Abstract:
A piezoelectric generator is provided which can improve an output current by almost two times by having an ultrathin metal electrode bonded between piezoceramic elements, with polarizations of the piezoceramic elements oriented in the same direction. In the piezoelectric generator for generating electricity by causing deformations to a platelike piezoceramic member, the piezoceramic member comprises two platelike piezoceramic elements and an ultrathin metal electrode disposed therebetween, the piezoceramic elements and the metal electrode being bonded together in layers, the piezoceramic elements having their polarizations oriented in the same direction. A central portion or end portions of one surface of the piezoceramic member is supported by a cushion member to form a flexible support structure that makes it difficult for a natural vibration of the piezoceramic member to be transmitted to other structures.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a piezoelectric generator capable of developing an output current almost two times that produced by conventional piezoelectric generators using prior art piezoelectric ceramic elements.  
           [0003]    2. Description of the Prior Art  
           [0004]    Piezoelectric materials have a wide range of applications as a transducer for transforming energy from a mechanical energy to an electrical energy and vice versa. Many materials have been known, both inorganic and organic, which exhibit the piezoelectric effect. Among materials currently in practical use are lead zirconium titanate (PZT)-based ceramic materials (piezoelectric ceramics).  
           [0005]    The piezoelectric ceramic element or piezoceramic element is a polycrystalline ceramic with a piezoelectricity that is produced by subjecting it to a high DC voltage to cause a residual polarization. Since a basic piezoelectric constant of the piezoceramic element can be varied substantially freely depending on its composition, its application range is wide. A piezoceramic element of lead zirconate titanate in particular has a wide selection range of composition ratio and additives and therefore a variety of applications.  
           [0006]    [0006]FIG. 3 is an explanatory view showing a conventional piezoelectric generator. This piezoelectric generator comprises a piezoceramic plate  31  bonded to a base plate  32  of acrylic material whose ends are held by holders  33 ,  34  made of a hard material such as metal. A steel ball  35  is dropped onto the piezoceramic plate  31  to apply a mechanical impact energy to the piezoelectric ceramic plate  31  to excite a deflective oscillation in the plate  31  thus producing an electric energy.  
           [0007]    Although there are expectations for its practical applications, the conventional PZT-based piezoceramic element described above has a drawback of a small generating capacity and cannot meet its expectations. Further, in piezoelectric generators using a piezoceramic element, it is essential that a natural vibration of the piezoceramic element plate  31  be continued for as long as possible. To meet this requirement, a support structure must ensure that the natural vibration of the piezoceramic element plate  31  is not accompanied by a mechanical resistance. Further, in the conventional technique described above, because the base plate  32  and the piezoceramic element plate  31  are made of different materials, it is difficult to bring the center of vibration (part not expanded or contracted) at a joint surface between the base plate  32  and the piezoceramic element plate  31 . When this center emerges in the piezoceramic element plate  31 , a polarization cancellation occurs reducing the generation efficiency.  
           [0008]    To solve these problems of the conventional piezoelectric generator, the applicant of this invention previously proposed a piezoelectric generator (Japanese Patent Application No. 11-322280) as shown in FIG. 4. This piezoelectric generator comprises layered piezoceramic members  1  (PZT 1 ),  1  (PZT 2 ), each of which has two platelike piezoceramic elements  1   a,    1   b  bonded together with their polarizations directed in opposite directions. These piezoceramic members  1 ,  1  are impacted on one side to generate electricity. This piezoelectric generator is suited for generating a small amount of electricity with a high generation efficiency.  
           [0009]    The present invention is intended to provide a piezoelectric generator in which, unlike the previous application, the polarizations are arranged in one and the same direction and an ultrathin metal electrode is bonded between the platelike piezoceramic elements to increase the current output to almost two times that of the piezoelectric generator of the above construction that this applicant proposed previously.  
         SUMMARY OF THE INVENTION  
         [0010]    To achieve the above objective, a first aspect of the present invention provides a piezoelectric generator for generating electricity by causing deformations to a platelike piezoceramic member; wherein the piezoceramic member  10  comprises two platelike piezoceramic elements  10   a,    10   b  and an ultrathin metal electrode  11  disposed therebetween, the piezoceramic elements and the metal electrode being bonded together in layers, the piezoceramic elements having their polarizations oriented in the same direction; wherein a central portion or end portions of one surface of the piezoceramic member  10  is supported by a cushion member  3  to form a flexible support structure that makes it difficult for a natural vibration of the piezoceramic member to be transmitted to other structures.  
           [0011]    Further, a second aspect of the present invention provides a piezoelectric generator wherein the piezoceramic members  10 ,  10  supported by the cushion members  3 ,  3  are opposed to each other and a hard impact member  4  is disposed between the piezoceramic members  10  to reciprocally move between and impact the piezoceramic members  10 .  
           [0012]    Further, a third aspect of the present invention provides a piezoelectric generator wherein a hard impact member  4  that reciprocally moves and impacts the piezoceramic member supported by the cushion member  3  is arranged on one side or both sides of the piezoceramic member  10 .  
           [0013]    Further, a fourth aspect of the present invention provides a piezoelectric generator wherein the two piezoceramic elements  10   a,    10   b  are formed in the same configuration to obtain a stable generation efficiency.  
           [0014]    Further, a fifth aspect of the present invention provides a piezoelectric generator wherein the two piezoceramic elements  10   a,    10   b  are each formed of a plurality of piezoceramic elements bonded together in layers.  
           [0015]    Further, a sixth aspect of the present invention provides a piezoelectric generator wherein a lead zirconate titanate material is used for the piezoceramic elements  10   a,    10   b.    
           [0016]    Further, a seventh aspect of the present invention provides a piezoelectric generator wherein the metal electrode is made from phosphor bronze or brass and formed 10-50 μm thick. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is an explanatory diagram showing one embodiment of a piezoelectric generator according to the present invention.  
         [0018]    [0018]FIG. 2 is a circuitry of a charging device in the embodiment.  
         [0019]    [0019]FIG. 3 is a schematic view showing a conventional piezoelectric generator.  
         [0020]    [0020]FIG. 4 is an explanatory diagram showing a piezoelectric generator previously proposed by the applicant of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    Now, the present invention will be described in detail in conjunction with the embodiments shown in the accompanying drawings.  
         [0022]    [0022]FIG. 1 shows one embodiment of this invention. Piezoceramic members  10  are arranged at both inner, closed ends of a cylindrical container  8  and are struck by a steel ball  4  to generate electricity.  
         [0023]    One of the end faces of the container  8  is bonded with a cushion member  3  using an adhesive  6  and a piezoceramic member  10  is bonded to only a central part of the cushion member  3  using an adhesive  5 . Similarly, the other end face of the container  8  is also bonded with the piezoceramic member  10  so that these two piezoceramic members  10  are opposed to each other.  
         [0024]    A protector plate  2  is secured to a central part of each of the opposing surfaces of the piezoceramic members  10 . Between the two piezoceramic members  10  is arranged a pipe  7  in which a steel ball  4  is rollably installed.  
         [0025]    In this invention, when the piezoceramic members  10  are disposed horizontally for example, it is possible to hold not only the central portions of the piezoceramic members  10  but also their end portions with the cushion member  3 , or to hold the central portions and end portions of the piezoceramic members  10  with the cushion member  3  on both the upper and lower surfaces of the piezoceramic members  10   
         [0026]    The piezoceramic members  10  each have two platelike piezoceramic elements  10   a,    10   b  of the same configurations (same material, same shape and same thickness) bonded together with an ultrathin metal electrode  11  sandwiched therebetween and with the polarizations of the piezoceramic elements  10   a,    10   b  set in the same direction. The metal electrode  11  is made from a conductive metal such as phosphor bronze or brass and has a very small thickness of 10 to 50 μm.  
         [0027]    Forming the metal electrode  11  ultrathin as described above can minimize a mechanical resistance of the metal electrode  11  so that even when deflective or bending vibrations occur at the joint surfaces between the two piezoceramic elements  10   a,    10   b  and the metal electrode  11  as motion centers (portions that do not expand or contract), attenuations by the metal electrode  11  of the bending vibrations can be kept as small as possible. In this construction when the ceramic element  10   a  on one side expands, the ceramic element  10   b  on the other side contracts and electrodes of their output voltages are in opposite directions. So, the piezoceramic elements  10   a,    10   b  form parallelly connected generators.  
         [0028]    Further, this embodiment can prevent the polarization cancellation during the bending vibrations, which would otherwise be caused by one ceramic element  10   a  (or  10   b ) performing both expanding and contracting motions, thus generating electricity efficiently. A current produced as an electric energy is extracted by lead wires  9  electrically connected to the ends of the metal electrode  11 .  
         [0029]    While in this embodiment two piezoceramic elements  10   a,    10   b  are stacked in layer through the metal electrode  11 , each ceramic element  10   a  ( 10   b ) itself may be formed in a layered structure. In the layered structure a plurality of piezoceramic elements are bonded together (in this case polarizations are oriented in the same direction) to form one piezoceramic element  10   a  (or  10   b ).  
         [0030]    When the piezoceramic element  10   a  (or  10   b ) itself is formed in a layered structure and these elements are bonded together with an adhesive that has an elasticity, for example, this elasticity effect can facilitate the bending of the piezoceramic member  10  that inherently lacks strength, thereby maintaining the bending strength. In this invention, the external shape of the piezoceramic member  10  is not limited to a particular shape but may take any desired geometry, such as circle, oval, triangle, square, or polygon, according to the application.  
         [0031]    The cushion member  3  used in this invention is formed from a soft material, such as synthetic resin, rubber or a sponge form of these materials. The reason that only the central portion or end portions of such a cushion member  3  is secured to the piezoceramic member  10  using the adhesive  5  is to prevent a possible attenuation of the vibrations of the piezoceramic member  10 . When the piezoceramic member  10  oscillates, those members supporting the piezoceramic member  10  tend to attenuate the oscillations of the piezoceramic member  10 . To remove this attenuation factor, the cushion member  3  is used to put the piezoceramic member  10  in as free a condition as possible.  
         [0032]    As in this invention, a strain of the piezoceramic member  10  becomes a natural vibration of the piezoceramic member and continues for a while. To have this natural vibration last long, it is important that this natural vibration not be transmitted to other structures than the piezoceramic member  10 . While the natural vibration of the piezoceramic member  10  is transformed into an electric energy, vibrations of other structures all constitute mechanical resistances absorbing the natural vibration energy, making it impossible to extract an electric energy from the natural vibration. Hence, in this embodiment, the cushion member  3  is used as a means to realize a flexible contact that prevents the natural vibration from being transferred between the piezoceramic member  10  and other structures. This arrangement allows the natural vibration of the piezoceramic member  10  to continue long, improving a generation efficiency. This cushion member  3  also acts to alleviate an impact on the piezoceramic member  10 . The protector plate  2  is made from a metal or synthetic resin and protects the piezoceramic member  10  from the impact of the steel ball  4 .  
         [0033]    While this embodiment uses the steel ball  4  placed in the pipe  7  as a member for impacting the piezoceramic member  10 , the material and shape of the impacting member are not limited to those of the steel ball. For example, it may be a cylindrical or egg-shaped heavy object. A member for accommodating the impacting member is also not limited to the pipe  7  but may take any other form that allows the impacting member to freely move. For example, it may be a rail on which the impacting member can travel. Further, instead of the pipe  7 , a spring member (extending upward or downward) may be fixed at one end and connected at the other end with the steel ball  4  so that the steel ball is struck against the piezoceramic members  10  on both sides by a lateral swinging motion of the spring member.  
         [0034]    When the piezoelectric generator of the above construction is placed in a predetermined motion environment that utilizes wind, wave or artificial actions, the steel ball  4  rolls in the pipe  7  striking the left and right piezoceramic members  10  to apply an impact energy to them. Subjected to the impacts, the piezoceramic members  10  are excited to oscillate, repeating expansion and contraction and generating an AC current. As for the amount of electricity generated in this way, the parallel type piezoelectric generator of this invention can produce almost two times the output current of the serial type piezoelectric generator previously proposed by the inventor of the present invention.  
         [0035]    [0035]FIG. 2 shows a circuit of a charger that uses the electricity generated by one of the piezoceramic members  10  of the above-described piezoelectric generator. In this embodiment, because two piezoceramic members  10  are opposed to each other, two such charging circuits are connected in parallel or in series. It is also possible to increase the number of the parallel circuits shown.  
         [0036]    This charging circuit has two piezoceramic elements  10   a,    10   b  as a piezoceramic member  10 , a metal electrode  11 , rectifying diodes D 1 -D 6 , a capacitor C for storing charge, a switch SW, and light emitting diodes L 1 -Le. The electricity generated by the piezoceramic element  10   a  is full-wave rectified by the diodes D 1 -D 3  and the electricity generated by the piezoceramic element  10   b  is full-wave rectified by the diodes D 4 -D 6 . These full-wave rectified electricity is charged to the capacitor C. In this parallel circuit only one capacitor C may be used. By operating the switch SW, the capacitor C is discharged to light up the light emitting diodes L 1 -L 3 .  
         [0037]    Using such a charging circuit to form a parallel type piezoelectric generator improves the generation efficiency by almost two times, making the piezoelectric generator of this invention a suitable power supply for a charging device and a light emitting device. Because of a simple construction, an ability to produce a large output current and an economical advantage, the piezoelectric generator described above is expected to find many practical applications. For example, this piezoelectric generator may be mounted on bicycles or attached to shoes to light up the light emitting diodes for their easy recognition at night. Also it may be mounted on battery-powered wrist watches or mobile cell phones and serve as a battery or a battery backup. Further, by using oscillations from waves to generate electricity, this piezoelectric generator can be expected to be used as a lighting device of a buoy.  
         [0038]    Since, with the piezoelectric generator of this invention, the ultrathin metal electrode is sandwiched between the two platelike piezoceramic elements with the polarizations of the piezoceramic elements oriented in the same direction and all these three members are bonded together to form a piezoceramic member of a layered structure, and since an impact load is applied to the piezoceramic member to generate electricity, this piezoelectric generator can produce almost two times the output current of the serial type piezoelectric generator previously proposed by the applicant of this invention, thereby significantly expanding the field of use and improving applicability.  
         [0039]    Further, since the piezoceramic member is held by the cushion member, the oscillation of the piezoceramic member can last long, improving the generation efficiency.  
         [0040]    Further, since the piezoceramic elements of the same configuration are combined to form a layered piezoceramic member, the expansion and contraction of the piezoceramic elements can be performed appropriately, improving the generation efficiency. Further, since each of the piezoceramic elements is formed of a plurality of piezoceramic elements bonded in layers, the strength of the piezoceramic elements are improved.  
         [0041]    Further, since the piezoceramic elements are made from a lead zirconate titanate material, they have a wide range of application as a piezoelectric generator.  
         [0042]    Furthermore, since the metal electrode is made from a conductive metal such as phosphor bronze or brass and formed 10-50 μm thick, when the piezoceramic elements undergo bending vibrations to generate electricity, the metal electrode produces almost no mechanical resistance, preventing the generation efficiency from deteriorating easily.