INKJET PRINTER

Abstract

 Provided is an inkjet printer, which limits reduction in work efficiency without deterioration of formed image quality. The inkjet printer is equipped with: a conveyor unit for conveying a print medium (P) on a conveyor surface in the conveyance direction; an ink-discharging section for discharging ink from nozzle openings on a nozzle surface that faces the conveyor surface; a vertical motion unit for changing the distance of the ink-discharging section from the conveyor surface; a measurement section (113) for measuring the floating distance of the print medium at a measurement position at a prescribed separation distance (L1) on the up-stream side of the ink-discharging section in the conveyance direction; a vertical motion control section for causing, when the floating distance reaches at least a first distance (H1) and after a prescribed period from the floating detection timing has elapsed, the ver - tical motion unit to perform, on the basis of the separation distance and the print medium conveyance speed, a retracting motion of moving the ink-discharging section to a position (H3) where the nozzle surface does not contact the floating portion while said floating portion is conveyed into a range that faces the ink-discharging section; and a discharge control section for continuing ink discharge until the retracting motion start timing.

Description

TECHNICAL FIELD

[0001] The present invention relates to an inkjet recording apparatus.

BACKGROUND ART

[0002] Heretofore, there have been inkjet recording apparatuses which eject ink to recording media to form images thereon. These inkjet recording apparatuses each cause ejecting units for ejecting ink and a recording medium to move relative to each other, so as to form an image on the recording medium, which is relatively larger than the ink ejection surface.

[0003] Unfortunately, a wrinkle in the recording medium causes the distance between the ink ejection surface and the facing recording medium to vary from place to place. This variation causes non-uniform deviations of landing points of the ink, resulting in insufficient image quality. In addition, a protrusion in the recording medium comes into contact with the ink ejection surface, leading to failure of the inkjet recording apparatus.

[0004]  To solve this problem, in a typical conventional inkjet recording apparatus that includes ejecting units for sequentially ejecting ink onto a moving recording medium, a sensor for detecting the height of the recording medium from the conveying surface is provided upstream of the ink landing points on the recording medium in the moving direction of the recording medium, and when it is detected that the recording medium has the height greater than a predetermined height, the surface having nozzle openings is protected (Patent Literature 1). Another conventional inkjet recording apparatus varies the volume of ink to be ejected depending on the height of a recording medium or decreases the conveyance rate to prevent low image quality, or halts the printing operation so as not to form any low-quality image (Patent Literature 2).

[0005] Another inkjet recording apparatus disclosed in Patent Literature 3 is provided with sensors at two points, in a sheet feeding unit and a conveying unit, upstream of the opposing portion between a recording medium and nozzle openings in the moving direction of the recording medium. The inkjet recording apparatus determines the availability of image formation and sheet feeding operation depending on the height of the recording medium detected by the sensor in the sheet feeding unit, and causes the recording medium to come to a sudden stop depending on the height of the recording medium detected by the sensor in the conveyer. This control can improve the throughput of image formation while preventing failure of the inkjet recording apparatus and formation of low-quality images.

CITATION LIST

Patent Literature

[0006] 

Patent Literature 1: Japanese Patent Application Laid-Open No. 2002-36525

Patent Literature 2: Japanese Patent Application Laid-Open No. 2008-246879

Patent Literature 3: Japanese Patent Application Laid-Open No. 2011-126131

SUMMARY OF INVENTION

PROBLEMS TO BE SOLVED BY INVENTION

[0007] Unfortunately, in rapid mass production of images on recording media, a stop of transfer of the medium or a simple interruption of image formation in response to every detection of a partial projection increases tasks for recovery and forms deficiency in the image on the recording medium, resulting in a decrease in operation efficiency.

[0008] An object of the invention is to provide an inkjet recording apparatus that can print images with satisfactory quality at sufficient operation efficiency.

MEANS FOR SOLVING PROBLEMS

[0009] The problems described above are accordingly solved by the present invention which is characterized as follows:

1. An inkjet recording apparatus comprising:

a conveying unit to transfer a recording medium on a conveying surface in a predetermined moving direction;

at least one ink ejecting unit to eject an ink droplet through a nozzle opening disposed on at least one nozzle surface facing the conveying surface;

an elevating unit to move the ink ejecting unit to change a distance between the nozzle surface and the conveying surface;

a measuring unit to measure a lift distance of the recording medium from the conveying surface at a measuring point disposed upstream of the ink ejecting unit in the moving direction so as to be distant from the ink ejecting unit by a predetermined separation distance;

an elevation control unit to cause the elevating unit to perform a retreating movement to change a distance between the ink ejecting unit and the conveying surface to a predetermined retreating distance to prevent the nozzle surface from coming into contact with a lifted portion of the recording medium on the basis of the separation distance and a moving rate of the recording medium transferred by the conveying unit when the lift distance measured by the measuring unit becomes a first predetermined distance or greater at a lift detection timing, after elapse of a predetermined time from the lift detection timing, before a timing of arrival of the lifted portion having the lift distance equal to or greater than the first predetermined distance at an area facing the ink ejecting unit; and

an ejection control unit to cause the ink ejecting unit to continue to eject the ink droplet from the lift detection timing at least until a timing ahead of a retreating movement start timing at which the retreating movement starts.

2. The inkjet recording apparatus of item 1., wherein the elevation control unit varies the retreating distance depending on the lift distance.

3. The inkjet recording apparatus of item 1. or 2., wherein when the lift distance becomes the first distance or greater and then falls below a second distance equal to or less than the first distance, the elevation control unit causes the elevating unit to perform a restart movement to change the distance between the nozzle surface and the conveying surface to a predetermined ink ejection distance for ejecting ink from the nozzle opening after the lifted portion having the lift distance equal to or greater than the second distance passes through the area facing the ink ejecting unit, and
the ejection control unit causes the ink ejecting unit for which the restart movement is performed to resume ejection of the ink droplet for image formation through the nozzle opening after completion of the restart movement.

4. The inkjet recording apparatus of item 3., wherein the elevation control unit enables the elevating unit to perform the restart movement when the lift distance remains less than the second distance for a predetermined waiting time.

5. The inkjet recording apparatus of item 4., wherein the waiting time is equal to or longer than a period required for reciprocating movement of the nozzle surface between the retreating distance and the ink ejection distance at a predetermined ascending/descending rate, the waiting time being equal to or shorter than a period during which the recording medium is transferred across the separation distance.

6. The inkjet recording apparatus of any one of items 3. to 5., wherein the ejection control unit causes the ink ejecting unit to perform a pre-ejection operation of the ink through the nozzle opening after completion of the restart movement, and then resume ink ejection for image formation through the nozzle opening.

7. The inkjet recording apparatus of item 5. or 6., wherein the ejection control unit causes a target image to be formed from a leading end of the target image after completion of the restart movement.

8. The inkjet recording apparatus of any one of items 1. to 7., wherein
when the lift distance becomes the first distance or greater, the ejection control unit determines whether an image being formed by the ink ejecting unit at the lift detection timing can be completely formed before the retreating movement start timing, and if the ejection control unit determines that the image cannot be completely formed, the ejection control unit causes the ink ejecting unit to halt ink ejection before the retreating movement start timing.

9. The inkjet recording apparatus of any one of items 1. to 8. , wherein
when the lift distance becomes the first distance or greater, the ejection control unit determines whether an image being formed by the ink ejecting unit at the lift detection timing can be completely formed before the retreating movement start timing, and if the ejection control unit determines that the image can be completely formed, the ejection control unit causes the ink ejecting unit to halt ink ejection after a completion timing of image formation and before the retreating movement start timing.

10. The inkjet recording apparatus of any one of items 1. to 7. wherein,
when the lift distance becomes the first distance or greater, the ejection control unit causes the ink ejecting unit to eject the ink for forming the target image which can be completely formed before the retreating movement start timing, and to halt ink ejection after a completion timing of image formation and before the retreating movement start timing.

11. The inkjet recording apparatus of any one of items 1. to 10., further comprising:

a conveyance control unit to cause the conveying unit to stop transfer of the recording medium when the lift distance is greater than a maximum value of the retreating distance.

12. The inkjet recording apparatus of any one of items 1. to 11. , wherein
the ink ejecting unit is composed of a plurality of ink ejecting units disposed in different positions in the moving direction,
the elevating unit changes the distances between the nozzle surfaces of the ink ejecting units and the conveying surface independently of one another, and
the elevation control unit causes the elevating unit to perform ascending/descending movements of the ink ejecting units depending on the distances between the ink ejecting units and the lifted portion, respectively, the movements being same as one another, to change each of the distances between the nozzle surfaces and the conveying surface to the retreating distance.

13. The inkjet recording apparatus of any one of items 1. to 12., wherein the recording medium is a fabric extending longer than the separation distance in the moving direction.

EFFECTS OF INVENTION

[0010] The present invention can provide an inkjet recording apparatus that can print images with satisfactory quality at sufficient operation efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] 

[FIG. 1] FIG. 1 is a perspective view of the entire configuration of an inkjet recording apparatus according to an embodiment of the present invention;

[FIG. 2] FIG. 2 is a block diagram illustrating a functional configuration of the inkjet recording apparatus;

[FIG. 3] FIG. 3 is a side view for explanation of changes in a height of a recording medium;

[FIG. 4] FIG. 4 is a flowchart illustrating steps of an image formation controlling process executed in the inkjet recording apparatus according to Embodiment 1;

[FIG. 5] FIG. 5 is a flowchart illustrating steps of an image formation interrupting process invoked in the image formation controlling process;

[FIG. 6] FIG. 6 is a flowchart illustrating steps of an image formation restart process invoked in the image formation interrupting process;

[FIG. 7] FIG. 7 is a time chart illustrating an example timing of interruption of image formation;

[FIG. 8] FIG. 8 is a flowchart illustrating an image formation interrupting process according to Modification 1;

[FIG. 9] FIG. 9 is a flowchart illustrating steps of an image formation restart process invoked in the image formation interrupting process according to Modification 1;

[FIG. 10] FIG. 10 is a flowchart illustrating an image formation interrupting process according to Modification 2;

[FIG. 11] FIG. 11 is a flowchart illustrating steps of an image formation interrupting process invoked in the image formation controlling process executed in the inkjet recording apparatus according to Embodiment 2;

[FIG. 12A] FIG. 12A illustrates an example timing of interruption of ink ejection in the inkjet recording apparatus according to Embodiment 2; and

[FIG. 12B] FIG. 12B illustrates an example timing of interruption of ink ejection in the inkjet recording apparatus according to Embodiment 2.

MODES FOR CARRYING OUT THE INVENTION

[0012] Embodiments of the invention will now be described with reference to the accompanying drawings.

[Embodiment 1]

[0013] FIG. 1 is a perspective view of the entire configuration of an inkjet recording apparatus 100 according to Embodiment 1 of the present invention. FIG. 2 is a block diagram illustrating the functional configuration of the inkjet recording apparatus 100.

[0014] The inkjet recording apparatus 100 includes a conveying unit 11, a plurality of carriages 120 (eight carriages 120 in this embodiment), and carriage elevating units 13 (elevating units) associated with the respective carriages 120. The carriages 120 extend in the width direction (y direction) orthogonal to the moving direction (x direction) of a recording medium transferred by the conveying unit 11, and face the conveying surface of the conveying unit 11 for transferring the recording medium. The carriages 120 each have a line head structure to which inkjet heads 12 are fixed so as to eject ink over the entire width of the conveyed recording medium. The carriages 120 disposed in the eight rows are located in mutually different positions in the moving direction and correspond to eight colors, respectively, of ink. Hereinafter the individual carriages 120 are referred to as carriage 120 in the first row, the second row, ... , and the eighth row in order from the upstream side in the moving direction, as required. The carriages 120 are moved by the respective carriage elevating units 13 to vary their distances from the conveying surface (in the z direction). Such movements of the carriages 120 also vary the distances of the respective inkjet heads 12 from the conveying surface.

[0015] The inkjet recording apparatus 100 has functional configurations including the conveying unit 11, the inkjet heads 12, the carriage elevating units 13, a control unit 14 (serving as an elevation control unit, an ejection control unit, and a conveyance control unit), an operational display unit 15, and a communication unit 16.

[0016] The conveying unit 11 is provided with a conveying motor 111, an encoder 112, a recording medium distance sensor 113 (measuring unit), a driving roller 114 , and a conveyor belt 115. The conveying motor 111 rotates the driving roller 114 at a predetermined rate. The conveyor belt 115 has no end and is wound around the driving roller 114 and a driven roller (not shown), to go around accompanied with rotation of the driving roller 114. The conveyor belt 115 has an outer peripheral surface called a conveying surface. A recording medium is placed on the conveying surface and is conveyed in the moving direction in accordance with movement of the conveyor belt 115.

[0017] The encoder 112 is of a rotary type and measures the rotational angle and rotational rate of the conveying motor 111. The rotational rate of the conveying motor 111 is used for calculation of the traveling rate of the conveyor belt 115 or the moving rate of the recording medium.

[0018] The recording medium distance sensor 113 measures the distance (lift distance) between the recording medium and the conveying surface. The recording medium distance sensor 113 measures the distance between the recording medium and the conveying surface at a measuring point, which is located upstream of the upstream end in the moving direction of the nozzle surface (which faces the conveying surface) of the first inkjet heads 12 (carriage 120) in the conveying path and is distant from the end by a predetermined distance L1 (med). Examples of the recording medium distance sensor 113 include an optical sensor, which detects reflection or shielding of light emitted to the recording medium to measure the distance between the recording medium surface and the conveying surface. In other words, the measured distance equal to or greater than the thickness of the recording medium indicates a lift of the recording medium due to a wrinkle or the like.

[0019] The inkjet heads 12 each include a driving circuit 121 and an ink ejecting unit 122. The ink ejecting unit 122 has an ink passage connecting an ink feeding unit (not shown) to individual nozzle openings for ejecting ink. The ink in the ink passage receives pressure with a predetermined driving pattern, so that the nozzle openings eject ink droplets. The nozzle openings are disposed on the surface (nozzle surface) of each inkjet head 12 facing the conveying surface. The ink flies in the substantially vertical direction to the recording medium (the conveying surface) and lands on the recording medium. The nozzle openings are disposed on the nozzle surface of the inkjet head 12 with predetermined intervals (pitches) in the width direction. The nozzle openings may be in any arrangement, such as simple one-dimensional arrangement or houndstooth check arrangement having multiple rows in the moving direction. In a line head structure where each carriage 120 includes multiple inkjet heads 12 for ejecting ink of the same color, it is desired that the nozzle openings of the adjacent inkjet heads 12 partially overlap with each other in the width direction, to certainly eject ink over the entire width of the recording medium.

[0020] The driving circuit 121 outputs driving signals to vary a pressure to be applied to the ink in the ink passage of the ink ejecting unit 122 and thus cause the ink to be ejected through the nozzle openings at an appropriate timing. The pressure to be applied to the ink may be varied by any well-known measure. For example, the ink may be pressurized by deformation (compression/expansion) of the ink passage through applying voltage having an appropriate waveform to a piezoelectric substance disposed along the ink passage, or by generation of bubbles through energizing an electric heating element (resistive element) and thus heating the wall of the ink passage.

[0021] The driving circuit 121 is integrated with the ink ejecting unit 122 and may be disposed in any position in the inkjet head 12.

[0022] The carriage elevating units 13 each include a motor driver 131, elevating motors 132, electromagnetic brakes 133, beam members 134, and supporting members 135. The carriage elevating unit 13 varies the distance between each carriage 120 and the conveying surface and fixes the carriage 120. Two beam members 134 are disposed above the conveyor belt 115 (on the conveying surface side for transferring the recording medium) in approximately parallel to each other in the direction across the moving direction, and are provided with two supporting members 135 fixed at both ends of the beam members 134. The elevating motors 132, the electromagnetic brakes 133, and the carriage 120 are mounted to the supporting members 135. The motor driver 131 drives the elevating motors 132 and the electromagnetic brakes 133 in response to control signals from the control unit 14 to determine the position of the carriage 120.

[0023] The elevating motors 132 move the carriage 120 at a predetermined ascending/descending rate depending on drive signals from the motor driver 131. Examples of the elevating motor 132 include servo motors and stepping motors. The elevating motors 132 can cause the nozzle surface of the inkjet heads 12 fixed to the carriage 120 to ascend and descend, such that the distance from the conveying surface varies within an ascending/descending width H0 between an ink ejection distance Hn and a maximum retreating distance H3.

[0024] The electromagnetic brakes 133 release the fixation of the carriage 120 in response to operational signals from the motor driver 131, to allow the elevating motors 132 to move the carriage 120. In other words, the electromagnetic brakes 133 lock the carriage 120 in normal modes including a power-off mode. A typical example of the electromagnetic brake 133 is a disk brake.

[0025] The control unit 14 comprehensively controls the entire operation of the inkjet recording apparatus 100 such that the individual components appropriately operate. The control unit 14 includes a memory 141, a central processing unit (CPU) 142, a read only memory (ROM) 143, a random access memory (RAM) 144, and a bus 145.

[0026] The memory 141 temporarily stores image data input from the outside through the communication unit 16. With respect to image data processed for image formation in the inkjet recording apparatus 100, the memory 141 stores the processed image data.

[0027] The CPU 142 executes various calculations for operational control of the inkjet recording apparatus 100. The CPU 142 also executes various processes for image formation under instructions of programs read from the ROM 143 on the basis of image data, and status signals and clock signals from the individual components. The CPU 142 may consist of a single CPU that comprehensively controls the entire operation of the inkjet recording apparatus 100, or may include a CPU for operational control and another CPU dedicated to the other functions, such as processing on image data, that operate independently of each other.

[0028] The ROM 143 stores initial setting data and control programs for image formation. The ROM 143 may be a mask ROM or a rewritable nonvolatile memory, such as a flash memory. The programs and setting data in the ROM 143 are read for execution/utilization by the CPU 142 as required.

[0029] The RAM 144 provides a working memory area to the CPU 142 and stores temporary data. The temporary data contains positional data on the carriages 120 related to the operation of the elevating motors 132 and distance data related to the measuring operation of the recording medium distance sensor 113.

[0030] The memory 141, the CPU 142, the ROM 143, and the RAM 144 are connected to each other through the bus 145 to transmit and receive data to and from each other. The bus 145 is also connected to the conveying unit 11, the inkjet heads 12, the carriage elevating units 13, the operational display unit 15, and the communication unit 16, which are disposed outside the control unit 14, so that these components can transmit and receive control signals and data to and from each other.

[0031]  The operational display unit 15 receives input from the outside, such as operation by a user, and outputs it as an input signal. The operational display unit 15 displays an operational menu and various statuses of the inkjet recording apparatus 100 in response to control signals from the CPU 142. The operational display unit 15 includes operational input means, such as push button switches and operational keys, and display means, such as a display screen (e.g., liquid crystal display (LCD)), a display driver for the display screen, and a light-emitting unit functioning as an indicator. The display screen may include touch sensors to serve as a touch panel.

[0032] The communication unit 16 is a communication interface, which receives commands and settings for print jobs and image data from the outside, and transmits status signals for image formation. Examples of the communication unit 16 include a network interface card (NIC), which contains drivers based on various communication standards.

[0033] The image formation operation in the inkjet recording apparatus 100 according to the embodiment will now be explained.

[0034] FIG. 3 is a side view illustrating a positional relationship among a recording medium P, the recording medium distance sensor 113, and the carriages 120 in the inkjet recording apparatus 100.

[0035] The inkjet recording apparatus 100 according to the embodiment performs printing on a recording medium, in specific, a fabric that extends longer than the distance L1 in the moving direction of the recording medium. In this embodiment, the inkjet recording apparatus 100 sequentially forms images based on the same image data or pieces of image data arrayed in a predetermined pattern or order, on the recording medium in intervals (margins) appropriately determined depending on the transfer of the fabric.

[0036] During the ink ejection for image formation, the carriages 120 are fixed in positions (ink ejection positions) in which the distance between the nozzle surfaces and the conveying surface becomes a predetermined ink ejection distance Hn.

[0037] A wrinkle in the recording medium P causes a portion (lifted portion) of the recording medium P to be lifted from the conveying surface, and thus decreases the gap between the nozzle surfaces and the surface of the recording medium P or the flying distance of ink, resulting in deviation of landing points of ink from the desired points. An inclined or folded/overlapping portion in the surface of the recording medium P relative to the nozzle surfaces narrows the range of the ink landing points in the moving direction, or fails to form the image. Furthermore, the recording medium P comes into contact with (crashes into) the nozzle surfaces (the inkjet heads 12 or the carriages 120). To solve this problem, the inkjet recording apparatus 100 performs a retreating movement in response to the detection of a lifted portion of the recording medium P having a reference lift amount H1 (first distance) or greater, which is expected to lower the image quality below a predetermined level. The retreating movement involves interrupting the image formation, and causing the carriage elevating unit 13 to move the carriage 120 away from the conveying surface to a position at a predetermined retreating distance from the conveying surface. In response to the detection of a lifted portion at a maximum retreating distance H3 of the carriages 120 or greater, the conveying unit 11 makes a sudden stop of transfer of the recording medium.

[0038] In the carriage 120 moving across the ascending/descending width H0 at an ascending/descending rate Va, the nozzle surface retreats in an ascending/descending time Ta = H0/Va. A traveling time Tt depends on the moving rate Vt of the recording medium P transferred by the conveying unit 11 (Tt = L1/Vt > Ta), where the traveling time Tt indicates a period from the detection of a lifted portion of the recording medium P by the recording medium distance sensor 113 to the arrival of the lifted portion at an area facing the carriage 120 (nozzle surface), i.e. , an area below the carriage 120 having a length Lh in the moving direction. For the moving rate Vt variable depending on an image formation mode, the difference between the traveling time Tt and the ascending/descending time Ta varies depending on the image formation mode.

[0039] To avoid the effects of such variation, the inkjet recording apparatus 100 sets a predetermined margin time Tm. The inkjet recording apparatus 100 halts the ink ejection for image formation and starts the retreating movement of the carriage 120 at a retreating movement start timing, which is Ta+Tm (the sum of the ascending/descending time and the margin time) earlier than the timing when the lifted portion of the recording medium P starts to face the nozzle surface of the inkjet heads 12 of the carriage 120 in the first row after the detection of the lifted portion having the reference lift amount H1 by the recording medium distance sensor 113. In other words, after the lifted portion passes through the measuring point of the recording medium distance sensor 113 and then moves by a distance (L1-La-Lm), the carriage 120 starts the retreating movement, where La indicates a moving distance of the recording medium P during the ascending/descending time Ta (La = Vt×Ta), and Lm indicates a margin, i.e. a moving distance of the recording medium P during the margin time Tm (Lm = Vt×Tm). The ink ejection for image formation can thus continue during the time (L1/Vt-Ta-Tm) (remaining ejection time) or by the distance (L1-La-Lm) after the detection of the lifted portion. In addition, the carriage 120 can complete the retreating movement just before the arrival of the lifted portion of the recording medium P at the position below the carriage 120.

[0040] The carriage 120 in the Nth row (N≥2) halts the ink ejection and starts the retreating movement, after the lifted portion of the recording medium P passes through the measuring point of the recording medium distance sensor 113 and then moves by a distance (L1 + (N-1) ×L2 - Vt (Ta+Tm)), where L2 indicates an interval between adjacent two carriages 120 in the moving direction. Accordingly, a part of the recording medium P that receives ink for image formation from the inkjet heads 12 fixed to the carriage 120 in the first row also receives ink for image formation from the inkjet heads 12 of all the other carriages 120, even when the recording medium distance sensor 113 detects the lifted portion of the recording medium P having the reference lift amount H1 or greater.

[0041] FIG. 4 is a flowchart illustrating steps of an image formation controlling process executed by the CPU 142 in the inkjet recording apparatus 100 according to the embodiment.

[0042] The CPU 142 starts the image formation controlling process, for example, when receiving a print job transmitted through the communication unit 16, or detecting an instruction for image formation of image data stored in the memory 141 that is input through the operational display unit 15.

[0043] At the start of the image formation controlling process, the CPU 142 outputs control signals to the motor drivers 131, as required, to move the carriages 120 so as to set the distance between the nozzle surfaces and a conveyed recording medium P to an ink ejection distance Hn for image formation (Step S101). After completion of the movement of the carriages 120, the CPU 142 starts image formation by outputting control signals to the driving circuits 121, controlling the memory 141 to output image data at an appropriate timing, and then controlling the nozzles to eject ink (Step S102).

[0044] The CPU 142 determines whether the image formation has been successfully completed (Step S103). If determining successful completion ("YES" in Step S103), the CPU 142 terminates the image formation controlling process. If determining no successful completion or determining that the image formation has not been completed ("NO" in Step S103), the CPU 142 acquires the measured value of the recording medium distance sensor 113, and then determines whether the lift amount of a lifted portion of the recording medium P is the maximum retreating distance H3 or greater at the measuring point of the recording medium distance sensor 113 (Step S104).

[0045] If determining the lift amount to be the maximum retreating distance H3 or greater ("YES" in Step S104), the CPU 142 outputs control signals to the driving circuits 121 to halt the ink ejection, and outputs signals to the conveying motor 111 to stop the transfer before the arrival of the lifted portion at the area facing the carriage 120 in the first row (Step S105). The CPU 142 controls the operational display unit 15 to display termination by an error (Step S106), and terminates the image formation controlling process.

[0046] If determining the lift amount to be smaller than the maximum retreating distance H3 in Step S104 ("NO" in Step S104), the CPU 142 invokes and executes an image formation interrupting process (Step S120). After completion of the image formation interrupting process, the CPU 142 returns to Step S103.

[0047] FIG. 5 is a flowchart illustrating steps of the image formation interrupting process invoked in the image formation controlling process by the CPU 142.

[0048] After invoking the image formation interrupting process, the CPU 142 determines whether the lift amount of the recording medium P acquired in Step S104 is the reference lift amount H1 or greater (Step S121). If determining the lift amount to be the reference lift amount H1 or greater ("YES" in Step S121), the CPU 142 determines whether a retreating flag of the carriage 120 is set (Step S122). The retreating flag is set during a period when a lifted portion having the reference lift amount H1 is detected and thereby the retreating movement is required, the corresponding carriage 120 retreats from the lifted portion, the retreated nozzle surface of the inkjet heads 12 of the carriage 120 returns to a position of the ink ejection distance Hn from the conveying surface, and ink ejection for image formation is resumed. The retreating flag is unset other than this period. The retreating flag is stored in the RAM 144 in this embodiment.

[0049] If detecting that the retreating flag is not set ("NO" in Step S122), the CPU 142 set the retreating flag (Step S123), and then starts counting of an elapsed time t1 from the timing (lift detection timing) of detection of the lifted portion having the reference lift amount H1 or greater (Step S124). The CPU 142 acquires the rotational rate of the conveying motor 111 from the encoder 112, to obtain the moving rate Vt of the conveying unit 11 (Step S125).

[0050] The CPU 142 determines whether the elapsed time t1 reaches the retreating movement start timing of each carriage 120 (Step S126). In specific, the CPU 142 determines whether any carriage number N (N indicates the row number of the carriage 120; 1≤N≤8 in this embodiment) currently satisfies Expression (1):

[0051] The timing of this determining process must not be significantly delayed from the original retreating movement start timing, because such delay causes the lifted portion to arrive at the area facing the carriage 120 before completion of the retreating movement of the carriage 120. That is, the margin time Tm needs to be larger than a time interval Δt1 in which the determining processes in Step S126 is performed.

[0052] If determining the carriage 120 of any carriage number N newly at the retreating movement start timing ("YES" in Step S126), the CPU 142 outputs control signals to the driving circuits 121 to halt the ink ejection from the inkjet heads 12 fixed to the carriage 120 of this carriage number N, and outputs control signals to the motor driver 131 to operate the elevating motors 132 and the electromagnetic brakes 133 corresponding to the carriage 120 in the Nth row, to move the carriage 120 to a position (retreating position) of the maximum retreating distance H3 from the conveying surface at the ascending/descending rate Va (Step S127). The margin time Tm may have any range before and/or after the ascending/descending time Ta, unless any delay in control processes causes the lifted portion to arrive at the area facing the carriage 120 before completion of the retreating movement of the carriage 120, as explained above. The CPU 142 then determines whether the carriage 120 newly at the retreating movement start timing belongs to the eighth row (Step S128). For the carriage 120 in the other row than the eighth row ("NO" in Step S128), the CPU 142 terminates the image formation interrupting process and returns to the image formation controlling process. For the carriage 120 in the eighth row ("YES" in Step S128), the CPU 142 terminates the counting of the elapsed time t1 (Step S129). The CPU 142 then terminates the image formation interrupting process and returns to the image formation controlling process.

[0053] If determining no carriage 120 newly at the retreating movement start timing in Step S126 ("NO" in Step S126), the CPU 142 terminates the image formation interrupting process and returns to the image formation controlling process.

[0054] If detecting that the retreating flag is set in Step S122 ("YES" in Step S122), the CPU 142 determines whether an elapsed time t2 is being counted (Step S131). The elapsed time t2 is used for determination of the timing of returning the retreated carriage 120 to a position at which a distance between the nozzle surface of the carriage 120 and the conveying surface becomes the ink ejection distance Hn to resume the ink ejection, as explained below. If determining no counting of the elapsed time t2 ("NO" in Step S131), the CPU 142 determines whether the elapsed time t1 is being counted (Step S132).

[0055] If determining no counting of the elapsed time t1 ("NO" in Step S132), the CPU 142 terminates the image formation interrupting process and returns to the image formation controlling process. If determining the counting of the elapsed time t1 ("YES" in Step S132), the CPU 142 proceeds to Step S126.

[0056] If determining the counting of the elapsed time t2 in Step S131 ("YES" in Step S131), the CPU 142 determines whether the elapsed time t2 is a predetermined waiting time (2Ta+Tp) or greater (Step S133). This condition is used for determination whether a restart movement of the carriage 120 from the retreating position to the ink ejection position has already been started in an image formation restart process (explained below) and whether the requirement for start of the restart movement is satisfied. If the elapsed time t2 is the waiting time (2Ta+Tp) or greater ("YES" in Step S133), the CPU 142 invokes and executes the image formation restart process (Step S135). The CPU 142 then proceeds to Step S136.

[0057] If determining that the elapsed time t2 is not the waiting time (2Ta+Tp) or greater ("NO" in Step S133), the CPU 142 interrupts the counting of the elapsed time t2 and cancels the restart movement of the carriage 120 before the start of the movement (Step S134). The CPU 142 then proceeds to Step S136.

[0058] In Step S136, the CPU 142 determines whether the elapsed time t1 is being counted (Step S136). In the counting of the elapsed time t1 ("YES" in Step S136), the CPU 142 proceeds to Step S126. In no counting of the elapsed time t1 ("NO" in Step S136), the CPU 142 proceeds to Step S124.

[0059] If determining the lift amount of the recording medium P to be smaller than the reference lift amount H1 in Step S121 ("NO" in Step S121), the CPU 142 invokes and executes the image formation restart process (Step S140). After completion of the image formation restart process, the CPU 142 determines whether the elapsed time t1 is being counted (Step S137). In no counting of the elapsed time t1 ("NO" in Step S137), the CPU 142 terminates the image formation interrupting process and returns to the image formation controlling process.

[0060] In the counting of the elapsed time t1 ("YES" in Step S137), the CPU 142 proceeds to Step S126.

[0061] FIG. 6 is a flowchart illustrating steps of the image formation restart process invoked in the image formation interrupting process by the CPU 142.

[0062] After invoking the image formation restart process in Step S135 or S140, the CPU 142 determines whether the retreating flag is set (Step S141). If detecting that the retreating flag is not set ("NO" in Step S141), which indicates that all the inkjet heads 12 have successfully formed the image, the CPU 142 terminates the image formation restart process and returns to the image formation interrupting process.

[0063] If detecting that the retreating flag is set ("YES" in Step S141), the CPU 142 determines whether another lifted portion is detected that has at least a restart lift amount H2 (second height) smaller than the reference lift amount H1 (Step S142). Cases of such detection of a lifted portion having the restart lift amount H2 or greater includes the detection of the lifted portion having the reference lift amount H1 or greater in Step S121 followed by the initiation of the image formation restart process in Step S135.

[0064] If detecting no lifted portion having the restart lift amount H2 or greater ("NO" in Step S142), the CPU 142 determines whether the elapsed time t2 is being counted (Step S143). In the counting of the elapsed time t2 ("YES" in Step S143), the CPU 142 proceeds to Step S148. In no counting of the elapsed time t2 ("NO" in Step S143), the CPU 142 starts the counting of the elapsed time t2 (Step S144), and then proceeds to Step S148.

[0065] If detecting the lifted portion having the restart lift amount H2 or greater in Step S142 ("YES" in Step S142), the CPU 142 determines whether the elapsed time t2 is being counted (Step S145). In no counting of the elapsed time t2 ("NO" in Step S145), which indicates no ongoing process for the restart movement of the carriage 120, the CPU 142 terminates the image formation restart process and returns to the image formation interrupting process.

[0066] In the counting of the elapsed time t2 ("YES" in Step S145), the CPU 142 determines whether the elapsed time t2 is the waiting time (2Ta+Tp) or greater (Step S146). In specific, the CPU 142 determines whether the sum of a reciprocating ascending/descending time 2Ta and a predetermined recording time Tp has elapsed, where the reciprocating ascending/descending time 2Ta is required for controlling the carriage elevating unit 13 to change the distance between the nozzle surface and the conveying surface from the maximum retreating distance H3 to the ink ejection distance Hn and then change the distance to the maximum retreating distance H3 again, and the recording time Tp indicates a period during which the carriage can be maintained at the ink ejection position or can eject ink.

[0067]  If determining that the elapsed time t2 is the waiting time (2Ta+Tp) or greater ("YES" in Step S146), the CPU 142 proceeds to Step S148. If determining that the elapsed time t2 is not the waiting time (2Ta+Tp) or greater ("NO" in Step S146), the CPU 142 interrupts the counting of the elapsed time t2 (Step S147), because the carriage 120 must move or may have to move away from the conveying surface to the retreating position before ejecting ink in the ink ejection position for at least a sufficient time for image formation. The CPU 142 then terminates the image formation restart process and returns to the image formation interrupting process. That is, the determination condition in Step S146 is the requirement for start of the restart movement of the carriage 120.

[0068] In Step S148 subsequent to Step S143, S144, or S146, the CPU 142 determines whether the elapsed time t2 reaches the timing (restart movement start timing) of newly returning any carriage 120 in the Nth row to the ink ejection position (Step S148).

[0069] In the inkjet recording apparatus 100 according to the embodiment, if the above requirement for start of the restart movement is satisfied regarding a portion of the recording medium P having a lift amount less than the restart lift amount H2, the carriage 120 starts the restart movement at the timing when this portion of the recording medium P passes through (exits) the area facing the inkjet heads 12.

[0070] That is, this condition is represented by Expression (2):

[0071] The recording medium distance sensor 113 is disposed such that the elapsed time t2 for the carriage 120 in the first row, which satisfies Expression (2) (i.e., t2 ≥ (L1+Lh)/Vt), is equal to or larger than the waiting time (2Ta+Tp) in the above comparison in Step S146. This configuration allows the restart movement to start without delay at the timing when the portion of the recording medium P having the lift amount less than the restart lift amount H2 passes through the area facing the inkjet heads 12.

[0072] If determining no carriage 120 in the Nth row newly at the restart movement start timing ("NO" in Step S148), the CPU 142 terminates the image formation interrupting process and returns to the image formation controlling process. If determining any carriage 120 in the Nth row newly at the restart movement start timing ("YES" in Step S148), the CPU 142 outputs control signals to the motor driver 131 to start operating the elevating motors 132 and the electromagnetic brakes 133, such that the distance between the nozzle surface of the carriage 120 in the Nth row and the conveying surface varies from the maximum retreating distance H3 to the ink ejection distance Hn (Step S149).

[0073] The CPU 142 then determines whether it is the timing (restart movement completion timing) of new completion of the restart movement of any carriage 120 in the Nth row to the ink ejection position (Step S150). In specific, the CPU 142 determines whether the sum of the ascending/descending time Ta and the margin time Tm has elapsed since the timing represented by Expression (2). If determining no carriage 120 at the restart movement completion timing ("NO" in Step S150), the CPU 142 terminates the image formation interrupting process and returns to the image formation controlling process. If determining any carriage 120 at the restart movement completion timing ("YES" in Step S150), the CPU 142 outputs control signals to the driving circuits 121 for all the inkjet heads 12 fixed to the carriage 120 in the Nth row, to cause all the nozzle openings to eject ink through a discharging operation (pre-ejection operation). After elapse of a predetermined time after the discharging operation, the CPU 142 resumes the ink ejection for image formation through the nozzles of the carriage in the Nth row to the recording medium P (Step S151) . The ink ejection is controlled to start image formation from the leading end of image data regardless of the position of the image data printed just before the previous halt of the ink ejection.

[0074] The CPU 142 determines whether the carriage number N of the carriage 120 including the inkjet heads 12 that resume ink ejection for image formation is eight (Step S152). If determining the carriage number N to be not eight ("NO" in Step S152), the CPU 142 terminates the image formation interrupting process and returns to the image formation controlling process. If determining the carriage number N to be eight ("YES" in Step S152), which indicates completion of the restart movement of all the carriages 120, the CPU 142 terminates the counting of the elapsed time t2 and unsets the retreating flag (Step S153). The CPU 142 then terminates the image formation interrupting process and returns to the image formation controlling process.

[0075] FIG. 7 is a time chart illustrating an example timing of interruption of image formation.

[0076] This time chart illustrates variations in the results of detection by the recording medium distance sensor 113 , the statuses of the respective carriages 120 (inkjet heads 12) in the first to fourth and eighth rows, the setting state of the retreating flag, and the counting states of the elapsed times t1 and t2, and is accompanied with a schematic diagram in which the position of a lifted portion of the recording medium is represented with solid lines relative to the measuring point (reference point defined as "0") in the moving direction (x direction).

[0077] At a time u1, in the initial state where the recording medium distance sensor 113 detects no lifted portion having the reference lift amount H1 or greater, the inkjet heads 12 of all the carriages 120 eject ink ("NO" in Step S121, and "NO" in Step S141).

[0078] At a time u2 of detection of a lifted portion having the reference lift amount H1 or greater, the retreating flag is set and the counting of the elapsed time t1 is started. The ascending/descending-time moving distance La is then calculated from the moving rate Vt and the ascending/descending time Ta of the carriages 120 ("YES" in Step S121, "NO" in Step S122, and Steps S123 to S125).

[0079] At a time u3 of arrival of the lifted portion of the recording medium P at a retreating movement start position corresponding to the retreating movement start timing of the carriage 120 in the first row (N=1), the carriage 120 in the first row starts the retreating movement ("YES" in Step S121, "YES" in Step S122, "NO" in Step S131, "YES" in Step S132, "YES" for N=1 in Step S126, and Step S127).

[0080] At a time u4 of arrival of the lifted portion of the recording medium P at the area facing the carriage 120 (N=1), the carriage 120 in the first row has already completed the retreating movement and is halting the ink ejection in a waiting mode. Furthermore, the carriage 120 in the second row newly starts the retreating movement ("NO" in Step S121, "YES" in Step S137, and "YES" for N=2 in Step S126). At the same time, the recording medium distance sensor 113 detects the distance less than the restart lift amount H2, and the counting of the elapsed time t2 is then started in the image formation restart process invoked in Step S140 ("YES" in Step S141, "NO" in Step S142, "NO" in Step S143, and Step S144).

[0081] At a time u5 when another lifted portion of the recording medium P having the restart lift amount H2 or greater is detected before the elapsed time t2 reaches the waiting time (2Ta+Tp) or the time corresponding to a distance (2La+Lp), the counting of the elapsed time t2 is interrupted in the image formation restart process invoked in Step S140, where Lp (Lp = Tp×Vt) indicates a recording distance by which the recording medium P moves at the moving rate Vt in the recording time Tp ("YES" in Step S141, "YES" in Step S142, "YES" in Step S145, "NO" in Step S146, and Step S147) .

[0082] If the detected other lifted portion has the reference lift amount H1 or greater, the counting of the elapsed time t2 is interrupted in Step S134 based on the determination in Steps S131 and S133.

[0083] At a time u6 when the lift amount of the recording medium P falls below the restart lift amount H2 again, the counting of the elapsed time t2 is resumed from the beginning (t2=0) ("NO" in Steps S142 and S143, and Step S144).

[0084] At a time u7 when the lifted portion having the reference lift amount H1 or greater passes through the area facing the carriage 120 in the first row, the carriage 120 in the first row is still in the waiting mode without returning to the ink ejection position, because the counting of the elapsed time t2 is not based on the tail end of the lifted portion ("YES" in Step S141, "NO" in Step S142, "YES" in Step S143, and "NO" in Step S148).

[0085] At a time u8 when the other lifted portion having the restart lift amount H2 or greater passes through the area facing the carriage 120 in the first row, the carriage 120 in the first row starts to return to the ink ejection position (i.e., starts the restart movement) on the basis of the elapsed time t2 ("YES" in Step S148, and Step S149).

[0086] At a time u9 when the carriage 120 in the first row returns to the ink ejection position after elapse of the ascending/descending time Ta since the start of the restart movement, the ink discharging operation is performed, followed by the resumption of the ink ejection for image formation from the leading end of the image data ("YES" in Step S150, and Step S151).

[0087] At this time, the carriage 120 in the eighth row, which is disposed on the most downstream side in the moving direction, is still not in the retreating movement in response to the lifted portion having the reference lift amount H1 or greater and continues to eject the ink ("NO" for N=8 in Step S126). In these steps, the retreating flag is still set and the elapsed time t1 is still counted.

[0088] If another lifted portion of the recording medium P having the reference lift amount H1 or greater is detected in this situation, the process on the elapsed time t1 for the previous lifted portion and the process on the elapsed time t1 for the current lifted portion are executed in parallel in the image formation interrupting process. In response to the detection of such another lifted portion, the process goes through the route "YES" in Step S133 and invokes the image formation restart process in Step S135. After the exit of the lifted portion from the measuring point of the recording medium distance sensor 113, the image formation restart process is invoked in Step S140. The elapsed time t2 for the previous lifted portion is maintained as it is ("YES" in Step S146), and the process on the elapsed time t2 for the previous lifted portion and the process on the elapsed time t2 for the current lifted portion are executed in parallel.

[0089] At a time u10 (retreating movement start timing) when the lifted portion having the reference lift amount H1 or greater arrives at the retreating movement start position for the carriage 120 in the eighth row, the carriage 120 in the eighth row starts the retreating movement. This situation indicates that all the carriages 120 have started the retreating movement. The counting of the elapsed time t1 is then terminated ("YES" for N=8 in Step S126, Step S127, "YES" in Step S128, and Step S129).

[0090] If another lifted portion having the reference lift amount H1 or greater is detected after the termination of the counting of the elapsed time t1 before the completion of the restart movement of all the carriages 120 (i.e., before the retreating flag is unset), the counting of the elapsed time t1 is newly started while the retreating flag is being set ("YES" in Steps S121, S122, S131, and S133, "NO" in Step S136, and Step S124).

[0091]  At a time u11 when the lifted portion having the restart lift amount H2 passes through the area facing the carriage 120 in the eighth row, the carriage 120 (N=8) starts the restart movement. At a time u12 of completion of the restart movement of the carriage 120 (N=8), the carriage 120 in the eighth row resumes ink ejection, the counting of the elapsed time t2 is terminated, and the retreating flag is unset ("YES" in Step S152, and Step S153).

[0092] As described above, the inkjet recording apparatus 100 according to the embodiment includes: the conveying unit 11 to convey the recording medium P on the conveying surface to the predetermined moving direction (x direction); the ink ejecting units 122 to eject ink droplets through the nozzle openings disposed on the nozzle surfaces facing the conveying surface; the elevating units 13 to move the ink ejecting units 122 for varying the distances between the respective nozzle surfaces and the conveying surface (i.e., head chip parts of the inkjet heads 12 that include at least the nozzle surfaces provided with the nozzle openings that eject ink) at the predetermined ascending/descending rate Va; the recording medium distance sensor 113 to measure the lift amount of a lifted portion of the recording medium P from the conveying surface at the measuring point, which is disposed upstream of the ink ejecting units 122 in the moving direction and is distant from the ink ejecting units 122 by the predetermined distance L1; and the CPU 142 serving as the elevation control unit and the ejection control unit. If the lift amount measured by the recording medium distance sensor 113 equals or exceeds the reference lift amount H1, the elevation control unit controls the elevating units 13 to perform the retreating movement based on the distance L1, the moving rate Vt of the recording medium P transferred by the conveying unit 11, and the ascending/descending rate Va, after elapse of a predetermined time from the timing (elapsed time t1=0) of detection of the lifted portion having the reference lift amount H1 or greater, before the timing (elapsed time t1 = traveling time Tt) of arrival of the lifted portion having the reference lift amount H1 or greater at the area facing the ink ejecting units 122. The retreating movement increases the distance between each ink ejecting unit 122 and the conveying surface to the maximum retreating distance H3 to prevent the lifted portion from coming into contact with the nozzle surface. The ejection control unit controls each ink ejecting unit 122 to continue to eject ink droplets from the timing (elapsed time t1=0) at least until a timing ahead of the retreating movement start timing of starting the retreating movement.

[0093] This configuration can interrupt the image formation for a shorter period than those in conventional configurations if the formed image may have insufficient quality, thereby ensuring sufficient operation efficiency.

[0094] In addition, the continuous image formation for a while after the detection of the lifted portion can increase the possibility of completion of the ongoing image formation. The configuration can thus achieve more effective use of the portions of the recording medium that must be discarded in the conventional configurations, resulting in improved throughput.

[0095] If the lift amount of the recording medium P increases to the reference lift amount H1 or greater and then falls below the restart lift amount H2 less than the reference lift amount H1, the CPU 41 controls each elevating unit 13 to perform the restart movement after the lifted portion of the recording medium P that has had the restart lift amount H2 or greater passes through the area (having the length Vh) facing the ink ejecting unit 122. The restart movement changes the distance between the nozzle surface and the conveying surface to the ink ejection distance Vn, at which the ink ejecting unit 122 ejects ink droplets through the nozzle openings. After completion of the restart movement, the CPU 122 controls the ink ejecting unit 122 completing the restart movement to resume the ejection of ink droplets for image formation through the nozzle openings.

[0096] The ink ejection can thus be immediately resumed based on information on the end position of a wrinkle, which can be located in advance. In specific, the restart lift amount H2, which is used for determination of resumption of the ink ejection, less than the reference lift amount H1 can improve the accuracy of the determination. The configuration can thus resume the ink ejection after ensuring the accuracy of image formation on the recording medium P.

[0097] In specific, if the lift amount remains less than the restart lift amount H2 for the predetermined waiting time (2Ta+Tp), the CPU 142 instructs the restart movement. This configuration can avoid such a wasteful case that the ink ejecting unit starts the restart movement but must retreat again before resuming the ink ejection. The configuration can also reduce the frequency of retreating movements of the ink ejecting units 122 through avoiding intermittent retreating movements in response to successive wrinkles in the recording medium P, thereby improving the efficiency.

[0098] In specific, the waiting time is equal to or longer than a period required for reciprocating movement by which the distance between the conveying surface and each nozzle surface changes between the retreating distance and the ink ejection distance. The ink ejection onto the recording medium P can be performed at least partially once the restart movement starts. Furthermore, the waiting time is equal to or shorter than a period of transfer of the recording medium P across the distance L1. The waste of the recording medium and time can thus be avoided that is caused by the ink ejecting unit not returning to the ink ejection position despite of the exit of the lifted portion from the area facing the nozzle surface and the detection of no subsequent lifted portion.

[0099] After completion of the restart movement, the CPU 142 controls the ink ejecting units to perform the discharging operation of ink droplets through the nozzle openings and then resume the formation of the target image. This configuration can avoid poor ink ejection through the nozzle openings in the resumption of the ink ejection after random interruption of the image formation, leading to rapid and efficient image formation.

[0100] In addition, the CPU 142 causes the target image to be formed from the leading end of the target image after completion of the restart movement. This configuration can prevent the formation of a useless fragmentary image just after the resumption of the ink ejection.

[0101] If the lift amount of the recording medium P is greater than the maximum retreating distance H3, the CPU 142 controls the conveying unit 11 to immediately stop the transfer of the recording medium P. This configuration can minimize the frequency of interrupting operations for avoiding the contact with the nozzle surfaces and the crash into the inkjet heads 12.

[0102] The carriages 120 each including the inkjet heads 12 having the ink ejecting units 122 are disposed in plural positions different from one another in the moving direction. The elevating units 13 vary the distances between the respective nozzle surfaces of the ink ejecting units 122 of the carriages 120 and the conveying surface independently of each other. The CPU 142 controls the respective ink ejecting units 122 to perform the same ascending/descending movement depending on the distances of the respective ink ejecting units 122 relative to the lifted portion of the recording medium P, such that the distance between each nozzle surface and the conveying surface increases to the maximum retreating distance H3 at an appropriate timing. That is, the ink ejecting units 122 of the carriages 120, which are disposed in different positions in the moving direction in printing of a color image, for example, interrupt the ink ejection for image formation only in a part of the recording medium P including the detected wrinkle. Accordingly, the image formation is interrupted in a range much narrower than that in the configuration interrupting all the image formation simultaneously in response to detection of a wrinkle. The configuration can thus significantly reduce the waste of the recording medium P.

[0103] In repetitive image formation on the recording medium P that is a fabric, in specific, extending longer than the distance L1 in the moving direction, the configuration can minimize the effects of wrinkles in the fabric, which even the inkjet recording apparatus 1 in successful operation cannot completely avoid. The configuration can thus achieve efficient image formation.

[Modification 1]

[0104] FIG. 8 is a flowchart illustrating the image formation interrupting process according to Modification 1 based on Embodiment 1. FIG. 9 is a flowchart illustrating steps of the image formation restart process invoked in the image formation interrupting process according to Modification 1.

[0105] In the image formation interrupting process and the image formation restart process according to Modification 1, the control is based on the moving distance of a recording medium transferred by the conveying unit 11, instead of the elapsed times t1 and t2.

[0106] In the image formation interrupting process, Steps S124 to S126, S129, S131 to S136, and S140 are replaced with Steps S124A to S126A, S129A, S131A to S136A, and S140A, respectively. In the image formation restart process, Steps S143 to S148, S150, and S153 are replaced with Steps S143A to S148A, S150A, and S153A, respectively. The other steps are identical to those in the image formation interrupting process and the image formation restart process according to Embodiment 1, and have the same reference signs without redundant description.

[0107] In the image formation interrupting process in FIG. 8, after the retreating flag is set in Step S123, the CPU 142 acquires the rotational rate and/or rotational position of the conveying motor 111 from the encoder 112 and starts the measurement of the moving amount of the recording medium P from this time point (Step S124A). The CPU 142 obtains the moving rate Vt, and then calculates the moving distance La during the ascending/descending time Ta of the carriage 120 moved by the elevating motors 132 on the basis of the moving rate Vt and the ascending/descending time Ta (Step S125A).

[0108]  The CPU 142 determines whether the moving distance Lt1 from the detection of a lifted portion of the reference lift amount H1 or greater newly becomes the distance from the measuring point of the recording medium distance sensor 113 to the position of the retreating movement of any carriage 120 in the Nth row or greater (Step S126A). This condition is represented by Expression (3):

where margin Lm indicates the moving distance of the recording medium at the moving rate Vt during the margin time Tm. Alternatively, margin Lm may be any constant value independent of the moving rate Vt.

[0109] If the carriage determined to be newly at the retreating movement start position in Step S126A belongs to the eighth row (Step S128), the CPU 142 terminates the measurement of the moving distance Lt1 (Step S129A).

[0110] In addition, in Steps S132A, S136A, and S137A, the CPU 142 determines whether the moving distance Lt1 is being measured. In Step S131A, the CPU 142 determines whether a moving distance Lt2 of the recording medium at the moving rate Vt during the elapsed time t2 is being measured.

[0111] If determining that the moving distance Lt2 is being measured in Step S131A ("YES" in Step S131A), the CPU 142 determines whether the moving distance Lt2 is the distance (2La+Lp) or greater (Step S133A). The recording distance Lp may be any value independent of the moving rate Vt. If the moving distance Lt2 is not the distance (2La+Lp) or greater ("NO" in Step S133A), the CPU 142 interrupts the measurement of the moving distance Lt2 (Step S134A). If the moving distance Lt2 is the distance (2La+Lp) or greater, the CPU 142 invokes and executes the image formation restart process according to Modification 1 in FIG. 9 (Step S135A).

[0112] In the image formation restart process in FIG. 9, if detecting no lifted portion having the restart lift amount H2 or greater in Step S142 ("NO" in Step S142), the CPU 142 determines whether the moving distance Lt2 is being measured (Step S143A). If determining no measurement ("NO" in Step S143A), the CPU 142 starts the measurement of the moving distance Lt2 of the recording medium transferred by the conveying unit 11 from this time point (Step S144A). The CPU 142 then proceeds to Step S148A. If determining the measurement of the moving distance Lt2, the CPU 142 proceeds to Step S148A.

[0113] If determining the detection of the lifted portion having the restart lift amount H2 or greater in the determining process in Step S142 ("YES" in Step S142), the CPU 142 determines whether the moving distance Lt2 is being measured (Step S145A). If determining no measurement ("NO" in Step S145A), the CPU 142 terminates the image formation restart process and returns to the image formation interrupting process.

[0114] If determining the measurement of the moving distance Lt2 ("YES" in Step S145A), the CPU 142 determines whether the moving distance Lt2 is the distance (2La+Lp) or greater (Step S146A). If determining that the moving distance Lt2 is not the distance (2La+Lp) or greater ("NO" in Step S146A), the CPU 142 interrupts the measurement of the moving distance Lt2 (Step S147A). The CPU 142 then terminates the image formation restart process and returns to the image formation interrupting process.

[0115] If determining that the moving distance Lt2 is the distance (2La+Lp) or greater ("YES" in Step S146A), the CPU 142 proceeds to Step S148A.

[0116] In Step S148A subsequent to any of Steps S143A, S144A, and S146A, the CPU 142 determines whether the moving distance Lt2 newly becomes the distance (L1+(N-1)L2+Lh) or greater for any of the carriages 120 in the eight rows (Step S148A). This distance indicates the distance from the measuring point of the recording medium distance sensor 113 to the downstream end of the carriage 120 in the Nth row in the moving direction. In other words, after the recording medium distance sensor 113 detects no portion of the recording medium P having the restart lift amount H2 or greater, the CPU 142 determines whether the portion of the recording medium P, which has been being at the measuring point, passes through the area facing the carriage 120 in the Nth row.

[0117] If determining that the distance of the lifted portion of the recording medium P having the restart lift amount H2 or greater does not newly become the distance up to the downstream end of any carriage 120 in the moving direction or greater ("NO" in Step S148A), the CPU 142 terminates the image formation restart process and returns to the image formation Interrupting process.

[0118] If determining that the distance newly becomes the distance up to the downstream end of any carriage 120 in the Nth row in the moving direction, or greater ("YES" in Step S148A), the CPU 142 proceeds to Step S149. After Step S149, the CPU 142 determines whether the moving distance Lt2 newly becomes the distance (L1+(N-1)L2+Lh+La+Lm) or greater for any of the carriages 120 in the eight rows (Step S150A). This distance indicates the moving distance of the recording medium P from the start of measurement of the moving distance Lt2 until the elapse of the ascending/descending time Ta and the margin time Tm that correspond to the restart movement being started at the downstream end of the carriage 120 in the Nth row in the moving direction toward the ink ejection position. In other words, the CPU 142 determines whether the restart movement of the carriage 120 in the Nth row is completed.

[0119] If determining that there is no carriage 120 whose distance newly becomes this distance or greater ("NO" in Step S150A), the CPU 142 terminates the image formation restart process and returns to the image formation interrupting process. If determining that there is the carriage 120 whose distance newly becomes this distance or greater ("YES" in Step S150A), the CPU 142 executes the process in Step S151.

[0120] If the determining process in Step S152 determines that the carriage 120, for which Step S150A has determined that the distance newly has become the above distance or greater, belongs to the eighth row, the CPU 142 terminates the measurement of the moving distance Lt2 and unsets the retreating flag (Step S153A).

[Modification 2]

[0121] FIG. 10 is a flowchart illustrating the image formation interrupting process according to Modification 2.

[0122] The image formation interrupting process according to Modification 2 involves additional Step S160 in the image formation interrupting process according to Modification 1. The other steps of the image formation interrupting process according to Modification 2 are identical to those of Modification 1, and have the same reference signs without redundant description.

[0123] In the image formation interrupting process according to Modification 2, while a lifted portion of the recording medium P having the reference lift amount H1 or greater is being detected, the process goes from any of Steps S132, S136, and S125A to Step S160. The CPU 142 acquires a lift distance H at that time and sets a retreating distance H4 based on the lift distance H. The CPU 142 then calculates the ascending/descending time Ta and the moving distance La during the ascending/descending time Ta, on the basis of the ascending/descending width H0 and the ascending/descending rate Va (Step S160), where H0 (H0=H4-Hn) indicates the range of distance between the nozzle surface and the conveying surface, from the ink ejection distance Hn to the retreating distance H4. The CPU 142 then proceeds to Step S126A.

[0124] The retreating distance H4 may be a constant value based on the maximum value of the lift distance H acquired within the lifted portion including the reference lift amount H1 or greater, or may be obtained by applying a certain low-path filter to the transition of the lift distance H, for example.

[0125] As described above, the image formation interrupting process according to Modifications 1 and 2 allows the interruption and restart of image formation to be directly controlled using the moving distance based on the data measured by the encoder 112, instead of counted time.

[0126] Furthermore, in the image formation interrupting process in the inkjet recording apparatus 100 according to Modification 2, the CPU 142 can vary the retreating distance depending on the lift amount of the recording medium P. In general, an increase in the retreating distance extends the ascending/descending time in the retreating movement, resulting in undesirable long interruption of ink ejection. To solve this problem, the CPU 142 can minimize the retreating distance while ensuring the sufficient ascending/descending time, thereby achieving efficient image formation.

[Embodiment 2]

[0127] The inkjet recording apparatus according to Embodiment 2 will now be described.

[0128] The internal configuration of the inkjet recording apparatus 100 according to Embodiment 2 is identical to that of the inkjet recording apparatus 100 according to Embodiment 1, and the same reference signs are assigned to the respective components without redundant description.

[0129] In the image formation in the inkjet recording apparatus 100 according to the embodiment, some steps of the image formation interrupting process are modified, which is invoked and executed in the image formation controlling process illustrated in FIG. 4. The processes other than the image formation interrupting process thus will not be described.

[0130] FIG. 11 is a flowchart illustrating steps of the image formation interrupting process invoked in the image formation controlling process executed by the CPU 142 in the inkjet recording apparatus 100 according to the embodiment.

[0131] This image formation interrupting process involves additional Steps S161 to S164 and substitutional Step S127B for Step S127 in the image formation interrupting process (FIG. 5) according to Embodiment 1. The other steps and the image formation restart process (FIG. 6), which is invoked in the image formation interrupting process, are identical to those of Embodiment 1 and have the same reference signs without redundant description.

[0132] After the moving rate Vt is obtained in Step S125, the CPU 142 further acquires an output image length Le, which indicates the length of an output image F in the ongoing formation in the moving direction, an interval Ld between repeatedly formed images, and an output distance x1e from the leading end of the image in the ongoing formation (Step S161).

[0133] The CPU 142 then calculates a remaining distance LR_N, which indicates the distance to the tail end of the output image F that can be completed by the inkjet heads 12 of the carriage 120 in the Nth row before the start of the retreating movement of the carriage 120 in response to the detection of a lifted portion (Step S162). The remaining distance LR_N for the carriage 120 is represented by Expression (4):

[0134] The determination factor B indicates whether the output image F in the ongoing formation by the inkjet heads 12 of the carriage 120 in the first row can be completely formed to its tail end (i.e. , across the distance (Le-x1e)). The determination factor B is represented by Expression (5a) or (5b) depending on conditions:

[0135] The total output number k indicates the number of output images F that can be output by the inkjet heads 12 of the carriage 120 in the first row across the entire output image length Le in addition to the image in the ongoing formation described above. That is, the total output number k is represented by Expression (6) :

where int indicates the quotient (an integral number equal to or larger than 0) of the division.

[0136] If the distance L1 is not much larger than the output image length Le and thereby k does not reach 1 (k ≥ 1), k may be preset to 0 to simplify the process in the inkjet recording apparatus 100.

[0137] The output distance x1e may be calculated from the preliminarily-held time difference between the timing of transmission of data from the memory 141 and the timing of ink ejection from the ink ejecting units 122 based on the transmitted data, for example. Alternatively, the output distance x1e may be indirectly calculated, for example, on the basis of an elapsed time from the timing of output of the leading end of each image.

[0138] In the inkjet recording apparatus including an imaging unit for checking for output images, the output distance x1e may be calculated based on image data taken by the imaging unit.

[0139] The output distance x1e may also be an estimated value.

[0140] The CPU 142 determines whether the elapsed time t1 for any carriage 120 in the Nth row newly becomes the value LR_N/Vt or greater (Step S163). If determining so for any carriage 120 ("YES" in Step S163), the CPU 142 outputs control signals to the driving circuits 121 to halt the ink ejection from the inkjet heads 12 of the carriage 120 in the Nth row (Step S164). The CPU 142 then proceeds to Step S126. If no carriage 120 having the elapsed time t1 newly becoming the value LR_N/Vt or greater ("NO" in Step S163) is present, the CPU 142 proceeds to Step S126.

[0141] If determining any carriage 120 in the Nth row newly at the retreating movement start timing in Step S126 ("YES" in Step S126), the CPU 142 starts the control for moving the carriage 120 in the Nth row to the retreating position (Step S127B). In this case, the ink ejection from the inkjet heads 12 of the carriage 120 in the Nth row has been already halted in Step S164. The CPU 142 then proceeds to Step S128.

[0142] FIGS. 12A and 12B each illustrate an example setting of interruption timing of the ink ejection in the inkjet recording apparatus 100 according to the embodiment.

[0143]  In the case illustrated in FIG. 12A, the inkjet heads 12 fixed to the carriage 120 in the first row have formed the output image F (having the output image length Le) across the output distance x1e at the timing of detection of a lifted portion having the reference lift amount H1 or greater by the recording medium distance sensor 113. In this case, the length of the remaining part of the output image F equals the distance (Le-x1e). This distance is shorter than the distance (L1-La-Lm) from the measuring point of the recording medium distance sensor 113 to the position of start of the retreating movement of the carriage 120 in the first row. Meanwhile, the distance required for formation of the entire subsequent output image F (i.e., across the output image length (Le+Ld)) is longer than the distance up to the retreating movement start position of the carriage 120 in the first row. Thus, the determination factor B and the total output number k are set to 1 and 0, respectively (B=1, k=0), and the ink ejection is interrupted after the completion of the ongoing image formation to the tail end of the image.

[0144] The inkjet heads 12 fixed to the carriage 120 in the second row eject ink across the sum of the length of the formed image and the interval L2 between the carriage 120 in the first row and the carriage 120 in the second row in the moving direction. That is, the ink ejection from the inkjet heads 12 fixed to the carriage 120 in the second row on the recording medium P continues to the end position of the ink ejection from the inkjet heads 12 of the carriage 120 in the first row.

[0145] In contrast, in the case illustrated in FIG. 12B, the length (Le-x1e) of the remaining part of the output image F in the ongoing formation by the inkjet heads 12 of the carriage 120 in the first row is longer than the distance (L1-La-Lm) from the measuring point of the recording medium distance sensor 113 to the position of the retreating movement of the carriage 120 in the first row. In this case, the output image F cannot be completely formed. Thus, the determination factor B and the total output number k are both set to 0 (B=0, k=0), and the ink ejection from the inkjet heads 12 of the carriage 120 in the first row is immediately halted at the current position.

[0146] In this case, the inkjet heads 12 of the carriage 120 in the second row may be controlled to eject ink across the distance (L2+Lh), which is the sum of the interval between the carriages 120 and the width of each carriage 120, before starting the movement to the retreating position.

[0147] In addition, for a deficient output image F formed by the incomplete ink ejection from the inkjet heads 12 of the carriage 120 in the first row, no ink ejection may be performed from the inkjet heads 12 of the carriages 120 in the second and subsequent rows. In other words, the inkjet heads 12 of the carriage 120 in the second row may be controlled to eject ink across the distance (Le-x2e).

[0148] As described above, in the inkjet recording apparatus 100 according to Embodiment 2, if the lift amount is the reference lift amount H1 or greater, the CPU 142 determines whether the ink ejecting units 122 can complete the ongoing image formation at the lift detection timing before the retreating movement start timing. If the ink ejecting units 122 cannot complete the ongoing image formation, the ink ejecting units 122 are controlled to halt the ink ejection before the retreating movement start timing, in specific, at the time of this determination. In other words, if the ink ejection until the retreating movement start timing is not sufficient for the image formation, the image formation is not started. This configuration can reduce the ink consumption and thus improve the efficiency.

[0149] In contrast, if determining that the ongoing image formation can be completed before the retreating movement start timing, the CPU 142 controls the ink ejecting units to halt the ink ejection after the timing of completion of the ongoing image formation before the retreating movement start timing, in particular, at the timing of completion of the ongoing image formation. This configuration can reduce the disposal of the recording medium P having a fragmentary image and the waste of ink.

[0150] If the ink ejecting units 122 can complete not only the ongoing image formation but also the formation of one or more other images before the retreating movement start timing, the CPU 142 controls the ink ejecting units 122 to eject ink for the formation of these images, and then halt the ink ejection after the timing of completion of all the image formation before the retreating movement start timing, in particular, at the timing of completion of all the image formation. This configuration can also reduce the disposal of the recording medium P and the waste of ink.

[0151] The above embodiments should not be construed to limit the invention, and may be modified in various manners.

[0152] For example, although the recording medium is a fabric extending in the moving direction in the embodiments, the recording medium may be recording paper or any other material. The invention is more suitable for media that readily get wrinkles, for example, thin films. The recording medium may also be a cut sheet other than a continuous recording medium (continuous sheet). In this case, the invention is more suitable for the image formation of multiple images in a certain pattern on a single cut sheet at least in the moving direction, for example.

[0153] The type of the elevating motors 132, and the positions and structures of the inkjet heads 12 and the elevating motors 132 are appropriately determined, as long as the elevating motors 132 can vary the distance of the nozzle surface of the inkjet heads 12 from the conveying surface. For example, the inkjet heads 12 may be directly mounted on the carriage. Alternatively, the inkjet heads 12 may be integrated in an inkjet head unit, which is then mounted on the carriage.

[0154] The number and size of the inkjet heads 12 may be appropriately determined.

[0155] Each nozzle surface is not necessarily completely flat. For a non-flat nozzle surface, the operation is based on the minimum distance between the nozzle surface and the conveying surface. The movements of the individual components of the inkjet heads 12 may have some variations as long as the position of the nozzle surface facing the conveying surface can be correctly shifted. For example, in the inkjet recording apparatus including a flexible printed circuit (FPC), the FPC and the nozzle surface do not necessarily have a constant distance in the z direction.

[0156] Although the carriages 120 move at a certain ascending/descending rate in the embodiments, the ascending/descending rate may be variable. For example, the moving (ascending) rate of the carriages 120 to the retreating position may differ from the moving (descending) rate of the carriages 120 to the ink ejection position, depending on the type of the elevating motors 132.

[0157] The type and position of the recording medium distance sensor 113 are appropriately determined. For example, the sensor may face the recording medium P and emit light to the recording medium P to measure the distance to the surface of the recording medium P on the basis of the reflected light. Alternatively, the sensor may use incident light in the lateral direction and detect the light not shielded by a lifted portion. The sensor may also use electromagnetic waves having various wavelengths and scattering rates that can be detected with sufficient accuracy other than the visible light.

[0158] Instead of direct acquisition of the moving rate from the encoder 112, a set rate prestored in the memory 141 may be used, for example. The rate and moving distance can be flexibly and accurately acquired from the encoder 112 regardless of a variation in the moving rate during the measurement of an elapsed time or moving distance.

[0159] In the embodiments, the interruption and restart of image formation are determined based on the distance (lift distance) between a lifted portion of the recording medium and the conveying surface caused by a wrinkle in the recording medium. Examples of the lifted portion described in the specification may include not only portions of the recording medium actually distant from the conveying surface but also uneven portions of a recording medium having irregular thickness. In this case, the interruption and restart of image formation may be determined based on a variation in the thickness of the recording medium.

[0160]  In the embodiments, after the interruption of the image formation and the retreating movement of each carriage 120, the carriage 120 performs the restart movement to resume the image formation if the time of continuous detection of lifted portions having lift amounts less than the restart lift amount H2 less than the reference lift amount H1 reaches the predetermined waiting time (2Ta+Tp). Alternatively, the restart lift amount H2 may be equal to the reference lift amount H1 in this determination. The reference lift amount H1 may be appropriately determined depending on, for example, the material of the recording medium P, as long as the reference lift amount H1 is less than the maximum retreating distance H3.

[0161] The image formation may be also resumed after the detection of a lifted portion having a lift amount less than the restart lift amount H2 and the subsequent detection of another lifted portion having the restart lift amount H2 or greater in the waiting time (2Ta+Tp), unless the distance of the other lifted portion equal to or larger than a predetermined distance equal to or less than the reference lift amount H1.

[0162] The recording time Tp of the waiting time (2Ta+Tp) may be set to 0 and the waiting time may be invalidated, so that the carriage 120 can perform the restart movement without continuous detection of lifted portions having lift amounts less than the restart lift amount H2 for the waiting time (2Ta+Tp). This configuration can simplify the determination, but may cause a problem in that the carriage 120 starts the restart movement but must return to the retreating position before the distance between the conveying surface and the nozzle surface achieves the ink ejection distance Hn (i.e., before resuming the ink ejection).

[0163] Although the image formation is resumed from the leading end of the image after the interruption of the ink ejection in the embodiments, the image formation may be resumed from the position of the image corresponding to the interruption of the ink ejection as long as a part of the recording medium having a fragmentary image before the interruption can be connected to another part of the recording medium having a fragmentary image after the interruption.

[0164] Although the description of the embodiments is focused on an example line-head inkjet recording apparatus equipped with eight color inks, the inkjet recording apparatus may use any number of inks. For example, the inkjet recording apparatus may be dedicated to image formation with a single color. The techniques of the invention can also be applied to an inkjet recording apparatus 100 having no line head structure on the basis of the traveling rate of the inkjet heads 12 in the width direction.

[0165] Although the identical image is repeatedly formed in the embodiments, images maybe different on the condition of repetitive image formation. For example, a serial number may be added to a corner of each image, or a predetermined number of images are repeatedly formed in cycles.

[0166] Although the inkjet recording apparatus in the embodiments uses the moving rate measured by the encoder 112 and the fixed distance L1, the inkjet recording apparatus may use a fixed moving rate and/or a variable distance L1. The fixed value can be stored in the ROM 143 or a storage (not shown) of the control unit 14 as required. The variable distance L1 can be measured by, for example, a linear encoder.

[0167] The specific details, such as the configurations, the positions of the components, and the contents and order of the controls, in the embodiments may be appropriately modified within the gist of the invention.

INDUSTRIAL APPLICABILITY

[0168] The invention can be applied to an inkjet recording apparatus.

REFERENCE SIGNS LIST

[0169] 

11

conveying unit

111

conveying motor

112

encoder

113

recording medium distance sensor

114

driving roller

115

conveyor belt

12

inkjet head

120

carriage

121

driving circuit

122

ink ejecting unit

13

carriage elevating unit

131

motor driver

132

elevating motor

133

electromagnetic brake

134

beam member

135

supporting member

14

control unit

141

memory

142

CPU

143

ROM

144

RAM

145

bus

15

operational display unit

16

communication unit

100

inkjet recording apparatus

B

determination factor

F

output image

H0

ascending/descending width

H1

reference lift amount

H2

restart lift amount

H3

maximum retreating distance

H4

retreating distance

Hn

ink ejection distance

k

total output number

L1

length

L2

interval

La

ascending/descending-time moving distance

Ld

interval

Le

output image length

Lm

margin

Lp

recording distance

Lt1

moving distance

Lt2

moving distance

P

recording medium

t1

elapsed time

t2

elapsed time

Ta

ascending/descending time

Tm

margin time

Tp

recording time

Tt

traveling time

Va

ascending/descending rate

Vt

moving rate

x1e

output distance

Claims

1. An inkjet recording apparatus comprising:

a conveying unit to transfer a recording medium on a conveying surface in a predetermined moving direction;

at least one ink ejecting unit to eject an ink droplet through a nozzle opening disposed on at least one nozzle surface facing the conveying surface;

an elevating unit to move the ink ejecting unit to change a distance between the nozzle surface and the conveying surface;

a measuring unit to measure a lift distance of the recording medium from the conveying surface at a measuring point disposed upstream of the ink ejecting unit in the moving direction so as to be distant from the ink ejecting unit by a predetermined separation distance;

an elevation control unit to cause the elevating unit to perform a retreating movement to change a distance between the ink ejecting unit and the conveying surface to a predetermined retreating distance to prevent the nozzle surface from coming into contact with a lifted portion of the recording medium on the basis of the separation distance and a moving rate of the recording medium transferred by the conveying unit when the lift distance measured by the measuring unit becomes a first predetermined distance or greater at a lift detection timing, after elapse of a predetermined time from the lift detection timing, before a timing of arrival of the lifted portion having the lift distance equal to or greater than the first predetermined distance at an area facing the ink ejecting unit; and

an ejection control unit to cause the ink ejecting unit to continue to eject the ink droplet from the lift detection timing at least until a timing ahead of a retreating movement start timing at which the retreating movement starts.

2. The inkjet recording apparatus of claim 1, wherein the elevation control unit varies the retreating distance depending on the lift distance.

3. The inkjet recording apparatus of claim 1 or 2, wherein
when the lift distance becomes the first distance or greater and then falls below a second distance equal to or less than the first distance, the elevation control unit causes the elevating unit to perform a restart movement to change the distance between the nozzle surface and the conveying surface to a predetermined ink ejection distance for ejecting ink from the nozzle opening after the lifted portion having the lift distance equal to or greater than the second distance passes through the area facing the ink ejecting unit, and
the ejection control unit causes the ink ejecting unit for which the restart movement is performed to resume ejection of the ink droplet for image formation through the nozzle opening after completion of the restart movement.

4. The inkjet recording apparatus of claim 3, wherein the elevation control unit enables the elevating unit to perform the restart movement when the lift distance remains less than the second distance for a predetermined waiting time.

5. The inkjet recording apparatus of claim 4, wherein the waiting time is equal to or longer than a period required for reciprocating movement of the nozzle surface between the retreating distance and the ink ejection distance at a predetermined ascending/descending rate, the waiting time being equal to or shorter than a period during which the recording medium is transferred across the separation distance.

6. The inkjet recording apparatus of any one of claims 3 to 5, wherein the ejection control unit causes the ink ejecting unit to perform a pre-ejection operation of the ink through the nozzle opening after completion of the restart movement, and then resume ink ejection for image formation through the nozzle opening.

7. The inkjet recording apparatus of claim 5 or 6, wherein the ejection control unit causes a target image to be formed from a leading end of the target image after completion of the restart movement.

8. The inkjet recording apparatus of any one of claims 1 to 7, wherein
when the lift distance becomes the first distance or greater, the ejection control unit determines whether an image being formed by the ink ejecting unit at the lift detection timing can be completely formed before the retreating movement start timing, and if the ejection control unit determines that the image cannot be completely formed, the ejection control unit causes the ink ejecting unit to halt ink ejection before the retreating movement start timing.

9. The inkjet recording apparatus of any one of claims 1 to 8, wherein
when the lift distance becomes the first distance or greater, the ejection control unit determines whether an image being formed by the ink ejecting unit at the lift detection timing can be completely formed before the retreating movement start timing, and if the ejection control unit determines that the image can be completely formed, the ejection control unit causes the ink ejecting unit to halt ink ejection after a completion timing of image formation and before the retreating movement start timing.

10. The inkjet recording apparatus of any one of claims 1 to 7 wherein,
when the lift distance becomes the first distance or greater, the ejection control unit causes the ink ejecting unit to eject the ink for forming the target image which can be completely formed before the retreating movement start timing, and to halt ink ejection after a completion timing of image formation and before the retreating movement start timing.

11. The inkjet recording apparatus of any one of claims 1 to 10, further comprising:

a conveyance control unit to cause the conveying unit to stop transfer of the recording medium when the lift distance is greater than a maximum value of the retreating distance.

12. The inkjet recording apparatus of any one of claims 1 to 11, wherein
the ink ejecting unit is composed of a plurality of ink ejecting units disposed in different positions in the moving direction,
the elevating unit changes the distances between the nozzle surfaces of the ink ejecting units and the conveying surface independently of one another, and
the elevation control unit causes the elevating unit to perform ascending/descending movements of the ink ejecting units depending on the distances between the ink ejecting units and the lifted portion, respectively, the movements being same as one another, to change each of the distances between the nozzle surfaces and the conveying surface to the retreating distance.

13. The inkjet recording apparatus of any one of claims 1 to 12, wherein the recording medium is a fabric extending longer than the separation distance in the moving direction.

Drawing