Control of mechanical systems in data storage

Collaborations in Data Storage

STMicroelectronics – Agrate – MI

Computer Mechanics Lab - UC Berkeley

Prof. Masayoshi Tomizuka – prof. Roberto Horowitz

Center for Magnetic Recording Research (CMRR) -UC San Diego

Prof. Frank Talke

Data Storage Center – Carnegie-Mellon University

Prof. William Messner

HITACHI-IBM Almaden Research Center

Data stored on concentric circular tracks

Operating modes

• Track Following when performing R/W operations

• Track Seek when changing track

Actuators:

• Spindle motor at a constant speed, between 3600 and 18000 rpm

• Brushless

• Provides the rotation of the disk stack assembly

• Voice Coil Motor (VCM)

• DC motor

• Moves the heads over the disk surface

Hard Disk Drives

Capacity indexes:

• TPI: Tracks Per Inch from 25k to 60k

• BPI: Bits Per Inch up to 500kb

• BPIxTPI: 30Gb per in2

Spindle

VCM

R/W Heads

ChannelChannel

•Microelectronics (analog and digital)

•Code theory

•Digital filters

BER < 10BER < 10-9-9SpindleSpindle •Electric Drives

•Mechanics

ElasticityElasticity

EccentricityEccentricity

Torque rippleTorque ripple

BearingsBearings

Disk modesDisk modes

Technologies, problems and objectives

Subsystem Technologies Problems and objectives

SuspensionSuspension •Mechanics

•Materials

•Aerodynamics

Resonant Resonant modesmodes

Fly height: Fly height: 15 nm15 nm

Speed: 120 Speed: 120 km/hkm/h

HeadsHeads

•Electric Drives

•Digital control

Seek Time: Seek Time: 5512ms12ms

PES: 5PES: 57% Tr.7% Tr.

(i.e. 50 nm (i.e. 50 nm precision in precision in servo-servo-positioning)positioning)

ServopositioningServopositioning

Head Servo-positioning

Servo sectorSector

Servo sector

• synchronization signals

• Track number

• Head postion w.r.t. track center (PES)

Servo sector

• synchronization signals

• Track number

• Head postion w.r.t. track center (PES)

Sampling

F = N x rpm / 60 ; N = #Servo sectors

F = 8 30 kHz

Sampling

F = N x rpm / 60 ; N = #Servo sectors

F = 8 30 kHz

The head servopositioning system

NRRO Non Repetitive Run Outs

WindageWindage

Effect of air turbulence on head Effect of air turbulence on head support support

(wind speed may reach up to 100 km/h)(wind speed may reach up to 100 km/h)

WindageWindage

Effect of air turbulence on head Effect of air turbulence on head support support

(wind speed may reach up to 100 km/h)(wind speed may reach up to 100 km/h)

-4 -3 -2 -1 0 1 2 3 4

-3

-2

-1

0

1

2

3

-4 -3 -2 -1 0 1 2 3 4

-3

-2

-1

0

1

2

3

-4 -3 -2 -1 0 1 2 3 4

-3

-2

-1

0

1

2

3

Ideal track center

Actual track center

n

n-1

RRO Repetitive Run OutsTrack deformationTrack deformation

•Due to initialization, heating, bearing Due to initialization, heating, bearing imperfectionsimperfections

•Track pitch: <1 Track pitch: <1 m (HDD Low End)m (HDD Low End)

•RRO: repetitive disturbance, locked in RRO: repetitive disturbance, locked in phase with disk rotation phase with disk rotation

•Amplitude may be more than track pitch Amplitude may be more than track pitch

•Frequencies: harmonics of rotational Frequencies: harmonics of rotational frequency (5400 rpm frequency (5400 rpm 90 Hz) 90 Hz)

Track deformationTrack deformation

•Due to initialization, heating, bearing Due to initialization, heating, bearing imperfectionsimperfections

•Track pitch: <1 Track pitch: <1 m (HDD Low End)m (HDD Low End)

•RRO: repetitive disturbance, locked in RRO: repetitive disturbance, locked in phase with disk rotation phase with disk rotation

•Amplitude may be more than track pitch Amplitude may be more than track pitch

•Frequencies: harmonics of rotational Frequencies: harmonics of rotational frequency (5400 rpm frequency (5400 rpm 90 Hz) 90 Hz)

Hard Disk

Interesting, multi-disciplinary case of study:

Modeling of complex mechanical systemsIdentification and control Power electronics and electric drivesVibration suppressionData coding, magnetic materials, aerodynamics, signal processing …

Research Activities in HDD Servo

Modeling and Simulation

Digital control algorithms design and test

Active vibration suppression

VCM voltage command

Modeling and SimulationExperimentally tuned simulator:

[rad ]

Arm Resonances

Arm Resonances

1/s1/s 1/s1/s1/J1/J

..

[Nm] [rad/s2] [rad/s]

τ Φ Φ Φ.

[rad]

Non L inearFrictionModel

Non L inearFrictionModel

TPRTPR

RRORRO NRRONRRO

PES

[Tr ] [Tr]

WindageWindageBias &

Flat CableBias &

Flat Cable

[rad]

Arm Resonances

Arm Resonances

1/s1/s 1/s1/s1/J1/J

..

[Nm] [rad/s2] [rad/s]

τ Φ Φ Φ.

[rad]

Non L inearFrictionModel

Non L inearFrictionModel

TPRTPR

RRORRO NRRONRRO

PES

[Tr ] [Tr]

WindageWindageBias &

Flat CableBias &

Flat Cable

0 10 20 30 40 50 60 70 80 90-0.06

-0.04

-0.02

0

0.02

0.04

0.06

0.08

[Samples]

[Tracks]

PESrro

Average data1 data2 data3 data4 data5 data6 data7 data8 data9 data10 data11 data12 data13 data14 data15 data16

Dual Stage Actuation - Piezo

LDVsuspension

spindle

gold-coatedslider

Dual stage actuator can be simulatedPiezo (experimentally tuned)Mems (multi-body mechanical system)

Dual Stage Actuation - Piezo

103

10410

-3

10-2

10-1

100

milli-actuator data

f [Hz]

ma

g [ m

u m

/V]

103

104-200

-100

0

100

200

f [Hz]

ph

ase

[de

g]

Characterization of piezo suspension (also vs. fly height)  

• “Modeling Product Variabilities of Dual-Stage Suspensions for Robust Control” - M. Rotunno, R. Oboe, R.A. de Callafon - ISPS 2002 – Santa Clara (USA) – June 2002

• “LQG / LTR control of a dual stage actuator hard disk drive with piezoelectric secondary actuator” – A.Beghi, R.Oboe – European Control Conference ECC 2001 – Porto (Portugal) – September 2001

Windage modellingHead position measured with LDV

Closed loop and open loop identification

R(s) P(s) P(s)

w wy yBias

Digital servo control design and test

VCMVCM

PreampChannel

Controller

DACPowerDrive

External BoardExternal Board

Controller

DAC

Estimated state feedback controller with disturbance observer

HDDPLANT

ESTIMATOR

K2 K3 K4

xs2 xs3 xs4

uTarget Track

+−

+ + +

− − −

External Controller

Head Position

K 1

xs1

HDDPLANT

ESTIMATOR

K 2 K 3 K 4

xs2 xs3 xs4

uTarget Track

+−

+ + +

− − −

External Controller

Head Position

K 1

xs1

Xs1: estim. position

Xs2: estim. velocity

Xs3: estim. disturbance

Xs4: u(k-1)

Xs1: estim. position

Xs2: estim. velocity

Xs3: estim. disturbance

Xs4: u(k-1)

• “Loop shaping issues in hard disk drive servo system design” - A.Beghi, R.Oboe, P.Capretta, F.Chrappan Soldavini - Advanced Intelligent Mechatronics AIM 2001 – Como (Italy) – July 2001

• “Optimal Estimation for Disk Drive Head Positioning System” - D.Ciscato, R.Oboe, G.Picci, E.Colecchia, G.P.Maccone, G.Traversa - The 2nd Annual Magnetic Recording Conference on Recording Systems - Hidden Valley, Pittsburgh PA (USA), June 12-15 1991

• “Loop shaping issues in hard disk drive servo system design” - A.Beghi, R.Oboe, P.Capretta, F.Chrappan Soldavini - Advanced Intelligent Mechatronics AIM 2001 – Como (Italy) – July 2001

• “Optimal Estimation for Disk Drive Head Positioning System” - D.Ciscato, R.Oboe, G.Picci, E.Colecchia, G.P.Maccone, G.Traversa - The 2nd Annual Magnetic Recording Conference on Recording Systems - Hidden Valley, Pittsburgh PA (USA), June 12-15 1991

Servo algorithmsRepetitive control

Gm Kt Planttorque

Head position

RRO&NRROTarget

U(t)PES

Repetitive Control Filter

UfEstimator -Controller 10

110

210

310

4-10

0

10

20

30

[Hz]

Magnitude

[dB

]

101

102

103

104

-10

0

10

20

30

[Hz]

Magnitude

[dB

]

G m K t P l a n t

torqueHead position

RRO&NRROTarget

U(t)PES

Repetitive Control Filter

UfEstimator -Controller 10

110

210

310

4-10

0

10

20

30

[Hz]

Magnitude

[dB

]

101

102

103

104

-10

0

10

20

30

[Hz]

Magnitude

[dB

]

0 90180

270360

450540

630720

810900

9901080

11701260

13501440

15301620

17101800

18901980

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Frequency (Hz)

0 90180

270360

450540

630720

810900

9901080

11701260

13501440

15301620

17101800

18901980

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 90180

270360

450540

630720

810900

9901080

11701260

13501440

15301620

17101800

18901980

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Frequency (Hz)

FFT PES

Standard Controller

Repetitive Controller

0 500 1000 1500 2000 2500 3000 35000

0.5

1

1.5

2

2.5

3

FFT PES

Frequency [Hz]

Tra

cks/

Hz

102

103

-50

-40

-30

-20

-10

0

10

Frequency (Hz)

Mag

nitu

de (

dB)

Larger Estimator BandwidthLarger Estimator Bandwidth

Estimator Bandwidth : 500 to 900 HzEstimator Bandwidth : 500 to 900 Hz

Controller Bandwidth : 500 Hz Controller Bandwidth : 500 Hz

102

103

-50

-40

-30

-20

-10

0

10

Frequency (Hz)

Mag

nitu

de (

dB)

Larger Estimator BandwidthLarger Estimator Bandwidth

Estimator Bandwidth : 500 to 900 HzEstimator Bandwidth : 500 to 900 Hz

Controller Bandwidth : 500 Hz Controller Bandwidth : 500 Hz

Sensitivity Function

Ts=m_Tc

Tc

u (k,0)

u (k,1)

u (k,i)

u (k,m-1)

x (k,0)

x (k,1)

x (k,i)x (k,m-1)

k_Ts (k+1) _Tsi_Tc (i+1)Tc

Ts=m_Tc

Tc

u (k,0)

u (k,1)

u (k,i)

u (k,m-1)

x (k,0)

x (k,1)

x (k,i)x (k,m-1)

k_Ts (k+1) _Tsi_Tc (i+1)Tc

Ts=m_Tc

Tc

u (k,0)

u (k,1)

u (k,i)

u (k,m-1)

x (k,0)

x (k,1)

x (k,i)x (k,m-1)

k_Ts (k+1) _Tsi_Tc (i+1)Tc

Ts=m_Tc

Tc

u (k,0)

u (k,1)

u (k,i)

u (k,m-1)

x (k,0)

x (k,1)

x (k,i)x (k,m-1)

k_Ts (k+1) _Tsi_Tc (i+1)Tc

Pubblications:“Disturbance rejection in hard disk drives with multi-rate estimated state feedback”R. Oboe, F. Marcassa - To appear in Mechatronics 2002 – Berkeley (USA) – December 2002

Pubblications:“Disturbance rejection in hard disk drives with multi-rate estimated state feedback”R. Oboe, F. Marcassa - To appear in Mechatronics 2002 – Berkeley (USA) – December 2002

Servo algorithmsMultirate control

Objectives (from literature):Objectives (from literature):

Reduce command discontinuitiesReduce command discontinuities

Reduce phase delayReduce phase delay

Enlarge control BWEnlarge control BW

Reduce TMRReduce TMR

Objectives (from literature):Objectives (from literature):

Reduce command discontinuitiesReduce command discontinuities

Reduce phase delayReduce phase delay

Enlarge control BWEnlarge control BW

Reduce TMRReduce TMR

Results:Results:

•Short Seek improvementShort Seek improvement

•Analytical evaluation of closed-loop Analytical evaluation of closed-loop sensitivity functionsensitivity function

•Worsening in sensitivity observed Worsening in sensitivity observed (analytically and experimentally)(analytically and experimentally)

Results:Results:

•Short Seek improvementShort Seek improvement

•Analytical evaluation of closed-loop Analytical evaluation of closed-loop sensitivity functionsensitivity function

•Worsening in sensitivity observed Worsening in sensitivity observed (analytically and experimentally)(analytically and experimentally)

K Zoh plant

Estimator

HTc

RRO&NRRO

u(k,i)

x(k,i))

Plant H

Ts

PES

PositionTarget

Ts = mTcTs = mTc

2 3 4 5 6 7 8

x 10-3

1.2288

1.2288

1.2288

1.2288

1.2289

1.2289

1.2289

1.2289

1.2289

1.229x 10

4 short seek (1 track): sr vs mr

time [sec]

head position [tracks]

multi-rate

single-rate

2 3 4 5 6 7 8

x 10-3

1.2287

1.2288

1.2289

1.229

1.2291

1.2292

1.2293

1.2294x 10

4 short seek (5 tracks)

time [t]

head position [tracks] single-rate

multi-rate

“DISTURBANCE REJECTION IN HARD DISK DRIVES WITH MULTI-RATE ESTIMATED STATE FEEDBACK”Federico Marcassa and Roberto Oboe

Control Engineering Practice 2003 – In press

IVCΔΔxxSSIVCΔΔxxSS

Mode Switching ControlMode Switching ControlMode Switching ControlMode Switching Control

yR

xS1

u

Disturbance

PROCESS

ESTIMATOR

-

a

y

yn

TRACK FOLL.

CONTROLLER

SEEK &

SETTLINGCONTROLLER

ut

us

+-

y a

>YD

xSn

xSi

yR

xS1

u

Disturbance

PROCESS

ESTIMATOR

-

a

y

yn

TRACK FOLL.

CONTROLLER

SEEK &

SETTLINGCONTROLLER

ut

us

+-

y

>YD

x

x

yR

xS1

u

Disturbance

PLANT

ESTIMATOR

-

y

yn

TRACK FOLL.

CONTROLLER

SEEK &

SETTLINGCONTROLLER

ut

us

SWITCH+

-

y

>YD

x

εThreshold

yR

xS1

u

Disturbance

PROCESS

ESTIMATOR

-

a

y

yn

TRACK FOLL.

CONTROLLER

SEEK &

SETTLINGCONTROLLER

ut

us

+-

ya

>Y D

xSn

xSi

yR

xS1

u

Disturbance

PROCESS

ESTIMATOR

-

a

y

yn

TRACK FOLL.

CONTROLLER

SEEK &

SETTLINGCONTROLLER

ut

us

+-

ya

>Y D

xSn

xSi

yR

xS1

u

Disturbance

PROCESS

ESTIMATOR

-

a

y

yn

TRACK FOLL.

CONTROLLER

SEEK &

SETTLINGCONTROLLER

ut

us

+-

y

>Y D

x

x

yR

xS1

u

Disturbance

PLANT

ESTIMATOR

-

y

yn

TRACK FOLL .

CONTROLLER

SEEK &

SETTL INGCONTROLLER

ut

us

SWITCH+

-

y

>Y D

x

εThreshold

140 160 180 200 220 240 260 280 300

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

Samples

Tracks

MSC+IVCMSC

140 160 180 200 220 240 260 280 300

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

Samples

Tracks

140 160 180 200 220 240 260 280 300

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

Samples

Tracks

MSC+IVCMSC

140 160 180 200 220 240 260 280 300

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

Samples

Tracks MSCMSCMSC+IVCMSC+IVC

Seek: Settling time phaseSeek: Settling time phase

Servo algorithmsInitial Value Compensation

“Initial value compensation applied to disturbance observer-based servo control in HDD” – R. Oboe, F. Marcassa - Advanced Motion Control 2002 (AMC2002) – Maribor (Slovenia) – July 2002

“Initial value compensation applied to disturbance observer-based servo control in HDD” – R. Oboe, F. Marcassa - Advanced Motion Control 2002 (AMC2002) – Maribor (Slovenia) – July 2002

•Objectives:• Bumpless switching

• Limitation of transients during settling phase

•Objectives:• Bumpless switching

• Limitation of transients during settling phase

•Solution:•Act on estimated states in order not to have a transient in both command and position estimate

•Solution:•Act on estimated states in order not to have a transient in both command and position estimate

Vibration compensation

MEMS rotational accelerometer

Feedforwardcompensator

controllerPES

13 kTPI, 5400 RPM HDDmounted on a shaker

VCM

Amp

iff

MEMS rotational accelerometer

Feedforwardcompensator

controllerPES

13 kTPI, 5400 RPM HDDmounted on a shaker

VCM

Amp

iff

Standard Controller

FF Filter

FF Gain

Compensation up to 600 Hz

Active vibration damping

Active suspension with two piezo strips:ActuationSensing

Active damping of resonant modes

ProsPros

Bandwidth 50kHz I=GURobustness against variations in RT LVCM

ProsPros

Bandwidth 50kHz I=GURobustness against variations in RT LVCM

ConsCons

Dissipation: R shunt – Linear AmplifierSilicon area: Linear AmplifierDigital current loop expensive A/D

ConsCons

Dissipation: R shunt – Linear AmplifierSilicon area: Linear AmplifierDigital current loop expensive A/D

1RT+sLVcm

[V] [A]

J*s

Kt

KtBemf

-

+ I(s)U(s)

R

Linear Power Amplifier& Phase Shaping[V]

Command

VCM control: Current Mode

Controllo VCM: Voltage Mode

ProsProsGood performance without current meas.Cost reductionMigration toward SOC

ProsProsGood performance without current meas.Cost reductionMigration toward SOC

1RT+sLVcm

[V] [A]

J*s

Kt

KtBemf

-

+ I(s)U(s)

PowerPSM[V]

Command DigitalPrefilter

Multi-rate

1RT+sLVcm

[V] [A]

J*s

Kt

KtBemf

-

+ I(s)U(s)

PowerPSM[V]

Command DigitalPrefilter

Multi-rate

ConsConsPre-filter cancels out the electrical dynamic

of VCMR varies ±30%On-line estimation of R required

ConsConsPre-filter cancels out the electrical dynamic

of VCMR varies ±30%On-line estimation of R required

Solution: Extended Kalman Filter to estimate R

On-line pre-filter adaptation

Presently developed for Seagate, IBM and STM

VCM control: Voltage Mode

From servo controller

H1

HoVCM+

voltage dr.

EKF

Rt

Tc

Tc

Ts Head position

Ts = 8 Tc

Gain matrix and ff compensator

Pos, vel, current…

xref

xest

+

-