EP0578189A1 - Optic smoke detector - Google Patents

Abstract

To increase the protection against false alarm by optical smoke detectors in accordance with the principle of extinction having two radiation sources (S1, S2) and two radiation receivers (D1, D2), a measuring system (T1) and a reference system (U) are arranged in an integrated optical chip, the reference system (U) being arranged in the form of waveguides (U) in a glass plate (G) and the light from the radiation source (S1) being conducted to the measuring system (T1) and from the measuring system (T1) to the radiation receiver (D1) through waveguides (U) formed in the glass plate (G). To improve the sensitivity whilst retaining at least the same resolution, the two radiation sources (S1, S2) emit light with different modulation and the optical bridge circuit is preceded by an electrical time-division multiplex bridge which alternately transmits the modulation signals straight (stage b) and then crossed over (state c) and the evaluating circuit contains special switching elements which eliminate the frequency responses Ri(fj) from the end result. <IMAGE>

Description

The invention relates to an optical smoke detector according to the preamble of claim 1. Smoke detectors of this type are generally known. They are used in particular as automatic fire detectors for the early detection of fires.

Smoke detectors occupy a special position among the multitude of types of automatic fire detectors on the market, since they are best suited to detect fires at such an early stage that countermeasures can still be successfully initiated.

There are two main types of smoke detectors: ionization smoke detectors and optical smoke detectors. In the case of ionization smoke detectors, the accumulation of smoke particles in air ions is used. Since a radioactive source is required to adequately ionize the air molecules, the handling of which during production and disposal is problematic, optical smoke detectors are increasingly being used. These optical detectors use the optical properties of aerosols to detect smoke. Here, either the weakening of a light beam by smoke ("extinction smoke detector '') or the scattering of light on smoke particles is used.

In the latter “scattered light smoke detectors”, the electromagnetic radiation scattered on smoke particles, preferably visible light or infrared radiation, is measured by a scattered radiation receiver arranged outside the direct beam path. Such a radiation receiver can be set up very sensitively, since no scattered radiation occurs in the absence of aerosol particles in the measuring section, the signal is practically zero and, in the presence of aerosol in the measuring room, a signal occurs that is very large compared to the zero value and therefore easy to determine is. A disadvantage of scattered-light smoke detectors is that they only respond to types of smoke that strongly scatter light. White smoke that contains a lot of water vapor, for example, scatters light very strongly, whereas black smoke, which contains a lot of soot, probably absorbs light, but scatters it only to a small extent.

Since the attenuation, or extinction, of light through smoke is relatively small, the measuring distance for extinction smoke detectors must be quite long in order to enable clear and reliable detection of smoke. In the past, they were therefore mostly designed as "line detectors", in which e.g. Light source and receiver are arranged separated from each other by a longer measuring section and the light beam passes through the measuring section once. In a similar example, the light source and receiver are located in the same housing and the light beam passes through the measurement section twice by being reflected back to the housing by a reflector. This already enables the length of the measuring section to be halved; nevertheless, a reflector to be installed outside the smoke detector is required, the alignment and mechanical stabilization of which can be problematic. With both types of detectors there is a risk of false alarms due to accidental interruption of the measuring section, mechanical displacement of the components or similar faults.

It has therefore long been the aim to also designate the extinction detectors as so-called "point detectors", ie detectors in which the light source and receiver are arranged in the same housing and no further components are required outside of this housing The main problem with punctiform extinction smoke alarms is that it is relatively difficult to reliably determine the slight attenuation of a light beam that occurs over a shorter measuring distance due to a fire aerosol. The signal evaluated for the alarm is made relatively different by the difference between the two The requirements for the stabilization of the light source and the light beam as well as for the sensitivity of the light receiver are accordingly very high.

It can be assumed that with the usual line extinction detectors where there is a measuring section of many meters, the sensitivity must be so great that a weakening of the signal of 4% per meter can be reliably detected. In point extinction detectors, however, the length of the measuring section is only about 5 to 20 cm. If the measuring distance is e.g. 10 cm, the alarm level is 0.4% attenuation. So that this value can be measured reliably, a resolution or drift of less than 0.04% over the desired temperature range of the detection system is required. Due to the temperature response of the individual elements of the system, this problem can only be solved with a special reference measurement method.

Es bedeuten:

T =

Transmission der Luftstrecke T1 = 10 ^-m·d/10 , wobei m = Extinktionsmodul (ISO ''optical density'') in [dB/m] und d = Länge der optischen Strecke

f1, f2 =

Modulationsfrequenz der Strahlungsquellen S1 bzw. S2

K1, K2 =

die optischen Teilverhältnisse bei den Strahlungsteilern

R1(f), R2(f) =

Signalverstärkung folgend den Strahlungsempfängern D1 bzw. D2 als Funktion der Modulationsfrequenz f

SG =

Synchrongleichrichter mit ausgangsseitiger Tiefpaßfilterung

Ai(fj) =

resultierendes Ausgangssignal vom Strahlungsempfänger i entsprechend dem Signalanteil bei der Modulationsfrequenz fj,
   wobei i=1,2 und j=1,2.

One possible solution is the optical bridge circuit (see Figure 1):
Are the output signals of the optical bridge A1 (f1), A1 (f2), A2 (f1) and A2 (f2) expressed by products from the transmission powers of the radiation sources S1 and S2, the beam split ratios K1 and K2 and the sensitivities of the radiation receivers D1 and D2 and if the quotient Q is formed from these four bridge signals, the transmission powers of the radiation sources and the sensitivities of the radiation receivers drop out:

It means:

T =

Transmission of the air path T1 = 10 ^{-m · d / 10} , where m = extinction module (ISO '' optical density '') in [dB / m] and d = length of the optical path

f1, f2 =

Modulation frequency of the radiation sources S1 and S2

K1, K2 =

the optical sub-ratios in the radiation splitters

R1 (f), R2 (f) =

Signal amplification following the radiation receivers D1 or D2 as a function of the modulation frequency f

SG =

Synchronous rectifier with low-pass filtering on the output side

Ai (fj) =

resulting output signal from the radiation receiver i corresponding to the signal component at the modulation frequency fj,
where i = 1.2 and j = 1.2.

The extinction module m in [dB / m] and the light opacity module D in [% / m] can easily be calculated from the transmission T:

The extinction calculated from the measured value Q is thus a function of the sub-ratios K1, K2 and the amplification factors Ri (fj) of the signal amplification, which are generally dependent on the modulation frequency. In practice, it turns out that the latter can hardly be kept constant with the required precision of 0.04% over a larger temperature range. Problems also arise from electronic offsets, channel crosstalk and the sub-ratios K1 and K2, in particular the drifting of all these variables over time and the temperature range.

A solution for realizing a similar reference measurement method for determining the transmission of a route filled with smoke or other fluid is set out, for example, in US Pat. No. 4,018,513 and CH-A-643,061. These systems make it possible to compensate for the changes in the properties of the light sources and the receivers by means of reference measurements. However, they are constructed in such a way that only a limited resolution in the measurement is possible, and they are therefore intended for purposes which permit a longer measuring distance or require a higher resolution.

Starting from this prior art, the object of the invention is to create an extinction smoke detector which avoids the disadvantages of the known smoke detectors and in particular has increased stability and can detect the same extinction values over a considerably shorter measuring distance.

This object is achieved in an optical smoke detector of the type mentioned by the characterizing features of claim 1. Preferred embodiments of the invention and refinements are defined in the dependent claims.

• Figur 1 eine optische Brückenschaltung gemäss dem Stand der Technik,
• Figur 2 ein praktisches Ausführungsbeispiel eines erfindungsgemäßen, punktförmigen Extinktionsrauchmelders,
• Figur 3 eine optische Brückenschaltung gemäß vorliegender Erfindung mit vorgeschalteter elektronischer Zeit-Multiplexbrücke,
• Figur 4 eine weitere Ausführungsform einer erfindungsgemäßen Brückenschaltung mit verdoppelter Sensitivität,
• Figur 5 eine weitere Ausführungsform einer erfindungsgemäßen Brückenschalltung mit aktiver Offset- und Übersprechkompensation,
• Figur 6 zeigt eine weitere Ausführungsform einer erfindungsgemäßen Brückenschaltung mit einer Kombination der Erweiterungen.

The invention is explained in more detail below with reference to the figures. Show it

• FIG. 1 shows an optical bridge circuit according to the prior art,
• FIG. 2 shows a practical exemplary embodiment of a punctiform extinction smoke detector according to the invention,
• FIG. 3 shows an optical bridge circuit according to the present invention with an upstream electronic time multiplex bridge,
• FIG. 4 shows a further embodiment of a bridge circuit according to the invention with doubled sensitivity,
• FIG. 5 shows a further embodiment of a bridge circuit according to the invention with active offset and crosstalk compensation,
• FIG. 6 shows a further embodiment of a bridge circuit according to the invention with a combination of the extensions.

FIGS. 2 and 3 show a practical application example for a punctiform extinction smoke detector according to the invention. The smoke detector has a measuring section T1 accessible to the outside atmosphere and comparison sections U which are not accessible to the outside atmosphere and which are arranged in an integrated optical chip in the form of a glass plate G. This integrated optical component is shown in FIG. It contains all the optical and optoelectronic elements necessary for the sensor. The most important component is a 60 mm x 30 mm x 1.5 mm glass substrate, in which the waveguides U are contained, which serve to guide light in the direction of the arrow.

The light is generated by two radiation sources S1 and S2, the light of which is modulated with different frequencies f1 and f2. The light is coupled into the waveguide U, which is contained within the glass chip, on the principle of "end-fire coupling". AlGaAs LEDs are expediently used as light sources S1 and S2. The light from one radiation source S1 is guided through a Y-shaped division of the waveguide (bifurcation) into two waveguides, the division of the light beam at this bifurcation being given by the division ratio K1. Through the one waveguide it is guided in a 180 ° bend with a radius of 10 mm to a radiation receiver D2, e.g. a photodiode, and it is guided through the second waveguide in a 90 ° arc to the measuring section T1, which is sawn into the side of the glass plate G as a 34 mm long space.

The light is emitted at the beginning of the measuring section T1 through a collimation lens L1, e.g. a gradient-index lens, coupled out of the waveguide U and, after passing through the measurement section T1, coupled through a second collimation lens L2 of the same type as the lens L1 into the second waveguide U, through which it is guided to the other radiation receiver D1.

After coupling into the waveguide U, the light from the second radiation source S2 is likewise guided through a Y-shaped bifurcation into two waveguides, the division being given by the division ratio K2, and in 180 ° arcs with a radius of curvature of 10 mm each to the radiation receivers D1 and D2. The four light signals from S1 and S2 detected by the radiation receivers D1 and D2 are amplified in a first amplifier stage R1 and R2 and synchronously rectified in a second stage, a synchronous rectifier SG with a low-pass filter on the output side; this distinguishes the signals by their different modulation frequencies f1, f2.

The waveguides U are gradient-index multimode strip waveguides which are introduced into the substrate glass by an ion exchange process and are buried under the substrate surface by approximately 10 μm by ion exchange with an applied electric field. This protects the waveguides from surface influences. For the purpose of additional protection and also increased mechanical stability, a second substrate of the same glass type and size is applied to the first glass substrate using an adhesive which has the same refractive index as the glass. The glass is a sodium glass, the sodium ions of which are locally replaced by silver ions and increase the refractive index at these local points. Since the waveguides must have a strip geometry, the ion exchange is only carried out locally according to the desired geometry of the strips. For this purpose, a mask made of evaporated aluminum is applied to the substrate by means of a photolithographic process before the ion exchange process. The aluminum mask just releases the strips where the waveguides are to be formed. After the ion exchange, the mask is removed again and the second protective glass plate is stuck on.

FIG. 3 shows an optical bridge circuit according to the present invention with an upstream electronic time-division multiplex bridge. In operation, there is an alternation between switch positions b and c.

$\mathit{\text{T}}\text{=}\overline{\text{√}\mathit{\text{Qb}}\text{·}\mathit{\text{Qc}}}\text{·}\mathit{\text{K}}\text{1·}\mathit{\text{K}}\text{2}$

Figur 4 zeigt eine weitere Ausführungsform einer erfindungsgemässen Brückenschaltung mit verdoppelter Sensitivität in der symmetrischen Brücke. Hier wird das Licht von beiden Strahlungsquellen S1 und S2 durch Messstrecken T1 bzw. T2 geführt, welche beide der Atmosphäre zugänglich sind und eine Transmission T haben. Die Empfindlichkeit der Brücke wird so verdoppelt wie aus folgender Gleichung ersichtlich ist.

Figur 5 zeigt eine weitere Ausführungsform einer erfindungsgemässen Brückenschaltung mit aktiver Kompensation von elektronischen Offsets und Kanalübersprechen und deren Drift. Die dadurch hinzugefügten Schalter, die durch die logischen Signale SE-f1, SE-f2 (send enables) gesteuert werden, erlauben es, die jeweilige Sende-Quelle fi ein- und auszuschalten. Im folgenden Ablaufdiagramm bezeichnet Ai(fj) den jeweiligen aktuellen Messwert am entsprechenden Ausgang. Die Variablen aij, bij (i = 1, 2; j = 1, 2) dienen der Zwischenspeicherung, Oij ist der aktuelle Kompensationswert. Das korrigierte Messignal erhält man später durch Subtraktion von Oij:
Ai(fj) (korrigiert): = Ai(fj) (gemessen) - Oij.

• 1. SE-f1 = off
  SE-f2 = off
     aij: = Ai(fj); i =1, 2; j = 1,2;
• 2. SE-f1 = on
  SE-f2 = on
     bij: = Ai(fj); i = 1, 2; j = 1, 2;
• 3. SE-f1 = off
  SE-f2 = on
     Oi2: = bi2 - Ai(f2) + ai2; i = 1, 2;
• 4. SE-f1 = on
  SE-f2 = off
     Oi1: = bi1 - Ai(f1) + ai1; i = 1, 2.

The frequency response Ri (fj) can be eliminated from the end result by connecting an electrical time-division multiplex bridge that passes the modulation signals alternately (state b) and then crosses over (state c). Two quotients Ob and Qc are formed in accordance with these two states. Aib (fj) and Aic (fj) denote the output signals Ai (fj), measured in the state b or c:

$\mathit{\text{T}}\text{=}\overline{\text{√}\mathit{\text{Qb}}\text{·}\mathit{\text{Qc}}}\text{·}\mathit{\text{<figure-callout id="1" label="K" filenames="imgf0001.png,imgf0002.png" state="{{state}}">K</figure-callout>}}\text{1·}\mathit{\text{K}}\text{2nd}$

FIG. 4 shows a further embodiment of a bridge circuit according to the invention with doubled sensitivity in the symmetrical bridge. Here, the light from both radiation sources S1 and S2 is guided through measuring sections T1 and T2, both of which are accessible to the atmosphere and have a transmission T. The sensitivity of the bridge is doubled as can be seen from the following equation.

FIG. 5 shows a further embodiment of a bridge circuit according to the invention with active compensation of electronic offsets and channel crosstalk and their drift. The switches added thereby, which are controlled by the logic signals SE-f1, SE-f2 (send enables), make it possible to switch the respective transmission source fi on and off. In the following flow chart, Ai (fj) denotes the respective current measured value at the corresponding output. The variables aij, bij (i = 1, 2; j = 1, 2) are used for temporary storage, Oij is the current compensation value. The corrected measurement signal can be obtained later by subtracting from Oij:
Ai (fj) (corrected): = Ai (fj) (measured) - Oij.

• 1. SE-f1 = off
  SE-f2 = off
  aij: = Ai (fj); i = 1, 2; j = 1.2;
• 2. SE-f1 = on
  SE-f2 = on
  bij: = Ai (fj); i = 1, 2; j = 1, 2;
• 3. SE-f1 = off
  SE-f2 = on
  Oi2: = bi2 - Ai (f2) + ai2; i = 1, 2;
• 4. SE-f1 = on
  SE-f2 = off
  Oi1: = bi1 - Ai (f1) + ai1; i = 1, 2.

• 1. SE-f1 = off
  SE-f2 = on
     Oi1: = Ai(f1); i = 1, 2,
• 2. SE-f1 = on
  SE-f2 = off
     Oi2: = Ai(f2); i = 1, 2.

Four measurements are therefore required to determine the compensation parameters Oij. Less powerful, simplified procedures are also possible. For example, offset and crosstalk can also be determined with the following two measurements. However, the corrections due to distortion and intermodulation are not exact.

• 1. SE-f1 = off
  SE-f2 = on
  Oi1: = Ai (f1); i = 1, 2,
• 2. SE-f1 = on
  SE-f2 = off
  Oi2: = Ai (f2); i = 1, 2.

FIG. 6 shows a further embodiment of a bridge circuit according to the invention with a combination of the extensions a) to c) as were implemented in a bridge circuit according to the invention. It is the overall circuit of an optical bridge with two extinction paths, an upstream electronic time-multiplex bridge and active offset and crosstalk compensation.

Modifications of the above-described circuits for optical smoke detectors are possible within the scope of the invention according to the claims and are familiar to the person skilled in the art.

Claims (3)

Optical smoke detector based on the extinction principle, which contains means for guiding light over a measuring section (T1) accessible to the outside atmosphere and a comparison section (U) inaccessible to the outside atmosphere, which also has two radiation sources (S1, S2) and two radiation receivers (D1, D2) and this downstream input amplifier (R1, R2), which generates receive pulses proportional to the received radiation intensity, and an evaluation circuit, which contains switching elements which are set up in such a way that they are able to form the quotient of the received pulses, and contains further switching elements which are set up in this way that they are able to emit an alarm signal when the received pulses indicate that a predetermined amount of smoke is exceeded in the measuring section (T1), characterized in that the measuring section (T1) and the comparison section (U) are arranged in an integrated optical chip, whereby the comparison route (U) in the form of We lllenleitern are arranged in a glass plate (G) and the light from the radiation source (S1) to the measuring section (T1) and from the measuring section (T1) to the radiation receiver (D1) is guided by waveguides formed in the glass plate (G). Optical smoke detector according to claim 1, characterized in that the two radiation sources (S1, S2) emit light of different modulation, that the optical bridge circuit is preceded by an electrical time-division multiplex bridge which alternately passes the modulation signals straight (state b) and then crosses (state c) and that the evaluation circuit contains switching elements which eliminates the frequency responses Ri (fj) from the end result by forming two quotients Qb and Qc corresponding to these two states, with Aib (fj) and Aic (fj) the output signals Ai ( fj) denote measured in state b or c.

$\mathit{\text{T}}\text{= √}\overline{\mathit{\text{Qb}}\text{·}\mathit{\text{Qc}}}\text{·}\mathit{\text{K}}\text{1·}\mathit{\text{K}}\text{2nd}$

Optical smoke detector according to claim 1, characterized in that the two radiation sources (S1, S2) can be switched on and off by logic signals and an offset value can be determined by switching off both radiation sources (S1, S2) and crosstalk compensation values are determined can by first switching off the first radiation source and switching on the second and then switching on the first radiation source and switching off the second.

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