Improvement of protection process using observer for soild imersion lens-based near-field recording

In the near field recording (NFR) system, there always exists the possibility of collision between solid imersion lens (SIL) and media owing to dust, scratches on the media, and external shock because of the extremely small gap distance. The kinetic energy associated with damage from shock is directly proportional to the square of velocity, and the phase of velocity is faster than the phase of displacement with 90°. Thus, it is effective to prevent a collision on the basis of velocity information. The velocity of the actuator in the NFR system should be estimated by an observer without using a velocity sensor. In this paper, we propose an improved protection process with a mode switching servo method using a Luenberger observer. The gap distance state can be estimated earlier by detecting the velocity state yielded by external shock. Through simulations and experiments, we confirm that the protection process based on velocity and gap distance is more powerful than the protection process based only on the gap distance.