Apparatus For Article Theft Detection

Abstract

In combination with article theft detection apparatus which employs a signal generator coupled to an antenna at a checkpoint and means for detecting and indicating the change in signal energy caused by the presence of a resonant circuit tuned to the signal frequency adjacent the antenna, a second antenna adjacent the checkpoint, a second detecting system connected thereto and means controlled by the second detecting system for disabling the indicating means when the energy change is due to spurious radiation, line voltage changes, etc., or due to the presence of such a resonant circuit at a different checkpoint to prevent false or ambiguous indications.

Description

This invention relates to detection systems and more particularly it concerns novel arrangements for reliably indicating the passage of articles past given checkpoints.

Various techniques have been developed in the past for monitoring checkpoints, such as the exits of stores, in order to prevent the unauthorized taking of articles of merchandise out of the store or other protected area. Some of these techniques utilize radiating electromagnetic energy which is reflected, absorbed, or otherwise transformed by miniature electronic circuits embedded in, or otherwise affixed to, the "protected" articles. The effect which the miniature electronic circuit produces on the transmitted energy is monitored continuously, and when a change occurs due to the movement past a checkpoint of an article bearing the electronic circuit, an alarm is sounded. One example of such a system is shown and described in U.S. Pat. No. 3,500,373, and in such system a simple resonant circuit is used as the electronic circuit and is attached to a "protected" article.

If the system relies upon a change in energy within the field of the radiating coil or antenna, such as in the system shown in said U.S. Pat. No. 3,500,373, it may frequently happen that the alarm will be activated by events which do not represent the unauthorized removal of an article out of the protected area. Thus, in order to obtain proper operation and protection, the detection equipment is adjusted so that it is very sensitive to movement of an article bearing a lable containing a properly tuned resonant circuit past the egress facility, but as a result, the system becomes responsive to energy changes caused by other nearby electrical equipment, line voltage changes, spurious electrical energy received over the power supply lines and the passage of a resonant circuit carrying label through another nearby egress facility protected by another protection system. Accordingly, false alarms, with the accompanying problems, will occur or an unnecessary number of alarms will be activated making it difficult to determine the location of the facility involved.

One object of the invention is to minimize the number of false alarms in systems of the type hereinbefore described.

Another object of the invention is to limit the operation of an alarm to that alarm controlled by the system protecting the egress facility nearest to the article bearing a resonant circuit, i.e. the "protected" article, and being removed from the protected area without authorization.

In one preferred embodiment of the invention intended to reduce the number of false alarms due to radiation from sources other than the protecting system, a second radiation energy change detecting system, substantially identical to the detection system used to operate the alarm, is coupled to the radiation source of the protecting system and to a control antenna located adjacent to the protecting antenna at the egress facility or checkpoint but in a position such that its radiation field is substantially unaffected by the movement of a resonant circuit past the egress facility and preferably, in a position such that its field cannot be affected appreciably by normal movement in the protected area of such resonant circuit.

In another preferred embodiment where there are two closely adjacent egress facilities or checkpoints with protecting antennas, the fields of which are affected by the passage of a resonant circuit past either of them, each detecting system is provided with a pulse discriminator and a gate circuit and interconnections therebetween so that the alarm of the protecting system which detects the greater energy change will be operated whereas the alarm of the other protecting system will not. In such embodiment, the alarm which is operated will indicate the egress facility nearest the resonant circuit.

In a further embodiment of the invention where there are two closely adjacent egress facilities with protecting antennas, the fields of which are affected by the passage of a resonant circuit past either of them, a pair of control antennas, connected to detecting systems, are located adjacent to the protecting antennas, and their associated detecting systems are cross-connected with the alarms of the detecting systems so as to permit operation of the alarm of the protecting system having its protecting antenna nearest the resonant circuit on an article being removed without authorization and so as to prevent operation of the other alarm. The control antennas are located so that the fields thereof are significantly affected by the movement of a resonant circuit past one of the protecting antennas but not past the other. In such embodiment, false alarms will be kept to a minimum and ambiguity with respect to the location of the resonant circuit will be substantially eliminated.

Other objects and advantages of the invention will be apparent from the following description of presently preferred embodiments thereof, which description should be considered in connection with the accompanying drawings in which:

FIG. 1 is a block diagram of an embodiment of the invention useful in minimizing false alarms;

FIG. 2 is a partial block diagram of an embodiment of the invention useful in restricting alarm operation to the system protecting the egress facility nearest the "protected" article and to some extent, in reducing false alarms; and

FIG. 3 is a block diagram of an embodiment of the invention useful in minimizing false alarms and restricting the operation of an alarm to that of the system protecting the egress facility nearest the "protected" article.

In FIG. 1, the apparatus included within the dotted rectangle designated by the reference numeral 1 corresponds to the protecting system shown in FIG. 1 of U.S. Pat. 3,500,373, and as described in such patent, the system comprises a coil or antenna 10 which is located in the vicinity of the checkpoint or egress facility to be monitored and protected. The antenna 10 may be the single coil illustrated in FIG. 1 or it may include secondary coils as described in said patent.

The antenna 10 receives energized current from a main oscillator 11, the output energy of which is periodically shifted in frequency by means of the tuning circuit 12 connected to a frequency control oscillator 13 through an amplifier 14.

The output energy from the main oscillator 11 is also fed to a detecting system comprising a first signal detector 15 which monitors the level of the energy at the junction 16. The output of the first signal detector 15 is connected to the radio frequency filter 16 which in turn is connected to a notch filter 17. The output of the notch filter 17 is connected to a pulse detector 18 through an amplifier 18a, and when pulses appear at the output of the pulse detector 18, a relay 19 operates which energizes an alarm 20, provided that the connection between the relay 19 and the alarm 20 is not interrupted as set forth hereinafter. As described in said patent, a reduction in the energy level at the junction 16 such as by reason of the movement of a properly tuned resonant circuit past the antenna 10, causes the appearance of pulses at the output of a pulse detector 18. In this way, the passage of a "protected" article bearing such a resonant circuit through or past the antenna 10 is detected, and the alarm 20 is operated.

However, it has also been found that when there are energy changes due, for example to spurious radiations from nearby electrical equipment which are received by the antenna 10 and which are within the frequency range detected by the first signal detector 15, when there are rapid changes in the supply line voltage or when spurious signals of the proper frequency enter the apparatus within the rectangle 1, such as by way of the power supply lines connected thereto, pulses will be produced at the output of the pulse detector 18 under certain circumstances. Such pulses will similarly activate the relay 19 and energize the alarm 20 thereby causing operation of the alarm 20 even when a "protected" article is not within the effective range of the antenna 10. Such false alarms are undesirable not only from the standpoint of causing unnecessary investigation and possible embarrassment, but also if there are frequent false alarms, the apparatus is considered less reliable and the attendants will pay less attention thereto, sometimes permitting the unauthorized removal of "protected" articles.

In one preferred embodiment of the invention for reducing the number of false alarms, a control detecting system which is a duplicate of the detecting system described hereinbefore is connected to the main oscillator 11 and to a control antenna 21 which may have the same in construction as the antenna 10. In FIG. 1, the control detecting system is enclosed within the dotted rectangle 2 and comprises a first signal detector 22, the output of which is connected to a radio frequency filter 23. The output of the radio frequency filter 23 is connected to a notch filter 24, and its output is fed to the pulse detector 25 through an amplifier 26. Whenever a pulse is present at the output of the pulse detector 25, a relay 27 is operated which opens the circuit between the relay 19 and the alarm 20 by opening the contacts 28, and such opening of the contacts 28 prevents operation of the alarm 20 by the relay 19.

Although a separate oscillator could be used, preferably the main oscillator 11 of the protecting system 1 is coupled to the junction point 29 of the control detecting system through a driver 30 and an amplifier 31 so that energy of the frequency radiated by the antenna 10 is also radiated by the control antenna 21 and is received by the first signal detector 22. Accordingly, if a "protected" article carrying a resonant circuit were placed adjacent to the antenna 21, pulses would be produced at the output of the pulse detector 25 which would operate the relay 27 and prevent operation of the alarm 20. However, the antenna 21 is not located at an egress facility or checkpoint but is located in a position with respect to the antenna 10 such that the field thereof is either unaffected by the movement of a resonant circuit past the antenna 10, or is affected to a substantially smaller degree than the field of the antenna 10. In the latter event, the control detecting system is adjusted in a well-known manner so that the change in the field of the antenna 21 must exceed a predetermined level before the relay 27 will operate. Preferably, the antenna 21 is located in a position such that within the normal area of movement of the "protected" article the field of antenna 21 will be substantially unaffected by a resonant circuit carried by a "protected" article.

From the foregoing, it will be apparent that when a "protected" article is moved past the antenna 10, the relay 27 will not be energized by such movement, thereby maintaining the continuity of the relay and alarm circuits 19 and 20, and the alarm 20 will be operated. However, the antenna 21 is located in a position with respect to the antenna 10 so that it will be affected by spurious radiation from nearby electrical equipment in essentially the same manner as antenna 10. Similarly, the control detecting system 2 is connected to the same power supply lines as the protecting system 1 so that it will be affected by spurious radiation on the power supply lines or by line voltage changes in the same manner as the protecting apparatus 1. Accordingly, whenever one of these events occurs which will affect the protecting system 1 so as to produce a pulse or pulses at the output of the pulse detector 18, a similar pulse or pulses will be produced by the output of the pulse detector 25, causing operation of the relay 27, opening the contacts 28 and thereby preventing operation of the alarm 20. Accordingly, false alarms will be reduced.

The system described in FIG. 1 will also provide substantial reduction of false operation of the alarm even if the interconnection between the main oscillator 11 and the junction 29 is omitted. Thus, by omitting the driver 30 and the amplifier 31 and their interconnections with the junction point 29 and the main oscillator 11, the control detecting system 2 and the antenna 21 will not be energized by the main oscillator 11 and therefore, the energization of the control detecting system will not be identical to that of the protecting apparatus 1. Accordingly, the control detecting apparatus may not be affected by spurious radiation etc. in exactly the same manner as the protecting apparatus 1 is affected thereby so that under some conditions the alarm 20 may be operated because of the failure of the control detecting apparatus 2 to produce an inhibiting pulse. However, in many cases, the spurious radiation, line voltage changes, etc. which will produce an alarm operating pulse in the protecting apparatus 1 will also produce an inhibiting pulse in the control protecting apparatus 2 even in the absence of the interconnection between the main oscillator 11 and the junction point 29.

FIG. 2 represents the relevant portions of a pair of protecting systems 3 and 4 including antennas 10 positioned at a pair of checkpoints or egress facilities located closely enough normally to cause operation in many instances, of the alarms of both protecting systems whenever a "protected" article is moved past or through the field of either of the antennas. For example, in some cases there are two exit doors located within a few feet of each other and each door is protected by a separate protecting system with the intent of indicating which doorway the protected article is being moved through. However, it has been found that in some instances the movement of a "protected" article through one of the doorways which will cause the operation of the alarm corresponding to such doorway will also cause operation of the alarm corresponding to the other doorway. This, of course, makes the indication of the doorway through which the "protected" article is being removed ambiguous and makes it difficult to ascertain the "protected" article being removed without authorization.

In FIG. 2 each of the protecting systems 3 and 4 comprises an amplifier 18a a pulse detector 18, a relay 19 and an alarm 20. Protecting systems 3 is connected to an antenna 10 located at a first doorway or checkpoint and protecting system 4 is connected to an antenna 10 located adjacent a nearby doorway or checkpoint. Normally it is desired that when a protected article is moved past the first doorway, the alarm 20 of the protecting system 3 will be activated and that when a protected article is moved past the other doorway, the alarm 20 of the protecting system 4 will be activated. However, it has been found that under some conditions the alarms 20 of both protecting systems 3 and 4 will be activated when a protected article is moved through either the first doorway or the other doorway. An examination of the output of the pulse detectors 18 shows that unless the alarms 20 are activated by the presence of a protected article almost midway between the two doorways, the pulse output of one pulse detector 18 will differ from the pulse output of the other detector pulse detector 18. For example, if the protected article is moved through the doorway protected by the system 3, the pulse at the output of the detector 18 forming a part thereof, will have an amplitude and shape different from the amplitude and shape of the pulse at the output of the pulse detector 18 forming part of the system 4 and vice versa. Thus, the pulses can be differentiated on the basis of their amplitude, rise time and wave shape. In the embodiment shown in FIG. 2 one or more of the differences is utilized to cause operation of only the alarm 20 associated with the system 3 or 4 having its antenna 10 closest to the protected article being removed without authorization.

Accordingly, in each of the systems 3 and 4, conventional gate circuits 32 and 33 controlled by conventional pulse discriminators 34 and 35 are interposed between the pulse detectors 18 and the relays 19. The outputs of the two pulse detectors 18 are also fed to both of the pulse discriminators 34 and 35 so that, for example, when the output of the pulse detector 18 of the system 3 has a predetermined relationship with the pulse output of the pulse detector 18 in the other system 4, the gate 32 will permit the relay 19 associated therewith to operate and hence, will cause operation of the alarm 20 of system 3. Similarly, when the output pulse of the pulse detector 18 of the system 4 has a predetermined relationship with the pulse output of the detector 18 of the other system 3, the gate 33 will permit operation of the relay 19 associated therewith and hence, operation of the alarm 20 in the system 4.

The gates 32 and 33 and the pulse discriminators 34 and 35 may take several conventional forms depending upon the type of operation it is desired to obtain. In the preferred embodiment, the gates 32 and 33 are normally closed and require pulses of a predetermined amplitude and polarity from the pulse discriminators 34 and 35 to cause opening of the gates 32 and 33 and activation of the associated alarms. For example, let it be assumed that positive going pulses must be applied to the gates 32 and 33 to cause opening thereof and hence, transmission of pulses from the pulse detectors 18 to the relays 19 and activation of the alarms 20. Let it also be assumed that the pulse discriminators 34 and 35 are conventional pulse differential amplifiers which are connected to the pulse detectors 18 so that when the pulse output of the pulse detector 18 of the system 3 is greater than the pulse output of the pulse detector 18 of the system 4, the pulse discriminator 34 will provide a positive going pulse sufficient to open the gate 32. Under the assumed conditions, this represents movement of a protected article nearer to the antenna 10 connected to the system 3 and therefore, the alarm 20 of such system will be activated.

However, when the pulse outputs of the two pulse detectors 18 are equal or the pulse output of the pulse detector 18 of the system 4 is greater in amplitude than the pulse output of the pulse detector 18 of the system 3, the pulse discriminator 34 will either produce no output pulse or will produce a negative going pulse. Such conditions represent either movement of a protected article substantially midway between the two doorways protected by the two systems or movement of a protected article nearer to the antenna 10 of the system 4. In such a situation, the gate 32 will not be opened, and the alarm 20 will not be operated.

Assuming that gate 33 and the pulse discriminator 35 are similar to and operate in the same way as gate 32 and pulse discriminator 34, it will be seen that when the amplitude of the pulse at the output of the pulse detector 18 of the system 4 is greater than the output pulse of the pulse detector 18 of the system 3, the alarm 20 of the system 4 will be operated indicating that the protected article is nearer to the antenna 10 of the system 4. Similarly, if the pulse outputs of the two detectors 18 are equal in amplitude or if the output pulse of the pulse detector 18 in the system 3 is greater than the output pulse of the detector 18 of the system 4, the alarm 20 of the system 4 will not be operated.

Therefore, in the embodiment shown in FIG. 2, only the alarm 20 of the protecting systems 3 and 4 which has its antenna 10 nearest the protected article being moved will be activated. Although, neither of the alarms 20 will be operated if the protected article is maintained substantially midway between the two doorways such conditions will not persist if a person tries to move the article through one or the other of the doorways. Also, the area midway between the doorways can be obstructed so at to prevent the presence of a protected article substantially midway between the two doorways.

It will also be apparent from the foregoing description of the embodiment in FIG. 2 that when both detecting systems are substantially similarly affected such as by spurious radiation from nearby electrical equipment, line voltage fluctuations etc., the gates 32 and 33 will not be open thereby reducing false operation of the alarms 20.

In the event that it is desired merely to prevent simultaneous operation of both alarms 20 when a protected article is moved past one of the doorways, the gates 32 and 33 may be conventional gates circuits which are continuously open and which are turned off only when a pulse of the proper polarity and amplitude is applied thereto by one of the pulse discriminators 34 and 35. In this situation the pulse discriminator 34 would turn off the gate 32 only when the output pulse of the detector 18 of the system 4 has an amplitude greater than the output pulse of the pulse detector 18 of the system 3. Similarly, the pulse discriminator 35 would turn off the gate 33 when the output pulse of the pulse detector 18 in the system 3 is greater in amplitude than the output pulse of the pulse detector 18 of the system 4. In this way, the alarm 20 of the system 3 would not be activated when the protected article is nearer the antenna 10 associated with the system 4 and the alarm 20 of the generating and detecting system 4 would not be activated when the protected article is nearer the antenna 10 associated with the system 3. However, if the output pulses of the detection 18 are substantially equal, such as is the case when the protected article is substantially midway between the doorways or when the systems are affected by spurious radiation, both alarms 20 will be activated.

Although the embodiment of FIG. 2 has been described on the basis that the pulse discriminators 34 and 35 distinguish between the pulse outputs on the basis of amplitude, it will be apparent to those skilled in the art that the pulse discriminators may differentiate between the pulses and activate or de-activate the gates 32 and 33 on the basis of other characteristics of the pulses, such as, rise time or wave shape.

FIG. 3 illustrates a further embodiment of the invention which may also be used to minimize false alarms and at the same time prevent ambiguity in alarm activation. The embodiment illustrated in FIG. 3 is in effect a combination of a pair of the systems shown in FIG. 1 with the control detecting system associated with one of the protecting systems cross-connected with the protecting system associated with an adjacent separate doorway or checkpoint.

In FIG. 3, a pair of protecting antennas 10 are located at closely spaced doorways or checkpoints and are in positions such that the fields of both are affected sufficiently by the passage of a protected article past either checkpoint, or through either doorway, to cause operation of the associated alarms 20. Thus, one of the antennas 10 is connected to the generating and detecting system 36 similar to the generating and detecting apparatus of system 1 shown in FIG. 1, and the output thereof controls an alarm 20 through a pair of contacts 37 which may be open by a relay 38. When the protected article is moved past the antenna 10 the system 36 operates the alarm 20 provided that contacts 37 are closed.

Similarly, the other antenna 10 is connected to a generating and detecting system 39 similar to the generating and detecting apparatus of system 1 shown in FIG. 1 and energizes the alarm 20 through the contacts 40 when the protected article is within the field of the antenna 10 associated therewith. When relay 41 is energized, contacts 40 open preventing operation of the alarm 20. In the absence of energization of the relays 38 and 41 the alarms 20 of both protecting systems would be operated whenever a protected article moves past either of the antennas 10.

The relay 41 is energized by the output of a control detecting system 42 similar to the controlled detecting system 2 shown in FIG. 1, and such system 42 is connected to a control antenna 21 which is located in a position such that the relay 41 is energized by the system 42 whenever a protected article is moved past the antenna 10 connected to the system 36, but is not energized when a protected article is moved past the antenna 10 connected to the detecting system 39. Similarly, the relay 38 is energized by the control detecting system 43, which is similar to the control detecting system 2 shown in FIG. 1 and to the control detecting system 42. Whenever a protected article is moved past the antenna 10 connected to the detecting system 39, the relay 38 is energized but it is not energized when a protected article is moved past the antenna 10 connected to the detecting system 36. However, all the antennas 10 and 21 may be located relatively close to each other so that they are substantially equally affected by spurious radiation from nearby electrical equipment, and all the detecting systems may be connected to the same supply lines so that they will be substantially equally affected by spurious radiation, line voltage changes, etc.

Accordingly, when a protected article is moved past the antenna 10 connected to the detecting system 36 the relay 38 will not be energized and the alarm 20 associated therewith will be sounded. At the same time, the antenna 21 connected to the system 42 will cause energization of the relay 41 preventing activation of an alarm 20 connected to the detecting system 39 even though pulses are produced therein by virtue of the movement of such protected article. Similarly, when a protected article is moved past the antenna 10 connected to the detecting system 39 the alarm 20 associated therewith will be operated and the control detecting system 43 will energize the relay 38 preventing operation of the alarm 20 associated with the detecting system 36 even though there is a pulse output from such system. In this way, only the alarm 20 of the detecting system past which the protected article is moved and nearest thereto will be activated.

Also, since all antennas are substantially similarly affected by spurious radiation, line voltage changes, etc. the relays 38 and 41 will both be energized at the time such radiation or changes occur so that neither alarm 20 will be activated. In this way, false alarms are reduced.

Although each of the embodiments hereinbefore described have been described as activating and alarm 20, which may be an audible or visible alarms, it will be apparent to those skilled in the art that instead of such alarms, or in addition thereto, the detecting systems may be used to activate automatic door closing mechanisms which will close and lock the exit door associated with the protecting system as a protected article is moved toward such door, such closing and locking of the doors indicating the attempted removal of a protected article and therefore, being indicating means. Also, although in each of the embodiments the indicating means is disabled subsequent to the pulse detector, i.e. either before or after the relay which normally operates the alarm, it will be apparent to those skilled in the art that the indicating means may be disabled at an earlier or later point in the protecting system.

Claims

What is claimed is:

1. In a system for detecting the unauthorized removal of articles from an enclosure comprising means for producing electromagnetic waves in the vicinity of a limited egress facility from said enclosure including first antenna means at said facility, signal generating means connected to said first antenna means, and means coupled to said generating means for indicating the passage of a resonant circuit tuned to the frequency of said signal past said facility, second antenna means located adjacent said facility, disabling means having an input connected to said second antenna means, said disabling means being responsive to variation of the electromagnetic energy within the field of said second antenna means to produce a disabling output and means applying the disabling output from said disabling means to said indicating means for disabling said indicating means when said energy varies.

2. A system as set forth in claim 1 wherein said second antenna means is coupled to signal generating means.

3. A system as set forth in claim 1 wherein said second antenna means is coupled to said signal generating means and is so located with respect to said facility that the field thereof is affected by the passage of said resonant circuit past said facility to an extent less than that required to operate said disabling means.

4. A system as set forth in claim 1 wherein said second antenna means is coupled to second signal generating means and is so located with respect to said facility that the field thereof is affected by the passage of said resonant circuit past said first-mentioned antenna means sufficiently to cause operation of said disabling means.

5. In a system for detecting the unauthorized removal of articles from an enclosure having first and second closely adjacent egress facilities comprising first and second protecting systems, one for each facility, each protective system comprising means for producing electromagnetic waves in the vicinity of its associated facility including antenna means and signal generating means and means including pulse generating means coupled to said generating means for indicating the passage of a resonant circuit tuned to the frequency of said signal past either of said facilities, the pulse generated by the pulse generating means of one said system being different from the pulse generated by the pulse generating means of the other system when said resonant circuit is nearer to the egress facility associated with said pulse generating means of said one system than to the egress facility associated with said pulse generating means of said other system and vice versa, the combination therewith of first gate means having two states, one of which permits and the other of which prevents operation of the indicating means of said first system, second similar gate means for permitting and preventing operation of the indicating means of said second system, first pulse discriminating means coupled to the pulse generating means of both said systems and to said first gate means for controlling the state of the latter and permitting operation of said indicating means of said first system when the pulses generated by said pulse generating means have a first predetermined difference but preventing operation thereof when said pulses have a second predetermined difference, second pulse discriminating means coupled to the pulse generating means of both said systems and to said second gate means for controlling the state of the latter and permitting operation of said indicating means of said second system when the pulses generated by said pulse generating means have said second predetermined difference but preventing operation thereof when said pulses have said first predetermined difference.

6. A system as set forth in claim 5 wherein each of said gate means is normally in a state which prevents operation of the indicating means controlled thereby and each of said pulse discriminating means comprises means for comparing the amplitudes of the pulses and changing the state of the associated gate, said first discriminator changing the state of said first gate when the amplitude of the pulse generated by the pulse generating means of said first system is greater than the pulse amplitude generated by the pulse generating means of said second system and said second discriminating means changing the state of said second gate when the pulse amplitude relationship is the opposite.

7. A system as set forth in claim 5 wherein each of said gate means is normally in a state which permits operation of the indicating means controlled thereby and each of said pulse discriminating means comprises means for comparing the amplitudes of the pulses and changing the state of the associated gate, said first discriminator changing the state of said first gate when the amplitude of the pulse generated by the pulse generating means of said first system is greater than the pulse amplitude generated by the pulse generating means of said second system and said second discriminating means changing the state of said second gate when the pulse amplitude relationship is the opposite.

8. In a system for detecting the unauthorized removal of articles from an enclosure having first and second closely adjacent egress facilities comprising first and second protecting systems, one for each facility, each protective system comprising means for producing electromagnetic waves in the vicinity of its associated facility including antenna means and signal generating means and means including pulse generating means coupled to said generating means for indicating the passage of a resonant circuit tuned to the frequency of said signal past either of said facilities, the combination therewith of a first and second control antenna means located respectively adjacent said first and second egress facilities, means coupling said first control antenna means to said signal generating means of said first system, means connecting said second control antenna means to said signal generating means of said second system, first means responsive to variation of the electromagnetic energy within the field of said first control antenna means, first disabling means connected to said first responsive means and to the indicating means of said second system for disabling the latter when said energy varies, second means responsive to variation of the electromagnetic energy within the field of said second control antenna means and second disabling means connected to said second responsive means and to the indicating means of said first system for disabling the latter when the energy within the field of said second control antenna means varies, said first control antenna also being so located with respect to said first and second egress facilities that the field thereof is affected by the passage of said resonant circuit past said first facility to an extent sufficient to cause operation of said first disabling means but is affected by the passage of said resonant circuit past said second facility to an extent less than that required to operate said first disabling means and said second control antenna means being so located with respect to said first and second egress facilities that the field thereof is affected by the passage of said resonant circuit past said second facility to an extent sufficient to cause operation of said second disabling means but is affected by the passage of said second resonant circuit past said first facility to an extent less than that required to operate said second disabling means.