Self-Servo Write Correction for HDD Coherent Runout at Reduced RPM
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Solution Overview
Problem
Existing self-servo writing processes in hard disk drives (HDDs) face challenges in accurately writing product servo patterns at rotational speeds different from the nominal operational speed, leading to increased coherent runout and track squeeze due to varying disk behavior at different rotational speeds.
Innovation Solution
Characterize the natural uncontrolled path of the magnetic head at both the operational and self-servo write (SSW) rotational speeds, and inject the difference between these paths into the servo loop to correct for coherent runout, using non-circularity values to ensure the product servo patterns follow the uncontrolled path accurately at both speeds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If self-servo write is performed at reduced RPM, then written-in non-repeatable runout is reduced, but written-in coherent runout increases significantly
Solution Approach 1:
The patent applies parameter changes by measuring the uncontrolled head path at both the reduced SSW RPM and the nominal operational RPM, then using these measurements to compute correction factors that account for the difference in disk behavior at different rotational speeds. This allows the system to write servo patterns at reduced RPM while compensating for the speed-dependent changes in coherent runout characteristics.
Solution Approach 2:
The patent implements feedback by measuring the actual uncontrolled head path during self-servo write operations and using these measurements to compute and apply correction factors to the position error signal. This closed-loop approach ensures that the written servo patterns accurately represent the natural head path despite variations in rotational speed.
2Manufacturing precision
If spiral RRO correction factors are computed at nominal RPM, then coherent runout is corrected at operational speed, but the correction factors are inaccurate when writing at reduced SSW RPM
Solution Approach 1:
The patent directly addresses this limitation by measuring the uncontrolled head path at the actual SSW RPM being used, rather than relying on correction factors computed at nominal RPM. This ensures that the correction factors accurately reflect the disk behavior at the specific rotational speed being used for self-servo writing.
Solution Approach 2:
The patent performs preliminary measurement of the uncontrolled head path at the reduced SSW RPM before computing the correction factors. This preliminary action ensures that the correction factors are based on actual measurements taken at the operating conditions, rather than assuming behavior similar to nominal RPM operation.
3Measurement precision
If product servo patterns follow uncontrolled head path at reduced RPM, then written-in non-repeatable runout is minimized, but coherent runout increases when rotated at nominal operational speed
Solution Approach 1:
The patent resolves this contradiction by measuring the uncontrolled head path at both reduced SSW RPM and nominal operational RPM, then computing correction factors based on the difference between these two measurements. This allows the system to write patterns that follow the natural head path at reduced RPM while compensating for the speed-dependent changes in coherent runout.
Solution Approach 2:
The patent uses feedback from measurements taken at both rotational speeds to compute correction factors that are applied to the position error signal. This ensures that the written servo patterns account for the different disk behavior at reduced versus nominal RPM, achieving low non-repeatable runout at write speed while maintaining low coherent runout at operational speed.
Data Source
AI summary
Self-servo write (SSW) is performed in a disk drive while rotating disks at an SSW rotational speed, such that the resultant product servo patterns have near-zero coherent runout when rotating the disks at an operational rotational speed. The natural uncontrolled path of a magnetic head of the drive is characterized at the operational rotational speed and at the SSW rotational speed, and the difference between the two uncontrolled paths is injected into the servo loop to modify the path of the magnetic head during the SSW process. Specifically, the natural uncontrolled path of the magnetic head of the drive at the normal operating rotational speed is characterized with a first set of non-circularity values, and the natural uncontrolled path of the magnetic head of the drive at the SSW rotational speed is characterized with a second set of non-circularity values.