U.S. patent number 3,909,813 [Application Number 05/374,310] was granted by the patent office on 1975-09-30 for ionization-type fire sensor.
This patent grant is currently assigned to Cerberus AG. Invention is credited to Andreas Scheidweiler, Hanspeter Thalmann.
United States Patent |
3,909,813 |
Scheidweiler , et
al. |
September 30, 1975 |
Ionization-type fire sensor
Abstract
An ionization chamber includes an ionizing element which, in
combination with an electrical circuit, is so arranged that the
electrical resistance of the chamber increases when smoke, or fire
aerosols are detected; the sensor further includes an electrical
circuit which has a first threshold detector responsive to a first
threshold value of increase of resistance to permit generation of a
warning signal, and a second threshold level with a second
threshold detector, which responds upon exceeding of the second
threshold level to provide an alarm signal different from the first
warning signal arising at the first threshold level. Preferably,
the warning signal is a constantly arising current, and the alarm
signal is a signal of varying intensity.
Inventors: |
Scheidweiler; Andreas (Stafa,
CH), Thalmann; Hanspeter (Urdorf, CH) |
Assignee: |
Cerberus AG (Mannedorf,
CH)
|
Family
ID: |
4365446 |
Appl.
No.: |
05/374,310 |
Filed: |
June 28, 1973 |
Foreign Application Priority Data
|
|
|
|
|
Jul 17, 1972 [CH] |
|
|
10655/72 |
|
Current U.S.
Class: |
340/517; 340/533;
340/629; 340/815.45 |
Current CPC
Class: |
G08B
17/11 (20130101); G08B 25/04 (20130101) |
Current International
Class: |
G08B
25/04 (20060101); G08B 17/11 (20060101); G08B
17/10 (20060101); G08B 25/01 (20060101); G08b
017/12 () |
Field of
Search: |
;340/237S,378,213R,331
;250/381,382,384,385 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Myer; Daniel
Attorney, Agent or Firm: Flynn & Frishauf
Claims
We claim:
1. Ionization-type fire sensor comprising
an ion chamber (3) accessible to atmosphere subject to fire
aerosols, smoke or the like, said ion chamber having electrical
resistance which, upon detection of fire aerosols or smoke therein
changes in resistance value,
wherein the improvement comprises
two threshold detectors of different threshold levels,
a first threshold detector (7) connected to the ion chamber (3)
sensitive to a first resistance condition of the chamber and having
a first threshold level to provide a first warning output signal
when the resistance value of the ion chamber changes and passes a
first threshold level; and
a second threshold detector (9, 10, 12) connected to the ion
chamber sensitive to a second resistance condition, higher than the
first condition, and having a second threshold level, higher than
the first, to provide a second alarm output signal when the
resistance value of the ion chamber passes a second level, higher
than said first level;
and wherein said second alarm output signal provided by said second
threshold detector differs in characteristic from said first
warning output signal.
2. Sensor according to claim 1 wherein the warning output signal is
a continuous signal, and the alarm output signal is a signal of
changing intensity.
3. Sensor according to claim 1 wherein the first threshold detector
comprises an FET (7), the gate electrode (6) of the FET being
controlled by the voltage drop across said ionization chamber.
4. Sensor according to claim 3 wherein said FET (7) is connected to
the ionization chamber to become conductive when said first
threshold level is passed;
and a light-emitting indicating means (11) connected in the
source-drain path of the FET.
5. Sensor according to claim 3 wherein the second threshold
detector comprises a transistor (12), the base of which being
controlled by the current through the FET (7), the transistor being
connected to become conductive when the voltage drop across the
ionization chamber, and thus the current through the FET (7) passes
said second threshold level.
6. Sensor according to claim 5 further comprising a second
ionization chamber (4) forming a reference ionization chamber,
connected in series with said ionization chamber accessible to
atmosphere;
the collector-emitter path of said transistor (12) forming a
parallel circuit to the series circuit formed by said ionization
chambers;
a resistor in series with said series connection of said chambers
(3, 4) and the parallel connected transistor (12), and voltage
supply lines (1, 2) connected to the free terminals of said
parallel circuits, and said resistor (5).
7. Sensor according to claim 6 wherein the resistor (5) is a
variable resistor to adjustably set said first warning threshold
level.
8. Sensor according to claim 1 further comprising an alarm
indicator (11), said alarm indicator providing a visual alarm
indicating output.
9. Sensor according to claim 8 wherein said alarm indicator is a
light emitting diode (LED 11, 16, 17).
10. Sensor according to claim 1 further comprising a light source
(16) controlled to be illuminated when said first threshold level
is passed;
a photosensitive sensor (21) disposed in light receiving
relationship to said light emitting element, and a warning control
line (20) connected to said photosensitive sensor, said warning
control line having a signal placed thereon when said
photosensitive sensor changes resistance due to illumination by
said light emitting element.
11. Sensor according to claim 1 further comprising a current sensor
(W);
a warning output signal line (20), said current sensor (W) being
connected in an electrical circuit to said warning signal line,
said warning signal line being activated and having current passing
therethrough when said first threshold is passed, and providing a
signal to said current, independently of the alarm signal, and in
advance thereof.
12. Sensor according to claim 1 further comprising a signal
central, and discrete signalling transmission links between the
individual sensors and said signal central, said links being
different for the warning output signal and the alarm output
signal.
13. Sensor according to claim 1 comprising means (34, 35, 36) to
suppress the warning output signal upon response of the second
threshold detector which generates said second alarm output
signal.
14. Sensor according to claim 1 further comprising a self holding
or self locking circuit connected to the second threshold detector
to hold the second alarm output signal even if said second
threshold detector no longer senses conditions in excess of said
second threshold level.
15. Sensor according to claim 1 wherein the first threshold
detector comprises a FET (7), the gate electrode of which is
controlled by the voltage drop across the ionization chamber;
a switching transistor (22) is provided, the base of which is
controlled by current through the FET (7), said switching
transistor becoming conductive after the first threshold level is
passed.
16. Sensor according to claim 15 further comprising an oscillator
controlled by conduction of said transistor (22) to provide a-c
signals indicative that said first threshold level has been
passed.
17. Sensor according to claim 16 wherein said oscillator (FIG. 5)
comprises a multivibrator-type oscillator (23, 24).
18. Sensor according to claim 15 further comprising (FIG. 6) a
circiut (35) of low a-c impedance connected between the supply
lines (1, 2) to the sensor;
and a controlled switch (34) connected to and controlled by said
transistor (22) and switching said low a-c impedance circuit (35)
in parallel to the sensor upon conduction of said transistor
(22).
19. Sensor according to claim 15 comprising an additional
transistor (12), the base of which is controlled by the current
through the FET (7), and voltage divider means in the source-drain
path of the FET (7) setting the threshold response levels of said
switching transistor (22) and said additional transistor (12) to be
at different levels to provide said first and second threshold
levels, conduction of said transistors, respectively, controlling
said first and second output signals.
20. Sensor according to claim 1 further comprising (FIG. 6) a
circuit (35) of low a-c impedance connected between the supply
lines (1, 2) to the sensor;
and a controlled switch (34) connected to and controlled by said
first threshold detector (7) and switching said a-c low impedance
circuit (35) in parallel to the sensor when the first threshold
level of the first threshold detector (7) is exceeded.
Description
Cross reference to related applications
U.s. pat. No. 3,767,917, Oct. 23, 1973
U.s. application Ser. No. 374,795 filed June 28, 1973
The present invention relates to an ionization-type fire alarm
sensor, and more particularly to this type of sensor in which an
ionization chamber is provided in which a radioactive substance is
located. The electrical resistance of the ionization chamber rises
when fire is sensed; the sensor further includes an evaluation
circuit including at least one threshold detector, as well as an
alarm indicator.
Ionization type fire sensors, in which a radioactive substance
generates ions are so arranged that, upon application of an
electric voltage between the electrodes of the ionization chambers,
a current is generated which decreases upon penetration of smoke,
or fire aerosols into the chamber. Decrease of ion current in the
ionization chamber is detected by an electrical circuit which
includes a threshold detector. Upon detection, an alarm circuit can
be activated. In one form, the ionization chamber is connected in
series with a resistance element, for example another ionization
chamber which may be termed a reference ionization chamber since it
is so constructed that it is substantially separated from ambient
air, and thus not subject to smoke or fire aerosols. The relative
voltage drops across the ionization chambers are sensed and applied
to a threshold detector, for example to a field effect transistor
(FET). If the voltage drop across the sensing ionization chamber
rises, due to increase in its resistance, then the threshold level
of the FET is exceeded, it begins to become conductive, and
provides a fire alarm signal.
Known ionization-type fire sensors are connected to a signal
central. The increased FET current is conducted directly to the
signal central, or over a further switching element, such as a
relay, a SCR, or the like. The signal central provides an alarm
signal. The sensor further includes, in its structure, or in its
vicinity, an alarm indicator, such as a lamp or other illuminating
device which permits an indication that a specific sensor has
responded. This is of particular advantage when a plurality of
sensors are connected, in parallel, over a common line to a signal
central. The signal central, in such a case, can determine that one
of the sensors on the line was responded to give an alarm, but it
is difficult to determine which one of the sensors has responded.
By checking the alarm indicators of the various sensors, the
location of the responding sensor can be determined.
Known ionization fire sensors utilize electrical components with
very high resistances. For example, the inherent or inner
resistance of the ionization chamber is in the order of 10.sup.10
ohms. The input resistance of the connected electrical circuit,
particularly of the FET, must be higher by an order of magnitude.
To further increase sensitivity, and to decrease the activity of
the radioactive substances, it has recently been tried to still
further decrease the ion current, that is, to further increase the
resistance of the ionization chamber. It has been found to be very
difficult, in actual practice, to maintain such high isolation
resistances in an ionization fire sensor over long periods of time,
for example over years. The resistance may change, particularly due
to precipitation of dust within the fire sensor, slow changes of
some materials, and the like. As a result, the voltage drop across
the ionization chamber will change slowly. In many instances it has
been found that the voltage drop, in time, slowly approaches the
alarm threshold level. An erroneous fire alarm may thus result.
It is an object of the present invention to provide an alarm
indicator for fires of the ionization-type, which provides an
indication that the voltage in the ionization chamber approaches
the alarm threshold. This permits identification of alarm sensors
which may have a tendency to give a false alarm, so that such
sensors may be replaced, exchanged, or cleaned and maintained
before a false alarm has been signalled. It is well known that
false alarms are as dangerous as undetected real fires.
SUBJECT MATTER OF THE PRESENT INVENTION
Briefly, the ionization fire alarm sensor has more than one
threshold level. A first, lower threshold detector is provided
which, when a first lower threshold level of resistance is exceeded
provides a pre-warning signal; when a second threshold level is
exceeded, indicative of fire, or the like, then a second alarm
signal, clearly distinguishable from the pre-warning signal is
given.
The ionization fire sensor of the present invention has the
advantage that an incipient fire provides, already in a very early
stage, a pre-warning signal, before the real fire alarm signal is
generated. A certain warning will thus be provided between the real
fire alarm, and quiescent conditions. This warning time permits
investigation whether, really, a fire is beginning, or whether
other reasons are present which might cause the sensor to provide
the pre-warning signal, such as, for example, excessive cigarette
smoke, welding gases, high dust or particle concentrations, or the
like. The pre-warning signal may be used, for example, to prepare
an otherwise deactivated fire extinguishing system, that is, to
bring the system into a "get ready" state, the system itself being
activated only when the final alarm signal, indicative of fire, is
received. Thus, it is possible to avoid costly steps, or possible
damage by the fire extinguishing system itself, before one is sure
that a fire really has started, while, however, an indication is
being made available that a possibly suspicious situation should be
investigated.
The invention will be described by way of example with reference to
the accompanying drawings, wherein:
FIG. 1 is a highly schematic circuit diagram of an ionization-type
fire sensor in accordance with the present invention;
FIGS. 2a and 2b illustrate voltage and current characteristics
useful in connection with the explanation of the operation of the
device; and
FIGS. 3-6 are schematic circuit diagrams of various other
embodiments of the invention.
Embodiment of FIG. 1 and principal system:
Construction of sensor in accordance with the present invention,
with reference to FIGS. 1, 2a, 2b: A fire sensor D is connected by
means of lines 1, 2 to a signal central S. The lines extend, as
schematically shown by lines 1', 2', for parallel connection of
further sensors, for example of the same type.
Each sensor has an ionization chamber 3 which is open to ambient
air. It includes two electrodes and a radioactive source. The
chamber is in series with a reference ionization chamber 4, which
is essentially closed off against ambient air. A controllable
resistor 5 is connected in series with the chamber 3, 4, across
lines 1, 2.
The voltage drop over ionization chamber 3, which varies, is
applied to the control electrode 6 of FET 7, or an equivalent
electronic element. As an equivalent element, an entire integrated
network may, for example, be used. The source-drain path of the FET
is connected with the series circuit of resistors 9, 10, and a
light-emitting diode (LED) 11, the series circuit being likewise
connected across lines 1, 2. LED 11 may be a gallium arsenide, or a
gallium phosphide diode, or may include other light-emitting
material.
The input voltage, that is, the voltage on gate electrode 6 of FET
7 is so controlled by means of resistor 5 that, under ordinary
quiescent condition, the FET is blocked. Under such conditions, no
fire has been sensed, there are no smoke or fire aerosols in the
air penetrating into chamber 3. Below an input voltage across the
gate electrode 6 of FET 7, indicated as S.sub.1 (FIG. 2a) no
current will flow through LED 11. If, however, the input voltage at
the control electrode 6 rises above the warning level S.sub.1, FET
7 becomes conductive and, depending on input voltage across lines
1, 2, current will flow through the FET 7. The LED 11 becomes
luminescent. The brightness of LED 11 is a measure for the extent
by which the voltage drop across ionization chamber 3 has exceeded
the warning threshold level.
The junction point between resistors 9 and 10 is connected to the
base of a further transistor 12, the emitter of which is connected
over LED 11 to the supply line 2. The collector is connected to the
controlled resistor 5. If the current through the FET 7 and
resistor 10 exceeds a predetermined threshold level I.sub.2 (FIG.
2b), corresponding to a threshold voltage S.sub.2 at the input 6 of
the control electrode of the FET 7 (FIG. 2a) transistor 12 becomes
conductive, which increases the voltage drop across resistor 5. The
voltage at the gate electrode 6 of FET 7 increases, which causes a
still further increase in current through transistor 12.
Resistor 5 thus has a dual function: first, it sets the first, or
warning threshold level and, secondly, it acts as a feedback
resistor to ensure that when the alarm threshold level S.sub.2 is
reached, current through the sensor D rises abruptly and
extensively. The circuit will then, also, be self holding; in other
words, the sensor changes state to an alarm condition which cannot
be terminated by reversion of the ionization chamber to its normal
lower value.
Lines 1, 2, will thus, suddenly, have current flow therethrough of
suddenly increased amplitude, which current is supplied by signal
central S. A current detector 13 (block A) is included in the
signal central S which responds when such a current is detected, in
order to provide an external alarm system (for example an alarm
notification to a fire signalling station, an acoustical signal,
optical indication or the like, or direct alarm to a fire
department). The current detector 13 further causes periodic
changes in the supply voltage supplied to lines 1, 2, so that the
LED 11 is supplied with a pulsed supply voltage and will flash in
the same rhythm. A suitable frequency is about 1 Hz.
The alarm central may have other circuits, for example voltage
evaluation circuits and the like, as referred to in the cross
referenced application Ser. No. 374,795 filed June 28, 1973,
assigned to the assignee of the present application.
A Zener diode 14 is connected across lines 1, 2 to protect against
over voltages and inverse polarity upon connection of the sensor to
the lines 1, 2. Resistor 15 forms, together with resistor 5, a
voltage divider to adjust the bias on the gate control electrode 6
of FET 7.
Operation (with reference to FIGS. 2a, 2b): Under normal, quiescent
conditions, when no fire is being sensed, the total current drawn
by the sensor is very small, and essentially the current which
flows through the two ionization chambers 3, 4, and the practically
negligible current through the parallel resistor 15. Upon
penetration of smoke, or fire aerosols into ionization chamber 3,
the resistance of the chamber 3 will increase. As soon as this
resistance has increased to such an extent that the voltage at the
input 6 of FET 7 exceeds the lower, or warning threshold levels
S.sub.1 (W), current starts to flow, and the LED 11 begins to light
with a brightness which depends on the amplitude by which the input
voltage exceeds the threshold level. If the voltage increases
further and reaches the second or alarm threshold level S.sub.2
(A), the sensor changes suddenly into alarm condition and a highly
increased alarm current I.sub.a will flow to the signal central S.
The signal central will thereupon command periodic dropping of the
supply voltage, so that the LED 11 will blink, or flash, thus
unambiguously indicating an alarm condition, and differentiating an
alarm condition from a warning condition.
In the further examples, similar parts, having similar functions
will not be described again, and have been given the same reference
numerals.
Embodiment of FIG. 3: The drain electrode of FET 7 is connected
over two LEDS 16, 17 and over a voltage divider formed by resistors
18, 19, to line 2. The tap point of the voltage divider 18, 19,
controls the base of transistor 12, the collector-emitter path of
which is in parallel to the series circuit formed of the two
ionization chambers 3, 4. When the gate voltage on gate electrode 6
of FET exceeds the warning threshold S.sub.1, then FET 7 becomes
conductive, and both LEDs 16, 17 become luminescent. Upon further
increase of input voltage, to the gate 6 of FET 7, that is, when
the alarm threshold S.sub.2 is passed, transistor 12 will become
conductive which switches the sensor into alarm condition, as in
FIG. 1. LED 17 will indicate if the fire sensor is in normal
condition, warning condition or alarm condition. Under alarm
conditions, a highly increased alarm current will flow over line 2
to signal central S, which can be identified in the signal central,
for example by current detector A, and utilized to provide an
alarm. The sensor further includes a photoresistor 21 which is in
light receiving relationship to the light emitted from LED 16. One
terminal of the photoresistor 21 is connected to line 1, and the
other terminal to a line 20, likewise supplied from signal central
S. The photoresistor is so arranged that it can be controlled only
by light from LED 16. Under normal, quiescent condition when the
diode does not emit light, photoresistor 21 has a very high dark
resistance. When diode 16, upon passing the warning threshold level
S.sub.1 begins to luminesce, the resistance of resistor 21 changes
suddenly and warning current flows over line 20 to signal central
S, to be there evaluated by a further current detector W to provide
a warning signal, as described. In this example, alarm indicator A
indicates by a discrete signal if the sensor is under normal,
warning or alarm conditions; additionally, the signal central can
separately distinguish if one of the sensors connected thereto is
in warning conditions or quiescent conditions, thus providing a
remote indication. This circuit may also use a plurality of
ionization fire sensors, connected in parallel over lines 1, 2, and
20 to signal central S.
Instead of two diodes 16, 17, a single diode can be used, so
located that its light can be used simultaneously for visual
indication, as well as for optical transmission to the
photoresistor 21. The optical path to the photoresistor must, of
course, be shielded against stray or extraneous illumination or
light penetration.
Embodiment of FIG. 4: The warning signal is electrically indicated,
the second LED 16 and photoresistor 21, and the optical path are
eliminated. Rather, a resistor 19 is connected to third line 20
(instead of to line 2). When current flows through FET 7 upon
passing of the warning level, the warning relay W in the signal
central will have current flowing therethrough, so that not only
LED 17 in detector D will provide a warning signal but,
additionally, a warning signal will appear at the signal central S.
If the alarm threshold of the second transistor 12 is exceeded,
transistor 12 becomes conductive and line 2 will have an alarm
current flowing through signal central S which, as before, provides
pulsed voltage supply to effect flashing of LED 17, and
additionally provides an alarm output. The embodiments of FIG. 3
and 4 require a three-wire circuit to provide separate alarm and
warning indication. The embodiment of FIG. 5 provides discrete
signals representative of warning condition and alarm conditions
over two lines. A third transistor 22 is provided, having a
threshold below the alarm threshold level of the second transistor
12. Its base is controlled over the voltage divider formed by
resistors 18, 19, 21, by the current of the FET 7. When the warning
level of the third transistor 22 is exceeded, a multivibrator
oscillator is brought into oscillations. The oscillator includes
transistors 23, 24, capacitors 25, 26 and resistors 27, 28, 29, 30.
The multivibrator is of conventional construction, and coupled to
transistor 22 over coupling resistor 31. Lines 1, 2, therefore will
have a-c signals applied thereover when the warning threshold is
exceeded. These signals can be coded by suitable dimensioning or
construction of the multivibrator. When the alarm threshold is
exceeded, however, transistor 12 will provide d-c over the lines 1,
2, to the signal central. The signal central includes circuits
which can distinguish between a-c components and d-c warning
currents, by well known isolating and filtering networks. Two lines
can thus carry, separately, a warning signal and alarm signal from
a sensor to a signal central. In many cases, the lines may already
be subject to a-c in the kiloHertz range, particularly in cases in
which a plurality of sensors are connected, in parallel, over a
long line to the signal central, and continuity of the lines up to
the last sensor is sensed by a terminating member 37, connected at
the end of the line and providing pulsed output, or change of its
internal resistance, in pulsed steps. FIG. 6 illustrates a circuit
which still permits transmission of a warning signal, separate from
an alarm signal over only two lines. Third transistor 22 controls a
controlled switching element 34, which may be a transistor, an SCR,
or the like, over collector resistor 32, connected between supply
lines 1 and 2, and a resistance 33. The controlled switch 34 is
connected in series with capacitor 35 which connects between lines
1 and 2. A resistor 36 is connected in parallel to switching
element 34. When the warning threshold is exceeded, transistor 22
controls the switching element 34 to close, thus inserting
capacitor 35 between lines 1, 2, and short circuiting pulses by the
oscillator 37. The transistor 12 is triggered into conduction as
previously explained.
Various changes and modifications may be made in the circuits of
the present invention; for simplicity, a simple central station has
been indicated, connected to the sensors by wire communication
links. Wireless communication may also be used.
* * * * *