Kobe, Japan--Hard-disk drive capacity governs the amount of data a user can store on a computer. Adding storage capacity without increasing size means denser radial spacing of tracks on disk drives. Industry has gone from 1,000 to 10,000 tracks per inch (tpi), and expects to pack in as many as 50,000 tpi in the near future. As a result the read/write-head element's magnetic sensitivity must also increase, which makes the manufacturing process even more demanding and acceptance testing more critical.
At the same time competitive price pressure drives manufacturers to boost throughput. One way to increase throughput is to cut testing cycle times. Hewlett Packard's Kobe Instrument Div. accomplished this by increasing the read-channel bandwidth of the motion platforms it uses to test read/write heads and head/media interfaces, with a high-stability spin stand from Dover Instrument Corp. (Westboro, MA). The result is increased testing quality and throughput.
The split-axis design with fast vacuum lockdown is patent-pending. At test velocities of up to 20,000 rpm, it offers position stability of less than 10 nm between the stages. The company claims several advantages over traditional spin-stand designs:
Excellent position stability under test
High throughput for coarse and fine moves
High rigidity and advanced controls for fast micro-positioning moves.
Most existing spin stands use leadscrew drives in a stacked stage configuration. These aren't stiff enough to reject the cyclic loading from the spindle's rotational imbalance at high speeds, according to Dover's Vice President of Engineering Phil Greene. In contrast, Dover's split-axis configuration moves both the spindle and the micro-positioning stage in the same plane.
|New spin on disk testing|
|The spin stand uses two separate, orthogonal vacuum pre-loaded air-bearing stages. One moves the piezo micro-positioning stage and head gimbal assembly, while the other moves the air bearing spindle that rotates the test disk.|
Getting the vacuum to lock down fast enough was the real challenge. The solution: "We came up with some clever pneumatic circuitry and an innovative air-bearing design that lets the air film escape in less than 50 msec," explains Greene. "Following the completion of a stage move, the XY stage reaches a stability of 10 nm in less than 100 msec." The high duty cycle required for the spindle to reach 10,000 rpm in 1 sec, 15,000 rpm in 1.5 sec, and 20,000 rpm in less than 2 sec, involved some creative techniques for dissipating the heat. Isolating the spindle motor from the bearing provides a thermally conductive path to a heat sink mounted below the spindle. Fans blow air over the heat sink, and direct heat out of the enclosure through vents.
To reduce spindle noise and turbulence, exposed bolt and balance holes have been eliminated. "We now offer lifetime warranties on our crash-proof high-speed spindles," explains Director of Sales Mike Townsend. "We don't expect any failures." Users can also expect improved reliability because of stationary linear-motor and encoder cables. "Because of the limited stroke, we can move the magnet track instead of the coil," says Townsend. "Likewise we move the scale on the encoder instead of the readhead."
"As a result of Dover's vacuum lock down spin stand and its unmatched stability during testing, we are able to increase the quality of testing and throughput to our customers," says HP Marketing Manager Matt Bellis. "The motion platform is no longer sensitive to environmental disturbances during head/media testing because of Dover's patent-pending vacuum lockdown. This feature combined with our high read-channel bandwidth provides the best performing head/media tester on the market."
|Frictionless motion...rigid lockdown|
|Both linear-motor-driven stages are frictionless during motion. However, when the stages reach the test position, air is removed quickly from the bearings and the stages become rigidly attached to the granite base plate by vacuum force. Since air is removed from both stages, there is no relative height change between the disk and head.|
Additional details...Contact Matt Giza, Dover Instrument Corp., Box 200, Westboro, MA 01581; (508) 366-1456.
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Test and measurement