Assembly in Action: Trackers Achieve Unprecedented Accuracy
March 20, 2008
The $1.4 billion Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL) in Tennessee is used to create high-energy neutron beams to investigate the molecular structures of everything from superconductors to proteins in living cells. An extraordinary feat of precision engineering, the 3,000-foot-long SNS system is comprised of hundreds of magnets and other components distributed within a series of underground tunnels. A work-in-progress, engineers are continually upgrading the system with an eye toward eventually being able to generate some 1.4 megawatts of beam power-eight times more than any other pulsed neutron source in the world.
Critical to the success of the project has been the work of the SNS survey and alignment group, which positions the many pieces of equipment making up the system in cooperation with the other groups comprising the overall design team. When a component arrives at the SNS loading dock, it is the survey and alignment team that creates any necessary fiduciaries, pinpoints the component’s location within the interior network, and then sets and aligns it within the overall system. The survey and alignment group is also responsible for integrating the many engineering drawings that make up the SNS into a single 3D coordinate system.
To ensure accurate placement and alignment, the team has been using a set of four LTD500 portable laser tracking systems from Leica Geosystems (Miamisburg, OH) in conjunction with some 1,400 “monument”reflectors, which serve as fixed reference points throughout the system. Equipped with a high-speed tracking 3D laser interferometer and precision angular encoder, the LTD500s provide measuring rates of 1,000 points per second at a range of 100 feet. The group is also using an LTD640 laser tracker with a measuring rate of 3,000 points per second and a measurement range of 131 feet. Together, the trackers make it possible for both group members and suppliers to capture 3D coordinate data on-demand, validate designs, build-and-inspect, confirm close tolerance work, and perform alignments and part mating.
“Modeling components within a single coordinate system is the way of the future for building networks,”says Joseph Error, group leader of the SNS alignment team. “The concept is not new, but what is really new is the tolerances in which we work. With the use of laser trackers, our goal is to position any component... within 50 microns or less with respect to beam.”
As an example of the laser trackers’ precision, one of the systems was used to locate 147 separate quadrupole magnets within six distinct segments making up a 116-foot length of accelerator tubing. The laser trackers were then used to position the tunnel segments-each of which is about the width of a 55-gallon drum-within the framework of the SNS. At the end of the process, the beam went right through on the first take.
“SNS may well be one of the most successful alignment endeavors in our field, particularly on this large a scale,”Error says of his team’s success to date. “As each segment of the SNS was made operational, the quality of the beam was exceptional. A machine this size consists of hundreds of magnets of all sizes and shapessome the size of your fist, some as large as an automobile, some weighing in excess of 20 tons. A number of these magnets are referred to as ‘correctors,’ as they help steer the beam and compensate for alignment deficiencies. As each section of the machine was commissioned, corrector magnets were not initially used. This was an amazing feat.”
For more on laser trackers, visit www.hexagonmetrology.com.