Issue: Volume: 24 Issue: 5 (May 2001)

Space-age Scanner

When it comes to space travel, the heat is on. During each of their Earth-orbiting missions, NASA's four space shuttles are subjected to temperatures ranging from minus 250 degrees Fahrenheit in space to nearly 3000 degrees during reentry into the Earth's atmosphere. Thus the ability of the orbiter to take the heat is critical to ensuring the safety of the crew and on-board equipment. Because of this, the surface of the orbiter, which is covered by 24,000 protective thermal tiles, is carefully scrutinized after each flight. Typically, the process of assessing the quality of each and every one of the tiles is a painstaking manual effort. Fortunately for the workers in the trenches, there is a laser light at the end of the tedious tunnel.

Engineers from NASA's Ames Research Center and the Boeing Company recently delivered a first-of-its-kind, portable 3D laser scanner to NASA's Kennedy Space Center. The scanner, which uses a digital camera and lasers in a measurement technique called laser triangulation, is the first component of what will eventually become an Electronic Inspection and Mapping System (EIMS). The EIMS objective will be to increase the accuracy and reliability of shuttle damage estimates, and in so doing reduce vehicle turn-around time.
A flaw in one of the space shuttle's protective thermal tiles is detected using a new laser scanning device. Custom software generates a 3D image of the damage.

When placed over a thermal tile, the new scanner detects flaws within a 3- by 3-inch area and transmits the collected data to a laptop computer. Custom software locates and characterizes the damage and generates a 3D image that indicates the size and depth of the flaw. The results of each scan are then stored in a database containing all of the fabrication and maintenance information for every tile, so that the latest damage history and maintenance information for each of NASA's four shuttles is readily available and easily accessible.

At the heart of the 3D digitizing technology is its laser triangulation technique in which laser diodes project a line of light onto a target object. A digital camera embedded in the scanner detects the light reflected off the object and records the position of the reflected beam to determine object height measurements. As the camera and lasers move over the object, the position of the reflection on the camera detector changes. The sensor calculates the amount of change based on the new laser line position on the detector. The process is continued until a complete description is achieved.
NASA's new portable 3D laser scanner is small enough to get into tight spots to collect data, which it then transmits to a laptop computer for 3D analysis.

Although laser triangulation itself is not new, NASA engineers put a novel spin on the technique in order to compensate for one of its common drawbacks: a shadowing effect on scanned images caused by the way the laser has to be angled to the target. "We solved this problem by adding a second laser at a complementary angle to the first and combining the images from both lasers into the final dataset for the scan," says Joseph Lavelle, a senior project engineer at NASA Ames. The complete scan of a 3- by 3-inch area, including the flaw identification and measurement processes, takes less than 20 seconds.

By far, the most significant challenge the engineers faced in the design of the new scanner was getting it down to size. Because it had to be small enough to fit around the scaffolding that surrounds the shuttle orbiter during its post-landing maintenance, portability was a key design objective. "We started off with a very large bench-top system that weighed about 50 pounds," says Lavelle, which is typical for scanning devices capable of the speed and resolution of the new device. But thanks to advances in digital camera laser diode technology, as well as "clever circuitry and custom packaging," the design team was able to shrink the scanner down to about 5 by 9 inches and approximately three pounds, including on-board batteries.

Although the device was designed specifically to measure the volume of surface flaws on the shuttle's thermal protection tiles, the underlying technology is expected to be generalized to suit countless other manufacturing applications in which scanner speed, accuracy, and portability are vital. To date, says Lavelle, "We've had inquiries from industries wanting a device to inspect the seals on their hardware, and from the integrated circuit industry needing a way to measure the depth and volume of components as well as the placement of components on printed circuit boards." He notes that plans are on tap for testing the scanner's suitability for those tasks.

The new scanning technology is still a work in progress, and as such there are a number of potential enhancements on the agenda. "We are currently addressing the difficulty of measuring flaws on significantly curved surfaces," says Lavelle. In addition, he notes, "We would like to be able to scan over a larger area than 3 by 3 inches, but this presents a major problem considering the size of the instrument and its ability to fit into small areas around the maintenance scaffolding encompassing the orbiters."
Flaw analysis tools characterize the damage to a given space shuttle tile based on its 3D scan information. The software locates the damage and generates a 3D image that indicates the size and depth of the flaw.

Toward this objective, the design team envisions the attachment of a scanner head to the end of a robot that scans over the entire orbiter surface. On the short-term "to do" list, planned enhancements include wireless communication between the scanner and the computer to eliminate the cable and the addition of rechargeable batteries and a more ergonomic case.

Diana Phillips Mahoney is chief technology editor of Computer Graphics World.