HDD Written Position Error Signal Compensation for Track Misregistration
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Solution Overview
Problem
Magnetic recording hard disk drives (HDDs) face track misregistration (TMR) issues due to disturbances during writing, leading to readback data errors and reduced areal densities and throughput, as the fixed track spacing and servo track pitch differ, causing misalignment between written and read positions.
Innovation Solution
The HDD writes recent estimated read head positions as written position error signal (WPES) values into data sectors, which are encoded and used during readback to predict the written positions and aid in data recovery, and in two-dimensional magnetic recording (TDMR), the difference between WPES and read PES values is used to select the best 2D equalizer for electronic tracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If fixed track spacing is used during manufacturing, then manufacturing precision is improved, but track misregistration occurs during readback due to disturbances during writing
Solution Approach 1:
The system performs preliminary action by writing the estimated read head position (PES value) into the data sector during the writing operation. This preliminary recording of position information allows the readback operation to compensate for any track misregistration that occurs during writing, thereby maintaining readback accuracy despite disturbances.
Solution Approach 2:
The system implements feedback by using the written PES values during readback to adjust and maintain the read head position. The read head position is continuously monitored and adjusted based on the stored PES information, creating a feedback loop that compensates for track misregistration and ensures accurate data retrieval.
2Adaptability or versatility
If servo track pitch differs from data track pitch, then positioning flexibility is improved, but misalignment between written and read positions occurs
Solution Approach 1:
The PES value serves as an intermediary that bridges the difference between servo track positioning and data track positioning. By storing the estimated read head position relative to the servo track, the system creates a reference that allows accurate positioning on data tracks even when the servo and data track pitches differ, thus maintaining position alignment despite the pitch mismatch.
3Reliability
If robust encoding is used for WPES values, then reliability of reading off-track values is improved, but device complexity increases
Solution Approach 1:
The system applies parameter changes by using different encoding schemes (repetition code, Reed-Muller code, or combination) for WPES values depending on the required robustness. By changing the encoding parameters based on the specific application requirements, the system achieves reliable off-track reading without unnecessarily increasing device complexity in all cases.
Data Source
AI summary
A hard disk drive is adapted to take the most recent estimated read head positions (the position error signal or PES values) that are obtained during writing and write those values into the data sectors as written PES (WPES) values. The WPES values are available on readback for use as a predictor of the positions in which subsequent data sectors were written and are also available for use in a data recovery procedure should a particular data sector fail to be recovered. In two-dimensional magnetic recording, the difference between the WPES value and the read PES value is a quantity required to rapidly select the best 2D equalizer. The PES values may be encoded prior to being written into the data sectors. The WPES values are appended to the data sectors after the preamble and sync fields, which occur after every servo sector.