EP0015348A1 - Fire detector - Google Patents

Abstract

1. A fire detector equipped with a reference element and an analysing element which is constituted by an open radioactive ionization chamber (1) the impedance of which varies in the presence of combustion gases, said elements being coupled in a bridge and connected to two inputs of two voltage comparators (A1 and A2) each connected to an alarm system (S1, S2) such that imbalance of the bridge produces a signal at the output of one or other comparator depending on the direction of the imbalance and operates the corresponding alarm system, characterized in that the voltage applied at the terminals of the analysing chamber lies within the so-called recombination zone of its curve for ionization current as a function of the voltage applied.

Description

The invention relates to a fire detector.

There are many types of fire detectors or alarms such as temperature rise detectors generally comprising thermistors, detectors of radiation emitted by a flame and comprising photocells or a corona tube and smoke or flue gas detectors. The latter are based on optical phenomena or else comprise at least one ionization chamber traversed by a current which varies as a function of the gases which pass through it.

All these devices are electrically connected to an alert or alarm system controlled by a device which establishes the difference between a measurement value and a reference value corresponding to a normal or tolerable level. These devices generally require a large consumption of current which is particularly expensive given their continuous operation. In addition, they are sensitive to disturbing external phenomena and use, for ionization detectors, significant radioactive sources.

The invention provides electronics which are particularly reliable and of low consumption. This is based on a comparison measurement between a measured value and a referenced value. In addition, the invention makes it possible to produce an alert system in the event of device malfunction. The device according to the invention is intended to be adapted to many types of electric detectors which include an alert or alarm system, a reference element and an analysis element whose impedance varies in the presence of a temperature and / or radiation and / or combustion gas rise, said reference and analysis elements being mounted in series or in parallel and constituting two adjacent branches of a Wheastone bridge assembly, the other two branches of which are constituted by possibly variable resistors while the measurement diagonal or bridge, joined, either the common connection points of the resistors and of said elements, or respectively the ends of the detector elements.

The device according to the invention is remarkable in that the two ends of the bridge are connected respectively to the two inputs of two voltage comparators, mounted in parallel and in reverse, each connected to an alert system so that an imbalance du pont creates a signal at the output of one or the other comparator depending on the direction of imbalance and controls the corresponding alert system.

Advantageously in this case a hysteresis is created between the two trips by means for example of a voltage divider bridge arranged at one of the inputs of one of the comparators.

Experience shows that comparators are advantageously constituted by linear amplifiers whose gain is programmable.

It has also been found after numerous researches and tests that the amplifiers which allow a good functioning are integrated operational amplifiers known under the references LM 4250 or µ A 776.

It is clear that this arrangement is adaptable to many types of detectors and the reference and analysis elements can be constituted by corona tubes sensitive to radiation or thermistors sensitive to temperature rises or ionization chambers sensitive to combustion gases.

The ionization chambers can furthermore include electrodes under reduced voltage, one of which is provided with a radioactive source or electrodes subjected to a high voltage with or without radio-element.

It is also possible that at least one of the comparators is preceded by at least one impedance adapter device such as a field effect transistor.

In the case of a detector whose reference and analysis elements consist of two radioactivity ionization chambers possibly provided with a common electrode, it appeared particularly important that the heights of the analysis and reference chambers are respectively 12.5 mm and 7.5 mm.

A device according to the invention has a high sensitivity due to its measurement by comparison, and makes it possible to control a fire alert system or a fault alert system according to the direction of the imbalance of the bridge. In its application to ionization detectors, the invention makes it possible to use weakly radioactive sources, which is of considerable interest given the strict regulations which are the subject of such sources and the risks of contamination which exist with sources of strong radioactivity.

To further reduce the radioactive sources used in the detectors, it has been imagined to increase all the more the voltage applied to the chambers.

However, contrary to popular belief, it appeared that this tension should be limited. A study of the ionization phenomenon shows that the curve of the ionization current as a function of the voltage applied to the system has several zones.

In the first zone, the so-called reboost zone, some of the ions contained in the chamber recombine with each other. This area has so far been used very little unlike other areas such as the ionization chamber area, the proportional counter area or the Geiger-Muller area. The inventor has discovered that this recombination zone which therefore corresponds to a poor ionization chamber was particularly sensitive to smoke or combustion gases.

Consequently, according to an embodiment of the invention in which at least the analysis element is constituted by a radioactive ionization chamber, the voltage applied to the terminals of this analysis chamber is located in the so-called recombination of its ionization current curve as a function of the applied voltage.

In addition, it has been found particularly advantageous for the reference element to be constituted by the internal resistance of a field effect transistor arranged in the bridge assembly. Indeed this very important internal resistance varies depending on external parameters, other ₅ that the combustion gases, in substantially the same manner as the analysis chamber and is therefore a good reference.

In this case it is advantageous for the source of the field effect transistor to be connected to ground via a variable resistor which makes it possible to vary the internal resistance of the field effect transistor.

It is then possible by means of this variable resistance to adjust the voltage applied to the analysis chamber and in the case of a parallel assembly to adjust the balance of the bridge.

To save energy, according to one embodiment, the detector is provided with a low voltage general supply source and with a higher voltage generator for the analysis and reference elements controlled in voltage by a regulator designed and arranged so as to deliver a voltage to said generator only when the voltage of the latter remains below a reference voltage.

In this case, the regulator is advantageously constituted by a comparator, one of the inputs of which is connected to a reference voltage source and the other input to a point on the circuit located downstream of the generator.

• - La figure 1 montre schématiquement un premier mode de réalisation d'un détecteur selon l'invention muni de deux chambres d'ionisation ;
• - La figure 2 montre une disposition en série semblable à celle de la figure 1 dans laquelle l'élément de référence est constitué par la résistance interne d'un transistor à effet de champ.;
• - La figure 3 montre un mode de réalisation semblable à celui de la figure 2 mais avec un montage en parallèle et,
• - La figure 4 montre un mode de réalisation monté en parallèle et muni d'un générateur de tension et d'un régulateur.

The invention will be better understood and other features will appear on reading the description which follows with reference to the accompanying drawing in which

• - Figure 1 shows schematically a first embodiment of a detector according to the invention provided with two ionization chambers;
• - Figure 2 shows a series arrangement similar to that of Figure 1 in which the reference element is constituted by the internal resistance of a field effect transistor .;
• FIG. 3 shows an embodiment similar to that of FIG. 2 but with a parallel mounting and,
• - Figure 4 shows an embodiment mounted in parallel and provided with a voltage generator and a regulator.

Ionization detectors are known and include at least one chamber in which is disposed a radioactive material intended to ionize said chamber. When smoke or combustion gases enter the chamber, the ionization state is modified so that the number of mobile charges decreases, which amounts to an increase in the internal impedance of the chamber.

In the example shown in FIG. 1, the device comprises two ionization chambers 1 and 2 respectively for analysis and for reference mounted in series. Each of the latter is provided with a radioactive material 3 and 4. The chambers are arranged in such a way that the positive electrode 5 of the analysis chamber 1 and the negative electrode 6 of the reference chamber are connected to the point common B while a current source is connected between points A and C, that is to say between the positive and negative electrodes of chambers 2 and 1. respectively (18 to 30 V source for example).

The common point B is electrically connected to the input of a quadrupole Q ₁ itself followed by a quadrupole Q ₂ . The quadrupoles Q and Q ₂ are impedance adapters intended to lower the large output impedance of the chambers. It is obviously possible to use only one quadrupole impedance adapter, such as a field effect transistor for example or any other equivalent component.

Three resistors in series R ₁ , R ₂ and R ₃ are mounted in parallel between points A and C, the resistor R ₂ being arranged as a potentiometer. The output X of the quadrupole Q ₂ and a variable point Y on the resistor R ₂ are connected respectively to the negative and positive inputs of an operational amplifier A ₁ . The output X is also connected via a resistor R ₄ to the positive input of an operational amplifier A ₂ while the point Y is electrically connected to the negative input of the latter. The point Y is therefore connected to the positive and negative inputs of the amplifiers A ₁ and A _{2 respectively} , through the example shown, of two input resistors R ₆ and R ₇₀

The operational amplifiers A ₁ and A ₂ used in comparator are advantageously programmable by means of resistors R _B and R _B , o Resistor R ₄ disposed between point X and the input of amplifier A ₂ constitutes one of the resistors of a divider bridge R ₄₉ and R ₅ , the mid point Z of which is directly connected to the above-mentioned input.

It is clear that the chambers 1 and 2 constitute two adjacent branches of a Wheastone bridge, the other two branches being formed by the resistors R ₁ , R ₃ and the variable resistance R ₂ placed between them while the measurement diagonal or bridge is between points X and Y.

Amplifiers A ₁ and A ₂ are respectively connected to two alert systems S ₁ and S ₂ comprising in particular quadrupoles Q ₃ , Q ₄ and resistors or relay coils 7 and 8 intended to actuate any suitable light and / or source means and / or other relays.

When the voltage at X is lower than the input voltage of amplifier A ₁ , the latter switches and a signal triggers the alert system S ₁ . On the other hand, when the voltage Z is greater than the input voltage of the amplifier A ₂ , the latter switches over and actuates the system S ₂ .

Figure 1 shows two chambers 1 and 2 shown schematically. In practice, these are arranged in a possibly common box, the analysis chamber 1 being open and the reference chamber 2 completely or partially closed.

The operation is simple to understand. The potentiometer R ₂ is adjusted so that the bridge is balanced. If external phenomena other than smoke emissions modify the impedances of the two chambers, the bridge is not unbalanced and no warning system is controlled. On the other hand, if combustion gases enter the analysis chamber, the current flowing through it is modified and the voltage at point Z becomes higher than the input voltage of amplifier A ₂ , which triggers the system of fire alarm S ₂ . On the contrary, if the imbalance of the bridge takes place in the other direction, i.e. if the voltage at point X becomes lower than the input voltage of amplifier A ₁ , it is the fault or failure alert system S ₁ which is triggered. This triggering takes place for example following a deterioration of the quadrupole adapters of impedance Q ₁ , Q ₂ or a rupture of the branches with resistances or even following a bad adjustment of the potentiometer R ₂ etc.

The divider bridge R ₄₉ R ₅ makes it possible to establish a triggering threshold by shifting the equilibrium point of the bridge, so as to produce a hysteresis between the triggering of the fire and fault alerts respectively. This arrangement avoids the sudden passage from one state to another.

In the device described, it appeared particularly advantageous to use as electronic components for the amplifiers A ₁ and A ₂ the operational amplifiers mentioned above. Similarly, the impedance adapters can be produced in the form of a single field effect transistor, of the 2N 4416 type for example. In addition, the height of each of the chambers is important and it has already been said that the analysis and reference chambers 1 and 2 advantageously have heights of 12.5 mm and 7.5 mm.

It is clear that the series connection of chambers 1 and 2 is not compulsory and it is possible to imagine a parallel connection on the power supply, the chambers 1 and 2 nonetheless constituting two adjacent branches of the bridge assembly as FIG. 4 shows it for example, to which we will return later.

Figures 2 and 3 show two very important embodiments according to the invention. They schematically represent a series connection (Figure 2) similar to that of Figure 1 and a parallel connection (Figure 3) similar to that of figure 4.

However here the reference element (chamber 2 of Figures 1 and 4) is replaced by the internal resistance Ri of a TEC field effect transistor, the resistance Ri being shown in dotted lines since it is a resistance fictitious.

Figure 2 shows the series arrangement of Figure 1 and we find the resistors R ₁ , R ₂ and R _{3 as} well as the ends of the bridge X and Y. This arrangement is particularly advantageous because it allows the use of a very radioactive source low on the order of, for example, 0.05 μC, that is to say 1850 disintegrations per second. Indeed, this low radioactivity causes a high impedance of the analysis chamber 1, compensated by the internal resistance Ri of great value. Furthermore, if the bridge is adjustable by means of the variable resistance R ₂ , it is possible to adjust the equilibrium point by means of a variable load resistance R _s arranged at the source of the transistor TEC which makes it possible to modify the internal resistance Ri. This adjustment thus authorizes the use of a large range of radioactive sources.

FIG. 3 also shows a bridge arrangement with an analysis chamber 1, a reference element constituted by the internal resistance Ri of the transistor TEC and resistors Ra and Rr constituting the two other branches of the bridge. However, to adjust the balance of the bridge, instead of having a variable resistance Rr, it is advantageous to provide, as in FIG. 2, a variable load resistance Rs at the source of the TEC which makes it possible to vary the resistance Ri.

Furthermore, as has been said, the voltage applied to the terminals of the analysis chamber is located in the recombination zone of the latter.

Figure 4 shows an internal embodiment operating with two ionization chambers, respectively for analysis and reference 1 and 2 as in FIG. 1, mounted in parallel. In this embodiment, the point Y is adjustable on the resistance Rr of the reference chamber (similar to that of FIG. 3) while the resistance R _A of the analysis chamber of FIG. 3 is constituted by two resistors R ₈ and R ₉ . This embodiment comprises, like that of FIG. 1, comparators A ₁ and A ₂ and alert systems shown diagrammatically in S ₁ and S ₂ . However, this embodiment is provided with a fairly low voltage supply represented by small triangles while the chambers 1 and 2 are supplied by a higher voltage generator (of the order for example of 15 to 30 volts) which comprises in the example shown, an oscillator and a transformer Tf with high permeability, for example μ> 1000 gauss, provided with two primary windings in series and a secondary winding. The oscillations are maintained by a transistor T _R1 ^p i-lot in voltage by a regulator comprising an operational amplifier A ₃ mounted as a comparator and a transistor T _R2 . The inverted input of amplifier A ₃ is connected to a divider bridge constituted by resistors R ₁₀ , R ₁₁ , R ₁₂ and diode Z which delivers a reference voltage, while the other input of said amplifier is connected to circuit between the resistors R ₈ and R ₉ which constitute a divider bridge with the analysis chamber 1. With this arrangement, the amplifier A ₃ only delivers a voltage to the oscillator when the voltage thus sampled does not reach the reference voltage applied to its inverted input, so that the secondary voltage to the oscillator is controlled while its consumption is dependent on the secondary consumption of use, particularly low in this embodiment.

It is obvious that the embodiments described can undergo a large number of modifications or be combined without departing from the scope of the invention. Furthermore, the analysis elements described consist of radioactive ionization chambers and it is clear that these could be replaced by electrodes subjected to a high voltage using a generator of the type described on the subject. in Figure 4 or by other heat or radiation detectors. Finally, the embodiments of FIGS. 1, 2 and 3 can obviously use a generator and a regulator of the type described in connection with FIG. 4.

Claims (12)

1 - Fire detector fitted with an alert system, a reference element and an analysis element whose impedance varies in the presence of a rise in temperature and / or radiation and / or of combustion gases, said reference and analysis elements being mounted in series or in parallel and constituting two adjacent branches of a Wheastone bridge assembly, the other two branches of which are formed by possibly variable resistances while the measurement diagonal or bridge, joint, either the common connection points of the resistors and of said elements, or respectively the ends of the detector elements, characterized in that the two ends of the bridge (X and Y) are connected respectively to the two inputs of two voltage comparators (A ₁ and A ₂ ), mounted in parallel and in reverse, each connected to an alert system (S ₁ , S ₂ ) so that an imbalance in the bridge creates a signal at the output of the one or the other comparator depending on the direction of imbalance and controls the corresponding alert system. 2 - Detector according to claim 1, characterized in that a hysteresis is created between the two trips by means for example of a voltage divider bridge arranged at one of the inputs of one of the comparators. 3 - Detector according to one of claims 1 and 2, characterized in that the comparators are constituted by linear amplifiers whose gain is programmable, 4 - Detector according to claim 3, characterized in that the amplifiers are integrated operational amplifiers known under the references LM 4250 or µ A776. 5 - Detector according to any one of the reven dications 1 to 4, characterized in that the reference and analysis elements consist of corona tubes sensitive to radiation or thermistors sensitive to temperature rises or ionization chambers (1 and 2) sensitive to combustion. 6 - Detector according to any one of claims 1 to 5 characterized in that at least one of the comparators is preceded by at least one impedance adapter device (Q ₁ , Q ₂ ) ₉ such as an effect transistor field. 7 - Detector according to any one of claims 1 to 6, the reference and analysis elements of which consist of two ionization chambers by radioactivity (1 and 2) possibly provided with a common electrode, characterized in that the heights of the analysis and reference chambers are respectively and substantially 12.5 mm and 7.5 mm. 8 - Detector according to any one of claims 1 to 7 of which at least the analysis element is constituted by an ionization chamber (1) by radioactivity characterized in that the voltage applied across the terminals of this analysis chamber is located in the so-called recombination zone of its ionization current curve as a function of the applied voltage. 9 - Detector according to any one of claims 1 to 8, the analysis element of which consists of a radioactivity ionization chamber (1) characterized in that the reference element consists of the internal resistance (Ri ) a field effect transistor (TEC) arranged in the bridge assembly. 10 - Detector according to claim 9 characterized in that the source of the field effect transistor (TEC) is connected to ground via a variable resistance (Rs) which makes it possible to vary the internal resistance of the transistor field effect. 11 - Detector according to any one of claims 1 to 10 characterized in that it is provided with a source, low general supply voltage and a generator of higher voltage for the analysis and reference elements voltage-controlled by a regulator designed and arranged so as to deliver a voltage to said generator only when the voltage of the latter remains below a reference voltage. 12 - Detector according to claim 11 characterized in that the regulator is constituted by a comparator (A ₃ ), one of the inputs is connected to a reference voltage source and the other input to a point in the circuit located downstream of the generator.

Images