Abstract:
An electroactive device incorporating the invention includes an electroactive ceramic annular substrate having a pair of opposed planar annular surfaces, a hollowed interior region and a thickness aspect. A first cap having a concave shape that extends into the hollowed interior region includes a rim portion, bounding the hollowed interior region, and joined to a first one of the planar surfaces. A second cap having a concave shape that extends into the hollowed interior region includes a rim portion, bounding the hollowed interior region, and joined to a second one of the planar surfaces. A potential measured across the ceramic substrate enables a field change in the ceramic substrate to be sensed, the field change caused by flexure of the ceramic substrate as a result of a pressure applied to the first and second caps.

Description:
PRIORITY 
     This Application claims priority from U.S. Provisional Application Serial No. 60/096,964, filed Aug. 18, 1998. 
    
    
     GOVERNMENT SUPPORT 
     This invention was made with Government support awarded by the U.S. Department of the Navy. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to electroactive ceramic transducers (piezoelectric, electrostrictive, etc.) and, more particularly, to a metal-ceramic electroactive actuator/sensor exhibiting large positional displacements. 
     BACKGROUND OF THE INVENTION 
     Flextensional transducers were first developed in the 1920s and have been used as underwater transducers since the 1950s. They consist of an active piezoelectric or magnetostrictive drive element and a mechanical shell structure. The shell is used as a mechanical transformer which transforms the high impedance, small extensional motion of the ceramic into low impedance, large flexural motion of the shell. According to the shape of the shell, flextensional transducers are divided into five classes. Flextensional transducers 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. 
     A new type of high performance flextensional transducer called the “moonie,” was developed by an inventor hereof, and is shown in U.S. Pat. No. 4,999,819. The moonie acoustic transducer is of sandwich construction and is particularly useful for the transformation of hydrostatic pressures to electrical signals. A pair of metal plates are positioned to sandwich a piezoelectric element, with each plate having a cavity formed adjacent to the piezoelectric element. The plates are bonded to the piezoelectric element to provide a unitary structure. The cavities provide a stress-transforming capability which amplifies an incoming compressive stress and converts it to a radial extensional stress in the ceramic. 
     U.S. Pat. No. 5,276,657, assigned to the same assignee of this application, describes a ceramic actuator that includes a piezoelectric or electrostrictive ceramic disk with conductive electrodes bonded to its major surfaces. A pair of metal end caps include rims that are bonded to ceramic conductive electrodes, respectively. Each end cap is comprised of a solid metal disk having a cavity formed in one surface. 
     If the ceramic disk is a piezoelectric material, it is poled, during manufacture, across its thickness dimension. If it exhibits an electrostrictive effect or a field-induced antiferroelectric-to-ferroelectric transformation, then it need not be poled. When a potential is applied across the electrodes, the ceramic disk expands in the thickness dimension. At the same time, the ceramic disk contracts in the x and y dimensions causing the end caps to bow outwardly, amplifying the actuation distance created by the contraction of the ceramic disk. 
     To improve the displacements achievable through actuation of the ceramic disk, an inventor hereof (in U.S. Pat. No. 5,729,077) utilized sheet metal caps (convex outward) joined to opposed planar surfaces of the ceramic substrate. In a sensor embodiment, the sheet metal caps were subjected to a displacement by an instrumentality (i.e., pressure), and a resulting change in voltage across the ceramic substrate was sensed. Due to the shape of the sheet metal caps, the transducer was dubbed a “cymbal” transducer. 
     Both the moonie and cymbal transducers use a piezoelectric disk (poled in the thickness direction) sandwiched between two metal end caps. The caps contain a shallow cavity on their inner surface. The presence of the cavities allows the caps to convert and amplify the small radial displacement of the disk into a much larger axial displacement normal to the surface of the caps, which contributes to a much larger acoustic pressure output than would occur in the uncapped ceramic. 
     The cymbal transducer shown in U.S. Pat. No. 5,729,077 is intended for shallow water use when employed as a hydrostatic sensor. If the applied hydrostatic pressure exceeds a certain threshold, the bounding metal caps will deform and collapse, destroying the pressure amplification effect. Accordingly there is need for a cymbal-type transducer that will operate at high depths, without collapsing. 
     SUMMARY OF THE INVENTION 
     An electroactive device incorporating the invention includes an electroactive ceramic annular substrate having a pair of opposed planar annular surfaces, a hollowed interior region and a thickness aspect. A first cap having a concave shape that extends into the hollowed interior region includes a rim portion, bounding the hollowed interior region, and joined to a first one of the planar surfaces. A second cap having a concave shape that extends into the hollowed interior region includes a rim portion, bounding the hollowed interior region, and joined to a second one of the planar surfaces. A potential measured across the ceramic substrate enables a field change in the ceramic substrate to be sensed, the field change caused by flexure of the ceramic substrate as a result of a pressure applied to the first and second caps. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side sectional view of a transducer embodying a first version of the invention. 
     FIG. 2 is a side sectional view of a transducer embodying a second version of the invention. 
     FIG. 3 is a plot of admittance spectrum of a single transducer in accord with the invention and a nine element array. 
     FIG. 4 is a plot of pressure dependence of the effective d h  coefficients for both the prior art cymbal transducer and the transducer of the invention. 
     FIG. 5 a  is a plot of measured and calculated Transmitting Voltage Response (TVR) for a transducer incorporating the invention hereof. 
     FIG. 5 b  is a plot of measured and calculated Free-Field Voltage Sensitivity (FFVS) for a transducer incorporating the invention hereof. 
     FIG. 6 a  is a plot of measured and calculated TVR for a nine element array of transducers incorporating the invention hereof. 
     FIG. 6 b  is a plot of measured and calculated FFVS for a nine element array of transducers incorporating the invention hereof. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a first embodiment of the transducer of the invention includes a piezoelectric or electrostrictive ceramic annulus  10  with conductive electrodes bonded  12  and  14  bonded to opposed surfaces of annulus  10 . A pair of formed sheet metal end caps  16  and  18  take the shape of opposed truncated cones and include rims that are bonded to respective electrodes  12  and  14 . The cones are positioned such that they respectively extend into hollowed out area  20  within ceramic annulus  10 . To prevent electrical contact between end caps  16  and  18 , insulating layers  22  may be placed on the opposed surfaces thereof. Hereafter, the transducer configuration of FIG. 1 will be referred to as the inverted cymbal transducer to distinguish it from the standard cymbal transducer shown in U.S. Pat. No. 5,729,077. 
     Each end cap is preferably produced by a stamping process wherein a metal sheet is both cut and shaped to a “cymbal” form in a stamping action. The metal sheet preferably exhibits a relatively constant thickness dimension that is thin in relation to the overall surface area of the end cap structure. The thickness of the metal sheet is dependent upon the actuation pressure required and will, as a result, be thicker when higher actuation pressures are needed. 
     Ceramic annulus  10  is preferably poled across its thickness dimension (i.e., along the direction shown by arrow  24 ). It is to be understood however, that ceramic annulus  10  can be poled in a radial direction. A sensor  26  is coupled to conductive electrodes  12  and  14  and is adapted to measure a voltage thereacross that enables pressure-induced changes in polarization to be sensed. 
     If the transducer of FIG. 1 is subjected to a pressure wave P, end caps  16  and  18  are caused to move towards each other and as a result create radially inward-directed stresses on ceramic annulus  10 , causing compressive forces thereon. The arrangement of end caps  16  and  18  enables them to withstand substantially higher pressures than the arrangement of the cymbal transducer of U.S. Pat. No. 5,729,077 (which operates by having radially-outward stresses induced into the ceramic substrate). The compression of ceramic annulus  10  causes a reorientation of the polarization dipoles that is sensed by resulting changes induced in the voltage applied by sensor  26 . 
     While not as preferred as the first embodiment, a second embodiment of the invention is shown in FIG. 2, wherein a thin region  30  of ceramic has been left within ceramic annulus  10 . Region  30  prevents the cymbal end caps  16  and  18  from touching each other under extreme pressure conditions. Further, it provides additional stability for ceramic annulus  10  under such conditions. 
     While many ceramics, as diskussed below, are appropriate for application to the cymbal actuator, it is preferred that the ceramic material exhibit high d 31  and d 32  coefficients so as to enable, for a given applied force, the greatest available voltage alterations to be experienced. 
     Piezoelectric transducer materials that are used with the structures shown and described above are based primarily on the lead zirconate titanate (PZT) family including PLZT ((Pb,La) (Zr,Ti) O 3 ). Also PVDF and its copolymer may be used. Electrostrictive ceramic disks utilize lead magnesium niobate (PMN)-based ceramics. Lead titanate-modified PMN (PMN-PT) is preferred. Pb(Sn,Zr,Ti)O 3  ceramics exhibiting antiferroelectric-to-ferroelectric transitions with applied field may also be used. 
     Experimental Procedure 
     A. Fabrication 
     Cymbal-type end caps are produced with a dimension depending on application. Preferred materials for endcaps  16  and  18  are dependent on application and can be chosen from metal, polymer, polymer-based composites and glass-based materials. If low level pressure changes are to be sensed, aluminum or copper-based metals may be used. If high pressures are to be encountered, a stiffer metal such as tungsten is preferred. Other metals for end caps  16  and  18  can be brass, bronze, kovar, zirconium, etc. 
     If end caps  16  and  18  are comprised of metal, to prevent shorting thereof under high pressure conditions, the inner surfaces of the end caps may be coated with an insulating layer  22  (e.g., a polymer). 
     Epoxy is used to bond the inverted end caps to a piezoelectric or electrostrictive ring  10 , which has the same outer diameter as the end caps. 
     Single transducer elements can be mounted into a multi-element planar or conformal array to further improve the performance. They are connected electrically in parallel in the array. 
     EXAMPLE 1 
     Brass caps were punched from a metal foil of 0.25 mm thickness and shaped using a special die. The shaped caps had a diameter of 12.7 mm. The cavity diameter was 9.0 mm at the bottom and 3.2 mm at the top. The cavity depth was 0.2 mm. The caps were then bonded to a piezoelectric ceramic ring (PKI 552, Piezokinetics Inc., Bellefonte, Pa.) having a thickness of 1 mm, outer diameter of 12.7 mm, and inner diameter of 9.0 mm. The PZT rings were poled in the thickness direction. The bonding material was an Emerson and Cuming insulating epoxy. A ratio of three parts 45 LV epoxy resin to one part 15LV hardener was used. The thickness of the epoxy bonding layer was approximately 20 mm. The entire assembly was kept under uniaxial stress in a special die for 24 hours at room temperature to allow the epoxy time to cure. 
     EXAMPLE 2 
     Single element inverted cymbal transducers were incorporated into a 9-element square planar array. The nine transducers were sandwiched between two copper-clad PC boards each 1.5 mm thick. Holes 11 mm in diameter were drilled equidistant from one another through the boards. Each element had a center-to-center spacing of 13.5 mm. Plastic posts 1.5 mm thick were used to maintain a uniform distance between the upper and lower panels, which were screwed together tightly to keep the transducers in place. The elements were electrically in parallel because of the PC board. After assembly, the array had a dimension of 5.5 cm×5.5 cm and a thickness of 0.4 cm. Its weight was about 30 g. 
     Underwater calibration tests of single inverted cymbals and arrays were performed at the Applied Research Laboratory at Penn State University. The tank measured 5.5 m in depth, 5.3 m in width and 7.9 m in length. A pure tone sinusoidal pulse signal of 2 msec duration was applied to the test transducer and its acoustic output was monitored with a standard F33 hydrophone. The test transducer and the standard were positioned at a depth of 2.74 m and separated by a distance of 3.16 m. The parameters measured for these underwater devices were resonance frequency, mechanical Q, Transmitting Voltage Response (TVR), Free-Field Voltage Sensitivity (FFVS), and beam pattern. 
     B. Finite Element Analysis 
     The Finite Element Analysis code ATILA was used in the evaluation and in further improvement of the inverted cymbal transducer designs. ATILA was developed at the Acoustics Department at Institut Superieur d&#39;Electronique du Nord (ISEN) to model underwater transducers. A static analysis was performed to provide information concerning pre-stresses and the transducer&#39;s response under hydrostatic pressure. Modal analysis was carried out to determine the vibration modes, their resonance and antiresonance frequencies, and associated coupling factors. 
     Through harmonic analysis, the in-air and in-water impedance and displacement field can be computed as a function of frequency, together with the Transmitting Voltage Response, Free Field Voltage Sensitivity and the directivity patterns. ATILA was used to estimate the TVR and FFVS of several cymbal transducer designs. 
     Results and Diskussion 
     It has previously been shown for non-inverted cymbal transducers that the flextensional resonance frequency can be tailored by changing the cap material and geometry. The inverted cymbal utilizes the same amplification mechanism as the standard cymbal, but its flextensional resonance frequency is slightly lower than its standard counterpart as is shown in Table 1 below, and it is also easily tailored. Other in-air characteristics of inverted cymbal transducers are also shown and compared with standard cymbal transducers in Table 1. The effective d 33  and capacitance of inverted cymbal transducers are smaller than those of standard cymbal transducers due to less piezoelectric material used in the device. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Typical in-air characteristics of standard and 
               
               
                 inverted cymbal transducers 
               
             
          
           
               
                   
                 standard cymbal 
                 inverted cymbal 
               
               
                   
                   
               
             
          
           
               
                   
                 flextensional frequency 
                 24 
                 kHz 
                 23 
                 kHz 
               
               
                   
                 coupling coefficient 
                 20% 
                   
                 15% 
               
               
                   
                 effective d 33   
                 12000 
                 pC/N 
                 4900 
                 pC/N 
               
               
                   
                 capacitance 
                 2.9 
                 nF 
                 1.2 
                 nF 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 3 shows a representative admittance spectrum of an inverted cymbal transducer. The first peak, at 22 kHz, is associated with the flextensional mode of the composite transducer. However, for illustrative purposes without limiting this invention, a frequency range of 1 kHz to 100 kHz can be obtained for PZT5 based rings with outer radius from one inch to a quarter inch. The higher resonance peaks come from the coupling between the radial mode of the ceramic ring and high order flextensional modes. 
     FIG. 3 also shows the admittance spectrum of a nine-element inverted cymbal array. Its resonance is damped and broadened due to element interactions and the fact that the individual elements have slightly different frequencies. 
     FIG. 4 shows the pressure dependence of the effective d h  of an inverted cymbal transducer and standard cymbal transducers. Effective d h  for bulk PZT as a function of pressure is also shown and compared. Both standard and inverted cymbal transducers have effective d h  values over fifty times larger than that of bulk piezoelectric material. The standard cymbal transducer can withstand 2 MPa, but when the pressure increases over 2 MPa, it fails catastrophically due to the permanent deformation of the end caps. 
     In the case of the inverted cymbal transducer, it has a slightly lower d h  than the standard cymbal transducer but it survives up to 6 MPa without significant degradation in its properties. It is anticipated that this design may withstand much greater pressures, depending upon the materials and dimensions used. The inverted cymbal exhibits much improved pressure tolerance under hydrostatic loading. Under some higher pressures, the inverted cymbal may fail due to a shorting of the two end caps, but such can be avoided by applying a thin insulating layer between the end caps. 
     The TVR and FFVS of a single inverted cymbal transducer are shown in FIGS. 5 a  and  5   b . Like the standard cymbal, the inverted cymbal shows narrow band characteristics with a mechanical Q around 9. The TVR and FFVS calculated with the ATILA code are also shown for comparison with the experimental results. Fairly good agreement is obtained indicating that ATILA adequately models the cymbal behavior in water. The small diskrepancies between the calculated and the experimental results may arise from imperfections in the ring, from stresses in the metal end caps, or from the fact the contribution of the epoxy bonding layer and the fixtures used to hold the sample were not taken into account in modeling. 
     Individual inverted cymbals may not be powerful enough, nor sufficiently broadband for a desired use. It may thus be necessary to incorporate a plurality of transducers into arrays to improve underwater performance. The TVR and FFVS of a nine-element array of inverted cymbal transducers are illustrated in FIG. 6 a  and  6   b . The array shows broader band characteristics in both TVR and FFVS. It has a TVR of around 125 dB re 1 mPa/V @ 1 m and is rather flat over the frequency range from 20-50 kHz. Its FFVS is not as flat as the TVR, but a wider bandwidth can be obtained by incorporating more cymbal transducers into the array and by precisely controlling and arranging the resonance frequencies and position of the array transducers. 
     Inverted cymbal transducers demonstrate a much improved pressure tolerance over standard cymbals (i.e., as shown in U.S. Pat. No. 5,729,077) and can go much deeper underwater, extending the range of operation for cymbal transducers. The simple fabrication process makes it easy to mass-produce these transducers and greatly reduces their cost. Their thin profile, light weight and low cost makes it possible to assemble a very large array of inverted cymbal transducers, either in a planar or conformal array. It is expected that the array can be further optimized to work as well as a standard Tonpilz array when a high source level is bit required. 
     The inverted cymbal transducer may be a good transducer for conformal arrays where drastically reducing the vehicle&#39;s weight and cost is important. Hard, PZT4, instead of soft, PZT5, piezoelectric drive elements can be used to achieve a high source level. 
     The low-cost inverted cymbal array can also be used as a receiver for acoustic imaging. The most useful frequency range for underwater imaging appears to be the 5-50 kHz band where the inverted cymbal transducer excels. When fully optimized, an array of inverted cymbal hydrophones may serve as a multi-element receiver capable of functioning as an acoustic camera. Phased arrays of a thousand or more hydrophones are feasible using these small low-cost, mass-produced components with high hydrostatic piezoelectric coefficients. The improved pressure performance allows the inverted cymbal transducer array to work both in shallow water and in deep submergence. 
     To obtain the desired directivity and impedance match, the transducers are mounted in arrays. The single transducers in an array are connected electrically using fine electrical wires. They can be connected in parallel or serially, or individually for beamforming or velocity control. Conductive epoxy is preferably used to bond the wires to the flanges of the transducers. The skeleton of transducers, along with their electrical connections can be pre-bent to a desired geometry, either spherical, cylindrical etc., and then potted in polymer. 
     After electrical connection, the assembled transducers are potted in polyurethane or another polymer. The introduction of the polymer coating, along with the thin profile and small size of the transducers, makes it possible to build flexible arrays. The arrays can be linear, planar, or curved, depending on the applications. 
     In summary, the inverted cymbal transducer has a much higher pressure tolerance than the standard cymbal transducer. Its thin profile and small size make it easy to mount in a planar or curved array. When incorporated into a planar nine-element array, it shows medium output power and broad band characteristics combined with a small radiating area and light weight, making it a promising candidate as an underwater sound projector and receiver. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.