Patent Publication Number: US-10791275-B2

Title: Methods for measuring and inspecting structures using cable-suspended platforms

Description:
RELATED PATENT APPLICATION 
     This application is a continuation-in-part of and claims priority from U.S. patent application Ser. No. 15/714,662 filed on Sep. 25, 2017. 
    
    
     BACKGROUND 
     The disclosure relates to position measurement for repair and maintenance management of large structures such as aircraft. More particularly, the disclosure relates to a local positioning system and methods for non-destructive measurement and inspection of vehicles such as aircraft that do not require physical contact with the vehicle. 
     When repair work is required on vehicles, such as on the skin of an aircraft, it may be necessary to take into account the size, shape and location of structural anomalies for optimum repair of the vehicle. Photographs of the structural anomalies may be made but may not be precisely located or sized on a target object or may not be useful for repair planning. During the analysis of a location of interest on the target object, it may be desirable to obtain measurement information without contacting the target object. Due to accessibility and/or contact constraints, it may be difficult to reach the location of interest to obtain position measurements. Therefore it is advantageous for a local positioning system to be able to take measurements without contacting the target object and from moderate to large distances from the target object. 
     SUMMARY 
     Finding and accurately measuring the locations of structural anomalies on a structure or vehicle, such as a large commercial airplane, can be a laborious task. An efficient and automated process for addressing this problem would be valuable to many organizations involved in building and maintaining large vehicles and structures. 
     The subject matter disclosed in some detail below is directed to systems and methods for measuring the distance to a target object and acquiring three-dimensional coordinates, scale information, and point-to-point distance information for that target object in an environment using a remotely operated cable-suspended platform. The measurement system uses on-board sensors to acquire data and then uses computer processing techniques to provide discrete or continuous measurements of three-dimensional coordinates of points or the distances between points on a target object or the scale of the target object. 
     Although various embodiments of systems and methods for acquiring three-dimensional coordinate information for points or scale and point-to-point distance information for target objects undergoing non-destructive inspection using a cable-suspended platform are described in some detail later herein, one or more of those embodiments may be characterized by one or more of the following aspects. 
     One aspect of the subject matter disclosed in detail below is a method for calculating a position of a point of interest on a target object in a frame of reference using a cable-suspended platform comprising an equipment support member, a pan-tilt mechanism mounted to the equipment support member, a camera mounted to the pan-tilt mechanism, and a laser range meter affixed to the camera, the method comprising: (a) suspending the cable-suspended platform from a plurality of cables connected to the equipment support member and supported at respective anchor points of a plurality of anchor points; (b) calibrating the pan-tilt mechanism relative to the frame of reference; (c) controlling lengths of paid-out portions of the plurality of cables in a manner that causes the equipment support member to move so that the camera is within range of a target object; (d) measuring the respective distances of a center point of the equipment support member from the respective anchor points based in part on the lengths of the paid-out portions of the plurality of cables; (e) controlling the pan-tilt mechanism to cause the laser range meter to aim at a point of interest on the target object; (f) measuring the pan and tilt angles of the pan-tilt mechanism while the laser range meter is aimed at the point of interest; (g) measuring the distance separating the laser range meter and the point of interest; and (h) converting the distance and angle measurements into a Cartesian coordinate vector representing the position of the point of interest in the frame of reference. 
     In accordance with one embodiment of the method described in the preceding paragraph, the frame of reference is a frame of reference of the target object, and step (b) comprises: aiming the laser range meter at three or more calibration points on the target object at different times while the equipment support member is stationary; and computing a calibration matrix representing a transformation from a frame of reference of the pan-tilt mechanism to the frame of reference of the target object. In one proposed implementation, step (b) further comprises: measuring the pan and tilt angles of the pan-tilt mechanism while the laser range meter is aimed at each calibration point; and measuring the distance separating the laser range meter and each calibration point while the laser range meter is aimed at each calibration point. 
     In accordance with an alternative embodiment, the frame of reference is a frame of reference of a plurality of at least three anchor points supporting respective cables of the plurality of cables, and step (b) comprises: aiming the laser range meter at three or more anchor points of the plurality of anchor points at different times while the equipment support member is stationary; and computing a calibration matrix representing a transformation from a frame of reference of the pan-tilt mechanism to the frame of reference of the plurality of anchor points. In one proposed implementation, step (b) further comprises: measuring the pan and tilt angles of the pan-tilt mechanism while the laser range meter is aimed at each anchor point; and measuring the distance separating the laser range meter and each anchor point while the laser range meter is aimed at each anchor point. 
     Another aspect of the subject matter disclosed in detail below is a method for inspecting and measuring a structure comprising: (a) suspending a cable-suspended platform from cables in a vicinity of a structure; (b) controlling the cable-suspended platform to move toward the structure; (c) using first and second laser range meters on-board the cable-suspended platform to repeatedly measure first and second distances respectively separating the first and second laser range meters from respective first and second spots on a surface of the structure while the cable-suspended platform is moving; (d) calculating a first separation distance separating the cable-suspended platform from the structure based at least on the first and second distances; (e) determining whether the first separation distance equals a goal offset; (f) controlling the cable-suspended platform to stay at a first location separated from the structure by the first separation distance in response to a determination in step (e) that the separation distance is equal to the goal offset; (g) using a camera on-board the cable-suspended platform to capture a first image of the structure while the cable-suspended platform is at the first location; and (h) displaying the first image on the display screen. In accordance with one embodiment, the method further comprises: computing an orientation angle of a focal axis of the camera relative to a line connecting the first and second spots on the surface of the structure based on the first and second distances; calculating a scale factor for the first image when displayed on the display screen based at least in part on the separation distance and the orientation angle; and displaying a scale indicator overlaid on the image, a value or a length of the scale indicator representing the scale factor. 
     A further aspect of the subject matter disclosed in detail below is a method for inspecting and measuring a structure comprising: (a) suspending a cable-suspended platform from cables in a vicinity of a structure; (b) controlling the cable-suspended platform to move to and then stay at a location separated from the structure; (c) directing first and second laser pointers pivotably mounted on-board the cable-suspended platform in parallel toward a surface of the structure, the respective pivot axes of the first and second laser pointers being separated by a fixed distance; (d) using the mutually parallel first and second laser pointers to transmit mutually parallel laser beams onto first and second spots respectively while the cable-suspended platform is at the location; (e) using a camera on-board the cable-suspended platform at a first time to capture a first image of a portion of the surface of the structure that includes the first and second spots; (f) pivoting the first and second laser pointers by a predefined angle while the cable-suspended platform is at the location so that the first and second laser pointers are no longer parallel; (g) using the pivoted first and second laser pointers to transmit non-parallel laser beams onto respective third and fourth spots on the surface of the structure while the cable-suspended platform is at the location; (h) using the camera at a second time to capture a second image of the portion of the surface of the structure that includes the third and fourth spots; and (i) processing the first and second images to calculate a first separation distance separating the cable-suspended platform from the structure based on the positions of the third and fourth spots in the images, the predefined angle and the fixed distance separating the pivot axes of the laser pointers. In accordance with one embodiment, step (h) further comprises calculating a second separation distance separating respective centers of the third and fourth spots, the method further comprising calculating a scale factor for the first and second images when displayed on a display screen based on the second separation distance. 
     Yet another aspect of the subject matter disclosed in detail below is a method for sizing a feature of a structure using a cable-suspended platform comprising a pan-tilt mechanism that supports a camera and a laser range meter, the method comprising: (a) suspending the cable-suspended platform from a plurality of cables supported at respective anchor points of a plurality of anchor points; (b) controlling lengths of paid-out portions of the plurality of cables in a manner that causes the cable-suspended platform to move to a first location; (c) measuring the respective distances of a center point of the cable-suspended platform from the respective anchor points based in part on the lengths of the paid-out portions of the plurality of cables when the cable-suspended platform is at the first location; (d) aiming the laser range meter at a first point corresponding to a first visible feature on a surface of the structure while the cable-suspended platform is at the first location and acquiring a first distance measurement; (e) using the pan-tilt mechanism to measure the respective pan and tilt angles of the laser range meter when the laser range meter is aimed at the first point; (f) converting the distance and angle measurements acquired in steps (d) and (e) into a first vector representing the location of the first point in the frame of reference of the cable-suspended platform; (g) controlling lengths of paid-out portions of the plurality of cables in a manner that causes the cable-suspended platform to move from the first location to a second location; (h) measuring the respective distances of the center point of the cable-suspended platform from the respective anchor points based in part on the lengths of the paid-out portions of the plurality of cables when the cable-suspended platform is at the second location; (i) aiming the laser range meter at a second point corresponding to a second visible feature on the surface of the structure while the cable-suspended platform is at the second location and acquiring a second distance measurement; (j) using the pan-tilt mechanism to measure the respective pan and tilt angles of the laser range meter when the laser range meter is aimed at the second point; (k) converting the distance and angle measurements acquired in steps (i) and (j) into a second vector representing the location of the second point in the frame of reference of the cable-suspended platform; (l) generating a transformation matrix representing a position difference and an orientation difference between the first and second locations of the cable-suspended platform based on information acquired in steps (c) and (h); (m) multiplying the second vector by the transformation matrix to form a third vector representing the location of the second point in the frame of reference of the cable-suspended platform at the first location; and (n) calculating a distance between the first and second points using the first and third vectors. 
     In accordance with one embodiment, the method described in the preceding paragraph further comprises: (o) transmitting one or more messages containing measurement data from the cable-suspended platform; (p) receiving the one or more messages at a control station; and (q) extracting the measurement data from the message, wherein steps (f) and (k) through (n) are performed by a computer system at the control station. This method may further comprise: using the camera to capture an image of a portion of the surface of the structure that includes the first and second visible features while the cable-suspended platform is at the first location; and displaying the image and symbology representing a value of the distance calculated in step (n) overlaid on the image on a display screen. For example, the first and second visible features may be respective endpoints of an anomaly in the structure. 
     A further aspect of the subject matter disclosed herein is a system for inspecting and measuring a structure comprising: at least two anchor points; at least two pulleys, each pulley being supported by a respective anchor point; at least two spools; at least two spool motors, each spool motor being operatively coupled for driving a respective spool to rotate; at least two rotational encoders, each rotational encoder being operatively coupled to detect incremental angular movements of a respective spool; a platform comprising a rigid support structure, a pan-tilt mechanism mounted to the rigid support structure, a camera mounted to the pan-tilt mechanism, and a laser range meter mounted to the camera; at least two cables connected to the rigid support structure, each cable having a first portion wrapped around a respective spool and a second portion in contact with a respective pulley; a computer system configured to control operation of the pan-tilt mechanism and the spool motors and to selectively activate the camera and laser range meter; and a transceiver configured to enable communication between the computer system and a control station. The computer system is further configured to: receive image data from the camera, pan and tilt angle data from the pan-tilt mechanism, distance data from the laser range meter, and rotation data from the rotational encoders; determine a first location of the platform relative to a structure; and send commands for controlling the spool motors in a manner that causes the platform to move from the first location to a second location whereat the camera is separated from a surface of the structure by a goal offset. 
     In accordance with one embodiment of the system described in the preceding paragraph, the rigid support structure comprises an equipment support member and a cable attachment ring affixed to or integrally formed with the equipment support member, and the number of cables attached to the cable attachment ring is at least three. In accordance with another embodiment of the system described in the preceding paragraph, the rigid support structure comprises an equipment support member and a trolley affixed to or integrally formed with the equipment support member, the number of cables attached to the trolley is two, and the number of anchor points is two, further comprising a third cable connecting the two anchor points, and wherein the trolley comprises first and second rollers that roll on the third cable. 
     Other aspects of systems and methods for acquiring three-dimensional coordinates of points or scale and point-to-point distance information for objects in an environment using a remotely operated cable-suspended platform are disclosed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features, functions and advantages discussed in the preceding section can be achieved independently in various embodiments or may be combined in yet other embodiments. Various embodiments will be hereinafter described with reference to drawings for the purpose of illustrating the above-described and other aspects. None of the diagrams briefly described in this section are drawn to scale. 
         FIG. 1  is a diagram showing a system for inspecting and measuring a target object using a camera-equipped cable-suspended platform in accordance with some embodiments. 
         FIG. 2A  is a block diagram identifying some components of the camera-equipped cable-suspended platform depicted in  FIG. 1 . 
         FIG. 2B  is a block diagram identifying other components of the system depicted in  FIG. 1 . 
         FIG. 2C  is a diagram representing a top view of a platform suspended from four cables supported by respective anchor points that lie in a plane in a rectangular arrangement. Various dimensions of interest are indicated. 
         FIG. 3  is a diagram showing a top view of a video camera having a pair of laser pointers directed at a target object in accordance with one embodiment. 
         FIG. 4  is a block diagram identifying some components of a system for performing non-destructive inspection of a structure using a remote-controlled cable-suspended platform having two or more laser pointers. The configuration of the laser pointers may be selected from the alternative embodiments depicted in  FIGS. 3, 6 and 8 . 
         FIG. 5A  is a diagram showing a video camera and a pair of laser pointers separated from a target object by a distance D, which laser pointers produce respective laser spots separated by a distance d on the surface of the target object. 
         FIG. 5B  is a diagram representing an image acquired by the video camera depicted in  FIG. 5A , which image contains a representation of the target object. 
         FIG. 6  is a diagram showing a top view of a video camera having a pair of pivotable laser pointers directed at a target object in accordance with another embodiment. 
         FIG. 7A  is a diagram showing a video camera and a pair of pivotable laser pointers separated from a target object by a distance D, which laser pointers produce respective laser spots separated by a distance don the surface of the target object. 
         FIG. 7B  is a diagram representing an image acquired by the video camera depicted in  FIG. 7A , which image contains a representation of the target object. 
         FIG. 8  is a diagram showing a top view of a video camera having a pair of laser pointers (a first color) and a pivotable (about a single axis) third laser pointer (a second color) directed at a target object in accordance with a further embodiment. 
         FIG. 9A  is a diagram showing a video camera and three laser pointers configured as depicted in  FIG. 8  and separated from a target object by a distance D, which laser pointers produce respective laser spots, the furthest apart of which are separated by a distance d on the surface of the target object. 
         FIG. 9B  is a diagram representing an image acquired by the video camera depicted in  FIG. 9A , which image contains a representation of the target object. 
         FIG. 10  is a diagram illustrating steps of a method for processing images to determine the distance in pixels between laser spots on a target object in accordance with one embodiment. 
         FIG. 11  is a diagram illustrating steps of a method for processing images to determine the distance in pixels between laser spots on a target object in a manner that improves the image processing efficiency. 
         FIG. 12  is a diagram showing a top view of a video camera having a pair of laser range meters directed at a target object in accordance with another embodiment. 
         FIG. 13  is a flowchart identifying steps of a method for operating a cable-suspended platform during non-destructive inspection of a target object in accordance with one embodiment. 
         FIG. 14  is a block diagram identifying some components of a system for performing non-destructive inspection of a structure using a remote-controlled cable-suspended platform having an on-board local positioning system. 
         FIG. 15  is a diagram showing a top view of a video camera having a laser range meter directed at a target object. 
         FIG. 16  is a flowchart identifying steps of a method for sizing a feature of a structure using a cable-suspended platform carrying a local positioning system. 
         FIG. 17  is a vector diagram illustrating a method for generating a vector representing the distance and direction from a first point on a target object to a second point on the target object using a cable-suspended platform that incorporates the video camera and laser range meters depicted in  FIG. 15 . 
         FIG. 18  is a block diagram identifying steps of a feedback control process for controlling the motion of a cable-suspended platform based on measurement data acquired by on-board equipment. 
         FIG. 19  is a diagram showing a system for inspecting and measuring a target object using a camera-equipped cable-suspended platform in accordance with alternative embodiments. 
         FIG. 20  is a diagram referred to in the Appendix and showing a position vector  A P extending from the origin of an instrument coordinate system {A}, substantially along the aim point axis of the instrument, to a point of interest P and showing a position vector  B P extending from the origin of a target object coordinate system {B} to the point of interest P. 
         FIGS. 21-23  are diagrams referred to in the Appendix, where an illustrative method for calculating a calibration matrix for coordinate system transformation is described. 
     
    
    
     Reference will hereinafter be made to the drawings in which similar elements in different drawings bear the same reference numerals. 
     DETAILED DESCRIPTION 
     For the purpose of illustration, systems and methods for acquiring three-dimensional coordinate information, scale, or point-to-point distance information for objects undergoing measurement or non-destructive inspection using a cable-suspended platform will now be described in detail. However, not all features of an actual implementation are described in this specification. A person skilled in the art will appreciate that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
       FIG. 1  is a diagram showing some components of a measurement system  2  in accordance with some embodiments for inspecting and measuring a target object  102 . The measurement system  2  includes a cable-suspended platform  16  that may be moved around a structure (e.g., target object  102 ) requiring periodic inspection. The measurement system  2  can be adapted for use in inspecting a wide range of target objects, including aircraft. The measurement system  2  is particularly well suited for use inside large buildings such as manufacturing facilities and warehouses. 
     In accordance with the measurement system  2  depicted in  FIG. 1 , the cable-suspended platform  16  is suspended from four cables  6   a - 6   d . More specifically, the cable-suspended platform  16  comprises a rigid support structure  4   a . In accordance with one embodiment, the rigid support structure  4   a  comprises an equipment support member  36  having a cable attachment ring  35  affixed thereto or integrally formed therewith. The ends of the cables  6   a - 6   d  are attached to the cable attachment ring  35 . The cables  6   a - 6   d  are paid out or pulled in by respective motor-driven spools  8   a - 8   d  of respective winch units (described in some detail below with reference to  FIG. 2B ), with the paid-out portions of the cables being passed over respective pulleys  12   a - 12   d . The pulleys  12   a - 12   d  are in turn rotatably coupled to respective yokes  14   a - 14   d  which are rigidly connected to respective anchor points  10   a - 10   d.    
     The anchor points  10   a - 10   d  may be attached to walls or ceilings for indoor applications or mounted on poles or cranes for outdoor applications. The anchor points  10   a - 10   d  need not be located in the same plane. As long as the locations of the spools  8   a - 8   d  and the lengths of the paid-out portions of cables  6   a - 6   d  are known, the computer system (not shown in  FIG. 1 ) that controls movement of the cable-suspended platform  16  can be configured to track the location of the center of the cable attachment ring  35  in the frame of reference of the anchor points. Nor is the measurement system  2  limited to the use of four anchor points. For example, three anchor points are sufficient and more than four can be utilized. 
     In accordance with the embodiment depicted in  FIG. 1 , the cable-suspended platform  16  further comprises a local positioning system  38  attached to a bottom end of the equipment support member  36 . The local positioning system  38  comprises: a pan-tilt mechanism  120  mounted to the bottom end of the equipment support member  36 ; a camera  20  mounted to the pan-tilt mechanism  120  and having a camera aperture  26 ; and a laser range meter  138  for projecting a laser beam along an aim direction vector  134  (indicated by a dashed line in  FIG. 1 ) onto the target object  102  to form a laser spot (not shown in  FIG. 1 ) on a surface of the target object  102 . The pan-tilt mechanism  120  comprises a pan unit  126  and a tilt unit  128 . The camera  20  comprises a housing  22  to which the laser range meter  138  is mounted. 
     The camera  20  may comprise a still camera (color and/or black and white) to obtain still images, a video camera to obtain color and/or black and white video, or an infrared camera to obtain infrared still images or infrared video of the target object  102 . It may be possible to have a single camera that can perform the functions of all of these types. The local positioning system  38  comprises a computer system (not shown in  FIG. 1 , but see computer system  162  in  FIG. 14 ) which is configured to measure coordinates of points on the target object  102  defined in the local coordinate system of the target object  102 . In particular, the computer system is programmed to control motions of the pan-tilt mechanism  120  to rotationally adjust the camera  20  to selected angles around the vertical, azimuth (pan) axis and the horizontal, elevation (tilt) axis. The computer system is also programmed to control operation of the camera  20  and receive image data therefrom for transmission to the control station. The computer system is further programmed to control operation of the laser range meter  138  and receive range data therefrom for transmission to the control station. 
     The on-board system of the cable-suspended platform  16  may further comprise a wireless transceiver and an antenna (not shown in  FIG. 1 ) to enable bidirectional, wireless electromagnetic wave communications with a control station  150  (see  FIG. 2B ). 
     The measurement system  2  disclosed herein leverages existing local coordinate measurement and remote operation techniques, specifically the capabilities of the local positioning systems described U.S. Pat. Nos. 9,285,296, 8,447,805 and 7,859,655, the disclosures of which are incorporated by reference herein in their entireties. The image data acquired by the video camera of the local positioning system may undergo image processing as disclosed in U.S. Pat. No. 8,744,133. 
     Still referring to  FIG. 1 , an aim direction vector  134  that describes the orientation of the laser range meter  138  relative to a fixed coordinate system of the cable-suspended platform  16  is determined from the azimuth and elevation angles. Using the laser range meter  138 , the orientation of the aim direction vector  134  in the instrument coordinate system is measured when the laser beam emitted by the laser range meter  138  is in turn aligned with each of three calibration points  5   a - 5   c  on the surface of the target object  102 , wherein positions of the three calibration points  5   a - 5   c  in the target object coordinate system are known. This method also includes measuring a distance (i.e., range) substantially along the aim direction vector  134  from the laser range meter  138  to each of the three calibration points  5   a - 5   c . This method also includes calculating a calibration matrix which transforms a position defined in the instrument coordinate system to a position defined in the target object coordinate system using at least the measured orientation and distance in the instrument coordinate system corresponding to the three calibration points  5   a - 5   c  and the known positions of the three calibration points  5   a - 5   c  in the target object coordinate system. 
     The control station  150  (see  FIG. 2B ) comprises a computer system that is programmed with three-dimensional localization software that is used to process the range data received from the computer system  162  onboard the cable-suspended platform  16 . For example, the three-dimensional localization software may be of a type that uses multiple calibration points  5   a - 5   c  on the target object  102  (such as points or features on a surface on an aircraft) to define the location (position and orientation) of camera  20  relative to target object  102 . The calibration points  5   a - 5   c  may be visible features of known position (such as the corner of a window frame, a screw used to attach the pitot tube, etc.) in the local coordinate system of the target object  102  as determined from a three-dimensional database of feature positions (e.g., a CAD model) or other measurement technique. During the process of calibrating the local positioning system, X,Y,Z data for at least three non-collinear points are extracted from the CAD model or other source of three-dimensional data. Typically calibration points  5   a - 5   c  are selected which correspond to features that can be easily located on the target object. The three-dimensional localization software utilizes the calibration points  5   a - 5   c  and the pan and tilt data from the pan-tilt mechanism  120  to define the relative position and orientation of the camera  20  with respect to the local coordinate system of the target object  102  (described in more detail below). The measured distances to the calibration points  5   a - 5   c  may be used in coordination with the azimuth and elevation angles from the pan-tilt mechanism  120  to solve for the camera position and orientation relative to the target object  102 . Further details concerning a methodology for generating a camera pose transformation matrix reflecting the position and orientation of a camera relative to a coordinate system of a target object are set forth in the Appendix. 
     Once the position and orientation of the camera  20  with respect to the target object  102  have been determined, the onboard computer system  162  may be operated to rotate and zoom the optical image field of the camera  20  to a point of interest of unknown coordinate position on the target object  102 , which may be a damage/repair location on an aircraft, for example. At this position of the aim direction vector  134 , the orientation of the camera  20  (which may include the respective angles of the camera  20  along the azimuth axis and the elevation axis) may be recorded. By using the azimuth and elevation angles from the pan-tilt mechanism  120  and the relative position and orientation (i.e., relative location) of the camera  20  determined in the calibration process, the location of the point of interest can be determined relative to the coordinate system of the target object  102 . The damage/repair location on the target object  102  may be sized using techniques which are described in some detail below. In the case of a crack, the length of the crack may be measured. 
     The reverse process, in which the position of a point of interest may be known in the target object&#39;s coordinate system (from a previous data acquisition session, a CAD model, or other measurement), can also be performed. In this situation, the camera  20  may be placed in any location on the work area where calibration points  5   a - 5   c  are visible (which may be in a different location than the location where the original data was recorded) and the instrument-to-target calibration step may be performed. This calibration is referred to herein as “the camera pose”, but it is associated with more than just the camera; for example, it may also include instrumentation for measuring distance (such as a laser range meter). The direction vector from the point of interest to the camera  20  may be calculated in the target object&#39;s coordinate system. The inverse of the camera pose transformation matrix may be used to convert the direction vector into the coordinate system of the cable-suspended platform  16 . The azimuth and elevation angles may then be calculated and used by the pan-tilt mechanism  120  to aim the camera  20  at the point of interest on the target object  102 . 
     In a typical implementation, the local positioning system may be set up within about 10-50 feet of the target object  102 . The target object  102  may, for example, be a structure such as a storage tank or a large vehicle such as an aircraft. The calibration points  5   a - 5   c  on the target object  102  may be selected and used by the three-dimensional localization software in conjunction with the pan and tilt data (i.e., azimuth and elevation angles) from the pan-tilt mechanism  120  and distance data from the laser range meter  138  to determine the position and orientation of the camera  20  with respect to target object  102 . The calibration points  5   a - 5   c  may be feature points of known position in the local coordinate system of the target object  102  as determined from a three-dimensional CAD model or other measurement technique. 
     As should be apparent from the above description of the process for calibrating the location of the cable-suspended platform  16  in the frame of reference of a target object  102  having three calibration points  5   a - 5   c  with known local coordinates, in the alternative the location of the cable-suspended platform  16  can be calibrated in the frame of reference of the measurement system  2 . In this case, the anchor points  10   a - 10   d  may be used as calibration points. To calibrate the system using the known locations of the anchor points  10   a - 10   d , the cable-suspended platform  16  is moved to a position where at least three of the anchor points  10   a - 10   d  can be targeted by the laser range meter  138 . Distance and pan-tilt angle data is acquired for the three anchor points and then compared to the known coordinates of those anchor points using a vector-based approach. 
     An enhanced type of calibration for the entire working volume (or a portion of the working volume) of the measurement system  2  can also be performed in order to compensate for cable stretch. For example, the calibration process described above can be performed for the eight corners of a virtual bounding box surrounding the expected measurement volume. This data is then compared to data for the same points calculated by a cable length-based 3-D positioning method (described by equations set forth below). One simple way to use the eight-corner calibration data is to perform a tri-linear interpolation at run-time to provide a correction amount to adjust the value measured by the cable length-based process. (There are other interpolation approaches using cable length data that could also be used.) 
     The laser range meter  138  (also called “a laser range finder” and “laser distance meter”) is affixed to the camera  20  to create a laser hybrid system. Measurement data from the laser range meter  138  can be used to obtain estimates of the respective distances from the laser range meter  138  (and from the camera  20  to which the laser range meter is fixed) to calibration points on a target object. A typical laser range meter comprises a laser diode which transmits a bundled, usually visible, laser beam toward a surface of a target object. The light which is backscattered and/or reflected by the target object is imaged on the active surface of a photoreceiver by receiving optics. The laser diode has a position and an orientation which are fixed relative to the position and orientation of the video camera; the photoreceiver has a position and an orientation which are fixed relative to the position and orientation of the laser diode. The time-of-flight between transmission and reception of the light can be used to calculate the distance between the laser range meter and the portion of the target object surface on which the transmitted beam impinged. The laser range meter also functions as a laser pointer. Alternatively, a distance meter which directionally projects wave energy other than a laser beam could be utilized. 
     For the sake of completeness, it may be noted that the foregoing methods for determining the three-dimensional coordinates of a point of interest on a target object relative to a frame of reference using the cable-suspended platform  16  depicted in  FIG. 1  have the following steps in common: (a) suspending the cable-suspended platform  16  from a plurality of cables  6   a - 6   d  connected to the equipment support member  36  and supported at respective anchor points of a plurality of anchor points  10   a - 10   d ; (b) calibrating the pan-tilt mechanism  120  relative to the frame of reference; (c) controlling lengths of paid-out portions of the plurality of cables  6   a - 6   d  in a manner that causes the equipment support member  36  to move so that the camera  20  is within range of a target object  102 ; (d) measuring the respective distances of a center point of the equipment support member  36  from the respective anchor points  10   a - 10   d  based in part on the lengths of the paid-out portions of the plurality of cables  6   a - 6   d ; (e) controlling the pan-tilt mechanism  120  to cause the laser range meter  138  to aim at a point of interest on the target object  102 ; (f) measuring the pan and tilt angles of the pan-tilt mechanism  120  while the laser range meter  138  is aimed at the point of interest; (g) measuring the distance separating the laser range meter  138  and the point of interest; and (h) converting the distance and angle measurements into a Cartesian coordinate vector representing the location of the point of interest in the frame of reference. 
     In accordance with one embodiment of the method described in the preceding paragraph, the frame of reference is a frame of reference of the target object  102 , and step (b) comprises: aiming the laser range meter  138  at three or more calibration points  5   a - 5   c  on the target object  102  at different times while the equipment support member  36  is stationary; and computing a calibration matrix representing a transformation from a frame of reference of the pan-tilt mechanism  120  to the frame of reference of the target object  102 . In one proposed implementation, step (b) further comprises: measuring the pan and tilt angles of the pan-tilt mechanism  120  while the laser range meter  138  is aimed at each calibration point  5   a - 5   c ; and measuring the distance separating the laser range meter  138  and each calibration point  5   a - 5   c  while the laser range meter  138  is aimed at each calibration point  5   a - 5   c.    
     In accordance with an alternative embodiment, the frame of reference is a frame of reference of a plurality of at least three non-collinear anchor points of the plurality of anchor points  10   a - 10   d  supporting respective cables of the plurality of cables  6   a - 6   d , and step (b) comprises: aiming the laser range meter  138  at three or more anchor points of the plurality of anchor points  101 - 10   d  at different times while the equipment support member  36  is stationary; and computing a calibration matrix representing a transformation from a frame of reference of the pan-tilt mechanism  120  to the frame of reference of the plurality of anchor points  10   a - 10   d . In one proposed implementation, step (b) further comprises: measuring the pan and tilt angles of the pan-tilt mechanism  120  while the laser range meter  138  is aimed at each anchor point; and measuring the distance separating the laser range meter  138  and each anchor point while the laser range meter  138  is aimed at each anchor point. 
       FIG. 2A  identifies some components of the cable-suspended platform  16 , including a counterbalance weight  30  not shown in  FIG. 1 . The counterbalance weight  30  is connected to the top end of the equipment support member  36 , while the pan-tilt mechanism  120  is connected to the bottom end of the equipment support member  36 . The counterbalance weight  30  provides an inert mass to balance the rotational inertia of the camera  20 . Preferably the center of the cable attachment ring is approximately collocated with the center of gravity of cable-suspended platform  16  to minimize undesired rotational or pendular motion of cable-suspended platform  16  as it is being moved by the extension and/or retraction of one or more of the supporting cables  6   a - 6   d . In addition, a gyroscope may be incorporated in the counterbalance weight  30  or other locations on the platform to stabilize the orientation of the equipment support member  36 . The system stays vertical because of gravity. While there is a counterbalance weight  30  on top, it is not as heavy as the payload below the cable supports. The gyroscope ensures the orientation does not change as the system is moving. A mechanical gyroscope has one or more fast spinning disks that allow the system to maintain orientation. In basic implementations, this type of gyroscopic stabilization can be accomplished passively without computer control. The spinning disks could also be part of the counterbalance weight  30 . In solid-state gyroscopic devices (such as MEMS-based inertial measurement units), the gyroscope measures rotation rates and then motors are used to correct the orientation of the system (a microprocessor or computer of some type would be configured to send commands to respective motor controllers in this variation).  FIG. 2B  is a block diagram identifying additional components of each winch unit incorporated in the measurement system  2  depicted in  FIG. 1 . Each winch unit comprises a motor-driven spool  8  for supporting a respective one of the cables  6   a - 6   d  and a cable guide  24  which guides the cable onto and along the length of the spool  8  during winding. Each spool  8  is driven to rotate by a respective spool motor  116 . Each cable guide  24  is a mechanical device that moves based on gearing to the same spool motor  116  that drives the spool  8 . The spool motors  116  are under the control of respective motor controllers  114  that receive commands from a winch unit control computer  112 . The winch unit control computer  112  coordinates the rotation of each spool  8  by sending appropriate commands to the associated motor controllers  114 . 
     Each spool  8  has an associated rotational encoder  118  that outputs signals representing incremental angular rotations of the spool  8 . The length of each cable which has been paid out from each spool  8  can be determined by the winch unit control computer  112  based in part on feedback from the respective rotational encoder  118 . 
     The winch unit control computer  112  communicates (via wired connections or wirelessly) with a control station  150  by way of transceivers  160   a  and  160   b . In response to commands input by a system operator via a user interface at the control station  150 , the control station  150  sends commands to the winch unit control computer  112  via transceivers  160   a  and  160   b  for controlling the movement of the cable-suspended platform  16 . The winch unit control computer  112  in turn sends commands to the motor controllers  114  for extending and/or retracting the cables  6   a - 6   d  by operation of the motor-driven spools  8 . Preferably all of the spool motors  116  are run by the same high-level computer/operator interface. When all motors  116  are operated by the same computer (e.g., winch unit control computer  112 ), the computer can be configured to use inverse kinematics to direct the position and orientation of the cable-suspended platform  16  simultaneously—which means that one can specify a location (position and orientation), such as with a 4×4 transformation matrix, and then have the system move everything at once to exactly the right place. 
     The exact position of the center of the cable attachment ring  35  can be tracked by configuring the winch unit control computer  112  to calculate the respective lengths of the paid-out portions of cables  6   a - 6   d  by counting the pulses output by the respective rotational encoders associated with each spool  8  (i.e., spools  8   a - 8   d ). Regarding the cable winding on each spool  8 , if done correctly it is possible to accurately compute the length of the paid-out (i.e., unwound) portion of the cable. As the spool  8  rotates, the associated rotational encoder generates a pulse for each incremental change in angular position during spool rotation in either direction. However, the amount of cable paid out or wound up per incremental rotation will vary as a function of the number of layers of cable wound on the spool. One way that this can be addressed is by managing the way the cable wraps on the spool so that the cable wraps are right next to each other instead of randomly wrapped on the spool  8 . This is usually managed by the cable guide  24 , which moves back and forth along the width of the spool  8 . When the cable arrives at one end of the spool  8 , the cable guide  24  reverses direction and puts the cable on the next level above the previous level, but if the system counts the number of levels of cable on the spool  8 , then the system can modify the effective diameter variable associated with the spool  8  (with cable on it) which allows the system to calculate the correct value for the incremental length of cable paid out from or wound up on the spool  8  per incremental rotation of the spool  8 . 
       FIG. 2C  is a diagram representing a top view of a center point C of a platform suspended from four cables supported by respective anchor points AP 1  through AP 4  that lie in a plane in a rectangular arrangement. In this example, anchor point AP 1  is separated from anchor point AP 2  by a distance d 2 ; anchor point AP 2  is separated from anchor point AP 3  by a distance d 1 ; anchor point AP 3  is separated from anchor point AP 4  by distance d 2 ; and anchor point AP 4  is separated from anchor point AP 1  by distance d 1 . The cables connecting the cable-suspended platform to the four anchor points AP 1  through AP 4  have respective cable lengths L 1 , L 2 , L 3  and L 4 . (Note: In the case where the ends of the cables are attached to a ring having a radius R, the respective cable lengths L 1 , L 2 , L 3  and L 4  as used in the equations will each be the sum of the actual length of the cable extending from the respective anchor point to the respective attachment point on the cable attachment ring and the radius R.) 
     At any given instant in time, the control station  150  has information from the rotational encoders  118  indicating the cable lengths L 1 , L 2 , L 3  and L 4 . These measurements can be used to compute the three-dimensional position (x, y, z) of the center point C (note that with the z (vertical) dimension is not shown in  FIG. 2C ) in the frame of reference of the measurement system  2 . To accomplish the foregoing, the following forward kinematics (constraint) equations are used:
 
 L   1   2   =x   2   +y   2   +z   2  
 
 L   2   2   =x   2 +( d   2   −y ) 2   +z   2  
 
 L   3   2 =( d   1   −x ) 2 +( d   2   −y ) 2   +z   2  
 
 L   4   2 =( d   1   −x ) 2   +y   2   +z   2  
 
These equations form a set of non-linear simultaneous equations. From inverse kinematics, one can derive the resulting closed-form x, y, z solution:
 
     
       
         
           
             x 
             = 
             
               
                 
                   L 
                   2 
                   2 
                 
                 - 
                 
                   L 
                   3 
                   2 
                 
                 + 
                 
                   d 
                   1 
                   2 
                 
               
               
                 2 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   d 
                   1 
                 
               
             
           
         
       
       
         
           
             y 
             = 
             
               
                 
                   L 
                   1 
                   2 
                 
                 - 
                 
                   L 
                   2 
                   2 
                 
                 + 
                 
                   d 
                   2 
                   2 
                 
               
               
                 2 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   d 
                   2 
                 
               
             
           
         
       
       
         
           
             z 
             = 
             
               ± 
               
                 
                   
                     L 
                     4 
                     2 
                   
                   - 
                   
                     d 
                     1 
                     2 
                   
                   + 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       d 
                       1 
                     
                     ⁢ 
                     x 
                   
                   - 
                   
                     x 
                     2 
                   
                   - 
                   
                     y 
                     2 
                   
                 
               
             
           
         
       
     
     In a right-handed coordinate system (with z in the up direction), the z equation that should be used is the negative one:
 
 z =−√{square root over ( L   4   2   −d   1   2 +2 d   1   x−x   2   −y   2 )}
 
     Thus for any point of interest on the target object  102 , the computer system at the control station  150  may be configured with position computation software that enables determination of the three-dimensional coordinates of that point of interest in the coordinate frame of reference of the measurement system, and also enables determination of the position of the center point C of the cable-suspended platform  16  in the coordinate frame of reference of the measurement system, and the pan and tilt angles of the pan-tilt mechanism  120  which would result in the generation of a laser beam by the laser range meter  138  that impinges on that point of interest. 
     Cable-suspended platforms of the type depicted in  FIG. 1  may further include the capability to acquire scale, and point-to-point distance information for objects undergoing measurement or non-destructive inspection. The cable-suspended platform may be provided with on-board sensors and processing techniques to provide discrete or continuous measurements of the distances between points on a target object or the scale of the target object. The acquisition of such distance and scale measurement data will be divided into the following three categories. 
     In one category of embodiments, a gimbaled video camera and two or more laser pointers mounted thereto are used to acquire the information to compute: distance to the target, a reference scale for the view of the target, and in some embodiments, distance between points of interest on the target. This category of embodiments is applicable to situations where the target surface is relatively flat and perpendicular to the aim direction of the laser pointers and camera. As used herein, the term “laser pointer” means a device that emits a laser beam and does not detect returned laser light. 
     Another category of embodiments of the concept are configurations where two or more laser range meters are mounted to a gimbaled video camera to enable: three-dimensional coordinate measurement of points of interest on the target, direct measurement of distance to the target, reference scale, as well as one or more orientation angle of the video camera relative to the target. If three non-collinearly mounted laser range meters are used, more than one orientation angle can be measured (for example, yaw and pitch). As used herein, the term “laser range meter” (also known as “laser rangefinder”) means a device that emits a laser beam and detects returned laser light. 
     A third category of embodiments includes a gimbaled video camera and a single laser range meter mounted thereto used to acquire distance and aim direction information from the cable-suspended platform to target objects in the environment. This concept leverages some aspects of the vector-based measurement algorithms disclosed in U.S. Pat. No. 7,859,655 (the disclosure of which is incorporated by reference herein in its entirety), along with the addition of sensors, such as cable length rotational encoders and system configuration (kinematics), to determine the relative position of the cable-suspended platform relative to the environment. This platform motion data along with the aim direction and distance data from the laser range meter can be used to acquire measurements of objects in the environment. 
     Example embodiments of the foregoing types of enhanced-capability cable-suspended platforms will now be described in some detail. For the purpose of illustration, it will be assumed that the platform comprises a video camera  130  having various laser devices affixed thereto (e.g., affixed to a camera housing  22 ). 
       FIG. 3  is a diagram showing a top view of a video camera  130  having a pair of laser pointers  132   a  and  132   b  affixed thereto and directed at a target object  102  in accordance with one embodiment, which video camera  130  can be incorporated in the cable-suspended platform  16  depicted in  FIG. 1 . Such a system is capable of acquiring scale and point-to-point distance information for objects undergoing non-destructive inspection. The laser pointers  132   a  and  132   b  are mounted to the housing  22  of the video camera  130  in a parallel configuration. Preferably the focal axis of the video camera  130  and the aim directions of the laser pointers  132   a  and  132   b  are mutually parallel. The laser pointers  132   a  and  132   b  and the video camera  130  are used to calculate distance to target object  102  and reference scale. This embodiment is used for situations where the cable-suspended platform  16  is relatively close to the target object  102 . 
     When activated, the laser pointers  132   a  and  132   b  direct respective mutually parallel laser beams along respective optical paths indicated by respective aim direction vectors  134   a  and  134   b  in  FIG. 3 , which laser beams form respective laser spots on a surface of the target object  102 . The video camera  130  may be activated to capture an image in which the two laser spots are visible. This image data can be processed (as described in some detail below) to derive pixel information which, in conjunction with the known distance separating the axes of the two laser pointers  132   a  and  132   b , can be used to determine a scale factor. That scale factor can then be used to display a scale indicator on any subsequent image captured by the video camera  130  while the cable-suspended platform  16  is at the same location. More specifically, one goal is to determine the distance D between the pointers  132   a  and  132   b  and the target object  102 , as will be described in more detail below with reference to  FIGS. 5A and 5B . 
       FIG. 4  is a block diagram identifying some components of a system for performing non-destructive inspection of a structure using a remote-controlled cable-suspended platform  16  having two or more laser pointers  132  (e.g., a first laser pointer  132   a  and a second laser pointer  132   b  as seen in  FIG. 2 ) mounted thereon. In this example, the components of the local positioning system  38  are controlled by an on-board computer system  162 , which may be configured with programming stored in a non-transitory tangible computer-readable storage medium (not shown). In particular, the computer system  162  may be programmed to execute radiofrequency commands received from the control station  150 . Those radiofrequency commands are received by a transceiver  160  on-board the cable-suspended platform  16 , converted into the proper digital format and then forwarded to the computer system  162 . 
     The control station  150  may comprise a general-purpose computer system configured with programming for controlling operation of the cable-suspended platform  16  by sending commands to the winch unit control computer  112  (not shown in  FIG. 4 , but see  FIG. 2A ), as previously described, and for controlling various components of the local positioning system  38 . For example, the control station  150  may send commands controlling the movements of the cable-suspended platform  16  and the pan-tilt mechanism  120  and commands for activation of the laser pointers  132  and the video camera  130 . In addition, the computer system at the control station  150  is configured with programming for processing data received from the cable-suspended platform  16  during an inspection operation. In particular, the computer system of the control station  150  may comprise a display processor configured with software for controlling a display monitor  152  to display images captured by the video camera  130 . The optical image field, as sighted by the video camera  130 , can be displayed on the display monitor  152 . More specifically, in response to commands from the control station  150 , the computer system  162  sends control signals for activating the video camera  130  and the laser pointers  132  (e.g., via electrical cables). The video camera  130  may have automated (remotely controlled) zoom capabilities. The computer system  162  also controls the rotations of the pan unit  126  and tilt unit  128  (see  FIG. 1 ) of the pan-tilt mechanism  120  by sending commands to the motor controllers  168  which respectively control a pan motor  122  and a tilt motor  124 . 
     In accordance with one embodiment, the pan-tilt mechanism  120  comprises a pan unit  126  configured to rotate the video camera  130  (and laser devices mounted thereto) about a pan axis and a tilt unit  128  configured to rotate the video camera  130  about a tilt axis (orthogonal to the pan axis) in response to control signals received from the computer system  162 . Actuators (not shown in the drawings), such as servo-motors or the like, in the pan-tilt mechanism  120  may receive and respond to control signals from the computer system  162  by adjusting the angular rotation of the video camera  130  about the pan and tilt axes, as well as the angular speed at which the video camera  130  rotates about the pan and tilt axes. The pan-tilt mechanism  120  further comprises pan and tilt rotational encoders (not shown in the drawings) that send signals representing current angular position data back to the computer system  162 . The control signals applied to the pan-tilt mechanism  120  may be computed by the computer system  162  in response to user instructions (e.g., manipulation of an input device that is part of the control station  150 ) or an automatic scan path generator. 
       FIG. 5A  is a diagram showing a video camera  130  and a pair of laser pointers  132   a  and  132   b  separated from a target object  102  by the distance D, which laser pointers produce respective laser spots on the surface of the target object  102 . These laser spots on the target object surface are separated by the distance d.  FIG. 5B  is a diagram representing an image  70  acquired by the video camera  130  depicted in  FIG. 5A , which image  70  includes a representation  102 ′ of the target object  102  and representations of the respective positions  106  and  108  of the laser spots. 
     In accordance with the situation depicted in  FIGS. 5A and 5B , the known variables are the current field-of-view of the video camera  130  (i.e., “FoV” in  FIG. 5A ), the maximum number of pixels in the width direction of the image  70  (i.e., “maxPx” in  FIG. 5B ), the number of pixels in the image  70  between the respective groups of pixels representing positions  106  and  108  of the laser spots produced by laser pointers  132   a  and  132   b  (i.e., “nPx,” in  FIG. 5B ); and the distance separating the laser pointers  132   a  and  132   b  (i.e., “L 1 ” in  FIG. 5A ). The unknown variables are the viewing angle α between the laser spots and the distances D and d. 
     The viewing angle α between the laser spots can be computed using the camera field-of-view (FoV) and image pixel data: 
                   α   =     2   *     atan   ⁡     (       nPx     max   ⁢           ⁢   Px       *     tan   ⁡     (     FoV   2     )         )                 (   1   )               
where nPx is the measured number of pixels between laser spots, and maxPx is the image width in pixels. Then the distances d and D can be computed using the following equations:
 
             d   =     L   1                 D   =         L   1     /   2       tan   ⁡     (     α   /   2     )               
Substituting Eq. (1) for the viewing angle α, one obtains:
 
             D   =         L   1     /   2         nPx     max   ⁢           ⁢   Px       *     tan   ⁡     (     FoV   2     )                 
In accordance with the embodiment depicted in  FIGS. 3 and 5A  (and other embodiments described hereinafter), the value of the distance D is updated continuously.
 
     In accordance with one possible implementation, the value of the distance d may be included anywhere in the image  70  displayed on the display monitor  152 . In accordance with another possible implementation, a scale factor can be calculated based on a ratio of the distance d and the number of pixels nPx and a scale bar or other scale indicator indicating the scale factor can be included as part of the image  70 . This scale indicator will be accurate so long as the cable-suspended platform-target object separation distance D is up-to-date. As that separation distance changes, the operations described above can be repeated to generate an updated scale factor. Over time, the scale indicator is repeatedly adjusted as a function of the variable distance separating the cable-suspended platform  16  and the target object  102 . 
     For the purpose of non-destructive inspection, preferably the acquired images of the inspected structure do not include representations of laser spots. Accordingly, following the initial sizing of the imaged surface area of the target object  102 , the video camera  130  can be activated to capture additional images (e.g., a video sequence of images) while the laser pointers  132   a  and  132   b  are de-activated. In this case, the video camera  130  preferably captures images while the separation distance D is up-to-date. 
       FIG. 6  is a diagram showing a top view of a video camera  130  having a pair of pivotable laser pointers  132   a  and  132   b  directed at a target object  102  in accordance with another embodiment, which video camera  130  can be incorporated in the cable-suspended platform  16  depicted in  FIG. 1 . Like the embodiment partly depicted in  FIG. 3 , the embodiment partly depicted in  FIG. 6  is also capable of acquiring scale and point-to-point distance information for objects undergoing non-destructive inspection. When activated, the laser pointers  132   a  and  132   b  direct respective laser beams at respective laser spots on a surface of a target object  102 . The laser pointers  132   a  and  132   b  may be independently pivotable or their pivoting mechanism may be coupled so that the laser pointers  132   a  and  132   b  are oppositely pivotable. As used herein, the phrase “oppositely pivotable” means that the angular positions of the laser pointers  132   a  and  132   b  relative to the focal axis (not shown in  FIG. 6 ) of the video camera  130  pivot are equal and opposite at all times. 
     The laser pointers  132   a  and  132   b  can be rotated on-board the cable-suspended platform  16  by a known amount relative to the parallel configuration. This creates additional separation between the laser spots on the target object  102 , which is useful for situations where the cable-suspended platform  16  is further from the target object  102  than may be the case for the embodiment depicted in  FIG. 3 . For example, initially the laser pointers  132   a  and  132   b  are positioned in parallel to emit respective laser beams along mutually parallel optical paths indicated by respective aim direction vectors  134   a  and  134   b . Then the laser pointers  132   a  and  132   b  are rotated by a known angle and then activated again to emit respective laser beams along optical paths indicated by respective aim direction vectors  134   a ′ and  134   b ′. The distance to the target object  102  can be determined using images captured by the on-board video camera (not shown), which images include the groups of pixels representing the laser spots. More specifically, this embodiment is configured to determine the distance d between the laser spots respectively produced on the target object  102  by the laser pointers  132   a  and  132   b ; and the distance D between the laser pointers  132   a  and  132   b  and the target object  102 , as will be described in more detail below with reference to  FIGS. 7A and 7B . 
       FIG. 7A  is a diagram showing a video camera  130  and a pair of pivotable laser pointers  132   a  and  132   b  separated from a target object  102  by the distance D, which laser pointers produce respective laser spots on the surface of the target object  102 . These laser spots on the target object surface are separated by the distance d.  FIG. 7B  is a diagram representing an image  70  acquired by the video camera  130  depicted in  FIG. 7A , which image  70  includes a representation  102 ′ of the target object  102  and respective groups of pixels representing respective positions  106  and  108  of the laser spots. 
     In accordance with the situation depicted in  FIGS. 7A and 7B , the known variables are the field-of-view of the video camera  130  (i.e., “FoV” in  FIG. 7A ), the maximum number of pixels in the width direction of the image  70  (i.e., “maxPx” in  FIG. 7B ), the number of pixels between the respective groups of pixels representing positions  106  and  108  of the laser spots produced by laser pointers  132   a  and  132   b  (i.e., “nPx,” in  FIG. 7B ); the angle between the laser beams transmitted by the laser pointers  132   a  and  132   b  (i.e., “β” in  FIG. 7A ); and the distance separating the respective pivot axes of the laser pointers  132   a  and  132   b  (i.e., “L 1 ” in  FIG. 7A ). The unknown variables are the viewing angle α between the laser spots and the distances D and d. 
     The viewing angle α between the laser spots can again be computed using Eq. (1). Then the distances d and D can be computed using the following equations: 
     
       
         
           
             d 
             = 
             
               
                 2 
                 * 
                 
                   L 
                   1 
                 
                 * 
                 
                   sin 
                   ⁡ 
                   
                     ( 
                     
                       α 
                       / 
                       2 
                     
                     ) 
                   
                 
                 * 
                 
                   cos 
                   ⁡ 
                   
                     ( 
                     
                       β 
                       / 
                       2 
                     
                     ) 
                   
                 
               
               
                 sin 
                 ⁡ 
                 
                   ( 
                   
                     
                       α 
                       - 
                       β 
                     
                     2 
                   
                   ) 
                 
               
             
           
         
       
       
         
           
             D 
             = 
             
               d 
               
                 2 
                 * 
                 
                   tan 
                   ⁡ 
                   
                     ( 
                     
                       α 
                       / 
                       2 
                     
                     ) 
                   
                 
               
             
           
         
       
     
     In accordance with one possible implementation, a scale factor can be calculated based on a ratio of the distance d and the number of pixels nPx and a scale bar or other scale indicator indicating the scale factor can be displayed on subsequent images captured by the video camera  130  while the cable-suspended platform  16  stays at the same location. 
       FIG. 8  is a diagram showing a top view of a video camera  130  having a pair of laser pointers  132   a  and  132   b  and a pivotable (about a single axis) third laser pointer  132   c  directed at a target object  102  in accordance with a further embodiment, which video camera  130  can be incorporated in the cable-suspended platform  16  depicted in  FIG. 1 . In accordance with this variation, the two laser pointers  132   a  and  132   b  are affixed to the video camera  130  and are mutually parallel and the third laser pointer  132   c  is rotatably mounted to the video camera  130  for rotation to a fixed or controllable angle relative to the other two laser pointers  132   a  and  132   b . The third laser pointer  132   c  may emit laser light having a different color than the laser light emitted by laser pointers  132   a  and  132   b  to help differentiate the laser spots from each other on the target object  102 . (In the alternative, this computational method can be made to use three laser pointers of the same color.) The laser pointers  132   a  and  132   b  emit respective laser beams along mutually parallel optical paths indicated by respective aim direction vectors  134   a  and  134   b , while the third laser pointer  132   c  emits a laser beam along the optical path indicated by aim direction vector  134   a  in  FIG. 8 . 
       FIG. 9A  is a diagram showing a video camera  130  and three laser pointers  132   a - c  configured as depicted in  FIG. 8  and separated from a target object  102  by the distance D, which laser pointers  132   a - c  produce respective laser spots, the furthest apart of which are separated by a distance d on the surface of the target object  102 . The laser spots produced on the target object surface by mutually laser pointers  132   a  and  132   b  are separated by the distance L 1 , which is also the physical distance separating the axes of laser pointers  132   a  and  132   b . The laser spots produced on the target object surface by laser pointers  132   a  and  132   b  are separated by the distance d.  FIG. 9B  is a diagram representing an image  70  acquired by the video camera  130  depicted in  FIG. 9A , which image  70  includes a representation  102 ′ of the target object  102  and respective groups of pixels representing respective positions  106 ,  107  and  108  of the laser spots. 
     In accordance with the situation depicted in  FIGS. 9A and 9B , the known variables are the field-of-view of the video camera  130  (i.e., “FoV” in  FIG. 9A ), the maximum number of pixels in the width direction of the image  70  (i.e., “maxPx” in  FIG. 9B ), the number of pixels between the respective groups of pixels representing respective positions  106  and  108  of the laser spots produced by laser pointers  132   a  and  132   b  (i.e., “nPx 1 ” in  FIG. 9B ); the number of pixels between the respective groups of pixels representing respective positions  108  and  107  of the laser spots produced by laser pointers  132   b  and  132   c  (i.e., “nPx 2 ,” in  FIG. 9B ); the angle between the laser beams transmitted by the laser pointers  132   b  and  132   c  (i.e., “β” in  FIG. 9A ); and the distance separating the respective axes of the laser pointers  132   a  and  132   b  (i.e., “L 1 ” in  FIG. 9A ). The unknown variables are the viewing angle α between the laser spots and the distances D and d. 
     The viewing angle α between the laser spots produced by laser pointers  132   a  and  132   b  can again be computed using Eq. (1). Then the distances d and D can be computed using the following equations: 
             D   =         L   1     /   2           nPx   1       max   ⁢           ⁢   Px       *     tan   ⁡     (     FoV   2     )                       d   =       L   1     +     D   *   tan   ⁢           ⁢   β                 or             d   =       L   1     +       (       nPx   1     +     nPx   2       )     /     nPx   1               
Thus there are two ways to calculate d: one uses the angle β and the other uses nPx 2 . Having two separate ways of calculating the value for distance d serves as a process check to improve reliability.
 
     In accordance with one possible implementation, a scale factor can be calculated based on a ratio of the distance d and the sum (nPx 1 +nPx 2 ). Thereafter a scale bar or other scale indicator indicating the scale factor can be displayed on subsequent images captured by the video camera  130  while the cable-suspended platform  16  stays at the same location. 
     In accordance with the embodiments partly depicted in  FIGS. 5A, 5B, 7A, 7B, 9A and 9B , an image processing method is used to determine the distance in pixels between the images of the laser spots displayed on the image  70 . The main goal of the image processing step is to determine the distance in pixels between laser spots on the target object. There are several methods that could be used for this, such as those using pixel color, but that approach is not very robust in environments that might have a wide variety of lighting conditions. For this application, a process involving sequential images with the laser spots on in one image and off in the next is used. The method involves cycling the laser pointers on and off at a rate that is half the rate (or other integer divisor) of the video camera frame rate, and then performing an image subtraction step to identify contiguous clusters pixels that have changed. The difference between the centroids of the contiguous clusters will be the pixel distance (nPixels), which pixel distance can be used in conjunction with the distance d to calculate a scale factor and later display a scale indicator that graphically depicts that scale factor. 
     Change detection is a process used to determine the difference between two or more images. For example, regions of change can be determined using digital image processing techniques. One such process may involve image subtraction, blur filters, and image segmentation steps. The term “digital image processing” means a computer-based analysis of an image or series of images. The term “pixels” refers to picture elements that make up a 2-D digital image. Segmentation is the process of identifying pixels that have similar properties in a digital image. 
       FIG. 10  is a diagram illustrating steps of a method for processing images to determine the distance in pixels between laser spots on a target object in accordance with one example. The video camera  130  captures respective frames  140   a - d  at times T=0.00, 0.01, 0.02 and 0.03. The laser pointers  132   a  and  132   b  are OFF when the frames  140   a  and  140   c  are captured at times T=0.00 and 0.02, but ON when the frames  140   b  and  140   d  are captured at times T=0.01 and 0.03. Frame  140   b  is subtracted from frame  140   a  to produce a subtraction image  142   a ; frame  140   d  is subtracted from frame  140   c  to produce a subtraction image  142   b . The locations of differences in the subtraction images  142   a  and  142   b  are then determined. The centroid of each area is found and converted into pixel coordinates (x, y). A distortion correction is performed to compensate for the optics of the camera, where two-dimensional image corrections are applied resulting in (x′, y′). This correction may depend on, for example, the lens optics, zoom, and focus levels. The corrections are determined experimentally in one embodiment, and recalled at run-time using a table lookup. After lighting correction is applied, the differences (i.e., the images of the laser spots) which appear in each subtraction image are determined. In one embodiment, a pixel-by-pixel difference operation is performed, followed by a blur filter operation, and then an image segmentation operation. An N×N blur filter (such as a 5×5 kernel) can be used to smooth out most of the high-frequency noise associated with the images, and can be adjusted to discard areas of various sizes. The blurred image is then segmented into distinct, non-touching areas. The centroid of each of the separate regions is computed and is stored in a list associated with each of the image pairs. The number of pixels separating the two centroids corresponding to the two laser spots is then calculated. 
     Since the groups of pixels representing the respective positions  106  and  108  of the laser spots will be in the same horizontal strip of each image, only that part of the image is needed for the image processing.  FIG. 11  is a diagram illustrating steps of a method for processing images to determine the distance in pixels between laser spots on a target object in a manner which improves the image processing efficiency. Horizontal strips  144   a  and  144   b , for example, can be created from by frames  140   a  and  140   b  respectively. Horizontal strip  144   b  is then subtracted from horizontal strip  144   a  to form a subtraction image  146 . 
     Another category of embodiments of the concept are configurations where the cable-suspended platform  16  incorporates two or more laser range meters that enable: measurement of distance to the target, reference scale, as well as one or more orientation angle of the cable-suspended platform  16  relative to the target. If three non-collinearly mounted laser range meters are used (not shown here), more than one orientation angle can be measured (for example, yaw and pitch). 
       FIG. 12  is a diagram showing a top view of a video camera  130  having a pair of laser range meters  138   a  and  138   b  directed at a target object  102  in accordance with another embodiment, which video camera  130  can be incorporated in the cable-suspended platform  16  depicted in  FIG. 1 . The laser range meters  138   a  and  138   b  are affixed to and rotate with the video camera  130 . The embodiment partly depicted in  FIG. 12  is capable of acquiring scale information for objects undergoing non-destructive inspection. It is also capable of measuring the separation distance D between the cable-suspended platform  16  and the target object  102  and the orientation angle of the cable-suspended platform  16  relative to target object  102 . The laser range meters  138   a  and  138   b  are arranged in a mutually parallel configuration. For example, the pair of laser range meters  138   a  and  138   b  having mutually parallel axes are fixedly mounted to the housing  22  of the video camera  130 . Preferably the focal axis of the video camera  130  and the aim directions of the laser range meters  138   a  and  138   b  are mutually parallel. When activated, the laser range meters  138   a  and  138   b  direct respective mutually parallel laser beams that form respective laser spots on a surface of the target object  102 . 
     In instances wherein the axes of the laser range meters  138   a  and  138   b  are not perpendicular to the portion of the surface of target object  102  where the laser beams impinge, the respective distances separating the laser range meters  138   a  and  138   b  from that surface will not be equal and the cable-suspended platform  16  will have a non-zero orientation angle relative to that surface. In instances wherein the axes of the laser range meters  138   a  and  138   b  are perpendicular to the portion of the surface of target object  102  where the laser beams impinge, the respective distances separating the laser range meters  138   a  and  138   b  from that surface will be equal and the orientation angle will be zero. Thus measurements of the respective separation distances of the laser range meters  138   a  and  138   b  from the target object  102  can be used to determine the current offset of the cable-suspended platform  16  from the target object  102  and the current orientation angle and then control the cable-suspended platform  16  to move in a manner that reduces both the deviation of the current offset from a goal offset and the deviation of the current orientation angle from a target orientation angle (e.g., an angle of zero degrees). 
     The video camera  130  may be activated to capture an image in which the two laser spots formed by the laser beams emitted by laser range meters  138   a  and  138   b  are visible. This image data can be processed (as described in some detail below) to derive pixel information which, in conjunction with the known distance separating the axes of the two laser range meters  138   a  and  138   b , can be used to determine a scale factor. That scale factor can then be used to display a scale indicator on any subsequent image captured by the video camera  130  while the cable-suspended platform  16  is at the same location. 
     For the multiple-laser-range-meter embodiments, since the information associated with the distances to the target object  102  from the respective laser range meters has been measured, and since the field-of-view of the video camera  130  is known, it is possible to determine the scale factor without the need for the image processing step. The part that can be used from the image processing step is nPx, but that can be computed as a function of FoV, average distance D/n, L 1 , and maxPx (where n is the number of laser range meters) using the following equation: 
             nPx   =         L   1     *   max   ⁢           ⁢   Px   *   n         ∑     i   =   0     N     ⁢       D   i     *     tan   ⁡     (     FoV   /   2     )                   
(Note: The foregoing computation also needs an image distortion correction step, or more accurately the inverse of it.)
 
     In response to commands from the control station  150  (see  FIG. 4 ), the video camera  130  and the laser range meters  138   a  and  138   b  can be activated by control signals (e.g., via electrical cables) output by the on-board computer system  162 . The computer system  162  also controls the movements of the pan unit  126  and tilt unit  128  by sending commands to the pan motor  122  and tilt motor  124  (see  FIG. 4 ). 
     In accordance with alternative embodiments, the cable-suspended platform  16  comprises more than one laser range meter to enable measurement of distance to the target object, as well as one or more orientation angle. If two laser range meters are used (as in the embodiment shown in  FIG. 12 ), one orientation angle can be measured (e.g., yaw). If three non-collinearly mounted laser range meters are used (not shown in the drawings), more than one orientation angle can be measured (e.g., yaw and pitch). From this information, a scale factor can be displayed to the user, or a motion constraint can be applied for vehicle control. 
       FIG. 13  is a flowchart identifying steps of a method  40  for operating a cable-suspended platform during non-destructive inspection of a structure in accordance with one embodiment in which three non-collinearly mounted laser range meters  38  are used and yaw and pitch orientation angles are measured. Method  40  comprises the following steps: (a) controlling a cable-suspended platform to move toward a structure to be inspected (step  42 ); (b) using the three laser range meters  38  on-board the cable-suspended platform to repeatedly measure (i.e., calculate) respective distances separating the laser range meters from respective spots on a surface of the structure while the cable-suspended platform is moving (step  44 ); (c) calculating a first separation distance separating the cable-suspended platform from the structure based at least on the distances calculated in step  44  (step  46 ); (d) controlling the cable-suspended platform to stop at a specified separation distance (e.g., equal to a goal offset) relative to the structure (step  48 ); (e) computing yaw and pitch orientation angles of a focal axis of the video camera  130  relative to a plane intersecting the three laser spots on the surface of the structure based on the distances calculated in step  44  (step  50 ); (f) controlling the pan-tilt mechanism  120  to reorient the video camera  130  so that the focal axis of the video camera  130  is normal to the surface of the structure (step  52 ); (g) using the video camera  130  to capture an image of the structure while the cable-suspended platform is at the specified separation distance (e.g., at a first location) (step  54 ); (h) calculating a scale factor for the image when displayed on a display screen based at least in part on the separation distance and a field of view of the video camera  130  (step  56 ); (i) displaying the image with a scale indicator overlaid thereon, a value or a length of the scale indicator representing the scale factor (step  58 ); and (j) determining with to continue the feedback control mode or not (step  60 ). If a determination is made in step  60  that the feedback control mode should be continued, the process returns to step  44 . If a determination is made in step  60  that the feedback control mode should not continue, the prior cable-suspended platform movement mode is resumed (step  62 ). 
     In accordance with the configuration depicted in  FIG. 1 , the data acquired by the equipment (i.e., the measurement data acquired by laser range meters  138   a  and  138   b  and the image data acquired by video camera  130 ) on-board cable-suspended platform  16  is transmitted by a transceiver  160 . That message is received by a control station  150  on the ground. The computer system at the control station  150  extracts the image data representing the image from the message and causes it to be displayed on the screen of display monitor  152  by controlling the states of the pixels of the display screen in accordance with the image data. 
     In accordance with one aspect of the motion control function, the cable-suspended platform  16  can be controlled to translate to a second location while maintaining the separation distance. Then the video camera  130  is activated to capture a second image of the structure while the cable-suspended platform  16  is at the second location, which second image can be displayed on the display screen. The two images may be stitched together for display. In some instances, the first and second images may respectively comprise first and second sets of image data representing partially overlapping or contiguous areas on a surface of the structure. 
     In accordance with another aspect of the motion control function, the computer system at the control station  150  may be programmed to detect a deviation of the separation distance from the goal offset after the cable-suspended platform  16  has moved from the first location to a second location, and then control the cable-suspended platform  16  to move to a third location at which the separation distance equals the goal offset, thereby reducing the deviation to zero. The computer system at the control station  150  may be further programmed to execute the following operations: computing an orientation angle of the focal axis of the video camera  130  relative to the surface of the structure based on the first, second and third distances measured by three laser range meters  138  while the cable-suspended platform  16  is at the second location; detecting a deviation from the desired orientation angle while the cable-suspended platform  16  is at the second location; and controlling the pan-tilt mechanism  120  to change the orientation of the video camera  130  so that the orientation angle equals the desired orientation angle while the cable-suspended platform  16  is at the second location. 
       FIG. 14  is a block diagram identifying some components of a system for performing non-destructive inspection of a structure using a remote-controlled cable-suspended platform  16  in accordance with an alternative embodiment. The only difference between the systems respectively depicted in  FIGS. 4 and 14  is that the system shown in  FIG. 4  has laser pointers  132  mounted to the housing  22  of the video camera  130 , whereas the system shown in  FIG. 14  has a laser range meter  138  mounted to the housing  22  of the video camera  130 . 
     As seen in  FIG. 14 , the equipment on-board the cable-suspended platform  16  comprises a pan-tilt mechanism  120 , a video camera  130  and a laser range meter  138 , all of which can be activated by control signals (e.g., via electrical cables) transmitted by the computer system  162 . The computer system  162  can also adjust the orientation of the video camera  130  by sending commands to the motor controllers  168  which respectively control pan motor  122  and tilt motor  124 . The pan-tilt mechanism  120  is controlled to rotationally adjust the laser range meter  138  and the video camera  130  to selected angles around the pan and tilt axes. The aim direction vector  134 , which describes the orientation of the laser range meter  138  (and the focal axis of the video camera  130 ) relative to the fixed coordinate system of the cable-suspended platform  16 , is determined from the pan and tilt angles when the laser range meter  138  is aimed at a point of interest on the target object  102 . 
     The laser range meter  138  may be incorporated inside the housing  22  of video camera  130  or mounted to the outside of housing  22  in such a way that it transmits a laser beam along the aim direction vector  134 . The laser range meter  138  is configured to measure the distance to any visible feature on or any marker attached to the target object  102 . In accordance with some embodiments, the laser range meter  138  uses a laser beam to determine the distance to the target object  102 . The most common form of laser range meter operates on the time-of-flight principle by sending a laser pulse in a narrow beam towards the target object  102  and measuring the time taken by the pulse to be reflected off the target object  102  and returned to a photodetector incorporated inside the laser range meter  138 . With the speed of light known and an accurate measurement of the time made, the distance from the laser range meter  138  to the spot on the surface of the target object  102  where the laser beam impinges can be calculated. Many pulses are fired sequentially while the cable-suspended platform  16  is at a location and the average response is most commonly used. 
       FIG. 15  is a diagram showing a top view of a video camera  130  having a laser range meter  138  directed at a target object  102 , which video camera  130  can be incorporated in the cable-suspended platform  16  depicted in  FIG. 1 . The laser beam transmitted by the laser range meter  138  impinges on a surface of the target  102  at a laser spot  104 . The angle of the field-of-view  136  (indicated by a pair of dashed lines) of the video camera  130  is indicated by the arc labeled “ang” in  FIG. 15 . The aim direction vector  134  extends from the laser range meter  138  to the laser spot  104  and has a length D (also referred to below as the “distance D” separating the laser range meter  138  and the target object  102 ). 
     In accordance with one embodiment, the distance D is measured by the laser range meter  138  while the angle of the field-of-view  136  is known. This information can be used to overlay or superimpose a size scale indicator on the screen of display monitor  152  (see  FIG. 14 ) when an image captured by the video camera  130  is being displayed. If the distance D to the target object  102  is known, scale information displayed in the image on the screen of display monitor  152  allows a user to gage the size of objects in the displayed image. The scale indicator could be in the form of the overall horizontal and vertical dimensions of the image on the display or an on-screen overlay showing scale factor data on a portion of the screen. This provides the size context for the scene captured by the video camera  130  and displayed in the image on the screen of display monitor  152 . 
     The known camera field-of-view angle is given by the following equation:
 
ang=2* a  tan(SCR x /(2 D ))
 
The image X and Y values are given by the following equations:
 
SCR x=D *tan(ang/2)
 
SCR y =ratio*SCR x  
 
where D is the distance to the target object surface measured by the laser range meter  138 , and “ratio” is the image aspect ratio (known), i.e., the ratio of the image width w to image height h.
 
     In accordance with further embodiments, the fully motorized pan-tilt mechanism  120  can be used for aiming the laser range meter  138  independently of the cable-suspended platform movement controls to acquire a direct measurement of the distance separating two points on the surface of the target object  102 . Assuming that the translational offset is zero or can be measured, then all of the basic features of the local positioning system  38  can be used. 
       FIG. 16  is a flowchart identifying steps of a method  170  for sizing (i.e., measuring a point-to-point distance of) a feature on the surface of a structure to be inspected using a cable-suspended platform  16  carrying a local positioning system  38 . The method  170  comprises the following steps: (a) controlling the cable-suspended platform  16  to move toward and then stay at a first location which is separated from a structure to be inspected (step  172 ); (b) aiming the laser range meter  138  at a first point corresponding to a first visible feature on a surface of the structure while the cable-suspended platform is at the first location (step  174 ) and acquiring a first distance measurement (step  176 ); (c) using the pan-tilt mechanism  120  to measure the respective pan and tilt angles of the laser range meter  138  when the latter is aimed at the first point (step  178 ); (d) converting the distance and angle measurements acquired in steps  176  and  178  into a first vector representing the location of the first point in the frame of reference of the cable-suspended platform  16  at the first location (step  180 ); (e) aiming the laser range meter  138  at a second point corresponding to a second visible feature on the surface of the structure while the cable-suspended platform  16  is at a second location (step  182 ) and acquiring a second distance measurement (step  184 ); (f) using the pan-tilt mechanism  120  to measure the respective pan and tilt angles of the laser range meter  138  when the latter is aimed at the second point (step  186 ); (g) converting the distance and angle measurements acquired in steps  184  and  186  into a second vector representing the location of the second point in the frame of reference of the cable-suspended platform  16  at the second location (step  188 ); (h) using an IMU  166  to measure acceleration and rotational rate of the cable-suspended platform during movement from the first location to the second location (step  190 ); (i) generating a transformation matrix representing a position difference and an orientation difference between the first and second locations of the cable-suspended platform  16  based on information acquired in step  190  (step  192 ); (j) multiplying the second vector by the transformation matrix to form a third vector representing the location of the second point in the frame of reference of the cable-suspended platform  16  at the first location (step  194 ); and (k) calculating a distance between the first and second points using the first and third vectors (step  196 ). 
     In accordance with one embodiment, the method described in the preceding paragraph further comprises: (l) transmitting one or more messages containing measurement data acquired in steps  176 ,  178 ,  184 ,  186  and  190  from the cable-suspended platform  16 ; (m) receiving the one or more messages at a control station  150  (see  FIG. 14 ); and (n) extracting the measurement data from the message, wherein steps  180 ,  188 ,  192 ,  194  and  196  are performed by a computer system at the control station  150 . This method may further comprise: using the video camera  130  to capture an image of a portion of the surface of the structure that includes the first and second visible features while the cable-suspended platform  16  is at the first location; and displaying the image and symbology representing a value of the distance calculated in step  196  overlaid on the image on a display monitor  152  at the control station  150 . For example, the first and second visible features may be respective endpoints of an anomaly (such as a crack) in the structure. 
       FIG. 17  is a vector diagram illustrating the above-described method for generating a vector representing the distance and direction from a first point on a target object  102  to a second point on the target object  102  using the above-described cable-suspended platform  16 . Because a single laser range meter  138  is used to directly measure coordinates for two points, a common reference location is used to determine the distance between the two points. In this situation, the user determines the difference between the first location of the cable-suspended platform  16  during acquisition of the coordinates of the first point in a first frame of reference of the local positioning system  38  (and of the cable-suspended platform  16 ) and the second location of the cable-suspended platform  16  during acquisition of the coordinates of the second point in a second frame of reference of the local positioning system which is offset from the first frame of reference. Using the acquired coordinate position data, a transformation matrix representing a position difference and an orientation difference between the first and second frames of reference of the local positioning system  38  (i.e., the differences between the first and second locations of the cable-suspended platform  16  at the instants in time when the first and second measurements were made) is generated. 
     The vector diagram seen in  FIG. 17  shows the configuration described in the preceding paragraph. Two pairs of mutually orthogonal arrows that meet at respective vertices graphically depict respective frames of reference (a respective third mutually orthogonal axis for each frame of reference is not shown to avoid clutter in the drawing). The left-hand pair of arrows represents a frame of reference A of the cable-suspended platform  16  at the first location, while the right-hand pair of arrows represents a frame of reference B of the cable-suspended platform  16  at the second location. The location offset of frame of reference B relative to frame of reference A is represented in  FIG. 17  by the transformation matrix B   A T, which is a 4×4 homogeneous transformation matrix that describes reference frame {B} relative to reference frame {A}. In this situation the position and orientation of reference frame {B} relative to reference frame {A} may be determined from data acquired using the previously described cable length position measurement process. 
     The distance from the laser range meter  138  to a first point P 1  on a surface of a target object  102  when the cable-suspended platform  16  is at the first location is represented by the length of a vector  A P 1  extending from the origin of frame of reference {A}. The distance from the laser range meter  138  to a second point P 2  on the surface of target object  102  when the cable-suspended platform  16  is at the second location is represented by the length of a vector  B P 2  extending from the origin of frame of reference {B} to second point P 2 . The vector  B P 2  is then multiplied by the transformation matrix  B   A T to convert it into a vector defined in reference frame A. The resulting product is:
 
 B   P   2 = A   P   2  
 
The magnitude (i.e., length) of vector  A P 2  represents the distance from the laser range meter  138  to the second point P 2  when the cable-suspended platform  16  was at the first location. The distance d is determined from the difference between those two vectors, which operation can be expressed as follows:
 
 d=|   A   P   2 − A   P   1 |
 
In an equivalent manner, the distance d between points P 1  and P 2  is the magnitude (i.e., the Euclidean norm) of the three-dimensional vector connecting the two points. It is computed as the square root of the sum of the squares of the differences of the individual components of the measured point coordinates (i.e., x, y and z values). The general form of this equation is:
 
 d =√{square root over (( x   2   −x   1 ) 2 +( y   2   −y   1 ) 2 +( z   2   −z   1 ) 2 )}
 
The resulting distance value is displayed (e.g., superimposed or virtually overlaid) on the screen of the display monitor  152  along with the camera image of the portion of the surface of the target object  102  that includes points P 1  and P 2 . Optionally, a line can be drawn between the two points to show context.
 
     The movement of the cable-suspended platform  16  during a non-destructive inspection operation may be subjected to various motion constraints which are designed to make the cable-suspended platform  16  easier for a user to control for specific types of tasks. The term “motion constraints” should be given the ordinary kinematic definition. In general, motion constraints remove one or more degrees of freedom (DoF) from the motion of an object. For example, a single rigid body object in free space has six degrees of freedom (i.e., x, y, z, roll, pitch and yaw), but when that rigid body object is constrained, for example, by placing it on a table (in a location with gravity), the number of degrees of freedom is reduced to three (i.e., x, y and yaw). In this example, the planar surface of the table introduces motion constraints that remove three degrees of freedom from the system. In another example, if a rotational (revolute) joint is attached between a 6-DoF object and another fixed-location object, the rotational joint constrains the motion of the object to one degree of freedom (i.e., rotation about the axis of the revolute joint), by removing five degrees of freedom from the system. These examples are physical motion constraints, but motion constraints can also be applied in software to remove one or more degrees of freedom from controlled motion. 
     For the system involving a cable-suspended platform and its operator, which in standard operation can control six degrees of freedom in free space, the distance measurement information is used to constrain the motion of the cable-suspended platform so that one or more of the degrees of freedom of the cable-suspended platform is not directly available to the operator to control. For example, if a motion constraint is applied to the distance to the target object (using real-time measurement data from the laser range meter), the system will attempt to keep the cable-suspended platform at that specified distance. This does not mean that the low-level controller cannot still control six degrees of freedom. Instead, it means that from the operator&#39;s point of view, there is one (or more) axis that they are not controlling directly. If a wind gust attempts to push the cable-suspended platform in the direction of the motion constraint, the low-level controller will provide the motion control to compensate for this without requiring user input. This is useful in conditions where it is desirable to maintain a specific offset from a target object. It is also useful in providing virtual boundaries or for collision avoidance. 
     Once the measurement data has been acquired, it can be displayed to the user or used for additional capabilities, such as providing motion constraints that can be used for vehicle control. This extension enables motion control capabilities for the cable-suspended platform  16  based on feedback of the data from the sensors and derived measurement data. This results in the ability to provide for semi-automated control to the system, as well as more intuitive manual control. 
     For the embodiments that employ laser pointers, the only types of motion constraints that can be added to the control system are those associated with position, since these embodiments do not measure orientation. The embodiments that have two or more laser range meters have the ability to measure orientation of the cable-suspended platform  16  relative to the target object  102 , in addition to determining the distance. This allows the embodiments with more than one laser range meter to control both position and orientation of the cable-suspended platform  16  relative to the target object  102 . 
       FIG. 18  is a block diagram identifying steps of a feedback control process  80  for controlling the motion of a platform  88  based on measurement data acquired by the equipment on-board the platform  88  in accordance with one embodiment. First, the user or agent inputs commands regarding the target distance and orientation of the platform  88  (step  82 ), which inputs are received by a summing junction  84 . The summing junction  84  also receives distance and orientation data from a distance and orientation computation software module which is configured to compute distance and orientation (step  94 ). The summing junction  84  subtracts the computed distance from the commanded distance and subtracts the computed orientation from the commanded orientation. The resulting deviations are output to a control signal computation software module that is configured to compute control signals calculated to reduce the deviations (step  86 ). Based on the output from the summing junction  84 , the control signal computation software module outputs control signals to the motion actuators  90  (e.g., motor controllers  114 ). During movement of the platform  88 , the sensors acquire sensor data (step  92 ), which sensor data is used to compute the distance and orientation (step  94 ). 
     In accordance with some embodiments, the computer system  162  uses an on-board alignment methodology to determine relative location (position and orientation) offsets of the video camera  130  relative to the target object  102 . This process uses distance information from three laser range meters  138  to compute relative location in real-time. The computer system  162  then uses that data to produce the desired feedback-based motion of the pan-tilt mechanism  120 . 
     One form of control that this process enables is semi-automated control to assist an operator in some aspect of alignment, such as orientation of the video camera  130  to make sure that its focal axis is always perpendicular to the surface of the target object  102  or making sure that it is always a specific distance from the surface. 
     More specifically, the computer system  162  is configured (e.g., programmed) to determine what movements are needed to align the focal axis of the video camera  130  with a vector normal to the surface of the target object  102  based on the distance information received from the laser range meters  138 . The computer system  162  sends command signals to selected motor controllers  168  to activate the pan and tilt motors  122  and  124  as needed to orient the video camera  130  so that its focal axis is aligned with the surface normal. 
     In addition to using the three laser range meters  138  to determine distance to the target object  102 , they are also used to determine the yaw and pitch orientation angles (hereinafter “yaw angle” and “pitch angle”). For the purpose of illustration, assume that the three laser range meters  138  are disposed at the vertices of an isosceles triangle such that the distance separating the two laser range meters disposed at the vertices of the base of the isosceles triangle is a and the distance separating the third laser range meter and a midpoint of the base of the isosceles triangle (i.e., the height of the isosceles triangle) is b. Assume that d 1 , d 2 , and d 3  are the respective measured distances of the respective laser range meters to the surface of the target object. Equations (2) and (3) can be used to calculate the pitch and yaw angles:
 
PitchAngle= a  tan 2( d   1 −( d   2   +d   3 )/2, b )  (2)
 
YawAngle= a  tan 2( d   2   −d   3   ,a )  (3)
 
where PitchAngle and YawAngle are the current computed orientation angles relative to the surface of the target object  102 , and a tan 2 is the two argument arctangent inverse trigonometric function. The goal for these angles, which are measured relative to the surface normal at the current location, is to be equal to zero; and the process to achieve the goal angles is described below.
 
     With the current yaw and pitch angles calculated, the system motion controller can use a velocity control method for the controlled motions: pan, tilt, and distance. A feedback controller, such as a proportional-integral-derivative (PID) controller, can be used to drive to zero the error between the current angle and the desired angle. Equations (4) and (5) can be used to compute the pitch and yaw motion control:
 
PitchRate= Kp   pitch *(PitchAngle−PitchAngle goal )  (4)
 
YawRate= Kp   yaw *(YawAngle−YawAngle goal )  (5)
 
where PitchRate and YawRate describe the angular rotation rates about the pitch axis of the alignment apparatus and yaw axis of the base, respectively; Kp pitch  and Kp yaw  are the proportional feedback gains associated with the pitch and yaw axes, respectively; PitchAngle and YawAngle are the angles computed from Eqs. (2) and (3), respectively; and PitchAngle goal  and YawAngle goal  are the desired goal angles to which the controller is driving the system toward (as mentioned earlier, these are both zero for this example). Integral and derivative feedback may also be used, but are not shown here.
 
     The various embodiments partly depicted in  FIGS. 3, 6, 8, 12 and 15  may be used in conjunction with cable suspension systems different that the example cable suspension system depicted in  FIG. 1 . For example,  FIG. 19  shows a system for inspecting and measuring a target object  102  using a camera-equipped cable-suspended platform  18  in accordance with alternative embodiments. 
     In accordance with the measurement system depicted in  FIG. 19 , the cable-suspended platform  18  is suspended from a fixed-length cable  28  that has ends attached to respective anchor points  10   a  and  10   b . In this case the cable-suspended platform  18  comprises a rigid support structure  4   b . In accordance with one embodiment, the rigid support structure  4   b  comprises an equipment support member  36  that is fixedly connected to and depends from a trolley  32 . The trolley  32  comprises a pair of rollers  34   a  and  34   b  that roll on and along the fixed-length cable  28 . The ends of the cables  6   a  and  6   b  are attached to the trolley  32 . The cables  6   a  and  6   b  are paid out or pulled in by respective motor-driven spools  8   a  and  8   d , with the paid-out portion of the cable being passed over respective pulleys  12   a  and  12   b . The pulleys  12   a  and  12   b  are in turn rotatably coupled to respective yokes  14   a  and  14   b  which are rigidly connected to respective anchor points  10   a  and  10   b . The anchor points  10   a  and  10   b  may be attached to walls or ceilings for indoor applications or mounted on poles or cranes for outdoor applications. As long as the locations of the spools  8   a  and  8   b  and the lengths of the paid-out portions of cables  6   a  and  6   b  are known, the computer system (not shown in  FIG. 19 ) that controls movement of the trolley  32  can be configured to track the location of the center of the trolley  32  in the frame of reference of the anchor points. 
     In accordance with the system depicted in  FIG. 19 , the cable-suspended platform  18  further comprises a local positioning system  38  attached to a bottom end of the equipment support member  36 . The local positioning system  38  comprises a pan-tilt mechanism  120  mounted to the bottom end of the equipment support member  36 , a camera  20  mounted to the pan-tilt mechanism  120 , and a laser range meter  138 , as previously described. A wireless transceiver and an on-board antenna (not shown in  FIG. 19 ) enable bidirectional, wireless electromagnetic wave communications with a control station similar to control station  150  as previously described. In the alternative, the communications could be via wires. Rotation of the spools of spools  8   a  and  8   b  may be controlled in the manner previously depicted in and described with reference to  FIG. 2B . For example, the exact position of the center of the trolley  32  can be tracked by configuring the winch unit control computer  112  to calculate the respective lengths of the paid-out portions of cables  6   a  and  6   b  by counting the pulses output by the respective rotational encoders associated with each spool  8   a  and  8   b.    
     In accordance with one alternative embodiment, a measurement system similar to the system depicted in  FIG. 1  can be provided that uses three cables instead of four cables for supporting the platform. In accordance with another alternative embodiment, a measurement system similar to the system depicted in  FIG. 19  can be provided in which the fixed-length cable  28  is eliminated, so that the platform is supported by cables  6   a  and  6   b  only. 
     While methods for controlling the operation of a cable-suspended platform during non-destructive inspection of a structure have been described with reference to various 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 teachings herein to a particular situation without departing from the scope thereof. Therefore it is intended that the claims not be limited to the particular embodiments disclosed herein. 
     The embodiments disclosed above use one or more computer systems. As used in the claims, the term “computer system” comprises a single processing or computing device or multiple processing or computing devices that communicate via wireline or wireless connections. Such processing or computing devices typically include one or more of the following: a processor, a controller, a central processing unit, a microcontroller, a reduced instruction set computer processor, an application-specific integrated circuit, a programmable logic circuit, a field-programmable gated array, a digital signal processor, and/or any other circuit or processing device capable of executing the functions described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term “computer system”. 
     The methods described herein may be encoded as executable instructions embodied in a non-transitory tangible computer-readable storage medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processing or computing system, cause the system device to perform at least a portion of the methods described herein. 
     As used in the claims, the term “location” comprises position in a three-dimensional coordinate system and orientation relative to that coordinate system. 
     The process 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 unless the claim language explicitly specifies or states conditions indicating a particular order in which some or all of those steps are performed. Nor should the process claims be construed to exclude any portions of two or more steps being performed concurrently or alternatingly unless the claim language explicitly states a condition that precludes such an interpretation. 
     APPENDIX 
       FIG. 20  shows a position vector  A P extending from the origin of an instrument coordinate system {A}, substantially along the aim point axis of the instrument, to a point of interest P and showing a position vector  B P extending from the origin of a target object coordinate system {B} to the point of interest P. 
     Referring to  FIG. 20 , when the coordinates of a point P in the instrument coordinate system  622  are spherical coordinates of pan (i.e., the pan angle  634  in  FIG. 13  of a vector  A P to the point P), tilt (the tilt angle  636  in  FIG. 20  of the vector  A P to the point P), and range (the distance along the vector  A P to the point P in  FIG. 20 ), the position of the point P represented as spherical coordinates in the instrument coordinate system  622  is related to the position of the point P in X,Y,Z Cartesian coordinates in the instrument coordinate system  622  from the following equations for the forward kinematics of the instrument  618 :
 
 X =Range*cos(pan)*cos(tilt)
 
 Y =Range*sin(pan)*cos(tilt)
 
 Z =Range*sin(tilt)
 
where pan (azimuth) is rotation about the Z axis and tilt (elevation) is rotation about the Y axis in the instrument coordinate system  622 .
 
     It is noted that the position of the point P represented as Cartesian coordinates (X,Y,Z) in the instrument coordinate system  622  is related to the position of the point P represented as spherical coordinates (pan, tilt, range) in the instrument coordinate system  622  from the following equations for the inverse kinematics of the instrument  618 :
 
pan=tan( Y,X ) −1  
 
tilt=tan( Z ,√{square root over ( X   2   +Y   2 )}) −1  
 
Range=tan √{square root over ( X   2   +Y   2   +Z   2 )}
 
     In one implementation, a position  B P (which is represented as a column vector in the form [X,Y,Z,1] T ) in the target object coordinate system  616  is calculated from a position  A P (also a column vector in the form [X,Y,Z,1] T ) in the instrument coordinate system  622  from the equation:
 
 B   P=   A   B   T   A   P  
 
where T is the calibration matrix. In one example, the calibration matrix is a 4×4 homogeneous transformation matrix having the form:
 
     
       
         
           
             
               
                   
                 A 
                 B 
               
               ⁢ 
               T 
             
             = 
             
               [ 
               
                 
                   
                     
                       r 
                       11 
                     
                   
                   
                     
                       r 
                       12 
                     
                   
                   
                     
                       r 
                       13 
                     
                   
                   
                     X 
                   
                 
                 
                   
                     
                       r 
                       21 
                     
                   
                   
                     
                       r 
                       22 
                     
                   
                   
                     
                       r 
                       23 
                     
                   
                   
                     Y 
                   
                 
                 
                   
                     
                       r 
                       31 
                     
                   
                   
                     
                       r 
                       32 
                     
                   
                   
                     
                       r 
                       33 
                     
                   
                   
                     Z 
                   
                 
                 
                   
                     0 
                   
                   
                     0 
                   
                   
                     0 
                   
                   
                     1 
                   
                 
               
               ] 
             
           
         
       
     
     It is noted that a position  A P in the instrument coordinate system  622  can be calculated from a position  B P in the target object coordinate system  616  using the inverse of the calibration matrix from the equation:
 
 A   P =( A   B   T ) −1 B   P=   B   A   T   B   P  
 
     In one example, the three calibration points are non-collinear, and the calibration matrix is calculated as follows:
 
{right arrow over ( n   A )} =V   A12   ×V   A13  
 
{right arrow over ( n   B )} ={right arrow over (V)}   B12   ×{right arrow over (V)}   B13  
 
{right arrow over ( k   1 )}={right arrow over ( n   A )}×{right arrow over ( n   B )}
 
θ 1 =α cos(|{right arrow over ( n   A )}|·|{right arrow over ( n   B )}|)
 
 R   1   =f   1 (|{right arrow over ( k   1 )}|,θ 1 )
 
{right arrow over ( k   2 )}={right arrow over ( V )} A12   ×{right arrow over (V)}   B12  
 
θ 2 =α cos(| {right arrow over (V)}   A12   |·|{right arrow over (V)}   B12 |)
 
 R   2   =f   1 (|{right arrow over ( k   2 )}|,θ 2 )
 
 R   12   =R   1   R   2  
 
 A   B   T =[ R   12 ,[ R   1 ( {right arrow over (V)}   B12   −{right arrow over (V)}   A12 )] T ]
 
 B   A   T =( A   B   T ) −1  
 
wherein, referring to  FIGS. 21-23 :
 
     {right arrow over (V)} A12  is the vector in coordinate system A that extends from point P A1  to P A2 ; 
     {right arrow over (V)} A13  is the vector in coordinate system A that extends from point P A1  to P A3 ; 
     {right arrow over (V)} B12  is the vector in coordinate system A that extends from point P B1  to P B2 ; 
     {right arrow over (V)} B13  is the vector in coordinate system A that extends from point P B1  to P B3 ; 
     {right arrow over (n)} A  and {right arrow over (n)} B  are the normals created from the vector cross products; 
     {right arrow over (k)} 1  and {right arrow over (k)} 2  are axes of rotation; 
     θ 1  and θ 2  are rotation angles about axes {right arrow over (k)} 1  and {right arrow over (k)} 2 , respectively; 
     R 1 , R 2 , and R 12  are 3×3 symmetric rotation matrices; and 
     ƒ 1 ( ) is the function (known to those skilled in the art and described, for example, in “Introduction to Robotics: Mechanics and Control”, 3rd edition, by John J. Craig and published July 2004 by Prentice Hall Professional Technical Reference) which generates a 3×3 rotation matrix from the angle-axis definition described below: 
                 f   1     ⁡     (       k   ^     ,   θ     )       =     [               k   x     ⁢     k   x     ⁢   v   ⁢           ⁢   θ     +     c   ⁢           ⁢   θ                 k   x     ⁢     k   y     ⁢   v   ⁢           ⁢   θ     -       k   z     ⁢   s   ⁢           ⁢   θ                 k   x     ⁢     k   z     ⁢   v   ⁢           ⁢   θ     +       k   y     ⁢   s   ⁢           ⁢   θ                     k   x     ⁢     k   y     ⁢   v   ⁢           ⁢   θ     +       k   z     ⁢   s   ⁢           ⁢   θ                 k   y     ⁢     k   y     ⁢   v   ⁢           ⁢   θ     +     c   ⁢           ⁢   θ                 k   y     ⁢     k   z     ⁢   v   ⁢           ⁢   θ     -       k   x     ⁢   s   ⁢           ⁢   θ                     k   x     ⁢     k   z     ⁢   v   ⁢           ⁢   θ     -       k   y     ⁢   s   ⁢           ⁢   θ                 k   y     ⁢     k   z     ⁢   v   ⁢           ⁢   θ     +       k   x     ⁢   s   ⁢           ⁢   θ                 k   z     ⁢     k   z     ⁢   v   ⁢           ⁢   θ     +     c   ⁢           ⁢   θ             ]           
where cθ=cos(θ), sθ=sin(θ), vθ=1−cos(θ), and {circumflex over (k)}=[k s ,k y ,k z ].
 
     Note that the 4×4 homogeneous calibration matrix  A   B T only is computed once for any position of the pointing instrument relative to the target object, and  A   B T can then be used to convert any number of vectors from coordinate system A (the instrument coordinate system  622 ) into coordinate system B (the target object coordinate system  616 ). It is also noted that the inverse calibration matrix  B   A T can be calculated by calculating the inverse of the calibration matrix  A   B T or can be calculated directly by switching the order of the vectors in the first equations of the previous paragraph.