Patent Publication Number: US-10324410-B2

Title: Image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Applications No. 2016-088871 and No. 2016-088874 filed Apr. 27, 2016, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     The present disclosure relates to an image forming apparatus that prints an image on a paper sheet. 
     Conventionally, there is known an image forming apparatus in which a paper sheet is conveyed along a paper conveying path, and an image is printed on the paper sheet that is being conveyed. 
     The conventional image forming apparatus is equipped with a paper feeding unit such as a paper feed roller, which feeds a paper sheet stored in a paper cassette to a paper conveying path so as to convey the paper sheet to a registration position. In addition, a registration unit such as a registration roller is disposed at the registration position so as to convey the paper sheet from the registration position to a printing position. Then, an image forming unit prints an image on the paper sheet that is being conveyed when the paper sheet passes the printing position. 
     SUMMARY 
     An image forming apparatus according to a first aspect of the present disclosure includes a paper conveying path, a registration unit, a first paper feeding unit, a second paper feeding unit, and a control unit. The paper conveying path is for conveying a paper sheet to a printing position via a registration position. The registration unit operates to convey the paper sheet after reaching the registration position to the printing position. The first paper feeding unit operates to convey the paper sheet to the registration position. The second paper feeding unit operates to convey the paper sheet to the registration position. The control unit selects one of the first paper feeding unit and the second paper feeding unit as a target paper feeding unit for conveying the paper sheet to be printed, sets a preceding feeding time, performs a primary paper feeding in which the registration unit is not operated but the target paper feeding unit is operated so that the paper sheet is conveyed to the registration position, finishes the primary paper feeding at timing when the paper sheet reaches the registration position, and then after operating the target paper feeding unit without operating the registration unit until the preceding feeding time elapses, performs a secondary paper feeding in which the registration unit is operated so that the paper sheet after reaching the registration position is conveyed to the printing position. The control unit sets different values of the preceding feeding time for the case where the first paper feeding unit is the target paper feeding unit and the case where the second paper feeding unit is the target paper feeding unit. 
     An image forming apparatus according to a second aspect of the present disclosure includes a paper conveying path, a paper feeding unit, a registration unit, an image forming unit, and a control unit. The paper conveying path is for conveying a paper sheet from a paper feeding position to a printing position via a registration position placed above the paper feeding position. The paper feeding unit operates to convey the paper sheet from the paper feeding position to the registration position. The registration unit operates to convey the paper sheet after reaching the registration position to the printing position. The image forming unit forms an image to be printed on the paper sheet and prints the image on the paper sheet conveyed to the printing position. The control unit performs a primary paper feeding in which the registration unit is not operated but the paper feeding unit is operated so that the paper sheet is conveyed from the paper feeding position to the registration position, finishes the primary paper feeding at a predetermined time after the paper sheet reaches the registration position, and then after a printing preparation is done for printing the image on the paper sheet, performs a secondary paper feeding in which the registration unit and the paper feeding unit are operated so that the paper sheet after reaching the registration position is conveyed to the printing position. The control unit measures waiting time after the primary paper feeding is finished until the printing preparation is done. When the printing preparation is done, if the measured time as the waiting time is longer than a predetermined threshold time, the control unit starts to operate the paper feeding unit first, and then starts to operate the registration unit while continuing the operation of the paper feeding unit, so as to start the secondary paper feeding. If the measured time is the threshold time or less, the control unit starts to operate both the paper feeding unit and the registration unit at the same time, so as to start the secondary paper feeding. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an overall structure of an image forming apparatus according to a first embodiment of the present disclosure. 
         FIG. 2  is a diagram illustrating a hardware structure of the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG. 3  is a timing chart for explaining a paper conveying operation (of a first paper feeding unit) of the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG. 4  is a timing chart for explaining the paper conveying operation (of a second paper feeding unit) of the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG. 5  is a diagram illustrating a paper sheet position at a time point when primary paper feeding is finished in the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG. 6  is a diagram illustrating a paper sheet position at a time point when primary paper feeding is finished in the image forming apparatus according to the first embodiment of the present disclosure (a diagram illustrating a first state and a second state). 
         FIG. 7  is a table showing an execution time of preceding paper feeding (preceding feeding time) performed in the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG. 8  is a table showing the execution time of the preceding paper feeding (the preceding feeding time) performed in the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG. 9  is a flowchart for explaining the paper conveying operation of the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG. 10  is a diagram illustrating an overall structure of an image forming apparatus according to a second embodiment of the present disclosure. 
         FIG. 11  is a diagram illustrating a hardware structure of the image forming apparatus according to the second embodiment of the present disclosure. 
         FIG. 12  is a timing chart for explaining a paper conveying operation of the image forming apparatus according to the second embodiment of the present disclosure. 
         FIG. 13  is a diagram for explaining a backslide of the front edge of the paper sheet that occurs at a registration position in the image forming apparatus according to the second embodiment of the present disclosure. 
         FIG. 14  is a timing chart for explaining the paper conveying operation of the image forming apparatus according to the second embodiment of the present disclosure. 
         FIG. 15  is a table showing timing information stored in a storage unit of the image forming apparatus according to the second embodiment of the present disclosure. 
         FIG. 16  is a flowchart for explaining the paper conveying operation of the image forming apparatus according to the second embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an image forming apparatus according to an embodiment of the present disclosure is described with an example of a laser printer. 
     First Embodiment 
     &lt;Structure of Image Forming Apparatus&gt; 
     As illustrated in  FIG. 1 , an image forming apparatus  100  ( 101 ) of a first embodiment has a paper conveying path  1  (shown by a broken line) for conveying a paper sheet P. This paper conveying path  1  extends from a paper feeding position P 1  to a discharge tray ET via a registration position P 2 , a printing position P 3 , and a fixing position P 4  in this order. Here, the registration position P 2  is positioned above the paper feeding position P 1 , the printing position P 3  is positioned above the registration position P 2 , and the fixing position P 4  is positioned above the printing position P 3 . In other words, the paper conveying path  1  is formed to extend from the lower part to the upper part in the apparatus main body (the paper sheet P is vertically conveyed). Note that a part of the paper conveying path  1 , which extends from the downstream position of the paper feeding position P 1  in the conveying direction to the registration position P 2 , is formed to extend in a substantially vertical direction (in an up and down direction). 
     In addition, the image forming apparatus  101  includes a printing unit  2  that feeds the paper sheet P to the paper conveying path  1  to convey the same and prints an image on the paper sheet P that is being conveyed. This printing unit  2  is constituted of a paper feeding unit  3  (a first paper feeding unit  3 A and a second paper feeding unit  3 B), a paper conveying unit  4 , an image forming unit  5  and a fixing unit  6 . 
     The paper feeding unit  3  includes a paper cassette  31  and a paper feed roller  32 , feeds the paper sheet P to the paper conveying path  1 , and conveys the paper sheet P along the paper conveying path  1 . The paper cassette  31  stores the paper sheets P to be used for printing. This paper cassette  31  is disposed at the lower part of the image forming apparatus  100  and is attachable and detachable with respect to the image forming apparatus  100 . Further, when the paper cassette  31  runs out of the paper sheets P, the paper cassette  31  is drawn out of the image forming apparatus  100 , and the paper sheets P are supplied to the paper cassette  31 . 
     In the paper cassette  31 , there is disposed a lift plate  33  that can move in the up and down direction. Further, the paper sheets P in the paper cassette  31  are placed on the lift plate  33 . The lift plate  33  is supported by walls of the paper cassette  31  so as to pivot at an upstream side end in a paper feed direction. For example, although not illustrated, the lift plate  33  moves (rotates) upward by a biasing force of a biasing member such as a spring. Alternatively, the lift plate  33  moves (rotates) upward by a drive force from a motor. In addition, a friction pad  34  made of cork or rubber is disposed on the upper surface of the lift plate  33  at a downstream side end in the paper feed direction. 
     In addition, the paper cassette  31  is provided with a regulating guide  35  that abuts sides of the paper sheets P so as to position the paper sheets P. This regulating guide  35  is slidable, and a size (conveying direction length) of the paper sheets P that can be stored in the paper cassette  31  is changed by sliding the regulating guide  35 . Note that the regulating guide  35  includes a first cursor for positioning in a width direction of the paper sheet P (perpendicular to the paper feed direction) and a second cursor for positioning the rear edge of the paper sheet P. 
     The paper feed roller  32  is disposed at the paper feeding position P 1  (feed port of the paper sheet P in the paper conveying path  1 ). Further, the lift plate  33  is disposed below the paper feed roller  32  so that the downstream side end thereof in the paper feed direction (the friction pad  34 ) faces the paper feed roller  32 . Therefore, when the lift plate  33  moves upward, the downstream side end of the lift plate  33  in the paper feed direction becomes close to the paper feed roller  32  so that the top paper sheet P among a plurality of paper sheets P placed on the lift plate  33  contacts with the paper feed roller  32 . In other words, the plurality of paper sheets P are sandwiched between the paper feed roller  32  and the friction pad  34 . Note that, although not illustrated, the paper feed roller  32  is biased toward the lift plate  33  (biased downward) by a biasing member such as a spring. 
     For example, the paper sheets P are set in the paper cassette  31 , and the paper cassette  31  is attached to the image forming apparatus  100 . Then, the lift plate  33  moves upward, and the top one of the paper sheets P on the lift plate  33  contacts with the paper feed roller  32 . In other words, the paper feed roller  32  always contacts with the top one of the paper sheets P on the lift plate  33 . Note that, if there is no paper sheet P on the lift plate  33 , the paper feed roller  32  contacts with a paper sheet placing surface (upside) of the lift plate  33 . 
     When feeding the paper sheet P to the paper conveying path  1 , the paper feed roller  32  rotates (the paper feeding unit  3  operates) in the state where the top one of the paper sheets P on the lift plate  33  contacts with the paper feed roller  32 . When the paper feed roller  32  rotates, the top one of the paper sheets P on the lift plate  33  is drawn out and is fed to the paper conveying path  1  (sent in the paper feed direction). In other words, the paper sheet P is conveyed from the paper feeding position P 1  to the registration position P 2 . In this case, because the bottom paper sheet P on the lift plate  33  contacts with the friction pad  34 , the bottom paper sheet P is hardly displaced, and firmness of the paper sheet bundle (the bundle of the plurality of paper sheets P on the lift plate  33 ) is secured. 
     Here, in the first embodiment, an optional sheet feeding device  10 B is additionally attached to an apparatus main body  10 A of the image forming apparatus  101 . In the structure of the apparatus main body  10 A to which the optional sheet feeding device  10 B is attached, the paper conveying path  1  ( 1   a ) formed in the apparatus main body  10 A and the paper conveying path  1  ( 1   b ) formed in the optional sheet feeding device  10 B are connected to each other. 
     In addition, in the structure of the apparatus main body  10 A to which the optional sheet feeding device  10 B is attached, the paper feeding unit  3  is separated into the first paper feeding unit  3 A and the second paper feeding unit  3 B. The first paper feeding unit  3 A is a paper feeding unit of the apparatus main body  10 A (a paper feeding unit that is normally provided to the apparatus main body  10 A), and the second paper feeding unit  3 B is a paper feeding unit of the optional sheet feeding device  10 B. Note that the paper feeding position P 1  of the first paper feeding unit  3 A is denoted by P 11 , and the paper feeding position P 1  of the second paper feeding unit  3 B is denoted by P 12  in  FIG. 1 . 
     For example, the optional sheet feeding device  10 B is attached to the lower part of the apparatus main body  10 A. Therefore, the second paper feeding unit  3 B is positioned below the first paper feeding unit  3 A. Further, the first paper feeding unit  3 A conveys the paper sheet P from the paper feeding position P 11  to the registration position P 2 . The second paper feeding unit  3 B conveys the paper sheet P from the paper feeding position P 12  positioned below the paper feeding position P 11  to the registration position P 2 . 
     The first paper feeding unit  3 A and the second paper feeding unit  3 B have substantially the same structure. In other words, each of the first paper feeding unit  3 A and the second paper feeding unit  3 B includes the paper cassette  31 , the paper feed roller  32 , the lift plate  33 , the friction pad  34 , and the regulating guide  35 . However, the second paper feeding unit  3 B further includes an intermediate roller pair  36  unlike the first paper feeding unit  3 A. 
     The intermediate roller pair  36  is disposed in the paper conveying path  1   b  of the optional sheet feeding device  10 B. Specifically, the intermediate roller pair  36  is disposed at a position on the downstream side in the conveying direction of the paper feed roller  32  of the second paper feeding unit  3 B. In this structure, the paper feed roller  32  corresponds to an “upstream side roller unit”, and the intermediate roller pair  36  corresponds to a “downstream side roller unit”. 
     The intermediate roller pair  36  is a pair of rollers  36   a  and  36   b  that are pressed to contact with each other. The one roller  36   a  is attached to an open/close cover  37 . Note that the open/close cover  37  is a cover that is attached in a rotatable (openable and closable) manner about a rotation shaft  37   a . When this open/close cover  37  is opened, the inside of the paper conveying path  1   b  of the optional sheet feeding device  10 B can be exposed. Therefore, if jamming (paper jam) occurs, a user opens the open/close cover  37  to perform unjamming treatment. 
     In the case where the one roller  36   a  is attached to the open/close cover  37 , when the open/close cover  37  is opened, the one roller  36   a  moves to separate from the other roller  36   b . Therefore, if jamming occurs, by opening the open/close cover  37 , the intermediate roller pair  36  nipping the paper sheet P is released, and hence the paper sheet P can be easily taken out. Then, when the open/close cover  37  is closed, the one roller  36   a  is pressed to contact with the other roller  36   b.    
     Note that the smallest length in the conveying direction of the paper sheet P that can be fed by the first paper feeding unit  3 A is equal to or more than the conveying distance of the paper sheet P from the paper feeding position P 11  to the registration position P 2  (for example, it is the conveying distance plus a margin). In addition, the smallest length in the conveying direction of the paper sheet P that can be fed by the second paper feeding unit  3 B is equal to or more than the conveying distance of the paper sheet P from the position of the intermediate roller pair  36  to the registration position P 2  (for example, the conveying distance plus a margin). 
     The paper conveying unit  4  includes a plurality of conveying roller pairs  41 , so as to convey the paper sheet P along the paper conveying path  1 . The plurality of conveying roller pairs  41  are disposed in the paper conveying path  1   a  of the apparatus main body  10 A. Here, among the plurality of conveying roller pairs  41 , the conveying roller pair  41  disposed at the registration position P 2  is a registration roller pair (hereinafter denoted by  42 ). Note that the registration roller pair  42  corresponds to a “registration unit”. 
     The registration roller pair  42  is stopped to rotate (operate) at a time point when the paper sheet P reaches the registration position P 2 . Then, after the paper sheet P reaches the registration position P 2 , the registration roller pair  42  rotates. In this way, the paper sheet P that has reached the registration position P 2  is conveyed to the printing position P 3 . 
     The image forming unit  5  includes a photosensitive drum  51 , an electrifying device  52 , an exposing device  53 , a developing device  54 , a transfer roller  55 , and a cleaning device  56 . Further, the image forming unit  5  forms a toner image (image) based on image data of an image to be printed and transfers (prints) the toner image onto the paper sheet P that is conveyed to the printing position P 3 . 
     When the toner image is formed, the photosensitive drum  51  rotates, and the electrifying device  52  electrifies the surface of the photosensitive drum  51  at a predetermined potential. In addition, the exposing device  53  includes a light emitting element (not shown) that emits a light beam for exposure, so as to scan and expose the surface of the photosensitive drum  51  while turning on and off the light emitting element. In this way, an electrostatic latent image is formed on the surface of the photosensitive drum  51 . The developing device  54  supplies toner to the electrostatic latent image formed on the surface of the photosensitive drum  51  so as to develop the electrostatic latent image into a toner image. 
     The transfer roller  55  is pressed to contact with the surface of the photosensitive drum  51  so that a transfer nip is formed between the transfer roller  55  and the photosensitive drum  51  at the printing position P 3 . Further, when the paper sheet P passes through this transfer nip, the toner image on the surface of the photosensitive drum  51  is transferred onto the paper sheet P. The cleaning device  56  removes toner and the like remaining on the surface of the photosensitive drum  51 . 
     The fixing unit  6  includes a fixing roller pair (a heating roller  61  and a pressure roller  62 ), so as to heat and press the toner image on the paper sheet P for fixing the toner image. The heating roller  61  includes a built-in heat source. The pressure roller  62  is pressed to contact with the heating roller  61  so that a fixing nip is formed between the pressure roller  62  and the heating roller  61  at the fixing position P 4 . Further, when the paper sheet P passes through this fixing nip, the toner image on the paper sheet P is fixed. After that, the paper sheet P goes out of the fixing unit  6  and is discharged onto the discharge tray ET. 
     Note that the image forming apparatus  101  is equipped with an operation panel  7 . The operation panel  7  includes a display unit  71  and an operation unit  72 . The display unit  71  is constituted of a liquid crystal display panel and displays a message indicating a status of the image forming apparatus  101 , and the like. The operation unit  72  is constituted of hardware keys and the like so as to receive various settings from the user. 
     In addition, the image forming apparatus  101  includes a control unit  110  as illustrated in  FIG. 2 . The control unit  110  includes a CPU  111 , a storage unit  112  (memories such as a ROM and a RAM), and an image processing unit  113 . 
     The CPU  111  operates based on control programs and data so as to control individual portions of the first paper feeding unit  3 A and the second paper feeding unit  3 B. In addition, the control unit  110  controls individual portions of the paper conveying unit  4 , the image forming unit  5 , and the fixing unit  6 . Further, the control unit  110  controls the operation panel  7 . The control programs and data are stored in the storage unit  112  (are stored in the ROM in advance and are loaded into the RAM). 
     The image processing unit  113  includes, for example, an image processing circuit, an image processing memory, and the like. Further, the image processing unit  113  performs image processing such as scaling and density conversion on the image data. In addition, the image processing unit  113  performs image processing for converting the image data to be processed into image data for exposure (data for turning on and off the light emitting element of the exposing device  53 ). 
     In addition, the control unit  110  is connected to a communication unit  120 . The communication unit  120  is connected to a personal computer (PC)  200  as a user terminal that is used by the user of the image forming apparatus  100 . The PC  200  transmits an instruction to execute a print job, image data of an image to be printed, job data including a job execution condition, and the like. For example, the job data includes information indicating the paper cassette  31  designated by the user (hereinafter may be referred to as a designated cassette  31 ), information indicating a paper sheet size designated by the user, and the like. When the communication unit  120  receives the job data, the control unit  110  determines that it has received the instruction to execute a print job from the user. Then, the control unit  110  executes the print job based on the job data. 
     Here, the control unit  110  controls to drive motors for various rotating members for conveying the paper sheet P and controls the operation for conveying the paper sheet P when the job is executed. The motors include, for example, paper feed motors M 1  (M 11  and M 12 ) and a conveying motor M 2 . 
     The paper feed motor M 11  is a motor for rotating (operating) the paper feed roller  32  of the first paper feeding unit  3 A. For example, the paper feed roller  32  of the first paper feeding unit  3 A rotates when receiving a drive force from the paper feed motor M 11  via a paper feed clutch (not shown). Then, the control unit  110  controls on and off of the paper feed clutch so as to switch the paper feed roller  32  of the first paper feeding unit  3 A between rotation and stop. 
     The paper feed motor M 12  is a motor for rotating (operating) each of the paper feed roller  32  and the intermediate roller pair  36  of the second paper feeding unit  3 B. For example, the paper feed roller  32  of the second paper feeding unit  3 B rotates when receiving a drive force from the paper feed motor M 12  via the paper feed clutch (not shown). Then, the control unit  110  controls on and off of the paper feed clutch so as to switch the paper feed roller  32  of the second paper feeding unit  3 B between rotation and stop. Using this paper feed clutch, it is possible to rotate the intermediate roller pair  36  in a state where the paper feed roller  32  of the second paper feeding unit  3 B is stopped to rotate. 
     The conveying motor M 2  is a motor for rotating (operating) the plurality of conveying roller pairs  41 . For example, among the plurality of conveying roller pairs  41 , the registration roller pair  42  rotates when receiving the drive force from the conveying motor M 2  via a registration clutch (not shown). Then, the control unit  110  controls on and off of the registration clutch so as to switch the registration roller pair  42  between rotation and stop. In this way, because the conveying motor M 2  and the registration roller pair  42  are connected via the registration clutch, it is possible to rotate the other conveying roller pair  41  in a state where the registration roller pair  42  is stopped to rotate. Note that the photosensitive drum  51  and the transfer roller  55  are also rotating members for conveying the paper sheet P, and these rotating members may also be rotated by the conveying motor M 2 . 
     In addition, the control unit  110  is connected to a registration sensor RS for detecting presence or absence of the paper sheet P at a position DP before the registration position P 2  (see  FIG. 1 ). The registration sensor RS is a sensor that changes its output value in accordance with presence or absence of the paper sheet P at the detection position DP. Further, the control unit  110  detects presence or absence of the paper sheet P at the detection position DP (arrival of the front edge of the paper sheet P and passage of the rear end of the same) based on an output value of the registration sensor RS. For example, based on a result of the detection, the control unit  110  determines whether or not the front edge of the paper sheet P has reached the registration position P 2 . 
     Further, the control unit  110  is connected to a size sensor SS for detecting a size of the paper sheets P stored in the paper cassette  31 . The size sensor SS is a sensor that changes its output value in accordance with a position of the regulating guide  35 . Further, on the basis of an output value of the size sensor SS, the control unit  110  detects a size (conveying direction length) of the paper sheets P stored in the paper cassette  31  attached to each of the apparatus main body  10 A and the optional sheet feeding device  10 B. 
     &lt;Primary Paper Feeding and Secondary Paper Feeding&gt; 
     When the control unit  110  receives the instruction to execute a print job (When the communication unit  120  receives job data), the control unit  110  performs image processing (such as conversion into the image data for exposure) on the image data. In addition, the control unit  110  recognizes the designated cassette  31  based on the job data. For example, the control unit  110  recognizes the paper cassette  31  storing the paper sheets P of the paper sheet size designated by the user, as the designated cassette  31 . Then, the control unit  110  selects the paper feeding unit  3 , from the first paper feeding unit  3 A and the second paper feeding unit  3 B, for feeding the paper sheet P in the designated cassette  31  (the paper sheet P to be printed) to the paper conveying path  1  so as to convey the same, as the paper feeding unit to be operated when executing the job. Hereinafter, the paper feeding unit selected by the control unit  110  is referred to as a target paper feeding unit  3 . 
     When selecting the target paper feeding unit  3 , the control unit  110  sets the preceding feeding time. Further, the control unit  110  performs the primary paper feeding. After that, the control unit  110  performs secondary paper feeding. Hereinafter, with reference to timing charts illustrated in  FIGS. 3 and 4 , the primary paper feeding and the secondary paper feeding are described. 
     First, with reference to  FIG. 3 , a case where the first paper feeding unit  3 A is selected as the target paper feeding unit  3  is described. When receiving the instruction to execute a print job (time point TP 1 ), the control unit  110  starts the primary paper feeding. In this case, the control unit  110  drives the paper feed motor M 11  so as to rotate the paper feed roller  32  of the first paper feeding unit  3 A. In addition, the control unit  110  drives the conveying motor M 2  so as to rotate the plurality of conveying roller pairs  41 . However, the control unit  110  turns off the registration clutch. In other words, the control unit  110  does not drive the registration roller pair  42  to rotate but drives only the other conveying roller pair  41  to rotate. 
     When the paper feed roller  32  of the first paper feeding unit  3 A rotates, the paper sheet P in the designated cassette  31  is fed to the paper conveying path  1 , and the paper sheet P is conveyed along the paper conveying path  1 . Then, the front edge of the paper sheet P reaches the detection position DP (time point TP 2 ). In this way, the output value of the registration sensor RS changes from L level indicating absence of the paper sheet P to H level indicating presence of the paper sheet P. In other words, the control unit  110  detects that the front edge of the paper sheet P has reached the detection position DP. 
     After detecting that the front edge of the paper sheet P has reached the detection position DP, when time necessary for the paper sheet P to travel the distance between the detection position DP and the registration position P 2  elapses (TP 3 ), the control unit  110  stops rotation of the paper feed roller  32  and finishes the primary paper feeding. In other words, the control unit  110  finishes the primary paper feeding at the timing when the front edge of the paper sheet P reaches the registration position P 2 . Note that it is possible to finish the primary paper feeding just before the front edge of the paper sheet P reaches the registration position P 2  (it is not necessary that the front edge of the paper sheet P reaches the registration position P 2  at the time point when the primary paper feeding is finished). 
     After finishing the primary paper feeding, the control unit  110  determines whether or not printing preparation for printing an image on the paper sheet P is done. For example, the control unit  110  determines that the printing preparation is done if the image processing on the image data of an image to be printed on the paper sheet P is completed so that the exposing device  53  is ready to expose the photosensitive drum  51  (to form an electrostatic latent image). 
     When the printing preparation is done (time point TP 4 ), the control unit  110  drives the paper feed roller  32  to rotate. In this case, the control unit  110  turns off the registration clutch and maintains its state (does not drive the registration roller pair  42  to rotate). In other words, at the time point when the printing preparation is done, conveyance of the paper sheet P from the registration position P 2  to the printing position P 3  is not started. 
     When rotating the paper feed roller  32  first without rotating the registration roller pair  42 , the front edge of the paper sheet P is pressed to the registration roller pair  42  that is stopped to rotate, and hence a skew of the paper sheet P is corrected. In this case, a deflection is formed in the front edge part of the paper sheet P. 
     Next, when a preceding feeding time T 1  elapses after rotation start of the paper feed roller  32  (time point TP 5 ), the control unit  110  turns on the registration clutch so that the registration roller pair  42  rotates. In other words, the control unit  110  starts the secondary paper feeding. In this way, a part of the paper sheet P after reaching the registration position P 2 , which remains at the paper feeding position P 1 , is sent out by the paper feed roller  32  in the paper feed direction, and the front edge of the paper sheet P enters the nip of the registration roller pair  42 . Then, the paper sheet P that has entered the nip of the registration roller pair  42  is conveyed by the registration roller pair  42  to the printing position P 3  (the secondary paper feeding is performed). After starting the secondary paper feeding, the control unit  110  starts the exposure of the photosensitive drum  51  by the exposing device  53  (the formation of the electrostatic latent image). 
     Further, when a predetermined time elapses from the rotation start of the registration roller pair  42 , the control unit  110  turns off the paper feed clutch so as to stop the rotation of the paper feed roller  32 . This rotation stop of the paper feed roller  32  is performed before the rear edge of the paper sheet P passes the position of the paper feed roller  32 , in order to prevent double feeding of the paper sheets P. Note that, even if the rotation of the paper feed roller  32  is stopped before the rear edge of the paper sheet P passes the position of the paper feed roller  32 , the paper sheet P has already entered the nip of the registration roller pair  42 , and hence the registration roller pair  42  continues to convey the paper sheet P. 
     Next, with reference to  FIG. 4 , the case where the second paper feeding unit  3 B is selected as the target paper feeding unit  3  is described. When receiving the instruction to execute a print job (time point TP 11 ), the control unit  110  drives the paper feed motor M 12  for starting the primary paper feeding, so as to rotate the paper feed roller  32  of the second paper feeding unit  3 B and the intermediate roller pair  36 . In addition, the control unit  110  drives the conveying motor M 2  in the state where the registration clutch is turned off, so as to rotate the conveying roller pair  41  that is other than the registration roller pair  42 . In this way, the front edge of the paper sheet P reaches the detection position DP (time point TP 12 ). 
     After it is detected that the front edge of the paper sheet P has reached the detection position DP, when the time necessary for the paper sheet P to travel the distance between the detection position DP and the registration position P 2  elapses (TP 13 ), i.e., when the front edge of the paper sheet P reaches the registration position P 2 , the control unit  110  stops to rotate the paper feed roller  32  and the intermediate roller pair  36  so as to finish the primary paper feeding. Note that if the conveying direction length of the paper sheet P is short (if the rear edge of the paper sheet P passes the position of the paper feed roller  32  before the primary paper feeding is finished), even before the primary paper feeding is finished, rotation of the paper feed roller  32  is stopped before the rear edge of the paper sheet P passes the position of the paper feed roller  32  in order to prevent the double feeding of the paper sheets P. However, the intermediate roller pair  36  is continued to rotate until the front edge of the paper sheet P reaches the registration position P 2 . 
     After the primary paper feeding is finished, when the printing preparation is done (time point TP 14 ), the control unit  110  drives the paper feed roller  32  and the intermediate roller pair  36  to rotate. However, if the rotation of the paper feed roller  32  is stopped before the primary paper feeding is finished (if the conveying direction length of the paper sheet P is short), the control unit  110  does not drive the paper feed roller  32  to rotate but drives the intermediate roller pair  36  to rotate. In this way, a skew of the paper sheet P is corrected (a deflection is formed in the front edge part of the paper sheet P). 
     Next, a preceding feeding time T 2  passes from the rotation start of the paper feed roller  32  (or the intermediate roller pair  36 ) (time point TP 15 ), the control unit  110  turns on the registration clutch so that the registration roller pair  42  rotates, for starting the secondary paper feeding. In this way, the paper sheet P that has reached the registration position P 2  is conveyed to the printing position P 3  (the secondary paper feeding is performed). 
     In the first embodiment, as illustrated in  FIGS. 3 and 4 , when the secondary paper feeding is performed, the paper feeding operation by the paper feeding unit  3  is first started. In other words, if the first paper feeding unit  3 A is the target paper feeding unit  3 , the rotation of the paper feed roller  32  is first started, and if the second paper feeding unit  3 B is the target paper feeding unit  3 , rotation of the paper feed roller  32  and rotation of the intermediate roller pair  36  (or rotation of the intermediate roller pair  36 ) is first started. Then, after that (after a skew of the paper sheet P is corrected), rotation of the registration roller pair  42  is started (the secondary paper feeding is started). Hereinafter, the control of starting the paper feeding operation by the paper feeding unit  3  prior to the start of the secondary paper feeding may be referred to as preceding paper feeding. 
     &lt;Setting of Preceding Feeding Time&gt; 
     As illustrated in  FIG. 5 , at a time point when the primary paper feeding is finished (a time point just before the preceding paper feeding is started), a part of the paper sheet P on the conveying direction downstream side of the intermediate roller pair  36  of the second paper feeding unit  3 B (see the lower part in  FIG. 5 ) becomes larger than a part of the paper sheet P on the conveying direction downstream side of the paper feed roller  32  of the first paper feeding unit  3 A (see the upper part in  FIG. 5 ). Therefore, in the preceding paper feeding in the case where the second paper feeding unit  3 B is the target paper feeding unit  3 , the paper feed roller pair  32  or the intermediate roller pair  36  easily slips on the paper sheet P. If the paper feed roller pair  32  or the intermediate roller pair  36  slips, the correction of a skew of the paper sheet P becomes insufficient. 
     In order to prevent occurrence of this inconvenience, out of the first paper feeding unit  3 A and the second paper feeding unit  3 B, the paper feeding unit that is more apt to slip on the paper sheet P when the preceding paper feeding is performed is determined in an experimental manner or a simulating manner. In this embodiment, the second paper feeding unit  3 B is determined to be the paper feeding unit that is apt to slip on the paper sheet P when the preceding paper feeding is performed. 
     Further, the control unit  110  sets the preceding feeding time in the case where the second paper feeding unit  3 B is the target paper feeding unit  3  (corresponding to the time T 2  illustrated in  FIG. 4 ) to be longer than the preceding feeding time in the case where the first paper feeding unit  3 A is the target paper feeding unit  3  (corresponding to the predetermined time T 1  illustrated in  FIG. 3 ). In this way, in the case where the second paper feeding unit  3 B is the target paper feeding unit  3 , time for correcting a skew of the paper sheet P (time for forming deflection in the front edge part of the paper sheet P) is set to be longer than in the case where the first paper feeding unit  3 A is the target paper feeding unit  3 . However, in the case where the second paper feeding unit  3 B is the target paper feeding unit  3 , start timing of the secondary paper feeding is later than that in the case where the first paper feeding unit  3 A is the target paper feeding unit  3 . 
     Although not limited particularly, in the case where the second paper feeding unit  3 B is the target paper feeding unit  3 , the start timing of the secondary paper feeding is set to be a few milliseconds to a few tens of milliseconds later than that in the case where the first paper feeding unit  3 A is the target paper feeding unit  3 . For example, the preceding feeding time is set to 90 ms in the case where the first paper feeding unit  3 A is the target paper feeding unit  3 , and the preceding feeding time is set to a time longer than 90 ms (for example, 100 ms or more) in the case where the second paper feeding unit  3 B is the target paper feeding unit  3 . 
     In addition, as illustrated in  FIG. 6 , in the case where the second paper feeding unit  3 B is the target paper feeding unit  3 , depending on the conveying direction length of the paper sheet P, there is a case of a first state where the rear edge of the paper sheet P has not yet passed the position of the paper feed roller  32  (the paper feeding position P 12 ) at the time point when the primary paper feeding is finished (the time point just before the preceding paper feeding is started), or a case of a second state where the rear edge of the paper sheet P has already passed the position of the paper feed roller  32 . In other words, there is a case where a part on the rear edge of the paper sheet P remains at the position of the paper feed roller  32  (the first state), or a case where the part on the rear edge of the paper sheet P does not remain at the position of the paper feed roller  32  (the second state). 
     For example, if the first state (see the upper part in  FIG. 6 ) is the current state at the time point when the primary paper feeding is finished, the preceding paper feeding is performed by both the paper feed roller  32  and the intermediate roller pair  36 , and hence the slip occurs more hardly when the preceding paper feeding is performed than in the case of the second state (see the lower part in  FIG. 6 ) at the time point when the primary paper feeding is finished. In other words, if the second state is the current state at the time point when the primary paper feeding is finished, the preceding paper feeding is performed by only the intermediate roller pair  36 , and hence the slip easily occurs. 
     Note that, because the one roller  36   a  of the intermediate roller pair  36  is attached to the open/close cover  37 , nip pressure of the intermediate roller pair  36  is apt to be low. Thus, if the preceding paper feeding is performed by only the intermediate roller pair  36 , the slip occurs more easily. 
     Therefore, when selecting the second paper feeding unit  3 B as the target paper feeding unit  3 , the control unit  110  sets the preceding feeding time in the case of the second state (see the lower part in  FIG. 6 ) at the time point when the primary paper feeding is finished to be longer than the preceding feeding time in the case of the first state (see the upper part in  FIG. 6 ) at the time point when the primary paper feeding is finished. In other words, in the case of the second state at the time point when the primary paper feeding is finished, the time for correcting a skew of the paper sheet P is set to be longer (so that the start timing of the secondary paper feeding is later) than that in the case of the first state at the time point when the primary paper feeding is finished. 
     Here, if the conveying direction length of the paper sheet P to be printed is longer than the conveying distance of the paper sheet P from the position of the paper feed roller  32  of the second paper feeding unit  3 B (the paper feeding position P 12 ) to the position of the registration roller pair  42  (the registration position P 2 ), it is considered that the first state is realized at the time point when the primary paper feeding is finished. On the other hand, if the conveying direction length of the paper sheet P to be printed is shorter than the conveying distance of the paper sheet P from the position of the paper feed roller  32  of the second paper feeding unit  3 B to the position of the registration roller pair  42 , it is considered that the second state is realized at the time point when the primary paper feeding is finished. 
     Therefore, the conveying distance of the paper sheet P from the position of the paper feed roller  32  of the second paper feeding unit  3 B to the position of the registration roller pair  42  is determined in advance, and the conveying distance is stored as a threshold length in the storage unit  112  in advance. Alternatively, the conveying distance plus a margin is stored as the threshold length in the storage unit  112  in advance. 
     Further, when selecting the second paper feeding unit  3 B as the target paper feeding unit  3 , the control unit  110  sets the preceding feeding time in the case where the conveying direction length of the paper sheet P to be printed is the threshold length (for example 200 mm) or less to be longer than the preceding feeding time in the case where the conveying direction length of the paper sheet P to be printed is more than the threshold length. In this way, in the case where the conveying direction length of the paper sheet P to be printed is the threshold length or less (in the case of the second state at the time point when the primary paper feeding is finished), the time for correcting a skew of the paper sheet P is set to be longer than that in the case where the conveying direction length of the paper sheet P to be printed is more than the threshold length (in the case of the first state at the time point when the primary paper feeding is finished). 
     For example, as illustrated in  FIG. 7 , if the conveying direction length of the paper sheet P to be printed is more than the threshold length, the preceding feeding time is set to 100 ms. Further, if the conveying direction length of the paper sheet P to be printed is the threshold length or less, the preceding feeding time is set to 110 ms. 
     Here, the rollers for conveying the paper sheet P are deteriorated with time. In other words, as rotation time (operation time) of the roller is longer, the roller is deteriorated more. As the deterioration of the roller becomes larger, the roller is more apt to slip on the paper sheet P. Therefore, it is preferred to control so that the preceding feeding time is longer as an accumulated value of the operation time of the target paper feeding unit  3  is larger. 
     In order to perform this control, the storage unit  112  stores operation time information indicating the accumulated values of the operation time of the first paper feeding unit  3 A and the second paper feeding unit  3 B. For example, the storage unit  112  stores information indicating the accumulated value of the rotation time of the paper feed roller  32  of the first paper feeding unit  3 A as the operation time information corresponding to the first paper feeding unit  3 A. In addition, the storage unit  112  stores information indicating the accumulated value of the rotation time of the intermediate roller pair  36  of the second paper feeding unit  3 B as the operation time information corresponding to the second paper feeding unit  3 B (information indicating the accumulated value of the rotation time of the paper feed roller  32  of the second paper feeding unit  3 B may be stored as the operation time information corresponding to the second paper feeding unit  3 B). 
     Further, the control unit  110  determines the accumulated value of the operation time of the target paper feeding unit  3  based on the operation time information and sets the preceding feeding time to a longer value as the determined accumulated value is larger. For example, the control unit  110  sets the preceding feeding time in the case where the accumulated value of the operation time of the target paper feeding unit  3  is longer than a predetermined threshold time (for example 10,000 minutes) to be longer than the preceding feeding time in the case where the accumulated value of the operation time of the target paper feeding unit  3  is the threshold time or less. 
     As an example, as illustrated in  FIG. 8 , in the case where the first paper feeding unit  3 A is the target paper feeding unit  3 , the preceding feeding time is set to 90 ms if the accumulated value of the operation time of the first paper feeding unit  3 A is the threshold time (10,000 minutes) or less, and the preceding feeding time is set to 100 ms if the accumulated value of the operation time of the first paper feeding unit  3 A is longer than the threshold time. 
     In the case where the second paper feeding unit  3 B is the target paper feeding unit  3 , the preceding feeding time is set in consideration of not only the accumulated value of the operation time of the second paper feeding unit  3 B but also the conveying direction length of the paper sheet P to be printed. In other words, if the accumulated value of the operation time of the second paper feeding unit  3 B is the threshold time (10,000 minutes) or less, and if the conveying direction length of the paper sheet P to be printed is the threshold length (200 mm) or less, the preceding feeding time is set to 110 ms, and if the conveying direction length of the paper sheet P to be printed is longer than the threshold length, the preceding feeding time is set to 100 ms. In addition, if the accumulated value of the operation time of the second paper feeding unit  3 B is longer than the threshold time, and if the conveying direction length of the paper sheet P to be printed is the threshold length or less, the preceding feeding time is set to 120 ms, and if the conveying direction length of the paper sheet P to be printed is longer than the threshold length, the preceding feeding time is set to 110 ms. 
     Note that number-of-fed-sheet information indicating the accumulated value of the number of fed paper sheets of each of the first paper feeding unit  3 A and the second paper feeding unit  3 B may be stored in the storage unit  112 , and the preceding feeding time may be set based on the number-of-fed-sheet information. In this case, as the accumulated number of fed paper sheets of the target paper feeding unit  3  is larger, the control unit  110  determines that the accumulated value of the operation time of the target paper feeding unit  3  is larger. In other words, as the accumulated number of fed paper sheets of the target paper feeding unit  3  is larger, the control unit  110  sets the preceding feeding time to be longer. 
     Hereinafter, with reference to the flowchart illustrated in  FIG. 9 , a process flow of the paper feeding operation is described. The flowchart illustrated in  FIG. 9  starts when the instruction to execute a print job is received. 
     In Step S 1 , the control unit  110  recognizes the designated cassette  31 . In this case, the control unit  110  selects the paper feeding unit for feeding the paper sheet P in the designated cassette  31  to the paper conveying path  1  so as to convey the same, from the first paper feeding unit  3 A and the second paper feeding unit  3 B, as the target paper feeding unit  3 . Further, in Step S 2 , the control unit  110  sets the preceding feeding time. After that, in Step S 3 , the control unit  110  starts the primary paper feeding. 
     Next, in Step S 4 , the control unit  110  determines whether or not the front edge of the paper sheet P has reached the registration position P 2 . As a result, if the control unit  110  determines that the front edge of the paper sheet P has reached the registration position P 2 , the process proceeds to Step S 5 . If the control unit  110  determines that the front edge of the paper sheet P has not reached the registration position P 2 , the determination process of Step S 4  is repeated. 
     When proceeding to Step S 5 , the control unit  110  finishes the primary paper feeding. After that, in Step S 6 , the control unit  110  determines whether or not the printing preparation is done. As a result, if the control unit  110  determines that the printing preparation is done, the process proceeds to Step S 7 . If the control unit  110  determines that the preparation is not done, the determination process of Step S 6  is repeated. 
     When proceeding to Step S 7 , the control unit  110  starts the preceding paper feeding. After that, in Step S 8 , the control unit  110  determines whether or not the preceding feeding time (set in Step S 2 ) has elapsed from the start of the preceding paper feeding. As a result, if the control unit  110  determines that the preceding feeding time has elapsed, the process proceeds to Step S 9 . If the control unit  110  determines that the preceding feeding time has not elapsed, the determination process of Step S 8  is repeated. Further, when proceeding to Step S 9 , the control unit  110  starts the secondary paper feeding. 
     As described above, the image forming apparatus  100  ( 101 ) of the first embodiment includes the paper conveying path  1  for conveying the paper sheet P to the printing position P 3  via the registration position P 2 , the registration roller pair  42  (registration unit) that operates to convey the paper sheet P after reaching the registration position P 2  to the printing position P 3 , the first paper feeding unit  3 A that operates to convey the paper sheet P to the registration position P 2 , the second paper feeding unit  3 B that operates to convey the paper sheet P to the registration position P 2 , and the control unit  110 , which selects the paper feeding unit for conveying the paper sheet P to be printed as the target paper feeding unit  3  from the first paper feeding unit  3 A and the second paper feeding unit  3 B, sets the preceding feeding time, performs the primary paper feeding in which the registration roller pair  42  is not operated but the target paper feeding unit  3  is operated so as to convey the paper sheet P to the registration position P 2 , finishes the primary paper feeding at the timing when the paper sheet P reaches the registration position P 2 , and then performs the secondary paper feeding in which the registration roller pair  42  is not operated but the target paper feeding unit  3  is operated until the preceding feeding time elapses, and afterward the registration roller pair  42  is operated while continuing to operate the target paper feeding unit  3  so that the paper sheet P after reaching the registration position P 2  is conveyed to the printing position P 3 . Further, the control unit  110  sets different values of the preceding feeding time for the case where the first paper feeding unit  3 A is the target paper feeding unit  3  and the case where the second paper feeding unit  3 B is the target paper feeding unit  3 . 
     In the first embodiment, after the primary paper feeding, before the secondary paper feeding, the registration roller pair  42  is not operated, but the target paper feeding unit  3  is operated until the preceding feeding time elapses (the preceding paper feeding is performed), and hence a skew of the paper sheet P can be corrected. Here, the control unit  110  sets different values of the preceding feeding time for the case where the first paper feeding unit  3 A is the target paper feeding unit  3  and the case where the second paper feeding unit  3 B is the target paper feeding unit  3 . Specifically, the preceding feeding time in the case where the second paper feeding unit  3 B (that slips more easily on the paper sheet P when the preceding paper feeding is performed) is the target paper feeding unit  3  is set to be longer than the preceding feeding time in the case where the first paper feeding unit  3 A is the target paper feeding unit  3 . In this way, even if the second paper feeding unit  3 B, which easily slips on the paper sheet P when the preceding paper feeding is performed, is the target paper feeding unit  3 , it is possible to prevent occurrence of inconvenience that the large slip on the paper sheet P causes insufficient skew correction of the paper sheet P (insufficient pressing of the front edge of the paper sheet P to the registration roller pair  42 ). In other words, it is possible to suppress positional deviation of an image printed on the paper sheet P (to prevent the paper sheet P from being conveyed in a skewed state to the printing position P 3 ). 
     In addition, in the first embodiment, as described above, the storage unit  112  stores the accumulated values of the operation time of the first paper feeding unit  3 A and the second paper feeding unit  3 B. Further, the control unit  110  determines the accumulated value corresponding to the target paper feeding unit  3  and sets the preceding feeding time to a larger value as the determined accumulated value is larger. In this structure, because the preceding feeding time becomes longer as the accumulated value of the operation time of the target paper feeding unit  3  is larger, it is possible to prevent the skew correction of the paper sheet P from being insufficient even if the target paper feeding unit  3  is apt to slip on the paper sheet P due to temporal deterioration of the target paper feeding unit  3  (wearing deterioration or the like of the paper feed roller  32  or the intermediate roller pair  36 ). 
     In addition, in the first embodiment, as described above, the storage unit  112  stores the conveying distance of the paper sheet P from the paper feed roller  32  of the second paper feeding unit  3 B to the registration roller pair  42  or the conveying distance plus a margin, as a threshold length. Then, in the case where the second paper feeding unit  3 B is the target paper feeding unit  3 , the control unit  110  sets the preceding feeding time in the case where the conveying direction length of the paper sheet P to be printed is the threshold length or less to be longer than the preceding feeding time in the case where the conveying direction length of the paper sheet P to be printed is longer than the threshold length. In this structure, because the preceding feeding time in the case of the second state at the start time point of the preceding paper feeding (see the lower part in  FIG. 6 ) is longer than the preceding feeding time in the case of the first state (see the upper part in  FIG. 6 ), it is possible to prevent insufficient skew correction of the paper sheet P regardless of the conveying direction length of the paper sheet P. 
     Note that, depending on mounting structures (mounting positions and the like) of the paper feed roller  32  and the intermediate roller pair  36 , a shape of the paper conveying path  1  from the paper feed roller  32  to the registration roller pair  42 , or the like, the slip when the first paper feeding unit  3 A performs the preceding paper feeding may be larger than the slip when the second paper feeding unit  3 B performs the preceding paper feeding. In this case, the preceding feeding time in the case where the first paper feeding unit  3 A is the target paper feeding unit  3  may be set to be longer than the preceding feeding time in the case where the second paper feeding unit  3 B is the target paper feeding unit  3 . 
     Second Embodiment 
     &lt;Structure of Image Forming Apparatus&gt; 
     As illustrated in  FIGS. 10 and 11 , the image forming apparatus  100  ( 102 ) of a second embodiment is different from that of the first embodiment in that the optional sheet feeding device  10 B is removed from the apparatus main body  10 A. In other words, the paper conveying path  1  of the image forming apparatus  102  includes only the paper conveying path  1   a . In addition, the paper feeding unit  3  of the image forming apparatus  102  includes only the first paper feeding unit  3 A. Other structures of the second embodiment are the same as those of the first embodiment. 
     &lt;Primary Paper Feeding and Secondary Paper Feeding&gt; 
     When the control unit  110  receives the instruction to execute a print job (When the communication unit  120  receives the job data), the control unit  110  performs the image processing (such as conversion into the image data for exposure) on the image data. In addition, the control unit  110  performs the primary paper feeding. After that, the control unit  110  performs the secondary paper feeding. Hereinafter, with reference to the timing chart illustrated in  FIG. 12 , the primary paper feeding and the secondary paper feeding are described. 
     First, the primary paper feeding is described. When receiving the instruction to execute a print job (time point TP 21 ), the control unit  110  starts the primary paper feeding. In this case, the control unit  110  drives the paper feed motor M 1  to rotate the paper feed roller  32 . In addition, the control unit  110  drives the conveying motor M 2  to rotate the plurality of conveying roller pairs  41 . However, the control unit  110  turns off the registration clutch. In other words, the control unit  110  does not drive the registration roller pair  42  to rotate but drives only the other conveying roller pair  41  to rotate. 
     When the paper feed roller  32  rotates, the paper sheet P is fed to the paper conveying path  1 , and the paper sheet P is conveyed along the paper conveying path  1 . Then, the front edge of the paper sheet P reaches the detection position DP (time point TP 22 ). In this way, the output value of the registration sensor RS changes from L level indicating absence of the paper sheet P to H level indicating presence of the paper sheet P. In other words, the control unit  110  detects that the front edge of the paper sheet P has reached the detection position DP. 
     Next, after detecting that the front edge of the paper sheet P has reached the detection position DP, when a predetermined deflection forming time elapses (time point TP 23 ), the control unit  110  turns off the paper feed clutch so that the rotation of the paper feed roller  32  is stopped, and thus the primary paper feeding is finished. Here, the deflection forming time is the sum of the time necessary for the paper sheet P to travel the distance between the detection position DP and the registration position P 2  and a predetermined time. In other words, when the predetermined time elapses after the front edge of the paper sheet P reaches the registration position P 2 , the primary paper feeding is finished. 
     Next, the secondary paper feeding is described. After finishing the primary paper feeding, the control unit  110  determines whether or not the printing preparation is done for printing an image on the paper sheet P. For example, the control unit  110  determines that the printing preparation is done if the image processing on the image data of the image to be printed on the paper sheet P is completed so that the exposing device  53  is ready to perform exposure of the photosensitive drum  51  (formation of an electrostatic latent image). 
     When the printing preparation is done (time point TP 24 ), the control unit  110  starts the secondary paper feeding. In this case, the control unit  110  turns on the paper feed clutch so that the paper feed roller  32  rotates. In addition, the control unit  110  turns on the registration clutch so that the registration roller pair  42  rotates. 
     In this way, a part of the paper sheet P whose front edge has reached the registration position P 2 , which remains at the paper feeding position P 1 , is sent out by the paper feed roller  32  in the paper feed direction, and the front edge of the paper sheet P enters the nip of the registration roller pair  42 . Then, the paper sheet P that has entered the nip of the registration roller pair  42  is conveyed to the printing position P 3  by the registration roller pair  42  (the secondary paper feeding is performed). After starting the secondary paper feeding, the control unit  110  starts the exposure of the photosensitive drum  51  by the exposing device  53  (the formation of the electrostatic latent image). 
     &lt;Start Timing of Secondary Paper Feeding&gt; 
     In the second embodiment, there is no rollers (roller pair) for conveying the paper sheet P on the conveying path of the paper sheet P between the paper feeding position P 1  and the registration position P 2 . Therefore, if time after the primary paper feeding is finished until the printing preparation is done becomes long (if a period of time after the primary paper feeding is finished until the secondary paper feeding starts becomes long), the front edge part of the paper sheet P that has reached the registration position P 2  causes backslide due to its weight as illustrated in  FIG. 13 . In other words, the front edge of the paper sheet P moves downward from the registration position P 2  (the front edge part of the paper sheet P sags down). If the secondary paper feeding starts in the state where the front edge of the paper sheet P is below the registration position P 2 , there occurs an inconvenience that the timing when the front edge of the paper sheet P reaches the printing position P 3  is delayed from the target timing (the timing when the secondary paper feeding is started in the state where the front edge of the paper sheet P is at the registration position P 2 ). 
     Therefore, when the primary paper feeding is finished, the control unit  110  measures waiting time after the primary paper feeding is finished until the printing preparation is done, and determines whether or not the measured time as the waiting time (hereinafter may be simply referred to as the measured time) is longer than a predetermined threshold time (for example 50 ms). For example, time after the front edge of the paper sheet P reaches the registration position P 2  until the front edge part of the paper sheet P starts to backslide is determined in an experimental manner or a simulating manner in advance, and the determined time is set as the threshold time. 
     Further, when the printing preparation is done, if the measured time is the threshold time or less, the control unit  110  starts the secondary paper feeding at once. In other words, when the printing preparation is done, the control unit  110  starts to rotate the paper feed roller  32  and the registration roller pair  42  at the same time. Therefore, when the printing preparation is done, the paper sheet P is conveyed from the registration position P 2  to the printing position P 3 . Note that, in this case, the secondary paper feeding is started at the same timing as shown in  FIG. 12 . 
     On the other hand, when the printing preparation is done, if the measured time is longer than the threshold time (if the front edge of the paper sheet P is below the registration position P 2 ), the secondary paper feeding is started at timing as shown in  FIG. 14 . In the timing chart illustrated in  FIG. 14 , it is supposed that waiting time T 11  after the primary paper feeding is finished until the printing preparation is done is the threshold time or longer. 
     Specifically, when the printing preparation is done (time point TP 24 ), the control unit  110  does not start the rotation of the registration roller pair  42  but starts the rotation of the paper feed roller  32  first. In this way, even if the front edge of the paper sheet P is below the registration position P 2 , the front edge of the paper sheet P returns to the registration position P 2 . After that, when time T 12  elapses (time point TP 25 ) after the rotation of the paper feed roller  32  is started, the control unit  110  starts the rotation of the registration roller pair  42  while continuing the rotation of the paper feed roller  32  (the secondary paper feeding is started). 
     Here, as the waiting time after the primary paper feeding is finished until the printing preparation is done is longer, backslide amount of the paper sheet P when the printing preparation is done becomes larger. Therefore, it is preferred to set a longer time for returning the front edge of the paper sheet P to the registration position P 2  as the waiting time is longer. 
     Therefore, the time from the rotation start of the paper feed roller  32  to the rotation start of the registration roller pair  42  (denoted by T 12  in  FIG. 14 ) is changed in accordance with the waiting time (denoted by T 11  in  FIG. 14 ). For example, the storage unit  112  stores timing information  80  for measuring rotation start timing of the registration roller pair  42  (see  FIG. 11 ). In this timing information  80 , the waiting time is classified into a plurality of time ranges, and time differences respectively corresponding to the plurality of time ranges are determined in advance, so that the time difference between the rotation start timing (operation start timing) of the paper feed roller  32  and the rotation start timing (operation start timing) of the registration roller pair  42  becomes larger as the waiting time is longer. 
     As an example, as illustrated in  FIG. 15 , the time difference in the case where the waiting time is 51 ms or more to 100 ms or less is set to 45 ms, and the time difference in the case where the waiting time is 101 ms or more is set to 90 ms. Note that the time difference in the case where the waiting time is 50 ms or less (namely in the case where the waiting time is the threshold time or less) is 0 ms. 
     Further, if the measured time as the waiting time is longer than the threshold time, the control unit  110  determines the time difference corresponding to the measured time based on the timing information  80 , and delays the rotation start timing of the registration roller pair  42  from the rotation start timing of the paper feed roller  32  by the determined time difference. In other words, if the measured time as the waiting time is longer than the threshold time, the rotation start timing of the registration roller pair  42  is delayed longer from the rotation start timing of the paper feed roller  32  (the period of time between the rotation start timing of the paper feed roller  32  and the rotation start timing of the registration roller pair  42  becomes longer) as the measured time is longer. As a result, as the measured time as the waiting time is longer, a longer time is set for returning the front edge of the paper sheet P to the registration position P 2 . With reference to the timing chart illustrated in  FIG. 14 , the time T 12  becomes longer so that the rotation start timing of the registration roller pair  42  is delayed longer as the waiting time T 11  is longer. 
     Hereinafter, with reference to the flowchart illustrated in  FIG. 16 , the process flow after the primary paper feeding is finished is described. The flowchart illustrated in  FIG. 16  starts when the primary paper feeding is finished. Note that, when the primary paper feeding is finished, the control unit  110  starts to measure the waiting time. 
     In Step S 11 , the control unit  110  determines whether or not the printing preparation is done. As a result, if the control unit  110  determines that the printing preparation is done, the process proceeds to Step S 12 . If the control unit  110  determines that the printing preparation is not done, the determination process of Step S 11  is repeated. 
     When proceeding to Step S 12 , the control unit  110  determines whether or not the measured time as the waiting time is the threshold time or shorter. As a result, if the control unit  110  determines that the measured time (waiting time) is the threshold time or shorter, the process proceeds to Step S 13 . If the control unit  110  determines that the measured time (waiting time) is longer than the threshold time, the process proceeds to Step S 14 . 
     When proceeding to Step S 13 , the control unit  110  starts to rotate the paper feed roller  32  and the registration roller pair  42  at the same time. In this case, the secondary paper feeding is started at the time point when the printing preparation is done. 
     When proceeding to Step S 14 , the control unit  110  does not start the rotation of the registration roller pair  42  but starts the rotation of the paper feed roller  32 . In other words, the secondary paper feeding is not started at the time point when the printing preparation is done. In this case, if the backslide of the front edge part of the paper sheet P has occurred at the registration position P 2 , the front edge of the paper sheet P is returned to the registration position P 2 . 
     Next, in Step S 15 , the control unit  110  determines the time difference corresponding to the measured time based on the timing information  80 . After that, in Step S 16 , the control unit  110  determines whether or not the time corresponding to the time difference determined in Step S 5  has elapsed after the rotation of the paper feed roller  32  is started. As a result, if the control unit  110  determines that the time corresponding to the time difference has elapsed, the process proceeds to Step S 17 . If the control unit  110  determines that the time corresponding to the time difference has not elapsed, the determination process of Step S 16  is repeated. 
     When proceeding to Step S 17 , the control unit  110  starts the rotation of the registration roller pair  42 . In this case, the control unit  110  continues the rotation of the paper feed roller  32 . In this way, the secondary paper feeding is started in which both the paper feed roller  32  and the registration roller pair  42  are rotated. 
     As described above, the image forming apparatus  100  ( 102 ) of the second embodiment includes a paper conveying path  1  for conveying the paper sheet P from the paper feeding position P 1  to the printing position P 3  via the registration position P 2  positioned above the paper feeding position P 1 , the paper feeding unit  3  for feeding the paper sheet P from the paper feeding position P 1  to the registration position P 2  by upward movement of the lift plate  34  and rotation of the paper feed roller  32 , the registration roller pair  42  (registration unit) that rotates to convey the paper sheet P after reaching the registration position P 2  to the printing position P 3 , the image forming unit  5  that forms an image to be printed on the paper sheet P and prints the image on the paper sheet P conveyed to the printing position P 3 , and the control unit  110  that performs the primary paper feeding in which the registration roller pair  42  is not rotated but the paper feed roller  32  is rotated so that the paper sheet P is conveyed from the paper feeding position P 1  to the registration position P 2 , finishes the primary paper feeding at a predetermined time after the paper sheet P reaches the registration position P 2 , and then after the printing preparation for printing the image on the paper sheet P is done, performs the secondary paper feeding in which the paper feed roller  32  and the registration roller pair  42  are rotated so that the paper sheet P after reaching the registration position P 2  is conveyed to the printing position P 3 . The control unit  110  measures the waiting time after the primary paper feeding is finished until the printing preparation is done. When the printing preparation is done, if the measured time as the waiting time is longer than the predetermined threshold time, the rotation of the paper feed roller  32  is started first, and then the rotation of the registration roller pair  42  is started while continuing the rotation of the paper feed roller  32 , so that the secondary paper feeding is started. If the measured time is the threshold time or shorter, both the paper feed roller  32  and the registration roller pair  42  are started to rotate at the same time, so that the secondary paper feeding is started. 
     In the second embodiment, when the printing preparation is done, the rotation of the paper feed roller  32  is started first, and hence even if the front edge of the paper sheet P is moved to a position below the registration position P 2 , the front edge of the paper sheet P returns to the registration position P 2 . Therefore, the secondary paper feeding can be started in the state where the front edge of the paper sheet P is returned to the registration position P 2 . In this way, it is possible to prevent the timing when the front edge of the paper sheet P reaches the printing position P 3  after the secondary paper feeding is started from being later than the target timing (the timing when the secondary paper feeding is started in the state where the front edge of the paper sheet P is at the registration position P 2 ). In other words, it is possible to suppress positional deviation of an image printed on the paper sheet P. Therefore, in order to suppress positional deviation of an image printed on the paper sheet P, it is not necessary to change the shape of the paper conveying path  1  or to dispose an additional roller and sensor (it is possible to suppress an increase in size of the apparatus or in the number of components). 
     Here, if the waiting time after the primary paper feeding is finished until the printing preparation is done is long, the backslide of the paper sheet P is apt to occur. On the other hand, if the waiting time is short, the backslide of the paper sheet P hardly occurs (or the backslide of the paper sheet P does not occur). In this case, the process of staring the rotation of the paper feed roller  32  first is not necessary. 
     Therefore, in the first embodiment, as described above, when the printing preparation is done, if the measured time as the waiting time is longer than the threshold time (if the backslide of the paper sheet P has occurred), the process of staring the rotation of the paper feed roller  32  first is performed. On the other hand, if the measured time is the threshold time or shorter (if the backslide of the paper sheet P has not occurred), the process of staring the rotation of the paper feed roller  32  first is not performed. In other words, the secondary paper feeding is started at once when the printing preparation is done. In this way, it is possible to suppress execution of the process of staring the rotation of the paper feed roller  32  first (unnecessary process) when the backslide of the paper sheet P has not occurred. 
     In addition, in the first embodiment, as described above, the storage unit  112  stores the timing information  80 , in which the waiting time is classified into a plurality of time ranges, and time differences between the rotation start timing of the paper feed roller  32  and the rotation start timing of the registration roller pair  42 , which respectively correspond to the plurality of time ranges, are determined in advance, so that the time difference is larger as the waiting time is longer. Further, when the printing preparation is done, if the measured time as the waiting time is longer than the threshold time, the control unit  110  determines the time difference corresponding to the measured time based on the timing information  80 , and delays the rotation start timing of the registration roller pair  42  from the rotation start timing of the paper feed roller  32  by time corresponding to the determined time difference. In other words, when the printing preparation is done, if the measured time as the waiting time is longer than the threshold time, the control unit  110  delays the rotation start timing of the registration roller pair  42  from the rotation start timing of the paper feed roller  32  more as the measured time is longer. In this structure, as the measured time as the waiting time measured by the control unit  110  is longer (as the backslide amount of the paper sheet P when the printing preparation is done is larger), the time for returning the front edge of the paper sheet P to the registration position P 2  is set to be longer. Therefore, even if the backslide amount of the paper sheet P when the printing preparation is done is large, the front edge of the paper sheet P can be returned to the registration position P 2 . 
     The embodiments described above are merely examples in every aspect and should not be interpreted as limitations. The scope of the present disclosure is defined not by the above description of the embodiments but by the claims and should be understood to include all modifications within the meaning and scope equivalent to the claims.