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 PurdhamPhysics Department, Gustavus Adolphus College

Mostafa Fatemi, Randy Kinnick, James GreenleafUltrasound 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 excitedusing ultrasound radiation force

HGA Suspension was clampedand simply supported

Confocal ultrasound transducer usedto excite modes from 1 kHz to 50 kHz

Vibrometer measured resonance frequencies and deflection shapesat several ultrasound focus positions

Brüel & Kjær mechanical shakerused 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 kHz6.0 kHz 7.2 kHz 10.8 kHz

(Click on each image for animation of deflection shape)

Results for MEMS Gyroscope Analog Devices

MEMS Gyroscope

Pair of Test Masses¾ mm squareseparated by 1.5 mm

14 kHz resonance frequency

Variation of Ultrasound Transducer Position Mechanical ShakerCan Produce Selective Excitation of Masses Shakes Entire Structure

ConclusionsUltrasound 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