Abstract:
A system and a method for detecting surface pressure on a surface is provided. A plurality of transponders are located on the surface for transmitting electromagnetic surface waves and for receiving the electromagnetic surface waves upon being reflected, diffracted, refracted, scattered, or otherwise altered by pressure variations on the surface. A controller is coupled to the plurality of transponders. The controller is adapted to coordinate the plurality of transponders for imaging the pressure variations on the surface. The surface includes a surface-wave medium and the surface-wave medium is pressure-sensitive.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is related to the following patent applications, all of which are incorporated herein by reference: U.S. patent application Ser. No. 12/144,073, filed Jun. 23, 2008, now U.S. Pat. No. 7,719,694, issued May 18, 2010, entitled “System and Method of Surface Wave Imaging to Detect Ice on a Surface or Damage to a Surface”; U.S. patent application Ser. No. 12/144,134, filed Jun. 23, 2008, now U.S. Pat. No. 7,931,858, issued Apr. 26, 2011, entitled “System and Method for Surface Decontamination Using Electromagnetic Surface Waves”; U.S. patent application Ser. No. 12/144,170, filed Jun. 23, 2008, entitled “System and Method for De-icing Using Electromagnetic Surface Waves”; and U.S. patent application Ser. No. 12/144,123, filed Jun. 23, 2008, entitled “System and Method for Large Scale Atmospheric Plasma Generation.” This application is also related to U.S. Pat. No. 7,307,589 entitled “Large-Scale Adaptive Surface Sensor Arrays,” which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electromagnetic surface waves, and in particular, to a system and a method of surface wave imaging to map pressure on a surface. 
     2. Description of Related Art 
     A conventional method of measuring surface pressure is to apply pressure taps or transducers to the surface, but these approaches have some significant disadvantages. Taps and transducers only allow measurements at discrete points on the surface. The surface pressures at other locations can only be interpolated from the known points. Another disadvantage is that taps and transducers are intrusive to the flow. Measurements cannot be taken downstream of other taps or transducers, since the flow is altered once it passes over the upstream disturbances. Finally, taps and transducers are time-consuming and expensive to use. Determining surface loads in aircraft design typically cost $500,000 to $1 million, with approximately 30% of that cost going towards the pressure taps and their installation. 
     Another method of measuring surface pressure utilizes pressure sensitive paint (PSP). PSP is essentially a luminescent dye dispersed in an oxygen permeable binder. The dye is excited by absorbing UV light; it then emits visible light. The intensity of the emitted light is dependent on the pressure of oxygen in the surrounding atmosphere. As the pressure of the oxygen above the PSP increases, the intensity of the emitted radiation will decrease. PSP provides a much greater spatial resolution than pressure taps, and disturbances in the flow are immediately observable. PSP also has the advantage of being a non-intrusive technique, since it does not affect the air flow across the surface. 
     PSP has the following disadvantages. 1) PSP coating degrades with time; 2) PSP response is temperature dependent; and 3) It requires excitation by an external source of UV, is only used in a controlled laboratory environment, and can&#39;t be used as a real time diagnostic. 
     Due to the disadvantages of using pressure taps to measure pressure on a surface and the undesirable characteristics of PSP, a need exists for an improved system and method for measuring pressure on a surface. 
     SUMMARY OF THE INVENTION 
     A system for detecting surface pressure on a surface is provided. A plurality of transponders are located on the surface for transmitting electromagnetic surface waves and for receiving the electromagnetic surface waves upon being reflected, diffracted, refracted, scattered, or otherwise altered by pressure variations on the surface. A controller is coupled to the plurality of transponders. The controller is adapted to coordinate the plurality of transponders for imaging the pressure variations on the surface. 
     In an exemplary embodiment of the present invention, the plurality of transponders are located at a perimeter of the surface. 
     In an exemplary embodiment of the present invention, a surface-wave medium is laminated to the surface, the surface-wave medium is pressure-sensitive. 
     In an exemplary embodiment, the surface-wave medium includes a compressible dielectric. 
     In an exemplary embodiment of the present invention, the surface-wave medium includes a conductive ground plane between the surface and the dielectric. 
     In an exemplary embodiment of the present invention, the surface-wave medium includes a metallic pattern on the dielectric for increasing an inductive reactance of the surface-wave medium. 
     In an exemplary embodiment of the present invention, the metallic pattern is a periodic. 
     In an exemplary embodiment of the present invention, the metallic pattern is a periodic metallic pattern of squares, rectangles, parallel or perpendicular hash marks, or Jerusalem crosses. 
     In an exemplary embodiment of the present invention, the received electromagnetic surface waves are analyzed and the received electromagnetic surface waves are compared to baseline signals for imaging the pressure variations on the surface. 
     A method of detecting surface pressure on a surface is provided. Electromagnetic surface waves are transmitted onto the surface. The electromagnetic surface waves are received upon being reflected, diffracted, refracted, scattered, or otherwise altered by pressure variations on the surface. The transmitting and the receiving of the electromagnetic surface waves is coordinated for imaging the pressure variations on the surface. 
     In an exemplary embodiment of the present invention, the electromagnetic surface waves are transmitted from a plurality of transmitters located at a perimeter of the surface and receiving the electromagnetic surface waves by the plurality of transmitters. 
     In an exemplary embodiment of the present invention, the plurality of transmitters are coordinated to transmit and to receive the electromagnetic surface waves for imaging the pressure variations on the surface. 
     A method of forming a surface pressure detection system on a surface is provided. A plurality of transponders are located on the surface for transmitting electromagnetic surface waves and for receiving the electromagnetic surface waves upon being reflected, diffracted, refracted, scattered, or otherwise altered by pressure variations on the surface. The plurality of transponders are coupled to a controller. The controller is adapted to coordinate the plurality of transponders, to analyze the received electromagnetic surface waves, and to compare the received electromagnetic surface waves to baseline signals for imaging the pressure variations on the surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts fields of a transverse magnetic surface wave on a flat metal surface. 
         FIG. 2  depicts a periodic frequency-selective surface-wave guide having high impedance. 
         FIG. 3  depicts another periodic frequency-selective surface-wave guide having an array of Jerusalem Crosses. 
         FIG. 4  is a schematic of an array of Jerusalem Crosses. 
         FIG. 5  is a circuit diagram depicting the equivalent circuit for the frequency selective surface-wave guide of  FIG. 3 . 
         FIG. 6  is a diagram depicting surface pressure detection with electromagnetic surface waves. 
         FIG. 7  depicts a surface-wave medium. 
     
    
    
     DETAILED DESCRIPTION 
     In the description below, an introduction to electromagnetic surface-wave technology, including surface-wave communication and power technology is provided. Systems and methods are then provided for imaging pressure on a surface using electromagnetic surface waves. 
       FIG. 1  depicts a transverse magnetic (TM) surface wave  10  on a flat metal surface  11 . A TM wave requires a surface with a surface impedance having an inductive term, while, in order to support a transverse electric (TE) surface wave, the reactive part of the surface impedance must be capacitive. 
     At optical frequencies, surface waves are known as surface plasmons. Surface waves are waves that are bound to the interface between a metal or other material and the surrounding space. The surface waves are characterized by longitudinally oscillating charges on the metal surface and associated fields in free space. On a flat metal surface, surface waves typically extend many thousands of wavelengths into the surrounding space. At low microwave frequencies, surface waves can extend many hundreds of meters into the surrounding space. Surfaces that allow surface waves to extend too far out into the surrounding space are not useful for wave guiding. Traditional techniques for creating surface wave media that confine fields closer to the surface generally involve thick dielectric coatings, which are not suitable for many military applications. Recent research has shown, however, that it is possible to produce thin, light-weight structures with textured-impedance surfaces that can have strong surface-wave guiding effects where the fields are confined close to the surface, do not readily leak power into free space, can follow curves in the surface, and have negligible propagation loss. 
       FIG. 2  and  FIG. 3  are two examples of textured-impedance surface geometries. A textured-impedance surface typically consists of a series of resonant structures tiled onto a thin flexible substrate. The complex geometry creates a medium that supports highly localized surface wave propagation by altering the surface impedance, such that the decay constant into free space is rapid, thus binding the wave to less than within a wavelength of the surface. A closely bound surface wave may be propagated along the surface with a small attenuation if the inductive reactance (i.e., reactive part of the surface impedance) is large and the resistance (i.e., real part of the surface impedance) is small.  FIG. 2  depicts a two-layer high impedance surface-wave guide  20 .  FIG. 3  depicts a periodic frequency-selective surface-wave guide  30  having an array of Jerusalem Crosses  31 . The surfaces depicted in  FIG. 2  and  FIG. 3  are inexpensive to manufacture and are readily integrated within structures. 
       FIG. 4  is a schematic of an array  40  of Jerusalem Crosses  41 .  FIG. 5  is a circuit diagram depicting the equivalent circuit for the frequency selective surface-wave guide  30  of  FIG. 3 . 
       FIG. 6  is a diagram depicting surface pressure detection with electromagnetic surface waves. In an exemplary embodiment of the present invention, the system and method provide for remotely mapping pressure profiles in real time. Such a system and method could be useful for pressure sensing on aircraft control surfaces, especially in conjunction with active material control surfaces. 
     According to an exemplary embodiment of the present invention, a system and a method of remotely mapping the pressure profile on a surface are provided. The method includes launching an electromagnetic surface wave across a surface and mapping its propagation characteristics. Pressure variations  140  squeeze a compressible layer on the surface. A pressure-induced variation  140  on the surface-wave material  141  leads to local changes in the surface wave propagation characteristics, causing the surface waves to reflect, refract, and diffract around the variations. The surface wave propagation characteristics are then a function of the pressure across the surface. The pressure mapping is correlated to the surface wave propagation. The system is realized using a thin layer of surface-wave medium  141  that is laminated to the surface under evaluation. The surface-wave medium  141  has several surface wave transponders  142 ,  143  located at its perimeter. A minimum of two transponders are required. The resolution of the surface imaging increases with the number of transponders. 
     The pressure is imaged by having each of the surface wave transponders  142  transmit an electromagnetic pulse that propagates along the surface and is measured by the other transponders  143  for time of flight, phase difference, and amplitude. Any pressure variation on the surface will modify the transmitted surface-wave propagation by reflecting, diffracting, and scattering the surface wave. The signals measured at each transponder  143  are the combination of the transmitted, reflected, and scattered waves. The amplitude and phase characteristics of the measured signals are dependent on the pressure variation across the surface. Each transponder  142  transmits an electromagnetic pulse that is measured by all of the other transponders  143 . If there are N transponders, then there are N(N+1)/2 unique signals that are detected and analyzed for the image of the damage. The measured signals are analyzed and compared to the baseline signals by the controller  144  to create an image of the pressure on the surface. 
       FIG. 7  depicts a surface-wave medium  153 . A surface wave medium  153  is created by printing a periodic metallic pattern  150  on a dielectric material  151 . The periodic metallic pattern  150  may be squares as depicted in  FIG. 7 , as depicted in  FIG. 3 , or some other periodic metallic pattern such as parallel or perpendicular hash marks. The metallic pattern  150  imposes a complex impedance boundary condition to the surface which traps electromagnetic radiation into waves tightly bound to the surface. In an exemplary embodiment, a thin compressible dielectric substrate  151  sits between the textured metallic layer  150  and a metallic ground plane  152 . When the pressure changes, the compressible dielectric layer  151  compresses or expands, thereby changing the separation between the top layer  150  and the ground plane layer  152 . The change in separation between the layers  150 ,  152  effectively changes the surface wave impedance and modifies the surface wave propagation characteristics. The surface wave propagation characteristics can be correlated to the pressure variation or changes in the pressure variation. Alternatively, the dielectric substrate  151  may be a pressure-sensitive dielectric material, such as a piezo-electric material that exhibits changes in permittivity when exposed to pressure changes. 
     In summary, the surface being monitored for surface pressure is coated with a laminated surface-wave medium  141 . The surface is surrounded with a set of surface wave transponders  142 ,  143  along its perimeter. Each transponder is capable of transmitting and receiving surface waves on the surface. The pressure variation  140  is detected by measuring the N(N+1)/2 signals generated by “pulsing” each transponder  142  in turn and measuring the phase and amplitude of the transmitted “pulse” on all of the other detectors  143 . The signal measured at each transponder  143  is a combination of the transmitted signal and the components reflected by and scattered off the pressure variation. The surface-wave medium  153  may be formed by laminating a pressure-sensitive dielectric or a thin compressible dielectric  151  on top of a metallic ground plane  152  in order to produce the property of complex surface impedance. The surface impedance is determined by the size and spacing of the metal patches and the thickness of the dielectric  151  and its electrical properties, such as its permittivity, resistivity and permeability. When pressure is applied to the surface, the thickness of the compressible dielectric  151  changes or the permittivity of the pressure-sensitive dielectric changes and therefore causes a local change in the surface-wave impedance. 
     For some applications, it is desirable to place a pattern of metallization  150  on top of the dielectric in order to tailor the surface impedance for optimum propagation characteristics and confinement of the surface wave energy. As depicted in  FIG. 7 , the metallic pattern, in its simplest form, is a periodic pattern of square or rectangular patches arranged in an array. The size of the patches and their spacing determines the surface impedance and surface wave propagation characteristics of the surface-wave medium  153 . The geometry of the patches can take on any shape desired, which may include complicated interlocking shapes or Jerusalem Crosses as depicted in  FIG. 3  and  FIG. 4 . The array can be a periodic or a periodic rectangular, hexagonal, or any other tiling geometry. 
     While the invention has been described in terms of exemplary embodiments, it is to be understood that the words which have been used are words of description and not of limitation. As is understood by persons of ordinary skill in the art, a variety of modifications can be made without departing from the scope of the invention defined by the following claims, which should be given their fullest, fair scope.