Patent Publication Number: US-7717529-B2

Title: Image recording apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application claims priority from Japanese Patent Application No. 2007-270965, which was filed on Oct. 18, 2007, 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 configured to eject liquid to record an image on a recording medium, and more particularly to an image recording apparatus equipped with an image sensor which picks up a recorded image. 
   2. Description of the Related Art 
   Recently, there has been developed an ink-jet printer using a line head having a recording range larger than a width of a recording sheet as a recording medium. In such a line printer, recording for each extremely narrow portion of the recording sheet extending in a direction in which the recording sheet is conveyed is carried out only by a corresponding one of nozzles. Thus, even where there are only a small number of nozzles in which an ink ejection failure occurs such as non-ejection of inks (liquids), variation of an ejected ink amount, and a deviation of a direction in which the inks are ejected, a white patch or line, or unevenness is formed or developed on the recording sheet, thereby deteriorating an image quality by a relatively large degree. 
   To solve this problem, in an ink-jet recording apparatus disclosed in Patent Document 1 (Japanese Patent Application Publication No. 2006-205742), image sensors of line type operable to read a test pattern recorded on a recording sheet are provided at respective positions each of which is adjacent to and located downstream of a corresponding one of recording heads in a direction in which the recording sheet is conveyed. Any nozzle in which an ink ejection failure occurs is detected on the basis of image data obtained by the image sensors of line type, and a prescribed recovering operation is performed for the nozzle. By a method like this, the nozzle in which the ink ejection failure occurs can be precisely detected to recover such nozzle. 
   SUMMARY OF THE INVENTION 
   In the ink-jet recording apparatus disclosed in Patent Document 1, lower surfaces of the respective image sensors are located below a height level of ejection surfaces of the respective recording heads. That is, the lower surfaces of the respective image sensors are located nearer to a belt for conveying the recording sheet than to the ejection surfaces of the respective recording heads. Thus, in both of a test pattern recording and a normal recording, ink mist tends to adhere to the lower surfaces (light-entering surfaces) of the respective image sensors. That is, the lower surfaces of the respective image sensors tend to be soiled. In addition, where the recording sheet is brought into contact with the lower surfaces of the respective image sensors due to jamming just after the inks have attached to the recording sheet, undried ink on the recording sheet unfortunately adheres to the lower surfaces of the respective image sensors. Where the inks have adhered to the lower surfaces of the respective image sensors, reliability of image data obtained by reading is lowered, so that unnecessary recovering operation is performed. 
   This invention has been developed in view of the above-described situations, and it is an object of the present invention to provide an image recording apparatus in which a lower surface of an image sensor that picks up an image recorded by recording heads is less soiled. 
   The object indicated above may be achieved according to the present invention which provides an image recording apparatus comprising: a medium-convey belt which conveys a recording medium; a recording head having an ejection surface in which a plurality of ejection openings for ejecting liquid are provided and which faces a surface of the medium-convey belt; an image sensor being integral with the recording head and being movable together with the recording head, and including an image-pickup element, a light-entering surface which faces the surface of the medium-convey belt, and which is more distant from the surface of the medium-convey belt than the ejection surface of the recording head, and an image-forming optical system which guides, to the image-pickup element, a light entered through the light-entering surface and which is for forming, on the image-pickup element, an optical image of an object, a raising and lowering mechanism which positions the recording head at a selected one of a plurality of heights including a first height and a second height, wherein the first height is a height at which the ejection surface is distant from the surface of the medium-convey belt, and the optical image of an image, as the object, recorded on the surface of the medium-convey belt or recorded on the recording medium on the medium-convey belt is formed on the image-pickup element by the image-forming optical system, and wherein the second height is a height at which the ejection surface is more distant from the surface of the medium-convey belt than at the first height; and a controller configured to execute controls for operations of the image recording apparatus, wherein the controller includes: a mode indicating section configured to indicate one of a normal recording mode and an inspecting mode, wherein in the normal recording mode, a desired image is recorded on the recording medium conveyed on the medium-convey belt while in the inspecting mode, the recording head is inspected; a raising and lowering controlling section configured to control the raising and lowering mechanism such that the recording head is positioned at the second height when the mode indicating section indicates the normal recording mode, and such that the recording head is positioned at the first height when the mode indicating section indicates the inspecting mode; a head controlling section configured to control the recording head such that the recording head ejects the liquid; and an image-pickup controlling section configured to control the image sensor such that the image sensor picks up the image recorded on the medium-convey belt or recorded on the recording medium on the medium-convey belt when the recording head is positioned at the first height. 
   In the image recording apparatus constructed as described above, since the image sensor is moved upward and downward together with the recording head in a state in which the image sensor is more distant from the surface of the medium-convey belt than the recording head, the light-entering surface of the image sensor is less soiled. Further, when the liquid is ejected as a test in the inspecting mode, since the distance between the ejection surface and the medium-convey belt or the recording medium on the medium-convey belt is shorter than that in the normal recording mode, an ejected-liquid-attaching accuracy upon recording can be improved. In addition, the light-entering surface of the image sensor can be less soiled due to flying liquid mist. Furthermore, since the ejected-liquid-attaching accuracy upon the recording can be improved, a liquid ejection failure such as a non-ejection of the liquid from any of the ejection openings can be detected with a relatively high accuracy. 

   
     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 a preferred embodiment of the invention, when considered in connection with the accompanying drawings, in which: 
       FIG. 1  is a side view of an ink-jet printer that is an image recording apparatus as an embodiment of the present invention; 
       FIG. 2  is a plan view partially showing the ink-jet printer; 
       FIG. 3  is a cross-sectional view taken along a line III-III in  FIG. 2 ; 
       FIG. 4  is a bottom view showing four ink-jet heads of the ink-jet printer; 
       FIG. 5  is a cross-sectional view schematically showing an image sensor of the ink-jet printer; 
       FIG. 6  is a block diagram showing the ink-jet printer; 
       FIGS. 7A and 7B  are side views each schematically showing one of the ink-jet heads and the image sensor; 
       FIG. 8  is a flow-chart showing a processing of a controller of the ink-jet printer; and 
       FIGS. 9A and 9B  are side views each for explaining a maintenance operation of the ink-jet printer. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Hereinafter, there will be described a preferred embodiment of the present invention by reference to the drawings. It is to be understood that the following embodiment is described only by way of example, and the invention may be otherwise embodied with various modifications without departing from the scope and spirit of the invention. 
   General Structure of Ink-Jet Printer 
     FIG. 1  shows an ink-jet printer  1  that is an image recording apparatus as an embodiment of the present invention. The ink-jet printer  1  is a color ink-jet printer having ink-jet heads  2  respectively ejecting inks (liquids) of four colors, namely, magenta, cyan, yellow, and black. The ink-jet printer  1  is provided with a sheet-supply mechanism  11  located at a left portion of  FIG. 1  and a sheet-discharge portion  12  located at a right portion of  FIG. 1 . 
   In the ink-jet printer  1 , there is formed a sheet-convey path through which a recording sheet as a recording medium is conveyed from the sheet-supply mechanism  11  toward the sheet-discharge portion  12 . The sheet-supply mechanism  11  is provided with a pickup roller  22 . The pickup roller  22  is rotated by driving of a pickup motor  132  (shown in  FIG. 6 ), whereby an uppermost one of the recording sheets in a sheet tray  21  is supplied and conveyed from the left side of  FIG. 1  toward the right side of  FIG. 1 . At a middle portion of the sheet-convey path, a sheet-convey mechanism is disposed. The sheet-convey mechanism includes two belt rollers  6 ,  7  and an endless sheet-convey belt  8  wound around the belt rollers  6 ,  7  to bridge the belt rollers  6 ,  7 . The endless sheet-convey belt  8  is for conveying the recording sheet. 
   An outer peripheral surface  8   a  of the sheet-convey belt  8  is subjected to a silicone processing to have a viscosity. Just on a downstream side of the sheet-supply mechanism  11  in a sheet-convey direction B in which the recording sheet is conveyed, a sheet-press roller  5  is disposed at a position facing the sheet-convey belt  8 . The sheet-press roller  5  presses, toward the outer peripheral surface  8   a  of the sheet-convey belt  8 , the recording sheet supplied from the sheet-supply mechanism  11 . Thus, the recording sheet pressed toward the outer peripheral surface  8   a  is conveyed in the sheet-convey direction B while being held by the outer peripheral surface  8   a  owing to the viscosity thereof. In this conveying of the recording sheet, a drive power of a sheet-convey motor  133  (shown in  FIG. 6 ) is applied to the belt roller  6  located downstream of the belt roller  7  in the sheet-convey direction B, whereby the belt roller  6  is rotated in a clockwise direction in  FIG. 1  (i.e., a direction indicated by arrow A). 
   A peeling plate  13  is provided just on a downstream side of the sheet-convey belt  8  in the sheet-convey direction B along the sheet-convey path. The peeling plate  13  peels, from the outer peripheral surface  8   a , the recording sheet held by the outer peripheral surface  8   a . Then, the recording sheet is conveyed on the peeling plate  13  toward the sheet-discharge portion  12  located on a right side of the peeling plate  13 . 
   A platen  9  having a generally rectangular parallelepiped shape is disposed in an area surrounded by the sheet-convey belt  8 . The platen  9  is held in contact with an inner surface of an upper portion of the sheet-convey belt  8 , which upper portion faces the ink-jet heads  2 , whereby the platen  9  supports the sheet-convey belt  8  from an inner side thereof. Thus, as shown in  FIG. 1 , there is formed a space defined between an outer surface of the upper portion of the sheet-convey belt  8  and lower surfaces of the ink-jet heads  2 . 
   At a position located on an obliquely right and lower side of the belt roller  6 , there is disposed an absorber  151  which always contacts with the outer peripheral surface  8   a  of the sheet-convey belt  8 . The absorber  151  has a rectangular parallelepiped shape whose longitudinal direction coincides with a direction perpendicular to a sheet surface of  FIG. 1 . A length of the absorber  151  in the longitudinal direction thereof is substantially the same as a width of the sheet-convey belt  8 . The absorber  151  is connected to a tank (not shown) via an opening and closing valve (not shown). When the opening and closing valve is opened by a command of a controller  101  which will be described below, cleaning liquid in the tank is supplied to the absorber  151 . On the other hand, when the opening and closing valve is closed by the command of the controller  101 , the supply of the cleaning liquid to the absorber  151  is stopped. In this way, the absorber  151  absorbs the cleaning liquid and applies the absorbed cleaning liquid to the outer peripheral surface  8   a.    
   At a position located on a downstream side of the absorber  151  in a direction in which the sheet-convey belt  8  is circulated (that is, on an upstream side of the belt roller  6  and a downstream side of the belt roller  7  which is located just on an upstream side of the ink-jet heads  2 ) and on an obliquely left and lower side of the belt roller  6  in  FIG. 1 , there is disposed a blade  152  such that a distal end of the blade  152  always contacts with the outer peripheral surface  8   a . The blade  152  scrapes or wipes, by the distal end thereof, the cleaning liquid and the inks adhering to the outer peripheral surface  8   a . The cleaning liquid and the like scraped by the blade  152  are discarded into a discarding portion  153  provided under the blade  152 . In this way, the absorber  151 , the blade  152 , and the discarding portion  153  constitutes a cleaning device  154  for cleaning the outer peripheral surface  8   a  of the sheet-convey belt  8 . 
   As shown in  FIG. 2 , each of the four ink-jet heads  2  extends in a main scanning direction which is perpendicular to the sheet surface of  FIG. 1 . The ink-jet heads  2  are arranged in a sub-scanning direction which is perpendicular to the main scanning direction and coincides with the sheet-convey direction B. That is, the ink-jet printer  1  is of a line-type. As shown in  FIGS. 1 and 3 , each of the ink-jet heads  2  has, at a lower end thereof, a corresponding one of head bodies  3 . Each of the head bodies  3  includes an ink channel unit and an actuator fixed to each other. The ink channel unit includes a plurality of individual ink channels including pressure chambers, and the actuator applies the inks in the pressure chambers. Each head body has a rectangular parallelepiped shape elongated in the main scanning direction. In a lower surface of each head body  3 , that is, an ejection surface  3   a  of each head body  3 , there are provided and arranged a large number of ejection openings  3   b  (shown in  FIG. 4 ) which are fine openings each for ejecting the inks and each as one end of a corresponding one of nozzles. 
   As shown in  FIGS. 1 and 3 , reservoir units  10  for temporarily storing the respective inks are fixed to respective upper surfaces of the head bodies  3 . Each of the reservoir units  10  is longer than a corresponding one of the head bodies  3  and projected from opposite ends of the corresponding head body  3  in a longitudinal direction thereof. The reservoir units  10  are fixed, at the projected portions thereof, to a frame  4  having a rectangular opening such that the ejection surfaces  3   a  can be seen through the opening from a lower side of the ejection surfaces  3   a . More specifically, a pair of flanges  4   a  each of which supports a corresponding one of opposite ends of the reservoir units  10  from lower sides thereof are projected from respective opposite end portions of the frame  4  toward a middle portion of the opening in a longitudinal direction thereof. The flanges  4   a  and longitudinal end portions of each reservoir unit  10  are fixed to each other by screws  50 . The ejection surfaces  3   a  are the same in height as a bottom surface of the frame  4 . 
   The head bodies  3  are disposed such that the ejection surfaces  3   a  face and are parallel to a portion of the sheet-convey belt  8  which is supported by the platen  9  and such that the small space or clearance is formed between the ejection surface  3   a  and the sheet-convey belt  8 . The small space constitutes a part of the sheet-convey path. When the recording sheet is conveyed on the outer peripheral surface  8   a  just under the four head bodies  3  in order, the inks of the four colors are ejected toward an upper surface or a print surface of the conveyed recording sheet, whereby a desired color image is recorded on the recording sheet. 
   In this ink-jet printer  1 , an image sensor  201  is attached to a downstream side face of one of the ink-jet heads  2  which is located at the most downstream position. A detailed explanation of the image sensor  201  will be described below. 
   Raising and Lowering Mechanisms 
   As shown in  FIGS. 2 and 3 , the frame  4  is supported by a pair of raising and lowering mechanisms  51  provided in the ink-jet printer  1 , so as to be movable in a vertical direction. The pair of raising and lowering mechanisms  51  are disposed on both sides of the four ink-jet heads  2  in the sub-scanning direction. Each of the raising and lowering mechanisms  51  includes a head motor  52  as a drive source for moving the frame  4  in the vertical direction, a pinion gear  53  fixed to a shaft of the head motors  52 , a rack gear  54  meshed with the pinion gear  53 , and a guide  56  disposed at a position at which the rack gear  54  is interposed between the pinion gears  53  and the guide  56 . As will be described below, the raising and lowering mechanisms  51  are driven on the basis of a control of the controller  101 . 
   The head motors  52  included in the respective raising and lowering mechanisms  51  are respectively fixed to a pair of body frames  1   a  of the ink-jet printer  1 . The pair of body frames  1   a  are disposed so as to face each other in the sub-scanning direction. The rack gears  54  extend in the vertical direction and are fixed, at lower ends thereof, to respective opposed side faces of the frame  4 . Each of the rack gears  54  slidably contacts with a corresponding one of the guides  56  at one of opposed side faces of each rack gear  54 , which one is opposite to the other that is mashed with a corresponding one of the pinion gear  53 . The guides  56  are fixed to the respective body frames  1   a.    
   When the pinion gears  53  are rotated forwardly or reversely with the two head motors  52  synchronized with each other, the rack gears  54  are moved upward or downward. With the movement of the rack gears  54 , the frame  4  is moved upward and downward in the vertical direction together with the four ink-jet heads  2 . 
   A pair of guide units  59  are disposed on respective opposed side faces of the frame  4  which extend in the sub-scanning direction. Each of the guide units  59  includes a bar  58  and a pair of guides  57  between which the bar  58  is interposed. As shown in  FIG. 3 , the two pairs of guides  57  extend in the vertical direction, and each of the pairs is fixed to a corresponding one of a pair of body frames  1   b  of the ink-jet printer  1 . The pair of body frames  1   b  are disposed so as to face each other in the main scanning direction. The bars  58  extend in the vertical direction like the guides  57  and are respectively fixed to the side faces of the frame  4  which respectively face the body frames  1   b . Each of the bar  58  is slidably contacted with a corresponding pair of the guides  57 . 
   The guide units  59  prevent the ejection surfaces  3   a  from inclining with respect to the portion of the sheet-convey belt  8  which is supported by the platen  9 , when the frame  4  is move upward and downward in the vertical direction by the raising and lowering mechanisms  51 . That is, even when the frame  4  and the ink-jet heads  2  are moved upward and downward in the vertical direction by the raising and lowering mechanisms  51 , the ejection surfaces  3   a  are always parallel to an upper surface of the platen  9 . As a result, an ejected-ink-attaching accuracy upon recording can be improved. 
   Normally, the frame  4  is disposed at a recording position (where the frame  4  is located in  FIG. 3 ) at which the ink-jet heads  2  eject the inks onto the recording sheet for the recording, by being moved by the raising and lowering mechanisms  51  in a direction indicated by arrow C in  FIG. 3 . Only when the ink-jet heads  2  are subjected to a maintenance operation such as a purging operation in which the inks are forced to be ejected from the ink-jet heads  2 , a wiping operation in which the inks adhering to the ejection surfaces  3   a  are wiped, and a covering operation in which the ejection surfaces  3   a  are covered or capped by caps, the frame  4  is moved in the direction indicated by the arrow C in  FIG. 3  by the raising and lowering mechanisms  51 , so as to be disposed at a maintenance position at which the four ink-jet heads  2  are above the height level of the recording position. 
   In view of the above, the raising and lowering mechanisms  51  are for positioning the ink-jet heads  2  at a selected one of a plurality of heights. 
   Image Sensor 
   Here, there will be explained, with reference to  FIG. 4 , a positional relationship among the ink-jet heads  2 , the image sensor  201 , and the recording sheet. As shown in  FIG. 4 , the plurality of ejection openings  3   b  which are the fine openings each as the one end of the nozzle are formed in the ejection surfaces  3   a . In the ejection surfaces  3   a , there are formed a plurality of nozzle rows (ejection openings rows)  3   c  each of which is constituted by ones of the ejection openings  3   b  that are arranged so as to be equally spaced from each other in the main scanning direction. The plurality of nozzle rows  3   c  are parallel to each other. Any of the ejection openings  3   b  formed in each of the ink-jet heads  2  is not located at the same positions as the other of the ejection openings  3   b  in the main scanning direction. The ejection openings  3   b  formed in each ink-jet head  2  are arranged so as to be equally spaced from each other in the main scanning direction. 
   The image sensor  201  is attached to the downstream side face of the one of the ink-jet heads  2  which is located at the most downstream position in the sub-scanning direction. That is, the image sensor  201  is integral with the one of the ink-jet heads  2 . Thus, the image sensor  201  is movable by the driving of the raising and lowering mechanisms  51  together with the four ink-jet heads  2  and the frame  4 . The image sensor  201  is disposed so as to be corresponded to a space formed between the frame  4  and the one of the ink-jet heads  2  which is located at the most downstream position in the sub-scanning direction. Thus, a lower surface of the image sensor  201  faces the outer peripheral surface  8   a  of the sheet-convey belt  8  without being intercepted by the frame  4 . 
   The image sensor  201  has an image-pickup range in which the image sensor can pick up an image. The image-pickup range is equal to or slightly larger than, in the main scanning direction, an image-recordable range of the ink-jet heads  2  in which the ink-jet heads  2  can record the image on the recording sheet. That is, the image sensor  201  is of a line type. In contrast, a width of a recording sheet P conveyed through the sheet-convey path (i.e., a length of the recording sheet P in the main scanning direction) is somewhat smaller than the image-recordable range of the ink-jet heads  2 . Accordingly, even when the recording sheet P is conveyed in a state in which the recording sheet P is slightly shifted in a widthwise direction thereof, the image sensor  201  can pick up the image recorded on the recording sheet P in its full width. As will be described below, the image sensor  201  is used for picking up a test image recorded on the outer peripheral surface  8   a  (or on the recording sheet). 
   As shown in  FIG. 5 , the image sensor  201  includes an image-pickup element  202 , an image-forming optical system  203 , an LED (a light-emitting diode)  204 , and a light guide  205 . The image-pickup element  202  is provided by a CCD or a CMOS, for example. The image-forming optical system  203  is provided between the image-pickup element  202  and a light-entering surface  201   a  as a lower surface of the image sensor  201  which faces the outer peripheral surface  8   a  of the sheet-convey belt  8 . The image-forming optical system  203  guides, to the image-pickup element  202 , a light entered through the light-entering surface  201   a  and is for forming, on the image-pickup element  202 , an optical image of an object (in this ink-jet printer  1 , the image recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8 ). The LED  204  is a source of light with which the object is irradiated. The light guide  205  guides the light emitted from the LED  204  such that the light is emitted from the light-entering surface  201   a  toward the object. Each of the image-pickup element  202 , the image-forming optical system  203 , the LED  204 , and the light guide  205  extends in the main scanning direction. 
   Structure of Maintenance Unit 
   There will be explained, with reference to  FIGS. 2 and 3 , a maintenance unit  70  for performing the maintenance operation for the ink-jet heads  2 . As shown in the figures, the maintenance unit  70  is disposed in a left side of the ink-jet heads  2 . The maintenance unit  70  includes two trays  71 ,  75  movable horizontally. The tray  71  has a generally square box-like shape having an opening opened upward. The tray  71  encloses the tray  75 . In other words, the tray  75  is in the tray  71 . The tray  71  and the tray  75  are connectable to and disconnectable from each other. A state in which the tray  71  and the tray  75  are connected to each other and a state in which the tray  71  and the tray  75  are disconnected from each other are changeable on the basis of whether recessed portions  74   b  and projecting portions  83   a  which will be described below are engaged with each other or disengaged from each other. 
   When the maintenance unit  70  is horizontally moved toward a right side thereof, the frame  4  is moved upward to the maintenance position in advance in the direction indicated by the arrow C in  FIG. 3 , so that the space for the maintenance unit  70  is assured between the four ejection surfaces  3   a  and the sheet-convey belt  8 . Thereafter, the maintenance unit  70  is horizontally moved in a direction indicated by arrow D in  FIG. 3 . One of side faces of the tray  71  which is further from the ink-jet heads  2  is opened, so that when the recessed portions  74   b  and the projecting portions  83   a  are disengaged from each other (in the purging operation, for example), only the tray  71  is horizontally moved rightward with the enclosed tray  75  remaining at its original position. 
   A waste-ink receiving tray  77  is disposed just below the maintenance unit  70 . The waste-ink receiving tray  77  has a size enclosing the tray  71  in plan view. Even when the tray  71  is moved to an right end of the ink-jet printer  1  in  FIG. 2 , the waste-ink receiving tray  77  overlaps with one of opposite end portions of the tray  71 , which one is located on an opposite side of the ink-jet heads  2 . An ink-discharge opening  77   a  is formed in one of opposite end portions of the waste-ink receiving tray  77  which one is nearer to the ink-jet heads  2 . The ink-discharge opening  77   a  guides or discharges, to a waste-ink accumulating portion (not shown), the ink flown onto the waste-ink receiving tray  77 . 
   A wiper  72  and the tray  75  are disposed in the tray  71  with the wiper  72  located nearer to the ink-jet heads  2  than the tray  75 . In the tray  75 , four caps  76  each having a rectangular shape in plan view are arranged side by side in correspondence with the respective ink-jet heads  2 . A longitudinal direction of the caps  76  is parallel to a longitudinal direction of the ink-jet heads  2 . The caps  76  are disposed in the sub-scanning direction with pitches which are the same as pitches with which the ink-jet heads  2  are disposed in the sub-scanning direction. 
   Each of the caps  76  includes a plate-like member  76   b  and a circular projection  76   a . Each plate-like member  76   b  has, in plan view, a rectangular shape which is substantially the same size as a corresponding one of the ejection surfaces  3   a , and each circular projection  76   a  projects upward from a peripheral portion of a corresponding one of the plate-like members  76   b . Each circular projection  76   a  is formed of an elastic material such as rubber and has a size and a shape in which each circular projection  76   a  faces a peripheral portion of a corresponding one of the ejection surfaces  3   a . Each cap  76  defines an air-tight space when the corresponding circular projection  76   a  contacts with the peripheral portion of the corresponding ejection surface  3   a . In this manner, each cap  76  can cover the corresponding ejection surface  3   a . The caps  76  are forced upward by two springs (not shown) while being supported by a bottom surface of the tray  75 . 
   To the tray  71 , there are fixed, in addition to the wiper  72 , a holding member  74  on which a mount member  78  is disposed. The holding member  74  has a three-sided rectangular shape in plan view. A groove  74   a  extending in the sub-scanning direction is formed in an upper surface of a portion of the holding member  74  which extends in the sub-scanning direction. In the groove  74   a , the mount member  78  and the wiper  72  are disposed. The above-described recessed portions  74   b  are respectively formed in respective upper surfaces of two portions of the holding member  74  which extend in the main scanning direction. 
   The wiper  72  is formed of an elastic material such as rubber. The wiper  72  has a length slightly larger than an entire width of the four ink-jet heads  2  in the sub-scanning direction and is fixed to the mount member  78  such that a longitudinal direction of the wiper  72  coincides with the sub-scanning direction. The mount member  78  has a rectangular parallelepiped shape which extends in the sub-scanning direction like the wiper  72 . 
   The recessed portions  74   b  and hooking members  83  are respectively provided near opposite ends of the trays  71 ,  75 . Each of the hooking members  83  extends in the main scanning direction and is pivotable about a central portion thereof. Each of the projecting portions  83   a  is provided on one of opposite end portions of a corresponding one of the hooking members  83 , which one is nearer to the ink-jet heads  2 . When the hooking members  83  are pivoted in a clockwise direction in  FIG. 3 , the projecting portions  83   a  are respectively engaged with the recessed portions  74   b . Above the maintenance unit  70 , contacting members  84  are disposed in correspondence with the respective two hooking members  83 . The state in which the tray  71  and the tray  75  are connected to each other and the state in which the tray  71  and the tray  75  are disconnected from each other are changeable on the basis of whether recessed portions  74   b  and projecting portions  83   a  which will be described below are engaged with each other or disengaged from each other. 
   The contacting members  84  are supported so as to be pivotable. When each of the contacting members  84  is pivoted in the clockwise direction in  FIG. 3 , one of opposite ends of each contacting member  84  is brought into contact with the other end portion  83   b  of a corresponding one of the hooking members  83 . When the contacting members  84  are further pivoted in the clockwise direction in  FIG. 3 , the hooking members  83  are pivoted in a counterclockwise direction in  FIG. 3 , whereby the projecting portions  83   a  and the respective recessed portions  74   b  are disengaged from each other. On the other hand, when the contacting members  84  are pivoted in the counterclockwise direction in  FIG. 3  to move away from the other end portions  83   b , the projecting portions  83   a  and the respective recessed portions  74   b  are engaged with each other by own weights of the respective hooking members  83 . 
   When the maintenance operation of the ink-jet heads  2  is not performed, the maintenance unit  70  is, as show in  FIGS. 2 and 3 , at rest at a retracted position at which the maintenance unit  70  does not face the ink-jet heads  2 . When the maintenance operation is performed, the maintenance unit  70  is horizontally moved from the retracted position to a maintenance position at which the maintenance unit  70  faces the ejection surfaces  3   a  of the respective ink-jet heads  2 . In this movement, distal ends of the wiper  72  and the caps  76  are not brought into contact with the ejection surfaces  3   a  because the frame  4  is disposed at the maintenance position. 
   It is noted that even when the maintenance operation is performed, only the tray  71  is moved, in the purging operation, from the retracted position to the maintenance position to receive the discharged inks, with the tray  75  remaining at its original position. When the ejection surfaces  3   a  are covered by the respective caps  76 , the tray  71  and the tray  75  are connected to each other by the engagement of the recessed portions  74   b  and the respective projecting portions  83   a , thereby moving the maintenance unit  70  to a position at which the caps  76  and the ejection surface  3   a  are respectively face each other. 
   The trays  71 ,  75  are slidably supported by a pair of guide shafts  96   a ,  96   b  extending in the main scanning direction. Two bearing members  97   a ,  97   b  are provided on the tray  71 . The bearing members  97   a ,  97   b  are projected from respective opposed outside faces of the holding member  74 . Two bearing members  98   a ,  98   b  are provided on the tray  75 . The bearing members  98   a ,  98   b  are projected from respective opposite side faces of the tray  75 . Each of the pair of guide shafts  96   a ,  96   b  is fixed, at opposite ends thereof, to the body frames  1   b ,  1   d . The pair of guide shafts  96   a ,  96   b  are disposed between the body frames  1   b ,  1   d  so as to be parallel to each other. 
   Here, there will be explained a horizontally moving mechanism  91  for horizontally moving the trays  71 ,  75  along the guide shafts  96   a ,  96   b  in the direction indicated by the arrow D. As shown in  FIG. 2 , the horizontally moving mechanism  91  includes a tray motor  92 , a motor pulley  93 , an idle pulley  94 , a timing belt  95 , the guide shafts  96   a ,  96   b , and so on. 
   The tray motor  92  is fixed to a mount portion  1   c  formed at one of end portions of the body frames  1   b  extending in the sub-scanning direction. The motor pulley  93  is connected to the tray motor  92 , and rotated in accordance with driving of the tray motor  92 . The idle pulley  94  is rotatably supported by a body frame  1   d  located at the most left side of the ink-jet printer  1  in  FIG. 2 . The timing belt  95  is disposed so as to be parallel to the guide shaft  96   a  and wound around the motor pulley  93  and the idle pulley  94  to bridge the motor pulley  93  and the idle pulley  94 . The timing belt  95  is connected to the bearing member  97   a  provided on the holding member  74 . 
   In this construction, when the tray motor  92  is driven, the timing belt  95  is moved in accordance with forward and reverse rotations of the motor pulley  93 . By the movement of the timing belt  95 , the tray  71  connected to the timing belt  95  via the bearing member  97   a  is horizontally moved. In a state in which the recessed portions  74   b  and the projecting portions  83   a  are respectively engaged with each other, the wiper  72  in the tray  71  and the caps  76  in the tray  75  are moved together with each other. On the other hand, in a state in which the projecting portions  83   a  and the recessed portions  74   b  are not engaged with each other, only the wiper  72  in the tray  71  is moved. 
   Controlling System 
   There will be next explained a controlling system of the ink-jet printer  1  with reference to  FIG. 6 . The ink-jet printer  1  includes the controller  101  configured to execute controls for operations of the ink-jet printer  1 . The controller  101  includes a Central Processing Unit (CPU), a Read Only Memory (ROM), and a Random Access Memory (RAM). The CPU functions as an arithmetic processing unit, the ROM stores controlling programs executed by the CPU and data used for the controlling programs, and the RAM is for temporarily storing data when the programs are executed. The CPU, the ROM, and the RAM function as a mode storing section  111 , a head controlling section  112 , a sheet-convey controlling section  113 , a maintenance controlling section  114 , a raising and lowering controlling section  115 , an image-pickup controlling section  116 , a sheet-thickness storing section  117 , a focus judging section  118 , an image analyzing section  119 , and so on. 
   The mode storing section  111  stores whether the ink-jet printer  1  is in a normal recording mode or an inspecting mode, and indicates, as a mode indicating section, one of the normal recording mode and the inspecting mode. Here, the normal recording mode is a mode in which a desired image is recorded on the recording sheet, and the inspecting mode is a mode which is for inspecting the ink-jet heads  2  and in which the test image is recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8 . A content stored in the mode storing section  111  is changed by a signal transmitted from a personal computer (PC)  100  on the basis of an operation of a user. As a modification, there may be provided, instead of the mode storing section  111 , an analog circuit in which a level of an outputted signal varies in accordance with whether the ink-jet printer  1  is in the normal recording mode or the inspecting mode. In short, a means or the like which indicates whether the ink-jet printer  1  is in the normal recording mode or the inspecting mode needs only to be provided. 
   When the ink-jet printer  1  is in the normal recording mode, the head controlling section  112  controls a head driving circuit  121  such that the inks are ejected, on the basis of a recording data received from the PC  100 , by one or ones of the ink-jet heads  2  which is or are corresponded to the received recording data. When the ink-jet printer  1  is in the inspecting mode, the head controlling section  112  controls the head driving circuit  121  such that the test image is recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8 . Here, image data relating to the test image may be data that is received from the PC  100 , or may be data that is stored in the ROM of the ink-jet printer  1 . 
   When the ink-jet printer  1  is in the normal recording mode, the sheet-convey controlling section  113  controls a motor driver  122  such that the uppermost one of the recording sheets in the sheet tray  21  is conveyed onto the sheet-convey belt  8  by rotating of the pickup roller  22  which is caused by driving of the pickup motor  132 , and controls a motor driver  123  such that the recording sheet is conveyed while being held on the outer peripheral surface  8   a  by the rotating of the belt roller  6  which is caused by driving of the sheet-convey motor  133 . Further, the sheet-convey controlling section  113  controls the motor driver  123  such that the rotation of the belt roller  6  is stopped by stopping the driving of the sheet-convey motor  133  after the recording sheet on the sheet-convey belt  8  has reached the sheet-discharge portion  12 . Furthermore, the sheet-convey controlling section  113  controls the motor driver  123  which drives the sheet-convey motor  133 , such that, when the ink-jet printer  1  is in the inspecting mode, the belt roller  6  is rotated in a state in which the recording sheet is not held on the outer peripheral surface  8   a.    
   The maintenance controlling section  114  controls the maintenance unit  70 . Specifically, the maintenance controlling section  114  controls a motor driver  127  such that the tray  71  is horizontally moved from the retracted position to the maintenance position by the driving of the tray motor  92  when the inks are initially introduced into the ink-jet heads  2 , and when the purging operation is performed. Further, the maintenance controlling section  114  controls the motor driver  127  such that the tray  71  is horizontally moved from the maintenance position to the retracted position by the driving of the tray motor  92  when the purging operation is finished. Furthermore, the maintenance controlling section  114  controls the motor driver  127  such that the trays  71 ,  75  are horizontally moved from the retracted positions to their capping positions by the driving of the tray motor  92  when the ejection surfaces  3   a  are covered or capped. Furthermore, the maintenance controlling section  114  controls the motor driver  127  such that the trays  71 ,  75  are horizontally moved from the capping positions to the retracted positions by the driving of the tray motor  92  when the recording data has been received from the PC  100 . Furthermore, the maintenance controlling section  114  controls a pump driver (not shown) such that the inks in respective ink cartridges (not shown) are forced to be respectively sent to the head bodies  3  by pumps (not shown) when the inks are initially introduced into the ink-jet heads  2 , and when the purging operation is performed. 
   The raising and lowering controlling section  115  changes a height of the frame  4  and the four ink-jet heads  2  by controlling the raising and lowering mechanisms  51 . Specifically, the raising and lowering controlling section  115  controls a motor driver  125  which is a portion of the raising and lowering mechanisms  51  and which drives the head motors  52  shown in  FIG. 2 . In the normal recording mode, the raising and lowering controlling section  115  controls the motor driver  125  such that the frame  4  is located at the maintenance position when the inks are initially introduced into the ink-jet heads  2 , and when the maintenance operation of the ink-jet heads  2  is performed. Further, in the normal recording mode, the raising and lowering controlling section  115  controls the motor driver  125  such that the frame  4  is located at the recording position when the image is recorded on the recording sheet. Furthermore, in the inspecting mode, the raising and lowering controlling section  115  controls the motor driver  125  such that the frame  4  is located at an inspecting position which is lower in height than the recording position. 
     FIG. 7A  is a side view schematically showing the image sensor  201  and the one of the ink-jet heads  2  which is located at the most downstream position, when the frame  4  is located at the inspecting position. As shown in  FIG. 7A , a distance from the outer peripheral surface  8   a  of the sheet-convey belt  8  to the ejection surfaces  3   a  of the respective ink-jet heads  2  is defined as a distance H 1  (hereinafter, may be referred to as a first height of the ink-jet head  2 ). The distance H 1  is predetermined such that an optical image of the test image recorded on the outer peripheral surface  8   a  is formed on the image-pickup element  202  by the image-forming optical system  203  of the image sensor  201 . That is, the distance H 1  is predetermined such that an image based on image data obtained when the image sensor  201  picks up the test image recorded on the outer peripheral surface  8   a  is a focused image. In other words, the first height is a height at which the ejection surfaces  3   a  are distant from the outer peripheral surface  8   a , and the optical image of the test image recorded on the outer peripheral surface  8   a  is formed on the image-pickup element  202  by the image-forming optical system  203 . 
     FIG. 7B  is a side view schematically showing the image sensor  201  and the one of the ink-jet heads  2  which is located at the most downstream position, when the frame  4  is located at the recording position. As shown in  FIG. 7B , a distance from the outer peripheral surface  8   a  of the sheet-convey belt  8  to the ejection surfaces  3   a  of the respective ink-jet heads  2  is defined as a distance H 2  (hereinafter, may be referred to as a second height of the ink-jet head  2 ). A distance from the ejection surface  3   a  to the light-entering surface  201   a  is defined as a distance H 3 . The distance H 2  is longer than the distance H 1 . In other words, the second height is a height at which the ejection surfaces  3   a  are more distant from the outer peripheral surface  8   a  than at the first height. The optical image of the test image recorded on the outer peripheral surface  8   a  is not formed on the image-pickup element  202  by the image-forming optical system  203  of the image sensor  201 . That is, a focal length of the image-forming optical system  203  is shorter than the sum of the distance H 2  and the distance H 3  (the distance H 2 +the distance H 3 ), and thus the sum of the distance H 2  and the distance H 3  is deviated from a range of a focal depth of the image-forming optical system  203 . 
   In view of the above, the raising and lowering controlling section  115  is configured to control the raising and lowering mechanisms  51  such that the ink-jet heads  2  are positioned at the second height when the ink-jet printer  1  is in the normal recording mode, that is, the mode storing section indicates the normal recording mode, and such that the ink-jet heads  2  are positioned at the first height when the ink-jet printer  1  is in the inspecting mode, that is, the mode storing section indicates the inspecting mode. 
   In this ink-jet printer  1 , the raising and lowering controlling section  115  further makes fine adjustments of the first height and the second height, the detail explanation of which will be described later. 
   The image-pickup controlling section  116  controls the image sensor  201  such that the image sensor  201  picks up the test image recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8  when the ink-jet heads  2  are located at the first height. Image data obtained by this image-pickup operation of the image sensor  201  is analyzed by the image analyzing section  119 . The test image recorded on the outer peripheral surface  8   a  is speedily cleaned or removed by the cleaning device  154 . 
   The sheet-thickness storing section  117  stores thicknesses of various types of the recording sheets conveyed by the sheet-convey belt  8 , and indicates the thickness as a medium-thickness indicating section. As a modification, there may be provided, instead of the sheet-thickness storing section  117 , an analog circuit in which a level of an outputted signal varies in accordance with the thickness of the recording sheet. In short, a means or the like which indicates the thickness of the recording sheet needs only to be provided. When the mode storing section  111  stores that the ink-jet printer  1  is in the normal recording mode, the raising and lowering controlling section  115  controls the raising and lowering mechanisms  51  such that the clearance or the space between the ejection surfaces  3   a  and the recording sheet takes a predetermined value on the basis of the thickness of the recording sheet which is stored in or indicated by the sheet-thickness storing section  117 , thereby making the fine adjustment of the second height. Thus, when a relatively thick recording sheet such as an envelope and a thick paper is used, the ink-jet heads  2  are moved upward in recording, thereby leading to less occurrence of jamming of the recording sheet. 
   The focus judging section  118  judges whether the image based on the image data obtained by the image sensor  201  is an unfocused image or not. When the focus judging section  118  has judged that the image based on the image data obtained by the image sensor  201  is the unfocused image, the raising and lowering controlling section  115  controls the raising and lowering mechanisms  51  to make the fine adjustment of the first height of the ink-jet heads  2  such that the image based on the image data obtained by the image sensor  201  becomes the focused image. More specifically, the raising and lowering controlling section  115  adjusts the first height by repeating a process of changing a height of the image sensor  201  and picking up the test image recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8  until the image based on the image data obtained by the image sensor  201  becomes the focused image. At this time, the raising and lowering controlling section  115  controls the raising and lowering mechanisms  51  to make the fine adjustment of the second height such that the second height is higher than the adjusted first height of the ink-jet heads  2  by a predetermined height, when the mode storing section  111  stores or indicates that the ink-jet printer  1  is in the normal recording mode. 
   The image analyzing section  119  analyzes the image data obtained by the image sensor  201  which has picked up the test image recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8 . The image analyzing section  119  judges, on the basis of a result of the analysis, whether an ink ejection failure, such as non-ejection of the inks, variation of an ejected ink amount, and a deviation of a direction in which the inks are ejected, occurs in any of the ejection openings  3   b  of the respective head bodies  3 . Where the image analyzing section  119  has judged that the ink ejection failure occurs, there is determined that the maintenance operation (in this case, the purging operation and the wiping operation of the wiper  72  for the ejection surfaces  3   a ) is needed. 
   Operations of Ink-Jet Printer 
   There will be next explained a processing of the controller  101  with reference to a flow-chart shown in  FIG. 8 . When the processing of the controller  101  is started, the controller  101  initially judges, in S 1 , the mode stored in the mode storing section  111  is the normal recording mode or the inspecting mode. Where the mode is the inspecting mode (S 1 : YES), the processing goes to S 2 . In S 2 , a sensor (not shown) confirms that the frame  4  is located at the inspecting position, that is, the ink-jet heads  2  are located at the first height. Where the ink-jet heads  2  are not located at the first height, the raising and lowering controlling section  115  controls the raising and lowering mechanisms  51  such that the ink-jet heads  2  are moved to the first height. 
   In S 3 , the sheet-convey controlling section  113  controls the motor driver  123  which drives the sheet-convey motor  133 , such that the belt roller  6  is started to be rotated in a state the recording sheet is not held on the outer peripheral surface  8   a  of the sheet-convey belt  8 . Further, the head controlling section  112  controls the head driving circuit  121  such that the test image is recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8 . At this time, a distance between the ejection surfaces  3   a  and the sheet-convey belt  8  is shorter than that in the normal recording mode. Thus, the ejected-ink-attaching accuracy is relatively high. Accordingly, the ink ejection failure such as the non-ejection of the inks from the ejection openings  3   b  can be detected with a relatively high accuracy. Moreover, since the distance between the ejection surfaces  3   a  and the sheet-convey belt  8  is shorter than that in the normal recording mode, the light-entering surface  201   a  of the image sensor  201  is less soiled due to flying ink mist. 
   In S 4 , the image-pickup controlling section  116  controls the image sensor  201  such that the image sensor  201  picks up the test image recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8 . The test image recorded on the outer peripheral surface  8   a  is speedily cleaned or removed by the cleaning device  154 . 
   In S 5 , the focus judging section  118  judges whether the image based on the image data obtained by the pickup operation in S 4  is the unfocused image. Originally, since the first height of the ink-jet heads  2  is a height determined such that the image based on the image data obtained by the image sensor  201  which has picked up the test image recorded on the outer peripheral surface  8   a  is the focused image, the image based on the image data obtained by the pickup operation in S 4  should be the focused image. However, there is a possibility that the image based on the image data obtained by the pickup operation in S 4  is the unfocused image where the first height varies from its initial height with time due to mechanical and structural defects of the raising and lowering mechanisms  51 . To solve this problem, in this ink-jet printer  1 , the focus judging section  118  judges whether the image based on the image data obtained by the pickup operation in S 4  is the unfocused image. Here, on the outer peripheral surface  8   a  of the sheet-convey belt  8 , there is recorded a focus judging chart in which black areas and white areas each having a width equal to pixel pitch (resolution) of the image sensor  201  are alternately arranged, for example. When the focus judging chart is picked up, a spatial frequency can be obtained. Whether the image based on the image data obtained by the pickup operation in S 4  is the unfocused image is judged on the basis that the spatial frequency is equal to or greater than a threshold value which is near the highest frequency depending upon a form of the focus judging chart. 
   Where the image based on the image data obtained by the pickup operation in S 4  is the unfocused image (S 5 : NO), the processing goes to S 6 . Where the image based on the image data obtained by the pickup operation in S 4  is the focused image (S 5 : YES), the processing goes to S 8 . In S 6 , the raising and lowering controlling section  115  controls the raising and lowering mechanisms  51  such that the height of the image sensor  201  is changed. Then, in S 7 , the image-pickup controlling section  116  controls the image sensor  201  such that the image sensor  201  again picks up the test image recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8 . The raising and lowering controlling section  115  adjusts the first height by repeating transactions of S 7  and S 8  until the image based on the image data obtained by the image sensor  201  becomes the focused image. That is, the raising and lowering controlling section  115  adjusts the first height by repeating the process of changing the height of the image sensor  201  and picking up the test image recorded on the outer peripheral surface  8   a  of the sheet-convey belt  8  until the image based on the image data obtained by the image sensor  201  becomes the focused image. In this ink-jet printer  1 , the fine adjustment of the first height allows the first height to be maintained to its initial height such that the image based on the image data is always the focused image. Thus, even when the focal depth of the image-forming optical system  203  is shallow, there is no need that an accuracy with which the ink-jet heads  2  are mounted with respect to height (i.e., an accuracy of a height position of the ink-jet heads  2 ) sets to be extremely high. After the transaction of S 7  is completed, the processing returns to S 5 . In this manner, even where the first height is changed from its initial height with time due to the mechanical and structural defects of the raising and lowering mechanisms  51 , the process of changing the height of the image sensor  201  and picking up the test image recorded on the outer peripheral surface  8   a  is repeated until the image based on the image data obtained by the image sensor  201  becomes the focused image, whereby the first height is maintained to its initial height such that the image based on the image data obtained by the image sensor  201  is always the focused image. Thus, even when the focal depth of the image-forming optical system  203  is shallow, there is no need that the accuracy with which the ink-jet heads  2  are mounted with respect to height sets to be extremely high. 
   In S 8 , the image analyzing section  119  analyzes the image data obtained in S 4  or S 7 . The image analyzing section  119  judges, on the basis of the result of the analysis, whether the ink ejection failure occurs in any of the ejection openings  3   b  of the ink-jet heads  2 . Where the image analyzing section  119  has judged that the ink ejection failure occurs, there is determined, in S 9 , that the maintenance operation for the ink-jet heads  2  is needed (S 9 : YES). At this time, where the ink ejection failure occurs in one of the ink-jet heads  2 , there may be determined that the maintenance operation is needed only for the one of the ink-jet heads  2 . Then, the processing goes to S 10  in which the maintenance unit  70  performs the maintenance operation for the ink-jet heads  2 . After the maintenance operation is completed, the processing returns to S 1 . 
   Here, there will be explained the maintenance operation of the ink-jet heads  2  performed in S 10  with reference to  FIGS. 9A and 9B .  FIG. 9A  is a side view showing that the frame  4  is moved from the recording position to the maintenance position, and the tray  71  of the maintenance unit  70  is moved to the maintenance position.  FIG. 9B  is a side view showing that the wiper  72  wipes the inks adhering to the ejection surfaces  3   a.    
   When performing the purging operation which is for recovering the ink-jet heads  2  in which the ink ejection failure occurs, the frame  4  is moved to the maintenance position by the raising and lowering mechanisms  51 . At this time, the two head motors  52  are driven so as to be synchronized with each other, thereby rotating the pinion gears  53  forwardly (in the clockwise direction in  FIG. 3 ). As a result, the rack gears  54  are moved upward in accordance with the rotations of the pinion gears  53 . The frame  4  fixed to the rack gears  54  is moved upward together with the four ink-jet heads  2 . When the frame  4  has reached the maintenance position, the rotations of the head motors  52  are stopped. 
   As a result, between the ejection surfaces  3   a  and the sheet-convey belt  8 , there is formed the space in which the maintenance unit  70  can be disposed. When the frame  4  is located at the maintenance position, the respective ejection surfaces  3   a  of the ink-jet heads  2  and the bottom surface of the frame  4  are located at a height position at which the distal ends of the wiper  72  and the circular projections  76   a  are not brought into contact with the respective ejection surfaces  3   a  of the ink-jet heads  2  and the bottom surface of the frame  4  when the maintenance unit  70  is moved to the maintenance position. 
   Then, the contacting members  84  are respectively brought into contact with the other end portions  83   b  of the respective hooking members  83 , whereby the projecting portions  83   a  are respectively moved away from the recessed portions  74   b . As a result, the recessed portions  74   b  and the respective projecting portions  83   a  are disengaged from each other. That is, the connection of the tray  71  and the tray  75  is released. Then, in this state, the timing belt  95  is moved by the driving of the tray motor  92  of the horizontally moving mechanism  91  such that the tray  71  is moved to the maintenance position. Then, when the tray  71  has reached the maintenance position as shown in  FIG. 9A , the tray motor  92  is stopped to be driven. 
   Subsequently, the pumps (not shown) by which the inks in the respective ink cartridges (not shown) are forced to be introduced into the respective ink-jet heads  2  are driven, thereby performing the purging operation in which the inks are ejected from the ejection openings  3   b  of the ink-jet heads  2  into the tray  71 . By performing the purging operation, clogging of the inks in the ejection openings  3   b  in which the ink ejection failure occurs and thickening of the inks in the ejection openings  3   b  are solved. The inks ejected into the tray  71  are moved toward the left side of  FIG. 9A  on and along a bottom surface of the tray  71 , and then flow into the waste-ink receiving tray  77 . Then, the inks used in the purging operation are discharged from the ink-discharge opening  77   a  of the waste-ink receiving tray  77 . A part of the inks, however, remains on the ejection surfaces  3   a  as ink droplets. 
   Thereafter, the ink-jet heads  2  are moved downward by the raising and lowering mechanisms  51 . At this time, the ink-jet heads  2  are located at a height position at which the distal end of the wiper  72  can be brought into contact with the ejection surfaces  3   a  and the bottom surface of the frame  4  when the tray  71  is moved toward the left side (that is, to the retracted position). Then, as shown in  FIG. 9B , the tray  71  is moved toward the left side by the horizontally moving mechanism  91  (that is, the tray  71  is moved from the maintenance position to the retracted position). 
   By this operation, the wiper  72  is moved in a wiping direction (a first direction) that is directed from the right side toward the left side in  FIG. 9A , whereby the wiping operation of the wiper  72  for the ejection surfaces  3   a  is performed. At this time, since the distal end (an upper end) of the wiper  72  is located above a height level of the bottom surface of the frame  4 , the wiper  72  contacts with the bottom surface of the frame  4  and the ejection surfaces  3   a  while being deformed or warped, thereby wiping the inks adhering to the ejection surfaces  3   a  by the purging operation. By performing the maintenance operation, the ink-jet heads  2  can be recovered from the ink ejection failure to a normal condition. Then, the processing returns to S 1 . 
   As shown in  FIG. 8 , where the controller  101  judges, in S 1 , that the ink-jet printer  1  is in the normal recording mode (S 1 : NO), the processing goes to S 11 . In S 11 , the sensor (not shown) confirms that the frame  4  is located at a normal recording position at which the ink-jet heads  2  are located at the second height. Where the ink-jet heads  2  are not located at the second height, the raising and lowering controlling section  115  controls the raising and lowering mechanisms  51  such that the raising and lowering mechanisms  51  move the ink-jet heads  2  to the second height. At this time, the light-entering surface  201   a  of the image sensor  201  is located above a height level of the ejection surfaces  3   a  of the respective ink-jet heads  2 . In other words, the light-entering surface  201   a  is more distant from the outer peripheral surface  8   a  than the ejection surfaces  3   a . Thus, the light-entering surface  201   a  is hardly soiled due to the ink mist. Also, undried inks adhering to the recording sheet hardly adhere to the light-entering surface  201   a.    
   Further, in S 11 , the raising and lowering controlling section  115  controls the raising and lowering mechanisms  51  such that the clearance or the space between the ejection surfaces  3   a  and the recording sheet takes the predetermined value on the basis of the thickness of the recording sheet, thereby making the fine adjustment of the second height. Thus, even where a plurality of the recording sheets of different types which are different from each other in thickness are used, the clearance or the space between the ejection surfaces  3   a  and the recording sheet can be always kept constant. In addition, where a transaction of S 6  has been executed, the raising and lowering controlling section  115  controls the raising and lowering mechanisms  51  to make the fine adjustment of the second height such that the second height is located at the position higher than the first height adjusted in S 6  by the predetermined height. As a result, the second height is determined to the position higher, by the predetermined height, than the first height, as a reference height, which has been adjusted and does not vary with time. Thus, even when the unadjusted second height varies with time due to the mechanical and structural defects of the raising and lowering mechanisms  51 , the second height can be always maintained to its initial height by the fine adjustment of the second height. Accordingly, satisfactory quality of the recording can be sustained. 
   In S 12 , the controller  101  waits to receive, from the PC  100 , a command for the recording. Where the controller  101  has received the command for the recording, the image is recorded, in S 13 , on the basis of the controls of the head controlling section  112  and the sheet-convey controlling section  113 . Then, after the image has been recorded on the recording sheet, the processing returns to S 1 . 
   In this ink-jet printer  1 , as described above, the ink-jet heads  2  and the image sensor  201  are moved upward and downward together with each other in a state in which the image sensor  201  is more distant from the outer peripheral surface  8   a  of the sheet-convey belt  8  than the ink-jet heads  2 . Thus, the light-entering surface  201   a  of the image sensor  201  is less soiled. When the image is recorded as a test in the inspecting mode, the distance between the ejection surfaces  3   a  and the sheet-convey belt  8  is shorter than in the normal recording mode. Thus, the ejected-ink-attaching accuracy becomes relatively high. In addition, the light-entering surface  201   a  of the image sensor  201  is hardly soiled with the flying ink mist. Further, since the ejected-ink-attaching accuracy becomes relatively high, the ink ejection failure such as the non-ejection of the inks from the ejection openings  3   b  can be detected with relatively high accuracy. 
   It is to be understood that the present 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 present invention. For example, the test image may be recorded on the recording sheet placed on the outer peripheral surface  8   a  of the sheet-convey belt  8 . In this case, the first height of the ink-jet heads  2  is predetermined such that the optical image of the test image recorded on the recording sheet on the outer peripheral surface  8   a  is formed on the image-pickup element  202  by the image-forming optical system  203  of the image sensor  201 . That is, the first height of the ink-jet heads  2  is predetermined such that the image based on the image data obtained when the image sensor  201  picks up the test image recorded on the recording sheet on the outer peripheral surface  8   a  is the focused image. 
   Further, the image sensor  201  may be disposed on an upstream side of the ink-jet heads  2  in the sheet-convey direction B. Where the image sensor  201  is thus disposed, the sheet-convey belt is circulated once in a state that the recording sheet on which the image has been recorded is placed on the sheet-convey belt, or the image sensor  201  picks up the image after the sheet-convey belt is reversely rotated. A position at which the image sensor  201  is attached is not limited to the side faces of the respective ink-jet heads  2 . That is, the image sensor  201  may be attached to the frame  4 , and may be attached to other components as long as the image sensor  201  cooperates with and is movable together with the ink-jet heads  2 . Further, the above-described function for making the fine adjustment may be omitted.