Thomas M. Huber Physics Department, Gustavus Adolphus College

1. Selective modal excitation using phase-shifted ultrasound radiation forceAcoustical Society of America MeetingJune 2006 Thomas M. Huber Physics Department, Gustavus Adolphus College Mostafa Fatemi, Randy Kinnick, James Greenleaf Ultrasound Research Laboratory, Mayo Clinic and Foundation

2. 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

3. 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 Ultrasound Stimulated Radiation Force Excitation

4. 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

5. 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

6. 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 frequencyDf leads to excitation from 100 Hz – 10 kHz • Modulation phase difference of 90 degrees leads to 180 degree phase difference in radiation force

7. Photos of phase-shift excitation of hard-drive suspension

8. 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

9. 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)

10. 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

11. 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

12. 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)

13. 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

14. Partial cancellation occurs even with non-symmetric geometry • Oblique geometry: one transducers not aimed directly at mirror (sidelobe only) • Transducers 8 cm and 13 cm from 3mm square mirror (λ=0.88 mm at 40 kHz)

15. 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

16. 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