Communications: electrical – Condition responsive indicating system – Specific condition
Patent
1992-11-30
1994-11-22
Swann, Glen
Communications: electrical
Condition responsive indicating system
Specific condition
340551, G08B 13187
Patent
active
053672917
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an electromagnetic shoplifting detection system of the high-frequency type. In known shoplifting detection systems, a transmitting coil of a transmitter or a transmitter/receiver generates a magnetic alternating field with a periodically varying frequency. This frequency is generally in the range of 1 to 10 MHz. To the articles to be protected so-called detection labels are attached. These labels each comprise a resonant circuit consisting of an air-core coil tuned with a capacitor. If such a label is carried into the magnetic alternating field of the transmitter coil, at those instants when the frequency of the alternating field is equal to the resonant frequency of the resonant circuit in the detection label, that resonant circuit will absorb energy and then oscillate. This oscillation can be detected by a receiver circuit, which is either connected to the transmitting antenna, as is the case in a so-called absorption system, or is coupled with a second (receiving) antenna, as is the case in a so-called transmission system. Such shoplifting detection systems are known inter alia from applicant's Netherlands patent applications 82.02951 (corresponding to U.S. Pat. No. 4,686,517) and 89.00658. The transmitting/receiving antennas of a complete shoplifting detection system can for instance form a row of upright panels or self-supporting antenna coil constructions, sometimes referred to as pillars. Heretofore, the pillars have mainly been used in clothes stores, with the pillars being positioned directly in front of the exits. In such surrounding, the pillars are mounted directly on the floor, the direct vicinity of each pillar being free of obstacles. A new application of shoplifting detection systems of the high-frequency type concerns their use in supermarkets, where the articles purchased are paid for at so-called checkout units. Checkout units are normally constructed with metal beams, metal or wooden surfaces, provided with a conveyor belt, an (electronic) cash register and sometimes a barcode scanner. A cashier is seated in such a checkout unit, the customers pass in front of the unit and deposit the articles to be paid for on the conveyor belt. The belt carries the articles to the cashier, who enters them in the cash register using a barcode scanner, if provided, and finally payment is effected. The articles have meanwhile passed on to a location on the checkout unit behind the cashier, in the direction of the exit of the supermarket, i.e. a separation occurs of the stream of articles to be paid for and the customers involved. In a typical supermarket situation a series of these checkout units are lined up in a row and the customers pass between the checkout units in the direction of the exit. This system is sometimes referred to by the term "checkout system". To check whether customers who pass the checkout unit take out articles without depositing them on the belt, i.e. without paying for them, the customers must pass a detection field. For that purpose, one or two detection pillars are arranged in the passage which is formed between two checkout units and which the customers pass through.
Since the interspace is small with a view to efficient use of space, the pillars are arranged as close to the checkout units as possible; accordingly, in practice, the pillars are affixed to the checkout units. The detection pillars generate a magnetic alternating field on both sides. Similarly, the detection pillars have a sensitivity area extending on both sides of the pillar. The field on one side covers the passage the customers pass through; the field on the other side extends into the checkout unit. There are many conductors in the checkout unit. In the first place, metal beams, which form the mechanical structure of the checkout unit, can be conductors. In the second place, there may be disposed in the checkout unit many electricity lines, such as lines for the electricity supply to the checkout unit, for the conveyor belt, the cash register, the scanner
N.V. Nederlandsche Apparatenfabriek Nedap
Swann Glen
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