An Experimental Investigation of the Nearfield Pressures of Ultrasonic Transducers and Arrays

131 Pages·2016·4.33 MB·English

by  George Anthony Fisher

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Open Access Master's Theses
1978
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George Anthony Fisher
University of Rhode Island
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Fisher, George Anthony, "An Experimental Investigation of the Nearfield Pressures of Ultrasonic
Transducers and Arrays" (1978). Open Access Master's Theses. Paper 878.
https://digitalcommons.uri.edu/theses/878
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AN EXPERIMENTAL INVESTIGATION OF THE NEARFIEI.D
PRESSURES OF ULTRASONIC TRANSDUCERS
AND ARRAYS
BY
GEORGE ANTHONY FISHER
1 THESIS IN PARTIAL OF ?HE
SOB~ITTED FOLFII.L~ENT
REQUIREaENTS OF THE DEGREE OF
OF SCIENCE
!ASTE~
IN
OCEAN ENGINEERING
UNIVERSITY OF aHODE ISLAND
1978
OF SCIENCE THESIS
~ASTER
OF
GEORGE ANTHONY FISHER
Approved:
Thesis
Committee 7?'~ ~tfh1
.Major P-rof P.ssor--/ ---------------------
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Dean of the Graduate School
UNIVERSITY OF RHODE ISLAND
1978
ABSTRACT
This thesis describes the procedures and resul.ts of
research conducted on pulsed ultrasonic piezoceramic
transducers and arrays which operate in the megahertz
range. These transducers and arrays are typical of the
types employed currently i.n biomedical diagnostic systems.
Two areas vere addressed: The verification of the Impulse
Response Approach as a method to predict the spatial and
temporal characteristics of the acoustic field pressures of
ultrasonic transducers of general shapes and excitations,
and the investigation of the element interaction and
spatial resolution of field pressures of arrays of
piezoceramic elements.
An automated computer controlled high frequency
acoastic measuring system was constructed. Several high
frequency transducers were buiit to obtain acoustic
nearfield data with this system. The data were
subsequently processed using computer programs which were
created to obtain impulse responses via the deconvolution
of the far field on axis and nearfield time dependent
pressures. The acoustic fields of an array driven under
focussed and unfocussed conditions were also extensively
mapped.
The experimentally derived impulse responses were in
good agreement with the theoretical responses and thus
provided important new verification of the Impulse Response
Approach. The results of the experiment also indicated
that extraneous modes of vibration of piezoceramic discs
i~t
were, in this case, insignificant. A multi-element
transducer array was constructed and operated in such a
manner as to focus the acoustic energy, confining it to a
narrow as opposed to a continuous beam. The element
regi~n
to element interaction effects caused by electrical and
acoustic coupling were found to be of significant
importance in the single driven element case. The effects
of interaction were not great enough to cause a
perturbation from the expected field pressure pattern vhen
the array was driven for focussing.
iv
Grateful. Acknowledgment for the Support of this work
is made to:
THE NATIONAL INSTITUTE OF HEAL'lli
and
THE UNIVERSITY OF RHODE ISLAND
ACADEMIC COMPUTER CENTER
v.
TABLE of CONTENTS
I. INTRODUCTION l
II. SYSTE! 6·
!EASURE~ENT
l/D Converter 9
Positioning system 19
Analog Signal Source 21
Receive Amplifiers and Probes 22
III. THEORY of the RESPONSE APPROACH 27
I~PULSE
IV. MATHEMATICAL TECHNIQUES for DECONVOLUTION 40
Evaluation Criteria 43
Specific Techniques 45
Description of Implemented Method 53
v. .
EXPERiaENTAL PROCEDURES and RESULTS
?OR SINGLE TRANSDUCERS 64
Transducer Construction 64
!easurements and Results 67
VI. PROCEDURES and RESULTS for ARRAYS 87
EIPERI~ENTAL
Introduction 87
Array construction 89
!easurement Procedures 90
and Results 92
~easurements
VII. CONCLUSIONS 108
LIST of REFERENCES 111
lPPENDII A - COMPUTER PROGRAM LISTINGS 113
vi.
LIST of FIGURES
TITLE page no:
a
Block Diagram of !easurement System
Positioning carriage 10
1/D Converter: Timing Diagram 12
1/D Converter: Block Diagram 16
Geometry of General Integral Equation 28
Geometry of Impulse Response Function 32
Geometry of Coordinate Transformation 33
Theoretical Impulse Response: On Axis 36
Theoretical Impulse Response: Inside Disc 37
Theoretical Impulse Response: outside Disc 38
Theoretical Impulse Response: Par Field Outside Dis= 39
Box !odel of Typical. !easurement System 42
Test Result of Frequency Division Technique 49
Test Result of Time Domain Approach 52
Test Result of Iterative Technique: No Dynamics 59
Test .Result of Iterative Technique: With Dynamics 61
Convolution Check of Iterative Technique 63
Experimental Impulse Response: Insufficient Range 69
Experimental Impulse Response: Steel and Poam Baffla 7l
Experimental Impu.lse Responses: lead Metaniobate on
Aluminum Bar, outside Disc 74
Lead !etaniobate on Tungsten Backing, Outside Disc 75
Lithium Sulphate on Tungsten Backing, outside Disc 76
vii.
Reconvolution Check, Outside Disc 77
Lead !etaniobate on Tungsten Backing, Inside Disc 78
Lithium Su1phate on Tungsten Backing, Inside Disc 79
Reconvolution Check, Inside Disc 80
Lead aetan1obate on A1uminum Bar, On Axis 81
Lead !etanioba te on Tungsten Backing, On Axis 82
Lithium Su1phate on Tungsten Backing, On Axis 83
\
Reconvolution Check, on Axis 84
Circuit Diagram of Digita1 Delay Pulser 91
Field·of Array: One E1ement, Others Open 94
Field of lrray: One Element, Others Open 95
Fie1d of Array: End Element, Others Clamped 97
Fie1d of Array: 111 Elements 98
Pressure Time History: Not Focussed 100
Pressure Time History: Focal Point 101
Geometry for Delay Calculation 102
Field of Array: Focussed at R=O Z=20 104
Field of Array: Focussed at R=10 Z=30 106
viii.
1.
I. INTRODOCTION
This thesis describes procedures and results of
research carried out at the Oniversity of Rhode Island,
under a grant from the National Institute of Health, GM
23104. The objectives of this study are: The
investigation of the nearfield characteristics of certain
ultrasonic transducers and arrays which operate in the
megahertz range, and the . verification of the impulse
response approach to evaluate the spatial and temporal
characteristics of the acoustic field which results from
the pulsed wide bandwidth excitation of ultrasonic
transducers and arrays. Ultimately the implementation of
this knowledge will be used to design and construct an
ultrasonic biomedical diagnostic system which operates in
real time, with a higher order of resolution than
previously attained • .
Reports generated by the National Science Foundation
have su.mmarized the present state of the art in the use of
aedical ultrasonic devices for diagnostic purposes. These
reports have also made explicit recommendations concerning
the type of research which is required in order to advance
this area. ( See references (1) through (5) ], In
particular, the need for research on evaluating the