Patent Publication Number: US-2009219434-A1

Title: Method and Device for Position Sensing of an Optical Component in an Imaging System

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to optical position sensing in an imaging system and, more particularly, to position sensing for auto-focus optics and/or an optical zoom module in the imaging system. 
     BACKGROUND OF THE INVENTION 
     Auto-focus optical systems require high precision in position sensing. In general, needed accuracy is in the order to a few microns. Sensor output linearity and immunity to external disturbances is important. Furthermore, the operation mode for position sensing also requires non-contact operation to avoid mechanical wear. When considering optics for use in a small electronic device, such as mobile phone, the size and cost of the optical sensing components and the suitability to mass production are important issues. 
     Typically, position determination in a commercial auto-focus module is carried out by counting stepper motor steps. For that purpose, the motor can have an embedded position encoder. In order to reduce the size of the optical modules, miniature piezoelectric motors or actuators are generally used. These motors and actuators require a separate position sensor. 
     In fulfilling the need for an auto-focus optical system or an optical zoom system with movement in the order of a few microns, the present invention provides a simple method and device for position sensing. 
     SUMMARY OF THE INVENTION 
     The present invention uses a reflection surface to reflect light, and a photo-emitter and photo-sensor pair to illuminate the reflection surface and to detect the reflected light from the reflection surface. In particular, the reflection surface is provided near the edge of a first mounting member and the photo-emitter/sensor pair is disposed on a second mounting member. The first and second mounting members are moved relative to each other when the first mounting member is used to move a lens element in an auto-focus system or an optical zoom system. The photo-emitter/sensor pair is positioned at a distance from the reflection surface such that the light cone emitted by the photo-emitter only partly hits the reflection surface. Part of the light cone misses the reflection surface because it falls beyond the edge. As the photo-emitter/sensor pair and the reflection surface move relative to each other, the area on the reflection surface illuminated by the photo-emitter changes. Accordingly, the amount of light sensed by the photo-sensor also changes. The change in the reflected light amount causes a near-linear output signal response in a certain travel range of the reflection surface. Preferably, the reflectivity of the reflection surface within the illuminated area is substantially uniform and the distance between the photo-emitter/sensor pair and the reflection surface is substantially fixed. As such, the output signal response is substantially proportional to a portion of a circular area of a fixed radius and the portion is reduced or increased as a function of a moving distance as the photo-emitter/sensor pair and the reflection surface move relative to each other. 
     In one of the embodiments of the present invention, the diameter of the illuminated area is smaller than the width of the reflection surface. 
     In another embodiment of the present invention, the diameter of the illuminated area is equal to or greater than the width of the reflection surface. 
     In yet another embodiment of the present invention, the reflection surface has a wedge shape. 
     In a different embodiment of the present invention, two photo-emitter/sensor pairs disposed at two reflection surfaces for sensing the relative movement in a differential way. 
     The present invention will become apparent upon reading the description taken in conjunction with  FIGS. 2   a  to  9 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of an imaging system wherein one or more lens elements are moved relative to the image sensor along the optical axis for focusing or zooming purposes. 
         FIG. 2   a  shows a photo-emitter/sensor pair positioned in relationship with a reflection surface near an edge of a mounting beam. 
         FIG. 2   b  is another view of the photo-emitter/sensor pair and the reflection surface. 
         FIG. 2   c  shows another embodiment of the present invention. 
         FIG. 3   a  shows a lens carrier having a mounting beam for mounting the photo-emitter/sensor pair. 
         FIG. 3   b  shows a lens carrier having a mounting beam for mounting the reflection surface. 
         FIG. 4   a  is a schematic representation of a camera having a photo-emitter/sensor pair fixedly mounted on a stationary part of the camera body. 
         FIG. 4   b  is a schematic representation of a camera having a reflective surface for folding the optical axis. 
         FIG. 5  shows a plot of output signal against the relative position between a photo-emitter/sensor pair and the reflection surface. 
         FIG. 6  shows another embodiment of the present invention. 
         FIG. 7  shows yet another embodiment of the present invention. 
         FIG. 8  shows two photo-emitter pairs positioned in relationship to two separate reflection surfaces near two edges of a mounting beam. 
         FIG. 9  shows a position sensing system, according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Imaging applications such as auto-focus lens systems and optical zoom systems require high precision in position sensing. In such applications, at least one lens element is moved along the optical axis of the imaging system so as to change the focal plane of the lens or the magnification of the image formed on an image sensor. As shown in  FIG. 1 , the movement of the lens element is substantially along the optical axis which is parallel to the Z axis. The image sensor is located in an image plane which is substantially parallel to the XY plane. The imaging system may have one or more stationary lens elements as depicted in dotted lines. 
     In auto-focus or optical zoom applications, it is required to determine the position of the lens element relative to a reference point or a home position. According to the present invention, a photo-emitter/sensor pair is used to sense the displacement of the lens element along the Z-axis. As shown in  FIG. 2   a , a reflection surface  70  is provided on a mounting member or mounting beam  30  and the photo-emitter/sensor pair  60  is disposed on a mounting member or mounting beam  20 . The photo-emitter/sensor pair  60  has a photo-emitting element, such as an LED  62 , for illuminating part of the reflection surface  70 . The emitter/sensor pair  60  also has a photo-sensor  64  to sense the amount of light reflected by the reflection surface  70 . Preferably, the reflectivity of the reflection surface within the illuminated area is substantially uniform and the distance, d, between the photo-emitter/sensor pair  60  and the reflection surface  70  is also fixed. 
     As shown in  FIG. 2   b , the reflection surface  70  is provided next to an edge of the mounting beam  30 . The distance and position between the emitter/sensor pair  60  and the reflection surface  70  is chosen such that the light cone  162  emitted by the photo-emitting element  62  only partially hits the reflection surface  70 . Part of the light cone  162  misses the reflection surface  70  as it falls beyond the edge  32  of the mounting beam  30 . As such, the output signal response from the photo-sensor  64  is substantially proportional to a portion of a circular area of a fixed radius and the portion is reduced or increased as a function of a moving distance as the photo-emitter/sensor pair and the reflection surface move relative to each other. 
     It should be noted that the edge of a mounting beam is not necessarily formed at an end of the mounting beam, as shown in  FIGS. 2   a  and  2   b . The edge can be made with a slot on the beam, for example. As shown in  FIG. 2   c , the beam  30  has a slot  34  with an edge  36 . The photo-emitter/sensor pair  60  is positioned on its mounting beam near the slot  34  so that the light cone emitted by the photo-emitter  62  hits only part of the reflection surface  70 . 
       FIG. 3   a  shows one embodiment of the present invention where the mounting beam  30  is fixedly mounted on a lens carrier  110  or is an integral part of the lens carrier. The lens carrier  110  is used to move the lens element  100  along the optical axis for auto-focus or optical zoom purposes.  FIG. 3   b  shows another embodiment of the present invention where the mounting beam  20  is fixedly mounted on the lens carrier  110 . 
       FIG. 4   a  is a schematic representation of an imaging system or camera  10  of the present invention. The imaging system  10  has a stationary body  14  for fixedly mounting the photo-emitter/sensor pair  60 . The lens element  100  is movable together with the lens carrier  110  along the optical axis in order to form an image at a focal plane on the image sensor  120 . As shown, the mounting beam  30  is fixedly mounted on the lens carrier  110 . It should be noted that the position sensing system of the present invention can also be used in an imaging system where the optical axis is folded by a reflective surface  130 , as shown in  FIG. 4   b.    
     It is understood by a person skilled in the art that the photo-emitter/sensor pair  60  is operatively connected to a power supply for providing electrical power to the photo-emitter  62  and to an output measurement device so that the output signal from the photo-sensor  64  can be measured for determining the relative movement between the photo-emitter/sensor  60  pair and the reflection surface  70 . The measured output signal from the photo-sensor  64 , in terms of collector voltage as a function of movement distance, is shown in  FIG. 5 . As shown, a near-linear range of approximately 1 mm can be found in the middle section of the curve. Within this range, the measurable movement in the order of a few microns is attainable. 
     It should be appreciated by a person skilled in the art that the edge  32 ,  36  and  26  as depicted in  FIGS. 2   a  to  3   b  is part of a beam surface that is substantially perpendicular to the reflection surface. However, the angle between the beam surface and the reflection surface is not necessarily a right angle. The angle can be larger than 90 degrees or small than 90 degrees, so long as the part of the light beam from the photo-emitter  62  falling beyond the edge does not yield a significant amount of detectable light as compared to the reflected light from the reflection surface. Furthermore, in  FIGS. 2   b  and  2   c , the width of the reflection surface  70  is greater than the diameter of the light cone  162  on the reflection surface. However, the width w of the reflection surface  70  can be equal to or smaller than the diameter D of the light cone  162  on the reflection surface, as shown in  FIG. 6 . Moreover, the reflection surface  70  can also be a wedge-shaped surface, as shown in  FIG. 7 . 
     In a different embodiment of the present invention, two separate optical sensors are used on one motion axis to form a differential position sensing system. As shown in  FIG. 8 , a photo-emitter/sensor pair  60  has a photo-emitter  62  for projecting a light cone  162  on a reflection surface  70 , and a photo-sensor  64  for sensing the amount of light reflected by the reflection surface  70 . A separate photo-emitter/sensor pair  60 ′ has a photo-emitter  62 ′ for projecting a light cone  162 ′ on a different reflection surface  70 ′, and a photo-sensor  64 ′ for sensing the amount of light reflected by the reflection surface  70 ′. As shown in  FIG. 8 , the reflection surface  70  is provided near an edge  32  of the mounting beam  30 , and the reflection surface  70 ′ is provided near another edge  32 ′ of the same beam  30 . The distance between the photo-emitter pair  70  and the photo-emitter pair  70 ′ is fixed so that when the position signal of one photo-emitter pair is increased due to the relative movement between mounting member  30  and the photo-emitter pairs, the position signal of the other photo-emitter pair is decreased. As such, the final position signal is the difference of the two separate position signals. With the arrangement as shown in  FIG. 8 , external influences such as temperature changes can be substantially eliminated. Furthermore, the effect of mechanical tilting is reduced. 
     It should be noted that optical sensors such as photo-emitter/sensor pairs are low-end components and, thus, the performance variation is generally quite large. It would be advantageous and desirable to calibrate the position system during start-up of the auto-focus or optical zoom system. This can be done by driving the lens element  100  over the entire available motion range, for example. During this stroke, the sensor output is measured at both extremes of the motion range. When the output signals at the two extremes are known, all the intermediate positions can be accurately determines from the intermediate output signals. 
     It should be appreciated by a person skilled in the art that the position sensing system  200  of the present invention also includes a movement mechanism  230 , such as a piezoelectric actuator or a motor, for moving the lens carrier  110  and a signal processing module  210  operatively connected to the photo-emitter/sensor pair  60  for determining the position of the lens element  100  based on the reflection from the reflection surface  70 . The position sensing system  200  also includes a control module  220  to control the movement amount of the lens element  100  via the movement mechanism  230 , based on the information provided by the signal processing module  210 . For auto-focus purposes, the signal processing module  210  may be required to receive image data from the image sensor  120  for checking the focus in part of the image formed on the image sensor  120 . It should be noted that, however, the signal processing module  210 , the control module  220  and the movement mechanism  230  are known in the art. They are not part of the present invention. The present invention is concerned with using at least one photo-emitter/sensor pair to sense the position of a reflection surface which is fixedly positioned in relationship to a lens element for auto-focusing or optical zoom purposes. 
     In an auto-focus system, it is possible to move the image sensor relative to the lens element. In that case, the position sensing system is used to sense the position of the image sensor, instead of sensing the position of the lens element. 
     Thus, although the invention has been described with respect to one or more embodiments thereof, it will be understood by those skilled in the art that the foregoing and various other changes, omissions and deviations in the form and detail thereof may be made without departing from the scope of this invention.