Patent Publication Number: US-7900918-B2

Title: Sheet conveying system, as well as image forming apparatus and sheet conveying apparatus thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet conveying system, as well as an image forming apparatus and a sheet conveying apparatus thereof, more particularly, to a sheet conveying system in which multiple sheet conveying apparatuses, each having a conveying roller pair which conveys sheets, are coupled together as in a configuration in which an image forming apparatus such as a copying machine or a printer is coupled with an optional apparatus, as well as an image forming apparatus and a sheet conveying apparatus thereof. 
     2. Description of the Related Art 
     In some sheet conveying apparatuses in which a plurality of conveying roller pairs nip and convey a sheet, a conveying roller pair on the downstream side may control acceleration, deceleration, or stop the conveyance of the sheets. 
     In such an apparatus, if the conveying velocity of the conveying roller pair on the downstream side is lower than that on the upstream side, a sheet being conveyed can be looped and physical damage to the paper, such as a bend, can be caused. 
     On the contrary, if the conveying velocity of the conveying roller pair on the downstream side is higher than that on the upstream side, both ends of a sheet is pulled in the opposite directions with respect to the conveying direction and the sheet would be torn. 
     To avoid these problems, techniques have been proposed including a technique that, while conveying roller pairs on the both of the upstream and downstream sides are nipping a sheet, a conveying roller pair on the upstream side has a one-way clutch so as to prevent the both ends of a sheet from pulling and a technique that controls to release the nipping of the sheet by separating the conveying roller pair on the upstream side (for example, see Japanese Laid-Open Patent Publication (Kokai) NO. 11-208939, No. 09-077299, and No. 05-270694). 
     However, the use of the one-way clutch for the conveying roller pair causes roller traces left on the surface of the sheet, which deteriorates image formed on the sheet. 
     Further, in the above conventional technique, the separation of the conveying roller pair on the upstream side is controlled having regard to only the fact that all the conveying roller pairs included in one sheet conveying apparatus are controlled by one controller in the sheet conveying apparatus. That is, the technique does not consider controlling separation of the conveying roller pair on the upstream side, in the case where a plurality of sheet conveying apparatuses each having a controller and a conveying roller pair are coupled together. 
     Further, it may solve the above problems that the velocity of the conveying roller pair on the upstream side is controlled so as to synchronize with the velocity of that on the downstream side. However, when the various types of sheet conveying apparatuses, each having sheet conveyance control means, are coupled together in various combinations, it is difficult to control each of the conveying roller pairs in these sheet conveying apparatuses to bring acceleration and deceleration of their conveying velocities into synchronization with each other. 
     SUMMARY OF THE INVENTION 
     The present invention provides a sheet conveying system in which when a sheet is passed from a sheet conveying apparatus to another, each having a sheet conveyance controller, the receiving apparatus can smoothly control acceleration, deceleration, or stop of conveyance of the sheet, as well as an image forming apparatus and sheet conveying apparatus thereof. 
     According to a first aspect of the present invention, there is provided a sheet conveying system comprising a first sheet conveying device including a first conveying roller pair adapted to nip and convey a sheet, a releasing mechanism adapted to release nipping of the sheet by the first conveying roller pair, and a first controller adapted to control the first conveying roller pair and the releasing mechanism, and a second sheet conveying device including a second conveying roller pair adapted to nip and convey the sheet conveyed by the first sheet conveying device, a second controller adapted to control the second conveying roller pair, and a detecting device adapted to detect position of a sheet, wherein in response to the detection by the detecting device, the second controller is adapted to cause the releasing mechanism to release the nipping of the sheet by the first conveying roller pair. 
     Thus, when a sheet is passed through a plurality of sheet conveying apparatuses of which each has a sheet conveyance controller, one of the sheet conveying apparatuses that receives the sheet can smoothly control acceleration, deceleration, or stop of conveyance of the sheet. Consequently, damage to sheets being conveyed, such as bends and tears of sheets, can be prevented. 
     The sheet conveying system can further comprise a communicating device adapted to communicate between the first controller and the second controller, wherein the second controller can be adapted to transmit an instruction of controlling the first controller to operate the releasing mechanism via the communicating device. 
     The releasing mechanism can be adapted to separate the first conveying roller pair to release the nipping of the sheet. 
     After the nipping of the sheet by the first conveying roller pair is released by the releasing mechanism, the second controller can be adapted to decelerate or stop the second conveying roller pair. 
     The first sheet conveying device can further include a third conveying roller pair disposed upstream from the first conveying roller pair and a second releasing mechanism adapted to release nipping of a sheet by the third conveying roller pair, and the second controller can be adapted to cause the second releasing mechanism to release nipping of a sheet by the third conveying roller pair when the releasing mechanism releases the nipping of the sheet by the first conveying roller pair. 
     The sheet conveying system can further comprise a size detecting device adapted to detect the size of the sheet, and wherein when the size of the sheet detected by the size detecting device is shorter than a distance from the first conveying roller pair to the third conveying roller pair, the second controller can be adapted to stop causing the second releasing mechanism to release the nipping of a sheet. 
     The sheet conveying system can further comprise a communicating device adapted to communicate between the second controller and the releasing mechanism, wherein the second controller can be adapted to communicate with the releasing mechanism via the communicating device to directly control thereof, without being adapted to communicate with the first controller. 
     According to a second aspect of the present invention, there is provided a sheet conveying system comprising a first sheet conveying device including a first conveying roller pair adapted to nip and convey a sheet, a releasing mechanism adapted to release nipping of the sheet by the first conveying roller pair, a first controller adapted to control the first conveying roller pair and the releasing mechanism, and a detecting device adapted to detect the sheet, a second sheet conveying device including a second conveying roller pair adapted to nip and convey the sheet from the first sheet conveying device to be conveyed, and a second controller adapted to control the second conveying roller pair, and a communicating device adapted to communicate between the first controller and the second controller, wherein the first controller is adapted to communicate with the second controller to estimate the time required to nip the sheet by the second conveying roller pair after the detecting device detects the sheet, and when the estimated time has elapsed after the detecting device detects the sheet, the first controller is adapted to cause the releasing mechanism to release the nipping of the sheet by the first conveying roller pair. 
     The first controller can be adapted to acquire distance information indicating a distance from a sheet inlet of the second sheet conveying device to the conveying roller pair to determine the time based the distance information. 
     According to a third aspect of the present invention, there is provided an image forming apparatus including a first roller pair adapted to nip and convey a sheet, a releasing mechanism adapted to release nipping of the sheet by the first conveying roller pair, and a first controller adapted to control the first conveying roller pair and the releasing mechanism, the image forming apparatus comprising a second conveying roller pair adapted to nip and convey the sheet conveyed by the first sheet conveying device, a second controller adapted to control the second conveying roller pair, and a detecting device adapted to detect a sheet, wherein in response to the detection by the detecting device, the second controller can be adapted to cause the releasing mechanism to release the nipping of the sheet by the first conveying roller pair. 
     According to a fourth aspect of the present invention, there is provided a sheet conveying apparatus including a sheet conveying apparatus connected with an image forming apparatus, a first conveying roller pair adapted to receive a sheet conveyed from the sheet conveying apparatus, a first controller adapted to control the first conveying roller pair, a detecting device adapted to detect a sheet, the sheet conveying apparatus comprising a second conveying roller pair adapted to nip and convey a sheet toward the image forming apparatus, a releasing mechanism adapted to release nipping of the sheet by the first conveying roller pair, a second controller adapted to control the second conveying roller pair and the releasing mechanism, and wherein the second controller can be adapted to cause the releasing mechanism to release the nipping of the sheet by the second conveying pair based on an instruction from the first controller in response to the detection by the detecting device. 
     Further features of the invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a sheet conveying system according to a first embodiment of the present invention. 
         FIGS. 2A to 2E  are diagrams illustrating an operation of the sheet conveying system shown in  FIG. 1 ;  FIG. 2A  shows a state in which a sheet is being discharged,  FIG. 2B  shows a state in which the sheet is being fed,  FIG. 2C  shows a state in which a conveying roller pair is releasing the nipping of the sheet, and  FIGS. 2D and 2E  are flowcharts showing the procedure of a sheet passing process. 
         FIG. 3  is a timing chart showing an operation for passing a sheet. 
         FIG. 4  is a schematic diagram showing a color image forming apparatus coupled with an optional paper deck. 
         FIG. 5  is a block diagram illustrating connection between a controller of a printer section and a reader section, all of which are included in the color image forming apparatus shown in  FIG. 4 . 
         FIG. 6  is a schematic cross-sectional view schematically showing the color image forming apparatus. 
         FIG. 7  is a diagram illustrating an example of control for separating a conveying roller pair. 
         FIG. 8  is a diagram illustrating a first variation of control for separating a conveying roller pair. 
         FIG. 9  is diagram illustrating a second variation of control for separating a conveying roller pair. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention will now be described in detail with reference to the drawings showing preferred embodiments thereof. It should be noted that the relative arrangement of the components, the numerical expressions and numerical values set forth in these embodiments do not limit the scope of the present invention unless it is specifically stated otherwise. 
       FIG. 1  is a diagram illustrating a sheet conveying system according to a first embodiment of the present invention.  FIGS. 2A to 2C  are diagrams illustrating an operation of the sheet conveying system shown in  FIG. 1 .  FIG. 3  is a timing chart showing an operation for passing a sheet. 
       FIG. 4  is a schematic diagram showing a color image forming apparatus coupled with an optional paper feed apparatus.  FIG. 5  is a block diagram illustrating connection between a controller of a printer section and a reader section, all of which are included in the color image forming apparatus shown in  FIG. 4 .  FIG. 6  is a schematic cross-sectional view schematically showing the color image forming apparatus. 
     The color image forming apparatus which represents a second sheet conveying apparatus of the present invention will be described first with reference to  FIG. 6  for convenience of explanation. 
     The color image forming apparatus (the second sheet conveying apparatus)  900  includes a reader section  901  arranged at its top and a printer section  902  arranged at its bottom. 
     Provided in the reader section  901  are components such as a CCD  921 , a substrate  922  on which the CCD  921  is mounted, a digital image processor  923 , a platen  911 , a document feeder (DF)  912 , mirrors  918  and  919 , and light sources  913  and  914 . 
     The light sources  913  and  914  have reflection umbrella  915  and  916 , respectively, for converging light onto an original. The light source  914  and reflection umbrella  915  and  916  are contained in a carriage  924  together with a mirror  917 . The mirrors  918  and  919  are contained in a carriage  925 . 
     The carriage  924  moves at a velocity of V and the carriage  925  moves at a velocity of V/2 in the direction (in the direction indicated by arrows “a” and “b” in  FIG. 6 ) perpendicular to the electric scanning (main scanning) direction of the CCD  921  to scan (sub-scan) throughout the surface of an original on the platen  911 . 
     A lens  920  is arranged in the vicinity of the CCD  921  that converges light reflected from an original or projected light onto the CCD  921 . The original on the platen  911  reflects light emitted from the light sources  913  and  914 . The reflected light is guided to the CCD  921 , where it is converted into an electric signal, and the converted electric signal (analog image signal) is output to a digital image processor  923 , where image processing is performed. 
     Also provided in the reader section  901  are a controller  910 , an external interface (I/F)  926  that provides an interface to an external device such as a computer, an external interface (I/F) that provides an interface to another device, and a printer control interface (I/F)  930 . 
     Provided in the printer section  902  are a laser scanner  931 , a photosensitive drum  932 , an electrostatic charger  980 , a rotary developing device  933 , a cleaning blade  951 , a blade  952 , a waste toner box  953 , and cassettes  938  to  941 . The rotary developing device  933  has developing devices  947  to  950 . 
     Also provided in the printer section  902  are a primary transfer roller  934 , an intermediate transfer member  935 , a secondary transfer roller  936 , a pressure roller  937 , pickup rollers  942  to  945 , a manual feed roller  946 , a first discharge roller  954 , and a second discharge roller  955 . 
     Further provided in the printer section  902  are an inverting roller  956 , a third discharge roller  957 , a first discharge flapper  958 , a second discharge flapper  959 , a third discharge flapper  960 , a manual feed sheet tray  961 , and feed rollers  962  to  965 . 
     Also provided in the printer section  902  are vertical path conveying rollers  966  to  969 , a registration roller  970 , an HP (home position for the intermediate transfer member  935 ) sensor  971 , a mark  972  indicating the HP, and a controller  909 . 
     As shown in  FIG. 5 , the controller  909  includes a CPU  101 , an operating section  603 , and a memory  602 . The CPU  101  has an interface for communicating information to and from components of the reader section  901 , such as the digital image processor  923 , the printer control interface  930 , and the external interface  926 , and an optional paper deck interface  402 . Thereby the CPU  101  controls these components respectively. 
     The memory  602  includes a RAM  605  which provides a work area for the CPU  101  and a ROM  604  which stores a control program of controlling the CPU  101 . 
     The user operating section  603  includes a liquid-crystal display equipped with a touch panel which is used by an operator to input instructions to execute processing, and notifies information concerning processing and alerts to the operator, etc. 
     An optional paper feed apparatus (first sheet conveying apparatus)  401  (hereinafter referred to as the “optional paper deck  401 ”) includes a CPU  102 , a ROM  404 , and RAM  405 , as will be described later. 
     A control signal from the CPU  101  of the controller  909  is received at the printer control interface  930 , and the printer section  902  operates in accordance with the control signal from the printer control interface  930 . 
     The surface of the photosensitive drum  932  is uniformly charged by the electrostatic charger  980  and an electrostatic latent image of each color component is formed on the photosensitive drum  932  in accordance with an image exposure pattern by a laser beam emitted from the laser scanner  931 . 
     The photosensitive drum  932  is 130 mm in diameter and is set so that the photosensitive drum  932  rotates in the direction indicated by arrow “c” at 137 mm/seconds in the present embodiment. The surface of the photosensitive drum  932  is charged to an electric potential in the range between −300 V and −900 V by the electrostatic charger  980  and the surface electric potential is monitored with an electric potential sensor, not shown. 
     The rotary developing device  933  has developing devices  947  to  950  arranged around the rotation shaft  933   a  and corresponding to black, yellow, magenta, and cyan, respectively. In the present embodiment, the developing devices  947  to  950  are readily detachable from the rotary developing device  933  and each of the developing devices  947  to  950  is attached in a specified position in accordance with the specified color. 
     When a black monochrome image is to be developed to form a toner image on the photosensitive drum  932 , only the black developing device  947  is used and the rotary developing device  933  is rotated to the position at which the developing sleeve of the black developing device  947  faces the photosensitive drum  932 . 
     Then, toner is transferred from the black developing device  947  to the surface of the photosensitive drum  932 , in an amount corresponding to an electric potential difference between the surface of the photosensitive drum  932  on which an electrostatic latent image is formed and the surface of the developing sleeve to which a developing bias is applied, whereby the electrostatic latent image on the surface of the photosensitive drum  932  is developed. 
     When a color image is to be formed, the rotary developing device  933  is rotated by a stepping motor (not shown), to thereby selectively bring one of the developing devices  947  to  950  closer to (or into contact with) the photosensitive drum  932  according to an associated color component to be developed, thereby developing an image. The toner image formed on the photosensitive drum  932  is primary-transferred to the intermediate transfer member  935 . 
     A sheet in one of the cassettes  938  to  941  is picked up by the associated pickup roller  942  to  945  and is then conveyed to the registration roller  970  through the associated feed rollers  962  to  965  and vertical path conveying rollers  966  to  969 . In the case of manual feed, a sheet stacked in the manual feed sheet tray  961  is conveyed to the registration roller  970  by the manual feed roller  946 . 
     At the timing of completion of the transfer to the intermediate transfer member  935 , the sheet is conveyed to between the intermediate transfer member  935  and the secondary transfer roller  936 . Then the sheet passes between the secondary transfer roller  936  and the intermediate transfer member  935  and the toner image is secondary-transferred from the intermediate transfer member  935  to the sheet. 
     The toner image transferred onto the sheet is heated and pressed by fixing roller (not shown) and pressure roller  937 , and the image is fixed on the sheet. Residual toner that has not been transferred to the sheet and remains on the intermediate transfer member  935  is scraped off the surface of the intermediated transfer member  935  with the cleaning blade  951  that rubs the surface under post-processing control at the later stage of the image formation sequence, thereby cleaning the surface of the intermediate transfer member  935 . 
     On the other hand, toner remaining on the surface of the photosensitive drum  932  is scraped by the blade  952  and is conveyed to the waste toner box  953  integrated with the photosensitive drum  932 . 
     In a first discharge mode, the first discharge flapper  958  is switched to the direction of the first discharge roller  954 , so that the sheet on which the image is fixed is discharged toward the first discharge roller  954 . 
     In a second discharge mode, the first and second discharge flappers  958  and  959  are switched to the direction of the second discharge roller  955  so that the sheet is discharged toward the second discharge roller  955 . 
     In a third discharge mode, the first and second discharge flappers  958  and  959  are switched to the direction of the inverting roller  956  so that the sheet is first inverted by the inverting roller  956 . After the sheet is inverted by the inverting roller  956 , the third discharge flapper  960  is switched to the direction of the third discharge roller  957  so that the sheet is discharged toward the third discharge roller  957 . 
     In a double-sided discharge mode, the sheet is first inverted by the inverting roller  956  as in the third discharge mode. Then, the third discharge flapper  960  is switched to the direction of a double-sided unit (not shown) and the sheet having an image formed on a first side is conveyed to the double-sided unit. When a predetermined time period has elapsed after the sheet was detected by a double-sided sensor (not shown), the conveyance of the sheet is temporarily stopped. As soon as the image forming apparatus becomes ready for the subsequent image forming sequence, the sheet is fed again, whereafter a toner image is formed on a second side of the sheet. 
     A sheet conveying system, which is one exemplary embodiment of the present invention, will be described with reference to  FIGS. 1 to 4  and  7 . 
       FIG. 4  shows an example in which the optional paper deck  401  capable of holding 5,000 sheets of paper is coupled to a color image forming apparatus (second sheet conveying apparatus)  900 . 
     When the optional paper deck  401  is coupled to the color image forming apparatus  900 , an optional unit  410  for receiving sheets conveyed from the optional paper deck  401  is attached to a sheet tray  961  (see  FIG. 6 ) of the color image forming apparatus  900 . The optional unit  410  is controlled by the CPU  901  of the image forming unit  900 . 
     A conveying roller pair  411  and sensors  412  and  413  for detecting a sheet are provided in the optional unit  410 . A conveying roller pair  423 , another conveying roller pair  421  and a sensor  422  for detecting a sheet are provided in the optional paper deck  401 . The sensors  422 ,  412 , and  413  detect position of the sheet to be conveyed. 
     A sheet fed from a cassette (not shown) of the optional paper deck  401  passes through the conveying roller pairs  423  and  421  and is passed to the color image forming apparatus  900  through the sheet inlet of the optional unit  410 . 
     If the image forming speed (sheet conveying velocity) of the color image forming apparatus  900  is slower than the sheet feeding speed of the optional paper deck  401 , the difference in speed between them must be accommodated. That is, control must be performed that reduces the sheet conveying velocity or stops sheet conveyance immediately after the optional unit  410  receives the sheet from the optional paper deck  401 . 
     An example in which this control is implemented will be described with reference to  FIG. 1 . 
     A CPU (a second controller)  101  in  FIG. 1  is a controller that controls the conveying roller pair (a second conveying roller pair)  411 . A CPU (first controller)  102  is a controller that controls the conveying roller pair (a first conveying roller pair)  412 . The CPUs  101  and  102  transmit and receive signals each other through a communication line  103 . 
     While the communication line  103  used in this example is a well-known ARCNET line, the communication line  103  is not limited to ARCNET, but it may be any means that enables the CPUs  101  and  102  to communicate with each other. 
     A conveying roller separating mechanism  430  (releasing mechanism) is provided for the conveying roller pair  421  of the optional paper deck  401 . A well-known technology is used for the structure of the conveying roller separating mechanism  430  and therefore the description of the structure will be omitted. The conveying roller separating mechanism  430  is controlled by the CPU  102  which is operated based on instructions of the CPU  101 . 
       FIG. 2A  shows a state in which the front end of a sheet P being conveyed in the optional paper deck  401  has reached the sheet discharge section of the optional paper deck  401 . In this state, the conveying roller pairs  421  and  411  are controlled so that they convey the sheet at a constant speed. 
       FIG. 2B  shows a state in which the front end of the sheet P has reached the sheet feed section of the optional unit  410  and the sensor  412  is turned on. In this state, the sheet P is not yet nipped by conveying the roller pair  411  of the optional unit  410  and is nipped and conveyed by the conveying roller pair  421  of the optional paper deck  401 . 
     Then, the sheet P is nipped by the conveying roller pair  411  of the optional unit  410 . Thereafter, when the front end of the sheet P is detected by the sensor  413 , the detected state of the sensor  413  switches from off mode to on mode as shown in  FIG. 2C . When the detected state of the sensor  413  set to on mode, the CPU  101  recognizes that the sheet P is firmly nipped by the conveying roller pair  411  of the optional unit  410 . 
     Subsequently, the CPU  101  transmits an instruction of controlling the conveying roller separating mechanism  430  to the CPU  102  ( FIG. 7 ). The CPU  102  receiving the instruction separates the conveying roller pair  421  by controlling the conveying roller separating mechanism  430 . Because the conveying roller pair  421  of the optional paper deck  401  are separated in this way, the conveyance of the sheet P is controlled in synchronization with the nipping by the conveying roller pair  411  of the optional unit  410  (see P 301  in  FIG. 3 ). 
     Thus, the conveying roller pair  411  can be smoothly controlled to reduce the conveying velocity on the optional unit  410  side and damage to the sheet P which would otherwise be caused by a difference in conveying velocity between the optional paper deck  401  and the optional unit  410  can be prevented. 
     The CPU  102  detects the off mode of the sensor  422  (absence of a sheet) and determines that the sheet P has passed through the conveying roller pair  421 , and therefore releases the separation of the conveying roller pair  421  and stops driving thereof (see P 303  in  FIG. 3 ). 
     Referring to  FIGS. 7 ,  2 D, and  2 E, an example will be described next in which the CPU  101  controls the separating mechanism through the CPU  102 . The procedures of a sheet passing process shown in the following flowcharts of  FIGS. 2D and 2E  are respectively executed by the CPUs  101  and  102  based on a program stored in the ROM  604 . 
     The CPU  102  first starts conveying a sheet by controlling the conveying roller pair  421  (step S 2004  in  FIG. 2E ). Then, when the CPU  101  detects the front end of the sheet with the sensor  413  (the CPU  101  receives C 1101 ) (YES to step S 2001  in  FIG. 2D ), the CPU  101  outputs a request (C 1102 ) for causing the conveying roller separating mechanism  403  to release nipping to the CPU  102  through a communication line  103  (step S 2002  in  FIG. 2D ). In response to the request (YES to step S 2005  in  FIG. 2E ), the CPU  102  controls (outputs C 1103 ) the conveying roller separating mechanism  430  to separate the conveying roller pair  421  (step S 2006  in  FIG. 2E ). Then, the CPU  101  controls acceleration and deceleration of the velocity of the conveying roller pair  411  (step S 2003  in  FIG. 2D ). 
     Thus, when a sheet is passed from a sheet conveying apparatus having a sheet conveyance controller to another sheet conveying apparatus having a sheet conveyance controller, the receiving apparatus can smoothly control acceleration, deceleration, or stop of conveyance of the sheet. Consequently, damage to a sheet being conveyed such as a bend or tear can be prevented. 
     After that, when the detected state of the sensor  422  switches from on mode to off mode (YES to step S 2007  in  FIG. 2E ), the CPU  102  releases the separation of the conveying roller pair  421  and stops driving thereof (step S 2008  in  FIG. 2E ). Thus, the conveying roller pair  421  can prepare for conveying the next sheet smoothly as soon as it finishes conveying the sheet P. 
     As the velocity of conveying the sheet is faster, the conveying roller separating mechanism  430  of the optional paper deck  401  needs to be controlled promptly to separate the conveying roller pair  421  as soon as the sensor  413  detects the front end of a sheet P. 
     Referring to  FIG. 8 , an example will be described in which the CPU  101  directly controls the conveying roller separating mechanism  430 . 
     The CPU  101  of the optional unit  410  has a control signal line  1001  for directly controlling the conveying roller separating mechanism  430  for the conveying roller pair  421  of the optional paper deck  401 . The conveying roller separating mechanism  430  controlling the conveying roller pair  421  receives control signals from the CPUs  101  and  102  at an OR circuit so that the conveying roller separating mechanism  430  can be controlled by both of the CPUs  101  and  102  independently. 
     When the CPU  101  of the optional unit  410  detects a change P 301  in the signal shown in  FIG. 3  described above with the sensor  413 , the CPU  101  controls the conveying roller separating mechanism  430  of the optional paper deck  401  through the control signal line  1001  to separate the conveying roller pair  421 . 
     Thus, the conveying roller pair  421  of the optional paper deck  401  can be separated in real time after the front end of a sheet P is nipped by the conveying roller pair  411  of the optional unit  410 . 
     In this way, after a sheet P conveyed from the optional paper deck  401  to the optional unit  410  is nipped by the conveying roller pair  411 , the nipping of the sheet P by the conveying roller pair  421  can be released in substantially real time in the present embodiment. 
     Referring to  FIG. 9 , an example will be described in which, the CPU  102 , rather than the CPU  101 , controls separation of the conveying roller pair  421  without being instructed by the CPU  101 . 
     In the example shown in  FIG. 9 , when predetermined time “t” has elapsed after the front end of a sheet being conveyed reached the sensor  422  of the optional paper deck  401 , rollers comprising of the conveying roller pair  421  are separated. 
     Before a sheet is passed, the CPU  101  communicates with the CPU  102  to acquire the information indicating the distance (L 1202 ) from the sheet inlet of the optional unit  410  to the conveying roller pair  411 . Thus, the CPU  102  decides the timing “t” at which the conveying roller pair  421  is to be separated (C 12011 ). 
     Specifically, the time “t” is calculated on the basis of not only the distance (L 1202 ) but also the distance (L 1201 ) from the position of the sensor  422  of the optional paper deck  401  to the sheet outlet and the sheet conveying velocity (V 12 ). That is, “t”=(L 1201 +L 1202 )/V 12 . 
     Then, the CPU  102  monitors the sensor  422 . When the CPU  102  detects (receives C 1202 ) that a sheet being conveyed turns on the sensor  422 , the CPU  102  starts measuring time “t”. When time “t” has elapsed, the CPU  102  controls (outputs C 1203 ) the conveying roller separating mechanism  430  to separate the conveying roller pair  421 . 
     In this example, once time “t” is predetermined between the optional unit  410  and the optional paper deck  401 , the subsequent processing is performed in the optional paper deck  401 . 
     Therefore, nipping of a sheet P by the conveying roller pair  421  can be released in real time after the sheet is nipped by the conveying roller pair  411  while the cost of the system can be reduced. 
     The predetermined time “t” varies depending on the types of apparatus coupled because the distances L 1201 ,  1202  varies depending on the types. However, a common mechanism can be used for any types of sheet conveying apparatuses respectively comprising a CPU for controlling thereof coupled because the time “t” is predetermined through communication between the two CPUs of adjacent sheet conveying apparatuses. 
     While the embodiment above is described with respect to the example in which nipping of a sheet by the conveying roller pair  421  nearest to the sheet outlet of the optional paper deck  401  is released, nipping of a sheet by another conveying roller pair of the optional paper deck  401 , in addition to the conveying roller pair  421 , may be released. 
     For example, assuming that a conveying roller separating mechanism (second releasing mechanism), not shown, may also be provided for a conveying roller pair  423  (see  FIG. 4 ) of the optional paper deck  401 . When a sheet longer than the distance from the conveying roller pair  411  of the optional unit  410  to the conveying roller pair  423  of the optional paper deck  401  is conveyed, both of the conveying roller pairs  421  and  423  may be separated at the same time. The size of the sheet can be detected by a sheet size detecting unit  406  provided in the optional paper deck  401 . 
     The conveying roller separating mechanism for the conveying roller pair  423  is controlled in the same way that the conveying roller separating mechanism  430  associated with the conveying roller pair  421  is controlled. Separation of the conveying roller pair  423  may be controlled by the CPU  101  or the CPU  102 , as described above. 
     When a sheet shorter than the distance from the conveying roller pair  411  of the optional unit  410  to the conveying roller pair  423  of the optional paper deck  401  is conveyed, rollers comprising of the conveying roller pair  421  are separated and rollers comprising of the conveying roller pair  423  are not separated. By selecting and controlling a conveying roller pair to release nipping of a sheet in this way, extra power consumption can be avoided. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions. 
     This application claims priority from Japanese Patent Application No. 2006-262720 filed Sep. 27, 2006, which is hereby incorporated by reference herein in its entirety.