Abstract:
A control method is provided to reduce the spring resonance of a voice coil motor when the coil current of the voice coil motor is changed. Each time a total variation for the coil current to be changed is identified and divided into a plurality of step variations applied one by one with a time step equal to one half of the spring resonant period of the voice coil motor. Due to reduction of the spring resonance, the control method speed up the voice coil motor to a steady state. With this control method, a lens focusing system has a shorter focusing time.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is related generally to a control method for a voice coil motor and, more particularly, to a control method for a voice coil motor for a lens focusing system. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a lens focusing system, as shown in  FIG. 1 , a control circuit  10  controls the coil current IL of a voice coil motor (VCM)  12  in order to adjust the position of a lens  14 . In the voice coil motor  12 , a spring  16  has one end fixed immovably and the other end fixed on a movable electromagnet  18  at whose opposite side is arranged a magnet  20 , so that the magnetic force produced by the coil current IL flowing through the coil  22  of the electromagnet  18  will interact with the magnet  20  to move the electromagnet  18  frontward or backward by controlling the value of the coil current IL. The lens  14  is driven by the electromagnet  18  in the manner that the movement of the electromagnet  18  will change the position of the lens  14 . To move the lens  14  frontward or backward, the control circuit  10  changes the coil current IL to change the magnetic force produced by the electromagnet  18 , thereby suddenly applying a force to the spring  16 , and then a new balance between the magnetic force and the recovery force of the spring  16  will be built up and thus determine the displacement d of the lens  14 . However, referring to  FIGS. 1 and 2 , when the coil current IL is changed by a variation A suddenly, the spring  16  will resonate with a gradually decayed amplitude over time and thus swing the lens  14  for a time interval. Only when the spring  16  stops resonating, the lens  14  becomes steady in position. Consequently, each time the lens  14  is moved, it requires waiting for a long focusing time. 
         [0003]    To reduce the resonance of the spring  16 , a popular solution is to change the coil current IL with a slower varying speed to decrease the instant force acting on the spring  16  and thereby reduce the resonant amplitude of the spring  16 . However, since the coil current IL is changed with a fixed varying slope, it will take a longer time for moving the lens  14  with a larger displacement d, due to the coil current IL requiring more time to reach the larger variation A. Recently, the volume of the lens  14  is more and more small and the weight of the lens  14  is more and more light. As a result, a small force acting on the spring  16  would cause the spring  16  to resonate with a large amplitude and thereby, the coil current IL needs a more slower varying speed to reduce the instant force acting on the spring  16 , and the voice coil motor  12  requires more time to reach a steady state accordingly. 
         [0004]    Therefore, it is desired a control method for a voice coil motor that can dramatically reduce the spring resonance of the voice coil motor and significantly speed up the voice coil motor to a steady state. 
       SUMMARY OF THE INVENTION 
       [0005]    An objective of the present invention is to provide a control method for a voice coil motor that can dramatically reduce the spring resonance of the voice coil motor. 
         [0006]    Another objective of the present invention is to provide a control method a voice coil motor that can significantly speed up the voice coil motor to a steady state. 
         [0007]    A further objective of the present invention is to provide a lens focusing system having a shorter focusing time. 
         [0008]    According to the present invention, a control method for a voice coil motor involves dividing a total variation for the coil current of the voice coil motor into a plurality of step variations applied one by one with a time step equal to one half of the spring resonant period of the voice coil motor. 
         [0009]    According to the present invention, a lens focusing system includes a voice coil motor and a control circuit to divide a total variation for the coil current of the voice coil motor into a plurality of step variations applied one by one with a time step equal to one half of the spring resonant period of the voice coil motor. 
         [0010]    In virtue of the characteristics of the spring resonance, the inventive method can dramatically reduce the spring resonance of a voice coil motor and significantly speed up a voice coil motor to a steady state, and thereby shorten the focusing time of a lens focusing system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other objectives, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic diagram of a lens focusing system; 
           [0013]      FIG. 2  is a schematic diagram showing a conventional control method for a voice coil motor and the lens displacement it causes; 
           [0014]      FIG. 3A  is a schematic diagram of a first embodiment according to the present invention; 
           [0015]      FIG. 3B  is a schematic diagram of the embodiment shown in  FIG. 3A  where n=1; 
           [0016]      FIG. 3C  is a schematic diagram of the embodiment shown in  FIG. 3A  where n=2; 
           [0017]      FIG. 4  is a schematic diagram showing how the control method of  FIG. 3A  reduces the spring resonance; 
           [0018]      FIG. 5  is a schematic diagram showing the lens displacement when using the control method of  FIG. 3A ; 
           [0019]      FIG. 6  is an HSPICE simulation result when using the control method of  FIG. 2 ; 
           [0020]      FIG. 7  is an HSPICE simulation result when using the control method of  FIG. 3B ; 
           [0021]      FIG. 8A  is a schematic diagram of a second embodiment according to the present invention; 
           [0022]      FIG. 8B  is a schematic diagram of the embodiment shown in  FIG. 8A  where n=1; 
           [0023]      FIG. 8C  is a schematic diagram of the embodiment shown in  FIG. 8A  where n=2; 
           [0024]      FIG. 9  is a schematic diagram showing how the control method of  FIG. 8A  reduces the spring resonance; 
           [0025]      FIG. 10A  is a schematic diagram of a third embodiment according to the present invention; 
           [0026]      FIG. 10B  is a schematic diagram of the embodiment shown in  FIG. 10A  where n=1; 
           [0027]      FIG. 10C  is a schematic diagram of the embodiment shown in  FIG. 10A  where n=2; and 
           [0028]      FIG. 11  is a schematic diagram showing how the control method of  FIG. 10A  reduces the spring resonance. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    For each voice coil motor, referring to  FIG. 2 , the spring thereof has a specific resonant period Tres, and the present invention uses the characteristics to reduce the spring resonance of the voice coil motor. 
         [0030]      FIG. 3A  is a schematic diagram of a first embodiment according to the present invention. Referring to  FIGS. 1 and 3A , in order to move the lens  14  with a target displacement d which requires a coil current IL, the control circuit  10  identifies the needed variation A for the coil current IL to be changed, and divides the total variation A into 2n+1 step variations, where n is a positive integer, and applies the 2n+1 step variations one by one with a time step equal to one half of the resonant period Tres of the spring  16 . In this embodiment, the first and the last step variations both have a value equal to 
         [0000]    
       
         
           
             
               A 
               
                 4 
                  
                 n 
               
             
             , 
           
         
       
     
         [0000]    while the rest of the step variations have a value equal to 
         [0000]    
       
         
           
             
               A 
               
                 2 
                  
                 n 
               
             
             . 
           
         
       
     
         [0000]    For example, referring to  FIG. 3B , when n=1, the number of the step variations for the coil current IL is three (2×1+1), the first and the third step variations both have a value equal to A/4, and the second step variation has a value equal to A/2; when n=2, as shown in  FIG. 3C , the total variation A for the coil current IL is divided into five (2×2+1) step variations, the first and the fifth step variations both have a value equal to A/8, and the second to the fourth step variation each has a value equal to A/4. The value of the parameter n is determined by the designer and may be constant or programmable. 
         [0031]      FIG. 4  illustrates the principles on which how the control method of  FIG. 3A  eliminates the spring resonance. Taking the case of n=1 for example, at time t 1 , the coil current IL is changed by the first step variation A/4, and as shown by waveform  24 , this step variation brings a resonance with an amplitude Amp 11  to the spring  16 ; at time t 2 , the coil current IL is changed by the second step variation A/2, and as shown by waveform  26 , this step variation brings a resonance with an amplitude Amp 12  to the spring  16 ; at time t 3 , the coil current IL is changed by the third step variation A/4, and as shown by waveform  28 , this step variation brings a resonance with an amplitude Amp 13  to the spring  16 . Since the first and third step variations both have a value equal to A/4, and the second step variation has a value equal to A/2, the corresponding amplitudes have the relationship 
         [0000]    
       
         
           
             
               Amp 
                
               
                   
               
                
               11 
             
             = 
             
               
                 Amp 
                  
                 
                     
                 
                  
                 13 
               
               = 
               
                 
                   
                     Amp 
                      
                     
                         
                     
                      
                     12 
                   
                   2 
                 
                 . 
               
             
           
         
       
     
         [0000]    Referring to  FIG. 4 , taking the third time step T 3  for example, waveform  24  has the amplitude +Amp 11 , waveform  26  has the amplitude −Amp 12 , and waveform  28  has the amplitude +Amp 13 , so that all of them mutually cancel in amplitude, and as shown in  FIG. 5 , there is almost no resonance in the spring  16 . As a result, the lens  14  becomes steady almost as soon as it is moved to the set position. 
         [0032]    Using HSPICE to simulate a same voice coil motor with the control methods as depicted in  FIGS. 2 and 3B  respectively, the results are shown in  FIGS. 6 and 7  respectively. Referring to  FIG. 6 , the conventional method where the total variation for the coil current is applied at one time always causes a spring resonance that has a very large amplitude and requires a very long time for the spring to become steady. Referring to  FIG. 7 , the control method of  FIG. 3B  achieves the target of the coil current with three step variations, and brings almost no resonance to the spring. By comparing the simulation results shown in  FIGS. 6 and 7 , it is evidenced that the control method according to the present invention can effectively eliminate the spring resonance and speed up a voice coil motor to a steady state. 
         [0033]      FIG. 8A  is a schematic diagram of a second embodiment according to the present invention. Referring to  FIGS. 1 and 8A , in order to move the lens  14  with a target displacement d which requires a coil current IL, the control circuit  10  identifies the needed variation A for the coil current IL to be changed, and divides the total variation A into 2(n+1) step variations, where n is a positive integer, and applies the 2(n+1) step variations one by one with a time step equal to one half of the resonant period Tres of the spring  16 . The first and the last step variations both have a value equal to 
         [0000]    
       
         
           
             
               A 
               
                 2 
                  
                 
                   ( 
                   
                     
                       2 
                        
                       n 
                     
                     + 
                     1 
                   
                   ) 
                 
               
             
             , 
           
         
       
     
         [0000]    while the rest of the step variations each has a value equal to 
         [0000]    
       
         
           
             
               A 
               
                 
                   2 
                    
                   n 
                 
                 + 
                 1 
               
             
             . 
           
         
       
     
         [0000]    As shown in  FIG. 8B , when n=1, the number of the step variations for the coil current IL is four (2(1+1)), the first and the fourth step variations each has a value equal to A/6, and the second and the third step variations each has a value equal to A/3; when n=2, as shown in  FIG. 8C , the total variation A for the coil current IL divided into six (2(2+1)) step variations, the first and the sixth step variations each has a value equal to A/10, and the second to the fifth step variation each has a value equal to A/5. 
         [0034]      FIG. 9  is a schematic diagram showing how the control method of  FIG. 8A  reduces the spring resonance. Taking the case of n=1 for example, at time t 1 , the coil current IL is changed by the first step variation A/6, and as shown by waveform  30 , this step variation brings a resonance with an amplitude Amp 21  to the spring  16 ; at time t 2 , the coil current IL is changed by the second step variation A/3, and as shown by waveform  32 , this step variation brings a resonance with an amplitude Amp 22  to the spring  16 ; at time t 3 , the coil current IL is changed by the third step variation A/3, and as shown by waveform  34 , this step variation brings a resonance with an amplitude Amp 23  to the spring  16 ; at time t 4 , the coil current IL is changed by the fourth step variation A/6, and as shown by waveform  36 , this step variation brings a resonance with an amplitude Amp 24  to the spring  16 . Since the first and fourth step variations both have a value equal to A/6, and the second and the third step variations both have a value equal to A/3, the corresponding amplitudes have the relationship 
         [0000]    
       
         
           
             
               Amp 
                
               
                   
               
                
               21 
             
             = 
             
               
                 Amp 
                  
                 
                     
                 
                  
                 24 
               
               = 
               
                 
                   
                     Amp 
                      
                     
                         
                     
                      
                     22 
                   
                   2 
                 
                 = 
                 
                   
                     
                       Amp 
                        
                       
                           
                       
                        
                       23 
                     
                     2 
                   
                   . 
                 
               
             
           
         
       
     
         [0000]    Taking the fourth time step T 4  for example, waveform  30  has the amplitude −Amp 21 , waveform  32  has the amplitude +Amp 22 , waveform  34  has the amplitude −Amp 23 , and waveform  36  has the amplitude +Amp 24 , so that all of them mutually cancel in amplitude, and thereby the spring  16  almost does not resonate. 
         [0035]      FIG. 10A  is a schematic diagram of a third embodiment according to the present invention. Referring to  FIGS. 1 and 10A , in order to move the lens  14  with a target displacement d which requires a coil current IL, the control circuit  10  identifies the needed variation A for the coil current IL to be changed, and divides the total variation A into 2n step variations having a same value 
         [0000]    
       
         
           
             
               A 
               
                 2 
                  
                 n 
               
             
             , 
           
         
       
     
         [0000]    where n is a positive integer, and applies the 2n step variations one by one with a time step equal to one half of the resonant period Tres of the spring  16 . When n=1, as shown in  FIG. 10B , the number of the step variations for the coil current IL is two (2×1), and each step variation has a value equal to A/2; when n=2, as shown in  FIG. 10C , the number of the step variations for the coil current IL is four (2×2), and each step variation has a value equal to A/4. 
         [0036]      FIG. 11  is a schematic diagram showing how the control method of  FIG. 10A  reduces the spring resonance. Taking the case of n=2 for example, at time t 1 , the coil current IL is changed by the first step variation A/4, and as shown by waveform  38 , this step variation brings a resonance with an amplitude Amp 31  to the spring  16 ; at time t 2 , the coil current IL is changed by the second step variation A/4, and as shown by waveform  40 , this step variation brings a resonance with an amplitude Amp 32  to the spring  16 ; at time t 3 , the coil current IL is changed by the third step variation A/4, and as shown by waveform  42 , this step variation brings a resonance with an amplitude Amp 33  to the spring  16 ; and at time t 4 , the coil current IL is changed by the fourth step variation A/4, and as shown by waveform  44 , this step variation brings a resonance with an amplitude Amp 34  to the spring  16 . Since each step variation has the same value equal to A/4, the corresponding amplitudes will have the relationship Amp 31 =Amp 32 =Amp 33 =Amp 34 . Taking the fourth time step T 4  for example, waveform  38  has the amplitude −Amp 31 , waveform  40  has the amplitude +Amp 32 , waveform  42  has the amplitude −Amp 33 , and waveform  44  has the amplitude +Amp 34 , so that all of them mutually cancel in amplitude, and thereby the spring  16  almost does not resonate. 
         [0037]    While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.