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The
science-fiction fantasy of nanotechnology — building
novel structures, devices, and materials at the atomic
or molecular scale — is becoming a reality. For the
great potential of nanoscience and nanotechnology to be
fully realized, however, research efforts must cross many
disciplines, from electrical engineering, mechanical engineering,
materials science, and computer science to bioengineering,
chemistry, and physics.
Nowhere
is this cross-disciplinary approach fostered more than
at UC Berkeley. Each month, Lab Notes is proud
to present the work of nanotechnology researchers from
the College of Engineering and our collaborators across
the campus.
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Solving the Hard Problems of Hard Disks
by David Pescovitz
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version
Roberto Horowitz also conducts research on automated highway systems.
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Moore's Law has
been outpaced by the explosion in data storage. Every time you open
a computer catalog, it seems, you can buy a bigger hard drive for
less money. The price/performance curve of hard drives is now steeper
than that of microprocessors.
Most recently, the storage industry demonstrated hard disks that
can pack 100 gigabits of data into one square inch of magnetic media.
Within the next few years, manufacturers promise a whopping one
terabit per square inch storage density. To help the industry reach
that milestone, UC Berkeley professor of mechanical engineering
Roberto Horowitz and his students are building microscopic actuators
and sensors that enable drives to pack bits just nanometers apart.
The actual disk in any PC's hard drive consists of an aluminum alloy
plated with a magnetizable material. A motor spins the disk at up
to 10,000 revolutions per minute. Another electromagnetic "voice
coil" motor controls the position of the suspension system that
holds the read/write head, not unlike a needle on the end of a record
player's tone arm. As the platter spins, a cushion of air is created
that lifts the head just above the platter so it literally flies
across the media as it reads and writes data in a circular track.
The closer the tracks are together, the more data can be stored
in the disk.
Only a few millimeters in size, the microactuator positions a hard drive's read/write head with nanoscale accuracy.
Courtesy
the researchers
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"The problem is that the track is not stationary because of vibration
and the wobble of the disk," Horowitz says. "In order to maintain
high track density, you need to reject those disturbances."
That's no easy task, he explains. It's essentially like trying to
place an object in a precise location using a very long fishing
pole. In a hard drive, the voice coil motor is the equivalent of
your hand, while the read/write head is the tip of the rod. In between
is the pole, the flexible suspension system.
The researchers' first innovation was to add a microactuator to
the end of the suspension. Fabricated in UC Berkeley's microlab
using the same processes by which semiconductors are manufactured,
the micro-electromechanical system (MEMS) actuator, just a few millimeters
in size, is mounted just above the read/write head.
"The voice
coil motor still handles the gross motion, while the microactuator
does the fine positioning at the very tip," Horowitz explains.
In
this experimental configuration, one of the two piezoelectric
actuators that positions the read/write head above the disk
was used as a sensor to provide feedback about the head's
position.
Courtesy
the researchers
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After proving
the effectiveness of the microactuator, the next step was to close
the feedback loop. Feeding the microactuator data about how much
the read/write head is shifting as a result of the vibration would
enable it to compensate for the disturbances.
To experiment, Horowitz
modified another industry approach for a dual-actuator suspension
system. The system employs two piezoelectric actuators that change
shape when current runs through them and generate a voltage as their
shape shifts. Horowitz hijacked one of the actuators as a strain
sensor to measure deflection, or the motion of the head. That measurement
was then used to determine how much the voice coil motor and the
other piezoelectric actuator should shift the head's position.
"It's a way to
actively dampen the vibration," Horowitz says.
Currently, Horowitz and his team are developing a MEMS strain sensor
to couple with their MEMS microactuator. The combined system, he
explains, will enable the read/write head to track with less than
3 nanometers of error.
"If the microsensor can detect that the suspenion is bending one
way, the microactuator can push back the other way," he says. "The
motion will not only be extremely precise but much faster, so you
can place tracks even closer together."
And that micromechanical solution, in conjunction with other hard
drive innovations developed at Berkeley and elsewhere, should keep
data storage right on track.
Roberto Horowitz's Home Page
Lab Notes is published online by the Public Affairs Office of the UC Berkeley College of Engineering. The Lab Notes mission is to illuminate groundbreaking
research underway today at the College of Engineering that will dramatically change our lives tomorrow.
Editor, Director of Public Affairs: Teresa Moore
Writer, Researcher: David Pescovitz
Designer: Robyn Altman
Subscribe or send comments to the Engineering Public Affairs Office: lab-notes@coe.berkeley.edu.
© 2003 UC Regents.
Updated 5/30/03.
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