Patent Publication Number: US-10322593-B2

Title: Ink jet printer and movement control method for carriage

Description:
TECHNICAL FIELD 
     This invention relates to an inkjet printer and a movement control method for a carriage. 
     BACKGROUND 
     Conventionally, an inkjet printer is designed to perform printing by moving a carriage in the main scanning direction, supplying ink from an ink cartridge to a recording head mounted on the carriage, ejecting ink droplets from the recording head and letting them adhere to (land on) a recording medium, thereby forming an image such as a character or picture consisting of multiple dots. 
     By the way, in moving the carriage in the inkjet printer, a target velocity that is the velocity targeted by the carriage is set as a target value, and the target velocity is categorized into an acceleration region where the carriage is accelerated from a stopped state, a uniform velocity region where the carriage is moved at a constant velocity, and a deceleration region where the carriage is decelerated and stopped. 
     In the uniform velocity region, by stably moving the carriage at a constant velocity, the tracks of ink droplets from their ejection from the recording head to their adhesion to the recording medium become uniform, and as the result, dots can be formed in target dot positions on the recording medium. However, if the carriage cannot be stably moved and unevenness occurs in its velocity for example, dots cannot be formed in their target dot positions on the recording medium, degrading the image quality. 
     Then, in the inkjet printer, feedback control is performed so that the carriage can stably move at a constant velocity in the uniform velocity region. 
     In the feedback control, based on the above-mentioned target velocity of the carriage, an actual velocity that is the actual velocity of the carriage, a gain, etc., control values including a proportional component and an integral component for the feedback, in other words, feedback control values, are calculated, and the feedback control values are supplied to a carriage driving part for driving the carriage, thereby performing carriage movement control so that the actual velocity becomes the target velocity. 
     By the way, if an abnormality occurs to the carriage movement, an image cannot be normally formed on the recording medium. For example, if the recording medium hits the carriage, causing a jam, the carriage cannot be moved, and an image cannot be formed on the recording medium. Also, if a foreign body enters inside a chassis of the inkjet printer, the recording medium could become warped, and if the warped recording medium hits the carriage, the carriage cannot be moved at a constant velocity, and dots cannot be formed in the target dot positions on the recoding medium, degrading the image quality. 
     Then, an inkjet printer has been offered, the inkjet printer being designed to judge whether a deviation between the target velocity and the actual velocity of the carriage has exceeded a threshold value, and if the deviation has exceeded the threshold value, judges that an abnormality has occurred to the carriage movement (see Patent Document 1 for example). 
     RELATED ART 
     Patent Document(s) 
     [Patent Doc. 1] JP Laid-Open Patent Publication 2010-64442 
     One Subject to be Solved 
     However, in the above-mentioned conventional inkjet printer, because carriage movement control is performed by feedback control, the actual velocity is delayed relative to the target velocity, and the deviation between the target velocity and the actual velocity becomes large, which could cause an error in judging whether an abnormality has occurred to the carriage movement. 
     Then, in order to judge accurately whether an abnormality has occurred to the carriage movement, one idea thought of is to increase the threshold value according to the above-mentioned deviation. However, if the threshold value is increased, when a minor abnormality such that the recording medium becomes distorted and slightly touches the carriage has occurred, the deviation may not exceed the threshold value. In that case, it becomes impossible to judge that an abnormality has occurred to the carriage movement. 
     As the result, blurring occurs in a formed image, degrading the image quality. 
     The objective of this invention is to offer an inkjet printer and a carriage movement control method that can solve the above-mentioned problem of the conventional inkjet printer and accurately judge whether an abnormality has occurred to the carriage movement. 
     SUMMARY 
     Considering the above objects, a controller of the invention controls a movement of carriage using not only a feedback value but a feedforward value also. Further, the feedback control value is composed with two components, which are a proportional component (Wp) and a integral component (Wi). Using the proportional component, the controller accurately determines if an abnormality of the carriage happens. 
     More specifically noted, an inkjet printer includes a carriage motor that moves a carriage, a carriage driving part that drives the carriage motor, a control value calculating part that calculates a total control value (Wt) composed with a control value for a feedback control and a control value for a feedforward control, which are respectfully referred as a feedback control value (Wb) and a feedforward control value (Wf), and an abnormality judging part that judges whether an abnormality has occurred to the carriage movement based on whether a proportional component (Wp) of the feedback control value has exceeded a threshold value. 
     In this case, because carriage movement control is performed using both feedback control and feedforward control, control values for feedback control can be decreased by the amount that control values for the feedforward control are calculated, and therefore the proportional component can be decreased. 
     Therefore, in judging whether the proportional component has exceeded the threshold value, because the threshold value can be decreased, even if the abnormality in the carriage movement is small, whether the proportional component has exceeded the threshold value can be securely judged. 
     As the result, whether an abnormality has occurred to the carriage movement can be accurately judged. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a control device of an inkjet printer in an embodiment of this invention. 
         FIG. 2  is a perspective view of the inkjet printer in the embodiment of this invention. 
         FIG. 3  is a time chart for explaining the operation of the inkjet printer when carriage movement control is performed using feedback control only. 
         FIG. 4  is a time chart for explaining the operation of the inkjet printer if no abnormality has occurred to the carriage movement while performing carriage movement control using feedback control only. 
         FIG. 5  is a time chart for explaining the operation of the inkjet printer if an abnormality has occurred to the carriage movement while performing carriage movement control using feedback control only. The above  FIGS. 3-5  are directed to prior art. 
         FIG. 6  is a flow chart showing the operation of the inkjet printer in the embodiment of this invention. 
         FIG. 7  is a time chart for explaining the operation of the inkjet printer when performing carriage movement control in the embodiment of this invention. 
         FIG. 8  is a time chart for explaining the operation of the inkjet printer if no abnormality has occurred to the carriage movement while performing carriage movement control in the embodiment of this invention. 
         FIG. 9  is a time chart for explaining the operation of the inkjet printer if an abnormality has occurred to the carriage movement while performing carriage movement control in the embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Below, an embodiment of this invention is explained in details referring to drawings. In this case, an inkjet printer as an image forming apparatus of an inkjet system is explained. 
       FIG. 2  is a perspective view of the inkjet printer in the embodiment of this invention. 
     In the figure, indicated as  10  is the inkjet printer, Fr is a frame of the inkjet printer  10 , and the frame Fr comprises an upper frame Fr 1  and an under frame Fr 2 . 
     The above-mentioned upper frame Fr 1  is provided with a receiving plate PB arranged extending from the left end to the right end when seen from the front side of the inkjet printer  10  (front side in the figure), an outer side plate PL 1  as a first main supporting part formed standing up from the left end of the receiving plate PB, an outer side plate PR 1  as a second main supporting part formed standing up from the right end of the receiving plate PB, an inner side plate PL 2  as a first sub supporting part formed standing up from the receiving plate PB at a prescribed distance rightward of the above-mentioned outer side plate PL 1 , and an inner side plate PR 2  as a second sub supporting part formed standing up from the receiving plate PB at a prescribed distance leftward of the above-mentioned outer side plate PR 1 . 
     A rail  15  is erected between the above-mentioned outer side plates PL 1  and PR 1 , and a carriage  17  is arranged in a freely movable manner in the left-right direction along the rail  15 . Therefore, arranged in a freely rotatable manner are a driving-side pulley  18  on the above-mentioned outer side plate PR 1  and a driven-side pulley  19  on the outer side plate PL 1 , an endless belt  21  is stretched in a freely travelable manner between the pulleys  18  and  19 , and the carriage  17  is attached to a prescribed place of the endless belt  21 . Then, connected to the above-mentioned pulley  18  is a below-mentioned carriage motor M 2  ( FIG. 1 ) as a drive part for moving the carriage. Also, attached to the above-mentioned carriage  17  are below-mentioned multiple, four in this embodiment, recording heads Hi (i=1, 2, . . . , 4) ( FIG. 1 ). 
     Then, the carriage  17  is moved in the left-right direction (main scanning direction) by driving the above-mentioned carriage motor M 2 , and by the individual recording heads Hi being moved in the left-right direction accompanying the movement of the carriage  17 , printing can be performed on an unshown recording medium. 
     In this embodiment, the above-mentioned recording heads Hi are recording heads of an inkjet system, where four recording heads Hi that eject black (Bk), yellow (Y), magenta (M), and cyan (C) ink droplets are mounted on the carriage  17  so that a color image can be formed. Note that the color reproducibility can be improved by using recording heads of other colors than the recording heads Hi of the above-mentioned colors. Also, used as an organic solvent as a solvent in this embodiment is a solvent ink containing a pigment as a colorant. 
     Unshown multiple nozzles are formed in an array shape on the nozzle face of each of the recording heads Hi, and a color image can be formed by attaching the recording heads Hi to the carriage  17  so that the nozzle faces oppose the recording medium, and having ink droplets of individual colors ejected through the nozzles according to image data and adhere onto target dot positions on the above-mentioned recording medium while reciprocating the carriage  17 . 
     Note that as the recording medium, other than sheets of paper, films made of resins such as vinyl chloride and PET can be used, and the recording medium is carried in a direction perpendicular to the moving direction of the carriage  17 . 
     Also, a linear scale  23  is arranged along the above-mentioned rail  15 , and by reading the divisions of the linear scale  23  using a below-mentioned encoder Se ( FIG. 1 ) arranged on the carriage  17 , the position of the carriage  17  can be detected. 
     Then, a medium center guiding part  25  is arranged between the inner side plates PL 2  and PR 2  on the above-mentioned receiving plate PB. The medium center guiding part  25  is provided with a platen unit u 1  as a supporting unit that is arranged extending between the inner side plates PL 2  and PR 2 , has a plate shape, and supports the recording medium, and an unshown air suction mechanism for drawing from below the recording medium carried on the platen unit u 1  toward the platen unit u 1  side with a negative pressure. 
     The above-mentioned platen unit u 1  is provided with a platen  45  as a supporting member having a plate shape, an unshown linear heater that is laid on the rear face of the platen  45  and heats the recording medium by heating the platen  45 , and an unshown platen cover as a covering member that has a plate shape, is attached to the rear face of the platen  45 , and covers the above-mentioned heater. 
     When the platen  45  is heated by the above-mentioned heater to make the platen  45  become a constant temperature, the recording medium carried on the platen  45  is heated. In this case, by making the temperature of the platen  45  uniform, an image of uniform quality can be formed. Also, by maintaining the temperature of the recording medium at the most appropriate temperature according to the property of the recording medium and the properties of the inks, ink droplets can be let adhere well to the recording medium, and thus the image quality can be enhanced. If there is unevenness in temperature of the recording medium, differences in drying speed of the individual ink droplets cause differences in their wetting and spreading, and as the result, differences in fusability occur, thereby a specific pattern such as a linear one occurs on the image, degrading the image quality. Furthermore, if the temperature of the platen  45  is too high, the recording medium becomes warped, making it impossible to carry the recording medium stably. 
     Also, multiple suction holes are formed on the platen  45 , a suction chamber is arranged in the above-mentioned air suction mechanism, and a fan is arranged on the suction chamber. By exhausting air inside the suction chamber with the fan and generating a negative pressure in the suction chamber, air can be sucked through each of the suction holes, and the recording medium can be drawn toward the platen unit u 1  side. 
     On the rear side (deeper side in the figure) of the above-mentioned upper frame Fr 1 , an unshown rear paper guide as a medium rear guiding part is arranged. The rear paper guide guides the recording medium, that is forwarded from an unshown forwarding roll and carried up, to the above-mentioned medium center guiding part  25 . Therefore, between the above-mentioned rear paper guide and the medium center guiding part  25  in the carrying direction of the recording medium, a carrying roller pair  30  as a carrying member is arranged in a freely rotatable manner. 
     The carrying roller pair  30  comprises a carrying roller  31  as a first roller arranged in a freely rotatable manner extending along the platen unit u 1 , and multiple pinch rollers  32  as second rollers that are arranged in a freely rotatable manner above the carrying roller  31  and press the recording medium onto the carrying roller  31 . Once a below-mentioned carrying motor M 1  as a drive part for carrying is driven to rotate the carrying roller  31 , the individual pinch rollers  32  are rotated by drag turning, thereby the recording medium is forwarded from the forwarding roll and carried in a state sandwiched between the carrying roller  31  and the individual pinch rollers  32 . Note that the above-mentioned individual pinch rollers  32  are supported in a freely swingable manner by an unshown arm, and each of them is independently biased toward the carrying roller  31  by an unshown spring as a bias member, constituting a pinch roller unit. 
     In this case, the above-mentioned carrying motor M 1  is driven to carry the recording medium by a prescribed distance, afterwards the carrying motor is stopped, the carriage  17  is moved in that state, and ink droplets of individual colors are ejected from the individual recording heads Hi, thereby one scan is performed to form one line of image. Once one scan is finished, the above-mentioned carrying motor M 1  is driven again to carry the recording medium by a prescribed distance, afterwards the carrying motor M 1  is stopped, the carriage  17  is moved in that state, and ink droplets are ejected from the individual recording heads Hi, thereby performing one scan and forming one line of image. By repeating this operation, an image is formed on the recording medium. 
     Note that although printing is performed by a single-pass system in this embodiment, if printing is performed by a multi-pass system, the distance to carry the above-mentioned recording medium is made shorter than the nozzle array of the recording head Hi, and one line of image is formed by performing multiple scans. 
     Arranged on the front side of the above-mentioned upper frame Fr 1  is a front paper guide  33  as a medium front guiding part for guiding the recording medium after printing is performed. The front paper guide  33  has a curved shape for guiding downwards the recording medium ejected horizontally from the above-mentioned medium center guiding part  25 . Note that an unshown heater is arranged on the rear face of the above-mentioned front paper guide  33 , and the front paper guide  33  is heated by the heater to heat the recording medium. 
     The above-mentioned under frame Fr 2  is provided with pedestals  35  and  36  arranged in parallel at a prescribed distance between the left-end vicinity and the right-end vicinity of the inkjet printer  10 , props  37  and  38  formed standing up from the center of the respective pedestals  35  and  36 , and holding plates PL 3  and PR 3  attached to the respective props  37  and  38 . 
     Between the holding plates PL 3  and PR 3 , an unshown paper tube (winding roll) is arranged in a freely rotatable manner for winding the recording medium that is ejected from the medium center guiding part  25  and guided by the above-mentioned front paper guide  33 . In order to hold the paper tube at the left end and the right end, roll bearings  26  and  27  are arranged in a freely rotatable manner on the above-mentioned holding plates PL 3  and PR 3 , respectively. By rotating the above-mentioned paper tube by an unshown winding device, the recording medium can be wound. 
     Also, in the front side of the above-mentioned paper tube, a tension roller  28  is arranged in a freely movable manner in the up-down direction extending between the holding plates PL 3  and PR 3 . The tension roller  28  gives tension to the recording medium that is guided by the front paper guide  33  and wound up by the paper tube so that the recording medium should not develop any slack. Therefore, a groove mt is formed extending in the up-down direction on the inner face of each of the above-mentioned holding plates PL 3  and PR 3 , and the tension roller  28  is moved in the up-down direction along the grooves mt. 
     By the way, as mentioned above, in this embodiment, the above-mentioned rail  15  is erected between the outer side plates PL 1  and PR 1 , and the medium center guiding part  25  is arranged between the inner side plates PL 2  and PR 2 . 
     Therefore, printing by the individual recording heads Hi is performed while the carriage  17  is being moved over the medium center guiding part  25 , and printing by the individual recording heads Hi is not performed while the carriage  17  is placed between the outer side plate PL 1  and the inner side plate PL 2  or between the outer side plate PR 1  and the inner side plate PR 2 . 
     Then, in this embodiment, the space between the outer side plate PR 1  and the inner side plate PR 2  is made a home position for performing the origin adjustment of the position of the carriage  17  and performing the maintenance of the individual recording heads Hi, and the space between the outer side plate PL 1  and the inner side plate PL 2  is made a retreat position for letting the carriage  17  retreat from over the medium center guiding part  25 . 
     Then, in the above-mentioned home position, a maintenance unit  41  is arranged opposing the individual recording heads Hi, and an operation panel  43  is arranged as an operation/display part for operating the inkjet printer  10 . In order to maintain the nozzles of the recording heads Hi in a fine state, the above-mentioned maintenance unit  41  is provided with a wiper to wipe the nozzle face of each of the recording heads Hi, a cap to prevent drying of the nozzles, a suction mechanism to suck ink whose viscosity increased inside a nozzle, etc. Also, the above-mentioned operation panel  43  is provided with an operation part where operation buttons etc. are arranged, and a display part where a display lamp etc. are arranged. 
     Next, explained is the control device of the above-mentioned inkjet printer  10 . 
       FIG. 1  is a block diagram showing the control device of the inkjet printer in the embodiment of this invention. 
     In the figure, indicated as  80  is a controller that controls the whole inkjet printer  10 ,  81  is ROM as a first memory part that records a control program, initial values, etc., and  82  is RAM as a second memory part used as a work area for the controller  80  to perform arithmetic operations and also as a buffer for temporarily recording various kinds of data. 
     Also, indicated as  83  is a setting value storing part as a third memory part for recording various kinds of setting values in the inkjet printer  10 . Recorded in the setting value storing part  83  are target values in moving the carriage  17  such as target velocity Vs of the carriage  17  and target position of the carriage  17  at every unit time (every 1 ms in this embodiment), and a control value for below-mentioned feedforward control, that is, a feedforward control value Wf at every unit time. The individual target values are set by performing experiments or the like in advance. Note that because movement control of the carriage  17  is performed at every unit time, the shorter the unit time that regulates the individual setting values, the better trackability to the target velocity Vs can become. 
     Furthermore, recorded in the above-mentioned setting value storing part  83  are threshold values εj (j=1, 2, 3) for judging whether an abnormality has occurred to the movement of the carriage  17 . The threshold values εj are set differently according to the peak values of the proportional component Wp in the acceleration region, the uniform velocity region, and the deceleration region of the target velocity Vs, and recorded in the setting value storing part  83 . 
     Note that in this embodiment, in order to judge whether an abnormality has occurred to the movement of the carriage  17 , it is judged whether the proportional component Wp calculated based on the deviation between the target position and the actual position of the carriage  17 , the deviation between the target velocity Vs and the actual velocity of the carriage  17 , the gain, etc. has exceeded the threshold value εj (whether the proportional component Wp is greater than the threshold value εj). If delays occur in the actual position and the actual velocity of the carriage  17 , thus the deviation between the target position and the actual position of the carriage  17  and the deviation between the target velocity Vs and the actual velocity of the carriage  17  increase, because the proportional component Wp also increases, it is possible to judge whether an abnormality has occurred to the movement of the carriage  17  based on whether the proportional component Wp has exceeded the threshold value εj. 
     Then, indicated as  84  is a timer as a clocking part that is operated by the controller  80  and measures time as time information. 
     The above-mentioned controller  80  is provided with a control value calculating part Pr 1  and an abnormality judging part Pr 2 , and following the control program recorded in the above-mentioned ROM  81 , an unshown CPU of the controller  80  operates the control value calculating part Pr 1  and the abnormality judging part Pr 2  mentioned above, and also operates a head driving part  85 , a carrying motor driving part  86 , a carriage driving part  87 , a carriage position detection circuit  88  as a carriage position detecting part, a carriage velocity calculation circuit  89  as a carriage velocity detecting part, etc. to perform various kinds of processes. One of the carriage position detection part and the carriage velocity detection part or both of these parts function as a carriage detection part. The carriage detection part functions to detect an actual status of the carriage. The actual status means an indicator to represent the actual status of the carriage, and may be composed with an actual position, an actual velocity and/or an actual travel time f the carriage, which are measured at a certain moment. 
     The above-mentioned head driving part  85  drives the recording heads Hi according to print data to have ink droplets ejected from the recording heads Hi. 
     Also, the above-mentioned carrying motor driving part  86  drives the carrying motor M 1  to rotate the carrying roller  31 , thereby carrying the recording medium, and the above-mentioned carriage driving part  87  drives the carriage motor M 2  to move the carriage  17  in the main scanning direction. 
     Then, the above-mentioned carriage position detection circuit  88  reads the divisions of the linear scale  23  with the encoder Se, thereby detecting the actual position of the carriage  17  as position information, the above-mentioned carriage velocity calculation circuit  89  calculates the period of a signal output from the encoder Se and divide the distance between the divisions of the linear scale  23  by the period of the signal, thereby calculating the actual velocity of the carriage  17  and detecting it as velocity information. Therefore, the above-mentioned actual position is the position detected by the carriage position detection circuit  88 , and the actual velocity is the velocity detected by the carriage velocity calculation circuit  89 . 
     Note that although in this embodiment the actual velocity Vr of the carriage is detected by dividing the distance between the divisions of the linear scale  23  by the period of the signal, the moving distance of the carriage  17  can be calculated based on the position of the carriage  17  detected last time and the position of the carriage  17  detected this time by the carriage position detection circuit  88 , elapsed time can be calculated based on the time when the position of the carriage  17  was detected last time and the time when the position of the carriage  17  is detected this time, and the actual velocity of the carriage  17  can be calculated by dividing the moving distance by the elapsed time. 
     Next, explained is an example of movement control of the carriage  17 . 
     First, explained is the operation of the inkjet printer  10  when carriage movement control is performed using feedback control only. 
       FIG. 3  is a time chart for explaining the operation of the inkjet printer when carriage movement control is performed using feedback control only. Note that the horizontal axis indicates time (unit: ms), the left vertical axis the proportional component Wp and a total control value (final control value) Wt that is the final control value, and the right vertical axis the target velocity Vs and the actual velocity Vr. Also, the proportional component Wp, the total control value Wt, the target velocity Vs, and the actual velocity Vr are dimensionless quantities. 
     In the figure, indicated as Ar 1  is the acceleration region where the carriage  17  is accelerated from a stopped state, Ar 2  is the uniform velocity region where the carriage  17  is moved at a constant velocity, and Ar 3  is the deceleration region where the carriage  17  is decelerated and stopped. Note that the acceleration region Ar 1 , the uniform velocity region Ar 2 , and the deceleration region Ar 3  are all categorized by preset changes of the target velocity Vs, where the target velocity Vs is increased at a constant slope (or constant rate) from  0  to a prescribed value in the acceleration region Ar 1 , maintained at the above-mentioned prescribed value in the uniform velocity region Ar 2 , and decreased at a constant slope from the above-mentioned prescribed value to 0 in the deceleration region Ar 3 . 
     In this case, because movement control of the carriage  17  is performed using feedback control only, and the feedback control is performed with PI control, the total control value Wt is equal to a control value for feedback control, that is, a feedback control value Wb, which becomes the proportional component Wp with an unshown integral component Wi added and can be expressed as
 
 Wt=Wb=Wp+Wi  
 
Note that the proportional component Wp constitutes a proportional element control value in the feedback control value Wb, and the integral component Wi an integral element control value in the feedback control value Wb. Also, because the integral component Wi has smaller variation than the proportional component Wp, it is not shown in  FIG. 3 .
 
     By the way, the above-mentioned total control value Wt is a command value for driving the carriage motor M 2 , and upon calculating the above-mentioned total control value Wt, the controller  80  sends it to the carriage driving part  87 . The carriage driving part  87  is provided with an unshown pulse width modulation signal generating part and a switching circuit, and upon receiving the total control value Wt, has a PWM control signal according to the total control value Wt generated in the above-mentioned pulse width modulation signal generating part, has current according to the duty of the PWM control signal generated in the above-mentioned switching circuit, and sends the current to the carriage motor M 2  to drive the carriage motor M 2 . In this case, the greater the above-mentioned total control value Wt is, the greater the value of the current sent to the carriage motor M 2  becomes, and the smaller the above-mentioned total control value Wt is, the smaller the value of the current sent to the carriage motor M 2  becomes. 
     In feedback control, based on the actual position and the actual velocity Vr of the carriage  17  detected at the current timing, the feedback control value Wb is calculated at the next timing, and the carriage motor M 2  is driven based on the feedback control value Wb, therefore delays in the actual position relative to the target position and in the actual velocity Vr relative to the target velocity Vs occur. 
     For example, as shown in the figure, when the target velocity Vs is in the acceleration region Ar 1 , separation between the target velocity Vr and the actual velocity Vs, that is, deviation δV gradually increases, and at the timing when the target velocity Vs shifts from the acceleration region Ar 1  to the uniform velocity region Ar 2 , it becomes an extremely large value δV 1 , thereby trackability to the target velocity Vs declines. 
     Therefore, after the target velocity Vs shifted to the uniform velocity region Ar 2 , prescribed time becomes necessary until the delays in the actual position and the actual velocity Vr are dissolved and the actual velocity Vr becomes the target velocity Vs in the uniform velocity region Ar 2 , and during that time ink droplets cannot be ejected from the recording heads Hi. That is, a first waiting region Ar 21  is formed for waiting until it becomes possible to eject ink droplets form the recording heads Hi, and during that time the carriage  17  is uselessly moved. Accompanying it, a print region Ar 22  where actual printing is possible becomes fairly shorter than the uniform velocity region Ar 2 . 
     Also, after the target velocity Vs shifted from the uniform velocity region Ar 2  to the deceleration region Ar 3 , when it has become 0, prescribed time becomes necessary until the actual velocity Vr becomes 0, and during that time the carriage  17  need be moved continuously. That is, a second waiting region Ar 31  is formed for waiting for the actual velocity Vr to become 0, and during that time the carriage  17  is uselessly moved. 
     In this manner, if movement control of the carriage  17  is performed using feedback control only, delays in the actual position relative to the target position and in the actual velocity Vr relative to the target velocity Vs occur, thereby trackability to the target velocity Vs declines. Therefore, for example, in order to have the carriage  17  shift from the acceleration region Ar 1  to the uniform velocity region Ar 2  or from the uniform velocity region Ar 2  to the deceleration region Ar 3 , necessity to perform excess control such as providing an excess margin occurs, thereby the movement time, one-scan distance, etc. of the carriage become longer, worsening the printing throughput. 
     Next, explained are changes in the proportional component Wp while performing movement control of the carriage  17  using feedback control only if no abnormality has occurred to the movement of the carriage  17 , or if an abnormality has occurred to the movement of the carriage  17 . 
       FIG. 4  is a time chart for explaining the operation of the inkjet printer if no abnormality has occurred to the carriage movement while performing carriage movement control using feedback control only, and  FIG. 5  is a time chart for explaining the operation of the inkjet printer if an abnormality has occurred to the carriage movement while performing carriage movement control using feedback control only. Note that the horizontal axis indicates time (unit: ms), the left vertical axis the proportional component Wp and the total control value Wt, and the right vertical axis the target velocity Vs and the actual velocity Vr. 
     In the figure, indicated as Vs is the target velocity, Vr is the actual velocity, Wp is the proportional component, and Wt is the total control value. The proportional component Wp is calculated based on the deviation between the target position and the actual position, the deviation δV between the target velocity Vs and the actual velocity Vr, the gain, etc. Relationships between the deviation values and the control values, which correspond to the deviation values, are determined in advance and stored in ROM  81 . For example, an equation for obtaining a control value corresponding to a deviation value, or a table in which control values and corresponding deviation values are aligned are stored in ROM  81 . The control part  80  obtains an intended control value Wp by a calculation using a target value and a deviation value obtained from an actual measured value. 
     When the target velocity Vs is in the acceleration region Ar 1  ( FIG. 3 ), and if no abnormality has occurred to the movement of the carriage  17 , as shown in  FIG. 4 , the proportional component Wp gradually increases according to delays occurring in the actual position and the actual velocity Vr, and after reaching a peak value Wpmax 1 , gradually decreases. In this case, because the peak value Wpmax 1  never exceeds a threshold value β, it is not judged that an abnormality has occurred to the movement of the carriage  17 . 
     As opposed to this, as shown in  FIG. 5 , if an abnormality occurs to the movement of the carriage  17  at timing t 1 , the actual velocity Vr of the carriage  17  decreases, the deviation δV increases, and accompanying it, the proportional component Wp increases. 
     At this time, if the abnormality in the movement of the carriage  17  is a small one to the extent that the recording medium is distorted and slightly touches the carriage, a peak value Wpmax 2  of the proportional component Wp never exceeds the threshold value β, therefore it is not judged that an abnormality has occurred to the movement of the carriage  17 . 
     On the other hand, if the movement of the carriage  17  is greatly delayed as in the case where the recording medium has hit the carriage and caused a jam, the proportional component Wp becomes large and exceeds the threshold value β as shown in a broken line, therefore it is judged that an abnormality has occurred to the movement of the carriage  17 . 
     Based on this, this embodiment is designed so that the proportional component Wp is suppressed not to increase beyond the threshold value β, and that it is accurately judged whether an abnormality has occurred to the movement of the carriage  17 . 
       FIG. 6  is a flow chart showing the operation of the inkjet printer in the embodiment of this invention, and  FIG. 7  is a time chart for explaining the operation of the inkjet printer when performing carriage movement control in the embodiment of this invention. Note that in  FIG. 7 , the horizontal axis indicates time (unit: ms), the left vertical axis the proportional component Wp, the integral component Wi, an acceleration corresponding control value Wα, a velocity corresponding control value Wv, and the total control value Wt, and the right vertical axis the target velocity Vs and the actual velocity Vr. Also, the proportional component Wp, the integral component Wi, the acceleration corresponding control value Wα, the velocity corresponding control value Wv, the total control value Wt, and the target velocity Vs and the actual velocity Vr of the carriage  17  are dimensionless quantities. 
     In this embodiment, the carriage  17  is designed to be reciprocated. In this case, explained here is a one-scan process of the carriage  17  when moving the carriage  17  from a starting point Ps for turning back the carriage  17  in the home position side toward (or to) an ending point Pe for turning back the carriage  17  in the retreat position side shown in  FIG. 7 . Because a one-scan process when moving the carriage  17  from the ending point Pe to the starting point Ps is the same except having the moving direction of the carriage  17  reversed, its explanation is omitted. 
     First of all, the controller  80  ( FIG. 1 ) reads and acquires the actual position of the carriage  17  detected by the carriage position detection circuit  88 . 
     Subsequently, the controller  80  reads and acquires time measured by the timer  84 . In this case, the time measured by the timer  84  is regarded as elapsed time while the carriage  17  moves from the starting point Ps to the ending point Pe. 
     Then, the controller  80  reads and acquires the actual velocity Vr of the carriage  17  that is calculated and detected by the carriage velocity calculation circuit  89 . 
     Next, the control value calculating part Pr 1  of the controller  80  calculates the feedback control value Wb. In this case, because feedback control is performed with PI control, the feedback control value Wb becomes a value that is the proportional component Wp with the integral component Wi added, that can be expressed as
 
 Wb=Wp+Wi  
 
     In this embodiment, the proportional component Wp and the integral component Wi are calculated based on the deviation between the target position and the actual position and the deviation between the target velocity Vs and the actual velocity Vr of the carriage  17 , the gain, etc. so that the actual position becomes the target position and the actual velocity Vr the target velocity Vs. 
     Therefore, the above-mentioned control value calculating part Pr 1  reads the target velocity Vs and the target position from the setting value storing part  83 , calculates the deviation between the target position and the actual position and the deviation δV between the target velocity Vs and the actual velocity Vr, and calculates the proportional component Wp and the integral component Wi. 
     By the way, the target velocity Vs in this embodiment is set to change as shown in  FIG. 7 , which is made different from the target velocity Vs when movement control of the carriage  17  is performed using feedback control only as shown in  FIGS. 3-5 . 
     Then, set for the above-mentioned target velocity Vs are the acceleration region Ar 1  where the carriage  17  is accelerated from a stopped state, the uniform velocity region Ar 2  where the carriage  17  is moved at a constant velocity, and the deceleration region Ar 3  where the carriage  17  is decelerated and stopped. 
     The above-mentioned acceleration region Ar 1  consists of a first acceleration region portion from the starting point Ps to an intermediate point P 1  where the actual velocity Vr is increased by gradually increasing the target acceleration of the carriage  17 , that is, target acceleration, in this case positive target acceleration, from the state where the carriage  17  is stopped, a second acceleration region portion from the intermediate point P 1  to an intermediate point P 2  where the actual velocity Vr is increased by maintaining the positive target acceleration at a constant value, and a third acceleration region portion from the intermediate point P 2  to an intermediate point P 3  where the actual velocity Vr is increased by gradually decreasing the positive target acceleration, and once the third acceleration region portion is finished, a shift occurs from the acceleration region Ar 1  to the uniform velocity region Ar 2 , where the target velocity Vs is kept constant. 
     Then, the deceleration region Ar 3  consists of a first deceleration region portion from an intermediate point P 4  to an intermediate point P 5  where the actual velocity Vr is decreased from the target velocity Vs by gradually decreasing (increasing in the absolute value) negative target acceleration, a second deceleration region portion from the intermediate point P 5  to an intermediate point P 6  where the actual velocity Vr is decreased by maintaining the negative target acceleration at a constant value, and a third deceleration region portion from the intermediate point P 6  to the ending point Pe where the actual velocity Vr is decreased by gradually increasing (decreasing in the absolute value) the negative target acceleration, and once the third deceleration region portion is finished, the carriage  17  is stopped. 
     Subsequently, the abnormality judging part Pr 2  of the controller  80  reads the threshold value εj corresponding to each region of the acceleration region Ar 1 , the uniform velocity region Ar 2 , and the deceleration region Ar 3  mentioned above for the target velocity Vs, compares the proportional component Wp and the threshold value εj, and judges whether an abnormality has occurred to the movement of the carriage  17  based on whether the proportional component Wp has exceeded the threshold value εj. Threshold value ε 1  is shown in  FIG. 8 . 
     If the proportional component Wp has exceed the threshold value εj (the proportional component Wp is greater than the threshold value εj) and it is judged that an abnormality has occurred to the movement of the carriage  17 , the above-mentioned abnormality judging part Pr 2  sends a stop instruction to the carriage driving part  87 , and upon receiving the stop instruction, the carriage driving part  87  stops the carriage motor M 2  to stop the carriage  17 . Then, the controller  80  shows a prescribed display on the display part of the operation panel  43  ( FIG. 2 ) to notify the user of the inkjet printer  10  that an abnormality has occurred to the movement of the carriage  17 . 
     Subsequently, the controller  80  moves the carriage  17  to the home position or the retreat position, ends the process, and waits for the user of the inkjet printer  10  to remove the cause of the abnormality occurrence and release the abnormality in the movement of the carriage  17 . 
     On the other hand, if the proportional component Wp has not exceeded the threshold value εj (the proportional component Wp is equal to or smaller than the threshold value εj) and it is judged that no abnormality has occurred to the movement of the carriage  17 , the above-mentioned control value calculating part Pr 1  reads and acquires the feedforward control value Wf from the setting value storing part  83 . 
     The above-mentioned feedforward control value Wf is a control value that combines the acceleration corresponding control value Wα and the velocity corresponding control value Wv. The value Wα is a control value that is set based on a variation amount of the target acceleration, and corresponds to the acceleration component. The value Ww is another control value that is set based on the target velocity Vs, and corresponds to the velocity component. The acceleration corresponding control value Wα and the velocity corresponding control value Wv are independently recorded in the setting value storing part  83 . 
     Then, the above-mentioned acceleration corresponding control value Wα is used in the acceleration region Ar 1  and the deceleration region Ar 3 , and the velocity corresponding control value Wv is used in the acceleration region Ar 1 , the uniform velocity region Ar 2 , and the deceleration region Ar 3 . 
     Therefore, the feedforward control value Wf is expressed as
 
 Wf=Wα+Wv  
 
in the acceleration region Ar 1  and the deceleration region Ar 3 , and as
 
 Wf=Wv  
 
in the uniform velocity region Ar 2 .
 
     The acceleration corresponding control value Wα linearly increases at a constant slope in the first acceleration region portion, becomes a constant value in the second acceleration portion, and linearly decreases at a constant slope in the third acceleration region portion of the acceleration region Ar 1 . Also, the acceleration corresponding control value Wα linearly decreases at a constant slope in the first deceleration region portion, becomes a constant value in the second deceleration portion, and linearly increases at a constant slope in the third deceleration region portion of the deceleration region Ar 3 . 
     In this embodiment, the acceleration corresponding control value Wα is set corresponding to the variation amount of the target acceleration, that is, a jerk that is a value calculated by differentiating the target acceleration, and consists of a part where the jerk takes a positive value, a part where it takes 0, and a part where it takes a negative value. 
     Note that although the target velocity Vs is set to draw a curve in the first and the third acceleration region portions of the acceleration region Ar 1  and the first and the third deceleration region portions of the deceleration region Ar 3  in this embodiment, it can be set not to draw a curve in the acceleration region Ar 1  and the acceleration region Ar 3 . In that case, it is preferred to set the acceleration corresponding control value Wα so that the jerk takes a negative value in a part immediately before shifting from the acceleration region Ar 1  to the uniform velocity region Ar 2 , and a part immediately after shifting from the uniform velocity region Ar 2  to the deceleration region Ar 3 . 
     Also, the velocity corresponding control value Wv is set corresponding to the target velocity Vs so as to draw a curve in the first and the third acceleration region portions of the acceleration region Ar 1  and the first and the third deceleration region portions of the deceleration region Ar 3 , and draw a line in the second acceleration region portion of the acceleration region Ar 1  and the second deceleration region portion of the deceleration region Ar 3 . 
     That is, the velocity corresponding control value Wv has its positive slope gradually increased in the first acceleration region portion, its positive slope kept constant in the second acceleration region portion, and its positive slope gradually decreased in the third acceleration region portion of the acceleration region Ar 1  of the target velocity Vs, and once a shift occurs from the acceleration region Ar 1  to the uniform velocity region Ar 2 , its value is kept constant. 
     Then, the negative slope is gradually increased in the first deceleration region portion, the negative slope is kept constant in the second deceleration region portion, and the negative slope is gradually decreased to make the value 0 in the third deceleration region portion of the deceleration region Ar 3  of the target velocity Vs. 
     Next, the above-mentioned control value calculating part Pr 1  calculates the total control value Wt based on the feedback control value Wb and the feedforward control value Wf. 
     Subsequently, the controller  80  sends the total control value Wt to the carriage driving part  87 . Upon receiving the total control value Wt, the carriage driving part  87  has a PWM control signal generated according to the total control value Wt in the above-mentioned pulse width modulation signal generating part, has current generated according to the duty of the PWM control signal in the above-mentioned switching circuit, and sends the current to the carriage motor M 2  to drive the carriage motor M 2 . 
     Then, the controller  80  judges whether one scan is finished, if one scan is not finished, sets the next scan and acquires the actual position of the carriage  17  again, and if one scan is finished, ends printing one line and places the carriage  17  at the starting point Ps. 
     As the result, the occurrences of delays in the actual position relative to the target position and in the actual velocity Vr relative to the target velocity Vs can be suppressed, therefore trackability to the target velocity Vs can be improved. 
     Also, because movement control of the carriage  17  is performed using both feedback control and feedforward control, and the feedforward control value Wf is not set to a constant value such as an offset value but consists of the acceleration corresponding control value Wα and the velocity corresponding control value Wv, the feedforward control value Wf can be easily changed according to the elapsed time while the carriage  17  is moved from the starting point Ps to the ending point Pe. 
     For example, if there is a region where the carriage  17  is desired to be rapidly accelerated or rapidly decelerated, or the actual velocity Vr is desired to be smoothly changed, by setting the acceleration component control value Wα in a prescribed pattern, the deviation δV between the target velocity Vs and the actual velocity Vr can be reduced. As the result, the occurrences of delays in the actual position relative to the target position and in the actual velocity Vr relative to the target velocity Vs can be suppressed, therefore trackability of the control can be improved. 
     Also, because the target acceleration a is set to 0 at the timing of shifting from the acceleration region Ar 1  to the uniform velocity region Ar 2 , the total control value Wt needs to be rapidly reduced. However, in this embodiment, because the acceleration corresponding control value Wα is reduced in the third acceleration region portion of the acceleration region Ar 1  and the first deceleration region portion of the deceleration region Ar 3 , variation of the feedback control value Wb can be reduced, therefore variation of the total control value Wt can be reduced. 
     As the result, because the actual velocity Vr of the carriage  17  can be prevented from varying greatly, there is no waiting region formed for waiting until it becomes possible to eject ink droplets from the recording heads Hi, allowing an improvement in the printing throughput. 
     Furthermore, because the velocity corresponding control value Wv changes drawing a curve immediately before the target velocity Vs shifts from the acceleration region Ar 1  to the uniform velocity region Ar 2  and immediately after it shifted from the uniform velocity region Ar 2  to the deceleration region Ar 3 , the peak value Wpmax of the proportional component Wp in the acceleration region Ar 1  and the peak value Wpmin of the proportional component Wp in the deceleration region Ar 3  can be reduced, and also variation of the proportional component Wp when shifting from the acceleration region Ar 1  to the uniform velocity region Ar 2  and variation of the proportional component Wp when shifting from the uniform velocity region Ar 2  to the deceleration region Ar 3  can be reduced. 
     Next, explained is the flow chart. 
     S 1 : The controller  80  acquires the actual position of the carriage  17 . 
     S 2 : The controller  80  acquires time. 
     S 3 : The controller  80  acquires the actual velocity Vr of the carriage  17 . 
     S 4 : The control value calculating part Pr 1  calculates the feedback control value Wb. 
     S 5 : The abnormality judging part Pr 2  compares proportional component Wp with the threshold value εj. 
     S 6 : The abnormality judging part Pr 2  judges whether an abnormality has occurred to the movement of the carriage  17 . Specifically, it is determined if proportional component Wp is larger than threshold value εj. If an abnormality has occurred to the movement of the carriage  17 , it proceeds to S 7 , and if no abnormality has occurred to the movement of the carriage  17 , it proceeds to S 9 .
 
S 7 : The carriage driving part  87  stops the carriage  17 .
 
S 8 : The controller  80  moves the carriage  17  to the home position or the retreat position, and ends the process.
 
S 9 : The control value calculating part Pr 1  acquires the feedforward control value Wf.
 
S 10 : The control value calculating part Pr 1  calculates the total control value Wt.
 
S 11 : The carriage driving part  87  drives the carriage motor M 2 .
 
S 12 : The controller  80  judges whether one scan is completed. If one scan is completed, it ends the process, and if one scan is not finished, proceeds to S 13 .
 
S 13 : The controller  80  sets the next scan and returns to S 1 .
 
     Next, explained are changes in the proportional component Wp if no abnormality has occurred to the movement of the carriage  17  or if an abnormality has occurred to the movement of the carriage  17  while performing movement control of the carriage  17  using both feedback control and feedforward control in this embodiment. 
       FIG. 8  is a time chart for explaining the operation of the inkjet printer if no abnormality has occurred to the carriage movement while performing carriage movement control in the embodiment of this invention, and  FIG. 9  is a time chart for explaining the operation of the inkjet printer if an abnormality has occurred to the carriage movement while performing carriage movement control in the embodiment of this invention. Note that in the figures, the horizontal axis indicates time (unit: ms), the left vertical axis the proportional component Wp, the integral component Wi, the acceleration corresponding control value Wα, the velocity corresponding control value Wv, and the total control value Wt, and the right vertical axis the target velocity Vs and the actual velocity Vr. Also, the proportional component Wp, the integral component Wi, the acceleration corresponding control value Wα, the velocity corresponding control value Wv, the total control value Wt, and the target velocity Vs and the actual velocity Vr of the carriage  17  are dimensionless quantities. 
     In the figures, denoted as Vs is the target velocity, Vr is the actual velocity, Wp is the proportional component, Wt is the total control value, Wα is the acceleration corresponding control value, and Wv is the velocity corresponding control value. The proportional component Wp is calculated based on the deviation between the target position and the actual position, the deviation δV between the target velocity Vs and the actual velocity Vr, the gain, etc. 
     In this embodiment, as mentioned above, because the deviation between the target position and the actual position and the deviation δV between the target velocity Vs and the actual velocity Vr can be reduced in the acceleration region Ar 1  and the deceleration region Ar 3 , the proportional component Wp can be reduced. 
     Also, variation of the proportional component Wp when the target velocity Vs shifts from the acceleration region Ar 1  to the uniform velocity region Ar 2 , and variation of the proportional component Wp when it shifts from the uniform velocity region Ar 2  to the deceleration region Ar 3  can be reduced. 
     Therefore, in the acceleration region Ar 1 , the uniform velocity region Ar 2 , and the deceleration region Ar 3 , the threshold value εj can be set small to the extent that it is not misjudged whether an abnormality has occurred to the movement of the carriage  17 , considering scatters among different units of the inkjet printer  10 . 
     When the target velocity Vs in the acceleration region Ar 1 , and if no abnormality has occurred to the movement of the carriage  17 , as shown in  FIG. 8 , the proportional component Wp gradually increases according to delays occurring in the actual position and the actual velocity Vr, and after reaching a peak value Wpmax 11 , gradually decreases. In this case, because the peak value Wpmax 11  never exceeds a threshold value ε 1 , it is never judged that an abnormality has occurred to the movement of the carriage  17 . 
     Also, in the uniform velocity region Ar 2 , because a peak value Wpmax 12  of the proportional component Wp is lower than the peak value Wpmax 11 , a threshold value ε 2  is set smaller than the threshold value ε 1 . 
     As opposed to this, as shown in  FIG. 9 , if an abnormality has occurred to the movement of the carriage  17  at timing t 11 , the actual velocity Vr of the carriage  17  decreases, the deviation δV increases, and accompanying it, the proportional component Wp increases and exceeds the threshold value ε 1 . 
     At this time, even if the abnormality in the movement of the carriage  17  is small to the extent that the recording medium is distorted and slightly touches the carriage, if the delay in the actual velocity Vr relative to the target velocity Vs of the carriage  17  increases even slightly, it is judged that an abnormality has occurred to the movement of the carriage  17 . 
     In this manner, in this embodiment, because movement control of the carriage  17  is performed using both feedback control and feedforward control, the feedback control value Wb can be reduced, and the proportional component Wp can be reduced. 
     Therefore, in judging whether the proportional component Wp has exceeded the threshold value εj, because the threshold value εj can be reduced, even if the abnormality in the movement of the carriage  17  is small, it can be securely judged whether the proportional component Wp has exceeded the threshold value εj. 
     As the result, it can be accurately judged whether an abnormality has occurred to the movement of the carriage  17 . 
     Then, because there will be no case where an image is formed on the recording medium without judging that an abnormality has occurred to the movement of the carriage  17 , degradation in the image quality can be prevented. 
     Also, because the above-mentioned threshold value εj is set according to the peak value of the proportional component Wp for each of the acceleration region Ar 1 , the uniform velocity region Ar 2 , and the deceleration region Ar 3 , it can be even more accurately judged whether an abnormality has occurred to the movement of the carriage  17 . 
     Furthermore, in this embodiment, as mentioned above, because the target velocity Vs changes so as to draw a curve in the first and the third acceleration region portions of the acceleration region Ar 1  and the first and the third deceleration region portions of the deceleration region Ar 3 , the deviation δV between the target velocity Vs and the actual velocity Vr can be reduced, and the proportional component Wp can be reduced. 
     Therefore, it can be accurately judged whether an abnormality has occurred to the movement of the carriage  17 . 
     Furthermore, in this embodiment, as mentioned above, because the acceleration corresponding control value Wα is reduced in the third acceleration region portion of the acceleration region Ar 1  and the first deceleration region portion of the deceleration region Ar 3 , variation of the feedback control value Wb can be reduced, and variation of the proportional component Wp can be reduced. 
     Therefore, it will never be misjudged that an abnormality has occurred to the movement of the carriage  17 . 
     Also, in this embodiment, as mentioned above, because the velocity corresponding control value Wv changes drawing a curve immediately before the target velocity Vs shifts from the acceleration region Ar 1  to the uniform velocity region Ar 3  and immediately after it shifted from the uniform velocity region Ar 2  to the deceleration region Ar 3 , the proportional component Wp in the acceleration region Ar 1  and the deceleration region Ar 3  can be reduced, and also variation of the proportional component Wp when shifting from the acceleration region Ar 1  to the uniform velocity region Ar 2  and shifting from the uniform velocity region Ar 2  to the deceleration region Ar 3  can be reduced. 
     Therefore, it can be accurately judged whether an abnormality has occurred to the movement of the carriage  17 , and there will be no misjudgment that an abnormality has occurred to the movement of the carriage  17 . 
     In this embodiment, the proportional component Wp and the integral component Wi are calculated based on the deviation between the target position and the actual position and the deviation δV between the target velocity Vs and the actual velocity Vr of the carriage  17 , the gain, etc. However, they can be calculated based on the deviation between the target position and the actual position, the gain, etc., or based on the deviation δV between the target velocity Vs and the actual velocity Vr, the gain, etc. 
     Also, the proportional component Wp and the integral component Wi can be calculated having as parameters the deviation between the target position and the actual position, and the deviation δV between the target velocity Vs and the actual velocity Vr. In this case, if those two parameters are assigned with prescribed weights in calculating the proportional component Wp and the integral component Wi, trackability to the target velocity Vs can be further improved. 
     Note that this invention is not limited to the above-mentioned embodiment but can be modified in various kinds of manners based on the purpose of this invention, and they are not excluded from the scope of this invention. Especially, the total control value Wt, feedback control value Wb, feedforward control value Wf are not limited by the disclosed components above. In view of conventional control technology, various types of components are available to compose these values. Also, proportional component Wp, integral component Wi, velocity and acceleration corresponding control values Wv and Wα are not limited within the above embodiments. In the light of conventional technologies, various components including velocity and acceleration are available. 
     In the invention, the target values, thresholds (or threshold values) and feedforward control values are determined in advance (or prior to the image forming process) from experiences under various conditions or try and error experiences. For example, when moving the carriage from one end of the reciprocation area, the one end is used as a staring point and an elapsed time counts after the carriage leaves form the starting point. The elapsed time is segmented with a predetermined interval so that many measured points are created. A velocity of the carriage is measured at each of the measured points, making the velocity the target value at the measured point. The velocity is illustrated as a line with Vs. The predetermined interval may be ranged within 0.001 to 0.1 seconds. 
     The threshold values are determined through experiences in which a carriage is caused to collide against a sheet as the control values change. In these experiences, it is observed whether or not any deficiency occurs. When an inacceptable control value is found, the value or near value may be used as the threshold value. 
     The feedforward control values are determined through many experiences that are performed under various value Wα. When a value Wα that is following well to the target is found, such a value may be used as the feedforward control value. 
     These above three values are determined at each of the measured points, the points being repeatedly disposed and separated one another with an interval. As described above, the interval may be based on an elapsed time (or a time unit that is obtained based on a measured time), but it may be based on a travel distance (or a moving distance unit) for which the carriage moves from the starting point.