Smoke detector for aspiration smoke detector system

Abstract

A smoke detector (100) for use with an aspiration smoke detector (ASD) is described. The smoke detector includes a light source (104) configured to emit a beam of light (108); a reflector (102) including an aperture (110), the aperture aligned with a direction of propagation of the beam of light when no scattering occurs; and a photodetector (106); the reflector configured to reflect light scattered from the beam of light received at the reflector to a single focal point; and the photodetector located at the single focal point. An aspiration smoke detector (ASD) system (2) includes the smoke detector and a method of detecting smoke using the smoke detector.

Claims

1. A smoke detector for use with an aspiration smoke detector (ASD) system, the smoke detector comprising: a light source configured to emit a beam of light; a reflector comprising an aperture, wherein the aperture is aligned with a direction of propagation of the beam of light when no scattering occurs; and a photodetector; wherein the reflector is configured to reflect light scattered from the beam of light received at the reflector to a single focal point; and wherein the photodetector is located at the single focal point; wherein the light source is configured to alternate a wavelength of the beam of light between a first wavelength and a second wavelength; and wherein a change in wavelength of the beam of light between the first wavelength and the second wavelength is a discrete transition.

2. A smoke detector as claimed in claim 1, wherein the reflector is a spherical mirror or a parabolic mirror.

3. A smoke detector as claimed in claim 1, wherein the light source is configured such that a divergence of the beam of light is such that the beam of light passes entirely through the aperture when no scattering occurs.

4. A smoke detector as claimed in claim 1, wherein the photodetector is configured to be synchronised with the light source such that it can be determined which wavelength of emitted light the light detected by the photodetector corresponds to.

5. A smoke detector as claimed in claim 4, wherein the smoke detector further comprises a controller in communication with the light source and the photodetector, and wherein the controller is configured to synchronise the light source and the photodetector.

6. A smoke detector as claimed in claim 1, wherein the smoke detector further comprises a light trap, and wherein the light trap is configured to absorb and/or capture any light passing through the aperture.

7. An aspiration smoke detector (ASD) system, the ASD system comprising: the smoke detector of claim 1; wherein the ASD system is configured to intake a sample and pass the sample into the smoke detector.

8. A method of detecting smoke using a smoke detector, wherein the smoke detector is the smoke detector as claimed in claim 1.

9. A method of detecting smoke using a smoke detector for an aspiration smoke detector (ASD) system, the method comprising: providing the smoke detector, wherein the smoke detector comprises a detection chamber with a reflector that is configured to reflect light received at the reflector to a single focal point; passing a sample into the detection chamber; passing a beam of light into the detection chamber; reflecting light scattered by the sample received at the reflector to the single focal point; and detecting, at the single focal point, the scattered light; alternating a wavelength of the beam of light between a first wavelength and a second wavelength; and wherein a change in wavelength of the beam of light between the first wavelength and the second wavelength is a discrete transition.

10. A method of detecting smoke as claimed in claim 9, wherein the reflector is a spherical mirror or a parabolic mirror.

11. A method as claimed in claim 9, further comprising the steps of: synchronising the detection of the scattered light with the alternating of the wavelength of the beam of light; and determining which wavelength of the beam of light emitted the detected light corresponds to.

12. A method as claimed in claim 9, further comprising the step of: determining a size of a smoke particle based on the scattered light detected.

13. A method as claimed in claim 9, further comprising the step of: absorbing and/or capturing any light not scattered.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Certain example embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which:

(2) FIG. 1 shows a schematic diagram of an ASD system; and

(3) FIG. 2 shows a cross-section of a smoke detector.

DETAILED DESCRIPTION OF THE INVENTION

(4) With reference to FIG. 1, an ASD system 2 comprises a fan 4, a filter 6, a smoke detector 100, an inlet 8, an exhaust 12, and a plurality of sampling points 16 dispersed along a network of pipes 18.

(5) The network of pipes 18 covers an area/environment to be monitored for smoke generation. Dispersed along the network of pipes are a plurality of sampling points 16. Whilst three sampling points 16A, 16B, 16C are shown in FIG. 1 any number of sampling points could be implemented into the ASD system 2 as appropriate. The sampling points 16 comprise an inlet, for example a small hole, which allows air to flow into the network of pipes 18 and be drawn into the ASD system 2. To draw the air in to be sampled, the ASD system 2 also comprises the fan 4. The fan 4 creates an airflow which leads to the intake of air and also exhausts air and/or samples 14 from the ASD system once no longer needed.

(6) The fan 4 shown in FIG. 1 diverts a portion of the air drawn in to use as a sample 14, to determine if smoke is present in the environment. The remaining air may be immediately exhausted from the ASD system 2 via the exhaust pipe 12. Whilst FIG. 1 shows a sample of air 14 being diverted from the air drawn in, in some cases all the air drawn in may be analysed for the presence of smoke.

(7) The sample 14 is passed through a filter 6. The filter 4 is configured to remove dust and/or moisture from the ASD system 2 before being passed to the smoke detector 100. The removal of dust and/or moisture (i.e. water droplets) may prevent particles that do not constitute smoke entering the smoke detector 100 and potentially generating a false alarm. Similarly, filtering the sample may protect sensitive equipment housed in the smoke detector 100 from being damaged over time. Once filtered, the sample 14 is passed into the smoke detector 100.

(8) Turning to FIG. 2, the smoke detector 100 comprises a reflector 102, a light source 104, a photodetector 106, an aperture 110, a light trap 112 and a plenum 114. The general function of the smoke detector 100 will now be briefly described.

(9) The sample 14 is directed into the plenum 114. The light source 104 emits a beam of light 108, the beam of light 108 passing through the sample 14 occupying the plenum 114. Whilst some of the light will pass through the aperture 110 in the reflector 102 and be evacuated from the plenum 114, some of the light may be scattered if particles (e.g. smoke) are present in the plenum 114. The scattered light is collected by the reflector 102. The reflector is configured such that any light it reflects will be focused to a single focal point. The single focal point is known, and the photodetector 106 is located at the single focal point such that is will receive any scattered light which is reflected by the reflector 102. The photodetector 106 then measures an intensity of light received at the photodetector 106, such that it may be determined whether or not particulate and/or smoke is present in the sample 14.

(10) The mechanisms by which light is scattered by particulate and/or smoke in the sample 14 will be readily understood by the skilled person, so will not be discussed in detail. The main source of scattering of the beam of light 108 is Rayleigh scattering, which is dependent on the wavelength of light scattered and the size of the particle which scatters the light.

(11) The light source 104 emits the beam of light 108 through the plenum 114. The light source 104 shown is a single light source 104. However, a plurality of light sources 104 may be used in the smoke detector 100, if required. The light source 104 may be an LED, laser diode or laser. The beam of light 108 is aligned with the aperture 110, such that when no scattering occurs (e.g. when there are clean conditions in the plenum 114) the beam of light 108 passes fully out the plenum 114. To prevent any light not scattered from returning into the plenum 114, once the beam of light 108 has passed through the aperture 110 it is configured to be incident on the light trap 112. The light trap 112 absorbs and/or captures the light incident on it, preventing the light from returning to the plenum 114. This may aid in reducing a background of light detected by the photodetector 106.

(12) To further ensure that when no scattering occurs a maximum, and preferably an entirety, of the beam of light 108 passes through the aperture 110, the beam of light 108 emitted by the light source 104 may be configured such that it is collimated for the path length it travels through the plenum 114. In other words, the divergence of the beam of light 108 may be minimised such that a diameter of the beam spot of the beam of light 108 at the aperture 110, is smaller than a diameter of the aperture 110 itself.

(13) When scattering of the beam of light 108 does occur, at least some (if not a majority) of the scattered light is collected by the reflector 102. The reflector 102 reflects the scattered light to a single focal point, increasing the efficiency of detection at the photodetector 106 which is located at the single focal point. This beneficially means that the light source 104 may be of a lower power and/or produce a beam of light 108 of lower initial intensity and/or a detection may be made when less scattering occurs (e.g. when the concentration of particles in the plenum 114 is very low). In the case of FIG. 2 a spherical mirror is shown. However, any reflective member which is configured to focus light to a single focal point may be used as appropriate. This may include, but is not limited to, other curved reflectors 102 such as parabolic mirrors.

(14) The focal point of the reflector 102, especially for a curved reflector such as a spherical or parabolic mirror, will often lie on a plane of symmetry of the curvature of the reflector 102. As such the light source 102 should not be aligned such that the beam of light 108 is shone through the single focal point when no scattering occurs. Thus, the light source 104 should ideally be located to the side of the single focal point, as shown in FIG. 2, in such a way that the light path of the scattered light reflected by the reflector 102 is not obstructed and/or interfered with, or at least minimised.

(15) The light source 104 may emit a beam of light 108 of a single wavelength, or may be capable of emitting beams of light 108 of different wavelengths. For example, the light source 104 may emit a beam of light 108 of a first wavelength for a period of time T.sub.1. The light source 104 may then emit a beam of light 108 of a second wavelength for a period of time T.sub.2. T.sub.1 and T.sub.2 may be the same length of time, or may differ. The light source 104 may alternate between the two wavelengths cyclically, or as required. The first wavelength may consist of generally red/IR light of wavelength 620 nm≤λ.sub.1≤1000 nm, whilst the second wavelength may consist of generally blue/UV light of wavelength 100 nm≤λ.sub.2≤490 nm.

(16) As will be readily understood by the skilled person, particles of different sizes scatter wavelengths of different lengths differently. For example, under a Rayleigh scattering regime the intensity I of the scattered light will be governed by the following relationship:

(17) $\begin{array}{cc}I\alpha \frac{r^6}{{\lambda }^4}& \left(1\right)\end{array}$

(18) wherein r is the radius of the particle and λ is the wavelength of the light scattered. As such, larger particles will generate a higher intensity of scattering for the same wavelength, whilst shorter wavelengths will experience more scattering than a longer wavelength for the same particle size.

(19) Thus, by knowing an expected scattering relationship for particles of different sizes, the size of the particle may be determined based on how light of different wavelengths is scattered in the smoke detector 100. As such the use of a light source 104 emitting light of two different wavelengths may allow the smoke detector 100 to be able to discriminate between particles of different sizes detected in the sample 14.

(20) Further, the photodetector 106 may be synchronised with the light source 104 such that when the light source 104 emits the beam of light 108 of the first wavelength, the photodetector 106 matches its measurements to the first wavelength. When the light source 104 emits the beam of light 108 of the second wavelength, the photodetector 108 then matches its measurements to the second wavelength. By synchronising the measurements of the photodetector 106 (i.e. gating the measurement window of the photodetector 106 accordingly) to the alternation of the wavelength of the beam of light 108 emitted by the light source 104, the intensity of scattered light to each wavelength may be clearly matched and thus the analysis of the sample improved.

(21) The difference of the intensity of scattered light related to the differing particle sizes and/or differing wavelengths may be slight. Thus, by utilising the reflector 102 in combination with the light source 104 emitting beams of light 108 of differing wavelengths the overall ability of the smoke detector 100 to detect and discriminate smoke from various sources, such as cooking sources and/or nuisance sources may be improved.

(22) To maintain synchronicity between the photodetector 106 and the light source 104, a controller (not shown) may be in communication with the components. The controller may be a ‘common controller’. The controller may receive information from the light source and the photodetector such that the reading from the photodetector can be synchronised (i.e. matched) with the light source that caused that reading.