Abstract:
In order to detect a position of an optical recording medium on which printing is performed, it is provided an optical sensor provided with include a photo emitter operable to emit light and a photo receiver operable to receive light and output a first signal in accordance with an amount of the received light. The optical sensor is transported to a position above a marker provided on the tray. Light is emitted from the photo emitter to irradiate the marker. Light reflected from the marker is received by the photo receiver. A reference value is determined based on a first value of the first signal outputted when the marker is irradiated. A predetermined calculation is executed with respect to the reference value to determine a threshold value. The optical sensor is transported above the tray, while emitting light from the photo emitter and comparing the first signal outputted from the photo receiver with the threshold value. A position of the marker is identified based on the comparison of the first signal and the threshold value. It is determined a position at which the printing begins based on the identified position of the marker.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a method of detecting a position of a printing medium which is performed in a printing apparatus.  
         [0002]     In recent years, printers are commercially distributed to enable printing information such as texts, images, etc. on a labeled surface of an optical recording medium, for example, CD-R (Compact Disk Recordable), etc. In the case where such printer is used to print on an optical recording medium in the form of a disk, a disk tray is in some cases used to hold the optical recording medium to feed the same into the printer.  
         [0003]     Such disk tray comprises a part for supporting an optical recording medium, and the disk tray moves in a secondary scanning direction (a direction of conveyance of a printing medium such as paper) of the printer to enable a recording head to print on a labeled surface of an optical recording medium.  
         [0004]     By the way, in case of printing on such optical recording medium, correct printing cannot be performed unless the printer recognizes a size and a position of the optical recording medium. Hereupon, Japanese Patent Publication No. 2004-114357A discloses a method of using a printer to print scales on an adjustment medium having the same shape of CD-R, on which a reference line is beforehand printed, identifying shift of a printed position according to how the reference line and the scales overlap, and adjusting a printed position according to the identified shift.  
         [0005]     Alternatively, there is proposed a method of applying a marker in a predetermined position on a disk tray, reading the marker with an optical sensor, indirectly finding a position of an optical recording medium from the positional relationship between the marker and a support part for the optical recording medium, and adjusting a printed position according to the position as found.  
         [0006]     By the way, with the former technique disclosed in Japanese Patent Publication No. 2004-114357A, it is necessary to print scales on an adjustment medium in order to perform the positional adjustment, and it is also necessary for a user to visually confirm results of printing, thus causing a problem that an operation is complicated.  
         [0007]     Also, with the latter technique, a marker provided on a disk tray is sometimes varied in light optical reflectance due to secular change, in which case there is caused a problem that it is not possible to correctly detect a position of a marker.  
       SUMMARY OF THE INVENTION  
       [0008]     It is therefore an object of the invention to provide a method of easily and stably detect a position of a printing medium in spite of secular change, which is performed in a printing apparatus.  
         [0009]     In order to achieve the above object, according to the invention, there is provided a method of detecting a position of an optical recording medium on which printing is performed, comprising:  
         [0010]     providing an optical sensor comprising a photo emitter operable to emit light and a photo receiver operable to receive light and output a first signal in accordance with an amount of the received light;  
         [0011]     transporting the optical sensor to a position above a marker provided on the tray;  
         [0012]     emitting light from the photo emitter to irradiate the marker;  
         [0013]     receiving light reflected from the marker with the photo receiver;  
         [0014]     determining a reference value based on a first value of the first signal outputted when the marker is irradiated;  
         [0015]     executing a predetermined calculation with respect to the reference value to determine a threshold value;  
         [0016]     transporting the optical sensor above the tray, while emitting light from the photo emitter and comparing the first signal outputted from the photo receiver with the threshold value;  
         [0017]     identifying a position of the marker based on the comparison of the first signal and the threshold value; and  
         [0018]     determining a position at which the printing begins based on the identified position of the marker.  
         [0019]     The calculation may be magnification with a value greater than 1.  
         [0020]     The calculation may be based on the first value of the first signal and a second value of the first signal outputted when another part of the tray is irradiated by the light emitted from the photo emitter.  
         [0021]     The calculation may include: obtaining a third value which is a difference between the first value and the second value; obtaining a fourth value by multiplying a predetermined value and the third value; and obtaining the threshold value by adding the first value and the fourth value.  
         [0022]     According to the invention, there is also provided a program product comprising a program operable to cause a computer to execute the above method.  
         [0023]     According to the invention, there is also provided a printing apparatus, adapted to perform printing on an optical recording medium placed on a tray provided with a marker, comprising:  
         [0024]     a tray, comprising a body adapted to mount the optical recording medium, and a marker provided on the body;  
         [0025]     an optical sensor, comprising a photo emitter operable to emit light and a photo receiver operable to receive light and output a first signal in accordance with an amount of the received light;  
         [0026]     a transporter, operable to transport the optical sensor above the tray;  
         [0027]     a reference value provider, operable to determine a reference value based on a first value of the first signal outputted when the marker is irradiated with the light emitted from the photo emitter;  
         [0028]     a calculator, operable to execute a predetermined calculation with respect to the reference value to determine a threshold value;  
         [0029]     a comparator, operable to compare the first signal and the threshold value when the optical sensor is transported above the tray while emitting the light from the photo emitter; and  
         [0030]     a position identifier, operable to identify a position of the marker based on the comparison of the first signal and the threshold value, and to determine a position at which the printing begins based on the identified position of the marker.  
         [0031]     The body of the tray may have a first optical reflectance and the marker may have a second optical reflectance which is higher than the first optical reflectance.  
         [0032]     The body of the tray may be formed with a through hole located adjacent to the marker. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein:  
         [0034]      FIG. 1  is a perspective view of an inside of a printer according to one embodiment of the invention;  
         [0035]      FIG. 2A  is a schematic side view showing a positional relationship among a carriage, a disk tray and a platen in the printer;  
         [0036]      FIG. 2B  is a schematic plan view showing the positional relationship;  
         [0037]      FIG. 3  is a plan view of a disk tray;  
         [0038]      FIG. 4A  is a circuit diagram of an optical sensor in the printer;  
         [0039]      FIG. 4B  is a schematic view showing the optical sensor and a marker provided on the disk tray;  
         [0040]      FIG. 5  is a block diagram showing a control system of the printer;  
         [0041]      FIG. 6  is a flowchart showing a processing for detecting a position of a printing medium performed in the printer;  
         [0042]      FIG. 7A  is a diagram showing how to detect the marker in the processing when the condition of the maker is normal;  
         [0043]      FIG. 7B  is a diagram showing how to detect the marker in the processing when the condition of the maker is deteriorated by secular change;  
         [0044]      FIG. 8A  is a diagram showing how to detect the marker in a related-art processing when the condition of the maker is normal; and  
         [0045]      FIG. 8B  is a diagram showing how to detect the marker in the related-art processing when the condition of the maker is deteriorated by secular change. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0046]     Embodiments of the invention will be described below in detail with reference to the accompanying drawings.  
         [0047]     A printing apparatus referred to in this specification comprehends a printer  10  shown in  FIG. 1 , or a combination of the printer  10  and a personal computer  120  shown in  FIG. 5 . The printer  10  comprises a chassis  11 , and a carriage  40  which is reciprocately movable relative to the chassis  11  in a primary scanning direction.  
         [0048]     The carriage  40  comprises an ink cartridge  42  that stores black ink and color ink (yellow, cyan, magenta, etc.), and a mounting part  41  that mounts the ink cartridge. A recording head (not shown) is provided below the mounting part  41  to be opposed to a disk tray  60  or a printing sheet (not shown). A lower end face of the recording head defines a nozzle formation face, from which ink can be ejected.  
         [0049]     A part of a timing belt  27  is fixed to the mounting part  41 . Also, the mounting part  41  is formed with an insertion hole  47 , through which an elongated guide shaft  25  can be inserted. The timing belt  27  is stretched between a drive pulley  28  of a carriage motor  26 , and a follower pulley  29 . Accordingly, when the carriage motor  26  rotates, the timing belt  27  is driven and the carriage  40  is moved along the guide shaft  25 . At this time, since an encoder  43  outputs a signal corresponding to a position of the carriage  40 , it is possible to know the position of the carriage  40  with reference to the signal.  
         [0050]     A platen  32  having a plurality of ribs is provided in a position opposed to the nozzle formation face of the carriage  40  on the chassis  11 , and the disk tray  60  holding thereon an optical recording medium  80  is conveyed above the ribs. Provided on an upstream side (a side, to which a printing sheet is fed) of the chassis  11  is a support frame  22  having a shielding plate portion  23  and side plate portions  24  on both ends of the shielding plate portion  23  to be bent toward a downstream side (a side, from which a printing sheet is ejected). Fixed to the side plate portions  24  are the driven pulley  29 , around which the timing belt  27  is stretched, and the guide shaft  25 . The carriage motor  26  is fixed to the shielding plate portion  23 .  
         [0051]     A sheet feeding motor  51  is provided on an upstream side of the shielding plate portion  23 , a roller  20  is provided and rotated whereby the disk tray  60  is moved in the secondary scanning direction.  
         [0052]     As shown in  FIG. 2A , the carriage  40  is provided in a position opposed to the platen  32  with the disk tray  60  therebetween. A plurality of ribs  32   a  are provided on a top of the platen  32 , and the disk tray  60  is conveyed above the ribs  32   a  by the roller  20 . An optical sensor  45  is provided on a bottom face of the carriage  40  to detect a position of a marker as described later, thereby calculating a position of the optical recording medium  80  and identifying a printing start position. Media, for example, CD-R or DVD-R (Digital Versatile Disk Recordable), etc., are used as the optical recording medium  80 . In addition, while the optical recording medium  80  in an example shown in  FIG. 2A  is put in a state of projecting from the disk tray  60  for the simplification of the figure, the optical recording medium  80  is actually positioned in substantially the same level as a top of the disk tray  60  since a recess, into which the optical recording medium  80  is fitted, is provided on the top of the disk tray  60  (described later in detail).  
         [0053]     As shown in  FIG. 2B , the plurality of ribs  32   a  are provided on the top of the platen  32 . Also, the optical sensor  45  is provided on the bottom face of the carriage  40  on an upstream side to detect a position of a marker.  
         [0054]     As shown in  FIGS. 2A and 2B , the disk tray  60  is a flat plate-shaped member having a predetermined thickness. As shown in  FIG. 3 , the disk tray  60  is formed with a recess  61 , in which the optical recording medium  80  is placed. Provided centrally of the recess  61  is a disk support  61   a,  which is inserted into a drive hole formed centrally of the optical recording medium  80  to support the optical recording medium  80 . Formed around the disk support  61   a  are holes  61   b,    61   c,    61   d  and  61   e,  into which fingers are inserted when an optical recording medium  80  is to be removed.  
         [0055]     Formed on a right and lower part of the disk tray  60  are a marker  62   a  and a hole  62   b,  which detect the fact that the disk tray  60  has been inserted to a predetermined position of the printer  10 . Also, formed at upper and lower ends of the recess  61  are markers  63   a,    65   a  and holes  63   b,    65   b,  by which a position of the optical recording medium  80  in the secondary scanning direction is detected. Further, formed on the left and right of an upper part of the recess  61  are markers  64   a,    66   a  and holes  64   b,    66   b,  by which a position of the optical recording medium  80  in the primary scanning direction is detected. In addition, the markers  62   a  to  66   a  are formed from a material (for example, a white plastic), which is higher in optical reflectance than a material (for example, a black plastic), which forms the disk tray  60 . Also, the holes  62   b  to  66   b  are bored adjacent to the markers  62   a  to  66   a.  In addition, the disk tray  60  is inserted into the printer  10  in a direction indicated by an arrow  67 .  
         [0056]     As shown in  FIG. 4A , the optical sensor  45  comprises an optical sensor element  451 . The optical sensor element  451  comprises a photo emitter  451   a  and a photo receiver  451   b  to irradiate light on the markers  62   a  to  66   a  to convert intensity of a reflected light into corresponding electric signals to output the same. Here, the photo emitter  451   a  is formed from, for example, a light emitting diode or the like to emit infrared rays. The photo receiver  451   b  is formed from, for example, a phototransistor or the like to permit a reflected light, which is emitted from the photo emitter  451   a  and reflected by the markers  62   a  to  66   a,  to be made incident thereupon, and thus is changed in resistance corresponding to the intensity of reflected light. In addition, the photo receiver  451   b  comprises, at a light incident part, a filter that attenuates visible light in order to lessen influences of ambient light (mainly, visible light).  
         [0057]     The optical sensor element  451  is placed on, for example, a printed board, and connects thereto terminals  452 ,  453 ,  454  provided on the printed board, and an electric source and grounding of the printer  10 . That is, the terminal  452  is connected to an anode of the photo emitter  451   a,  the terminal  452  being connected to one end of a resistor  90  arranged in a sensor controlling circuit  112  (described later) of the printer  10 . Also, the terminal  453  is connected to a cathode side of the photo emitter  451   a,  an emitter side of the photo receiver  451   b,  and grounding of the sensor controlling circuit  112 . Also, the terminal  454  is connected to a collector side of the photo receiver  451   b  and a resistor  91  arranged in the sensor controlling circuit  112 .  
         [0058]     As shown in  FIG. 4B , the optical sensor element  451  comprises the photo emitter  451   a  and the photo receiver  45   b,  both of which are placed inside a housing  460  with a partition  461 . Here, the housing  460  prevents an ambient light from being incident upon the photo receiver  451   b.  The partition  461  prevents light, which is emitted from the photo emitter  451   a,  from being made incident directly upon the photo receiver  45   b.  As shown in  FIG. 4B , when the markers  62   a  to  66   a  being detected objects are present, light emitted from the photo emitter  451   a  is reflected by surfaces of the markers  62   a  to  66   a  to be made incident upon the photo receiver  45   b.  As a result, the photo receiver  451   b  is activated and an electric current flows through the resistor  91  from an electric source Vcc, so that Vs being an output voltage is put in a low state. On the other hand, when the markers  62   a  to  66   a  are not present, a reflected light is not made incident upon the photo receiver  451   b  and the photo receiver  451   b  is deactivated, so that an output voltage Vs is put in a high state.  
         [0059]     Subsequently, an explanation will be given to a control system of the printer  10  shown in  FIG. 1 . As shown in  FIG. 5 , the control system of the printer  10  comprises a CPU (Central Processing Unit)  100 , a ROM (Read Only Memory)  101 , a RAM (Random Access Memory)  102 , a EEPROM (Electrically Erasable and Programmable ROM)  103 , an I/F (Interface)  104 , an I/O (Input and Output) unit  105 , a bus  106 , an I/O circuit  107 , a motor controlling circuit  110 , stepping motors  111 , the sensor controlling circuit  112 , the optical sensor  45 , a head driving circuit  113 , and a recording head  46 , and the personal computer (PC)  120  is connected to the I/F  104 .  
         [0060]     Here, the CPU  100  executes various arithmetic processings according to programs stored in the ROM  101  and the EEPROM  103  and controls respective parts of the apparatus including the stepping motors  111 .  
         [0061]     The ROM  101  comprises a semiconductor memory that stores various programs executed by the CPU  100  and various data.  
         [0062]     The RAM  102  comprises a semiconductor memory that temporarily stores programs executed by the CPU  100  and data.  
         [0063]     The EEPROM  103  comprises a semiconductor memory, in which predetermined data obtained as a result of the arithmetic processings in the CPU  100  are stored and the data are held also after the electric source of the printer  10  is interrupted.  
         [0064]     The I/F  104  comprises a device for appropriate conversion of the form of data presentation when it gives and receives information from the personal computer  120 . The I/O  105  comprises a device that gives and receives information from the input/output circuit  107 .  
         [0065]     The bus  106  comprises a signal conductor group that connects the CPU  100 , the ROM  101 , the RAM  102 , the EEPROM  103 , the I/F  104 , and the I/O  105  mutually and enables giving and receiving information among these elements.  
         [0066]     The motor controlling circuit  110  comprises, for example, a logic circuit and a drive circuit and controls the stepping motors  111  according to control by the CPU  100 .  
         [0067]     The stepping motors  111  comprises, for example, the carriage motor  26  and the sheet feeding motor  51  and drives the carriage  40  and the roller  20  according to control by the motor controlling circuit  110 .  
         [0068]     The sensor controlling circuit  112  is one that controls the optical sensor  45  and comprises the resistors  90 ,  91  shown in  FIG. 4A  and a buffer, which supplies the output voltage Vs from the optical sensor  45  to the input/output circuit  107 .  
         [0069]     The optical sensor  45  detects the markers  62   a  to  66   a  being detected objects as described above with reference to  FIGS. 4A and 4B .  
         [0070]     The head driving circuit  113  comprises a driver connected to the recording head  46 , which executes a recording processing on a labeled surface of an optical recording medium  80 , and exercises control of a recording processing on the recording head  46 . As described above, the recording head  46  ejects ink of various colors from the plurality of nozzles according to control by the head driving circuit  113  and prints desired images and texts on a labeled surface of an optical recording medium  80 .  
         [0071]     Subsequently, an explanation will be given to an operation of the printer  10  with reference to  FIG. 6 .  
         [0072]     Programs for execution of the flowchart are stored in the ROM  101 , etc., read and executed by the CPU  100  at need. When the processings in the flowchart are started, the following steps are executed.  
         [0073]     Step S 10 : A user places an optical recording medium  80  into the recess  61  of the disk tray  60  to fix the same with the disk support  61   a,  and thereafter inserts the disk tray  60  between the platen  32  and the carriage  40  of the printer  10  in the direction indicated by the arrow  67  in  FIG. 3 .  
         [0074]     Step S 11 : The CPU  100  uses the optical sensor  45  to detect the marker  62   a  for confirmation of the fact that the disk tray  60  has been inserted to the predetermined position. In addition, the CPU  100  refers to control data (data indicative of a position of the marker  62   a  on the disk tray  60 ) stored in the ROM  101  to drive the carriage motor  26  to move the same to a position, in which the optical sensor  45  can detect the marker  62   a.    
         [0075]     Step S 12 : Referring to whether the marker  62   a  has been enabled to be detected in Step S 12 , the CPU  100  judges whether the disk tray  60  has been inserted to the predetermined position, proceeds to Step S 13  in the case where the disk tray has been inserted, and repeats the same processing in a case except that. In addition, the case where the marker  62   a  cannot be detected in the processing conceivably includes, for example, the case where the disk tray  60  is inadequately inserted, or the case where the marker  62   a  is decreased in reflectance due to secular change, or the like. In the former case, the state of insertion can be confirmed by providing a switch, which is actuated when the disk tray  60  has been inserted to the predetermined position. Also, since the situation in the latter case can be avoided by a processing described later, the procedure may proceed to the processing in Step S 13  in the case where the marker  62   a  cannot be detected even when the processings in Step S 11  and Step S 12  are repeated predetermined times.  
         [0076]     Step S 13 : Referring to the control data stored in the ROM  101 , the CPU  100  controls the carriage motor  26  and the sheet feeding motor  51  to move the optical sensor  45  above a predetermined marker. In addition, selected as a marker being an object is the marker  63   a  closest to the marker  62   a  being made an object of detection in Step S 11  and smallest in moving distance in the primary scanning direction and in the secondary scanning direction, or the markers  64   a,    66   a  existent in an end, which is contacted by a user&#39;s fingers to be liable to become dirty. Alternatively, the marker  62   a  can be selected. An explanation is given below to an example, in which the marker  63   a  is made an object. In addition, the reason why a marker liable to become dirty is selected is that all the markers can be surely detected by selecting a marker being worst in property.  
         [0077]     Step S 14 : The CPU  100  measures an output voltage Vb of the optical sensor  45  according to a reflected light from the marker  63   a.  That is, the CPU  100  controls the sensor controlling circuit  112  to have the photo emitter  451   a  of the optical sensor  45  irradiating a light. As a result, the light irradiated from the photo emitter  451   a  is reflected by the marker  63   a  to be made incident upon the photo receiver  45   b.  Since the photo receiver  45   b  is varied in resistance according to the intensity of a reflected light, an output voltage Vs according to the intensity of a reflected light appears in the resistor  91 . The CPU  100  inputs thereinto the output voltage Vs through the sensor controlling circuit  112  to make the same a voltage Vb corresponding to a reflected light from the marker  63   a.    
         [0078]     Step S 15 : The CPU  100  calculates a discriminant threshold Vth in detecting a marker. That is, the CPU  100  doubles the voltage Vb corresponding to the marker  63   a  and found in Step S 14  to provide Vth. Here, the magnification may be an arbitrary value greater than 1. Such value is determined individually and specifically according to the sensitivity of the optical sensor  45 , the optical reflectance of the marker  63   a,  or the like.  
         [0079]     Step S 16 : The CPU  100  controls the carriage motor  26  and the sheet feeding motor  51  to use the optical sensor  45  to detect positions of the markers  62   a  to  66   a  in a predetermined order. Specifically, in case of, for example, detecting the marker  64   a  and the marker  66   a  in the primary scanning direction, the CPU  100  controls the sheet feeding motor  51  to move the marker  64   a  and the marker  66   a  of the disk tray  60  to a position just below the optical sensor  45 . The CPU  100  drives the carriage motor  26  to move the carriage  40  to a left end to move the carriage  40  to a right end while having the photo emitter  451   a  emitting light. At this time, the CPU  100  compares a voltage output from the optical sensor  45  with the discriminant threshold Vth, thereby identifying positions of the marker  62   a  and the marker  66   a.    
         [0080]      FIG. 7A  shows a change in the output voltage Vs of the optical sensor  45  in the case where the carriage  40  is scanned in the primary scanning direction. Here, an abscissa indicates a position of the optical sensor  45  in the primary scanning direction and an ordinate indicates the output voltage Vs of the optical sensor  45 . This figure shows a case where the disk tray  60  has not undergone any secular change. When the optical sensor  45  is moved, the optical sensor  45  first reaches a position above the hole  66   b.  Since the hole  66   b  transmits therethrough light irradiated by the photo emitter  451   a,  reflected light does not reach the photo receiver  45   b  and the photo receiver  451   b  is put in a state of being deactivated, so that an output voltage becomes Vp. When the optical sensor  45  is moved to reach a position above the marker  66   a,  light irradiated by the photo emitter  451   a  is reflected with a high rate to reach the photo receiver  451   b,  so that the photo receiver  451   b  is put in a state of being activated and an output voltage becomes Vb. Here, since Vb&lt;Vth is established, the CPU  100  judges that one end of the marker has been detected, and has the RAM  102  storing an output of the encoder  43  at that time.  
         [0081]     When the carriage  40  is further moved, the optical sensor  45  moves onto the flat plate member of the disk tray  60  from the marker  66   a.  At this time, since the flat plate member reflects light to some extent, the optical sensor  45  outputs a voltage Vm somewhat lower than that with the hole  66   b.  Here, because of Vm&gt;Vth, the CPU  100  judges that the other end of the marker  66   a  has been detected, and stores an output of the encoder  43  at that time.  
         [0082]     When the carriage  40  is further moved, the optical sensor  45  reaches one end of the marker  64   a.  As a result, an output of the optical sensor  45  changes to Vb from Vm. Here, because of Vb&lt;Vth, the CPU  100  judges that one end of the marker  64   a  has been detected, and has the RAM  102  storing an output of the encoder  43  at that time.  
         [0083]     When the carriage  40  is further moved, the optical sensor  45  reaches the hole  64   b.  As a result, an output of the optical sensor  45  changes to Vp from Vb. Here, because of Vb&gt;Vth, the CPU  100  judges that the other end of the marker  64   a  has been detected, and stores an output of the encoder  43  at that time.  
         [0084]     In the processings described above, respective ends of the marker  66   a  and the marker  64   a  are detected and outputs of the encoder  43  in respective occasions are stored in the RAM  102 .  
         [0085]     In this embodiment, since the marker  64   a  ( 66   a ) and the hole  64   b  ( 66   b ) are located adjacent to each other, the first signal is largely varied at the boundary between the hole and the marker, so that the marker can be surely detected.  
         [0086]     In this embodiment, an output voltage Vb of the optical sensor  45  for a marker is magnified to calculate a discriminant threshold Vth, and presence and absence of a marker are judged on the basis of the discriminant threshold Vth. In a related art, since a discriminant threshold is a fixed value (in an example shown in  FIGS. 8A and 8B , Vth=1.1 V), no problem is caused in the case where a marker is normal in optical reflectance as shown in  FIG. 8A . That is, assuming that an output voltage in the markers  64   a,    66   a  is 0.5 V and an output voltage in the holes  64   b,    66   b  is 0.3 V, 1.1 V existent therebetween is set as a discriminant threshold. As shown in  FIG. 8B , however, in the case where a marker is decreased in optical reflectance due to secular change, etc., an output voltage Vs from the optical sensor  45  does not fall below the discriminant threshold Vth, so that it becomes impossible to detect a marker. Also, a similar situation is thought to occur in the case where not only an optical reflectance of a marker but also the sensitivity of the optical sensor  45  varies due to secular change, etc.  
         [0087]     On the other hand, in this embodiment, Vth is set according to variation in Vb, so that it is possible to surely detect a marker even in the case where a marker undergoes secular change to be decreased in optical reflectance, or the optical sensor  45  is varied in sensitivity.  FIG. 7B  shows the case a marker undergoes secular change to be decreased in optical reflectance. As shown in this figure, since the discriminant threshold Vth is set on the basis of Vb even in the case where the markers  64   a,    66   a  are decreased in optical reflectance and Vb is decreased (since Vb&lt;Vth is established), it is possible to surely detect the markers  64   a,    66   a.    
         [0088]     Although an explanation has been given taking, as an example, the case where the markers  64   a,    66   a  are detected, the markers  63   a,    65   a  are also detected through the same processings and an output of the encoder  43  at the time of detection is stored in the RAM  102 .  
         [0089]     Step S 17 : The CPU  100  refers to information representative of a position of a marker detected in Step S 16  to identify a position of an optical recording medium  80 . That is, since the positional relationship between the markers  63   a  to  66   a  and an optical recording medium  80  is beforehand known, a position of an optical recording medium  80  can be identified indirectly by detecting positions of the markers  63   a  to  66   a.    
         [0090]     Step S 18 : The CPU  100  refers to a position of an optical recording medium  80  detected in Step S 18  to identify a printing start position. Specifically, since the positional relationship between an optical recording medium  80  and the recording head  46  is made apparent by the processing in Step S 17 , a printing start position is identified on the basis of the positional relationship.  
         [0091]     Step S 19 : The CPU  100  adjusts a position of picture data on the basis of a printing start position identified in Step S 18 , and thereafter executes the print processing. Specifically, after a printing start position is adjusted referring to a position of an optical recording medium  80  detected in Step S 18 , the print processing is executed on the basis of print data supplied from the personal computer  120 , and a predetermined picture or the like is printed on a labeled surface of an optical recording medium  80 .  
         [0092]     Step S 20 : The CPU  100  drives the sheet feeding motor  51  to execute the processing of ejecting the disk tray  60 . As a result, a user can take out the disk tray  60  from the printer  10  and remove an optical recording medium  80  from the disk tray  60 .  
         [0093]     The discriminant threshold Vth set in the manner described above can be stored in, for example, the EEPROM  103  to be made use of for a subsequent printing. In this case, since it is thought that a marker or the like has possibly undergone secular change in the case where a long time (for example, in the case where one year or longer elapses) elapses since a previous printing, a date, when printing is carried out at the last time, may be stored together with the discriminant threshold. A discriminant threshold may be set again in the case where a predetermined term or longer has elapsed (for example, in the case where one month or longer has elapsed).  
         [0094]     As described above, according to the invention, since a value obtained by increasing (amplifying) an output voltage of the optical sensor  45  for a predetermined marker with a predetermined magnification greater than 1 is made use of as a discriminant threshold and presence or absence of a marker is judged, a marker can be surely detected even in the case where a marker is varied in optical reflectance due to secular change or the like.  
         [0095]     Also, it is known that the optical sensor  45  can be varied in element sensitivity due to secular change, even in which case a marker can be stably detected in spite of secular change since a discriminant threshold is updated each time.  
         [0096]     Also, while it is known that the optical sensor  45  involves dispersion in element sensitivity every one, a marker can be stably detected in spite of dispersion since an appropriate discriminant threshold is set every optical sensor.  
         [0097]     In this embodiment, a value corresponding to an output of the optical sensor  45  for a marker is increased with a predetermined magnification greater than 1 to provide a discriminant threshold. However, a discriminant threshold may be determined by using a value corresponding to an output of the optical sensor  45  for a marker and other parts (for example, a hole or a flat plate member).  
         [0098]     Specifically, in the case where, for example, an output voltage of the optical sensor  45  for a hole or a flat plate member is made Vp, a discriminant threshold may be provided assuming Vth=k·(Vp−Vb)+Vb. In addition, a predetermined value (for example, “0.2” or the like), which provides for k&lt;1 according to an environment in use and a state of a printer as used is set as k. According to such method, a discriminant threshold Vth is set to a predetermined position between Vb and Vp, so that it is possible to surely detect a marker even in case of variation in element sensitivity.  
         [0099]     In this embodiment, the disk tray  60  includes five markers  62   a  to  66   a.  However, the number of markers may be greater than the above.  
         [0100]     In this embodiment, the marker  62   a  is made use of in order to set a discriminant threshold. However, this may be applied to other markers than the above. In short, it suffices to select a marker suited to a purpose of use in a range free of erroneous detection.  
         [0101]     In this embodiment, the program for execution of the processing shown in  FIG. 6  is stored in the ROM  101 . However, the program may be stored in the personal computer  120 .  
         [0102]     In this embodiment, print data is supplied from the personal computer  120 . However, a removable recording medium may be connected to the printer  10  and print data may be supplied directly from the recording medium.  
         [0103]     In this embodiment, only the single optical sensor  45  is provided. However, a plurality of optical sensors may be provided and a marker may be detected by the respective sensors. In this case, a discriminant threshold may be set every optical sensor, or a discriminant threshold may be set for a specific optical sensor and a discriminant threshold may be calibrated and used taking account of a difference in sensitivity for other optical sensors.  
         [0104]     The processing can be executed by a computer. In this case, a program is provided to describe the content of a processing that the printing apparatus executes. A computer executes the program whereby the processing is performed in the computer. The program, which describes the content of the processing, can be recorded in a recording medium, which can be read by a computer. A recording medium, which can be read by a computer, includes a magnetic recording system, an optical disk, a magneto-optical recording medium, a semiconductor memory, etc. The magnetic recording system includes a hard disk drive (HDD), a floppy disk (FD), a magnetic tape, etc. The optical disk includes a DVD, a DVD-RAM, a CD-ROM, a CD-R/RW (Rewritable), etc. The magneto-optical recording medium includes an MO (magneto-Optical disk), etc.  
         [0105]     In case of distribution of programs, portable recording media, such as DVD, CD-ROM, etc., with the programs recorded are sold. Also, programs are stored in a storage device of a server computer, and the programs can be transferred to other computers from the server computer.  
         [0106]     A computer that executes programs stores in its own storage device programs recorded in a portable recording medium, or programs transferred from the server computer. The computer reads the programs from its own storage device to execute a processing according to the programs. In addition, the computer can read the programs directly from a portable recording medium to execute a processing according to the programs. Also, the computer can also execute a processing sequentially according to the received programs each time a program is transferred from the server computer.  
         [0107]     Although the present invention has been shown and described with reference to specific preferred embodiments, various changes and modifications will be apparent to those skilled in the art from the teachings herein. Such changes and modifications as are obvious are deemed to come within the spirit, scope and contemplation of the invention as defined in the appended claims.