GB2586283A

GB2586283A - Optical smoke detector

Info

Publication number: GB2586283A
Application number: GB1911810.8A
Authority: GB
Inventors: Martyn Crick Jason; Andrew Lenn Hugh; Watson Philip
Original assignee: Apollo Fire Detectors Ltd
Current assignee: Apollo Fire Detectors Ltd
Priority date: 2019-08-16
Filing date: 2019-08-16
Publication date: 2021-02-17
Anticipated expiration: 2039-08-16
Also published as: WO2021032942A1; GB201911810D0; GB2586283B

Abstract

A smoke detector comprises a base 130, housing 140, and light directing means 150. A light source 160 directs light through openings 230a-c via a prism 270 and lens 280 across the field of view of sensor 170. Baffles 250a, 250b are provided in the line of sight of the light source 160 and the sensor 170 and are arranged to provide a quiescent light directing means through the housing 140. The quiescent light directing means is configured to direct quiescent light indirectly towards sensor 170 via a quiescent light path (165,fig.4 & 165,fig.6). A vertical protrusion 190 is provided to prevent a portion of the light beam emitted from entering the region directly above the field of view of the sensor 170. This obstruction of light makes the sensor less vulnerable to false alarms caused by foreign bodies around the opening 230c. The quiescent light may be used to confirm operation of the detector and determine a level of contamination of the sensor directly across or on the sensor. The gain or baseline of the sensor may be adjusted to compensate for drift due to contamination of the sensor.

Description

Optical Smoke Detector

Field of the Invention

The present invention concerns smoke detectors. In particular, the present invention 5 concerns optical smoke detectors including a light source and sensor for detecting light scattered by smoke particles in the detector.

Background of the Invention

Optical smoke detectors can take many forms but typically fall into two categories. Some optical smoke detectors operate on the principle of light beam obscuration, whereby smoke particles reduce the amount of light detected by a sensor as a consequence of the smoke particles blocking the light via either light absorbance or light scattering, thereby triggering an alarm. Other forms of optical smoke detectors operate on the principle of light beam sensing, whereby the smoke-induced light scattering directs light toward an otherwise unilluminated sensor, to be sensed, thereby setting off an alarm.

In both modes of operation, the smoke detectors usually contain a light source of infrared, visible or ultraviolet light such as a light emitting diode (LED), an incandescent light bulb or, another form of light emitting means. The smoke detector will also contain a sensor that can convert a light signal into an electrical signal e.g. a photodiode, as well as other optical components and optical directing means including, but not limited to, lenses and reflective surfaces.

Many challenges in the design of optical smoke detectors exist. An important consideration 25 is the conflict between the requirements to have a detector sensitive enough to detect the relatively low intensity of smoke scattered light, against the requirement to try to avoid false detections of smoke caused by other non-smoke events.

With reference to the low intensity of smoke scattered light, where the method of 30 operation is light beam sensing, the light source and sensor are typically arranged to function such that the source and sensor's respective fields of emission and view intersect at a point within a chamber or cavity, as opposed to there being a direct line of view between the source and sensor. However, in such arrangements there is a risk of insufficient scattering intensity, in which the detector fails to detect light being scattered by the smoke as it is of too low an intensity.

In particular, light scattered on to a sensor from a light beam by smoke particles is typically a very small fraction of the total light emitted by the source. Although dependent on smoke density, a typical value would be of the order of 0.001 % at the alarm point of a general-purpose fire detector. Reliably sensing such a small signal is difficult and thus it is desirable to design the smoke detector so as to maximise the amount of scattered light falling on the sensor.

A known solution in the art is to use a more intense light source. However this has its own disadvantages. Firstly, the inclusion of a light source that emits more energy means the smoke detector will itself operate at a higher power thus making the device more energy demanding. Secondly, the lifespan of the light source will be reduced. Thirdly, if the light source is too intense then the quiescent (background) light in the detector, which is present by virtue of there being a light source in the detector chamber, could be detected by the sensor at an intensity that would trigger the alarm, causing a false alarm event.

Methods and means of increasing the amount of scattered light falling on the sensor have previously been disclosed in GB2404731 B wherein an enclosure, within the smoke detector chamber is described that directs a light beam across a field of view of the sensor within the smoke detector using beam forming apparatus such as lenses. Smoke detections in this set up occur when light from the light source is scattered onto the sensor, by smoke.

With reference to the possibility of false detections as a result of the sensitivity of the detector, GB2404731 B further teaches a method and design for reducing the effect of reflected light from the light source through the use of a light source with a narrow beam having small divergence. The reflection from a narrow beam can be more easily controlled within a small optical chamber, thus allowing the detector size to be kept small. GB2404731 B uses a small light source (preferably in the form of a LED chip device) in Surface-Mount Device (SMD) technology, in association with a high quality lens of relatively-short focal length. In that case, the divergence of the light beam is fixed by the ratio of the lens' focal length to the diameter of the light source.

Methods and means in accordance with 6B2404731 B also have the additional advantage of ensuring that an amount of quiescent light remains in the chamber of the smoke detector. This is desirable, as quiescent light in the chamber that can be detected by the sensor is known to have its own advantages. For example, the presence of quiescent light provides a non-zero background reading allowing detection of whether the sensor is functioning correctly. Secondly, the quiescent light can provide a rudimentary means of assessing and measuring the level of contamination, such as dust, dirt and other debris, in the smoke detector. The ability to measure contamination levels, even to a rudimentary level is of benefit, as contamination is a known source of many false alarm events (false detections).

False detections from non-smoke events can arise due to conditions both external and internal to the smoke detector. Externally, changes in ambient light in the local environment of the smoke detector can lead to false detections, especially in instances where the design of the detector comprises a sensor of high sensitivity.. Internally, false detections can occur as a consequence of stray light events when light from the light source is reflected off contamination in the smoke detector, and reaches the sensor. Contamination can comprise dust, debris, small insects, or any other form of non-smoke derived object or substance that can enter the smoke detector to cause reflection and/or non-smoke scattering events.

Aspects of the prior art such as GB2531495 B attempt to address some of these problems through the inclusion of a light trap in the chamber of the smoke detector. The light trap being designed to absorb light in the chamber which has passed the field of view of the sensor. The use of a light trap in this form has the advantage of reducing, and in some instances, preventing stray light events from light reflected in, and around, the chamber. However the light trap has been found to create an unexpected disadvantage, in that it also reduces, or in some instances completely prevents, the presence of quiescent light in the chamber, which is known to have benefits associated with the performance of the smoke detector as outlined previously. Consequently, there still remains a need for smoke detectors of improved accuracy and performance which are designed so as to remove as much stray light from the chamber of the detector as possible, while maintaining quiescent light to benefit from its advantageous functionality, as outlined previously.

Aspects of the present invention address these issues and provide a solution to this aim.

Summary of the Invention

Aspects of the invention are defined by the accompanying claims.

According to a first aspect of the invention, a smoke detector is provided comprising: a light source; a sensor; a light directing means for directing light from the light source across a field of view of the sensor, the light directing means being configured to form a real image of the light source in the field of view of the sensor and such that in use smoke in the field of view of the sensor scatters at least some of the light in the field of view of the sensor onto the sensor; and a housing unit, the light source and the sensor being within the housing unit. The housing unit is configured to provide an indirect light path from the light source to the sensor such that in use a quiescent light is provided to the sensor.

The at least one baffle is preferably in the direct line of sight light path between the light source and the sensor such that direct illumination of the sensor by the light source is prevented. The indirect light path is such that light travels around the at least one baffle.

The smoke detector preferably has a protrusion located between the light directing means and the field of view of the sensor, configured to prevent light from the light directing means passing proximate, and directly above the sensor.

The light source and the sensor are preferably fixed to a base. The base preferably has 30 means for the housing unit to be secured to the base and/or the base is formed integrally with the housing.

The base may be a printed circuit board. The light source and the sensor may be surface-mount devices on the printed circuit board.

The smoke detector of any preceding claim, wherein the sensor is located at a first end of 5 the housing and the light source is located at a second end of the housing.

The light directing means is preferably located adjacent to the light source at the second end of the housing, and comprises a light-reflective surface, a prism and/or a beam-forming lens configured to direct the light beam across the field of view of the sensor.

The light directing means preferably has a roof having an aperture configured to direct light across the field of view of the sensor.

Preferably, signal-processing means are provided for detecting, based on a sensor input: (ii) a presence of smoke to trigger an alarm, and, separately, (ii) a build-up of dirt or contamination.

Brief Description of the Drawings

There now follows, by way of example only, a detailed description of preferred 20 embodiments of the present invention, with reference to the figures identified below. Figure 1 shows a plan view of an aspect of a smoke detector; Figure 2 shows a side view of the aspect of the smoke detector shown in Figure 1; Figure 3 shows a plan view of an aspect of a smoke detector; Figure 4 shows a side view of the aspect of the smoke detector shown in Figure 3; Figure 5 shows a side view of an aspect of a smoke detector; Figure 6 shows a plan view of the aspect of the smoke detector of Figure 5. Figure 7 shows a processing unit of the smoke detector.

Detailed description

In the following description, functionally similar parts carry the same reference numerals between figures. Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings. In this description 'upper', 'lower', 'top', bottom' and similar terms are defined with reference to the orientation of the smoke detector as shown in the Figures.

It will be apparent to those of ordinary skill in the art that various modifications and variations can be made in the present invention without departing from the scope of the invention. For instance, features described as part of one embodiment can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents Figure 1 shows a plan view of an aspect of a smoke detector according to the present invention, and Figure 2 shows a side view of the aspect of the smoke detector shown in Figure 1 along line A-A.

In embodiments of the present invention the smoke detector comprises a base 130, a housing unit 140, and a light directing means 150. The light directing means 150 is located adjacent to the housing unit 140. In particular, the light directing means 150 may be located at one end of the housing unit 140.

In embodiments of the present invention, the smoked detector is configured to advantageously provide a level of quiescent light onto the sensor 170 whilst in use. In particular, in existing smoke detectors, and as will be described further below, there is a light source which provides a light beam into a field of view of the sensor such that the light beam may be scattered onto the sensor by smoke which enters the smoke detector. In embodiments of the present invention, various advantageous configurations of the smoke detector are provided which harness the presence of the light source 160, which primarily provides the light beam into a field of view of the sensor 170 as described above, to additionally provide a quiescent level of light onto the sensor 170. This quiescent level of light, being sensed by the sensor 170 whilst the light source 160 is on, and the smoke detector is in use. The presence of this quiescent light provides three advantages. Firstly, it provides a non-zero background reading which allows detection of whether the sensor is functioning correctly. Secondly, the quiescent light provides a means of assessing and measuring the level of contamination on the sensor which may have a deleterious impact on the smoke detector's functionality, and/or smoke detection capabilities. Importantly, and advantageously, the contamination level being measured is the level of contamination directly across or on the sensor, as opposed to within the smoke detector as a whole (including the chamber). In this way, the present invention advantageously provides a smoke detector of improved accuracy, performance and reliability, which is better able to detect the presence of smoke and prevent false alarm events.

Compared with the prior art, the smoke detector of the present embodiment, possesses the additional advantage of excluding from consideration, when assessing and measuring contamination levels, contamination build-up in the chamber, which is of a lesser concern than contamination directly on the sensing surface of the sensor. This may result in the reduction in the frequency of alerts that the smoke detector needs to be cleaned/cleared of contamination.

Further advantageously, this improved smoke detector is provided in a manner that does not require any more light sources and thereby does not require any further energy consumption above the single light source 160 already present.

In a preferred embodiment the base 130 is a printed circuit board (PCB). The circuit board comprises a light source 160 and a sensor 170 for sensing light. The light source comprises one of a light emitting diode (LED), a surface-mount device LED (SMD LED), a SMD LED chip package, a laser diode, an evanescent light bulb or another light emitting device. The sensor comprises one of a photodiode, a photo-voltaic cell, a photo-emissive cell, a photoconductive cell, a photo-junction cell, or another light detecting device. It will be apparent to those of ordinary skill in the art that various other forms of light source and sensor, not here listed or described, can be used in an optical smoke detector.

In a preferred embodiment the light source 160, as shown in Figure 2, may be a SMD LED. The SMD LED device is a chip device in which the LED is mounted on a small insulating substrate and is over-moulded with a small beam-forming lens (not shown in Figure 2). The main optical axis of the device is normal to the surface of the circuit board upon which it is mounted. Hence, as shown in Figure 2, the light that is emitted from the light source 160 in use will enter the light directing means 150 located adjacent to the light source 160.

In the instance where the light source 160 is the SMD LED, as the SMD LED lens is very small, typically 1.8 mm in diameter, a supplementary beam-forming lens with for instance a focal length of only 8 mm will give a beam of the required diameter and divergence. The overall assembly is advantageously therefore only one third of the size that is possible using a 5 mm diameter LED. The fact that the LED can be placed by machine is also a great advantage when compared with through-hole devices that must be fitted manually.

The circuit board may further comprise one or more holes (not shown in Figure 2) configured to provide the housing unit 140 securing means to affix the housing unit 140 to the circuit board 130 over the light source 160 and the sensor 170.

The smoke detector as shown in Figure 2 further comprises a housing unit 140 configured to provide quiescent light directing means. The housing unit 140 comprises an opening through which a light source 160 can be inserted (not shown in Figure 2) and an opening through which a sensor can be inserted (not shown in Figure 2). The housing unit 140 further comprises one or more openings 230 to allow the light directing means 150 to form a real image of the light source across the field of view of the sensor 170.

The housing unit 140 may further comprise one or more resilient legs (not shown in Figure 2), wherein the one or more resilient legs each fitting into a respective hole in the base/printed circuit board for retaining the housing unit on the base/board.

In a preferred embodiment (partly shown in Figure 2) the housing unit 140 further comprises two baffles 250, in the line of sight of the light source 160 and the sensor 170 configured to provide a quiescent light directing means. The quiescent light directing means configured to direct quiescent light indirectly toward the sensor. However, it is equally envisaged that the housing unit 140 can have one or more than two baffles 250 in the direct line of sight of the light source 160 configured to direct quiescent light indirectly toward the sensor.

In a preferred embodiment the one or more baffles 250 in the housing unit 140 are positioned (as shown in Figure 2) so as to prevent a beam, or beams of light from the light source, propagating directly (via a direct line of sight pathway) toward the sensor, through the housing unit. The presence of the baffles thereby provide the advantageous effect of preventing erroneous detection of smoke that could occur if a light beam were to travel across, and very proximate to, the sensing surface of the sensor.

Further advantageously, the quiescent light directing means is configured to provide a means for quiescent light to be incident on the sensor at multiple angles. This has the advantage of reducing the effect that contamination on the surface of the sensor will have on the amount of quiescent light being sensed by the sensor.

The housing unit 140 further comprises a vertical protrusion 190 with an extending form, perpendicular to the top surface of the housing unit 140. The vertical protrusion being partially in the line of sight of the light beam emitted from the light directing means 150, and configured to obstruct / prevent a portion of the light beam from the light directing means, passing through and illuminating, the region directly above the field of view of the sensor. The obstruction of light passing directly above the field of view of the sensor has the advantageous effect of making the sensor less susceptible to false alarms from erroneous scattering events caused by small insects or other foreign bodies in the vicinity of the top of the housing unit, especially around the opening 230c through which light is scattered to the sensor.

The light directing means 150 as shown in Figure 2 further comprises one or more openings 230 providing light directing means for forming a real image of the light source across the field of view of the sensor.

In a preferred embodiment as shown in Figure 2 the light directing means 150 further comprises at least one of a light reflecting surface (not shown in Figure 2) , a beam-forming lens 280 and, a prism 270. Both the lens 280 and the prism 270 may be formed of at least one of glass, plastic, polycarbonate, polymer, or any other optically suitable material that is apparent to those of ordinary skill in the art. The lens 280 may be formed to be one of biconvex, biconcave, piano-convex, piano-concave (shown in Figure 2), meniscus or any other beam forming design suitable for beam forming and directing, apparent to those of ordinary skill in the art. The lens 280 may be formed as part of the prism 290 or, formed separately.

In operation, the light source 160 emits light into the light directing means 150, whereby the light directing means 150 directs the light in a substantially perpendicular direction such that it enters an area of the smoke detector within the field of view of the sensor 170. As such, the light directing means 150 directs light from the light source 160 across the field of view of the sensor 170. When smoke enters the smoke detector, the light in the field of view of the sensor 170 is scattered by the presence of the smoke, and a least a portion of the scattered light in the field of view of the sensor enters the sensor 170 and is detected as an increase in the amount of light entering the sensor 170. The detection of the increased amount of light is processed by electronic circuitry, for instance located on the base 130, whereby the electronic circuitry is configured to output a signal indicating the detection of smoke, whereby for instance such a signal is sent to an audible alarm such that the presence of smoke is indicated to people in the vicinity of the smoke detector.

Figure 3 shows a plan view of an aspect of a smoke detector. The FF cross-section of the smoke detector as outlined in Figure 3 is depicted in Figure 4. As in previous Figure 2, the quiescent light directing means comprises a base 130, and a housing unit 140, with the light directing means 150 configured to be overlaid atop of the housing unit 140, for instance as a removable element, or is integrally formed with the housing unit 140. Figure 4 further shows a prospective quiescent light path 165 emanating from the light source 160. As can be seen, the prospective light path 165 is an indirect light path between the light source 160 and the sensor 170. In operation the quiescent light passes along the length of the housing unit 140, around the one or more baffles 250, and is incident on the sensor 170. To assist in the directing of the quiescent light to the sensor, the quiescent light directing means may further comprise one or more reflective surfaces 200.

As previously disclosed, the one or more baffles 250 in the housing unit 140 provide a means for quiescent light to be incident on the sensor at multiple angles. This has the advantage of reducing the effect that contamination on the surface of the sensor will have on the amount of quiescent light being sensed by the sensor.

The housing unit 140 further comprises a vertical protrusion 190 with an extending form, and perpendicular to, the top surface of the housing unit 140. The vertical protrusion being partially in the line of sight of the light beam emitted from the light directing means 150, and configured to obstruct / prevent a portion of the light beam from the light directing means, passing through and illuminating, the region directly above the field of view of the sensor. The advantages of the obstruction of this portion of the light as described previously.

Figure 5 shows a side view of an aspect of a smoke detector, and Figure 6 shows a plan 15 view of the aspect of the smoke detector along the CC cross section of Figure 5.

Figure 6 provides a top down view into the housing unit 140 to show the quiescent light directing means. The quiescent light directing means comprises a base 130, and a housing unit 140, the light directing means 150 configured to be overlaid atop of the housing unit 20 140, for instance as a removable element, or is integrally formed with the housing unit 140.

The housing comprises an opening 210 through which a light source 160 can be overlaid and an opening 220 through which a sensor can be overlaid.

In operation the smoke detector comprises a quiescent light path 165 from the light source 160 to the sensor 170. The quiescent light passing along the length of the housing unit 140, around the one or more baffles 250, which may optionally provide quiescent light directing means.

The quiescent light directing means can further comprise one or more reflective surfaces 200 for directing the quiescent light across the field of view of the sensor to provide quiescent light whilst the smoke detector is in use as previously described.

Referring to Figure 7 the smoke detector preferably comprises signal processing means 700 for detecting smoke to trigger an alarm and to measure a level of contamination on the sensor. The signal processing means is shown as having sensor input 710 from the sensor 170 and outputs 720, 730 and 740.

The signal processing means further comprises electronic circuitry whereby the electronic circuitry and processing means are configured to output one of a number of signals, based on the sensor input signal 710, indicating one or more of (i) detecting smoke thereby triggering an alarm 720, and (ii) measuring 730, 740, a level of contamination on the sensor.

An alarm signal 720 is generated when there are fluctuations in sensor input signal 710 over very short periods and/or over certain thresholds.

Measuring of a level of contamination on the sensor based on the sensor input signal 710 comprises measuring the level of contamination on the sensor in a time period r1 to identify drift in the sensor input signal caused by the gradual build up of contamination on the sensor 730. This time period rj is longer than that used for detecting very short term fluctuations that trigger an alarm. Drift due to a build up of contamination on the sensor is detected and identified as either one of (i) a reduction in sensitivity of the sensor caused by a reduction in the amount of quiescent light being detected by the sensor due to an obfuscation of quiescent light entering the sensor by contamination or (ii) an increase in the amount of quiescent light being detected by the sensor due to additional reflection events directing additional quiescent light into the sensor, caused by the quiescent light reflecting from contamination on the sensor.

When drift is detected, a sensor baseline and/or gain adjustment can be made to offset the effects of such drift. Signal 730 could be entirely internal to the processor 700.

If the level of contamination on the sensor indicates excessive drift in the sensor input signal 710 caused by the gradual build up of contamination on the sensor 170, a further output signal 740 is generated, indicating a need to clean the sensor 170.

Long-term drift due to a build up of contamination on the sensor is detected and identified when a reduction in sensor sensitivity reaches a threshold (e.g. over multiple short-term drifts of time periods t1 or over a longer time period T2).

Signal 740 triggers a separate response e.g. LED light, indicative of a need for the smoke detector to be cleaned to remove contamination.

Thus, the signal processing means is preferably configured to sense long-term drift in the output signal caused by the presence of contamination on the sensor that has built up over time, whereby the long-term drift sensing is indicative of a contamination level above a threshold, wherein the contamination level above a threshold is indicative of the need for the smoke detector to be cleaned.

The embodiments described above are illustrative of, rather than limiting to, the present invention. Alternative embodiments apparent on reading the above description may nevertheless fall within the scope of the invention.

Claims

Claims 1. A smoke detector comprising: a light source; a sensor; a light directing means for directing light from the light source across a field of view of the sensor, the light directing means being configured to form a real image of the light source in the field of view of the sensor and such that in use smoke in the field of view of the sensor scatters at least some of the light in the field of view of the sensor onto the sensor; a housing unit, the light source and the sensor being within the housing unit wherein the housing unit is configured to provide an indirect light path from the light source to the sensor such that in use a quiescent light is provided to the sensor.
2. The smoke detector of claim 1, further comprising at least one baffle in the direct line of sight light path between the light source and the sensor such that direct illumination of the sensor by the light source is prevented.
3. The smoke detector of claim 2, wherein the indirect light path is such that light 20 travels around the at least one baffle.
4. The smoke detector of claim 3, further comprising a protrusion located between the light directing means and the field of view of the sensor, wherein the protrusion is configured to prevent light from the light directing means passing proximate and directly 25 above the sensor
5. The smoke detector of any preceding claim, wherein the light source and the sensor are fixed to a base, the base having means for the housing unit to be secured to the base and/or the base being formed integrally with the housing
6. The smoke detector of any preceding claim, wherein the base is a printed circuit board, the light source and the sensor being surface-mount devices.
7. The smoke detector of any preceding claim, wherein the sensor is located at a first end of the housing and the light source is located at a second end of the housing.
8. The smoked detector of claim 7, wherein the light directing means is located adjacent to the light source at the second end of the housing, and comprises a light-reflective surface, a prism, and/or a beam-forming lens configured to direct the light beam across the field of view of the sensor.
9. The smoke detector of claim 8 wherein the beam-forming lens forms part of the prism.
10. The smoke detector of claim 8 or 9, wherein the light directing means further comprises a roof having an aperture, the aperture configured to direct light across the field 15 of view of the sensor.
11. The smoke detector of any preceding claim wherein the smoke detector further comprises a signal-processing means for detecting, based on a sensor input: a presence of smoke to trigger an alarm, and, separately, (ii) a build-up of dirt or contamination.