Abstract:
An actuator, including: a substrate; a middle plate supported above the substrate to be rotatable, about a rotation axis, with respect to the substrate; a stage connected to and spaced above the middle plate; a pair of first driving electrodes located on the substrate around the rotation axis; and a pair of second driving electrodes located on the substrate surrounding the first driving electrodes. Torsion springs connect opposite sides of the middle plate to support the middle plate above the substrate. A connecting member connects the stage and middle plate.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION  
       [0001]     This application is based upon and claims the benefit of priority from Korean Patent Application No. 10-2005-0021846, filed on Mar. 16, 2005, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Devices, systems, and methods consistent with the invention relate to an actuator with a double plate structure.  
         [0004]     2. Description of the Related Art  
         [0005]     An actuator is used as an optical scanner for reflecting a laser beam, in a display appliance such as a projection television. The optical scanner can be driven by electrostatic force and is manufactured from a micro-electromechanical system (MEMS).  FIG. 1  is a perspective view illustrating an example of the structure of a flat type electrostatically driven optical scanner according to the related-art. As shown in  FIG. 1 , the optical scanner of the related-art includes a stage  20  supported above a substrate  10 , a mirror  22  on the stage  20 , torsion springs  30  extending from both sides of the stage  20 , anchors  40  holding the ends of the torsion springs  30  to the substrate  10 , and a pair of driving electrodes  51  and  52  on the substrate  10 . The driving electrodes  51  and  52  are spaced on either side of the torsion spring  30 , which is the rotation axis of the mirror  22 .  
         [0006]      FIG. 2  is a cross-sectional view for explaining the operation of the related-art optical scanner with the above construction. When a voltage is applied to the driving electrode  51 , the stage  20  is rotated to one side by the electrostatic force between the driving electrode  51  and the stage  20 . That is, the stage rotates by a driving angle θ. The restoring force of the torsion spring  30  returns the stage  20  to its original position. Thus, the voltage applied to the driving electrodes  51  and  52  can be controlled to give the stage  20  a periodic motion with a certain driving angle and velocity (i.e., driving frequency).  
         [0007]     The driving angle varies with the driving voltage. The electrostatic force is expressed by the following equation 1.
 
 F =(ε AV   2 )/ D   2   (Equation 1)
 
         [0008]     where ε is the dielectric constant, A is the area of the stage, and D is the distance between the stage and the electrode.  
         [0009]     Accordingly, the driving force on the stage  20  increases in proportion to the square of the distance D as the stage  20  moves closer to the driving electrodes  51  and  52 . However, since the restoring force of the torsion spring  30  is proportional to the angle of rotation, if the stage is rotated too far, the restoring force becomes smaller than the driving force, and the stage  20  can contact the driving electrode  51  or  52 , thereby making it difficult to adjust the driving angle.  
         [0010]     Therefore, there are limits to how much the driving angle of the optical scanner can be increased. Also, in order to increase the restoring force of the torsion spring and the driving angle, the higher driving voltage must be applied.  
       SUMMARY OF THE INVENTION  
       [0011]     According to an aspect of the invention, an actuator is provided with a double plate structure by which a driving angle is increased with a low driving voltage.  
         [0012]     According to another aspect of the invention, there is provided an actuator with a double plate structure, including a substrate; a middle plate supported above the substrate by torsion springs extending from its opposite sides, and which is rotatable about a rotation axis linking the torsion springs; a stage connected to and spaced above the middle plate by a connecting member; a pair of first driving electrodes located on the substrate around the rotation axis; and a pair of second driving electrodes located on the substrate surrounding the first driving electrodes.  
         [0013]     According to another aspect of the invention, the area of the middle plate is smaller than that of the stage.  
         [0014]     According to another aspect of the invention, the area of the first driving electrode is smaller than that of the second driving electrode.  
         [0015]     According to another aspect of the invention, the actuator may further include an electrical conductor connecting the first and second driving electrodes.  
         [0016]     According to another aspect of the invention, the stage may be parallel to the middle plate.  
         [0017]     According to another aspect of the invention, there is provided an actuator with a double plate structure, including a substrate; a stage supported above the substrate by torsion springs extending from its opposite sides, and which is rotatable about a rotation axis linking the torsion springs; a middle plate connected to and spaced below the stage by a connecting member; a pair of first driving electrodes located on the substrate around the rotation axis; and a pair of second driving electrodes located on the substrate surrounding the first driving electrodes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The above and/or other aspects of the invention will become more apparent by describing in detail exemplary embodiments of the invention with reference to the attached drawings in which:  
         [0019]      FIG. 1  is a perspective view illustrating an example of the structure of a related-art optical scanner;  
         [0020]      FIG. 2  is a cross-sectional view explaining the operation of the related-art optical scanner;  
         [0021]      FIG. 3  is a perspective view illustrating an actuator with a double plate structure according to a first exemplary embodiment of the invention;  
         [0022]      FIG. 4  is a cross-sectional view taken along a line IV-IV of  FIG. 3 ;  
         [0023]      FIG. 5  is a view explaining the operation of the invention;  
         [0024]      FIG. 6  is a view explaining the horizontal movement of the center point of a stage and the vertical movement of a reflecting surface according to a double plate structure of the invention;  
         [0025]      FIG. 7  is a cross-sectional view illustrating an actuator according to a second exemplary embodiment of the invention;  
         [0026]      FIG. 8  is a plan view showing electrodes on a substrate;  
         [0027]      FIG. 9  is a view explaining the operation of an actuator according to the second exemplary embodiment of the invention; and  
         [0028]      FIG. 10  is a cross-sectional view illustrating an actuator with a double plate structure according to a third exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0029]     Exemplary embodiments of the invention will now be described below by reference to the attached Figures. The described exemplary embodiments are intended to assist the understanding of the invention, and are not intended to limit the scope of the invention in any way. Like numbers refer to like elements throughout the specification.  
         [0030]      FIG. 3  is a perspective view illustrating an actuator with a double plate structure according to a first exemplary embodiment of the invention, and  FIG. 4  is a cross-sectional view taken along a line IV-IV of  FIG. 3 .  
         [0031]     Referring to  FIGS. 3 and 4 , the actuator includes a middle plate  120  supported above a substrate  110 , a stage  130  fixed above the middle plate  120 , torsion springs  140  extending from opposite sides of the middle plate  120 , anchors  142  holding the ends of the torsion springs  140  above the substrate  110 , and driving electrodes  151  and  152  formed on the substrate  110 . A mirror  132  with a light-reflecting surface may be further formed on the stage  130 .  
         [0032]     The driving electrodes  151  and  152  consist of a pair of first driving electrodes  151  spaced around the rotation axis of the torsion springs  140 , and a pair of second driving electrodes  152  surrounding the first driving electrodes  151 . A circuit is constructed such that different voltages may be applied to the first and second driving electrodes  151  and  152 . The substrate  110  may be made from pyrex glass or silicon (or similar material), and the driving electrodes  151  and  152  may be made from conductive metal such as chromium, Indium-Tin Oxide (ITO) (or similar material).  
         [0033]     The stage  130  is connected to and spaced apart from the middle plate  120  by a connecting member  124 . The stage  130 , the connecting member  124 , the middle plate  120 , the torsion springs  140 , and the anchors  142  may be made from a conductive material (e.g., polysilicon). The stage  130  is parallel to the middle plate  120 . The area of the stage  130  is larger than that of the middle plate  120 .  
         [0034]     The distance d between the first driving electrode  151  and the middle plate  120  is less than the distance D between the second driving electrode  152  and the stage  130 .  
         [0035]     When a first voltage V 1  is applied to one of the first driving electrodes  151 , the stage  130  is rotated to one side by an electrostatic force F 1  between the first driving electrode  151  and the middle plate  120 . The electrostatic force can be expressed by the following equation 2.
 
 F   1 =(ε× A   1   ×V   1   2 )/ d   2   (Equation 2)
 
         [0036]     where ε is the dielectric constant, A 1  is the area of the middle plate  120 , and d is the distance between the middle plate  120  and the first driving electrode  151 .  
         [0037]     Compared with the electrostatic force (F of Equation 1) of the related-art actuator without the middle plate  120 , if F 1 =F to exert the same driving angle, and A=2×A 1  and D=2×d, then V 1  is equal to V/√{square root over (2)}. Accordingly, the driving voltage of the actuator with the middle plate  120  of the invention is lower than that of the related-art actuator. The driving voltage V 1  may vary with the area and position of the middle plate  120 . That is, as the distance d between the substrate  110  and the middle plate  120  is less, the driving voltage is lower.  
         [0038]      FIG. 5  is a view for explaining the operation of the invention. Referring to  FIG. 5 , when the first driving voltage V 1  is applied to the middle plate  120 , a torque T 1 (T 1 =F 1 ×r 1 ) is generated, depending upon the electrostatic force F 1  between the first driving electrode  151  and the middle plate  120 , thereby rotating the stage  130  by an angle θ 1 . Then, when a second voltage V 2  is applied to the second driving electrode  152 , an electrostatic force F 2  is generated. The electrostatic force F 2  can be expressed by the following Equation  3 .
   F   2 =(ε× A   2   ×V   2   2 )/ D   1   2   (Equation 3) 
         [0039]     where ε is the dielectric constant, A 2  is the area of the stage  130 , and D 1  is the distance between the stage  130  and the second driving electrode  152 .  
         [0040]     A torque T 2  (T 2 =F 2 ×r 2 ) is exerted, depending upon the electrostatic force F 2  between the second driving electrode  152  and the stage  130 . At an initial state, the electrostatic force F 1  is larger than F 2 , but as the driving angle increases, F 2  also increases. Since r 2  is much larger than r 1 , T 2  becomes larger than T 1 . Here, r 1  and r 2  are the respective lengths of the pivot arms of the middle plate  120  and the stage  130 . Accordingly, with an increase of the driving force and torque, the stage  130  and the middle plate  120  rotate further up to an angle θ 2 .  
         [0041]     Therefore, by controlling the voltage applied to the first and second driving electrodes  151  and  152 , it is possible to increase the driving angle of the stage  130 . Also, by applying the first voltage to initially rotate the stage  130 , and then applying a low voltage to the second driving electrode  152 , the torque exerted on the stage  130  is increased, so that the driving angle can be increased even with a low voltage.  
         [0042]      FIG. 6  is a view for explaining the horizontal movement of the center point of the stage  130  and the vertical movement of a reflecting surface, according to a double plate structure of the invention.  
         [0043]     Referring to  FIG. 6 , when the stage  130  rotates by an angle θ, the rotation axis of the middle plate  120  remains at the center point P 0 , which is fixed. However, the center point P 1  of the stage  130  rotates to a position P 2  along the circumference of a circle as the middle plate  120  rotates. If the distance between the middle plate  120  and the stage  130  is r, the moving distance of the center point of the stage  130  can be expressed by the following Equation 4.
 
 Dm =2 πr ×θ/360  (Equation 4)
 
         [0044]     where Dm is the moving distance (the length of an arc) of the center point between P 1  and P 2 , r is the distance between the stage  130  and the middle plate  120 , and θ is the driving angle.  
         [0045]     When r is 50 μm and θ is 10 degrees, Dm is 8.7 μm. Meanwhile, the mirror  132  of the stage  130  may have a length of 1 to 1.5 mm, so that the movement of the center point of the mirror  132  does not cause a problem.  
         [0046]     Meanwhile, when rotated by the angle shown in  FIG. 6 , the solid line indicates the top of the stage  130  rotated about the rotation axis of the middle plate  120 , and the dotted line indicates the position of the stage  130  rotated about the center point P 1  of the stage  130 . A vertical length-g between the positions of the stage  130  indicated by the dotted line and the solid line can be expressed by the following Equation 5.
 
 g=r−r  cos θ  (Equation 5)
 
         [0047]     In Equation 5, if r is 50 μm and θ is 10 degrees, g is 0.76 μm. This value is a negligible value in the optical scanner.  
         [0048]     It can be therefore known that even when the center point of the reflecting surface is rotated, the actuator with the double plate structure of the invention can be used as an optical scanner.  
         [0049]     The operation of the actuator according to a first exemplary embodiment of the invention will now be explained with reference to the drawings.  
         [0050]     First, when the first voltage V 1  is applied to the first driving electrode  151 , the stage  130  rotates by a first angle θ 1  about the rotation axis of the torsion spring  140 . Then, when the second voltage V 2  is applied to the second driving electrode  152 , the stage  130  rotates further, from the first angle θ 1  to a second angle θ 2 . After that, the first voltage V 1  can be turned off.  
         [0051]     Then, if the second voltage V 2  is turned off, the stage  130  returns to its rest state.  
         [0052]     Next, if voltages are applied in sequence to the first and second driving electrodes  151  and  152  on the other side, the stage  130  rotates in the opposite direction, and then when those voltages are turned off, the stage  130  returns again to its rest state. Accordingly, by adjusting the voltage applied to the driving electrodes, it is possible to rotate the mirror  132  periodically with a certain driving angle and driving velocity (driving frequency).  
         [0053]      FIG. 7  is a cross-sectional view illustrating an actuator according to a second exemplary embodiment of the invention, and  FIG. 8  is a plan view showing electrodes on the substrate. In  FIGS. 7 and 8 , elements which are substantially the same as those of the first exemplary embodiment are identified with similar terms, and a detailed explanation thereof will be omitted.  
         [0054]     Referring to  FIGS. 7 and 8 , the actuator includes a middle plate  220  supported above a substrate  210 , a stage  230  fixed above the middle plate  220 , a mirror  232  on the stage  230 , torsion springs  240  extending from opposite sides of the middle plate  220 , anchors  242  supporting the ends of the torsion spring  240  above the substrate  210 , and driving electrodes  250 .  
         [0055]     The driving electrodes  250  consist of a pair of first driving electrodes  251  spaced around the rotation axis of the torsion springs  240 , and a pair of second driving electrodes  252  surrounding the first driving electrodes  251 . The first and second driving electrodes are electrically connected together by an electric conductor  254 .  
         [0056]     The stage  230  is connected to and spaced apart from the middle plate  220  by a connecting member  224 . The stage  230 , the connecting member  224 , the middle plate  220 , the torsion springs  240  and the anchors  242  may be made from a conductive material (e.g., polysilicon). The stage  230  is parallel to the middle plate  220 . The area of the stage  230  is larger than that of the middle plate  220 .  
         [0057]      FIG. 9  is a view for explaining the operation of an actuator according to the second exemplary embodiment of the invention.  
         [0058]     Referring to FIGS.  7  to  9 , when a voltage is applied to one of the driving electrodes  250 , a torque T 1  (T 1 =F 1 ×r 1 ) is exerted according to the electrostatic force F 1  between the particular driving electrode  250  and the middle plate  220 , and a torque T 2  (T 2 =F 2 ×r 2 ) is exerted according to the electrostatic force F 2  between the particular driving electrode  250  and the stage  230 . At the rest position, the electrostatic force F 1  is larger than F 2 , but as the driving angle increases, F 2  increases. Since r 2  is much larger than r 1 , T 2  becomes larger than T 1 . Here, r 1  and r 2  are the respective lengths of the pivot arm of the middle plate  220  and the stage  230 .  
         [0059]     Meanwhile, the driving electrodes  250  consist of the first driving electrode  151  and the second driving electrode  252 , and the area of the first driving electrode  251  is smaller than that of the second driving electrode  252 . The electrostatic force between the middle plate  220  and the first driving electrode  251  begins to rotate the stage  230 , and the electrostatic force between the stage  230  and the second driving electrode  252  rotates the stage further. Accordingly, although the driving voltage V 1  is lower than the threshold voltage for the maximum driving angle of the middle plate  220 , the driving angle is increased by driving the stage  230 , thereby controlling and increasing the driving angle of the stage  230  within the threshold voltage.  
         [0060]     The operation of the second exemplary embodiment of the actuator will now be explained with reference to the drawings.  
         [0061]     First, when the first voltage V 1  is applied to the driving electrodes  250 , the stage  230  is driven mainly by the force F 1  on the middle plate  220 , which is larger than the force F 2  on the stage  230 . Then, after a point, the force F 2  becomes larger than the force F 1  and the distance r 2  to the center of rotation is larger, so that the torque increases. Thus, if the driving angle exceeds a certain angle, the torque is increased mainly by the force F 2 , allowing a larger driving angle. Therefore, the driving angle may be increased even using a low driving voltage.  
         [0062]     Then, if the first voltage is turned off, the stage  230  returns to its rest position.  
         [0063]     Next, if a voltage is applied to the other side driving electrodes, the stage  230  rotates in the opposite direction, and then if the voltage to those electrodes is turned off, the stage  230  returns to its rest position. Accordingly, by controlling the voltage applied to the driving electrodes  250 , it is possible to rotate the mirror  232  periodically with a certain driving angle and driving velocity (driving frequency).  
         [0064]      FIG. 10  is a cross sectional view illustrating an actuator with a double plate structure according to a third exemplary embodiment of the invention. In  FIG. 10 , elements which are substantially the same as those of the first exemplary embodiment are identified with similar terms, and a detailed explanation thereof will be omitted.  
         [0065]     Referring to  FIG. 10 , the actuator includes a middle plate  320  supported above a substrate  310 , a stage  330  fixed above the middle plate  320 , a mirror  332  on the stage  330 , torsion springs  340  extending from opposite sides of the stage  330 , an anchor  342  holding the ends of the torsion springs  340  above the substrate  310 , and driving electrodes  350  formed on the substrate  310 .  
         [0066]     The driving electrodes  350  consist of a pair of first driving electrodes  351  spaced around the rotation axis of the torsion springs  340 , and a pair of second driving electrodes  352  surrounding the first driving electrodes  151 . A circuit is constructed such that separate voltages may be applied to the first and second driving electrodes.  
         [0067]     The stage  330  is connected to and spaced apart from the middle plate  320  by a connecting member  324 . The stage  330 , the connecting member  324 , the middle plate  320 , the torsion springs  340  and the anchors  342  may be made from a conductive material (e.g., polysilicon).  
         [0068]     The actuator according to the third exemplary embodiment of the invention differs from that of the first exemplary embodiment in that the rotation axis is at the stage  330 , but the operation thereof is substantially identical to that of the first exemplary embodiment, and a detailed explanation thereof will be omitted.  
         [0069]     As described before, according to the actuator of the invention, the middle plate is installed between the stage and the substrate, and can be driven with low driving voltage. In addition, if one side of the stage approaches the substrate, the driving angle can be increased by the electrostatic force between the stage and the driving electrode. Accordingly, the actuator of the invention can be used as an optical scanner in display devices.  
         [0070]     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the following claims.