Device for detecting strains and transmitting detected data and method for manufacturing the same

Abstract

A device for detecting strain and transmitting detected data, as well as a method for the manufacturing of a device of the type indicated above are provided. The device can be either applied to the surface of a structure to be monitored or inserted inside the structure and it allows to reliably acquire and transmit the data relating to the strains suffered by the structure, avoiding errors due to disturbances in the positioning of the strain sensor of the device or in the electronics associated with the sensor. The device includes a middle layer, in which at least one strain sensor made by using a composite material containing electrically conductive impregnable fibers, an electronic circuit and at least an antenna are provided, and a first and a second outer layers made by using a composite material containing electrically insulating impregnable fibers, between which the middle layer is placed.

Claims

1. A device for detecting strains and transmitting detected data, comprising a first outer layer, a second outer layer and a middle layer arranged between said first outer layer and said second outer layer, wherein said middle layer comprises at least one strain sensor, at least one antenna and an electronic circuit electrically connected to said at least one strain sensor and to said at least one antenna, wherein each of said first outer layer and said second outer layer consist of a composite material consisting of an electrically insulating matrix in which impregnatable, electrically insulating fibers are contained such that said first and second outer layers completely insulate and electrically isolate the middle layer, wherein said at least one strain sensor is made of a composite material containing impregnatable, electrically conductive fibers that provide an electrical resistance which changes when subjected to strains, and wherein said electronic circuit is configured to detect and measure a change in electrical resistance of said conductive fibers.

2. The device according to claim 1, wherein said first outer layer, said second outer layer and said middle layer are catalyzed together to form a single body.

3. The device according to claim 1, wherein said electronic circuit and said antenna are arranged on a single printed circuit board, and wherein said printed circuit board is made as a flexible printed circuit board.

4. The device according to claim 1, wherein said middle layer further includes a shielding layer.

5. The device according to claim 4, wherein a further conductive layer is coupled to said shielding layer.

6. The device according to claim 1, wherein said middle layer comprises one or more sensors for detecting environmental conditions in the surroundings of said device.

7. A method for manufacturing a device for detecting strains and transmitting detected data comprising the steps of: providing a first outer layer, made by using a composite material containing impregnatable, electrically insulating fibers; arranging, on said first outer layer, at least one strain sensor made by using a composite material containing impregnatable, electrically conductive fibers that provide an electrical resistance which changes when subjected to strains; placing an electronic circuit and at least one antenna on said first outer layer; electrically connecting said at least one strain sensor and said at least one antenna to said electronic circuit, the electronic circuit being configured to detect and measure a change in electrical resistance of the conductive fibers of the at least one strain sensor; and covering with a second outer layer, made by using a composite material containing impregnatable, electrically insulating fibers; wherein the composite material of each of said first outer layer and said second outer layer completely insulate and electrically isolate the at least one strain sensor from external disturbances.

8. The method according to claim 7, wherein said first outer layer, said at least one strain sensor and said second outer layer, after being assembled together, are subjected to a catalysis step.

9. The method according to claim 7, wherein said at least one strain sensor is obtained by the steps of: providing a layer of electrically conductive fibers; impregnating said layer with a resin or a glue; catalyzing; cutting the layer thus obtained according to a desired pattern.

10. The method according to claim 7, wherein said at least one strain sensor is obtained by the steps of: providing a layer of electrically conductive fibers; impregnating said layer with a resin or a glue; cutting the layer thus obtained according to a desired pattern; and wherein said at least one strain sensor is subjected to catalysis only after being assembled with said outer layers.

11. The method according to claim 7, wherein said at least one strain sensor is obtained by the steps of: providing a middle layer made of electrically insulating fibers; impregnating said layer with a resin or a glue; making the electrically insulating fibers of said middle layer locally become electrically conductive by deposition of a metal on said electrically insulating fibers of said middle layer, on both faces of said middle layer and according to a desired pattern.

12. The device according to claim 4, wherein said shielding layer is made of a ferritic material.

13. The device according to claim 1, wherein said middle layer comprises one or more sensors for detecting temperature and humidity in the surroundings of said device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features and advantages of the invention will become more evident from the following detailed description of a preferred embodiment, given by way of non-limiting example with reference to the accompanying drawings, in which:

(2) FIG. 1 schematically shows a cross-section of a device for detecting strains and transmitting detected data according to a preferred embodiment of the present invention;

(3) FIG. 2 schematically shows a section along the plane II-II of the device of FIG. 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

(4) With reference to FIG. 1, a device for detecting strains and transmitting detected data 1 according to the invention is shown.

(5) According to the invention, said device comprises: a first outer layer 3, made of a composite material containing electrically insulating impregnable fibers, and more particularly consisting of an electrically insulating matrix containing electrically insulating impregnable fibers; a middle layer, indicated as a whole with reference 5; a second outer layer 7, made of a composite material containing electrically insulating impregnable fibers, and more particularly consisting of an electrically insulating matrix containing electrically insulating impregnable fibers.

(6) With reference also to FIG. 2, said middle layer 5 comprises at least: at least one strain sensor 9, made of a composite material containing electrically conductive impregnable fibers, and more particularly consisting of a matrix—electrically conductive or electrically insulating, preferably electrically insulating—containing electrically conductive impregnable fibers, the dimensional changes (due to an applied load) of said at least one sensor being converted into variations of its electrical resistance; an electronic circuit 11 comprising means for detecting said variations of said electrical parameter of said strain sensor 9; at least one antenna 13;
wherein said at least one strain sensor 9 and said electronic circuit 11 are electrically connected at corresponding contacts or pads 10 and wherein said electronic circuit 11 and said at least one antenna 13 are electrically connected at corresponding contacts or pads 12.

(7) The strain sensor 9 is preferably made of a composite material comprising carbon fibers, titanium fibers and/or polyester fibers with deposition of a metal (for example nickel).

(8) Said strain sensor 9 can have the desired size and geometry; in particular it can have a size much larger than that of the strain gauges used in the prior art and its geometry can follow complex patterns.

(9) The electronic circuit 11 and the antenna 13 can be made according to any suitable technology within the common knowledge of the person skilled in the art.

(10) In the preferred embodiment illustrated in the Figures, said electronic circuit 11 and said antenna 13 can be implemented on a single printed circuit board (PCB) 15.

(11) Specifically, said printed circuit board 15 is preferably made in the form of a flexible printed circuit board, made for example of a polyamide film or a synthetic fabric such as PEEK, so that the corresponding device 1 as a whole will show a certain flexibility, which will allow said device to adapt to structures with complex surfaces.

(12) In an alternative embodiment, the antenna 13—like the strain sensor 9—may also be made of a composite material containing electrically conductive impregnable fibers, i.e. consisting of an electrically conductive or electrically insulating (preferably electrically insulating) matrix containing electrically conductive impregnable fibers. According to this embodiment, the same element made of composite material containing electrically conductive impregnable fibers can be designed to perform both the functions of strain sensor and antenna.

(13) This solution may be advantageous when the device 1 is intended to be integrated inside a structure made of composite material, since it allows to limit the disturbance to the physical properties of the structure itself.

(14) The middle layer 5 may also comprise one or more sensors (not shown) capable of detecting environmental conditions in the environment surrounding the device 1, such as temperature and humidity.

(15) If provided, such additional sensors will also be electrically connected to the electronic circuit 11 by means of corresponding contacts or pads.

(16) The electronic circuit 11 may also include a memory unit that allows to store information about the device 1 and the data detected by said device during its operation.

(17) Specifically, said memory unit allows to permanently store in the sensor all the necessary information for interpreting the carried out measurements (including the calibration parameters of the sensor) and for identifying the sensor.

(18) In the case of monitoring of a complex structure, which requires the use of a large number of devices 1, this greatly simplifies the management, eliminating the need to maintain specific external documentation, which is complex to manage and potentially subject to failures or losses.

(19) Still with reference to the embodiment illustrated in the Figures, the middle layer 5 of the device 1 also comprises a shielding layer 17 associated to said middle layer 5, and in particular to the printed circuit board 15. It may be arranged, for example, below the printed circuit board 15 (as in the example of FIG. 1) or above it.

(20) Said shielding layer 17 is preferably made of ferritic material and its function will be clear from the description of the operation of the device 1 according to the invention which is provided below.

(21) As mentioned above, said device 1 can be applied to a new structure or to an already existing structure.

(22) In particular, it can be laminated on said structure, so as to become an integral part thereof and avoid any deterioration in the accuracy of data detection due to a loss of adhesion.

(23) The sensitive part of the device, consisting of the middle layer 5 that carries the strain sensor 7 and the electronic circuit 11 connected thereto, is protected from external agents by the outer layers 3, 7. In particular, said outer layers 3, 7 not only protect the middle layer 5 from the atmospheric agents, but also electrically isolate it, thanks to the fact that they are made of a composite material consisting of an electrically insulating matrix containing electrically insulating fibers, thus completely insulated from the electrical point of view.

(24) The antenna 13 allows to communicate in a wireless way—for example through radio-frequency—with an external instrument 100.

(25) In a particularly simple embodiment of the invention, the instrument 100 is capable of receiving data transmitted by the antenna 13. In this case, the device 1 must be provided with supplying means (batteries) for exciting the strain sensor 9 and supplying power to the electronic circuit 11.

(26) However, in the illustrated preferred embodiment of the invention, the wireless communication between the device 1 and the instrument 100 is made in both directions, as also shown in FIG. 1.

(27) In this way, it is possible to avoid equipping the device 1 with an internal power source, as the energy required for its operation is provided by the external instrument 100, through radio-frequency or similar wireless mode.

(28) Therefore, when detection of strains of the structure to which the device 1 is applied is required, the external instrument 100 provides the device 1 with the energy required for exciting the strain sensor 9.

(29) The dimensional changes suffered by said strain sensor as a consequence of the load (stress) to which it is subjected result in a corresponding variation of its electrical resistance; said variation of said electrical resistance is detected by the electronic circuit 11 and transmitted to the external instrument 100 through the antenna 13.

(30) When additional sensors suitable for detecting environmental conditions (temperature, humidity, etc.) are provided, the data detected by said additional sensors are also processed by the electronic circuit 11 and transmitted to the external instrument 100 by the antenna 13.

(31) It is to be noted that the excitation energy can be supplied to the strain sensor 9 at the same time as its interrogation; alternatively it is possible to provide the supplying energy to the strain sensor 9 and interrogate it at different times, providing it at the same time with means for accumulating energy (always supplied from the outside in a wireless way).

(32) It will be evident that the operations of detection and transmission of data described above can take place either continuously or in a discrete manner, and in the latter case they can occur at regular and predetermined time intervals or upon input by the user.

(33) The importance of providing the shielding layer 17 associated to the middle layer 5 of the device 1 according to the invention will also be evident from the foregoing description.

(34) Since the radio-frequency communication between the device 1 and the external instrument 100 takes place at short distance, the magnetic component in the radio-frequency emission is of major importance. The presence of conductive materials (carbon, reinforced concrete, metals, and so on) in the structure to which the device 1 is applied near the antenna 13 disturbs or cancels the communication, because of eddy currents generated by the radio-frequency emission in such materials. These eddy currents in turn generate a magnetic field symmetrical and opposite to that of the radio-frequency emission, which is therefore attenuated or canceled.

(35) In the case of the device according to the invention, the problem posed by the eddy currents is even more serious, since not only the radio-frequency communication between the antenna 13 and the external instrument 100 must be preserved from the influence of said eddy currents in order to correspondingly preserve the accuracy of the transmitted data, but it is also necessary that the external instrument 100 transmits to said device 1 enough energy for correctly exciting the strain sensor 9 without any negative influence by such eddy currents.

(36) Hence the importance of the shielding layer 17.

(37) As mentioned above, said shielding layer is made of ferritic material.

(38) In this respect it is to be noted that said ferritic material should preferably be chosen on the basis of the conductive materials contained in the structure to which the device 1 is applied, so that the shielding effect is optimized according to the specific characteristics of the magnetic field generated by the eddy currents. This is possible when the final destination of the device 1—i.e. the type of structure to which it will be applied and the materials that compose such structure—is already known at the manufacturing stage.

(39) However, in some cases it is desirable to obtain a device for detecting strains which is “universal”, i.e. whose behavior is effective whatever its final destination is.

(40) In these cases it is possible to provide for associating to the shielding layer 17 a further conductive layer (not shown) which has known characteristics and which is used for selecting the ferritic material chosen for the shielding layer 17. Said additional conductive layer can be made (for example) of carbon.

(41) Since said conductive layer is closer to the shielding layer 17 than the conductive materials contained in the structure to be monitored, the magnetic field generated in said conductive layer is much stronger than the one generated in said structure. As a result—since the ferritic material is chosen on the basis of the characteristics of said conductive layer—the device 1 according to the invention is effectively shielded, independently from the characteristics of the structure to which it is associated.

(42) It is clear from the above that, thanks to the structure of the device 1 according to the invention, it is therefore possible to correctly detect the strains suffered by the associated structure and remotely transmit the detected data to an external instrument 100.

(43) The absence of wire connections allows to apply to a same structure a large number of devices for detecting strains according to the invention and ensures a considerable freedom in the choice of the positions at which said devices are placed.

(44) It also ensures a high flexibility, as the number and position of the devices for detecting strains according to the invention can be varied over time according to the specific needs that may arise each time.

(45) It is possible to associate a corresponding external instrument to each of the devices according to the invention (for example in the case of applications where the detection of strains has to take place in a continuous manner), or to use only a single external instrument in association with all the devices according to the invention (for example in the case of applications in which such devices are interrogated only at discrete intervals of time).

(46) As mentioned above, the device 1 according to the invention can be made according to a method comprising at least the steps of: preparing the first outer layer 3, made by using a composite material containing electrically insulating impregnable fibers, and more particularly consisting of an electrically insulating matrix containing electrically insulating impregnable fibers; arranging, on said first outer layer 3, at least one strain sensor 9 made by using a composite material containing electrically conductive impregnable fibers, and more particularly consisting of a matrix—electrically conductive or electrically insulating, preferably electrically insulating—containing electrically conductive impregnable fibers; placing, on said first outer layer 3, the electronic circuit 11 and the at least one antenna 13; electrically connecting said electronic circuit 11 to said strain sensor 9 and to said antenna 13; covering with the second outer layer 7, which is also made by using a composite material containing electrically insulating impregnable fibers, and more particularly consisting of an electrically insulating matrix containing electrically insulating impregnable fibers.

(47) The different layers can be catalyzed separately or together.

(48) According to a preferred embodiment of the invention, the layers are catalyzed together, after being assembled.

(49) Thanks to this expedient, the different layers of the device 1 according to the invention are integrated in a single piece, which allows to avoid any slipping of the strain sensor 9 relative to the outer layers 3, 7, which could lead to errors in the strain detection during the operation of the device.

(50) As far as the manufacturing of the strain sensor 9 is concerned, it is possible to envisage several possibilities.

(51) According to a first option, the strain sensor 9 is obtained by the steps of: providing a layer of electrically conductive fibers; impregnating said layer with a resin or glue (electrically conductive or—preferably—electrically insulating); catalyzing; cutting the obtained layer according to the desired pattern.

(52) The strain sensor 9 thus obtained has a large hysteresis and tends to maintain its shape when not stressed. Moreover, if the layer of impregnated fibers is catalyzed between two “peel ply” layers, after their removal it has the optimal surface for the following assembling step.

(53) According to a second option, said strain sensor 9 is obtained by the steps of: providing a layer of electrically conductive fibers; impregnating said layer with a resin or glue (electrically conductive or—preferably—electrically insulating); cutting the obtained layer according to the desired pattern.

(54) In this case, the strain sensor 9 is arranged between the outer layers of 3, 7 without being catalyzed (so-called “fresh”) and it is catalyzed only after being assembled to said outer layers.

(55) In this way, the risk of displacement or slipping of the thus obtained strain sensor 9 relative to the outer layers during operation is completely eliminated. In this case it is preferable to provide positioning marks in the outer layers for the correct positioning of the strain sensor before catalysis.

(56) According to a third option, the strain sensor 9 is obtained by providing a middle layer of composite material consisting of a matrix containing electrically insulating fibers and by making the electrically insulating fibers of said middle layer locally become electrically conductive by deposition of a metal according to a desired pattern.

(57) In particular, the metal—such as nickel—is deposited on both faces of said middle layer so as to achieve the effect of electric conduction, and said middle layer is then placed between the two outer layers 3, 7.

(58) For example, it is possible to manufacture the middle layer by using polyester fibers (as the outer layers 3,7) and subsequently deposit nickel on both faces of said middle layer according to a pattern corresponding to the desired geometry for the strain sensor.

(59) The advantage of this solution mainly consists in the fact that the middle layer is much easier to handle as it is not complex in shape and its positioning with respect to the outer layers can take place with greater ease.

(60) It is evident from the above description that the invention reaches the objects set forth above, as it provides a device allowing to detect strains and to transmit the detected data with high accuracy and reliability, and showing at the same time a great versatility in terms of practical applications.

(61) It will also be evident that the embodiment described above with reference to the accompanying drawings has been given by way of example only, without any limiting purpose, and that several modifications and variations within the common knowledge of the person skilled in the art can be made without departing from the scope of protection defined by the appended claims.