Abstract:
An actuator comprises: an electro-mechanical conversion element; a driving frictional member that is mounted onto one side in an extension/contraction direction of the electro-mechanical conversion element; a driven member that is frictionally engaged with the driving frictional member; a fixed frame; an elastic supporting member that is mounted onto the other side in the extension/contraction direction of the electro-mechanical conversion element and displaceably supports the other side of the electromechanical conversion element with respect to the fixed frame; and a pressing member that presses the elastic supporting member in the extension/contraction direction to maintain the elastic supporting member in an elastically deformed state.

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 Invention  
         [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, the piezoelectric element frequently undergoes resonant vibration, and because, the piezoelectric element or the driving shaft undergoes lateral vibration or torsion in directions that differ from the displacement direction due to this resonant vibration, a driven member does not accurately move in the displacement direction. The actuator of Japanese Patent No. 2633066 is thus unstable in driving amount with respect to driving frequency.  
         [0006]     As a method for resolving the above issue, a method for displaceably supporting the other side of the piezoelectric element and attaching a weight member to the other side of the piezoelectric element may be considered.  
         [0007]     However, with this method, because when the weight of the weight member is small, the moving distance, moving speed, and thrust of the driven member are inadequate, a weight member of large weight must be used and the device thus becomes large. Especially when a driven member is to be moved across a large moving distance as in the case of zoom lens, a weight member of extremely large weight is necessary and the device becomes large.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been made in view of these circumstances, and an object thereof is to provide an actuator that can perform stable drive control and yet can be made compact.  
         [0009]     To achieve the above object, according to a first aspect of the invention, there is provided an actuator comprising: an electro-mechanical conversion element; a driving frictional member that is mounted onto one side in an extension/contraction direction of the electro-mechanical conversion element; a driven member that is frictionally engaged with the driving frictional member; a fixed frame; an elastic supporting member that is mounted onto the other side in the extension/contraction direction of the electro-mechanical conversion element and displaceably supports the other side of the electro-mechanical conversion element with respect to the fixed frame; and a pressing member that presses the elastic supporting member in the extension/contraction direction to maintain the elastic supporting member in an elastically deformed state.  
         [0010]     The inventor of the present invention made the finding that by supporting the other side of the electromechanical conversion element via the elastic supporting member and maintaining the elastic supporting member in the elastically deformed state, the same effect as attaching a large weight to the other side of the electro-mechanical conversion element, that is, improvement of the moving distance, moving speed, and thrust of the driven member can be achieved.  
         [0011]     Because the first aspect of the invention is made in view of these circumstances and the elastic supporting member is held in the elastically deformed state, the moving distance, moving speed, and thrust of the driven member can be improved. Furthermore, according to the first aspect of the invention, because the elastic supporting member is simply pressed, the device can be made more compact than when a large weight member is used.  
         [0012]     A second aspect of the invention provides the actuator according to the first aspect of the invention, wherein the pressing member is integrally formed with the fixed frame. Thus with the second aspect of the invention, the number of parts can be reduced and the device can be made even more compact.  
         [0013]     A third aspect of the invention provides the actuator according to the first aspect of the invention, wherein the pressing member is an adjusting screw that is screwed onto the fixed frame, and the elastic supporting member is pressed by a tip of the adjusting screw. With the third aspect of the invention, the pressing force can be adjusted by the degree of tightening of the adjusting screw.  
         [0014]     A fourth aspect of the invention provides the actuator according to any of the first to third aspects of the invention, wherein the elastic supporting member is a plate member having a spring property and has both of its ends supported by the fixed frame, and a rear end face of the electromechanical conversion element is affixed to a central portion of the elastic supporting member.  
         [0015]     A fifth aspect of the invention provides the actuator according to any 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  
       [0016]      FIG. 1  is a perspective view of a lens device to which an actuator according to the present invention is applied;  
         [0017]      FIG. 2  is a perspective view of an internal arrangement of the lens device of  FIG. 1 ;  
         [0018]      FIG. 3  is a perspective view of the lens device as viewed from a different direction as  FIG. 2 ;  
         [0019]      FIG. 4  is a schematic diagram of an arrangement of the actuator of the first embodiment;  
         [0020]      FIG. 5  is a sectional view of a connection portion of a driving shaft and a connecting block;  
         [0021]      FIGS. 6A and 6B  show diagrams of examples of voltage drive pulses that are applied to a piezoelectric element;  
         [0022]      FIG. 7  is a schematic diagram of an arrangement of an actuator of a second embodiment; and  
         [0023]      FIG. 8  is a diagram of an arrangement of an actuator of a comparative example. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     Preferred embodiments of an actuator according to the present invention shall now be described in detail with reference to the attached drawings.  
         [0025]      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.  
         [0026]     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 .  
         [0027]     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.  
         [0028]     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.  
         [0029]     Symbols  72  and  74  in FIGS.  1  to  3  indicate position detectors that detect positions of the holding frame  18  and the holding frame  20 . The position detector  72  is a reflection type photointerrupter that is disposed opposite a plate-like reflecting portion  78 , formed integral to the holding frame  18  (or the holding frame  20 ), and is fitted and fixed in an opening  12 A of the main body  12  (see  FIG. 1 ). On the reflecting portion  78 , a plurality of reflecting bodies (not shown) are disposed at fixed intervals along the driving direction. Light is thus projected from the position detector  72  to the reflecting portion  78 , and by receiving the reflected light and detecting the variation of the light amount, the movement amount of the reflecting portion  78  (that is, the holding frame  18  or  20 ) can be detected. Meanwhile, the position detector  74  has a light projecting portion  74 A and a light receiving portion  74 B, and a plate-like light blocking portion  76  that is formed integral to the holding frame  18  (or the holding frame  20 ) is inserted into and drawn out from in between the light projecting portion  74 A and the light receiving portion  74 B. Thus by the light blocking portion  76  being inserted between the light projecting portion  74 A and the light receiving portion  74 B and the light amount detected by the light receiving portion  74 B varying accordingly, that the light blocking portion  76  (that is, the holding frame  18  or  20 ) has moved to a predetermined position can be detected. By thus detecting reference positions of the holding frames  18  and  20  by means of the position detector  74  and detecting the moving amounts of the holding frames  18  and  20  by means of the position detector  72 , the positions of the holding frames  18  and  20  can be accurately determined. The actuators  34  and  36  are driven and controlled based on measurement values of the position detectors  72  and  74 .  
         [0030]      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 .  
         [0031]     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.  
         [0032]     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. The driving shaft  44  is formed to a cylindrical 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.  
         [0033]     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.  
         [0034]      FIG. 5  is a sectional view of the connection 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 connection 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.  
         [0035]     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 .  
         [0036]     A pressing spring  56  is mounted onto the connecting block  46 . The pressing spring  56  is arranged by bending a metal plate 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. Here, the frictional force (sliding resistance) is preferably no less than 10 gf and no more than 30 gf and more preferably no less than 15 gf and no more than 25 gf.  
         [0037]     As shown in  FIG. 4 , a weight member  58 , formed of a soft material, is fixed by adhesion onto the end face  42 B of the piezoelectric element  42 . 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.  
         [0038]     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 .  
         [0039]     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 .  
         [0040]     A pressing member  60  is disposed to the rear of the mounting bracket  48 . The pressing member  60  is formed integral to the main body  12  of the lens barrel  10  and is disposed so as to be in planar contact with the mounting bracket  48  and press the mounting bracket  48  forward (that is, toward the piezoelectric element  42  side). The mounting bracket  48  is thus maintained in a state of being slightly elastically deformed forward (for example, by approximately 0.1 mm). The pressing member  60  is not restricted in particular in terms of shape and, for example, may be formed to a semispherical shape and put in point contact with the mounting bracket  48 . The position of pressing by the pressing member  60  preferably lies along the extension of the central axis of the driving shaft  44 .  
         [0041]     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.  
         [0042]     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.  
         [0043]     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.  
         [0044]     The actions of the actuator  34  arranged as described above shall now be described.  
         [0045]     Because the zoom lenses (groups)  14  and  16  are large in movement amount in comparison to a focusing lens, etc., the driving shaft  44  becomes long and the vibration of the piezoelectric element  42  is not readily transmitted to the tip of driving shaft  44 . In the related art, to resolve this problem, a large weight member  64  of large weight had to be mounted as shown in  FIG. 8 . The related-art lens device thus required a larger weight member the longer the moving distance of the connecting block  46 , and the device thus became large.  
         [0046]     On the other hand, with the embodiment shown in  FIG. 4 , the pressing member  60  is disposed to the rear of the mounting bracket  48  and by this pressing member  60  pressing the mounting bracket  48 , the mounting bracket  48  is maintained in an elastically deformed state. By this arrangement, the same effect as the arrangement shown in  FIG. 8  (that is, the attachment of a large weight member  64  to the end face  42 B of the piezoelectric element  42 ) is obtained by the elastic force of the mounting bracket  48 . That is, the moving distance, moving speed, and thrust of the connecting block  46  are thus improved, and even when the moving amount of the connecting block  46  is large, stable drive control can be performed.  
         [0047]     Also, with the present embodiment, because the mounting bracket  48  is simply pressed by the pressing member  60 , the device can be made compact in comparison to the case where the large weight member  64  is provided. Especially with this embodiment, because the pressing member  60  is formed integral to the main body  12 , the number of parts can be reduced and the device can be made compact.  
         [0048]     A second embodiment of an actuator according to the present invention shall now be described. The actuator of the second embodiment shown in  FIG. 7  has an adjusting screw  62  disposed to the rear of the mounting bracket  48 . The adjusting screw  62  is screwed into the main body  12  of the lens device  10 , and a tip thereof is made to press the mounting bracket  48 . Also, the adjusting screw  62  is positioned along the extension of the central axis of the driving shaft  44  and is made to press the mounting bracket  48  in the driving direction.  
         [0049]     With the second embodiment arranged as described above, because the mounting plate  48  is pressed and maintained in an elastically deformed state by the adjusting screw  62 , the moving distance, moving speed, and thrust of the connecting block  46  can be improved.  
         [0050]     Also, with the second embodiment, by adjusting the amount of tightening of the adjusting screw  62 , the pressing force can be readily adjusted. The moving distance, moving speed, and thrust of the connecting block  46  can thus be adjusted to appropriate values.  
         [0051]     Though in the above-described embodiments, the piezoelectric element  42  is supported via the mounting bracket  48  with a spring property, any member that provides an elastic force may be used, and for example, the rear end face of the piezoelectric element  42  may be supported via a rubber material, etc. Even in this case, by pressing the rubber material, etc., in the driving direction and maintaining an elastically deformed state, the same effects as those described above are obtained.  
         [0052]     Also, as applications of the actuator according to the present invention, application, for example, to digital cameras, mobile phones, and other compact precision apparatuses is possible. Especially with a mobile phone, driving must be performed at a low voltage of no more than 3V, and by using the actuator according to the present invention, driving at a high frequency of approximately 20 kHz is enabled and the holding frame  20  can be moved at a high speed of no less than 2 mm/s. Thus even a zoom lens requiring movement of approximately 10 mm can be rapidly moved. Applications of the actuator according to the present invention are not restricted to applications of moving focusing lenses, zoom lenses, and other movable lenses, and use in applications in which a CCD is moved is also possible.  
         [0053]     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 as table 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 .  
         [0054]     With the actuator according to the present invention, because the other side of the electromechanical conversion element is supported via the elastic supporting member and the elastic supporting member is maintained in the elastically deformed state, the moving distance, moving speed, and thrust of the driven member can be improved without using a large weight member. Stable drive control can thus be performed and the device can be made compact.  
         [0055]     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.