NASA Uses 'Polka Dots' For Precision Measurements

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.