 Galvanometer Accuracy and Ruggedness Helped Ensure Safe Return of NASA Shuttle Discovery
What was originally planned as a test of imaging technology designed to evaluate the condition of the Shuttle Discovery’s heat shield became a critical part of last July’s mission. Launch photographs showed foam from the fuel tank broke away and struck the Shuttle during lift off. The three-dimensional laser scanner developed by Neptec, Inc., Ottawa, Ontario, was then deployed once the Shuttle was in orbit to seek out damage amidst the thousands of ceramic panels that protect the Shuttle from the heat of reentry. Galvanometers provided by General Scanning Inc., Billerica, Massachusetts (a member of GSI Group’s Precision Motion Division), were critical to effective operation of the laser scanner. These galvos positioned the laser beam with an accuracy of 15 to 20 microradians, playing a critical role in delivering the required 350 to 500 micron imaging resolution. Such high resolution was necessary for detail that could identify potentially dangerous cracks in the heat shield, even when viewed from a distance of 15 feet.
 Once in orbit, the scanner showed that there were indeed no cracks in the heat shield, but it did identify 11 sites with nicks, scuffs, or blemishes on the nose cap and Reinforced Carbon Ceramic panels along the leading edge of the wings. It also identified loose strips of filler protruding between the tiles. This potential threat posed by the loose filler was then fixed in a subsequent space walk.
 Columbia Loss Spurs Focus on Heat Shield Inspection
Improving orbiter tile inspection techniques and tools has been a major focus for NASA since the 2003 loss of the Columbia. Neptec had already developed a general-purpose laser scanner that was tested on an earlier Shuttle flight, so, NASA then asked the company to develop an optimized version that would be mounted on a boom to scan the Shuttle’s entire heat shield.
 Click on this link, to see video of the NASA VR Laboratory Simulation
(Windows Streaming Media File: no audio).
Triangulation Demands Rotational Accuracy
Like laser scanners used in industrial applications, Neptec’s new Laser Camera System (LCS) operates on a principle of triangulation, projecting laser light onto surfaces while cameras continuously triangulate the changing distance and profile of the laser as it sweeps along. But while industrial scanners operate an inch or two from the surface, Neptec’s scanner provides similar accuracy from the end of a 50-foot orbital boom.
 Beam Positioning Accuracy, Key to Mission Success
The mirror positioning challenge was heightened by the large size of the mirrors which, in turn, created high inertial loads. One of the key factors in achieving the necessary camera resolution was the use of a six-inch long y-axis mirror and a large, ultra-rigid polished aluminum x-axis mirror. Neptec looked at a number of rotary motion technologies and settled on galvanometers because of their proven success in many laser positioning and focusing applications. Two M2 galvanometers provided the axes of rotation. Weighing only 210 grams, the M2 was smaller and lighter than other options. According to Iain Christie, Neptec’s director of research and development, “One of the most critical requirements of this application was for a rotary motion device that could meet our inertial loading and accuracy requirements while surviving launch loads as well as the vacuum and temperature conditions of space.”
 Selection of General Scanning Model M2 Galvanometer
Neptec engineers considered a number of galvanometers and selected General Scanning’s M2 model over alternative technologies. This galvo uses moving-magnet technology to provide high torque while consuming relatively little power. These features were critical because of the large inertial loads as well as the power constraints in space-flight applications.” It looked like the M2 would meet the needs of the application so we put it through our launch qualification process,” Christie said. “First we put it on a vibration table at David Florida Labs and shook it to simulate the vibrational bending loads experienced during launch. Then we tested the device to make sure that it would still meet our accuracy requirements. Not only did the M2 galvo meet NASA’s vibration specification, our tests showed it was pretty much indestructible. We shook it to the limits of our equipment and it passed with flying colors.” In conventional earthbound applications, when the M2 galvo is used with General Scanning’s innovative servo-driver, it can achieve high bandwidth, fast mirror settling, and low drift. As a result, it offers optical repeatability to 5 microradians, nonlinearity as low as 0.1% over its range of rotation, plus low thermal drift, wobble, and jitter.
 Need to Avoid Out-Gassing-
A Customized Approach
The need to avoid lubricant out-gassing was critical in this application because the vacuum of space can extract - “out-gas” - some of the lighter constituents of the lubricant. The out-gassed materials might then be deposited on the mirrors, reducing their reflectivity affecting the system’s operation. The high cost of power in space dictates the use of relatively low powered lasers, so high reflectivity mirrors are very important. Even a small reduction in reflectivity can be crippling. Any reduction in reflectivity might also cause the mirror to absorb more laser energy than it was designed for, possibly enough power to damage the mirror. The galvos also had to maintain accuracy in the vacuum of space, which causes conventional lubricants to quickly evaporate. By reducing lubricant efficiency, out-gassing could cause the galvanometer bearings to fail, clearly unacceptable given the nature of the mission.
 In this regard, General Scanning’s ability to provide design customization was critical to the success of the mission. “The lubricant normally used in the galvo was subject to out-gassing so our director of engineering worked directly with the people at General Scanning to qualify an alternative bearing set and dry lubricant,” Christie said. General Scanning also insured that the printed circuit boards inside the galvo met the requirements of space flight - which are much stricter than those used in Earthbound applications for which the galvos were originally designed. In the end, Neptec’s LCS confirmed that the Shuttle’s heat shield was intact. A collective sigh of relief was heard around the world and the Discovery returned safely with the help of the Neptec imaging system and General Scanning galvanometers.
 The LCS will ride on future Shuttle missions.
|
|
Neptec Laser Camera System, LCS, operates at the end of a 50’ boom, inspecting the entire Shuttle heatshield.
Neptec LCS detects and profiles defect
in Shuttle heat shield.
Neptec’s LCS is a
2 axis system with large inertial mirror loads. The M2 galvo delivered the required accuracy and operated flawlessly during the mission.
Safe return of NASA Shuttle Discovery.
At left: The Neptec Mission team.
|
