Patent Publication Number: US-7583564-B2

Title: Piezoelectric actuator and electronic equipment with piezoelectric actuator

Description:
FIELD OF THE INVENTION 
   The present invention relates to piezoelectric actuator, and to an electronic device comprising the same. 
   BACKGROUND OF THE INVENTION 
   In conventional practice, piezoelectric actuators for driving driven objects by the vibration of a vibrator having a piezoelectric element have been used in the calendar mechanisms of timepieces, the zoom mechanisms or auto-focus mechanisms of cameras, the drive mechanisms of mobile toys, and the like. 
   With timepieces, cameras, mobile toys, or other electronic devices provided with such piezoelectric actuators, the operation may be affected by the fact that the vibrator of the piezoelectric actuator and the rotor or other objects driven by the piezoelectric actuator may be brought out of alignment by external impact that occurs when the device is dropped, and damages may occur if the impact is severe. Particularly, the component that supports and secures the vibrator is generally thin and fragile so as not do disrupt the vibration of the vibrator Therefore, problems are encountered in that the deformation of the vibrator due to impact causes this supporting part to break or causes the wiring provided to the supporting part to be cut. 
   Methods in which the vibrator is merely supported in a direct manner in order to provide an impact-resistant structure for the piezoelectric actuator are subject to problems in that the support interferes with vibration and reduces drive efficiency. 
   Therefore, in one proposed impact-resistant structure in which the vibrator is not directly supported, the rotor and the vibrator are prevented from separating during impact. This is achieved by providing a spring member that pushes the rotor towards the vibrator, and providing a pin to a position adjacent to the spring member to regulate the movement of the spring member (Patent Reference 1). 
   The pin in Patent Reference 1 is provided so as to come into contact with the spring member in cases in which the rotor moves a specific distance in the direction away from the vibrator during impact, and also so as to maintain the state of contact between the piezoelectric actuator and the rotor. The pin is also disposed at a position whereby the rotor will not collide with any other members in cases in which the rotor separates from the vibrator during impact, and whereby the meshing between the rotor and the teeth of the next transmission wheel will not disengage. 
   [Patent Reference 1] Japanese Laid-open Patent Application No. 2004-301627 (Specification, paragraph no. [0013], FIG. 2) 
   SUMMARY OF THE INVENTION 
   [Problems the Invention is Intended to Solve] 
   However, in cases in which a pin such as the one described in Patent Reference 1 is used, design proves difficult because it is difficult to determine a position suitable for positioning the pin. 
   Also, in Patent Literature 1, the movement of the rotor can be regulated during impact and damage to the rotor can be prevented, but the movement of the vibrator is not directly regulated. Therefore, there still remains the possibility that the vibrator will separate from the rotor and be damaged as a result of impact. 
   In view of these problems, an object of the present invention is to provide a piezoelectric actuator whereby design can be facilitated and impact resistance can be greatly improved without causing a reduction in drive efficiency; and to provide an electronic device comprising this piezoelectric actuator. 
   [Means for Solving These Problems] 
   The piezoelectric actuator of the present invention is a piezoelectric actuator that has piezoelectric elements to which electrodes are provided, and that comprises a vibrator vibrated by the application of voltage to the electrodes; wherein the vibrator has the piezoelectric elements and a reinforcing member that are stacked on each other; the reinforcing member has a free end that has the piezoelectric elements disposed thereon and that has a protuberance in contact with a driven object, and also has a fixed part that is fixedly attached to an attachment part to which the piezoelectric actuator is attached; and the free end, when in a state in which no impact is applied to the attachment part from the outside, is disposed in relation to the attachment part via spaces that have specific dimensions in the stacking direction of the piezoelectric elements and the reinforcing member, and in an in-plane direction that intersects with the stacking direction. 
   According to this invention, spaces having specific dimensions are formed between the attachment part and the vibrator in an electronic device or the like when the piezoelectric actuator is incorporated into the electronic device. Therefore, the vibrator can be prevented from moving beyond the dimensions of the spaces. This is because the free end is in a completely free and movable state within the space portions, and the vibrator is secured in place by the attachment part to the free end side when subjected to external impact. The stacking direction of the vibrator is designated as the Z direction. Spaces are provided in both the Z direction and the XY in-plane direction that intersects with the Z direction; i.e., in all of the XYZ directions, between the attachment part and the free end. Because of these spaces, the free end is cushioned when subjected to an external force having an arbitrary direction, and the external force is therefore reduced. 
   It is thereby possible to drive the driven object without hindering vibration, and it is also possible to prevent incidents in which the protuberance of the vibrator becomes separated from the driven object due to impact, the amount by which the driven object is rotated fluctuates, or the vibrator collides with and damages other components. The effect of impact resistance is significant, because the vibrator directly faces the attachment part and the vibrator is securely captured by the attachment part when the vibrator moves or deforms during impact, and also because the vibrator is captured at the free end, where the amount of bending reaches a maximum during impact. 
   Therefore, resistance against impact from falling or the like can be greatly improved without reducing drive efficiency, and the usual problems with the difficulty of reconciling drive efficiency and impact resistance can be resolved. 
   Also, the design used to obtain these effects is not difficult, because spaces of specific dimensions need be provided only between the vibrator and the attachment part. 
   The dimensions of the spaces are appropriately determined according to the amplitude and direction of vibration in the vibrator, the strength of the external force during impact, and other factors. 
   For example, it is possible to design the conditions of the load on the vibrator during impact, to find the maximum bending amount of the free end of the vibrator, and to set the specific space dimensions to be equal to or less than the maximum dimensions that are determined based on the maximum bending amount and on other factors. Impact resistance under specific load conditions can thereby be ensured. 
   The tolerances between the vibrator and the attachment part due to nonuniformities in shape and assembling errors may be kept to a minimum. 
   Furthermore, one significance of forming these spaces between the vibrator and the attachment part while the attachment part is not undergoing external impact is that vibration noise is not likely to occur in the drive electrodes provided to the piezoelectric elements while the piezoelectric actuator is driven, and drawbacks such as the drive control circuit being destroyed by electric shocks can thereby be prevented in advance. 
   Vibration noise is also not likely to occur in detection electrodes in cases in which detection electrodes for detecting the vibrating state of the piezoelectric elements as voltage signals are provided to parts of the piezoelectric elements. The vibrating state can thereby be accurately and securely detected, and the piezoelectric actuator can be driven in a stable manner on the basis of this vibration detection. Since the vibration can be accurately and securely detected, the detection electrodes can be disposed in the vicinity of the vibration nodes of the vibrator or at other locations having a low vibration detection output, and the layout of electrodes in the piezoelectric elements can be simplified. 
   In the piezoelectric actuator of the present invention, it is preferable that the free end have a vibrating part that is provided with the fixed part on one side in the longitudinal direction, is formed into a flat substantially rectangular shape, and is made to vibrate in a mixed mode involving both longitudinal vibration that expands and contracts in a longitudinal direction, and bending vibration that bends in a transverse direction that intersects with the longitudinal direction, and that the free end further have an arm part that extends from the other side of the vibrating part in the planar direction of the vibrating part and that is disposed on the attachment part via the spaces; and that the arm part act as a cushion for the piezoelectric actuator. 
   According to this invention, longitudinal vibration and bending vibration can be securely induced in the vibrating part, and the entire free end can be cushioned by the arm part provided separately from the vibrating part. The drive efficiency and impact resistance of the piezoelectric actuator can be further improved by dividing functions between the vibrating part and the arm part in this manner. 
   The impact resistance against external stress, particularly stress that acts in a direction aligned with the planar direction of the vibrating part, can be improved, because the arm part faces the attachment part via the spaces in the planar direction of the vibrator. Also, these spaces in the planar direction are no obstruction to longitudinal vibration that is displaced in a direction aligned with the plate surface of the vibrator. 
   Furthermore, bending vibration is accompanied by a mechanical moment but is not obstructed because a space is also provided in the stacking direction (Z direction) of the vibrator. 
   It is preferable that the vibrating part (including the protuberances), the fixed part, and the arm part be integrally formed on the reinforcing plate. 
   In the piezoelectric actuator of the present invention, it is preferable that the characteristic frequency of the arm part be different by a specific value from the characteristic frequency of the longitudinal vibration of the vibrating part; and also be set so as not to interfere with the longitudinal vibration of the vibrating part. 
   According to this invention, there is no obstacle to inducing bending vibration as secondary vibration because the vibration of the arm part does not interfere with the longitudinal vibration of the vibrating part, and the longitudinal vibration as primary vibration induced by the vibrating part does not diminish. 
   In the piezoelectric actuator of the present invention, it is preferable that the dimensions of the arm part be set so that the length of the arm part in the longitudinal direction of the vibrator is less than the length of the vibrator in the longitudinal direction; and that the dimensional ratio for the length of the arm part is set so that the ratio of the length of the arm part to the width of the arm part in the transverse direction of the vibrating part is less than the ratio of the length of the vibrating part to the width of the vibrating part in the transverse direction. 
   According to this invention, the length of the arm part is less than the length of the vibrating part and the dimensional ratio of the arm part is such that the shape of the arm part is not long and thin in comparison with the vibrating part. Therefore, the vibration mode of the arm part is predominantly longitudinal vibration that expands and contracts in the longitudinal direction of the vibrating part, and there is no secondary, tertiary, or other high-order vibration modes in the vibration of the arm part. The vibration mode of the arm part can be considered to be only longitudinal vibration, and characteristic frequency of the arm part is greater than the characteristic frequency of the vibrating part because the dimensions of the arm part are smaller than the vibrating part, and the characteristic frequencies of the arm part and the vibrating part are different. An interference-induced reduction in the vibration of the vibrating part can thereby be satisfactorily prevented. Consequently, there is no reduction in the range of shapes that the arm part can assume so as not to interfere with the vibration of the vibrating part. 
   In the piezoelectric actuator of the present invention, it is preferable that the surface area obtained by multiplying the length of the arm part in the longitudinal direction of the vibrating part by the width of the arm part in the transverse direction of the vibrating part be less than the surface area of the vibrating part. 
   According to this invention, the arm part is more lightweight than the vibrating part in cases in which the surface area of the arm part is less than the surface area of the vibrating part and the arm part and in which vibrating part are formed from the same material Therefore, the characteristic frequency of the arm part can be made to be sufficiently different from the characteristic frequency of the vibrating part, and a reduction in the vibration of the vibrating part due to interference from vibration of the arm part can be satisfactorily reduced. 
   In the piezoelectric actuator of the present invention, it is preferable that a hole be formed in the arm part, that the attachment part have a protrusion that extends towards the hole and that is inserted through the hole, and that the spaces be formed between the protrusion and the inner peripheral edge of the hole in the arm part. 
   According to this invention, impact resistance can be improved because the inner peripheral edge of the hole in the arm part interlocks with the protrusion of the attachment part during impact, and movement of the vibrating part from the inside of the hole to the outside is securely regulated. 
   Since the vibrating part is capable of rotating within the hole in the arm part when the vibrator is driven, it is possible to avoid obstruction by the attachment part to bending vibration, which vibrates in a direction that intersects with longitudinal vibration and which is induced by an imbalance in the arrangement of the electrodes or in the positions of the protuberances in relation to the centroid of the vibrator. 
   In the piezoelectric actuator of the present invention, it is preferable that the protrusion have a stepped part in the middle of the direction of protrusion; and that the spaces be formed between the mutually opposing surfaces of the stepped part and the arm part. 
   According to this invention, inserting the protrusion into the hole in the arm part allows spaces to be formed not only in the XY direction between the protrusion and the inner peripheral edge of the hole in the arm part, but also in the Z direction between the mutually opposing surfaces of the arm part and the stepped part of the protrusion. The structures created by these spaces function as cushions for the vibrator during impact. Thus, owing solely to the configuration of the arm part, it is possible to provide both impact resistance against external forces acting in the planar direction (XY direction) of the vibrator, and impact resistance against external forces acting in the stacking direction (thickness direction, Z direction) of the vibrator. 
   In the piezoelectric actuator of the present invention, it is preferable that the arm part be provided in the vicinity of a node of the bending vibration in the vibrator. 
   According to this invention, since the arm part is provided in the vicinity of a node of bending vibration, the effects of the arm part on the vibration characteristics can be greatly reduced. 
   Since displacement due to vibration is extremely small in the vicinity of the node, vibration is not obstructed if the dimensions of the spaces between the protrusion and the inner peripheral edge of the hole are reduced, and impact resistance can be further improved by reducing the dimensions of the spaces. In this case, it is possible to use the tolerances between the vibrator and the attachment part as specific space dimensions. 
   Three nodes of bending vibration in the vibrating part are found to pass through the planar center and to lie on the center line in the longitudinal direction of the vibrating part, and the term “the node of bending vibration in the vibrator” to which the arm part is provided refers to any of these three nodes. It is more preferable that the arm part be provided in the vicinity of the planar center of the vibrating part. This is because the bending vibration node that is located in the planar center of the vibrating part and that is one of the three bending vibration nodes is at the same position as the node of longitudinal vibration of the vibrating part. 
   The term “the vicinity of the node of bending vibration” includes positions where lines that extend in the transverse direction of the vibrating part from the position of the bending vibration node intersect with the external periphery of the vibrating part (the sides of the vibrating part in the longitudinal direction), and it is preferable that the arm part be provided to such a position. 
   In the piezoelectric actuator of the present invention, it is preferable that the arm part be provided to the vibrating part via a constricted neck part that is on the side of the arm part and that connects to the vibrating part; and that the neck part on the side of the arm part be provided in the vicinity of the node of bending vibration in the vibrating part. 
   According to this invention, greater freedom is allowed with the shape of the arm part by providing the neck part on the side of the arm part in the vicinity of the node of the vibrating part, and the vibration energy that is dissipated by the vibrating part through the arm part can be reduced by providing the neck part on the side of the arm part, thus allowing drive efficiency to be improved. 
   In the piezoelectric actuator of the present invention, it is preferable that the fixed part be provided to the vibrating part via a constricted neck part that is on the side of the fixed part and that connects to the vibrating part; and that the neck part on the side of the fixed part be provided in the vicinity of the node of bending vibration in the vibrating part, at a position that faces the neck part on the side of the arm part in the transverse direction of the vibrating part. 
   Specifically, in cases in which the vibrating part, the fixed part, and the arm part are integrally formed on the reinforcing plate, which is made of a steel plate from a single material, the neck part on the side of the arm part and the neck part on the side of the fixed part are provided to either side of the vibrating part in the longitudinal direction, on both sides of the center line that passes through the planar center of the vibrating part and is aligned with the transverse direction. 
   According to this invention, the portions where the arm part and fixed part are provided to the vibrating part via neck parts are in point symmetry about the planar center of the vibrating part. Therefore, during external impact from falling or the like, the vibrating part can be securely captured by the cushioning structure of the arm part from the side directly opposite the fixed part that supports and secures the vibrating part. The vibrating part supported in cantilever fashion by the fixed part is thereby vibrated by impact, making it possible to prevent twisting displacement from the fixed part in radial directions from being added to the vibrating behavior of the vibrating part. Specifically, normal vibration is maintained in the vibrating part even during such disturbances, and fluctuations in the driven amount of the piezoelectric actuator can be prevented. 
   In the piezoelectric actuator of the present invention, it is preferable that the attachment part have an opposing part that faces the free end from a direction aligned with the stacking direction of the vibrator; and that the spaces be formed between the opposing part and the vibrator. 
   According to this invention, since the opposing part of the attachment part is disposed on the vibrator in the stacking direction of the vibrator, impact resistance can be improved particularly against external forces acting in a direction aligned with the stacking direction (thickness direction) of the vibrator. 
   In the piezoelectric actuator of the present invention, it is preferable that the attachment part have a base member to which the vibrator is fixedly attached, and also have a plate member disposed on the other side of the vibrator across from the base member; and that the opposing parts be provided at positions where the base member and the plate member face each other. 
   According to this invention, the opposing parts provided to the base member and the plate member face towards the free end of the vibrator, and these opposing parts make it possible to further improve impact resistance because the free end is prevented from moving towards the base member as well as towards the plate member during impact. 
   In the piezoelectric actuator of the present invention, it is preferable that the driven object have a contact surface that is in contact with the protuberance and that is formed into a substantially flat shape. 
   According to this invention, since the vibrator is captured by the attachment part during impact and the protuberance of the vibrator does not separate from the driven object as previously described, there is no need to form an indentation or the like for holding the protuberance. The driven object can thereby be easily manufactured by press punching or another method. 
   The flat shape of the driven object can be circular, rectangular, or any other arbitrary shape. 
   The electronic device of the present invention comprises the piezoelectric actuator previously described. 
   The piezoelectric actuator can be used in the zoom mechanism or auto-focus mechanism of a camera, or other mechanisms. 
   According to this invention, providing the above-described piezoelectric actuator makes it possible to achieve the same operations and effects as those previously described. 
   It is preferable that the electronic device of the present invention be a timepiece comprising timekeeping means and a timekeeping information display unit for displaying the information timed by the timekeeping means. 
   According to this invention, it is possible for the gears constituting the timekeeping means and the timekeeping information display unit to be driven by the above-described piezoelectric actuator. It is thereby possible to eliminate drawbacks such as those wherein the rotating amount of the driven object fluctuates due to impact and the indicated positions of the date, month, and day of the week become misaligned. 
   Additionally, it is possible to utilize the advantages of the piezoelectric actuator, which are that the actuator is not affected by magnetism, is very responsive and can be driven in extremely small amounts, can be easily made small and thin, and has high torque. 
   EFFECTS OF THE INVENTION 
   According to the present invention, the vibrator is securely cushioned during impact without obstructing the vibration of the vibrator merely by designing spaces having specific dimensions between the vibrator and the attachment part, and it is therefore possible to greatly improve impact resistance without complicating the design or causing a reduction in drive efficiency. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an external perspective view of a timepiece in the first embodiment of the present invention; 
       FIG. 2  is a plan view of the movement in the present embodiment; 
       FIG. 3  is an enlarged partial view of  FIG. 2 , depicting a piezoelectric actuator; 
       FIG. 4  is a cross-sectional side view of the piezoelectric actuator in the present embodiment; 
       FIG. 5  is a cross-sectional side view of the piezoelectric actuator in the present embodiment; 
       FIG. 6  is a diagram showing the dimensions of components in the piezoelectric actuator shown in  FIG. 3 ; 
       FIG. 7  deals with the vibrator in the present embodiment, wherein  7 A is a graph depicting the relationship between drive frequency and impedance, and  7 B is a graph depicting the relationship between drive frequency and the amplitude of both longitudinal and bending vibration; and 
       FIG. 8  is a cross-sectional side view of the piezoelectric actuator in the second embodiment of the present invention. 
   

   KEY 
   1: wristwatch (timepiece),  14 : main plate (base member),  31 : piezoelectric actuator,  32 : rotor (driven object),  50 : vibrator,  51 : reinforcing plate (reinforcing member),  52 ,  53 : piezoelectric elements,  54 : pressing plate (plate member),  142 : screw pin (attachment part),  144 : protrusion,  144 A: large part (stepped part),  144 B: small part (stepped part),  144 C: opposing surface,  322 : contact surface,  511 : vibrating part,  511 A: protuberance,  512 : fixed part,  512 A: neck part (neck part on the side of the fixed part),  513 : arm part,  513 A: neck part (neck part on the side of the arm part),  513 B: hole,  513 C: opposing surface,  541 : stepped-down part (opposing part),  74 : main plate (base member),  741 : pin part (opposing part),  75 : pressing plate (plate member),  751 : convexity (opposing part), FR: free end, SP 1 , SP 2 , SP 3 , SP 4 : spaces, A: node position, L 1 : length (length of vibrating part), L 3 : length (length of arm part), W 1 : width of vibrating part), W 3 : width (width of arm part) 
   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The first embodiment of the present invention will now be described with reference to the diagrams. In the present embodiment, an example is described in which the present invention is applied to a wristwatch. 
   First Embodiment 
     FIG. 1  is a diagram showing the external configuration of a wristwatch  1  according to the first embodiment of the present invention. 
   The wristwatch  1  (watch) comprises a movement  10  ( FIG. 2 ) and a case  20  for housing the movement  10 , and has a wristband  21  provided to the 12:00 position and the 6:00 position of the wristwatch  1 . The timepiece may be a quartz timepiece, a mechanical timepiece, or an electronically controlled mechanical timepiece, but the wristwatch  1  of the present embodiment is configured as an analog quartz timepiece. 
   The wristwatch  1  has a disc-shaped dial  11 , a seconds hand  121 , a minute hand  122 , an hour hand  123 , and a crown  13 . 
   Also, a circular 24-hour display unit  111  is provided to the dial  11 , and the hours “0”through “23” are indicated by the rotation of a 24-hour display hand  111 A in this 24-hour display unit  111 . 
   Furthermore, the wristwatch  1  comprises a calendar mechanism  30  for displaying the date, and this calendar mechanism  30  includes a rectangular day display window  112 , a circular month display unit  113 , and a fan-patterned year display window  114 , all of which are provided in the dial  11 . 
   The numerals “1” through “31” for indicating the “day” of the date are displayed in the day display window  112 . The digits for the ones column and the digits for the tens column in the “day” are shown on separate date wheels, as will be described later. 
   Displays of “JAN” through “DEC” for indicating the “month” of the date are disposed at equal intervals around the circumference of the month display unit  113 , and these displays are indicated by a month display hand  113 A to display the “month” of the date. 
   The digits “0” through “4” for indicating how many years the date is from leap year are displayed in the year display window  114 . 
     FIG. 2  is a diagram depicting the movement  10 . 
   A flat, substantially circular main plate  14  is disposed in the movement  10 . The main plate  14  of the present embodiment is made from a resin, but may also be made from a metal or the like. 
   The mechanism for driving the seconds hand  121 , the minute hand  122 , and the hour hand  123  ( FIG. 1 ) is provided to the main plate  14  on the reverse side of the wristwatch  1  and is similar to that of a normal analog quartz watch, and therefore is not shown in the diagrams. The mechanism comprises a circuit board into which a crystal oscillator is incorporated; a stepping motor having a coil, a stator, and a rotor; a drive gear train; and a battery as a power source. In this configuration, the stepping motor is driven by a pulse signal that is generated by the crystal oscillator and that is divided in frequency by means of a circuit block. The drive force of the stepping motor is transmitted to the drive gear train, whereby the seconds hand  121 , the minute hand  122 , and the hour hand  123  are each driven. The number of stepping motors is not important, and one possibility, for example, is to provide one stepping motor for driving the seconds hand  121  and one for driving the minute hand  122  and hour hand  123 , for a total of two stepping motors. 
   The calendar mechanism  30  is disposed on the main plate  14  on the front side of the wristwatch  1 , as shown in  FIG. 2 . 
   The drive means for the calendar mechanism  30  is a piezoelectric actuator  31  that uses the vibration resulting from the inverse piezoelectric effects of the piezoelectric element. The piezoelectric actuator  31  comprises a vibrator  50  that has a piezoelectric element, and the vibration of this vibrator  50  repeatedly applies pressure to the outer periphery of a rotor  32  as the driven object, causing the rotor  32  to be rotatably driven. An intermediate wheel  33  meshes with the rotor  32 , and intermediate wheels  33  through  35  are meshed in sequence. The intermediate wheel  35  is meshed with a control wheel pinion  36 , and this control wheel pinion  36  is integrally formed on a control wheel  37  that controls the turning of the date. These components constitute a reduction gear train for turning the control wheel  37 . 
   The control wheel  37  comprises ratchet wheels (not shown) that have differing numbers of ratchets, and these ratchet wheels are each meshed with a ones-column day indicator driving wheel  40 , a tens-column day indicator driving wheel  42 , and a month indicator intermediate wheel  44 . 
   The digits “0” through “9” are displayed at equal intervals in the circumferential direction on the outer peripheral surface of a ones-column day wheel  41  that is turned by the ones-column day indicator driving wheel  40 . A “blank space” and the digits “1” through “3” are displayed at equal intervals in the circumferential direction on the outer peripheral surface of a tens-column day wheel  43  that is turned by the tens-column day indicator driving wheel  42 . The “blank space” is a space on which no digits are written. 
   A ones-column ratchet from among the ratchets of the control wheel  37  causes the ones-column day indicator driving wheel  40  and a ones-column day pinion  41 A to rotate, thus rotating the ones-column day wheel  41  integrally, and causing the digits “0” through “9”on the outer peripheral surface to be turned in the circumferential direction at rate of once per day. The ones-column day wheel  41  rotates in accordance with the rotation of the control wheel  37 . When this rotation reaches the date where the tens column is advanced, the tens-column turning ratchet of the control wheel  37  causes the tens-column day indicator driving wheel  42  and a tens-column day pinion  43 A to rotate, thus rotating the tens-column day wheel  43  integrally, and causing a “blank space” or the digits “1” through “3” on the outer peripheral surface to be turned in the circumferential direction at a rate of once every ten days. 
   The digits “1” through “31” indicating the “day” of the date are displayed through the day display window  112  ( FIG. 1 ) as a result of the combination of the digits “0” through “9” on the ones-column day wheel  41  and the “blank space” or the digits “1” through “3” on the tens-column day wheel  43 . 
   The rotation of the control wheel  37  causes the ones-column day wheel  41  and the tens-column day wheel  43  to rotate. When this rotation reaches the date at which the display of the “month” is advanced, the month turning ratchet of the control wheel  37  causes the month indicator intermediate wheel  44  and a month determination wheel  45  to rotate, and a month wheel  46  rotates integrally with these two wheels. The month display hand  113 A attached to the month wheel  46  then rotates and indicates the displays “JAN” through “DEC” on the month display unit  113  ( FIG. 1 ). 
   A year display intermediate wheel  47  is meshed with the month determination wheel  45 , and a year turning wheel  48  is meshed with the year display intermediate wheel  47 . A year wheel  49  for turning a year display wheel  114 A is meshed with the year turning wheel  48 . The year display wheel  114 A is turned once per year by the year wheel  49 , and the digits “0” through “4” are displayed in sequence through the year display window  114  ( FIG. 1 ). The number of years of the current date from leap year is thereby displayed. 
   The calendar mechanism  30  is provided with a date correction gear train (the gears  391 ,  392 , and  393  in  FIG. 2 ), and when the crown  13  is pulled out towards the outside of the case  20 , the crown  13  meshes with the intermediate wheel  35  via this date correction gear train, and the date can be corrected by rotating the crown  13 . 
   Meanwhile, the structure for driving the 24-hour display hand  111 A provided to the 24-hour display unit  111  ( FIG. 1 ) has a 24-hour determination wheel  62  that meshes with an hour wheel  61  to which the hour hand  123  is attached, and that determines the location of the 24 hour mark (12:00 am) from the amount by which the hour wheel  61  has rotated; and also has a 24-hour wheel  63  that meshes with the 24-hour determination wheel  62  and that is attached to the 24-hour display hand  111 A. 
   The 24-hour determination wheel  62  has an encoder for determining when the hand is at the “24 hour” mark from the amount by which the hour wheel  61  has rotated, and also has a spring switch that turns on according to the timing with which the “24 hour” mark is determined by the encoder. 
   The 24-hour determination wheel  62  is provided with a spring switch connected to a control block that is mounted on a circuit board  15  ( FIG. 5 ) provided to the main plate  14 , and the calendar mechanism  30  is driven when this spring switch is turned on. At this time, a one-day turning process is first conducted to rotatably drive the calendar mechanism  30  by an amount equivalent to one day. A calendar determination process is conducted to determine the turned day and to determine whether the day coincides with the current day. If the day does not coincide with the current day, then a calendar correction process is conducted to perform a so-called month-end correction by driving the calendar mechanism  30  in order to display the actual current day. 
   The most characteristic piezoelectric actuator  31  of the present invention, as well as the peripheral structure, will now be described in detail. 
     FIG. 3  is a diagram depicting the piezoelectric actuator  31  together with the peripheral structure, and  FIG. 4  is a cross-sectional view as seen from the direction of arrows IV-IV in  FIG. 2 .  FIG. 5  is a cross-sectional view of the piezoelectric actuator  31 . 
   The rotor  32  driven by the piezoelectric actuator  31  is rotatably supported by a rotor supporting member  320 . 
   The rotor supporting member  320  is disposed so as to be able to oscillate about a pin  321 , and a pressure spring  325  wound around a shaft  141  provided in the main plate  14  urges the rotor supporting member  320  in the counterclockwise direction in  FIG. 3 ; that is, towards the piezoelectric actuator  31 , whereby the rotor  32  comes into contact with the vibrator  50 . The contact pressure between the rotor  32  and the vibrator  50  is thereby kept at a suitable pressure that allows the rotor  32  to rotate efficiently when the piezoelectric actuator  31  is driven, and the amount by which the rotor  32  is turned by the vibrator  50  per unit time is kept at a sufficient level. 
   The rotor  32  is made by press punching or another means, and the contact surface  322  of the rotor  32  against which the vibrator  50  comes into contact is formed into a flat shape devoid of unevenness, as shown in  FIG. 4 . 
   The vibrator  50  of the piezoelectric actuator  31  comprises a reinforcing plate (reinforcing member)  51  formed by rolling stainless steel or another metal material, and also comprises rectangular plate-shaped piezoelectric elements  52 ,  53  laid over either surface of the reinforcing plate. 
   A pressing plate  54  (illustrated by the double-dashed lines in  FIG. 3 ) as a plate member is disposed so as to cover the vibrator  50 . 
   A spring pin  142  as an auxiliary attaching part is formed on the main plate  14  so as to intersect with the planar direction (the XY direction in  FIG. 3 ) of the reinforcing plate  51  as shown in  FIG. 5 , and the vibrator  50  is fixedly attached to the spring pin  142 . 
   A pair of positioning pins  143  ( FIG. 3 ) for positioning the vibrator  50  are provided at the sides of the spring pin  142 , and a substantially cylindrical protrusion  144  is similarly formed at a position that encloses the vibrator  50  with the positioning pins  143 , so as to protrude in the planar direction of the reinforcing plate  51 . 
   The protrusion  144  is provided in the vicinity of a position that divides the lengthwise dimensions of the piezoelectric elements  52 ,  53  in two. 
   The protrusion  144  also has a large part  144 A (on the side of the main plate  14 ) and a small part  144 B that are disposed concentrically to each other, and a stepped part is formed by the large part  144 A and the small part  144 B. 
   The degree to which the protrusion  144  extends is less than that of the spring pin  142 . The pressing plate  54  is formed so as to drop towards the main plate  14  at the portion near the protrusion  144 , and is mounted at the distal end of the protrusion  144 . 
   The material of the piezoelectric elements  52 ,  53  is not particularly limited, and can be lead zirconate titanate (registered trademark), crystal, lithium niobate, barium titanate, lead metaniobate, polyvinylidene fluoride, zinc lead niobate, scandium lead niobate, or another material, for example. 
   Also, electrodes are formed over both entire front and reverse sides of the piezoelectric elements  52 ,  53  by plating, sputtering, vapor deposition, or another method using nickel or gold. The entire electrodes (not shown) formed on the front and reverse sides of the piezoelectric elements  52 ,  53  are superposed over the reinforcing plate  51  and are made electrically conductive with the reinforcing plate  51 . The electrodes on the front sides of the piezoelectric elements  52 ,  53  are divided by grooves  511 E ( FIG. 6 ) formed by etching, and detection electrodes  511 C for detecting the vibrations of the piezoelectric elements  52 ,  53  as voltage signals by means of the piezoelectric effects are formed, as are drive electrodes  511 D to which a drive voltage is applied. 
     FIG. 6  depicts a detection electrode  511 C and a drive electrode  511 D with double-dashed lines. The areas of the electrodes formed on the surfaces of the piezoelectric elements  52 ,  53  excluding the detection electrodes  511 C constitute the drive electrodes  511 D. The arrangements of the detection electrodes  511 C and the drive electrodes  511 D are identical in both the piezoelectric elements  52 ,  53  that enclose the reinforcing plate  51 , wherein the detection electrode  511 C of the piezoelectric element  53  is formed on the reverse side of the detection electrode  511 C of the piezoelectric element  52 , for example. In  FIG. 6 , the double-dashed lines are drawn as being inside of the outer peripheral edges of the piezoelectric element  52 , but the detection electrodes  511 C and the drive electrodes  511 D are formed to extend to the outer peripheral edges of the piezoelectric elements  52 ,  53 , as shown in  FIG. 3 . 
   The detection electrodes  511 C extend from the vicinity of the planar centers (refer to position A) of the piezoelectric elements  52 ,  53  in the longitudinal direction of the piezoelectric elements  52 ,  53  up to a length equivalent to about ⅕ to ⅓ of the long sides of the piezoelectric elements  52 ,  53  toward one of the widthwise ends of the piezoelectric elements  52 ,  53  (toward one of the long sides). The widths of the detection electrodes  511 C are equivalent to about ⅙ to ⅓ of the piezoelectric elements  52 ,  53 . Thus, the surface areas of the detection electrode  511 C are considerably smaller than those of the drive electrodes  511 D. Position A is a node position of bending vibration in the vibrator  50  as will be later described. Generally speaking, the detection electrodes  511 C are disposed in the vicinity of the node position A of the bending vibration in the vibrator  50 . 
     FIG. 3  is a schematic depiction of a drive control circuit  514  (IC chip) that supplies drive voltage to the piezoelectric elements  52 ,  53 . The drive control circuit  514  is installed on the circuit board  15  ( FIG. 5 ), and conduction is established with the reinforcing plate  51  and a lead substrate  561 . A drive voltage is applied between the reinforcing plate  51  and the drive electrodes  511 D by means of the reinforcing plate  51  and lead substrate  561 , and a vibration signal is detected as a differential signal of the detection electrodes  511 C in relation to the electric potential of the reinforcing plate  51 . 
   The specific electrical relationship to the piezoelectric actuator  31  will be described later with reference to  FIG. 5 . 
   Next, the reinforcing plate  51  has an integrally formed fixed part  512  that is fixedly attached to the spring pin  142 , and a free end FR on which the piezoelectric elements  52 ,  53  are disposed, as shown in  FIG. 3 . 
   The free end FR is formed into a flat, substantially rectangular shape corresponding to the shapes of the piezoelectric elements  52 ,  53 , and has a vibrating part  511  to which the fixed part  512  is provided on one side in the longitudinal direction, and an arm part  513  that extends from the other side of the vibrating part  511 . 
   The vibrating part  511  has, on both surfaces along the short sides, protuberances  511 A,  511 B that protrude in arcuate shapes in the longitudinal direction. These protuberances  511 A,  511 B both have arcuate shapes with a radius of 0.25 mm and are disposed at positions that are in point symmetry in relation to the node position A (planar center) of bending vibration of the vibrating part  511 , and one protuberance  511 A is in contact with the rotor  32 . 
   The positions where the protuberances  511 A,  511 B are formed are disposed at a distance from the center line C aligned with the longitudinal direction of the vibrating part  511 , and are formed at positions that are unbalanced with respect to the longitudinal primary vibration in the longitudinal direction of the vibrating part  511 . When longitudinal primary vibration is induced by the application of voltage to the piezoelectric elements  52 ,  53 , secondary bending vibration is induced in a direction that intersects the longitudinal direction of the vibrating part  511 . 
   The fixed part  512  is provided in the vicinity of the node position A of the bending vibration induced in a direction that intersects with the longitudinal direction of the vibrating part  511 . The fixed part has a neck part  512 A that connects with the vibrating part  511  on the side of the fixed part. 
   The fixed part  512  has holes  512 B,  512 C,  512 D provided in alignment in the longitudinal direction of the vibrating part  511 , as shown in  FIG. 3 . Positioning pins  143  are inserted through the holes  512 B,  512 D at the ends, and a screw pin  142  ( FIG. 5 ) is inserted through the center hole  512 . 
   The fixed part  512  is provided with spacers  551 ,  552 , and lead substrates  561 ,  562 . These as elements that ensure electrical conduction with the piezoelectric elements  52 ,  53 , as shown in  FIG. 5 . 
   The spacers  551 ,  552  are formed so that their planar shapes are substantially identical to that of the fixed part  512 , and are bonded at both sides around the hole  512 C of the fixed part  512 . The lead substrates  561 ,  562  are bonded to the spacers  551 ,  552 , respectively. 
   Copper foils (wiring patterns)  561 A,  562 A that overhang from the lead substrates  561 ,  562  are soldered to the piezoelectric elements  52 ,  53 , respectively. 
   The lead substrates  561 ,  562  are disposed so that the copper foils  561 A,  562 A face to the outer side (the side opposite the reinforcing plate  51 ). 
   When the fixed part  512  is fixed to the main plate  14 , a spring member  57  is assembled and disposed on the lead substrate  561 , and the spring member  57  is in electroconductive contact with the circuit board  15 . This spring member  57  provides the same electric potential to the electrodes on the outer sides of the piezoelectric elements  52 ,  53  so as to enclose the reinforcing plate  51 . 
   Furthermore, an insulating plate  58  is disposed on the spring member  57 , and the screw pin  142  is inserted through holes formed in the spacers  551 ,  552 , the lead substrates  561 ,  562 , and the insulating plate  58 . The vibrator  50  is fixedly attached to the main plate  14  by threading a screw  512 E into a female screw hole formed in the screw pin  142  via the pressing plate  54 . 
   The restricted state of the reinforcing plate  51  in the present embodiment is such that the rotation of the reinforcing plate  51  is regulated by inserting the positioning pins  143  and the screw pin  142  through the holes  512 B through  512 D ( FIG. 3 ), and the reinforcing plate  51  is prevented from chattering in a direction that intersects its plane by fastening a screw  512 E. 
   The pressing plate  54  has a spring part  59  that is electroconductive contact with the reference potential of the circuit board  15 , and also with the reinforcing plate  51 . The pressing plate  54  is made conductive with the inner sides of the piezoelectric elements  52 ,  53  via the spring part  59  and the reinforcing plate  51 . The pressing plate  54  and the spring member  57  are kept at different electric potentials by the insulating plate  58 . 
   As a result of the vibrator  50  being fixedly attached to the main plate  14  by the fixed part  512 , the vibrator is secured in cantilever fashion. With the vibrator  50  as a cantilevered beam, the vibrating part  511  and the arm part  513  that are not secured to the main plate  14  constitute a free end FR of the vibrator  50 . 
   The arm part  513  is formed into a substantially rectangular shape that extends towards the protrusion  144  from a neck part  513 A, which is disposed on the side of the arm part and is connected to the vibrating part  511 , as shown in  FIG. 3 . This arm part  513  is provided in the vicinity of the node position A of bending vibration in the vibrating part  511 , similar to the fixed part  512 , and the neck parts  513 A,  512 A of the arm part  513  and fixed part  512  are provided at opposing positions in the transverse direction of the vibrating part  511 . 
   A hole  513 B through which the small part  144 B of the protrusion  144  is inserted is also formed in the arm part  513 . 
   Referring to  FIG. 6 , specific numerical values are given for the dimensions of the above-described neck parts  513 A,  512 A of the arm part  513  and the fixed part  512 . In a case in which the width W 1  (short sides) of the vibrating part  511  in the transverse direction is 1.98 mm and the length L 1  (long sides) in the longitudinal direction is 7 mm, the widths w of the neck parts  513 A,  512 A in the longitudinal direction of the vibrating part  511  are preferably set to about 0.4to 0.6 mm. It is preferable that the widths w be set to about 5 to 8% of the length L 1  of the vibrating part  511 , or, even more preferable, to about 6 to 7%. If these widths are less than 5%, then the neck parts will not be strong enough to endure the normal vibration of the vibrating part  511 , and if the widths are greater than 8%, then the neck parts will hinder the longitudinal vibration of the vibrating part  511 . 
   Furthermore, the lengths  1  of the neck parts  513 A,  512 A in the longitudinal direction of the vibrating part  511  are preferably less than the widths w. It is preferable that the lengths  1  be about 90%±5% of the widths. 
   The characteristic frequency of the arm part  513  in the vibration of the vibrating part  511  must be set so as not to interfere with the vibration of the vibrating part  511 , but if the arm part  513  is long and thin, then the vibration of the arm part  513  is secondary, tertiary, or another high-order vibration mode, narrowing the range of shapes that the arm part  513  can assume so as not to interfere with the vibration of the arm part  513 . 
   Therefore, the arm part  513  is not designed to be long and thin. Specifically, the shape is designed so that the length of the arm part  513  in the longitudinal direction of the vibrating part  511  is less than that of the vibrating part  511  and the fixed part  512 , whose length is less than that of the vibrating part  511 . In other words, referring to  FIG. 6 , the length L 3  of the arm part  513  is less than both the length L 2  of the fixed part  512  and the length L 1  of the vibrating part  511 . The width W 3  of the arm part  513  is less than the length L 3  of the arm part  513 , but the ratio of the length L 3  to the width W 3  is less than both the ratio of the length L 1  to the width W 1  of the vibrating part  511  and the ratio of the length L 2  to the width W 2  of the fixed part  512 . In other words, the arm part  513  is not longer and thinner than the fixed part  512  or the vibrating part  511 . 
   The vibration mode induced by the arm part  513  is thereby predominantly a vibration in the longitudinal direction of the vibrating part  511 . The interference-induced reduction in vibration can be preventing by placing the characteristic frequency of the arm part  513  at a sufficient distance from the characteristic frequency of the vibrating part  511 . 
   These dimensions for the arm part  513  result in the surface area of the arm part  513  being less than the surface areas of both the fixed part  512  and the vibrating part  511 , and also in the arm part  513  being lighter in weight than both the fixed part  512  and the vibrating part  511 . Therefore, it is possible to place the characteristic frequency of the arm part  513  at a sufficient distance from the characteristic frequency of the vibrating part  511 , and a reduction in the vibration of the vibrating part  511  resulting from interference with the vibration of the arm part  513  can be satisfactorily reduced. 
   With the arm part  513  thus configured, the vibration of the arm part  513  can be considered as a problem that involves mass and a spring constant. The characteristic frequency P of the arm part  513  is expressed by the following mathematical equation (1), wherein W 3  is the width of the arm part  513  ( FIG. 6 ), t is the thickness of the arm part  513 , L 3  is the length of the arm part  513  ( FIG. 6 ), m is the mass of the arm part  513 , and E is the Young&#39;s modulus of the reinforcing plate  51 . 
   
     
       
         
           
             
               
                 [ 
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 ] 
               
             
             
               
                   
               
             
           
           
             
               
                 
                   
                     
                       P 
                       = 
                       
                         
                           
                             
                               
                                 W 
                                 3 
                                 3 
                               
                               ⁢ 
                               t 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               E 
                             
                             
                               4 
                               ⁢ 
                               m 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 L 
                                 3 
                                 3 
                               
                             
                           
                         
                         ⁡ 
                         
                           [ 
                           
                             rad 
                             ⁢ 
                             
                               / 
                             
                             ⁢ 
                             sec 
                           
                           ] 
                         
                       
                     
                   
                   
                     
                         
                     
                   
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   The specific value of the characteristic frequency of the arm part  513  is determined from the relationship of this frequency with the characteristic frequency of the longitudinal vibration of the vibrating part  511 , or the frequency of the drive voltage applied to the piezoelectric actuator  31 . The frequency of the drive voltage (the drive frequency) is determined with consideration to the resonance point of longitudinal vibration and the resonance point of bending vibration in the vibrating part  511 . 
     FIG. 7A  depicts the relationship between the drive frequency and impedance in the vibrating part  511 , and  FIG. 7B  depicts the relationship between the drive frequency and amplitude of longitudinal vibration, and also between the drive frequency and amplitude of bending vibration in the vibrating part  511 . As shown in  FIG. 7A , two resonance points occur at which the impedance is extremely low in relation to the drive frequency and at which the amplitude reaches a maximum. The lower of these frequencies is the resonance point of longitudinal vibration, while the higher is the resonance point of bending vibration. 
   Specifically, when a vibrator  20 A is driven at a frequency that lies between the longitudinal resonance frequency fr 1  of longitudinal vibration and the bending resonance frequency fr 2  of bending vibration, the amplitudes of both the longitudinal vibration and bending vibration are maintained, and the piezoelectric actuator  31  is driven with high efficiency. A drive frequency at which the amplitudes of both the longitudinal vibration and bending vibration are even greater can be set by bringing the longitudinal resonance frequency fr 1  and the bending resonance frequency fr 2  close to each other. 
   In the present embodiment, the longitudinal resonance frequency (characteristic frequency of the longitudinal vibration) of the vibrating part  511  is about 200 kHz. Referring to  FIG. 7 , the drive frequency is set in a range of about 230 kHz to about 300 kHz. It is assumed that if the characteristic frequency of the arm part  513  is about 5% of the characteristic frequency of 200 kHz of longitudinal vibration, i.e., 10 kHz or greater as a specific value, then the vibration of the arm part  513  does not interfere with the vibration of the vibrating part  511 . This specific value of 10 kHz is about 4% of the drive frequency in cases in which the drive frequency is 250 kHz. In this case, the vibration of the arm part  513  does not interfere with the longitudinal vibration of the vibrating part  511 . If the longitudinal vibration as the primary vibration induced by the vibrating part  511  does not diminish, then there is no obstacle to inducing bending vibration as secondary vibration. The characteristic frequency settings of the arm part  513  can be expressed by the following conditional equations (2-1) and (2-2). 
   
     
       
         
           
             
               
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                     ( 
                     
                       
                         
                           
                             longitudinal 
                             ⁢ 
                             
                                 
                             
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                             frequency 
                             ⁢ 
                             
                                 
                             
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                             ⁢ 
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                   - 
                   
                     10 
                     ⁢ 
                     
                         
                     
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                     ( 
                     
                       
                         
                           
                             longitudinal 
                             ⁢ 
                             
                                 
                             
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                             resonance 
                           
                         
                       
                       
                         
                           
                             frequency 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
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                   + 
                   
                     10 
                     ⁢ 
                     
                         
                     
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                     kHz 
                   
                 
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                           3 
                         
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                         4 
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   However, in cases in which the wristwatch  1  is subjected to external impact from being dropped or the like, there is a danger that the vibrator  50  or the rotor  32  will be damaged if the force of this external impact acts in a direction in which pressure is applied to the rotor  32  and the vibrator  50 . Also, if the force of external impact acts in this direction when the piezoelectric actuator  31  is being driven, then the amplitude of the piezoelectric actuator  31  is reduced, and it might not be possible for the rotor  32  to be turned sufficiently. Furthermore, the impact may cause the vibrator  50  to collide with and damage other components. 
   In view of this, to provide the piezoelectric actuator  31  with an impact-resistant structure, spaces SP 1 , SP 2  are formed between the arm part  513  and the protrusion  144  of the main plate  14 , as shown in  FIG. 5 , when the piezoelectric actuator  31  is incorporated into the main plate  14 . Specifically, a space SP 1  is formed in the planar direction (XY direction) of the reinforcing plate  51  between the inner peripheral edge of the hole  513 B in the arm part  513  and the small part  144 B of the protrusion  144 , and a space SP 2  is formed in the thickness direction Z of the wristwatch  1  (substantially the same direction as the direction in which the piezoelectric elements  52 ,  53  and the reinforcing plate  51  are stacked) between the mutually opposing surfaces  513 C,  144 C of the arm part  513  and the large part  144 A. The space SP 2  is also formed in the thickness direction Z of the wristwatch  1  between a stepped-down part  541  of the pressing plate  54  and an opposing surface  513 D of the arm part  513  that faces this stepped-down part  541 . 
   These spaces SP 1 , SP 2  are formed in a manner that does not cause external impact, and the presence of these spaces SP 1 , SP 2  allows the free end FR to move, and vibration in the vibrating part  511  to be maintained. During impact, the movement of the vibrating part  511  is kept within the range of the spaces SP 1 , SP 2  and the vibrating part  511  does not move beyond the dimensions of the spaces SP 1 , SP 2 . 
   Therefore, the vibrating part  511  can be prevented from colliding with the rotor  32  and other components. 
   The dimensions of the spaces SP 1 , SP 2  are set according to the directions and amplitudes of both the longitudinal vibration and bending vibration, and also are set according to the relationship between the maximum amount of bending in the free end FR when the wristwatch  1  undergoes impact, the internal stress that occurs in the neck part  512 A due to a uniformly distributed load in the vibrator  50  (shown as a beam) when the free end FR undergoes maximum bending, and the strength of the stainless steel that constitutes the reinforcing plate  51 . In other words, if the vibrator  50  is displaced or deformed during impact, the vibrating part  511  does not move outside of the range of the spaces SP 1 , SP 2 , and the neck part  512 A is prevented from breaking or being otherwise damaged. 
   The impact resistance can be improved by reducing the dimensions of the spaces SP 1 , SP 2  to an extent that does not inhibit vibration. Therefore, the tolerances between the vibrator  50  and the protrusion  144  that result from assembling errors or nonuniformities in the shapes of the reinforcing plate  51  or the piezoelectric elements  52 ,  53  can be established as the dimensions of the spaces SP 1 , SP 2 . 
   The conditions of the load applied to the wristwatch  1  during impact vary depending on the orientation and position of the wristwatch  1  when the wristwatch  1  is dropped or bumped, but the force of impact acting on the vibrator  50  when the wristwatch falls and lands on the floor with the 9:00 to 3:00 direction aligned vertically is greater than in a case in which the wristwatch falls to the floor with the 6:00 to 12:00 direction aligned vertically and in which the impact is absorbed by the wristband  21  provided to the 6:00 and 12:00 positions. Therefore, in the present embodiment, the maximum amount of bending in the free end FR is calculated from the external force that acts on the vibrator  50  when the wristwatch  1  is dropped from a height of several meters, and the wristwatch  1  strikes the floor on the side of the case  20  while the 9:00 to 3:00 direction is aligned vertically. 
   This calculation is not limited to cases in which the side of the case  20  of the wristwatch  1  strikes the floor, and the maximum amount of bending when the front (cover glass side) or back (back lid side) of the case  20  of the wristwatch  1  strikes the floor may also be used. 
   To achieve maximum bending in the vibrator  50 , the load on the vibrator  50  is configured as a point load rather than a linear load in which bending is distributed by the uniform load distribution properties of a beam, and the position at which the load is applied is moved to the vicinity of the arm part  513  of the vibrating part  511 . 
   The piezoelectric actuator  31  in the present embodiment is driven by the application of single-layer alternating-current voltage. When the piezoelectric actuator  31  is driven, a drive voltage with a frequency of 250 kHz, for example, is applied to the piezoelectric elements  52 ,  53  by a voltage application device (not shown). The piezoelectric elements  52 ,  53  are thereby simultaneously expanded and contracted using the reinforcing plate  51  as a common electrode, inducing longitudinal primary vibration that expands and contracts in the longitudinal direction. The presence of a transverse moment in the vibrating part  511  caused by an imbalance between the protuberances  511 A,  511 B causes the vibrating part  511  to undergoes secondary bending vibration in which the vibrating part  511  bends along the transverse direction in point symmetry about the planar center (node position A), and the vibrating part  511  to vibrate in a mixed mode of both longitudinal vibration and bending vibration. The protuberance  511 A of the vibrator  50  is thereby caused to move in a substantially elliptical route R ( FIG. 3 ). The movement of the protuberance  511 A in the route R allows the rotor  32  to be rotatably driven in the counterclockwise direction in  FIG. 2 . 
   The protuberance  511 B that is not in contact with the rotor  32  functions as a balancer at a position symmetrical to the protuberance  511 A, whereby the path of the protuberance  511 A has the desired route R. 
   When the piezoelectric actuator  31  is driven in this manner, the free end FR of the vibrator  50  can move within the confines of the space SP 1  and the space SP 2 , and these spaces SP 1 , SP 2  are set on the basis of the amplitudes of the longitudinal and bending vibrations. Therefore, the vibration of the vibrating part  511  is not obstructed by the protrusion  144 , and the drive efficiency is not reduced even if the protrusion  144  is inserted through the hole  513 B in the arm part  513 . Specifically, because the space SP 1  is formed, there is no obstruction to longitudinal vibration in which displacement occurs in the direction aligned with the plate surface of the vibrating part  511 . Also, a mechanical moment accompanies the bending vibration, but the presence of the space SP 2  prevents the bending vibration from being obstructed. 
   When the wristwatch  1  is dropped or otherwise struck, the vibrator  50  begins to move and deform in an arbitrary direction that includes both the planar direction (XY direction) and thickness direction Z of the wristwatch  1  due to the external force of the impact. Depending on the magnitude of the impact force, the vibrating part  511  either moves within the spaces SP 1 , SP 2  or moves through the spaces SP 1 , SP 2 , after which the protrusion  144  interlocks with the hole  513 B in the arm part  513 . 
   Specifically, the movement of the vibrating part  511  is captured by the space SP 1  when external force acts in the planar direction (XY direction) of the wristwatch  1 , and the movement of the vibrating part  511  is captured by the space SP 2  when external force acts in the thickness direction Z of the wristwatch  1 . 
   Since the neck part  513 A of the arm part  513  and the neck part  512 A of the fixed part  512  are in point symmetry about the planar center (node position A) of the vibrating part  511 , the vibrating part  511  can be securely captured by the cushioning structure of the arm part  513  from the side exactly opposite the fixed part  512  when the wristwatch is dropped or subjected to other external impact. The vibrating part  511  supported in cantilever fashion by the fixed part  512  is thereby vibrated by the impact, preventing twisting displacement from the fixed part  512  in radial directions from being added to the vibrating behavior of the vibrating part  511 . Specifically, normal vibration is maintained in the vibrating part  511  even during such disturbances. 
   Therefore, it is possible to prevent the vibrating part  511  from moving beyond the spaces SP 1 , SP 2  and colliding with and damaging the rotor  32  and other components, to prevent the vibrating part  511  from separating too far from the rotor  32  and causing the rotational amount of the rotor  32  to fluctuate, and to prevent the pressurized engagement with the rotor  32  from becoming completely disengaged. 
   Also, since the spaces SP 1 , SP 2  are determined while giving consideration to the material strength of the stainless steel in the neck part  512 A of the fixed part  512 , it is possible to prevent the area in the vicinity of the neck part  512 A from breaking due to impact, and to prevent the copper foils  561 A,  562 A provided to the neck part  512 A from tearing. 
   Specifically, according to the present embodiment, the following effects are achieved. 
   (1) In the piezoelectric actuator  31  for driving the calendar mechanism  30  of the wristwatch  1 , the vibrator  50  has an arm part  513 , and spaces SP 1 , SP 2  are formed between the arm part  513  and the protrusion  144  when the vibrator  50  is incorporated into the main plate  14 . Therefore, the free end FR moves freely within these spaces SP 1 , SP 2  during driving, but the protrusion  144  is captured by the arm part  513  when the wristwatch is subjected to external impact. Since the vibrating part  511  is secured in place at both sides where the fixed part  512  and the arm part  513  are provided, the vibrating part  511  can thereby be prevented from moving beyond the dimensions of the spaces. The space SP 1  is formed in the XY direction and the space SP 2  is formed in the Z direction, and the presence of these three-dimensional spaces cushions the arm part  513  when external force that can act in an arbitrary direction is applied, reducing the external force. 
   Therefore, it is possible for the rotor  32  to be driven without hindering the vibration of the reinforcing plate  511 , and it is also possible to prevent impact from causing fluctuations in the rotation of the rotor  32  by the vibrating part  511 , and to prevent the vibrating part  511  from colliding with and damaging the rotor  32 , the rotor supporting member  320 , and other components. 
   (2) The effect of impact resistance is significant because the arm part  513  faces the protrusion  144  from the other side of the spaces SP 1 , SP 2  and the vibrator  50  is securely captured by the protrusion  144  when the vibrator moves and deforms during impact, and also because the vibrator  50  is captured at the free end FR, where the amount of bending reaches a maximum during impact. 
   Therefore, resistance against impact from falling or the like can be greatly improved without reducing drive efficiency, and the usual problems with the difficulty of reconciling drive efficiency and impact resistance can be resolved. 
   The design used to obtain these effects is not difficult to accomplish because spaces SP 1 , SP 2  having specific dimensions need be provided only between the free end FR and the protrusion  144  on the basis of the maximum bending amount of the free end FR, the internal stress in the neck part  512 A, the material strength of the reinforcing plate  51 , and other factors, as previously described. 
   (3) Furthermore, impact resistance can be improved because of a structure in which the protrusion  144  is inserted through the hole  513 B formed in the arm part  513  of the vibrator  50 , and a space SP 1  is formed by the gap between the protrusion  144  and the hole  513 B in the arm part  513 . Another reason for the improved impact resistance is that the inner peripheral edge of the hole  513 B in the arm part  513  interlocks with the protrusion  144  during impact, whereby the vibrating part  511  is securely controlled in its ability to move from the inside of the hole  513 B to the outside. 
   (4) Since the protrusion  144  inserted through the hole  513 B in the arm part  513  is configured with a large part  144 A and a small part  144 B, not only is a space SP 1  formed between the inner peripheral edge of the hole  513 B and the outer periphery of the protrusion  144  (the small part  144 B), but a space SP 2  is also formed between the opposing surfaces  144 C,  513 C of the large part  144 A and the arm part  513 , and the structures of these spaces SP 1 , SP 2  can be made to function as a cushion for the vibrator  50  during impact. A pressing plate  54  is also provided, and the space SP 2  formed between the stepped-down part  541  and the opposing surface  513 D of the arm part  513  also serves as a cushioning structure for the vibrator  50 . 
   Thus, impact resistance can be improved even further because spaces SP 1 , SP 2  are formed in the planar direction (XY direction) and the thickness direction Z of the wristwatch  1 , and the structures of these spaces SP 1 , SP 2  make it possible to confine the movement of the vibrator  50  caused by external forces acting in the planar direction and thickness direction of the wristwatch  1 , as well as in inclined directions that intersect with the planar direction or thickness direction. 
   (5) Since the fixed part  512  and arm part  513  are provided to the vibrating part  511  via the constricted neck parts  512 A,  513 A, and these neck parts  512 A,  513 A are provided in the vicinity of the node position A of bending vibration in the vibrating part  511 , the effects of the fixed part  512  and arm part  513  on the vibrating characteristics of the vibrating part  511  can be greatly reduced. 
   Also, the drive efficiency of the piezoelectric actuator  31  can be improved because the vibration energy that is dissipated by the vibrating part  511  through the arm part  513  and the fixed part  512  can be reduced. 
   (6) As described above, since the vibrator  50  is captured by the protrusion  144  during impact, and the protuberance of the vibrator  50  does not separate from the rotor  32 , there is no need to form an indentation or the like for holding the protuberance  511 A in the contact surface  322  of the rotor  32 , and the contact surface  322  can be formed into a substantially flat shape with no irregularities. The rotor  32  can thereby be easily manufactured by press punching or another method. 
   (7) Applying the piezoelectric actuator  31  that has superior impact resistance to a wristwatch  1  that has a calendar mechanism  0  does not cause the rotation of the rotor  32  to fluctuate during impact, and eliminates drawbacks such as the calendar display being off-center in the day display window  112  or the month display unit  113 , producing a remarkable effect. 
   (8) Additionally, one significance of the spaces SP 1 , SP 2  is that vibration noise is not likely to occur in the drive electrodes  511 D provided to the piezoelectric elements  52 ,  53  while the piezoelectric actuator  31  is driven, and drawbacks such as the drive control circuit  514  being destroyed by electric shocks can therefore be prevented in advance. Since vibration noise is also not likely to occur in the detection electrodes  511 C, the vibrating state of the vibrating part  511  can be accurately and securely detected regardless of whether the detection electrodes  511 C are made significantly smaller than the drive electrodes  511 D in the vicinity of the node position A so as not to interfere with the vibration of the vibrating part  511 . The piezoelectric actuator  31  can be driven in a stable manner by the drive control circuit  514  on the basis of this vibration detection. 
   Second Embodiment 
   Next, the second embodiment of the present invention will be described. 
   In the following descriptions, element that are similar to those of the above-described embodiment are denoted by the same numerical symbols, and descriptions thereof are omitted or simplified. 
   The present embodiment differs from the first embodiment in that spaces of specific dimensions are formed between the surface of the vibrator  50  and the main plate and pressing plate that are disposed on either side of the vibrator  50 . 
     FIG. 8  is a cross-sectional side view of a piezoelectric actuator  71 , a main plate  74  as a base member, and a pressing plate  75  in the present embodiment. 
   A pin part  741  is formed in the main plate  74  as an opposing part that extends in the thickness direction (substantially the same direction as the stacking direction of the reinforcing plate  51  and the piezoelectric elements  52 ,  53 ) of the wristwatch  1  towards a free end FR of the vibrator  50 . 
   Also, a convexity  751  is formed by press molding or the like in the pressing plate  75  as an opposing part that extends in a direction aligned with the thickness direction Z of the wristwatch  1 , and the protruding distal end surface of the convexity  751  constitutes an opposing surface  751 A that faces the vibrator  50 . 
   When the vibrator  50  is incorporated into the main plate  74  and the pressing plate  75 , the pin part  741  and the opposing surface  751 A face each other from either side of the free end FR, a space SP 3  is formed between the pin part  741  and the reverse surface of the vibrating part  511 , and a space SP 4  is formed between the opposing surface  751 A of the pressing plate  75  and the front surface of the vibrating part  511 . 
   According to the present embodiment, the following effects are achieved in addition to the operational effects described in the first embodiment. 
   (9) Since a pin part  741  and a convexity  751  are formed respectively in the main plate  74  and the pressing plate  75 , and the pin part  741  and convexity  751  face the free end FR of the vibrator  50  in the thickness direction Z of the wristwatch  1  via the spaces SP 3 , SP 4 , the free end FR is prevented from moving into the main plate  74  as well as into the pressing plate  75  during impact, and impact resistance can be further improved. 
   (10) Moreover, the impact resistance of the vibrator  50  can be even further improved because the pin part  741  and the convexity  751  are provided so as to face the free end FR where bending reaches a maximum during impact. 
   [Modifications of the Invention] 
   The preferred configurations for working the present invention were described in detail above, but the present invention is not limited thereto. Specifically, the present invention is particularly illustrated and described primarily with reference to specific embodiments, but those skilled in the art can make various modifications and improvements to the shapes, materials, quantities, and other specific details of the embodiments described above without deviating from the scope of the technical ideas and objects of the present invention. 
   The descriptions that are disclosed above and that refer to specific shapes, materials, and other aspects are given solely with the intent of making the present invention easy to understand and are not intended to limit the present invention. For this reason, descriptions that contain names of members in which some or all of the limitations on shapes, materials, and other items have been removed are also included in the present invention. 
   For example, in the previous embodiments, spaces SP 1 , SP 2  were formed between the protrusion  144  and the inner peripheral edge of the hole  513 B in the arm part  513 , but the structure is not limited thereto and may also be designed so that a concavity instead of a hole is formed in the arm part and spaces are formed between the attachment part and the inner surfaces of this concavity in the arm part, for example. 
   Furthermore, the free end need not have an arm part, and instead, the side surface of the flat rectangular free end may be disposed in the space next to the attachment part, for example. 
   The shape of the free end is not limited to a rectangular shape, and may also be a diamond, a parallelogram, a trapezoid, a truss, or various other shapes. 
   The free end can be in a portion of the vibrator that is not fixedly attached to the attachment part; i.e., a portion other than the fixed part, and the space between the free end and the attachment part can be positioned in the substantial center of the free end instead of the end or side surface of the free end as in the previous embodiments. 
   Furthermore, in the second embodiment, a convexity  751  facing the free end FR was formed in the pressing plate  54  that was separate from the main plate  14  on which the vibrator  50  was fixedly attached, but the present invention is not limited thereto, and the opposing part that faces the free end may also be formed integrally with the member on which the attachment part is formed. For example, another possibility is to laterally insert a plate-shaped vibrator into a case that has a U-shape in cross section and that includes the attachment part, and to integrally form the opposing part that faces the free end of the vibrator inside the case. 
   In the previous embodiments, the thickness direction Z of the wristwatch  1  and the stacking direction of the piezoelectric elements  52 ,  53  and the reinforcing plate  51  were substantially the same, but the present invention is not limited thereto, and the piezoelectric elements and the reinforcing plate may also be stacked in the planar direction of the timepiece, for example. 
   The object driven by the piezoelectric actuator is not limited to a rotor that is rotatably driven, and may also be a driven object that is driven so as to move in linear fashion. 
   In the previous embodiments, a calendar mechanism of a wristwatch was depicted as an example of applying the piezoelectric actuator, but the present invention is not limited thereto, and the piezoelectric actuator of the present invention can also be applied as drive means for a seconds hand or other component that is driven almost continuously and turned by a greater amount per unit time than the calendar. The type of the timepiece is not limited to a wristwatch, and the present invention is also suitable for a personal pocket watch or the like. 
   Furthermore, aside from timepieces, the piezoelectric actuator of the present invention can also be suitably used in the zoom or auto-focus mechanisms of cameras, film winding mechanisms, paper rolling mechanisms in printers, and mechanisms for driving mobile toys such as cars or dolls. In other words, the piezoelectric actuator of the present invention is not limited to timepieces, and can also be incorporated into cameras, printers, mobile toys, and various other electronic devices. 
   Furthermore, the length, width, and other dimensions of the vibrating part depicted in the previous embodiments, as well as the lengths and widths of the fixed part and arm part as compared with the vibrating part are merely one example, and the characteristic frequency of the arm part may be appropriately determined according to the characteristic frequency or drive frequency of the vibrating part. Specifically, in the previous embodiments, the characteristic frequency of the arm part was 10 kHz as a value that is different from the characteristic frequency of the vibrating part, but the present invention is not limited thereto, and the characteristic frequency of the arm part can be made different from the characteristic frequency of the vibrating part by about 4% to 7% of the characteristic frequency of the longitudinal vibration in the vibrating part. 
   The terms “front,” “back,” “up,” “down,” “perpendicular,” “horizontal,” “slanted,” and other direction-related terms used above indicate the directions in the diagrams used. Therefore, the direction-related terminology used to describe the present invention should be interpreted in relative terms as applied to the diagrams used. 
   “Substantially,” “essentially,” “about,” and other terms that are used above and represent an approximation indicate a reasonable amount of deviation that does not bring about a considerable change as a result. Terms that represent these approximations should be interpreted so as to include a minimum error of about ±5%, as long as there is no considerable change due to the deviation. 
   This specification claims priority to Japanese Patent Application Nos. 2005-184454 and 2006-119951. All the disclosures in Japanese Patent Application Nos. 2005-184454 and 2006-119951 are incorporated herein by reference. 
   The embodiments described above are only some of possible embodiments of the present invention, but it is apparent to those skilled in the art that it is possible to add modifications to the above-described embodiments by using the above-described disclosure without exceeding the range of the present invention as defined in the claims. The above-described embodiments furthermore do not limit the range of the present invention, which is defined by the accompanying claims or equivalents thereof, and are designed solely to provide a description of the present invention.