Disk Drive Fly Height Measurement via Capacitive Noise Correction
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Solution Overview
Problem
Current methods for measuring the fly height of a disk drive head over a disk, such as using thermal sensors, are unreliable and prone to errors due to ground noise, thermal drift, and spin speed variations, which can lead to inaccurate capacitance measurements and potential head crashes.
Innovation Solution
A capacitive sensor system that utilizes a second head as a reference to correct for ground noise and thermal drift, measuring capacitance changes between the selected head and disk, and adjusting the fly height to improve measurement accuracy by compensating for these errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If thermal sensors are used to measure fly height, then measurement capability is provided, but measurement precision deteriorates due to ground noise, thermal drift, and spin speed variations
Solution Approach 1:
The patent introduces a capacitive sensor as an intermediary measurement mechanism that directly measures the capacitance between the head and disk to determine fly height. This capacitive measurement approach serves as a more reliable intermediary than thermal sensors, as it is not affected by ground noise, thermal drift, or spin speed variations. The capacitive sensor provides a direct electrical measurement path that isolates the fly height determination from the harmful environmental factors that plague thermal sensing methods.
Solution Approach 2:
The patent replaces the thermal sensing mechanism (which relies on thermal conduction and is susceptible to thermal drift) with a capacitive sensing mechanism (which relies on electrical field effects). This substitution transitions from a thermal-based measurement system to an electrical-based system, eliminating the fundamental vulnerabilities of thermal sensors to ground noise and thermal drift while maintaining the ability to measure fly height through the relationship between capacitance and distance.
2Measurement precision
If fly height distance is reduced to improve read/write accuracy, then read and write operation accuracy is improved, but the risk of head physical contact with the disk increases
Solution Approach 1:
The patent implements a feedback control system where the capacitive sensor continuously measures the actual fly height by monitoring capacitance changes, and this measurement is fed back to the fly height control mechanism. The feedback loop enables real-time adjustment of the head's position relative to the disk, allowing the system to maintain an optimal fly height that maximizes read/write accuracy while preventing contact. The continuous monitoring and adjustment create a self-correcting system that responds to any tendency toward head-disk contact.
Solution Approach 2:
The patent employs dynamic fly height adjustment capabilities, allowing the fly height to be varied during operation rather than being fixed. The system can dynamically change the head's vertical position in response to capacitive sensor measurements, enabling adaptation to different operating conditions. This dynamic control allows the system to optimize performance by adjusting fly height while maintaining safety margins to prevent contact, transforming a static configuration into a responsive, adaptive system.
3Ease of operation
If thermal elements are used to adjust head position, then fly height control capability is provided, but measurement precision deteriorates due to thermal drift affecting capacitance measurements
Solution Approach 1:
The patent introduces a reference capacitive sensor as an intermediary measurement device that measures capacitance in a region unaffected by the thermal element's heating. This reference sensor serves as a stable baseline that is not influenced by thermal drift, allowing the system to distinguish between capacitance changes caused by fly height variations and those caused by thermal effects. The reference sensor acts as a mediator that isolates the measurement system from thermal interference.
Solution Approach 2:
The patent extracts the thermal drift effect from the measurement system by using a separate reference capacitive sensor that is physically isolated from the thermal element's influence. By taking out the thermal interference component through spatial separation and using it as a reference, the system can subtract or compensate for thermal drift effects from the main measurement, leaving only the fly height-related capacitance changes. This extraction approach separates the harmful thermal effect from the useful measurement signal.
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
The capacitive sensor system significantly reduces errors in fly height measurement, achieving accuracy within 0.2% to 1%, thereby enhancing the reliability and precision of disk drive operations and preventing head crashes.
Implementation Method 1
measuring a first capacitance between the first head and the first disk
Implementation Method 2
heating the writer element 20 with a thermal element 24
Data Source
AI summary
A system for determining a fly height includes a first head of a disk drive, a second head of the disk drive, a capacitive sensor circuit coupled to the first head and the second head, and a logic device coupled to the capacitive sensor circuit. The capacitive sensor circuit is configured to measure a first capacitance between the first head and the first disk, remove noise from the first capacitance using a second capacitance between the second head and the second disk, and based thereon determine a corrected first capacitance. The logic device is configured to determine the fly height between the first head and the first disk using the corrected first capacitance.