Abstract:
A system for printing an image on a printing media, comprising: a) an impression roller; b) a gripper which receives the printing media when said gripper is open, closes to hold the printing media to the impression roller while the image is printed, and opens to release the printing media from the impression roller; and c) at least one sensor which senses whether the gripper is open or closed.

Description:
FIELD OF THE INVENTION  
       [0001]     The field of the invention is printers and copiers, especially apparatus and methods for monitoring malfunctioning of grippers.  
       BACKGROUND OF THE INVENTION  
       [0002]     Printers and copiers often have a photo-sensitive member which receives an image, and an intermediate transfer member, often with a heated blanket, which receives the image from the photo-sensitive member and transfers it to sheet of paper or other printing media on an impression roller. (The printing media will henceforth be referred to as “paper,” but any printing media should be understood.) The intermediate transfer member often has a delicate surface, for example the heated blanket may comprise a release surface with a soft conforming layer underneath, which allows the surface to press against the impression roller with uniform pressure. These characteristics of the intermediate transfer member produce good images on the printing media, but also mean that its surface may be damaged relatively easily, and such damage may require a time-consuming and expensive replacement of the member. Even for different structures, the intermediate transfer member (often in the form of a blanket) is subject to damage from excess pressure and/or from toner that is left on the release surface too long.  
         [0003]     One cause of such damage is paper sticking to the blanket. Normally, paper is fed onto the impression roller and held there with grippers. If the paper is misfed for any reason, then the paper may stick to the blanket of the intermediate transfer member after the image is printed. The printer must then be stopped, opened up, the paper removed, and the ink on the blanket (which in normal operation gets completely transferred to the paper) must be cleaned off. Delay in removing the ink from the blanket may result in the ink drying onto the (generally heated) blanket, which must then be replaced. Ink may also remain on the intermediate transfer member if, as a result of the paper misfeeding, at least part of the intermediate transfer member presses directly against the surface of the impression roller, without any paper in between. If the paper is folded or wrinkled as a result of the misfeed, it may dent the blanket, also making it necessary to replace the blanket. If paper is not released on time from the impression roller, then two sheets of paper may end up on the impression roller, which can hurt the blanket.  
         [0004]     Because paper misfeeds are potentially so damaging, it would be desirable to prevent any events which could cause a misfeed, even relatively rare events.  
       SUMMARY OF THE INVENTION  
       [0005]     An aspect of an embodiment of the invention concerns an apparatus and method for preventing some misfeeds which can damage the blanket, by sensing when the grippers fail to open properly or fail to close properly. Optionally, this is done by two proximity sensors, for example capacitive, inductive or optical sensors, which sense the proximity of a target element which is in a different position depending on whether the grippers are open or closed. Alternatively, the state of the grippers is sensed directly, but sensing the proximity (or lack of proximity) of a target element potentially provides a simpler way to determine the state of the grippers, since the grippers are not generally accessible.  
         [0006]     In an embodiment of the invention, the target element rotates with the impression roller. The target element changes its position as the impression roller rotates, while the sensors are fixed in place and sense the target element as it passes them, if the grippers are open. The first sensor is located in a position such that, in normal operation, the grippers would already be open when the target passes the first sensor, if the grippers are supposed to open on that cycle of the impression roller. The second sensor is located in a position such that, in normal operation, the grippers would already be closed when the target passes the second sensor (and hence the second sensor would not sense the target), if the grippers are supposed to close on that cycle of the impression roller. Alternatively, one or both of the sensors are instead located at a position such that they would sense the target only if the grippers were closed. Alternatively, only one of these sensors is present, and only one type of malfunction of the grippers (failure to open, or failure to close) is sensed.  
         [0007]     Optionally, the target element is, or is attached to, an element which causes the grippers to open and close. For example, the target element is a cam follower, which follows a cam while rotating with the impression roller, opening and closing the grippers at the proper points in the cycle.  
         [0008]     In printers where only one transfer to paper takes place at each print engine, the cam is often fixed in place. The grippers always open and close at the same points in the cycle of the impression roller, which picks up a sheet of paper during each cycle, and releases it before picking up the next sheet during the next cycle. In such a printer, the grippers virtually never fail to open or close properly. However, in printers which print more than one image on each sheet of paper, for example color printers which print several color separations on each sheet of paper, the grippers do not open and close on every cycle, and the cam is not fixed in place. Instead, as in the HP3000 Twister printer using a cam, there is a locking mechanism, in this example a hook which hooks onto the cam, which locks the cam in place when the grippers are supposed to open and close, to release a sheet of paper which is finished being printed, and to pick up the next sheet.  
         [0009]     When the grippers are not supposed to open, for example when additional images are going to be printed on the sheet that is being held by the grippers, then the locking mechanism is released, and the cam rotates together with the impression roller and the cam follower. The grippers then remain closed, until the locking mechanism is re-engaged, and the cam follower starts to move relative to the cam again. This process is illustrated in  FIGS. 2A-2I , and described in detail below.  
         [0010]     In printers with a moveable and lockable cam, the grippers are much more likely to fail to open or fail to close at the proper time. This can occur, for example, if the locking mechanism fails to release, or fails to engage, or if the cam fails to rotate with the impression roller when the locking mechanism is released, or if the cam stops rotating at the wrong orientation when an attempt is made to engage the locking mechanism. While gripper malfunction is very rare, the consequences are harmful and it is desirable to sense gripper malfunction as soon as it occurs.  
         [0011]     If the sensors sense that the grippers have failed to open, or failed to close, then optionally the printer is automatically stopped, before damage has been done to the blanket of the intermediate transfer member, and a diagnostic message is issued to the operator.  
         [0012]     There is thus provided, in accordance with an exemplary embodiment of the invention, a system for printing an image on a printing media, comprising: 
        a) an impression roller;     b) a gripper which receives the printing media when said gripper is open, closes to hold the printing media to the impression roller while the image is printed, and opens to release the printing media from the impression roller; and     c) at least one sensor which senses whether the gripper is open or closed.        
 
         [0016]     Optionally, the impression roller rotates on a rotation axis, the closing of the grippers occurs within a first angular range in the rotation of the impression roller, the opening of the grippers to release the printing media occurs within a second angular range in the rotation of the impression roller, and the at least one sensors comprise one or both of: 
        a) a first sensor which senses whether the gripper is open or closed at a first sensor angle in the rotation of the impression roller, which first sensor angle follows the first angular range and precedes the second angular range; and     b) a second sensor which senses whether the gripper is open or closed at a second angle in the rotation of the impression roller, which second angle follows the second angular range and precedes the first angular range.        
 
         [0019]     Optionally, the at least one sensors comprise only the first sensor.  
         [0020]     Alternatively, the at least one sensors comprise only the second sensor.  
         [0021]     Alternatively, the at least one sensors comprise both the first and second sensors.  
         [0022]     In an embodiment of the invention, there is a sensor target which has a first position when the gripper is open and a second position when the gripper is closed, and at least one of the at least one sensors is a proximity sensor which detects the target only in one of the first and second positions.  
         [0023]     Optionally, the proximity sensor detects the target only in the first position.  
         [0024]     Optionally, the proximity sensor is an inductive sensor.  
         [0025]     Alternatively, the proximity sensor is a capacitive sensor.  
         [0026]     Alternatively, the proximity sensor is an optical sensor.  
         [0027]     Optionally, the target is attached to a control element which controls the opening and closing of the gripper.  
         [0028]     Alternatively, the target comprises a control element which controls the opening and closing of the gripper.  
         [0029]     In an embodiment of the invention, there is a cam, and the control element comprises a cam follower which follows the perimeter of the cam, the grippers being open when the cam follower is on a first portion of said perimeter, and closed when the cam follower is on a second portion of said perimeter.  
         [0030]     Optionally, there is a control rod attached to the gripper, and a lever joining the control rod to the cam follower, and the cam follower is on the first portion of the perimeter of the cam, the lever rotates the control rod into an orientation where the gripper is open, and when the cam follower is on the second portion of the perimeter of the cam, the lever rotates the control rod into an orientation where the gripper is closed.  
         [0031]     Optionally, there is a cam stopper, the cam does not rotate when the cam stopper is engaged, and the cam rotates substantially in synchrony with the impression roller when the cam stopper is disengaged, thereby preventing the gripper from opening when the cam stopper is disengaged and the cam follower is on the first portion of the perimeter of the cam.  
         [0032]     Optionally, friction keeps the cam rotating substantially in synchrony with the impression roller when the cam stopper is disengaged.  
         [0033]     Alternatively or additionally, there is a cam attachment mechanism which attaches the cam to the impression roller when the cam stopper is disengaged, thereby keeping the cam rotating substantially in synchrony with the impression roller.  
         [0034]     Optionally, the cam stopper comprises a first cam stopper element which is attached to the cam, and a second cam stopper element which is fixed in place, and the cam stopper elements engage each other to engage the cam stopper, and disengage to disengage the cam stopper.  
         [0035]     Optionally, there is an actuator which moves one of the cam stopper elements into a first position where it engages the other cam stopper element, and into a second position where it disengages the other cam stopper element.  
         [0036]     Optionally, the cam only stops at a stopping position, and the cam stopper element moved by the actuator does not interfere with the rotation of the cam when said element is in the first position, until the cam is substantially in the stopping position.  
         [0037]     Optionally, one of the cam stopper elements is a hook.  
         [0038]     There is thus also provided, in accordance with an exemplary embodiment of the invention, a method of preventing damage to an intermediate transfer member in a printer with a rotating impression roller, and a gripper which has an open position and a closed position, which gripper holds a printing media to the impression roller when the gripper is closed but not when the gripper is open, the method comprising: 
        a) feeding the printing media onto the impression roller when the gripper is open and the impression roller is oriented at a first range of angles in its rotation;     b) closing the gripper to hold the printing media onto the impression roller when the impression roller is oriented at a second range of angles in its rotation;     c) transferring an image from the intermediate transfer member to the printing media while the printing media is held onto the impression roller;     d) opening the gripper to release the printing media from the impression roller after the image is transferred, when the impression roller is oriented at a third range of angles in its rotation; and     e) sensing at least one of: a failure of the gripper to be open to receive the printing media, a failure of the gripper to close to hold the printing media, a failure of the gripper to stay closed until after the image is transferred, and a failure of the gripper to open to release the printing media.        
 
         [0044]     Optionally, sensing comprises sensing, a failure of the gripper to be open to receive the printing media.  
         [0045]     Alternatively or additionally, sensing comprises sensing a failure of the gripper to close to hold the printing media.  
         [0046]     Alternatively or additionally, sensing comprises sensing a failure of the gripper to stay closed until after the image is transferred.  
         [0047]     Alternatively or additionally, sensing comprises sensing a failure of the gripper to open to release the printing media.  
         [0048]     In an embodiment of the invention, the method includes stopping the printer after at least one of the failures has been sensed, before said failure causes damage to the intermediate transfer member.  
         [0049]     Optionally, sensing comprises sensing whether the gripper is open or closed when the impression roller is oriented at an angle that it reaches in its rotation after passing the second range of angles and before reaching the second range of angles, thereby sensing failure of the gripper to close and failure of the gripper to stay closed.  
         [0050]     Additionally or alternatively, sensing comprises sensing whether the gripper is open or closed when the impression roller is oriented at an angle that it reaches in its rotation after passing the third range of angles and before reaching the first range of angles, thereby sensing failure of the gripper to open and failure of the gripper to be open.  
         [0051]     Optionally, sensing whether the gripper is open or closed when the impression roller is oriented at an angle comprises detecting the proximity or lack of proximity of a target to a proximity sensor, and the target passes near the sensor only when the impression roller is oriented at said angle, and only when the gripper is in one of an open state and a closed state, but not when the gripper is in the other of said states. 
     
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
       [0052]     Exemplary embodiments of the invention are described in the following sections with reference to the drawings. The drawings are generally not to scale and the same or similar reference numbers are used for the same or related features on different drawings.  
         [0053]      FIG. 1  is a side perspective view of one end of an impression roller with grippers, showing a mechanism for opening and closing the grippers, according to an exemplary embodiment of the invention;  
         [0054]      FIGS. 2A-2I  are a time sequence of schematic axial views showing the operation of the mechanism, according to the same embodiment of the invention; and  
         [0055]      FIG. 3  is an axial view showing a situation in which the grippers fail to close, according to the same embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0056]      FIG. 1  shows one end of an impression roller  102  in a printer or copier, in accordance with an embodiment of the invention. The impression roller rotates continuously around an axis  104  during normal operation of the printer or copier. A gripper  106  is opened and closed by the rotation of a gripper control rod  108 , which is mounted in a bearing  109  that is attached to the impression roller. Optionally there are one or more additional grippers not shown in the drawing, located for example further to the left of the gripper shown, which are also opened and closed by control rod  108 . A lever  110  attaches control rod  108  to a cam follower  112 . When cam follower  112  moves radially outward from axis  104 , lever  110  causes control rod  108  to rotate in a direction which opens gripper  106 . When cam follower  112  moves radially inward, then lever  110  causes control rod  108  to rotate in the other direction, which closes gripper  106 . Alternatively, cam follower  112 , lever  110 , contrail rod  108 , and gripper  106  are configured so that moving cam follower  112  outward causes gripper  106  to close, and moving cam follower  112  inward causes gripper  106  to open. In describing the drawings, we will assume that moving cam follower  112  inward causes gripper  106  to close, but the appropriate changes in the description, if the reverse is true, will be obvious to one skilled in the art.  
         [0057]     In an embodiment of the invention, a sensor target  114  is situated at or near the end of cam follower  112 , opposite the end of cam follower  112  that is attached to lever  110 . Optionally, sensor target  114  is not a separate element added to cam follower  112 , but is just the far end of cam follower  112 , made of the same material as the rest of cam follower  112 .  
         [0058]     Cam follower  112  follows the surface of a cam  116 , as impression roller  102  rotates around axis  104 . Although cam  116  is shown some distance to the right of impression roller  102 , in  FIG. 1 , for clarity, cam  116  is optionally directly in contact with the end of impression roller  102 . Cam  116  has a depression  118  on one side. When cam follower  112  moves into depression  118 , then gripper  106  closes, by the mechanism described above. When cam follower  112  moves out of depression  118 , then gripper  106  opens.  
         [0059]     A first position sensor  120  is located directly to the right of, and somewhat above, depression  118 . When cam follower  112  falls into depression  118 , then sensor target  114  at the end of cam follower  112  will be located radially inward from sensor  120 , and sensor  120  will fall to detect target  114 . However, if cam follower  112  fails to fall into depression  118 , but remains at the same distance from axis  104  as it normally is when gripper  106  is open, then target  114  at the end of cam follower  112  will be located at the same radius as sensor  120 , which will detect target  114  as it sweeps by sensor  120  in the course of the rotation of impression roller  102 .  
         [0060]     A second position sensor  122  is optionally located at the same radial distance from axis  104  as sensor  120 , but at a different azimuthal position. As cam follower  1112  follows outside of cam  116 , target  114  will go past sensor  122 , which will detect it. However if, as will be described below, cam  116  and depression  118  rotate together with impression roller  102 , and cam follower  112  remains in depression  118  as impression roller  102  rotates, then target  114  will be too far inward radially to be sensed by sensor  122 , as cam follower  112  goes past sensor  122 .  
         [0061]     Alternatively, sensor  120  and/or sensor  122  are located in positions so that they will only detect target  114  if cam follower  112  is in depression  118  when it passes the sensor.  
         [0062]     Optionally, sensors  120  and  122  are inductive sensors. Alternatively, they are capacitive sensors, or optical sensors, or any other kind of proximity sensor known to the art. Optionally, the two sensors are not the same kind of sensor, although, using the same kind of sensor for both sensor  120  and sensor  122  has the potential advantage of making the design and operation of the apparatus simpler. Optionally, only one of sensor  120  and sensor  122  is present but having both sensors present has the potential advantage that the sensor can detect both failure of the grippers to open and failure of the grippers to close.  
         [0063]     A rod  124 , attached to cam  116 , is restrained by a hook  126 , to keep cam  116  fixed in place while impression roller  102  rotates. Hook  126  is attached to an axle  128 , which is turned by a control motor  130 . The control motor brings hook  126  upward, where it catches rod  124 , to lock cam  116  in place, and brings hook  126  downward to a position where hook  126  does not interfere with rod  124 , to unlock cam  116 . Alternatively, any other actuator known to the art, for example a solenoid, is used instead of control motor  130  and axle  128 , to move hook  126  back and forth. Alternatively, another reversible attachment mechanism known to the art, such as a clamp or a latch, or a rod which fits into a hole, serves to keep cam  116  fixed in place instead of hook  126  and rod  124 . A potential advantage of using hook  126  and rod  124  over some other attachment mechanisms is that the hook can be moved into a position to catch rod  124  any time after rod  124  has passed by that position on the previous rotation of the cam, and the hook will stop the cam at the proper time, but will not interfere with the rotation of the cam until rod  124  reaches the hook.  
         [0064]     Although rod  124  is shown at the bottom of cam  116 , optionally rod  124 , or whatever attachment mechanism is used, is located anywhere where it can conveniently hold the cam in place, and where it does not interfere with other elements, for example the cam follower.  
         [0065]     When hook  126  is moved away from rod  124 , then cam  116  is free to rotate around axis  104 , and does rotate around axis  104 , in synchrony with impression rotor  102 . For example, friction between cam  116  and impression roller  102  keeps them rotating together when hook  126  does not prevent cam  116  from rotating. Alternatively, cam  116  is attached to impression roller  102  by another pin or a latch or any other reversible attachment mechanism, when hook  126  does not interfere with rod  124 . Alternatively, separate synchronized motors are used to drive impression roller  102  and cam  116 . Using friction to keep cam  116  and impression roller  102  rotating together has the potential advantage that it is not necessary to actively drive cam  116 , or to activate a separate attachment mechanism, but simply removing hook  126  from rod  124  makes cam  116  rotate together with impression, roller  102 . Alternatively, cam follower  112  and depression  118  are used to keep cam  116  rotating together with impression roller  102 , as described below.  
         [0066]     The details of how impression roller  102  and cam  116  are mounted are not shown, for clarity. Optionally, impression roller  102  and cam  116  are both mounted on a common axle along axis  104 , for example, or any kind of rotary bearing known to the art is used. Similarly, the mechanism used to drive impression roller  102  is not shown in  FIG. 1 , and may be any kind of rotary drive mechanism known to the art.  
         [0067]     The normal operation of the cam and cam follower in an exemplary embodiment of the invention will be described with reference both to  FIG. 1 , a side view, and  FIGS. 2A through 2I , which provide a time sequence of axial views. The operation may be easier to understand by looking at both  FIG. 1  and the series of  FIGS. 2A-2I  together. In the axial view of  FIGS. 2A-2I , the outer surface of impression roller  102  is shown as a dashed circle surrounding cam  116 , although in fact, as seen in  FIG. 1 , cam  116  is located at a different axial position than impression roller  102 . In  FIGS. 2A-2I , gripper  106  is in an open state when it is oriented at an angle to the surface of impression roller  102 , and in a closed state when it is tangent to the surface of impression roller  102 .  
         [0068]     In a first mode of operation, cam  116  is held in place by hook  126  when the impression roller is waiting for paper. This is shown in  FIG. 2A , where hook  126  is shown hooked around rod  124 . Depression  118  in cam  116  is located at a position such that gripper  106  will be in the open position when paper is fed into gripper  106 , as in  FIG. 2B , and gripper  106  then closes around the paper, as in  FIG. 2C , holding the paper to impression roller  102 . Note that when cam follower  112  falls into depression  118  and gripper  106  closes, as in  FIG. 2C , cam follower  112  does not pass next to sensor  120 , but passes below sensor  120 , and sensor  120  does not detect target  114  (not shown in  FIGS. 2A-2I ) which is attached to cam follower  112 . Optionally, depression  118  is positioned so that gripper  106  closes shortly after the paper is fed into the gripper, and the paper does not have time to slip out of the gripper after it is fed in. Although  FIGS. 1, 2A , and  2 B show depression  118  located on the top of cam  116  when cam  116  is held in place by hook  126 , the actual position of depression  118  need not be on the top of cam  116  when cam  116  is held in place by hook  126 .  
         [0069]     Once gripper  106  closes around the paper, hook  126  starts to swing downward, away from rod  124 , releasing cam  116 , as shown in  FIG. 2C , and cam  116  begins to rotate with impression roller  102 , as shown in  FIGS. 2D, 2E , and  2 F. Cam follower  112  thus remains in depression  118 , and gripper  106  remains closed around the paper, as impression roller  102  rotates. Note that, because cam follower  112  remains in depression  118 , cam follower  112  does not pass next to sensor  122 , but passes to the side of sensor  122  in  FIG. 2E , and target  114  is not detected by sensor  122 , indicating that gripper  106  is closed.  
         [0070]     A printed image is then transferred to the paper from the intermediate transfer member, not shown in the drawing. Optionally, this is done in a single rotation of the impression roller. Optionally, if only a single image is being printed on the paper, then hook  126  is not removed from rod  124  at all, so  FIGS. 2D, 2E  and  2 F are skipped, and, the image is printed during, the fraction of a cycle when the grippers are closed. Alternatively, for example in color printing, two or more rotations of the impression roller are used in order to print the full image on the paper. Cam  116  continues to rotate synchronously with impression roller  102 , and gripper  106  remains closed around paper, while the image is printed. Optionally, the trapping of cam follower  112  in  112  in depression  118  is sufficient to keep cam  116  rotating in synchrony with impression roller  102 . Alternatively, other mechanisms are used, as described previously.  
         [0071]     When the image has been printed and it is desired to remove the paper from the impression roller, then hook  126  swings back upward, as shown in  FIG. 2F , until it is in position to catch rod  124 , as shown in  FIG. 2G . Cam  116  then stops rotating and remains fixed in place with depression  118  at its original location, for example on top of cam  116  as shown in  FIG. 2G . As impression roller  102  and cam follower  112  continue to rotate around axis  104 , cam follower  112  goes out of depression  118  and moves outward from axis  104 , as shown in  FIG. 2H . This causes lever  110  to rotate control rod  108 , opening gripper  106  when impression roller  102  is in a proper orientation to release the paper. As impression roller  102  continues to rotate, cam follower  112  continues to follow the surface of cam  116 . This time, as shown in  FIG. 2I , cam follower  112  passes right by sensor  122 , and sensor  122  detects target  114  on cam follower  112 , indicating that gripper  106  is open. Impression roller  102  and cam follower  112  return to the position shown in  FIG. 2A , where gripper  106  is ready to receive the next piece of paper.  
         [0072]     If hook  126  fails to catch rod  124  when impression roller  102  is in the position shown in  FIG. 2G , then cam  116  will continue to rotate with impression roller  102 , as shown in  FIGS. 2D, 2E  and  2 F, and gripper  106  will fail to open. This means that the paper will not be released, and that gripper  106  will not be able to receive the next sheet of paper. This condition will be detected because sensor  122  will fail to sense cam follower  112 , which will be positioned as in  FIG. 2E , rather than as in  FIG. 2I  as it is supposed to be. The printer is then optionally stopped, before the next piece of paper can misfeed and possibly damage the blanket of the intermediate transfer member.  
         [0073]     If hook  126  fails to disengage properly from rod  124  when it is supposed to, then cam  116  will not rotate with impression roller  102 , but will remain in the orientation shown in  FIG. 2C . Then cam follower  112  will go out of depression  118  and gripper  106  will open prematurely, as in  FIGS. 2H and 2I , rather than remaining closed as it is supposed to, as shown in  FIGS. 2D and 2E . This could lead to the paper sticking to the blanket of the intermediate transfer member. It could also lead to the paper slipping out of place before all of the image has been printed on the paper, with the result that part of the image is printed directly on the impression roller. Since the impression roller may not absorb all of the ink from the intermediate transfer member, as the paper does, this can result in some of the ink remaining on the blanket of the intermediate transfer member and drying, damaging the blanket. The premature opening of gripper  106  will be detected because sensor  122  will detect target  114  when cam follower  112  passes by sensor  122 , as in  FIG. 2I . The printer is then optionally stopped before any damage is done to the blanket. In the case of damage due to ink drying on the blanket, it is specially important to detect the problem quickly, so that the ink can be cleaned off the blanket before it dries. Using sensor  122  to detect the problem potentially allows the ink to be cleaned off the blanket in time. If the problem is not detected until a paper jam occurs later, for example, then it may be too late to save the blanket because on the ink on the blanket may already be dry.  
         [0074]      FIG. 3  shows an example in which gripper  106  fails to close when impression roller  102  reaches the orientation, shown in  FIG. 2C , at which gripper  106  is supposed to close. In  FIG. 3 , cam follower  112  follows cam  116 , but cam  116  is oriented in the wrong direction, with depression  118  on the side instead of on top, when the grippers are supposed to close. For example, cam  116  stops rotating when it is at a wrong orientation for stopping, due to a problem with the bearing of cam  116 . Such a condition could also cause gripper  106  to open prematurely. The condition shown in  FIG. 3 , where gripper  106  fails to close, could also be caused by a failure of hook  126  to hold cam  116  in place, causing cam  116  to start rotating with impression roller  102  before cam follower  112  has reached depression  118 . The failure of gripper  106  to close, or the premature opening of gripper  106 , can cause paper to misfeed or to stick to the blanket of the intermediate transfer member, possibly damaging the blanket.  
         [0075]     The condition shown in  FIG. 3 , whatever its cause, and whether it involves a failure of the gripper to close, or a premature opening of the gripper, will result in sensor  120  detecting target  114  as cam follower  112  passes sensor  120 , indicating that gripper  106  is open when it is supposed to be closed. The printer then optionally is stopped, before a paper misfeed does any damage.  
         [0076]     Although this description and the claims refer sometimes to paper, the invention may also be used with any other printing media, and the claims cover the apparatus and the method when any printing media is used. Similarly, the term “printer” used in the description or the claims covers any apparatus which prints an image on a printing media, including a copier, for example. The invention has been described in the context of the best mode for carrying it out. It should be understood that not all features shown in the drawings or described in the associated text may be present in an actual device, in accordance with some embodiments of the invention. Furthermore, variations on the method and apparatus shown are included within the scope of the invention, which is limited only by the claims. Also, features of one embodiment may be provided in conjunction with features of a different embodiment of the invention. As used herein, the terms “have”, “include” and “comprise” or their conjugates mean “including but not limited to.”