Abstract:
An optical encoder includes a planar surface comprising an insulator material. An optical encoder pattern partially occupies the planar surface. The encoder pattern has at least one continuous geometry and is made from a conductive material.

Description:
BACKGROUND  
         [0001]    Optical encoding is often used in mechanical systems as an inexpensive and reliable way to measure and track motion among moving components. For instance, printers, scanners, photocopiers, fax machines, plotters, and other imaging systems commonly use optical encoding to track the position of an image media, such as paper, as an image is printed on the media or an image is scanned from the media.  
           [0002]    One common technique for optical encoding uses an optical sensor and an optical encoder. The optical sensor focuses on a surface of the optical encoder. As the sensor moves with respect the encoder, or the encoder moves with respect to the sensor, the sensor reads a pattern on the encoder to detect the motion.  
           [0003]    A typical encoder pattern is an alternating series of features. As the encoder and sensor move relative to the one another, transitions from one feature to the next in the pattern are optically detected. For instance, an encoder pattern could be an alternating pattern of holes, or optically transmissive windows, in an opaque material. In which case, an optical sensor can detect transitions from darkness to light passing through the holes or windows.  
           [0004]    Optical encoders, like many other mechanical components, are often made of insulating materials, such as plastics. Friction between a plastic encoder and another material, such as a plastic sensor housing or guide, generates an electric surface charge on the encoder. This charge is called a triboelectric charge. The electric charge tends to attract airborne particles, such as paper dust and ink aerosol in an ink jet printer. Over time, the particulate matter can accumulate on the encoder and interference with the accuracy of the optical encoding process by obscuring transitions between features in the encoding pattern. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    Examples of the present invention are illustrated in the accompanying drawings. The accompanying drawings, however, do not limit the scope of the present invention. Similar references in the drawings indicate similar elements.  
         [0006]    [0006]FIG. 1 illustrates an exemplary mechanical system in which one embodiment of the present invention can be used.  
         [0007]    [0007]FIG. 2 illustrates another perspective of the mechanical system of FIG. 1.  
         [0008]    [0008]FIG. 3 illustrates another exemplary mechanical system in which one embodiment of the present invention can be used.  
         [0009]    [0009]FIG. 4 illustrates another perspective of the mechanical system of FIG. 3.  
         [0010]    [0010]FIG. 5 illustrates one embodiment of an encoder wheel.  
         [0011]    [0011]FIG. 6 illustrates an encoder wheel incorporating one embodiment of the present invention.  
         [0012]    [0012]FIG. 7 illustrates one embodiment of the present invention having a ground path to a center mount of an encoder wheel.  
         [0013]    [0013]FIG. 8 illustrates one embodiment of the present invention having a ground path to a contact path on an encoder wheel.  
         [0014]    [0014]FIG. 9 illustrates an encoder strip incorporating one embodiment of the present invention having a ground path.  
         [0015]    [0015]FIG. 10 illustrates one embodiment of the present invention having a ground path to a contact path on an encoder strip. 
     
    
     DETAILED DESCRIPTION  
       [0016]    In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, those skilled in the art will understand that the present invention may be practiced without these specific details, that the present invention is not limited to the depicted embodiments, and that the present invention may be practiced in a variety of alternative embodiments. In other instances, well known methods, procedures, components, and circuits have not been described in detail.  
         [0017]    Various operations will be described as multiple discrete steps performed in turn in a manner that is helpful for understanding the present invention. However, the order of description should not be construed as to imply that these operations are necessarily performed in the order they are presented, nor even order dependent. Lastly, repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment, although it may.  
         [0018]    In various embodiments, the present invention distributes and/or discharges electric charge from an optical encoder. Embodiments of the present invention can substantially reduce the accumulation of particulate matter on an optical encoder over time that could otherwise occur without a substantial additional cost. In general, embodiments of the present invention use an encoder pattern having a continuous geometry of conductive material to allow electric charge to migrate from areas of high voltage potential to areas of low voltage potential.  
         [0019]    [0019]FIG. 1 illustrates an example of a mechanical system in which an embodiment of the present invention can be used. Feed roller  120  is in contact with imagining media  110  so as to rotate when imagining media  110  moves perpendicularly relative to feed roller  120 . Optical encoder  140  is attached to an end of feed roller  120  and rotates in unison with feed roller  120 . Optical sensor  130  is situated relative to optical encoder  140  to detect rotational motion of encoder  140 .  
         [0020]    Guide  150  is an example of a source of electric charge. In the illustrated embodiment, encoder  140  comprises a thin and flexible film. Guide  150  presses on encoder  140  to maintain a predetermined spacing between encoder  140  and sensor  130 . If both encoder  140  and guide  150  are insulators, guide  150  will generate triboelectric charge along a friction path on a surface of encoder  140  as encoder  140  rotates. In alternate embodiments, electric charge may accumulate on encoder  140  from any number of additional sources.  
         [0021]    [0021]FIG. 2 illustrates the mechanical system of FIG. 1 as seen from an end of feed roller  120 . Encoder  140  comprises a wheel shape and includes an encoding pattern  210  along the edge of the wheel. Sensor  130  straddles encoder  140  over encoding pattern  210 . Assuming the path of friction from guide  150  is over encoding pattern  210 , the triboelectric charge will preferentially attract particulate matter to pattern  210  unless the charge is discharged or distributed.  
         [0022]    [0022]FIG. 3 illustrates another simplified example of a mechanical system in which one embodiment of the present invention can be used. Instead of a rotating encoder wheel, as illustrated in FIGS. 1 and 2, the embodiment of FIG. 3 uses a fixed optical encoder  350  in the shape of a strip or tape. Like encoder  140  discussed above, encoder  350  is a thin and flexible film seen in FIG. 3 from an edge.  
         [0023]    Encoder  350  and lateral carriage axis  320  are both mounted between two support mounts  310 . Imaging element  330  is coupled to lateral carriage axis  320  so as to move from side to side over imaging media  110 . Imaging element  330  may be, for instance, a print head, scan head, or the like. Optical sensor  340  is coupled to imaging element  330  so as to straddle encoder  350 . As imaging element  330  moves along lateral carriage axis  320 , optical sensor  340  detects the motion relative to encoder  350 .  
         [0024]    [0024]FIG. 4 illustrates the mechanical system of FIG. 3 as seen from a perspective showing the surface of optical encoder  350 . Encoder  350  includes encoding pattern  410 . Sensor  340  straddles encoder  350  over encoding pattern  410 . Any rubbing between encoder  350  and sensor  340  could generate electrical charge along the path of the friction and attract particulate matter to encoding pattern  410  unless it is discharged or distributed.  
         [0025]    The mechanical systems depicted in FIGS. 1 through 4 are greatly simplified for purposes of highlighting the embodiments of the present invention. Several components are not shown, including a support structure for feed roller  120  and sensor  130 , gears and a motor assembly to drive feed roller  120  to move imagining media  110 , gears and a motor assembly to drive lateral carriage axis  320  to move imaging element  330 , as well as control systems to operate the motor assemblies based on output from sensors  130  and  340 .  
         [0026]    The mechanical system of FIG. 3 could be used in conjunction with the mechanical system of FIG. 1. For instance, if the mechanical systems were in a printer, feed roller  120  could advance media  110  in increments measured by sensor  130 . Meanwhile, imaging element  330  could move across media  110  in increments measured by sensor  340  to print an image with one horizontal pass for each incremental movement of feed roller  120 .  
         [0027]    [0027]FIG. 5 illustrates one embodiment of an optical encoder wheel that could accumulate sufficient particulate matter to interfere with its proper operation. The encoder wheel comprises an optically transmissive substraight  510 . Substraight  510  has an opaque coating  520  which has been applied to form a home coding pattern  540  and a fine coding pattern  530 . Any number of approaches can be used to form opaque coating  520  into the illustrated patterns, including photolithographic processing, mechanical etching, laser etching, and the like. In alternate embodiments, the opaque coating  520  is painted onto substraight  510  or deposited within substraight  510  in the illustrated patterns.  
         [0028]    Home coding pattern  540  has just one transition from optically transmissive to opaque, or one transition from opaque to optically transmissive, per revolution. In which case, home coding pattern  540  can be used to detect and control large-scale rotational motion, such as counting revolutions or finding a home reference point. Fine coding pattern  530  comprises numerous isolated opaque bars  550 , as shown in an enlarged view. The isolated opaque bars  550  are tightly packed to provide transitions at very small increments. Fine coding pattern  530  can be used to detect and control minute rotational motion.  
         [0029]    Transmissive substraight  510  is commonly made from an insulating material, such as Mylar or some other form of plastic. Triboelectric charge builds-up on the surface of substraight  510  as it rubs against other materials, such as a housing for an optical sensor or a guide. Since the surface is an insulator, the charge has no where to go. In other words, electric charge on an insulator is rather like a drop of water on a glass table top. Left isolated, the charge attracts and collects particulate matter until it eventually dissipates into the atmosphere, leaving the particles behind. Over time, the particles can obscure pattern transitions and cause errors in optical encoding, especially in fine coding pattern  530 .  
         [0030]    [0030]FIG. 6 illustrates one embodiment of the present invention for an encoding wheel. The number of pattern transitions per revolution in home coding patterns  640  and  540 , and in fine coding patterns  630  and  530 , are identical. However, rather than applying the opaque coating in such a manner as to leave isolated opaque bars, as shown in the encoder of FIG. 5, opaque coating  620  is applied in such a way as to leave optically transmissive windows through substraight  510 , such as isolated bars  650 . That is, opaque coating  620  retains a continuous geometry, with no isolated sections among home coding pattern  640  or fine coding pattern  630 .  
         [0031]    Furthermore, opaque coating  620  includes a conductive material, such as silver or carbon. Accumulated surface charge, that had no where to go in FIG. 5, is conducted away in FIG. 6 and distributed throughout the continuous geometry of opaque coating  620 . That is, the “drops” of electric charge that are deposited within the continuous geometry of coating  620  are rather like drops falling onto the surface of a pool of water. The amount of charge is evenly distributed over the entire coating  620 , allowing it to dissipate over a larger surface area. In the illustrated embodiment, large sections of opaque coating  620  are left in place to form distribution field  660 , surrounding coding patterns  640  and  630 , to conduct and dissipate accumulated charge. In other words, opaque coating  620  acts like a ground plane.  
         [0032]    Of course, electric charge is also deposited on the islands of insulated substraight  510 . However, electric charge is naturally attracted to areas of lower voltage potential. Rather like drops of rain falling on an island and draining into the surrounding sea, electric charge deposited on the islands of insulated substraight  510  may migrate into the ground plane of coating  620 .  
         [0033]    If coating  520  in FIG. 5 were made of the same conductive material as coating  620 , the same migration of charge from insulator to conductor could occur. However, in FIG. 5, coating  520  does not have a continuous geometry. Rather than islands of insulators in a sea of conductor, FIG. 5 would be oases of conductor in a dessert of insulator. Charge deposited on the insulator could migrate to the conductors and likely accumulate to a high enough level to attract and accumulate particulate matter. With a sea of conductor, however, as in FIG. 6, the charge is much more widely spread, keeping the charge level, or “water” level, so to speak, at any one place to a much lower level, dissipating the charge over a larger area and reducing the likelihood of attracting and accumulating particulate matter. The illustrated embodiment accomplishes this reduction with little or no additional cost compared to the embodiment of FIG. 5, and without introducing any new parts to a mechanical system.  
         [0034]    In some mechanical systems, the combination of a conductive surface and a continuous geometry may not be sufficient to dissipate enough charge to sufficiently reduce particulate accumulation. FIG. 7 illustrates another embodiment of the present invention, however, having ground path  710  leading to a mounting hole at the center of the encoder wheel. In which case, by mounting the encoder wheel to a conductive material, charge build-up on the encoder wheel can be discharged to the conductive material. For instance, referring back to FIG. 1, where encoder  140  couples to feed roller  120 , feed roller  120  can be made out of a metal or a plastic impregnated with a conductor, such as carbon. In turn, the conductive part of roller  120  can be grounded to other components (not shown) in the mechanical system.  
         [0035]    [0035]FIG. 8 illustrates another embodiment of the present invention for discharging an encoder. In FIG. 8, contact path  810  is illustrated as a white dashed circular path within the conductive distribution field of an encoder. For instance, referring back to FIG. 1, guide  150  may rub on the encoder wheel along contact path  810 . In which case, by using a conductive material for guide  150 , charge build-up on the encoder wheel could be discharged to the conductive material. In other words, rather than guide  150  being a source of triboelectric charge, guide  150  could be used to discharge the encoder.  
         [0036]    In the embodiments of FIGS. 7 and 8, no additional parts are needed to discharge the encoder wheels, and any number of conductor geometries could be used to connect to ground paths. Furthermore, the levels of charge involved are likely to be very low in most situations, so strong ground connections are not usually needed. In which case, the small amount of conductivity needed in the grounding components that couple to an encoder wheel is unlikely to add significant cost or complexity in most mechanical systems. Any number of axle or gear assemblies, contact guides, mounting pads, or the like, could be made of metal or impregnated with a conductor.  
         [0037]    [0037]FIGS. 9 and 10 illustrate two embodiments of the present invention for encoder strips. Both embodiments use the same conductive opaque coating  920 , having a continuous geometry and including coding pattern  910 . As discussed above, charge is distributed throughout the continuous geometry. If distributing the charge is insufficient, FIG. 9 includes mounting holes  930  that are punched through the encoder strip. Opaque coating  920  extends up to mounting holes  930 , providing a ground path area around the holes. By mounting the encoder to at least one conductive mount, such as support mounts  310  in FIG. 3, the encoder can be discharged. Similarly, in FIG. 10, contact path  1010  goes through opaque coating  920 . By using a conductive contact along contact path  1010 , such as a guide, the encoder can be discharged.  
         [0038]    As discussed above, using various embodiments of the present invention, no additional parts or cost are needed in most situations to reduce charge build-up, and hence reduce particulate matter accumulation, in optical encoders. Those skilled in the art will appreciate that embodiments of the present invention can be used with encoders having a wide variety of form factors in addition to round and rectangular films. Those skilled in the art will also appreciate that embodiments of the present invention can be used with a wide variety of conductive geometries and encoding patterns. For instance, in certain embodiments, rather than using a continuous geometry covering an entire encoder surface, multiple continuous geometries could be used on an encoder surface, covering different parts of the surface, but still providing areas for charge distribution and/or dissipation. Alternatively, in some embodiments of the present invention, some sections of the encoder surface may remain isolated, with other sections of the encoder surface being covered with one or more continuous, conductive geometries. For instance, charge accumulation may be irrelevant in areas of an encoder surface that are not covered by an encoder pattern. So, leaving those sections isolated may have no performance impact on the encoder. Even in areas covered by an encoder pattern, some embodiments may be designed to include some isolated pattern features, or some pattern features may become isolated during the operational life of an encoder if, for instance, the conductive coating is scratched. The extent to which charge accumulation is reduced tends to depend on the size, location, and/or number of isolated sections relative to the continuous, conductive section(s). In other words, performance benefits of embodiments of the present invention tend to increase with fewer and/or smaller isolated pattern features, as well as with isolated pattern features placed closer to a continuous, conductive geometry.  
         [0039]    Thus, a modified optical pattern for an optical encoder is described. Whereas many alterations and modifications of the present invention will be comprehended by a person skilled in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Therefore, references to details of particular embodiments are not intended to limit the scope of the claims.