Fault monitoring event detection device

Details Fault monitoring event detection device Fault monitoring event detection device

1997-03-13

1998-08-18

Horabik, Michael

Communications: electrical

Continuously variable indicating

34087009, 34087016, 34082516, 340567, G08C 1916

Patent

active

057963533

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to event detection devices and more particularly to a combined technology event detection device having an improved fault monitoring system.


BACKGROUND ART

Combined technology event detection devices, including, for example, passive infrared sensors, Doppler shift microwave sensors, acoustic detectors and vibration detectors are known in the art. Typically they are used to detect unauthorised entry or intrusion into a protected space. Examples of such devices including specifically a combination of a photoelectric sensor and a microwave sensor are shown in U.S. Pat. Nos. 3,725,888 and 4,401,976, the entire disclosures of which are incorporated herein by reference.
In a typical combination, the outputs of two independent sensing means, responding to different physical stimuli, are supplied to an AND gate, and if both sensing means register an event within a specified period of time, then an alarm is triggered. In this manner the incidence of false alarms occurring when only a single sensing means is used can be greatly reduced.
A problem with combined technology event detection devices is that both sensing means need to be operating, properly in order for a true event to be detected. If one sensing means is disabled, either due to an electronic failure, sabotage, or faulty installation, and does not detect the event, then an alarm condition will not be triggered. Combined technology event detection devices thus need to be provided with a fault monitoring system. Examples of such monitoring systems are disclosed in U.S. Pat. Nos. 5,216,410 and 4,660,024, the entire disclosures of which are incorporated herein by reference.
In European Patent No. 259015, the entire disclosure of which is incorporated herein by reference, there is described an intrusion detection apparatus having dual sensing means which comprises first and second sensing means, logic means for receiving output signals from the first and second sensing means and for generating an alarm in response thereto, first and second means for storing the number of output signals received respectively from the first and second sensing means, and logic means for comparing the number of first output signals from the first storing means and the number of second output signals from the second storing means and generating an output signal indicative of fault in the apparatus, in response to said comparison.
A drawback to this prior art system is that, unless both sensing means are equally sensitive, there will inevitably be a difference between the number of output signals generated by the first and second sensing means. Since these output signals are counted, stored and compared, it follows that there will Inevitably come a time when the comparison indicates a fault condition even when no fault has occurred, whatever comparison ratio is chosen.


SUMMARY OF THE INVENTION

The invention provides an improved fault detection system for a combined technology event detection device in which a counter associated with a sending means which has become active is incremented and all other counters associated with all other sensing means are reset to a base level. A fault condition is Indicated when any one counter reaches a pre-sat threshold limit level.
Thus, according to the invention, there Is provided a combined technology event detection device which comprises a first sensing means for generating a first output signal in response to the detection of an event, a second sensing means for generating a second output signal in response to the detection of an event, logic means for receiving the first and second output signals and for generating an alarm in response thereto, and a fault monitoring system comprising:
a first means for storing the number of first output signals received from the first sensing means;
a second means for storing the number of second output signals received from the second sensing means: and
means for detecting an output signal from either the first or the second sensing means and for increme

REFERENCES:
patent: 4660024 (1987-04-01), McMaster
patent: 5216410 (1993-06-01), Pildner et al.

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