The present invention relates to field of intrusion detection and, more particularly, it relates to improvements in multizone intrusion detection systems of the type which include a supervisory circuit for detecting the operability of the various intrusion-detecting sensors which define the different zones of protection.
An intrusion detection system in which the various intrusion-sensing elements are non-functioning is, of course, of psychological value only. Obviously, in a multizone system in which each zone of protection is defined by the field of view or detection range of each of a plurality of sensors (e.g., microwave, passive-infrared, photoelectric, ultrasonic, passive-acoustic, etc.), the level of security depends on the percentage of sensors which are functioning at any given time. Since a non-functioning sensor is not easy to detect without actually "walk-testing" the sensor to determine whether it produces an alarm output, it is becoming increasingly common to incorporate a so-called "supervisory" circuit in such systems to monitor the operating status of each sensor (or at least those which are particularly prone to fail). Such circuit operates to activate a "supervisory" alarm (e.g., a light-emitting diode) to alert the user of any sensor failure. Detection systems incorporating such supervisory circuits are disclosed, for example, in the commonly assigned U.S. Pat. No. 4,660,024 to R. L. McMaster.
In the commonly assigned U.S. application Ser. No. 492,482, filed on Mar. 12, 1990 in the name of W. S. Dipoala, there is disclosed a dual-technology (passive-infrared/microwave) intruder detection system in which both sensors are "actively" supervised by periodically simulating, within the system, a target of interest. In the event of either sensor failure, a supervisory alarm is given. While such "active" supervision provides optimal protection against sensor failure, it does so at the expense of requiring target-simulation apparatus within each sensor device.
Recently, it has become known to "passively" supervise the various sensors of a multizone system by monitoring the pedestrian-produced activity of the sensors during the period that the system is disarmed, e.g., during the daylight hours in which the protected premises are being used by the owner of the system. The supervisory apparatus includes a display which indicates which of the several sensors have been activated during the disarm period and, hence, are functional; it also, of course, indicates those which have not been activated. To prevent the system from being re-armed without having the operability of those non-activated sensors verified (e.g., by walk-testing), such control device can be programmed to inhibit re-arming until it detects that all sensors have been activated. While this arrangement provides a high degree of security, it can be a nuisance to a user who, for example, arms the system after verifying that all sensors are functional and then realizes that he forgot something inside the protected premises. To re-enter such premises, even for a moment, means that he must walk-test all sensors, since there is no intervening traffic to do this job for him. Because of this inconvenience, there may be some reluctance on the part of the security customer to opt for this very effective passive supervisory feature.
In the commonly assigned U.S. application Ser. No. 07/576,055, filed concurrently herewith in the name of James E. Berube and entitled "Intruder Detection System With Passive Self-Supervision", there is disclosed a multizone intruder detection system which includes a timing circuit for establishing a time interval during which the system user can re-arm a disarmed system without paying the afore-mentioned penalty of having to walk-test all sensors. So long as the user re-arms the system in, say, one hour after disarming, he need not cause an arm-enable signal to be generated from the passive supervisory circuit. While this apparatus renders the system far more "user-friendly" there is some risk that one or more of the sensors may be sabotaged, or inadvertantly masked, during the disarm period, after the time it has produced a sensor alarm for the supervisory circuit. For example, after a passive-infrared sensor has been activated, someone may set a box or the like directly in front of the sensor. While the supervisory circuit is satisfied that the sensor operates, the sensor will be totally ineffective in sensing intrusion during the succeeding arm period. Thus, it will be appreciated that the afore-described passive supervision technique may be convenient, but not provide optimal protection. To assure maximum security, each sensor must be walk-tested as close to the time of arming as possible.