Abstract:
An ultrasonic transducer comprises a lamination structured piezo-electric element (1) to which center shaft (2) of a diaphragm (3) is connected, a housing (7) mounted integral with a horn (11), a buffer member (10) disposed between said housing (7) and said diaphragm (3) for holding said diaphragm (3) in vibratable manner; thereby a sharp rise up and fall down characteristics of ultrasonic wave and sharp directivity are attained.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an improvement in an ultrasonic transducer using a laminated piezo-electric element and more particularly to an ultrasonic transducer with improved directivity characteristics and improved transient characteristics (pulse characteristics). 
     2. Description of the Prior Art 
     Ultrasonic transducer for use in the air has been proposed and includes laminated piezo-electric ceramic elements which are designed to work at resonance point or anti-resonance point. Further, since the mechanical impedance of air is very smaller than that of the piezo-electric ceramic element, the laminated element is connected to a diaphragm for attaining mechanical impedance matching therebetween. 
     In video camera having automatic focussing mechanism for its objective lens by means of ultrasonic distance measurement, the measurement must be continuously made. Such continuous measurement requires a good transient characteristic in order to avoid error of measurement. For such good transient measurement, short rise up and falling down time are necessary. On the other hand, in such video camera uses zoom lens as objective lens, and distance measurement for such zoom lens must be made with a sharp directivity corresponding to narrowest picture angle of the zoom lens. 
     Hitherto, ceramic ultrasonic transducer is known as the apparatus of a high sensitivity, high durability against moisture or acidic or salty atmosphere and high S/N ratio due to its resonance characteristic. But the ceramic ultrasonic transducer has had bad transient characteristic due to its very high mechanical Q value. 
     A typical example of conventional ultrasonic transducer is shown in FIG. 1, which is a sectional elevation view along its axis. As shown in FIG. 1, a lower end of a coupling shaft 2 is fixed passing through a central portion of a laminated piezo-electric element 1 with the upper part secured to a diaphragm 3. The laminated piezo-electric element 1 such as a ceramic piezo-electric element is mounted at positions of nodes of oscillation via a flexible adhesive on tips of supports 4. Lead wires 9,9&#39; of the laminated piezo-electric element is connected to terminals 6,6&#39; secured to base 71 of a housing 7, which has a protection mesh 8 at the opening thereof. 
     FIG. 2 is a graph showing envelope of radiated ultrasonic wave transmitted when the transducer is supplied with the ultrasonic wave during the time of 0 to 2 m sec of time graduated on the abscissa. As is observed in FIG. 2, the response of the transducer, i.e., the rise time and fall time are relatively long, both being of the order of 2 m sec. When data signal is sent and received by use of such ultrasonic transducer, time density of the data, or data transmission speed is limited by such relatively long rise time and fall time. If a high density data signal is sent and received via such transducer, for example, in ultrasonic wave distance measurement, data become mixed with the tailing part of the preceding data. Accordingly accurate sending and receipt of data is not attained. 
     Furthermore, when it is intended to obtain a sharp directivity with such device as shown in FIG. 1, use of larger laminated piezo-electric element 1, larger diaphragm 3, and larger supports 4 must be made much large, and pure piston disc motion of such large diaphragm, if used, become hard to realize. Therefore, sharp directivity has been hard to realize. When, in order to attain a sharp directivity, a horn is intended to be combined to such apparatus with large components, then, improvement of the transient characteristic through lowering of the mechanical Q value of the ultrasonic vibration system becomes more difficult. 
     SUMMARY OF THE INVENTION 
     Therefore the purpose of the present invention is to provide an improved ultrasonic transducer wherein both sharp directivity and high sensitivity are obtainable without losing the sharp transient characteristic, thereby a high speed data sending and receiving or ultrasonic distance measurement in a very short time is attainable. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     FIG. 1 is the sectional elevation view of the conventional ultrasonic transducer. 
     FIG. 2 is the graph of the envelope of ultrasonic wave radiation showing the transient characteristic of the transducer shown in FIG. 1. 
     FIG. 3 is a sectional elevation view of an example embodying the present invention. 
     FIG. 4 is a graph of an envelope of ultrasonic wave radiation showing the transient characteristic of the transducer shown in FIG. 3. 
     FIG. 5(a) and FIG. 5(b) are graphs of relations between inner diameter of the buffer member 10 of the apparatus of FIG. 3 and half acoustic pressure angle (directivity) and rise time, respectively. 
     FIG. 6(a) and FIG. 6(b) are graphs of relations between sizes of a laminated piezo-electric element 10 of the apparatus of FIG. 3 and half acoustic pressure angle and rise time (transient time), respectively. 
     FIG. 7 is a graph of relation between aperture angle of a horn and half acoustic pressure angle. 
     FIG. 8 is a graph of relation between length of waveguide part and the half acoustic pressure angle. 
     FIG. 9 is a graph of relation between inner diameter of opening of the horn and the half acoustic pressure angle. 
     FIG. 10 is a sectional elevation view of another example embodying the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3 is a sectional elevation view at the axis of an example embodying the present invention. As shown in FIG. 3, a lower end of a coupling shaft 2 is fixed passing through a central portion of a laminated piezo-electric element 1 with the upper part secured to a diaphragm 3 of metal or resin. Peripheral end part of the diaphragm 3 is held by an inner end of a ring shaped buffer member 10 of elastic and vibration absorbing substance, such as rubber or silicone rubber, and the outer face of the buffer member 10 is fixed to the inner wall of the cylindrical housing 7 of hard plastic or metal. By bonding the periphery of the diaphragm 3 onto the upper face of the buffer member 10, the space on the front face side of the diaphragm is isolated from the space of the rear face side of the diaphragm 3. The housing 7 is further fixed to the inner face of a horn 11 at the bottom part thereof. The horn 11 is made of metal or a hard plastic, and the housing 7 is fixed by force fit, or alternatively, the housing 7 and the horn 11 may be formed continuously and integrally with the same material. Anyway, the housing and the horn should be mechanically integral with each other. The housing 7 has two terminals 6, 6&#39; to which lead wires 9, 9&#39; from the laminated piezo-electric element 1 is connected. Bonding of the buffer member 10 to the housing 7 and bonding of the diaphragm to the buffer member 10 are made preferably with an electrically conductive bond in order to discharge undesirable electric charges due to ultrasonic vibration. 
     The details of the example apparatus are as follows: 
     
         ______________________________________diameter of the laminated            10 mmpiezo-electric element 1substance of the laminated            PbTiO.sub.3.PbZrO.sub.3.Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3            -piezo-electric element            mixed crystaldiameter of the diaphagm 3            17 mmsubstance of the diaphragm 3            Al 0.1 mm thicktop angle of the cone of the            112°diaphragm 3diameter of the opening of the            55 mmhorn 11substance of the horn            ABS resinshape of the horn is            conical horn with cylindrical            throat partdriving ultrasonic frequency            about 50-70 KHz            depending on thickness            of piezo-electric            element.______________________________________ 
    
     Transient characteristic of the ultrasonic transducer is satisfactory as shown by FIG. 4 which is a graph of envelope curve of ultrasonic radiation when the ultrasonic transducer of FIG. 3 is driven by an ultrasonic signal for a period of 0 m sec to 2 m sec. 
     As shown by FIG. 4, the rise and fall transient time is only less than 0.15 m sec. 
     FIG. 5(a) and FIG. 5(b) show relations of inner diameter (in mm) of the buffer member 10 vs. half width of main lobe (in degree) of the directivity curve and rise time (in m sec) i.e., transient characteristic, respectively, of the example of FIG. 3. As shown in FIG. 5(a) and FIG. 5(b), it is understood that as the inner diameter decreases the rise time become shorter but the half width of the main lobe increases. When the inner diameter is made far smaller, the side lobes of the directivity curve also increase. From many experiments, it is found that the inner diameter of the buffer member 10 should be 80% to 85% of that of the diaphragm in order to obtain desirable half width of main lobe as well as desirable rise time. 
     FIG. 6(a) and FIG. 6(b) show relation of thickness of laminated piezo-electric element 1 vs. half width of main lobe (in degree) of the directivity curve and rise time (in m sec) i.e., transient characteristic, respectively, of the above-mentioned example. As shown in FIG. 6(a) and FIG. 6(b), as the thickness of the laminated piezo-electric element increases, the rise time becomes longer and also the half width of main lobe increases. Of course, as the thickness decreases, the driving frequency becomes higher. 
     FIG. 7 and FIG. 8 show relations of the half width of main lobe (degree) vs. angle θ of horn (degree) and length L of throat (mm), respectively, shown in FIG. 3. The second example apparatus used for the experiments is as follows: 
     
         ______________________________________diameter of the laminated            10 mmpiezo-electric element 1thickness of the laminated            0.6 mmpiezo-electric element 1substance of the laminated            PbTiO.sub.3.PbZrO.sub.3.Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3            -piezo-electric element 1            mixed crystaldiameter of the diaphragm 3            17 mmsubstance of the diaphragm 3            Al 0.1 mm thickinner diameter of the buffer            13 mmmember 10substance of the buffer member            silicone rubber10driving ultrasonic frequency            about 50-70 KHz.______________________________________ 
    
     As shown in FIG. 7, for both of horns of the diameters D of opening of 40 mm and 50 mm, the directivity is the best when the angle θ is about 23°, and for desirable directivity the angle θ should be between 20° and 26°. 
     FIG. 8 shows that optimum directivities are obtainable, at the throat length L of 4-8 mm for the horn of 40 mm opening diameter D and at 5-10 mm for the horn of 50 mm opening diameter D. Experiments show that throat length L of 10-20% of the horn opening diameter D is preferable. 
     FIG. 9 shows relation of diameter D of opening of the horn 11 vs. half width of main lobe (degree) of the above-mentioned second example, wherein parameter is driving frequency f. FIG. 9 shows that the larger diameter D produces better directivity. 
     Instead of the above-mentioned conical shape horn 11, a parabola-shaped horn as shown in FIG. 10 is also effective in the same manner. 
     As has been elucidated in detail citing much experimental data, the ultrasonic transducer embodying the present invention is characterized by acoustically integral structure of the housing 7 and horn 11 and peripheral holding of the diaphragm by the ring-shaped buffer member 10 of resilient and absorbing substance fixed with its outer face to the housing 7, thereby isolating the rear side space of the diaphragm from the front side space in the horn of the diaphragm. Such characterized configuration produces a synergistic effect which results in compatibility of good directivity and good transient characteristic at the same time. Therefore, the ultrasonic transducer of the present invention is useful when used in continuous distance measuring apparatus for movie camera or TV camera, and especially suitable for use in cameras for video tape recorder wherein very quick distance measuring is required with a very high directivity corresponding to use of automatic zoom objective lens.