GB2207510A

GB2207510A - Magnetic sensor

Info

Publication number: GB2207510A
Application number: GB08817238A
Authority: GB
Inventors: Emmanuel Cohen
Original assignee: MANCHESTER LASERS; University of Manchester Institute of Science and Technology (UMIST); University of Manchester
Current assignee: MANCHESTER LASERS; University of Manchester Institute of Science and Technology (UMIST); University of Manchester
Priority date: 1987-07-21
Filing date: 1988-07-20
Publication date: 1989-02-01
Anticipated expiration: 2008-07-20
Also published as: GB8817238D0; AU2081488A; WO1989000702A1; GB2207510B; GB8717201D0

Abstract

A magnetic sensor comprising a body defining two spaced apart limbs, a central member extending from an intermediate portion of the body to define two gaps with the limbs of the body, and a magnetic flux detector positioned between the central member and the said intermediate portion of the body. The body and central member being of a high permeability so as to provide low reluctance magnetic paths. A first pair of spaced apart permanent magnets are positioned on opposite sides of one gap, a second pair of spaced apart permanent magnets are positioned on opposite sides of the other gap, the polarities of the magnets of each pair are opposed so that magnetic fields exist in the spaces therebetween and so that the magnetic flux in the central member resulting from one pair of magnets is in the opposite direction to the magnetic flux in the central member resulting from the other pair of magnets. The output of the flux detector is monitored to detect differences in the magnetic permeability of the spaces defined between the two pairs of magnets.

Description

MAGNETIC SENSOR The present invention relates to a magnetic sensor and in particular to a magnetic sensor in the form of a differential magnetic bridge.

A wide range of transducers are known which enable various parameters to be monitored by monitoring changes in magnetic flux resulting from, for example, the movement of ferromagnetic cores.

One example is the well known differential transformer which can -be used to detect the position of a ferromagnetic core relative to the transformer.

Other transducers are known which rely upon the direct measurement of a magnetic field, the magnetic field being sensed by, for example, a Hall effect probe There are many applications, however, where it is desirable to be able to detect changes in magnetic permeability where the known devices are either not applicable or are difficult to apply given accurate calibration requirements.

A device is known which comprises a U-shaped permanent magnet having soft iron bodies positioned on the ends of the magnet. A flux detector was positioned between metal rods located midway between the legs of the permanent magnet and extending from the neutral point of the magnet to a third soft iron body positioned between the other two iron bodies.

Two gaps were defined between the third soft iron body and the other two iron bodies. The mass magnetic susceptibilities of chemicals placed in one of the gaps were measured by monitoring the imbalance of the resultant magnetic circuits.

The flux densities in the gaps were relatively low and the device was relatively large. As a result, the known device was not widely used.

Alternative approaches were suggested using two magnetic circuits and comprising a flux detector, the outputs of the detectors being subtracted one from the other. The size and sensitivity of the devices were still not found to be satisfactory.

It is an object of the present invention to provide a magnetic sensor which is able to readily monitor variations in the magnetic permeability of a medium.

According to the present invention, there is provided a magnetic sensor comprising a body defining two spaced apart limbs, a central member extending from an intermediate portion of the body to define two gaps with the limbs of the body, the body and central member being of a high permeability so as to provide low reluctance magnetic paths, a magnetic flux detector positioned between the central member and the said intermediate portion of the body, a first pair of spaced apart permanent magnets positioned on opposite sides of one gap, a second pair of spaced apart permanent magnets positioned on opposite sides of the other gap, the polarities of the magnets of each pair being opposed so that magnetic fields exist in the spaces therebetween and so that the magnetic flux in the central member resulting from one pair of magnets is in the opposite direction to the magnetic flux in the central member resulting from the other pair of magnets, and means for monitoring an output of the flux detector to detect differences in the magnetic permeabilAity of the spaces defined between the two pairs of magnets.

Preferably, the magnets are rare earth magnets, e.g. neodymium-boron-iron, or samarium-cobalt, as such maGnets of small size can provided a high flux density.

The present invention thus provide a differential magnetic bridge which is sensitive to any differences in the permeability of the spaces between the magnets. The bridge can be balanced either electrically or by mechanical adjustment, so as to be highly sensitive to relatively small changes in the permeability of the space defined between one of the pairs of magnets.

The body may be generally U-shaped defining two parallel spaced apart limbs. The central member may be of stibstantially the same length as each of the limbs and terminate in a wedge-shaped tip located close to an intermediate portion of the U-shaped body. A detector, for example, a Hall effect probe, can be positioned between the central member and the body.

When the medium occupying the spaces defined by the pairs of magnets is the same, the bridge can be easily balanced, for example, by electrically inducing compensating magnetic fields in one or both of the body limbs or by mechanical adjustment of, for example, iron screws inserted into threaded bores defined in the limbs. Once balanced, the output of the Hall effect probe indicates no magnetic field.

The insertion of a different medium into one of the spaces defined between the magnets will then disturb the balance of the magnetic circuit, enabling an output to be derived from the Hall effect probe which is directly related to the change in permeability.

Alternatively, the bridge may be rebalanced by, for example, adjusting the current supplied to a coil mounted on one of the limbs and monitoring the change in permeability by the change in current through that coil.

The facing surfaces of the pairs of magnets may taper towards or away from the intermediate portion of the body, and the body may define two pairs of spaced apart limbs, a central member and detectors being located between each pair.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic sectional view of a first embodiment of the invention; and Figs. 2, 3, 4 and 5 are schematic sectional views of second, third, fourth and fifth embodiments of the invention, the same reference numerals being used for equivalent components in Figs. 1 to 5.

Referring to Fig. 1, the illustrated sensor comprises a body 1 which in section is U-shaped, defining two spaced apart limbs 2. A central member 3 is positioned midway between the limbs 2 and extends to a wedge-shaped tip adjacent a portion of the body intermediate to the limbs 2. A Hall effect magnetic flux detector 4 is sandwiched between the wedge-shaped tip of the central member 3 and the body 1.

A first pair of permanent magnets 5, 6 is arranged in the gap defined between one of the limbs 2 and the central member 3. A second pair of permanent magnets 7, 8 is positioned in the gap defined between the central member 3 and the other limb 2. Each pair of permanent magnets is supported one on the central member 3 and the other on a limb 2, the polarities of the permanent magnets being such that a magnetic field exists between the adjacent poles and such that the flux in the central member resulting from one pair of magnets is in the opposite direction to the flux resulting from the other pair.

For example, the north pole of the magnet 5 can face the south pole of the magnet 6, and the north pole of the magnet 7 can face the south pole of the magnet 8. Two spaces 9 and 10 are defined between the pairs of permanent magnets.

The body 1 and central member 3 are fabricated from a high permeability material, for example iron.

Hitherto it was appreciated that conventional permanent magnets would have a length to thickness ratio that would be large and, if inserted in a structure such as that illustrated in the accompanying drawings, would make the whole differential bridge large, cumbersome and unwieldy for many applications. However, the powerful rare earth magnets which are now available which are of high coercive force and magnetic energy are such that demagnetization effects are negligible. The emphasis is that the rare earth magnets have a very small length to thickness ratio. Thus, despite the unfavourable configuration of the magnetic circuit, high flux densities exist in the spaces 9 and 10.

Suitable materials for the permanent magnet are, for example, neodymium-boron-iron or samarium-:obalt. In the arrangement illustrated, the magnetic circuit is symmetrical and accordingly the flux densities of the magnetic fields in the gaps 9 and 10 are substantially the same. This means that the flux density in the central member 3 is substantially zero. In practice, it will not generally be possible to manufacture a sensor which is perfectly balanced and accordingly, it is necessary to correct for any small imbalance. One possible method is to energise balancing coils 11 and 12 mounted on the limbs 2, so as to inject appropriate ampere-turns to annul the imbalance. An alternative method would be to drill and tap holes in the iron adjacent to the two outside magnets 5 and 8 and to insert iron screws into the tapped holes.Adjustment of the length to which the screws penetrate the holes will enable the circuit to be balanced.

It will be appreciated that the body 1 and central member 3 are a relatively large cross-sectional area to reduce the reluctance of the magnetic path.

Once balance has been achieved, the introduction of a body into, for example, the gap 10, results in the magnetic permeability in the gap 10 being different from that in the gap 9. This will unbalance the bridge and the degree of imbalance will be a measure of the change in the magnetic permeability of the gap 10. Hence, the magnetic susceptibility of the body introduced into the gap 10 can be measured by monitoring the output of the probe 4.

The described arrangement can be used for many applications, for example measuring the mass susceptibility of blood taken from leukaemia patient, determining the iron content of glass, and determining the mass susceptibility of diamagnetic and paramagnetic materials. It is also apparent that the approach to one of the gaps defined between the permanent magnets of a ferromagnetic material will cause the bridge to move rapidly out of balance.

Another possible application is in the measuring of water flow. If one of the spaces is occupied by a non-ferrous tube through which water is passing, then as the water has finite electrical conductivity, eddy currents will be induced in the water giving rise to magnetic fields which by Lenz's Law will tend to oppose the field creating them. The net effect is that the bridge becomes unbalanced and the degree of imbalance is a measure of the flow rate of the water. Such an arrangement could also be used to measure the speed of vessels moving through water.

The illustrated device could also be used in the measurement of the flow rate of domestic fuel gas.

It is, however, necessary to alter the magnetic neutrality of the gas, but this can be achieved by passing the gas through an intense non-homogeneous magnetic field so as to temporarily endow the gas with a magnetic movement. The bridge is then unbalanced and as described before, the degree of imbalance is a measure of the flow rate.

A device is known whereby electric power may be conventionally~ measured using. a Hall effect device.

However, embodiments of the present invention may be adapted to measure this parameter by passing the load current conductor of the electric power supply through one of the gaps and using electronic feedback arrangements to balance the bridge. Thus the future needs of the gas, water and electricity supply utilities for a common metering approach can be met with comparative low cost benefits.

Still further applications are possible in, for example, displacement transducers where a moving member traverses the magnetic fields in one of the gaps to lead to an increasing disturbance of the balance of the bridge, A further application is in the assessment of the thickness of thin films of, for example, nickel deposited on a plastic substrate, or on a non-ferrous metallic substrate. This could be used in, for example, production processes where the product is caused to pass through one of the gaps defined by the sensor.

The sensor can also be used to monitor the quality of produced materials, for example, stainless steel which is substantially non-ferromagnetic, together with any other future applications which suggest themselves to engineers.

The illustrated embodiment of the invention shows four independent magnets arranged in pairs to define the two gaps of the bridge. Each of the magnets shown in the illustration could be replaced however by a stack of magnets enabling various magnet sizes and strengths to be provided using a common small size magnet as the basic building block. For example each of the single magnets shown in the drawing could be replaced by a stack of two magnets.

In the embodiment of Fig. 1, the facing surfaces of the permanent magnets are substantially parallel.

This results in relatively small flux leakage. There are circumstances however where flux leakage is desirable.

Fig. 2 shows an arrangement in which the facing surfaces of the magnets 5, 6 and 7, 8 taper inwards towards the intermediate portion of the body. This results in leakage magnetic flux extending widely outside the magnetic system. The resultant remote magnetic field will now interact with any external magnetic field, e.g. that resulting from movement of an intruder or material in the remote magnetic field. Thus the sensor has applications in intruder alarm systems. Tests have shown that particularly high sensitivities are obtainable if the angle A is in the range of 80 to 100 degrees.

Alternatively, as shown in Fig. 3, the facing surfaces of the magnets can taper away from the intermediate portion of the body. A re-entrant magnet arrangement of this type has applications in for example electricity, gas, and water flow measurement. For example, the pair of magnets on one side of the sensor can be arranged in contact with a pipe conveying water positioned between the central marker 3 and one limb of the body, the pipe diameter being greater than the minimum spacing between the magnets. The sensor can thus be used with large diameter pipes and still provide high sensitivity.

As a still further alternative, as shown in Fig.

4, one pair of magnets can taper towards the body and the other taper away from the body. This maximises the differentiation between the two magnetic flux paths affecting the detector.

Finally, as shown in Fig. 5, two or more bridges could be fabricated from a single flat piece of high permeability material, as shown in Fig. 5.

Claims

1. A magnetic sensor comprising a body defining two spaced apart limbs, a central member extending from an intermediate portion of the body to define two gaps with the limbs of the body, the body and central member being of a high permeability so as to provide low reluctance magnetic paths, a magnetic flux detector positioned between the central member and the said intermediate portion of the body, a first pair of spaced apart permanent magnets positioned on opposite sides of one gap, a second pair of spaced apart permanent magnets positioned on opposite sides of the other gap, the polarities of the magnets of each pair being opposed so that magnetic fields exist in the spaces therebetween and so that the magnetic flux in the central member resulting from one pair of magnets is in the opposite direction to the magnetic flux in the central member resulting from the other pair of magnets, and means for monitoring an output of the flux detector to detect differences in the magnetic permeability of the spaces defined between the two pairs of magnets.

2. A magnetic sensor according to claim 1, wherein the magnets are rare earth magnets.

3. A magnetic sensor according to claim 1 or 2, comprising one or more electrical coils supported on the body or the central member, and means to energise the or each coil to balance the magnetic fluxes of the two pairs of magnets.

4. A magnetic sensor according to claim 1 or 2, comprising one or more iron screens supported on the body, the position of the or each screens relative to the body being adjustable to balance the magnetic fluxes of the two pairs of magnets.

5. A magnetic sensor according to any preceding claim, wherein the body is U-shaped, defining two parallel spaced apart limbs.

6. A magnetic sensor according to claim 5, wherein the central member is of substantially the same length as each of the limbs and terminates in a wedge-shaped tip located close to an intermediate portion of the U-shaped body, the detector being positioned between the tip of the central member and the body.

7. A magnetic sensor according to any preceding claim, wherein the facing surfaces of both pairs of permanent magnets are substantially parallel.

8. A magnetic sensor according to any one of claims 1 to 6, wherein the facing surfaces of both pairs of permanent magnets taper outwards away from the intermediate portion of the body.

9. A magnetic sensor according to any one of claims 1 to 6, wherein the facing surfaces of one pair of permament magnets taper outwards away from the intermediate portion of the body and the facing surfaces of the other pair of permanent magnets taper inwards towards the intermediate portion of the body.

10. A magnetic sensor according to -any one of claims 1 to 6, wherein the facing surfaces of both pairs of permanent magnets taper inwards towards the intermediate portion of the body.

11. A magnetic sensor according to any preceding claim, comprising a single body defining two pairs of spaced apart limbs, a said central member and detector being positioned between each pair of spaced apart limbs.

12. A magnetic sensor substantially as hereinbefore described with reference to Fig. 1, Fig.

2, Fig. 3, Fig. 4 or Fig. 5 of the accompanying drawings.