Abstract:
An encoder including a light source for emitting light, a photodetector for detecting at least part of the light emitted by the light source, and an encoder film including a plurality of alternating bars and windows and being located between the light source and the photodetector. The encoder includes a mask including a plurality of alternating bars and windows, the mask being located between the light source and the photodetector. The encoder further includes a spacer located between the mask and the encoder film, the encoder film contacting the spacer to maintain a predetermined spacing between the mask and the encoder film.

Description:
TECHNICAL FIELD 
     The present invention is directed to an encoder, and more particularly, to an encoder having a spacer to maintain a predetermined distance between the encoder mask and the encoder film. 
     BACKGROUND OF THE INVENTION 
     Optical encoders are used to transform the motion of various components into an electronic waveform to track the location and velocity of the components. For example, printers and other business machines may include encoders to track, for example, the rotational position and velocity of a feedroll shaft, or the linear position and velocity of a printhead. Such encoders typically include a light source (such as a light emitting diode (“LED”)) and a photosensor or photodetector (such as a photodiode) to detect light emitted by the light source. The encoder further typically includes an encoder film (such as an encoder strip or encoder wheel) located between the light source and the photodetector, and a mask located between the light source and the photodetector. The encoder film is typically coupled to the shaft, printhead or other component to be tracked such that the encoder film rotates with the shaft or moves with the printhead. The encoder and the mask each usually include a series of alternating bars and windows printed thereon. 
     In many existing encoders, the mask is embedded in the photodetector. For example, the mask may be etched over the detecting element of the photodetector. However, when the mask is located internally to the photodetector, the encoder typically must include optics to process (i.e. “columnize”) the light entering the photodetector. Because optics can be expensive, it may be desired to move the mask externally of the photodetector and adjacent to the encoder strip, which eliminates the need for optics to process and columnize the light. 
     However, when the mask is moved externally of the photodetector, a tight tolerance must be maintained between the mask and the encoder film to ensure high overall accuracy for the encoder. For example, it is desirable to maintain the mask and encoder film as close as possible to each other without contact in order to ensure that the light passing through the mask and film is colonized. However, the mask and encoder film should remain spaced apart to avoid rubbing off the bars printed on the mask or encoder film, and to avoid other undesired effects that may be caused by friction between the mask and encoder film. 
     SUMMARY OF THE INVENTION 
     The present invention is an encoder which includes an encoder film, a mask, and a spacer located between the mask and the encoder film to maintain a set spacing between the mask and the encoder film. In one embodiment the invention is an encoder including a light source for emitting light, a photodetector for detecting at least part of the light emitted by the light source, and an encoder film including a plurality of alternating bars and windows and being located between the light source and the photodetector. The encoder includes a mask including a plurality of alternating bars and windows, and the mask is located between the light source and the photodetector. The encoder further includes a spacer located between the mask and the encoder film, the encoder film being biased against the spacer to maintain a predetermined spacing between the mask and the encoder film. 
    
    
     Other objects and advantages of the present invention will be apparent from the following description and the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a front perspective partially exploded view of one embodiment of he encoder of the present invention; 
     FIG. 2 is a front perspective exploded view of the encoder of FIG. 1; 
     FIG. 3 is a rear perspective partially exploded view of the encoder of FIG. 1; 
     FIG. 4 is a rear perspective exploded view of the encoder of FIG. 1; 
     FIG. 5 is a front perspective view of the light source housing of the encoder of FIG. 1; 
     FIG. 6 is a side view of the light source housing of FIG. 5; 
     FIG. 7 is a front perspective view of the mask support of the encoder of FIG. 1; 
     FIG. 8 is a side view of an upper portion of the mask support of FIG. 7; 
     FIG. 9 is a front perspective view of the mask of the encoder of FIG. 1; 
     FIG. 9A is a front view of the mask support of FIGS. 7-8 with the mask of FIG. 9 mounted thereon; 
     FIG. 10 is a side view of the assembled encoder of FIG. 1; and 
     FIG. 11 is a detailed view of the encoder of FIG. 10, with the size of the spacing protrusions exaggerated for illustrative purposes. 
    
    
     DETAILED DESCRIPTION 
     As shown in FIGS. 1-4, in one embodiment the encoder of the present invention, generally designated  10 , includes a shaft  12  and a gear  14  mounted to a distal end of the shaft  12 . The shaft  12  can be rotatably driven by a motor (not shown). An encoder film  18  is mounted to the gear  14 , such as by an adhesive, and the gear  14  provides support and stiffness to the encoder film  18 . As shown in FIGS. 1 and 3, the encoder film  18  includes a printed pattern  20  that includes a plurality of narrow, alternating bars and windows formed about an outer periphery of the encoder film. The bars are preferably black printed matter that block light, and the windows are transparent portions that allow light therethrough. Each of the bars and windows of the printed pattern  20  preferably include a uniform thickness. The encoder film  18  includes a central opening  22  which is shaped to receive the shaft  12  therethrough. 
     It should be understood that although the encoder film  18  is referred to herein as a “film”, the encoder film can be made from a wide variety of materials (i.e. not necessarily films) that includes a plurality of transparent or translucent portions (windows) and opaque portions (bars). The encoder film  18  is preferably made of a relatively transparent, flexible material such as MYLAR® film or polyester film with the plurality of bars printed thereon. Furthermore, the encoder film  18  may have a variety of shapes, such as circular (as illustrated) as well as other acceptable shapes such as rectangular (for a linear encoder), portions of a circle, etc. 
     The encoder  10  includes a mask  30 , as shown in FIG. 9 (the size of the various components in FIG. 9 are not necessarily to scale). The mask  30  includes a pair of mask segments  32 ,  34 , each mask segment  32 ,  34  including a printed pattern  36 . The pattern  36  of each mask segment  32 ,  34  includes a plurality of bars and openings that preferably correspond in spacing and orientation to the pattern  20  of bars and windows on the encoder film  18 . For example, the mask  30  may be made of the same film material as the encoder film  18 , and each mask segment  32 ,  34  can be identical to a segment of the pattern  20  of the encoder film  18 . The spacing of the pattern  36  of bars and windows on mask segment  32  is preferably misaligned with the pattern  36  of bars and windows of mask segment  34 . For example, if the encoder  10  is a quadrature encoder, each segment  32 ,  34  may be offset by a quarter of the resolution of the pattern  20  of the encoder film  18  in a manner well known in the art. Thus, for example, the “junction”  40  between the two segments  32 ,  34  may be a bar that has a thickness of about 1½ times that of the other bars of the patterns  20 ,  36 , in a manner well known in the art. 
     The mask  30  include s a set of four alignment openings  42  located about the mask segments  32 ,  34 , and a distal opening  44  shaped to receive the shaft  12  therethrough. The mask  30  further includes an alignment notch  43  formed in an upper edge of the mask  30 . 
     Returning to FIGS. 1-4, the encoder  10  further includes a light source  50 , such as an LED, mounted to a light source housing  52 . The light source housing  52  includes a front portion  54  and a back plate  56 , which may be a printed circuit board, coupled to the front portion  54  to capture the light source  50  inside the light source housing  52 . As shown in FIG. 5, the light source housing  52  includes a pair of outer opposed attachment tabs  58  and a pair of opposed fastener openings  60 . The light source housing  52  further includes a light source opening  62  that is shaped to receive the light source  50  therethrough. 
     The light source housing  52  further includes a pair of generally ramp-shaped biasing protrusions  64  located on opposed sides of the light source opening  62 , and a set of four generally cylindrical biasing protrusions  66  located about the light source opening  62 . Each cylindrical biasing protrusion has a generally hemispherical tip  69  The ramp-shaped biasing protrusions  64  and cylindrical biasing protrusions  66  are located around the light source opening  62  in a generally rectangular pattern. As shown in FIG. 6, the biasing protrusions  64 ,  66  are coupled to, extend and generally outwardly from, the light source housing  52 . 
     As best shown in FIG. 7, the encoder  10  includes a mask support  70 . The mask support  70  includes a pair of apertures  72 ,  74 , and is shaped to receive a photodetector  80 , such as photo diode, on its back side (see FIGS.  1  and  2 ). The photodetector  80  includes a set of output lines  82 , and is mounted to the mask support  70  such that the photodetector can detect light that passes through the apertures  72 ,  74 . 
     The mask support  70  includes a set of four spacing protrusions  76  that are coupled to and extend outwardly from the mask support  70  (also see FIG.  8 ). The spacing protrusions  76  are located about the apertures  72 ,  74  in a generally rectangular pattern. The spacing protrusions  76  may include a variety of shapes at their tips, such as flat, hemispherical, conical, etc. The mask support  70  includes a distal opening  78  shaped to receive the shaft therethrough, and a locating tab  45  located above the apertures  72 ,  74 . The mask support  70  further includes a pair of opposed attachment openings  77  and a pair of opposed fastener openings  79 . 
     As best shown in FIGS. 1 and 3, when the encoder  10  is assembled, the light source  50  is coupled to the light source housing  52  such that light source  50  protrudes through the light source opening  62 . The mask  30  is mounted to the mask support  70  such that the spacing protrusions  76  of the mask support  70  are received through the alignment openings  42  of the mask  30 , and the alignment tab  45  is received in the alignment notch  43  (see FIG.  9 A). The mask  30  is preferably aligned on the mask support  70  such that each mask segment  32 ,  34  is located over one of the apertures  74 ,  72 . The cooperation between the alignment openings  42  and the spacing protrusions  76 , and between the alignment tab  45  and alignment notch  43 , ensures that the mask  30  is located in the desired location on the mask support  70 . The mask  30  may be secured to the mask support  70  by various means, such as by an adhesive. The photodetector  80  is mounted to the mask support  70  such that the photodetector  80  can detect light emitted by the light source  50  that passes through the encoder film  18 , mask  30  and apertures  72 ,  74 . 
     The shaft  12  is received through the central opening  24  of the gear  14 , the central opening  22  of the encoder film  18 , the distal opening  44  of the mask  30  the distal opening  78  of the mask support  70 . The encoder film  18  is mounted about the shaft  12  such that the encoder film  18  rotates with the shaft  12  and the pattern  20  of bars and windows is located in front of the light source  50 . The attachment tabs  58  of the light source housing  52  are received through the attachment openings  77  of the mask support  70  to align the light source housing  52  relative to the mask support  70  such that the apertures  72 ,  74  of the mask support  70  are aligned with the light source  50 . Next, a set of screws  86  are passed through to the fastener openings  79  of the mask support  70  and the fastener openings  60  of the light source housing  52  to couple the support  70  and housing  52  together. As shown in FIG. 10, when the encoder  10  is fully assembled, the spacing protrusions  76  of the mask support  70  extend through the mask  30  and engage the inner surface  82  of the encoder film  18 . 
     In operation, the shaft  12  is rotated by the motor and the light source  50  is activated to emit light during rotation of the shaft  12 . As the shaft  12  is rotated, the gear  14  and encoder film  18  are rotated along with the shaft  12 . In this manner, light emitted by the light source  50  and passing through the encoder film  18  is selectively blocked, and permitted to pass through, the mask segments  32 ,  34  of the mask  30  in a manner well known in the art. For example, when the bars of the pattern  20  of the encoder film  18  are aligned with the bars of the pattern  36  of a mask segment  32 ,  34 , the emitted light passes through the encoder film  18  and mask segment  32 ,  34  and is detected by the photodetector  80 . In contrast, when the bars of the pattern  20  of the encoder film  18  are aligned with the windows of the pattern  36  of a mask segment  32 ,  34 , the encoder film  18  and mask segment  32 ,  34  block light from passing through that aperture  72 ,  74  of the mask support  70 . 
     As noted earlier, the two mask segments  32 ,  34  of the mask  30  may provide for two output signals that are 90° out of phase with each other, which can provide information relating to the direction of rotation of the shaft  12 . In other words, when the bars of the pattern  36  of mask segment  32  are aligned with the bars of the pattern  20  of the encoder  18  (and light can thereby pass through the mask segment  32  and aperture  74 ), the bars of the pattern  36  of mask segment  34  are aligned with the windows of the pattern  20  of the encoder  18  (and light is blocked from passing through the mask segment  34  and aperture  72 ). 
     The photodetector  80  can be coupled to a processor or controller (not shown) by the output lines  82 . The controller can process the signals outputted by the photodetector  80  to thereby determine the angular location and velocity of the shaft  12 . 
     As noted earlier, when the encoder  10  is fully assembled, the spacing protrusions  76  of the mask support  70  extend through the mask  30  and engage the inner surface  82  of the encoder film  18  (see FIG.  10 ). As shown in FIG. 10, the spacing protrusions  76  may extend forwardly a relatively small distance (such as, for example, between 50-100 microns) to ensure close spacing between the mask  30  and encoder film  18 . FIG. 11 illustrates the encoder of FIG. 10 with the size of the spacing protrusions  76  exaggerated for illustrative purposes. While the spacing protrusions  76  of the mask support  70  engage the inner surface  82  of the encoder film  18 , the biasing protrusions  64 ,  66  of the light source housing  52  engage an outer surface  84  of the encoder film  18  to urge or bias the encoder film  18  against the spacing protrusions  76 . In this manner, as shown in FIGS. 10 and 11, the mask  30  and encoder film  18  are in close, parallel alignment, but remain spaced apart to avoid contact between the encoder film  18  and mask  30 . In other words, the encoder film  18  is “captured” between the spacing protrusions  76  and biasing protrusions  64 ,  66 . In this manner, when the encoder film  18  is rotated by the shaft  12 , the encoder film  18  can slide between the biasing protrusions  64 ,  66  and spacing protrusions  76 , and a desired spacing between mask  30  and encoder film  18  can be maintained. Thus, the encoder film  18  is maintained in a position close to the mask  30 . This ensures that the rays of light that pass through the mask  30  and strip  18  have sharp, well-defined outer edges, or a sharp resolution. Furthermore, the encoder film  18  is prevented from contacting the mask  30 , which avoids the adverse effects discussed earlier. 
     It may be desired to place a set of biasing protrusions as closed as possible to the light detecting element  50 , and therefore the ramp-shaped protrusions  64  are located on either side of the light detecting element. Although it may be desired to use, in general, cylindrical biasing protrusions with hemispherical tips such as the protrusions  66 , the ramp-shaped protrusions  64  include the sharp inner edges  65  to ensure that the tips  67  of the protrusions  64  are located in the proper position such that the tips  67  engage the encoder film  30  at the proper location. It should be further understood that the biasing protrusions may take a variety of shapes beyond the tabs or pins shown herein, and may include springs, plates, air jets, and the like without departing from the scope of the present invention. 
     As noted earlier, the encoder film  18  and gear  14 , as well as mask support  70  and mask  30 , are each mounted to the shaft  12 . This feature helps to ensure that the mask  30  is properly aligned with the encoder film  18 . For example, manufacturing tolerances for locating the distal opening  44  in the mask  30  and the central opening  22  of the encoder film  18  are relatively tight (i.e. manufacturing clearance is small). Thus, because the distal opening  44  and central opening  22  can both be accurately located, when the mask  30  and encoder film  18  are both mounted onto the shaft  12 , the mask  30  is properly and precisely aligned relative to the encoder film  18 . This feature ensures precise alignment between the mask  30  and encoder film  18 , and ensures that the optical center of the patterns  36  on the mask  30  are aligned with the optical center of the pattern  20  on the encoder film  18 . 
     Although the encoder  10  is described and shown with reference to a rotary encoder, it should be understood that the present invention may also be used with a linear encoder or other encoders without departing from the scope of the present invention. For example, in the case of a linear encoder, a linear encoder strip that slides laterally can be pressed against a set of spacing protrusions by a set of biasing protrusions, similar to those described above. The linear encoder strip will simply slide laterally between the biasing protrusions spacing protrusions instead of being Attorney rotated between the biasing protrusions and spacing protrusions in a manner that would be apparent to one of ordinary skill in the art upon a reading of this specification. 
     Having described the invention in detail and by reference to the preferred embodiments, it will be apparent that modifications and variations thereof are possible without departing from the scope of the invention.