Fire Detector And Method Employing Assymetrical Integrator

Abstract

A fire indication signal, derived from a sensor, is tested whether it exceeds a first threshold level; if it does, the signal is integrated in an integrator which has a slow, or long time constant to integrate a rising signal, but a short, rapid time constant to discharge, upon drop of the signal; in one form, the integrator includes a diode in the integration circuit, so that it will discharge an integrating capacitor rapidly, upon drop in signal level exceeding the first threshold. When the integrated signal exceeds a second threshold, an alarm is given. Providing for rapid discharge of the integrated signal prevents false alarms resulting from repetitive short pulses exceeding a first threshold level which, however, do not persist.

Description

BACKGROUND OF THE INVENTION

The present invention relates to sensing, and indicating 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 generated 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 electrically. 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, shrouds, 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 connected 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 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 other in relatively short periods of time, the known fire sensors and systems will integrate the signal, and an average value will result which may well be 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 smoking 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 essentially 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 steps: 1. 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.

2. 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.

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 integrating 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 provide a low resistance, short time constant discharge path for the integrating capacitor of the integrator.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described by way of example with reference to the accompanying drawings, wherein:

FIG. 1 is a highly 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.

DESCRIPTION OF THE PREFERRED EMBODIMENT

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 connected in series with a reference chamber 2. The ionization chamber 1 can be referred to as the sensing chamber, and is accessible 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 concentration 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 junction 5, exceeds a 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 resistance R. The capacitor 8 is charged with a time constant of RC, in which C is the capacitance of capacitor 8. When the voltage across capacitor 8 reaches the alarm threshold of a second threshold detector 10, threshold detector 10 will generate 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 provides 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 governed 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 disappears 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 which, for example, may arise in an office upon 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 first alarm sensor, without time delay, would provide an alarm when the first pulse exceeds the alarm threshold, indicated at A.sub.O, and as shown at curve S. A known fire sensor with time delay would have different characteristics; in accordance with curve V, the voltage at the input to the threshold detector would, first, increase slowly; after termination 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, providing a false alarm, as indicated at A.sub.1.

A fire sensor in accordance with the present invention, under conditions of smoke pulses, would provide signals as shown in 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 of the desired delay time, that is, at A.sub.z.

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 charge circuit by a factor of 10, 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 10 to 60 seconds; the discharge time constant, rC, may be in the order of from 0.1 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 first 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 changes and modifications may be made within the scope of the inventive concept.

Claims

1. Method to indicate occurrence of a fire having means generating a signal indicative of fire comprising

detecting a first threshold level of said fire indication signal, and providing a first signal representative of the fire indication signals having exceeded said first threshold level;

integrating rise in the level of said first signal with a first integration time constant, and drop in level of the first signal with a second integration time constant which is small with respect to said first integration time constant;

and detecting when the integrated signal reaches a second threshold value,

2. Method according to claim 1, wherein the first and second time constants

3. Method according wherein claim 1, whereien the first time constant is in the order of 10-60 seconds and the second time constant in the order of

4. Fire alarm system carrying out the method of claim 1

having fire sensing means (1, 2) providing an output signal indicative of a fire,

characterized by a first threshold detector (6) having the fire indicating signal applied thereto and providing a first signal representative of the fire indicating signal's having exceeded the first threshold of said first threshold detector;

a non-symmetrical integrator (I) integrating the first threshold signal with the first integrating time constant (RC) and a drop of the first signal with a second integrating time constant (rC) which is small with respect to said first time constant;

and a second threshold detector (10) sensing when the integrated signal has

5. System according to claim 4, wherein the fire sensing means comprises an ionization chamber (1), in which the ion current changes upon presence of

6. System according to claim 5, further comprising a resistance means (2) connected in series with the ionization chamber, the voltage drop across

7. System according to claim 4, wherein the integrator (I) comprises a capacitor (8) and a resistor (9) connected to charge the capacitor (8), the signal indicative of fire being connected through the resistor (9) to

8. System according to claim 7, further comprising a diode (11) connected in parallel to the resistor (9) to charge the capacitor, and poled oppositely the polarity of the signal indicative of fire charging the capacitor, and to permit rapid discharge of the capacitor upon cessation

9. System according to claim 4, wherein the integrator (I) comprises a capacitor (8), a first resistor (9) connected between the output of the first threshold detector and one terminal of the capacitor, and a second resistor (7) connected between the output of the first threshold detector (6) and the other terminal of the capacitor, and a diode (11) connected in parallel to the first resistor (9) and poled to block signals from the first threshold detector (6) charging the capacitor (8) so that said signals flow through the first resistor, but pass discharge current from the capacitor and permit discharge of the capacitor through the second resistor (7), the resistance value of said second resistor (7) being small with respect to the resistance value of said first resistor (9);

the second threshold detector (10) being connected across the capacitor (8) and providing an alarm signal when the charge across the capacitor (8)

10. System according to claim 9, wherein the relative values of resistance of said first resistor (9) and said second resistor (8) are selected to provide a charge time constant of the first resistor - capacitor R/C circuit which is at least about tan times the discharge time constant of the R/C circuit formed of the second resistor (7) and the capacitor (8).

11. System according to claim 4, wherein the integrator comprises a first timing circuit (8, 9) having a time constant in the order of 10-60 seconds, a second timing circuit (7, 8) having a time constant in the order of 0.1-5 seconds, and a diode (11) connected and poled in the integrator to render the timing circuit with the shorter time constant effective upon decrease of the signal applied to the integrator, to provide for non-symmetrical integration of signals exceeding the first threshold level, in increasing, and decreasing direction of said signals.