (see

Figure 3). For this purpose, a divider circuit 8 is provided which, on the one hand, taps the voltage U of the controllable current source 7 and, on the other hand, supplies a measurement signal proportional to the current intensity I. By dividing the voltage value ü by the current value I, the dividing

REPLACEMENT LEAF circuit the resistance R of the magnetic field sensor and transmits the result to a microcomputer 9 and to a controller 10. The microcomputer 9 also receives a voltage signal proportional to the current I, digitizes the resistance signal transmitted to it in an analog / digital converter 11 and the signal representing the current value I in an analog / digital converter 14, checks the resistance value and the current value for plausibility and gives the controller 10 a new one if the measuring range is exceeded or undershot via a digital / analog converter 12 Working point R ₌ des

Magnetic field sensor before. From the comparison of R and R, the controller 10 derives a manipulated variable S with which the voltage u of the controllable current source 7 is set such that the magnetic field sensor 1 has the resistance R 3.

If there is only the magnetic field generated by the electromagnet 3 itself at the location of the magnetoresistor 4, but no external magnetic field, then R = R and S = 0. However, if the magnetic field generated by the electromagnet 3 itself becomes an external magnetic field 13 superimposed, then the resistance of the magnetic field sensor is either greater or less than R, depending on whether the outer magnetic field 13 is rectified or opposed to the magnetic field generated in the magnetic field sensor itself. The changed resistance is determined by the dividing circuit 8, communicated to the controller 10 as the actual value, which then forms a manipulated variable S by comparison with the setpoint value R 3, by means of which the voltage of the controllable current source 7 is changed such that the resistance of the magnetic field sensor again assumes the value R_. The manipulated variable S is proportional to the force flow of the external magnetic field 13 at the location of the magnetoresistor 4. Die

The manipulated variable is therefore a further input of the microcomputer

REPLACEMENT LEAF 9 supplied, there digitized in an analog / digital converter 14 and evaluated according to a predetermined program. The result of the evaluation can be shown numerically on a display. However, it is also possible to control an analog display, for example a signal lamp 16, via a further digital / analog converter 15, which can indicate that a limit value has been reached.

If, instead of the external field 13 to be determined, the magnetic field sensor 1 is exposed from time to time to the known magnetic field of a permanent magnet 17 by approaching the permanent magnet 17 from time to time to the magnetic field sensor 1, one can see from the comparison of the measured force flow that that of the permanent magnet 17 goes out, determine and compensate for a possible temperature drift of the magnetic field sensor with its known, actual power flow.

Figure 4 shows the arrangement of such a magnetic field sensor for monitoring the pressure in pneumatic tires of vehicles. The pneumatic tire 20 sits on the rim 21 of a wheel 22 which is screwed to a hub 23 which is part of a wheel suspension 24. A pressure sensor 25 is installed in the rim 21 parallel to the wheel axis 26, which projects with one end into the air space 27 of the tire and with its other end lies outside the air space 27. The structure of the pressure sensor is shown in detail in FIG. 5. It consists of a tube 28, at the end of which is located in the air space 27, the open end of a bellows 29 is fastened, which inside is subjected to the pressure P in the air space 27. The opposite end of the bellows 29 is closed and carries a permanent magnet 30, the position of which in

T Pipe 28 depends on the pressure P. At the same center distance as the pressure sensor 25, a magnetic field sensor according to the invention is attached to part of the wheel suspension 24, namely to a spring strut 31, on which the magnet 30 moves once with each revolution of the wheel 22. The pressure P can be determined from the measured force flow of the magnet 30 at the location of the magnetic field sensor 1.

At a diametrically opposite location of the rim 21 of the pressure sensor 25, a permanent magnet 17 is firmly attached as a reference magnet, which enables temperature compensation. Changes in the ambient temperature, but also heat, which is temporarily emitted by the brake disk 32 and heats the magnetic field sensor 1, do not lead to a falsification of the measurement result in this way.

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Claims

Claims:

1. Device for determining the strength of a magnetic field of a predetermined orientation with a magnetic field sensor (1) and with an associated electrical evaluation circuit (2),

characterized in that the magnetic field sensor (1) is a fixed arrangement comprising an electromagnet (3) and an electrical resistor (4) (hereinafter referred to as a magnetoresistor) connected in series therewith and having a magnetic field-dependent conductivity,

that the evaluation circuit (2) has a current source (7) which feeds the electromagnet (3) and which, based on its basic resistance R in the absence of magnetic flux (B = 0) on the R (B) - the resistance R of the magnetoresistor (4) - Shifts characteristic to a working point with higher resistance R 3,

that the evaluation circuit (2) contains a control circuit (10), which by controlling the voltage U or the

Current I of the current source (7) regulates the resistance R of the magnetoresistor (4) constantly to the resistance value R 3.

and that a tapping circuit (14) is provided which taps a signal proportional to the current I and for further

REPLACEMENT LEAF Offers processing in the evaluation circuit (2, 9) or for display (16).

2. Device according to claim 1, characterized in that a reference magnet (17) is provided which generates a magnetic field of known strength and orientation at the location of the magnetic field sensor (1),

and that means (22) are provided which expose the magnetic field sensor (1) from time to time to the known magnetic field of the reference magnet (17) instead of the magnetic field to be determined,

and that a correction circuit (9) is provided, which corrects the tapped signal, which is proportional to the manipulated variable (S), in accordance with a determined deviation of the manipulated variable (S) occurring when determining the reference field from a normal value.

3. Device according to claim 1 or 2, characterized in that the electromagnet (3) has a cylindrical coil (6) on a rod core (5) and that the magnetoresistor (4) is arranged at one end of the rod core (5).

4. Device according to one of the preceding claims, characterized in that the electromagnet (3) and the reference magnet (17) are arranged parallel or antiparallel to each other.

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5. Device according to one of the preceding claims, da¬ characterized in that the magnetoresistor (4) is a field plate magnetic sensor.

6. The device according to claim 5, characterized in that the magnetoresistor (4) as a magnetic field sensitive material contains a semiconductor based on InSb / NiSb.

7. Use of a device according to one of the preceding claims as part of a tire pressure monitor, which pressure sensor (25) attached to the wheel (22) with a permanent magnet (30), the distance from the wheel axis (26) is constant, the position of which is parallel to the wheel axis (26) but depends on the tire pressure (P) and has a magnetic field sensor (1) which is arranged in a constant position on the wheel suspension (24, 31).

8. Tire pressure monitor according to claim 7 and claim 2, characterized in that the reference magnet (17) is arranged at a point of the wheel (22) diametrically opposite the pressure sensor (25).

9. Tire pressure monitor according to claim 7 or 8, characterized ge indicates that the magnets (3, 17, 30) are oriented parallel to the wheel axis (26).

10. Use of a device according to one of claims 1 to 6 for the detection of ferromagnetic objects with the proviso that the electromagnet (3) has a ferromagnetic core (5), in particular made of soft iron.

11. Use according to claim 10, characterized in that the objects are the teeth of a ring gear on an anti-lock motor vehicle wheel.

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