control of moving coil actuators in disk drives: c. …...control of moving coil actuators in disk...
TRANSCRIPT
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Control of Moving Coil
Actuators in Disk Drives:
c. 1980 to 2013
Presented to the IEEE Control
Systems Society, Silicon Valley
6/5/2013 by
J. Arthur Wagner, Ph.D.
Prof. Emeritus SJSU
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Contents
• The magnetics and load parameters of
moving coil actuators c. 1981
• Philosophy of control c. 1981
• Seek control c. 1981
• Historical cases of actuators and control
• Changed things and unchanged things 2013
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IBM type, “Short Coil” steel
Cu shorted turn
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IBM type, “Long Coil”
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ISS Ferrite Magnet Linear
Actuator
• Rectangular coil
• Shorted turn
• Magnets on two sides (vs. four sides)
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ISS Magnetics and Disk
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ISS Airgap
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Flux Lines: Cylindrical and
Rectangular
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Linear Actuator ISS
Cabinet with Four EMAs
spindle
motor--
”direct
drive”
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IBM 3370 - Double Actuator
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Cylindrical Actuator
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Linear Actuator Cylindrical 8”
c. 1986
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8” Linear Actuator, Coil, and
Shorted Turn
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Control Modes
• Seek -- Velocity feedback
• Track-follow -- Position feedback
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Linear Actuator Models State variables, position X, velocity U, current I
K = force factor
U = velocity
Z = coil impedance
I1 = Z^(-1)*(Vs - K*U)
coil
shorted turn
magnetizing
inductance s
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Kf Variation
Control Design Lived with
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Servo Surface: Bottom and
Middle of Disk Stack
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Velocity Generator (Oswald)
z = arbitrary variable
can be used for velocity and
acceleration (proportional to
current)
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Phase Plane -- Seek
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Trajectory Generator
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Beginning of a Seek ISS
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128 Track Seek Waveforms ISS
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Seek--Showing Velocity
Command too steep
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Seek Waveforms IBM 3350
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Early Rotary
Actuator
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Early Rotary, c. 1985 head arm mounts pivot shorted turn coil magnet
1st
torsional
mode
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The rotary actuator head mimicked the
linear actuator head. head orientation pivot balance weight flat coil
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Rotate the Head 90 deg:
New pivot point, flat coil actuator,
c. 1989
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Early Flat Coil Design
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Flat Coil Actuator, Introduced c. 1989
Shorted Turn not required
Shown: 3 1/2 inch c. 2006
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3 1/2 inch Actuator and Disk
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3 1/2 inch Actuator--no Shorted
Turn Necessary
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Servo Data
• Servo surfaces (one disk surface) c. 1970s until c. 1995
– Very high sample rate -- “linear” control system theory
– A-B bursts -- A half track right, B half track left
• Embedded servo data c. 1995 until today, 2013
– Sampled data control system theory
– Sample rates, lower mechanical modes due to rotary actuator
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Embedded Servo -- c. 1995
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Servo Writing (position data onto disks)
• Servo surface and sector servo -- 1970s -- c. 1999
– “Servowriter” -- External device positioned head(s)
– One clock track head and
– VCM current biased actuator against a pin
– e.g. laser interferometer guides VCM to servo track
• Self servo-track writing -- c. 1999 -- today, 2013
– Position reference is regenerated from previously
written tracks -- close VCM loop and position offset
– Servo pattern propagates (error propagates)
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Fundamentally Unchanged
by 2013
• Seek Mode
• Track Follow Mode
– Control Laws in Each
• Flat coil rotary actuator (since c. 1989)
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Incremental Changes
by 2013
• Flat coil, rotary actuator
– 3D Magnetics Analysis
– Coil arm manufacturing techniques
• Seek mode
– Acoustics
– Smoother seek profiles--access time trade off
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Incremental Changes
by 2013
• 2 1/2 in. HDD in 3 1/2 in. footprint
• Four disks
– Thinner base plate and cover
– Lower frequency resonances
– Filter out resonances in track following
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2013 Advancement:
Dual (Two Stage) Actuator
• Track following accuracy
• Vibration suppression –Primary actuator
• conventional electromagnetic actuator (VCM)
• coarse displacement
– Secondary actuator • Piezoelectric push-pull
• To fine tune the head position
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Dual (Two Stage) Actuator in an
HDD, 2013
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Summary
• Showed the status of the actuator and the
control system c. 1980
• Compared with 2013
• Basic control functions unchanged
• Basic flat coil actuator unchanged from c.
1989
• Significant incremental improvements
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References
• Boettcher, De Callafon, Talke, “Modeling and Control of a Dual Stage
Actuator Hard Disk Drive”, Journal of AMDSM, 2010.
• D. Abramovitch and G. Franklin, “A brief history of disk drive
control”, IEEE Control Systems Magazine, June 2002. (60 references)
• R.K. Oswald, “Design of a disk file head-positioning servo,” IBM J
Res Development, vol. 18, pp. 506-512, Nov. 1974.
• J. Arthur Wagner, “The shorted turn in the linear actuator of a high
performance disk drive”, IEEE Transactions on Magnetics, vol. 18,
issue:6, pp. 1770-1772, Nov. 1982.
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