Abstract:
A piezoelectric motor including a base member; a stator disposed on the base member and comprising at least one piezoelectric element; a rotor configured to rotate by a wave motion of the stator, the wave motion being generated by the piezoelectric element; a cover member attached to the base member; a bearing arranged between the cover member and the rotor; and an elastic member configured to press the rotor toward the stator.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 10-2010-0021009, filed on Mar. 9, 2010, in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The invention is a piezoelectric motor and method of operation thereof, and more particularly, a piezoelectric motor having an elastic member for pressing a rotor against a piezoelectric element or an element driven by the piezoelectric element. 
     2. Description of the Related Art 
     The piezoelectric motor is a new type of motor that does not require a magnet or coil winding. A piezoelectric motor is a motor having a piezoelectric element that is driven by a high frequency voltage. The piezoelectric element vibrates in response to the high frequency voltage and the vibration is used to drive the rotor. 
     The piezoelectric motor needs to be compact and reliable to be used in portable electronic devices. 
     SUMMARY 
     Therefore, there is a need in the art for a piezoelectric motor including a base member; a stator disposed on the base member and comprising at least one piezoelectric element; a rotor configured to rotate by a wave motion of the stator, the wave motion being generated by the piezoelectric element; a cover member attached to the base member; a bearing arranged between the cover member and the rotor; and an elastic member configured to press the rotor toward the stator. 
     A plurality of operation protrusions may be formed on the stator and a part of the rotor is configured to contact the operation protrusions. 
     The piezoelectric motor may include stator installation member for attaching the stator onto the base member. 
     A plurality of operation protrusions may be formed on the stator, and a part of the stator installation member may be inserted in a space between the operation protrusions, and the other part of the stator installation member may be fixed to the base member. 
     A coupling groove may be formed in the stator and a coupling protrusion formed on the base member may be configured to couple with the coupling groove. 
     A coupling protrusion may be formed on the stator and a coupling groove formed in the base member may be configured to couple with the coupling protrusion. 
     A mounting groove may be formed in the base member and a mounting protrusion formed on the cover member may be configured to couple to the mounting groove. 
     A mounting groove may be formed in the cover member and a mounting protrusion may be formed on the base member and configured to couple with the mounting groove. 
     At least one pressure control member may be arranged between the cover member and the bearing. 
     At least one pressure control hole may be formed in the cover member and a pressure control bolt may be installed in the pressure control hole. 
     The elastic member may be arranged between the cover member and the bearing. 
     The elastic member may be a leaf spring having a ring shape. 
     The elastic member may include a cover member installation portion configured to be installed onto the cover member; a base member installation portion configured to be installed onto the base member; and a middle portion configured to connect the cover member installation portion with the base member installation portion. 
     The cover member installation portion and the base member installation portion may be configured to elastically deform during installation of the elastic member, and the elastic member may be configured to operate an elastic force in a direction to decrease the distance between the cover member installation portion and the base member installation portion. 
     An installation protrusion may be formed on at least one of the cover member installation portion and the base member installation portion. 
     A piezoelectric motor is disclosed. The piezoelectric motor may include a base member comprising a cover member; a stator disposed on the base member and comprising at least one piezoelectric element; a rotor configured to rotate by a wave motion of the stator, the wave motion being generated by the piezoelectric element; a bearing arranged between the cover member and the rotor; and an elastic member configured to urge the rotor toward the stator. 
     The cover member may be installed onto the base member. 
     A method of rotating a rotor is disclosed. The method including applying an electrical current to a piezoelectric element supported by a base member causing the piezoelectric element to move in a wave motion; pressing the rotor towards the piezoelectric element with the force from an elastic member, wherein the force acts between a cover above the base and the base and wherein the force presses on a bearing arranged between the cover member and the base, and wherein the bearing rotates in response to the rotor rotating. 
     The piezoelectric element may be a circular ring shape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a perspective view of a piezoelectric motor according to an embodiment of the invention; 
         FIG. 2  is an exploded perspective view of a piezoelectric motor of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along a line III-III of  FIG. 1 ; 
         FIG. 4  is an enlarged view of a portion of the piezoelectric motor of  FIG. 3 ; 
         FIG. 5  is a perspective view of a stator installation member according to an embodiment of the invention; 
         FIG. 6  is a perspective view of a base member according to an embodiment of the invention; 
         FIG. 7  is a perspective view of a cover member according to an embodiment of the invention; 
         FIG. 8  is a perspective view illustrating a mounting protrusion of the cover member inserted in a coupling groove of the base member, according to an embodiment of the invention; 
         FIG. 9  illustrates a contact operation between operation protrusions of a stator and a rotor according to an embodiment of the invention, in which the other parts except for the operation protrusions of a stator and the rotor are omitted; 
         FIG. 10  is an enlarged view of a portion A of  FIG. 9 ; 
         FIG. 11  is a perspective view schematically illustrating a mutual operation of the operation protrusions and the rotor of  FIG. 9  according to an embodiment of the invention, in which the other parts except for a contact portion of the rotor and the operation protrusions of a stator are omitted; 
         FIG. 12  is a perspective view of a piezoelectric motor according to another embodiment of the invention; 
         FIG. 13  is an enlarged cross-sectional view of a portion of the piezoelectric motor of  FIG. 12 ; 
         FIG. 14  is a front perspective view of a base member according to another embodiment of the invention; 
         FIG. 15  is a rear perspective view of the base member of  FIG. 14 ; 
         FIG. 16  is a perspective view of a cover member according to another embodiment of the invention; 
         FIG. 17  is a perspective view illustrating a part of a stator according to another embodiment of the invention; and 
         FIG. 18  is a perspective view of an elastic member according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The attached drawings for illustrating exemplary embodiments of the invention are referred to in order to understanding of the invention, the merits thereof, and the objectives accomplished by the implementation of the invention. Hereinafter, the invention will be described in detail by explaining exemplary embodiments of the invention with reference to the attached drawings. Like reference numerals in the drawings denote like elements. 
     First Embodiment 
       FIG. 1  is a perspective view of a piezoelectric motor  100  according to an embodiment of the invention.  FIG. 2  is an exploded perspective view of the piezoelectric motor  100  of  FIG. 1 .  FIG. 3  is a cross-sectional view taken along a line III-III of  FIG. 1 .  FIG. 4  is an enlarged view of a portion of the piezoelectric motor of  FIG. 3 .  FIG. 5  is a perspective view of a stator installation member  123  according to an embodiment of the invention.  FIG. 6  is a perspective view of a base member  110  according to an embodiment of the invention.  FIG. 7  is a perspective view of a cover member  140  according to an embodiment of the invention.  FIG. 8  is a perspective view illustrating a mounting protrusion  141   a  of the cover member  140  inserted in a mounting groove  112   a  of the base member  110 , according to an embodiment of the invention. 
     Referring to  FIGS. 1-4 , the piezoelectric motor  100  according to the current embodiment includes the base member  110 , a stator  120 , a rotor  130 , the cover member  140 , a bearing  150 , an elastic member  160 , and a pressure control member  170 . 
     Referring to  FIGS. 2 and 6 , the base member  110  has a circular ring shape. The base member  110  includes a support portion  111  where the stator  120  is arranged, and an extending portion  112  extending from the support portion  111  toward the cover member  140 . The mounting groove  112   a  is formed in the extending portion  112  and the cover member  140  may be installed onto the base member  110  via the mounting groove  112   a.    
     The stator  120  has a circular ring shape and includes a piezoelectric element  121  on one surface thereof and a plurality of operation protrusions  122  on another surface which is opposite to the surface on which the piezoelectric element is disposed. The piezoelectric element  121  has a circular ring shape and is divided into a plurality of polarized portions that are electrically polarized. A main expansion/contraction direction of each polarized portion of the piezoelectric element  121  is the same as a direction of polarization. The polarization direction of each polarized portion of the piezoelectric element  121  is arranged to be perpendicular to a rotation surface of the rotor  130 . 
     Although the piezoelectric element  121  of the current embodiment includes a single piezoelectric ceramic layer, that is, a mono-layered piezoelectric ceramic, the invention is not limited thereto. In other words, the piezoelectric element according to the invention may have a structure in which a plurality of piezoelectric ceramic layers are deposited. 
     The operation protrusions  122  of the stator  120  protrude toward the rotor  130  to form a contact frictional surface between the stator  120  and the rotor  130 . The operation protrusions  122  amplify a curved wave generated by the piezoelectric element  121 . The movement of each of the operation protrusions  122  follows an oval motion track. That is, the operation protrusions  122  which participate in forming consecutive curved waves follow an oval motion track. Also, points of the operation protrusions  122  having positional differences have phase differences from each other and follow the oval motion track, thereby forming consecutive curved waves. 
     The stator  120  is installed at the base member  110  by using the stator installation member  123  in the current embodiment. Referring to  FIG. 5 , the stator installation member  123  includes a hook portion  123   a , a connection portion  123   b , and an installation portion  123   c . The hook portion  123   a  is inserted in a groove  122   a  between the operation protrusions  122 . The installation portion  123   c  is fixed to the base member  110 . The connection portion  123   b  connects the hook portion  123   a  and the installation portion  123   c  to thus install the stator  120  at the base member  110 . 
     The stator  120  and the base member  110  are not completely fixed to each other. That is, the stator  120  is installed at the base member  110  in such a way that the stator  120  can slightly move in a direction perpendicular to the rotation direction of the rotor  130 , that is, in a z-axis direction. However, this installation structure is to allow the wave motion of the stator  120 . The wave motion of the stator  120  is possible because of the motion of the piezoelectric element  121  expanding and contracting in the z-axis direction. 
     In the current embodiment, the stator  120  is installed at the base member  110  by using the stator installation member  123 , but the invention is not limited thereto. That is, the stator installation structure according to the invention has no special limitation except that the movement of the stator  120  in the rotation direction of the rotor  130  is restricted and simultaneously the stator  120  is allowed to move in the z-axis direction to thus enable the wave motion of the stator  120 . 
     As illustrated in  FIG. 2 , the rotor  130  has a cylindrical ring shape. The rotor  130 , as illustrated in  FIG. 4 , includes a rotor main body  131 , a contact portion  132  extending from the rotor main body  131  toward the stator  120 , and an elastic connection portion  133  elastically connecting the rotor main body  131  and the contact portion  132 . 
     The rotor main body  131  supports most of the mass of the rotor  130  and performs a rotation motion by receiving a driving force from the stator  120 . The contact portion  132  contacts the operation protrusions  122  of the stator  120 . That is, the contact portion  132  contacts the operation protrusions  122  of the stator  120 , receives consecutive curved waves from the operation protrusions  122 , and converts the received consecutive curved waves into the rotation motion of the rotor  130 . 
     The elastic connection portion  133  elastically connects the rotor main body  131  and the contact portion  132 . The structure of the elastic connection portion  133  allows the elastic contact between the contact portion  132  and the operation protrusions  122  and stably maintains the contact by pressing the contact portion  132  against the operation protrusions  122  at an appropriate pressure. Thus, loss in the transfer of power may be prevented, the control of rotation may be made easy, and abrasion due to the friction between parts may be reduced. 
     Referring to  FIGS. 2 and 7 , the cover member  140  has a circular ring shape. The cover member  140  includes an inner wall portion  141  and a rim portion  142 . A mounting protrusion  141   a  is formed on the inner wall portion  141 . During assembly, as illustrate in  FIG. 8 , the mounting protrusion  141   a  of the cover member  140  is inserted in the mounting groove  112   a  of the base member  110  to install the cover member  140  onto the base member  110 . 
     According to the current embodiment, the mounting protrusion  141   a  is formed on the inner wall portion  141  of the cover member  140  and the mounting groove  112   a  is formed on the base member  110 , and thus, during assembly, the mounting protrusion  141   a  is inserted into the mounting groove  112   a  to install the cover member  140  onto the base member  110 . However, the invention is not limited thereto. That is, according to the invention, in a manner opposite to the above-described embodiment, a mounting groove is formed in the inner wall portion  141  of the cover member  140  and a mounting protrusion is formed on the extending portion  112  of the base member  110 . Accordingly, during assembly, the mounting protrusion of the base member  110  is inserted in the mounting groove of the cover member  140  to install the cover member  140  onto the base member  110 . 
     The rim portion  142  performs a function of pressing against the pressure control member  170 . The pressing operation will be described later with the description of the pressure control member  170 . Thus, a detailed description on the pressing operation will be omitted herein. 
     Four pressure control holes  142   a  are formed in the rim portion  142 . As illustrated in  FIGS. 2 and 3 , a pressure control bolt  142   b  that directly presses against the pressure control member  170  is coupled to each of the pressure control holes  142   a . When an assembler rotates the pressure control bolt  142   b , the pressure control bolt  142   b  is moved to be closer to or away from the stator  120  so as to push the pressure control member  170  toward the elastic member  160  or to the opposite direction. According to the structure, assembly performance may be improved by finely controlling a pressing force against the rotor  130  according to a degree of the rotation of the pressure control bolt  142   b , which will be described later. 
     Although, in the current embodiment, the pressure control holes  142   a  are formed at four positions on the rim portion  142  and the pressure control bolt  142   b  is arranged at each pressure control hole  142   a , the invention is not limited thereto. That is, according to the invention, there is no special limit in the number of the pressure control bolt  142   b  and the pressure control holes  142   a  formed in the rim portion  142 . For example, the number of the pressure control holes  142   a  and the pressure control bolt  142   b  may be three or five. 
     The bearing  150  enables the rotation of the rotor  130  and is arranged between the cover member  140  and the rotor  130 . In the current embodiment, a rolling bearing is used as the bearing  150 . That is, the bearing  150  includes a first wheel  151 , a second wheel  152 , a plurality of rolling members  153 , and a support member  154 . 
     The first wheel  151  is fixed to the rotor main body  131  and rotates with the rotor  130 . The second wheel  152  contacts the elastic member  160  and is arranged to face the first wheel  151 . The rolling members  153  are arranged between the first wheel  151  and the second wheel  152  and perform a function of rolling during the operation of the bearing  150 . Balls, cylindrical rollers, conic rollers, or needles may be used as the rolling members  153 . The support member  154  supports the rolling members  153  and maintains constant distances between the rolling members  153 . 
     Although a rolling bearing is used as the bearing  150  of the current embodiment, the invention is not limited thereto. That is, the bearing according to the invention may use a journal bearing. 
     The elastic member  160  of the current embodiment, as illustrated in  FIGS. 2-4 , is arranged between the cover member  140  and the bearing  150  and presses the rotor  130  against the stator  120 . The elastic member  160 , as illustrated in  FIG. 2 , has a circular ring shape and is of a leaf spring type. 
     Although, in the current embodiment, the elastic member  160  has a circular ring shape and is of a leaf spring type, the invention is not limited thereto. That is, the elastic member  160  according to the invention has no special limit in the shape or type, except that the elastic member can press the rotor  130  against the stator  120 . 
     The pressure control member  170  is arranged between the elastic member  160  and the cover member  140 . As illustrated in  FIG. 2 , the pressure control member  170  has a circular ring shape having a predetermined thickness. The pressure control member  170  performs a function of controlling a pressing force to press the rotor  130 . That is, the pressure control member  170  has a predetermined thickness, and the elastic member  160  is further pressed by the existence of the pressure control member  170  and thus the pressing force may be controlled. A designer may control the amount of a pressing force applied to the rotor  130  by the number of the pressure control member  170 . That is, when a designer wishes to have a large pressing force, a large number of the pressure control member  170  is arranged. When the designer wishes to have a small pressing force, a small number of the pressure control member  170  is arranged, or no pressure control member may be arranged. When the pressure control member  170  is not arranged at all, the cover member  140  is designed to directly contact the elastic member  160 . 
     Although in the current embodiment a single pressure control member  170  is employed, the invention is not limited thereto. That is, the designer may adjust the number of the pressure control member  170  to be employed according to the required amount of a pressing force. 
     According to the current embodiment, the relatively fine control of a pressing force is performed by using the pressure control bolt  142   b , and the general control of a pressing force is performed by the number of the pressure control member  170 . That is, a designer or assembler calculates in advance the amount of a pressing force to be applied to the rotor  130 , determines the number of the pressure control member  170 , and arranges the pressure control member  170  between the cover member  140  and the bearing  150  during assembly. When a fine pressing force is additionally needed after the pressure control member  170  is arranged, the pressure control bolt  142   b  is rotated to move the pressure control member  170  toward the elastic member  160  or in the opposite direction, thereby performing a fine control of a pressing force. 
     According to the current embodiment, the pressure control holes  142   a  are formed in the rim portion  142  of the cover member  140 , and the pressure control bolt  142   b  is coupled to each of the pressure control holes  142   a . However, the invention is not limited thereto. That is, a piezoelectric motor according to the invention may not include the pressure control holes  142   a  and the pressure control bolt  142   b.    
     The overall structure of the piezoelectric motor  100  according to the current embodiment will be described below with reference to the above-described constituent elements and the accompanying drawings. 
     The stator  120  is arranged on the base member  110 , and the contact portion  132  is arranged on the operation protrusions  122  of the stator  120 . Also, the first wheel  151  of the bearing  150  is fixedly installed on an inner surface of the rotor main body  131 , and the elastic member  160  is arranged on an upper surface of the second wheel  152  of the bearing  150 . The pressure control member  170  is arranged on and above an upper surface of the elastic member  160 , and the rim portion  142  of the cover member  140  is disposed above the pressure control member  170 . The mounting protrusion  141   a  of the cover member  140  is inserted in the mounting groove  112   a  of the base member  110  to fixedly install the cover member  140  onto the base member  110 . 
     The pressing force to the rotor  130  is applied by the cover member  140 , the pressure control member  170 , and the elastic member  160 . A designer or assembly may easily control the pressing force to the rotor  130  by adjusting the number of the pressure control member  170  and/or the amount of rotation of the pressure control bolt  142   b . Then, the contact portion  132  of the rotor  130  may always contact the operation protrusions  122  of the stator  120  at an appropriate pressing force and thus rotation may be easily controlled, power loss may be decreased, and friction and abrasion between the stator  120  and the rotor  130  may be reduced. 
     Next, referring to  FIGS. 9-11 , the operation of the piezoelectric motor  100  according to an embodiment of the invention will be described. 
       FIG. 9  illustrates a contact operation between the operation protrusions  122  of the stator  120  and the rotor  130  according to an embodiment of the invention, in which the other parts except for the operation protrusions  122  of the stator  120  and the rotor  130  are omitted.  FIG. 10  is an enlarged view of a portion A of  FIG. 9 .  FIG. 11  is a perspective view schematically illustrating the mutual operation of the operation protrusions  122  and the rotor  130  according to an embodiment of the invention, in which the other parts except for the contact portion  132  of the rotor  130  of the operation protrusions  122  of the stator  120  are omitted. 
     After a user applies power to the piezoelectric motor  100  and turns a switch on, a driving apparatus (not shown) applies a driving voltage to the piezoelectric element  121 . A cosine wave voltage and a sine wave voltage having predetermined frequencies are applied to the piezoelectric element  121  as the driving voltage. 
     When the driving voltage is applied to the piezoelectric element  121 , the polarized portions of the piezoelectric element  121  are expanded and contracted mainly in the z-axis direction according to the frequency of the driving voltage. The expansion and contraction operation of the polarized portions of the piezoelectric element  121  causes consecutive curved waves. As a result, as illustrated in  FIG. 9 , the rotational force is transferred to the rotor  130  by the contact between the rotor  130  and the operation protrusions  122 . 
     That is, as illustrated in  FIG. 10 , a motion trajectory F of the operation protrusions  122  is oval shaped. The operation protrusions  122  transfer the rotational force to the rotor  130  by the movement of the operation protrusions  122  in a tangential direction f t  on the motion trajectory F. 
     As illustrated in  FIG. 10 , the curved wave movement of the stator  120  acts on all of the contact portion  132  of the rotor  130 . Accordingly, the rotor  130  performs a rotation motion to drive the piezoelectric motor  100 . 
     According to the piezoelectric motor  100  of the current embodiment, according to the structure of the pressure control member  170  and the pressure control bolt  142   b , the pressing force of the rotor  130  acting on the stator  120  may be easily controlled as necessary. When the pressing force is appropriately controlled, the rotation of the rotor  130  may be easily controlled, power loss may be decreased, and friction and abrasion may be reduced. 
     Also, according to the piezoelectric motor  100  of the current embodiment, since the cover member  140  is easily installed onto the base member  110  by using the mounting protrusion  141   a  and the mounting groove  112   a , the number of assembly steps and the manufacturing cost may be reduced. 
     Also, according to the piezoelectric motor  100  of the current embodiment, since the stator  120  is easily installed onto the base member  110  by using the stator installation member  123 , the number of assembly steps and the manufacturing cost may be reduced. 
     Second Embodiment 
     Referring to  FIGS. 12-18 , a piezoelectric motor  200  according to another embodiment of the invention will now be described mainly based on the differences from the piezoelectric motor  100  according to the above-described embodiment of the invention. 
       FIG. 12  is a perspective view of the piezoelectric motor  200  according to another embodiment of the invention.  FIG. 13  is an enlarged cross-sectional view of a portion of the piezoelectric motor  200  of  FIG. 12 .  FIG. 14  is a front perspective view of a base member  210  according to the current embodiment.  FIG. 15  is a rear perspective view of the base member  210  of  FIG. 14 .  FIG. 16  is a perspective view of a cover member  240  according to the current embodiment.  FIG. 17  is a perspective view illustrating a part of a stator  220  according to the current embodiment.  FIG. 18  is a perspective view of one of a plurality of elastic members  260  according to the current embodiment. 
     As illustrated in  FIGS. 12 and 13 , the piezoelectric motor  200  according to the current embodiment includes the base member  210 , the stator  220 , a rotor  230 , the cover member  240 , a bearing  250 , the elastic members  260 , and a plurality of pressure control members  270 . 
     As illustrated in  FIGS. 12 ,  14 , and  15 , the base member  210  has a circular ring shape. The base member  210  includes a support portion  211  where the stator  220  is arranged and an extension portion  212  extending from the support portion  211  toward the cover member  240 . 
     As illustrated in  FIG. 15 , a first installation groove  211   a  is formed in a rear surface of the support portion  211 , in which one end of each of the elastic members  260  is inserted. Three coupling protrusions  212   a  are formed on the extension portion  212 . The coupling protrusions  212   a  allow for installation of the stator  220 , which will be described in detail later. Also, three opening portions  212   b  for the installation of the elastic members  260  are formed in the extension portion  212 . Parts of the elastic members  260  are located at the opening portions  212   b  during assembly. 
     The stator  220  has a circular ring shape. As illustrated in  FIG. 17 , a piezoelectric element  221  is arranged on one surface of the stator  220 , a plurality of operation protrusions  222  are formed on the other surface thereof which is opposite to the surface on which the piezoelectric element  221  is disposed, and a coupling groove  223  is formed in an inner circumferential surface thereof. 
     Since the shape, type, and polarization state of the piezoelectric element  221  are the same as those of the piezoelectric element  121  of the above-described embodiment, a detailed description thereof will be omitted herein. The operation protrusions  222  of the stator  220  protrude toward the rotor  230  and form a contact friction surface between the stator  220  and the rotor  230 . 
     The stator  220  is installed onto the base member  210 . In the current embodiment, the stator  220  is installed onto the base member  210  by using the coupling groove  223  formed in the inner circumferential surface of the stator  220  and the coupling protrusions  212   a  of the base member  210 . That is, as each of the coupling protrusions  212   a  of the base member  210  is inserted in the coupling groove  223  of the stator  220 , the stator  220  is installed onto the base member  210 . 
     The stator  220  and the base member  210  are not completely fixed to each other. That is, as each of the coupling protrusions  212   a  of the base member  210  is coupled to the coupling groove  223  of the stator  220  while being capable of sliding, the stator  220  is installed onto the base member  210  while being capable of moving a predetermined distance in a direction perpendicular to the rotation direction of the rotor  230 , that is, the z-axis direction. This structure allows the rotor  230  to be rotated, by embodying the wave motion of the stator  220 , as described above in the above-described embodiment. 
     In the current embodiment, the coupling groove  223  is formed in the inner circumferential surface of the stator  220  and the coupling protrusions  212   a  are formed on the base member  210 , each of the coupling protrusions  212   a  is coupled to the coupling groove  223  to install the stator  220  onto the base member  210 . However, the invention is not limited thereto. That is, according to the invention, in a manner opposite to the description above, coupling protrusions are formed on the inner circumferential surface of the stator  220  and a coupling groove is formed in the base member  210  and then the coupling protrusions are coupled to the coupling groove to install the stator  220  onto the base member  210 . 
     The rotor  230  has a circular ring shape. The rotor  230  includes a rotor main body  231  and a contact portion  232  extending from the rotor main body  231  toward the stator  220 . The rotor main body  231  is a part that takes most of mass of the rotor  230  and performs a rotation motion by receiving power from the stator  220 . 
     The contact portion  232  is a portion that contacts the operation protrusions  222  of the stator  220 . That is, the contact portion  232  contacts the operation protrusions  222  of the stator  220  and receives consecutive curved waves from the operation protrusions  222  and performs a function to convert the consecutive curved waves into the rotation motion of the rotor  230 . 
     The cover member  240  has a circular ring shape and performs a function of pressing against the pressure control members  270 . Since the pressing operation will be described with the description on the pressure control members  270 , a detailed description of which is omitted herein. 
     The cover member  240  is installed onto the base member  210 . To this end, as illustrated in  FIG. 16 , second installation grooves  241  in which the elastic members  260  are inserted and fixed are formed in the cover member  240 . 
     The bearing  250  facilitates the rotation of the rotor  230  and is arranged between the cover member  240  and the rotor  230 . A rolling bearing is used for the bearing  250  according to the current embodiment. The bearing  250  includes a first wheel  251 , a second wheel  252 , a plurality of rolling members  253 , and a support member  254 . 
     The first wheel  251  is fixed to the rotor main body  231  and rotates with the rotor  230 . The second wheel  252  contacts the pressure control member  270  and is arranged to face the first wheel  251 . The rolling members  253  are arranged between the first wheel  251  and the second wheel  252  and a function of rolling during the operation of the bearing  250 . Balls, cylindrical rollers, conic rollers, or needles may be used as the rolling members  253 . The support member  254  supports the rolling member  253  and maintains constant distances between the rolling members  253 . 
     The elastic members  260  install the cover member  240  onto the base member  210  in an elastic manner. As illustrated in  FIG. 18 , the elastic members  260  have a shape of “Π” and are arranged at three positions to correspond to the opening portions  212   b  of the base member  210 . 
     According to the current embodiment, three elastic members  260  are installed with an interval of 120° between each thereof, but the invention is not limited thereto. That is, according to the invention, there is no limit in the number of the elastic members and also in the arrangement shape. However, it is preferable that the elastic members are symmetrically arranged to provide a uniform pressing force to the rotor  230  over all of the piezoelectric motor  200 . 
     Each of the elastic members  260  includes a cover member installation portion  261 , a base member installation portion  262 , and a middle portion  263 . The cover member installation portion  261  is a part to be inserted in each of the second installation grooves  241  of the cover member  240 . An installation protrusion  261   a  to prevent slippage and provide an elastic force is formed on an inner surface of the cover member installation portion  261 . 
     The base member installation portion  262  is a part to be inserted in the first installation groove  211   a  of the base member  210 . An installation protrusion  262   a  to prevent slippage and provide an elastic force is formed on an inner surface of the base member installation portion  262 . The middle portion  263  is a part to connect the cover member installation portion  261  and the base member installation portion  262 . 
     The elastic member  260  installs the cover member  240  onto the base member  210 , and simultaneously, presses against the cover member  240  so as to press against the pressure control member  270  and the rotor  230  toward the stator  220 . The operation of a pressing force will be described later with the description of the pressure control member  270 . 
     The elastic members  260  of the current embodiment have a shape of “Π”, but the invention is not limited thereto. That is, there is no special limit in the shape or type of an elastic member according to the invention as long as the elastic member presses against the cover member  240  so as to press against the rotor  230  toward the stator  220 . 
     Although the installation protrusions  261   a  and  262   a  are formed on the cover member installation portion  261  and the base member installation portion  262  of the elastic member  260  of the current embodiment, the invention is not limited thereto. That is, the installation protrusions  261   a  and  262   a  may not be formed on the cover member installation portion  261  and the base member installation portion  262  of the elastic member  260  of the current embodiment, and an installation protrusion may be formed on any one of the cover member installation portion  261  and the base member installation portion  262 . When the installation protrusions  261   a  and  262   a  are not formed, by configuring the distance between end portions of the cover member installation portion  261  and the base member installation portion  262  to be relatively shorter than that when the installation protrusions  261   a  and  262   a  are formed, a required elastic force may be implemented. 
     Two pressure control members  270  are arranged between the cover member  240  and the bearing  250 . The pressure control members  270  have a circular ring shape having a predetermined thickness. 
     The pressure control members  270  control a pressing force for pressing against the rotor  230 . That is, since the pressure control members  270  have a predetermined thickness, the rotor  230  is further pressed due to the existence of the pressure control members  270  and thus a pressing force may be controlled. A designer by adjusting the number of the pressure control members  270  may control the amount of the pressing force applied to the rotor  230 . That is, when a large pressing force is desired, the designer may arrange a large number of the pressure control members  270 . When a small pressing force is desired, a less number of the pressure control members  270  may be arranged or no pressure control member may be arranged. When no pressure control member is arranged, the cover member  240  directly contacts the second wheel  252  of the bearing  250 . 
     Although in the current embodiment two pressure control members  270  are arranged, the invention is not limited thereto. That is, as described above, the number of the pressure control members  270  to be applied may be controlled according to the required amount of a pressing force. 
     The overall structure of the piezoelectric motor  200  of the current embodiment will be described below with respect to the respective constituent elements. Referring to the respective above-described constituent elements and drawings, the overall structure of the piezoelectric motor  200  of the current embodiment will be described below. 
     The stator  220  is arranged on the base member  210 . The contact portion  232  of the rotor  230  is arranged on the operation protrusions  222  of the stator  220 . Also, the first wheel  251  of the bearing  250  is fixed to an inner surface of the rotor main body  231 . The two pressure control members  270  are arranged on an upper surface of the second wheel  252  of the bearing  250 . The cover member  240  is located on an upper surface of the pressure control members  270 . 
     The cover member installation portion  261  of the elastic member  260  is inserted in each of the second installation grooves  241  of the cover member  240  and the base member installation portion  262  is inserted in the first installation groove  211   a  of the base member  210  and fixed thereto, and thus the cover member  240  is installed at the base member  210 . When the cover member installation portion  261  is inserted in each of the second installation grooves  241  and the base member installation portion  262  is inserted in the first installation groove  211   a , elastic bending is generated by the structures of the installation protrusions  261   a  and  262   a , which protrude inwardly, in directions indicated by arrows of  FIG. 18 . As the elastic bending is generated, an elastic force is applied in a direction to decrease the distance between the cover member installation portion  261  and the base member installation portion  262  after the elastic member  260  is installed. Due to the application of the elastic force, the elastic member  260  presses against an upper surface of the cover member  240  and thus the rotor  230  is pressed at a predetermined pressure. 
     That is, the elastic force by the elastic member  260  presses against the rotor  230  sequentially via the cover member  240 , the pressure control member  270 , and the bearing  250 . A designer or assembler may easily control the pressing force to the rotor  230  by adjusting the number of the pressure control member  270 . Then, since the contact portion  232  of the rotor  230  always contacts the operation protrusions  222  of the stator  220  at an appropriate pressure, the rotation of the rotor  230  may be easily controlled, power loss may be decreased, and friction and abrasion between the stator  220  and the rotor  230  may be reduced. 
     Since the structure, operation, and effect of the piezoelectric motor  200  according to the current embodiment other than those described above are similar as those of the piezoelectric motor  100  of the above-described embodiment, the detailed descriptions thereon will be omitted herein. 
     According to the piezoelectric motor  200  of the current embodiment, according to the structure of the pressure control member  270 , the pressing force of the rotor  230  acting on the stator  220  may be easily controlled as necessary. When the pressing force is appropriately controlled, the rotation of the rotor  230  may be easily controlled, power loss may be decreased, and friction and abrasion may be reduced. 
     Also, according to the piezoelectric motor  200  of the current embodiment, since the cover member  240  is easily installed onto the base member  210  by using the elastic member  260 , the number of assembly steps and the manufacturing cost may be reduced. 
     Also, according to the piezoelectric motor  200  of the current embodiment, since the stator  220  is easily installed onto the base member  210  as each of the coupling protrusions  212   a  of the base member  210  is inserted in the coupling groove  223  of the stator  220 , the number of assembly steps and the manufacturing cost may be reduced. 
     As described above, a piezoelectric motor having improved performance may be embodied. 
     In the description above, the term formed has been used to describe physical shapes of members of the piezoelectric motor. Formed should be understood to mean that a single piece may formed into a shape or that multiple pieces may be joined together to achieve the desired shape. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.