Selective modal excitation using phase-shifted ultrasound radiation force Acoustical Society of America Meeting June 2006

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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

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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 ?f 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 201 to smaller than 13.
• 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 (?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
• ?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 doesnt
  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