Patent Publication Number: US-6667753-B2

Title: Device for detecting a leading edge of a recording paper

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for detecting a leading edge of a recording paper. This device is employed in a printer and a paper magazine using a paper roll of a thermosensitive recording paper, a printing paper and so forth. 
     2. Description of the Related Art 
     It is known that some of photographic printers and color thermal printers use a paper roll formed by rolling a strip of a recording paper. The paper roll is contained in a paper supply section of the printer, and is rotated by a driving mechanism so as to be rotated forward in a paper feeding direction and so as to be rotated backward in a rewinding direction. A flange having a rotary shaft is attached to the paper roll. As to ways for supporting the rotary shaft, there are a fixation type and a movement type. In the fixation type, the rotary shaft of the paper roll is rotatably supported at a fixed position. In the movement type, a drive roller abutting on a periphery of the paper roll is provided, and the rotary shaft is moved toward the drive roller in accordance with a diameter change of the paper roll. 
     Upon rotating the paper roll forward, the recording paper is advanced from the paper supply section to a print section to execute a printing process. After printing, the printed recording paper is cut in a predetermined size and is discharged to the outside of the printer. A region of the recording paper, which has been advanced to the print section but has not been used for printing, is rewound by rotating the paper roll backward. When rewinding the recording paper, there arises a problem in that looseness of winding is caused because the printing paper and the thermosensitive recording paper have a paper thickness and a paper strength. 
     In order to prevent the looseness of winding, Japanese Patent Laid-Open Publication No. 2000-169013 discloses a method in which the paper roll is rotated backward so as to be securely wound before feeding the paper. After securely winding the paper roll, the recording paper is fed. This method employs a detecting device for a leading edge of the recording paper. This detecting device (hereinafter, leading-edge detecting device) abuts on a peripheral surface of the paper roll to detect the leading edge thereon. Secure winding of the paper roll is judged when a detection interval of the leading-edge detecting device becomes constant during the reverse rotation of the paper roll. 
     The leading-edge detecting device comprises a basic roller, an actuation lever, a driven roller, and a micro switch. The basic roller abuts on the periphery of the paper roll. The actuation lever rotates around the basic roller. The driven roller is disposed on the actuation lever. When the driven roller overleaps the leading edge of the recording paper, the micro switch outputs a detection signal in response to the movement of the actuation lever. 
     The actuation lever is urged by a first spring so as to contact the driven roller with the periphery of the paper roll. Meanwhile, the basic roller is supported by a roller supporting lever to which a second spring is attached to contact the basic roller with the periphery of the paper roll. One end of the first spring is attached to the actuation lever, and the other end thereof is attached to the roller supporting lever. The micro switch is disposed at a fixed position in the paper supply section. 
     However, the above unit for detecting the leading edge has problems in that a number of parts thereof is large and a structure thereof is complicated. Particularly, in the rotary-shaft fixing type, a moving amount of the actuation lever is large in comparison with the rotary-shaft moving type. Thus, there arises another problem in that it is necessary to separately provide a mechanism for moving the micro switch together with the actuation lever. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is a primary object of the present invention to provide a device for detecting a leading edge of a recording paper, in which the reading edge is detected with a simple structure. 
     It is a second object of the present invention to provide a device for detecting a leading edge of a recording paper, in which a number of parts thereof is reduced. 
     In order to achieve the above and other objects, the leading-edge detecting device according to the present invention comprises an abutting member contacting with a periphery of a paper roll, and a vibration detecting sensor for detecting a vibration of the abutting member. The vibration detecting sensor outputs a signal in accordance with a scale of the vibration, and is preferable to be attached to the abutting member. 
     The paper roll is formed by rolling a strip of a recording paper. While the paper roll is rotated backward in a winding direction, the abutting member overleaps a leading edge of the recording paper every rotation thereof. When the abutting member has overleapt the leading edge, the vibration detecting sensor outputs the signal having a higher level. On the basis of the signal of the higher level, disengagement of the leading edge and the abutting member is judged to detect the leading edge. 
     In a preferred embodiment, the abutting member is a separator for separating the leading edge from the paper roll. When the paper roll is rotated forward in a paper feeding direction, a head portion of the separator abuts on the lading edge to separate it from the paper roll. 
     A rotary shaft is set to the paper roll. In a case that the rotary shaft is adapted to be held at a fixed position, it is preferable to provide a drive roller, a first supporting arm, and a first spring. The drive roller abuts on the periphery of the paper roll to rotate it. The first supporting arm moves the drive roller in accordance with a diameter change of the paper roll. The first spring urges the first supporting arm so as to contact the drive roller with the paper roll. 
     Further, it is preferable to provide a second supporting arm and a second spring. The second supporting arm supports the abutting member. The second spring urges the second supporting arm so as to contact the abutting member with the periphery of the paper roll. The second spring is attached to the first supporting arm so as to move together with the second supporting arm. 
     According to the present invention, a structure of the leading-edge detecting device may be simplified, and a number of parts thereof may be reduced by utilizing the separator used for separating the leading edge from the periphery of the paper roll. Further, the vibration detecting sensor is attached to the separator so that it is unnecessary to provide a special mechanism for moving the vibration detecting sensor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments of the invention when read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is an explanatory illustration of a printer; 
     FIGS. 2A and 2B are section views of a paper supply section of the printer; 
     FIG. 3 is a perspective view showing a moving mechanism for a separator and a drive roller; 
     FIG. 4 is an explanatory illustration showing a movement of the separator when a leading edge of a recording paper passes the separator; 
     FIG. 5 is a graph showing a relationship between an output level of a signal, which is sent from a vibration detecting sensor, and a cycle T of detecting the leading edge when a paper roll is reversed; 
     FIG. 6 is a block diagram showing a controller of the printer; 
     FIG. 7 is an illustration showing a looseness state of the paper roll; 
     FIG. 8 is a flow chart showing a process for securely winding the paper roll; and 
     FIGS. 9A and 9B are explanatory illustrations showing another embodiment of the paper roll in which a rotary shaft is movably provided. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     A color thermal printer  2  shown in FIG. 1 comprises a paper supply section  6 , a print section  7 , and a printer controller  8 . The paper supply section  6  contains a paper roll  4  formed by rolling a strip of a recording paper  3 . The print section  7  executes a printing process on the recording paper  3  fed from the paper supply section  6 . The printer controller  8  controls the print section  7  and so forth. The recording paper  3  is a well-known color thermosensitive recording paper including a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, and a yellow thermosensitive coloring layer, which are formed on a base in order. 
     The print section  7  is provided with a conveyor roller pair, a thermal head, a fixing unit, and so forth. The conveyor roller pair nips a leading edge of the recording paper  3  and conveys the recording paper  3  inside the print section  7 . The thermal head performs thermal recording on the respective thermosensitive coloring layers of the recording paper  3 . The fixing unit optically fixes the respective thermosensitive coloring layers, on which thermal recording has been performed, with ultraviolet rays. 
     Upon instruction of printing, the paper roll  4  contained in the paper supply section  6  is rotated backward in a winding direction to carry out secure winding. After that, the paper roll  4  is rotated forward in a paper feeding direction. The recording paper  3  is fed to the print section  7  through a paper mouth  17 . The fed recording paper  3  is conveyed inside the print section  7  by means of the conveyor roller pair. When the recording paper  3  passes the thermal head and the fixing unit, the printing process is performed in a three-color frame-sequential manner to print an image in full color. The printed recording paper  9  is cut by a cutter in a predetermined size to be discharged to the outside of the printer  2 . Meanwhile, the recording paper  3  from which the printed recording paper  9  has been separated is rewound into the paper supply section  6  through the paper mouth  17  by rotating a drive roller  14 . 
     As to the printer  2 , it is possible to assign a print number at the time of print instruction. When the assigned print number is a plural number, the printing process is continuously performed plural-number times corresponding to the assigned print number. After the continuous processes have been over, the recording paper  3  is rewound. 
     The paper supply section  6  has a light-shielding ability and a moisture-retention ability. As shown in FIGS. 2A and 2B, the paper supply section  6  comprises a case  10  containing the paper roll  4 , and the drive roller  14  abutting on a periphery of the paper roll  4 . The drive roller  14  rotates the paper roll  4  forward in the paper feeding direction, and rotates it backward in the winding direction. 
     A top face of the case  10  is adapted to be a detachable lid  18 , for example. The paper roll  4  is loaded into the case  10  after detaching the lid  18 . A flange  23  provided with a rotary shaft  22  is attached to both ends of the paper roll  4 . An inner wall of the case  10  is formed with a support member (not shown) for rotatably supporting the rotary shaft  22  at a fixed position. The paper roll  4  is contained in the case  10  such that the rotary shaft  22  is supported by the support member. 
     The drive roller  14  comprises a metal shaft and a covering material applied on a peripheral surface thereof. The covering material prevents the drive roller  14  from slipping. As the covering material, a rubber is used for instance. The drive roller  14  is rotated by a drive motor  21  and is attached to a first supporting arm  31 , which comprises a pair of L-shaped arm members  31   a  and  31   b  (see FIG.  3 ). The drive roller  14  is attached to ends of the respective arm members  31   a  and  31   b . The other ends of the respective arm members  31   a  and  31   b  are attached to pivots  29  formed on a supporter  28 . Owing to this, the first supporting arm  31  is rotatable around the pivot  29 . 
     FIG. 2A shows an initial state of the paper roll  4 , and FIG. 2B shows a state thereof in that a half or more of the recording paper  3  is used. As shown in FIGS. 2A and 2B, a diameter of the paper roll  4  changes as the recording paper  3  is used. The first supporting arm  31  moves the drive roller  14  in accordance with the change of the diameter. The first supporting arm  31  is urged by a first spring  32  so as to be rotated in a clockwise direction in FIGS. 2A and 2B. One end of the first spring  32  is fixed to the case  10 , and the other end thereof is fixed to the first supporting arm  31 . Alternatively, two ends of the first spring  32  may be respectively attached to the supporter  28  and the first supporting arm  31 . With respect to the first spring  32 , a coiled spring is used for instance. The first spring  32  supports the drive roller  14  so that a spring possessing a strong urging force is used. 
     The drive roller  14  is merely moved between the maximum diameter of the paper roll  4  and the minimum diameter thereof. In view of this, a rotational range of the first supporting arm  31  is regulated by a stopper, which is not shown, so as to rotate the first supporting arm  31  within a required range. 
     The drive roller  14  is pressed against the periphery of the paper roll  4  by means of the first spring  32 . In virtue of this, the drive roller  14  always abuts on the periphery of the paper roll  4  regardless of the diameter change thereof. Thus, the paper roll  4  is adapted to be driven from beginning to end. By the way, when the paper roll  4  is reversed, the drive roller  14  contributes to the secure winding of the paper roll  4 . 
     A separator  34  is disposed above the drive roller  14 . The separator  34  elastically contacts with the peripheral surface of the paper roll  4 . When the paper roll  4  is rotated forward in the paper feeding direction, the separator  34  abuts on a leading edge  3   a  of the recording paper  3  to separate it from the periphery of the paper roll  4 . The separated leading edge  3   a  is guided to the paper mouth  17 , and the recording paper  3  is fed into the print section  7 . 
     The separator  34  is a plastic thin plate having elasticity and is attached to a second supporting arm  36  comprising a pair of L-shaped arm members  36   a  and  36   b  (see FIG.  3 ). The separator  34  is attached to ends of the respective arm members  36   a  and  36   b . The other ends of the respective arm members  36   a  and  36   b  are attached to the pivots  29  formed on the supporter  28 . Owing to this, the second supporting arm  36  is rotatable around the pivots  29 . Incidentally, similarly to the first supporting arm  31 , a rotational range of the second supporting arm  36  is also regulated by a stopper, which is not shown, so as to rotate the second supporting arm  36  within a required range. 
     The second supporting arm  36  moves the separator  34  in accordance with the diameter change of the paper roll  4 . The second supporting arm  36  is urged by a second spring  37  so as to be rotated in the clockwise direction in FIGS. 2A and 2B. In virtue of this, the separator  34  always abuts on the periphery of the paper roll  4  regardless of the diameter change thereof. It is possible to separate the leading edge  3   a  of the recording paper  3  from beginning to end. 
     The second spring  37  is attached in a state that one end thereof is put on the second supporting arm  36  and the other end thereof is put on the first supporting arm  31 . As to the second spring  37 , are used a coiled spring, a torsion spring, and so forth. In a case the torsion spring is used, the torsion spring is held by the pivot  29  for example, and both ends thereof are engaged with the first supporting arm  31  and the second supporting arm  32  respectively. 
     The urging force of the second spring  37  is weaker than that of the first spring  32 . Owing to this, the second spring  37  is moved by the urging force of the first spring  32  together with the first supporting arm  31 . 
     The second spring  37  is moved together with the first supporting arm  31  such as mentioned above so that the urging force of the second spring  37  for urging the second supporting arm  36  is prevented from largely changing. Thus, the separator  34  is pressed against the periphery of the paper roll  4  with constant force from beginning to end while the paper roll  4  is used. It is possible to surely separate the leading edge  3   a  of the recording paper  3  regardless of the diameter change of the paper roll  4 . 
     As shown in FIG. 3, the first supporting arm  31  and the second supporting arm  36  are provided with guide plates  41  and  42  respectively. The guide plates  41  and  42  abut on the recording paper  3  in an up-and-down direction so as to guide the leading edge  3   a  toward the paper mouth  17 . In virtue of the guide plates  41  and  42 , the leading edge  3   a  is surely led to the paper mouth  17  even though the separator  34  is moved in accordance with the diameter change of the paper roll  4 . 
     For the purpose of easily separating the recording paper  3  from the paper roll  4 , the separator  34  is contrived so as to have some advantages. For instance, a thickness of the separator  34  is adapted to be half or less of a thickness of the recording paper  3 . In doing so, even if the recording paper  3  closely contacts with the paper roll  4 , they are easily separated. Moreover, a head portion  34   a  of the separator  34  may be formed in an arc shape so as to contact the head portion  34   a  with the leading edge  3   a  at a sole point in a width direction. In doing so, it is possible to deal with rigidity of the color thermosensitive recording paper having a paper thickness and a paper strength. 
     A vibration detecting sensor  46 , which constitutes a leading-edge detecting device, is attached to an upper face of the separator  34 . When the paper roll  4  is rotated backward in the winding direction, a vibration of the separator  34  contacting with the periphery of the paper roll  4  is converted into an electric signal by the vibration detecting sensor  46 . The converted electric signal is sent to the printer controller  8 . The vibration detecting sensor  46  outputs the electric signal whose level corresponds to a scale of the vibration. When the separator  34  vibrates, deformation and strain are caused thereon. In view of this, as the vibration detecting sensor  46 , are used a piezoelectric element, a strain sensor, and so forth for detecting the deformation and the strain of the separator  34 . 
     As shown in FIG. 4, the head portion  34   a  of the separator  34  is pressed against the peripheral surface of the paper roll  4  by means of the second spring  37 . Thus, the separator  34  contacts with the paper roll  4  in a curvature state. When the leading edge  3   a  of the recording paper  3  passes the head portion  34   a  during the backward rotation of the paper roll  4 , the head portion  34   a  overleaps the leading edge  3   a  owing to the elasticity of the separator  34  and the urging force of the second spring  37 . 
     FIG. 5 is a waveform showing the output level of the vibration detecting sensor  46  under a condition that the paper roll  4  is rotated backward. While the paper roll  4  is reversed, the separator  34  slightly vibrates in a state that the leading edge  3   a  does not pass the separator  34 . In this state, the vibration detecting sensor  46  outputs the signal having a lower level corresponding to the scale of the vibration. Meanwhile, when the separator  34  overleaps the leading edge  3   a  of the recording paper  3 , the separator  34  largely vibrates because the head portion  34   a  thereof is moved by an amount corresponding to the thickness of the recording paper  3 . At this time, the output level of the vibration detecting sensor  46  is much higher than the others. 
     A memory included in the printer controller  8  stores a level L 1  in advance as a threshold value, which is set between the output level of the large vibration and the other output levels. When the output level of the vibration detecting sensor  46  exceeds the level L 1 , the printer controller  8  judges that the separator  34  has overleapt the leading edge  3   a  of the recording paper  3 . In this way, the leading edge  3   a  is detected. 
     The separator  34  is moved by the second supporting arm  36  and the second spring  37  in accordance with the diameter change of the paper roll  4 . Since the second spring  37  is attached to the first supporting arm  31 , the urging force thereof does not change largely from beginning to end while the paper roll  4  is used. Thus, the output level of the vibration detecting sensor  46  is substantially kept at a constant level from beginning to end while the paper roll  4  is used. Consequently, the output level is prevented from varying in accordance with the diameter change so that it is possible to stably detect the leading edge. 
     Further, since the separator  34  contacts with the paper roll  4  in the curvature state, the head portion  34   a  of the separator  34  comes into contact with the peripheral surface of the paper roll  4  actively when the head portion  34   a  has overleapt the leading edge  3   a  of the recording paper  3 . As an impact of this occasion becomes greater, the scale of the vibration becomes larger. The output level obtained at the moment of the disengagement of the separator  34  and the leading edge  3   a  is more greatly different from the other output levels in comparison with a case in that the separator  34  comes into contact with the paper roll  4  in a non-curvature state. Owing to this, it is possible to surely detect the leading edge  3   a  of the recording paper  3 . 
     Incidentally, the separator  34  is utilized as an abutting member of the leading-edge detecting device, and the vibration detecting sensor  46  is attached to this separator  34  so that the structure is simplified. 
     As shown in FIG. 6, the printer controller  8  is connected to a motor driver  62  and a display  63 . The motor driver  62  controls the rotation of the drive motor  21 . The display  63  reads an error message and a residual amount of the paper roll  4 . By the way, printing is instructed with a print start button  64 . 
     The printer controller  8  includes a CPU  66 , a memory  67 , a time counter  68 , and a loop counter  69 . The CPU  66  and the memory  67  execute the operation and the relative judgement. The time counter  68  measures a detection cycle of the leading edge  3   a  of the recording paper  3 . The loop counter  69  counts the rotations of the paper roll  4 . 
     The time counter  68  measures the detection cycle of the leading edge  3   a  while the paper roll  4  is rotated backward. The measured detection cycle is stored in the memory  67  and is compared with the detection cycle to be newly measured. When it is concluded from the comparison that the detection cycles of the leading edge  3   a  are substantially identical, it is judged that the secure winding of the paper roll  4  is completed. 
     The loop counter  69  is for rightly judging the completion of the secure winding. The loop counter  69  counts the rotations of the paper roll  4  after a difference between the last detection cycle and the current detection cycle has been included within a predetermined value. When a count number of the loop counter  69  exceeds a predetermined number “N” set in advance, completion of the secure winding is judged. Incidentally, the predetermined value of the difference is set such that the winding state of the paper roll  4  does not affect the operation of the printer. 
     The loop counter  69  is reset when printing is instructed and the backward rotation of the paper roll  4  is commenced in the winding direction. After that, upon detecting the leading edge  3   a  of the recording paper  3 , “1” is added. In case the difference between the last detection cycle and the current detection cycle is not included in the predetermined value, the loop counter  69  is reset. When the difference is included in the predetermined value, the loop counter  69  is not reset and “1” is added at the time of the next detection of the leading edge  3   a.    
     A predetermined period is set in an LUT  71  as a timer value. This predetermined period is set so as to exceed a period during which the leading edge  3   a  of the recording paper  3  makes one revolution on condition that the paper roll  4  has the maximum diameter. When the timer value or more has passed without detecting the leading edge  3   a  after commencing the backward rotation of the paper roll  4 , the drive motor  21  is stopped. At the same time, the display  63  reads the error message to notify a trouble to a user. 
     The printer controller  8  calculates the residual amount of the paper roll  4  on the basis of the detection cycle of the leading edge  3   a  in the state that the secure winding of the paper roll  4  is completed. The residual amount is represented as a remaining printable number. The calculation is carried out, referring to the LUT  71  storing a relationship between the detection cycle T of the leading edge of the securely wound paper roll  4  and the remaining printable number. This relationship is obtained in advance from the diameter of the paper roll  4 , the thickness of the recording paper  3 , and the predetermined cut size. 
     In the meantime, when the paper roll  4  is wound in the looseness state such as shown in FIG. 7, it takes a longer time to make one revolution of the leading edge  3   a . Thus, in the time counter  68  for measuring the detection cycle of the leading edge  3   a , a count value T thereof becomes larger. When the paper roll  4  is reversed and is securely wound, the diameter of the paper roll  4  becomes smaller and the detection cycle of the leading edge  3   a  becomes shorter. 
     Consequently, the count value T of the time counter  68  becomes smaller every rotation. After completing the secure winding, the count value T substantially becomes a constant value. For instance, such as shown in FIG. 5, the count value T 1  of the first rotation is larger than the count value T 2  of the second rotation. Upon completion of the secure winding, the detection cycle T of the reading edge  3   a  substantially becomes a constant cycle. In other words, T(n−1) becomes equal to Tn. When this condition is satisfied, it is judged that the secure winding has been completed. After judging the completion of the secure winding, the rotation of the paper roll  4  is changed to the forward rotation in the paper feeding direction. As a matter of fact, the completion of the secure winding is judged when the detection cycle is substantially becomes a fixed cycle, since there are a measurement error and so forth. In view of this, a prescribed tolerance is predetermined from experiment. When the detection cycle is included within the tolerance, it is judged that the detection cycle is fixed. 
     Next, an operation of the above structure is described below, referring to FIG.  8 . Upon depressing the print start button  64 , the printer controller  8  commences, via the motor driver  62 , actuating the drive motor  21  so as to rotate the drive roller  14  in a counterclockwise direction in the drawings. Owing to this, the paper roll  4  is rotated backward in the clockwise direction. Since the paper roll  4  contacts with the drive roller  14 , the paper roll  4  of the looseness state is securely wound. By the way, the loop counter  69  is reset at the time of commencement of the backward rotation. 
     When the leading edge  3   a  of the recording paper  3  passes the separator  34  during the backward direction, they are disengaged from each other so that the output level of the vibration detecting sensor  46  exceeds the level L 1 . In response to this, the printer controller  8  judges the disengagement of the leading edge  3   a  and the separator  34  to detect the leading edge  3   a.    
     Upon detecting the leading edge  3   a , the printer controller  8  adds “1” to the loop counter  69 . At the same time, the printer controller  8  makes a timer start. The paper roll  4  is successively rotated backward. When the leading edge  3  is detected again, “1” is further added to the loop counter  69 . The count value T 1  of the timer taken for one rotation is written in the memory  67 . When the paper roll  4  is loosely wound, the difference between the last count value T(n−1) and the current count value Tn is not included in the predetermined range. In this case, the paper roll  4  is successively rotated backward. At the same time, the loop counter  69  is reset. 
     When the count value Tn of the timer substantially satisfies a condition of Tn=T(n−1), a loop-count value of the loop counter  69  is referred. If the loop-count value does not exceed the predetermined number N, the backward rotation is continued. At this time, since the count value Tn satisfies the condition of Tn=T(n−1), the loop counter  69  is not reset. And then, when the loop-count value exceeds the predetermined number N, it is judged that the secure winding of the paper roll  4  has been completed. 
     When the completion of the secure winding is judged, the paper roll  4  is stopped at a predetermined rotational position. After that, the rotation of the paper roll  4  is changed to the forward rotation. Upon rotating the paper roll  4  forward, the leading edge  3   a  of the recording paper  3  is separated from the paper roll  4  by the separator  34 . The separated recording paper  3  is guided by the guide plates  41  and  42  to lead the leading edge  3   a  thereof to the paper mouth  17 . In this way, the recording paper  3  is fed into the print section  7 . The print section  7  executes the printing process. The printed recording paper  9  is cut in the prescribed size to be discharged to the outside of the printer  2 . Incidentally, the non-printed recording paper  3  is rewound toward the paper roll  4 . 
     Upon depressing the print start button  64  again, winding the paper roll  4  is performed. After completing the secure winding of the paper roll  4 , the printing process is commenced. These operations are repeated until the paper roll  4  is used up. In virtue of the above-described structure, the leading edge  3   a  is stably detected until the paper roll  4  is used up. Hence, the leading edge  3   a  is correctly detected so that the secure winding of the paper roll  4  is properly performed. 
     The above embodiment is described regarding the rotary-shaft fixation type in which the rotary shaft of the paper roll is rotatably supported at the fixed position. The present invention, however, may be applied to another type shown in FIG.  9 . This type is a rotary-shaft movement type in which a drive roller  81  abutting on the periphery of the paper roll  4  is disposed at a fixed position. In this type, the rotary shaft  22  is moved toward the drive roller  81  in accordance with the diameter change of the paper roll  4 . In FIGS. 9A and 9B, members being identical with the foregoing embodiment are denoted by the same reference numerals. 
     In FIG. 9A, a support plate  82  rotatably supports the paper roll  4 . The support plate  82  is formed with a slit  82   a  for moving the paper roll  4  toward the drive roller  81 . The rotary shaft  22  is inserted into the slit  82   a . The paper roll  4  is pressed against the drive roller  81  along the slit  82   a  by means of a press member (not shown) abutting on the rotary shaft  22 . 
     As the recording paper  3  is used, the diameter of the paper roll  4  becomes smaller. In accordance with the diameter, the rotary shaft  22  moves within the slit  82   a  to always contact the periphery of the paper roll  4  with the drive roller  81 . Owing to this, the paper roll  4  is properly rotated from beginning to end while it is used. 
     The separator  34  is attached to a second supporting arm  84  and is disposed near the drive roller  81 . The second supporting arm  84  is rotatably attached to a supporter  83  and is urged by a second spring  86  in the clockwise direction in FIG.  9 B. Thus, the separator  34  is moved in accordance with the diameter change of the paper roll  4 . The vibration detecting sensor  46  is attached to the separator  34 . 
     In the rotary-shaft movement type, it is unnecessary to provide the first supporting arm, which is for supporting the drive roller, and the first spring. By disposing the separator  34  near the drive roller, it is possible to reduce a movement amount of the separator moving in accordance with the diameter change of the paper roll  4 . Owing to this, the urging force of the second spring  86  is prevented from largely changing, regardless of the diameter of the paper roll  4 . Consequently, the vibration detecting sensor  46  stably works so that the leading edge is correctly detected. 
     Incidentally, when the separator  34  is approached more close to the drive roller, the separator  34  is hardly affected by the diameter change of the paper roll  4 . In this case, the second supporting arm may be omitted. Moreover, it is possible to contact the separator  34  with the paper roll  4  only by flexibility of the separator itself, from beginning to end while the paper roll  4  is used. Hence, the second spring may be also omitted. In doing so, the structure of the leading-edge detecting device is simplified. 
     In the above embodiment, the leading edge is detected on the basis of the signal level outputted from the vibration detecting sensor. However, as the vibration detecting sensor, it is possible to employ a sensor which outputs a pulse signal when detecting the vibration of a predetermined scale. On the basis of the pulse signal, the leading edge may be detected. 
     The above embodiment is described regarding the color thermal printer using the color thermosensitive recording paper. The present invention, however, may be applied to a photo printer in which printing is performed for a photographic paper. Further, the present invention may be applied to another printer using the other paper roll. 
     Moreover, in the above embodiment, the paper roll is contained in the chamber provided in the printer. However, the paper roll may be contained in a paper magazine, which is removably set to the printer. 
     Although the present invention has been fully described by way of the preferred embodiments thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.