Disk Drive Servo Phase Offset Distortion Cancellation

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Solution Overview

Problem

Existing disk drive servo control systems face challenges in accurately positioning the transducing head due to distortion in the position error signal (PES) caused by environmental conditions and changes in the fly-height of the transducing head, which affects the linearization of the PES signal.

Innovation Solution

The solution involves shifting the phase of the first servo burst with respect to the second servo burst by half a sample time, effectively introducing a 45-degree phase offset, which cancels out the distortion introduced by the third harmonic, thereby reducing the variation in the position error signal gain.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If post-processing is used to linearize the PES signal, then positioning accuracy is improved, but the system becomes sensitive to distortion caused by environmental conditions and fly-height changes

Engineering Contradiction:

Improvepositioning accuracyVSAvoidsignal sensitivity to distortion

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent applies preliminary action by pre-compensating for distortion in the servo pattern itself, rather than attempting to correct distortion after it occurs. The servo pattern is designed with built-in compensation for third harmonic distortion, environmental variations, and fly-height changes, so that the distortion is canceled out before it affects positioning accuracy.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The patent changes the parameters of the servo pattern by introducing phase shifts between multiple servo bursts and adjusting the timing relationships between servo wedges. These parameter modifications in the servo pattern structure enable the system to inherently compensate for distortion without requiring post-processing corrections.

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If the third harmonic distortion is present in the raw PES signal, then positioning can be achieved, but the distortion varies with environmental conditions and fly-height changes

Engineering Contradiction:

Improveposition error signal accuracyVSAvoidenvironmental condition sensitivity

Core Design Contradiction:

Measurement precisionVSAdaptability or versatility

Solution Approach 1:

The patent converts the harmful third harmonic distortion into a beneficial effect by using the distortion characteristics to design the servo pattern compensation. The servo pattern is specifically structured to generate distortion that cancels the harmful third harmonic, turning what was previously a problem into a solution for improving positioning accuracy across varying environmental conditions.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

Inventive Principle:

Principle #10Preliminary action

3Device complexity

If single-phase servo bursts are used, then the system is simpler, but distortion in the PES signal increases

Engineering Contradiction:

Improveservo pattern structureVSAvoidPES signal distortion

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent applies segmentation by dividing the servo pattern into multiple phases with multiple bursts. Instead of using a single-phase servo burst, the system uses multiple phases (e.g., phase A and phase B) with multiple bursts per phase. This segmentation allows the system to cancel distortion through phase shifting while maintaining manageable complexity through structured organization of the servo wedges.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent merges multiple servo bursts and phases into a unified servo pattern structure. By combining the information from multiple phases and bursts and processing them together through the voice coil motor control system, the patent achieves distortion cancellation while maintaining system coherence and avoiding excessive complexity.

Inventive Principle:

Principle #5Merging (Combining)

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 approach significantly reduces the distortion in the position error signal, leading to more accurate head positioning and reduced variations in the signal used to control the voice coil motor, resulting in improved tracking accuracy and reduced data erosion during writing operations.

Implementation Method 1

The rotary actuator is driven by a voice coil motor

Methodology Applied

Scientific EffectElectromagnetic force: Lorentz Force

Implementation Method 2

The servo pattern is read by the transducing head

Methodology Applied

Scientific EffectMagnetic field detection: Magnetic Field

Data Source

PatentUS7440224B2Disk drive servo

Publication Date: 2008.10.21 KK TOSHIBA

AI summary

A disk drive includes a disk having a first servo burst written with advancing phase as a radial distance on the disk increases, and a second servo burst written with decreasing phase as a radial distance on the disk increases, and a transducing head, and a read channel. The disk drive also includes an element for shifting the phase of the signal produced by the first burst pattern with respect to the signal produced by the second burst pattern by an amount sufficient to substantially cancel distortion in a signal produced by the first servo burst with distortion in a signal produced by the second servo burst when the signal from the first servo burst is added to the signal from the second servo burst.