Transcript Non-contact Modal Testing of Hard

Noncontact Modal Excitation of Small Structures
Using Ultrasound Radiation Force
Society for Experimental Mechanics Annual Meeting
Springfield, MA
June 4, 2007
Thomas M. Huber, Scott D. Hagemeyer, Eric T. Ofstad
Physics Department, Gustavus Adolphus College
Mostafa Fatemi, Randy Kinnick, James Greenleaf
Ultrasound Research Laboratory, Mayo Clinic and Foundation
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 simple cantilever

Results for MEMS Gyroscope

Results for MEMS mirror
Conclusions
2
Ultrasound Stimulated Radiation Force Excitation

Vibro-Acoustography
Developed 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

We have also used the ultrasound radiation
force for modal testing of organ reeds and
hard drive suspensions (IMAC 2006)
3
Ultrasound Stimulated Amplitude Modulated Excitation

Dual sideband, carrier suppressed
amplitude modulated signal centered,
for example, at 550 kHz

Difference frequency of
Δf of 100 Hz to over 50 kHz

Difference frequency Δf 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

Transducer used in this experiment
had 1.5 mm diameter focus spot size
4
Photos of Setup
5
Selective Excitation using Phase-Shifted Pair of Transducers


To illustrate this technique, consider first a simple cantilever in air
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 cantilevers, MEMS mirror and hard drive suspensions
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 frequency Δf leads to excitation
from 100 Hz – 10 kHz

Modulation phase difference of 90 degrees
leads to 180 degree phase difference in
radiation force
7


Phase Shifted Selective Excitation
Use scanning vibrometer to measure deflection shape
Adjust amplitudes of two 40 kHz transducers to give roughly equal response
8


Phase Shifted Selective Excitation
Adjust amplitudes of two 40 kHz transducers to give roughly equal response
When both transducers turned on simultaneously with same modulation phase
 Enhanced Transverse Mode
 Suppressed Torsional Mode
9
Phase Shifted Selective Excitation


Driving in-phase excites transverse but suppresses torsional modes (dashed blue curve)
Driving out-of-phase (phase difference near 90 degrees) excites torsional while suppressing
transverse modes (red curve)
 This technique allows information about mode shape to determined even from a

single point vibrometer
Can differentiate two overlapping modes (if, for example, 2nd transverse and 1st
torsional mode were at nearly identical frequencies)
10
Case Study: MEMS Gyroscope

Analog Devices ADXRS
MEMS Gyroscope

Pair of Test Masses ¾ mm square
separated by ½ mm

Test masses have in-plane
resonance frequency of 14 kHz.

Question: What about out-of-plane motion
11
Ultrasound Excitation of MEMS Gyroscope

Scanning vibrometer detects motion
of test masses & nearby regions


Ultrasound transducer focused on
gyroscope.
Central frequency of 600 kHz, with
Δf = 13.5 kHz

Maximum velocity of 250 μm/s

Measured out-of-plane displacement
amplitude of 2.5 nm!
12
Ultrasound Excitation of MEMS Gyroscope
Ultrasound transducer
centered
Base Excitation with
Mechanical Shaker
Ultrasound transducer
Moved ½ mm right
Vibrates entire structure
Demonstrates capability
of this technique for noncontact selective
excitation without
exciting the base
Ultrasound transducer
Moved ½ mm left
13
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
 Y Axis torsion mode: 829 Hz
 Transverse mode (forward/back): 329 Hz
(incidental – not used for operation of mirror)
14
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
15
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
16
Phase-Shifted Selective Excitation of MEMS Mirror

Driving in-phase excites transverse and
Y-Torsion modes but suppresses Xtorsional 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
17
Conclusions

Ultrasound excitation allows non-contact modal testing
of MEMS mirror, MEMS gyroscope and other devices

Selective excitation
 Insensitive to vibration of base or other parts of system
 Selectively excite modes by moving ultrasound focus point
 Phase-shifted pair of transducers allows transverse/torsional selectivity

May be especially useful for devices with nearly overlapping modes

Future possibilities:
 Other MEMS devices???
 In-plane excitation
18
Acknowledgements
This material is based upon work supported by the National Science
Foundation under Grant No. 0509993
Thank You
19