Abstract:
A liquid ejecting head is operable to eject liquid toward a target medium. A platen opposes to the liquid ejecting head, and has a first region formed with a plurality of projections. A first detector includes a light emitter operable to emit light toward the platen and a light receiver adapted to receive light reflected from the platen. The first detector is operable to generate a detection signal in accordance with an amount of the light received by the light receiver. A controller is operable to obtain the detection signal at a plurality of positions in the first region to determine a threshold value of the detection signal for detecting whether the target medium exists on the platen.

Description:
BACKGROUND  
       [0001]     1. Technical Field  
         [0002]     The present invention relates to a method of determining a threshold of detection for an edge of a printing medium, and to a printer operable to execute the same.  
         [0003]     2. Related Art  
         [0004]     There have been known ink jet printers operable to perform printing on a prescribed printing medium such as paper. As such printers, there has been known a printer which comprises: a printing head operable to eject ink toward the printing medium; a carriage on which the printing head is mounted; a platen facing the printing head; and an optical sensor for detecting an edge of the paper in the moving direction of the carriage. Such a printer is disclosed in Japanese Patent Publication No. 2004-351898A (JP-A-2004-351898). In this printer, an optical sensor is fixed to a carriage and detection of the edge of the printing medium is performed with the movement of the carriage. Also, in the printer, a protrusion (rib) for supporting a printing medium during the printing operation is formed on the platen.  
         [0005]     In general, in order to detect the edge of the printing medium, a prescribed threshold is set with respect to a detection signal from an optical sensor. As a method of setting the threshold, there has been proposed a method of setting a threshold on the basis of an output signal of an optical sensor when the optical sensor senses a protrusion formed on a platen. Such a method is disclosed in Japanese Patent Publication No. 2003-260829A (JP-A-2003-260829).  
         [0006]     However, JP-A-2003-260829 is silent about how to specifically process the output signal of the optical sensor in order to set the threshold.  
       SUMMARY  
       [0007]     It is therefore one advantageous aspect of the invention to provide a specific method of determining a threshold of detection for an edge of a printing medium and a printer operable to execute the method.  
         [0008]     According to one aspect of the invention, there is provided a liquid ejecting apparatus, comprising:  
         [0009]     a liquid ejecting head, operable to eject liquid toward a target medium;  
         [0010]     a platen, opposing to the liquid ejecting head, and having a first region formed with a plurality of projections;  
         [0011]     a first detector, including a light emitter operable to emit light toward the platen and a light receiver adapted to receive light reflected from the platen, the first detector operable to generate a detection signal in accordance with an amount of the light received by the light receiver; and  
         [0012]     a controller, operable to obtain the detection signal at a plurality of positions in the first region to determine a threshold value of the detection signal for detecting whether the target medium exists on the platen.  
         [0013]     The projections may include first projections and second projections having smaller sizes than the first projections. The first regionmay be a region in which the second protrusions are formed.  
         [0014]     The first projections may be adapted to support the target medium.  
         [0015]     Each of the second projections may have a slope face.  
         [0016]     Signal levels of the detection signal obtained at the plurality of positions may be different from each other.  
         [0017]     The controller may be operable to determine the threshold value based on an average of the signal levels.  
         [0018]     The liquid ejecting apparatus may further comprise: a carriage, operable to carry the liquid ejecting head and the first detector in a first direction; and a position detector, operable to detect a position of the carriage in the first direction to determine the plurality of positions.  
         [0019]     The controller may be operable to detect whether a foreign substance exists on the platen, based on a signal level of the detection signal, and is operable to determine the threshold value except the signal level indicative of the existence of the foreign substance.  
         [0020]     The plurality of positions may include a first position at which a first signal level of the detection signal is obtained and a second position at which a second signal level of the detection signal is obtained. The first signal level may be greater than a signal level of the detection signal obtained at a position shifted from the first position by a first distance in the first direction. The second signal level may be less than a signal level of the detection signal obtained at a position shifted from the second position by the first distance in the first direction.  
         [0021]     According to one aspect of the invention, there is provided a method of determining a threshold value of a detection signal for detecting whether a target medium to which a liquid ejecting head ejects liquid exists on a platen which opposes the liquid ejecting head and has a first region formed with a plurality of projections. The method comprises:  
         [0022]     emitting light from a light emitter to the platen;  
         [0023]     receiving light reflected from the platen and generating a detection signal in accordance with an amount of the light received; and  
         [0024]     obtaining the detection signal at a plurality of positions in the first region to determine the threshold value.  
         [0025]     The projections may include first projections and second projections having smaller sizes than the first projections. The first region may be a region in which the second protrusions are formed.  
         [0026]     Signal levels of the detection signal obtained at the plurality of positions may be different from each other.  
         [0027]     The threshold value may be determined based on an average of the signal levels.  
         [0028]     The method may further comprise: carrying the liquid ejecting head, the light emitter and the light receiver in a first direction; and detecting a position of the carriage in the first direction to determine the plurality of positions.  
         [0029]     The method may further comprise: detecting whether a foreign substance exists on the platen, based on a signal level of the detection signal; and determining the threshold value except the signal level indicative of the existence of the foreign substance.  
         [0030]     The plurality of positions may include a first position at which a first signal level of the detection signal is obtained and a second position at which a second signal level of the detection signal is obtained. The first signal level may be greater than a signal level of the detection signal obtained at a position shifted from the first position by a first distance in the first direction. The second signal level may be less than a signal level of the detection signal obtained at a position shifted from the second position by the first distance in the first direction. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1  is a perspective view of a printer according to one embodiment of the invention.  
         [0032]      FIG. 2  is a schematic section view showing an internal configuration of the printer.  
         [0033]      FIG. 3  is a schematic view showing detection mechanisms in the printer.  
         [0034]      FIG. 4  is a schematic view of a sheet edge detector in the printer.  
         [0035]      FIG. 5  is a block diagram showing a control system in the printer.  
         [0036]      FIG. 6  is a partial plan view of a platen in the printer.  
         [0037]      FIG. 7  is a diagram showing a relationship between the shapes of the platen (cross sectional shape along the line VII-VII in  FIG. 6 ) and signals output from the sheet edge detector.  
         [0038]      FIG. 8  is a diagram showing an enlarged view of the part VIII in  FIG. 7 .  
         [0039]      FIG. 9  is a flowchart showing how to set a threshold for the signals output from the sheet edge detector. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0040]     Exemplary embodiments of the invention will be described below in detail with reference to the accompanying drawings.  
         [0041]     A printer  1  according to one embodiment of the invention is an ink jet printer operable to perform printing by ejecting ink onto a printing medium P. As shown in FIGS.  1  to  3 , the printer  1  comprises: a carriage  3  on which a printing head  2  for ejecting ink drops is mounted; a carriage motor  4  for driving the carriage  3  in a primary scanning direction PS; a sheet feeding motor  5  for feeding the printing medium P in a secondary scanning direction SS; a sheet transporting roller  6  connected to the sheet feeding motor  5 ; a platen  7  opposing an ink ejecting face  2   a  of the printing head  2 ; and a main body chassis  8  on which those components are mounted. In this embodiment, both the carriage motor  4  and the sheet feeding motor  5  are DC motors. The printing medium P includes plain paper used to ordinary document printing, photographic paper used for photo printing, paper thicker than the plain paper or photographic paper, and transparent films, such as seals or OHP sheets.  
         [0042]     As shown in  FIG. 2 , the printer  1  further comprises: a hopper  11  on which blank printing media P are mounted; a sheet feeding roller  12  and a separating pad  13  for feeding the printing media P mounted on the hopper  11  into the printer  1  in a one by one manner; a sheet guiding plate  30  for guiding a leading edge of the printing medium P fed from the hopper  11  to the sheet transporting roller  6 ; a sheet detector  14  for detecting the passage of the printing medium P fed from the hopper  11 ; and a sheet ejecting roller  15  for ejecting the printing medium P from the printer  1 .  
         [0043]     In the printer  1 , the right side of  FIG. 1  is a side closer to a home position HP of the carriage  3 . On the other hand, the left side of  FIG. 1  is a side closer to a turning point of a reciprocal movement of the carriage  3  which is referred to as an away position AP.  
         [0044]     The carriage  3  can be moved by a timing belt  18  and a guide shaft  17  supported by a supporting frame  16  fixed to the main body chassis  8 . That is, the timing belt  18  is wound under prescribed tension on a pulley  19  and a pulley  20  in a state where a portion of the timing belt  18  is fixed to the carriage  3  (see  FIG. 2 ). The pulley  19  is attached to an output shaft of the carriage motor  4  and the pulley  20  is rotatably attached to the supporting frame  16 . The guide shaft  17  slidably supports the carriage  3  so as to guide the carriage  3  in the primary scanning direction PS. An ink cartridge  21  containing various kinds of ink to be supplied to the printing head  2  is mounted on the carriage  3 .  
         [0045]     Although not shown, a plurality of nozzles (not shown) are provided in the printing head  2 . Further, a plurality of piezoelectric elements (not shown) which have excellent responsibility as a kind of an electrostrictive element are provided in the printing head  2 , for example, to correspond to the individual nozzles. More specifically, the piezoelectric elements are disposed at positions being in contact with a wall face forming ink passages (not shown). When the wall face is pressed by the operation of the piezoelectric element, the printing head  2  ejects ink drops from the nozzle provided at the end of the ink passage. More specifically, the ink head  2  ejects ink from the ink ejecting face  2   a . The ink cartridge  21  contains, for example, dye-based ink with a good color appearance property for excellent image quality or pigment-based ink with excellent water resistibility or light resistibility.  
         [0046]     The sheet feeding roller  12  is connected to the sheet feeding motor  5  through a gear (not shown) and is driven by the sheet feeding motor  5 . As shown in  FIG. 2 , the hopper  11  is a plate-shaped member on which the printing medium P is mounted and can pivot about a pivot shaft supporting the upper portion of the hopper  11  by a cam mechanism (not shown). The pivot causes the lower end of the hopper  11  to be elastically brought into contact with the sheet feeding roller  12  or be separated from the sheet feeding roller  12 . The separating pad  13  is formed of a member having a high friction coefficient and is disposed at a position facing the sheet feeding roller  12 . When the sheet feeding roller  12  rotates, the surface of the sheet feeding roller  12  is pressed against the separating pad  13 . Therefore, when the sheet feeding roller  12  rotates, an uppermost printing medium P mounted on the hopper  11  passes through a nip made between the separating pad  13  and the surface of the sheet feeding roller  12  and is transported to a downstream side, while transporting of the second or lower printing media P is blocked by the separating pad  13 .  
         [0047]     The sheet transporting roller  6  is connected to the sheet feeding motor  5  directly or through a gear (not shown). As shown in  FIG. 2 , a sheet feeding follower roller  23  for transporting the printing medium P together with the sheet transporting roller  6  is provided in the printer  1 . The sheet feeding follower roller  23  is held on the downstream side of a follower roller holder  24  capable of pivot about a pivot shaft  25 . The follower roller holder  24  is urged (counterclockwise in  FIG. 2 ) by a spring (not shown) such that the sheet feeding follower roller  23  is always subjected to urging force directed toward the sheet transporting roller  6 . When the sheet transporting roller  6  is driven, the sheet feeding follower roller  23  also rotates together with the sheet transporting roller  6 .  
         [0048]     As shown in  FIG. 2 , the sheet detector  14  comprises a detection lever  26  and a photoelectric sensor  27  and is provided in the vicinity of the driving roller holder  24 . The detection lever  26  can pivot about a pivot center  28 . When the printing medium P completely passes by the lower side of the detection lever  26 , the detection lever  26  pivots counterclockwise in this figure. When the detection lever  26  pivots, light from a light emitting element (not shown) to a light receiving element (not shown) of the photoelectric sensor  27  is blocked, thereby detecting the passage completion of the printing medium P.  
         [0049]     The sheet ejecting roller  15  is disposed on the downstream side of the printer  1  and is connected to the sheet feeding motor  5  through a gear (not shown). As shown  FIG. 2 , a sheet ejecting follower roller  29  for ejecting the printing medium P together with the sheet ejecting roller  15  is provided in the printer  1 . Similar to the sheet feeding follower roller  23 , the sheet ejecting follower roller  29  is always subjected to urging force directed to the sheet ejecting roller  15  by a spring (not shown). Therefore, when the sheet ejecting roller  15  is driven, the sheet ejecting follower roller  29  also rotates together with the sheet ejecting roller  15 .  
         [0050]     As shown in  FIGS. 2 and 3 , the printer  1  comprises a linear encoder  33  having a linear scale  31  and a photoelectric sensor  32  as a position detector for detecting the speed of the carriage  3  or the position of the carriage  3  in the primary scanning direction PS. Further, as shown in  FIG. 3 , the printer  1  includes a rotary encoder  36  having a rotary scale  34  and a photoelectric sensor  35  as a position detector for detecting the transported speed of the printing medium P or the position of the printing medium P in the secondary scanning direction SS (more specifically, the rotation position or the rotation speed of the sheet transporting roller  6 ). Position detection signals output from the linear encoder  33  and the rotary encoder  36  are input to a controller  37  as shown in  FIG. 3 , and the controller  37  performs various kinds of control of the printer  1 . In  FIG. 1 , for convenience, the linear scale  31  and so on are omitted.  
         [0051]     The photoelectric sensor  32  constituting the linear encoder  33  includes a light emitter  41  and a light receiver  42 , as shown in  FIGS. 2 and 3 . The photoelectric sensor  32  is fixed to the rear face of the carriage  3 . The linear scale  31  is formed of a thin plate made of, for example, a transparent resin, or a thin and elongated stainless steel plate. The linear scale  31  is attached to the supporting frame  16  in parallel with the primary scanning direction PS. In the linear scale  31 , light transmitting portions (not shown) for transmitting light from the light emitter  41  of the photoelectric sensor  32  and light blocking portions (not shown) for blocking light from the light emitter  41  are alternately formed along the longitudinal direction thereof. When the carriage  3  moves, the linear scale  31  relatively moves between the light receiver  42  and the light emitter  41  of the photoelectric sensor  32 . According to the relative movement of the linear scale  31 , the photoelectric sensor  32  outputs a position detection signal having a rectangular waveform with a cycle according to the moving speed of the carriage  3 .  
         [0052]     As shown in  FIG. 3 , the photoelectric sensor  35  constituting the rotary encoder  36  has a light emitter  81  and a light receiver  82 , and is fixed to, for example, the main body chassis  8  through a bracket (not shown). The rotary scale  34  is formed of a circular thin stainless steel plate or a circular thin transparent resin plate. The rotary scale  34  in this embodiment is attached to the sheet transporting roller  6  and rotates integrally with the sheet transporting roller  6 . In other words, when the sheet transporting roller  6  rotates 360° degrees, the rotary scale  34  also rotates 360° degrees. In the rotary scale  34 , a plurality of light transmitting units (not shown) for transmitting light from the light emitter  81  of the photoelectric sensor  35  and a plurality of light blocking units (not shown) for blocking light from the light emitter  81  are alternately formed along the circumference thereof. When the sheet transporting roller  6  rotates, the rotary scale  34  relatively rotates between the light receiver  82  and the light emitter  81  of the photoelectric sensor  35 . According to the relative rotation of the rotary scale  34 , the photoelectric sensor  35  outputs a position detection signal having a rectangular waveform with a cycle according to the rotation speed of the sheet transporting roller  6 . In this embodiment, the photoelectric sensor  35  outputs a position detection signal for a resolution of 180 dpi.  
         [0053]     As shown in FIGS.  2  to  4 , the printer  1  further includes an edge detector  65  for detecting the edge of, for example, the printing medium P in the primary scanning direction PS (the moving direction of the carriage  3 ). The edge detector  65  is fixed to the carriage  3 , as shown in  FIG. 2 . More specifically, the edge detector  65  is fixed to the lower face of the carriage  3  on the upstream side of the printing head  2  in the secondary scanning direction SS (on the right side of  FIG. 2 ). Further, as shown in  FIG. 3 , the edge detector  65  is fixed to the away position AP side of the carriage  3  in the primary scanning direction PS. As shown in  FIG. 4 , the edge detector  65  includes a light emitter  66  for emitting light toward the platen  7  and a light receiver  67  receiving light that is emitted from the light emitter  66  and then reflected by the platen  7 . In this edge detector  65 , according to the movement of the carriage  3  in the primary scanning direction PS, the light emitter  66  emits light toward the platen  7  and the light receiver  67  receives the light reflected by the platen  7 . Then, the edge detector  65  outputs to the controller  37  an edge detection signal having a level corresponding to the amount of light received by the light receiver  67  (see  FIG. 3 ).  
         [0054]     As shown in  FIG. 5 , the controller  37  includes a bus  48 , a CPU  49 , a ROM  50 , a RAM  51 , a character generator (CG)  52 , a nonvolatile memory  53 , an ASIC  54 , a DC unit  55 , a sheet feeding motor driving circuit  56 , a carriage motor driving circuit  57 , a head driving circuit  61 , etc.  
         [0055]     The CPU  49  performs an operating process for executing a control program of the printer  1  stored in, for example, the ROM  50  or the nonvolatile memory  53 , or other necessary operating processes. The ROM  50  stores a control program for controlling the printer  1 , data necessary for the processes, and so on.  
         [0056]     The RAM  51  temporarily stores, for example, a program being executed by the CPU  49  or data being operated. The CG  52  stores an extended dot pattern corresponding to a print signal input to the ASIC  54 . The nonvolatile memory  53  stores various kinds of data required to be held after the printer  1  is deactivated.  
         [0057]     The ASIC  54  performs, for example, the control of the carriage motor  4  and the sheet feeding motor  5  or the control of the printing head  2  through the DC unit  55  or the head driving circuit  61 . This ASIC  54  is provided with a parallel interface circuit and can receive a print signal supplied from, for example, a host computer through an interface  62 . As shown in  FIG. 5 , the ASIC  54  is supplied with signals from the linear encoder  33 , the rotary encoder  36 , and various detectors, such as the sheet detector  14  and the edge detector  65 .  
         [0058]     The ASIC  54  computes the position of the carriage  3  on the basis of the number of rectangular pulses of the position detection signal (that is, digital signal) input from the linear encoder  33  and computes the speed of the carriage  3  on the basis of a pulse interval of the position detection signal. Further, the ASIC  54  computes the rotation position of the sheet transporting roller  6  on the basis of the number of rectangular pulses of the position detection signal (that is, digital signal) input from the rotary encoder  36  and computes the rotation speed of the sheet transporting roller  6  on the basis of a pulse interval of the position detection signal. Furthermore, the ASIC  54  sets a threshold to a sheet detection signal from the sheet detector  14  and detects the passage of the printing medium P fed into the printer  1  on the basis of the threshold and the sheet detection signal. In addition, the ASIC  54  sets a threshold to an edge detection signal from the edge detector  65  and detects the edge of the printing medium P in the primary scanning direction PS on the basis of the threshold and the edge detection signal. A method of detecting the edge of the printing medium P will be described later in detail.  
         [0059]     The DC unit  55  is composed of a control circuit for controlling the speed of the carriage motor  4  or the sheet feeding motor  5 , which is a DC motor. This DC motor  55  performs various operations for controlling the speed of the carriage motor  4  or the sheet feeding motor  5  on the basis of an operation instructing signal transmitted from the ASIC  54  and outputs a motor control signal to the sheet feeding motor driving circuit  56  or the carriage motor driving circuit  57  on the basis of the operation results. In this embodiment, a PID control method of controlling the current rotation speed of the carriage motor  4  or the sheet feeding motor  5  to be converged to a target rotation speed by a combination of proportional control, integral control, and derivative control is used as a method of controlling the carriage motor  4  and the sheet feeding motor  5 .  
         [0060]     The sheet feeding motor driving circuit  56  drives the sheet feeding motor  5  on .the basis of the motor control signal from the DC unit  55 . In this embodiment, a PWM (pulse width modulation) method is used as an example of a method of controlling the sheet feeding motor  5 , and the sheet feeding motor driving circuit  56  outputs a PWM driving signal. Similarly, the carriage motor driving circuit  57  also drives the carriage motor on the basis of the motor control signal from the DC unit  55 . In this embodiment, the carriage motor  4  is also driven by the PWM control method.  
         [0061]     The head driving circuit  61  drives the nozzles (not shown) of the printing head  2  on the basis of, for example, a control instruction transmitted from the CPU  49  or the ASIC  54 .  
         [0062]     The bus  48  is a signal line for connecting the above-mentioned individual components of the controller  37 . The CPU, the ROM  50 , the RAM  51 , the CG  52 , the nonvolatile memory  53 , and the ASIC  54  are connected to one another by the bus  48  so as to transmit and receive data thereamong.  
         [0063]      FIG. 6  shows a portion of the platen  7 . For convenience, the left side thereof is referred to as the home position HP side and the right side thereof is referred to as the away position AP side. Also, a direction from the lower side to the upper side of this figure is referred to as the transported direction of the printing medium P.  
         [0064]     As shown in  FIG. 6 , the platen  7  has a plurality of first protrusions  71  for supporting the printing medium P when printing is performed on the printing medium P (that is, when the printing head  2  ejects ink onto the printing medium P), and ink absorbers  72  for absorbing the ink. Further, groove portions  73  are formed between the first protrusions  71 . As shown in  FIG. 7 , in the groove portion  73 , a plurality of second protrusions  74  smaller than the first protrusions  71  are formed on the upstream side (the lower side of  FIG. 6 ) of the ink absorbers  72  relative to the secondary scanning direction SS. Specifically, the plurality of second protrusions  74  are arrayed in the primary scanning direction PS at a portion on which light is emitted from the light emitter  66  of the edge detector  65  fixed in the carriage  3  according to the movement of the carriage  3 .  
         [0065]     The first protrusion  71  has at the top thereof a horizontal supporting face  71   a  on which the printing medium P is mounted. In the first protrusion  71 , both sides of the supporting face  71   a  in the primary scanning direction PS and both sides of the supporting face  71   a  in the secondary scanning direction SS are inclined to the bottom portion of the first protrusion  71  such that the first protrusion  71  is widen toward the bottom portion thereof. The ink absorber  72  is provided to absorb ink drops of ink ejected from the printing head  2  that has not been impacted on the printing medium P. This ink absorber  72  is formed of, for example, a water-absorbing sponge.  
         [0066]     As shown in  FIG. 8 , the second protrusion  74  includes: a first flat face  74   a  formed at the top thereof; second flat faces  74   b  between adjacent second protrusions  74 ; and slope faces  74   c  inclined such that the protrusion  74  is widen from the first flat face  74   a  toward the second flat faces  74   b . The first flat face  74   a , the second flat faces  74   b , and the slope faces  74   c  have substantially the same width in the primary scanning direction PS. Further, the second protrusions  74  are formed on the groove portion  73  at regular interval K. For example, the regular interval K is 2 mm and the first flat face  74   a n, the second flat faces  74   b , and the slope faces  74   c  all have a width of 0.5 mm. Furthermore, the first flat face  74   a  and the second flat faces  74   b  are formed in parallel with the supporting faces  71   a.    
         [0067]     As described above, in order to detect the edge of the printing medium P in the primary scanning direction PS, the light emitter  66  of the edge detector  65  emits light according to the movement of the carriage  3  toward the platen  7 , and the light receiver  66  receives the light reflected by the platen  7  and outputs to the controller  37  an edge detection signal having a level according to the amount of the received light. When the printing medium P is mounted on the platen  7 , the edge detector  65  outputs an edge detection signal SG 1 , as shown by a solid line in  FIG. 7 . In other words, the edge detector  65  outputs an edge detection signal SG 1  having a substantially sine-wave shape of which the level varies according to the shape of, for example, the first protrusion  71  or the second protrusion  74 , as shown by the solid line in  FIG. 7 . The edge detection signal SG 1  is a signal of which the level becomes lower as the amount of light received by the light receiver  67  increases.  
         [0068]     More specifically, since light from the light emitter  66  is reflected by the first flat faces  74   a  and the second flat faces  74   b  of the groove portions  73  and thus it is easy for the reflected light to be returned to the light receiver  67 , as shown in  FIG. 8 , in portions corresponding to the first flat faces  74   a  and the second flat faces  74   b , the edge detection signal SG 1  becomes a low level. Since the light from the light emitter  66  is diffusely reflected by the slope faces  74   c  of the groove portions  73  and thus it is difficult for the diffusely reflected light to be returned to the light receiver  67 , in portions corresponding to the slope faces  74   c , the edge detection signal SG 1  becomes a high level. Similarly, in a portion corresponding to the supporting face  71   a , the edge detection signal SG 1  becomes the low level, and in portions corresponding to the slope faces inclined from both sides of the supporting face  71   a  in the primary scanning direction PS toward the bottom portion of the first protrusion  71 , the edge detection signal SG 1  becomes the high level.  
         [0069]     Further, since the first flat face  74   a  is closer to the edge detector  65  than the second flat face  74   b , a larger amount of light is reflected by the first flat face  74   a  than the second flat face  74   b  such that a large amount of light is incident on the light receiver  67 . Therefore, as shown in  FIG. 8 , a signal level L 1  of the edge detection signal SG 1  in a portion corresponding to the center of the first flat face  74   a  in the primary scanning direction PS is lower than a signal level L 2  of the edge detection signal SG 1  in a portion corresponding to the center of the second flat face  74   b  in the primary scanning direction PS. Also, since the supporting face  71   a  is closer to the edge detector  65  than the first flat face  74   a , as shown in  FIG. 7 , a signal level L 3  of the edge detection signal SG 1  in a portion corresponding to the center of the supporting face  71   a  in the primary scanning direction PS is lower than the signal level L 1  of the edge detection signal SG 1  in a portion corresponding to the center of the first flat face  74   a  in the primary scanning direction PS.  
         [0070]     Here, as shown in  FIG. 8 , the level of the edge detection signal SG 1  in a portion corresponding to the groove portion  73  varies with a constant cycle T corresponding to the regular interval K at which the second protrusions  74  are formed. In other words, when the signal level of the edge detection signal SG 1  in a portion corresponding to the center of the slope face  74   c  in the primary scanning direction PS is a signal level L 5 , a portion of the edge detection signal SG 1  corresponding to the groove portion  73  varies in the order of the signal level L 1 , the signal level L 5 , the signal level L 2 , the signal level L 5 , and the signal level L 2  with a constant cycle T corresponding to the regular interval K.  
         [0071]     Further, in order to detect the edge of the printing medium P in the primary scanning direction PS, as shown in  FIG. 7 , a prescribed threshold (edge detection threshold) t is set to the edge detection signal SG 1 . As shown in  FIG. 7 , a value lower than the level L 3  of the edge detection signal SG 1  in a portion corresponding to the supporting face  71   a  is set as the threshold t. Then, as shown by a dashed line in  FIG. 7 , when the printing medium P is mounted on the supporting faces  71   a  during the printing of the printing medium P, the level of the edge detection signal SG 1  in a portion corresponding to the printing medium P is lower than the threshold t, as an edge detection signal SG 11  shown by a chain line in  FIG. 7 . When variation occurs such that the level of the edge detection signal SG 11  goes across the threshold t, it is possible to detect the edge of the printing medium P in the primary scanning direction PS. A method of setting the threshold t will be described below in detail.  
         [0072]     In the printer  1  having the above-mentioned configuration, the carriage  3  driven by the carriage motor  4  reciprocates in the primary scanning direction PS while the printing medium P introduced from the hopper  11  into the printer  1  by the sheet feeding roller  12  or the separating pad  13  is transported in the secondary scanning direction SS by the sheet transporting roller  6  rotated by the sheet feeding motor  5 . When the carriage  3  reciprocates, the printing head  2  ejects ink drops, thereby performing printing on the printing medium P. When printing on the printing medium P is finished, the printing medium P is ejected to the outside of the printer  1  by the sheet ejecting roller  15 .  
         [0073]     When the carriage  3  moves, the linear encoder  33  outputs the position detection signal. The ASIC  54  receives the output position detection signal and detects the position or speed of the carriage  3  from the received position detection signal. Then, various control of the printer  1  is performed on the basis of the detected position or speed of the carriage  3 . Further, when the carriage  3  moves, the edge detector  65  outputs the edge detection signal SG 1 . The ASIC  54  receives the output edge detection signal SG 1  and detects the edge of the printing medium P in the primary scanning direction PS from the received edge detection signal SG 1  and the threshold t set to the edge detection signal SG 1 . Then, various control of the printer  1  is performed on the basis of the detection result of the edge of the printing medium P. For example, on the basis of the detection result of the edge of the printing medium P, for printing on the edge of the printing medium P, control of the printing head  2  (control of, for example, the amount of ink ejected by the printing head  2  or ejection timings) is performed by the head driving circuit  61 .  
         [0074]     Furthermore, the ASIC  54  detects, for example, the rotation position or rotation speed of the sheet transporting roller  6  from the position detection signal output according to the rotation of the sheet transporting roller  6  from the rotary encoder  36 , and then various control is performed on the printer  1  on the basis of the detection result. In addition, the ASIC  54  detects the passage of the printing medium P introduced into the printer  1  from the sheet detection signal from the sheet detector  14  and the threshold set to the sheet detection signal and then various control is performed on the printer  1  the basis of the detection result.  
         [0075]     As described above, in order to perform the control of the printing head  2 , etc., the detection of the edge of the printing medium P in the primary scanning direction PS is performed. In order to detect the edge of the printing medium P, a method of setting the threshold (edge detection threshold) t to the edge detection signal SG 1  will be described below with reference to  FIG. 9 .  
         [0076]     When the carriage  3  moves in a state where no printing medium P is mounted on the first protrusions  71 , the light emitter  66  emits light, the platen  7  (specifically, the first protrusions  71  and the portions of the groove portions  73  in which the second protrusions  74  are formed) reflects the emitted light, and the light receiver  67  receives the reflected light. In the case, the setting of the threshold t is performed on the basis of an output signal of the edge detector  65  output according to the amount of light received the light receiver  67 . While the output signal of the edge detector  65  is a signal that is substantially the same as the edge detection signal SG 1  shown by the solid line in  FIG. 7 , hereinafter, the output signal used for setting the threshold t is represented as a threshold setting signal SG 21 .  
         [0077]     The setting or updating of the threshold t is performed, for example, when the printer  1  is activated. A case of setting the threshold t when the printer  1  is activated will be described below. However, a time for setting or updating of the threshold t is not limited to the time when the printer  1  is activated. The setting or updating of the threshold t may be performed when a prescribed time period elapses after the activation or after printing on a prescribed number of sheets is finished. Accordingly, it is possible to setting the threshold t in view of chronological changes such as variation in reflectance of the platen  7  or a stain on the edge detector  65 .  
         [0078]     As shown in  FIG. 9 , when the printer  1  is activated (Step S 1 ), the carriage  3  reciprocates in a state where no printing medium P is mounted on the first protrusions  71  (that is, in a state where no printing medium P is introduced into the printer) (Step S 2 ). In Step S 2 , according to the movement of the carriage  3 , the linear encoder  33  outputs the position detection signal, and the ASIC  54  receives the position detection signal and counts the number of pulses of the received position detection signal. That is, the ASIC  54  acquires information on the position of the carriage  3 . Further, in Step S 2 , according to the movement of the carriage  3 , the edge detector  65  outputs the threshold setting signal SG 21  and the ASIC  54  receives the output threshold setting signal SG 21 .  
         [0079]     In this embodiment, the threshold t is set on the basis of three signal levels of the threshold setting signal SG 21  when light which is emitted by the light emitter  66  and then reflected at three positions of the groove portion  73  is incident on the light receiver  67 . More specifically, as shown in  FIG. 8 , the threshold t is set on the basis of a first signal level L 11  of the threshold setting signal SG 21  when a light component reflected at a position approximate to the center of a slope face  74   c  (hereinafter, this position is referred to as a first position X 1 ) is incident on the light receiver  67 , a second signal level L 12  of the threshold setting signal SG 21  when a light component reflected at a position approximate to the center of a first flat face  74   a  (hereinafter, this position is referred to as a second position X 2 ) is incident on the light receiver  67 , and a third signal level L 13  of the threshold setting signal SG 21  when a light component reflected at a position approximate to the left edge of the first flat face  74   a  shown in  FIG. 8  (hereinafter, this position is referred to as a third position X 3 ) is incident on the light receiver  67 . That is, the threshold t is set using the signal level L 5 , the signal level L 1 , and an intermediate level between the signal level L 5  and the signal level L 1  as the signal levels L 11 , L 12 , and L 13 , respectively.  
         [0080]     Consequently, when the threshold setting signal SG 21  is input to the ASIC  54 , the ASIC  54  obtains the first signal level L 11 , the second signal level L 12 , and the third signal level L 13  of the threshold setting signal SG 21  (Step S 3 ).  
         [0081]     In this case, the first position X 1 , the second position X 2 , and the third position X 3  are set according to the position detection signal from the linear encoder  33 . That is, the number of pulses of the position detection signal corresponding to the first position X 1  (hereinafter, referred to as a first number of pulses), the number of pulses of the position detection signal corresponding to the second position X 2  (hereinafter, referred to as a second number of pulses), and the number of pulses of the position detection signal corresponding to the third position X 3  (hereinafter, referred to as a third number of pulses) are stored in the ASIC  54  beforehand. Then, the signal levels of the threshold setting signal SG 21  when the number of pulses of the position detection signal input from the linear encoder  33  according to the movement of the carriage  3  is equal to the number of first pulses to the number of third pulses become the signal levels of the threshold setting signal SG 21  when the light components reflected at the first position X 1  to the third position X 3  are incident on the light receiver  67 , respectively.  
         [0082]     For this reason, in Step S 3 , the ASIC  54  obtains the signal levels of the threshold setting signal SG 21  when the number of pulses of the position detection signal input from the linear encoder  33  according to the movement of the carriage  3  is equal to each of the number of first pulse to the number of third pulses, as the first signal level L 11 , the second signal level L 12 , and the third signal level L 13 , respectively.  
         [0083]     For example, in this embodiment, when it is assumed that the first number of pulses, which is the number of pulses corresponding to the first position X 1 , is Y, the second number of pulses becomes (Y+32) and the third number of pulses becomes (Y+16). As described above, in this embodiment, since the resolution of the position detection signal output from the linear encoder  33  is 180 dpi, the interval between the first position X 1  and the second position X 2  becomes, for example, 4.515 nm (=25.4 (inches)/180 (dpi)×32 (pulses)) and the interval between the first position X 1  and the third position X 3  becomes, for example, 2.257 mm (=25.4 (inches)/180 (dpi)×16 (pulses)).  
         [0084]     Also, the positions of the groove portion  73  corresponding to the first number of pulses to the third number of pulses become the first position X 1  to the third position X 3 , respectively. For this reason, according to component accuracy of the platen  7  and the accuracy of assembly of the platen  7  into the main body of the printer  1 , the first position X 1  may not be the position approximate to the center of the slope face  74   c , the second position X 2  may not be the position approximate to the center of the first flat face  74   a , and the third position X 3  may not be the position approximate to the left edge of any one first flat face  74   a  shown in  FIG. 8 . That is, practically, the first position X 1  to the third position X 3  may be positions deviating from the positions shown in  FIG. 8  to the home position HP side or the away position AP side. Therefore, the first signal level L 11  is not always coincident with the signal level L 5 , and similarly, the second signal level L 12  is not always coincident with the signal level L 1 .  
         [0085]     However, in this embodiment, as shown in  FIG. 8 , the interval between the first position X 1  and the second position X 2  is constant. That is, the first signal level L 11  and the second signal level L 12  become signal levels of the threshold setting signal SG 21  at two points separated by about 9/4T thereon. For this reason, as shown in  FIG. 8 , when the first position X 1  is a position approximate to the center of the slope face  74   c , the second position X 2  is a position approximate to the center of the first flat face  74   a . However, for example, when the first position X 1  is a position approximate to the center of the first flat face  74   a , the second position X 2  may be a position approximate to the center of the slope face  74   c.    
         [0086]     Therefore, in a case where the first position X 1  is a position approximate to the center of the slope face  74   c  and the second position X 2  is a position approximate to the center of the first flat face  74   a , when the carriage  3  moves from a position corresponding to the first position X 1  to, for example, the away position AP side, the level of the threshold setting signal SG 21  exhibits a tendency to be lower than the first signal level L 11 . When the carriage  3  moves from a position corresponding to the second position X 2  to the away position AP side, the level of the threshold setting signal SG 21  exhibits a tendency to be higher than the second signal level L 12 .  
         [0087]     On the other hand, in a case where the first position X 1  is a position approximate to the center of the first flat face  74   a  and the second position X 2  is a position approximate to the center of the slope face  74   c , when the carriage  3  moves from the position corresponding to the first position X 1  to, for example, the away position AP side, the level of the threshold setting signal SG 21  exhibits a tendency to be higher than first the signal level L 11 . In addition, when the carriage  3  moves from the position corresponding to the second position X 2  to the away position AP side, the level of the threshold setting signal SG 21  exhibits a tendency to be lower than the second signal level L 12 .  
         [0088]     That is, in this embodiment, in a case where the level of the threshold setting signal SG 21  when the carriage  3  moves from the position corresponding to the first position X 1  to one side exhibits a tendency to be lower than the first signal level L 11 , when the carriage  3  moves from the position corresponding to the second position X 2  to one side, the level of the threshold setting signal SG 21  exhibits a tendency to be higher than the second signal level L 12 . Also, in a case where the level of the threshold setting signal SG 21  when the carriage  3  moves from the position corresponding to the first position X 1  to one side exhibits a tendency to be higher than the first signal level L 11 , when the carriage  3  moves from the position corresponding to the second position X 2  to one side, the level of the threshold setting signal SG 21  exhibits a tendency to be lower than the second signal level L 12 . As described above, in this embodiment, when one of the first signal level L 11  and the second signal level L 12  becomes higher, the other signal level becomes lower.  
         [0089]     In this embodiment, as shown in  FIG. 8 , the third signal level L 13  becomes the signal level of the threshold setting signal SG 21  at a point separated from a point corresponding to the first signal level L 11  by about 9/8T. The third signal level L 13  becomes an intermediate signal between the first signal level L 11  and the second signal level L 12 .  
         [0090]     When the three signal levels, that is, the first signal level L 11  to the third signal level L 13  are computed in Step S 3 , the ASIC  54  determines whether a signal level, which becomes 50% or less of the maximum signal level of the first signal level L 11  to the third signal level L 13 , exists among the first signal level L 11  to the third signal level L 13  (Step  4 ).  
         [0091]     When a signal level, which is 50% or less of the maximum signal level of the first signal level L 11  to the third signal level L 13 , does not exist among the first signal level L 11  to the third signal level L 13 , the ASIC  54  calculates the average of the three signal levels, that is, the first signal level L 11  to the third signal level L 13 , and sets, for example, 60% of the average as the threshold t (Step S 5 ).  
         [0092]     Meanwhile, when a signal level, which is 50% or less of the maximum signal level of the first signal level L 11  to the third signal level L 13 , exists among the first signal level L 11  to the third signal level L 13 , the ASIC  54  determines that the signal level is an abnormal value due to existence of a foreign substance, such as paper, on the platen  7 . Then, the ASIC  54  obtains two signal levels other than the signal level which is 50% or less of the maximum signal level, and sets, for example, 60% of the average as the threshold t (Step S 6 ).  
         [0093]     When the threshold t is set in Step S 5  or S 6 , a series of operations for setting the threshold t is finished.  
         [0094]     Due to the transfer of the printing medium P, as time goes on, the first protrusion  71  is worn down as shown by a dashed chain line in  FIG. 7 . When the first protrusion  71  is worn down, since the reflection area or reflectance of light from the light emitter  66  increases in the supporting face  71   a , the signal level of the edge detection signal SG 1  (the threshold setting signal SG 21 ) is lower than the threshold t. For this reason, when the edge of the printing medium P is detected or when the threshold t is updated, it is preferable that the ASIC  54  performs a masking process on a portion of the edge detection signal SG 1  (the threshold setting signal SG 21 ) corresponding to the supporting face  71   a  and determines the relationship between the edge detection signal SG 1  (the threshold setting signal SG 21 ) and the threshold t.  
         [0095]     As described above, in this embodiment, when one of the first signal level L 11  and the second signal level L 12  becomes higher, the other signal level becomes lower. More specifically, in a case where the level of the threshold setting signal SG 21  when the carriage  3  moves from a position corresponding to the first position X 1  to, for example, the away position AP side exhibits a tendency to be lower than the first signal level L 11 , when the carriage  3  moves from a position corresponding to the second position X 2  to the away position AP side, the level of the threshold setting signal SG 21  exhibits a tendency to be higher than the second signal level. On the other hand, in a case where the level of the threshold setting signal SG 21  when the carriage  3  moves from the position corresponding to the first position X 1  to, for example, the away position AP side exhibits a tendency to be higher than the first signal level L 11 , when the carriage  3  moves from the position corresponding to the second position X 2  to the away position AP side, the level of the threshold setting signal SG 21  exhibits a tendency to be lower than the second signal level.  
         [0096]     In other words, in this embodiment, since the first flat face  74   a , the second flat face  74   b , and the slope face  74   c  are formed with substantially the same width in the moving direction of the carriage  3  and the threshold setting signal SG 21  with the cycle T is output from the edge detector  65 , in two points of the threshold setting signal SG 21 , which are points for obtaining the first signal level L 11  and the second signal level L 12  and are separated from each other by about 9/4T, when the threshold setting signal SG 21  at one of the two points has a tendency to be higher, the threshold setting signal SG 21  at the other point has a tendency to be lower. The threshold t is set on the basis of the first signal level L 11  and the second signal level L 12 . Therefore, it is possible to reduce variation in the threshold t and to stably detect the edge of the printing medium P.  
         [0097]     As described above, according to the component accuracy of the platen  7  or the accuracy of assembly of the platen into the main body of the printer  1 , the first signal level L 11  or the second signal level L 12  actually computed may not be the signal level of the threshold setting signal SG 21  when light reflected at the position approximate to the center of the slope face  74   c  or the first flat face  74   a  shown in  FIG. 8  is incident on the light receiver  67 . Even in this case, as the first signal level L 11  becomes higher, the second signal level L 12  becomes lower, and as the first signal level L 11  becomes lower, the second signal level L 12  becomes higher. Therefore, it is possible to offset variation in the first signal level L 11  and variation in the second signal level L 12 . As a result, it is possible to reduce variation in the threshold t and to stably detect the edge of the printing medium P.  
         [0098]     Particularly, in this embodiment, the threshold t is set on the basis of the three signal levels, that is, the first signal level L 11 , the second signal level L 12 , and the third signal level L 13  which is the signal level of the threshold setting signal SG 21  at a point separated from the point corresponding to the first signal level L 11  by about 9/8T. Since the third signal level L 13  becomes an intermediate level between the first signal level L 11  and the second signal level  12 , when the threshold t is set on the basis on the three signal levels, it is possible to set the threshold t having a small variation.  
         [0099]     In this embodiment, when a signal level which is 50% or less the maximum signal level does not exist among the first signal level L 11  to the third signal level L 13 , the threshold t is set on the basis of the average of the three signal levels, that is, the first signal level L 11  to the third signal level L 13 . Therefore, it is possible to offset variation in the first signal level L 11  and variation in the second signal level L 12 . Further, the calculation of the average makes it possible to reduce the influence of variation in the third signal level L 13 . Therefore, it is possible to effectively reduce variation in the threshold t. Furthermore, since the threshold t is set using not only the first signal level L 11  and the second signal level L 12  but also the third signal level L 13  that is an intermediate level between the first signal level L 11  and the second signal level L 12 , it is possible to stabilize the threshold t.  
         [0100]     Meanwhile, in this embodiment, when a signal level which is 50% or less the maximum signal level exists among the first signal level L 11  to the third signal level L 13 , the threshold t is set on the basis of the average of two signal levels other than the signal level that is an intermediate level between the first signal level L 11  and the second signal level L 12 . Therefore, it is possible to eliminate the influence of the abnormal signal level and to effectively reduce variation in the threshold t.  
         [0101]     Although one exemplary embodiments of the invention has been described in detail above, those skilled in the art will readily appreciated that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the invention. Accordingly, all such modifications are intended to be included within the scope of the invention.  
         [0102]     In the above-mentioned embodiment, the threshold t is set on the basis of the three signal levels, that is, the first signal level L 11 , the second signal level L 12 , and the third signal level L 13 . However, the threshold t may be set on the basis of, for example, two signal levels, that is, the first signal level L 11  and the second signal level L 12 . Even in this case, it is possible to offset variation in the first signal level L 11  and variation in the second signal level L 12  and to reduce variation in the threshold t. Also, the threshold t may be set on the basis of four or more signal levels including not only the first signal level L 11  to the third signal level L 13  but also at least one signal level.  
         [0103]     In the above-mentioned embodiment, the first signal level L 11  and the second signal level L 12  used for setting the threshold t are signal levels of the threshold setting signal SG 21  at two points separated from each other by about 9/4T. However, the first signal level L 11  and the second signal level L 12  may be signal levels of the threshold setting signal SG 21  at two points separated from each other by about (¼+n 1 /2)T (n 1  is an integer equal to or greater than 0). In the case where the first signal level L 11  and the second signal level L 12  are signal levels of the threshold t at two points separated from each other by about (¼+n 1 /2)T, when the threshold signal SG 21  at the point corresponding to one of the first and second levels has a tendency to be higher, the threshold signal SG 21  at the point corresponding to the other level has a tendency to be lower. Therefore, it is possible to reduce variation in the threshold t.  
         [0104]     In the above-mentioned embodiment, the third signal level L 13  is the signal level of the threshold setting signal SG 21  at a point separated from the point corresponding to the first signal level L 11  by about 9/8T. However, the third signal level L 13  may be a signal level of the threshold setting signal SG 21  at a point separated from the point corresponding to the first signal level L 11  by about (⅛+n 2 /2)T (n 2  is an integer equal to or greater than  0).    
         [0105]     In the above-mentioned embodiment, the first flat face  74   a , the second flat face  74   b , and the slope face  74   c  have substantially the same width in the primary scanning direction PS. However, the first flat face  74   a , the second flat face  74   b , and the slope face  74   c  may have different widths in the primary scanning direction PS, or two of the first flat face  74   a , the second flat face  74   b , and the slope face  74   c  may have the same width in the primary scanning direction PS.  
         [0106]     Also, at least one of the first flat face  74   a  and the second flat face  74   b  may be parallel with the supporting face  71   a . Even in this case, when the threshold setting signal SG 21  at a point corresponding to the first signal level L 11  has a tendency to be higher, the threshold setting signal SG 21  at a point corresponding to the second signal level L 12  has a tendency to be lower. In addition, when the threshold setting signal SG 21  at the point corresponding to the first signal level L 11  has a tendency to be lower, the threshold setting signal SG 21  at the point corresponding to the second signal level L 12  has a tendency to be higher. When the threshold t is set on the basis of the first signal level L 11  and the second signal level L 22  having the above-mentioned relationship, it is possible to obtain the same advantages as that in the above-mentioned embodiment.  
         [0107]     In the above-mentioned embodiment, when a signal level, which is 50% or less of the maximum signal level, does not exist among the first signal level L 11  to the third signal level L 13 , the threshold t is set on the basis of the three signal levels, that is, the first signal level L 11  to the third signal level L 13 . However, the threshold t may be set on the basis of the maximum, minimum, or intermediate signal level of the first signal level L 11  to the third signal level L 13 .  
         [0108]     In the above-mentioned embodiment, when a signal level, which is 50% or less of the maximum signal level, exists among the first signal level L 11  to the third signal level L 13 , the threshold t is set on the basis of the average of two signal levels other than the signal level that is 50% or less of the maximum signal level. However, the threshold t may be set on the basis of one of two signal levels other than the signal level that is 50% or less of the maximum signal level.  
         [0109]     Although the explanation is made as to the ink jet printer as an example of the liquid ejecting apparatus, the invention can also be applied to a facsimile apparatus, a copying apparatus etc. Further, the invention can be applied not only to the printing apparatus but also to an apparatus provided with a color material ejecting head used for manufacturing color filters for liquid crystal displays etc., an electrode material (conductive paste) ejecting head used for forming the electrodes of organic EL displays or field emission displays (FED) etc., a bio-organic material ejecting head used for manufacturing biochips, a sample ejecting head as an accurate pipette, and so on as the liquid ejection apparatus which ejects liquid from a liquid ejecting head toward a target medium thereby to land the liquid onto the target medium.  
         [0110]     The disclosure of Japanese Patent Application No. 2005-331444 filed Nov. 16, 2006 including specification, drawings and claims is incorporated herein by reference in its entirety.