A pulse-echo transducer system which uses an ultrasonic generating element and an optical detection technique is described. The transmitting transducer consists of a concentric ring electrode pattern deposited on a circular, X-cut quartz substrate with a circular hole in the center. The rings are independently pulsed with a sequence high voltage signals phased in such a way that the ultrasonic waves generated by the separate rings superimpose to produce a composite field which is focused at a controllable distance below the surface of the specimen. The amplitude of the field reflected from this focus position is determined by the local reflection coefficient of the medium at the effective focal point. By processing the signals received for a range of ultrasonic transducer array focal lengths, the system can be used to locate and size anomalies within solids and liquids. Applications in both nondestructive evaluation and biomedical scanning are suggested

Claus, R. O.

Wade, J. C.

English

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(NASA-CH-175919) ACOUSTOOPTIC PULSE-ECHO	 N85-28224
TRANSDUCER SYSTEM (Virginia Pclytechnic
Inst. and State Univ.) 21 p EC A02/dF A01
CSCL 09A	 U.nclas
G3/33 23760
ACOUSTOOPTIC PULSE-EC110
TRANSDUCER SYSTEM
-	 n
Richard 0. Claus	 r
Department of Electrical Engineering Cr. ^?	 ^•^
Virginia Polytechnic Institute and State University
	
Blacksburg, VA 24061 	 ^{
Janet C. Wade
Nondestructive Evaluation Center
Electric Power Research Institute
J. A. Jones Applied Research Company
P. 0. Box 217097
Charlotte, NC 28221
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Abstract
A pulse-echo transducer system which uses an ultrasonic generating
element and an optical detection technique is described. The trans-
mitting transducer consists of a concentrLe ring electrode pattern
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"	 deposited on a circular, a-cut quartz substrate with a circular hole
in the center.	 The rings are independently pulsed with a sequence
high-voltage signals phased in such a way that the ultrasonic waves
by the	 rings superimpose to	 agenerated	 separate	 produce	 composite
field which is focused at a controllable distance below the surfnce,of
the specimen.	 The amplitude of the field reflected from this focus
position is determined by the local reflection coefficient of the medium
at the effective focal point. 	 This reflected wave produces both normal
displacement
	
the surface of theand parallel components of particle 	 on
speeLmen at the location of the transmitting transducer and a sensitive,
wideband optical system not affected by low-frequency background vibra-
tion is used to measure the normal displacement through the center hole.
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1 By processing the signals received for a range of ultrasonic transducer
array focal lengths,	 the system can be used to locate and size nnomaltus
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within solids and liquids.	 Applications in both nondestructive evaluation
1.1
and biomedical scanning are suggested.
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UI. INTRODUCTION
In many ultrasonic NDE applications, it is importnnL to be able Lo
Internally scan a specimen with a well charneLerl •r.od InpuL acuntcLl1,.
field nod to detect the resulting output field with a sensitive and
calibrated receiving transducer. Conventional pulse-echo systems simplify
this process in one sense by taking advantage of the nearly reciprocal
properties of some transducer designs, but rarely can the properties of
a single transducer simultaneously optimize both input and output perform-
ance. In consequence, ultrasonic pitch-catch systems which utilize
different input and output devices having specific design advantages
can usually be employed for overall better system operation f1]. In
this paper, such a hybrid pitch-catch system is considered which uses
a concentric ring array acoustic input transducer and an interferometric
optical output transducer. The independent operation of each transducer
and the combined operation of the entire system is described.
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II.	 FOCUSING INPUT ULTRASONIC TRANSDUCER
Increased resolution in ultrasonic NDC scanning systems may be
li achieved by appropriately controlling the amplitude and phase profiles
of the generated acoustic field.	 For a piezoelectric transducer, such
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^ y a field may be passively shaped by altering 1) the geometry of the entire
transducer crystal, 2) the piezoelectric coefficient as a function of
position on the surface of the crystal, 3) the shapes and/or spacings
of individual electrodes which combined form the transducer, or by
4) external transmitting or reflecting elements to modify the
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field after it has been launched [2-4].	 Passive focusing may similarly
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be achieved using curved transducer surfaces, proper electrode shapes
and spacings, or external acoustic lenses [5 , 6].	 Alternatively, scanning
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via adaptive focusing can be implemented electronically by phasing the
r signals which are applied to individual elements in a transducer array
[7,8].	 Array phasing theory has been well developed in electromagnetics
f! for the case of steady state excitation, but these results must be
modified in acoustics to account for spatially localized disturbances
associated with pulses or pulse-gated continuous wave signals [9].
(
Using such a modified approach reported by Hildebrand [ 10], a focusing
piezoelectric input transducer having concentric ring electrodes was
Cdesigned [11].
The input transducer was fabricated from a circular x-cut, 2.25 MHz
diameter,	 1	 diameter holecrystal quartz substrate 1.81 cm in 	 with a	 mm
drilled normally through the center. 	 Concentric ring electrodes around
this hole were constructed on the upper flat surface of the transducer
- 107 -
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using photographic etching techniques. Electrode widths and spacings
were determined by considering the transducer diameter and frequency and
the desired focal length and gated pulse length and repetition rate.
Individual leads were attached to the electrodes as shown in Figure 1
and a thin layer of non-co;iducting epoxy around thu center hole used to
strengthen the lead-electrode bond.	 Coaxial cables were attached to
each lead and the entire transducer potted in a PVC case to provide
rId additional strain relief.	 The bottom wear plate surface of the trans-
ducer served as a ground plane.
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To achieve field focusing, fast-rising, high-voltage gated pulses
were applied to the correct sequence of electrodes at the proper times.
Ii This	 the	 in block diagrampulse control was achieved using 	 circuitry shown
form in Figure 2.	 Since the transducer was designed to include as many
rings as could be fit on the surface without electrical breakdown occur-
Fillx ring between adjacent rings, this circuitry was used to select the ring
electrodes necessary to achieve focusing at a particular distance, and
to initiate	 Thell the pulse sequencing to those rings.	 microprocessor
^ controls a two step process which involves electrode addressing and
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pulse timing.
	 Addresses of electrodes identified as necessary to
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contribute to the focused field at a particular focal distance are
first transferred to the tri-state buffer latches.	 Second, the internal
Hclock low level;provides appropriately phased leading edge signals at
these signals are then amplified and applied to the selected electrodes
in phase sequence.
Phased switching signals of 425 V amplitude and risetimes of less
than 10 ns produce individual shock-excited electrode fields similar. to
-108-
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- 109 -
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those shown in Figure 3. Superimposed electrode fields produce the
focusing field shown in cross-section in Figure 4. Minimum half power
beam radii of less than 0.65 mm were obtained at a focal length of
10 cm in water.
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-`	 III. OPTICAL DETECTION SYSTEM
Ej The transducer and associated pulsing electronics discussed above
are used to generate a selectably focusable acouSLlc field.	 if LIwL
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field is incident upon a bulk material anomaly, pa r t of the reflected
and scattered field will propagate back to the transducer and produce
a displacement of the free surface of the specimen at the center of the
transducer.	 The normal component of this surface displacement may bu
{ detected using the optical Michelson interferometer shown inFil;uro 5.
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Here, light from the monochromatic light source is split by the beam-
C splitter and focused on the surfaces of both the reference mirror and
the specimen.
	
'fhe reflected fields are then recombined by the beam-
splitter and focused on an optical detector after being spatially
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filtered by a Rc .nchi ruling.	 The normal component of displacement
at the center of the hole in the transducer alters the uprleal
	 paLh-
length of the light in the sample arm, causing a fringe shift in the
`-, detected output interference pattern.	 The detector output is propor-
tional to the amplitude of this displacement [12].
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The Michelson interferometer is inherently sensitive to random
environmental noise which can mask the small signals created by the
surface particle displacements. 	 To eliminate the low frequency noise
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components, the reference mirror may be mounted on a piezoelectric
translation stage rather than a stationary mount [13,141.	 This piezo-
electric transducer is then fed with an amplified low frequency signal
from the detector. 	 Additionally, this feedback correction circuit
fixes the gain of the interferometer and may therefore be adjusted to
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maximize the gain. Although the feedback technique has non-linear
stability limitations because the voltage compensation cannot be made
infinite, it enables improved system operation in acuusticully hostile
environments.
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IV. EXPERIMENT
The transducer system described above was used to detect a 3.18 mm
diameter stainless steel sphere suspended by crossed thread supports In
an 80 cm cubic water tank as shown in Figure G. The sphere was located
10 cm below the center of the bottom of the input ultrasonic transducer
which was positioned so it Just made contact with the surface of the
water in the tank. Under microprocessor control, the transducer varied
its focal length from 2.5 cm to 17.5 cm in discrete 1.0 em steps and the
maximum detected optical signal amplitude for each acoustic focus
location is shown in Figure 7. The optically detected signal corres-
ponding to an acoustic focal length of 10 cm is shown in Figure 8.
Agreement between measured and actual positions of the sphere is better
than two percent.
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- 112 -
V. SUMMARY
	
is
	 The generation of a focused acoustic field using a pulsed concentric
	
7
	 electrode array transducer has been examined. The focusing phenomena is
based on the geometric superposition of the pulsed fields from each of
the rings at the position and time where and when the fields interact.
The focused field may be used as a point probe to interrogate internal
media structure which enables the tomographic reconstruction of the
acoustic impedance field of the medium.
Internal media reconstruction is possible because the amplitude of
the reflected field is determined by the local reflection coefficient of
the medium. Variation in this reflection coefficient distribution due
to material anomalies causes a field amplitude variation which may be
used to infer local structure. The feedback stabilized optical receiving
11
	
transducer detects the normal component of surface displacement which
results when the reflected field reaches the surface. Tito system has
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applications in both biomedical scanning and the characterization of
solid material specimens.
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-113-
un	 VI. ACKNOWLEDGEMENTS
^y	 The authors thank P. S. Zerwekh, G. U. Dockery, and K. 11. Gcheltree
for assistance with the experiment. This work was partially supported
by NASA Crant NAG-1-263 and was performed when J. C. Wade was a student
.._._.__-_.__.-_, .- " .
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at the Virginia Polytechnic Institute and Stare University.
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Bibliography
[1] W. Sachae and N. N. Hsu, "Ultrasonic transducers fur materials
testing," Physical Acoustics, Vol. 14, W. P. Mason and R. N.
Thurston, ads. New York: Academic Press, 1979.
[2] A. D. Vopilkin, et. al., "Wide-band ultrasonic trnnsducer and its
uses," United States Patent #3968680, July 1976.
[3] F. D. Martin and M. A. Breazeale, "A simple way to eliminate
diffraction lobes emitted by ultrasonic transducers," J. Acoust.
Soc. Am. 49, 1668 (1971).
a
[4] P. S. 2erwekh and R. 0. Claus, "Ultrasonic transducer with Gaussian
radial velocity distribution," Proc. IEEE Ultrasonics Symposium 2,
974 (1981).
[5] G. W. Willard, "Focusing ultrasonic radiators," J. Acoust, Soc.
E^i Am.	 21,	 360	 (1949).
[6] H. T. O'Neil,
	 "Theory of focusing radiators," J. Acoust. Soc. Am.
21,	 516	 (1949).
[7] A. Freedman, "Transient fields of acoustic radiators," J. Acoust.
Il
Soc. Am.
	
48,	 195	 (1970).
[8] P. R. Stephanishen, "Impulse response and radiation impedance of
Fill
an annular piston," J. Acoust. Soc. Am. 56, 305 (1974).
[9] Y. A. Kondrat'ev, "Pulsed operation of a focusing piezoelectric
transducer," Soviet J. of Nondestructive Testing 14, 688 (1978).
[10] B. P. Hildebrand, "An analysis of pulsed ultrasonic arrays," in
Acoustical Holography, Vol. 8, A. F. Metherell, ad., New fork:
p,
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Plenum Press (1980).
[11] J. C. Wade and R. 0. Claus, "Interferometric pulse-echo ultrasonic
transducer system," J. Acoust. Soc. Am. S1, 71, S109 (1982).
n
[12] C. H. Palmer, "Ultrasonic surface wave detection by optical
interferometry," J. Acoust. Soc. Am. 53, 948 (1978).
[13] C. H. Palmer and R. E. Green, "Optical detection of acoustic
emission waves," Appl. Opt. 16, 2333 (1977).
[14] R. A. Kline, R. E. Green, and C. H. Palmer, "A comparison of.
optically and piezoelectrically sensed acoustic emission signals,"
J. Acoust. Soc. Am. 64, 1633 (1978).
Iii
	
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- 115 -
Pigure Titles
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Figure 1. Piezoelectric ring array transducer eleetroding and construction.
Figure 2. Adaptive transducer focusing electronics.
Figure 3. Acoustically detected response ( top) of one transducer ring
due to computer—timed electrical pulse (bottom).
Figure G. Acoustically measured crossection of focused transducer field.
Half power points are plotted.
Figure 5. Optical detection of reflected acoustic field.
Figure 6. Experimental system for detection of 3.18 min diameter stainless
steel sphere.
Figure 7. Optically detected surface displacement caused by acoustic wave
reflected from stainless steel sphere in water.
Figure 8. Time domain signal produced by optical detector for system
focal length of 10 cm.
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TRANSDUCER
MATERIAL
LEADS
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Acoustooptic pulse-echo transducer system

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