Abstract:
Disclosed is an actuator that realizes reciprocal rotational motion of an electric toothbrush, for example, without utilizing a drive transmission mechanism that is separate from the drive source. In the actuator, a fixed body ( 120 ) has a coil ( 128 ) that is disposed around a magnet ( 160 ) and faces the magnetic planes with different polarities within the magnet ( 160 ) at individual prescribed distances, and an outer yoke ( 150 ) that covers the outer periphery of the coil ( 128 ). The fixed body ( 120 ) rotatably supports a movable body ( 110 ), which holds the magnet ( 160 ), via a coil spring that is an elastic member ( 130 ) made from a wire. An alternating current supplying part ( 140 ) supplies an alternating current having roughly the same resonance frequency as that of the fixed body ( 120 ) to the coil ( 128 ) to cause the movable body ( 110 ) to vibrate in a reciprocal rotational motion. The coil spring that is the elastic member ( 130 ) uniformly disperses stress generated by the vibration.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an actuator used in, for example, an electric toothbrush or an electric sonic wave toothbrush. 
       BACKGROUND ART 
       [0002]    Electric toothbrushes including electric sonic wave toothbrushes known heretofore include a bass brushing tooth brush places the brush in the part between the tooth and the gum at an angle (at an angle of approximately 45 degrees) and vibrates the brush to the right and left in back-and-forth linear motion, a rolling brushing toothbrush that rotates back and forth (forward and backward) over a predetermined angle range around a shaft and moves from the gum to the tooth rotating, and so on. 
         [0003]    The drive of toothbrushes like these involves many structures for converting the rotation of a rotating DC motor that rotates regularly around a shaft into back-and-forth linear motion or back-and-forth rotating motion, via a motion direction converting mechanism. Furthermore, besides these structures, a structure to move a toothbrush in back-and-forth linear motion by means of a linear drive actuator, and a structure to move a toothbrush in back-and-forth rotating motion by making a resonance vibrating mechanism apart from the drive source resonate by the vibration of an actuator, are known. 
         [0004]    With an electric toothbrush structured to move the brush part in back-and-forth linear motion by means of a linear drive actuator, as shown in patent literature 1, the linear actuator directly produces back-and-forth vibration in the axial direction of an output shaft that is directly connected with the brush part, and makes possible bass brushing. With this structure, there is little power loss due to a motion converting mechanism, and makes possible fast vibration. 
         [0005]    Furthermore, with an electric toothbrush of a structure having an actuator and resonance vibrating mechanism apart from the drive source, as shown in patent literature 2, a drive means with an electro magnet and permanent magnet vibrates the resonance vibrating mechanism having a lever arm. By this means, the lever arm that is coaxially connected with the brush part moves in swinging motion, making possible rolling brushing. 
       CITATION LIST 
     Patent literature 
       [0006]    PTL 1 
         [0007]    Japanese Patent Application Laid-Open No. 2002-078310 
         [0008]    PTL 2 
         [0009]    Japanese Patent Publication No. 3243529 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0010]    Now, to make possible rolling brushing with an electric toothbrush and to make the handle part in which the drive part to drive a rolling brushing toothbrush is accommodated as thin as possible, there is a demand to miniaturize the toothbrush drive part. 
         [0011]    However, to realize rolling brushing using a regular motor that rotates around a shaft, apart from this motor, a motion direction converting mechanism to covert the rotation of this motor into back-and-forth rotating motion is necessary. Also, to realize rolling brushing using a linear drive actuator as shown in patent literature 1, apart from this linear drive actuator, a torque generating mechanism (drive source) is necessary. 
         [0012]    Also, the structure shown in patent literature 2 requires a drive source as well as a resonance vibrating mechanism apart from the drive source. 
         [0013]    Consequently, with conventional structures, if a motor or a linear drive actuator is used as a drive source of an electric toothbrush, it is necessary to secure a space for placing a drive source, and, in addition, a motion direction converting mechanism, a torque generating mechanism, or a resonance vibrating mechanism, apart from the drive source, and there is therefore problem that it is difficult to miniaturize the toothbrush. 
         [0014]    In addition, in the event a drive transmitting mechanism (e.g. motion direction converting mechanism) is provided apart from an actuator (e.g. motor) as a toothbrush drive part, there is a threat of producing noise in the drive transmitting mechanism and there is furthermore a threat that the drive transmitting mechanism suffers poor efficiency due to power loss, and it is necessary to take measures against these. 
         [0015]    It is an object of the present invention to provide a small actuator and electric toothbrush that can realize back-and-forth rotating motion of an electric toothbrush or the like without using a drive transmitting mechanism apart from a drive source. 
       Solution to Problem 
       [0016]    An actuator according to the present invention adopts a configuration having: an outer yoke having inner wall planes that are placed a predetermined interval apart opposing each other; a permanent magnet that is placed to oppose the opposing inner wall planes over an air gap and that has different magnetic pole planes that oppose the inner wall planes respectively; and a coil that is placed in the air gap and surrounds the permanent magnet, and this actuator further has: a fixed body that has one of the permanent magnet and the coil; a movable body that has the other one of the permanent magnet and the coil and that has an output shaft that is perpendicular to both a direction in which the magnetic pole planes and the inner wall planes oppose each other and an axial direction of winding of the coil; an alternating current supplying section that supplies an alternating current of approximately a same frequency as a resonance frequency of the movable body to the coil; and a linear elastic member that has its one end fixed to the fixed body and the other end fixed to the movable body, and that supports the movable body on the fixed body to be able to rotate about an axis along the output shaft. 
         [0017]    An electric toothbrush according to the present invention adopts a configuration having: an actuator of the above configuration; and a toothbrush part that is coaxially coupled with the output shaft of the actuator, at a head of the toothbrush part a hair bundle part being provided to be perpendicular to an axial direction. 
       Advantageous Effects of Invention 
       [0018]    According to the present invention, it is possible to achieve back-and-forth rotating motion of an electric toothbrush or the like without using a drive transmitting mechanism apart from a drive source. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a perspective view showing an actuator according to the first embodiment of the present invention; 
           [0020]      FIG. 2  is a perspective view showing a state an outer yoke is removed from this actuator; 
           [0021]      FIG. 3  is a schematic cross-sectional view showing configurations of a movable body and fixed body of this actuator; 
           [0022]      FIG. 4  is an arrow cross sectional view along line A-A in  FIG. 1 ; 
           [0023]      FIG. 5  is a perspective view of an elastic member; 
           [0024]      FIG. 6  is a schematic view for explaining operation of this actuator; 
           [0025]      FIG. 7  shows a cycle of alternating current to be supplied to a coil in this actuator; 
           [0026]      FIG. 8  is a perspective view showing an actuator according to a second embodiment of the present invention; 
           [0027]      FIG. 9  is an exploded perspective view of this actuator; 
           [0028]      FIG. 10  is an arrow cross sectional view along line C-C in  FIG. 8 ; 
           [0029]      FIG. 11  is a perspective view showing an actuator according to a third embodiment of the present invention; 
           [0030]      FIG. 12  is an exploded perspective view of this actuator; 
           [0031]      FIG. 13  is an arrow cross sectional view along line D-D in  FIG. 11 ; 
           [0032]      FIG. 14  is a rear view of a wire-shaped spring body of an actuator according to a third embodiment of the present invention; 
           [0033]      FIG. 15  is a perspective view showing an actuator according to a fourth embodiment of the present invention; 
           [0034]      FIG. 16  is a principal-part exploded perspective view of this actuator; 
           [0035]      FIG. 17  is schematic cross-sectional view showing a principal-part configuration of this actuator; 
           [0036]      FIG. 18  is a schematic view for explaining operation of an actuator according to the fourth embodiment of the present invention; 
           [0037]      FIG. 19  is exploded perspective view showing a configuration of an actuator according to a fifth embodiment of the present invention; 
           [0038]      FIG. 20  is schematic cross-sectional view showing configurations of a movable body and fixed body of this actuator; 
           [0039]      FIG. 21  is a principal-part exploded perspective view of an actuator according to a sixth embodiment of the present invention; and 
           [0040]      FIG. 22  is schematic cross-sectional view showing a movable body and fixed body of this actuator. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0041]    Now, embodiments of the present invention will be described below in detail with reference to the accompanying drawings. 
       First Embodiment 
       [0042]      FIG. 1  is a perspective view showing actuator  100  according to the first embodiment of the present invention, and  FIG. 2  is a principal-part perspective view of this actuator  100 . 
         [0043]    Actuator  100  shown in  FIG. 1  and  FIG. 2  has movable body  110 , fixed body  120 , elastic member  130  that supports movable body  110  on fixed body  120  in a movable fashion, and alternating current supplying part  180  (see  FIG. 2 ). Movable body  110  has outer yoke  150 , magnet  160 , yoke holder  171 , and output shaft  180 , which is a back-and-forth rotating vibration transmission shaft, and fixed body  120  has base plate  122 , support wall parts  124  and  126  and coil  128  (see  FIG. 2 ). 
         [0044]    With actuator  100  shown in  FIG. 1 , in fixed body  120 , an alternating current is supplied from alternating current supplying part  140  to coil  128  that is provided in a center part on the surface of base plate  122 . By this means, movable body  110 , which has magnet  160  that is placed on the inner side of coil  128  and which is supported by fixed body  120  via linear elastic member  130 , is driven (moves) in a resonant state. When this movable body  110  moves, output shaft  180  of movable body  110  rotates in forward and backward directions (the directions of arrow B in  FIG. 1 ) in a predetermined angle range, and outputs back-and-forth rotating vibration outside. 
         [0045]      FIG. 3  is a schematic cross-sectional view showing configurations of movable body  110  and fixed body  120  of actuator  100 . 
         [0046]    As shown in  FIG. 1  to  FIG. 3 , outer yoke  150  has a cross section that is virtually U-shaped and is open downward, and is formed by bending a flat magnetifc body. Outer yoke  150  has yoke center part  151  of a flat rectangular shape, and mutually opposing sidewall parts  152  and  153  that hang from the side parts of yoke center part  151 . 
         [0047]    In the center area on the back of yoke center part  151  of outer yoke  150 , magnet  160  is placed via non-magnetic body  170 , such that air gaps are formed between magnet  160  and opposing sidewall parts  152  and  153  of outer yoke  150 . 
         [0048]    Magnet  160  is provided to hang from yoke center part  151 , via non-magnetic body  170 , and different magnetic poles face inner wall planes  152   a  and  153   a  of sidewall parts  152  and  153 . 
         [0049]    That is to say, here, the S-pole end of magnet  160  faces inner wall plane  152   a  of sidewall part  152  of outer yoke  150 , and the N-pole side faces inner wall plane  153   a  of sidewall part  153  of outer yoke  150 . 
         [0050]    Furthermore, magnet  160  is a cuboid having a length to match the length of the extension direction of outer yoke  150 , and is attached to the back of yoke center part  151 , via non-magnetic body  170  having the same bottom surface shape, along the extension direction of yoke center part  151 . 
         [0051]    Magnet  160  thus turns planes of different magnetic poles to all of inner wall planes  152   a  and  153   a  of side wall parts  152  and  153  that extend in the long direction of outer yoke  150 . Magnet  160  may also be placed in outer yoke  150 , without involving non-magnetic body  170 , such that air gaps are formed between magnet  160  and opposing sidewall parts  152  and  153  of outer yoke  150 . 
         [0052]    In air gaps between magnet  160  and sidewall parts  152  and  153  of outer yoke  150 , coil  128  that surrounds magnent  160  is placed spaced apart from all of side wall planes (magnetic pole planes)  160   a  and  160   b  of magnet  160 , inner wall planes  152   a  and  153   a  of sidewall parts  152  and  153 , and the back of yoke center part  151 . That is to say, coil  128  of fixed body  120  is placed, in a non-contact fashion, in air gaps G between sidewall parts  152  and  153  and magnet  160 . 
         [0053]    Furthermore, as shown in  FIG. 1  and  FIG. 2 , outer yoke  150 , to which magnet  160  is attached, is fixed on yoke holder  171  on the surface of yoke center part  151 . 
         [0054]      FIG. 4  is an arrow cross sectional view along line A-A in  FIG. 1 . 
         [0055]    As shown in  FIG. 2  and  FIG. 4 , in a long flat member that extends in the long direction of outer yoke  150  (corresponding the direction of the extension of output shaft  180 ), edge parts that are spaced part in the log direction are bent downward and form the shape of a letter U that is placed sideways on a side view. Output shaft attaching part  174 , to which output shaft  180  is attached, is connected to the front end part of yoke holder  171 . By this means, output shaft  180  is provided to project. from the front end part of yoke holder  171 , in the same direction as the direction of extension of outer yoke  150 , that is, in a direction that is virtually perpendicular to the direction magnet  160  and sidewall parts  152  and  153  oppose each other. 
         [0056]    Furthermore, joint part  172 , which connects linear elastic member  130  that is connected to support wall part  124 , is attached to the rear end part of yoke holder  171 . Fitting hole  1721  is formed in joint part  172 , and, in this fitting hole  1721 , opposite end part  132  of a twisted coil spring, which is elastic member  130 , is inserted. By this means, joint part  172  connects opposite end part  132  of elastic member  130  and yoke holder  171 . Joint part  172  and output shaft attaching part  174  are preferably non-magnetic bodies. 
         [0057]    Output shaft  180  is fixed to outer yoke  150  via output shaft attaching part  174  and yoke holder  171 , and, by this means, is attached to movable body  110  to be located on an axis to pass the center of gravity of movable body  110 . By this means, when movable body  119  moves in back-and-forth rotating vibration, output shaft  180  is able to transmit the vibration to the outside. 
         [0058]    When actuator  100  is used for an electric toothbrush, a toothbrush part is coaxially coupled with output shaft  180 , and, at the head of this toothbrush part, a hair bundle part is provided to be perpendicular to the axial direction. By this means the toothbrush part moves in the same motion as shaft  125 , that is, moves in rolling motion, which is back-and-forth rotating vibration. 
         [0059]    Coil  128  of fixed body  120  is a voice coil here, and is wound to surround magnet  160 . To be more specific, in each air gap, coil  128  is wound in a direction perpendicular to the direction in which magnet  160  and sidewall parts  152  and  153  oppose each other. 
         [0060]    Coil  128  is provided in fitting channel part  1221  (see  FIG. 4 ) formed in the surface of base plate  122 . Base plate  112  is a flat rectangular shape that is long in the direction in which output shaft  180  of movable body  110  extends, and, from the end parts (rear end part  122   a  and front end part  122   b ) of this base plate  122  spaced apart along the long direction, support wall parts  124  and  126  are erected. 
         [0061]    As shown in  FIG. 1 ,  FIG. 2  and  FIG. 4 , support wall parts  124  and  126  are spaced apart in the long direction of base plate  122 , and are provided in front and rear end parts  122   a  and  122   b  that project upward beyond the center part of base plate  122  where coil  128  is erected. 
         [0062]    Support wall part  126  has opening part  126   a  in which output shaft  180  of movable body  110  is inserted, and, by inserting output shaft  180  in this opening part  126   a,  movable body  110  is supported to be able to rotate about output shaft  180 . 
         [0063]    Support wall part  124  supports elastic member  130  that is provided between support wall part  124  and joint part  172  of movable body  110 . Via this elastic member  130 , in a normal state, movable body  110  is supported virtually horizontally (that is, virtually parallel to base plate  122 ) by means of support wall parts  124  to be capable of back-and-forth rotating vibration. 
         [0064]    In the area between opposing support wall parts  124  and  126 , elastic member  130  supports movable body  110  in the twisting directions of magnent  160  and output shaft  180 , such that movable body  110  is able to move in the front, back, left and right directions. 
         [0065]      FIG. 5  is a perspective view showing elastic member  130 . 
         [0066]    As shown in  FIG. 5 , elastic member  130  is a coil spring that is formed with a linear wire element (linear member) that can be deformed elastically, has its end parts  131  and  132  bent in parallel and has active coil part  133  placed in the center part. 
         [0067]    One end part  131  is inserted in insertion hole  124   b  formed in fixed block  124   c  of support wall part  124  shown in  FIG. 2 , and opposite end part  132  is inserted in fitting hole  1721  formed in joint part  172 . By this means, in a state in which parts other than active coil part  133  are prevented from moving in peripheral directions and axial direction, elastic member  130  supports movable body  110  in fixed body  120  so as to be able to rotate about an axis along output shaft  180 . 
         [0068]    Also, as shown in  FIG. 4 , opening part  124   a  that is open toward movable body  110  is formed in support wall part  124 , and, in this opening part  124   a,  guide shaft  125  that projects from support wall part  124  toward support wall part  126  is attached. In this guide shaft  125 , projection part  1251  that projects from support wall part  124  forms a bar shape, and is inserted in a coil spring (i.e. elastic member  130 ) from one end. One end of the coil spring projects from the outer periphery on the base end side of projection part  1251 , and contacts flanges  1252  which contact the inner wall plane of support wall part  124 . By this means, support wall part  124 , with guide shaft  125 , receives one end part  131  of the coil spring, which is elastic member  130 , and limits the movement of the coil spring (elastic member  130 ) in the radial direction. 
         [0069]    By this means, the coil spring, which is elastic member  130 , has its one end part  131  and opposite end part  132  fixed to support wall part  124  and joint part  172  attached to rear wall part  1712  of yoke holder  171 . By this means, the coil spring, which is elastic member  130 , is placed such that it can be compressed in the winding direction of the coil—that is, in the twisting directions—between support wall part  124  and joint part  172 . 
         [0070]    Via elastic member  130  configured in this way, movable body  110  is supported to be able to rotate in twisting directions. 
         [0071]    Assuming that the inertia of movable body  110  is J and the spring constant in a twisting direction is k sp , as compared with fixed body  120 , movable body  110  vibrates in a resonance frequency calculated based on equation 1 below: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0072]    In actuator  100  of the present embodiment, an alternating current of substantially the same frequency as resonance frequency f 0  of movable body  110  is supplied from alternating current supplying part  140  to coil  128 . By this means, it is possible to drive movable body  110  efficiently. 
         [0073]    In fixed body  120  and movable body  110 , outer yoke  150 , magnet  160  and coil  128  form a magnetic circuit. 
         [0074]    As shown in  FIG. 3 , actuator  100  has a magnetic circuit where magnetic fluxes produced from magnet  160  (designated by outline arrows) pass an air gap where coil  170  is placed, sidewall part  153  of outer yoke  150 , yoke center part  151 , sidewall part  152  and the opposite air gap, in order. 
         [0075]    Movable body  120  of this actuator  100  is a spring mass system structure that is supported by fixed body  120  via elastic member  130 , and, when an alternating current of the same frequency as resonance frequency f 0  of movable body  110  is supplied to coil  128 , movable body  110  is driven in a resonant state. The back-and-forth rotating vibration that is produced then is transmitted to output shaft  1280  of movable body  110 . 
         [0076]    Actuator  100  is driven based on the equation of motion represented by equation 2 below and based on the circuit equation represented by equation 3 below. 
         [0000]    
       
         
           
             
               
                 
                   
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         J: Inertia moment [Kgm2] 
         Θ(t): Angle [rad] 
         K t : Torque constant [Nm/A] 
         i(t): Current [A] 
         K sp : Spring constant [Nm/rad] 
         D: Attenuation coefficient [Nm/(rad/s)] 
         T LOAD : Load torque [Nm] 
       
     
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         e(t): Voltage [V] 
         R: Resistance [Ω] 
         L: Inductance [H] 
         K e : Counter electromotive force multiplier [V/(rad/s)] 
       
     
         [0088]    That is to say, the inertia moment, rotation angle, torque constant, current, spring constant, attenuation coefficient, and load torque in actuator  100  can be changed as adequate in a range to satisfy equation 2, and the voltage, resistance, inductance, and counter electromotive force multiplier can be changed as adequate in a range to satisfy equation 3. 
         [0089]    Actuator  100  of this embodiment uses a coil spring as elastic member  130  to support movable body  110  in a movable fashion. 
         [0090]    For example, when an elastic member such as flat spring is used as a member to support movable body  110  on fixed body  120  in a movable fashion, the distortion of the elastic member, (ε), increases as the rotation angle of the movable body increases. Also, the stress to apply to the elastic member increases following equation σ=Eε (E: the modulus of longitudinal elasticity of material) representing the relationship between stress (σ) and distortion (ε). When stress increases thus and the maximum stress value of the elastic material such as a flat spring becomes large, the elastic member is more likely to be fatigued. Consequently, it is possible to take measures by, for example, replacing the flat spring itself at an early time, applying processing such as drilling and bending to the flat spring to spread the stress and lower the maximum stress value, and so on. However, in the event the flat spring itself is to be replaced, the replacement may become more frequent and is burdensome, or, in the event the flat spring is subject to processing, it requires an increased number of steps and there is a threat of increasing the cost and making the spring constant unstable. Furthermore, considering making the diameter of actuator  100  itself small, the space for placing the flat spring decreases, and it becomes difficult to spread the stress by processing the flat spring. 
         [0091]    By contrast with this, actuator  100  uses a coil spring as an elastic member to support movable body  110  in a movable fashion, and the coil spring is placed such that its axial core virtually matches the center of rotation when movable body  110  moves in resonance vibration. 
         [0092]    Consequently, when movable body  110  moves in resonance vibration and moves in back-and-forth rotating motion, the stress which increases following the increase of the angle of rotation is produced uniformly in active coil part  133  of the coil spring. That is to say, unlike a case where a flat spring is used as an elastic material to support movable body  110  that moves in resonance vibration, it is possible to spread stress uniformly without applying special ingenuity to the shape in order to spread required stress. Consequently, as a member to support movable body  110  in a movable fashion, a structure provided that prevents stress from being concentrated on a location specific basis, that prevents the maximum stress value from increasing, and that therefore is robust against fatigue fracture. 
         [0093]    Furthermore, the structure to support movable body  110  using a coil spring is likely to make possible miniaturization and can be made using a forming machine used in general, so that it is possible to lower the cost of making. Furthermore, the coil spring being elastic member  130  can practically absorb the load in the direction of thrust, so that it is possible to improve the anti-shock robustness of actuator  100 . 
         [0094]    The operation of actuator  100  will be described next. 
         [0095]      FIG. 6  is a schematic view for explaining the operation of actuator  100  according to the first embodiment. Although the flow of magnetic fluxes from magnet  160  is shown by outline arrows in  FIG. 6A , the same flow applies to  FIG. 6B  to  FIG. 6D , and illustration is omitted in  FIG. 6B  to  FIG. 6D . 
         [0096]    When an alternating current is supplied from alternating current supplying part  140  to coil  128 , thrusts F 1 , F 2 , F 3  and F 4 , represented by arrows in the drawing, are produced in coil  128 , following Fleming&#39;s left hand rule. By this means, in movable body  110  that is attached to base plate  112  having coil  128 , via support wall part  114 , elastic member  130  and in a movable fashion, a rotating force about an axial center at the center of rotation is produced. 
         [0097]    One operation cycle of actuator  100  will be described. 
         [0098]    When a current flows in coil  128  in the direction shown in  FIG. 6A  (a current to flow in this direction will be hereinafter referred to as “forward current”), upward thrust F 1  (directed toward outer yoke  150 ) is produced in part  128   a  of coil  128  opposing S-pole plane  160   a  of magnet  160 . Meanwhile, in part  128   b  of coil  128  opposing N pole plane  160   b  of magnet  160 , downward thrust F 2  (directed toward base plate  112 ) is produced. 
         [0099]    By this means, as shown in  FIG. 1 ,  FIG. 2  and  FIG. 4 , relative rotating force is produced in movable body  110  supported by support wall part  124  that is erected from base plate  122  having coil  122 , guide shaft  125 , elastic member  130  and support wall part  126 . This relative rotating force places movable body  110  in the position shown in  FIG. 6B . 
         [0100]    In the state shown in  FIG. 6B , actuator  100  produces reaction forces, designated by arrows R 1  and R 2 , by the restoring force of elastic member  130 . From the state shown in  FIG. 6B  to the state shown in  FIG. 6D , a reverse current is supplied to coil  128  as compared with  FIG. 6A . By this means, from the state shown in  FIG. 6B  to the state shown in  FIG. 4C , movable body  110  rotates anticlockwise with respect to fixed body  120  by the reaction forces designated by arrows R 1  and R 2  and by the thrusts designated by arrows F 3  and F 4 . From the state shown in  FIG. 6C  to the state shown in  FIG. 6D , movable body  110  rotates anticlockwise with respect to fixed body  120  by the thrusts designated by arrows F 3  and F 4 . 
         [0101]    In the state shown in  FIG. 6D , actuator  100  produces reaction forces, designated by arrows R 3  and R 4 , by the restoring force of elastic member  130 . From the state shown in  FIG. 6D , passing the state shown in  FIG. 6A , to the state shown in  FIG. 6B , a forward current is supplied to coil  128 . By this means, from the state shown in  FIG. 6D  to the state shown in  FIG. 6A , movable body  110  rotates anticlockwise with respect to fixed body  120  by the reaction forces designated by arrows R 3  and R 4  and by the thrusts designated by arrows F 1  and F 2 . From the state shown in  FIG. 6A  to the state shown in  FIG. 6B , movable body  110  rotates anticlockwise with respect to fixed body  120  by the thrusts designated by arrows F 1  and F 2 . 
         [0102]    Next, the alternating current to be supplied in each state shown in  FIG. 6  will be described briefly with reference to  FIG. 7 . 
         [0103]    The alternating current to flow in coil  128  may be a pulse wave of frequency f 0  as shown in  FIG. 7A  or may be a sine wave of frequency f 0  as shown in  FIG. 7B . 
         [0104]    In the state of  FIG. 6A , the forward current at time point t 1  shown in  FIG. 7  is supplied. In the state of  FIG. 6B , the direction of the current is switched as shown at time point t 2  in  FIG. 7 . In the state of  FIG. 6C , the reverse current at time point t 3  shown in  FIG. 7  is supplied. Also, in the state of  FIG. 6D , the direction of the current is switched as shown at time point t 4  in  FIG. 7 , and, in the state of  FIG. 6D , the forward current at time point t 5  shown in  FIG. 7  is supplied. This is one operation cycle, and, by repeating these operations, movable body  110  produces back-and-forth rotating vibration. 
         [0105]    In actuator  100 , movable body  110  produces back-and-forth rotating motion (that is, back-and-forth rotating vibration), and this back-and-forth rotating vibration is sent outside via output shaft  180 . When a toothbrush part is coupled with output shaft  180  and a hair bundle part is provided to be perpendicular to the axial direction at the head of this toothbrush part, the toothbrush part moves in back-and-forth rotating vibration and makes possible rolling brushing. 
         [0106]    By this means, actuator  100  satisfies equations 2 and 3 and is driven by a resonance phenomenon using the resonance frequency represented by equation 1. By this means, in actuator  100 , the power to be consumed in a static state is only the loss due to load torque and the loss due to friction and the like, so that low power drive is possible—that is, it is possible to move movable body  110  in back-and-forth rotating vibration at low power consumption. As described above, with actuator  100  of the present embodiment, it is possible to realize back-and-forth rotating motion of an electric toothbrush or the like without using a drive transmitting mechanism apart from a drive source, and furthermore make possible back-and-forth rotating motion at low power consumption. 
         [0107]    Furthermore, with this actuator  100 , movable body  110  is driven using coil  128  which is a voice coil, so that magnetic attraction (detent force) is not produced, and therefore excellent controllability is provided. To be more specific, the position of movable body  110  while stopped is secured at the center location by the restoring force of elastic member  130 , so that there is little power loss when the drive stops. 
         [0108]    For the configuration of actuator  100 , such a magnetic circuit is possible in which magnent  160  is replaced with a magnetifc body of the same shape and in which two magnets are placed to turn differing magnetic pole planes to inner wall planes  152   a  and  153   a  of sidewall parts  152  and  153 . In actuator  100 , magnet  160  is attached to outer yoke  150 , between sidewall parts  152  and  153 , to turn different magnetic pole planes to sidewall parts  152  and  153 , and is placed on the inner side of coil  128 , thereby forming a magnetic circuit. By forming this magnetic circuit, compared to the configuration of making magnet  160  a magnetic body and attaching a plurality of magnets to the inner wall planes of sidewall parts  152  and  153 , it is possible to reduce the number of magnets, improve the assembility and reduce the cost. An electric toothbrush having actuator  100  provides the same advantage, so that it is possible to miniaturize the electric toothbrush itself. 
       Second Embodiment 
       [0109]      FIG. 8  is a perspective view showing actuator  100 A according to a second embodiment of the present invention, and  FIG. 9  is an exploded perspective view of this actuator  100 A.  FIG. 10  is an arrow cross sectional view along line C-C in  FIG. 8 . This actuator  100 A basically has the same configuration as actuator  100  according to the first embodiment, shown in  FIG. 1 , and therefore parts in actuator  100 A that are the same as in actuator  100  will be assigned the same reference numerals and codes as in actuator  100  and their explanations will be omitted. 
         [0110]    Based upon actuator  100 , actuator  100 A has a configuration in which output shaft  180  of movable body  110  is axially supported, in a rotatable fashion, on fixed body  120 , via bearing  127 —that is, axially supported in a rotatable fashion coaxially with output shaft  180  in the configuration of actuator  100 . 
         [0111]    That is to say, based upon the configuration of actuator  100 , actuator  100 A attaches bearing  127  in opening part  126   a  in support wall part  116  in which output shaft  180  is inserted. Support wall part  126  supports output shaft  180  to be coaxial with guide shaft  125 , in a rotatable fashion, via bearing  127 . By this means, output shaft  180  transmits and outputs the movement/motion of movable body  110 , and functions as a bearing to axially support movable body  110  on fixed body  120 . 
         [0112]    In this way, with actuator  100 A, movable body  110  is axially supported by support wall parts  124  and  126 , in a rotatable fashion, by means of a coil spring, which is elastic member  130  attached to guide shaft  125  outwardly, and output shaft  180 . Consequently, with actuator  100 A, when an alternating current is supplied from alternating current supplying part  140  to coil  128 , movable body  110  moves in stable back-and-forth rotating vibration, in both directions shown by arrow B in  FIG. 8 , about an axial center of output shaft  180 , with respect to fixed body  120 . 
         [0113]    Consequently, the degree of freedom is improved with respct to rotation and in the axial direction, and, by improving the anti-shock robustness of actuator  100 A itself, it is possible to move movable body  110  stably in back-and-forth rotating vibration. 
       Third Embodiment 
       [0114]      FIG. 11  is a perspective view showing actuator  100 B according to a third embodiment of the present invention and  FIG. 12  is an exploded perspective view of this actuator  100 B. Also,  FIG. 13  is an arrow cross sectional view along line D-D in  FIG. 11 . Actuator  100 B basically has the same configuration as actuator  100  according to the first embodiment, shown in  FIG. 1 , and therefore parts in actuator  100 B that are the same as in actuator  100  will be assigned the same reference numerals and codes as in actuator  100  and their explanations will be omitted. 
         [0115]    Based upon the configuration of actuator  100 A, actuator  100 B uses wire-shaped spring body  190 , instead of a coil spring being elastic member  130 . 
         [0116]    To be more specific, in the configuration of actuator  100 A shown in  FIG. 8  to  FIG. 10 , the coil spring being elastic member  130 , support wall part  124 , guide shaft  125  and joint part  172  are removed, and wire-shaped spring body  190  is attached. 
         [0117]    Wire-shaped spring body  190  is provided in the rear end side of actuator  100 B, between base plate  122  where coil  128  is erected in the center part on the surface, and yoke holder  171  of movable body  110 B having magnet  160  that is placed a predetermined space apart in coil  128 . Movable body  110 B has a configuration removing joint part  172  from rear wall part  1712  of yoke holder  171  in mobile body  110  of actuator  100  or  100 A. 
         [0118]      FIG. 14  is a rear view of wire-shaped spring body  190  of actuator  100 B according to a third embodiment of the present invention. 
         [0119]    As shown in  FIG. 11  to  FIG. 14 , wire-shaped spring body  190  has base plate fixing part  191  to be attached to base plate  122 , yoke fixing part  192  to be fixed to yoke holder  171 , and linear arm part  193  that is elastically deformable and that connects between base plate fixing part  191  and yoke fixing part  192 . 
         [0120]    As shown in  FIG. 14 , base plate fixing part  191  has a flat shape here and is attached to rear end part  122   b  of base plate  122 . The front side of base plate fixing part  191  opposes rear wall part  1712  of yoke holder  171  of movable body  110 B that is placed in a movable fashion. 
         [0121]    Yoke fixing part  192  has a flat shape, placed above base plate fixing part  191  spaced apart, and placed in the front beyond base plate fixing part  191 . Yoke fixing part  192  has projection part  194  that projects forward, in the center part of the front surface. As shown in  FIG. 13 , projection part  194  is inserted in opening part  1712   a  formed in rear wall part  1712  of yoke holder  171 , thereby fixing yoke fixing part  192  to rear wall part  1712  of yoke holder  171 . Projection part  194  is placed on the rotation axis of movable body  110 B, and yoke fixing part  192  is fixed to yoke holder  171  in a bilateral position with respect to the center of rotation. 
         [0122]    Arm parts  193  to project in horizontal directions are provided between both side parts of yoke fixing part  192  and both side parts of base plate fixing part  191 . 
         [0123]    Arm parts  193  are formed with a linear wire element that is elastically deformable (i.e. linear material). One end part of each arm part  193  is fixed to base plate  122  of fixed body  120 B, and the other end is fixed to movable body  110 B, thereby supporting movable body  110 B on fixed body  120 B to be capable of back-and-forth rotating motion about an axis along output shaft  180  (here, the axial center of output shaft  180 ). 
         [0124]    Arm parts  193  are made by processing a linear material such that, when movable body  110 B moves in back-and-forth rotating motion, the stress that is produced accompanying increasing distortion is spread or distributed over the entirety and is produced uniformly from the whole of arm parts  193 . 
         [0125]    Here, as shown in  FIG. 14 , in arm part  193 , from the tip part of one side part  1931  that projects in directions (here, horizontal directions) to cross output shaft  180  from both sides of base plate fixing part  191 , curved part  1932  to draw an upward curve is provide in a continuous fashion. Curved part  1934  is provided between the upper end of this curved part  1932  and opposite side part  1933 , which is fixed in both side parts of yoke fixing part  192 . Based on the degree of curve of these curved part  1932  and bent part, the stress that is produced in arm part  193  when movable body  110 B is driven, is distributed uniformly. That is to say, the stress that applies to arm part  193  is distributed over the entirety, and the maximum stress value is made lower. By this means arm part  193  is not likely to be subject to fatigue fracture and does not have to be replaced frequently. 
         [0126]    Furthermore, arm part  193  is formed by processing a linear material and therefore can be made by a forming machine, which is cost effective. Furthermore, since arm part  192  is formed by processing a linear material, it can be provided in small space, thereby improving the degree of freedom in terms of the design of actuator  100 B itself. For example, compared to actuators  100  and  100 A actuator  100 B does not require a coil spring being elastic member  120 , guide shaft  125  and joint part  172 , so that it is possible to reduce the number of parts or and reduce the cost of parts. 
         [0127]    Furthermore, unlike the configuration of actuators  100  and  100 A, actuator  100 B does not have to provide guide shaft  125  and joint part  172  before and after the coil spring being elastic member  130 . By this means, compared to actuators  100  and  100 A, actuator  100 B is able to shorten its length in the direction of output shaft  180 , which defines the axis of rotation, and allow further miniaturization. 
       Fourth Embodiment 
       [0128]      FIG. 15  is a perspective view showing actuator  100 C according to a fourth embodiment of the present invention, and  FIG. 16  is a principal-part exploded perspective view of this actuator. 
         [0129]    Actuator  100 C according to this fourth embodiment has virtually the same magnetic circuit as in actuators  100  and  100 A. By this means, the magnetic circuit of actuator  100  satisfies above equations 2 and 3, and, assuming that the inertia of movable body  110 C is J and the spring constant in a twisting direction is k sp , is driven by the resonance frequency calculated by above equation 1, with respect to fixed body  120 C. Actuator  100 C basically has the same configuration as actuator  100  according to the first embodiment, shown in  FIG. 1 , and therefore parts in actuator  100 C that are the same as in actuator  100  will be assigned the same reference numerals and codes as in actuator  100  and their explanations will be omitted. 
         [0130]    Actuator  100 C shown in  FIG. 15  and  FIG. 16  has fixed body  120 C, movable body  110 C, coil spring  130  that supports movable body  110 C on fixed body  120 C in a movable fashion, and alternating current supplying part  140 . 
         [0131]    As shown in  FIG. 16 , with this actuator  100 C, when movable body  110 C that is supported in fixed body  120 C via a coil spring (i.e. elastic member  130 ) moves, output shaft  180  of movable body  110 C rotates in forward and backward directions (both directions of arrow B) in a predetermined angle range, and outputs back-and-forth rotating vibration outside. 
         [0132]    As shown in  FIG. 16 , fixed body  120  has base plate  122 C, support wall parts  124  and  126 C, outer yoke  150 C, and magnet  160  that is attached to outer yoke  150 C via non-magnetic body (spacer)  170 C. 
         [0133]    In fixed body  120 , base plate  122 C forms a flat rectangular shape that is long in the direction in which output shaft  180  extends, and is formed of a non-magnetic body here. Above a center area on the surface of base plate  122 C, coil  128  of movable body  110 C is placed, and outer yoke  150 C having a U-shaped cross section (including the shape of a letter U placed sideways) is attached to base plate  122 C, to cover this coil  128 . 
         [0134]    Furthermore, support wall parts  124  and  126 C are erected from edge parts of base plate  112 C that are spaced apart in the long direction. 
         [0135]    Support wall part  126 C has opening part  126   a  in which output shaft  180  movable body  110 C is inserted. 
         [0136]    Also, support wall part  124  supports movable body  110 C in a movable fashion via a coil spring, which is elastic member  130 . That is to say, support wall parts  124  and  126 C hold movable body  110 C in a movable fashion via the coil spring being elastic member  130  in a state in which output shaft  180  is inserted in opening part  126   a  of support wall part  126 C. In a normal state, movable body  110 C is supported virtually horizontally (that is, virtually parallel to base plate  122 C) by means of support wall parts  124  and  126 C, elastic member  130 , and so on. The structure for supporting movable body  110 C on base plate  122 C via elastic member  130  is the same as described above. That is to say, elastic member  130  (coil spring) is provided between guide shaft  125  attached to support wall part  124  that is erected in rear end part  122   a  of base plate  122 C, and joint part  172  attached to the movable body  110 C side. By this means, movable body  110  is supported to be able to move in back-and-forth rotating motion about output shaft  180  via elastic member  130 . 
         [0137]    Outer yoke  150 C is placed between these support wall parts  124  and  126 C to cover the main part of movable body  110 C. 
         [0138]    Outer yoke  150 C has a cross section approximately in the shape of a letter U that is placed sideways, and is formed by bending a flat magnetic body. Outer yoke  150 C has yoke center part  151  of a flat rectangular shape, and mutually opposing sidewall parts  152  and  153  that hang from the side parts of yoke center part  151 . Here, between support wall parts  124  and  126 , outer yoke  150 C is placed from above to cover coil  128  and coil holder  171 C of movable body  110 C. Outer yoke  150 C has its openings in the tip parts of sidewall parts  152  and  153  closed by base plate  122 C, and, with base plate  122 C and support wall parts  124  and  126 C, forms a box shape to accommodate movable body  110 C. 
         [0139]    Outer yoke  150  constitutes a magnetic circuit with coil  128  of movable body  110 C to be placed inside and magnet  160  that is attached in the back of yoke center part  151  of outer yoke  150 . 
         [0140]      FIG. 17  is a schematic cross-sectional view showing a principal-part configuration of actuator  100 C according to the fourth embodiment. 
         [0141]    As shown in  FIG. 17 , magnet (permanent magnet)  160  is placed in the center area on the back of yoke center part  151  of outer yoke  150 C, via non-magnetic body  170 C, such that air gaps G are formed between magnet  160  and opposing sidewall parts  152  and  153  of outer yoke  150 C. 
         [0142]    Magnet  160  is provided to hang from yoke center part  151 , via non-magnetic body  170 C, and different magnetic poles face the inner wall parts of sidewall parts  152  and  153 . 
         [0143]    That is to say, here, the S-pole end (S magnetic pole plane  160   a ) of magnet  160  faces the inner wall plane of sidewall part  152  of outer yoke  150 C, and the N-pole side (N magnetic pole plane  160   b ) faces the inner wall plane of sidewall part  153  of outer yoke  150 C. 
         [0144]    Furthermore, magnet  160  is a cuboid having a length to match the length of the extension direction of outer yoke  150 C, and is attached in yoke center part  151 , via non-magnetic body  170 C having the same outer shape, along the extension direction of yoke center part  151 . 
         [0145]    By this means, magnet  160  (see  FIG. 16  and  FIG. 17 ) has virtually the same length as the length of the long direction of outer yoke  150 C, and is placed in yoke center part  151  in a state the inner wall planes of opposing sidewall parts  152  and  153  all face planes of different magnetic poles. 
         [0146]    In air gaps G between magnet  160  and sidewall parts  152  and  153  of outer yoke  150 C, coil  128  of movable body  110 C is placed spaced apart from side wall planes (magnetic pole planes)  160   a  and  160   b  of magnet  160 , inner wall planes of sidewall parts  152  and  153 , and the back of yoke center part  151 . 
         [0147]    Coil  128 , with coil holder  171 C, output shaft  180 , and joint part  172 , constitutes movable body  110 C. 
         [0148]    To be more specific, in each air gap G, coil  128  is wound in a direction to be perpendicular to the direction in which magnet  160  and sidewall parts  152  and  153  oppose each other and surround the periphery of magnet  160 . Similar to the first embodiment, from alternating current supplying part  140 , an alternating current supply (AC voltage) is supplied to coil  128 , as shown in  FIG. 16  and  FIG. 17 . 
         [0149]    This coil  128  is placed in coil holder  171 C and held, and this coil holder  171 C is supported by fixed body  120 C via elastic member  130 . 
         [0150]    As shown in  FIG. 16 , this coil holder  171 C is formed in the shape of a letter U placed sideways on a side view, and has bottom plate part  1711  on which coil  128  is placed, and front wall part  1713  and rear wall part  1712  that erect from edge parts of bottom plate part  1711  that are spaced apart along the long direction (that is, along the direction in which output shaft  180  extends). 
         [0151]    This coil holder  171 C is formed of a non-magnetic body. 
         [0152]    In front wall part  1713 , output shaft  180  is attached perpendicular, and, in rear wall part  1712 , joint part  172  is attached. That is to say, output shaft  180  is placed approximately along the center of magnet  160 , approximately parallel to varying magnetic pole planes  160   a  and  160   b  of magnet  160  (see  FIG. 17 ). 
         [0153]    Elastic member  130  supports movable body  110 C between support wall part  124  and rear wall part  1712  such that movable body  110 C is able to move in the front, back, left and right directions. Via this elastic member  130 , in the area surrounded by base plate  122 C and outer yoke  150 C, movable body  110 C is supported on fixed body  120 C to be able to move in the twisting directions of magnet  160  and output shaft  180  about the axis of output shaft  180 . 
         [0154]    Incidentally, as shown in  FIG. 15  and  FIG. 16 , output shaft  180  of movable body  110 C is provided to project outward from support wall part  126 C in the same direction as the direction of extension of outer yoke  150 C. Incidentally, with actuator  100 C, output shaft  180  is provided to project is a direction to be virtually perpendicular to the direction in which magnent  160  and sidewall parts  152  and  153  oppose each other. 
         [0155]    Output shaft  180  is fixed in front wall part  1713  of coil holder  171 C in this way, and, by this means, is attached to movable body  110 C to be located on an axis to pass the center of gravity of movable body  110 C. By this means output shaft  180  is able to move in back-and-forth rotating vibration with coil  128  and coil holder  171 C constituting the main body of movable body  110 C, and transmit this vibration outside. 
         [0156]    When actuator  100 C is used for an electric toothbrush, a toothbrush part is coaxially coupled with output shaft  180 , and, at the head of this toothbrush part, a hair bundle part is provided to be perpendicular to the axial direction. By this means the toothbrush part moves in the same motion as output shaft  180 , that is, moves in rolling motion, which is back-and-forth rotating vibration. 
         [0157]    As shown in  FIG. 17 , with fixed body  120 C and movable body  110 C, outer yoke  150 C, magnet  160  and coil  128  form a magnetic circuit. 
         [0158]    Actuator  100 C has a magnetic circuit where magnetic fluxes produced from magnet  160  (designated by outline arrows) pass an air gap where coil  128  is placed, sidewall part  153  of outer yoke  150 C, yoke center part  151 , sidewall part  152  and the opposite air gap, in order, and continue to the opposite pole of magnet  160 . 
         [0159]    Similar to movable body  110  of actuator  100 , movable body  110 C of this actuator  100 C is supported by a spring mass system structure supported by fixed body  120 C via elastic member  130 . When an alternating current of the same frequency as resonance frequency f 0  of movable body  110 C is supplied to coil  128  from alternating current supplying part  140 , movable body  110 C is driven in a resonant state efficiently. The back-and-forth rotating vibration that is produced then is transmitted from output shaft  180  to the outside. 
         [0160]    Actuator  100 C is driven based on the equation of motion represented by equation 2 below and based on the circuit equation represented by equation 3 below. Consequently, the inertia moment, rotation angle, torque constant, current, spring constant, attenuation coefficient, and load torque in actuator  100 C can be changed as adequate in a range to satisfy equation 2, and the voltage, resistance, inductance, and counter electromotive force multiplier can be changed as adequate in a range to satisfy equation 3. 
         [0161]    Next, the operations of actuator  100 C will be described in detail. 
         [0162]      FIG. 18  is a schematic view for explaining operation of actuator  100 C according to the fourth embodiment. Although the flow of magnetic fluxes from magnet  160  is shown by outline arrows in  FIG. 18A , the same flow applies to  FIG. 18B  to  FIG. 18D , and illustration is omitted in  FIG. 18B  to  FIG. 18D . Also, although  FIG. 18A  shows alternating current supplying part  140  that supplies an AC voltage to coil  128 , the same applies to  FIG. 18B  to  FIG. 18D , and illustration is omitted in  FIG. 18B  to  FIG. 18D . 
         [0163]    When an alternating current is supplied from alternating current supplying part  140  to coil  128 , thrusts F 1 , F 2 , F 3  and F 4  in the drawing are produced in coil  128 , following Fleming&#39;s left hand rule. By this means, in movable body  110 C that is attached to fixed body  120 C in a movable fashion, a rotating force about an axial center at the center of rotation is produced. 
         [0164]    One operation cycle of actuator  100 C will be described. 
         [0165]    When a current flows in coil  128  in the direction shown in  FIG. 18A  (a current to flow in this direction will be hereinafter referred to as “forward current”), upward thrust F 1  (directed toward outer yoke  150 C) is produced in part  128   b  of coil  128  opposing N-pole plane  160   b  of magnet  160 . Meanwhile, in part  128   a  of coil  128  opposing S pole plane  160   a  of magnet  160 , downward thrust F 2  (directed toward base plate  122 C) is produced. 
         [0166]    By this means, a rotating force is produced in movable body  110 C that has coil  128  and that is supported by support wall parts  124  and  126 C that erect from base plate  122 C of fixed body  120 C (see  FIG. 16  and  FIG. 17 ), via elastic member  130 . Movable body  110 C moves anticlockwise to assume the position shown in  FIG. 18B  by thrusts F 1  and F 2  of coil  128 . 
         [0167]    In the state shown in  FIG. 18B , actuator  100 C produces reaction forces, designated by arrows R 1  and R 2 , by the restoring force of elastic member  130  (see  FIG. 15  and  FIG. 16 ). From the state shown in  FIG. 18B  to the state shown in  FIG. 18D , a reverse current is supplied to coil  128  as compared with  FIG. 18A . By this means, from the state shown in  FIG. 18B  to the state shown in  FIG. 18C , movable body  110 C rotates anticlockwise with respect to fixed body  120 C by the reaction forces designated by arrows R 1  and R 2  and by the thrusts designated by arrows F 3  and F 4 . From the state shown in  FIG. 18C  to the state shown in  FIG. 18D , movable body  110 C rotates anticlockwise with respect to fixed body  120 C by the thrusts designated by arrows F 3  and F 4 . 
         [0168]    In the state shown in  FIG. 18D , actuator  100 C produces reaction forces, designated by arrows R 3  and R 4 , by the restoring force of elastic member  130 . From the state shown in  FIG. 18D  to the state shown in  FIG. 18A , a forward current is supplied to coil  128 . By this means, from the state shown in  FIG. 18D  to the state shown in  FIG. 18A , movable body  110 C rotates anticlockwise with respect to fixed body  120 C by the reaction forces designated by arrows R 3  and R 4  and by the thrusts designated by arrows F 1  and F 2 . 
         [0169]    From the state shown in  FIG. 18A  to the state shown in  FIG. 18B , movable body  110 C rotates anticlockwise with respect to fixed body  120 C by the thrusts designated by arrows F 1  and F 2 . Although movable body  110 C operates in back-and-forth rotating vibration about magnet  160 , but movable body  110 C is also able to operate in the same way as shown in  FIG. 18  by thrusts F 1  to F 4 , without using the reaction force of elastic member  130 . 
         [0170]    The alternating current to be supplied to coil  128  in each state shown in  FIG. 18  may be a pulse wave of frequency f 0  as shown in  FIG. 7A  or may be a sine wave of frequency f 0  as shown in  FIG. 7B . 
         [0171]    The cycle of alternating current supplied from alternating current supplying part  140  to coil  128  of movable body  110 C in the actuator according to the present embodiment is the same as in actuator  100 . 
         [0172]    In the state of  FIG. 18A , the forward current at time point t 1  shown in  FIG. 7  is supplied. In the state of  FIG. 18B , the direction of the current is switched as shown at time point t 2  in  FIG. 7 . In the state of  FIG. 18C , the reverse current at time point t 3  shown in  FIG. 7  is supplied. Also, in the state of  FIG. 18D , the direction of the current is switched as shown at time point t 4  in  FIG. 7 , and, in the state of  FIG. 18D , the forward current at time point t 5  shown in  FIG. 7  is supplied. This is one operation cycle, and, by repeating these operations, movable body  110 C produces back-and-forth rotating vibration. 
         [0173]    Actuator  100 C uses a coil spring for elastic member  130  to support movable body  110 C to be able to move in back-and-forth rotating motion. That is to say, unlike a case where a flat spring is used as an elastic member to support movable body  110  that moves in resonance vibration, it is possible to spread stress uniformly without applying special ingenuity to the shape in order to spread required stress. Consequently, as a member to support movable body  110  in a movable fashion, a structure is provided that prevents stress from being concentrated on a location specific basis, that prevents the maximum stress value from increasing, and that therefore is robust against fatigue fracture. Furthermore, this structure is likely to make possible miniaturization and can be made using a forming machine used in general, so that it is possible to lower the cost of making. Furthermore, given that a coil spring can practically absorb the load in the direction of thrust, it is possible to improve the anti-shock robustness of actuator  100  itself. 
         [0174]    Actuator  100 C configured thus has the same working effects as actuator  100 . 
         [0175]    In addition, movable body  110 C is formed with coil  128  and coil holder  171 C, without outer yoke  150 C. Consequently, the scale of the inertia moment of movable body  110 C does not depend on the outer shape and is determined based upon the shape of coil  128 . Coil  128  is placed in a position on the inner side outer yoke  150  and therefore is unlikely to be a factor to increase the inertia. The increase of inertia moment due to change of the outer shape of actuator  100 C is reduced, so that constraints are removed in terms of design, and it is therefore possible to improve the freedom of design with respct to actuator  100 C itself. 
         [0176]    An electric toothbrush having actuator  100 C provides the same advantage, so that it is possible to miniaturize the electric toothbrush itself. 
         [0177]    In the configuration of actuator  100 C, base plate  122 C may be formed by a magnetic body. With this configuration, actuator  100 C forms two paths for magnetic fluxes by magnet  160  in fixed body  120 C. That is to say, as shown in  FIG. 20 , in the magnetic circuit of actuator  100 C, magnetic fluxes that are produced from magnet  160  pass an air gap where coil  128  is placed, and, through sidewall part  153  of outer yoke  150 C and yoke center part  151 , arrive at sidewall part  152 . Next, from sidewall part  153  of outer yoke  150 C, the magnetic fluxes pass base plate  112 C on the opposite side from yoke center part  151 , and then arrive at sidewall part  152 . Magnetic fluxes pass sidewall part  152  and the opposite air gap from the above air gap, in order, and continue to the opposite pole of magnet  160 . By this means, the magnetic saturation in the magnetic circuit is reduced, so that it is possible to increase the thrust of movable body  110 C that is produced when an AC voltage is supplied from alternating current supplying part  140  to coil  128 . Furthermore, in the event base plate  122 C in actuator  100 C is made a magnetic body, the outer periphery part of fixed body  120 C accommodating movable body  110 C in a movable fashion—that is, a magnetic circuit including magnetic  160 —is formed by outer yoke  150 C, which is a magnetic body, and base plate  122 C, which is also a magnetic body. That is to say, by forming the outer surface of actuator  100 C using a magnet body, in actuator  100 C, it is possible to prevent magnetic fluxes from leaking from the magnetic circuit including base plate  112 C, outer yoke  150 C, magnet  160  and coil  128 . 
       Fifth Embodiment 
       [0178]      FIG. 19  is exploded perspective view showing a configuration of actuator  100 D according to a fifth embodiment of the present invention.  FIG. 20  is schematic cross-sectional view showing configurations of movable body  110 D and fixed body  120 D of this actuator  100 D. This actuator  100 D basically has the same configuration as actuator  100  according to the fourth embodiment, shown in  FIG. 15  and  FIG. 16 , and therefore parts in actuator  100 D that are the same as in actuator  100  will be assigned the same reference numerals and codes as in actuator  100  and their explanations will be omitted. 
         [0179]    Base upon the configuration of actuator  100 C, actuator  100 D of the fifth embodiment is configured by, maintaining the magnetic circuit configuration, removing magnet  160  from outer yoke  150 C and fixing it on the base plate  122 D side via a non-magnetic body (spacer). In addition, movable body  110 D is formed by turning movable body  110 C having coil  128  in actuator  100 C upside down and attaching this movable body  110 D to fixed body  120 D via elastic member  130  so as to be able to move in back-and-forth rotating vibration in twisting directions. 
         [0180]    To be more specific, actuator  100 D has fixed body  120 D, movable body  110 D, elastic member  130  that supports movable body  110 D on fixed body  120 D to be able to move in twisting directions about output shaft  180  of movable body  110 D, and alternating current supplying part  140 . 
         [0181]    As shown in  FIG. 19  and  FIG. 20 , fixed body  120 D has base plate  122 D, magnet  160  that is placed on base plate  122 D via projection part  170 D of a non-magnetic body (spacer), and U-shaped outer yoke  150 C that is attached to base plate  122 D to cover magnet  160 . Furthermore, fixed body  120 D has support wall parts  124  and  126 C that are spaced apart between the front side and rear side of movable body  110 D. Movable body  110 D connects joint part  172  to elastic member  130  that is attached outwardly to guide shaft  125  of support wall part  124  and inserts output shaft  180  in opening part  126   a  of support wall part  126 , and, by this means, is supported on fixed body  120 D to be capable of back-and-forth rotating motion. 
         [0182]    As shown in  FIG. 20 , in fixed body  120 D, base plate  122 D of a flat rectangular shape is formed by a non-magnetic body, and magnet  160  is attached via non-magnetic projection part  170 D that is projected in the center part on the surface to project upward. 
         [0183]    Magnet  160  is placed on non-magnetic projection part  170 B such that air gaps are formed between its differing magnetic pole planes and opposing sidewall parts  152  and  153  of outer yoke  150 C. 
         [0184]    Like magnet  160  of the above embodiments, the magnetic pole planes of magnet  160  are spaced apart in a direction perpendicular to output shaft  180  and oppose sidewall parts  152  and  153  of outer yoke  150 C. 
         [0185]    Projection part  170 D is formed on base plate  122 D integrally and has the same outer shape as magnet  160 . Here, projection part  170 D is a cuboid to extend, with magnet  160 , in the long direction of base plate  122 D. Projection part  170 D places magnet  160  apart from base plate  122 D, thereby securing an area to allow coil  128  of movable body  110 D located in the surroundings of magnet  160  to move in back-and-forth rotation about magnet  160 . 
         [0186]    Thus, movable body  110 D is placed on fixed body  120 D such that coil  128  and upper plane part  1714  of coil holder  171 D are placed over magnet  160  attached on projection part  170 D projecting from base plate  122 D via an air gap. 
         [0187]    Movable body  110 D is placed in an air gap formed between opposing inner wall planes of outer yoke  150 C and magnet  160 , and is formed with coil  138  that surrounds magnet  160 , and coil holder  171 D that holds coil  128 . 
         [0188]    In coil holder  171 D where front wall part  1713  and rear wall part  1712  hang from edge parts that are spaced part in the log direction, coil  128  is attached on the back of upper plane part  1714 . 
         [0189]    Coil holder  171 D has joint part  172  that is attached to rear wall part  1712 , and, via this joint part  172 , opposite end part  132  of elastic member  130 , provided between coil holder  171 D and support wall part  124  of fixed body  120 D is fixed. Coil holder  124 B is attached to support wall parts  114  and  116  of fixed body  120 B, via elastic member  130 , to be able to move in twisting directions about shafts  125  and  126  provided perpendicular to the axial direction of coil  170 . By this means, movable body  110 D is attached to fixed body  120 D to be able to move in twisting directions about output shaft  180 . 
         [0190]    Similar to actuators  100  and  100 C, an alternating current having approximately the same frequency as a resonance frequency is supplied to coil  128  from alternating current supplying part  140  that supplies an AC voltage. By this means, movable body  110 D, supported in fixed body  120 C by means of elastic member  130  to be able to move in twisting directions of output shaft  180 , moves in back-and-forth rotating vibration by the thrust by coil  128  in fixed body  120 D. 
         [0191]    As shown in  FIG. 20 , actuator  100 D has a magnetic circuit where magnetic fluxes produced from magnet  160  (designated by outline arrows) pass air gap G where coil  128  is placed, sidewall part  153  of outer yoke  150 C, yoke center part  151 , sidewall part  152  and the opposite air gap, in order, and continue to the opposite pole of magnet  160 . In  FIG. 20 , the flow of magnetic fluxes in the magnetic circuit of actuator  100 D is shown by outline arrows. 
         [0192]    When an alternating current is supplied from alternating current supplying part  140  to coil  128  in actuator  100 D (see  FIG. 7 ) as in the case of actuator  100 C, following Fleming&#39;s left hand rule, the thrusts designated by arrows F 1  and F 2  in the drawing and reverse thrusts to these thrusts designated F 1  and F 2  are produced alternately. By this means, a rotating force about an axial center being output shaft  180 , which is the center of rotation, is produced in coil  128 , and movable body  110 D repeats the same operation (see  FIG. 10 ) as coil  128  of actuator  100 C shown in  FIG. 18 , and produces back-and-forth rotating vibration. 
         [0193]    Furthermore, although actuator  100 D of this embodiment places magnet  160  differently compared to actuator  100 C, the magnetic circuit configuration is the same and the same effect as actuators  100  and  100 C described above can be provided. In particular, with actuator  100 B, it is possible to achieve back-and-forth rotating motion of an electric toothbrush or the like without using a drive transmitting mechanism apart from a drive source. 
         [0194]    Furthermore, since actuator  100 D directly places magnet  160  on projection part  170 B that is formed on non-magnetic base plate  122 D integrally, so that, compared to actuator  100 C, it is not necessary to use a separate non-magnetic body and it is therefore possible to reduce the number of parts and make actuator  100 D more cost effective. 
         [0195]    Furthermore, upon assembly, magnet  160  is placed on projection part  170 D that projects from the surface of flat base plate  122 D, so that, compared to the case of placing magnet  160  in the denting interior of U-shaped outer yoke  150 , it is possible to perform positioning and assembling operations easily. 
         [0196]    In the configuration of actuator  100 D, it is equally possible to form base plate  122 D by a different magnetic body from that of projection part  170 D and provide projection part  170 D in base plate  122 D of this magnetic body. With this configuration, compared to actuator  100 , actuator  100 A forms two paths for magnetic fluxes by magnetic  150  in fixed body  120 . That is to say, as shown in  FIG. 7 , in the magnetic circuit of actuator  100 D, magnetic fluxes (shown by outline arrows) that are produced from magnet  160  pass an air gap where coil  128  is placed, pass from sidewall part  153  of outer yoke  150  to yoke center part  151 , and, in addition, pass from sidewall part  153  to base plate  122 D on the opposite side of yoke center part  151 , and then arrive at sidewall part  152 . Magnetic fluxes passing this sidewall part  152  then pass the opposite air gap and continue to the opposite pole of magnet  160 . By this means, it is possible to achieve the same working effect as in the case where base plate  122 C is made a magnetic body in the fourth embodiment. 
       Sixth Embodiment 
       [0197]      FIG. 21  is a principal-part exploded perspective view of actuator  100 E according to a sixth embodiment of the present invention, and  FIG. 22  is schematic cross-sectional view showing movable body  110 E and fixed body  120 E of this actuator  100 E.  FIG. 22  shows the flow of magnetic fluxes, from magnet  160  as a magnetic circuit of actuator  100 E, with outline arrows. 
         [0198]    Actuator  100 E according to a sixth embodiment has the same magnetic circuit as in actuators  100  and  100 A. 
         [0199]    Assuming that the inertia of movable body  110 E is J and the spring constant in a twisting direction is k sp , actuator  100 E satisfies equations 2 and 3 and is driven by the resonance frequency calculated by equation 1 above, with respect to fixed body  120 E. This actuator  100 E basically has the same configuration as actuator  100 C according to the fourth embodiment, shown in  FIG. 15  and  FIG. 160 , and therefore parts in actuator  100 E that are the same as in actuator  100 C will be assigned the same reference numerals and codes as in actuator  100 C and their explanations will be omitted. This actuator  100 E has basically the same magnetic circuit as in actuator  100 C, except that magnet  160  is provided as a magnetic body, unlike actuator  100 C in which coil  128  is the movable body. 
         [0200]    Actuator  100 E shown in  FIG. 21  and  FIG. 22  has fixed body  120 E, movable body  110 E, a twisted coil spring (hereinafter referred to as “coil spring”), which is elastic member  130  to support movable body  110 E on fixed body  120 E in a movable fashion, and alternating current supplying part  140 . 
         [0201]    As shown in  FIG. 21 , with this actuator  100 E, when movable body  110 E that is supported in fixed body  120 E via elastic member  130  moves, output shaft  180  of movable body  110 E rotates in forward and backward directions in a predetermined angle range, and outputs back-and-forth rotating vibration outside. 
         [0202]    Fixed body  120 E has base plate  122 C, support wall parts  124  and  126 C, outer yoke  150 C, and coil  128  that is attached to outer yoke  150 C. Meanwhile, movable body  110 E has magnet (permanent magnet)  160 , magnet holder  171 E that is supported by support wall part  124  via a coil spring as elastic member  130  and that holds magnet  160 , and output shaft  180 . 
         [0203]    In fixed body  120 E, in outer yoke  150 C, magnet  160  of movable body  110 E is placed in an air gap on the inner side of coil  128 . In actuator  100 , by receiving as input an alternating current supply (AC voltage) from alternating current supplying part  140  in coil  128 , movable body  110 E is driven in a resonant state. The cycle of alternating current to be supplied is the same between embodiments (see  FIG. 7 ) and overlapping explanations will be omitted. 
         [0204]    Above the surface of base plate  122 C, magnet  160  of movable body  110 E is placed, and, surrounding this magnet  160 , coil  128  is attached, via its outer periphery part, to opposing inner wall planes  152   a  and  153   a  of outer yoke  150 C having a U-shaped cross section (including the shape of a letter U placed sideways). 
         [0205]    Furthermore, support wall parts  124  and  126 C are erected from edge parts of base plate  122 C that are spaced apart in the long direction. The structure for supporting movable body  110 E on fixed body  120 E using support wall parts  124  and  126 C, guide shaft  125 , elastic member  130 , joint part  172  and output shaft  180  is the same as with actuator  100 C, and so descriptions will be omitted. 
         [0206]    That is to say, in the coil spring being elastic member  130 , one end part  131  is inserted in insertion hole  124   b  formed in fixed block  124   c  of support wall part  124 , and opposite end part  132  is inserted in fitting hole  1721  formed in joint part  172 . By this means, in the area surrounded by base plate  122 C and outer yoke  150 C, support wall part  124  is supported via elastic member  130 , such that movable body  110 E is able to move in twisting directions, about the axis of output shaft  180 . 
         [0207]    Outer yoke  150 C is attached to base plate  122 C in the same way as in the configuration of actuator  100 C, and, with support wall parts  124  and  126 C, forms a box shape to accommodate movable body  110 E. Inside this box—to be more specific, in opposing inner wall planes  152   a  and  153   a  of side wall parts  152  and  153  of outer yoke  150 —coil  128  that is wound to surround the periphery of magnet  160  of movable body  110 E via an air gap is fixed. 
         [0208]    Coil  128  is a voice coil here, and is placed such that its outer diameter parts are fixed on inner wall planes  142   a  and  143   a  of side wall parts  142  and  143  of outer yoke  150 , and magnet  160  is placed on the inner side from the inner diameter parts, via air gaps from the inner periphery parts. That is to say, the inner periphery parts of coil  128  are placed to oppose the outer periphery planes of different poles of magnet  160  at a certain distance. 
         [0209]    Also, between side wall parts  152  and  153  of outer yoke  150 C, coil  128  has a square cylindrical shape formed by winding a coil wire around an axis to extend in a direction virtually perpendicular to yoke center part  151  of outer yoke  150 , base plate  122 C and output shaft  180 . An alternating current of substantially the same frequency as a resonance frequency f 0  of movable body  110 E is supplied from alternating current supplying part  140  to coil  128 . 
         [0210]    This coil  128  is attached on inner wall planes of outer yoke side wall parts  152  and  153  closer to yoke center part  151  and is placed in locations to face different magnetic poles of magnet  160  (magnetic pole planes  160   a  and  160   b ). 
         [0211]    Magnet (permanent magnet)  160 , which is placed on the inner side of coil  128  via air gaps, is a cuboid having magnetic pole planes  160   a  and  160   b  that are long in the direction in which outer yoke  150 C extends. Here, magnet  160  is held in a rotatable fashion in an air gap on the inner side of coil  170 , by means of magnet holder  171 E held rotatably by support wall parts  124  and  126 C via elastic member  130 . 
         [0212]    This magnet holder  171 E is formed in the shape of a letter U that is placed sideways on a side view, and that is open upward, as shown in  FIG. 21 . Magnet holder  171 E has bottom plate part  1715  having a flat rectangular shape and front wall part  1713  and rear wall part  1712  that are erected from end parts that are spaced apart in the long direction of bottom plate part  1715  (that is, along the direction of extension of output shaft  180 ). 
         [0213]    This magnet holder  171 E is formed of a non-magnetic body. In front wall part  1713  of magnet holder  171 E, output shaft  180  is attached perpendicularly. Furthermore, in rear wall part  1712  of magnet holder  171 E, joint part  172  is attached such that the axial center of the coil spring of elastic member  130  that is connected to joint part  172  is placed to be virtually coaxial with output shaft  180 . That is to say, output shaft  180  is attached to movable body  110 E, approximately along the center of magnet  160 , approximately parallel to varying magnetic pole planes  160   a  and  160   b  of magnet  160  (see  FIG. 22 ), and to be located on an axis to pass the center of gravity movable body  110 E. 
         [0214]    Magnet holder  171 E places magnet  160  apart from coil  128  and the back of yoke center part  151  of outer yoke  150 C, and holds magnet  160  to be able to rotate in twisting direction about the axis of output shafts  180  and  126 . In movable body  110 E, coil  170  is placed between front wall part  1713  of magnet holder  171 E and magnet  160  and between rear wall part  1712   c  and magnet  160 , without making coil  128  touch these wall parts or magnet  160 , so that movable body  110 E is able to move on the inner side and outer side of coil  128 . 
         [0215]    Magnetic pole planes  160   a  and  160   b  of magnet  160 , held by magnet holder  171 E, are placed to oppose, entirely, the inner wall planes of outer yoke sidewall parts  152  and  153  via coil  128 . 
         [0216]    Here, the S-pole end (S magnetic pole plane  160   a ) of magnet  160  faces the inner wall plane  152   a  of sidewall part  152  of outer yoke  150 C, and the N-pole side (N magnetic pole plane  160   b ) faces the inner wall plane  153   a  of sidewall part  153  of outer yoke  150 C. 
         [0217]    As shown in  FIG. 21 , output shaft  180  is provided to project outward from support wall part  126 C in the same direction as the direction in which outer yoke  150 C extends. By this means, in actuator  100 , output shaft  180  is provided to project in a direction that is virtually perpendicular to the direction in which magnet  160  and sidewall parts  152  and  153  oppose each other over coil  128 , from the center of sidewall parts  152  and  153 . 
         [0218]    When actuator  100  is used for an electric toothbrush, a toothbrush part is coaxially coupled with output shaft  180 , and, at the head of this toothbrush part, a hair bundle part is provided to be perpendicular to the axial direction. By this means the toothbrush part moves in the same motion as output shaft  180 , that is, moves in rolling motion, which is back-and-forth rotating vibration. 
         [0219]    As shown in  FIG. 22 , with fixed body  120 E and movable body  110 E, outer yoke  150 C, magnet  160  and coil  128  form a magnetic circuit. 
         [0220]    To be more specific, actuator  100 E has a magnetic circuit where magnetic fluxes produced from magnet  160  (designated by outline arrows) pass an air gap where coil  128  is placed, sidewall part  153  of outer yoke  150 C, yoke center part  151 , sidewall part  152  and the opposite air gap, in order, and continue to the opposite pole of magnet  160 . 
         [0221]    Similar to movable body  110 C of actuator  100 C, movable body  110 E of this actuator  100 E is supported by a spring mass system structure supported by fixed body  120 E via elastic member  130 . When an alternating current of the same frequency as resonance frequency f 0  of movable body  110 E is supplied to coil  128  from alternating current supplying part  140 , movable body  110 E is driven in a resonant state efficiently. The back-and-forth rotating vibration that is produced then is transmitted from output shaft  180  to the outside. 
         [0222]    Actuator  100 E is driven based on the equation of motion represented by equation 2 above and based on the circuit equation represented by equation 3 above. That is to say, similar to actuator  100 , the inertia moment, rotation angle, torque constant, current, spring constant, attenuation coefficient, and load torque in actuator  100 C can be changed as adequate in a range to satisfy equation 2, and the voltage, resistance, inductance, and counter electromotive force multiplier can be changed as adequate in a range to satisfy equation 3. 
         [0223]    The operation principle of movable body  110 E of this actuator  100 E is the same as actuator  100 C and therefore will not be described in detail.  FIG. 22  shows thrusts F 1  and F 2  of coil  128  when a forward current is applied, and thrusts R 1  and R 2  of magnet  160 , which are reaction forces to these. When thrusts R 1  and R 2  are produced, movable body  110 E moves in the directions of thrusts R 1  and R 2 . When the direction of current changes, reverse thrusts to F 1  and F 2  work on coil  128 , and, by this means, opposite thrusts to R 1  and R 2  work on magnet  160 , and, consequently, movable body  110 E moves in directions designated by reverse thrusts to R 1  and R 2 . By repeating these, similar to the first embodiment, actuator  100 E moves mobile body  120 E in back-and-forth rotating vibration. 
         [0224]    In actuator  100 E, movable body  110 E produces back-and-forth rotating motion (that is, back-and-forth rotating vibration), and this back-and-forth rotating vibration is sent outside via output shaft  180 . When a toothbrush part is coupled with output shaft  180  and a hair bundle part is provided to be perpendicular to the axial direction at the head of this toothbrush part, the toothbrush part moves in back-and-forth rotating vibration and makes possible rolling brushing. 
         [0225]    By this means, actuator  100 E satisfies equations 2 and 3 and is driven by a resonance phenomenon using the resonance frequency represented by equation 1. 
         [0226]    Furthermore, movable body  110 E is formed with magnet  160  and magnet holder  171 E, without using large-sized components like outer yoke  150 C. Consequently, the scale of the inertia moment of movable body  110 E does not depend on the outer shape and can be determined based upon the shape of magnet  160 . Furthermore, given that magnet  160  is placed such that its center of gravity is located near output shaft  180  in movable body  110 E, and, to be more specific, approximately on the axis of output shaft  180 , so that magnet  160  is unlikely to be a factor to increase the inertia of movable body  110 E. The increase of inertia moment due to change of the outer shape of actuator  100  is reduced, so that constraints are removed in terms of design, and it is therefore possible to improve the freedom of design with respct to actuator  100  itself. An electric toothbrush having actuator  100  provides the same advantage, so that it is possible to miniaturize the electric toothbrush itself. 
         [0227]    Also, although with the configuration of actuator  100 E according to the sixth embodiment base plate  122 C is a non-magnetic body, this is by no means limiting, and it is equally possible to use a magnetic body. If base plate  122 C in the configuration of actuator  100 E is formed by a magnetic body, actuator  100 E forms two paths for magnetic fluxes by magnet  160 . That is to say, if base plate  122 C in the configuration of actuator  100 E is formed by a magnetic body, magnetic fluxes that are produced from magnet  160  reach sidewall part  153  of outer yoke  150 C, from magnetic pole plane  160   b,  passing an air gap where coil  128  is placed. Next, from sidewall part  153 , the magnetic fluxes pass both yoke center part  151  and base plate  112 G on the opposite side from yoke center part  151 , and then arrive at sidewall part  153 . Magnetic fluxes pass sidewall part  152  and the opposite air gap in order, and continue to the opposite pole of magnet  160  (magnetic pole plane  160   a ). By this means, the magnetic saturation in the magnetic circuit is reduced, so that it is possible to increase the thrust of movable body  110 E that is produced when an AC voltage is supplied from alternating current supplying part  140  to coil  128 . That is to say, in actuator  100 E, it is possible to prevent magnetic fluxes from leaking from the magnetic circuit including base plate  122 C, outer yoke  150 C, magnet  160  and coil  128 . 
         [0228]    Furthermore, outer yoke  150  according to the above embodiments can be configured in any way as long as there are inner wall planes to oppose different magnetic poles of magnet  160 , and a magnetic circuit is formed with coil  128  and magnet  160 , and it is possible to, for example, form the entirety of outer yoke  150  to have an arc-shaped cross section or make the main body of the yoke a arc shape. 
         [0229]    Various changes can be made to the present invention without departing from the spirit of the present invention, and such changes are certainly within the scope of the present invention. 
         [0230]    The disclosure of Japanese Patent Application No. 2008-292631, filed on Nov. 14, 2008, including the specification, drawings, and abstract, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0231]    An actuator according to the present invention provides an advantage of realizing back-and-forth rotating motion of an electric toothbrush or the like without using a drive transmitting mechanism apart from a drive source and allowing miniaturization of an electric toothbrush or the like, and therefore is suitable for use as an actuator that is used for an electric toothbrush or the like to produce back-and-forth rotating vibration. 
       REFERENCE SIGNS LIST 
       [0232]      100 ,  100 A,  100 B,  100 C,  100 D,  100 E Actuator 
         [0233]      110 ,  110 B,  110 C,  110 D,  110 E Movable body 
         [0234]      120 ,  120 B,  120 C,  120 D,  120 E Fixed body 
         [0235]      122 ,  122 C,  122 D Base plate 
         [0236]      127  Bearing 
         [0237]      128  Coil 
         [0238]      130  Elastic member 
         [0239]      131  One end part 
         [0240]      132  Opposite end part 
         [0241]      140  Alternating current supplying part 
         [0242]      150 ,  150 C Outer yoke 
         [0243]      151  Yoke center part 
         [0244]      152 ,  153  Sidewall part 
         [0245]      152   a,    153   a  Inner wall part 
         [0246]      160  Magnet 
         [0247]      160   a,    160   b  Magnetic pole plane 
         [0248]      170 ,  170 C Non-magnetic body 
         [0249]      170 B,  170 D Projection part 
         [0250]      171  Yoke holder 
         [0251]      171 C,  171 D Coil holder 
         [0252]      171 E Magnet holder 
         [0253]      172  Joint part 
         [0254]      180  Output shaft 
         [0255]      190  Wire-shaped spring body 
         [0256]      191  Base plate fixing part 
         [0257]      192  Yoke fixing part 
         [0258]      193  Arm part