Disk Drive Fly Height Measurement Using Capacitive Reference Sensing
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Solution Overview
Problem
Current methods for measuring the fly height of a disk drive head over a disk are unreliable due to issues with thermal conductivity and are prone to errors from ground bounce, thermal drift, and spin speed variations, which can lead to inaccurate measurements and potential catastrophic head crashes.
Innovation Solution
The use of capacitive sensors to measure fly height, incorporating a second head as a reference to compensate for ground noise and thermal drift, and implementing current mode interconnects to reduce noise coupling and simplify routing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If thermal conductivity methods are used to measure fly height, then measurement capability is provided, but measurement precision deteriorates due to ground bounce, thermal drift, and spin speed variations
Solution Approach 1:
The patent replaces thermal conductivity-based measurement methods with capacitive sensing. The capacitive sensor measures fly height by detecting capacitance changes between the head and disk, eliminating dependence on thermal conductivity which is affected by ground bounce, thermal drift, and spin speed variations. This substitution of measurement physics fundamentally resolves the reliability issues of the thermal method.
Solution Approach 2:
The patent introduces a capacitive sensor as an intermediary measurement mechanism. Instead of directly measuring thermal properties, the system uses capacitance as an intermediate physical quantity that correlates with fly height but is independent of thermal disturbances. The capacitive sensor acts as a mediator that translates mechanical distance into an electrical signal without being affected by thermal interference.
2Measurement precision
If fly height is reduced to improve read/write accuracy, then read and write performance improves, but the risk of head-disk contact increases causing catastrophic damage
Solution Approach 1:
The patent implements a feedback control system where capacitive sensors continuously measure the actual fly height and provide real-time data to the control mechanism. This feedback enables dynamic adjustment of head position to maintain optimal fly height while preventing contact. The system can respond to deviations and correct them before catastrophic contact occurs, enabling safer operation at reduced fly heights.
Solution Approach 2:
The patent employs preliminary measurement and warning mechanisms that detect approaching critical fly height thresholds before contact occurs. The capacitive sensing system provides advance notice of potentially dangerous conditions, allowing the system to take preventive action (such as adjusting head position or alerting operators) before catastrophic head-disk contact happens.
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
Accurate fly height measurements are achieved with reduced errors, enhancing the reliability and performance of disk drives by minimizing the impact of ground bounce, thermal drift, and spin speed variations.
Implementation Method 1
the fly height can be determined by measuring a capacitance of a capacitor formed by the head with the disk
Implementation Method 2
The distance D2 is achieved by heating the writer element 20 with a thermal element 24 (e.g., a coil)
Data Source
AI summary
A method for determining a fly height includes measuring a first differential voltage between a first head of a disk drive and a reference voltage with a first front end circuit, converting the first differential voltage to a first analog current signal with the first front end circuit, and converting the first analog current signal to a second differential voltage with a first back end circuit. The first front end circuit is coupled with the first head. The first back end circuit is coupled with the first front end circuit. The method further includes determining a first capacitance between the first head and a first disk of the disk drive based on the second differential voltage and determining the fly height between the first head and the first disk using the first capacitance.