Abstract:
A pinch control apparatus in a printer for controlling a pinch force exerted on a medium which is being fed into a printing zone is provided. The pinch control apparatus includes a camshaft rotatably mounted across a width of the medium, at least one cam attached to the camshaft, a plunger and a biasing rod. The cam has a predefined profile and is able to rotate with the camshaft. The plunger abuts the predefined profile of the at least one cam. The biasing rod extends from a pinch plate to the plunger to bias the pinch plate to a linefeed roller for exerting the pinch force on the medium therebetween. The pinch force exerted on the medium is controllable by the rotation of the camshaft.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to printers, and more particularly to a pinch control in a printer for controlling a pinch force exerted on a medium.  
       BACKGROUND OF THE INVENTION  
       [0002]     A printer generally uses a linefeed roller and an output roller to drive a medium in the printer during a printing process. The linefeed roller and the output roller are driven by a servo motor. A pick motor controls a pick system to pick up the medium, for example a paper, from an input tray and feeds it to the linefeed roller. The linefeed roller drives the paper into a printing area where droplets of ink are sprayed onto the paper from an ink cartridge.  
         [0003]     One or more pinch rollers are biased against the linefeed roller so that the paper is driven between the pinch rollers and the linefeed roller. Since the pinch rollers are biased against the linefeed roller, a pinch force is exerted on the paper. The linefeed roller and the pinch rollers control the advancement of the paper during most of the printing process.  
         [0004]     In some printing processes, once a bottom of form (BOF) edge of the paper leaves the linefeed roller and the pinch rollers (the pinching point), the output roller drags the paper from the printing area to an output tray. One or more star wheels are normally used together with the output roller to drag the paper from the printing area. The star wheels are located adjacent to the output roller, with the spikes of the star wheels touching the output roller. The paper is dragged out of the printing area between star wheels and the output roller.  
         [0005]     The configuration of the printer described above allows the printer to continue to print on the paper even when the paper has left the pinching point. This enables the printed image on the paper to have very small BOF margin, or even full bleed printing.  
         [0006]     However, paper positioning errors normally occur when the control of the driving of the paper is changed from the linefeed roller to the output roller. Such positioning errors are called BOF transition error (BOFTE). The BOFTE are more prominent in high quality photo printing. One of the main causes of BOFTE is the result of pinch rollers squeezing the bottom edge of the paper when the paper leaves the pinching point.  
         [0007]     Special print mode may be applied during or after transition from the linefeed roller to the output roller to smoothen printing defects caused by BOFTE. It is also possible to use special print head swath shifting corresponding to the paper movement during the transition to minimize such printing defects. However, the printing defects caused by BOFTE still could not be eliminated using such methods, and these methods may also cause additional printing defects.  
         [0008]     It is desirable to provide a method and a system to reduce BOFTE in small BOF margin and full bleed printing.  
       SUMMARY OF THE INVENTION  
       [0009]     In an embodiment, a pinch control apparatus in a printer for controlling a pinch force exerted on a medium which is being fed into a printing zone is provided. The pinch control apparatus includes a camshaft rotatably mounted across a width of the medium, at least one cam attached to the camshaft, a plunger and a biasing rod. The cam has a predefined profile and is able to rotate with the camshaft. The plunger abuts the predefined profile of the at least one cam. The biasing rod extends from a pinch plate to the plunger to bias the pinch plate to a linefeed roller for exerting the pinch force on the medium therebetween. The pinch force exerted on the medium is controllable by the rotation of the camshaft. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The embodiments of the invention will be better understood in view of the following drawings and the detailed description.  
         [0011]      FIG. 1  shows a cross-sectional view of a part of a paper driving mechanism in a printer.  
         [0012]      FIG. 2  shows an isometric view of a pinch control apparatus with a pinch support holder and a transmission gear train according to an embodiment.  
         [0013]      FIG. 3   a  shows a cross-sectional view of the pinch control apparatus with a cam in a home position according to an embodiment.  
         [0014]      FIG. 3   b  shows the cross-sectional view of the pinch control apparatus with the cam in a position where the pinch force exerted by the pinch roller on the linefeed roller is zero according to an embodiment.  
         [0015]      FIG. 4  shows a cross-sectional view of a protrusion of the pinch support holder acting as a stopper for the cam according to an embodiment.  
         [0016]      FIG. 5  shows the transmission gear train with a selector gear disengaged from a connecting gear according to an embodiment.  
         [0017]      FIG. 6  shows the transmission gear train with the-selector gear engaged with the connecting gear according to an embodiment.  
         [0018]      FIG. 7  shows a cross-sectional of a pick motor and its relation with an idler gear of the transmission gear train according to an embodiment.  
         [0019]      FIG. 8  shows a flow chart of a printing process with pinch control according to an embodiment.  
         [0020]      FIG. 9   a  shows a cross-sectional view of the pinch control apparatus having a second cam according to an embodiment.  
         [0021]      FIG. 9   b  shows the cross-sectional view of the pinch control apparatus with the second cam in a position pushing the pinch plate, resulting in the pinch roller to be lifted away from the linefeed roller according to an embodiment.  
         [0022]      FIG. 10  shows a cross-sectional view of a protrusion of the pinch plate acting as a stopper for the second cam according to an embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]      FIG. 1  shows a cross-sectional view of a part of a paper driving mechanism in a printer. The paper driving mechanism includes a linefeed roller  101 , an output roller  102 , a servo motor  103 , a pinch roller  104  and a star wheel  105 . The servo motor  103  drives the linefeed roller  101  and the output roller  102 . The pinch roller  104  is mounted at one end of a pinch plate  106 . The other end of pinch plate  106  is attached to a spring  107 . The pinch plate  106  is pivoted  108  between the two ends. The pinch roller  104  is biased by the spring  107  to the linefeed roller  101 .  
         [0024]     Although only one pinch plate  106  is shown, and one spring  107  is attached to the pinch plate  106 , it should be noted that it is possible that the paper driving mechanism includes more than one pinch plates  106  with one or more springs  107  attached to each pinch plate  106  in other embodiments. Also, each pinch plate  106  may include one or more mounted pinch rollers  104 . Similarly, the paper driving mechanism may also include more than one star wheels  105  in other embodiments.  
         [0025]     The spring  107  is attached to the pinch plate  106  at one end, and to a plunger  109  at the other end. The plunger  109  sits or abuts on a cam  120  which is attached to a camshaft  110 . The structure of the plunger  109 , the cam  120  and the camshaft  110  will be described in greater detail later.  
         [0026]      FIG. 2  shows an isometric view of the pinch control apparatus with a pinch support holder  130  and a transmission gear train  131 . It can be seen that the pinch control apparatus includes four plungers  109 , four springs  107  and four pinch plates  106 . Each spring  107  is attached to one of the plungers  109 . It should however be noted that the pinch control apparatus may include any number of plungers  109 , springs  107 , and pinch plates  106  in other embodiments. The camshaft  110  is supported by the pinch support holder  130 , and is rotatable with respect to the pinch support holder  130 . In an embodiment, the pinch plate  106  is pivoted  108  on the pinch support holder  130 .  
         [0027]     The transmission gear train  131  transfers a torque or rotation from a pick motor  145  (see  FIG. 7 ) to the camshaft  110 . The transmission gear train  131  has a reduction ratio of 18.9 in one embodiment. The pick motor  145  is normally engaged to a pick system in a printer to pick paper from an input tray and feeds it to a turn roller  113 . The turn roller  113  then feeds the paper to the linefeed roller  101  which drives the paper for printing.  
         [0028]     The transmission gear train  131  includes an idler gear  132  rotatably mounted on a same shaft  129  of the linefeed roller  101 , a selector gear  133 , two connecting gears  134 ,  135  and a camshaft gear  136 . The idler gear  132  is able to rotate independently from the shaft  129  of the linefeed roller  101 . The selector gear  132  is engaged to the idler gear  133 , and can be selected using a selector mechanism  137  to be engaged with the connecting gear  134  or with another system such as the pick system.  
         [0029]      FIG. 3   a  shows a cross-sectional view of the pinch control apparatus according to an embodiment. A cam  120  is provided on the camshaft  10 , and has a predefined profile  120   a . The plunger  109  sits on or abuts the profile  120   a  of the cam  120 . The profile  120   a  of the cam  120  is defined in such a manner that a rotation of the camshaft  110  in a counter-clockwise direction causes the distance between the plunger  109  and the camshaft  110  to decrease.  FIG. 3   b  shows a cross-sectional view of the pinch control apparatus when the cam  120  is rotated in a counter-clockwise direction. As can be seen from  FIG. 3   b , the distance between the plunger  109  and the camshaft  110  has decreased compared to that in  FIG. 3   a . As a result, the biasing force of the spring  107 , and hence, the force exerted by the pinch rollers  104  on the linefeed roller  101  is decreased.  
         [0030]     The pinch support holder  130  may also include a protrusion  140  for the cam  120  as shown in  FIG. 4 . Similarly, the cam  120  also includes a corresponding protrusion  142 . When the camshaft  110  is rotated in the clockwise direction beyond a certain point, the protrusion  142  of the cam  120  is restrained by the protrusion  140  of the pinch support holder. Therefore, any further clockwise rotation of the camshaft  110  is prevented.  
         [0031]     Accordingly, the protrusion  140  of the pinch support holder  130  acts as a stopper for the cam  120  and prevents the rotation of the camshaft  110  in the clockwise direction beyond an end point. Therefore the protrusion  140  may be used as a hard stop for firmware identification and counts reset for the rotation of the camshaft in the clockwise direction. The hard stop is also referred as a home position of the camshaft  110 .  
         [0032]     In an embodiment, each spring  107  delivers approximately 650 grams of force on the pinch plate  106  when the camshaft  110  is in the home position. To keep the home position of the camshaft  110  and the force exerted by spring  107  on the pinch plate  106  stable, a 10 degrees counter-clockwise rotation of the camshaft  110  from the home position keeps the plunger  109  in the same position with respect to the camshaft  110 . Therefore, the design of the pinch control apparatus according to the embodiment is robust to any undesirable changes in the force exerted by the spring  107  due to any slight movement of the camshaft  110  at the home position. Such design robustness of the pinch control apparatus is advantageous as the constant force of approximately 650 grams exerted by the pinch plate  106  can be ensured without the need for a precise calibration of the position of the camshaft  110  to the home position.  
         [0033]     As the camshaft  10  is further rotated 180 degrees in the counter-clockwise direction, the force exerted by the spring  107  on the pinch plate  106 , and hence the pinch force exerted on the linefeed roller  101 , decreases to approximately 0 grams. When the camshaft  100  is rotated a further 10 degree in the counter-clockwise direction beyond this point, the pinch force exerted on the linefeed roller  101  is maintained as zero.  
         [0034]     It should be noted that the degrees of rotation of the camshaft  110  and its corresponding force exerted by the springs  107  in the above-described embodiment only illustrate one manner of implementation. Any combination of the degrees of rotation of the camshaft  110  and the corresponding forces exerted by the springs  170  are possible in other embodiments.  
         [0035]      FIG. 5  shows the transmission gear train  131  with the selector gear  133  disengaged from the connecting gear  134 .  FIG. 6  shows the transmission gear train  131  with the selector gear  133  engaged with the connecting gear  134 .  
         [0036]      FIG. 7  shows a cross-sectional view of the pick motor  145  and its relation to the idler gear  132  in an embodiment. The pick motor  145  drives a pick motor gear  146  using a rotating shaft  147 . The pick motor gear  146  is engaged with the idler gear  132 . The rotation of the pick motor gear  146  causes the idler gear  132  to rotate. It can be seen that a clockwise rotation of the pick motor gear  146  by the pick motor  145  results in the counter-clockwise rotation of the camshaft  10 . Similarly, a counter-clockwise rotation of the pick motor gear  146  results in the clockwise rotation of the camshaft  110 .  
         [0037]     It should be noted that it is also possible to use a separate motor in another embodiment for directly rotating the camshaft  110 . In this embodiment, the rotation of the camshaft  110  is not controlled by the pick motor  145 . Therefore, the transmission gear train  131  for connecting the pick motor  145  to the camshaft  110  is not needed.  
         [0038]     When a print job is initiated, a medium, such as a paper, is picked from an input tray  111 . The paper travels along a path indicated by the arrow  112  (see  FIG. 1 ) and is driven by a turn roller  113  into a paper guiding zone  114 . A paper sensor  115  senses the presence of the paper in the guiding zone  114  and an Out Of Paper Sensor (OOPS)  116  senses the Bottom of Form (BOF) edge of the paper.  
         [0039]     During a printing process, the paper in the guiding zone  114  is driven into a printing zone  117  by the linefeed roller  101  and the pinch rollers  104 . In the printing zone  117 , droplets of ink are ejected from an ink cartridge  118  onto the paper. Once the OOPS detects that the paper BOF edge of the paper is leaving the linefeed roller  110 , the linefeed pinching force is released. Hence, the output roller  102  and the star wheels  105  drive the paper from the printing zone  117  into an output tray (not shown).  
         [0040]      FIG. 8  shows a flow chart of a printing process for printing on paper according to an embodiment. Step  800  includes picking a paper by the pick motor  145 . Step  801  includes detecting the presence of paper using a sensor  115  provided in the guiding zone  114  of the printer. Step  802  includes switching the selector mechanism  137  to engage the selector gear  133  to the connecting gear  134  when the paper is detected.  
         [0041]     Step  803  includes rotating the pick motor  145  in the counter-clockwise direction until the cam  120  touches the protrusion  140  of the pinch support holder  130 . This step  803  ensures the camshaft  110  is in its home position.  
         [0042]     Step  804  includes advancing the paper by the linefeed roller  101  into the printing area  117  to be printed. The paper is advanced into the printing area  117  in a series of paper advancement steps. Step  805  includes detecting the bottom of form (BOF) edge of the paper. The BOF edge of the paper can be detected using the Out-Of-Paper Sensor (OOPS)  116  in an embodiment. Step  806  includes rotating the pick motor  145  in the clockwise direction corresponding to the paper advancement. Specifically, the pick motor  145  is rotated in the clockwise direction in predefined steps or counts for every certain number of paper advancement steps. Each predefined step or count of the pick motor  145  translates to a counter-clockwise rotation of the camshaft  110 . The pinch force exerted on the paper between the pinch rollers  104  and the linefeed roller  101  decreases when the pick motor  145  is rotated in the clockwise direction. The pinch force exerted on the paper gradually becomes zero when the paper has advanced a predetermined number of steps.  
         [0043]     Step  807  includes checking if the paper advancement has exceeded the predetermined number of steps for the pinch force to become zero. Step  808  includes advancing the paper into the printing zone  117  by the output roller  102  and the star wheels  105  for BOF printing when the pinch force becomes zero. When the printing is completed, the paper is ejected by the output roller  102  in step  809 .  
         [0044]     Step  810  includes resetting the position of the camshaft  110  to the home position by rotating the pick motor  145  in the counter-clockwise direction. This sets the camshaft  110  back to its home position so that the spring  170  delivers a biasing force of 650 g to the pinch plate  106 . Step  811  includes switching the selector mechanism  137  to engage the selector gear  133  to the pick system for picking another paper into the guiding zone. Steps  800  to  810  are repeated for controlling the pinch force on another paper during the printing process.  
         [0045]     The pinch control apparatus as described in the above embodiments allows the pinch force exerted on the paper to be gradually reduced to zero before the BOF edge of the paper leaves the pinching point. Therefore, a watermelon seed effect causing paper feeding error during a transitional point from the linefeed roller to the output roller is eliminated. The watermelon seed effect is a phenomenon when the pinch rollers squeezes the bottom edge of the paper during printing, and causing the paper to over advance (pushed forward suddenly) when the bottom edge leaves the pinch point. The watermelon seed effect is one of the main causes of Bottom of Form Transition Error (BOFTE) as already described earlier.  
         [0046]     In an embodiment, the camshaft  110  further includes a second cam  121  arranged adjacent to each cam  120 . The second cam  121  has a profile  121   a  which abuts the pinch plate  106  as shown in  FIG. 9   a  and  FIG. 9   b.    
         [0047]     The profile  121   a  of the second cam  121  is defined in a manner such that when the camshaft  110  is rotated in the counter-clockwise direction beyond the position when the pinch force exerted on the linefeed roller  101  has decreased to zero, the end of the pinch plate  106  where the springs  107  are attached to are pushed away from the camshaft  110  by the second cam  121  as shown in  FIG. 9   b . As a result, the pinch plate  106  is rotated about its pivoted point  108 , causing the other end of the pinch plate  106  where the pinch rollers  104  are mounted on to be lifted from the linefeed roller  101 .  
         [0048]     A final hard stop may be provided as an end point for the rotation of the camshaft  110  in the counter-clockwise direction. The final hard stop may be provided as a protrusion  141  extending from the pinch plate  106  as shown in  FIG. 10  in one embodiment. In this embodiment, the second cam  121  includes a corresponding protrusion  143 . When the camshaft  110  is rotated in the counter-clockwise direction beyond the end point, the protrusion  143  of the second cam  121  is restrained by the protrusion  141  of the pinch plate  106 . Therefore, any further counter-clockwise rotation of the camshaft  110  is prevented. The final hard stop may also be controlled by motor stall torque values using firmware in another embodiment.  
         [0049]     Thus the embodiment described above not only is able to control the pinch force exerted on the medium during printing, but is also able to control the lifting of the pinch rollers  104  from the linefeed roller  101 . The lifting of the pinch rollers  104  from the linefeed roller  101  allows the paper to be reversed into the guiding zone  114  even when the paper has left the pinching point. This allows small margin or even borderless duplex printing even when a duplexer is arranged at a rear end of the printer. The lifting of pinch rollers  104  may also allow a thick medium, such as a CD, to be fed into the paper guiding zone  114  from a front end of the printer (the same end where the input and output tray are) for printing.  
         [0050]     It should also be noted that the pinch force exerted on the medium may be adjusted to any desired level according to different media properties for different print jobs. Also, by varying the profiles  120   a  of the cams  120 , different pinch force may be applied on the medium from different pinch plates  106  in accordance to any special print requirements. Furthermore, pinch rollers may be separately controlled to be lifted from the linefeed roller, and hence from the medium, during printing by varying the profiles  120   a  of the cams  120  of the different pinch plates  106 . This separate control of pinch plates  106  lifting can be used to prevent certain area of printed media from being contacted by the pinch rollers  104 .  
         [0051]     Although the present invention has been described in accordance with the embodiments as shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.