Integration of piezoelectric sensing and control for nano-scale vibration suppression in hard disk drives

Industry goals for magnetic recording are currently in the terabit-per-square-inch regime, with hard disk drives (HDDs) remaining a competitive technology for server level data storage. Such ultrahigh data density requires a multifaceted approach to HDD servo systems, which need to provide nanometer-scale positioning of the read/write head. Dual-stage actuation has been extensively studied and advanced actuation schemes have been deployed in commercial prototypes. However, sensing technology remains limited, since servo sectors on the disk provide the only position information in state-of-the-art commercial drives. This work takes an integrated mechatronic approach to combine a novel sensing scheme with appropriate control methods to improve vibration suppression and tracking.;The major contribution of this work is fabrication, implementation, and evaluation of a novel piezoelectric strain sensor integrated into a PZT-actuated HDD suspension. The sensors consisted of thin-film ZnO fabricated directly onto the steel suspension structure that carries the read/write head. This technology required extensive process development, but allowed for the addition of sensors at arbitrary locations without significantly altering the dynamics of the suspension design. Moreover, the sensors had excellent resolution in the high frequency range where mechanical modes in the suspension are problematic. As a benchmark comparison, a similar PZT-actuated suspension was implemented using self-sensing, wherein a bridge circuit was used to extract a sensing signal from the PZT actuators. Both strategies were deployed in experimental disk drives and used to measure and model system dynamics.;Feedback control was explored using the auxiliary ZnO sensors, with the objective of targeting high-frequency structural vibrations excited by airflow. Optimal closed-loop control simulations with a nominal plant model predicted up to 30% improvement in suppression of windage-induced vibrations with high-resolution strain sensing at increasing sampling rates. A simple feedback damping controller was successfully operated experimentally on instrumented suspension prototypes. Finally, an adaptive lattice filter was formulated that utilized strain sensing to respond to uncertain disturbance conditions and attain optimal performance online. Simulations demonstrated that the filter achieved rapid recovery of optimal performance when varying windage disturbed the suspension.