GB2328281A - Magnetic reading devices - Google Patents

Abstract

Non-contact reading devices for magnetic media (e.g. those used in identification and anti-counterfeit applications) comprise a pair of air-cored coils which are connected in series and in anti-phase; and are positioned so as (i) to be contiguous along at least one dimension and (ii) to be disposed symmetrically in relation to each other. The coils may be arranged coaxially, or side by side, or in a stacked configuration.

Description

MAGNETIC READING DEVICES This application relates to novel non-contact reading devices for magnetic media. The devices are particularly appropriate for reading discrete magnetic features such as those currently used for identification and anti-counterfeit applications, for example in certain security documents, cheques, and banknotes. They are also suitable for non-contact reading of conventionally-recorded data where the spatial extent and spacing of features is sufficiently large.

BACKGROUND Features formed from discrete segments of magnetic media have been used for some years as security and identification features in certain documents, cheques, and banknotes. Unlike conventional magnetic recording, which relies on storing a varying magnetic pattern along a continuous layer, this form of magnetic pattern uses the shapes and spacing of magnetic material segments to encode data. Data in this form cannot be erased or permanently corrupted by external magnetic fields. The density of information, however, is determined by the process which deposits the magnetic material, and this is generally much lower than that for conventional recording. Using normal printing processes, for example, to deposit magnetic ink, spatial features are generally greater than around 0.2 mm in dimension. In some security printing applications, feature lengths can be several millimetres or more.

The traditional way of detecting patterns of discrete magnetic elements of the type described is by first magnetising the elements in the appropriate direction, and then passing them over a magnetic replay head (or passing a head over them). Magnetisation can easily be achieved using a magnet. For magnetic materials with high remanence, passing the elements close to a suitably oriented permanent magnet or electromagnet of sufficient strength to overcome the coercivity of the magnetic material will leave the elements permanently magnetised. For low-remanence materials a permanent magnet or electromagnet can be positioned close to the head to ensure the elements remain magnetised when they pass the head. In both cases the moving magnetised elements induce voltages in the coil of the replay head, thereby enabling their detection by subsequent electronic processing.

At their most basic, conventional magnetic replay heads consist of a gapped ring of high permeability magnetic material such as permalloy, mu-metal or ferrite wound with a coil of wire. Magnetised material passing close to the gap in the ring causes a change in the magnetic flux in the ring and induces a voltage in the coil. Very high spatial resolution is achievable with this arrangement, because external magnetic elements can only influence the flux in the ring when they pass over the gap in the ring, and the gap can be made very small. In general, resolution is comparable to the size of the gap in the ring. The drawback, however, is that the coupling between the magnetised element and the gap falls of very rapidly with lift-off of the element from the gap, and even with lowresolution heads it is generally difficult to extend the reading range beyond a small fraction of a millimetre. In practice, conventional magnetic replay heads are generally regarded as contact reading devices.

Conventional magnetic heads are widely used for reading magnetic media in security documents and banknotes. However, the need for contact (or nearcontact) with the document is often a problem in high speed automated machinery because of wear and debris build up, and because it creates a constriction, a common source of jams.

A newer type of magnetic reading head is based on the phenomenon of magnetoresistance. Such "MR" heads employ a material which, when magnetically biased, has an electrical resistance which varies with applied magnetic field. Such heads can detect magnetic features of the kind described at greater distances than is the case with conventional replay heads. They suffer, however, from certain drawbacks: 1. The read distances which can be achieved are not always sufficient to eliminate the jamming problem.

2. Simple heads are microphonic as a result of the magneto-resistive material employed also having piezo-electric properties. This can be a serious problem in automated machinery, and more complex and expensive differential head designs are necessary to overcome it.

3. The sensitivity of MR heads falls off as the track width is increased. This means that in applications where wide track width is required e.g.

reading across an entire banknote, multiple heads and electronic processing channels are needed, making the overall reading system expensive.

The present invention relates to simple, low-cost replay heads which can detect small magnetisable elements at a distance comparable with or greater than that of typical MR heads, which are not microphonic, and which can provide very wide track widths without loss of sensitivity.

THE INVENTION Simple air-cored coils of wire will generate voltages when magnetised element pass them in an orientation which allows the flux lines form the element to couple with the windings of the coil. This is a very well known phenomenon. Simple air cored coils are not practical as general purpose replay heads, however, for at least two reasons: 1. the spatial resolution which can be achieved is low, because the sensitive regions around the coil are relatively diffuse; and 2. the sensitivity of the coil to external interference is high, giving rise to unacceptable signal to noise ratio in most practical situations.

This invention is based on air-cored coils, and overcomes the two problems described by novel coil design. For reading relatively coarse magnetic features of the kind discussed above, the first objection is overcome with suitably-configured compact coils. The second objection is overcome by using balanced coil configurations which are sensitive to nearby magnetic sources, but reject signals from greater distances. Three designs for heads according to the invention are shown in Figures 1, 2 and 3.

These are intended to illustrate the general principles of the invention, and other designs employing the same principles are possible.

In Figure 1, a head design employing coaxial coils is illustrated. This design of head is suitable for applications where it is convenient to pass the magnetic media through the central aperture of the arrangement. The two coils are wound such that their extent in the direction of travel of the magnetic material is comparable with the size of the elements it is desired to detect. The numbers of turns in the coils, and the diameter of the coil wire are optimised such that the number of turns is as great as possible commensurate with the coil dimensions not being excessive, and the coil resistance not being a dominant source of electrical noise. The sizes and number of turns of the two coaxial coils are matched such that the turns x area products are equal, and the two coils are connected in series in anti-phase. Far field onaxis signals are cancelled by this arrangement, while magnetised elements passing through the coils create a differential output.

In some situations the sensitivity of the head to external interference can be reduced further by surrounding the coils, at a small spacing, with a coaxial continuous loop of metal. To be effective, the metal should have very high conductivity, and a suitable material is, for example, copper or highpurity aluminium. The required electrical resistance of the loop is inversely related to the interference frequency, and the technique is only practical for frequencies of greater than a few hundred Hz.

In a typical implementation the inner coil has 100 turns of 0.1 mm enamelled wire and an internal aperture of dimensions of 80 x 6 mm. The coil width is 5 mm.

The outer coil has 57 turns of 0.1 mm diameter wire, and has internal dimensions of 84 x 10 mm. This coil is again 5 mm wide.

A screen for high-frequency interference was formed from a plate of 6 mm thick 99.5% pure aluminium.

The coil assembly is mounted centrally in a slot 88 x 14 mm machined in the aluminium.

The response of the head to a small magnetised element is indicated in Figure 1.

The sensitivity of the complete head assembly is relatively constant for magnetic elements passing at constant speed through the head at any position in the aperture.

Figure 2 shows another balanced coil arrangement, this time using a pair of identical coils arranged side by side. Such an arrangement is useful in situations where an enclosed loop is not convenient or where access to the magnetic feature is only available from one side. Again the coils are connected in anti-phase and in series, so that on-axis far field signals are cancelled to a high degree.

In a typical design of this sort, the two coils are each pile-wound with 100 turns of 0.1 mm wire on formers with dimensions 80 mm x 1 mm. The coils are placed as close together as the winding thickness will allow, which in the prototype gave a coil centre to coil centre distance of about 5 mm.

The response of this head for magnetic elements which are short compared with the coil spacing is indicated diagrammatically in Figure 2. This is more complex than that of the arrangement in Figure 1, and consists of 4 peaks of alternating polarities as the magnetic element passes sequentially over the two coils.

The sensitivity of this single-sided head, at constant spacing between the head surface and the magnetic element, is essentially constant over the central 75 mm of the head width. Sensitivity diminishes with distance form the coil surface. Features as small as 2 mm x 1 mm made from 15 micron thick 300 oersted magnetised magnetic recording tape moving at 1 metre/sec could be detected with good signal to noise ratio at a distance of 3 mm from the coil surface, and were still just detectable at 5 mm from the surface.

Figure 3 illustrates a further embodiment using the same general principles. In this case the two coils are stacked one on top of the other. Once again, on-axis far field signals are cancelled, while signals from nearby magnetic elements produce a differential output. In a prototype head the coils were wound from 100 turns of 0.1 mm diameter wire on formers 80 mm x 2 mm, and had a width of 5 mm. Magnetic elements may pass through either of the two coil apertures, or over the top or bottom of the overall assembly. In both cases, however, the design is non-preferred compared with the implementations illustrated in Figures 1 and 2. The response to a small element passing through or over the assembly is similar to that shown in Figure 1.

Screening of the designs shown in Figures 2 and 3 against external interference is possible using highconductivity metal between the source of external interference and the heads.

In all the examples illustrated the coils are shown as being wound from enamelled copper wire. An alternative technique is to manufacture the coils using well-known printed circuit board techniques widely used in electronic circuit manufacture. This is particularly appropriate for the implementation illustrated in Figure 2. A head using this topology, and with similar length and width to those described above, has been produced using 10 layer printed circuit board. Each layer contains a pattern of 2 x 7 turns etched from 1 oz copper, and the individual patterns are suitably interconnected by conductive vias. The 10 layer laminated board is 1.5 mm thick. The advantage of this form of construction is that the dimensional accuracy of the coils is very high, so balance between the coils is also high. This form of construction is also very convenient for applications requiring multiple channels in a single head assembly, since no extra processing steps are required to produce multiple coils. Furthermore, at the expense of doubling the number of layers in the laminate, it is also possible to interleave coils from adjacent channels by placing them on alternate layers. This allows continuous coverage across the full head width to be achieved, with no "dead" bands between channels. In volume manufacture, printed circuit coils are cheaper to make and more reproducible than conventionally wound coils.

In all examples described, the output from the coils is connected to a high-gain amplifier with sufficiently low self-generated noise not to degrade significantly the signal-to-noise ratio of the basic coil assembly. The amplified signals are then suitable for band-pass filtering, digitisation and further processing as required by the application. For optimum signal to noise ratio the filter bandwidth is matched to that of the signal resulting from passage of the magnetic media through/over the heads. Suitable lownoise amplifiers, in integrated circuit format, are available commercially at low cost from suppliers such as Analogue Devices (USA).

Claims (12)

CLAIMS:

1. A magnetic reading device which comprises a pair of air-cored coils, characterised in that the coils: (a) are connected in series and in anti-phase; and (b) are positioned so as: (i) to be contiguous along at least one dimension and (ii) to be disposed symmetrically in relation to each other.

2. A magnetic reading device as claimed in claim 1, wherein said coils are generally rectangular in section.

3. A magnetic reading device as claimed in claim 1 or 2, wherein said coils are arranged co-axially, the inner coil defining the extent of a slot through which a laminar magnetic element may be passed.

4. A magnetic reading device as claimed in claim 3, wherein the inner coil (CI) has N1 turns and is of area AI and the outer coil (cho) has N2 turns and is of area Ag, such that the products N1.AI and N2vAo are equal.

5. A magnetic reading device as claimed in claim 2, wherein the two coils are identical and are positioned side by side along their major axis.

6. A magnetic reading device as claimed in claim 2, wherein the two coils are identical and are positioned in a stacked configuration.

7. A magnetic reading device as claimed in claim 3 or 4, wherein said coils are surrounded by a coaxially arranged continuous loop of a highconductivity metal so as to provide shielding from sources of external interference.

8. A magnetic reading device as claimed in claim 7, wherein said coils are positioned within a slot formed in a plate of aluminium.

9. A magnetic reading device as claimed in any preceding claim, which further comprises a high-gain amplifier and signal processing means.

10. A magnetic reading device substantially as hereinbefore described with reference to, and as illustrated in, Figure 1 of the accompanying drawings.

11. A magnetic reading device substantially as hereinbefore described with reference to, and as illustrated in, Figure 2 of the accompanying drawings.

12. A magnetic reading device substantially as hereinbefore described with reference to, and as illustrated in, Figure 3 of the accompanying drawings.