Note: Descriptions are shown in the official language in which they were submitted.
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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-
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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
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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
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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
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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
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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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