Class V flextensional transducer with directional beam patterns

Details Class V flextensional transducer with directional beam patterns Class V flextensional transducer with directional beam patterns

2001-08-29

2003-09-02

Budd, Mark (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S317000, C310S328000

Reexamination Certificate

active

06614143

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electro active devices, and in particular, to a directional flextensional transducer.
2. Description of the Prior Art
Electro active devices in the form of flextensional transducers were first developed in the 1920s and have been found to be particularly useful for underwater acoustic detection and transmission since the 1950s. They typically comprise an active piezoelectric or magnetostrictive drive element coupled to a mechanical shell structure. The shell is used as a mechanical transformer which transforms the high impedance, small extensional motion of the ceramic into a low-impedance, large flexural motion of the shell. The term “flextensional” is derived from the concept of the extensional and contractional vibration of the drive element causing a flexural vibration of the shell. Flextensional transducers have been divided into seven classes according to the shape of the shell and the configuration of the drive elements. For example, a Class I transducer has a shell similar to an American football in shape. The drive motor is typically a stack of drive elements oriented along the major axis of the shell. A Class II transducer is essentially a modified Class I shape having extensions along the major axis. A Class V transducer, applicable to this application, typically includes a radially vibrating ring or disk as a drive element, as opposed to a linear stack of drive elements oriented along a major axis of the shell. The radially vibrating ring or disk is usually sandwiched between two spherical cap shells.
Flextensional transducers may range in size from several centimeters to several meters in length and can weigh up to hundreds of kilograms. They are commonly used in the frequency range of 300 to 3000 Hz. Such transducers can operate at high hydrostatic pressures, and have wide bandwidths with high power output.
Two electro active devices, versions of the Class V flextensional transducer, called the “moonie” and the “Cymbal™” have been developed at the Materials Research Laboratory at the Pennsylvania State University (Cymbal™ is a trademark of the Pennsylvania State University). The moonie and Cymbal™ can be constructed using bonding and fabrication processes that are very simple, therefore, they can be inexpensive and easy to mass-produce.
An example of a moonie transducer is described in U.S. Pat. No. 4,999,819. The moonie acoustic transducer utilizes a sandwich construction and is particularly useful for the transformation of hydrostatic pressures to electrical signals.
U.S. Pat. No. 5,276,657 describes a moonie ceramic actuator similar to that shown in
FIG. 1. A
piezoelectric or electrostrictive element
100
is sandwiched between a pair of endcaps
105
,
110
, with each endcap having a cavity
115
,
120
formed adjacent to the piezoelectric element
100
. The endcaps
105
,
110
are bonded to the piezoelectric element
100
to provide a unitary structure. Conductive electrodes
125
and
130
are bonded to the piezoelectric element's major surfaces. When a potential is applied between electrodes
125
and
130
, the piezoelectric element
100
expands in its thickness dimension and contracts in its axial dimension, causing endcaps
110
and
105
to bow outward as shown by lines
135
and
140
, respectively. The bowing action amplifies the actuation distance created by the contraction of the piezoelectric element
100
, enabling the use of the element as an actuator.
U.S. Pat. No. 5,729,077 describes another Class V transducer having sheet metal caps with an outward convex shape, joined to opposed planar surfaces of the ceramic substrate to improve the displacements achievable through actuation of the ceramic disk. Due to the shape of the sheet metal caps, the transducer is commonly known as a Cymbal™ transducer, as mentioned above. An example of a Cymbal™ transducer is shown in
FIG. 2. A
multi-layer ceramic substrate
200
is interposed between two end caps
205
and
210
. The multi-layer substrate
200
includes a plurality of interspersed electrodes
215
and
220
. Electrodes
215
are connected together by end conductor
225
to endcap
210
and electrodes
220
are connected together by end conductor
230
to endcap
205
. Both endcaps are bonded to multi-layer substrate
200
about their periphery. Application of a potential across electrodes
215
and
220
causes an expansion of multi-layer substrate
200
in its thickness dimension, and contraction in its axial dimension, in a fashion similar to the moonie piezoelectric element
100
described above. As a result, endcaps
205
and
210
pivot about bend points
235
,
240
and
245
,
250
, respectively. As a result of such pivoting, substantial displacement of end surfaces
255
and
260
occurs.
Thus, the structure of piezoelectric element
100
or multi-layer substrate
200
in combination with their respective endcaps convert and amplify the small radial displacement of the element or substrate into a much larger axial displacement normal to the surface of the caps. For underwater applications, this contributes to a much larger acoustic pressure output than would occur when using piezoelectric element
100
or multi-layer substrate
200
alone.
The moonie and Cymbal™ transducers are capable of being constructed so as to be small compared to the wavelength of sound they produce in a usable frequency range, which is usually near their first resonance frequency. In addition, most of the radiating surface area of the shells moves in phase. As a result, the resulting acoustic radiation pattern is nearly omni directional, resembling an acoustic monopole. The omni directional characteristics of flextensional transducers create significant problems in projection transducer and array applications designed to transmit in one direction. At the present time, rows of transducers are carefully arranged and phased, or large baffles are used to produce the desired beam patterns. This is expensive, time-consuming and cumbersome. It would be desirable to construct and operate a Class V flextensional transducer that would be capable of generating a directional radiation pattern.
Butler et al., in “A Low Frequency Directional Flextensional Transducer,” J. Acoust. Soc. Am., vol. 102, July 1997, pp. 308-314, propose a method for generating a directional beam using a Class IV flextensional transducer by exciting both an extensional mode and a bending mode simultaneously. Butler et al. is directed to operating a Class IV transducer, in the 900 Hz range. The shell has an elliptical shape and the transducer is driven by a linear, rectangular stack of drive elements oriented along the major axis of the shell. The transducer disclosed by Butler et al. has overall dimensions of 19.4 inches long, 9.5 inches wide, and 20.3 inches high, and an in air weight of 350 lbs. In addition, Butler et al. discloses assembling six transducers in a line array with 20 inch center to center spacing. Thus the assembled array measures 10 feet long and weighs approximately 2100 lbs.
Prior to this application, there is no known method or apparatus for driving a Class V flextensional transducer to produce a directional beam.
SUMMARY OF THE INVENTION
An electro active device for generating a directional beam includes first and second electro active substrates each having first and second opposed continuous planar surfaces wherein each of the first opposed surfaces have a polarity and each of the second opposed surfaces have an opposite polarity. The first opposed surfaces of the first and second electro active substrates are in close contact. A first electrode is coupled to a junction formed by the first opposed surfaces having the same polarity, a second electrode is coupled to the second opposed surface of the first electro active substrate, and a third electrode is coupled to the second opposed surface of the second electro active substrate. A first endcap is joined to the second opposed surface of the first electro active substrate and a second

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