AU725418B2 - Smoke detector - Google Patents

Abstract

The smoke detector has a fixing socket in which a detector insert with an optical module is fitted. The optical module has a light source (6) and a photodetector (7) contained within a measuring chamber having a base (11). The module further has a labyrinth system provided by plates (9) projecting inwards from the periphery of the measuring chamber. The base of the measuring chamber has a conical or pyramid shape so that its centre lies at a greater distance from the plane containing the light source and the photodetector than its periphery.

Description

Cerberus AG, CH-8708 Mannedorf C-426 Cerberus AG, CH-8708 M~nnedorf C-426 Smoke detector The invention relates to a smoke detector with a detector unit that can be secured in a base, with an optical module having a light source, an optical receiver, a measuring chamber, a central screen, a base and a labyrinth system with screens arranged on the periphery of the measuring chamber.

In smoke detectors of this kind which are known as scatteredlight smoke detectors and which may optionally contain, in addition to the optical module, a further sensor, for example, a temperature sensor, it is well known that the optical module is designed in such a way that interfering light from an external source cannot penetrate the measuring chamber and smoke can penetrate very easily. The light source and optical receiver are arranged in such a way that no light rays can arrive at the receiver on a direct path from the source. In the presence of smoke particles in the beam path, the light of the light source is scattered on said particles and part of this scattered light falls on the optical receiver and actuates an electrical signal.

It is obvious that the reliability of such scattered-light smoke detectors in terms of preventing false alarms depends, inter alia, to a very large extent on the fact that the light of the light source is scattered only on smoke particles, so that with the exception of smoke particles, no other particles are able to enter the measuring chamber, the term particle being understood in the widest sense and including, for example, insects. The problem with insects has been recognised for some time and is solved by an insect screen surrounding the measuring chamber.

A scattered-light smoke detector known from DE-A-44-12-212 contains a measuring chamber in the form of a round box which is secured with a front side to a plate connected to the ceiling of the room to be monitored and whose side wall is formed by an insect screen. The front of the measuring chamber turned away from the said plate and facing the detector dome is covered by a flat base.

The practical use of such scattered-light smoke detectors with a flat, cylindrical measuring chamber has shown that the frequency of false alarms with said detectors can increase with increasing period of use, an important cause thereof being the fact that the light of the light source is scattered on dust particles deposited in the measuring chamber thereby simulating the presence of smoke particles. In order to exclude the occurrence of such false alarms, the detectors have to be cleaned to remove the dust, which represents unwanted additional expenditure.

The object of the invention is to provide a smoke detector of the type mentioned at the beginning in which the actuation of false alarms due to light scattering on dust particles is avoided completely, if possible, or at any rate greatly reduced and a prolonged maintenance interval is thereby achieved.

The said object is achieved according to the invention in that the base is designed in such a way that it has in its centre a greater distance from the plane defined by the light source and optical receiver than at its edge.

The solution according to the invention brings about a drastic reduction in the interfering effect of dust particles because these are now much further away from the actual measuring zone than hitherto, as a result of which the probability of light scattered on dust particles reaching the measuring chamber has become much lower.

As is known, the optical axes of light source and optical receiver intersect in the region of the centre of the measuring chamber and hence also of the middle of the base. As the base in this very place in the region of its cone or peak has its greatest distance from the plane of measurement, and as dust is deposited predominantly in this base region, it is very unlikely that light radiated by the light source will reach a dust particle deposited on the cone of the base and be scattered back into the measuring chamber.

A first preferred embodiment of the smoke detector according to the invention is characterised in that the base is funnelshaped and has the form of a cone or a pyramid.

In a second preferred embodiment of the smoke detector according to the invention, the base has a sieve-like or gridlike structure and acts as an insect screen. This embodiment has the advantage that the smoke detector has one component less than hitherto, which is associated with a corresponding cost advantage.

A third preferred embodiment of the smoke detector according to the invention is characterised in that the base is provided on its inner surface facing the measuring chamber with a plurality of lamellae projecting vertically upwards and that the arrangement, number, height and mutual distance of said lamellae are chosen such that light falling against the base reaches one of the lamellae before reaching said base, and that all the optical receiver sees of the base are the lamellae.

The design of the base according to the invention with the upwardly directed lamellae again reduces the likelihood of light from the path of rays being scattered in the measuring chamber on dust particles deposited on the base. This is because the dust particles are deposited not on the lamellae but at the foot thereof on the inner surface of the base, and that is a region which is screened by the lamellae from light from the measuring chamber. Moreover, the lamellae also serve as a screen from light from external sources, which further increases the reliability of measurement of the smoke detector according to the invention.

A further important advantage which results from the said absorption of the light originating from unwanted secondary scattering or reflection, known as background light, are lower requirements in respect of the production tolerances. That is, as the absorption of background light increases, the requirements in respect of positional accuracy of the light source and optical receiver fall.

The invention is illustrated in more detail below on the basis of an embodiment and the drawings, in which: Figure 1 shows a cross-section through a scattered-light smoke detector at the level of the optical axis of its optical module, looking towards the base of the optical module, and Figure 2 is a schematic section along line II-II of Figure 1 on a reduced scale compared with Figure 1.

The scattered-light smoke detector shown comprises in the known manner a detector unit 1, which can be secured in a base (not shown) mounted preferably on the ceiling of the room to be monitored, and a detector cover 2 upturned over the detector unit 1, which cover is provided with smoke inlet slits 3 in the region of its cone directed at the room to be monitored when the detector is in the operating state. The detector unit 1 comprises substantially a box-like base body on whose side facing the detector cone is arranged an optical module 5 surrounded by a side wall 4 and on whose side facing the detector base is arranged a printed circuit board with analysis electronics (not shown). This detector construction is well known and is not described in more detail here.

Reference is made in this connection, for example, to the detectors of the AlgoRex series (AlgoRex registered trade mark of Cerberus AG) and to European patent application no.

95117405.1.

The optical module 5 comprises substantially a light source 6, an optical receiver 7, a measuring chamber 8, a labyrinth system consisting of peripheral screens 9 arranged on the inside of the side wall 4, a central screen 10 and a base 11.

The optical axes of the light source 6 formed by an infrared light-emitting diode (IRED) and of optical receiver 7 lie not on a common line but have a kinked path, the central screen being arranged near the point of intersection. Side wall 4 and base 11 screen the measuring chamber 8 from external light from outside, and the peripheral screens 9 and the central screen 10 prevent light rays reaching optical receiver 7 on a direct path from light source 6. The peripheral screens 9 also serve to suppress the so-called background light which is caused by unwanted scattering or reflection.

The better the background light is suppressed, the lower the base pulse, that is, the signal that is detected when no smoke is present in the measuring chamber 8. The intersection region of the bundle of rays emitted from the light source 6 and of the visual field of the optical receiver 7 form the actual measuring region referred to hereinafter as the scatter area.

The light source 6 emits short, intensive light pulses into the scatter area, whereby the optical receiver 7 "sees" the scatter area but not the light source 6. The light from light source 6 is scattered by smoke penetrating the scatter area, and a part of this scattered light falls on the optical receiver 7. The receiver signal thereby produced is processed by the electronics. Naturally, the smoke detector may contain, in addition to the optical sensor system contained in the optical module 5, further sensors, for example a temperature and/or a gas sensor.

If smoke is produced in the room to be monitored and rises up to the smoke detector, it then penetrates the smoke inlet slits 3 and flows in these in a horizontal direction to the funnel-shaped base 11. The base 11 has a sieve-like or gridlike structure and is provided on its outside with ribs 12 arranged in a star-form, through which the smoke is guided to the base. As a result, the smoke flows in a vertical direction into the measuring chamber 8 and into the scatter area. As a result of the funnel-shaped design, base 11 is much further away from the measuring chamber than it would be if the base were flat. Dust particles that have penetrated the measuring chamber 8 which scatter the light from light source 5 and therefore act like smoke particles are deposited in the cone of the base 11 where they lie outside the incident region of the radiation from light source 6, as a result of which the interfering effect of these smoke particles is drastically reduced.

As can be derived from the Figures, the funnel-shaped area of base 11 takes the form of a pyramid or a truncated pyramid, all the side surfaces of the pyramid having the sieve-like or grid-like structure already mentioned. For reasons of clarity, one such grid-like structure 13 of only one of the pyramid surfaces is shown diagrammatically in Figure 1. The ribs 12 on the outside of base 11 are arranged preferably along the side edges of the pyramid.

The likelihood of the interfering effect of dust particles deposited on the base 11 is further reduced by a special design of the base. This consists in the fact that the base 11 is provided on its inner surface with a plurality of lamellae 14, 15 projecting vertically upwards, whereby their arrangement, number, height and mutual distance is chosen such that light from the measuring chamber falling onto the base reaches one of the lamellae before reaching the base, and in that the optical receiver 7 sees only the lamellae 14, 15 of base 11. As a result, the risk of light being scattered on dust particles is much lower since the dust is much more likely to remain on the base than to adhere to the vertical walls of the lamellae. In addition to the screening of the base 11 from light from measuring chamber 8, the lamellae 14, screen the optical receiver 7 from external light from outside.

As shown, not all the pyramid surfaces are provided with lamellae, only the pyramid surface opposite light source 6 and the one opposite optical receiver 7 and the pyramid surface enclosed between these two surfaces. The pyramid surfaces opposite light source 6 and optical receiver 7 are provided with longitudinal lamellae 14 oriented parallel to the base edge of the pyramid, and the pyramid surface enclosed between said surfaces is provided with at least one longitudinal lamella 14 and with several transverse lamellae 15 oriented perpendicularly thereto. The longitudinal lamellae 14 run at least approximat'ely perpendicular to the optical axis of the opposite light source and to the opposite optical receiver.

The primary purpose of the transverse lamellae 15 is optical decoupling of light source 6 and optical receiver 7.

The base 11 which, like the entire detector unit 1 (with the exception of light source 6 and optical receiver is composed of a suitable plastic and is produced as an injection moulded part, has on its edge several snap organs (not shown) which are intended to connect base 11 in a removable manner with the side wall 4 of the optical module 5 (Figure 2).

In order to achieve even better absorption of background light, at least certain parts of optical module particularly the peripheral screens 9, the central screen and the cover of measuring chamber 8 opposite the base 11, have glossy, i.e. reflecting surfaces instead of the hitherto customary matt surfaces. Naturally, further parts or the entire inside of optical module 5 may also have a glossy surface.

It had been assumed hitherto that background light can best be destroyed by absorption on matt surfaces, but this consideration overlooked the fact that the light is scattered in a diffuse manner on the matt surfaces and arrives in an uncontrolled manner in the measuring chamber. If, on the other hand, glossy surfaces are used, these act like black mirrors and reflect the non-absorbed light in a defined, noninterfering direction, for example, onto the adjacent peripheral screen. As the reflecting surfaces are black and therefore reflect only about 5% of the incident radiation, this can be destroyed almost completely by repeated reflection between such surfaces. The glossy surfaces are produced by means of an injection mould which has a suitable, preferably polished surface at least on the surfaces which are intended to be glossy.

A further very important feature for increasing the measuring reliability of the smoke detector shown consists in the fact that the peripheral screens 9 or at least most of them are arranged not in a rotationally symmetrical manner but in such a way that the angle of incidence of the light beams emitted by light source 6 and received by optical receiver 7 is constant on said screens. Peripheral screens 9 arranged in a rotationally symmetrical manner would be those that are formed by rotation of a screen about the centre. In Figure 1, the four peripheral screens 9 adjacent to the light source 6 and the four adjacent to optical receiver 7 are not designed in a rotationally symmetrical manner. The angle of incidence is chosen such that the incident and non-absorbed light is reflected as often as possible between the peripheral screens 9.

The peripheral screens 9 are each composed, as shown, of two angled partial surfaces, whereby their mutual inclination and distance and the length of the peripheral screens 9 are chosen such that the light radiated to the peripheral screens 9 cannot reach the inner surface of side wall 4 directly but arrives in each case at a peripheral screen 9 and is reflected by said screen onto the adjacent peripheral screen. Moreover, the non-rotationally symmetrical arrangement of the majority of peripheral screens 9 leads to better absorption of the background light and thus to less stringent requirements in respect of the positional and component accuracy of light source 6 and optical receiver 7 and to a detector which is less susceptible to fouling.

As may be derived from Figure 1, the peripheral screens 9 are designed with edges as sharp as possible on their inner edge directed at the central screen 10. This has the advantage that only a little light falls on such a sharp edge and thus less light is reflected in a plurality of directions.

In the production of the injection mould by erosion, a limit is imposed on the sharpness of an edge by the thickness of the wire used, and this limit does not satisfy the requirements of the inner edges of peripheral screens 9. In detector unit 1, the desired sharpness of the inner edges is achieved in that a core is inserted in the injection mould which has a graduated (toothed or zigzag) contour on its periphery provided for forming the inner edges mentioned. The individual gradations of said contour fit inside the grooves formed in the injection mould for forming peripheral screens 9, and close these off towards the centre. As a result, very sharp edges can be formed between the grooves of the injection mould and the gradations of the core.

Practical tests have shown that the simultaneous use of peripheral screens 9 with sharp internal edges and .of optical module parts (peripheral screens 9, central screen 10, cover of measuring chamber 8) with a glossy surface leads to a marked reduction in the base pulse, and that the detector is less susceptible to dust and condensation.

As may also be derived from the Figures, the light source 6 and the optical receiver 7 are each arranged in a housing 16 and 17 respectively. The two housings 16 and 17, which are fixed to the cover of measuring chamber 8, are open towards the bottom and are covered on their open side by base 11. On their front side facing the central screen 10, the housings 16 and 17 are each closed off by a window with a light outlet and light inlet aperture respectively. These windows differ from the housing windows of known scattered-light smoke detectors in that they are designed as a single part.

In the known scattered-light smoke detectors, the windows are composed of two parts, one of which is fixed to the cover of the measuring chamber and the other is fixed to the base.

When the base is placed in position, problems of fit occur repeatedly, and a light gap is formed between the two halves of the window thus resulting in unwanted disturbances in the emitted and received light. In one-piece housing windows, disturbances of this kind are excluded and no problems can arise with the positional accuracy of the two window halves.

As shown in Figure 2 in the case of window 18 of housing 16, the upper and the lower half of the one-piece windows are offset in the manner of the two blades of scissors. As a result, the injection mould can be designed without a mould slide such that a separate moulding element is provided for each of the two offset halves of the light outlet and inlet aperture, so that a precisely defined shape and clean surface of these apertures is obtained.

Claims (10)

1. A smoke detector with a detector unit which can be secured in a base, with an optical module which has a light source, an optical receiver, a measuring chamber, a bottom, and a labyrinth system with screens arranged on the periphery of the measuring chamber, characterised in that the bottom is designed in such a way that it has in its centre a greater distance from the plane defined by light source and optical receiver than at its edge.

2. A smoke detector according to claim 1, characterised in that the bottom is funnel-shaped and has the form of a cone or a pyramid.

3. A smoke detector according to claim 2, characterised in that the bottom has a sieve-like or grid like structure and is designed as an insect screen. 20

4. A smoke detector according to claim 1 to 3, characterised in that the bottom is provided on its inner surface facing the measuring chamber with a plurality of upwardly projecting lamellae, and that the arrangement, number, height and mutual distance of said lamellae are chosen such that light falling against the bottom reaches one of the lamellae before reaching said bottom, and the optical receiver sees only the lamellae of bottom. S'

5. A smoke detector according to claim 4, 30 characterised in that the optical receiver is screened by the lamellae from external light penetrating the measuring chamber from outside.

6. A smoke detector according to one of claims 2 and 4, characterised in that the lamellae are oriented parallel and perpendicular to the bottom edge of the respective pyramid side surfaces. S\\melbfiles\home$\Priyanka\Keep\speci\ 33 3 27 97 .doc 1/08/00 13

7. A smoke detector according to claim characterised in that ribs arranged in the form of a star are provided on the outside of the bottom, which form the side walls of smoke-conducting channels.

8. A smoke detector according to one of claims 1 to 3, characterised in that the peripheral screens are arranged in such a way that the angle of incidence of the light beam radiated by the light source and received by the optical receiver is constant on the majority of said screens.

9. A smoke detector according to one of claims 1 to 3, characterised in that the peripheral screens have as sharp an edge as possible on their front side directed at the central screen.

10. A smoke detector substantially as herein 20 described with reference to the accompanying drawings. Dated this 1" t day of August 2000 CERBERUS AG By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia o• o coo•* \\melbfiles\homeS\Priyanka\Keep\speci\33 3 2 7 9 7 .doc 1/08/00