GB2342987A

GB2342987A - Improved smoke dectector with fault monitoring capability

Info

Publication number: GB2342987A
Application number: GB9822756A
Authority: GB
Inventors: David Appleby; Stephen Henry Ellwood
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-10-20
Filing date: 1998-10-20
Publication date: 2000-04-26
Anticipated expiration: 2018-10-20
Also published as: GB9822756D0; GB2342987B

Abstract

Smoke may enter a partly enclosed chamber 1, and is sensed by an increased signal from receiver 8 due to scattering and reflection of light from source 7. Barrier 10, and the design of the chamber's internal surfaces 12, 13, 14 minimise background scattering and reflection. An indirect light path from source 7 to receiver 8 is formed by a first bright scattering and/or reflective surface 15 within the field of emission of source 7 but outside of the field of view of receiver 8, and by a second bright surface 16 within the field of view of receiver 8 but outside of the field of emission of source 7. The quiescent signal is dominated by the indirect light path, and is similar to the signal increase in the presence of smoke at the fire detection threshold. The quiescent signal is predictable, is relatively unaffected by contamination or by insects, and may be used to fault monitor the sensor.

Description

IMPROVED SMOKE SENSOR WITH A FAULT MONITORING CAPABILITY This invention relates to an improved sensor suitable for sensing the presence of smoke particles in air. It is well known that the presence of smoke may be sensed by means of sensing light at visible or near infra-red wavelengths which is scattered and reflected from smoke particles. Sensors based on this principle find widespread application in optical point smoke detectors, which are generally mounted on the ceiling of a protected space and sample airflow past the detector through a mesh and a chamber. A light emitting diode source emits pulses of near infrared radiation into the chamber, and light is sensed by a receiver, generally a silicon photodiode or photo transistor mounted so that it does not receive radiation from the source by a direct path.

Light scattered and reflected from smoke particles (the smoke signal) adds to that scattered and reflected from the inner surfaces of the chamber (the quiescent signal), and the presence of smoke is sensed by an increase in the total signal. A forward light scattering angle of typically 45 degrees and a light wavelength of approximately 900nm are generally used, and this combines a good sensitivity with a convenient and low cost mechanical and electronic arrangement. The sensitivity of the sensor is defined in terms of the smoke signal in a standardised oil mist aerosol.

The smoke signal to detect a fire (the standard smoke signal) is that measured in a standard aerosol concentration having a light extinction coefficient of typically 0.15dB/m, measured at a wavelength of 900nm. The fire detection threshold is generally set to equal the standard smoke signal.

In the aforementioned sensor it is important that the quiescent signal is minimised as a proportion of the standard smoke signal. If the quiescent signal dominates, this may lead to sensor instability. since the signal level is dependent on the efficiency of the electro-optic components and their associated electronics circuitry, and these are susceptible to drifts, for example as a result of temperature changes and ageing. It is generally found necessary to manufacture the walls of the chamber from a black material, usually injection moulded plastic, and to equip the inner surfaces in those areas which are most sensitive to light reflection with anti-reflection features. Generally, the smaller the chamber, the more difficult it is to achieve a low quiescent signal as a proportion of the standard smoke signal. Nevertheless, by employing the best design practice, it is possible to achieve a very low quiescent signal, for instance corresponding to a standard aerosol concentration having a light extinction coefficient of less than 0.05dB/m.

Ideally, the quiescent signal will result from a combination of very small amounts of scattering and reflection from many points on the chamber inner surfaces. An important feature of optical point smoke detectors, especially those which may be individually identified within a fire detection installation, is that they should be capable of automatically monitoring the functioning of the sensor, and of detecting and signalling a fault both in the event of total failure, or a reduction in sensitivity to the point where the fire detector is out of specification, such as could occur if the optical surfaces of the source and receiver become coated by contaminants borne by the atmosphere. A commonly used method is to arrange that there is sufficient quiescent signal to be able to detect a fault when the signal falls in some defined manner from its normal or expected level. To achieve this in practice, it is preferable that the quiescent signal is of a similar order to the standard smoke signal.

Residual light scattered or reflected in the chamber is generally used to provide the required quiescent signal, and to achieve this the aforementioned the best design practice is compromised. One consequence is that the quiescent signal level proves to be unpredictable, and differs from detector to detector as a result of variations in manufacturing and component tolerances. This makes it difficult to monitor the sensor quantitatively for a reduction in sensitivity. A more serious problem is that the features within the chamber from which light scattering and reflection occur are more vulnerable to changes in their optical properties than would otherwise be the case. This is generally true if the surfaces become contaminated with dust particles, or as a result of other environmental effects. In extreme cases such effects are known to cause fire detectors to false alarm.

Known techniques exist whereby a more stable quiescent signal may be achieved, for example with the aid of a light pipe arrangement. However, this adds significantly to the complexity of the optical arrangement, and therefore to the size and cost of the sensor. It is the main object of the present invention to provide for an improved fire detector, having a manufacturing cost similar to that of existing detectors, and incorporating a light scattering sensor which has a stable and predictable quiescent signal in order to permit reliable fault monitoring.

According to the present invention there is provided a sensor wherein the presence of smoke which may enter a partly enclosed chamber is sensed by means of light scattered and reflected from smoke particles, and wherein the optical axes of a light source and a light receiver are so disposed and the features of the chamber are so arranged that there is substantially no direct light path from source to receiver, and that stray scattering and reflection of light from the inner surfaces is minimised, characterised in that a first bright scattering and/or reflective surface is disposed substantially within the field of emission of a source but outside of the field of view of a receiver, and that a second bright scattering and/or reflective surface is disposed substantially within the field of view of the aforementioned receiver but outside of the field of emission of the aforementioned source, and wherein light is transferred from source to receiver by an indirect path which includes the bright surfaces.

The invention will now be described by way of example with reference to the accompanying drawing, in which figure 1 shows a first embodiment of the sensing chamber and its optical arrangement, and figure 2 is a plan view of a second embodiment.

As shown schematically in figure 1, smoke may enter from the atmosphere into a partly enclosed chamber 1, through apertures 2 in an outer case 3, through a mesh 4 intended to exclude larger particles and insects, and past a light trap structure 5 and 6. Source 7 emits infrared or visible light into chamber 1, and this light may reach receiver 8 through lens 9 via indirect paths through chamber 1. A barrier 10 prevents light from reaching receiver 8 by a direct path from source 7. The signal from receiver 8 results from scattering and reflection by any smoke particles present in chamber 1, for example via path 11, and also from stray scattering and reflection via multiple paths (not shown) from the inner surfaces 12,13,14 of chamber 1. The overall layout of the sensor, and the nature of barrier 10 and surfaces 12,13,14, reduces the stray scattering and reflection to the minimum level which is practical for the size of the sensor. A first bright scattering and/or reflective surface 15 is disposed substantially within the field of emission of source 7, but outside of the field of view of receiver 8. A second bright scattering and/or reflective surface 16 is disposed substantially within the field of view of receiver 8, but outside of the field of emission of source 7. A direct light path 17 exists between bright surfaces 15 and 16. Light may therefore travel from source 7 to receiver 8, by an indirect path 18,17,19, which includes scattering and/or reflection from bright surfaces 15 and 16.

The quiescent signal in the absence of smoke derives from both scattering and/or reflection from the inner surfaces of chamber 1, and from the bright surfaces 15 and 16. According to a further feature of the invention it is arranged that the contribution from the bright surfaces is substantially greater than that from the inner surfaces of the chamber. By ensuring that the majority of the quiescent signal derives from highly scattering and/or reflective surfaces, the signal is relatively stable and predictable, and is much less likely to be affected by changes in the optical properties of the surfaces. This could otherwise be the case should they become contaminated with a lighter coloured material, for example dust particles, or if a small insect alights on the surface. In a sensor according to the present invention the quiescent signal may even be observed to fall in response to some environmental influences, for example an increase in the relative humidity of the air. It will be understood that the sensitivity of the sensor to smoke will vary, mainly as a result of differences in the efficiency and the angular emission of the light source, the efficiency of the receiver, and variations in the electronics circuitry. In general, the sensor sensitivity must be calibrated and set up during the manufacturing process. By ensuring that the bright surfaces lie well within the field of emission of the source or the field of view of the receiver, the light scattered and/or reflected along the indirect path from source to receiver is in a reasonably constant ratio to the light scattered and reflected by a standard aerosol concentration. The quiescent signal is therefore substantially proportional to the standard smoke signal. The quiescent signal level is arranged to be of a similar size to the standard smoke signal. By this means the quiescent signal may be used to monitor the functioning of the sensor.

As shown schematically in figure 2 in a second embodiment of the sensor the overall mechanical design is similar to that of embodiment 1. However, two sources 20,21 emit infrared or visible light into substantially different volumes 22,23 of a chamber, and this light may reach receiver 24 through lens 25 via indirect paths 26,27 through chamber volumes 22,23. A barrier 28 prevents light from reaching receiver 24 by direct paths from sources 20,21. One first bright surface 29 is disposed substantially within the field of emission of source 20, and another first bright surface 30 substantially within the field of emission of source 21, both surfaces being outside of the field of view of the receiver 24. A second bright surface 31 is disposed substantially within the field of view of receiver 24, but outside of the fields of emission of sources 20,21. Direct light paths 32, 33 exist between the first and second bright surfaces 29 and 31, and between the first and second bright surfaces 30 and 31 respectively. Light may therefore travel from source 20 to receiver 24 by one indirect path which includes scattering and/or reflections from bright surfaces 29 and 31, and also from source 21 to receiver 24 by another indirect path which includes scattering and/or reflections from bright surfaces 30 and 31. The principle of operation is similar to that of the first embodiment, except that for each combination of source and receiver so arranged to sense light scattered and reflected from smoke particles there is a stable quiescent signal which may be used to monitor the functioning of each source and receiver. According to a further feature of the invention either a first or a second bright surface may be associated with more than one combination of source and receiver. For example, in the second embodiment, surface 31 is shared by the indirect paths from source 20 to receiver 24, and from source 21 to receiver 24. The presence of smoke may be sensed in two substantially different volumes of the chamber. Also, environmental influences which affect scattering or reflection of light from the surfaces of only one volume of he chamber, for example the presence of a insect or a length of fibre, will influence the quiescent signal for only one combination of source and receiver.

These features may be used to improve the reliability and false alarm performance of the sensor.

The combination of first and second bright scattering and/or reflective surfaces is arranged to contribute an appropriate quiescent signal level for the sensor application. Where the sensor is applied to a very high sensitivity smoke detector, the appropriate quiescent signal level will be significantly less than that for a normal point smoke detector. Within the constraints of the sensor size and layout, the required level of the quiescent signal dictates the sizes of the bright surfaces and their distances from their associated source or receiver. It is preferred that the proportion of the field of emission of a source which is included by its associated first bright surface, is smaller than the proportion of the field of view of the corresponding receiver which is included by its associated second bright surface. This is to minimise the quantity of light which is scattered or reflected back into the chamber from the first bright surface. This light may be subject to secondary scattering or reflection into the receiver from other parts of the chamber, which could adversely influence the stability of the sensor. As shown in figure 1, the feature which creates the first bright surface 15 is smaller than the feature which creates the second bright surface 16, and surface 15 is more distant from the source 7 than is surface 16 from the receiver 8. According to a further feature of the invention the preferred shape for each bright surface is curved in the plane of the optical axis of its associated source and receiver. This is to ensure that at least one light path exists from a source to a receiver which includes direct reflections from bright surfaces. As shown in figure 1, where there is a direct light path between the first and the second bright surface, the features which create the bright surfaces 15,16 have a round cross section.

The chamber body of the sensors described in the foregoing could be constructed using plastic or metal components of suitable design, such as would be obvious to one skilled in the design of light scattering smoke sensors. A preferred material for the bulk of the chamber body is black or darkly coloured plastic, manufactured by means of an injection moulding process. A suitable black plastic material is polystyrene with appropriate additives. It is preferred that the inner surfaces of the chamber from which a significant amount of light reflection may take place, and which thereby may contribute strongly to the quiescent signal, are equipped with anti-reflection features, for example vanes, grooves, or ridges with sharp edges. A preferred material for the bright surfaces is white or lightly coloured plastic, manufactured by means of an injection moulding process. A suitable white plastic material is ABS (a proprietary butyl styrene) with appropriate additives. A preferred method of construction is that the white plastic material is part of a moulding external to the body of the chamber, which may conveniently form part of the outer case of the fire detector, and the bright surfaces are formed by projections from this moulding which pass through holes in the chamber body. This is shown in figure 1, where the features which create the bright surfaces 15,16 are round pegs projected from outer case 3 which intrude into the chamber 1.

Preferably, each source is a visible or infrared semiconductor light emitting diode, for example a GaAlAs device which emits efficiently at wavelengths around 880nm.

Preferably, each receiver is a silicon photodiode, which operates efficiently at corresponding wavelengths. Both sources and receivers are widely available in various forms from a number of suppliers. Amplification and signal capture means are associated with each receiver, and electronic control means is arranged to control a pulsed electrical current through each source and to monitor the signal from each receiver. Preferably, the control and monitoring means is implemented with the aid of a microcomputer. Suitable single chip CMOS microcomputers are available from a number of suppliers. In embodiments where there is one source and one receiver a fire alarm is detected if there is a defined increase in the signal above the quiescent level.

Preferably, in embodiments where there is more than one source and/or receiver, a fire alarm is sensed if there is a defined increase in the combined signals from more than one source and receiver pair. Preferably, a fault is sensed in the event of a defined decrease in the signal below the quiescent level for each combination of source and receiver, so that all critical components of the sensor are monitored.

It will be understood that the sensor embodiments herein described are given by way of example only. It is not precluded to use bright surfaces in addition to the first and second bright surfaces herein described in order to create an indirect light path between a source and a receiver.

Additional optical elements, for example lenses associated with the sources, and additional sources and/or receivers, including those working at different light wavelengths, could be employed to further improve the sensitivity or reliability of the sensor. The principle of operation is not fundamentally dependent on the choice of microcomputer or other electronic components, or the detailed implementation of the electronics circuit, or of any operational software.

These could be realised in a variety of ways by one skilled in the art. It will be further understood that elements in addition to those described in the foregoing would be necessary to construct a practical fire detector, as would be known or obvious to one skilled in the art. These would be include mechanical and electronic elements to physically enclose the sensors, mount the detector, and to provide the interface to a fire detection system. The ambient atmosphere may be permitted to reach the sensor either by natural or forced convection, or by forced flow with the assistance of a fan. Additional sensors could also be included in the detector, for example a thermal sensor in order to improve the detection of some types of fire.

Claims

CLAIMS 1 A sensor wherein the presence of smoke which may enter a partly enclosed chamber is sensed by means of light scattered and reflected from smoke particles, and wherein the optical axes of a light source and a light receiver are so disposed and the features of the chamber are so arranged that there is substantially no direct light path from source to receiver, and that stray scattering and reflection of light from the inner surfaces is minimised, characterised in that a first bright scattering and/or reflective surface is disposed substantially within the field of emission of a source but outside of the field of view of a receiver, and that a second bright scattering and/or reflective surface is disposed substantially within the field of view of the aforementioned receiver but outside of the field of emission of the aforementioned source, and wherein light is transferred from source to receiver by an indirect path which includes the bright surfaces.
2 A sensor according to claim 1, wherein the quantity of light which is transferred from a source to a receiver by an indirect path which includes two or more bright surfaces is substantially greater than that which is transferred as a result of stray scattering and reflection from the rest of the inner surfaces of the chamber, but is of a similar order to that which is transferred as a result of light scattered or reflected from smoke particles at a concentration characteristic of the detection of a fire, such that there is a substantially stable and predictable signal level in the absence of smoke which may be used to monitor the functioning of the sensor.
3 A sensor according to claim 1 or claim 2, wherein more than one source and/or more than one receiver are disposed so as to form two or more substantially separate light paths to sense the presence of smoke, and wherein for each combination of source and receiver so arranged to sense light scattered and reflected from smoke particles there are two or more associated bright surfaces, such as to provide indirect light paths for the purpose of monitoring the functioning of each source and receiver in the sensor.
4 A sensor according to claim 3 wherein any one bright surface may be associated with more than one combination of source and receiver.
5 A sensor according to any one of claims 1 to 4, wherein the proportion of the field of emission of a source which is included by its associated first bright surface, is smaller than the proportion of the field of view of the corresponding receiver which is included by its associated second bright surface.
6 A sensor according to any one of claims 1 to 4, wherein a bright surface is curved in the plane of the optical axis of an associated combination of source and receiver, such as to guarantee that at least one light path exists from source to receiver which includes direct reflection from the bright surface.
7 A sensor according to any one of claims 1 to 4, wherein the inner surfaces of the chamber substantially consist of black or darkly coloured plastic, manufactured by means of an injection moulding process, and are equipped with antireflection features, and wherein the bright surfaces are formed by intrusions into the chamber of white or lightly coloured plastic, manufactured by means of an injection moulding process.
8 A sensor according to any one of claims 1 to 7, wherein each source is a visible or infrared light emitting diode and each receiver is a silicon photodiode, and associated electronic control means are arranged to control a pulsed electrical current through each source and to monitor the signal from each receiver, and wherein a fire alarm is sensed in the event of defined increases above the quiescent level, either in one signal or in combined signals from more than one combination of source and receiver, and a fault is sensed in the event of a defined decrease in any signal below the quiescent level.
9 A sensor according to any one of claims 1 to 8 included as part of a fire detector, contained within a mechanical enclosure appropriate for use in sampling smoke resulting from a fire, and into which the ambient atmosphere may enter either by natural or forced convection, or by forced flow.
10 A sensor according to any one of claims 1 to 8 substantially as herein described with reference to figure 1 or figure 2 of the accompanying drawing.