EMBEDDABLE REMOTELY IDENTIFIABLE IDENTIFICATION SYSTEM

Abstract: An embeddable, non-invasively and non-destructivelly measurable identification marker (10) is described which can be read out despite a cover layer (2) which substantially reduces radio frequency and mechanical access. The marker (10) can store data in excess of 16 bit. The marker (10) is preferably remotely readable, is not erasable by electromagnetic radiation, is visually camouflageable and its stored information survives exposure to high temperatures which makes it suitable for use in safety critical applications. A system of detection of the identification marker (10) is also described.

Description: EMBEDDABLE REMOTELY IDENTIFIABLE IDENTIFICATION SYSTEM The present invention relates to a system for remote identification of objects, an identification marker for use in the system, methods for remote identification of objects, a device for generating a moving magnetic field for use with the invention and a detection device for detecting the marker. The marker in accordance with the present invention may be embedded underneath a cover layer which allows limited mechanical and radio frequency (RF) access to the marker. The marker in accordance with the present invention may also be embedded and processed at high temperature without its information content being destroyed or modified. The marker in accordance with the present invention may be used with safety critical components. Background of the Invention There is a general need for an authentication and identification system for high value and/or critical objects such as replacement aircraft parts. The system should provide a marking device compatible preferably with the manufacturing process for such parts, not easily copied, modified or transferred to other parts of inferior quality, having a large enough information storage capacity that the necessary data for confirming the origin of the part (manufacturer, date of manufacture, certification mark and date, etc.) can be stored and preferably the marking device does not influence the mechanical properties, in particular the lifetime of the part under working stress conditions. Aircraft wings and fuselage may include an outer skin of an aluminium laminate as described, for instance, in EP-A-056 289. Such structures rely for their strength on the intactness of all the layers of the laminate. Further, for aerodynamic reasons, it is not desirable to have protrusions above the surface of the laminate. US 4,733,079 and US 4,795,906 propose a method of altering the infra-red reflection properties of an aircraft part to form a code and to detect these differences using an infra-red source and a detector. The disadvantage of this method and any similar method using any other form of optical or near-optical detection method such as a bar-code is that the markings must be on the surface of the part. They are easily accessible and can be modified or easily transferred many times to inferior parts. They are also subject to the abrasive action of wind, sand, and de-icing and washing processes, all of which individually or in combination can result in obliteration of the marking with time unless it is etched deeply into the outer surface which may be undesirable. US 5,524,758 proposes an authentication system not for the part but for the packaging of the part. A code is placed underneath a tear tape which is located within the multi-layered material which forms the outer walls of the packaging. Authentication can only be performed by destroying the package. This system has the disadvantage that once the method is known it can be duplicated. Further, the package can be opened and the contents exchanged for others and the package carefully re-sealed. A skilled person would not attempt application of this known method to the aircraft part itself as authentication would require destruction or severe damage of the part. Transponders are known which require no battery and receive their energy from a radio frequency ("RF") signal from the detector. To place an electromagnetic marker under the surface of a wing or fuselage part would make electromagnetic detection extremely difficult as the aluminium layers act as an RF barrier thus preventing signal transfers between the marker and the detector. In addition such transponders include semiconductor memory and processing elements. Such devices are limited in the processing temperature they can tolerate. The temperatures used to bake the aluminium laminate of an aircraft wing are much too high for such devices to function reliably afterwards. Also, any data which is stored on a semiconductor device can be destroyed by electromagnetic radiation if this is of high enough energy. During a lightning strike on an aircraft very high electrical currents can flow in a thin outer layer of the fuselage and wings. These currents can produce large local electric and magnetic fields which could seriously affect any semiconductor device located just underneath the surface. Hence, such transponders are not considered as providing sufficiently permanent storage of their data particularly for safety critical applications. A discussion of the effects of lightning strikes on aircraft components and the need for shielding even within the aircraft is given in the article "First realistic simulation of effects of EM coupling in a commercial aircraft wiring", by Jean-Philippe Parmentier, Computing & Control Engineering Journal, April 1998, IEE. One method of providing a marker under an RF barrier is described in US 5,523,750 but this requires a metallic part which penetrates the barrier to act as a conductor for the signals between the inside and the outside. Such penetration through the outer skin of an aircraft wing or fuselage part is undesirable. An electronic tag for attachment to an aircraft part is described in US 5,469,363. This tag includes an antenna for operation at megahertz frequencies which eliminates the possibility of secreting this tag below the metal surface of the part. Instead, the tag is applied to the outer surface of the part and removed before use. This has the disadvantage that another part may be substituted for the authenticated part after the tag has been removed but before installation on the final equipment. The authenticated part may be resold. Further, in case of an accident there is no permanent record of the authentication of the aircraft part. It is thus seen that there is a need to provide a marker and a system of marking which overcomes at least some of the disadvantages of the prior art. Such a marker preferably is robust, can be read out remotely, and does not require batteries to operate it. Additionally it is preferred that such a marker can withstand the temperatures during many manufacturing processes and which also may survive high temperatures which may occur during an accident. It is also desirable to provide a remotely readable marker which has a low manufactirung cost. Summary of the Invention The present invention may include an article including an identification marker, the marker including stored information, said article comprising a first conductive layer, said conductive layer restricting mechanical and RF access to said marker; and said marker being visually camouflageable beneath said conductive layer and being non-destructively and non-invasively measurable through said conductive layer. The present invention may also include use of an embeddable identification marker for the verification and tracing of safety critical components, said marker comprising only passive elements and being visually camouflageable. The present invention may also include use of an embeddable identification marker for the verification and tracing of safety critical components, said marker being processable at temperatures exceeding 120°C without loss of information content and being visually camouflageable. The present invention may also include use of an embeddable verification marker for the verification and tracing of safety critical components, said marker including stored information which can be non-destructively and non-invasively read out through a mechanical and RF barrier; and said marker being visually camouflageable. The present invention may include a system for identifying objects each including an identification marker, said marker including stored information which can be non- destructively and non-invasively read out through a mechanical and RF barrier; and said marker being visually camouflageable. The markers in accordance with the present invention mentioned above may be preferably read out remotely. The preferred method uses a rotating magnetic field having a substantially constant magnitude. The elements which make up the marker as well as the marker and detection system in accordance with the present invention preferably differ considerably from a transponder, or transponder systems, although the present invention does not necessarily exclude transponders and is limited only by the claims. Transponders rely on setting up a communication channel over which messages, e.g. interrogative and answer messages, are sent in accordance with a communication protocol. The preferred markers in accordance with the present invention may be described by one or more of the following terms: the elements of the marker are passive, the markers are non-protocol, noncommunication channel, non-interrogative message based. By passive is meant that the markers do not include active components which, for instance, could answer interrogative questions over a communication channel. The present invention may also include a device for generating a magnetic field within a predetermined interrogation zone, comprising: means for generating a magnetic vector of said magnetic field having a substantially constant magnitude; means for varying the orientation of said magnetic field vector within said interrogation zone; and means for determining the magnitude of said magnetic field vector, wherein the interrogation zone lies outside and is not bounded by the generating and varying means. The present invention also includes a marker comprising soft ferromagnetic material having a plurality of substantially straight edges, the edges being arranged in a pre-formed spatial relationship to each other, said spatial relationship being pre-selected to define a code, the straight edges being linked together; the soft ferromagnetic material having a low magnetic resistance (high magnetic susceptibility); and each straight edge of the soft ferromagnetic material having at least one state in which the magnetising direction is reversible when the relative orientation between said marker and a magnetic field vector of a magnetic field having a substantially constant magnitude is varied. This type of marker may be fabricated from wire or from sheets of material by stamping or cutting which reduces manufacturung cost. The invention can provide the advantages of an embeddable verification marker for an object which includes a code having an information content similar to that of a bar code but without having to use customised integrated circuits or antenna coils. Further, the identification of the marker is not disturbed by high temperatures. The marker can also be made optically neutral so that it is difficult to determine the presence of the marker or the code which it carries. Further advantages and embodiments are described in the following with respect to the following drawings. Brief Description of the Drawings Fig. 1 shows a part of a component including a verification marker in accordance with an embodiment of the present invention, Fig. 2 shows identification pulses produced by an identification system in accordance with an embodiment of the present invention, Fig. 3 shows a schematic representation of a material for components for use with an embodiment of the present invention, Fig. 4 and Fig. 5 show laminated specimens for load cycle testing, Figs. 6A to C show a magnetic field generator, receiver windings and electronic circuitry in accordance with an embodiment of the present invention, Figs. 7A, B show schematically a further embodiment of a marker in accordance with the present invention, Figs. 8 A and B show a schematic representation of a lock with which the marker of Fig. 7 may be used. Fig. 9 A and Fig. 9C show schematic representations of further embodiments of a marker in accordance with the present invention and Fig. 9B shows an equivalent to the marker of Fig. 9 A using elongate strips. Fig. 9D is a side-view of the marker shown in Fig. 9C: Figs. 10A is a schematic representation of a further embodiment of a marker in accordance with the present invention and 10B shows its equivalent using elongate strips. Figs. 1 1 A to 1 IE show marker elements for use with the present invention. Detailed Description of the Preferred Embodiments In the following the invention is described in detail with respect to a planar arrangement of information elements but the invention is not restricted thereto. The invention also includes a two dimensional linear or a three dimensional sequence of elements the means for generating the magnetic field therefor and means for detecting the code stored in the marker. Fig. 1 is a schematic representation of an embodiment of part of a component including a verification marker 10 in accordance with the present invention. For example, the component can be an aircraft wing. Safety critical systems and components relate to a separate field of engineering which has its own unique regulations and standards. Components which may be acceptable for normal uses may not be suitable for safety critical applications. Some aspects of safety critical systems may be found in the book "Safety- critical systems: current issues, techniques and standards", editors F. Redmill and T. Anderson, Chapman & Hall, 1993. Verification markers for safety critical components have the following requirements: 1) the possibility of verification, i.e. identification that the component is a true component of the alleged manufacturer, during the time that the component is being shipped from its place of manufacture or storage to its final destination; 2) the possibility of verification after the component has been installed. 3) the possibility of identification of the marker for tracing purposes after accidents whereby it is accepted that the marker cannot be expected to function after serious damage to, or destruction of the component with which it is associated. Anything less than the above is not acceptable for verification and tracing of safety critical components. It is preferred if the marker is visually camouflaged - this makes duplication more difficult. It is also preferred if the marker may not be transferable from one component to another without destruction thereof. In accordance with this embodiment the component includes a laminate 1 having one or more layers 2, 4, 6, 8 which are joined together with high performance curable adhesive layers 3, 5, 7 which may include reinforcing fibres. The layers 2, 4, 6, 8 are typically aluminium or an aluminium alloy and may be used in the construction of aircraft wings or fuselage. The thickness of at least the outer conductive layer 2 may be typically, when aluminium, in a range 0. 1 to 2 millimetres. The conductive layers 2, 4, 6, 8 are not restricted to aluminium and need not be conductive in accordance with the present invention. The laminate 1 may be an effective block to radio frequency electromagnetic radiation. The lowest RF frequencies, e.g. long wave radio frequencies, are generally considered to be 148.5 kHz or above. In accordance with the present invention "RF" may include frequencies somewhat lower than this. The terms RF barrier, RF block or having restricted RF access are an indication that the layer 2 attenuates an RF electromagnetic wave at about 100 kHz or above significantly more than the energy attenuation of a magnetic field at 1 kHz or below, e.g. by a factor of about 10 or more. This restricted RF access may be considered to be fulfilled by a sheet of aluminium of 0.15 mm thickness which is a typical thickness for RF suppression by conductive shielding, see for example, the "Architectural Electromagnetic Shielding Handbook", Leland H. Hemming, IEEE press. For other materials than aluminium, the RF blocking effect of the outer conductive layer 2 may be defined as the conductive layer 2 having a value V of greater than 300, where V is given by (thickness of conductive layer in mm) 2 x (relative magnetic permeability of the conductive layer) / (specific resistivity of conductive layer in ohm. mm). Thicknesses of conductive materials with V greater than 300, and in particular greater than 1500 are effective RF barriers. The formula for V is derived from the absorption of electromagnetic radiation as given for example in formula 3-10 on page 18 of the above handbook. The good mechanical properties of laminate 1 which make it suitable for use in highly stressed applications such as aircraft components relies on the good adhesion between the adhesive layers 3, 5, 7 and the layers 2, 4, 6, 8. Any interference, distortion, slit, tear or hole in one of the layers 2, 4, 6, 8 is undesirable. In accordance with an embodiment of the present invention a marker 10 is included below the outer layer of a conductive material 2, for instance embedded in the adhesive layer 3. The marker 10 defines a code which may be read out using a detector external to the laminate 1. The outer layer 2 need not be penetrated or otherwise damaged to be able to read out the information stored in the marker 10, i.e. it may be read out remotely and non-invasively. The marker 10 may be a read only memory which can be read out non-invasively despite the fact that the marker 10 may be surrounded on both sides by conductive layers 2, 4 which severely restrict or prevent mechanical or RF access to the marker 10. The marker 10 may include thin soft ferromagnetic elongate elements 9 of the kind generally described in EP-A-713195 but excluding the novel features of the present invention. These elements 9 are preferably sufficiently thin that they do not create significant distortion of the conductive layers 2 and 4 and do not seriously effect the mechanical properties of the complete laminate 1. The elements 9 may be arranged radially such that the angular differences between the elongate elements 9 represent coded information or data, for instance, each element 9 may be placed at one of two angular separations with respect to the next element. The first angular separation may represent a binary "zero" and a second angular separation may represent a binary "one" but the invention is not limited to a binary system. It is sufficient if the elongate elements 9 are arranged in part of a circle, e.g. a semicircle. A 5° angular difference of the elongate elements 9 shown in Fig. 1 may be easily distinguished by the detection device in accordance with the present invention so that the marker 10 may have 36 individual elongate elements 9 and thus a storage capacity of a binary number of 2 36 . This is adequate to be able to code items individually with one or more references, e.g. manufacturer, manufacturing date and/or location. The minimum size of the angular or linear difference between the elongate elements 9 of the marker 10 which can be detected is determined by size and material of the elongate elements 9, the detection coil arrangement and the frequency of rotation of the external magnetic field. If the elements 9 are arranged in a full circle, it is preferable if an elongate element 9 in one semicircle does not have the same orientation as an elongate element 9 in the opposing semicircle. This may be achieved by differing angular separations enabling different coding techniques. For example, if two different angular spacings of 5.5° and 8.5° are chosen to represent a binary code, the axes of any two elongate elements 9 in two different semicircles would be at least 0.5° apart independent of the value the code represents. Other methods and arrangements of the elements 9 may be used to form a readable code and are included within the scope of the present invention. For instance the code could be a presence/absence single width bar code as described in EP-A-0472842. The soft ferromagnetic elements 9 preferably have a low magnetic resistance, i.e. they have a high relative permeability or high magnetic susceptibility, χ m . The soft ferromagnetic elements 9 may be surrounded by electrically conductive layers 2, 4 of high magnetic resistance (low relative permeability or low magnetic susceptibility), i.e. non- magnetizable material such as plastic, aluminium or copper or their alloys or magnetizable materials defined by the value of the parameter V being greater than 300, where V is given by (thickness of conductive layer 2 in mm) 2 x (relative magnetic permeability of the conductive layer 2) / (specific resistivity of conductive layer 2 in ohm. mm). The value of V is preferably 12 x 10 4 or less. It is preferable but not necessary for the invention if the ratio of the length of an elongate element 9 to the square root of its cross-sectional area is 150 or more. The elongate soft ferromagnetic element 9 has the property of reaching saturation at low magnetic field strengths. Suitable soft magnetic materials have a low coercive force and a high permeability (high susceptibility), e.g. Permalloy. When the marker 10 as shown in Fig. 1 is placed in a substantially uniform magnetic field of sufficient intensity whose magnetic vector has a particular first direction, the elongate element 9 will be magnetised along its own longitudinal axis in the direction of the component of the magnetic field vector in this direction. If the orientation of the magnetic field vector is changed to a second direction such that it sweeps through a direction perpendicular to the longitudinal axis of the elongate soft ferromagnetic element 9, the magnetised direction of this elongate element 9 will reverse. The elongate element 9 now has a component of the magnetic field vector along the longitudinal axis of the soft ferromagnetic element 9 but in the reverse direction. As the material is soft ferromagnetic the reversal will occur shortly after or simultaneously with the movement of the magnetic field vector through the direction perpendicular to the longitudinal axis of each relevant elongated element 9. By the choice of suitable materials with magnetic properties similar to those indicated below the reversal of magnetic direction of the elongate soft ferromagnetic element 9 can be made to occur within a very short period of time, so that a detection coil placed in the region of the marker 10 would detect a narrow pulse of induced voltage created by the change in magnetic orientation of the particular elongate element 9. An individual pulse normally has a wide frequency spectrum including a lot of high frequency components. When a series of elongate elements 9 are arranged radially and the imposed magnetic field having a substantially constant magnitude is made to rotate in the plane of the marker 10, a series of pulses is obtained as shown schematically in Fig. 2. The separation in time of the pulses, one from another, is a result of, and proportional to the angular separations of the elements 9. By analysing the times between pulses the code defined by marker 10 may be identified. However, detection of the change in magnetising direction of any element 9 is made difficult or impossible by an outer conductive layer 2. All high frequencies in the frequency spectrum of the signal generated by the change in direction of magnetisation of element 9 are absorbed by the layer 2. Thus, even with a low frequency of rotation of the imposed magnetic field (e.g. 400 Hz.), the received signal from each elongate element 9 is highly attenuated and often unmeasurable. Table 1 shows the results of tests done using an aluminium cover layer 2 and 100 mm long x 0.75 mm wide elongate elements 9 of Vitrovac 6030Z material from VAC, Hanau, Germany. Table 1 It can be seen that in air, i.e. without a conductive cover (cover = 0 mm), the signal was 2.2 units whereas with a 0.5 mm aluminium cover layer 2 the signal reduced to about one tenth of this and for a cover layer 2 of 1 mm or more there was no measurable signal. With thicknesses of aluminium layer 2 used in aircraft parts, the marker 10 is effectively RF shielded from any device for detecting the change of magnetising direction of strips 9. Surprisingly, in accordance with the present invention strong signals may be obtained by placing a second conductive layer 4 beneath the marker 10 to form a laminate 1 as shown schematically in Fig. 1, i.e. so that the marker 10 is sandwiched between two conductive layers 2, 4. Table 2 shows the results in this case for various thicknesses of aluminium cover layer 2 and underneath layer 4 using the same elongate elements 9 as mentioned above with respect to table 1. Table 2 The effect of increasing thickness of the underneath layer 4 is to increase the received signal, such that for a cover layer of 0.5 mm, an underneath layer of 1 mm or more is capable of increasing the signal strength almost back to the value measured in air. For a cover layer 2 of 1.5 mm, a similar underneath layer 4 causes received signal strengths which are about 25% of the strength in air and twice the signal strength obtained when a cover layer of 0.5 mm without an underneath layer is used. Bearing in mind that the signal strengths from signals which pass through a conductive metal layer should be approximately proportional to the thickness of the RF barrier, this means that for the cover layer 2 with a thickness of 1.5 mm and no underneath layer, the received signal strength is estimated as one thousandth of the signal in air. Unexpectedly the sandwiching of the marker 10 between two such 1.5 mm aluminium layers 2, 4 increases the received signal strength by a rather large factor of 250 times. Thus, in accordance with the present invention, when use is made of an electromagnetically detectable and readable marker, the received signal from the marker is preferably at least 1% of the intensity of the imposed measurement field, more preferably 5% or more and most preferably 10% or more of the imposed field intensity. A preferred embodiment of the present invention includes providing an identification system having an object (such as an aircraft part) with a marker 10 including at least one elongate soft ferromagnetic element 9 (with or without a control or keeper element), the marker 10 being embedded in the object underneath a conductive layer which restricts RF and mechanical access. The conductive layer 2 may have a value V of greater than 300, where V is given by: (thickness of conductive layer in mm) 2 x (relative magnetic permeability of the conductive layer) / (specific resistivity of conductive layer in ohm. mm). The object may be disposed to pass through an interrogation zone in which a magnetic field having a magnetic field vector which has a substantially constant magnitude is generated, and varying the orientation of the identification marker 1 with respect to the magnetic field vector. Any change in magnetising direction of the soft ferromagnetic element 9 can be detected by a detection system adjacent to the interrogation zone. The marker 10 stores a useful amount of information, e.g. above 16 bit, and can be read out by a non-invasive method despite the fact that it is embedded beneath a cover layer 2 which substantially prevents mechanical and RF access. The material for the elongate elements 9 may be selected from any suitable soft magnetic material. The elongate elements 9 may be made from a soft ferromagnetic material such as amorphous magnetic metal alloys having the property of being relatively easily magnetizable along the longitudinal direction of the elongate element 9 when placed in a magnetic field having a component parallel to this first direction and of reversing the magnetisation direction when a component of the magnetic field is in the direction which is the reverse of the first direction. It is preferable that the soft ferromagnetic material is chosen so that the magnetising direction of the elongate elements 9 can be changed easily and rapidly. Some suitable materials are listed in Table 3. Further suitable materials are the alloys 2705M and 2714Z made by Allied Signal Corp., USA. Table 3 The term "ferromagnetic material" includes conductive and non-conductive ferromagnetic materials, e.g. ferrites, amorphous metal alloys. The magnetic properties for such materials are preferably a saturation flux density (B s ) of a magnetic induction B (= μ 0 (H+M)) of 0.5 to 1.0 Tesla, a coercivity (H c ) of about 0.025 to 1 A/cm, as well as a relative permeability (μ re ι = ΔB/ΔH| H =o) of greater than 10,000. Suitable materials may have a permeability of 250,000 or even 400,000. It is also advantageous if the material has some remanence, in particular in the range 50 to 95% of the magnetic saturation. The results obtained for signal strengths and received pulse durations with the materials of Table 3 are shown in Table 4. In this table a comparison is made between signal strength and pulse duration in air and then when sandwiched between two 0.5 mm thick layers of aluminium. It is noticeable that there are differences in amplitude and pulse duration with the various materials. Ideally the amplitude of the signal generated by a marker 10 sandwiched between two aluminium layers 2, 4 should be high and the pulse duration should be short. A high absolute amplitude of returned signals assists in accurate determination of the information stored in the marker 10 and to distinguish a true signal compared to noise signals. The shortness of the pulse determines the minimum detectable angular separation between the elongate elements 9 in marker 10 and therefore the maximum amount of data which can be stored by the marker 10. Preferably, the ratio of amplitude over pulse duration should also be as high as possible. The most preferred material is Vitrovac 6018 which gives the highest absolute amplitude and the second highest amplitude/pulse duration ratio. Other preferred materials are Vitrovac 6030Z and 6006. Using these materials a marker 10 may be sandwiched between the outer layer 2 and the next layer 4 of aluminium in the laminate 1 of Fig. 1 and the marker 10 having a useful amount of data may be read using external electromagnetic detecting equipment despite the fact that layers 2, 4 are RF barriers. Table 4 A further advantage of the marker 10 in accordance with the present invention is that it can withstand the temperatures for forming the laminate 1 shown in Fig. 1 without loss or corruption of the stored information. Typically the magnetic materials of Table 4 may withstand temperatures in excess of 120°C during production of the laminate 1 without change. Temperatures in excess of 150°C or even 250 °C can be used in preparing the laminate 1 without affecting the data stored in the marker 10. Suitable soft ferromagnetic materials may be chosen so that their Curie temperatures are in excess of 200°C. The marker 10 may experience considerably higher temperatures for a short time during manufacture provided the magnetic properties are not affected irreversibly. Suitable soft and remanent ferromagnetic materials can be chosen which may experience temperatures above 350° C without loss of the data. Hence, the marker 10 may also be included within an injection moulding, blow moulding, autoclave curing or other manufacturing machine or process so that the marker can be included in, moulded in or formed in any object produced by such machines. Further, as the elongate elements 9 may be made of soft ferromagnetic materials, high electromagnetic fields may change the magnetisation direction of each elongate element 9 but cannot destroy the stored information except by complete destruction or severe distortion or rupture of the laminate 1. Still further, as the marker 10 may be buried beneath a relatively thick and stable layer 2, such as an aluminium or similar metal layer, the marker 10 is visually camouflaged and is difficult to detect. Further, as the marker 10 is buried it cannot be altered by mechanical abrasion by sand, wind or de-icing procedures. Thus in accordance with the present invention, the cover layer 2 also may provide restricted mechanical access to the buried marker, in particular attempts to reach the marker can only be done by destroying or severely damaging at least a part of the cover layer 2. Yet a further advantage is that the mechanical properties of the laminate 1 are not seriously affected by marker 10. Clad aluminium alloy 2024-T3 is nowadays the most used material for fuselage skin material and is also a frequently used material for GLARE ™ fibre metal laminate. For the aluminium laminates both clad and bare material may be used with a thickness of 0.3, 0.4 and 0.5 mm. "Clad" means that an extra, nearly pure thin aluminium layer is provided on the alloy for protection of the aluminium alloy against environmental influences. "Bare" means that the protecting clad layer is not present. GLARE ™ is a material, based on thin layers of aluminium 2, 4, 6 combined with layers of glass-fibre 3, 5 (Fig. 3). In GLARE ™ 3 the same amount of glass fibres is orientated in 0° and 90° to the rolling direction of the aluminium layers. GLARE material is reported to out-perform aluminium with respect to fatigue and residual strength, although it has a slightly lower stiffness. Therefore, it makes the material desirable not only for repairs, but also for new constructions. Two types of GLARE™ 3 were used in the tests: GLARE 3-3/2-0.2. GLARE 3-2/1-0.3. The difference is the number of aluminium/prepreg layers (3/2 or 2/1) and the thickness of the aluminium layers (0.2 mm or 0.3 mm). All GLARE material used was standard commercially available material and delivered by the Structural Laminates Company, Delft, Holland. The adhesive bond used in the metal laminates 12 (specimen 5-7, Table 5) was the AF-163-2-OST made by the Minnesota Mining and Manufacturing Co., St. Paul, USA (3M). The patches 13 (specimens 1-4, table 5) were bonded with AF 163-2K, also made by 3M. Both adhesives are produced with a carrier, the difference is that the 2K-version has a woven carrier, where the OST-version has randomly orientated fibres. For the markers 10 an area of 170 mm by 100 mm was used. For testing, it was found that the undisturbed material next to the markers 10 should be at least 25 mm on each side of the label, resulting in an aluminium laminated specimen and a patch 13 of at least 150 mm wide. The lay-out of the specimens was as follows (see Table 5): Aluminium laminated specimens (Fig. 4): 160 mm wide, 420 mm high, marker attached to the outer aluminium layer, Patched-skin specimens (Fig. 5) : patch 150 mm wide, 220 mm high, skin sheet 400 mm wide, 900 mm high, marker attached to the patch 13. As the patches 13 should simulate a repair, a 50 mm cut was made using a jewellers saw in the underlying aluminium. This cut was repaired by drilling holes (3 mm diameter) at the crack tips and bonding a patch over the cracked area. The patch 13 had a corner radius of 25 mm (Fig. 5). The GLARE 3-2/1-0.3 patches were tested in LT-direction of the aluminium layers, whereas the GLARE 3-3/2-0.2 patches were tested in L-direction of the aluminium layers. The L-direction of a material is the rolling direction of the material, whereas LT is perpendicular to the L-direction in the plane of the L-direction. Table 5 Specimen preparation for the aluminium laminates was done as follows : - degreasing and cleaning with acetone - degreasing for 30 minutes in a bath at 70 degrees including an alkaline degreasing liquid such as P3RST from Henkel, DE - pickling for 20 minutes in a chromic-acid/sulphuric-acid bath at 60 degrees - bonding the soft ferromagnetic strips (100mm) in the pattern shown in Fig. 4 at three points with cyanoacrylate adhesive such as type 770 from Hottinger Baldwin MeBtechnik, DE - laminating the material with AF-163-2-OST - curing the material under pressure and temperature in an autoclave Specimen preparation for the repaired aluminium skin was done as follows : - saw cut of 50 mm with jewellers saw in the skin sheets - drilling 3 mm diameter holes at the ends of each cut - degreasing and cleaning the patches with acetone - bonding the soft ferromagnetic strips (100mm) in the pattern shown in figure 5 at three points with NOGO C20 cyanoacrylate adhesive - sanding the patch area of the skin with sandpaper 600 - sanding the patch area of the skin with sandpaper 2000 - degreasing and cleaning the skin area with acetone - bonding the patches with AF-163-2K from Minnesota Mining and Manufacturing Co., St Paul, USA to the skin-sheet, fixing the position of the patches with tape - curing the material under pressure and temperature in an autoclave The curing cycle in the autoclave was: application of vacuum for 30 minutes release vacuum pressure and temperature rise in 30 minutes up to 3 bar and 120°C cooling down in 30 minutes to 40°C releasing the pressure to 1 bar The specimens were then subjected to mechanical load cycles in tension. All the laminated specimens (5-7) reached the pre-set life of 200,000 cycles at a loading of 120 mPa. For the patch specimens 1-4, the test time was limited by crack propagation, but all specimens exceeded 170,000 cycles. No delaminations were found between the soft ferromagnetic elements 9 and the layer 2. No influence was determined which could be attributable to the elements 9. Electromagnetic testing before and after the load cycling showed that received signal strength from the marker 10 had not changed. A further advantage of the marker 10 in accordance with the present invention is that it need only include passive elements 9 rather than active elements such as transistors. This may make the marker 10 in accordance with the present invention easy and inexpensive to make, reliable, not affected by electromagnetic radiation or high temperatures (in excess of 125°C, e.g. in excess of 150°C) nor dependant upon the transfer of energy from an externally imposed magnetic field to power up any active devices. Nor need marker 10 include hard magnetic materials (but this is not excluded from the present invention) which may change their magnetisation direction semi-permanently in a high magnetic field. Figs. 6A and B are schematic representations of a coil arrangement suitable for generating a planar rotating magnetic field within a two dimensional interrogation area 29 equalling the size the field generator-detector 20 and parallel to its outer surface. A useful part of the magnetic field lies outside the generator-detector 20 so that the rotating magnetic field may be imposed upon a laminate 1 with a marker 10 such as shown in Fig. 1. The field generator-detector 20 includes two flat transmission coils 21, 22 and two flat receiving coils 23, 24. The interrogation zone 29 of the field generator-detector 20 lies outside the field generator-detector 20. The transmission coils 21 and 22 are wound at 90° to each other around a laminated metal core 25 which may be made of transformer steel plates. The core 25 increases the magnetic field around the generator-detector 20 in proportion to the ratio of the permeability of the core 25 compared with air. Associated with the transmission coils 21, 22 are two perpendicularly arranged receiving coils 23 and 24. The direction of the magnetic field vector 26 in the interrogation zone 29 is determined by the relative phase of sinusoidal currents in the coils 21 and 22. When the phase difference between the currents in the two coils is 90° a uniformly rotating field is obtained at any position in the interrogation zone 29. As the field rotates uniformly at any position within the interrogation zone 29 it is not important at which position the marker 10 is placed within the interrogation zone 29 to obtain a satisfactory receipt of the signals from the marker 10. The magnetic field generator-detector 20 in accordance with the invention may be designed to produce a magnetic field which may have a magnitude in the range 5 to 400 Gauss. The coils 21, 22 of the magnetic field generator-detector 20 may be driven so that they generate a magnetic field whose magnetic field vector has a substantially constant magnitude and moves through the 2- dimensional interrogation zone 29 in a sequence of different orientations or rotates smoothly. The output of the receiving coils 23, 24 may be fed to a controller and electronic signal processing device for segregation of the detected pulses from stray noise. Pulse wave shaping circuits may improve pulse quality to output the signal sequence. The orientations of the magnetic field vector may be selected randomly or may follow any particular regular sequence. The magnetic field vector may also oscillate through a small angle at each orientation. The controller and electronic signal processing device may include means to record each particular orientation of the magnetic field vector with which a pulse is generated. From this information it is then possible to identify magnetic field vector directions which produced pulses in the receiving coils and to reconstruct the stored data therefrom. If the field rotates uniformly, a sequence of pulses can be detected which follows a time pattern which is related in a one-to-one relationship with the spatial separation of the elongate elements 9 on the marker 10. If the magnetic field vector moves regularly through the interrogation zone 29 it is then merely necessary to record the time sequence of the pulses to infer the corresponding directions of the magnetic field vector. The pulse sequence may be fed to logic circuits to convert the time sequence of pulses into a binary number or other code. An embodiment of a detector circuit in accordance with the present invention will be described with reference to Fig. 6C. The controller 1 1 generates currents which are fed to the coils 21, 22 via amplifiers 36, 38. The required values of current are read from the EPROMS 35, 33 by the address circuits 30 to 31. The magnetic field generated by the coils 21, 22 rotates continuously in a plane to form the 2-D interrogation zone 29. As shown in Fig. 6C the outputs of the two orthogonal detection coils 23, 24 are used for detection and appear as distorted sine waves when a marker 10 is in the interrogation zone 29. The individual pulses generated from a particular elongate soft ferromagnetic element 9 are superimposed on the fundamental sine wave along with random noise and pulses generated from discharge across small gaps, etc. The magnitude of the magnetic field vector is calculated from the squares of the outputs of the coils 23, 24 generated in circuits 45 - 47. The outputs of the circuits 45, 47 are summed in the adder 48 to produce a combined signal. The combined signal is then fed to a superheterodyne receiver circuit 49 - 54. A first oscillator 53 generates a signal with an intermediate frequency (typically 455 kHz). The product of the intermediate frequency and the combined signal is generated in the mixer 49. The output of the mixer 49 is fed to a ceramic filter 50. The output of the ceramic filter 50 is demodulated in the demodulator circuit 51, 52, 54 by multiplying the output of the ceramic filter 50 with the intermediate frequency from the oscillator 54 in the mixer 51 and supplying the multiplied signal to a low-pass filter 52. The procedure described above is well known to the person skilled in AM-Receiver technology (see for example introduction to Communication Systems" by Ferrel G. Stremler, Addison-Wesley Publishing Company, Third Edition, 1990). The pulse chain from the low-pass filter 52 is fed to the controller 11 from which it may be sent to an external computer (not shown) for further analysis and decoding. Any particular direction of the magnetic field vector 26 may be determined from the phases of the detection coils 23, 24 (not shown). Means for determining the orientation of the magnetic field vector 26 may be a circuit for comparing the relative phases of the outputs of coils 23, 24 and may be included in the controller 11 or in a separate microprocessor or computer (not shown). A further embodiment of the present invention is shown schematically in Figs. 7A and B. It consists of a series of soft ferromagnetic elongate elements 9 similar to those described above sandwiched between two aluminium plates 27 and 28. The aluminium plates 27, 28 may be adhered together using a curing adhesive. The aluminium plates 27, 28 are preferably 0.1 to 2 mm thick or may be made of any conductive material with a value of V greater than 300 where V is given by V = (thickness of conductive layer in mm) 2 x (relative magnetic permeability of the conductive layer) / (specific resistivity of conductive layer in ohm. mm). The elements 9 are arranged with angular differences between them in order to provide a code when read out with a suitable detection device, e.g. as has been described with respect to Fig. 6. The identification device 25 may be used as an electronic key suitable for hotel room doors or for private house doors. Due to the fact that the electronic key is made of a laminate of two aluminium plates 27, 28, the device 25 is extremely robust and may replace metallic keys conventionally used for locks. A suitable lock 40 is shown in Fig. 8. The lock includes a space 41 for receipt of the device or electronic key 25. The space 41 forms an interrogation zone of a detector 42 similar to the detector described with respect to Fig. 6. The lock 40 also includes suitable electronics 43 and power leads 32 and 34 for controlling and powering of the detector 42. The electronics 43 are also capable of detecting the code of any device 25 and comparing the code with a code stored within its own memory. If the code of the device 25 matches with the stored code in electronics 43 the solenoids 39 and 37 operate to first of all retract the locking pin 44 out of the recess 46 in bolt 55 against the force of spring 56 and then to withdraw bolt 55 from its position in the lock against the force of spring 57. The identification marker 10 shown in Fig. 1 is of the radial type but in accordance with the present invention the elongate soft ferromagnetic elements 9 may be arranged in any spatial arrangement which is suitable for generating an identification code. In particular it is not necessary that the elongate elements 9 are arranged radially. Another embodiment of the invention includes a linear arrangement of elongate elements 9 similar in appearance to a conventional linear optical bar code. In this embodiment the linear distances between the elongate elements 9 may define the code. The marker 10 in accordance with the present invention may include various shapes of soft ferromagnetic materials and the present invention is not limited to separate elongate elements 9 for forming the code of marker 10. Surprisingly, it has been found that a single strip of soft ferromagnetic material bent to various angles as shown schematically in Fig. 9A produces a series of signals similar to that produced by the arrangement of ferromagnetic strip elements shown in Fig. 9B. Hence, the elements 9 of ferromagnetic material do not have to be electrically or magnetically separate from each other. In particular, the arrangement as shown in Fig. 9A can be produced by laying a wire 59 of soft ferromagnetic material onto an aluminium tape 60 covered with adhesive 58 on one side as shown in Fig. 9C and in cross-section in Fig. 9D. Further, it has been determined that a multi-sided sheet of soft ferromagnetic material as shown schematically in Fig. 10A generates a signal similar to that from the strip arrangement shown in Fig. 10B. From these examples it can assumed that it is each straight edge region of any shape or figure of a soft ferromagnetic material which will produce a signal whose position within one cycle of the rotating external magnetic field is determined by its angle. The magnitude of the signal is determined the length of the edge portion. Thus, the present invention includes a marker 10 comprising ferromagnetic material having a polyhedral shape including both sheet and strip products. The above description of the present invention relates to moving the magnetic field vector relative to the marker 10. However the present invention also includes moving the marker 10 relative to a magnetic field. For instance the marker 10 of the type shown in Figs. 1, 7, 8 A, 9A or 10A may be placed on an object mounted on a rotating turntable in a static magnetic field. Any form of relative movement between the marker 10 and the magnetic field vector which results in a change in the orientation of the magnetic field vector relative to the marker 10 such that a plurality of such orientations is generated in a sequence is included within the present invention. In accordance with the present invention it is preferred if the marker 10 includes elements 9 which record a certain information as a read only memory. However, the present invention also includes a read/writable memory. The soft ferromagnetic elements shown schematically in Figs. 1 1 A to 1 IE may be laminated with a conductive layer 2 to form a camouflagable, non-invasively measurable, embeddable marker in accordance with the present invention. A marker 10 can be made programmable by adapting the elongate soft ferromagnetic elements 9 so that they lose their ability to reverse their magnetised direction without causing permanent damage. This may be done by providing keeper elements associated with one or more of the elongate magnetic elements 9. By changing the state of the keeper element the associated elongate element 9 can be activated or de- activated. These elements are known as such and they are not described in detail here. The keeper element is preferably made of a material with remanent magnetic properties. The keeper element preferably includes a material having a higher coercivity and a higher saturation flux density than the material of the elongate element 9. The keeper element may include gamma Fe 2 O 3 , Vicalloy, Remendur, Arnochrome III made by Arnold Engineering, USA or any similar material. As long as the keeper element is magnetised it prevents the change in the magnetic direction of the associated elongate element 9 or at least severely alters the change so that the pulses detected can be distinguished from each other. If the keeper element is placed in a powerful (above the coercive force of the keeper element) but decreasing oscillating magnetic field it becomes demagnetised and the associated elongate element 9 may then respond to an external magnetic field. The keeper element may also be a relatively wide laminate of the soft ferromagnetic and the remanent material. The keeper element may be superimposed on, or lie underneath the elongate soft ferromagnetic element 9 and preferably includes a remanent magnetic material. The marker 10 of Fig. 11 A is known in principle from US-A-3665449. Adjacent to the elongate soft ferromagnetic element 9 is a keeper or control element 15 having remanent magnet properties and magnetically coupled to the elongate soft magnetic element 9 because of its close proximity. The magnetic material of the keeper element 15 has preferably a higher coercive force and saturation flux density than that of the soft magnetic material. The keeper element should preferably not become saturated in the magnetic field required for detection of the marker 10. As long as the keeper element is magnetised it influences the elongate element 9 so that a change of magnetic direction of the elongate element 9 can not occur. Complete suppression of magnetic reversal is not necessary provided the pulses generated in the detecting coils differ sufficiently to be distinguished from each other. Each element 9 may also consist of a laminate of a soft ferromagnetic 19 and a remanent material 15 as described in US-A-4746908 and shown schematically in Fig. 1 IB. If the marker 10 is placed in a powerful decreasing oscillating magnetic field the keeper element 15 is de-magnetised and each elongate element 9 can now generate pulses. For instance, when embedded within articles in a shop to prevent pilfering, the keeper element 15 is first de-magnetised. After purchase it is magnetised so that the marker 10 does not activate the detection system. As the marker 10 is buried underneath a conductive cover layer 2 it cannot be seen visually, hence is camouflaged. The marker 10 shown in Fig. 11C is known from US-A-4746908. It consists of a relatively large piece of soft ferromagnetic elongate elements 9, 19 and keeper elements 15, 25 which have a magnetic pattern. This marker 10 works on the principle that a large area of soft magnetic material produces little or no pulses. Accordingly when the keeper elements 15, 25 are demagnetised the soft ferromagnetic elongate elements 9, 19 do not produce pulses. When the keeper elements 15, 25 are magnetised they cancel out the area of soft ferromagnetic material lying beneath them leaving an active elongate soft ferromagnetic elongate element 9. The marker 10 shown in Fig. 1 ID is known in principle from US-A-3983552. Instead of placing the keeper element 15 alongside the soft magnetic elongate element 9 the two are superimposed. The areas of the keeper element 15 and the elongate element 9 may be the same and the two may be laminated together. Depending on the relative size and magnetic strengths of the keeper element 15 and the elongate element 9, pulses may be suppressed or distorted which can be used for detection purposes. The marker 10 shown schematically in Fig. 1 IE is known from US-A-3747086 and consists of a soft ferromagnetic elongate element 9 and two keeper elements 15, 25 having differing remanent magnetic properties. Depending upon the magnetic state of the keeper elements 15 and/or 25 the nature of the pulses generated by the elongate element 9 can be used for detection purposes. As described above markers 10 are used in a detection system in accordance with the present invention including means to generate a magnetic field having a substantially constant magnitude and the markers 10 are detected by varying the relative orientation of the marker 10 and the magnetic field vector having a substantially constant magnitude. The detection system may also include a conventional detection system as described in US-A- 4746908, 3983552, 3747086 or 3665449 in addition to the detection system in accordance with the present invention. The conventional system can be used to locate the approximate position of the marker 10 and the detection system in accordance with the present invention may be used to read out the code. The use of the keeper element 15 in accordance with Fig. 11 will now be described with reference to improving the security of marker 10 in accordance with a further embodiment of the present invention. The marker 10 in accordance with this embodiment may include a radial sequence of soft ferromagnetic elongate elements 9 as shown schematically in Fig. 1. The first three strips may define a start zone so that the start of the sequence may be determined, e.g. the first strip is at 0°, the second strip at 7 and the third strip at 10°. The remainder of the strips define a binary number and are located at 5° intervals. The sequence 7° and 3° of the start zone is therefore uniquely identifiable. The remaining elements 9 up to 185° are located at any of the 5° spacings from 15° up to 85°. The presence of a strip 9 indicates a " 1 " in the binary number. The largest number which can be represented is 2 35 . For each part produced a number is allocated by a single authorising body and registered in a publicly available database, e.g. on the Internet protected if necessary by passwords. Hence, once a number has been assigned, any new part which has a marker 10 which represents the same number as an assigned number is a fake. Additionally each element 9 is made up of a lamination of two materials, all looking identical. From the number allocated a smaller number is generated by a secure algorithm, e.g. a one-way function which is a function from which a smaller number may be generated easily from a larger number, but with which knowledge of several of the larger and smaller numbers still does provide the possibility of calculating the smaller number for any other larger number. Once the smaller number is obtained which may be a hash number for instance, a count is started at the first element 9 present in the marker 10. Let us say that there are 15 elements 9 in the marker 10 and the hash number is 6. Then the count is started from the first strip next to the start zone and the sixth element 9 is a special strip which is a laminate of the kind shown in Fig. 11 B. This element 9 has a keeper element 15 laminated with a soft magnetic material. The marker 10 now behaves differently depending upon the magnetic treatment of the marker 10. After subjecting the marker 10 to a large decreasing oscillating magnetic field the marker 10 reads out with the correct number when placed in a rotating magnetic field with a magnetic vector of substantially constant magnitude. If the marker 10 is then subjected to a strong constant magnetic field it reads out a different number as the response of the sixth element 9 is suppressed. Confirmation of the correctness of such a comparison assists in detecting a valid marker 10. The present invention has been described with reference to certain embodiments and drawings, but the skilled person will appreciate that modifications and additions may be made to the present invention while still remaining within the scope of the present invention as defined in the attached claims. For instance, the present invention includes an embedded marker 10 which may be read out non-invasively and non-destructively by using ultrasonic test equipment. The marker 10 may be made of a material which has ultrasound contrast compared with the surrounding materials. For instance, it has been found that with the test samples mentioned above (Figs. 4 and 5) it was possible to detect the embedded marker 10 and the elements 9 using ultrasonic test equipment having a probe generating wideband 10 MHz transducer producing pulses of ultrasound whose reflection from the marker 10 were recorded. It is not necessary to make the elements 9 from soft ferromagnetic material for this form of detection. Further, the elements 9 could be of any shape, e.g. they could be alphanumeric characters. Ultrasound testing can be used as an additional method of reading out markers 10 which can also be read out using the rotating magnetic field mentioned above. For instance a marker 10 may be made up of a mixture of soft ferromagnetic elements 9 and other similar but non-magnetic elements. Hence, the number read out by the magnetic method would differ from that seen by ultrasound, which difference could be used as an additional security check. An embeddable non-invasively and non-destructively readable marker may also be made using materials which are more opaque to γ-rays than the surrounding materials. By shining a γ-ray source though the component with sensitive photographic material on the other side a γ-ray photo of the embedded marker 10 is obtained. The γ-ray and ultrasound methods of reading out a marker 10 are less preferred than the magnetic method of the earlier embodiments as γ-ray sources require special safety handling requirements and ultrasound probes work best with a gel film between the probe and the object to be examined which is more clumsy than the simple hand-held test equipment using detector coils as shown in Fig. 6 which allows marker 10 to be read out remotely.

Claims: C L A I M S 1. An article including an identification marker, the marker including stored information, said article comprising a first conductive layer, said conductive layer restricting mechanical and RF access to said marker; and said marker being visually camouflageable beneath said conductive layer and being non-destructively and non-invasively measurable through said conductive layer. 2. An article according to claim 1, wherein said marker is non-communication channel based. 3. An article according to claim 1, wherein said marker includes a soft ferromagnetic element having a plurality of substantially straight edges, the edges being arranged in a pre-formed spatial relationship to each other, said spatial relationship being pre-selected to define the stored information, the soft ferromagnetic element having a low magnetic resistance (high magnetic susceptibility); and each edge of the soft ferromagnetic element having at least one state in which the magnetising direction is reversible when the relative orientation between said marker and a magnetic field vector of a magnetic field having a substantially constant magnitude is varied. 4. An article according to claim 3, wherein the edges are in an angular relationship to each other, the angular separations between the edges defining said information. 5. An article according to claim 1, wherein said marker is sandwiched between the first conductive layer and a second conductive layer. 6. An article according to claim 1, wherein the information stored in said marker is not electromagnetically erasable. 7. An embeddable identification marker for the verification and tracing of components, said marker consisting essentially of passive elements and being visually camouflageable. 8. An embeddable identification marker for the verification and tracing of components, said marker being processable at temperatures exceeding 120°C without loss of information content and being visually camouflageable. 9. An embeddable verification marker for the verification and tracing of components, said marker including stored information which can be non-destructively and non-invasively read out through a mechanical and RF barrier; and said marker being visually camouflageable. 10. A system for identifying objects including an identification marker, said marker including stored information which can be non-destructively and non-invasively read out through a mechanical and RF barrier; and said marker being visually camouflageable. 1 1. A system according to claim 10, wherein said marker is non-communication channel based. 12. A system according to claim 10, wherein said marker includes a soft ferromagnetic element having a plurality of substantially straight edges, the edges being arranged in a pre-formed spatial relationship to each other, said spatial relationship being pre-selected to define the stored information; the soft ferromagnetic element having a low magnetic resistance (high magnetic susceptibility); and each edge of the soft ferromagnetic element having at least one state in which the magnetising direction is reversible when the relative orientation between said marker and a magnetic field vector of a magnetic field having a substantially constant magnitude is varied. 13. A system according to claim 12, wherein edges are in an angular relationship to each other, the angular separations between the edges defining the information. 14. A system according to claim 10, wherein said marker is sandwiched between the RF barrier and another RF barrier. 15. A system according to claim 10, wherein the information stored in said marker is not electromagnetically erasable. 16. A system according to claim 12, wherein the magnetic field is a rotating field with a substantially constant magnitude. 17. A device for generating a magnetic field within a predetermined interrogation zone, comprising: means for generating a magnetic vector of said magnetic field having a substantially constant magnitude; means for varying the orientation of said magnetic field vector within said interrogation zone; and means for determining the magnitude of said magnetic field vector, wherein the interrogation zone lies outside and is not bounded by the generating and varying means. 18. Generating device according to claim 17, wherein said generating means includes two pairs of quadratic mutually orthogonal windings each having a rectangular cross section. 19. A marker comprising a soft ferromagnetic element having a plurality of substantially straight edges, the edges being arranged in a pre-formed spatial relationship to each other, said spatial relationship being pre-selected to define a code, the straight edges being linked together, the soft ferromagnetic element having a low magnetic resistance (high magnetic susceptibility); and each straight edge of the soft ferromagnetic element having at least one state in which the magnetising direction is reversible when the relative orientation between said marker and a magnetic field vector of a magnetic field having a substantially constant magnitude is varied.

Inventors: LACAVE DANIEL, SCHENKE KAY-UWE, VANDESTEEG GREGG A, LACAVE, DANIEL, SCHENKE, KAY-UWE, VANDESTEEG, GREGG, A.
Applicants: MINNESOTA MINING & MFG, MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1999
Main classification: G06K19
IPC codes: B64D47, G06K19, H04B1