GB2259761A

GB2259761A - Smoke detector

Info

Publication number: GB2259761A
Application number: GB9119942A
Authority: GB
Inventors: Peter Varney; Frederick John James; David Nicholas Ball
Original assignee: Kidde Graviner Ltd
Current assignee: Kidde Graviner Ltd
Priority date: 1991-09-18
Filing date: 1991-09-18
Publication date: 1993-03-24
Anticipated expiration: 2011-09-18
Also published as: EP0604502A1; WO1993006462A1; GB2259761B; GB9119942D0

Abstract

A smoke detector has a chamber (12) through which air or another gaseous medium in which smoke or other particles to be detected flows along a flow path (14). A light source (16) emits a first part (18) of its light towards the flow path (14) and a second part (32) of its light directly towards the sensor (20). A light obscuring plunger (24) normally blocks direct light (32) from the sensor (20) which can thus only respond to light scattered by smoke or other particles. Periodically the plunger (24) is withdrawn so that the sensor (20) produces an output dependent on the light received directly from the source (16). The latter output can be used to adjust the light emitted by the source (160) to compensate for any degradation of the detector e.g. obscuration of the source or the sensor, or their respective windows. <IMAGE>

Description

SMOKE AND PARTICLE DETECTOR The invention relates to smoke and particle detectors.

According to the invention, there is provided a detector of smoke or other particles in a gaseous medium, comprising means defining a flow path for the gaseous medium, a source emitting light and light sensing means mutually juxtaposed in relation to the flow path, the source emitting the electromagnetic radiation in a beam a first part of which intersects the flow path but substantially cannot reach the sensing means except when scattered by smoke or other particles in the gaseous medium and a second part of which is directed towards the sensing means, and light blocking means mounted in the second part of the beam and switchable between a first setting in which it blocks the second part of the beam so as to enable the sensing means to produce an output dependent on the said scattered light and thus the presence of the smoke or particles, and a second setting in which it permits the second part of the beam to reach the sensing means whereby the latter produces an output dependent on factors other than smoke or particles.

According to the invention, there is further provided a smoke detector, comprising a housing defining a chamber and a flow path through the chamber for a gaseous medium in which smoke may be present, light source means emitting into the chamber a light beam, a first part of which intersects the flow path and light therein is scattered by smoke particles in the gaseous medium, and a second part of which does not intersect the flow path, light sensing means mounted in the housing and positioned so as to receive light from the first part of the light beam when scattered by the said smoke particles but not substantially receiving any other light from the first part of the light beam and positioned directly in alignment with the second part of the light beam, and light blocking means mounted on the housing and switchable between a first setting in which it blocks light in the second part of the light beam from the light sensing means, so as to enable the light sensing means to produce an output in dependence on the light scattered by any smoke in the flow path, and a second setting in which it permits light in the second part of the beam to pass directly to the light sensing means, whereby the latter produces an output dependent on factors other than smoke in the gaseous medium.

Smoke and particle detectors embodying the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawing which is a cross-section through one of the detectors.

The detector comprises a casing 10 made of suitable solid material, such as plastics or metal, defining a hollow chamber 12. The chamber 12 has inlet and exit passageways (not shown) defining a flow path across the chamber and through which a gaseous medium, for example air, which may carry smoke or other particles to be detected, may be passed through the chamber. The flow path for the gaseous medium is indicated diagrammatically at 14, though it will be appreciated that the gaseous medium may not in practice flow in such a well-defined way. The gaseous medium may be caused to flow along the path 14 by means of a pump or blower, or may diffuse along the path.

At one side of the chamber, a light source 16 is mounted, this being an electrically energised light source of any suitable type. It is arranged to produce a beam directed across the chamber 12 and indicated generally at 18, the beam 18 intersecting the flow path 14.

On the opposite side of the chamber 10 a light sensor 20 is mounted, this being of any suitable type capable of detecting the light from the light source 16 and producing a corresponding electrical signal.

In addition, the detector incorporates a bore 22 in one wall of the chamber 12 in which is slidably mounted a solid plunger 24, the plunger having a portion 26 extending into the hollow chamber 12 and a portion 28 extending into an actuating unit 30. In a manner to be described, the actuator unit 30 can move the plunger 24 into and out of the chamber 12. The Figure shows the plunger 24 in its limit position in the chamber 12.

During normal operation, the plunger 24 is in the position illustrated. In this position, the plunger 24 prevents any of the light in the beam 18 from directly reaching the light sensor 20 along the straight line path indicated at 32. Therefore, in the absence of any smoke or other particles in the gaseous medium in the flow path 14, the sensor 20 will receive substantially no light from the source 16. In order to prevent any light from being reflected by the wall of the chamber 12 towards the sensor 20, all internal surfaces of the chamber are painted matt black. Furthermore, the geometry of the chamber may be designed such that any minimal amount of light that is reflected from the chamber wall is reflected, or multiply reflected away from the sensor 20.

If any smoke or other particles are present within the gaseous medium in the flow path 14, however, these will scatter some of the light in the light beam 18 and some of this scattered light will be scattered towards and sensed by the sensor 20, thus producing an electrical output indicating the presence of the smoke or other particles.

In order to increase the efficiency of operation and to prevent the sensor 20 from producing a false smoke alarm in response to stray light from other sources, the light source 16 may be caused to product light modulated at a particular frequency. Any of this light which is scattered by smoke or particles in the flow path 14 will produce an electrical output from the sensor 20 which is correspondingly modulated and can thus be detected and distinguished, such as by filtering or phase discrimination, from stray light from other sources.

Over a period of time, various factors may operate to degrade the relationship between the electrical output of the sensor 20 and the amount of smoke or other particles present in the gaseous medium in the flow path 14. For example, contamination from dirt or other deposits (such as from the gaseous medium in the flow path 14) may become present on the surface of the light source 16 and/or the sensor 20, or on windows covering them. Such contamination will reduce the overall sensitivity of the detector. Furthermore, ageing of the light source and/or light sensor will change the output of the sensor 20, as will other defects in them or their associated circuitry.

In order to test for any such degrad-a.tion in performance, the plunger 24 is momentarily withdrawn from the chamber 12 so as to open the direct path 32 between the light source 16 and the light sensor 20. The light beam 18 from the light sensor is designed such that some of the light from the source is directed along this path 32 and will consequently reach the light sensor when the plunger is withdrawn in this way. The light sensor will produce a resultant output, and the level of this output, in comparison with a predetermined datum, will be a measure of the total degradation in performance (due to the factors previously mentioned).

In this way, the degradation in performance can be monitored. If it becomes excessive (if the degradation, as measured by the reduction in the output of the sensor 20, exceeds a predetermined value), compensating steps may be taken; for example, the light output of the source 16 may be increased. This may be done manually or automatically.

After completion of the degradation test, the plunger 24 is moved back to the position illustrated.

The plunger 24 can be moved into the degradation-testing position under manual control or (preferably) automatically at predetermined instances.

The plunger 24 is moved into and out of the position illustrated by the actuating unit 30 which may take any suitable form. For example, it may comprise a solenoid which can be electrically energised so as to attract and repel the plunger 24 and thus move it between its two positions. Instead, for example, it could be pneumatically operated. Another possible form could incorporate an electric motor connected to move the plunger 24 by means of a crank or a cam and cam follower arrangement.

Instead of the plunger 24, a mechanical shutter arrangement could be used which would alternately block and open the direct light path 32.

However, a non-mechanical arrangement could be used for controlling the direct light path 32, such as some suitable electrical device which can be switched, by means of an electrical or other non-mechanical signal, between a light obscuring and a light transmitting setting.

In this specification and its claims, light" includes other suitable electromagnetic radiation.

Claims

1. A detector of smoke or other particles in a gaseous medium, comprising means defining a flow path for the gaseous medium, a source emitting light and light sensing means mutually juxtaposed in relation to the flow path, the source emitting the electromagnetic radiation in a beam a first part of which intersects the flow path but substantially cannot reach the sensing means except when scattered by smoke or other particles in the gaseous medium and a second part of which is directed towards the sensing means, and light blocking means mounted in the second part of the beam and switchable between a first setting in which it blocks the second part of the beam so as to enable the sensing means to produce an output dependent on the said scattered light and thus the presence of the smoke or particles, and a second setting in which it permits the second part of the beam to reach the sensing means whereby the latter produces an output dependent on factors other than smoke or particles.

2. A smoke detector, comprising a housing defining a chamber and a flow path through the chamber for a gaseous medium in which smoke may be present, light source means emitting into the chamber a light beam, a first part of which intersects the flow path and light therein is scattered by smoke particles in the gaseous medium, and a second part of which does not intersect the flow path, light sensing means mounted in the housing and positioned so as to receive light from the first part of the light beam when scattered by the said smoke particles but not substantially receiving any other light from the first part of the light beam and positioned directly in alignment with the second part of the light beam, and light blocking means mounted on the housing and switchable between a first setting in which it blocks light in the second part of the light beam from the light sensing means, so as to enable the light sensing means to produce an output in dependence on the light scattered by any smoke in the flow path, and a second setting in which it permits light in the second part of the beam to pass directly to the light sensing means, whereby the latter produces an output dependent on factors other than smoke in the gaseous medium.

3. A detector according to claim 1 or 2, in which the blocking means is mechanical blocking means physically movable into and out of the second part of the beam.

4. A detector according to claim 3, in which the blocking means is moved electromagnetically between its two settings.

5. A detector according to claim 1 or 2, in which the blocking means comprises means positioned in the second part of the beam and switchable by means of an externally applied non-mechanical signal between the two said settings.

6. A detector according to any preceding claim, including means responsive to the output produced by the sensing means when the blocking means is in its second setting for adjusting the emission of the source so as to tend to bring that output of the sensing means to a predetermined value.

7. A detector according to any preceding claim, including means for modulating the emission of the source in a predetermined manner, and processing means responsive only to corresponding modulation in an output signal of the sensing means to produce the said output.

8. A detector of smoke or other particles, substantially as described with reference to the accompanying drawings.