CA2319237C - Magnetic markers or tags - Google Patents

Abstract

A magnetic marker or tag which is characterised by a substrate carrying a plurality of discrete magnetically active regions disposed in one or more linear arrays, the magnetically active regions being formed of a thin film or spin-melt material having a preferential axis of magnetisation, and the preferential axes of magnetisation being aligned within the (or each) linear aray.

Description

MAGNETIC MARKERS OR TAGS
This application is divided out of Application No.

2,217,762.
The parent application relates to the exploitation of magnetic properties in a range of practical techniques, and utilises a new technique of spatial magnetic interrogation in conjunction with a magnetic to marker or identification tag. More particularly, it relates to methods of determining the presence and/or the location of a magnetic marker or tag within an interrogation zone; to methods of identifying a magnetic tag (e.g. identifying a given tag in order to i5 discriminate that tag from others); to systems for putting these methods into practice; to magnetic tags for use in such methods and systems; and to the storage of data in such tags, and the subsequent remote retrieval of data from such tags.
2o It should be understood that the terms "tag" and marker" are used herein interchangeably; such devices may be used in many different applications and, depending on the magnetic qualities of the device, may serve to denote (a) the mere presence of the tag (and hence that of an 25 article to which the tag is attached); or (b) the identity of the tag (and hence that of an article to which it is attached); or they may serve to define the precise position of the tag with respect to predetermined co-ordinates (and hence that of an article to which it is 3o attached); or they may serve to provide access codes (e. g. for entry into secure premises; or for ticketing purposes, e.g. on public transport networks); or they may serve generally to discriminate one article or set of articles from other articles.

In addition, the terms "ac field" and "DC field" are used herein to denoted magnetic fields whose characteristics are, respectively, those associated with an electrical conductor carrying an alternating current (ac) or a direct current (DC) .
The tags, methods and systems of this invention have a wide variety of applications as indicated above. These include (but are not restricted to) inventory control, ticketing, automated shopping systems, monitoring work-io in-progress, security tagging, access control, anti-counterfeiting, and location of objects (in particular the precise positioning of workpieces [e.g. probes in surgery]).
Prior Art There are a number of passive data tag systems currently available. The most widely-used is based on optically-read printed patterns of lines, popularly known as barcodes. The tag element of such .systems is very low-cost, being typically just ink and paper. The readers are also relatively low cost, typically employing scanning laser beams. For many major applications the only real drawback to barcodes is the need for line-of-sight between the reader and the tag.
For applications where line-of-sight is not possible, systems not employing optical transmission have been developed. The most popular employ magnetic induction for coupling between the tag and the interrogator electronics. These typically operate with 3o alternating magnetic fields in the frequency range of 50kHz to lMHz, and generally employ integrated electronic circuits ("chips") to handle receive and transmit functions, and to provide data storage and manipulation.
In order to avoid the need for a battery, power for the chip is obtained by rectification of the interrogating signal received by an antenna coil. In order to increase the power transferred, and to provide discrimination against unwanted signals and interference, the coil is s usually resonated with a capacitor at the frequency of the interrogation signal carrier frequency. A typical product of this type is the TIRTS system manufactured by Texas Instruments Ltd.
Other mufti-bit data tag systems have employed to conventional h.f. radio technology, or technologies based on surface acoustic waves or magnetostriction phenomena.
The Invention The present invention involves, inter alia, the use 15 of a new type of passive data tag system which employs small amounts of very high-permeability magnetic material, and a scanned magnetic field for interrogation.
Since the magnetic material can be in the form of a thin foil, wire or film, it can be bonded directly to a 2o substrate, e.g. paper or a plastics material, to form self-supporting tags.
Alternatively, the magnetic material may be incorporated into the structure of an article with which the tag is to be associated; thus a tag may be formed in 25 situ with the article in question by applying the magnetic material to the surface of the article, or by embedding the magnetic material within the body of the article.
The invention exploits magnetic fields which contain 3o a "magnetic null" - this term is used herein to mean a point, line, plane or volume in space at or within which the component of the magnetic field in a given linear direction is zero. The volume in space over which this condition is met can be very small and this gives rise to certain embodiments of the invention in which precise position is determine a. Typically the magnetic null will be extant over a relatively small linear range. It should be understood that, where there is a magnetic null, it is possible (and is often the case) that the magnetic field component in a direction orthogonal to the given linear direction will be substantial. In some embodiments of this invention, such a substantial orthogonal field is desirable.
to One way of creating the magnetic null is to employ opposing magnetic field sources. These may be current-carrying coils of wire, or permanent magnets (these being well suited to small-scale systems), or combinations of coils) and permanent magnet(s). It is also possible to exploit the magnetic nulls which exist in specific directions when a single coil or permanent magnet is used.
For large scale applications, the magnetic field sources are preferably coils carrying direct current.
2o The invention also utilises the relative movement between a magnetic marker and an applied magnetic field in order to affect passage over of the marker of the magnetic null. This can be achieved by moving the marker with respect to the applied magnetic field, or by holding 2s the marker in a fixed position while the magnetic field is scanned over it. Generally, the invention exploits the difference between the magnetic behaviour of the marker in (i) a zero field (at the magnetic null), and (ii) in a high, generally saturating, magnetic field.
3o According to an aspect of the present invention a magnetic marker or tag for encoding information which is characterised by a substrate carrying a plurality of discrete magnetically active regions having identical magnetic characteristics and being disposed in one or _ 5 _ more linear arrays, the magnetically active regions being formed of a thin film or spin-melt material having a preferential axis of magnetisation, wherein the preferential axes of magnetisation of the magnetically active regions of the or each array are aligned with each other, and wherein the lengths of and/or the spaces between, the magnetically active regions serves to encode information.
In the methods according to this invention, io detection of magnetic response of the magnetic element advantageously comprises observation of harmonics of the applied AC field which are generated by the magnetic element as its magnetisation state is altered by passing through the magnetic null.
i5 As indicated above, the system operates with a zero or very low frequency scanning field, and an HF (high frequency) in the range 50Hz - 50kHz. This allows for good signal penetration through most materials including thin metal foils. In addition, international regulations zo allow high fields for transmission at these low frequencies.
Preferred embodiments of the invention provide multi-bit data tag system which employs low-frequency inductive magnetic interrogation, and avoids the need for z5 complex, expensive tags.
According to another aspect of the present invention, there is provided a method of coding and/or labelling individual articles within a predetermined set of articles by means of data characteristic of the 3o articles, e.g. article price and/or the nature of the goods constituting the articles, which method is characterised by applying to each article a magnetic tag or marker carrying a predetermined arrangement of magnetic zones unique to that article or to that article and others sharing the same characteristic, e.g. article price or the nature of the goods constituting the article, said magnetic tag or marker being susceptible to interrogation by an applied magnetic field to generate a s response indicative of the magnetic properties of the tag or marker and hence indicative of the nature of the article carrying the magnetic tag or marker.
Fundamentals of the Invention to Before describing further embodiments, it will be helpful to explain some fundamental aspects of the invention giving reference, where appropriate, to relatively simple embodiments.
A key aspect of the invention is the form of the is magnetic field created in the interrogation Zone; as will become apparent later, this field allows very small spatial regions to be interrogated. The means for generating this magnetic field will be termed hereinafter an "interrogator".
2o In one simple form, the interrogator consists of a pair of closely-spaced identical coils arranged with their axes coincident. The coils are connected together such that their winding directions are opposed in sense, and a DC current is passed through them. This causes z5 opposing magnetic fields to be set up on the coils axis, such that a position of zero field - a magnetic null - is created along the coil axis, mid-way between the coils.
The level of current in the coils is such as to heavily saturate a small sample of high permeability magnetic 3o material placed at the centre of either of the two coils.
A much lower amplitude AC current is also caused to flow in opposite directions through the two coils, so that the AC fields produced sum together mid-way between the coils. This can easily be arranged by connecting a suitable current source to the junction of the two coils, with a ground return. The frequency of this AC current may typically be about 2 kHz, but its value is not critical, and suitable frequencies extend over a wide range. This AC current generates the interrogating field which interacts with a magnetic tag to generate a detectable response. Another effect of this AC current is to cause the position of zero field - the magnetic null -to oscillate about the mid-way position along the coils to axis by a small amount (this is a wobble or oscillation rather than an excursion of any significant extent) In addition, a further, low frequency AC current may be fed to the coils so as to generate a low frequency scanning field (which may be zero). The frequency of the scanning field (when present) should be sufficiently low to allow many cycles of the relatively high frequency interrogation field to occur in the time that the magnetic null region passes over the tag; typically, the frequency ratio of interrogating field (w~) to the zo scanning field (wb) is of the order of 100:1, although it will be appreciated that this ratio can vary over a considerable range without there being any deleterious effect on the performance of the invention.
When a tag containing a piece of high-permeability z5 magnetic material is passed along the coils axis through the region over which oscillation of the magnetic zero plane occurs, it will initially be completely saturated by the DC magnetic field. It will next briefly be driven over its B-H loop as it passes through the zero field 3o region. Finally it will become saturated again. The region over which the magnetic material is "active", i.e.
is undergoing magnetic changes, will be physically small, and is determined by the amplitude of the DC field, the amplitude of the AC field, and the characteristics of the magnetic material. This region can easily be less than 1 mm in extent. If the level of the alternating field is well below that required to saturate the magnetic material in the tag, then harmonics of the AC signal will be generated by the tag as it enters the zero field region of interrogator field and responds to the changing field. As the tag straddles the narrow zero field region the tag will be driven on the linear part of its B-H
loop, and will interact by re-radiating only the to fundamental interrogation frequency. Then, as the tag leaves the zero field region, it will again emit harmonics of the interrogation field frequency. A
receiver coil arranged to be sensitive to fields produced at the zero field region, but which does not couple directly to the interrogator coils, will receive only these signals. The variation of these signals with time as the tag passes along the coils axis gives a clear indication of the passage of the ends of the magnetic material through the zero field region.
zo It will be appreciated that because the interrogation zone can be very narrow, each individual piece of magnetic material can be distinguished from its neighbours, from which it is separated by a small distance. Naturally, the magnetic material will be selected to suit the particular application far which the tag is intended. Suitable magnetic materials are commercially available, as described hereinbefore.
If a tag containing a number of zones or pieces of magnetic material placed along the axis of the label is 3o now considered, it will be appreciated that as each zone or piece of magnetic material passes through the zero-field region, its presence and the positions of its ends can be detected. It then becomes a simple matter to use the lengths and spacing of individual zones or pieces of _g_ magnetic material to represent particular code sequences.
Many different coding schemes are possible: one efficient arrangement is to use an analogue of the coding scheme used for optical barcodes, where data is represented by s the spacing and widths of the lines in the code.
The system so far described allows for the scanning of a single-axis tag (e.g. a wire or a thin strip of anisotropic material, having a magnetic axis along its length) as it physically moves through the coil assembly.
to It will be appreciated that relative movement between the tag and the interrogating field can be achieved either with the field stationary and the tag moving, or vice versa. Tf required, the arrangement can be made self-scanning, and thus able to interrogate a stationary tag, i5 e.g, by modulating the d.c. drive currents to the two interrogator coils, so that the zero field region scans over an appropriate portion the axis of the coils. The extent of this oscillation needs to be at least equal to the maximum dimension of a tag, and should preferably be 2o considerably greater, to avoid the need for precise tag positioning within the interrogation zone.
By using extra coils arranged on the 2 axes orthogonal to the original, tags in random orientations can be read by sequentially field scanning. This involves 25 much greater complexity in the correlation of signals from the three planes, but because of the very high spatial resolution available would be capable of reading many tags simultaneously present in a common interrogation volume. This is of enormous benefit for 3o applications such as tagging everyday retail shopping items, and, for example, would allow automated price totalisation of a bag of shopping at the point of sale.
Thus the invention has applicability to the price labelling of articles and to point-of-sale systems which generate a sales total (with or without accompanying inventory-related data processing).
The size of a simple linear tag is dependent on the length of the individual elements, their spacing and the number of data bits required. Using strips of the highest permeability material commercially available, such as the "spin-melt" alloy foils available from suppliers such as Vacuumschmeltze (Germany) and Allied Signal (USA), the minimum length of individual elements which can be used to is probably of the order, of a few millimetres. This is because the extrinsic permeability will be dominated by shape factors rather than by the very high intrinsic permeability (typically 10~), and shorter lengths may have insufficient permeability for satisfactory operation.
For this reason it is attractive to use very thin films of high permeability magnetic material. Provided it is very thin, (ideally less than lam), such material can be cut into small 2 dimensional pieces (squares, discs 2o etc) with areas of just 20 mm2 or less, yet still retain high permeability. This will enable shorter tags than possible with elements made from commercially available high-permeability foils. Suitable thin film materials are available commercially from IST (Belgium).
An extension to this type of programming can also be used to prevent the composite tag producing an alarm in a retail security system (such an alarm would be a false indication of theft, and would thus be an embarrassment both to the retailer and to the purchaser). If different 3o regions of the tag are biased with different static field levels, they will produce signals at different times when they pass through retail security systems. This will complicate the label signature in such systems and prevent an alarm being caused. In the present invention, the reading system will be able to handle the time-shifted signals caused by such magnetic biassing.
Thus far tag coding has been described on the basis of physically separated magnetic elements. It is not s essential, however, to physically separate the elements;
programming of data onto a tag may be accomplished by destroying the high-permeability properties of a continuous magnetic element in selected regions thereto.
This can be done, for example, by local heating to above 1o the recrystallisation temperature of the amorphous alloy, or by stamping or otherwise working the material. Of even more importance is the ability to magnetically isolate regions of a continuous element of high permeability material by means of a magnetic pattern stored on an i5 adjacent bias element made from medium of high coercivity magnetic material. Such a composite tag could then be simply coded by writing a magnetic pattern onto the bias element using a suitable magnetic recording head. If required, the tag could then be erased (by de-gaussing 2o with an AC field) and re-programmed with new data.
The scheme described can also be extended to operate with tags storing data in two dimensions. This allows for much more compact tags, since as well as being a more convenient form, a tag made up from an N x N array of 2s thin-film patches has much more coding potential than a linear array of the same number of patches. This is because there are many more unique patch inter-relationships that can be set up in a given area.
3o Further Embodiments Use of Spatial Magnetic Scanning for Position Sensing In addition to interrogating space to read data tags, this new technique of moving planes of zero field through space (or moving things through the planes) can be used to provide accurate location information for small items of high permeability magnetic material.
Thus, according to another aspect, the invention provides a method of determining the precise location of an object, characterised in that the method comprises:
(a) securing to the object a small piece of a magnetic material which is of high magnetic permeability;
(b) applying to the region in which said object is to located a magnetic field comprising two opposed field components, generated by magnetic field sources, which result in a null field at a position intermediate said magnetic field sources; (c) applying a low amplitude, high frequency interrogating field to said region; (d) i5 causing the position of the null field to sweep slowly back and forth over a predetermined range of movement;
(e) observing the magnetic interaction between said applied magnetic field and said small piece of magnetic material; and (f) calculating the position of the object 2o from a consideration of said magnetic interaction and form the known magnetic parameters relating to said applied field and to said small piece of magnetic material. Advantageously, the small piece of high permeability magnetic material is in the form of a thin z5 foil, a wire or a thin film.
This aspect of the invention is of particular interest when the object whose location is to be determine is a surgical instrument, for example a surgical probe or needle. The invention allows precise 3o determination of the location of, for example, a surgical probe during an operation.
This technique is ideal for accurate location of very small markers within relatively confined volumes; it can separately resolve multiple markers. It also displays low sensitivity to extraneous metal objects.
The magnetic tag or marker can typically be a 1 cm length (longer if desired) of amorphous wire (non-corrosive, diameter 90 micron or less) similar to that used in EAS tags or, with suitable process development, a short length (e. g. 1 cm) of a needle sputter-coated with a thin layer of soft magnetic material.
In use around the head of a patient, resolution to l0 0.1 mm with the described markers can be achieved.
Accuracy should also have the potential to approach this value if some precautions about calibration and use of other magnetic materials are observed, but for optimum performance a rigid but open structure close to the head would be desired. The magnetic field levels employed will be lower than those generated by every day magnets (e. g.
kitchen door catches etc).
This technique has particular application to brain surgery, where there is the requirement to locate the 2o position of probes in three dimensions and with high precision. It is therefore possible, in accordance with this invention, to use small magnetic markers on such probes or needles. In this case, a key advantage is that the signal from the marker need only be detected and resolved in time; the resolution is determined by the location of the zero field plane, not by the signal to noise ratio of the detected marker signal. This permits a very small marker to be used.
A single axis position sensor may be implemented 3o with a set of coils similar to the tag reading system described above. This comprises: a pair of opposed coils carrying DC current to generate a DC field gradient; a means of applying a relatively uniform low level AC field to drive the marker in and out of saturation in the small region where the DC field is close to zero; and a means of applying a relatively uniform DC field of variable strength and polarity to move the location of the plane of zero DC field around the volume to be interrogated.
An anisotropic marker - i.e. one having a preferential axis of magnetisation - resolves the magnetic field along its length. Such a marker can be obtained, for example, by using a long, thin element of a magnetic material or by suitable treatment of an area of to magnetic material having a much lower aspect ratio, e.g.
by longitudinally annealing a generally rectangular patch of a spin-melt magnetic material. In the context of the single axis position sensor under discussion there are five degrees of freedom (x, y, z and two angles (rotation of the marker about its axis has no effect)). Three orthogonal complete sets of coils can capture sufficient information by doing three scans of the uniform DC field on each of the sets of coils in turn. The first scan with no field from the other sets, the second with a uniform 2o DC field from one of the other sets, and the third with DC field from the other set. This gives nine scans in all; these may be represented as in the following table, in which the magnetic field sources are identified as a, b and c and the scans are numbered from 1-9 (scanning order being of no significance) Orthogonal f field 1 2 3 4 5 6 7 8 9 source a ON ON ON OFF OFF ON OFF OFF ON

b OFF ON OFF ON ON ON OFF ON OFF

c OFF OFF ON OFF ON OFF ON ON ON

The only information required from each scan is the position of the centre of the harmonic output from the marker within that scan. These nine DC field values can then be converted into the xyz-theta-phi co-ordinates of the marker. To start with, the system can simply be used by holding the marker in the desired position before the head is put into the coils; and then when the head is placed in the coils the marker can be moved until the same signals are obtained.
An alternative to sequential interrogation which has the advantage of requiring less time to scan the region to of interest is to rotate the magnetic field gradient continuously so as to scan all directions of interest.
This can be accomplished by driving three sets of coils with appropriate continuous waveforms. For example, a suitable scanning field will be created if coils in the x, y and z planes are driven with currents IX, Iy, and IZ, given by the equations:
IX = cos wat (A cos cobt - sin wbt . sin c~~t ) - Sln CJat . COS CJ~t 2o Iy = sin coat (A cos wbt - sin wbt . sin wit) + cos wat . cos wit IZ - A sin c~bt + cos c.~bt . sin wit Where: wa = overall frequency of rotation of applied magnetic field wb = null scanning frequency c~c = interrogation frequency A = amplitude ratio cub : wC-Typical (but non-limiting) values of these 3o parameters are: A10;
frequency rat io c~a . cob = 1 . 10 ; and f requency raio wb . c~~ = 1 . 4 0 0 .

Description of the Drawinas The invention will now be illustrated with reference to the accompanying drawings, in which:
FIGURE 1 illustrates the fundamental elements of a tag reading system of the invention;
FIGURE 2 is a circuit diagram illustrating one mode of generating the desired magnetic field pattern with the arrangement of Fig. 1;
io FIGURE 3 relates the magnetic response of a tag to its position within the reading system of Fig. 1;
FIGURE 4 illustrates where magnetic nulls occur with a permanent magnet;
FIGURE 5 illustrates an embodiment of the invention i5 which utilises a coil and a permanent magnet to generate the desired field pattern;
FIGURE 6 illustrates an embodiment of the invention which utilises a pair of permanent magnets to generate the desired field pattern;
2o FIGURE 7 illustrates an embodiment of the invention which utilises a plurality of permanent magnets disposed in an annular array with a coil to generate the desired field pattern;
FIGURE 8 is a schematic circuit diagram for one 25 embodiment of a tag interrogator in accordance with the invention;
FIGURE 9 illustrates a selection of tags in accordance with this invention; and FIGURE 10 illustrates an embodiment of the invention 3o as applied to surgical operations.
Referring to Figure 1, a schematic arrangement is shown in which a tag 1 is positioned mid-way between two coils Txl and Tx2. ~The tag is of the type shown in Figure 9a, i.e. a simple linear tag carrying a plurality of magnetic elements each of which is a high-permeability magnetic alloy material, for example Vacuumschmeltze 6025 spin melt ribbon having an intrinsic permeability of about 105. The reader will appreciate that the values given in this description for the various parameters associated with the elements shown in Figure 1 are given merely by way of example, and illustrate one working embodiment. The values of these parameters will inevitably vary according to the overall size of the to system and its intended function. The magnetic elements which constitute the discrete magnetically active regions of the tag have dimensions lOmm x lmm x 25 microns; the spacing between adjacent elements is lmm. The two coils are spaced apart by approximately 20cm and each comprise 450 turns of 0.56mm copper wire wound in a square configuration typically 45cm x 45cm. Each coil has a resistance of 6S2 and an inductance of 100mH. Each of the coils Txl and Tx2 carries a direct current I superimposed upon which is a smaller alternating current i; typically, 2o the direct current I is of the order of 3A while the superimposed alternating current i is of the order of 50mA. The alternating current i is of relatively high frequency, typically about 2kHz.
With a system such as that just described, the alternating and direct currents in the two coils generate a magnetic field pattern in which there is a magnetic null in the direction of arrow x at points lying in a plane parallel to the two coils and mid-way between them.
In Figure 1, the x- and y- coordinates of this mid-way 3o plane are represented by the lines 2 and 3, respectively.
If a magnetic tag of this invention is passed through the two coils shown in Figure l, travelling in direction x and generally along the longitudinal axis defined between the centre points of the two coils, it will pass through a magnetic field polarity inversion at the mid-way plane defined by coordinates 2 and 2. The change in polarity of the magnetic field comes about because the DC current flows in one sense in the first of s the coils and in the opposite sense in the other of the coils, as indicated by the bold arrows in Figure 1. At the mid-way plane, the magnetic field component generated by the direct current flowing in the first coil exactly cancels the magnetic field component generated by the to direct current flowing in the other coil.
As the tag travels through the centre of the first coil, it experiences a high magnetic field which is sufficient to saturate its magnetically active elements;
as the field strength decreases on moving towards the 15 mid-way plane, the magnetic material is influenced by the decreasing magnetic field in a way dictated by its hysteresis curve. In the vicinity of the magnetic null, the direction of magnetisation of the magnetic elements of the tag is reversed.
2o The relatively high frequency alternating current i shown in Figure 1 is identical in each of the coils Txl and Tx2.
The alternating current can have a frequency within a wide range, as indicated hereinbefore; a typical 25 operating value with the arrangement of Figure 1 is about 2kHz. The effect of this relatively low amplitude alternating current is to cause the mid-way plane defined by coordinates 2, 3 to oscillate about the geometric midpoint along the longitudinal axis defined between the 3o midpoints of the two coils. In other words, the plane containing the magnetic null oscillates or wobbles back and forth over a small spatial region at the frequency of the alternating current.

Figure 2 shows a simple circuit for providing opposed DC fields combined with AC fields. Capacitor C1 is selected to resonate with the inductance of coils Txl and Tx2 at the AC drive frequency; each of these coils has a resistance of 6 ohms and an inductance of 100 millihenries. A typical value for C1 is 0.1~,F. C2 is a capacitor selected to behave as an effective short-circuit at the AC drive frequency; a typical value for this component is 22~F. The DC power supply will to typically provide 30 volts at 3 amps; and the AC source will typically deliver an alternating current at a frequency of 2kHz at 2v rms.
Figure 3 illustrates how the magnetisation of a single magnetic element varies with time at different positions within the magnetic field pattern defined between the coils Txl and Tx2 of Figure 1. For ease of illustration, the oscillation of the plane containing the magnetic null is represented by the bold double-headed arrow (H) 4, the extreme positions of the plane being 2o represented by dashed lines 5 and 6, respectively, and the mid-point between limiting planes 5 and 6 being represented by dashed line 7. In the right hand portion of Figure 3, the applied AC field is shown varying with time between positive (H+) and negative (H-) field values. Beneath the graph of the applied AC field, there are five graphs depicting how the net magnetisation of the magnetic element varies with time in each of five geometric positions indicated to the left as Position 1, Position 2, etc. Planes 5 and 6 define the limits of 3o regions within which magnetic field polarity reversals occur. In practice, the separation between planes 5 and 6 is typically of the order of lmm; for a given magnetic material, this distance can be increased or decreased at will within certain limits by varying the amplitude of the AC current and/or the DC current in the coils.
At all times, the magnetic element has a linear magnetic axis which is orthogonal to the planes 5, 6 and 7.
In Position 1, the end of the magnetic element is adjacent to plane 6; in this condition, it experiences a positive magnetic field at all times and its net magnetisation is time-invariant. In Position 2, the to leading end of the element has reached the mid-way plane 7. Most of the magnetic material, however, still remains outside limiting plane 6. In consequence, the null plane is able to interact with only a portion of the magnetic material, resulting in a time-variable net magnetisation i5 having the repeat pattern shown, i.e. a straight line positive-value portion followed by a generally sinusoidal arc which dips towards zero and then rises to its original positive value.
In Position 3, the magnetic material is positioned zo symmetrically with respect to the mid-way plane 7. Here, the net magnetisation versus time plot consists of a sine wave whose frequency corresponds to that of the applied AC field. In Position 4, the majority of the magnetic element experiences a negative field at all times, while z5 a smaller part of the element experiences polarity reversals; this leads to the net magnetisation versus time plot as shown. The fact that Position 4 is in effect the inverse of Position 2 is reflected in the relationship between the magnetisation plots for these 3o two positions; as can be seen, the plot for Position 4 is effectively a mirror image of that for Position 2 but with the curved portions time-shifted.
Finally, that Position 5, all of the tag experiences the negative field, and no part of the tag experiences field polarity reversal. In consequence, the net magnetisation is time-invariant, being a constant negative value as shown.
When a tag containing such a magnetic element is passed along the coils' axis through the region of zero field, it will initially be completely saturated by the DC magnetic field. It will next briefly be driven over its B-H loop as it passes through the zero field region.
Finally it will become saturated again. The portion of to the traverse over which the magnetic material is "active", i.e. is undergoing magnetic changes, is physically small, and is determined by the amplitude of the DC field, the amplitude of the AC field, and the characteristics of the magnetic material. This region can easily be less than mm an extent. If the level of the alternating field is well below that required to saturate the magnetic material in the tag, then harmonics of the AC signal will be generated by the tag as it enters the zero field region (Positions 1 to 2) and responds to the zo changing field. As the tag straddles the narrow zero field region (Position 3) the tag will be driven on the linear part of its B-H loop, and will interact by re-radiating only the fundamental interrogation frequency.
Then, as the tag leaves the zero field region, (Positions 4 to 5) it will again emit harmonics of the interrogation field frequency.
A receiver (Rx) coil arranged to be sensitive to fields produced at the zero field region, but which does not couple directly to the interrogator (TX) coils, will 3o receive only these signals. Such an arrangement can be achieved by using separate Tx and Rx coils physically arranged to have low mutual coupling; or by using a single coil (having both Tx and Rx functions) together with suitable filtering in the Tx and Rx paths. The variation of these signals with time as the tag passes along the coils' axis gives a clear indication of the passage of the ends of the magnetic material through the zero field region.
The result of this interaction between the tag and the magnetic field it experiences is shown in Figure 3b.
Here, the region 4 over which the magnetic null oscillates is shown on a smaller scale, and the numbered dots represent the location of the mid-point of the tag to in each of Positions 1-5. The generation of a harmonic signal by the tag (illustrated by the second harmonic of the applied frequency) is apparent at positions where the tag enters the region defined by limiting planes 5 and 6, i.e. the zone where magnetic field polarity reversals i5 occur. Because of the symmetry of the system, a single magnetic element will generate a doublet peak 8a. and 8b since Positions 2 and 4 are redundant.
Referring now to Figure 4, this illustrates the lines of force (i.e. the magnetic contours) existing with 2o a simple bar magnet. The plane X-Y which intersects the longitudinal axis of the bar magnet and which is orthogonal to the plane of the paper constitutes a magnetic null plane. Thus a magnetic element possessing a sensitive magnetic axis aligned orthogonally with respect 25 to the null plane will experience a magnetic null as it traverses either path A-B or path C-D. Consequently a simple bar magnet can be used as part of an interrogation system to detect the presence of such a magnetic tag, or to read information carried by such a tag.
3o The generation of second harmonic signal can form the basis of a tag detection system. If, instead of just a single magnetic element, the tag includes a linear array of n magnetic elements, the second harmonic output from the tag will comprise n duplet peaks, each of the type shown in Figure 3b. If the size and magnetic characteristics of the magnetic elements are all the same, the peaks will have the same profile and each peak will define an envelope of constant area. The spacing between individual magnetic elements will influence the relative positions of the duplet peaks on an amplitude versus time plot. It will be appreciated that the present invention is not restricted to the use of such simple tags as just described. The use of magnetic elements of to different sizes and magnetic characteristics, and with non-uniform spacing along the length of the magnetic tag, will generate more complex signal patterns which nevertheless are characteristic of the given tag construction. By varying the number, the magnetic characteristics, and the, positioning of a series of magnetic elements, it is possible to manufacture a very large number of magnetic tags each with its own unique characteristics which will accordingly generate a unique signal when used in conjunction with the system of 2o Figures 1-3.
It will also be appreciated that the invention is not limited to observing the second harmonic of the applied alternating frequency; this particular harmonic has been selected for the purposes of illustration since it is relatively easy, to generate a transmit signal (Tx output) which has no (or very little) second harmonic content, thus permitting good discrimination between the Tx signal and the response of the tag; and since it also contains a relatively high proportion of the total 3o harmonic energy output from the tag.
Referring next to Figure 5, there is shown a schematic arrangement for a simple tag reader in accordance with this invention, the reader utilising a permanent magnet 10 and a coil 11 located adjacent to one face of the magnet. In this embodiment, a tag which is to be read can be passed along path C-D through coil 11. or along path A-B above the coil. The rags must be oriented with their magnetic axis aligned with the direction of tag movement. In Figure 5, the magnetic null plane is positioned at 12 as shown.
Referring next to Figure 6, the use of two permanent magnets positioned with their magnetic axes aligned and with like poles opposing one another is illustrated. Such io an arrangement generates a null plane 13; the direction of tag motion required is indicated by arrows 14. Again, the magnetic axis of the tag must be aligned with the direction of movement.
Figure 7 shows a simple realisation of a tag reader i5 head using a plurality of permanent magnets to generate a magnetic null plane. As illustrated, ten polymer-bonded ferrite magnets are disposed in an annular array with like poles facing inwards. A common transmit/receive coil L1 sits within the annulus of magnets in the manner 2o indicated. The tag is read as it passes through the null plane in the centre of the loop of magnets.
Referring next to Figure 8, there is shown one embodiment of an interrogation system in accordance with this invention. This is based on the use of a single coil 25 Li to act as both transmitter (TX) coil, which generates the desired magnetic field pattern, and as the receiver (Rx) coil. The system uses the second harmonic output of the tag as the basis for tag detention/identification.
Circuit components C1 and L2 form a resonant trap at 3o frequency 2f to reduce signals at this frequency in the Tx output to a very low level; C2 resonates with L1 at frequency f; and components C3, C4, L1 and L3 form a filter to pass wanted signals from the tag at frequency 2f while rejecting signals at the transmitted frequency f.
The output obtained from this circuit passes through a low pass filter to an analogue to digital converter (ADC) and thence to a digital signal processor. These components, and in particular the signal processor, will be configured to suit the intended application of the interrogation unit. The nature of the signal processing, and. the means by which it is achieved, are all io conventional and therefore will not be described further here.
Figure 9 illustrates the basic structure of magnetic tags in accordance with the invention. Fig. 9a shows a tag 100 which comprises a support medium 101 (e. g. paper i5 or a plastics material) and a linear array of magnetically active regions 102, 103, 104, 105 and 106 Each magnetically active region is formed from a patch of high-permeability magnetic material (e. g. Vacuumschmeltze 6025) having its magnetic axis aligned along the length 20 of the tag. Each patch is about lOmm2 in area and is adhesively secured to the substrate 101.
Patches 101-105 are identical in dimensions and magnetic properties, and are uniformly spaced apart, gaps 110, 111 and 112 all being the same. The gap between 25 patches 105 and 106, however, is larger - as though there were one patch missing at the position indicated by dotted lines at 113.
Tag 100 behaves as a six-bit tag, coded 111101 (the zero being area 113).
3o A functionally equivalent tag 120 is formed of a substrate 121 carrying magnetic elements 122-126 and having a "gap" 127; in this embodiment, the magnetic elements are in the form of a strip or wire of high-permeability magnetic material (e. g. Vacuumschmeltze 6025), typically being about 5mm long, lmm wide and about 15 microns in thickness.
Figure 9b illustrates an alternative construction for a six-bit, laminated tag 130. This tag is coded 111101, as in Fig. 9a. Here, a continuous layer or length of high permeability magnetic material 131 (in the form of wire, strip, thin film or foil) and a substrate 133 have sandwiched between them a magnetic bias layer 132.
The bias layer is magnetised in predetermined areas which to influence the overlying high permeability material to generate magnetically active regions indicated as 134, 135, 136, 137 and 138. Region 139 is not active, and thus constitutes a magnetic zero. When read by an interrogation system such as that of Fig. 8, the output generated by tags 100, 120 and 130 will be as shown in Figure 9d.
A more complex tag is shown in Fig. 9c. Here there are a series of parallel linear arrays of magnetically active material, generating a 4 x 4 array of sites where 2o the magnetically active material may be present (coding as '1') or absent (coding as '0').
Figure 10 illustrates the general arrangement of three sets of coils as used in accordance with this invention for surgical applications. The three sets o~
coils are all mutually orthogonal and define a cavity into which the head 200 of a patient may be positioned.
The first coil set consists of coils 201a and 201b; the second set consists of coils 202a and 202b; and the third set consists of coils 203a and 203b. In the drawing, two 3o surgical probes 204 and 205 are shown schematically in position within the patient's cranium. The probes each have, at their distal ends, a magnetic tag 206, 207 such as one of those described with reference to Fig. 9 above.
Because the magnetic element of the tag is only required to provide information of its presence (rather than hold extensive data) relatively simple tags are preferred. A
single magnetic element of high permeability magnetic material located at the tip of the probe is sufficient.
The coils are operated in the manner described in detail hereinabove. By means of the present invention, it is possible to determine the positions of the ends of the probes with high precision and thus to carry out delicate surgical procedures with accuracy and with minimum damage to to healthy tissue.

Claims (6)

What We Claim Is:

1. A magnetic marker or tag for encoding information which is characterised by a substrate carrying a plurality of discrete magnetically active regions having identical magnetic characteristics and being disposed in one or more linear arrays, the magnetically active regions being formed of a thin film or spin-melt material having a preferential axis of magnetisation, wherein the preferential axes of magnetisation of the magnetically active regions of the or each array are aligned with each other, and wherein at least one of the lengths of and the spaces between, the magnetically active regions serves to encode information.

2. A magnetic marker or tag as claimed in claim 1, characterised in that the preferential axes of magnetisation are generated by longitudinal annealing of a spin-melt material.

3. A magnetic marker or tag as claimed in claim 1 or 2, characterised in that said discrete magnetically active regions are formed from a continuous area of magnetisable material discrete regions of which are magnetised in a manner that generate space(s) between said magnetically active regions.

4. A magnetic marker or tag as claimed in any one of claims 1 to 3, characterised in that the tag has two linear arrays disposed orthogonally.

5. A magnetic marker or tag as claimed in any one of claims 1 to 4, characterised in that it is in the form of a relatively long, thin strip having a preferential axis of magnetisation along its length.

6. A magnetic marker or tag as claimed in any one of claims 1 to 5 characterised in that at least one of the magnetically active regions, and the spaces between said magnetically active regions is non-uniform.