Abstract:
A system and method for tracking belts disposed on rollers in a photoreceptor apparatus. The system includes a movable, belt edge guide that operates in combination with an encoder, a belt edge sensor, a belt hole sensor for detecting a hole in the belt surface, wherein the profile of the edge of the belt is learned as a function of the belt position on the rollers. The encoder and belt hole sensor are used to actuate the edge guide system to compensate for the contours of the belt edge and to maintain a constant lateral position of the belt at any given point on the belt.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to electrophotographic printing machines, and more particularly to a tracking system for laterally moving belts in electrophotographic printing machines. 
     Color registration in electrophotographic printing machines requires that images are precisely registered in the lateral direction. As an endless photoreceptor belt or intermediate transfer belt turns around a set of rollers similar to a continuous conveyer belt, there is typically an undesired motion of the belt that occurs back and forth in the lateral direction of the belt travel. This lateral belt motion is caused by lateral forces that are generated by misalignment of the rollers as well as belts that are conical (one belt edge longer than the other) and rolls that do not have constant diameters. Heretofore, one solution to the problem of lateral belt motion was addressed by using stationary edge guides and low lateral force rolls wherein fixed edge guides are located on either side of the belt to prevent it from walking off the rolls completely. These edge guides supply an equalizing force to that generated by the misshapened belt and misaligned rollers. However, in using stationary edge guides, the belt will still move back and forth according to the contour profile of the belt edge. When this type of belt guidance system is used in a single pass color xerographic application, the lateral belt motion due to the belt edge profile causes a misregistration of the colors. 
     A feature of the disclosed embodiment is an apparatus for tracking belts in an electrophotographic printing machine. That includes an actuated (i.e. movable) edge guide system that operates in combination with a belt edge sensor, a belt hole sensor and low lateral force rollers. The P/R, or IBT module is racked (making the long axis of the rollers not parallel) slightly so that the belt has a tendency to walk toward the edge guide. The profile of the belt edge is learned as a function of belt position. This is accomplished by the use of an encoder and a belt hole sensor. Once the belt edge profile is learned, the encoder, belt edge sensor and belt hole sensor are used to actuate the edge guiding system to compensate for the contours of the belt edge. The movement of the edge guide will be prescribed to mimic the belt edge profile, which will result in a constant force being applied on the belt. In this way, a constant lateral position of any given point on the belt can be maintained. 
     Prior Art Statement 
     The following Xerox Corporation U.S. patents disclose some examples of belt tracking apparatus. U.S. Pat. Nos. 3,500,694, 5,510,877, 6,137,517, 6,141,526 issue to Ikeda discloses a color printer belt meander control method. 
     All the references cited herein are incorporated by reference for their teachings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present embodiment will become apparent upon reading the following detailed description and upon reference to the drawings, in which: 
     FIG. 1 illustrates a schematic elevational view of a belt tracking apparatus for an electrophotographic printing machine incorporating the features of the present invention therein. 
     FIG. 2 is a portion of the belt tracking apparatus of FIG. 1 showing the actuator device in more detail. 
     FIG. 3 is a detailed illustration of a side view of the actuation device of FIG.  1 . 
     FIG. 4 is a detailed illustration of an end view of the actuation device of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, an embodiment of a belt tracking apparatus according to the present invention is illustrated wherein a photoreceptor belt module includes an endless belt  10  that is disposed on rollers  12  and  14 . Rollers  12  and  14  are low, lateral force rollers. Generally, a LLF roller is similar to a standard drive roller which has an elastomer coating of a predetermined thickness that is sliced to reduce the axial stiffness between the belt wrapped on the roller and the rigid roller shaft. The elastomer material normally has a high coefficient of friction so that the belt will not slip with respect to the roller. A LLF roller limits the lateral force the edge guide must develop to prevent the belt from further lateral motion. The edge guide is driven by a movable edge guide system  16  that includes a stepping motor  18 , that drives cam  20 , and a moveable edge guide  22  connected to arm  38 . The edge guide  22  must be placed at a roller that has a significant amount of belt wrap in order to maximize the edge force that can be generated before belt edge damage will occur. The moveable edge guide  22  is shaped such that the guide supports the edge of the belt  10 . There is a raised section of the guide  22  that acts as a stop to prevent lateral motion of the belt  10  past this point. The edge guide  22  is spring loaded by spring  24  about a pivot  26  so that the natural position of the movable edge guide  22  is away from the edge of belt  10 . The stepping motor  18  drives cam  20 , thus pushing the movable edge guide  22  to apply force to the belt  10 . 
     There are four sensors on the apparatus. The first is a revolution sensor device that detects the presence of an indicium located on the belt. The indicium is shown as a hole  30  in the belt in the present embodiment, but may also be magnetic or optically detected indicia and the like. In FIG. 1, a belt hole sensor  28  is shown which detects the passing of a single belt hole  30  that is located in the outer edge of the belt. This allows the number and time of each belt revolution to be monitored. The second is a belt edge sensor  32 , which detects lateral motion of the edge of the belt  10 . The belt edge sensor  32  is located in advance of edge guide  22 . The third sensor is a rotary encoder  34  mounted on the shaft of roller  14 , which senses the process motion and position of the belt  10 . The fourth is a cam flag sensor that is located on the stepping motor  18  and is used to locate the cam  20  at its nominal (halfway) position. Referring to FIG. 2, the movable edge guide system  16  of FIG. 1 is shown in more detail wherein the movable edge guide  22  on arm  38  rotates around pivot point  26  as cam  20 , driven by stepping motor  18 , rotates and thus moves edge guide  22  laterally. An actuation head portion  40  of edge guide  22  is in contact with and applies lateral force to belt  10  at location  36  such that belt  10  also moves laterally. Spring  24  pulls on the arm  38  of edge guide  22  and keeps the actuation head portion  40  of edge guide  22  loaded against cam  20 . Element  42  is a belt support for belt  10  on acuation head  40 . 
     Referring to FIG. 3 a side view of the edge guide  22  is shown illustrating arm  38 , pivot point  26 , actuation head  40 , belt support  42  and belt  10 . 
     In FIG. 4, an end view of the edge guide of FIG. 3 is shown sharing the relationship of arm  38  pivot point  26 , belt support  42  and belt  10 . 
     The following algorithm is used by the movable edge guide system  16  to minimize the lateral motion of any point on belt  10 , while also accommodating any irregularities that may exist in the belt edge profile. First, belt  10  begins to be driven on rollers  12  and  14 . Cam  20  is held in its nominal position and then, over a small number of belt revolutions, is driven to move the belt towards the center of the belt module. Once the belt has been moved a few mm away from its normal operating location, the edge guide is quickly returned to the normal position. The belt is then free to slowly walk back toward the movable edge guide. While the belt is walking back toward its printing position, the belt edge learning can take place. When belt hole sensor  28  detects moving belt hole  30 , the learning of the edge profile of the belt  10  begins. The belt edge sensor  32  measures the lateral position of belt  10  as a function of position for one revolution of belt  10 . Rotary encoder  34  is used to sample belt  10  at equal distances. This edge position versus belt length position is stored for as many belt revolutions until the belt again comes in contact with the edge guide (this is determined by the belt edge sensor readings). The method of processing this stored belt edge information can be processed similar to that which is described in previous patents by Xerox. 
     Once the profile of the belt edge has been learned, tracking of belt  10  can start. The learned edge profile becomes the reference signal that the controller will follow. To insure that the feed forward technique works, movable edge guide  22  has to move the same amount of belt edge movement. This is insured by using rotary encoder  34  to sample belt  10  and sense the belt&#39;s longitudinal motion. For example, if the edge was learned every N encoder pulses, the movable edge guide system will sample every N encoder pulses, and adjust its position to compensate for the change in reference. 
     The next time that belt hole  30  is sensed, cam  20 , and thus, moveable edge guide  22 , are returned to the nominal position and the procedure repeats itself. After one complete belt revolution, the motion of edge guide  22  (laterally) mimics the belt edge profile—thus applying a constant force to the belt edge and keeping all points on the surface of the belt  10  in a constant lateral position. 
     This learning and tracking algorithm leads to a more robust design and a lower lateral belt motion than if the belt edge profile were ignored. The controller is set up as a function of belt position in the process direction (as measured by the rotary encoder  34 ). This allows it to be insensitive to different process running speeds. 
     It should be noted that the rollers supporting the belt must be skewed slightly with respect to the belt, so that the belt has a tendency to walk towards the moveable edge guide  22  as it travels in the process direction. 
     It should be understood that the foregoing description is only illustrative. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.