FR2694810A1 - Device for detecting the variations of the heat exchange coefficient by convection with a fluid, in particular for anti-intrusion and anti-theft alarm systems. - Google Patents

Abstract

The invention relates to a device making it possible to detect with great sensitivity the variations of the heat exchange coefficient by convection of a detector with a fluid, in particular in the case of a change from a natural convection regime to a regime of. forced convection or a change in the nature of the ambient fluid, the detector consisting of a metal wire of very small section heated by the passage of an electric current and installed in an electric and electronic circuit carrying out the detection functions as well than the detector self-test. The device according to the invention is particularly intended for anti-intrusion, anti-theft, glass breakage alarm systems for homes, public or private buildings, vehicles.

Description

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  The present invention relates to a device for detecting variations in the convective heat exchange coefficient with a fluid intended particularly for alarm systems for homes, public or private buildings and vehicles. The known alarm systems combine zpr_- = specific devices 4 specific to each type of liformation or event sought as example

and in a non-limiting way -

  volumetric detection of presence detection of openness Lre of doors or windows of breakage of windows detection of movement of objects Each of the devices providing these various detections often use particular processes and techniques It is sometimes possible to devise parades to thwart their action Some

can be activated remotely.

  The device according to the invention makes it possible to remedy these inconveniences by using the fact that each movement of human beings or of objects, breaking glass and in a manner such that all human action causes

  Slowly putting in the surrounding air.

  The detection of this setting in motion with a high sensitivity 4 allows to ensure all the i functions of an alarm system as described

  above, through a single device.

  The device according to the invention comprises a detector itself, an electronic and electronic circuit

associates 4.

  The drawings annexss illustrate the invention.

  Figure n shows the schematic diagram of

the invention.

  Figure No. 2 represents a variant comprising

  several detectors in series in the same circuit.

  Figures N 03 to 8 show by way of example some details of realization or configurations

possible for the detector.

î

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  With reference to FIG. 11, the device comprises a detector (1) consisting of a wire of very small section operating according to the principle

known as "hot wire".

  This principle is generally used for the production of anemometers that is to say to measure the speed of a fluid This use imposes to compensate the anemometer of the fluctuations of the temperature of the fluid, to look for a rather wide measuring range and to restore at the hot wire aerodynamic flow if possible laminar The use that is made of this principle in the present invention differs significantly in that it is not a question of measuring the absolute speed of the fluid but of detecting either its setting in motion whatever the regime, laminar or turbulent is a modification of its naturej with an operation of the type "all or nothing" where the sensitivity will be privileged around

  from the point of equilibrium to a natural convection regime.

  Finally, this device does not provide for

  temperature compensation system.

  The principle is recalled hereinafter: A homogeneous conductive material of section S and of length L has an electrical resistance R expressed by the formula L R = x S

  ol is electrical resistivity.

  the electrical resistivity depends on the material used and its temperature e according to the approximate formula: o çx (1 + rs t) to Oëlap St rsistivit & C C

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  when this material is crossed by a current I

  its temperature reaches an equilibrium value e.

  such as: 2 e G 6 Rxi Kx: pL ai with K = convective heat exchange coefficient p = conductor perimeter a = ambient fluid temperature This equation represents the balance between the energy dissipated by the Joule effect and the convection-exchanged energy, neglecting conduction and radiation exchanges Any variation in the coefficient K, especially during a transition from a natural convection regime to a forced convection regime, results in a variation of the temperature of the driver so d his resistance that it is about

  detect-r by a suitable electrical circuit.

  With reference to FIG. Nel, the detector (1) consisting of such a conductor is placed in series with a resistor (2) whose value is close to that of (1) in the natural convection equilibrium regime. This allows you to change the resistance of (1) given, to obtain the greatest variation of the voltage across (2). The resistance (5) placed in parallel of (1) has a value of very big compared

  to that of (1) and (2) Its r 8 will be made clearer by far.

  It can be neglected for the following.

  Under these conditions, the equilibrium detector temperature is: YI e O C + a SE i 1 = 4 = / (- 3 (Ga) 4 AA 4 K pp LA 2 A vi-4- If one study the variations of I as a function of the variations of the coefficient K, we obtain the formula: dl Id K =

2 K P S

  The sensitivity of the detector is therefore even greater than the product (t A) is large and the product (p S) is small It is therefore advantageous for this application to use a detector

  made of iron, steel, titanium or one of its alloys.

  In addition to their electrical characteristics, these materials have excellent mechanical and corrosion resistance. It is also possible to dispose of them in the form of wire with a diameter of less than eight hundredths of a millimeter, which is quite suitable for Other materials can be validly used provided that (% A) is greater than about 35. 10 "ohm meter / OC so that the length of wire required for the realization of the detector is not prohibitive and allows overall dimensions

  less than a few centimeters.

  The device which is the subject of the invention can also be effectively used to detect a modification of the nature of the ambient fluid in particular, a liquid / gaseous fluid transition or vice versa. This application relates in particular to the cooling circuits, for example of an automobile engine, to detect a pipe break and trigger an alarm well before the temperature reaches an excessive value, or to detect the level of a liquid For all these applications, the detector is optimized to detect relatively rapid changes in the temperature coefficient. Thus, it is not necessary to equip the device with any temperature compensation as is generally provided for in the anemometers using the hot wire principle. The temperature variations in the present applications of the invention, are slow. This is the reason for the presence, in the circuit of the conde-nsator (3) which allows to detect at the terminals of the resistor (4) that the variations

  sufficiently fast voltage at> terminals of (2).

  This function of suppressing the continuous component can also be achieved by other conventional devices not described here. The resistor (5) in parallel with (1) makes it possible to carry out a simple test on the state of the detector (1). If the metallic wire breaks, the voltage across (2) will become much smaller than E / 2 "in the event of a short-circuit, it will become much larger than E / 2. A voltage monitoring across (2) to Using 2 comparators (9) and (9) which compares, for example, respectively to 3 E / 4 and E / 4, thus allows with states Al and A 2 to know continuously the state of the detector The voltage at the terminals of (4) is optionally amplified by means of (6) and compared by means of (7) to an adjustable reference voltage Uref which allows the adjustment of the overall sensitivity of the device The output S will be, for example,

a logical level.

  The principle of the invention allows in a same circuit, to mount in series several detectors (1) (10) (11)

  as shown in Figure No. 2.

  Figure N 03 shows the principle of realization of the detector It consists of the wire (12) connected at each end to electrical contacts (13) all fixed on an electrically insulating support and thermiq Lluement The wire (12) is protected

  by a grid (15) against mechanical aggression.

  This grid is permeable to fluids. It can be metallic to form a cage-like protection.

  Faraday against electromagnetic radiation.

  It is advantageous if the hot wire (12) is moved away from the insulating support (14), however, it is possible that it is directly in contact with it as shown in FIG. N 05, at the cost of a slight loss of sensitivity. This particular configuration may be preferred for the detection of the nature of ambient fluid for, for example, a level switch The hot wire (12) can be installed either horizontally or vertically or in any intermediate position and be installed on a support (14) of any shape, for example and without limitation, planar, concave or convex, round or polyhedral, spiral To reduce the dimensions of the detector it is possible, as shown in Figure N 04, to fold the wire in the form of "S" around insulating pads (16) optionally mounted on a voltage device of which one of the possible solutions is shown in Figure N 06 This voltage device comprises the iso pad Sliding and guided lant (16) provided with a spring (17) which holds the wire in tension to catch the thermal expansions and any possible length dispersions The electrical contacts - (1 3) can be carried on any side of the detector, either on a different side as in Figure N04 or both on the same side as shown in Figure N07 and this in the plane of the support (14) or perpendicular thereto To increase the sensitivity of the detector in a particular direction, a variant consists in adding a convergent device of the "Venturi" type as illustrated in principle in FIG.

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Claims (10)

  1) Device for detecting the variations of the coefficient of heat exchange by convection with a fluid, these variations being able to be due to the setting in motion of this fluid whichever is the regime, laminar or turbulentent to a change of the nature of the ambient fluid a variation of its speed, characterized in that it comprises a detector (; 1) consisting of a wire of very small section heated by passage of an electric current and placed in series with an adjustable resistor (2) the all being supplied with a DC voltage E, the value of the resistor (2) being chosen so that the voltage at its terminals is close to E / 2 in the equilibrium situation with respect to which the variations will be detected, the detector (1) being connected in parallel with a resistor (5) of large value with respect to the sensor (1), the voltage variations across (2) which correspond to the variations of the heat exchange coefficient being observed after removal of the component continuedpar example, through a capacitor (3) and a resistor (4) in series with said capacitor, possibly amplified by means of an amplifier (6) and compared to an adjustable reference voltage E corresponding to the threshold of   detection through a comparator (7).   2) Device according to claim i characterized in that the metal wire used for the detector is made of a material whose electrical characteristics are such that the product: electrical resistivity at O CC by the temperature coefficient (C x A) is greater at 35 10 ohm meter / C, and especially the Vacier, titanium or an alloy thereof.   3) Device according to any one of the claims   above characterized in that the wire used in the detector has a diameter less than eight hundredths of a millimeter.   4) Device according to any one of the claims   above characterized in that several detectors can be mounted in series in the me circuit electric.   ) Device according to any one of the claims   above characterized in that it can be used in alarm systems for the detection of: presence, opening of doors or windows, movement of objects, breakage of ice and In general, in any system, the detection of the setting in motion of the air is necessary, and that for this application the value of the resistor (2) is close to the resistance of the detector (1) in natural convection mode with 1 ambient air   6) Device according to any one of the claims   above characterized in that it can be used to detect a change in the nature of the fluid in contact with the detector (1), and for this application the value of the resistor (2) is close to that of the detector (1). ) in convection mode   natural with the starting fluid.   7) Device according to any one of the claims   above characterized in that the presence of an electrical resistor (5) in parallel with the detector (1) makes it possible, by comparing the voltage across the resistor (2) with two predetermined values, to detect either a rupture a short-circuiting of the wire (12) of the detector (1). 9 _ q 2694810   8) Device according to any one of the claims   above characterized in that the wire m Stallique (12) can be installed in contact with the insulating support (14)   or on the contrary by sparing a space.   9) Device according to any one of the claims   above characterized in that the wire (12) can be folded around insulating supports (15) optionally mounted on a thread tensioning device ea e extremiits being connected to contacts lecterique (13) f these contacts can be brought back to different ends of the detector in a plane parallel to the support (14) O Lu in a plane perpendicular to this one.   ) Device according to any one of the claims   above characterized in that the wire (12) is protected by a grid, possibly metallic and connected to the electrical mass   grid being permeable to fluids of any kind.   11) Device according to any one of the claims   above characterized in that the detector may be provided with a convergent device of the "Venturi" type (16) to increase its sensitivity in one direction particular.   12) Device according to any one of the claims   above characterized in that the metal wire (12) of the detector (1) can be oriented in any one of the directions of the space, and be installed on a support (14) of any shape for example> : example and this in a non-limiting way 5 plan,   concave or convexerond or plyèdriqueen spiral.