Abstract:
A tag for use with an electromagnetic identification system includes a magnetic control component 1, having alternating regions 10, 20 of, respectively, high and medium coercivity material, and an element 30 of magnetically soft material. The element 30 may be polarized so as to deactivate the tag by, for example, selectively demagnetizing the regions 20 of medium coercivity, such as by exposing the tag to an alternating magnetic field having an initial field strength which is intermediate between the coercivity of the regions 10, 20, which may be formed as a striped coating or from a number of discrete magnetic components.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an electromagnetic identification system of a kind which can be employed, for example, in electronic article surveillance (E.A.S.), as it may be used for in-store security, in access control, or in other applications such as the protection of documents or data storage devices at a place of work. 
     In any environment, an element is used which influences an electromagnetic field, so causing a disturbance in a characteristic of the field as detected by a detector device. The element is frequently referred to as a &#34;tag&#34; and needs to exhibit a number of characteristics one of which, especially relevant for in-store usage, is the capability of being readily de-activated by check-out personnel whilst being difficult for a would-be thief to de-activate. 
     It is usual for the element, or tag, to contain a strip of highly permeable magnetic material. Such material is easily influenced by a relatively weak interrogating field, causing the generation of a number of harmonic frequencies which are readily detected by a suitable detection device. Unless it is proposed to physically fracture or mechanically strain the strip in order to de-activate it, and such activities would place severe limitations upon the form which a tag could take, de-activation is usually achieved by magnetising a control element consisting of a normally unmagnetised, magnetically hard material disposed in the tag and configured so as to produce a series of poles along the length of the strip of highly permeable material when so magnetised. This alters the magnetic &#34;profile&#34; of the tag, as presented to the interrogating field, and inhibits or characteristically alters the generation of the aforementioned harmonics, thus permitting the detection device to distinguish between activated and de-activated tags. 
     It has been usual hitherto for the magnetically hard material constituting the control element to be provided either in the form of discontinuous lengths running close to the strip of magnetically soft material, or in continuous lengths similarly disposed. In the first case, de-activation is relatively straightforward for check-out personnel to accomplish, but the same can be said for would-be thieves as a single swipe along the tab with a permanent magnet of appropriate strength is all that is required to magnetise the magnetically hard material. In the second case, more care has to be taken by the check-out personnel because the continuous length of magnetically hard material has to be selectively magnetised to produce a pole pattern sufficient to change the electromagnetic profile of the tag, but at the same time de-activation is made more difficult for the would-be thief. It is an object of the present invention to provide an improved tag. 
     SUMMARY OF THE INVENTION 
     In accordance with this invention, a tag for use in an electromagnetic identification system comprises an element of magnetically soft material and, in magnetic communication therewith for enabling polarisation of the element, a magnetic control component including first and further regions having, respectively, first and further magnetic characteristics for enabling magnetisation or demagnetisation of first or further regions independently of the other regions. The magnetic characteristic may comprise magnetic coercivity or Curie temperature. Preferably, the element of magnetically soft material is of striplike form. 
     The invention provides for an arrangement in which the control element is such as to leave the tag in an activated condition whether said control element is unmagnetised or uniformly magnetised but can, with an appropriately configured deactivating device, be selectively magnetised to de-activate the tag. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be clearly understood and readily carried into effect, some embodiments thereof will now be described by way of example only with reference to the accompanying drawings, of which: 
     FIGS. 1(a) to 1(e) shows a schematic view of a tag and various active and deactivated states thereof, 
     FIG. 2 shows another form of tag, and 
     FIGS. 3(a) and 3(b) show signals as detected with a tag in active and deactivated conditions respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One way of implementing the invention is to provide a tag that is active when the control element (magnetically hard material) is magnetised and deactivated by a.c. demagnetisation, as this is very difficult to defeat. Such a tag can be constructed, in accordance with one example of the invention, as shown in FIG. 1a. Here the magnetically hard material of the control component 1 consists of alternating regions 10, 20 respectively, of high and medium coercivity magnetic material, the length of the repeat pattern of the regions 10 and 12 being the wavelength of the tag, as shown in FIG. 2. The magnetically soft material (i.e. with low coercivity) is provided in the form of a ribbon or film 30 spaced from the control element 1 by a magnetically inert spacing layer 40 (the thickness of which is small relative to the pitch of alternating hard regions and can be zero). An adhesive layer 50 may be provided to cause the tag as a whole to adhere to a chosen substrate and a bar code or other printed layer 60 may be provided on top of the control element if desired. 
     When the tag is uniformly magnetised, as shown in FIG. 1(b), the control element 1 will produce a small bias field which fluctuates only to a limited extent along the length of ribbon or film 30 and the tag will remain active. At a checkout, or other point of sale or validation, the tag is exposed to a decaying, alternating magnetic field, the initial (peak) strength of which is intermediate between the coercivities of the regions 10 and 20 of the control element 1, and this results in demagnetisation of the regions 20 of medium coercivity as illustrated in FIG. 1(c), creating the pole pattern in the control element 1 necessary to deactivate the tag. 
     If necessary, a small d.c. component of the appropriate polarity can be superimposed on the alternating field to prevent anhysteretic magnetisation of the regions 20 of lower coercivity. The same technique can also be used to achieve reverse magnetisation in the regions 20 of lower coercivity, as shown in FIG. 1(d). 
     The tag can alternatively be deactivated using an alternating and decaying magnetic field of initial (peak) value higher than that of the regions 10 of higher coercivity, to demagnetise both sets of regions 10 and 20 in the control element 1, succeeded by a d.c. magnetic field of intermediate magnitude sufficient to remagnetise only the regions 20 of lower coercivity, as illustrated in FIG. 1(e). 
     It is further possible to deactivate the tag 1 without the use of alternating fields by applying a large pulse of d.c. magnetic field, to magnetise the regions of both high and low coercivity in the same direction, followed by a smaller d.c. magnetic field in the opposite direction to reverse the magnetisation of the regions 20 of lower coercivity, achieving the effect shown in FIG. 1(d). 
     It will thus be understood that the tag 1 can be proximity deactivated at the checkout, using coils to generate an alternating magnetic field. It is also possible to combine detection and deactivation circuitry to give positive confirmation of deactivation. 
     Thus far, the two sets of regions 10, 20 have been described as having different coercivities, but it will be appreciated that regions having differences in other magnetic parameters or characteristic could alternatively be used. For example, two hard magnetic materials of differing Curie temperatures could be used; deactivation of such tags requiring a momentary elevation of the temperature of the tag. This may be achieved by the use of a hand held laser scanner possibly in combination with a conventional barcode reader. 
     In one arrangement, as shown in FIG. 2, a ribbon 30&#39; of magnetically soft material is disposed alongside a control element consisting of alternate regions 10&#39;, 20&#39; of material of high and medium coercivities respectively. 
     Each of the regions 10&#39; comprises six layers of three micron-thickness metal particle (mp) tape of width 8 mm, whilst each of the regions 20&#39; comprises two layers of fifty micron-thickness of an iron oxide (Fe 2  O 3 ) tape, also of width 8 mm. The element 30&#39; may comprise typically either a ribbon of thickness twenty-six microns and width 1 mm or a permalloy film of thickness two microns and width 4 mm. 
     Dimensional and magnetic characteristics of the above-described elements may conform to the values shown in the following table: 
     
         ______________________________________   Coerci-          Br or Bs  total   vity   Flux      coating  Width Flux   Hc     Density   thickness                             W     Bt   (Oerst.)          B (Gauss) t (mm)   (mm)  W______________________________________10&#39; mp Tape   1400     2400    0.018  8     35020&#39; Fe.sub.2 O.sub.3         100       750    0.100  8     60030&#39; ribbon    0.1      9600    0.026  1     250    or    permalloy 0.75     8800    0.002  4      70    film______________________________________ 
    
     When using a ribbon as the magnetically soft element 30&#39;, the available steady state flux and field levels are sufficient to de-activate the tag in accordance with the arrangement shown in FIG. 1 (d). This may be achieved by effecting a single swipe along the tag with a neodymium-iron-boron magnet block generating a field of about +5000 Oe, and with the block in wiping contact with the tag, to fully magnetise regions 10&#39; and 20&#39; in the same direction, followed by a reverse swipe with the block spaced from the tag by about 5 mm, reducing the peak field to about -500 Oe, to reverse magnetise the regions 20&#39;. 
     If the metal particle and iron oxide tapes have insufficient tape coating thickness, it may not be possible to create the de-activated conditions shown in FIGS. 1(c) and 1(e) with the ribbon 30&#39;. However, improved results may be obtained by the use of the thin permalloy film for the element 30&#39;, for all methods of deactivation, over a range of wavelengths up to λ=20 mm. 
     To enable the arrangements shown in FIGS. 1c and 1e to be successfully implemented using a ribbon for the element 30&#39;, a better match between the available fluxes from regions 10&#39; and 20&#39; is required, and this flux should preferably be sufficient to ensure that enough flux enters the ribbon 30&#39; to saturate it, after allowing for leakage. Leakage will be more for a small wavelength, but effective field will be reduced at large wavelength. The optimum for a narrow ribbon and wide tape is likely to be such that λω2W, where W is the tape width. 
     With a thin film low coercivity magnetic layer, instead of a narrow ribbon, the element 30&#39; of FIG. 2 may be made the same, or nearly the same width (W) as the high and medium coercivity components 10&#39; and 20&#39;. The optimum wavelength is then reduced with less allowance for flux leakage required. It may not be necessary to fully saturate the ribbon or film 30&#39; to ensure satisfactory de-activation, as long as the bias field values and magnetic field variations in the element 30&#39; provided by layer 1 in FIGS. 1c, d or e are sufficiently high to spread out the switching of element 30&#39; over a large time interval, instead of giving sharp pulses. 
     Approximate calculations for 2 cases are as follows: 
     EXAMPLE 1 
     8 mm wide tape with λ=16 mm 
     1 mm×0.026 mm RIBBON with B s  =9600 gauss 
     where B s  is the saturation flux density 
     Flux to Saturate=250 gauss mm 2   
     Flux Required=1000 gauss mm 2   
     (upper limit to saturate, with leakage allowance of 4×) 
     10&#39; MP-tape with B r  =2400 gauss requires t=52 micron 
     20&#39; Fe 2  O tape with B r  =1000 gauss requires t=125 micron 
     where B r  is the remanent flux density and t is the thickness of the magnetic tape coating. 
     The calculated average field produced by either region 10&#39; or region 20&#39; alone is then: 20 Oe for λ=16 mm. 
     EXAMPLE 2 
     8 mm wide tape 
     8 mm×0.002 PERMALLOY with B s  =8800 gauss 
     Flux to Saturate=140 gauss mm 2   
     Flux Required=280 gauss mm 2  (allowing for leakage) 
     (upper limit to saturate, with leakage allowance of 2×) 
     Regions 10&#39;: MP-tape with B r  =2400 gauss requires t=15 micron 
     Regions 20&#39;: --Fe 2  O 3  with B r  =1000 gauss requires t=35 micron 
     The calculated average field Hav produced by either 10&#39; or 20&#39; alone is: ##EQU1## 
     A wavelength of λ=5 mm would give H av  =20 Oe, in this case, but this is not necessarily the optimum wavelength to use. The optimum wavelength may be anywhere between 4 and 16 mm, depending on thicknesses, spacing, pulse shape requirements etc. 
     Experimental EAS tags have been made using one layer of soft and one layer of hard magnetic material. The soft layer comprised of NiFe, approximately 1 μm thick, with a coercivity of 0.7 Oe and a well defined easy axis of magnetisation. The NiFe film was deposited onto a flexible polyester substrate by electro-plating, thin films of Cr and Cu having first been deposited in vacuum as a &#34;strike layer&#34; for the NiFe. A rectangle 30 mm by 20 mm was then cut out and used as the switching element 30 in the experimental tag. A hard magnetic layer of the same area was made by cutting out strips of two different magnetic tapes and arranging the strip pieces as shown in FIG. 1. This high coercivity material used was from a 3 μm thick Metal Particle video tape coating, coercivity 1450 Oe; the lower coercivity material used was from a 40μm thick coating of isotropic 100 Oe iron oxide particles. In the case of the high coercivity material, the strips were cut at 90 degrees to the easy axis and a stack of three layers was used for each stripe (i.e. 9 μm magnetic coating thickness) to give the same remanent magnetisation in both materials, when magnetised across their width. 
     The tag was activated by a transverse swipe with a magnet and deactivated with a hand held a.c. tape demagnetiser; FIGS. 3(a) and 3(b) show respectively the detected signal pattern with the tag active and de-activated. 
     With regard to the manufacture of tags in accordance with this invention, the following process may be adopted: 
     1. The soft layer is deposited in a semi-continuous process by passing a roll of flexible substrate (e.g. polyester) through a series of plating baths to build up the necessary strike and soft layers. 
     2. The hard layer is deposited in a semi-continuous process using a modification of a conventional magnetic tape coating method, in which a flexible substrate (e.g. polyester) is passed under a coating trough fitted internally with a number of separating panels. The channels in between the separators are fed alternately with dispersions of the two magnetic particles to generate a striped coating of uniform thickness and required magnetic characteristics. The wet coating is then passed, before drying, over an orientating magnetic field arranged at approximately 90 degrees to the direction of pass. 
     Cobalt doped iron oxide and gamma iron oxide may be used, respectively, for the magnetic particles used to form the high and medium coercivity regions 20, 30 of the control element 1. 
     Although the present invention has been described with respect to specific embodiments it should be understood that modifications may be effected whilst remaining within the scope of the invention. For example, the magnetically soft element need not necessarily be of strip form. The element may be fabricated in any shape, the requirement being that the length and width are much larger than its thickness. Hence the tag shape can be made compatible with a price or bar code label and the tag may, therefore, be provided with a boundary layer for displaying the bar code, price or other label information. 
     Additionally, the magnetically soft element may be coated directly onto a copper foil, thereby obviating the need for an intermediate strike layer. 
     Furthermore, the tag as described has the magnetically hard and soft layers formed integrally with a substrate. The tags may also be of a form that can be transferred from the substrate onto an item to be tagged. 
     Also, it will be appreciated that other magnetic materials may be used to form the medium and high coercivity regions of the control element.