Abstract:
The present invention has been made to prevent density irregularity from appearing in a print result and to thereby improve image quality by controlling a motor so that the rotational speed of a drive shaft that feeds printing media becomes constant. The present invention provides a printing media feeding apparatus including a feeding section that includes a motor, a drive transmission mechanism which transmits a driving force of the motor, and a drive shaft to which the driving force is transmitted by the drive transmission mechanism, the drive shaft being rotated to feed the printing media, a detection section that is provided in the drive shaft and detects the rotational speed of the drive shaft, and a controller that controls the motor, the controller controlling the motor based on an input from the detection section so that the rotational speed of the drive shaft becomes constant.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]     The present invention contains subject matter related to Japanese Patent Application JP 2004-242797 filed in Japanese Patent Office on Aug. 23, 2004, the entire contents of which being incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a printing media feeding apparatus, a printing apparatus provided with the feeding apparatus, a printing media feeding speed control method, and a computer program.  
         [0004]     2. Description of the Related Art  
         [0005]     In related art a thermal printer with a thermal head has a thermal head in which a plurality of heat elements are linearly arranged and controls power distribution to the heat elements depending on the tone level to heat a heat-sensitive recording layer, thereby printing an image on printing media.  
         [0006]     In such a thermal printer, color density depends on the energy applied to the printing media. That is, to obtain deep color, the heating value of the heat elements is increased; to obtain light color, the heating value thereof is reduced.  
         [0007]     The energy to be applied to the printing media is increased when the feeding speed of the printing media is low. In this case, the obtained color may become deeper than a desired level. On the other hand, the energy to be applied to the printing media is reduced when the feeding speed of the printing media is high, with the result that the obtained color may become lighter than a desired level.  
         [0008]     As shown in  FIG. 1 , speed irregularity of the printing media appears as density irregularity in an image printed on the printing media, the density irregularity being formed by a plurality of lines running in the direction perpendicular to the feeding direction of the printing media. In  FIG. 1 , an area  101  is a low-density part, and area  102  is a high-density part. It is therefore demanded in the thermal printer that the speed irregularity in the feeding operation of the printing media be eliminated.  
         [0009]     In general, the printing media feeding apparatus uses a stepping motor as a drive source to rotate a capstan of the final stage through a drive transmission mechanism such as a pulley, an endless belt, and a gear. The capstan feeds the printing media while holding the printing media together with a roller provided opposite thereto. Since a means for directly feeding the printing media is the capstan, the rotational speed of the capstan needs to be constant in order to feed the printing media at a constant speed.  
         [0010]     However, it is difficult to make the rotational speed of the capstan constant, since it depends on the mechanical accuracy of the drive transmission mechanism. For example, the rotational speed of the capstan comes under the influence of the rotational accuracy of the pulley, the rotational accuracy of the capstan itself, and the like. However, even if the drive transmission mechanism has been mechanically assembled with high accuracy, it is still difficult to make the feeding speed of the printing media constant and eliminate the density irregularity.  
         [0011]     As prior art documents related to a technique of eliminating the density irregularity, the following patent documents are known: Jpn. Pat. Appln. Laid-Open Publication Nos. H11-334160, H5-169708, 2001-239686, and S63-296976.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention has been made to solve the above problem, and it is desirable to provide a printing media feeding apparatus capable of controlling a motor so that the rotational speed of a drive shaft that allows the printing media to run become constant to prevent the density irregularity from appearing in the printing result to thereby increase image quality, a printing apparatus provided with the feeding apparatus, a printing media feeding speed control method, and a computer program.  
         [0013]     According to the present invention, there is provided a printing media feeding apparatus comprising: a feeding means that includes a motor, a drive transmission mechanism which transmits a driving force of the motor, and a drive shaft to which the driving force is transmitted by the drive transmission mechanism, the drive shaft being rotated to feed the printing media; a detection means that is provided in the drive shaft and detects the rotational speed of the drive shaft; and a control means for controlling the motor, the control means controlling the motor based on an input from the detection means so that the rotational speed of the drive shaft becomes constant.  
         [0014]     According to the present invention, there is provided a printing apparatus comprising: a print head that prints visual data on printing media; a feeding means that includes a motor, a drive transmission mechanism which transmits a driving force of the motor, and a drive shaft to which the driving force is transmitted by the drive transmission mechanism, the drive shaft being rotated to feed the printing media; a detection means that is provided in the drive shaft and detects the rotational speed of the drive shaft; and a control means for controlling the motor, the control means controlling the motor based on an input from the detection means so that the rotational speed of the drive shaft becomes constant.  
         [0015]     According to the present invention, there is provided a printing media feeding speed control method that rotates a drive shaft through a drive transmission mechanism transmitting a driving force of a motor to feed printing media, comprising the steps of: detecting the rotational speed of the drive shaft; and controlling the rotation number of the motor based on the detected rotational speed so that the rotational speed of the drive shaft becomes constant.  
         [0016]     According to the present invention, there is provided a computer program used for a printing media feeding speed control method that rotates a drive shaft through a drive transmission mechanism transmitting a driving force of a motor to feed printing media, comprising the steps of: detecting the rotational speed of the drive shaft; and controlling the rotation number of the motor based on the detected rotational speed so that the rotational speed of the drive shaft becomes constant.  
         [0017]     The present invention having the configuration as described above detects the rotational speed of the drive shaft that feeds the printing media and controls, based on the detected rotational speed, the rotation number of the motor so that the rotational speed of the drive shaft becomes constant, thereby making the feeding speed of the printing media constant. As a result, in a printed image, the density irregularity caused due to irregularity of the feeding speed of the printing media can be reduced or eliminated. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a view showing an image in which density irregularity has occurred;  
         [0019]      FIG. 2  is a perspective view of a printer apparatus according to the present invention, the printer apparatus being placed flat;  
         [0020]      FIG. 3  is a perspective view of the printer apparatus, which is placed upright;  
         [0021]      FIG. 4  is a cross-sectional view of a print sheet;  
         [0022]      FIG. 5  is a view showing an internal configuration of the printer apparatus;  
         [0023]      FIG. 6  is a view showing a configuration of a printing block of the printer apparatus;  
         [0024]      FIG. 7  is a perspective view showing the drive mechanism of a capstan;  
         [0025]      FIG. 8  is an exploded perspective view showing a detection mechanism that detects the rotation of the capstan;  
         [0026]      FIG. 9  is a view showing irregularity strength obtained by reading out a gray print with a scanner and applying Fast Fourier Transform to the readout density data;  
         [0027]      FIG. 10  is a view showing a noise component appearing as a result of extracting the speed fluctuation component of the capstan at printing time and unloaded time;  
         [0028]      FIG. 11  is a view showing a noise component appearing as a result of extracting the speed fluctuation component of the capstan in the case where images of different densities are printed;  
         [0029]      FIG. 12  is a view showing the circuit configuration that removes a noise component included in the output from an encoder;  
         [0030]      FIG. 13  is a view obtained by comparing noise removal effects of a filter;  
         [0031]      FIGS. 14A and 14B  are views showing correlation between the density irregularity in a printed image and speed irregularity of the capstan;  FIG. 14A  is a view showing spectrum strength (irregularity strength) for each frequency obtained by applying Fast Fourier Transform to the density data same as  FIG. 9 , and  FIG. 14B  is a view obtained by applying Fast Fourier Transform to clock number which is a speed fluctuation component of the capstan  14   d  in the feeding direction and plotting frequency on the abscissa and spectrum strength (irregularity strength) on the ordinate;  
         [0032]      FIGS. 15A and 15B  are views showing clock number based on pulse number;  FIG. 15A  shows the case before feedback control, and  FIG. 15B  shows the case after feedback control; and  
         [0033]      FIGS. 16A and 16B  are views showing irregularity strength obtained by reading out a gray print with a scanner and applying Fast Fourier Transform to the readout density data;  FIG. 16A  shows the case before feedback control, and  FIG. 16B  shows the case after feedback control. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     A printer apparatus according to the present invention will be described below with reference to the accompanying drawings.  
         [0035]     As shown in  FIGS. 2 and 3 , a printer apparatus  1  according to the present invention uses, as a print sheet, a print film on which CT (Computerized Tomography) image data and the like taken in a hospital is printed. The printer apparatus  1  prints the image data with thermal transfer technology. A print sheet  2  used in the printer apparatus  1  is, as shown in  FIG. 4 , obtained by laminating a heat-sensitive layer  2   b  on a resin sheet  2   a  and further laminating a protection layer  2   c  on the heat-sensitive layer  2   b . The print sheet  2  thus obtained has rigidity higher than that of fine paper or coated paper and has elasticity.  
         [0036]     As shown in  FIGS. 2 and 3 , the printer apparatus  1  has a rectangular casing  3 . The front face  3   a  of the casing  3  serves as an operation face. Various operation buttons  4  such as a power button, a reset button, and a paper eject button, as well as a display section  5  constituted by an LCD (Liquid Crystal Display) that indicates an operation state and the like are arranged on the front face  3   a . Further arranged on the front face  3   a  of the casing  3  are a detachable housing tray  6  in which the print sheets  2  are stacked and an ejection port  7  from which the print sheet  2  is ejected. The housing tray  6  and ejection port  7  are arranged adjacently to each other.  
         [0037]     An outer lid  8  for opening/closing an opening of the casing  3  is provided on one side face  3   b  of the casing  3 . A positioning block for positioning the print sheet  2  being fed and the like are provided in the interior of the casing  3  that is covered by the outer lid  8 . When paper jam occurs, the outer lid  8  is opened to apply maintenance.  
         [0038]     The printer apparatus  1  can be placed flat, as shown in  FIG. 2 , such that the sheet surface of the print sheet  2  is horizontally set, as well as, can be placed upright, as shown in  FIG. 3 , such that the sheet surface of the print sheet  2  is vertically set. That is, a user can select whether the printer apparatus  1  is to be placed flat or upright depending on the install location, thereby increasing usability.  
         [0039]     The internal configuration of the printer apparatus  1  will be described with reference to  FIG. 5 . The printer apparatus  1  uses a pick-up block  11  constituted by a plurality of rollers  11   a  and the like for picking up the stacked print sheet  2  from the housing tray  6  housed in the casing  3  to pick up one sheet and then uses a plurality of feeding rollers  12   a  that constitute a feeding block  12  to feed the picked up print sheet  2 .  
         [0040]     After positioning the print sheet  2  with the positioning block  13  before printing, the printer apparatus  1  perform printing on the print sheet  2  with a printing block  14  based on print data, reverses the printed print sheet  2  with a reverse roller  15   a  that constitutes a feeding block  15 , further reverses the print sheet  2  with a reverse roller  15   b  that constitutes the feeding block  15 , and ejects the print sheet  2  from the ejection port  7 .  
         [0041]     As shown in  FIG. 6 , in the printing block  14  used here, a print head  14   a  such as a thermal head for heating the print sheet  2 , in which a plurality of heat elements are arranged in the direction perpendicular to the feeding direction of the print sheet  2  is supported by a head support member. A platen roller  14   c  is disposed opposite to the print head  14   a . The print sheet  2 , which is guided by guide rollers  14   b  and held between a capstan  14   d  and roller  14   e , is fed by the rotation of the capstan  14   d .  
         [0042]     In the printing block  14 , the print head  14   a  and platen roller  14   c  sandwich the print sheet  2 , and the print head  14   a  heats the print sheet  2  to thereby forming an image on the print sheet  2 . In this printer apparatus  1 , the platen roller  14   c  is not in a driven state at the printing time. The platen roller is rotated in the feeding direction of the print sheet  2  when the print sheet  2  is fed without being printed.  
         [0043]     A drive mechanism  20  of the capstan  14   d  will be described with reference to  FIG. 7 . A stepping motor  21  is used as a drive source of the drive mechanism  20 . A driving force of the stepping motor  21  is transmitted through a drive transmission mechanism  20   a  to the capstan  14   d . The drive transmission mechanism  20   a  includes first to third pulleys  22 ,  23 , and  24 .  
         [0044]     The first pulley  22  is fitted to the drive shaft of the stepping motor  21  and is coupled to the second pulley  23  through a first endless belt  25 . The second pulley  23  is coupled to the third pulley  24  to which the capstan  14   d  is fitted through a second endless belt  26 .  
         [0045]     The first to third pulleys  22  to  24  are rotatably fitted to spindles provided on a base  27 . When the first pulley  22  is rotated, the driving force of the stepping motor  21  is transmitted to the second pulley  23  through the first endless belt  25  and further transmitted to the third pulley  24  through the second endless belt  26 , thereby causing the capstan  14   d  integrally fitted to the third pulley  24  to be rotated.  
         [0046]     A detection mechanism  30  for detecting the rotational speed of the capstan  14   d  is provided in the third pulley  24  to which the capstan  14   d  is integrally fitted. In the detection mechanism  30 , as shown in  FIG. 8 , a sensor substrate  33  is fixed to a bracket  31  by screws  34 , and the bracket is fixed to the base  27  by screws  32 .  
         [0047]     The sensor substrate  33  has an encoder  35  constituted by a light emitter and light receiver disposed opposite to each other. An encoder disc  36  is provided between the light emitter and light receiver constituting the encoder  35 . The encoder disc  36  has a plurality of slits  36   a  formed radially and is rotated together with the capstan  14   d . In order to perform the speed control of the capstan  14   d  (to be described later) correctly, the number of the slits  36   a  is determined so that two or more pulse signals can be output from the encoder  35  while one line that constitutes an image is being printed.  
         [0048]     The encoder  35  detects the rotation of the capstan  14   d  by detecting a light that has emitted from the light emitter and passed through the slits  36   a  with the light receiver. The encoder  35  outputs, for example, 2000 pulses during one rotation of the capstan  14   d  and outputs 3.6 pulses while one line is printed (for example, 6.25 ms/one line).  
         [0049]     The detection mechanism  30  further has an attachment member  37  to which the capstan  14   d  is press-fitted. Provided in the center of the attachment member  37  is a sleeve  37   a , to which the capstan  14   d  is press-fitted. The sleeve  37   a  is inserted through a center hole  36   b  of the encoder disc  36  and further inserted through a bearing  38  so that the attachment member  37  to be integrally fitted to the capstan  14   d  can smoothly be rotated relative to the bracket  31  fixed to the base  27 .  
         [0050]     The bearing  38  is press-fitted to a through hole  31   a  of the bracket  31  that has been fixed to the base  27 . A cover  39  is fixed to the bracket  31  by screws or the like in such a manner to house the encoder disc  36  fixed to the attachment member  37  and bearing  38  in the space between the cover  39  and bracket  31 . A main body  37   b  of the attachment member  37  projects from a through hole  39   a  of the cover  39  and is press-fitted to an inner concave-portion  24   a  of the third pulley  24  of the drive transmission mechanism  20   a.    
         [0051]     The capstan  14   d  is press-fitted to the attachment member  37  to be integrally fitted to the third pulley  24 , and thereby the capstan  14   d  is rotated relative to the bracket  31  that has been fixed to the base  27 . The encoder  35  detects the rotation of the capstan  14   d  by detecting a light that has passed through the slits  36   a  of the encoder disc  36  which is rotated integrally with the capstan  14   d  through the attachment member  37 . The capstan  14   d  goes into the feeding path of the print sheet  2  through a through hole  27   a  of the base  27  to feed the print sheet  2  in corporation with the roller  14   e.    
         [0052]     In an image printed by a conventional printer apparatus, the density irregularity occurs, as shown by the areas  101  and  102  in  FIG. 1 . To verify this, grey is printed on the entire surface of the print sheet  2  as shown in  FIG. 1 , the gray print is read out with a scanner, and the read out density data is subjected to the Fast Foirier Transform along the feeding direction of the print sheet  2 . The obtained data is shown in  FIG. 9 .  
         [0053]      FIG. 9  shows that the abscissa denotes frequency, and the ordinate denotes spectrum strength (irregularity strength). As can be seen from  FIG. 9 , peak frequency components  41   a  to  41   c  appear at a plurality of frequency levels, which appears as the density irregularity. The peak frequency component  41   a  corresponds to unsteady component of the rotation of the first pulley  22 , the peak frequency component  41   b  corresponds to unsteady component of the rotation of the second pulley  23 , and the peak frequency component  41   c  corresponds to unsteady component of the ½ rotation of the capstan  14   d.    
         [0054]     Fluctuation in the rotational speed of the capstan  14   d  that causes the density irregularity will be verified. Here, the encoder  35  is fitted to the capstan  14   d  like the configuration of the printer apparatus  1 , and CPU clock number corresponding to the pulse count number of the output from the encoder  35  is measured.  FIG. 10  shows the relationship between the clock number and the pulse count number at printing time where the print sheet  2  is allowed to run and at non-feeding time where the print sheet is not allowed to run, that is, unloaded time. A line  43  denotes the printing time, and a line  44  denotes the unloaded time. However, the output from the encoder  35  contains an enourmous amount of noise components. Further, the noise level greatly differs between the printing time and unloaded time.  
         [0055]      FIG. 11  shows the relationship between the clock number and the pulse count number of the output from the encoder  35  when images of different densities (black 100%, black 50%, black 1%) are printed. Aline  45   a  denotes black 1%, a line  45   b  denotes black 50%, and a line  45   c  denotes black 100%. However, also in  FIG. 11 , the output from the encoder  35  contains an enourmous amount of noise components. Further, the noise level greatly differs depending on the densities.  
         [0056]     In the printer apparatus  1  according to the present invention, a pulse signal indicating the rotation number of the capstan  14   d  is input to the controller  42  from the encoder  35 , as shown in  FIG. 6 . The controller  42  removes noise components from the data shown in  FIGS. 10 and 11  and thereby extracts only a speed fluctuation component of the capstan  14   d.    
         [0057]     That is, the controller  42  includes, as shown in  FIG. 12 , a filter  42   a  to which the pulse signal from the encoder  35  is input, a moving-average circuit  42   b  which performs moving-average of a filtering result, and a comparing control circuit  42   c  which generates a control signal for the stepping motor  21 .  
         [0058]     The filter  42   a  removes the noise components from the signal as shown in  FIGS. 10 and 11  and thereby extracts the speed fluctuation component of the capstan  14   d . In order to make the rotational speed of the capstan  14   d  constant, the filter  42   a  is required to perform sequential real-time processing. Further, it is preferable to decrease computation amount. Consequently, a dynamic Kalman filter is used as the filter  42   a . The Kalman filter can sufficiently perform computation within one cycle of the input pulse from the encoder  35 .  
         [0059]      FIG. 13  shows the relationship between the CPU clock number and the pulse count number obtained by filtering the output from the encoder  35 . In  FIG. 13 , a line  46  denotes characteristics obtained by 3 moving-average processing, a line  47  denotes characteristics obtained by 20 moving-average processing, a line  48  denotes characteristics obtained by filtering with the Kalman filter, and a line  49  denotes characteristics obtained by Kalman filtering and 3 moving-average processing. As can be seen from  FIG. 13 , the noise can be removed more effectively with the Kalman filtering (line  48 ) than with n moving average processing (lines  46  and  47 ).  
         [0060]     As shown by the line  48 , the Kalman filter cannot remove the noise component completely. To cope with this, as shown in  FIG. 12 , the moving-average circuit  42   b  is connected to the rear stage of the filter  42   a  in the controller  42  to perform moving-average processing for the output from the Kalman filter. As shown in  FIG. 13 , by adding the 3 moving-average processing to the Kalman filtering (line  49 ), it is possible to remove the noise more effectively than in the case where only Kalman filtering is applied (line  49 ), thereby extracting the speed fluctuation component of the capstan  14   d.    
         [0061]     The number of moving-average processing is not limited to 3. Further, the moving-average processing may be performed at the front stage of the Kalman filter.  
         [0062]      FIG. 14  is a view for comparing the density irregularity in a printed image and speed irregularity of the capstan. More specifically,  FIG. 14A  is a view showing the relationship between frequency (abscissa) and spectrum strength (irregularity strength) (ordinate) obtained by applying Fast Fourier Transform to the density data same as  FIG. 9 , and  FIG. 14B  is a view showing the relationship between frequency (abscissa) and spectrum strength (ordinate) obtained by applying Fast Fourier Transform to clock number which is a speed fluctuation component of the capstan  14   d  in the feeding direction. The comparison between  FIG. 14A  and  FIG. 14B  reveals that the peak frequency components  41   a  to  41   c  in  FIG. 14A  and peak frequency components  40   a  to  40   c  in  FIG. 14B  appear at the same frequency levels. This indicates that there exists a correlation between the density irregularity and rotational speed of the capstan  14   d.    
         [0063]     The controller  42  determines the peak frequencies  40   a  to  40   c  shown in  FIG. 14B  as a factor of the density irregularity in a printed image and controls the stepping motor  21  with the comparing control circuit  42   c  so that the peak frequencies  40   a  to  40   c  is reduced or eliminated. The comparing control circuit  42   c  compares a signal output from the moving-average circuit  42   b  and a reference signal stored in a memory.  
         [0064]     To be more specific, when the signal (line  49  in  FIG. 13 ) that has been subjected to the filter processing in the filter  42   a  and moving-average circuit  42   b  is greater than the reference signal, that is, when the speed of the capstan  14   d  is less than a reference speed, the comparing control circuit  42   c  makes the cycle of the pulse signal that drives the stepping motor  21  shorter than a reference pulse signal to increase the rotation number of the stepping motor  21 , thereby increasing the rotational speed of the capstan  14   d.    
         [0065]     When the signal (line  49  in  FIG. 13 ) that has been subjected to the filter processing in the filter  42   a  and moving-average circuit  42   b  is smaller than the reference signal, that is, when the speed of the capstan  14   d  is greater than a reference speed, the comparing control circuit  42   c  makes the cycle of the pulse signal that drives the stepping motor  21  longer than a reference pulse signal to reduce the rotation number of the stepping motor  21 , thereby reducing the rotational speed of the capstan  14   d.    
         [0066]      FIG. 15  shows a result obtained when the controller  42  performs the above control. In  FIG. 15 ,  FIG. 15A  shows a result obtained before feedback control of the controller  42 , and  FIG. 15B  shows a result obtained after feedback control of the controller  42 . As can be seen from the comparison between  FIGS. 15A and 15B , the fluctuation of the CPU clock becomes smaller in  FIG. 15B  (after feedback control) than in  FIG. 15A , that is, the fluctuation of the CPU clock is substantially eliminated to make the rotational speed of the capstan  14   d  nearly constant.  
         [0067]     As in the case of  FIG. 9 , grey is printed on the entire surface of the print sheet  2 , the gray print is read out with a scanner, and the read out density data is subjected to the Fast Fourier Transform (FFT) along the feeding direction of the print sheet  2 . The obtained density data is shown in  FIG. 16 . In  FIG. 16 ,  FIG. 16A  shows a result obtained before feedback control, and  FIG. 16B  shows a result obtained after feedback control. As can be seen from the comparison between  FIGS. 16A and 16B , the entire curve including the peak frequency components  41   a  to  41   c  becomes flat. This indicates the density irregularity has been reduced in a printed image.  
         [0068]     In the printer apparatus  1  having the configuration as described above, the controller  42  extracts the rotational speed fluctuation component of the capstan  14   d  and controls the stepping motor  21  so that the speed fluctuation component is reduced or eliminated, thereby making the rotational speed of the capstan  14   d  constant while allowing for a mechanical error of the drive transmission mechanism  20   a  and the like. Therefore, in a printed image, the density irregularity caused due to the fluctuation of the feeding speed of the print sheet  2  can be reduced or eliminated. Further, it is possible to make it easier to design and assemble the drive transmission mechanism  20   a.    
         [0069]     In the encoder disc  36  provided for the capstan  14   d , the encoder  35  can output a plurality of pulses while the print head  14   a  prints one line. Further, the Kalman filter is used as the filter  42   a , so that the controller  42  can extract the fluctuation component of the rotational speed of the capstan  14   d  in real time. Therefore, the controller  42  has excellent response characteristics to the speed fluctuation and thereby can control the rotational speed of the capstan  14   d  in real time.  
         [0070]     The above control performed by the controller  42  can be realized by hardware as well as by software. In the case where software is used, the above control can be realized by storing software to which the present invention is applied in a memory such as a hard disc or semiconductor memory and by performing computation with a CPU.  
         [0071]     The print sheet  2  is fed by the capstan  14   d  in the printer apparatus  1  described above. In the case of using a printer apparatus in which the platen roller  14   c  is rotated in printing time to feed the print sheet  2 , the encoder  35  may be fitted to the drive shaft of the platen roller  14   c . In this configuration, the controller  42  extracts the rotational speed fluctuation component of the platen roller  14   c  to thereby control the stepping motor  21  so that the platen roller  14   c  is rotated at a constant speed. By this, it is possible to obtain the same effect as that in the case of the printer apparatus  1 .  
         [0072]     Although the heat-sensitive layer  2   b  is formed on the print sheet  2  and the print head  14   a  prints an image on the heat-sensitive layer  2   b  in the above example, the present invention is also applied to a thermal printer apparatus in which a print head allows ink of an ink ribbon to sublime to thereby thermally transfer an image on a print sheet, or an inkjet printer that discharges ink to print an image on a print sheet.  
         [0073]     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alternations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.