Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to sensing, and indica-
ting presence of a fire, and more particularly to indicate
presence of a fire reliably, while rejecting spurious responses,
which might lead to false alarms. Specifically, the present
invention relates to a method to reliably detect a fire, and to
a system to carry out the method, in which electrical signals
are processed.
Fire alarm systems in which an alarm signal is gene-
rated as soon as a characteristic of a fire exceeds a certain
threshold value are known. Various types of fire sensors respond,
specifically, to certain characteristics of fires. Difficulties
arise when such fire sensors are used in rooms, or spaces in
which the specific characteristic of the fire may occur for a
short period of time, intermittently, or in pulses, without a
fire actually being present. One such sensor uses smoke as the
sensing characteristic. In offices, garages, automotive repair
shops, and the like, smoke may arise, intermittently, without a
fire actually being present. Fire sensors installed in such
locations may provide false alarms. It has been tried to avoid
the undesirable influence of such pulsed smoke on the sensors
by introducing a time delay, either mechanically, or electric-
ly. A mechanical time delay can be introduced by partly masking,
or interfering with the accessibility of a measuring chamber of
the fire sensor to free ingress of air, for example by means
of covers, shrounds, or the like. Time delays can also be
introduced electrically by electrically delaying a signal derived
from the sensor. Delaying the response of the sensor has the
result that the threshold value of the response circuit connec-
ted to the sensor will be reached only after a predetermined
time corresponding to the delay time.
Interfering with free air circulation as a way to
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introduce a time delay has a decided disadvantage. Slowly
developing fires such as smoldering fires are sensed only
with difficulty, or poorly. Usually, therefore, the second
solution has been used, that is, a signal derived from the
sensor is delayed in time. Disturbances due to single, short-
time smoke pulses, which might have led to a false alarm, can
be avoided by this solution. If, however, sequential smoke
pulses follow each otherin relatively short periods of time,
the known fire sensors and systems will integrate the signal,
ln and an average value will result which may well he above the
alarm threshold of the response circuit so that a false alarm
is triggered, although with delay. Sequential smoke pulses
are frequent in actual practice, for example due to heavy smo-
king by various people in a small space.
It is an object of the present invention to provide
a fire alarm system and method, in which false alarms are essen-
tially avoided although the sensors are subjected to pulsed
influences, characteristic of fires, and to which the sensors
can respond.
Subject matter of the present invention: Briefly,
the method includes a plurality of step: tl) The electrical
signal from the sensing element is processed to determine if it
exceeds a first threshold level. If it does, an output signal
is generated, independent of the input signal, so long as
the input signal exceeds the threshold value.
. .
t2) The output signal which exceeds the first
threshold is applied to an integrator which is non-symmetrical,
that is, has two time constants, one long or slow time constant
if the signal is increasing, and a short or rapid time constant
if the signal is decreasing; thus, the charge time constant
of the integrator will be substantially greater, or longer, than
the discharge time constant.
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(3) The output signal from the integrator is applied
to a second threshold detector which provides the alarm signal
when its threshold level is exceeded.
In accordance with a feature of the invention, the
system includes an integrator which has a diode in its integra-
ting circuit so poled that it is blocked upon a rising signal
applied thereto, so that the time constant of the integrator
will be determined by circuit parameters without consideration
of the diode and, for decreasing input signals, the diode
becomes conductive to procide a low resistance, short time
constant discharge path for the integrating capacitor of the
integrator.
The invention will be described by way of example
with reference to the accompanying drawings, wherein:
Fig. 1 is a high~y schematic diagram of a fire alarm
system in accordance with the present invention; and
Fig. 2 is a series of graphs illustrating the operation
of the circuit of Fig. 1, and the method, in accordance with
the present invention.
The fire alarm sensor selected for the example is an
ionization-type fire sensor. Referring to the circuit of Fig. 1,
an ionization chamber 1, acting as the fire sensor, is connec-
ted in series with a reference chamber 2. The ionization cham-
ber 1 can be referred to as the sensing chamber, and is accessi-
ble to outside, ambient atmosphere. Chambers 1, 2 are series
connected between energized conductors 3, 4. The junction 5
between the chambers 1, 2 will have a voltage arise thereat
which changes continuously as the smoke or aerosol concentra-
tion in the sensing chamber 1 changes. This voltage is applied
to a threshold detector 6. The threshold detector 6 will have
an output only if the input thereto, from unction 5, exceeds a
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certain threshold value. The output from threshold detector
6 is applied to an integrator I, which includes a resistor 7,
having a resistance value r. The voltage across resistor 7 is
applied to capacitor 8 over a resistor 9, which has a resistan-
ce R. The capacitor 8 is charged with a time constant of
RC, in which C is the capacitance of capacitor 8. ~hen the
voltage across capacitor 8 reaches the alarm threshold of a
second threshold detector 10, threshold detector ln will gene-
rate an alarm signal. The time required after the first
threshold detector 6 provided an output signal exceeding its
first threshold value until the second threshold detector provi-
des an alarm is determined by the time constant of the R/C
circuit determined by resistor 9 and capacitor 8. This is the
alarm delay time.
A diode 11 is connected in parallel to resistor 9~
The diode 11 is so poled that the voltage across capacitor 8
can discharge through the diode. As soon as the voltage from
the first threshold detector 6 drops, and specifically when the
signal from the sensor drops below the threshold level of the
first threshold detector 6, capacitor 8 can discharge through
diode 11. This discharge will occur with the time constant
rC, in which the time constant of the discharge circuit is gover-
ned by the resistance value of resistor 7 and the capacitance
of capacitor 8. resistor 7 is selected to be substantially less
than resistor 9. The capacitor discharge will, then, occur
much more rapidly when the signal from the sensor 1 ~sappears
than the charge build-up on the capacitor 8 when the first
threshold level is exceeded.
The operation of the circuit is best seen by reference
to Fig. 2, in which also curves showing the operation of known
systems are illustrated. Curve S is indicative of separate
smoke pulses whieh, for example, may arise in an office upon
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presence of heavy smokers. Similar pulses may arise in an
automobile garage. Clearly, no fire is present, and the pulses,
essentially, are disturbance pulses. In case of a fire, there
would be a continuous increase in smoke concentration.
A simple fire alarm sensor, without time delay,
would provide an alarm when the first pulse exceeds the alarm
threshold, indicated at Ao~ and as shown at curve S. A known
fire sensor with time delay would have different characteri~tics;
in accordance with curve V, the voltage at the input to the
threshold detector would, first, increase slowly; after termi-
nation of the pulse S, the voltage would drop slowly, with the
same time constant. As can be seen by consideration of curve
V, several sequential smoke pulses would, eventually, cause the
level of the signal to rise until, finally, the alarm threshold
is exceeded, providin~ a flase alarm, as indicated at Al.
A fire sensor in accordance with the present invention,
under conditions of smoke pulses, would provide signals as shown
to curve B. Upon sensing of the first smoke pulse, the input
voltage to the second threshold detector would follow curve B,
and rise. After the smoke pulse stops, however, the curve
drops rapidly, and with a short time constant. Thus, although
sequential smoke pulses occur, following each other rapidly,
the voltage at the input to the second threshold detector
cannot build up, or accrete, and reach the alarm threshold.
After each smoke pulse, the voltage of the integrator, that is,
across capacitor 8, drops practically again to zero or null.
Thus, false alarms due to sequential smoke pulses are avoided.
A real fire which persists would, however, cause continuous
charge of the integrating capacitor, as seen in curve C. An
alarm will be given, when the threshold level of the first
threshold detector has been exceeded by the duration Gf the
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desired delay time, that is, at A2.
Experience has shown that false alarms can be
avoided if the second time constant, that is, the discharge
time constant, is less than the time constant of the first, or
char~e circuit by a factor of ln, at least. A typical
example would be:
RC : 30 seconds; rC : 1 second.
These time periods are not critical; the charge
time, that is, the RC time constant may be in the order of
from between lp to 60 seconds; the dischar~e time constant, rC,
may be in theorder of from O.l to 5 seconds, the two respective
time constants having a relationship of 1 to 10, at least.
Various types of fire sensors may be used, and the
present invention is not limited to ionization-type sensors,
with which it has been described. The fire sensors may be
responsive to other parameters than smoke, for example
combustion gases, or other indicia of fire, and suitable
sensors of known types may be used. Various chan~es and
modifications may be made within the scope of the inventive
concept.
