AU1760400A - Apparatus for interrogating magnetic tags - Google Patents

Description

S&F Ref: 398004D3

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Address of Applicant: Actual Inventor(s): Address for Service: Invention Title: Flying Null Limited Harston Mill, Harston Cambridgeshire CB2 United Kingdom Andrew Nicholas Dames Spruson Ferguson St Martins Tower 31 Market Street Sydney NSW 2000 Apparatus for Interrogating Magnetic Tags

S

The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845c -1- APPARATUS FOR INTERROGATING MAGNETIC TAGS This application is divided out of Application No.

AU52806/96 (hereinafter termed "the parent application").

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 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 identifying a given tag in order to 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.

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 the mere presence of the tag (and hence that of an article to which the tag is attached); or 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 attached); or they may serve to provide access codes 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 -2articles.

In addition, the terms "ac field" and "DC field" are used herein to denote 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 the invention disclosed in the parent application have a wide variety of applications as indicated above. These include (but are not restricted to) inventory control, ticketing, automated shopping systems, monitoring work-in-progress, security tagging, access control, anti-counterfeiting, and location of objects (in particular the precise positioning of workpieces [e.g.

probes in surgery]). Full details of the magnetic interrogation technique on which these various practical applications are based are found in the parent application (AU52806/96) and in International Patent Application No. PCT/GB96/00823 (published as 96/31790 and hereinafter referred to as "the PCT application") from which it is derived.

25 The parent application describes and claims a method of interrogating a magnetic element having nonlinear magnetic properties, which is characterised by the steps of: applying a magnetic field to an S•interrogation zone where the magnetic element is, or is expected to be, located, said magnetic filed being: (i) generated by magnetic field generating means positioned independently of said magnetic element; and (ii) such that a magnetic null as herein defined is generated within said interrogation zone, the magnetic null being contiguous with regions where the applied magnetic field is sufficient to saturate the, or a part of the, magnetic element; causing relative movement between said magnetic field and said magnetic element such that at least a portion of the magnetic element becomes magnetically saturated and then enters the magnetic null; and detecting the magnetic response of the magnetic element during said relative movement.

o* 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 alternating magnetic fields in the frequency range of to 1MHz, 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 usually resonated with a capacitor at the frequency of the interrogation signal carrier frequency. A typical product of this type is the TIRIS system manufactured by Texas Instruments Ltd.

Other multi-bit data tag systems have employed conventional h.f. radio technology, or technologies based on surface acoustic waves or magnetostriction phenomena.

The invention disclosed in the PCT application involves, inter alia, the use 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 substrate, e.g.

paper or a plastics material, to form self-supporting **25 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 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 described and claimed in the parent application exploits magnetic fields which contain 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 parent invention in which precise position is determined.

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 the interrogation system, such a substantial orthogonal 15 field is desirable.

It will, however be understood that materials with a soft magnetic axis, can, under very high field conditions, be magnetised in a direction orthogonal to that of the easy axis; if the orthogonal component of the magnetic field exceeds the limiting value for a particular element material, the element will remain in saturation. Clearly this would be undesirable since the magnetically active elements would not be subjected to o *25 a changing magnetic field and would not produce a detectable response I One way of creating the magnetic null is to employ opposing magnetic field sources. These may be currentcarrying coils of wire, or permanent magnets (these being well suited to small-scale systems), or combinations of coil(s) 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.

-6- For large scale applications, the magnetic field sources are preferably coils carrying direct current.

The invention described and claimed in the parent application also utilises the relative movement between a magnetic marker and an applied magnetic field in order to effect 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 the marker in a fixed position while the magnetic field is scanned over it. Generally, the interrogation technique exploits the difference between the magnetic behaviour of the marker in a zero field (at the magnetic null), and (ii) in ahigh, generally saturating, magnetic field.

The magnetic interrogation systems described in PCT application can function with magnetic tags which are characterised by carrying a plurality of discrete magnetically active regions in a linear array.

The discrete magnetically active regions may be ~supported on a substrate, e.g. paper or a plastics material, or they may be self-supporting.

Alternatively, the magnetic elements may be o "25 incorporated directed into or onto articles during o **manufacture of the articles themselves. This is appropriate, for example, when the articles are goods, e.g. retail goods, which carry the tags for inventory purposes; or when the articles are tickets or security passes.

A tag as described above can also be formed from a continuous strip of high permeability material, discrete regions of which have their magnetic properties permanently or temporarily modified. It will be appreciated that such a process can begin with a high permeability strip selected regions of which are then treated so as to modify their magnetic properties, generally by removing or reducing their magnetic permeability; or with a strip of high permeability magnetic material accompanied by a magnetisable strip positioned close to the high permeability magnetic material, e.g. overlying it or adjacent to it, selected regions of which are magnetised. In relatively simple embodiments, each magnetically active region has the same magnetic characteristics; in more complex embodiments, each magnetically active region can possess a different magnetic characteristic, thus making it possible to assemble a large number of tags each with unique magnetic properties and hence with a 15 unique magnetic identity and signature (when processed S.by a suitable reader device) Because the invention of the parent application utilises relative movement between a tag and an applied magnetic field, it will be appreciated that there will be a correspondence between the time domain of output signals from a tag reading device and the linear dimensions of the magnetically active regions of a tag and of the gaps between the magnetically active regions. In this sense, the active regions and the gaps between them function analogously to the elements of an optical bar code (black bar or white gap between adjacent bars). It follows from this that, just as variability of magnetic characteristics in the active regions can be used to generate part of a tag "identity", so can the linear spacing between adjacent magnetically active regions. It will readily be understood that a vast number of tags, each with its own unique identity, can thus be produced.

Although the tags have been described as possessing a linear array of magnetically active regions, the tags may in fact have two or more such linear arrays. These may be disposed mutually parallel, or mutually orthogonal, or in any desired geometrical arrangement. For simplicity of reading such tags, arrays which are parallel and/or orthogonal are preferred.

Appropriate techniques for manufacturing the tags just described are well-known in conventional label magnetic marker) manufacture. Suitable magnetic materials are also well-known and widely available; they are high-permeability materials whichpreferably have an extrinsic relative permeability of at least 103 The coercivity of the magnetic material will depend on the tag's intended use. The magnetic .material is preferably in the form of a long thin strip or of a thin film; these formats avoid major internal demagnetisation effects. Suitable strip materials are readily available from commercial suppliers such as Vacuumschmeltze (Germany), Allied.Signal Corp. (USA), and Unitika (Japan). Thin film material currently o manufactured in high volume by IST (Belgium) for retail security tag applications is also suitable for use.

In the magnetic interrogation techniques described in the PCT application, the magnetic null is preferably caused to sweep back and forth over a predetermined S" region within the interrogation zone. The scanning frequency the sweep frequency of the magnetic null) is preferably relatively low, e.g. 1 500Hz.

Conveniently, the field pattern is arranged so that (a) said magnetic null lies in a plane; and the saturating field occurs adjacent to said plane.

The relative movement between the magnetic element and the magnetic field may advantageously be produced by sweeping the applied magnetic field over the magnetic element. Alternatively, the relative movement can be achieved by the application of an alternating magnetic field to a generally static magnetic field pattern.

In carrying out the methods described above, preferred embodiments of the magnetic element are either elongate, and the magnetic null is then arranged to extend along the major axis of said magnetic element; or they are in the form of a thin film, in which case the magnetic null is arranged to extend to 15 be aligned with the axis of magnetic sensitivity of the *eo thin film material.

oooo The magnetic field or field pattern utilised in the methods described above may be established by the means of two magnetic fields of opposite polarity.

S.This can conveniently be achieved by use of one or more e coils carrying direct current; or by the use of one or more permanent magnets; or by a combination of coil(s) and magnet(s) Where a coil is used, it may be arranged to carry a substantially constant current so as to maintain the magnetic null at a fixed point. Alternatively, the coil(s) carry/carries a current whose magnitude varies in a predetermined cycle so that the position of the magnetic null is caused to oscillate in a predetermined manner. We describe this as a "flying null". A similar arrangement can be used to give a flying null when both a coil or coils and a permanent magnet are used.

Relative movement between the magnetic field and the magnetic element may by achieved by applying a relatively low amplitude alternating magnetic field superimposed on the DC field. Typically, such a low amplitude alternating magnetic field has a frequency in the range from 10Hz to 100kHz, preferably from 50Hz to and most advantageously from 500Hz to In one embodiment, the coils carry a substantially constant current so as to maintain the magnetic null at a fixed point. In another embodiment, the coils carry a current whose amplitude varies in a predetermined cycle so that the position of the magnetic null is caused to oscillate in a predetermined manner.

In the methods described above, detection of the .0 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.

ooe• As indicated above, the methods 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 allow high fields for transmission at these low frequencies.

These interrogation methods may thus provide a multi-bit data tag system which employs low-frequency inductive magnetic interrogation, and avoids the need for complex, expensive tags.

Before describing the present invention, it will -11be helpful to explain some fundamental aspects of the interrogation system described in the PCT application giving reference, where appropriate, to relatively simple embodiments.

A key aspect of the interrogation system is the form of the 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". 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 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 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 25 coils, so that the AC fields produced sum together midway 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 axis by a small amount (this -12is 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 to the scanning field (Cb) 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 technique.

*99 When a tag containing a piece of high-permeability 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 region. Finally it will become 25 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 a 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 -13region 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, 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.

It will be appreciated that because the interrogation zone can be very narrow, each individual o piece of magnetic material can be distinguished from o. its neighbours, from which it is separated-by a small distance. Naturally, the magnetic material will be selected to suit the particular application for which the tag is intended. Suitable magnetic materials are commercially available, as described hereinbefore If a tag containing a numberof zones or pieces of magnetic material placed along the axis of the label is 25 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 spacings of individual zones or pieces of 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 the spacing and widths of the lines in the code.

-14 The system so far described allows for the scanning of a single-axis tag a wire or a thin strip of anisotropic material, having a magnetic axis along its length) as it physically moves through the coil assembly. 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. If required, the arrangement can be made self-scanning, and thus able to interrogate a stationary tag, 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 of 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 considerably greater, to avoid the need for precise tag positioning within the interrogation zone- 20 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 S" involves much greater complexity in the correlation of signals from the three planes, but because of the very S. 25 high spatial resolution available would be capable of "reading many tags simultaneously present in a common interrogation volume. This is of enormous benefit for "'"applications such as tagging everyday retail shopping S•items, and, for example, would allow automated price totalisation of a bag of shopping at the point of sale.

Thus the technique 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 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 105), 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 lm), such material can be cut into small 2 dimensional pieces (squares, discs etc) with areas of just 20 mm 2 or less, yet still 20 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 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 technique, the reading -16system 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 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 thereof. This can be done, for example, by local heating to above 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 adjacent bias element made from medium or 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 with an AC field) and S•re-programmed with new data.

25 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 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.

The present invention is concerned with apparatus for interrogating magnetic tags. More particularly, -17the present invention provides apparatus for interrogating a magnetically coded tag, which apparatus is characterised by: means for generating a magnetic field, said magnetic field comprising a first region at which the component of the magnetic field resolved in a first direction is zero, and being characterised in that within regions contiguous with said first region the component of the magnetic field resolved in said first direction is sufficient to saturate the, or a part of the, tag; an electrical circuit which comprises a signal source at frequency 2f and output means for providing an electrical output, said source being coupled to said .e output means by means for halving the-frequency of said source signal; (ii) first filter means arranged substantially to reject signal at frequency 2f; (iii) 20 tuned circuit means tuned to energy at frequency f and including a transmitter coil adapted to radiate energy towards a magnetically coded tag positioned so as to be influenced by said radiated energy and a receiver coil (which may be the same coil as saidtransmitter coil) e 25 for receiving energy radiated by the tag in response to the received energy; and (iv) second filter means ~arranged substantially to reject energy at frequency f and to pass energy at frequency 2f, and wherein said output means is arranged to provide an output which is a function of the amplitude of the output of said second filter means and the phase difference between the source and the output of said second filter means; wherein the magnetic field generating means is associated with said transmitter and receiver coils such that, in use, a magnetically coded tag moving -18through the first region will also interact with said transmitter and receiver coils.

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 parent application; FIGURE 2 is a circuit diagram illustrating one mode of generating the desired magnetic field pattern with the arrangement of Fig. 1; FIGURE 3 relates the magnetic response of a tag to its position within the reading system of Fig. 1; and FIGURE 4 is a schematic circuit diagram for one embodiment of a tag interrogator in accordance with the present invention.

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 20 Figure 9a, i.e. a simple linear tag carrying a plurality of magnetic elements each of which is a highpermeability magnetic alloy material, for example Vacuumschmeltze 6025 spin melt ribbon having an intrinsic permeability of about l05. The reader will 25 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 system and its intended function.

The magnetic elements which constitute the discrete magnetically active regions of the tag have dimensions x Imm x 25 microns; the spacing between adjacent elements is imm. The two coils are spaced apart by approximately 20cm and each comprise 450 turns of 0.56mm copper wire wound in a square configuration -19typically 45cm x 45cm. Each coil has a resistance of 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, the direct current I is of the order of 3A while the superimposed alternating current i is of the order of 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 15 between them. In Figure 1, the x- and y-coordinates of 0 eo this mid-way plane are represented by the lines 2 and 3, respectively.

If a magnetic tag is passed through the two coils shown in Figure 1, 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 midway plane defined by coordinates 2 and 3. The change Soe 25 in polarity of the magnetic field comes about because the DC current flows in one sense in the first of 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 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 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.

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 operating value with the arrangement of Figure :i 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 20 defined between the midpoints of the two coils. In other words, the plane containing the magnetic null S. oscillates or wobbles back and forth over a small S- spatial region at the frequency of the alternating current.

o .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 0.1F.

C2 is a capacitor selected to behave as an effective short-circuit at the AC drive frequency; a typical value for this component is 22AF. The DC power supply will typically provide 30 volts at 3 amps; and the AC source will typically deliver an alternating current at -21a 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 doubleheaded arrow 4, the extreme positions of the plane being represented by dashed lines 5 and 6, respectively, and the mid-point between limiting planes 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 and negative 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 and 6 define the limits of regions within which magnetic field polarity reversals occur. In practice, the separation between planes 5 and 6 is typically of the order of 1 mm; 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 i, 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 -22leading 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 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 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 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 two positions; as can be seen, the plot S. for Position 4 is effectively a mirror image of that for Position 2 but with the curved portions timee :25 shifted.

*e Finally, at 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 -23its B-H loop as it passes through the zero field region. Finally it will become saturated again. The portion of 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 Imm 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 (Positions 1 to 2) and responds to the changing field.

As the tag straddles the narrow zero field region 15 (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 e oo will again emit harmonics of the interrogation field 20 frequency.

A receiver (Rx) coil arranged to be sensitive to o• fields produced at the zero field region, but which does not couple directly to the interrogator (Tx) coils, will 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 -24- 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 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 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.

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 S. profile and each peak will define an envelope of S- constant area. The spacing between individual magnetic elements will influence the relative positions of the 25 duplet peaks on an amplitude versus time plot. It will *be appreciated that the present invention is not restricted to use with such simple tags as just *"described. The use of magnetic elements of different sizes and magnetic characteristics, and with nonuniform 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 Figures 1-3.

It will also be appreciated that the interrogation system 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 harmonic energy output from the tag.

too.

Referring next to Figure 4, there is shown one embodiment of an interrogation system in accordance with this .invention. This is based on the use of a single coil L1 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 detection/identification. Circuit components C1 and L2 25 form a resonant trap at 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 -26intended application of the interrogation unit. The nature of the signal processing, and the means by which it is achieved, are all conventional and therefore will not be described further here.

o*t* e 96 *4

Claims (2)

1. Apparatus for interrogating a magnetically coded tag, which apparatus is characterised by: a) means for generating a magnetic field, said magnetic field comprising a first region at which the component of the magnetic field resolved in a first direction is zero, and being characterised in that within regions contiguous with said first region the component of the magnetic field resolved in said first direction is sufficient to saturate the, or a part of the, tag; an electrical circuit which comprises a signal source at frequency 2f and output means for providing an electrical output, said source being coupled to said output means by means for halving the frequency of said source signal; (ii) first filter means arranged *0Se substantially to reject signal at frequency 2f; (iii) S" 20 tuned circuit means tuned to energy at frequency f and including a transmitter coil adapted to radiate energy see, towards a magnetically coded tag positioned so as to be 6666 influenced by said radiated energy and a receiver coil (which may be the same coil as said transmitter coil) 25 for receiving energy radiated by the tag in response to o* S. the received energy; and (iv) second filter means arranged substantially to reject energy at frequency f 0o and to pass energy at frequency 2f, and wherein said OOc•r SS output means is arranged to provide an output which is a function of the amplitude of the output of said second filter means and the phase difference between the source and the output of said second filter means; wherein the magnetic field generating means is associated with said transmitter and receiver coils such that, in use, a magnetically coded tag moving -28- through the first region will also interact with said transmitter and receiver coils.

2. Apparatus, for interrogating a magnetic tag, as claimed in claim 1 and substantially as hereinbefore described. DATED this eighteenth Day of February, 2000 Flying Null Limited Patent Attorneys for the Applicant/Nominated Person SPRUSON FERGUSON a e e e e