SPECIFICATIONComposite TransducerThis invention relates to transducers, and more particularly to a composite transducer for sonar applications which has separate, non-interfering transmit and receive transducers.
A longitudinal vibrator-type transducer is widely used in the prior art as a transmitter and receiver in sonar applications. The transducer consists essentially of an electromechanically active element 11 (typically a piezoelectric ceramic), a head mass 12, a rear mass 13, a bias rod 14, a pressure release system 1 5 and a waterproof housing 1 6, (see Fig. 1). The bias rod 14 provides a bias compressive stress on both the active element 11 and the pressure release system 1 5. Acoustic decoupling of the assembly of these components and the housing 16 is provided by the pressure release system 1 5.
There are many variations of the longitudinal vibrator-type transducer, but transducers of this general type have two characteristic frequencies that adversely affect receiving response. The two frequencies are the head and tail mount resonances.
Because of the phase shifts associated with resonances and the deterioration of a beam produced by an array of transducers as a consequence of phase shift differences between the transducers, a relatively flat receiving response is desired over a wide bandwidth.
However, a typical transducer receiving response has uncontrolled head and tail mount resonances 20 and 21, respectively, as shown in Fig. 2. Fig. 2 shows a plot of receiving sensitivity versus normalized frequency n=f/fr, where fr is the opencircuit (constant-current) resonant frequency 21.
The peak 22 in the response below resonance is due to the head mass-tie rod resonance. Similarly, the response minimum 23 is caused by the resonance of the spring-mass formed by the pressure release pad 1 5 and the rear mass 13.
In order to achieve a uniform or flat receiving response, the head and tail mount resonance frequencies must be equal, as well as the amplitudes of their resonances. Because of the difficulty of obtaining this balance in high volume production, damping is generally employed to compensate for any unbalance. The damping is often obtained by rubber bumpers 1 7 that are attached to the rear mass 13 and make frictional contact to the housing 1 6. A closely balanced transducer requires tight tolerances on both the material parameters and physical dimensions of the transducer. This adds significantly to the cost of the transducer, particularly in high volume production.
In addition to uniformity of receiving sensitivity, transducer self-noise is an extremely important performance parameter. Noisy transducers in a sonar array can cause a degradation of sonar system performance, as well as reveal the presence of the sonar platform.
Longitudinal vibrator-type transducers such as that described hereinbefore, in particular have been found to generate extraneous noise when exposed to a changing hydrostatic pressure head.
Typically, the extraneous noise is determined by measuring the open-circuit transducer voltage developed during pressure cycling. Polished contacting surfaces of the head mass 12, ceramic 11 and rear mass 13, very close tolerances on machine parts, and well-controlled alignment procedures have been found to be necessary to produce quiet transducers of the longitudinal vibrator type. These noise-quieting features have also added significantly to the cost of the transducer.
Also known in the prior art is a piezoelectric polymer which has low mass density and is mechanically flexible. These properties make the polymer more shock resistant than the prior art piezoceramics. Additionally, the characteristic impedance of the polymer more nearly matches that of water. Piezoelectric polymer film is presently made of polyvinylidene fluoride and is often referred to as PVF2. A polarization procedure must be used to render the polymer usefully piezoelectric. In one method of polarizing, both surfaces of the film are metallized to provide electrodes and a high d-c voltage is applied to the electrodes and heid for about one hour at 10000.
Subsequent cooling to room temperature under the applied field results in permanent polarization with the strongest piezoelectric effect in the direction transverse to the metallized surfaces of the film.
The polymer PVF2 has been used previously as a transducer for the transmission and reception of ultrasound signals. Since the acoustic power which may be transmitted with this material is limited, its use has been confined to low power applications such as in medical ultrasound.
Summary of the InventionIt is therefore an object of this invention to provide a composite transducer which utilizes the prior art longitudinal vibrator-type transducer for the transmitting function, but not for the receiving function, employing instead a separate transducer of piezoelectric polymer type for receiving signals.
More particularly, the composite transducer of this invention comprises a prior art longitudinal vibrator-type transducer 10 with a piezoelectric polymer 60 attached to its radiating face 121 for the receiving function as shown in Fig. 1. During transmission the receiver transducer is shortcircuited, and during reception the transmitter transducer 10 is terminated in an electrical impedance that optimizes receiver response and minimizes transducer self-noise. The associated switching circuitry for switching between reception and transmission can be internal to the transducer.
It is a feature of the invention that the composite transducer occupies substantially the same space as required by the longitudinal vibrator-type transducer of the prior art, and can be retrofitted into a sonar system without modification of the transducer mounting arrangement.
Brief Description of the DrawingsThe objects and features of this invention are explained in the following description taken in conjunction with the accompanying drawings, in which:Fig. 1 is an isometric view of the composite transducer of the invention;Fig. 2 is a frequency response curve for a longitudinal vibrator-type transducer of the prior art;Fig. 3 is an isometric exploded view of the PVF2 film; andFig. 4 is a cross-sectional isometric view of the assembled PVF2 hydrophone.
Description of the Preferred EmbodimentThe composite transducer 100 of Fig. 1 employs separate elements for the transmission and reception of sound. Transmission is provided by applying an electrical voltage from transmitter 101 through a transmit/receive (T/R) switch 102 to a piezoelectric ceramic 11. A transformer 111 is used to impedance match the ceramic 11 to the transmitter 101. During the time that transmitter 101 is providing power to ceramic 11,the transmit/receive (T/R) switch 103 is providing a short circuit across lines 631 connected to the polymer assembly 601. At the termination of transmitter power, T/R switch 103 connects the receiver 104 across polymer assembly 601 andT/R switch 102 connects an impedance 105 across lines 632 to thereby terminate ceramic 11.
The composite transducer 100 transmitterportion is the well-known longitudinal electromechanical vibrator-type 10 which in the prior artis used for both transmitting and receiving.
However, in this invention the receiving transducer (hydrophone) is a layer of thick filmpiezoelectric polymer 601, polyvinylidene fluoride(PVF2), used in a modified hydrostatic mode. In ahydrostatic mode, the acoustic pressure actsequally in all three axes so there is no need for apressure release mechanism and associatedhousing for decoupling one side of the sensingelement from the sound field. In the hydrostaticmode, there is an absence of pressure differential across the hydrophone, resulting in virtuallyunlimited operating pressure capability. In thisinvention, there is modified hydrostatic operation since one surface of the polymer is in directcontact with the radiating face 121 of thetransducer 10, and hence, not exposed to the water pressure.Because PVF2 closely matchesthe impedance of water, the transmitted sound from transducer 10 will pass through the PVF2 sheet 601 with negligible attenuation. During transmission, the hydrophone 601 is shortcircuited. The polymer hydrophone 601 attached to the radiating face 121 of transducer 10 doesnot adversely affect its transmission properties.
During reception. the transducer 10 is preferably terminated in an impedance which minimizes the noise in the hydrophone 601 signal or transducer10 may be short-circuited with no loss of received signal-to-noise ratio. Moreover, the useful receiving response of the hydrophone 601 extends up to 100 KHz. Measured polymer hydrophone response for the open-circuit and short-circuit transducer 10 conditions indicates that the short-circuit condition yields the more uniform response essentially flat from 10 KHz to100 KHz. Greater uniformity is expected, especially below 10 KHz, for the optimum termination of transducer 10.
Velocity control, noise cancellation, and intercept receivers, among other uses, are potential applications for this invention.
The measured directivity pattern of the 4x4 inch sheet of PVF2 hydrophone 601 at 100 KHz had a 3 db beamwidth of approximately 4.50 which is indicative of the excellent beam pattern available from polymer hydrophones at high frequencies. The results of high power transmission tests from transducer 10 indicate that exposure to high intensity sound fields has no measurable effect on the polymer hydrophone performance properties. In addition, exposure to hydrostatic pressure cycling during noise testing did not adversely affect the polymer hydrophone.
The fabrication of the composite transducer 100 of Fig. 1 was accomplished by removing a square of the vulcanized rubber 34 which is adjacent the face 121 of the head mass 12 of the prior art transducer 10 in order to expose the aluminum head mass 12. An isometric crosssectional view of the face 121 is shown in Fig. 1 where the square pocket 123 produced by removing rubber 34 extends over a substantial portion of the rectangular face 121. In order to obtain a flat, smooth surface on face 121, the face was machined and a few thousandths of the face removed. A hole 124 was drilled through head mass 12 from face 121 to an interior space 125 within housing 16 of the transducer 10. A rectangular channel 126 was machined into face 121 at the location of the hole 124.The exposed surface of the face 121 was grit-blasted as was the mating surface of a sheet of glass-fiber, epoxy-impregnated, electrical insulating mat 32.
A suitable mat, commercially available, is known as G-1 0 mat. The mat 32 and face 121 were bonded to each other by an epoxy 50' under heat and pressure to exclude trapped air and to provide a rigid bond.
The piezoelectric polymer PVF2 assembly 601 was assembled of two PVF2 films 60', 60", each being 4x4x0.023 inches, with opposed faces covered with epoxy 50", as shown in Fig. 3. Each film 60 has a metallic coating 62 on both sides of the sheet to which wires 63 are electrically connected by vacuum welding or by low temperature soldering. Typically, the metallic coating 62 is copper and the wire 63 is a Kovar ribbon, .003x.010 inches. Other metallic materials would be satisfactory for each provided electrical connection can be made between the selected coating and wire materials at a temperature which is nondestructive to the PVF2.
Using suitable fixturing, the PVF2 films 60 were bonded together by the epoxy 50" and air cured to form the PVF2 polymer assembly 601 as shown in Fig. 4.
The attachment of the PVF2 assembly 601 to the glass fiber mat 32 is the next step in the fabrication of the composite transducer. To prevent the PVF2 assembly from electrically shorting to the face 121 or to the wires 63, electrical-insulating tape 64 was attached to the four edges of the PVF2 square as shown in Fig. 4.
The wires 63 were also covered with a lightweight polyolefin shrink sleeving material.
The exposed face of fiberglass mat 32 was roughened to remove shiny glaze and cleaned by air dusting. The surface 602 of PVF2 assembly 601 was cleaned by wiping with methylethylketone (MEK) which did not adversely affect the copper surface 62 of the PVF2.
The mat 32 and the PVF2 assembly 601 surface 602 were coated with epoxy 50"' and placed in contact with each other to air cure with the insulated wires 63 folded in the channel 126 and threaded through the head mass hole 124 as shown in Fig. 1. Channel 126 allows the PVF2 assembly 601 to lay flat against the face 121 by providing space for the connecting wires 63.
The final steps in the assembly of the composite transducer 100 of Fig. 1 comprise cleaning the outermost surface of the PVF2 assembly 601 with MEK, coating with liquid neoprene, and air drying. A square of rubber 33, substantially of the thickness and area of the remaining depth of the pocket 123 and the exterior surface of the PVF2, are coated with neoprene and bonded to each other by air curing. The resulting exterior surface of the rubber 33 and rubber head mass cover 34 are machine-ground to provide a flat exterior surface.
The wires 63 pass through the space 1 6 between the transducer shroud 1 7 and the head mass 12, ceramic 11, and rear mass 13 to the cable 18. Cable 18 is a four-conductor cable containing also the two conductors 632 to the transducer ceramic 11 from the transmitter 101 located at some remote location. A receiver preamplifier (not shown) may be contained within the composite transducer 10 to amplify the signal prior to connection to the two wires of cable 18 which are connected to a receiver.
The PVF2 material 62 has an internal polarization when used as a piezoelectric element.
The polarization is represented by the voltage polarity designations of the PVF2 shown in Fig. 3.
A parallel connection is obtained by connecting wire 63' to wire 63"", and wire 63" to wire63"', as shown in Fig. 4, to thereby provide one pair of wires 631 to which the receiver is connected. The parallel connection doubles the capacitance provided by each PVF2 sheet 60 and thereby provides a better impedance match to thecable. If a preamplifier is used within the transducer 11, the capacitance of the PVF2 assembly 601 is of less importance. The thickness of each of the PVF2 sheets, the number of sheets, and their serial/parallel electrical interconnection are design choices.
Modifications of the preferred embodiment will be apparent to those skilled in the art. For example, the copper electrode on both sides of each PVF2 sheet could be removed at its edges by etching or some other suitable technique. This would eliminate possible short circuits between the PVF2 assembly and the connecting wires, thereby eiiminating the need for taping. The use of a heavier copper wire may replace the relatively fragile Kovar material of the preferred embodiment. Also, an electrically conductive epoxy might be used for fastening wires 63 to the PVF2 metallic film 62 instead of welding or soldering.
Having described a preferred embodiment of the invention, it will now be apparent to one of skill in the art that other embodiments incorporating its concept may be used. It is felt, therefore, that this invention should not be restricted to the disclosed embodiment, but rather should be limited only by the spirit and scope of the appended claims.