AN ELECTRONIC ALARM SYSTEM

Abstract: An electronic alarm system of the kind used in commodity surveillance systems, wherein the system includes an alarm element which is adapted to receive a magnetic alternating field transmitted from a transmitter by means of a transmitter coil and also adapted to re-transmit a magnetic alternating field, wherein a receiver which includes a receiver coil is adapted to receive and detect the re-transmitted alternating field. The invention is characterized in that the transmitter coil and the receiver coil are one and the same coil (1), wherein between the transmitter (TX) and the receiver (RX) on the one side and the transmitter-receiver coil (1) on the other side, there is provided an electronic unit (14) which includes a transformer and a resistance (2) which are so adapted that when transmitting, a first part of the transmitter power will be applied to the transmitter coil (1) and the remaining part, a second part, will be applied to the resistance (2), and when receiving a first part of the received signal will be applied to the receiver (RX) and the remaining part, the second part, will be applied to a circuit or to a resistance. Only a small part of the transmitter power will reach the receiver.

Description: AN ELECTRONIC ALARM SYSTEM The present invention relates to an electronic alarm system of the kind used in commodity surveillance systems. Many different types of commodity surveillance alarm systems are described in the patent literature. These alarm systems are intended to prevent commodities from being removed from stores and the like without being paid for. The commodity surveillance alarm systems normally include some type of alarm element that is attached to the commodities to be protected, and a detection arrangement which is installed permanently at a store exit and which with the aid of some form of remote sensing device is able to produce an alarm when an alarm element is brought into the vicinity of the store exit. This remote sensing of a commodity is normally effected by transmitting a magnetic alternating field, wherein the presence of an alarm element can be detected as a result of a change in the alternating field that is characteristic to the alarm element concerned. Alarm elements may have the form of long, narrow thin strips of highly permeable material capable of transmitting high order harmonics when exposed to a magnetic alternating field. Using this basic principle, it is possible to detect the presence of small and inexpensive alarm elements with the aid of complicated and relatively expensive detector arrange- ments. This type of commodity surveillance alarm system is particularly well-suited for use in stores that sell everyday commodities, utilities or convenience goods and is described, inter alia, in European Patent Specification EP 0 153 286. Also known to the art are alarm elements which consist of a simple electric resonance circuit. In this case, the detector /arrangement can be made simple and inexpensive if the coil in the resonance circuit is made relatively large, wherewith a good Q-value can be readily obtained at the same time as coupling to external fields becomes large. In this case, the coil is placed in an alarm plate which is secured to the commodity to be protected with the aid of some kind of fastener means. As before mentioned, the detector arrange¬ ments for this type of alarm may be made relatively uncompli¬ cated and inexpensive. However, it is difficult to avoid a false alarm, because the store environment will often include loops of conductive material which give rise to resonances similar to the resonances obtained with the alarm elements. The detector arrangement of such alarm systems will often include two mutually spaced alarm arches which delimit the investigation zone. The alarm arches will normally include separate transmitter and receiver coils. There is a strong desire to construct alarm systems that are both simpler and less expensive than existing systems. It is also desirable to lower the cost of installing an alarm system, among other things by simplifying the manner in which cables are drawn between the alarm arches and the electronic unit of the alarm system. The present invention fulfils these desiderata and enables one and the same coil to be used both as a transmitter coil and as a receiver coil, and reguires only one coaxial cable to be drawn up to respective alarm arches. The present invention thus relates to an electronic alarm system of the kind used in a commodity surveillance system, said system including an alarm element which functions to receive a magnetic alternating field transmitted from a transmitter by means of a transmitter coil and which also functions to re-transmit a magnetic alternating field, wherein a receiver which includes a receiver coil is adapted to receive and detect the re-transmitted alternating field, wherein the system is characterized in that the transmitter coil and the receiver coil consists of one and the same coil, wherein a transformer and a resistance between the transmit¬ ter and the receiver on one side and the transmitter-receiver coil on the other side are so arranged that when transmitting a first part of the transmitter power is applied to the transmitter coil and the remaining part, a second part, of said transmitter power is applied to said resistance, and wherein when receiving, a first part of the received signal is applied to the receiver and the remaining part, the second part, of said signal is applied to a circuit or to said resistance. The invention will now be described in more detail with reference to an exemplifying embodiment thereof and also with reference to the accompanying drawing, in which Figure 1 illustrates an embodiment of the invention. Figure 1 illustrates an embodiment of a transmitting and receiving arrangement for use in an electronic alarm system of the kind used in a commodity surveillance system. Such a system includes an alarm element which is adapted to receive a magnetic alternating field which is transmitted from a transmitter TX by means of a transmitter coil and which is also adapted to re-transmit a magnetic alternating field, wherein a receiver RX connected to a receiver coil is adapted to receive and detect the re-transmitted alternating field. The transmitted alternating field has a frequency below 30 MHz, preferably a frequency of 8.2 MHz. According to the invention, the transmitter coil and the receiver coil are one and the same coil, there being provided between the transmitter TX and the receiver RX on the one hand and the transmitter-receiver coil 1 on the other hand an electronic unit which includes a transformer 5 and a resistance so arranged that when transmitting, a first part of the transmitter power will be applied to the transmitter coil 1 and the remaining part, a second part, will be applied to the resistance 2, and that when receiving, a first part of the received signal will be applied to the receiver RX and the remaining part, second part, will be applied to a circuit or to a resistance. This function whereby only a small part of the transmitter power reaches the receiver is dependent on the impedance that the electronic unit experiences or encounters towards the transmitter-and-receiver coil being the correct impedance, i.e. has the value for which the transformer and resistance are intended. The transmitter-and-receiver coil may be connected in parallel to a capacitor and a resistor, such as to form a resonance circuit that has a low Q-value. In this way, the impedance is essentially constant, i.e. equal to the resis¬ tive value of the resistor over a sufficiently wide frequency range for the application in question, for instance a frequency of 8.2 ± 0.5 MHz. By arranging for the impedance of the transmitter-and- receiver coil to be equal to the characteristic impedance of the cable which extends between the electronic unit and the transmitter-and-receiver coil, for instance by transforming, the impedance at the electronic unit terminal to which the coil is connected will be independent of the length of the cable. In a commodity surveillance system of this kind, the trans- mitted power is low, for instance in the order of one or a few watts. The present invention is based on the realization that even if half the transmitter power is lost in heat in the resis- tance 2, all that is needed is to increase the transmitter power, for instance to double the transmitter power, with no actual cost. The saving afforded by and the simplicity of a common transmitter-and-receiver coil is obvious. However, the present invention is also based on the realiza¬ tion that the sensitivity of the receiver is limited by external interference and disturbances from other electrical apparatus in the area in which the alarm system is used, and not by the signal/noise ratio of the alarm system itself. The function of the system will not be influenced to any appre¬ ciable extent by lowering the energy content of the received signal to, e.g., 50% of its typical energy content. When the impedance of the transmitter-and-receiver coil is adapted to the cable 4 that extends between the electronic unit and said coil, the length of the cable has no signifi- cance, or only negligible significance. The cable may be, for instance, a 50 Ohm coaxial cable. This means that the transmitter and receiver equipment, including the electronic unit and the transmitter-and-receiver coil, can be made the same for all installations, or at least for a large number of installations, where the length of said cable is then adapted to the store area concerned. Both the manufacturing cost and the installation cost will therewith be relatively low. The present invention thus fulfils the desiderata mentioned in the introduction. Figure 1 illustrates one embodiment of the invention. This embodiment includes a transformer 5 having a ferrite core and a ratio of 1:1. The two windings 6, 7 of the transformer are connected together on one side of the transformer to form a terminal 8 to which the transmitter-receiver coil 1 is connected by means of a coaxial cable 15. On the other side of the transformer, one of the transformer windings, six in this case, is connected to the transmitter TX and the other winding 7 is connected to the receiver RX. A resistance 2 is connected between the windings on said other side of the transformer. Provided that the resistance 4 has an impedance which is four times higher than the impedance which the electronic unit on the left of the terminal 8, in Figure 1, experiences or encounters in the terminal 8, half of the transmitter power will be lost and also half of the received power. When the characteristic impedance of the cable 15 is equal to the impedance Z of the load 1, the impedance in terminal 8 will be independent of cable length. Furthermore, the electronic unit can be designed so that more than 50% of the transmitter power will be lost and thereby enable more than 50% of receiver power to be utilized, or vice versa. The present invention is therefore not restricted to the aforedescribed embodiments thereof, since variations and modifications can be made within the scope of the following Claims.

Claims: CLAIMS 1. An electronic alarm system of the kind used in commodity surveillance systems, said systems including an alarm unit which is adapted to receive a magnetic alternating field transmitted from a transmitter by means of one transmitter coil and to re-transmit a magnetic alternating field, wherein a receiver which includes a receiver coil is adapted to receive and detect the re-transmitted alternating field, characterized in that the transmitter coil and the receiver coil are one and the same coil (1), wherein between the transmitter (TX) and the receiver (RX) on the one side and the transmitter-receiver coil (1) on the other side, there is provided an electronic unit (14) which includes a trans- former and a resistance (2) which are so adapted that when transmitting, a first part of the transmitter power is applied to the transmitter coil (1) and the remaining part, a second part, is applied to said resistance (2), and when receiving, a first part of the received signal is applied to the receiver (RX) and the remaining part, the second part, is applied to a circuit or to a resistance. 2. A system according to Claim 1, characterized in that the transmitter-and-receiver coil (1) is impedance-adapted to a coaxial cable that extends between the electronic unit (14) and the coil (1), such that the system will be independent of cable length. 3. A system according to Claim 1 or Claim 2, characterized in that the two transformer windings (6, 7) are connected together on one side of the transformer to form a terminal (8) to which the transmitter-receiver coil (1) is connected; in that on the other side of the transformer (5) one of the transformer windings (6) is connected to the transmitter (TX) and the other winding (7) is connected to the receiver (RX) ; and in that said resistance (2) is connected between the windings (6, 7) on said other side of the transformer.

Inventors: AASBRINK LEIF, AASBRINK, LEIF
Applicants: AASBRINK LEIF, AASBRINK, LEIF
Application year: 1995
Main classification: G08B13
IPC codes: G08B13