Improved Gap Control System Using a Disturbance Observer for Near-Field Recording

Abstract

In a solid immersion lens (SIL) based near-field recoding (NFR) system which is one of the emerging technologies for nextgeneration optical data storage systems, it is essential that the air gap between the SIL and the rotating disk is maintained at less than 50 nm without collision in order to obtain the proper coupling efficiency of evanescent waves. To fabricate a reliable near-field air gap servo system, various disturbances such as disk vibration, external shock and overshoot have to be considered, and these possible disturbances have to be prevented effectively. We propose an improved gap servo system using a disturbance observer (DOB), which has a reduced overshoot and rejection performance for the previously mentioned disturbances. The effectiveness of the proposed controller is verified by experimentally. The experiment results, show that the overshoot was reduced using the proposed near-field air gap servo system with a DOB. In the case of the ramp approach mode with and without the hand-over mode in the mode-switching servo, the overshoots were decreased to 50.9% and collision was avoided, respectively. In addition, in the case of the modified approach mode with and without the hand-over mode in the mode-switching servo, the overshoot was decreased to 2.9 and 3.7%, respectively. Consequently, the access time was decreased in each approach case without the hand-over mode using the DOB-based controller. In addition, the disturbance rejection performance of the external shock was improved to 9.11%.

Control Methods in Data-Storage Systems

“…Because of the delay of the air-gap error signal, the introduction of an appropriate open-loop control signal is desirable to reduce the pull-in time and to prevent overshoot [188]. Use of a DOB to reduce the overshoot at the switching point between open-loop and closed-loop modes was demonstrated in [189].…”

Effect of Air Bearing Surface on Shock Resistance of Optical Head for Solid Immersion Lens Based Near Field Recording System

“…The and are stiffness and damping coefficient of wires, and the and are stiffness and coefficient by the ABS. We carried out this study in half-sinusoidal shock with the amplitude of 10 G and the duration of 10 ms to satisfy the safety shock level about 1 G at 10 ms in operating condition and the protected shock level about 1 G at 10 ms that is researched using a disturbance observer-based near field air gap servo controller [3].…”

“…To protect the NFR system from the external shock, Yoon et al [2] proposed the control algorithm so that can resist 243 G (the acceleration of gravity) shock for the external shock using the protector and the safe mode. Kim et al [3] evaluated the performance of the proposed a disturbance observer-based near field air gap servo controller upon receiving an external shock with a magnitude of 1 G and a time duration of 10 ms. However, these methods were difficult to realize a commercial optical disc drive because of a high sensitivity accelerometer and short shock duration that is different from actual shock specification of 6 G and 10 ms.…”

Application of Solid Immersion Lens-Based Near-Field Recording Technology to High-Speed Plasmonic Nanolithography

“…The SIL-based nanolithography method can precisely control the nanogap between the optical head and the substrate with an active gap controller that uses the gap error signal (GES); [12][13][14] thus the line patterning with a several tens nanometer-size width can be performed at high speed. To realize an optical head beyond the diffraction limit, we designed and fabricated a plasmonic SIL optical head with an optimized sharp-ridged hightransmission nanoaperture on an Al film.…”