Abstract:
An ultrasonic transducer utilizes an acoustic transmission line to increase the sensitivity of both the transmitting response and the receiving response. The resonant element of the transducer could employ a ceramic disc operating in the radial resonance mode for its transduction means, or it could be a half wavelength resonator utilizing a forward transmission line, a rear transmission line, and a non-resonant ceramic for transduction. The shape of the forward transmission line is designed to generate the desired acoustic radiation pattern at the frequency of resonance, which could be a broad or narrow conical beam or a fan shaped beam. A sealed housing structure is disclosed that contains the forward transmission line that is acoustically disconnected from the rest of the housing. This housing provides improved environmental protection and allows for easy mounting of the transducer without effecting its electroacoustic response characteristics.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
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     BACKGROUND OF THE INVENTION 
     This invention is concerned with electroacoustic transducers and, more specifically, with electroacoustic transducers for transmitting or receiving sound in a fluid medium. Although not limited to the ultrasonic frequency region, this improving the performance characteristics of electroacoustic transducers to be used in the ultrasonic frequency region in a gaseous medium. 
     The teachings of this invention can be used by one skilled in the art in a wide variety of transducer designs using many different methods of transduction for sound radiation or reception in fluid or gaseous mediums. Transducers using the teachings of this invention can be designed using a wide variety of transduction materials, such as magnetostrictive rods, piezoelectric crystals, and polarized ceramic elements. However, the teachings of this invention can be best used in conjunction with ultrasonic transducers designed with polarized ceramics for use in a gaseous medium. Such a transducer is described in U.S. Pat. No. 3,928,777. 
     U.S. Pat. No. 3,928,777 described an ultrasonic transducer employing a ceramic disc as the transduction material. For optimum performance, the transducer is operated in the vicinity of resonance. This disc could operate in the thickness mode of resonance, but in the preferred embodiment the ceramic disc operates in the radial mode of resonance, since a transducer of this design is smaller and less costly. The transducer further employs a novel acoustic transformer in the form of an acoustic impedance matching material inserted between the ceramic and the gaseous medium. This acoustic transformer material is characterized in that its acoustic impedance is less than the acoustic impedance of the ceramic, but greater than the acoustic impedance of the gaseous medium. The acoustic impedance of a material is its density, ρ, times the velocity of sound, c, in the material. In addition, the thickness of the acoustic transformer material is approximately one quarter of a wavelength. 
     As shown in the referenced patent, a transducer utilizing the teachings of that invention will produce significant increase in sensitivity over a broader frequency response, and the resultant acoustic radiation pattern will invention is particularly useful for contain reduced secondary lobes. However, the beam angle from the transducer is fixed and controlled at the resonant frequency by the diameter of the ceramic disc. To overcome that limitation, this invention provides a novel design of an acoustic transformer with dimensions that are not the same as the dimensions of the ceramic disc. This acoustic transformer can therefore be designed to produce a wide variety of different acoustic radiation patterns. These radiation patterns can be conical in shape with different beam angles, or they can be fan shaped containing one beam angle in the horizontal plane and a different beam angle in the vertical plane. The transducer can also be designed to utilize a plastic housing that provides improved environmental protection. 
     BRIEF SUMMARY OF THE INVENTION 
     The primary object of this invention is to provide an acoustic transmission line between the vibrating surface of the transduction material and the medium into which the vibrations are to be transmitted, with the dimensions of the transmission line designed to create an acoustic radiating pattern that has a desired beam angle. 
     Another object of this invention is to produce an acoustic radiation pattern that is fan shaped containing one beam angle in the horizontal plane and a different beam angle in the vertical plane. 
     An additional object of this invention is to provide an acoustic transmission line that is part of a transducer resonant structure that is a half wavelength in length. 
     A still further object of this invention is to provide a means of housing the transducer structure, where a portion of the housing structure is part of the acoustic transmission line, but this radiating portion of the housing is acoustically disconnected from the rest of the housing. 
     Another object of this invention is to provide improved environmental protection for the transducer by utilizing a sealed housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features which are characteristic of the invention are set forth with particularity in the appended claims. However, the invention itself, both as to its organization and method of operating, together with further objects and advantage thereof, will best be understood by reference to the description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic cross-sectional representation of a cylindrical transducer resonant element using a thin radial resonant ceramic disc for the transduction material with an acoustic transmission line that radiates sound into the medium that has a diameter that is equal to the diameter of the ceramic disc. 
         FIG. 2  is a schematic cross-sectional representation showing the radial resonant structure of  FIG. 1 , except the diameter of the acoustic transmission line that radiates sound into the medium is smaller than the diameter of the radial resonant disc, therefore producing a conical radiation pattern with a larger beam angle. 
         FIG. 3  is a top view of the structure shown in  FIG. 2 . 
         FIG. 4  is a plot showing how the beam angle of the acoustic radiation pattern changes as a function of the dimension of the diameter of the acoustic transmission line radiator divided by the wavelength of the sound in the medium. 
         FIG. 5  is a schematic cross sectional representation of a cylindrical transducer resonant element that is a half wavelength resonator, which uses a thin non-resonant ceramic disc as the transduction material with an acoustic transmission line that radiates sound into the medium that has a diameter that is equal to the diameter of the ceramic disc. 
         FIG. 6  is a schematic cross-sectional representation showing the half wavelength resonant structure of  FIG. 5 , except the acoustic transmission line for radiating sound into the medium is rectangular in shape, which enables it to produce a fan shaped acoustic radiation pattern containing one beam angle in the horizontal plane and another angle in the vertical plane. 
         FIG. 7  is a top view of the structure shown in  FIG. 6 . 
         FIG. 8  is a schematic cross-sectional representation of a rectangular transducer resonant element that is a half wavelength resonator and produces a fan shaped acoustic radiation pattern. 
         FIG. 9  is a top view of the structure shown in  FIG. 8 . 
         FIG. 10  is a schematic cross-sectional view of a transducer in a housing employing the teachings of this invention. 
         FIG. 11  is a sectional view taken along the line A-A of  FIG. 10 . 
         FIG. 12  is a plot showing the measured horizontal acoustic radiation pattern of a transducer similar to the structure shown in  FIGS. 10 and 11 . 
         FIG. 13  is a plot showing the measured vertical acoustic radiation pattern of a transducer similar to the structure shown in  FIGS. 10 and 11 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more particularly to the figures,  FIG. 1  shows a schematic cross-section of a representation of a cylindrical resonant element of a transducer that uses the teachings of U.S. Pat. No. 3,928,777. The transduction material consists of a thin piezoelectric ceramic disc  1 , which may be any one of the well known polarized ceramic materials such as, for example, lead-zirconate-titanate or barium titanate. The flat surfaces of the ceramic disc  1  are coated with metallic electrodes  2  and  3 . They could be silver, electroless nickel, or other material as well known in the art. The ceramic disc  1  will resonate in the radial mode, also called the planar mode, and the frequency of the radial resonance, f R , will be inversely proportional to its diameter, D C , as is also well known in the art. To keep the thickness resonant frequency of the ceramic from interfering with the transducer operating at f R , the thickness of ceramic disc  1  must be kept small enough so that the frequency of the thickness resonance of the ceramic is much higher than the frequency of the radial resonance. Electrical lead  4  is attached to electrode surface  2 . This connection could be made using solder, electrically conducting cement, or any other means well known in the art. Electrical lead  5  is similarly connected to electrode surface  3 . 
     A cylindrical acoustic transmission line  6  is attached to the front of ceramic disc  1 . This attachment can be by any method that produces a strong mechanical bond, such as vulcanizing or cement. In the resonant element embodiment of  FIG. 1 , this transmission line  6  can be made of a wide variety of materials, such as potting compounds, rubbers, or plastics, but it is important that its specific acoustic impedance is greater than the specific acoustic impedance of the fluid or gas transmission medium into which the sound is to radiate, and less than the specific acoustic impedance of the ceramic material. The specific acoustic impedance is defined as the product of its density times the velocity of sound in the material. As was described in U.S. Pat. No. 3,928,777, the length of the transmission line  6  should be a quarter wavelength, λ/4, at frequency f R , where for this structure λ is the wavelength of sound in the transmission line material. 
     When an electrical voltage is applied across the electrical leads  4  and  5  at a frequency equal to the radial resonant frequency, f R , of the ceramic disc  1 , electrode surfaces  2  and  3  will alternately move towards each other and away from each other with maximum amplitude at that same frequency. The quarter wavelength transmission line  6  will then amplify the magnitude of the vibration that occurs at its surface that is attached to the front of electrode  2  of ceramic disc  1  and produce a much larger magnitude of vibration at its opposite surface  7 . This large vibration of surface  7  will then radiate the sound from the transducer into the transmission medium at a higher sound pressure than would be produced by the ceramic radiating into the transmission medium by itself. Because the transducer is reciprocal, the same increase in sensitivity is obtained when the transducer is used as a receiver. The response magnitude and Q of the transducer can be changed by using materials with different properties for the transmission line. If the material has greater internal mechanical losses, it will produce a transducer with a lower Q and lower sensitivity. 
     It is often desirable to design transducers to produce different acoustic radiation patterns. A transducer could be designed so that the radiation pattern of the sound it produces is omni-directional, or it could be designed to produce higher levels of sound pressure in a particular direction, usually perpendicular to the vibrating surface. This lobe of high sound pressure output can be made very broad, very narrow, or anywhere in between. The beam angle of the acoustic radiation pattern produced by a transducer is defined as the angle subtended by the two points on the lobe where the sound pressure radiated is 3 dB lower than the maximum level of the lobe. The beam angle at any specific frequency is a function of the dimensions of the radiating surface divided by the wavelength of sound in the transmission medium, λ M , at that frequency. A transducer with a circular radiating surface with a diameter D will produce a conical acoustic radiation pattern. The beam angle of the conical pattern is inversely proportional to the ratio D/λ M . Therefore, for a given frequency the beam angle will decrease as the diameter increases. 
     The radiating surface of the resonant element shown in  FIG. 1  is surface  7  of the transmission line  6 . This radiating surface is circular and has a diameter equal to D T , which is also equal to the diameter of the ceramic, D C . Since the radial resonant frequency, f R , of the ceramic is inversely proportional to the diameter of the ceramic, D C , and since D T  is equal to D C , both diameters will increase as the frequency f R  decreases, but λ M  will also increase by the same proportion as the frequency decreases. Therefore, the ratio of D T /λ M  will stay the same for an element that is designed to resonate at any frequency, provided it uses the same ceramic material and radiates into the same transmission medium. The radiation patterns for all resonant frequencies will be conical with the same beam angles. It was shown in U.S. Pat. No. 3,928,777 that for the range of the most common types of ceramic materials, the beam angle of the transducer radiating into air will be between approximately 8° to 12° at the radial resonant frequency of the ceramic discs. 
     It is possible to change the radiation pattern at its resonant frequency of a transducer using a resonant element similar to the structure illustrated in  FIG. 1 .  FIG. 2  shows a schematic cross-sectional representation of a modification of the transducer resonant element of  FIG. 1  that would change the beam angle of the transducer.  FIG. 3  is a top view of the structure shown in  FIG. 2 . This structure contains the same ceramic disc that was used in the resonant element of  FIG. 1 ; therefore, the diameter of the ceramic disc  1 , D C , is the same for both. The length of the transmission line  6   a  of  FIGS. 2 and 3  is a quarter wavelength long at f R  and is also approximately the same as the length of transmission line  6  of  FIG. 1 . The diameter of the transmission line  6   a , Da T , however, is less than the diameter of the transmission line  6 , D T , of  FIG. 1 . 
     Since the ceramic disc is the same for both structures, the radial resonant frequency f R  is the same and λ M  is the same. Because the diameter of the transmission line is decreased, the ratio D T /λ M  also decreases, and the beam angle of the conical acoustic radiation pattern of the transducer therefore increases.  FIG. 4  shows a plot of the beam angle of the acoustic radiation pattern produced by a transducer as a function of D T /λ M . 
     By changing the diameter of the transmission line  6   a  of  FIGS. 2 and 3 , it is therefore possible for one skilled in the art to fabricate transducers that use the same ceramic disc  1  in their resonant elements, and operate at the same resonant frequency, f R , but produce different conical beam angles. To obtain the desired beam angle, the diameter of the transmission line  6   a  is adjusted to produce the required value of D T /λ M  shown in the graph of  FIG. 4 . 
     For some applications it is often desirable to make the resonant element of the transducer a half wavelength resonator. This type of device produces a single clean resonance with one resonant frequency and one anti-resonant frequency. In the transducer resonant elements of  FIGS. 1 ,  2 , and  3 , the radial resonance of the ceramic produce one resonant frequency and one anti-resonant frequency; however, the quarter wavelength resonance of the transmission line  6  is superimposed onto the resonant frequency of the ceramic. These two different resonant frequencies in the resonant element can change at different rates as the temperature of the transducer is changed. This is not the case with a half wavelength element. Half wavelength transducers are therefore more temperature stable and consistent, and their electroacoustic responses are more immune to degradation if dirt or other material attaches to the radiating surface. 
       FIG. 5  shows a schematic cross-section of a representation of one preferred embodiment of a cylindrical resonant element that uses a half wavelength resonator. The thin ceramic disc  1   a  has a radial resonant frequency and a thickness resonant frequency that are both higher than the half wavelength resonant frequency of the transducer element. Two cylindrical acoustic transmission lines are used in the structure, the forward transmission line  8  and the rear transmission line  9 . When designing half wavelength resonating structures such as this, a wide variety of materials can be used for the two transmission lines. Different materials will produce transducers with different characteristics, such as different Qs, different sensitivities, different maximum stresses, different front to back vibration ratios, and different temperature responses. The resonant element is designed to operate at the frequency of half wavelength length resonance, f L . To accomplish this, the length of the forward transmission line  8  plus half of the thickness of the ceramic disc  1   a  is designed to be a quarter of a wavelength long at f L , and the length of the rear transmission line  9  plus half the thickness of the ceramic disc  1   a  is also made a quarter wavelength. This makes the structure one half wavelength long at frequency f L . 
     It has been found that different plastic and rubber materials similar to those used in the transmission lines for the radial resonant elements are preferred for use in the forward transmission line  8  of  FIG. 5 . Metals such as aluminum or steel are best used for the rear transmission line  9  to ensure that there is a much larger vibration amplitude produced at surface  7   b  that radiates the sound into the transmission medium than occurs at the open surface of the rear transmission line  9 . Since the resonant frequency of the element shown in  FIG. 5  is controlled by the lengths and material choices of the forward transmission line  8 , and the rear transmission line  9 , and the thickness of the ceramic disc  1   a , the diameters of the forward and rear transmission lines and the ceramic disc can be made any value desired, provided all other resonances in these structures are kept well outside the operating frequency region of f L . Therefore, the transducer element can be designed to produce any reasonable conical beam angle for the acoustic radiation pattern by making the diameter of the forward transmission line  8  conform with the curve of  FIG. 4 . 
     Since the rear transmission material is usually metallic, it is an electrical conductor. Therefore, to make fabrication easier, the rear transmission line  9  can be attached to the bottom electrode of the ceramic disc  3   a  using conductive cement, and the electrical lead  5  can then be electrically attached to the bottom of the rear transmission line  9 , as shown. The lead  5  could obviously also be attached to the ceramic directly. In a structure such as that shown in  FIG. 5 , the diameters of the forward transmission line  8 , the rear transmission line  9 , and the ceramic can all be different, but it is usually easier to assemble the transducer if all three diameters are approximately the same. 
     It is often desirable to have an ultrasonic transducer produce a fan shaped radiating beam. For example, in an obstacle detection system for a robot or a vehicle, a transducer with a broad horizontal radiation pattern and a narrow vertical radiation pattern is ideal because the narrow vertical angle will not detect back scatter from the road surface and the broad horizontal pattern will require fewer transducers to cover detection of objects over the desired angular azimuth. A transducer with a rectangular radiating surface will produce a fan shaped acoustic radiation pattern that is broad in the plane around the width of the radiating surface, which I will call the horizontal plane, and narrow in the plane around its length, which I will call the vertical plane. If the width and length dimensions of the radiating surface are Wand L respectively, then the beam angle of the acoustic radiation pattern in the horizontal plane, which is perpendicular to the radiating surface, bisects the length and is parallel to the planes formed by the two shorter ends, will produce a beam angle that is inversely proportional to the ratio W/λ M . The beam angle in the vertical plane which is perpendicular to the radiating surface, bisects the width and is paralleled to the planes formed by the two longer ends, will produce a beam angle that is inversely proportional to the ratio L/λ M . The beam angle of the radiation pattern in the horizontal plane will therefore be broader than the beam angle in the vertical plane, thus producing the fan shaped beam. For a given frequency the beam angle in each plane will decrease as W and L increase, and vice versa. The relationship of beam angle to W/λ M  and L/λ M  is very similar to the relationship of beam angle to D T /λ M  shown by the curve in  FIG. 4 . 
       FIG. 6  shows a cross sectional view of a modification to the resonant element structure of  FIG. 5  that will produce a fan shaped radiation beam.  FIG. 7  shows a top view of the structure of  FIG. 6 . In this embodiment the forward transmission line  8   a  has been shaped into a rectangular structure that is L long and W wide. The surface  7   c  radiates sound into the transmission medium. Typically L would be the same dimension as the diameter of the ceramic disc  1 , but it can be smaller as shown. If L is equal to the ceramic diameter, the short ends of the forward transmission line  8   a  could form an arc that follows the circular curve of the ceramic. This would typically make the transducer easier to fabricate. Since W is smaller than L, the ratio L/λ M  is less than the ratio W/λ M . This will therefore produce a large beam angle for the radiation pattern in the horizontal plane around the width of the rectangular radiating surface, and a narrow beam angle in the vertical plane around its length. 
       FIG. 8  and  FIG. 9  show a schematic cross-sectional and planar view of another embodiment of the resonant element structure shown in  FIGS. 6 and 7 , in which the front transmission line  8   b , the rear transmission line  9   a , and the ceramic  1   b , are made rectangular. The surface  7   d  of the forward transmission line  8   b  radiates sound into the transmission medium. A structure of this nature would be a little more expensive to produce, but the transducer would also fit into a smaller size envelop. 
     The schematic resonator element structures shown in  FIGS. 1 ,  2 ,  3 ,  5 ,  6 ,  7 ,  8  and  9  illustrate how using the technology of this invention the basic resonator element design can be modified by one skilled in the art to produce transducers with different radiation patterns. However, these structures are not usable unless they can be incorporated into a housing that will allow the transducer to be protected and mounted without affecting its electroacoustic responses.  FIG. 10  is a cross-section of one preference embodiment of a complete cylindrical transducer employing the teachings of this invention, and  FIG. 11  is a sectional view of the structure taken along line A-A of  FIG. 10 . This transducer employs a similar resonator assembly to the one shown in  FIGS. 6 and 7 , except the forward transmission line  8   c  is incorporated into the transducer housing  10 . The housing  10  would typically be a molded plastic piece. The length of the forward transmission line  8   c  plus half of the thickness of the ceramic  1   a  is equal to a quarter wavelength at the transducer resonant frequency, f L . Likewise, the length of the metallic rear transmission line  9  plus half of the ceramic thickness is equal to a quarter wavelength at f L . The top electrode of the ceramic disc  2   a  can be attached to the forward transmission line  8   c  by using any of a wide variety of cements that are commercially available. The rear transmission line  9  is also attached to the bottom electrode of the ceramic disc  3   a  using a commercially available cement, but in this configuration, the cement should be electrically conducting, or conducting particles should be mixed into the cement. This will allow lead  5  to be electrically attached to the rear transmission line  9  which will make it also electrically attached to electrode  3   a  of ceramic disc  1   a . Lead  4   a  is electrically attached to electrode  2   a  of the ceramic disc  1   a . It contains an insulating jacket to insure that an electrical connection cannot be inadvertently made to electrode  3   a  or rear transmission line  9 . Leads  4   a  and  5  are electrically attached to each conductor of cable  12 , as shown. 
     The structure of  FIG. 10  contains a separation disc  11  which fits over the back of the resonating element structure and mounts into the shoulder in housing  10 . Separation disc  11  contains holes that will allow leads  4   a  and  5  to pass through. It can be made from a wide variety of materials, but, it is usually best for it to be made from a material with high acoustic losses, such s certain plastics, rubbers, or corprene. The purpose of separation disc  11  is to form a dam to allow potting compound  13  to be poured into the back of the transducer while keeping it from flowing into the interior space of the transducer. After the potting compound cures, it forms a seal over the back of the transducer and also provides a strain relief for the leads  4   a  and  5  and the cable  12 . The walls of housing  10  are made relatively thin to reduce the amount of acoustic reverberation within the structure after the transducer transmits a sound pulse. 
     It is necessary that the rectangular forward transmission line  8   c  is acoustically isolated from the rest of housing  10 . This is accomplished by designing the peripheral portions of the front of housing  10  to have a large thickness to ensure that it is very stiff at the resonant frequency f L . The rectangular forward transmission line  8   c  is disconnected from the stiff front of the housing by undercut  13 . This undercut has a width w, and the thickness of the plastic in front of the undercut is t, as shown in  FIG. 10 . It is important that t and w be designed so that the resonance between the stiff portion of housing  10  and the resonant element is well below f L , thus causing a complete mechanical decoupling of the forward transmission line  8   c  from the rest of housing  10  when the transducer is driven at frequency f L . 
     If undercut  13  is properly designed, only the front portion of the housing directly in front of rectangular forward transmission line  8   c  will vibrate, and the rest of the front surface of the housing will be relatively stationary. The acoustic radiation pattern from the transducer will therefore be the same as that produced by the resonant element shown in  FIGS. 6 and 7 . If the undercut is not properly designed, there will also be motion in the rest of the front surface of the housing beyond the surface that is in the front of the rectangular forward transmission line  8   c , and the radiation pattern will therefore become distorted. 
     A transducer similar to the structure shown in  FIG. 10  was fabricated and tested. In this unit that resonated at 62 kHz, the width and length of rectangular forward transmission line  8   c  were 0.2 inches wide by 0.54 inches long. The width w of undercut  13  was 0.062 inches and the thickness t was 0.012 inches. The thickness of the front of the housing beyond the area in front of transmission line  8   c  was 0.18 inches. The measured broad horizontal radiation pattern in the plane around the width of the forward transmission line  8   c  is shown in  FIG. 12 , and the measured narrow vertical radiation pattern in the plane around its length is shown in  FIG. 13 . 
     It is possible for anyone skilled in the art to employ the technique of this invention to design an ultrasonic transducer to operate at any desired frequency and to produce any reasonably desired acoustic radiation pattern. The transducer housing forms a continuous surface in its front portion, thus providing optimal environmental protection. This design also will be relatively unaffected by dirt attaching to the radiating surface, particularly if the resonant element is a half wavelength resonator. It is obvious to one skilled in the art that any of the resonant elements shown in  FIGS. 1 ,  2 ,  3 ,  5 ,  6 ,  7 ,  8  and  9  can be substitutes for the resonator in the embodiment shown in  FIGS. 10 and 11 . 
     While a few specific embodiments of the present invention have been shown and described, it should be understood that various additional modifications and alternative constructions may be made without departing from the true spirit and scope of the invention. Therefore, the appended claims are intended to cover all such equivalent alternative construction that fall within their true spirit and scope.