Patent Publication Number: US-8995855-B2

Title: Printing device that executes calibration at frequency suited to user demand

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2009-271592 filed Nov. 30, 2009. The entire content of this priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a printing device, and more particularly to a printing device having a function that allows a user to specify the usage amount of print agent. 
     BACKGROUND 
     When printing images with a printing device having a function to adjust image density, a user can reduce the amount of toner or ink to be used during printing by setting to a lower image density. 
     Also, Japanese Patent-Application Publication No. H10-198222 discloses a printing device having a save mode. If a user selects the save mode, then the usage amount of toner or ink during printing is reduced by reducing the image density or by thinning out dots or lines. 
     There is also provided a printing device that automatically executes a calibration process for calibrating a density level or a printing position when a predetermined condition is met. In this calibration process, the printing device prints a predetermined pattern, detects the pattern, and calibrates the density level or the printing position based on detection results. 
     SUMMARY 
     If a user emphasizes image quality, then it is desirable that the calibration process be performed prior to printing so as to ensure the appropriate density level and the like. If the user puts not much emphasis on image quality, on the other hand, then frequent execution of the calibration process undesirably consumes more ink or toner. 
     In view of the foregoing, it is an object of the invention to provide a printing device capable of executing the calibration process at a frequency suited to user demand. 
     In order to attain the above and other objects, the invention provides a printing device including a printing unit that prints an image with print agent, a calibration unit that executes a calibration process, a specifying unit that specifies a usage amount of the print agent, and a control unit that controls the calibration unit to execute the calibration process at a lower frequency when the specifying unit specifies a lower amount as the usage amount. The calibration process is for printing a mark with the print unit, detecting the mark, and calculating a calculation value based on a detection result. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a cross-sectional side view of a printer according to an embodiment of the invention; 
         FIG. 2  is a block-diagram of the printer according to the embodiment of the invention and an external computer connected thereto; 
         FIG. 3  is a flowchart representing a printing/calibration process executed in the printer according to the embodiment of the invention; 
         FIG. 4  is a flowchart representing a calibration execution process executed in the printer according to the embodiment of the invention; 
         FIG. 5  is a view showing an example of density pattern printed with the printer; and 
         FIG. 6  is a table based on which frequency of calibration is determined in the embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     A printer  1  according to an embodiment of the invention will be described while referring to the accompanying drawings. The terms “right,” “left,” “beneath,” and the like will be used throughout the description assuming that the printer  1  is disposed in an orientation in which it is intended to be used. In use, the printer  1  is disposed as shown in  FIG. 1 . 
     The printer  1  of the embodiment is a direct-tandem-type color printer for forming color images with toner (print agent) of four colors (black, yellow, magenta, and cyan). Note that some reference signs denoting components identical for each color are omitted in  FIG. 1 . 
     As shown in  FIG. 1 , the printer  1  includes a main casing  2  formed with a cover  2 A on top thereof. The cover  2 A is capable of selectively opening and closing. The printer  1  also includes a supply tray  4 , a sheet-feed roller  5 , registration rollers  6 , and a printing unit  10 . The printing unit  10  includes a belt unit  11 , exposure units  17 K,  17 Y,  17 M, and  17 C, process units  19 K,  19 Y,  19 M, and  19 C, and a fixing unit  31 . 
     The supply tray  4  is disposed in the bottom section of the main casing  2  for accommodating a stack of sheets  3  (recording medium). The sheets  3  stacked in the supply tray  4  are fed by the sheet-feed roller  5  and conveyed to the belt unit  11  by the registration rollers  6  one at a time. 
     The belt unit  11  includes a pair of belt-support rollers  12 A and  12 B opposing each other in a front-to-rear direction and a ring-shaped belt  13  wound about and spanning between the belt-support rollers  12 A and  12 B. The belt  13  supports the sheet  3  thereon by electrostatic adsorption, and circulation of the belt  13  conveys the sheet  3  rearward. Four transfer rollers  14  are disposed within the belt  13  at positions confronting photosensitive drums  28  (described later) of the process units  19 K,  19 Y,  19 M, and  19 C with an upper part of the belt  13  interposed therebetween. 
     The exposure units  17 K,  17 Y,  17 M, and  17 C respectively correspond to black, yellow, magenta, and cyan, and each includes an LED head  18  having an array of a plurality of LEDs (not shown) at the bottom. Each of the exposure units  17 K,  17 Y,  17 M, and  17 C emits a light beam with each LED based on print data and scans the light beam across a surface of the corresponding photosensitive drum  28  one line at a time. 
     The printer  1  further includes a pattern sensor  15  and a cleaner  16 . The pattern sensor  15  is disposed beneath the belt  13  for detecting patterns formed on the surface of the belt  13 . The pattern sensor  15  irradiates a light to the surface of the belt  13 , receives a reflected light with a phototransistor or the like, and outputs a single of a level corresponding to a received amount of reflected light. The cleaner  16  is disposed beneath the belt unit  11  for collecting toner, paper dust, and the like clinging on the surface of the belt  13 . 
     The process units  19 K,  19 Y,  19 M, and  19 C respectively correspond to black, yellow, magenta, and cyan, and each includes a frame  21  and a developing cartridge  22 . Opening the cover  2 A withdraws the exposure units  17 K,  17 Y,  17 M, and  17 C upward, enabling a user to remove or mount each of the exposure units  17 K,  17 Y,  17 M, and  17 C from or into the main casing  2  individually. Also, the belt unit  11  can be mounted into or removed from the main casing  2  when all of the process units  19 K,  19 Y,  19 M, and  19 C are removed from the main casing  2 . 
     Each developing cartridge  22  includes a toner chamber  23 , a supply roller  24 , and a developing roller  25 . The toner chamber  23  accommodates toner of corresponding color. Toner discharged from the toner chamber  23  is supplied to the developing roller  25  by rotation of the supply roller  24 , and is tribocharged to a positive polarity at a position between the supply roller  24  and the developing roller  25 . 
     The photosensitive drum  28  and a charger  29  are disposed in the lower section of each frame  21 . The photosensitive drum  28  is formed of an electrically-grounded column-shaped main body having a surface coated with a positively-chargeable photosensitive layer. The charger  29  generates a discharge to uniformly charge the surface of the rotating photosensitive drum  28  to a positive polarity. Thus uniformly charged surface of the photosensitive drum  28  is exposed to a scanning of the light beam emitted from the exposure unit  17 K,  17 Y,  17 M, or  17 C. As a result, the surface electric potential of the photosensitive drum  28  partially decreases by an amount corresponding to the intensity of the light beam. In this manner, an electrostatic latent image corresponding to an image to be formed on the sheet  3  is formed on the photosensitive drum  28 . 
     Then, positively-charged toner held on the developing roller  25  is selectively supplied to the electrostatic latent image formed on the photosensitive drum  28  by a developing bias applied to the developing roller  25 . As a result, the electrostatic latent image on the photosensitive drum  28  is transformed into a visible toner image. 
     The toner images formed on the photosensitive drums  28  in this manner are sequentially transferred onto the sheet  3  so as to be superimposed over each other by a transfer bias applied to the transfer rollers  14  when the sheet  3  passes through transfer positions between the photosensitive drums  28  and the transfer rollers  14 . The sheet  3  with the toner images formed thereon is conveyed to the fixing unit  31  located in the rear section of the main casing  2 . The fixing unit  31  thermally fixes the toner images onto the sheet  3  and discharges the sheet  3  upward. Thereafter, the sheet  3  is discharged onto a top surface of the main casing  2  by discharge rollers  32 . 
     As shown in  FIG. 2 , the printer  1  further includes a CPU  40 , a ROM  41 , a RAM  42 , a non-volatile RAM (NVRAM)  43 , and a network interface (I/F)  44 . The ROM  41  stores various programs for executing various operations in the printer  1 . The CPU  40  executes overall control of the printer  1  based on programs read from the ROM  41  while storing processed results into either the RAM  42  or the NVRAM  43 . The network I/F  44  (a specifying unit, an input unit, an accepting unit) is connected to an external computer  60  and the like through a communication circuit  50  to establish data communication therebetween. 
     The printer  1  also includes a display unit  45 , an operation unit  46 , and a cover sensor  47 . Although not shown in the drawings, the display unit  45  includes a display panel and a lamp, and is capable of displaying various setting screens and operation status of the printer  1 . The operation unit  46  (a specifying unit, an input unit, an accepting unit) includes various buttons (not shown) through which a user can input various instructions. The cover sensor  47  outputs a detection signal indicating an open or closed status of the cover  2 A. 
     The printer  1  has a toner-save mode, and a user can manipulate the operation unit  46  to selectively turn ON and OFF the toner-save mode. The user also can specify a desired image density (usage amount of print agent) and input a calibration execution command through the operation unit  46 . Details will be described later. 
     The computer  60  includes a CPU  61 , a ROM  62 , a RAM  63 , a hard disk drive (HDD)  64 , an operation unit  65  including a keyboard and a pointing device (not shown), a display unit  66  including a display panel (not shown), and a network I/F  67  connected to the communication circuit  50 . The HDD  64  stores various programs including an application software for generating image data to be printed and a printer driver for controlling the printer  1 . 
     When a user instructs print execution on the operation unit  65  of the computer  60 , a print command is transmitted from the computer  60  to the printer  1  and received by the CPU  40  through the network I/F  44 . When instructing the print execution, the user can have the display unit  66  display a print setting screen of the printer driver and input various print-condition settings on the print setting screen. These settings are transmitted to the printer  1  along with the print command. In this embodiment, a user can set a print quality (resolution) to either normal quality (normal resolution) or high quality (high resolution), and this quality setting is transmitted to the printer  1  along with the print command. 
     Next, a printing/calibration process executed in the printer  1  will be described with reference to the flowchart of  FIG. 3 . In this printing/calibration process, the printer  1  selectively executes a printing process and a calibration process. The calibration process is a process to perform calibration that affects image quality. In this embodiment, the calibration process includes a density level calibration process and a printing position calibration process. The printer  1  executes the printing/calibration process of  FIG. 3  repeatedly while a main power of the printer  1  is ON. 
     First in S 101 , the CPU  40  determines whether or not any of the process units  19 K,  19 Y,  19 M, and  19 C has been detached and attached. In this embodiment, the CPU  40  makes a positive determination in S 101  if opening/closing of the cover  2 A is detected by the cover sensor  47 . If a negative determination is made in S 101  (S 101 : No), then the CPU  40  directly proceeds to S 103 . On the other hand, if a positive determination is made in S 101  (S 101 : Yes), then the CPU  40  executes both the density level calibration process and the printing position calibration process in S 102  and then proceeds to S 103 . 
     In the density level calibration process, first the CPU  40  controls the printing unit  10  to print a density pattern P shown in  FIG. 5 , for example, on the belt  13 . The density pattern P includes a plurality of patches (marks) aligned along the circulation direction of the belt  13 . More specifically, the density pattern P includes five patches for each of the colors black, yellow, magenta, and cyan formed in different densities (black patches K 1  to K 5 , cyan patches C 1  to C 5 , magenta patches M 1  to M 5 , and yellow patches Y 1  to Y 5 , some of which are omitted from  FIG. 5 ). 
     Next, the CPU  40  measures the density of each patch with the pattern sensor  15 . Then, based on the measurement results, the CPU  40  calculates density calibration data for realizing an ideal image density when printing images on the sheet  3 , for each of 256 tones (density of 0% to 100% is equally divided into 256 tones) of each color, and stores each density calibration data into the NVRAM  43  for later use. That is, when printing images, the CPU  40  retrieves the density calibration data from the NVRAM  43  and sets the intensity of the light beam and the developing bias voltage based on the density calibration data to calibrate image density. The toner forming the density pattern P is removed from the belt  13  by the cleaner  16 . 
     In the printing position calibration process, on the other hand, first the CPU  40  prints on the belt  13  a position-calibration pattern (not shown) including a plurality of marks of each color with the printing unit  10  and then detects a position of each mark with the pattern sensor  15 . Based on the detection results, the CPU  40  calculates a positional offset amount from an ideal position for each color, calculates position calibration data for compensating the positional offset for each color, and stores the position calibration data into the NVRAM  43  for later use. That is, when printing images, the CPU  40  retrieves the position calibration data from the NVRAM  43  and controls the scanning start timings of the light beams from the exposure units  17 K,  17 Y,  17 M, and  17 C based on the retrieved position calibration data so as to calibrate the printing positions. 
     In S 103 , the CPU  40  determines whether or not a calibration execution command is received. As described above, the calibration execution command may be input by a user through the operation unit  46 . However, the user may alternatively input the calibration execution command by using such programs as the printer driver executable on the computer  60 . In this case, the calibration execution command is transmitted from the computer  60  to the printer  1  through the network I/F  44 . 
     If a negative determination is made in S 103  (S 103 : No), then the CPU  40  directly proceeds to S 105 . On the other hand, if a positive determination is made in S 103  (S 103 : Yes), then the CPU  40  executes the density level calibration process and the printing position calibration process described above in S 104 , and then proceeds to S 105 . 
     Note that the CPU  40  may execute only either one of the density level calibration process and the printing position calibration process in S 104 . Also, the user may specify either the density level calibration process or the printing position calibration process to be executed in S 104 . 
     In S 105 , the CPU  40  executes a calibration execution process, which will be described next with reference to the flowchart of  FIG. 4 . 
     In S 201  of  FIG. 4 , first in S 201  the CPU  40  determines whether or not a predetermined condition to execute the printing position calibration process is met. In this embodiment, an elapse of a predetermined time since the printing position calibration process was executed last time is set as the condition to execute the printing position calibration process. Thus, a positive determination is made in S 201  if the predetermined time has elapsed since the printing position calibration process was executed last time. However, the condition to execute the printing position calibration process may be changed as needed. For example, it is possible to set the condition such that a positive determination is made in S 201  if images have been printed on a predetermined number of sheets  3  since the printing position calibration process was executed last time or if the amount of temperature change since the printing position calibration process was executed last time has reached a predetermined amount. 
     If a positive determination is made in S 201  (S 201 : Yes), then the CPU  40  executes the printing position calibration process in S 202 , and then proceeds to S 203 . On the other hand, if a negative determination is made in S 201  (S 201 : No), then the CPU  40  directly proceeds to S 203 . 
     In S 203 , the CPU  40  determines whether or not a condition to execute the density level calibration process is met. In this embodiment, a positive determination is made in S 203  if a time interval T has elapsed since either when the density level calibration process was executed last time or when execution of the density level calibration process was deferred last time. Note that deferment of the density level calibration process means ending the calibration execution process of  FIG. 4  without executing the density level calibration process when a positive determination is made in S 203  as described later (process to proceed to S 205  after S 204  (S 204 : Yes)). 
     The time interval T is determined based on a user-specified density, an ON/OFF setting of the toner-save mode, and a table TB shown in  FIG. 6 . The table LB will be described in detail. The table TB indicates a print density, the time interval T, and deferment of calibration, for each combination of the user-specified densities and the ON/OFF settings of the toner-save mode. As described above, a user can set a desired image density (hereinafter referred to as “user-specified density”) and ON/OFF of the toner-save mode through the operation unit  46 . 
     However, the user may alternatively set the user-specified density and ON/OFF of the toner-save mode on the computer  60 . For example, the computer  60  displays on the display unit  66  a print-setting screen of the printer driver to enable the user to specify a desired user-specified density and to indicate an ON/OFF of the power-save mode. In this case, when the user instructs print execution, then these settings made on the print-setting screen are transmitted to the printer  1  through the communication circuit  50  together with a print command and various print-condition settings. 
     In this embodiment, the user can set the user-specified density to a level between +5 and −5 (some of which are omitted from  FIG. 6 ), and the print density can be set to a level between +5 to −5. When the toner-save mode is ON, then the print density is the same as the user-specified density. On the other hand, when the toner-save mode is OFF, then the print density is equal to a difference obtained by subtracting 4 from the user-specified density. However, if the difference is −6 or less, then the print density is set to −5. In actual printing, the CPU  40  controls the intensity of the light beam from the LEDs and the like based on one of the print densities listed on the table TB corresponding to the current combination of the user-specified density and the ON/OFF setting of the toner-save mode to realize the target print density. 
     The time interval T is set longer for a lower user-specified density when the toner-save mode is OFF. For example, the time interval T is 60 minutes when the user-specified density is 0, 90 minutes when the user-specified density is −1, 120 minutes when the user-specified density is −2 or −3, and 150 minutes when the user-specified density is −4 or −5. In other words, the condition to execute the density level calibration process is stricter (met less often) when a lower toner usage amount is specified by the user. On the other hand, when the toner-save mode is ON, the time interval T is always 150 minutes regardless of the user-specified density. 
     Also, the table TB indicates either “defer” or “not defer” in a deferment column. In this embodiment, “deter” is set when the print density is −4 or less, and “not defer” is set when the print density is −3 or greater. 
     Thus, in S 203  of  FIG. 4 , the CPU  40  refers to the table TB and determines the time interval T corresponding to the current combination of the user-specified density and the ON/OFF setting of the toner-save mode that have been previously input by the user, and determines whether or not the time interval T has elapsed since either when the density level calibration process was executed last time or when execution of the density level calibration process was deferred last time. 
     If a positive determination is made in S 203  (S 203 : Yes), then in S 204  the CPU  40  refers to the table TB and determines whether or not the deferment of calibration is set to “defer” for the current combination of the user-specified density and the ON/OFF setting of the toner-save mode. If so (S 204 : Yes), then the CPU  40  increments a counter N by 1 and ends the calibration execution process without executing the density level calibration process. That is, the CPU  40  defers execution of the density level calibration process. On the other hand, if a negative determination is made in S 204  (S 204 : No), then the CPU  40  executes the density level calibration process in S 206  and ends the calibration execution process. Note that the CPU  40  resets the counter N to 0 each time the CPU  40  executes the density level calibration process, not only in S 206 , but also in S 102 ,  104 , and S 108  of  FIG. 3 . 
     If a negative determination is made in S 203  (S 203 : No), then in S 207  the CPU  40  determines whether or not the counter N is greater than 0. If not (S 207 : No), then this means that the density level calibration process has not been deterred after the density level calibration process was executed last time. In this case, the CPU  40  ends the calibration execution process. On the other hand, if so (S 207 : Yes), then this means that the density level calibration process has been deterred after the density level calibration process was executed last time. In this case, the CPU  40  determines in S 208  whether or not the counter N is 5 or greater. If so (S 208 : Yes), then the CPU  40  proceeds to S 206  to execute the density level calibration process. That is, if the density level calibration process has been deferred five times, then the density level calibration process is executed. 
     If a negative determination is made in S 208  (S 208 : No), on the other hand, then the CPU  40  determines in S 209  whether or not the toner-save mode has been switched from ON to OFF. If so (S 209 : Yes), then the CPU  40  proceeds to S 206  to execute the density level calibration process. In other words, the density level calibration process is executed at a timing of when the toner-save mode is switched from ON to OFF. However, if not (S 209 : No), then the CPU  40  ends the calibration execution process without executing the density level calibration process. 
     After executing the calibration execution process in S 105  of  FIG. 3 , the CPU  40  determines in S 106  whether or not a print command is received. If not (S 106 : No), then the CPU  40  ends the process of  FIG. 3 . On the other hand, if so (S 106 : Yes), then in S 107  the CPU  40  determines whether or not the quality setting received together with the print command is high quality. If so (S 107 : Yes), then the CPU  40  executes the density level calibration process in S 108  and proceeds to S 109 . On the other hand, if not (S 107 : No), then the CPU  40  directly proceeds to S 109 . In other words, execution of the density level calibration process in S 108  is allowed if the image quality is set to high quality, but is not allowed if the image quality is set to normal quality. 
     In S 109 , the CPU  40  controls the printing unit  10  to execute printing based on the print command. Then, the CPU  40  ends the printing/calibration process. 
     As described above, according to the present embodiment, the printing position calibration process is executed in S 202  of  FIG. 4  once each time the predetermined time elapses unless the printing position calibration process is executed in either S 102  or S 104 . On the other hand, the frequency of the density level calibration process executed in S 206  is determined based on the combination of the user-specified density and the ON/OFF of the toner-save mode, unless the density level calibration process is executed in either S 102 , S 104 , or S 108 . 
     That is, if “not defer” is set in the table TB, then the time interval T is longer when the user-specified density is lower, so the density level calibration process is executed less frequently. Also, when the toner-save mode is ON, execution of the density level calibration process may be deferred. Thus, the density level calibration process is executed less frequently compared to when the toner-save mode is OFF. 
     Also, the printing position calibration process and the density level calibration process are executed each time detachment/attachment of the process units  19 K,  19 Y,  19 M, and  19 C is detected (S 101 : Yes) and each time the calibration execution command is received (S 103 : Yes). Further, the density level calibration process is executed when a print command is received if the image quality is set to high quality (S 107 : Yes). That is, in these cases, the density level calibration process is executed regardless of any determination results made in the calibration execution process of  FIG. 4 . 
     As described above, according to the present embodiment, a calibration process (density level calibration process) is executed less frequently when the specified usage amount (image density) of print agent (toner) is smaller. Generally, it is assumed that a user more likely emphasizes saving the print agent over image quality when a lower usage amount of print agent is specified (when the user-specified density is lower). Thus, setting lower frequency for the density level calibration process when a smaller usage amount is specified enables to execute the density level calibration process at a frequency suited to user demand. 
     Also, according to the above-described embodiment, a condition to execute the calibration process is stricter when a lower usage amount of print agent is specified. This enables to lower the frequency of the density level calibration process when the usage amount of print agent is set to a lower amount. 
     Also, when the usage amount of print agent specified by a user is equal to or less than a predetermined amount, then the execution of the calibration process may be deferred even if the condition to execute the calibration process is met. This lowers the frequency of the calibration process, and thus reduces the consumption of print agent. 
     Also, when the toner-save mode is turned OFF (in other words, when the specified usage amount of print agent is increased; S 209 : Yes) after the calibration process was deferred (S 207 : Yes), then the calibration process is executed. That is, it is highly likely that the image quality is emphasized over saving the print agent if the usage amount of print agent is increased. Thus, in this case, the calibration process is executed upon detecting turning OFF of the toner-save mode to meet user demand. 
     Further, the calibration process is executed upon increase of the usage amount of print agent, regardless of whether or not a print command has been received. Thus, when a print command is received afterward, printing based on the print command can be executed quickly without being delayed by execution of the calibration process, while securing high-quality image. 
     Moreover, when the calibration execution command is input, then the calibration process is always executed, without regard to settings regarding execution of the calibration process (i.e., the time interval T and/or deferment of calibration). With this configuration, the calibration process is executed based on a user command even if the setting is to lower the frequency of the calibration process, and thus it is possible to enhance the convenience. 
     Also, when a print command for a high-quality image is received, then the calibration process is always executed, without regard to the settings regarding execution of the calibration process. This configuration ensures high-quality images to meet user demand. 
     When attachment conditions of the components of the printing unit  10  to the main casing  2  are changed, there is a danger of greatly degrading image quality, if images are printed without executing the calibration process. Thus, according to the above-described embodiment, if the cover sensor  47  detects the attachment/detachment of the process units  19 K,  19 Y,  19 M, and  19 C, the calibration process is always executed, without regard to the settings regarding execution of the calibration process. This configuration prevents degradation of image quality. 
     Generally, it is expected that positional deviations affect image quality more than density deviations do. Thus, in the above-described embodiment, the frequency of the printing position calibration process is not lowered even when the usage amount of print agent is set low, although the frequency of the density level calibration process is lowered in accordance with the usage amount of print agent. Thus, it is possible to reduce the overall frequency of the calibration process (the printing position calibration process and the density level calibration process combined) while maintaining a certain level of image quality. 
     While the invention has been described in detail with reference to the embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     For example, the printer  1  is not limited to an electrophotographic color printer, but may be a monochrome printing device, an inkjet printing device, or different type of printing device. Also, the print agent may be ink or the like. 
     In the above-described embodiment, the usage amount of print agent is specified by a user specifying the image density and turning ON/OFF the toner-save mode. However, it may be configured to enable the user to either specify the image density or turn ON/OFF the toner-save mode. Also, in the above-described embodiment, the usage amount of print agent is decreased by lowering the image density. However, the usage amount of print agent may be decreased by thinning out dots or lines. 
     In the above-described embodiment, the frequency of the calibration process is adjusted based on the time interval T and “defer” or “not deter” set in the table TB shown in  FIG. 6 . However, the frequency may be adjusted based on only one of these two. Also, the printing position calibration process also may be executed at a frequency determined based on the usage amount of print agent. 
     In the above-described embodiment, the calibration process is executed after the calibration process has been deferred a predetermined number of times (five times in the embodiment). However, the predetermined number can be changed arbitrary or may not be used. For example, it is possible to set a larger number for a lower usage amount of print agent, such that the frequencies are changed in steps. It is also possible to prohibit execution of the calibration process when the frequency of the calibration process is lowered. 
     In the above-described embodiment, the calibration process is executed upon increase of the usage amount of print agent after deferment of the calibration process. However, when the usage amount of print agent is increased, it is possible to wait for a next print command without immediately executing the calibration process and to execute the calibration process upon receiving a next print command immediately before printing based on the next print command. It is also possible to enable a user to select either to execute the calibration process immediately or to wait until a next print command is received. 
     In the above-described embodiment, it is determined that the process units  19 K,  19 Y,  19 M, and  19 C are detached/attached when the cover sensor  47  detects the opening/closing of the cover  2 A. However, it is possible to determine that the process units  19 K,  19 Y,  19 M, and  19 C are detached/attached when an amount of remaining toner is detected to be changed with a toner sensor, for example. Also, the calibration process can be executed when detachment/attachment of such other components as the belt unit  11  of the printing unit  10  is detected. 
     Oscillation or impact applied to the printer  1  also may change the attachment condition of the components, which in turn affects image quality. Thus, it is possible to automatically execute the calibration process when vibration greater than a predetermined level is detected in the printer  1  with a vibration sensor, regardless of the settings regarding the frequency of the calibration process.