Patent Publication Number: US-8121510-B2

Title: Printer with duplex circulation route speed control

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to printers adapted for a sheet transfer control, and in particular to a printer adapted for a sheet transfer control with a sheet circulating transfer route including a sheet invert portion for inverting a sheet between front side and back side. 
     2. Description of Related Art 
     There has been known printers configured with a sheet circulating transfer route including a sheet transfer portion of a printing mechanism and a sheet invert portion. The printers are adapted for a sheet with a print made on one side thereof by the printing mechanism to be inverted at the sheet invert portion, to make a print on the opposite side by the same printing mechanism. A recent trend of demands for printers with increased printing speeds is directed to implement a high productivity of print material by a high speed printing not only simply in one-side printing but also in both-side printing. 
     The productivity of print material mainly depends on a transfer speed of sheet commensurate with a printing speed at a sheet transfer portion of a printing mechanism. The transfer speed has been ruling transfer speeds of sheets at other sections of a sheet circulating transfer route in the past. Japanese Patent Application Laid-open Publication No. 2005-280897 discloses a printing technique that both-side printing of sheets is alternately done to a front side of a sheet and a back side of another sheet and transfer speeds in a sheet circulating transfer route are controlled in accordance with an associated sheet size, independently from a transfer speed at a printing mechanism. This technique permits a sufficient inter-sheet spacing to be secured with an enhanced productivity of print material. 
     SUMMARY OF THE INVENTION 
     However, for some sheet sizes, the technique cannot control transfer speeds in the sheet circulating transfer route independently from the transfer speed at the printing mechanism. 
     For instance, a normal printing is typically done on a “sheet of standard size (referred herein to as “regular sheet)” fed from a tray employed in a printer. However, the printer may be used for a “sheet with a different size from the standard size (referred herein to as an “irregular sheet)”, in particular, a “sheet longer than the regular sheet in a transfer direction (referred herein to as a “long sheet)”. On the way of a printing such a long sheet by a printing mechanism, the leading edge may be engaged to a transfer drive member in a section of a sheet circulating transfer route with a different transfer speed than the printing mechanism. In this situation, the printing will go wrong. 
     The present invention has been made in view of such an issue and has an object of providing a printer adapted to control a transfer speed at a sheet transfer portion of a printing mechanism and a transfer speed at a section of a sheet circulating transfer route extending downstream the sheet transfer portion in consideration of a length of sheet in a sheet transfer direction. 
     To achieve the object, according to an aspect of the present invention, a printer comprises a printing mechanism comprising a sheet transfer portion configured with a first drive member for transfer of a sheet as positioned thereto, and an image former configured to form an image on the sheet as positioned to provide a sheet as image-formed, a sheet circulating transfer route including the sheet transfer portion of the printing mechanism, a transfer section having a second drive member for transfer of the sheet as image-formed configured in a position at a prescribed path distance from the image former of the printing mechanism, and a sheet invert portion configured to invert the sheet as image-formed, a printing condition determiner configured to determine a set of printing conditions including a first condition that a sheet to be positioned in the printing mechanism is equal in length to or longer than the prescribed path distance and a long sheet to be printed on both sides thereof, and a drive controller configured to control the first drive member to establish a first transfer speed of the sheet as positioned and the second drive member to establish a second transfer speed of the sheet as image-formed in accordance with the set of printing conditions determined by the printing condition determiner, the drive controller being adapted, in conformity to the first condition, to set the second transfer speed as identical to the first transfer speed, and in non-conformity to the first condition, to set the second transfer speed as greater than the first transfer speed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of configuration of a printer  100  according to an embodiment of the present invention. 
         FIG. 2  is a schematic diagram of a sheet circulating transfer route CR and a system of feed routes FR of the printer  100 . 
         FIG. 3  is a block diagram of functional configuration of a controller  300  of the printer  100 . 
         FIG. 4  is an illustration of a transfer belt  160 , a head unit  110 , a first transfer roller  260 , and a top roller  265  arranged in the printer  100 . 
         FIG. 5A  is an illustration of a positional relationship between a path distance S from the head unit  100  to the top roller  265  and a sheet P with a length Lp,  FIG. 5B  an illustration of the positional relationship in printing to a regular sheet, and  FIG. 5C  an illustration of the positional relationship in printing to a long sheet. 
         FIG. 6A  is an explanatory time chart of sheet transfer control for one-side printing to a regular sheet in the printer  100 , and  FIG. 6B  an explanatory time chart of sheet transfer control for both-side printing to a regular sheet. 
         FIG. 7A  and  FIG. 7B  are explanatory time charts of printing schedules for regular sheets in the printer  100 , respectively. 
         FIG. 8A  is an explanatory time chart of sheet transfer control for one-side printing to a long sheet in the printer  100 , and  FIG. 8B  an explanatory time chart of sheet transfer control for both-side printing to a long sheet. 
         FIG. 9A  and  FIG. 9B  are explanatory time charts of printing schedules for long sheets in the printer  100 , respectively. 
         FIG. 10  is a flowchart of actions for sheet transfer control in the printer  100 . 
         FIG. 11A  is a time chart of control pulses for starting register rollers  240  and feed rollers  220  in one-side printing to regular sheets in the printer  100 , and  FIG. 11B  a time chart of control pulses for starting register rollers  240  and feed rollers  220  in both-side printing to regular sheets. 
         FIG. 12A  is a time chart of control pulses for starting register rollers  240  and feed rollers  220  in one-side printing to long sheets in the printer  100 , and  FIG. 12B  a time chart of control pulses for starting register rollers  240  and feed rollers  220  in both-side printing to long sheets. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     There will be described the preferred embodiments of the present invention with reference to the accompanying drawings. 
       FIG. 1  is an illustration of configuration of a printer  100  according to an embodiment of the present invention. As illustrated in the figure, the printer  100  has a sheet feeding mechanism including: a specified number of internal feed trays  130   a ,  130   b ,  130   c  and  130   d  (referred herein collectively to  130 ) incorporated in an equipment casing and adapted for feed of regular sheets; a side feed rack  120  exposed outside at a lateral side of the casing and adaptive for feed of irregular sheets; and a sheet discharging mechanism including a discharge port  140  for discharging any sheets as-printed. The printer  100  has a controller  300  composed of a substrate with a mounted CPU, memories, etc., and an operation panel  400  for interfacing user operations at a top side of the printer housing. 
     The printer  100  is a line color printer of an inkjet type for printing by lines. The line color printer has a printing mechanism including an array of print heads, each of which is formed with multiple nozzles in the direction perpendicular to a sheet transfer direction and operable to propel droplets of black or color ink for printing. It is noted that the present invention is applicable to printers such as of a serial inkjet system or a laser system. It is applicable to printers implementing individual printing processes including not only simply a printing based on transmitted data from a host computer but also a copy printing, facsimile printing, etc. 
     The printer  100  has a sheet transfer system that includes: a system of feed routes FR, as a part of the sheet feeding mechanism, for feeding a selective one of different types of regular sheets and irregular sheets; a sheet discharge route DR, as a part of the sheet discharging mechanism, for discharging any type of regular or irregular sheet; a normal transfer route PR (with a later-described sheet transfer portion of the printing mechanism inclusive) for transferring any sheet received from the system of feed routes FR to the sheet discharge route DR; and an inverting route SR, as a dropping switchback type sheet invert portion, branched from the normal transfer route PR, for inverting any sheet, received from the normal transfer route PR, between front side and back side, to re-feed to the nominal transfer route PR. The inverting route SR cooperates with the normal transfer route PR to constitute a looped sheet circulating transfer route CR. 
     A sheet is fed one by one from the side feed rack  120  or any feed tray  130  of the sheet feeding mechanism, and transferred along one route of the system of feed routes FR, by an associated drive mechanism such as rollers, to a register R constituting the sheet transfer portion (R,  160 ) of the printing mechanism (R,  160 ,  110 ). The register R is configured with a pair of register rollers  240  (see  FIG. 2 ) for positioning a front edge of a fed sheet to a transfer belt  160  of the sheet transfer portion (R,  160 ), to avoid giving an oblique position to the sheet to be carried by the transfer belt  160 . The fed sheet enters the register R, where it is once put in a pause, before being carried downstream in a sheet transfer direction at a controlled timing. 
     Past the register R, the sheet being carried by the transfer belt  160  proceeds along the normal transfer route PR, and comes under a head unit  110  that is an assembly of print heads constituting an image former ( 110 ) of the printing mechanism (R,  160 ,  110 ). The transfer belt  160  is looped, and has a top side thereof facing an ink droplet-propelling side of the head unit  110 . On the top side of the transfer belt  160 , the sheet is vacuum-contacted at the back side to be carried at a constant transfer speed-depending on a set of printing conditions described later on, while on the front-side of the sheet an image is formed (printed) by ink droplets propelled from the print heads, by the line or lines. 
     The sheet as printed is transferred by an associated drive mechanism such as rollers inside the casing. For one-side printing, the printed sheet is guided directly to the sheet discharge port  140 , where it is discharged to stack, with a printed side down, on a discharge rack  150  provided as a sheet receiver at the sheet discharge port  140 . The discharge rack  150  is set in the form of a tray protruding from the casing, with a certain thickness. The discharge rack  150  is inclined to a lateral wall of the casing, so the printed sheet once discharged from the sheet discharge port  140  is slid down along an inclination of the discharge rack  150 , and trimmed to pile up on the discharge rack  150  in due course. 
     For both-side printing of any type of sheet, assuming “a front side” thereof as the side to be printed first and “a back side” thereof as the side to be printed next a sheet as printed on the front side is to be routed inside the casing without being guided to the sheet discharge port  140 . This is implemented in the printer  100  by a route selecting mechanism  170  provided to select a sheet transfer route for back side printing. With this route selected by the route selecting mechanism  170 , the sheet as printed on the front side is transferred to the inverting route SR. On the inverting route SR, the sheet is switched back in a dropping manner for inversion between front side and back side, and contacts at the (printed) front side with an upside of the sheet transfer route. This sheet is transferred along the route SR by an associated drive mechanism such as rollers, to re-feed to the register R, where it is put in a pause, before being carried downstream in the printing mechanism at a controlled timing, to have an image formed on the back side in a similar manner to the front side. The sheet now image-formed on both sides with the back side printed, is guided to the sheet discharge port  140 , where it is discharged to stack on the discharge rack  150 . It is noted that the sheet discharge port, which is single in this embodiment, may be substituted by a plurality of selective sheet discharge ports. 
     For the printer  100 , an internal space of the discharge rack  150  is availed to implement a dropping switch back for both-side printing. The space in the discharge rack  150  is enclosed to keep a sheet or sheets from being taken from outside in the course of switchback. This prevents the sheet or sheets from being pulled out by a mistake of user in the course of switchback. The discharge rack  150 , as an inherent member to the printer  100 , affords to eliminate provision of an extra space for switchback in the casing of the printer  100 . This permits the casing to be kept from being enlarged in size. The inverting route SR, separated from the sheet discharge port  140 , allows for parallel operations between a sheet to be switched back and another sheet to be discharged. 
     In the printer  100 , the register R is adapted to position in the front edge of an unprinted sheet fed thereto and a sheet printed up on the front side and re-fed thereto for both-side printing. The sheet circulating transfer route CR thus has, at a location just before the register R, a junction between the route CR for circulation of a sheet as printed on the front side and a feed route for transfer of an unprinted sheet. This junction is a reference to define the above-noted system of feed routes FR. That is, the feed route system FR is defined as a system of feed routes for interconnections from the respective feed trays  130  and the side feed rack  120  of the sheet feeding mechanism to the junction. 
       FIG. 2  is a schematic diagram of the system of feed routes FR and the sheet circulating transfer route CR. For simplicity, main drive rollers are illustrated. The system of feed routes FR includes a transverse pair of side feed rollers  220  for feeding a sheet from the side feed rack  120 , and a set of specified number of transverse pairs of tray rollers  230   a ,  230   b , . . . (referred herein collectively to “ 230 ”) for feeding a sheet from a selected one of feed trays  130 . Those roller pairs are each operable to take up a sheet one by one from a stack of sheets in the side feed rack  120  or any feed tray  130 , to transfer to the register R. Each roller pair is independently controllable. 
     The sheet circulating transfer route CR includes in order: a pair of front and rear register rollers  240 ; the transfer belt  160  facing the head unit  110 ; a pair or set of transverse pairs of front and rear first transfer rollers  260 ; a pair or set of transverse pairs of front and rear top rollers  265 ; a pair or set of transverse pairs of front and rear discharge rollers  270  for transferring a sheet as printed to the sheet discharge port  140 ; a pair or set of transverse pairs of front and rear switchback rollers  280  for pulling a sheet as printed up on the front side into the inverting route SR to invert in a dropping manner; and a pair or set of transverse pairs of front and rear re-feed rollers  285  for re-feeding an inverted sheet to the register R. Each roller pair or set is independently controllable. For instance, for a long sheet that has, in a course of printing thereon, a front end thereof reaching the top roller pair or set  265  (refer to  FIG. 5C ), whether one-side printing or both-side printing, the first transfer roller pair or set  260  is controllable to set a sheet transfer speed thereof up as consistent with a sheet transfer speed of the top roller pair or set  265 , or to set free to employ as follower rollers, in accordance with an associated set of printing conditions. 
     According to the present embodiment, the printer  100  is operable not simply to feed a sheet after a previous fed sheet is printed and discharged, but also to feed a sheet before discharge of a previous fed sheet or previous fed sheets to be consecutively printed at specified intervals. That is, for a consecutive printing of sheets, the printer  100  is adapted for transfer of sheets on the sheet circulating transfer route CR. 
     The system of feed routes FR as well as the sheet circulating transfer route CR has unshown sheet sensors arranged in positions to detect presence or absence of sheet, feed errors, transfer jams, discharge errors, etc. 
       FIG. 3  is a block diagram of functional configuration of the controller  300  of the printer  100 . The controller  300  sequentially receives one or more printing jobs transmitted from the PC connected to the printer  100 , as well as one or more printing jobs interfaced through the operation panel  400 . Each printing job includes a set of given printing conditions such as specifications for a color printing and a sheet to be fed, and a set of printing data such as pixel data for the color printing. In order for each printing job to be adjusted to a high-speed printing of the printer  100 , the controller  300  is adapted to implement determination (identification and decision, see  FIG. 10 ) of and on given printing conditions, to generate a new set of printing conditions including results of determination and necessary printing conditions. Based on this set of printing conditions, the controller  300  creates a series of sheet feed commands and a schedule of timings for sending those commands to drives (roller motors) for the system of feed routes FR, and a series of sheet transfer commands and a schedule of timings for sending these commands to drives (belt motors, roller motors) for the sheet circulating transfer route CR. Further, based on a combination the set of printing conditions and an associated set of printing data, the controller  300  creates a combination of a series of sequences of frames of image data and a series of sequences of print commands, and a schedule of timings for sending those sequences to a temporary storage portion (frame registers for heads) and a circuit drive portion (head drive circuits) of the printing mechanism. 
     The controller  300  has a printing condition determiner  320  adapted for determination of and on given printing conditions to provide a new set of printing conditions, a frame processor  310  adapted for processing given print data in accordance with the set of printing conditions to provide a sequence of frames of image data, and a drive controller  330  adapted on basis of the set of printing conditions for control of drives such as those of feed rollers  220  and  230 , register rollers  240 , transfer belt  160 , and top rollers  265 . Note here that a flame is defined as a unit making up image data. Although a sequence of frames makes up image data in the present embodiment, one frame may make up the image data in the present invention. 
     The printing condition determiner  320  is adapted on the basis of a set of given printing conditions for an associated printing job, to provide a new set of printing conditions adjusted to a high-speed printing of the printer  100 . The set of given printing conditions includes, among others, a type and a size of a sheet to be positioned in the printing mechanism (specifically, to the sheet transfer portion, or more specifically, to the transfer belt  110 ) for the associated printing job, identification of the printing job to be one-side printing or both-side printing, a required quality of the printing, and the like. The new set of printing conditions includes, besides given printing conditions, conformity and non-conformity of a first condition ( FIG. 10 , step S 104 , Yes, No) that the sheet to be positioned is equal in length or longer than a later-described path distance S and a long sheet to be printed on both sides thereof, conformity or non-conformity of a second condition ( FIG. 10 , step S 103 , No) that the sheet to be positioned is a sheet to be printed on one side thereof, whether a regular sheet or an irregular sheet, conformity or nonconformity of a third condition ( FIG. 10 , step S 104 , No) that the first condition is non-conforming with a collateral condition that the sheet to be positioned is a sheet to be printed on both sides thereof, and a data on a fourth condition ( FIG. 10 , step S 105 , S 107 , or S 110 ) that defines the number N of sheets to be simultaneously identified for control from feed to discharge of sheet. 
     The frame processor  310  responds to a set of printing conditions of an associated printing job, by generating a sequence of frames of image data on a sheet-side basis, to output to the head unit  110  of the printing mechanism. For a printing job of both-side printing, if a length Lp (refer to  FIG. 5C ) in a transfer direction of a sheet to be positioned in the printing mechanism is equal to or longer than a path distance S between the head unit  110  and paired upper rollers  265  (i.e. conformity of the 1 st  condition), the frame processor  310  generates in order a sequence of frames of image data for a front side of the sheet, and a sequence of frames of image data for a back side of the sheet (refer to  FIG. 9B ). If the length Lp in the transfer direction of the sheet to be positioned is shorter (refer to  FIG. 5B ) than the path distance S between the head unit  110  and paired upper rollers  265  (i.e. conformity of the 3 rd  condition), the frame processor  310  generates in order a sequence of frames of image data for a front side of the sheet, a sequence of frames of image data for a back side of a previous sheet of the sheet, and a sequence of frames of image data for a front side of a subsequent sheet of the sheet to be positioned (refer to  FIG. 7B ). 
     The drive controller  330  is adapted to set up a sheet transfer speed of the transfer belt  160  while printing in accordance with a set of printing conditions for an associated printing job. For a printing job of both-side printing, if the length Lp in transfer direction of a sheet is equal to or longer than the path distance S between the head unit  110  and paired upper rollers  265  (i.e. conformity of the 1 st  condition), the drive controller  330  sets a sheet transfer speed of top rollers  265  up as an identical speed to the sheet transfer speed of the transfer belt  160  while printing (refer to  FIG. 8B ). If the length Lp in transfer direction of the sheet is shorter than the path distance S between the head unit  110  and paired upper rollers  265  (i.e. conformity of the 3 rd  condition), the drive controller  330  sets a sheet transfer speed of top rollers  265  up as a greater speed than the sheet transfer speed of the transfer belt  160  while printing (refer to  FIG. 6B ). For a printing job of one-side printing (i.e. conformity of the 2 nd  condition), the drive controller  330  sets a sheet transfer speed of top rollers  265  up as an identical speed to the sheet transfer speed of the transfer belt  160  while printing (refer to  FIG. 6A  and  FIG. 8A ), irrespective of the length Lp in transfer direction of sheet. 
       FIG. 4  is an illustration of the transfer belt  160 , the head unit  110 , a first transfer roller  260 , and a top roller  265  arranged along the sheet circulating transfer route CR. An image is formed by the head unit  110  on an upside of a sheet being transferred by the transfer belt  160 . The sheet as image-formed is to be transferred by paired first transfer rollers  260  and paired top rollers  265 .  FIG. 4  is an exploded view illustrating a path distance, and really the sheet circulating transfer route CR is curved, excepting principal parts of the printing mechanism and sheet invert portion. The path distance S is defined as a distance along the transfer route CR from a downstream end of the head unit  110  to a center position of upper rollers  265 . This distance S is preset as an ex-factory specification of the printer  100 , which is longer than a longitudinal dimension of a A4 size and shorter than a longitudinal dimension of a A3 size in this embodiment, while instead a fixing position of upper rollers  265  may well be optional or variable, so the path distance S can be set to a regular position of the user, permitting a registration through the operation panel  400 . 
     A printing is now supposed on a sheet P with a length Lp in a transfer direction.  FIG. 5A  illustrates the printing. While forming an image, the head unit  110  continues propelling ink droplets, requiring a constant transfer speed of the sheet P. Letting Vg be the sheet transfer speed in a course of printing, the drive controller  330  is operated to drive the transfer belt  160  at the speed Vg while printing. The sheet transfer speed Vg depends on a subset of the above-noted set of printing conditions including a printing quality such as resolution, and a sheet type. 
     Paired first transfer rollers  260  are disposed nearer to the transfer belt  160  than paired top rollers  160 . On the way of image formation by the head unit  110 , first transfer rollers  260  may cooperate with the transfer belt  160  for transfer of a sheet P, and the drive controller  330  may drive the first transfer rollers  260  at the same transfer speed Vg as the transfer belt  160 . 
     On the other hand, paired top rollers  160  are disposed at a path distance S from the head unit  110 . 
     For use of a regular sheet P with a shorter length Lp in transfer direction than the path distance S, as illustrated in  FIG. 5B , the head unit  110  completes formation of image before a leading edge of the sheet P reaches a position of top rollers  265 . Paired top rollers  265  can thus have a sheet transfer speed Vr set independently of the above-noted sheet transfer speed Vg while printing. However, the transfer speed Vr of sheet P by top rollers  265  should be consistent with a sheet transfer speed of first transfer rollers  260 . To avoid differences in between, first transfer rollers  260  may be each provided with a clutch mechanism. 
     For use of a long sheet P with an equal or greater length Lp in transfer direction to or than the path distance S, as illustrated in  FIG. 5C , the head unit  110  is still on the way of image formation when a leading edge of the sheet P has reached a position of paired top rollers  265 . The top rollers  265  should be driven to transfer the sheet P at the same speed as a transfer speed Vg the transfer belt  160  then has. 
     Accordingly, this embodiment implements a sheet transfer control depending on a length Lp in transfer direction of sheet, as follows. 
     Description is now made of modes of transfer control for one-side printing and both-side printing of regular sheets in the printer  100 , with reference to  FIG. 6A  and  FIG. 6B . For the mode of one-side printing, each sheet as printed on the front side is discharged as it is straightly transferred, so the number of as-printed sheets that can be discharged per unit time is limited even with an increased transfer speed of top rollers  265 . For the mode of both-side printing, a sheet as printed on the front side is routing the sheet circulating transfer route CR, while another sheet is fed and printed before the sheet as printed on the front side is re-fed, so the printing interval can be optimized by adjusting a transfer speed of top rollers  265 , allowing for an increased number of output sheets per unit time. 
       FIG. 6A  illustrates a varying sheet transfer speed in a one-side printing of a regular sheet Sheet feeding is now assumed to be from the side feed rack  120 . As illustrated in the figure, at a time a 1 , the sheet is transferred by side feed rollers  220  at a prescribed transfer speed, and is fed to the register R, where it is put in a pause by register rollers  240 . At a time a 2 , the sheet is transferred at a transfer speed Vg, while a printing is made thereon by the head unit  110 . In due course, at a time a 3 , a leading edge of the sheet reaches a position of top rollers  265 , while the sheet transfer speed is unchanged, so the transfer of sheet by the speed Vg is maintained, and at a time a 4 , the sheet is discharged. The printing is completed before the leading edge of sheet reaches the position of top rollers  265 , and the sheet transfer speed of top rollers  265  can be increased, so the sheet can be transferred by the top rollers  265  at an increased speed suitable for discharge with a build momentum, for instance, while the number of output sheets per unit time remains unchanged. 
       FIG. 6B  illustrates a varying sheet transfer speed in a both-side printing of a regular sheet. As illustrated in the figure, at a time b 1 , the sheet is transferred by side feed rollers  220  at a prescribed transfer speed, and is fed to the register R, where it is put in a pause by register rollers  240 . At a time b 2 , the sheet is transferred at a transfer speed Vg, while a printing is made thereon by the head unit  110 . In due course, at a time b 3 , a leading edge of the sheet reaches a position of top rollers  265 , when the sheet transfer speed is changed to a speed Vr higher than the speed Vg. The printing is completed before the leading edge of sheet reaches the position of top rollers  265 , and the sheet transfer speed of top rollers  265  can be increased, without effects on a result of printing. After a dropping switchback for inversion of sheet, the sheet is re-fed to the register R, where it is put in a pause by register rollers  240 . At a time b 4 , the sheet is transferred at a transfer speed Vg for printing, where the head unit  110  makes a printing on the back side of the sheet. In due course, at a time b 5 , a leading edge of the sheet reaches the position of top rollers  265 , when the sheet transfer speed is changed to a speed Vr higher than the speed Vg, and at a time b 6 , the sheet is discharged. 
     The sheet transfer speed Vr is determined in a manner illustrated in  FIG. 7A  and  FIG. 7B . In the figures, designated at integers are identification numbers of sheets, black integers on white background each representing a printing on the front side, white integers on black background each representing a printing on the back side. For one-side printing of sheets, as illustrated in  FIG. 7A , a possible printing is assumed at intervals of time dA. For implementation of this printing speed in both-side printing, a printing to the back side of a first sheet is made at a timing for a printing to a fourth sheet in one-side printing. As a presumption for this example, N=3, the illustration is for a pattern of both-side printing that makes, between a printing on the front side of a sheet (e.g., a 2 nd  sheet) and a printing on the back side of the sheet, a printing on the backside of a previous sheet (e.g., a 1 st  sheet) and a printing on the front side of a subsequent sheet (e.g., a 3 rd  sheet) fed anew. However, even if “N” takes another number, the sheet transfer speed Vr is determined in the same manner too. 
     Letting T 4  be an interval of time from a start of printing on a first sheet to a start of printing on a fourth sheet in the one-side printing, the both-side printing is to enter a transfer for discharge of a first sheet with a lapse of time T 4  from an initiation of printing to the first sheet. The sheet transfer speed Vr is determined in dependence on an associated set of printing conditions including information on a total path length of the sheet circulating transfer route CR, in considerations such as of a necessary time for switchback. 
     Description is now made of modes of transfer control for one-side printing and both-side printing of long sheets in the printer  100 , with reference to  FIG. 8A  and  FIG. 8B .  FIG. 8A  illustrates a varying sheet transfer speed in a one-side printing of a long sheet. As illustrated in the figure, at a time c 1 , the sheet is transferred by side feed rollers  220  at a prescribed transfer speed, and is fed to the register R, where it is put in a pause by register rollers  240 . At a time c 2 , the sheet is transferred at a transfer speed Vg, while a printing is made thereon by the head unit  110 . In due course, at a time c 3 , a leading edge of the sheet reaches a position of top rollers  265 , while the sheet transfer speed is unchanged, so the transfer of sheet by the speed Vg is maintained, and at a time c 4 , the sheet is discharged. For the long sheet, when the leading edge of sheet has reached the position of top rollers  265 , the printing is still on the way, and it is avoided to change the sheet transfer speed of top rollers  265 . A resultant transfer control mode is identical to the one-side printing of regular sheet. 
       FIG. 8B  illustrates a varying sheet transfer speed in a both-side printing of a long sheet. As illustrated in the figure, at a time d 1 , the sheet is transferred by side feed rollers  220  at a prescribed transfer and is fed to the register R where it is put in a pause by register rollers  240 . At a time d 2 , the sheet is transferred at a transfer speed Vg, while a printing is made thereon by the head unit  110 . In due course, at a time d 3 , a leading edge of the sheet reaches a position of top rollers  265 , while the sheet transfer speed is unchanged, so the transfer of sheet by the speed Vg is maintained. For the long sheet, when the leading edge of sheet has reached the position of top rollers  265 , the printing is still on the way, and it is avoided to change the sheet transfer speed of top rollers  265 . After a dropping switchback for inversion of sheet, the sheet is re-fed to the register R, where it is put in a pause by register rollers  240 . At a time d 4 , the sheet is transferred at a transfer speed Vg for printing, where the head unit  110  makes a printing on the back side of the sheet. In due course, at a time d 5 , a leading edge of the sheet reaches the position of top rollers  265 , while the sheet transfer speed is unchanged, so the transfer of sheet by the speed Vg is maintained, and at a time d 6 , the sheet is discharged. 
       FIG. 9A  and  FIG. 9B  illustrate resultant printing schedules for long sheets in the printer  100 , respectively. For one-side printing of long sheets, as illustrated in  FIG. 9A , a possible printing is assumed at intervals of time dC. In both-side printing of long sheets, as illustrated in  FIG. 9B , after an initiation of a printing to the front side of a sheet, with a lapse of time dD till the sheet comes around at a sheet transfer speed Vg for the printing, there is a printing to the back side of the sheet, and with an additional lapse of time identical to the interval dC in the one-side printing, there is a printing to the front side of a subsequent sheet. That is, this embodiment implements a sheet transfer control to make a both-side printing of long sheets on a sheet basis. (N=1). 
     Description is now made of actions for sheet transfer control in the printer  100 , with reference to a flowchart of  FIG. 10 . The sheet circulating transfer route CR has: a section directly associated with printing, with the transfer belt  160  and top rollers  265  inclusive; and the rest, which is referred herein sometimes to a speed variable section. At a step S 101 , the controller  300  receives a printing job from the operation panel  400  or a PC connected to the printer  100 , when the sheet transfer control starts. 
     At a step S 102 , given printing conditions in the printing job are referenced to set up a sheet transfer speed Vg for printing. The sheet transfer speed Vg for printing is a speed to be consistent with an ink-droplets propelling for image formation of the head unit  110 , and depends on a maximal ink droplet number per pixel, resolution, and the like to be defined by information in given printing conditions. Given such printing conditions, for the sheet transfer speed Vg for printing, a maximum value is uniquely determined in accordance with performances of an ink propelling mechanism of the printing mechanism, in particular of the head unit  110 , ink properties, and the like, irrespective of whether one-side printing or both-side printing, whether regular sheet or long sheet. This embodiment assumes a sheet transfer at a maximum speed the printing mechanism is afforded to provide for a sufficient exhibition of its performance, and a commensurate sheet transfer speed is set as the sheet transfer speed Vg for printing, as it is an operational maximal speed in consideration of prescribed margins and the like, that is not always in accord with a physical maximum speed. 
     At a step S 103 , on the basis of given printing conditions, the printing condition determiner  320  determines whether or not the printing job is both-side printing. As a result of this determination, unless the printing job is both-side printing (that is, if it is one-side printing), the control flow goes to a step S 105 , where a sheet transfer speed Vr for the speed variable section is set up as identical to the sheet transfer speed Vg for printing, and an associated N is set. In the speed variable section, the sheet transfer speed will be unchanged. Then, at a step S 106 , for feed rollers  220  and register rollers  240 , N sets of start timings are set, and at a step S 113 , courses of printing are executed at set timings. 
       FIG. 11A  is a time chart of control pulses for starting feed rollers  220  and register rollers  240  in a one-side printing of regular sheets. This figure corresponds to  FIG. 6A  and  FIG. 7A . As illustrated in the figure, register rollers  240  start a transfer of sheet on a sheet-side basis at an interval of time dA. Feed rollers  220  start a feed of sheet at an anterior α to a start timing of the register rollers  240 , for guiding the sheet to the register rollers  240 . For the printing job of one-side printing, the sheet feed by feed rollers  220  is made at an interval of time dA every time for the register rollers  240  to start. 
       FIG. 12A  is a time chart of control pulses for starting feed rollers  220  and register rollers  240  in a one-side printing to long sheets. This figure corresponds to  FIG. 8A  and  FIG. 9A . As illustrated in the figure, register rollers  240  start a transfer of sheet on a sheet-side basis at an interval of time dC. Feed rollers  220  start a feed of sheet at an anterior α to a start timing of the register rollers  240 , for guiding the sheet to the register rollers  240 . For the printing job of one-side printing, the sheet feed by feed rollers  220  is made at an interval of time dC every time for the register rollers  240  to start. 
     As a result of determination at the step S 103 , if the printing job is both-side printing (‘Yes’), the control flow goes to a step S 104 , where the printing condition determiner  320  determines whether the printing job is a printing of long sheet (i.e., whether the length in transfer direction of sheet is equal in length to or longer than the path distance from the head unit  110  to a position of top rollers  265 ). As a result of this determination, if the printing job is a printing of long sheet (‘Yes’ at the step S 104 ), the control flow goes to a step S 110  for setting n=1, and to a step S 111 , where a sheet transfer speed Vr for the speed variable section is set up as identical to the sheet transfer speed Vg for printing. In the speed variable section, the sheet transfer speed will be unchanged. Then, at a step S 112 , for feed rollers  220  and register rollers  240 , their start timings are set, and at the step S 113 , courses of printing are executed at set timings. 
     On the other hand, as a result of determination at the step S 104 , unless the printing job is a printing of long sheet (‘No’, that is a printing of regular sheet), the control flow goes to a step S 107 , where N is determined in accordance with a sheet size and a path length of the sheet circulating transfer route CR. More specifically, N is determined in accordance with the number of sheets that can be simultaneously transferred along the sheet circulating transfer route CR, subject to an odd number as N in this case to permit a required alternate printing of front side and back side of different sheets. 
     Then, at a step S 108 , sheet transfer speeds Vr for the speed variable section are determined by calculation to implement a both-side printing of N regular sheets with an even productivity of printing material to one-side printing, as illustrated in  FIG. 6B  and  FIG. 7B . Then, at a step S 109 , for feed rollers  220  and register rollers  240 , N sets of start timings are set, and at the step S 113 , courses of printing are executed at set timings. 
       FIG. 11B  is a time chart of control pulses for starting register rollers  240  and feed rollers  220  in a both-side printing to regular sheets. This figure corresponds to  FIG. 6B  and  FIG. 7B . As illustrated in the figure, register rollers  240  start a transfer of sheet at an interval of time dA in accordance with a schedule for both-side printing. The illustration is for a pattern of both-side printing for N=3 that makes, between a printing on the front side of a sheet (e.g., K-th sheet) and a printing on the back side of the sheet, a printing on the backside of a previous sheet (e.g., K−1-th sheet) and a printing on the front side of a subsequent sheet (e.g., K+1-th sheet) fed anew. 
     For the printing job of both-side printing, feed rollers  220  start a feed of sheet at an anterior α to a start timing for front-side printing of register rollers  240 , for guiding the sheet to the register rollers  240 . The sheet feed by feed rollers  220  is made at an interval of time dB every time for the register rollers  240  to start for front-side printing, such that dB=dA×2. 
       FIG. 12B  is a time chart of control pulses for starting register rollers  240  and feed rollers  220  in a both-side printing to long sheets. This figure corresponds to  FIG. 8B  and  FIG. 9B . As illustrated in the figure, register rollers  240  start transfer of a sheet for front-side printing, and thereafter, transfer of the sheet for back-side printing at an interval of time dD. After that, they start transfer of a subsequent sheet for front-side printing at an interval of time dC. 
     For the both-side printing, feed rollers  220  start a feed of sheet at an anterior α to a start timing for front-side printing of register rollers  240 , for guiding the sheet to the register rollers  240 . The sheet feed by feed rollers  220  is made at an interval of time dF every time for the register rollers  240  to start for front-side printing, such that dF=dD+dC. 
     As will be seen from the foregoing description, according to the present embodiment, a length in transfer direction of sheet is taken into consideration to control a sheet transfer speed for printing and a sheet transfer speed for circulation. More specifically, upon a determination that the length in transfer direction of sheet is shorter than a path distance S, where the printing is completed before a leading edge of sheet reaches a position at the path distance S, the sheet transfer speed for circulation is set up as higher than the sheet transfer speed for printing, thereby allowing for an enhanced productivity. Upon a determination that the length in transfer direction of sheet is equal to or longer than the path distance S, where the printing is still on the way when the leading edge of sheet has reached the position at the path distance S, the sheet transfer speed for circulation is set up as identical to the sheet transfer speed for printing, thereby allowing for a prevented deterioration of printing quality due to a deviation. 
     For both-side printing, upon the determination that the length in transfer direction of sheet is shorter than the path distance S, an alternate printing is implemented between front side and back side of different sheets, allowing for an enhanced productivity, and upon the determination that the length in transfer direction of sheet is equal to or longer than the path distance S, an associated printing is implemented on a sheet-side basis, allowing for an enhanced integrity of the process. 
     While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims. 
     This application is based upon the Japanese Patent Application No. 2008-184170, filed on Jul. 15, 2008, the entire content of which is incorporated herein by reference.