Abstract:
An image recording apparatus includes an image forming section, a transport belt, a fixing roller, a discharge roller, and a controller. The image forming section forms a toner image on a print paper. The transport belt transports the print paper onto which the toner image has been transferred. The fixing roller is disposed downstream of the transport belt with respect to a direction of travel of the print paper, and then cooperates with the discharge roller to further advance the paper toward the stacker. The controller controls the speeds of the transport belt, the fixing roller, and discharge roller. The speed of the fixing roller and discharge roller are changed relative to the speed of the transport belt in accordance with a distance from a reference position over which the print paper advanced by the fixing roller and discharge roller toward the paper stacker.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to an image forming apparatus.  
           [0003]    2. Description of the Related Art  
           [0004]    Conventional color image forming apparatuses include printers, copying machines, and facsimile machines. A color image forming apparatus includes image-forming sections that form yellow, magenta, cyan, and black images. Image forming sections are aligned in a direction in which a medium-transporting belt runs. As the medium-transporting belt runs through the image forming sections, yellow, magenta, cyan, and black toner images are transferred onto a print medium in registration with one another to form a full color toner image on the print medium. Then, the print medium is further advanced to a fixing unit where the print medium passes between a heat roller and a backup roller so that the full color toner image is fused into a permanent full-color image.  
           [0005]    With a conventional image recording apparatus, the medium-transport belt can advance the print medium to the fixing unit at a somewhat higher speed than the circumferential speed of the heat roller. This small difference in speed creates slack in the print medium between the heat roller and the backup roller. The amount of slack depends on the length of the print medium. A long print medium will have a significant amount of slack therein and there is a possibility of the toner image formed on the print medium touching a chassis of the fixing unit. Thus, the toner image on the print medium can be damaged with the result that print quality is low.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention was made in view of the aforementioned problems.  
           [0007]    An object of the invention is to provide an image recording apparatus in which the print medium is prevented from contacting the chassis of the fixing unit and image quality is prevented from deteriorating.  
           [0008]    An image recording apparatus includes an image forming section, a first medium-transporting section such as transport belt, a second medium-transporting section such as fixing roller and a discharge roller, and a controller. The image forming section forms a toner image on a medium. The first medium-transporting section transports the medium onto which the toner image has been transferred. The second medium-transporting section receives the medium transported from the first medium transporting section. The second medium-transporting section further transports the medium. The second medium-transporting section is disposed downstream of the first medium-transporting section with respect to a direction of travel of the medium. A controller controls at least one of a first transport speed of the first medium-transporting section and a second transport speed of the second medium-transporting section, so that a relative speed of the first medium-transporting section and the second medium-transporting section is changed. The second transport speed is changed relative to the first transport speed in accordance with a distance from a reference position over which the medium is advanced by the second transporting section toward the stacker.  
           [0009]    The controller sets the second transport speed higher than the first transport speed when the medium has traveled over a distance longer than a predetermined length from the reference position.  
           [0010]    If the medium has traveled over a predetermined distance after the controller sets the second transport speed higher than the first transport speed, the controller sets the second transport speed lower than the first transport speed.  
           [0011]    The controller determines whether the medium has a length greater than a predetermined medium length. The controller changes the second transport speed by a predetermined fraction when the medium has a length greater than the predetermined medium length.  
           [0012]    The image recording apparatus may further include a medium detector and a counter. The medium detector is disposed in a transport path of the medium to detect the medium, and the counter that counts operation parameters of the first medium-transporting section after the medium is detected by the medium detector. The controller checks an output of the counter to determine whether the medium is longer than the predetermined medium length.  
           [0013]    The controller sets a second transport speed higher than the first transport speed when the medium has traveled over a distance longer than a predetermined length from the reference position.  
           [0014]    The first medium-transporting section has a transfer section that transfers the toner image onto the medium. The second medium-transporting section has a fixing section that fixes the toner image on the medium.  
           [0015]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limiting the present invention, and wherein:  
         [0017]    [0017]FIG. 1 illustrates a general configuration of a first embodiment;  
         [0018]    [0018]FIG. 2 is a block diagram of a printer according to the first embodiment;  
         [0019]    [0019]FIG. 3 illustrates the relationship between the discharge distance and the speed ratio;  
         [0020]    [0020]FIG. 4 is a flowchart, illustrating a medium-size determining operation according to the first embodiment;  
         [0021]    [0021]FIG. 5 is a flowchart, illustrating a print mode setting operation;  
         [0022]    [0022]FIG. 6 is a subroutine for the long-medium mode in which printing is performed on a long print medium;  
         [0023]    [0023]FIG. 7 illustrates the relationship between the discharge distance and the speed ratio;  
         [0024]    [0024]FIG. 8 is a flowchart, illustrating the operation of changing the discharge speedup and down repetitively after the discharge speed has reached to a maximum value;  
         [0025]    [0025]FIG. 9 is a flowchart, illustrating the medium-discharging operation according to the third embodiment;  
         [0026]    [0026]FIG. 10 illustrates medium discharge modes according to the third embodiment; and  
         [0027]    [0027]FIG. 11 is a flowchart, illustrating the setting of parameters of discharge mode according to the fourth embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    By way of example, embodiments of a recording apparatus according to the invention will be described with respect to a printer.  
         [0029]    First Embodiment  
         [0030]    [0030]FIG. 1 illustrates a general configuration of a first embodiment of the invention.  
         [0031]    [0031]FIG. 2 is a block diagram of a printer according to the first embodiment.  
         [0032]    Referring to FIG. 1, a multi-purpose cassette  11  holds a stack of print medium such as paper and transparency.  
         [0033]    When the printer is in a standby condition, the leading end of the stack of print medium  12  is raised by a hopping plate  14  so that the leading end of the top page of the stack is in pressure contact with a hopping roller  13 .  
         [0034]    A feed sensor  15  is disposed between the registry roller  17  and the hopping roller  13 , and detects the leading end of the print medium  12  to generate a detection signal. The detection signal is sent to the controller  32 . The registry roller  17  rotates in contact with a pinch roller  16  to feed the print medium  12  to a transport belt  23 . The print medium  12  is placed on the transport belt  23 , passing through transfer points defined by the photoconductive drums  19 K  19 Y,  19 M, and  19 C and corresponding transfer rollers  20 K,  20 Y,  20 M, and  20 C. Then, the print medium  12  passes through a fixing unit  25 . The fixing unit  25  includes a heat roller  30  and a backup roller  31  in pressure contact with the heat roller  30 . When the print medium  12  passes through the fixing unit  25 , the heat roller  30  applies heat to the toner image on the print medium  12  and the backup roller  31  applies pressure to the toner image, thereby fusing the toner image into the print medium  12 . Then, the print medium  12  is pulled in between a discharging roller  27  and a pinch roller  28  to be discharged into a stacker  29 . The print medium  12  travels over a distance Lt=500 mm from the start sensor  18  to the fixing unit  25 .  
         [0035]    The discharge roller  27  rotates in synchronism with the heat roller  30 . An end sensor  26  is disposed between the fixing unit  25  and the discharge roller  27  to detect the trailing end of the print medium  12  when the print medium  12  is discharged into the stacker  29 .  
         [0036]    Upon a print command, the controller  32  generates first data used for the hopping roller  13  to feed the print medium  12  from the multi-purpose cassette  11  into the printer. The first data is sent to a first converter  41 , which in turn converts the first data into pulses used for driving a drive source  51 . The drive source  51  is driven by the pulses to rotate the hopping roller  13 .  
         [0037]    Upon receipt of the detection signal from the feed sensor  15 , the controller  32  generates second data for transporting the print medium  12  from the hopping roller  13  to the registry roller  17 . The second data is sent to the first converter  41 . Then, the converter  41  converts the second data into pulses to control the drive source  51  to further rotate the hopping roller  13 , thereby advancing the print medium  12  to the registry roller  17  and pinch roller  16 .  
         [0038]    Then, the controller  32  generates third data for advancing the print medium  12  from the registry roller  17  and pinch roller  16  to the transport belt  23 . The third data is sent to a second converter  42 . Then, the second converter  42  converts the third data into pulses to rotate the registry roller  17 . The pulses are sent to a drive source  52 , which in turn causes the registry roller  17  to rotate.  
         [0039]    Upon receiving the detection signal from the start sensor  18 , the controller  32  generates fourth data based on which the transport belt  23  advances the print medium  12 . The fourth data is sent to a converter  43 , which in turn converts the fourth data into pulses to drive a drive source  53 . The drive source  53  is then driven by the pulses to drive the belt drive roller  24 , so that the transport belt  23  runs along the row of the photoconductive drums  19 BK,  19 Y,  19 M, and  19 C.  
         [0040]    Disposed around each photoconductive drum are a charging unit, an exposing unit, a developing unit, and a cleaning unit, not shown. The charging unit charges the surface of the photoconductive drum uniformly. Each image forming section performs an electrophotographic process including charging, exposing, developing, transferring and cleaning.  
         [0041]    Referring to FIG. 2, shortly after activation of printing, the controller  32  reads fifth data from a memory  45  and sends the fifth data to a converter  44 . The fifth data describes the length Lm of print medium  12  and is used for advancing the print medium  12  by means of the heat roller  30  toward the stacker  29 .  
         [0042]    The converter  44  converts the fifth data into pulses that control a drive source  54 . The drive source  54  drives the heat roller  30  in rotation by a rotation amount specified by the number of pulses, thereby discharging the print medium  12  through the fixing unit  25 .  
         [0043]    Upon receiving a detection signal from an end sensor  26 , the controller  32  generates sixth data for discharging the print medium  12  out of the printer. The sixth data is sent to a converter  44 , which in turn converts the sixth data into pulses that controls a drive source  54 . The drive source  54  then drives the discharge roller  27  in rotation, thereby discharging the print medium  12  out of the printer.  
         [0044]    The print medium  12  is discharged face up onto the stacker  29 . The heat roller  30  and discharge roller  27  are rotated through a sufficient number of rotations to completely discharge the print medium  12 , and are then stopped.  
         [0045]    When printing is performed on a long print medium, the print medium  12  may have a large amount of slack therein during transportation if the print medium  12  is fed to the fixing unit  25  at a speed slightly higher than the circumferential speed of the heat roller  30 . As a result, a large amount of slack may cause the print medium  12  to contact the chassis of the fixing unit  25  inadvertently, so that the toner image formed on the print medium is broken. This results in poor print quality.  
         [0046]    In the first embodiment, the rotational speed of the heat roller  30  is changed to transport the print medium  12  at a different discharge speed Vd to prevent the print medium  12  from having a large amount of slack.  
         [0047]    The controller  32  changes the discharge speed Vd in accordance with a discharge distance Ld over which the leading edge of print medium  12  has traveled from a nip created between the heat roller  30  and the backup roller  31  toward stacker  29 . For this purpose, the heat roller  30  is rotated at rotational speeds increased stepwise as the print medium  12  passes through the fixing unit  25 . The discharge speed Vd is increased in a stepwise fashion at points P 1 , P 2 , and P 3  (FIG. 3), i.e., when the print medium  12  has traveled distances Ld=400 mm, 600 mm, and 800 mm, respectively, from the nip between the heat roller  30  and the backup roller  31 .  
         [0048]    Thus, when the print medium  12  reaches points P 1 , P 2 , and P 3 , the discharge speed Vd is changed with respect to a reference speed Vs by predetermined values of the speed ratio γ.  
         [0049]    [0049]FIG. 3 illustrates the relationship between the discharge distance Ld and the speed ratio γ. FIG. 3 plots Ld as the abscissa and γ as the ordinate.  
         [0050]    Referring to FIG. 3, when the discharge distance Ld in millimeters is in the range of 0≦Ld≦800, the speed ratio γ is smaller than 1.0000 and the discharge speed Vd is lower than the reference speed Vs. When the discharge distance Ld is in the range of 800≦Ld, the speed ratio γ is larger than 1.0000 and the discharge speed Vd is higher than the reference speed Vs. The speed ratio γ is increased progressively by 0.20%, as the leading end of the print medium  12  reaches points P 1 , P 2 , and P 3 , respectively.  
         [0051]    When the discharge distance Ld is in the range of 0≦Ld≦400, the γ is 0.9955. When the discharge distance Ld is in the range of 400≦Ld≦600, they is 0.9975. When the discharge distance Ld is in the range of 800≦Ld, the γ is 1.0015. The initial value γx is 0.9955.  
         [0052]    The initial value γx is such that the print medium  12  has no significant slack therein regardless of the medium thickness.  
         [0053]    The operation of the controller  32  will now be described with reference to FIGS.  4 - 6 .  
         [0054]    [0054]FIG. 4 is a flowchart, illustrating a medium-size determining operation according to the first embodiment.  
         [0055]    The operator places a stack of long medium in the multipurpose cassette  11  (FIG. 1) and operates the operation panel, not shown, to input information indicative of a long medium. The controller  32  carries out the medium-size determining process (FIG. 4) to detect the medium size inputted by the operator. The process determines whether the print medium  12  is of A4 size, letter size, B5 size, postcard size, or envelope size. If the print medium  12  is none of these sizes, then the process determines whether the print medium  12  has a long medium size. If the print medium  12  has a long medium size, the controller selects a long medium mode. In the specification, the term long medium size is used to cover a medium that has a length larger than that of any of the aforementioned mediums, i.e., longer than 600 mm (e.g., 900 mm, 1200 mm).  
         [0056]    [0056]FIG. 5 is a flowchart, illustrating a print mode setting operation.  
         [0057]    The controller  32  performs the print setting process (step S 11 , FIG. 5) in which a check is made to determine whether the long medium mode is selected. If the long medium mode is selected (YES at step S 12 ), the controller  32  performs printing in the long medium mode (step S 13 ). If the long medium mode is not selected (NO at step S 12 ), the controller  32  performs printing in an ordinary-medium mode (step S 13 ).  
         [0058]    [0058]FIG. 6 is a subroutine for the long-medium mode in which printing is performed on a long print medium.  
         [0059]    The long-medium mode will be described with reference to FIG. 6. If the long medium mode has been selected, the controller  32  sets the speed ratio γ to the initial value γx (i.e., 0.9955) and the discharge speed Vd to an initial value Vx for the long medium mode, Vx being Vx=0.9955 Vs. Then, printing is initiated so that the hopping roller  13  rotates to feed the print medium  12  into the printer. The start sensor  18  detects the leading end of the print medium  12  and sends the detection signal to the controller  32 . The controller  32  includes a counter  32   a  that takes the form of a memory area in which the number of the rotational pulses is overwritten. The counter  32   a  counts the rotational pulses of the drive source sent to the converter  43 , the pulses being representative of the medium length Lm.  
         [0060]    Upon receiving the fifth data, the converter  43  converts the fifth data into the pulses, which in turn are sent to the drive source  53  and the controller  32 . The controller  32  converts the number of rotational pulses into an amount of travel of the print medium  12 , thereby calculating the length Lm of long medium that is transported from the start sensor  18  into the printer.  
         [0061]    When the counter  32   a  counts up to a value α0 indicating that the leading end of the print medium  12  has traveled over a distance Lt to reach the fixing unit  25 , the controller  32  controls the drive source  54  so that the discharge speed Vd is equal to the initial value Vx=0.9955 Vs at step S 12 - 1 . Then, the discharge of the print medium  12  is begun.  
         [0062]    At step S 12 - 3 , when the counter  32   a  counts up to a value α1 indicating that the leading end of the print medium  12  traveled 400 mm from the fixing unit  25  to reach point P 1 , the controller  32  increases the discharge speed Vd by 0.20% from the reference Vx at step S 12 - 4 .  
         [0063]    At step S 12 - 5 , when the counter  32   a  counts up to a value α2 indicating that the leading end of the print medium  12  further travels over 200 mm to reach point P 2 , the controller  32  increases the discharge speed Vd by 0.20% from Vx(1+0.002) at step S 12 - 6 .  
         [0064]    At step S 12 - 7 , when the counter  32   a  counts up to a value α3 indicating that the leading end of the print medium  12  further travels over 200 mm to reach point P 3 , the controller  32  increases the discharge speed Vd by 0.20% from Vx(1+0.004) at step S 12 - 8 .  
         [0065]    When the end sensor  26  detects the trailing end of the print medium  12  at step S 12 - 9 , the controller  32  sets the discharge speed Vd to the reference speed Vs at step S 12 - 10 .  
         [0066]    As described above, the discharge speed Vd is increased as the medium length Lm increases. The discharge speed Vd is also increased as the output of the counter  32   a  increases. Increasing the discharge speed Vd in this manner prevents the print medium from having an excess slack therein.  
         [0067]    In the embodiment, the discharge speed Vd can be automatically changed only in accordance with the output of the counter  32   a  that counts the rotational pulses, eliminating the need for manually setting the medium length Lm from the operation panel.  
         [0068]    The stepwise increases of the discharge speed Vd after the print medium has passed the fixing unit  25  prevents not only the color shift of toner image on the print medium  12  but also insufficient developing, transferring, and medium discharging.  
         [0069]    The paper discharging operation can be simplified by setting the initial speed ratio γx to a value less than 1.0000 and increasing little by little so that the speed ratio γ exceeds 1.0000.  
         [0070]    {Modification} 
         [0071]    If the medium length Lm is over 1200 mm, the speed ratio y may be changed in such a way that the discharge speed Vd is increased and decreased repetitively after the print medium  12  has been transported through the fixing unit  25  over a discharge distance Ld=1200 mm.  
         [0072]    [0072]FIG. 7 illustrates the relationship between the discharge distance Ld and the speed ratio γ.  
         [0073]    [0073]FIG. 8 is a flowchart, illustrating the operation of changing the discharge speed Vd up and down repetitively after the discharge speed has reached to a maximum value.  
         [0074]    Just as in the long-medium mode described with reference to FIG. 6, the discharge speed Vd is increased stepwise by the speed ratios γ with respect to the reference speed Vs at points P 1 , P 2 , and P 3 . The operation of the modification is the same as the first embodiment except for steps S 12 - 8  to S 12 - 10  shown in FIG. 6, and therefore only steps different from the first embodiment will be described.  
         [0075]    The operation of the modification will be described with reference to FIGS. 7 and 8.  
         [0076]    Referring to FIG. 8, at step S 12 - 7 , a check is made to determine whether the leading end of the print medium  12  has reached the third point P 3 . If YES at step S 12 - 7 , then the program proceeds to step S 14 - 1  where a check is made to determine whether the print medium  12  has reached a point P 3 . If YES at step S 14 - 1 , a check is made to determine whether the print medium  12  has reached point P N . If YES at step S 14 - 1 , the program proceeds to step S 14 - 2  where the speed ratio γ is decreased to 0.9995 to decrease the discharge speed Vd. Then, the program proceeds to S 14 - 3  where a check is made to determine whether the trailing end of the print medium has been detected. If YES at step S 14 - 3 , the program proceeds to step S 14 - 4  where a check is made to determine whether the print medium  12  has reached point P N+1 . If NO at step S 14 - 3 , then the program jumps to step S 14 - 7  where Vd is reset. If YES, at step S 14 - 5  where the speed ratio γ is increased to 1.0015 to increase the discharge speed Vd. Then, the program proceeds to step S 14 - 6  where a check is made to determine whether the trailing end of print medium  12  has been detected. If YES at step S 14 - 6 , the program proceeds to step S 14 - 7  where Vd is reset. If NO at step S 14 - 6 , then the program proceeds to step S 14 - 8  where 2 is added to N. The initial value of N is 2. Then, the program jumps back to step S 14 - 1 . The modification has an advantage that the print medium is not taut nor does it have slack in it during the long medium mode. Moreover, repetitive changing the discharge speed Vd up and down as the print medium is discharged toward the paper stacker allows adjustment of taut and slack in the print medium between the transfer unit and the fixing unit. The modification may also be applied to second, third, and fourth embodiments which will be described later.  
         [0077]    Second Embodiment  
         [0078]    The printer according to a second embodiment has substantially the same construction as that according to the first embodiment and differs only in the medium-discharging operation.  
         [0079]    [0079]FIG. 9 is a flowchart, illustrating the medium-discharging operation according to the third embodiment.  
         [0080]    In the second embodiment, the controller  32  performs the medium-discharging operation for a long medium without a command indicative of a long medium inputted by the operator. In other words, the controller  32  performs the setup for printing where the length Lm of print medium  12  is detected and a check is made to determine whether the print medium  12  is a long-medium.  
         [0081]    The controller  32  performs the medium discharging operation in which the speed ratio γ is set to 1.0000 and the discharge speed Vd is set to the reference speed Vs. The hopping roller  13  is rotated to feed the print medium  12 . When the start sensor  18  detects the leading edge of the print medium  12 , the counter  32   a  in the controller  32  receives the rotational pulses from the converter  43  (FIG. 2) and counts the pulses.  
         [0082]    The controller  32  checks the output of the counter  32   a  to determine whether the length Lm of the print medium is equal to or less than a predetermined value β1 (step S 21 ). In this embodiment, a medium having a length greater than β1 is assumed to be a long medium. If the start sensor  18  detects the trailing end of the print medium  12  before the output of the counter  32   a  becomes β1, then it is determined that the length Lm of the print medium  12  is equal to or less than β1 and therefore the print medium is not long. Then, the controller  32  performs printing in the normal medium mode (step S 22 ). The controller  32  sets the speed ratio γ to 1.0000 and transports the print medium  12  at a discharge speed Vd=Vs.  
         [0083]    If the count of the counter  32   a  is more than β1 when the print medium travels 400 mm after the start sensor  18  detects the trailing end of the print medium  12 , then the controller  32  determines that the print medium  12  is longer than β1 and therefore is a long medium. The steps S 23  to S 33  are the same as steps S 12 - 1  to S 12 - 10  and therefore the description thereof is omitted.  
         [0084]    While the second embodiment has been described with respect to a case where the setting β1 indicative of the print medium  12  is preset, the setting β1 may be set to an arbitrary value by the operator. For that purpose, the parameters for long-mediums are stored in a firmware, so that the operator operates the operation panel to set desired parameters.  
         [0085]    Third Embodiment  
         [0086]    The printer according to a third embodiment has substantially the same construction as that according to the first embodiment and differs only in the medium-discharging operation.  
         [0087]    [0087]FIG. 10 illustrates medium discharge modes according to the third embodiment.  
         [0088]    There are provided three discharge modes for long print mediums. Parameter data that constitute different discharge modes are stored in the memory (FIG. 2). The operator accesses these discharge modes from the operation panel, not shown, to set a desired discharge mode.  
         [0089]    The first discharge mode includes an initial value γx of 0.9900, an increment Δγ of 0.30% at points P 1 , P 2 , and P 3 , respectively, and Ld at points P 1 , P 2 , and P 3  of 300 mm, 500 mm, and 700 mm.  
         [0090]    The second discharge mode includes an initial value γx of 0.9955, an increment Δγ of 0.20% at points P 1 , P 2 , and P 3 , respectively, and Ld at points P 1 , P 2 , and P 3  of 400 mm, 600 mm, and 800 mm.  
         [0091]    The third discharge mode includes an initial speed ratio γx of 0.9990, an increment γΔ of 0.10% at points P 1 , P 2 , and P 3 , respectively, and Ld at points P 1 , P 2 , and P 3  of 500 mm, 700 mm, and 900 mm.  
         [0092]    A plurality of print patterns allows setting of optimum discharge speeds Vd for long mediums of various sizes.  
         [0093]    Fourth Embodiment  
         [0094]    The printer according to the fourth embodiment is of substantially the same construction as the first embodiment. The fourth embodiment allows setting of arbitrary parameters of discharge mode.  
         [0095]    [0095]FIG. 11 is a flowchart, illustrating the setting of parameters of discharge mode according to the fourth embodiment.  
         [0096]    The operator operates the operation panel, not shown, to input an initial speed ratio γx, an increment Δγ at points P 1 , P 2 , and P 3  in FIG. 3 and discharge distance Ld at points P 1 , P 2 , and P 3  in FIG. 3.  
         [0097]    Arbitrary parameters allow setting of optimum discharge speeds Vd for long mediums of various sizes.  
         [0098]    The operation of the fourth embodiment will be described with reference to FIG. 3 and FIG. 11.  
         [0099]    Referring to FIG. 11, at step S 41 , a check is made to determine whether the initial speed ratio γx should be changed. If YES at step S 41 , the program proceeds to step S 42  where the initial speed ratio γx is set to a new value. If NO at step S 41 , the program proceeds to step S 43  where a check is made to determine whether the increment Δγ should be set to a new value. If YES at step S 43 , the program proceeds to step S 44  where the increment Δγ is set to a new value, the increment Δγ being a fraction in percent of the reference speed Vs. If NO at step S 43 , the program proceeds to step S 45  where a check is made to determine whether the first point P 1  should be set. If YES at step S 45 , the program proceeds to step S 46  where Discharge distance Ld is set to an arbitrary desired value of LA. LA is a distance between points P 0  and P 1  in FIG. 3. If NO at step S 45 , the program proceeds to step S 47  where a check is made to determine whether the second point P 2  should be set. If YES at step S 47 , the program proceeds to step S 48  where LB is set to an arbitrary desired value, then the program proceeds to step S 49  where a check is made to determine whether LA&lt;LB. LB is a distance between points P 1  and P 2  in FIG. 3. If NO at step S 49 , the program jumps back to step S 48  where another larger value of LB is set. If NO at step S 47 , the program proceeds to step S 50  where a check is made to determine whether the third point should be set. If YES at step S 50 , then the program proceeds to step S 51  where LC is set to an arbitrary desired value and then the program proceeds to step S 52  where a check is made to determine whether LB&lt;LC. LC is a distance between points P 2  and P 3  in FIG. 3. If NO at step S 52 , then the program jumps back to step S 51  where another larger value of LC is set. If YES at step S 52 , the program ends.  
         [0100]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art intended to be included within the scope of the following claims.