Excitation of Vibrational Eigenstates of Coupled Microcantilevers Using Ultrasound Radiation Force ASME 2 nd International Conference on Micro and Nanosystems Brooklyn, NY August 6, 2008 Thomas M. Huber, Brad Abell, Sam Barthell, Dan Mellema, Eric Ofstad Physics Department, Gustavus Adolphus College Arvind Raman, Matthew Spletzer Department of Mechanical Engineering, Purdue University
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Excitation of Vibrational Eigenstates of Coupled Microcantilevers Using Ultrasound Radiation Force ASME 2 nd International Conference on Micro and Nanosystems.
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Excitation of Vibrational Eigenstates of Coupled Microcantilevers Using Ultrasound Radiation
Force
ASME 2nd International Conference on Micro and Nanosystems
Brooklyn, NY August 6, 2008Thomas M. Huber, Brad Abell, Sam Barthell, Dan
Mellema, Eric Ofstad
Physics Department, Gustavus Adolphus College
Arvind Raman, Matthew SpletzerDepartment of Mechanical Engineering, Purdue University
Introduction Ultrasound Radiation Force Excitation
Excitation of microcantilevers using ultrasound radiation force Resonance frequency and mode shapes Higher order modes
Selective excitation by phase shift
Conclusions
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.
Technique has been used for imaging in water and tissue, andmode excitation of objects in air
Organ Reed Hard Drive MEMS Coupled AFM Suspension Gyroscope Microcantilevers Microcantilever
12 mm x 5 mm 10 mm x 2 mm 3mm x 0.8mm 0.5 mm x 0.1 0.3 mm x 0.02 mm
100 Hz – 10 kHz Up to 30 kHz 18 kHz Up to 80 kHz Up to 200 kHz
Organ Reed
12 mm x 5 mm
100 Hz – 10 kHz
Modal Excitation Using Ultrasound Radiation Force Originally demonstrated in 2004 for Pipe Organ Reeds
Have since used for ever smaller devices and higher frequencies
The same ultrasound transducer has been used to excite from 100 Hz up to 200 kHz!
Acoustic Radiation Force Excitation Consider two sound waves impinging on an object
This radiation force will have component at the difference frequency Δf
FDifference = F0 sin [2π Δf t + (φ2 - φ1) ] Δf =f2 - f1
Ultrasound Radiation Force Excitation
Suppressed carrier AM signal
Centered at, for example, 450 kHz
Radiation Force Excitation: Advantages
Non-Contact Does not have driver resonances and does not excite fixture modes Wide Bandwidth
Using our 500 kHz transducer, can excite structures with resonances from 100 Hz to over 200 kHz Focused
The transducer used has focal spot of about 2 mm diameter Capability for selective excitation using multiple transducers
Generation of Excitation Signal
Can also generate a chirp waveform For example, fMod=4.5 kHz to 5.5 kHz in 0.6 seconds
Leads to excitation frequency chirp from 9 kHz to 11 kHz
Radiation Force Excitation: Experimental Setup
Microcantilever Pair using Ultrasound Radiation Force Gold Microcantilevers (500 micron by 100 micron, 250 micron separation) Ultrasound 450 kHz central frequency
Modulation chirp frequency of 4950 Hz to 5150 Hz Difference frequency of 9900 Hz to 10300 Hz
Measure motion using laser Doppler vibrometer Comparison with scanning probe microsystem (Blue Triangles)
Microcantilever Pair using Ultrasound Radiation Force
Measure amplitude & phase at multiple points to determine operating deflection shapes
2nd Transverse Modes of Au pair (about 60 kHz)
First Torsional Mode of Au Pair (about 87 kHz)
Excitation of AFM Cantilever Tipless Silicon AFM Microcantilever (300 micron by 20 micron) Ultrasound 450 kHz central frequency
Modulation chirp frequency of 4500 Hz to 6750 Hz Difference frequency of 9000 Hz to 13500 Hz
Smallest structure excited using ultrasound radiation force in air
Excitation using Ultrasound Radiation Force Silicon AFM Cantilever (300 micron by 20 micron) Vibrometer response using Piezo base excitation (Cyan Triangles) Nearly identical frequency response obtained using Ultrasound Excitation
Excitation using Ultrasound Radiation Force Silicon AFM Cantilever (300 micron by 20 micron) Repeat for 2nd bending mode (72 kHz) Ultrasound data taken at single frequencies using lock-in amplifier
Excitation using Ultrasound Radiation Force Repeat for 3rd bending mode (204 kHz) Highest frequency excited using ultrasound radiation force in air Note: Additional peaks in base excitation spectra due to fixture/piezo
resonances
Selective Excitation using Phase-Shifted Pair of Transducers
Instead of using a single transducer, use a pair of ultrasound transducers to allow selective excitation If radiation force from both transducers are in phase, selectively
excites symmetric mode while suppressing antisymmetric mode If radiation force is out of phase, selectively excites antisymmetric
mode while suppressing symmetric mode Previously demonstrated for selectively exciting transverse and
torsional modes of cantilevers, and hard drive suspensions
Phase Shifted Selective Excitation Adjust amplitudes of two 40 kHz transducers to give roughly equal response
Phase Shifted Selective Excitation Adjust amplitudes of two 40 kHz transducers to give roughly equal response When they are driven together in phase, strong enhancement of the
symmetric peak, while some cancellation of the antisymmetric peak
Phase Shifted Selective Excitation Adjust amplitudes of two 40 kHz transducers to give roughly equal response When they are driven out of phase, strong suppression of the the symmetric
peak, while some enhancement of the antisymmetric peak
Phase Shifted Selective Excitation Driving in-phase excites symmetric but suppresses antisymmetric mode Driving out-of-phase excites antisymmetric while suppressing symmetric mode
Can differentiate two overlapping modes. This capability may be very valuable for coupled cantilevers. High mass sensitivity requires weak coupling, but this implies that the symmetric
and antisymmetric would nearly overlap By using ultrasound excitation, the symmetric mode can be highly
suppressed
Conclusions Ultrasound excitation allows non-contact excitation of microcantilever
Excitation demonstrated up to 200 kHz
Selective excitation of symmetric versus antisymmetric modes Using phase-shifted pair of transducers Allows overlapping modes to be individually excited May increase sensitivity of mass sensing
Future possibilities: Other MEMS devices New transducers should allow about 300 kHz or more of bandwidth Excitation of microcantilevers in water In-plane excitation
Acknowledgements
This material is based upon work supported by the National Science Foundation under Grant No. 0509993
Thank You
Brad Abell, Dan Mellema, Physics Department, Gustavus Adolphus College
Mostafa Fatemi and James GreenleafUltrasound Research Laboratory, Mayo Clinic and Foundation