Abstract:
An apparatus for recording an image onto a sheet medium ( 12 ) has an entrance drive roller ( 16 ) paired with a corresponding entrance pressure roller ( 18 ) to form an entrance nip ( 14 ) for transporting the sheet medium into an image recording section ( 20 ). The image recording section has a write head ( 56 ) for recording onto a portion of the sheet medium being transported between the entrance nip ( 14 ) and exit nip ( 24 ). The exit nip is formed by a drive roller ( 26 ) paired with a corresponding exit pressure roller ( 28 ) for transporting the sheet medium out from the image recording section. A motor ( 60 ) provides rotary motion to a pinion roller ( 40 ) mechanically coupled to the entrance and exit drive rollers ( 16, 26 ). A loading mechanism provides a loading force to nest the pinion roller into rotational contact against a portion of the entrance and exit drive rollers ( 16, 26 ).

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 10/977,841, filed Oct. 29, 2004, entitled SHEET RECORDING APPARATUS WITH DUAL NIP TRANSPORT by Hawver et al., the disclosure of which is incorporated herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention generally relates to sheet media transport apparatus and more particularly relates to an image recording apparatus with a precision media transport apparatus that uses a dual nip system having precision drive roller motion provided by a pinion drive.  
       BACKGROUND OF THE INVENTION  
       [0003]     Nip-fed sheet media transport systems using paired rollers are widely used in various printing applications. In a nip-fed system, a drive roller is pressed against a backing roller to form a nip and provides drive motion at the nip. A nip-fed transport can be engineered to perform with a suitable degree of accuracy in devices such as printers and office copiers. However, conventional nip-fed media transport mechanisms do not provide sufficient precision for imaging applications that require high resolution. For example, many types of medical imaging apparatus print onto a sheet of recording medium at resolutions well exceeding 600 dots per inch. For such devices, a sheet media transport must provide extremely accurate motion when moving the sheet through the image recording mechanism. This problem becomes even more pronounced with full-sheet imaging, in which little or no margin is to be provided at the leading or trailing edges of a sheet. As is well appreciated by those skilled in media transport arts, the dynamics of handling and urging a sheet of recording medium through a printing mechanism can be much more complex at the leading and trailing edges than along more central portions of the sheet.  
         [0004]     Dual nip apparatus provide advantages where it is necessary to provide more precise motion control for sheet media. By using two pairs of rollers in series along the transport path, a more stable sheet media transport is provided, since the motion of the medium is controlled through at least one nip at any point during the image recording process.  FIG. 1  shows, in schematic form, a conventional dual nip transport apparatus  10  as used for a sheet of recording medium  12 . In the travel path, recording medium  12  is fed through an entrance nip  14  formed between an entrance drive roller  16  and a pressure roller  18 , then through an exit nip  24  formed between an exit drive roller  26  and a pressure roller  28 . Image data is recorded by a printhead  56  onto recording medium  12  in an imaging area  20  between entrance nip  14  and exit nip  24 , typically using a laser or other source of electromagnetic radiation. In order to provide uniform speed with dual nip media transport apparatus  10 , it is necessary to couple the speed of entrance drive roller  16  at entrance nip  14  with the speed of exit drive roller  26  at exit nip  24 . The conventional method for coupling entrance and exit drive rollers  16  and  26  is using a belt  22 , as shown in  FIG. 1 .  
         [0005]     While the use of belt  22  for synchronizing entrance and exit drive rollers  16  and  26  works well in many applications, the precision afforded by this arrangement falls short of what is needed for high resolution imaging. Problems such as disturbance of uniform velocity or flutter cause variation in the transport velocity of recording medium  12 , particularly during leading-edge and trailing-edge handling intervals in which recording medium  12  is gripped only at entrance nip  14  or exit nip  16 . Other problems related to compliance and tracking render the use of belt  22  as an unsatisfactory solution, particularly for media such as film that is generally thicker and more rigid than paper media or for sheet media that can vary in thickness. Furthermore, belt  22  is a wear item that may require replacement and whose performance can be degraded by age, usage, and dust or dirt.  
         [0006]     There are a number of alternatives for providing rotational motion to entrance and exit drive mechanisms. As one alternative, either entrance drive roller  16  or exit drive roller  26  could be directly coupled to a motor shaft, with coupling mechanisms provided between these rollers. However, due to inherent coupling losses and mechanical tolerances, it can be difficult to obtain a coupling arrangement that provides highly efficient coupling with minimum flutter. As another alternative, a third roller can be driven by the motor and used to couple rotation to entrance and exit rollers. While this option offers some advantages, its implementation is complicated by the need to maintain efficient coupling under load and to compensate for unwanted mechanical effects caused by motor rotation.  
         [0007]     Thus, it can be seen that there is a need for a transport mechanism that provides precision handling of single sheet media at a constant transport speed, allowing full sheet imaging from leading to trailing edge.  
       SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to provide a sheet media transport apparatus capable of improved precision. With this object in mind, the present invention provides an apparatus for recording an image onto a sheet medium, comprising: 
        a) an entrance drive roller paired with a corresponding entrance pressure roller to form an entrance nip for transporting the sheet medium into an image recording section;     b) an image recording section comprising a write head for recording onto a portion of the sheet medium being transported between the entrance nip and an exit nip;     c) an exit nip formed by a drive roller paired with a corresponding exit pressure roller, for transporting the sheet medium out from the image recording section;     d) a motor for providing rotary motion to a pinion roller, the pinion roller mechanically coupled to the entrance and exit drive rollers; and     e) a loading mechanism providing a loading force to nest the pinion roller into rotational contact against a portion of the entrance and exit drive rollers.        
 
         [0014]     It is a feature of the present invention that it employs a coupling roller to transfer rotational energy to both driver rollers. Unlike prior art arrangements, the coupling roller does not form a nip or directly transport the medium, but is used to provide continuous, smooth motion between the entrance and exit drive rollers, each of which forms its corresponding nip with a separate pressure roller.  
         [0015]     It is an advantage of the present invention that it provides a sheet media transport solution with higher mechanical coupling stiffness than is conventionally available using belt devices. This increased coupling stiffness, in turn, improves mechanical resonance characteristics of the media transport apparatus of the present invention. The apparatus and method of the present invention minimize the need for replaceable components and provide a self-aligning coupling, minimizing the need for synchronization adjustment to the sheet transport apparatus.  
         [0016]     It is an advantage of the present invention that it provides improved velocity uniformity, with a design that inherently averages surface noise from system components.  
         [0017]     These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed that the invention will be better understood from the following description when taken in conjunction with the accompanying drawings, wherein:  
         [0019]      FIG. 1  is a schematic diagram showing a conventional, prior art, dual-nip media transport apparatus;  
         [0020]      FIG. 2  is a perspective view of an apparatus for image recording, using a dual-nip media transport according to the present invention;  
         [0021]      FIG. 3  is a perspective view showing the dual nip transport apparatus of the present invention;  
         [0022]      FIGS. 4A and 4B  are perspective and partially exploded views, respectively, of drive components of the dual nip media transport;  
         [0023]      FIG. 5  is a top view showing the dual nip media transport;  
         [0024]      FIG. 6  is a side view showing the dual nip media transport;  
         [0025]      FIG. 7  is a cutaway end view showing drive components of the dual nip media transport;  
         [0026]      FIG. 8  is a cutaway side view showing drive components of the dual nip media transport;  
         [0027]      FIG. 9  is a cutaway top view showing dual nip media transport components;  
         [0028]      FIG. 10  is a cutaway end view showing drive components of the dual nip media transport;  
         [0029]      FIG. 11  is a side view of drive mounting components for the dual nip media transport;  
         [0030]      FIG. 12  is a rear view of motor pinion assembly components;  
         [0031]      FIGS. 13A and 13B  are side and front views, respectively, of drive components in one embodiment; and,  
         [0032]      FIGS. 14A and 14B  are side and front views, respectively, of drive components in an alternate embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The present description is directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0034]     Referring to  FIG. 2 , there is shown an image recording apparatus  58  for full sheet imaging, utilizing a dual nip media transport apparatus  30  according to an embodiment of the present invention. A sheet of recording medium  12 , transported in direction Q, has a leading edge  32  and a trailing edge  34 . Pressure rollers  18  and  28  cooperate with corresponding entrance and exit drive rollers  16  and  26  to form entrance nip  14  and exit nip  24 , respectively. In the embodiment shown, a motor  60  provides rotational energy to a pinion roller  40 . Pinion roller  40  couples rotation to both entrance and exit drive rollers  16  and  26  at speed reduction wheels  42  and  44 . For reference, conventional Cartesian coordinate x, y, z axes are as shown in  FIG. 2 , with direction Q in parallel with the y axis and rollers extended in the direction of the x axis.  
         [0035]     Imaging area  20  is in a widthwise strip of recording medium  12  between entrance and exit nips  14  and  24 . Printhead  56  directs exposure energy from a laser or other source, in a scanned fashion, onto that portion of recording medium  12  that is within imaging area  20 . A control logic processor  62  controls the flow of image data to printhead  56 , controls operation of motor  60 , which may be provided with an encoder, and controls other internal and interface functions of image recording apparatus, using components, algorithms, and techniques familiar to those skilled in the electronic imaging arts.  
         [0036]     Referring to  FIG. 3 , there is shown a portion of dual nip transport apparatus  30  without recording medium  12  or pressure rollers  18 ,  28 . This provides a clearer view of pinion roller  40  and the support mechanisms for driving entrance and exit drive rollers  16  and  26 . Speed reduction wheels  42 ,  44  are in continuous rotational contact with pinion roller  40 . The use of speed reduction wheels  42  enables motor  60  to operate more efficiently, running at a higher rotational speed.  
         [0000]     Mounting Arrangement for Motor  60   
         [0037]     Still referring to  FIG. 3 , a motor pinion assembly  82  is designed to provide a number of functions in dual nip transport apparatus  30 . Pinion roller  40  must be maintained in continuous rotational contact against speed reduction wheels  42 ,  44 . Force is required to nest pinion roller  40  in position against speed reduction wheels  42 ,  44 . At the same time, components of motor pinion assembly  82  must counter the rotational torque of motor  60 .  
         [0038]     Referring to  FIG. 4A , there is shown a cutaway perspective view of the drive portion of dual nip transport apparatus  30  and motor pinion assembly  82 . An optional counter roller  46  may also be provided, for reasons described subsequently.  FIG. 4B  shows a partially exploded view of the components shown in  FIG. 4A . Motor  60  is mounted in a mounting bracket  70  that serves as a motor body mount. Mounting bracket  70  has extended portions: a left and right arm  80 L and  80 R extending in directions away from rotational axis R. Mounting bracket  70  fits onto a base  72  at two seats  78  and is configured to counter the rotational torque of motor  60  and to provide a substantially balanced support for the mass of motor  60 . A constraining member  84 , spherically shaped in the embodiment shown, provides one seat along left arm  80 L. A spring  76  provides a counter force providing element at the second seat along right arm  80 R. This counter force effectively balances the weight of motor  60  in its mounting bracket  70 . In the embodiment shown, spherical constraining member  84  provides a contact surface  74  as the interface between bracket  70  and base  72 . Constraining member  84  is captive in left arm  80 L of mounting bracket  70  in the embodiment described here; however, an equivalent component could be captive or supported within base  72  or formed as a machined or molded portion of mounting bracket  70 , for example.  
         [0039]     A better understanding of the design and function of motor pinion assembly  82  in one embodiment is given by the reference top and side views, respectively, of  FIGS. 5 and 6  and the corresponding sectional views of  FIGS. 7, 8 ,  9 , and  10  taken from various perspectives with reference to  FIGS. 5 and 6 . In order to counteract the torsional force exerted by motor  60 , motor pinion assembly  82  advantageously maintains a symmetric relationship with the center of gravity, CG, of motor pinion assembly  82 . In one embodiment, the approximate center of gravity CG of motor pinion assembly  82 , shown most clearly in  FIGS. 4B, 5 ,  8 , and  11 , is in a substantially common plane, shown as a vertical plane, with the points of contact at constraining member  84  and at spring  76 . Common plane V containing these points is shown in the top view of  FIG. 5  and in the side view of  FIG. 11 . Common plane V is orthogonal to the axis of rotation R of motor  60 . This arrangement is advantaged for achieving the balanced weight condition just described.  
         [0040]     As is best shown in the sectional view of  FIG. 10 , spring  76  applies a force F that opposes the weight of motor pinion assembly  82  at base  72 . Constraining member  84  can be captive in either mounting bracket  70  or base  72 . A Vee fitting  90 , shown in  FIGS. 10 and 11 , provides a seat that restricts constraining member  84  from movement in both the x-direction, that is, parallel to the axis of rotation R and in the upward or z-direction ( FIG. 4A ). This, in turn, constrains unwanted forward, backward, and vertical movement of motor  60 .  
         [0041]     As is shown in  FIG. 12 , an encoder  86  may be mounted on mounting bracket  70  for providing feedback signals on motor speed and position.  
         [0000]     Applying Force to Nest Pinion Roller  40   
         [0042]     As is shown in  FIG. 4A , pinion roller  40  nests between speed reduction wheels  42  and  44  in one embodiment. Alternately, pinion roller  40  may nest against some other portion of entrance and exit drive rollers  16  and  26 . In order to drive entrance and exit drive rollers  16  and  26 , a nesting force is applied by some type of loading mechanism as a loading force to press pinion roller  40  into position.  FIGS. 13A and 13B  show the general direction of the nesting force N on pinion roller  40 . One goal of the loading mechanism is to apply nesting force N evenly, so that the full contact surface of pinion roller  40  applies substantially equal pressure at all points of contact against speed reduction wheels  42  and  44 . With reference to  FIG. 13A , for example, this means that the effective nesting force applied at a contact point  41  is the same as the effective nesting force applied at a contact point  43 .  
         [0043]     In the embodiment of  FIG. 8 , nesting force N is applied directly to pinion roller  40  using an arrangement of magnets  50  as a loading mechanism. Magnetic attraction pulls pinion roller  40  into its nesting position and maintains pinion roller  40  in continuous contact for driving entrance and exit drive rollers  16  and  26 . The use of magnets  50  as a loading mechanism is particularly advantaged for providing an even amount of pressure along the areas of contact, allowing nesting force N to be applied directly to the body of pinion roller  40 .  
         [0044]      FIGS. 14A and 14B  show side and front views, respectively, of an alternative approach for applying nesting force N. Here, a bearing  88  is used, providing a surface for application of force using a spring apparatus of some kind (not shown) as the loading mechanism. Those skilled in the mechanical design arts will readily recognize that deployment of a spring as a loading mechanism requires additional mounting hardware and may require means for adjustment and/or replacement. However, there may be applications for which use of a spring is an advantageous alternative.  
         [0045]     Using the two-point balance arrangement of mounting bracket  70  within base  72 , as described hereinabove, simplifies the design task of selecting appropriate magnets  50  or spring components. By balancing bracket  70  with respect to the center of gravity CG of motor pinion assembly  82 , substantially all of the nesting force applied to pinion roller  40  is, in turn, applied to speed reduction wheels  42  and  44 .  
         [0046]     The arrangement of motor pinion assembly  82  described with reference to  FIGS. 4A through 14B  provides two key functions: maintaining pinion roller  40  in continuous contact for driving speed reduction wheels  42  and  44  of entrance and exit drive rollers  16  and  26  and preventing x-axis (R-axis) rotation of the housing of motor  60 .  
         [0047]     There are a number of options for providing one more magnet  50 . One or more stationary magnets  50  can be installed along or within a holder such as a bar  52  ( FIG. 8 ) to attract pinion roller  40 . Alternately, pinion roller  40  could itself be magnetized and attracted toward bar  52 , where bar  52  is made of a ferromagnetic material, to produce the same effect. Magnets  50  could be replaceable magnets, for example. Possible types of magnet  50  include Alnico, Samarium cobalt, Neodymium Iron Boron, or ceramic, for example. In another embodiment, magnets  50  can be electromagnets. This arrangement would allow printer control logic (from control logic processor  62  in  FIG. 2 ) to apply nesting force to pinion roller  40  only when needed.  
         [0000]     Providing Additional Coupling Stiffness  
         [0048]     The present invention provides a further refinement to the use of pinion roller  40  whereby additional coupling stiffness and reduced flutter are achieved. Even though recording medium  12  is transported in a single direction, providing coupling stiffness in both directions is be advantageous. That is, there is quantifiable improvement of movement uniformity and reduction of flutter when coupling stiffness is provided in both forward and reverse directions. A belt could be provided to increase coupling stiffness between entrance and exit drive rollers  16  and  26 . Alternately, counter roller  46 , shown particularly in  FIGS. 4A, 4B , and  7 , can be provided for this reason.  
         [0049]     Some type of loading force is required for counter roller  46 . In the embodiment of  FIG. 7 , counter roller  46  is nested into position using magnetic attraction, in a manner similar to the attraction of pinion roller  40 . Otherwise, springs, bearings, and other supporting mechanical components would be necessary to provide a loading force to seat counter roller  46  properly into position against at least some portions of speed reduction wheels  42  and  44  or entrance and exit drive rollers  16  and  26 .  
         [0050]     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention as described above, and as noted in the appended claims, by a person of ordinary skill in the art without departing from the scope of the invention. For example, the use of speed reduction wheels  42  and  44  as enlarged portions of entrance and exit drive rollers  16  and  26 , although advantageous for allowing highs motor speeds and improved torque, is optional for the present invention. In another embodiment, pinion roller  40  is nested directly against the main body of entrance and exit drive rollers  16  and  26 . Rollers themselves could be formed from a number of materials, suitably selected according to roller function. In one embodiment, for example, drive rollers  16  and  26  are urethane-coated rollers. A combination of spring force and magnetic or electromagnetic attraction could be used to nest pinion roller  40  into position. Multiple counter rollers  46  or a segmented counter roller  46  could be used. One or more rollers could be hollow, particularly where magnetic attraction is used for nesting.  
         [0051]     Various types of printhead  56  could be employed, such as using lasers, LEDs, or other light sources, wherein the light emitted may be outside the visible spectrum. Other types of printhead, utilizing thermal or inkjet printing mechanisms, could be used. Sheet medium  12  could be a photosensitive medium or some other type of recording medium. Either entrance drive roller  16  or exit drive roller  26  could serve as the driving roller in an embodiment.  
         [0052]     Thus, what is provided is an apparatus and method for an image recording apparatus with a precision media transport apparatus that uses a dual nip system having precision drive roller motion provided using a pinion roller.  
       Parts List  
       [0000]    
       
           10  dual nip transport apparatus  
           12  recording medium  
           14  entrance nip  
           16  entrance drive roller  
           18  pressure roller  
           20  imaging area  
           22  belt  
           24  exit nip  
           26  exit drive roller  
           28  pressure roller  
           30  dual nip transport apparatus  
           32  leading edge  
           34  trailing edge  
           40  pinion roller  
           41  contact point  
           42  speed reduction wheels  
           43  contact point  
           44  speed reduction wheels  
           46  counter roller  
           50  magnet  
           52  bar  
           56  printhead  
           58  image recording apparatus  
           60  motor  
           62  control logic processor  
           70  bracket  
           72  base  
           74  contact surface  
           76  spring  
           78  seat  
           80 L left arm  
           80 R right arm  
           82  motor pinion assembly  
           84  constraining member  
           86  encoder  
           88  bearing  
           90  Vee fitting