Abstract:
Apparatus for monitoring structural health conditions of structures. The apparatus includes a piezoelectric device that has a bottom cover plate, piezoelectric rings concentrically disposed with respect to an axis and positioned on the bottom cover plate, and concentric filler rings made from an electrically insulating material. Each of the piezoelectric rings is interposed between two adjacent filler rings. The piezoelectric device also includes electrical connection means for communicating electrical signals to the piezoelectric rings and a top cover plate that is positioned over the piezoelectric rings and filler rings. The electrical connection means are operative to transmit electrical signals that cause one or more of the piezoelectric rings to generate waves or transmit electrical signals developed by one or more of the piezoelectric rings in response to the waves.

Description:
CROSS REFERENCE TO RELATED APPLICTIONS  
       [0001]     This application is a continuation-in-part of application Ser. No. 10/942,366, filed on Sep. 16, 2004, which claims the benefit of U.S. Provisional Applications No. 60/505,120, filed on Sep. 22, 2003. 
     
    
     BACKGROUND  
       [0002]     The present invention relates to diagnostics of structures, and more particularly to diagnostic network patch (DNP) systems for monitoring structural health conditions.  
         [0003]     As all structures in service require appropriate inspection and maintenance, they should be monitored for their integrity and health condition to prolong their life or to prevent catastrophic failure. Apparently, the structural health monitoring has become an important topic in recent years. Numerous methods have been employed to identify fault or damage of structures, where these methods may include conventional visual inspection and non-destructive techniques, such as ultrasonic and eddy current scanning, acoustic emission and X-ray inspection. These conventional methods require at least temporary removal of structures from service for inspection. Although still used for inspection of isolated locations, they are time-consuming and expensive.  
         [0004]     With the advance of sensor technologies, new diagnostic techniques for in-situ structural integrity monitoring have been in significant progress. Typically, these new techniques utilize sensory systems of appropriate sensors and actuators built in host structures. However, these approaches have drawbacks and may not provide effective on-line methods to implement a reliable sensory network system and/or accurate monitoring methods that can diagnose, classify and forecast structural condition with the minimum intervention of human operators. For example, U.S. Pat. No. 5,814,729, issued to Wu et al., discloses a method that detects the changes of damping characteristics of vibrational waves in a laminated composite structure to locate delaminated regions in the structure. Piezoceramic devices are applied as actuators to generate the vibrational waves and fiber optic cables with different grating locations are used as sensors to catch the wave signals. A drawback of this system is that it cannot accommodate a large number of actuator arrays and, as a consequence, each of actuators and sensors must be placed individually. Since the damage detection is based on the changes of vibrational waves traveling along the line-of-sight paths between the actuators and sensors, this method fails to detect the damage located out of the paths and/or around the boundary of the structure.  
         [0005]     Another approach for damage detection can be found in U.S. Pat. No. 5,184,516, issued to Blazic et al., which discloses a self-contained conformal circuit for structural health monitoring and assessment. This conformal circuit consists of a series of stacked layers and traces of strain sensors, where each sensor measures strain changes at its corresponding location to identify the defect of a conformal structure. The conformal circuit is a passive system, i.e., it does not have any actuator for generating signals. A similar passive sensory network system can be found in U.S. Pat. No. 6,399,939, issued to Mannur, J. et al. In Mannur &#39;939 patent, a piezoceramic-fiber sensory system is disclosed having planner fibers embedded in a composite structure. A drawback of these passive methods is that they cannot monitor internal delamination and damages between the sensors. Moreover, these methods can detect the conditions of their host structures only in the local areas where the self-contained circuit and the piezoceramic-fiber are affixed.  
         [0006]     One method for detecting damages in a structure is taught by U.S. Pat. No. 6,370,964 (Chang et al.). Chang et al. discloses a sensory network layer, called Stanford Multi-Actuator-Receiver Transduction (SMART) Layer. The SMART Layer® includes piezoceramic sensors/actuators equidistantly placed and cured with flexible dielectric films sandwiching the piezoceramic sensors/actuators (or, shortly, piezoceramics). The actuators generate acoustic waves and sensors receive/transform the acoustic waves into electric signals. To connect the piezoceramics to an electronic box, metallic clad wires are etched using the conventional flexible circuitry technique and laminated between the substrates. As a consequence, a considerable amount of the flexible substrate area is needed to cover the clad wire regions. In addition, the SMART Layer® needs to be cured with its host structure made of laminated composite layers. Due to the internal stress caused by a high temperature cycle during the curing process, the piezoceramics in the SMART Layer® can be micro-fractured. Also, the substrate of the SMART Layer® can be easily separated from the host structure. Moreover, it is very difficult to insert or attach the SMART Layer® to its host structure having a curved section and, as a consequence, a compressive load applied to the curved section can easily fold the clad wires. Fractured piezoceramics and the folded wires may be susceptible to electromagnetic interference noise and provide misleading electrical signals. In harsh environments, such as thermal stress, field shock and vibration, the SMART Layer® may not be a robust and unreliable tool for monitoring structural health. Furthermore, the replacement of damaged and/or defective actuators/sensors may be costly as the host structure needs to be dismantled.  
         [0007]     Another method for detecting damages in a structure is taught by U.S. Pat. No. 6,396,262 (Light et al.). Light et al. discloses a magnetostrictive sensor for inspecting structural damages, where the sensor includes a ferromagnetic strip and a coil closely located to the strip. The major drawback of this system is that the system cannot be designed to accommodate an array of sensors and, consequently, cannot detect internal damages located between sensors.  
         [0008]     Thus, there is a need for an efficient, accurate, and reliable system that can be readily integrated into existing and/or new structures and provide an on-line methodology to diagnose, classify and forecast structural condition with the minimum intervention of human operators.  
       SUMMARY OF THE DISCLOSURE  
       [0009]     A diagnostic network patch (DNP) system that is attached to a host structure for monitoring the health conditions thereof is provided. The DNP system contains actuators/sensors and is capable of detecting and monitoring flaws/damages of the host structure. Like the nerve system of human body, the DNP system forms an internal wave-ray communication network in the host structure by establishing signal paths between actuators and sensors, wherein acoustic waves or impulses (such as, Lamb waves) travel through the signal paths.  
         [0010]     According to one embodiment, a device for monitoring structural health conditions of an object includes a piezoelectric device that has a bottom cover plate, piezoelectric rings concentrically disposed with respect to an axis and positioned on the bottom cover plate, and concentric filler rings made from an electrically insulating material. Each of the piezoelectric rings is interposed between two adjacent filler rings. The piezoelectric device also includes electrical connection means for communicating electrical signals to the piezoelectric rings and a top cover plate that is positioned over the piezoelectric rings and filler rings. The bottom and top cover plates are adapted to hold the piezoelectric rings and filler rings in place. The electrical connection means are operative to transmit electrical signals that cause one or more of the piezoelectric rings to generate waves or transmit electrical signals developed by one or more of the piezoelectric rings in response to the waves.  
         [0011]     According to another embodiment, a device for monitoring structural health conditions of an object includes a piezoelectric device, wherein the piezoelectric device includes filler rings concentrically disposed with respect to an axis and formed from an electrically insulating material and piezoelectric rings. Each piezoelectric ring is concentrically interposed between two adjacent filler rings and has flat top and bottom surfaces that are perpendicular to the axis and covered with top and bottom conductive flakes, respectively. The piezoelectric device also include: top conductive rings concentrically disposed with respect to the axis, each top conductive ring having a flat bottom surface in contact with one of the top conductive flakes and being electrically insulated from the other top conductive rings; one or more top covering layers and top base layers alternatively stacked on the filler rings and configured to cover at least one of top conductive rings; top electrical connection means for communicating electric signals to the top conductive rings; bottom conductive rings concentrically disposed with respect to the axis, each bottom conductive ring having a flat top surface in contact with one of the bottom conductive flakes and being electrically insulated from the other bottom conductive rings; one or more bottom covering layers and bottom base layers alternatively stacked beneath the filler rings and configured to cover at least one of the bottom conductive rings; and bottom electrical connection means for communicating electric signals to the bottom conductive rings. The top and bottom electrical connection means are operative to transmit electrical signals that cause one or more of the piezoelectric rings to generate the waves or transmit electrical signals developed by one or more of the piezoelectric rings in response to the waves.  
         [0012]     These and other advantages and features of the invention will become apparent to those persons skilled in the art upon reading the details of the invention as more fully described below.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1A  is a schematic top cut-away view of a pickup unit of a patch sensor in accordance with one embodiment of the present teachings.  
         [0014]      FIG. 1B  is a schematic side cross-sectional view of the patch sensor shown in  FIG. 1A .  
         [0015]      FIG. 1C  is a schematic top view of a typical piezoelectric device.  
         [0016]      FIG. 1D  is a schematic side cross-sectional view of the typical piezoelectric device in  FIG. 1C .  
         [0017]      FIG. 1E  is a schematic top cut-away view of a patch sensor in accordance with another embodiment of the present teachings.  
         [0018]      FIG. 1F  is a schematic side cross-sectional view of the patch sensor shown in  FIG. 1E .  
         [0019]      FIG. 1G  is a schematic cross-sectional view of a composite laminate including the patch sensor of  FIG. 1E .  
         [0020]      FIG. 1H  is a schematic side cross-sectional view of an alternative embodiment of the patch sensor of  FIG. 1E .  
         [0021]      FIG. 2A  is a schematic top cut-away view of a pickup unit of a hybrid patch sensor in accordance with one embodiment of the present teachings.  
         [0022]      FIG. 2B  is a schematic side cross-sectional view of the hybrid patch sensor shown in  FIG. 2A .  
         [0023]      FIG. 2C  is a schematic top cut-away view of a hybrid patch sensor in accordance with another embodiment of the present teachings.  
         [0024]      FIG. 2D  is a schematic side cross-sectional view of the hybrid patch sensor shown in  FIG. 2C .  
         [0025]      FIG. 3A  is a schematic top cut-away view of a pickup unit of an optical fiber patch sensor in accordance with one embodiment of the present teachings.  
         [0026]      FIG. 3B  is a schematic side cross-sectional view of the optical fiber patch sensor shown in  FIG. 3A .  
         [0027]      FIG. 3C  is a schematic top cut-away view of the optical fiber coil contained in the optical fiber patch sensor of  FIG. 3A .  
         [0028]      FIG. 3D  is a schematic top cut-away view of an alternative embodiment of the optical fiber coil shown in  FIG. 3C .  
         [0029]     FIGS.  3 E-F are schematic top cut-away views of alternative embodiments of the optical fiber coil of  FIG. 3C .  
         [0030]      FIG. 3G  is a schematic side cross-sectional view of the optical fiber coil of  FIG. 3E .  
         [0031]      FIG. 4A  is a schematic top cut-away view of a pickup unit of a diagnostic patch washer in accordance with one embodiment of the present teachings.  
         [0032]      FIG. 4B  is a schematic side cross-sectional view of the diagnostic patch washer shown in  FIG. 4A .  
         [0033]      FIG. 4C  is a schematic diagram of an exemplary bolt-jointed structure using the diagnostic patch washer of  FIG. 4A  in accordance with one embodiment of the present teachings.  
         [0034]      FIG. 4D  is a schematic diagram of an exemplary bolt-jointed structure using the diagnostic patch washer of  FIG. 4A  in accordance with another embodiment of the present teachings.  
         [0035]      FIG. 5A  is a schematic diagram of an interrogation system including a sensor/actuator device in accordance with one embodiment of the present teachings.  
         [0036]      FIG. 5B  is a schematic diagram of an interrogation system including a sensor in accordance with one embodiment of the present teachings.  
         [0037]      FIG. 6A  is a schematic diagram of a diagnostic network patch system applied to a host structure in accordance with one embodiment of the present teachings.  
         [0038]      FIG. 6B  is a schematic diagram of a diagnostic network patch system having a strip network configuration in accordance with one embodiment of the present teachings.  
         [0039]      FIG. 6C  is a schematic diagram of a diagnostic network patch system having a pentagon network configuration in accordance with one embodiment of the present teachings.  
         [0040]      FIG. 6D  is a schematic perspective view of a diagnostic network patch system incorporated into rivet/bolt-jointed composite laminates in accordance with one embodiment of the present teachings.  
         [0041]      FIG. 6E  is a schematic perspective view of a diagnostic network patch system incorporated into a composite laminate repaired with a bonding patch in accordance with another embodiment of the present teachings.  
         [0042]      FIG. 6F  is a schematic diagram illustrating an embodiment of a wireless communication system that controls a remote diagnostic network patch system in accordance with one embodiment of the present teachings.  
         [0043]      FIG. 7A  is a schematic diagram of a diagnostic network patch system having clustered sensors in a strip network configuration in accordance with one embodiment of the present teachings.  
         [0044]      FIG. 7B  is a schematic diagram of a diagnostic network patch system having clustered sensors in a pentagonal network configuration in accordance with another embodiment of the present teachings.  
         [0045]      FIG. 8A  is a schematic diagram of a clustered sensor having optical fiber coils in a serial connection in accordance with one embodiment of the present teachings.  
         [0046]      FIG. 8B  is a schematic diagram of a clustered sensor having optical fiber coils in a parallel connection in accordance with another embodiment of the present teachings.  
         [0047]      FIG. 9  is a plot of actuator and sensor signals in accordance with one embodiment of the present teachings.  
         [0048]      FIG. 10A  is an exploded partial cutaway view of a piezoelectric device in accordance with one embodiment of the present teachings.  
         [0049]      FIG. 10B  is a cross sectional diagram of the piezoelectric device in  FIG. 10A , taken along the line  10 - 10 .  
         [0050]      FIG. 11A  is an exploded partial cutaway view of a piezoelectric device in accordance with another embodiment of the present teachings.  
         [0051]      FIG. 11B  is a cross sectional diagram of the piezoelectric device in  FIG. 11A , taken along the line  11 - 11 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0052]     Although the following detained description contains many specifics for the purposes of illustration, those of ordinary skill in the art will appreciate that many variations and alterations to the following detains are within the scope of the invention. Accordingly, the following embodiments of the invention are set forth without any loss of generality to, and without imposing limitation upon, the claimed invention.  
         [0053]      FIG. 1A  is a schematic top cut-away view of a pickup unit of  100  of a patch sensor in accordance with one embodiment of the present teachings. Hereinafter, the terms “pickup unit of a patch sensor” and “patch sensor” are used interchangeably.  FIG. 1B  is a schematic cross-sectional view of the patch sensor  100  taken along a direction A-A of  FIG. 1A . As shown in FIGS.  1 A-B, the patch sensor  100  may include: a substrate  102  configured to attach to a host structure; a hoop layer  104 ; a piezoelectric device  108  for generating and/or receiving signals (more specifically, Lamb waves); a buffer layer  110  for providing mechanical impedance matching and reducing thermal stress mismatch between the substrate  102  and the piezoelectric device  108 ; two electrical wires  118   a - b  connected to the piezoelectric device  108 ; a molding layer  120  for securing the piezoelectric device  108  to the substrate  102 ; and a cover layer  106  for protecting and sealing the molding layer  120 . The piezoelectric device  108  includes: a piezoelectric layer  116 ; a bottom conductive flake  112  connected to the electrical wire  118   b;  and a top conductive flake  114  connected to the electrical wire  118   a.  The piezoelectric device  108  may operate as an actuator (or, equivalently, signal generator) when a pre-designed electric signal is applied through the electric wires  118   a - b.  Upon application of an electrical signal, the piezoelectric layer  116  may deform to generate Lamb waves. Also, the piezoelectric device  108  may operate as a receiver for sensing vibrational signals, converting the vibrational signals applied to the piezoelectric layer  116  into electric signals and transmitting the electric signals through the wires  118   a - b.  The wires  118   a - b  may be a thin ribbon type metallic wire.  
         [0054]     The substrate  102  may be attached to a host structure using a structural adhesive, typically a cast thermosetting epoxy, such as butyralthenolic, acrylic polyimide, nitriale phenolic or aramide. The substrate  102  may be an insulation layer for thermal heat and electromagnetic interference protecting the piezoelectric device  108  affixed to it. In some applications, the dielectric substrate  102  may need to cope with a temperature above 250° C. Also it may have a low dielectric constant to minimize signal propagation delay, interconnection capacitance and crosstalk between the piezoelectric device  108  and its host structure, and high impedance to reduce power loss at high frequency.  
         [0055]     The substrate  102  may be made of various materials. Kapton® polyimide manufactured by DuPont, Wilmington, Del., may be preferably used for its commonplace while other three materials of Teflon perfluoroalkoxy (PFA), poly p-xylylene (PPX), and polybenzimidazole (PBI), can be used for their specific applications. For example, PFA film may have good dielectric properties and low dielectric loss to be suitable for low voltage and high temperature applications. PPX and PBI may provide stable dielectric strength at high temperatures.  
         [0056]     The piezoelectric layer  116  can be made of piezoelectric ceramics, crystals or polymers. A piezoelectric crystal, such as PZN-PT crystal manufactured by TRS Ceramics, Inc., State College, Pa., may be preferably employed in the design of the piezoelectric device  108  due to its high strain energy density and low strain hysteresis. For small size patch sensors, the piezoelectric ceramics, such as PZT ceramics manufactured by Fuji Ceramic Corporation, Tokyo, Japan, or APC International, Ltd., Mackeyville, Pa., may be used for the piezoelectric layer  116 . The top and bottom conductive flakes  112  and  114  may be made of metallic material, such as Cr or Au, and applied to the piezoelectric layer  116  by the conventional sputtering process. In  FIG. 1B , the piezoelectric device  108  is shown to have only a pair of conductive flakes. However, it should be apparent to those of ordinary skill that the piezoelectric device  108  may have the multiple stacks of conductive flakes having various thicknesses to optimize the performance of the piezoelectric layer  116  in generating/detecting signal waves. The thickness of each flake may be determined by the constraints of thermal and mechanical loads given in a particular host structure that the patch sensor  100  is attached to.  
         [0057]     To sustain temperature cycling, each layer of the piezoelectric device  108  may need to have a thermal expansion coefficient similar to those of other layers. Yet, the coefficient of a typical polyimide comprising the substrate  102  may be about 4-6×10 −5  K −1  while that of a typical piezoelectric ceramic/crystal comprising the piezoelectric layer  116  may be about 3×10 −6  K −1 . Such thermal expansion mismatch may be a major source of failure of the piezoelectric device  108 . The failure of piezoelectric device  108  may require a replacement of the patch sensor  100  from its host structure. As mentioned, the buffer layer  110  may be used to reduce the negative effect of the thermal coefficient mismatch between the piezoelectric layer  116  and the substrate  102 .  
         [0058]     The buffer layer  110  may be made of conductive polymer or metal, preferably aluminum (Al) with the thermal expansion coefficient of 2×10 −5  K −1 . One or more buffer layers made of alumina, silicon or graphite may replace or be added to the buffer layer  110 . In one embodiment, the thickness of the buffer layer  110  made of aluminum may be nearly equal to that of the piezoeletric layer  116 , which is approximately 0.25 mm including the two conductive flakes  112  and  114  of about 0.05 mm each. In general, the thickness of the buffer layer  110  may be determined by the material property and thickness of its adjacent layers. The buffer layer  110  may provide an enhanced durability against thermal loads and consistency in the twofold function of the piezoelectric device  108 . In an alternative embodiment, the piezoelectric device  108  may have another buffer layer applied over the top conductive flake  114 .  
         [0059]     Another function of the buffer layer  110  may be amplifying signals received by the substrate  102 . As Lamb wave signals generated by a patch sensor  100  propagate along a host structure, the intensity of the signals received by another patch sensor  100  attached on the host structure may decrease as the distance is between the two patch sensors increases. When a Lamb signal arrives at the location where a patch sensor  100  is located, the substrate  102  may receive the signal. Then, depending on the material and thickness of the buffer layer  110 , the intensity of the received signal may be amplified at a specific frequency. Subsequently, the piezoelectric device  108  may convert the amplified signal into electrical signal.  
         [0060]     As moisture, mobile ions and hostile environmental condition may degrade the performance and reduce the lifetime of the patch sensor  100 , two protective coating layers, a molding layer  120  and a cover layer  106  may be used. The molding layer  120  may be made of epoxy, polyimide or silicone-polyimide by the normal dispensing method. Also, the molding layer  120  may be formed of a low thermal expansion polyimide and deposited over the piezoelectric device  108  and the substrate  102 . As passivation of the molding layer  120  does not make a conformal hermetic seal, the cover layer  106  may be deposited on the molding layer  120  to provide a hermitic seal. The cover layer  120  may be made of metal, such as nickel (Ni), chromium (Cr) or silver (Ag), and deposited by a conventional method, such as electrolysis or e-beam evaporation and sputtering. In one embodiment, an additional film of epoxy or polyimide may be coated on the cover layer  106  to provide a protective layer against scratching and cracks.  
         [0061]     The hoop layer  104  may be made of dielectric insulating material, such as silicon nitride or glass, and encircle the piezoelectric device  108  mounted on the substrate  102  to prevent the conductive components of the piezoelectric device  108  from electrical shorting.  
         [0062]      FIG. 1C  is a schematic top view of a piezoelectric device  130 , which may be a conventional type known in the art and can be used in place of the piezoelectric device  108 .  FIG. 1D  is a schematic cross-sectional view of the piezoelectric device  130  taken along the direction B-B of  FIG. 1D . As shown FIGS.  1 C-D, the piezoelectric device  130  includes: a bottom conductive flake  134 ; a piezoelectric layer  136 ; a top conductive flake  132  connected to a wire  138   b;  a connection flake  142  connected to a wire  138   a;  and a conducting segment  144  for connecting the connection flake  142  to the bottom flake  134 . The top conductive flake  132  may be electrically separated from the connection flake  142  by a groove  140 .  
         [0063]      FIG. 1E  is a schematic top cut-away view of a patch sensor  150  in accordance with another embodiment of the present teachings.  FIG. 1F  is a schematic side cross-sectional view of the patch sensor  150  shown in  FIG. 1E . As shown in FIGS.  1 E-F, the patch sensor  150  may include: a bottom substrate  151 ; a top substrate  152 ; a hoop layer  154 ; a piezoelectric device  156 ; top and bottom buffer layers  160   a - b;  two electrical wires  158   a - b  connected to the piezoelectric device  108 . The piezoelectric device  156  includes: a piezoelectric layer  164 ; a bottom conductive flake  166  connected to the electrical wire  158   b;  and a top conductive flake  162  connected to the electrical wire  158   a.  The functions and materials for the components of the patch sensor  150  may be similar to those for their counterparts of the patch sensor  100 . Each of the buffer layers  160   a - b  may include more than one sublayer and each sublayer may be composed of polymer or metal. The top substrate  152  may be made of the same material as that of the substrate  102 .  
         [0064]     The patch sensor  150  may be affixed to a host structure to monitor the structural health conditions. Also, the patch sensor  150  may be incorporated within a laminate.  FIG. 1G  is a schematic cross-sectional view of a composite laminate  170  having a patch sensor  150  therewithin. As illustrated in  FIG. 1G , the host structure includes: a plurality of plies  172 ; and at least one patch sensor  150  cured with the plurality of plies  172 . In one embodiment, the plies  172  may be impregnated with adhesive material, such as epoxy resin, prior to the curing process. During the curing process, the adhesive material from the plies  172  may fill cavities  174 . To obviate such accumulation of the adhesive material, the hoop layer  154  may have a configuration to fill the cavity  174 .  
         [0065]      FIG. 1H  is a schematic side cross-sectional view of an alternative embodiment  180  of the patch sensor  150  of  FIG. 1E . As illustrated, the patch sensor  180  may include: a bottom substrate  182 ; a top substrate  184 ; a hoop layer  198 ; a piezoelectric device  190 ; top and bottom buffer layers  192  and  194 ; and the piezoelectric device  196 . For simplicity, a pair of wires connected to the piezoelectric device  190  is not shown in  FIG. 1H . The piezoelectric device  190  may include: a piezoelectric layer  196 ; a bottom conductive flake  194 ; and a top conductive flake  192 . The functions and materials for the components of the patch sensor  180  may be similar to those of their counterparts of the patch sensor  150 .  
         [0066]     The hoop layer  198  may have one or more sublayers  197  of different dimensions so that the outer contour of the hoop layer  198  may match the geometry of cavity  174 . By filling the cavity  174  with sublayers  197 , the adhesive material may not be accumulated during the curing process of the laminate  170 .  
         [0067]      FIG. 2A  is a schematic top cut-away view of a pickup unit  200  of a hybrid patch sensor in accordance with one embodiment of the present teachings. Hereinafter, the terms “pickup unit of a hybrid patch sensor” and “hybrid patch sensor” are used interchangeably.  FIG. 2B  is a schematic cross-sectional view of the hybrid patch sensor  200  taken along a direction C-C of  FIG. 2A . As shown in FIGS.  2 A-B, the hybrid patch sensor  200  may include: a substrate  202  configured to attach to a host structure; a hoop layer  204 ; a piezoelectric device  208 ; an optical fiber coil  210  having two ends  214   a - b;  a buffer layer  216 ; two electrical wires  212   a - b  connected to the piezoelectric device  208 ; a molding layer  228 ; and a cover layer  206 . The piezoelectric device  208  includes: a piezoelectric layer  222 ; a bottom conductive flake  220  connected to the electrical wire  212   b;  and a top conductive flake  218  connected to the electrical wire  212   a.  In an alternative embodiment, the piezoelectric device  208  may be the same as the device  130  of  FIG. 1C . The optical fiber coil  210  may include; a rolled optical fiber cable  224 ; and a coating layer  226 . Components of the hybrid patch sensor  200  may be similar to their counterparts of the patch sensor  100 .  
         [0068]     The optical fiber coil  210  may be a Sagnac interferometer and operate to receive Lamb wave signals. The elastic strain on the surface of a host structure incurred by Lamb wave may be superimposed on the pre-existing strain of the optical fiber cable  224  incurred by bending and tensioning. As a consequence, the amount of frequency/phase change in light traveling through the optical fiber cable  224  may be dependent on the total length of the optical fiber cable  224 . In one embodiment, considering its good immunity to electromagnetic interference and vibrational noise, the optical fiber coil  210  may be used as the major sensor while the piezoelectric device  208  can be used as an auxiliary sensor.  
         [0069]     The optical fiber coil  210  exploits the principle of Doppler&#39;s effect on the frequency of light traveling through the rolled optical fiber cable  224 . For each loop of the optical fiber coil  210 , the inner side of the optical fiber loop may be under compression while the outer side may be under tension. These compression and tension may generate strain on the optical fiber cable  224 . The vibrational displacement or strain of the host structure incurred by Lamb waves may be superimposed on the strain of the optical fiber cable  224 . According to a birefringence equation, the reflection angle on the cladding surface of the optical fiber cable  224  may be a function of the strain incurred by the compression and/or tension. Thus, the inner and outer side of each optical fiber loop may make reflection angles different from that of a straight optical fiber, and consequently, the frequency of light may shift from a centered input frequency according to the relative flexural displacement of Lamb wave as light transmits through the optical fiber coil  210 .  
         [0070]     In one embodiment, the optical fiber coil  210  may include 10 to 30 turns of the optical fiber cable  224  and have a smallest loop diameter  236 , d i , of at least 10 mm. There may be a gap  234 , d g , between the innermost loop of the optical fiber coil  210  and the outer periphery of the piezoelectric device  208 . The gap  234  may depend on the smallest loop diameter  236  and the diameter  232 , d p , of the piezoelectric device  208 , and be preferably larger than the diameter  232  by about two or three times of the diameter  230 , d f , of the optical fiber cable  224 .  
         [0071]     The coating layer  226  may be comprised of a metallic or polymer material, preferably an epoxy, to increase the sensitivity of the optical fiber coil  210  to the flexural displacement or strain of Lamb waves guided by its host structure. Furthermore, a controlled tensional force can be applied to the optical fiber cable  224  during the rolling process of the optical fiber cable  224  to give additional tensional stress. The coating layer  226  may sustain the internal stress of the rolled optical fiber cable  224  and allow a uniform in-plane displacement relative to the flexural displacement of Lamb wave for each optical loop.  
         [0072]     The coating layer  226  may also be comprised of other material, such as polyimide, aluminum, copper, gold or silver. The thickness of the coating layer  226  may range from about 30% to two times of the diameter  230 . The coating layer  226  comprised of polymer material may be applied in two ways. In one embodiment, a rolled optic fiber cable  224  may be laid on the substrate  202  and the polymer coating material may be sprayed by a dispenser, such as Biodot spay-coater. In another embodiment, a rolled optic fiber cable  224  may be dipped into a molten bath of the coating material.  
         [0073]     Coating layer  226  comprised of metal may be applied by a conventional metallic coating technique, such as magnetron reactive or plasma-assisted sputtering as well as electrolysis. Specially, the zinc oxide can be used as the coating material of the coating layer  226  to provide the piezoelectric characteristic for the coating layer  226 . When zinc oxide is applied to top and bottom surfaces of the rolled optical fiber cable  224 , the optical fiber coil  210  may contract or expand concentrically in radial direction responding to electrical signals. Furthermore, the coating material of silicon oxide or tantalum oxide can also be used to control the refractive index of the rolled fiber optical cable  224 . Silicon oxide or tantalum oxide may be applied using the indirect/direct ion beam-assisted deposition technique or electron beam vapor deposition technique. It is noted that other methods may be used for applying the coating layer  226  to the optical fiber cable  224  without deviating from the present teachings.  
         [0074]     The piezoelectric device  208  and the optical fiber coil  210  may be affixed to the substrate  202  using physically setting adhesives instead of common polymers, where the physically setting adhesives may include, but not limited to, butylacrylate-ethylacrylate copolymer, styrene-butadiene-isoprene terpolymer and polyurethane alkyd resin. The adhesive properties of these materials may remain constant during and after the coating process due to the lack of cross-linking in the polymeric structure. Furthermore, those adhesives may be optimized for wetting a wide range of substrate  202  without compromising their sensitivity to different analytes, compared to conventional polymers.  
         [0075]      FIG. 2C  is a schematic top cut-away view of a hybrid patch sensor  240  in accordance with another embodiment of the present teachings.  FIG. 2D  is a schematic side cross-sectional view of the hybrid patch sensor  240  shown in  FIG. 2C . As shown in FIGS.  2 C-D, the hybrid patch sensor  240  may include: a bottom substrate  254 ; a top substrate  242 ; a hoop layer  244 ; a piezoelectric device  248 ; an optical fiber coil  246  having two ends  250   a - b;  top and bottom buffer layers  260   a - b;  and two electrical wires  252   a - b  connected to the piezoelectric device  248 . The piezoelectric device  248  includes: a piezoelectric layer  264 ; a bottom conductive flake  262  connected to the electrical wire  252   b;  and a top conductive flake  266  connected to the electrical wire  252   a.  The optical fiber coil  246  may include; a rolled optical fiber cable  258 ; and a coating layer  256 . Components of the hybrid patch sensor  240  may be similar to their counterparts of the hybrid patch sensor  200 .  
         [0076]     As in the case of the patch sensor  150 , the hybrid patch sensor  240  may be affixed to a host structure and/or incorporated within a composite laminate. In one embodiment, the hoop layer  244  may be similar to the hoop layer  198  to fill the cavity formed by the patch sensor  240  and the composite laminate.  
         [0077]      FIG. 3A  a schematic top cut-away view of a pickup unit  300  of an optical fiber patch sensor in accordance with one embodiment of the present teachings. Hereinafter, the terms “pickup unit of an optical fiber patch sensor” and “optical fiber patch sensor” are used interchangeably.  FIG. 3B  a schematic side cross-sectional view of the optical fiber patch sensor  300  taken along the direction D-D of  FIG. 3A . As shown in FIGS.  3 A-B, the optical fiber patch sensor  300  may include: a substrate  302 ; a hoop layer  304 ; an optical fiber coil  308  having two ends  310   a - b;  a molding layer  316 ; and a cover layer  306 . The optical fiber coil  308  may include; a rolled optical fiber cable  312 ; and a coating layer  314 . The material and function of each element of the optical fiber patch sensor  300  may be similar to those of its counterpart of the hybrid patch sensor  200  in  FIG. 2A . The diameter  313  of the innermost loop may be determined by the material property of the optic fiber cable  312 .  
         [0078]      FIG. 3C  a schematic top cut-away view of the optical fiber coil  308  contained in the optical fiber patch sensor of  FIG. 3A , illustrating a method for rolling the optical fiber cable  312 . As shown in  FIG. 3C , the outermost loop of the optical fiber coil  308  may start with one end  310   a  while the innermost loop may end with the other end  310   b.    FIG. 3D  a schematic top cut-away view of an alternative embodiment  318  of the optical fiber coil  308  shown in  FIG. 3C . As shown in  FIG. 3D , the optical fiber cable  322  may be folded and rolled in such a manner that the outermost loops may start with both ends  320   a - b . The rolled optical fiber cable  322  may be covered by a coating layer  319 .  
         [0079]     It is noted that the optical fiber coils  308  and  318  show in FIGS.  3 C-D may be attached directly to a host structure and used as optical fiber coil sensors. For this reason, hereinafter, the terms “optical fiber coil” and “optical fiber coil sensor” will be used interchangeably. FIGS.  3 E-F are alternative embodiments of the optical fiber coil  308 . As illustrated in  FIG. 3E , the optical fiber coil  330  may include: an optical fiber cable  334  having two ends  338   a - b  and being rolled in the same manner as the cable  312 ; and a coating layer  332 . The coil  330  may have a hole  336  to accommodate a fastener as will be explained later. Likewise, the optical fiber coil  340  in  FIG. 3F  may include: an optical fiber cable  344  having two ends  348   a - b  and being rolled in the same manner as the cable  322 ; and a coating layer  342 . The coil  340  may have a hole  346  to accommodate a fastener.  FIG. 3G  is a schematic side cross-sectional view of the optical fiber coil  330  taken along the direction DD of  FIG. 3E .  
         [0080]     It should be noted that the sensors described in  FIG. 3A -G may be incorporated within a laminate in a similar manner as described in  FIG. 1G .  
         [0081]      FIG. 4A  a schematic top cut-away view of a pickup unit  400  of a diagnostic patch washer in accordance with one embodiment of the present teachings. Hereinafter, the terms “pickup unit of a diagnostic patch washer” and “diagnostic patch washer” are used interchangeably.  FIG. 4B  a schematic side cross-sectional view of the diagnostic patch washer  400  taken along the direction E-E of  FIG. 4A . As shown in FIGS.  4 A-B, the diagnostic patch washer  400  may include: an optical fiber coil  404  having two ends  410   a - b ; a piezoelectric device  406 ; a support element  402  for containing the optical fiber coil  404  and the piezoelectric device  406 , the coil  404  and the device  406  being affixed to the support element  402  by adhesive material; a pair of electrical wires  408   a - b  connected to the piezoelectric device  406 ; and a covering disk  414  configured to cover the optical fiber coil  404  and the piezoelectric device  406 . The optical fiber coil  404  and piezoelectric device  406  may be include within a space or channel formed in the support element  402 .  
         [0082]     The material and function of the optical fiber coil  404  and the piezoelectric device  406  may be similar to those of the optical fiber coil  210  and the piezoelectric device  208  of the hybrid patch sensor  200 . In one embodiment, the piezoelectric device  406  may be similar to the device  130 , except that the device  406  has a hole  403 . The optical fiber coil  404  and the piezoelectric device  406  may be affixed to the support element  402  using a conventional epoxy. The support element  402  may have a notch  412 , through which the ends  410   a - b  of the optical fiber coil  404  and the pair of electrical wires  408   a - b  may pass.  
         [0083]     In FIGS.  4 A-B, the diagnostic patch washer  400  may operate as an actuator/sensor and have the optical fiber coil  404  and the piezoelectric device  406 . In an alternative embodiment, the diagnostic patch washer  400  may operate as a sensor and have the optical fiber coil  404  only. In another alternative embodiment, the diagnostic patch washer  400  may operate as an actuator/sensor and have the piezoelectric device  406  only.  
         [0084]     As shown in FIGS.  4 A-B, the diagnostic patch washer  400  may have a hollow space  403  to accommodate other fastening device, such as a bolt or rivet.  FIG. 4C  is a schematic diagram of an exemplary bolt-jointed structure  420  using the diagnostic patch washer  400  in accordance with one embodiment of the present teachings. In the bolt-jointed structure  420 , a conventional bolt  424 , nut  426  and washer  428  may be used to hold a pair of structures  422   a - b , such as plates. It is well known that structural stress may be concentrated near a bolt-jointed area  429  and prone to structural damages. The diagnostic patch washer  400  may be incorporated in the bolt-joint structure  420  and used to detect such damages.  
         [0085]      FIG. 4D  is a schematic cross-sectional diagram of an exemplary bolt-jointed structure  430  using the diagnostic patch washer  400  in accordance with another embodiment of the present teachings. In the bolt-joint structure  430 , a conventional bolt  432 , nut  434  and a pair of washers  436  and  438  may be used to hold a honeycomb/laminated structure  440 . The honeycomb and laminate structure  440  may include a composite laminate layer  422  and a honeycomb portion  448 . To detect the structural damages near the bolt-joint area, a pair of diagnostic patch washers  400   a - b  may be inserted within the honeycomb portion  448 , as illustrated in  FIG. 4D . A sleeve  446  may be required to support the top and bottom patch washers  400   a - b  against the composite laminate layer  442 . Also, a thermal-protection circular disk  444  may be inserted between the composite laminate layer  422  and the diagnostic patch washer  400   b  to protect the washer  400   b  from destructive heat transfer.  
         [0086]     As shown in  FIG. 4B , the outer perimeter  415  of the covering disk  414  may have a slant angle to form a locking mechanism, which can keep optical fiber coil  404  and the piezoelectric device  406  from excessive contact load by the torque applied to the bolt  424  and nut  426 .  
         [0087]      FIG. 5A  is a schematic diagram of an interrogation system  500  including a sensor/actuator device in accordance with one embodiment of the present teachings. Hereinafter, the terms “sensor” and “pickup unit of a sensor” are interchangeably used. As shown in  FIG. 5A , the system  500  may include: a sensor/actuator device  502  for generating and/or receiving Lamb wave signals; a two-conductor electrical wire  516 ; a conditioner  508  for processing signals received by the device  502 ; analog-to-digital (A/D) converter  504  for converting analog signals to digital signals; a computer  514  for managing entire elements of the system  500 ; an amplifier  506 ; a waveform generator  510  for converting digital signals into the analog Lamb wave signals; and a relay switch array module  512  configured to switch connections between the device  502  and the computer  514 . In general, more than one device  502  may be connected to the relay switch  512 .  
         [0088]     The device  502  may be one of the sensors described in  FIGS. 1A-2D  and FIGS.  4 A-D that may include a piezoelectric device for generating Lamb waves  517  and receiving Lamb waves generated by other devices. To generate Lamb waves  517 , a waveform generator  510  may receive the digital signals of the excitation waveforms from computer  514  (more specifically, an analog output card included in the computer  514 ) through the relay switch array module  512 . In one embodiment, the waveform generator  510  may be an analog output card.  
         [0089]     The relay switch array module  512  may be a conventional plug-in relay board. As a “cross-talks” linker between the actuators and sensors, the relay switches included in the relay switch array module  512  may be coordinated by the microprocessor of the computer  514  to select each relay switch in a specific sequencing order. In one embodiment, analog signals generated by the waveform generator  510  may be sent to other actuator(s) through a branching electric wire  515 .  
         [0090]     The device  502  may function as a sensor for receiving Lamb waves. The received signals may be sent to the conditioner  508  that may adjust the signal voltage and filter electrical noise to select meaningful signals within an appropriate frequency bandwidth. Then, the filtered signal may be sent to the analog-to-digital converter  504 , which may be a digital input card. The digital signals from the analog-to-digital converter  504  may be transmitted through the relay switch array module  512  to the computer  514  for further analysis.  
         [0091]      FIG. 5B  is a schematic diagram of an interrogation system  520  including a sensor in accordance with another embodiment of the present teachings. The system  520  may include: a sensor  522  having an optical fiber coil; optical fiber cable  525  for connections; a laser source  528  for providing a carrier input signal; a pair of modulators  526  and  534 ; an acoustical optic modulator (AOM)  530 ; a pair of coupler  524  and  532 ; a photo detector  536  for sensing the light signal transmitted through the optical fiber cable  525 ; an A/D converter  538 ; a relay switch  540 ; and a computer  542 . The sensor  522  may be one of the sensors described in  FIGS. 2A-4D  that may include an optical fiber coil. In one embodiment, the coupler  524  may couple the optical fiber cable  525  to another optical fiber  527  that may be connected to another sensor  523 .  
         [0092]     The sensor  522 , more specifically the optic fiber coil included in the sensor  522 , may operate as a laser Doppler velocitimeter (LDV). The laser source  528 , preferably a diode laser, may emit an input carrier light signal to the modulator  526 . The modulator  526  may be a heterodyne modulator and split the carrier input signal into two signals; one for the sensor  522  and the other for AOM  530 . The sensor  522  may shift the input carrier signal by a Doppler&#39;s frequency corresponding to Lamb wave signals and transmit it to the modulator  534 , where the modulator  534  may be a heterodyne synchronizer. The modulator  534  may demodulate the transmitted light to remove the carrier frequency of light. The photo detector  536 , preferably a photo diode, may convert the demodulated light signal into an electrical signal. Then, the A/D converter  538  may digitize the electrical signal and transmit to the computer  542  via the relay switch array module  540 . In one embodiment, the coupler  532  may couple an optical fiber cable  546  connected to another sensor  544 .  
         [0093]      FIG. 6A  is a schematic diagram of a diagnostic network patch system (DNP)  600  applied to a host structure  610  in accordance with one embodiment of the present teachings. As illustrated in  FIG. 6A , the system  600  may include: patches  602 ; transmission links  612 ; at least one bridge box  604  connected to the transmission links  612 ; a data acquisition system  606 ; and a computer  608  for managing the DNP system  600 . The patches  602  may be a device  502  or a sensor  522 , where the type of transmission links  612  may be determined by the type of the patches  602  and include electrical wires, optical fiber cables, or both. Typically, the host structure  610  may be made of composite or metallic material.  
         [0094]     Transmission links  612  may be terminated at the bridge box  604 . The bridge box  604  may connect the patches  602  to admit signals from an external waveform generator  510  and to send received signals to an external A/D converter  504 . The bridge box  604  may be connected through an electrical/optical cable and can contain an electronic conditioner  508  for conditioning actuating signals, filtering received signals, and converting fiber optic signals to electrical signals. Using the relay switch array module  512 , the data acquisition system  606  coupled to the bridge box  604  can relay the patches  602  and multiplex received signals from the patches  602  into the channels in a predetermined sequence order.  
         [0095]     It is well known that the generation and detection of Lamb waves is influenced by the locations of actuators and sensors on a host structure. Thus, the patches  602  should be properly paired in a network configuration to maximize the usage of Lamb waves for damage identification.  
         [0096]      FIG. 6B  is a schematic diagram of a diagnostic network patch system  620  having a strip network configuration in accordance with one embodiment of the present teachings. As shown in  FIG. 6B , the system  620  may be applied to a host structure  621  and include: patches  622 ; a bridge box  624  connected to a computer  626 ; and transmission links  632 . The patches  622  may be a device  502  or a sensor  522 , where the type of transmission links  632  may be determined by the type of the patches  622 . The transmission links  632  may be electrical wires, optical fiber cables, or both.  
         [0097]     The computer  626  may coordinate the operation of patches  622  such that they may function as actuators and/or sensors. Arrows  630  represent the propagation of Lamb waves generated by patches  622 . In general, defects  628  in the host structure  621  may affect the transmission pattern in the terms of wave scattering, diffraction, and transmission loss of Lamb waves. The defects  628  may include damages, crack and delamination of composite structures, etc. The defects  628  may be monitored by detecting the changes in transmission pattern of Lamb waves captured by the patches  622 .  
         [0098]     The network configuration of DNP system is important in Lamb-wave based structural health monitoring systems. In the network configuration of DNP system  620 , the wave-ray communication paths should be uniformly randomized. Uniformity of the communication paths and distance between the patches  622  can determine the smallest detectible size of defects  628  in the host structure  621 . An optimized network configuration with appropriate patch arrangement may enhance the accuracy of the damage identification without increasing the number of the patches  622 .  
         [0099]     Another configuration for building up wave ‘cross-talk’ paths between patches may be a pentagonal network as shown in  FIG. 6C .  FIG. 6C  is a schematic diagram of a diagnostic network patch system  640  having a pentagon network configuration in accordance with another embodiment of the present teachings. The system  640  may be applied to a host structure  652  and may include: patches  642 ; a bridge box  644  connected to a computer  646 ; and transmission links  654 . The patches  642  may be a device  502  or a sensor  522 . As in the system  630 , the patches  642  may detect a defect  650  by sending or receiving Lamb waves indicated by the arrows  648 .  
         [0100]      FIG. 6D  is a schematic perspective view of a diagnostic network patch system  660  incorporated into rivet/bolt-jointed composite laminates  666  and  668  in accordance with another embodiment of the present teachings. As illustrated in  FIG. 6D , the system  660  may include: patches  662 ; and diagnostic patch washers  664 , each washer being coupled with a pair of bolt and nut. For simplicity, a bridge box and transmission links are not shown in  FIG. 6D . The patches  662  may be a device  502  or a sensor  522 . In the system  660 , the patches  662  and diagnostic patch washers  664  may detect the defects  672  by sending or receiving Lamb waves as indicated by arrows  670 . Typically, the defects  672  may develop near the holes for the fasteners. The diagnostic patch washers  664  may communicate with other neighborhood diagnostic patches  662  that may be arranged in a strip network configuration, as shown in  FIG. 6D . In one embodiment, the optical fiber coil sensors  330  and  340  may be used in place of the diagnostic patch washers  664 .  
         [0101]      FIG. 6E  is a schematic perspective view of a diagnostic network patch system  680  applied to a composite laminate  682  that may be repaired with a bonding patch  686  in accordance with one embodiment of the present teachings. As illustrated in  FIG. 6E , the system  680  may include patches  684  that may be a device  502  or a sensor  522 . For simplicity, a bridge box and transmission links are not shown in  FIG. 6E . In the system  680 , the patches  684  may detect the defects  688  located between the repair patch  686  and the composite laminate  682  by sending or receiving Lamb waves as indicated by arrows  687 .  
         [0102]      FIG. 6F  is a schematic diagram illustrating an embodiment of a wireless data communication system  690  that controls a remote diagnostic network patch system in accordance with one embodiment of the present teachings. As illustrated in  FIG. 6F , the system  690  includes: a bridge box  698 ; and a ground communication system  694  that may be operated by a ground control  692 . The bridge box  698  may be coupled to a diagnostic network patch system implemented to a host structure, such as an airplane  696 , that may require extensive structural health monitoring.  
         [0103]     The bridge box  698  may operate in two ways. In one embodiment, the bridge box  698  may operate as a signal emitter. In this embodiment, the bridge box  698  may comprise micro miniature transducers and a microprocessor of a RF telemetry system that may send the structural health monitoring information to the ground communication system  694  via wireless signals  693 . In another embodiment, the bridge box  698  may operate as a receiver of electromagnetic waves. In this embodiment, the bridge box  698  may comprise an assembly for receiving power from the ground communication system  694  via wireless signals  693 , where the received power may be used to operate a DNP system applied to the structure  696 . The assembly may include a micro-machined silicon substrate that has stimulating electrodes, complementary metal oxide semiconductor (CMOS), bipolar power regulation circuitry, hybrid chip capacitors, and receiving antenna coils.  
         [0104]     The structure of the bridge box  698  may be similar to the outer layer of the host structure  696 . In one embodiment, the bridge box  698  may have a multilayered honeycomb sandwich structure, where a plurality of micro strip antennas are embedded in the outer faceplate of the multilayered honeycomb sandwich structure and operate as conformal load-bearing antennas. The multilayered honeycomb sandwich structure may comprise a honeycomb core and multilayer dielectric laminates made of organic and/or inorganic materials, such as e-glass/epoxy, Kevlar/epoxy, graphite/epoxy, aluminum or steel. As the integrated micro-machining technology evolves rapidly, the size and production cost of the micro strip antennas may be reduced further, which may translate to savings of operational/production costs of the bridge box  698  without compromising its performance.  
         [0105]     The scope of the invention is not intended to limit to the use of the standard Wireless Application Protocol (WAP) and the wireless markup languages for a wireless structural health monitoring system. With a mobile Internet toolkit, the application system can build a secure site to which structural condition monitoring or infrastructure management can be correctly accessed by a WAP-enable cell phone, a Pocket PC with a HTML browser, or other HTML-enabled devices.  
         [0106]     As a microphone array may be used to find the direction of a moving source, a clustered sensor array may be used to find damaged locations by measuring the difference in time of signal arrivals.  FIG. 7A  is a schematic diagram of a diagnostic network patch system  700  having clustered sensors in a strip network configuration in accordance with one embodiment of the present teachings. As illustrated in  FIG. 7A , the system  700  may be applied to a host structure  702  and include clustered sensors  704  and transmission links  706 . Each clustered sensor  704  includes two receivers  708  and  712  and one actuator/receiver device  710 . Each of the receivers  708  and  712  may be one of the sensors described in  FIGS. 1A-4D , while the actuator/receiver device  710  may be one of the sensors described in  FIGS. 1A-2D  and FIGS.  4 A-D and have a piezoelectric device for generating Lamb waves. When the actuator/receiver  710  of a clustered sensor  704  sends Lamb waves, the neighboring clustered sensors  704  may receive the Lamb waves using all three elements, i.e., the actuator/receiver device  710  and receivers  708  and  712 . By using all three elements as a receiver unit, each clustered sensor  704  can receive more refined Lamb wave signals. Also, by measuring the difference in time of arrivals between the three elements, the direction of the defect  714  may be located with enhanced accuracy.  
         [0107]      FIG. 7B  is a schematic diagram of a diagnostic network patch system  720  having clustered sensors in a pentagonal network configuration in accordance with another embodiment of the present teachings. As illustrated in  FIG. 7B , the system  720  may be applied to a host structure  722  to detect a defect  734  and include clustered sensors  724  and transmission links  726 . Each clustered sensor  724  may be similar to the clustered sensor  704 .  
         [0108]      FIG. 8A  shows a schematic diagram of a clustered sensor  800  having optical fiber coils in a serial connection in accordance with one embodiment of the present teachings. The clustered sensor  800  may be similar to the clustered sensor  704  in  FIG. 7A  and include two sensors  804  and  808  and an actuator/sensor  806 . In this configuration, an input signal may enter the sensor through one end  810   a  and the output signal from the other end  810   b  may be a sum of the input signal and contribution of the three sensors  804 ,  806  and  808 . In one embodiment, the signal from each sensor may be separated from others using a wavelength-based de-multiplex techniques.  
         [0109]      FIG. 8B  a schematic diagram of a clustered sensor  820  having optical fiber coils in a parallel connection in accordance with one embodiment of the present teachings. The clustered sensor  820  may be similar to the clustered sensor  704  in  FIG. 7A  and include two sensors  824  and  828  and an actuator/sensor  826 . In this configuration, input signals may enter the three sensors through three end  830   a,    832   a  and  834   a,  respectively, while output signals from the other ends  830   b,    832   b  and  834   b  may be a sum of the input signal and contribution of the three sensors  824 ,  826  and  828 , respectively.  
         [0110]     It is noted that, in FIGS.  8 A-B, the sensors  804 ,  808 ,  824  and  828  have been illustrated as optical fiber coil sensors  308 . However, it should apparent to those of ordinary skill in the art that each of the sensors  804 ,  808 ,  824  and  828  may be one of the sensors described in  FIGS. 1A-4D , while each of the middle sensors  806  and  826  may be one of the sensors described in  1 A- 2 D and FIGS.  4 A-D and have a piezoelectric device for generating Lamb waves. Also, the clustered sensors  800  and  820  may be incorporated within a composite laminate in the same manner as described in  FIG. 1G .  
         [0111]      FIG. 9  shows a plot  900  of actuator and sensor signals in accordance with one embodiment of the present teachings. To generate Lamb waves, an actuator signal  904  may be applied to an actuator, such as a patch sensor  100 . The actuator signal  904  may be a toneburst signal that has several wave peaks with the highest amplitude in the mid of waveform and has a spectrum energy of narrow frequency bandwidth. The actuator signal  904  may be designed by the use of Hanning function on various waveforms and have its central frequency within 0.01 MHz to 1.0 MHz. When the actuator receives the actuator signal  904 , it may generate Lamb waves having a specific excitation frequency.  
         [0112]     Signals  912   a - n  may represent sensor signals received by sensors. As can be noticed, each signal  912  may have wave packets  926 ,  928  and  930  separated by signal extracting windows (or, equivalently envelops)  920 ,  922  and  924 , respectively. These wave packets  926 ,  928  and  930  may have different frequencies due to the dispersion modes at the sensor location. It is noted that the signal partitioning windows  916  have been applied to identify Lamb-wave signal from each sensor signal. The wave packets  926 ,  928  and  930  correspond to a fundamental symmetric mode S 0 , a reflected mode S 0     —     ref  and a fundamental antisymmetric mode A 0 , respectively. The reflected mode S 0     —     ref  may represent the reflection of Lamb waves from a host structure boundary. A basic shear mode, S 0 ′, and other higher modes can be observed. However, they are not shown in  FIG. 9  for simplicity.  
         [0113]     Portions  914  of sensor signals  912  may be electrical noise due to the toneburst actuator signal  904 . To separate the portions  914  from the rest of sensor signals  912 , masking windows  916 , which may be a sigmoid function delayed in the time period of actuation, may be applied to sensor signals  912  as threshold functions. Then, moving wave-envelope windows  920 ,  922  and  924  along the time history of each sensor signal may be employed to extract the wave packets  926 ,  928  and  930  from the sensor signal of  912 . The envelope windows  920 ,  922  and  924  may be determined by applying a hill-climbing algorithm that searches for peaks and valleys of the sensor signals  912  and interpolating the searched data point in time axis. The magnitude and position of each data point in the wave signal may be stored if the magnitude of the closest neighborhood data points are less than that of the current data point until the comparison of wave magnitude in the forward and backward direction continues to all the data points of the wave signal. Once wave envelopes  918  are obtained, each envelope may break into sub envelope windows  920 ,  922  and  924  with time spans corresponding to those of Lamb-wave modes. The sub envelop windows  920 ,  922  and  924  may be applied to extract wave packets  926 ,  928  and  930  by moving along the entire time history of each measured sensor signal  912 .  
         [0114]      FIG. 10A  is an exploded partial cutaway view of a piezoelectric device  1000  in accordance with one embodiment of the present teachings.  FIG. 10B  is a cross sectional diagram of the piezoelectric device in  FIG. 1A , taken along the line  10 - 10 . The piezoelectric device  1000  may be used in place of the previously described exemplary embodiments  108  ( FIG. 1B ),  156  ( FIG. 1F ),  190  ( FIG. 1H ),  208  ( FIG. 2B ),  248  ( FIG. 2D ), and  406  ( FIG. 4B ), for example. More detailed descriptions of the sensors and systems that include the previous embodiments can be found in U.S. patent application Ser. No. 10,942,366, and its divisional application Ser. Nos. 11/304,441, 11/391,351, 11/414,166, and 11/445,452, which are herein incorporated by reference in their entirety. It is noted that the piezoelectric device  1000  may be compatible with the sensors and systems disclosed in these applications.  
         [0115]     As depicted in  FIGS. 10A-10B , the piezoelectric device  1000  may include: a top base layer  1002 ; a top covering layer  1003  positioned beneath the top base layer; one or more top conductive rings  1004  formed beneath the top base layer  1002 ; a top layer tab  1006  formed on the side of the top base layer  1002 ; a top electrode or electrical node  1022  formed beneath the top layer tab  1006  and the top base layer  1002 , and electrically connected to the top conductive rings  1004 ; one or more piezoelectric rings  1008 ; one or more filler rings  1010  formed between the piezoelectric rings  1008 ; top/bottom conductive flakes  1018  formed on the top/bottom surfaces of the piezoelectric rings; a bottom covering layer  1005 ; a bottom base layer  1014 ; a bottom layer tab  1024  formed on the side of the bottom base layer  1014 ; one or more bottom conductive rings  1012  and a bottom electrode or electrical node  1016  formed on the bottom layer tab  1024  and the bottom base layer  1014 , and electrically connected to the bottom conductive rings  1012 .  
         [0116]     The top cover plate  1060  may include the top base layer  1002 , top covering layer  1003 , and top conductive rings  1004 , while the bottom cover plate  1062  may include the bottom base layer  1014 , bottom covering layer  1005 , and bottom conductive rings  1012 . The top base layer  1002  and top layer tab  1006  may be fabricated by, but not limited to, cutting out a polyimide or polyester sheet having a metal coating thereon. The metal coating may be formed from copper, silver, gold, or other suitable metallic materials. Then, the metal coating may be etched to form a pattern of rings thereby generating the top conductive rings  1004 . The pattern may also include the top electrode node  1022 , wherein the node  1022  may include extensions  1017  for connecting to the top conductive rings  1004 . The top base layers  1002  may be secured to the top covering layers  1003  by use of a thermo-setting adhesive, such as acrylic resin or epoxy resin. The top covering layer  1003 , which fills the spacing between adjacent top conductive rings  1004 , may be formed from polyimide or polyester. The bottom cover plate  1062  may be fabricated in the same manner as the top cover plate  1060 . Likewise, the bottom layer tab  1024 , the bottom electrical node  1016 , and extensions  1019  may be generated in the same manner as their counterparts in the top cover plate  1060 .  
         [0117]     The conductive flakes  1018  may provide firm contact between the piezoelectric rings  1008  and top/bottom conductive rings  1004 ,  1012 . Each of the conductive flakes  1018  may have a flat disk ring shape, and preferably fabricated by coating a metal layer on the piezoelectric rings  1008 . The filler rings  1010  may be formed from glass-epoxy or carbon-epoxy. Each of the filler rings  1010  may be also generated by winding glass or carbon fiber impregnated with epoxy around a dummy rod to form a ring shape and baking the fiber ring. Thermo-setting adhesives, such as acrylic or epoxy resin, may be used to attach the filler rings  1010  to the top and bottom covering layers  1003 ,  1005  thereby form an integrated body of the piezoelectric device  1000 . The top and bottom electrical nodes  1022 ,  1016  may respectively have holes  1007 ,  1026  for coupling to two electrical wires through which actuator signals or sensor signals may be transmitted to or from the piezoelectric rings  1008 .  
         [0118]     The piezoelectric device  1000  may have a hole  1020  such that it can be used in a diagnostic patch washer  400  of  FIG. 4A . As a variation, the center hole may be filled with epoxy. As another variation, the piezoelectric device  1000  may not have a hole and, instead, a piezoelectric disk that is covered with conductive flakes, top/bottom base layers, and top/bottom covering layers and is coupled to the electrical nodes may be included in place of the hole. In  FIGS. 10A-10B , only three piezoelectric rings are shown for the purpose of illustration. However, it should be apparent to those of ordinary skill that the present disclosure may be practiced with any suitable number of piezoelectric rings.  
         [0119]      FIG. 11A  is an exploded partial cutaway view of a piezoelectric device  1100  in accordance with another embodiment of the present teachings.  FIG. 11B  is a cross sectional diagram of the piezoelectric device  1100  in  FIG. 11A , taken along the line  11 - 11 . As in the case of the piezoelectric device  1000  depicted in  FIGS. 10A-10B , the piezoelectric device  1100  may be used in place of the previously described exemplary embodiments  108  ( FIG. 1B ),  156  ( FIG. 1F ),  190  ( FIG. 1H ),  208  ( FIG. 2B ),  248  ( FIG. 2D ), and  406  ( FIG. 4B ), for example. Likewise, the piezoelectric device  1100  may be compatible with the sensors and systems disclosed in U.S. patent application Ser. No. 10,942,366 and its divisional application Ser. Nos. 11/304,441, 11/391,351, 11/414,166, and 11/445,452.  
         [0120]     As depicted in  FIGS. 11A-11B , the piezoelectric device  1100  may have a top cover plate  1101 , a middle portion  1103 , and a bottom cover plate  1105 . The top cover plate  1101  may include three pairs of top base layers  1122  and top covering layers  1128 , a top layer tab  1132 , and three top conductive rings  1104 . Each of the covering layers  1128  may include a top electrode or electrical node  1124  coupled to a corresponding one of the top conductive rings  1104 . Except the portions occupied by the top electrical nodes  1124 , the covering layers  1128  may be formed from polyimide or polyester to insulate one of the top conductive rings  1104  from the others. The base layers  1122  may be formed from polyimide or polyester. Some of the top conductive rings  1104  may be simply metal rings attached to the top covering layers  1124  by a conductive epoxy. The top conductive rings  1104  may be also generated by winding carbon or glass fiber impregnated with conductive epoxy, such as epoxy having boron nitride particles, around a dummy rod to form a ring shape and baking the fiber ring. Some of the top conductive rings  1104 , such as the outermost of three, may be generated by etching a metal coating formed on a top base layer. The bottom conductive rings  1144  may be fabricated in the same way as the top conductive rings  1104 . The top base layers  1122  may be secured to the top covering layers  1128  by use of a thermo-setting adhesive, such as acrylic resin or epoxy resin.  
         [0121]     The middle portion  1103  may include three piezoelectric rings  1108  and top/bottom conductive flakes  1130  formed on the top/multiplex bottom surfaces of the piezoelectric rings  1108 . The conductive flakes  1130  may have similar structures as the flakes  1018 . The middle portion  1103  may also include filler rings  1106 , wherein the height of each filler ring may be such that the protruding portions of the filler ring may fit into the corresponding recesses formed in the top and bottom cover plates  1101 ,  1105 . The filler rings  1106  may be formed of glass-epoxy or carbon-epoxy. The filler rings  1106  may be also fabricated in the same way as the filler rings  1010 .  
         [0122]     The bottom cover plate  1105  may have the same structure as the top cover plate  1101  and include bottom base layers  1142 , bottom covering layers  1140 , a bottom layer tab  1134 , and bottom conductive rings  1144 . Likewise, each of the bottom covering layers  1140  may include one of the bottom electrical nodes  1136 . Thermo-setting adhesives, such as acrylic or epoxy resin, may be used to attach the filler rings  1106  to the top and bottom covering layers  1128 ,  1140  thereby to form an integrated body of the piezoelectric device  1100 . The bottom base layers  1142  may be secured to the bottom covering layers  1140  by use of a thermo-setting adhesive, such as acrylic resin or epoxy resin.  
         [0123]     The top and bottom electrical nodes  1124 ,  1136  may have three holes for coupling to three pairs of electric wires, respectively. Each pair of electric wires may be coupled to one of the piezoelectric rings  1108  and operative to transmit actuator signals to or sensor signals from the piezoelectric ring. As such, each of the three piezoelectric rings  1008  may simultaneously function as an actuator or a sensor, i.e., the piezoelectric device  1100  may operate in dual mode in a point in time.  
         [0124]     The piezoelectric device  1100  may have a hole  1120  such that it can be used in a diagnostic patch washer  400  of  FIG. 4A . As a variation, the center hole may be filled with epoxy. As another variation, the piezoelectric device  1100  may not have a hole and, instead, may include a piezoelectric disk that is covered with additional set of conductive flakes, top/bottom base layers, and top/bottom covering layers and is coupled to another pair of electric wires and electrical nodes. In this case, the piezoelectric device  1100  may include four piezoelectric rings. In  FIGS. 11A-11B , only three piezoelectric rings are shown. However, it should be apparent to those of ordinary skill that the present disclosure may be practiced with any suitable number of piezoelectric rings.  
         [0125]     While the present invention has been described with reference to the specific embodiments thereof, it should be understood that the foregoing relates to preferred embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.