Patent Publication Number: US-8995901-B2

Title: Image forming apparatus and rotation control method for motor driving rotation of timing rollers

Description:
This application is based on application No. 2011-221918 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention pertains to a image forming apparatus such as a printer or copier, and specifically to technology for the transportation of a recording sheet by a pair of timing rollers. 
     (2) Description of the Related Art 
     In an image forming apparatus forming images through electrophotography, for example, the transportation of a recording sheet is temporarily stopped when a leading edge of the recording sheet abuts a nip portion formed in a pair of timing rollers, which are not rotating, in order to transfer a toner image formed on a photosensitive drum to the recording sheet at a correct position. The recording sheet is then transported by starting the rotation of the timing rollers with such timing that the leading edge of the toner image (including whitespace therein) formed on the photosensitive drum and the leading edge of the recording sheet meet at a transfer position. The toner image is then transferred to the recording sheet at the correct position. 
     The rotation drive source for the pair of timing rollers is frequently a motor that also drives the rotation of the photosensitive drum and other components. Rotational force is transmitted from the motor to the timing rollers via a power transmission mechanism. Also, the starting and stopping of rotation by the timing rollers is controlled by the motor, via a clutch provided immediately before the timing rollers in the power transmission mechanism that is switched ON and OFF. 
     In recent years, the system speed has been increased in order to improve the number of images formed per unit time, and the gap for transporting the recording sheet (i.e., the paper gap) has been made smaller. Thus controlling the starting and stopping of the rotation by the timing rollers by controlling the clutch has thus become difficult to execute while still having the timing rollers handle the recording sheet with precision. 
     Conventional technology having a common motor driving the photosensitive drum and the pair of timing rollers has progressed by providing a motor for the timing rollers that is separate from the motor for the photosensitive drum, and removing the clutch from the power transmission mechanism. In such a system, the start and stop of rotation by the timing rollers is controlled by switching the separate motor ON (activation) and OFF (stopping). 
     In such a configuration, a situation may arise where, after one set of image formation operations (hereinafter, a job), a job (hereinafter, later job) using a different system speed (i.e., the transport speed for the recording sheets) than the first job (hereinafter, earlier job) is executed. A relative discrepancy was then observed to arise in terms of image formation position between the first page and subsequent pages of the later job. 
     Upon research into the discrepancy, the inventor has identified backlash in the power transmission mechanism as the cause. The power transmission mechanism between the motor and the timing rollers is made up of a plurality of components, such as gears, for transmitting rotational force. Each of these components has a degree of slack in the direction of rotation, or in other words, has backlash. 
     Thus, due to inertia, the components rotate within the range of the backlash after the motor is stopped. The rotation of the components caused by inertia after the motor is stopped is hereinafter termed momentum-driven rotation. 
     Accordingly, when the motor is restarted, the timing rollers begin to rotate only after the motor has rotated by an amount equivalent to the momentum-driven rotation. That is, a lag occurs between the activation of the motor and the beginning of rotation by the timing rollers, corresponding to the extent of the momentum-driven rotation (this lag is hereinafter termed rotation delay time). 
     The momentum-driven rotation is greater when the rotation speed of the motor is fast (i.e., when the transport speed for the recording sheets is fast), and is smaller when the rotation speed is slow (i.e., when the transport speed for the recording sheets is slow). Accordingly, the rotation delay is greater for jobs at a fast transport speed and is smaller for jobs at a slow transport speed. 
     As such, when, for example, an image formation job on a thick sheet at a slow transport speed is followed by an image formation job on a regular sheet at a fast transport speed, the image formation position for the first recording sheet of the later job is further upstream in the sheet transport direction than the image formation position for subsequent recording sheets. 
     In order to solve this problem, an approach has been devised that involves narrowing the range of tolerance for the component dimensions, so as to reduce the backlash for each component. 
     However, this approach reduces the yield rate of the components, and decreases manufacturability as a result of greatly reduced tolerances during assembly. Also, a certain degree of backlash between gears and the like is indispensible for the ensuring smooth rotation of engaged gears. Backlash can thus never be completely removed. 
     The above-discussed problem is not restricted to situations where a stop occurs at the conclusion of a job and the system speed (i.e., the transport speed for the recording sheets) is changed for a subsequent job. The problem also occurs when the transport speed for the recording sheets is changed during a single job. 
     SUMMARY OF THE INVENTION 
     In consideration of the above-described problem, the present invention seeks to effectively reduce the influence of backlash on the power transmission mechanism by providing a image forming apparatus capable of reducing image discrepancy as much as possible, and providing a rotation control method for the motor driving the rotation of the timing rollers. 
     In a first aspect of the present invention, an image forming apparatus having a transport device including a pair of timing rollers, operable to cause a leading edge of a recording sheet to abut a nip portion of the pair of timing rollers, which are not rotating, and to initiate rotation so as to transport the recording sheet toward a toner image transfer position, the transport device comprising: a motor transmitting rotational force to the pair of timing rollers via a power transmission mechanism such that the pair of timing rollers rotate; and a control unit controlling rotation by the motor, wherein the control unit activates the motor, causes the pair of timing rollers to transport a first recording sheet by rotating at a first rotation speed, and stops the motor once transportation is complete, and when a second recording sheet is to be subsequently transported at a second rotation speed that differs from the first rotation speed, the control unit causes the pair of timing rollers to execute an idle rotation operation of rotating at the second rotation speed, or at another speed closer to the second rotation speed than to the first rotation speed, and then stopping, before beginning transportation of the second recording sheet. 
     In a second aspect of the present invention, an image forming apparatus having a transport device including a pair of timing rollers operable to cause a leading edge of a recording sheet to abut a nip portion of the pair of timing rollers, which are not rotating, and to initiate rotation driving the pair of timing rollers to rotate at a first rotation speed or at a second rotation speed that differs from the first rotation speed, so as to transport the recording sheet toward a toner image transfer position, the transport device comprising: a motor transmitting rotational force to the pair of timing rollers via a power transmission mechanism such that the pair of timing rollers rotate; and a control unit controlling rotation by the motor, wherein the transport device defines transporting the recording sheet at the second rotation speed as a default, and when a final recording sheet of a given image formation job has been transported at the first rotation speed, upon concluding the first image formation job, the control unit causes the pair of timing rollers to execute an idle rotation operation of rotating at the second rotation speed and then stopping. 
     In a third aspect of the present invention, a rotation control method for a motor in an image forming apparatus operable to cause a leading edge of a recording sheet to abut a nip portion of a pair of timing rollers, which are not rotating, and to initiate rotation such that the pair of timing rollers rotate at a first rotation speed, or at a second rotation speed that differs from the first rotation speed, so as to transport the recording sheet toward a toner image transfer position, the motor causing the pair of timing rollers to rotate via a power transmission mechanism, the rotation control method comprising: a first step of activating the motor such that the pair of timing rollers rotate at the first rotation speed, causing the pair of timing rollers to transport a first recording sheet; a second step of stopping the motor once transportation of the first recording sheet is complete; a third step of activating the motor and causing the pair of timing rollers to execute an idle rotation operation at the second rotation speed, or at another speed that is closer to the second rotation speed than to the first rotation speed, and then stopping; and a fourth step of activating the motor such that the pair of timing rollers rotate at the second rotation speed, causing the pair of timing rollers to transport a second recording sheet. 
     In a fourth aspect of the present invention, a rotation control method for a motor in an image forming apparatus operable to cause a leading edge of a recording sheet to abut a nip portion of a pair of timing rollers, which are not rotating, and to initiate rotation such that the pair of timing rollers rotate at a first rotation speed or at a second rotation speed that is a default rotation speed, so as to transport the recording sheet toward a toner image transfer position, the motor causing the pair of timing rollers to rotate via a power transmission mechanism, the rotation control method comprising: a first step of activating the motor such that the pair of timing rollers rotate at the first rotation speed, causing the pair of timing rollers to transport a final recording sheet of a given image formation job; a second step of stopping the motor once transportation of the final recording sheet is complete; and a third step of, once the given image formation job is complete, activating the motor and causing the pair of timing rollers to execute an idle rotation at the second rotation speed, then stopping the motor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other objects, advantages, and features of the invention will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the invention. 
       In the drawings: 
         FIG. 1  is an overall configuration diagram of a tandem printer pertaining to an Embodiment; 
         FIG. 2  is a perspective view diagram illustrating the overall configuration of a pair of timing rollers and a drive mechanism therefor; 
         FIG. 3  is an expanded perspective view diagram illustrating a motor, a power transmission mechanism, and an end portion of the timing rollers; 
         FIG. 4  illustrates helical gears as connecting components of the power transmission mechanism; 
         FIGS. 5A ,  5 B, and  5 C each illustrate a pin connection in connecting components of the power transmission mechanism; 
         FIGS. 6A ,  6 B, and  6 C each illustrate a spur gear and a shaft as connecting components in the power transmission mechanism;  FIGS. 6D and 6E  illustrate spur gears as connecting components in the power transmission mechanism; 
         FIG. 7  illustrates the cause of relative image formation discrepancies between recording sheets; 
         FIG. 8  is a block diagram indicating the overall configuration of a control unit for a printer; 
         FIG. 9  is a flowchart of a program executed by a CPU of a motor drive unit of the control unit; 
         FIG. 10  is a sequence diagram illustrating communication between two CPUs in the control unit; 
         FIGS. 11A ,  11 B,  11 C, and  11 D illustrate specific examples of motor drive control executed by the motor drive unit; and 
         FIG. 12  is a flowchart of a variant program executed by a CPU of a motor drive unit of the control unit. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     An Embodiment of the image forming apparatus pertaining to the present invention is described below, with reference to the accompanying drawings. 
     (Overall Configuration of Image Forming Apparatus) 
       FIG. 1  is an overall configuration diagram of a tandem printer  10  (hereinafter, printer  10 ) pertaining to the present Embodiment. Although this example describes a printer, the present invention is also applicable to another image forming apparatus, such as a copier or FAX machine. 
     As shown in  FIG. 1 , the printer  10  includes a transfer belt  14  suspended horizontally within a housing  12  and running in the direction indicated by arrow A, four imaging units  16 C,  16 M,  16 Y, and  16 K aligned in the running direction of the transfer belt  14 , primary transfer rollers  18 C,  18 M,  18 Y, and  18 K provided in one-to-one correspondence with the imaging units, and a secondary transfer unit  20 . The printer  10  is an intermediate-transfer image forming apparatus in which toner images created by the imaging units  16 C,  16 M,  16 Y,  16 K in one of each component colour are overlaid and temporarily transferred onto the transfer belt  14 , then transferred onto a recording sheet S to form a colour image. 
     Each of the imaging units  16 C,  16 M,  16 Y, and  16 K includes a photosensitive drum  22 C,  22 M,  22 Y, or  22 K serving as an image carrier, as well as a charging unit  24 C,  24 M,  24 Y, or  24 K and a developing unit  26 C,  26 M,  26 Y, or  26 K disposed therearound. Also, an exposure unit  28  is disposed below the imaging units  16 C,  16 M,  16 Y, and  16 K, emitting a modulated laser LB toward the photosensitive drums  22 C,  22 M,  22 Y, and  22 K. The photosensitive drums  22 C,  22 M,  22 Y, and  22 K each rotate in the direction indicated by the respective arrows B. The surface of each photosensitive drum  22 C,  22 M,  22 Y, and  22 K is uniformly charged by the respective charging units  24 C,  24 M,  24 Y, and  24 K, then exposed to the laser LB so as to form a latent image. Each latent image is developed into a toner image by the respective developing units  26 C,  26 M,  26 Y, and  26 K. The developing units  26 C,  26 M,  26 Y, and  26 K supply toner, which is a developing agent, in a respective colour C (cyan), M (magenta), Y (yellow), or K (black) to the photosensitive drums  22 C,  22 M,  22 Y, an  22 K, in accordance with a modulation component of the laser. 
     The toner images formed on the photosensitive drums  22 C,  22 M,  22 Y, and  22 K are sequentially transferred onto the running transfer belt  14  through the effect of a magnetic field produced between the primary transfer rollers  18 C,  18 M,  18 Y, and  18 K and the photosensitive drums  22 C,  22 M,  22 Y, and  22 K. 
     Meanwhile, a recording sheet of a desired type and size is supplied by one of a first paper take-up cassette  30  and a second paper take-up cassette  32  in a recording sheet transport device  29  (hereinafter, a transport device  29 ). 
     The recording sheet S is delivered from the first paper take-up cassette  30  by a first pick-up roller  34 . The recording sheet S so delivered is then transported to a pair of timing rollers  42  by a first vertical transport roller  38  and a second vertical transport roller  40 . The recording sheet S is delivered from the second paper take-up cassette  32  by a second pick-up roller  36 , then transported to the pair of timing roller  42  by the second vertical transport roller  40 . 
     A leading edge of the recording sheet S so transported abuts a nip portion in the timing rollers  42 , which are not rotating. Upon abutting, the recording sheet S is transported downstream in the transport direction, away from the timing rollers  42 , for a predetermined time by rollers sandwiching the recording sheet S therebetween. As a result, the recording sheet S traces a loop. Accordingly, any skew in the transport direction of the recording sheet S is corrected. 
     Then, a later-described motor  64  for the timing rollers  42  is started, thus beginning the rotation of the timing rollers  42 . The recording sheet S is transported to a transfer position in the secondary transfer unit  20  with timing such that the leading edge of the recording sheet S and the toner image (including whitespace therein) transferred onto the transfer belt  14  meet at the transfer position. 
     With respect to the pair of timing rollers  42 , a sensor  44  provided directly upstream in the transport direction serves to detect the leading and trailing edges of the recording sheet S passing through a detection position while being transported along the transport path. The aforementioned skew correction occurs during a predetermined interval that begins when the sensor  44  detects the leading edge of the recording sheet S and involves the rotation of rollers sandwiching the recording sheet S during transport upstream by the timing rollers  42 . 
     The secondary transfer unit  20  transfers the toner images overlaid on the transfer belt  14  to the recording sheet S. The toner images so transferred to the recording sheet S are then fixed by a fixing device  46 . Once fixing is complete, exit rollers  48  cause the recording sheet S to exit onto an exit tray  50 . 
     In the present Embodiment, the first paper take-up cassette  30  contains thick sheets, while the second paper take-up cassette  32  contains regular sheets. The regular sheets are, for example, recording sheets each having a weight ranging from 64 g/m 2  to 90 g/m 2 . The thick sheets are recording sheets having more weight than the regular sheet and having a thickness that is greater than that of the regular sheets. The speed at which the recording sheet S is transported by the timing rollers  42  is a low transport speed LS when one of the thick sheets is used, and is a high transport speed HS when one of the regular sheets is used, the high transport speed HS being faster than the low transport speed LS. The speed at which the timing rollers  42  transport the recording sheet is none other than the system speed of the printer  10 . Thus, the transport speed and the system speed are hereinafter defined as being identical. Accordingly, the low transport speed LS is also a low system speed LS, and the high transport speed HS is also a high system speed HS. 
     The printer  10  comprises an operation panel  52 , disposed such that an upper face thereof is easily operated. The operation panel  52  includes a liquid crystal display unit, a menu selection key, cursor keys, a cancel key, and the like (none diagrammed). Operating the menu selection key and the cursor keys enables selection of a menu to be displayed on the liquid crystal display unit, and enables the execution of various settings. For example, these settings include setting the type and size of the recording sheets contained in the first paper take-up cassette  30  and the second paper take-up cassette  32 . 
     The printer  10  further comprises a control unit  54 . The control unit  54  controls the above-described units and devices in unison to execute smooth printing operations. 
     (Timing Rollers) 
       FIG. 2  is a perspective view diagram illustrating the overall configuration of the pair of timing rollers  42  and a drive mechanism therefor. 
     The pair of timing rollers  42  is made up of a driving roller  56  and a driven roller  58 , operating as a pair. The driving roller  56  has a core  60  made from an aluminium pipe having a plurality of rolls  62 , each made of rubber, fitted thereover and separated from each other by gaps in the length direction. The driven roller  58  is radially pressed toward the driving roller  56  by a spring or similar resilient material (not diagrammed). Thus, a nip portion is formed at the point of contact between the driving roller  56  and the driven roller  58 . 
     A spur gear  78  is attached to one end of the core  60 . 
     The driving roller  56  in the pair of timing rollers  42  is driven to rotate by rotational force imparted thereto by the motor  64  via a power transmission mechanism that includes spur gear  78 . In the present example, a stepping motor is used as the motor  64 . 
     (Power Transmission Mechanism) 
       FIG. 3  is an expanded perspective view diagram illustrating the motor  64 , the power transmission mechanism  66 , and end portions of the pair of timing rollers  42 . 
     The motor  64  has an output shaft  68  with helical gear  70  attached thereto. 
     Helical gear  70  meshes with another helical gear  72 , which has a larger radius. 
     An axial bore (not diagrammed in  FIG. 3 ) is provided at the centre of helical gear  72 , and has a shaft  74  inserted therein. The rotational force of helical gear  72  is transmitted to the shaft  74  by a later-described connection pin  86 . 
     An end portion of the shaft  74  is cut at the circumference so as to have a D-shaped cross section, as described later (the end portion of the shaft  74  so cut is hereinafter referred to as a D-cut portion  74 A). 
     Another spur gear  76  having a D-shaped axial bore is attached to the D-cut portion  74 A of the shaft  74 , by having the D-cut portion  74 A be inserted into the axial bore. 
     Spur gear  78  meshes with spur gear  76  at the end portion of the driving roller  56 . 
     In the power transmission mechanism  66 , configured as described above, when the motor  64  is activated and causes the output shaft  68  to rotate in the direction indicated by arrow C, helical gear  70  is enjoined to also rotate in the direction of arrow C. As helical gear  70  rotates in the direction of arrow C, helical gear  72  engaged therewith rotates in the direction of arrow E. 
     Then, helical gear  72  enjoins the shaft  74 , and subsequently, spur gear  76 , to also rotate in the direction of arrow E. As spur gear  76  rotates in the direction of arrow E, spur gear  78  engaged therewith rotates in the direction of arrow F, and the driving roller  56  is enjoined to also rotate in the direction of arrow F. The driven roller  58 , which is in contact with the driving roller  56 , then rotates in the direction of arrow G. 
     According to the above sequence, the rotational force of the motor  64  is transmitted to drive the rotation of the timing rollers  42 . 
     (Power Transmission Delay Due to Slack in Power Transmission Mechanism) 
     Delays to power transmission caused by slack in the direction of rotation at neigbouring components (any connecting components) of the power transmission sequence of the power transmission mechanism  66  are discussed below with reference to  FIGS. 4 ,  5 A through  5 C, and  6 A through  6 E. 
     (i) Helical Gear  70  and Helical Gear  72   
     Slack between connecting components is further described below with reference to  FIG. 4 . Among these connecting components, the driver is helical gear  70 , which rotates in the direction of arrow C, and helical gear  72  is driven thereby, receiving power from helical gear  70  in the axial direction. As described later, helical gear  72  loosely engages with the shaft  74  and thus displaced in the direction of arrow H upon receiving the force in the axial direction. A retaining ring  80  is attached to the shaft  74  and serves as a stopper preventing the separation of helical gear  72  and helical gear  70  and restricting the displacement of helical gear  72  during driving. Accordingly, a gap  82  formed in the axial direction between the two components in contact with the retaining ring  80  is eliminated on the side facing helical gear  72 , during driving. 
     When the rotation of helical gear  70  stops, the rotation of helical gear  72  continues due to momentum. Upon rotating by an amount equivalent to the backlash between the two gears, a counterforce in the thrust direction is imparted to helical gear  72  by helical gear  70 , causing displacement in the directions of arrow H and the opposing arrow J (this displacement is hereinafter termed momentum displacement). The momentum displacement increases with faster rotational speed of helical gear  72  prior to stopping the rotation (due to greater inertia). A retaining ring  84  is provided opposite retaining ring  80  in order to constrain the maximum momentum displacement. 
     Once the rotation of helical gear  70  is restarted, helical gear  72  begins to rotate and to be displaced in the direction of arrow H only after helical gear  70  has rotated by the amount equivalent to the backlash. Then, once helical gear  72  comes into contact with retaining ring  80 , helical gear  72  begins to rotate normally in accordance with the reduction ratio. 
     That is, once helical gear  70  resumes rotation, helical gear  72  begins normal rotation only after displacement in the direction of arrow H corresponding to the momentum displacement. Accordingly, a lag corresponding to the momentum displacement occurs between the beginning of rotation by helical gear  70  and the beginning of normal rotation by helical gear  72 . As described above, the momentum displacement depends on the speed of rotation prior to stopping. As a result, the faster the speed of rotation prior to stopping, the longer the lag, and conversely, the slower the speed, the shorter the lag. 
     (ii) Helical Gear  72  and Shaft  74   
     Slack between connecting components is further described below with reference to  FIG. 5 . Among the following components, the driver is helical gear  72  and the shaft  74  is driven thereby. 
     As shown in  FIG. 5A , the shaft  74  has a hole  74 B passing radially therethrough, and the connection pin  86  is loosely inserted in the hole  74 B. A groove  72 A is formed on one side of helical gear  72  so as to be elongated in the radial direction. An exposed portion of the connection pin  86  is implanted in the groove  72 A. The clearance between the radius of the hole  74 B and the connection pin  86  and the clearance between the connection pin  86  and the groove  72 A is set as appropriate, in consideration of the assemblage connected thereto. 
     In the above configuration, as shown in  FIG. 5B , the inner walls of the groove  72 A and two corners of the connection pin  86  come into contact once helical gear  72  begins to rotate in the direction of arrow E, such that the connection pin  86  also rotates in the direction of arrow E. Once the connection pin  86  begins to rotate, the circumferential surface of the connection pin  86  comes into contact with the circumferential edge of the hole  74 B in the shaft  74 . Accordingly, the shaft  74  also rotates in the direction of arrow E. 
     When the rotation of helical gear  72  stops, the rotation of the connection pin  86  and the shaft  74  continues due to momentum. The magnitude of this rotation due to momentum increases with faster rotational speed (due to increased inertia) of the connection pin  86  and the shaft  74 , prior to stopping.  FIG. 5C  shows a state of maximum rotation. 
     When helical gear  72  resumes rotation, and after helical gear  72  has rotated by an amount corresponding to the magnitude of the above-described momentum-driven rotation, power is transmitted to the shaft  74 , which begins to rotate. 
     Accordingly, a lag corresponding to the momentum-driven rotation occurs between the beginning of rotation by helical gear  72  and the beginning of normal rotation by the shaft  74 . As described above, the magnitude of the momentum-driven rotation depends on the speed of rotation prior to stopping. As a result, the faster the speed of rotation prior to stopping, the longer the lag, and conversely, the slower the speed, the shorter the lag. 
     (iii) Shaft  74  and Spur Gear  76   
     Slack between connecting components is further described below with reference to  FIGS. 6A ,  6 B, and  6 C. Among the following components, the driver is the shaft  74  and spur gear  76  is driven thereby. 
     As described above and shown in  FIG. 6A , spur gear  76  has a D-shaped axial bore  76 A and the shaft  74  has the D-cut portion  74 A. The D-cut portion  74 A is inserted into the axial bore  76 A so as to be able to transmit rotational force from spur gear  76  to the shaft  74 . 
     As shown in  FIG. 6B , once the shaft  74  begins to rotate in the direction of arrow E, the right-hand edge portions of the respective D-cut faces come into contact. Power is thus transmitted to spur gear  76 , causing spur gear  76  to rotate in the direction of arrow E. 
     When the rotation of the shaft  74  stops, the rotation of spur gear  76  continues due to momentum. The magnitude of this momentum-driven rotation increases with faster rotational speed (due to increased inertia) of spur gear  76 , prior to stopping.  FIG. 6C  shows a state of maximum rotation. 
     Upon resuming rotation, the shaft  74  rotates by an amount corresponding to the magnitude of the above-described momentum-driven rotation. The rotation of spur gear  76  resumes only when the state illustrated in  FIG. 6B  is reached. 
     Accordingly, a lag corresponding to the momentum-driven rotation occurs between the beginning of rotation by the shaft  74  and the beginning of normal rotation by spur gear  76 . As described above, the magnitude of the momentum-driven rotation depends on the speed of rotation prior to stopping. As a result, the faster the speed of rotation prior to stopping, the longer the lag, and conversely, the slower the speed, the shorter the lag. 
     (iv) Spur Gear  76  and Spur Gear  78   
     Slack between connecting components is further described below with reference to  FIGS. 6D and 6E . Among these components, the driver is spur gear  76  and spur gear  78  is driven thereby. The slack between these components is in the form of backlash between spur gear teeth on each of the gears. 
       FIG. 6D  illustrates a situation where spur gear  76  rotates, power is transmitted to spur gear  78  engaged therewith, and spur gear  78  rotates in the direction of arrow F. 
     When the rotation of spur gear  76  stops, the rotation of spur gear  78  continues due to momentum, within the backlash range. The magnitude of this momentum-driven rotation increases with faster rotational speed (due to increased inertia) of spur gear  78 , prior to stopping.  FIG. 6E  shows a state of maximum rotation. 
     Upon resuming rotation, spur gear  76  rotates by an amount corresponding to the magnitude of the above-described momentum-driven rotation. The rotation of spur gear  78  resumes only when the state illustrated in  FIG. 6D  is reached. 
     Accordingly, a lag corresponding to the momentum-driven rotation occurs between the beginning of rotation by spur gear  76  and the beginning of normal rotation by spur gear  78 . As described above, the magnitude of the momentum-driven rotation depends on the speed of rotation prior to stopping. As a result, the faster the speed of rotation prior to stopping, the longer the lag, and conversely, the slower the speed, the shorter the lag. 
     Although the above describes a lag occurring between the beginning of rotation by the driver and the beginning of rotation by the driven component for each set of connected components, the lag is also perceivable as a cumulative lag of the power transmission mechanism  66  as a whole. The cumulative lag is equivalent to the sum of the above-described lags, and occurs between the activation of the motor  64  (i.e., the beginning of rotation) and the beginning of rotation by the pair of timing rollers  42 . Accordingly, the faster the rotational speed of the motor (and consequently, of the timing rollers) prior to stopping, the longer the lag between the next activation of the motor and the beginning of rotation by the timing rollers. Likewise, the slower the speed of the motor (and consequently, of the timing rollers) prior to stopping, the shorter the lag between the next activation of the motor and the beginning of rotation by the timing rollers. 
     The phenomenon of the lag between the activation of the motor  64  and the beginning of rotation by the timing rollers  42  is hereinafter termed rotation delay, and the lag itself is called a rotation delay time. 
     (Image Position Discrepancies) 
     When the speed at which the recording sheet is transported by the timing rollers (i.e., the rotation speed of the timing rollers) is changed, a mutual discrepancy, caused by the above-described rotation delay, arises between the toner image formed on the recording sheet immediately after the change and the formation of subsequent toner images on the recording sheet. The cause is further described below. 
       FIG. 7  is a graph in which the horizontal axis represents time, the vertical axis represents rotation speed of the timing rollers (solid lines), and an operation diagram of the motor driving the timing rollers is superimposed (dashed lines) thereon. The vertical axis of the operation diagram is the aforementioned rotation speed of the motor. The vertical axis represents the rotation speeds of the pair of timing rollers and of the motor at different scales. Note that  FIG. 7  is a schematic diagram provided in order to explain the aforementioned discrepancies, and is not an accurate representation of the rotation speeds. 
       FIG. 7  illustrates a case in which a thick sheet image formation job is followed by a regular sheet image formation job. The (steady) rotation speed of the timing rollers during image formation on a thick sheet is denoted PL, and the (steady) rotation speed of the timing rollers during image formation on a regular sheet is denoted PH (where PH&gt;PL). 
     As indicated by the portion of the graph for the final thick sheet, the timing rollers begin to rotate after the rotation delay time T 1  has elapsed since motor activation. Also, due to momentum, the timing rollers continue to rotate for time D 1  after the motor is stopped. 
     For the first recording sheet of a regular sheet job executed after completing the thick sheet job, the rotation of the timing rollers begins after rotation delay time T 1  has elapsed since the motor activation, in accordance with rotation speed PL. Conversely, once the motor is stopped, the timing rollers continue to rotate for a time (time D 2 ) that is longer than time D 1 , proportional to the extent to which the rotation speed is greater than that used for the thick sheet (i.e., PH&gt;PL). 
     Then, for the second regular sheet, the rotation of the timing rollers begins after rotation delay time T 2  (where T 2 &gt;T 1 ) has elapsed since motor activation, in accordance with the rotation speed PH of the first sheet. Likewise, for the third sheet and subsequent sheets, the rotation of the timing rollers begins after rotation delay time T 2  has elapsed since motor activation, in accordance with the rotation speed PH of the preceding (regular) recording sheet. 
     The time at which the recording sheet begins to be transported from the timing rollers to the transfer position for the toner image is measured beginning at the activation of the motor. Thus, the longer the rotation delay time, the later the recording sheet arrives at the transfer position. Thus, the toner image arrives at the transfer position relatively sooner. As a result, the toner image is formed on the recording sheet closer to the leading edge, with respect to the direction of transport. 
     Consideration of the first and second regular sheets reveals that the rotation delay time differs therebetween (such that T 1 &lt;T 2 ). The toner images formed (transferred) on the first and second regular sheets differ in that the image formed on the second sheet is closer to the leading edge than that formed on the first sheet. Thus, a relative discrepancy arises between sheets. No such discrepancy exists between the second sheet and subsequent recording sheets, given that the rotation delay time remains constant (i.e., is T 2 ). 
     Although omitted from the drawings, the opposite case, i.e., a case where an image formation job on thick sheets follows an image formation job on regular sheets, may also occur. In such a case, respective images are formed closer to the trailing edges the second and subsequent thick sheets than to the trailing edge of the first thick sheet, such that a relative discrepancy arises between the first sheet and the subsequent sheets. 
     In response to the above described problem, the present Embodiment has the timing rollers execute an idle rotation operation when the transport speed for the recording sheet is changed, such that the timing rollers stop and perform an idle rotation at the post-change rotation speed before beginning the transportation of the recording sheet at the post-change rotation speed. 
     With reference to  FIG. 7 , the timing rollers are made to rotate at rotation speed PH and then stop between the final thick sheet and the first regular sheet. Accordingly, the rotation delay time for the first regular sheet is time T 2 , thus matching the rotation delay time for the second and subsequent sheets. As such, the above-described image formation discrepancy is constrained as much as possible. 
     (Control Unit) 
     The control unit  54  executing the above-described controls, including the idle rotation, is described below with reference to  FIG. 8 . 
       FIG. 8  is a block diagram indicating the overall configuration of the control unit  54 . 
     As indicated, the control unit  54  includes an image data reception unit  102 , an image data writing unit  104 , an image memory  106 , a laser diode drive unit  108 , a motor drive unit  110 , CPU  112 , and ROM  114 . 
     In accordance with an instruction from CPU  112 , the image data reception unit  102  applies various correction processes, such as edge enhancement, to image data in an image formation job received from a personal computer or the like, then transmits the image data to the image data writing unit  104 . 
     In accordance with the instruction from CPU  112 , the image data writing unit  104  writes the image data transmitted thereto by the image data reception unit  102  to the image memory  106 . 
     In accordance with the instruction from CPU  112 , the laser diode drive unit  108  reads the image data from the image memory  106  and according to the data so read, drives the modulation of (non-diagrammed) laser diodes provided for each colour C, M, Y, and K with respect to the exposure unit  28 . 
     The motor drive unit  110  controls the activation, stopping, and rotation speed of the motor  64 . The motor drive unit  110  has CPU  116  and executes control upon receiving instructions from the CPU  112 . The motor drive unit  110  also includes a speed setting storage unit  118  storing the transport speed (i.e., the system speed) for recording sheets recently transported by the timing rollers  42 . 
     (Motor Drive Control) 
     Next, the rotation control executed by the motor drive unit  110  on the motor  64  is described with reference to  FIGS. 9 and 10 . 
       FIG. 9  is a flowchart of a motor rotation control program executed by CPU  116  (see  FIG. 8 ) of the motor drive unit  110 .  FIG. 10  is a sequence diagram representing exchanges between CPU  116  and CPU  112  during program execution. In the sequence diagram, the CPU  116  side is labeled with step numbers corresponding to the flowchart. 
     The program under discussion is activated by an instruction from CPU  112  upon reception of a new image formation job. As the following explanation clarifies, when the program is activated, the speed setting storage unit  118  stores the transport speed for the last recording sheet of the most recent previously-completed image formation job. 
     For the image formation job received by the image data reception unit  102 , CPU  112  reads header information for each page, determines a system speed (i.e., the transport speed) for the next page (i.e., recording sheet) using the header information, and notifies CPU  116  of the system speed (i.e., the transport speed) so determined (q 1 ). Specifically, the header information includes information specifying whether the recording sheet to be used for printing the page is a regular sheet or a thick sheet. The determination results in using the high system speed HS (i.e., the high transport speed HS) when a regular sheet is to be used, and using the low system speed (i.e., low transport speed LS) when a thick sheet is to be used. 
     When a particular page (i.e., recording sheet) is the last page (i.e., the final recording sheet) of a given image formation job, the header information also includes information to such effect. For the final recording sheet, CPU  112  notifies CPU  116  that the recording sheet is final, along with the system speed (i.e., the transport speed) therefor. 
     Upon receiving such a notification (Yes in step S 1 ), CPU  116  compares the system speed (i.e., the transport speed) so received and the transport speed in the speed setting storage unit  118  (step S 2 ). If the two are identical (Yes in step S 2 ), CPU  116  waits for a motor  64  rotation instruction (q 3 ) from CPU  112  (step S 3 ). 
     CPU  112  issues a motor  64  rotation instruction to CPU  116  which such timing that the leading edge of the recording sheet abutting the nip portion of the timing rollers  42 , which are not currently rotating, matches the leading edge of the toner image (including whitespace therein) formed on the transfer belt  14 , which is currently rotating, at the transfer position in the secondary transfer unit  20  (q 3 ). Upon receiving the instruction (Yes in step S 3 ), CPU  116  activates the motor  64  (step S 4 ), causing the motor  64  to rotate at a rotation speed corresponding to the transport speed stored in the speed setting storage unit  118 . 
     Accordingly, the recording sheet begins to be transported by the timing rollers  42 . 
     Once the trailing edge, with respect to the transport direction, of the transported recording sheet is deemed to have passed through the timing rollers  42  (Yes in step S 5 ), the transport of the recording sheet by the timing rollers is deemed complete. CPU  116  then stops the motor  64  (step S 6 ). In step S 5 , the trailing edge of the recording sheet is deemed to have passed through the timing rollers  42  once a predetermined interval elapses after the sensor  44  (see  FIG. 1 ) detects the trailing edge. The predetermined interval is the time needed for the trailing edge to move from the detection position of the sensor  44  to the nip portion of the timing rollers  42 . This time is calculated from the transport speed and the distance between the detection position of the sensor  44  and the nip portion of the timing rollers  42 , and is stored in the ROM  114  (see  FIG. 8 ) in advance. 
     If CPU  116  has received a final recording sheet notification from CPU  112  during step S 1  (Yes in step S 7 ), the program is ended. If no such notification has been received (No in step S 7 ), CPU  116  waits for a notification from CPU  112  of the system speed (i.e., the transport speed) from the next page (i.e., recording sheet) on which to perform image formation (step S 1 ). 
     Also, if the result of step S 2  is that the received system speed (i.e., the transport speed) and the transport speed stored in the speed setting storage unit  118  are different (No in step S 2 ), CPU  116  changes the transport speed stored in the speed setting storage unit  118  to the received transport speed (step S 8 ), and makes an idle rotation instruction request to CPU  112  (step S 9 ). 
     Upon receiving the idle rotation instruction request, CPU  112  makes an idle rotation instruction (q 2 ) to CPU  116 , timed such that the idle rotation is completed before the leading edge of the recording sheet supplied by the first paper take-up cassette  30  or by the second paper take-up cassette  32  arrives at the nip portion of the timing rollers  42 . Upon receiving the idle rotation instruction (q 2 ) from CPU  112  (Yes in step S 10 ), CPU  116  activates the motor  64  (step S 11 ) and causes the motor  64  to rotate at a speed corresponding to the transport speed stored in the speed setting memory unit  118 . CPU  116  stops the motor  64  once predetermined time Tk has elapsed since activation (Yes in step S 12 ). Predetermined time Tk is beneficially set to the minimum value needed for ordinary rotation of the timing rollers  42  at the speed corresponding to the transport speed stored in the speed setting memory unit  118 . 
     The process then advances to step S 3 . CPU  116  waits for a motor  64  rotation instruction (q 3 ) from CPU  112 , intended for starting the transportation of the recording sheet by the timing rollers  42 . 
     Then, the above-described process is repeated until the transportation of the final recording sheet is completed (Yes in step S 7 ). 
     (Specific Example of Motor Driving) 
     A specific example of driving the motor  64  by executing the above-described control is given below, with reference to  FIGS. 11A through 11D . 
       FIGS. 11A through 11D  are line drawings each showing the operations of the motor  64 , with the horizontal axes representing time and the vertical axes representing the rotation speed (rotations per unit time) of the motor  64 . The label RL on the vertical axes indicates the rotation speed used for image formation on a thick sheet, when the timing rollers  42  rotate at rotation speed PL. Similarly, the label RH indicates the rotation speed used for image formation on a regular sheet, when the timing rollers  42  rotate at rotation speed PH. Needless to say, the values of RL and RH are such that RH&gt;RL. 
     The following example is illustrated by  FIGS. 11A through 11D  and is described by correspondence to the flowchart of  FIG. 9 . 
     In  FIG. 11A , an image formation job on a thick sheet (hereinafter, a thick sheet job) is followed by an image formation job on a regular sheet (hereinafter, a regular sheet job). 
     When the transport of the final recording sheet of the thick sheet job is complete, the low transport speed LS is stored in the speed setting memory unit  118  (see  FIG. 8 ). For the first page of the regular sheet job, CPU  112  notifies CPU  116  of the high transport speed HS (i.e., the high system speed HS). The transport speed in the notification differs from the previously-used (for the final recording sheet of the thick sheet job) transport speed (i.e., the transport speed stored in the speed setting memory unit  118 ) (No in step S 2 ). Thus, before transportation begins for the first recording sheet of the regular sheet job (step  4 ), the timing rollers  42  execute idle rotation at the transport speed for the regular sheets (i.e., the high transport speed HS) (steps S 11 -S 13 ). 
     Accordingly, the rotation delay time for the first page of the regular sheet job is T 2  (see  FIG. 7 ). The transport speed does not change for the second and subsequent sheets of the regular sheet job. Thus, the rotation delay time therefor is also T 2  (see  FIG. 7 ). This has the effect of constraining the possibility of a relative discrepancy arising, with respect to the transport direction, between the first page and subsequent pages of the regular sheet job, in terms of the formation (i.e., the transfer) of the toner image on the recording sheet. 
     Thus, according to the present Embodiment, the possibility of a relative discrepancy arising between the first page and subsequent pages of later regular sheet jobs is constrained, despite the difference in system speed (i.e., in the speed at which the timing rollers  42  transport the recording sheet) between successive image formation jobs. 
       FIG. 11B  indicates an image formation job in which some images are formed on thick sheets and other images are formed on regular sheets (hereinafter, a mixed job). In this example, the first and second sheets are thick sheets, while the third and fourth sheets are regular sheets. 
     Once the transportation of the second recording sheet (a thick sheet) is complete, the low transport speed LS is stored in the speed setting memory unit  118  ( FIG. 8 ). For the third sheet, CPU  112  notifies CPU  116  of the high transport speed HS (i.e., the high system speed HS). The transport speed in the notification differs from the previously-used (for the thick sheet of the second page) transport speed (i.e., the transport speed stored in the speed setting memory unit  118 ) (No in step S 2 ). Thus, before transportation begins for the regular sheet third sheet (step  4 ), the timing rollers  42  execute idle rotation at the transport speed for the regular sheets (i.e., the high transport speed HS) (steps S 11 -S 13 ). 
     Accordingly, the rotation delay time for the third page is T 2  (see  FIG. 7 ). The transport speed does not change for the fourth sheet. As such, the rotation delay time therefor is also T 2  (see  FIG. 7 ). This has the effect of constraining the possibility of a relative discrepancy arising, with respect to the transport direction, between the third and fourth page, in terms of the formation (i.e., the transfer) of the toner image on the recording sheet. 
     As such, according to the present Embodiment, the possibility of a relative discrepancy arising between the first page and subsequent pages of later regular sheet jobs is constrained when the system speed (i.e., the speed at which the recording sheet is transported by the timing rollers  42 ) is changed during a mixed job. 
       FIG. 11C  shows an example like that of  FIG. 11A , differing in that time Ta, from the end of the idle rotation of the motor  64  to the activation of the motor  64  for the transport the first recording sheet, is equal to time Tb, from the end of the transportation of a given recording sheet by the motor  64  to the activation of the motor  64  for the transport of the next recording sheet. 
     When Ta is significantly longer than Tb, the members of the connecting components in the power transmission mechanism are prone to rotation during time Ta, caused for example by vibrations within the housing  12 , despite the motion of the connecting components being stopped after completing the idle rotation. Consequently, a situation may arise in which the state of the connecting components in the power transmission mechanism at motor  64  activation time for the transportation of the first recording sheet differs from the state of the connecting components in the power transmission mechanism at motor  64  activation time for the transportation of the second recording sheet. 
     However, the above-described approach allows matching of the state of the connecting components in the power transmission mechanism when the motor  64  is activated for the transport of the first recording sheet (in terms of momentum-driven rotation and so on) and the state of the connecting components in the power transmission mechanism when the motor  64  is activated for the transport of the second recording sheet (in terms of momentum-driven rotation and so on). Thus, the rotation delay time for the first and second recording sheets can be equalized. As a result, the relative discrepancy between the first sheet and subsequent sheets is further constrained. 
     Ta and Tb are equalized by having CPU  112  and CPU  116  cooperate to plan the timing of each instance where the motor is stopped (step S 13 , step S 6 ) and started (step S 4 ), such that the time (Ta) between one instance where the motor is stopped (step S 13 ) and started (step S 4 ) is equal to the time (Tb) between another instance where the motor is stopped (step S 5 ) and started (step S 4 ). 
     (Variations) 
       FIG. 11D  is a line diagram of operations pertaining to rotation control executed in a variation. 
     The present variation describes a situation in which preparations are made for using a regular sheet as the recording sheet. In other words, the high transport speed HS is being set up as the transport speed for the recording sheet. This preparation is predicated on the regular sheets being more frequently used than the thick sheets. 
     Once the above-described settings are in place, the timing rollers  42  perform an idle rotation at rotation speed PH after the final recording sheet has been transported whenever the final recording sheet is a thick sheet. 
     Accordingly, when the next job is executed using regular sheets some time after the thick sheet is used, there is no need to perform a further idle rotation. The transportation of the first recording sheet by the timing rollers  42  can begin sooner, as the image formation time (FCOT or FPOT) corresponding thereto is not needlessly extended. 
     A program for executing the above-described control is described with reference to the flowchart of  FIG. 12 . 
       FIG. 12  is a flowchart of a program pertaining to the present variation. The steps described by  FIG. 12  are executed after step S 7  of the flowchart from  FIG. 9 . That is, the program pertaining to the present variation is made up of steps S 1  through S 13  from  FIG. 9  and steps S 14  through S 18  from  FIG. 12 . 
     When CPU  116  has received a notification from CPU  112  during step S 1  concerning the final recording sheet (Yes in step S 7 ), CPU  116  makes a determination as to whether or not the high transport speed HS is stored in the speed setting storage unit  118  (step S 14 ). 
     When the high transport speed HS is so stored (Yes in step S 14 ), the program ends. 
     Conversely, when the low transport speed LS is stored (No in step S 14 ), the speed setting stored in the speed setting storage unit  118  is rewritten with the high transport speed HS (step S 15 ). Afterward, the timing rollers  42  perform an idle rotation at the high rotation speed PH (steps S 16 , S 17 , S 18 ). The program then ends. The process of steps S 16  through S 18  is identical to that of steps S 11  through S 13  from  FIG. 9 . The details thereof are thus omitted. 
     When the program has ended and the next job uses regular sheets (regardless of whether the final recording sheet of the preceding job was a thick sheet), the idle rotation is not repeated before the transportation of the first recording sheet for the next job begins (Yes in step S 2  (see  FIG. 9 )). Thus, the image formation time (FCOT or FPOT) for the first recording sheet is not needlessly extended. 
     When the thick sheets are used more frequently than the regular sheets, the inverse of the above applies, such that the thick sheets are considered to be the default recording sheets. For such cases, the determination made in step S 14  ( FIG. 12 ) concerns whether or not the low transport speed LS is stored in the speed setting storage unit  118 . Then, when the high transport speed HS is stored (No in step S 14 ), the speed setting stored in the speed setting storage unit  118  is re-written with the standard low transport speed LS (step S 15 ). Afterward, the timing rollers  42  execute an idle rotation at the low rotation speed PL (steps S 16 , S 17 , S 18 ). The program then ends. 
     Most importantly, when the thick sheets are established as the default recording sheets, then when the final recording sheet of a job is a regular sheet, an idle rotation at the low rotation speed PL is not necessarily required at the conclusion of the regular sheet job. 
     When image formation on a thick sheet follows image formation on a regular sheet, the fixing temperature of the fixing device  46  is raised higher than that used for a regular sheet. The idle rotation at the low rotation speed PL can be executed concurrently with this temperature rise. As such, there is little need for an idle rotation to be performed ahead of time. 
     The program for the above-described variation includes steps S 1  through S 13  from  FIG. 9  and steps S 14  through S 18  from  FIG. 12 , and the control unit  54  has been described as executing steps S 1  through S 18 . However, no limitation is intended. The control unit  54  may also execute only the steps corresponding to the indications given by  FIG. 12 . 
     That is, when the regular sheets are established as the default recording sheets to be used, i.e., when the default speed at which the recording sheets are transported is set to the high transport speed HS, and the final recording sheet of a given job is a thick sheet (i.e., when the speed at which the final recording sheet is transported is the low transport speed LS), then the timing rollers  42  perform an idle rotation at the rotation speed PH once the transportation of the final recording sheet is complete (i.e., the timing rollers  42  perform an idle rotation at rotation speed PH when the given job ends). 
     The merits of this approach are the same as those of the above-described variation. 
     Although the present invention is described above with reference to the Embodiment, no limitation thereto is intended. For example, the following variations are also possible. 
     (1) In the above-described Embodiment, two paper take-up cassettes are provided, thick sheets and regular sheets are used as the recording sheets, and the recording sheets are transported at two speeds. However, no limitation is intended. Three or more paper take-up cassettes may be provided, the varieties of recording sheets may be increased in number, and the recording sheets may be transported at three or more different speeds. 
     In such circumstances, when the transport speed for the recording sheets (i.e., the rotation speed for the timing rollers) is changed, the timing rollers perform an idle rotation at the post-change rotation speed, prior to beginning the transportation of the first recording sheet after the change. This control can be executed by the program of the flowchart from  FIG. 9 . 
     (2) In the above-described Embodiment, the transport speed for the recording sheets (i.e., the system speed) is changed according to the type of recording sheet being used. However, no limitation is intended. The transport speed for the recording sheets (i.e., the system speed) may also be changed according to whether a monochrome image or a colour image is being formed. In such circumstances, the transport speed for the recording sheets (i.e., the system speed) used for colour image formation is slower than that used for monochrome image formation.
 
(3) In the above-described Embodiment, whenever the system speed (the transport speed for the recording sheets) is changed, the timing rollers  42  perform an idle rotation at a system speed corresponding to the post-change system speed before beginning the transportation of the recording sheet at the post-change system speed. In other words, the motor  64  is activated, causes the timing rollers  42  to rotate at a first speed, completes the transportation of a first recording sheet, then is stopped. Subsequently, when a second recording sheet is transported at a second rotation speed that differs from the first rotation speed, the timing rollers  42  perform an idle rotation at the second rotation speed before beginning the transportation of the second recording sheet.
 
     However, the rotation speed of the timing rollers  42  for the idle rotation is not limited to the second rotation speed. Any speed that is closer to the second rotation speed than the first rotation speed may be used. 
     Accordingly, the state of the connecting components in the power transmission mechanism  66  when the motor  64  is activated to transport the second recording sheet (e.g., the momentum-driven rotation) is more similar to the state of the connecting components in the power transmission mechanism  66  when the motor  64  is activated to transport subsequent recording sheets, relative to a case where no idle rotation occurs. Thus, the rotation delay time before the transportation of the second recording sheet is made more similar to the rotation delay time before the transportation of the subsequent recording sheet than is the case in the absence of the idle rotation. As a result, the relative discrepancy between the images formed on the second recording sheet and on the next recording sheet can be made smaller than is the case in the absence of the idle rotation. 
     (4) The above Embodiment is described as a printer. However, for a copier, the image data reception unit (see  FIG. 8 ) may receive an image formation job from an image reading device (i.e., a scanner). Also, the type of recording sheet (e.g., thick sheet or regular sheet) is specified via the operation panel, and the transport speed for the recording sheet (i.e., the system speed) is set in accordance with the type of recording sheet so specified. 
     The transport speed for the recording sheet (i.e., the system speed) may also be determined in accordance with an instruction made via the operation panel specifying a monochrome copy or a colour copy. 
     (5) The above Embodiment describes a stepping motor being used as the rotation drive source for the timing rollers. However, no limitation is intended. A DC motor may also be used, for example. 
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.