GB1566624A

GB1566624A - Optical fire-detector

Info

Publication number: GB1566624A
Application number: GB35588/77A
Authority: GB
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1976-09-01
Filing date: 1977-08-24
Publication date: 1980-05-08
Also published as: DE2736224C2; CH621425A5; DK148226B; NO140519B; SE395554B; NO773020L; FR2363840A2; FI772470A; FR2363840B2; FI63125C; BE858192A; DE2736224A1; NL7709640A; DK148226C; AU2838477A; NO140519C; CA1102429A; DK386877A; FI63125B; AU510627B2

Abstract

A radiation transmitter (20) generates a beam, generated by a modulator (22), with phase-inverted relationship in a first wavelength band and a second wavelength band. After passing through the room to be monitored, the said beam is received by a radiation detector (26). The latter has a first device (27, 28) for individual measuring of the radiation intensity in the two wavelength bands and a second device (38, 40, 42) for determining the changes in intensity. Between the first device and the second device there is connected a demodulator (30), in which a summing device (31) is connected to two radiation-sensitive elements (27, 28) of the first device. A multiplying device (34) shifts the gain in a single path synchronously with the mutually phase-inverted modulation in the two wavelength bands. This achieves the effect that the flickering caused, for example by the ambient electric lighting or by mechanical effects (heavy traffic on wet road or the like), is suppressed well. <IMAGE>

Description

(54) AN OPTICAL FIRE-DETECTOR (71) We, TELEFoNAKTIEBoLAGET L M ERICSSON, a company organised under the laws of Sweden, of SH126 25 Stockholm, Sweden, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to an optical firedetector.

According to this invention there is provided an optical fire-detector comprising: a) radiation - emitting means for emitting a beam of radiation which comprises first and second sub-beams, the first subbeam being in a first wave-length band and the second sub-beam being in a second wave-length band, the radiation - emitting means including modulator means for modulating the sub-beams in a phase-inverted relationship; and b) radiation-detecting means for receiving the beam of radiation after this has passed through an intermediate air medium which may contain a fire, the radiation-detecting means comprising:--- 1) first means, for measuring the intensity of the received beam in the said first and second wave-length bands and including first and second radiation-sensitive devices with associated filter means; 2) demodulator means coupled with the output side of the first means and including summation means and amplifying means for shifting the gain in the demodulator means between a positive and a negative value in synchronism with the mutually phase-inverted modulation; and 3) second means, coupled with the output side of the demodulator means for detecting variations of the demodulated measured intensities characteristic of a fire.

The summation means could be coupled with the said first and second radiation sensitive devices via an inverting and a noninverting input respectively, the said amplifying means being connected in cascade with the summation means.

Alternatively, the summation means could be coupled with the said first and second radiation sensitive devices via first and second indentical inputs and first and second amplifier devices included in the said amplifier means respectively, the amplifier devices being controlled in phase with each other.

In one example, the amplifier means has a control input coupled with the said first means via pulse shaping means.

The amplifier means could have a control input coupled with the said first means via a phase-locked oscillator circuit.

In another example, the amplifier means has a control input coupled with a control oscillator for driving the said modulator means of the radiation-emitting means.

The invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 shows an embodiment of an optical fire-detector which detects heat from a fire and Figure 2 shows an embodiment of an optical fire-detector which detects heat and smoke from a fire.

Figure 1 shows an optical fire-detector in which 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 phase inverter 3, a mutually phase-inverted modulation of the radiation intensity of a green wavelength band of radiation produced by a light emitting diode 4 and an infra-red wavelength band of radiatioi produced by a light emitting diode 5. A radiation detector 6 is placed at a distance from the radiationemitting means 1 for receiving the 'beam of radiation after this beam has passed through an intermediate air medium. This radiation detector 6 comprises two photo-transistors 7 and 8 which are arranged to achieve sepa rate measurements of the radiation intensity in the green and the infra-red wavelength bands respectively.For this purpose, a dich- roic filter 9 is placed in front of the phototransistor 7, in the path of the incoming beam of radiation, and is arranged at an angle of 45 degrees with respect to this path.

The second photo-transistor 8 is placed in the path of a part of the incoming beam of radiation which part is reflected by the dich roic filter 9. The dichroic filter 9, which is known per se, in this case transmits the green part of the beam of radiation to the photo-transistor 7 and reflects the infra-red part of the beam of radiation to the phototransistor 8.

In the radiation-emitting means 1, a second dichroic filter 10 is placed in the path of the outgoing green radiation from the light emitting diode 4 and is arranged in an angle of 45 degrees with respect to this path.

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 that of 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, achieving therefore a practically lossless summation of the radiation produced by the light emitting diodes 4 and 5.

The radiation detector 6 comprises a demodulator 11 in which summation means 12 has an inverting input and a non-inverting input connected with the photo-transistors 7 and 8 respectively, via alternating voltage amplifiers 13 and 14 respectively.

The summation means 12 produces a summation signal derived from the mutually phase-inverted modulation of the green and infra-red wavelength sub-beams in the beam of radiation produced by the radiation emitting means 1. The summation signal is supplied to an amplifier 15 which is arranged to shift the gain of the demodulator 11 between a positive and a negative value in synchronism with the mutually phase-inverted modulation of the green and infra-red wavelength sub-beams in the beam of radiation.

The method utilized for modulation and demodulation gives the demodulated summation signal the property of good discrimination against flicker generated by ambient electrical illumination.

The amplifier 15 has a control input connected with an output of the summation means 12 via pulse shaping means 16. A suitable embodiment for the amplifier 15 is described in the publication "Electronics" January 9, 1975, p. 113. The pulse shaping means 16 comprises a voltage comparator with an earthed reference input.

The demodulator 11 is connected with an AM-detector 17 for detection of such an amplitude modulation for the incoming beam of radiation which is representative of the presence of heat. For this purpose, the AMdetector 17 comprises a band-pass filter which, in this example, is arranged to pass the frequency range: 10-100 Hz. The AMdetector 17 is connected with a heat alarm output 18 via integrating and threshold detecting means 19.

Referring to Figure 2, radiation-emitting means 20 is arranged to generate an outgoing beam of radiation and comprises the same means as the radiation-emitting means 1 shown in Figure 1. These means comprise a sine-wave oscillator 21, a phase inverter 22 which is controlled by the sine-wave oscillator 21 and arranged to provide a phaseinverted modulation of the radiation intensity produced by a green light-emitting diode 23 and an infra-red light-emitting diode 24.

The means further comprise a dichroic filter 25 for summation of the radiation from the light-emitting diodes 23 and 24 into an outgoing beam of radiation which corresponds to the outgoing beam of radiation produced by the radiation-emitting means 1 shown 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 radiation provided by reflection of the outgoing beam of radiation by means of a remote reflector (not shown). Like the radiation detector 6 shown in Figure 1, the radiation detector 26 comprises two phototransistors 27 and 28, a dichroic filter 29 and a demodulator 30. The demodulator 30 comprises summation means 31 which has two identical inputs connected with the phototransistors 27 and 28 respectively via alternating voltage amplifiers 32 and 33 respectively and amplifier means 34. The amplifier means 34 comprises two amplifiers 35 and 36 which are controlled in phase with each other and arranged to shift the gain of the demodulator 30, between the photo-transistors 27 and 28 and their respective inputs to the summation means 31, between a positive and a negative value in synchronism with the mutually phase-inverted modulation of the green and infra-red wavelength sub-beams in the beam of radiation produced by the radiation-emitting means 20.

The amplifiers 35 and 36 have respective control inputs connected with the sine-wave oscillator 21 of the radiation-emitting means 20 via common pulse shaping means 37, which is included in the demodulator 30 and comprises a voltage comparator with an earthed reference input.

The radiation detector 26 also comprises an AM-detector 38 for detection of such amplitude variations in the incoming beam of radiation which are representative of the presence of heat. The AM-detector 38, which is connected with the photo-transistors 27 and 28 via the amplifiers 35 and 36 respec tively of the amplifier means 34 and via the identical inputs of the summation means 31, is fed with a difference signal derived from the mutually phase-inverted modulation of the green and infra-red wavelength sub beams in the beam of radiation produced by the radiation-emitting means 20. The AM detector 38 comprises, besides a band-pass filter for the frequency range: 10-100 Hz, amplification means for raising the signal level before detection.

The AM-detector 38 is connected with a heat alarm output 39 via integrating and threshold detecting means 40.

In the radiation detector 26, the summa tion means 31 is further connected with a smoke alarm output 41 via integrating and threshold detecting means 42 - which thus is fed with the same diflerence signal as that fed to the AIM-detector 38. The polarity of the difference signal for producing a smoke alarm is normally 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 suitable example is described in "Electronics", September 5, 1974 p. 113-114.

In the fire detector shown in Figure 2, the design of the heat alarm as well as that of the smoke alarm is based on the experience that fire influences a beam of radiation to a different degree within two different wave length bands and therefore can be detected by a difference measurement. This enables the design of a heat and smoke alarm with good discrimination against flicker generated by ambient electrical illumination and, furthermore, good discrimination against such flicker which is generated when mech anical vibrations caused by, for example, heavy street traffic vary the outgoing direc tion of the beam of radiation produced by the radiation emitting means 20.

The photo-transistors 7 and 8 in Figure 1 and 27 and 28 in Figure 2 could be of Aphoto-Darlington type with two or even three transistor elements. This is possible because the principle utilized for modulation and demodulation permits good discrimination against flicker generated by ambient electri cal illuamination at a low modulation fre quency, for example of the order of 1 kHz, which means that the total rise time may amount to the order of 100 jays. The integra ting and threshold detecting means 19 in Figure 1 and 40 in Figure 2 could be provided with such means, for more effective heat detection, which are described in German Patent No. 2,051,640.At low intensities of the beam of radiation received by the radiation detector 6 in Figure 1, the pulse shaping means 16 could 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.

WHAT WE CLAIM IS: 1. An optical fire-detector comprising: a) radiation-emitting means for emitting a beam of radiation which comprises first and second subeams, the first sub-beam being in a first wave-length band and the second sub-beam being in a second wave-length band, the radiation-emitting means including modulator means for modulating the sub-beams in a phase-inverted relationship; and b) radiation-detecting means for receiving the beam of radiation after this has passed through an intermediate air medium which may contain a fire, the radiation-detecting means comprising: 1) first means, for measuring the intensity of the received beam in the said first and second wave-length bands and including first and second radiation-sensitive devices with associated filter means; 2) demodulator means coupled with the output side of the first means and including summation means and amplifying means for shifting the gain in the demodulator means between a positive and a negative value in synchronism with the mutually phase-inverted modulation; and 3) second means, coupled with the output side of the demodulator means for detecting variations of the demodulated measured intensities characteristic of a fire.

2. An optical fire-detector according to claim 1, wherein the said summation means is coupled with the said first and second radiation-sensitive devices via an inverting and a non-inverting input respectively, the said amplifying means being connected in cascade with the summation means.

3. An optical fire-detector according to claim 1, wherein the said summation means is coupled with the said first and second radiation sensitive devices via first and second identical inputs and first and second amplifier devices included in the said amplifier means respectively, the amplifier devices being controlled in phase with each other.

4. An optical fire-detector according to any of claims 1 to 3, wherein the said ampli

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Claims

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of radiation which are representative of the presence of heat. The AM-detector 38, which is connected with the photo-transistors 27 and 28 via the amplifiers 35 and 36 respec tively of the amplifier means 34 and via the identical inputs of the summation means 31, is fed with a difference signal derived from the mutually phase-inverted modulation of the green and infra-red wavelength sub beams in the beam of radiation produced by the radiation-emitting means 20. The AM detector 38 comprises, besides a band-pass filter for the frequency range: 10-100 Hz, amplification means for raising the signal level before detection.

The AM-detector 38 is connected with a heat alarm output 39 via integrating and threshold detecting means 40.

In the radiation detector 26, the summa tion means 31 is further connected with a smoke alarm output 41 via integrating and threshold detecting means 42 - which thus is fed with the same diflerence signal as that fed to the AIM-detector 38. The polarity of the difference signal for producing a smoke alarm is normally 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 suitable example is described in "Electronics", September 5,

1974 p. 113-114.

In the fire detector shown in Figure 2, the design of the heat alarm as well as that of the smoke alarm is based on the experience that fire influences a beam of radiation to a different degree within two different wave length bands and therefore can be detected by a difference measurement. This enables the design of a heat and smoke alarm with good discrimination against flicker generated by ambient electrical illumination and, furthermore, good discrimination against such flicker which is generated when mech anical vibrations caused by, for example, heavy street traffic vary the outgoing direc tion of the beam of radiation produced by the radiation emitting means 20.

The photo-transistors 7 and 8 in Figure 1 and 27 and 28 in Figure 2 could be of Aphoto-Darlington type with two or even three transistor elements. This is possible because the principle utilized for modulation and demodulation permits good discrimination against flicker generated by ambient electri cal illuamination at a low modulation fre quency, for example of the order of 1 kHz, which means that the total rise time may amount to the order of 100 jays. The integra ting and threshold detecting means 19 in Figure 1 and 40 in Figure 2 could be provided with such means, for more effective heat detection, which are described in German Patent No. 2,051,640.At low intensities of the beam of radiation received by the radiation detector 6 in Figure 1, the pulse shaping means 16 could 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.

WHAT WE CLAIM IS: 1. An optical fire-detector comprising: a) radiation-emitting means for emitting a beam of radiation which comprises first and second subeams, the first sub-beam being in a first wave-length band and the second sub-beam being in a second wave-length band, the radiation-emitting means including modulator means for modulating the sub-beams in a phase-inverted relationship; and b) radiation-detecting means for receiving the beam of radiation after this has passed through an intermediate air medium which may contain a fire, the radiation-detecting means comprising: 1) first means, for measuring the intensity of the received beam in the said first and second wave-length bands and including first and second radiation-sensitive devices with associated filter means; 2) demodulator means coupled with the output side of the first means and including summation means and amplifying means for shifting the gain in the demodulator means between a positive and a negative value in synchronism with the mutually phase-inverted modulation; and 3) second means, coupled with the output side of the demodulator means for detecting variations of the demodulated measured intensities characteristic of a fire.
2. An optical fire-detector according to claim 1, wherein the said summation means is coupled with the said first and second radiation-sensitive devices via an inverting and a non-inverting input respectively, the said amplifying means being connected in cascade with the summation means.
3. An optical fire-detector according to claim 1, wherein the said summation means is coupled with the said first and second radiation sensitive devices via first and second identical inputs and first and second amplifier devices included in the said amplifier means respectively, the amplifier devices being controlled in phase with each other.
4. An optical fire-detector according to any of claims 1 to 3, wherein the said ampli

fier means has a control input coupled with the said first means via pulse shaping means.
5. An optical fire-detector according to any of claims 1 to 4, wherein the said amplifier means has a control input coupled with the said first means via a phase-locked oscillator circuit.
6. An optical fire-detector according to any of claims 1 to 3, wherein the said amplifier means has a control input coupled with a control oscillator for driving the said modulator means of the radiation-emitting means.
7. An optical fire-detector substantially as herein described with reference to Figure 1 or Figure 2 of the accompanying drawing.