GB2324155A - Method and apparatus for scanning a magnetic structure - Google Patents

Abstract

A method of scanning a magnetic structure consisting of at least two components having different orientations, such as plies of steel reinforcing cords 4 of a vehicle tyre 3, comprises the steps of magnetising the structure by applying an external magnetic field in a direction such that the remanent magnetic fields of the separate components will have different polarisations, removing the external field and detecting the remanent magnetic fields, discrimination between the separate components of the structure being possible on the basis of the direction of the remanent fields.

Description

A METHOD AND APPARATUS FOR SCANNING A MAGNETIC STRUCTURE The present invention relates to a method and apparatus for magnetic scanning of a magnetic structure comprising components which have easy axes lying in different directions.

Particularly, but not exclusively, the invention relates to non-invasive corrosion and flaw detection in ferrous based structures.

Many engineering structures and industrial products contain components of steel or other ferrous and thus magnetic material. In many cases the steel component serves a strengthening or reinforcing function, such as, for example, in reinforced concrete and vehicle tyres. In such cases the structural integrity of the structure as a whole will depend upon the condition of the reinforcing steel elements. In other words, any corrosion or mechanical damage to the steel reinforcing structure can have a significant detrimental effect on the integrity of the structure as a whole.

It is therefore highly desirable to have effective means for testing the integrity of such reinforcing structures. Moreover, given that such reinforcing structures cannot generally be accessed for visual inspection without destroying the structure as a whole. it is desirable to develop non-invasive, and thus non-destructive, testing methods.

Testing methods and apparatus have been proposed which rely on the magnetic properties of steel and other ferrous materials often used in such structures. For example UK patent specification number 1 542 933 describes a method of detecting mechanical defects (such as breaks) in steel wire ropes. The method proposed involves magnetising a portion of the cable being tested (using either a permanent or electro-magnet) to the point of saturation and monitoring the cable for magnetic leakage using an inductive device. A magnetic leakage field will be produced by any breaks in the wire and thus the detection of such a leakage field indicates the presence of a defect. A further example of a method of defect detection in magnetic structures from the conductive detection of flux leakage (having established a saturated magnetic field in the object being tested) is disclosed in UK patent specification number 2 157 439 A.

U.S. Patent No. 5,559,437 discloses a process and apparatus for checking the condition of the magnetic wire reinforcement of vehicle tyres. The tyre is rotated past an external magnetic field source and a remanent magnetic field detector which are angularly displaced from one another. As the magnetic wire reinforcement passes the external magnetic field it is magnetised and then as it subsequently passes the detector the remanent magnetic field is detected, variations in the detected magnetic field inducing electromotive force in the detector. The induced electromotive force is detected and recorded for the whole magnetic reinforcement and is used to make a determination of the condition of the reinforcement. That is, the root mean square of the electromotive force recorded for the whole tyre is compared with reference values to determine whether or not the tyre reenforcement is in good or bad condition. Whilst this method offers a simple good/bad determination of the condition of the tyre it provides no detailed information on the structure of the tyre.

It is an object of the present invention to provide a new non-destructive method of scanning a magnetic structure, for instance to determine the integrity of the structure. which can differentiate between different parts of the structure.

According to the present invention there is provided a method of scanning a magnetic structure having at least two components with magnetic easy axes in different directions, the method comprising: magnetising a region of the structure to be scanned by applying an external magnetic field thereto; ii removing the external magnetic field and allowing the region to be scanned to assume a remanent state; iii detecting the magnetisation over said region to provide information representative of the structure over said region being scanned: wherein the direction of the applied external magnetic field is selected so that the remanent magnetisation in each of said at least two components will have different magnetic polarizations; and wherein information provided by said detection of the magnetisation is separated by reference to the different polarities of the detected magnetisation to thereby differentiate between the respective components.

The invention utilises the fact that where a magnetic structure has different components (e.g. different layers) which have respective magnetic easy axes in different directions, it is possible to magnetise the structure in such a way that when the magnetic field is removed the remanent magnetisation established in each component will lie along the respective easy axes and thus the polarity of magnetisation of each component will be different. Similarly the structure could be magnetised so that the remenant magnetisation within one component has no definate polarity but the magnetisation in the other component is polarised in a definate direction along the magnetic easy axis. This difference in polarities can then be used to identify which elements of the detected magnetisation apply to which component and thus differentiate between the different components. For instance, vehicle tyres generally comprise a re-inforcing sub-structure comprising two or more plies of reinforcing cords (which are generally ferrous based and therefore magnetic). In a typical construction, the cords in one ply run in a different direction to the cords in the other ply and each ply will have an magnetic easy axis which runs along the length of the respective cords.

The invention can thus be used to extract information specific to each ply (each ply constituting a "component" referred to above). This provides an advantage over the prior art methods and apparatus discussed above.

The direction of detection of the remanent magentisation is preferably selected so that the magnetisation of said at least two different components of the structure has a different polarity along the direction of detection. This enables detection of the magnetisation to be performed using a single detector, or a group of detectors with a single direction of detection, orientated to detect the magnetisation in said direction of detection such that said detector(s) output(s) a single signal representative of the structure of both of said at least two components and the information carried by the signal is separated by reference to the different polarities.

The information derived from the detected magnetisation may be processed (e.g.

using conventional signal processing techniques) to provide separate two dimensional representations of each of said at least two components of the structure. In addition, if the relative physical arrangement of said at least two components is basically known the information may be processed to provide a three dimensional image of the whole magnetic structure being scanned.

In addition, a tyre might have more than two plies of re-inforcing cords with, for example, the first and third ply lying in the same direction. In this case, the induced remanent magnetisation in the first and third plies would have the same polarity. However, if the basic structure of the tyre is known, a further parameter, such as the intensity of the magnetic field due to the remanent magnetisation. may be used to differentiate between the signals received relating to the first and third plies respectively. That is, a detector could be positioned closer to one ply than the other and the known expected fall-off rate of the intensity in the magnetic field used to differentiate between the two plies. Alternatively. two or more magnetic detectors could be used and the outputs of each detector combined, the relative intensities of the magnetic fields detected being compared to differentiate between different components within the structure that have substantially the same polarity.

It may also be useful to use more than one detector in scanning structures which have only two components with different polarities. For instance two magnetic detectors may be used to measure the magnetisation from different positions and in different directions so that if the resultant field at one detector is zero through a combination of the opposite polarity fields emanating from each component of the structure, a measurement may nevertheless be taken from the other detector.

The method may be used to monitor changes in the magnetic field produced by each of said at least two components over the region being scanned due to localised differences in the remanent state within each component.

In a preferred embodiment of the invention the method is used to detect localised defects in each of said at least two components, said defects being characterised by the localised remanent state of the magnetised component.

This relies on the fact that the nature of the remanent state will be different for regions of the magnetic material in good condition, regions containing mechanical defects. and regions which are corroded. The magnetic field emanating from the scanned region will depend upon the localised remanent state. Thus monitoring variations in the magnetic field provides information about the localised variations in remanent state over the region being tested and thus the localised integrity of each component in the scanned region. Using this method it is possible to detect very small areas of corrosion or mechanical damage.

It will be appreciated that the method according to the present invention can be used to detect defects in any magnetic material. The method is particularly suited for testing ferrous materials such as steel, since the remanence of unoxidised metal falls quickly to a small value when the external field is removed, whereas the remanence of the oxidised metal remains at a larger value. By detecting the remanent magnetisation it is therefore easier to differentiate between corroded and un-corroded areas than by measuring the saturated field, i.e. a greater sensitivity is possible.

A variety of different methods and apparatus could be used to generate the necessary external magnetic field (which is preferably a uniform linear field). For instance, one or more permanent magnets or electro-magnets could be used (or a combination of both).

The strength and orientation of the applied external magnetic field may be varied to suit different test structures and different magnetic materials, to maximise the difference in remanent states between good, damaged and corroded areas of the structure being tested. For instance, the required strength of the magnet may depend upon the volume of the desired field, the type of material to be magnetised and the decay rate of the remanent field (i.e. the decay in the remanent field between removal of the external magnetic field and the monitoring of the remanent magnetisation).

If the external magnetic field is generated by an electro-magnet. the field can. if required, be removed from the region being scanned (in accordance with step (ii) above) simply by de-energising the electro-magnets. In other cases either the field source or the test structure (or both) could be moved to remove the external magnetic field from the test structure. For example, in the case of large test structures, such as reinforced concrete, it would clearly be more practical to move the external magnetic field source than to move the subject being tested.

Any suitable means may be used to monitor the magnetisation of the magnetised region such as an inductive detection device (which can be used to detect changes in the magnetic field emanating from the magnetised region, a Hall effect device or a magnetoresistive device. An inductive detection device may preferably be used as such devices are relatively cheap. Such a device may typically be an inductive head (similar to the type used in magnetic digital storage devices or analogue tape recorders) comprising one or more conductive coils in which a current is induced as the device is moved over the region to be tested (or vice versa). Magnetic induction detection devices of this type output a linearly varying analogue signal representative of the changes in localised strength of the magnetic field emanating from the test region. One or more of such detection devices may be used and the detailed structure and configuration could vary widely depending upon the application.

For instance, a smaller detection head gap could be used to increase spatial resolution.

It is preferable that the analogue information from the or each detection head is processed digitally, particularly in cases in which a plurality of detection heads are used.

This allows relatively conventional digital processing techniques to be used to extract desired information from the detection head. For instance, the analogue information may first be processed into a digital representation of the relative magnetisation at each localised position within each component throughout the region of the structure being scanned (within the limitations of the digital resolution of the system). The digital information may then be processed to enhance the signal to noise ratio, for instance using conventional techniques of bandwidth limiting and noise floor reduction.

The enhanced data can then be pieced together in a linear manner to provide a twodimensional representation of each component the region of the structure being tested, or a three-dimensional representation of the whole structure if there is a basic knowledge of the structural relationship of the different components of the structure. This data can be further processed using a threshold detection algorithm to provide the information in a suitable format for the controlling operator (or machine). For instance. the output of the threshold detection algorithm may be an almost transparent representation of each individual item of data collected (subject to the spatial resolution of the detection device).

The algorithm may also include routines for taking decisions based on the monitored integrity of the structure under test. For instance the "safety" of the structure, or of each component of the structure, (relative to predetermined reference criteria) may be indicated as a "go/no-go" decision.

The threshold detection algorithm may of course be varied depending upon the nature of the structure being scanned.

As mentioned above, the detection method according to the present invention can be used to scan a variety of structures and in one preferred embodiment of the invention the method is used for testing the integrity of vehicle tyres which comprise a reinforcing mesh of steel (or other such ferrous and thus magnetic material) cords. The present invention thus provides a method in which the integrity of such tyres can be tested without damage to the tyre. The method will detect both areas of corrosion and mechanical damage such as breaks in the individual steel strands making up the cords.

When used for testing tyres, it is preferable to mount the tyre so that it is rotatable within a fixed external magnetic field (for instance. produced by electro-magnets) and such that the tyre is rotated past one or more fixed detection heads.

According to a second aspect of the present invention, there is provided apparatus for scanning a magnetic structure comprising means for magnetising a region of the structure to be scanned by applying an external magnetic field thereto, and detection means for detecting the localised magnetisation of the magnetised region.

Where the apparatus is to be used to test the integrity of vehicle tyres as mentioned above, the apparatus preferably comprises means for rotatably supporting the tyre being tested, and means for supporting the detection means in an appropriate position relative to the tyre whereby the whole of the magnetised sub-structure may be monitored by rotating the tyre past the detection means.

In a preferred embodiment of the apparatus, means are provided for supporting the external magnetic field generating means in a fixed position relative to the tyre so that the whole of said sub-structure may be magnetised by rotating the tyre through the magnetic field.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures la to Id illustrate graphically the method of magnetising a region of the structure to be tested by applying an external magnetic field thereto in a direction relative to the structure such that, following the subsequent removal of the external magnetic field, the different layers or parts of the structure would assume different magnetic polarizations, using the example of a vehicle tyre reinforcing steel mesh structure which consists of two layers of steel mesh; Figure 2 is a part sectioned illustration of apparatus in accordance with the present invention adapted for testing the integrity of a vehicle tyre reinforcing steel mesh structure; and Figure 3 is a schematic illustration of a detection head used in the apparatus of Figure 2.

The basic principles by which the method according to the present invention can be used to obtain information relating to different sections (or components) of a magnetic structure which have different magnetic easy axes are now explained with reference to Figures la to ld. Figure la schematically illustrates a structure comprising two sets 1 and 2 of magnetic wires which cross at an angle. The first set of wires 1, can be regarded as a first section or component of the magnetic structure and the second set of wires 2 can be regarded as a second section or component of the magnetic structure. The angle of orientation of wires 1 with respect to wires 2 need not be specified and the wires of set 1 may be touching the wires of set 2 or may be spaced therefrom, as in, for instance. the reinforcing structure of a vehicle tyre. Referring to Figure lb. according to the present invention. the wires 1 and 2 may be magnetised by applying an external magnetic field in the direction indicated by arrows A which has a component in the direction of the magnetic easy axes of at least one set of wires (which in each case lies along the length of the wires). Thus. when the external magnetic is removed, the remanent magnetisation in each set of wires 1 and 2 will resort to the magnetic easy axes as illustrated in Figure ic. It will be appreciated that the direction of the applied magnetic field need not be exactly that indicated in Figure 1 b but could be varied provided it has a component along the easy axes of at least one set of wires 1 and 9. There are therefore a variety of angles at which the external magnetic field could be applied.

Having induced a remanent magnetisation in each set of wires 1 and 9 which has a definite and different polarity it is possible to measure the magnetisation across the structure and separate out the components of the detected magnetisation by reference to the different polarities of the fields produced by the wires 1 and wires 2. That is, in accordance with the present invention the magnetisation of the set of wires 1 can be readily differentiated from the magnetisation of the set of wires 2 by reference to the different field polarities and thus information specific to each set of wires can be obtained.

For instance, if the magnetisation is measured in a single direction, such as one of the directions shown by arrows B I to B3 in Figure 1 d (using, in each use, one or more detectors), the detected magnetisation within the set of wires 1 will have a different polarity in the direction of detection to the magnetisation within the set of wires 2. Thus, it is possible in accordance with the present invention to differentiate between the magnetised states of the set of wires 1 and the set of wires 2 using only a single detector (or array of detectors) with a single direction of detection since the detected magnetisation of one polarity will relate to one set of wires and the detected magnetisation of the other polarity will relate to the other set of wires and it will be relatively straightforward using conventional signal processing teclmiques to separate out the different polarity signals.

It will be appreciated that the direction of detection need not be exactly one of those shown in Figure id but could be varied provided the direction is selected such that the magnetic fields emanating from each set of wires have different polarities in the direction of detection. In addition in some cases it may be desirable to monitor the magnetisation in two or more different directions of observation. For instance, referring to Fig id, if the direction of detection is perpendicular to wires 1 the polarity of the magnetisation in those wires in the direction of observation will be zero so that only magnetisation in wires 2 will be detected.

Such a direction of detection may be desirable if information relating to wires 1 is not required. However, if in this case information on the whole structure is required a second measurement must be made in a different direction so that magnetisation withn the wires 1 is dectected (an appropriate direction would be perpendicular to wires 2 to so that magnetisation within wires 2 is eliminated from the second detection).

The direction of detection may thus be selected to suit the particular application to which the invention is being put. For instance, where the invention is being used in a system to detect only faults in the magnetic structure, rather than to produce a detailed image of the entire structure, the direction of observation indicated by arrow B2 is particularly advantageous. That is because this direction of observation has an equal but opposite orientation relative to the polarities of the two sets of wires and thus the resultant magnetic field detected by the detector will be zero unless there is a difference between the magnetisation (at the observed location) in each set of wires. If the two sets of wires are assumed to have the same structure and composition they should therefore have the same magnetisation, unless there is a localised fault providing a localised difference in the magnetisation in one set of wires from the other. The detection of resultant magnetisation at the detector will therefore indicate the presence of a fault, e.g. a break or corrosion. Thus, for instance, if an inductive detection head is being used to monitor changes in the magnetic field emanating from the magnetised structure in a direction of observation corresponding to direction B2 of Figure id, the detection head will only produce a signal when there is a change in the magnetic field which will correspond to the location of a fault giving rise to a difference in the magnetisation between each set of wires. Furthermore. by reference to the polarity of the detected change in the magnetic field it is possible to identify which set of wires the fault lies in.

Once the information relating to each set of wires has been extracted a two dimensional image of each set of wires can be constructed. Furthermore, with an a priori knowledge of the physical construction of the structure, i.e. the relationship of the two sets of wires to one another, the apparent two dimensional images of each set of wires can be extrapolated into a three-dimensional representation using conventional processing techniques.

As mentioned above, more than one detector may be used and indeed more than one detector may be desirable where the structure being scanned has more than two components in which case at least two components are likely to have a polarisation in substantially the same direction relative to a single direction of observation. In this instance, a second detector located at a different position from the first detector could be used. the outputs of the detectors being combined to differentiate between such components, for instance by reference to the position of the two detectors relative to the structure and the expected fall-off rate of the intensity in the magnetic fields emanating from the different components of the structure.

As mentioned above, the method according to the present invention is particularly suited for scanning the reinforcing steel wire mesh structure of vehicle tyres. Such reinforcing mesh structures typically comprise two or more crossed plies of reinforcing wires as schematically illustrated in Figures la to Id. Apparatus in accordance with the present invention suitable for such an application of the above method is illustrated in Figure 2.

Referring to Figure 2, the illustrated apparatus is designed for testing the integrity of the reinforcing steel wire mesh structure of conventional vehicle tyres. As illustrated in cross-section, the tyre being tested, which is indicated generally by the reference 3, comprises a reinforcing steel mesh 4 which is embedded within an inner layer 5 of a rubber compound tyre body.

The testing apparatus comprises a table 6 upon which the tyre ; is mounted for rotation (the detail of the mounting structure can readily be adapted by the skilled person to suit a particular type and size of tyre and thus will not be described here). Two pole pieces 7 of a magnet 8 are mounted so as to face one another with a gap therebetween within which the rim of the tyre 3 is received. The arrangement is such that as the tyre 3 is rotated upon the table 6 the tyre rim, and in particular the steel mesh structure 4, passes between the magnetic poles 7.

An array of magnetisation detectors 9 is suspended close to the tyre body (the supporting structure is not illustrated but it will be appreciated that the skilled person will readily be able to construct an appropriate mounting), a fixed distance from the position of the steel mesh structure 4. The orientation of the detection heads 9 relative to the mesh 4 will be dependent on the detailed form of the mesh 4 and magnetising field due to magnet 7.

However, where the system is to be used to indicate the locations of faults only, and not, for instance, to produce a detailed "picture" of the mesh structure, the detection heads 9 should preferably be located so that the polarities within the two sets of wires are exactly opposite in the direction of detection (i.e. a direction corresponding to direction B2 of Fig. Id) so that only changes in the magnetisation within the structure which are indicative of the presence of a fault will be detected.

A single detection head 9 is illustrated in greater detail in Figure 3. Each head comprises a wire coil 10 wound around a core 11 of soft magnetic material. The coil 10 is housed within the magnetic circuit of the head structure 12 which is constructed of a magnetically soft material. The gap in the head structure 12 is placed in close proximity to the outer layer 1 of the tyre 3 and hence the steel mesh structure 4.

The procedure for testing the integrity of the mesh structure 4 will now be described.

The tyre 3 is rotated within the external magnetic field so that the steel mesh 4 is magnetised so that the remanent magnetisation in the mesh 4 will lie along the easy axes of the respective wires. As mentioned in the introduction to this specification. the strength and orientation of the external magnetic field necessary to sufficiently magnetise the mesh 4 is dependent on a number of factors (including the detailed structure of the tyre 3 and its component materials) and can be calculated and/or if necessary be determined by a process of empirical deduction.

Rotation of the tyre 3 is continued so that the magnetised steel mesh structure 4 is moved past the detection heads 9. In doing so, the remanent magnetic field emanating from the steel mesh 4 is incident on the head gap and is coupled to the detection coil 10 by the magnetic circuit of the head structure 12, inducing a voltage therein dependent upon variations in the localised strength and the polarity of the field. That is, each of the detection heads 9 outputs a linearly varying analogue signal representative of the variations in remanent magnetic field over each ply of the tyre mesh structure 4. As discussed above, the localised strength of the remanent magnetic field depends upon the localised condition of the steel mesh 4 within each ply and thus the signals output from the detection heads 9 can be processed to give desired information as to the condition of the steel mesh structure 4, the information relating to each ply being differentiated by the different polarities. A signal processing procedure which may be used is described below.

In order to obtain an accurate "picture" of the condition of the steel mesh structure 4 it is necessary for the signals from each of the detection heads 9 to be processed simultaneously. The signals are therefore processed digitally using a computer (not shown) with a relatively large data storage capacity. The signal output by each detection head 9 is transformed into the digital domain by sample and hold and quantisation stages (at a conversion rate

The processing system may be provided with input means by way of which an operator can input data which may be required for combination with the monitored data. For instance, the system can be designed so that the operator can input details of, for example, tyre make or size.

It will be appreciated by the skilled person that the method of signal processing described above is by way of example only. That is, it will be appreciated that alternative procedures and devices for processing the data, and representing the processed data, could readily be designed to suit particular applications and operating requirements.

For instance, the structure (eg size) number and orientation of the detection heads could be varied. Indeed, means could be provided to generate an external magnetic field sufficiently large to magnetise the entire tyre at once, without the need to rotate the tyre.

It will be understood that the method of testing the integrity of a magnetic structure, in accordance with the present invention. is not limited in application to the testing of tyres.

Suitable apparatus could readily be devised to enable the method to be used to test the integrity of a wide variety of structures varying considerably in size and nature. For instance, the method could be used for testing the integrity of the steel support members of reinforced concrete structures. Clearly in this case the apparatus would have to be substantially different from that described above, particularly given that for large unmanageable structures the external magnetic field and detection heads would have to be moved relative to the test structure rather than vice versa.

Other possible modifications to the apparatus and method described above, and other possible applications of the invention, will be apparent to the skilled person.

Claims (27)

1. A method of scanning a magnetic structure having at least two components with magnetic easy axes in different directions. the method comprising: magnetising a region of the structure to be scanned by applying an external magnetic field thereto; ii removing the external magnetic field and allowing the region to be scanned to assume a remanent state; iii detecting the magnetisation over said region to provide information representative of the structure over said region being scanned; wherein the direction of the applied external magnetic field is selected so that the remanent magnetisation in each of said at least two components will have different magnetic polarizations; and wherein information provided by said detection of the magnetisation is separated by reference to the different polarities of the detected magnetisation to therebv differentiate between the respective components.

2. A method according to claim 1, wherein the direction of detection of the remanent magnetisation is selected so that the magnetisation within said at least two different components of the structure have different polarities along the direction of detection.

3. A method according to claim 1, wherein the direction of detection of the remanent magnetisation is selected so that the magnetisation within said at least two different components of the structure have opposite polarities along the direction of detection.

4. A method according to claim 2 or claim 3. wherein detection means comprises at least one detector orientated to detect the magnetisation in said direction of detection such that said detector outputs a single signal representative of the structure of both of said at least two components and the information carried by the signal is separated by reference to the opposite polarities.

5. A method according to any preceding claim, wherein said information derived from the detected magnetisation is processed to provide separate two dimensional representations of each of said at least two components of the structure.

6. A method according to claim 5, wherein the relative physical arrangement of said at least two components is known and said information is processed to provide a three dimensional image of the whole magnetic structure being scanned.

7. A method according to any preceding claim, wherein said magnetic structure is a magnetic reinforcing structure of a vehicle tyre comprising at least two plies of reinforcing members, each ply constituting one of said at least two components.

8. A method according to any preceding claim and in which the magnetic structure has more than said at least two components, wherein information derived from said detection is separated by reference to a further parameter in addition to the polarity of the detected magnetisation to differentiate between components which have substantially the same polarity relative to the direction of detection.

9. A method according to claim 8, wherein said further parameter is the intensity of the detected magnetic field.

10. A method according to any preceding claim, wherein means for detecting the magnetisation comprises at least two magnetic detectors spaced apart from one another and the information from each detector is combined.

11. A method according to any preceding claim, wherein the method is used to monitor changes in the magnetisation of each of said at least two components over the region being scanned due to localised differences in the remanent state within each component.

12. A method according to claim 11, wherein the method is used to detect localised defects in each of said at least two components, said defects being characterised by the localised remanent state of the magnetised component.

13. A method according to any preceding claim, wherein the external magnetic field is generated by one or more permanent magnets and/or one or more electro-magnets.

14. A method according to any preceding claim, wherein the external magnetic field is fixed in location and said region being scanned is placed in or moved through the magnetic field.

15. A method according to any preceding claim, wherein the external magnetic field is moved relative to said region being scanned in order to magnetise the whole of said region.

16. A method according to any preceding claim, wherein detection means are moved over the region being scanned to detect said magnetisation.

17. A method according to any one of claims 1 to 15, wherein detection means for detecting the magnetisation are fixed in position and the region being scanned is moved past the detection means.

18. A method according to any preceding claim, wherein detection means for detecting the magnetisation includes at least one inductive detection device.

19. A method according to any preceding claim. wherein means for detecting the magnetisation includes at least one Hall effect device.

20. A method according to any preceding claim, wherein means for detecting the magnetisation includes at least one magneto-resistive device.

21. A method according to any one of claims 16 to 20, wherein digital processing techniques are used to process the signal or signals output from the detection means.

22. Apparatus for scanning a magnetic structure by the method of any preceding claim, the apparatus comprising means for magnetising a region of the structure to be scanned by applying an external magnetic field thereto, and detection means for detecting the magnetisation of the magnetised region.

23. Apparatus according to claim 22, wherein said detection means is adapted for detecting changes in the remanent magnetic field due to localised differences in the remanent state of the magnetised region.

24. Apparatus according to claim 22 or claim 23 adapted for testing the integrity of a magnetic re-inforcing sub-structure of a vehicle tyre. wherein means are provided for rotatably supporting the tyre being tested, and means are provided for supporting the detection means in an appropriate position relative to the tyre whereby the whole of the magnetised sub-structure is scanned by rotating the tyre past the detection means.

25. Apparatus according to claim 24, wherein means are provided for supporting the external magnetic field generating means in a fixed position relative to the tyre so that the whole of said sub-structure may be magnetised by rotating the tyre through the magnetic field.

26. A method of scanning magnetic structures, substantially as hereinbefore described, with reference to the accompanying drawings.

27. Apparatus for scanning magnetic structures, substantially as hereinbefore described, with reference to the accompanying drawings.