CA1102429A

CA1102429A - Optical fire-detector

Info

Publication number: CA1102429A
Application number: CA285,212A
Authority: CA
Inventors: Erik Lindgren
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1976-09-01
Filing date: 1977-08-22
Publication date: 1981-06-02
Also published as: BE858192A; AU510627B2; CH621425A5; DE2736224C2; IT1088004B; NL7709640A; FR2363840A2; DK386877A; NO140519C; DK148226B; FI772470A; FI63125B; NO140519B; FR2363840B2; SE395554B; FI63125C; NO773020L; AU2838477A; GB1566624A; DK148226C

Abstract

ABSTRACT OF THE DISCLOSURE
The invention relates to an optical fire-detector in which a radiation-emitting means is arranged to emit a beam of radiation and comprises modulator means for modulation of the beam of radiation in a phase-inverted relationship within a first and a second wavelength band and a radiation-detecting means is arranged to receive the beam of radiation after that this has passed through an intermediate air medium and comprises first means for an individual sensing of intensity in said two wavelength bands and second means for detection of such variations in the sensed intensities which are representative for fire.

Description

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:: The present invention relates to an op-tical fire-detector in which a radiation-emitting means is arranged to emit a beam of radiation and comp:rises modulator means for modulation of the beam of radia-tion in a phase-inverted relationship within a first and a second wavelength band and a radiation-~: detecting means is arranged to receive the beam af radiation after this has passed throuyh an intermediate air medium and - comprises first means for an indi.vidual sensing of intensity `- in said two wavelength bands and second means for detection of such variatlons in the measured intensities which are represent--:- ative for fire.
- ~ photoelectric detecting appara-tus which can be used as a fire detector is described in the Canadian Patent No.
; 662,224, Ap.ril 30, 1963 Steele et al. The apparatus comprises . at least two light responsi.ve elements, one element being predominantly responsive to a first band of frequencies of the light spectrum and the other element being predominantly similarly responsive to a substantially different band of frequencies. The fire detector cân obtain a good discrimination against flicker generated by the surrounding electrical illumination by a method which is described in British Ratent No. 1,405,615, August 2, 1973 (.Chubb Fire Security Limited) and according to which the beam of radiation is emitted in the form of a series of thin high-power pulses, the radiation-detecting means being arranged to be fre~uency-sel.ective for the rise time of the pulses.
. One drawback with this known method is, however, that the fire detector achieves the desired discrimination against flicker generated by the surrounding electrical illumination j 30 only if the radiation detector as well as the radiation-emitting : means has a short rise time of the order of ~s. Then, this method does not enable an efficient use of such radiation-- 1 - ~i Z~

sensitive elements in which a high sensitivity is achieved at the cost of a long rise time of the order of 100 ~s.
The first wavelength band of radiation is the green light band and the second wavelength band of radiation is the .~ infrared light band. Since the shor-ter wavelength green light is attenuated more by presence o~ smoke in the air than does the longer wavelength infrared light, differences in the intensities : of the s~nsed radiations may be an indication of smoke in the air and thus of fire.
Air heated by fire has a refractive index different from that of cold air. Movements in the air caused by fire induces flickering in the transmitted radiation with a frequency - characteristic of open fire~ Sorting out this frequency enables avoiding undue fire alarms.
: The optical fire detector according to the invention achieves a good discrimination against flicker generated by surrounding electrical illumination without demanding a short rise time neither at the radiation detector nor at the radiation emitting means and enables an improved discrimination against such flicker which is generated when mechanical vibrations for example caused by heavy street traffic vary the outgoing direction of the beam of radiation from the radiation-emitting means.
According to the present lnvention there is provided an optical fire-detector in which a radiation-emitting means is arranged to emit a beam of radiation and comprises modulator means for modulation of the beam of radiation in a phase-inverted relationship within a first and a second wavelength band and a radiation-detecting means is arranged to receive the beam of : 30 radiation after this has passed through an intermediate air medium and comprises ~irst means for an individual sensing of the radiation intensity in said two wavelength bands and . ~

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second means for detection of such variations in the sensed intensities which are representative for fire, the improvement in which the radiation detector comprises a demodulator which is connected between said first and second means and in which a summation means is connected to a First and second radiation-sensitive element which are included in said first means and are arranged to selectively sense a respective wavelength band of sai.d two wavelenyth bands in the beam of radi.ation and a rnultiplier means is arranged to shift the yain in a si.gnal path in the demodulator between a positive and a negative value in synchronism with the mutually phase-inverted modulation within said two wavelength bands in the beam of radiation.
In one embodiment of the present invention the summation means is connected to said first and second radiation-sensitive ~' .
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element via an inverting and a not-inverting input respectively, said multiplier means being connected in cascade with the ~ummation means.
In another embodiment of the present inVention the summation means is connected to said first and second radiation sensitive element via two iden-tically equal inputs and two multiplier elements included in said multiplier means and controlled in phase with each other.
In a further embodiment of the present invention the multiplier means has a control input connected to said radiation~
measuring irst means via a pulse shaping means.
In a still further embodiment of the pxesent invention the multiplier means has a control input connected to said radiation-measuring first means via a phase-locked oscillator circuit.
In a further embodiment of the present invention the multiplier means has a control input connected to a control oscillator which is arranged to drive said modulator means of the radiation-emitting means.
The present invention will be further illustrated by way of the accompanying drawing where Figure 1 shows a preferred embodiment of an optical heat-detector and Figure 2 shows a preferred embodiment of an optical heat- and smoke~detector.
Figure 1 shows a preferred embodiment of an optical heat-detector according to the invention. A radiation-emitting means 1 is arranged to generate an outgoing beam of radiation and - comprises a sine-wave oscillator 2 arranged to achieve via a ~0 phase inverter 3 a mutually phase-inverted modulation of the radiation intensity within a green wavelength band of a radiation contribution from a light emitting diode 4 and an lnfra-red .' .
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wavelength band of a radia-tion contribution from a light emi-tting diode 5, respectively. A radiation detector 6 is placed at a distance from the raaiation-emitting means 1 for receiving the beam of radiation after that this has passed an intermediate air medium and comprises two photo-tran~istors 7 and 8 which are arranged to achieve a separate measurement of the intensity in the green and the infra-red wavelength band respectively in the beam of radiation. For this purpose a dichroic filter 9 is placed in front of the photo-transistor 7 in the path of the received beam of radiation and is arranged in an angle of 45 degree relatively this path, the second photo-transistor 8 being placed-in the path of a part of khe received beam of radiation reflected by the : dichroic filter 9. The filter 9, which is known per se, transmits according to the example the green part of the beam o~ radiation to the photo-transistor 7 and reflects the infra-red part of the beam of radiation to the photo-transistor 8.
In the radiation-emitting means 1, a second dichroic filter 10 is placed in the path for the outgoing green radiation . from the light emitting diode 4 and is arranged in an angle of 45 degree relatlvely this path, and the second light emitting diode 5 is placed so that its outgoing infra-red radiation is reflected by the filter 10 out into the same path as the outgoing green radiation from the light emitting diode 4. The green radiation from the light emitting diode 4 and the infra-red radiation from the~light emitting diode 5 are transmitted and reflected respectively by the filter 10 substantially without any loss. This achieves thus a practically lossless summation of the radiation from the light emitting diodes 4 and 5.
: ' According to the invention the radiation detector 6 comprises a demodulator 11 in which a summation means 12 according to the example has an înverting and a not-inverting input connected to the photo-transistor 7 and to the photo-transistor 8 respectively ~f z~
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via ~ alternating voltage ~i~i-er 13 and 14 respectively.
The summation means 12 produces a summation signal derived from - the mutually phase-inverted modulation within the green and infxa-red wavelength band in the beam of radiation from the radiation emittinq means 1. The summation signal is supplied to a multipli.er : 15 which is arranged to shift the gain of the demodulator 11 between a positive an-d a negative value in synchronism wlth the mutually phase-lnverted modul.ati.on within the yreen and infra-red wavelength band in the beam of radiatlon from the radiation emltting means 1. The utilized method for modulation and demodula-tion gives the demodulated summation signal the property of a good . discrimination against fli.cker generated by surrounding electrical - illumination.
According to the example the multipller 15 has a control input connected -to an output of the summatlon means 12 via a pulse shaping means 16. A sultable embodiment for the multiplier 15 is described in the publication Electronics, January 9, 1975, p. 113. The pulse shaping means 16 consists according to the example of a voltage comparator with an earthed reference ~- 20~ input.
- The demodulator is connected to an AM detector 17 for detection of such an amplitude modulation in the received beam . of radiation which is representative for heat. For this purpose the'AM-detector 17 comprises a band-pass filter which according to the example is arranged to pass the frequency interval 10-100 H~.
-: The AM-detector 17 is connected to a heat alarm output 18 via an integrating and threshold detecting means 19.
. Figure 2 shows a preferred embodiment for an optical . ' heat and smoke detector accord.ing to the invention. A radiation-. 30 emitting means 20 is arranged to genera~e an outgoing beam of : radiation and comprises the same means as the radiation-emitting means 1 in Figure 1, namely a sine-wave oscillator 21, a phase . -- 5 .'-''' '- .

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inverter 22 controlled by the sine-wave oscillator 21 and arranged . -to provide a phase-inverted modulation of the radiation intensity of a green emitting light emitting diode 23 and an infra red emitting light emitting diode 24 and a dichroic filter 25 for summation of the radiation from the light emitting diodes 23 and 24 into an outgoing beam of radiation which completely corresponds to the outgoing beam of radiation in Figure 1.
A radiation detector 26 is placed side by side with -the radiation-emitting means 20 and is arranged to receive an incoming beam of radia-tion generated by reflection of the outgoing beam of radiation by means of a remote reflector (not shown). The radiation detector 26 comprises like the radiation detector 6 in Figure 1 two photo-transistors 27and 28, a dichroic filter 29 and a demodulator 30. In the demodulator 30 a summation means 31 is .
included which according to the example has two identically equal inputs connected to the photo-transistor 27 and to the photo-transistor 28 respectively via an alternating voltage amplifier 32 and 33 respectively and a multiplier means 34 which comprises - two multipliers 35 and 36 controlled in phase with each other and arranged to shift the gain between the photo-transistors 27 and 28 and their respective connected inputs of the summation means 31 between a positive and a negative value in synchron.ism !~ with the mutually phase-inverted modulation within the green and infra-red wavelength band in the beam of radiation from the radiation-emitting means 20.

According to the example the multipliers 35 and 36 have ~ s respective control ~ ~t connected to the sine-wave oscillator 21 in the radiation-emitting means 20 via a common pulse shaping means 37 which is included in the demodu].ator 30 and consists of a voltage comparator with an earthed reference input.
The radiation de-tector Z6 comprises an AM-detector 38 for detection of such amplitude variations in the received beam

2~;29 of radia-tion which is representative for heat. The AM-dekector 38, which according to the example ls connected to the photo-transistors 27 and 28 via said multipliers 35 and 36 of the multiplier means 34 and via said identically equal inputs of -the summation means 31, is fed with a difference signal derived from the mutually phase-inverted modulation within the green and infra-red ~avelength band in the beam of radiation from the radiation-emitting means 20. The AM-detector 38 comprises besides a band pass filter for the frequency interval 10-100 Hz an amplification means for raising the signal level before de-tectionO 'rhe AM-detector 38 is connected to a heat alarm output 39 via an integrating and threshold detecting means ~0.
In the radiation detec-tor 26 the summation means 31 is further connected to a smoke alarm output ~1 via an integrating and threshold detecting means 42 which thus is fed with the same difference signal as the AM-detector 38. The polarity of the difference signal for smoke alarm is norma]ly predetermined but if this is not the case then said threshold detection can be carried out by means of-a window comparator for which a ', 20 suitable embodiment is described.in Electronics, September 5, `.' p. 113-114.
In the heat and smoke detector in Figure 2 the function : for heat alarm as well as for smoke alarm is based on the experience that f`ire influences a beam o radiation in a diferent degree within two different wavelength band and therefore can be detected by a difference measurement. This principal function , enables heat and smoke alarm with a very good discrimination , . agains-t flicker generated by surrounding electrical illumination . , and gives furthermore a good discrimination against such flicker ,~ 30 which is generated when mechanical vibrations caused by,for .`. example heavy street traffic vary the outgoing direction of the beam of radiation from the radiation emitting means 20.

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.~ The invention is not limi-ted to the described embodiment but can be modified in many ways within the scope of the appended claims. For example, the photo-transistors 7 and 8 in Figure 1 and 27 and 28 in Figure 2 can be of photo-.darlington type with two or even three transistor elements thanks to the fact that the utilized principle for modulation and demodulation enables a good discrimination against flicker possible generated by surrounding electrical illumination also at a low modulation frequency or example of the order of 1 kHz which means that the entire rise -time is allowed to amount to the order of 100 lIS. The integrating and threshold detecting means 19 in Figure 1 and 40 in Figure 2 can ~e provided with such means for a more effective heat detection which are described in the German Pa-tent 2 051 640. At a low intensity of the beam of radiation received by the radiation detector 6 in Figure 1 the - pulse shaping means 16 can suitably be connected to the output . of the summation means 12 via a phase-locked oscillator of a - . known construction for providing a phase shift of 0 degrees between the outgoing and the incoming signal.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In an optical fire-detector in which a radiation-emitting means is arranged to emit a beam of radiation and comprises modulator means for modulation of the beam of radiation in a phase-inverted relationship within a first and a second wave-length band and a radiation-detecting means is arranged to receive the beam of radiation after that this has passed through an intermediate air medium and comprises first means for an individual sensing of the radiation intensity in said two wavelength bands and second means for detection of such variations in the sensed intensities which are representa-tive for fire, the improvement in which the radiation detector comprises a demodulator which is connected between said first and second means, and in which a summation means is connected to first and second radiation-sensitive elements which are included in said first means and are arranged to selectively sense a respective wavelength band of said two wavelength bands in the beam of radiation and a multiplier means is arranged to shift the gain in a signal path in the demodulator between a positive and a negative value in synchronism with the mutually phase-inserted modulation within said two wavelength bands in the beam of radiation.

2. Optical fire-detector according to claim 1, in which said summation means is connected to said first and second radiation-sensitive elements via an inverting and a not-inverting input respectively, said multiplier means being connected in cascade with the summation means.

3. Optical fire-detector according to claim 1, in which said summation means is connected to said first and second radiation sensitive elements via two identically equal inputs and two multiplier elements included in said multiplier means and controlled in phase with each other.

4. Optical fire-detector according to claim 1, 2 or 3, in which said multiplier means has a control input connected to said radiation-sensitive first means via a pulse shaping means.

5. Optical fire-detector according to claim 1, 2 or 3, in which said multiplier means has a control input connected to said radiation-sensitive first means via a phase-locked oscillator circuit.

6. Optical fire-detector according to claim 1, 2 or 3, in which said mulitplier means has a control input connected to a control oscillator which is arranged to drive said modulator means of the radiation-emitting means.