GB2504523A

GB2504523A - Intruder alarm system incorporating at least one passive infrared sensor and at least one microwave sensor

Info

Publication number: GB2504523A
Application number: GB201213708A
Authority: GB
Inventors: Martin James Haines Fenner
Original assignee: Individual
Current assignee: Individual
Priority date: 2012-08-01
Filing date: 2012-08-01
Publication date: 2014-02-05
Also published as: GB201213708D0

Abstract

An intruder alarm system comprises a first housing 2a containing a first sensor 4a, and a second housing 2b containing a second sensor 4b. A control circuit 10 receives signals from the first and second sensors 4a,4b and determines an alarm condition only when both sensors detect an intrusion within a predetermined time interval. The respective sensors 4a,4b can be installed in locations that are spaced apart, while monitoring a single zone 6. Thereby both the sensors 4a,4b will respond to an intruder entering the zone but they will respond differently to environmental factors such as flying birds that are local to just one of the sensors so that false alarm conditions can be distinguished and ignored. The sensors may comprise at least one passive infra-red sensor and at least one Doppler microwave sensor.

Description

TITLE

Intruder alarm systems

DESCRIPTION

s Technical field

The invention relates to intruder alarm systems, which trigger an alarm condition when an unidentified object, which may be a person, intrudes into a zone that is monitored by the alarm system. The invention has particular application to alarm systems that monitor zones in outdoor locations, where there is a heightened risk of false alarms caused by environmental factors.

Background of the invention

Intruder alarm systems employ one or more sensors to monitor a zone of interest within the field of view of the sensor and to generate a detection signal when the sensor detects a significant change in the field of view. Passive infra-red (PIR) detectors respond to a change in the pattern of infra-red radiation received by the sensor, which may be caused by the body heat of a person entering the zone or, conversely, by a person or other object in the zone obscuring the background pattern of infra-red emissions. Doppler microwave sensors direct a beam of microwave radiation at a known frequency into the zone of interest and monitor the reflected microwave radiation. Differences between the frequencies of the emitted and reflected microwaves indicate a person or object moving within the field of view.

It is known to provide muhiple sensors within a single housing to monitor the same zone. Multiple sensors of the same type (e.g. two PIR detectors) may provide redundancy in case one detector fails, or the system may be arranged to trigger an alarm condition only when both sensors simultaneously generate detection signals in order to cut down false alarms. Multiple sensors of different types (e.g. one PIR detector and one Doppler microwave detector) may complement each other because they are sensitive to different kinds of intrusion. Again, the system may be arranged to trigger an alarm condition when either sensors generates a detection signal in order to increase the sensitivity of the system, or only when both sensors simultaneously generate detection signals in order to cut down false alarms.

False alarms are a particular problem when alarm systems are used to monitor zones s in outdoor locations because the environment surrounding outdoor zones is usually less predictable than indoor zones. For example, tree branches or other foliage adjacent to the detector may be blown by the wind to create movement that is wrongly sensed as an intrusion. In another example, wildlife may move through the zone and trigger an alarm, even though it poses no threat to security. Both indoor and outdoor detectors can be ananged so that they are not triggered by pets and other small animals moving at ground level but outdoors there always remains a risk that birds may fly in front of the sensor and trigger a false alarm. The control circuitry of alarms intended for outdoor use is programmed to try to identify such events and avoid false alarms but it can be difficult, for example, to distinguish a bird flying is close to the sensor from a larger person moving past it at a greater distance.

As a result, outdoor installations of intruder alarm systems arc notorious for generating false alarms, which pose a nuisance to property owners and to neighbours.

They can also be expensive if the alarms trigger automatic calls to security services, who will often make a charge for attending in thc case of a false alarm. The unreliability of outdoor alarm systems is a particular problem because property in outdoor locations is often vulnerable to theft and vandalism. It also prevents a quality standard being agreed for outdoor alarm systems, without which purchasers have no independently assessed basis for evaluating and choosing between rival systems.

There is thus a great demand for an intruder alarm system that operates more reliably than the known systems when installed in an outdoor location.

Summary of the invention

The invention provides an intruder alarm system as defined in claim 1.

The invention provides a method of operating an intruder alarm system as defined in claim 10.

Features of the invention that are preferred but not essential are defined in the s dependent claims.

In accordance with the invention, the first and second sensors are provided in different housings. This allows the respective sensors to be installed in locations that are spaced apart, while monitoring the same zone. Accordingly, both sensors will respond to an intruder entering the zone but most environmental factors will be local to only one of the sensors and will not cause a detection in the other sensor. This enables false &arm conditions to be distinguished and ignored.

The drawings is Figure 1 shows an intruder alarm system according to the invention in a side-by-side installation.

Figure 2 shows an intruder alarm system according to the invention in a face-to-face installation.

Figure 3 is a schematic diagram of a first configuration of a control circuit for an intruder alarm system according to the invention.

Figure 4 is a schematic diagram of a second configuration of a control circuit for an intruder alarm system according to the invention.

The intruder alarm system comprises a first housing 2a that contains a first set of sensors 4a and a second housing 2b that contains a second set of sensors 4b. The sensors 4a,4b may be passive infra-red (PIR) detectors, Doppler microwave detectors or any other suitable type of sensor that is capable of detecting the presence of an intruder within a field of view 8a,Sb of the sensor. In a preferred embodiment of the invention, each housing 2a,2b contains a pair of PIR detectors and a pair of microwave detectors. Because the Doppler microwave detectors emit a beam and respond to its reflections, there is a risk that the beam emitted from one housing could fall on the detector of the other housing and confuse its measurements. It can therefore be arranged that the two beams (or more) differ in a way that can be distinguished by the respective sensors, for example by being tuned to different frequencies.

s The first and second sensor housings 2a,2b and a control circuit 10 (not shown in Figures 1 and 2) are connected together by cables 12. (Wireless communication could be used instead.) The control circuit 10 receives the signals output by all the sensors 4a,4b in the system and processes them to determine whether an alarm condition exists. If so, then the circuit 10 may cause an audible or visible alarm to be emitted; it may send a signal to a remote location, for example to alert the property owner or a security firm to the presence of an intruder; it may also switch on lighting or recording equipment; or it may trigger any other action that is thought desirable in the event of an intrusion.

is Figures 1 and 2 show two possible installations of the intruder alarm system. In each case, the first and second housings 2a,2b are mounted so as to be spaced apart from one another but with the zone of interest 6 falling within the fields of view 8a,8b of both sets of sensors 4a,4b. In the Figures, dot-dash lines mark the boundaries of the fields of view 8a,8b. It will be understood each field of view extends over a vertical range as well as over the horizontal range shown. Each sensor or each type of sensor may have a slightly different field of view provided that they all cover the whole zone of interest 6.

In Figure 1, the two sensor housings 2a,2b are mounted on the same side of the zone of interest 6 in a side-by-side arrangement. Thus they may both be mounted on a wall overlooking an adjacent yard, for example.

In Figure 2, the two sensor housings 2a,2b are mounted on opposite sides of the zone of interest 6 in a face-to-face arrangement. Thus they may monitor persons or vehicles passing along a road 13 that runs between them, for example.

It should be noted that the zone of interest 6 faIls within the respective fields of view 8a,8b of the sensors 4a,4b in both housings 2a,2b. This allows both sets of sensors 4a,4b to assess independently from their different locations whether there has been an intrusion into the zone of interest 6. The system can thereby correct for errors s that are specific to one set of sensors, for example because of environmental causes that are local to one of the housings 2a,2b or because the particular viewing angle of one set of sensors 4a,4b causes the respective detection algorithm to misinterpret the signals that it receives.

For example, Figure 2 illustrates a bird 15 flying close to the second housing 2b. As shown by the dashed lines 14b that form a wide angle at the second sensor 4b, because the bird is close to the sensor 4b it occupies a large sector of the field of view 8b and can be mistaken for a larger object at a greater distance, within the zone 6. The sensor 4b may thus wrongly generate an intruder detection signal.

is However, as shown by the dashed lines 14a that form a narrow angle at the first sensor 4a, the bird 15 occupies only a small sector of the field of view 8a of that sensor. Thus the first sensor will not generate an intruder detection signal.

It is sufficient for the correction of errors that the two sets of sensors 4a,4b should independently monitor the whole zone of interest 6 from different locations.

However, it is preferred that the respective housings 2a,2b should be installed such that, as illustrated in each example, the first housing 2a falls within the field of view 8b of the second set of sensors 4b and that the second housing 2b falls within the field of view 8a of the first set of sensors 4a. This allows each set of sensors 4a,4b effectively to monitor the other set and thus prevent an intruder approaching one housing 2a,2b while remaining out of view of the sensors in the other housing 2b,2a.

As shown in Figure 3, both sets of sensors 4a,4b are connected to a control circuit 10, which receives the signals output by all the sensors 4a,4b and processes them to determine whether an alarm condition exists. The control circuit 10 is arranged so that it determines an alarm condition to exist based on the received signals only when it receives intruder detection signals from all of the sensors simultaneously. Thus, in the case of an intruder entering the zone 6, all the sensors 4a,4b in both housings 2a,2b detect the intrusion and the control circuit 10 correctly determines that an alarm condition is present. However, in the ease of a bird 15 flying close to one housing 2b, as shown in Figure 2, the control circuit 10 receives detection signals only from the s sensors 4b in the second housing 2b and it correctly determines that no alarm condition is present.

A similar result occurs for other environmental factors that are local to one housing 2a,2b, such as the swaying of a tree branch in strong wind. Although, even in the prior art, each sensor or its control circuit may be programmed with algorithms to recognize inputs that are typical of the movement of birds, insects or branches, such algorithms are sometimes defeated by unusual patterns of movement. However, it is very unlikely that the algorithms of two sets of widely spaced sensors 4a,4b viewing from different angles in accordance with the invention will both be defeated at the same time.

In order to allow for the different response times of the different types of sensor 4a,4b, and for the different positions of the sensors relative to the zone of interest 6, the control circuit 10 need not receive all of the signals strictly simultaneously in order to trigger an alarm. It is sufficient that they all be received within a predetermined time interval of one another, for example 2 seconds. Depending on the situation where the system is installed, flirther conditions may be programmed into the control circuit in order to reduce false alarms, for example it may be decided that action should be taken only if an alarm condition is determined twice within a period of 30 seconds.

It is known to provide intruder sensors with additional functions such as the detection of tampering and with anti-masking capabilities. Such functions are not affected by the present invention: they may be carried out within each housing 2a,2b or they may be carried out by the control circuit 10. However, it is clearly necessary that an alarm should be triggered if tampering is detected at either housing and that this does not require simultaneous detection at both housings.

In Figure 3, the control circuit 10 is shown as a unit separate from the first and second sensor housings 2a,2b. However, as shown in Figure 4, the control circuit may alternatively be contained within the first housing 2a. The first housing 2a thus s becomes a "master" unit and the second housing 2b becomes a "slave", although in most cases the signals received from the respective sensors 4a,4b will continue to carry equal weight in the determination of an alarm condition.

As described above, the signal output by each individual sensor is transmitted to the control circuit 10, which combines all of the signals (i.e. eight signals in total if each housing 2a,2b contains a set of four sensors) and determines an alarm condition only if all of the signals are received within a predetermined time interval of each other. Tn an alternative embodiment of the invention, some of the processing could be devolved to the individual housings 2a,2b so that each housing transmits a single signal to the is control circuit 10 only if all the sensors contained in that housing agree that there is an intrusion event. Thus the control circuit has to combine only two signals rather than eight and does not need to handle so many channels of information. This may make the system easier to scale up and it allows each housing to behave more like a conventional intruder detection unit.

It will be understood that in particularly sensitive applications the system could employ more than two sets of sensors 4a,4b monitoring a single zone 6. An alarm condition might then be determined only in the event of agreement by the sensors from all the housings or instead by a majority verdict. The described system can also be integrated into a larger security system that may contain also sensor housings that operate independently as well as other types of detection and monitoring devices. In that case some of the functions of the control circuit 10 described herein may be carried out by a central controller of the larger system.

Claims

CLAIMS1. An intruder alarm system comprising: a first housing containing a first sensor that generates a first detection signal when it detects an intrusion condition; a second housing containing a second sensor that generates a second detection signal when it detects an intrusion condition; and a control circuit arranged to receive the detection signals from the first and second sensors, thc control circuit dctcrmining an alarm condition based on the received signals only when it receives both the first detection signal and the second detection signal separated by no more than a predetermined interval of time.
2. An intruder alarm system according to daim 1, wherein the control circuit is contained within the first housing.
3. An intruder alarm system according to claim 1 or claim 2, wherein: the first sensor is one of a first set of sensors contained in the first housing, each of the first set of sensors generating a respective detection signal when it detects an intrusion condition; the second sensor is one of a second set of sensors contained in the second housing, each of the second set of sensors gcnerating a respective detection signal when it detects an intrusion condition; and the control circuit determining the alarm condition based on the received signals only when it receives detection signals from all of the sensors in the first set and the second set spread over no more than the predetermined interval of time.
4. An intruder alarm system according to claim 3, wherein each set of sensors comprises at least one passive infra-red sensor and at least one Doppler microwave sensor.
5. An intruder alarm system according to claim 4, wherein the Doppler microwave sensor in the first set and the Doppler microwave sensor in the second set respectively employ different frequencies of microwaves.
6. An intruder alarm system according to claim 4 or claim 5, wherein each set of sensors comprises two passive infra-red sensors and two Doppler microwave sensors.
7. An intruder alarm system according any preceding claim, which is installed such that the first housing and the second housing are mutually spaced apart but the first and second sensors have overlapping fields of view.
8. An intrudcr alarm systcm according to claim 7, wherein: the second housing is within the field of view of the first sensor; and the first housing is within the field of view of the second sensor.
9. An intruder alarm system according to claim 7 or claim 8 which is installed outdoors.
10. A method of operating an intruder alarm system, the method comprising: generating a first dctcction signal when a first scnsor contained in a first housing detects an intrusion condition; generating a second detection signal when a second sensor contained in a second housing dctccts an intrusion condition; rccciving thc dctcction signals from thc first and sccond scnsors; and determining an alarm condition based on the received signals only upon receiving both the first detection signal and the second detection signal separated by no morc than aprcdctcrmincd interval of timc.
11. A method according to claim 10, wherein: thc first scnsor is onc of a first sct of scnsors containcd in thc first housing; and thc sccond scnsor is onc of a sccond sct of scnsors containcd in thc sccond housing; the method further comprising: generating a respective detection signal when each of the first set of sensors detects an intrusion condition generating a respective detection signal when each of the second set of sensors detects an intrusion condition; and determining the alarm condition based on the received signals only upon receiving detection signals from all of the sensors in the first set and the second set spread over no more than the predetermined interval of time.
12. An intruder alarm system substantially as described herein with reference to the drawings.
13. A method of operating an intruder alarm system substantially as described herein with reference to the drawings.