Abstract:
A device for stabilizing images acquired by a digital-image sensor includes a motion-sensing device, for detecting quantities correlated to pitch and yaw movements of the digital-image sensor, and a processing unit, connectable to the digital-image sensor for receiving a first image signal and configured for extracting a second image signal from the first image signal on the basis of the quantities detected by the motion-sensing device. The motion-sensing device includes a first accelerometer and a second accelerometer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of International Patent Application No. PCT/EP2006/066387, filed Sep. 14, 2006, now pending, which application is incorporated herein by reference in its entirety. 
     This application claims the benefit under 35 U.S.C. §119(a) of Italian Patent Application No. TO2005A 000628, filed Sep. 15, 2005, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an image stabilizing device, in particular for image acquisition using a digital-image sensor. 
     2. Description of the Related Art 
     As is known, shots taken using non-professional portable apparatuses, such as camcorders or digital cameras, either stand-alone or incorporated in telephone apparatuses, suffer from flickering caused by minor movements of the operator. In particular, portable apparatuses are supported only by the hands of the operator, and the lack of a firm point of rest makes it practically impossible to keep the framing stable. The resulting image is hence unstable and consequently unpleasant to the eye. The same problem also regards cameras during the acquisition of single images. A movement can render acquisition imprecise, especially for long exposure times. 
     The use of image stabilizers has thus been proposed. By appropriate processing, in digital apparatuses it is possible “to cut out” a portion (hereinafter referred to as “usable frame”) of the image effectively acquired (hereinafter referred to as “complete image”). Only the usable frame is made available for display, whereas an outer frame is eliminated from the complete image. Stabilizer devices enable estimation of the movements of the equipment and recalculation of the co-ordinates of the usable frame so as to compensate for the movements and render the image stable. 
     Image stabilizers of a first type are based upon the content of the images to be stabilized. After identification of reference elements in a scene, in practice the displacement of the apparatus and the position of the usable frame are estimated by comparing the positions of the reference elements in successive frames. Systems of this type are not satisfactory when the scene framed contains elements that are effectively moving, such as, for example, a person who is walking. 
     According to a different solution, image stabilizers include gyroscopes, which measure angular velocity of the apparatus with respect to axes transverse to an optical axis thereof (normally, two axes perpendicular to one another and to the optical axis). The rotations about said axes cause in fact the greatest disturbance. By means of temporal integration of the data detected by the gyroscopes, it is possible to trace back to the instantaneous angular position of the optical axis of the apparatus and from here to the position of the centre of the usable frame. The image can then be corrected accordingly. In this way, the stabilization of the image is independent of its content. Gyroscopes, however, absorb a lot of power, because they use a mass that must be kept constantly in oscillatory or rotational motion. Their use is hence disadvantageous in devices that are supplied autonomously because they markedly limit the autonomy thereof. 
     BRIEF SUMMARY 
     One embodiment of the present invention provides an image stabilizer device that is free from of the above referred drawbacks. 
     A stabilizer device of images acquired by a digital-image sensor device is provided, as defined in claim  1 . 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the invention, an embodiment thereof is now described purely by way of non-limiting example and with reference to the attached drawings, wherein: 
         FIG. 1  is a right side view of a digital camera in a first operating configuration; 
         FIG. 2  is a front view of the camera of  FIG. 1 ; 
         FIG. 3  is a simplified block diagram of the camera of  FIG. 1 ; 
         FIGS. 4   a - 4   c  are front views of an image sensor incorporated in the camera of  FIGS. 1 and 2 , in different operating configurations; 
         FIG. 5  is a block diagram of a, image stabilizer device according to the present invention, incorporated in the camera of  FIG. 1 ; 
         FIG. 6   a  is a right view of the camera of  FIG. 1  and shows a movement of pitch in the first operating configuration; 
         FIG. 6   b  is a bottom view of the camera of  FIG. 1  and shows a movement of yaw in the operating configuration of  FIG. 6   a;    
         FIG. 6   c  is a right view of the camera of  FIG. 1  and shows a movement of pitch in a second operating configuration, in which the camera is rotated substantially through 90° about a horizontal axis with respect to the first operating configuration; 
         FIG. 6   d  is a bottom view of the camera in the second operating configuration of  FIG. 6   c  and shows a movement of yaw in the second operating configuration; 
         FIG. 7  is a cross-sectional view through a first portion of a semiconductor chip incorporating the image stabilizer device of  FIG. 5 , taken along line VII-VII of  FIG. 8 ; 
         FIG. 8  is a front view of the first portion of the semiconductor chip of  FIG. 7 ; 
         FIGS. 9 and 10  schematically show the responses of a component incorporated in the image stabilizer device of  FIG. 5  to linear and, respectively, angular accelerations; 
         FIG. 11  shows a cross-section through a second portion of the semiconductor chip of  FIGS. 7 and 8 , taken along line XI-XI of  FIG. 8 ; 
         FIG. 12  is a front view of the second portion of the semiconductor chip of  FIG. 11 ; 
         FIG. 13  is a front view of the semiconductor chip of  FIGS. 7 ,  8 ,  10  and  11 , in the first operating configuration; 
         FIG. 14  is a more detailed block diagram of a first portion of the image stabilizer device of  FIG. 5 ; 
         FIG. 15  is a front view of the semiconductor chip of  FIGS. 7 ,  8 ,  10 ,  11  and  13 , in the second operating configuration; and 
         FIG. 16  is a more detailed block diagram of a second portion of the image-stabilizer device of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIGS. 1-3 , a digital camera  1 , adapted for shooting digital films, comprises a body  2 , a lens  3 , a digital-image sensor  5 , a non-volatile storage unit  6 , a display  8 , and an image stabilizer device  10 . 
     The body  2  comprises a base  2   a , normally facing downwards, and houses inside it the image sensor  5 , the storage unit  6 , and the image stabilizer device  10 . 
     The image sensor  5  is, for example, a CCD or CMOS sensor and is arranged perpendicular to an optical axis OA of the lens  3 . Furthermore, the optical axis OA intercepts the centre of the image sensor  5 . Note that a sensitive portion  5   a  of the image sensor  5  has a rectangular shape (see  FIGS. 4   a  and  4   b ) and, during use of the camera  1 , is normally arranged in a “landscape” configuration or in a “portrait” configuration. More precisely, in the “landscape” configuration ( FIG. 4   a ), the larger sides L 1  of the sensitive portion  5   a  are substantially horizontal and the smaller sides L 2  are frequently, but not necessarily, vertical; in the “portrait” configuration ( FIG. 4   b ), the smaller sides L 2  are substantially horizontal and the larger sides L 1  are frequently, but not necessarily, vertical. With reference to the orientation of the larger sides L 1  and of the smaller sides L 2  of the sensitive portion  5   a  of the sensor  5 , by “yaw” movements (and angles) are meant rotations (and angles) of the optical axis OA with respect to a yaw axis parallel to those sides, between the larger sides L 1  and the smaller sides L 2 , which are less inclined with respect to the vertical. In particular, in the “landscape” configuration, a yaw movement is a rotation of the optical axis about a yaw axis parallel to the smaller sides L 2 ; in the “portrait” configuration, instead, the yaw axis is parallel to the larger sides L 1 . By “pitch” movements (and angles) are meant rotations of the optical axis OA about a pitch axis perpendicular to the yaw axis (and to the optical axis OA itself). Consequently, in the “landscape” configuration, the pitch axis is parallel to the larger sides L 1 , whereas in the “portrait” configuration the pitch axis is parallel to the smaller sides L 2 . 
     With reference to  FIGS. 3 and 4   c , the stabilizer device  10  receives from the image sensor  5  a first image signal IMG regarding a complete image  11  detected by the image sensor  5  itself, and generates a second image signal IMG′ regarding a usable frame  12  obtained from the complete image  11  and stabilized. The second image signal IMG′ is supplied to the storage unit  6  and to the display  8 . 
     As illustrated in  FIG. 5 , the stabilizer device  10  comprises a processing unit  14 , a first accelerometer  15 , a second accelerometer  16 , and a third accelerometer  17 , all of a microelectromechanical type and preferably integrated in a single semiconductor chip  19  (see also  FIGS. 13 and 15 ). Furthermore, the stabilizer device  10  includes a pre-processing stage  18 , which supplies variations of a pitch angle Δφ P  and variations of a yaw angle Δφ Y  ( FIGS. 6   a ,  6   b , which refer to the “landscape” configuration, and  FIGS. 6   c ,  6   d , which refer to the “portrait” configuration, in which the base  2   a  of the body  2  is facing sideways and not downwards; for reasons of simplicity,  FIGS. 6   a - 6   d  show only the sensitive portion  5   a  of the image sensor  5  and, moreover, in  FIG. 6   d  the stabilizer device  10  is not illustrated) on the basis of signals detected by the first, second, and third accelerometers  15 ,  16 ,  17 . 
     The processing unit  14  receives the first image signal IMG and extracts the second image signal IMG′ therefrom, using the variations of the pitch angle Δφ P  and the variations of the yaw angle Δφ Y . In practice, the processing unit  14  is configured for determining displacements of the body  2  and of the optical axis OA on the basis of the variations of the pitch angle Δφ P  and the variations of the yaw angle Δφ Y , for positioning the usable frame  12  within the complete image  11 , so as to compensate for the detected displacements of the body  2  and of the optical axis OA, and for generating the second signal image IMG′ on the basis of the portion of the complete image  11  corresponding to the usable frame  12 . 
     As illustrated in  FIGS. 7 and 8 , the first accelerometer  15  has a specularly symmetrical oscillating-beam structure. In greater detail, the first accelerometer  15  comprises two beams  20  of semiconductor material, constrained to a substrate  21  of the semiconductor chip  19  by return torsional springs  22 , fixed to respective anchorages  23 . The torsional springs  22  are shaped so that the beams  20  are free to oscillate about respective first rotation axes R 1  in response to external stresses. In particular, the first rotation axes R 1  are parallel to one another and to the surface  21   a  of the substrate  21 , and perpendicular to longitudinal axes L of the beams  20  themselves and to the optical axis OA. The longitudinal axes L of the beams  20  are mutually aligned at rest. The first axes of rotation R 1  intercept the longitudinal axes L in points staggered with respect to the centroids G of the respective beams  20 , dividing each of them into a larger portion  20   a , containing the respective centroid G, and into a smaller portion  20   b.    
     At rest, the beams  20  are arranged in a specularly symmetrical way with respect to one another. In the embodiment of the invention described herein, the beams  20  have their respective smaller portions  20   b  facing one another, whereas the larger portions  20   a  project outwards in opposite directions. In the absence of external stresses, moreover, the torsional springs  22  tend to keep the beams  20  parallel to the surface  21   a  of the substrate  21 . 
     A first electrode  25  and a second electrode  26  are associated to each beam  20 , and are housed in the substrate  21  (insulated therefrom) in positions that are symmetrical with respect to the respective first axes of rotation R 1 . The larger portion  20   a  and the smaller portion  20   b  of each beam  20  are capacitively coupled with the respective first electrode  25  and the respective second electrode  26  and form first and second capacitors  27 ,  28 , having variable capacitance. In this way, a rotation of a beam  20  about the respective first rotation axis R 1  causes a corresponding differential capacitive unbalancing between the first capacitor  27  and the second capacitor  28  associated thereto. In  FIGS. 7 and 8 , the capacitances of the first capacitors  27  are designated by C 1A  and C 1C , respectively, whereas the capacitances of the second capacitors  28  are designated by C 1B  and C 1D , respectively. 
     In the presence of linear accelerations AL having a component perpendicular to the surface  21   a  of the substrate  21  (in practice, parallel to the optical axis OA), the two beams  20  are subject to rotations of equal amplitude, one in a clockwise direction and one in a counterclockwise direction ( FIG. 9 ). Consequently, the capacitances of both of the first capacitors  27  increase (decrease) by an amount +ΔC (−ΔC), whereas the capacitances of both of the second capacitors  28  decrease (increase) by an amount −ΔC (+ΔC). The variations are hence of equal absolute value and of opposite sign. Instead, when the semiconductor chip  19  is subjected to a rotational acceleration α, both of the beams  20  undergo rotations in the same direction, whether clockwise or counterclockwise ( FIG. 10 ). Consequently, for one of the beams  20 , the capacitance of the first capacitor  27  increases by an amount +ΔC and the capacitance of the second capacitor  28  decreases by an amount −ΔC, while, on the contrary, for the other beam  20  the capacitance of the first capacitor  27  decreases by the amount −ΔC, and the capacitance of the second capacitor  28  increases by the amount +ΔC. 
     Capacitance variations ΔC 1A , ΔC 1B , ΔC 1C , ΔC 1D  are detectable by means of a sense interface  30  having terminals connected to the first electrodes  25 , to the second electrodes  26 , and to the beams  20  (through the substrate  21 , the anchorages  23 , and the torsional springs  22 , made of semiconductor material). 
     The second accelerometer  16  has a structure identical to the first accelerometer  15  and is rotated by 90° with respect thereto, as illustrated in  FIGS. 11 and 12 . More precisely, the beams  20  of the second accelerometer  16  are free to oscillate, in response to external stresses, about second rotation axes R 2  parallel to one another and perpendicular to the first rotation axes R 1  and to the optical axis OA. Also the second rotation axes R 2  intercept the longitudinal axes L of the respective beams  20  in points staggered with respect to the centroids G. For the second accelerometer  16  ( FIG. 11 ), the capacitances of the first capacitors  27  are designated by C 2A  and C 2C , whereas the capacitances of the second capacitors  28  are designated by C 2B  and C 2D ; the corresponding capacitance variations are designated by ΔC 2A , ΔC 2B , ΔC 2C , ΔC 2D . The response of the second sensor  16  to linear accelerations AL, perpendicular to the second rotation axes R 2  and to angular accelerations about axes parallel to the second rotation axes R 2 , is altogether similar to the response of the first accelerometer  15  to linear accelerations AL, perpendicular to the first rotation axes R 1  and to angular accelerations about axes parallel to the first rotation axes R 1  (as represented in  FIGS. 9 and 10 ). 
     The semiconductor chip  19  is mounted in the body  2  of the camera  1  so that, in the absence of external stresses, the beams  20  of the first accelerometer  15  and of the second accelerometer  16  are perpendicular to the optical axis OA ( FIG. 13 ). Furthermore, when the optical axis OA and the base  2   a  of the body  2  are horizontal, the first rotation axes R 1  of the first accelerometer  15  are horizontal, whereas the second rotation axes R 2  of the second accelerometer  16  are vertical. 
     The third accelerometer  17  ( FIG. 13 ) is of a biaxial type with comb-fingered electrodes, as illustrated schematically in  FIG. 13 , and is a low-resolution accelerometer. The third accelerometer  17  has a first detection axis X and a second detection axis Y, both perpendicular to the optical axis OA. Furthermore, the first detection axis X is parallel to the first rotation axes R 1  of the beams  20  of the first accelerometer  15 , whereas the second detection Y is parallel to the second rotation axes R 2  of the beams  20  of the second accelerometer  16 . In practice, according to how it is oriented, the third accelerometer  17  is able to discriminate along which one of the first detection axis X and the second detection axis Y the force of gravity prevalently acts and is thus able to provide an indication of how the body  2 , the optical axis OA, and the semiconductor chip  19 , the relative positions whereof are constant, are oriented. An orientation signal S XY , of a logic type, supplied by a sense interface (not illustrated in detail) of the third accelerometer  17  is sent to the pre-processing stage  18  (see  FIG. 5 ). 
       FIG. 14  shows in greater detail the pre-processing stage  18 , which comprises a first computation module  31 , a selector module  32 , and an integrator module  33 . The first computation module  31  is connected to the first accelerometer  15  and to the second accelerometer  16  for receiving sensing signals representing the capacitance variations ΔC 1A , ΔC 1B , ΔC 1C , ΔC 1D , ΔC 2A , ΔC 2B , ΔC 2C , ΔC 2D  of the respective first capacitors  27  and second capacitors  28  (see also  FIGS. 7 and 12 ). The first computation module  31  is moreover configured to calculate the variations of the pitch angle Δφ P  and an acceleration of yaw α Y  on the basis of the capacitance variations ΔC 1A , ΔC 1B , ΔC 1C , ΔC 1D , ΔC 2A , ΔC 2B , ΔC 2C , ΔC 2D , selectively according to one of two modalities, according to whether the camera  1  is used in the “landscape” configuration or in the “portrait” configuration. The selection of the calculation mode is made by the selector module  32  on the basis of the orientation signal S XY  supplied by the third accelerometer  17 . 
     In practice, when the camera is in the “landscape” use configuration, the force of gravity acts prevalently on the second detection axis Y, and the orientation signal S XY  has a first value. In this case, the first calculation mode of the first computation module  31  is selected, in which the first accelerometer  15  is used as inclinometer for measuring variations of the pitch angle Δφ P , and the second accelerometer  16  is used as rotational accelerometer for determining angular accelerations due to the variations of the yaw angle Δφ Y  (accelerations of yaw α Y ; in this case, the yaw axis is parallel to the second detection axis Y). The calculation is carried out according to the equations: 
                             sin   ⁢           ⁢   Δ   ⁢           ⁢     φ   P       ≅       ⁢     Δ   ⁢           ⁢     φ   P         =               =       ⁢       K   1     ⁡     [       (       Δ   ⁢           ⁢     C     1   ⁢   A         -     Δ   ⁢           ⁢     C     1   ⁢   B           )     +     (       Δ   ⁢           ⁢     C     1   ⁢   D         -     Δ   ⁢           ⁢     C     1   ⁢   C           )       ]                     (   1   )                 α   Y     =       K   2     ⁡     [       (       Δ   ⁢           ⁢     C     2   ⁢   A         -     Δ   ⁢           ⁢     C     2   ⁢           ⁢   B           )     -     (       Δ   ⁢           ⁢     C     2   ⁢           ⁢   D         -     Δ   ⁢           ⁢     C       2   ⁢   C     ⁢                   )       ]               (   2   )               
where K 1  and K 2  are coefficients of proportionality.
 
     As regards Eq. (1), a movement of pitch of the camera  1  in the “landscape” configuration modifies the effect of the force of gravity on the first accelerometer  15  and is equivalent, in practice, to a linear acceleration AL directed perpendicularly to the surface  21   a  of the substrate  21  (as in the example of  FIG. 9 ). Furthermore, the variation of the effect of the force of gravity is proportional to the sine of the variation in the pitch angle Δφ P . However, for small oscillations, as in the present application, the approximation sin Δφ P ≅Δφ P  is justified. Alternatively, the first computation module  31  of  FIG. 14  calculates the value of the function arcsin Δφ P . For the first accelerometer  15 , Eq. (1) enables an amplification of the capacitive variation due to linear accelerations AL and the selective rejection of the effects of angular accelerations α due to rotations (in particular, following upon variations of the yaw angle Δφ Y ) to be obtained. From  FIG. 9 , in which the effects of a linear acceleration AL are illustrated, from Eq. (1) we obtain
 
Δφ P   =K   1 [(Δ C −(−Δ C ))+(Δ C −(−Δ C ))]=4 K   1   ΔC  
 
     In the case of angular accelerations α, illustrated in  FIG. 10 , we obtain instead:
 
Δφ P   =K   1 [(Δ C −(−Δ C ))+(−Δ C−ΔC )]=0
 
     Instead, for the second accelerometer  16 , Eq. (2) enables amplification of the effects of angular accelerations due to the accelerations of yaw α Y  and the selective rejection of linear accelerations perpendicular to the surface  21   a  of the substrate  21  to be obtained. Again, with reference to  FIG. 10 , from Eq. (2) we obtain
 
α Y   =K   2 [(Δ C −(−Δ C ))−(−Δ C−ΔC )]=4 K   2   ΔC  
 
whereas, in the case of  FIG. 9  (effect of linear accelerations AL), we have
 
α Y   =K   2 [(Δ C −(−Δ C ))−(Δ C −(−Δ C ))]=0
 
     In practice then, the first accelerometer  15  senses only linear accelerations or forces having a component parallel to the optical axis OA and perpendicular to the second axis of detection Y (yaw axis) and is used as inclinometer for evaluating the variations of the pitch angle Δφ P . The second accelerometer  16  is selectively sensitive to the angular accelerations and is used as rotational accelerometer for determining the yaw accelerations α Y . 
     When the camera  1  is in the “portrait” configuration, the force of gravity acts prevalently along the first detection axis X of the third accelerometer  17 , and the orientation signal S XY  has a second value. 
     In this case, the second calculation mode of the first computation module  31  is selected, in which the second accelerometer  16  is used as inclinometer for measuring variations of the pitch angle Δφ P , and the first accelerometer  15  is used as rotational accelerometer for determining angular accelerations caused by the variations of the yaw angle Δφ Y  (yaw accelerations α Y ; in this case, the yaw axis coincides with the first detection axis X). In practice, the second detection axis Y is substantially horizontal, as illustrated in  FIG. 15 . The calculation is carried out according to the following equations: 
     
       
         
           
             
               
                 
                   
                     
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     In practice, the functions performed by the first and second accelerometers  15 ,  16  are swapped on the basis of the information supplied by the third accelerometer  17 . Consequently, the first accelerometer  15  is selectively sensitive to the angular accelerations about the yaw axis (first detection axis X) and rejects the linear accelerations. Instead, the second accelerometer  16  selectively rejects the angular accelerations and reacts to the linear accelerations and to the forces having a component parallel to the optical axis and perpendicular to the first detection axis X. 
     Returning to  FIG. 14 , the values of the yaw acceleration α Y , determined by the first computation module  31 , are supplied to the integrator module  33 , which integrates them twice to trace back to the variations of the yaw angle Δφ Y . 
       FIG. 16  shows, in greater detail, the processing unit  14 , which comprises a second computation module  35  and an image-processing module  36 . The second computation module  35  receives from the pre-processing stage  18  the variations of the pitch angle Δφ P  and the variations of the yaw angle Δφ Y  and accordingly calculates compensated co-ordinates X C , Y C  of the usable frame  12  ( FIG. 4   a ), so as to compensate for pitch and yaw movements and to stabilize the corresponding image. Stabilization is carried out according to criteria in themselves known. 
     The image-processing module  36  receives the first image signal IMG from the image sensor  5 , and the compensated co-ordinates X C , Y C  of the usable frame  12  from the second computation module  35 . On the basis of these compensated co-ordinates X C , Y C , the image-processing module  36  extracts the usable frame  12  from the complete image  11  ( FIGS. 4   a  and  4   b ) and generates the second image signal IMG′, stabilized. 
     The stabilizer device is mainly advantageous because accelerometers are used. Image stabilization can then be performed based on the detected movement, rather than on the content of the image itself, and, moreover, power absorption is minimal and in any case much lower than the consumption of stabilizer devices based on gyroscopes. Consequently, also the autonomy is improved, and the stabilizer device is particularly suited for being integrated in appliances for which power absorption is a critical factor, such as, for example, cellphones equipped with a camera. The stabilizer device described is moreover advantageous because the accelerometers used are simple and robust and, moreover, can be integrated in a single semiconductor chip. Also this feature renders the stabilizer device suitable for being incorporated within cellphones and other appliances of small dimensions. 
     Finally, it is evident that modifications and variations can be made to the stabilizer device described herein, without departing from the scope of the present invention. In particular, instead of oscillating-beam accelerometers, rotational accelerometers or linear accelerometers with comb-fingered electrodes may be used. In the first case, two rotational accelerometers with rotation axes perpendicular to one another and to the optical axis are sufficient. In the second case, two pairs of linear accelerometers with comb-fingered electrodes are necessary, arranged so as to differentially react to the accelerations directed along two axes perpendicular to one another and to the optical axis. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.