NASA Uses 'Polka Dots' For Precision Measurements
Rob Black, senior applications engineer with Shape Fidelity Inc., of Huntsville, Ala., a contractor with the Ares I Upper Stage team, sets up for photogrammetry process. Rob Black, senior applications engineer with Shape Fidelity Inc., of Huntsville, Ala., a contractor with the Ares I Upper Stage team, sets up for photogrammetry process. Image credit: NASA/MSFC/David Higginbotham
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Computer-aided design model of a B52 cockpit section and a cockroach At left, a computer-aided design model of a B-52 cockpit section and a cockroach captured with the same system for comparison purposes using the photogrammetry process. Image credit: Shape Fidelity Inc.
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What weighs 600 pounds, is shiny-silver with black and white polka dots and shaped like an upside-down saucer? If you guessed some sort of mod, fancy looking UFO, you are close. It's a fuel tank dome being developed for NASA's next-generation launch vehicles.

But why polka dots? They are part of an engineering tool called photogrammetry, the practice of determining the geometric properties of objects from photographic images. It is a process used by engineers at NASA's Marshall Space Flight Center in Huntsville, Ala., to accurately measure most everything from hardware to the tools used to make the hardware. Analytical photogrammetry is now routinely employed in tasks as diverse as machine tool inspection, fixture checking and structural deformation monitoring.

"This is a reasonably cheap process that provides engineers with a precise, three-dimensional measuring tool," said Sandeep Shah, upper stage manufacturing and assembly subsystem manager for Ares Projects at the Marshall Center. "It's a novel application of an existing technology that allows us to capture the true geometry of parts and components as they are produced, and provides immediate feedback to our team."

So How Does It Work?
The system typically requires only two engineers, a computer, a camera, targets or dots, two scale bars -- used as points of reference because of their exact length -- and a specially designed 3-D scanner.

"That’s what makes photogrammetry such a great tool," Shah said. "It's simple, mobile, fast, cheap and extremely accurate. Though we've only used photogrammetry for a couple of years, I can't imagine future development and production of flight hardware without it."

First, black and white target dots are irregularly placed several inches apart on the test object. The irregular spacing is designed to assist the computer software in identifying each individual target. Next, the engineer takes pictures of the test article from every angle, using a standard, 10-megapixel camera. The number of photographs needed varies depending on the size and shape of the test article. The photos then are transferred to a computer, where the software identifies the targets to produce a skeleton-like outline, referred to as an optical global framework.

Finally, a three-dimensional, white-light scanner is used to scan small sections of the test article -- producing accurate surface definitions and thus a near-perfect computer-aided design, or CAD, model.

"CAD systems allow engineers to view a design from any angle, with the push of a button, to zoom in or out for close-ups or long-distance views," said Rob Black, senior applications engineer with Shape Fidelity Inc., of Huntsville, Ala, a contractor for Ares Projects at Marshall. "NASA is one of the very few organizations worldwide that employs this technology on large-scale precision hardware."

Photogrammetry is often used for large terrestrial applications such as architecture or shipbuilding, but NASA is unique in its routine use of close-range, precision photogrammetry and scanning on large aerospace structures and tooling.

"We have used this process to build CAD models of everything from an airplane to a roach -- that's right, a bug -- just to demonstrate the flexibility of the system," Black said. "When engineers needed a computer model of a C-130 aircraft, we used the photogrammetry process to provide an exact computer model.

"But it's important to understand that with photogrammetry we are providing a fully functional, 3D engineering model of the test article," he said. "Take the roach for example -- once photographed and scanned into the system, the software is capable of providing exact measurements of every detail, from the length of its antenna to the exact width of its wing. How cool is that?"

This technology provides an additional application called reverse engineering, a process that allows engineers to put a completed product through the photogrammetry process, then compare it to the original engineering model.

"We have a project involving valves that need to be replaced, but no drawings, models or other documentation exists," Shah said. "This technology allows us to rebuild these items digitally and generate data necessary to manufacture new ones or define analysis models."

"The larger vision for photogrammetry is that we can quickly develop manufacturing definitions of major vehicle elements while they are still at their respective fabrication sites," he said. "These elements can be assembled digitally to find integration, alignment or any other problems before they are shipped to the assembly site. Problems can be detected early, addressed and fixed prior to shipment -- saving tremendously on both schedule and cost."

Media Contact:
Jennifer Morcone Stanfield, Marshall Space Flight Center, Huntsville, Ala.