Abstract:
A structure subjected to stresses that can lead to structural failure. The structure includes first and second conductive layers and an intermediate layer therebetween formed of a dielectric, semiconductive, or resistive material, such that the first, second, and intermediate layers form in combination an electrical element, namely, a capacitive or resistive element. The electrical element is located within the structure so as to be physically responsive to transitory and permanent distortions of the structure resulting from extrinsic and intrinsic sources. The structure further includes applying an electrical potential to at least one of the first and second conductive layers so as to generate an electrical signal from the electrical element, sensing changes in the electrical signal generated by the electrical element in response to the electrical element physically responding to the transitory and permanent distortions, and transmitting the changes in the electrical signal to a location remote from the structure.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/595,318, filed Jun. 22, 2005, and is a continuation-in-part patent application of U.S. patent application Ser. No. 11/276,500, filed Mar. 2, 2006, now U.S. Pat. No. 7,555,936, which claims the benefit of U.S. Provisional Application No. 60/658,932, filed Mar. 4, 2005. The contents of these prior applications are incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   The present invention generally relates to structures subject to stresses that can lead to structural failure, such as structures that contact or contain static or flowing fluids, examples of which include tires, airfoils, and pipes of types used in mobile machinery, automotive, aerospace, manufacturing, and process equipment. More particularly, this invention relates to structures equipped with life-sensing means in terms of wear, fatigue, and/or other structural breakdowns within the structure, and means for transmitting an output of the sensing means to detect an impending structural failure. 
   Ongoing interest exists in developing methods for detecting the failure of conduits that transport fluids. For example, U.S. Pat. No. 5,634,497 to Neto, U.S. Pat. No. 6,386,237 to Chevalier et al., and U.S. Pat. No. 6,498,991 to Phelan et al. disclose the detection of a worn hose by sensing the electrical resistivity in one or more wires embedded in the wall of the hose. These patents focus on detecting a discontinuity in the embedded wires, such as would result from breakage of the wires due to wear as opposed to sensing a gradual increase in resistivity attributable to wear or deformation of the hose or its wires. 
   U.S. Pat. No. 5,343,738 to Skaggs differs by disclosing a method for capacitively sensing the failure of a hose. In Skaggs, a fuel leakage through an inner layer of a hose is sensed on the basis of the leaked fuel altering the dielectric properties of an insulating material between a pair of copper wires embedded in the hose. Similar to Skaggs, U.S. Pat. No. 5,992,218 to Tryba et al. discloses sensing water leakage through a hose on the basis of the leaked water increasing the conductivity of an electrical insulating layer between a pair of conductor layers separated by the insulating layer. U.S. Pat. No. 5,969,618 to Redmond also discloses a method for detecting the failure of a hose on the basis of electrical conductivity. Redmond&#39;s hose is formed to have an annulus containing separated wires, and the failure of the inner layer of the hose is sensed when fluid leaks into the annulus and closes an electric circuit containing the wires. 
   Another approach to sensing an impending failure of a hose is disclosed in U.S. Pat. No. 4,446,892 to Maxwell. Maxwell discloses a fluid (oil) transport hose formed by at least two plies and a sensing element therebetween. In one embodiment of Maxwell, the sensing element is responsive to the electromagnetic properties of fluid present between the plies as a result of a failure of an inner ply of the hose. In a second embodiment of Maxwell, the sensing element is responsive to the failure of an inner ply of the hose by presenting an open circuit. The sensing element is said to preferably be a coil of fine wire wrapped around the inner ply and connected to means responsive to changes in the electrical impedance (AC) of the coil. Such changes are said to occur from fluid seepage into the material contacting with the coil or deformation of the inner ply, both of which change the inductance of the coil. In an alternative embodiment in which the sensing element is primarily intended to be responsive to the seepage of fluid (oil) between the plies of the hose, Maxwell employs parallel non-touching wires connected to means responsive to a change in conductance between the individual wires or to a change in the capacitance between the wires. 
   The prior art discussed above is particularly concerned with conduits through which a fluid is conveyed from one location to another, as opposed to fluid vessels such as hydraulic hoses, pipes, and tires in which little if any flow may occur and/or in which structural fatigue of a vessel wall from pressure cycles is often the most important factor in the life of the vessel. Furthermore, sensing systems of the type suggested by Maxwell are generally useful in relatively low pressure systems where the detection of seepage within the hose wall could provide an adequate warning of impending failure. However, in vessels subjected to fluids at relatively high pressures, once seepage occurs catastrophic failure is likely to occur in a matter of seconds, not hours or even minutes. Therefore, it would be desirable to sense an imminent fatigue failure of a relatively high-pressure vessel, as well as other structures subjected to high cyclical pressures. It would also be desirable to predict when a structural failure of such structures will occur, so that the structure can be safely used for its full life and then replaced before any damage occurs to any fluid system containing the structure or to any objects surrounding the structure. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a structure of a type that is subjected to stresses which can lead to structural failure. Such structures particularly include those that contact or contain a static or flowing fluid at high pressures, including tires, airfoils, and pipes of types used in mobile machinery, automotive, aerospace, manufacturing, and process equipment. The structure is equipped with means responsive to distortions within the structure caused by extrinsic and intrinsic sources, such as the result of external forces applied to the structure and internal forces created as a result of wear, fatigue, and/or other structural breakdowns within the structure, so as to be capable of detecting an impending structural failure. 
   According to the invention, the structure includes first and second conductive layers and an intermediate layer therebetween formed of a dielectric, semiconductive, or resistive material, such that the first, second, and intermediate layers form in combination an electrical element, namely, a capacitive or resistive element. The electrical element is located within the structure so as to be physically responsive to transitory and permanent distortions of the structure resulting from extrinsic and intrinsic sources. The structure further includes means for applying an electrical potential to at least one of the first and second layers so as to generate an electrical signal from the electrical element, means for sensing changes in the electrical signal generated by the electrical element in response to the electrical element physically responding to the transitory and permanent distortions, and means for transmitting the changes in the electrical signal to a location remote from the structure. 
   As applied to particular structures, such as tires, airfoils, pipes, etc., the conductive layers can be in the form of structural reinforcement layers, as well as passive layers that do not positively or negatively affect the overall structural integrity of the structure. Various sensing techniques can be utilized with the invention that are responsive to distortions in the first, second, and/or intermediate layers. In tire applications, such responsiveness can be used to monitor regular cyclic loading as the tire rotates, as well as irregular loading or load distributions that occur from changing road and vehicle dynamics, cuts and punctures in the tire, excessive speed or load, tire imbalance, bruising, impacts with curbs, hardening, improper mounting and damage during mounting, impending tread separation, impending burst failure, etc. As such, by monitoring distortions resulting from a variety of sources, the present invention provides the capability of continuously monitoring a structure and eventually removing the structure from service before a catastrophic failure occurs. 
   Other objects and advantages of this invention will be better appreciated from the following detailed description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-section through a bead of a tire sealed against a rim, and shows a pair of conductive layers within the tire separated by an intermediate layer to form an electrical circuit in accordance with an embodiment of the invention. 
       FIG. 2  is a perspective view of a tire with partial cutaways to expose conductive and intermediate layers located within the tread of the tire in accordance with an embodiment of the present invention. 
       FIGS. 3 and 4  schematically represent cross-sectional views through the tread of a tire and show sensor inserts that can be used to electrically connect to conductive layers within the tread of the tire in accordance with yet another embodiment of the present invention. 
       FIG. 5  is a cross-section through the tread of a tire and shows belt wires that can be used as conductive layers within the tire of  FIG. 2  in accordance with another embodiment of the present invention. 
       FIGS. 6 and 7  show two alternative patterns for belt wires that can be used as conductive layers for the tire of  FIG. 2  in accordance with the invention. 
       FIG. 8  schematically represents the inclusion of a grid to assist in monitoring the sidewalls of a tire in accordance with the present invention. 
       FIG. 9  schematically represents the inclusion of proximity sensors to assist in monitoring a tire in accordance with the present invention. 
       FIGS. 10 and 11  show different views of a pipe with a pair of conductive layers separated by a intermediate layer to form an electrical circuit in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As represented in  FIGS. 1 through 11 , the present invention involves creating an electrical circuit within a manufactured structure subjected to high cyclical or intermittent forces, including but not limited to relatively high-pressure vessels such as tires, pipes, etc., and sensing changes in the electrical circuit that occur in response to transitory and permanent distortions of the structure. Such distortions can be the result of extrinsic and intrinsic sources, including extraneously applied forces and internal forces resulting from wear, fatigue, and other structural breakdowns within the structure. The electrical circuit contains conductive layers separated by dielectric, semiconductive, or resistive layers to form one or more capacitive or resistive elements by which changes in capacitance, resistance, or inductance can be sensed. The layers of the circuit are configured to enable sensing of an imminent fatigue failure, remaining life, and damage to the high-pressure structure, and can be coupled to data processing circuitry capable of predicting when a structural failure of the structure will occur, so that the structure can be safely used for its full life and then replaced before any damage occurs to any system containing the structure or to any objects surrounding the structure. 
   The invention can be applied to essentially any structure capable of having a multilayer wall construction for contacting or containing a flowing or static fluid under relatively high pressure. Notable examples include pneumatic tires and pipes. In the context of tires, the invention is also applicable to solid tires, such as solid rubber tires that can be constructed to contain multiple layers near their treads. Other applications include composite aircraft wings made up of multiple layers, whose outer airfoil surfaces are subjected to cyclical or intermittent forces resulting from changes in air pressure, etc. 
     FIG. 1  is a cross-section through a bead  12  of a tire  10  sealed against a rim  14  (either the inside or outside rim side), and shows a pair of conductors  16  and  18  within the tire  10  separated by an intermediate layer  20 . The conductors  16  and  18  are individually connected to conductive strips  22  and  24  on the rim  14  that are electrically insulated from the rim  14 , assuming the rim  14  is formed of an electrically conductive material. The intermediate layer  20  may be formed of a dielectric material such that the conductors  16  and  18  and intermediate layer  20  form a capacitor, or may be formed of a semiconductive or electrically resistive material such that the conductors  16  and  18  and intermediate layer  20  form a resistive circuit. As will be evident from the following discussion, the conductors  16  and  18  and intermediate layer  20  can be a functionally active or passive components of the tire construction. 
     FIG. 2  is a partial cutaway view of the tire  10 , in which the conductors  16  and  18  and intermediate layer  20  of  FIG. 1  are represented as including or otherwise electrically integrated with three layers immediately adjacent the tread  26  of the tire  10 . As such, these layers are hereinafter referred to as conductive layers  16  and  18  and intermediate layer  20 . As evident from  FIG. 2 , essentially the entire tire  10  is a circuit containing an electrical inductive or resistive sensing of changes in the thickness or electrical conductivity of the layer  20 . By applying a voltage across the layers  16  and  18  via the conductive strips  22  and  24  ( FIG. 1 ), an electrical signal is generated that can be detected and wirelessly transmitted by a transmitting device  34  to a receiving unit (not shown) installed on the vehicle to which the tire  10  is mounted. The transmitting device  34  can use, for example, existing pressure sensor and chip technology developed to monitor tire pressure. The transmitting device  34  preferably contains circuitry to process the raw analog data resulting from the electrical signal, perform analog-to-digital data conversion, and transmit the digital data wirelessly to the remote receiving unit, such as a node of a vehicle controller network. Conditions within and beneath the tread  26  are not desirable for the transmitting device  34  because of, for example, excessive accelerations. Therefore, the transmitting device  34  is shown embedded in one of the sidewalls  28 , preferably adjacent the rim  14 , so that the device  34  is located where acceleration levels are minimized. Conductive transmission elements  36  preferably interconnect the transmitting device  34  with the conductive and intermediate layers  16 ,  18 , and  20  that form the sensing element of this invention. To inhibit failure from fatigue, the transmission elements  36  can be formed from extremely thin metal components in the sidewalls  28 . One such example would be the use of coated photo-etched strands that can be made in thicknesses of as little as about 0.0005 inch (about ten micrometers) from a wide variety of metals. Alternatively, the transmission elements  36  can be defined by conductive rubber paths selectively formed in the sidewalls  28 . 
     FIGS. 3 and 4  schematically represent another alternative transmission technique that makes use of sensor/transmitter inserts  44  and  46  to electrically connect to the conductive layers  16  and  18  within the tire  10  of  FIG. 2 . The inserts  44  and  46  preferably contain the circuitry required to receive, process, and transmit the electrical signals from the conductive layers  16  and  18 , and may be powered by a battery (not shown), an induced current, or another less conventional method. The inserts  44  and  46  are shown as capable of being installed in the tire  10  through the exterior of the tire tread  26 , though other locations are possible, including the sidewalls  28  and beads  12 . For a better understanding of  FIGS. 3 and 4 , the layers of the tire  10  are generally depicted to include the tread  26 , an under-tread material  38 , the conductive layers  16  and  18 , the intermediate layer  20 , a body ply  40 , and an inner liner  42 . In  FIG. 3 , the insert  44  is represented as being of a type that is preferably forced into a preformed hole and secured with adhesive. The insert  44  has multiple contact points  45  for contact with the conductive layers  16  and  18 , and a blunt end to avoid puncturing the inner liner  42  of the tire  10 . In  FIG. 4 , the insert  46  has two pin-like contacts  47 , each going to a different conductive layer  16  or  18  in the tire  10 . The insert  46  is configured to be forcibly pushed into the tire  10  and held in place with barbs on the contacts  47 . The head of the insert  46  on the tire exterior preferably contains the circuitry and transmitter and can serve to limit the penetration of the contacts  47  to prevent puncturing of the inner liner  42 . An advantage of the inserts  44  and  46  is that they can be installed in the tire  10  after constructing and curing the tire  10  to protect their electronic components from the harsh conditions experienced during the curing process. Additional advantages with this approach include the ability to check the tire  10  for defects before shipping, and the ability for replacement in case of a malfunction or defect. 
   A dielectric intermediate layer  20  formed of a silicon-based dielectric material has been shown to achieve a capacitive sensitivity of ten to one when placed between layers  16  and  18  formed of a metal. Though rubber materials of the type conventionally used in tire manufacture would exhibit reduced sensitivity, a sensitivity of even two to one (or possibly less) is believed to be attainable with such materials and acceptable for use with this invention. As such, each of the conductive and intermediate layers  16 ,  18 , and  20  may be formed of a base material of rubber, steel, or other materials that are conventionally used in tire construction, and whose electrical properties can be modified as necessary to obtain the desired conductive/resistive electrical properties for the particular layer  16 ,  18 , and  20 . For example, materials of the type conventionally used as steel reinforcement bands in tires can be used as the conductive layers  16  and  18 . Concentric conductive layers  16  and  18  of this type can have a conventional construction, size, and shape similar to steel reinforcement bands widely used in tire construction, or differ in any of these characteristics. As an alternative, either or both of the conductive layers  16  and  18  could be formed by increasing the conductivity of an elastomeric (e.g., rubber) layer of the tire  10  through additions of conductive materials during rubber compounding. By applying an electric current to one of the conductive layers  16  or  18 , capacitance can be measured to capture changes in the distance between the conductive layers  16  and  18 . 
   Another alternative is available with existing tire constructions reinforced with steel wire belts whose individual wires are electrically isolated from each other. An example of this type of tire construction is depicted in  FIG. 5 , which shows the two conductive layers  16  and  18  formed by two sets of wires  30  and  32 , in which each wire  30  and  32  is electrically insulated from the other wires  30  and  32 , and the layers  16  and  18  formed by the sets of wires  30  and  32  are separated by a dielectric (e.g., rubber) intermediate layer  20 .  FIG. 6  depicts a plan view of a typical arrangement for this type of reinforcement, in which the multiple wires  30  of the conductive layer  16  are orthogonal to the multiple wires  32  of the other conductive layer  18 . Pairs of these wires  30  and  32  within either or both conductive layers  16  and  18  can be coupled to form multiple capacitors within the tire  10 . Another alternative is to modify this type of reinforcement belt by electrically connecting the wires  30  or  32  in series as depicted in  FIG. 7 , so that either or both conductive layers  16  and  18  define continuous conductive paths around the tire  10 . 
   The performance and condition of the tire  10  can also be monitored by locating sensing structures within the sidewalls  28  of the tire  10 . For example, sidewall performance and loading can be monitored with measurements taken from the sidewalls  28  to directly or indirectly observe road and vehicle dynamics that may provide an indication of loss of control and, in the case of freight trucks, indicate unsafe conditions due to overloading. For this purpose, another optional feature of the present invention is to provide one or more capacitive grids in the sidewalls  28  of the tire  10  that are separate from the conductive and intermediate layers  16 ,  18 , and  20 . As represented in  FIG. 8 , each grid  29  can be located similar to the transmission elements  36  seen in  FIG. 2 , and powered and sensed in a manner similar to the layers  16 ,  18 , and  20  as represented in  FIGS. 1 and 2 . The grids  29  can have a variety of alternative configurations, and the size and number of grids  29  can be tailored to achieve the desired level of sensitivity. For example, twelve individual grids  29  could be incorporated into each sidewall  28  around the perimeter of the tire  10 , with the grids  29  spaced about thirty degrees apart. Side-loading of the sidewalls  28  can be further monitored with optical or other types of proximity sensors  48  to measure the distance between, for example, the tire beads  12  and the corners of the tread  26 , as schematically represented in  FIG. 9 . Pressure and temperature may also be measured with appropriate sensors (not shown) to further monitor the condition of the tire  10 . 
   The resulting combination of a dielectric intermediate layer  20  with the conductive layers  16  and  18  forms a capacitor consistent with the previous embodiments. This approach also permits detection of electrical currents sent separately through the conductive layers  16  and  18 , with changes in conductivity (resistance) evidencing strain and eventual breakage of the reinforcement wires  30  and  32  within these layers  16  and  18 . 
   Those skilled in the art will appreciate that the conductive layers  16  and  18  as configured in  FIGS. 2 through 7  are conducive to being incorporated into a variety of structures subjected to cyclical or intermittent loading, including aircraft wings and other airfoils capable of having laminate constructions. For example, the orthogonal sets of wires  30  and  32  that define the conductive layers  16  and  18  can be formed of an electrically conductive material that may contribute to the strength or toughness of a wing, or at least have negligible adverse impact on the structural properties of the wing. 
   As noted above, an alternative to capacitive sensing involves forming the intermediate layer  20  of a semiconductive or resistive material. For example, the two conductive layers  16  and  18  (e.g., of a type discussed above) can be separated by a semiconductive intermediate layer  20  formed of a conductive adhesive or a rubber material whose conductivity is increased with carbon or another conductive material. By passing a current through the three conductive and intermediate layers  16 ,  18 , and  20 , resistance can be measured, with the resistance level depending on the condition of the three materials that form the conductive and intermediate layers  16 ,  18 , and  20 . 
   By locating the conductive and intermediate layers  16 ,  18 , and  20  immediately beneath the tread  26  of the tire  10  (or, for example, within the sidewalls  28  of the tire  10 ), the conductive and intermediate layers  16 ,  18 , and  20  are subjected to regular cyclic loading as the tire  10  rotates, as well as irregular loading or load distributions that occur from changing road and vehicle dynamics, cuts, excessive speed, punctures, imbalance, bruising, impacts with curbs, hardening, improper mounting or damage during mounting, impending tread separation, and impending burst failure. As a result, the conductive and intermediate layers  16 ,  18 , and  20  are subject to physical distortions, both transient and permanent, that alter the electrical signal generated when a voltage is applied across the layers  16  and  18 . By detecting and appropriately processing the electrical signal, trends and abrupt changes in the condition of the tire  10  can be sensed that indicate such things as vehicle control characteristics (skidding, swerving, etc.), vehicle loading characteristics (overloading), condition of the tread  26  (tread life, separation, and/or damage), etc. 
     FIGS. 10 and 11  show cutaway views of the invention applied to a pipe  50 , and particularly a multilayer pipe  50  formed of plastic, rubber, or other relatively flexible materials that are susceptible to fatigue failure. In  FIGS. 10 and 11 , the pipe  50  is represented as having two conductive layers  56  and  58  separated by an intermediate layer  60  formed of a dielectric, semiconductive, or resistive material. Together, these layers  56 ,  58 , and  60  form capacitive, inductive, or resistive circuits as described for the embodiments of  FIGS. 1 through 8 .  FIG. 11  shows a connector  52  for the two conducting layers  56  and  58  at one end of the pipe  50 . The connector  52  is preferably integrated into a coupling flange (not shown), and can be attached with a termination coupler (not shown) to measure fatigue and breakdown of the pipe wall on the basis of the same sensing capabilities as the tire  10  of  FIGS. 1 through 8 . As an example, a length of the pipe  50  could be equipped with sensing circuitry at one of its ends, with additional sensing circuitry periodically located along the length of the pipe  50  to sense individual sections of the layers  56 ,  58 , and  60 , or to sense two sections of the layers  56 ,  58 , and  60  with a multiplexer between. A hose coupler can be adapted to make electrical contact with the conductive layers  56  and  58  within the pipe  50 . 
   While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the tire  10  and pipe  50  could differ from that shown, and materials and processes other than those noted could be use. Therefore, the scope of the invention is to be limited only by the following claims.