1. Control of mechanical systems in data storage

2. 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

3. 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 in 2 Spindle VCM R/W Heads

4. Channel Microelectronics (analog and digital) Code theory Digital filters BER < 10 -9 Spindle Electric Drives Mechanics ElasticityEccentricity Torque ripple Bearings Disk modes Technologies, problems and objectives SubsystemTechnologiesProblems and objectives Suspension Mechanics Materials Aerodynamics Resonant modes Fly height: 15 nm Speed: 120 km/h Heads Electric Drives Digital control Seek Time: 5  12ms PES: 5  7% Tr. (i.e. 50 nm precision in servo- positioning) Servopositioning

5. Head Servo-positioning Servo sector Sector 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

6. The head servopositioning system

7. NRRO Non Repetitive Run Outs Windage Effect of air turbulence on head support (wind speed may reach up to 100 km/h) Windage Effect of air turbulence on head support (wind speed may reach up to 100 km/h)

8. Ideal track center Actual track center n n-1 RRO Repetitive Run Outs Track deformation Due to initialization, heating, bearing imperfectionsDue to initialization, heating, bearing imperfections Track pitch: <1  m (HDD Low End)Track pitch: <1  m (HDD Low End) RRO: repetitive disturbance, locked in phase with disk rotationRRO: repetitive disturbance, locked in phase with disk rotation Amplitude may be more than track pitchAmplitude may be more than track pitch Frequencies: harmonics of rotational frequency (5400 rpm  90 Hz)Frequencies: harmonics of rotational frequency (5400 rpm  90 Hz) Track deformation Due to initialization, heating, bearing imperfectionsDue to initialization, heating, bearing imperfections Track pitch: <1  m (HDD Low End)Track pitch: <1  m (HDD Low End) RRO: repetitive disturbance, locked in phase with disk rotationRRO: repetitive disturbance, locked in phase with disk rotation Amplitude may be more than track pitchAmplitude may be more than track pitch Frequencies: harmonics of rotational frequency (5400 rpm  90 Hz)Frequencies: harmonics of rotational frequency (5400 rpm  90 Hz)

9. Hard Disk Interesting, multi-disciplinary case of study: Modeling of complex mechanical systems Identification and control Power electronics and electric drives Vibration suppression Data coding, magnetic materials, aerodynamics, signal processing …

10. Research Activities in HDD Servo Modeling and Simulation Digital control algorithms design and test Active vibration suppression VCM voltage command

11. Modeling and Simulation Experimentally tuned simulator:

12. Dual Stage Actuation - Piezo LDVsuspension spindle gold-coated slider Dual stage actuator can be simulated Piezo (experimentally tuned) Mems (multi-body mechanical system)

13. Dual Stage Actuation - Piezo 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

14. Windage modelling Head position measured with LDV Closed loop and open loop identification R(s)P(s) ww y y Bias

15. Digital servo control design and test VCM PreampChannel Controller DAC Power Drive External Board Controller DAC

16. Estimated state feedback controller with disturbance observer 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

17. Servo algorithms Repetitive control FFT PES Standard Controller Repetitive Controller

18. 0500100015002000250030003500 0 0.5 1 1.5 2 2.5 3 FFT PES Frequency [Hz] Tracks/Hz 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 algorithms Multirate control Objectives (from literature):  Reduce command discontinuities  Reduce phase delay  Enlarge control BW  Reduce TMR Objectives (from literature):  Reduce command discontinuities  Reduce phase delay  Enlarge control BW  Reduce TMR Results: Short Seek improvementShort Seek improvement Analytical evaluation of closed-loop sensitivity functionAnalytical evaluation of closed-loop sensitivity function Worsening in sensitivity observed (analytically and experimentally)Worsening in sensitivity observed (analytically and experimentally)Results: Short Seek improvementShort Seek improvement Analytical evaluation of closed-loop sensitivity functionAnalytical evaluation of closed-loop sensitivity function Worsening in sensitivity observed (analytically and experimentally)Worsening in sensitivity observed (analytically and experimentally) KZoh plant Estimator H Tc RRO&NRRO u(k,i) x(k,i)  Plant H Ts PES Position Target Ts = mTc 2345678 x 10 -3 1.2287 1.2288 1.2289 1.229 1.2291 1.2292 1.2293 1.2294 x 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

19. Mode Switching Control MSCMSC+IVC Seek: Settling time phase Servo algorithms Initial 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 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

20. Vibration compensation Standard Controller FF Filter FF Gain Compensation up to 600 Hz

21. Active vibration damping Active suspension with two piezo strips: Actuation Sensing Active damping of resonant modes

22. Pros  Bandwidth 50kHz  I=GU  Robustness against variations in R T L VCMPros  Bandwidth 50kHz  I=GU  Robustness against variations in R T L VCMCons  Dissipation: R shunt – Linear Amplifier  Silicon area: Linear Amplifier  Digital current loop  expensive A/DCons  Dissipation: R shunt – Linear Amplifier  Silicon area: Linear Amplifier  Digital current loop  expensive A/D VCM control: Current Mode

23. Controllo VCM: Voltage Mode Pros  Good performance without current meas.  Cost reduction  Migration toward SOCPros  Good performance without current meas.  Cost reduction  Migration toward SOCCons  Pre-filter cancels out the electrical dynamic of VCM  R varies ±30%  On-line estimation of R requiredCons  Pre-filter cancels out the electrical dynamic of VCM  R varies ±30%  On-line estimation of R required

24. Solution: Extended Kalman Filter to estimate R On-line pre-filter adaptation Presently developed for Seagate, IBM and STM VCM control: Voltage Mode Gain matrix and ff compensator Pos, vel, current… x ref x est + -