Abstract:
A method of controlling a moving part of a voice coil motor to move from an first position to a second position, wherein the position of the moving part is controlled by the level of an electrical signal applied to a coil of the voice coil motor, a first level of the electrical signal corresponding to the first position, and a second level of the electrical signal corresponding to the second position, the method including: at a first time, changing the electrical signal from the first level to an intermediate level, the intermediate level being chosen such that a peak overshoot of the moving part corresponds to the second position; and at a second time calculated to correspond to a delay of half an oscillation period of the moving part after the first time, changing the electrical signal to the second level.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application number 07/58472, filed on Oct. 22, 2007, entitled “VCM Control Circuit,” which is hereby incorporated by reference to the maximum extent allowable by law. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and a control circuit for controlling a voice coil motor. 
       BACKGROUND OF THE INVENTION 
       [0003]      FIG. 1  illustrates schematically in cross-section a mobile lens unit  100  comprising a cylindrical lens barrel  102  with a number of fixed lenses  104 ,  106  and  108  mounted therein, the lens barrel  102  having a conducting coil  110  mounted within its walls. The coil  110  is formed of a wire represented in cross-section by small circles with dots to represent a current flow one way in the wire, and crosses to represent a current flow in the opposite direction. Mobile lens unit  100  also comprises a housing  112 , which is for example formed of plastic, and has rims  113 ,  114  at the top and bottom respectively, which extend some way towards the center of the housing, and act as stoppers for the lens barrel  102 . A permanent magnet  115 , which is for example also cylindrical, is mounted against the inner surface of the housing  112  and lies adjacent to the lens barrel  102 . The coil  110 , and the permanent magnet  115  together form a voice coil motor (VCM). Springs  116  and  118  are connected between the housing  112  and the lens barrel  102 . 
         [0004]    Coil  110  is connected to a current source, in this example by conductive springs  116  and  118 . In particular, springs  116  and  118  connect opposite ends of coil to a control circuit  120 , which comprises a variable current source coupled between spring  116  and ground, and a supply voltage Vdd connected to spring  118 . 
         [0005]    In operation, the mobile lens unit  100  is mounted over an image sensor (not shown), and the positioning of the lenses with respect to the image sensor can be adjusted by moving the lens barrel  102  up and down within the housing, which can be achieved by passing a current signal through the coil  110 . Springs  116 ,  118  hold the lens barrel  102  at an first position at the bottom end of the housing resting against rim  113  when no current is applied to the coil. When current is applied to the coil, due to electromagnetic force generated by the current flow in a magnetic field, the lens barrel moves towards the top of the housing, counteracting the force of the springs  116 ,  118 . Springs  116 ,  118  have an increasing restoring force the further the lens barrel is from its first position, meaning that the lens barrel rests at in equilibrium at a certain position depending on the current level applied to the coil. 
         [0006]      FIG. 2A  illustrates the current I applied to the coil for changing the position of the lens barrel  102  in the housing  112 . As illustrated, the current is initially at an value I i , which maintains the lens barrel at the first position. The first position could be with the lens barrel  102  at the bottom of the housing  112 , or at any position within the lens housing. At a time t 0 , a new current I f  is applied, which has a value corresponding to the current that is needed to keep the lens barrel at the final position. 
         [0007]      FIG. 2B  illustrates the position x of the lens barrel in response to the change in the current at time t 0 . As illustrated, from the initial position x i , the lens barrel moves towards the final position x f . However, the lens barrel  102  and springs  116 ,  118  form a mass spring system that oscillates before settling at the final position x f . Damping in the system is very low, as the lens barrel is designed with very little friction, in order to reduce energy consumption. Thus the oscillations can continue for some time before the lens barrel settles at the final position. These oscillations cause problems, as the mobile lens unit cannot be used to capture images until the oscillations of the lens barrel have settled to within a certain amplitude, below which they do not affect the focusing of the lens. This settling time depends on the decay rate of the system, and can be several hundred milliseconds, meaning that there is a considerable delay before the lens can be used. In many optical applications, this delay is unacceptable. 
         [0008]    This problem occurs when a lens barrel is moved upwards in the lens unit, by a step increase in the current, and also when the lens barrel is moved downwards in the lens unit, by a step decrease in the current. A further problem that can occur when the lens barrel  102  is to be moved to a final position close to the end stops provided by rims  113 ,  114  is that the oscillations cause an impact between the lens barrel and the end stops, which could damage the lens barrel and create unwanted noise. 
         [0009]    One solution to these problems that has been proposed is to slowly change the current in the coil from the initial current to the final current. While this can help reduce the magnitude of oscillations, there is still a considerable delay before the lens unit can be used, particularly in smaller lens barrels designed for miniature application such as mobile telephones in which the friction of the lens barrel is extremely low. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an aim of embodiments of the present invention to at least partially address one or more problems in the prior art. 
         [0011]    According to one aspect of the present invention, there is provided a method of controlling a moving part of a voice coil motor to move from a first position to a second position, wherein the position of the moving part is controlled by the level of an electrical signal applied to a coil of the voice coil motor, a first level of the electrical signal corresponding to the first position, and a second level of the electrical signal corresponding to the second position, the method comprising: at a first time, for example when the moving part is stationary with respect to the rest of the voice coil motor, changing the electrical signal from the first level to an intermediate level, the intermediate level being chosen such that a peak overshoot of the moving part corresponds to the second position; and at a second time calculated to correspond to a delay of half an oscillation period of the moving part after the first time, changing the electrical signal to the second level. 
         [0012]    According to an embodiment of the present invention, the electrical signal is a current signal, and the first level, second level and intermediate level are current levels. 
         [0013]    According to another embodiment of the present invention, the intermediate electrical signal is calculated based an electrical signal corresponding to an intermediate position of the moving part, according to the following formula: 
         [0000]    
       
         
           
             
               x 
               s 
             
             = 
             
               
                 
                   ( 
                   
                     
                       x 
                       f 
                     
                     · 
                     
                       - 
                       
                         x 
                         i 
                       
                     
                   
                   ) 
                 
                 · 
                 Φ 
               
               + 
               
                 x 
                 i 
               
             
           
         
       
       
         
           
             
               wherein 
                
               
                 : 
               
                
               
                   
               
                
               Φ 
             
             = 
             
               1 
               
                 
                   
                     - 
                     
                       t 
                       d 
                     
                   
                   
                      
                     τ 
                   
                 
                 + 
                 1 
               
             
           
         
       
     
         [0014]    and wherein x i  is the first position, x S  is the intermediate position, x f  is the second position, t d  is the delay of half an oscillation period after the first time, and τ is a time constant of decay of oscillations of the moving part. 
         [0015]    According to an embodiment of the present invention, the intermediate level and the second level of the electrical signal are determined from look-up tables based on the intermediate position and the new position respectively, wherein there is preferably a linear relationship between the levels of the electrical signal and the positions of the moving part. 
         [0016]    According to an embodiment of the present invention, the intermediate electrical signal and the delay of half an oscillation period of the moving part are calculated using constant values relating to a time constant of decay of oscillation of the moving part and on an oscillating frequency of the moving part, the constant values being stored in a memory. 
         [0017]    According to an embodiment of the present invention, the method further comprises determining, during a calibration phase, the constant values based on measurements of oscillations of the moving part in response to a change in the electrical signal, wherein the constant values are, for example, stored in a non-volatile memory which is accessible during control of the voice coil motor. 
         [0018]    According to an embodiment of the present invention, the moving part comprises lens of a mobile lens unit. 
         [0019]    According to another aspect of the present invention, there is provided control circuitry for controlling a moving part of a voice coil motor to move from a first position to a second position, wherein the position of the moving part is controlled by the level of an electrical signal applied to a coil of the voice coil motor, a first level of the electrical signal corresponding to the first position, and a second level of the electrical signal corresponding to the second position, the control circuitry comprising: a processor arranged to determine an intermediate level of the electrical signal between the first and second levels, the intermediate level calculated based on the second position, an oscillating frequency of the moving part and on a time constant of decay of the oscillations of the moving part, and being calculated to correspond to a level that causes a first overshoot of the moving part having a peak displacement corresponding to the new position; and driving circuitry coupled to the coil and arranged to apply the intermediate level to the coil at a first time and to apply the second level at a second time calculated to correspond to a delay of half an oscillation period of the moving part after the first time. 
         [0020]    According to a further aspect of the present invention, there is provided an electronic device comprising a voice coil motor, a non-volatile memory arranged to store at least one parameter relating to a time constant of decay of oscillation of the moving part and on an oscillating frequency of the moving part, and the above control circuitry. 
         [0021]    According to a further aspect of the present invention, there is provided an optical device comprising a mobile lens unit comprising a lens barrel and a voice coil motor, an image sensor, a memory, and the above control circuitry. 
         [0022]    According to a further aspect of the present invention, there is provided a digital camera and/or a mobile telephone comprising the above optical device. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0023]    The foregoing and other purposes, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which: 
           [0024]      FIG. 1  (described above) illustrates a mobile lens unit; 
           [0025]      FIG. 2A  (described above) is a graph illustrating a current change applied to the coil of  FIG. 1 ; 
           [0026]      FIG. 2B  (described above) is a graph illustrating displacement of the lens barrel of  FIG. 1  in response to the current change of  FIG. 2A ; 
           [0027]      FIG. 3  is a graph illustrating current changes applied to the coil of  FIG. 1  according to an embodiment of the present invention; 
           [0028]      FIG. 4  is a graph illustrating displacement of the lens barrel of  FIG. 1  in response to the current changes of  FIG. 3 ; 
           [0029]      FIG. 5  illustrates circuitry for controlling position of a lens barrel according to an embodiment of the present invention; and 
           [0030]      FIG. 6  illustrates an optical device according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 3  is a graph illustrating current changes applied to the coil  110  of  FIG. 1  in order to change the position of the lens barrel  102 . In this example the current is to be increased from an initial value I i  to a final value I f , to move the lens barrel upwards, from a first position to a second position. As illustrated, the current is increased in two steps. At time t 0 , the current is increased from the initial current I i  to a step value I S , which is between the initial current I i  and the final current I f . Then, at time t 1 , the current is again increased to bring it to the final current I f , which has a level corresponding to the current for maintaining the lens barrel at the second position. 
         [0032]    In alternative embodiments in which the position of the lens barrel is to be lowered in the housing, the initial current I i  is higher than the final current I f . In this case, the current is decreased at time t 0  from I i  to an intermediate value I S  between I i  and I f , and then at time t 1  it is decreased to I f . 
         [0033]    The value I S  and the time of t 1  at which the final current I f  is applied are chosen to avoid oscillations of the lens barrel as will now be explained with reference to  FIG. 4 . 
         [0034]      FIG. 4  is a graph illustrating displacement x of the lens barrel  102  in response to the current changes of  FIG. 3 . As illustrated, the lens barrel  102  starts at a first position x i . At time t 0 , when current I S  is applied, the position of the lens barrel changes rapidly, accelerating until it reaches its maximum velocity at position x S , which is the position associated with current I S , and then overshooting. As soon as the lens barrel passes x S , it starts decelerates, until it reaches the peak of the first oscillation, when its speed falls to zero. Current I S  is chosen such that this maximum displacement is the desired final position x f  of the lens barrel. At this point, the final current I f  is applied, at time t 1 . This change to I f  prevents the lens barrel accelerating back towards position x S , and the final current I f  maintains the lens barrel at the second position x f , with little or no oscillation. Thus the lens barrel is moved very quickly from the initial position to the final position, with a very short settling time. 
         [0035]    According to the above method, there are two parameters that are controlled to change the position of the lens barrel. Firstly, the value of the current I S  applied at to is chosen such that the peak of the first oscillation caused by I S  corresponds to the desired second position x f  of the lens barrel. Secondly, the time t 1  that the final current I f  is applied is chosen such that it corresponds to the peak displacement of the lens barrel, when it reaches position x f . Time t 1  is preferably determined using time to as the reference, and thus in the following the delay t d  between t 0  and t 1 , which is equal to t 1 −t 0 , will be referred to. 
         [0036]    In order to calculate I S  and t d , the response of a mass spring system to a step-change in force applied to a mass can be used to model the displacement of the lens barrel in response to the electromagnetic force generated by a change in current. This gives a displacement x(t) of the type: 
         [0000]    
       
         
           
             
               
                 
                   
                     x 
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     
                       x 
                       end 
                     
                     + 
                     
                       
                         ( 
                         
                           
                             x 
                             ini 
                           
                           - 
                           
                             x 
                             end 
                           
                         
                         ) 
                       
                       · 
                       
                          
                         
                           - 
                           
                             t 
                             τ 
                           
                         
                       
                       · 
                       
                         ( 
                         
                           
                             cos 
                              
                             
                               ( 
                               
                                 ω 
                                 · 
                                 t 
                               
                               ) 
                             
                           
                           + 
                           
                             
                               1 
                               
                                 ω 
                                 · 
                                 τ 
                               
                             
                             · 
                             
                               sin 
                                
                               
                                 ( 
                                 
                                   ω 
                                   · 
                                   t 
                                 
                                 ) 
                               
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0037]    where t is time, x end  is the second position, x ini  is the first position, τ is the time constant of decay of the oscillation of the mass, which results from friction in the system, and ω is the oscillating frequency of the system. 
         [0038]    The first peak in the displacement when the speed falls to zero corresponds to a time after the change in force equal to half the time period of an oscillation, which is equal to 2π/ω. Thus t d =π/ω, which can be calculated by knowing the oscillating frequency, which is substantially constant for a given lens barrel. 
         [0039]    The peak displacement is x end +(x end −x ini )*e −t/π . If the above model is applied to the application of the intermediate current I S  to the voice coil motor, the first position x ini  corresponds to position x i  of the lens barrel, the end position x end  corresponds to the intermediate position x S , and the peak displacement corresponds to the desired second position x f  of the lens barrel. Thus the intermediate position x S , adapted for a start position x i  at t 0 , and to incur a peak of x f  at delay t d  after t 0 , can be expressed as follows: 
         [0000]    
       
         
           
             
               
                 
                   
                     x 
                     s 
                   
                   = 
                   
                     
                       
                         
                           x 
                           f 
                         
                         + 
                         
                           
                              
                             
                               
                                 - 
                                 
                                   t 
                                   d 
                                 
                               
                               τ 
                             
                           
                           · 
                           
                             x 
                             i 
                           
                         
                       
                       
                         
                            
                           
                             
                               - 
                               
                                 t 
                                 d 
                               
                             
                             τ 
                           
                         
                         + 
                         1 
                       
                     
                     = 
                     
                       
                         
                           
                             x 
                             f 
                           
                           · 
                           
                              
                             
                               
                                 t 
                                 d 
                               
                               τ 
                             
                           
                         
                         + 
                         
                           x 
                           i 
                         
                       
                       
                         
                            
                           
                             
                               t 
                               d 
                             
                             τ 
                           
                         
                         + 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0040]    This can we rewritten as: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       x 
                       s 
                     
                     = 
                     
                       
                         
                           ( 
                           
                             
                               x 
                               f 
                             
                             · 
                             
                               - 
                               
                                 x 
                                 i 
                               
                             
                           
                           ) 
                         
                         · 
                         Φ 
                       
                       + 
                       
                         x 
                         i 
                       
                     
                   
                    
                   
                     
 
                   
                    
                   
                     
                       wherein 
                        
                       
                         : 
                       
                        
                       
                           
                       
                        
                       Φ 
                     
                     = 
                     
                       1 
                       
                         
                           
                             - 
                             
                               t 
                               d 
                             
                           
                           
                              
                             τ 
                           
                         
                         + 
                         1 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0041]    x S  can be determined based on the first position x i , final desired position x f , and parameter Φ, which is based on the time delay t d  and the time constant of decay τ. As shown in formula (3) above, when based on Φ, the calculation becomes only a subtraction, multiplication and addition, which can be easily performed, for example by a low powered processor. Once x S  has been calculated, the value of I S  can be determined, for example using a lookup table or known relationship between the current in the coil and corresponding displacement of the lens barrel. The current/displacement relationship is for example linear, and of the form x i =aI S +x 0 , where “a” is a proportionality factor and x 0  an offset. 
         [0042]    Thus, to change the position of the lens barrel from an first position x i  to a second desired position x f , and knowing the time delay t d  and the time constant of decay τ of the oscillation of the mobile lens unit, a current I S  is first applied at time t 0 , calculated based on the associated position x S  determined using formula (2) above, and then a time delay t d  after t 0 , at time t 1 , the final current I f  is applied, determined based on the second desired position x f  of the lens barrel. 
         [0043]    The values for the constants t d  and τ can, for example, be stored in a non-volatile memory associated with the mobile lens unit. Assuming that manufacturing processes for producing the mobile lens units have a high level of precision, it may be sufficient to use the same constants for a series of mobile lens units. However, as manufacturing variations are possible, it is preferable that these constants are determined and stored during a calibration phase for a particular device. There are a number of ways they can be determined. One possible solution would be measuring the oscillations of a lens unit after applying a step change in the current, using a position sensor. Alternatively, the position of the lens barrel could be tracked by measuring the back EMF (electromotive force) voltage in the coil generated by the moving lens barrel. 
         [0044]    In some embodiments, when the initial position of the lens barrel is in contact with one of the end stops provided by rims  113 ,  114  of  FIG. 1 , the algorithm is adapted to compensate for the current level that should be reached to separate the lens barrel from either of these end stops. For example, a certain minimum current I min  should be applied to start linear movement of the lens barrel when the lens barrel is in contact with the rim  114  at the bottom of the lens housing. Likewise, the current should fall to I max  before the lens barrel leaves the upper rim  113  of the lens housing. The values of I min  and I max  could be stored in memory, or alternatively, as these values can vary according to the orientation of the lens barrel due to gravity, they can be recalculated at regular intervals, using sensors to detect when the barrel is in contact with the rims. The currents I S  and I f , which are determined based on positions x S  and x f  respectively can thus be determined taking into account these offsets. 
         [0045]    If the final position of the lens barrel x f  is determined as being in contact with one of the end stops provided by rims  113 ,  114 , an alternative algorithm to this described above could be applied to prevent impact between the lens barrel and one of the end stops. For example, the above method could be used to move the lens barrel to a position close to one of the end stops, allowing a small margin for any small oscillations, and then a final current corresponding to the position in contact with the end stop can be applied shortly afterwards. 
         [0046]      FIG. 5  illustrates a lens barrel control circuit  500  arranged to perform the steps described above. Circuit  500  comprises a processor  502 , which receives a signal x f  indicating a final displacement of the lens which is required, a clock signal CLK, for timing the changes in current, and an input for receiving reference values τ and t d . The first position x i , of the lens barrel is, for example, known from the initial current I i  presently applied to the coil, or from a second position x f  of a previous lens movement. 
         [0047]    The processor  502  generates an output signal in digital form which is provided to a digital to analog converter (DAC)  504 . The output of the DAC is provided to an amplifier  505 , which drives a coil  506  of a voice coil motor. Coil  506  is for example the coil  110  of  FIG. 1 , connected to amplifier  505  via the springs  116 ,  118 . 
         [0048]      FIG. 6  illustrates a device  600  comprising a processor  602 , connected to a mobile lens unit  604 , which is for example the mobile lens unit  100  of FIG.  1 . Mobile lens unit  604  also comprises a digital to analog converter and an amplifier for driving the coil of the lens barrel. In this embodiment, processor  602  comprises processing means for generating the current control signals for driving the lens barrel. Processor  602  is connected to a non-volatile memory  606  which stores the constants t d  and τ relating to the lens barrel. Processor  602  is also connected to an autofocus block  608 , which is arranged to determine focusing of the lens, for example using a focusing sensor, or determined based on images from an image sensor. The autofocus block  608  is coupled to the image sensor  610 , which is in turn coupled to a further memory  612 , which stores images captured by the image sensor  610 . 
         [0049]    In operation, the autofocus block  608  determines what displacement of the lens barrel is needed to focus images formed on the image sensor  610 , for example based on an algorithm. The autofocus block  608  communicates this, for example in the form of a final position x f , to the processor, which determines the current steps to be applied to the lens unit  604  and controls the lens unit  604  accordingly. 
         [0050]    Device  600  is for example a digital camera, mobile telephone, PDA (Personal Digital Assistant), PC with webcam, or alternative image capturing device. 
         [0051]    Various alterations, modifications and improvements will readily occur to those skilled in the art. 
         [0052]    While the embodiments described above relate to a current controlled voice coil motor, in alternative embodiments voltage could equally be used to control VCM, in which case the driver  505  of  FIG. 5  generates a voltage signal which is applied to the coil. In such embodiments the voltage could be adjusted to compensate for temperature variations in the coil. 
         [0053]    While embodiments of the present invention have been described in relation to a mobile lens unit, it would be apparent to those skilled in the art that this invention could be applied to other applications of a VCM motor in which low settling times are desirable, such as lens positioning devices in optical storage drives or mechanical translation stages. 
         [0054]    It would be apparent to those skilled in the art that depending on the accuracy of the constants t d  and τ, the lens barrel may still oscillate to some extent, while to a much lesser extent than in prior known methods of VCM control. However, as explained in the background section, a minimum amplitude of oscillation is acceptable in many applications, and in particular, in the case of a lens system, oscillations having an amplitude less than the depth of focus do not affect the focusing of the image on the image sensor. The depth of focus is for example in the region of 5 μm for relatively small lens barrels, and thus oscillations having amplitude of up to 5 μm may be allowable in such systems. Thus oscillations of the moving part of a voice coil motor are preferably lower than such a minimum amplitude level in the embodiments described herein. 
         [0055]    Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.