Patent Publication Number: US-7591602-B2

Title: Gap detector, liquid ejecting apparatus incorporating the same, and gap detecting method executed in the apparatus

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a gap detector for detecting a gap between a liquid ejecting head and a target medium by detecting a rotating state of a carriage guide shaft. 
   The present invention also relates to a liquid ejecting apparatus incorporating such a gap detector and to a gap detecting method executed in such a liquid ejecting apparatus. 
   The liquid ejecting apparatus is not limited to a recording apparatus, such as a printer, a copier, or a facsimile, which employs an ink jet recording head and ejects ink from the recording head to a recording medium, to thus effect recording. The liquid ejecting apparatus is employed to encompasses an apparatus that ejects a liquid appropriate to an application, in place of ink from a liquid ejecting head corresponding to the ink jet recording head onto a target medium corresponding to a recording medium, thereby causing the liquid to adhere to the medium. 
   In addition to the recording head, the liquid ejecting head encompasses a coloring material ejecting head used for manufacturing a color filer such as a liquid-crystal display or the like; an electrode material (conductive paste) ejecting head used for forming electrodes, such as an organic EL display or a field emission display (FED) or the like; a bio-organic substance ejecting head used for manufacturing a bio-chip; a sample ejecting head serving as a precision pipette; and the like. 
   Some of the liquid ejecting apparatus are configured to be able to adjust a gap between a liquid ejecting head and a target medium or a platen for supporting the target medium (hereinafter abbreviated as “the platen gap”), as described in Japanese Patent Publication No. 2002-67428A, in order to appropriately perform recording (liquid ejection) on a variety of types of target media (recording media); more specifically, recording media of different thicknesses. 
   In the recording apparatus disclosed in the above publication, a carriage guide shaft is formed into an eccentric shaft structure, so that the carriage is vertically actuated in association with rotation of the carriage guide shaft, to thus control the platen gap. Further, the control of the platen gap is not manually performed but automatically controlled by rotating the carriage guide shaft with a driving force of a motor. 
   With this configuration, several positions for defining different platen gaps can be determined. For instance, there may be a case where four positions are set; namely, a position for recording information on plain paper, a position for recording information on a cardboard, a position for recording data on an optical disk, and a position where a tray having an optical disk set thereon is inserted. In order to achieve a superior recording result, appropriate print control must be performed for each position. 
   Accordingly, when recording is carried out, a control section of a printer must recognize the current platen gap. To this end, a detector for detecting the current platen gap is required. Japanese Patent Publication Nos. 2003-211778A and 2004-314591A disclose a technique for detecting the current platen gap. For instance, Japanese Patent Publication No. 2003-211778A discloses a device capable of identifying four positions by a combination of activation/deactivation of two sensors. Japanese Patent Publication No. 2004-314591A discloses a stable area sensor. In this sensor, a disc, which has four light-shielding plates provided around an outer periphery thereof, is attached to a carrier guide shaft, to thus detect a light-shielding plate that is now blocking the optical sensor. Since each of the light-shielding plates corresponds to one of the platen gaps, the current platen gap can be determined based on the detection result. 
   However, the above technique requires a plurality of exclusive sensors for detecting a plurality of platen gaps, which has become a factor for incurring a cost hike. 
   This technique also raises the following problem. Provision of an exclusive motor for switching the platen gap entails a cost hike. Accordingly, the recording apparatus is desirably constructed to cause the motor that drives a roller for transporting a recording medium to double as a motor for switching the platen gap; and to transmit or cut off power from the motor to the carriage guide shaft, as required. Here, operation for switching transmission and cut-off of power from the motor to the carriage guide shaft can also be performed by a carriage. Specifically, the recording apparatus is provided with an actuated member and the actuated member is pressed by the carriage, to thus switch transmission of power from the motor to the carriage guide shaft. 
   With this configuration, the number of components can be curtailed, which is preferable in terms of cost. However, the construction raises the following problem. Specifically, when a recording sheet is present in a position where a carriage opposes a recording head during the course of the carriage proceeding to press the actuated member, the recording head may rub against the recording sheet when switching to the platen gap appropriate to the recording sheet has not yet been performed, or the recording head may collide with a side edge of the recording sheet, to thus cause paper jamming. When a tray having a plate-shaped member, such as an optical disk, set therein is inserted into a transporting path while sufficiently-large the platen gap is not ensured, the tray collides with a recording head in the same manner, which in turn may induce a failure. If a distance, over which the carriage travels, is long when the carriage proceeds to press the actuated member, operation for switching the platen gap requires consumption of much time. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to provide a gap sensor capable of accurately detecting a plurality of platen gaps with a low-cost configuration. 
   It is also an object of the invention to provide a liquid ejecting apparatus capable of preventing occurrence of interference between a liquid ejecting head and a target medium. 
   It is also an object of the invention to provide a liquid ejecting apparatus capable of adjusting the platen gap quickly. 
   In order to achieve at least one of the above objects, according to the invention, there is provided a gap detector, operable to detect a distance between a liquid ejecting head adapted to eject liquid toward a target medium and a platen adapted to support the target medium, comprising: 
   a circular plate, adapted to be provided on a guide shaft which is rotated together with the rotary plate to selectively determine the distance as one of a plurality of distances, the circular plate comprising a plurality of flags formed on an outer periphery, each of which is associated with one of the distances; and 
   a sensor, disposed in the vicinity of the outer periphery of the circular plate and operable to output a signal indicative of a passage of each of the flags in accordance with the rotation of the circular plate, 
   wherein at least one of intervals between the flags in a circumferential direction of the circular plate is made different from the other. 
   One of the flags forming the at least one of the intervals may be associated with a minimum one of the distances. 
   In order to achieve at least one of the above objects, according to the invention, there is also provided a liquid ejecting apparatus, comprising: 
   a liquid ejecting head, adapted to eject liquid toward a target medium; 
   a platen, adapted to support the target medium; 
   a rotatable guide shaft, extending in a first direction; 
   a carriage, mounting the liquid ejecting head and being supported by the guide shaft so as to reciprocately movable thereon; 
   a gap adjuster, operable to rotate the guide shaft, thereby moving an axis of the guide shaft in a second direction orthogonal to the first direction to selectively determine a distance between the liquid ejecting head and the platen as one of a plurality of distances; 
   a circular plate, provided on the guide shaft and rotatable together with the guide shaft, the circular plate comprising a plurality of flags formed on an outer periphery thereof, each of which is associated with one of the distances; and 
   a sensor, disposed in the vicinity of the outer periphery of the circular plate and operable to output a signal indicative of a passage of each of the flags in accordance with the rotation of the circular plate, 
   wherein at least one of intervals between the flags in a circumferential direction of the circular plate is made different from the other. 
   With this configuration, the rotation amount of the rotary plate (i.e., the first motor), which is required after a certain flag has passed by the sensor before the next flag is detected, corresponds to the unique interval, whereby the flag detected at that time can be determined to be at least one of the two flags whose interval is unique. On the basis of the rotational direction of the rotary plate (i.e., the first motor) obtained at that time, the detected flag can be identified as one of the two flags; namely, one of the plurality of flags. 
   So long as the thus-detected flag can be identified as any one of a plurality of the flags (as corresponding to any one of the distances), a positional relationship between the identified flag and another flag is determined in advance. Accordingly, the position of the other flag; i.e., the rotation amount of the first motor required to change the distance between the liquid ejecting head and the platen (platen gap) can be determined. 
   Thus, an exclusive sensor to detect the platen gap can be embodied by one sensor. For this reason, cost can be curtailed. Detection of the unique interval requires a sensor that detects the rotation amount of the first motor used for rotating the guide shaft; namely, the rotation amount of the rotary plate. 
   Moreover, since the flags are provided such that each of those is associated with one of the selectable distances, the respective distances can be detected without fail. Namely, selection of the platen gap can be elected without fail. 
   One of the flags forming the at least one of the intervals may be associated with a minimum one of the distances. 
   By taking this flag as a reference flag, the guide shaft is rotated, to thus sequentially increase the platen gap, and hence control is facilitated. 
   The liquid ejecting apparatus may further comprise: a first motor, operable to rotate the guide shaft; a detector, operable to detect a rotation amount of the first motor; and a controller, operable to identify one of the flags forming the at least one of the intervals with reference to the signal and the rotation amount. 
   Here, the liquid ejecting apparatus may further comprise: a second motor, operable to move the carriage along the guide shaft; and a transmitter; configured to be actuated by the carriage so as to be either a first state for transmitting a driving force from the first motor to the guide shaft, or a second state for not transmitting the driving force from the first motor to the guide shaft. In this case, the controller is configured to drive the second motor to move the carriage so as to place the transmitter in the first state before the identification of the one of the flags. 
   Here, the transmitter may comprise: a gear train, coupling the first motor and the guide shaft, the gear train including a spur gear having a rotary shaft extending parallel to the first direction; and an urging member, urging the spur gear toward the carriage. In this case, the spur gear is slidable in the first direction between a first position meshing with an adjacent gear in the gear train to place the transmitter in the first state and a second position separating from the adjacent gear to place the transmitter in the second state. The spur gear is placed in the first position when the carriage presses the rotary shaft against an urging force of the urging member. The spur gear is placed in the second position in accordance with the urging force when the carriage is separated from the rotary shaft. The controller is configured to drive the second motor to move the carriage so as to separate from the rotary shaft, after the identification of the one of the flags. The controller is configured to drive the first motor so as to reverse a rotating direction thereof, after the separation of the carriage from the rotary shaft 
   Teeth of the gears may remain in press contact with each other even if the carriage has separated from the spur gear, thereby failing to move the spur gear to the second position. Accordingly, there may arise a case where the transmitter is not placed in the second state. However, with the above configuration, the press contact between the teeth of the gears is lessened or cancelled, whereby the engagement section can be displaced smoothly from the first position to the second position. Specifically, the transmitter can be brought into the second state without fail. 
   The liquid ejecting apparatus may further comprise a transmitter, configured to be actuated by the carriage so as to be either a first state for transmitting a driving force from the first motor to the guide shaft, or a second state for not transmitting the driving force from the first motor to the guide shaft, the transmitter disposed in an opposite side to a transporting path of the target medium relative to a home position of the carriage. 
   With this configuration, the carriage does not need to straddle the transporting path of the target medium during the course of the carriage proceeding from the home position to actuate the transmitter; namely, the carriage does not interfere with the target medium. Consequently, even when the platen gap is set in an inappropriate state, occurrence of interference occurs between the liquid ejecting head and the target medium. Moreover, the travel distance of the carriage required to proceed to actuate the transmitter can be shortened, and operation for changing the platen gap can be performed within a short period of time. 
   Here, the transporting path may be adapted to transport a tray member mounting the target medium. 
   With this configuration, during the course of proceeding from the home position to actuate the transmitter, the carriage does not interfere with the tray. Occurrence of interference between the liquid ejecting head and the tray can be prevented, and there is no potential risk of inducing a failure or the like. 
   Moreover, the liquid ejecting apparatus may further comprise a first roller, adapted to transport the target medium in a third direction orthogonal to the first direction a the second direction, the roller arranged in an upstream side of the liquid ejecting head relative to the third direction. In this case, the transmitter is configured to transmit the driving force by way of the roller. 
   With this configuration, when compared with a configuration which employs an exclusive motor for rotating the guide shaft, an attempt to achieve cost-saving can be enabled. 
   Here, the liquid ejecting apparatus may further comprise a second roller, adapted to transport the target medium toward the first roller. In this case, the transmitter is configured to rotate the second roller in cooperation with the first roller. 
   With this configuration, the first roller, the second roller, and the guide shaft are driven by a single motor. Therefore, further cost-saving can be attained. 
   The transmitter may comprise a gear train, coupling the first motor and the guide shaft, the gear train including a spur gear having a rotary shaft extending parallel to the first direction. In this case, the spur gear is slidable in the first direction between a first position meshing with an adjacent gear in the gear train to place the transmitter in the first state and a second position separating from the adjacent gear to place the transmitter in the second state. 
   With this configuration, the switching between the first state and the second state of the transmitter can be attained with a simple and inexpensive configuration. 
   In order to achieve at least one of the above objects, according to the invention, there is also provided a gap detecting method executed in the above liquid ejecting apparatus, comprising: 
   rotating the guide shaft with a first motor; 
   detecting a rotation amount of the first motor; and 
   identifying one of the flags forming the at least one of the intervals with reference to the signal and the rotation amount. 
   Here, the detecting method may further comprise: 
   providing a transmitter, configured to be actuated by the carriage so as to be either a first state for transmitting a driving force from the first motor to the guide shaft, or a second state for not transmitting the driving force from the first motor to the guide shaft; and 
   moving the carriage along the guide shaft with a second motor so as to place the transmitter in the first state before the identification of the one of the flags. 
   Here, the detecting method may further comprise: 
   configuring the transmitter so as to comprise:
         a gear train, coupling the first motor and the guide shaft, the gear train including a spur gear having a rotary shaft extending parallel to the first direction; and   an urging member, urging the spur gear toward the carriage;       

   configuring the spur gear so as to be slidable in the first direction between a first position meshing with an adjacent gear in the gear train to place the transmitter in the first state and a second position separating from the adjacent gear to place the transmitter in the second state; 
   driving the second motor to move the carriage so as to press the rotary shaft against an urging force of the urging member, thereby placing the spur gear in the first position; 
   driving the second motor to move the carriage so as to separate from the rotary shaft, after the identification of the one of the flags, thereby placing the spur gear in the second position in accordance with the urging force when the carriage is separated from the rotary shaft; and 
   driving the first motor so as to reverse a rotating direction thereof, after the separation of the carriage from the rotary shaft. 
   In order to achieve at least one of the above objects, according to the invention, there is also provided a program product comprising a program causing a computer to execute the above detecting method. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a schematic section view of a printer according to one embodiment of the invention; 
       FIG. 2  is a block diagram showing a control section of the printer; 
       FIG. 3  is a perspective view of an internal structure of the printer; 
       FIG. 4  is a perspective view of a part of a platen gap adjuster in the printer; 
       FIG. 5  is an enlarged perspective view of a power transmitter in the platen gap adjuster; 
       FIG. 6  is an enlarged perspective view of the power transmitter, showing a state that a gear cover is removed; 
       FIGS. 7A and 7B  are enlarged perspective views showing how to operate the platen gap adjuster; 
       FIGS. 8A through 9A  are side views showing how to operate the platen gap adjuster; 
       FIG. 9B  is an enlarged perspective view showing a tooth shape of a gear in the power transmitter; 
       FIG. 10  is a perspective view of an internal structure of the printer, showing a platen gap detector; 
       FIG. 11  is a side view of the platen gap detector; 
       FIG. 12  is a diagram showing a relationship among a rotation angle of a carriage guide shaft, a sensor output from the platen gap detector and a platen gap; 
       FIGS. 13 and 14  are flow charts showing operations for determining a reference platen gap; and 
       FIG. 15  is an enlarged perspective view of a front section of the printer. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
   Embodiments of the invention will be described below in detail with reference to the accompanying drawings. 
   An ink jet printer (hereinafter simply called a “printer”)  1 , which is employed as an example of a recording apparatus or a liquid ejecting apparatus, will be described as one embodiment. In the following description, the right direction (the front side of the printer) in  FIG. 1  will be referred to as “downstream” relative to a transporting direction of a recording medium (target medium). The left direction (the rear side of the printer) will be referred to as “upstream” relative to the transporting direction. 
   A feeder  2 , into which recording media (target media) P in a cut-sheet form (hereinbelow referred to as “sheet P”) can be set in an inclined posture, is provided in a rear section of the printer  1 . A sheet P is fed from the feeder  2  to a transporter  4  located downstream. The thus-fed sheet P is transported downstream to a recording section  3  by the transporter  4 , whereby recording is effected. The sheet P recorded by the recording section  3  is ejected forward of the recording apparatus by an ejector  5  located downstream. 
   As shown in  FIG. 15 , the printer  1  is constructed to be able to transport a tray  50  adapted to carry an optical disk D, by the transporter  4 , and to be able to perform recording of information on a label surface of the optical disk D set in the tray  50  by the recording section  3 . Reference numeral  7  denotes a tray guide for supporting the tray  50 , and the tray  50  is inserted into the printer  1  from the front to the rear by way of the tray guide  7  and is transported to the recording section  3  by the transporter  4 . 
   Constituent elements disposed in the medium transporting path of the printer  1  will be described in more detail. The feeder  2  is constructed of a hopper  11 , a feeding roller  12 , a retard roller  13 , and a returning lever  14 . 
   The hopper  11  is formed from a plate-shaped element and constructed to be able to-pivot about a pivot center (not shown) provided in an upper part of the hopper  11 . As a result of the hopper  11  pivoting, the inclined sheet P supported on the hopper  11  is brought into press contact with the feeding roller  12  or caused to retract from the feeding roller  12 . The feeding roller  12  essentially assumes the shape of the letter D when viewed from the side. The feeding roller  12  feeds downstream the top sheet P, which remains in press contact with an arcuate portion of the D-shaped cross section of the feeding roller  12 . During the course of the sheet P being transported by the transporter  4  after feeding of the sheet P, the transporter  4  is controlled such that a flat portion of the D-shaped cross section of the feeding roller  12  opposes the sheet P as illustrated, so as to prevent occurrence of transport load. 
   The retard roller  13  is provided so as to be able to come into press contact with the arcuate portion of the feeding roller  12 . When overlapping transfer of the sheets P does not arise and only one sheet P is being fed, the retard roller  13  comes into contact with the sheet P to be thus rotated (clockwise in  FIG. 1 ). When a plurality of the sheets P are present between the feeding roller  12  and the retard roller  13 , the retard roller  13  does not rotate and remains stationary, because the frictional coefficient between the sheets P is lower than the frictional coefficient between the sheet P and the retard roller  13 . Consequently, sheets P, which are subsequent to the top sheet P and are about to be delivered in an overlapping manner along with the top sheet P to be fed, do not proceed downstream from the retard roller  13 , thereby preventing overlapped delivery of the sheets P. The returning lever  14  is pivotably provided and performs operation for returning to the hopper  11  sheets P, which are subsequent to the top sheet and have been prevented from being delivered in an overlapping manner. 
   A sensor (not shown in  FIG. 1 ) for detecting passage of the sheet P and a guide roller  26 , which sets an attitude for feeding the sheet P and prevents the sheet P from contacting the feeding roller  12  to thus lessen transport load, are interposed between the feeding device  2  and the transporter  4 . In the present embodiment, the guide roller  26  is rotatably supported by an upstream end of an upper guide  24 . 
   The transporter  4  provided downstream of the feeder  2  comprises a transporting roller  30  rotationally driven by a power feeding motor  52  (see  FIG. 2 ) and a transporting follower-roller  31  which comes into press contact with the transporting roller  30  and rotates in a following manner. The transporting roller  30  is formed from an adhesion layer which is formed by dispersing, in an essentially uniform manner, wear-and-brasion resistant particles over an outer peripheral surface of a metal shaft extending in the widthwise direction of the sheet. An outer peripheral surface of the transporting follower roller  31  is formed from low frictional material, such as an elastomer or the like. A plurality of the transporting follower rollers  31  are provided around the transporting roller  30  in the axial direction thereof. 
   In the present embodiment, two transporting follower rollers  31  are axially supported by a downstream end portion of one upper paper guide  24  so as to be able to freely rotate. As shown in  FIG. 1 , three transporting follower rollers  31  are arranged side by side on the paper guide  24  in the widthwise direction of the sheet. A shaft  24   a  is axially supported by a main frame  23 , and hence the sheet guide  24  is pivotable about the shaft  24   a  when the medium transporting path is viewed from the side. The transporting follower rollers  31  are urged by a coil spring  25  in a direction to come into press contact with the transporting roller  30 . 
   The sheet P having reached the transporter  4  is transported to the recording section  3  located downstream, by rotation of the transporting roller  30 , while being nipped between the transporting roller  30  and the transporting follower rollers  31 . 
   The recording section  3  comprises an ink jet recording head (hereinafter referred to as a “recording head”)  36  and a lower guide (platen)  37  provided so as to oppose the recording head  36 . The recording head  36  is provided at the bottom of a carriage  33 , and the carriage  33  is driven so as to move back and forth in the primary scanning direction by a carriage driving motor  113  (see  FIG. 2 ) while being guided by a carriage guide shaft  34  extending in the primary scanning direction. The carriage  33  carries ink cartridges  35  of a plurality of colors which are independent of each other, and supplies ink to the recording head  36 . 
   Ribs (not shown) and a recess (not shown), where ink is discarded, are formed on a surface of the lower guide  37  opposing the recording head  36 , which defines a distance between the sheet P and the recording head  36 . Ink ejected to an area outside the edges of the sheet P is discarded to the recess, thereby effecting so-called marginless printing where printing is carried out so as not to leave margins on the sheet P. 
   The position of the axial core of the carriage guide shaft  34  can be displaced by a platen gap adjuster  51  (described later in detail). Thereby, a gap between the sheet P (or the platen  37 ) and the recording head  36  (platen gap) can be adjusted. This adjustment will be described in detail later. 
   Subsequently, a guide roller  43  and the ejector  5  are provided downstream of the recording head  36 . The guide roller  43  exhibits the function of preventing lift of the sheet P from the lower guide  37 , to thus maintain the distance between the sheet P and the recording head  36  constant. The ejector  5  comprises an ejecting roller  41  to be rotationally driven by the power feeding motor  52 ; and an ejecting follower roller  42  which comes into contact with the ejecting roller  41  and is rotated in a driven manner. 
   In the present embodiment, the ejecting roller  41  is formed from a rubber roller, and a plurality of the ejecting rollers  41  are provided around a shaft element, which is rotationally driven, in an axial direction thereof. The ejecting follower roller  42  is formed from a toothed roller having a plurality of teeth provided along an outer periphery thereof. A plurality of the ejecting follower rollers  42  are provided in a frame assembly  45 , which is elongated in the primary scanning direction, so as to correspond to the plurality of ejecting rollers  41 . The sheet P, on which information is recorded by the recording section  3 , is rotationally driven by the ejecting rollers  41  while being nipped between the ejecting rollers  41  and the ejecting follower rollers  42 , and is ejected forward of the recording apparatus to an ejected sheet stacker (not shown). 
   The frame assembly  45  is provided in such a way that the assembly can be displaced by a releaser  6  (described later) so as to be able to assume a contact position where the ejecting follower rollers  42  come into contact with the ejecting rollers  41 , and a retracted position where the ejecting follower rollers  42  are retracted from the ejecting rollers  41 . 
   Next, the control section  100  that executes various control operations of the printer  1  and the peripheral configuration of the control section  100  will now be described by reference to  FIG. 2 . The control section  100  is constructed so as to be able to exchange data with a host computer  200  which transmits print information (print data) to the printer  1 . The control section comprises an IF  101  which is an interface with the host computer  200 ; an ASIC  102 ; a RAM  103 ; a PROM  104 ; an EEPROM  105 ; a CPU  106 ; a timer IC  107 ; a DC motor controller  108 ; a first motor driver  110 ; a second motor driver  111 ; and a head driver  109 . 
   The CPU  106  performs arithmetic operation for executing the control program of the printer  1  and other required arithmetic operations, and the timer IC  107  causes the CPU  106  to generate periodic interrupt signals required for various types of processing operations. The ASIC  102  controls print resolution or a drive waveform of the recording head  36  in accordance with print data transmitted from the host computer  200  by way of the IF  101 . The RAM  103  is used as a work area for the ASIC  102  and the CPU  106  or a primary storage area for other data. Various control programs (firmware) required to control the printer  1 , data required for processing, and the like, are stored in the PROM  104  and the EEPROM  105 . 
   The DC motor controller  108  is a circuit for controlling the speed of DC motors (the carriage driving motor  113  and the power feeding motor  52 ), and has a PID control circuit, an acceleration control section, and a PWM control circuit, all of which are omitted from the drawings. The DC motor controller  108  executes various operations for controlling the speed of the DC motor in accordance with a control command output from the CPU  106  and signals output from sensors such as a rotary encoder  118 , a linear encoder  119 , a sheet detector  116  for detecting passage of the sheet P, a sheet width sensor  115 , a platen gap detector  80 , and the like. The DC motor controller  108  sends a signal to the first motor driver  110  and the second motor driver  111 . 
   Under control of the DC motor controller  108 , the second motor driver  111  controls driving of the power feeding motor  52 . The power feeding motor  52  serves as a common power source for a plurality of objects of driving in the present embodiment; namely, the feeding roller  12 , the transporting roller  30 , the ejecting roller  41 , the carriage guide shaft  34 , and the like. 
   The first motor driver  110  controls driving of the carriage driving motor  113  under control of the DC motor controller  108 , thereby moving the carriage  33  back and forth in the primary scanning direction or halting and holding the carriage  33 . The head driver  109  controls driving of the recording head  36  under control of the CPU  106  in accordance with the print data transmitted from the host computer  200 . 
   The CPU  106  and the DC motor controller  108  are provided with a detection signal output from the sheet detector  116  that detects the leading end and trailing end of the transported sheet P; a signal output from the rotary encoder  118  for detecting the number of rotation steps, rotating direction, and rotational speed of the power feeding motor  52 ; and a signal output from the linear encoder  119  that detects the absolute position of the carriage  33  in the primary scanning direction. The CPU  106  and the DC motor controller  108  are provided with a signal output from the sheet width sensor  115 , as well. 
   This sheet width sensor  115  is an optical sensor provided on the bottom of the carriage  33 ; and has a light-emitting section (not shown) for emitting light toward the sheet P and a light-receiving section (not shown) for receiving the light reflected from the sheet P. The sheet width sensor  115  detects a difference in reflectivity on the sheets P. By the thus-detected difference in reflectivity, the control section  100  detects presence/absence of the sheet P and the width of the sheet P in association with sensing activity of the sheet width sensor  115 . 
   The rotary encoder  118  has a disc-shaped scale (not shown) having in an outer peripheral section thereof a plurality of light transmission sections and a sensing section (not shown) which has a light-emitting section for emitting light to the light transmission sections and a light-receiving section for receiving the light having passed through the light transmission sections. In accordance with rotation of the disc-shaped scale, the sensing section outputs a rise signal and a fall signal, which are formed as a result of light having passed through the light transmission sections. Upon receipt of the signal output from such a rotary encoder  118 , the control section  100  detects the number of rotation steps, rotational speed, and rotating direction of an object to be driven by the power feeding motor  52  (the transporting roller  30 , the ejecting roller  41 , the carriage guide shaft  34 , and the like). Thereby, intended various control operations can be executed. 
   The linear encoder  119  has a sign board  119   b  elongated in the primary scanning direction and a sensing section  119   a  that has a light-emitting section for emitting light toward a plurality of light transmission sections formed in the sign board  119   b  in the primary scanning direction and a light-receiving section for receiving the light having passed through the light transmission sections. The sensing section  119   a  outputs a rise signal and a fall signal, which are formed as a result of light having passed through the light transmission sections, and the control section  100  detects the position of the carriage  33  in the primary scanning direction by receiving the signal output from such a sensing section  119   a.    
   Next, the platen gap adjuster will be described in detail with reference to  FIGS. 3 through 11 . 
   As shown in  FIG. 4 , the platen gap adjuster  51  comprises the power feeding motor  52  and a power transmitter  67  for transmitting power from the power feeding motor  52  to the carriage guide shaft  34 . 
   As shown in  FIG. 3 , the carriage guide shaft  34  is supported by a left side frame  23   b  and a right side frame  23   a . More specifically, as shown in  FIGS. 7A and 7B , the carriage guide shaft  34  is formed so as to penetrate through vertically-elongated guide grooves  75  formed in the respective right side frame  23   a  and left side frame  23   b . Therefore, only vertical movement of the carriage guide shaft  34  is permitted, and horizontal movement of the same is not allowed. 
   Turning again to  FIG. 4 , a gear  66  is attached to a part of the carriage guide shaft  34  close to the right side frame  23   a . As a result of power being transmitted from the power feeding motor  52  to the gear  66 , the carriage guide shaft  34  rotates (described in detail later). As shown in  FIG. 11 , the gap adjusting cam  76  is attached to a shaft end  34   a  of the carriage guide shaft  34 , and a cam follower pin  77  is fixedly provided in a position adjacent to the gap adjusting cam  76 . 
   When power is transmitted from the power feeding motor  52  to the gear  66 , the gap adjusting cam  76  rotates, whereupon the position of the shaft center of the carriage guide shaft  34  is moved up and down by interaction of the outer peripheral face of the gap adjusting cam  76  with the cam follower pin  77 , so that the platen gap can be adjusted.  FIG. 11  shows the gap adjusting cam  76  and the cam follower pin  77 , which are attached to the shaft end  34   a  of the carriage guide shaft  34  close to the left side frame  23   b . An analogous gap adjusting cam  76  and an analogous cam follower pin  77  are provided on the shaft end of the right side frame  23   a.    
   The power transmitter  67  that transmits power from the power feeding motor  52  to the carriage guide shaft  34  will now be described in detail. As shown in  FIG. 4 , the power feeding motor  52  is provided in a position close to the left side frame  23   b , and the gear  54  is attached to the rotary shaft of the power feeding motor  52 . The gear  54  meshes with the gear  55  attached to the shaft end of the transporting roller  30 . Thus, the transporting roller  30  is rotationally driven. 
   The gear  56  is attached to the other shaft end of the transporting roller  30 , and power is transmitted to the feeding device  2  (the feeding roller  12 ) by way of gears  57 ,  58 , and  59 . Moreover, power of the power feeding motor  52  is transmitted from the gear  59  to the gear  60  into which a spur gear  60   b  and a spur gear  60   c  are accumulated together by a shaft  60   a ; and further to the gear  66  attached to the shaft end of the carriage guide shaft  34  by way of gears  61 ,  62 ,  63 ,  64 , and  65 . 
   Constituent elements of a power transmission path which extends from the power feeding motor  52  to the carriage guide shaft  34  form the power transmitter  67  that transmits power from the power feeding motor  52  to the carriage guide shaft  34 . The gear  58  transmits power to an unillustrated pump, as well. Thereby, negative pressure is fed to a cap (not shown) which seals the recording head  36 . As mentioned above, the power feeding motor  52  acts as the common drive source among the transporting roller  30 , the feeding roller  12 , the unillustrated pump, and the carriage guide shaft  34 . 
   Next, a gear train  68  constituting a power transmission path that extends from the gear  60  to the gear  66  will be described in detail with reference to  FIGS. 5 through 10  and other drawings, as required.  FIG. 10  shows a state where the carriage  33  is situated in the vicinity of the home position. As illustrated, the right side of the travel area of the carriage  33  (the right side of the recording apparatus when viewed from front) serves as the home position of the carriage  33 . As shown in  FIGS. 5 and 6 , the gear train  68  is attached to the right side frame  23   a  close to the home position in the travel area of the carriage  33 . Specifically, the gear train  68  is disposed at the end of the travel area of the carriage  33  close to the home position of the carriage  33 . Reference numeral  70  designates a cover which covers the gear train  68  and retains the gears. 
   The gears  60  to  66  that constitute the gear train  68  are attached to the right side frame  23   a  so as to be able to rotate around a rotary shaft parallel to the primary scanning direction. Of the gears, the gear  63  and the gear  64  are provided so as to be slidable in the axial direction of the rotary shaft of the gears  63 ,  64 . First, the gear  64  is described in detail, as follows. 
   As shown in  FIGS. 7A and 7B , a hole  74  is formed in the right side frame  23   a . As shown in  FIGS. 7 and 8 , an engagement section  64   a , which can engage with an engagement section  33   a  formed in a sidewall close to the home position of the carriage  33 , is formed in the gear  64  as a result of projecting toward the inside of the right side frame  23   a  from the hole  74 . The gear  64  is provided with the engagement section  64   a  being urged by a spring  78  in a direction to protrude to the inside of the right side frame  23   a  (toward the carriage  33 ). 
   The gear  64  slidably moves axially along the rotary shaft thereof, so that the gear  64  can be displaced between a non-mesh position (a position shown in  FIG. 8A ) where the gear  64  does not mesh with the adjacent gear  63 , and a mesh position (a position shown in  FIG. 8B ) where the gear  64  meshes with the gear  63 . In a state where the carriage  33  does not press the engagement section  64   a , the gear M assumes the non-mesh position by urging force of the spring  78  as shown in  FIG. 8A . Specifically, the gear  64  enters a power disconnection state where the power transmitter  67  does not transmit power from the power feeding motor  52  to the carriage guide shaft  34 . 
   When the carriage  33  has moved to a position adjacent to the right side frame  23   a , the carriage  33  presses the engagement section  64   a , as shown in  FIG. 8B , whereupon the gear  64  is displaced to a mesh position. Namely, there is achieved a power transmission state where the power transmitter  67  transmits power from the power feeding motor  52  to the carriage guide shaft  34 . 
   Positioning pins  64   b  are formed in the surrounding area of the engagement section  64   a  so as to protrude toward the inside of the right side frame  23   a . As shown in  FIGS. 7B and 8B , as a result of the positioning pins  64   b  fitting into the fitting holes  71 , the phase of the gear  64  is set to a predetermined position (the gear  64  becomes non-rotatable). In the present embodiment: the platen gap is configured to be switched among four head positions according to the size of the platen gap (described later in detail). The fitting holes  71  are formed in positions corresponding to the four head positions. 
   As mentioned above, when the platen gap is adjusted; namely, when switching among the head positions is performed, the carriage  33  is first moved to a position adjacent to the right side frame  23   a  so as to push the gear  64 , thereby displacing the gear  64  into the mesh position. The power feeding motor  52  is rotated to a certain extent in this state, thereby rotating the carriage guide shaft  34  and switching the platen gap to a desired head position. The carriage  33  is caused to depart from the right side frame  23   a , to thus bring the gear  64  into the non-mesh position and fitting the positioning pins  64   b  into the fitting holes  71 , to thus fix the platen gap. 
   The reason why the gear  63  adjacent to the gear  64  is provided so as to be slidable in the direction of the rotary shaft of the gear  63  is as follows. Specifically, when the gear  64  moves from the non-mesh position ( FIG. 8A ) to the mesh position ( FIG. 8B ), there may arise a case where teeth of the gear  64  collide with the teeth of the gear  63 , to thus end in a failure to attain proper meshing. When the carriage  33  forcefully presses the gear  64 , there may also arise a case where the teeth are damaged. Consequently, the gear  63  is provided so as to be slidable in the direction of the rotary shaft thereof so that, even when the teeth of the gear  64  and the teeth of the gear  63  collide with each other, the gear  64  can be displaced to the mesh position ( FIG. 9A ). 
   As shown in  FIG. 9B , a tooth “c” of the gear  63  is formed so as to have tapered faces “b.” By the tapered faces, the gear  63  and the gear  64  smoothly mesh with each other in association with rotation of the gear  63 . In  FIGS. 8 and 9A , reference numeral  78  designates an spring for urging the gear  64  toward the non-mesh position. Reference numeral  79  designates another spring for urging the gear  63  to an ordinary position (a position where the gear  63  meshes with an adjacent gear). 
   In the printer  1  constructed as mentioned above, the gear  64 , which acts as an engagement member for receiving a trigger from the carriage  33 , is located in a position within the travel area of the carriage  33  close to the home position (i.e., the right side frame  23   a ). The gear  64  is located in a position with reference to the transporting path for the sheet P or the transporting path of the tray  50  with the home position of the carriage  33  being sandwiched therebetween. Accordingly, during the course of proceeding from the home position to press the gear  64 , the carriage  33  does not need to straddle the transporting path for the sheet P or the transporting path of the tray  50 ; namely, does not interfere with the sheet P or the tray  50 . Thereby, even when the platen gap is set to an inappropriate state, occurrence of interference between the recording head  36  and the sheet P or the tray  50  can be prevented. Further, the distance over which the carriage  33  travels when proceeding to press the gear  64  can be shortened by a great extent, so that switching of the platen gap can be performed within a short period of time. 
   Next, the configuration of the platen gap detector  80  and a sequence for detecting the reference platen gap will now be described in detail by reference to  FIGS. 10 to 14 . 
   As shown in  FIG. 10 , the left side frame  23   b  is provided with the platen gap detector  80  for detecting the current platen gap. As shown in  FIG. 11 , the platen gap detector  80  comprises a rotary plate  81  and a sensor  85 . The rotary plate  81  has four flags  82   a ,  82   b ,  82   c , and  82   d  which are provided along the outer periphery of the rotary plate  81  and project in a radial direction of the same. The rotary plate  81  is attached to the shaft end  34   a  of the carriage guide shaft  34  and rotates along with the carriage guide shaft  34 . 
   The sensor  85  is an optical sensor having a light-emitting section (not shown) and a light-receiving section (not shown). The sensor  85  is provided in a predetermined position (a position which is between the carriage guide shaft  34  and the main frame  23  and is adjacent to the left side frame  23   b ) such that the flags  82   a  to  82   d  provided on the rotary plate  81  cross the optical axis of the sensor  85 . Accordingly, when the carriage guide shaft  34  rotates, the control section  100  ( FIG. 2 ) can detect passage of the flags  82   a  to  82   d  by a detection signal sent from the sensor  85 . When the carriage guide shaft  34  is not rotating, presence/absence of-a flag can be detected; namely, the control section can detect the current head position where the flag is located or that the flag is not situated in any of the head positions, by a detection signal transmitted from the sensor  85 . 
   The flags  82   a  to  82   d  are located in correspondence with the respective head positions. Specifically, in the present embodiment, the head positions are set in four steps, 1, 2, 3, and 4, in increasing sequence of the platen gap. By way of example, the platen gap in position  1  is set to 1.2 (mm); the platen gap in position  2  is set to 1.7 (mm); the platen gap in position  3  is set to 2.35 (mm); and the platen gap in position  4  is set to 4.2 (mm). 
   When the head position assumes “1” in association with rotation of the carriage guide shaft  34 , the flag  82   a  blocks the optical axis of the sensor  85  as shown in  FIG. 11 . Similarly, when the head position assumes “2,” the flag  82   b  blocks the optical axis of the sensor  85 . When the head position assumes “3,” the flag  82   c  blocks the optical axis of the senor  85 . When the head position assumes “4,” the flag  82   d  blocks the optical axis of the sensor  85 . The control section  100  ( FIG. 2 ) of the printer  1  determines which one of the flags is now blocking the optical axis of the sensor  85 , by a method to be described later, and performs print control operation appropriate to the head position. A method for determining a head position will be described in detail later. 
   Start point “a” in  FIG. 12  represents the state shown in  FIG. 11 . When the carriage guide shaft  34  rotates through 360° from this state and reaches a point “t” in  FIG. 12 , the state returns to that shown in  FIG. 11  (i.e., the point “a” is equal to the point “t”). Circled numerals ( 1  to  4 ) in  FIG. 12  denote the respective head positions. 
   In  FIG. 12 , a segment during which the platen gap assumes the size of position  1  ranges from point “a” to point “d.” A segment during which the flag  82   a  assigned to position  1  blocks the optical axis of the-sensor  85  is set to points “b” and “c.” In this way, an area where the sensor  85  detects the flag  82   a  is set to the center of the segment where the platen gap actually assumes the size of position  1 ; namely, an area where the platen gap becomes most stable. This also applies to the remaining positions  2  to  4 . Accordingly, the platen gap can be maintained stably. 
   Next, a sequence for detecting the reference platen gap will now be described by reference to  FIGS. 13 and 14 . The term “number of rotation steps of the power feeding motor  52 ” used herein signifies the number of pulses (the number of steps) transmitted from the rotary encoder  118  ( FIG. 2 ) in association with rotation of the power feeding motor  52 . In the present embodiment, the “reference platen gap” corresponds to the head position  1 . The sequence, which will be described hereunder, is for the control section  100  ( FIG. 2 ) detecting the flag  82   a  assigned to head position  1 . 
   In order to detect the reference platen gap, a variable P (described in detail later) showing the number of retries is first reset to zero (step S 101 ), and the carriage  33  is then moved to a position adjacent to the right side frame  23   a ; i.e., the right end position (step S 102 ). Thereby, the gear  64  is displaced to the mesh position shown in  FIG. 8B , and the power transmitter  67  enters the power transmission state where power is transmitted from the power feeding motor  52  to the carriage guide shaft  34 . 
   Next, the power feeding motor  52  is rotated forward by α1 steps and backward by α1 steps (step S 103 ). As mentioned above, when the gear  64  moves from the non-mesh position ( FIG. 8A ) to the mesh position ( FIG. 8B ), there may arise a case where the teeth of the gear  64  collide with the teeth of the gear  63 , to thus fail to attain proper meshing. When the teeth of the gears have failed to mesh properly as mentioned above, the gear  63  is rotated slightly (forward and backward), so that the teeth of the respective gears can be meshed with each other. Accordingly, α1 is preferably set to such a value that a gear is rotated to at least an amount corresponding to a product which is determined by multiplying a tooth pitch of the gears by 0.5 or more. In the present embodiment, the thus-determined value is further multiplied by a safety factor of 1.2, and consideration is paid to backlash of the gear. Specifically, given that the pitch of teeth is “p” and a backlash is taken as “j,” α1 is set such that the teeth are fed by an amount of (0.5 p+j). 
   Next, a variable K showing the number of detected flags is reset to zero (step S 104 ), and a variable L showing the total number of rotational steps (an accumulated value) of the power feeding motor  52  is reset to zero (step S 105 ). The power feeding motor  52  is rotated forward by one step, and the variable L is incremented (steps S 106 , S 107 ). Consequently, when the sensor  85  is determined to have been activated; namely, when any one of the flags  82   a  to  82   d  can be determined to have blocked the optical axis of the sensor  85  (Yes in step S 108 ), a determination is made as to whether or not the variable L achieved at this time is β1 steps or more (step S 109 ). Alternatively, when the sensor  85  can be determined to have been deactivated; namely, when any one of the flags  82   a  to  82   d  can be determined to have failed to block the optical axis of the sensor  85  (No in step S 108 ), a determination is made as to whether or not the variable L achieved at this time is β2 steps or more (step S 114 ). 
   The significance of β1 steps and that of β2 steps will be described in detail. Of the flags  82   a  to  82   d , a circumferential interval between the rotation detection members  81  of two adjacent flags is expressed by symbols A, B, C, and D as shown in  FIGS. 11 and 12 . For instance, point “c” in  FIG. 12  shows a position where the flag  82   a  comes off the optical axis of the sensor  85 . When the rotary plate  81  rotates from this position by angle A, point “f” is reached, where the next flag  82   b  blocks the optical axis of the sensor  85 . The same also applies to angles B, C, and D. 
   In the present embodiment, angle D is set to become the largest, and angle C is set to become the second largest. Symbol β1 denotes the number of rotation steps of the power feeding motor  52  required when the rotary plate  81  is rotated through an angle which is determined by multiplying angle C by a safety factor of 1.2. Symbol β2 denotes the number of rotation steps of the power feeding motor  52  required when the rotary plate  81  is rotated through an angle which is determined by multiplying angle D by a safety factor of 1.2. 
   When the variable L, which is an accumulated value of the number of rotation steps of the power feeding motor  52 , is determined to be smaller than β1 (No in step S 109 ), the flag detected in step S 108  can be determined to be any one of the flags  82   b ,  82   c , and  82   d  (the next flag has been detected as a result of the power feeding motor  52  having rotated through any of angles A, B, and C). In this case, the power feeding motor  52  is rotated by α2 steps, and the flag (in this case, any one of the flags  82   b ,  82   c , and  82   d ) is caused to come off the optical axis of the sensor  85  without fail (step S 110 ). 
   Symbol α2 denotes the number of rotation steps of the power feeding motor  52  for causing the flag to come off the sensor  85  without fail; namely, the number of rotation steps of the power feeding motor  52  required when the rotary plate  81  is rotated by an angle which is determined by multiplying an angle, which is double angle E in  FIG. 11 , by the safety factor. 
   Consequently, when the sensor  85  still remains activated despite operation for causing the flag to come off the optical axis of the sensor  85  without fail having been performed in step S 110  (Yes in step S 111 ), any anomaly has arisen. In that case, processing is terminated as a fatal error. When the sensor  85  has become deactivated (No in S 111 ), the flag has properly come off the optical axis of the sensor  85 , and hence variable K showing the number of flags detected is incremented (step S 112 ). 
   A determination is made as to whether or not K=4 is achieved. When K=4 is achieved (Yes in step S 113 ), all of the flags have been detected despite a failure to detect the flag  82   a  serving as a reference; namely, a failure to the largest interval between the flags (the interval between the flags  82   d  and  82   a ). Even in this case, processing is terminated as a fatal error. When K assumes a value of less than four (No in step S 113 ), processing returns to step S 105 , and the next flag is successively detected. 
   When the number of rotation steps of the power feeding motor  52  has exceeded β2 steps in step S 114 , none of the flags have been detected even when the rotary plate  81  has been rotated through angle D, which corresponds to the largest interval between the flags (the interval between the flags  82   d  and  82   a ), or more. In this case, processing is terminated as a fatal error (Yes in step S 114 ). 
   When the sensor  85  has become activated (Yes in step S 109 ), the flag  82   a  has blocked the optical axis of the sensor  85  (point “b” in  FIG. 12 ). In this case, processing proceeds to step S 115  in  FIG. 14 , where the power feeding motor  52  is rotated forward by α3 steps. Here, symbol α3 denotes the number of rotation steps of the power feeding motor  52  for causing the optical axis of the sensor  85  to be situated at the center of the flag  82   a . Specifically, symbol α3 signifies the number of rotation steps of the power feeding motor  52  in order to rotate the rotary plate  81  through angle E in  FIG. 11 . With this configuration, the optical axis of the sensor  85  comes to the center of the flag  82   a.    
   Next, in this state, the power transmitter  67  ( FIG. 4 ) remains in a power transmission state where power is transmitted from the power feeding motor  52  to the carriage guide shaft  34  (a state in  FIG. 8B ). Hence, the carriage  33  is caused to recede from the right side frame  23   a  (step S 116 ), to thus disengage the teeth of the gear  64  from the teeth of the gear  63 , as shown in  FIG. 8A . 
   By the forward rotation of the power feeding motor  52  in step S 115 , the teeth of the gear  64  and the teeth of the gear  63  remain in compressed contact with each other. Accordingly, even when the carriage  33  has receded in step S 116 , there may arise a case where the teeth of the gear  63  and the teeth of the gear  64  remain meshed with each other. When the power feeding motor  52  has rotated in this state, there may also arise a case where the optical axis of the sensor  85  comes off the center of the flag  82   a  and further from the flag  82   a . Accordingly, in step S 117  the power feeding motor  52  is slightly rotated backward by α4 steps corresponding to a backlash of the gear. Thereby, the teeth of the gear  63  and the teeth of the gear  64  are released from a compressed contact, and the gear  64  can be reliably displaced to the non-mesh position by urging force of the spring  78 . 
   The teeth of the gear  64  and the teeth of the gear  63  can also be reliably disengaged from each other, or the power transmitter  67  can also be brought into a power disconnected state, by sufficiently increasing the urging force of the spring  78  that urges the gear  64  to the non-mesh position in  FIG. 8A . 
   There is a case where the gear  64  is displaced to the non-mesh position by the urging force of the spring  78  with a slight lag behind the moment when the carriage  33  has receded in step S 116 . In such a case, collision sound, which would arise when the engagement section  64   a  collides with the carriage  33 , may arise. However, as a result of an increase in the urging force of the spring  78 , the gear  64  is immediately displaced to the non-mesh position as a result of the receding action of the carriage  33 , thereby preventing occurrence of collision sound. 
   Turning again to  FIG. 14 , the power feeding motor  62  is forwardly rotated by α5 steps in step S 118 . In the present embodiment, α5 denotes the number of rotation steps of the power feeding motor  52  required when the rotary plate  81  is rotated through an angle which is double angle E in  FIG. 11 ; namely, the number of rotation steps of the power feeding motor  52  required when the rotary plate  81  is rotated through an angle corresponding to one flag. 
   However, at this point in time, the gear  64  and the gear  63  should have been disengaged from each other. Accordingly, even when the power feeding motor  52  has been rotated forward by α5 steps, no changes should arise in the sensor  85 . Step S 119  is for determining that no changes have arisen in the sensor  85 . If the sensor  85  still remains activated (Yes), the detected platen gap is determined as the reference platen gap. In other words, it is determined that the flag  82   a  corresponding to the head position  1  is detected (step S 120 ). 
   Meanwhile, when the sensor  85  has become deactivated in step S 119 , the rotary plate  81  has been rotated by forward rotation of the power feeding motor  52 ; namely, the gear  64  and the gear  63  still remain meshed with each other. In this case, all the above-mentioned processing operations are again performed (No in step S 123 ). When variable P has assumed a value of 2 (Yes in step S 123 ) as a result of increment in variable P which shows the number of retries in S 122 , processing is terminated as a fatal error. 
   The flags  82   a  to  82   d  are provided along the outer periphery of the rotary plate  81  in correspondence with the respective head positions ( 1  to  4 ). By detection of the flags  82   a  to  82   d , the current position of the platen gap can be determined to be any one of a plurality of head positions. At least one circumferential interval between the flags is set uniquely such that circumferential intervals between the other flags become different from each other. In the present embodiment, the circumferential interval is set to a unique value such that circumferential intervals among all flags become different from each other (see angles A to D shown in  FIGS. 11 and 12 ). 
   When the largest interval between the flags (angle D) has been detected during the course of rotation of the carriage guide shaft  34  in the clockwise direction (as indicated by an arrow in  FIG. 11 ) in the present embodiment, the detected flag is determined to be the flag  82   a  corresponding to head position  1  set to the minimum the platen gap. So long as the flag  82   a  is detected, the positions of other flags ( 82   b ,  83   c , and  82   d ) in relation to the flag  82   a ; namely, the numbers of rotations of the power feeding motor  52  required to switch the platen gap from position  1  to positions  2 ,  3 , and  4 , are determined. Hence, the platen gap can be accurately switched to a desired position by a single sensor  85 . 
   the platen gap can also be switched to another position by solely managing the number of rotation steps of the power feeding motor  52  after detection of the flag  82   a  that serves as a reference. However, since the respective flags are provided in correspondence with the respective positions, the platen gap can be switched to a desired position with higher accuracy by monitoring activation/deactivation of the sensor  85  in conjunction with the number of rotation steps of the power feeding motor  52 . 
   Incidentally, in the printer  1  of the present embodiment, the power feeding motor  52  drives a plurality of objects to be driven, in view of cost saving. Consequently, when the power feeding motor  52  is rotated backward, power is transmitted to another object to be driven, which may in turn induce a failure. For these reasons, even when the carriage guide shaft  34  is rotated, the essential requirement is to rotate the carriage guide shaft  34  in only one direction. Therefore, a desire to rotate such a carriage guide shaft  34  in only one direction can be met. 
   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.