HAMR Head NFT Touchdown Detection via Dither Signal Monitoring

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Solution Overview

Problem

In magnetic disk devices employing heat-assisted magnetic recording (HAMR) heads, it is challenging to detect when the near-field transducer (NFT) touches down on the disk due to thermal expansion, leading to potential damage as the NFT may unintentionally contact the disk, making it difficult to determine the exact point of contact and causing degradation.

Innovation Solution

Applying a direct current followed by an alternating current to dither the HAMR head, allowing for the detection of NFT touchdown by monitoring variations in the produced signal, which enhances the signal-to-noise ratio (SNR) and prevents NFT degradation.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If the NFT is made smaller to improve recording precision, then the measurement precision of NFT touchdown deteriorates

Engineering Contradiction:

Improverecording precisionVSAvoidtouchdown detection precision

Core Design Contradiction:

Manufacturing precisionVSMeasurement precision

Solution Approach 1:

A dither signal is introduced as an intermediary to amplify the detection of NFT touchdown. The dither signal causes the head to oscillate, and when the NFT contacts the disk, this oscillation is disrupted, creating a detectable signal change that amplifies the otherwise imperceptible touchdown event of the tiny NFT.

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent applies mechanical vibration through dithering of the head assembly. By vibrating the head at a known frequency and detecting changes in this vibration pattern when the NFT contacts the disk, the system can detect touchdown events that would be invisible due to the NFT's small size.

Inventive Principle:

Principle #18Mechanical vibration

2Reliability

If the back-off technique is used to achieve high SNR, then the clearance measurement becomes inaccurate due to unintentional NFT contact

Engineering Contradiction:

Improvesignal-to-noise ratioVSAvoidclearance measurement accuracy

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The system uses feedback from the dither signal monitoring to detect when the NFT contacts the disk during the back-off process. This feedback mechanism allows the system to identify unintentional contact events and adjust the back-off distance accordingly, ensuring accurate clearance measurements while maintaining high SNR.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent applies preliminary dithering before and during the back-off process to proactively detect potential NFT contact. By continuously monitoring the dither signal, the system can prevent unintentional contact from occurring or can immediately identify it, ensuring accurate clearance measurement.

Inventive Principle:

Principle #10Preliminary action

3Productivity

If the NFT is made significantly smaller than other head elements, then the recording performance improves, but the detection of NFT touchdown becomes difficult

Engineering Contradiction:

Improverecording performanceVSAvoidtouchdown detection difficulty

Core Design Contradiction:

ProductivityVSDifficulty of detecting and measuring

Solution Approach 1:

The patent employs mechanical vibration through dithering to make the invisible NFT touchdown visible. The vibration-induced signal changes provide a detection mechanism that works effectively regardless of the NFT's small size, allowing high recording performance with tiny NFT while maintaining detectability.

Inventive Principle:

Principle #18Mechanical vibration

Solution Approach 2:

The system changes the operational parameters by introducing dithering at specific frequencies and monitoring signal variations. This parameter change transforms the detection problem from trying to directly sense the tiny NFT contact to detecting the characteristic signal changes caused by the dithered head's interaction with the disk during contact.

Inventive Principle:

Principle #35Parameter changes

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 method effectively detects NFT touchdown, preventing damage by improving the signal-to-noise ratio and allowing for accurate clearance calculation, thereby ensuring the longevity and performance of the magnetic disk device.

Implementation Method 1

a direct current is applied to an element in a HAMR head. An alternating current is then applied to the element over top of the direct current to cause the HAMR head to dither

Methodology Applied

Scientific EffectElectromagnetic actuation: Electromagnet

Implementation Method 2

By monitoring the head signal at the dither frequency, a touchdown or contact of a NFT on a disk may be detected based upon variations in the produced signal

Methodology Applied

Scientific EffectContact detection through signal variation:

Data Source

PatentUS9595281B2Tunable contact detection sensitivity to directly measure clearance of protrusions in magnetic recording heads

Publication Date: 2017.03.14 WESTERN DIGITAL TECHNOLOGIES INC

AI summary

Embodiments disclosed herein generally relate to contact at the disk by the recording head in a hard disk drive. In one embodiment, a direct current is applied to an element in a HAMR head. An alternating current is then applied to the element over top of the direct current to cause the HAMR head to dither. By monitoring the head signal at the dither frequency, a touchdown or contact of a NFT on a disk may be detected based upon variations in the produced signal.