Patent Publication Number: US-7725269-B2

Title: Sensor infrastructure

Description:
RELATED APPLICATIONS 
   This application is a continuation of U.S. application Ser. No. 11/437,539, now U.S. Pat. No. 7,627,439, filed May 18, 2006, which is a continuation of U.S. application Ser. No. 11/071,129, filed Mar. 3, 2005, now U.S. Pat. No. 7,373,260, which claims the benefit of priority to U.S. Provisional Application Ser. No. 60/549,668 filed Mar. 3, 2004, U.S. Provisional Application Ser. No. 60/616,748, filed Oct. 7, 2004 and U.S. Provisional Application Ser. No. 60/616,705, filed Oct. 7, 2004, each of which is herein incorporated by reference in its entirety. 

   GOVERNMENT GRANT 
   The Government of the United States of America has certain rights in this invention pursuant to contract No. FA9550-05-C-0024 awarded by the Air Force Office of Scientific Research (AFOSR). 

   FIELD OF THE INVENTION 
   The invention generally relates to the field of sensing, monitoring, damage detection and structural health monitoring systems defused in aerospace, automotive, naval, civil or other applications. 
   BACKGROUND OF THE INVENTION 
   Known methods of laboratory non-destructive structural testing (NDT) methods, such as X-ray detection and C-scans, are impractical for service inspection of built-up structures due to the size and complexity of their infrastructure. Structural Health Monitoring (SHM) involves the incorporation of non-destructive test methods into a structure to provide continuous remote monitoring for damage. SHM systems are systems with the ability to detect and interpret adverse changes in a structure, such as an airplane or other aircraft, automobiles, and naval applications, for example. SHM systems that have been implemented in diverse industries generally include the adhesion of strain gauges or thermocouples to monitor changes in strain, frequency and temperature. Known forms of SHM are “black-boxes” on aircraft that collect critical flight data. 
   SUMMARY OF THE INVENTION 
   The invention relates to a damage detection sensor to provide packaged components to facilitate damage detection using a variety of sensors and sensing methods. An embodiment of the invention includes a device for use in detecting an event in a structure, the device comprising a sensor encapsulation, the encapsulation containing a sensor, an actuator positioned substantially in-plane to the sensor within the housing and a printed circuit board in communication with at least one of the sensor and the actuator. The printed circuit board includes a microprocessor constructed and arranged to collect data from at least one of the sensor and the actuator, a signal generator constructed and arranged to provide excitation to at least one of the sensor and the actuator, and an amplifier to condition the excitation. 
   Implementations of the invention can include one or more of the following features. The sensor can include a sensor that measures at least one of stress, strain, vibration, acoustics, temperature, humidity, pressure, acceleration, location, rotation, radiation, electric fields, magnetic fields, light or motion. The device may include a connector to provide a power and a data connection between the device and a sensor network bus. The connector can include a micro-USB connector. The device may also include a wireless chip positioned in the sensor encapsulation. The device may include a thin film lithium ion battery to supply power to the device. The microprocessor can be configured to collect analog data from at least one of the sensor and the actuator, and further configured to convert the analog data to digital data. The digital data can be stored locally. 
   Further implementations of the invention may include one or more of the following features. The sensor encapsulation can include an outer cylindrical ring and a lid, and wherein the sensor and the actuator can be positioned in the cylindrical ring. The sensor can be at least one of a geometry including triangular, circular, semi-circular, square, rectangular, octagonal, hexagonal, and pie-shaped. The actuator can be at least one of a geometry including triangular, circular, semi-circular, square, rectangular, octagonal, hexagonal, and pie-shaped. The actuator can substantially completely surround the sensor. The sensor can substantially completely surround the actuator. The device can include a plurality of sensors co-located on at least one piezoelectric wafer, wherein the plurality of sensors are collectively at least partially surrounded by the actuator. The device can include a plurality of actuators co-located on at least one piezoelectric wafer, wherein the plurality of actuators are collectively at least partially surrounded by the sensor. The sensor can provide substantially a 360-degree radial detection of structural occurrences in a material. 
   In order to practically attain the full economic and design benefits of SHM, several components that amount to sensor infrastructure are integrated into a small package, and an architecture is developed. The components can include a microprocessor to command testing, a function generator to excite actuators, an amplifier to increase signal strength, an acquisition chip to collect data, power, connectors, a communication standard, shielding from electric and magnetic interference, and casing to protect and package the components. 
   The invention provides one or more of the following capabilities. The damage detection infrastructure can be mass-produced at a low cost, and customized for any application in software. The infrastructure can be broadly defused in aerospace, automotive, naval and civil applications, or any field in which a single sensor or a distributed network of sensors is required to collect data. The infrastructure can be integrated into ageing structures or integrated into newly designed structures. The invention can enable the elimination of scheduled inspections. Structural design can be improved with increased reliability and reduced life-cycle costs. Embodiments of the invention can be constructed without the use of solder and exposed wires. Fewer sensors can accomplish detection without limiting the range over which detecting is desired. Embodiments of the invention can be implemented as a continuously monitoring system, which can require less human intervention. Other capabilities will be apparent upon a review of the Figures and Detailed Description that follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an assembly view of a damage detection device. 
       FIG. 2  is a top perspective view of the internal portion of an assembled damage detection device. 
       FIG. 3  is an assembly drawing of the piezoelectric stack contained in the casing of a damage detection device. 
       FIG. 4A  is a portion of the piezoelectric stack of  FIG. 3 . 
       FIG. 4B  is a side perspective view of a portion of the piezoelectric stack of  FIG. 3 . 
       FIG. 5A  includes alternative geometries for a sensor substantially surrounded by an actuator. 
       FIG. 5B  includes alternative geometries for an actuator substantially surrounded by a sensor. 
       FIG. 6A  is a schematic of a wired system attached to a structure. 
       FIG. 6B  is a schematic of a wireless system attached to a structure. 
       FIG. 7  is a flow chart of a process of using a device as in  FIG. 1 . 
       FIG. 8  is a schematic showing certain features of an exemplary embodiment of the PCB  22 . 
       FIG. 9  is a schematic of a wired system attached to a structure. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The features and other details of the invention will now be more particularly described. It will be understood that particular embodiments described herein are shown by way of illustration and not as limitations of the invention. The principal features of this invention can be employed in various embodiments without departing from the scope of the invention. 
   Embodiments of the invention are directed to concentrically positioned sensors and actuators. Embodiments of the invention can be directed to a piezoelectric-based sensor and actuator for use in facilitating damage detection, non-destructive testing (“NDT”) and structural health monitoring (“SHM”) using a variety of sensors and sensing methods. Embodiments of the invention can include damage detection systems employing one or more than one piezoelectric damage detector. Embodiments of the invention can be directed to an infrastructure for use in monitoring a structure. Embodiments of the invention relate to a collection of electrical and mechanical components necessary to conduct in-situ damage detection methods. Embodiments of the invention can be implemented as wired systems or as wireless systems. Embodiments of the invention can be used in SHM of aircraft, spacecraft, naval vessels and automobiles. Embodiments of the invention may be used in other structures using sensor networks and to conduct testing procedures other than NDT and SHM procedures. For example, embodiments of the invention can be used for non-destructive evaluation, measurement, usage monitoring (HUMS), security, surveillance, condition monitoring or quality control. Embodiments of the invention can be used for other applications. 
   Referring to  FIG. 1 , a sensor node  5  includes a top lid  18  having an o-ring  20 , a printed circuit board (PCB)  22 , a plunger  24 , a casing  16 , a flexible circuit having a top portion  26  and a bottom portion  28  and a sensor  50  and actuator  51  pair. For purposes of the following, the sensor  50  and actuator  51  pair may also be referred to as a piezoelectric wafer  29 . The lid  18  and the casing  16  join to form a housing for the electronic components of the node  5 . The PCB  22  is protected by the sealing o-ring  20  and the plunger  24  within the casing  16 . The top portion of the flexible circuit  26  is positioned above the piezoelectric wafer  29 . The bottom portion  28  is positioned below the wafer  29 . The flexible circuit  26  and  28  provides power and data connections to and from the wafer  29 . A piezoelectric stack  30  (shown in  FIG. 3 ) includes the flexible circuit  26 ,  28  and the wafer  29 . 
   The PCB  22  collects data via a connection to the sensor  50 . For example, the PCB  22  includes electronic components to collect analog data (e.g., microprocessor  515 ), convert the analog voltage data to digital data (e.g., A/D Converter  500 ), and locally store the data in a buffer at a high update rate and bit-resolution. Embodiments of the PCB  22  could also collect other analog current or resistance measurements, as well as direct digital signals. The PCB  22  can include an integrated data logger capable of, for example, 1 MHz acquisition on 2 channels of 1000 points per channel, with a dynamic range of 10 mV-10V. The PCB  22  also includes electronic components to facilitate excitation of the sensor  50  or actuator  51  with programmable or fixed arbitrary waveforms. The PCB  22  can contain a signal generator (e.g., signal generator  505 ) capable of 1 MS/s and 20 Vpp arbitrary function generation of up to 1000 points on 2 channels, for example. An amplifier or other electronic components can be included to condition the signal (e.g., signal conditioner  510 ), such as to condition the excitation and resulting sensor voltages. 
   The nodes may have a unique nodal address that can be accessed either directly or via the internet in order to collect the critical information regarding the sensor node, including version, date fabricated, design revision, operating, reliability and certification data. Some embodiments of the invention may include a microprocessor within the sensor node  5  located on PCB  22  that can receive command data remotely to initiate damage detection checks, as well as uploading new firmware to control internal components or software to locate and interpret data. 
   Referring to  FIG. 2 , a housing  10  provides an interface between the sensor  50  and the structure to which the node  5  is connected for monitoring. When assembled, the node  5  is capable of providing an integrated sensing unit for conveying information about a structure. The sensor node including the piezoelectric wafer  29  is assembled in the housing  10 . The housing  10  is comprised of the cylindrical casing  16  and the top lid  18 . The casing  16  includes the inner o-ring  20  and an o-ring groove  32 , a grounding ring  34 , mini USB connector and apertures  36 . The apertures  36  are positioned to accept micro-connectors, such as connector  12  and connector  14 . The USB connector apertures  36  accept USB connectors that complete mating connection with the internal portion of the housing  10  and extend to an external portion of the node  5 . The apertures  36  can be positioned on opposite sides of the cylinder  16 . The o-ring groove  32  is positioned on a top face of the cylinder  16  and accommodates an o-ring  61 . The o-ring  20  provides a seal that is preferably watertight to keep moisture from entering the housing  10 . 
   A top portion of the casing  16  can be threaded on an internal face of the casing  16 , for example. The top lid  18  can be a flat portion having a threaded rim to engage with the threads of the cylinder  16 . Alternatively, the top lid  18  and the casing  16  can be fitted in a number of known means of closure. The lid  18  can be alternatively designed to complete the housing  10  including glue-on press fits, screw top, and cam-lock, preferably incorporating o-rings to provide a seal. 
   The housing  10  provides a barrier for the electronic components of the node  5 . The housing  10  can include a low moisture absorbing plastic casing. For example, a low density, low moisture absorbing and moldable plastic such as an Acetal (e.g. Delrin) can be used as a casing material. The housing  10  provides an enclosure to package each component of the infrastructure of the node  5 , protecting the components against incidental impact damage, sealing the components from moisture, and isolating the sensor  50  from large induced strains on the structure or cables. The housing  10  can provide additional protections or barriers for node  5 . Nominal dimensions for this housing  10  can be, for example, approximately 1.5″ in diameter and 0.3″ in height with a 0.1″ wall thickness, however depending on the nature of the application, the housing  10  can be smaller or larger in any dimension. Preferably, the housing  10  of the detection device has an outer diameter of approximately 1.6 inches and a total volume less than 1 cubic inch. The height of the housing can be approximately 0.5 inches. 
   The housing  10  is survivable to a large variety of common solvents, including fuels, oils, paint, acetone and cleaning solutions, as well as other chemicals. The housing  10  can operate under thermal conditions between −50° F. and 250° F. The housing  10  may be designed to operate under thermal conditions below −50° F. or above 250° F. The housing  10  containing the node  5  can be adhered to a structure using a thermoset or thermoplastic film adhesive, or by using a traditional epoxy. Other adhesives are possible. The housing  10  is further preferably constructed to withstand a strain of 2000 microstrain and can have a vibration resonance tolerance of 66 Hz or greater. 
   In an expanded view, in  FIG. 3 , a piezoelectric stack  30  is contained in the housing  10  and includes a copper-coated Kapton™ shield  40 , an adhesive film  42 , a copper-coated Kapton™ electrode  44 , an electrically conductive adhesive  46 , a second film adhesive layer  48 , the piezoelectric sensor  50  and actuator  51 , a third film adhesive layer  52  having an electrically conductive portion  53 , a polyester film layer  54  and a fourth film adhesive layer  56 . The copper-coated Kapton™ shield  40  is a layer of copper-coated Kapton™ that provides an insulating surface on the topside and an EMI shield on the underside. The adhesive film  42  can be an insulator capable of bonding to copper-coated Kapton™. For example, the adhesive film  42  can be 3M™ 3132 film adhesive. The copper-coated Kapton™  electrode    44  is a layer of copper-coated Kaptom™. The electrode pattern can be created using Ferric Chloride. The copper-coated Kapton electrode  44  provides contacts to both the sensor  50  and the actuator  51 . The copper-coated Kapton™ electrode  44  can also provide a shielding ground loop between the sensor  50  and the actuator  51 . The ground loop can prevent in-plane parasitic noise. The electrically conductive adhesive  46  and the second film adhesive layer  48  connect the leads to the piezoelectric sensor  50  and actuator  51 . The adhesive  46  and the second film adhesive layer  48  can be provided to avoid a short circuit. The third film adhesive layer  52  provides an electrically conductive layer of adhesive and is positioned beneath the sensor  50 /actuator  51  layer to provide a common ground. The film layer  54  and the fourth film adhesive layer  56  provide a semi-rigid backing for mounting to a structure that the sensor node  5  is monitoring. 
   The sensor  50 /actuator  51  is controlled by the flexible circuit electrode  44 . Adhesive layers between the electrode  44  and the sensor  50 /actuator  51  connect each layer of the piezoelectric stack  30 . Adhesive layers can be electrically conductive. Alternatively, adhesive layers can connect other layers without electrical conductivity. The piezoelectric sensor  50  measures reflected waves in a material on which the sensor is positioned. Sensors can record, for example, variables such as strain, acceleration, sound waves, electrical or magnetic impedance, pressure or temperature. The actuator  51  excites waveforms in a material to create reflected waves that the sensor  50  measures. 
   The node  5  can be used as a wireless device, as shown in  FIG. 3 . In  FIG. 3 , the housing  10  includes a wireless transceiver  58  and a battery  59 . The transceiver  58  receives commands and transmits data. The battery  59  can be a rechargeable thin-film Lithium ion polymer battery to provide power. A wireless inductive loop or energy harvesting may be used to recharge the battery  59 . As described, however, the sensor node  5  can be used as a wired embodiment. The sensor infrastructure used in a wired system includes a communication standards chip, either USB, RS485 or CAN bus, and a mini-USB connector to provide power and digital data transfer, all of which is contained on the PCB  22 , for example, as described with respect to  FIG. 1 . 
     FIG. 4A  is an exploded assembly view showing each of the layers of the flexible circuit surrounding the piezoelectric elements. Included are a conducting layer on top with a shield layer above that, and a bottom grounding layer. Also displayed are the layered wings that carry the power and sensor signal with shields on either side.  FIG. 4B  is a collapsed assembled version of  FIG. 4A . 
   The electrode flexible circuit, shown in  FIG. 4A , controls the sensor  50  and actuator  51 . The electrode flexible circuit is positioned above the sensor  50 /actuator  51  layer. Each of the layers of the flexible circuit is connected by the contact of the side tabs, shown in  FIG. 4B . The flexible circuit  180  provides electrical connections. A copper-coated Kaptom™ element is printed so that there are separate grounds for the actuator and sensor, and separate ground traces to provide in and out-of-plane signal shielding. Wings on the side of the flexible circuit  180  fold up. The wings can provide an electrical connection in a substantially convenient location during manufacture and integration. The wings are shielded in and out-of-plane. The wings terminate in heat bonded or soldered connections with the PCB  22 , and serve to transfer power and data to and from the PCB  22  to the sensor  50 /actuator  51  layer. 
   The sensor  50 /actuator  51  layer of the node  5  comprises a concentric, circular sensor  50  having an outer ring comprising the actuator  51 . The sensor  50  and the actuator  51  are in-plane components capable of connection to the circuit without the use of wires. Referring to  FIG. 5A , the in-plane sensor  50  and actuator  51  can be a number of alternative shapes. For example, the sensor  50  can be circular, semicircular, square, triangular, rectangular, pie-shaped, hexagonal, octagonal, and any of a number of other shapes. The actuator  51  can also be any of a number of shapes configured to substantially surround the sensor  50 . The substantially concentric design of the sensor  50  and actuator  51  provide omni-directional operation of the node  5 . The substantially concentric design of the sensor  50  and actuator  51  provide a pulse/echo method of sensing. By having an actuator that surrounds a sensor or set of sensors (or vice versa) this allows excited signals (electrical, magnetic, acoustic, vibrational or otherwise) to be emanated omni-directionally from a nearly point source, and for response measurements to be taken from nearly the same location. 
   Each of the sensor  50  and the actuator  51  can surround, or substantially surround the other. In each of the alternative configurations shown in  FIG. 5B , the center portion can be the actuator  51 , surrounded by one or more than one sensor  50 . Thus, a sensor or a set of sensors can be surrounded by an actuator or a set of actuators. Alternatively, an actuator or a set of actuators can be surrounded by a sensor or a set of sensors in the concentric design. In some systems, at least one of the piezoelectric nodes includes a sensor  50  surrounded by an actuator  51 , and at least one of the piezoelectric nodes includes an actuator  51  surrounded by a sensor  50  where each of the nodes works in tandem with the other or others to accomplish material sensing. 
   The in-plane configuration of the actuator  51 /sensor  50  pair achieves contact with a material to be monitored or tested using thermoset or thermoplastic tape, epoxy, using a couplant material, or with an externally applied force. Other room temperature or elevated cure methods of contact are possible and envisioned. In some applications, the sensor  50  and actuator  51  pair are not encapsulated in a housing  10 , but are substantially directly positioned on a material or structure for use. The actuator  51 /sensor  50  pair can be actuated with an electrical or magnetic field being applied so as to excite through-thickness, axial, shear or radial modes in the actuator. This field can be applied to a parallel face of the actuator  51 , or using interdigitated electrode patterns. Sensor voltage data can be measured using any of these fields. Preferably, the sensor  50  and actuator  51  are constructed of a piezo-ceramic material. Other known materials can be used, however, such as other piezoelectric materials (PVDF, PMA, etc), piezoresistive materials or magnetorestrictive materials, for example. A variety of other sensor may be used within the infrastructure, including, but not limited to sensors that measure, stress, strain, temperature, moisture, acceleration, motion, radiation or electrical and magnetic fields. 
   The sensor  50 /actuator  51  pair can comprise a single piezoelectric wafer, or more than one piezoelectric wafer. The particular piezoelectric material used for the wafer  29  can be PZT-5A in order to reduce the dependency of performance on temperature, however other grades of PZT such as PZT-5H would also be acceptable. The piezoelectric elements are either injection molded, machined or micro-fabricated in either addition or subtraction processes into the desired geometry, typically less than 1″ in diameter. Other dimensions are possible and envisioned, and may vary depending on optimizing an application. 
   Sensors  50  embodied in nodes  5  are used in an infrastructure to monitor defects in a material or structure. One or more than one node  5  can be used. A variety of sensor types can be placed into an infrastructure together or separately, including, but not limited to, sensors that measure stress, strain, vibration, acoustics, temperature, humidity, acceleration, radiation, electric or magnetic fields, light and/or motion. Further, infrastructure systems of nodes  5  can be surface-mounted or embedded for applications that include, for example, structural health monitoring, non-destructive evaluation, health usage monitoring, surveillance or security. 
   Referring to  FIGS. 6A and 6B , one or more than one node  5  can be positioned in a system or structure to detect damage in a material or structural configuration. In  FIG. 6A , a wired damage detection architecture  120  is positioned in a structure  122 . For example, the structure  122  can be a panel in the body of an airplane. The nodes  5  are positioned throughout the structure  122 . Connectors  124  allow communication between nodes  5 . 
   In  FIG. 6A , a “wired” version of an infrastructure includes a communication standard chip to provide power and digital data transfer between a central computer, which controls each node in the system, and the sensor infrastructure. This is achieved by using a USB, RS485 or CAN bus. Other embodiments that support digital data transfer over long distances are also possible. A mini-USB connector is used to mount the PCB  22  into the housing  10 , and to connect each sensor node  5  to each other and to the central processor. The wired nodes  5  can be daisy chained to each other to transfer data or power, or connected to a computer USB port through a protocol adapter. The sensors can be daisy-chained in order to reduce cable length, complexity and cost, as well as for other purposes. For larger structures, (e.g., referring also to FIG.  9 ),clusters of nearby sensor nodes are daisy chained, and connected to a hub (e.g., hub  150  in  FIG. 9 ), then relayed to the central processor, (e.g., central processor  155  in  FIG. 9 ), which is the computer that collectively controls each node in the system, where a node is defined as a single sensor infrastructure unit. 
   Internally, the electronic components of the sensor infrastructure are connected to mini-USB connectors on either side of the sensor node to facilitate CAN bus connectivity and daisy chaining to the nearest neighbor node. The standard USB 10 Vpp power supply is used, as well as a standard USB cable, however other suitable cable may be used to support various harsh and specialized conditions. The damage detection sensors can be surface mounted on structures in which health monitoring is conducted. Alternatively, the damage detection sensors can be embedded into structures to accomplish health monitoring. 
   Referring to  FIG. 6B , nodes  5  are positioned throughout a structure  142 . The wireless version  140  of the infrastructure includes a wireless transceiver to receive commands and transmit data, and may not have a need for any other type of connector. Software and firmware can be remotely loaded onto the microprocessor using the wireless protocol. The wireless nodes  5  can pass information to the nearest neighbor nodes, or other nearby nodes, in a “fire brigade” fashion until the data reaches the central processor. The data can be passed from node to node redundantly. 
   Damage detection methods use the wired or the wireless infrastructure to determine the presence of damage in a structure. Damage detection methods may also be used to determine the size, shape, type, location and extent of damage in a structure or material, as well as the criticality of maintenance, repair or replacement. For example, methods include lamb waves, modal analysis, acoustic emission, strain/stress monitoring, temperature and acceleration measurement. Each of the damage detection methods can use a single actuator  51 /sensor  50  pair measuring at different frequencies and time samples. Methods of detection can be accomplished by changing frequency of actuation, frequency of acquisition and filters. Further, the use of passive methods (such as strain and/or acoustic emission) to trigger active methods (such as frequency response and lamb waves) can be used to conserve power. Active modes can be used at set intervals or upon user command tests. Methods of detection can include intermittent active methods, which can seek detailed information. Passive methods can be listening for events that can trigger active methods of detection. 
   In operation, referring to  FIG. 7 , with further reference to  FIGS. 1-6 , a process  200  for detecting damage in a material or structure using a node  5  includes the stages shown. The process  200 , however, is exemplary only and not limiting. The process  200  may be altered, e.g., by having stages added, removed, or rearranged. 
   At stage  202 , a node  5  is positioned on the surface of a material or a structure for which structural integrity is to be tested or monitored. Preferably, a plurality of nodes are distributed throughout a structure and work in tandem. The node  5  can alternatively be embedded in a material or structure to conduct detection. Although the system can operate continuously, the system can be accessed by individuals to perform inspections on demand. 
   At stage  204 , the node  5  collects data related to the structure to which it is affixed. The node  5  can collect data passively, for example, using strain and acoustic emission methods. Passive damage detection methods can be used continuously to sense the presence of damage in the structure. Passive methods are generally those that operate by detecting responses due to perturbations of ambient conditions. Strain monitoring is used to record strains over design limits, and can also be used to trigger more sophisticated detection methods. By analyzing the data at smaller time scales, acoustic emission can be performed passively to detect and record impact events and approximate the energy of impact. The nodes  5  pass the collected information to a local processing unit at stage  206 . 
   Abnormal strain and/or acoustic events are recorded, as shown at stage  208 . Conditions that differ from the ambient conditions of a structure can be recorded and further analyzed. To determine damage, comparison is made with a baseline measurement. 
   Where abnormal events have been detected, an active sensing method is triggered at the node  5 , stage  210 . When abnormal data is encountered, active methods such as frequency response and Lamb wave techniques are initiated. Active methods are used to give more information about the type, severity and location of damage. Active methods, for example, use an externally supplied energy in the form of a stress or electromagnetic wave to function. Examples of active methods include, but are not limited to, electrical and magnetic impedance measurements, eddy currents, optical fibers that use a laser light source, modal analysis and Lamb wave propagation. Active methods can be triggered by an event detected by the passive methods. Alternatively or concurrently, active methods can be performed at pre-set time intervals or initiated by an operator. 
   At stage  212 , data from the active sensing mode is collected to verify damage. In a system that employs more than one node  5  for detection, once a single node  5  has collected damage, data is collected by nearby nodes in order to help confirm the presence and severity of damage, stage  214 . At stage  216 , the data is passed from node  5  to node  5 , and to a central processing unit to be interpreted. For example, all of the data can be passed from each node  5 . Data can be passed in a fire brigade fashion, such that substantially all data is passed from node to node within the system. The damage type, severity, and location can be communicated to other individuals, as can suggested actions. 
   In some methods of the invention, fixed spacing between the actuator  51  in a first node  5  and the sensor  50  in a second node  5  can be used to calculate wave speed in a material at the material&#39;s present state. The wave speed calculation self-calibrates the system and may reduce the need for analytically derived wave speed calculations to be determined. The calibration process can take place prior to each test measurement. Based on the calibration process, the system is self-compensating for the effects of temperature, humidity, strain or creep. For example, the fixed distance between the actuator and the sensor divided by the time of flight of the wave between the actuator and the sensor determines wave speed. The wave can be, for example, a surface, shear, Raleigh, Lamb or other type of wave for use in calculating wave speed. Self-compensation can be used to determine the state of the structure, e.g., thermal, hygral or strain. Also, by measuring the impedance and other signature data such as total energy and frequency spectrum of the actuator while being excited, a self-diagnostic can be performed to detect irregular operation. 
   Active Damage detection methods can be performed by using either a single damage detection node  5 , or a network of several devices  5  working independently or in collaboration. When using a single node  5 , a pulse-echo type of operation is used, where the structure being monitored or tested is excited by an actuator, and a response or reflections are measured by a co-located sensor. In the case of using multiple nodes  5 , damage detection can also be performed by pulse-echo, whereas each node  5  independently collects response or reflection data, which is fused together to map out damage locations. Alternatively, when using more than one node  5 , a pitch-catch method can also be used, whereas an actuator from one node  5  excites the structures being monitored or tested, and sensors from one or more other device nodes  5  measure the transmitted response to determine the state of the structure. The device  5  at which the actuation occurs is referred to as the master node. When using the pitch-catch method, the master node designation is iteratively cycled through each of the various nodes  5  so that combinations of transfer functions can be collected. The preferred method is to employ both of the pulse-echo and pitch-catch methods simultaneously. This case is similar to the previously described pitch-catch only method, however in this case reflected data from the master node sensor is also collected to be fused with all of the other data. 
   The structural monitoring tests are facilitated with the electronics on the PCB  22 . The microprocessor initiates testing by triggering the arbitrary function generator to excite the actuator in the node  5  and initiating data collection by the datalogger on the PCB  22 . The tests can be initiated remotely by a user, pre-programmed to be executed at certain intervals, or be triggered by passive methods. Digital data from the buffer is collected by a central processor via the wired or wireless data link. The data is processed by the central processor. The microprocessor on the PCB  22  can provide processing to locally assess damage. 
   Once voltage data has been collected by one of the methods previously described, there are a variety of ways this data can be decomposed in order to ascertain the state of the structure. First data can be filtered and de-noised using bandpass filters in order to remove high frequency electrical noise and low frequency drift and mechanical vibrations. Algorithms can be used that compare the integrated energy levels received at the sensors to determine if damage is present; increased reflected energy and decreased transmitted energy are both metrics of damage. This is followed by an evaluation of reflection time of flight, in order to determine the damage location by multiplying these results by the wave velocity. A fast-Fourier-transform can be performed to inspect the resulting frequency bandwidth, which is used to determine the type of damage present in the structure. By using three separate sensor physics to evaluate the damage, for example, one can minimize the occurrence of false positives. 
   In a system of damage detection devices embodied in a structure, actuation of the detection device  5  occurs at a master node. Sensing occurs at all other nodes. Use of a single master node distance to damage can be estimated, and the angle from the master node at which damage has been detected can also be estimated. The damage detection procedure can be run iteratively making each node a master node. With the use of multiple nodes, damage can be triangulated. 
   The invention provides an infrastructure to a sensor or actuator that operates using a compatible power source, such as piezoelectric, foil resistive, MEMS or eddy current, for example. The infrastructure described herein provides a “black-box” infrastructure allowing data and power to flow in and out of a distributed network of daisy-chained sensors, reducing cabling, time of installation, and cost. The infrastructure can be mass-produced at a low cost, and customized for applications in software. The device can be used to perform damage detection methods such as Lamb wave, Frequency response, Acoustic emission and strain/stress monitoring for any known material or structure. 
   Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents are considered to be within the scope of the invention. Various substitutions, alterations, and modifications may be made to the invention without departing from the spirit and scope of the invention. Other aspects, advantages, and modifications are within the scope of the invention. The contents of all references, issued patents, and published patent applications cited throughout this application are hereby incorporated by reference. The appropriate components, processes, and methods of those patents, applications and other documents may be selected for the invention and embodiments thereof.