Noncontact modal testing of hard-drive suspensions using ultrasound radiation

force

Acoustical Society of America Meeting: October 18, 2005Thomas M. Huber

Physics Department, Gustavus Adolphus CollegeDan Calhoun

Advanced Product Development, Hutchinson Technology, Incorporated

Mostafa Fatemi, Randy Kinnick, James GreenleafUltrasound Research Laboratory, Mayo Clinic

Introduction Overview of hard drives and head-gimbal-assembly suspensions

Non-contact, ultrasound stimulated excitation Overview Selective excitation by varying focus position Selective excitation of neighboring parts Selective excitation by phase shift In-Situ measurements of suspension vibration

Conclusions

Hard Drive HGA Suspension

HGA (Head Gimbal Assembly) suspension holds hard drive read/write heads Read/write head is attached to the flexure Flexure can gimbal around dimple Head flies over spinning disk surface Hinge and load beam provide downward force to balance lift from flying head

Suspension length about 10-14.5 mm , thickness of 25-100 μm Typical width about 4-6 mm

Hard Drive HGA Suspension

Head slider flies about ~10 nm above surface of the disk Human hair ~50µm (or 50,000 nm) in diameter Scale to macroscopic size – equivalent to 747 flying about 1mm above ground

In operating hard drive, vibration of head (pitch, roll, or sway) may cause loss of data or head crash

Instead of damping vibrations, suspensions are engineered to have specific vibrational frequencies

Leading Manufacturer: Hutchinson Technology

Headquarters in Hutchinson, MN (about 50 miles west of Minneapolis)

Manufacturing plants in Hutchinson and Plymouth, MN, Sioux Falls, SD, Eau Claire, WI.

Typical production rate of 14 million suspensions per week

Worldwide market leader of suspension assemblies Virtually all shipped to other countries for integration into hard

drives

Production monitoring involves resonance testing of small fraction of suspensions Suspension mounted on mechanical shaker for excitation (1-20

kHz) Laser doppler vibrometer used for non-contact measurement

R&D measurements of resonance frequency and deflection shapes

Weaknesses in current shaker/vibrometer test protocol

Smaller hard drives require smaller suspensions Requires modal testing between 1 kHz to about 50 kHz Existing mechanical shakers not useful above 20 kHz

Fixture modes: vibrations of support assembly unrelated to suspension Use of shaker assembly eliminates possibility of in-situ

testing of operating hard drive

Can ultrasound radiation force be used for non-contact excitation?

Ultrasound Stimulated Radiation Force Excitation Vibro-AcoustographyDeveloped in 1998 at Mayo Clinic Ultrasound Research Lab by Fatemi & Greenleaf

Difference frequency between two ultrasound sources causes excitation of object. Detection by acoustic re-emission

Technique has been used for imaging in water and tissue(Thursday AM Session 4aBB:Acoustic Radiation Force Methods for Medical Imaging and Tissue Evaluation)

Recently, we have also used the ultrasound radiation force for modal testing of organ reeds and MEMS devices in air

Ultrasound Stimulated Vibrometry for Suspensions

Pair of ultrasound frequencies directed at suspension

One ultrasound frequency differs from the other by frequency Δf that may be in the audio range or higher frequency

Difference frequency Δf between ultrasound beams produces radiation force that causes vibration of object

Vibrations were detected using a Polytec laser Doppler vibrometer

In some experiments, comparison of ultrasound excitation and mechanical shaker

Experiment Details: Dual Element Confocal Transducer 600 kHz broadband (>100 kHz

bandwidth)

70 mm focal length; 1 mm focus spot size

Confocal (concentric elements with different frequencies)

Inner disk fixed at f1=550 kHz Outer ring swept sine

f2=551–570 kHz

Difference frequency of Δf = 1 kHz – 20 kHz Caused excitation of suspension

Dual beams mean essentially silent operation since frequencies only combine at small spot on suspension

Experiment Details: Amplitude Modulated Excitation

Dual sideband, carrier suppressed amplitude modulated signal centered, for example, at 550 kHz

Difference frequency of Δf = 1 kHz – 20 kHz between the two frequency components caused excitation

Better for excitation since entire transducer producing the same signal (no need for mixing near surface).

Unfortunately, small fraction of both frequencies are combined in transducer, so some audio emitted

Instead of two transducer elements producing the two frequencies, an alternate method is an amplitude-modulated signal to cause excitation

Ultrasound excitation of HGA Suspension

Goal: To determine whether vibrationalresonances of suspension can be excitedusing ultrasound radiation force

To simulate an operational disk, end of suspension clampedthe gimbal head was simply supported on flat surface

Confocal ultrasound transducer usedto excite modes from 1 kHz to 50 kHz

Vibrometer measured resonance frequencies and deflection shapesat several ultrasound focus positions

Brüel & Kjær mechanical shakerused for comparison

Photos of Setup

Comparison of Shaker and Ultrasound Excitation Ultrasound excitation: 501 – 520 kHz

swept sine and 500 kHz fixed tone (red curve)

Brüel & Kjær mechanical shaker (blue curve)

Ultrasound excitation reproduces the resonances measured using mechanical shaker

Ultrasound excitation produces a cleaner spectrum than shaker Shaker has fixture modes

(resonances of supports or shaker) 2 kHz to 4 kHz

Ultrasound focused only on suspension, so little excitation of supports

High Frequency Ultrasound Excitation

Current resonance testing of suspensions to 20 kHz Limited by 20 kHz upper limit

of mechanical shakers used

As suspensions get smaller, desire resonance testing up to 50 kHz

Ultrasound excitation: Amplitude modulated swept sine with 550 kHz central frequency

Resonances clearly seen up to 50 kHz

Should be possible to measure resonances to over 100 kHz with this transducer

Selective excitation: Changing ultrasound focus position Ultrasound focus (ellipse of about 1mm by 1.5 mm)

centered on suspension (red curve) and towards edge of suspension (blue curve)

Selective Excitation: For ultrasound focus towards the edge (blue curve), large increase in amplitude of torsional modes at 6, 11, 13 and 15 kHz relative to the transverse modes at 2, 7, and 16 kHz.

Mode shapes determined using ultrasound excitation

2.0 kHz6.0 kHz 7.2 kHz 10.8 kHz

Pair of Unsupported Hard Drive Suspensions

Suspensions clamped at one end and free at other; 7.25 mm separation

Transducer mounted perpendicular and behind suspensions

Resonances up to 50 kHz 1mm focus leads to little cross

excitation (focused ultrasound allows selective excitation of single suspension)

Technique may be useful for analyzing suspensions before they are separated during manufacturing process

406 Hz 4.7 kHz 6.1 kHz 25 kHz

Selective Excitation using Phase-Shifted Pair of Transducers

Uses a pair of ultrasound transducers to allow selective excitation of transverse or torsional modes

Radiation force from two transducers with phase difference If driving forces are in phase, selectively excites transverse

modes while suppressing torsional modes If driving forces are out of phase, selectively excites torsional

modes while suppressing transverse modes

Technique used for phase-shifted selective excitation

Trials to date: pair of low-cost 40 kHz diverging transducers

Each transducer has dual sideband suppressed carrier AM waveform (software generated)

Modulation frequency swept

from 100 – 5000 Hz

Difference frequency Df leads to excitation from 200 Hz – 10 kHz

Variable phase shift between modulation signal Df applied to transducers

A 90 degree phase shift in signal results in 180 degree phase shift of radiation (driving) force

Phase-shifted selective excitation

Adjust amplitudes of two transducers to give roughly equal response

The pair of 40 kHz transducers not exactly matched (note different amplitudes near 5 kHz)

When both transducers turned on simultaneously with same modulation phase

Enhanced Transverse Mode

Suppressed Torsional Mode

Can give nearly 100% cancellation when transducers are matched

Driving in-phase excites transverse but suppresses torsional mode (blue curve)

Driving out-of-phase excites torsional while suppressing transverse mode (red curve)

Selective excitation may be useful for systems where transverse and torsional modes nearly overlap in frequency.

Phase-Shifted Selective Excitation of Suspension

Selective Excitation of Torsional/Transverse Modes

The maximum amplitude for the transverse modes is at angles near 0 degrees, with a minimum near 90 degrees

The maximum amplitude for torsional mode is at angles near 90 degrees, with minimum near 0 degrees.

By shifting the phase by 90 degrees, the ratio of the lowest transverse divided by torsional mode can change from above 20:1 to smaller than 1:3.

Selective excitation via phase shifted ultrasound has been demonstrated for several other types of devices, including rectangular cantilevers and a MEMS mirrors

In Situ Testing For Rotating Disk•Resonance testing of suspension in operating hard drive not possible because of attachment of shaker

•Ultrasound excitation non-contact; needs no fixture

•Allows for in-situ testing

•May be useful for diagnosing integrated system problems

•Red curve: Ultrasound off

•Vibration due to windage of flying head

•Blue curve: Ultrasound on

•Vibration in excess of windage

Conclusions Ultrasound allows excitation of resonances and deflection shapes

Completely non-contact for both excitation and measurement Produces same resonances of suspension as mechanical shaker

Does not excite fixture modes Useful for frequencies up to 50 kHz or more

Selective excitation Localized excitation can excite part without exciting neighboring parts Select transverse/torsional modes by moving ultrasound focus point Select transverse/torsional modes using phase shift between two transducers

Ultrasound excitation can be used for in-situ testing in a hard drive

Ultrasound excitation shown to be feasible for resonance testing of hard drive suspensions

Acknowledgements

This project includes support from the The Gustavus Presidential Research Award program

Student assistant John Purdham (GAC ’06)

This material is based upon work supported by the National Science Foundation under Grant No. 0509993

Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF)

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