Wednesday, May 9, 2007, 11am - 12pm, MDEA

Dual-Stage Servo Systems and Vibration Compensation in Computer Hard Disk Drives
 

Roberto Horowitz

Professor of Mechanical Engineering

Compute Mechanics Laboratory

University of California at Berkeley

 Abstract

A current goal of the magnetic disk drive industry is to break the 1 Terrabit per sqin storage density barrier.  It is predicted that the necessary track density for 1 Terabit/sqin recording will be 500,000 tracks-per-inch (TPI), which requires a track miss-registration (TMR) budget of less than 5nm (3-sigma value).  To achieve such high TPI's, a new class of dual-stage actuation for hard disk drives has been proposed: a microactuator is placed at the end of the voice coil motor (VCM) suspension and moves the magnetic head (or slider) relative to the suspension, allowing increase servo bandwidth. However, we have found in experimental tests that the slider motion due to airflow induced suspension vibration is significant and becoming more important as the RMP of the drives increases. Since the major energy component of this motion is located at a frequency range that higher than the expected servo bandwidth of even dual-stage servo systems, it cannot be compensated by the servo loop and, in fact, it may be amplified. This finding has been supported by the industry’s own testing. As a consequence, we are currently investigating suspension vibration compensation control schemes using instrumented suspensions and dual-stage servo systems to overcome this problem.

The first part the talk discusses the design, fabrication, testing and control of microactuators for dual-stage servo systems in disk drives, particularly electrostatic microactuatrors fabricated using MEMS technologies. The second part of this talk discusses the design, fabrication and experimental testing of instrumented suspensions. The incorporation of these two micro-mechatronic devices into HDDs requires the development of significantly more complex servo architecture than what exists in current units, since the control system must now simultaneously control two actuators and feed back three sensing signals, at different sampling rates. Moreover, this servo control system must be able to perform robustly and at an extremely high level of performance in a huge number of units within a HDD product line. Thus, the third part of the presentation will focus on the development of robust control synthesis techniques for Multi-input Multi-output (MIMO) and multi-rate servo systems for HDDs.  In particular, we will present a discussion and comparison of two major categories of controller design methodologies. The first methodologies are based on sequential single-input single-output (SISO) design techniques. The second category includes methodologies based on multirate and multi-input multi-output (MIMO) design techniques, including mixed H2/H-infinity, mixed H2/mu, and robust H2 synthesis. The final part of this talk discusses a novel systematic, semi-automated method for identifying a nominal reduced-order state space model, as well as a description of the plant uncertainty with a minimal number of parameters, from a batch of experimentally obtained frequency response plots, which can be used in the design of robust H2 controllers.

Biographical sketch

Roberto Horowitz received a B.S. degree with highest honors in 1978 and a Ph.D. degree in 1983 in mechanical engineering from the University of California at Berkeley. In 1982 he joined the Department of Mechanical Engineering at the University of California at Berkeley, where he is currently a Professor.  Dr. Horowitz teaches and conducts research in the areas of adaptive, learning, nonlinear and optimal control. His current research interests include: Micro-Electromechanical Systems (MEMS), computer disk file systems, robotics, mechatronics of smart exercise machines and paper handling devices, and Intelligent Vehicle and Highway Systems (IVHS).