Patent Publication Number: US-8989648-B2

Title: Image recording apparatus, recording-media aligning method executed by the same, and non-transitory storage medium storing instructions readable by the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2012-218404, which was filed on Sep. 28, 2012, the disclosure of which is herein incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image recording apparatus including a tray capable of supporting a plurality of recording media on which images have been respectively recorded, a method of aligning the plurality of recording media which is executed by a processor of the image recording apparatus, and a non-transitory storage medium storing a plurality of instructions readable by a computer of the image recording apparatus. 
     2. Description of the Related Art 
     There is conventionally known a technique of applying an external force to sheets stacked on a sheet-output tray to align the sheets. For example, there is known a sheet-discharge apparatus configured to discharge recording media in the form of sheets on which images are respectively formed or recorded, onto a sheet-output tray and align the sheets stacked on the sheet-output tray. In this sheet-discharge apparatus, trailing edges of the respective sheets stacked on the sheet-output tray are in abutment against a sheet-trailing-edge abutment fence. A vibration generator applies a vibration wave to the sheet-trailing-edge abutment fence to align the trailing edges of the respective sheets stacked on the sheet-output tray. 
     SUMMARY OF THE INVENTION 
     Incidentally, there is known an image recording apparatus including: a recording unit configured to record an image on a recording medium; and an output tray on which image-recorded recording media are discharged and stacked. In a case where the above-described sheet-discharge apparatus is applied to this image recording apparatus, when a vibration wave is applied to the sheet-trailing-edge abutment fence during image recording. This vibration wave may be transmitted to the recording unit, resulting in deterioration of a quality of an image recorded on the recording medium. 
     This invention has been developed to provide an image recording apparatus capable of aligning a plurality of recording media stacked on a tray while preventing deterioration of a quality of an image recorded on a recording medium, a method of aligning the plurality of recording media which is executed by a processor of the image recording apparatus, and a non-transitory storage medium storing a plurality of instructions readable by a computer of the image recording apparatus. 
     The present invention provides an image recording apparatus including: a recording unit configured to record an image on a recording medium; a storage configured to store a recording job containing image data representative of a plurality of images to be recorded on a plurality of recording media by the recording unit; a tray configured to support the recording medium on which an image has been recorded by the recording unit; a conveyor mechanism configured to convey, to the tray, the recording medium on which the image has been recorded by the recording unit; an alignment mechanism configured to align a plurality of recording media stacked on the tray, by application of an external force; a controller configured to control the recording unit, the conveyor mechanism, and the alignment mechanism, the controller being configured to control the recording unit to record the plurality of images respectively on a plurality of recording media based on the image data contained in the recording job stored in the storage, the controller being configured to control the conveyor mechanism to convey, to the tray, the plurality of recording media on which the plurality of images have been respectively recorded by the recording unit, in a period from a start to an end of recording based on one recording job, the controller being configured to control the alignment mechanism to align the plurality of recording media stacked on the tray in a period in which image recording is not performed by the recording unit and control the alignment mechanism not to align the plurality of recording media stacked on the tray in a period in which image recording is being performed by the recording unit. 
     The present invention also provides A method of aligning a plurality of recording media which is executed by a processor of an image recording apparatus, the image recording apparatus including: a recording unit configured to record an image on a recording medium; a storage configured to store a recording job containing image data representative of a plurality of images to be recorded on a plurality of recording media by the recording unit; a tray configured to support the recording medium on which an image has been recorded by the recording unit; a conveyor mechanism configured to convey, to the tray, the recording medium on which the image has been recorded by the recording unit; and an alignment mechanism configured to align a plurality of recording media stacked on the tray, by application of an external force, the method including: causing the recording unit to record the plurality of images respectively on a plurality of recording media based on the image data contained in the recording job stored in the storage; causing the conveyor mechanism to convey, to the tray, the plurality of recording media on which the plurality of images have been respectively recorded by the recording unit; and in a period from a start to an end of recording based on one recording job, causing the alignment mechanism to align the plurality of recording media stacked on the tray in a period in which image recording is not performed by the recording unit and controlling the alignment mechanism not to align the plurality of recording media stacked on the tray in a period in which image recording is being performed by the recording unit. 
     The present invention also provides A non-transitory storage medium storing a plurality of instructions readable by a computer of an image recording apparatus, the image recording apparatus including: a recording unit configured to record an image on a recording medium; a storage configured to store a recording job containing image data representative of a plurality of images to be recorded on a plurality of recording media by the recording unit; a tray configured to support the recording medium on which an image has been recorded by the recording unit; a conveyor mechanism configured to convey, to the tray, the recording medium on which the image has been recorded by the recording unit; and an alignment mechanism configured to align a plurality of recording media stacked on the tray, by application of an external force, the plurality of instructions, when executed by a processor of the image recording apparatus, causing the image recording apparatus to perform: recording the plurality of images respectively on a plurality of recording media based on the image data contained in the recording job stored in the storage; conveying, to the tray, the plurality of recording media on which the plurality of images have been respectively recorded by the recording unit; and in a period from a start to an end of recording based on one recording job, aligning the plurality of recording media stacked on the tray in a period in which image recording is not performed by the recording unit and controlling the alignment mechanism not to align the plurality of recording media stacked on the tray in a period in which image recording is being performed by the recording unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects, features, advantages, and technical and industrial significance of the present invention will be better understood by reading the following detailed description of one embodiment of the invention, when considered in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic side view illustrating an overall structure of a printer according to one embodiment of the present invention; 
         FIG. 2  is a top view illustrating the printer illustrated in  FIG. 1 ; 
         FIG. 3  is a top view illustrating a portion of the printer illustrated in  FIG. 1 ; 
         FIG. 4A  is a block diagram illustrating a configuration of a controller illustrated in  FIG. 1 , and  FIG. 4B  is a block diagram illustrating functional portions of the controller; 
         FIG. 5  is a table contained in a first predetermined number determiner illustrated in  FIG. 4B ; 
         FIG. 6  is a flow chart illustrating one example of a procedure of processings executable by the controller illustrated in  FIG. 1 ; and 
         FIG. 7  is a schematic side view illustrating an overall structure of a printer according to a modification of the one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     Hereinafter, there will be described one embodiment of the present invention by reference to the drawings. 
     As illustrated in  FIG. 1 , a printer  101  according to the present embodiment includes a housing  101   a  having generally a rectangular parallelepiped shape. The housing  101   a  contains from its upper side four recording heads  1 , a conveyor mechanism  16 , a sheet-supply unit  17 , and a tank unit  18 . The conveyor mechanism  16  is configured to convey a sheet P in a conveying direction that is a direction directed from the left side toward the right side in  FIG. 1 . The sheet-supply unit  17  is configured to supply the sheet P, and the tank unit  18  stores inks. A top of the housing  101   a  over the recording heads  1  includes a sheet-output portion  10  onto which the sheet P is discharged. The housing  101   a  further contains a controller  100  configured to control operations of the printer  101 . 
     The four recording heads  1  are configured to respectively eject the inks of different four colors, namely, cyan, magenta, yellow, and black. Each of the recording heads  1  has generally a rectangular parallelepiped shape elongated in a main scanning direction. These recording heads  1  are fixed to a support frame  3  so as to be arranged in the conveying direction. That is, this printer  101  is a line printer, and accordingly the main scanning direction is a direction perpendicular to the conveying direction. Each of the recording heads  1  includes a head main body  2  whose lower face serves as an ejection face  2   a  having a multiplicity of ejection openings, not shown, formed therein. 
     The conveyor mechanism  16  includes two belt rollers  6 ,  7 , a conveyor belt  8 , a tension roller  9 , and a platen  19 . The conveyor belt  8  is an endless belt looped over the rollers  6 ,  7  and tensioned by the tension roller  9 . The platen  19  is disposed in the conveyor belt  8  to support the conveyor belt  8  at a position opposite the recording heads  1 . The belt roller  7  is a drive roller that is rotated by a motor, not shown. When the conveyor mechanism  16  drives the belt roller  7 , the conveyor belt  8  is moved to convey the sheet P placed on a conveyor surface  8   a  of the conveyor belt  8 . Also, the conveyor mechanism  16  is provided with a sheet sensor  15  for sensing the presence of the sheet P in a conveyance path. This sheet sensor  15  is provided mainly for sensing a leading edge of the sheet P being conveyed by the conveyor mechanism  16 , i.e., a downstream edge of the sheet P in the conveying direction in order for the recording heads  1  to eject the inks at appropriate timings. In addition, the sheet sensor can sense a jam of the sheet P at an area opposite the ejection faces  2   a  of the respective recording heads  1  by comparing, with a predetermined length of time, a length of time elapsed from a timing when the sheet sensor  15  senses the leading edge of the sheet P, i.e., the downstream edge of the sheet P in the conveying direction, to a timing when the sheet sensor  15  senses a trailing edge of the sheet P, i.e., an upstream edge of the sheet P in the conveying direction. When this length of time is longer than the predetermined length of time, the controller  100  determines that the jam of the sheet P has occurred. 
     The sheet-supply unit  17  is removably installed in the housing  101   a  and includes: a sheet-supply tray  17   a  configured to accommodate a plurality of sheets P; and a sheet-supply roller  17   b  rotatable to supply an uppermost one of the plurality of sheets P stacked on the sheet-supply tray  17   a . The sheet P supplied from the sheet-supply tray  17   a  is conveyed to the conveyor mechanism  16  by a conveyor roller pair  14  along guides  13   a ,  13   b.    
     The tank unit  18  contains four ink tanks  18   a . The ink tanks  18   a  are removably mounted in the tank unit  18 . The ink tanks  18   a  respectively store the inks of the cyan, magenta, yellow, and black, which are respectively supplied to the recording heads  1  through ink tubes, not shown. 
     As illustrated in  FIG. 1 , the conveyance path extending along the black arrows is formed in the printer  101 . The sheet P fed from the sheet-supply unit  17  to the conveyor mechanism  16  is pressed against the conveyor surface  8   a  by a pressure roller  4 . When the sheet P passes through the area opposite the ejection faces  2   a  of the respective recording heads  1 , a desired color image is formed on an upper side of the sheet P. The sheet on which the image has been formed is peeled from the conveyor surface  8   a  by a peeling member  5  disposed just downstream of the conveyor mechanism  16 . The sheet P is then conveyed upward along guides  29   a ,  29   b  by a conveyor roller pair  28  and discharged onto the sheet-output portion  10  through an output opening  22  formed in an upper portion of the housing  101   a.    
     The sheet-output portion  10  includes: an output tray  11  on which the sheets P discharged from the output opening  22  are stacked; and an alignment mechanism  30  configured to jog and align the sheets P stacked on the output tray  11 . The output tray  11  is provided with a number-of-stacked-sheets obtaining sensor  12  that is an optical sensor configured to obtain the number of sheets P stacked on the output tray  11 . 
     As illustrated in  FIG. 2 , the alignment mechanism  30  includes a pair of limiting walls arranged parallel to each other on the output tray  11 , and the pair of limiting walls is constituted by a left limiting wall  31   a  and a right limiting wall  31   b  in  FIG. 2 . The alignment mechanism  30  further includes a limiting-wall displacement mechanism  33  configured to displace or move the left limiting wall  31   a  with respect to the right limiting wall  31   b . Each of the limiting walls  31   a ,  31   b  is a plate member that extends in a direction in which the sheet P is discharged from the output opening  22 , i.e., in a direction indicated by the white arrow in  FIG. 2 . The limiting walls  31   a ,  31   b  limit or constrain opposite edge portions of the sheets P stacked on the output tray  11  in their widthwise direction, A left portion of the output tray  11  has a guide groove  11   a  extending in the widthwise direction of the sheet P. Fitted in the guide groove is a support portion  32  for supporting a lower end portion of the limiting wall  31   a.    
     The limiting-wall displacement mechanism  33  includes: a pair of pulleys  35   a ,  35   b  spaced apart from each other in the widthwise direction of the sheet P; a driving belt  37  looped over the pulleys  35   a ,  35   b ; and a motor  39  for rotating the pulley  35   b . The support portion  32  supporting the limiting wall  31   a  is holding the driving belt  37 , so that the limiting wall  31   a  is moved together with the driving belt  37 . Thus, when the motor  39  rotates the driving belt  37 , the limiting wall  31   a  is moved along the guide groove  11   a  in the widthwise direction of the sheet P. That is, the limiting wall  31   a  can be moved toward or away from the limiting wall  31   b  depending upon a direction of rotation of the motor  39  to change a distance between the limiting walls  31   a ,  31   b . Thus, when the sheets P are aligned, the limiting wall  31   a  is moved toward the limiting wall  31   b  as indicated by the black arrow in  FIG. 2 , and an external force is applied to the sheets P such that right edges of the sheets P stacked on the output tray  11  in  FIG. 2  are pressed against the limiting wall  31   b.    
     The printer  101  further includes a maintenance mechanism  40  configured to perform maintenance on the recording heads  1 . As illustrated in  FIG. 3 , when the maintenance is not performed, the maintenance mechanism  40  is located on a side of the recording heads  1  in the main scanning direction, specifically, the maintenance mechanism  40  is located at the left of the recording heads  1  in  FIG. 3 . The maintenance mechanism  40  includes a cap unit  41  and a wiping unit  45 . 
     The cap unit  41  is mounted on a support plate  43  that is movable in the main scanning direction. The cap unit  41  includes four purging caps  42  that face the respective recording heads  1  when the support plate  43  is moved to a position opposite the recording heads  1 . The recording heads  1  are lowered by an elevating and lowering mechanism, not shown, in a state in which the four purging caps  42  face the respective recording heads  1 , so that the four purging caps  42  respectively cover the ejection faces  2   a  of the respective recording heads  1 . A pressure pump, not shown, is provided between the recording heads  1  and the ink tanks  18   a . When this pressure pump is driven in the state in which the ejection faces  2   a  of the respective recording heads  1  are covered with the respective purging caps  42 , high-viscosity inks and foreign matters in the recording heads  1  are forcibly discharged from the ejection openings of the recording heads  1  (noted that this operation may be hereinafter referred to as “pressure purging”). Also, the purging caps  42  communicate with a waste-ink tank, not shown, such that the inks and the foreign matters discharged are stored in the waste-ink tank. 
     The wiping unit  45  is disposed at the right of the cap unit  41  in  FIG. 3  and includes a wiper  46  mounted on a support plate  47  that is movable in the main scanning direction. When the recording heads  1  are lowered by an elevating and lowering mechanism, not shown, in a state in which the wiper  46  is located at a position opposite the recording heads  1 , a distal end portion of the wiper  46  is brought into contact with the ejection faces  2   a  of the respective recording heads  1 . When the support plate  47  is moved in the main scanning direction in this state, the wiper  46  wipes the ejection face  2   a . The maintenance mechanism  40  performs this wiping after the pressure purging. 
     As illustrated in  FIG. 4A , the controller  100  includes: a central processing unit (CPU)  111 ; an electrically erasable and programmable read only memory (EEPROM)  112  configured to rewritably store programs executable by the CPU  111  and data used for the programs; and a random access memory (RAM)  113  configured to temporarily store data upon execution of the programs. Upon execution by the CPU  111 , a control program in the present invention provides various functional portions of the controller  100  which are illustrated in  FIG. 4B . 
     As illustrated in  FIG. 4B , the controller  100  includes a storage  51 , a counter  52 , a head controller  53 , a conveyance controller  54 , an alignment controller  55 , and a first predetermined number determiner  56 . 
     The storage  51  stores one recording job containing a plurality of sets of image data respectively representative of a plurality of images. The one recording job is transferred from a device coupled to the printer  101  such as a personal computer (PC) to the storage  51 . In a period from a start to an end of recording based on the one recording job, the counter  52  is configured to count the number of recorded sheets N which is the number of sheets P for which image recording is completed after the preceding alignment of the sheets P stacked on the output tray  11  is performed by the alignment mechanism  30 . 
     The head controller  53  controls the recording heads  1  such that a plurality of images respectively based on a plurality of sets of image data contained in the recording job stored in the storage  51  (hereinafter may be simply referred to as “the plurality of images”) are respectively recorded on a plurality of sheets P. This printer  101  includes a normal image quality mode and a high image quality mode each as an image quality mode, and a higher quality image is recorded on the sheet P in the high image quality mode than in the normal image quality mode. The head controller  53  can select one of the normal image quality mode and the high image quality mode each as the image quality mode. Also, in the period of the recording based on the one recording job, the head controller  53  interrupts the recording of the image on the sheet P when the number of recorded sheets N which is counted by the counter  52  has reached a first predetermined number determined by the first predetermined number determiner  56 . That is, after the plurality of images for the recording job start to be recorded on the respective sheets P by the recording heads  1 , when recording on an Nth sheet P is finished, the head controller  53  interrupts recording on an N+1th sheet P. However, in a case where the number of sheets P to be recorded after the preceding alignment of the sheets P by the alignment mechanism  30  in the period of the recording based on the one recording job is larger than the first predetermined number and smaller than a second predetermined number (larger than the first predetermined number), the head controller  53  does not interrupt the recording of the image on the sheet P even when the number of recorded sheets N has reached the first predetermined number. The conveyance controller  54  controls the conveyor mechanism  16  to convey, to the output tray  11 , the plurality of sheets P on which the plurality of images are respectively recorded by the recording heads  1 . 
     In the period of the recording based on the one recording job, the alignment controller  55  controls the alignment mechanism  30  to align the sheets P stacked on the output tray  11  in a period in which the image recording is not being performed on the sheet P by the recording heads  1  (noted that this period includes a period in which the recording is interrupted) and controls the alignment mechanism  30  not to align the sheets P stacked on the output tray  11  in a period in which the image recording is being performed on the sheet P by the recording heads  1 . 
     Also, the alignment controller  55  controls the alignment mechanism  30  to align the sheets P stacked on the output tray  11  when the number of recorded sheets N counted by the counter  52  has reached the first predetermined number determined by the first predetermined number determiner. However, in the case where the number of sheets P to be recorded after the preceding alignment of the sheets P by the alignment mechanism  30  in the period of the recording based on the one recording job is larger than the first predetermined number and smaller than the second predetermined number, the sheets P are not aligned even when the number of recorded sheets N has reached the first predetermined number. In the case where the plurality of images have been recorded on the respective sheets P by the recording heads  1 , the sheets P are aligned. In the present embodiment, the second predetermined number is a number that is obtained by adding five to the first predetermined number. 
     Also, the alignment controller  55  controls the alignment mechanism  30  not to align the sheets P in an interruption period in which the recording of the image on the sheet P by the recording heads  1  is interrupted owing to a first event, and the alignment controller  55  controls the alignment mechanism  30  to align the sheets P in an interruption period in which the recording of the image on the sheet P is interrupted owing to a second event that differs from the first event. Here, examples of the first event in the present embodiment include: a case where lowered performance of the recording heads  1  for ink ejection has created a need to cause the maintenance mechanism  40  to perform the maintenance on the recording heads  1 ; and a case where the sheet sensor  15  has sensed the jam of the sheet P at the area opposite the ejection faces  2   a  of the respective recording heads  1 . Examples of the second event include: a case where the printer  101  runs out of sheet or ink; a case where a recorded sheet P needs to be dried; and a case where the number of sheets P stacked on the output tray  11  has reached a predetermined number, in other words, the output tray  11  becomes full. 
     In the case where the image quality mode is the high image quality mode, the alignment controller  55  controls the alignment mechanism  30  not to align the sheets P when the conveyor mechanism  16  is conveying the sheet P printed by the recording heads  1  to the output tray  11 , and the alignment controller  55  controls the alignment mechanism  30  to align the sheets P when the conveyor mechanism  16  is not conveying the sheet P printed by the recording heads  1  to the output tray  11 . 
     The first predetermined number determiner  56  is configured to determine the first predetermined number based on the number of sheets P stacked on the output tray  11  which is obtained by the number-of-stacked-sheets obtaining sensor  12  when the preceding alignment of the sheets P stacked on the output tray  11  is performed by the alignment mechanism  30 . That is, the first predetermined number determiner  56  has a table as illustrated in  FIG. 5  in which the first predetermined number and the number of sheets P stacked on the output tray  11  are associated with each other. As illustrated in  FIG. 5 , the larger the number of sheets P stacked on the output tray  11 , the smaller the first predetermined number is. 
     There will be next explained, with reference to  FIG. 6 , one example of a procedure of processings that are executed by the controller  100  when the printer  101  records a plurality of images respectively on a plurality of sheets P based on a plurality of sets of image data contained in one recording job. It is noted that, at the start of this flow, the first predetermined number is set at 50 based on the table illustrated in  FIG. 5 . 
     This flow begins with S 1  at which the head controller  53  controls the recording heads  1  to start recording one of the plurality of images respectively based on the plurality of sets of image data stored in the storage  51 , on a sheet P in the normal image quality mode or the high image quality mode. At S 2 , the alignment controller  55  determines whether the first event has occurred or not. 
     When the first event has occurred (S 2 : YES), the head controller  53  at S 3  interrupts the image recording by the recording heads  1 . At S 4 , the controller  100  controls the maintenance mechanism  40  to perform the maintenance on the recording heads  1  or executes a processing for a user to clear the jam of the sheet P. The controller at S 5  determines whether the event has been solved or not. When the event has been solved (S 5 : YES), this flow returns to S 1 , and the recording heads  1  restart the image recording. On the other hand, when the event has not been solved (S 5 : NO), this flow ends with an error. 
     On the other hand, when the first event has not occurred (S 2 : NO), the controller  100  at S 6  determines whether the second event has occurred or not. When the second event has occurred (S 6 : YES), the head controller  53  at S 7  interrupts the image recording by the recording heads  1 . The controller  100  at S 8  executes a processing for the user to replenish sheets or ink, for example, and at the same time the alignment controller  55  controls the alignment mechanism  30  to align the sheets P stacked on the output tray  11 . The controller  100  at S 9  determines whether the event has been solved or not. When the event has been solved (S 9 : YES), the counter  52  at S 10  resets the number of recorded sheets N (N=0). This flow then returns to S 1 , and the recording heads  1  restart the image recording. On the other hand, when the event has not been solved (S 9 : NO), this flow ends with an error. 
     When the second event has not occurred (S 6 : NO), the controller  100  at S 11  determines whether the recording of the image on the one sheet P has been completed or not. When the recording of the image on the one sheet P is not completed (S 11 : NO), this flow returns to S 1  at which the recording heads  1  continue to record the image. When the recording of the image on the one sheet P is completed (S 11 : YES), the controller  100  at  812  determines whether the image quality mode selected by the head controller  53  is the high image quality mode or not. 
     When the image quality mode is not the high image quality mode (S 12 : NO), this flow goes to S 14 . On the other hand, when the image quality mode is the high image quality mode (S 12 : YES), the controller  100  at S 13  determines whether the sheet P for which the image had been recorded has been discharged onto the output tray  11  or not, that is, the controller  100  determines whether the conveyance of the sheet to the output tray  11  has been completed or not. The determination at S 13  is repeated until the conveyance of the sheet to the output tray  11  is completed. That is, this flow does not go to S 14  until the conveyance of the sheet to the output tray  11  is completed. 
     The counter  52  at S 14  increments the number of recorded sheets N by one. The controller  100  at S 15  determines whether or not the recording heads  1  have recorded all the images based on the plurality of sets of image data contained in the one recording job. When all the images have not been recorded (S 15 ; NO), the controller  100  at S 16  determines whether the number of recorded sheets N is equal to the first predetermined number or not. When the number of recorded sheets N is not equal to the first predetermined number (S 16 : NO), this flow returns to S 1  at which the recording heads  1  start recording the next image. 
     On the other hand, when the number of recorded sheets N is equal to the first predetermined number (S 16 : YES), the controller  100  at S 17  determines whether or not the number obtained by adding the number of unrecorded images among the plurality of images based on the plurality of sets of image data contained in the one recording job stored in the storage  51 , to the number of recorded sheets N (i.e., the first predetermined number) is smaller than the second predetermined number. When the number obtained by adding the number of unrecorded images to the first predetermined number is smaller than the second predetermined number, that is, when the number of sheets P to be recorded after the preceding alignment of the sheets P in the period of the recording based on the one recording job is smaller than the second predetermined number (S 17 : YES), this flow returns to S 1  at which the recording heads  1  start recording the next image. 
     On the other hand, when the number obtained by adding the number of unrecorded images to the first predetermined number is equal to or larger than the second predetermined number (S 17 ; NO), the alignment controller  55  at S 18  controls the alignment mechanism  30  to align the sheets P stacked on the output tray  11 . It is noted that the image is not recorded by the recording heads  1  throughout this alignment. The first predetermined number determiner  56  at S 19  determines the first predetermined number based on the table illustrated in  FIG. 5  and the number of sheets P stacked on the output tray  11  which is obtained by the number-of-stacked-sheets obtaining sensor  12 . Then, the counter  52  at S 10  resets the number of recorded sheets N (N=0) 
     When all the images have been recorded (S 15 : YES), the controller at S 20  determines whether or not the number of recorded sheets N is equal to or larger than the first predetermined number. When the number of recorded sheets N is equal to or larger than the first predetermined number (S 20 : YES), the alignment controller  55  at S 21  controls the alignment mechanism  30  to align the sheets P stacked on the output tray  11 , and this flow ends. On the other hand, when the number of recorded sheets N is smaller than the first predetermined number (S 20 : NO), this flow ends. 
     In the present embodiment described above, the printer  101  includes the alignment mechanism  30  configured to apply an external force to the sheets P stacked on the output tray  11  to jog and align the sheets P stacked on the output tray  11 . In the period from the start to the end of the recording of the plurality of images by the recording heads  1  based on the image data contained in the one recording job, the alignment controller  55  controls the alignment mechanism  30  to align the sheets P stacked on the output tray  11  in the period in which the image recording is not being performed on the sheet P by the recording heads  1  and controls the alignment mechanism  30  not to align the sheets P stacked on the output tray  11  in a period in which the image recording is being performed on the sheet P by the recording heads  1 . This configuration makes it possible to align the sheets P stacked on the output tray  11  while preventing that vibrations generated during the alignment performed by the alignment mechanism  30  deteriorate the quality of the image recorded on the sheet P. 
     In the printer  101  according to the present embodiment, when the number of sheets P conveyed to the output tray  11  by the conveyor mechanism  16  after the preceding alignment of the sheets P stacked on the output tray  11  has reached the first predetermined number, the recording of the image on the sheet P is interrupted to align the sheets P stacked on the output tray  11 . Accordingly, the sheets P stacked on the output tray  11  can be aligned each time when the first predetermined number of the sheets P have been conveyed to the output tray  11 . 
     In the printer  101  according to the present embodiment, in the case where the number of sheets P to be recorded after the preceding alignment of the sheets P by the alignment mechanism  30  in the period of the recording based on the one recording job is larger than the first predetermined number and smaller than the second predetermined number, the sheets P stacked on the output tray  11  are not aligned when the first predetermined number of the sheets P are conveyed to the output tray  11  by the conveyor mechanism  16 , but the sheets P are aligned after the plurality of images have been recorded on the respective sheets P by the recording heads  1 . Accordingly, even in a case where the first predetermined number of the sheets P have been conveyed to the output tray  11 , when the recording of the plurality of images on the respective sheets P is close to completion, the completion of the recording of the plurality of images on the respective sheets P is given a higher priority, so that the recorded sheets P are stacked on the output tray  11  early. This configuration allows the user to selectively collect the sheets P from the output tray  11  before or after the alignment of the sheets P. 
     In the printer  101  according to the present embodiment, the sheets P stacked on the output tray  11  are aligned in the interruption period in which the recording of the image on the sheet P is interrupted owing to the second event. This configuration makes it possible to shorten a length of time required for the alignment or reduce the number of alignments of sheets P stacked on the output tray  11  after the interruption period. 
     In the printer  101  according to the present embodiment, when the maintenance mechanism  40  performs the maintenance on the recording heads  1 , the sheets P stacked on the output tray  11  are not aligned in the maintenance. This configuration can prevent that the vibrations generated during the alignment performed by the alignment mechanism  30  cause a failure of the printer  101  due to, e.g., a collision between the maintenance mechanism  40  and the ejection faces  2   a  of the respective recording heads  1 . 
     In the printer  101  according to the present embodiment, when the sheet sensor  15  has sensed the jam of the sheet P at the area opposite the ejection faces  2   a  of the respective recording heads  1 , the sheets P stacked on the output tray  11  are not aligned. This configuration can prevent that the vibrations generated during the alignment performed by the alignment mechanism  30  cause a collision between the jammed sheet P and the ejection faces  2   a.    
     In the printer  101  according to the present embodiment, the output tray  11  is provided above the height level of the recording heads  1 . Since the vibrations generated during the alignment of the sheets P stacked on the output tray  11  do not deteriorate the quality of the image recorded on the sheet P in the present invention, the printer  101  can be made compact by providing the output tray  11  above the recording heads  1  as in the above-described construction. 
     In the printer  101  according to the present embodiment, in the case where the image quality mode is the high image quality mode, the sheets P are not aligned when the conveyor mechanism  16  is conveying the sheet P printed by the recording heads  1  to the output tray  11 , and the sheets P are aligned when the conveyor mechanism  16  is not conveying the sheet P printed by the recording heads  1  to the output tray  11 . This configuration can prevent that an image-recorded side of the sheet P being conveyed by the conveyor mechanism  16  comes into contact with components of the conveyor mechanism  16  in the high image quality mode, resulting in deterioration of the quality of the image recorded on the sheet P. 
     In the printer  101  according to the present embodiment, the larger the number of sheets P stacked on the output tray  11  which is obtained by the number-of-stacked-sheets obtaining sensor  12  when the preceding alignment of the sheets P stacked on the output tray  11  is performed by the alignment mechanism  30 , the smaller the first predetermined number is determined. In other words, the sheets P are aligned at shorter intervals with increase in the number of sheets P stacked on the output tray  11 . Accordingly, even if the number of sheets P stacked on the output tray  11  has been increased, the sheets P can be aligned reliably. 
     In the printer  101  according to the present embodiment, the alignment mechanism  30  includes: the pair of limiting walls  31   a ,  31   b  disposed parallel to each other to constrain the opposite edge portions of the sheets P stacked on the output tray  11 ; and the limiting-wall displacement mechanism  33  that can move the limiting wall  31   a  with respect to the limiting wall  31   b  to change the distance between the pair of limiting walls  31   a ,  31   b . Accordingly, the limiting-wall displacement mechanism  33  can displace the limiting wall  31   a  with respect to the limiting wall  31   b  to align the opposite edge portions of the sheets P stacked on the output tray  11  in the widthwise direction. 
     While the embodiment of the present invention has been described above, it is to be understood that the invention is not limited to the details of the illustrated embodiment, but may be embodied with various changes and modifications, which may occur to those skilled in the art, without departing from the spirit and scope of the invention. 
     For example, the alignment mechanism  30  includes: the pair of limiting walls  31   a ,  31   b  disposed parallel to each other; and the limiting-wall displacement mechanism  33  that can move the limiting wall  31   a  with respect to the limiting wall  31   b  to change the distance between the pair of limiting walls  31   a ,  31   b  in the above-described embodiment, but the present invention is not limited to this configuration. That is, the limiting-wall displacement mechanism  33  may be configured to displace both of the limiting walls  31   a ,  31   b.    
       FIG. 7  illustrates a printer  201  as a modification of the above-described embodiment. This printer  201  includes a housing  201   a  having an output opening  222  formed in a side wall thereof. The side wall of the housing  101   a  is provided with an output tray  211  on which sheets P discharged from the output opening  222  by a conveyor roller pair  228  are stacked. The output tray  211  has a support face  211   a  that is inclined from a direction in which the sheets P are discharged from the output opening  222 , such that an end portion of the support face  211   a  nearer to the output opening  222  is located at a position lower in height than an end portion of the support face  211   a  farther from the output opening  222 . Thus, a side wall of the output tray  211  near the output opening  222  serves as an abutment member  211   b  with which one edges of the sheets P stacked on the output tray  211  come into contact. In the present modification, an alignment mechanism  230  configured to align the sheets P stacked on the output tray  211  is a vibrator for vibrating the output tray  211  such as a vibration motor. The alignment mechanism  230  vibrates the output tray  211  to apply an external force to the sheets P to align the one edges of the sheets P being in abutment against the abutment member  211   b.    
     In the above-described embodiment, when the first predetermined number of the sheets P are conveyed to the output tray  11  after the preceding alignment of the sheets P, the recording of the image on the sheet P is interrupted to align the sheets P stacked on the output tray  11 . In the case where the number of sheets P to be recorded after the preceding alignment of the sheets P by the alignment mechanism  30  in the period of the recording based on the one recording job is larger than the first predetermined number and smaller than the second predetermined number, the sheets P stacked on the output tray  11  are not aligned when the first predetermined number of the sheets P are conveyed to the output tray  11 , but the sheets P are aligned after the plurality of images have been recorded on the respective sheets P by the recording heads  1 . However, the present invention is not limited to this configuration. That is, even where the recording of the plurality of images on the respective sheets P is close to completion, the sheets P may or may not be aligned each time when the first predetermined number of the sheets P have been conveyed to the output tray  11 . 
     While the second predetermined number is the number obtained by adding five to the first predetermined number in the above-described embodiment, the present invention is not limited to this configuration. That is, the second predetermined number may be a number obtained by adding any number or constant other than five to the first predetermined number. For example, the second predetermined number may be the number obtained by adding ten percent of the first predetermined number to the first predetermined number. 
     In the above-described embodiment, the sheets P are aligned in the interruption period in which the recording of the image on the sheet P is interrupted owing to the second event such as the case where the printer  101  runs out of sheet or ink. However, the sheets P may not be aligned when the second event has occurred. 
     In the above-described embodiment, the sheets P are not aligned in the interruption period in which the recording of the image on the sheet P is interrupted owing to the first event such as the case where the maintenance on the recording heads  1  is required and the case where the sheet jam has occurred. However, the sheets P may be aligned when the first event has occurred. 
     While the output tray  11  is provided on the top of the housing  101   a  in the above-described embodiment, the present invention is not limited to this construction. For example, as in the printer  201  according to the modification illustrated in  FIG. 7 , the output tray  211  may be disposed on a side of the housing  201   a.    
     In the above-described embodiment, in the case where the image quality mode is the high image quality mode, the sheets P are not aligned when the recorded sheet P is being conveyed to the output tray  11 , and the sheets P are aligned when the recorded sheet P is not being conveyed to the output tray  11 . However, the present invention is not limited to this configuration. For example, the sheets P may be aligned at any appropriate timing independently of the image quality mode. 
     In the above-described embodiment, the larger the number of sheets P stacked on the output tray  11  upon the preceding alignment of the sheets P, the smaller the value of the first predetermined number is determined, but the present invention is not limited to this configuration. For example, the printer may be configured such that the first predetermined number is fixed, and the larger the number of sheets P stacked on the output tray  11 , the larger external force is applied to the sheets P by the alignment mechanism  30 . Also, the alignment mechanism  30  may be controlled independently of the number of sheets P stacked on the output tray  11 . 
     While the number of sheets P stacked on the output tray  11  are obtained by the optical sensor in the form of the number-of-stacked-sheets obtaining sensor  12  in the above-described embodiment, the present invention is not limited to this configuration. For example, the number of sheets P stacked on the output tray  11  may be obtained by a sensor for sensing a weight of the sheets P stacked on the output tray  11 . Also, the controller  100  may have software serving as a counter configured to count the cumulative total of the number of sheets P conveyed to the output tray  11  to obtain the number of sheets P stacked on the output tray  11 . 
     While the sheets P are aligned by the mechanical external force such as collision and vibration in the above-described embodiment, the alignment mechanism  30  may be controlled to align the stacked sheets P by applying an external force such as air and pressure to the stacked sheets P. 
     While the controller  100  is configured by the single CPU in the above-described embodiment, the controller  100  may be configured by a plurality of CPUs, an application-specific integrated circuit (ASIC), or a combination of the CPU(s) and the ASIC.