The Development of Affordable Constant Beamwidth Transduce~
1-3 Piwoelectric Composite
Urn C. Benja@
Using Injection Molded
AL. Van Burem and S. Petrie
Naval Undersea Warfare Center Division Newport, Code 216, Bldg. 1171A, 1176 Howell St., Newport, M 02841
Abstract: The desi~ fabrimtio% and dhation rdts for a new class of constant beamwidth &ansducm (CBT) is pre=ted.
This study extends pRviously reported work [J. Acowt SW. h. 73, 2200-2209(1983)] in fich a CBT was constructed using
hundreds of indiviM end-pped PZT ceramic cyhders. Mch cemrnic element assembly was wapped in corpme to provide
mechanical isolation from the massive meti housing md a pressure relm boundry condition. This presmtation describesa new
design approach that employs injection molded 1-3 piezocompsite which is curved to forma spherical mp. Of key importance
to a successful CBT design is the proper implementation of velocity amplitude shading. The current approach us= an area
shading techni~e which is achieved through selective deckoplating of COP.
~nODUC~ON
The constant kmwidth transducer (CBT) concept introdumd by Rogers and Van Buren (1) promises uniform
acoustic loading and a constant directivity pattern with low side-lobe levels above a certain cutoff frequency. This is
accomplished by controlkg the surface velocity of a radiating spherical cap with Legendre funtion shading. The
velocity shading a be implemented by dividing the spherical cap into seved discrete concentric bank each band
with its own prescribed volume velocity. Theoretid expressions appropriate for the CBT design are given in
references (1) and (2).
Figure 1 shows one of the electrde patterns used in this study to reake velocity shading h addition to their
spatial fidelity, the CBTS described here are intrinsically broadband devices due to the use of injection molded 1-3
piezocomposite (3). This new type of transduction material consists of an army of several thousand cemrnic pillars,
ach 1 mm (.04 inches) in diameter, emkdded in a viscoel~c polymer matrik thus providing a better acoustic
match to water than solid PZT wrmnic. The material is available in sheet form with panel sizes nominally 250 mm x
250 m x 6.3 mm (10 x 10 x .25 inches). Larger panels can k produced by using spke joints to connect tvvo or
more of the smaller pels.
Figure 2 shows one of the largest 1-3 piezocomposites used in this study. The
diameter of this piezoelectic panel was 762 mm (30 inches), and it was tsimrned from a 3 x 3 configuration of
smtier panels. Its ceramic rods were PZT-5H and comprised 25°A of the piezocomposite’s total volume
HG~
1. Plated elwtrode pattern used for velocity shading
HG~
1687
2, hge
piezocomposite panel prior to plating
TRANSDUCER DESIGN AND FADWCA~ON
The fabrication process begins by selectively electroplating copper on to the flat circdar 1-3 piezocomposite
substrate to form the two electrode surfaces. tie surface of the substrate is completely covered with copper and the
other is photoetched to obtain the shading bands. me plated disc can now & curved to the desired spherid cap
geometry. This is done by warming the active byer to approtimtely 65° C (150° ~ and slowly deforming the
cirdar disc between two aluminum spherical mold surfaces. ~er cookg back to room te~rature
the part is
again structura~y stable. However, care must& taken to properly support the curved part during subs~ent heat
cycles in the fabrication process. Wwes are attached to the curved compnent with conductive epo~, and it is
mountd in the cirtiar housing ring using 5-minute epo~. Using the concave M of the deformation tooling an
acoustic window is molded over the curved piezocomposite. The window matend selected has a room temperature
cure in order to avoid heat rehted distortion. The absorptive transducer backing matefi consists of lad loaded
rubber chunks interspersed within a syntactic foam-we matrix Due to the spherica~y curved geometry, the backing
is cast in place. Figure 3 shows a cross-sectioti view of the Mshd transducer.
.
~ASURED
~SULTS
me acoustic calibration of a smaller 330 mm (13 inch) CBT was made at the Naval Undersea Warfare Center’s
Acoustic Pressure Tank Facfity (prior to its relocation from Orhdo to Newport. Acoustic measurements
were made over a range of temperature and pressure combinations that simtite various ~
environments. h
general the transducer performance was stable with tem~rature and pressure. Meawements taken in both the
transmit and receive modes indicate the transducer’s performance is reciprocal Directivity patterns measured over
the broad fieWenq range from 40 to 230 kHz, Fig. 4, demowte
the constant bearnwidth nature of the transducer.
Reference (4) provides additio~l details on the performance of this prototype CBT.
1
HG~
3. Cross-sectiod
view of CBT construction
~G~
4. M_ed
patt~
40,50,60,
100,200, and 230 kHz
CONCLUSIONS
A new type of CBT has been designed , builg and tested. The device utihes a conformable 1-3 piezocomposite
material which has kn electropbted with copper to rdize a velocity amplitude distribution that approximates a
Ugendre function via area shading. With this arm shading approach, reciprocal constant beamwidth characterisdcs
were observed for a bandwidth greater than two octaves,
RE~RENCES
(1) RogersP.H., and VanBuren AL., J Acoust. Sot. h. 64, 3843(1978)
(2) Van Buren A.L., Luker L.D,, Jevnager MD. and Tires A.C., 1 Acmt. Sot. Am. 73, 22~-2209 (1983)
(3) Bowen L., and French K., ‘Fabrication of piezoelectric tidplymer
composites by injection molding; Pm.
Intmtiono[
S~potium
on Applic&ons
of Fmoelec~’cs
8* EEE
160-164 (1992)
(4) Benjamin K. C., Walden A.K.,rmd Van Buren A.L,, ‘~ign and development of a constant beamwidth transducer for sub’97, 1054-1059 (1997)
bottom acoustic Profiltig,” Proc. BEE OCW
1688