Abstract:
A portable thermal imaging apparatus uses an ultrasonic acoustical source and an infrared camera to examine a flat or curved specimen such as the surface of the fuselage of an aircraft for defects such as subsurface disbonds, delaminations, cracks, corrosion, embedded contaminants, inclusions, and voids. The apparatus includes a base framework removably attachable to the specimen by a set of vacuum cups and a pair of guide rails along which the imaging apparatus can travel to allow multiple images to be captured without relocating the apparatus on the specimen. The acoustical signal from the ultrasonic source sweeps over a range of frequencies in order to excite defects of greatly differing size to exhibit local heating.

Description:
CLAIM FOR PRIORITY 
   This application is a continuation-in-part of, and Applicants hereby claim the benefit of the filing date of, U.S. patent application Ser. No. 10/324,014, Ultrasonic Thermography Inspection Method and Apparatus, filed Dec. 20, 2002, the disclosure of which is incorporated herein in its entirety by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to the use of ultrasonic thermography for inspecting the structural integrity of certain components, materials and/or structures. More particularly, the present invention relates to an apparatus and method for positioning an ultrasonic thermography test apparatus to ease and speed field inspections of aircraft fuselages and other structural components. 
   BACKGROUND OF THE INVENTION 
   Establishing and verifying the structural integrity of components and structures is important in many industries, such as aviation, automobiles, petroleum, and construction. Loss of structural integrity can be caused by material defects, such as disbonds, delaminations, cracks, corrosion, embedded contaminants, inclusions, and voids that can exist in a composite or monolithic component or structure. For example, it is important in the aviation industry that reliable nondestructive inspection (NDI) techniques exist to examine the structural integrity of each aircraft&#39;s fuselage and other structural components to assure that the aircraft will not experience structural failure during operation. Point-by-point inspection of airplanes may thus be advisable, and may be required to be performed, at routine service intervals. Similarly, by way of example, non-invasive inspection and analysis of automotive components such as load-bearing panels, and of petroleum and other chemical transport pipelines, can be of value in the detection of minor flaws, allowing them to be repaired and preventing them from growing into potentially harmful ruptures. 
   One current method for non-invasive analysis of materials and/or components for defects includes treating the material or component with a dye penetrant such that the dye enters any crack or defect that may exist. The component is then cleaned and then treated with a powder that causes the dye remaining in the defects to wick into powder. Next, ultraviolet light is applied to the material or component causing the residual dye remaining in any cracks or defects to fluoresce. This technique has drawbacks however. The dye sometimes is not suitable to identify cracks that located in areas other than the surface of the component. In addition, this technique is can be operator dependent in that the person performing this technique should be adequately trained and skilled. 
   Other methods currently utilized for the non-invasive analysis and inspection of materials and components include use of an electromagnetic current and use of thermal imaging including ultrasonic excitation or ultrasonic thermography. 
   The non-invasive analysis method of using an electromagnetic current is carried out by employing an electromagnetic coil to induce eddy currents in the test material or component. The current pattern changes at the location of a defect or crack. This technique requires point by point inspection, which can be labor intensive and is to some extent limited to only specific types of defects. In addition, the evaluator must be properly trained and skilled. 
   Ultrasonic thermography is a non-invasive analysis method by which a component, material, or structure, or a portion thereof, is excited with an ultrasonic pulse using an ultrasonic transducer pressed against the surface of the test subject. The resulting mechanical vibration of the subject under test tends to feature differential motion across the face of any defects that may be present, producing friction and causing the defects to heat up sharply, while defect-free areas of the test subject tend to be only minimally and uniformly heated by the vibration. Heat diffusing to the surface from a defect within the volume of a test subject causes a transient local surface temperature increase that can be detected as a bright spot in an image captured shortly after the ultrasonic pulse using, for example, an infrared camera. This ultrasonic thermography technique can identify disbonds, delaminations, cracks, corrosion, embedded contaminants, inclusions, voids, and other types of defects within a broad range of metallic, fiber reinforced plastic, composite, and other structural materials used in the transportation, construction, and other manufacturing industries. It is to be understood that finding defects is not the same as correcting them; ultrasonic thermography as applied herein is primarily a tool for detection. 
   Ultrasonic thermography has proven successful for detecting defects in materials and components in research, production, and operational maintenance environments. Some presently available analysis systems employing ultrasonic thermography techniques have drawbacks and limitations, however. For example, some presently available ultrasonic thermography systems can analyze only small specimens, and are generally restricted to laboratory use. Some other such systems require manual placement, orientation, and application of force between the transducer and the test surface, which requirements can degrade the repeatability, accuracy, and noninvasive nature of the technique. That is, too much or too little pressure applied to the transducer during a pulse may degrade the repeatability of detection, while misalignment of the transducer can cause the part or surface being inspected to be cut or burned, or can cause damage to surface finishes. Such systems are thus not always well suited for field inspection of materials or components, or for inspection of large objects, such as fuselages and flight control structures of in-service airplanes. 
   Accordingly, it is desirable to provide an apparatus and method for detecting defects of multiple types in both metal and composite structures. It is also desirable to provide an apparatus and method for effectuating the quick and efficient inspection and analysis of large components and/or large amounts of materials, such as entire airplane fuselages and structures, in real time. It is further desirable to provide a repeatable analysis apparatus and method using ultrasonic thermography to effectuate inspection of large components or areas to detect disbonds, delaminations, cracks, corrosion, embedded contaminants, inclusions, voids, and other defect types. It is desirable as well that an ultrasonic thermography apparatus should be readily transportable, usable under field as well as laboratory conditions, and usable by a small team of operators. 
   SUMMARY OF THE INVENTION 
   The foregoing needs are met, at least in part, by the present invention, where, in one aspect, a portable thermal imaging analysis apparatus for analyzing a specimen having a surface comprises a base framework that removably attaches to the specimen, a frame that slideably attaches to the base framework, a sound source that mounts to the frame and couples its energy to the specimen, a thermal imaging camera directed toward the specimen, and a controller connected to the sound source and the thermal imaging camera. 
   In accordance with another aspect of the present invention, a portable thermal imaging analysis apparatus for analyzing a specimen having a surface comprises means for attaching a thermal imaging analysis apparatus to a specimen, means for moving a thermal imaging analysis apparatus across a region of a specimen, means for generating an acoustic signal with energy content along a motional axis perpendicular to a surface of a specimen, means for detecting thermal response to stimulation by the generating means, and means for controlling the generating means and the detecting means. 
   In accordance with yet another aspect of the present invention, a method for portable thermal imaging analysis of a specimen having a surface comprises the steps of attaching a thermal imaging apparatus to a specimen, repositioning a thermal imaging apparatus at a multiplicity of sites across a region of a specimen, generating an acoustical signal with energy content along a motional axis generally perpendicular to the surface of the specimen, detecting thermal response to stimulation by the acoustical signal, and controlling the acoustical signal generation and image detection operations. 
   In accordance with still another aspect of the present invention, a portable thermal imaging analysis apparatus for analyzing a specimen having a surface comprises a base framework that removably attaches to the specimen, a sound source that mounts to the frame and couples acoustical energy into the specimen, wherein the acoustical energy is characterized by a principal frequency that changes with time, a thermal imaging camera that captures infrared images of the specimen, and a controller connected to the sound source and the thermal imaging camera. 
   In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
   As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of an ultrasonic thermography inspection apparatus in accordance with a preferred embodiment of the present invention, showing a partially perspective view and a partially block diagram view. 
       FIG. 2  is a schematic view of a pneumatic assembly utilized in a preferred embodiment of the present invention. 
       FIG. 3  is an oblique view of a preferred embodiment of the present invention, incorporating the apparatus of  FIG. 1  and resting on a novel base that permits x-axis translation. 
       FIG. 4  is an oblique view of a motorized version of the manually positioned apparatus of  FIG. 2 . 
       FIG. 5  is a time vs. frequency plot of two waveforms for different embodiments of the test apparatus. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides a translation frame to increase the speed for survey use of an ultrasonic thermography apparatus in the analysis and inspection of the structural integrity of metal and composite structures. The preferred embodiment is particularly suitable for use with airplanes and is suitable for inspecting subassemblies and in-service airplanes for defects such as disbonds, delaminations, cracks, corrosion, embedded contaminants, inclusions, and voids. It should be understood, however, that the present invention is not limited in its use with aerostructures, being usable, for example, for the inspection and analysis of pipelines, railroad rolling stock, architectural and civil engineering construction, and other structures where human safety or costly failure is a concern. 
   Detailed description of the operation of the mechanism within the frame referenced herein is contained in patent application Ser. No. 10/324,014, Ultrasonic Thermography Inspection Method and Apparatus, filed Dec. 20, 2002, incorporated herein in its entirety by reference. 
   Referring now to the figures, wherein like reference numerals indicate like elements,  FIG. 1  shows a partial perspective view of an ultrasonic thermography inspection apparatus  10 . The ultrasonic thermography inspection apparatus  10  includes a frame  12  having multiple frame members  20 – 54 , a biasing element  13 , a sound source, preferably an ultrasonic transducer  14  connected to an actuator  15 , a thermal imaging camera  16 , and a system controller  17 . The apparatus  10  further includes a vacuum attachment assembly that includes four vacuum cups  108  (See  FIG. 2 ) that are attached in the present inventive apparatus to the base framework  102 . The system controller  17  can have an associated display  19 . 
   As depicted in  FIG. 1 , the frame  12  preferably includes four vertical frame members  20 ;  22 ,  24  and  26  to which the vacuum cups  18   a ,  18   b ,  18   c    18   d  are attached. The frame  12  additionally includes lower transverse frame members  28 ,  30 ,  32 ,  34 . Lower transverse frame member  28  extends between and is attached to vertical frame members  26  and  20 . Lower transverse frame member  30  extends between and is attached to vertical frame members  20  and  22 . Lower transverse frame member  32  extends between and is attached to vertical frame members  22  and  24 . And lower transverse frame member  34  extends between and is attached to vertical frame members  24  and  26 . The frame  12  also includes upper transverse frame members  36 ,  38 ,  40 ,  42  that are coupled to and extend between the vertical frame members  20 ,  22 ,  24 ,  26 . Upper transverse frame member  36  extends between and is attached to vertical frame members  20  and  26 . Upper transverse frame member  38  extends between and is attached to vertical frame members  20  and  22 . Upper transverse frame member  40  extends between and is attached to vertical frame members  22  and  24 . Upper transverse frame member  42  extends between and is attached to vertical frame members  24  and  26 . 
   The frame  12  of the apparatus  10  additionally includes a first cross bar  44  that is attached to and extends between vertical frame members  22  and  24  and a second cross bar  46  that is attached to and extends between vertical frame members  20  and  26 . The frame  12  has a first slider bar  48  having a first end  50  slidably coupled to the first cross bar  44  and a second end  52  slidably coupled to the second cross bar  46  such that it can translate back and forth between vertical frame members  20 ,  22  and members  24 ,  26 . The frame  12  also has a second slider bar  54  having a first end  56  slidably coupled to the first cross bar  44  and a second end  58  slidably coupled to the second cross bar  46  such that it can translate back and forth between vertical frame members  20 ,  22  and members  24 ,  26 . 
   The slider bars  48 ,  54  are preferably coupled to the cross bars  44 ,  46  via linear bearings  60  or other suitable slidable coupling means known in the art that can enable it to slide along cross bars  44  and  46 . 
   The actuator  15  is attached to a vertical holder bar  62  that is slidably coupled to the second slider bar  54 . The vertical holder bar  62  is preferably rigidly attached to the holder bar  62  via mechanical attachment means such as bolt and/or clamp. Alternatively, the vertical holder may be coupled to the slider bar  54  via linear bearing or other suitable slidable coupling means known in the art such that it can translate between cross bars  44  and  46 . Similarly, as depicted in  FIG. 1 , the thermal imaging camera  16  is adjustably mounted to the second vertical holder bar  68 . The vertical holder bar  68  is rigidly attached to the holder bar  62  via mechanical attachment such as a bolt and/or clamp first slider bar  48 . Alternatively, the vertical holder  68  may be coupled to the slider bar  48  via linear bearing or other suitable slideable coupling means known in the art such that it can translate between cross bars  44  and  46 . 
   Referring now to  FIGS. 1 and 2 , the sound source  14  is preferably an ultrasonic transducer attached to the actuator  15 . The transducer  14  preferably includes a piezoelectric element that generates ultrasonic energy within a desired ultrasonic or sonic frequency band for a certain length of time. The transducer  14  can be any transducer capable of generating ultrasonic energy preferably at varying ultrasonic frequencies, power levels and pulse durations. As depicted in  FIG. 1 , the biasing element  13  is preferably a spring that attaches to and extends between one of the upper frame members  36 ,  38 ,  40 ,  42  and the ultrasonic transducer  14 . The spring  13  biases the transducer  14  in the downward direction or in the direction of the specimen to be analyzed. The spring  13  provides adjustable pre-load force between the ultrasonic transducer  14  and the specimen during attachment of the apparatus to the specimen. The pre-load force provided by the spring  13  is less than the suctioned force generated by the vacuum cups  18   a ,  18   b ,  18   c ,  18   d  allowing the transducer to translate in the upward direction or direction away from the specimen during attachment. 
   As depicted in  FIG. 1 , the ultrasonic energy from the transducer  14  can be coupled to a specimen, part or area to be tested through a coupler  66 . The coupler  66  is a mechanical contact that is in contact with both the ultrasonic transducer  14  and the specimen. The coupler  66  is preferably a thin piece of soft metal, such as copper, that effectively couples the ultrasonic energy to the specimen. Alternatively, other materials known in the art other than copper may be used, for example, any material that is soft and malleable that can be deformed against the end of the transducer and prevent the transducer from bouncing and/or sliding along the specimen during operation. Alternatively, various applications, such as the analysis of composite materials, may not require the use of a coupler  66 . 
   The actuator  15  is preferably a linear stroke, double action pneumatic actuator that functions to translate the ultrasonic transducer  14  in a generally upward and downward direction with respect to the specimen. The pneumatic actuator  15  can be powered by compressed air and can translate the ultrasonic transducer  14  such that the transducer  14  preferably applies approximately 10 lbs. to approximately 25 lbs. of force on the test surface or test specimen. More preferably, the actuator  15  exerts approximately 15 lbs. to approximately 20 lbs. of force. Use of the actuator  15  enables the analysis method herein described to be repeated and provides a consistent placement of the transducer  14  against the specimen and/or coupler  66 . 
   The actuator  15  can be any actuator suitable for the purposes described herein. For example, actuation need not be pneumatic. While a representative pneumatic actuator may use an external air supply, a pressure regulator, and a fixed piston size to establish a fixed application force, an electrical actuator, driven by a motor and gear reducer, for example, can apply force, a strain gauge-based sensor can provide feedback, and a control circuit can regulate the applied force at the correct level. 
   Referring now to  FIG. 2 , a pneumatic assembly  100  employed in an embodiment of the present invention to power the pneumatic actuator  15  and vacuum cups  18   a ,  18   b ,  18   c ,  18   d  is schematically depicted. The assembly  100  is preferably a parallel system that includes the actuator system, generally designated  102 , and an attachment assembly, generally designated  104 . Each system,  102 ,  104  has a common compressed or pressured air inlet  106  where the pressurized air enters the pneumatic assembly  100 . 
   As depicted in  FIG. 2 , the actuator system  102  has an air inlet conduit  108  for carrying pressurized air from the inlet  106  to the actuator system  102 . The actuator system  102  further includes air conduits  109  and  110  which function to carry pressurized air to and from the actuator  15  during operation. The actuator system  102  also includes an air regulator  112 , a controller  114 , a first air control valve  116  and second air control valve  118 . 
   The controller  114  is preferably a two position, four way spool valve that is toggle activated that controls the up and down movements of the actuator  15 . Alternative controllers may also be utilized. When the controller  114  is in a first position, it pressurizes the air conduit  109  while it vents air conduit  110 , causing the actuator  15  to translate the transducer  14  in the direction towards the specimen and contact the specimen. In this position, the first air control valve  116  functions to adjust the air pressure provided to the actuator  15 , controlling the movement of the actuator  15  and transducer  14  toward the specimen. It also senses the pressure being exerted by the transducer  14  on the specimen and regulates the pressure being applied to specimen by the ultrasonic transducer  14 . 
   Alternatively, when the controller  114  is in a second position, it pressurizes the air conduit  110  while it vents air conduit  109 , causing the actuator  15  to translate the transducer  14  in the upward direction away from the specimen. In this position, the second air control valve  118  functions to adjust the air pressure provided to the actuator, controlling the movement of the actuator  15  and transducer  14  away from the specimen. 
   The air regulator  112 , controller  114 , first control valve  116  and second control valve  118  combine to control the amount of compressed air powering the actuator  15 , in turn controlling the translation of the ultrasonic actuator  14 , thereby controlling the distance at which the ultrasonic transducer  14  is positioned with respect to the specimen. In addition, the aforementioned components also combine to control the amount of pressure applied to the specimen by the transducer  14  via the actuator  15 . The apparatus  10  is generally preferably arranged so that the actuator  15  moves the transducer  14  vertically downwards toward the specimen. However, other orientations are possible. 
   The preferred force level is a function of the properties of the ultrasonic transducer  14  and the physical properties of the expected test specimens. For typical transducers  14  capable of coupling power at levels on the order of a kilowatt at twenty kilohertz, and for typical aircraft production materials, for example, it has been demonstrated that twenty pounds of force is effective to fully couple the excitation into the specimen, both preventing the transducer  14  from lifting off during its stroke and allowing essentially full excursion of the transducer  14 . Force levels significantly below about 10 pounds have been shown to allow the transducer  14  to lift free of the specimen, causing impacts that may damage the specimen and may produce anomalous out-of-band signal content. Force levels significantly above about 30 pounds tend to suppress the operation of the transducer  14 , potentially preventing detection of significant defects. 
   It is to be understood from this discussion that the appropriate actuator  15  force level for various styles of transducers  14  may differ, and that some styles of ultrasonic transducers  14  may be better suited or entirely unsuited to apparatus of the type described herein. This may be determined by, among other issues, contact face size and the ability of the transducer  14  to maintain oscillation when coupled to the types of specimens for which the preferred embodiment is intended. It is to be further understood that testing of other materials, such as those that may be more highly internally damped, more massive, more brittle, or otherwise significantly different from typical aerostructures materials, may require different transducers  14  or different coupling pressures and methods for successful outcomes. 
   As illustrated in  FIG. 2 , the attachment assembly  104  is a vacuum attachment assembly having a venturi  120  coupled to the air inlet  106 . The venturi  120  includes an exhaust muffler  122 . The attachment assembly  104  also includes a vacuum conduit  124 , a vacuum switch  126  and a two-way splitter  128  that splits the vacuum conduit  126  into two vacuum cup series, generally designated  130  and  132 . Each of the series  130 ,  132  include two of the vacuum cups  18   a ,  18   b ,  18   c ,  18   d . Alternative embodiments covered by the present invention may include more or less vacuum cups coupled to one another in multiple series or in a single series. 
   Each vacuum cup series  130 ,  132  includes a vacuum air conduit  134 , a two-way splitter  136 , a pair of check valves  138  and a pair of air conduits  140 . The conduit  134  provides the vacuum suction to the two-way splitter  136  which splits the conduit and provides suction the assemblies&#39;  130 ,  132  and their respective vacuum cups via the conduits  140  and the check valves  138 . The check valves  138  allow vacuum air to be provided to individual vacuum cups  18   a ,  18   b ,  18   c ,  18   d  or removed from individual vacuum cups  18   a ,  18   b ,  18   c ,  18   d  whereas the switch  126  functions to turn the vacuum air “on” or “off” in the entire attachment assembly  104 . 
   During operation, compressed air from the inlet  106  enters the venturi  120  and exits the exhaust  122 , creating a vacuum. When the vacuum switch  126  is in the “on” position, vacuum air is provided to the vacuum cups  18   a ,  18   b ,  18   c ,  18   d  via the above described conduits, splitters and check valve components, enabling the apparatus  10  to be attached to the specimen, part and/or component to be analyzed. Alternatively, when the vacuum switch  126  is in the “off” position, vacuum air is not provided to the vacuum cups  18   a ,  18   b ,  18   c ,  18   d  and the apparatus cannot be attached to the specimen, etc. 
   As illustrated in  FIG. 1 , the thermal imaging camera  16  is preferably an infrared camera that generates images of the specimen, part or area being tested in association with ultrasonic excitations of the test specimen, part or area. In addition, the infrared camera  16  can convert the heat energy detected to grayscale information if desired. The images and grayscale information are preferably displayed on the image display  19 . The image information is captured preferably by using a digital storage unit  21 , located within the apparatus  10  or located remotely and accessed electronically. Preferably, digital storage unit  21  allows for the infrared images to be recorded digitally and/or by standard video, depending upon application and the size of the defect. 
   Alternatively, the image data and grayscale data may stored in databases located within the ultrasonic thermography inspection apparatus  10 , located within storage media or located remotely. The remotely located databases can be accessible by corded and/or wireless communication including the Internet, Ethernet, or other remote memory storage facility. The storage media upon which the image and grayscale information is stored can include, but is not limited to, floppy disc (including ZIP); tape drive cartridge (such as DAT); optical media (such as CD-ROM, DVD-ROM, etc.); flash memory (such as smart media, compact flash, PC card memory, memory sticks, flash SIMMs and DIMMS, etc.); magnetic based media, magneto optical; USB drives; Nanotechnology memory or any other storage media that an operator can store or retrieve information from it. A person skilled in the art will recognize that any suitable storage media can be used. 
   As previously described, the infrared camera  16  is mounted to the vertical holder bar  64  that extends from the slider bar  48 . The infrared camera  16  is preferably mounted to the vertical holder bar  64  via linear bearing or other suitable coupling means known in the art such that the camera  16 , enabling it to be positioned at varying distances from the specimen, test part or area. The aforementioned positioning of the infrared camera allows the camera  16  to move along a X, Y and Z axis, allowing the camera to be positioned at multiple locations with respect to the part or area being tested. 
   The system controller  17  may be disposed within the frame  12  where it directly communicates with the ultrasonic transducer  14  and thermal imaging camera  16  via wires. The system controller  17  can be any computer suitable for carrying out the analysis process described herein. Alternatively, the controller  17 , may be remotely located away from the apparatus  10  and communicate in either a corded or wireless fashion with the camera  16  and ultrasonic transducer  14 . Similarly, the image and grayscale data may be stored locally on the unit on a hard drive, compact disc, other storage media or may be stored remotely via corded or wireless communication. 
   During operation, analysis and/or inspection is initiated by first attaching the ultrasonic thermography apparatus  10  to the specimen to be analyzed by switching the previously described vacuum switch  126  “on,” activating the attachment apparatus  104 . Next, a baseline image of the test area is taken by the infrared camera  16  and stored in the digital storage unit  21  or other storage media previously described, providing a reference point for analysis. The ultrasonic transducer  14  is then positioned and adjusted such that it is held in contact with the test surface using the pneumatic actuator  15  and controller  114 , providing proper pressure between the specimen and/or coupler  66  and the transducer  14 . 
   Also during operation, the system controller  17  provides timing between the transducer  14  and the infrared camera  16 . Once the analysis process is initiated, the controller  17  causes the camera  16  to begin taking sequential images of the specimen, test part or area at a predetermined rate. Once the image sequence begins, the controller  17  sends a signal to the transducer  14  to generate the ultrasonic signal. The ultrasonic energy is in the form of a pulse at a predetermined frequency. The pulse time periods and frequencies and input power of the apparatus  10  can vary depending on the apparatus being used and the composition of the area or part being tested. 
   Upon application of the ultrasonic energy, the specimen becomes “excited” and the areas of the test area that contain defects vibrate with greater amplitude and cause the surface to heat up, which is detected by the camera  16  and can be viewed on the display and/or stored. The camera  16  may also convert the heat energy images is to grayscale information. The grayscale information is then sent to the display and/or stored for later review and analysis. If the grayscale level of the test area during the application of ultrasonic energy exceeds the baseline threshold level previously recorded, the peak store image capture unit records and retains this new level on the image display, immediately notifying the operator of the existence of defects. The above-described process may now be repeated on the same area or the apparatus may be transferred to a new area to be tested. 
     FIG. 3  illustrates a frame  100  equivalent to that of  FIG. 1  carried on a base  102 , the latter comprising longitudinal guide rails  104  and cross members  106 .  FIG. 2  further shows vacuum cups  108  located on adjustable legs  110  on the base  102 , where the vacuum cups  108  can attach to a test specimen, while the interface between the frame  100  and the base  102  can be comprised of low-friction sliding fittings  112 , such as linear bearings sized and configured to move smoothly along the longitudinal guide rails  104 . A clamp, such as those integral with each of the sliding fittings  112  in the exemplary embodiment, can be actuated to hold the frame at any preferred location along the longitudinal guide rails  104 . 
     FIG. 4  illustrates an apparatus  116  similar to that of  FIG. 3 , but with a positioning motor  118  to provide torque, a drive screw  120  to translate torque to linear force, a ball nut  122  to couple the linear force to the frame, an end bearing  124  to contain the drive screw  120 , and a turns counter  126  to sense position in turns of the drive screw  120 . This configuration permits a single operator to command the apparatus  116  to reposition the test frame  128  remotely, while remaining at a viewing console throughout a series of test operations. Positioning motor  118  design may be such as to hold the frame  100  in place when deenergized, to use the reduction ratio in an integral or external gearbox (not shown) to perform the holding function, to use an external braking device similar to that of the apparatus of  FIG. 3 , or an equivalent function to permit the frame  100  to remain fixed during a test event for any orientation of the base  102 . Other positive positioning technologies include chain and rack-and-pinion drives. 
   Contemporary applications for the apparatus call for controlled positioning along a single axis perpendicular to the head pressurization axis, with the third perpendicular axis fixed. This allows a lightweight system to be assembled that permits several tests to be performed between repositionings of the apparatus. An apparatus that can be positioned or can position itself over both of the axes perpendicular to the head pressurization axis may be of use in examining large, nearly flat surfaces such as aircraft wings, architectural construction panels, and the like. Such an apparatus has a second base framework interposed between the head assembly and the vacuum pads, with guide rails perpendicular to those of the guide rails of a single-axis apparatus. 
   It is to be understood that the spatial orientation of the exemplary apparatus described herein may be any orientation without significant effect on function. For example, in performing examination of an aircraft fuselage, the exemplary apparatus may be attached to the top, the bottom, and all other possible regions of the subject fuselage, with the transducer pointed generally toward the central axis of the fuselage. Thus the actuator  15  mechanism will press the transducer  14  downward in some orientations, upward in others, and at any intermediate angle in some application. It has been shown that the low mass of an exemplary ultrasonic transducer  14  and the high retention force of a multiplicity of vacuum cups  108  allow an actuator  15  to provide a substantially invariant force level at all orientations of the exemplary apparatus, affording relatively uniform imaging performance. 
   For applications such as pipelines, a configuration with straight guide rails as shown herein in the flow direction and curved guide rails in the circumferential direction may be appropriate if for example many lengths of pipe of a single diameter are to be examined. For such an application, helical frame advancement may be preferable, as may a self-contained power system and wireless communication. 
   Similarly, positioning by systems other than a linear guide frame adhered to the test object with vacuum pads may be preferable, including, for example, polar instead of rectangular coordinate positioning (i.e., r-θ instead of x-y), and multiple-degree-of-freedom robotic arm positioning with the transducer/imager assembly serving as the so-called end effector of the robotic arm. 
     FIG. 5  shows a constant-frequency waveform  150  and a swept-frequency waveform  152  for two embodiments of the preferred apparatus. The constant-frequency waveform  150  is relatively straightforward to generate at the high signal level desirable for the preferred apparatus and reasonably effective for detection. The waveform  150  may be realized using, for example, a quartz crystal of appropriate dimensions, excited by an electronic drive circuit at its working frequency. The swept-frequency waveform  152  is readily produced at low power levels, although it remains comparatively costly at high power levels at the current state of the technology for ultrasonic transducers. 
   The swept-frequency transducer has the advantage of providing in the test subject excitation of defects that resonate at many frequencies, the effect of which is to cause appreciable increase in the induced motion within defects of many sizes of interest. The rapid heating of defects caused by this phenomenon may increase detection speed or likelihood, particularly at greater depths, where detection is otherwise less assured. As an alternative to a swept frequency transducer, a multiplicity of transducers can be used that interoperate to excite at multiple frequencies over a range, so that the resonant frequencies of defects of many sizes are approximated, with an effect that may be intermediate between that of a single-frequency transducer and that of a swept frequency transducer. 
   The many features and advantages of the invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and, accordingly, all suitable modifications and equivalents may be resorted to that fall within the scope of the invention.