Abstract:
A transducer apparatus is disclosed including a spool member having a body portion and first and second elevated regions formed on the body portion. A piezoelectric film such as a PVDF film surrounds the spool member and is spaced apart from the body portion of the spool member by an elevation of the elevated region, thereby forming a predetermined gap between the piezoelectric film and the body portion of the spool member. The predetermined gap is at least 0.1 mm and enables a predetermined resonance frequency in the piezoelectric film to control the resonance frequency of the transducer. Opposite lateral ends of the piezoelectric film are secured together such that secured ends of the piezoelectric film have substantially the same resonance frequency as a remainder of the piezoelectric film.

Description:
RELATED APPLICATIONS 
     This application is related to co-pending provisional patent application serial number 60/080,101 filed on Mar. 31, 1998 entitled OMNI-DIRECTIONAL ULTRASONIC TRANSDUCER APPARATUS and to co-pending commonly assigned patent application Ser. No. 09/281,247, filed on Mar. 30, 1999, entitled OMNI-DIRECTIONAL ULTRASONIC TRANSDUCER APPARATUS AND STAKING METHOD. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of transducers. More particularly, the present invention relates to an omni-directional ultrasonic transducer apparatus. 
     DESCRIPTION OF RELATED ART 
     In the environment of transducers, it is known that an ultrasonic transducer may be formed with either a linear or curved film incorporated therein. Each of the types of film is described in the following. 
     Referring first to FIG. 4, a linear polymer piezoelectric film  50  is shown. When an AC voltage is applied to electrodes  52  on surfaces of the film  50 , the film length in the molecular chain direction shrinks or expands. In other words, the PVDF (polymer piezoelectric material) is stretched during the process, and molecular chains are aligned in parallel. This is due to excitation in the linear direction. 
     Alternatively, a cylindrical piezoelectric film  54  is shown in FIG. 5 whereby the stretched axis is wrapped around a cylinder (not shown). Here, when an AC voltage is applied to electrodes  56  on surfaces of the cylindrical film  54 , the length vibration is converted to radial vibration. This is the principle of PVDF tweeter as disclosed in “Electroacoustic Transducers with Piezoelectric High Polymer Films”, J. Audio Eng. Soc. Vol. 23, No.1, pp. 21-26, (1975) by M. Tamura et al. The high polymer element in the piezoelectric film is a poly-vinylidene fluoride) (PVDF) in film form. 
     The cylindrical PVDF vibrator has a certain mass and stiffness for radial expansion or shrinkage, and this mass and stiffness enable a resonance whose frequency is 
     
       
         f 0 =(½ pR)ÖY/r where R is the radius in meters, Y is Young&#39;s modulus (N/m 2 ), and r is density (Kg/m 3 ).  (1) 
       
     
     This equation is shown in a paper by A. S. Fiorillo entitled “Design and Characterization of a PVDF Ultrasonic Range Sensor”, IEEE Trans. Ultrasonics, Ferroelectrics and Frequency Control”, Vol. 39, No. 6, pp. 688-692 (1992), which is for semi-circularly curved film with both ends clamped, but it has the same resonance frequency as a cylinder. 
     In IEEE paper, the cylindrical PVDF film is mounted on a smooth-surfaced spool. The radius of the spool determines the resonance frequency through equation (1). The PVDF film can be directly wrapped around a cylindrical surface of the spool with almost no gap between the surface of the film and the surface of the spool. Even though the appearance is of no gap, the film is actually supported on the spool by many tiny points of surface roughness. It has been determined that most of the supported area has gaps of from 2-20 microns between the contacts of the many tiny points of surface roughness. Since actual vibration amplitudes are about 1 micron peak to peak for a 150 Vpp drive, there are enough spaces to vibrate and actually permit the device to work. 
     However, in the known application of a film to a spool as described, two problems have been discovered by the inventors of the instant application. First, it has been discovered that with the “gap-free” wrapping attempted in the known art, there is a problem of uncontrollable resonance frequency. Secondly, in the “gap-free” wrapping, there is a reduced vibration of the PVDF film. 
     In other words, since the air found in the 2˜20 micron region (the “back air space”) has a stiffness and spring effect, this also increases the effective stiffness of the PVDF film and in turn increases the resonance frequency of the film. Also, many points of contact are present between the cylinder and the PVDF film such that energy is lost due to friction, and the vibration of the PVDF film is thereby reduced. Since a thickness of the back air space is not controlled in the known art, nor recognized that it could or should be controlled, the resonance frequency and reduction in vibration also can not be controlled. Instead, it has been discovered by the inventors that if back air thickness exceeds a certain value, the spring effect of back air becomes less and even becomes negligible, thereby solving both problems of uncontrollable resonance frequency and reduction in vibration. 
     Accordingly, a need in the art exists for an ultrasonic transducer apparatus in which a thickness of a space between a PVDF film and a spool supporting the film is controlled. Controlling of the thickness of the space between the PVDF film and the spool has been discovered by the inventors to reduce a spring effect of air trapped therebetween and ultimately controls resonance frequency and improves vibration in a manner not heretofore known in the art. 
     OBJECTS AND SUMMARY OF AN EMBODIMENT OF THE INVENTION 
     It is an object of an embodiment of the invention to provide an ultrasonic transducer apparatus having a controlled resonance frequency. 
     It is another object of an embodiment of the invention to provide an ultrasonic transducer apparatus having an air thickness of a predetermined value between a spool and a film surrounding the spool. 
     It is yet another object of an embodiment of the invention to provide an ultrasonic transducer apparatus in which the air thickness of a predetermined value between the spool and the film surrounding the spool is selected to substantially negate a spring effect of the air therebetween. 
     It is a still further object of an embodiment of the invention to provide a cost effective ultrasonic transducer apparatus for eliminating the problems found in the known art of ultrasonic transducer. 
     These and other objects of the present invention are achieved by providing a transducer apparatus including a spool member having a body portion and first and second elevated regions formed on the body portion. A piezoelectric polymer film such as a PVDF film surrounds the spool member and is spaced apart from the body portion of the spool member by an elevation of the elevated region, thereby forming a predetermined gap between the electrode film and the body portion of the spool member. The predetermined gap is at least 0.1 mm to enable a predetermined resonance frequency in the piezoelectric film. Opposite lateral ends of the piezoelectric film are secured together such that secured ends of the piezoelectric film have substantially the same resonance frequency as a remainder of the electrode film. 
     Advantages of an embodiment of the invention as described more fully hereinbelow include a cost effective assembly for providing an ultrasonic transducer assembly having improved resonance. This is accomplished by reducing a spring effect between a film surrounding a spool in an ultrasonic transducer assembly by forming a predetermined back air space between the film and the spool. 
     Additionally, the ultrasonic transducer of the instant disclosure reduces the complexity and cost previously associated with the use of ultrasonic transducers. The stored coils are easily accessible and manageable in a manner not previously known in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a perspective view of a spool for an ultrasonic transducer; 
     FIG. 2 is a side view of the spool shown in FIG. 1 with a film wrapped around the spool; 
     FIG. 3 is a perspective view of the combined spool and film showing a general location of joining of the film to itself; 
     FIG. 4 is a perspective view of a conventional straight PVDF film prior to forming a cylindrical shape with the film; and 
     FIG. 5 is a perspective view of the PVDF film of FIG. 5 after forming the cylindrical shape and applied to a conventional spool; and 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present invention may have many applications, an exemplary application and related description follows. Specifically, a purpose of the present invention is to provide an ultrasonic transducer apparatus having improved resonance. To that end, the following is a detailed description of an embodiment according to the teachings of the present invention. 
     Referring first to FIG. 1, there is illustrated a spool  10  for use with an ultrasonic transmitter (FIG. 3) in connection with the present invention. The spool  10  is of a unique shape and includes a cylindrical body portion  12  and a pair of elevated regions  14  surrounding the cylindrical body portion  12 . The cylindrical body portion  12  has an outer peripheral surface  16 , an inner surface  18 , and opposite ends  20 . The inner surface  18  defines a longitudinal opening  22  of a uniform cylindrical shape corresponding to the shape of the cylindrical body portion  12 . 
     The elevated regions  14  of the spool  10  are integrally formed with the body portion  12  of the spool  10  and may either be of a one-piece construction with the body portion  12  or attached to the body portion by suitable securing methods. As shown, there are two elevated regions  14 . Each elevated region  14  is coextensive with one of the opposite ends  20  of the cylindrical body portion  12  so as to extend therefrom and terminates in an outer edge  24  of the elevated region  14 . The positioning of the elevated region  14  at opposite ends  20  of the cylindrical body portion  12  has been found to be optimal for the ultrasonic transmitter of the present invention. However, this arrangement should not be construed to eliminate the possibility of the elevated region  14  being set in from one or more opposite ends  20  of the cylindrical body portion  12  of the spool  10 . Further, the outer peripheral edge  24  of the elevated region  14  is shown to be at least 0.1 mm from the outer peripheral surface  16  of the body portion  12 . The determination of that optimum distance and its effect will be described in the following. 
     Referring now in further detail to FIG. 2, there is shown a film  26  wrapped around the spool  10 . In particular, the film  26  is a PVDF film similar to the type used in the conventional art but applied to a cylinder in a different manner than known in the art. As shown here, the film  26  is of a sheet type having opposite longitudinal edges and opposite lateral edges. The longitudinal edges are positioned to surround the outer peripheral edge  24  of the elevated region  14  rather than being in direct surface contact with the body portion  12  of the spool. The distance between the outer peripheral surface  16  of the cylindrical body portion  12  and the outer edge  24  of the elevated region is at least 0.1 mm. The positioning of the film around the outer edge  24  creates a back air area  28  between a back surface of the film  26  and the outer peripheral surface  16  of the cylindrical body portion  12 . 
     The reason for the distance between the outer peripheral surface  16  of the body portion  12  and the outer peripheral edge  24  of the elevated region  14  is to provide an effective spring constant between the body portion  12  of the spool and the wrapped film  26 . The effective spring constant of the back air area  28  is given by 
      K a =2 pRHrV S   2 /d where d is the back air gap in meters, R is the radius of the film, H is the height of the cylinder in meters, here shown at approximately 12 mm, r is the air density measured by 1.3 Kg/m 3 , and V s  is the sound velocity at 344 m/s.  (2) 
     The effective spring of the PVDF cylinder is 
     
       
         K p =2 pHYt/R where Y is the effective Young&#39;s modulus of PVDF with approximately [5˜6×10 9 N/m 2 ] Ag/C electrodes (6×10 9 N/m 2 ), and t is the total thickness with electrodes at approximately 30˜50 mm.  (3) 
       
     
     In order for K a  to become ⅕ of K p , d has to be greater than 0.9×10 −4  lt (where 1 is the wavelength) which is 0.1 mm for the above parameters. Therefore the film  26  has to be held with a certain space between the film  26  and the outer peripheral surface  16  of the cylindrical body  12 . Accordingly, the opposite ends  20  of the spool  10  have the elevated regions  14  as shown. 
     The film  26  has a uniform radial vibration motion from top to bottom (longitudinal edge to longitudinal edge of the film  26 ) if the film  26  is not bonded to anything. If the longitudinal edge areas of the film  26  are bonded to the elevated regions  14 , respectively, the bonded regions  14  will not vibrate but the remaining non-bonded area will vibrate. Although the transducer characteristics such as the resonance frequency and the output pressure are not much different for either case, it is preferred that there is no bonding between the film  26  and the outer longitudinal edges  24  of the elevated regions  14 . Not only are production and a processing of the transducer apparatus simplified when an extra step of bonding is eliminated, but the resonance frequency is improved and vibration is reduced. 
     Turning now to FIG. 3, the film  26  must be secured in some fashion to itself when wrapped around the spool  10 . As an example, one end  30  (lateral end) of the film  26  is joined to the opposite end  30  by overlapping the opposite ends and securing the same together. In this instance, securing of the lateral edges together is by an adhesive or the like. 
     A radius of the spool  10  can be determined by its ultimate application to an end product. For example if the size of the end product to which the PVDF film  26  is mounted has a diameter of 7˜15 mm, the resonance frequency can be determined by Equation (1) above. Young&#39;s modulus of PVDF and density are modified by Ag-carbon ink formed on the surface of the film  26 . Accordingly, the parameters to be used for Equation (1) are 
     Young&#39;s modulus of PVDF, Y p =4×10 9  N/m 2    
     Young&#39;s modulus of Ag/C ink, Y AgC =8×10 9  N/m 2    
     Thickness of PVDF t p =18-35 micron 
     Thickness of Ag/C ink, t AgC =5-10 micron per one side (actually on both sides) 
     Density of PVDF P p =1800 Kg/m 3    
     Density of Ag/C ink P AgC =2000 Kg/m 3    
     Thickness weighted Young&#39;s modulus 
     Y=(Y p t p +2Y Agc t Agc )/(t P +2t AgC )=6.1˜5×10 9  where 6.1×10 9  is the thickest Ag/C and 5×10 9  is the thinnest Ag/C 
     Thickness weighted density r=(r p t p +2r AgC t Agc )/(t p +2t AgC )=1900˜1850 Kg/m 3  where 1900 is the thinnest PVDF and 1850 is the thickest PVDF 
     and R=3.5˜7.5×10 3 m. 
     Using these parameters, the resonance frequency ranges from 35˜81 Khz with 35 Khz being the lowest possible frequency and 81 Khz being the highest possible frequency from the above parameters. 
     It should be noted that carbon ink is commercially available, however the resistivity thereof is too high such that the electrode resistance is not negligible compared to the transducer impedance which becomes lower at a high frequency. Therefore, carbon ink can be used only for a low frequency device. At an ultrasonic frequency region (high frequency), silver ink is better because of its much lower resistance, but silver tarnishes due to sulfurization. Therefore silver needs surface coating which is an extra process. Further, the color of a silver carbon mixture is dark, and tarnished silver is invisible. Thus, a silver-carbon mixture is necessary for high-frequency applications. 
     The invention having been described, it is clear that certain modifications and variations of the ultrasonic transducer apparatus can be made without departing from the spirit and scope of the invention. These modifications may include the application of various materials for the film, spool, and related components, and is intended to include variations in size and shape of the recited components to the extent that they are still able to perform as described. These obvious modifications and variations are within the theme and spirit of the invention and are considered within the scope of the following claims.