Abstract:
An apparatus for verifying the integrity of a wall automatically sets off an intervention in the case of an act committed against the wall. A capacitive sensor is formed on the external or internal surface of the wall and consists of a thin planar condenser matching the shape of the wall. An oscillator having variable frequency has its control input connected to a plate of the condenser, the other plate being connected to ground, and a circuit detects the variation of frequency of the output signal of the oscillator resulting from a variation of condenser capacitance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a monitoring apparatus for verifying the integrity of any wall, metallic or nonmetallic, and is adapted to automatically set off an intervention in the case of an act committed against this wall. 
     2. Description of Background and Relevant Information 
     There presently exists a large number of articles produced, documents, or valuable articles which must be protected. Examples are bank notes, checks or check books, credit cards or other cards usable as money, secret documents, microfilm, magnetic and/or optical recordings, etc. All of these objets to be protected are generally stored in safety containers, such as enclosures which are each defined by a wall separating the environment from the internal volume in which the protected objects are stored. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an apparatus making it possible to permanently monitor the state of such a wall and to automatically control an intervention, as soon as an aggression is committed against this wall, this intervention being, for example an alteration or even a destruction of the objects located within the protected container. 
     The invention comprises, on the external or internal surface of the wall, a capacitive sensor constituted by a thin planar condenser matching the shape of the wall and, on the internal side of this wall, an electronic surveillance circuit comprising a variable frequency oscillator, to the control input of which is connected a plate of the condenser carried by the surface of the wall and whose other plate is connected to ground, and means for detecting the variation of frequency of the output signal of the oscillator resulting from a variation in capacitance of the condenser, and for then delivering an alarm signal when an external aggression on the wall causes a variation of the surface of the condenser carried by this wall and consequently the capacitance thereof. 
     Preferably, the external plate of the condenser pressed against the wall is connected to ground and this plate serves as a Faraday cage protecting the electronic surveillance circuit with respect to external radio electric aggressions. 
     According to a further characteristic of the invention, the electronic surveillance circuit comprises a temperature detection probe placed in the vicinity of the internal surface of the wall and this temperature detection probe is connected to a temperature correction circuit to provide compensation for variations of temperature, in a range preferably from -40° C. to +60° C. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described below, by way of non-limiting examples, with reference to the annexed drawings in which: 
     FIG. 1 is a diagram of an apparatus for monitoring the integrity of a wall according to the invention. 
     FIG. 2 is a cross-sectional view of the wall carrying the capacitive sensor, formed as a planar condenser. 
     FIG. 3 is a diagram of an alternative embodiment of the apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The apparatus, according to the invention which is shown in its entirety in FIG. 1, is adapted to permanently monitor the integrity of a wall 1. This wall forms a surface which is or is not closed on itself, to define a safety enclosure containing objects which must be protected. The apparatus utilizes an external capacitive sensor formed as a planar condenser 2 which is applied directly or indirectly on the external surface 1a of the wall 1. This condenser comprises an external metallic plate 2a, an internal metallic plate 2b in contact with the external surface 1a of the wall 1 and an intermediate dielectric layer 2c, in the case where this wall 1 is made of a non-metallic material. If the wall 1 is metallic, it can itself serve as a plate which is then coated with an adhesive and insulation lacquer separating it from the external plate 2a. Preferably, the external plate is grounded in a manner so as to constitute a Faraday cage surrounding and protecting the various elements constituting the electronic circuit, which will be described below. This condenser is utilized as a surface integrity sensor, it being given that the capacitance of the condenser is directly proportional to the instantaneous area of the surface of the condenser plates, if one assumes that the thickness of the dielectric layer 2c is constant. According to the invention, one utilizes this property to detect an aggression against the wall, the aggression being indicated by a variation in the metallic surface of condenser 2. This aggression can be of two different types, namely a slow aggression, for example as a result of elevation of local temperature by means of a blowtorch, so as to pierce the wall and a rapid aggression, for example, as a result of the impact of a bullet perforating condenser 2 and the wall 1, the reduction of the surface area of condenser 2 resulting from the hole made by the bullet. 
     The apparatus according to the invention utilizes, for detecting the aggression committed against wall 1, a variable frequency oscillator 3 at the input of which is connected condenser 2 having a capacitance which varies in response to potential aggressions. More particularly, the internal plate 2b is connected to the input of variable frequency oscillator 3, it being given that the external plate 2a constitutes a ground plane for all of the electronic circuits. The variable frequency oscillator 3 delivers at its output an alternating frequency signal f which is inversely proportional to the capacitance C of condenser 2, otherwise stated f=k/C, k being constant. In this embodiment, the output of the variable frequency oscillator 3 is connected to an input of a frequency demodulator stage 4 whose output is itself connected to a level detector 5. The frequency demodulator 4 is preferably constituted by a circuit having a phase lock loop. One then adjusts the oscillation frequency of this phase lock loop on the variable frequency oscillator 3 and all of the frequency offsets are then caught up by the control of the phase look loop. One thus obtains, at the output of the frequency demodulator 4, an amplitude signal which is proportional to the spacing of the frequency with respect to the reference frequency. 
     In fact, one distinguishes in practice two types of aggression namely: 
     (a) the rapid crossing of wall 1 or rapid aggression which creates, in the output signal of the variable frequency oscillator 3, a sudden offset in phase in a period; 
     (b) the slow aggression or wear (for example attack by chemical product of low reaction velocity of a blowtorch) which translates into a progressive frequency offset in time and which is directly proportional to the variation of the area of surface 2a of condenser 2. These two types of aggression can be detected by a single circuit namely, the frequency demodulator 4. The demodulator 4 is connected to the level detector 5 which can be constituted by an operational amplifier with a window having two detection levels. In fact an aggression by boring or wearing causes a diminution of the surface S and thus the capacitance C of the condenser 2, while on the contrary, a thermal aggression (by a blowtorch, for example) causes, in the first place, an increase in the surface S and of the capacitance C as a result of the dilation of the metallic plates of the condenser 2, which results in the necessity of having two detection thresholds. The output signal of the operational amplifier causes, as soon as one of the two thresholds is crossed, the engaging of a circuit, such as a flip-flop in a logic circuit so that the crossing of one of the thresholds is translated by the engaging of the circuit and the emission of a corresponding signal. Furthermore, the engaging of this circuit assures the storage and memory of this aggression. 
     The output signal of the level detector 5 can be utilized for various purposes, for example, to cause the destruction or at least the irremediable alteration of the objects protected by the wall 1 (destruction or coloration of bank notes, for example). 
     The apparatus according to the invention, comprises an apparatus which assures the correction of the frequency measurement with respect to the room temperature which itself causes a expansion or contraction of surface S of condenser 2 and would therefore cause a variation of capacitance C. The temperature correction apparatus comprises a temperature detection sensor 6 which is situated immediately adjacent the internal surface 1b of the wall 1. This sensor 6 is connected to a temperature correction circuit 7 which is itself connected to frequency demodulator 4. The circuit 7 intervenes to take into account the slow variations of ambient temperature, by distinguishing them from the rapid variations due to a thermal aggression. For this reason, the circuit 7 has a slower response time than the condenser 2 so as to be able to effect this difference. 
     As can be better seen in FIG. 2, the condenser 2 formed on the external surface 1a of the wall 1 can be obtained by application of a first layer of metal (for example aluminum), by vacuum metallization or galvanoplasty, to constitute the internal plate 2b of the condenser, then application of an insulating varnish layer of constant thickness to constitute the intermediate dielectric layer 2c; then application of a second metallic layer, by metallization under vacuum or galvanoplasty, to constitute the external plate 2a of the condenser 2; and finally, application of an exterior varnish protection layer 8. The electric connection of the two metallic plates 2a and 2b to the internal electronic circuit can occur easily, in the manner illustrated, by means of conductor wires 9, 10 passing through holes bored in the wall 1. The wire 9 passes through a hole 11 formed in the metallic layer 2b constituting the internal plate, when this layer is made, thanks to the provision, at the location of hole 11, of an appropriate guide. The wire 9 consequently traverses the internal plate 2b without being in contact with it. The other wire 10 is connected directly to the internal plate 2b. The varnish 2c constituting the intermediate dielectric layer is selected so as to have a similar expansion-contraction coefficient as that of wall 1 supporting the condenser 2 forming the sensor. 
     In the embodiment of the invention shown in FIG. 3, the condenser 2 is supposed to be used only as a wear detector. In this case one can utilize, in place of the frequency demodulator stage 4, a frequency meter 12 connected, by an interface, to a microcomputer 13 which analyzes by its program, the frequency offsets. In this embodiment of the invention, the level detector 5 is likewise unnecessary because the detection thresholds are defined by the program of the microcomputer 13. Furthermore the temperature correction circuit 7 is connected to the microcomputer 13 which corrects the frequency with respect to the temperature. 
     Of course, the invention is not limited to the embodiments described and shown by way of examples, but includes all equivalent techniques as well as their combinations.