Abstract:
Improved apparatus, systems, and methods for inspecting a structure are provided that use a probe with sled appendages and an axial braking system. The probe uses pulse echo ultrasonic signals to inspect the structure. The sled appendages permit the probe to contact and ride along the surface of the structure and are rotatably connected and curved away from the surface of the structure to compensate for contoured surfaces and inspection around holes and edges. The axial braking system, in coordination with a motion control system moving the probe, fixes the positions of the sled appendages just before the probe travels over a hole or off an edge of the structure to prevent the probe from falling through the hole or off an edge and to permit the probe to return to the surface of the structure to continue inspection of the structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The contents of U.S. Pat. Nos. 6,722,202; 7,231,826; application Ser. No. 10/943,088, entitled “Magnetically Attracted Inspecting Apparatus and Method Using a Ball Bearing,” filed Sep. 16, 2004; application Ser. No. 10/943,135, entitled “Magnetically Attracted Inspecting Apparatus and Method Using a Fluid Bearing,” filed Sep. 16, 2004; and application Ser. No. 11/178,584, entitled “Ultrasonic Inspection Apparatus, System, and Method,” filed Jul. 11, 2005, are incorporated by reference. 
   FIELD OF THE INVENTION 
   The present invention relates generally to an apparatus, system, and method for inspecting a structure and, more particularly, to an apparatus, system, and method for non-destructive pulse echo ultrasonic inspection of a structure and inspection near holes and edges of the structure. 
   BACKGROUND 
   Non-destructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is typically preferred to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or defects (flaws) in the structure. Inspection may be performed during manufacturing or after the completed structure has been put into service, including field testing, to validate the integrity and fitness of the structure. In the field, access to interior surfaces of the structure is often restricted, requiring disassembly of the structure, introducing additional time and labor. 
   Among the structures that are routinely non-destructively tested are composite structures, such as composite sandwich structures and other adhesive bonded panels and assemblies and structures with contoured surfaces. These composite structures, and a shift toward lightweight composite and bonded materials such as using graphite materials, dictate that devices and processes are available to ensure structural integrity, production quality, and life-cycle support for safe and reliable use. As such, it is frequently desirable to inspect structures to identify any defects, such as cracks, discontinuities, voids, or porosity, which could adversely affect the performance of the structure. For example, typical defects in composite sandwich structures, generally made of one or more layers of lightweight honeycomb or foam core material with composite or metal skins bonded to each side of the core, include disbonds which occur at the interfaces between the core and the skin or between the core and a buried septum. 
   Various types of sensors may be used to perform non-destructive inspection. One or more sensors may move over the portion of the structure to be examined, and receive data regarding the structure. For example, a pulse-echo (PE), through transmission (TT), or shear wave sensor may be used to obtain ultrasonic data, such as for thickness gauging, detection of laminar defects and porosity, and/or crack detection in the structure. Resonance, pulse echo or mechanical impedance sensors are typically used to provide indications of voids or porosity, such as in adhesive bondlines of the structure. High resolution inspection of aircraft structure is commonly performed using semi-automated ultrasonic testing (UT) to provide a plan view image of the part or structure under inspection. While solid laminates and some composite structures are commonly inspected using one-sided pulse echo ultrasonic (PEU) testing, composite sandwich structures are commonly inspected using through-transmission ultrasonic (TTU) testing for high resolution inspection. In through-transmission ultrasonic inspection, ultrasonic sensors such as transducers, or a transducer and a receiver sensor, are positioned facing the other but contacting opposite sides of the structure. An ultrasonic signal is transmitted by at least one transducer, propagated through the structure, and received by the other transducer. Data acquired by sensors is typically processed and then presented to a user via a display as a graph of amplitude of the received signal. To increase the rate at which the inspection of a structure is conducted, a scanning system may include arrays of inspection sensors, i.e., arrays of transmitters and/or detectors. As such, the inspection of the structure can proceed more rapidly and efficiently, thereby reducing the costs associated with the inspection. However, it has traditionally not always been possible to perform continuous scanning of a structure with holes and off the edges of the structure. For example, inspection probes which contact and ride along the surface of the structure under inspection and are typically supported against the structure by the pull of gravity or by pressure exerted by a motion control system, referred to as part-riding probes, may fall through a hole in a structure or off the edge of the structure. Although a structure can be inspected in a manner to scan around holes, a second inspection method typically must be performed for inspecting the edges of the structure and edges defining holes in the structure. For example, a technician can manually scan around the edges of the structure and the edges of holes in a structure using a pulse-echo or through transmission ultrasonic hand probe. 
   Non-destructive inspection may be performed manually by technicians who typically move an appropriate sensor over the structure. Manual scanning requires a trained technician to move the sensor over all portions of the structure needing inspection. While manual scanning may be required around the edges of the structure and the edges of holes in a structure, manual scanning may also be employed for scanning the remainder of the structure. 
   Semi-automated inspection systems have been developed to overcome some of the shortcomings with manual inspection techniques. For example, the Mobile Automated Scanner (MAUS®) system is a mobile scanning system that generally employs a fixed frame and one or more automated scanning heads typically adapted for ultrasonic inspection. A MAUS system may be used with pulse-echo, shear wave, and through-transmission sensors. The fixed frame may be attached to a surface of a structure to be inspected by vacuum suction cups, magnets, or like affixation methods. Smaller MAUS systems may be portable units manually moved over the surface of a structure by a technician. However, for through-transmission ultrasonic inspection, a semi-automated inspection system requires access to both sides or surfaces of a structure which, at least in some circumstances, will be problematic, if not impossible, particularly for semi-automated systems that use a fixed frame for control of automated scan heads. 
   Automated inspection systems have also been developed to overcome the myriad of shortcomings with manual inspection techniques. For single sided inspection methods, such as pulse echo ultrasonic inspection, a single-arm robotic device, such as an R-2000iA™ series six-axis robot from FANUC Robotics of Rochester Hills, Mich., or an IRB 6600 robot from ABB Ltd. of Zurich, Switzerland, may be used to position and move a pulse echo ultrasonic inspection device. For through transmission inspection, a device such as the Automated Ultrasonic Scanning System (AUSS®) system may be used. The AUSS system has two robotically controlled probe arms that can be positioned proximate the opposed surfaces of the structure undergoing inspection with one probe arm moving an ultrasonic transmitter along one surface of the structure, and the other probe arm correspondingly moving an ultrasonic receiver along the opposed surface of the structure. Conventional automated scanning systems, such as the AUSS-X system, therefore require access to both sides or surfaces of a structure for through transmission inspection which, at least in some circumstances, will be problematic, if not impossible, particularly for very large or small structures. To maintain the transmitter and receiver in proper alignment and spacing with one another and with the structure undergoing inspection, the AUSS-X system has a complex positioning system that provides motion control in ten axes. The AUSS system can also perform pulse echo inspections, and simultaneous dual frequency inspections. 
   Many structures, however, incorporate holes through which a part-riding probe may fall through and edges over which a part-riding probe may fall off. Further, most structures require inspection of edges around the structure and defining holes in the structure. Accordingly, improved apparatus, systems, and methods for inspecting structures with holes and inspecting structures at edges are desired. 
   SUMMARY OF THE INVENTION 
   The present invention provides an improved apparatus, systems, and methods for inspecting a structure using an inspection probe that includes sled-like appendages, referred to herein as sled appendages or sleds, an axial braking system and a probe extension braking system. Inspection probes according to the present invention may be used in conjunction with a motion control system that both moves the probe over the structure for inspection and operates with the axial and extension braking systems for when the probe travels over holes or off edges of the structure. An inspection probe may also be used with an extension coupling device between the motion control system and the probe to press the probe against the structure for adjusting to changes in surface contours of the structure, rather than requiring the motion control system to make detailed changes in orientation and movement of the probe to adjust to changes in surface contours. Either the motion control system or a separate device, such as an extension coupling device, would be used to press the inspection probe against the structure so the inspection probe will ride across the structure on the sled appendages. Embodiments of the present invention combine the physical structure of the sled appendages with the axial braking system to fix the position of the sled appendages for traveling over holes or off an edge of the structure, including large holes or cut-outs in the structure which are also referred to herein as holes. Embodiments of the present invention can be used for various inspection applications but are particularly useful for inspection of structures that include holes and require inspection of the edges around the structure or defining a hole or have contoured surfaces. A probe will include one or more sensors, typically pulse echo ultrasonic transducers, possibly defining an array of pulse echo ultrasonic transducers. Such devices can be used for high resolution defect detection in structures of varying shapes and sizes. Embodiments of apparatus, systems, and methods of the present invention can be used for inspection of structures during manufacture or in-service. Further, embodiments of the present invention provide new inspection capabilities for non-destructive inspection of large and small structures, particularly including the edges of structures and structures with holes. 
   Embodiments of apparatus, systems, and methods of the present invention typically operate in array modes using an array of pulse echo ultrasonic transducers, thereby increasing inspection speed and efficiency while reducing cost. Apparatus, systems, and methods of the present invention are also capable of operating with a single or a plurality of pulse echo ultrasonic transducers. 
   For continuous scanning applications, embodiments of apparatus, systems, and methods of the present invention permit the probe to contact and ride along the surface of the structure using one or more sled appendages, thereby reducing the necessary sophistication of a motion control system that is typically required by conventional scanning systems to maintain the probe in a predefined orientation and predefined position with respect to the surface of the structure. By allowing the probe to ride across the structure, the motion control system, or a separate device such as an extension coupler, only needs to press the probe against the structure, but does not need to know the surface contours of the structure because the act of pressing the probe against the surface combined with the sled appendages having freedom of motion and the axial motion of the probe compensate for surface contours. In addition to sled appendages, the probe may also use contact members to support the probes against the respective surfaces of the structure, such as roller bearings along the bottom of the sled appendages. The sled appendages are rotatably connected to permit freedom of motion of the sled appendages for riding along contoured surfaces. Contact with the surface ensures consistent orientation of transducers with respect to the structure for pulse echo ultrasonic inspection. Contact with the surface also permits accurate position measurement of the inspection device during continuous scanning, such as keeping an optical or positional encoder in physical and/or visual contact with the surface of the structure under inspection. Contact with the surface also permits the probe to disperse a couplant between the surface of the structure and the pulse echo ultrasonic transducers. Where a couplant is used, a probe may also include a bubbler shoe that disperses the couplant around each pulse echo ultrasonic transducer to independently couple the signal from each transducer to the surface of the part. By individually coupling each transducer to the surface of the part, the bubbler shoe compensates for when the probe travels over a hole or off an edge of the structure where all of the transducers are not over the surface of the structure. In such a manner, only the probes over the hole or off the edge of the structure will lose the coupling with the surface, but the transducers remaining over the surface of the structure will continue to be independently coupled. 
   The axial and extension braking systems of a probe are used to fix the position of the sled appendages for traveling over holes or off an edge of the structure. Thus, for continuous scanning applications, the probe contacts and rides along the surface of the structure on the sled appendages, but as the probe approaches a hole or edge, the axial and extension braking systems, either using data of the hole and edge positions for the structure and the current location of the probe or using braking signals from a motion control system, fixes the current position of the sled appendages for traveling over the hole or off an edge and again contacting and riding along the surface of the structure after passing the hole or retracting from the edge at which time the axial braking system releases to permit the sled appendages to follow the contour of the surface of the structure. An axial braking system of an embodiment of a probe of the present invention can operate in more than one axis, and typically operates in two perpendicular axes referred to herein as the x-axis perpendicular to the distal length of the sled appendages to control the front-to-back tilt, or pitch, of the sled appendages and the y-axis parallel to the distal length of the sled appendages to control the side-to-side slant, or roll, of the sled appendages. 
   According to one aspect of the present invention, an apparatus, system, and method for non-destructive inspection of a structure includes a probe which is configured for traveling over a surface of the structure along sled appendages and using an axial braking system for traveling over holes and off edges of the structure. The probe includes at least one pulse echo ultrasonic transducer. A plurality of pulse echo ultrasonic transducers may be arranged in an array for faster and more complete scanning of the structure. If a couplant is used to couple the transducers to the surface of the structure, the probe may include a bubbler shoe to individually couple each transducer to the surface of the structure to prevent loss of coupling of transducers remaining over the surface of the structure when one or more transducers are over a hole or off an edge. The probe may also include a visual inspection sensor for providing position or optical information related to the location of the probe or transducers thereof. 
   According to another aspect of the present invention, a method may include providing a probe with at least one pulse echo ultrasonic transducer, at least one sled appendage for contacting a surface of a structure, and axial and extension braking systems; transmitting pulse echo ultrasonic signals from the transducer into the structure; receiving pulse echo ultrasonic signals at the transducer from the structure; and fixing the position of the sled for scanning a portion of the structure where only a portion of the probe is over the surface of the structure. 

   
     BRIEF DESCRIPTION OF THE DRAWING(S) 
       FIG. 1  is a schematic diagram of an embodiment of an inspection apparatus of the present invention. 
       FIG. 2  is another view of the schematic diagram of the inspection apparatus of  FIG. 1 . 
       FIG. 3A  is a schematic diagram of another embodiment of an inspection apparatus of the present invention. 
       FIG. 3B  is a top plan view of the inspection apparatus of  FIG. 3A . 
       FIG. 3C  is a top plan view of the bubbler shoe of the inspection apparatus of  FIG. 3A . 
       FIG. 4  is a cross-section of a schematic diagram of yet another embodiment of an inspection apparatus of the present invention. 
       FIG. 5  is a block diagram of an embodiment of an inspection system of the present invention. 
   

   DETAILED DESCRIPTION 
   The present invention will be described more fully with reference to the accompanying drawings. Some, but not all, embodiments of the invention are shown. The invention may be embodied in many different forms and should not be construed as limited to the described embodiments. Like numbers and variables refer to like elements and parameters throughout the drawings. 
   The term “holes” refers to holes of varying sizes in a structure, including features described as “cut-outs” in the structure. The term “edges” refers generally to the sides of the structure, but also includes reference to the perimeter of holes, particularly large holes or cut-outs through which a conventional part-riding probe might fall through. Thus, holes may be described as having edges, and the term edges is inclusive of both an external perimeter of a structure and perimeters of internal holes in the structure. Although being characteristically different, for purposes of the present invention holes and edges differ primarily by the manner in which a probe of the present invention operates near these features. For example, the probe typically travels over a hole or cut-out but travels off an edge of the structure, and possibly returning over the structure from an edge. Further, while in some instances in the description below using only one of the two terms holes and edges may be sufficient, typically both terms are used to emphasize that the described function or operation applies to both holes in the structure and edges of the structure, and not merely one of these features. 
   The term “rotatably” refers to a characteristic of angular motion in at least one plane, and typically only one plane as may be defined by a connection about an axis-line as described in the examples below. However, a rotatable connection may also be defined by a connection that provides angular motion in more than one plane, such as a ball-and-socket joint connection that permits motion of the joint without permitting rotation in at least one plane, such as to provide freedom of motion to pitch and roll, but not yaw. 
   The present invention provides apparatus and methods for an ultrasonic array probe for inspecting a structure while riding on a surface of the structure. The probe has the ability to travel over holes and off edges of the structure during inspection. Typically a probe according to the present invention would be moved over a structure by a motion control system, such as an R-2000iA™ series six-axis robot from FANUC Robotics, an IRB6600 robot from ABB, or similar automated robotic motion control system, and possibly also using an extension coupler to compensate for surface contours rather than requiring the motion control system to compensate for surface contours. An example motion control system with an extension coupler for manipulating an inspection apparatus of the present invention is described in application Ser. No. 11/178,584, entitled “Ultrasonic Inspection Apparatus, System, and Method,” which is incorporated by reference. The combination of sled appendages and an axial braking system provide the configuration for the probe to be able to travel over holes and off edges of the structure during inspection. By comparison, conventional part-riding probes, probes which contact and ride along the surface of the structure under inspection, may fall through a large hole or off the side of a part rather than having the ability to travel over holes and off the edge of a part for inspection. Using conventional part-riding probes, a structure typically is scanned in a manner to go around holes and to not inspect near edges, leaving the edges of the structure to be inspected by a second inspection method, such as by a technician using a manual pulse echo scanning device. Sled appendages, or sleds, of a probe according to the present invention are linear extensions rotatably attached to the bottom of the probe and upon which the probe rides over a surface of the structure. An axial braking system according to the present invention operates to temporarily fix the current positions of the sled appendages to maintain those positions while the probe travels over a hole or off an edge of the structure. An axial braking system may operate in one or more axes. For example, the braking system may lock simply in an x-axis, in both x- and y-axes, or in x-, y-, and z-axes. The axial braking system fixes the position of the sled appendages by locking the axes of motion of the sled appendages before traveling over a hole or off an edge of the structure. 
   Although in some instances the length of sled appendages may be sufficient to pass over a small hole without needing to use the axial braking system of the probe, the combination of sled appendages and axial braking system are generally provided and used for instances when the probe would otherwise fall through a large hole or off an edge of a structure like a conventional part-riding probe were it not for the operation of the axial braking system to maintain the position of the sled appendages while the probe moves over a hole or off an edge of the structure. Further, by using a probe according to the present invention, a motion control system does not need to maintain or know the precise shape or contour of the structure, but merely the location of holes and edges of the structure so the axial braking system can fix the position of the sled appendages before the probe is passed over a hole or off an edge of the part. Further, although the inspection apparatus described and depicted herein includes two sled appendages located on opposing sides of the inspection apparatus, and an inspection apparatus according to the present invention typically includes two sled appendages, an inspection apparatus of an embodiment of the present invention might include only a single sled appendage such as a sled appendage with a broad surface width for providing side-to-side balance to the inspection apparatus. Alternative embodiments of an inspection apparatus may include a plurality of sled appendages extending below the inspection apparatus and/or to the sides of the inspection apparatus. 
   A probe may also include a bubbler shoe. A bubbler shoe according to the present invention provides a couplant around each transducer for individually coupling each transducer of the probe that remain over the structure for inspection even when other transducers may be over holes or off an edge of the structure. By comparison, conventional coupling shoes typically provide a cavity that surrounds all of the transducers to act as a single couplant for all of the transducers. Thus, if a conventional probe travels over a large hole or off an edge of the part, the water cavity will empty and the ultrasonic signals of all of the transducers may be lost or will be degraded due to the lack of coupling between the structure and the transducers. However, when using a bubbler shoe of an embodiment of the present invention, only the transducers that are over the hole or off the edge of the structure may lose coupling for ultrasonic signals while the transducers remaining over the structure retain the coupling provided by the bubbler shoe. 
     FIGS. 1 and 2  are schematic diagrams of an embodiment of an inspection apparatus according to the present invention, also generally referred to as a probe or inspection probe. The inspection apparatus  10  includes two sled appendages  12 ,  13  located on opposite sides of the inspection apparatus  10 . The sled appendages  12 ,  13  are rotatably attached to a frame member  14  of the inspection apparatus  10  about a first axis  24  defining a first direction of motion for the sled appendages  12 ,  13 , also referred to as an x-axis, front-to-back tilt axis, or pitch axis. The frame of the inspection apparatus  10  also includes a second frame member  16  which is rotatably connected to the first frame member  14  about a second axis  26  defining a second direction of motion for the sled appendages  12 ,  13 , also referred to as a y-axis, side-to-side slant axis, or roll axis. By having two rotational axes, the sled appendages  12 ,  13  are capable of rotating in at least two directions of motion with respect to a motion control system connected to the inspection apparatus  10 , such as by way of an attachment at the opening  18  and securing screws  19 , to compensate for surface variations of the structure, such as shape and contour characteristics of the surface. Further, because as described below, a transducer holder or bubbler shoe for an inspection apparatus of the present invention is connected to sled appendages, rather than the frame, the transducers maintain the same position and orientation as achieved by the sled appendages, thereby providing the transducers a consistent orientation with respect to the surface of the structure over which the inspection apparatus rides on the sled appendages. Maintaining a consistent orientation, distance and angle, of the transducers with respect to the surface of the structure ensures consistent quality of inspection by the transducers. 
   At least one of the sled appendages  12 ,  13  includes an upper portion  22 ,  23  that functions as a stationary brake plate against which a brake disc  30  of the axial braking system can be applied to fix the position of the sled appendage about the first axis of motion  24 . An axial braking system of an embodiment of the present invention may also include a pneumatic brake cylinder  32  with an extendable piston arm  34  to which a brake disc  30  is attached at the distal end of the extendable piston arm  34  protruding from the brake cylinder  32 . A brake cylinder  32  may be activated by any conventional method, such as by compressing a fluid, typically air, through a supply line  38  into a valve  36  attached to the brake cylinder  32 . When the brake mechanism is activated, the compression of fluid causes a piston inside the brake cylinder  32  and attached to the distal end of the extendable piston arm  34  inside the brake cylinder  32  to force the extendable piston arm  34  out of the brake cylinder  32  to force the brake disc  30  to press against the stationary brake plate  22 ,  23  of one or more sled appendages  12 ,  13 . 
   To fix the position of the sled appendages in the second axis of motion  26 , a second brake plate  28  may be affixed to the first frame member  14  to permit a second brake mechanism  40 ,  42 ,  44 ,  46 ,  48 , to engage the second stationary brake plate  28  in the same manner that the first brake mechanism  30 ,  32 ,  34 ,  36 ,  38  engages the first stationary brake plate  22 ,  23  to fix the position of the sled appendages  12 ,  13  about the first axis of motion  24 . The first frame member  14  may include a vertical support member  15  connected to the second stationary brake plate  28  to provide stability between the first frame member  14  and the second stationary brake plate  28 , such as when a brake disc  40  is pressed against the second stationary brake plate  28  to fix the position of the sled appendages in the second axis of motion  26 . An axial braking system of an alternative embodiment may also include a brake mechanism in a third direction of motion, such as a vertical z-axis with respect to the surface of the structure, and may be incorporated into an attachment to a motion control system. 
   To improve braking capabilities of a braking system, brake discs and/or stationary brake plates may be coated with or include an attached layer of material, such as being coated with rubber, to cause increased friction between a brake disc and stationary brake plate for fixing the positions of sled appendages and preventing slippage of the positions of the sled appendages. 
   The inspection apparatus  10  includes at least one pulse echo ultrasonic transducer  50 . If not using a couplant between the transducers  50  of the inspection apparatus  10  and the structure, a transducer holder may be attached to the sled appendages  12 ,  13  to support the transducers  50 , such as supported in an array where a plurality of transducers are used to increase the inspection coverage area. As mentioned above, by attaching the transducer holder, or bubbler shoe as described below, to the sled appendages  12 ,  13  the transducer holder and transducers  50  supported thereby also maintain constant orientation with the surface of the structure over which the inspection apparatus  10  rides because the inspection apparatus  10  rides over the surface of the structure on the sled appendages  12 ,  13 . Because inspection of a structure typically requires ensuring that the transducers maintain constant orientation, distance and angle, with respect to the surface of the structure, attaching a transducer holder, or bubbler shoe, to sled appendages ensures that the transducer holder, or bubbler shoe, and transducers supported thereby also maintain constant orientation with respect to the surface of the structure for consistent quality of inspection by the transducers. 
   If a couplant is to be used to couple the ultrasonic signals from the transducers  50  into the structure and reflected from the structure back to the transducers  50 , a bubbler shoe  60  may be incorporated into the inspection apparatus  10 . The bubbler shoe  60  individually couples each transducer  50  rather than using a single cavity to couple all of the transducers  50 . A bubbler shoe may include a top (or first) layer  62  that includes holes  64  to permit access to the transducers  50 , such as by the transducer protruding through the holes  64  in the top layer  62  or by permitting a wired connection through the holes  64  in the top layer  62  for communication with the transducers  50 . The top layer  62  may also include one or more fluid inlets  68 ,  69  through which a couplant may be injected into the bubbler shoe  60 . The bubbler shoe  60  may also include a bottom (or second) layer that, together with the top layer  62 , define a cavity through which a couplant from the fluid inlet  68 ,  69  can flow to individually couple each transducer  50 . By way of example, such cavities may be a single open cavity providing a fluid path to each transducer or may be a cavity structured with a manifold configuration whereby the couplant passes into separate subcavities that lead to the individual transducers. The bottom layer includes holes through which the couplant passes to couple the transmission of ultrasonic signals from the transducers  50 . The transducers  50  may pass through the holes in the bottom layer, may terminate inside the cavity, or may terminate within the bottom layer. 
     FIG. 3A  is a schematic diagram of another embodiment of an inspection apparatus of the present invention.  FIG. 3B  is a top plan view of the inspection apparatus of  FIG. 3A .  FIG. 3C  is a top plan view of the bubbler shoe of the inspection apparatus of  FIG. 3A . The inspection apparatus  310  of  FIGS. 3A ,  3 B, and  3 C differs from an inspection apparatus  10  of  FIGS. 1 and 2  in that the inspection apparatus  310  of  FIGS. 3A ,  3 B, and  3 C provides only one axis of motion  324  for the sled appendages  312 , 313 , while the inspection apparatus  10  of  FIGS. 1 and 2  provides two axes of motion  24 ,  26  for the sled appendages  12 ,  13 . Although a bubbler shoe  60  with a transducer array is present in the inspection apparatus  10  of  FIGS. 1 and 2 ,  FIGS. 3A ,  3 B, and  3 C clearly show an example configuration for an array of transducers in the bubbler shoe  360  of the inspection apparatus  310 . While the internal construction of the bubbler shoe  360  is visible to some extent in  FIG. 3A ,  FIG. 4  clearly shows an example internal construction of another bubbler shoe  460 . 
     FIG. 4  is a cross-section of a schematic diagram of yet another embodiment of an inspection apparatus of the present invention. The cross-section represents an approximate mid-point through a first axis of rotation  424  corresponding to the front-back tilt of the sled appendages  412 ,  413 . The cross-sectional view shows the internal structure of one embodiment of a bubbler shoe  460  for individually coupling each transducer  450  according to the present invention. The bubbler shoe  460  includes a top layer  462  and a bottom layer  464  configured together to form a cavity  461  into which a couplant is injected for being dispersing about the cavity  461  and, after filling the cavity  461 , being evenly dispersed around each of the transducers  450  to couple the ultrasonic signals from the transducers  450  to the structure. A fluid couplant path  472  passes through a supply line  470  into and through a fluid inlet  486  into the bubbler shoe  460 . The couplant path continues to disperse throughout the cavity  461  as indicated by the fluid couplant path  478 . The ejection of the couplant from the cavity  461  of the bubbler shoe  460  around each of the transducers  450  is indicated by fluid couplant paths  476 . Typically water may be used for a couplant, but other fluids may be used, including a gas, such as air. 
   The cross-section of the inspection apparatus of  FIG. 4  also shows how the bubbler shoe  460  may be connected to the sled appendages  412 ,  413  to maintain constant orientation with respect to the structure by the bubbler shoe  460  and transducers  450  supported thereby. The connection  474  between the sled appendages  412 ,  413  and the bottom layer  464  of the bubbler shoe  460  provides a non-rotational connection between the bubbler shoe  460  and the sled appendages  412 ,  413 . By comparison to the first axis of motion  424 , the connection  474  is not a rotational axis that provides a direction of motion but is fixed to provide the same orientation with respect to the structure that the sled appendages  412 ,  413  have to the bubbler shoe  460  and transducers  450  supported thereby. 
     FIG. 5  is a block diagram of an inspection system of the present invention. The block diagram shows communication between a motion control system  512  and an axial braking system  514 . In addition, electronic data  510  representing the configuration of the structure under inspection, including position information for holes in edges of the structure, is provided to the motion control system  512 . An alternative embodiment for an inspection system may include an axial braking system that incorporates hardware and software to interpret the position of the inspection apparatus with respect to holes and edges of the structure, referred to as a smart axial braking system. For example, a smart axial braking system may include some form of a position encoder or positioning system that operates to identify the location of the inspection apparatus with respect to the structure and electronic data representing the configuration of the structure, such as the electronic data  510  provided to the motion control system in the embodiment shown in  FIG. 5 . 
   The axial braking system  514  may be activated based on data provided by the motion control system  512 . For example, the motion control system  512  may incorporate software that interprets the position of the inspection apparatus with respect to holes in edges of the structure and indicate to the axial braking system  514  when to activate the braking mechanisms on an inspection apparatus to fix the positions of sled appendages on the inspection apparatus and when to deactivate the braking mechanisms. For example, when the motion control system  512  identifies that the inspection apparatus is about to travel over a hole, the motion control system  512  can communicate to the axial braking system  514  to fix the current position of the sled appendages for while the inspection apparatus travels over the hole. When the motion control system  512  determines that the inspection apparatus has passed over the hole, the motion control system  512  may communicate to the axial braking system  514  to release the sled appendages so they may continue to ride along and follow the contoured surface of the structure. For example, a solenoid actuated pneumatic switch of the axial braking system  514  may activate to apply pressure to a pneumatic brake cylinder to extend brake discs against stationary brake plates on the sled appendages. The activation of the solenoid actuated pneumatic switch may be controlled by output signals provided by the motion control system  512  to indicate to the axial braking system  514  to fix the positions of the sled appendages. 
   Alternatively, the motion control system  512  may provide location data of the inspection apparatus with respect to a structure being inspected to the axial braking system  514 , and the axial braking system  514  may use the location data, in addition to electronic data  510  representing the configuration of the structure either provided through the motion control system  512  or directly to the axial braking system  514 , to determine when the axial braking system  514  should activate braking mechanics on the inspection apparatus to fit the positions of sled appendages, such as before traveling over a hole or off an edge of the structure. 
   The invention should not be limited to the specific disclosed embodiments. Specific terms are used in a generic and descriptive sense only and not for purposes of limitation.