United States Patent 3624631

An antipilferage system comprising a cooperating radio frequency transmitter and receiver pair. The transmitter directs swept radio frequency energy to the cooperating receiver circuit wherein balanced oscillating conditions are maintained in a normally steady state. Passive tuned circuits, preferably in the form of printed circuit elements are placed on the merchandise in a store and if an attempt is made to remove unpurchased merchandise from the store without first destroying a fusible link in the passive tuned circuit an unbalance is caused in the receiver oscillator circuits which operates to actuate a suitable alarm. A fully automatic deactivation circuit is also included as part of the invention.

Chomet, Marc (East Northport, NY)
Ellison, Donald E. (Sea Cliff, NY)
Watterson, Robert F. (Lake Ronkonkoma, NY)
Application Number:
Publication Date:
Filing Date:
Sanders Associates, Inc. (Nashua, NH)
Primary Class:
Other Classes:
334/39, 340/572.3, 340/572.5
International Classes:
G08B13/24; (IPC1-7): G08B13/00; G08B21/00
Field of Search:
340/258R,280 343
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US Patent References:

Primary Examiner:
Yusko, Donald J.
Assistant Examiner:
Slobasky, Michael
Having described what is new and novel and desired to secure by Letters Patent, what is claimed is

1. A pilferage control system for detecting the unauthorized removal of goods from an area comprising

2. Apparatus as recited in claim 1 wherein said transmitting means comprises

3. Apparatus as recited in claim 1 wherein said detecting means comprises

4. Apparatus as recited in claim 3 wherein said rejecting means includes

5. Apparatus as recited in claim 4 wherein said rejecting means includes correlation means coupled to said dual time constant detector means comprising

6. Apparatus as recited in claim 1 further including

7. Apparatus as recited in claim 1 wherein said passive tuned tank circuit comprises in series

8. Apparatus as recited in claim 1 wherein said tuned circuit deactivation means comprises

9. Apparatus as recited in claim 8 further including

10. A tuned passive tank circuit for use with a radiofrequency pilferage control system comprising in series

11. Apparatus as recited in claim 10 wherein

12. Apparatus as recited in claim 10 wherein:


The present invention relates generally to the field of radiofrequency circuits and more specifically to a radiofrequency antipilferage system.


Pilferage of merchandise from business establishments has become a matter of great concern and many devices have been devised in an attempt to minimize losses. One such device is a Detecting Means for Stolen Goods described in U.S. Pat. No. 2,774,060 which issued to T. F. Thompson on Dec. 11, 1956. In this system one or more fixed frequency oscillators are employed to set up a radiation field of predetermined shape and size and precision resonant circuits are concealed in sales tags on the merchandise. When merchandise with a resonant circuit is passed through the radiation field the potential of the oscillator is changed and actuation of an alarm results. In order to prevent false alarms by legitimately purchased merchandise it is necessary that the sales tag containing the circuit be physically destroyed.

Several practical problems attend the use of an antipilferage system such as that described by Thompson. The use of a fixed frequency oscillator requires that the resonant tuned circuit be precision tuned to that frequency. In order to prevent actuation of the alarm by an object of appropriate size carried by a legitimate customer it is desirable to use three different fixed frequency oscillators and three precision tuned circuits, one for each frequency. In such an arrangement wherein frequency precision is required a serious problem may arise in detuning of the tuned circuit by body capacity, i.e. the electrical capacity of the human body in close proximity to the tuned circuit is in many cases sufficient to detune the circuit to the extent that detection by a single frequency system is precluded. It will be noted that even in the three oscillator embodiment of the Thompson apparatus detuning of any of the precision circuits will preclude detection. The fact that the Thompson apparatus necessitates a manual destruction of the tuned circuit bearing tag increases the potential for inadvertent and embarrassing triggering of the alarm should an employee forget to remove the tag. An accomplished pilferer may also become aware of the gross nature of the system from observation of consistent removal of such tags and/or the consequences of nonremoval. The apparatus described by Thompson further involves the use of one antenna for both transmitting the radiation pattern and detecting an absorption by the resonant circuit. Such an arrangement has an inherently low-volumetric efficiency and the tuned circuit must pass in close proximity to the antenna for reliable detection. Many business establishments are not physically laid out such as to assure such proximity.


From the foregoing it will be understood that among the objectives of the present invention are the following:

To provide a new and novel radiofrequency antipilferage system.

To provide apparatus of the above-described character using a swept radiofrequency.

To provide apparatus of the above-described character having a cooperating transmitter and receiver remotely disposed with respect to one another.

To provide apparatus of the above-described character wherein a passive tuned circuit disposed on merchandise may be automatically and surreptitiously deactivated.

To provide apparatus of the above-described character having improved false alarm rejection.

The foregoing objectives are accomplished through the practice of the present invention by providing a swept radiofrequency oscillator and transmitting antenna and a remotely disposed cooperating broad band radiofrequency receiver and antenna coupled to a post detection signal processor. A passive tuned circuit having a fusible link is affixed to articles of merchandise preferably within a price tag or other item affixed by the store. When pilfered merchandise bearing the tuned circuit is carried between the transmitter and receiver there is absorption of energy from the radiation field and an unbalance is produced in the receiver which is used to actuate a suitable alarm. To preclude actuation of the alarm by tags or legitimately purchased merchandise each passive tuned circuit is provided with a fusible link which is opened when the circuit is exposed to radiofrequency energy above a preselected level. To accomplish the deactivation swept radiofrequency energy above the preselected level is transmitted for example through the surface of a check out counter at which purchased merchandise is wrapped. This energy destroys the fusible link in the passive tuned circuit and automatically checks for the continued presence of an active circuit. Thereafter the merchandise with the tag bearing the deactivated tuned circuit, and in which there has been no change which is readily discernable to the casual observer, may pass between the transmitter and receiver without actuating the alarm. The deactivation portion of the invention is fully automatic in operation and under normal circumstances completely eliminates any requirement for manual participation of sales employees in the deactivation of the tuned circuit.

These and other objects, features and advantages of the present invention will become more apparent from the detailed discussion taken in conjunction with the appended drawings.


FIG. 1 is a block diagram of a pilferage control system in accordance with the present invention.

FIG. 2 is a schematic diagram of a fusible tuned circuit for use with the present invention.

FIG. 3 is a schematic block diagram of a deactivation circuit for use in the practice of the present invention.


Turning now to FIG. 1 there is illustrated a block diagram of the cooperating transmitter and receiver portions of the present invention which would be placed across each exit of a store. The transmitter generally designated 10 comprises a voltage controlled oscillator 12 the output frequency of which is controlled by a sweep control means 14. The swept radiofrequency output signal from the oscillator 12 is coupled through a driver 16 to a transmitter antenna 18. A radiation field 20 over a desired area defined by the characteristics of the transmitter antenna 18 is thus established. The transmitter 10 is preferably placed on one side of the entrance/exit to the place of business.

The receiver designated generally at 22 is placed on the opposite side of the store entrance/exit from the transmitter. A receiver antenna 24 picks up the swept radiofrequency energy and is coupled to a passband radio receiver 26. The output of the receiver 26 is applied to a post detection processor 28 which comprises in sequence a dual time constant detector 30, amplifier 32, passband filter 34 and correlation circuit 36.

The receiver 26 output in the absence of a tuned circuit in the radiation field will be substantially at a continuous level as the transmitter oscillator 12 is swept in frequency. When a tuned circuit enters the radiation field 20 there will result a discernable dip in the receiver 26 output due to energy absorption by the circuit at the frequency for which it is tuned. The dual time constant detector 30 operates to reject false absorption signals of two types; first, that caused by an individual merely passing through the exit and which is manifested as an absorption at all frequencies within the sweep range of the oscillator 12 and second, the random noise within any electronic system which is characterized by very sharp spikes. In that the frequency is swept rather than fixed the dual time constant detector 30 may simply reject any detected absorption which is of too long or short a duration; i.e. corresponding to either too wide or too narrow frequency bands to be produced by a tuned circuit in the radiation field 20.

Thus receiver signals which have the proper frequency (and thus time) characteristics are coupled to amplifier 32 and thence through a passband filter 34 to a correlation circuit 36. The correlation circuit 36 may, for example, comprise a plurality of one shot multivibrators or any other of the recognized means for dividing the frequency sweep of oscillator 12 into a given number of bins or windows. Once the presence of a tuned circuit is detected in the radiation field 20 by a proper absorption characteristic the correlation circuit 36 establishes a frequency window or bin; e.g. 10 percent of the total frequency sweep range, within which the absorption was detected. Only after a predetermined number of sequential detections have occurred in that frequency window will the correlation circuit 36 produce an output signal. A transient signal in the system is thus rejected since it would not appear in the requisite number of successive sweeps and random signals would not have the frequency stability to appear in the same window for a long enough period to actuate the alarm. The correlation circuit output signal could be coupled directly to a suitable alarm 40, however, as an added false alarm rejection feature this signal may be applied to an AND-gate 38 to which is also coupled a signal from a foot switch (not shown) concealed for example under a floor mat in an exit doorway. In this fashion the alarm 40 would only be actuated by a person in the radiation field 20 who was carrying merchandise bearing an active tuned circuit.

FIG. 2 is a schematic illustration of a passive tuned circuit which is useful in the practice of the present invention. The circuit is essentially a simple tank circuit comprising a coil 42 and capacitors 44 and 46. A significant difference, however, which is of great practical utility in the practice of this invention is the incorporation of a fusible link 48 in the circuit. This link is shown for the purpose of illustration as being disposed between capacitors 44 and 46, however, it is to be understood that disposition anywhere in the circuit is equally suitable in the practice of the invention. The link 48, when placed in series in the tuned circuit, has negligible current induced therein by the radiation field across the entrance/exit of the store and thus has substantially no effect upon the detection process discussed above. The fusible link 48 is formed of a fine conductor such as steel wire which may be "burned out" by a preselected level of radiofrequency energy. The passive tuned circuit may be formed using printed circuit techniques well known in the art and incorporated into any of the variety of cards, tags or the like which are normally found in or on merchandise displayed for sale. Initially the circuit is complete and when placed in a radiation field 20 operates to absorb energy in the field. On the application of a relatively higher power RF field the induction field overloads the fusible link, opening the circuit and thus rendering it ineffective as an absorber.

FIG. 3 is a schematic block diagram of a fully automatic deactivation circuit which is preferred in the practice of the present invention. This circuit may be placed beneath the checkout and wrapping counter usually found in business establishments and operates to detect the tuned circuit carried by merchandise being legitimately purchased, switch to an output power sufficient to "burnout" or open the fusible link and check to assure that deactivation has actually been accomplished. A voltage controlled oscillator 50 having substantially the same frequency range as oscillator 12 of FIG. 1 and which is controlled by a frequency sweep control means 52 is coupled through a buffer amplifier 54 to a power driver 56. The power driver 56 is coupled to a transmitter antenna 58 which may be flush-mounted in or under the wrapping counter. Mounted concentrically with the transmitter antenna 58 is a receiver antenna 60. When a piece of merchandise bearing a tuned circuit is placed on the wrapping counter an unbalance is produced in the receiver 62 to which the receiver antenna 60 is coupled. The receiver output is coupled through receiver logic circuitry 64 to a deactivation control logic circuit 66. When the presence of a tuned circuit is detected the receiver logic circuit 64 generates an output signal which is applied to the deactivation control logic circuit 66 which operates to switch the power driver 56 into a relatively high-power mode (e.g., 100 mw.) for a predetermined period of time. At the end of this time the power drive 56 is returned to its normal lower level output. If the continued presence of a tuned circuit is indicated the deactivation process is automatically repeated. If, after a predetermined number of deactivation cycles, as determined by feeding back the output of the deactivation control logic circuit 66 to the receiver logic circuit 64, a tuned circuit continues to be detected a manual destruct indicator 68 such as a light, buzzer or the like, may be activated to instruct the employee to manually remove and destroy the circuit-bearing tag or label. To further discourage the employee from simply ignoring the manual tag destruction indicator a reset switch (not shown) may be provided which must be closed by the employee before the indicator will be deactivated. The deactivation circuit is thus both fully automatic and self-checking in operation, and substantially precludes the actuation of the entrance/exit alarm by legitimately purchased merchandise. It is only in rare instances that the manual involvement of an employee is required and even in this event the employee need have little if any knowledge of what the under-counter unit is or how it operates.

It will thus be seen that the objectives set forth hereinabove, among those made apparent from the preceding description are efficiently attained and since certain changes may be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the appended drawings shall be interpreted as illustrative and not in a limiting sense.