Intrusion alarm system

Abstract: An intrusion alarm system provided with a passive sensor with a detector for detecting energy of light (electromagnetic radiation) from an object in a location to be monitored, and with an alarm for generating an alarm signal, dependent on whether a detection signal is emitted by the detector or not, wherein an active sensor is provided, said sensor having a source for emitting light at least partially onto an aperture of the passive sensor and a detector for detecting reflected light from the source.

Description: The invention relates to an intrusion alarm system as outlined in the preamble of claim 1. Such an intrusion alarm system is known from EP-A-0 186 226. The known intrusion alarm system is provided with reflection means in the form of a mirror (20) positioned at the outer end of the region to be monitored for detecting long distance sabotage attempts. The known intrusion alarm system is provided with a further emitter (18) fixed to the exterior of the housing and directing radiation through the window towards a sensor in the housing in order to detect sabotage attempts at a short distance of the intrusion alarm system. The further emitter tests the functioning of the passive sensor and detects masking of the window. It is a disadvantage of the known intrusion alarm system that it is complex (it has two emitters and two detectors) and in addition it requires an exact positioning of the mirror at the outer end of the region to be monitored which positioning is difficult. Furthermore repositioning of the system is difficult because it requires readjustment of the mirror. The present invention has the object to provide a simple and less expensive but effective intrusion alarm system, which makes it possible to detect sabotage to the passive sensor thereof. Thereto the intrusion alarm system according to the invention is characterized in that the radiation reflection means are in the form of radiation scattering means fixed to the housing in the vicinity of the window, and that electromagnetic radiation originating from the light source is scattered by the entire window itself as well. The scattering means are formed as wings (claim 9) and/or as a texture covering the window (claim 8). The entire window itself is scattering as well, also in the case when it is not covered with the texture. The scattering means only locally provide a minimum background radiation level and kind of self tests the system in that if the minimum background radiation level underspends the minimum level in the alarm means an alarm is fired. The scattering means does not require the adjustment in case the alarm system is repositioned, because the radiation scattering means is fixed to the housing. Furthermore the alarm system according to the invention, apart from the passive sensor, only has one light source and one light detector. In addition the reflection means may be simple scattering means. èhen it comes to the reflection properties lower demands are posed on those properties with the latter means. It is noted that the intrusion alarm system according to the invention knows no restrictions with regard to the type of light being used, i.e. not only infrared light, but also visible light (for example with a wavelength between 0.35 to 0.8 um) may be used. From a marketing point of view it may even be interesting to use visible blue, green or red light. Furthermore it is noted that an important advantage of the intrusion alarm system according to the invention is the fact that the active (whether or not infrared) sensor has a limited range, so that (sabotaging) manipulations on the aperture of the passive sensor and in the vicinity thereof are detected, whereas an authorized person when passing by the active sensor during the daytime does not generate an alarm signal. It is further noted that the intrusion alarm system according to the invention may include a passive infrared sensor coupled to a radio-frequency sensor, all this in accordance with European patent application No. 0 255 812 in the name of Elkron S.p.A. It is furthermore noted that with the intrusion alarm system according to the invention the active (whether or not infrared) sensor may also include more than one source (light source) and/or more than one detector (light diode). The specific advantage of this is that the alarm of the intrusion alarm system is not activated when for example insects come near the aperture of the passive sensor. It is to be preferred hereby that the sources and the associated detectors are sequentially driven in pairs. European patent document No. 0 289 261 in the name of Takenaka Engineering Co.,Ltd. describes an alarm system in accordance with the preamble of claim 1. This prior art document discloses an edge at the periphery of the window, wherein a small amount of light is scattered by the edge after being radiated by electromagnetic light originating from the light sources. However, scattered light does not pass via the entire window/cover, so that removal of the window by a burglar can be detected, but other sabotage attempts concerning the window, such as spraying, are left undetected. The invention will be further explained with reference to Figures illustrated in drawing, wherein: Figure 1 shows a housing of a passive infrared sensor associated with a prior art intrusion alarm system; Figure 2 shows the passive infrared sensor of Figure 1 in exploded view; Figure 3 shows a housing of a passive infrared sensor associated with an intrusion alarm system according to the invention; Figure 4 shows the housing of Figure 3, whereby infrared radiation as emitted or received by the source or the detector of the active infrared sensor respectively is drawn in full lines; Figure 5 shows the housing of Figure 3, illustrating an area covered by the active infrared sensor (with conical envelopes of emitted and received beams of infrared radiation); and Figure 6 schematically shows a block diagram of an electric circuit of an intrusion alarm system according to the invention. In figure 1 a housing of a passive infrared sensor associated with a prior art intrusion alarm system is shown, said housing including a window 1 for the detector of the passive infrared sensor, a cover 2, a mounting base 3, an alarm light 4 which will light up when the alarm is activated and means of attachment 5 for securing the cover 2 and the mounting base 3 together. figure 2 shows the passive infrared sensor of figure 1 in disassembled condition, whereby besides the aforesaid parts also the following parts are depicted: an insulation plate 6, a sticker 7 with connection data, an insulation sticker 8, a metal radio-frequency shield 9, an amplifier circuit board 10, a pyroelectric element 11 with a holding fixture, a circuit board 12 with control functions, a far infrared focusing mirror 13, a masking plate 14 for long detection fields, a type-indication sticker 15 and masking plates 16 for short detection fields. In figure 3 a housing of a passive infrared sensor associated with an intrusion alarm system according to the invention is depicted. Said housing indudes a window 1 for the detector of the passive infrared sensor, a cover 2, a mounting base 3, an alarm light 4, means of attachment 5 for securing the cover 2 and the mounting base 3 together, windows 17 and 18 for the source and the detector of the active infrared sensor respectively, and an alarm light 19 which lights up when it is attempted to sabotage the passive infrared sensor. It is noted that from a purely technical point of view the windows 17 and 18 are not absolutely necessary, but in principle function to make the unit look more attractive. In figure 4 the infrared radiation emitted by the source 20 or received by the detector 21 of the active infrared sensor respectively is illustrated in full lines 22. Figure 5 shows the area covered by the active infrared sensor with conical envelopes 23 of emitted and received beams of infrared radiation. Those parts in figures 4 and 5 that correspond with parts shown in figure 3 are indicated by the same reference numerals. The operation of the intrusion alarm system according to the invention will be explained in more detail with reference to the block diagram of figure 6 of an electric circuit of said intrusion alarm system. The source of the active infrared sensor consists of a photoemitter 24 (near infrared transmitter (NIR-TX)) having an angle of opening between 60° and 120°, said photoemitter 24 emitting radiation in the near infrared wave range onto and around the aperture of the passive infrared sensor (see the preceding figures). The passive infrared sensor is actually sensitive to infrared light of the far infrared wave range. The photoemitter 24 is connected to a power driver amplifier 25, which generates pulse flows with peak currents in the order of about 1 A, so that the photoemitter 24 emits short infrared pulses at a high intensity. A master oscillator 26, with a pulse repetition time in the millisecond range and a pulse time in the microsecond range, provides the timing of the photoemitter 24. The aforesaid window of the passive infrared sensor is possibly covered with a (very) fine texture, in such a manner that infrared light from the photoemitter 24 that falls thereon is scattered in numerous directions. The advantage of this is that a certain amount of background light is received by the detector of the active infrared sensor at all times, so that a "reference reflection signal of a constant low value" is present at all times. For the same reason objects may be placed (as a testing method) near the passive infrared sensor, in order to effect that the scattered light is directly passed from the photoemitter 24 to the detector of the active infrared sensor. The detector of the active infrared sensor consists of a photodiode 27 (near infrared receiver (NIR-RX)), likewise with an angle of opening between 60° and 120°, which photodiode 27 is receptive to infrared light scattered by the window and adjacent wings (indicated at V in figure 3) of the passive infrared sensor, as well as by objects located in the immediate vicinity of the window. The photodiode is connected to an amplifier/filter 28, which amplifies pulses at a high rate and which rejects signals having a low frequency, such as signals caused by fluctuations in the ambient light. A peak detector 29 detects the peak amplitude of the fast infrared pulses received by the photodiode 27 and amplified by the amplifier/filter 28. In this connection it is noted that a system of transmitting and amplifying short infrared pulses with a high intensity has been opted for, on the one hand in order to conserve energy and on the other hand in order to retain the possibility of distinguishing the pulses emitted by the photoemitter 24 from fluctuations in the ambient light. The peak detector 29 is followed by a band pass filter 30 which only amplifies variations in the peak amplitude ranging from slow to very slow (0.001 - 1 Hz). This was opted for in order to filter out ultra-slow amplitude variations, such as caused in particular by ageing of used semiconductors or by thermal drift, and in order to keep detecting in a reliable manner the slow movement of objects towards the window of the passive infrared sensor during an attempt at sabotage . The peak detector 29 may be synchronized by means of the master oscillator 26. As a result of the addition of a synchronisation signal the peak detector 29 will only be operative for a short time, during which also a transmission pulse of the photoemitter 24 takes place. As a result of this the signal-noise ratio of the intrusion alarm system according to the invention will be improved considerably. The following improvements will be possible in that case: a. a greater immunity to daylight (the system continues to operate in a reliable manner, even with direct incident sunlight), b. a much smaller consumption of emitter current and yet an adequate functionality, and c. greater reliability and a longer life of the intrusion alarm system due to the reduced load of active semiconductor devices. A window comparator 31 with a logic alarm circuit connected thereto, which is linked to the band pass filter 30, is activated when predetermined limiting values are exceeded, which indicate that the quality of the intrusion alarm system according to the invention is affected as a result of an attempt at sabotage. A low limiting value indicates that there is less scattering of infrared light in the direction of the photodiode 27. This points for example to changes with regard to the scattering by the aforesaid fine texture or by the aforesaid wings, which may be caused by varnish or paint being sprayed on the window of the passive infrared sensor. This will also be the case when the windows of the photoemitter 24 and the photodiode 27 are covered or when the photoemitter 24 or the photodiode 27 does not function optimally. A high limiting value indicates that a reflecting object must be present in the vicinity of the window, which object increases the amount of infrared light travelling from the photoemitter 24 to the photodiode 27. This will inter alia be the case when a glass pane is used to cover the detector of the passive infrared sensor or when an intruder attempts to cover the aperture of the passive infrared sensor with his hands, a sheet of paper or a piece of plastic. The sensitivity of the intrusion alarm system according to the invention with regard to the detection of reflecting materials, absorbent materials and attempts at spraying paint can be optimized by placing the photoemitter 24 and the photodiode 27 at an acute angle, in particular an angle of less than 20°, with respect to the window of the passive infrared sensor; optimizing the characteristics of the (very) fine texture on the window of the passive infrared sensor, so that light scattered therefrom can be optimally transmitted to the photodiode 27; using more than one path along which infrared light can travel from the photoemitter 24 to the photodiode 27, especially by introducing wings V (see figure 3).

Claims: An intrusion alarm system containing a passive sensor for detecting an object intruding in a location to be monitored, a housing (3) including a window (1) for the passive sensor, a light source (20, 24) with at least one source for emitting electromagnetic radiation, a light detector (21, 27) for generating a signal containing a value for detected radiation originating from the light source (20, 24), alarm means (28, 29, 30, 31) coupled to the light detector (21, 27) for providing an alarm in case the detected value exceeds a maximum level or underspends a minimum level, and radiation reflection means which are radiated by electromagnetic radiation originating from the light source (20, 24) such that under normal operating conditions the value detected by the light detector (21, 27) lies within the minimum and maximum level, characterized in that the radiation reflection means are in the form of radiation scattering means fixed to the housing (3) in the vicinity of the window (1), and that electromagnetic radiation originating from the light source (20, 24) is scattered by the entire window (1) itself as well. An intrusion alarm system according to claim 1, characterized in that the passive sensor is a passive infrared sensor and the light detector (21,27) is an infrared light detector based on the detection of infrared light. An intrusion alarm system according to claim 1 or 2, characterized in that the intrusion alarm is provided with an operation synchronization means (26) for generating a synchronization signal in dependence on whether a transmission pulse is issued by the light source (20,24) or not. An intrusion alarm system according to one of the claims 1-3, characterized in that said light source (20,24) can emit radiation onto and around the aperture of the passive sensor. An intrusion alarm system according to daim 2, 3 or 4, characterized in that said passive infrared sensor is sensitive to infrared light having a wavelength between 6 and 50 µm. An intrusion alarm system according to any of the preceding claims, characterized in that said light detector (21,27) is sensitive to light having a wavelength between 0.35 and 4 µm. An intrusion alarm system according to any of the preceding daims, characterized in that the at least one source in the light source (20,24) and the light diode in the light detector (21,27) both have an angle opening between 60° and 120°. An intrusion alarm system according to any of the claims 1-7, characterized in that the scattering means contains a texture covering the window (1). An intrusion alarm system according to any of the claims 1-8, characterized in that the scattering means are formed as wings (V). An intrusion alarm system according to any of the claims 1-9, characterized in that the intrusion alarm comprises a band-pass filter (28) coupled to the light detector (21,27), a peak-detector coupled to the band-pass filter (28) and a further band-pass filter (30) connected between the peak-detector (29) and the alarm means (31).

Inventors: PANTUS MATHIAS MARIA JOZEF, PANTUS, MATHIAS MARIA JOZEF
Applicants: ARITECH BV, ARITECH B.V.
Application year: 1993
Main classification: G08B29
IPC codes: G08B29