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Good Calibrations

(Photo)Like most men with work trucks, SLAC Geodetic Engineer Georg Gassner uses his for hauling equipment. However, when he transports the most accurate field instrument in the world, he takes extraordinary precaution. He places the $140,000 laser tracker in a strapped down case, padded with a 5-inch layer of foam.

Portable laser trackers provide incredibly accurate measurements—up to 30 micrometers for distance and 1 arc second for angles. Laser trackers' measurements make them ideal tools for aligning large mechanisms in endeavors demanding extreme precision. At SLAC, laser trackers will allow the heavyweight machinery of the Linac Coherent Light Source (LCLS) to be positioned down to 80 micrometers over short distances and to 300 micrometers over the machine's 140-meter length.

"It would be virtually impossible to meet the geometrical requirements for the LCLS without the help of laser trackers," Gassner said.

In the field, trackers project laser beam pulses at various prisms that act as landmarks. The LCLS network already consists of 393 such "monuments," which reflect pulses back to the trackers. The distance of the reflector from the laser tracker can be determined by measuring the phase shift of the outgoing signal to the incoming signal. Another method uses the onboard interferometer to determine distance differences with a slightly higher accuracy. Monument coordinates are calculated from the combination of the distance measurements and the angle measurements.

Before laser trackers can be used in the field, their accuracy must be tested. It takes a special tool—ten times more precise than the laser trackers themselves—to calibrate laser tracker angle measurements. Enter the rotary calibration table, manufactured by machinists in Germany and adapted by SLAC technicians.

"It isn't trivial to prove the accuracy of an instrument that will be more accurate than any we've ever seen," said Gassner.

The first key to calibration is to ensure the rotary table and laser tracker move smoothly. To do this, technicians pump 120 pounds per square inch of pressurized air into the table. A semi-spherical air bearing keeps laser trackers locked dead center and counters a horizontal "air bed," which supports the 26-kilogram, roughly 60-pound laser trackers. The air bed also prevents friction, which would decrease the accuracy of the devices.

Relative angle measurements between an encoder found in the laser tracker and an encoder in the rotary table are compared on an interface. Angles correspond with lines, or gradations, in radial arrays inside both instruments. Correct measurements match up to 0.2 arc seconds, or about five decimal places.

After calibration is achieved, laser trackers are ready for field use, although they are usually left onsite overnight to acclimatize. Variations in temperature can disturb a laser tracker's accuracy by expanding and contracting internal steel components—a risk found in any high-precision tool.

"Achieving the highest possible accuracies with equipment like the rotary table allows us to achieve even greater measurement accuracies than specified by the instrument manufacturers," said Metrology Department Head Robert Ruland. "This will improve the capabilities of our laser tracker measurements at SLAC and elsewhere."

Matt Cunningham, SLAC Today, March 18, 2008

Above image: Georg Gassner, at the SLAC Calibration Facility, with a laser tracker mounted on a calibration device. (Click on image for larger version.)