Improved air gap controller for solid immersion lens-based near-field recording servo system

The near-field air-gap controller for solid immersion lens (SIL)-based near-field recording (NFR) systems has been required to improve its robustness to dynamic disturbances associated with rotating conditions such as periodic disk resonance and external shock. These dynamic disturbances usually exist in optical disk drive systems as a result of the operating condition. Moreover, SIL based NFR servo systems should be considered to realize stable servo systems because it is essential that the air gap between the SIL and the rotating disk should be less than 100nm and that the SIL and the disk should be protected. Hence, we propose improved air-gap control methods using internal model principle (IMP)-based and dead-zone nonlinear controllers. To increase air gap control performance with respect to dynamic disturbances, the IMP-based control method, which is one of the improved control methods for eliminating periodic disturbances at some frequencies, is implemented in the original NFR servo system. In addition, in case of anti-shock control performance, the original air gap servo system should be improved in terms of the anti-shock control such that the actuator head with SIL optics has to maintain on extremely small air gap to avoid collision between the SIL and the media when external shock is applied to the NFR system. Consequently, the experimental results show that the gap error signal is reduced by 76% during vibration of the disk using an IMP-based controller, and antishock control performance under external shock is improved by 60% using a dead-zone nonlinear controller.