WO1997035287A1 - Electric air ionisation smoke detector - Google Patents

Abstract

An air ionisation smoke detector including a detection chamber (1) with an anode (2) and a cathode (3) interconnected via a bias circuit (16) having a current strength measurement device (18), as well as an ionisation chamber (6) communicating with the detection chamber. Ions are electrically generated in the ionisation chamber then transferred into the detection chamber by means of a continuous stream of air with a low flow-rate.

Description

Electric air ionization smoke detector.

The present invention relates to smoke detectors with air ionization by electrical means. Known smoke detectors with air ionization, such as for example that described in the document FR-A-2 594 953, comprise a detection chamber which contains air charged with ions and which communicates with the atmosphere, this detection chamber being the seat of an electrostatic field generated between an anode and a cathode which are connected to each other by an electric bias circuit provided with a voltage generator creating between the cathode and the anode a DC voltage of less than 100 volts, so that the ions contained in the detection chamber are attracted, one towards the cathode and the other towards the anode, thereby generating an electric current in the bias circuit, which circuit includes in addition to a device for measuring electric current and for alarm for measuring said electric current and for triggering an alarm reaction when this electric current undergoes an abnormal variation.

In these smoke detectors, the presence of smoke in the detection chamber results in the fixation of the ions present in this chamber on the constituent particles of this smoke, which weighs down these ions: this phenomenon slows down the ions, so that the intensity of the electric current flowing in the bias circuit is reduced.

When this intensity falls below a certain threshold, the detector triggers an alarm to signal the presence of smoke in the atmosphere.

Usually, in smoke detectors of the kind in question, the air is ionized directly in the detection chamber by means of a source of ionizing radiation, generally consisting of a radioactive substance α or β. In order to avoid using such a radioactive source, it has already been proposed to generate the ions in the detection chamber by electrical means.

But the ionization of the air cannot be carried out between the cathode and the anode of the detection chamber, because that would imply applying between these two electrodes a voltage generally higher than 1000 volts, tension which would attract the ions respectively towards the anode and the cathode with electrostatic forces so strong that the detector would become insensitive to the presence or not of smoke particles in the detection chamber.

To remedy this drawback, it has been proposed to give the cathode of the ionization chamber the form of a grid delimiting an ionization chamber which adjoins the detection chamber and which communicates with this detection chamber through the holes of the grid.

The ionization chamber contains an electrode which generally has a point shape and which is brought to a potential difference generally greater than 1000 volts with respect to the grid, so that electric discharges are emitted between the grid and the electrode, ionizing the air inside the ionization chamber.

The ions generated in the ionization chamber are attracted at relatively high speeds, some towards the grid, and the others towards the tip-shaped electrode: a small proportion of the ions which are attracted to the grid pass through the holes of this grid, and is found in the detection chamber, thus allowing the creation in this chamber of an ion current of relatively low speed between the anode and the cathode, under the effect of the voltage less than 100 volts which is applied between the anode and the cathode.

However, these electrically ionized air smoke detectors have the serious drawback that the ion concentration in the ionization chamber is much higher than the ion concentration in the ionization chamber. detection, due to the fact that few ions manage to pass through the holes in the grid.

Therefore, when the smoke detector is swept by an external stray air flow, the ions contained in large quantity in the ionization chamber are displaced in the detection chamber, which can considerably increase the circulating current. in the bias circuit, for example by a factor of 10 or 100, for the duration of said external parasitic air current. During all this time, if smoke particles enter the detection chamber generating a certain drop in the current flowing in the bias circuit, this drop is not enough to compensate for the considerable increase in this current caused by the current d air, so that no alarm is triggered.

Thus, the smoke detector no longer fulfills its role as soon as it is swept by a parasitic outside air stream. The object of the present invention is to propose an air ionization smoke detector which does not use a radioactive source and which does not have the abovementioned drawbacks.

To this end, according to the invention, a device of the kind in question is essentially characterized in that the ions are generated in at least one ionization chamber which is separate from the detection chamber and which communicates only with said detection, the ionization chamber also communicating with the atmosphere and comprising at least two electrodes connected together by an electrical supply circuit capable of generating between these two electrodes electrical discharges ionizing the air, and in that air displacement means are further provided to create a weak continuous air current from the ionization chamber towards the detection chamber, so as to transfer the generated ions into said detection chamber in the ionization chamber.

Thanks to these provisions, on the one hand, it is no longer necessary to use a radioactive source to ionize the air, and on the other hand, it avoids creating in any part of the detector an area where the ions would be very concentrated, so that, even when the detector is swept by a parasitic external air current, this air current does not generate a significant increase in the ionic concentration in the detection chamber, and therefore does not unduly disturb the operation of the detector. In preferred embodiments, use is also made of one and / or the other of the following arrangements:

- the air flow between the ionization chamber and the detection chamber has a speed between

1 and 20 cm per second;

the air flow between the ionization chamber and the detection chamber arrives in the detection chamber by a channeling device which comprises at least one duct and which offers to said air stream a small total cross-section, the detection chamber having, in the vicinity of this channeling device, an internal section which, taken perpendicular to the air stream, is at least five times greater than said total passage section, and the air contained in the detection chamber tion having an average renewal time of 1 to 10 minutes under the effect of the permanent air current coming from the ionization chamber;

- The air displacement means comprise a ventilation device displacing the air by mechanical action;

- The ventilation device is arranged upstream of the ionization chamber;

- The ventilation device comprises rotary blades driven by an electric motor;

- the ventilation device includes a slat substantially flat which extends longitudinally between on the one hand a fixed upstream end and on the other hand a free downstream end, this strip being actuated by an electric actuator so that the free end of this strip moves perpendicular to the plane of said strip;

- the electric actuator is chosen from electrostatic actuators, magnetic actuators, and piezoelectric actuators; - The air displacement means comprise a heating element for heating the air upstream of the detection chamber, so as to move this air by natural convection, the smoke detector being shaped so that the continuous stream of air follows an ascending path from the heating element to the detection chamber;

- The device comprises control means designed to operate intermittently the air displacement means and the ionization chamber;

- The device further comprises control and command means for measuring the speed of the air current which goes from the ionization chamber to the detection chamber, and for controlling the air displacement means so as to maintain constant said speed;

the device also comprises a sensor for measuring the speed of the air current which goes from the ionization chamber to the detection chamber, this sensor being connected to the device for measuring electrical intensity and for alarm, which device is designed to take into account the speed measured by the sensor and to correct as a function of this speed at least some of the data which it takes into account to trigger an alarm reaction, so that this device for measuring electrical intensity and alarm is freed from variations in the measured electrical intensity which are due to variations in the speed of the air current;

- the air flow between the ionization chamber and the detection chamber opens at least partially into an ion reserve volume which is not subjected to the electrostatic field prevailing between the anode and the cathode of the detection chamber and which communicates with the detection chamber so as to be swept by any disturbing outside air stream which would also sweep this detection chamber, before the passage of this air stream in at least part of said detection chamber.

Other characteristics and advantages of the invention will appear during the following detailed description of one of its embodiments, given by way of nonlimiting example, with reference to the accompanying drawings.

In the drawings:

FIG. 1 is a schematic view of a smoke detector according to an embodiment of the invention,

FIGS. 2 to 4 are detailed diagrammatic views illustrating variants of ventilation devices usable in the detector of FIG. 1,

- and Figure 5 shows schematically another variant of the smoke detector of Figure 1.

In the different figures, the same references designate identical or similar elements.

As shown in FIG. 1, the smoke detector according to the invention conventionally comprises a detection chamber 1 which is delimited between an anode 2 and a cathode 3 forming the outer wall of said detection chamber. The detection chamber communicates with the external atmosphere via orifices 4, 5 made in the cathode 3. The cathode and the anode are connected to each other by an electrical bias circuit 16 which includes a voltage generator 17 generating between the anode and the cathode a potential difference V for example of the order of 20 volts and in general less than 100 volts, so that the detection chamber 1 is the seat of an electrostatic field. Furthermore, the air contained in the detection chamber 1 is charged with ions which are generated in an ionization chamber 6 separate from the detection chamber and which are then transferred to said detection chamber via a or several transfer conduits 7, 8, by means of a low permanent air flow going from the ionization chamber 6 to the detection chamber 1.

The ions are generated in the ionization chamber 6 by electrically ionizing the air between two electrodes 9,

10, at least one of which preferably has one or more points directed towards the other electrode, these two electrodes being connected to a voltage generator 11.

This voltage generator applies between the two electrodes 9, 10 an alternating or direct electric voltage, which is sufficient to cause the appearance of electric discharges in the air between these two electrodes, which ionizes the air: the voltage in question can be for example of the order of 6000 volts, and it is generally at least a few hundred volts.

Furthermore, the permanent air current which goes from the ionization chamber 6 to the detection chamber 1 is obtained, in the example considered, by a small fan 12 constituted by an electric motor 13 driving a propeller 14, this fan being arranged in an air inlet duct 15 upstream of the ionization chamber 6.

The ions thus transferred into the detection chamber 1 are subjected to the electrostatic field prevailing in this chamber: this field attracts the negative ions or the electrons contained in the chamber 1 towards the anode 2 and the positive ions contained in said chamber towards the cathode 3.

These ions discharge respectively on the cathode and on the anode, so that an electric current i is generated in the bias circuit 16.

The intensity of this electric current is measured by an electronic device 18 for measuring intensity and alarm, known per se. When the electric current i drops below a certain threshold, the device 18 triggers an alarm reaction on an output 19 connected for example to an alarm center.

So that the permanent air current coming from the ionization chamber does not disturb the operation of the detection chamber, this air current preferably has a speed of the order of 1 to 20 centimeters per second in the conduits 7, 8, this speed being for example of the order of 1 to 5 centimeters per second and in particular of approximately 2 centimeters per second.

For the same purpose, advantageously, the conduits 7, 8 together provide a continuous flow section of the continuous air flow which is very much smaller (for example 10 times lower) than the internal section of the ionization chamber taken in the vicinity of these conduits perpendicular to the direction of arrival of the air stream carrying ions.

In addition, the average air renewal time included in the detection chamber is preferably between 1 and 10 minutes: in other words, the ratio between the interior volume of the detection chamber and the volume flow rate of the permanent air charged with ions is preferably between 1 and 10 minutes. Furthermore, preferably, the smoke detector has an ion reserve volume 20 which is supplied with ions by at least one duct 8 through which at least part of the permanent air stream charged with ions passes.

As taught in document FR-A-2 594 953, this volume of ion reserve 20 is not subjected to the electrostatic field which prevails in the detection chamber 1, and it communicates with this detection chamber so as to be swept by any disturbing external air current which would sweep the detection chamber, before the passage of this air current in at least part of said detection chamber. This prevents such a disturbing air current from causing an excessive drop in the current i flowing in the bias circuit 16.

Advantageously, the ion reserve volume 20 is entirely delimited between on the one hand a metal plate

21 and on the other hand an openwork cage 22 in electrical contact with the plate 21, this plate and this cage constituting

1 anode 2.

Furthermore, the smoke detector may optionally further include a speed sensor 41 for measuring the speed of the continuous air current generated by the fan 12. This speed sensor can be arranged in particular in one of the conduits 7 , 8, or possibly in the air inlet duct 15, or the like. The air speed sensor 41 may consist, for example, of a small propeller which is rotated by the stream of air charged with ions and whose speed of rotation is detected by an electro-optical or electric system. This air speed sensor 41 is connected to an electronic control circuit 42 which is designed to control the speed of rotation of the fan 12 as a function of the measurement made by said sensor 41, so as to maintain the speed of the current substantially constant continuous air which goes from the ionization chamber 6 to the detection chamber 1.

Furthermore, the electronic circuit 42 can optionally also control the voltage generator 11 and be designed to operate this voltage generator as well as the fan 12 intermittently, in order to limit the energy consumption of the smoke detector.

For example, the control circuit 42 can be designed to operate the voltage generator 11 and the fan 12 by generating the current of continuous air charged with ions for periods each lasting a few seconds, for example approximately 5 seconds, these periods of being separated from each other by periods of stopping "lasting for example about 20 seconds.

In addition, the circuit 42 is connected to the device 18 to control the stopping of the measurement of the current i during the periods of stopping of the fan 12.

Optionally, the circuit 42 could have the sole function of controlling the intermittent operation of the smoke detector, in which case this detector does not include an air speed sensor 41. It will also be noted that the fan 12 could be replaced by any other ventilation system, mechanical or not.

For example, the fan 12 can be replaced by a substantially flat strip 23 which extends longitudinally between on the one hand an upstream end 24 secured to the inlet duct 15 and on the other hand a downstream end 25 free, this strip being actuated by an electric actuator so that the free end of this strip moves perpendicular to the plane of said strip, thereby generating the above-mentioned permanent air current.

In this case, as in the example in FIG. 1, the smoke detector could possibly include an electronic control circuit 42 associated if necessary with an air speed sensor 41, to control the intermittent operation of the detector smoke and / or to control the beat frequency as a function of the air speed measured by the sensor 41, of the strip 23 so as to maintain constant the speed of the continuous air stream charged with ions. In the example shown in FIG. 2, the action of the strip 23 is piezoelectric.

To this end, the strip 23 is constituted by two layers 26, 27 of piezoelectric material separated by a conductive layer 28 which is connected to an alternating voltage source 29.

In addition, the two layers 26, 27 of material piezoelectric are covered to the outside of the lamella 23 respectively by two conductive layers 30, 31 connected to ground (zero potential).

The layers 26, 27 of piezoelectric material are oriented to deform longitudinally (lengthening or shortening) in phase opposition relative to each other under the effect of the alternating voltage generated by the source 29, and the end fixed 24 of the strip is embedded in a support 32 integral with the inlet duct 15.

Thus, under the effect of the alternating voltage generated by the source 29, the contrary longitudinal deformations undergone by the layers 26, 27 of piezoelectric material deform the strip 23 alternately in one direction and in the other by bimetallic effect, this which creates alternating beats of the free end of the slide 25 by generating the permanent air current which brings the ions into the detection chamber.

In the example shown in FIG. 3, the actuation of the strip 23 is obtained by electrostatic means.

To this end, the strip 23 is metallic and is connected to ground (zero potential), the fixed end 24 of this strip being pivotally mounted on a support 33 secured to the inlet duct 15. Furthermore, in the duct d the inlet 15 are arranged two metal plates 34, 35 which are arranged so as to form a divergent corner downstream from the fixed end 24, without however being in contact with said fixed end 24. These two plates 34, 35 are preferably pierced with holes 36 and they are respectively connected to a voltage generator device 37 which is designed to apply alternately to either of the two plates 34, 35 a positive voltage, the other metal plate - that then being brought to zero potential.

Thus, the strip 23 is alternately applied against one or the other of the plates 34, 35 by electrostatic effect, so that its free end 25 undergoes an alternating beat which creates the abovementioned permanent air flow, in particular through the holes 36. In the example of Figure 4, the actuation of the strip 23 is obtained by magnetic means.

For this, the strip 23 is made of a magnetic material such as iron, and the fixed end 24 of this strip can for example be rigidly secured to a support 38 fixed inside the inlet duct 15.

An electromagnet 39 is arranged opposite the strip 23: this electromagnet is alternately supplied then not supplied by a circuit 40, so that it attracts the strip 23 towards it during each feeding period, thus forcing this lamella to deform elastically, then the lamella returns to its initial position by elasticity during the following period of non-supply of the electromagnet.

The free end 25 of the strip thus moves in an alternating beat, which generates the abovementioned permanent air current.

Furthermore, as shown in FIG. 5, the fan 12 could be replaced by an electric heating resistor 43 traversed by an electric current generated by a power supply 44.

In this case, the smoke detector is arranged so that the continuous air stream follows an upward path from the resistor 43 to the detection chamber 1, so that this continuous air stream is generated by a phenomenon natural correction due to the heating of the air produced by the resistor 43.

In particular, the smoke detector is preferably arranged so that the detection chamber 1, the ionization chamber 6 and the inlet duct 13 are aligned vertically.

In the example shown in Figure 5, the smoke detector includes an air speed sensor 41 similar to that already mentioned with reference to FIG. 1, but this sensor is here connected to the electronic circuit 18 for measuring intensity and alarm instead of being connected to a control circuit 42.

The electronic circuit 18 is designed to correct the measurement of the electric current i or to correct the alarm detection threshold as a function of the air speed measurement carried out by the sensor 41, so that the smoke detection is made substantially independent of the variations in the electric current i due to the variations in the speed of the continuous air current charged with ions.

For example, the electronic device 18 can have in memory a correspondence table giving the corrected values of the electric current i or of the alarm detection threshold as a function of the air speed measured by the sensor 41.

These correspondence tables can be determined experimentally for a given type of smoke detector, for example by measuring the variations in the electric current i as a function of the variations in the measurement carried out by the sensor 41 in the absence of smoke. The correspondence table can then give, as a function of the measurement of the sensor 41: - either a correction coefficient to be applied to the actual measurement of the current i in order to obtain a corrected measurement imperceptibly independent of the variations in the speed of the direct air current charged with ions,

- Or a value of the alarm detection threshold which can be equal to a certain percentage (for example around 80%) of the normal value of the electric current i in the absence of smoke.

It will be noted that in the embodiments of FIGS. 1 to 4, the sensor 41 could possibly be connected to the electronic device 18 so as to correct the operation of this device as described above: in this case, it is no longer necessary to control the operation of the fan 12 or of the strip 23 to the measurement made by the sensor 41.

Furthermore, it will also be noted that in the embodiment of FIG. 5, it would be possible to provide an electronic control circuit 42 connected to the sensor 41 and to the power supply 44, to control the operation of the electrical resistance 43 to the measurement carried out by the sensor 41, so as to keep the speed of the continuous air stream charged with ions substantially constant: in this case, the sensor 41 would generally no longer be connected to the electronic device 18.

As it goes without saying, and as it follows from the above, the invention is in no way limited to those of its embodiments which have been specially envisaged: on the contrary, it embraces all variants.

Claims

1. Air ionization smoke detector, comprising a detection chamber (1) which contains air charged with ions and which communicates with the atmosphere, this detection chamber being the seat of an electrostatic field generated between an anode (2) and a cathode (3) which are connected to each other by an electric bias circuit (16) provided with a voltage generator (17) creating a voltage between the cathode and the anode continuous (V) less than 100 volts, so that the ions contained in the detection chamber (1) are attracted one towards the cathode (3) and the other towards the anode (2), thus generating an electric current (i) in the bias circuit (16), which circuit further comprises an electrical intensity measurement and alarm device (18) for measuring said electrical current and for triggering an alarm reaction when this current electric undergoes an abnormal variation, characterized in that the ions are generated in at least one ionization chamber (6) which is separate from the detection chamber (1) and which communicates with said detection chamber, the ionization chamber also communicating with the atmosphere and comprising at least two electrodes (9, 10) connected together by an electrical supply circuit capable of generating between these two electrodes electric discharges ionizing the air, and in that air displacement means (12, 23, 43) are further provided to create a low continuous air flow from the ionization chamber (6) to the detection chamber (1), so as to transfer into said detection chamber the ions generated in the ionization chamber. 2. Smoke detector according to claim 1, wherein the air flow between the ionization chamber (6) and the detection chamber (1) has a speed between 1 and 20 cm per second. 3. Smoke detector according to any one of Claims 1 and 2, in which the air flow between the ionization chamber (6) and the detection chamber (1) arrives in the detection chamber by a channeling device which comprises at least one duct (7, 8 ) and which offers said stream of air a small total passage section, the detection chamber (1) having, in the vicinity of this channeling device, an interior section which, taken perpendicular to the stream of air, is at least five times greater than said total passage section, and the air contained in the detection chamber (1) having an average renewal time of 1 to 10 minutes under the effect of the permanent air current coming from the ionization chamber . i 4. Smoke detector according to any one of the preceding claims, in which the air displacement means comprise a ventilation device (12, 23) displacing the air by mechanical action. 5. Smoke detector according to claim 4, wherein the ventilation device (12, 23) is arranged upstream of the ionization chamber. 6. Smoke detector according to any one of claims 4 and 5, in which the ventilation device (12) comprises rotary blades (14) driven by an electric motor (13). 7. smoke detector according to any one of claims 4 and 5, in which the ventilation device comprises a substantially planar strip (23) which extends longitudinally between on the one hand an upstream end (24) fixed and on the other hand a free downstream end (25), this strip being actuated by an electric actuator (26, 27; 34, 35; 39), so that the free end (25) of this strip moves perpendicular to the plane of said strip. 8. Smoke detector according to claim 7, in which the electric actuator (26, 27; 34, 35; 39) is chosen from electrostatic actuators, the action- magnetic sensors, and piezoelectric actuators. 9. Smoke detector according to any one of claims 1 to 3, wherein the air displacement means comprise a heating element (43) for heating the air upstream of the detection chamber (1), so moving this air by natural convection, the smoke detector being shaped so that the continuous air flow follows an upward path from the heating element (43) to the detection chamber (1). 10. Smoke detector according to any one of the preceding claims, comprising control means (42) designed to operate intermittently the air displacement means (12, 23, 43) and the ionization chamber. (6). 11. Smoke detector according to any one of the preceding claims, further comprising monitoring and control means (41, 42) for measuring the speed of the air flow which goes from the ionization chamber towards the detection, and to control the air displacement means (12, 23, 43) so as to maintain constant said speed. 12. Smoke detector according to any one of claims 1 to 10, further comprising a sensor (41) for measuring the speed of the air current which goes from the ionization chamber to the detection chamber, this sensor being connected to the electrical intensity measurement and alarm device (18), which device is designed to take into account the speed measured by the sensor (41) and to correct as a function of this speed at least some of the data that it takes into account to trigger an alarm reaction, so that this device for measuring electrical intensity and alarm (18) overcomes variations in 1 measured electrical intensity (i) which are due to variations in the air speed. 13. Smoke detector according to any one of the preceding claims, in which the air stream _e etween the ionization chamber (6) and the detection chamber (1) opens at least partially in a reserve volume _of ions (20) which is not subjected to the electrostatic field existing between the anode (2 ) and the cathode (3) of the detection chamber and which communicates with the detection chamber (1) so as to be swept by any disturbing outside air current which would also sweep this detection chamber, before the passage of this current air in at least part of said detection chamber.