Transcript Reed-Resonator Interactions in Reed Organ Pipes

Non-contact mode excitation of small
structures in air using ultrasound
radiation force
Acoustical Society of America Meeting: May 17, 2005
Thomas M. Huber, John Purdham
Physics Department, Gustavus Adolphus College
Mostafa Fatemi, Randy Kinnick, James Greenleaf
Ultrasound Research Laboratory, Mayo Clinic and Foundation
Introduction

Background material on ultrasound stimulated excitation

Study of different devices
 MEMS mirror
 Hard Drive HGA Suspension
 MEMS Gyroscope

Conclusions
Ultrasound Stimulated Vibrometry

Pair of ultrasound beams directed at
object

One ultrasound transducer differed
from other by audio-range frequency

Difference frequency between
ultrasound beams produces radiation
force that causes vibration of object

Vibrations were detected using a
Polytec laser Doppler vibrometer

In some experiments, comparison of
ultrasound excitation and mechanical
shaker
Experiment Details
Confocal ultrasound transducer used
 600 kHz broadband (>100 kHz
bandwidth)
 30 mm focal length; 1 mm focus spot
size
 Confocal (concentric elements with
different frequencies)
 Mounted on 3-D translation stage

Inner disk fixed at 500 kHz

Outer ring sweeps 501 – 520 kHz

Difference frequency of
1 kHz – 20 kHz
Caused excitation of object
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: 829 Hz
Transverse mode (forward/back): 329 Hz
(incidental – not used for operation of mirror)
Selective Ultrasound Excitation of MEMS Mirror


Ultrasound focus ellipse about 1x1.5 mm
Focus position can be moved horizontally or
vertically
 Changing transducer position
allows selective excitation

Upper figure: All modes present
when focus near center of mirror.



Red line shows excitation using
mechanical shaker.
Middle: X-torsional mode increases
when ultrasound focus near top of
mirror.
Bottom: Z-Torsional mode increases
when focus near right edge
Selective Ultrasound Excitation of MEMS Mirror


X-Torsional mode peaks when focus
near top/bottom of mirror
Transverse mode decreases as
transducer moved vertically
(smaller fraction of beam on mirror)
 Ratio of amplitudes of X-Torsional to
Transverse modes changes by over
factor of 10x as vertical position is
varied
Hard Drive HGA Suspension

HGA (Head Gimbal Assembly) suspension holds heads as they fly over the disk

Leading manufacturer: Hutchinson Technology, in Hutchinson, MN

Length about 5-10 mm, max. width about 2 mm, thickness of 25-100 μm

The suspensions are engineered to have specific vibrational modes.

Quality control involves measuring mode frequencies and deflection shapes
(using mechanical shaker for excitation and vibrometer for measurement)
Ultrasound excitation of HGA Suspension

Goal: To determine whether vibrational
modes of suspension can be excited
using ultrasound radiation force

HGA Suspension was clamped
and simply supported

Confocal ultrasound transducer used
to excite modes from 1 kHz to 50 kHz

Vibrometer measured resonance
frequencies and deflection shapes
at several ultrasound focus positions

Brüel & Kjær mechanical shaker
used for comparison
Photos of Setup
Results for focused ultrasound excitation: HGA Suspension


Interference between ultrasound frequencies between 501 – 520 kHz and 500 kHz
Ultrasound focus (ellipse of about 1mm by 1.5 mm)
centered on suspension (red curve) and towards edge of suspension (blue curve)
Demonstrates feasibility of noncontact excitation using ultrasound radiation
force.
Selective Excitation: For ultrasound focus towards the edge (blue curve), large
increase in amplitude of torsional modes at 6, 10, 13 and 15 kHz relative to
the transverse modes at 2, 7, and 16 kHz.
Mode shapes determined using ultrasound excitation
2.0 kHz
6.0 kHz
7.2 kHz
(Click on each image for animation of deflection shape)
10.8 kHz
Results for MEMS Gyroscope

Analog Devices
MEMS Gyroscope

Pair of Test Masses
¾ mm square
separated by 1.5 mm

14 kHz resonance
frequency
Variation of Ultrasound Transducer Position
Can Produce Selective Excitation of Masses
Mechanical Shaker
Shakes Entire Structure
Conclusions
Ultrasound excitation shown to be feasible for modal analysis

Allows excitation of resonances from below 1 kHz to over 40 kHz
 Parts such as MEMS mirror, gyroscope and HGA suspension
 Completely non-contact for both excitation and measurement

Selective excitation of torsional/transverse modes
 Selective excitation by moving ultrasound focus point
 Similar selective excitation using phase shift between two transducers

Special thanks to
 Hutchinson Technology
 Applied MEMS
 Analog Devices
 Polytec Incorporated