Gap Servo Control for Near-Field Optical Disc Drives
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Solution Overview
Problem
Conventional gap servo control in near-field optical disc drives is limited in disturbance cancellation capability and prone to overshoot during pull-in, due to sensitivity to disc scratches, contamination, and electrical noise, which increases the risk of collision between the solid immersion lens and the disc.
Innovation Solution
A gap servo control method and apparatus that utilize multiple disturbance observers operating within different frequency bands to generate actuator driving signals, transitioning from a first band to a second band as the gap error signal reaches specific levels, thereby reducing overshoot and enhancing disturbance cancellation capability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the loop gain of the feedback controller is increased to control the pickup to react to disturbances of the disc, then the disturbance cancellation capability is improved, but the servo bandwidth increases making the system sensitive to disc scratches, contamination, and electrical noise, thereby increasing the occurrence of overshoot during pull-in
Solution Approach 1:
The patent segments the feedback control into two distinct paths: a high-gain feedback controller for disturbance cancellation and a low-gain feedback controller for stable operation. This segmentation allows the system to achieve both high disturbance rejection capability and low sensitivity to harmful factors by selectively using appropriate control paths based on operating conditions
Solution Approach 2:
The patent dynamically switches between different control configurations based on the gap error signal level. When the gap error signal exceeds a threshold, the high-gain feedback controller is activated for aggressive disturbance cancellation. When the threshold is not exceeded, the low-gain controller is used for stable, low-sensitivity operation. This dynamic adaptation resolves the contradiction by adjusting control aggressiveness to actual system needs
2Reliability
If the loop gain is increased to ensure sufficient disturbance cancellation capability, then the disturbance rejection is improved, but the occurrence of overshoot during pull-in increases, raising the possibility of collision between the SIL and the disc
Solution Approach 1:
The patent implements dynamic control gain adjustment based on the gap error signal threshold. During pull-in when disturbances are significant, the high-gain controller provides strong disturbance cancellation. When the system stabilizes and the gap error signal remains below the threshold, the low-gain controller prevents overshoot and ensures stable operation, thereby eliminating the collision risk while maintaining disturbance rejection capability when needed
3Manufacturing precision
If a single high-gain feedback controller is used to maintain the gap within tenths of a nm, then the gap control precision is improved, but the system becomes unstable and prone to overshoot when transitioning from open loop to closed loop state
Solution Approach 1:
The patent segments the feedback control into two distinct controllers with different gain characteristics. The high-gain feedback controller provides precise gap control when needed, while the low-gain feedback controller ensures stable transitions and steady-state operation. This segmentation allows the system to achieve both precision and stability by using the appropriate controller for each operating phase
Solution Approach 2:
The patent changes the feedback gain parameter based on the operating state, specifically based on whether the gap error signal exceeds a threshold. This parameter change allows the system to switch from aggressive high-gain control (for precision) to conservative low-gain control (for stability), thereby resolving the contradiction between precision and stability during state transitions
Data Source
AI summary
A gap servo control method and apparatus for increasing a disturbance cancellation capability and decreasing the occurrence of an overshoot in a near-field optical disc drive include: generating an actuator driving signal of a transient response process by using a signal obtained by removing a disturbance contained in the gap error signal using a first band when a level of a gap error signal reaches a first level; and generating the actuator driving signal of the transient response process by using a signal obtained by removing the disturbance contained in the gap error signal using a second band when the level of the gap error signal reaches a second level.