DK145169B - PLANT FOR REGISTERING A PASSAGE OF AN OBJECT THROUGH A PRESCRIBED AREA - Google Patents

Description

1451Βδ

The invention relates to a system for recording a passage of an object through a predetermined area and of the nature comprising a transmitter and a receiver with a common antenna for alternately transmitting and receiving electromagnetic signals, and a marker attached to each of the objects. receiving said signals and transmitting other signals upon passage of the region where the transmitting and receiving periods change at a predetermined frequency, the transmitted signals having a constant signal frequency throughout the transmitting period, and an analysis circuit counting a given number of continuously received signals from the pointer constituted by a tuned circuit and compares the frequency of these with the frequency of the signals transmitted by the transmitter during a transmission period.

15 From U.S.A. Patent No. 3,818,472 discloses an alarm system which uses a single loop antenna. For background noise, it is necessary to use a complicated signal processing and analysis circuit containing a super-heterodyne receiver. The plant therefore becomes relatively complicated and expensive.

It is also known to use two loops in that an electrical circuit provides the difference between the signals emitted by the loops, thereby compensating any external distant fields.

The system according to the invention is characterized in that the antenna for alternately transmitting and receiving is constituted by a dipole formed as one to eight octagonal loop lying in one plane.

Thereby, the plant is cheapened without reducing the probability of detection for that reason. The system according to the invention will also - all else equal - be able to cope with a simpler analysis circuit.

2 145169

The invention will be explained in more detail below with reference to the drawing, in which 1 is a diagram of a plant according to the invention for recording a passage of an object through a predetermined area; FIG. 2 is an illustration of an application; FIG. 3 is an illustration of another application indicating the structure of an antenna belonging to the system; and FIG. 4 is an illustration of a third application.

The FIG. 1 for recording a passage of an object through a predetermined range comprises a transmitter 2 and a receiver 3, 4. An antenna 1 for alternately transmitting and receiving electromagnetic signals is connected to either transmitter via a switch 5 2 or receiver 3,4. Switch 5 is controlled by a synchronization circuit 6 operating at a suitably high frequency. By transmitting a short-duration signal from the transmitter 2 at a suitable frequency, an alternating current is induced in an LC circuit in a marker 18, the latter of which is within the electromagnetic field formed by the antenna 1. At the end of this signal transmission, ie. when the antenna 1 is connected to the receiver 3, 4 by means of the switch 5, the marker LC circuit emits a signal of the same frequency until the oscillation in the LC circuit dies out.

This signal is received by the antenna 1, the signal being transmitted via the receiver 3, 4 to a signal processing and analysis circuit 7. This circuit 7 simultaneously receives pulses from the synchronization circuit 6, thereby being able to count how many signals are received. from a marker in 30 an unbroken order. This can be achieved by resetting a counter if no signal is received in a receiver period immediately after a transmission period. It has been found that a requirement for 256 signals received from a marker in an unbroken sequence provides sufficient assurance that this is a marker passage and not random electrical noise. The signal processing circuit 7 further checks whether the received 5 signals have the same frequency as the transmitted signals. This provides additional security against false alarms. If an alarm is to be set equal to 256 signals received in an uninterrupted sequence at the correct frequency, the signal processing circuit 7 sends an alarm signal to an auditory or visual alarm system.

In FIG. 2, it is shown how a number of antennas 8 can be arranged to control a wide passageway. The system is built into a frame-shaped stand. The antenna itself in the form of a eight to eight loop is located inside the parts 9, 10, 11, 12, 13, 14, 15 and 16 of the rack 15, while the transmitter 2, the receiver 3, 4 and the signal processing and analysis circuit 7 are incorporated. in a box 17.

The FIG. 3 systems are equipped with an additional antenna.

One antenna 19 is lodged in a door, while the other antenna 20 is lodged in the floor at the doorway. Since these two antennas are perpendicular to each other, the door passage is completely secured, with the marker 18 with any orientation being detected, contrary to the arrangement of FIG. 2, where the cursor can pass unobstructed if it is oriented perpendicular to both the antenna and the floor.

Finally, in FIG. 4 shows a location of the plant at a cash register in a shop.

The system according to the invention can be used wherever an object removal control is desired. Thus, the plant 30 may also be used in e.g. libraries, in that the pointer in the form of thin films can be attached or inserted invisibly in the volumes of the books.