Intruder detection arrangements and methods

Abstract: An intruder detection arrangement has a passive infra-red sensor (10) for detecting the presence of an intruder in a predetermined zone of protection and an IR anti-masking detection unit (20) for detecting the presence of masking material M in view of sensor (10). A signal processing circuit (30) is responsive to outputs from the sensor (10) and the detection unit (20) producing an intruder alarm signal in response to an output from sensor (10) and an anti-masking alarm signal in response to an output from unit (20). The signal processing circuit is arranged to cancel the anti-masking alarm signal in response to an output from sensor (10) produced after the output from unit (20) that gave rise to the anti-masking alarm signal.

Description: This invention relates to intruder detection arrangements and methods. More specifically, though not exclusively, the invention relates to intruder detection arrangements and methods employing passive infra-red (IR) sensors. Intruder detection arrangements employing passive infra-red sensors have a field of view directed into one, or more, zones to be protected. Infra-red radiation emitted by an intruder entering the protected zone will be detected by the IR sensor and an intruder alarm signal is produced. However, such arrangements are rendered ineffective if an object or material opaque to infra-red radiation is accidentally, or intentionally placed in view of the sensor. Known intruder detection arrangements designed to deal with this problem employ a source of IR radiation associated with the IR sensor. This source directs radiation into the field of view with the intention that it will be reflected by any masking material present. The so-reflected radiation would then be detected either by the passive IR sensor itself (see e.g. EP-A-289 621) or a separate, dedicated sensor, giving rise to an anti-masking alarm signal. If a separate, dedicated sensor is used, the source and sensor may operate in a wavelength range different from that of the passive IR sensor. Indeed, radiation other than IR radiation (e.g. visible or UV radiation) could be used. In use, the intruder detection arrangement is connected to a control unit. However, known control units do not have a separate input for the anti-masking alarm signal. It would be possible to route the anti-masking alarm signal to the tamper input of the control unit. However, this input is usually monitored continuously and, in such circumstances, the anti-masking alarm signal could give rise to an alarm at any time of the day or night, even in response to accidental obscuration of the passive IR sensor. This arrangement therefore is unsatisfactory. Instead, it is preferable to latch the anti-masking alarm signal and connect the latched signal to the alarm input of the control unit. This arrangement has the advantage that the anti-masking alarm signal would be discovered when the system operator attempts to set the control unit in the alarm mode in order to make the unit responsive to an intruder alarm signal, and the operator would need to cancel the anti-masking alarm signal before the alarm mode could be set. The present invention provides an intruder detection arrangement enabling the system operator to cancel the anti-masking alarm signal but only if the passive IR sensor is not being obscured. According to one aspect of the invention, there is provided an intruder detection arrangement comprising first detection means for detecting the presence of an intruder in a predetermined zone of protection, second detection means for detecting obscuration of the first detection means and signal processing means responsive to outputs from the first and second detection means indicating detection of an intruder and an obscuration respectively, characterised in that the signal processing means is capable of latching an anti-masking alarm signal in response to an output from the second detection means, and in that said signal processing means cancel the latched anti-masking alarm signal provided the first detection means produces a respective output after production by the second detection means of the output giving rise to the latched anti-masking alarm signal. With this arrangement, the anti-masking alarm signal can be cancelled by the subsequent detection of an intruder in the protected zone, and the system operator can accomplish this by simply performing a walk-test in the protected zone. It will be understood, therefore, that the term "intruder" is intended to include a system operator performing such a walk-test. Cancellation of the anti-masking alarm signal would enable the system operator to set an associated control unit in the alarm mode so that the latter can respond to an intruder alarm signal produced by the signal processing means in response to an output signal from the first detection means. The first detection means may be a passive infra-red sensor, and the second detection means may comprise a source of radiation (e.g. infra-red radiation) and a sensor responsive to radiation emitted by the source. In an embodiment, the signal processing means is operable in an operator-selectable test mode and has the capability to cancel the anti-masking alarm signal provided said test mode has been selected by the user. In another embodiment, the signal processing means has said capability to cancel a said anti-masking alarm signal after a preset delay following production of a respective output signal by the second detection means. In accordance with another aspect of the invention, there is provided a method of detecting intruders comprising the steps of using a first detection means to detect for intruders, using a second detection means for detecting for obscuration of the first detection means and being characterised by using signal processing means responsive to said first and second detection means to latch an anti-masking alarm signal in response to detection of an obscuration and to cancel the latched anti-masking alarm signal provided an intruder is detected after detection of the obscuration. Intruder detection arrangements embodying the invention, and intruder detection methods according to the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a block diagram of the arrangements; and Figures 2(a) and 2(b) are flow diagrams illustrating two alternative modes of operation of the arrangements. Referring to Figure 1, the intruder detection arrangement includes a passive IR sensor 10 and an active IR anti-mask detection unit 20. The passive IR sensor 10 views a field of view 11. If an intruder moves into the field of view, heat emitted by the intruder is detected by sensor 10, this heat resulting from the inevitable difference in temperature between the intruder and the immediate surroundings. When sensor 10 detects heat due to the presence of an intruder or other unexpected or unauthorised heat source, it produces a first detection signal D1 on line 12 which is connected to a first input I 1 of a signal processing circuit 30. In response, the processing circuit 30 produces an intruder alarm signal on a first output line 13 which is connected to the alarm input of an associated control unit (not shown). Provided the control unit has been set in the alarm condition by the system operator the intruder alarm signal on line 13 will trigger an alarm relay in the unit. The active IR anti-masking detection unit 20 detects for the presence of masking material (represented at M in Figure 1) such as paper, or paint from a spray, placed in the field of view of sensor 10. To that end, the detection unit 20 has a source 21 of IR radiation and a further sensor 22 for detecting source radiation reflected at an obscuration. Sensor 22 produces a second detection signal D2 in response to such reflected radiation and this signal is routed on line 23 to a second input I 2 of the signal processing circuit 30. In response, circuit 30 produces an anti-masking alarm signal on line 13 and also on a second output line 24, although the latter output line could, optionally, be omitted. The signal processing circuit 30 is programmed to latch the anti-masking alarm signal on line 13 until the system operator causes its cancellation in the manner to be described. Until such cancellation occurs the associated control unit cannot be set in the alarm mode by the operator. In order to cancel the anti-masking alarm signal, the processing circuit 30 is suitably programmed to respond to a detection signal D1 on line 12 indicating that the sensor 10 is not being obscured. The system operator can generate this detection signal by simply performing a walk-test in the field of view of the sensor. This arrangement has the advantage that it requires the operator to check that any obscuration of the IR sensor 10, that gave rise to the anti-masking alarm signal, is no longer present. When the anti-masking alarm signal has been cancelled in this manner, the associated control unit to which the detection arrangement is connected can be set in the alarm mode. Figures 2(a) and 2(b) are flow diagrams illustrating the operational modes of two arrangements according to the invention having additional, optional security features. Referring to Figure 2(a), the signal processing circuit 30 may be programmed to operate in an operator-selectable test mode and would only have the capability to cancel the anti-masking alarm signal provided the test mode has been selected. This arrangement requires the active intervention of an authorised operator ensuring that the signal processing circuit 30 cannot be reset inadvertently or by unauthorised personnel. In one implementation of this arrangement, the signal processing circuit 30 would have a test line, shown at L in Figure 1, and the test mode would be selected by activating this line. Known intruder detection arrangements are already provided with such test lines. However, they are used for an entirely different purpose; that is, to enable an LED provided to assist in the initial placement of the sensors when the detection arrangement is being installed. Referring to Figure 2(b), the signal processing circuit 30 can be so programmed to have the capability to cancel the anti-masking alarm signal but only after a preset delay following production of the second detection signal D2. Although, the invention has been described in relation to an intruder detection arrangement having an infra-red radiation detector, more specifically a passive infra-red sensor, it will be understood that the invention is also applicable to arrangements using other forms of detector, for example, microwave detectors.

Claims: An intruder detection arrangement comprising first detection means (10) for detecting the presence of an intruder in a predetermined zone of protection, second detection means (20) for detecting obscuration of the first detection means (10) and signal processing means (30) responsive to outputs from the first and second detection means (10,20) indicating detection of an intruder and an obscuration respectively, characterised in that the signal processing means (30) is capable of latching an anti-masking alarm signal in response to an output (D2) from the second detection means (20), and in that said signal processing means (30) cancel the latched anti-masking alarm signal provided the first detection means (10) produces a respective output (D1) after production by the second detection means (20) of the output (D2) giving rise to the latched anti-masking alarm signal. An arrangement as claimed in claim 1, wherein the first detection means (10) comprises a passive infra-red sensor. An arrangement as claimed in claim 1 or claim 2, wherein the second detection means (20) comprises a source of radiation (21) and a sensor (22) responsive to radiation emitted by the source (21). An arrangement as claimed in claim 3, wherein the source (21) of radiation is a source of infra-red radiation. An arrangement as claimed in claim 3 or claim 4, wherein the sensor (22) responsive to radiation emitted by the source (21) is a passive infra-red sensor. An arrangement as claimed in any one of claims 1 to 5, wherein the signal processing means (30) is operable in an operator-selectable test mode, and has said capability to cancel said latched anti-masking alarm signal provided the test mode has been selected. An arrangement as claimed in any one of claims 1 to 5, wherein the signal processing means (30) has said capability to cancel a said latched anti-masking alarm signal after a preset delay following production of a respective output signal (D2) by the second detection means (20). A method of detecting intruders comprising the steps of using a first detection means (10) to detect for intruders, using a second detection means (30) for detecting for obscuration of the first detection means (10) and being characterised by using signal processing means (30) responsive to said first and second detection means to latch an anti-masking alarm signal in response to detection of an obscuration and to cancel the latched anti-masking alarm signal provided an intruder is detected after detection of the obscuration.

Inventors: GALLOWAY JOHN LINDSAY, HENNESS ALAN, OWERS IAN ALEXANDER, GALLOWAY, JOHN LINDSAY, HENNESS, ALAN, OWERS, IAN ALEXANDER
Applicants: GUARDALL LTD, GUARDALL LIMITED
Application year: 1991
Main classification: G08B13
IPC codes: G08B29, G08B13