Abstract:
An actuator comprises: an electromechanical conversion element; a driving frictional member mounted onto one side in an extension/contraction direction of the electromechanical conversion element; a driven member frictionally engaged with the driving frictional member; an urging section that urges the driven member to frictionally engage the driven member with the driving frictional member; and an urging force adjusting section that adjusts an urging force of the urging section.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an actuator, and particularly relates to an actuator that is installed in a digital camera, mobile phone, or other compact precision apparatus and drives a zoom lens.  
         [0003]     2. Description of the Related Art  
         [0004]     As a driving device for a lens unit of a digital camera, etc., an actuator that employs a piezoelectric element is used. For example, with an actuator of Japanese Patent No. 2633066, a driving shaft is affixed to one side of a piezoelectric element, and the other side of the piezoelectric element is fixed to a main device body. A lens barrel is slidably supported on the driving shaft, and the lens barrel is frictionally engaged with the driving shaft by making use of an urging force of a plate spring. Drive pulses of substantially sawtooth-like waveform are applied to the piezoelectric element, and the piezoelectric element deforms at different speeds in an extension direction and a contraction direction. For example, when the piezoelectric element gradually deforms, the lens barrel moves along with the driving shaft. Oppositely, when the piezoelectric element rapidly deforms, the lens barrel stays at the same position due to its mass inertia. Thus, by repeatedly applying drive pulses of substantially sawtooth-like waveform to the piezoelectric element, the lens barrel can be moved intermittently at a fine pitch.  
         [0005]     However, with the actuator of Japanese Patent No.  2633066 , because the moving distance, moving speed, and thrust of the driven member vary according to the position of the driven member and the operating environment, stable drive control cannot be performed.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention has been made in view of the above circumstances, and an object thereof is to provide an actuator that can perform stable drive control regardless of the position of the driven member and the operating environment.  
         [0007]     To achieve the above object, according to a first aspect of the invention, there is provided an actuator comprising: an electromechanical conversion element; a driving frictional member mounted onto one side in an extension/contraction direction of the electro-mechanical conversion element; a driven member frictionally engaged with the driving frictional member; an urging section that urges the driven member to frictionally engage the driven member with the driving frictional member; and an urging force adjusting section that adjusts an urging force of the urging section.  
         [0008]     With the first aspect of the invention, because the urging force can be adjusted by the urging force adjusting section, the driven member and the driving frictional member can be engaged at an appropriate frictional force. The driven member can thus be driven over a fixed moving distance at a fixed moving speed and thrust, and stable drive control can be performed.  
         [0009]     A second aspect of the invention provides the actuator according to the first aspect of the invention, further comprising: a position detecting section that detects a position of the driven member; and a controlling section that controls the urging force adjusting section based on a detection value of the position detecting section.  
         [0010]     With the second aspect of the invention, because the urging force of the urging section can be adjusted according to the position of the driven member, stable drive control can be constantly performed regardless of the position of the driven member.  
         [0011]     A third aspect of the invention provides the actuator according to the first aspect of the invention, further comprising: an operating environment measuring section that measures an operating environment; and a controlling section that controls the urging force adjusting section based on measurement values of the operating environment measuring section.  
         [0012]     With the third aspect of the invention, because the urging force of the urging section can be adjusted according to the operating environment (for example, the temperature, humidity, atmospheric pressure, etc.), stable drive control can be constantly performed regardless of the operating environment.  
         [0013]     A fourth aspect of the invention provides the actuator according to any one of the first to third aspects of the invention, wherein the urging section is a plate spring comprising a shape memory alloy, and the urging force adjusting section is a heating device that passes an electric current through the plate spring to heat the plate spring. With the fourth aspect of the invention, the shape of the plate spring can be changed by heating the plate spring, and the urging force can thereby be adjusted.  
         [0014]     A fifth aspect of the invention provides the actuator according to any one of the first to fourth aspects of the invention, wherein a holding frame of a zoom lens is mounted onto the driven member. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is a perspective view of a lens device to which an actuator according to the present invention is applied;  
         [0016]      FIG. 2  is a perspective view of an internal arrangement of the lens device of  FIG. 1 ;  
         [0017]      FIG. 3  is a perspective view of the lens device as viewed from a different direction as  FIG. 2 ;  
         [0018]      FIG. 4  is a schematic diagram of an arrangement of the actuator of the first embodiment;  
         [0019]      FIG. 5  is a sectional view of a connecting portion of a driving shaft and a connecting block;  
         [0020]      FIG. 6  shows diagrams of examples of voltage drive pulses that are applied to a piezoelectric element;  
         [0021]      FIG. 7  is a front view of an arrangement of a reflecting portion;  
         [0022]      FIG. 8  is a front view of a reflecting portion of an arrangement that differs from that of  FIG. 7 ;  
         [0023]      FIG. 9  is a schematic diagram of an arrangement of an actuator of a second embodiment;  
         [0024]      FIG. 10  is a diagram of relationships between a position of a driven member along the driving shaft and moving speed and thrust; and  
         [0025]      FIG. 11  is a diagram of a relationship between operating environment and moving speed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]     Preferred embodiments of an actuator according to the present invention shall now be described in detail with reference to the attached drawings.  
         [0027]      FIG. 1  is a perspective view of a lens device  10  to which an actuator according to the present invention is applied, and  FIGS. 2 and 3  are perspective views of an internal arrangement of this device.  
         [0028]     As shown in  FIG. 1 , the lens device  10  has a main body  12 , formed to a substantially rectangular shape, and has zoom lenses (groups)  14  and  16  in the interior of the main body  12  as shown in  FIGS. 2 and 3 . Of the zoom lenses (groups)  14  and  16 , one is a variator lens and the other is a compensator lens. The zoom lenses (groups)  14  and  16  are respectively held in holding frames  18  and  20 , and these holding frames  18  and  20  are supported by two guide shafts  22  and  24  in a manner enabling sliding along an optical axis P direction. The two guide shafts  22  and  24  are disposed in parallel to the optical axis P at diagonal positions inside the main body  12  and are fixed to the main body  12 .  
         [0029]     The holding frame  18  has a guide portion  26 , having an insertion hole  26 A through which the guide shaft  22  is inserted, and an engaging portion  28 , having a U-shaped groove  28 A that is engaged with the guide shaft  24 . The holding frame  18  is thereby guided by the two guide shafts  22  and  24 , and the zoom lens (group)  14  is supported in a manner enabling movement along the optical axis P direction. Likewise, the holding frame  20  of the zoom lens  16  has a guide portion  30 , having an insertion hole (not shown) through which the guide shaft  24  is inserted, and an engaging portion  32 , having a U-shaped groove  32 A, with which the guide shaft  22  is engaged. The holding frame  20  is thereby guided by the two guide shafts  22  and  24 , and the zoom lens (group)  16  is supported in a manner enabling movement along the optical axis P direction.  
         [0030]     The zoom lenses (groups)  14  and  16  are driven along the optical axis P direction by means of actuators  34  and  36 , respectively. The actuators  34  and  36  are disposed at opposing faces of the main body  12 . Specifically, the actuator  34  for the zoom lens (group)  14  is disposed on an upper face of the main body  12  in  FIG. 1 , and the actuator  36  for the zoom lens (group)  16  is disposed on a lower face of the main body  12 . Though a description of the actuator  34  shall be provided below, the actuator  36  is arranged in the same manner.  
         [0031]      FIG. 4  is a perspective view of an arrangement of the actuator  34 . As shown in this figure, the actuator  34  is mainly arranged from a fixed frame  40 , a piezoelectric element (corresponding to being an electromechanical conversion element)  42 , a driving shaft (corresponding to being a driving frictional member)  44 , a connecting block (corresponding to being a driven member)  46 , and a mounting bracket (corresponding to being an elastic supporting member)  48 , and the fixed frame  40  is fixed to the main body  12  of the lens device  10  of  FIG. 1 .  
         [0032]     The piezoelectric element  42  is layered along the optical axis P direction (hereinafter, “driving direction”) of the lens device  10  and is arranged to deform (extend or contract) along the driving direction upon application of voltage. Thus with the piezoelectric element  42 , end faces  42 A and  42 B in the longitudinal direction become displaced along the driving direction upon application of voltage.  
         [0033]     Of the end faces  42 A and  42 B of the piezoelectric element  42 , a base end of the driving shaft  44  is affixed to one end face  42 A, and a weight member  58 , formed of a soft material, is fixed by adhesion onto the other end face  42 B.  
         [0034]     By applying a load to the end face  42 B, the weight member  58  prevents the end face  42 B from becoming displaced more than the end face  42 A. Thus as the weight member  58 , a member that is greater in weight than the driving shaft  44  is preferable. Also, the weight member  58  is formed of a material with a Young&#39;s modulus less than that of each of the piezoelectric element  42  and the driving shaft  44  and, for example, is formed of a material with a Young&#39;s modulus of no more than 300 MPa. For example, the weight member  58  is formed of urethane rubber or urethane resin, etc., and is manufactured by mixing a powder of tungsten or other metal into the rubber or resin to make the specific gravity high. To achieve compact size, the specific gravity of the weight member  58  is preferably made as high as possible and is set, for example, to approximately 8 to 12.  
         [0035]     The weight member  58  is adhered onto the mounting bracket  48  at the side opposite the piezoelectric element  42 . The mounting bracket  48  is formed by bending a thin metal plate to a square C-shape and openings  48 B are formed at the bent portions at both ends. The mounting bracket  48  is mounted onto the fixed frame  40  by fitting protrusions  40 B of the fixed frame  40  into the openings  48 B. The piezoelectric element  42  is thus supported on the fixed frame  40  via the weight member  58  and the mounting bracket  48 .  
         [0036]     The piezoelectric element  42  that is supported as described above is supported in a manner in which the end face  42 B can be displaced along the driving direction. That is, the end face  42 B can be displaced along the driving direction by the expansion or contraction of the soft weight member  58  or the warping of the mounting bracket  48 . By thus supporting so that the end face  42 B of the piezoelectric element  42  can be displaced, resonant vibration of the device arrangement system within a driving frequency range can be prevented to enable stable drive control to be performed.  
         [0037]     Meanwhile, the driving shaft  44  which is affixed to the end face  42 A of the piezoelectric element  42  is formed to a rectangular columnar shape and is positioned so that its central axis lies along the driving direction. The driving shaft  44  is inserted through two holes  40 A formed in the fixed frame  40  and is thereby guided and supported in a manner enabling sliding along the central axis direction. As the material of the driving shaft  44 , a graphite crystal composite, such as carbon graphite, in which graphite crystals are firmly compounded is used.  
         [0038]     A connecting block  46  is engaged with the driving shaft  44 . The connecting block  46  is connected to the above-described holding frame  18  of the zoom lens  14  and is supported in a manner enabling sliding along the optical axis P direction (the driving direction) along with the holding frame  18 . The connecting block  46  is formed to a rectangular shape, and upwardly protruding protrusions  46 A are respectively provided on the four corners thereof.  
         [0039]      FIG. 5  is a sectional view of the connecting portion of the connecting block  46  and the driving shaft  44 . As shown in this figure, a first sliding member  52  and a second sliding member  54  are provided at the connecting portion of the connecting block  46  and the driving shaft  44 . The first sliding member  52  is disposed at an upper side of the driving shaft  44 , and the second sliding member  54  is disposed at a lower side of the driving shaft  44 . The first sliding member  52  and the second sliding member  54  are members that are provided to obtain a stable frictional force between the connecting block  46  and the driving shaft  44 , and are formed, for example, from stainless steel.  
         [0040]     The second sliding member  54  is formed to a V-shape and is fixed to the connecting block  46 . Meanwhile, the first sliding member  52  is formed to an inverted V-shape and is positioned in a region surrounded by the four protrusions  46 A of the connecting block  46 . The first sliding member  52  is notched at its respective corner portions in accordance with the protrusions  46 A of the connecting block  46 . Thus when the first sliding member  52  is positioned in the region surrounded by the protrusions  46 A, the first sliding member  52  is prevented from falling off from the connecting block  46 .  
         [0041]     A pressing spring  56  is mounted onto the connecting block  46 . The pressing spring  56  is arranged by bending a metal plate, formed of a shape memory alloy, and is mounted onto the connecting block  46  by hitching a pawl  56 A onto a lower portion of the connecting block  46 . The pressing spring  56  also has a pressing portion  56 B that is positioned on an upper side of the first sliding member  52  and is arranged to urge the first sliding member  52  downward by the pressing portion  56 B. The driving shaft  44  is thereby put in a state of being sandwichingly pressed by the first sliding member  52  and the second sliding member  54 , and the connecting block  46  is frictionally engaged with the driving shaft  44  via the first sliding member  52  and the second sliding member  54 . The frictional force between the connecting block  46  and the driving shaft  44  is set so that when drive pulses of a gradual voltage variation is applied to the piezoelectric element  42 , the frictional force is greater than the driving force, and when drive pulses of a rapid voltage variation is applied to the piezoelectric element  42 , the frictional force is less than the driving force.  
         [0042]     A heating device  66  is electrically connected to the pressing spring  56 . That is, lead wires  68  are connected to both ends of a pressing spring  56 , and these lead wires  68  are connected to the heating device  66 . The heating device  66  is arranged to apply a predetermined voltage and thereby pass an electric current through the pressing spring  56  and heat the pressing spring  56 . The pressing spring  56  is arranged from a shape memory alloy as mentioned above, and deforms to a memorized shape when heated to a predetermined temperature or higher. Specifically, the pressing spring  56  is arranged so that when it is heated, the pressing spring  56  deforms in a manner such the pressing portion  56 B, shown in  FIG. 5 , becomes lowered and the frictional force (sliding resistance) between the first sliding member  52  and the driving shaft  44  and the frictional force (sliding resistance) between the second sliding member  54  and the driving shaft  44  increase. The sliding resistances in this state are preferably adjusted within a range of no less than 10 gf and no more than 30 gf and more preferably within a range of no less than 15 gf and no more than 25 gf.  
         [0043]     The heating device  66  is connected to a controller  70 , and the heating device  66  is controlled by the controller  70 . The controller  70  is connected to position detectors  72  and  74 , and the heating device  66  is controlled based on detection values of the position detectors  72  and  74 .  
         [0044]     The position detector  72 , shown in  FIG. 2 , is a reflection type photointerrupter that is fitted and fixed in an opening  12 A of the main body  12  (see  FIG. 1 ). This position detector  72  is disposed opposite a plate-like reflecting portion  78 , formed integral to the holding frame  18 . Additionally, the position detector  72  has a light projecting portion and a light receiving portion (not shown), and is arranged so that the light projecting portion projects light toward the reflecting portion  78  and the reflected light reflected from the reflecting portion  78  is received by the light receiving portion, which detects the light amount of the reflected light.  
         [0045]     As shown in  FIG. 7 , on the reflecting portion  78 , a plurality of reflecting bodies  80  are disposed at fixed intervals along the driving direction. Thus when the actuator  34  is driven and the reflecting portion  78  moves along the optical axis P direction, the light amount received by the light receiving portion of the position detector  72  changes and the movement amount of the reflecting portion  78  (that is, the movement amount of the holding frame  18 ) can thereby be detected. The arrangement of the reflecting portion  78  is not restricted to that of the embodiment described above, and it is sufficient that the reflecting portion  78  have an arrangement with which the reflection amount changes when the reflecting portion  78  is moved in the driving direction. Thus for example, a triangular reflecting body  82  may be disposed as shown in  FIG. 8 .  
         [0046]     Meanwhile, the position detector  74  is a transmitting type photoreflector and is fixed to the main body  12  (see  FIG. 1 ). An upper portion of the position detector  74  is divided into two portions with one being a light projecting portion  74 A and the other being a light receiving portion  74 B. The position detector  74  is arranged so that the light projecting portion  74 A projects light toward the light receiving portion  74 B and the light receiving portion  74 B receives this light and detects the received light amount.  
         [0047]     A plate-like light blocking portion  76 , formed integral to the holding frame  18 , is inserted into and drawn out from in between the light projecting portion  74 A and the light receiving portion  74 B. That is, when the actuator  34  is driven and the holding frame  18  is moved along the optical axis P direction, the light blocking portion  76  is inserted into and drawn out from in between the light projecting portion  74 A and the light receiving portion  74 B of the photointerrupter  74 . The amount of light received by the light receiving portion  74 B thus varies and that the light blocking portion  76  has been inserted between the light projecting portion  74 A and the light receiving portion  74 B can be detected. That the holding frame  18  has moved to a reference position can thus be detected.  
         [0048]     Thus by the reference position of the holding frame  18  being detected by the position detector  74  and the moving amount of the holding frame  18  being detected by the position detector  72 , the position of the holding frame  18  can be accurately determined.  
         [0049]     The controller  70  controls the heating device  66  according to the detection values of the position detectors  72  and  74  (that is, the position of the holding frame  18  or the connecting block  46 ). For example, when the connecting block  46  is positioned near a base end or near a front end of the driving shaft  44 , current is passed through the pressing spring  56  to heat the pressing spring  56  and thereby make the pressing spring  56  deform to the memorized shape. The urging force due to the pressing spring  56  is thereby increased and the frictional forces between the driving shaft  44  and the first sliding member  52  and the second sliding member  54  increase. Thus even when the connecting block  46  is positioned at the base end or front end of the driving shaft  44 , an adequate moving distance, movement speed, and thrust can be secured for the connecting block  46 .  
         [0050]     The voltages of the drive pulses shown in  FIGS. 6A and 6B  are applied to the piezoelectric element  42 .  FIG. 6A  shows the drive pulses for moving the connecting block  46  of  FIG. 4  in the left direction, and  FIG. 6B  shows the drive pulses for moving the connecting block  46  of  FIG. 4  in the right direction.  
         [0051]     In the case of  FIG. 6A , substantially sawtooth-like drive pulses, each of which gradually rises from a time α 1  to a time α 2  and rapidly drops at a time α 3 , are applied to the piezoelectric element  42 . Thus from the time α 1  to the time α 2 , the piezoelectric element  42  gradually extends. Because in this process, the driving shaft  44  moves at a gradual speed, the connecting block  46  moves along with the driving shaft  44 . The connecting block  46  of  FIG. 4  can thereby be moved in the left direction. At the time α 3 , because the piezoelectric element  42  rapidly contracts, the driving shaft  44  moves in the right direction. Because in this process, the driving shaft  44  rapidly moves, the connecting block  46  remains stopped at the same position due to inertia and just the driving shaft  44  moves. Thus by repeatedly applying the sawtooth-like drive pulses shown in  FIG. 6A , the connecting block  46  of  FIG. 4  is made to repeat movement in the left direction and stoppage, and can thus be moved in the left direction.  
         [0052]     In the case of  FIG. 6B , substantially sawtooth-like drive pulses, each of which gradually drops from a time β 1  to a time β 2  and rapidly rises at a time β 3 , are applied to the piezoelectric element  42 . Thus from the time β 1  to the time β 2 , the piezoelectric element  42  gradually contracts. Because in this process, the driving shaft  44  is displaced at a gradual speed, the connecting block  46  moves along with the driving shaft  44 . The connecting block  46  of  FIG. 4  can thereby be moved in the right direction. At the time β 3 , because the piezoelectric element  42  rapidly extends, the driving shaft  44  moves in the left direction. Because in this process, the driving shaft  44  rapidly moves, the connecting block  46  remains stopped at the same position due to inertia and just the driving shaft  44  moves. Thus by repeatedly applying the sawtooth-like drive pulses shown in  FIG. 6B , the connecting block  46  of  FIG. 4  is made to repeat movement in the right direction and stoppage, and can thus be moved in the right direction.  
         [0053]     The actions of the actuator  34  arranged as described above shall now be described.  
         [0054]     By driving the actuator  34  as described above, the connecting block  46  and the holding frame  18  are moved along the driving shaft  44 . In this process, with the related-art actuator, the moving distance, moving speed, and thrust of the connecting block  46  greatly varies as the connecting block  46  moves.  
         [0055]      FIG. 10  shows a relationship between the position of the driving shaft  44  and the moving speed and the thrust of the connecting block  46 . With a comparative example shown in this figure, a fixed urging force is applied by the pressing spring  56  regardless of the position of the connecting block  46 .  
         [0056]     As shown in  FIG. 10 , with the comparative example with which a fixed urging force is applied, the speed and thrust of the connecting block  46  decrease at a base end and a front end of the driving shaft  44 , and the speed and thrust improve at a central portion of the driving shaft  44 . Because with the related-art device, the speed and thrust vary according to the position of the connecting block  46 , stable drive control cannot be performed. As a cause, it is considered that when the piezoelectric element  42  is made to extend or contract by applying pulse-form voltages to the piezoelectric element  42 , the driving shaft  44  undergoes minute torsion or warping and the entirety of the driving shaft  44  is not uniformly displaced.  
         [0057]     Thus with the present embodiment, the position of the connecting block  46  is detected and the pressing spring  56  is heated and deformed to adjust the urging force due to the pressing spring  56  according to the detection value. Specifically, the pressing spring  56  is heated and deformed to increase the urging force of the pressing spring  56  when the connecting block  46  is positioned near the front end or the base end of the driving shaft  44 . The frictional force between the connecting block  46  and the driving shaft  44  is thereby increased and an adequate moving speed and thrust are secured for the connecting block  46 . As shown in  FIG. 10 , because even when the connecting block  46  is positioned at the front end or the base end of the driving shaft  44 , substantially the same moving speed and thrust as those obtained when the connecting block  46  is positioned at a central portion of the driving shaft  44  are obtained, stable drive control that is not dependent on the position of the connecting block  46  can be performed.  
         [0058]     Though with the above-described embodiment, the pressing spring  56  is heated and adjusted in urging force when the connecting block  46  is positioned near the front end or the base end of the driving shaft  44 , the timing of adjustment of the urging force (that is, at which positions along the driving shaft  44  the pressing spring  56  is to be heated) is not restricted to that of the above-described embodiment. For example, the urging force may be adjusted only when the connecting block  46  is positioned at the front end of the driving shaft  44 . Also, the pressing spring  56  may be heated and adjusted in urging force when the connecting block  46  is positioned at the central portion of the driving shaft  44 .  
         [0059]     Also, though with the above-described embodiment, the pressing spring  56  is made to memorize a shape such that the urging force increases when the pressing spring  56  is heated, the pressing spring  56  may oppositely be made to memorize a shape such that the urging force decreases (that is, the urging force disappears) upon heating.  
         [0060]     Also, though with the above-described embodiment, a unidirectional shape memory alloy that deforms to the memorized shape when the pressing spring  56  is heated is used, the present invention is not restricted thereto, and a bidirectional shape memory alloy that deforms to another shape when cooled may be used instead.  
         [0061]     Also, though with the above-described embodiment, the urging section is arranged from a pressing spring  56  formed of a shape memory alloy, the arrangement of the urging section is not restricted thereto and, for example, the urging section may be arranged from a spring formed of a shape memory alloy or a shape memory resin.  
         [0062]     Furthermore, though with the above-described embodiment, the position detector  72 , arranged from a photointerrupter, and a position detector  74 , arranged from a photoreflector, are used as the position detecting section for the holding frame  18  (that is, the connecting block  46 ), just one of either may be used or a position detecting section of another arrangement may be employed.  
         [0063]     A second embodiment of an actuator  34  according to the present invention shall now be described. The actuator  34  according to the second embodiment shown in  FIG. 9  is an example in which the urging force is adjusted according to the operating environment and is equipped with a temperature sensor  84  that measures the temperature inside a fixed frame  40 . As the temperature sensor  84 , for example, a thermocouple is used, and a front end thereof is disposed near a piezoelectric element  42 . This temperature sensor  84  is connected to a controller  70 , and the controller  70  controls a heating device  66  in accordance with detection values of the temperature  84  to adjust the temperature of a pressing spring  56 . The pressing spring  56  thus deforms and the urging force due to the pressing spring  56  is adjusted according to the temperature of the surroundings in which the actuator  34  is installed. The temperature at which the pressing spring  56  deforms to the memorized shape and the memorized shape are set based on experimental results and are set so that even when the temperature of the operating environment varies, the connecting block  46  moves at a fixed moving speed or thrust. For example, settings are made so that the urging force due to the pressing spring  56  increases as the temperature decreases.  
         [0064]     With the second embodiment, because the pressing spring  56  is deformed by heating to adjust the urging force due to the pressing spring  56  according to the temperature of the operating environment, a fixed moving speed or thrust can be constantly maintained regardless of the temperature. For example,  FIG. 11  shows a relationship of the temperature of the operating environment and the moving speed of the connecting block  46 . With a comparative example shown in this figure, a constant urging force is applied by a pressing spring  56  regardless of the operating environment. As can be understood from this comparative example, when a fixed urging force is applied, the speed of a connecting block  46  decreases as the temperature decreases. Therefore, stable driving control cannot be performed. On the other hand, with the present embodiment, because the urging force due to the pressing spring  56  is made to increase as the temperature decreases, an adequate speed can be maintained even when the temperature decreases. With the present embodiment, because the speed of the connecting block  46  can thus be kept substantially fixed, stable drive control that is not dependent on the operating environment can be performed.  
         [0065]     Though with the second embodiment, the urging force is adjusted so that the moving speed of the connecting block  46  is substantially fixed, the urging force may instead be adjusted so that the thrust of the connecting block  46  is substantially fixed. In this case, settings are made so that the urging force increases as the temperature increases.  
         [0066]     Also, though with the second embodiment, the urging force is adjusted according to temperature, another operating environment parameter, such as humidity or atmospheric pressure, may be measured and the urging force may be adjusted based on the measured value.  
         [0067]     Also, though the urging force is adjusted according to the position of the connecting block  46  in the above-described first embodiment, and the urging force is adjusted according to the operating environment in the second embodiment, arrangements may be made to adjust the urging force based on both the position of the connecting block  46  and the operating environment.  
         [0068]     Furthermore, with the first and second embodiments, the urging force may be adjusted in multiple stages of two stages or three or more stages or may be adjusted so as to vary in a continuous manner.  
         [0069]     Though the material of the weight member  58  in the present invention is not restricted to the above-described soft material and a hard material may be used, the use of a soft material is preferable from the following points. That is, by using the weight member  58  formed of a soft material, the resonance frequency of the system arranged from the piezoelectric element  42 , the driving frictional member  44 , and the weight member  58  is made low. By the resonance frequency being made low, effects due to scattering among arrangements of the piezoelectric element  42 , the driving driving shaft  44 , and the weight member  58  are lessened, and a stable driving force can be obtained. Also, by the resonance frequency f 0  being made low, the driving frequency f can be readily set in a vibration-proof region of f≧2 1/2 ·f 0  to lessen the effects of resonant vibration and enable a stable driving force to be obtained. Because the driving force due to extension and contraction of the piezoelectric element  42  is thereby reliably transmitted to the driven member, the driven member can be accurately moved in the extension/contraction direction of the piezoelectric element  42 . Also, because the effects due to resonant vibration are lessened by the resonance frequency f 0  being made low, the supporting position and supporting method of the actuator can be arbitrarily selected, and for example, the actuator can be supported at the end face  42 A or a side face of the piezoelectric element  42  and a side face or an end face of driving shaft  44 .  
         [0070]     With the actuator according to the present invention, because the urging force of the urging section can be adjusted to adjust the frictional force between the driven member and the driving frictional member, the driven member can be driven over a fixed moving distance at a fixed moving speed and thrust, and stable drive control can be performed.  
         [0071]     The entire disclosure of each and every foreign patent application from which the benefit of foreign priority has been claimed in the present application is incorporated herein by reference, as if fully set forth.