Patent Publication Number: US-7212358-B2

Title: Digital camera system with piezoelectric actuators

Description:
RELATED APPLICATIONS 
   This application is a continuation in part application based on U.S. application for patent, Ser. No. 10/163,111, filed on Jun. 5, 2002, now U.S. Pat. No. 6,710,950 and Applicant claims priority thereof. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an assembly of optical components for a miniature digital camera, including piezoelectric actuators for moving the optical components to provide focusing, zoom, and other functions. In particular an arrangement of the components is described that reduces the overall size and facilitates assembly. 
   2. Brief Description of Related Developments 
   The components of electronic cameras require low power consumption, low weight and cost efficiency. These design criteria are challenged by the demand for optically adjustable cameras that provide autofocus, zoom optics, or both. These features require the relative movement of optical elements to provide the adjustment. The required motion is typically linear but may use a rotating motor combined with a motion-converting mechanism such as a lead-screw. The motion range is often in the order of millimeters. It is a purpose of this invention to provide a mechanism for adjusting the position of the optical elements in an electronic camera. 
   One component that has been used in numerous applications is a bimorph piezoelectric element, such elements are constructed of multiple layers of piezoelectric material wherein each layer is connected for independent excitation. In U.S. Pat. No. 4,291,958, a bimorph piezoelectric cantilever beam is used in combination with a magnifying lever for focusing a camera. However, the necessary stroke of such a focusing device results in a poor stiffness of the device. In electronic camera applications, space is a crucial factor. There is thus a need for simple drive elements that can operate in narrow spaces with limited mechanical support. It is a purpose of this invention to utilize a bimorph piezoelectric element to adjust the position of a lens in an optical system of a digital camera. 
   A camera system using a piezoelectric actuator is described in commonly owned, related application for patent, Ser. No. 10/163,111 referenced above. The disclosure of this application is incorporated herein by reference. In this application, a lens element is mounted within a camera on a tubular member. The lens tube is in turn mounted on a support tube for movement along the longitudinal axis of the tubular member. The adjustment movement is provided by means of multiple bimorph piezoelectric elements, for example by three elements, spaced symmetrically around the circumference of the support tube. The piezoelectric elements are connected to and mounted on a flexible printed circuit board which may contain other electronic components associated with the lens drive system. The flexible printed circuit board is mounted on the support tube and is in turn connected to a voltage source such as a battery. The flexibility of the printed circuit board allows it to be formed to the shape of the support tube and for the piezoelectric element to be positioned in engagement with the lens tube. 
   The bimorph piezoelectric element used in the mechanism of the cited application is constructed of at least two layers of piezoelectric material which are independently energized to provide relative deformation between the two layers. This piezoelectric element is formed in the shape of a beam having an engagement pad extending transverse to the plane of the element from its midpoint. The beam is fixed to the circuit board close to ends or nodal positions. The beam comprises a pair of bimorph piezoelectric elements extending to either side of the engagement pad. Each of the bimorph elements has dual active layers. The differential deformation generated by energizing only one of the two layers will cause the piezoelectric elements to bend, moving the outer end of the engagement pad into contact with the movable lens tube. By altering the excitation of the piezoelectric elements, the engagement pad causes movement in an axial direction, thereby adjusting the position of the lens. A pattern of excitation is devised to provide movement in discrete steps. 
   In the system of the cited application, a processor is connected in the printed circuit board to provide the main control for the digital camera and is constructed to generate a drive voltage pattern in accordance with the desired movement of the lens. 
   The movement generated by the piezoelectric element provides a high resolution, but there are no structural features that provide a reference in order to obtain accurate repeatability. The step length provided by the piezoelectric element can vary with operational and environmental conditions. In order to obtain the precision required in some optical designs, a position sensor is used to monitor the position of the movable tubes. An optical sensor is used to view a reflecting surface, which is mounted on the moveable tube. The reflecting surface consists of a gray-scale incorporated into the surface treatment of the moveable tube. This configuration will provide accurate positional monitoring of a moveable tube. 
   It is a purpose of this invention to provide a miniature digital camera system of the type described above in which the components are arranged to facilitate their assembly in an over all package that is smaller. It is another purpose of this invention to provide a rail system external to the lens support tubes for engagement by piezoelectric actuators. 
   SUMMARY OF THE INVENTION 
   A digital camera is constructed with multiple lenses mounted in a pair of tubular elements, which are nested together for relative axial movement to provide a zoom function. The lens tube assembly is in turn mounted on a support tube. The lens tube assembly is moveable within said support tube with respect to an image plane to provide an autofocus function. Movement is provided by piezoelectric actuators mounted externally to the support tube on flexible printed circuit board elements. Each lens tube is provided with a drive rail which extends at least partially along the length of the lens tube and project radially outward from the periphery of each of the tubes. The rails are accessible to the engagement pads of the piezoelectric actuators to allow the transmission of drive forces to each of the tubes. Slots are constructed in the support tube to allow the drive rails to project through the support tube. The piezoelectric actuators are mounted to permit engagement by a pair of actuators on either side of the rail. 
   The rails are constructed having a wedge shaped cross section which narrows radially inward towards the axis of the system. The piezoelectric elements are mounted on opposing surfaces of U-shaped flexible printed circuit boards with a rail in between. In this manner, the piezoelectric elements are positioned for engagement with the tapered sides of the rail. A mounting bracket is provided in which is secured the printed circuit board. The clip like mounting bracket exerts a spring force on the printed circuit board or the piezoelecric element itself to bias the piezoelectric elements into engagement with the rail. This bias force also has a radial component which is applied outward on the tube element to maintain the tubes in axial alignment and minimize function effects. Position sensors are mounted on the U-shaped printed circuit board in optical communication with reflective surfaces of respective lens tubes. 
   In one embodiment, the rails are positioned concentrically on the tube assembly. In a second embodiment the rails are positioned in the same quadrant of the tube assembly circumference. In the latter embodiment the engagement forces are applied eccentrically to the tube elements. To assist in maintaining tube alignment, wedge shaped grooves and mating surfaces are formed on the engaging surfaces of the lens tube assembly and the support tube. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The digital camera system of this invention is explained in more detail below with reference to the accompanying drawing, in which: 
       FIG. 1  is an axially exploded, perspective view of an embodiment of a camera system, according to this invention; 
       FIG. 2  is an enlarged, exploded perspective view of the lens elements of  FIG. 1 ; 
       FIG. 3  is an enlarged exploded, perspective view of the piezoelectric drive system of  FIG. 1 ; 
       FIG. 4   a  is a transverse sectional view of the embodiment of this invention shown in  FIG. 1 , taken along section lines  4 A— 4 A of  FIG. 4   b;    
       FIG. 4   b  is a side view of the assembled optical assembly of  FIG. 1 ; 
       FIG. 5  is an axially exploded, perspective view of an embodiment of a camera system, according to an alternative embodiment of this invention; and 
       FIG. 6  is a transverse sectional view of the alternate embodiment of this invention shown in  FIG. 5 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a digital camera  1  is constructed with a pair of lens systems  2  and  3  mounted in a pair of tubular elements  4  and  5 . Lens tubes  4  and  5  are nested together for relative movement along axis x—x to provide a multiple optical functions, such as auto focus and zoom. The assembled lens tubes  4  and  5 , assembly  6 , is in turn mounted on a support tube  7  for axial movement within said support tube  7  with respect to an image plane  8 . A filter  9  may be positioned in front of image plane  8 . An image sensor  10  is mounted on main circuit board  11  on which may also be mounted the control and processing components of the digital camera. Lens tubes  2  and  3  may have multiple cooperating lens mounted within. 
   Support tube  7  is constructed to receive the lens tubes  4  and  5  in an interior passage  12 . Movement is provided by piezoelectric actuators  13 – 16  mounted externally to the support tube on flexible printed circuit board elements  17  and  18 . Spring clip mounting brackets  19  and  20  secure the piezoelectric modules in place within a cover tube  21 . 
   Referring to  FIG. 2 , lens tubes  4  and  5  are nested together for sliding movement. As shown lens tube  5  is constructed with rounded surfaces  22  provided on the outer periphery of a trio of projections  23 – 25 . The projections  23 – 25  extend through mating slots  26 – 28  in lens tube  4 . Slots  26 – 28  are constructed with tapered surfaces  29  which engage opposing tapered surfaces  30  on projections  23 – 25 . 
   Each of lens tubes  4  and  5  is provided with a drive rail  31  and  32  respectively which extends at least partially over the length of the lens tubes  4  and  5  and project radially outward from the periphery of each of the tubes. As shown in  FIG. 3 , the rails  31  and  32  are accessible to the engagement pads  33 – 36  of the piezoelectric actuators  13 – 16  to allow the transmission of drive forces to each of the tubes  4  and  5 . Slots  37  are constructed in the support tube  7  to allow the drive rails  31  and  32  to project through the support tube  7 . 
   As shown in  FIG. 3 , the piezoelectric actuators  13 – 16  are independently mounted on flexible printed circuit boards  17  and  18 . Printed circuit boards  38  and  39  are formed in the shape of a U having opposing arms  40  and  41 . The printed circuit board is constructed having circuit paths (not shown) to supply power to the piezoelectric actuators  13 – 16 . Optical sensors  42  and  43  may also be fixed to the printed circuit boards  17  and  18 . As shown, the actuators  13 – 16  are mounted in pairs on opposing arms  40  and  41 . The assembly is fixed to the support tube  7  in a position to permit engagement by a pair of actuators, such as  15  and  16 , on either side of the rails  31  and  32 . 
   Lens tubes  4  and  5  have to be aligned with the support tube  7  with minimum friction forces. The friction forces will result in a torque that tend to rotate the lens tube away from axial alignment. If the supporting surfaces between the lens tubes  4  and  5  and the supporting tube  7  are very close to, or centered with, the engagement pads of the drive elements, the torque will be reduced due to a shorter torque lever. To maintain these advantageous conditions the lens tubes  4  and  5  have to be pressed against the supporting tube  7  with a controlled force. This desired bias force in the radial direction can be created by several means with their respective advantages and disadvantages. The bias force can be achieved by a flexible spring (not shown) of e.g. steel or rubber that forces the lens tubes against the support tube with a minimum of axial forces. The flexible spring could either be in frictional contact with the lens tube, e.g. the lens tube is moving within a slot in the support tube, or fixed directly to the lens tube, e.g. a thin metal wire that can easily bend in the axial direction. Another solution that could be used in particular cases is bias forces created by permanent magnets. 
   In one embodiment, shown in  FIG. 4   a , the rails  31  and  32  can be constructed having a wedge shaped cross section which narrows radially inward towards the axis of the system. The piezoelectric elements  13 – 16  are mounted on opposing surfaces of U-shaped flexible printed circuit boards  17  and  18  with a rail  31  or  32  in between. In this manner, the piezoelectric elements are positioned for engagement with the tapered sides of the rail. Spring clips  19  and  20  are provided in which are secured the printed circuit boards  17  and  18 . The spring clips  19  and  20  exert a spring force on the printed circuit boards to bias the piezoelectric elements into engagement with their respective rail. This bias force also has a radial component which is applied outward on the tube element to maintain the tubes in axial alignment and minimize binding. Optical sensors  42  and  43  are mounted on the U-shaped printed circuit boards in optical communication with reflective surfaces  44  and  45  of respective lens tubes  4  and  5 . As shown in  FIG. 3 , sensor ports  46  (not shown) and  47  are provided in support tube  7  to provide access for the optical sensors  42  and  43 . The optical sensors are responsive to provide a position indication for the lens tubes as they are moved by actuators  13 – 16 . Optical sensors are shown for illustration, but other types of position sensors may be adapted for the same purpose, for example, a resistive position sensor. 
   For illustration of the basic structure of this invention, two lens tubes are shown, however, it should be understood, that more complex lens configurations may be constructed which would require a greater number of lens tubes. Multiple lens tubes may be nested for relative movement and driven as shown and described in this application. Each of the lens tubes may contain lens system comprised of multiple lens elements. 
   In one embodiment, the rails are positioned concentrically on the tube assembly, as shown in  FIG. 1 . In a second embodiment the rails are positioned in the same quadrant of the tube assembly circumference, as shown in  FIGS. 5 and 6 . In the latter embodiment the engagement forces are applied eccentrically to the tube elements. To assist in maintaining tube alignment, wedge shaped grooves  150  and  154  and mating surfaces  151  and  153  are formed on the engaging surfaces of the lens tube assembly and the support tube, as shown in  FIG. 6 . 
   Referring to  FIG. 5 , the optical system  101  of the second embodiment is constructed with dual lens tubes  104  and  105 . Lens tubes  104  and  105  contain the lenses of the optical system  101  mounted within support tube  107 . A cover tube  121  encloses the optical assembly. More lens tubes could be employed if a more complex optical system, i.e., more lenses, is desired. 
   Similarly to the first embodiment, the optical system  101  is mounted on printed circuit board  108  to transmit light from an image (not shown) to an image sensor  110  at an image plane  111 . The printed circuit board may also contain the control circuit  162  for the digital camera and provide means to connect the optical system to a power source such as a battery. 
   Lens tubes  104  and  105  are nested for movement along axis y—y within support tube  107 . This movement includes sliding movement of lens tube  105  relative to lens tube  104  and movement of the lens tube  104  within support tube  107 . In the embodiment of  FIGS. 5 and 6 , the lens tubes are nested in a different manner. Lens tube  104  has only a partial tubular surface  152  which has an outer surface  151  for engagement with a groove  150  on the inner surface of passage  112  in support tube  107 . The engaging surfaces provide a track in which the lens tube  104  is moved by the action of piezoelectric elements  115  and  116 . A rail  131  is constructed on the surface  152 , extending radially upward according to  FIG. 6 . Rail  131  extends through a slot  137  constructed for this purpose in support tube  107 . Rail  131  is shaped with a wedge shaped cross section which provides surfaces for the engagement of the piezoelectric actuators  115  and  116 . These slanted surfaces  155  and  156  cause the engagement force of the piezoelectric actuators  115  and  116  to apply a radial outward component of force, which tends to maintain alignment of the lens tube  104  within the groove  150 . 
   Lens tube  105  may be assembled by insertion through opening  157  in support tube  107  in a motion transverse to the axis y—y. Lens tube  105  is also constructed with an engagement surface  153  which extends axially on its outer periphery. A groove  154  is constructed in the inner surface of passage  112  approximately 90° from groove  150  to receive engagement surface  153 . A rail  132  extends radially outward from surface  154  through a slot  158  constructed in support tube  107 . 
   In a manner similar to the first embodiment piezoelectric actuators  113 – 116  are mounted on the opposing surfaces of U-shaped printed circuit boards  117  and  118 . Circuit boards  117  and  118  are secured to support tube  107  by spring clips  119  and  120 . A pair of outward extending flanges  160  and  161  may be formed on support tube  107  to receive printed circuit boards  117  and  118 . Clips  119  and  120  engage the flanges to secure the circuit boards in place. As shown in  FIG. 6 , piezoelectric actuator&#39;s  115  and  116  engage slanted surfaces  155  and  156  of rail  131  and are urged into engagement by bias force exerted by spring clip  119 . The radial component of this force urges the optical tube  104  outward and into engagement with groove  150 . An identical assembly secures the piezoelectric actuators  113  and  114  on support tube  107  and urges the actuators into engagement with rail  132 . 
   The configuration and operation of the piezoelectric elements are described in more detail in the parent application referenced above and incorporated herein.