Noncontact Modal Excitation of Small Structures Using Ultrasound Radiation

Force

Society for Experimental Mechanics Annual Meeting Springfield, MA

June 4, 2007Thomas M. Huber, Scott D. Hagemeyer, Eric T. Ofstad

Physics Department, Gustavus Adolphus College

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

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Introduction Overview of Ultrasound Stimulated Excitation

Uses ultrasound radiation force for non-contact modal excitation

Selective Excitation by Phase Shifted Pair of Transducers Results for simple cantilever

Results for MEMS Gyroscope

Results for MEMS mirror

Conclusions

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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

We have also used the ultrasound radiation force for modal testing of organ reeds and hard drive suspensions (IMAC 2006)

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Ultrasound Stimulated Amplitude Modulated Excitation

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

Difference frequency of Δf of 100 Hz to over 50 kHz

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

Transducer used in this experiment had 1.5 mm diameter focus spot size

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Photos of Setup

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Selective Excitation using Phase-Shifted Pair of Transducers

To illustrate this technique, consider first a simple cantilever in air Instead of using a single transducer, use a pair of ultrasound transducers

to allow selective excitation of transverse or torsional modes If radiation force from both transducers are in phase, selectively

excites transverse modes while suppressing torsional modes If radiation force is out of phase, selectively excites torsional modes

while suppressing transverse modes Demonstrated for cantilevers, MEMS mirror and hard drive

suspensions

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Phase-shifted selective excitation: Detailed Description

Two 40 kHz transducers, each with dual sideband suppressed carrier AM waveform

Modulation frequency swept from 50 – 5000 Hz

Difference frequency Δf leads to excitation from 100 Hz – 10 kHz

Modulation phase difference of 90 degrees leads to 180 degree phase difference in radiation force

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Phase Shifted Selective Excitation Use scanning vibrometer to measure deflection shape Adjust amplitudes of two 40 kHz transducers to give roughly equal response

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Phase Shifted Selective Excitation Adjust amplitudes of two 40 kHz transducers to give roughly equal response

When both transducers turned on simultaneously with same modulation phase Enhanced Transverse Mode Suppressed Torsional Mode

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Phase Shifted Selective Excitation Driving in-phase excites transverse but suppresses torsional modes (dashed blue

curve) Driving out-of-phase (phase difference near 90 degrees) excites torsional while

suppressing transverse modes (red curve)

This technique allows information about mode shape to determined even from a single point vibrometer

Can differentiate two overlapping modes (if, for example, 2nd transverse and 1st torsional mode were at nearly identical frequencies)

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Case Study: MEMS Gyroscope

Analog Devices ADXRSMEMS Gyroscope

Pair of Test Masses ¾ mm squareseparated by ½ mm

Test masses have in-planeresonance frequency of 14 kHz.

Question: What about out-of-plane motion

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Ultrasound Excitation of MEMS Gyroscope Scanning vibrometer detects

motion of test masses & nearby regions

Ultrasound transducer focused on gyroscope.

Central frequency of 600 kHz, with Δf = 13.5 kHz

Maximum velocity of 250 μm/s

Measured out-of-plane displacement amplitude of 2.5 nm!

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Ultrasound Excitation of MEMS Gyroscope

Ultrasound transducer centered

Ultrasound transducer Moved ½ mm right

Ultrasound transducer Moved ½ mm left

Base Excitation withMechanical Shaker

Vibrates entire structure

Demonstrates capability of this technique for non-contact selective excitation without exciting the base

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Another Device Tested: 2-d MEMS Mirror

Manufactured by Applied MEMS Mirror is 3mm on Side - Gold plated Silicon Three vibrational modes

X Axis torsion mode: 60 Hz Y Axis torsion mode: 829 Hz Transverse mode (forward/back): 329 Hz

(incidental – not used for operation of mirror)

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Selective Ultrasound Excitation of MEMS Mirror

Ultrasound focus ellipse about 1x1.5 mm Focus position can be moved horizontally

or vertically

Changing transducer position allows selective excitation

Upper figure: All modes present when focus near center of mirror. Red line shows excitation

using mechanical shaker. Middle: X-torsional mode

increases when ultrasound focus near top of mirror.

Bottom: Z-Torsional mode increases when focus near right edge

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Selective Ultrasound Excitation of MEMS Mirror

X-Torsional mode peaks when focus near top/bottom of mirror

Transverse mode decreases as transducer moved vertically(smaller fraction of beam on mirror)

Ratio of amplitudes of X-Torsional to Transverse modes changes by over factor of 10x as vertical position is varied

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Phase-Shifted Selective Excitation of MEMS Mirror

Driving in-phase excites transverse and Y-Torsion modes but suppresses X-torsional mode (blue curve)

Driving with 90 degree phase shift excites X-torsional mode while suppressing other modes (red curve)

By varying phase, the relative amplitude of the modes can be adjusted

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Conclusions Ultrasound excitation allows non-contact modal testing

of MEMS mirror, MEMS gyroscope and other devices

Selective excitation Insensitive to vibration of base or other parts of system Selectively excite modes by moving ultrasound focus point Phase-shifted pair of transducers allows transverse/torsional selectivity

May be especially useful for devices with nearly overlapping modes

Future possibilities: Other MEMS devices??? In-plane excitation

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Acknowledgements

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

Thank You