1. 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

2. 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.

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

4. 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.

5. 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”

6. 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

7. 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.

8. Location of the actuator
Memorex 360

9. 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

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

11. Crucial Design Parameter Airgap (allows the coil to move)

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

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

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

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

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

17. IBM Double Actuator

18. 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.

19. Enter 8” Disks

20. Linear Cylindrical Actuator 8” c.1986

21. 8” Linear Actuator, Coil, and Shorted Turn
Flat aluminum wire
Ferrite magnet in three arc segments

22. 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.

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

24. 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

25. 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

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

27. 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

28. 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).

29. 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.

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

31. DEC, Colorado Springs, 5 ¼”, full high, c
DEC, Colorado Springs, 5 ¼”, full high, c 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

32. 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.

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

34. 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

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

36. (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.

37. 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

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

39. 3 1/2 inch Actuator and Disk

40. 3 1/2 inch Actuator--no Shorted Turn Necessary
Magnets on both sides of the coil.
Parking latch. Worked using leakage fields.

41. 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.

42. 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

43. Dual (Two Stage) Actuator in an HDD, 2013

44. 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.

45. 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 (60 references)
R.K. Oswald, “Design of a disk file head-positioning servo,” IBM J Res Development, vol. 18, pp , Nov
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 , Nov

46. Back Up

47. 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 until today, 2015
Sampled data control system theory
Sample rates, lower mechanical modes due to rotary actuator

48. Embedded Servo -- c. 1995

49. Servo Writing (position data onto disks)
Servo surface and sector servo s -- 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 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)

50. 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

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

52. 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

53. 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

54. 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.

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

56. SanDisk (SSDs) cont.
” 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

57. 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) never
Heated-dot Magnetic Recording (HAMR+BPM) 2022

58. 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 to seconds.  The announced product had an average seek time of about seconds.

59. 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)

60. 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)

61. Kf Variation Control Design Lived with

62. Servo Surface: Bottom and Middle of Disk Stack

63. Velocity Generator (Oswald)
z = arbitrary variable
can be used for velocity and acceleration (proportional to current)

64. Phase Plane -- Seek

65. Trajectory Generator

66. Beginning of a Seek ISS

67. 128 Track Seek Waveforms ISS

68. Seek--Showing Velocity Command too steep

69. Seek Waveforms IBM 3350

70. IBM type, “Short Coil”
steel
Cu shorted turn