AU2003200391A1

AU2003200391A1 - Optical smoke alarm based on the extinction principle and use of the said alarm

Info

Publication number: AU2003200391A1
Application number: AU2003200391A
Authority: AU
Inventors: Peter Kunz; Markus Loepfe; Kurt Muller
Original assignee: Siemens Building Technologies AG
Current assignee: Siemens Building Technologies AG
Priority date: 2002-03-28
Filing date: 2003-02-06
Publication date: 2003-10-23
Also published as: NO20030680D0; EP1349127A1; HU0300792D0; HUP0300792A2; NO20030680L; CZ2003886A3; CN1448896A; KR20030078664A; HUP0300792A3; PL359358A1

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant: SIEMENS BUILDING TECHNOLOGIES AG Invention Title: OPTICAL SMOKE ALARM BASED ON THE EXTINCTION PRINCIPLE AND USE OF THE SAID ALARM The following statement is a full description of this invention, including the best method of performing it known to me/us: Optical smoke alarm based on the extinction principle and use of the said alarm Description The present invention relates to an optical smoke alarm based on the extinction principle, with an optical bridge which has a light source and a measuring section and reference section each having a receiver, and with an evaluating circuit.

In the extinction method of measurement, as is known, a light beam is transmitted through the measuring section, which is accessible to the ambient air and thereby to any smoke, and through the reference section which is not accessible to the smoke, and the two reception signals are compared with one another. Since both the light diffusion on the smoke particles and also the absorption by the latter contribute to the extinction, and the light is mostly diffused by bright particles and mostly absorbed by dark particles, the extinction method of measurement possesses relatively uniform sensitivity to different smoke particles and is therefore equally well suited to detecting smouldering fires (bright particles) and open fires (dark particles). When the extinction method of measurement is used in localised alarms, that is to say smoke alarms which are entirely accommodated in a single housing, the extinction of the aerosols in the air can be determined only over a very short measuring section, as a result of which the demands on the sensitivity of the transmission measurement rise accordingly.

EP-A-1 017 034 describes a localised extinction-type alarm which utilises a simple optical bridge which has, in addition to the light source and the two receivers as the only optical elements, two apertured diaphragms which are disposed in front of the light source. This extinction-type alarm has the advantage, compared with other known extinction-type alarms (EP-A-0 740 146, EP-A-0 578 189) having parabolic mirrors and lenses, that the elimination of the said mirrors and lenses leads to a marked reduction in the temperature-dependence of the optical bridge and thereby to an improvement in the stability of the said extinction-type alarm. However, this very stability is very important because, in the case of a measuring section with a length of, for example, 10 cm, the alarm threshold of 4% per metre lies at a transmission of 99.6% of the reference transmission. And if transmission values below the alarm threshold are to be triggered, then values of, for example, 99.96 transmission must be identifiable, a fact which makes extremely high demands on the stability of the electronics, the optoelectronics and the mechanics of these alarms.

Other potential sources of errors are the temperature drift of the receivers and their susceptibility to bedewing, and what is more the latter applies to all optical smoke alarms. This should be understood to mean that the formation of water vapour that occurs under certain circumstances, for example when showers or baths are taken in hotel rooms, can lead to bedewing of the receiver in the measuring section, whereas the receiver in the reference section remains unbedewed since the said section is, of course, sealed off from the ambient atmosphere. The bedewing of the measuring receiver brings about a decline in its photocurrent, which the alarm may interpret as smoke and which may therefore lead to the triggering of a false alarm.

The invention is now intended to indicate a localised extinction-type alarm which permits the detection of, and compensation for, bedewing and temperature drift of the receivers.

This object is achieved, according to the invention, through the fact that a second light source is provided and that, from each light source, an outwardly open measuring section leads to one receiver, and a reference section which is screened against the outside leads to the other receiver, each receiver acting as a measuring receiver for one light source and as a reference receiver for the other, and vice versa.

The smoke alarm according to the invention therefore uses a double optical bridge with two light sources and two receivers. A localised extinction-type alarm with two light sources and two receivers is known, in principle, from EP-A-O 578 189. In this alarm, however, only one measuring section is provided and compensation for bedewing is therefore not possible.

A first preferred form of embodiment of the smoke alarm according to the invention is characterised in that the emission from the two light sources is regulated to a stable photocurrent in the particular reference receiver, and that the evaluating circuit has means for evaluating the difference in the signals from the two receivers.

A second preferred form of embodiment of the smoke alarm according to the invention is characterised in that the two light sources are activatable sequentially and form a channel with the associated receivers in each case, and that a measuring-sectionminus-reference-section differential signal is formed for one channel, and a referencesection-minus-measuring-section differential signal is formed for the other channel.

A third preferred form of embodiment of the smoke alarm according to the invention is characterised in that the two aforesaid differential signals are tested for any changes, and that a change in the differential signals in opposite directions is interpreted as the occurrence of smoke. A change in the aforesaid differential signals in the same direction is interpreted as an indication of an unequal variation in the sensitivity of the receivers which is caused-bya ch-ange in the ambient temperature or by bedewing.

Another preferred form of embodiment of the smoke alarm according to the invention is characterised in that the aforesaid unequal variation in the sensitivity of the receivers is compensated for by regulating the emission of the light sources, and that regulation which exceeds a certain extent triggers a fault indication.

When the double bridge is in a state of equilibrium without smoke or dew, the output voltages of the two receivers, and thereby the differential signal of the two channels, will be in equilibrium. If one light source is activated, the photocurrent in the reference receiver of the one light source will be stable, and if the other light source is activated, the photocurrent in the reference receiver of the other light source will be stable. If smoke now enters the measuring sections, the photocurrents in the measuring receiver decrease and the differential signal will decrease in one channel and increase in the other. If, on the other hand, the receivers become bedewed, or their sensitivity changes as a result of a change in the ambient temperature, the two differential signals vary in the same direction.

A change of this kind in the sensitivity of the receivers can be identified by the automatically occurring follow-up regulation of the photocurrents in the particular reference receiver. If the drifting of the receivers is so great that it can no longer be adequately compensated for by the automatic follow-up regulation, heavy bedewing exists and a fault indication is initiated.

Another preferred from of embodiment of the smoke alarm according to the invention is characterised in that the light sources and receivers are carried by a common component which can be inserted in a housing. The said common component preferably has the shape of an elongated prism, on one of whose end faces the light sources are mounted, and on the other of whose end faces the receivers are mounted, namely along one of the two diagonals of the end faces in each case.

Another preferred form of embodiment of the smoke alarm according to the invention is characterised in that the prismatic component has, in its centre, an opening which forms a measuring chamber and through which the two measuring sections pass, and that the two reference sections extend in regions of the prismatic component which are spatially separate from the said opening.

The invention also relates to the use of the smoke alarm according to the invention in bathrooms and/or rooms connected to the latter or in freight compartments of aircraft.

The invention will be explained in greater detail below, with the aid of an exemplified embodiment and of the drawings, in which: figure 1 shows a block diagram-type representation of a smoke alarm according to the invention; figure 2 shows a partially cut-away, perspective representation of a detail of the alarm in figure 1; figure 3 shows a view in the direction of the arrow III in figure 2; and figure 4 shows a view in the direction of the arrow IV in figure 3.

The smoke alarm which is diagrammatically illustrated in figure 1 is a so-called "localised extinction-type alarm" or "transillumination-type alarm" which consists of a base, an alarm insert with a measuring module and evaluating electronics, and a hood.

The representation in figure 1 relates to the measuring module and evaluating electronics, and the base and hood are not represented. The alarm insert is intended, in known manner, for fastening in the base which is preferably mounted on the ceiling of a room which is to be monitored. The alarm hood which covers the alarm insert and, optionally, the base too, is inverted over the said alarm insert and interlocked with the base. This alarm structure is known and will therefore not be described in any greater detail here; in this connection, the reader is referred to the fire alarms of the AlgoRex series by Siemens Building Technologies AG of Mannedorf (formerly Cerberus AG) (AlgoRex is a registered trade mark of Siemens Building Technologies AG or of Cerberus AG).

The measuring module designated by the reference symbol 1 contains a double optical bridge with two light sources L 1 and L 2 formed by light-emitting diodes (LED's) or infrared LED's (IRED's) and two receivers E 1 and E 2 formed by photodiodes. From each light source L 1

L

2 one measuring section in each case which is open in relation to the ambient atmosphere and is therefore accessible to aerosols, and one reference section in each case which is screened from the environment, R 1 and M 2

R

2 respectively, lead to the two receivers, each receiver El, E 2 forming the measuring receiver for one light source and the reference receiver for the other. As represented, the receiver E, is the reference receiver for the light source L, and the measuring receiver for the light source L 2 and the receiver E 2 is the reference receiver for the light source L 2 and the measuring receiver for the light source L,.

The two light sources L, and L 2 are activated sequentially in a rhythm of about 1 second and, in the activated mode, each emit one pulse sequence in each case of 8 individual pulses of 50 psec duration, with just such gaps between them. The circuit part 2 which is drawn in on the right-hand side in figure 1 and which forms part of the evaluating electronics 3, measures the difference in the photocurrents in through the receivers E,.

The output signal of the circuit part 2, which signal is designated by the reference symbol Sm, is a voltage which is proportional to the difference in the photocurrents in. In the state of equilibrium, without any smoke in the measuring sections, without bedewing and a change in temperature, the output signals Sm are in equilibrium.

If the measuring-section-minus-reference-section difference is formed in the channel of one LED, and the reference-section-minus-measuring-section difference is formed in the channel of the other LED, the two output signals Sm of circuit stage 2 should be of equal size:

S

1 i 2 (LI) i,(L 1 (measuring section minus reference section)

L

2 S2 i 2

(L

2 i,(L 2 (reference section minus measuring section) The emission from L, is regulated in such a way that the photocurrent of its reference beam on the photodiode that is to say amounts to a very stable 5 pA. The same applies to LED L 2 and its reference beam on the photodiode E 2 that is to say i 2

(L

2 Thus, the following applies: 5 pA and i 2

(L

2 5 pA.

If smoke penetrates into the measuring sections M, and M 2 the corresponding photocurrents i 2 and i,(L 2 then respectively decrease and that means that S, decreases and S2 increases, namely by: A_S, -|A_i 2

(L

1 0. A_i(L) 0

A_S

2 +IAi(L 2 0. A_i 2

(L

2 0 By subtraction in a microprocessor 4, which likewise forms part of the evaluating electronics 3, the following is obtained: S2- S, AS 2 A_S, +IAi,(L 2 )l 14Ai 2 (L)I 0.

The result of this subtraction is twice the signal change caused by the smoke in the single channel.

If the ambient temperature changes and, as a consequence thereof, the sensitivity of the two photodiodes E, and E 2 vary in a different manner, or if unequal bedewing of the said photodiodes occurs, then S, and S2 change in the same direction, so that the value zero is produced when subtraction takes place in the microprocessor 4:

S

2

S

1 A S 2

AS

1 0 A variation of this kind in the sensitivity of the photodiodes E, and E 2 regardless of whether it is caused by a temperature change or bedewing, is indicated with the aid of the automatically occurring follow-up regulation of the photodiode currents in the reference channels.

If the situation arises that the bedewing is so heavy, or occurs so abruptly, that the drifting of the photodiodes in the manner described can no longer be adequately eliminated, something which manifests itself, for example, in the fact that the drifting cannot be eliminated within a predetermined time, the issuing of an alarm is blocked by the microprocessor 4 and replaced by a fault indication.

Figures 2 to 4 show an actual construction of the alarm insert which carries the double optical bridge. Figure 2 shows the alarm insert designated by the reference numeral partially cut away, the sectional plane marked by hatching extending double-diagonally through the alarm insert and containing the two light sources L, and L 2 and also the photodiode E 2 The alarm insert 5 has the shape of a prism of, for example, square cross-section and having arched (convex) end faces 6 and 7. As can be seen from figures 2 and 4, the prism has, in the centre, a clearance 8 which passes through from one side wall to the other and which forms the actual measuring chamber and has the measuring sections M, and M 2 passing through it.

Adjoining the clearance 8 above and below, there are located regions which are closed off towards the outside and have the reference sections R 1

R

2 In principle, the latter regions could consist of solid material and merely have one bore in each case for the reference section in question. However, it has proved more practical to incorporate an approximately triangular depression 9 into these regions and to then insert in the said depression a precisely fitting plate 10 which can be removed for the purpose of cleaning the bore forming the reference section (see figure 2 in which the upper plate 10 is 7 removed).

The two light sources L, and L 2 are disposed on the end face 6 which is on the left in the figures, and the photodiodes E 1 and E 2 on the end face 7 which is on the right, namely diagonally in each case on the end face in question and crosswise relative to one another. The alarm insert 5, which consists of a material having a good heat-conducting capacity, for example aluminium, is manufactured in one piece, with the exception of the two plates 10, and is thus robust and easy to handle. The dimensions of the alarm insert 5 are chosen in such a way that the latter can be built into the housing of an optical smoke alarm of the Algorex type. That means that the length of the alarm insert is, at any rate, below 10 cm.

The smoke alarm according to the invention can be employed, without any risk of a false alarm caused by drifting of the photodiodes, in places where the formation of dew or major fluctuations in temperature can occur as a result of severe environmental conditions. Examples of such places are bathrooms and anterooms/entrances connected to the latter, particularly in hotel rooms, and particularly too, the freight compartments of aircraft, in which a major rise in temperature, and an associated formation of dew, occurs during the landing approach.

For the purposes of this specification it will be clearly understood that the word "comprising" means "including but not limited to", and that the word "comprises" has a corresponding meaning.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

Claims

1. Optical smoke alarm based on the extinction principle, with an optical bridge which has a light source (L 1 and a measuring section (M 1 and reference section each having a receiver (E 2 and with an evaluating circuit characterised in that a second light source (L 2 is provided and that, from each light source L 2 an outwardly open measuring section leads to-one receiver, and a reference section which is screened against the outside leads to the other receiver, each receiver acting as a measuring receiver for one light source and as a reference receiver for the other, and vice versa.

2. Smoke alarm according to claim 1, characterised in that the emission from the two light sources L 2 is regulated to a stable photocurrent in the particular reference receiver E 2 and that the evaluating circuit has means for evaluating the difference in the signals from the two receivers.

3. Smoke alarm according to claim 2, characterised in that the two lightsources (L, L 2 are activatable sequentially and form a channel with the associated receivers E 2 in each case, and that a measuring-section-minus-reference-section differential signal is formed for one channel, and a reference-section-minus- measuring-section differential signal is formed for the other channel.

4. Smoke alarm according to claim 3, characterised in that the two aforesaid differential signals are tested for any changes, and that a change in the differential signals in opposite directions is interpreted as the occurrence of smoke.

Smoke alarm according to claim 4, characterised in that a change in the aforesaid differential signals (Sm) in the same direction is interpreted as an indication of an unequal variation in the sensitivity of the receivers E 2 which is caused by a change in the ambient temperature or by bedewing.

6. Smoke alarm according to claim 5, characterised in that the aforesaid unequal variation in the sensitivity of the receivers E 2 is compensated for by regulating the emission of the light sources L 2 and that regulation which exceeds a certain extent triggers a fault indication.

7. Smoke alarm according to one of claims 2 to 6, characterised in that the light sources (L 1 L 2 and receivers E 2 are carried by a common component which can be inserted in a housing.

8. Smoke alarm according to claim 7, characterised in that the common component has the shape of an elongated prism, on one of whose end faces the light sources L 2 are mounted, and on the other of whose end faces the receivers E 2 are mounted, namely along one of the two diagonals of the end faces (6 and 7 respectively) in each case.

9. Smoke alarm according to claim 8, characterised in that the prismatic component has, in its centre, an opening which forms a measuring chamber and through which the two measuring sections (M 1 M 2 pass, and that the two reference sections (R 1 R 2 extend in regions of the prismatic component which are spatially separate from the said opening Use of the smoke alarm according to one of claims 1 to 9 in bathrooms and/or rooms connected to the latter or in freight compartments of aircraft. Dated this 6th day of February 2003 SIEMENS BUILDING TECHNOLOGIES AG By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia