Abstract:
A device and method of enhancing contact between a photoconductive member of a electrophotographic printing machine and the paper to which an electrostatic latent image is to be transferred uses an array of wiper blade segments mounted on a common shaft. Each segment is attached to the shaft for limited rotational movement on the shaft in opposition to a torsion spring. The torsion spring biases the blade segment towards the paper. Some of the blade segments are operatively associated with a stop mechanism to control the length of the wiper blade array in accordance with the size of the paper being processed. The stop mechanism prevents movement of the blade segment into engagement with the paper by restraining movement of the segment against its torsion spring.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to a color or monochrome electronic reprographic printing system, and more particularly concerns apparatus for optimizing the contact between paper or other copy media and a photoconductive surface. 
     In an electrophotographic printing machine, a photoconductive member (often a photoreceptor belt) is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is treated with toner particles and is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration. 
     Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated in a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. Alternately, a plurality of images may be superimposed on the photoreceptor surface, and transferred simultaneously to the sheet. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material. 
     Surface irregularities in the paper may occur prior to use or during handling. Such irregularities are often caused by exposure to moisture, mishandling, duplexing, etc and create localized deformities in the copy paper. As a result, air gaps may form between the paper and the photoreceptor belt. Such gaps result in poor transfer of toner from the belt to the paper, which may, in turn, cause deletions or distortions in the printed copy. Flipping the paper over, or discarding the old paper and adding fresh paper offer possible solutions to this problem, but require the labor of frequent monitoring. The resulting rotation of paper stock is inherently expensive in paper costs, labor, and down time. Therefore, a means for reducing the need for operator involvement and reducing the amount of paper that is wasted is needed. 
     A device which applies a force against the back of a sheet and flattens it against the photoreceptor belt is one possible solution to the problem. U.S. Pat. No. 5,247,335, owned by Xerox Corp., describes a machine having such a device. The device described in the &#39;335 patent employs a cam to move a wiper blade against the copy paper to facilitate engagement of the paper and photoreceptor belt. 
     Another Xerox Corporation patent, U.S. Pat. No. 5,227,852, describes another embodiment of a wiper blade which uses four flexible blade segments, each of which is deflected back away from the photoreceptor belt by solenoid actuated mechanisms. One or more of the solenoids are activated by the passage of a sheet, depending on the paper size being used. Since the blades of these machines are held in a deflected-back state both during standby and between each copy, the blade may tend to take on a permanent set over time, decreasing the force applied. This may result in the degradation in performance, over time, of the blades, and the need to replace the blades frequently. 
     There remains a need for a device that will provide enhanced contact between a copy sheet and a photoreceptor belt that is reliable and requires little maintenance. 
     SUMMARY OF THE INVENTION 
     In the method and apparatus of this invention, a series of wiper blades are provided which are mounted on a common shaft and are spring biased against the paper in operation. The wiper blades are operated individually or in pairs by steeping motors which drive a linkage system to rotate the blades into and out of engagement with the paper. The blades pivot about a common pivot rod which is mounted transverse to the path of the paper. Each blade is equipped with an additional elastic plastic contact edge that is less rigid than the body of the supporting blade segment. 
     Each blade is fixed to the rod for rotation therewith through a torque spring. The torque spring allows the blade to pivot on the rod through a limited arc of motion. The pivot motion of the blades on the rod is biased by the torque spring towards engagement with the paper. The torque spring thereby provides a gradual and consistent loading of the paper to provide accurate and effective toner transfer when the blades are rotated into engagement. 
     The actuation mechanism of the blades involves a lever and crank assembly which applies a stepped rotation of a stepping motor to rotate the blades between two positions. All of the blades operate on the same rod and are actuated simultaneously towards and away from engagement with the paper. Depending on the size of the paper, all of the blades may not be necessary to apply uniform pressure to the paper. In order to avoid contamination of the wiper blades with toner and wear to the photoconductor element, a mechanism is needed to select the combination of blades suitable for the particular size paper in process. Accordingly, each of the blades is operatively associated with a decision stop which may be constructed as a cam sector. The cam sector engages a pawl shaped extension on the blade assembly to selectively limit movement of a selected blade against the paper. As the blade rod rotates, the cam sector holds the engaged blade assembly against the torque spring, while the rod continues to rotate to engage the unrestrained blades into contact with the paper. The blade array may consist of multiple pairs of outboard blades and a single central inboard blade to service paper in the required range of sheet widths. In a center registered configuration only the outboard blades would be associated with a stop mechanism. Alternately, the blade array may consist of a single outboard blade and a plurality of inboard blades. In this edge-registered configuration, only the inboard blades would be associated with a stop mechanism. 
     The cam sector is mounted on a second shaft which is driven by a second stepping motor. The second stepping motor rotates the cam sector between positions which provide the desired range of restraint to the associated blade assembly. The stop stepping motor is controlled by sensors that monitor the size of the paper as it passes through the copier. A separate control actuates the blade motor in response to a sensor which senses the leading edge of the paper prior to its arrival at the photoconductive element. The timing of the stepping motors and their motion may be determined by reference to a table of electronically stored actuation and deactuation timing values. These values are referenced to data regarding blade mechanism position which is acquired from sensors within the blade mechanism, and sheet position, which is acquired either from sensors within the blade mechanism or elsewhere in the paper path. 
     The invention offers the following advantages: 
     A large, if not limitless, number of sheet sizes may be accommodated by only two driving members (motors). Previous designs required one driving member for each size accommodated. 
     A flexible blade tip provides a gentle application of the load and prevents the image from being disturbed when the blade touches down. 
     The flexible tip also conforms to the photoreceptor belt position, thereby providing a uniform pressure to the sheet, despite tolerances in its alignment to the surface of the belt. 
     A spring loaded blade support provides a more consistent applied load. 
     Selecting the blade segment for restraint rather than actuation, simplifies the mechanism and the adjustment of the blade system to multiple paper sizes. 
     The use of stepping motors as the driving mechanism provides an accurate and easily controllable motion. 
    
    
     DESCRIPTION OF THE DRAWING 
     The invention is described in more detail below with reference to the attached drawings in which: 
     FIGS. 1 a  and  1   b  are schematic illustrations of a copier system employing contact enhancement; 
     FIG. 2 is a perspective view of the blade wiper assembly; 
     FIG. 3 is a perspective view of the blade wiper assembly showing the blade actuation mechanism; 
     FIG. 4 is a perspective view of the blade wiper assembly showing the decision stop actuation mechanism; 
     FIGS. 5 a  and  5   b  are end views of the blade wiper actuation mechanism in the engaged and disengaged positions respectively; 
     FIG. 6 is an end view of the blade assembly; and 
     FIG. 7 is a block diagram of the control system for operation of the blade system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 a  and  1   b  illustrate the general arrangement of a contact enhancing mechanism  28 . The photoconductive member is entrained about a plurality of rollers (only one roller  12  is shown). The photoconductive member  10  is advanced in the direction of arrow  14  in a recirculating path of movement with a developed image (or toner image)  26  electrostatically secured thereto. A sheet  20  is electrostatically attracted to the photoconductive member  10  and is drawn in the direction of arrow  18 . FIGS. 1 a  and  1   b  further show the developed image  26  interposed between the advancing photoconductive member  10  and the advancing sheet  20 . Photoconductive member  10  could take the form of a belt in some systems or a drum in others without appreciably altering the function of this invention. 
     The contact enhancing mechanism  28  functions to enhance contact between the sheet  20  and the developed image  26  so as to improve the quality of transfer of the developed image  26  from the photoconductive member  10  to the sheet  20 . The contact enhancing mechanism  28  includes a blade  32  which is pivotable on a rotatable rod  34 . A single sensor  70  is shown to monitor the position of the blade  32 . Depending on the application, it may be desirable to use multiple sensors to detect the various positions of the parts of the mechanism  28 . 
     FIGS. 1 a  and  1   b  depict the movement of the sheet  20  as it is transported, by the electrostatic attraction, through the transfer zone  24 . More specifically, FIG. 1 a  shows sheet  20  just prior to passing over the contact enhancing mechanism  28 . Without a contact enhancing mechanism, a number of gaps  30  between the sheet  20  and the developed image  26  may develop. The gaps  30  define areas of poor contact between the sheet and the developed image. These areas of poor contact may hinder the transfer of developed image  26  from the photoconductive member  10  to the sheet  20 . With continued advancement of the sheet  20 , a timed signal triggers the actuation of the enhancing mechanism  28  to pivot the blade  32  on the rotatable rod  34  from its position shown in FIG. 1 a  to its position shown in FIG. 1 b . The blade  32  contacts the sheet  20  so as to cause the sheet to be urged toward and into contact with the developed image  26 , as shown in FIG. 1 b , thereby reducing the undesirable presence of gaps  30 . This signal may be timed based on the paper length run as detected in the paper tray. 
     As a result, contact between the sheet and the developed image is enhanced as successive portions of the sheet are advanced by and in contact with the blade  32 . With further advancement, the sheet passes over the corona generating device  22 . The corona generating device establishes a transfer field that is effective to attract the developed image from the photoconductive member  10  to the sheet  20 . The contact enhancing mechanism, in response to a second timed signal, pivots the blade  32  on the rotatable rod  34  from its position shown in FIG. 1 b  back to its position shown in FIG. 1 a.    
     An actuation and support assembly  1  for wiper blade  32  is shown in FIG.  2 . Assembly  1  is constructed with mounting brackets  4  for installation in a copier machine (not shown). Stepping motors  5  and  6  are fixed on brackets  4 . Motor  5  drives blade pivot rod  34 , as shown in FIG. 2, on which is mounted an array of wiper blades  8 . Motor  6  drives rod  9  on which is mounted decision stops  11   a ,  11   b , and  11   c . Motor  5  is connected to rod  34  through a crank and lever assembly  15  and motor  6  is connected to rod  9  by a gear system  17 . As an alternative, a single motor may be used which is connected to the rods  9  and  34  through appropriate clutches which allow rotation of one of the rods while the other slips. 
     For the purpose of illustration, wiper blades  8  are constructed of multiple blade segments  32 . In particular, to allow adjustment to accommodate different sized paper, there is a central blade segment  32   a  and a pair of outboard segments  32   b . As shown in FIG. 2, each of the blade segments  32   a  and  32   b  are independently mounted on the pivot rod  34  for rotation therewith. Each of the blade segments  32  are connected to the rod  34  by means of torsion springs  7 . The springs  7  are constructed and attached between the rod  34  and the respective blade segments  32  to generate a torque on the blade segments  32  that tends to rotate the blade segment towards the paper. In this manner a limited rotation of the blade segments  32  is permitted on the pivot rod  34  against the torsion spring  7 , otherwise the blade segments  32  move with the pivot rod  34 . 
     As shown in more detail in FIG. 6, each blade segment  32  consists of a body  19  having a central bore  21 . Body  19  includes a blade edge holder portion  33  and a pawl shaped extension portion  35 . The bore  21  is constructed with opposing key slots  23 . The blade segment  32  is fitted onto the pivot rod  34  through the bore  21 . Pin  25  is inserted through a transverse passage  27  to seat within the key slots  23 . Key slots  23  are arcuate segments which allow a limited range of movement of the blade segment  32  on the rod  34 . The key slots in the blade holder allows the blade to rotate with respect to the pivot rod in the direction away from the photoreceptor belt and sheet. The sheet  20  is pressured into engagement with photoconductive member  10  by a force exerted by blade edge  29  which may be constructed of a flexible sheet material. Blade edge  29  is mounted on blade edge holder portion  33  and extends outward to form an engaging surface for contact with sheet  20 . 
     Pivot rod  34  is driven by stepping motor  5  through a crank and lever assembly shown at  15  in FIG.  2  and FIG.  3 . Crank and lever assembly  15  is an operatively associated assembly of a crank  40  and lever  41 . Crank  40  is fixed for rotation on drive shaft  42  of stepping motor  5  and is constructed having a body which extends radially outward from the shaft  42 . A pin  44  is fixed transversely to the crank  40  in a position which is displaced radially outward from the axis of rotation of the shaft  42 . Lever  41  is fixed to pivot rod  34  to transmit rotary motion of drive shaft  42  to the rod  34 . Lever  41  is constructed having an elongated body which extends to meet crank  40 . The outer end  46  of lever  41  is constructed with a longitudinal slot  47  extending partially down the lever  41 . Slot  47  engages pin  44  allowing pin  44  to freely move within slot  47 . As shown in FIGS. 5 a  and  5   b , crank  40  is in the position in which the blade edges  29  of the blade segments  32   a  and  32   b  are in contact with the sheet on member  10 . As crank  40  rotates counterclockwise with drive shaft  42  to a new position  49 , as shown in FIG. 5 b , pin  44  will pivot lever  41  through an angle  48 . In this position the blades are disengaged. Blade position may be monitored by a sensor  45  which generates a signal triggered by flag  43  mounted on the crank  40 . 
     As shown in FIGS. 5 a  and  5   b , because of the limited range of movement allowed by the mounting arrangement of the blade body  19  to the pivot rod  34 , the force exerted by the blade edge  29  is dependent on the spring constants of the torsion spring  7 . Torsion spring  7  is fixed between pivot rod  34  and blade body  19 . 
     Because of the varied size of paper  20  processed by the copier device, the length of the wiper blades  8  must be adjustable. As previously stated, the blades  8  consist of inboard central blade segment  32 a and a pair of outboard blade segments  32   b . It should be noted that any combination of segments may be used to accommodate the degree of adjustment required by the particular application. For this purpose, each of the blade segments  32  are operatively associated with a decision stops  11   a ,  11   b , or  11   c . The decision stops  11  are constructed with a cam sector  37  and an open sector  39 . Decision stops  11   a ,  11   b , and  11   c  are mounted on a common rod  9  for rotation therewith. Rod  9  is driven by stepping motor  6  through a gear system  17  consisting of a drive gear  50  connected to drive shaft  53 , a transmission gear  51  and a driven gear  52  attached to rod  9 . 
     In operation, in response to a signal from, for example, a paper size monitor  72  within the paper tray of the copier system, a decision stop signal is generated by the control computer  71  to operate the decision stop  11 . If for example the paper size indicator  72  reflects the most narrow width, only the inboard blade segment  32   a  is needed. As shown in FIG. 5 b , when the cam sector  37  engages the pawl  35  of the blade body  19 , it restrains movement of the blade segments  32   b  against the torsion spring  7 . When paper having a larger width is detected, the decision stop is rotated so that the open sector  39  aligns with the pall  35  and the blade segment  32   b  is allowed to rotate with blade segment  32   a , as shown in FIG. 5 a.    
     Movement of decision stop  11  is accomplished by stepping motor  6  which moves through a series of steps that rotate the cam sector  37  or open sector  39  into engagement with the pawl  35  to adjust the position of blade segments  32   b , i.e. restrained or unrestrained. 
     In response to another timed signal generated by sensors in the system, for example upon the entrance of the leading edge of the sheet  20  into the transfer zone, stepping motor  5  receives a signal from control computer  71  to pivot the blade segment  32  into engagement with the sheet  20 . Stepping motor  5  rotates pivot rod  34  through a programmed series of stepped increments at which the edge  29  of the blade segment  32  engages the sheet  20 . It should be noted that this movement will move all of the blade segments  32   a  and  32   b  towards the sheet  20  unless one or more of the stops  11   a ,  11   b , or  11   c  is engaged. 
     In this manner, a simple and precise mechanism is provided to adjust the width of the contact enhancing assembly. This prevents contact of the edge  29  with the photoconductive member, thereby avoiding blade contamination and damage to the member. Significantly, the contact enhancing mechanism  28  operates with only two motors to drive the elements of the mechanism  28 .