Abstract:
An optical encoder includes an emitter, a first lens, a detector, a second lens, and a protrusion. The emitter emits light which is directed by the first lens to a code scale for reflection. The reflected light is directed by the second lens to the detector. The detector detects the reflected light from the code scale. The protrusion is between the first lens and the second lens. The protrusion defines at least one surface that refracts stray light from the emitter away from the detector. Accordingly, the stray light does not reach the detector; thus the detector can operate more effectively.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to optical encoders. More particularly, the present invention relates to improved optical encoders having higher contrast than prior art encoders.  
         [0002]     Optical encoders detect motion and typically provide closed-loop feedback to a motor control system. When operated in conjunction with a code scale, an optical encoder detects motion (linear or rotary motion of the code scale), converting the detected motion into digital signal that encode the movement, position, or velocity of the code scale. Here, the phrase “code scale” includes code wheels and code strips.  
         [0003]     Usually, motion of the code scale is detected optically by means of an optical emitter and an optical detector. The optical emitter emits light impinging on and reflecting from the code scale. A typical code scale includes a regular pattern of slots and bars that reflect light in a known pattern. Light is either reflected or not reflected from the code scale. The reflected light is detected by the optical detector. As the code scale moves, an alternating pattern of light and dark corresponding to the pattern of the bars and spaces reaches the optical detector. The optical detector detects these patterns and produces electrical signals corresponding to the detected light, the electrical signals having corresponding patterns. The electrical signal, including the patterns, can be used to provide information about position, velocity and acceleration of the code scale.  
         [0004]      FIG. 1A  illustrates a cross sectional side view schematic of a known optical encoder  100  and a code scale  120 .  FIG. 1B  is the code scale  120  as viewed from the optical encoder  100 .  FIGS. 1A and 1B  include orientation axes legend for even more clarity.  
         [0005]     Referring to  FIGS. 1A and 1B , the encoder  100  includes an optical emitter  102  and an optical detector  104  mounted on a substrate  106  such as a lead frame  106 . The optical emitter  102  and the optical detector  104  as well portions of the lead frame  106  are encapsulated in an encapsulant  108  including, for example, clear epoxy. The encapsulant  108  defines a first dome-shaped surface  110  (first lens  110 ) over the optical emitter  102  and a second dome-shaped surface  112  (second lens  112 ) over the optical detector  104 .  
         [0006]     The optical emitter  102  emits light  114  that leaves the encapsulant  108  via the first lens  110 . The first lens  110  concentrates or directs the emitted light  114  toward the code scale  120 , the light reflecting off of the code scale  120 . The reflected light  116  reaches the optical detector  104  via the second lens  112 . The second lens  112  concentrates or directs the reflected light toward the optical detector  104 . The optical detector  104  can be, for example only, photo detector that converts light into electrical signals.  
         [0007]     The shape and the size of the first lens  110  and the second lens  112  are dictated by various factors such as, for example only: the distance of the code scale  102  from the lenses  110  and  112  and the characteristics of the emitter  102  and the detector  104 .  
         [0008]     Often, space  118  between the lenses  110  and  112  is filled with the same encapsulant  108  material and has a flat surface  117 . The flat surface  117  presents a surface from which stray light such as stray light  119  from the emitter  102  reflects to impinge on the detector  102  as reflected stray light  121 . Such stray light  119  is not desired because stray light that reach the detector  102  introduces false signals, lowers resolutions at which the desired signals can be analyzed.  
         [0009]     Accordingly, there remains a need for improved optical encoder that alleviates or overcomes these shortcomings.  
       SUMMARY  
       [0010]     The need is met by the present invention. In a sample embodiment of the present invention, an optical encoder includes an emitter, a first lens, a detector, a second lens, and a protrusion. The emitter emits light which is directed by the first lens to a code scale for reflection. The reflected light is directed by the second lens to the detector. The detector detects the reflected light from the code scale. The protrusion is between the first lens and the second lens. The protrusion defines at least one surface that refracts stray light from the emitter away from the detector. Accordingly, the stray light does not reach the detector; thus the detector can operate more effectively.  
         [0011]     The protrusion can be formed in many different shapes. For example, the protrusion can have frustum shape including, but not limited to, a frustum of a circular cone. Alternatively, the protrusion can have, as additional examples only, pyramid shape or a generally hemispherical shape. The protrusion connects the first lens and the second lens. In fact, the protrusion and the two lenses can be made from the same encapsulant material. The encapsulant material is formed to include surfaces that define the first lens, the second lens, the protrusion, or any combination of these. Further, the encapsulant material encapsulates the emitter, the detector, or both, with the first lens being proximal to the emitter and the second lens being proximal to the detector.  
         [0012]     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1A  illustrates a cross sectional side view schematic of a known optical encoder and a code scale;  
         [0014]      FIG. 1B  is the code scale of  FIG. 1A  as viewed from the optical encoder of  FIG. 1A ;  
         [0015]      FIG. 2  illustrates an optical encoder according to one embodiment of the present invention;  
         [0016]      FIG. 3  illustrates an optical encoder according to another embodiment of the present invention;  
         [0017]      FIG. 4  illustrates an optical encoder according to yet another embodiment of the present invention; and  
         [0018]      FIG. 5  includes a graph including curves useful for illustrating operating characteristics of an optical encoder of the present invention as compared with those of a prior art optical encoder.  
     
    
     DETAILED DESCRIPTION  
       [0019]     The present invention will now be described with reference to the Figures which illustrate various embodiments of the present invention. In the Figures, some sizes of structures or portions may be exaggerated and not to scale relative to sizes of other structures or portions for illustrative purposes and, thus, are provided to illustrate the general structures of the present invention. Furthermore, various aspects of the present invention are described with reference to a structure or a portion positioned “on” or “above” relative to other structures, portions, or both. Relative terms and phrases such as, for example, “on” or “above” are used herein to describe one structure&#39;s or portion&#39;s relationship to another structure or portion as illustrated in the Figures. It will be understood that such relative terms are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures.  
         [0020]     For example, if the device in the Figures is turned over, rotated, or both, the structure or the portion described as “on” or “above” other structures or portions would now be oriented “below,” “under,” “left of” “right of” “in front of,” or “behind” the other structures or portions. References to a structure or a portion being formed “on” or “above” another structure or portion contemplate that additional structures or portions may intervene. References to a structure or a portion being formed on or above another structure or portion without an intervening structure or portion are described herein as being formed “directly on” or “directly above” the other structure or the other portion. Same reference number refers to the same elements throughout this document.  
         [0021]     Referring to  FIG. 2 , a cross sectional side view schematic of an optical encoder  200  in accordance with one embodiment of the present invention is illustrated. The optical encoder  200  includes an emitter  102  operable to emit light. The emitted light is directed by a first lens  110  toward a code scale  120  for reflection. The reflected light is directed by a second lens  112  toward a detector  104 . The detector  104  is adapted to detect the reflected light directed by the second lens  112 . The emitter  102  and the detector  104  may be mounted on a substrate  106  such as a lead frame  106 .  
         [0022]     The optical emitter  102  is encapsulated in an encapsulant material  108  including, for example, clear epoxy. The encapsulant  108  includes a dome-shaped surface  110  that defines the first lens  110 . The first lens  110  is proximal to the emitter. The same encapsulant material  108  is used, in the illustrated sample embodiment, to encapsulate the detector  104  and form a dome-shaped surface  112  that defines the second lens  112 . The second lens  112  is proximal to the detector.  
         [0023]     The same encapsulant material  108  is used, in the illustrated sample embodiment, to form a protrusion  202 . The protrusion  202  is between the first lens  110  and the second lens  112 . In fact, the protrusion  202  connects the first lens  110  and the second lens  112 . The protrusion  202  defines protrusion surfaces  203  that refract the stray light  119  such that the refracted stray light  205  does not reach the detector  104 . Thus, the stray light  119  is prevented from reaching the detector  104 .  
         [0024]     Again,  FIG. 2  illustrated the optical encoder  200  in a cross sectional side view. In three dimensions, the protrusion  202  is, as illustrated in  FIG. 2 , a frustum shape—frustum of a pyramid or frustum of a circular cone.  
         [0025]      FIG. 3  illustrates cross sectional side view of another embodiment of the optical encoder of the present invention as an optical encoder  300 . Referring to  FIG. 3 , portions of the optical encoder  300  are similar to corresponding portions of the optical encoder  200  of  FIG. 2 . The optical encoder  300  includes a protrusion  302  that has pyramid shape that present surfaces  303  at an angle  307  different than the angle  207  of the surfaces  203  of the protrusion  202  of the optical encoder  200  of  FIG. 2 . With the optical encoder  300 , similar desired result in achieved. That is, the protrusion  302  and its surfaces  303  refract the stray light  119  such that the refracted stray light  305  does not reach the detector  104 . Thus, the stray light  119  is prevented from reaching the detector  104 .  
         [0026]      FIG. 4  illustrates cross sectional side view of yet another embodiment of the optical encoder of the present invention as an optical encoder  400 . Referring to  FIG. 4 , portions of the optical encoder  400  are similar to corresponding portions of the optical encoder  200  of  FIG. 2 . The optical encoder  400  includes a protrusion  402  that generally has hemispherical shape that present a curved surface  403 . With the optical encoder  400 , similar desired result in achieved. That is, the protrusion  402  and its surface  403  refracts the stray light  119  such that the refracted stray light  405  does not reach the detector  104 . Thus, the stray light  119  is prevented from reaching the detector  104 .  
         [0027]     Referring to  FIGS. 2, 3 , and  4 . The each of the protrusions  202 ,  302 , and  402  connect the first lens  110  and the second lens  112 . In fact, the protrusions  202 ,  302 , and  402  are made with the same encapsulant material  108  as the first lens  110  and the second lens  112 .  
         [0028]      FIG. 5  illustrates two curves  500  and  502 . The first curve  500  demonstrates measured image contrast at various resolutions measured using the prior art optical encoder  100  of  FIG. 1 . The measured image contrast is in percentages; the resolutions are measured as lines per inch. As shown by the first curve  500 , the measured contrast is at slightly over 40 percent at 100 lines per inch, and decreases at higher resolutions. At the resolution of 250 lines per inch, the measured contrast is only at approximately 20 percent.  
         [0029]     The second curve  502  demonstrates measured image contrast at various resolutions measured using the optical encoder  200  of  FIG. 2 . As shown by the second curve  502 , the measured contrast is easily over 90 percent at 100 lines per inch. Even at higher resolutions, the measured contrast for the optical encoder  200  is much higher than the measured contrast of the optical encoder  100  of  FIG. 1 . At the resolution of 250 lines per inch, the measured contrast is near 50 percent for the optical encoder  200 . Such improvement in contrast results from the fact that stray light is prevented from reaching the detector  104 .  
         [0030]     From the foregoing, it will be apparent that the present invention is novel and offers advantages over the current art. Although specific embodiments of the invention are described and illustrated above, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. For example, differing configurations, sizes, or materials may be used but still fall within the scope of the present invention. The invention is limited by the claims that follow.