Patent Publication Number: US-9411293-B2

Title: Image forming apparatus with automatic document feeder

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a document reading apparatus, such as an automatic document feeder (ADF) unit, and an image forming apparatus such as a copying machine and a laser beam printer provided with the ADF unit. 
     2. Description of the Related Art 
     With a conventional image forming apparatus of this type, a document conveyance path for conveying a document to a document reading unit, and a recording material conveyance path for conveying a recording material to an image forming unit are configured independently of each other. More specifically, a sheet feed unit, guide members forming a predetermined conveyance path, a plurality of conveyance rollers, a motor for driving conveyance rollers, and a sheet discharge unit are disposed separately for each of a document and a recording material. 
     For this reason, it is difficult for the image forming apparatus to avoid the increase in complexity of the overall mechanical configuration, the increase in cost, and the increase in size. To solve these problems, for example, Japanese Patent Application Laid-Open No. 2000-185881 discusses a technique for simplifying the configuration and reducing cost and size by using a document conveyance path and a recording material conveyance path as common conveyance paths. A document reading unit is disposed in the recording material conveyance path ranging from a sheet feed unit to a sheet discharge unit. 
     However, in an image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2000-185881, the document reading unit is disposed in the recording material conveyance path, for example, between a fixing device and the sheet discharge unit. Therefore, there has been a problem that, during a document reading operation during which a document exists in the recording material conveyance path, the printing operation for a recording material is suspended, resulting in degraded productivity of the image forming apparatus. 
     Further, in a case where a plurality of document sheets is read and printed on a plurality of recording materials with the configuration discussed in Japanese Patent Application Laid-Open No. 2000-185881, alternately performing the document reading operation and the printing operation will degrade the productivity of recording material printing because of the above-described reason. To avoid this problem, the document reading operation may be performed for all of document sheets prior to the printing operation for recording materials. In this case, however, sufficient printing productivity cannot be obtained. Further, a high-capacity image memory is required to store all of image data after the document reading operation, resulting in a cost increase. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an image forming apparatus capable of preventing degradation of the productivity in a case where a document reading operation and a printing operation are performed in parallel. 
     According to an aspect of the present invention, to solve the above-described problem, the image forming apparatus includes an image forming unit configured to form an image on a recording material, a conveyance path configured to convey the recording material having the image formed thereon by the image forming unit, and reverse the recording material, a reading unit configured to read an image of a document sheet conveyed from a document sheet feed unit to the conveyance path, and a control unit configured to perform control to parallelly achieve document image reading by the reading unit and image formation by the image forming unit for forming on a recording material the document image read by the reading unit. The control unit controls timing for starting document sheet conveyance from the document sheet feed unit so that document sheets and recording materials are alternately conveyed in the conveyance path. 
     According to another aspect of the present invention, the image forming apparatus includes an image forming unit configured to form an image on a recording material, a conveyance path configured to convey the recording material having the image formed thereon by the image forming unit, and reverse the recording material, a reading unit configured to read an image of a document sheet conveyed from a document sheet feed unit to the conveyance path, and a control unit configured to perform control to parallelly achieve document image reading by the reading unit and image formation by the image forming unit for forming on a recording material the document image read by the reading unit. The control unit controls timing for reversing recording material for conveying to the conveyance path the recording material having the image formed thereon by the image forming unit so that document sheets and recording materials are alternately conveyed in the conveyance path. 
     Further features of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are cross sectional views illustrating a configuration and a two-sided printing process of an image forming apparatus according to a first exemplary embodiment of the present invention. 
         FIGS. 2A and 2B  illustrate operations of two-sided reading of a document sheet and two-sided printing on a recording material according to the first exemplary embodiment,  FIGS. 2C and 2D  illustrate operations of two-sided reading of a document sheet and two-sided printing on a recording material according to the first exemplary embodiment, and  FIG. 2E  illustrates operations of two-sided reading of a document sheet and two-sided printing on a recording material according to the first exemplary embodiment. 
         FIG. 3  is a block diagram illustrating a configuration of a control unit according to the first exemplary embodiment. 
         FIG. 4A  is a block diagram illustrating a circuit configuration of a document reading unit according to the first to fourth exemplary embodiments, and  FIG. 4B  illustrates the order of conveyance of document sheets and recording materials to a common conveyance path. 
         FIG. 5A  illustrates a state where a document sheet is ready for conveyance, and  FIG. 5B  illustrates an operation of the document sheet to enter a two-sided conveyance path according to the first exemplary embodiment. 
         FIG. 6  is a timing chart illustrating recording material and document conveyance operations according to the first exemplary embodiment. 
         FIG. 7  is a flowchart illustrating an operation of a central processing unit (CPU) according to the first exemplary embodiment. 
         FIG. 8A  is a block diagram illustrating a configuration of a control unit, and  FIG. 8B  illustrates the order of conveyance of document sheets and recording materials to a common conveyance path, according to a second exemplary embodiment of the present invention. 
         FIG. 9A  illustrates a state where a document sheet is ready for conveyance,  FIG. 9B  illustrates an operation of the document sheet to enter the two-sided conveyance path according to the second exemplary embodiment, and  FIG. 9C  illustrates a document sheet conveyance operation in the two-sided conveyance path according to the second exemplary embodiment. 
         FIG. 10  is a timing chart illustrating recording material and document conveyance operations according to the second exemplary embodiment. 
         FIG. 11  is a flowchart illustrating an operation of the CPU according to the second exemplary embodiment. 
         FIG. 12A  illustrates a state where a recording material is ready for conveyance, and  FIG. 12B  illustrates a recording material conveyance operation, according to a third exemplary embodiment of the present invention. 
         FIG. 13  is a timing chart illustrating recording material and document conveyance operations according to the third exemplary embodiment. 
         FIG. 14  is a flowchart illustrating an operation of the CPU according to the third exemplary embodiment. 
         FIG. 15A  illustrates a state where a recording material is ready for conveyance,  FIG. 15B  illustrates an operation of the recording material to enter the two-sided conveyance path, and  FIG. 15C  illustrates a recording material conveyance operation in the two-sided conveyance path, according to the fourth exemplary embodiment. 
         FIG. 16  is a timing chart illustrating recording material and document conveyance operations according to the fourth exemplary embodiment. 
         FIG. 17  is a flowchart illustrating an operation of the CPU according to the fourth exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will be described in detail below. 
     &lt;Image Forming Process Performed in Image Forming Apparatus&gt; 
     First of all, an image forming process will be described.  FIG. 1A  is a cross sectional view illustrating an image forming apparatus  1  according to a first exemplary embodiment. Referring to  FIG. 1A , the image forming apparatus  1  includes a rotatable photosensitive drum  10  as an image bearing member, and a developing roller  11  that rotates while holding toner, in contact with the photosensitive drum  10 . The photosensitive drum  10  and the developing roller  11  are disposed at the center of the image forming apparatus  1 . Upon reception of an image forming instruction, a light emitting unit  21  included in an optical unit  2  irradiate the surface of the rotating photosensitive drum  10  with a laser beam. A charge-based latent image is formed on the surface of the photosensitive drum  10  which has been irradiated with a laser beam. When the developing roller  11  applies the toner held thereon to the latent image formed on the surface of the photosensitive drum  10 , development is performed and a toner image is formed on the surface of the photosensitive drum  10 . 
     A plurality of recording materials S is stored in a first sheet feed unit  30  (first sheet feed unit). The plurality of recording materials S is conveyed one by one in a conveyance path for image formation (first conveyance path), configured between a conveyance roller pair  40  and a discharge roller pair  60 , and then subjected to image formation. Upon reception of an image forming instruction, the plurality of recording materials S is conveyed one by one to the conveyance roller pair  40  by a cassette (hereinafter referred to as CST) pickup roller  31  and a separation member  32 . The conveyance roller pair  40  conveys a recording material S to a transfer unit (transfer roller)  15  while adjusting the conveyance timing so that the toner image on the photosensitive drum  10  is transferred onto a predetermined position on the recording material S. 
     The toner image on the photosensitive drum  10  is transferred onto the recording material S by a transfer voltage and pressure applied to the transfer unit  15 . Then, the recording material S is conveyed to a fixing unit  50 . The fixing unit  50  applies heat and pressure to the toner image on the recording material S to fix the toner image thereon. In this case, heat is generated by a heating roller  51 , and pressure is generated by a pressure roller  52  facing the heating roller  51 . After the toner image has been fixed onto the recording material S, the recording material S is conveyed to the discharge roller pair  60 . 
     In the case of one-sided printing, the discharge roller pair  60  conveys the recording material S to the outside of the image forming apparatus  1  as it is, and the recording material S is stacked onto a first discharge unit  70 . In the case of two-sided printing, the discharge roller pair  60  conveys the recording material S until the trailing edge of the recording material S in the conveyance direction (hereinafter simply referred to as “trailing edge”) passes a two-sided flapper  61 . Then, upon detection of the recording material S having passed the two-sided flapper  61 , the two-sided flapper  61  changes the conveyance destination of the recording material S to the side of the two-sided conveyance path  80  (second conveyance path) to which both the recording material S and a document sheet G are conveyed. The discharge roller pair  60  rotates in the reverse direction to convey the recording material S to the two-sided conveyance path  80 . As illustrated in  FIG. 1B , the switchbacked recording material S is conveyed to a document reading unit  100  by the conveyance roller pair  41 . Further, the recording material S is conveyed again to the transfer unit  15  by the conveyance roller pairs  42  and  40 . At the transfer unit  15 , a toner image is transferred onto the other side of the recording material S. The fixing unit  50  fixes the toner image onto the recording material S. Then, the discharge roller pair  60  discharges and stacks the recording material S onto the first discharge unit  70 . 
     &lt;Operations of Two-Sided Feeding-Reading of Document Sheet and Two-Sided Printing on Recording Material&gt; 
     Next, a process of reading an image of a document image and making two-sided printing on a recording material, will be described.  FIG. 2A  illustrates a state where reading of the front side of the document sheet G is started. A plurality of document sheets G stored in a second sheet feed unit  90  (second sheet feed unit) disposed on the upstream side of the two-sided conveyance path  80  in the conveyance direction is conveyed one by one to the conveyance roller pair  41  by a document pickup roller  91  and a separation member  92 . Before the document reading unit  100  starts reading the first side (front side) of the document sheet G conveyed from the second sheet feed unit  90 , the document reading unit  100  emits light to a white reference member  101 , corrects the white reference value, and then rotates to face the two-sided conveyance path  80 . The conveyance roller pair  41  conveys the document sheet G to the document reading unit  100 . Upon detection of the leading edge of the document sheet G in the conveyance direction (hereinafter simply referred to as “leading edge”), the document reading unit  100  reads an image on the document sheet G. The image read by the document reading unit  100  is stored in an image memory  804  (described below) as document image data for the first side of the document sheet G. Referring to  FIG. 2A , the white reference member  101  is disposed downward in consideration of dust adherence. Although a white reference plate is used as a reference member, the color of a reference member is not limited to white. 
       FIG. 2B  illustrates a state where reading of the first side (front side) of the document sheet G is completed. After passing through the document reading unit  100 , the document sheet G is conveyed to the conveyance roller pair  42 . Upon detection of the trailing edge of the document sheet G having passed the switchback flapper  82 , the conveyance roller pair  4  stops. Accordingly, the document sheet G is stopped while being pinched by the conveyance roller pair  42 . 
       FIG. 2C  illustrates a state where the document reading unit  100  starts reading the second side (back side) of the document sheet G. When the switchback flapper  82  changes the conveyance path of the document sheet G from the two-sided conveyance path  80  to a document conveyance path  81  (third conveyance path), the document reading unit  100  rotates to face the document conveyance path  81 . Then, the conveyance roller pair  42  starts rotating in the reverse direction. The document sheet G is conveyed in the opposite direction along the document conveyance path  81  to the document reading unit  100 . Upon detection of the leading edge of the document sheet G, the document reading unit  100  reads an image of the second side (back side) of the document sheet G, and stores in the image memory  804  the read image as document image data for the second side of the document sheet G. When the back side of the document sheet G is not to be read, the document sheet G is conveyed in the document conveyance path  81  by the conveyance roller pairs  43  and  44 , and then stacked onto the second discharge unit  110 . 
     The plurality of recording materials S fed from the first sheet feed unit  30  is conveyed one by one to the conveyance roller pair  40 . When the light emitting unit  21  irradiates the photosensitive drum  10  with a laser beam, a latent image based on the document image data for the second side (back side) of the document sheet G stored in the image memory  804  is formed on the photosensitive drum  10 . Then, when the toner image formed by developing the latent image is transferred onto the recording material S at the transfer unit  15 , and the recording material S is conveyed to the fixing unit  50 . Thus, the image formation for the second side of the document sheet G is completed. Referring to  FIG. 2C , feeding of the recording material S is started at the same time when reading of the image on the second side (back side) of the document sheet G is started. However, feeding of the recording material S may be started after reading of the image on the second side of the document sheet G is completed. 
       FIG. 2D  illustrates a state where reading of the back side of the document sheet G is completed. Upon completion of reading the document sheet G, the document sheet G is conveyed by the conveyance roller pairs  43  and  44 , and then stacked onto the second discharge unit  110 . When the trailing edge of the document sheet G has passed the switchback flapper  82 , the switchback flapper  82  changes the conveyance path of the recording material S from the document conveyance path  81  to the two-sided conveyance path  80  so that the recording material S currently being conveyed in the two-sided conveyance path  80  is conveyed toward the conveyance roller pair  40 . Then, the discharge roller pair  60  disposed on the downstream side of the recording material S in the conveyance direction rotates in the reverse direction. The recording material S that has been completed image formation for the second side of the document sheet G is conveyed to the two-sided conveyance path  80  selected by the two-sided flapper  61 . 
       FIG. 2E  illustrates a state where the recording material S is currently being conveyed to the image forming unit to be subjected to image formation for the first side of the document sheet G. The two-sided conveyance path  80  is a two-sided conveyance path for image formation on the second side of the recording material S that has been completed image formation on the first side. The recording material S conveyed in the two-sided conveyance path  80  passes the document reading unit  100 , in which a sensor faces the side of the document conveyance path  81 , and then is conveyed to the conveyance roller pair  42 . Then, the recording material S is conveyed again to the transfer unit  15  by the conveyance roller pairs  40  and  42 , as illustrated by the broken line. A toner image based on the document image data for the first side of the document sheet G stored in the image memory  804  is formed on the recording material S that completed image formation for the second side of the document sheet G. The recording material S is stacked onto the first discharge unit  70 . 
     &lt;Overview of Control Unit of Image Forming Apparatus&gt; 
       FIG. 3  is a block diagram illustrating a configuration of a control unit  800  including the CPU  801  for controlling the image forming apparatus  1 . Referring to  FIG. 3 , a light emitting unit  21  including a rotational polygon mirror, a motor, and a laser light emitting element is connected to an application specific integrated circuit (ASIC)  802 . To irradiate the photosensitive drum  10  with a laser beam to form a desired latent image, the CPU  801  outputs control signals to the ASIC  802  to control the light emitting unit  21  included in an optical unit  2 . A main motor  830  drives the CST pickup roller  31 , the conveyance roller pair  40 , the photosensitive drum  10 , the transfer unit  15 , the heating roller  51 , and the pressure roller  52  to convey the recording material S. When feeding rollers for feeding the recording material S are started to be driven, a CST feeding solenoid  822  is turned ON to drive the CST pickup roller  31 . A two-sided drive motor  840  drives the document pickup roller  91  and the conveyance roller pairs  41  to  44 . The discharge roller drive motor  850  drives the discharge roller pair  60 . The CPU  801  controls drive systems, such as the main motor  830 , the CST feeding solenoid  822 , the two-sided drive motor  840 , and the discharge roller drive motor  850  via the ASIC  802 . 
     The CPU  801  controls a high-voltage power source  810  for controlling a charging voltage, a development voltage, and a transfer voltage required for the electrophotographic process, a low-voltage power source  811 , and the fixing unit  50 . The CPU  801  further detects temperature by using a thermistor (not illustrated) provided in the fixing unit  50 , and performs control to maintain the temperature of the fixing unit  50  constant. 
     A program memory  803  is connected to the CPU  801  via a bus (not illustrated). The program memory  803  stores programs and data used by the CPU  801  to perform processing. The CPU  801  controls operations of the image forming apparatus  1  based on the programs and data stored in the program memory  803 . 
     The ASIC  802  performs speed control for the motor in the light emitting unit  21 , and speed control for the main motor  830 , the two-sided drive motor  840 , and the discharge roller drive motor  850  based on instructions from the CPU  801 . In motor speed control, the ASIC  802  detects a tack signal (a pulse signal output from each motor each time the motor rotates), and outputs an acceleration or deceleration signal to each motor so that the tack signal is output at predetermined intervals. Performing motor control via hardware circuit, such as the ASIC  802 , in this way enables reducing control load on the CPU  801 . 
     Control operations performed by the control unit  800  at the time of printing on a recording material will be described. Upon reception of a print command for instructing printing on a recording material from a host computer (not illustrated), the CPU  801  drives the main motor  830 , the two-sided drive motor  840 , and the CST feeding solenoid  822  via the ASIC  802  to convey the recording material S. The toner image formed on the photosensitive drum  10  is transferred onto the recording material S at the transfer unit  15 . The fixing unit  50  fixes the toner image onto the recording material S. The discharge roller pair  60  discharges the recording material S onto the first discharge unit  70  as a recording material stacking unit. To improve the alignment characteristics of recording materials, the first discharge unit  70  is provided with a gentle rising slope from the vicinity of the discharge port toward the recording material discharge direction. The CPU  801  supplies predetermined power from the low voltage power source  811  to the fixing unit  50  to cause the fixing unit  50  to generate a desired heating value to heat the recording material S so that the toner image thereon is melted and fixed onto the recording material S. 
     Next, control operations performed by the control unit  800  at the time of document reading will be described. Upon reception of a scanning command for instructing reading of the document sheet G from a host computer (not illustrated), the CPU  801  drives a two-sided flapper solenoid  820  and the two-sided drive motor  840  via the ASIC  802  to operate a document feeding solenoid  823 . As a result, the torque of the two-sided drive motor  840  is transmitted to the document pickup roller  91 , and the document sheet G is conveyed. The document reading unit  100  reads the document sheet G based on a CISSTART signal  902 , a CISLED signal  903 , a Sl_in signal  912 , a Sl_select signal  913 , and a SYSCLK signal  914 , which are control signals output from the ASIC  802 . These control signals will be described in detail below. The CPU  801  stores in the image memory  804  connected to the ASIC  802  the read document image data output as a Sl_out signal  910  from the document reading unit  100  through control via the ASIC  802 . Then, the CPU  801  operates the switchback solenoid  821  to turn over the switchback flapper  82  toward the side of the document conveyance path  81 , and reverses the two-sided drive motor  840  to convey the document sheet G to the second discharge unit  110 . 
     &lt;Overview of Document Reading Unit&gt; 
     The document reading unit  100  will be described in detail with reference to  FIG. 4A .  FIG. 4A  is a circuit block diagram of the document reading unit  100 . Referring to  FIG. 4A , a contact image sensor (CIS) unit  901  includes, for example, photo diodes including 10,368 pixels arranged in array form with a certain main scanning density (for example, 1200 dots per inch (dpi)). The CISSTART signal  902  is a document reading start pulse signal input to the CIS sensor unit  901 . The CISLED signal  903  is a control signal for controlling a light emitting element  907 . A current amplifier unit  906  controls the current to be supplied to the light emitting element  907  based on the CISLED signal  903 . The light emitting element  907  uniformly irradiates the document sheet G. A timing generator  917  receives the SYSCLK signal  914 , and generates an ADCLK signal  916  and a CISCLK signal  915 . The SYSCLK signal  914  is a system clock for determining the operation speed of the document reading unit  100 . The ADCLK signal  916  is a sampling clock for determining the sampling rate of an analog-to-digital (A/D) converter  908 . The CISCLK signal  915  is used as a transfer clock for a CISSNS signal  918  which is an output signal of a shift register  905 . 
     Next, the document reading operation will be described. When the CISSTART signal  902  becomes active, the CIS sensor unit  901  starts accumulating charges based on the light emitted from the light emitting element  907  and reflected by the document sheet G, and sequentially sets charge data accumulated in an output buffer  904 . The timing generator  917  outputs to the shift register  905 , for example, the CISCLK signal  915  having a clock frequency of about 500 kHz to 1 MHz. The shift register  905  outputs the charge data set in the output buffer  904  to the A/D converter  908  as the CISSNS signal  918  in synchronization with the input CISCLK signal  915 . Since the CISSNS signal  918  includes a predetermined data guaranteed region, the A/D converter  908  needs to perform sampling of the CISSNS signal  918  when a predetermined time has elapsed since the rising timing of the CISCLK signal  915  (transfer clock). The CISSNS signal  918  is output from the shift register  905  in synchronization with both the rising and falling edges of the CISCLK signal  915  (transfer clock). Therefore, the timing generator  917  generates the ADCLK signal  916  and the CISCLK signal  915  so that the frequency of the ADCLK signal  916  (a clock for sampling the CISSNS signal  918 ) become twice the frequency of the CISCLK signal  915 . Then, the CISSNS signal  918  is sampled on the rising edge of the ADCLK signal  916 . The timing generator  917  divides the SYSCLK signal  914  (input system clock) to generate the ADCLK signal  916  and the CISCLK signal  915  (transfer clock). The phase of the ADCLK signal  916  lags behind the CISCLK signal  915  (transfer clock) by the amount of the above-described data guaranteed region. 
     The A/D converter  908  converts the CISSNS signal  918  to a digital signal, and outputs the digital signal to an output interface circuit  909  as a CISSNS_D signal  919 . The output interface circuit  909  outputs the CISSNS_D signal  919  at a predetermined timing as the Sl_out signal  910  (serial data). In this case, an analog output reference voltage is output to the CISSNS_D signal  919  for a predetermined number of pixels from the CISSTART signal  902  (start pulse). These pixels cannot be used as effective pixels. 
     Via the ASIC  802 , the control circuit  911  controls the A/D conversion gain of the A/D converter  908  based on the Sl_in signal  912  and the Sl_select signal  913  from the CPU  801 . For example, if enough contrast of the read document image cannot be acquired, the CPU  801  increases the A/D conversion gain of the A/D converter  908  to increase the contrast so that the document can be read constantly with the best contrast. 
     Although the image forming apparatus  1  is configured to output image information of all pixels as the CISSNS_D signal  919  (output signal), the configuration is not limited thereto. The image forming apparatus  1  may be configured to divide pixels into a plurality of areas and simultaneously apply A/D conversion to the plurality of areas to achieve high-speed document reading. Although a CIS sensor is used for the document reading unit  100  herein, the CIS sensor may be replaced with a complementary metal-oxide semiconductor (CMOS) sensor or a charge-coupled device (CCD) sensor. 
     &lt;Document Conveyance Timing&gt; 
     Next, conveyance timing control for document sheets G when performing two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S, according to the present exemplary embodiment, will be described. When a plurality of recording materials S or document sheets G exists, the following designation rule is applied in descriptions to distinguish each of the plurality of recording materials S or document sheets G. That is, the plurality of recording materials S stored in the first sheet feed unit  30  is referred to as S 1 , S 2 , S 3 , and so on in order of conveyance at the time of image formation. Likewise, the plurality of document sheets G stored in the second sheet feed unit  90  is referred to as G 1 , G 2 , G 3 , and so on in order of conveyance at the time of document reading. When it is not necessary to describe a plurality of sheets distinctly, one sheet is referred to as a recording material S or a document sheet G in descriptions. 
     A conveyance path ranging from the discharge roller pair  60  to the conveyance roller pair  41  is defined as a recording material conveyance path  83 . A conveyance path ranging from the document pickup roller  91  to the conveyance roller pair  41  is defined as a document conveyance path  84 . A conveyance path ranging from the conveyance roller pair  41  to the conveyance roller pair  42  is defined as a common conveyance path  85 . These definitions will be used in the following descriptions.  FIG. 4B  illustrates the order of conveyance of the document sheets G and the recording materials S in the common conveyance path  85  according to the present exemplary embodiment. In a case of a process of two-sided reading of n document sheets and two-sided printing on n recording materials, the document sheets G and the recording materials S are alternately conveyed in the common conveyance path  85  in order of the document sheet G 1 , the recording material S 1 , the document sheet G 2 , the recording material S 2 , the document sheet G 3 , . . . , the recording material Sn−1, the document sheet Gn, and the recording material Sn. While alternately conveying the document sheets G and the recording materials S in this order, the CPU  801  continuously performs reading of the document sheets G and image formation on the recording materials S. The present exemplary embodiment is characterized in controlling the time when the document sheet G starts to be conveyed in the document conveyance path  84  to achieve conveyance control for alternately conveying the document sheet G and the recording material S in the common conveyance path  85 . 
     Operations performed according to the present exemplary embodiment will be described below particularly focusing on conveyance order control for the recording material S 1  and the document sheet G 2  out of conveyance order control for the document sheets G and the recording materials S. Therefore, in the following descriptions, subsequent operations will be described in detail below on the premise that two-sided reading of the document sheet G 1  is completed, the second side (back side) of the document sheet G 1  has been printed on the recording material S 1 , and the recording material S 1  is currently being conveyed toward the common conveyance path  85 . 
       FIG. 5A  illustrates a state where the document sheet G 2  is ready for conveyance. Referring to  FIG. 5A , the document sheet G 2  is drawn with a bold solid line without an arrow head at both ends. Hereinafter, such a bold solid line indicates that the document sheet G 2  is stopped. The recording material S 1  is once suspended at the first discharge unit  70 , and then starts to be conveyed by the discharge roller pair  60  from the position of the discharge roller pair  60  (reversing start position). Then, the recording material S 1  is conveyed in the recording material conveyance path  83  by the discharge roller pair  60 , and then enters the common conveyance path  85 . The document sheet G 2  is stored in the second sheet feed unit  90 , and waits till the time when the document sheet G 2  starts to be conveyed in the document conveyance path  84 , calculated by the CPU  801  (described below). The time when the document sheet G 2  starts to be conveyed in the document conveyance path  84  refers to time with reference to a printing start time of the image forming apparatus  1 . The time when the document sheet G 2  starts to be conveyed also refers to the time when, during conveyance of the document sheet G 2  through the document conveyance path  84  to the common conveyance path  85 , it becomes possible for the document sheet G 2  to be conveyed by following the trailing edge of the recording material S 1  while avoiding collision between the recording material S 1  and the document sheet G 2  in the common conveyance path  85 . 
       FIG. 5B  illustrates an operation of the document sheet G 2  to enter the common conveyance path  85 . When the elapsed time from the reference time reaches the time when the document sheet G 2  starts to be conveyed in the document conveyance path  84 , the document sheet G 2  starts to be conveyed from the second sheet feed unit  90  through the document conveyance path  84  toward the common conveyance path  85  by the document pickup roller  91  and the separation member  92 . When the document sheet G 2  enters the common conveyance path  85 , the document sheet G 2  is pinched by the conveyance roller pair  41 . When the leading edge of the document sheet G 2  is pinched by the conveyance roller pair  41 , the trailing edge of the recording material S 1  has already passed the conveyance roller pair  41 . Then, the document sheet G 2  and the recording material S 1  are conveyed in the common conveyance path  85  with a sufficient conveyance interval therebetween so that the document sheet G 2  and the recording material S 1  do not collide with each other. In the meantime, by the time when the document sheet G 2  enters the common conveyance path  85  and reading of the first side (front side) of the document sheet G 2  is started, the document reading unit  100  completes preparation for document reading to prepare for reading the document sheet G 2 . Preparation for document reading refers to operations for securing an image storage area in the image memory  804  and rotating the image reading unit  100  to face the common conveyance path  85 . While the document sheet G 2  is passing the document reading unit  100 , the document reading unit  100  reads image information of the first side (front side) of the document sheet G 2 , and stores the read image information in the image memory  804 . 
     The recording material S 1  is conveyed in the recording material conveyance path  83  at a conveyance speed Vp1, and conveyed in the common conveyance path  85  at a conveyance speed Vp2. The document sheet G 2  is conveyed in the document conveyance path  84  at a conveyance speed Vs1, and conveyed in the common conveyance path  85  at a conveyance speed Vs2. In the present exemplary embodiment, the conveyance speed Vp1 is determined by an optimum conveyance speed for image formation, and the conveyance speeds Vs1 and Vs2 are determined by an optimum conveyance speed for document reading. The conveyance speeds Vp1, Vs1, and Vs2 do not change during printing operation. After the document sheet G 2  and the recording material S 1  enter the common conveyance path  85 , the CPU  801  controls the conveyance speed Vp2 of the recording material S 1  (Vp2=Vs2) so that the document sheet G 2  and the recording material S 1  are conveyed while maintaining constant distance between the leading edge of the document sheet G 2  and the trailing edge of the recording material S 1 . 
     In the present exemplary embodiment, to avoid collision between the recording material S 1  and the document sheet G 2  in the common conveyance path  85 , the CPU  801  controls only the time when the document sheet G 2  starts to be conveyed in the document conveyance path  84 , and does not control the conveyance speed Vs1 of the document sheet G 2  in the document conveyance path  84 . Specifically, the CPU  801  controls only the timing for starting conveyance of the document sheet G 2  from the second sheet feed unit  90 . Subsequently, the CPU  801  reads the second side (back side) of the document sheet G 2 , prints the first side (front side) of the document sheet G 1  on the recording material S 1 , and prints the second side (back side) of the document sheet G 2  on the recording material S 2 . However, subsequent operations are performed in the above-described process illustrated in  FIGS. 2C and 2E , and detailed descriptions thereof will be omitted. This completes descriptions of conveyance timing control for conveying the document sheet G in the common conveyance path  85  when performing two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S. 
     &lt;Descriptions of Recording Material and Document Conveyance Operations&gt; 
       FIG. 6  is a timing chart illustrating conveyance operations for the recording material S 1  and the document sheet G 2 . Referring to  FIG. 6 , timing (a) indicates that the recording material S 1  is passing through the recording material conveyance path  83 , timing (b) indicates that the recording material S 1  is passing the conveyance roller pair  41 , timing (c) indicates that the recording material S 1  is passing through the common conveyance path  85 , timing (d) indicates that the document sheet G 2  is passing through the document conveyance path  84 , timing (e) indicates that the document sheet G 2  is passing the conveyance roller pair  41 , and timing (f) indicates that the document sheet G 2  is passing through the common conveyance path  85 . With the printing start time defined as a reference time 0, t 131  is defined as the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83 , and t 132  is defined as the time when the document sheet G 2  starts to be conveyed in the document conveyance path  84 . Further, with the printing start time defined as a reference time, t 133  is defined as the time when the recording material S 1  has passed the conveyance roller pair  41 , and t 134  is defined as the time when the document sheet G 2  starts passing the conveyance roller pair  41 . T 135  is defined as a required time since the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  till the time when the recording material S 1  starts passing the conveyance roller pair  41 . T 136  is defined as a required time since the time when the recording material S 1  starts passing the conveyance roller pair  41  till the time when the recording material S 1  has passed the conveyance roller pair  41 . T 137  is defined as a required time since the time when the document sheet G 2  starts to be conveyed in the document conveyance path  84  till the time when the document sheet G 2  starts passing the conveyance roller pair  41 . 
     The time when the document sheet G or the recording material S starts passing the conveyance roller pair  41  refers to the time when the leading edge of the document sheet G or the recording material S reaches the conveyance roller pair  41 . The time when the document sheet G or the recording material S has passed the conveyance roller pair  41  refers to the time when the trailing edge of the document sheet G or the recording material S leaves the conveyance roller pair  41 . According to the configuration of the present exemplary embodiment, the CPU  801  controls the time t 132  when the document sheet G 2  starts to be conveyed in the document conveyance path  84  to control the order of the recording material S 1  and the document sheet G 2  passing through the common conveyance path  85 . Since the CPU  801  performs control to convey the recording material S 1  in the common conveyance path  85  earlier than the document sheet G 2 , the time t 133  when the recording material S 1  has passed the conveyance roller pair  41  precedes the time t 134  when the document sheet G starts passing the conveyance roller pair  41 , as illustrated in  FIG. 6 . In other words, the trailing edge of the recording material S 1  passes the conveyance roller pair  41  before the leading edge of the document sheet G 2  reaches the conveyance roller pair  41 , thus avoiding collision between the recording material S 1  and the document sheet G 2  in the common conveyance path  85 . The CPU  801  calculates the required times T 135 , T 136 , and T 137 , and the times t 132 , t 133 , and t 134  based on the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83 , the recording material conveyance speeds Vp1 and Vp2, and the document conveyance speeds Vs1 and Vs2 which are known information. 
     &lt;Processing for Determining Start of Document Conveyance&gt; 
     The processing by the CPU  801  for determining whether conveyance of the document sheet G 2  is started will be described with reference to  FIG. 7 . The CPU  801  receives from the host computer (not illustrated) an instruction for two-sided reading of a plurality of document sheets G and two-sided printing on a plurality of recording materials S. In the process of reading control and printing control, the CPU  801  performs the processing illustrated in  FIG. 7  ranging from the conveyance start standby state to the start of conveyance of the document sheet G. The processing for determining whether conveyance of the document sheet G 2  is started will be described. 
     In step S 1401 , the CPU  801  acquires the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83 , which is known information. The CPU  801  acquires as the time t 131  information equivalent to the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83 , such as a reverse rotation start time of the discharge roller pair  60 , stored in a random access memory (RAM) of the CPU  801 . In step S 1402 , the CPU  801  calculates the required time T 135  from the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  to the time when the recording material S 1  starts passing the conveyance roller pair  41 , and the required time T 136  from the time when the recording material S 1  starts passing the conveyance roller pair  41  to the time when the recording material S 1  has passed the conveyance roller pair  41 . The CPU  801  further calculates the required time T 137  from the time when the document sheet G 2  starts to be conveyed in the document conveyance path  84  to the time when the document sheet G 2  starts passing the conveyance roller pair  41 . 
     The CPU  801  can calculate the required times T 135 , T 136 , and T 137  based on known information, such as the conveyance path length, the conveyance speeds Vp1 and Vp2 of the recording material S 1 , or the conveyance speed Vs1 of the document sheet G 2 . More specifically, the required time T 135  can be calculated based on the conveyance path length between the discharge roller pair  60  and the conveyance roller pair  41 , and the conveyance speed Vp1 of the recording material S 1 . The required time T 136  can be calculated based on the length of the recording material S 1  in the conveyance direction, and the conveyance speed Vp2 (=Vs2) of the recording material S 1 . The required time T 137  can be calculated based on the conveyance path length between the document pickup roller  91  and the conveyance roller pair  41 , and the conveyance speed Vs1 of the document sheet G 2 . Further, the CPU  801  may refer to each required time in a table generated and stored in the program memory  803 . This table indicates the correspondence between the conveyance speeds Vp1, Vp2, Vs1 and the required times T 135 , T 136 , and T 137 . 
     In step S 1403 , the CPU  801  adds the required times T 135  and T 136  to the time t 131  to calculate the time t 133  when the recording material S 1  has passed the conveyance roller pair  41  (t 133 =t 131 +T 135 +T 136 ). As described above, it is necessary to control the time t 132  when the document sheet G 2  starts to be conveyed in the document conveyance path  84  so that the time t 133  precedes the time t 134  when the document sheet G starts passing the conveyance roller pair  41 . In step S 1404 , therefore, the CPU  801  calculates the time t 134  that satisfies t 133 ≦t 134 . The CPU  801  calculates the time t 134  to provide a certain fixed margin for the time difference between the times t 133  and t 134  in consideration of conveyance speed variation and mechanical accuracy. For example, this condition is given by t 134 =t 133 +Δt where Δt indicate the time difference to provide a certain fixed margin in consideration of conveyance speed variation and mechanical accuracy. 
     In step S 1405 , the CPU  801  subtracts the required time T 137  from the time t 134  (calculated in step S 1404 ) to calculate the time t 132  when the document sheet G 2  starts to be conveyed in the document conveyance path  84  (t 132 =t 134 −T 137 ). In step S 1406 , referring to a timer (not illustrated), the CPU  801  determines whether the present time has passed the time t 132 . When the CPU  801  determines that the present time has not yet passed the time t 132  (NO in step S 1406 ), the CPU  801  waits for a predetermined period of time in step S 1407 , and the processing returns to step S 1406 . On the other hand, when the CPU  801  determines that the present time has passed the time t 132  (YES in step S 1406 ), the CPU  801  starts conveyance of the document sheet G 2 . This completes descriptions of the processing by the CPU  801  for determining whether conveyance of the document sheet G 2  is to be started. 
     According to the configuration of the present exemplary embodiment, the CPU  801  stores information about the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  with reference to the printing start time, and uses the information to calculate the time t 132  when the document sheet G 2  starts to be conveyed in the document conveyance path  84 . However, it is also possible to provide a sensor in the middle of the conveyance path of the recording material S 1 , and calculate the time t 132  based on the time when the recording material S 1  has passed the sensor. Further, the CPU  801  may calculate the time t 132  based on the time when the recording material S 1  starts to be fed from the first sheet feed unit  30 . In the present exemplary embodiment, the CPU  801  performs two-sided reading of the document sheet G in order of the first side (front side) of the document sheet G and the second side (back side) of the document sheet G. Further, the CPU  801  performs two-sided printing on the recording material S in order of the second side (back side) of the recording material S and the first side (front side) of the recording material S. However, the present invention is not limited the above-described order of reading and printing. The present invention is applicable even if the order of the two-sided reading of the document sheet G and the order of two-sided printing on the recording material S are changed. 
     Thus, according to the configuration of the present exemplary embodiment, conveyance timing control for conveying the document sheet G 2  in the document conveyance path  84  can be performed. As a result, the conveyance order and the conveyance interval of the document sheet G and the recording material S can be accurately controlled. Thus, it is possible to achieve a process of two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S with high productivity, without requiring a complicated apparatus configuration. As described above, according to the present exemplary embodiment, it is possible to prevent degradation of the productivity in a case where a document reading operation and a printing operation are performed in parallel based on a configuration in which a recording material and a document sheet share a common conveyance path. 
     In the first exemplary embodiment, the recording material S passes through the common conveyance path  85  earlier than the document sheet G, and the timing for conveying the document sheet G in the common conveyance path  85  is adjusted only with the relevant document conveyance start time. A second exemplary embodiment differs from the first exemplary embodiment in that the document sheet G passes through the common conveyance path  85  earlier than the recording material S, and the conveyance timing for the common conveyance path  85  is adjusted with both the relevant document conveyance speed and the relevant document conveyance start time. The overall configuration of the image forming apparatus  1  is similar to that in the first exemplary embodiment, and detailed descriptions thereof will be omitted. However, the present exemplary embodiment differs from the first exemplary embodiment in that the image forming apparatus  1  includes a control unit  1500  instead of the control unit  800 . 
     &lt;Overview of Control Unit of Image Forming Apparatus&gt; 
     The control unit  1500  will be described below.  FIG. 8A  illustrates the control unit  1500  centering on the CPU  801  of the image forming apparatus  1  according to the present exemplary embodiment. The present exemplary embodiment differs from the first exemplary embodiment in the use of the document inversion driving motor  860 , the document feeding drive motor  870 , and the document dedicated conveyance path drive motor  880  controlled by the ASIC  802 . Rollers driven by each motor will be described below. The two-sided drive motor  840  that drives the document pickup roller  91  and the conveyance roller pairs  41  to  44  in the first exemplary embodiment drives only the conveyance roller pair  41  in the present exemplary embodiment. The document inversion driving motor  860  drives the conveyance roller pair  42 . The document feeding drive motor  870  drives the document pickup roller  91 . The document dedicated conveyance path drive motor  880  drives the conveyance roller pairs  43  and  44 . Similar to the first exemplary embodiment, the discharge roller drive motor  850  drives the discharge roller pair  60 . Components having the same function as the control unit  800  are assigned the same reference numeral, and redundant descriptions thereof will be omitted. 
     &lt;Document Conveyance Timing&gt; 
     A conveyance timing control method for conveying the document sheet G in the common conveyance path  85  when performing two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S, according to the prevent exemplary embodiment, will be described. Also in the present exemplary embodiment, when a plurality of recording materials S or document sheets G exists, a similar designation rule to that in the first exemplary embodiment is applied in descriptions to distinguish each of the plurality of recording materials S and document sheets G. 
       FIG. 8B  illustrates the order of conveyance of the document sheets G and the recording materials S in the common conveyance path  85  according to the present exemplary embodiment. The document sheets G and the recording materials S are alternately conveyed in the common conveyance path  85  in order of the document sheet G 1 , the document sheet G 2 , the recording material S 1 , the document sheet G 3 , the recording material S 2 , . . . , the document sheet Gn−1, the recording material Sn−2, the document sheet Gn, the recording material Sn−1, and the recording material Sn. The CPU  801  continuously performs reading the document sheets G and image formation on the recording materials S while alternately conveying the document sheets G and the recording materials S in the above-described order. The present exemplary embodiment is characterized in controlling both the relevant conveyance start time and the relevant conveyance speed of the document sheet G to achieve conveyance control for alternately conveying the document sheet G and the recording material S in the common conveyance path  85 . 
     Operations performed according to the present exemplary embodiment will be described below particularly focusing on conveyance order control for the recording material S 1  and the document sheet G 2  out of conveyance order control for the document sheets G and the recording materials S. Therefore, in the following descriptions, subsequent operations will be described in detail below on the premise that two-sided reading of the document sheet G 1  is completed, the recording material S 1  has started to be conveyed from the first sheet feed unit  30  to print the second side (back side) of the document sheet G 1  on the recording material S 1 . 
       FIG. 9A  illustrates a state where the document sheet G 2  is ready for conveyance. The recording material S 1  passes the transfer unit  15  and the fixing unit  50 , and the second side (back side) of the document sheet G 1  is printed on one side of the recording material S 1  in a known image formation process. The document sheet G 2  is stored in the second sheet feed unit  90 , and waits till the time when the document sheet G 2  starts to be conveyed in the document conveyance path  84 , calculated by the CPU  801  (described below). The time when the document sheet G 2  starts to be conveyed in the document conveyance path  84  refers to time with reference to the printing start time of the image forming apparatus  1 . The time when the document sheet G 2  starts to be conveyed also refers to the time when, during conveyance of the document sheet G 2  from the second sheet feed unit  90  to the common conveyance path  85 , it becomes possible for the recording material S 1  to be conveyed by following the trailing edge of the document sheet G 2  while avoiding collision between the recording material S 1  and the document sheet G 2  in the common conveyance path  85 . 
       FIG. 9B  illustrates an operation of the document sheet G 2  to enter the common conveyance path  85 . When the elapsed time from the reference time reaches the time when the document sheet G 2  starts to be conveyed from the second sheet feed unit  90  through the document conveyance path  84  toward the common conveyance path  85  by the document pickup roller  91  and the separation member  92 . In this case, the document sheet G 2  is conveyed in the document conveyance path  84  at the conveyance speed Vs1. The conveyance speed Vs1 is adjusted to a conveyance speed at which the recording material S 1  can be conveyed by following the trailing edge of the document sheet G 2  while avoiding collision between the recording material S 1  and the document sheet G 2  in the common conveyance path  85 . When the document sheet G 2  enters the common conveyance path  85 , the document sheet G 2  is pinched and conveyed by the conveyance roller pair  41 . The recording material S 1  is pinched by the discharge roller pair  60 , and conveyed toward the first discharge unit  70 . In the meantime, by the time when the document sheet G 2  enters the common conveyance path  85  and reading of the first side (front side) of the document sheet G 2  is started, the document reading unit  100  completes preparation for document reading to prepare for reading the document sheet G 2 . 
       FIG. 9C  illustrates a conveyance operation for the document sheet G 2  in the common conveyance path  85 . The document sheet G 2  moves in the common conveyance path  85 . While the document sheet G 2  is passing the document reading unit  100 , the document reading unit  100  reads image information of the first side (front side) of the document sheet G 2 , and stores the read image information in the image memory  804 . When the document sheet G is conveyed in the common conveyance path  85  at the conveyance speed Vs2, the conveyance speed Vs2 is adjusted to an optimum conveyance speed in consideration of the reading speed of the document reading unit  100 . The recording material S 1  is switchbacked while being pinched by the discharge roller pair  60 , passes through the recording material conveyance path  83 , and advances toward the common conveyance path  85 . When the recording material S 1  enters the common conveyance path  85  and the leading edge is pinched by the conveyance roller pair  41 , the trailing edge of the document sheet G 2  has already passed the conveyance roller pair  41 . The CPU  801  controls the conveyance roller pair  41  and the conveyance roller pair  42  so that the document sheet G 2  and the recording material S 1  are conveyed in the common conveyance path  85  with a sufficient conveyance interval therebetween so that the document sheet G 2  and the recording material S 1  do not collide with each other. 
     The recording material S 1  is conveyed in the recording material conveyance path  83  at the conveyance speed Vp1, and conveyed in the common conveyance path  85  at the conveyance speed Vp2. Similar to the first exemplary embodiment, the conveyance speed Vp1 is determined by the optimum conveyance speed for image formation, and the conveyance speed Vp2 is controlled to satisfy Vp2=Vs2. However, in the present exemplary embodiment, to avoid collision between the recording material S 1  and the document sheet G 2  in the common conveyance path  85 , the CPU  801  controls both the time when the document sheet G 2  starts to be conveyed in the document conveyance path  84 , and the conveyance speed of the document sheet G 2  in the document conveyance path  84 . In other words, the CPU  801  controls the timing for starting conveyance of the document sheet G 2  from the second sheet feed unit  90 , and controls the conveyance speed Vs1 of the document sheet G 2  during conveyance from the second sheet feed unit  90  to the common conveyance path  85 . 
     Subsequently, the CPU  801  reads the second side (back side) of the document sheet G 2 , prints the first side (front side) of the document sheet G 1  on the recording material S 1 , and prints the second side (back side) of the document sheet G 2  on the recording material S 2 . However, subsequent operations are performed in the above-described process illustrated in  FIGS. 2C and 2E , and detailed descriptions thereof will be omitted. This completes descriptions of conveyance control for conveying the document G in the common conveyance path  85  when performing two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S. 
     &lt;Descriptions of Recording Material and Document Conveyance Operations&gt; 
       FIG. 10  is a timing chart illustrating conveyance operations for the recording material S 1  and the document sheet G 2 . The timings (a) to (f) are similar to those in the first exemplary embodiment. The present exemplary embodiment differs from the first exemplary embodiment in that t 201  is defined as the time when the document sheet G 2  has passed the conveyance roller pair  41 , t 202  is defined as the time when the recording material S 1  starts passing the conveyance roller pair  41 , and T 203  is defined as the time required by the document sheet G 2  to pass the conveyance roller pair  41 . According to the configuration of the present exemplary embodiment, the time t 202  when the recording material S 1  starts passing the conveyance roller pair  41  is later than the time t 201  when the document sheet G has passed the conveyance roller pair  41 . The CPU  801  calculates the required times T 135 , T 137 , and T 203 , and the times t 132 , t 201 , and t 202  based on the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83 , the recording material conveyance speeds Vp1 and Vp2, and the document conveyance speed Vs2, which are known information. Other processing is similar to that in the first exemplary embodiment, and detailed descriptions thereof will be omitted. 
     &lt;Processing for Determining Start of Document Conveyance&gt; 
     The processing by the CPU  801  for determining whether conveyance of the document sheet G 2  is started, will be described with reference to  FIG. 11 . Basic control is similar to that in the first exemplary embodiment. Steps having the same processing as those in the first exemplary embodiment illustrated in  FIG. 7  are assigned the same step numbers, and redundant descriptions thereof will be omitted. The points different from the first exemplary embodiment will be described. In step S 1401 , by using a similar method to the one in the first exemplary embodiment, the CPU  801  acquires the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  which is known information. Then, the CPU  801  performs processing in step S 2102  and subsequent steps. In step S 2102 , the CPU  801  calculates the required time T 135  from the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  to the time when the recording material S 1  starts passing the conveyance roller pair  41 , and the required time T 203  from the time when the document sheet G 2  starts passing the conveyance roller pair  41  to the time when the document sheet G 2  has passed the conveyance roller pair  41 . When calculating the required time T 203 , the CPU  801  uses the length of the document sheet G 2  in the conveyance direction, and the previously acquired conveyance speed Vs2 (=Vp2) of the document sheet G 2  in the common conveyance path  85 . 
     In step S 2103 , the CPU  801  adds the required time T 135  to the time t 131  to calculate the time t 202  when the recording material S 1  starts passing the conveyance roller pair  41  (t 202 =t 131 +T 135 ). As described above, it is necessary to control the time when the document sheet G 2  starts to be conveyed so that the time t 202  is later than the time t 201  when the document sheet G has passed the conveyance roller pair  41 . In step S 2104 , therefore, the CPU  801  calculates the time t 201  that satisfies t 201 ≦t 202 . In step S 2105 , the CPU  801  calculates the conveyance speed Vs1, the required time T 137 , and the time t 132  when the document sheet G 2  starts to be conveyed in the document conveyance path  84 . In the present exemplary embodiment, it is necessary to perform control so that the time t 132  is later than the time t 131  (t 131 ≦t 132 ). In step S 2105 , the CPU  801  calculates the conveyance speed Vs1 adjusted so that the time t 132  is later than the time t 131 , and then calculates the required time T 137  based on the calculated conveyance speed Vs1 and the conveyance path length between the document pickup roller  91  and the conveyance roller pair  41 . Then, the CPU  801  subtracts from the time t 201  the required time T 137  and the required time T 203  (calculated in step S 2102 ) to calculate the time t 132  (t 132 =t 201 −T 203 −T 137 ). Subsequent processing is similar to that in the first exemplary embodiment, and redundant descriptions thereof will be omitted. This completes descriptions of the processing by the CPU  801  for determining whether conveyance of the document sheet G 2  is to be started. 
     According to the configuration of the present exemplary embodiment, conveyance timing control and conveyance speed control for conveying the document sheet G in the document conveyance path  84  can be performed. As a result, the conveyance order and the conveyance interval of the document sheet G and the recording material S can be accurately controlled. Further, in the present exemplary embodiment, the CPU  801  adjust both the time t 132  when the document sheet G starts to be conveyed in the document conveyance path  84 , and the conveyance speed Vs1 in the document conveyance path  84 . Thus, it is possible to achieve a process of two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S with higher productivity than in the first exemplary embodiment, without requiring a complicated apparatus configuration. As described above, according to the present exemplary embodiment, it is possible to prevent degradation of the productivity in a case where a document reading operation and a printing operation are performed in parallel based on a configuration in which a recording material and a document sheet share a common conveyance path. 
     In the first and second exemplary embodiments, the CPU  801  controls the timing for conveying the document sheet G in the common conveyance path  85  in synchronization with the conveyance operation for the recording material S. The third exemplary embodiment differs from the first and second exemplary embodiments in that the CPU  801  controls the time when the recording material S passes through the common conveyance path  85  in synchronization with the conveyance operation for the document sheet G. The overall configuration and the control unit  1500  of the image forming apparatus  1  are similar to those in the above-described first and second exemplary embodiments, and detailed descriptions thereof will be omitted. 
     &lt;Controlling Time when Recording Material Passes Through Common Conveyance Path&gt; 
     A conveyance timing control method for conveying the document sheet G in the common conveyance path  85  when performing two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S according to the exemplary embodiment, will be described. Also in the present exemplary embodiment, when a plurality of recording materials S or document sheets G exists, a similar designation rule to that in the first exemplary embodiment is applied in descriptions to distinguish each of the plurality of recording materials S or document sheets G. 
     The order of conveyance of the document sheets G and the recording materials S in the common conveyance path  85  according to the present exemplary embodiment is similar to the order of conveyance illustrated in  FIG. 8B . In the present exemplary embodiment, the CPU  801  continuously performs reading of the document sheets G and image formation on the recording materials S while alternately conveying the document sheets G and the recording materials S in order of the document sheet G 1 , the document sheet G 2 , the recording material S 1 , the document sheet G 3 , the recording material S 2 , and so on. The present exemplary embodiment is characterized in controlling the time when the recording material S starts to be conveyed to achieve this conveyance control. 
     Operations performed by the present exemplary embodiment will be described below particularly focusing on conveyance order control for the recording material S 1  and the document sheet G 2  out of conveyance order control for the document sheets G and the recording materials S. Therefore, in the following descriptions, subsequent operations will be described in detail below on the premise that two-sided reading of the document sheet G 1  is completed, and the second side (back side) of the document sheet G 1  has been printed on the recording material S 1 . 
       FIG. 12A  illustrates a state where the recording material S 1  is ready for conveyance. Referring to  FIG. 12A , the recording material S 1  is drawn with a bold solid line without an arrow head at both ends. Hereinafter, such a bold solid line indicates that the recording material S 1  is stopped. After printing is made on the second side (back side), the recording material S 1  is conveyed toward the first discharge unit  70 , and then suspended while being pinched by the discharge roller pair  60 . Then, the recording material S 1  waits till the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  calculated by the CPU  801  (described below). The time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  refers to time with reference to the printing start time of the image forming apparatus  1 . The time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  also refers to the time when, during conveyance of the recording material S 1  from the discharge roller pair  60  through the recording material conveyance path  83  toward the common conveyance path  85 , collision between the recording material S 1  and the document sheet G 2  in the common conveyance path  85  can be avoided. The time when the recording material S 1  starts to be conveyed in recording material conveyance path  83  also refers to the time when it becomes possible for the recording material S 1  to be conveyed by following the trailing edge of the document sheet G 2 . The document sheet G 2  starts to be conveyed from the second sheet feed unit  90  to the document conveyance path  84  so that the document reading unit  100  reads image information. When the document sheet G 2  enters the common conveyance path  85  from the document conveyance path  84 , the document sheet G 2  is pinched and conveyed by the conveyance roller pair  41 . On the other hand, by the time when the document sheet G 2  enters the common conveyance path  85  and reading of the first side (front side) of the document sheet G 2  is started, the document reading unit  100  completes preparation for document reading to prepare for reading the document sheet G 2 . 
       FIG. 12B  illustrates a conveyance operation for the recording material S 1 . When the time when the recoding material S 1  starts to be conveyed in the recording material conveyance path  83  comes, the recoding material S 1  is pinched and conveyed by the discharge roller pair  60 , and advances toward the recording material conveyance path  83  and the common conveyance path  85 . The document sheet G 2  moves in the common conveyance path  85 . While the document sheet G 2  is passing the document reading unit  100 , the document reading unit  100  reads image information of the first side (front side) of the document sheet G 2 , and stores the read image information in the image memory  804 . When the recording material S 1  enters the common conveyance path  85  and the leading edge of the recording material S 1  is pinched by the conveyance roller pair  41 , the trailing edge of the document sheet G 2  has already passed the conveyance roller pair  41 . Then, the document sheet G 2  and the recording material S 1  are conveyed in the common conveyance path  85  with a sufficient conveyance interval therebetween so that the document sheet G 2  and the recording material S 1  do not collide with each other. 
     The recording material S 1  is conveyed in the recording material conveyance path  83  at the conveyance speed Vp1, and conveyed in the common conveyance path  85  at the conveyance speed Vp2. The document sheet G 2  is conveyed in the document conveyance path  84  at the conveyance speed Vs1, and conveyed in the common conveyance path  85  at the conveyance speed Vs2. In the present exemplary embodiment, the conveyance speed Vp1 is determined by an optimum conveyance speed for image formation, and the conveyance speeds Vs1 and Vs2 are determined by the optimum conveyance speed for document reading. The conveyance speeds Vp1, Vs1, and Vs2 do not change during printing operation. After the document sheet G 2  and the recording material S 1  enter the common conveyance path  85 , the CPU  801  controls the conveyance speed Vp2 of the recording material S 1  (Vp2=Vs2) so that the document sheet G 2  and the recording material S 1  are conveyed while maintaining constant distance between the leading edge of the document sheet G 2  and the trailing edge of the recording material S 1 . 
     In the present exemplary embodiment, to avoid collision between the recording material S 1  and the document sheet G 2  in the common conveyance path  85 , the CPU  801  controls only the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83 , and does not control the conveyance speed of the recording material S 1  in the recording material conveyance path  83 . In other words, the CPU  801  controls only the timing for starting conveyance of the recording material S 1  from the reversing start position. Subsequently, the CPU  801  reads the second side (back side) of the document sheet G 2 , prints the first side (front side) of the document sheet G 1  on the recording material S 1 , and prints the second side (back side) of the document sheet G 2  on the recording material S 2 . However, subsequent operations are performed in the above-described process illustrated in  FIGS. 2C and 2E , and detailed descriptions thereof will be omitted. This completes descriptions of the conveyance timing control method for conveying the recording material S in the common conveyance path  85  when performing two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S. 
     &lt;Descriptions of Recording Material and Document Conveyance Operations&gt; 
       FIG. 13  is a timing chart illustrating conveyance operations for the recording material S 1  and the document sheet G 2 . Components having the same function as those in the first and second exemplary embodiments are assigned the same reference numeral, and redundant descriptions thereof will be omitted. According to the configuration of the present exemplary embodiment, the CPU  801  controls the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  to control the order of the recording material S 1  and the document sheet G 2  passing through the common conveyance path  85 . Since the CPU  801  performs control to convey the document sheet G 2  in the common conveyance path  85  earlier than the recording material S 1 , the time t 202  when the recording material S 1  starts passing the conveyance roller pair  41  is later than the time t 201  when the document sheet G 2  has passed the conveyance roller pair  41 , as illustrated in  FIG. 13 . The CPU  801  calculates the required times T 135 , T 137 , and T 203 , and the times t 131 , t 201 , and t 202  based on the time t 132  when the document sheet G 2  starts to be conveyed in the document conveyance path  84 , the recording material conveyance speeds Vp1 and Vp2, and the document conveyance speeds Vs1 and Vs2, which are known information. Other processing is similar to that in the above-described exemplary embodiments, and detailed descriptions thereof will be omitted. 
     &lt;Processing for Determining Start of Recording Material Conveyance&gt; 
     The processing by the CPU  801  for determining whether conveyance of the recording material S 1  is started will be described with reference to  FIG. 14 . Basic control is similar to that in the first exemplary embodiment. Steps having the same processing as those in the first exemplary embodiment illustrated in  FIG. 7  are assigned the same step numbers, and redundant descriptions thereof will be omitted. The points different from the first exemplary embodiment will be described. In step S 2501 , by using a similar method to that in the first exemplary embodiment, the CPU  801  acquires the time t 132  when the document sheet G 2  starts to be conveyed in the document conveyance path  84 , which is known information. In step S 2502 , similar to the first exemplary embodiment, the CPU  801  calculates the required time T 135  from the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  to the time when the recording material S 1  starts passing the conveyance roller pair  41 . Similar to the first and second exemplary embodiments, the CPU  801  calculates the required time T 137  from the time when the document sheet G 2  starts to be conveyed in the document conveyance path  84  to the time when the document sheet G 2  starts passing the conveyance roller pair  41 , and the required time T 203  since the time when the document sheet G 2  starts passing the conveyance roller pair  41  till the time when the document sheet G 2  has passed the conveyance roller pair  41 . 
     In step S 2503 , the CPU  801  adds the required times T 137  and T 203  to the time t 132  to calculate the time t 201  when the document sheet G has passed the conveyance roller pair  41  (t 201 =t 132 +T 137 +T 203 ). As described above, it is necessary to control the time when the recording material S 1  restarts to be conveyed so that the time t 202  when the recording material S 1  starts passing the conveyance roller pair  41  is later than the time t 201 . In step S 2504 , therefore, the CPU  801  calculates the time t 202  that satisfies t 201 ≦t 202 . In step S 2505 , the CPU  801  subtracts from the time t 202  (calculated in step S 2504 ) the required time T 135  (calculated in S 2502 ) to calculate the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  (t 131 =t 202 −T 135 ). 
     In step S 2506 , referring to a timer (not illustrated), the CPU  801  determines whether the present time has passed the time t 131 . When the CPU  801  determines that the present time has not yet passed the time t 131  (NO in step S 2506 ), the CPU  801  waits for a predetermined period of time in step S 1407 , and the processing returns to step S 2506 . On the other hand, when the CPU  801  determines that the present time has passed the time t 131  (YES in step S 2506 ), the CPU  801  starts conveyance of the recording material S 1 . This completes descriptions of the processing by the CPU  801  for determining whether conveyance of the recording material S 1  is started. 
     According to the configuration of the present exemplary embodiment, the CPU  801  stores information about the time t 132  when the document sheet G 2  starts to be conveyed in the document conveyance path  84  with reference to the printing start time, and uses the information to calculate the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83 . However, it is also possible to provide a sensor in the middle of the conveyance path of the document sheet G 2 , and calculate the time t 131  based on the time when the document sheet G 2  has passed the sensor. In addition, according to the configuration of the present exemplary embodiment, the CPU  801  suspends the recording material S 1  being pinched by the discharge roller pair  60  during conveyance, and controls the time t 131  when the recording material S 1  subsequently starts to be conveyed in the recording material conveyance path  83 . However, the present exemplary embodiment may be implemented with a configuration in which the CPU  801  controls the time when the recording material S 1  starts to be supplied from the first sheet feed unit  30 . 
     According to the configuration of the present exemplary embodiment, conveyance timing control for conveying the recording material S 1  in the recording material conveyance path  83  can be performed. As a result, the conveyance order and the conveyance interval of the document sheet G and the recording material S can be accurately controlled. Thus, it is possible to achieve a process of two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S with high productivity, without requiring a complicated apparatus configuration. As described above, according to the present exemplary embodiment, it is possible to prevent degradation of the productivity in a case where a document reading operation and a printing operation are performed in parallel based on a configuration in which a document reading unit is provided in the middle of a recording material conveyance path. 
     In the third exemplary embodiment, the document sheet G passes through the common conveyance path  85  earlier than the recording material S, and the timing for conveying the document sheet G in the common conveyance path  85  is adjusted only with the relevant recording material conveyance start time. A fourth exemplary embodiment differs from the first exemplary embodiment in that the recording material S passes through the common conveyance path  85  earlier than the document sheet G, and the conveyance timing for the common conveyance path  85  is adjusted with both the relevant recording material conveyance speed and the relevant recording material conveyance start time. The overall configuration and the control unit  1500  of the image forming apparatus  1  are similar to those in the above-described configurations, and detailed descriptions thereof will be omitted. 
     &lt;Conveyance Timing Control for Recording Material Conveyance in Common Conveyance Path&gt; 
     A conveyance timing control method for conveying the recording material S G in the common conveyance path  85  when performing two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S, will be described. Also in the present exemplary embodiment, when a plurality of recording materials S or document sheets G exists, a similar designation rule to that in the first exemplary embodiment is applied in descriptions to distinguish each of the plurality of recording materials S or document sheets G. 
     The order of conveyance of the document sheets G and the recording materials S in the common conveyance path  85  according to the present exemplary embodiment is similar to the order of conveyance illustrated in  FIG. 8B . The document sheets G and the recording materials S are conveyed in order of the document sheet G 1 , the document sheet G 2 , the recording material S 1 , document sheet G 3 , the recording material S 2 , the document sheet G 4 , and so on. In the second exemplary embodiment, the document sheets G and the recording materials S are conveyed in order of a combination of the document sheet G 2  and the recording material S 1 , a combination of the document sheet G 3  and the recording material S 2 , and so on. In the present exemplary embodiment, the recording material S is conveyed in the common conveyance path  85  earlier than the document sheet G, i.e., in order of a combination of the recording material S 1  and the document sheet G 3 , a combination of the recording material S 2  and the document sheet G 4 , and so on. The CPU  801  continuously performs reading of the document sheets G and image formation on the recording materials S while alternately conveying the document sheets G and the recording materials S in the above-described order. The present exemplary embodiment is characterized in controlling both the relevant recording material conveyance start time and the relevant conveyance speed of the recording material S to achieve this conveyance control. 
     Operations performed according to the present exemplary embodiment will be described below particularly focusing on conveyance order control for the recording material S 1  and the document sheet G 3  out of conveyance order control for the document sheets G and the recording materials S. Therefore, in the following descriptions, subsequent operations will be described in detail below on the premise that two-sided reading of the document sheet G 1  is completed, the first side (front side) of the document sheet G 2  has been read, and the second side (back side) of the document sheet G 1  has been printed on one side of the recording material S 1 . 
       FIG. 15A  illustrates a state where the recording material S 1  is ready for conveyance. After printing is made on the second side (back side), the recording material S 1  is conveyed toward the first discharge unit  70 , and then suspended while being pinched by the discharge roller pair  60 . Then, the recording material S 1  waits till the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  calculated by the CPU  801  (described below). The time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  refers to the time when, during conveyance of the recording material S 1  from the discharge roller pair  60  through the recording material conveyance path  83  toward the common conveyance path  85 , collision between the recording material S 1  and the document sheet G 3  in the common conveyance path  85  can be avoided. The time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  refers to the time when it becomes possible for the document sheet G 3  to be conveyed by following the trailing edge of the recording material S 1 . The document sheet G 2  moves in the document conveyance path  81 , and advances toward the second discharge unit  110 . The image reading unit  100  reads image information of the second side (back side) of the document sheet G 2 , and stores the read image information in the image memory  804 . 
       FIG. 15B  illustrates an operation of the recording material S 1  to enter the common conveyance path  85 . When time has elapsed, and the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  comes, the recording material S 1  is conveyed in the recording material conveyance path  83  while being pinched by the discharge roller pair  60 , and then enters the common conveyance path  85 . The document sheet G 2  is discharged onto the second discharge unit  110 . When the recording material S 1  is conveyed at the conveyance speed Vp1, the conveyance speed Vp1 is adjusted to a conveyance speed at which the document sheet G 3  can be conveyed by following the trailing edge of the recording material S 1  while avoiding collision between the recording material S 1  and the document sheet G 3  in the common conveyance path  85 . 
       FIG. 15C  illustrates a conveyance operation for the recording material S 1 . The document sheet G 3  starts to be conveyed from the second sheet feed unit  90  through the document conveyance path  84  toward the common conveyance path  85  by the document pickup roller  91  and the separation member  92 . When the document sheet G 3  enters the common conveyance path  85 , the document sheet G 3  is pinched by the conveyance roller pair  41 . When the leading edge of the document sheet G 3  is pinched by the conveyance roller pair  41 , the trailing edge of the recording material S 1  has already passed the conveyance roller pair  41 . Then, the document sheet G 3  and the recording material S 1  are conveyed in the common conveyance path  85  with a sufficient conveyance interval therebetween so that the document sheet G 3  and the recording material S 1  do not collide with each other. On the other hand, by the time when the document sheet G 3  enters the common conveyance path  85  and reading of the first side (front side) of the document sheet G 3  is started, the document reading unit  100  completes preparation for document reading to prepare for reading the document sheet G 3 . While the document sheet G 3  is passing the document reading unit  100 , the document reading unit  100  reads image information of the first side (front side) of document sheet G 3 , and stores the read image information in the image memory  804 . 
     The document sheet G 3  is conveyed in the document conveyance path  84  at the conveyance speed Vs1, and conveyed in the common conveyance path  85  at the conveyance speed Vs2. Similar to the third exemplary embodiment, the conveyance speeds Vs1 and Vs2 are controlled based on the optimum conveyance speed for document reading. However, in the present exemplary embodiment, to avoid collision between the recording material S 1  and the document sheet G 3  in the common conveyance path  85 , the CPU  801  controls both the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83 , and the conveyance speed of the recording material S 1  in the recording material conveyance path  83 . More specifically, the CPU  801  controls the timing for starting conveyance of the recording material S 1  from the reversing start position, and controls the conveyance speed Vp1 of the recording material S 1  during conveyance from the reversing start position to the common conveyance path  85 . Subsequently, the CPU  801  reads the second side (back side) of document sheet G 3 , and prints the first side (front side) of the document sheet G 1  on the recording material S 1 . However, subsequent operations are performed in the above-described process illustrated in  FIGS. 2C and 2E , and detailed descriptions thereof will be omitted. This completes descriptions of a conveyance control method for conveying the recording material S in the common conveyance path  85  when performing two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S. 
     &lt;Descriptions of Recording Material and Document Conveyance Operations&gt; 
       FIG. 16  is a timing chart illustrating conveyance operations for the recording material S 1  and the document sheet G 3 . In the present exemplary embodiment, timings (d) to (f) indicate states of the document sheet G 3 . According to the configuration of the present exemplary embodiment, the time t 133  when the recording material S 1  has passed the conveyance roller pair  41  precedes the time t 134  when the document sheet G 3  starts passing the conveyance roller pair  41 . The CPU  801  calculates the required times T 135 , T 136 , and T 137 , and the times t 131 , t 133 , and t 134  based on the time t 132  when the document sheet G 3  starts to be conveyed in the document conveyance path  84 , the recording material conveyance speed Vp2, and the document conveyance speeds Vs1 and Vs2 which are known information. Other processing is similar to that in the above-described exemplary embodiments, and detailed descriptions thereof will be omitted. 
     &lt;Processing for Determining Start of Recording Material Conveyance&gt; 
     The processing performed by the CPU  801  for determining whether conveyance of the recording material S 1  is started will be described with reference to  FIG. 17 . Basic control is similar to that in the third exemplary embodiment. Steps having the same processing as those in the third exemplary embodiment illustrated in  FIG. 14  are assigned the same step numbers, and redundant descriptions thereof will be omitted. The points different from the third exemplary embodiment will be described. Steps having the same processing as those in the first exemplary embodiment illustrated in  FIG. 7  are assigned the same step numbers, and redundant descriptions thereof will be omitted. In step S 3001 , by using a similar method to that in the third exemplary embodiment, the CPU  801  acquires the time t 132  when the document sheet G 3  starts to be conveyed in the document conveyance path  84 , which is known information. In step S 3002 , the CPU  801  calculates the required time T 136  from the time when the recording material S 1  starts passing the conveyance roller pair  41  to the time when the recording material S 1  has passed the conveyance roller pair  41 , and the required time T 137  from the time when the document sheet G 3  starts to be conveyed in the document conveyance path  84  to the time when the document sheet G 3  starts passing the conveyance roller pair  41 . The CPU  801  calculates the required times T 136  and T 137  in a similar way to the first to third exemplary embodiments. 
     In step S 3003 , the CPU  801  adds the required time T 137  (calculated in step S 3002 ) to the time t 132  to calculate the time t 134  when the document sheet G 3  starts passing the conveyance roller pair  41  (t 134 =t 132 +T 137 ). As described above, it is necessary to control the time when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  so that the time t 133  when the recording material S 1  has passed the conveyance roller pair  41  precedes the time t 134 . In step S 3004 , therefore, the CPU  801  calculates the time t 133  that satisfies t 133 ≦t 134 . In step S 3005 , the CPU  801  calculates the conveyance speed Vp1, the required time T 135 , and the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83 . In the present exemplary embodiment, it is necessary to perform control so that the time t 131  is later than the time t 132 . In step S 3005 , the CPU  801  calculates the conveyance speed Vp1 adjusted so that the time t 131  is later than the time t 132 , and then calculates the required time T 135  based on the calculated conveyance speed Vp1 and the conveyance path length between the discharge roller pair  60  and the conveyance roller pair  41 . Then, the CPU  801  subtracts from the time t 133  the required time T 135  and the required time T 136  (calculated in step S 3002 ) to calculate the time t 131  when the recording material S 1  starts to be conveyed in the recording material conveyance path  83  (t 131 =t 133 −T 136 −T 135 ). Subsequent processing is similar to that in the third exemplary embodiment, and redundant descriptions thereof will be omitted. This completes descriptions of the method for determining the conveyance timing for the recording material S 1  performed by the CPU  801 . 
     According to the configuration of the present exemplary embodiment, conveyance timing control and conveyance speed control for conveying the recording material S 1  in the recording material conveyance path  83  can be performed. As a result, the conveyance order and the conveyance interval of the document sheet G and the recording material S can be accurately controlled. The CPU  801  adjusts both the time t 131  when the recording material S 1  starts to be conveyed and the conveyance speed Vp1. Thus, it is possible to achieve a process of two-sided reading of image information of a plurality of document sheets G and continuous two-sided printing of the read image information on a plurality of recording materials S with higher productivity than in the third exemplary embodiment, without requiring a complicated apparatus configuration. As described above, according to the present exemplary embodiment, it is possible to prevent degradation of the productivity in a case where a document reading operation and a printing operation are performed in parallel based on a configuration in which a recording material and a document sheet share a common conveyance path. 
     Although the above-described exemplary embodiments premise a configuration of a monochrome image forming apparatus, the present invention is also applicable to a color image forming apparatus. The present invention is applicable to a color image forming apparatus in which photosensitive drums (image bearing members) for forming images of respective colors (yellow, magenta, cyan, and black) are arranged in a row, and images formed on the respective photosensitive drums are transferred onto a recording material or an intermediate transfer member. The present invention is also applicable to a color image forming apparatus in which images of respective colors are sequentially formed on one image bearing member (photosensitive drum), and a color image formed on an intermediate transfer member is transferred onto a recording material. 
     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 such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2012-205856, filed Sep. 19, 2012, which is hereby incorporated by reference herein in its entirety.