Abstract:
The present disclosure describes an optical displacement sensor having a dense multi-axis array of photosensitive elements. Generally, the sensor includes a two dimensional array of multiple photosensitive elements. In one embodiment, the array includes multiple linear arrays of photosensitive elements arranged along three or more axes in a space-filling, close-packed multi-axis array. The photosensitive elements are connected to each other in such a way that motion is determinable along each of the axes by measuring differential photocurrents between photosensitive elements along each of the axes. The inventive architecture advantageously increases signal redundancy, and reduces signal drop-out or low signals due to random fluctuations in the incident or absorbed light or in the signals from the photosensitive elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of U.S. provisional patent application No. 60/629,733, entitled “Dense Multi-Axis Array Architectures for Motion Sensing,” filed Nov. 19, 2004, by inventors Michael J. Dueweke and Douglas A. Webb. The disclosure of which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates generally to an Optical Postioning Device (OPD).  
       BACKGROUND  
       [0003]     Pointing devices, such as computer mice or trackballs, are well known for inputting data into and interfacing with personal computers and workstations. Such devices allow rapid relocation of a cursor on a monitor, and are useful in many text, database and graphical programs. A user controls the cursor, for example, by moving the mouse over a surface to move the cursor in a direction and over distance proportional to the movement of the mouse. Alternatively, movement of the hand over a stationary device may be used for the same purpose.  
         [0004]     Computer mice come in both optical and mechanical versions. Mechanical mice typically use a rotating ball to detect motion, and a pair of shaft encoders in contact with the ball to produce a digital signal used by the computer to move the cursor. One problem with mechanical mice is that they are prone to inaccuracy and malfunction after sustained use due to dirt accumulation, etc. In addition, the movement and resultant wear of the mechanical elements, particularly the shaft encoders, necessarily limit the useful life of the device.  
         [0005]     One solution to the above problems has been the development of optical mice. Optical mice have become very popular, because they provide better pointing accuracy. One approach for measuring linear displacements uses an array of photo-sensors or detectors, such as photodiodes, in which the output of the individual detectors in the array are wired together in a repeating pattern spanning two or more detectors, referred to here as the M value of the array. Generally, the distribution of spatial frequencies in an image on the array is roughly centered around the spatial frequencies of the detectors, and signal processing techniques are used to track the movement of this image back and forth across the array.  
         [0006]     The dominant architecture used in previous embodiments used two or more one dimensional (1D) arrays at substantially orthogonal angles to one another to track motion in two directions or dimensions. The drawback of this approach includes limited accuracy along directions that deviate significantly from the detector-array orientations. This is due to the fact that an off-axis image enters and leaves the array too quickly before the image has a chance to build-up an unambiguous signal. Thus, this approach does not provide sufficient redundancy for low-noise operation. In particular, for linear comb arrays having axes oriented orthogonally, motion primarily along one axis leads to a small signal in the orthogonal axis, and hence a small signal to noise ratio along the orthogonal axis.  
         [0007]     An alternative implementation uses correlation uses a square two dimensional (2D) array consisting of pixel architectures that take differences along orthogonal axes, and therefore also do not provide needed redundancy in the outputs.  
         [0008]     Accordingly, there is a need for an optical pointing device that is capable of detecting movement with an acceptable path error in both on-axis and off-axis directions. It is further desirable that the array have an architecture that simplifies data processing requirements.  
       SUMMARY  
       [0009]     The present disclosure provides a solution to these and other problems, and offers further advantages over conventional optical displacement sensors or systems for sensing relative movement between the sensor and a surface by detecting changes in optical features of light reflected from the surface.  
         [0010]     The present disclosure describes an optical displacement sensor having a dense multi-axis array of photosensitive elements. Generally, the sensor includes a two dimensional array of multiple photosensitive elements. In one embodiment, the array includes multiple linear arrays of photosensitive elements arranged along three or more axes in a space-filling, close-packed multi-axis array. The photosensitive elements are connected to each other in such a way that motion is determinable along each of the axes by measuring differential photocurrents between photosensitive elements along each of the axes. The inventive architecture advantageously increases signal redundancy, and reduces signal drop-out or low signals due to random fluctuations in the incident or absorbed light or in the signals from the photosensitive elements. In some embodiments, each of the axes form non-orthogonal angles to the remaining axes. In other embodiments, certain pairs of axes may form orthogonal angles.  
         [0011]     The present disclosure also describes a method of sensing relative movement between an optical displacement sensor or system and a surface by detecting changes in optical features of light reflected from the surface.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     These and various other features and advantages of the present invention can be understood more fully from the detailed description that follows and from the accompanying drawings, which however, should not be taken to limit the appended claims to the specific embodiments shown, but are for explanation and understanding only, where:  
         [0013]      FIG. 1  is a functional block diagram of an optical mouse for which a multi-axis array architecture according to an embodiment of the present invention is particularly useful;  
         [0014]      FIG. 2  is a schematic block diagram including a one-dimensional (1D) linear comb-detector array in a ‘4N’ configuration;  
         [0015]      FIGS. 3A, 3B , and  3 C are diagrams of a multi-axis array architecture according to an embodiment of the present invention showing a dense packed hexagonal lattice of photodiodes;  
         [0016]      FIGS. 4A and 4B  are layout diagrams of a three-axis photodiode array architecture having concentric, tapered linear arrays according to an embodiment of the present invention; and  
         [0017]      FIGS. 5A, 5B , and  5 C are alternative layout diagrams of multi-axis array architectures according to embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     The present disclosure relates generally to a sensor for an Optical Positioning Device (OPD), and to methods for sensing relative movement between the sensor and a surface based on displacement of a random intensity distribution pattern of light generated from the surface. The random distribution pattern of light can include, for example, speckle that is generated by scattering of light from a coherent source scattered off of a rough surface. OPDs can include, for example, optical mice or trackballs for inputting data to a personal computer.  
         [0019]     Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification do not necessarily all refer to the same embodiment.  
         [0020]     The following description describes a photodetection technique which combines the 2D displacement measurement accuracy of a correlator with the signal processing simplicity of a comb-array.  
         [0000]     Optical Positioning Device Architecture  
         [0021]     Generally, the sensor includes an illuminator having a light source and illumination optics to illuminate a portion of the surface, a detector having a number of photosensitive elements and imaging optics, and signal processing or mixed-signal electronics for combining signals from each of the photosensitive elements to produce an output signal from the detector.  
         [0022]     An exemplary embodiment of an optical mouse for which the signal processing algorithm according to the present invention is particularly useful will now be described with reference to  FIG. 1 .  FIG. 1  is functional diagram of an optical positioning device  100  having at least two sets of multiple CMOS photodiode arrays, or Linear Comb Arrays (LCAs)  110 , configured to provide displacement measurements along two orthogonal axes, x and y. Groups of the photodiodes in each array are passively combined to produce group signals, which are subsequently algebraically combined to produce an (x, y) signal providing information on the magnitude and direction of displacement of the OPD in x and y directions.  
         [0023]     The system  100  shown in  FIG. 1  includes a laser source  102 , illumination optics  104 , imaging optics  108 , at least two sets of multiple CMOS photodiode arrays  110 , front-end electronics  112 , signal processing circuitry  114 , and interface circuitry  118 . As mentioned above, the photodiode arrays  110  may be configured to provide displacement measurements along two orthogonal axes, x and y. Groups of the photodiodes in each array may be combined using passive electronic components in the front-end electronics  112  to produce group signals. The group signals may be subsequently algebraically combined by the signal processing circuitry  114  to produce an (x, y) signal providing information on the magnitude and direction of displacement of the OPD in x and y directions. The (x,y) signal may be converted by the interface circuitry  118  to x,y data  120  which may be output by the OPD. Sensors using this detection technique may have arrays of interlaced groups of linear photodiodes known as “differential comb arrays.” 
         [0024]     Although described in detail with reference to LCAs of CMOS photodiodes, it will be understood that the method of the present invention is not limited to any one particularly type of photosensor, i.e., CMOS photodiodes, but can be applied equally well to any type of LCA photodetector CMOS, CCD, etc.  
         [0000]     Linear Comb Arrays (LCAs)  
         [0025]     As noted above, the output of the individual detectors in the LCA are wired together in a repeating pattern spanning two or more detectors, referred to as the M value of the array. An example of an LCA with an M value of  4  is shown in  FIG. 2 .  
         [0026]      FIG. 2  shows a general configuration (along one axis) of such a photodiode array  202 , wherein the surface  204  is illuminated by a coherent light source, such as a Vertical Cavity Surface Emitting Laser (VCSEL)  206  and illumination optics  208 , and wherein the combination of interlaced groups in the array  202  serves as a periodic filter on spatial frequencies of light-dark signals produced by the speckle images. Speckle generated by the rough surface  204  is mapped to the detector plane with imaging optics  210 .  
         [0027]     In one embodiment, the comb array detection is performed in two independent, orthogonal arrays to obtain estimations of displacements in x and y. A small version of one such array  202  is depicted in  FIG. 2 .  
         [0028]     Each array in the detector consists of a number, N, of photodiode sets, each set having a number, M, of photodiodes (PD) arranged to form an MN linear array. In the embodiment shown in  FIG. 2 , each set consists of four photodiodes (4 PD) referred to as 1,2,3,4. The PD 1 s from every set are electrically connected (wired sum) to form a group, likewise PD 2 s, PD 3 s, and PD 4 s, giving four signal lines coming out from the array. Their corresponding currents or signals are I 1 , I 2 , I 3 , and I 4 . These signals (I 1 , I 2 , I 3 , and I 4 ) may be called group signals. Background suppression (and signal accentuation) may be accomplished by using differential analog circuitry  212  to generate an in-phase differential current signal  214  (I 13 )=I 1 -I 3  and differential analog circuitry  216  to generate a quadrature differential current signal  218  (I 24 )=I 2 -I 4 . These in-phase and quadrature signals may be called line signals. Comparing the phase of I 13  and I 24  permits detection of the direction of motion.  
         [0029]     Referring to  FIG. 2 , the image captured on such a detector  202  may be magnified or de-magnified to achieve matching and so that the distribution of spatial frequencies in the image is roughly centered around the spatial frequencies of the detector. Through use of signal processing, it is possible to track the movement of this image as it moves back and forth across the detector  202  and from that tracking derive the motion of the surface  204  relative to the detector  202  along the long axis of the detector. Current processing techniques require that detector  202  outputs be weighted with coefficients derived from sine and cosine waves, combined together, and processed to produce two quasi-sinusoidal outputs representing separate in-phase and quadrature signals. These signals are then used to track motion.  
         [0000]     Multi-Axis Array Detector for Motion Sensing  
         [0030]     In accordance with an embodiment of the present invention it is desirable to have at least three 1-D detections to obtain both components of motion. This is particularly useful for speckle based sensors, since speckle is random, and any given eigenfunction could fade out, causing the loss of a signal. Note that no amount of filtering, e.g., Kalman filtering, can provide the correct motion if the user changes direction while the signal has faded. Thus, it is desirable to provide at least three signals to give some level of resistance to fading; more, of course, would be better, although the strategies for handling the extra information get a bit more complicated.  
         [0000]     Strategies for Multi-Axis Arrays  
         [0031]     There are a couple of different strategies or approaches that can be taken in placement of the arrays.  
         [0032]     The present disclosure provides a means for embodying several dependent axes within a confined space. In accordance with an embodiment of the disclosed invention, photodiode elements arranged along three or more axes in a space-filling, close-packed array, are connected to each other in such a way that motion may be determined along each of the three or more axes by measuring differential photocurrents between diode array elements along each of the three or more axes.  
         [0033]     In one embodiment, the array consists of a hexagonal close-packed array that may provide the densest configuration of multiple dependent axes for motion detection. An example of a three-axis hexagonal array consisting of six types of photodiodes (two for each axis) is shown in  FIG. 3A .  FIG. 3A  is a diagram of a test structure, showing a unit cell  300  of a dense packed hexagonal lattice. The unit cell  300  consists of twelve hexagonal photodiode cells of six different types. The photodiode types are labeled 1, 2, 3, 4, 5, and 6, and they are used to detect motion along three dependent axes. Photodiode cells of a same type are electrically coupled by conductive connections  302 . Implementation of metal routing  304  along hexagonal pixel edges as shown in  FIG. 3A . also provides light blocking between pixels to better delineate individual pixels and reduce cross-talk.  
         [0034]      FIG. 3B  illustrates three neighboring unit cells  300  of the array, and how they adjoin each other. Referring to  FIG. 3C , motion along each axis is determined by the differential current between i) types  1  and  2  (60 degree axis  312 ), ii) types  3  and  6  (120 degree axis  314 ), and iii) types  4  and  5  (180 degree axis  316 ).  
         [0035]     Another embodiment of a space-filling, multi-axis array is shown in  FIG. 4A .  FIG. 4A  is a schematic diagram of a unit cell  400  of a compact, space-filling 3-axis axis photodiode array, consisting of three “concentric,” tapered linear arrays. In this embodiment, the three concentric tapered linear arrays  402  are placed with coincident centers  403  and tapered toward the common center  403  in a ‘pin-wheel’ type configuration. Each concentric tapered linear array comprises two opposing triangular sections  404 . As shown in  FIG. 4A , the triangular sections in this example are labeled 1, 2, 3, 4, 5,and 6. The photodiode cells within each triangular section are electrically coupled by conductive connections. As illustrated in  FIG. 4B , motion along each of three axes is determined by the differential current between i) sections  1  and  2  (60 degree axis  412 ), ii) sections  3  and  6  (120 degree axis  414 ), and iii) sections  4  and  5  (180 degree axis  416 ).  
         [0036]     This configuration provides multiple dependent axes within a compact space.  
         [0037]     The ‘pin-wheel’ type configuration may be generalized to provide one or more additional axes as shown, for example, in  FIGS. 5A and 5B . Within a single axis there can also be redundant channels to provide for dropouts or low signal level.  
         [0038]     A generalization of ‘pin-wheel’ type configurations including additional numbers of axes is shown in  FIGS. 5A  to  5 C.  
         [0039]      FIG. 5A  shows a configuration with four concentric tapered linear arrays, each such array comprising two opposing triangular sections. Since the unit cell is in the shape of a square, it may be used to form a close-packed array. As shown in  FIG. 5A , the triangular sections in this example are labeled 1, 2, 3, 4, 5, 6, 7 and 8.  
         [0040]     The photodiode cells within each triangular section are electrically coupled by conductive connections. In this configuration, motion along each of four axes is determined by the differential current between i) sections  1  and  2 , ii) sections  3  and  4 , iii) sections  5  and  6 , and iv) sections  7  and  8 .  
         [0041]      FIG. 5B  shows a unit cell configuration with six concentric tapered linear arrays, each such array comprising two opposing triangular sections. Since the unit cell is in the shape of a hexagon, it may be used to form a close-packed array. As shown in  FIG. 5B , the triangular sections in this example are labeled 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12. The photodiode cells within each triangular section are electrically coupled by conductive connections. In this configuration, motion along each of six axes is determined by the differential current between i) sections  1  and  2 , ii) sections  3  and  4 , iii) sections  5  and  6 , iv) sections  7  and  8 , v) sections  9  and  10 , vi) sections  11  and  12 .  
         [0042]      FIG. 5C  shows a pin-wheel type configuration with five triangular sections. Since the unit cell is in the shape of a pentagon, it may not be used to form a close-packed array. As shown in  FIG. 5B , the triangular sections in this example are labeled 1, 2, 3, 4, and 5. The photodiode cells within each triangular section are electrically coupled by conductive connections. In this configuration, motion along each of five axes may be determined by the appropriate linear combinations of the currents from the five sections.  
         [0043]     An advantage of the apparatus and method of the present invention over previous or conventional approaches include: (i) fade resistant signal for off axis movement through the use of multiple dependent axes; (ii) reduced occurrence of dropouts or low signal level through use of redundant channels within a single axis; (iii) increased efficiency in the utilization of die space by enabling implementation of multiple dependent axes within a confined space; and (iv) metal routing between along pixel edges provides light blocking between pixels to better delineate individual pixels and reduce cross-talk.  
         [0044]     The foregoing description of specific embodiments and examples of the invention have been presented for the purpose of illustration and description, and although the invention has been described and illustrated by certain of the preceding examples, it is not to be construed as being limited thereby. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and many modifications, improvements and variations within the scope of the invention are possible in light of the above teaching. It is intended that the scope of the invention encompass the generic area as herein disclosed, and by the claims appended hereto and their equivalents.