Abstract:
A improved method of monitoring a structure by mounting a sensor within a cavity of the structure to measure at least one of strain experienced by the structure and vibration experience by the structure. Mounting a wireless communication unit mounted within the structure and connecting the wireless communication unit to the sensor to receive data from the sensor and transmit the data to a receiver outside the structure. Mounting a power supply within the structure and connecting the power supply to the sensor and the wireless communication unit to supply necessary electrical power to the sensor and the communication unit.

Description:
This application claims the benefit of and incorporates by reference U.S. Provisional Application No. 61/361,723 filed Jul. 6, 2010. 
    
    
     This invention was made with United States government support under U.S. Navy SBIR Award No. N68335-09-C-0176. The government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to structural monitoring systems, and more particularly to monitoring systems embedded within a structure that includes autonomous power supplies, sensors, and wireless communication. 
     Structural monitoring is a known field used to examine the integrity of structures and predict when maintenance activities should be performed. Most conventional prior art in this field pertains to sensor systems which use sensors that are wired to a central data acquisition and processing unit. Structural monitoring systems have been applied historically in situations where the cost to benefit ratio is low, because the sensor systems add significant cost, additional maintenance requirements, and a level of reliability that may not coincide with that of the structural system being monitored. For example, health monitoring sensors in power plants are often used because common failures cost millions of dollars in down time, while bridge monitoring sensors are not typically employed because bridges rarely fail and sensor installations are costly. The cost-benefit ratio for each application depends on both the cost savings that can be derived from early failure prediction and the lifecycle cost of the particular implementations of the structural monitoring system. The lifecycle cost for most monitoring systems is in large part determined by the wired connections between remote sensor locations and a central data acquisition and processing unit. Wiring faults are common and are often difficult to diagnose, which increases the maintenance required to sustain the monitoring system. Wire installation costs are high because the wire often needs to be protected. 
     In addition to wiring, the typical external mounting and means of attachment of a monitoring device to a structure strongly influences their accuracy, performance stability and ultimately lifecycle cost of such devices. Small degradation in adhesives or mechanical fastener boundary conditions due to corrosion leads to changes in the operation of sensor and power supply, which can radically compromise the value that they offer. Changes to the sensor mount may require recalibration of device, which erodes the maintenance reduction objectives of the monitoring system. Changes to the power supply mount can affect output, which compromises the performance of the whole monitoring system. External mounting of sensor systems presents further drawbacks in terms of vulnerability to external damage or tampering. 
     In the specific case of helicopter components such as rod ends, replacement is often initiated by the appearance of damage and blemishes in the external surface of the structural part. These blemishes may result from impacts with small debris during operation or from maintenance activities in the proximity of the structural part. Considering the robustness of metal components relative to the vulnerability of an externally mounted device of a monitoring system, there is an issue of whether the devices of the monitoring system must be replaced more often than the structure that is being monitored. 
     A common problem with prior load sensors such as wire foil strain gauges is induced noise from external electromagnetic sources. Foil strain gauges can behave similar to an antenna, because they are typically exposed and mounted to a metal component. This is a particularly relevant issue in structural monitoring systems, if wireless communication and wireless power transfer is used. 
     It is an object of the present invention to provide a structural monitoring system which can be embedded in a structure. 
     It is an object of the present invention to provide a structural monitoring system which does not require wired connections for power or communication. 
     It is an object of the present invention to provide a structural monitoring system with improved system robustness and reduced susceptibility to tampering. 
     It is an object of the present invention to provide a structural monitoring system with reduced maintenance and improved useful life of the monitoring system. 
     SUMMARY OF THE INVENTION 
     A improved method of monitoring a structure by mounting a sensor within a cavity of the structure to measure at least one of strain experienced by the structure and vibration experience by the structure. Mounting a wireless communication unit mounted within the structure and connecting the wireless communication unit to the sensor to receive data from the sensor and transmit the data to a receiver outside the structure. Mounting a power supply within the structure and connecting the power supply to the sensor and the wireless communication unit to supply necessary electrical power to the sensor and the communication unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic front view of an internal structural monitoring system embedded in a rod end according to the present invention. 
         FIG. 2  is a schematic representation of an internal structural monitoring system according to the present invention. 
         FIG. 3  is a schematic representation of an internal structural monitoring system embedded in a rod end according to the present invention. 
         FIG. 4  is a schematic representation of a load sensor embedded in a structure according to the present invention. 
         FIG. 5  is a schematic representation of a piezoelectric energy harvester embedded in a structure according to the present invention. 
         FIG. 6  is a schematic representation of an inductive coupling for wireless power transfer according to the present invention. 
         FIG. 7  is a schematic representation of an approach for electromagnetic radiation power transfer according to the present invention. 
         FIG. 8  is a schematic representation of an antenna mounted external to a structure according to the present invention. 
         FIG. 9  is a schematic representation of an antenna embedded in a structure according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is an internal structural monitoring system  8  for a structure as shown in  FIG. 1 . The present invention includes apparatus and methods associated with the internal structural monitoring system. The present invention includes structure modification and methods of how to modify a structure to receive the internal structural monitoring system. The present invention includes devices as part of the internal structural monitoring system, how to install the devices of internal structural monitoring system within a structure and how to use devices of the internal structural monitoring system within a structure. It further includes features to enable autonomous functionality without the use of wired connectivity to a central processing unit. 
     The present invention provides an autonomous internal monitoring system that interrogates a structure or conditions to which the structure is subject. The present invention provides for measuring parameters which characterize a structure&#39;s condition, use, or exposure to its operational environment without the use of external wires for power and communication. Such structural monitoring is used in support of maintenance activities. Structural monitoring is also used for evaluation of safety or control of a structure&#39;s use and performance. For example, monitoring of loading experienced by rod ends that are port of rods which interconnect components on helicopter main rotor assembly provides essential information regarding the accumulation of damage to the rotor assembly. Knowing the accumulation of damage can help determine the optimal time at which rotor assembly components should be replaced to maintain a high level of safety, while minimizing vehicle maintenance cost. In this specific example, structural monitoring provides an essential basis for generating value in terms of reductions in schedule and unscheduled maintenance, reduced work load, minimization of down time, and increased safety. 
     The present invention provides for the use of low power sensors and wireless features for use in structural monitoring that can be used in a broader range of applications, while reducing the monitoring system cost-benefit ratio. The low power consumption by a sensor, allows the internal structural monitoring system to be powered over a long period of time by devices such as small batteries or energy harvesters that derive energy from their surroundings. Wireless features reduce the high liability of hardwiring devices of the internal structural monitoring system. 
     The present invention includes four primary subsystems of devices, which include a sensor unit, master control circuit, wireless communication unit, and power supply, as shown in  FIG. 2 . These subsystems function integrally for realizing the benefits of monitoring of structures while they are in use. The sensor unit includes one or more sensors to interrogate the structure and environment surrounding the structure, in order to determine the structure&#39;s condition, use, or exposure to its environment. The sensors interface with the master control circuit for control and data acquisition. Analysis and data compression of the sensor measurements is performed at the master control circuit, which includes a processor for such duties. The master control circuit communicates with the wireless communication unit to deliver the structure&#39;s use data to a remote receiver location. The sensor measurements are also stored locally in data storage at the master control circuit. The power supply uses energy harvester transducers, energy receivers, and energy storage devices to power the internal structural monitoring system. The energy harvesters convert energy related to the structure&#39;s use or environment to electrical energy. During times when the structure is not in use or is stationary and statically loaded, communication to or from the sensor may be required. The present invention provides for energy to be supplied to the internal structural monitoring system using wireless energy transfer or an energy storage unit. Energy generated or received can be accumulated or permanently stored in the energy storage unit, if available. 
     The term “structure” refers to the physical object that is the primary subject that the internal structural monitoring system monitors and interrogates.  FIG. 2  shows a schematic of a monitoring system according to the present invention.  FIG. 2  shows the four primary subsystems of a sensor unit, master control circuit, wireless communication unit, and power supply. The embedding of the devices of these subsystems within the structure and their corresponding particular implementations are part of the present invention. It is desirable to have the four subsystems collocated or tethered together in a close proximity of one another for various reasons. The sensor unit includes one or more sensors that measure motion, temperature, strain and load on the structure. Motions can include vibration and acceleration. The master control circuit controls and communicates with each of the other subsystems. The master control circuit includes an analog/digital (A/D) interface and digital input/output (I/O) for interfacing with the sensors, wireless radio and flash memory for data storage. The wireless communication unit provides a wireless data link to a remote location and includes a wireless radio and an antenna. Control and data transmission to the radio is dictated by the master control circuit. The power supply functions to deliver and regulate electrical energy to all other subsystems. The power supply can include an energy harvester, wireless power receiver and energy storage unit. The implementation of each subsystem is described using an application for helicopter components as an example. Specifically, the example shows the use of the internal structural monitoring system in rod ends of rods that are used as linkages. 
       FIG. 1  shows a schematic drawing of a rod end extending from the rod with the internal structural monitoring system embedded within a cavity  10  of a rod end shaft  12  of the rod end. The rod end is an ideal platform for the internal monitoring system. This is because the rod ends are located at critical load links on main rotor dampers, pitch links or push rod assemblies of a helicopter. Together, the internal structural monitoring system in the rod end supplement existing helicopter Health and Usage Monitoring Systems (HUMS) by providing near real time load data on the rotor assembly. Since the rod end is a common aircraft component, the internal structural monitoring system is applicable to other locations on aircraft including landing gear. Rod ends are also used as structural components on many other types of vehicles and structures. The rod is usually a round tubular shape, but the rod could be other shapes. The rod end includes an eye  14  used as a connection point to other structural components. The rod end when connected to a rod is considered as the end of the rod. Extending from the eye is the rod end shaft  12 . The rod end shaft  12  connects to the rod and is typically threaded into an opening in the rod. The rod end shaft  12  of the present invention includes the cavity  10  to receive the internal monitoring system.  FIG. 3  shows a closer view of the internal structural monitoring system in the rod end and how it can be applied to any type of structure that is designed to primarily receive axial loads. Example structures designed to primarily receive axial loads include rods, beams, tubes, struts, cables, and trusses. The structure to receive the internal structural monitoring system should include some type of cavity to receive and enclose the internal structural monitoring system.  FIG. 3  shows a strain sensor, energy harvester, energy storage, master control circuit and RF circuit within the cavity  10  of the axial body rod shaft  12  of the rod end. On the outside of the axial body is an energy receiver that is linked to the energy storage of the power supply and an antenna that is linked to the RF circuit of the wireless communication unit. 
       FIG. 4  shows a detailed schematic view of a load sensor used as the strain sensor shown in  FIG. 3 .  FIG. 4  is one of many types of strain sensors that could be used. The load sensor is shown as a cylindrical shape within a cylindrical cavity of the rod end. The load sensor is placed against the closed end  16  of the cylindrical cavity  10  which acts as a first system support. The load sensor is held in place with the closed end  16  and a second system support  18 , as shown in  FIG. 3 . The load is applied along the axis of symmetry  20  of the cylindrical cavity  10  from the close end of the cylindrical cavity  10  downward towards the second system support  18 . Elongation or retraction of the structure about the cylindrical cavity  10  will be registered as strain along axis of symmetry  20 . The distance between the first system support  16  and the second system support  18  can be such that the load sensor is under a state of compression between the first system support  16  and the second system support  18 . The second system support  18  is mounted in the cylindrical cavity such that the second system support  18  moves with the structure along the axis of symmetry  20 . Because both the strain sensor and the structure have elastic compliance, a nominal state of compression on the load sensor enables it to register bidirectional loading. Compression of the load sensor is preferably greater than the maximum tensile load that is intended to be measured. The alignment of the center axis of the load sensor with the center axis of the cylindrical structure, enables accurate load measurements even if bending moments are introduced perpendicular to the structure&#39;s axis of symmetry. This is as opposed to sensor solutions mounted externally that require multiple sensing locations and averaging algorithms to deduce the uniaxial loading. Spherical contacts at the top and/or bottom of the load sensor prevent transfer of bending from the structure to the load sensor. 
     The sensor shown in  FIG. 4  as an example uses a unique an axi-symmetric magnetostrictive element and magnetic field sensor. The magnetic field sensor is shown as a Hall effect element for sensing changes in the magnetic field.  FIG. 4  shows the Hall effect sensor mounted in the center of a cylindrical magnetostrictive element and permanent magnets located about the sides of the cylindrical magnetostrictive element. Changes in the load on the magnetostrictive element, cause changes the magnetic field in the proximity of the Hall effect sensor. The Hall effect sensor provides an electrical signal that is related to the magnetic field to which it is exposed. The sensor has a ferromagnetic housing mounted about the cylindrical magnetostrictive element. The ferromagnetic housing acts as a shield to protect against changes in the magnetic environment around the load sensor including the proximity of ferromagnetic materials and other passive or active magnets which can strongly influence the load sensor&#39;s sensitivity. For example, if the load sensor is mounted in a rod end on a helicopter and the load sensor passes large steel fasteners as the blades rotate, the sensitivity could vary, if susceptible to external magnetic environments because of lack of shielding. Because both the stiffness of the magnetostrictive element and magnetostrictive responses of the magnetic field are nonlinear, it is advantageous to have their nonlinearity oppose each other. In this case, the magnetic field change due to displacement change of the magnetostrictive element tends to negate the high sensitivity of the magnetostricitve material for small strains. For large strains, the deflection contribution is minimal and the magnetostrictive material is less sensitive to displacement change in this region, which results in greater linearity over a wide range of strains. 
       FIG. 5  shows an example of an energy harvester power supply  22 . The energy harvester  22  shown in  FIG. 5  is a piezoelectric energy harvester with piezoelectric material. In this design, a piezoelectric transducer is directly attached or embedded in the host structure, where the host structure is the cylindrical cavity  10  of the rod end. The energy harvester  22  is positioned between the second system support  18  and a third system support  24  within the shaft  12  of the rod end. The second system support  18  and a third system  24  support hold the energy harvester  22  in position, so that straining of the rod end causes the piezoelectric material of piezoelectric element to strain with the rod end. Both the second system support  18  and a third system support  24  can be positioned in the cylindrical cavity  10  by threading them into a helical thread located on the inside of the cylindrical cavity  10 . Both the second system support  18  and a third system support  24  can be a fastener with outside threads. There can be the case where the bottom surface of the sensor acts as the second system support and the energy harvester  22  is positioned against the sensor, so that the sensor and the energy harvester  22  are between only the first system support  16  and third system support  24 . Other methods than threads can be used to mount the system supports  16 ,  18  within the host structure. Strain induced in the piezoelectric element in turn generates electrical charge at electrodes connected to the piezoelectric element. The charge is extracted by the master control circuit so it can be delivered to other electrical devices of the internal structural monitoring system. Since the electric field generated in the piezoelectric material is roughly proportional to strain, the voltage output of the piezoelectric device is controlled by its thickness dimension. To maintain useable voltages without adding large step-down converters to the power management circuitry, the piezoelectric thickness must be small. Thin piezoelectric layers are stacked to increase the strain energy in the piezoelectric material which in turn increases the power generation capability of the energy harvester  22 . The piezoelectric stack element is coupled mechanically to the rod end by placing it under a preload using the surrounding structure of the cylindrical cavity  10 , the second system support  18  and a third system support  24 . In this way, the piezoelectric element is in parallel with the primary stiffness of the rod end. Similarly to the load sensor, compression of the piezoelectric element between second system support  18  and a third system support  24  is preferably greater than the maximum tensile load that the rod end is designed to accommodate. The stack is preferably a cylindrical element fitted into the cylindrical cavity  10  in the structure. The stack can be ring shaped with a hollow center. It is preferably between 1 and 50 mm long and between 5 and 50 mm in outside diameter. 
     Information recorded about the structure remains with the structure due to the internal structural monitoring system being embedded within the structure. In this case, removing the structure of the rod end from its assembly will not disassociate it from the recorded data. This is important for air and land vehicle components that are periodically removed, refurbished and may remain in storage for a period of time. Since retrieval of recorded information from the sensor depends on the operation of the energy harvesting power supply, the power harvester must be subject to its intended environment. When a structure with an embedded sensor and energy harvester is in storage or removed from its assembly, the energy harvester is likely to be completely inert, making it difficult to extract recorded information from the structure. This is an issue because one of the main purposes of embedded sensors is to evaluate if a structure can be returned to service. In this case, energy must be delivered to the sensor system while it is not in its intended assembly. Since the sensor system is embedded and wireless, a wired connection for power would not be the most convenient solution. Energy delivery by wireless methods provides a simpler solution.  FIG. 6  shows an inductive coupling method that is used to transfer energy from an external source using a resonant driver to a tuned resonant receiver that is part of the internal structural monitoring system. Inductive coupling as depicted in  FIG. 6  involves generating a periodic magnetic field at the source and harnessing the electromotive force defined by Faraday&#39;s law to provide an electrical current in a receiving electrically conductive element. By adding capacitance to the receiving inductive receiving element, electrical resonance is used to amplify the impedance load and the power transfer. The power transfer is also enhanced for a given system by adding an additional set of electromagnetic resonators between the delivering and receiving coils. These resonators provide an impedance match condition that enables efficient power transfer in a compact package.  FIG. 7  shows a secondary power transfer method that uses an alternating electric field to deliver power from an external power generator to an energy receiver. This is generally achieved using electromagnetic radiation in the far field where the magnetic and electric fields are in phase. The electric field is generally initiated and received by antennas. Resonance is used to amplify the response of the antennas. The antennas include various topology including helical, Yagi, half wave dipole, quarter wave dipole, patch, collinear, conformal, fractal, aperture, etc. 
     Both the electromagnetic receiver for energy and the RF communication components, including antennas, require an unimpeded path for delivering or receiving electromagnetic radiation. The structure is typically fabricated out of electrically conductive materials, which inhibit line of site radiation. One solution is to use an antenna external to the structure. This requires a small port through the structure to connect the antenna to the electrical components that are embedded within the structure.  FIG. 8  shows an external antenna and a wiring conduit as the small port.  FIG. 9  shows another solution by having a plug with a low electrically conductivity to enable the antenna components to be embedded and still deliver and receive electromagnetic radiation. The communication antenna and electromagnetic energy receiver are preferably potted or enclosed in packages having low electrical conductivity. 
     The present invention allows for monitoring structures in support of maintenance, control, or safety evaluations or activities related to the structure. The present invention allows for sensing parameters relevant to monitoring the structure&#39;s use, condition, or exposure to its environment. The present invention allows for storing the sensed parameters at or near the measurement location on the structure. The present invention compresses and analyzes the measured parameters to extract useful information pertaining to the structure&#39;s use, condition, or exposure. The present invention transmits useful information wirelessly from the monitoring location to a remote receiver. The present invention allows for harvesting energy and supplying it to the monitoring system for operation when the structure&#39;s use or environment surrounding the structure provides an opportunity to generate energy. The present invention receives energy wirelessly from an external source or from an energy storage unit when the use or environment surrounding the structure provides an insufficient opportunity to generate energy. 
     The present invention addresses essential features lacking in prior state-of-the-art monitoring systems. In particular, it addresses the robustness and long term reliability issues described herein. The approach to addressing these issues involves integration of the sensor system directly with the host structure rather than simply attaching it to the host structure as is common in the prior art. The integration influences the particular implementation of each of the subsystems. It improves the performance of both the sensor and the energy harvester. It also reduces cost by eliminating expensive manufacturing steps associated with bonded piezoelectric fiber composites and foil strain gauges. The system reduces maintenance costs associated with the monitoring system by providing a platform that does not require periodic calibration or repair due to rigorous use. With these advancements, autonomous sensor nodes render a low cost and highly reliable structural monitoring solution, thereby expanding the opportunities for structural monitoring to a wide range of applications on vehicles, buildings, and other basic civilian infrastructure. 
     While different embodiments of the invention have been described in detail herein, it will be appreciated by those skilled in the art that various modifications and alternatives to the embodiments could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements are illustrative only and are not limiting as to the scope of the invention that is to be given the full breadth of any and all equivalents thereof.