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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
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
IBM type, “Short Coil” steel
Cu shorted turn
IBM type, “Long Coil”
ISS Ferrite Magnet Linear
Actuator
• Rectangular coil
• Shorted turn
• Magnets on two sides (vs. four sides)
ISS Magnetics and Disk
ISS Airgap
Flux Lines: Cylindrical and
Rectangular
Linear Actuator ISS
Cabinet with Four EMAs
spindle
motor--
”direct
drive”
IBM 3370 - Double Actuator
Cylindrical Actuator
Linear Actuator Cylindrical 8”
c. 1986
8” Linear Actuator, Coil, and
Shorted Turn
Control Modes
• Seek -- Velocity feedback
• Track-follow -- Position feedback
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
Kf Variation
Control Design Lived with
Servo Surface: Bottom and
Middle of Disk Stack
Velocity Generator (Oswald)
z = arbitrary variable
can be used for velocity and
acceleration (proportional to
current)
Phase Plane -- Seek
Trajectory Generator
Beginning of a Seek ISS
128 Track Seek Waveforms ISS
Seek--Showing Velocity
Command too steep
Seek Waveforms IBM 3350
Early Rotary
Actuator
Early Rotary, c. 1985 head arm mounts pivot shorted turn coil magnet
1st
torsional
mode
The rotary actuator head mimicked the
linear actuator head. head orientation pivot balance weight flat coil
Rotate the Head 90 deg:
New pivot point, flat coil actuator,
c. 1989
Early Flat Coil Design
Flat Coil Actuator, Introduced c. 1989
Shorted Turn not required
Shown: 3 1/2 inch c. 2006
3 1/2 inch Actuator and Disk
3 1/2 inch Actuator--no Shorted
Turn Necessary
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
Embedded Servo -- c. 1995
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)
Fundamentally Unchanged
by 2013
• Seek Mode
• Track Follow Mode
– Control Laws in Each
• Flat coil rotary actuator (since c. 1989)
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
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
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
Dual (Two Stage) Actuator in an
HDD, 2013
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
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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