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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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