BRIEF DESCRIPTION OF THE DRAWINGS
Numerous other objects, features and advantages of the invention should now become apparent upon a reading of the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1A is a schematic cross-sectional view of a low profile cylindrical embodiment showing the principles of the present invention applied to two piezoelectric discs with an attached intermediate member support disc and masses attached at the center with the periphery of the intermediate disc attached to the housing;
FIG. 1B is a schematic cross-sectional view showing the motion of the transducer of FIG. 1A under electrical drive with the piezoelectric discs moving oppositely causing bending motion which, in turn, causes increased relative motion between the pistons of the housing and the interior center masses;
FIG. 2 is a schematic cross-sectional view of an alternate embodiment of the present invention employing a rigid spherical housing allowing a stiffer housing structure and more internal room for accommodating greater size internal masses; and
FIG. 3 is a schematic cross-sectional view of still another alternate embodiment of the present invention illustrating a transducer housing in the shape of a circular cylinder with the piezoelectric bender operating in a 33 mode but in opposition on the right and left sides causing bending and, in turn, causing the cylinder to move relative to the two masses.
DETAIL DESCRIPTION
In accordance with the present invention, there is now described herein a number of different embodiments for practicing the present invention. There is provided a dipole transducer for obtaining increased source strength by means of the additional mass which causes greater translational motion of the radiating housing and also allows a lower resonant frequency and mechanical Q. A cross-sectional view with labeled parts for a cylindrical dipole transducer with additional mass is shown in FIG. 1A. FIG. 1B shows the dynamic motion of the transducer of FIG. 1A during part of a drive cycle. In FIG. 1A, parts 1 and 2 are piezoelectric disc, with polarization direction indicated by the arrows, together operating in a planar bending mode. The discs 1 and 2 may be constructed with many different shapes such as a rectangular shape. The two discs 1, 2 may be cemented to a substrate 3 (typically a metal such as brass or aluminum). This substrate 3, in turn, is cemented between two cylindrical housing cups, 4 and 5, (typically a low density metal such as magnesium or aluminum).
The inertial masses, 6 and 7, (typically a high density metal such as steel or tungsten) are attached to the center of the substrate 3, although they can also be attached to the piezoelectric discs 1 and 2. The discs 1 and 2 are provided with a through passage at their center so as to receive the respective masses 6 and 7 so that the masses can be attached to the substrate 3. The piezoelectric pieces 1 and 2 are energized by a voltage V at terminals 8 and 9 through wires connected to electrodes on the piezoelectric discs 1 and 2. The interior space 10 is typically, but not limited, to a gas such as air. The exterior is typically, but not limited to, a fluid such as water.
Once energized with voltage V at the terminals 8 and 9, the housing that is comprised of piezoelectric elements 4 and 5, moves along the direction of symmetry labeled as direction or axis A in FIG. 1A. This motion is illustrated in FIG. 1B where here the arrows now indicate the direction of relative motion for a half-cycle.
In the illustration shown in FIG. 1B the piezoelectric discs 1 and 2 bend because of opposite radial expansion as a result of opposite polarization direction shown in FIG. 1A by the arrows. The bending causes the substrate 3 to bend causing the housing to move to the right, for this half-cycle, along the axis of symmetry A causing a compression in the medium on the right side and a rarefaction in the medium on the left side creating a dipole radiator. The direction is reversed on the next half-cycle. The inertial masses 6 and 7, each of mass M, enhance this motion and also provide a lower resonance frequency and lower mechanical Q.
Some simple equations for the housing displacement, resonance frequency and mechanical Q illustrate the advantage to using these inertial members of mass, M. With x the displacement of the housing along the axis of symmetry, with m the mass of the housing comprised of piezoelectric elements 4 and 5 and any additional radiation mass, with m′ the dynamic mass of the bender section comprised of piezoelectric elements 1 and 2 and substrate 3, with K the short circuit dynamic stiffness of the bender, then the force is expressed as F=NV generated by the piezoelectric bender, where N is the electromechanical transduction transformer ratio. At low frequencies, below resonance, it can then be shown that the axial displacement of the housing x=(F/K)/[1+m/(M+m′)]. Now for M>>m the displacement is x=F/K while for M=0, x=F/2K for a typical case of m′=m; and consequently the inclusion of the inertial masses can increase the displacement by a factor of two for large values of M. The resonance frequency may be written as fr=f0[1+m/(M+m′)]1/2 where f0 is the ideal resonance frequency when the mass M is very large. Thus for M>>m, fr=f0 while for M=0, fr=fo√2 for the typical case of m′=m; and
consequently, the inclusion of the inertial masses can decrease the resonance frequency by the factor √2 for large values of M. Another advantage is the reduction in the mechanical Q which may be written as Qm=Q0[1+m/(M+m′)] where Q0 is the ideal Q for M>>m. Thus for M>>m, Qm=Q0 while for M=0, the Qm=2Q0 for the typical case of m′=m; and consequently, the inclusion of the inertial masses can decrease the mechanical Q by a factor 2 for large values of M.
The present invention is not limited to a cylinder and can take the form of a spherical structure as illustrated in FIG. 2 or other geometric shapes. Although the embodiment of FIG. 1A affords a low profile structure the spherical embodiment of FIG. 2 allows greater room for the inertial mass and a stiffer housing structure allowing deeper submergence with less interference from housing structural modes of vibration. In FIG. 2 parts 11 and 12 are piezoelectric discs with the polarization direction indicated by the arrows and together operating in a planar bending mode. The two discs are cemented to a substrate 13 (typically a metal such as brass or aluminum). This substrate 13 may be cemented between two hemispherical caps 14 and 15 (typically a metal such as magnesium or aluminum). The inertial masses 16 and 17 (typically a metal such as steel or tungsten) are attached to the center of the substrate 13, although they can also be attached to the piezoelectric discs 11 and 12. The discs 11 and 12 are provided with a through passage at their center so as to receive the respective masses 16 and 17 so that the masses can be attached to the substrate 3. The piezoelectric pieces 11 and 12 are energized by a voltage V at terminals 18 and 19 through wires connected to electrodes on the piezoelectric pieces 11 and 12. In addition to the spherical shape, the shell structure can also take on other forms such as a spheroid including oblate or prolate spheroids.
The transducer of the present invention can also take the form of a circular cylinder driven by segmented piezoelectric bender bars as shown in a schematic cross-sectional view in FIG. 3. Mechanically isolated end caps (not shown) prevent the medium and acoustic radiation from entering into the interior space 10. In this case the radiation is not from the cylinder end caps (not shown) but from the sides of the cylinder. The cylinder cross-section may also be elliptical.
In FIG. 3, parts 21 and 22 are piezoelectric bars with the polarization direction indicated by the arrows and wired in parallel for 33-mode bending mode operation. The two bars 21 and 22 are cemented to a substrate 23 (in this case a non conductor). The substrate 23 may be cemented between two hemi-cylinders (or hemi-ellipses) 24 and 25 (typically a metal such as magnesium or aluminum). The inertial masses 26 and 27 (typically a metal such as steel or tungsten) are attached to the center of the substrate 23, although they can also be attached to the respective bars 21 and 22. The piezoelectric bars 21 and 22 are provided with a through passage at their center so as to receive the respective masses 26 and 27 so that the masses can be attached to the substrate 3. The piezoelectric bars 21 and 22 are energized by a voltage V at terminals 28 and 29 through wires connected to electrodes on the piezoelectric bars 21 and 22. In operation, the motion is in the direction of the B axis. The piezoelectric drive section that is comprised of bars 21 and 22, as well as substrate 23 of FIG. 3 may be comprised of left and right sections that are not reverse polarized but yet move extensionally in opposite directions by wiring the left and right sections in series and thus out of phase. The bars 21 and 22 may be polarized in a direction perpendicular to that show by the arrows of FIG. 3 and operated in a 31 mode. Finite element models have been constructed to verify the performance of the transducer illustrated in FIG. 1A. A magnesium cylindrical housing was 3 inches in diameter and 2 inches long with a wall thickness of approximately 0.32 inches. The housing is driven with two piezoelectric ceramic discs that are each 2.25 inches diameter and 0.088 inches thick. The substrate is 0.07 inch thick and the two tungsten masses are each of a diameter of 0.56 inches and a length of 0.40 inches. The results show it produced an in-water resonant frequency of approximately 4,000 Hz and a source level of 80 dB/1 μPa @ 1 m at 1,000 Hz. Without the inertial masses the in-water resonant frequency was approximately 6,000 Hz with a source level of approximately 77.5 dB/1 μPa @ 1 m at 1,000 Hz. Transducer models were also fabricated with a housing constructed of aluminum. The measured results compared favorably with a corresponding finite element model.
Having now described a limited number of embodiments of the present invention, it should now become apparent to those skilled in the art that numerous other embodiments and modifications thereof are contemplated as falling within the scope of the present invention as defined in the appended claims. Examples of modification would be the use of other transduction devices or materials such as single crystal, magnetostriction or electrostriction material. The interior medium may be fluid. The exterior medium may be a mechanical load and in this case the transducer would be used as an actuator. As a result of reciprocity, the transduction device can be used as a receiver of sound as well as a transmitter of sound. As a receiver it produces an output voltage as a result of a pressure differential across the housing from an incoming acoustical wave or from a force producing an output voltage as an accelerometer.
Patent Application
20080079331
