Variable Servo Track Pitch for TDMR Disk Drive Skew Compensation
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Solution Overview
Problem
In two-dimensional magnetic recording (TDMR) hard disk drives, head skew causes an increase in cross-track sensor-to-sensor spacing, leading to track misregistration (TMR) due to fixed servo track pitch, which cannot be optimally compensated by multiple read heads.
Innovation Solution
The servo track pitch is varied across the radius of the disk based on the skew angle, allowing each read sensor to read a different servo half-track, thereby averaging out noise sources and reducing TMR through an optimal variable servo track pitch profile.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If fixed servo track pitch is used, then manufacturing is simplified, but track misregistration increases due to head skew
Solution Approach 1:
The patent applies dynamics by transitioning from a fixed servo track pitch to a variable servo track pitch that changes with radial position on the disk. The pitch is smaller at the inner diameter and larger at the outer diameter, allowing the track pitch to adapt to the varying geometry of the actuator's arcuate path at different radial positions, thereby reducing track misregistration while maintaining manufacturability.
Solution Approach 2:
The patent applies local quality by making the servo track pitch non-uniform across the disk surface. Different regions of the disk have different track pitches optimized for their specific radial positions. This allows each local region to have optimal track spacing for minimizing skew effects at that particular radius, improving overall tracking precision without requiring complete redesign of the entire track structure.
2Quantity of substance
If multiple read sensors are used, then areal data bit density increases, but cross-track sensor-to-sensor spacing increases due to head skew
Solution Approach 1:
The patent applies parameter changes by modifying the servo track pitch parameter as a function of radial position. By changing the track pitch parameter to match the geometric requirements at different radii, the system maintains optimal sensor spacing relationships even as the physical geometry changes due to head skew, thereby preserving areal data bit density benefits.
3Device complexity
If fixed servo track pitch is used, then device complexity is reduced, but noise averaging effect is compromised
Solution Approach 1:
The patent applies dynamics by implementing a variable servo track pitch that adapts to radial position, enabling the system to maintain optimal noise averaging performance across the entire disk surface. The dynamic adjustment of track pitch ensures that multiple read sensors consistently sample uncorrelated noise sources regardless of their radial position.
Solution Approach 2:
The patent applies local quality by optimizing the servo track pitch for each local region's specific requirements. This allows different regions of the disk to have tailored track spacing that maximizes the noise averaging effect for sensors at those particular radial positions, improving overall system reliability without excessive complexity.
Data Source
AI summary
A two-dimensional magnetic recording (TDMR) disk drive has a disk with servo tracks with a track pitch that varies across the radius of the disk. The servo track pitch (STP) is related to the cross-track spacing (CTS) of the multiple read sensors in the TDMR head structure. The CTS is given by the equation: CTS=(CTO)cos θ+(ATO)sin θ, where θ is the skew angle, and CTO is the cross-track spacing and ATO the along-the-track spacing of the read sensors at zero skew angle. The optimal variable STP profile results in track misregistration reduction because it allows each read sensor to read a different servo half-track and thus noise sources not correlated to noise sources read by the other read sensors. The servo tracks may be arranged into a plurality of annular bands, with the STP in each band being fixed and different from the STP in the other bands.