Seismic sensor for a security system and security system including such a sensor

A seismic sensor for a security system is described, the sensor comprising: a piezoelectric transducer suitable for converting mechanical stresses into electrical signals; a base structure including a seat suitable for receiving said transducer; a cover element that can be coupled with the base structure to define an inner chamber of said sensor. The sensor is wherein the cover element has an essentially bell-shaped body, is equipped with a bottom opening and includes side walls that extend around an axis of said opening, the base structure being provided with a collar that surrounds said seat and that crosses said bottom opening so as to define at least one channel communicating with said chamber between the side walls of said cover element and said collar.

BACKGROUND

1. Technical Field

The present invention refers, in general, to the technical field of security systems and, in particular, concerns a seismic sensor for a security system, as defined in the preamble of the first claim. The present invention also concerns a security system including such a sensor.

2. Description of the Related Art

As known, for a long time there has been a great need to protect the perimeter of buildings, with immediate indication, through suitable emission of an alarm, of a possible attempt to gain unwarranted access to a building being protected or of a possible attempt to escape from it.

Security systems, known as underground systems, which use various types of sensors, intended to be arranged in the ground or incorporated in the flooring and along the perimeter of a building to be protected, or along its potentially accessible areas are known and extensively used. In practice, the sensors of underground systems are seismic sensors generally sensitive to footsteps on the ground or on the flooring and deriving from a person approaching the perimeter or the protected area.

A particular security system of the type described above is disclosed by international patent application published as WO 96/10195, which in particular discloses a security system including a plurality of pressure sensors, or seismic sensors, equipped with a piezoelectric transducer.

In the seismic sensors disclosed by the aforementioned patent application, the seismic waves that propagate through the ground, caused by the passing of a person at the surface and near to the sensor, produce the movement of a suitable “receptor”, the pivot point of which is indirectly coupled through soft resin with a piezoelectric transducer housed in the base of the sensor.

The movement of the receptor with respect to the base of the sensor causes a weak mechanical stress at the piezoelectric membrane, which thus produces an electrical impulse that is then amplified and analyzed by suitable electronic circuits.

The piezoelectric transducer is characterized by a high conductivity of two of its conductive surfaces insulated from each other by a very fine insulating layer.

It has been observed that the correct operation of the piezoelectric transducer and its lifetime essentially depend upon two factors:a stable working position, which does not involve the application of high mechanical stress to the transducer, so as to prevent it from breaking; andperfect electrical insulation of the two conductive surfaces of the transducer, both between each other and against the surrounding environment (ground).

The mechanical protection of the transducer is obtained by designing the base of the sensor like a rigid container that houses the transducer leaving just its upper free surface accessible. The coupling between transducer and receptor occurs indirectly, through a thick layer of polyurethane or epoxy resin arranged on the free surface of the transducer, so as to prevent it from breaking due to a possible direct contact with the pin of the receptor.

The electrical insulation is ensured by the same resin that effectively also carries out the function of sealing and electrically insulating the transducer from the surrounding environment from infiltrations of water/humidity.

With this type of solution the protection from humidity relies totally on the integrity of the layer of sealing resin that, in the long term, due to thermal shifts and perhaps due to contact with particular chemical substances that may be present in the ground, can deteriorate and lose its adhesion.

Moreover, the different expansion of the materials (resin and plastic of the casing) caused by the continuous thermal cycles (night/day, winter/summer), can cause the resin to detach from the casing and consequently water channels to be formed on the edges.

For greater protection from infiltrations a thick layer of additional sealing material, sufficiently soft to allow the receptor to move with respect to the base, has been foreseen along the edges defined between the receptor and the base of the sensor. Such a sealing material is the only barrier against the penetration of water or humidity inside the sensor.

BRIEF SUMMARY

A purpose of the present invention is to provide a sensor that, with respect to the sensors of the prior art described above, is stronger with respect to possible infiltrations of water inside the sensor.

Such a purpose is accomplished through a sensor as defined in the attached claim1in its most general form and in the dependent claims in some particular embodiments.

A further purpose of the present invention is to provide a security system as defined in the attached claim13.

In the figures, identical or similar elements are indicated with the same reference numerals.

DETAILED DESCRIPTION

FIG. 1schematically shows a side section view of a particularly preferred embodiment of a seismic sensor, globally indicated with1, in accordance with one embodiment of the present invention. The seismic sensor1is intended to be installed below a layer of ground, preferably at a depth of 50-60 cm, to detect thrusts of pressure acting substantially near to the surface of the ground and that propagate in the form of seismic waves inside the layer of ground. For example, such thrusts of pressure are caused by the passing of a person or of a vehicle on the surface of the ground that incorporates the sensor1, in a range of the seismic sensor1that constitutes the working area of the sensor. The sensor1can be part of a system, or security apparatus, including many parts each including a plurality of sensors. A security apparatus of this type is, for example, described in the aforementioned European patent EP 1005003 and for this reason a description of the system of such an apparatus shall not be dealt with in any greater depth at this stage. A seismic sensor1in accordance with the present invention can also advantageously be used in a security apparatus of the type described in the international patent application published as WO 2006/051561.

The seismic sensor1includes a base structure2, for example made from hard plastic, including an assembly seat3that is such as to receive a piezoelectric transducer4suitable for supplying in output electrical signals in response to mechanical stresses to which it is subjected.

The piezoelectric transducer4is preferably a plate-shaped transducer, in this example disc-shaped, and in practice comprises a plate made from conductive material, for example brass or copper, coated with a thin layer of piezoelectric ceramic.

A flexing chamber7for the transducer4opens on the bottom of the assembly seat3. Preferably, but not limitingly, the flexing chamber7is defined by a circular bottom and by a cylindrical shell and preferably has a very low depth. For example, in a particularly preferred embodiment, the flexing chamber7has a depth roughly within the range 1 mm-3 mm. This shallow depth advantageously makes it possible to avoid the transducer4breaking when it is subjected to excessive mechanical stresses, since in these cases the bottom of the flexing chamber7, going into abutment against the transducer4, limits the possibility of inflection of the latter.

From the transducer4project conducting wires8through which the transducer4supplies in output electrical signals in response to mechanical stresses that are such as to cause its deformation/inflection. Such conducting wires are intended to be connected to electrical cables, not shown inFIG. 1, which allow the connection of the sensor1to a security system.

The seismic sensor1includes a cover element5, which can be coupled with the base structure2to define an inner chamber6of the seismic sensor1, communicating with the assembly seat3of the piezoelectric transducer4.

Advantageously, the cover element5has an essentially bell-shaped body and is equipped with a bottom opening10and includes side walls11that extend around an axis Z-Z of such a bottom opening10. As depicted in the example ofFIG. 1, advantageously, the base structure1includes a collar12that surrounds the assembly seat3of the sensor and that crosses the bottom opening10of the cover element so as to define, between the side walls11of the cover element5and the collar12at least one channel13communicating with the inner chamber6of the seismic sensor1. In the particular embodiment represented inFIG. 1, the channel13is in reality a continuous and tubular interspace that surrounds the collar12.

In the particular embodiment ofFIG. 1, the cover element5is essentially a bell-shaped element in the form of an inverted pot. In a possible alternative embodiment, the cover element5could be made essentially like a bell-shaped element in the form of a bell-mouthed dome.

It should be observed that, advantageously, since the collar12has a mouth19arranged at a greater height than the bottom opening10of the cover element5, by suitably sizing such a difference in height dH it is possible to ensure that the inner chamber6and the channel13contain a sufficient amount of air to prevent the penetration of water inside the seat of the transducer4, even when the ground inside which the sensor1is buried is completely flooded. In practice, to obtain this it is necessary to size the height of the collar12so that a possible rise of water along the channel13is blocked by the pressure inside the chamber6before the level of water in the channel13rises beyond the height of the mouth of the collar12. It has been observed that since the typical depths of installation of the seismic sensors do not generally exceed 70/80 cm, it is sufficient to foresee a collar of 4-5 centimeters to prevent the penetration of water inside the chamber6. More specifically, it is sufficient to foresee a difference in height dH between the mouth of the collar12and the bottom opening of the bell-shaped element5equal to about 4-5 cm.

It should also be observed that, in the particularly advantageous embodiment in which the bell-shaped cover element5is essentially dome-shaped, possible drops of condensation that form on the inner walls of the cover element5tend to run inside the channel13, thus avoiding the undesired falling of condensation into the assembly seat of the transducer.

It should also be observed that the cover element5represents a receptor member of pressure waves that propagate in the ground and it is provided with transmission means14suitable for transferring mechanical stresses to the transducer4, in response to the captured pressure waves. In the particular example represented inFIG. 1, the transmission means are in the form of a central pin, or stem,14that extends inside the chamber6between the upper wall of the bell-shaped element5, to which it is connected, until it reaches the assembly seat3of the transducer4.

Advantageously, inside the assembly seat3a layer of protective material15is foreseen defined between a lower contact surface with the face of the transducer4facing towards the inner chamber6and an opposite free surface16. Preferably, the layer of protective material15is a layer of resin that at least partly fills the assembly seat3. More preferably, the layer of resin15almost totally fills the assembly seat3of the base structure2. The resin used is preferably a bicomponent epoxy resin, or a bicomponent polyurethane resin.

The layer of resin15carries out a sealing action, preventing the formation of oxide on the face of the transducer4facing towards the chamber6, due to possible infiltrations of humidity from the outside, or else from internal condensation caused by thermal variations. Advantageously, the layer of resin15is such as to carry out the additional function of transmitting thrusts of pressure captured by the cover element5to the transducer4, so that the transducer4is subjected to corresponding mechanical stresses. For this reason, the layer of resin15has sufficient rigidity to ensure that such thrusts of pressure are essentially transmitted, in the example through the pin14, to the transducer4and not absorbed by the layer of resin15.

In a particularly advantageous embodiment, the pin14is a hollow tubular element. In the particular example described, in a non-limiting way, such a tubular element14has a circular section and has an opening18on the bottom that is such as to allow the resin of the protective layer15to penetrate inside the tubular element itself, at the assembly stage of the sensor1, i.e., when the resin15is not yet in solid state. Preferably, the opening18on the bottom has a smaller section than the inner section of the tubular element, so that once the resin of the protective layer15has thickened, the pin14and the cover element5are firmly fastened to the layer15of protective material. The opening18on the bottom having a small section therefore represents a particularly preferred embodiment of fastening means foreseen on the pin14and embedded in the layer of protective material15to fasten the cover element5to the layer of protective material15and therefore also to the base structure2.

In a particularly preferred embodiment, the seismic sensor1also includes a push rod20, for example having a length of about 8-10 cm, foreseen in the lower part of the base structure2to drive the seismic sensor1to a greater level of depth, preferably to the base of the excavated area and in a layer of ground not excavated. This provision allows the sensor1to be given a particularly stable position, holding it positioned vertically.

FIG. 2shows a variant embodiment of the seismic sensor in accordance with the present invention. The sensor1ofFIG. 2includes a protective lid21intended to attenuate excessively intense thrusts coming from above, in other words substantially perpendicular to the axis of the piezoelectric transducer4. Advantageously, this allows the sensitivity of the seismic sensor1to the movements on the surface to be made substantially homogeneous. In the example ofFIG. 2, the lid is equipped with feet27that allow the lid to be fixed to the base structure2of the sensor1. In greater detail, the end portions of the feet27are such as to engage in suitable housing seats22foreseen on the outer walls of the base structure2. In this way the thrusts collected by the lid21discharge onto the base structure2and not onto the receptor member5.

The cover element5, which in practice acts as a receptor member, is provided on the outside with a plurality of tabs23that are such as to increase the contact surface with the ground in order to obtain greater sensitivity.

In the example ofFIG. 2the seismic sensor1also includes a bottom cover25suitable for coupling with the base structure2on the opposite side with respect to the cover element5. The push rod20is integrated in the bottom cover25, and fastening elements26are foreseen for the coupling of the bottom cover25with the base structure2.

FIG. 3shows a side view with some parts in section of the seismic sensor1ofFIG. 2. As can be observed from such a figure, the bottom cover25when coupled with the base structure2closes a lower space30preferably foreseen in the base structure2for housing the end portions of the connection cables31intended for the connection of the seismic sensor1to the remaining part of the security system. Suitable holes or recesses24made in the walls of the base structure2, also visible inFIG. 2, are foreseen for the passage of the connection cables31.

Preferably, the lower space30is located below the assembly seat of the transducer4, for example foreseeing a partition34inside the collar12. More preferably, the partition34is shaped so as to have a concave region that is able to act as a flexing chamber7for the transducer4when the transducer is rested on an edge of the partition34defining the concave region.

In a particularly preferred embodiment, once the cabling of the seismic sensor1has been carried out the lower space30, which can possibly house electronic components like for example a printed circuit board32, is also filled with protective material, like for example the same protective resin15used to seal the assembly seat of the transducer4.

In a particularly advantageous embodiment, as shown inFIG. 3, the base structure2includes a groove33that surrounds the assembly seat3of the transducer4suitable for receiving a countershaped edge of the cover element5, defining the bottom opening10thereof. Preferably, a thin layer of sealing material (not shown in the figures) is arranged between the groove33and the countershaped edge of the cover element5. Preferably, such a sealing material has a sufficient softness to ensure the possibility of movement of the bell-shaped cover element5with respect to the base structure2.

Preferably, limit stop means are provided to limit the size of the movement of the cover element5with respect to the base structure2. In the example ofFIG. 3such limit stop means are represented by the same groove33and by the countershaped edge of the cover element5defining the bottom opening10. Alternatively, or in addition, it is possible to size the height of the collar12so that its mouth goes into abutment against the inner surface of the cover element5to stop the forward movement of the cover element5with respect to the base structure2.

As can be deduced from what has been outlined above, the purposes of the invention are fully accomplished, since a sensor in accordance with the present invention is able to resist infiltrations of water from the outside, even when the ground incorporating the sensor is completely flooded.

Of course, a man skilled in the art can make numerous modifications and variations to a sensor in accordance with the present invention, in order to satisfy contingent and specific requirements, all of which are in any case covered by the scope of protection of the invention, as defined by the following claims.