In just the past two years, the functionality of portable computer-aided metrology system (CAMS) in harsh factory environments has improved a great deal. A few examples of today’s improved instrumentation/measurement technology are laser trackers, digital photogrammetry systems, laser radars, accordion fringe interferometry, and a wider variety of white-light scanners.
Advanced composite assembly of today’s commercial aircraft requires faster and more cost-effective quality assurance verification within the production environment. Recent discoveries, in new “merged” targeting, allows for combining the best attributes of laser trackers, photogrammetry, laser radars, and white-light scanners within the same quality controlled survey. One major setback has been the contamination of the CAMS equipment and their special targets by carbon fiber dust and general foreign object debris.
The goal of the Boeing Company is to create fuselage and wing structures with robust and repeatable positional reference systems, defined in their respective computer-aided designed (CAD) engineering models. Once established, these stable reference holes and/or end-of-part features are “hard targeted” with CAMS-friendly targets. The photograph seen in figure 3 displays $1,500 laser tracker/photogrammetry-merged process targets. Rapid measurement of large complex structures is now possible, with only periphery targeting required. The setup time of attaching these special targets to this large structure requires one hour of “scissor lift” factory support. After the survey, another hour of scissor lift assistance is needed to remove and safely store these expensive targets. Boeing production experts would rather keep these targets on the support tooling and large interfacing engineering end-items. Accuracy of the finished assembly improves when the fuselage or large wing structures can be measured as they are assembled. Variation in the outside skin surface can be controlled to a very tight tolerance. Key features at Boeing are the maintenance interface critical holes and interface surfaces. These cornerstones of the Boeing assembly process can have portable-metrology-friendly targets installed throughout the entire assembly and final construction.
Figure 1
Key advantages to consider:
- Real-time fuselage shape control with simple “drift checks” of the protected targets
- Assembly accumulated tolerance error can be detected early and compensated for
- The four hours required to hot glue 1,500 steel spheres to the determinant assembly holes is eliminated.
Figure 2
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Digital photogrammetry used in testing is very accurate (to 0.002 in. X, Y, and Z within a 5-meter volumetric cube). Measuring dark, light-absorbing, carbon-fiber-composite fuselage and wing structures requires new targeting techniques. Often surface preparation reflective powder or a light shade of primer paint is needed.
Measuring bright and highly reflective Invar fuselage lay up mandrels with digital photogrammetry required the use of multiple flashes of different light intensities. The process is called Combo-Shot, patented by Geodetic Systems Inc. (GSI). Their laptop- friendly V-Stars software merges the three best photogrammetric exposures. The software captures the center, middle, and outer edge flashes of the 2 meter by 2 meter required measurement area. The program then assembles one perfect composite surface image from the three exposures.
Figure 3
Metrology equipment covers, opened by laser beam contact, are easy to design. Ironically, many portable metrology companies have considered this simple protection tool. The merging of two or more different but complimentary metrology systems with the appropriate target covers is becoming more commonplace. The graphics seen in figures 4 and 5 are a mix of simplified solutions.
Figure 4
This creative protective cover is simply a modified trash can found on the factory floor. Affordable automated covers from many electronic suppliers are now available with simple “off the shelf” solutions. There is no longer any need to create an expensive or overly complex cover.
Figure 5
More detailed analysis was required to create the computer-aided design models seen in figures 6 and 7. Laser trackers and laser radars are becoming a common quality control tool in aircraft manufacture. They are often used together in large-scale mapping alignment surveys. 75-meter surveys are commonplace when integrating fuselage and wing structure to very close tolerances.
Figure 6
Figure 7
Integrating the automated targets covers with the computer-aided metrology tool covers creates a strong factory-friendly solution. Millions of dollars of precision measurement equipment can be used more often. Return on investment of this expensive hardware is more quickly realized, well before the life of these special tools has ended.
The triggering devices for opening and closing the target covers have expanded to simple laser pointers and inexpensive, wireless, remote control activation. The measurement tools are the “master” machines, initiating the start of all measurement sessions. They trigger the targets to open only when needed. Once the data is captured, the “master” machines, automated by New River Kinematics’ Spatial Analyzer or similar software, direct the closing of the target covers.
These measurement sessions, or surveys, depend upon clean and ready-to-measure targets. Human interaction and constant target maintenance adds time and cost to these surveys.
Before this measurement-friendly environment was created, many potential customers were lost over the required time to gather this precise information. Early photogrammetry cameras, shooting actual film, required many days of post-processing computer measurements. Today’s digital photogrammetry systems are near real time in processing speed. The downside remains the fragile targets. A human thumbprint can destroy a photogrammetry retroreflective target’s ability to reflect the flashed light back to the camera.
With these new target covers, the added positional variation from removing and then installing the targets again is eliminated.
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