Noncontact modal testing of hard-drive suspensions using ultrasound radiation force Acoustical Society of America Meeting: October 18, 2005 Thomas M. Huber Physics Department, Gustavus Adolphus College Dan Calhoun Advanced Product Development, Hutchinson Technology, Incorporated Mostafa Fatemi, Randy Kinnick, James Greenleaf Ultrasound 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 mm, thickness of 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-Acoustography Developed 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 f 1 =550 kHz  Outer ring swept sine f 2 =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 vibrational resonances of suspension can be excited using ultrasound radiation force  To simulate an operational disk, end of suspension clamped the gimbal head was simply supported on flat surface  Confocal ultrasound transducer used to excite modes from 1 kHz to 50 kHz  Vibrometer measured resonance frequencies and deflection shapes at several ultrasound focus positions  Brüel & Kjær mechanical shaker used 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 kHz 6.0 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 Hz4.7 kHz 6.1 kHz25 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 D f leads to excitation from 200 Hz – 10 kHz  Variable phase shift between modulation signal D f 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 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) Thank You