GB2314618A - Smoke detector using light scatter and extinction - Google Patents

Abstract

A smoke detector has a partially closed chamber 8 into which smoke may penetrate; light reaches a receiver 4 via three paths: from source 1 via an indirect path 11 past barrier 9, as a result of scattering and reflection by smoke particles or from the chamber's surfaces; from source 2 via a transmission path 12 through the chamber; and from source 3 via a path 13 through a closed volume 7. A microcomputer plus electronic means 10 controls the light emitted by the sources, and analyses the signals from receiver 4. The presence of smoke increases light scattered in path 11 and the extinction in path 12. Path 13 is a reference path. Over shorter time periods, smoke detection relies on correlation of the two signals, the sensitivity being dominated by light extinction. Over longer time periods the sensitivity is dominated by light scattering. The detector is self-calibrating, and is stated to have a more even response to different types of fire and to be less susceptible to false alarms.

Description

SMOKE DETECTOR USING LIGHT SCATTER AND EXTINCTION This invention relates to an improved detector suitable for sensing the presence of particles in air, such as smoke resulting from a fire. It is well known that the presence of smoke may be sensed by means of the quantity of light at visible or near infra-red wavelengths which is scattered or reflected by smoke particles. Sensors based on this principle find widespread application in point smoke detectors, which are generally mounted on the ceiling of a protected space and sample part of any airflow past the detector through a mesh and a chamber. A light emitting diode source emits pulses of near infra-red radiation into the chamber and scattered and reflected 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 or reflected by smoke particles adds to that scattered or reflected by the walls of the chamber, and the presence of smoke is sensed by an increase in the received signal. The design of such sensors is well known, but varies from instance to instance. The majority use a forward light scattering angle of approximately 45 degrees, and a light wavelength around 900nm, which is found to combine good sensitivity with a convenient mechanical and electronic arrangement. Effective operation is possible with a low mean input of power to the light source ( < lmW). The typical detection alarm threshold in a standardised oil mist aerosol is equivalent to a light extinction coefficient of about 0.10dB/m, measured at a wavelength of 900nm. Such detectors may be constructed using low cost light source and receiver components, are relatively easy to manufacture, and are generally reliable. However, there are 2 main disadvantages which are discussed below.

The first disadvantage is that the sensitivity varies as a result of differences in the efficiency and the angular emission of the light sources, the efficiency of the receivers, and variations in the electronics circuitry. The sensitivity must be set up during the manufacturing process, and this can be an expensive and time consuming process in a standard aerosol concentration. The sensitivity is also difficult to maintain constant during the operating life of the sensor. This is because the electro-optic components, and their associated electronics circuitry are susceptible to drifts, eg as a result of ageing or temperature changes, and more significantly because the optical surfaces become coated by atmospheric borne contaminants. There would be a significant benefit if it were possible to reliably monitor and correct for such sensitivity variations. The second disadvantage is that the sensitivity is very dependent on the nature of the smoke, and this differs significantly according to the type of fire. Point smoke detectors are covered by the European standard EN54:part 7, which refers to certain tests in a series of standard fire tests (FT1 to FT6, described in EN54:part 9). The highest sensitivity tends to be achieved with light coloured smokes having a particle size of typically 0.5Mm to lm, which is a characteristic product of smouldering fires in materials such as wood (eg in FT2), or cotton (eg in FT3). The sensor is much less sensitive to the smaller particles produced in free burning wood (eg in FTl) and to dark coloured smokes such as those emitted by burning polyurethane foam (eg in FT4) or hydrocarbon mixtures (eg in FT5). It is known that the sensitivity to smaller smoke particles may be improved by combining a light scattering sensor with an ionisation chamber sensor. However, ionisation chamber sensors contain radioactive components, which have a high disposal cost.

They are less reliable than light scattering sensors, being subject to surface leakage and wind sensitivity effects, and their inclusion adds to the manufacturing cost, and increases the size of a fire detector.

It is known that smoke particles may be sensed by measuring the reduction in intensity (extinction) of a beam of visible or infrared light. In principle, light extinction sensors offer a significant improvement over light scattering sensors, since they do not suffer from the two disadvantages previously referred to. The extinction is a combination of absorption and several scattering and reflection mechanisms, the relative importance of which depends on the particle size and colour, the wavelength of the light, and the sensor geometry. It is found that for test fires FT1 to FT5 the light extinction at 900nm is much more constant than is the light scattered. Light extinction detectors may also be made self-calibrating, ie the smoke sensitivity will not vary as a function of time or the state of contamination of the sensor, and the sensor components are automatically monitored for continued operation. Various light extinction sensors are known, for example those disclosed in UK Patent GB 2 267 963 B, and in European Patent EP 0 631 263 Al.

These generally generally rely on the use of two light receivers and precision optical components. Hitherto it has not proved possible to realise a sensor suitable for widespread application which combines a fire detection performance equivalent to a combined light scattering sensor and ionisation chamber sensor, a very low false alarm rate, low quiescent power consumption, and a manufacturing cost comparable with that of existing point smoke detectors.

The principal technical difficulty with light extinction sensors is known to lie in achieving a very high level of signal stability under environmental influences. For a sensor to be sufficiently small it is difficult to incorporate a light path length longer than about 100mm. A good fire detection performance requires an alarm threshold corresponding to a light extinction value at 900nm of < 0.30dB/m. With a 100mm path length this threshold equates to a signal level change from clean air of -0.7%. In order to meet the requirements of EN54:part 7 the signal stability must be maintained over a time period of up to 12 hours and for temperature fluctuations from -100C to +500C. It can be shown that to achieve this with a light extinction sensor alone necessitates a signal drift with time of < +0.04 per hour, and a signal drift with temperature of < +0.01% per K.

There are also a number of other difficulties to be overcome, not least in the prevention of false alarms caused by insects obscuring part of a light beam.

It is the main object of the present invention to provide for an improved smoke detector, by means of combining a light scattering sensor and a light extinction sensor in such a manner as to provide a more even sensitivity for a range of fire types, an increase in the reliability of detection, and the minimisation of unwanted alarms. In addition, the sensitivity may be automatically self calibrating, both when the detector is newly manufactured, and to partly compensate for contamination during its life.

According to the present invention there is provided a smoke detector wherein the presence of smoke is sensed by means of light scattered or reflected from smoke particles which ingress into a partly enclosed chamber, characterised in that a second sensor measures the concentration of the smoke within the said chamber by means of the extinction of a beam of light, and wherein signals from the light scattering sensor and signals from the light extinction sensor are processed within a microcomputer or by similar means, and wherein the signals are correlated within different time domains such that for at least certain types of fire the detection of the fire is conditional on the presence of defined signals from both of the said sensors, but wherein the sensitivity of the detector to smoke is predominantly determined by signals from the light extinction sensor.

The invention will now be described by way of example with reference to the accompanying drawing, in which figure 1 shows the basic arrangement of a detector in schematic form, and figures 2 to 4 show respectively a first, second and third embodiment of the sensing chamber and the optical arrangement. As shown schematically in figure 1, there is a chamber 8, into which smoke may ingress from the atmosphere through a mesh (not shown) which is intended to exclude larger particles and insects. The assembly contains three light sources and one light receiver. Light from source 1 reaches receiver 4 via an indirect path 11 through the chamber, this being the active path for the light scattering sensor. A barrier 9 prevents light from reaching receiver 4 by a direct path from source 1, consequently the received signal results from scattering and reflection by smoke particles if present, or from the surfaces of chamber 8.

Light from source 2 reaches receiver 4 predominantly via path 12 through the chamber. Path 12 is the active path for the light extinction sensor. Light from source 3 reaches receiver 4 predominantly via path 13 which lies within a closed volume 7. Path 13 is the reference path for the light extinction sensor. The signals in path 12 or 13, or a combination of the two, may also be used as a reference to assist in calibrating the gain of the light scattering sensor. The preferred path or paths used for this function is dependent on the detailed embodiment, and this is referred to later. A microcomputer plus electronic means 10 controls the emission of light from sources 1, 2 and 3 and monitors the signals from receiver 4.

A first embodiment of the invention is shown in figure 2.

In this case, source 1, source 2 and source 3 and receiver 4 are disposed more or less in a single plane around the circular chamber 8. Light in path 12 and path 13 undergoes no reflection or substantial deviation before reaching receiver 4. Paths 11 and 12 lie within chamber 8. Path 13 lies within volume 7 which is closed off from the atmosphere. Lenses 5 and 6 focus the light on to the receiver and permit volume 7 to be isolated from chamber 8.

Lens 14 collimates the light emitted by source 1. In this arrangement there are two optical surfaces in each of paths 11 and 12 exposed to contamination in the chamber, one of these being on the common lens 5. Path 12 is therefore the preferred reference path for calibrating the light scattering sensor.

A second embodiment of the invention is shown in figure 3.

This is similar in concept to that shown in figure 2, except that source 2 and source 3 are disposed on the same side of the chamber 8 as is receiver 4. Light in path 12 and path 13 is reflected by mirrors 15 and 16 respectively before reaching receiver 4. As compared with the embodiment of figure 2, for a sensing chamber of equivalent size, this approximately doubles the length of the active path of the light extinction sensor, resulting in a greater relative signal change for a given smoke concentration. The mirrors 15 and 16 are arranged to form a diffuse focus at receiver 4, so as to minimise the effect of changes in the angular emission of the sources, for example due to temperature changes. With this arrangement, there are two optical surfaces in path 11, three in path 12, and none in path 13, exposed to contamination in the chamber. An average of the signals in paths 12 and 13 is therefore the preferred reference for calibrating the light scattering sensor.

A third embodiment of the invention is shown in figure 4.

As in figure 3 this employs folded paths 12 and 13, but instead of the mirrors reflective prisms 17 and 18 are used to deflect the light in paths 12 and 13. The light from paths 11, 12 and 13 passes through a common lens 19 to receiver 4. Although part of path 13 lies within chamber 8, the influence of smoke will be much smaller than on light in path 12 since the path length inside the chamber is smaller, and path 13 may still be used as a reference path. This arrangement demonstrates an important principle that the active path and the reference path for the light extinction sensor need not be the same length, or have exactly equivalent geometry. It has the advantage that the closed volume 7 is much smaller than that of chamber 8, which makes it easier to arrange an even airflow into the chamber from all directions. The receiver lens 19 is also common for all of the paths. With this arrangement, there are two optical surfaces in each of paths 11 and 13 exposed to contamination in the chamber. Path 13 is therefore the preferred reference path for calibrating the light scattering sensor.

Sources 1, 2 and 3 are preferably light emitting diodes.

These are pulsed in a known sequence, and the signals from receiver 4 are monitored coincident with the pulsing of each source, such that the signal intensity in each of paths 11, 12 and 13 may be separately analysed. Light emitting diodes convert electrical energy directly into either visible light or infrared radiation, the emitted intensity being approximately proportional to the electrical current which is passed through them. For a given source the main variable which affects the light output at a given current is the temperature, the typical coefficient at 20 degrees C being -0.8% per degree C. However, the coefficients are generally well matched for a given type of source, and the ratio of the pulsed charge passed through the sources may be controlled by the microcomputer to control the ratio of the light outputs over a wide temperature range.

In the case of the light extinction sensor the preferred method is to control the pulse charge ratio between source 2 and source 3 in an accurate ratio, such that the signals from each source at receiver 4 are substantially equal. The ratio of pulse charges and/or small differences in the received signals may then be used in order to calculate small changes in the light extinction in path 12. There are important benefits in sensing only the difference signals.

Because these can be maintained at a small proportion of the signal received from any one source, the stability of the receiver gain becomes relatively unimportant and the difference signals need not be sensed to a very high resolution. This permits the use of easily available, low cost components for the receiver, its electronics circuitry, and any necessary analogue to digital signal conversion means. A preferred method by which this may be achieved is disclosed in UK Patent GB 2 267 963 B.

In case of the light scattering sensor the preferred method is to control the pulse charge ratio between source 1, and either source 2 or source 3 as appropriate to the embodiment, such that the signals at receiver 4 are maintained substantially equal or in a known ratio. The pulsed charge passed through source 2 or source 3 in performing this function may be the same as that passed though the same source during the operation of the light extinction sensor, as is described in the foregoing.

Alternatively, it may preferable that the pulsed charges are different in order to optimise the performance of each sensor. The ratio of the pulse charges may be used to correct for short term variations in the absolute light emission efficiencies of the sources and in the efficiency of the receiver 4, for example due to temperature changes.

This technique offers an important advantage in this invention that the stability of the light scattering sensor may be significantly improved, and hence the sensitivity to smoke may be made higher than is normally possible. Changes in the properties of the light transmitting surfaces during the working life of the sensor, due to surface contamination within the chamber, may also be at least partly corrected for by using the reference signal as hitherto described for each embodiment. More details of the method by which this proportional mode of operation may be achieved are disclosed in UK Patent application GB 2 273 769 A.

The relatively simple, low cost light extinction sensor described in the foregoing may not be guaranteed to have a signal drift with temperature significantly better than +0.05% per K, because of the stability and matching of the components, and the simple 2 path optical arrangement. This is not stable enough to meet the very demanding requirements of EN54:part 7, intended to ensure that very slowly developing, smouldering fires are detected. However, such fires tend to produce larger, reflective smoke particles, which are readily sensed by the light scattering sensor.

The most important fire detection advantage which derives from the inclusion of the light extinction sensor is in sensing the smoke from more rapidly developing, free burning fires which produce smaller smoke particles, or in some cases black smoke. The light extinction sensor is therefore primarily used as an additional sensor to enhance the performance of the fire detector in just such instances.

The presence of smoke in the chamber increases light scattered in path 11 and the light extinction in path 12, ie the signal from the light scattering sensor tends to increase whilst that from the active path of the light extinction sensor tends to decrease. It is preferred that the signals from the two sensors are correlated within several time domains, using software in the microcomputer.

Within shorter time domains, for example up to 1 hour, the detection of the fire is conditional on the presence of signals from both sensors, consistent with the presence of smoke. However, the point at which a fire is detected, ie the sensitivity of the detector to smoke, is primarily determined by the signal from the light extinction sensor.

Within longer time domains for fire detection, for example from 1 to 12 hours, the software relies increasingly on signals from the optical scatter sensor. The fire detection algorithm employed can be represented in the simplest form by the following logical statements.

IF ((a rapid increase in the scatter signal is sensed) AND (a rapid increase in the extinction signal is sensed) AND (the extinction signal has exceeded a defined threshold)) THEN (a fire is detected).

IF ((a slow increase in the scatter signal is sensed) AND (the scatter signal has exceeded a defined threshold)) THEN (a fire is detected).

In practice, it is preferable that a more complicated fire detection algorithm is employed, in order to cater for the widest possible range of fire types and operational circumstances, as would be obvious to one skilled in the art of signal processing for fire detection. A preferable technique, which would tend to minimise the rate of false alarms, is to demand a high degree of correlation between the temporal pattern of the signals from the two sensors over the shortest operative time periods (eg tens of seconds), steadily decreasing the degree of correlation demanded as the time period increases (eg to 1 hour or more). This technique reflects not only the reducing absolute stability of the light extinction sensor as the timescale increases, but also the observed fact that the nature of smoke from a given fire, and hence the ratio of the scatter signal to the extinction signal, may change over time. A preferred method which permits this form of processing is to generate a series of time averages of the sensor signals using a number of different integration times, and to use differences between these averages to determine rates of signal change within different time domains.

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 smoke sensors. The light emitting sources could be semiconductor light emitting diodes, such as GaAlAs devices widely available from a number of suppliers, which emit efficiently at a wavelength around 880nm. The receiver could comprise a silicon photodiode and suitable amplification and signal capture means. Silicon photodiodes operate efficiently over a corresponding range of wavelengths and are widely available in different forms from a number of suppliers.

The scattering from sub-micron sized particles, such as those found in typical smokes, is known to be strongly dependent on the wavelength of the radiation in the range 500nm to 1000nm. It is not essential that sources 1, 2 and 3 operate at exactly the same wavelength. However, it is preferable that source 2 and source 3 do operate at a similar wavelength, so that efficiency changes relating to active path 12 and reference path 13 of the light extinction sensor are as well matched as possible. In all of the disclosed embodiments the majority of the light scattered due to smoke in path 11 takes place in a predominantly different part of the chamber to the extinction in path 12.

This feature, and the fact that the sensors employ contrasting mechanisms greatly increases the reliability of the fire detector as compared with those using only one sensing mechanism.

It will be further understood that the principle of operation of the embodiments described is not fundamentally dependent on the exact arrangement of the optical components, the details of the electronics circuitry, or on the specification of the components used, and as such could be realised in a variety of ways by one skilled in the art.

For example additional light sources and receivers, possibly working at different wavelengths could be employed to further improve the sensitivity or reliability of the detector. Additional sensors could also be present in the detector, for example a thermal sensor which could further improve the detection of some types of fire. It will also be understood that elements in addition to those described in the foregoing, would be necessary to construct a practical smoke detector, as would be known or obvious to one skilled in the art of smoke detector design. These would be primarily mechanical and electronic elements, to physically mount the detector and interface it to a fire detection and alarm system.

Claims (9)

1. A smoke detector wherein the presence of smoke is sensed by means of light scattered or reflected from smoke particles which ingress into a partly enclosed chamber, characterised in that a second sensor measures the concentration of the smoke within the said chamber by means of the extinction of a beam of light1 and wherein signals from the light scattering sensor and signals from the light extinction sensor are processed within a microcomputer or by similar means, and wherein the signals are correlated within different time domains such that for at least certain types of fire the detection of the fire is conditional on the presence of defined signals from both of the said sensors, but wherein the sensitivity of the detector to smoke is predominantly determined by signals from the light extinction sensor.

2. A detector according to claim 1 wherein the light scattering sensor and the light extinction sensor are physically disposed such that each sensor is influenced by smoke particles in a predominantly different part of the said chamber.

3. A detector according to claim 1 or claim 2 wherein the light scattering sensor and the light extinction sensor make common use of at least one light emitting source or light receiver.

4. A detector according to claim 3 wherein the said sensors in combination consist of three light emitting sources and one light receiver, in conjunction with suitable optical and mechanical components and electronic circuitry, and wherein light emitted from a first source reaches the receiver predominantly by means of scattering or reflection from inner surfaces of the chamber enclosure and by smoke particles if these are present within a part of the chamber, and wherein light emitted by a second source reaches the receiver predominantly via a transmission path in which the light extinction is affected by the presence of smoke, and wherein light emitted by a third source reaches the receiver via a reference path in which the light extinction is predominantly not affected by the presence of smoke.

5. A sensor according to claim 4 wherein the light sources are pulsed light emitting diodes and the ratio of the pulse charges passed through at least two of the sources is accurately controlled by a microcomputer in conjunction with suitable electronic means, so as to control the ratio of the light outputs from these sources.

6. A sensor according to claim 5, wherein the ratio of the pulse charges passed through the second and third sources is controlled such that the signal received from the second source is substantially equal to that received from the third source, and wherein the the ratio of the pulse charges and/or small differences in the received signals are used in order to calculate changes in the light extinction.

7. A sensor according to claim 5, wherein the ratio of the pulse charges passed through either the first and second source, or the first and third source, is controlled such that the signal received from the first source is maintained substantially equal or in a known ratio to that received from either the second or the third source, and wherein the ratio of the pulse charges and/or differences in the received signals are used in order to assist in calibrating the sensitivity of the light scattering sensor.

8. A detector according to any one of claims 1 to 7 contained within a mechanical enclosure appropriate for use in detecting smoke resulting from a fire, and into which the ambient atmosphere may ingress either by natural or forced convection, or by forced flow.

9. A detector according to any one of claims 3 to 7 substantially as herein described with reference to figure 1, figure 2, figure 3, or figure 4 of the accompanying drawing.

9. A detector according to any one of claims 4 to 7 substantially as herein described with reference to figure 1, figure 2, figure 3, or figure 4 of the accompanying drawing.

Amendments to the claims have been filed as follows CLAIMS 1. A smoke detector wherein the presence of smoke which may ingress into a partly enclosed chamber is sensed by a first sensor by means of light scattered or reflected from the smoke particles, and by a second sensor by means of the extinction of a beam of light, characterised in that the said sensors in combination employ three or more individually controlled light emitting sources and one light receiver, and wherein signals from the two sensors are processed within a microcomputer or by similar means and are correlated within different time domains, such that for at least some fire types the detection of the fire is conditional on the presence of defined signals from both sensors, but the sensitivity of the detector to smoke is predominantly determined by signals from the light extinction sensor.

2. A detector according to claim 1 wherein the algorithm for the detection of a fire may be represented by the following logical statement: IF ((a rapid increase in the light scatter signal is sensed) AND (a rapid increase in the light extinction signal is sensed) AND (the light extinction signal has exceeded a defined threshold)) OR IF ((a slow increase in the light scatter signal is sensed) AND (the light scatter signal has exceeded a defined threshold)) THEN (a fire is detected).

3. A detector according to claim 1 wherein the light scattering sensor and the light extinction sensor are physically disposed such that each sensor is influenced by smoke particles in a predominantly different part of the said chamber.

4. A detector according to claim 1 or claim 3 wherein the said sensors in combination consist of three light emitting sources and one light receiver, in conjunction with suitable optical and mechanical components and electronic circuitry, and wherein light emitted from a first source reaches the receiver predominantly by means of scattering or reflection from inner surfaces of the chamber enclosure and by smoke particles if these are present within a part of the chamber, and wherein light emitted by a second source reaches the receiver predominantly via a transmission path in which the light extinction is affected by the presence of smoke, and wherein light emitted by a third source reaches the receiver via a reference path in which the light extinction is predominantly unaffected by the presence of smoke.

5. A sensor according to claim 4 wherein the light sources are light emitting diodes through which an electrical current is pulsed, and wherein the ratio of the charges passed through at least any two of the sources is accurately controlled by a microcomputer in conjunction with suitable electronic means, so as to accurately control the ratio of the pulsed light outputs from these sources.

6. A sensor according to claim 5, wherein the ratio of the charges passed through the second source and the third source is controlled such that the signal received from the second source is substantially equal to that received from the third source, and wherein the the ratio of the charges and/or small differences in the received signals are used in order to calculate changes in the light extinction.

7. A sensor according to claim 5, wherein the ratio of the charges passed through the first source and the third source is controlled such that the signal received from the first source is maintained substantially equal or in a known ratio to that received from the third source, and wherein the ratio of the charges and/or differences in the received signals are used in order to assist in calibrating the sensitivity of the light scattering sensor.

8. A detector according to any one of claims 1 to 7 contained within a mechanical enclosure appropriate for use in detecting smoke resulting from a fire, and into which the ambient atmosphere may ingress either by natural or forced convection, or by forced flow.