Abstract:
A resonance type torsional vibrator capable of switching to an object driving frequency is provided, which comprises a frequency switching means capable of switching an excitation frequency between at least two levels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an optical deflector, which is fabricated by using the MEMS (Micro Electro Mechanical Systems) technology, and moreover, it relates to an image forming apparatus using this optical deflector.  
         [0003]     2. Related Background Art  
         [0004]     In an optical deflector having a mirror portion, in order to obtain a large deflection angle by a small power consumption, there is generally utilized a resonance phenomenon of a structural member having a mirror portion and an elastic support member for supporting the mirror portion. As an example of the optical deflector for making this resonance frequency variable, there is known an optical deflector disclosed in Japanese Patent Application Laid-Open No. 2002-202474.  
         [0005]      FIG. 8  is a schematic view explaining the optical deflector of a frequency variable type disclosed in the above patent document. This optical deflector comprises a mirror portion  1002 , a pair of elastic support beams  1003  integrally formed with the mirror portion  1002  along an oscillation axis which passes through a center of gravity of the mirror portion  1002 , a substrate  1005  for holding the pair of elastic support beams  1003 , and a drive means  1015  for oscillating the mirror portion  1002 . An excitation frequency generation means  1018  provides an excitation frequency to the drive means  1015 , and moreover, the frequency thereof is compared with an output of a resonance frequency detection means  1019  for detecting the resonance frequency of the mirror potion  1002  by a comparator  1017 . Further, a control means  1016 , by using a beam binding means  1007 , varies the binding state of the pair of elastic support beams  1003  to vary an intrinsic elastic constant of the elastic support beams  1003 , and performs a control in such a way that the output of the comparator  1017  becomes zero. In this manner, at an arbitrary frequency generated by the excitation frequency generation means  1018 , the mirror portion  1002  can mechanically be driven in a resonance state.  
         [0006]     The present invention is to solve the following problems in relation to the optical deflector of the frequency variable type.  
         [0007]     The structure is complicated and the production cost thereof is high.  
         [0008]     A separate frequency-varying mechanism other than a drive mechanism is required, and therefore, the power consumption is large.  
         [0009]     A friction loss is generated in the binding means and the elastic support beams, so that the Q value of resonance is lowered.  
         [0010]     Wear is generated in the binding means and the elastic support beams so that change in the resonance characteristics with time is generated.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention has been accomplished to solve the above-described problems, and a first aspect of the present invention is a torsional vibrator, comprising: 
        a plurality of torsion springs and a plurality of vibrators alternatively connected, torsional axes of all the plurality of torsion springs being arranged in the same straight line and an end portion of at least one of the plurality of torsion springs being fixed to a fixing portion;     an excitation (or driving) means for imparting a torsional vibration to at least one of the plurality of vibrators; and     a frequency switching means for switching an excitation frequency of the excitation means between at least two levels,     wherein the vibrator vibrates resonantly at the at least two levels of frequencies by being imparted with the torsional vibration.        
 
         [0016]     In the present invention, it is preferable that the excitation means is an electrostatic actuator.  
         [0017]     Further, it is preferable that the excitation means is an electromagnetic actuator.  
         [0018]     Moreover, it is preferable that the excitation means is a piezoelectric actuator.  
         [0019]     Further, a second aspect of the present invention is an optical deflector comprising the above-mentioned torsional vibrator wherein at least one of the plurality of vibrators has a light deflecting means.  
         [0020]     Moreover, a third aspect of the present invention is an image forming apparatus comprising a light source, a light source modulating means for modulating the light source, the above-mentioned optical deflector, and a control means for controlling the light source modulating means and the optical deflector.  
         [0021]     According to the present invention, because a complicated frequency-switching mechanism is not used, a frequency variable torsional vibrator and a resonance type optical deflector can be provided.  
         [0022]     Further, because a separate frequency-varying mechanism other than a drive mechanism is not required, the power consumption can be reduced.  
         [0023]     Moreover, because a binding means is not required, the friction loss is reduced and the Q value of resonance can be made high, thereby reducing the power consumption.  
         [0024]     Further, because there exists no wearing portion, the change in the resonance characteristics with time can be reduced.  
         [0025]     Moreover, by using the resonance type optical deflector of the present invention, a light scanning display capable of switching a scanning frequency can be provided. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1  is a view explaining a resonance type optical deflector of Example 1;  
         [0027]      FIGS. 2A and 2B  are views explaining a vibration mode of the resonance type optical deflector of Example 1;  
         [0028]      FIG. 3  is a view explaining a principle of operation of the present invention;  
         [0029]      FIGS. 4A and 4B  are views explaining a principle of operation of the present invention;  
         [0030]      FIG. 5  is a view explaining a resonance type optical deflector of Example 2;  
         [0031]      FIGS. 6A, 6B  and  6 C are views explaining a vibration mode of the resonance type optical deflector of Example 2;  
         [0032]      FIG. 7  is a view explaining a light scanning display of Example 3; and  
         [0033]      FIG. 8  is a view explaining a conventional resonance frequency variable type optical deflector. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     First, reference numerals shown in the figures will be described.  
         [0035]     Reference numeral  004  denotes a fixing portion, reference numerals  011  and  012  a torsional vibrator, reference numeral  021  and  022  a torsion spring, reference numeral  050  an excitation means, reference numeral  104  a fixing frame, reference numerals  111  to  112  a vibrator, reference numerals  121  to  124  a torsion spring, reference numeral  150  an excitation means, reference numeral  204  a fixing frame, reference numerals  211  to  215  a torsional vibrator, reference numerals  221  to  226  a torsion spring, reference numeral  301  a resonance type optical deflector, reference numeral  302  an optical deflector, reference numeral  303  a laser light source, reference numeral  304  a control means, reference numeral  310  a laser light, and reference numeral  320  a screen.  
         [0036]     A principle of operation of the resonance type vibrator of the present invention will be described.  FIG. 3  is a schematic view of the resonance type vibrator of the present invention. A torsion spring  021 , a torsional vibrator  011 , a torsion spring  022 , and a torsional vibrator  012  are connected in the mentioned order on the same axis, and the torsion spring  021  is connected to a fixing portion  004 .  
         [0037]     Where the moment of inertial about axis and the displacement angle of the vibrators  011  and  012  are represented by I 1 , θ 1 , I 2 , and θ 2 , respectively, and the spring constants of the torsion springs  021  and  022  are represented by k 1  and k 2 , and a damping term is disregarded, the dynamic equation of the vibrator  011  and the torsional vibrator  012  can be given as follows.  
                   (           I   1         0           0         I   2           )     ⁢     (             θ   ¨     1                 θ   ¨     2           )       +       (             k   1     +     k   2             -     k   2                 -     k   2             k   2           )     ⁢     (           θ   1               θ   2           )         =     (         0           0         )                   (             θ   ¨     1                 θ   ¨     2           )     =         -       (           I   1         0           0         I   2           )       -   1         ⁢     (             k   1     +     k   2             -     k   2                 -     k   2             k   2           )     ⁢     (           θ   1               θ   2           )       =     M   ⁡     (           θ   1               θ   2           )                     M   =       -       (           I   1         0           0         I   2           )       -   1         ⁢     (             k   1     +     k   2             -     k   2                 -     k   2             k   2           )                 
 
         [0038]     At this time, the eigenvalue and the eigenvector of M represent a square of an angular frequency ω and a vibration mode, respectively. Here, by appropriately designing the motion of inertia and the spring constant, it is possible to set the eigenvalue to a desired value. The state of this resonant vibration is shown in  FIGS. 4A and 4B .  FIGS. 4A and 4B  are views showing the state of vibration of the vibrator when observed in the direction of the arrow in  FIG. 3 . In this example, there exist two modes including mode  1  ( FIG. 4A ) of vibrating with θ 1  and θ 2  being in phase, and a mode  2  ( FIG. 4B ) of vibrating with θ 1  and θ 2  being in opposite phase.  
         [0039]     Further, as is easily seen, the number of vibration modes can be increased to two or more by additionally connecting vibrators and torsion springs.  
         [0040]     Moreover, by giving a driving torque at a driving frequency approximately equal to any one of these resonance modes by the excitation means  050 , the torsional vibrator can be driven resonantly. By switching this resonance frequency, the driving frequency of the torsional vibrator can be selected.  
         [0041]     Further, by providing an optical deflector component on at least one of the torsional vibrators, a resonance type optical deflector can be attained.  
         [0042]     Moreover, by using the resonance type optical deflector of the present invention, a light scanning display capable of switching a scanning frequency can be provided.  
       EXAMPLE 1  
       [0043]      FIG. 1  is a plan view showing a resonance type light scanner of Example 1. A frame shaped vibrator  111  is connected to a fixing frame  104  via torsion springs  121  and  124 , and a vibrator  112  is connected to the inner side of the vibrator  111  via torsion springs  122  and  123 . In this case, a configuration is adopted such that the torsional axes of the torsion springs  121 ,  122 ,  123 , and  124  are in line with the principal axes of inertial of the vibrators  111  and  112 , and these are formed integrally by etching a silicon wafer. On a surface of the vibrator  112  is formed a light deflecting layer. The excitation means  150  imparts a driving torque to the vibrators  111  and  112 . Specifically, examples of the excitation means include an electrostatic actuator using opposing electrodes, an electromagnetic actuator using an electromagnetic force which acts on a magnetic substance, a stacked piezoelectric element, and the like. Further, they may be vacuum-sealed to increase the Q value of resonance, thereby reducing the power consumption.  
         [0044]     The sizes of the vibrators  111  and  112  of the present example shown in  FIG. 1  are a 1 =2400 μm, a 2 =1600 μm, a 3 =1200 μm, b 1 = 3800  μm, b 2 =3000 μm, and b 3 =1000 μm. Where the thickness t of the silicon wafer is 150 μm, and the density ρ thereof is 2330 kgm −3 , then the moments of inertial about torsional axis I 1  and I 2  become I 1 =1.175×10 −12  [kgm 2 ], and I 2 =5.111×10 −14  [kgm 2 ]. Where the spring constants k 1  and k 2  of the torsion of the torsion springs  121  and  122  are k 1 =2.123×10 −2  [Nm/rad], and k 2=1.156 ×10 −3  [Nm/rad], then  
       M   =     (           1.905   ×     10   10               -   9.838     ×     10   8                   -   2.262     ×     10   10             2.262   ×     10   10             )         
 
 is established, and therefore, the eigenvalues and eigenvectors of M become as follow.  
                 λ   1     =     1.579   ×     10   10         ,                         v   1     =     (         0.3018           1         )                     λ   2     =     2.587   ×     10   10         ,                         v   2     =     (           -   0.1441             1         )               
 
         [0045]     Because an eigenvalue is a square of an angular frequency, resonance frequencies f 1  and f 2  become as follow. 
 
 f   1 ={square root}{square root over (λ 1 )}/2π=20.0×10 3  
 
 f   2 ={square root}{square root over (λ 2 )}/2π=25.6×10 3  
 
         [0046]     That is, this resonance type mirror has two vibration modes of 20.0 kHz and 25.6 kHz. When resonating at 20.0 kHz, the amplitude angle of the vibrator  111  is 0.3018 times that of the mirror  112 , and the vibrator  111  and the mirror  112  vibrate in phase, and when resonating at 25.6 kHz, the amplitude angle of the vibrator  111  is 0.1441 times that of the mirror  112 , and the vibrator ill and the mirror  112  vibrate in opposite phase.  
         [0047]     These two resonance frequencies are allowed to correspond to, for example, two display modes of SVGA (800×600 pixels) and XGA (1024×768 pixels) in a luster scanning display. That is, the resonance type optical deflector of the present example can be used while switching two vibration modes of the SVGA display and the XGA display.  
         [0048]     As described above, according to the present invention, a frequency variable, resonance type optical deflector can be provided without using a complicated frequency-switching mechanism.  
         [0049]     Further, because a separate frequency-varying mechanism other than a driving mechanism is not required, the power consumption can be reduced.  
         [0050]     Moreover, because a binding means is not required, the friction loss is reduced and the Q value of resonance can be increased, thereby reducing the power consumption.  
         [0051]     Further, because there exists no wearing portion, the change in the resonance characteristics can be reduced.  
       EXAMPLE 2  
       [0052]      FIG. 5  is a view explaining an optical deflector of Example 2 of the present invention. A fixing frame  204 , torsional vibrators  211 ,  212 ,  213 ,  214 , and  215 , and torsion springs  221 ,  222 ,  223 ,  224 ,  225 , and  226  are made integrally by etching a silicon wafer. The torsional vibrators  211  to  215  and the torsion springs  221  to  226  are connected in the order as shown in  FIG. 5 , and the torsion springs  221  and  226  are connected to the fixing frame  204 . Further, on the central torsional vibrator  213  is formed a light reflecting surface. Further, excitation is effected by a means similar to that of Example 1.  
         [0053]     The sizes of the torsional vibrators  211  to  215  are a 1 =4000 μm, b 1 =200 μm, a 2 =3000 μm, b 2 =200 μm, a 3 =1200 μm, and b 3 =1000 μm.  
         [0054]     The sizes of torsion springs  221  to  226  are I 1 =100 μm, I 2 =200 μm, I 3 =1000 μm, and w=50 μm.  
         [0055]     Assuming that the density and the shear modulus of the silicon material used are 2330 kgm −3  and 65 Gpa respectively and the thickness of the silicon wafer is 150 μm, the moments of inertia about axis I 1  to I 5  of the torsional vibrators  211  to  215  are I 1 =3.733×10 −13  [kgm 2 ], I 2 =1.577×10 −13  [kgm 2 ], I 3 =5.111×10 −14  [kgm 2 ], I 4 =1.577×10 −13  [kgm 2 ], and 15=3.733×10 −13  [kgm 2 ], and the spring constants k 1  to k 6  of the torsion springs  221  to  226  become k 1 =3.209×10 −3  [Nm/rad], k 2 =1.604×10 −3  [Nm/rad], k 3 =3.209×10 −4  [Nm/rad], k 4 =3.209×10 −4  [Nm/rad], k 5 =1.604×10 −3  [Nm/rad], and k 6 =3.209×10 −3  [Nm/rad]. Then,  
             M   =           (           I   1                                                                           I   2                                                                           I   3                                                                           I   4                                                                           I   5           )       -   1       ⁢     (             k   1     +     k   2             -     k   2                                                     -     k   2               k   2     +     k   3             -     k   3                                                     -     k   3               k   3     +     k   4             -     k   4                                                     -     k   4               k   4     +     k   5             -     k   5                                                     -     k   5               k   5     +     k   6             )       =                 (           4.018   ×     10   9               -   1.339     ×     10   9                                                       -   3.171     ×     10   9             3.805   ×     10   9               -   6.342     ×     10   8                                                       -   1.956     ×     10   9             3.913   ×     10   9               -   1.956     ×     10   9                                                       -   6.342     ×     10   8             3.805   ×     10   9               -   3.171     ×     10   9                                                       -   1.339     ×     10   9             4.018   ×     10   9             ⁢           )             
 
 is established, and since the eigenvalues λ 1-5  of M are λ 1 =4.160×10 9 , λ 2 =5.930×10 9 , λ 3 =1.268×10 10 , λ 4 =1.917×10 10 , and λ 5 =2.082×10 10 , the resonance frequencies are f 1 =10.26×10 3  [Hz], f 2 =12.26×10 3  [Hz], f 3 =17.92×10 3  [Hz], f 4 =22.04×10 3  [Hz], and f 5 =22.96×10 3  [Hz]. 
 
         [0056]     Further, the eigenvectors v 1-5  are given by  
                 v   1     =     (         1           2.032           3.039           2.032           1         )       ,               v   2     =     (           -   1               -   1.620             0           1.620           1         )       ,               v   3     =     (         1           0.0496             -   5.011             0.0496           1         )       ,                   v   4     =     (           -   1             1.461           0             -   1.461             1         )       ,             v   5     =     (         1             -   1.844             2.802             -   1.844             1         )                           
 
         [0057]     Of these five vibration modes, the mode that can be used for optical scanning is those modes in which the central torsional vibrator  213  is displaced, i.e., v 1 , v 3  and v 5 . The state of vibration at this time is shown in  FIGS. 6A, 6B  and  6 C. The  FIGS. 6A, 6B  and  6 C correspond to v 1 , v 3  and v 5 , respectively.  
         [0058]     Hence, by exciting the torsional vibrators  211  to  215  by the excitation means at frequencies approximately close to the frequencies of f 1 =10.26×10 3  [Hz], f 3 =17.92×10 3  [Hz], and f 5 =22.96×10 3  [Hz], resonance oscillation can be effected at these frequencies.  
         [0059]     As described above, according to the present invention, a frequency variable, resonance type optical deflector can be provided without using a complicated frequency-switching mechanism.  
         [0060]     Further, because a separate frequency-varying mechanism other than a drive mechanism is not required, the power consumption can be reduced.  
         [0061]     Further, because a binding means is not required, the friction loss is reduced and the Q value of resonance can be made high, thereby reducing the power consumption.  
         [0062]     Further, because there exists no wearing portion, the change in the resonance characteristics with time can be reduced.  
       EXAMPLE 3  
       [0063]      FIG. 7  is a schematic view for explaining a light scanning display in accordance with the present invention. A laser light  310  emitted from a laser light source  303  is scanned in a horizontal direction by a resonance type optical deflector  301  of the present invention, is then scanned in a vertical direction by an optical deflector  302  such as a galvano mirror and the like, and forms an image on a screen  320 . The resonance type optical deflector  301 , the optical deflector  302  and the laser light source  303  are controlled by a control means  304 .  
         [0064]     By using the resonance type optical deflector of the present invention, the light scanning display of the present invention can easily perform switching of a driving frequency when performing switching of resolution.  
         [0065]     This application claims priority from Japanese Patent Application No. 2003-417977 filed on Dec. 16, 2003, which is hereby incorporated by reference herein.