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.