Abstract:
A portable instrument for 3D surface metrology projects augmented-reality feedback directly on the measured target surface. The instrument generates structured-light measuring-patterns and projects them successively on a target surface. Features, contours, and textures of the target surface distort each projected measuring-pattern image (MPI) from the original measuring-pattern. The instrument photographs each MPI, extracts measurement data from the detected distortions, and derives a result-image from selected aspects of the measurement data. The instrument warps the result-image to compensate for distortions from the projector or surface and projects the result-image on the measured surface, optionally with other information such as summaries, instrument status, menus, and instructions. The instrument is lightweight and rugged. Accurate measurements with hand-held embodiments are made possible by high measurement speed and an optional built-in inertial measurement unit to correct for pose and motion effects.

Description:
RELATED APPLICATIONS 
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     FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
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     APPENDICES 
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     BACKGROUND 
     Related fields include optical measurement of surfaces by projecting structured-light patterns; projectors that project related information along with a main image; projection of images onto real objects; and real-time augmented reality. 
     Measurement of surface contours, textures, and discontinuous features such as pits or cracks often requires resolution on the order of microns to millimeters. Optical approaches such as laser scanning and structured-light projection are popular for their speed, non-invasiveness, and acceptable precision and accuracy. Objects routinely measured by these methods include (but are not limited to) aircraft and other vehicle bodies, precision mechanical parts, textiles, glass, sheet metal, granular substances such as abrasives and powders, and in-situ archaeological artifacts. The measurements may be part of fabrication, quality assurance and control, or forensic reconstruction of past events. Measurements of parts of the human body are applicable in a widening range of fields including security, medicine and dentistry, fabrication of prosthetics, fitting of apparel, and immersive games. 
     In many of these applications, quick return of measurement results is crucial to productivity, and sometimes even to human safety. A manufacturing line or medical procedure may need to be halted immediately upon discovery of an unacceptable error. Also, while some applications may use a metrology instrument in one location full-time, others may need to move the capability frequently between locations. 
     Many optical instruments that produce excellent measurement results in a quiet, protected laboratory are sorely challenged by the shocks, vibrations, temperature ranges and gradients, air currents, moisture, contaminants, and other variables found in factory and field environments. In these places, space is often cramped and the objects to be measured may be awkwardly positioned or in constant motion. Power outlets may be scarce, and trailing cables an unacceptable hazard. Wireless signals may be blocked or suffer from electromagnetic interference. 
     Typically, metrology results are displayed on a screen connected to, or integrated with, the instrument. If an operator must mark or repair problem areas on the object, looking (or in some cases walking) back and forth between the screen and the target object, or having the results communicated by a second person, consumes time and creates opportunities for mistakes. “See-through” displays, head-mounted or otherwise, alleviate some of these drawbacks. However, they may create parallax errors or obscure peripheral vision too much for safety. Also, if more than one person needs to look at the results, each of them needs a separate display or they need to take turns viewing. 
     These practical challenges have created a need for a 3D surface metrology instrument that displays the measurement results directly on the surface being measured. Such a display would remove ambiguity during in-situ repair work, could be viewed by several users simultaneously, and would not obscure their vision of other objects. Ideally, the instrument would be portable (e.g., compact, lightweight, and rugged), fast, accurate, versatile, and easy to use. 
     SUMMARY 
     A non-contact 3D surface metrology instrument displays the measurement results directly on a target surface being measured. A result-image generator creates, in various embodiments, false-color representations of the measurement data, local or statistical measurement values, text, pass/fail markers, fiducials, and other symbols. The projection of this image is corrected for distortions introduced by the projector and the target surface. In some embodiments, the result-image generator generates other information such as menus and instructions. Depending on the embodiment, the colors, character sizes, and layouts may be manually adjusted or may automatically adjust themselves for optimal legibility. 
     Embodiments of a non-contact 3D surface metrology instrument are portable, compact, lightweight, rugged, and in some cases self-contained. The same image generator and projection optics used to display the measurement results on the target surface may also project structured-light patterns for performing the measurements. The same camera used to capture measurement-pattern images may also capture projected result-images for archival. Some embodiments have few or no significantly moving parts, using liquid-crystal or microelectromechanical systems (MEMS) to generate the measurement patterns and the result-images. Light-emitting diodes (LEDs) providing the illumination are small, lightweight, durable, long-lasting, and require little or no cooling. Ruggedized lightweight instrument housings may be handheld or mounted by various portable means. Power sources and processing electronics may be inside or outside the housing. 
     Embodiments of the non-contact 3D surface metrology instrument are fast, delivering essentially real-time results. Structured-light projection illuminates the entire target surface at once and eliminates the lag-time associated with scanning LCD and MEMS-based image generators can change the structured-light patterns very quickly. The camera capturing the measurement-pattern images may be electronically triggered, eliminating the delay of activating mechanical shutters. High-performance techniques such as multi-threading and graphics processing unit (GPU) computing reduce processing time. 
     Embodiments of the non-contact 3D surface metrology instrument are accurate. The image generators can generate Gray-code and phase-shifted measurement patterns for improved precision and robustness. For many applications, the speed alone ensures acceptably accurate measurements even for hand-held operations. Other embodiments include inertial measurement units (IMUs) to collect data on instrument pose and motion during measurement. The data from the IMU may trigger a “do-over” instruction when excessive motion results in less-than-acceptable precision or accuracy. The processor may also use IMU data to internally correct measurement data or the arrangement of result-images, to document which part of a large target surface was measured, or to combine neighboring measurements of smaller areas into a map of a larger area. 
     Embodiments of the non-contact 3D surface metrology instrument are versatile and easy to use. Grips and switches are ergonomic. In some embodiments, measurement triggers are configured so that activating the switch does not cause the instrument to move. Some embodiments sense external factors affecting measurement accuracy (e.g. ambient light, target size, working distance) and warn the user if the conditions are too adverse for the instrument to compensate. Embodiments with IMUs can correct the effects of some motion, warn the operator if the effects cannot be corrected, and provide horizontal text in the result-image independent of instrument pose. The colors of the result-images may be adjusted for legibility in a variety of lighting conditions, as may the size and linewidth of fiducials and characters. The camera may be configured to store result images for later archival and statistical analysis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional block diagram of a generalized metrology instrument. 
         FIGS. 2A ,  2 B, and  2 C illustrate some examples of measurement patterns used in structured-light metrology. 
         FIG. 3  is a flowchart of a generalized measurement process. 
         FIG. 4  is a functional block diagram of a metrology instrument including an IMU and enclosed in a portable housing. 
         FIG. 5  is a flowchart of a measurement process using data from the IMU. 
         FIG. 6A  illustrates an example of a portable metrology instrument measuring a riveted section of an aircraft body. 
         FIG. 6B  illustrates an example of a portable metrology instrument displaying a result-image on a riveted section of an aircraft body. 
         FIG. 7  illustrates an example of a free-standing portable metrology instrument. 
         FIG. 8  illustrates an example of a standing portable metrology instrument with a working-distance spacer. 
         FIG. 9  illustrates an example of a hand-held portable metrology instrument. 
     
    
    
     DETAILED DESCRIPTION 
     This Description will explain the operation of the basic instrument, followed by the operation of the instrument with an IMU. A walk-through of operation in a specific application will be followed by discussion of alternate embodiments. 
       FIG. 1  is a functional block diagram of a generalized metrology instrument. Projection assembly  101  includes, at a minimum, light source  102 , image generator  103 , and projection optical train  104 . Image generator  103  generates images according to control signals from data processor  107 . Projection assembly  101  projects an image  105  from image generator  103  on a target surface being measured. Camera  106  photographs the projected image  105  from the target surface. Camera  106  is triggered and may be otherwise controlled (e.g. autofocus; zoom; gain; baffles, stops or filters to exclude ambient light that would otherwise cause errors in the measurement data) by processor  107 , and sends its captured images as measurement data to processor  107  for analysis, storage, manipulation, or rebroadcast. In some embodiments, processor  107  controls light source  102  to adjust brightness, color, pulse duty cycle, or other variables. In some embodiments, processor  107  controls focus, filtering, aperture, and optical corrections or compensations in projection optics  104 . 
     Two types of image are projected on surfaces to be measured: a measurement-pattern image (MPI) for structured-light metrology and a result image (RI) showing some form of the measurement results and, optionally, auxiliary information and features such as fiducial marks, instructions, instrument status, menus, and other user-interface display data. In general, each MPI is projected for as brief a time as the camera&#39;s response to the projection light levels will allow, to minimize image blurring from motion of the instrument or target. The RI, if intended for direct viewing by a user, either remains “on” until turned off or its refresh frequency and duty cycle exceed the flicker fusion threshold for the brightness and color settings being used. 
       FIGS. 2A ,  2 B, and  2 C illustrate some non-limiting examples of measurement patterns used in structured-light metrology. Square-wave spatial modulation  211  produces striped measurement pattern  212 . Sinusoidal spatial modulation  213  produces periodic-gradient measurement pattern  214  (typically grayscale, but illustrated here with variably spaced line shading). Stochastic noise patterns such as  215  are also sometimes used. Modulation amplitude may go from “black to white” as shown, or use intermediate gray levels. Embodiments of the metrology instrument may use any suitable type of measurement pattern. 
     The processor compares the camera&#39;s capture of the MPI on the target surface with a stored MPI measured or modeled on a theoretical or actual reference surface. The processor derives a three-dimensional (3D) “point cloud” from the deviation between each point of the captured target MPI and the corresponding point of the stored MPI. A measurement will often include the projection, capture, and analysis of several MPIs differing in frequency, phase, orientation, structure, or any parameter where one of the patterns reveals or clarifies a surface characteristic that the other(s) might miss or obscure. 
       FIG. 3  is a flowchart of a generalized measurement process. Preparatory steps  301 , to be completed before measuring, include calibration, setting the working distance from the instrument to the target surface, and entering or loading any other settings such as choice of measurement patterns, acquisition parameters such as brightness and ambient-light exclusion, tolerances, and visualization modes. In some embodiments, collections of these settings can be associated with a particular target type or test type and entered, edited, stored in and retrieved from the processor. After receiving a “Measure” command, the instrument executes a measure cycle  302 , projecting and capturing each MPI in the set. Preferably, this is done very quickly, e.g. 12 MPIs projected and captured in 0.1 s or less. In analysis cycle  303 , the processor generates the 3D point cloud for the target surface and reduces it to the results to be shown in the RI. The RI coordinates are transformed to compensate for any projector or target-surface distortion that might otherwise displace features in the RI from the corresponding part of the target surface. (For simplicity, this flowchart shows all the captures in measure cycle  302  occurring before any of the analysis of the captured images in analysis cycle  303 . However, some embodiments process the already-captured MPIs in parallel with acquiring new MPIs). 
     Many factory, field, and operating-room environments would benefit from these 3D metrology capabilities being made portable, even hand-held. This presents challenges related to keeping the measurements precise and accurate when the instrument is not kept perfectly still. Some structured-light measurements can avoid imprecision and inaccuracy associated with instrument motion simply by operating at high speed. Others (because they require higher resolution, or longer exposure because the projected MPI is dim, or for other reasons) benefit from an added capability for the instrument to “know” how it moved during the measurement. At a minimum, it could warn the user or force a re-measurement if the motion reduced the measurement precision or accuracy below a predetermined threshold of acceptability (one of the tolerances that some embodiments allow the user to select). A more advanced embodiment can adjust the three-dimensional point cloud by removing some of the effects of the detected motion from the captured MPI sets before analyzing them. Pose and motion tracking can also enable the instrument to adapt features and locations of the result-image while it is being projected. For example, fiducials or text strings can be displayed as horizontal even when the instrument is rotated off-horizontal, or the projected features could be constrained to stay in place on the target surface even if the instrument moves or tilts while the result-image is being projected. 
     Inertial measurement units (IMUs), often comprising accelerometers and gyroscopes, are available in very small sizes and light weights. Coupled with a processor, they can store a history of the instrument&#39;s pose and motion as well as keeping track of its current orientation. Current orientation is useful when projecting the RI; for instance, it can enable the processor to align characters or fiducials with the external horizon even when the instrument is held in a tilted position. 
       FIG. 4  is a functional block diagram of a metrology instrument including an IMU and showing a representation of the portable housing. Here, projection assembly  101 , camera  106 , processor  107 , and IMU  412  are enclosed in protective housing  411 . Housing  411  is designed for manual transport using lightweight materials (e.g., a shell of aluminum, carbon fiber, or hard polymer) and with shock-absorbing measures (e.g., stiff metal springs or polymer foam) for projection assembly  101  and camera  106 . Alternate configurations, such as that in  FIG. 1  without the IMU, can also be assembled into portable housings similar to  412 . Light from projection assembly  101  exits housing  411 , and light from projected MPI  105  enters housing  411 , through ports  413  which may or may not be fitted with windows or lenses. IMU  412  is connected to transmit measurements of instrument pose and motion to processor  107 . 
     In a self-contained embodiment, a power source such as a battery or, for some outdoor environments, a solar cell may also be in, on, or closely connected to housing  411 . Other alternatives include a data port or a wireless transmitter, receiver or transceiver for communication with an off-board processor (besides or instead of internal processor  107 ), or with an off-board controller. 
       FIG. 5  is a flowchart of a measurement process using data from the IMU. Throughout measurement cycle  302 , the IMU collects a pose and motion history  501  for use by the processor. Separately from (either before, as shown here, or in parallel with) analysis cycle  303 , the processor performs excessive-motion check  502 . Comparing the pose and motion history data with predetermined thresholds of unacceptable effects on measurement data, the system can display a warning or an instruction to re-measure the target surface if a threshold has been exceeded. Some embodiments may also use the IMU data to correct the captured MPIs for instrument pose angle,  503  (very useful if the references being compared were oriented differently). Some embodiments may also use the IMU data to offset successive MPIs to correct for motion between successive capture events. 
     As an instructive but non-limiting example, a workflow measuring fastener height on part of an aircraft body is described. 
       FIG. 6  illustrates an example of a portable metrology instrument measuring fasteners on a section of an aircraft body. Fasteners  621  attach body cover  620  to an underlying frame or other structure. Some fasteners  622  protrude, disturbing the air flow across body cover  620  and increasing resistance. The portable 3D metrology instrument in housing  611  projects PMI  605 M from projection assembly  601 . The stripes in PMI  605 M are curved by the overall curve of the body and disturbed to a greater or lesser extent by the fastener heads&#39; relief from the surface. The camera  606  captures PMI  605 M for processing. Here, the field of view of camera  606  is shown as slightly larger than PMI  605 M, but other embodiments could have the camera field of view slightly smaller than the PMI, or the same size, or with a small lateral offset. Substantial overlap between the fields is sufficient. 
     This embodiment has a wireless receiver  615  receiving a control signal  616  from a wireless remote control  617 . This is one way to enable a user to send “Measure” or other commands without mechanically engaging any part of housing  611 , avoiding the risk that the act of starting the measurement will cause an undesirable motion of the instrument. Control signal  616  may be radio-frequency, infrared, audio, or any other signal compatible with the work environment. 
     A non-limiting example of an algorithm to identify and characterize problem fasteners may run as part of the analysis cycle in the processor. After the 3D point cloud is extracted from the captured MPIs, fasteners are recognized within the point cloud using stored data on fastener sizes and shapes. For each of the found fasteners, a best-fit ellipse is calculated. A best-fit outer plane is calculated from three or more point-cloud points on a ring outside the ellipse. A best-fit inner plane is calculated from three or more point-cloud points on a ring inside the ellipse. The angle between the two planes represents the angle error of the fastener head, and the distance between the two planes at the center of the ellipse represents the relief error of the fastener head. The derived results are marked on each fastener in the result-image as a false color, grayscale, symbol, or label. 
     The result-image is warped (transformed into coordinates matched to projection conditions on the surface), using points stored during calibration of the projector and, in some cases, points sampled from the measurement. This ensures that result-image fastener marks land on the corresponding fasteners when the result-image is projected on the target surface. 
       FIG. 6B  illustrates an example of a portable metrology instrument displaying a result-image (RI) of measured fasteners on an aircraft body. The measurement shown in process in the previous figure is complete, and projection assembly  601  now projects result-image  605 R on the measured surface. Fastener  630  is within the predetermined tolerance and is marked differently from out-of-tolerance fasteners  631  (protrudes too far) and  632  (recessed too far, bending the surrounding surface as shown by the irregular blotch marked around it). RI  605 R may also include text  633  (related to the measurements or not), fiducial  634 , navigation menu  635 , and status indicators such as battery-charge indicator  636  monitoring on-board battery  626 . 
     In some embodiments, projected information such as  633 - 636  is automatically positioned, or adjusted in brightness or color, by the processor for best legibility on the current target surface. In some embodiments, an IMU inside the instrument keeps text  633  horizontal even if the instrument is rotated. In some embodiments, camera  606  captures result image  605 R to be stored for archival, statistics, or further manipulation. 
       FIG. 7  illustrates an example of a free-standing portable metrology instrument. Housing  711  may be temporarily or permanently attached to monopod  741 , stabilized by base  742 . Optionally, some of the weight required to stabilize base  742  may be a battery pack  726  accessible through hatch  725  and connected to a power input in housing  711  by a power cable  727 . 
       FIG. 8  illustrates an example of a standing portable metrology instrument with a working-distance spacer. Here, monopod  841  ends in a foot  842  that allows the assembly to lean toward target surface  820 . A battery  826  is mounted inside monopod  841 , near the top for easy access. Battery compartments inside mount legs can be implemented with any type of monopod or tripod stand; in some designs the batteries may be near the bottom of a mount leg for mechanical stability. A working-distance spacer mounted (detachably or interchangeably) to housing  811  comprises a pair of rods  843  ending in tips  844 . When foot  842  rests on the floor and tips  844  rest on target surface  820 , a leaning tripod is formed. Microphone  815  can receive spoken commands, another way to initiate a measurement without moving the instrument; this can be implemented on other embodiments as well. 
       FIG. 9  illustrates an example of a hand-held portable metrology instrument. Housing  911  may be conveniently carried, or held for measurement, by handle  945 . In non-autofocus embodiments, textured focusing rings  946  may be provided for manual focusing of the camera and projection optics. In some embodiments (and with other housing types as well) a flexible cord  927  may couple the instrument to a remote control  917  to initiate measurements without jostling the instrument, a battery pack  926  to provide sufficient power without excess weight in housing  911 , or a combination of both as shown here. This corded module may be, by way of non-limiting example, clipped to a belt or carried in a pocket to avoid dragging on housing  911 . 
     Double-projector embodiments are also contemplated. Reasons for this more complex approach could include a need for continuous simultaneous measurement and result-image display (although rapid image switching in a single projector may be satisfactory in many situations) or using a non-visible measuring wavelength. In medicine, for example, tissue-penetrating near-infrared wavelengths may be used to measure features under the outermost layer of skin, and near-ultraviolet wavelengths may be used to measure surfaces where a fluorescing marker has been applied (e.g., to identify cancerous cells). Any of the mounting and holding configurations described here, as well as their equivalents, may be adapted to single- or double-projector embodiments. The projectors could be adjacent to each other or on either side of a central camera, as long as the projected fields and the camera&#39;s field of view overlap. 
     Those skilled in the art will recognize that many variations are possible using equivalent parts or steps to those described. The scope of patent protection for this subject matter is not limited by anything in the abstract, description, or drawings, but only by the appended claims.