Abstract:
Devices for enabling navigation of a crawler vehicle along an airfoil-shaped body, such as a helicopter blade, in a low-cost fashion with high reliability, especially for swept configuration blades. Using the natural tendency of vacuum adherence devices to adhere to changing surface contours, the crawler vehicle can adhere itself to airfoil-shaped structures in a way that allows the crawler vehicle to easily translate along an airfoil-shaped body while accommodating extreme variations along the surface of the airfoil-shaped body. The crawler vehicle can be designed to eliminate any trailing edge follower wheel, which simplifies the crawler&#39;s ability to accommodate trailing edge protrusions, such as trim tabs.

Description:
RELATED PATENT APPLICATIONS 
       [0001]    This application is a continuation-in-part of and claims priority from U.S. patent application Ser. No. 14/036,464 filed on Sep. 25, 2013, which application is a continuation-in-part of and claims priority from U.S. patent application Ser. No. 13/663,709 filed on Oct. 30, 2012, which application is a continuation-in-part of and claims priority from U.S. patent application Ser. No. 12/657,424 filed on Jan. 19, 2010 and issued as U.S. Pat. No. 8,347,746 on Jan. 8, 2013. This application is also a continuation-in-part of and claims priority from U.S. patent application Ser. No. 13/615,862 filed on Sep. 14, 2012. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates generally to the field of automated maintenance (including nondestructive inspection) of aircraft structural elements such as airfoil-shaped bodies, and more particularly to an automated end effector-carrying apparatus that is coupled to and travels along an airfoil-shaped body having a relatively short chord length while performing a maintenance function. 
         [0003]    U.S. patent application Ser. No. 13/663,709 discloses automated apparatus for performing maintenance functions on airfoil-shaped bodies having short chord lengths, without the necessity of removing the airfoil-shaped body from the aircraft. One such apparatus comprises a platform, an end effector carried by the platform, the end effector being selected from a group of interchangeable end effectors, means for mounting the end effector-carrying platform on an airfoil-shaped body, means for moving the end effector-carrying platform in a spanwise direction along the airfoil-shaped body, and means for moving the end effector in a chordwise direction relative to the airfoil-shaped body when the platform is stationary. In one implementation, the automated apparatus comprises a blade crawler which is movable in a spanwise direction and comprises a traveling element (e.g., a slider) that is linearly translatable in a chordwise direction when the spanwise-movable blade crawler is stationary. The selected end effector (mounted to the aforementioned slider) can be moved in a chordwise direction when the blade crawler is stationary. The foregoing blade crawler was designed to use the leading and trailing edge features of the blade to maintain its alignment with the blade. In practice, however, it can be difficult to maintain crawler alignment on complexly curved blades with twist, camber and sweep. In addition, a blade crawler should be able to traverse over trailing edge protrusions such as trim tabs, trim tab covers, and other irregularities. 
         [0004]    Some proposed solutions are too complex, having too many components. For example, one proposed solution employed a multiplicity of alignment/follower wheels and a compression spring to induce crawler alignment using compression mechanisms. Further, an aft follower wheel was employed provide compression against the trailing edge. This creates a difficulty when the crawler encounters trailing edge protrusions (e.g. trim tabs) necessitating a host of complex mechanisms in order to accommodate these anomalies. Complex alignment systems of the foregoing type can be expensive to develop, manufacture and maintain. For example, the opposing compression mechanisms of the rollers and alignment wheels may require continuous fine tuning and adjusting. Unless the compressing forces are adjusted properly, the crawler may encounter misalignment, thus lowering the usage value of the apparatus. 
         [0005]    Some current solutions are limited in their effectiveness in that they do not accommodate swept blade configurations well. A crawler that can accommodate only moderate contour complexities may require either significant kinematics re-programming or manual operator intervention. Next generation helicopter blades and emerging blade designs will have significant swept tip designs. This will be a significant difficulty that may need to be overcome for maintenance blade crawlers to be successfully deployed to factories, depots and forward bases. 
         [0006]    A blade crawler design that improves the crawler&#39;s ability to navigate over swept helicopter blade configurations and simplifies the componentry that couples the crawler to the helicopter blade would be a technological advance. 
       SUMMARY 
       [0007]    The subject matter disclosed in detail below is directed to an automated end effector-carrying apparatus that is capable of being coupled to and then traveling along an airfoil-shaped body having a relatively short chord length while performing a maintenance function. As used herein, the term “maintenance” includes, but is not limited to, operations such as nondestructive inspection (NDI), drilling, scarfing, grinding (e.g., to remove bonded or bolted components), fastening, appliqué application, ply mapping, depainting, cleaning, and painting. Any one of a multiplicity of end effectors for performing a respective one of the foregoing maintenance functions can be attached to the apparatus disclosed herein. There are a number of types of blade components on aircraft that will benefit from maintenance automation, including rotorcraft blades, propeller blades, flaps, ailerons, trim tabs, slats, stabilators and stabilizers. As a whole, the automated apparatus disclosed herein reduces maintenance time, labor hours and human errors when robotic maintenance functions are performed on blade components. 
         [0008]    The apparatuses disclosed herein comprise devices for maintaining crawler alignment on complex-shaped blades while at the same time enabling the blade crawler to traverse over trailing edge protrusions. The disclosed devices enable robust and automatic motion where the crawler can track along complex curvature blades with twist, camber and sweep, and can also traverse over trailing edge protrusions. With the ability to track along complex-geometry rotor blades, propellers and other airfoils, and the ability to autonomously traverse over trailing edge protrusions without loss of functionality, a crawler equipped with the devices disclosed hereinafter can provide manufacturing and in-service automated NDI and repair functionality. 
         [0009]    The devices disclosed in detail below enable navigation of a crawler vehicle along an airfoil-shaped body, such as a helicopter blade, in a low-cost fashion with high reliability, especially for swept configuration blades. Using the natural tendency of vacuum manifolds to adhere to changing surface contours, the crawler vehicle can adhere itself to airfoil-shaped structures in a way that allows it to easily translate along the blade while accommodating extreme variations along the surface of the blade. The crawler vehicle can be designed to eliminate any trailing edge follower wheel, which simplifies the crawler&#39;s ability to accommodate trailing edge protrusions, such as trim tabs. 
         [0010]    One aspect of the subject matter disclosed in detail below is an apparatus comprising: a frame; and a vacuum adherence device coupled to the frame, the vacuum adherence device comprising a seal, the orientation of the seal relative to the frame being adaptable. The apparatus may further comprise: a carriage linearly displaceably coupled to the frame; a first motor for driving linear displacement of the carriage along the frame; an end effector coupled to the carriage, the end effector being configured to perform a maintenance function; a drive wheel rotatably coupled to the frame; and a second motor for driving rotation of the drive wheel. Also, the foregoing apparatus may further comprise a ball-and-socket bearing coupled to the frame or to the vacuum adherence device, wherein a ball of the ball-and-socket bearing is disposed within or in proximity to an area bounded by the seal and protrudes beyond the seal. 
         [0011]    In accordance with some embodiments of the apparatus described in the preceding paragraph, the vacuum adherence device comprises a vacuum plate having a channel, the seal is attached to and projects from a side of the vacuum plate, and the ball-and-socket bearing is coupled to the vacuum plate in an area bounded by the seal, the apparatus further comprising a vacuum port in flow communication with a space adjacent the area bounded by the seal via the channel. In one embodiment, the vacuum plate comprises flexible material capable of conforming to a shape of a confronting surface of an airfoil-shaped body. In another embodiment, the vacuum plate comprises a plurality of rigid segments, a plurality of continuous membranes connecting the plurality of rigid elements in series, and a plurality of hinges which pivotably couple adjacent rigid elements of the plurality of rigid elements. 
         [0012]    In accordance with other embodiments of the apparatus described above, the apparatus further comprises an attachment plate having a first channel, wherein the vacuum adherence device comprises a suction cup having a second channel in flow communication with the first channel, the suction cup and the ball-and-socket bearing being attached to the attachment plate, and the second channel having an opening surrounded by the seal. In one such embodiment, the suction cup comprises: a sleeve housing attached to the attachment plate; a sleeve comprising a first portion displaceably coupled to the sleeve housing and a second portion comprising a bearing surface; and a socket ring pivotably coupled to the second portion of the sleeve and comprising a bearing surface in contact with the bearing surface of the second portion of the sleeve, the seal being attached to the socket ring. 
         [0013]    Another aspect of the subject matter disclosed herein is an apparatus comprising: a frame; an attachment plate coupled to the frame; and a plurality of suction cups, each of the suction cups comprising a sleeve housing attached to the attachment plate, a sleeve comprising a first portion displaceably coupled to the sleeve housing and a second portion comprising a bearing surface, a socket ring pivotably coupled to the second portion of the sleeve and comprising a bearing surface in contact with the bearing surface of the second portion of the sleeve, and a seal attached to the socket ring. This apparatus may further comprise: a carriage linearly displaceably coupled to the frame; a first motor for driving linear displacement of the carriage along the frame; an end effector coupled to the carriage, the end effector being configured to perform a maintenance function; a drive wheel rotatably coupled to the frame; and a second motor for driving rotation of the drive wheel. In addition, the foregoing apparatus may further comprise first and second ball-and-socket bearings, each of the first and second ball-and-socket bearings comprising a socket attached to the attachment plate and a ball rotatably coupled to the socket. 
         [0014]    A further aspect is an apparatus comprising: a frame; a vacuum plate coupled to the frame, the vacuum plate comprising a channel; a seal attached to and projecting from a side of the vacuum plate; a vacuum port in flow communication with a space adjacent the area bounded by the seal via the channel of the vacuum plate; a carriage linearly displaceably coupled to the frame; a first motor for driving linear displacement of the carriage along the frame; an end effector coupled to the carriage, the end effector being configured to perform a maintenance function; a drive wheel rotatably coupled to the frame; and a second motor for driving rotation of the drive wheel. In accordance with some embodiments, the vacuum plate comprises flexible material capable of conforming to a shape of a confronting surface of an airfoil-shaped body. In accordance with other embodiments, the vacuum plate comprises a plurality of rigid segments, a plurality of continuous membranes connecting the plurality of rigid elements in series, and a plurality of hinges which pivotably couple adjacent rigid elements of the plurality of rigid elements. The apparatus may further comprise a plurality of ball-and-socket bearings coupled to the vacuum plate, wherein the vacuum plate can be positioned adjacent a portion of an airfoil-shaped body in a manner such that the seal is adjacent to and balls of the plurality of ball-and-socket bearings are in contact with that portion of the airfoil-shaped body. 
         [0015]    Yet another aspect of the subject matter disclosed in detail below is a method for coupling a crawler vehicle to an airfoil-shaped body, comprising: (a) equipping the crawler vehicle with vacuum adherence devices; (b) placing the vacuum adherence devices in positions such that respective seals of those vacuum adherence devices are adjacent to respective other portions of the surface of the airfoil-shaped body; and (c) partially evacuating respective channels of the vacuum adherence devices to produce floating adherence of the crawler vehicle to the surface of the airfoil-shaped body. This method may further comprise: equipping the crawler vehicle with ball-and-socket bearings, a drive wheel and a motor for driving rotation of the drive wheel; placing the ball-and-socket bearings and the drive wheel in contact with respective portions of the surface of the airfoil-shaped body, the driver roller being oriented to roll in a spanwise direction along the surface of the airfoil-shaped body; and driving the drive wheel to rotate. 
         [0016]    A further aspect is a system comprising: an airfoil-shaped body having a surface; a frame; a first vacuum adherence device comprising a channel and a seal capable of adapting to a contour of the surface; a vacuum system coupled to enable partial evacuation of the channel of the first vacuum adherence device; and a drive wheel in contact with the airfoil-shaped body, wherein the first vacuum adherence device, the first ball-and-socket bearing, and the drive wheel are coupled for concurrent movement with the frame. The vacuum system may comprise an electrically controllable valve. The system may further comprise: a carriage linearly displaceably coupled to the frame; an end effector carried by the carriage, the end effector being configured to perform a maintenance function; a first motor for driving linear displacement of the carriage; a second motor for driving rotation of the drive wheel; and a computer system programmed to control the electrically controllable valve and the first and second motors during a maintenance operation in which the end effector travels over the surface of the airfoil-shaped body. In addition, the foregoing system may further comprise a ball-and-socket bearing comprising a ball in contact with the surface of the airfoil-shaped body. 
         [0017]    In accordance with some embodiments of the system described in the preceding paragraph, the first vacuum adherence device comprises a flexible vacuum plate having the channel formed therein and the seal attached thereto, the flexible vacuum plate being capable of conforming to a contour of the surface of the airfoil-shaped body. In accordance with other embodiments, the first vacuum adherence device comprises a plurality of rigid segments, a plurality of continuous membranes connecting the plurality of rigid elements in series, and a plurality of hinges which pivotably couple adjacent rigid elements of the plurality of rigid elements. Optionally, a plurality of ball-and-socket bearings are coupled to the vacuum plate, the plurality of ball-and-socket bearings projecting from the vacuum plate and being in contact with the surface of the airfoil-shaped body, the seal and surfaces of the vacuum plate and the airfoil-shaped body forming a chamber in flow communication with the channel. 
         [0018]    In accordance with alternative embodiments, the first vacuum adherence device comprises a sleeve housing, a sleeve comprising a first portion displaceably coupled to the sleeve housing and a second portion comprising a bearing surface, and a socket ring pivotably coupled to the second portion of the sleeve and comprising a bearing surface in contact with the bearing surface of the second portion of the sleeve, the seal being attached to the socket ring. 
         [0019]    The system may further comprise a second vacuum adherence device comprising a channel and a seal capable of adapting to a contour of the surface, the second vacuum adherence device being carried by the frame, and the vacuum system comprising a manifold in flow communication with the channels of the first and second vacuum adherence devices. The system may further comprises first and second vacuum generators in fluid communication with the channels of the first and second vacuum adherence devices respectively. 
         [0020]    The floating suction cups described above provide adherence. The ball-and-socket bearings provide alignment that works in conjunction with vacuum adherence devices to keep the crawler attached to the blade at precise standoff distances. The floating vacuum plate (flexible or hinged) provides both adherence and alignment. 
         [0021]    Other aspects of blade crawlers capable of performing maintenance functions while traveling along an airfoil-shaped body having a relatively short chord length are disclosed and claimed below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a diagram showing a top plan view of a blade crawler comprising an end effector for performing a maintenance function on an airfoil-shaped body (e.g., a blade component) having a short chord length and means for scanning that end effector in a chordwise direction. 
           [0023]      FIG. 2  is a diagram showing an isometric view of some components of a blade crawler having a plurality of suction cups for maintaining adherence and a plurality of ball-and-socket bearings for maintaining alignment on a blade component in accordance with one embodiment. The end effector for performing a maintenance function and the means for scanning that end effector in a chordwise direction are not shown to avoid clutter in the drawing. 
           [0024]      FIG. 3  is a diagram showing an isometric view of some components of the same blade crawler depicted in  FIG. 2 , except that a portion of the chassis has been omitted to reveal structure that would otherwise be blocked from view. 
           [0025]      FIG. 4  is a diagram showing an isometric view of some components of a blade crawler having a plurality of floating suction cups for maintaining adherence and a plurality of ball rollers for maintaining alignment on a blade component in accordance with an alternative embodiment. The end effector for performing a maintenance function and the means for scanning that end effector in a chordwise direction are not shown to avoid clutter in the drawing. Also a portion of the chassis has been omitted to reveal structure that would otherwise be blocked from view. 
           [0026]      FIG. 5  is a diagram showing an isometric view of three suction cups and five ball-and-socket bearings of a blade crawler in relation to an airfoil-shaped body. In accordance with this embodiment, the blade crawler has five points of contact and, when the suction cups are evacuated, three zones of adherence. 
           [0027]      FIG. 6  is a diagram showing the location of three points of adherence of the blade crawler depicted in  FIG. 5  in a plane generally perpendicular to a spanwise axis of the airfoil-shaped body. 
           [0028]      FIG. 7A  is a diagram showing a cross-sectional view of a vacuum adherence device in accordance with one implementation. 
           [0029]      FIG. 7B  is a diagram showing a cross-sectional view of the vacuum adherence device depicted in  FIG. 7A  adhered to a non-planar surface. The air gap between the vacuum adherence device and the non-planar surface has been exaggerated for the purpose of illustration. [What is  FIG. 8   c?]   
           [0030]      FIGS. 8A through 8C  are diagrams showing isometric views of a floating suction cup assembly during a process of providing adhering contact between a blade component and a pair of ball-and-socket bearings for maintaining alignment of a crawler on the blade component. 
           [0031]      FIG. 9A  is a diagram showing an isometric view of some components of a blade crawler having a flexible vacuum plate to maintain adherence and assisting in alignment and a set of ball-and-socket bearings for assisting in alignment on a blade component in accordance with a further alternative embodiment. The end effector for performing a maintenance function and the means for scanning that end effector in a chordwise direction are not shown to avoid clutter in the drawing. 
           [0032]      FIG. 9B  is a diagram showing an isometric view of some components of a blade crawler having a flexible vacuum plate for maintaining both adherence and alignment on a blade component in accordance with an alternative embodiment. The end effector for performing a maintenance function and the means for scanning that end effector in a chordwise direction are not shown to avoid clutter in the drawing. 
           [0033]      FIG. 10  is a diagram showing a sectional view of a flexible vacuum plate designed to conform to the leading edge of a blade component in accordance with one embodiment. 
           [0034]      FIGS. 10A and 10B  are diagrams showing sectional views of a hinged vacuum plate designed to conform to the leading edge of a blade component in accordance with an alternative embodiment. In  FIG. 10A , the hinged vacuum plate is shown adhered to a small-radius leading edge; in  FIG. 10B , the hinged vacuum plate is shown adhered to a large-radius leading edge. 
           [0035]      FIG. 11  is a block diagram identifying some components of a system comprising a computer-controlled blade crawler having vacuum adherence devices for maintaining adherence and alignment on a blade component. 
       
    
    
       [0036]    Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals. 
       DETAILED DESCRIPTION 
       [0037]    Embodiments of vacuum-adhering blade crawlers capable of performing maintenance functions while traveling along an airfoil-shaped body will now be described for the purpose of illustration. The vacuum adherence functionality is provided by one or more vacuum adherence devices. Rolling elements are provided to assist in aligning the blade crawler with the airfoil-shaped body, which rolling elements are preferably capable of omnidirectional movement. The omnidirectional rolling elements (e.g., ball-and-socket bearings) work in conjunction with the vacuum adherence devices (e.g., one or more floating flexible vacuum plates and/or one or more floating suction cups) to enable the blade crawler to adhere to but still move freely over the surface of the airfoil-shaped body. Chassis position and angularity are maintained by vacuum adherence devices which float on the surfaces of the airfoil-shaped body during a maintenance operation. Each vacuum adherence device is designed to float due to the presence of an air cushion between a seal and the blade surface when the vacuum adherence device is partially evacuated. This air cushion enables lateral displacement of the crawler relative to the airfoil-shaped body because contact friction between the seal and body surface is avoided. The resulting total suction force is strong enough to adhere the crawler to the airfoil-shaped body, but not so strong as to inhibit lateral displacement. 
         [0038]    In the following disclosure, certain vacuum-adhering means will be referred to herein as vacuum plates and suction cups. In these contexts, the modifiers “vacuum” and “suction” should be treated as synonymous. The vacuum plates and suction cups disclosed herein each comprise a channel and a seal. When the channels are partially evacuated, the resulting partial vacuums produce adherence forces sufficient to adhere the blade crawler to a blade component. The seals are configured to adapt to the contour of the surface of the blade component. 
         [0039]      FIG. 1  is a top view showing aspects of one design for an autonomous, self-propelled blade crawler  10  for performing a maintenance function on an airfoil-shaped body  100  (such as a rotorcraft blade, an aircraft propeller, a small winglet, or a narrow tail section) by crawling along the length of the airfoil-shaped body (i.e., in a spanwise direction) using the airfoil-shaped body  100  as a track and scanning an end effector  16  (e.g., a non-destructive inspection sensor or sensor array or some other maintenance tool) in a chordwise direction. The airfoil-shaped body  100  has a leading edge  94  and a trailing edge  96 . Although not shown in  FIG. 1 , the airfoil-shaped body  100  may have trailing edge protrusions (such as trim tab  98  seen in  FIG. 2 ). The blade crawlers disclosed are designed such that trailing edge protrusions are not an impediment to spanwise scanning of the airfoil-shaped body. 
         [0040]    As seen in  FIG. 1 , the blade crawler  10  comprises a frame  12  having a length greater than a chordwise dimension of the airfoil-shaped body  100 . The end effector  16  is carried by a carriage  14  which is displaceably coupled to frame  12  in a manner that allows the carriage  14  to travel back and forth along the frame  12  in the chordwise direction. For example, the carriage  14  may be displaceably coupled to frame  12  by means of a pair of linear guide units (not shown), each linear guide unit comprising a respective linear guide track attached to a respective side of frame  12  and a respective slider attached to the carriage  14 . Each slider may comprise a pair of recirculating ball bearings, the balls of which roll along the corresponding linear guide track. Optionally, the position of the carriage  14  can be encoded to provide feedback to a motion control subsystem (not shown). In accordance with one embodiment, the end effector  16  may be pivotably coupled to the carriage  14  to enable the end effector  16  to follow the curved surface of the airfoil-shaped body  100  during chordwise displacement of the carriage  14 . A tool chordwise position encoder may be provided which works by encoding the position of the carriage  14  relative to the frame  12 , e.g., by outputting pulses representing incremental movements of carriage  14  along frame  12 . 
         [0041]    To translate the end effector  16  chordwise across the airfoil-shaped body  100 , the carriage  14  is attached to a belt  22  by a clamp or fastener  24 . The belt  22  circulates (in part) around a drive pulley  18  and a passive pulley  20 , these pulleys being rotatably coupled to and carried by the frame  12 . Preferably the drive pulley  18  and passive pulley  20  are sufficiently far apart from each other that the range of motion of carriage  14  includes the entire chordwise dimension of the airfoil-shaped body  100 . The drive pulley  18  is operatively coupled to a motor (not shown in  FIG. 1 , but see stepper motor  78  in  FIG. 11 ), which motor is also mounted to frame  12  and operates under the control of the motion control system. Alternatively, the carriage  14  may be driven to displace along the frame  12  by other well-known means, such as a frame-mounted lead screw coupled to a nut attached to the carriage  14 . 
         [0042]    To move the blade crawler  10  in a spanwise direction along the airfoil-shaped body  100 , a drive wheel  26  is mounted to an output shaft of another motor (not shown in  FIG. 1 , but see stepper motor  76  in  FIG. 11 ) which is mounted to frame  12 . The drive wheel  26  is positioned and oriented to engage a leading edge  94  of the airfoil-shaped body  100 . The drive wheel can be placed in other locations such as on the top, underside or trailing edge of the blade of the airfoil-shaped body. Rotation of drive wheel  26  while in frictional contact with the leading edge  94  can produce a tractive force sufficient to cause the blade crawler  10  to travel in a spanwise direction along the airfoil-shaped body  100 . In addition, a follower wheel  28  is rotatably mounted to frame  12  by means not shown in  FIG. 1 . The follower wheel  28  is displaced spanwise from the drive wheel  26  as shown. Additional means for maintaining crawler alignment with the airfoil-shaped body  100 , such as ball-and-socket bearings and vacuum adherence devices, are not shown in  FIG. 1 . 
         [0043]    Although not shown in  FIG. 1 , the blade crawler  10  may further comprise a crawler spanwise position encoder in the form of a rotary encoder mounted to frame  12  that carries an encoder wheel on a free end of a shaft. The spanwise position of the blade crawler  10  can be measured by the rotary encoder, which encodes rotation of the encoder wheel. The encoder wheel rides on the surface of the airfoil-shaped body  100  as the blade crawler  10  travels in the spanwise direction. The rotary encoder sends respective encoder pulses to an operations control center (e.g., via an encoder cable or a wireless connection) after each incremental movement of the blade crawler  10  in the spanwise direction. When the end effector  16  is an NDI scanner, these encoder pulses are used by a control computer (not shown) and by ultrasonic pulser/receiver devices (not shown) to determine the spanwise coordinate of each scan plane in a well-known manner. 
         [0044]    The alignment and movement of automated blade crawlers of the type shown in  FIG. 1  can be enhanced by the addition of vacuum adherence devices that assist in maintaining crawler alignment during spanwise motion. Various embodiments which employ a plurality of floating suction cups to produce adherence forces will now be described with reference to  FIGS. 2-4 . 
         [0045]      FIG. 2  shows an isometric view of some components of a blade crawler having a multiplicity of floating suction cups for maintaining alignment on a blade component in accordance with one embodiment. The end effector for performing a maintenance function and the means for scanning that end effector in a chordwise direction are not shown to avoid clutter in the drawing. The use of vacuum adherence devices eliminates the needs for an aft lower contact wheel and an aft follower wheel disposed at the trailing edge  96 , and also enables alternative placement of drive wheel (not shown). In addition, it should be understood that, although not shown in  FIG. 2 , the blade crawler comprises at least one drive motor/drive wheel for self-propulsion and means for scanning an end effector in a chordwise direction even though such components are not shown. This convention will also be adopted in  FIGS. 3 ,  4 ,  9 A and  9 B. 
         [0046]    The arrow in  FIG. 2  represents spanwise motion in a direction such that the blade crawler will need to pass a trim tab  98  coupled to a rear edge  96  of the airfoil-shaped body  100 . None of the blade crawler components depicted in  FIG. 2  is in a position that would interfere with such spanwise movement past a trim tab. The blade crawler partly shown in  FIG. 2  has a rigid support structure that comprises a frame  12  (which in turn comprises a cantilever beam  12   a  and a forward cross beam  12   b ), a vertical beam  34  having an upper end connected to forward cross beam  12   b  of frame  12 , an upper adherence attachment plate  30   a  attached to the side beams of frame  12 , and a lower adherence attachment plate  30   b  attached to a lower end of vertical beam  34 . This rigid support structure supports multiple devices which enable the frame  12  to travel in a spanwise direction along the airfoil shaped body  100 , including floating suction cups and ball-and-socket bearings. 
         [0047]      FIG. 3  is a partial cut-away view of the apparatus depicted in  FIG. 2  in a stationary position on an airfoil-shaped body  100 . A portion of one of the side beams of frame  12  has been removed to reveal the structure that would otherwise be hidden behind that removed portion. Referring to  FIG. 3 , the frame  12  supports a first plurality of floating suction cups  32   a , including a row of four attached to the upper adherence attachment plate  30   a  and two attached to the forward cross beam  12   b ; a second plurality of suction cups  32   b , in including four in a first row and two in a second row attached to the lower adherence attachment plate  30   b ; and a single suction cup  32   c  attached to the cantilever beam  12   a . In an alternative embodiment (not shown), the upper adherence attachment plate  30   a  could be enlarged so that the two suction cups attached to cross beam  12   b  in  FIG. 3  could instead be attached to the enlarged upper adherence attachment plate  30   a . Each suction cup may comprise a channel in flow communication with a vacuum system (not shown in  FIGS. 2 and 3 ) and a seal that contacts the surface of the airfoil-shaped body  100  when the vacuum system is not activated. 
         [0048]    In addition, the rigid support structure depicted in  FIG. 3  supports a multiplicity of ball-and-socket bearings  38 . In the implementation depicted in  FIGS. 2 and 3 , one ball-and-socket bearing is attached to cantilever beam  12   a , two ball-and-socket bearings are attached to forward cross beam  12   b , and two ball-and-socket bearings are attached to lower adherence attachment plate  30   b . Additional ball-and-socket bearings could be included. It should be noted that the ball-and-socket bearings are positioned in proximity to the floating suction cups to allow the former to work in conjunction with the latter to induce alignment of the blade crawler with the airfoil-shaped body  100 . 
         [0049]      FIG. 4  shows an isometric view of some components of a blade crawler having a plurality of floating suction cups for maintaining alignment on a blade component in accordance with an alternative embodiment. In this alternative embodiment, the lower adherence manifold assembly seen in  FIG. 3  is replaced by a ball-and-socket bearing assembly without suction cups. In the implementation depicted, the ball-and-socket bearing assembly comprises a lower attachment plate  36  connected to and extending transversely from the lower end of the vertical beam  34  of the support structure and a pair of ball-and-socket bearings  38  attached to the opposing arms of the attachment plate  36 . The ball-and-socket bearings  38  attached to attachment plate  36  contact a lower surface of the leading edge  94 , while an opposing pair of ball-and-socket bearings  38  attached to the forward cross beam  12   b  contact an upper surface of the leading edge  94  of the airfoil-shaped body  100 . 
         [0050]      FIG. 5  is an isometric view of three suction cups  32   a ,  32   c  and five ball-and-socket bearings  38  of a blade crawler in relation to an airfoil-shaped body  100 . Other portions of the blade crawler are not shown. In accordance with this configuration, the blade crawler has five points of contact and, when the suction cups are evacuated, three zones of adherence. This configuration is sufficient to adhere and align a blade crawler on an airfoil-shaped body  100  without including any rolling elements that contact the trailing edge of the airfoil-shaped body  100 , thereby enabling crawler scanning of airfoil-shaped bodies having trailing edge protrusions and/or swept blade designs. 
         [0051]    In accordance with a variation of the configuration shown in  FIG. 5 , two additional floating suction cups can be placed in proximity to the lower surface of the airfoil-shaped body, providing five points of adherence.  FIG. 6  is a side view of this configuration in which three suction cups  32   a - 32   c  and three ball-and-socket bearings  38  are visible. Fourth and fifth suction cups behind suction cups  32   a  and  32   b  respectively and fourth and fifth ball-and-socket bearings respectively behind the ball-and-socket bearings  38  in proximity to the upper and lower surfaces of the leasing edge of the airfoil-shaped body  100  are not visible in  FIG. 6 . This configuration is sufficient to induce alignment of the crawler with the airfoil-shaped body  100 . As seen in  FIG. 6 , a drive roller  42  is provided to induce crawler motion in a spanwise direction as the drive roller  42  frictionally contacts the leading edge of the airfoil-shaped body  100 . The suction cups  32   a - 32   c  will function when the airfoil-shaped body  100  has any one of a multiplicity of orientations that may be encountered at helicopter depots or aircraft maintenance facilities, including horizontal, vertical or at an acute angle. The blade crawler partly depicted in  FIG. 6  can also be reversed 180 degrees and coupled to the airfoil-shaped body  100  in a manner such the suction cup  32   b  is above and suction cups  32   a  and  32   c  are below the airfoil-shaped body  100 . 
         [0052]    Still referring to  FIG. 6 , the vertical distance separating the balls of the upper and lower ball-and-socket bearings  38  near the leading edge of the airfoil-shaped body  100  will be selected so that the opposing balls respectively contact the upper and lower surfaces of the airfoil-shaped body  100 . The length of the vertical beam  34  connecting an upper portion of frame  12  to a lower cantilever beam  36  (is this  12   a ?) may be adjustable. For example, the vertical beam  34  may comprise an upper and lower vertical beam sections arranged in a telescoping relationship such that the length of the vertical beam  34  is adjustable. A locking device (not shown) can be unlocked to allow length adjustment and then locked to set the length adjustment. For example, when the balls of the upper and lower ball-and-socket bearings  38  near the leading edge of the airfoil-shaped body  100  are separated by an optimum vertical distance, a set screw or other locking means can be used to prevent further relative movement of the vertical beam sections during crawler operation. 
         [0053]    The ball-and-socket bearings  38  enable motion of the apparatus along complex-shaped blades (i.e., in a spanwise direction) without causing misdirection. The ball-and-socket bearings  38  can be similar to any one of a plurality of commercially available types of ball-and-socket bearings, such as those used in the design of office furniture. When ball-and-socket bearings are used in conjunction with vacuum adherence devices, a nearly frictionless omni-directional alignment device is provided. The ball-and-socket bearings  38  maintain positive alignment of the crawler with the blade features without causing misdirection, so that complex-curvature blades with twist, camber and sweep can be accommodated. 
         [0054]    Returning to  FIG. 3 , each floating suction cup  32   a - 32   c  comprises a channel in flow communication with a vacuum system via a respective manifold. The upper and lower attachment plates  30   a ,  30   b , cantilever beam  12   a , and forward cross beam  12   b  each comprise a respective manifold (not shown) which is in fluid communication with channels of the suction cups attached to those support elements. The term “manifold” is used herein in the sense of a chamber or duct having several outlets through which a fluid can be distributed or gathered. These manifolds connect the channels in the suction cups to the vacuum system, which may comprise a vacuum pump and one or more electrically controllable valves between the vacuum pump and the manifolds. The vacuum system is connected to the crawler vehicle by way of an umbilical cable that may includes air lines, electrical lines, and even a water line (e.g., in cases where the end effector is an ultrasonic sensor or sensor array). In accordance with alternative embodiments, each individual floating suction cup has a respective vacuum motor (not shown). 
         [0055]    In accordance with one embodiment, all of the floating suction cups have a similar structure.  FIG. 7A  is a diagram showing a cross-sectional view of a suction cup  32  in accordance with one implementation. This suction cup  32  comprises a circular cylindrical sleeve housing  52  and a sleeve  54  having a circular cylindrical portion which is axially slidable along a center axis  66  inside the sleeve housing  52 . The sleeve  54  further comprises bearing portion  56  having an outer spherical bearing surface having a center point located along the center axis  66 . The bearing portion  56  may be integrally formed with the aforementioned circular cylindrical portion of sleeve  54 . The suction cup  32  further comprises a pivotable seal assembly  58  comprising a socket ring  60  that holds a seal  62 . The socket ring  60  also has an inner spherical bearing surface which is concentric with and pivotably coupled to the outer spherical bearing surface of bearing portion  56  of sleeve  54 . The pivot point of the socket ring  60  is collocated with the center point of the outer spherical bearing surface of bearing portion  56  of sleeve  54 . 
         [0056]    The pivotable seal assembly  58  is configured to rotate relative to the sleeve  54  about the pivot point to at least partially conform to a shape of a confronting surface. The floating suction cup  32  can adhere to such a confronting surface when air is drawn into a channel  64  formed in part by the channel of sleeve housing  52 , in part by the channel of sleeve  54 , and in part by the opening in the seal  62 . The pivotable seal assembly  58  is configured to rotate relative to the sleeve  54  independently of translational movement of the sleeve  54  in a direction parallel to the center axis  66  within the sleeve housing  52 . The amount of rotation of pivotable seal assembly  58  may be limited by the size and/or shape of the outer spherical bearing surface of the bearing portion  56  of sleeve  54 . 
         [0057]    Although not shown in  FIG. 7A , the floating suction cup preferably comprises a spring arranged to urge the sleeve  54  to extend out of the sleeve housing  52  by downward (as seen in the view of  FIG. 7A ) sliding along the center axis  66 . This sliding movement may be restricted to within a selected range of movement. However, sleeve  54  may “float” freely relative to sleeve housing  52  within this selected range of movement. This restriction of the translational motion of sleeve  54  can be implemented by providing a slot  68  in the wall of the circular cylindrical portion of sleeve  54  and by providing a pin  70  which extends radially inward from the wall of sleeve housing  52  and into the slot  68 . The pin  70  may also be used to hold sleeve  54  inside sleeve housing  52 . The length of slot  68  restricts the sliding movement of sleeve  54  relative to sleeve housing  52 . 
         [0058]    To generate vacuum adherence forces, the channel  64  is in fluid communication with a control valve (not shown in  FIG. 7A ), which control valve is in turn in flow communication with a vacuum pump (also not shown in  FIG. 7A ). The vacuum pump, control valve, channel  64 , and connecting conduits form a vacuum system which is configured to draw air into the channel  64  such that a vacuum adherence is formed between the pivotable seal assembly  58  and a confronting surface. The vacuum adherence is the result of a vacuum pressure generated inside the channel  64 . As a result of this partial vacuum inside the suction cup, ambient air can be sucked into channel  64 . The ambient air flows through any gap between the seal  62  and the confronting surface of the airfoil-shaped body. The flow of air radially inward through such gap has the effect of producing an air cushion. The height of the gap may vary along the periphery of the seal  62 . This gap height depends on the shape of the confronting surface and the degree of rotation of the seal  62  to conform to that shape. 
         [0059]    The seal  62  may be formed of any one of a number of different materials. For example, seal  62  may comprise silicone rubber or other elastomeric material, a viscoelastomeric material, or some other suitable flexible material. 
         [0060]    It may be appreciated that different embodiments may be designed to take into account different considerations. For example, a vacuum adherence system for a blade crawler may comprise a multiplicity of floating suction cups of the type depicted in  FIG. 7A , the totality of the resulting vacuum adherence forces being sufficient to enable the blade crawler to adhere to an airfoil-shaped body. The individual suction cups may be designed to adhere to a surface that is not flat and/or has inconsistencies using vacuum adherence forces with a desired level of strength while minimizing static friction between the blade crawler and the airfoil-shaped body. Further, the capability of each of a multiplicity of pivotable seal assemblies  58  to rotate about a corresponding pivot point and each of a multiplicity of sleeves  54  to float within a corresponding sleeve housing  52  may allow the blade crawler to move along a surface having varying shapes and/or surface inconsistencies. For example, the airfoil-shaped body may have a convex curved surface. In some cases, the surface may have inconsistencies such as, for example, without limitation, protrusions, protruding fastener joints, and/or other types of inconsistencies that may affect the width of the gaps between the suction cups and the confronting surface as the blade crawler moves over the surface. 
         [0061]      FIG. 7B  shows a cross-sectional view of the floating suction cup  32  depicted in  FIG. 7A  adhered to a convex curved surface  102 . The air gap between the suction cup  32  and the convex curved surface  102  has been exaggerated for the purpose of illustration. The air gap may function as an air bearing that holds the pivotable seal assembly  58  close to surface  102 , while reducing static friction to within selected tolerances. In other words, the air gap allows pivotable seal assembly  58  to “float” above surface  102  while maintaining vacuum adherence between pivotable seal assembly  58  and surface  102 . Further, the air gap allows pivotable seal assembly  58  to be moved over surface  102  with a reduced amount of static friction and without causing undesired effects to surface  102 . The height of the air gap may be within selected tolerances for maintaining the strength of the vacuum adherence force in a desired range. The air gap height can be varied as a function of the spring constant of the compression spring (not shown in  FIG. 7B ) and/or the vacuum pressure inside the channel  64  of the suction cup  32 . 
         [0062]    The gaps between the seals and the surface of the airfoil-shaped body allow the suction cups to float above the surface while the downward force provided by the vacuum system allows the drive wheel and alignment elements (e.g., ball-and-socket bearings) on the blade crawler to remain in contact with the surface. In this manner, the blade crawler may exert normal force on the drive wheel to create friction between the drive wheel and blade surface, thus obtaining traction for the drive wheel, which may be coupled to a motor that propels the blade crawler in a spanwise direction. Consequently, the blade crawler may adhere to the surface and move along the airfoil-shaped body with a reduced amount of friction between the blade crawler and the surface as the drive wheel propels the crawler spanwise along the blade. 
         [0063]    The widths of the gaps between the seals and the surface may determine the strength of the vacuum adherence formed between the blade crawler and the surface. When the gap is wider than some specified threshold, the vacuum adherence may not have the desired level of strength. Consequently, the blade crawler may lose traction and be unable to travel in the spanwise direction. When the gap is narrower than some specified threshold, the vacuum adherence may be stronger than desired. Consequently, the blade crawler may become stuck to the surface and unable to move. Accordingly, the system may be adjusted prior to performing a maintenance operation to produce vacuum adherence forces within a desired range. 
         [0064]    In one embodiment, the seal  62  may be corrugated in such a way as to allow small channels for airflow between the seal  62  and component surface  102 . In some instances, these corrugated channels have been shown to promote vacuum on surfaces of uneven profile or varying surface roughness. In accordance with this embodiment, the corrugations may comprise a low-friction material that further induces sliding such that crawler motion will be enabled, yet airflow is ensured by the corrugated channels. 
         [0065]    In another embodiment (not shown), a multiplicity of small ball-and-socket bearings may be arranged along a circular perimeter surrounding (i.e., radially outward of) the seal  62 . Alternatively, these ball-and-socket bearings can be disposed along a circular perimeter radially inward of the seal or actually embedded in the seal. The ball-and-socket bearings should be installed in such a way that a precise gap is always maintained between the seal and the surface of the component  102 . A similar arrangement involving the placement of ball-and-socket bearings in proximity to a seal of a vacuum plate will be described below with reference to  FIG. 10 . 
         [0066]    Referring again to  FIG. 7B , the pivotable seal assembly  58  is rotated relative to the convex curved surface  102  such that the pivotable seal assembly  58  at least partially conforms to that surface. Further, the sleeve  54  is moved in a direction along a center axis through the sleeve housing  52 . Thereafter, air is drawn into the channel  64  such that the pivotable seal assembly  58  adheres to the surface  102 . These operations may be performed during movement of the blade crawler along the airfoil-shaped body. Rotation of the pivotable seal assembly  58  and movement of the sleeve  54  allow the adherence system to adapt to changes in the shape of the surface  102  as the blade crawler moves along the surface. 
         [0067]    As shown in  FIG. 3 , multiple floating suction cup assemblies are configured with one or more ball-and-socket bearings to comprise respective adherence manifold assemblies. The floating suction cups contain a specific mechanism that allows them to extend toward the blade surface, then, once engaged, the suction cups retract, drawing the adherence manifold assembly and the blade surface together, such that adherence contact can be maintained between the ball-and-socket bearings and the blade surface. Because the suction cups are configured with a floating nature, the entire adherence manifold can still float relative to the blade surface while maintaining adherence forces. For the purpose of the following discussion, the lower adherence attachment plate  30   b  and the suction cups  32   b  and ball-and-socket bearings  38  attached to the lower adherence attachment plate  30   b  will be referred to as the lower adherence manifold assembly. 
         [0068]      FIGS. 8A-8C  show the lower adherence manifold assembly at three stages in the process of providing adhering contact between the ball-and-socket bearings  38  and the lower surface of the airfoil-shaped body  100  for maintaining alignment of a crawler on the airfoil-shaped body  100 . It may be appreciated that the upper and aft adherence manifold assemblies shown in  FIG. 3  are constructed to operate in a similar manner as will now be described. 
         [0069]      FIG. 8A  shows the lower adherence manifold assembly when the channels inside the suction cups  32   b  are at the ambient atmospheric pressure, not a vacuum pressure, i.e., the suction cups are not producing suction. Each of the six suction cups  32   b  of the lower adherence manifold assembly further comprises a compression spring (not shown), which can be arranged to urge the retractable sleeve (see sleeve  54  in  FIG. 7A ) of each suction cup to move relative to the sleeve housing (see sleeve housing  52  in  FIG. 7A ) in a direction of extension (indicated by an arrow in  FIG. 8A ). This compression spring also resists retraction of the sleeve into the housing, which resistance can be overcome by the production of a vacuum adherence force greater than the spring force.  FIG. 8A  shows the retractable sleeves of suction cups  32   b  in their respective extended position, with their respective retracted position being indicated by dashed ellipses  44 . 
         [0070]    In the state depicted in  FIG. 8B , the lower adherence manifold assembly is placed under the lower surface of the airfoil-shaped body  100  while the suction is turned off. In this state, the compression springs  44  urge the seals at the ends of the sleeves of the suction cups  32   b  into contact with the lower surface of the airfoil-shaped body  100  while the balls of the ball-and-socket bearings  38  are not in contact. 
         [0071]    When the channels of the suction cups  32   b  are partially evacuated, the sleeves of the suction cups  32   b  retract, thus bringing the balls of the ball-and-socket bearings  38  into contact with the airfoil, as depicted in  FIG. 8C . While the sleeves are retracting, the suction forces produced by the partially evacuated suction cups of the lower adherence manifold assembly produce adherence forces on the lower surface of the airfoil-shaped body  100 . The same operations apply to the upper and aft adherence manifold assemblies seen in  FIG. 3 , causing those assemblies to produce adherence forces on the upper surface of the airfoil-shaped body  100 . The result is a total suction force sufficient to cause adherence of the crawler vehicle to the surface of the airfoil-shaped body  100  while still allowing the crawler vehicle to move over the surface of the airfoil-shaped body  100  by rolling on the balls of the ball-and-socket bearings  38 . For example, the adherence forces will assist in holding the drive wheel  26  (see  FIG. 1 ) against the leading edge  94  of the airfoil-shaped body  100 , enabling the generation of a tractive force sufficient to overcome the small frictional forces exerted on the crawler vehicle by the vacuum adherence devices. 
         [0072]      FIG. 9A  shows an isometric view of some components of a blade crawler having a vacuum adherence device for maintaining alignment on a blade component in accordance with a further alternative embodiment. The embodiment shown in  FIG. 9A  differs from the embodiment shown in  FIG. 4  in that a flexible vacuum plate  40  is attached to a cantilever beam that connects to the vertical beam  34 . The flexible vacuum plate  40  performs the adherence function in place of the plurality of floating suction cups  32   a  that are part of the upper adherence manifold assembly seen in  FIG. 4 . The flexible vacuum plate  40  is designed with the capability to conform to the shape of a surface of the airfoil-shaped body  100 . In the implementation shown in  FIG. 9A , the flexible vacuum plate  40  is wrapped around a portion of the leading edge  94 . In this implementation, the upper portion of flexible vacuum plate  40  is situated between the balls of the pair of ball-and-socket bearings  38  attached to attachment plate  36 , while the lower portion of flexible vacuum plate  40  is situated between the balls of the pair of ball-and-socket bearings  38  attached to the forward cross beam  12   b.    
         [0073]    In accordance with yet another alternative embodiment shown in  FIG. 9B , the ball-and-socket bearings  38  disposed near the leading edge  94  of the airfoil-shaped body  100  can be eliminated. The blade crawler is adhered to the airfoil-shaped body  100  by the flexible vacuum plate  40  and the aft suction cup  32   c , which both float on the surfaces of the airfoil-shaped body  100  when they are partially evacuated. The aft ball-and-socket bearing  38  seen in  FIG. 9B  is capable of omnidirectional movement over the upper surface of the airfoil-shaped body  100  while maintaining the alignment of the crawler vehicle on the airfoil-shaped body  100 . 
         [0074]    The structure of a flexible vacuum plate in accordance with one implementation is shown in  FIG. 10 , which is a sectional view taken in a plane normal to the axis of the airfoil-shaped body  100 . The flexible vacuum plate  40  is an assembly comprising a flexible substrate  46  (made, e.g., of semi-rigid rubber optionally reinforced with carbon or nylon rods), a flexible vacuum seal  48  (made, e.g., of rubber) attached to the flexible substrate  46  along a perimeter, and a multiplicity of the ball-and-socket bearings  38 , the sockets of which are embedded in the flexible substrate  46 . When in a flattened state, the shape of the flexible substrate  46  is rectangular, while the ball-and-socket bearings  38  are arranged in rows and columns. Only one column of ball-and-socket bearings  38  is shown in  FIG. 10 . 
         [0075]    The flexible substrate  46  and opposing surfaces of the airfoil-shaped body  100  form a chamber  88  which is sealed along a perimeter by the vacuum seal  48 . This vacuum seal  48  is designed so that when the balls of the ball-and-socket bearings  38  are in contact with the surfaces of the airfoil-shaped body  100 , there will be a slight gap between the vacuum seal  48  and the confronting surface of the airfoil-shaped body  100  that allows some air to flow into chamber  88  when the latter is partially evacuated. 
         [0076]    The flexible substrate  46  can be formed by molding. The molded structure shown in  FIG. 10  includes a protuberance that has an attachment bushing  50  embedded therein for coupling the flexible vacuum plate  40  to the support structure of the crawler vehicle. The flexible substrate  46  further includes an opening that has a channel  86  embedded therein. The channel  86  connects to a vacuum port  84 , which is in turn connected to a vacuum pump by means not shown in  FIG. 10 . The distal end of the channel  86  is in flow communication with the chamber  88 . When the vacuum pump is activated, the resulting partial vacuum formed in chamber  88  will produce a suction force that adheres the flexible vacuum plate  40  to airfoil-shaped body  100 , but still allows the flexible vacuum plate  40  to float on airfoil-shaped body  100  due to the air cushion created by air being sucked through the slight gap between vacuum seal  48  and the airfoil-shaped body  100 . The flow of air inside chamber  88 , through channel  86  and out vacuum port  84  during evacuation is indicated by arrows in  FIG. 10 . 
         [0077]      FIGS. 10A and 10B  show sectional views of a hinged vacuum plate designed to conform to the leading edge of an airfoil-shaped body  100  in accordance with an alternative embodiment. In  FIG. 10A , the hinged vacuum plate is shown adhered to a small-radius leading edge; in  FIG. 10B , the hinged vacuum plate is shown adhered to a large-radius leading edge. In contrast to the flexible vacuum plate depicted in  FIG. 10 , the hinged vacuum plate comprises a plurality of rigid segments  46   a  connected by continuous membranes  46   b . The rigid segments  46   a , continuous membranes  46   b , confronting surface of the leading edge of the airfoil-shaped body  100 , and vacuum seal  48  form a chamber which, when partially evacuated, causes the hinged vacuum plate to adhere to the leading edge of the airfoil-shaped body  100 . The continuous membranes  46   b  maintain the integrity of that chamber. The hinged vacuum plate further comprises a plurality of hinges  90  (four in the implementation shown in  FIGS. 10A and 10B ) which flex to accommodate large variations in leading edge radius. The hinges  90  may incorporate biasing means, such as springs, arranged to urge the rigid segments  46   a  toward the confronting surface of the leading edge when the chamber is not evacuated. 
         [0078]      FIG. 11  is a block diagram identifying some components of a system comprising a computer-controlled blade crawler having vacuum adherence devices for maintaining alignment on a blade component in accordance with one embodiment. Various components of the end effector-carrying blade crawler communicate with a control computer  72  located at an operations command center. The control computer may be connected to the blade crawler by an electrical cable (not shown in the drawings). Alternatively, the control computer and the blade crawler could communicate wirelessly. 
         [0079]    The control computer  72  controls the operations of a pair of stepper motors  76  and  78 , which are mounted on the above-described support structure of the crawler vehicle. Stepper motor  76  drives rotation of the drive wheel  26  during spanwise movement of the crawler vehicle. Stepper motor  78  drives rotation of the drive pulley  18  during chordwise movement of the end effector. The control computer  72  controls stepper motors  76  and  78  in dependence on crawler position information derived from sensors  74 . When the blade crawler reaches a target spanwise position, the control computer can be programmed to shut off stepper motor  76  and then start stepper motor  78 . The sensors  74  may include position encoders that generate pulses in response to incremental movements of the crawler vehicle in the spanwise direction and position encoders that generate pulses in response to incremental movements of the end effector in the chordwise direction. 
         [0080]    In cases where the end effector is a rotary tool (such as a scarfer, drill, deburrer or reamer), when the rotary tool reaches a target chordwise position, the control computer  72  can be programmed to shut off the stepper motor  78  and then start an end effector motor (not shown), e.g., a drive motor which drives rotation of the rotary tool. It should be appreciated that in cases where the end effector is emitting or ingesting a liquid or particles, the control computer  72  will activate a pump. In cases where the end effector&#39;s elevational position is adjustable by operation of an actuator, such actuator may also be controlled by the computer. 
         [0081]    In addition, the control computer  72  can be programmed to control the state of an electrically controllable valve  80  that connects a vacuum pump  82  to a plurality of vacuum adherence manifolds  31 . Each vacuum adherence manifold  31  is in flow communication with one or more suction cups  32 , as described above with reference to  FIGS. 2 and 3 . The control computer  72  can be programmed to send a signal that causes the valve  80  to remain open during operation of the crawler vehicle. In the valve open state, the vacuum pump  82  will apply a partial vacuum to the vacuum adherence manifolds  31  and channels of the suction cups  32 , thereby adhering the crawler vehicle to the airfoil-shaped body. 
         [0082]    The control computer may also be programmed to control a cable management system (not shown). For example, motion control application software running on the control computer can control a cable motor of the cable management system. When the blade crawler is operated, one or more cables need to accompany the crawler down the length of the airfoil-shaped body, e.g., a helicopter blade. The motion control software running on the control computer synchronizes the movement of the cables with the movement of the blade crawler, extending or retracting the cables as appropriate. The control computer  72  can be programmed to control the cable motor (not shown) in dependence on crawler position information derived from sensors  74 . 
         [0083]    In accordance with the embodiments described above, the control computer is provided with information concerning the spanwise position of the crawler. This functionality can be provided by any one of a multiplicity of known positional tracking mechanisms. 
         [0084]    The blade crawler disclosed herein can be adapted for use in the automation of various maintenance functions, including but not limited to nondestructive inspection, drilling, grinding, fastening, appliqué application, scarfing, ply mapping, depainting, cleaning and painting. There are a number of types of blade components on aircraft that will benefit from maintenance automation, including rotorcraft blades, propeller blades, flaps, ailerons, trim tabs, slats, stabilators and stabilizers. 
         [0085]    The use of vacuum adherence devices to adhere a crawler vehicle to an airfoil-shaped body, such as a blade component, provides multiple benefits, including: (1) the ability to maintain reliable contact between the crawler vehicle and blade component; (2) the ability to accommodate trailing edge protrusions (e.g., trim tabs) without the need for an aft follower wheel; (3) the ability to accommodate swept blade configurations; (4) the ability to accommodate blade surface roughness and non-uniformities; (5) lower cost/skill to operate when adherence is automatic; and (6) the provision of a low-cost apparatus with elimination of an aft follower wheel and an aft lower ball-and-socket bearing. With the ability to track along complex-geometry rotor blades, propellers and other airfoils, autonomously translate over trailing edge protrusions without loss of functionality, and accommodate swept blade configurations, the crawler vehicles disclosed above can provide manufacturing and in-service automated NDI and repair functionality. 
         [0086]    While automated blade crawlers have been described with reference to particular embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the teachings herein. In addition, many modifications may be made to adapt the concepts and reductions to practice disclosed herein to a particular situation. Accordingly, it is intended that the subject matter covered by the claims not be limited to the disclosed embodiments. 
         [0087]    As used in the claims, the term “computer system” should be construed broadly to encompass a system having at least one computer or processor, and which may have multiple computers or processors that communicate through a network or bus. As used in the preceding sentence, the terms “computer” and “processor” both refer to devices comprising a processing unit (e.g., a central processing unit) and some form of memory (i.e., computer-readable medium) for storing a program which is readable by the processing unit. 
         [0088]    The method claims set forth hereinafter should not be construed to require that the steps recited therein be performed in alphabetical order (any alphabetical ordering in the claims is used solely for the purpose of referencing previously recited steps) or in the order in which they are recited. Nor should they be construed to exclude any portions of two or more steps being performed concurrently or alternatingly.