Selective modal excitation using phase-shifted ultrasound radiation

force

Acoustical Society of America MeetingJune 2006

Thomas M. HuberPhysics 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 hard-drive suspension Results for simple cantilever 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

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

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Ultrasound Stimulated Amplitude Modulated Excitation

Dual sideband, carrier suppressed amplitude modulated signal centered, at 40 kHz

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

Vibrations detected using a Polytec laser Doppler vibrometer

Completely non-contact modal testing for both excitation and detection

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Selective Excitation using Phase-Shifted Pair of Transducers

Current Experiment: 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 hard-drive suspensions, MEMS mirror and

cantilevers

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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 Df 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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Photos of phase-shift excitation of hard-drive suspension

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Phase-shifted selective excitation

Adjust amplitudes of two ultrasound 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

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Phase-Shifted Selective Excitation of Suspension

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

Driving out-of-phase (phase difference near 90 degrees) excites torsional while suppressing transverse mode (red curve)

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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

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Phase-Shifted Selective Excitation of Simple Cantilever Clamped-Free Brass Cantilever: 3 cm by 0.8 cm

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

Driving out-of-phase excites torsional mode, suppresses transverse modes (Dashed red curve)

Ratio of Fundamental divided by 1St Torsional mode amplitudes varies by over two orders of magnitude as modulation phase is shifted by 90 degrees

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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 Z Axis torsion mode: 827 Hz Transverse mode (forward/back): 330 Hz

(incidental – not used for operation of mirror)

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Phase-Shifted Selective Excitation of MEMS Mirror

Driving in-phase excites transverse and Z-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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Partial cancellation occurs even with non-symmetric geometry Transducers 8 cm and 13 cm from

3mm square mirror (λ=0.88 mm at 40 kHz)

Oblique geometry: one transducers not aimed directly at mirror (sidelobe only)

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Conclusions Ultrasound excitation allows non-contact modal testing

Using pair of phase-shifted transducers allows selective excitation of torsional versus transverse modes Works for variety of devices Dimensions of objects can be smaller than ultrasound wavelength

λ=0.88 mm at 40 kHz Suspension pad 2 mm square, MEMS Mirror 3 mm square

Partial cancellation can occur even for non-uniform geometries or non-matched transducers May be especially useful for devices with nearly overlapping modes

Future areas of research Better understanding of radiation distribution from diverging transducers Understanding why maximum cancellation doesn’t always occur at 0 degrees and 90 degrees Under development: 600 kHz transducer pair with high bandwidth and 2 mm focus diameter

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Acknowledgements

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)

Thank You