GB1605468A

GB1605468A - Sonar transducer

Info

Publication number: GB1605468A
Application number: GB4800665A
Authority: GB
Inventors: Graham Walton; De Filippis Tulio
Original assignee: Control Data Corp
Current assignee: Control Data Corp
Priority date: 1964-11-18
Filing date: 1965-11-11
Publication date: 2016-07-27
Anticipated expiration: 1981-11-11

Description

i

We, CONTROL DATA CORPORATION, a corporation organised and existing under the laws of the State of Minnesota, United Sates of America, having a place of business at 8100 34th Avenue. South Minneapolis, Minnesota, United States of America, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement:-

This invention relates to a sonar transducer of the conformal type in which the transducer is recessed into the hull of a ship with only the transducer transmitting and receiving surface exposed to the water.

The requirements for a transducer element used in conformal sonar systems are more exacting than for conventional sonars. The desire to "steer<">the conformal array to end-fire (directly<7>forward or aft) imposes a limit on the diameter of the individual elements in order to avoid significant system degradation due to element directivity'. This constraint makes the maximum ratio of element diameter to wavelength (at the highest operating frequency) about one-quarter.

The normalized radiation resistance varies as the square of this ratio, and has. therefore, a value of approximately one-quarter of that of a half-wave element. Both the efficiency and the bandwidth of the element decreases with the radiation resistance.

A lotv efficiency is undesirable because of the wasted pow'er and heating of the ceramic drive element. A low' bandwidth is undesirable because it restricts the number of frequencies available for multiple pulsing (for achieving a high data rate) or for minimizing the interference among a group of ASW ships.

Still another constraint on the design of the element is that it must have an internal impedance (the impedance seen looking into the acoustical terminals), which is high compared with the radiation impedance, in order that velocity control can be readily achieved in transmission, and beams can be readily formed in reception. Finally, the element must possess good shock and vibration characteristics and a high (cavitation-limited) acoustic pow'er output.

In a sonar device in accordance with the invention, there is provided the combination including a vibratory rigid mass for coupling electrically produced mechanical energy to a liquid medium and coupling mechanical energy from the liquid medium to a transducer, said transducer being an electro-mechanical transducer and coupled to said rigid mass for generating or seeing

In a sonar device in accordance with the invention, there is provided the combination including a vibratory rigid mass for coupling electrically produced mechanical energy to a liquid medium and coupling mechanical energy from the liquid medium to a transducer, said transducer being an electro-mechanical transducer and coupled to said rigid mass for generating or seeing vibrations of the mass corresponding to sonar signals, and a separate compliant mass interposed between said rigid mass and said liquid medium the compliance of which is chosen to extend the available bandwidth of frequency response of the transducer by tuning the acoustic impedance. The device can be housed in a sea chest built in the hull of a ship and may include a resonant cavity between the outer surface of the compliant mass and the outer wall of the sea chest. The compliant mass may be tuned in a manner described below to extend the frequency response of the transducer and to lessen the effects of load impedance variation.

In the preferred form, the compliance of said compliant mass is non uniform being less near the periphery thereof than at the centre thereof. This substantially raises the otherwise impedimentary cavitation limitation of the acoustic power output.

The invention .will be more fully described and understood in the following details description, which is to be read in connection with the accompanying drawings wherein:Figure 1 is an eievational view in partial cross-section of a sonar device made in accordance with the invention; and Figure 2 is a schematic representation of the device illustrated in Figure 1 to aid in explaining the mechanical relationship between the various elements.

Referring to Figure 1 , the device includes front compliance 10, rigid front mass 12, piezoelectric stack 14, and larger rigid back mass 16, all of which are held together as a unit inside shell 18 by tension rod 20. All of these elements are of circular transverse cross-section. Front compliance 10 is a circular member having annular rings 22 which support and space diaphragm 24 from front mass 12. The front compliance 10 may be machined from solid aluminium stock to provide an effective and inexpensive element.

Front compliance 10 is joined to the front end of front mass 12 by a suitable bonding agency such as an epoxy adhesive bond along the interfaces between rings 22 and front mass 12. Front mass 12 may likewise be machined or otherwise formed from solid aluminium stock.

Front mass 12 is held against piezoelectric stack 14 by tension rod 20, which is threaded at one end to front mass 12 and at the opposite end to spherical nut 25 which bears against washer 26 and through it against the periphery of hole 28 drilled through back mass 16.

<■>Connecting these elements in this manner by spherical nut 25 insures that rod 20 exerts only compressive stress on piezoelectric stack 14 with no attendant bending or shearing stresses on stack 14 or front or back mass 12 and 16. Cap 30 threads into the back end of back mass 16 may be satisfactorily fabricated from solid brass stock by machining or the like.

Piezoelectric stack 14 in the illustrated embodiment in Figure 1 is an assembly ofPZT 4 ceramic rings 32. Rings 32 are arranged in alternating polarity and are connected electrically and mechanically by nickel grids embedded in an epoxy boding agent. The ends of stack 14 are isolated from the front and rears masses 12 and 16 by thin MYLAR (Registered Trade Mark) films 33 which provide good electrical insulation and lubricity to allow' the stack 14 to expand radially when heated, thus avoiding shearing stresses at this surface. The danger of chipping stack 14 is also reduced by MYLAR films 33 which provide highly localized compliances which are negligible to the overall transducer characteristic. vibrations of the mass corresponding to sonar signals, and a separate compliant mass interposed between said rigid mass and said liquid medium the compliance of which is chosen to extend the available bandwidth of frequency response of the transducer by tuning the acoustic impedance. The device can be housed in a sea chest built in the hull of a ship and may include a resonant cavity between the outer surface of the compliant mass and the outer wall of the sea chest. The compliant mass may be tuned in a manner described below to extend the frequency response of the transducer and to lessen the effects of load impedance variation.

Piezoelectric stack 14 in the illustrated embodiment in Figure 1 is an assembly ofPZT 4 ceramic rings 32. Rings 32 are arranged in alternating polarity and are connected electrically and mechanically by nickel grids embedded in an epoxy boding agent. The ends of stack 14 are isolated from the front and rears masses 12 and 16 by thin MYLAR (Registered Trade Mark) films 33 which provide good electrical insulation and lubricity to allow' the stack 14 to expand radially when heated, thus avoiding shearing stresses at this surface. The danger of chipping stack 14 is also reduced by MYLAR films 33 which provide highly localized compliances which are negligible to the overall transducer characteristic. Stack 14 is mechanically preloaded by tension rod 20, the preload being applied by advancing spherical nut 25 while being measured by metering the electrical charge developed in stack 14 by a ballistic galvanometer. Electrical current is supplied to or extracted from stack 14 by cable 34 which is joined to cable terminal 38 in back mass 16 by water-tight cable clamp 36. Outer conductor 35 of cable 34 is conductively connected to a busbar 37a which interconnects alternate conductive grid interfaces (not shown) between piezoelectric elements 32. Center conductor 39 of cable 34 is conductively connected through busbar 37b to the remaining conductive grid interfaces to complete the parallel electrical connection of the series of piezoelectric elements 32. All of the above connected elements are supported radially and axially in shell 18 by a pair of “O'<5>rings 40 and 42 squeezed between oppositely inclined beveled surfaces of front and back masses 12 and 16. respectively; and matching axially-spaced beveled surfaces of shell 18 This arrangement physically aligns and isolates the front and back masses 12 and 16 from shell 18 and also results in a mechanically floating design which provides shock isolation and prevents the build up of internal stresses. In addition “0” rings 46 and 48 may be employed for further radial support.

To insure a watertight seal, boot 44. having a characteristic impedance close to that of water, is bonded to front compliance 10 and shelf 18. Experiments have shown RTV silicone rubber satisfactory for this application. Added protection against shock damage is provided by a second set of “0” rings 52 and 54 positioned between shell 18 and the sea chest (not shown) in which the transducer is positioned. Rings 52 and 54 are relatively soft and act as vibration mounts, whereas the earlier referred sets 40, 42. 46 and 48 are relatively hard and act as shock isolators.

Referring to Figure 2 the schematic relationship between the various components of the sonar device is shown. Front compliance 10 is represented as springs 50 supporting diaphragm 24 and front mass 12.

In order to explain certain important features of the invention, it is convenient to utilize the known analogy between mechanically vibrating structures and alternating current electrical circuits. In fact, heavy reliance is placed upon this technique in the design of the device. This design uses a mechanical compliance (the electrical analog of which is a capacitance) in series with the radiation load to “tune’<'>the radiation load, which is analogous to increasing the resistive component of the electrical impedance seen by an electrical radiating element. In the electrical impedance analog, the compliance (capacitance) appears in parallel with the radiation load, producing a parallel resonant circuit. The increase in the radiation resistance makes it possible to achieve both a high efficiency and a high bandwidth in an element with a diameter that varies approximately from only 1/8 and 1/4 the w-avelength over its operating frequency range. As an added advantage, it is possible to design the compliance to be more flexible at the centre than at the edge of the piston to vary the velocity (and pressure) distribution across the face of the piston in such a w<r>ay as to give a higher cavitation limitation on power output than for a rigid piston of the same size.

Stack 14 is mechanically preloaded by tension rod 20, the preload being applied by advancing spherical nut 25 while being measured by metering the electrical charge developed in stack 14 by a ballistic galvanometer. Electrical current is supplied to or extracted from stack 14 by cable 34 which is joined to cable terminal 38 in back mass 16 by water-tight cable clamp 36. Outer conductor 35 of cable 34 is conductively connected to a busbar 37a which interconnects alternate conductive grid interfaces (not shown) between piezoelectric elements 32. Center conductor 39 of cable 34 is conductively connected through busbar 37b to the remaining conductive grid interfaces to complete the parallel electrical connection of the series of piezoelectric elements 32. All of the above connected elements are supported radially and axially in shell 18 by a pair of “O'<5>rings 40 and 42 squeezed between oppositely inclined beveled surfaces of front and back masses 12 and 16. respectively; and matching axially-spaced beveled surfaces of shell 18 This arrangement physically aligns and isolates the front and back masses 12 and 16 from shell 18 and also results in a mechanically floating design which provides shock isolation and prevents the build up of internal stresses. In addition “0” rings 46 and 48 may be employed for further radial support.

In order to explain certain important features of the invention, it is convenient to utilize the known analogy between mechanically vibrating structures and alternating current electrical circuits. In fact, heavy reliance is placed upon this technique in the design of the device. This design uses a mechanical compliance (the electrical analog of which is a capacitance) in series with the radiation load to “tune’<'>the radiation load, which is analogous to increasing the resistive component of the electrical impedance seen by an electrical radiating element. In the electrical impedance analog, the compliance (capacitance) appears in parallel with the radiation load, producing a parallel resonant circuit. The increase in the radiation resistance makes it possible to achieve both a high efficiency and a high bandwidth in an element with a diameter that varies approximately from only 1/8 and 1/4 the w-avelength over its operating frequency range. As an added advantage, it is possible to design the compliance to be more flexible at the centre than at the edge of the piston to vary the velocity (and pressure) distribution across the face of the piston in such a w<r>ay as to give a higher cavitation limitation on power output than for a rigid piston of the same size. The explanation of how the compliant front mass leads to larger available bandwidth and greater efficiency can best be explained by consideration of the electrical circuit analogs for the acoustic transducer system.

The maximum attainable bandwidth for either a mechanical or electrical system is limited by the "Q<">(which for these purposes may be considered to be the ratio of the imaginary part of this impedance to its real part) of the load impedance; the smaller the “Q”, the larger the available bandwidth.

Known techniques allow' one to calculate the mechanical impedance of a rigid piston in an infinitely rigid flat baffle loaded on one side "which approximates the basic structure of acoustic transducers according to the prior, art. The non-directionality constraint of a λ /4 (approximately) piston face diameter results in a theoretical<!>'Q<‘>’ of approximately 2 for the acoustic load. This size element usually yields a narrow' operating bandwidth in a conventional design.

A study of the nature of this load impedance was made fro a 45 inch diameter element by considering electrical analogs. The acoustic impedance was normalized to a selected value of 5000 ohms. The electrical analogous component values for this load are relatively independent of frequency. A parallel resistance of 21.7ΚΩ and inductance of 0.54 henries characterize the acoustic load with adequate accuracy in the frequency range of interest (vicinity of 3.5kc).

Having found that the load can be represented as an inductor in parallel with a resistor, the simplest measure for achieving the largest bandwidth, is to parallel resonate this load with a "capacitor’'. This will yield, in the neighbourhood of resonance, a resistance which is approximately equal to 21.7ΚΩ over a rather large frequency bandwidth, it should be noted that parallel resonating the radiation load with a capacitor transforms the acoustic load impedance from 4.8ΚΩ to 21.7ΚΩ. This measure improves the efficiency by increasing the impedance level of the acoustic load without reducing the maximum bandwidth capabilities. As previously mentioned, further advantage can be obtained with the compliant front mass by designing it to be more compliant near the centre and less compliant near the periphery By this technique, a sonar device having a front mass of a given diameter can be driven at higher power before its operation becomes adversely affected by cavitation. An understanding of this advantageous feature can be gained by considering a prior art rigid front mass as an effectively rigid piston. The velocity' distribution across the face of such a piston necessarily is uniform. However, the pressure distribution is peaked at the centre of the transducer (which is assumed to be small compared with one wavelength) because the pressure is not as effectively·' concentrated around the edges of the piston as it is at the centre. it therefore becomes apparent that cavitation, which is a function of pressure, commences at the centre of the piston before the average pressure across the piston reaches a critical level. It is therefore desirable to render the pressure distribution across the face of the piston more nearly uniform. This is accomplished according to the present invention by creating a non-uniform velocity' distribution with greater velocity' at the edge of the front mass and a less velocity' at the centre. This is readily accomplished by causing the compliant front mass to be more compliant near its centre, either by decreasing the diaphragm 24 thickness in the centre of the front mass, or

The explanation of how the compliant front mass leads to larger available bandwidth and greater efficiency can best be explained by consideration of the electrical circuit analogs for the acoustic transducer system.

Having found that the load can be represented as an inductor in parallel with a resistor, the simplest measure for achieving the largest bandwidth, is to parallel resonate this load with a "capacitor’'. This will yield, in the neighbourhood of resonance, a resistance which is approximately equal to 21.7ΚΩ over a rather large frequency bandwidth, it should be noted that parallel resonating the radiation load with a capacitor transforms the acoustic load impedance from 4.8ΚΩ to 21.7ΚΩ. This measure improves the efficiency by increasing the impedance level of the acoustic load without reducing the maximum bandwidth capabilities. As previously mentioned, further advantage can be obtained with the compliant front mass by designing it to be more compliant near the centre and less compliant near the periphery By this technique, a sonar device having a front mass of a given diameter can be driven at higher power before its operation becomes adversely affected by cavitation. An understanding of this advantageous feature can be gained by considering a prior art rigid front mass as an effectively rigid piston. The velocity' distribution across the face of such a piston necessarily is uniform. However, the pressure distribution is peaked at the centre of the transducer (which is assumed to be small compared with one wavelength) because the pressure is not as effectively·' concentrated around the edges of the piston as it is at the centre. it therefore becomes apparent that cavitation, which is a function of pressure, commences at the centre of the piston before the average pressure across the piston reaches a critical level. It is therefore desirable to render the pressure distribution across the face of the piston more nearly uniform. This is accomplished according to the present invention by creating a non-uniform velocity' distribution with greater velocity' at the edge of the front mass and a less velocity' at the centre. This is readily accomplished by causing the compliant front mass to be more compliant near its centre, either by decreasing the diaphragm 24 thickness in the centre of the front mass, or increasing the spacing of rings 22, or by any other suitable expedient.

From the foregoing explanation, it wifi be seen that the transducer with compliant front mass produces valuable advantages among which are increased bandwidth, operating efficiency, and cavitation threshold power.

We wish therefore to be limited not by the foregoing description of a preferred embodiment of the invention but. on the contrary, solely by the claims granted to us. increasing the spacing of rings 22, or by any other suitable expedient.

We wish therefore to be limited not by the foregoing description of a preferred embodiment of the invention but. on the contrary, solely by the claims granted to us.

Claims

What we claim is: 1. In a sonar device, the combination including a vibratory rigid mass for coupling electrically produced mechanical energy to a liquid medium and coupling mechanical energy from the liquid medium to a transducer, said transducer being an electro-mechanical transducer and coupled to said rigid mass for generating or sensing vibralions of the mass corresponding to sonar signals, and a separate compliant mass interposed between said rigid mass and said liquid mediu the compliance of which is chosen to extend the available bandwidth of frequency response of the transducer by tuning the acoustic impedance.
2. A sonar device as claimed in claim 1 , in which the compliance of said compliant mass is selected to tune the transducer to substantially eliminate the reactive component of the impedance of the combined sonar device and liquid medium load.
3. A sonar device as claimed in claim 1 or claim 2. wherein said compliant mass has a small transverse dimension as compared to a wavelength of the vibration frequency of the rigid mass.
4. A sonar device as claimed in any one of claims 1 to 3, wherein the compliance of said compliant mass is nonuniform, being less near the periphery thereof than at the centre thereof.
5. A sonar device as claimed in any one of claims 1 to 4 . wherein said compliant mass includes a metal diaphragm secured to said rigid mass by at least one support projecting from said diaphragm.
6. A sonar device as claimed in claim 5, wherein the diaphragm of said compliant mass is secured to said rigid mass by a plurality of concentric support rings.
7. A sonar device as claimed in any one of the preceding claims, wherein said electro-mechanical transducer is coupled to said vibratory rigid mass by a rod exerting pressure against one end of said transducer through a fastener on said rod having a spherically-shaped bearing surface, there being an intermediate bearing plate having on one side a spherically-shaped surface for engaging said fastener surface and having its opposite side facing toward said end of said eleotro-mechanical transducer.
8. A sonar device as claimed in any one of the preceding claims, wherein said electro-mechanical transducer is secured between said vibrator)' rigid mass and a rigid back mass larger than said vibratory rigid mass.
9. A sonar device as claimed in claim 8, further including a first electrical insulating and lubricative film between said electro-mechanical transducer and said vibratory rigid mass and a second such film between said electro-mechanical transducer and said back mass.
10. A sonar device as claimed in any one of the preceding claims, wherein said electro-mechanical transducer includes a plurality' of piezoelectric elements.
11. A sonar device as claimed in claim 10, wherein said plurality of piezoelectric elements are connected physically in series and electrically in parallel.
12. A sonar device as claimed in any one of the preceding claims, further including a shell within which the assembly of said coupled transducer and masses is housed, said shell having two oppositelyinclined and axially spaced annular bearing bevels paired with two co-operating bearing bevels on said assembly, the bearing surfaces of each pair of bevels being disposed at angle to the axial orientation of said assembly, and bands of compressible material squeezed between the bearing surfaces of both pairs of bevels to position said assembly in said shell both radially and axially. What we claim is: 1. In a sonar device, the combination including a vibratory rigid mass for coupling electrically produced mechanical energy to a liquid medium and coupling mechanical energy from the liquid medium to a transducer, said transducer being an electro-mechanical transducer and coupled to said rigid mass for generating or sensing vibralions of the mass corresponding to sonar signals, and a separate compliant mass interposed between said rigid mass and said liquid mediu the compliance of which is chosen to extend the available bandwidth of frequency response of the transducer by tuning the acoustic impedance. 2. A sonar device as claimed in claim 1 , in which the compliance of said compliant mass is selected to tune the transducer to substantially eliminate the reactive component of the impedance of the combined sonar device and liquid medium load. 3. A sonar device as claimed in claim 1 or claim 2. wherein said compliant mass has a small transverse dimension as compared to a wavelength of the vibration frequency of the rigid mass. 4. A sonar device as claimed in any one of claims 1 to 3, wherein the compliance of said compliant mass is nonuniform, being less near the periphery thereof than at the centre thereof. 5. A sonar device as claimed in any one of claims 1 to 4 . wherein said compliant mass includes a metal diaphragm secured to said rigid mass by at least one support projecting from said diaphragm. 6. A sonar device as claimed in claim 5, wherein the diaphragm of said compliant mass is secured to said rigid mass by a plurality of concentric support rings. 7. A sonar device as claimed in any one of the preceding claims, wherein said electro-mechanical transducer is coupled to said vibratory rigid mass by a rod exerting pressure against one end of said transducer through a fastener on said rod having a spherically-shaped bearing surface, there being an intermediate bearing plate having on one side a spherically-shaped surface for engaging said fastener surface and having its opposite side facing toward said end of said eleotro-mechanical transducer. 8. A sonar device as claimed in any one of the preceding claims, wherein said electro-mechanical transducer is secured between said vibrator)' rigid mass and a rigid back mass larger than said vibratory rigid mass. 9. A sonar device as claimed in claim 8, further including a first electrical insulating and lubricative film between said electro-mechanical transducer and said vibratory rigid mass and a second such film between said electro-mechanical transducer and said back mass. 10. A sonar device as claimed in any one of the preceding claims, wherein said electro-mechanical transducer includes a plurality' of piezoelectric elements. 11. A sonar device as claimed in claim 10, wherein said plurality of piezoelectric elements are connected physically in series and electrically in parallel. 12. A sonar device as claimed in any one of the preceding claims, further including a shell within which the assembly of said coupled transducer and masses is housed, said shell having two oppositelyinclined and axially spaced annular bearing bevels paired with two co-operating bearing bevels on said assembly, the bearing surfaces of each pair of bevels being disposed at angle to the axial orientation of said assembly, and bands of compressible material squeezed between the bearing surfaces of both pairs of bevels to position said assembly in said shell both radially and axially. 13. A sonar device as claimed in claim 12, further including one or more bands of compressible material around said shell, to position said shell radially in a sea chest in which said shell is to be housed. 14. A sonar device substantially as described with reference to the accompanying drawings.
13. A sonar device as claimed in claim 12, further including one or more bands of compressible material around said shell, to position said shell radially in a sea chest in which said shell is to be housed.
14. A sonar device substantially as described with reference to the accompanying drawings.