Communications: electrical – Condition responsive indicating system – With particular coupling link
Reexamination Certificate
1998-06-15
2001-09-11
Pope, Daryl (Department: 2736)
Communications: electrical
Condition responsive indicating system
With particular coupling link
C340S506000, C340S511000, C340S526000, C340S541000
Reexamination Certificate
active
06288640
ABSTRACT:
DESCRIPTION
1. Technical Field
The invention relates to intrusion detection systems and is especially applicable to systems which comprise an “open” transmission line, for example a so-called “leaky” or “ported” cable, for receiving a radio frequency signal and a receiver attached to the open transmission line for processing the received radio frequency signal to detect perturbations caused by an intruder in proximity to the open transmission line.
2. Background Art
Examples of such intrusion detection systems are disclosed in U.S. Pat. No. 3,163,861 (Suter) issued Dec. 29, 1964, U.S. Pat. No. 3,794,992 (Gehman) issued Feb. 26, 1974, U.S. Pat. No. 4,419,659 (Harman et al) issued Dec. 6, 1983, U.S. Pat. No. 4,887,069 (Maki) issued Dec. 12, 1989 and international patent application number PCT/CA93/00366 (Harman et al) published Mar. 31, 1994.
To increase detection rates, the system disclosed by Gehman compares the signals from two adjacent cables, one via a quarter-wavelength section. Such duplication entails additional expense.
To avoid “null” problems which arise when an intruder crosses the line at a certain angular position, the system disclosed in international patent application number PCT/CA93/00366 uses two receivers, one at each end of the cable. The receivers are coupled to a reference antenna which receives a FM radio frequency signal directly from a nearby commercial radio transmitter and use synchronous detection to extract amplitude and phase modulation caused by the intruder and determine from them the presence of the intruder. At a fixed frequency, an intruder could cause a maximum amplitude modulation with minimum phase modulation or, conversely, maximum phase modulation with minimum amplitude modulation. Consequently, in order to maintain uniform detection along the line, the receivers use full vector demodulation of the in-phase (I) and quadrature (Q) components, where amplitude is {square root over ((I
2
+L +Q
2
+L ))} and phase is arc tg (Q/I).
The additional expense of such systems can be tolerated by “high end” users protecting very expensive property or high security areas such as military bases and correctional facilities. Such sites are likely to be serviced also by video surveillance systems or full time guards on site, so increased false alarm rates resulting from using sensors designed to give maximum probability of detection can be tolerated.
There is a need, however, for “low end” intrusion detection systems which are relatively inexpensive. For a particular site, system cost can be reduced by increasing the length of the open transmission line to limit the number of relatively expensive receivers and processors needed. A disadvantage of this approach, however, is that long sensor lines can increase the likelihood of undetected intrusion. Thus, attenuation along the length of the line may make it difficult to set the sensitivity so that the system will detect an intruder at the far end of the line while not being overloaded by perturbations caused by an intruder near to the receiver. Graded cables could be used to overcome this problem, but they are relatively expensive. Another disadvantage of long sensor lines concerns the need to allow legitimate access to a protected area such as a compound. When a sensor line across the entrance to a compound is switched off to allow a vehicle to enter, for example, the risk of an intruder gaining access at the same time is greater for longer sensor lines. Other problems which are exacerbated by longer sensor lines include variations in sensitivity caused by differing media along the length of the line; objects moving within the protected area; and increased range capability for any video monitors used in conjunction with the system.
DISCLOSURE OF INVENTION
The present invention seeks to eliminate, or at least mitigate, one or more of the disadvantages of known intrusion detection systems and to provide an intrusion detection system which is relatively inexpensive yet reliable.
According to the present invention, an intrusion detection system comprises a plurality of sensors coupled to a corresponding plurality of receivers, each receiver to receive a radio frequency signal from the associated sensor, the radio frequency signal having a multiplicity of transmissions at different frequencies within a predetermined frequency spectrum, the receiver being arranged to detect said transmissions and having computing means for determining, for each of said multiplicity of transmissions, corresponding signal amplitude measurements, comparing each of such signal amplitude measurements for a particular frequency with at least one preset threshold value and, if the amplitude exceeds the threshold for a predetermined time period, indicating a potential alarm condition.
The receiver may include means for scanning an FM radio spectrum and selecting a number of said transmission frequencies, and computing means for sampling the amplitude of the FM radio signal received from the associated sensor over a predetermined time interval, each sample being said signal amplitude measurement, derive statistics of a plurality of said samples over each of successive time periods, and adjust the preset threshold value periodically in dependence upon said statistics.
The computing means may also derive higher and lower variance values of the amplitudes of the plurality of samples and use such variance values to determine respective upper and lower thresholds delimiting a range of acceptable amplitude values, and generate the potential intruder alarm signal when said measurement of signal amplitude is outside the range. The computing means then updates the threshold values periodically on the basis of mean and variance values computed for a predetermined number of samples.
The intrusion detection system may further comprise a common processor for receiving station alarm signals from the plurality of receivers, comparing station alarm signals for a particular sensor and corresponding station alarm signals of at least one of its immediately neighbouring sensors, and generating a system intrusion alarm signal when the station alarm signals for the particular sensor do not occur contemporaneously with the corresponding station alarm signals for said at least one of the neighbouring sensors.
The common processor may be arranged to generate the station alarm signal only when the signal amplitude measurements for a predetermined proportion of the multiplicity of station transmissions exceed their respective threshold values in the same time interval.
Each sensor may comprise an open transmission line, the open transmission lines being concatenated by the plurality of receivers, a first of the receivers being connected to the common processor for processing signals from the different receivers, each of the receivers other than the first receiver interconnecting two of the open transmission lines, each receiver being arranged to transmit station alarm signals to the common processor by way of any intervening open transmission lines and receivers.
The common processor may supply power to the receivers by way of intervening transmission line(s) and/or receivers.
One or more of the sensors may comprise a localized antenna acting as a single point in space instead of a distributed antenna in the form of an open transmission line.
The intrusion detection system may comprise a plurality of sub-systems sharing the common processor, the sub-systems being physically separated from each other. The sub-systems and the common processor may then have respective transceivers for communicating station alarm signals and control signals between each sub-system and the common processor.
Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings of preferred embodiments of the invention, which are described by way of example only.
REFERENCES:
patent: 3163861 (1964-12-01), Suter
patent: 4562428 (1985-12-01), Harman et al.
patent
Adams Thomas
Pope Daryl
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