Patent Publication Number: US-7912393-B2

Title: Image-forming device with a density measuring unit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2007-020488 filed Jan. 31, 2007. The entire content of its priority application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an image-forming device and a method for measuring density of a test image. 
     BACKGROUND 
     Methods of determining the accuracy of density-calibrated images are well known in the art as means for preventing error in density measurements due to changes in properties over time or lot variations. One such method disclosed in Japanese unexamined patent application publication No. HEI-6-331630 involves performing density measurements on samples having known densities and correcting changes over time of image densities based on the results of these measurements. 
     In this conventional method, after first setting the samples used for calibrating image density, the densities of all samples are measured. If the measured density is determined to be within a preset range, then it is determined whether results of density measurements corresponding to all samples fall within a preset range. If the results of density measurements are found to fall within the preset range for all samples, the calibration results are determined to be normal and density calibration is performed based on these results. However, if the density measurement results for any sample are found to fall outside the preset range for that sample, then the calibration results are determined to be abnormal. 
     Accordingly, when repeating image density calibration, density measurements must be obtained not only for samples yielding abnormal results, but also for samples yielding normal results. Consequently, samples are unnecessarily consumed by repeating calibration for samples yielding normal results. 
     SUMMARY 
     In view of the foregoing, it is an object of the present invention to provide an image-forming device and a method capable of reducing the consumption of image-forming material when repeating image density calibration. 
     In order to attain the above and other objects, the invention provides an image-forming device including an image-forming unit, a test image memory, a test image forming unit, a density measuring unit, an abnormality determining unit, a test image re-forming unit, and a density re-measuring unit. The image-forming unit forms an image on a recording medium based on inputted image data. The test image memory stores image data of test image used for calibrating density of image to be formed by the image-forming unit. The test image forming unit controls the image-forming unit to form the test image by reading image data for test image stored in the test image memory and outputting the image data to the image-forming unit. The density measuring unit measuring the density of the test image that the image-forming unit forms on the recording medium. The abnormality determining unit compares the density of test image measured by the density measuring unit with prescribed values pre-stored in association with the test images to determine whether the measured density is abnormal. The test image re-forming unit controls the image-forming unit to re-form test image determined to be abnormal by the abnormality determining unit on the recording medium by outputting image data for the test image determined to be abnormal to the image-forming unit. The density re-measuring unit measures the density of the test image that the image-forming unit re-forms on the recording medium. 
     According to another aspect, the present invention provides a method for measuring density of a test image, including forming a test image based on an test image data; measuring a density of the test image; determining whether the measured density is abnormal with comparing the measured density of test image with prescribed values pre-stored in association with the test images; re-forming test image determined to be abnormal based on the test image data; and re-measuring a density of the test image re-formed. 
    
    
     
       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 vertical cross-sectional view of a color laser printer according to an embodiment; 
         FIG. 2  is a block diagram showing the electrical structure of the color laser printer; 
         FIG. 3  is a table showing the content of a density patch formation data memory area or an error density patch formation data memory area; 
         FIG. 4  is an explanatory diagram conceptually illustrating density patches formed on a conveying belt; 
         FIG. 5  is a table showing the content of a normal value table memory area; 
         FIG. 6(   a ) is a table showing the content of a cyan target data memory area and a cyan measured density memory area; 
         FIG. 6(   b ) is a graph plotting cyan target density data stored in the cyan target data memory area, and cyan measured density data stored in the cyan measured density memory area; 
         FIG. 6(   c ) is a table showing the content of a cyan calibration table memory area; 
         FIG. 7  is a flowchart illustrating steps in a conveying distance counter updating process executed by a CPU in a controller of the color laser printer; 
         FIG. 8  is a flowchart illustrating steps in a calibration process executed by the CPU; 
         FIG. 9  is a flowchart illustrating steps in a calibration retry process executed by the CPU; 
         FIGS. 10(   a ) and  10 ( b ) are explanatory diagrams conceptually illustrating density patches re-formed on the conveying belt; 
         FIGS. 11(   a ) and  11 ( b ) are explanatory diagrams conceptually illustrating density patches re-formed on the conveying belt; 
         FIG. 12(   a ) is an explanatory diagram conceptually illustrating an example in which the formation position of a density patch is shifted by a patch width; and 
         FIG. 12(   b ) is an explanatory diagram conceptually illustrating an example in which the formation position of a density patch will exceed a value Xmax if shifted the patch width. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a vertical sectional view showing an overall configuration of a color laser printer  1  as a first embodiment of the present invention. As shown in  FIG. 1 , the color laser printer  1  is of a transverse-mounting tandem type in which four image forming units  20  are provided in series in a horizontal direction. The laser printer  1  includes a paper feed section  9 , an image forming section  4 , a paper ejection section  6  and a control section  90 . The paper feed section  9  feeds sheets of recording paper  3  one sheet at a time as recording medium to the image forming section  4 . The image forming section  4  forms an image on the fed recording paper  3 . The paper ejection section  6  ejects the recording paper  3  on which the image has been formed. The controller  90  controls the color laser printer  1 . 
     The paper feed section  9  includes a paper feed tray  12 , a paper feed roller  83  and conveying rollers  14   a  and  14   b . The paper feed tray  12  is detachably mounted on the main casing  5  from the front side (right side in  FIG. 1 ) in the bottom of the main casing  5 . The paper feed roller  83  is provided at one end (at the front side) of the paper feed tray  12 . The conveying rollers  14   a  and  14   b  are provided on the downstream side in the conveying direction of the recording paper  3  with respect to the paper feed roller  83  at the front side of the paper feed roller  83 . 
     A plurality of sheets of the recording paper  3  is stacked in the paper feed tray  12 . The uppermost sheet of the recording paper  3  is fed towards the conveying rollers  14   a  and  14   b  by rotations of the paper feed roller  83  and is conveyed sequentially between a conveying belt  68  and each of photosensitive drums  62  ( 62 C,  62 M,  62 Y, and  62 K). 
     In the middle portion of the main casing  5 , the image forming section  4  includes four image forming units  20  ( 20 Y,  20 M,  20 C, and  20 K) for forming images, a transfer section  17 , and a fixing section  8 . The transfer section  17  transfers images formed by each of the image forming units  20  to the recording paper  3 . The fixing section  8  fixes the images transferred to the recording paper  3  by heating and pressurizing the same. The above-described subscripts Y, M, C, and K represent the colors of Yellow (Y), Magenta (M), Cyan (C), and Black (K), respectively. 
     Four image forming units  20  have the same configuration except for storing different colors of toners. Each image forming unit  20  ( 20 C,  20 M,  20 Y, or  20 K) has a photosensitive drum  62  ( 62 C,  62 M,  62 Y, or  62 K), a charger  31  ( 31 C,  31 M,  31 Y, or  31 K), an exposure unit  41  ( 41 C,  41 M,  41 Y, or  41 K), and a developing unit  51  ( 51 C,  51 M,  51 Y, or  51 K). Each charger  31  ( 31 C,  31 M,  31 Y, or  31 K) is provided adjacent to the corresponding photosensitive drum  62  ( 62 C,  62 M,  62 Y, or  62 K) for charging the same. Each exposure unit  41  ( 41 C,  41 M,  41 Y or  41 K) forms an electrostatic latent image on the corresponding photosensitive drum  62  ( 62 C,  62 M,  62 Y or  62 K). The developing unit  51  ( 51 Y,  51 M,  51 C, or  51 K) forms a toner image by providing toner as a developing agent to the photosensitive drum  62  ( 62 C,  62 M,  62 Y, or  62 K), using a development bias applied between the photosensitive drum  62  ( 62 C,  62 M,  62 Y, or  62 K) and the developing unit  51  ( 51 Y,  51 M,  51 C, or  51 K). 
     Each charger  31  ( 31 C,  31 M,  31 Y or  31 K) is, for example, a Scorotron charger generating corona discharge from a discharging wire made of tungsten and evenly charging the surface of the photosensitive drum  62  ( 62 C,  62 M,  62 Y or  62 K) in a positive polarity. Each exposure unit  41  ( 41 C,  41 M,  41 Y or  41 K) includes an LED array emitting light for forming an electrostatic latent image on the surface of the photosensitive drum  62  ( 62 C,  62 M,  62 Y or  62 K). In this exposure unit  41  ( 41 C,  41 M,  41 Y or  41 K), light emitted from the LED array is irradiated on the photosensitive drum  62  ( 62 C,  62 M,  62 Y or  62 K), and an electrostatic latent image is formed on the surface of the photosensitive drum  62  ( 62 C,  62 M,  62 Y or  62 K). The exposure unit  41  ( 41 C,  41 M,  41 Y or  41 K) need not be an LED array, but may be an exposure unit that emits laser light. 
     Each developing unit  51  ( 51 C,  51 M,  51 Y or  51 K) has a developing casing  55  ( 55 C,  55 M,  55 Y or  55 K), in which provided are a hopper  56  ( 56 C,  56 M,  56 Y or  56 K), a supply roller  32  ( 32 C,  32 M,  32 Y or  32 K), and a developing roller  52  ( 52 C,  52 M,  52 Y or  52 K). Each hopper  56  ( 56 C,  56 M,  56 Y or  56 K) is formed as an inner space of the developing casing  55  ( 55 C,  55 M,  55 Y or  55 K). Toner of Cyan is contained in the hopper  56 C in the image forming unit  20 C. Toner of Magenta is contained in the hopper  56 M of the image forming unit  20 M. Toner of Yellow is contained in the hopper  56 Y of the image forming unit  20 Y. Toner of Black is contained in the hopper  56 K of the image forming unit  20 K. 
     Each supply roller  32  ( 32 C,  32 M,  32 Y or  32 K) is provided in the lower section of the hopper  56  ( 56 C,  56 M,  56 Y or  56 K). A roller portion made of a conductive sponge member is covered on a metallic roller shaft of the supply roller  32  ( 32 C,  32 M,  32 Y or  32 K). Each supply roller  32  ( 32 C,  32 M,  32 Y or  32 K) is rotatably supported so as to rotate in a direction to move opposite to the developing roller  52  ( 52 C,  52 M,  52 Y or  52 K) at a nip portion in contact with the developing roller  52  ( 52 C,  52 M,  52 Y or  52 K). 
     Each developing roller  52  ( 52 C,  52 M,  52 Y or  52 K) is rotatably provided at a position in contact with the supply roller  32  ( 32 C,  32 M,  32 Y or  32 K). A roller portion made of an elastic member such as a conductive rubber material is covered on a metallic roller shaft of the developing roller  52  ( 52 C,  52 M,  52 Y or  52 K). A developing bias voltage is applied from a power source (not shown) to the developing rollers  52 C,  52 M,  52 Y and  52 K. 
     The transfer section  17  is provided so as to be opposed to the photosensitive drums  62  ( 62 C,  62 M,  62 Y or  62 K) in the main casing  5  and has a conveying belt driving roller  63 , a conveying belt follow roller  64 , the conveying belt  68  which is an endless belt, and transfer rollers  61  ( 61 C,  61 M,  61 Y or  61 K). 
     The conveying belt driving roller  63  is provided on the downstream side of the photosensitive drums  62  ( 62 C,  62 M,  62 Y and  62 K) in the conveying direction of the recording paper  3  as well as on the upstream side of the fixing section  8 . The conveying belt follow roller  64  is provided on the upstream side of the photosensitive drums  62  ( 62 C,  62 M,  62 Y and  62 K) with respect to the conveying direction of the recording paper  3  as well as at the upper front side of the paper feed roller  83 . The conveying belt  68  is wound around between the conveying belt driving roller  63  and the conveying belt follow roller  64 , with the outer surface thereof being in contact with all the photosensitive drums  62  of the image forming units  20 . The conveying belt  68  is circularly moved in a counter-clockwise direction between the conveying belt driving roller  63  and the conveying belt follow roller  64  by being driven by the conveying belt driving roller  63 . 
     Each transfer roller  61  ( 61 C,  61 M,  61 Y or  61 K) is provided inside the loop of the conveying belt  68  so as to be opposed to the corresponding photosensitive drum  62  ( 62 C,  62 M,  62 Y or  62 K) with interposing the conveying belt  68  therebetween. In a transfer operation, a predetermined voltage is applied between the transfer roller  61  ( 61 C,  61 M,  61 Y or  61 K) and the photosensitive drum  62  ( 62 C,  62 M,  62 Y or  62 K) to transfer toner images from the photosensitive drum  62  ( 62 C,  62 M,  62 Y or  62 K) to the recording paper  3 . 
     The fixing section  8  is provided on the downstream side of the image forming units  20  and the transfer section  17 , and has a heating roller  81  and a pressure roller  82 . The heating roller  81  is made of a metallic pipe, on the surface of which a release layer is formed. A halogen lamp (not shown) is provided in the heating roller  81  along the axial direction thereof, and the surface of the heating roller  81  is heated to a fixing temperature by the halogen lamp. The pressure roller  82  is provided so as to pressurize the heating roller  81 . Toner image on the recording paper  3  is fixed by heat, when conveying the recording paper  3  between the pressure roller  82  and heating roller  81 . 
     The paper discharging section  6  is provided on the downstream side of the fixing section  8  in the upper portion of the main casing  5 , and includes a pair of paper discharging rollers  11  and a paper discharging tray  10 . The pair of paper discharging rollers  11  discharges the recording paper  3  on which an image has been fixed to the paper discharging tray  10 . The paper discharging tray  10  is provided on the downstream side of the paper discharging rollers  11  for accumulating the sheets of the recording paper  3  having completed the image forming process. 
     A density sensor  80  is provided obliquely rearward below the conveying belt driving roller  63  so as to oppose the outer surface of the conveying belt  68 . The density sensor  80  is configured to detect patches formed on the conveying belt  68 . 
     A toner-collecting device  107  is provided obliquely forward below the conveying belt driving roller  63 . A cleaning blush  105  is provided in the toner-collecting device  107 , and is in contact with the outer surface of the conveying belt  68 . The cleaning blush  105  is for electrically scraping off toner (patches and the like described above) adhered to the conveying belt  68 . A toner-collecting roller  106  is further provided in the toner-collecting device  107  and is for collecting the toner scraped off by the cleaning blush  105 . A blade  106   a  is further provided in the toner-collecting device  107  and scrapes off toner collected by the toner-collecting roller  106 . The toner scraped off by the blade  106  is collected in the toner-collecting device  107 . 
     Operation keys  108  and a display device  109  are provided on the main casing  5  at a location above the discharging rollers  11 . The operation keys  108  enable a user to input a predetermined command to the color laser printer  1 . The display device  109  displays a processing state of the color laser printer  1  and a message for a user. 
     An electric configuration of the color laser printer  1  will be described with reference to  FIG. 2 . As shown in  FIG. 2 , the color laser printer  1  is provided with the controller  90  for controlling each component of the apparatus. An ASIC  26  which is part of the controller  90  is connected to the image forming units  20 , the paper supply roller  83 , the conveying rollers  14   a  and  14   b , the conveying belt driving roller  63 , the transfer rollers  61 , the heating roller  81 , the pressure roller  82 , the paper discharging rollers  11 , the density sensor  80 , a panel gate array  108   a , and a display controller  109   a.    
     The controller  90  includes a CPU  22 , a ROM  23 , a RAM  24 , a flash memory  25 , the ASIC  26 , and a network interface  28 . The CPU  22 , the ROM  23 , the RAM  24 , the flash memory  25 , and the network interface  28  are connected to the ASIC  26  via a bus line. The CPU  22  is a microprocessor for executing various programs stored in the ROM  23 . The ROM  23  is a read-only memory for storing programs executed by the CPU  22  and for storing constants and tables that the CPU  22  refers to when executing the programs. 
     The RAM  24  has a work area in which the CPU  22  temporarily stores variables and the like when executing programs. The flash memory  25  is a rewritable memory device storing various data that can be overwritten when the power is on and is capable of preserving the memory content when the power is off. The ROM  23 , RAM  24  and flash memory  25  will be described later. 
     The ASIC  26  is an integrated circuit that converts commands from the CPU  22  and outputs corresponding signals for driving components of the laser printer  1 , and that converts signals outputted from the density sensor  80  and panel gate array  108   a  and outputs the converted signals to the CPU  22 . 
     Each of the photosensitive drums  62  ( 62 C,  62 M,  62 Y, or  62 K), developing rollers  52 , feed roller  83 , conveying rollers  14   a  and  14   b , conveying belt driving roller  63 , heating roller  81 , pressure roller  82 , and ejection rollers  11  is connected to the ASIC  26 , and includes a motor (not shown) for applying a rotational force and thereto a power source (not shown) for supplying power to the corresponding motor. The ASIC  26  outputs a control signal to rotate each motor, and the rotational force of the motor drives the corresponding element to rotate. 
     Each of chargers  31  is connected to the ASIC  26  and charges the corresponding photosensitive drum  62  upon receiving a control signal from the ASIC  26 . 
     Each of exposure units  41  is connected to the ASIC  26 . The ASIC  26  outputs control signals to each exposure unit  41  to control irradiation of the light beam from the corresponding LED array and to control the irradiated position of the light beam. 
     The heating roller  81  heats the transmitted toner on the recording paper  3  based on a control signal outputted by the ASIC  26 . 
     The density sensor  80  outputs data of measured densities (described later) to the ASIC  26 , and the ASIC  26  stores data of the measured densities in the RAM  24 . 
     The panel gate array  108   a  is connected to the operation keys  108  and controls the operation keys  108 . Specifically, the panel gate array  108   a  detects when some operation key  108  is pressed (input) and outputs a prescribed code signal to the ASIC  26 . A plurality of code signals is assigned to the plurality of operation keys  108 . Upon receiving a prescribed code signal from the panel gate array  108   a , the ASIC  26  issues an interrupt to the CPU  22 . When an interrupt is issued, the CPU  22  performs a prescribed control process based on a key process table. The key process table includes control processes associated with code signals and is stored in the ROM  23 , for example. 
     The display controller  109   a  controls the display of data related to operations of the laser printer  1  and the like on the display unit  109 . The display controller  109   a  is connected to and the display unit  109 . 
     The network interface  28  is an interface communicating by means of USB standard and is connected to the PC  125 . The network interface  28  can convert image information input from the PC  125  and output the converted image information to the CPU  22 . 
     A PC  125  inputs image information or set densities to the laser printer  1 . The PC  125  is connected to and capable of communicating with the network interface  28 . The densities of an image formed on the recording paper  3  are determined based on the set densities inputted from the PC  125 . 
     Next, the content of the ROM  23  will be described with reference to  FIGS. 5 and 6(   a ). As shown in  FIG. 2 , the ROM  23  includes a calibration program memory  23   a , a density patch formation data memory area  23   b , a normal value table memory area  23   c , and a target memory area  23   d.    
     The calibration program memory area  23   a  stores a calibration program, which is a control program executed by the CPU  22  to implement the process described in the flowcharts of  FIGS. 8 and 9 . 
     The density patch formation data memory area  23   b  stores density patch formation data used to form density patches C 1 -K 5  shown in  FIG. 4  on the conveying belt  68 . Next, the content of the density patch formation data in the density patch formation data memory area  23   b  will be described with reference to  FIG. 3 . 
       FIG. 3  is a table showing the content in the density patch formation data memory area  23   b . The density patch formation data stored in the density patch formation data memory area  23   b  is used to form the density patches C 1 -K 5  on the conveying belt  68  initially. The density patch formation data for each patch C 1 -K 5  includes a density patch name, a color of the density patch, a density of the patch, X and Y coordinates for the position at which the density patch is formed, and width and height of the patch. The density patch formation data includes data for a total of 20 patches, for the color, density, position, and dimensions of the patch in association with the density patch name. 
     A density patch name is assigned to each of the density patches C 1 -K 5 . In the embodiment, a total of 20 density patch names are provided from cyan  1  to black  5 . The color indicates the color in which the density patches C 1 -K 5  are formed. For example, when the color is cyan, the density patch is formed in the color cyan on the conveying belt  68 . In the embodiment, density patches are formed in one of the four colors cyan, magenta, yellow, and black. 
     The density value specifies the density in which the density patches C 1 -K 5  are to be formed. The density value can have a maximum value of 100% and a minimum value of 0%, a higher percentage resulting in a greater density of the density patch. A total of five density values are used in the embodiment: 100%, 80%, 60%, 40%, and 20%. 
     The patch width specifies a width of each density patch C 1 -K 5 . In the embodiment, all patch widths are set to a. The patch height specifies a height of each density patch C 1 -K 5 . In the embodiment, the height of all patches is set to b. 
     The X coordinate specifies a coordinate on a X-direction for forming the density patches C 1 -K 5  (see  FIG. 4 ). The X coordinate is set to X 1  for all density patches in the embodiment. The Y coordinate specifies a coordinate on the Y-direction at which the density patches C 1 -K 5  are to be formed (see  FIG. 4 ). In the embodiment, the density patch formation data memory area  23   b  is preset with a total of 20 Y coordinates ranging from Y 1  to Y 20 . 
     As shown in  FIGS. 3 and 4 , the Y coordinates indicating the position for forming a density patch are set by adding the patch height b of a preceding density patch to the Y coordinate for forming the preceding density patch. For example, the Y coordinate Y 2  of the position for forming density patch M 1  (density patch name: magenta  1 ) is calculated by adding the patch height b of density patch C 1  (density patch name: cyan  1 ) to the Y coordinate Y 1  at which the density patch C 1  is formed. Accordingly, the density patches C 1 -K 5  are formed on the conveying belt  68  (see  FIG. 1 ) with no gaps therebetween. 
     The value of the Y coordinate is set as a counter value for a conveying distance counter  24   d  described later. The Y coordinate Y 1  for cyan  1  (density patch C 1 ) is set at least two times the patch height b away from a point at which the conveying distance counter  24   d  is zero (a starting point from which the conveying belt  68  is moved to form the density patches C 1 -K 5  thereon). The coordinate Y 1  is set in this way so that, when the conveying belt  68  completes one loop and returns to the starting point, at least one of the density patches C 1 -K 5  can be formed between the starting point and the initial position for forming the density patches C 1 -K 5  and so that this single density patch will not overlap the initial position in which the corresponding density patch was formed. 
     Here,  FIG. 4  is an explanatory diagram conceptually illustrating the density patches C 1 -K 5  formed on the conveying belt  68 . In  FIG. 12 , the Y direction corresponds to the direction in which the conveying belt  68  moves, indicated by the arrow “a” in  FIG. 1 , while the X direction corresponds to a direction orthogonal to the surface of the drawing in  FIG. 1  and parallel to the surface of the conveying belt  68 . A printable range in the X direction denotes the region of the conveying belt  68  in which the density patches C 1 -K 5  can be formed. Therefore, if the density patches C 1 -K 5  are formed with one edge aligned with an end of the printable range in the X direction (the end farthest from the density sensor  80  shown in  FIG. 4 ), the X coordinate for the density patches is an Xmax value calculated by subtracting the patch width a from the X coordinate at the end of the printable range. The Xmax value is the maximum value to which the X coordinate can be set. 
     Next, the formation of the density patches C 1 -K 5  using the density patch formation data (see  FIG. 3 ) stored in the density patch formation data memory area  23   b  (see  FIG. 2 ) will be described. For example, when forming the density patch C 1 , the CPU  22  reads data from the density patch formation data memory area  23   b  for density patch name cyan  1  and forms the density patch C 1  on the conveying belt  68  at the position indicated by X and Y coordinates X 1  and Y 1 , in the color cyan, at a density of 100%, and with a width and height of a and b, respectively. Here, the X and Y coordinates determines a point Pc 1  for positioning the density patch C 1 . Specifically, the X coordinate at the point Pc 1  is X 1 , while the Y coordinate is Y 1 . 
     Further, when forming the density patch M 3 , for example, the CPU  22  reads data from the density patch formation data memory area  23   b  for the density patch name magenta  3  and forms the density patch M 3  on the conveying belt  68  at the formation position indicated by X and Y coordinates X 1  and Y 10  (Y 9 +b), in the color magenta, at the density 60%, and with the patch width and height of a and b, respectively. The X and Y coordinates determine a point Pm 3  for forming the density patch M 3 . Specifically, the X coordinate at the point Pm 3  is X 1 , while the Y coordinate is Y 10  (Y 9 +b=Y 1 +8b). 
     Hence, the CPU  22  forms a total of twenty density patches C 1 -K 5  on the conveying belt  68 , as shown in  FIG. 4 . Subsequently, the density sensor  80  measures the density of each of the density patches C 1 -K 5 . 
     Returning to  FIG. 2 , the normal value table memory area  23   c  stores a normal value table that is used for determining whether the densities of the density patches C 1 -K 5  formed on the conveying belt  68  and measured by the density sensor  80  (see  FIG. 1 ) are normal or abnormal. 
     As shown in  FIG. 5 , the normal value table stored in the normal value table memory area  23   c  is configured of maximum and minimum values for the density measured by the density sensor  80  in association with each density patch name. Accordingly, the normal value table is configured of 20 records in the embodiment. 
     The normal value table memory area  23   c  is used in a process for determining whether the densities measured by the density sensor  80  are normal or abnormal. After the density sensor  80  has measured the density patch C 1 , for example, the CPU  22  reads data from the normal value table memory area  23   c  for density patch name cyan  1 . The CPU  22  determines whether the density of the density patch C 1  measured by the density sensor  80  falls between the minimum and maximum values of the measured density read from the normal value table memory area  23   c  for cyan  1 . Hence, if the density of the density patch C 1  measured by the density sensor  80  is 0.9, the CPU  22  determines that the measured density is normal. However, if the density measured by the density sensor  80  is 1.1 or 0.7, the CPU  22  determines that the measured density is abnormal. 
     Similarly, after the density sensor  80  measures the density patch M 3 , the CPU  22  reads data from the normal value table memory area  23   c  for the density patch name magenta  3 . If the density of the density patch M 3  measured by the density sensor  80  is 0.5, the CPU  22  determines that the measured density is normal. However, if the density measured by the density sensor  80  is 0.8 or 0.3, the CPU  22  determines that the measured density is abnormal. 
     Returning to  FIG. 2 , the target data memory area  23   d  stores target density data used as target of calibration for each of the density patches C 1 -K 5  formed on the conveying belt  68 . The target data memory area  23   d  includes a cyan target data memory area  23   d   1 , a magenta target data memory area  23   d   2 , a yellow target data memory area  23   d   3 , and a black target data memory area  23   d   4  corresponding to the four colors used to form the density patches C 1 -K 5 . Hence, target density data for cyan is stored in the cyan target data memory area  23   d   1 , and target density data for yellow is stored in the yellow target data memory area  23   d   3 . Calibration is performed using the target density data stored in the target data memory area  23   d  and measured densities for the density patches C 1 -K 5  measured by the density sensor  80  to create calibration tables. The calibration tables are created in S 26  of the calibration process of  FIG. 8  and are stored in the calibration table memory area  25   a.    
     Next, the content of the target data memory area  23   d  will be described with reference to  FIG. 6(   a ).  FIG. 6(   a ) shows the content of the cyan target data memory area  23   d   1  (see  FIG. 2)  and the content of a cyan measured density memory area  24   c   1  described later. While only the content of the cyan target data memory area  23   d   1  will be described with reference to  FIG. 6(   a ), the content of the magenta target data memory area  23   d   2 , yellow target data memory area  23   d   3 , and black target data memory area  23   d   4  has the same structure as that of the cyan target data memory area  23   d   1 , differing only in the values for the target density data stored therein. 
     The target density data for the color cyan stored in the cyan target data memory area  23   d   1  (hereinafter abbreviated to “cyan target density data”) is configured of the density patch name and cyan target density for each of the density patches C 1 -C 5 . Since the cyan target density is stored in association with the density patch name, the cyan target density data includes five records. The cyan target densities in the cyan target density data are set to the densities of the density patches C 1 -C 5  measured by the density sensor  80  in the factory prior to shipping. 
     The cyan target density data indicates the cyan target densities corresponding to the set densities of the density patches C 1 -C 5 . For example, since the density patch C 1  has a set density of 20%, the cyan target density is 0.20. Further, since the density patch C 3  has a set density of 60%, the cyan target density is set to 0.60. 
     After the density patches C 1 -K 5  are formed on the conveying belt  68 , the density sensor  80  measures the density of the density patch C 1 , which has the set density of 20%. 
     Returning to  FIG. 2 , the RAM  24  has calibration flags  24   a , an error density patch formation data memory area  24   b , a measured density memory area  24   c , and a conveying distance counter  24   d.    
     The calibration flags  24   a  indicate abnormal measured densities when the CPU  22  determines that the measured densities for the density patches C 1 -K 5  do no fall between the upper and lower limits of measured densities in the normal value table stored in the normal value table memory area  23   c  and are thus abnormal measured densities. The calibration flags  24   a  include cyan  1 - 5  error flags  24   a   11 - 24   a   15 , magenta  1 - 5  error flags  24   a   21 - 24   a   25 , yellow  1 - 5  error flags  24   a   31 - 24   a   35 , and black  1 - 5  error flags  24   a   41 - 24   a   45  corresponding to the colors of the density patches C 1 -K 5 . For example, the cyan  1  error flag  24   a   11  is in correspondence with the density patch C 1 , and the magenta  3  error flag  24   a   23  is in correspondence with the density patch M 3 . Accordingly, if the CPU  22  determines that the measured density of the density patch C 1  is abnormal, the CPU  22  sets the cyan  1  error flag  24   a   11  to ON. If the CPU  22  determines that the measured density of the density patch M 3  is abnormal, the CPU  22  sets the magenta  3  error flag  24   a   23  to ON. 
     If the CPU  22  subsequently determines that the measured density of one of the density patches C 1 -K 5  is abnormal, i.e., does not fall between the upper and lower limits of measured densities in the normal value table stored in the normal value table memory area  23   c , then the CPU  22  sets the corresponding calibration flag  24   a  to ON. On the other hand, if the CPU  22  determines that the density of one of the measured density patches C 1 -K 5  is normal, i.e., falls between the upper and lower limits of measured densities in the normal value table stored in the normal value table memory area  23   c , then the CPU  22  sets the corresponding calibration flag  24   a  to OFF. 
     The error density patch formation data memory area  24   b  stores positions on the conveying belt  68  for re-forming any of the density patches C 1 -K 5  whose measured density was determined to be abnormal by the CPU  22 . The error density patch formation data memory area  24   b  is initially identical to that of the density patch formation data memory area  23   b . If the CPU  22  subsequently determines that the measured density for one of the density patches C 1 -K 5  is abnormal, the CPU  22  updates the content in the error density patch formation data memory area  24   b  each time the density patch found to have an abnormal measured density is re-formed. 
     The measured density memory area  24   c  stores the measured density of each density patch C 1 -K 5 . The measured density memory area  24   c  includes a cyan measured density memory area  24   c   1 , a magenta measured density memory area  24   c   2 , a yellow measured density memory area  24   c   3 , and a black measured density memory area  24   c   4  corresponding to the colors in which the density patches C 1 -K 5  are formed. Hence, when the density sensor  80  measures the densities of the cyan density patches C 1 -C 5 , for example, the measured densities are stored in the cyan measured density memory area  24   c   1 , and when the density sensor  80  measures the densities of the yellow density patches Y 1 -Y 5 , for example, the measured densities are stored in the yellow measured density memory area  24   c   3 . Measured densities stored in the measured density memory area  24   c  are updated each time the density sensor  80  measures the density of one of the density patches C 1 -K 5 . 
     Next, the content of the measured density memory area  24   c  will be described with reference to  FIG. 6(   a ).  FIG. 6(   a ) shows the content of the cyan target data memory area  23   d   1  and the cyan measured density memory area  24   c   1 . While the content of the cyan measured density memory area  24   c   1  will be described with reference to  FIG. 6(   a ), the content of the magenta measured density memory area  24   c   2 , yellow measured density memory area  24   c   3 , and black measured density memory area  24   c   4  are identical in structure to that of the cyan measured density memory area  24   c   1 , except for the values of the measured density. 
     The cyan measured density data stored in the cyan measured density memory area  24   c   1  is configured of the density patch names for density patches C 1 -C 5  and their corresponding densities measured by the density sensor  80 . Since the measured densities are stored in association with the density patch names, the cyan measured density data indicates the measured densities of the density patches C 1 -C 5 . For example, the measured density of the density patch C 1  is 0.15, while the set density is 20%. Further, the measured density of the density patch C 3  is 0.40, while the set density is 60%. 
     Returning to  FIG. 2 , the conveying distance counter  24   d  is used for finding the rotated amount of the drive roller  63  (see  FIG. 1 ), i.e., the conveying distance of the conveying belt  68  in the Y direction (see  FIG. 4 ). The conveying distance counter  24   d  is reset to zero when the calibration process starts. The value of the conveying distance counter  24   d  (hereinafter, referred to “counter value”) is subsequently incremented each time the conveying belt  68  moves a prescribed distance in the Y direction. A maximum value for the conveying distance counter  24   d  is set to a distance in which the conveying belt  68  makes one complete circuit after the conveying distance counter  24   d  starts counting. When the conveying belt  68  makes a complete circuit, the value counter of the conveying distance counter  24   d  is reset to zero. The counter value of the conveying distance counter  24   d  is the Y coordinate for patch forming positions stored in the density patch formation data memory area  23   b  (see  FIG. 3 ) and the error density patch formation data memory area  24   b  (see  FIG. 3(   b )). The CPU  22  initiates formation of each of the density patches C 1 -K 5  when the counter value reaches the Y coordinate in each patch forming position. 
     The Y coordinate Y 1  indicating the position for forming the density patch C 1  (see  FIG. 3 ) is set by the counter value of the conveying distance counter  24   d . As an example, the Y coordinate Y 1  for forming the density patch C 1  is set to 5 in terms of the count value of the conveying distance counter  24   d . Therefore, since the Y coordinate Y 2  for forming the density patch M 1  is Y 1 +b (see  FIG. 3 ), if the patch height b is set to 3 in terms of count values of the conveying distance counter  24   d , then the Y 2  is set to a count value of 8 (Y 2 =Y 1 +b=5+3). In this way, Y coordinates stored in the density patch formation data memory area  23   b  are set based on count values of the conveying distance counter  24   d . Y coordinates stored in the error density patch formation data memory area  24   b  also are set based on count values of the conveying distance counter  24   d.    
     The flash memory  25  has a calibration table memory area  25   a . The calibration table memory area  25   a  stores calibration tables for determining the degree to which set densities for the density patches C 1 -K 5  should be calibrated based on measured densities of the density patches C 1 -K 5  formed on the conveying belt  68 . 
     The calibration table memory area  25   a  includes a cyan calibration table memory area  25   a   1 , a magenta calibration table memory area  25   a   2 , a yellow calibration table memory area  25   a   3 , and a black calibration table memory area  25   a   4  corresponding to the colors of each the density patches C 1 -K 5 . 
     Next, the content of the cyan calibration memory area  25   a   1  will be described with reference to  FIG. 6(   c ). The cyan calibration table stored in the cyan calibration memory area  25   a   1  is configured of set densities and calibrated set densities. Since the calibrated set densities are stored in association with the set densities, the cyan calibration table in the embodiment is configured of six records. The calibrated set density for the set density of 0% is fixed at the preset value of 0%. 
       FIG. 7  is a flowchart illustrating steps in a conveying distance counter updating process. The conveying distance counter updating process is performed to update the rotational amount of the drive roller  63  (see  FIG. 1 ), i.e., to update the counter value of the conveying distance counter  24   d  for finding the conveying distance in the Y direction of the conveying belt  68  (Y coordinate; see  FIG. 4 ). The CPU  22  of the controller  90  executes this process periodically at intervals of 2 ms, for example, while the power to the laser printer  1  is on. 
     In S 1  of the conveying distance counter updating process, the CPU  22  determines whether the drive roller  63  has rotated a prescribed amount. The CPU  22  reaches a YES determination in S 1  when a stepping motor (not shown) has been driven a prescribed number of steps for rotating the drive roller  63  with a rotational drive force. The CPU  22  detects an encoder waveform of the stepping motor (DC motor) as the DC motor is driven to rotate the drive roller  63  to determine whether the drive roller  63  has rotated the prescribed amount. 
     When the CPU  22  determines that the drive roller  63  has been rotated the prescribed amount (equivalent to a prescribed amount of the conveying distance of the conveying belt  68  in the Y direction indicated by the Y coordinate; S 1 : YES), then in S 2  the CPU  22  increments the count value of the conveying distance counter  24   d  by one (1). In S 3  the CPU  22  determines whether the counter value of the conveying distance counter  24   d  is greater than or equal to the max value. The max value is the maximum count value for the conveying distance counter  24   d  and denotes that the conveying belt  68  has moved in one complete circuit from the point that the conveying distance counter  24   d  began counting. If the CPU  22  determines that the conveying belt  68  has moved one complete circuit based on the value of the conveying distance counter  24   d  reaching or exceeding the max value (S 3 : YES), then in S 4  the CPU  22  resets the value of the conveying distance counter  24   d  to zero. Accordingly, in S 4  the CPU  22  initializes the conveying distance of the conveying belt  68  in the Y direction (Y coordinate). Subsequently, the CPU  22  ends the conveying distance counter updating process. 
     However, if the CPU  22  determines that the drive roller  63  has not rotated the prescribed amount (S 1 : NO) or if the CPU  22  determines that the counter value of the conveying distance counter  24   d  is less than the max value (S 3 : NO), indicating that the conveying belt  68  has not yet moved in a complete circuit, the CPU  22  ends the conveying distance counter updating process without resetting the counter value to zero. 
     Through the conveying distance counter updating process, the CPU  22  can find the rotational amount of the drive roller  63 , i.e., the conveying distance of the conveying belt  68  in the Y direction (Y coordinate). 
     Next, a calibration process executed by the CPU  22  of the controller  90  will be described with reference to  FIG. 8 .  FIG. 8  is a flowchart illustrating steps in the calibration process. The CPU  22  executes the calibration process when the user operates the operation keys  108 . 
     In S 10  at the beginning of the calibration process, the CPU  22  resets the counter value of the conveying distance counter  24   d  to zero, thereby initializing the conveying distance of the conveying belt  68  in the Y direction (Y coordinate). 
     In S 11  the CPU  22  initializes all of the calibration flags  24   a  to OFF. In S 12  the CPU  22  transfers the density patch formation data stored in the density patch formation data memory area  23   b  (see  FIG. 3 ) to the error density patch formation data memory area  24   b . In S 13  the CPU  22  reads the density patch formation data from the error density patch formation data memory area  24   b , and forms each of the density patches C 1 -K 5  on the conveying belt  68  based on the density patch formation data from the error density patch formation data memory  24   b . Since the Y coordinate stored in the error density patch formation data memory  24   b  is set based on the counter value of the conveying distance counter  24   d , the counter value is used as the Y coordinate when forming the density patches C 1 -K 5  on the conveying belt  68 . 
     In S 14  the CPU  22  measures the densities of the density patches C 1 -K 5  using the density sensor  80  and in S 15  stores the measured density data in the measured density memory area  24   c  corresponding to the colors of density patches C 1 -K 5 . Through the process of S 15 , the CPU  22  stores measured density data for each of the density patches C 1 -C 5  in the cyan measured density memory area  24   c   1 , measured density data for the density patches M 1 -M 5  in the magenta measured density memory area  24   c   2 , measured density data for the density patches Y 1 -Y 5  in the yellow measured density memory area  24   c   3 , and measured density data for the density patches K 1 -K 5  in the black measured density memory area  24   c   4 . 
     In S 16  the CPU  22  reads one of the measured density data records stored in the measured density memory area  24   c . For example, the CPU  22  reads measured density data for the density patch C 1  from the cyan measured density memory area  24   c   1 . In S 17  the CPU  22  reads the normal value table corresponding to the density patch name of the measured density data read in S 16  from the normal value table memory area  23   c . When measured density data for the density patch C 1  was read in S 16 , for example, the density patch name is cyan  1  (see  FIG. 6(   a )). Accordingly, the CPU  22  reads the normal value table corresponding to cyan  1  (see  FIG. 5)  from the normal value table memory area  23   c.    
     In S 18  the CPU  22  determines whether the measured density data read in S 16  falls within the range of values in the normal value table. For example, if the value of the measured density data for the density patch C 1  is 0.9 and the range of values in the normal value table is a range between the lower limit 0.8 and upper limit 1.0 (see  FIG. 5 ), then the CPU  22  determines in S 18  that the measured density data falls within the range of values in the normal value table (S 18 : YES). However, if the measured density data for the density patch C 1  is 0.7 or 1.1, then the CPU  22  determines in S 18  that the measured density data does not fall within the values in the normal value table (S 18 : NO). 
     If the CPU  22  determines that the measured density data read in S 16  falls outside the range of values in the normal value table (S 18 : NO), then in S 19  the CPU  22  sets the calibration flag  24   a  corresponding to the density patches C 1 -K 5  associated with the measured density data in question to ON. For example, if measured density data for the density patch C 1  is being processed, the cyan  1  error flag  24   a   11  corresponding to the density patch C 1  is set to ON. 
     In this way, the CPU  22  determines whether the density for the density patch measured by the density sensor  80  falls within the range of values in the normal value table stored in the measured density memory area  24   c  for the same density patch. Therefore, the CPU  22  can determine whether the measured density falls within a suitable range of densities, without performing a special data process. 
     On the other hand, if the CPU  22  determines that the measured density data read in S 16  falls within the normal range (S 18 : YES), or after the CPU  22  performs the process in S 19 , in S 20  the CPU  22  determines whether the process has been completed for all measured density data. Specifically, the CPU  22  determines whether the process in S 16 -S 19  has been performed for all measured density data corresponding to the density patches C 1 -K 5 . The CPU  22  returns to S 16  upon determining that the process has not been completed for all measured density data (S 20 : NO) and advances to S 21  upon determining that the process has been completed for all measured density data (S 20 : YES). 
     In S 21  the CPU  22  determines whether all of the calibration flags  24   a  are set to OFF. The CPU  22  reaches a NO determination in S 21  if an error occurred for any of the calibration flags  24   a  (for example, even if only the cyan  2  error flag  24   a   12  is ON). If the CPU  22  determines that any one of the calibration flags  24   a  is not off (S 21 : NO), then in S 22  the CPU  22  displays a message on the display unit  109  indicating that calibration was abnormal. Since the CPU  22  must perform the calibration retry process in S 23  described later when an error has been indicated by any of the calibration flags  24   a , resulting in a relatively long processing time for calibration, the CPU  22  displays a message on the display unit  109  indicating this situation so that the user does not misinterpret the extended processing time as a malfunction of the laser printer  1 . 
       FIG. 9  is a flowchart illustrating steps in S 23  of the calibration retry process. In S 30  the CPU  22  acquires data for one of the calibration flags  24   a . As an example, the CPU  22  acquires data for the cyan  1  error flag  24   a   11  in S 30 . 
     In S 31  the CPU  22  determines whether the acquired calibration flag  24   a  is ON. For example, the CPU  22  reaches a NO determination in S 31  if the CPU  22  acquired the cyan  1  error flag  24   a   11  in S 30  and the cyan  1  error flag  24   a   11  is set to OFF. However, the CPU  22  reaches a YES determination in S 31  if the CPU  22  acquired the cyan  2  error flag  24   a   12  in S 30  and the cyan  2  error flag  24   a   12  is set to ON. 
     If the CPU  22  determines that the calibration flag  24   a  acquired in S 30  is OFF (S 31 : NO), then in S 40  the CPU  22  determines whether all calibration flags  24   a  have been checked. If not all calibration flags  24   a  have been checked (S 40 : NO), the CPU  22  returns to S 30 . However, if the CPU  22  determines that all calibration flags  24   a  have been checked (S 40 : YES), indicating that all calibration flags  24   a  are off, the CPU  22  ends the calibration retry process. 
     When the CPU  22  determines in S 31  that the acquired calibration flag  24   a  is ON (S 31 : YES), then in S 32  the CPU  22  reads density patch formation data for the density patch in question (the single density patch corresponding to the calibration flag  24   a  acquired in S 30 ) from the error density patch formation data memory area  24   b . In S 33  the CPU  22  adds the patch height b for forming the density patch to the Y coordinate in the density patch formation data read in S 32 , updates the density patch formation data, and stores the updated data in the error density patch formation data memory area  24   b . In S 34  the CPU  22  acquires the density patch formation data from the error density patch formation data memory area  24   b  that was newly updated for the density patch in question and forms this density patch on the conveying belt  68  based on the density patch formation data. 
     As an example, the CPU  22  acquires the cyan  2  error flag  24   a   12  in S 30  and determines that the cyan  2  error flag  24   a   12  is ON (S 31 : YES). In S 32  the CPU  22  reads density patch formation data for density patch C 2  (cyan  2 ) from the error density patch formation data memory area  24   b . In S 33  the CPU  22  adds the patch height b to the Y coordinate in the density patch formation data (Y 5 ), updates the Y coordinate to (Y 6 =Y 5 +b), and stores the updated density patch formation data in the error density patch formation data memory area  24   b . In S 34  the CPU  22  acquires the updated density patch formation data for the density patch C 2  from the error density patch formation data memory area  24   b  and forms the density patch C 2  on the conveying belt  68  based on this updated density patch formation data stored in the cyan density patch formation data memory area  23   d.    
     Through the process of S 32  and S 33 , the CPU  22  modifies only the Y coordinate in the position for forming the density patch C 2  on the conveying belt  68 , without changing the X coordinate. Hence, in S 33  the position on the conveying belt  69  at which the density patch C 2  is re-formed is adjusted by the patch height b from the position on the conveying belt  68  at which the density patch C 2  is formed in S 13  of  FIG. 8 . In S 34  the CPU  22  forms the density patch C 2  at the new position modified from the original position by the patch height b. 
     Next, the process of S 30 -S 34  will be described while referring to  FIG. 10(   a ).  FIG. 10(   a ) conceptually illustrates a density patch C 2  that has been re-formed on the conveying belt  68 . The region from density patches C 1  through C 2  outlined by a dotted line indicates the position at which the density patches C 1  through C 2  were formed in S 13  of  FIG. 8 . When executing the calibration retry process after forming density patches C 1 -K 5  in S 13  of  FIG. 8 , the conveying belt  68  has moved a complete circuit (see S 33  and S 34 ) while the cleaning brush  105  (see  FIG. 1)  scrapes off all the density patches C 1 -K 5 . However, density patches C 1  through C 2  have been indicated with a dotted line to reveal their formation positions. 
     Since the cyan  2  error flag  24   a   12  is set to ON in this example, in S 32 -S 34  of  FIG. 9  the CPU  22  re-forms the density patch C 2  at a new position (position of the density patch C 2  indicated with hatch marks in  FIG. 10(   a )) shifted exactly the patch height b from the position at which the density patch C 2  was initially formed on the conveying belt  68  (position of the density patch C 2  displayed above the density patch C 2  with hatch marks in FIG.  10 ( a )). 
     The position at which the density patch C 2  is re-formed on the conveying belt  68  is shifted the patch height b from the initial position of the density patch C 2  to prevent the new density patch C 2  from overlapping the position of the previous density patch C 2 . Accordingly, if the density patch C 2  was previously formed in a region of the conveying belt  68  having tears or deflection, the new density patch C 2  can be re-formed at a different position on which the density patch C 2  was formed, thereby increasing the probability that the density sensor  80  will properly measure the density of the density patch C 2 . 
     Returning to  FIG. 9 , in S 35  the density sensor  80  measures the density of the density patch that was formed in S 34  (one of the density patches C 1 -K 5  corresponding to the calibration flag  24   a  of ON judged in S 31 ) and the CPU  22  stores the measured density data in the measured density memory area  24   c . For example, when the density patch C 2  was formed in S 34  (see  FIG. 10(   a )), in S 35  the density sensor  80  measures the density of the density patch C 2  and the CPU  22  stores the measured density data in the cyan measured density memory area  24   c   1 . 
     In S 36  the CPU  22  reads the measured density data for the density patch currently undergoing processing from the measured density memory area  24   c  and in S 37  reads the normal value table corresponding to the density patch name of the measured density data read in S 36  from the normal value table memory area  23   c . In S 38  the CPU  22  determines whether the measured density data read in S 36  falls within the range of values indicated in the normal value table. 
     As an example of  FIG. 10(   a ), in S 36  the CPU  22  reads the cyan measured density data for the density patch C 2  from the cyan measured density memory area  24   c   1  and in S 37  reads the normal value table (see  FIG. 5)  corresponding to the density patch C 2  (cyan  2 ) from the normal value table memory area  23   c . Since the range of values in the normal value table for the density patch C 2  has a tower limit of 0.6 and an upper limit of 0.9 for the measured density, if the value of the cyan measured density data for the density patch C 2  is 0.7, then the CPU  22  determines that the cyan measured density data read in S 36  falls within the range of normal values (S 38 : YES). However, if the cyan measured density data for the density patch C 2  is 0.5 or 1.0, for example, then the CPU  22  determines that the cyan measured density data falls outside the range of normal values (S 38 : NO). 
     When the CPU  22  determines that the measured density data falls within the range of normal values in the table (S 38 : YES), then in S 39  the CPU  22  sets the calibration flag  24   a  corresponding to the density patch whose measured density data is being processed to OFF, and advances to S 40 . For example, if the CPU  22  determines in S 38  that the measured density data for the density patch C 2  falls within the range of values in the normal value table (S 38 : YES), then in S 39  the CPU  22  sets the cyan  2  error flag corresponding to the density patch C 2  to OFF. 
     The CPU  22  determines in S 40  all the calibration flags  24   a  has been checked (S 40 : YES), the CPU  22  ends the calibration retry process. However, if the CPU  22  determines in S 40  that not all the calibration flags  24   a  have been checked (S 40 : NO), the CPU  22  returns to S 30 . 
     After returning to S 30 , the CPU  22  again acquires data from one of the calibration flags  24   a  and in S 31  determines whether the calibration flag  24   a  is ON. As an example, in S 30  the CPU  22  acquires the magenta  2  error flag  24   a   22 . If the magenta  2  error flag  24   a   22  is ON (S 31 : YES), then the CPU  22  performs the process from S 32 . Here, the magenta  2  error flag  24   a   22  corresponds to the density patch M 2 . 
     The process of S 32 -S 34  will be described with reference to  FIG. 10(   b ) for the case in which the CPU  22  returns to S 30  and determines in S 31  that the magenta  2  error flag  24   a   22  is ON (S 31 : YES).  FIG. 10(   b ) conceptually illustrates the density patch M 2  formed on the conveying belt  68 . The example in  FIG. 10(   b ), assumes that the magenta  2  error flag  24   a   22  was acquired in S 30  of  FIG. 9  after the density patch C 2  was re-formed according to the process in S 30 -S 34  of  FIG. 9 . 
     When the magenta  2  error flag  24   a   22  acquired in S 30  of  FIG. 9  is ON, the CPU  22  forms the density patch M 2  at a new position (Y 7 =Y 6 +b) (indicated by the density patch M 2  with hatching marks in  FIG. 10(   b )) shifted the patch height b from the position (Y 6 ) at which the density patch M 2  was initially formed on the conveying belt  68  (the position of the density patch C 2  directly above the density patch M 2  shown with hatching marks in  FIG. 10(   b ), i.e., the position at which the density patch M 2  was initially formed on the conveying belt  68 ). 
     In this way, if a plurality of calibration flags  24   a  are ON (at least the cyan  2  error flag  24   a   12  and magenta  2  error flag  24   a   22  in the above example), the CPU  22  re-forms each of the density patches for which the calibration flag  24   a  acquired in S 30  were ON. Accordingly, the density sensor  80  can measure the density for the re-formed density patches one at a time. Hence, with the laser printer  1  of the embodiment, the density sensor  80  need not recognize the formation positions of the re-formed density patches, thereby simplifying density measurements of the density patches with the density sensor  80 . 
     When the CPU  22  determines in S 38  of  FIG. 9  that the measured density data does not fall within the range of values in the normal value table (S 38 : NO), then in S 41  the CPU  22  determines whether the Y coordinate stored in the error density patch formation data memory area  24   b  for the position at which the density patch in question was formed is less than the Y coordinate stored in the density patch formation data memory area  23   b  for the position at which the same density patch was initially formed. The process of S 41  will be described in detail with reference to  FIGS. 11(   a ) and  11 ( b ). 
       FIG. 11(   a ) shows an example in which the CPU  22  reaches a NO determination in S 41  of  FIG. 9 , and  FIG. 11(   b ) shows an example in which the CPU  22  reaches a YES determination in S 41  of  FIG. 9 . Both examples in  FIGS. 11(   a ) and  11 ( b ) are described for the density patch C 2 . In  FIG. 11(   a ), it is assumed that the density patch C 2  has already been re-formed twice in S 32 -S 34  of  FIG. 9 . In  FIG. 11(   b ), it is assumed that the density patch C 2  has already been re-formed a plurality of times in S 32 -S 34  of  FIG. 9 . 
     First, the example in which the CPU  22  reaches a NO determination in S 41  will be described with reference to  FIG. 11(   a ). In this example, the Y coordinate Y 7  stored in the error density patch formation data memory area  24   b  for the formation position of the density patch C 2  (the density patch C 2  with hatching marks in  FIG. 11(   a )) is greater than the Y coordinate Y 5  stored in the density patch formation data memory area  23   b  for the formation position of the density patch C 2  (the density patch C 2  initially printed on the conveying belt  68 ). Accordingly, the CPU  22  reaches a NO determination in S 41  of  FIG. 9 . In other words, the CPU  22  determines that the density patch C 2  re-formed twice by repeatedly performing the process in S 32 -S 41  of  FIG. 9  has not yet surpassed the starting point on the conveying belt  68  (the point at which the value of the conveying distance counter  24   d  counting the amount of movement of the conveying belt  68  is 0). Therefore, there is no chance that the formation position of the next density patch C 2  will overlap the Y coordinate Y 5  at which the density patch C 2  was initially printed on the conveying belt  68 . By reaching a NO determination in S 41  of  FIG. 9 , the CPU  22  returns to S 32  in order to re-form the density patch C 2  a third time. 
     Next, the case in which the CPU  22  reaches a YES determination in S 41  of  FIG. 9  will be described. In this example, the Y coordinate Yα stored in the error density patch formation data memory area  24   b  where the formation position of the density patch C 2  (the density patch C 2  with hatching marks in  FIG. 11(   b )) is smaller than the Y coordinate Y 5  stored in the density patch formation data memory area  23   b  for the formation position of the density patch C 2  (the density patch C 2  initially printed on the conveying belt  68 ). Therefore, the CPU  22  reaches a YES determination in S 41  of  FIG. 9 . In other words, the CPU  22  determines that the density patch C 2  re-formed a plurality of times by repeatedly performing the process in S 32 -S 41  of  FIG. 9  has exceeded the starting point of the conveying belt  68 . Hence, there is a high probability that the formation position of the next density patch C 2  will overlap the Y coordinate Y 5  of the position at which the density patch C 2  was initially printed on the conveying belt  68 . By reaching a YES determination in S 41  of  FIG. 9 , the CPU  22  does not return to S 32  but advances to S 42  in order to confirm the position for re-forming the next density patch C 2 . As will be described in greater detail later, the CPU  22  re-forms the density patch C 2  one more time by executing the process in S 32  of  FIG. 9  in the example of  FIG. 11(   b ). 
     There is a high likelihood that the measured density data of the density patch C 2  will not fall within the range of values in the normal value table (S 38 : NO) if the formation position of the re-formed density patch C 2  overlaps the Y coordinate Y 5  of the position at which the density patch C 2  was initially printed on the conveying belt  68 . Accordingly, in S 41  and S 42  of  FIG. 9  the CPU  22  determines the position for re-forming the next density patch C 2  so that the formation position does not overlap the Y coordinate Y 5  of the original density patch C 2 . As described earlier, the Y coordinate Y 1  in the formation position of the cyan  1  density patch is set to a value at least two times the patch height b from the starting point of the conveying belt  68  (the point at which the value of the conveying distance counter  24   d  is 0). This functions to ensure that at least one of the density patches C 1 -K 5  can be formed between the starting point and the formation position for the corresponding density patches C 1 -K 5  initially formed on the conveying belt  68  after the conveying belt  68  has moved one complete circuit and returned to the starting point, and to ensure that the density patch being re-formed does not overlap the formation position of the initially formed density patch. By setting the Y coordinate Y 1  in this way, it is possible to perform the processes in S 41  and S 42  without overlapping the initial formation positions of the density patches C 1 -K 5 . 
     When the CPU  22  reaches a YES determination in S 41  of  FIG. 9 , in S 42  the CPU  22  determines whether the Y coordinate of a formation position found by adding two times the patch height b to the Y coordinate of the formation position stored in the error density patch formation data memory area  24   b  for the density patch in question is greater than or equal to the Y coordinate in the formation position stored in the density patch formation data memory area  23   b  for the density patch in question. The process of S 42  will be described in greater detail with reference to  FIGS. 11(   b ) and  12 ( a ). 
       FIG. 11(   b ) shows the example in which the CPU  22  reaches a NO determination in S 42  of  FIG. 9 , and  FIG. 12(   a ) shows an example in which the CPU  22  reaches a YES determination in S 42  of  FIG. 9 . The examples of  FIGS. 11(   b ) and  12 ( a ) are for the density patch C 2 . Further, the examples in  FIGS. 11(   b ) and  12 ( a ) assume that the density patch C 2  has been re-formed a plurality of times through the process of S 32 -S 34  in  FIG. 9 . 
     First, an example in which the CPU  22  reaches a NO determination in S 42  of  FIG. 9  will be described with reference to  FIG. 11(   b ). The Y coordinate for the formation position is found by adding two times the patch height b to the Y coordinate Yα of the formation position for the density patch C 2  stored in the error density patch formation data memory area  24   b  (the density patch C 2  with hatch marks in  FIG. 11(   b ); Yα+2b). Since this Y coordinate is smaller than the Y coordinate Y 5  of the formation position for the density patch C 2  stored in the density patch formation data memory area  23   b  (the position at which the density patch C 2  was initially printed on the conveying belt  68 ), the CPU  22  reaches a NO determination in S 42  of  FIG. 9 . In other words, the CPU  22  determines that the formation position of the density patch C 2  to be re-formed next in S 32 -S 34  of  FIG. 9  does not overlap the Y coordinate Y 5 . Therefore, the CPU  22  reaches a NO determination in S 42  and returns to S 32  in order to re-form the density patch C 2 . 
     Next, the example in which the CPU  22  reaches a YES determination in S 42  of  FIG. 9  will be described with reference to  FIG. 12(   a ). In this example, the Y coordinate of the formation position is found by adding two times the patch height b to the Y coordinate Yβ of the formation position for the density patch C 2  stored in the error density patch formation data memory area  24   b  (the density patch C 2  with hatch marks in  FIG. 12(   a ); Yβ+2b). Since this Y coordinate is greater than the Y coordinate Y 5  of the formation position for the density patch C 2  stored in the density patch formation data memory area  23   b  (the position at which the density patch C 2  was initially printed on the conveying belt  68 ), the CPU  22  reaches a YES determination in S 42  of  FIG. 9 . In other words, the CPU  22  determines that the formation position of the density patch C 2  to be re-formed next in S 32 -S 34  of  FIG. 9  overlaps the Y coordinate Y 5 . Therefore, the CPU  22  reaches a YES determination in S 42  and rather than returning to S 32  advances to S 43  to modify the formation position of the next density patch C 2  so as not to overlap the original formation position. 
     In S 43  of  FIG. 9  the CPU  22  adds the patch width a to the X coordinate of the density patch stored in error density patch formation data memory area  24   b  (the density patch corresponding to the calibration flag  24   a  acquired in S 30 ) and stores the result in error density patch formation data memory area  24   b . This process shifts the position for re-forming the density patch in the X direction so that the formation position does not overlap the position at which the density patch was initially formed on the conveying belt  68 . 
     In S 44  the CPU  22  reads density patch formation data for the density patch currently being processed from the error density patch formation data memory area  24   b  and in S 45  determines whether the X coordinate of the formation position in the density patch formation data exceeds Xmax. As described above, Xmax is a value obtained by subtracting the patch width a from an end  68   e   1  of the printable range in the X direction. The end  68   e   1  of the printable range is a downstream end  68   e   2  of the printable range in the X direction, which is opposite end  68   e   2  (upstream end) nearest the density sensor  80  in  FIG. 4 . Xmax therefore indicates the maximum X coordinate that can be used for forming the density patches C 1 -K 5 . 
     Hence, when the CPU  22  determines that the X coordinate of the formation position in the density patch formation data for the density patch read in S 44  exceeds Xmax (S 45 : YES), then the CPU  22  can not re-form the density patch read in S 44  on the conveying belt  68  and ends the calibration retry process. 
     On the other hand, if the CPU  22  determines that the X coordinate does not exceed Xmax (S 45 : NO), indicating that the formation position for the density patch to be re-formed falls within the printable range in the X direction, the CPU  22  advances to S 46  in order to form the density patch. 
     In S 46  the CPU  22  reads the Y coordinate of the formation position for the density patch in question from the density patch formation data memory area  23   b , updates the Y coordinate of the formation position for the density patch in question stored in the error density patch formation data memory area  24   b  to the Y coordinate read from the density patch formation data memory area  23   b , stores the updated Y-coordinate data in the error density patch formation data memory area  24   b , and returns to S 34 . Through this process, the Y coordinate of the formation position for the density patch being re-formed is set to the Y coordinate used when initially forming the same density patch on the conveying belt  68  in S 13  of the calibration process in  FIG. 8  (see  FIG. 4 ). 
     The process of S 43 -S 46  will be described with reference to  FIGS. 12(   a ) and  12 ( b ). In these examples, the density patch in question is the density patch C 2  (cyan  2 ). Further, the examples in  FIGS. 12(   a ) and  12 ( b ) assume that the density patch C 2  has been re-formed a plurality of times in S 32 -S 34  of  FIG. 9 . 
     In the example of  FIG. 12(   a ), the X coordinate of the formation position in the density patch formation data for the density patch C 2  obtained by adding the patch width a to the X coordinate Xβ of the formation position for the density patch C 2  stored in the error density patch formation data memory area  24   b  (Xβ+a) does not exceed Xmax. Accordingly, the X coordinate for the position at which the density patch C 2  is to be re-formed is set to Xβ+a. Further, the Y coordinate for the position at which the density patch C 2  is to be re-formed is set to the Y coordinate Y 5  of the position at which the density patch C 2  was initially formed in S 13  of  FIG. 8 . The density patch C 2  having hatch marks in  FIG. 12(   a ) is formed based on the density patch formation data updated in this way. 
     However, in the example of  FIG. 12(   b ), the X coordinate of the formation position in the density patch formation data for the density patch C 2  obtained by adding the patch width a to the X coordinate Xγ for the formation position of the density patch C 2  in question (the density patch C 2  having hatch marks in  FIG. 12(   b )) stored in the error density patch formation data memory area  24   b  (Xγ+a) exceeds Xmax. Hence, the formation position for the next density patch C 2  to be re-formed goes beyond the printable region in the X direction. Hence, the density patch C 2  cannot be re-formed, and the CPU  22  ends the calibration retry process of  FIG. 9 . 
     Accordingly, when the CPU  22  reaches a NO determination in S 45  of  FIG. 9 , the formation position for the density patch to be re-formed next is shifted by the patch width a in S 43 . Since the conveying belt  68  is configured to move circularly, the density patches C 1 -K 5  can overlap positions in which the density patches C 1 -K 5  were formed before as the conveying belt  68  circulates and the density patches C 1 -K 5  are repeatedly formed. Accordingly, there is a high probability the CPU  22  will determine in S 38  that the measured density falls outside the range of values in the normal value table when the density patches C 1 -K 5  are repeatedly formed. However, by shifting the formation position of density patches C 1 -K 5  by the patch width a in S 43 , it is possible to re-form the density patches C 1 -K 5  at positions shifted in the X direction. As a result, the density sensor  80  can measure the densities of the density patches C 1 -K 5  formed at these shifted positions, increasing the probability that the CPU  22  will determine the measured densities fall within the range of values in the normal value table in S 38 . 
     On the other hand, the CPU  22  ends the calibration retry process of  FIG. 9  when reaching a YES determination in S 45 . Hence, if the surface of the conveying belt  68  contains tears or deflection, the CPU  22  will not continue forming the density patches C 1 -K 5  in the calibration retry process, despite the density sensor  80  not being able to properly measure the densities of the density patches C 1 -K 5 , thereby preventing the unnecessary consumption of toner. 
     Returning to  FIG. 8  after ending the calibration retry process in S 23 , in S 24  the CPU  22  determines whether all calibration flags  24   a  are off. If the CPU  22  determines that all calibration flags  24   a  are off (S 24 : YES), then all measured density data for the density patches C 1 -K 5  fall within the range of values in the normal value table stored in the normal value table memory area  23   c . Therefore, in S 25  the CPU  22  reads the measured density data from the measured density memory area  24   c , reads the target density data from the target data memory area  23   d , creates calibration tables (see  FIG. 6 ) from the measured density data and target density data, and stores the calibration tables in the calibration table memory area  25   a . Subsequently, the CPU  22  ends the calibration process. 
     In S 25  the CPU  22  creates a calibration table for each of the colors and stores the calibration tables in the calibration table memory area  25   a . When creating the cyan calibration table, for example, the CPU  22  reads the cyan measured density data from the cyan measured density memory area  24   c   1 , reads the cyan target density data from the cyan target data memory area  23   d   1 , creates the cyan calibration table, and stores the cyan calibration table in the cyan calibration memory area  25   a   1 . In this way, the cyan calibration table shown in  FIG. 6(   c ) can be created. 
     Here, the process in S 25  will be described in greater detail with reference to  FIGS. 6(   a ) through  6 ( c ).  FIG. 6(   a ) is a table showing relation between the target density data stored in the cyan target data memory area  23   d   1  and the cyan measured density data stored in the cyan measured density memory area  24   c   1 .  FIG. 6(   b ) is a graph plotting interpolated data of the target data, and interpolated data of the measured density data.  FIG. 6(   c ) is a table showing the cyan calibration table created and stored in the standard mode cyan calibration table memory area  24   d   1  through the process of S 25 . 
     The CPU  22  first performs linear interpolation on the measured density data stored in the cyan measured data memory area  24   c   1  and the target density data stored in the cyan target data memory area  23   d   1 . 
     In the graph of  FIG. 6(   b ), the horizontal axis represents the interpolated set density (%), and the vertical axis represents the interpolated measured density. Hence, the horizontal axis indicates the interpolated values of the set densities, while the vertical axis indicates the interpolated measured cyan densities and the interpolated target densities shown in  FIG. 6(   a ). 
     As shown in  FIG. 6(   b ), when the set density is 20%, the target density (target value for measured density) is 0.20, while the density of the cyan interpolation data is 0.15. Hence, it is known that even when a cyan image is printed on the recording paper  3  based on a 20% set density, the measured density of the printed cyan image outputted by the density sensor  80  will be 0.15, which is lower than the density of 0.20 in the cyan target data. In this case, the CPU  22  reads the set density that is required for achieving a measured density for the printed cyan image equivalent to the 0.20 target density from  FIG. 6(   b ). As can be seen from  FIG. 6(   b ), the set density must be corrected from 20% to 30% in order to achieve a measured density of 0.20 for the printed cyan image. 
     As another example, when the set density is 80%, the target value is 0.80, while the density in the interpolation data is 0.62. 
     In this case, the CPU  22  reads the set density from  FIG. 6(   b ), which is required for achieving a measured density for the printed cyan image equivalent to the target density of 0.80. As can be seen in  FIG. 6(   b ), the set density must be calibrated from 80% to 90% in order to obtain a 0.80 measured density for the printed cyan image. 
     In this way, the CPU  22  creates a cyan calibration table from the cyan target density data and the cyan measured density data. If the measured density of an image printed in cyan is less than the cyan target density in the target density data, or if the measured density of the printed cyan image is greater than the cyan target density, the cyan calibration table can appropriately calibrate the set density so that the measured density of the printed cyan image is equal to the cyan target density in the target density data. 
     By creating each of the calibration tables in S 25 , the CPU  22  can appropriately calibrate the set densities using the calibration tables so that the measured densities of the density patches C 1 -K 5  match the target densities in the target density data, even when the measured densities of the density patches C 1 -K 5  formed on the conveying belt  68  are less than or greater than the target densities in the target density data stored in the reference density memory area  23   d.    
     On the other hand, if the CPU  22  determines in S 24  that not all calibration flags  24   a  are off (S 24 : NO), indicating that the calibration retry process of  FIG. 9  was canceled because the density patches C 1 -K 5  could not be re-formed on the conveying belt  68 , in S 26  the CPU  22  displays a message on the display unit  109  indicating that the results of the calibration retry process were abnormal, and subsequently ends the calibration process without creating calibration tables. 
     According to the calibration process of  FIG. 8  and the calibration retry process of  FIG. 9  described above, the density patches C 1 -K 5  are formed on the conveying belt  68  when performing calibration, after which the density sensor  80  measures the density of each of the density patches C 1 -K 5 . The densities measured by the density sensor  80  are compared to values in a normal value table stored in the normal value table memory area  23   c  to determine whether the measured densities fall within the range of normal values. If the measured density for any of the density patches is found to fall outside the range of values in the normal value table, then the density patch determined to have a value outside the normal values is repeatedly re-formed on the conveying belt  68  until the measured density of the density patch falls within the range of values in the normal value table. When the measured densities for all density patches repeatedly formed on the conveying belt  68  are found to fall within the range of normal values, all calibration flags  24   a  are set to OFF, at which time the CPU  22  reads the measured density data from the measured density memory area  24   c , reads the target density data from the target data memory area  23   d , creates calibration tables from the measured density data and target density data, and calibrates the image densities using these calibration tables. Hence, if the density of one of the density patches C 1 -K 5  is found to fall outside the range of normal values in the normal value table when calibrating image density, the CPU  22  can re-form the density patch having the abnormal measured density on the conveying belt  68  in the calibration retry process of  FIG. 9 , without re-forming all the density patches C 1 -K 5 . This method reduces the number of density patches re-formed in the calibration process, thereby reducing the amount of toner consumed when repeating calibration. 
     Further, in the calibration retry process of  FIG. 9 , the density patch whose measured value is found to be outside the range of normal values is formed repeatedly on the conveying belt  68  until the measured density for this density patch falls within the normal values in the normal value table, thereby increasing the probability that the density of the re-formed density patch can be measured properly and increasing the reliability of the calibration process. 
     While the invention has been described in detail with reference to specific embodiment thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
     In the embodiment, the present invention is applied to a tandem color laser printer  1 . However, the present invention may also be applied to a transfer drum-type color laser printer, a transfer belt-type color laser printer, or a direct transfer-type color laser printer. 
     Further, while the density sensor  80  measures the densities of density patches C 1 -K 5  formed on the conveying belt  68  in the embodiment, an image scanner provided in the laser printer  1  may be used to scan and measure the densities of the density patches C 1 -K 5  formed on the paper  3  in order to perform calibration. Since the scanner performs the function of the density sensor  80  for measuring the densities of the density patches C 1 -K 5 , the density sensor  80  is not necessary in this example. 
     Further, while density patch formation data for the density patches C 1 -K 5  is stored in the density patch formation data memory area  23   b  in the embodiment, density patch formation data for the density patches C 1 -K 5  may be stored on the PC  125  connected to the laser printer  1 . When executing the calibration process in this case, the PC  125  outputs the density patch formation data for the density patches C 1 -K 5  to the laser printer  1 , and the laser printer  1  stores the density patch formation data in the RAM  24 . The CPU  22  subsequently executes the calibration process by reading the density patch formation data from the RAM  24 . This method can reduce the capacity required for the ROM  23 . 
     Further, while the laser printer  1  forms square density patches C 1 -K 5  on the conveying belt  68  in the embodiment, but the density patches may be formed in a polygonal shape or other shape. 
     Further, in the embodiment, the laser printer  1  repeatedly forms the error density patch while successively shifting the position of the density patch in the Y direction until the conveying belt  68  moves one complete circuit, and then shifting the position of the density patch in the X direction. However, the laser printer  1  may repeatedly form the error density patch while successively shifting the position of the density patch in the X direction until the density patch is formed at a location near to the end of printable range of the conveying belt in the X direction and then shifting the position in the Y direction. In this case, the calibration process shown in  FIG. 9  is modified by exchanging the process of adding b to Y coordinate in S 33  with the process of adding a to X coordinate in S 43 , and exchanging the process of Y coordinate determination in S 41  and S 42 , with the process of X coordinate determination in S 45 . 
     In the embodiment, when reforming the test image, the position of the test image is shifted the length b in the Y direction or the length a in the X direction. However, the position of the test image may be shifted a distance grater than the length b in the Y direction or a distance grater than the length a in the X direction. 
     Although the present invention has been described with respect to specific embodiments, it will be appreciated by one skilled in the art that a variety of changes may be made without departing from the scope of the invention.