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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to electroacoustic transducer and, in
particular, to transducers of the type utilizing piezoelectric bender
members as active transducer elements.
Bender-type piezoelectric or piezoceramic transducers are rather well known
and have been used to advantage in a variety of forms. One such type is
provided by the use of a circular, piezoelectric disc that is securely
bonded to a circular metal plate so as to vibrate in an umbrella-like mode
when mechanically or electrically energized. Such discs, however, have
limited power capabilities. Also, because they are disc-shaped, a maximum
packing density is not easily achieved. Compared with square or
rectangular configurations having sides equal to the disc diameter,
considerably less exposed surface area is formed of piezoelectrically
active material.
Another type of bender transducer utilizes a square or rectangular
piezoelectrical bender in the form of a single, solid plate. Such a
geometry clearly is more favorable, but the use of the single plate still
results in the umbrella-like vibratory motion of the disc configuration.
Since the piezoelectric coupling is in all directions, all edges move and,
to produce the umbrella motion, they require support at their corners. The
need for this support presents significant constructional difficulties and
further reduces the strength of the transducer element when subjected to
pressure loads across its surface.
The present invention has as one of its objects the provision of a bender
transducer which is capable of utilizing the square or rectangular bender
configuration to facilitate construction and provide greater structural
strength.
Another important object is to provide a transducer element structure that
results in relatively higher electroacoustical efficiencies due,
particularly, to the nature of its piezoelectric coupling.
In a manner to be described, the objects of the invention are achieved by
using elongate, thin and narrow, piezoelectric bars or strips. The strips
extend lengthwise of a flexible support plate to which they are securely
bonded in a tightly-compacted side-by-side arrangement. The plate itself
is rigidly engaged and supported at its longitudinal ends. Consequently,
forces tending to cause the piezoelectric bars to lengthen or shorten
result in a bending or flexing of the bars and, of course, the
end-restricted plate to which the bars are bonded. To energize the bars
and transduce electroacoustical signals, a circuit is coupled across each
bar so that, in effect, the bars become a plurality of parallel
capacitors. In a preferred form, the strips or bars are bonded to both
sides of the plate although a single sided arrangement is operable. To
improve flexibility, the plate itself can be hinged in a manner which will
be described.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated in the accompanying drawings of which:
FIG. 1 is a somewhat schematic side view of one form of the transducing
element of the present bender bar transducer;
FIG. 2 is a face view of the transducer element of FIG. 1;
FIG. 3 is an end view of the element showing in particular the manner in
which the bender bars are electrically connected to input or output
terminals;
FIG. 4 is a face or plan view of a wire screen device used for the
electrical coupling;
FIG. 5 is a view in perspective illustrating a modified form of the bender
bar construction of the transducing element;
FIG. 6 is a schematic view showing in section one manner in which the
transducing element can be employed in a transducer assembly;
FIGS. 7 and 8 are face and end views of a bender block arrangement
employing four individual transducing elements, and
FIG. 9 is a planar bender arrangement including the four element blocks of
FIGS. 7 and 8 as well as other two element blocks.
DETAILED DESCRIPTION OF THE INVENTION
The present description primarily will be with reference to a particular
transducing element or, in other words, to the element which provides the
active or piezoelectrically-responsive component of the transducer
assembly in which it is mounted. This element is identified generally by
numeral 1, FIGS. 1-5. FIG. 6, in turn, illustrates the manner in which
element 1 is incorporated into a transducer assembly. Although this
particular assembly is clearly advantageous, other arrangments, of course,
are contemplated.
Transducer element 1 generally includes a plurality of thin, narrow
piezoelectric or piezoceramic strips or bars 2 bonded to a flexible metal
support plate 3 formed, for example, of aluminum, brass or steel. At its
end the plate is engaged by rigid walls 4 and 6. The wall, in addition to
providing support, also functions to rigidly maintain the fixed length of
the plate or, in other words, to prevent the plate from expanding or
stretching lengthwise. Instead, tendencies to stretch are translated into
a bending mode as indicated by the dotted lines in FIG. 1. To facilitate
the flexing or bending motion, the end portions of the plate are reduced
in thickness to form, in effect, hinges 7 and 8.
The arrangement of bars 2 on the support is shown in FIG. 2. As noted, they
are disposed in side-by-side disposition extending substantially the width
of the plate. Most suitably, the bars are square or rectangular in cross
section to provide flat surfaces for providing a tightly-compacted, side
wall engagement. In the dotted line bending mode shown in FIG. 1, the bars
and the plate on which they are mounted bend or flex as a unit. For this
purpose, bars 2 may be securely bonded to the plate by an appropriate
epoxy layer 10 or by other suitable means, and for electrical purposes,
the epoxy may be a conductive material. However, as will be described, the
requisite conductivity can be provided in other manners. It also is to be
noted that the bar arrangements can be provided either on one or both
sides of the plate. A double-side bender is shown in FIG. 1 while FIG. 5
shows a single-side arrangement. Functionally, a double-side arrangement
increases the binding or flexing action although the single side is fully
capable of operating as a transducer element.
FIG 3. illustrates the electrical connections for transducer element 1 or,
more specifically, for piezoelectric bars 2. As shown, the circuit
utilizes a single wire 9 for coupling each bar to input and output
terminals 11 and 12. Input terminal 11 is coupled to the outer surface of
each of the bars and terminal 12 is coupled to the inner or plate-engaging
surface of the bars, a conductive wire screen member 13 can, if desired,
be sandwiched in the epoxy bond between the bars and the plate. Such a
screen avoids the need for a conductive bonding material and is preferred
in most arrangments. Another coupling technique is simply to roughen the
surface of the central support plate 3. If the piezoelectric bars then are
clamped sufficiently during assembly, the roughened surface will contact
the bars at various points intermixed with glue contact.
Wire 9 is soldered in the usual manner to each of the bars so that, in
effect, electrodes are provided across the bar thickness and the bars thus
become a series of parallel capacitors. With such an electrical
connection, the polarization of the ceramic or piezoelectric material is
as shown in FIG. 3. More particularly, the connection is such that the
polarization of the bars on opposite sides of plate 3 is mutually
opposite. Another feature apparent in FIG. 3 is the fact that the bar
arrangment on one side of the plate is staggered relative to the bar
arrangement on the other. In other words, the bars are alternately spaced
from top to bottom for the purpose for suppressing the vibration modes of
the plate with nodal lines parallel to the -X- axis i.e. lateral modes of
vibration are suppressed.
The use of the thin, elongate strips of piezoelectric material is to permit
the utilization of the g.sub.31.sup.l coupling and to suppress the
g.sub.31.sup.w coupling. Although obviously both couplings are present,
g.sub.31.sup.w is not dominant for l >> w and l >> t. When such bars are
separated from the plate or, in other words, when their ends are not
fixedly supported, they function as thin, length-extenders. Obviously, the
length extension also is maintained when they are coupled or bonded to
plate 3. As a result, when an AC signal is applied to the bars, the
resulting movement is as shown in the dotted line bending mode of FIG. 1.
More specifically, the length extension property of the bar results in a
thickness descrease or increase (Poisson's ratio) tending to pull the
plate apart in the X direction (FIG. 5). Since the plate is supported at
its ends, it will bend parallel to the X axis in the Z direction. The
application of the AC signal to the top bars (FIG. 3) tends, for example,
to increase the length and decrease the thickness while, with the
application of the same AC signal, the connection to the bottom bar
produces a shortening which increases the desired bending action. The same
bending motion occurs with the single-side FIG. 5 arrangement although it
may not be quite as pronouced. In other words, the use of piezoelectric
bars on both sides of the plate is not essential to operation since a
single-side arrangement also functions as a transducer. In the illustrated
forms of the transducing element it will be noted that the fixed support
for plate 3 is provided only at its longitudinal ends.
FIG. 6 demonstrates the bender plate assembled as a transducer element. As
shown, the transducer has a bender element body member 14 formed at its
upper end into a cavity 16 by upwardly-extending walls 17 and 18.
Transducing element 1, in turn, is mounted at the upper or outer end of
cavity 16 and walls 17 and 18 provide the desired rigid support for plate
3. Also, the cavity preferably is a complaint member filled with a
suitable fluid which, as shown, is in communication with an expansion,
fluid-filled cavity 19. A rubber diaphragm 21 separates cavity 19 from a
fluid reservoir 22 which may simply be the ambient pressure of the fluid
media. In some situations, it may be desirable to eliminate the rubber
diaphragm and have the fluid common to one large reservoir. Functionally,
compliant cavity 16 provides a termination for the backside of the bender
as well as a means of stopping the radiation from this face and hence
suppressing a dipole response pattern. It also is desirable to seal the
moving edges of the support plate with an appropriate elastic adhesive to
operate as a monopole source. The adhesive may be a room temperature
vulcanizing compound selected for its compatability with the fluid used
for pressure relief in cavity 16. In some situations a dipole response may
be desirable in which case no compliant cavity becomes a valid transducer
configuration. In general, the sound radiated from the front face is that
due to a square piston with some average velocity equivalent to the
average velocity of a square or rectangular member moving in its first
mode. Expansion cavity 19 also be included for temperature pressure
effects on the field.
FIGS. 7, 8 and 9 illustrate some of the advantages achievable by the use of
the present square or rectangular bender element. Thus, as seen best in
FIG. 7, the transducing element itself may be formed as a four element
square bender with the bender elements 1 disposed side-by-side to form
block 23. In this configuration, the boundary or end support of the plate
of each of the elements is provided by appropriate walls of the transducer
assembly body portion. Such a square or rectangular shape also lends
itself to the fabrication of the group arrays which, as shown in FIG. 9,
may include such configurations as two or three elements in line or four
in a square. The resulting array composed of such groups of modules
advantageously consolidates inner connections and makes pressure relief,
such as that provided by the fluid reservoir, easier to construct and
utilize. In particular, the square or rectangular configuration results in
a higher power handling ability due to an increase in the active material
volume. In comparison to a circular configuration, it utilized more of the
surface area of the array for active piezoelectric material. The motion in
such an array is more like that of a square or rectangular piston and is
closer to the radiation characteristics of rigid pistons used in current
Tonpilz transducer elements than that of a circular umbrella-type bender
disc. The rectangular configuration also should result in higher
efficiencies due to the piezo coupling which is involved. In other words,
the electrical to acoustical or vice versa conversion efficiency is
improved.
The operation of the transducer as well as its particular advantages should
be readily apparent from the foregoing description. Obviously many
modifications and variations of the present invention are possible in the
light of the above teachings. It is therefore to be understood that within
the scope of the appended claims the invention may be practiced otherwise
than as specifically described.
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