Abstract:
A retard feeder adapted to separate and advance media sheets comprising a media sheet advancing device including a drive roll and a retard roll. The drive roll and the retard roll include a feed nip therebetween for driving the media sheets at a velocity. A first drive system is provided to selectively drive the drive roll and the at least one nudger roll in a forward direction. A second drive system drives the retard roll through a slip clutch having a torque wherein the slip clutch torque allows the retard roll to rotate at substantially the same velocity as the drive roll when only one sheet is in the feed nip. The retard feeder further includes a motion sensor for detecting a signal when the retard roll stops rotating at the velocity of the drive roll corresponding to when more than one sheet is in the feed nip. The second drive system can selectively vary the velocity of the retard roll in response to the signal from the retard roll motion sensor.

Description:
BACKGROUND 
   This disclosure relates generally to a sheet feeder, and more particularly concerns sheet feeder having a reversing retard roll utilizing an integral torque limiting slip clutch having a reversing bias. 
   In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light to selectively dissipate the charges 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. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated to permanently affix the powder image to the copy sheet. 
   In a commercial printing machine of the foregoing type, a sheet misfeed or multi-fed sheets can seriously impair the operation of the machine. It is advantageous in many of today&#39;s machines to provide for the in seriatim feeding of sheets from the top of the stack. Many devices have been developed to attempt to alleviate problems associated with feeding sheets and prevent multi-fed sheets. The present disclosure improves over past systems by providing a simple integral device to separate multi-feeds quickly and effectively. 
   CROSS REFERENCE TO RELATED PATENTS AND APPLICATIONS 
   The following disclosures may be relevant to various aspects of the present disclosure: U.S. Pat. No. 5,039,080 to Kato et al.; U.S. Pat. No. 4,368,881 to Landa; U.S. Pat. No. 4,203,586 to Hoyer; and U.S. Pat. No. 5,435,538 to Billings, et al. 
   Portions of the foregoing disclosures may be briefly summarized as follows. U.S. Pat. No. 5,039,080 describes a sheet feeding apparatus having a feed roller and a separating roller forming a nip utilizing a rotation resisting torque limiter and a spring to resiliently urge the separating roller in the reverse direction when a double fed sheet is in the nip. U.S. Pat. No. 4,368,881 discloses a top feed friction retard feeder that utilizes a spring loaded retard roll and a torque limiter to bias the reverse rotation at a predetermined torque level. U.S. Pat. No. 4,203,586 describes a multi-feed detection system including a drag roll in contact with a feed belt wherein a slip clutch applies a torque to the drag roll. A double fed sheet causes the drag roll to hesitate which is then detected by a sensor to activate a shut down as a result of the double fed sheet. 
   Active friction retard feeding is the most common feeding technology used in copiers/printers in the 20-100 pages per million (ppm) range. The primary drawback to active friction retard systems is the limited life of the roller materials and the slip clutch. These must be replaced every 200,000 to 500,000 sheets which increases the parts and labor/service costs of the machine. The roller life can be improved by using larger diameter rolls and newly developed elastomer materials, however, the life of the slip clutch can still be a limiting factor. It is desirable to increase the life of the slip clutch thereby reducing the frequency of replacing said clutch, to be described in more detail hereinafter. 
   SUMMARY 
   Aspects of the present disclosure in embodiments thereof include a retard feeder adapted to separate and advance media sheets comprising a media sheet advancing device including a drive roll and a retard roll. The drive roll and the retard roll include a feed nip therebetween for driving the media sheets at a velocity. A first drive system is provided to selectively drive the drive roll in a forward direction. A second drive system drives the retard roll through a slip clutch having a torque wherein the slip clutch torque allows the retard roll to rotate at substantially the same velocity as the drive roll when only one sheet is in the feed nip. The retard feeder further includes a motion sensor for detecting when the retard roll stops rotating at the velocity of the drive roll corresponding to when more than one sheet is in the nip feed. The second drive system can selectively vary the velocity of the retard roll when the more than one sheet is in the feed nip. 
   Pursuant to other aspects of the disclosure, there is provided an apparatus adapted to separate and advance media sheets comprising a media sheet advancing device including a drive roll and a retard roll wherein the drive roll and the retard roll include a feed nip therebetween for driving the media sheets at a velocity. A first drive system is provided to selectively drive the drive roll in a forward direction. A second drive system drives the retard roll through a slip clutch having a torque wherein the slip clutch torque allows the retard roll to rotate at substantially the same velocity as the drive roll. A motion sensor is provided for detecting when the retard roll stops rotating at the velocity of the drive roll. Further, a control system is provided for changing the velocity of the second drive system when the sensor detects that the retard roll has stopped turning at substantially the same velocity as the drive roll. 
   Yet still in accordance with other aspects, there is provided an apparatus adapted to separate and advance media sheets comprising a media sheet advancing device including a drive roll and a retard roll. The drive roll and the retard roll include a feed nip therebetween for driving the media sheets at a velocity. A first drive system is provided to selectively drive the drive roll in a forward direction. A second drive system drives the retard roll through a slip clutch having a torque wherein the slip clutch torque allows the retard roll to rotate at substantially the same velocity as the drive roll. A motion sensor is provided for detecting when the retard roll stops rotating at the velocity of the drive roll. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic elevational view of an exemplary electrophotographic printing machine including a retard roll feeder; 
       FIG. 2  is an elevational view of a friction retard feeder device in a first position according to the present disclosure; 
       FIG. 3  is an elevational view of the friction retard feeder device second position; and, 
       FIG. 4  is a flowchart displaying a control approach for the retard feeder device. 
   

   DETAILED DESCRIPTION 
   For a general understanding of the features of the present disclosure, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to identify identical elements.  FIG. 1  schematically depicts an exemplary electrophotographic printing machine. It will become evident from the following discussion that the sheet feeding apparatus of the present disclosure may be employed in a wide variety of devices and is not specifically limited in its application to the particular embodiment depicted herein. 
     FIG. 1  schematically illustrates an electrophotographic printing machine which generally employs a belt  10  having a photoconductive surface  12  deposited on a conductive ground layer  14 . Preferably, photoconductive surface  12  is made from a photoresponsive material, for example, one comprising a charge generation layer and a transport layer. Conductive layer  14  can be made preferably from a thin metal layer or metallized polymer film which is electrically grounded. Belt  10  moves in the direction of arrow  16  to advance successive portions of photoconductive surface  12  sequentially through the various processing stations disposed about the path of movement thereof. Belt  10  is entrained about stripping roller  18 , tensioning roller  20  and drive roller  22 . Drive roller  22  is mounted rotatably in engagement with belt  10 . Motor  24  rotates roller  22  to advance belt  10  in the direction of arrow  16 . Roller  22  is coupled to motor  24  by suitable means, such as a drive belt. Belt  10  is maintained in tension by a pair of springs (not shown) resiliently urging tensioning roller  20  against belt  10  with the desired spring force. Stripping roller  18  and tensioning roller  20  are mounted to rotate freely. 
   Initially, a portion of belt  10  passes through charging station A. At charging station A, a corona generating device, indicated generally by the reference numeral  26  charges the photoconductive surface  12  to a relatively high, substantially uniform potential. After photoconductive surface  12  of belt  10  is charged, the charged portion thereof is advanced through exposure station B. 
   At exposure station B, a controller or electronic subsystem (ESS), indicated generally by reference numeral  28 , receives the image signals representing the desired output image and processes these signals to convert them to a continuous tone or greyscale rendition of the image which is transmitted to a modulated output generator, for example the raster output scanner (ROS), indicated generally by reference numeral  30 . Preferably, ESS  28  is a self-contained, dedicated minicomputer. The image signals transmitted to ESS  28  may originate from a computer, thereby enabling the electrophotographic printing machine to serve as a remotely located printer for one or more computers. Alternatively, the printer may serve as a dedicated printer for a high-speed computer. The signals from ESS  28 , corresponding to the continuous tone image desired to be reproduced by the printing machine, are transmitted to ROS  30 . ROS  30  can include a laser with rotating polygon mirror blocks. The ROS can illuminate the charged portion of photoconductive belt  20  at a resolution of about 300 or more pixels per inch. The ROS will expose the photoconductive belt to record an electrostatic latent image thereon corresponding to the continuous tone image received from ESS  28 . As an alternative, ROS  30  may employ a linear array of light emitting diodes (LEDs) arranged to illuminate the charged portion of photoconductive belt  20  on a raster-by-raster basis. 
   In another embodiment, ESS  28  may be connected to a raster input scanner (RIS). The RIS has an original document positioned thereat. The RIS can include document illumination lamps, optics, a scanning drive, and photosensing elements, such as an array of charge coupled devices (CCD). The RIS captures the entire image from the original document and converts it to a series of raster scanlines which are transmitted as electrical signals to ESS  28 . ESS  28  processes the signals received from the RIS and converts them to greyscale image intensity signals which are then transmitted to ROS  30 . ROS  30  exposes the charged portion of the photoconductive belt to record an electrostatic latent image thereon corresponding to the greyscale image signals received from ESS  28 . 
   After the electrostatic latent image has been recorded on photoconductive surface  12 , belt  10  advances the latent image to a development station C, where toner, in the form of liquid or dry particles, is electrostatically attracted to the latent image using commonly known techniques. At development station C, a magnetic brush development system, indicated by reference numeral  38 , advances developer material into contact with the latent image. Magnetic brush development system  38  includes two magnetic brush developer rollers  40  and  42 . Rollers  40  and  42  advance developer material into contact with the latent image. These developer rollers form a brush of carrier granules and toner particles extending outwardly therefrom. The latent image attracts toner particles from the carrier granules forming a toner powder image thereon. As successive electrostatic latent images are developed, toner particles are depleted from the developer material. A toner particle dispenser, indicated generally by the reference numeral  44 , dispenses toner particles into developer housing  46  of developer unit  38 . 
   With continued reference to  FIG. 1 , after the electrostatic latent image is developed, the toner powder image present on belt  10  advances to transfer station D. A print sheet  48  is advanced to the transfer station D by a sheet feeding apparatus  50 . The sheet feeding apparatus  50  can include a nudger roll  52  contacting the uppermost sheet of stack  54 . Nudger roll  52  rotates to advance the uppermost sheet from stack  54  to a retard feeder assembly  80  which includes a drive roll or feed roll  82  and a retard roll  84  for forwarding sheets into chute  56 . Chute  56  directs the advancing sheet of support material into contact with photoconductive surface  12  of belt  10  in a timed sequence so that the toner powder image formed thereon contacts the advancing sheet at transfer station D. Transfer station D includes a corona generating device  58  which sprays ions onto the back side of sheet  48 . This attracts the toner powder image from photoconductive surface  12  to sheet  48 . After transfer, sheet  48  continues to move in the direction of arrow  60  onto a conveyor (not shown) which advances sheet  48  to fusing station E. The fusing station E includes a fuser assembly, indicated generally by the reference numeral  62 , which permanently affixes the transferred powder image to sheet  48 . Fuser assembly  60  includes a heated fuser roller  64  and a back-up roller  66 . Sheet  48  passes between fuser roller  64  and back-up roller  66  with the toner powder image contacting fuser roller  64 . In this manner, the toner powder image is permanently affixed to sheet  48 . After fusing, sheet  48  advances through chute  68  to catch tray  72  for subsequent removal from the printing machine by the operator. After the print sheet is separated from photoconductive surface  12  of belt  10 , the residual toner/developer and paper fiber particles adhering to photoconductive surface  12  are removed therefrom at cleaning station F. Cleaning station F includes a rotatably mounted fibrous brush in contact with photoconductive surface  12  to disturb and remove paper fibers and a cleaning blade to remove the nontransferred toner particles. The blade may be configured in either a wiper or doctor position depending on the application. Subsequent to cleaning, a discharge lamp (not shown) floods photoconductive surface  12  with light to dissipate any residual electrostatic charge remaining thereon prior to the charging thereof for the next successive imaging cycle. 
   It is believed that the foregoing description is sufficient for purposes of the present application to illustrate the general operation of an electrophotographic printing machine incorporating the features of the present disclosure therein. 
   The description hereinafter proposes a method of increasing the life of a slip clutch by significantly reducing the number of slip revolutions the slip clutch sees during normal operation. This can be done by leaving a retard roll drive shaft  88  either stationary or turning at a slow velocity during normal feeder operation. It is to be appreciated that instead of a dedicated step motor or servo motor being applied to the retard drive  86 , a simple clutch or other temporary drive engagement system (not illustrated) tied into the main drives could also be used. Examples include multiple engaging and disengaging gear sets for altering speed and direction of the retard roll relative to the feed roll. 
   Turning now to  FIGS. 2 and 3 , the details of the retard roll assembly generally referred to by reference  80  with an integral reverse biasing device will be discussed. The retard roll  84  is formed from a cylindrical section  86  supported for rotation on a fixed shaft  88 . The cylindrical portion  86  can be coated with an elastomeric surface such as silicone rubber which provides good frictional contact with the sheets being fed and is impervious to commonly used silicone release agents. Although not illustrated, the retard roll can have an integral slip clutch that acts between the shaft and drive roll. A wear ring can be fixed to one end of the roll portion  86 . A wrapped spring can be fixed at one end to a disk so that the end of the spring remains fixedly attached to the shaft  88 . As the retard roll assembly  80  rotates by frictional contact with a single sheet being driven by the feed roll  82 , the spring can be unwound from the outer diameter of the wear ring. Once a predetermined torque is reached, the inner diameter of the spring can slip relative to the wear ring, allowing the retard roll  84  to overrun in the feed direction. Once again, if a double sheet enters the nip, the torque imparted to the retard roll  84  is less than that stored by the spring and the spring winds and through frictional contact with roll wear ring which is attached to roll portion  86 , the retard roll  84  can be caused to reverse direction and again feed the double sheets back toward the feed tray. This embodiment could also be utilized with a tapered spring and a conical wear ring attached to the retard roll. It should be appreciated that a limited torque slip-clutch type mechanism could be used to provide the retard roll function described above. 
   An encoder  100 , i.e., motion sensor, can be mounted to the retard roll  84  to monitor the motion of the retard roll  84 . If the encoder  100  detects that the retard roll  84  has stopped rotating (indicating that more than one sheet has entered the feed nip), the retard drive shaft  88  is driven backward  102  via step motor  90  to reverse shingle the sheets and prevent a multi-feed. Since the retard roller  84  is not rotating in a reverse direction (i.e.  104 ) except when a slug (plurality of sheets) is being reversed shingled back into the tray, the life of the slip clutch  106  can be extended significantly. Variations of the above operation are possible in which the retard shaft  88  can be rotated slowly in the reverse  102  or forward  104  direction (instead of being stopped) until a slug is detected. This method and system also allows more latitude in selecting the reverse velocity of the retard shaft  88 . In current active friction retard feeders, the reverse velocity of the drive shaft has a large affect on the life (number of slip revolutions) of the slip clutch  106 . In the present system, the reverse drive velocity has a minimal affect on clutch life so a higher reverse drive velocity can be used (within other functional limits.) The size of the slug (number of sheets) that can be reversed shingled without causing a multi-feed is proportional to this reverse velocity. 
   By way of example, a control approach for the retard drive described above is shown in the flowchart of  FIG. 4 . The encoder signal  120  indicates whether the retard roll  84  is turning at substantially the speed of the feed roll. If the retard roll  84  is turning, the retard drive remains off  122  (motor stopped (stationary) or reverse drive clutch not engaged). If the retard roll  84  is turning, then the retard reverse drive is turned on  124  (motor run in reverse direction or clutch engaged). It is to be appreciated that even greater slip clutch life improvements can be realized if the retard shaft  88  is driven in a forward direction  104  (at a speed less than the feed roll) until a slug is detected (by sensing a change in the retard roll speed). Other variations are also possible, for example, the control system changing the velocity of the retard drive system from a first velocity to a second velocity. The first velocity can be substantially zero and the second velocity can be in a reverse direction  102 . Alternatively, the first velocity can be in a forward direction  104  but slower than that of the feed roll, and the second velocity can be in a reverse direction  102 . Also, alternatively, the first velocity can be in a reverse direction and the second velocity can be in a higher reverse direction  102 . 
   After the retard reverse drive has been turned on, a wait period  128  is measured (the specific wait time dependent on the velocities used in the system) in which to provide sufficient time to reverse shingle the worst case slug. By way of example only, one such wait period can be about 200 milliseconds. Once the sufficient period time has elapsed, the retard reverse drive is turned off  130  (motor stopped or clutch disengaged). The above approach is repeated until the job has been completed. If a quadrature encoder is used, then the reverse drive can be left on until the retard roll stops turning in a reverse direction. This could minimize drive time and unnecessary slip clutch revolutions, but increase cost associated with a more expensive encoder. 
   The retard feeders described have been shown in use as copy sheet feeders but are equally well adapted for use in document handlers to feed original documents for imaging or in any other use in which sequential single sheet feeding is desired. 
   It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.