Abstract:
An apparatus for opening a platen in an impact printer is implemented. The apparatus permits rapid loading of a paper supply while maintaining the required tolerance in the spacing of the platen and a printhead. The mechanism also accommodates the printing of form documents in which the thickness of the document material may be variable. The mechanism adjusts to the varying thickness of the document medium while maintaining the required tolerance in the spacing between the platen and printhead.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 09/478,684 filed on Jan. 6, 2000, which in turn is a divisional of application Ser. No. 09/041,172 filed on Mar. 12, 1998, now U.S. Pat. No. 6,102,590. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to impact printers, and in particular, to a cover-platen opening mechanism in such printers. 
     BACKGROUND INFORMATION 
     Printers used in point-of-sale applications frequently need to have a paper supply reloaded by an operator who may be new to the job, or otherwise untrained. Moreover, it is often necessary that the paper be reloaded while customers are waiting to be served. Impact printers require close tolerances between the printhead and the platen. As a consequence, impact printers according to the prior art require the threading of paper through the printing mechanism, and a simultaneous manipulation of feed actuating mechanisms to load the paper in order that the relationship of the printhead and paper maintain the required tolerance. Moreover, the implementation of alternate paper feed paths in order to provide for the printing of form documents, as well as printing onto paper supplied in bulk, is difficult to implement in point-of-sale impact printers according to the prior art. The close tolerances between the printhead and the platen must be maintained in the presence of forms having different thickness paper. This is precluded in the impact printing mechanisms in printers having a platen and printhead with a fixed relative position, according to the prior art 
     Thus, there is a need in the art for a mechanism that allows simple drop-and-load paper loading while maintaining tight head gap tolerances, and in which form thickness compensation is accommodated. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the previously mentioned needs by providing a cover-platen opening mechanism that permits drop-in replacement of a paper supply roll without the necessity of threading the paper through the platen and printhead mechanism. At the same time, the cover-platen opening mechanism according to the principles of the present invention maintains the required platen-printhead spacing tolerances. 
     In a cover-platen opening mechanism according to the principles of the present invention, a platen shaft longitudinally affixed to the platen forms a pivotal attachment to a cover frame. A protrusion on a first end portion of the platen provides a bearing surface for engaging a spring. The platen has a stop attached to a pre-determined one of a first and a second end portion. The stop engages positioning means that displaces in response to the thickness of a form document onto which printing is to be performed. The spacing between the platen and a printhead is thereby adjusted in response to the thickness of the form document. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates, in perspective view, a cover-platen opening mechanism in accordance with an embodiment of the present invention; 
     FIG. 2 illustrates, in side view, a cover-platen opening mechanism in accordance with an embodiment of the present invention; 
     FIG. 3 illustrates, in side view, a cover-platen opening mechanism in accordance with an embodiment of the present invention; 
     FIG. 4A illustrates, in side view, in closed position, a cover-platen opening mechanism in accordance with an embodiment of the present invention; 
     FIG. 4B illustrates, in side view, in partially open position, a cover-platen opening mechanism in accordance with an embodiment of the present invention; 
     FIG. 4C illustrates, in side view, in fully open position, a cover-platen opening mechanism in accordance with an embodiment of the present invention; and 
     FIG. 5 illustrates, in perspective view, a printer according to an embodiment of the present invention. 
     FIG. 6 illustrates a normal and rotated view of the Tee bushing accordance to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Refer now to FIG. 1 in which is depicted in perspective view cover-platen opening mechanism  100  in accordance with an embodiment of the present invention. Cover-platen opening mechanism  100  includes platen  101 , platen shaft  102 , torsion spring  103 , cover frame  104 , primary stop  105 , and document feed roller Tee bushing  106 . 
     Platen  101  is pivotally attached to platen shaft  102  which is supported in V-notch  107  in a first side of frame  108 , as illustrated in FIG.  1 . The second end of platen shaft  102  is similarly supported in a second V-notch in an opposite side of frame  108 . Platen shaft  102  together with V-notch  107  provide a first point of suspension for platen  101 . 
     A second point of suspension is provided by torsion spring  103 . In FIG. 1, torsion spring  103  is viewed through cut-away  109  which is not a part of frame  108 . A first end of torsion spring  103  bears on protrusion  111  on an end of platen  101 . A second end of torsion spring  103 , torsion spring end  110  is attached to cover frame  104  via a hole in a side portion thereof. This will be more clearly illustrated in FIG. 2, subsequently to be discussed. 
     A third point of suspension for platen  101  is provided by primary stop  105  which is fixedly attached to an end of platen  101 . Primary stop  105  provides a third point of suspension in conjunction with document feed roller Tee bushing  106 . Document feed roller Tee bushing  106  is supported in T-notch  112  in the side of frame  108 . This will be more clearly illustrated in FIG. 2, to be discussed. Primary stop  105  bears against a cylindrical portion of Tee bushing  106  thereby forming the third point of suspension of platen  101 . An end of document feed roller  113  is rotatably inserted into document feed roller Tee bushing  106 . A second document feed bushing, not illustrated in FIG. 1, is fixed in the opposite side of frame  108 , and likewise provides rotatable and pivotal support for a second end of document feed roller  113 . Moreover, Tee bushing  106 , and T-notch  112  into which it is inserted, together form a slidable support for document feed roller  113  on one side of frame  108 . A detailed illustration, in normal and rotated views, of Tee bushing  106  is shown in insert “A” in FIG.  6 . 
     Refer now to FIG. 2 showing an illustration of cover-platen opening mechanism  100  in a side elevation view. FIG. 2 clearly shows Tee bushing  106  slidably supported in T-notch  112  within frame  108 . In FIG. 2, platen  101  and cover frame  104  are shown in the closed position. In the closed position, platen  101  is proximal to printhead  201 , and separated therefrom by paper gap  202 . The medium on which printing is to take place passes through paper gap  202  wherein printing is effected by printhead  201 . Neither the printing medium nor inked ribbon are shown in FIG. 2 for clarity. It is necessary that the width of paper gap  202  be held within close, pre-determined tolerances while accommodating print media of varying thickness. 
     Varying thicknesses of print media are accommodated by the action of Tee bushing  106  and primary stop  105 . This has been previously described in conjunction with FIG. 1, and may be clearly seen in FIG.  2 . Recall that Tee bushing  106  provides a rotatable support for document feed roller  113 . Tee bushing  106  is free to move in the a-b direction within T-notch  112 . The maximum distance that Tee bushing  106  can move is determined by a width of T-notch  112 - Document feed roller  113  forms one of a pair of pinch rollers that control the motion of a form document on which printing is to occur. The second roller has not been illustrated in FIG. 2 for clarity, but will be subsequently if described in conjunction with FIG.  3 . 
     The displacement of Tee bushing  106  within T-notch  112  causes a slight rotation of platen  101  in the “c” direction (shown by the arrow) about platen shaft  102 . This is accomplished through the action of primary stop  105  which bears on a cylindrical surface (not shown in FIG. 2) of Tee bushing  106 , as has been previously described in conjunction with FIG.  1 . Concomitant with the rotation of platen  101  about platen shaft  102  is a slight upward displacement of platen pivot  203  within J-notch  204  in cover frame  104 . Thicker form documents cause Tee bushing  106  to displace further into T-notch  112  thereby producing a rotation, in the “c” direction, of platen  101  about patent shaft  102 . As a consequence of the rotation, the width of paper gap  202  increases. Conversely, for thinner form documents, Tee bushing  106  displaces a shorter distance into T-notch  112 , reducing the width of paper gap  202  when platen  101  rotates back in the “d” direction (shown by arrow) about platen shaft  102 . In an embodiment of the present invention, document thickness is a range of from at least 0.004 inches to 0.019 inches may be accommodated. The present invention will work with other ranges of document thicknesses. 
     The rotation of platen  101  about platen shaft  102  is resisted by torques produced by platen torsion spring  103 . These torques also tend to hold cover frame  104  in the closed position when cover-platen opening mechanism  100  is closed. The action of the torques acting on cover-platen opening mechanism  100  will now be discussed. 
     Refer now to FIG. 3 also depicting cover-platen opening mechanism  100  in a side view in which the torque producing forces acting on cover-platen opening mechanism  100  are also illustrated. The significant reaction forces acting on platen  101  are denoted “A”, “B”, “C”, and “D”. Compressive forces in platen torsion spring  103  produce reaction force “C” acting on a line between the point of contact of the end of torsion spring  103  on protrusion  111 , and platen torsion spring end  110  retained in cover frame  104 . Because of the displacement between the point of contact of platen torsion spring  103  on protrusion  111  and platen shaft  102 , reaction force “C” produces a torque about platen shaft  102  in the “d” direction as indicated by the arrow in FIG.  3 . This torque is countered by a torque produced by reaction force “B” produced by document pressure roller  301  acting on document feed roller  113 . A paper path for the feeding of form documents is formed between document pressure roller  301  and document feed roller  113  (as shown in FIG.  4 B). The displacement of reaction force “B” from platen shaft  102  produces a torque about platen shaft  102  that is in the “c” direction, as indicated by the arrow, in FIG.  3 . Reaction force “D” is a principally horizontal force produced by the tension in platen extension spring  302 . One end of platen extension spring  302  is attached to cover frame  104 , and a second end of platen extension spring  302  is attached to extension spring attachment  114  (obscured in FIG.  3 ), as illustrated in FIG.  1 . Reaction force “D” acts on a line passing through a center line of platen shaft  102 . Therefore reaction force “D” produces no torque about platen shaft  102 . Reaction forces “C” and “D” are balanced by reaction force “B” and reaction force “A”, which is produced by V-notch  107  acting on platen shaft  102 . Reaction force “A” is also directed through an axis of platen shaft  102 , thereby producing no torque about platen shaft  102 . The balancing of the reaction forces acting on platen  101 , and the torques they produce, maintain the relationship between platen  101 , and printhead  201 , and maintain the width of paper gap  202  within its pre-determined tolerance. 
     When cover-platen opening mechanism  100  is in the closed position, cover frame  104  is held closed by torque from platen torsion spring  103 . Compressive force in platen torsion spring  103  produces a reaction on cover frame  104  at the point of attachment of platen torsion spring end  110  in cover frame  104 . This force is shown as “E” in FIG.  3 . Because the point of attachment of platen tension spring end  110  in cover frame  104  is displaced from cover pivot  303 , it produces a torque about an axis through cover pivot  303 . This torque is indicated by the direction of the arrow, “e”, in FIG. 3, and tends to keep cover frame  104  in the closed position. The torque is countered by down stop  115  on cover frame  104 , resting on frame  108 . 
     Refer now to FIG. 4A, in which cover-platen opening mechanism  100  is illustrated in the closed position, at an instant before it opens in response to application of an opening force. Cover-platen opening mechanism  100  opens in response to the opening force applied at an end of cover frame  104 . The opening force is supplied by an operator. 
     As cover frame  104  is displaced upward, it produces reaction force “F” (FIG. 4B) on platen pivot  203 , resting in J-notch  204 . Reaction force “A” on platen shaft  102  now includes a vertical component from an upper portion of V-notch  107 , that balances reaction force “F”. 
     While platen shaft  102  is so vertically constrained by V-notch  107 , reaction force “A” produces a torque about an axis through pivot  203  causing platen  101  to rotate in the direction “c”, indicated on FIG.  4 A. As the platen is displaced vertically, platen shaft  102  begins to withdraw from V-notch  107 , and continues to rotate about platen pivot  203  under the action of torque produced by reaction force “A”. 
     The rotation of platen  101  in the direction “c” also causes a rotation of the line of force of reaction force “C”, which lies along the line between the point of contact of the end of torsion spring  103  and protrusion  111 , and the point of attachment of torsion spring end  110  in cover frame  104 . 
     The rotation of the line of force of reaction force “C” causes the torque about the axis through platen pivot  203  to change direction when the line of force of reaction force “C” passes through that axis. After passage of the line of force of reaction force “C” through the axis through platen pivot  203 , the torque produced by reaction force “C” now causes platen  101  to continue to rotate in the direction “c” about the axis through platen pivot  203 . This occurs prior to platen shaft  102  being withdrawn from V-notch  107 . Reaction force “A” is eliminated as a consequence. The rotation of platen  101  continues until platen shaft  102  is engaged by secondary platen stop  401  formed by a lower portion of cover frame  104 . This is the condition of cover-platen opening mechanism  100  illustrated in FIG.  4 B. 
     Also illustrated in FIG. 4B is the insertion of document  402  between document feed roller  113  and document pressure roller  301 . This shows the paper path for the printing of form documents, and the illustrates the displacement of Tee bushing  106  into T-notch  112  by the thickness of document  402 . It would be understood that this displacement, as depicted in FIG. 4B, is greatly exaggerated. Moreover, it would be understood that in normal operation, document  402  would be in position between document feed roller  113  and document pressure roller  301  for printing when cover-platen opening mechanism  100  is in the closed position, not in the open position illustrated in FIG.  4 B. For the purpose of clarity, document  402  has been shown in FIG. 4B, positioned between document feed roller  113  and document pressure roller  301 , as for printing. 
     The operator continues to apply an opening force to cover frame  104  until cover-platen opening mechanism  100  is in the fully open position, illustrated in FIG.  4 C. When cover-platen opening mechanism  100  is in the fully open position, the operator can insert a new roll of paper  403  into printer  404 . Loose end  405  then passes through paper gap  202  (not illustrated in FIG. 4C) formed between platen  101  and printhead  201  when cover-platen opening mechanism  100  is closed, as in FIG.  4 A. After insertion of new paper roll  403 , cover-platen opening mechanism  100  may be closed, and printer  404  is then ready for printing. 
     In FIG. 1, paper roll drive roller  114  and paper roll back-up roller  115  (partially obscured) are shown. A paper roll drive roller  114  in the “x” direction when it is desired to print on a paper roll. Drive means and paper roll paper are not shown for the sake of clarity. Paper roll drive roller  114  is rotatably supported by frame  108  and paper roll back-up roll  115  is rotatably and compliantly supported by platen  101 . Forces between paper roll drive roller  114  and paper roll back-up roll  115  are small as compared to the previously described reaction loads. FIG. 4C shows that loose end  405  is positioned between platen  101  and print head  201  as well as between paper roll drive roller  114  and paper roll back-up roller  115  when cover-platen mechanism  100  moves to the closed position. 
     As cover-platen opening mechanism  100  moves from the closed position shown in FIG. 4A, through the partially open position in FIG. 4B, to the fully open position in FIG. 4C, torsion spring  103  first compresses and then expands. This is a consequence of the distance between the point of contact with protrusion  111  and the point of attachment of spring end  110  first decreasing, and then increasing as cover fine  104  and platen  101  move through succeeding positions. Platen  101 , in combination with torsion spring  103  and cover frame  104 , has two stable positions of equilibrium, one where cover-platen opening mechanism  100  is closed, and the other where cover-platen opening mechanism  100  is fully open. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.