GB2515817A

GB2515817A - The measurement of magnetic fields in localised regions of space

Info

Publication number: GB2515817A
Application number: GB1312078.7A
Authority: GB
Inventors: John Dudding; Robin Cornelius
Original assignee: Hirst Magnetic Instruments Ltd
Current assignee: Hirst Magnetic Instruments Ltd
Priority date: 2013-07-05
Filing date: 2013-07-05
Publication date: 2015-01-07
Also published as: GB201312078D0

Abstract

A parameter of a rapidly varying magnetic field in a localised region of space, e.g. the residual magnetism in a ferrous object, is measured using an array of sensors spaced at known positions around the region. The sensors may be Hall sensors 302 in a linear array forming a probe 300 which is moved along the surface of the object 304. Alternatively, a single sensor may be moved to simulate an array. Three mutually orthogonal arrays may also be used. The probe measures the magnitude and direction of the magnetic field at known or determinable distances from the region of space and determines therefrom the parameter of the magnetic field in the region of space. The parameter may be displayed visually or audibly e.g. if it exceeds a threshold.

Description

THE MEASUREMENT OF MAGNETIC FIELDS IN LOCALISM) REGIONS

OF SPACE

The* present invention relates to the quantification of non-homogeneous magnetic fields and more specifically to the measurement of residual magnetism in ferrous or other magnetic artefacts.

The processes involved in the manufacture of ferrous artefàcts often result in them becoming magnetised. For the purposes of this specification, this magnetisation is referred to as residual magnetism. For many purposes, this residual magnetism is of little consequence, but in some cases, it is undesirable and efforts are made to remove it, or at least to reduce it to below a specified value. Before this can be done, it is necessary to characterise the magnetic field in the region surrounding the artefact concerned. There are a number.of techniques and instruments for doing this, but existing techniques use instruments which are intended to measure homogeneous magnetic fields. However, magnetic fields resulting from residual magnetism may be very non-homogeneous.

It is an object of the present invention to provide a method of and apparatus for measuring magnetic field parameters in localised regions of space.

According to the present invention there is provided a method of determining a characteristic of a magnetic field in a localised region of space, comprising the operations of measuring the magnitude of the magnetic field at a plurality of positions at known distances and orientations from the localised region of space in which the characteristic of the magnetic field is to be determined, deriving therefrom a function representative of the rate of variation of a parameter of the magnetic field with distance and direction from the said region of space and determining therefrom the characteristic of the magnetic field in the said region of space.

The determination of the said characteristic of the magnetic field can be achieved by the mathematical technique known as curve fitting, or, generally, utilising Maxwell's equations Also according to the present invention there is provided an apparatus for determining a characteristic of a magnetic field in a localised region of space, comprising a probe housing including an array of individual magnetic field sensors each adapted to produce a signal related to the magnitude of he magnetic field at the location of the sensor, a central signal processing unit to which the signals from the said sensors are applied and an output signal generator adapted to produce a signal indicative of the characteristic of the magnetic field in the said localised region of space, wherein the central signal processing unit is adapted to derive from signals received from the said sensors the rate of variation of the magnetic field with distance and direction from the said localised region of space and to determine therefrom the said characteristic of the

magnetic field in the said region of space.

Parameters of the magnetic field which may be determined include the strength of the magnetic field, the intensity of the magnetic field, the gradient of the intensity of the magnetic field, the magnetic flux density and the gradient of the magnetic flux density.

Preferably, the individual sensors are arranged in three orthogonal linear arrays of regularly spaced sensors.

Alternatively, the array of magnetic sensors can be simulated by moving a single sensor to a plurality of determined or determinable locations.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a representation of a portion of the magnetic field at an edge of an artefact which possesses magnetisation, 1 5 Figure 2 is a graph showing how the strength of a magnetic field decreases with

distance from a source of a magnetic field,

Figure 3 is a diagrammatic representation of the arrangement of individual magnetic field sensors within a probe forming part of an apparatus embodying the invention and Figure 4 is block diagram of an apparatus embodying the invention, of which the probe of Figure 3 forms a part.

Referring to figure 1 of the drawings, lines 100 are representations of the magnetic field at an edge 101 of an artefact 102, which possess residual magnetism. As can be seen, they are highly divergent, showing that the magnetic field is far from being homogeneous. Conventional magnetic field measuring devices use sensors which measure magnetic fields over relatively large regions of space and are not appropriate for use to measure non-homogeneous magnetic fields. Some forms of magnetic field sensor, such as Hall Effect devices, are small and can be used to measure magnetic in smaller regions of space, but such devices are encapsulated and the thickness of the encapsulation will introduce a displacement of unknown amount of the sensor from the accessible surface of the artefact 102 Figure 2 shows how the strength of a magnetic field decreases with distance from a source of the magnetic field. As can be seen, the initial rate of decrease of the strength of the magnetic field is very high and a relatively small variation in the distance of a magnetic field measuring device from the edge 101 of the artefact 102 can cause a large error in the measurement of the residual magnetism in the artefact 102.

Referring to Figure 3 of the drawings, there is shown schematically, a probe 300 embodying the invention, which is capable of determining the characteristics of a highly divergent magnetic field at a given position in space. The probe 300 consists of an housing 301 which contains a linear array of magnetic field sensors 302, of which, for convenience, only a few are shown. The fir st of the magnetic field sensors 302 is positioned at a fixed, but not necessarily known distance from the outer surface of the housing 301 and the other magnetic field sensors 302 are spaced apart by known distances. The probe 300 is shown in contact with a corner 303 of an artefact 304 made of a ferrous or magnetic material which possesses magnetism. Thus, the probe 300 provides output signals which are indicative of the variation of the magnetic field at various distances from the actual point of contact 303 of the pmbe 300 with the artefact 304.

In a more sophisticated version of the probe 300, which is not illustrated, there are three orthogonal sensors at each sensor position 302, so as to provide a three-dimensional indication of the magnetic fields at known locations, so offering information about the actual magnetic field the point of contact 303 of the probe 300 with the artefact 304.

Suitable magnetic field sensors for use in the probe 300 are Hall Effect devices.

Referring to Figure 4, an apparatus which can provide a measurement of the characteristics of the magnetic field at the actual point of contact 303 with the artefact 304, consists of a signal processing unit 400. which includes a central processor unit 401 to which output signals from the sensors3O2 within the probe 300 are applied, either directly or by a wireless link, and an output signal generator device 402. The central processor unit 401 is adapted to derive from the output signals from the probe 300 thnctions which represent the rate of change with distance from the point of contact 303 of the probe 300 with the artefact 304 of a parameter, or parameters of the magnetic field to be measured and to extrapolate from the values of the said parameter, or parameters, of the magnetic field at the magnetic field sensors 302 closest to the actual point of contact 303 of the probe 300 with the artefact 304 to give the value of these magnetic field parameters at the actual point of contact 303 of the probe 300 with the artefact 304, or at some other defined distance from the point of contact 303 with the artefact 304, so as to provide a measurement of the magnetism in the artefact 304. The output signal generator device 402 can be a calibrated visual display unit, a computer screen or, if the apparatus is to be used for quality control or monitoring purposes, an audible warning device, which is activated should the determined value of the magnetic field exceed a threshold value.

In use, to determine the magnetism in an artefact 304, the probe 300 is placed in contact with the surface of the artefact 304 and scanned over it and the strength of the flux leakage from the artefact 304 is observed as the scanning of the probe 300 over the surface of the artefact 304 progresses.

is Parameters which can be measured to characterise the magnetism in the artefact 404 include the magnetic field strength, the magnetic field strength gradient, the magnetic flux density, or the magnetic flux density gradient In a simpler version of the apparatus, which is not illustrated, the array of sensors is simulated by moving a single sensor within a defined or determinable space and measuring the magnitude of the parameter, or parameters, of the magnetic field at defined positions within that space Although the present invention has been described in relation to the determination of the residual magnetism in an artefact, the invention can be used in other circumstances where it is desired to know the characteristics of a magnetic field in a localised region of space, which is remote from the position at which the measurement is made.

Claims

CLAIMS1. A method of determining a characteristic of a parameter of a magnetic field in a localised region of space, comprising the operations of measuring the strength of the magnetic field at a plurality of positions at known distances and orientations from the localised region of space in which the characteristic of the parameter of the magnetic field is to be determined, deriving therefrom fUnctions representing the rate of variation of the magnetic field with distance and direction from the said region of space and determining therefrom the characteristic of the magnetic field in the said localised region of space.
2. An apparatus for determining a characteristic of a parameter of a magnetic field in a localised region of space, comprising a probe including an way of individual magnetic field sensors each of which is adapted to produce a signal related to the strength of the magnetic field at the location of that sensor, a central signal processing unit to which the signals from the said sensors are applied and an output signal generator adapted to produce a signal indicative of the characteristic of the parameter of the magnetic field in the said localised region of space, wherein the central signal processing unit is adapted to derive from the signals received from the said sensors the rate of variation of the magnetic field with distance and direction from the said localised region of space and to determine therefrom the characteristic of the parameter of the magnetic field in the said region of space and provide output signals related thereto.
3. An apparatus according to claim 2, wherein the individual field sensors are disposed in at least one regularly spaced linear array.
4. An apparatus according to claim 3, wherein the individual magnetic field sensors are disposed in three mutually orthogonal linear anays.
5. An apparatus according to claim 3 or claim 4 wherein the array of sensors is simulated by moving a single sensor from one known location to another.
6. An apparatus according to any of claims 2 to 5 wherein the output signal generator is adapted to display visually the characteristic of the magnetic field in the said region of space.
7. An apparatus according to any of claims 2, to 6 wherein the output signal generator is adapted to produce an audible signal when the strength of the magnetic field in the said region exceeds a threshold value.
8. A method of measuring a parameter of the magnetism in an artefact, including the operations of bringing the probe of an apparatus according to any of claims 2 to 7 in contact with the surface of the said artefact, scanning the probe over the surface of the said artefact and noting the distribution of any magnetism as shown by the display device or the audible signal.
9. A method of determining the characteristics of a magnetic field in a localised region of space substantially as hereinbefore described and with reference to the accompanying drawings.
10. An apparatus for determining the characteristics of a magnetic field in a localised region of space substantially as hereinbefore described and with reference to the accompanying drawings
11. An apparatus for determining the characteristics of a magnetic field in a localised region of space substantially as hereinbefore described and with reference to the accompanying drawings.