Fly-Height Control Using Asynchronous Sampling and Loop-Back Reference
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Solution Overview
Problem
Existing methods for controlling fly height in hard-disk drive systems are inaccurate due to environmental variations affecting the electronics in the HDD system read path, leading to deviations in harmonic ratios and subsequent inaccuracies in measuring fly height changes.
Innovation Solution
A method involving a read-back mode and a loop-back mode is used to control fly height, where the read path samples a first signal from pre-recorded data and the write path samples a second signal bypassing the medium, generating values to control fly height, with asynchronous sampling to address quantization error and frequency aliasing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the read path samples signals from pre-recorded data using synchronous sampling, then the measurement process is simple, but environmental variations cause deviations in harmonic ratios leading to inaccurate fly height measurements
Solution Approach 1:
The patent introduces a reference signal as an intermediary that bypasses the read path electronics. By comparing the signal processed through the read path with this reference signal, the method isolates and eliminates the effect of environmental variations on the read path electronics, thereby improving measurement accuracy without compromising reliability
Solution Approach 2:
The patent creates a copy of the periodic signal that bypasses the read path (reference signal) and uses this copy for comparison. This copying approach allows the system to distinguish between actual fly height changes and artifacts caused by environmental variations in the read path, resolving the contradiction between measurement precision and reliability
2Measurement precision
If asynchronous sampling is used to eliminate quantization error and frequency aliasing, then measurement accuracy improves, but the complexity of the sampling system increases
Solution Approach 1:
The patent implements asynchronous sampling where the sampling frequency is dynamically adjusted to be different from the signal frequency, preventing quantization error and frequency aliasing. This dynamic approach improves measurement precision while the system manages complexity through controlled frequency variation rather than complex hardware modifications
3Reliability
If fly height is reduced to improve bit-error rate, then reading and writing performance improves, but the risk of element contact with platter surface increases
Solution Approach 1:
The patent employs feedback control where fly height is continuously monitored using the harmonic ratio measurement method, and adjustments are made to maintain optimal spacing. This feedback mechanism allows the system to achieve low bit-error rates through precise fly height control while preventing harmful contact between the element and platter surface
Data Source
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AI summary
In one embodiment, a hard-disk drive system performs fly-height control using a read-back mode and a loop-back mode. The read-back mode measures first and second harmonics pre-recorded on the medium and divides the first measurement by the second to obtain a read-back mode harmonic ratio. The loop-back mode measures the same first and second harmonics; however, the harmonics are provided by a write precompensation circuit rather than the medium. Further, the loop-back mode measurements are performed using asynchronous sampling to address aliasing and quantization errors. The first measurement is divided by the second to generate a loop-back harmonic ratio. In logarithm domain, the loop-back ratio is subtracted from the read-back mode ratio to remove environment-induced variations in the read path electronic circuits. The resulting harmonic ratio is subtracted from an initial harmonic ratio determined, for example, during manufacturing, to determine how much the harmonic ratio has changed.