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Testing Positions to 4 Millionths of An Inch
Air-bearing stages and high-resolution scales drive extreme accuracy and repeatability
Al Presher, Contributing Editor -- Design News, January 8, 2007
Five air-bearing stages working in concert with high-resolution scales provide the extreme accuracies required for assembling small hydrogen targets used in testing thermonuclear ignition.
Weighing in at about 150 lb, the air-bearing system includes mechanical arms that slide — without friction — into position with accuracies of up to 4 millionths of an inch. “Each target is 28 thousandths of an inch in size, smaller than the top of a ballpoint pen,” says Ken Abbott, owner of AB Tech, the company that manufactures the system that builds the targets.
The machine is at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. With each test, a double shell implosion target is placed in a 30-ft diameter chamber with the laser beams fired simultaneously to explode it, demonstrating thermonuclear ignition and providing output for measurement and testing.
The overall components of this custom machine include three linear air bearings and two rotary air bearings, a PMAC motion controller from Delta Tau, a host PC and application software. The system is capable of positioning target shell halves to locations within 0.1 µm.
“The only way this is possible is with the use of today's ultra-precise linear scales for use on the linear slides,” says Abbott. “Because of the strict accuracies in the specifications, our only choice was the extremely high accuracy scales.”
The system utilizes LIP 481 scales from HEIDENHAIN Corp. that are exposed linear encoders characterized by high accuracy and measuring steps as small as 0.005 µm, depending on the model. Their measuring standard is a phase-grating, applied to a substrate of glass and is typically used in high-precision machines such as diamond lathes for optical components, facing lathes for magnetic storage disks and measuring microscopes and semiconductor equipment machines.
Abbott says the success of the system is all about maintaining positioning accuracy at very, very slow speeds. A fast speed for the system is .5 inches/sec for aligning components. Small parts measure 28 thousands of an inch in diameter and are positioned on the system, moved around slowly, and very accurately positioned to achieve proper alignment. The smallest incremental move on the system allows the operator to push a button and the x axis will move 4 millionths of an inch.
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 Five air-bearing stages and a high-resolution scale create a custom machine with high positioning accuracy and repeatability. |
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