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Nie J.,University of California at Berkeley | Sheh E.,Western Digital Corporation | Horowitz R.,University of California at Berkeley
IEEE Transactions on Control Systems Technology | Year: 2012

Missing position error signal sampling data in hard disk drives (HDDs) results in their servo systems having irregular sampling rates (ISRs).With the natural periodicity ofHDDs, which is related to the disk rotation, HDD servo systems with ISRs can be modeled as linear periodically time-varying systems. Based on our previous results, an explicit minimum entropy H ∞ controller for HDD servos with ISRs via discrete Riccati equations is first obtained. The resulting controller is subsequently demonstrated to be periodically time-varying and implementable. Simulation and experimental studies, which have been carried out on a set of hard disk drives, demonstrate that the proposed control synthesis technique is able to handle ISRs and can be used to conveniently design a loop-shaping track-following servo that achieves the robust performance of a desired error rejection function for disturbance attenuation. Experimental results on ten actual 2.5″ hard disk drives show around 27% improvement of the 3σ PES was obtained by the proposed control algorithm. © 2011 IEEE. Source

Huang K.,Western Digital Corporation | Lee D.,Seoul National University
IEEE Transactions on Robotics | Year: 2013

We propose a novel peer-to-peer (P2P) distributed control architecture for the shared haptic interaction among remotely located users over partially connected and undirected unreliable Internet communication network with varying delay, packet loss, data swapping/duplication, etc. Each user simulates and interacts with their own local copy of the shared deformable virtual object (for haptic responsiveness against latency), while these local copies are synchronized via a proportional-derivative type consensus control over the Internet communication network (for haptic experience consistency among the users). Our proposed architecture enforces passivity, thereby, rendering itself to be interaction stable, portable, and scalable for heterogeneous (passive) users and devices. Configuration consensus among the local copies and force balance among the users are also shown. The issue of optimizing communication network topology is also addressed with some relevant experimental results. © 2004-2012 IEEE. Source

Mastellone S.,ABB | Mejia J.S.,Western Digital Corporation | Stipanovi D.M.,Urbana University | Spong M.W.,University of Texas at Dallas
Automatica | Year: 2011

We study the problem of formation control and trajectory tracking for a group of robotic systems modeled by Lagrangian dynamics. The objective is to achieve and maintain a stable formation for a group of multi-agent systems, while guaranteeing tracking of a specified trajectory. In order to do so, we partition the state space for the collective system into coordinates of the geometric center of mass of the group and coordinates that describe the relative positions of the robots with respect to the center of mass, thus defining the formation shape. The relative positions can be further partitioned in coordinates which describe the absolute distances and orientation of each robot to the center of mass. We can rewrite the total system as dynamics of the center of mass of the formation, and dynamics of the shape, where the systems are, in general, coupled. By imposing holonomic constraints between the subsystems (i.e.; imposing a configuration constraint) and hence reducing the system's dimension, we guarantee that the group can be driven to follow a desired trajectory as a unique rigid body. Using high gain feedback, we achieve asymptotic decoupling between the center of mass and the shape dynamics and the analysis is performed using a singular perturbation method. In fact, the resulting system is a singularly perturbed system where the shape dynamics describe the boundary layer while the center of mass dynamics describes the reduced system. After an initial fast transient in which the robots lock to the desired shape, a slower tracking phase follows in which the center of mass converges to a desired trajectory while maintaining a stable formation. © 2011 Elsevier Ltd. All rights reserved. Source

Wu S.-C.,Western Digital Corporation | Tomizuka M.,University of California at Berkeley
Mechatronics | Year: 2010

This paper considers the modeling and compensator design for Self-ServoWriting (SSW) process in disk drives. An Iterative Learning Control (ILC) based scheme is established to deal with radial error propagation and improve the quality of written tracks. In the proposed scheme, a feedback controller for track following is first designed to achieve good disturbance attenuation. Then, an ILC structure is applied to generate an external signal, which is injected into the feedback loop in order to compensate for the written-in errors in the previous track while the next track is written. As a result, the error propagation can be contained. The learning controller is synthesized by solving Linear Matrix Inequality (LMI) equations to ensure the stability and monotonic convergence of the control algorithm. Simulation results show the effectiveness of the proposed scheme on the error containment which results in good quality written tracks. © 2009 Elsevier Ltd. All rights reserved. Source

Hossein-Babaei F.,Stanford University | Sinclair R.A.,Stanford University | Srinivasan K.,Western Digital Corporation | Bertero G.A.,Western Digital Corporation
Nano Letters | Year: 2011

The key component of a hard disk medium is a Co-based magnetic layer (ML) grown on a Ru seed layer. The ML nanostructure, composed of less than 10 nm grains, is believed to be controlled by this seed layer. We successfully used scanning transmission electron microscopy energy dispersive spectrometry simultaneous composition-based imaging and Moiré pattern analysis for determining the mutual structural and orientation relationship between the two layers revealing a grain-to-grain agreement. The method presented here can be utilized for observing structural correlations between consecutive polycrystalline thin film layers in general. © 2011 American Chemical Society. Source

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