Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-03-23
2001-07-31
Wu, Daniel J. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S568200, C340S568400, C340S572800, C340S573300, C340S539230
Reexamination Certificate
active
06268795
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a security device including a security tag for fixing to a surface such that relative movement between the tag and the surface can be detected. The device may also include a monitoring circuit for monitoring an electrical characteristic of the tag such that an alarm is triggered when relative movement is detected.
2. Description of Related Art
In shops where valuable items are on display there is a need to prevent or inhibit theft of the items. A known security tag used for this purpose comprises a flexible substrate with an adhesive backing for bonding a head portion of the substrate to the surface of an item being protected: and an electrically conductive track defining a circuit between two locations on the tag, at least part of the conductive track being located between the substrate and the adhesive backing.
The tag has a release layer adjacent the conductive track so arranged that, when an attempt is made to peel the tag away from a surface to which the head portion is fixed, some portions of the track remain bonded to the substrate while others remain bonded to the adhesive layer. The circuit is thus disrupted and the change in resistance is used to trigger an alarm.
This device has several problems. For example, it is designed as a disposable item and is relatively expensive to manufacture due to the number of manufacturing processes. Moreover, the device has to be manufactured within strict electrical tolerances for its most commonly used application, and this results in a relatively high proportion of rejects during manufacture. The device also relies on the properties of the release layer (ink) which can alter during drying.
SUMMARY OF THE INVENTION
According to the present invention from one aspect there is provided a security tag comprising an electrically conductive track formed on a flexible substrate, the substrate including an adhesive backing for bonding the substrate to a surface, the elasticity of the substrate and the bonding strength of the adhesive being selected such that, with the substrate adhesively bonded to the surface, the force required to stretch the substrate is less than the adhesive bonding force holding the substrate against the surface.
Accordingly, the substrate will stretch before it separates from the surface. The stretching of the substrate preferably changes an electrical characteristic, such as the resistance of the electrically conductive track by an amount which is detected by a monitoring circuit monitoring this characteristic of the track. If the monitoring circuit detects that the value of the characteristic lies outside predetermined upper and lower limit values, an alarm is triggered. For example, the width of the track may be narrowed as the track stretches with the substrate, and the resistance of the track thereby increases. If the increase exceeds a predetermined upper limit, the alarm is triggered.
Preferably, to increase sensitivity and to provide a more robust tag with improved handling properties, the substrate further includes a non-elastic, or at least less elastic, layer of material bonded to a portion of the substrate overlying the adhesive backing and preferably extending beyond the adhesive backing, the extended portion of the substrate preferably including terminal contacts at each end of the conductive track which are used to connect the tag to the monitoring circuit.
The substrate preferably comprises a soft stretchable material, such as self-cling PVC.
The non-stretch or less elastic portion of the substrate is preferably achieved by laminating a less elastic material to the material forming the elastic portion of the substrate.
The conductive track may have a substantially uniform width along its length, and a portion of the track is then elongated in response to the stretching of a corresponding portion of the substrate on which the track is formed. The elongation is accompanied by a corresponding narrowing of the track which increases its resistance.
Alternatively, improved performance is achieved by increasing the resistance of the conductive track along a predetermined portion of its length and by arranging that the substrate is stretched in an area that includes this predetermined portion of the track. The increased resistance is preferably achieved by reducing the cross-sectional area of a length of the track, either by narrowing its width or reducing its thickness. The substrate is preferably shaped so that the force tending to stretch the substrate is concentrated in the area that includes this length of track. Moreover, the track may be stretched to a point at which the track fractures and an open circuit is created.
In one particular embodiment, the conductive track has a first width over a first portion of the substrate and a second width over a second portion of the substrate, the first width exceeding the second width and the elasticity of the first portion of the substrate being less that of the second portion. The narrower width track therefore lies on the more elastic portion of the substrate.
The elasticity of the material forming the conductive track can be made less than that of the elastic substrate. For example, if the track consists of an ink printed on the substrate, the elasticity of the printed ink can be made less than that of the substrate whereby the ink fractures when the substrate is stretch beyond a predetermined limit. The monitoring circuit may then detect an open circuit. The ink preferably comprises a mixture of graphite and silver inks, and the amount of silver can be adjusted to achieve the required resistance and elasticity.
REFERENCES:
patent: 4000488 (1976-12-01), Ephraim
patent: 5574430 (1996-11-01), Ott et al.
patent: 5574470 (1996-11-01), De Vall
patent: 5644295 (1997-07-01), Connolly et al.
patent: 5751256 (1998-05-01), McDonough et al.
patent: 5910768 (1999-06-01), Ott
patent: 2405526 (1979-05-01), None
patent: 2287339 (1995-09-01), None
patent: 2291733 (1996-01-01), None
Advanced Technology Communications Limited
Dowell & Dowell , P.C.
Nguyen Tai T.
Wu Daniel J.
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