Abstract:
A printer includes a casing, a print mechanism disposed inside the casing, and a paper tray. A conveying route extends from the tray to the print mechanism, with a first roller disposed adjacent the tray to feed a sheet from the tray to the conveying route. A second roller is disposed in the conveying route and conveys the sheet fed by the first roller to the print mechanism. A driving source provides a driving source to the first roller and the second roller. A controller is configured to, in response to a printing request, apply the driving force from the driving source to the first and second rollers such that the first and second rollers rotate. In response to a magnitude of a load of the driving source, the controller applies the driving force to the second roller such that the second roller rotates, and the controller does not apply the driving source to the first roller such that the first roller does not rotate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2016-050481 filed on Mar. 15, 2016, the content of which is incorporated herein by reference in its entirety. 
       FIELD OF DISCLOSURE 
       [0002]    The disclosure relates to a printer and computer readable storage device. 
       BACKGROUND 
       [0003]    A known printer decides that, if a predetermined time elapses after a current flowing in a driving motor that feeds sheets has exceeded a reference value, there is a jam (more specifically, a paper jam). 
       SUMMARY 
       [0004]    In accordance with aspects of the present disclosure, a printer includes a casing, a print mechanism disposed inside the casing, and a tray configured to support a sheet stack including one or more sheets. A conveying route extends from the tray to the print mechanism, with a first roller disposed adjacent the tray to feed a sheet from the tray to the conveying route. A second roller is disposed in the conveying route and conveys the sheet fed by the first roller to the print mechanism. A driving source provides a driving source to the first roller and the second roller. A controller is configured to, in response to a printing request, apply the driving force from the driving source to the first and second rollers such that the first and second rollers rotate. In response to a magnitude of a load of the driving source, the controller applies the driving force to the second roller such that the second roller rotates, and the controller does not apply the driving source to the first roller such that the first roller does not rotate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a cross-sectional view illustrating the general structure of a printer in an embodiment. 
           [0006]      FIG. 2  is a block diagram illustrating the electrical structure of the printer in the embodiment. 
           [0007]      FIG. 3  illustrates how a motor load changes when a small sheet becomes jammed. 
           [0008]      FIG. 4  illustrates how a motor load changes when a large sheet becomes jammed. 
           [0009]      FIG. 5  is the first part of a two-part flowchart illustrating detection processing. 
           [0010]      FIG. 6  is the second part of the flowchart illustrating detection processing. 
           [0011]      FIG. 7  is a timing diagram illustrating correspondence between the motor load and detection processing steps. 
           [0012]      FIG. 8  is a flowchart illustrating reverse processing. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    In an image forming apparatus such as a printer, if a paper jam is detected as early as possible in continuous sheet feeding, the number of jammed sheets can be reduced. This can be expected to suppress the jam from becoming worse. Detection of a small degree jam is effective in early detection of a jam. The larger a jam is, the more a current tends to flow in the driving motor. Therefore, a possible solution to detecting a small degree jam is to reduce the above reference value. If, however, the reference value is reduced, an overcurrent due to a factor other than a jam is also detected. This may cause a jam to be incorrectly detected. If the operation of the printer is stopped in spite of incorrect detection, the throughput may be lowered. 
       Outline of the Printer 
       [0014]    The laser printer (simply referred to below as the printer)  1  in this application will be described with reference to  FIG. 1 . The directions in the description below are defined with respect to the user who uses the printer  1 . Specifically, in  FIG. 1 , the left side is “front”, the right side is “back”, the top side is “up”, and the bottom side is “down”. 
         [0015]    As illustrated in  FIG. 1 , the printer  1  includes a main device  2 , shaped substantially like a box, and three optional sheet feed units  70 . The main device  2  includes a sheet feed unit  10 , a print mechanism or image forming unit  20 , and a motor M 1 . A discharge tray  3 , on which discharged sheets P are stacked, is formed on the upper surface of the main device  2 . The discharge tray  3  is inclined upwardly at an oblique angle from the back side toward the front side. 
         [0016]    The sheet feed unit  10  includes a feed tray  11 , in which sheets P are stacked, a main body pressing plate  12 , a supply roller  13 , conveying rollers  14 , and the like. The main body pressing plate  12 , the rotational center of which is at the back end of the main body pressing plate  12 , can be rotated from a standby position indicated by dash-dot-dot lines in  FIG. 1  to a raised position indicated by solid lines. A sheet P is pressed against the supply roller  13  by the main body pressing plate  12  raised to the raised position. When the supply roller  13  is rotated while being in contact with the sheet P, the sheet P is fed out and conveyed to the image forming unit  20  through the conveying rollers  14 , conveying rollers  15 , and the like along a conveying route R. The conveying rollers  14  are disposed in the vicinity of the supply roller  13  at its downstream side in the conveying direction. The conveying rollers  15  are disposed upstream of a photosensitive drum  31  in the image forming unit  20  in the conveying direction. Conveying rollers  16  are disposed in the vicinity of the discharge tray  3 . 
         [0017]    The image forming unit  20  is disposed substantially at the center of the main device  2  and on the sheet feed unit  10 . The image forming unit  20  includes a process cartridge  30 , an exposure device  40 , a fixing unit  60 , and the like. 
         [0018]    The process cartridge  30  includes the photosensitive drum  31 , a transfer roller  32 , a developing roller  33 , a toner storage  34 , a charger (not illustrated), a layer thickness restricting blade (not illustrated), and the like. 
         [0019]    The exposure device  40 , which is disposed above the photosensitive drum  31 , includes a laser light source (not illustrated), a polygon mirror (not illustrated), and the like. A laser beam emitted from the laser light source is deflected by the polygon mirror and illuminates the surface of the photosensitive drum  31 . 
         [0020]    The fixing unit  60  is disposed behind the main device  2 . The fixing unit  60  includes a heating roller  61 , a pressurizing roller  62 , and the like. 
         [0021]    The motor M 1  is a driving source that drives the supply roller  13 , conveying rollers  14 , and the like. The driving force of the motor M 1  is transmitted through a driving force transmitting mechanism  80  to the supply roller  13  and conveying rollers  14 . The driving force transmitting mechanism  80  includes a first transmitting portion  81 , electromagnetic clutches  82  and  86 , a second transmitting portion  83 , a third transmitting portion  84 , a fourth transmitting portion  85 , a fifth transmitting portion  87 , and the like. The first transmitting portion  81  transmits the driving force of the motor M 1  to the conveying rollers  14 . The first transmitting portion  81  is connected to the input side of the electromagnetic clutch  82 , and the second transmitting portion  83  is connected to the output side of the electromagnetic clutch  82 . The electromagnetic clutch  82  is switched between a transmission state in which the electromagnetic clutch  82  can transmit the driving force to the supply roller  13  and a disconnected state in which the electromagnetic clutch  82  ceases the transmission of the driving force. The third transmitting portion  84 , which branches from the first transmitting portion  81 , is structured so that the third transmitting portion  84  can transmit the driving force to the optional sheet feed unit  70  connected behind the third transmitting portion  84 . The fifth transmitting portion  87  transmits the driving force of the motor M 1  through the electromagnetic clutch  86  to the developing roller  33 . The fourth transmitting portion  85  is connected to the input side of the electromagnetic clutch  86 , and the fifth transmitting portion  87  is connected to the output side of the electromagnetic clutch  86 . The electromagnetic clutch  86  is switched between a transmission state in which the electromagnetic clutch  86  can transmit the driving force to the fifth transmitting portion  87  and a disconnected state in which the electromagnetic clutch  86  ceases the transmission of the driving force. The fifth transmitting portion  87  transmits the driving force to the developing roller  33 . The developing roller  33  is structured so that it is rotated even if the electromagnetic clutch  86  is in the disconnected state. The rotational speed of the developing roller  33  with the electromagnetic clutch  86  in the disconnected state is lower than the rotational speed of the developing roller  33  with the electromagnetic clutch  86  in the transmission state. 
         [0022]    Each of three optional sheet feed units  70  disposed below the main device  2  includes an optional feed tray  71 , an optional pressuring plate  72 , an optional supply roller  73 , and conveying rollers  74 . The optional pressuring plate  72 , the rotational center of which is at the back end of the optional pressuring plate  72 , can be rotated from a standby position indicated by dash-dot-dot lines in  FIG. 1  to a raised position indicated by solid lines. A sheet P is pressed against the optional supply roller  73  by the raised optional pressuring plate  72 . When the optional supply roller  73  is rotated while being in contact with the sheet P, the sheet P is fed out toward the conveying route R. In the description below, the feed tray  11  will sometimes be referred to as tray T 1 , and the three optional feed trays  71  will sometimes be referred to as tray T 2 , tray T 3 , and tray T 4  sequentially from the top. In the description below, the feeding of a sheet P from the tray T 1  toward the conveying route R by the supply roller  13  and the feeding of a sheet P from each of the trays T 2  to T 4  toward the conveying route R by the relevant optional supply roller  73  will be denoted as sheet supply. 
         [0023]    The optional sheet feed unit  70  includes a driving force transmitting mechanism  90  in addition to the components described above. The driving force transmitting mechanism  90  includes a first transmitting portion  92 , a second transmitting portion  98 , a third transmitting portion  97 , a fourth transmitting portion  94 , and electromagnetic clutches  93  and  96 . If the main device  2  is disposed on one optional sheet feed unit  70 , its first transmitting portion  92  is connected to the driving force transmitting mechanism  80  in the main device  2 . If another optional sheet feed unit  70  is connected on the optional sheet feed unit  70 , its first transmitting portion  92  is connected to the second transmitting portion  98  in the other optional sheet feed unit  70 . The first transmitting portion  92  is connected to the input side of the electromagnetic clutch  96 , and the fourth transmitting portion  94  is connected to the output side of the electromagnetic clutch  96 . The electromagnetic clutch  96  is switched between a transmission state in which the electromagnetic clutch  96  can transmit the driving force to the third transmitting portion  97  and a disconnected state in which the electromagnetic clutch  96  ceases the transmission of the driving force. The third transmitting portion  97  transmits the driving force to the conveying rollers  74 . The third transmitting portion  97  is connected to the input side of the electromagnetic clutch  93 , and the fourth transmitting portion  94  is connected to the output side of the electromagnetic clutch  93 . The electromagnetic clutch  93  is switched between a transmission state in which the electromagnetic clutch  93  can transmit the driving force to the optional supply roller  73  and a disconnected state in which the electromagnetic clutch  93  ceases the transmission of the driving force. The second transmitting portion  98 , which branches from the third transmitting portion  97 , is structured so that the second transmitting portion  98  can transmit the driving force to another optional sheet feed unit  70  connected behind the second transmitting portion  98  through the electromagnetic clutch  96 . 
         [0024]    The first transmitting portion  81 , second transmitting portion  83 , third transmitting portion  84 , fourth transmitting portion  85 , and fifth transmitting portion  87  provided in the main device  2  are each implemented by using, for example, a plurality of gears. The first transmitting portion  92 , second transmitting portion  98 , third transmitting portion  97 , and fourth transmitting portion  94  provided in the optional sheet feed unit  70  are each also implemented by using, for example, a plurality of gears. 
         [0025]    Next, the operation of the printer  1  at the time of image forming will be described. Upon receipt of a print job, the printer  1  executes printing processing according to the print job and forms an image on a sheet P. The surface of the photosensitive drum  31  is positively charged uniformly by the charger and is exposed to laser beams emitted from the exposure device  40  according to print data. Thus, an electrostatic latent image is formed on the surface of the photosensitive drum  31 . Then, toner is supplied to the electrostatic latent image formed on the surface of the photosensitive drum  31  by the developing roller  33 . Thus, the electrostatic latent image on the surface of the photosensitive drum  31  becomes visible and a toner image is supported on the surface of the photosensitive drum  31 . 
         [0026]    The sheet P is fed out toward the conveying route R from the sheet feed unit  10  or optional sheet feed unit  70  at a certain time. The sheet P fed out toward the conveying route R is supplied to the image forming unit  20  along the conveying route R by the conveying rollers  74 ,  14 , and  15  and the like. The toner image supported on the surface of the photosensitive drum  31  is transferred to the sheet P by the transfer roller  32 . The sheet P is then conveyed to the fixing unit  60  along the conveying route R. Toner is thermally fixed to the sheet P, after which the sheet P is conveyed along the conveying route R and discharged to the discharge tray  3  by the conveying rollers  16 . 
         [0027]    In the printer  1 , a re-conveying route Ra is provided so that two-sided printing can be performed. The re-conveying route Ra is a conveying route along which the sheet P that has passed the fixing unit  60  is re-conveyed to the image forming unit  20  so that printing is performed on a second surface, which is the rear surface of the sheet P for which printing has been performed on a first surface, which is one surface of the sheet P. The re-conveying route Ra branches from the conveying route R at a branching point A located downstream of the heating roller  61  and upstream of the conveying rollers  16  in the conveying direction. The re-conveying route Ra starts from the branching point A, passes between the image forming unit  20  and the feed tray  11 , and joins the conveying route R at a joining point B located downstream of the conveying rollers  14  on the conveying route R. 
         [0028]    Next, the operation of the printer  1  at the time of two-sided printing will be described. After the sheet P has passed along the conveying route R and an image has been formed on the first surface, the sheet P is conveyed to the conveying rollers  16 . After the rear edge of sheet P has passed the branching point A, the conveying rollers  16  temporarily stop its rotation with the sheet P sandwiched between them. Then, the rotational direction of the conveying rollers  16  is changed to reverse the conveying direction of the sheet P. The sheet P is conveyed through the branching point A to the re-conveying route Ra. The sheet P is then returned toward the conveying route R through the joining point B on the upstream side on the conveying route R relative to the image forming unit  20 . Therefore, the front and back of the sheet P are reversed and an image is formed on the second surface. 
         [0029]    Rear-edge sensors Se 1  to Se 4  are respectively attached to the trays T 1  to T 4 . A pre-registration sensor Se 5  is attached upstream of the conveying rollers  15  in the conveying direction. A first post-registration sensor Se 6  is attached between the conveying rollers  15  and the photosensitive drum  31 . A second post-registration sensor Se 7  is attached downstream of the heating roller  61  in the conveying direction. The back-end sensors Se 1  to Se 4 , pre-registration sensor Se 5 , first post-registration sensor Se 6 , and second post-registration sensor Se 7  each output an ON signal when a sheet P is present and output an OFF signal when a sheet P is not present. 
         [0030]    Although not illustrated, a plurality of conveying rollers are disposed on the conveying route R in addition to the conveying rollers  14 ,  15 ,  16 , and  74 . In the description below, conveying rollers that are not illustrated and the conveying rollers  14 ,  15 ,  16 , and  74  will be collectively referred to as the conveying rollers. 
       Electrical Structure 
       [0031]    Next, the electrical structure of the printer  1  will be described with reference to  FIG. 2 . In addition to the sheet feed unit  10 , image forming unit  20 , fixing unit  60 , and the like illustrated in  FIG. 1 , the printer  1  includes a control unit  100 , a manipulation display unit  111 , a motor control circuit  112 , a clutch control circuit  113 , and the like. The control unit  100  includes a central processing unit (CPU)  101 , a random-access memory (RAM)  102 , a read-only memory (ROM)  103 , an application-specific integrated circuit (ASIC)  104 , and the like. The CPU  101  execute various types of programs including detection processing and reverse processing, which will be described later, the programs being stored in the ROM  103 , to control the sheet feed unit  10 , image forming unit  20 , fixing unit  60 , manipulation display unit  111 , and the like, which are mutually connected through a bus  105 . The RAM  102  is used as the main storage device with which the CPU  101  executes various types of processing. Control programs and various types of data are stored in the ROM  103 . The ASIC  104  is notified of the rotational speed of the motor M 1  by the CPU  101 , calculates a duty cycle of a pulse width modulation (PWM) signal according to the rotational signal, and outputs the duty cycle to the motor control circuit  112 . The manipulation display unit  111  includes various types of buttons, including a power supply button, and also includes a touch panel and the like. The manipulation display unit  111  displays a setting screen used for printing and other purposes, the operation state of the printer  1 , and the like. The manipulation display unit  111  also accepts settings for, for example, printing. 
         [0032]    The motor control circuit  112  controls the supply of a DC driving voltage to the motor M 1  (see  FIG. 1 ) to control the motor M 1 . The motor control circuit  112  also outputs a Hall signal and a frequency generator (FG) signal, which will be described later, to the control unit  100  as will be described later in detail. The clutch control circuit  113  controls the supply of control signals, such as DC voltages, to the electromagnetic clutches  86 ,  82 ,  93 , and  96  to control the electromagnetic clutches  86 ,  82 ,  93 , and  96 . For example, the electromagnetic clutches  86 ,  82 ,  93 , and  96  may be placed in transmission and disconnected states (further described below) based on receiving first and second control signals, respectively. The sensor Se is used to collectively refer to the back-end sensors Se 1  to Se 4 , pre-registration sensor Se 5 , first post-registration sensor Se 6 , and second post-registration sensor Se 7  illustrated in  FIG. 1 . The sensor Se outputs an ON or OFF signal to the control unit  100  depending on whether a sheet P is present, as described above. 
         [0033]    Next, the motor M 1  and motor control circuit  112  will be described in detail. 
         [0034]    The motor M 1  is, for example, a DC brushless motor of three phases, U, V, and W. The motor M 1  includes a stator in which a U-phase coil, a V-phase coil, and a W-phase coil are disposed so as to be star-connected at an intermediate point, a rotor, Hall elements, a frequency generator, and the like. The rotor, which is disposed around the stator, includes alternately disposed N-pole magnets and S-pole magnets. A total of three Hall elements, for example, are disposed in the vicinity of the rotor so as to be equally spaced at 120-degree intervals. Each Hall element detects a rotational phase of the rotor according to a magnetic field, which changes with the rotation of the rotor. The frequency generator generates an FG signal, which has a frequency proportional to the rotational speed of the rotor. 
         [0035]    The motor control circuit  112  includes an inverter  112   a  and is electrically connected to the Hall elements and frequency generator included in the motor M 1 . The motor control circuit  112  uses signals output from the Hall elements to acquire a Hall signal, which is a pulse signal related to the number of poles of the rotor, and outputs the Hall signal to the control unit  100 . The motor control circuit  112  also acquires an FG signal from the frequency generator and outputs the FG signal to the control unit  100 . 
         [0036]    The motor control circuit  112  controls current to each coil included in the motor M 1  under PWM control and drives the motor M 1 . The inverter  112   a  included in the motor control circuit  112  is connected to the coils in the motor M 1 . The inverter  112   a  includes three switch circuits. Each switch circuit is comprised of two transistors, which are connected in series between a power supply voltage and a ground voltage. The point at which the two transistors are connected is connected to an end of the relevant coil. The transistor is implemented by using, for example, a bipolar transistor or a metal oxide semiconductor field-effect transistor (MOSFET). The motor control circuit  112  inputs a PWM signal to the gate terminal or base terminal of each transistor in correspondence to the terminal. Each transistor is turned on while, for example, the PWM signal is turned on. For example, the motor control circuit  112  outputs a PWM signal that commands current to a transistor included in the switch circuit connected to the U-phase coil, the transistor being connected to the power supply voltage, and to a transistor included in the switch circuit connected to the W-phase coil, the transistor being connected to the ground voltage. Thus, a current flows from the U-phase coil to the V-phase coil, in which case, for example, the U-phase coil is excited to the N pole and the V-phase coil is excited to the S pole. As a result, the rotor rotates. The larger the duty cycle of the PWM signal is, the higher the rotational speed is. As described above, when the CPU  101  notifies the ASIC  104  of a target rotational speed, the ASIC  104  calculates the duty cycle of the PWM signal at which the target rotational speed is attained and outputs the duty cycle to the motor control circuit  112 . The motor control circuit  112  accepts the duty cycle and outputs a PWM signal at the duty cycle to the motor M 1 . The ASIC  104  compares the Hall signal or FG signal fed back from the motor control circuit  112  with the target rotational speed to adjust the duty cycle of the PWM signal. Thus, the control unit  100  can control the motor M 1  to the target rotational speed. 
         [0037]    If the conveying route R is blocked with a sheet P while being conveyed during an image forming operation, a so-called jam may occur. The control unit  100  can decide whether a jam has occurred, according to a signal output from a sensor Se. If, for example, a sheet P that has passed the pre-registration sensor Se 5  reaches the first post-registration sensor Se 6 , after a predetermined time has elapsed, the control unit  100  can decide that the sheet P has not jammed. The printer  1  includes three optional sheet feed units  70  as described above. The conveying route R from the tray T 4  of the lowest optional sheet feed unit  70  to the image forming unit  20  is longer than the conveying route R from the tray T 1  to the image forming unit  20 . However, no sensor is attached to the conveying route R from the rear-edge sensor Se 4  attached to the tray T 4  to the pre-registration sensor Se 5 . Therefore, if a decision is made based on only a signal from a sensor, when a jam occurs in the conveying route R from the tray T 4  to the image forming unit  20 , there has been a risk that detection of the jam will be delayed. If the control unit  100  decides that a jam has occurred, the control unit  100  suspends the image forming operation. At the same time, the control unit  100  causes the manipulation display unit  111  to display an error message to prompt the user to remove the jammed sheet P. If the image forming operation is continued without the jammed sheet P being removed, a subsequent sheet P is conveyed to the location at which the jam has occurred. This may worsen the jam. If the jam is worsened, it becomes difficult for the user to remove the jammed sheets P. In jam detection processing described later, therefore, a jam is detected according to an increase in the load of the motor M 1 . If a jam occurs, the load of the motor M 1  is increased because, for example, the jammed sheet P impedes the rotation of conveying rollers. An increase in the load of the motor M 1  at the occurrence of a jam will be described below in detail. 
         [0038]    It is also possible to detect the magnitude of the load of the motor M 1  by, for example, measuring a current flowing in the motor M 1 . However, a case in which the magnitude of the load is detected according to the duty cycle of the PWM signal will be described here. The duty cycle of the PWM signal reflects the magnitude of the load of the motor M 1 . If a jam occurs, the rotation of the conveying rollers at the position of the jam is impeded, so the torque of the motor M 1  is increased. Unless the current that is to flow into the motor M 1  is increased, the target rotational speed cannot be maintained. In view of this situation, the ASIC  104  increases the duty cycle of the PWM signal. Therefore, if the duty cycle of the PWM signal is large, the control unit  100  can decide that the load of the motor M 1  is large. In this arrangement, even if the printer  1  is not structured to detect the value of the current flowing in the motor M 1 , the control unit  100  can measure the magnitude of the load of the motor M 1  by acquiring the duty cycle of the PWM signal, the duty cycle being calculated by the ASIC  104 . 
         [0039]      FIGS. 3 and 4  illustrate results of experiments in which the conveying route R was blocked in the vicinity of the pre-registration sensor Se 5  to reproduce a jam and variations in the load of the motor M 1  were investigated. Sheets P in different sizes were used in the experiments in  FIGS. 3 and 4 ; the size of the sheet P in  FIG. 3  is smaller than in  FIG. 4 . The feed tray used in image forming was the tray T 4  and successive printing was performed on three sheets P. 
         [0040]    In  FIGS. 3 and 4 , the horizontal axis indicates elapsed time and the vertical axis indicates the motor load. Here, the magnitude of the duty cycle of the PWM signal corresponds to the magnitude of the load of the motor M 1 . After the printer  1  had been turned on, the control unit  100  caused the motor M 1  to start. The large peaks in  FIGS. 3 and 4  were due to the start of rotation of the motor M 1 . Times ta 1 , ta 2 , and ta 4  in  FIG. 3  respectively indicate times at which a first sheet P, a second sheet P, and a third sheet P were supplied. Time ta 3  is a time at which the electromagnetic clutch  86  that switches the transmission of the driving force to the developing roller  33  was turned on. The load was temporarily increased around time ta 3 , and the load was gradually increased around time ta 3  and later. Times tb 1 , tb 3 , and tb 4  in  FIG. 4  respectively indicate times at which a first sheet P, a second sheet P, and a third sheet P were supplied. Time tb 2  is a time at which the electromagnetic clutch  86  was turned on. The load was increased around time tb 2 . In  FIG. 4 , the load was increased at time tb 3  in a stepped manner. 
         [0041]    Comparing  FIG. 3  and  FIG. 4 , the load was increased in response to a sheet supply at different times. This is because the size of the sheet P differed. When the size of the sheet P was small (see  FIG. 3 ), the supply of the second sheet P was already complete before the first sheet P reached the vicinity of the pre-registration sensor Se 5  at which the conveying route R was blocked and an increase in load was started. By comparison, when the size of the sheet P was large (see  FIG. 4 ), the time at which an increase in load was started and the time of the supply of the second sheet P were almost the same. In  FIGS. 3 and 4 , although a tendency for the load to increase differs due to, for example, a difference in the size of the sheet P, the phenomena in which the loads are increased are the same, so the inventors found that variations in the load can be used to detect a jam. Detection processing to detect a jam will be described below with reference to  FIGS. 5 and 6 . 
         [0042]    When the printer  1  is turned on, it enters a standby state in which the printer  1  waits for a print job. The control unit  100  commands the motor control circuit  112  to place the motor M 1  in an operation state. Then, the motor M 1  enters the operation state. The control unit  100  also commands the clutch control circuit  113  to place the electromagnetic clutches  96  in the trays T 2  to T 4  in the transmission state in which the electromagnetic clutches  96  can transmit the driving force. Thus, the electromagnetic clutches  96  in the trays T 2  to T 4  enter the transmission state, and the driving force of the motor M 1  is transmitted to the conveying rollers  74  in the trays T 2  to T 4 , causing the conveying rollers  74  in the trays T 2  to T 4  to start rotating. 
         [0043]    Upon receipt of a print job, the control unit  100  sets to 0 a count N to be stored in the RAM  102  and starts detection processing. In an example described here, it will be assumed that the number of copies to be printed is 3, a print job specifying the tray T 4  as the feed tray is accepted, three sheets P are printed in succession, and the load of the motor M 1  is increased after a second sheet P is supplied. 
         [0044]    First, the control unit  100  decides whether to perform printing (Si). Printing as described here indicates processing to supply one sheet P and form an image on it. If the control unit  100  decides to perform printing (the result in S 1  is Yes), the control unit  100  increments the count N (S 3 ) to 1. The control unit  100  then decides whether the count N is 1 (S 4 ). Since the count N is 1, the control unit  100  decides that the count N is 1 (the result in S 4  is Yes) and measures a PWM duty cycle (S 5 ). To measure a PWM duty cycle is to acquire, in a predetermined period, the duty cycle of the PWM signal, the duty cycle having been calculated by the ASIC  104  described above. Since the ASIC  104  calculates the duty cycle of the PWM signal a plurality of times in the predetermined period, a plurality of duty cycles are obtained. Next, the control unit  100  calculates a first PWM duty cycle (S 13 ). Specifically, the control unit  100  calculates the average of the plurality of duty cycles of PWM signals that have been obtained in step S 5 . If, for example, the electromagnetic clutch  86 , which transmits the driving force to the developing roller  33 , is turned on, the reason for an increase in the load of the motor M 1  is clear, so the period in which the load is increased is excluded from the calculation. A calculation method in this case will be described later. Next, the control unit  100  decides whether the count N is 1 (S 15 ). Since the count N is 1, the control unit  100  decides that the count N is 1 (the result in S 15  is Yes) and issues a first ON command to the clutch control circuit  113  to have it place the electromagnetic clutch  93  included in the tray T 4  in the transmission state to start to supply a first sheet P (S 16 ). Thus, the clutch control circuit  113  places the electromagnetic clutch  93  included in the tray T 4  in the transmission state for a predetermined period. The control unit  100  then returns to step S 1  to supply a second sheet P. 
         [0045]    The steps described so far will be described with reference to  FIG. 7 . In  FIG. 7 , the motor load indicates the magnitude of the load of the motor M 1 , and M 1 ,  86 , and  93  (T 4 ) respectively indicate the operation states of the motor M 1 , the electromagnetic clutch  86 , and the electromagnetic clutch  93  in the tray T 4 . Se 4  indicates a signal output from the rear-edge sensor Se 4 , and Se 5  indicates a signal output from the pre-registration sensor Se 5 . 
         [0046]    When the printer  1  is turned on, the motor M 1  is turned on and start rotating in response to a command from the control unit  100 . A first peak of the motor load is caused by the start of the rotation of the motor M 1 . Then, the electromagnetic clutches  96  included in the trays T 2  to T 4  are placed in the transmission state in response to a command from the control unit  100 . Specifically, the control unit  100  commands the trays T 2  to T 4  so that the electromagnetic clutches  96  included in them are sequentially placed in the transmission state, starting from the top tray, that is, the tray T 2  followed by the trays T 3  and T 4  in that order. Accordingly, the motor load is gradually increased. When the control unit  100  starts detection processing, if the control unit  100  decides that there is printing processing to be performed (the result in S 1  is Yes), the control unit  100  obtains the duty cycles of PWM signals in a predetermined period taken before the first sheet P is supplied (S 5 ) and calculates the average of the duty cycles (S 13 ). The control unit  100  then supplies a first sheet P at time t 1  (S 16 ). 
         [0047]    The description of the flowchart will be continued with reference again to  FIG. 5 . Since the printing of a second sheet P is to be performed, the control unit  100  decides that there is printing processing to be performed (the result in S 1  is Yes) and increments the count N to 2 (S 3 ). Since the count N is 2, the control unit  100  decides that the count N is not 1 (the result in S 4  is No) and decides whether the rear-edge sensor Se 4  in the tray T 4 , which is a feed tray, has been turned on (S 7 ). If the control unit  100  decides that the rear-edge sensor Se 4  has not been turned on (the result in S 7  is No), the control unit  100  waits until the rear-edge sensor Se 4  is turned on. If the control unit  100  decides that rear-edge sensor Se 4  has been turned on (the result in S 7  is Yes), the control unit  100  starts a second PWM duty cycle measurement (S 9 ). The control unit  100  then decides whether the rear-edge sensor Se 4  has been turned off (S 11 ). If the control unit  100  decides that the rear-edge sensor Se 4  has not been turned off (the result in S 11  is No), the control unit  100  waits until the rear-edge sensor Se 4  is turned off. If the control unit  100  decides that rear-edge sensor Se 4  has been turned off (the result in S 11  is Yes), the control unit  100  terminates the second PWM duty cycle measurement, calculates a second PWM duty cycle, and stores the calculated duty cycle in the RAM  102  (S 13 ). 
         [0048]    Since the count N is 2, the control unit  100  decides that the count N is not 1 (the result in S 15  is No) and decides whether a difference between the first and second PWM duty cycles is equal to or larger than a certain value (S 17 ). Specifically, the control unit  100  decides whether a difference in the averages of duty cycles, which is obtained by subtracting the average of the first PWM duty cycles from the average of the second PWM duty cycles, is equal to or larger than a reference value stored in the ROM  103 . If the control unit  100  decides that the difference between the averages of the first and second duty cycles is smaller than the reference value (the result in S 17  is No), the control unit  100  decides that the load of the motor M 1  has not been increased and there is no jam. To supply a second sheet P, the control unit  100  then issues a second ON command to the clutch control circuit  113  to have it place the electromagnetic clutch  93  in the transmission state (S 16 ), after which the control unit  100  returns to step S 1 . 
         [0049]    The control unit  100  sets the count N to 3 and executes steps S 1  to S 4  and S 7  to S 17  in the same way as described above. If the control unit  100  decides, in step S 17 , that a value obtained by subtracting the second PWM duty cycle from a third PWM duty cycle is equal to or larger than a reference value (the result in S 17  is Yes), the control unit  100  decides that the load of the motor M 1  has been increased and there may be a jam and then proceeds to step S 19 . In step S 19 , the control unit  100  decides whether there is printing processing to be performed. Since there is printing processing to be performed for a third sheet P, the control unit  100  decides that there is printing processing to be performed (the result in S 19  is Yes) and then puts the supply of the third sheet P on hold (S 21 ). At that time, the electromagnetic clutch  96 , which transmits the driving force to the conveying rollers  74 , is kept in the transmission state. Therefore, the first and second sheets P, which have been already supplied, continue to be conveyed. The control unit  100  then decides whether the first and second conveyances of the sheets P have succeeded (S 23 ). 
         [0050]    The control unit  100  decides, according to a signal output from the pre-registration sensor Se 5 , whether a sheet P has been successfully conveyed. The control unit  100  executes conveyance confirmation processing to decide, according to a signal output from the pre-registration sensor Se 5 , whether a sheet P has been successfully conveyed, separately from detection processing. In conveyance confirmation processing, the control unit  100  uses the rear-edge sensor Se 4  in the tray T 4  specified as the feed tray to count a time elapsed after the rear edge of the sheet P has passed the rear-edge sensor Se 4 . The control unit  100  can calculate a conveyance time taken from when the rear edge of the sheet P has passed the rear-edge sensor Se 4  until the front edge of the sheet P reaches the pre-registration sensor Se 5 , according to the size of the sheet P and a conveyance speed stored in, for example, the ROM  103 . Therefore, even after the conveyance time is elapsed after the rear edge of the sheet P has passed the rear-edge sensor Se 4 , if the signal from the pre-registration sensor Se 5  is not changed to an ON signal in response to the arrival of the front edge of the sheet P, the control unit  100  decides that the conveyance has failed and a jam has occurred. If the signal from the pre-registration sensor Se 5  is changed to an ON signal in response to the arrival of the front edge of the sheet P, the control unit  100  decides that the conveyance has succeeded. 
         [0051]    In step S 23 , the control unit  100  produces a Yes result if the control unit  100  decides that the sheet P has been successfully conveyed in conveyance conformation processing, and produces a No result if the control unit  100  decides that the sheet P has been unsuccessfully conveyed in conveyance conformation processing. If the control unit  100  decides that the sheet P has been successfully conveyed (the result in S 23  is Yes), the control unit  100  resumes the supply of the third sheet P (S 25 ) and returns to step S 1 . Since the number of copies to be printed is 3, the control unit  100  decides that there is no more printing processing to be performed (the result in S 1  is No) and terminate the processing. 
         [0052]    The steps described so far will be described with reference to  FIG. 7 . After the first sheet P has been supplied at time t 1 , the rear-edge sensor Se 4  is turned on while the first sheet P is passing it. The control unit  100  acquires duty cycles of the PWM signals in a period TD 1  taken before the second sheet P is supplied (S 5 ) and calculates the average of the duty cycles (S 13 ). To eliminate an increase in the load due to a switchover of the electromagnetic clutch  93  to the transmission state, duty cycles in a predetermined time measured from time t 1  are excluded. If the control unit  100  decides that there is no increase in the load of the motor M 1  (the result in S 17  is No), the control unit  100  issues a second On command for the electromagnetic clutch  93  at time t 3 , which is a time after the elapse of a predetermined time after the first sheet P has passed the rear-edge sensor Se 4  and it has been turned off (S 16 ). Thus, the second sheet P is supplied. As with the first sheet P, after the second sheet P has been supplied, the rear-edge sensor Se 4  is turned on while the second sheet P is passing it. Then, the control unit  100  acquires the duty cycles of PWM signals in the period TD 1  taken before the second sheet P is supplied (S 9 ). Although not illustrated, a period from time t 4  to time t 7  is the same as the period TD 1 . In an example described here, the electromagnetic clutch  86 , which transmits the driving force to the developing roller  33 , is turned on at time t 5  and is switched to the transmission state. As described with reference to  FIGS. 3 and 4 , when the electromagnetic clutch  86  is turned on, the load of the motor M 1  is increased. Therefore, the average is calculated by excluding a period from time t 5  to time t 6  during which the load is increased because the electromagnetic clutch  86  has been turned on (S 13 ). Specifically, the average of the duty cycles of PWM signals output in a period TD 2   a  from time t 4  to t 5  and a period TD 2   b  from time t 6  to time t 7  is calculated. As a method of excluding a period, a predetermined period starting from when the electromagnetic clutch  86  is turned on is excluded. If the control unit  100  decides that a difference obtained by subtracting the second PWM duty cycle from the third PWM duty cycle is equal to or larger than a reference value (the result in S 17  is Yes), the control unit  100  puts a third ON command for the electromagnetic clutch  93  on hold (S 21 ); the third ON command is intended to be output at time t 7 , which is a time after the elapse of a predetermined time after the rear-edge sensor Se 4  has been turned off, if the control unit  100  decides that the difference is smaller than the reference value (the result in S 17  is No). If the pre-registration sensor Se 5  is turned on at time t 8 , the control unit  100  decides that the conveyance has succeeded and the first and second sheets P have arrived at the pre-registration sensor Se 5  (S 23 ) and resumes the third ON command for the electromagnetic clutch  93 , which has been put on hold, at time t 9  (S 25 ). 
         [0053]    The description of the flowchart will be continued with reference again to  FIG. 6 . If the control unit  100  decides that the sheet P has been unsuccessfully conveyed (the result in S 23  is No), the control unit  100  decides whether a time-out has occurred (S 27 ). Specifically, until a predetermined time elapses, the control unit  100  decides that a time-out has not occurred (the result in S 27  is No), in which case the control unit  100  returns to step S 23 . If the predetermined time has elapsed and the control unit  100  decides that a time-out has occurred (the result in S 27  is Yes), the control unit  100  terminates the processing. As a structure in which, if the control unit  100  produces a No result in step S 23  and produces a Yes result in step S 27 , the control unit  100  changes the value of a flag, which indicates the occurrence of a jam, stored in, for example, in the RAM  102 , the control unit  100  displays an error message on the manipulation display unit  111  according to the value of the flag. 
         [0054]    In detection processing, if the control unit  100  decides that the load of the motor M 1  has been increased, the control unit  100  puts the sheet supply on hold. If a first or second sheet P becomes jammed but a third sheet P is supplied, the number of jammed sheets P is increased, which may worsen the jam. When the supply of the third sheet P is put on hold, it is possible to suppress the jam from worsening. However, the reason of an increase in the load may not be a jam. In view of this, the first and second sheets P continue to be conveyed. This prevents a throughput in image forming from being lowered. Whether the first or second sheet P becomes jammed can be reliably checked with a signal from the pre-registration sensor Se 5 . If the control unit  100  confirmed that neither the first nor second sheet P becomes jammed, the control unit  100  resumes the supply of the third sheet P. 
         [0055]    Now, a variation of detection processing will be described. Although, in the above description, the electromagnetic clutch  96 , which transmits the driving force to the conveying rollers  74 , has been maintained in the transmission state in step S 21 , the conveyance of the second sheet P located upstream of the first sheet P may be put on hold. If the first sheet P has jammed, this suppresses the second sheet P from being conveyed to a position of the jam and thereby suppresses the conveying route R from being blocked. Specifically, the control unit  100  uses the rear-edge sensor Se 4  to count a time elapsed after the edge of the sheet P on the upstream side has passed the rear-edge sensor Se 4 . The control unit  100  can identify the position of the sheet P in the conveying route R from a time elapsed after the edge of the sheet P on the upstream side has passed the rear-edge sensor Se 4  until the control unit  100  executes step  21  and from the conveyance speed stored in, for example, the ROM  103 . The rotation of the conveying rollers located upstream of the identified position of the sheet P is put on hold. This suppresses the jam from worsening. 
         [0056]    Next, reverse processing will be described with reference to  FIG. 8 . 
         [0057]    Upon receipt of a print job, the control unit  100  starts reverse processing. The control unit  100  then decides whether reversing is specified (S 31 ). If two-sided printing is included in the print job, the control unit  100  decides that reversing is specified. If the control unit  100  decides that reversing is specified (the result in S 31  is Yes), the control unit  100  starts reversing (S 33 ). Specifically, the control unit  100  controls the relevant conveying rollers and starts processing involved in two-sided printing. The control unit  100  then measures the PWM duty cycle at intervals of a predetermined length of time (S 35 ). In reverse processing, a conveyance time taken to convey a sheet P from the branching point A on the re-conveying route Ra to the joining point B is preset. Therefore, the conveyance is divided by a predetermined division number to obtain the predetermined time. Then, the control unit  100  decides whether there is an abrupt change in the measured PWM duty cycles (S 37 ). Specifically, a duty cycle is obtained at intervals of the predetermined length of time, calculates an average, and calculates a difference between two segments, as in detection processing. If the difference is equal to or larger than a predetermined value, the control unit  100  decides that there is an abrupt change (the result in S 37  is Yes). If the control unit  100  decides that there is an abrupt change (the result in S 37  is Yes), the control unit  100  puts the supply of a next sheet from the feed tray on hold (S 39 ). The control unit  100  then checks whether the sheet P that was being re-conveyed has been successfully re-conveyed (S 41 ). Specifically, after the elapse of a predetermined time after it was checked with the second post-registration sensor Se 7  that the rear edge of the sheet P passed the second post-registration sensor Se 7 , if the presence of the sheet P is checked with the pre-registration sensor Se 5 , the control unit  100  decides that the sheet P has been successfully re-conveyed. If the control unit  100  decides that the sheet P has been successfully re-conveyed (the result in S 41  is Yes), the control unit  100  resumes the supply of the sheet P, which has been put on hold (S 43 ), and returns to step S 31 . If the control unit  100  decides that there is no abrupt change (the result in S 37  is No), the control unit  100  decides that no jam has occurred and returns to step S 31 . If the control unit  100  decides that the sheet P has not been successfully re-conveyed (the result in S 41  is No), the control unit  100  decides whether a time-out has occurred (S 45 ). Specifically, until a predetermined time elapses, the control unit  100  decides that a time-out has not occurred (the result in S 45  is No), in which case the control unit  100  returns to step S 41 . If the predetermined time has elapsed and the control unit  100  decides that a time-out has occurred (the result in S 45  is Yes), the control unit  100  terminates the processing. 
         [0058]    The control unit  100  is an example of a control apparatus. The feed tray  11  and optional feed tray  71  are an example of a tray. The conveying route R is an example of a conveying route. The supply roller  13  and optional supply roller  73  are an example of a first roller. The conveying rollers  14 ,  15 ,  16 , and  74  are an example of a second roller. The pre-registration sensor Se 5  is an example of a sensor. Steps S 5  and S 9  in detection processing are an example of acquisition processing. Steps S 17  in detection processing is an example of decision processing. Step S 21  in detection processing is an example of continuation processing and first on-hold processing. Step S 25  in detection processing is an example of resumption processing. The back-end sensors Se 1  to Se 4  are an example of a passage sensor. The fifth transmitting portion  87  in a state in which the electromagnetic clutch  86  is in the disconnected state is an example of a first transmission route. The fifth transmitting portion  87  in a state in which the electromagnetic clutch  86  is in the transmission state is an example of a second transmission route. The electromagnetic clutch  86  is an example of a developing electromagnetic clutch. The Hall signal and FG signal are an example of a rotation signal. The re-conveying route Ra is an example of a sheet conveying route. Step S 39  in reverse processing is an example of second on-hold processing. The PWM signal is an example of a PWM control signal. The first sheet P is an example of a second sheet and a fourth sheet. The second sheet P is an example of a third sheet. The third sheet P is an example of a first sheet. 
         [0059]    The embodiment described above has the following effects. 
         [0060]    If the control unit  100  decides in step S 17  that the load of the motor M 1  has been increased, a jam may have occurred. Therefore, the feeding of an Nth sheet from the feed tray is put on hold in step S 21 , so it possible to suppress the jam from worsening. Since (N−1)th and Nth conveyances are continued, image forming on (N−1)th and Nth sheets P can be continued, so it is possible to suppress a throughput in image forming from being lowered. Since the control unit  100  can resume the conveyance of the (N−1)th sheet P, which has been put on hold in step S 25 , it is possible to suppress the throughput in image forming from being lowered. Since the average of duty cycles is calculated in step S 13  and calculated averages are compared in step S 17 , effects due to variations in the magnitude of the load can be reduced and the control unit  100  thereby can accurately determine an increase in the load. In the calculation of the average in step S 13 , an increase in the load is excluded that is caused by a switchover of the electromagnetic clutch  93 , which transmits the driving force to the optional supply roller  73 , to the transmission state and by a switchover of the electromagnetic clutch  86 , which transmits the driving force to the developing roller  33 , to the transmission state. Therefore, precision in the detection of an increase in the load due to a jam is improved, so it is possible to prevent conveyance from being unnecessarily put on hold. If the control unit  100  decides in step S 37  that the load of the motor M 1  has been increased, a jam may have occurred in the re-conveying route Ra. Therefore, the control unit  100  puts the conveyance of the sheet P on hold in step S 39 . This can suppress the jam from worsening. 
         [0061]    The present invention is not limited to the embodiment described above. It will be appreciated that various improvements and modifications are possible without departing from the intended scope of the present invention. 
         [0062]    Although, in the above description, the load of the motor M 1  has been determined according to the duty cycle of the PWM signal, this is not a limitation; the load may be determined according to the value of a current flowing in the motor M 1 . Although, in the above description, the load of the motor M 1  has been determined by calculating the average of duty cycles of PWM signals, this is not a limitation; a duty cycle at an arbitrary point in time in the period TD 1  may be used or the sum of duty cycles at a plurality of points in time may be calculated. 
         [0063]    In step S 23 , only the Nth conveyance may be checked instead of checking the (N−1)th and Nth conveyances. In this case, the sheet P may be decided to have been successfully conveyed if the state of the pre-registration sensor Se 5  is changed from outputting an OFF signal to outputting an ON signal. This decision can be made when the sheet P has not yet reached the pre-registration sensor Se 5  during execution in step S 23 . If the signal from the pre-registration sensor Se 5  is changed to an ON signal, the control unit  100  can decide that the sheet P has reached the pre-registration sensor Se 5 . Alternatively, the sheet P may be decided to have been successfully conveyed if the state of the pre-registration sensor Se 5  is changed from outputting an ON signal to outputting an OFF signal. This decision can be made when the sheet P has already reached the pre-registration sensor Se 5  during execution in step S 23 . If the signal from the pre-registration sensor Se 5  is changed to an OFF signal, the control unit  100  can decide that the rear edge of the sheet P has passed the pre-registration sensor Se 5 . 
         [0064]    The method of deciding whether conveyance is being normally performed is not limited to the above. For example, this decision may be made on the basis of the magnitude of the load. Specifically, if the magnitude of the load is equal to or smaller than a reference value, it can be decided that conveyance is being normally performed. 
         [0065]    Although, in the above description, a predetermined period starting from when the electromagnetic clutch  86  is turned on has been excluded from the calculation of the average of duty cycles of PWM signals in step S 13 , this is not a limitation. For example, duty cycles may be compared with a reference value, and if a period during which a duty cycle is equal to or larger than the reference value is shorter than a predetermined period, the period may be excluded. 
         [0066]    Although, in the above description, the duty cycles of PWM signals have been acquired in a predetermined period taken before the first sheet P is supplied in step S 5 , this is not a limitation. The duty cycles of PWM signals may be acquired in a predetermined period that starts after the elapse of a predetermined time after the motor M 1  has been turned on. 
         [0067]    Although, in this embodiment, the laser printer  1  has been described as an example, this is not a limitation. A so-called multi-function peripheral, which includes a scanner function, a copy function, a facsimile function, and the like, may be used. Further, the laser printing components comprising the image forming unit are also described as an example. In other embodiments, the image forming unit could employ other printing technologies, such as ink jet printing. 
         [0068]    Although, in this embodiment, a case in which the control unit  100  includes the CPU  101  and ASIC  104  has been described as an example, this is not a limitation. The control unit  100  may include a plurality of CPUs or may include a plurality of ASICs. Alternatively, the control unit  100  may be an arbitrary combination of CPUs and ASICs.