NON-DIRECTIONAL ULTRASONIC TRANSDUCER

Abstract: A transducer is comprised of a piezoelectric ceramic cylindrical vibrator vibrating radially, and a sheet provided on an outer peripheral surface of the cylindrical vibrator and including a fiber reinforced composite material with fibers oriented only in the direction of central axis of the cylindrical vibrator. Non-piezo­ electric cylinder consisting of Aℓ alloy or Mg alloy may be usable instead of the sheet. Another type of transducer includes a cylindrical piezo-transducer vibrating radially, a cylindrical sound radiator with the central axis coincident with the cylindrical piezo-transducer, and a bending coupler provided at a predetermined interval on end surfaces of the two cylinders and coupling the cylindrical piezo-transducer and the cylindrical sound radiator. Other type of transducer is comprised of a cylindrical piezo-transducer vibrating radially, an outside cylindrical sound radiator with its central axis coincident with the central axis of the cylindrical piezo-transducer which contains the piezo-transducer therein, and a coupler extending radially from an outer peripheral surface of the cylindrical piezo-transducer to an inner peripheral surface of the cylindrical sound radiator, thereby coupling both the two.

Description: The present invention relates to a transducer and more particularly to a non-directional high power underwater ultrasonic transducer with a wide band characteristic. A cylindrical piezoelectric ceramic transducer, shown in Fig. 1, operating under a radial mode has been used as a non-directional transducer. In the transducer, a radial polarization is effected by applying a high DC voltage between silver- or gold-baked electrodes 101, 102 formed on the inner and outer surfaces. Application of an AC voltage through electric terminals 103, 104 causes a non-directional acoustic radiation, as indicated by arrows, from the outer surface of a cylinder with reference to the central axis 0 - 0' under a so-called radial extensional mode. The aforementioned conventional cylindrical piezoelectric ceramic transducer is all made of piezoelectric ceramics, therefore, the following problem may arise. That is, the piezoelectric ceramics are about 8.0 x 10³ kg/m³ in density, and a speed of sound under the radial extentional mode is 3,000 to 3,500 m/sec., so that a characteristic acoustic impedance (defined by the product of density and speed of sound) becomes 24 x 10⁶ - 28 x 10⁶ MKS rayls, which is extremely large, nearly 20 times as large as the characteristic acoustic impedance of a water medium. Thus, there arises a mismatch of the acoustic impedance between the water and the transducer, limiting fractional bandwidth to 15% to 30% at best. In a sonar system, the range resolution is affected by the transmitted pulse trails. The pulse trails becomes longer with the decrease of the bandwidth of the transducer. Therefore, a broad band transducer will be indispensable for the sonar system. In the cylindrical piezoelectric ceramic transducer, in order to improve the impedance matching with water, or to obtain a broad-band characteristic, it is necessary that a mechanical impedance of the transducer be minimized (or the mass of the transducer per acoustic radiation area be minimized). For this purpose it has been tried to reduce the wall thickness of the cylindrical transducer. However, a thin-wall transducer involves a difficulty in working the piezoelectric ceramics and results in a considerable deterioration of mechanical strength, making it impossible to realize a high power acoustic radiation. Another known transducer is shown in US-A-4 156 824 wherein a cylindrical transducer is adapted to vibrate radially, and drives a sound radiator consisting of a plurality of cantilevered leaves arranged in a cylindrical pattern. Each leaf vibrates independently in a bending mode. An object of the invention is to provide a non-directional transducer having a broad-band characteristic. Another object of the invention is to provide a non-directional transducer having a high efficiency acoustic radiation characteristic. A further object of the invention is to provide a non-directional transducer capable of transmitting a high power. Another object of the invention is to provide a miniaturized non-directional transducer having the aforementioned characteristics. According to the present invention, there is provided a transducer comprising: a cylindrical piezo-transducer adapted to vibrate radially; and characterised by a cylindrical sound radiator with its central axis coincident with the central axis of said cylindrical piezo-transducer, said sound radiator being positioned adjacent to said piezo-transducer; and a bending coupler extending at predetermined intervals between end surfaces of said two cylinders and coupling said cylindrical piezo-transducer and said cylindrical sound radiator. Other objects and features will be clarified from the following description with reference to the drawings. Fig. 1 is an illustration showing a conventional non-directional cylindrical piezoelectric ceramic transducer; Fig. 2 is a perspective view representing an embodiment of the invention; Fig. 3 is a diagram for illustrating an operation of the embodiment given in Fig. 2; Fig. 4 is an equivalent circuit diagram of the embodiment shown in Fig.2. ; Fig. 5 is a perspective view representing another embodiment of the invention; Fig. 6 is a perspective view representing a further embodiment of the invention; In Fig. 2, reference numeral 20 denotes a cylindrical piezo-transducer, 23 denotes a cylindrical sound radiator, and 24 denotes a bending coupler. The cylindrical piezo-transducer 20 comprises an inside piezoelectric ceramic cylindrical vibrator 22, an outside cylinder 21 made of metal or fiber-reinforced composite material, and the vibrator 22 and the cylinder 21 are bonded tightly by an adhesive. The piezoelectric ceramic cylindrical vibrator 22 has, for example, an electrode provided on both upper and lower surfaces or on inner and outer peripheral surfaces, whereby a piezoelectric displacement can be achieved by polarization through these electrodes. A radial extensional vibration under the lateral effect mode can be excited strongly. Then, in case the radial extensional vibration is excited strongly under a stiffened mode, the piezoelectric ceramic cylinder is divided radially by a plane rectangular to the circumference, as known well hitherto, electrodes are formed on planes rectangular to the circumference obtained through division, a polarization is carried out through the electrodes. The cylindrical piezo-transducer 20, the piezoelectric ceramic cylindrical vibrator 22 and the cylinder 21 must be unified for radial extensional vibration, and it is desirable that a compressive bias stress be applied on a portion of the piezoelectric ceramic vibrator 22. The reason is that the piezoelectric ceramics are fragile in tension and the strength in tension in several times less than the compressive strength. Therefore, when the vibrator 22 expands uniformly under the radial extensional mode, a fracture can be prevented. As stated before, by using a big difference of thermal expansion coefficients between the cylinder made of metal or fiber-reinforced composite material and the piezoelectric ceramic cylindrical vibrator 22, a compression bias stress is kept applied on the portion of the piezoelectric ceramic cylindrical vibrator 22 at all times at normal operating temperature, and hence a large amplitude drive can be realized as compared with the conventional cylindrical piezoelectric ceramic vibrator. Further, it is preferable that the cylindrical sound radiator 23 is lightweight for easy broad-band matching with water and made of a fiber-reinforced composite material with a rigidity large enough to cope with a flexure deformation for realizing a uniform radial extensional vibration, or an alloy with Al, Mg as main constituents or that for which these materials are compounded in a plural layer. The bending coupler 24 is made preferably of a high strength of metallic material such as, for example, Al alloy, Mg alloy, Ti alloy and steel alloy or of a fiber-reinforced composite material. Then, it goes without saying that the parts 21, 24, 23 can be integrated for construction. Next, the operating principle of the transducer according to the embodiment will be described. As described hereinabove, the transducer operates under two vibration modes, namely an in-phase mode and an antiphase mode. The in-phase mode is a vibration mode wherein the sound radiator 23 expands radially as indicated by a solid line arrow when the transducer 20 expands radially as indicated also by a solid line arrow, and a deformation is almost not caused on the bending coupler 24. The antiphase mode is a vibration mode wherein the sound radiator 23 contracts radially as indicated by a broken line arrow when the transducer expands radially as indicated by a solid line arrow. In this case, the flexure deformation arises such that, as shown symbolically in Fig. 3, the junction with the sound radiator 23 and the other junction with the transducer 20 each behave as roll ends. The antiphase mode may cause a flexure deformation on the coupler 24 as compared with the in-phase mode, and a resonance frequency becomes higher than that in-phase mode due to flexure stiffness of the coupler 24. That is, there exist the in-phase mode and the antiphase mode varying each other in resonance frequency. Then, it goes without saying that when the cylindrical piezoelectric vibrator 20 contracts radially uniformly, a vibration displacement in the sound radiator 23 becomes counter to the directions indicated by the arrows in Fig. 2. An equivalent circuit of the transducer according to the embodiment can be indicated by a lumped parameter approximated equivalent circuit shown in Fig. 4. As will be apparent from Fig. 4, the transducer according to the embodiment is totally different from a conventional single resonant transducer, and is a band pass filter with water as a sound load. In Fig. 4, C d denotes a damped capacity and -C d denotes that which appears when a stiff end mode ceramic vibrator is used, and -C d does not appear on an unstiffened mode vibrator. A reference character A denotes a power factor, m₁ and c₁ denote an equivalent mass and an equivalent compliance of the cylindrical piezoelectric vibrator 20 respectively, m₂ and c₂ denote an equivalent mass and an equivalent compliance of the cylindrical sound radiator 23 respectively, C c denotes a flexure compliance of the flexible coupler, S a denotes a sound radiation sectional area, and Z a denotes a sound radiation impedance of water in an acoustic system. It should be noted here that the present embodiment may apply to the transducer having not only the same equivalent mass and resonant frequency (m₁ = m₂, c₁ = c₂) but also different equivalent mass and resonant frequency (m₁ = m₂, c₁ = c₂). The latter transducer is called an asymmetric underwater ultrasonic transducer. Another construction of the transducer according to the embodiment will be exemplified in Fig. 5. Two cylindrical piezoelectric vibrators 20, 20a are disposed on both the portions of the cylindrical sound radiator 23 through bending couplers 24, 24a; the two vibrators 20, 20a, are then driven in-phase, thereby realizing a further uniform radial extensional mode as compared with Fig. 2. Consequently, a broad-band and non-directional high power transducer is obtainable. Here, parts 21a, 22a are constructed of the same members as 21, 22. In Fig. 6 representing an actually constructed embodiment, the piezoelectric ceramic cylinder 22 is polarized in the direction of thickness with silver-baked electrodes formed on the inner and outer peripheral surfaces. The cylinder 21 is made of Al alloy, which is bonded tightly through an epoxy adhesive at temperature of 150°C according to the aforementioned process Accordingly, a compression bias stress is applied and so kept on the piezoelectric ceramics at ordinary temperature. The reference numeral 25 denotes an inside cylinder of the sound radiator 23. The parts 21, 24, 25 are of an Al alloy made and so unified. A reference numeral 26 denotes a carbon fiber-reinforced plastics (C-FRP) with the epoxy resin in which fibers are disposed longitudinally of the cylindrical sound radiator 23 as a matrix, which functions as an outside cylinder. A glass fiber may be wound on an outer surface of the outside cylinder 26 to apply a compression bias stress on the cylindrical sound radiator 23, thus enhancing a bonding strength of the parts 25 and 26. A reinforced fiber such as carbon fiber, alamide fiber or the like other the the glass fiber may function likewise. The parts 25 and 26 thus vibrate integrally under the radial extensional mode, and a sound can be radiated intensively from the outer surface of the part 26. The C-FRP cylinder 26 has the fibers disposed longitudinally (0 - 0' direction) of the cylinder. A flexure stiffness to the longitudinal direction in the sound radiator 23 becomes large, and thus a flexure will almost not arise on the cylinder in a usual frequency band. On the other hand, in the circumferential direction, since fibers are not disposed except that the reinforced fiber is wound somewhat on the outside of the cylinder 26, the C-FRP cylinder 26 functions as lowering a resonance frequency of the sound radiator 23. Under the diametral vibration mode, the Al alloy is greater in speed of sound by 40% or so than piezoelectric ceramics. The speed of sound under the diametrical vibration mode of the C-FRP cylinder 26 is almost equal to a speed of sound in epoxy resin working as matrix, and the speed of sound is smaller by 40% or so than that in the piezoelectric ceramics. Accordingly, in the sound radiator 23, a resonance frequency of the sound radiator 23 under the diametrical vibration mode can be controlled by changing the ratio in thickness of the Al alloy cylinder 25 to the C-FRP cylinder 26, thus coordinating easily with an optimum design value for manufacture. The cylindrical vibrator 20 is covered with an acoustic decoupling material or cork rubber, both ends longitudinal of the transducer are capped with an Al alloy disk through the cork rubber and further molded with a neoprene rubber. A prototype transducer is 15.8 cm high and 10.5 cm diametral in outline dimensions. Since this embodiment can utilize two resonance modes, namely in-phase mode and antiphase mode, a considerably broader band is realizable as compared with a conventional transducer. Further, according to this embodiment a broad-band sound matching can easily be attained by using a lightweight material such as Al alloy and C-FRP as the sound radiator, and high power transmission is possible by using a high strength material such as Al alloy or the like as the base material. These features make it possible to provide a transducer capable of sending a broad-band, fractional band width 60% and a high power, 190 dB relµPa at 1m in output sound pressure with a superior sound matching efficiency with water. A formation of the cylinder 26 is so preferable but not necessarily indispensable. The bending coupler 24 may be formed directly on the piezoelectric ceramic cylindrical vibrator 22.

Claims: A transducer comprising: a cylindrical piezo-transducer (21,22) adapted to vibrate radially; and characterised by a cylindrical sound radiator (23) with its central axis coincident with the central axis of said cylindrical piezo-transducer, said sound radiator being positioned adjacent to said piezo-transducer; and a bending coupler (24) extending at predetermined intervals between end surfaces of said two cylinders and coupling said cylindrical piezo-transducer (21,22) and said cylindrical sound radiator (23). The transducer according to Claim 1, wherein said piezo-transducer, (21,22) sound radiator (23) and bending coupler (24) are formed integrally. The transducer according to Claim 1 or 2, wherein said cylindrical piezo-transducer includes an outside cylinder (21) consisting of a fiber reinforced material having a fiber axis coincident with the direction of the central axis, and a cylindrical piezoelectric ceramic vibrator (22) contained tightly within an inner peripheral surface of said outside cylinder. The transducer according to any preceding claim, wherein said cylindrical piezo-transducer (22) is subjected to a compression bias stress. The transducer according to Claims 3 and 4, wherein said cylindrical piezoelectric ceramic vibrator (22) is subjected to a compression bias stress by said outside cylinder (21). The transducer according to Claim 5, wherein said cylindrical sound radiator (23) consists of a material with large rigidity to flexural deformation. The transducer according to Claim 6, wherein said cylindrical sound radiator (23) consists of an alloy material with Al or Mg as a main constituent. The transducer according to Claim 1, wherein said bending coupler (24) consists of an Al alloy, Mg alloy, Ti alloy or steel alloy. The transducer according to Claim 1, wherein said cylindrical piezo-transducer (21,22) and cylindrical sound radiator (23) are configured to vibrate in phase. The transducer according to Claim 1, wherein said cylindrical piezo-transducer (21,22) and cylindrical sound vibrator (23) are configured to vibrate in antiphase. The transducer according to Claim 1, wherein said cylindrical sound radiator (23) consists of a fiber reinforced compound material with fibers orientated in the direction of central axis (0 - 0'). The transducer according to Claim 1, wherein there are: two said cylindrical piezo-transducers (21,22;21a,22a) adapted to vibrate radially; the cylindrical sound radiator (23) being disposed between said two cylindrical piezo-transducers with its central axis coincident with the central axis of said piezo-transducers; bending couplers (24,24a) extending at predetermined intervals between the end surfaces of said cylindrical piezo-transducers and the end surfaces of the said cylindrical sound radiator and coupling said piezo-transducers and sound radiator. The transducer according to Claim 12, said two cylindrical piezo-transducers each comprising an outer cylinder (21,21a) consisting of a fiber reinforced compound material having a fiber axis coincident with its central axis, and a cylindrical piezoelectric ceramic vibrator (22,22a) contained tightly within the inner peripheral surface of said outside cylinder. The transducer according to Claim 13, wherein said cylindrical piezoelectric ceramic vibrator (22,22a) is subjected to a compression bias stress by said outside cylinder (21,21a).

Inventors: INOUE, TAKASHI, KONNO, MASASHI
Applicants: NEC CORPORATION
Application year: 1987
Main classification: H04R17
IPC codes: B06B1, G10K13