CA1064139A

CA1064139A - Light scattering type smoke detector

Info

Publication number: CA1064139A
Application number: CA256,746A
Authority: CA
Inventors: Tsunehiko Araki; Yoshihiko Okuda
Original assignee: Matsushita Electric Works Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 1975-07-15
Filing date: 1976-07-12
Publication date: 1979-10-09
Also published as: IT1078734B; JPS5855446B2; DE2630843A1; SE416852B; US4103997A; FR2318417A1; FR2318417B1; GB1560421A; SE7607902L; JPS5210788A

Abstract

LIGHT SCATTERING TYPE SMOKE DETECTOR
Abstract of the Disclosure A light scattering type smoke detector effectively sensitive both to white and black smokes is provided. A wave length .lambda. of an incident light from a light source and a scattered light receiving angle .theta. defined by the direction of the incident light and the direction connecting between a smoke particle and alight receiving element are so determined as to satisfy .lambda. ? 950 mµ when .theta. ? 45°, .theta. ? - 0.18 .lambda. + 216 when 45° ? .theta. ? 135° and .theta. ? 135° when .lambda. ? 450 mµ.

Description

This inventicn relates to a light scattering type smoke detector wherein a detection of a scattering of emitted light from a light source due to a smoke is utilized for detections of firesO
- 5 In conventional light scattering type smoke detectors, mostly a sub-infrared light emitting diode iS used for the light source and a scattered light due to a presence of smoke in light beam is received by a photocell disposed at a position in front of the source and in a rectangular direction with respect to optical axis of the light beam from the sourceO In this case, generally, the smoke will be a white smoke produced by a smoldering or the like of the cellulose, or a black or dark smoke produced due to a combustion of a petroleum series fuel or the lîke aDd the light scattering type smoke detectors are 1 15 more highly responsive to the white smoke than to the black : smoke in such that the detectors are responsive to the white smoke about ten times as high as they do with respect to the black smoke in the case when, for example, an emitted light with a wave length of 940 ~ is received as scattered by the photocell at a fore side 135 relation to optical axis of the emitted lighto On the other hand, it is desirable to set the sensitivity of the detectors to the white smoke properly l~wer so that, for example, the detectors will be responsive generally to the white smoke showing a light extinction rate of about 10% per meter, whereby the detectors will be prevented from misoperating in response to any dusts froating in the atmosphere, the smoke of cigrettes or the like~ In this case, however, such detectors will become low sensitive remarkably with respect to the black smoke and they may be unable to act as the detector . .
against fires accompanled mostly by the black smoke. The present invention has been suggested to remove such defect of the known detectors of the kind referred to in such manner that the - difference in the sensitivity of the detector to the white smoke and that to the black smoke is made smaller by properly defining the relation between the wave length of incident light and the scattered light receiving angle in the detector and thereby the preventions of misoperations with respect to the white smoke and of any non-responses to the black smoke of the detectors will be simultaneously achieved.
In accordance with the present invention there is provided in a smoke detector of the light scattering type for actuating a warning device and comprising a housing containing a light source, for emitting incident light having a wave length ~ , a light receiving element, and a smoke collecting region so arranged that incident light from the source which contacts a smoke particle in the region is scattered such that the scattered light directed toward the receiving element forms an angle ~ with the direction of the incident light, the ratio of the intensity Is of the scattered light to the intensity Io of the incident light defining an intensity ratio Is/Io for the smoke; the ratio of the intensity ratio for a black smoke to the intensity ratio for a white smoke defining the sensitivity ratio of the detector; the improvement wherein the wave length A
and the angle ~ each bear a relationship to the intensity ratio and are determined as a function of one another within the limits ~ - 950 mJu when ~ - 45, ~ - -0.18~ + 216 when 45 -~ ~ -L 135, and ~ - 135 when ~ = 450 mJu, to define a preselected range of sensitivity ratios for the detector.

~ - 3 -As primary object of the present invention is to provide a smoke detector wherein the fluctuation in the sensitivity is little depending on the kind of smoke.
Another object of the present invention is to provide a smoke detector wherein the sensitivity to the black smoke is high and a non-warning less occurs in the case of fires generating a large amount of black smoke due to kerosene, foamed polystyrol or the like.

- 3a -~6 ~
A further object of the present invention is to provide a smoke detector wherein the sensitivity is high and the misoperation is little.
Other objects and advantages of the present invention will become clear upon the following explanation of the invention advances detailed with reference to a preferred embodiment of the present invention in conjunction with accompanying drawings, in which:
FIGURE 1 is an explanatory view of a principle of the light scattering type smoke detector of the present inventioa;
; FIG~ 2 is a characteristics diagram of the same;
FIG~ 3 is a block circuit diagram of an embodiment of the detector according to the present invention; -FI~S~ 4A through 4C show wave forms at respective - 15 parts in the circuit of FIG~ 3; and - FIGo 5 shows an exemplary structure of a smoke detecting chamber in the smoke detectorc The present inventors have perceived that, in the arrangement of a light source 1 and a light receiving element

2 such as a photocell as shown in FIG~ 1, the intensity ratio IS/Io of a scattered light Is by a smoke particle 3 to an incident light IO from the light source 1 has a certain relation to the wave length ~ of the incident light Io and the scattered light receiving angle ~ (which is an angle between the incident light Io and scattered light Is), and this has been experimentally confinmed by the inventors. The IS/Io ratio for each of a black smoke B (such as, for example, a kerosene burning smoke) and a white smoke W (such as, for example, a cellulose smoldering smoke) is determined by varying the optical wave length ;~ and scattered light receiving angle ~ and the sensitivity ratio B/W of the black smoke to the white smoke is determined as diagrammatically shown in FIGo 2, wherein the wave length ~ is taken on the abscissa, the scattered light receiving angle ~ is taken on the ordinate and five curves of the sensitivity ratio B/W = 0.1, 002, 0~3, 0.4 and 0.5 are shownO It is understood from this diagram that, the shorter the wave length ~ of the light from the source, the larger the ratio B/W and that, the smaller the scattered light receiving angle ~, the larger the ratio B/Wo : Considerations shall be made on the preferable value of the sensitivity ratio B/W.- Generally the sensitivity of the smoke detector of the kind referred to can not be said to be the higher the better, because a cigarette smoke or a slight dust floating in air may/apt to cause a miswarning of such detector~
Therefore, it is clear in experiences that, if the sensitivity is so set that the light extinction rate (which is a rate of reduction of a light by passing through a smoke) will be about 10% per meter, the reliability will be the highestO
~n the other ~and, since the smoke detector is related specifically with the beginning of a refuge of persons and a fire extinguishing activity, it must operate with a smoke of a light extinction rate of at least about 30% per meterO This means that the smoke to which the detector should be sensible is of a thickness through which 5 to 10 meters ahead can be seenO
Hence, it is preferable that the ratio B/W is 0.3 or moreO
From the above, the range defined as hatched in the diagram of FIGo 2 is preferable, that is:

:
:

(i) When ~ < 45, ,~ c 950 m~.
- (ii) When 45S ~ ~ 135, ~ S - 0.18 ~ ~ 216 (iii~ When,\ c 450 m~, Q ~ 135 The inclined line L in the diagram is represented by ~ = - 0.18 ,~ + 2160 In the actual s ke detector, the scattered light receiving angle ~ of the scattered light received by the light receiving element or photocell has a certain range but the angle ~ referred to here is a scattered light receiving angle contributing to the greater part of the amount of the received lighto - For the light source, such light emitting diode as GaAs, GaAsP or GaP or a xenon lamp is used. The light oÇ such -i source is not monochromatic in the strict snese of the word but has a certain wave length range and the wave length ~ defined here represents its typical value.
Further, the practically preferable range is of the wave length ~ = 300 to 700 m~ and scattered light receiving angle ~ = 30 to 75 which are found to be most preferable in view of the design of the smoke detector and the type of light sourceO In other words, with ~ = less than 300 ~u, the light will be impermeable to any glass, and from the viewpoint of the easiness of procurement of the light emitting diodes, ~ = less than 700 my is proper. Further, in the design of the smoke detecting chamber, the angle P = 30 is a limit and, from the intersection of the curve of B/W = 0O3 and ~ = 700 m~u, ~ = 75 is determined.
The st~ucture of the smoke detector most preferable to work the present invention shall be described in the following.

~3~

The embodiment shall be referred to with reference to the case where a light emitting GaAsP diode emitting a light of a wave length ~ = 640 is used and a scattered light receiving angle ~ = 60~ In this respect, the case of ~ = 30 to 75 is the same~
With this angle, the ratio IS/Io of the scattered light is so small in the experiment that it is difficult to obtain a - signal sufficient for a noise generated from a light receiving silicon photo diode itself and/or an amplifier connected with it or, for example, a heat- noise generated from a resistor as well as for a foreign noise leaking through a light labyrinth ` of the smoke detector.
This problem can be.reduced by a known method wherein a light emitting diode is driven with a pulse current source of a very small duty so as to momentarily obtain a large light output as taught, for example, in Swiss Patent No. 417,405.
That is, with the momentary feed of such electric power as will immediately bring about a destruction by an ordinary direct current feed, an amount of emitted light lO to 20 times as large will be obtained and, as a result, the signal per noise ratio can be elevated. In a detector emitting such pulsating light, in some case, there is carried out a method wherein a signal will be interrupted in an amplifying circuit in a light emission stopping period so as to reduce the possibility of a misoperation being m~de by a foreign noise. However, in such method, there is no effect on a constant noise generated from the light receiving element itself and/or amplifying circuit.
In the embodi~ient of the present invention, a CR
integrating circuit having a time constant several times as large as that of a light emitting period is loaded through a switch - in an amplifying circuit for definitely detecting a scattered light signal from various noises. This switch is closed as synchronized with the light emitting period. By integrating 5 the signal in this light emitting period over several periods, noises represented by the same positive and negative probabilities are to be canceled.
In FIGo 3 showing a practical circuit arrangement, 11 is a p~ilse current source for driving a light emitting 10 diode 121 in a detecting chamber 12, the pulse width is about 100 ~lS, the period is about 1 secon~ and 1 ampere is fed to the light emitting diode 121. An output of about lOO PA is obtained from a light receiving photocell 122, which is amplified by an amplii~ying circuit 13 and is given to a signal processing circuit .~
~- 15 14 ~ich is formed of a field effect transistor 141 and an integrating circuit of a resistance 143 and condenser 144. The ~-pulses from the pulse current source 11 are given through a condenser 142 to the gate of the field effect transistor 141 SQ that, whenever each pulse arrives, a switch (between the 20 source and the drain of the field effect transistor) will be ON. Therefore, during the light emitting period, the field effect transistor 141 will become a low resistance and a signal voltage including a noise will be accumulated in the condenser 144 through the resistance 143 This light emi tting period is 25 about a fraction of the time constant of CR and is so small that the voltage of the condenser will be elevated but slightly by the output from the light receiving element and, during the non-light emitting period, there will be a high resistance between the source and the drain of the field effect transistor 141 and the voltage of the condenser 144 will not be elevated by a noiseO
During the next light emitting period, the voltage of the condenser 144 will be again elevated by the output from the light receiving element, thus the condenser voltage will gradually become higher . and, when it reaches a certain value, it will operate a threshold value circuit 15 so that a warning device 16 or the like will be thereby operated.
In FIG. 4 showing voltage wave forms, FIG. 4A is of the pulse voltage, FIG. 4B is of the output voltage from the amplifier 13 when a smoke is present and FIG. 4C is of a terminal voltage of the condenser 144. According to the present invention, as an integrating circuit is provided, positive and negative noises will be added together and averaged to be zero so t~at their t 15 influence will remarkably reduced.
Referring next to FIG. 5 showing an example of the structure of smoke detecting chamber, a light source 22 and a photocell 23 are disposed as separated by a separator 24 within a cylindrical housing 21 so that a light projected onto smoke particles present in a region 26 shown as hatched through a window 25 will become a scattered light which reaches the photocell 23 through a window 27. In the arrangement, light intercepting plates 28 to 32 are provided within the housing 21 so that the relations of the respective tip edges "a" through "i" and base positions c' and e r Of the windows and light intercepting plates will be as follows~ The smoke is to come in and out in the axial directions of the cylindrical housing, the edge "a" of the window 25 is so determined that the light .~ from the light source 22 will not reach the edges "g" and "f", the edge "b" of the light intercepting plate 28 is so determined that the light will not directly reach a fore field of vision of the photocell 23 between the base c' of the wall 30 and the edge "d" of the plate 32, the edges "g" and "f" are so determined as to limit the field.of vision of the photocell and the edges "c", "d", "e", "h" and "i" are to prevent the second reflection within the housing, that is, to prevent the surfaces in the field -.: of vision of the photocell from being within the surfaces to which . 10 the light is irradiated directly from the light source 220 According to the present invention, as described above,even the black smoke can be positively detected by the detector because of the specified relations of the wave length of an incident light and the scattered light receiving angleO
Further, with the provision of the integrating circuit as a detecting circuit, an effect of preventing any misoperation of the detector from occurring due to any noise can be brought about.

- 10 - .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a smoke detector of the light scattering type for actuating a warning device and comprising a housing containing a light source for emitting incident light having a wave length .lambda. , a light receiving element, and a smoke collecting region so arranged that incident light from said source which contacts a smoke particle in said region is scattered such that the scattered light directed toward said receiving element forms an angle .theta. with the direction of the incident light, the ratio of the intensity Is of the scattered light to the intensity Io of the incident light defining an intensity ratio Is/Io for the smoke; the ratio of the intensity ratio for a black smoke to the intensity ratio for a white smoke defining the sensitivity ratio of the detector; the improvement wherein said wave length .lambda.
and said angle .theta. each bear a relationship to the intensity ratio and are determined as a function of one another within the limits .lambda. ? 950 m,µ when .theta. ? 45°, .theta. ? -0.18.lambda. + 216 when 45° ? .theta. ? 135°, and .theta. ? 135° when .lambda. = 450 mµ, to define a preselected range of sensitivity ratios for the detector.

2. A detector according to claim 1 wherein said wave length .lambda. is 300 to 700 mµ and said scattered light receiving angle .theta. is 30 to 75°.

3. A detector according to claim 2 wherein said light source is driven by a voltage of a pulse wave form.

4. A detector according to claim 3 wherein an output from said light receiving element is applied to an integrating circuit through a switch element conducted only while said light emitting element is excited by said pulse wave voltage and, when an integrated value in said integrating circuit reaches a threshold value, a warning signal is provided.

Page 1 of Claims

5. A detector according to claim 1 wherein the sensitivity ratio is 0.3 or more.