Dual Offset ZAP Values for HAMR Thermal Runout Correction
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Solution Overview
Problem
In heat-assisted magnetic recording (HAMR) devices, dynamically changing read/write offset due to thermal expansion and laser focal point shifts causes significant tracking errors and reliability issues, as conventional repeatable runout correction (ZAP) methods are ineffective when the reader/writer offset changes over time.
Innovation Solution
Computing and writing two repeatable runout correction (ZAP) values per virtual track, offset in opposite directions from the track center, to ensure effective compensation for repeatable runout even when the read/write offset changes dynamically, thereby maintaining accurate tracking and data integrity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a single ZAP value is written at the virtual track center, then the device complexity is low and manufacturing is simple, but tracking precision deteriorates when read/write offset changes due to thermal expansion
Solution Approach 1:
The single ZAP correction value is segmented into multiple ZAP values positioned at different radial offsets from the virtual track center. This segmentation allows the servo system to select the appropriate ZAP value based on the current read/write offset, maintaining tracking precision across varying thermal conditions without requiring a fundamentally complex system architecture.
Solution Approach 2:
Multiple ZAP values are pre-computed and pre-written to the disk at different radial positions during manufacturing. This preliminary action ensures that when thermal expansion occurs during operation, the servo system can immediately access the appropriate pre-computed correction value without real-time calculation, maintaining both precision and operational simplicity.
2Manufacturing precision
If multiple ZAP values are written at different radial offsets, then tracking precision is maintained under thermal variation, but the quantity of data written per track increases
Solution Approach 1:
Different ZAP values are written at different radial positions (local regions) corresponding to different read/write offset conditions. This local quality approach ensures that each radial position has the appropriate correction value for that specific offset condition, optimizing tracking precision locally while minimizing the total data quantity by only storing values where needed.
3Adaptability or versatility
If the read/write offset changes dynamically due to thermal expansion, then HAMR recording capability is achieved, but servo tracking reliability deteriorates with conventional single ZAP correction
Solution Approach 1:
The system transitions from a static single ZAP correction value to a dynamic selection mechanism that chooses among multiple pre-computed ZAP values based on the current read/write offset. This dynamic adaptation allows the servo system to maintain reliability under varying thermal conditions while preserving the thermal adaptation capability necessary for HAMR operation.
Solution Approach 2:
The servo system uses feedback from the actual read/write offset measurement to select the appropriate ZAP correction value. This feedback mechanism ensures that the correction applied is always matched to the current thermal state, maintaining servo tracking reliability despite dynamic thermal expansion during HAMR recording operations.
Data Source
AI summary
First and second repeatable runout (ZAP) values are both located on a first virtual track of a magnetic disk. The first ZAP value is offset from the first virtual track center in a first direction and the second ZAP value is offset from the first virtual track center in a second direction opposite the first direction. At least one of the first and second ZAP values are accessed when performing repeatable runout correction for a writer of the read/write head that is being positioned over a second virtual track of the magnetic disk.