History of Moving Coil Actuators in Disk Drives Presented at SJSU 5/19/18 IEEE Magnetics Monthly Meeting J. Arthur Wagner, Ph.D. Prof. Emeritus in Electrical Engineering SJSU

Basic Purpose of Disk drive Moving Coil Actuators Disk drives store data magnetically. The data is stored and located on concentric tracks on surfaces of disks. Magnetic heads “fly” over the disk surfaces to write or read data to or from magnetic media on a disk. The purpose of disk drive moving coil actuators is to position the magnetic heads over a track.

Contents The basic magnetics of a moving coil actuators Historical cases of actuators

IBM Started operations in San Jose in c. 1956 First location: downtown San Jose First product: RAMAC c.1956 IBM spawned other disk drive companies In the 1970s IBM introduced the moving coil actuator using a 14” diameter disk.

Disks 24” RAMAC 1956 50 disks 3.75 MB 14” flying heads 10” 8” 5 ¼” 3 ½” 2 ½” Today’s size 1.8”

Voice Coil Motor: Like the short air gap, long coil, linear disk drive actuator Leads attached to a moving Coil mounted onto a cylinder. Air gap at inner edge of plate. Axially magnetized ferrite magnet. End plate

IBM Linear Actuator, Like a voice coil motor, “Long Coil”, probably weighed 25 lbs There was a shorted turn on this actuator, not shown. Explain purpose.

Location of the actuator Memorex 360

ISS Ferrite Magnet Linear Actuator Exchange ferrite for Alnico to reduce the cost Rectangular coil Shorted turn Magnets on the two longer sides of the rectangular coil

ISS Magnetics and Disk Ferrite magnets Center pole Shorted turn Flux path completed through an end plate

Crucial Design Parameter Airgap (allows the coil to move)

ISS or Priam, c. 1981, 14” Disk, Rectangular Coil, Rectangular wire, Aluminum wire

ISS 14” Coil Edge view 6 layers Rectangular wire Aluminum Two coil sides in the air gap

Flux Lines: Cross section for the shown Rectangular Coil linear actuator f = el I X B Head carriage on this end (not shown)

Next step Multiple disk drives in one box HDA or EMA Spindle motors become “direct drive” on the shaft

Linear Actuator ISS Cabinet with Four EMAs ~500 MB spindle motor--”direct drive”

IBM 3370 - Double Actuator

Design of a Linear Cylindrical Actuator Design goal: For fixed outside dimensions, minimize seek time by adjusting the thicknesses of the steel, magnet, clearance, coil, and shorted turn.

Enter 8” Disks

Linear Cylindrical Actuator 8” c.1986

8” Linear Actuator, Coil, and Shorted Turn Flat aluminum wire Ferrite magnet in three arc segments

From 8” to 5 ¼” disks The linear actuator was known. In a 5 ¼” disk drive format, the linear actuator was too long. Resulted in linear dual coil actuators followed by rotary actuators.

CAST, Dual Coil Actuator 5 ¼” Disks “half high”, c CAST, Dual Coil Actuator 5 ¼” Disks “half high”, c. 1983 Go over geometry Bg = 0.20 T Ferrite magnets shorted turn coil carriage rails

Flux leakage from front plates toward disks Seagate, c. 1985, 5 ¼” full high, 20 MB, dual coil linear actuator, coils and carriage not shown Steel Magnets, Ferrite, Shorted turns Flux leakage from front plates toward disks Top rail clamped on (not shown) Bottom rail Bg = 0.18 T

Siemens (?) mid 80s, 5 ¼” full high, Race-track Coil, Circular ferrite magnets, fabricated in two halves. One coil was shared by the halves Bg = 0.28 T

Enter Samarium Cobalt Magnets Phased out the ferrite magnets SmCo magnets expensive Over about a 2-year span

5 ¼” Floppy Spindle motor access Media access One head, each side of media Dual coil Rectangular wire Aluminum Shorted turn (not shown) SmCo magnets Bg = 0.17 T

Enter the rotary moving coil actuator The rotary moving coil actuator did not protrude as did the linear actuator. Designers were wary initially, because of the first torsional resonance mode (low frequency).

Early Rotary Actuator: The magnetics still resembled magnetics of a linear actuator. A pivot was added. The coil had to clear the center pole over its arc. The head arm (flexure) and head were adapted from a linear actuator.

Early Rotary, c. 1985 Siemens head arm mounts (heads not rotated) pivot shorted turn coil magnet Bg = 0.22 T 1st torsional mode

DEC, Colorado Springs, 5 ¼”, full high, c DEC, Colorado Springs, 5 ¼”, full high, c. 1988 Pivot point, ferrite magnets, center pole, flux flow Rotary actuator Coil and shorted turn not shown. Coil expensive, but actuator was quite stiff and had a level Kt (torque factor) Bg = 0.31 T

Rotate the Head 90 deg: Actuator pivoted on an axis (more controllable than bearings on linear rods) c. 1989 Rotating the head and getting the head and flexures was a major, time-consuming redesign. The flex cable loop inserted a bias into the sum of torques on the actuator.

Syquest removable cartridge Syquest removable cartridge. Rotary actuator, Rectangular coil, Counterweight, Head ramp, Troublesome actuator modes, Removable disk format perpetuated the actuator design

Enter the flat coil rotary actuator The coil is in a plane. Disk drives were getting smaller. Fit better in the critical, height (z-) direction. Neodymium Iron Boron (NdFeB) magnet material made thin, flat magnets possible

Early Flat Coil Design Shorted turn not required because of a larger effective airgap, which extends through both magnets and the mechanical gap.

(Quantum) Plus, The rotary actuator head mimicked the linear actuator head. head orientation pivot balance weight flat coil Successful Product for Quantum. The actuator was non-symmetrical introducing vibration modes.

Flat Coil Actuator (Samsung) Flat Coil, molded to actuator Parking magnetics Bg = 0.73 T 3 1/2” Flat Coil on a bobbin Mounted the coil on arms

Flat Coil Actuator, Introduced c. 1989 Shown: Samsung, 3 1/2 inch c

3 1/2 inch Actuator and Disk

3 1/2 inch Actuator--no Shorted Turn Necessary Magnets on both sides of the coil. Parking latch. Worked using leakage fields.

Enter the 2 ½” disk Disk drives getting smaller. This disk size is the main disk now. Tweaks on the diameter for special disk drives. A 1.8” disk was introduced and used briefly in camcorders.

Iota (Syquest) 2 1/2” Removable Disk Late 80s Flat coil actuator Head ramp (uncommon at the time in fixed HDDs) Disk insertion Bg = 0.55 T

Dual (Two Stage) Actuator in an HDD, 2013

Summary Showed the development of the moving coil actuator from c. 1980 Linear voice coil motors were used on 14”, 8” disk drive products. The actuator transitioned to rotary beginning with the 5 ¼” disk drive. The actuator moved to a flat coil midway in the 5 ¼” product production. The flat coil actuator was used, and is now used, in the 3 ½” and 2 ½” disk drives.

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.

Back Up

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, 2015 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, 2015 “Seed” tracks are prewritten by the HDD or externally Position reference is regenerated from previously written tracks -- close VCM loop and position offset Servo pattern propagates (error propagates)

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

Fundamentally Unchanged by 2015 Seek Mode Track Follow Mode Control Laws in Each Flat coil rotary actuator (since c. 1989)

Incremental Changes by 2015 Flat coil, rotary actuator 3D Magnetics Analysis Coil arm manufacturing techniques Seek mode Acoustics Smoother seek profiles--access time trade off

Incremental Changes by 2015 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

Will SSDs replace HDDs? Yes, gradually, but not completely. SSDs more rugged, more expensive, limited rewrites, lighter weight, lower access time SSDs may place between RAM and HDD in memory hierarchy HDDs still lots cheaper. HDDs moving toward large, long-term, stationary storage One buys an SSD for its convenience. Tape storage is still used.

SanDisk (SSDs) 1991 1st SSD 20 MB, $1000, $50/MB went into cameras eventually replaced 1.8 HDDs in video cameras 2002 1 GB Compact Flash Card demise of film 2002 1st USB drive. replaced spinning flexible media like Iomega’s Clik and Zip drives

SanDisk (SSDs) cont. 2007 1.8” and 2.5” SSDs began replacing HDDs in notebook computers 2015 SSDs entering PCs and enterprise servers 2015 Fry’s: random sample HDD 3 TB $110 SSD 120 GB (0.12 TB) $95

Next HDD Technologies Flash memory caches interior to an HDD, now Helium-filled space (thinner disks), now Shingled Magnetic Recording (SMR) 2015 in “mostly-read” applications Heat-assisted Magnetic Recording (HAMR) 2018 Two Dimensional Magnetic Recording (TDMR) 2017 Bit Patterned Media (BPM) 2022 - never Heated-dot Magnetic Recording (HAMR+BPM) 2022

RAMAC Two heads per arm Head position detent at track Arms position detent at disk Arms move horizontally Carriage moves vertically Drive Capstan Magnetic powder counter rotating clutches Linear potentiometer – vertical position – wiper proportional to distance to go. Velocity control system until near disk Head positioning – same control system The RAMAC actuator is a motor driven pair of magnetic-powder counter-rotating clutches which have a common-output shaft driving a capstan which in turn uses a small steel cable to drive both the carriage vertically and the arms horizontally.  Depending upon the voltage applied to the clutches, the shaft can be made to rotate up to 100% of the motor speed which is about 100 inches per second cable speed.   The carriage moves vertically on a vertical way until it detents at a position for each of the fifty disks and then the arms move horizontally until they detent at the track location.  Mechanical interlocks assure only “ Manhattan ,” motion, that is, only x motion over the disk or only y motion outside the disks A linear potentiometer provides position information for vertical motion, its slider is on the carriage and the element is on the way.  Fifty taps are provided, one for each disk location, and one of which is grounded to indicate the desired disk.  A floating reference voltage supply is attached to the ends of the linear potentiometer element so the wiper generates a voltage proportional to the distance to go with a sign indicating the direction to go (+ is down).  This is used as a control signal in a velocity servo with feedback provided by the tachometer to cause the carriage to approach zero velocity as it approaches the target disk location.  When the target disk location is reached a pneumatic solenoid detents the carriage in place and releases the arms for horizontal motion. Horizontal motion operates in much the same way – the distance is less (6-inches vs 20-inches) and the mass lower (1:3).  A tapped rotary potentiometer provides the position information and is used with the same tachometer and actuator in a horizontal velocity servo.  When the target track location is reached a pneumatic solenoid detents the arm and after a settling delay the two heads are ready for use.  The engineers calculated that the maximum access time that is the time to travel from the inner track on a bottom disk to the inner track on a top disk would range from 0.600 to 0.720 seconds.  The announced product had an average seek time of about 0.600 seconds.

Control Modes Seek -- Velocity feedback Track-follow -- Position feedback Major Electromechanical Parameters Mass M [kg] or J [kg m^2] Force Factor Kf [N/A] or Torque Factor Kt [Nm/A] BEMF Factor Ke [V/m/s] [V/rad/s] (Kt = Ke)

Linear Actuator Models State variables, position X, velocity U, current I coil shorted turn magnetizing inductance s K = force factor U = velocity Z = coil impedance I1 = Z^(-1)*(Vs - K*U)

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

IBM type, “Short Coil” steel Cu shorted turn