Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1991-12-26
1994-09-06
Swann, III, Glen R.
Communications: electrical
Condition responsive indicating system
Specific condition
340551, 343841, 343842, 343894, G08B 1324
Patent
active
053452224
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This application relates to detection apparatus for security and surveillance systems, in particular but not necessarily exclusively for systems relying on magnetic detection of special markers or tags, which are often used in electronic article surveillance (EAS), e.g. in retail premises.
Detection systems in general use large, relatively flat, pile-wound, air-cored induction coils for reception of ac magnetic fields generated when tags pass through the detection zone. The coil axis is usually perpendicular to the direction of travel of persons walking through the detection zone, This type of detection system is prone to interference from external sources of ac magnetic fields such as cash registers, motors and electrical cables, since these will also induce voltages in the pick-up coils. These extraneous signals complicate the recognition of the signals from the markers, and generally cause false alarms or reduce the genuine detection rate. Additionally, this type of detection suffers from further unwanted signals which are generated by external (normally) `passive` objects such as iron and steel panels or other metal fixtures close to the detection volume, since these objects are driven to produce unwanted magnetic: signals by the magnetic field which is generated by the EAS system, which is used to interrogate the tags in and around the detection volume.
Screen material can be employed to shield the air-cored detection coils from unwanted external signals, but these have to cover at least the entire area of the coil, so are expensive, cumbersome, difficult to install and aesthetically undesirable.
SUMMARY OF THE INVENTION
This invention is concerned, inter alia, with methods for reducing or eliminating these problems, and with apparatus constructed accordingly.
In accordance with one aspect of the invention, detection coils are used which have a ferromagnetic core of high permeability and low coercive force, suitable exemplary materials being soft ferrite, transformer steel or mumetal.
In one embodiment of the invention, the detector coil is wound onto a rod or long block of the core material. This will produce substantially the same performance in the far- and mid-field as a dipole air-cored detection coil of diameter equivalent to the length of the core rod or block.
The solid cored coil has advantages of lower overall size, but the primary advantage in accordance with this invention is that the magnetic flux entry points to the detection coil are considerably more confined, being located at the tips of the core rather than spread out over the entire plane of the air-cored coil. This means that the position of flux entry and exit may be easily manipulated and moved around by moving or shaping the ends of the core. For example, the core ends may be pointed inwards to the detection zone to reduce sensitivity to external interference. The advantage of this well-defined flux control is that the receivers can be shielded more effectively from unwanted external fields, as described below.
Suitable core materials will generally have an effective relative magnetic permeability of between 1 and 10,000, preferably between 30 and 1000. The effective permeability may be governed either by intrinsic material properties or core shape, or a combination of the two. Typically, rod cross-sections will be a few cm.sup.2 and rod length from 5-50 cm, although these dimensions are given as typical examples only.
Furthermore in accordance with, and as a preferred component of, this aspect of the invention small areas of screening material may be placed behind or around the flux entry points at the tips of the rod; these provide effective screening of the receive system for unwanted external systems. The quantity, and hence the weight and cost, of screening material is considerably less than is required for an air-cored coil, and the ease with which it can be manipulated is improved. Since only a small amount of material is needed, there may be gaps between screens, allowing lines of sight in
REFERENCES:
patent: 4166264 (1979-08-01), Starr
patent: 4623877 (1986-11-01), Buckens
patent: 4769631 (1988-09-01), Copeland
patent: 5061941 (1991-10-01), Lizzi et al.
EPO Search Report on European counterpart of this application.
Davies Dafydd G.
sbrink Leif
Esselte Meto International Produktions GmbH
Swann III Glen R.
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