PID and Hinf control HDD

Technische Universiteit Eindhoven, The Netherlands University of California, San Diego, CA, USA

Reduction of flow induced vibrations in hard disk drives by active control of dual-stage suspensions

A.P. Teerhuis & S.J.M. Cools

Reportnumber: DCT-2003/60

Supervisors: Dr. Ir. R.A. de Callafon (UCSD) Prof. Dr. Ir. M. Steinbuch (TUE)

Technische Universiteit Eindhoven Department of Mechanical Engineering Section Control Systems Technology, Power Transmissions P.O. Box 513 5600 MB, Eindhoven

The Netherlands Contents 1 Introduction 1

1.1 Design of a Hard Disk Drive ................... 1

1.2 Goal of this research ....................... 2 ............................. 1.3 Approach 3

Non-collocated control 6

2.1 Introduction ............................ 6 2.2 Model identification ....................... 7

2.3 Prediction error method ..................... 8

2.4 Experimentally found models .................. 9

2.5 Designing a non-collocated controller based on experimental data ................................ 9

2.5.1 Designing a suitable discrete Ha-controller ....... 10 .................... 2.6 Reducing controller order 14

3 Collocated controller design 20

............................ 3.1 Introduction 20

..................... 3.2 The experimental setup 21

3.2.1 Experiments on non-flying suspensions ......... 21

3.2.2 Experiments and model fitting for flying suspensions . 21 ...................... 3.3 Designing a controller 23

3.3.1 Designing a PD-controller ................ 23

3.3.2 Designing an Hz-controller ................ 25

..................... 3.4 Implementing controllers 26

....................... 3.4.1 PD-controller 26

....................... 3.4.2 Hz-controller 26

4 Conclusion 2 9 CONTENTS 2

A Design of an analog PID-controller 32

........... A.l Dynamic behaviour of the PID-controller

32

.................. A.2 Electronic design and analysis 34

...................... A.2.1 Electric circuit 34

..................... A.2.2 Network analysis 34

....... A.3 Tuning of the controller and the frequency range 36 Abstract

The trend in future generation Hard Disk Drives (HDD) is to contain more data and access this data faster. The latter is achieved by increasing the rotational speed of the disks. This, however, also increases the wind inside the HDD housing, generated by the rotating disks. In this report, we focus on two control strategies to decrease the effects of these wind disturbances (windage) on the data read-out of a HDD with a dual-stage suspension. The suspension is produced with two (coupled) piezo elements. Experimentally found models form the basis for the robust controllers. In the first part, non-collocated control is used to decrease windage ef- fects. The readlwrite head is used to detect small position errors and the controller will actuate both piezos of the suspension at the same time. This method will show a great decrease within the range where windage plays the most significant role. However, this method relies on discrete position errors, so the bandwith of this controller is limited. The second part discusses the collocated control for windage. In this case the two piezos, present on the suspension, will be decoupled and used for both actuation and sensing. A new experimental model has to be found

on which a new robust controller is designed and implemented. The general idea is to make the (mechanically weak) suspension behave stiffer. Also this method of control shows an improvement in the region where windage is active. Chapter - 1

Introduction

1.1 Design of a Hard Disk

Drive

In this report, new control strategies are proposed that deal with a specific type of disturbances which occur inside a hard disk drive. Before getting into details, a short introduction in the terminology and the principles of a hard disk drive is given.

Magnetic head Rigid arm with suspension

Figure 1.1: Basic features of an Hard Disk Drive The Hard Disk Drive (HDD) consist of a stack of disks on a rotating spindle. Each separate disk contains data on both sides. This data is posi- tioned on tracks. A magnetic element, hovering (or flying) over the surface

of the disk, reads out the data. When the disks are rotating, a thin layer of air is rotating with the disks. This boundary layer between the magnetic head and the disk (typically 5-15 nm) forms an air bearing on which the