Systems are known for the prevention of theft of articles, e.g., books from libraries and products from stores, which generally comprise a marker element attached to the article and instruments adapted to sense a signal produced by the marker upon its passage through an interrogation zone. Typically the marker is a magnetic wire or strip and the interrogation zone is provided by transmitter antenna coils which generate an alternating magnetic interrogation field in the zone. The marker is driven into and out of saturation which disturbs the alternating magnetic interrogation field and produces alternating magnetic fields at frequencies which are harmonics of the alternating magnetic interrogation field. The harmonics are detected by receiver antenna coils, which are frequently housed in the same structure as the transmitter coils, which in turn are processed by electronic processing means to produce alarm signals.
On a more sophisticated level, for retail tagging, tagging used in the road/air-freight package industry, and pallet tagging in manufacturing processes, a tag (also known as an "identification tag", "marker" or "label") is useful for identifying a product or article in detail. Such tags include bar code labels and radio frequency tags based on silicon memory and logic circuits (also known as RFID tags). By providing the tag with a sufficient number of bits and interrogating the tag, the tag can provide information such as what the product is, when it was manufactured, its price, and whether the product has been properly passed through a check-out counter or kiosk. Tags can also be used to identify a variety of other animate and inanimate objects too numerous to mention. Identifying a product via an RFID tag may hasten a new type of checkout system for the retail industry giving rise to a so-called "no-wait checkout".
The present technology used for magnetic tags which are widely used for anti-theft purposes has several drawbacks when extended to multibit capabilities including that relatively large sized markers result. In addition, there are problems due to the method of the tag's operation which prevents them from being solidly embedded. An example of a large multi-bit marker is the tag of Pettigrew et al., U.S. Pat. No. 4,940,966. Pettigrew et al. describe a tag having a limited number of bits which includes a plurality of soft magnetic strips mounted on a substrate, biased by adjacent strips of hard magnetic material. Typical spacing between strips is on the order of one centimeter with some strip lengths greater than 4 cm. Here, each element or strip defining a bit must have dimensions that differ from one another in order to give them a distinct characteristic, here being a unique hysteresis curve.
The magnetic multibit tag described by Dames and Hyde (U.S. Pat. No. 5,420,569) uses magneto-mechanical coupling to cause resonances characterized by planar dimensional changes and enhanced magnetization of the strip elements. This tag is both large and has the disadvantage that the strip needs to be free-standing in order to resonate and therefore cannot be embedded.
The systems described above all have the capability of being interrogated remotely. In addition, there are means for storing many bits of information per unit area using magnetic stripes on items like credit cards. However, this type of storage device requires either direct or very close proximity of the reading head to the object and can be easily tampered with by use of a hard magnet. Furthermore, magnetic stripes cannot serve as a remotely sensed antitheft device.
There is a widespread problem with theft in many industries, e.g., in the computer industry where there is theft of both computer parts and components and entire computers. The theft is difficult to detect and if recovered after the theft, the recovered items are difficult if not impossible to identify. Thus, what is needed in addition to an anti-theft device is a means to identify the owner along with important data such as the warranty date. It is therefore important to 1) provide a means for preventing theft by concealing a type of sensor that can sound an alarm to an externally located receiver and 2) provide a set of elements that yield a code upon local interrogation, the elements being concealed and personalized to permit identification of the object should it be stolen and recovered.