PLL Calibration for Repetitive Phase Error in Servo Writing
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Solution Overview
Problem
Conventional hard disk drives face challenges in synchronizing self-servo-write processes due to variations in spindle speed and written-in rotational timing mark errors, leading to non-uniform placement of servo sectors and increased system noise when increasing the bandwidth of phase-locked loops.
Innovation Solution
The implementation of a phase-locked loop calibration system that includes an oscillator model, averaging and summing submodules, and a delay buffer to generate and refine repetitive phase error corrections, reducing phase errors and improving servo sector placement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the bandwidth of the phase-locked loop is increased to minimize phase errors, then the accuracy of servo sector placement is improved, but system noise increases and stability deteriorates
Solution Approach 1:
The patent segments the phase error into two distinct components: repetitive phase error (caused by periodic disturbances like spindle speed variations) and non-repetitive phase error (random noise and irregular disturbances). By separating these components, the system can apply different handling strategies to each, allowing the PLL to maintain stability while improving accuracy through targeted correction of repetitive errors.
Solution Approach 2:
The patent implements preliminary action by measuring and storing repetitive phase errors over multiple rotation periods before using them to correct future phase errors. The system accumulates phase error data over several revolutions, identifies the repetitive component, and applies correction in advance, allowing the PLL to maintain stability while compensating for periodic disturbances proactively rather than reactively.
2Measurement precision
If the bandwidth of the phase-locked loop is increased to minimize phase errors, then the accuracy of servo sector placement is improved, but system noise increases
Solution Approach 1:
The patent segments the phase error into repetitive and non-repetitive components, allowing the system to correct only the repetitive portion while leaving the non-repetitive (noisy) portion to be handled by the standard PLL with lower bandwidth. This segmentation enables noise reduction while maintaining accuracy for periodic errors.
Solution Approach 2:
The patent extracts the repetitive phase error component from the total phase error using averaging over multiple rotation periods. By taking out and separately processing the repetitive component, the system removes the source of periodic noise while maintaining correction effectiveness, thereby reducing overall system noise while preserving measurement precision.
3Device complexity
If conventional phase-locked loops are used without calibration, then the device complexity is low, but the accuracy of servo sector placement deteriorates due to repetitive phase errors
Solution Approach 1:
The patent applies preliminary action by implementing a calibration process that runs before normal operation. During calibration, the system measures repetitive phase errors over multiple rotations and stores correction values in lookup tables. This preliminary calibration enables accurate servo sector placement during normal operation without requiring complex real-time processing, thus maintaining low device complexity while improving precision.
Solution Approach 2:
The patent uses copying by creating lookup tables that store pre-measured repetitive phase error patterns. Instead of continuously analyzing and processing phase errors during operation, the system copies the calibrated correction values from the lookup tables based on the current rotational position, significantly reducing computational complexity while maintaining high placement accuracy.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution reduces phase errors, increases the accuracy and precision of servo sector placement, and enhances the stability of self-servo-write processes by iteratively calibrating phase-locked loops to compensate for repetitive phase disturbances.
Implementation Method 1
For example, a voltage controlled oscillator (VCO) can adjust the frequency and phase of the SSW clock.
Data Source
AI summary
Systems and techniques to calibrate a control loop include, in at least one implementation, a system including the control loop configured to generate a clock signal and lock the clock signal to timing marks detected on a machine readable medium, a repetitive error correction module configured to receive a predicted phase and a corrected phase error for the clock signal, generate a predicted repetitive phase disturbance using the predicted phase and the corrected phase error for the clock signal, and calibrate a phase error to compensate for variations in repetitive phase errors in the clock signal using the predicted repetitive phase disturbance; and a servo track generator configured to generate servo tracks using the clock signal.