Abstract:
In a platen gap adjustment device, a stable area detection device for a platen gap formed between ahead and an upper surface of a platen, wherein the carriage guide shaft is moved relatively to the platen so that the platen gap is adjusted by driving the drive motor to rotate the gap adjuster cam, the gap adjuster cam is configured so as to provide a plurality of stable areas and a plurality transition areas; and wherein a stable area detection sensor is provided so as to face to a rotational member which rotates synchronously with the gap adjuster cam, and a detection object in correspondence with the stable areas of the platen gap is provided on the rotational member.

Description:
[0001]    The present application is based on Japanese Patent Applications Nos. 2003-100638 and 2003-343646, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a recording apparatus. Further, the invention relates to a liquid ejecting device such as an ink jet recording apparatus for ejecting liquid such as ink from its head into an ejection medium.  
           [0004]    The liquid ejecting device is not restricted to a recording apparatus including a printer, a copy, and a facsimile which uses an inkjet recording head in order to discharge the ink therefrom into the recording medium, for performing a recording, but it includes a liquid ejection apparatus for ejecting the liquid corresponding to the same purpose, instead of ink, into the ejection medium corresponding to the recording medium, from a liquid ejection head corresponding to the recording head and attaching the above liquid to the ejection medium.  
           [0005]    The liquid ejection head includes a color material ejection head for use in color filter manufacture such as a liquid crystal display, an electrode material (conductive paste) ejection head for use in electrode formation such as an organic EL display and a field emission display (FED), a living organic matter ejection head for use in bio chip manufacture, and a sample ejection head as an accurate pipette, other than the recording head.  
           [0006]    2. Related Art  
           [0007]    In the recording apparatus having a recording head, it is necessary to change a space between the recording head and the upper surface of the platen, that is, a platen gap, depending on the thickness of a recording medium. As the conventional technique for changing the platen gap, there is a technique, as disclosed in Japanese utility model publication No. JP-U-H05-35311, in which the thickness of paper set at the printing section is detected, the gap amount of a printing head is corrected by using a correction value predetermined depending on the detected thickness of the paper, and a print head gap suitable for the paper to be printed is set.  
           [0008]    Further, in Japanese Patent publication No 3027974B2, there is an apparatus comprising: a stepping motor for moving a carriage on which a recording head is mounted in a vertical direction of a platen; a rotary encoder with a detection mark provided on its circumference for supplying pulse signals in proportion to the quantity of rotation of the motor, that is, the moving quantity of the carriage; time lag integrating means for moving the carriage from a reference position into the direction of the platen to calculate the integrated value of the time lag between the pulse signal from the rotary encoder and the drive pulse of the stepping motor; and contact judging means for detecting that the integrated value reaches a predetermined value, in which paper thickness calculating means calculates the thickness of the paper according to the number of pulses of the rotary encoder up to the time when the signal is supplied from the contact judging means.  
         SUMMARY OF THE INVENTION  
         [0009]    Although a platen gap has to be switched in several stages depending on the thickness of the recording medium to be used, in the case of switching it by using a cam, there occurs a transition area from the stable area to the next stable area, other than an area where the platen gap becomes stable.  
           [0010]    When the turning phase angle of the cam a little deviates because of tolerance, the platen gap is determined at the transition area and accordingly there is a possibility of failing to get the accurate platen gap. An object of the invention is to provide a stable area detection device of a platen gap and a recording apparatus in which a cam can rotate at such an accurate phase angle to get the platen gap in the stable area.  
           [0011]    In order to achieve the above object, the invention provides a stable area detection device for a platen gap formed between a head and an upper surface of a platen, in a platen gap adjustment device, the platen gap adjustment device including  
           [0012]    a carriage guide shaft,  
           [0013]    a guide shaft gear fixed to an end of the carriage guide shaft,  
           [0014]    a gap adjuster cam rotatable integrally with the guide shaft gear and formed in a shape to change the platen gap in a plurality of platen gap stages,  
           [0015]    a cam follower for the gap adjuster cam, and  
           [0016]    a drive motor for driving the guide shaft gear to rotate,  
           [0017]    wherein the carriage guide shaft is moved relatively to the platen so that the platen gap is adjusted by driving the drive motor to rotate the gap adjuster cam,  
           [0018]    the gap adjuster cam is configured so as to provide a plurality of stable areas corresponding to the platen gap stages where the platen gap is constant while a rotational phase of the gap adjuster cam varies in a predetermined range and  
           [0019]    a plurality transition areas where the platen gap changes between the stable areas as the rotational phase of the gap adjuster cam varies;  
           [0020]    wherein a stable area detection sensor is provided so as to face to a rotational member which rotates synchronously with the gap adjuster cam, and  
           [0021]    a detection object in correspondence with the stable areas of the platen gap is provided on the rotational member.  
           [0022]    According to the first aspect of the invention, since the gap adjuster cam is prevented from standing in the transition area where there is a change in the platen gap, it is possible to perform the recording on the recording medium at high quality.  
           [0023]    The stable area detection device of platen gap according to the second aspect of the invention is constituted in that  
           [0024]    in addition to the first aspect, the stable area detection sensor includes a light emitting portion and a light receiving portion and  
           [0025]    the detection object comprises a light shielding plate which passes between the light emitting portion and the light receiving portion. According to this aspect, since the light shielding plate prevents the light receiving portion from receiving the light emitted from the light emitting portion, the light shielding state or the light passing state can be detected as the stable area.  
           [0026]    The stable area detection device of platen gap according to the third aspect of the invention is constituted in that, in addition to the first aspect or the second aspect, the detection object detected by the detection sensor for the stable areas is formed in correspondence with a central portion in each stable area, other than adjacent portions to the transition areas formed in both ends of said stable area. According to this aspect, it is possible to prevent the stable area detection sensor from misidentifying the transition area to be the stable area.  
           [0027]    The stable area detection device of platen gap according to the fourth aspect of the invention is constituted in that, in addition to one of the first aspect to the third aspect, a home position detection sensor is provided so as to face to the rotational member, and  
           [0028]    the rotational member is provided with anther detection object for the home position detection sensor at a position where the gap adjuster cam is located in a home position. According to this aspect, since the home position of the gap adjuster cam can be detected easily, it can contribute to the improvement of throughput.  
           [0029]    The stable area detection device of platen gap according to the fifth aspect of the invention is constituted in that, in addition to the fourth aspect, the position where the gap adjuster cam is located in the home position is a boundary portion between the stable area of a maximum platen gap stage and the transition area adjacent to the stable area of the maximum platen gap stage. According to this aspect, even when a user turns on the printer without knowing there is foreign substance under the recording head, since the platen gap is enough, it is possible to decrease a possibility of damaging the recording head owing to the foreign substance, through the scanning operation of the recording head.  
           [0030]    The stable area detection device of platen gap according to the sixth aspect of the invention is constituted in that, in addition to one of the first aspect to the third aspect, the gap adjuster cam includes a restricting mechanism for restricting a rotation thereof so as to be rotatable in a range from the stable area of a minimum platen gap stage to the stable area of the maximum platen gap stage.  
           [0031]    In this aspect, according to the restricting mechanism for restricting the rotation range of the gap adjuster cam so as to be rotatable in a range from the stable area of the minimum platen gap to the stable area of the maximum platen gap, when the stable area sensor detects no change for a predetermined hour even when a driving force is given to the gap adjuster cam, it is possible to recognize that it means the minimum platen gap or the maximum platen gap, figuring out the current position without providing another sensor for the exclusive use.  
           [0032]    The recording apparatus of the invention for performing a recording on a recording medium comprises the stable area detection device of platen gap, according to one of the first aspect to the sixth aspect. According to this aspect, since the platen gap can be always kept at a stable distance, it is possible to perform the recording on the recording medium at high quality.  
           [0033]    The liquid ejection apparatus of the invention for ejecting a liquid on a liquid ejection medium comprises the stable area detection device of platen gap, according to one of the first aspect to the sixth aspect.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIG. 1 is a cross sectional side view of the recording apparatus having the stable area detection device of the platen gap according to the invention;  
         [0035]    [0035]FIG. 2 is a cross sectional side view showing the state of feeding the stiff recording medium;  
         [0036]    [0036]FIG. 3 is a perspective view of the transport-driven roller holder and its vicinity when feeding the flexible recording medium;  
         [0037]    [0037]FIG. 4 is a perspective view of a driving force transmission branch gear and its vicinity;  
         [0038]    [0038]FIG. 5 is a cross sectional side view showing the state of engagement of the driving force transmission branch gear and its vicinity;  
         [0039]    [0039]FIG. 6 is a perspective view showing the structure of vertically moving the carriage guide shaft;  
         [0040]    [0040]FIG. 7 is a front view showing the structure of a gap adjuster cam and its vicinity;  
         [0041]    [0041]FIG. 8 is a side view of the driving force transmission branch gear and its vicinity;  
         [0042]    [0042]FIG. 9 is a graph showing the PG displacement, the retreating operation of the transport-driven roller, and the sensor detection state;  
         [0043]    [0043]FIG. 10 is a top view showing a sensor provided in a disc coaxial with the guide shaft gear;  
         [0044]    [0044]FIGS. 11A and 11B are perspective views showing the sensor provided in a disc coaxial with the guide shaft gear according to the second embodiment;  
         [0045]    [0045]FIGS. 12A and 12B are a perspective view and a side view showing the structure of vertically moving the carriage guide shaft;  
         [0046]    [0046]FIG. 13A is a front view showing the structure of vertically moving the carriage guide shaft;  
         [0047]    [0047]FIG. 13B is a front view showing the structure of vertically moving the carriage guide shaft;  
         [0048]    [0048]FIG. 13C is a front view showing the structure of vertically moving the carriage guide shaft;  
         [0049]    [0049]FIG. 13D is a front view showing the structure of vertically moving the carriage guide shaft; and  
         [0050]    [0050]FIG. 14 is a graph showing the PG displacement and the sensor detection state. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       [0051]    Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a cross sectional side view showing an ink jet printer (hereinafter, referred to as a printer) as one example of a recording apparatus including a stable area detection device of platen gap, according to the invention, FIG. 2 is a cross sectional side view showing the state of feeding a stiff recording medium, FIG. 3 is a perspective view of a transport-driven roller holder and its vicinity when feeding a flexible recording medium. Although the invention can be applied to an ejection medium on which surface, instead of ink, liquids corresponding to other applications are ejected, other than the recording medium such as paper and the like, the recording medium will be hereafter described representatively.  
         [0052]    A printer  1  comprises a feeding section  2  for feeding a recording medium P, at the upstream side and the feeding section  2  keeps a stack of the recording mediums P in a slanting state and transports the recording mediums P one by one to the downstream. When the recording medium is flexible like paper, the recording medium is transported to the recording process through a feeding path as illustrated in the circled number  1  in FIG. 1, when the recording medium has rigidity (stiff recording medium), it is transported to the recording process through the feeding path as illustrated in the circled number  2  in FIG. 2.  
         [0053]    The feeding section  2  comprises a hopper  16  and the hopper  16  holds a stack of the recording mediums in a slanting state. The hopper  16  is provided with a rotational support point at the upstream side and by turning around the rotational support point, it is formed so as to release or contact with a feeding roller  14  formed in a substantially D-shape from a side view. The recording medium P is pushed up by the clamping operation toward the feeding roller  14  and the uppermost one of the recording mediums P is made into contact with the feeding roller  14  In this state, the feeding roller  14  is rotated so as to transport the recording medium P to the downstream.  
         [0054]    A plate-shaped guide  15  is provided almost horizontally in the downstream of the feeding roller  14 , and the distal end of the recording medium P transported from the feeding section  2  is in contact with the guide  5  and smoothly and flexibly directed to the downstream. A transporting roller  19  including a transport-driving roller  19   a  of rotationally moving by a driving unit not illustrated and a transport-driven roller  19   b  of rotating together in contact with the transport-driving roller  19   a  is provided in the downstream from the guide  15 , and the recording medium P is pressed by the transporting roller  19  and given a driving force to the downstream. The transport-driving roller  19   a  is formed in a cylindrical roller long in a main scanning direction and a plurality of the transport-driven rollers  19   b  are provided shortly in the main scanning direction at predetermined intervals in the main scanning direction.  
         [0055]    The transport-driven roller  19   b  is supported by a shaft on the downstream side of the transport-driven roller holder  18 . The transport-driven roller holder  18  is provided in a rotative way around the rotation axis  18   a,  and the transport-driven roller  19   b  is always urged rotationally into close contact with the transport-driving roller  19   a  by a helical torsion spring (not illustrated).  
         [0056]    The transport-driven roller  19   b  can be turned into a retreat state of retreating upwardly by the rotation of the transport-driven roller holder  18  around the rotational support point  18   a,  as illustrated in FIG. 2.  
         [0057]    Namely, a cam  36  is provided in a driven roller release shaft  31  in a way of coming into contact with a cam follower  18   b  on the upstream portion of the transport-driven roller holder  18 , the cam  36  is coming into contact with the cam follower  18   b  from top down, according to the rotation of the release shaft  31 , and the transport-driven roller holder  18  is rotated around the rotational support point  18   a.  Thus, the transport-driven roller  19   b  retreats upwardly, to thereby be in a retreat state as shown in FIG. 2. When the cam  36 &#39;s contact with the cam follower  18   b  is released, the transport-driven roller  19   b  is rotationally urged toward the transport-driving roller  19   a  by the helical torsion spring (not illustrated) and returned into a contact state as shown in FIG. 1. Here, a reference mark P G  is attached particularly to the stiff recording medium which is difficult for the transporting roller to nip, for distinction.  
         [0058]    Further, a recording section  26  for performing a recording on the recording medium P is provided on the downstream of the transporting roller  19 . A platen  28  and a recording head  13  are arranged in the recording section  26  so as to vertically oppose to each other. The platen  28  is formed long in the main scanning direction and supports the recording medium P transported to the recording section  26  upwardly.  
         [0059]    The recording head  13  is provided in the bottom of a carriage  10  capable of holding an ink cartridge  11  and the carriage  10  can reciprocate in the main scanning direction while being directed by a carriage guide shaft  12  extending in the main scanning direction. The distance between the upper surface of the platen  28  and the recording head  13 , that is, a platen gap (hereinafter, there may be some cases of abbreviating as PG) is an important element for determining the recording accuracy, and it is necessary to properly adjust it depending on the thickness of the recording medium P. The PG adjustment will be described later.  
         [0060]    The downstream portion from the recording section  26  forms a discharge portion of the paper P in the printer  1 , which is provided with a discharge roller  20  including a discharge-driving roller  20   a  of rotationally moving by the driving means not illustrated and a discharge-driven roller  20   b  of being driven while being lightly clamped with the discharge-driving roller  20   a.  The recording medium P on which the recording by the recording section  26  has been performed is clamped by the discharge roller  20  and discharged onto a stacker  50  according to the rotation (normal rotation) of the discharge-driving roller  20   a.    
         [0061]    The discharge-driven roller  20   b  is a toothed roller having a plurality of teeth around its outer periphery and supported by a discharge-driven roller holder  23  in a rotatable way. The discharge-driven roller holder  23  is formed in a plate shape long in the main scanning direction and it is fixed to a discharge sub frame  25  extending almost horizontally from the vicinity of the recording head  13  toward the downstream along the discharge path of the recording medium P. The discharge sub frame  25  is attached to a discharge main frame  24  long in the main scanning direction and formed in a plate shape extending substantially horizontally from the vicinity of the recording head  13  toward the downstream, by a coil spring  27  in a way of downwardly pressing.  
         [0062]    A discharge auxiliary roller  22  is provided in the upstream of the discharge-driven roller  20   b  and the recording medium P is slightly pressed downward by the discharge auxiliary roller  22 . The position of the core axis of the transport-driven roller  19   b  is positioned in the downstream further than that of the transport-driving roller  19   a,  and the position of the core axis of the discharge-driven roller  20   b  is positioned in the upstream further than that of the discharge-driving roller  20   a.  According to this structure, the recording medium P is a little curved and convexed downwardly between the transporting roller  19  and the discharge roller  20 , and the recording medium P facing the recording head  13  is pushed down on the platen  28 , to thereby prevent from floating up of the recording medium P and correctly perform the recording thereon.  
         [0063]    The driving mechanism of the cam  36  for retreating the PG adjusting mechanism and the transport-driven roller  19   b  upwardly will be described with reference to FIGS.  4  to  8 . FIG. 4 is a perspective view of the vicinity of a driving force transmission branch gear, FIG. 5 is a cross sectional side view showing the state of engagement of the driving force transmission branch gear and its vicinity, FIG. 6 is a perspective view showing the structure of vertically moving the carriage guide shaft, FIG. 7 is a front view showing the structure of a gap adjuster cam and its vicinity, and FIG. 8 is a lateral side view of the driving force transmission branch gear and its vicinity.  
         [0064]    As shown in FIG. 4 and FIG. 5, the printer  1  is provided with a drive motor  51  for driving the PG adjustment device and cam  36 . A driving pulley  52  of the drive motor  51  transmits a driving force to an input gear  55  through an input gear mechanism  53  consisting of a gear train and the input gear  55  is engaged in a driving force transmission branch gear  57 .  
         [0065]    As well illustrated in Fig;  5 , the driving force transmission branch gear  57  is formed in three gear stages including a main gear  59  to be engaged in the input gear  55 , a first output gear  61  and a second output gear  63  fixed to the main gear  59 , for integrally rotating together. A toothless portion  65  is formed on one of the outer peripheral portion of the first output gear  61  and the other teeth of the gear can be engaged in an intermediate gear  67  adjacent to the first output gear  61 . The function of the toothless portion  65  in the first output gear  61  will be described later.  
         [0066]    The intermediate gear  67  is engaged in a guide shaft gear  69  and a carriage guide shaft  12  is fixed at the center of the guide shaft gear  69 . A gap adjuster cam  71  which rotates synchronously with the guide shaft gear  69  is fixed to the carriage guide shaft  12  adjacent to the guide shaft gear  69  and a fixed pin  73  working as a cam follower is fixed in the vicinity of the gap adjuster cam  71 .  
         [0067]    As illustrated in FIG. 6, FIG. 7, and FIG. 8, the carriage guide shaft  12  penetrates into a guide groove  77  extending longitudinally, which is formed on the frame  75  of the printer  1 , and accordingly, only the vertical movement is permitted and the horizontal movement is not permitted. According to this structure, when a rotational driving force is given to the guide shaft gear  69  from the drive motor  51 , the gap adjuster cam  71  begins to rotate and according to the function of the outer peripheral surface of the gap adjuster cam  71  and the fixed pin  73 , the carriage guide shaft  12  moves vertically. As a result, the carriage  10  supported by the carriage guide shaft  12  also moves vertically, to thereby adjust the platen gap (PG).  
         [0068]    On the other hand, a toothless portion  79  is also formed in one of the outer peripheral portion of the second output gear  63  and the other teeth of the gear can be engaged in a cam driving gear  81  fixed to the end portion of the driven roller release shaft  31 . The function of the toothless portion  79  in the second output gear  63  will be described later.  
         [0069]    According to this structure, when a rotational driving force is given to the driven roller release shaft  31  from the drive motor  51 , the driven roller release shaft  31  and the cam  36  also begin to rotate, and the function of the cam  36  and the cam follower  18   b  as mentioned above can realize the state of retreating the transport-driven roller  19   b  upwardly and the state of keeping it into contact with the transport-driving roller  19   a.    
         [0070]    As mentioned above, use of the driving mechanism for the platen gap adjustment enables the retreat state and the contact state of the transport-driven roller  19   b,  and therefore, it is not necessary to prepare for another driving mechanism separately, which makes the structure simple and decreases the cost.  
         [0071]    Hereinafter, with reference to FIG. 9 and FIG. 10, adjustment of a platen gap, and the retreat state and the contact state of the transport-driven roller  19   b  realized by the above structure will be described. FIG. 9 is a graph showing a displacement of the platen gap, the retreating operation of the transport-driven roller  19   b,  and the detection state of a sensor, according to the rotation of the drive motor  51 , and FIG. 10 is a perspective view showing the sensor provided on a disc  70  (rotational member) coaxial with the guide shaft gear  69 .  
         [0072]    In FIG. 9, the horizontal axis indicates the rotational phase position of the drive motor  51 , the right is the direction of counterclockwise rotation from a viewpoint of the output shaft and the left is the direction of clockwise rotation. A solid line  83  in FIG. 9 indicates a displacement of the platen gap accompanying the rotation of the drive motor  51  and it shows that the displacement becomes larger according to the upper direction of the vertical axis. A broken line  85  continued to the solid line  83  at the right side shows the state in which the toothless portion  65  of the first output gear  61  faces the intermediate gear  67  and therefore the rotational driving force of the drive motor  51  is not transmitted to the gap adjuster cam  71 .  
         [0073]    The solid line  87  indicates the displacement of the driven-roller release shaft  31  at a time of performing the retreating and contact operation of the transport-driven roller  19   b,  and in this case, the upper direction of the vertical axis indicates how much the transport-driven roller  19   b  is removed upward from the contact state, and the horizontal portion  87   a  at the right end of the solid line  87  indicates the retreat completion state of the transport-driven roller  19   b.  The broken line  89  continued to the solid line  87  at the left side indicates the state in which the toothless portion  79  of the second output gear  63  faces the cam driving gear  81  and therefore the rotational driving force of the drive motor  51  is not transmitted to the driven roller release shaft  31 . The horizontal line indicated by the broken line  89  indicates the contact state of the transport-driving roller  19   a  and the transport-driven roller  19   b.    
         [0074]    In FIG. 9, as apparent from the positional relationship between the boundary point  91  of the solid line  83  and the broken line  85  and the boundary point  93  of the solid line  87  and the broken line  89 , the toothless portion  65  of the first output gear  61  is formed in the range of the second output gear  63  and the cam driving gear  81  being in mesh, and contrary, the toothless portion  79  of the second output gear  63  is formed in the range of the first output gear  61  and the intermediate gear  67  being in mesh.  
         [0075]    If the driving force of the drive motor  51  is transmitted also to the driven-roller release shaft  31 , when this driving force should be transmitted to the gap adjuster cam  71  through the first output gear  61 , the transport-driven roller  19   b  could retreat when it should not and the transport-driven roller  19   b  could come into contact with the transport-driving roller  19   a  when it should retreat. The reason for forming the toothless portion  79  in the second output gear  63  is to avoid such the draw back.  
         [0076]    On the other hand, the reason for forming the toothless portion  65  in the first output gear  61  is to decrease the load on the drive motor  51  by releasing the engagement of the first output gear  61  and the intermediate gear  67  by the toothless portion  65  because the load on the drive motor  51  is increased when the rotational driving force is transmitted to the driven roller release shaft  31 . When it is not necessary to decrease the load on the drive motor  51 , it is not necessary to form the toothless portion  65  in the first output fear  61 .  
         [0077]    As shown by the solid line  83  in FIG. 9, this example can select a platen gap in four stages. The horizontal portion of the solid line  83  indicates stable areas  95 ,  96 ,  97 , and  98  of PG (−, Typ, +, ++) in the four stages. The stable area  96  indicated by “Typ” corresponds to the PG for the paper having usual thickness, the stable area  95  indicated by “−” corresponds to the PG for thin paper, the stable area  97  indicated by “+” corresponds to the PG for the paper slightly thicker than the usual paper, and the stable area  98  indicated by “++” corresponds to the PG for the further thicker paper. Transition areas  99 ,  100 , and  101  for transiting to the respective stable areas are formed respectively between the stable areas  95  and  96 ,  96  and  97 ,  97  and  98 .  
         [0078]    In order to keep the platen gap constant during recording into the recording medium, it is necessary to fix the platen gap at one of the stable areas  95 ,  96 ,  97 , and  98  not at any of the transition areas  99 ,  100 , and  101 . As shown in FIG. 10, four light-shielding plates  103   a,    103   b,    103   c,  and  103   d  are formed in a protruding way at intervals on the outer periphery of a disc  70  coaxial with the guide shaft gear  69 , and an optical stable area detection sensor  105  is provided at a position adjacent to the outer periphery of the guide shaft gear  69 . The stable area detection sensor  105  has a light emitting portion and a light receiving portion, and it is to detect the presence of the light shielding plate depending on whether or not the light emitted from the light emitting portion is received by the light receiving portion.  
         [0079]    The respective positions of the four light shielding plates  103   a,    103   b,    103   c,  and  103   d  on the outer periphery of the disc  70  correspond to the respective stable areas  95 ,  96 ,  97 , and  98 , and when one of the four light shielding plates shields the light of the stable area detection sensor  105 , a judging unit, not illustrated, judges that the platen gap is in the stable area. The judging unit makes a judgment which light shielding plate is now shielding the light and which stable area the GP is standing in, through sequentially shielding the light of the stable area detection sensor  105  by the four light shielding plates  103   a,    103   b,    103   c,  and  103   d.    
         [0080]    In FIG. 9, the solid line  107  indicates the position where the light of the stable area detection sensor  105  is shielded, correspondingly to the solid line  83  indicating each stage of the platen gap. As for the solid line  107 , the stepped-up portion indicates “light shield state” and the stepped-down portion indicates “light pass state”. As is apparent from the comparison between the solid line  107  and the solid line  83 , the four light shielding plates  103   a,    103   b,    103   c,  and  103   d  do not completely conform to each length of the stables areas  95 ,  96 ,  97 , and  98 , but each circumferential length of the light shielding plates is determined in a way of corresponding to each central area of the stable areas  95 ,  96 ,  97 , and  98  excluding each transition area and each neighboring end portion. This can prevent the stable area detection sensor  105  from misidentifying the transition area to be the stable area, taking the tolerance into consideration.  
         [0081]    As illustrated in FIG. 10, an arc-shaped light shielding plate  109  is formed in predetermined length on one surface of the disc  70 , and a home position detection sensor  111  including a light emitting portion and a light receiving portion is provided on the same surface of the disc  70 . The home position detection sensor  111  is provided in order to determine the home position of the gap adjuster cam  71  and the solid line  113  of FIG. 9 indicates the light shield and the light pass by the home position detection sensor  111 , correspondingly to the solid line  83  indicating the stages of the platen gap.  
         [0082]    As for the solid line  113 , the stepped-up portion on the right indicates the “light shield state” and the stepped-down portion on the left indicates the “light pass state”. As is apparent from the comparison between the solid line  113  and the solid line  83 , it is found that the home position detection sensor  111  turns from the “light pass state” to the “light shield state” at the point when the transition area  101  moves to the stable area  98  as for the solid line  83 . Namely, in this example, the point of moving from the transition area  101  to the stable area  98  where the platen gap becomes the maximum is defined as a home position and the home position can be found by detecting the change from the “light pass state” to the “light shield state” in the home position detection sensor  111  or the inverse change. Further, by defining the point of moving from the transition area  101  to the stable area  98  where the platen gap becomes maximum, as the home position, even when a user turns on the power of the printer  1  without knowing there is foreign substance under the recording head  13 , since the platen gap is enough, it is possible to decrease the possibility of damaging the recording head  13  by the foreign substance through the scanning operation of the recording head  13 .  
       Second Embodiment  
       [0083]    Hereinafter, a second embodiment of the invention will be described with reference to FIGS. 11A to  14 . The second embodiment described later is made by changing the structure of the PG adjusting mechanism of the above mentioned first embodiment. Here, FIGS. 11A and 11B are perspective views showing a sensor provided on a disc coaxial with the guide shaft gear, FIGS. 12A and 12B are perspective view and side view showing the structure of vertically moving the carriage guide shaft, FIGS. 13A to  13 D are front views each showing the structure of vertically moving the carriage guide shaft, and FIG. 14 is a graph showing the PG displacement and the sensor detection state. In the second embodiment, the same reference numeral is attached to the same component as that in the above-mentioned first embodiment, and the description thereof is omitted.  
         [0084]    Although the PG adjusting mechanism according to this embodiment is provided on the left end portion of the carriage guide shaft  12 , the structure of the right end portion will be described at first. In FIGS. 11A and 11B, a guide groove  77  extending along the vertical direction, for supporting the carriage guide shaft  12  is formed on the right surface of the frame  75  formed in a substantially U-shape from lateral side view (the guide groove  77  is also formed on the left surface thereof), and the both ends of the carriage guide shaft  12  are inserted into the guide grooves  77 . The disc  70  is mounted on each shaft end portion of the carriage guide shaft  12 , and four light shielding plates  103  are formed on the outer periphery of the disc at predetermined intervals. Though these light shielding plates are formed in a way of standing at right angles to the disc, differently from the light shielding plates  103   a  to  103   d  according to the first embodiment shown in FIG. 10, the other structure and function and effect are the same and they are served for detecting the stable area by the sensor  105  including a light emitting portion and a light receiving portion.  
         [0085]    In FIG. 11B, the reference numeral  203  indicates a tension spring as urging means for holding the carriage guide shaft  12  stably, and the reference numeral  201  indicates a plate to be mounted on the right surface of the frame  75  at a predetermined inward angle, in order to hang the tension spring  203  with the carriage guide shaft  12 . The tension spring  203  is hung between a latch hook formed in the plate  201  and a groove formed in the carriage guide shaft  12  and the carriage guide shaft  12  is urged toward three directions including the vertical downward direction, the printer backward direction, and the axis line direction of-the carriage guide shaft  12 , to thereby obtain the following effects.  
         [0086]    At first, although the carriage guide shaft  12  is put into the guide groove  77  extending in the vertical direction, a clearance is formed between the guide groove  77  and the shaft in the horizontal direction to some degree. Accordingly, the tension spring  203  urges the carriage guide shaft  12  toward one side inside of the guide groove  77  (in this embodiment, on the printer backward side) so to stabilize the carriage guide shaft  12  within the guide groove  77  without chatter.  
         [0087]    At second, though the carriage guide shaft  12  is supported by the both lateral sides of the frames  75  (the details of the supporting portion is not described), it comes loose in the direction of the axis core. Accordingly, the tension spring  203  urges the carriage guide shaft  12  in the direction of the axis core, so to stabilize the above without chatter.  
         [0088]    At third, since the carriage guide shaft  12  is provided with a gap adjuster cam  216  (described later) on the left end, as illustrated in FIG. 13A, which comes into contact with the cam follower  211   b  (described later) from top down, so to define the platen gap, the tension spring  203  presses the gap adjuster cam  216  against the cam follower  211   b  so as not to upwardly displace the gap adjuster cam  216  from the cam follower  211   b.  Namely, the above cam serves a function of stabilizing the platen gap without any improper displacement.  
         [0089]    As mentioned above, one tension coil spring  201  can stabilize the carriage guide shaft  12  in multi directions at low cost with a little space. On the left end of the carriage guide shaft  12 , although a bar spring  213  shown in FIGS. 12A and 12B pushes the gap adjuster cam  216  against the cam follower  211   b  as well as urges the carriage guide shaft  12  to one side within the guide groove  77  so as not to make chatter, the tension spring  203  takes advantage of managing the load more easily than this bar spring  213 .  
         [0090]    Sequentially, the PG adjusting mechanism is provided on the left end of the carriage guide shaft  12  as illustrated in FIGS. 12A and 12B. The PG adjusting mechanism of this embodiment changes the PG by transmitting a driving force from the drive motor  51  that is the driving source of exclusive use to the guide shaft gear  215  mounted on the left end of the carriage guide shaft  12  through the first gear  205 , the second gear  207 , and the third gear  209  (these gears are two stepped gears), to thereby rotate the above guide shaft gear  215 . These are all mounted on the left surface of the frame  75  not illustrated.  
         [0091]    Hereinafter, the guide shaft gear  215  will be described in detail. The guide shaft gear  215  has a tooth portion to be engaged into the third gear  209 , on one portion of the outer circumference and a toothless portion where a tooth portion is lost, and a projection  218  protruding in the diameter direction is formed in the boundary between the tooth portion and the toothless portion. On the other hand, the gap adjuster cam  216  is formed on the disc surface of the guide shaft gear  215  and a projection  217  protruding in the diameter direction is formed on the cam surface.  
         [0092]    A bush  211  for parallelism adjustment is mounted on the vicinity of the guide shaft gear  215 . The parallelism adjustment bush  211  is to adjust the parallelism of the carriage guide shaft  12  and mounted on the both lateral sides of the frame  75 . A cam follower  211   b  is formed in the parallelism adjustment bush  211  and the platen gap is defined by the gap adjuster cam  216  pushing against the above cam follower  211   b  from top down. Namely, since the cam surface of the gap adjuster cam  216  is formed in a shape of varying the distance from the axis core of the carriage guide shaft  12  that is the rotation axis, the distance from the cam follower  211   b  of the carriage guide shaft  12  varies according to the rotation of the guide shaft gear  215 , as illustrated in FIG. 13A to FIG. 13D, thereby changing the platen gap. Further, the parallelism adjustment bush  211  can swing around a hole  211   a  for a shaft not illustrated to penetrate, and by this swing, similarly, the platen gap changes. Accordingly, by sliding the both parallelism adjustment bushes  211  on the right and left, adjustment of the parallelism of the carriage guide shaft  12  is possible.  
         [0093]    Hereinafter, the restricting mechanism for restricting the rotation range of the gap adjuster cam  216  between the stable area of the minimum platen gap and the stable area of the maximum platen gap will be described with reference to FIG. 14.  
         [0094]    In FIG. 14, the reference numerals  95  to  98  indicate the respective stable areas and the reference numerals  99  to  101  indicate the respective transition areas, similarly to FIG. 9. The solid line  107  indicates the position where the light of the stable area detection sensor  105  is shielded, correspondingly to the solid line  83  indicating each stage of the platen gap, similarly to FIG. 9.  
         [0095]    This embodiment is not provided with the home position detection sensor  111 , differently from the above-mentioned first embodiment. Namely, in the minimum platen gap shown in FIG. 13A, since the projection  217  comes into contact with the cam follower  211   b,  the further rotation of the gap adjuster cam  216  (guide shaft gear  215 ) is restricted by this. As mentioned above, the gap adjuster cam  216  will be restricted to the rotation range within the above range of from the stable area of the minimum platen gap to the stable area of the maximum platen gap.  
         [0096]    The “stopping position” shown in the both sides of FIG. 14 indicates the position of restricting the rotation of the gap adjuster cam  216  as mentioned above and at the reset operation, the drive motor  51  is rotated in the direction of bringing the projection  217  into contact with the cam follower  211   b.  Here, when the stable area detection sensor  105  does not change even if applying a drive current to the drive motor  51  for more than a predetermined hour, it is judged that the projection  217  comes into contact with the cam follower  211   b  as illustrated in FIG. 13A and namely, it is judged that the current platen gap is the minimum platen gap. Next, the platen gap is changed to the maximum while monitoring the detected signal of the stable area detection sensor  105  in order to seek the home position of the carriage (CR)  10  and again returned to the minimum platen gap, into a printing waiting state.  
         [0097]    As mentioned above, without using the home position detection sensor  111  as shown in the first embodiment, the current position of the platen gap can be judged by using the stable area detection sensor  105 , thereby saving the cost.  
         [0098]    The invention can be applied to a recording apparatus represented by a facsimile and a printer and a liquid ejecting device, that is, a liquid ejection apparatus for attaching liquid to an ejection medium from a head for ejecting the liquid.