Patent Publication Number: US-6703602-B1

Title: Optical encoder with at least one lenticular sheet

Description:
FIELD OF THE INVENTION 
     The present invention relates to an optical encoder used for measuring a displacement of a movable member, such as an optical shaft angle encoder or a position sensor. 
     BACKGROUND OF THE INVENTION 
     Various types of optical encoders that are used to resolve the position and the movement of an object are known. FIG. 1 illustrates an example of an optical encoder disclosed in a German Laid-Open Patent Application (DE A1) No.2,316,248. The optical encoder comprises a light source  200 , a lens  202  which collimates a light beam from the light source  200 , a first fixed diffraction grating  204  and a second movable diffraction grating  206 , a condenser lens  208  and light receiving elements  210 ,  212  and  214 . The collimated light source  200  is incident on the first fixed diffraction grating  204  and then to a second movable diffraction grating  206 . As the movable diffraction grating  206  is displaced in the direction indicated by an arrow R, the interference fringes are moved on the light receiving elements  210 ,  212 ,  214  via the condenser lens  208 , resulting in a sinusoidal change in the amount of light received by the light receiving element  210 ,  212  and  214 . Thus, if the movable diffraction grating  206  moves a single pitch of the grating, the level of output from the light receiving elements  210 ,  212 ,  214  varies like a single period of sine wave. By sensing this change, the amount of displacement of the movable diffraction grating  206  can be determined. 
     Another typical optical encoder which is illustrated in FIG. 2 uses a shaft encoder  300  which includes an encoding wheel  302  having plurality of slits  304  therein. A light source  306  is positioned on one side of the encoding wheel  302 , while a photosensor  308 , such as a phototransistor, is positioned on the other side of the encoding wheel  302  opposite to the light source  306 . The rotation of the encoding wheel  302  therebetween generate a series of light pulses to be received by the photosensor  308 , by which the displacement of the encoding shaft  300  can be measured. 
     Although such a prior art approach has worked well depending on the measuring apparatus and the precision required, the optical encoder which use diffraction gratings offers high resolution, but also expensive to manufacture and relatively complex in their design compare to the encoder wheel. However, the low cost and simplicity the encoding wheel does not generate a very high resolution that is required by some devices, there is an upper limit on the number of slits that can be incorporated in an encoding wheel. 
     Thus, it is desirable to provide an optical encoder apparatus that produces high resolution, low cost of manufacturing and simple in design. 
     SUMMARY OF THE INVENTION 
     It is a primary objective of this invention to provide an optical encoder in which the aforementioned disadvantages are eliminated. 
     Another object of the present invention is to provide a simple optical encoder without using a collimating lens. 
     Yet another object is to provide an optical encoder having components which can be easily manufacture. 
     Yet another object is to provide an optical encoder in which a light source having a wide light-emitting surface can be use. 
     Yet another object of the present invention is to provide a two dimensional optical encoder. 
     Yet another object is to provide an optical encoder that applies over a large surface area. 
     According to one aspect of the present invention, an optical encoder comprising: 
     a light source emitting a light beam; 
     a first array of lenslets to which the light beams is directed; 
     a second array of lenslets or lens to which the light beams exiting from the first array of lenslets are directed; and 
     a means for obtaining the displacement information of one of the first or second array of lenslets is being displaced. The displacement information is obtained by the changing position of the dark and bright patterns generated by the individual lenslets, which define the light beam into fine beams of periodic pattern or dark and bright fringes as the light beam pass through the first and second array of lenslets. 
     Additionally, applying the same principle can create a two-dimensional optical encoder. The method is to superpose two array of lenslets, the arrays of lenslets are arranged in such a manner that the longitudinal axis of the lenslets are perpendicular to each other. Thus, the light beam that is being defined by the first layer composes of arrays of lenslets superpose perpendicularly and the exiting beam is then directed to a second similar layer which defines the light beam into two sets of dark and bright fringes. Thus generating a two dimensional optical encoder. 
     Another feature of the present invention is that array of lenslets can be manufactured quite economically in a large array size. Thus, the present invention is ideal for applications that require an optical encoder that covers a large surface area. An example of such application is an optical pen or mouse. 
     According to another aspect of the present invention, an optical encoder comprising: 
     a light source emitting a light beam; 
     a movable lenticular array to which the light beams is directed. The light beam passes through each individual lenslets which defines the light beam into a fine beam of periodic pattern; 
     a means for detecting the light pattern and measuring the output signal as the lenticular array is being displaced. 
     Similarly, a two-dimensional optical encoder can be obtained by superposing two lenticular arrays having the longitudinal axis of the lenslets arrange perpendicularly to each other. 
    
    
     The foregoing and other objects, features and advantages of the present invention will become more readily apparent from the following detailed description of a preferred embodiment, which proceeds with reference to the drawings. 
     DESCRIPTION OF THE DRAWING 
     FIGS. 1-2 illustrates the prior art optical encoder; 
     FIG. 3 illustrates the first embodiment of an optical encoder according to the present invention; 
     FIG. 4 represents a cross-sectional view along the line R—R of FIG. 3; 
     FIGS. 4A-C is a detail description of the operation the first embodiment shown in FIG. 3; 
     FIG. 5 is graph showing the amount of light received by the light receiving element as the position of the movable array of lenslets is being displaced; 
     FIG. 7 is an illustration of the principle of operation of the present invention; 
     FIGS. 8,  9 - 9 D illustrate the possible variations of the first embodiment shown if FIG. 3; 
     FIG. 10 is an illustration of a rotary optical encoder according to the present invention; 
     FIG. 11 is an illustration of a rotary optical encoder according to the present invention; 
     FIG. 12 is an illustration of a second embodiment of a two-dimensional optical encoder according to the present invention; 
     FIG. 13 is an illustration of a variation of a second embodiment of a two-dimensional optical encoder according to the present invention 
    
    
     DETAIL DESCRIPTION OF THE PREFER EMBODIMENT 
     A detail description will now be given, with reference to FIG. 3, which illustrates the first embodiment of an optical encoder according to the present invention. 
     The optical encoder shown in FIG. 3 comprises a light source  12 , a first and second array of lenslets  14  and  16  on which the light beam  20  is incident and a displacement information obtaining means  18 . 
     Referring now to the diagrammatic view of FIGS. 4-4C for an understanding of the operation of the first embodiment of present invention. FIG. 4 illustrates a cross-sectional view along the line R—R of FIG.  3 . The first array of lenslets  14  is parallel to the second array of lenslets  16 . The first array of lenslets is fixed and the second array of lenslets is movable in the direction indicated by an arrow R. In the greatly enlarged views of FIG. 4A, the structure of the arrays of lenslets  14  and  16  may be seen to be comprised of arrays of thin convex lenslets  10   a . In this particular illustration of the first embodiment of the present invention, the lenslets are considered to be thin lens since the width W of the lens is much greater than the thickness of the lens. However, it should be understood that the lenslets can be compose of thick or thin convex or concave or cylindrical or spherical lens or can be of more complex optical system and imaging element, such as hologram and other diffractive lens. FIG. 4A illustrates the principle of operation of the first embodiment of the present invention using Paraxial optics. Paraxial optics is used to determine the angle of refraction and size of the light beam  20  after being refracted by the arrays of lenslets  14  and  16 . The paraxial quantities provide information about ideal image formation in the selected set of coordinate. It will be seen later that the paraxial quantities serve as a basis for the actual ray paths through the optical system. The paraxial equation for a ray that passes though a lens can be described by the following expression:                [         I           B         ]     =     MTL        [         y           α         ]               (   1.1   )                         
     Where α is the angle and y is the height of the incident ray relative to the optical axis Z, M is the refraction matrix, T is the matrix that transform y into the same optical axis X of the lenslet  10   a , L is the translation matrix, I is the height and B is the angle of the refracted ray relative to the optical axis X of the lenslet  10   a . Thus, the matrix equation for the thin convex lenslet  10   a  can be express as:                [         I           B         ]     =           [         1       0               -   1          /        f         1         ]          [         t       a           0       1         ]            [         1       l           0       1         ]            [         y           α         ]               (   1.2   )                 [           y   ′             α         ]     =         [         t       a           0       1         ]          [         1       l           0       1         ]            [         y           α         ]               (   1.4   )                         
     To simplify the equation by:                [         I           B         ]     =       [         1       0               -   1          /        f         1         ]          [           y   ′             α         ]               (   1.5   )                         
     Which is equivalent to the algebraic equations              I   =     y   ′             (   1.6   )               B   =         -     y   ′       /   f     +   α             (   1.3   )                         
     From the equations 1.5 and 1.6 it is clear that for an incident ray with positive angle α, if y′ is negative then the refraction angle B is greater than the incident angle α, which mean that the incident ray is being refracted away from the optical axis Z and if y′ is positive and the refracted angle B is negative then the incident ray is being refracted toward the optical axis Z. By applying these equations to the incident ray  20   a  on FIG. 4A, it is possible to trace the incident ray  20   a  through the arrays of lenslets  14  and  16 . At the point A, where the incident ray  20   a  meet the lenslet  14   a , the equation 1.3 is used to calculate the translation of the incident ray  20   a  at a distance l from the first arrays of lenslets  14  and the height y′ of the incident ray  20   a . At the point A, y′ is negative, the equation 1.4 shown that the incident ray  20   a  is being refracted away from the optical axis Z. Now to trace the ray  20   a  from point A to B, the same equations 1.3 and 1.4 is applied again, but now the reflected ray become the incident ray. Thus, substitute y with y′ and α with B. Using the equation 1.3 to calculate the translation of the refracted ray  20   a  at a distance d from the first arrays of lenslets  14  to the second array of lenslets  16  and the height y″ a point B. At point B, y″ is negative, and again the equation 1.4 show that the ray  20   a  is further refracted from the optical axis Z. Now referring to FIG. 4B, using the same principle applied to the incident ray  20   a  to the incident rays  20   b ,  20   c  and  20   d ,it is possible to show that at the point D the incident ray  20   b  is being refracted away from the optical axis Z and at the point H and F the incident ray  20   c  and  20   d  is being refracted toward the optical axis Z and the light receiving element  18 . Since all rays at substantially any given angle incident on the first and second arrays of lenslets  14  and  16  will either similarly refracted away from the optical axis Z, or similarly refracted toward the optical axis Z, which will generate dark and bright patterns. By displaying the second arrays of lenslets  16  in the direction indicated by an arrow R, the rays exiting from the first array of lenslets  14  is incident on the second movable array of lenslets  16  at a different position y″. Using equation 1.3, y″ change as the second array of lenslets is being displace in the direction indicated by an arrow R, which will result in a shift in the position of the dark and bright fringes as illustrated by the FIG.  4 C. More specifically, if the movable second array of lenslets  16  moves a small distance corresponding to a single width W of a lenslet, the level of output from the light receiving element  18  varies like a single period of sine wave as shown in FIG.  5 . By sensing this change, the amount of displacement of the movable array of lenslets  16  can be determined. 
     Additionally, the light receiving element  18  can be a CCD, photodetector, photodetector array, fiber optic or other light receiving mean for obtaining information of the changing light patterns. The output of the light receiving element  18  may then be converted to a analog or digital signal indicative of the position and direction of the movable array of lenslets  16  by a manner well know to the art. 
     FIG. 7 is an illustration of a principle of an operation of the present invention. Any ray that passes through a lens, or through a combination of lenses, can be described by two parameters: by its direction α 0 , with respect to the axis Z, and by its height y  0  above the axis Z. As show in FIG. 7 each array of lenslets is represented by the letter A: A 1  represents the first array of lenslets, A 2  represents the second array of lenslets . . . and A N  represents the N th  array of lenslets. Also, each lenslet in an array is treated as an optical lens and can be represented by the letter L: L 11  represent the first lenslets in first array A 1 , L, 12  represent the second lenslet in the first array A 1 , . . . and L NM  represents the M th  lenslet in the N th  array. Moreover, each lenslet L can be further represented by four other matrix: M 111  is the matrix that transforms a paraxial ray relative to the optical axis Z into a paraxial ray relative to the optical axis of the lens L 11 , M 112  represents a matrix of the first refraction surface of the optical lens L 11  at point B in FIG. 7, M 113  represents the translation matrix of the lens L 11  between point C and D, M 114  represents a matrix of the second refraction surface of the optical lens L 11  at point D. By combining matrices that represent individual lenslet, the position of an image of the ray starting from point B to I can be obtained by the following expressions. 
     
       
         
           L 
           11 
           =M 
           114 
           M 
           113 
           M 
           112 
           M 
           11 
           , L 
           21 
           =M 
           214 
           M 
           213 
           M 
           212 
           M 
           211 
           , . . . L 
           N1 
           =M 
           N14 
           M 
           N13 
           M 
           N12 
           M 
           N11 
         
       
     
     
       
         
           
             
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                       B 
                       0 
                     
                   
                 
                 
                   
                     
                       I 
                       0 
                     
                   
                 
               
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             = 
             
               
                 L 
                 N1 
               
                
               
                   
               
                
               … 
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                
               
                 L 
                 21 
               
                
               
                   
               
                
               
                 
                   L 
                   11 
                 
                  
                 
                   [ 
                   
                     
                       
                         
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                           0 
                         
                       
                     
                     
                       
                         
                           y 
                           0 
                         
                       
                     
                   
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     If the array of lenslets A N  is displaced in the direction indicated by the arrow R, the Matrix M N11  . . . M NM1  are the only matrices that are being changed, since the position of the lenslet L N1  . . . L NM  are displaced. Thus M N11 ″ . . . M NM1 ′ represent the new matrices. Therefore, the equation to find the image of the ray can be obtained by the following expressions. 
     
       
           L   11   =M   114   M   113   M   112   M   111   , L   21   =M   211   M   213   M   212   M   211   , . . . L   N1   ′=M   N14   M   N13   M   N12   M   N11 ′ 
       
     
     
       
         
           
             
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                       B 
                       
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                 21 
               
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     The matrix method lends itself to various computer techniques for tracing a ray through an optical system of arbitrary complexity. The above expressions make it possible to trace a ray and the image of a ray after an array of lenslets is being displaced. Thus, by combining the above equations with modern optics theory and computer programming, it is possible to design different type of arrays of lenslets such that when combine they will define light beam into dark and bright fringes. 
     As stated earlier paraxial optics is seen to describe the ray path through the lens for real rays in an ideal aberration-free imaging situation. Ray tracing is the best method used for a complete description of a ray path through an optical system. A light ray travels in a straight line until it encounters a surface, at which point its direction and in general its amplitude is changed and it proceeds further. If one know the shapes and positions of all of the optical surface, and specifies a bundle of rays coming from some object, one can develop a procedure to find the location and size of an image exactly. The procedure is to take each ray, find where it intersects the first surface and at what angle so that Snell&#39;s law can be applied to get the direction of the refracted ray and follow this new ray to the next surface, find the new intersection point and angle applied Snell&#39;s law, get the new direction of the refracted ray, et cetera, until the rays reach the final image. The theory for ray tracing can be obtained from a textbook on optics, such as Allen Nussbaum, “Optical System Design” and R. R. Shannon, “The Art and Science of Optical Design”. 
     As an example of the application of the above mention principle of operation, FIG. 8, FIG. 9, FIG. 9A, FIG. 9B is a cross-section view of the variations of an optical encoder using different combination of arrays of lenslets to define light beam into dark and bright fringes by applying the principle described above. The optical encoder in FIG. 8 comprises a light source  12 , a movable lenticular array having plurality of cylindrical lenses  28  and a light receiving element  18 . FIG. 9 illustrates an optical encoder comprising a light source  12 , a first and second lenticular array having plurality of cylindrical lenses  28  and  29 , a movable lenticular array having plurality of biconcave lenses  30  and a light receiving element  18 . FIG. 9A illustrates an optical encoder comprising a light source  12 , a first and second lenticular array  102  and  104  and a light receiving element  18 . The lenticular array  102  and  104  are composed of array of concave lenslets. FIG. 9B illustrates an optical encoder comprising a light source  12 , a first and second movable lenticular array  110  and  112  and a light receiving element  18 . The lenticular array  110  and  112  are composed of array of cylindrical lenslets. 
     FIG. 9C illustrates another example of an optical encoder comprises of a light source  12 , a first and second movable lenticular array  120  and  122 , each lenticular array are composed of plurality of spherical lenslets and a light receiving element  18 . The phasing in and out of axial alignment of the spherical lenslets either focus the light ray or refract the light ray, which generate a Moiré like pattern. As the second movable array is being displaced the Moiré like pattern change. The optical effect creates by the superposition of the lenticular arrays  120  and  122  are disclosed in the U.S. Pat. No. 3,357,772 entitles “Phased Lenticular Sheets for Optical Effect”. However, the application and the use of the lenticular sheets disclosed in the U.S. Pat. No. 3,357,772 are different and the lenticular sheet are not movable, but glue together to form a Moiré like pattern. 
     For optical encoder that comprises of two or more lenticular sheets, the distance between two lenticular sheets is quite important in obtaining a sharp pattern. For best result the distance that separate the two lenticular sheet should not be substantially more than three to four time the focal length of the lenslet. It has been found that a transparent coating on both side of the lenticular sheet can greatly enhance the optical pattern. For example, a clear epoxy coating employing a modified cycloaliphatic amine hardener and an epichlorohydrin and bisphenol—A product on both sides of the lenticular sheets has been found to provide good results with both cellulose propionate and polycarbonate lenticular sheet. Also, the distance that separates the two lenticular sheets can be made greater than four time the focal length of its lenslets by coating both side of the lenticular sheet. 
     The above mentioned lenticular sheets are generally manufactured by extrusion molding using thermoplastic resin. Beside glass, various synthetic plastic may be use for the plastic sheet material of the present invention, including acrylic acid esters and copolymers thereof, polystyrene, polycarbonates, cellulose nitrate, polypropylene and polyethylene. The manufacturing process of the lenticular is described in a Japanese Patent Application Laid-open No. Hei 3-200948 and Hei 2-146536. Because lenticular sheet can be manufacture inexpensively and the ability to produce lenticular lens having very fine pitch, such as more than 300 lines per inch, make it possible to fabricate inexpensive and high resolution optical encoder. Also, the clear plastic lenticular sheet allow the optical encoder to be used in conjunction with a mirror or reflective coating to create a reflective optical encoder. 
     Anyone skills in the art can create a program using the principle describe above that will simulate all the condition required to built a similar optical encoder to the present invention. Although, the first embodiment of the present invention only requires two arrays of lenslets, it can be easily deduced from the above principle and examples that any number of arrays of lenslets can be used, as long as at least one of the array of lenslets is a movable array. Moreover, lenslets of different sizes and shapes or of more complex optical system and imaging element, such as a hologram or other diffractive lens can be used as long as the combined optical system defines the light beam into dark and bright fringes. 
     Diffractive lens can be formed of a grating comprising plurality of line, either parallel or concentric which causes diffraction of incoming light to give the desire focusing effect. One such type of diffractive lens is described in the patent application of H. P. Kleinknecht, from United State with Ser. No. 754,134 entitled “Optical Imager With Diffractive Lenticular Array”. 
     Another type of lenticular array can be in the form of a hologram. A hologram is generated by the interference pattern between coherent beam passing through a mask and a reference beam. Thus, a hologram having plurality of patterns or holographic array of lenslets that define light beam into dark and bright fringes can be used as the lenticular array. 
     Another embodiment of the present invention is illustrated in FIG.  9 D. The optical encoder  140  comprises a light source  12 , a lenticular array having plurality of cylindrical lenslet  141 , a movable ball lens  142  and a movable light receiving element  18 . As the ball lens  142  and light receiving element  18  are being displaced in the direction of the line indicated by R, the ball lens  142  either focus light rays onto the light receiving element  18  or scatter light ray away from the light receiving element  18 . 
     Although, the above-mentioned embodiments are linear optical encoder due to the linear displacement of the movable array of lenslets  16 ,  28  or  30 , a rotary or spherical optical encoder can also be formed using the principle of the present invention. 
     FIGS. 10 and 11 illustrate examples of a rotary encoder using the above-mentioned principle. The rotary encoder of FIG. 10 comprises the light source  32 , a fixed array of lenslets  34 , a rotatable array of lenslets  33  which is formed on a cylindrical surface and a light receiving element  31 . The optical encoder of FIG. 11 comprises the light source  36 , a fixed array of lenslets  37 , a rotatable array of lenslets  35  which is formed on a plane circular surface and a light receiving element  38 . The rotatable arrays of lenslets  33  and  35  both rotate about an axis X. 
     One of the greatest advantages of the present invention using the principle described above is in the application of two-dimensional optical encoder. Conventional two-dimensional optical encoders are made of a combination of two linear one-dimensional optical encoder positions at 90 degree from each other on the same plane surface. Such a design can lead to a more complex and bulky apparatus and ultimately can increase the cost of production or in some case impossible to miniaturize the apparatus. Thus a simple two-dimensional optical encoder as illustrates in FIG. 12 will overcome the above-mentioned limitations. 
     A detail description will now be given, with reference to FIG. 12, which illustrates the second embodiment of the present invention. The two-dimensional encoder comprises a light source  50 , a first and second movable arrays of lenslets  51  and  52  which are placed on the top of each other forming a single layer two-dimensional lenslet array, a first and second fixed arrays of lenslets  48  and  49  and a light receiving elements  47  and  46 . In the two-dimensional lenslets array, the longitudinal axis of the lenslets  56  of the array of lenslets  51  extend along the X axis and the longitudinal axis of the lenslets  58  of the array of lenslets  52  extend along the Y-axis. The fixed array of lenslets  48  and  49  are positioned substantially perpendicular to each other and parallel above the two-dimensional lenslet array. The beam of light is first incident on the two-dimensional lenslets array, which is defined by the individual lenslet  56  and  58 , the exited light beam is then directed at the fixed array of lenslets  48  and  49 . Each array of lenslet  48  and  49  will further defines the exiting light beam into two set dark and bright fringes. As the two-dimensional lenslet array is displaced parallel to the X axis, only the position of the dark and bright fringes that are generated by the fixed array of lenslets  48  will change, for only the angle of the incident light beam exiting from the two-dimensional lenslet array will change relative to surface  60  of the individual lenslets  61 . Similarly, if the two dimensional lenslet array is displaced parallel to the Y axis, only the position of the dark and bright fringes that are generated by the fixed array of lenslets  49  will change position, for only the angle of the incident light beam exiting from the two dimensional lenslet array will change relative to the surface  62  of the individual lenslet  63 . That is, by sensing the change in the position of the two sets of dark and bright fringes, the light receiving element  47  and  48  will generate a corresponding electrical signal, which can be used to determine the displacement of the two-dimensional lenslet array in the XY plane. 
     FIG. 13 illustrates a two dimensional rotational optical encoder, which uses the same principle described for the second embodiment in FIG.  12 . The two-dimensional rotational encoder comprises a light source  45 , a first and second movable array of lenslet  43  and  44  which are formed on a cylindrical surface and inserted together forming a two layer cylindrical array of lenslets, and light receiving elements  39  and  40 . The longitudinal axis of the lenslets  68  of the array of lenslets  44  extend circularly along the X axis and the longitudinal axis of the lenslet  69  of the array of lenslets  43  extend circularly along the Y axis. In order to generate a two-dimensional rotational encoder, the movable array of lenslets  44  and  43  rotate around the X-axis and is displaced along the direction provided by the arrow R. 
     Additionally, it is possible to create variations of the two-dimensional encoder illustrate in FIGS. 12 and 13 by simply omitting the fixed arrays of lenslets  48  and  49  from the embodiment in FIG.  12  and the fixed arrays of lenslets  44  and  43  from the embodiment in FIG.  13 . 
     It should be noted that having fully described a preferred embodiment of the invention and various alternatives, those skilled in the art will recognize, given the teaching herein, that numerous alternatives and equivalent exist which do not depart from the scope of the invention. It is therefore intended that the invention not be limited by the foregoing description, but only by the claims. 
     Additionally, in the above-mention embodiments thereof having more than two arrays of lenslets, any one of the arrays of lenslets may be a movable array of lenslets while the other arrays are made to be fixed.