NL9300124A - Improved antenna configuration in supermarket shoplifting detection systems. - Google Patents

Description

P.J.W.M. van Breemen

Improved antenna configuration in supermarket shoplifting detection systems.

The invention relates to a shoplifting detection system of the high-frequency type. In the prior art shoplifting detection systems, a transmit coil ("transmit antenna column") generates a magnetic alternating field of varying frequency. This frequency is generally in the range between 1 and 10 MHz.

So-called detection labels are attached to the goods to be protected. These labels, also called wafers, contain a resonant circuit consisting of a coil tuned with a capacitor. For example, they can be designed as a plastic box containing an air coil, as a adhesive label, built up with a flat printed coil and a film capacitor, or as a circuit with distributed capacity and self-inductance.

When such a label is brought into the magnetic alternating field of the headpiece, at times when the frequency of the alternating field is equal to the resonant frequency of the circuit in the detection label, those circuit absorb and resonate circuit energy. This resonance can be detected by a receiver circuit which is either connected to the transmit antenna in a so-called absorption system or coupled to a second (receive) antenna in a so-called transmission system. These shoplifting detection systems are known, inter alia from the Dutch patent applications 8202951 and 8900658 of the applicant.

The transmit / receive antennas of a complete shoplifting detection system are realized, for example, in a row of upright or self-supporting antenna coil constructions, also referred to as columns. The transmitter and / or receiver electronics are usually located somewhere in the column, preferably in the base. Until now, these columns have mainly been used in shops, such as clothing stores, where the row of columns is placed just in front of the exit. In such an environment, the columns are mounted directly on the floor, the immediate area of each column being free from obstacles. The detection columns generate a magnetic alternating field on both sides. Likewise, the columns have a sensitivity area that extends on either side of the column. In order to be able to determine as best as possible who is carrying one when a label is detected, optical signaling per passage is desirable. A transmission system can pass a label to the left or right of a receiver column. The label signal in the receiver is identical in both cases. It makes no difference whether the label is emitted by the left or the right transmitter.

In order to make a distinction in the passage to the left or right of a receiver column, the two transmitters on the left and right of this column, ie the odd and even numbered transmitters in the row, are alternately switched on and off: multiplexed.

From the phase of the multiplex signal, the signal which alternately turns on the odd and even transmitters, it is deduced in the receiver whether the label is to the left or right of the antenna. The signaling associated with the passage is then controlled with this information. The multiplexing must be done at a frequency high enough not to affect the response speed of the system. In practice, the lower limit is 2 Hz and the upper limit is half the swing frequency (multiplexing per full swing period).

Another application of shoplifting detection systems of the high-frequency type concerns those in supermarkets, where the settlement of the purchased goods takes place on so-called checkout blocks. Figure 1 shows this arrangement.

Checkout blocks (1) are usually constructed using metal bars, metal or wooden surfaces, fitted with a conveyor belt (2), a (computer) cash register (3) and sometimes with a barcode scanner and / or a scale (5). In such a cash register block (1) there is a cashier (4); the customers walk in front of the block (9), and put the goods to be settled on the beginning of the conveyor belt (2). The belt delivers the goods to the cashier, who enters them into the cash register (3) with or without the help of the barcode scanner (5), after which the settlement takes place. In the meantime, the goods have been moved further to a place on the cash register behind the cashier, towards the supermarket exit. So there is a split of the flow of goods to be settled and the associated customers. In a typical supermarket situation, a series of these checkout blocks (1) are lined up, where customers walk in between (9), towards the exit. More than ten checkout blocks can easily be found in a medium-sized supermarket (1). This arrangement is also referred to as the "check-out system". In order to check whether customers who pass through the cash register (9) take goods that are not put on the belt and are therefore not paid, customers must pass a detection field. To this end, two detection columns (6) are placed in the aisle, which arises between two checkout blocks and where the customers pass: a column containing receiver electronics (7) and a column containing transmitter electronics (8). A detection system in this application is preferably built with two shielded antennas.

Figure 2 shows a more detailed embodiment of Figure 1. The aforementioned electronics units are interconnected via cables (12).

The use of antennas shielded at the rear with a shield (10) prevents undesired electromagnetic coupling between the transmitting or receiving antennas and the electrical conductors in, on or at the cash register block. Otherwise, multipath propagation phenomena and parasitic resonances in construction parts and cabling in the checkout blocks can lead to reduced detection sensitivity and false alarms.

Such antennas are known, inter alia from applicant's patent application 9001033. These antennas also have the advantage that no label detection takes place at the rear, where articles with not yet deprecated labels pass on the conveyor belt (2), so that no false alarm can be caused. A magnetic alternating field is only generated at the front of the antenna, in the area where the customers pass (9). Since the receiver in this configuration can only receive a label signal generated by the field of the associated transmitter across the aisle, the detection of labels is inherently selective per pass and no multiplexer!

A situation has now arisen with alternating passage - non-passage where the customers walk through the passage and the checkout blocks form the non-passage. Detection of labels is undesirable in the non-pass.

Because the antennas now only have a (desired) detection on one side - the front -, an alternating order of transmitters and receivers is no longer necessary.

The function of the shielding in a shielded column is not perfect. There always remains, through the shielding, a residual coupling of the detection field with conductors present in the checkout blocks.

The sensitivity of a system to the residual unwanted effects of multipath propagation, caused by, among other things, coupling of transmitters, via conductors and equipment, with receivers on the other side of a cash register, and by parasitic resonances of conductors in the cash blocks, can be further reduced by to mount the same column type on either side of a cash register block: alternately two transmitter columns or two receiver columns per cash register.

The detection range and the sensitivity for unwanted coupling via the cash register block between two columns of the same type is much lower than between a transmitter and a receiver column. As a result, gel abeldfi articles on the conveyor belt of the cash register block give less cause for alarm.

According to the current state of the art in a checkout system, each antenna is connected to the transmitter or receiver electronics associated with that antenna, whether or not mounted in the antenna column.

In another embodiment, a shielded antenna is connected with a coaxial cable (11, Fig. 1) to the associated receiver or transmitter electronics unit, which is then located in a separate cabinet outside the columns.

If a checkout system with alternating two transmit antennas or two receive antennas per checkout block is constructed in this way, a checkout block will alternate between two transmitter and two receiver electronics units.

In the antenna configuration according to the invention (see figure 2), a strong cost price reduction is realized by instead of placing two electronics units of the transmitter or receiver type per cash block, only one electronics unit that is connected to the two antennas on either side of the the checkout block. (Incidentally, the invention can also be applied in a system with unshielded columns.)

In its simplest embodiment (principle diagram in figure 3), the two antennas (6) with shielding (10) on a checkout block (1) are connected in parallel to the corresponding electronics unit, for example via a torque transformer, a power splitter / combiner (13) or a directional coupling. By now having the transmitter units (8) multiplexer, ie alternately switching on, according to the principle described for a free-standing row of columns, it is achieved that detection takes place simultaneously in the passages on both sides of a cash block containing a transmitter unit, where cash blocks with transmitters switched on and off alternate. If the passages are sequentially numbered according to the sequence 1, 2, 3, 4, 5, etc., the detection is alternately enabled in the passages 1, 2, 5, 6, 9, 10, etc. or in the passages 3, 4 , 7, 8, 11, 12, etc.

From the phase of the multiplex signal it is deduced in the receivers in which passage detection takes place. The signaling associated with the passage is controlled with this information. The detection in the passages is therefore switched on alternately two by two.

This embodiment results in some electrical losses caused by the parallel connection via an impedance transformer of two antennas, which results in a reduction of detection sensitivity of 2 x 3 dB.

The multiplexing of the transmitters themselves, as in a freestanding column row, means that they are switched on only half the time. As a result, the amount of label information per unit time is not maximum. This results in a slightly reduced reaction speed or sensitivity. In practice, this effect is hardly noticeable.

To avoid these losses, another embodiment of the system is preferred. Figure 4 shows the schematic diagram of this. In this variant, a preferably electronic switch (14) is placed between two antennas (6) and a transmitter unit (8) or receiver unit (7). This switch behaves like a fast relay with a changeover contact and can, for example, be implemented with PIN diodes or FETS as switching element. An electro-mechanical relay is also possible at low multiplex frequencies. This switch replaces the torque transformer used to connect two antennas in parallel.

In this way it is achieved that alternately only those antennas are connected that are actively used for detection.

This prevents losses caused by the parallel-connected antenna that is not currently participating in the detection. The channels are also switched on continuously.

The performance of a system constructed in this way is comparable to that of a freestanding, column array multiplexed by alternately switching on the odd and odd transmitters, while the number of electronics units is halved.

In this variant, multiplexing takes place between even and odd passages to the left and right of each cash register, and not between passages two to two as in the variant without antenna switches. This is not important for the operation of a system.

In principle, it is also possible to multiplex more than two-fold, for example 4-fold. This saves extra transmitters and receivers. In practice, this is at the expense of the reaction speed and the sensitivity of the system. These requirements place a lower limit on the time that first signal of minimum strength. must be present to enable reliable detection. Complicated cabling would also be required.

Depending on the width of a passage and the quality of the labels used, a trade-off can be made between cost price and system performance.

Claims (3)

A system for detecting shoplifting in the passageways between a row of checkout blocks in, for example, a supermarket, characterized in that one electronics unit is used per checkout counter, which contains only one receiver circuit or one transmitter circuit, whereby both the transmitter in the one checkout block as well as the receiver in the neighboring checkout block are simultaneously switched between two antenna systems mounted on either side of the respective checkout blocks, such that upon detecting a tag signal it can be determined in which passage between two adjacent checkout blocks the relevant tag is located. A system for detecting shoplifting in the passages between a row of checkout blocks in, for example, a supermarket according to claim 1, characterized in that the simultaneous switching referred to in this claim takes place continuously with the aid of electronic switches or relays, with such small time intervals, that detection certainly occurs during the passage of someone with a label. A system for detecting shoplifting in the passageways between a row of checkout blocks in, for example, a supermarket, according to one or more of the preceding claims, characterized in that one electronics unit is used per checkout block, containing only one receiver circuit or one transmitter circuit, with two antenna systems on each unit being connected in parallel, possibly using power splitters or transformers, the selectivity of the passage in which a label signal is detected being achieved by alternating the transmitter on the Left and the transmitter on the right of each receiver switch on and off. A system for detecting shoplifting in the passages between a row of checkout blocks in, for example, a supermarket, according to one or more of the preceding claims, characterized in that the antenna systems are each provided on one side with an electromagnetic shield, for instance consisting of metal mesh. A system for detecting shoplifting in the passageways between a row of checkout blocks in, for example, a supermarket, according to one or more of the preceding claims, characterized in that a similar division of the antenna systems as arranged on both sides of a checkout block is applied on both sides other obstacles present in the area to be protected, such as pillars. A system for detecting shoplifting in the passageways between a row of checkout blocks in, for example, a supermarket, according to one or more of the preceding claims, characterized in that a similar division of the antenna systems as arranged on either side of a checkout block is used to bridge an area where label detection is not needed or not desired.