Abstract:
A modular encoder has been developed that can be mounted to a motor shaft without requiring time consuming alignment or special tools. The encoder includes a base having an annular opening, the base including a plurality of resilient arms surrounding the annular opening with each arm having a terminal end that extends into the annular opening, and a hub having a horizontal flange with a top and a bottom surface and a generally vertical wall extending from the bottom surface of the horizontal flange, the wall is configured to fit within the annular opening of the base and is circumscribed with a groove for receiving the terminal ends of the resilient arms to secure the hub to the base and enable rotation of the hub within the annular opening of the base as the terminal ends of the resilient arms slide within the groove.

Description:
TECHNICAL FIELD 
     The encoder described below generally relates to angular position encoders used having rotary print drums and the like. More specifically, the encoder relates to the structure of encoders for mounting the encoder to a rotating shaft and for assembling encoders. 
     BACKGROUND 
     Modern printers use a variety of inks to generate images from data. These inks may include liquid ink, dry ink, also known as toner, and solid ink. So-called “solid ink” refers to ink that is loaded into a printer as a solid, which is typically in stick or pellet form. The solid ink is melted within the printer to produce liquid ink that is supplied to a print head for ejection onto media or an intermediate member to generate a printed image from image data. These solid ink printers typically provide more vibrant color images than toner or liquid ink jet printers. 
     A schematic diagram for a typical solid ink imaging device is illustrated in  FIG. 1 . The solid ink imaging device, hereafter simply referred to as a printer  108 , has an ink loader  110  that receives and stages solid ink sticks. The ink sticks progress through a feed channel of the loader  110  until they reach an ink melt unit  120 . The ink melt unit  120  heats the portion of an ink stick impinging on the ink melt unit  120  to a temperature at which the ink stick melts. The liquefied ink is supplied to one or more print heads  130  by gravity, pump action, or both. Printer controller  180  uses the image data to be reproduced to control the print heads  130  and eject ink onto a rotating print drum or image receiving member  140  as image pixels for a printed image. Media  170 , such as paper or other recording substrates, are fed from a sheet feeder  160  to a position where the image on the drum  140  can be transferred to the media. To facilitate the image transfer process, the media  170  are fed into a nip between the transfer, sometimes called transfix, roller  150  and the rotating print drum  140 . In the nip, the transfix roller  150  presses the media  170  against the print drum  140 . Offset printing refers to a process, such as the one just described, of generating an ink or toner image on an intermediate member and then transferring the image onto some recording media or another member. 
     Generation of images on the print drum may require several revolutions of the drum. In order to eject the ink in the proper position within a partially formed image, the precise position of the drum must be monitored. Additionally, the controller synchronizing the finished image with the feeding of a media sheet into a nip with the print drum for the transfer of the image from the drum to the media sheet needs accurate information regarding the position of the drum&#39;s surface as it rotates about its center. The printer  100 , therefore, includes a rotary encoder that generates an electrical signal corresponding to the angular position of the rotating drum  140 . 
     Rotary encoders may use optical, magnetic, or inductive sensing to generate a position signal. Optical encoders include a light source and sensor that are mounted on opposite sides of a flat disk. The disk is coupled to the rotating shaft of a print drum so the disk rotates with the shaft and drum. A plurality of spaced apart marks is located within a circumferential slot on the disk and this slot is positioned between the light source and light sensor. As the disk rotates, the light from the light source is interrupted by the marks. Consequently, the light sensor detects light in an on/off pattern corresponding to the marks on the disk as they pass between the light and its sensor. The resulting optical digital signal is converted by the sensor into an electrical digital signal. This signal may be used by a controller in a known manner for coordinating control of the print heads with an image on the print drum and for transferring an image from the print drum to a sheet of media. The disk bearing the series of spaced apart marks is sometimes known as a code wheel. 
     Optical encoders fall into two broad categories. The first category includes encoders that are assembled with a shaft extending from the center of the code wheel through the body or housing of the encoder. This type of encoder is delivered as a complete package for attachment via a coupler to the shaft about which the print drum rotates. The assembly of the encoder at the manufacturer&#39;s facility enables the code wheel, optical sensor, and light source to aligned and spaced from one another at the factory. The second category of encoders, sometimes referred to as modular encoders, do not have a shaft section built into the body or housing of the encoder. Instead, the code wheel typically has an annular opening at its center and the center of the housing so a collar to which the code wheel is mounted can be coupled to the shaft of the motor. The coupling may be accomplished using a set screw or the like. Various structures have been developed for axial and radial alignment of the code wheel so the code wheel is centered on the shaft and appropriate tolerances are provided for the placement of the code wheel within the gap between the light source and sensor of the encoder. These structures and tools require time during the installation of the encoder on the motor shaft for the alignment of the encoder components that are critical to accurate signal generation. Should the motor later require maintenance, the encoder must be removed and the alignment procedure repeated before returning the motor to service. 
     SUMMARY 
     In order to facilitate installation and service of encoders in printers, a modular encoder has been developed that can be mounted to a motor shaft without requiring time consuming alignment or special tools. The encoder includes a base having an annular opening, the base including a plurality of resilient arms surrounding the annular opening with each arm having a terminal end that extends into the annular opening, and a hub having a horizontal flange with a top and a bottom surface and a generally vertical wall extending from the bottom surface of the horizontal flange, the wall is configured to fit within the annular opening of the base and is circumscribed with a groove for receiving the terminal ends of the resilient arms to secure the hub to the base and enable rotation of the hub within the annular opening of the base as the terminal ends of the resilient arms slide within the groove. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings, in which: 
         FIG. 1  is a general schematic diagram of a printer having a print drum on which images are formed by a print head ejecting ink onto the print drum; 
         FIG. 2  is a cross sectional view of the hub, base and code wheel of a modular encoder  200   
         FIG. 3  is an exploded perspective view of a base and a hub of a modular encoder shown in  FIG. 2  depicting the assembly of the hub to the base. 
         FIG. 4  is a partial perspective view of a code wheel being mounted to the hub of  FIG. 3 . 
         FIG. 5  is a partial perspective view of a sensor unit being mounted to the base of the encoder for detection of a masking pattern on the code wheel shown in  FIG. 4 . 
         FIG. 6  is a perspective view of the modular encoder with a cover installed on the encoder. 
     
    
    
     DETAILED DESCRIPTION 
     The term “printer” refers, for example, to reproduction devices in general, such as printers, facsimile machines, copiers, and related multi-function products. While the specification focuses on a system that rotates the transfix roller in solid ink printers, the system may be used with any printer that uses a belt or roller to assist in transferring the image to media. 
       FIG. 2  depicts one embodiment of a modular encoder  200  having a base and hub that accurately position a code wheel within a sensor unit for generation of an electrical signal that corresponds with the angular position and speed of a shaft to which the hub is mounted. The encoder  200  includes a base  204  and a hub  220 . The base  204  has a mounting ring  208  that defines an annular opening in the floor of the base  204 . While the embodiment shown in  FIG. 2  uses a solid ring to define the annular opening, other structures may be used to define the annular opening. For example, a plurality of resilient arms configured to conform to the cross-sectional view of the ring  208  shown in  FIG. 2  may be used to define the annular opening in an alternative embodiment. The annular opening has a longitudinal axis  212  as shown in  FIG. 2 . 
     The hub  220  of the encoder  200 , as shown in  FIG. 2 , has a vertical wall  210  in which a groove  214  has been circumscribed. The mounting ring  208  or other structure defining the annular opening in the floor of the base  204  is received within the groove  214  to secure the hub  220  to the base  204 . The mounting ring  208  is sized and shaped so the hub  220  can be inserted into the annular opening as the external surface of wall  210  urges the mounting ring  208  away from the opening until the groove  214  is opposite the mounting ring  208 . The ring or other resilient structure rebounds to be received within the groove to secure the hub to the base. This type of fit is sometimes called a snap fit. 
     Also located in the vertical wall  210  is a plurality of axial slits  264 . The slits  264  enable the central bore  238  within the hub  220  to expand so a print drum shaft can more easily slide through the central bore. Once the hub  220  is in proper position on the shaft, the slits allow the wall  210  to constrict slightly and grip the shaft. Thereafter, rotating the print drum shaft also rotates the hub on the mounting ring. The slits are, preferably, equally spaced from one another and the number of slits may be, for example, 4, 6, 8, 10, or as shown in  FIG. 2 , 12 equally spaced apart axial slits  264 . The axial slits  264  may have a width SW and a length SL, which are chosen to enable the hub to be mounted to a print drum shaft with a frictional fit sufficient to grip the shaft as it turns during operation of the printer. The slot width SW may be greater at the bottom of a slit than it is nearer the top of a slit. 
     The vertical wall  210  also includes a flange  218  that extends perpendicularly from the top of the wall. In  FIG. 2 , the flange  218  extends from an upper juncture of the groove  214  with the wall  210 , although such a structural arrangement is not required. A code wheel  236  is mounted to the top surface of the flange  218 . The location of the groove on the external surface of the wall  210  as well as the height of the groove  214  and the height of the mounting ring portion that is received in the groove  214  secure the hub  220  so it rotates within the annular opening. Additionally, these components cooperate to keep the top surface of the flange  218  and the code wheel  236  within a vertical tolerance that corresponds to a sensor slot  222  in a sensor unit  226  mounted to the floor of the base  204  as shown in  FIG. 2 . By appropriately locating the groove  214  in the wall  210  and configuring the size of the groove and the mounting ring  208 , vertical displacement of the hub  220  is controlled for proper operation with the sensor unit  226 . By controlling the dimensions and positions of the groove  214 , the mounting ring  208 , and the code wheel  236 , the snap fitting hub  220  is positioned so properly mounting the code wheel  236  to the flange  218  enables the code wheel to be read by the sensor unit  226  when it is mounted to the base. That is, the location of the groove  214  in the wall  210  positions the code wheel mounted on the horizontal flange at a height above the base that corresponds with the slot in the sensor unit  226  so the code wheel rotates within the slot of the sensor unit. 
     The code wheel  236  and the sensor unit  226  cooperate to generate an electrical signal corresponding to the rotation of the hub  220 . For example, the code wheel  236  may include a slot having a masking pattern  228  located in the slot. Such code wheels are well-known in the industry. The code wheel  236  is mounted to the flange  218  so the code wheel is centered about the longitudinal axis of the hub  220 . The slot and masking pattern  228  are located in the outboard portion  230  of the code wheel  236  that rotates within the slot  222  of the sensor unit  226 . Included within the sensor unit  226  is a light source that directs light across the slot and a light sensor that receives light from the light source after the light has traversed the slot. The masking pattern in the code wheel selectively blocks the light so the light sensor detects light in an on/off manner. The light sensor generates an electrical signal that corresponds to the alternating light signal being received by the light sensor. Thus, the electrical signal corresponds to the rotation of the code wheel mounted to the hub. The masking pattern is configured so the optical signal provides information regarding the angular position and speed of the hub and the shaft to which it is mounted as they rotate. 
     The electrical signal generated by the sensor unit  226  in response to the optical signal may be provided to a controller for synchronizing movement of a transfer roller with the print drum, the ejection of ink by the print head onto the print drum, or the feeding of media into the nip formed between the transfer roller and the print drum. While the sensor unit  226  has been described as an optical sensor, other types of rotational movement sensors may be used. For example, the sensor unit  226  may be an inductive type sensor that detects the selective induction of a current in a conductor arising from an alternating pattern of magnetized areas on the code wheel passing through the slot  222 . 
     Construction of the hub and base as described above enables simple assembly of the modular encoder  200 . As shown in  FIG. 3 , the hub  220  is pushed into the annular opening  216  to snap fit the hub onto the mounting ring  208 . The dimensions of the mounting ring  208 , hub  220 , and the groove  214  centrally locate the hub within the annular opening  216  and limit the horizontal and vertical travel of the hub within the annular opening. As shown in  FIG. 3 , a mounting pad  240  for the sensor unit  226  is located on the base  204 . Additionally, recessed areas  246  accommodate a spring clip (not shown), which retains the sensor unit  226  in the base. Other mounting pad configurations may be used for sensor units having geometrical configurations different than the one shown in the figures. Base  204  also includes cover mounting holes  248  for installing a cover over the encoder as discussed in more detail below. 
     After the hub has been installed, a code wheel  236  having a mask pattern (not depicted) is mounted on the top surface of the hub flange  218  as shown in  FIG. 4 . The code wheel  236  may be mounted to the hub flange by adhesive applied to the surface of the flange  218 , the underside of code wheel  236 , or to both surfaces. Preferably, the central bore  238  of the hub  220  is sized to correspond to the central opening  252  of the code wheel  236  so the code wheel may be properly aligned for mounting to the hub by making the circumference of the bore  238  flush with the circumference of opening  252 , although other alignment methods may be used. 
     After the code wheel  236  has been mounted to the hub  220  and the adhesive has sufficiently cured that the code wheel position is not disturbed by handling, the sensor unit  226  may be installed. As shown in  FIG. 5 , the sensor unit  226  is slipped into position on the pad  240  and a spring clip (not shown) is installed to mount the sensor unit  226  on the pad  240 . Also, the end of the code wheel  236  is positioned within the slot  222  of the sensor unit  226 . The hub may be rotated to verify smooth rotation of the code wheel within the slot  222  of the sensor unit  226 . Electrical pins  258  provide electrical power to the sensor unit  226  and couple the electrical signal generated by the sensor unit to a controller. 
     To protect the sensor unit  226  and the code wheel  236  from paper debris and other particulate in the air surrounding the encoder  200 , a cover  260  is installed to the base  204 . As shown in  FIG. 6 , the cover  260  mates with the base  204  and fasteners  266  may be used to secure the cover  260  to the base  204 . The fasteners  266  are received within openings in the cover  260  that are aligned with holes  248  in the base  204 . The holes  248  may be threaded for receiving threads on the fasteners  266 . Alternatively, the holes may be smooth and nuts may be used to receive the ends of the fasteners and secure the cover to the base or the holes may be smooth and self-tapping screws used to secure the cover. The cover  260  also includes angular orientation features  270  formed about the central opening  272 . These features are provided to orient and retain a drum heater (not shown). After an encoder is pushed onto a print drum shaft so the axial slits  264  of the hub  220  grip the shaft, mounting holes  278  may be aligned with threaded bores on a printer frame or other structure. Fasteners may be placed into the aligned holes to secure the encoder to the printer. 
     Variations and modifications of the present invention are possible, given the above description. However, all variations and modifications which are obvious to those skilled n the art to which the present invention pertains are considered to be within the scope of the protection granted by this Letters Patent.