Patent Application: US-91392106-A

Abstract:
an air - coupled transducer includes a ultrasonic transducer body having a radiation end with a backing fixture at the radiation end . there is a flexible backplate conformingly fit to the backing fixture and a thin membrane conformingly fit to the flexible backplate . in one embodiment , the backing fixture is spherically curved and the flexible backplate is spherically curved . the flexible backplate is preferably patterned with pits or depressions .

Description:
the present invention includes a number of aspects all of which have broad and far - reaching application . although specific embodiments are described herein , the present invention is not to be limited to these specific embodiments . one aspect of the invention relates to the use of a flexible backplate in an air - coupled ultrasonic transducer . the flexible backplate allows it to conform to a number of different geometries , including , but not limited to a spherical shape . turning now to the drawings in which similar reference characters denote similar elements through the several views . illustrated in fig1 - 10 is the combination of various views and in - use configurations of the transducer . the transducer being described with particularity herein . fig1 is an isometric view of the transducer . the process of generating and receiving ultrasound is very similar to the working principles of a condenser microphone . as shown in fig1 , an air coupled transducer 10 has a metallized polymer film 16 is suspended above a micromachined copper / polyimide backplate 12 . for ultrasound generation , a dc bias v dc ( t ) is superimposed upon a transient voltage signal v ac ( t ) from a signal source . the superimposed voltage is applied between the backplate 12 and the grounded metalized surface 12 of the metalized polymer film 16 . as a result , electrostatic force is generated and attracts the film toward the bottom of the pits . based on the frequency content of the drive signal , the metalized polymer film 16 vibrates with certain amplitude . if the metalized film 16 vibrates near its mechanical resonance frequency , the maximum displacement will be generated , leading to a large ultrasonic signal . conversely , receiving the vibrating sound signals is achieved using the same transducer as a reciprocal device . in the receiver mode , the received signals are modulated into minute capacitance changes , owing to the displacements of the film . the capacitance is converted into electrical signals by the transducer . for both a transmitter and receiver , a dc bias voltage 42 is always applied to charge the polymer film 16 so that the transducer is sensitive to very small changes in charge . the dc voltage 42 is highly determinative in the performance of a capacitive air - coupled transducer . the transducer of the present invention is referred to generally as 10 . fig1 is an isometric view of the transducer . in fig1 , the component parts of the transducer 10 are shown . the transducer has a backplate 12 , a backplate fixture 14 , a metalized polymer film 16 , a bottom case 18 , an outer case 20 , an insulator 22 , and a top cover 24 . the backplate 12 is conformably deformed and attached to the backplate fixture 14 . the backplate fixture 14 is preferably spherical in shape so that the transducer functions like an ideal spherically focused piston radiator . the backplate fixture 14 is electrically connected to the center pin of a subminiature version a ( sma ) connector , ( not shown ). the shield contact of the sma connector is connected to the bottom case 18 . an insulator 22 is added between the backplate fixture 14 and the outer case 20 to isolate the electrical connection between the backplate 12 and the ground . it is preferred that all the bottom case 18 , outer case 20 , backplate fixture 14 , backplate 12 and top cover 24 be constructed of aluminum . a one - sided metallized polymer film 16 is positioned between the backplate 12 and top cover 24 , with the metallized side facing the top cover 24 . one type of film suitable for use is a commercially available polyethylene terephthalate ( pet ) polymer ( mylar ) film with a 200 nm deposited aluminum coating . in the preferred embodiment , the metalized polymer film 16 is placed over the entire backplate 12 , extending about 20 % beyond the circumference of the pit - etched area . fig2 a - 2d illustrate the backplate fabrication process . flexible copper ( cu )/ polyimide ( pi ) is one type of material having optimum attributes for fabricating a backplate 12 . the backplate 12 has a polyimide substrate 28 with a copper layer 26 . the cu / pi backplate 12 is a non - planar backplate 12 with curvature in two dimensions forming a spherically focused capacitive air - coupled transducer 10 . the fabrication of a cu / pi backplate 12 begins with cleaning the surface 27 , as shown in fig2 a . in fig2 b , the circular patterns 29 are defined on the surface 27 of the copper layer 26 in a square grid by photoresist 30 using lithography and wet etching processes . after etching , the pits 32 form a well - defined bowl shape on the surface 27 of the backplate 12 . one type of etchant used is a copper etchant ( ferric chloride etchant ) and the resulting pits 32 , as best shown in fig3 b , are approximately 42 μm in diameter 33 with a 9 - μm pit depth 34 . fig3 a and 3b show scanning electron microscope ( sem ) images of a cu / pi backplate 12 after the wet etching process , as shown in fig2 b . the patterns shown in fig3 a exhibit an 80 - μm center - to - center spacing 35 between adjacent pits 32 . the center - to - center spacing 35 corresponds to the distance between two adjacent pits 32 on a square grid , shown in fig3 a . the pit depth 34 represents the maximum distance from an unetched surface 31 to the bottom of a pit 34 . fig4 is a photograph of the spherically focused transducer . the cu / polyimide backplate 12 is attached to conform to the spherically curved backplate fixture 14 , whose preferred radius 36 is 5 - mm having a focal length of 25 . 4 mm ( 1 inch ). the metallized polymer film 16 is positioned on the spherically deformed backplate 12 to prevent wrinkles on the film 16 . the film 16 is conformed to the spherical backplate 12 , to prevent wrinkles . one method of preventing wrinkles in the film 16 is to stretch the film 16 over a stainless steel ball bearing where the radius of the ball bearing is approximately the same as the geometric focal length of a spherically deformed backplate 12 . the stretching of the film 16 is done using a panel member having an aperture with the same radius of the bearing . the edge of the aperture is used as a boundary to ; apply uniform hoop stress while stretching the film . to assist the film 16 in conforming to the spherical backplate 12 , the ball bearing is heated to 35 ˜ 40 ° c . holding the film 16 in the stretched position for 20 ˜ 30 seconds creates a half sphere , for fitting the spherical backplate 12 . the film 16 is attached to the backplate 12 by applying 20 ˜ 30 v dc biased to the transducer . this same process can be used to stretch and attach such films as 12 . 5 μm thick kapton polyimide film . fig5 is a graph illustration of amplitude response and corresponding frequency spectrum for the transducer . in fig5 , tests of the focused capacitive spherical transducer 10 show an ideal spherically focused piston transducer &# 39 ; s beam diameter . the sound pressure fields are measured by scanning a 200 - μm quasi - point receiver and recording its output voltage versus position . the focused capacitive spherical transducer 10 is driven at 250 v p - p using a broadband sweep signal . in particular , fig5 a shows a typical response of the focused transducer at the focal zone ( z ) of ˜ 50 . 8 mm when the quasi point receiver is applied at 250 dc v . fig5 b shows a corresponding frequency spectrum . the frequency spectrum is centered at 805 khz with 6 - db points and a bandwidth of approximately 800 khz , measured at a lower and upper frequency of 400 and 1200 khz , respectively . fig6 a - e are illustrations of measured sound pressure fields for the transducer . shown in fig6 a and 6b are pressure fields for broadband excitation in the x - z plane at y = 0 over an 8 - mm × 35 - mm area with a spatial resolution of 100 μm , starting at z = 15 mm . the origin of the coordinate system is located at the center 38 of the concave face of the spherical backplate 12 in the transducer 10 , as best illustrated in fig4 . darker regions near the center represent stronger sound pressure fields with the adjacent lying lighter regions representing a weakened sound pressure field . the pressure field 40 shows a starting and ending of the focal zone with − 6 db points located at 19 . 1 mm and 32 . 2 mm . fig6 c - e shows the sound pressure fields 40 in the cross - sectional regions ( x - y plane ) at the focal zone over 6 mm × 6 mm area , respectively . the beam diameter is approximately 2 . 1 mm at z = 18 mm , z = 25 mm and z = 35 mm respectively for fig6 c - e when the transducer is driven by a broadband signal . fig6 c shows the maximum amplitude for the measured sound field occurring at ˜ z = 25 mm . the maximum amplitude fades in signal strength as the focal length approaches ˜ z = 18 mm and z = 35 mm , as best illustrated by fig6 c and 6e , respectively . fig7 a - 7b are a graph illustration of the cross sections of the focal region of the measured and theoretical sound pressure fields for the transducer . in these graphs , the liner scan measurements and theoretical predictions are shown along the x - axis with a spatial resolution of 0 . 1 mm for 800 khz tone burst using broadband excitation . the performance of the transducer 10 is compared to the theoretical prediction using a modified rayleigh - sommelfield model 16 . the modified rayleigh - sommerfield model shows that the pressure , p , in a fluid for a circular piston transducer of radius , a , having uniform piston velocity ν 0 is : p ( r 0 , y , ω )=− iωρν 0 a 2 [ exp ( ikr 0 )/ r 0 ][ j 1 ( ka sin θ )/ ka sin θ ] ( 1 ) where ρ is the medium density , r 0 is the focal length and k is the wavenumber . the measurements are obtained at the focal zone for each excitation signal found in the x - z plane scan . the full width at half maximum ( fwhm ) value is measured approximately 1 . 38 mm and its theoretical predication is 1 . 37 mm . the transducer 10 is driven by a broadband excitation signal , the measured fwhm value is approximately 2 . 7 mm at the focal point , z = 24 . 9 mm . thus , beam diameters of the focused transducer are 1 . 38 mm and 2 . 7 mm using an 800 khz tone burst and broadband excitation . and , the transducer 10 exhibits sound pressures nearly identical to the ideal spherically focused piston transducer &# 39 ; s beam diameter . fig8 a - b are illustrations of measured signal intensity and theoretical predications for the transducer . fig8 a and 8b show a comparison between the airy disks of measured signal intensity and theoretical predications for an 800 khz tone burst . fig8 a illustrates the measured signal intensity , where the adjacent rings extending out from the center or origin represent a decease in signal intensity . fig8 b illustrates the theoretical predications calculated using a modified rayleigh - sommerfield model . comparison of fig8 a with fig8 b reveals little to no aberrations between the two plots . thus , inducer 10 of the present invention is proof of a simple , yet reliable , fabrication method to produce the natively focused micromachined capacitive air - coupled spherical ultrasonic transducer . by selecting , producing and integrating a flexible substrate with a curved backplate 12 fabrication into the transducer 10 solves the most difficult and unsolved problem plaquing transducers , especially air sound generation and detection . moreover , because the transducer 10 is natively focused , the transducer 10 eliminates the need for auxiliary devices , such as acoustic mirrors , to focus air - coupled acoustic beams , and still behaves identical to an ideal spherically focused piston radiator . the transducer 10 exhibits higher signal amplitude , wider bandwidth and better spatial resolution and significantly improves air - coupled ultrasonic nondestructive evaluation and imaging applications . fig9 a - e are the isometric views of the bottom case , backplate fixture , outer case , insulator , and top cover for the transducer according to one embodiment of the present invention . fig9 a - e illustrate the design for a 10 mm spherically focused capacitive air - coupled transducer having a 25 . 4 mm geometric focal length and active angular sensitivity of ± 15 ° with respect to the normal axis and according to one embodiment . the 10 mm diameter transducer is applicable to almost all phase - match angles for common engineering materials , including metals , plastics , carbon and glass fiber composites . in fig9 a the bottom case 18 is illustrated , as best shown in fig1 . the bottom case 18 is preferably manufactured by machining out of aluminum stock . dimensions , clearances , feature parts and tolerances for manufacturing are noted in each engineering drawing . fig9 b is the backplate fixture 14 , as best shown in fig1 . the backplate fixture 14 is also preferably manufactured from aluminum stock , having a high degree of machinability . the outer case 20 is shown in fig9 c . the outer case 20 is machined from aluminum stock . the outer case 20 is also shown in fig1 . fig9 d is an engineering drawing for the insulator . the insulator is preferably constructed of delrin and is also shown in fig1 . fig9 e shows the top cover 24 of the insulator 10 device . the top cover 24 is shown in fig1 , as well . the top cover 24 is preferably manufactured from aluminum stock and has an aperture with a radius 36 of 1 inch . fig1 a - e are the isometric views of the bottom case , backplate fixture , outer case , insulator , and top cover for the transducer according to one embodiment of the present invention . fig1 a - e illustrate the design for a 50 mm spherically focused capacitive air - coupled transducer having a 50 . 8 mm geometric focal length and active angular sensitivity of ± 33 ° with respect to the normal axis and , according to another embodiment . the 50 mm diameter transducer is applicable to almost all phase - match angles for common engineering materials , including metals , plastics , carbon and glass fiber composites . in fig1 a the bottom case 18 is illustrated , as best shown in fig1 . the bottom case 18 is preferably manufactured by machining out of aluminum stock . dimensions , clearances , feature parts and tolerances for manufacturing are noted in each engineering drawing . fig1 b is the backplate fixture 14 , as best shown in fig1 . the backplate fixture 14 is also preferably manufactured from aluminum stock , having a high degree of machinability . the outer case 20 is shown in fig1 c . the outer case 20 is machined from aluminum stock . the outer case 20 is also shown in fig1 . fig1 d is an engineering drawing for the insulator 22 . the insulator is preferably constructed of delrin and is also shown in fig1 . fig1 e shows the top cover 24 of the transducer 10 device . the top cover 24 is shown in fig1 , as well . the top cover 24 is preferably manufactured from aluminum stock and has an aperture with a radius 36 of 2 inches . various factors determine the performance characteristics of a capacitive air - coupled transducer . overall , both surface geometries of a backplate 12 and transducer &# 39 ; s operating conditions strongly affect performance characteristics . these include pit diameter 33 , center - to - center spacing 35 , pit depth 34 , bias voltage , and nature of a metalized polymer film 16 . based on the calibration results , the sensitivity of the capacitive transducer 10 is improved by utilizing a smaller pit diameter 33 , wider center - to - center spacing 35 , and increased pit depths 34 on the backplate geometry 12 , as best illustrated in fig3 a - b . in particular , the strongest sound pressure amplitude in air is measured when center - to - center spacing 35 is equal to “ 4 *( pit diameter )”. this same result holds true for both 40 μm and 80 μm pit diameters 33 . cross couplings between the pits 32 is a strong consideration if the center to center spacing 35 of the pits 32 is too close together . for example , the sensitivity of a backplate 12 with 40 μm pit diameter 33 and 60 μm center - to - center spacing 35 has a 30 % lower sensitivity than a backplate 12 with 40 μm pit diameter 33 and 160 μm center - to - center spacing 35 . this finding is evidence of the cross coupling effect when 80 μm pit diameter 33 is employed in the backplate design 12 . comparison between 120 μm and 320 μm center - to - center spacings 35 shows that the sensitivity decreases approximately 90 % when the center - to - center spacing 35 changes from 320 μm to 120 μm . accordingly , given a pit diameter 33 , the sensitivity of a capacitive transducer 10 is maximized in part by employing an optimal center - to - center spacing 35 which is 4 *( pit diameter ). cross coupling effects increase as pit diameter 33 is increased . when a backplate 12 has deeper pits 32 rather than shallower pits 32 , the sensitivity is much higher than employing shallower pits 32 on a backplate 12 design . when pit depth 34 varies from 5 . 5 μm to 11 . 7 μm , the sensitivity increases approximately two fold . thus , there exists an optimal point where the sensitivity is maximized . sensitivity is also increased by either applying high dc bias to the transducer 10 or utilizing a thinner metalized polymer film 16 . in particular , the sensitivity of a capacitive air - coupled transducer 10 increases as the applied bias voltage increases , where the applied bias is higher than the critical voltage and lower than the breakdown voltage of the metallized polymer film 16 . for example , the 6 μm thick mylar film 16 with a 20 nm thick aluminum layer on one side has a critical voltage around 100 v . the critical voltage is highly dependent on the nature of a metalized polymer film 16 , such as thickness and chemical structure of the polymer layer . a thinner metallized polymer film 16 improves the sensitivity of a capacitive transducer 10 . the resulting effect of thinning the metallized polymer film 16 correlates with the resulting effect of applying high dc bias to the transducers 10 . the correlation exists because the polymer film 16 over the pits 32 is vibrated by a high electric field , which is approximately inversely proportional to the thickness of the polymer layer 16 . at the same dc bias , a polymer film 16 with higher dielectric constant generates better sensitivity than a polymer film 16 with lower dielectric constant . for example , 0 . 5 mil kapton film 16 exhibits sensitivities 10 % higher than the 0 . 5 mil pet film 16 . similarly , the 0 . 3 mil kapton film 16 shows 20 % higher sensitivity than a 0 . 25 mil pet &# 39 ; s film 16 . the resulting sensitivities related to film thickness incrementally reduces the electrostatic force by 3 . 3 % while the difference in dielectric constant exhibits a 25 % increase in electrostatic force . thus , a thinner polymer layer with high dielectric constant generates higher sensitivity . in addition to these previously noted advantages , the present invention using the mylar film 16 can be polarized with a high voltage , and when this external voltage is removed a permanent bias voltage remains on the film 16 . this residual bias eliminates the need for an external biasing source during operation of the transducer 10 and allows the transducer 10 to be applied in just the same manner , from an electronic standpoint , as a conventional piezoelectric transducer 12 . this development makes the capacitive transducer easier and more convenient to use . the frequency characteristics of the capacitive air - coupled transducer 10 are controlled in part by the surface geometries of a backplate 12 and transducer &# 39 ; s operating conditions , as previously stated . moreover , the resonant frequency of a capacitive air - coupled transducer 10 significantly increases when a small pit diameter 33 , shallow pit 34 , high bias voltage or thin metalized polymer film 16 are used in the backplate 12 design . other considerations , such as center - to - center spacing 35 of the pits 32 , are not as influential to the resonant frequency as much as other factors . variation of center - to - center spacing 35 from 80 μm to 200 μm for a backplate 12 with 40 μm pit diameter 33 , the variation in the resonant frequency is approximately ± 23 . 7 khz . variations of approximately ± 12 . 7 khz result from use of the backplate 12 employing 80 μm pit diameters 33 and varied center - to - center spacing 35 from 120 μm to 400 μm . a backplate 12 with shallow pit depths 34 , exhibited higher resonant frequencies , such that the resonant frequency increases linearly as pit depth 34 decreases . more notably , for pit depths 34 less than 15 μm , the resonant frequency of a capacitive air - coupled transducer 10 is inversely increasing with respect to pit depth 34 . similar to center - to - center spacing 35 , bias voltage does not significantly change the resonant frequency . the lowest resonant frequency results when the applied bias voltage is at the critical voltage , 100 v . except for bias voltages around 100 v , other voltages in the range between 0 and 300 v produce a constant resonant frequency . at 0 v , the resonant frequency is approximately the same as at 300 v . utilizing a thin metalized polymer film 16 , the resonant frequency of a capacitive air - coupled transducer 10 is increased . particularly , using a 0 . 25 mil pet film 16 instead of a 0 . 5 mil pet film 16 results in 40 khz increase in the resonant frequency . further , increases in resonant frequency are increased for a kapton film 16 . the resonant frequency of a 0 . 3 mil kapton film is 180 khz higher than the 0 . 5 mil kapton film 16 . similar to the resulting resonant frequency , the bandwidth of a capacitive air - coupled transducer 10 increases with larger pit diameters 33 , shallower pits 34 , high bias voltage , and thinner polymer films 16 . however , center - to - center spacing 35 does not significantly change the bandwidth . as pit depth 34 increases , the bandwidth significantly decreases . when pit depth 34 is 5 . 5 μm on a copper / polyimide backplate 12 , the bandwidth increases approximately 300 khz as compared to the 11 . 7 μm deep pits 34 used in the backplate 12 design . in addition to shallow pits 32 , the pits with large diameter 33 also increases the bandwidth . the order of the variations is not so significant to be considered as minor variations in design . in fact , employing the thin metalized polymer film 16 attains a wider bandwidth . moreover , a polymer film 16 with a high dielectric constant exhibits a narrower bandwidth than a polymer film 16 with low dielectric constant . the present invention contemplates numerous other options in the design and use of air - coupled non - contact sensors . it is to be understood , for example , that the air - coupled non - contact sensor need not be spherical but can be of other shapes , including conical , cylindrical , or otherwise shaped depending upon the particular application . it is also to be understood that the flexible backplate can made of other materials , including , but not limited to , the types of materials used in making flexible circuit boards . also , the present invention contemplates variations in the type of polymer membrane used . although it is preferred that the membrane be metallized or otherwise have a conductive layer , the membrane need not . also , the present invention contemplates that an integral thin membrane can be used over the flexible backplate . where an integral thin membrane is used , there is no need to apply a polymer film such as mylar and the integral thin membrane would not be susceptible to dust particles or damage . these and other options , variations , are all within the spirit and scope of the invention . all the references as listed below are herein incorporated by reference in their entirety . r . stoessel , n . krohn , k . pflediderer , and g . busse , air - 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