Abstract:
A piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator. A piezoelectric vibrator has a multilayered piezoelectric actuator, with special internal and surface electrodes making the body vibrate in a longitudinal and bending direction when an electrical signal is inputted. It can be manufactured using conventional multilayer piezoelectric actuator manufacturing techniques, therefore, it has reduced manufacturing time and cost, as it is not necessary to go through two polarization processes, and may simplify and decrease the volume, while improving the vibration performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of Korean Patent Application No. 2005-42711 filed with the Korea Industrial Property Office on May 20, 2005, the disclosure of which is incorporated herein by reference.  
       BACKGROUND  
       [0002]     1. Technical Field  
         [0003]     The present invention relates to a piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator, more specifically to a piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator that have simple compositions and small volumes and can improve vibration performance.  
         [0004]     2. Description of the Related Art  
         [0005]     Recently, the ultrasonic motor has received attention as a motor that does not require wound coils, to be suitable, for example, for decreasing the size of a device. The ultrasonic motor is widely used, as it entails low power consumption, has a light weight, provides linear motion directly without gears, allows the control of speed and position electrically, and allows movement in either the forward or reverse direction.  
         [0006]      FIG. 1  is a plan view of a piezoelectric vibrator  10  used in a conventional ultrasonic motor.  
         [0007]     A conventional piezoelectric vibrator  10  comprises a rectangular piezoelectric element  13  made of piezoelectric ceramic, etc., and a protrusion part  11  formed on a side of the piezoelectric element  13 . The protrusion part  11  applies pressure on the object of vibration (not shown), where the protrusion part  11  moves the object of vibration due to the vibration of the piezoelectric element  13 . There are four polarization regions, i.e. a first polarization region  13   a , second polarization region  13   b , third polarization region  13   c , and fourth polarization region  13   d , formed on the piezoelectric element  13 , where all polarization regions  13   a ,  13   b ,  13   c ,  13   d  have the same polarization direction in the direction of thickness. The four polarization regions  13   a ,  13   b ,  13   c ,  13   d  have the same size and are arranged in two rows. On each of the four polarization regions  13   a ,  13   b ,  13   c ,  13   d  is formed an electrode.  
         [0008]     The first and fourth polarization regions  13   a ,  13   d  have the same polarization direction, while the second and third polarization regions  13   b ,  13   c  have a polarization direction opposite to that of the first polarization region  13   a . Also, the first and fourth polarization regions  13   a ,  13   d  and the second and third polarization regions  13   b ,  13   c  are connected respectively by a lead wire  17 .  
         [0009]     The piezoelectric element  13  vibrates in longitudinal and bending directions when an electric current is supplied to the first and fourth polarization regions  13   a ,  13   d . Similarly; when an electric current is supplied to the second and third polarization regions,  13   b ,  13   c , the piezoelectric element  13  vibrates in longitudinal and bending directions however, this time the direction of bending vibration is opposite to previous case.  
         [0010]     Since the conventional piezoelectric vibrator  10  has two polarization directions on one piezoelectric element  13 , as described above, two polarization processes are required. This entails the problems of increased manufacturing time and cost of the piezoelectric element. In particular, if the two polarization processes are performed separately on one piezoelectric element  13 , depolarization may occur on the portions where the polarization is performed first, to consequently lower the performance of the piezoelectric element  13 .  
         [0011]     Further, in the conventional piezoelectric vibrator  10 , only one pair of polarization regions  13   a ,  13   d  positioned diagonally are excited, while the other pair  13   b ,  13   c  are not, which lowers the vibration performance of the piezoelectric vibrator  10 . This means that a higher voltage must be supplied to improve the vibration performance of the conventional piezoelectric vibrator  10 . Moreover, to improve vibration performance, the conventional piezoelectric vibrator  10  was used with the piezoelectric elements  13  stacked in multiple layers, which incurs the problem of increased volume of the piezoelectric element.  
         [0012]     Also, polarization involves supplying a high DC voltage to a piezoelectric element  13  to arrange the dipoles within the piezoelectric element  13  into a desired orientation, and during the polarization process, large amounts of stress are concentrated on the boundaries of the electrodes positioned in-between the stacked piezoelectric elements  13 . Such stress becomes a major cause of cracks later during the operation of the piezoelectric vibrator  10 , and deteriorates the properties of the piezoelectric element  13 .  
       SUMMARY  
       [0013]     As a solution to the foregoing problems, an aspect of the invention provides a piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator, with which the manufacturing time and cost are reduced, as it is not necessary to go through two polarization processes.  
         [0014]     Another aspect of the invention provides a piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator, with which the volume may be decreased, while also providing a simple composition and improved vibration performance.  
         [0015]     Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept.  
         [0016]     A piezoelectric vibrator according to a first disclosed embodiment of the invention comprises an elastic member having a quadrilateral cross section, and a piezoelectric element attached to each side of the elastic member and vibrating the elastic member in a longitudinal direction and a bending direction when an electrical signal is inputted, where the piezoelectric elements may have the same size and may be shorter than the elastic member.  
         [0017]     Since a piezoelectric vibrator having such a composition uses piezoelectric elements having a single polarization direction, the manufacturing time and cost of the piezoelectric vibrator may be reduced. In addition, since the piezoelectric elements attached respectively to each side of the elastic member vibrate simultaneously, the vibration performance may be improved and the volume of the piezoelectric vibrator may be decreased. Also, since the piezoelectric elements vibrate the elastic member, a greater rigidity may be provided compared to conventional piezoelectric vibrators.  
         [0018]     The pair of piezoelectric elements attached to opposing sides of the elastic member may be polarized in the same direction, while the other pair may be polarized in opposite directions. Thus, one pair of piezoelectric elements may make the whole vibrator body vibrate in a longitudinal direction, while the other pair of piezoelectric elements may make the whole vibrator body vibrate in a bending direction. Therefore, the combination of longitudinal motion and bending motion may cause an end of the elastic member to move in an elliptical trajectory.  
         [0019]     One end of the piezoelectric element may be aligned with one end of the elastic member to concentrate the vibration on one end of the elastic member. Further, one edge of the piezoelectric element may be trimmed and the trimmed edge may be positioned to face outward, so as to prevent short circuits between the piezoelectric elements.  
         [0020]     Preferably, the length of the elastic member may be twice the length of the piezoelectric element, to maximize the vibration of the elastic member. Since voltages with a phase difference of 90° are supplied respectively to the pair of piezoelectric elements attached to opposing sides of the elastic member and the other pair of piezoelectric elements, one pair may vibrate in a bending direction, and the other pair may vibrate in a longitudinal direction.  
         [0021]     A piezoelectric vibrator according to a second disclosed embodiment of the invention comprises a pair of first piezoelectric elements, having the same rectangular parallelepiped shape and polarized in opposite directions, and a pair of second piezoelectric elements, having the same rectangular parallelepiped shape and respectively attached to a side of the first piezoelectric element, and polarized in the same direction, where the first piezoelectric elements may be longer than the second piezoelectric elements, and the first piezoelectric elements may vibrate in a longitudinal direction while the second piezoelectric elements may vibrate in a bending direction when electrical signals are inputted.  
         [0022]     Since the piezoelectric vibrator according to the disclosed embodiments of the invention uses piezoelectric elements having a single polarization direction, the manufacturing time and cost of the piezoelectric vibrator may be reduced. In addition, since the piezoelectric elements attached respectively to each side of the elastic member vibrate simultaneously, the vibration performance may be improved and the volume of the piezoelectric vibrator may be decreased. Also, since only the piezoelectric elements are used, the piezoelectric vibrator may be produced with greater ease in manufacture.  
         [0023]     By aligning one end of each of the second piezoelectric elements with one end of a first piezoelectric element, the displacement may be maximized on one end of each of the second piezoelectric elements, and by attaching each of the second piezoelectric elements to the center of a first piezoelectric element, both ends of the second piezoelectric element may be made to vibrate.  
         [0024]     It may be preferable for the length of the first piezoelectric element to be twice the length of the second piezoelectric element, to maximize the amount of vibration of the first piezoelectric element. Also, since voltages with a phase difference of 90° are supplied respectively to the first piezoelectric elements and the second piezoelectric elements, the first and second piezoelectric elements may vibrate in the longitudinal and bending directions simultaneously.  
         [0025]     A piezoelectric vibrator according to a fourth disclosed embodiment of the invention comprises multiple layers of piezoelectric elements having one polarization direction, conductive electrodes formed on both faces of the piezoelectric elements and interconnected to one another, and a protrusion part formed on a side of the piezoelectric elements, where the adjacent piezoelectric elements may be polarized respectively in two opposite directions, and the electrodes may be interconnected.  
         [0026]     Since the piezoelectric vibrator according to the fourth disclosed embodiment of the invention uses piezoelectric elements having a single polarization direction, the manufacturing time and cost of the piezoelectric vibrator may be reduced. Also, since the piezoelectric elements vibrate simultaneously, the vibration performance may be improved, and the volume of the piezoelectric vibrator may be decreased.  
         [0027]     By supplying a 4-phase electrical signal to the conductive electrode, the magnitude of the electrical signal may be increased further.  
         [0028]     An ultrasonic motor according to a fifth disclosed embodiment of the invention is equipped with a piezoelectric vibrator of any of the first to fourth disclosed embodiments, and comprises a case into which the piezoelectric vibrator is inserted, a slider inserted into the case to be movable in vertical directions and moving in contact with the piezoelectric vibrator, a first pressing member for pressing the piezoelectric vibrator towards the slider, and a second pressing member for pressing the slider towards the piezoelectric vibrator.  
         [0029]     The ultrasonic motor according to the fifth disclosed embodiment of the invention has a decreased volume, and can increase the amount of vibration with a lower voltage. Also, since the piezoelectric vibrator and the slider are held together steadily by the first and second pressing members, the vibration of the piezoelectric vibrator is efficiently transferred to the slider.  
         [0030]     The first pressing member may have a circular cross section and may be pressed towards the sliders by a flat spring inserted into the case, to hold the piezoelectric vibrator and the slider together more steadily.  
         [0031]     The case may comprise a vibrator housing part into which the piezoelectric vibrator is inserted, slider insertion holes leading to the vibrator housing part through which the sliders are inserted, first pressing member fitting grooves formed on one end of the case in a pre-determined depth into which the first pressing member is inserted to contact one end of the piezoelectric vibrator, second pressing member insertion holes formed perpendicularly to the slider insertion holes through which the second pressing member is inserted to contact the slider, and spring insertion grooves formed perpendicularly to the first pressing member fitting grooves through which the flat spring is inserted to contact the first pressing member. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]     These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the disclosed embodiments, taken in conjunction with the accompanying drawings of which:  
         [0033]      FIG. 1  is a plan view of a conventional piezoelectric vibrator.  
         [0034]      FIG. 2  is a perspective view of a piezoelectric vibrator according to a first disclosed embodiment of the invention.  
         [0035]      FIG. 3  is a schematic diagram illustrating the polarization directions of piezoelectric elements in a piezoelectric vibrator according to a first disclosed embodiment of the invention.  
         [0036]      FIG. 4  is a graph illustrating the admittances of the piezoelectric elements with respect to changes in frequency, in a piezoelectric vibrator according to a first disclosed embodiment of the invention.  
         [0037]      FIG. 5  is an illustration using the ATILA™ software of the longitudinal vibration of a piezoelectric vibrator according to a first disclosed embodiment of the invention.  
         [0038]      FIG. 6   a  is an illustration using the ATILA™ software of the bending vibration in the direction of the x-axis of a piezoelectric vibrator according to a first disclosed embodiment of the invention.  
         [0039]      FIG. 6   b  is an illustration using the ATILA™ software of the bending vibration in the direction of the y-axis of a piezoelectric vibrator according to a first disclosed embodiment of the invention.  
         [0040]      FIG. 7  is a perspective view of a piezoelectric vibrator according to a second disclosed embodiment of the invention.  
         [0041]      FIG. 8  is a schematic diagram illustrating the polarization directions of piezoelectric elements in a piezoelectric vibrator according to a second disclosed embodiment of the invention.  
         [0042]      FIG. 9   a  is an illustration using the ATILA™ software of the longitudinal vibration of a piezoelectric vibrator according to a second disclosed embodiment of the invention.  
         [0043]      FIG. 9   b  is an illustration using the ATILA™ software of the bending vibration of a piezoelectric vibrator according to a second disclosed embodiment of the invention.  
         [0044]      FIG. 10  is a perspective view of a piezoelectric vibrator according to a third disclosed embodiment of the invention.  
         [0045]      FIG. 11  is a schematic diagram illustrating the polarization directions of piezoelectric elements in a piezoelectric vibrator according to a third disclosed embodiment of the invention.  
         [0046]      FIG. 12   a  is an illustration using the ATILA™ software of the longitudinal vibration of a piezoelectric vibrator according to a third disclosed embodiment of the invention.  
         [0047]      FIG. 12   b  is an illustration using the ATILA™ software of the bending vibration of a piezoelectric vibrator according to a third disclosed embodiment of the invention.  
         [0048]      FIG. 13  is a perspective view of a piezoelectric vibrator according to a fourth disclosed embodiment of the invention.  
         [0049]      FIG. 14  is a perspective view of an example of conductive electrodes in a piezoelectric vibrator according to a fourth disclosed embodiment of the invention.  
         [0050]      FIG. 15  is a schematic diagram illustrating the polarization directions of piezoelectric elements in a piezoelectric vibrator according to a fourth disclosed embodiment of the invention.  
         [0051]      FIG. 16   a  is an illustration using the ATILA™ software of the longitudinal vibration of a piezoelectric vibrator according to a fourth disclosed embodiment of the invention.  
         [0052]      FIG. 16   b  is an illustration using the ATILA™ software of the bending vibration of a piezoelectric vibrator according to a fourth disclosed embodiment of the invention.  
         [0053]      FIG. 17  is an exploded perspective view of an ultrasonic motor according to a fifth disclosed embodiment of the invention.  
         [0054]      FIG. 18  is an assembled perspective view of an ultrasonic motor according to a fifth disclosed embodiment of the invention.  
         [0055]      FIG. 19  is a cross-sectional view of an ultrasonic motor according to a fifth disclosed embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0056]     Hereinafter, embodiments of the present invention will be described in more detail with reference to the accompanying drawings.  
         [0057]      FIG. 2  is a perspective view of a piezoelectric vibrator according to a first disclosed embodiment of the invention. The piezoelectric vibrator  30  according to the first disclosed embodiment comprises an elastic member  31 , having a constant length and a quadrilateral cross section, and four piezoelectric elements  33 , having the same size and attached respectively to each side of the elastic member  31 .  
         [0058]     When an electrical signal is inputted to the piezoelectric element  33  and vibration occurs, the elastic member  31  vibrates in a longitudinal direction or a bending direction, so that consequently the end of the elastic member vibrates in an elliptical trajectory. This causes the object of vibration (not shown) in contact with the end of the elastic member  31  to vibrate, due to the frictional force with the elastic member  31 .  
         [0059]     The elastic member  31  may be made from any material having elastic force, for example, brass or stainless steel, etc. It is preferable that the length of the elastic member  31  be twice the length of the piezoelectric elements to maximize the vibration generated on the elastic member  31 . Also, as illustrated in  FIG. 2 , by aligning the piezoelectric elements  33  with one end of the elastic member  31 , the vibration generated on the other end of the elastic member  31  may be maximized.  
         [0060]     Although in the first disclosed embodiment the elastic member  31  is described as having a quadrilateral cross section, the present invention is not thus limited, and an elastic member  31  having any cross section may be used that can vibrate in the longitudinal direction or bending direction using the vibration of the piezoelectric elements  33 . For example, an elastic member may be used that has an octagonal cross section, with the neighboring piezoelectric elements positioned respectively on each side of the elastic member arranged to have opposite polarization directions.  
         [0061]     All of the piezoelectric elements  33  have the same size and are attached respectively on each side of the elastic member  31  by means of epoxy resin, etc. The length of the piezoelectric elements  33  corresponds to half the length of the elastic member  31 . Also, when the piezoelectric elements  33  are attached respectively to each side of the elastic member  31 , as shown in  FIG. 2 , the assembled cross section of the four piezoelectric elements  33  forms a quadrilateral. The thickness of the piezoelectric elements  33  is determined depending on the size and shape of the piezoelectric elements  33 .  
         [0062]     The piezoelectric element  33  is formed from a material having a piezoelectric effect (a piezoelectric material). Suitable examples may include PZT-based ceramics and PbTiO3-based ceramics, etc. Suitable examples of PZT-based ceramics may include PZT and Pb(Ni⅓Nb⅔)O3-Pb(Zn⅓Nb⅔)O3-PbTiO3-PbZrO3 ceramics. The piezoelectric element  33  has unique vibration characteristics, where a strong vibration is generated when the frequency of the electrical signal inputted to the piezoelectric element  33  coincides with the characteristic frequency of the piezoelectric element.  
         [0063]     A conductive electrode (not shown) is attached on one face of the piezoelectric element  33 , and the electrical signal is inputted to the electrode. On one face of the piezoelectric element  33  is formed a polishing part  35 , trimmed by mechanical processing, etc. This prevents short circuits between the electrodes of each of the adjoining piezoelectric elements  33 .  
         [0064]      FIG. 3  is a schematic diagram illustrating the polarization directions of piezoelectric elements  33  in a piezoelectric vibrator  30  according to the first disclosed embodiment of the invention.  
         [0065]     As shown in  FIG. 3 , one pair of piezoelectric elements  33  attached to opposing sides of the elastic member  31  are polarized in the same direction, to vibrate in a bending direction with the input of an electrical signal. Also, the other pair of piezoelectric elements  33  attached to opposing sides of the elastic member  31  are polarized in opposite directions, to vibrate in a longitudinal direction with the input of an electrical signal. Electrical signals having a phase difference of 90° are inputted to each pair of the piezoelectric elements. For example, a voltage having a frequency of sin ωt (where ω is angular frequency) may be supplied to the pair of piezoelectric elements polarized in the same direction, while a voltage having a frequency of cos ωt (where w is angular frequency) may be supplied to the pair of piezoelectric elements polarized in opposite directions.  
         [0066]     As seen in  FIG. 3 , the piezoelectric element  33  has a single polarization direction, unlike the conventional piezoelectric element  13 , to provide the advantages of easy manufacture of the piezoelectric element and low manufacturing cost. Moreover, as will be described hereafter, since all of the piezoelectric elements  33  are excited during the vibration of the piezoelectric vibrator  30 , the magnitude of the vibration may be increased.  
         [0067]      FIG. 4  is a graph illustrating the admittances of the piezoelectric elements with respect to changes in frequency of the signals supplied to a piezoelectric vibrator  30  according to the first disclosed embodiment. Here, the elastic member  31  is made of brass, having a length of 8.0 mm and a square cross section with a length of 0.7 mm for each side. The piezoelectric elements have a length of 4.0 mm, a width of 1.0 mm, and a thickness of 0.3 mm. In  FIG. 4 , the horizontal axis represents the frequency of the signals supplied to the piezoelectric element  33 , and the vertical axis represents the admittance, in units of  
       S   =     A   V           
 [siemens]. Also, in  FIG. 4 , one of the overlapping curves that has two peaks around 110 kHz and 200 kHz represents the admittance measured by an impedance analyzer for the pair of piezoelectric elements polarized in opposite directions and the peaks represent second and third bending modes, while the other overlapping curve that has single peak at around 200 kHz represents the admittance of the other pair of piezoelectric elements polarized in the same direction and the single peak represent first longitudinal mode. The curve above the two overlapping curves is the total admittance of the vibrator, where first longitudinal and third bending modes are combined around 200 kHz. 
 
         [0068]     The higher the admittance of the piezoelectric element  33 , i.e. the lower the impedance of the piezoelectric elements, the greater is the vibration of the piezoelectric elements  33 . As seen in  FIG. 4 , the admittances of the piezoelectric elements  33  increase drastically at certain frequencies, and these frequencies at which the vibrations of the piezoelectric elements  33  increase drastically are the resonance frequencies.  
         [0069]     Table 1 was formed using the ATILA™ software to represent the resonance frequencies (Fr) at which the admittances increase drastically, the anti-resonance frequencies, the electromechanical coupling, and the vibration directions in  FIG. 4 .  
                                                     TABLE 1                                   Electromechanical   Direc-       Resonance Mode   Fr (Hz)   Fa (Hz)   Coupling (%)   tion                                first bending   36307.5   36307.8   0.37   B1_x       first bending   36307.5   36417.4   7.76   B1_y       second bending   119642   119645   0.69   B2_y       second bending   119642   120819   13.92   B2_x       first longitudinal   198706   199284   7.61   L1       third bending   199335   199393   2.43   B3_x       third bending   199335   201690   15.24   B3_y                  
 
         [0070]     As seen in  FIG. 4  and Table 1, the piezoelectric vibrator  30  according to the first disclosed embodiment vibrates drastically in the bending direction at resonance frequencies Fr=36307.5, 119642, and 199335 (Hz), and vibrates drastically in the longitudinal direction at Fr=198706 (Hz). Here, since the frequency range for the third bending and first longitudinal vibrations are very similar, electrical signals of this frequency are supplied to the piezoelectric elements  31  to generate vibration.  
         [0071]     Here, since the frequency range for the third bending and first longitudinal vibrations are very similar, the piezoelectric elements  31  vibrate in the bending direction and the longitudinal direction, simultaneously. In addition, since the electromechanical coupling is the greatest for the third bending and first longitudinal vibrations, maximum mechanical vibration may occur with electrical signals having the same magnitude. Electromechanical coupling represents the conversion rate between electrical and mechanical energy, and when a large mechanical output (e.g. displacement) is generated for a certain electrical input, it may be said that there is large electromechanical coupling.  
         [0072]      FIG. 5  is an illustration using the ATILA™ software of the longitudinal vibration of the piezoelectric vibrator  30  according to the first disclosed embodiment of the invention, and  FIGS. 6   a  and  6   b  are graphs illustrating the bending vibration of the piezoelectric vibrator  30 .  
         [0073]     When an electrical signal is inputted to the piezoelectric vibrator  30 , the elastic member  33  vibrates in the longitudinal direction (L1 longitudinal) through repetitions of elongation and contraction, as shown in  FIG. 5 . Here, the frequency number, found using the ATILA™ software, is Fr=198706 (Hz). Also, when a frequency of Fr=199335 (Hz) is inputted, the elastic member  33  undergoes a B3 bending motion with three bends, as shown in  FIGS. 6   a  and  6   b . Due to the combination of the L1 longitudinal and the B3 bending vibrations, one end of the elastic member  31  vibrates in an elliptical trajectory.  
         [0074]     In the piezoelectric vibrator  30  according to the first disclosed embodiment of the invention, all of the piezoelectric elements  33  vibrate when an electrical signal is inputted, unlike conventional piezoelectric elements, so that not only can the vibration be increased, but also the volume of the piezoelectric vibrator may be decreased.  
         [0075]      FIG. 7  is a perspective view of a piezoelectric vibrator  40  according to a second disclosed embodiment of the invention. Referring to  FIG. 7 , the piezoelectric vibrator  40  according to the second disclosed embodiment comprises first piezoelectric elements  41  having a pair of piezoelectric elements  41   a ,  41   b  of equal size, and second piezoelectric elements  43  having a pair of piezoelectric elements  43   a ,  43   b  of a shorter length compared to the first piezoelectric elements  41 .  
         [0076]     The first piezoelectric elements  41  are formed by stacking the pair of identical piezoelectric elements  41   a ,  41   b . The first piezoelectric element  41  is formed from the same piezoelectric ceramics as the piezoelectric element  33  of the first embodiment described above. A conductive electrode (not shown) is formed in-between the first piezoelectric element  41 , which supplies electrical signals inputted from an outside source to the first piezoelectric element  41 . Also, the attachment surface of the first piezoelectric element  41  is grounded. One end of the first piezoelectric element  41  is in contact with the object of vibration (not shown) to transfer driving power to the object of vibration by means of the longitudinal and bending vibrations.  
         [0077]     The second piezoelectric elements  43  are formed as a pair of identical piezoelectric elements  43   a ,  43   b  are each attached to a face of the first piezoelectric elements  41   a ,  41   b  by means of epoxy resin, etc. In the piezoelectric vibrator  40  of the second disclosed embodiment, one end of the second piezoelectric elements  43  is aligned with one end of the first piezoelectric elements  41 . Therefore, similar to the elastic member  31  of the first disclosed embodiment, only one end of the first piezoelectric elements  41  vibrate and transfer vibrational force to the object of vibration.  
         [0078]     The second piezoelectric elements  43  are formed from piezoelectric ceramics, with the same thickness and width as the first piezoelectric elements  41 . Also, forming the second piezoelectric elements  43  to have half the length of the first piezoelectric elements  41  may maximize the vibration. Conductive electrodes (not shown) are formed on the upper and lower faces of the second piezoelectric elements  43 , by which electrical signals supplied from an outside source is transferred to the second piezoelectric elements  43 .  
         [0079]      FIG. 8  is a schematic diagram illustrating the polarization directions of piezoelectric elements  41 ,  43  in the piezoelectric vibrator  40  according to the second disclosed embodiment of the invention. Referring to  FIG. 8 , the first piezoelectric elements  41  are polarized in opposite directions, and the second piezoelectric elements  43  are polarized in the same direction. When electrical signals having a phase difference of 90° are inputted to the first piezoelectric elements  41  and second piezoelectric elements  43 , the first piezoelectric elements  41  make the whole stator body vibrate in the longitudinal direction, while at the same time the second piezoelectric elements  43  make the whole stator body vibrate in the bending direction. Thus, due to the combination of the longitudinal and bending vibrations, both ends of the first piezoelectric elements  41  vibrate in an elliptical trajectory.  
         [0080]      FIGS. 9   a  and  9   b  are graphs using the ATILA™ software to illustrate the vibration of the piezoelectric vibrator according to the second disclosed embodiment of the invention. As seen in  FIGS. 9   a  and  9   b , the piezoelectric vibrator  40  according to the second disclosed embodiment is capable of L1 longitudinal and B3 bending vibrations, as in the first disclosed embodiment.  
         [0081]     With regards the piezoelectric elements  41 ,  43  in the piezoelectric vibrator  40  according to the second disclosed embodiment, it is seen that the manufacture of the piezoelectric elements is made easier, as each piezoelectric element  41   a ,  41   b ,  43   a ,  43   b  has the same polarization direction. Also, since the first piezoelectric elements  41  and the second piezoelectric elements  43  vibrate simultaneously, the piezoelectric vibrator may have a simple structure, with improved vibration performance, while the volume of the piezoelectric vibrator  40  may be decreased. Further, with the piezoelectric vibrator  40  according to the second disclosed embodiment, vibration is generated by the piezoelectric elements  41 ,  43  only, to allow high efficiency and easy manufacture.  
         [0082]      FIG. 10  is a perspective view of a piezoelectric vibrator  40 ′ according to a third disclosed embodiment of the invention. Referring to  FIG. 10 , the piezoelectric vibrator  40 ′ according to the third disclosed embodiment comprises first piezoelectric elements  41 ′ having a pair of piezoelectric elements  41   a ′,  41   b ′ of equal size, and second piezoelectric elements  43 ′ having a pair of piezoelectric elements  43   a ′,  43   b ′ of a shorter length compared to the first piezoelectric elements  41 ′.  
         [0083]     The composition of the first piezoelectric elements  41 ′ and the second piezoelectric elements  43 ′ are identical to the first piezoelectric elements  41  and the second piezoelectric elements  43  in the piezoelectric elements of the second disclosed embodiment described above. The difference from the second disclosed embodiment is that the second piezoelectric elements  43 ′ are positioned at the center of the first piezoelectric elements  41 ′.  
         [0084]     The lengthwise center of the second piezoelectric elements  43 ′ coincides with the lengthwise center of the first piezoelectric elements  41 ′. Therefore, when electrical signals are inputted to the first piezoelectric elements  41 ′ and the second piezoelectric elements  43 ′, both ends of the first piezoelectric elements  41 ′ vibrate in elliptical trajectories.  
         [0085]      FIG. 11  is a schematic diagram illustrating the polarization directions of the piezoelectric elements  41 ′,  43 ′ in the piezoelectric vibrator  40 ′ according to the third disclosed embodiment of the invention. Referring to  FIG. 11 , the first piezoelectric elements  41 ′ are polarized in opposite directions, and the second piezoelectric elements  43 ′ are polarized in the same direction, which is the same as the polarization directions of the piezoelectric elements of the second disclosed embodiment.  
         [0086]      FIGS. 12   a  and  12   b  are illustrations using the ATILA™ software of the vibration of the piezoelectric vibrator  40 ′ according to the third disclosed embodiment. As illustrated in  FIGS. 12   a  and  12   b , the piezoelectric vibrator  40 ′ according to the third disclosed embodiment undergoes L1 longitudinal and B3 bending vibrations, as do the piezoelectric vibrators of the first and second disclosed embodiments described above. Both ends of the first piezoelectric elements  41 ′ vibrate simultaneously in elliptical trajectories.  
         [0087]     With regards the piezoelectric elements  41 ′,  43 ′ in the piezoelectric vibrator  40 ′ according to the third disclosed embodiment, it is seen that the manufacture of the piezoelectric elements is made easier, as each piezoelectric element  41   a ′,  41   b ′,  43   a ′,  43   b ′ has the same polarization direction. Also, since the first piezoelectric elements  41 ′ and the second piezoelectric elements  43 ′ vibrate simultaneously, the piezoelectric vibrator may have a simple structure, with improved vibration performance, while the volume of the piezoelectric vibrator  40 ′ may be decreased. Further, with the piezoelectric vibrator  40 ′ according to the second disclosed embodiment, vibration is generated by the piezoelectric elements  41 ′,  43 ′ only, to allow high efficiency and easy manufacture.  
         [0088]      FIG. 13  is a perspective view of a piezoelectric vibrator  50  according to a fourth disclosed embodiment of the invention. The piezoelectric vibrator  50  according to the fourth disclosed embodiment comprises identical piezoelectric elements  51  stacked in multiple layers, conductive electrodes  53  formed between the piezoelectric elements  51 , and a protrusion part  55  protruded from one side of the piezoelectric elements  51 .  
         [0089]     The piezoelectric elements  51  have equal sizes and are stacked in multiple layers. The piezoelectric elements  51  are formed from the same piezoelectric ceramics as in the piezoelectric elements  33 ,  41 ,  41 ′ of the first to third disclosed embodiments described above. Electrodes are formed on both faces of the piezoelectric element  51  which supply electrical signals to the piezoelectric element  51 . Although in  FIG. 13  the piezoelectric elements  51  are formed in seven layers, the present invention is not thus limited, and a stack of six or less or eight or more layers may be used, depending on the size of the piezoelectric elements  51  and desired amount of vibration, etc.  
         [0090]     The protrusion part  55  protrudes from a side of the piezoelectric element  51  to the exterior by a predetermined length. The protrusion part  55  vibrates in an elliptical trajectory, due to the vibration of the piezoelectric elements  51  in the longitudinal and bending directions. As the protrusion part  55  is in contact with the object of vibration (not shown), the object of vibration is made to vibrate by means of the protrusion part  55 .  
         [0091]     Since the protrusion part  55  transfers driving power using frictional force with the object of vibration, a wear-resistant member may be formed on the protrusion part  55 . The wear-resistant member may include various materials containing glass materials, such as soda, lead, borates (e.g. Pyrex™), crowns, flint, heavy flint, and quartz glass, etc., or ceramic materials, such as alumina, zirconium oxides, silicon carbides, silicon nitrides, tungsten carbide, and titanium carbide, etc.  
         [0092]      FIG. 14  is a perspective view of an example of the conductive electrodes  53  in the piezoelectric vibrator  50  according to the fourth disclosed embodiment of the invention. Referring to  FIG. 14 , the electrodes  53  include an upper electrode  53   a  formed on the uppermost face of the piezoelectric elements  51 , a lower electrode  53   b  formed on the lowermost face, and a first to sixth inner electrodes  53   c   1 ,  53   c   2 ,  53   c   3 ,  53   c   4 ,  53   c   5 ,  53   c   6  stacked orderly on the faces of the piezoelectric elements  51 .  
         [0093]     As shown in  FIG. 14 , the upper electrode  53   a  and the second inner electrode  53   c   2  have identical patterns, and the first inner electrode  53   c   1  and the third inner electrode  53   c   3  have identical patterns. Also, the fourth inner electrode  53   c   4  and the sixth inner electrode  53   c   6  have identical patterns, and the fifth inner electrode  53   c   5  and the lower electrode  53   b  have identical patterns. Further, the first inner electrode  53   c   1  and third inner electrode  53   c   3  have a pattern symmetrical to the fourth inner electrode  53   c   4  and sixth inner electrode  53   c   6  in the length and width directions of the electrodes. Also, the upper electrode  53   a  and second inner electrode  53   c   2  are symmetrical to the lower electrode  53   b  and fifth inner electrode  53   c   5  in the length and width directions of the electrodes. The upper electrode  53   a , the lower electrode  53   b  and the first to sixth inner electrodes  53   c   1 ,  53   c   2 ,  53   c   3 ,  53   c   4 ,  53   c   5 ,  53   c   6  are used to polarize the piezoelectric element  51  and supply electrical signals. The electrodes  53  according to the fourth disclosed embodiment are not limited to those shown in  FIG. 14 , and it is apparent that a variety of modifications may be made by the skilled person.  
         [0094]     Sin and −sin are inputted to the upper electrode  53   a , and cos and −cos are inputted to the lower electrode  53   b . Thus, electrical signals having four phases are inputted to the piezoelectric vibrator  50  according to the fourth disclosed embodiment. Since sin and −sin and cos and −cos are inputted to the upper electrode  53   a  and the lower electrode  53   b , the input of electrical signals with a relative magnitude of 2 sin (or 2 cos) may be effected.  
         [0095]      FIG. 15  is a schematic diagram illustrating the polarization directions of piezoelectric elements  51  in the piezoelectric vibrator  50  according to the fourth disclosed embodiment. As shown in  FIG. 15 , all adjacent piezoelectric elements  51  are polarized in opposite directions. Also, each of the piezoelectric elements  51  has one polarization direction.  
         [0096]      FIGS. 16   a  and  16   b  are illustrations using the ATILA™ software of the vibration of the piezoelectric vibrator  50  according to the fourth disclosed embodiment. As shown in  FIGS. 16   a  and  16   b , the piezoelectric vibrator  50  according to the fourth disclosed embodiment vibrates in the longitudinal direction at 265 kHz, and vibrates in B2 mode in the bending direction at 267 kHz. Therefore, as the frequency band ranges are the same, the piezoelectric vibrator  50  vibrates simultaneously.  
         [0097]     Since all of the piezoelectric elements  51  in each layer of the piezoelectric vibrator  50  according to the fourth disclosed embodiment have a uniform polarization direction, it is seen that the manufacture of the piezoelectric elements is made easier. Moreover, since all of the piezoelectric elements  51  vibrate simultaneously, the piezoelectric vibrator may have a simple structure, with improved vibration performance, while the volume of the piezoelectric vibrator  50  may be decreased.  
         [0098]     Hereinafter, an ultrasonic motor  70  according to a fifth disclosed embodiment of the invention will be described with reference to FIGS.  17  to  19 .  
         [0099]      FIG. 17  is an exploded perspective view of the ultrasonic motor  70  according to the fifth disclosed embodiment of the invention, and  FIG. 18  is a perspective view representing the ultrasonic motor  70  illustrated in  FIG. 17  in its assembled state.  FIG. 19  is an assembled cross-sectional view of the ultrasonic motor of  FIGS. 17 and 18 .  
         [0100]     Referring to  FIG. 17 , an ultrasonic motor  70  based on an aspect of the present invention comprises a case  71 , a piezoelectric vibrator  80  inserted into the case, a first pressing member  73  pressing the rear end of the piezoelectric vibrator  80 , second pressing members  75  pressing the sliders  79   a ,  79   b , and a flat spring  77  pressing the first pressing member  73 .  
         [0101]     The case  71  houses the piezoelectric vibrator  80 , first pressing member  73 , second pressing members  75 , flat spring  77 , and sliders  79   a ,  79   b . The case  71  comprises a vibrator housing part  715  into which the piezoelectric vibrator  80  is inserted, slider insertion holes  713  holding the sliders  79   a ,  79   b , first pressing member fitting grooves  717  into which the first pressing member  73  is inserted, second pressing member insertion holes  711  through which the second pressing members  75  are inserted, and spring insertion grooves  719  through which the flat spring  77  is inserted.  
         [0102]     The vibrator housing part  715  is formed in the center of the case  71 . Although both ends of the piezoelectric vibrator  80  are isolated by the case  71  from the exterior, the other parts are exposed to the exterior. The piezoelectric vibrator  80  is inserted into and fixed in the vibrator housing part  715 . The slider insertion holes  713  lead to the vibrator housing part  715 .  
         [0103]     A portion of the sliders  79   a ,  79   b  is inserted through the slider insertion holes  713 . Since the diameter of the slider insertion holes  713  is somewhat greater than the diameter of the sliders  79   a ,  79   b , the sliders  79   a ,  79   b  may freely ascend and descend. The slider insertion holes  713  lead to the vibrator housing part  715  and are formed perpendicularly to the second pressing member insertion holes  711 .  
         [0104]     Both ends of the second pressing members  75   a ,  75   b  are inserted through the second pressing member insertion holes  711 . The first pressing member fitting grooves  717  are grooves formed at one end of the case  71  in the length direction having the shape of slots with the ends on one side open. The first pressing member  73  is fitted into the first pressing member fitting grooves  717  to press the rear side of the piezoelectric element  80  inserted into the vibrator housing part  715 . The spring insertion grooves  719  are grooves formed on the case  71  in a vertical direction, and the flat spring  77  inserted into the spring insertion grooves  719  presses the first pressing member  73 .  
         [0105]     A piezoelectric vibrator  30 ,  40 ,  40 ′  50  according to the first to fourth disclosed embodiments may be used for the piezoelectric vibrator  80 . A protrusion part  81  is formed on one end of the piezoelectric vibrator  80 , where the protrusion part  81  moves the sliders  79   a ,  79   b  in vertical directions using frictional force. The composition of the piezoelectric vibrator  80  is the same as in the first to fourth disclosed embodiments, so that detailed explanations are omitted.  
         [0106]     The sliders  79  include a first slider  79   a  inserted through the slider insertion holes  713  to contact the protrusion part  81  of the piezoelectric element  80 , and a second slider  79   b  which guides the first slider  79   a  to prevent it from rotating. Since the first slider  79   a , as shown in  FIG. 19 , is in contact with the protrusion part  81  of the piezoelectric vibrator  80 , it moves in vertical directions due to the vibration of the protrusion part  81 .  
         [0107]     The first pressing member  73  is a rod having a circular cross section. The first pressing member  73 , as shown in  FIG. 19 , is in line-contact with the piezoelectric vibrator  80 . Thus, the first pressing member  73  may press the piezoelectric vibrator  80  exactly perpendicularly. The first pressing member  73  is prevented from being dislodged from the first pressing member fitting grooves  717  by the flat spring  77 .  
         [0108]     The second pressing members  75  are rods having circular cross sections inserted through the second pressing member insertion holes  711  and, as shown in  FIG. 19 , presses the first slider  79   a  towards the piezoelectric vibrator  80 . The second pressing members  75  may be formed in numbers of three or greater. The flat spring  77  presses the first pressing member  73  towards the sliders  79  by means of elastic force. Thus, the protrusion part  81  of the piezoelectric vibrator  80  and the first slider  79   a  are always in contact.  
         [0109]     While the spirit of the invention has been described in detail with reference to particular embodiments, the embodiments are for illustrative purposes only and do not limit the invention. It is to be appreciated that those skilled in the art can change or modify the embodiments without departing from the scope and spirit of the invention.  
         [0110]     According to an aspect of the present invention, the embodiments of which are as set forth above, a piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator may be provided, with which the manufacturing time and cost are reduced, as it is not necessary to go through two polarization processes.  
         [0111]     According to another aspect of the present invention, a piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator may be provided, with which the decline in performance due to the depolarization of the piezoelectric elements is removed.  
         [0112]     According to yet another aspect of the present invention, a piezoelectric vibrator and an ultrasonic motor having the piezoelectric vibrator may be provided, with the volume decreased and the vibration performance improved.