Patent Publication Number: US-8523310-B2

Title: Printing apparatus and printing method

Description:
TECHNICAL FIELD 
     The present invention relates to a technique for controlling image forming position of a printing apparatus. More particularly, it relates to control of a conveying roller which conveys a printing medium. 
     BACKGROUND ART 
     An image forming apparatus of an ink jet type records on a printing medium by discharging ink from a print head during reciprocating motion in a main scanning direction. It forms an image by repeated recording in the main scanning direction while conveying the printing medium in a sub-scanning direction by means of a conveying roller. Generally, when conveying a printing medium, such as paper, on a conveying roller and the like, there are variations in the amount of conveyance (feed rate) depending on the mounting condition of the conveying roller, type of printing medium, and the like. Thus, patent document 1 discloses a technique for determining a correction value for the amount of conveyance based on printing results of a plurality of test patterns recorded using different correction values. That is, this technique selects the pattern which gives the best printing result from among the recorded test patterns and thereby determines a parameter for use to drive the conveying roller. 
     (Patent document 1) Japanese Patent Laid-Open No. 2003-011344 
     DISCLOSURE OF INVENTION 
     Problems it to be Solved by the Invention 
     However, the technique disclosed in the above document poses the following problems if there are variations in the amount of conveyance within one roller rotation (one cycle). One of the problems is that in order for the correction value that depends on the phase of the conveyance roller during execution of an adjustment operation to be set, a different correction value is determined each time the adjustment operation is executed, the result being that a stable image quality cannot be realized. Another problem is that this technique is incapable of correcting image forming irregularities known as white streaks and black streaks caused by variations during one roller rotation. 
     Variations in the feed rate caused by variations in roller profile, flexure of the roller, mounting of a roller support member, and the like with a period equal to one rotation of the roller have been negligible in conventional recording resolution. However, with recent improvement in recording resolution, the effect of feed rate variations with a period equal to one rotation of the roller has increased so much in a relative manner that the variations can no longer be ignored. Consequently, conveying control with a higher accuracy is required. 
     Naturally, with the improvement in recording resolution, machine accuracy has also been improved to ensure recording quality. However, it is technically difficult to increase machine accuracy to the extent that the effect of the feed rate variations with a period equal to one rotation of the roller will be negligible and it is not desirable in terms of cost performance. 
     The present invention has been made in view of the above problems and has an object to provide a technique which can reduce misregistration in a conveying direction of a printing medium. 
     Means for Solving the Problems 
     To achieve the above object, the present invention is configured as follows. 
     According to one aspect of the present invention, a printing apparatus which prints an image on a printing medium using a print head which discharges ink, the printing apparatus comprises: a conveyance unit for conveying the printing medium by rotating a roller; a detection unit for detecting a conveyance amount of a printing medium conveyed by rotating the roller in less than one rotation; an acquisition unit for acquiring a conveyance amount of the printing medium corresponding to a predetermined rotation amount of the roller by detecting the conveyance amount a plurality of times; and a setting unit for setting a rotation amount of the roller when forming an image on the printing medium based on a conveyance amount of a printing medium corresponding to the acquired predetermined rotation amount of the roller. 
     According to another aspect of the present invention, a printing method for a printing apparatus which has a print head which discharges ink, a conveyance unit which conveys a printing medium by rolling a roller, the printing apparatus prints an image on the printing medium using the print head, the printing method comprises: the detection step of detecting a conveyance amount of a printing medium conveyed by rotating the roller in less than one revolution; and the setting step of acquiring a conveyance amount of the printing medium corresponding to a predetermined rotation amount of the roller by detecting the conveyance amount a plurality of times, and setting a rotation amount of the roller when an image is formed on the printing medium based on an acquired conveyance amount of a printing medium. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
     Effects of the Invention 
     The present invention can provide a technique which can reduce misregistration in the conveying direction of a printing medium. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an external perspective view of a color ink jet printer according to a first embodiment; 
         FIG. 2A  is a perspective view illustrating a structure of an ink jet cartridge  150 ; 
         FIG. 2B  is a perspective view illustrating a structure of an ink jet cartridge  150 ; 
         FIG. 3  is a schematic diagram illustrating a reflective optical sensor  130 ; 
         FIG. 4  is a schematic block diagram of a control circuit of the color ink jet printer according to the first embodiment; 
         FIG. 5  is a diagram schematically showing variations in a feed rate in one cycle of a roller. 
         FIG. 6A  is a schematic diagram showing differences in the amount of paper conveyance according to roller shape; 
         FIG. 6B  is a schematic diagram showing differences in the amount of paper conveyance according to roller shape; 
         FIG. 7A  is a diagram illustrating the effect of variations in the amount of paper conveyance on recording, where the variations are dependent on the cycle of the roller; 
         FIG. 7B  is a diagram illustrating the effect of variations in the amount of paper conveyance on recording, where the variations are dependent on the cycle of the roller; 
         FIG. 8  is a diagram schematically showing changes in feed rate according to the position (phase) of a conveying roller; 
         FIG. 9  is a diagram schematically showing a print head according to the first embodiment; 
         FIG. 10A  is a diagram illustrating procedures for printing reference patterns; 
         FIG. 10B  is a diagram illustrating procedures for printing reference patterns; 
         FIG. 11A  is a schematic diagram of patterns printed one over another; 
         FIG. 11B  is a schematic diagram of patterns printed one over another; 
         FIG. 12  is a diagram illustrating adjustment patches (configuration example 1); 
         FIG. 13A  is a diagram illustrating adjustment patches (configuration example 2); 
         FIG. 13B  is a diagram illustrating adjustment patches (configuration example 2); 
         FIG. 14  is a diagram showing an example of detection of the adjustment patches (configuration example 2) shown in  FIG. 13B ; 
         FIG. 15A  is a diagram illustrating how nozzle columns are divided into two parts; 
         FIG. 15B  is a diagram illustrating how nozzle columns are divided into eight parts; 
         FIG. 16  is a flowchart showing procedures for deriving an average amount of conveyance and instruction pulse value during one rotation of a conveying roller; 
         FIG. 17A  is a diagram illustrating structures of a conveying roller and roller support member; 
         FIG. 17B  is a diagram illustrating structures of a conveying roller and roller support member; 
         FIG. 18  is a diagram showing measured values feed rate for approximately 2.5 rotations of the conveying roller when there is eccentricity; 
         FIG. 19  is a diagram illustrating nozzle positions when a nozzle column is divided into A to H intervals (eight parts); 
         FIG. 20  is a diagram showing detected amounts of displacement when there is no fluctuation in the amount of variation due to paper slippage or the like; 
         FIG. 21  is a flowchart showing procedures for deriving the amount of displacement and an instruction pulse value in each phase during one rotation of a conveying roller; 
         FIG. 22A  is a drawing explaining rotation of a conveyance roller when there is no misalignment of a rotation axis of a conveyance roller; 
         FIG. 22B  is a drawing explaining rotation of a conveyance roller when there is no misalignment of a rotation axis of a conveyance roller; 
         FIG. 22C  is a drawing explaining rotation of a conveyance roller when there is a misalignment of a rotation axis of a conveyance roller; 
         FIG. 22D  is a drawing explaining rotation of a conveyance roller when there is a misalignment of a rotation axis of a conveyance roller; 
         FIG. 23A  is a drawing explaining rotation due to bending of a conveyance roller; 
         FIG. 23B  is a drawing explaining rotation due to bending of a conveyance roller; 
         FIG. 24A  is a drawing explaining a possible application of an acquisition method for a printing medium conveyance amount to the present invention; 
         FIG. 24B  is a drawing explaining a possible application of an acquisition method for a printing medium conveyance amount to the present invention; and 
         FIG. 25  is a drawing explaining a sampling point of a conveyance roller during conveyance amount acquisition. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments of the present invention will be described in detail below in an exemplary manner with reference to the drawings. However, components of the embodiments are strictly exemplary and are not intended to limit the scope of the present invention. 
     In the embodiments described below, a recording apparatus which uses a print head of an ink jet type will be taken as an example. The term “record” (or “print”) herein not only means forming meaningful information such as characters or images, but also broadly means forming images, patterns, and the like on a printing medium or processing a printing medium regardless of whether the information is meaningful or meaningless or whether the information is visible to human vision. 
     Also, the term “printing medium” means not only paper used on typical recording apparatus, but also broadly means something that can receive ink, such as cloth, plastic film, metal plates, glass, ceramics, wood, and leather. 
     Furthermore, the term “ink” (also referred to as “liquid”) should be interpreted broadly as is the case with the term “record” (or “print”). That is, it means a liquid which may be used to form images, patterns and the like or to process a printing medium, or to process ink when applied to a printing medium. The phrase “process ink” means the process of solidifying or insolubilizing coloring material in the ink applied to the printing medium, for example. 
     Besides, the term “nozzle” refers collectively to discharge ports, flow paths connected to them, and elements which generate energy used to discharge ink unless otherwise specified. 
     (First Embodiment) 
     A first embodiment of an image forming apparatus according to the present invention will be described below by taking a color ink jet printer as an example. 
     &lt;Equipment Configuration&gt; 
       FIG. 1  is an external perspective view of a color ink jet printer according to a first embodiment. Incidentally, a front cover has been removed in  FIG. 1  to reveal an interior of the apparatus. 
     In  FIG. 1 , reference numeral  150  denotes a replaceable ink jet cartridge and  102  refers to a carriage unit which detachably holds the ink jet cartridge. Reference numeral  103  denotes a holder used to secure the ink jet cartridge  150  to the carriage unit  102 . A cartridge lock lever  104 , when operated after the ink jet cartridge  150  is mounted in the carriage unit  102 , brings the ink jet cartridge  150  into press contact with the carriage unit  102 . Consequently, the ink jet cartridge  150  is positioned and an electrical contact installed on the side of the carriage unit  102  to transmit necessary signals is brought into contact with an electrical contact on the side of the ink jet cartridge  150 . Reference numeral  105  denotes a flexible cable used to transmit electrical signals to the carriage unit  102 . Reference numeral  130  denotes a reflective optical sensor installed on the carriage unit  102 . The reflective optical sensor  130  functions to detect density of adjustment patterns recorded and formed on paper during an automatic registration adjustment according to this embodiment. By combining carriage scanning (in a main scanning direction) and paper conveyance (in a sub-scanning direction), the reflective optical sensor  130  can freely detect density of the adjustment patterns formed on paper. Incidentally, the reflective optical sensor  130  may be used to detect ends of the paper. 
     Reference numeral  106  denotes a pulley which rotates when receiving power from a carriage motor as a drive source for causing round-trip scanning of a carriage unit  102  in a main scanning direction. Reference numeral  107  denotes a carriage belt which transmits motor drive power received through contact with the pulley to the carriage unit  102 . Reference numeral  111  denotes a guide shaft located in the main scanning direction to support the carriage unit  102  and guide its movement. Reference numeral  109  denotes a transmissive photocoupler mounted on the carriage unit  102 , and  110  denotes a light shielding plate installed near a carriage home position. Reference numeral  112  denotes a home position unit (also called a recovery unit) including a cap member which caps the front face of an ink jet head and a suction unit which sucks ink by creating negative pressure in this cap, and further including a recovery system such as a member which wipes the front face of a head. 
     Reference numeral  113  denotes an ejection roller used to eject the printing medium such as paper. It sandwiches the printing medium in cooperation with a spur roller (not shown) and ejects it out of the printer. In addition, there is a line feed unit which delivers a printing medium by a predetermined amount in a sub-scan direction. 
       FIGS. 2A and 2B  are perspective views illustrating a structure of the ink jet cartridge  150 , where  FIG. 2A  is an exploded perspective view of the cartridge  150  and  FIG. 2B  is a schematic diagram showing a structure of the essence of a print head in the cartridge  150 . 
     In  FIG. 2A , reference numeral  215  denotes an ink tank containing black (Bk) ink and  216  denotes an ink tank containing cyan (C), magenta (M), and yellow (Y) inks. The ink tanks are detachable from the body of the ink jet cartridge. Reference numeral  217  denotes connection ports for the color inks contained in the ink tank  216 : they are to be connected with ink supply pipes  220  on the side of the ink jet cartridge body. Reference numeral  218  similarly denotes a connection port for the black ink contained in the ink tank  215 . The connection ports, when connected, allow the inks to be supplied to the print head  201  held in the ink jet cartridge body. Reference numeral  219  denotes an electrical contact which comes into contact with an electrical contact installed on the carriage unit  102 . The electrical contacts, when brought into contact, allow electrical signals to be received from a console on the printer body via the flexible cable  105 . 
     The print head  201  has a Bk-ink discharge unit consisting of an array of nozzles which discharge Bk ink as well as nozzle groups which respectively discharge Y, M, and C inks. The nozzle groups are arranged in line integrally composing a color-ink discharge unit. They are installed in the same range as Bk-ink discharge ports. 
     Discharge ports  222  are formed at a predetermined pitch on a discharge port surface  221  which faces a printing medium  108  such as paper with a predetermined clearance (e.g., approximately 0.5 to 2.0 mm). Also, electrothermal transducers (such as heaters) for generating energy used in ink discharge are installed along the wall of each flow path  224  connecting each of the discharge ports  222  with a common ink chamber  223 . 
     Further, a cartridge  150  is mounted to a cartridge unit  102  in positions such that the plurality of discharge ports  222  will intersect a scanning direction of the carriage unit  102 . Appropriate electrothermal transducers (hereinafter referred to as “discharge heaters”)  225  are driven based on an image signal or discharge signal inputted via the electrical contact  219 . Specifically, the inks in the flow paths  224  are caused to undergo film boiling and the inks are discharged through the discharge ports  222  under the pressure of generated bubbles. 
       FIG. 3  is a schematic diagram illustrating the reflective optical sensor  130 . 
     The reflective optical sensor  130  has a light-emitting unit  331  and light-receiving unit  332 . Light Iin  335  emitted from the light-emitting unit  331  is reflected by a surface of the printing medium  108 . Reflected light includes regularly reflected light and irregularly reflected light. To detect density of an image formed on the printing medium  108  more accurately, it is desirable to detect irregularly reflected light Iref  337 . For this reason, the light-receiving unit  332  is placed such that it receives reflected light at an angle different from an incident angle of light from the light-emitting unit  331  so as to detect scattered reflection in this embodiment. A detection signal resulting from the detection is transmitted to an electrical circuit board of the printer. 
     It is assumed here that a white LED or three-colored LED is used as the light-emitting unit and photodiode sensitive to a visible-light region is used as the light-receiving unit to make registration adjustments for all the head which discharge C, M, Y, or K inks. However, it is preferable to use a three-colored LED capable of selecting colors which have high detectivity if adjustments are made among different colors when detecting relationship between relative recording position and density of colors printed one over another. 
     Incidentally, in detecting the density of an image formed on the printing medium  108 , it is only necessary to detect a relative value rather than the absolute value of the density although details will be described later. Also, it is only necessary to have detection resolution enough to detect relative density differences among individual patterns belonging to an adjustment pattern group described later (hereinafter each pattern among adjustment patterns will be referred to as a patch). 
     Regarding stability of a detection system including the reflective optical sensor  130 , it is sufficient if detected density differences are not affected during detection of one adjustment pattern group. Sensitivity is adjusted, for example, by moving the reflective optical sensor  130  to a margin of the printing medium. A possible adjustment method involves adjusting emission intensity of the light-emitting unit  331  or gain of detection amplifier of the light-receiving unit  332  such that a detection level will reach an upper limit. Incidentally, sensitivity adjustment is not essential, but it is a suitable method for improving an S/N ratio and increasing detection accuracy. 
       FIG. 4  is a schematic block diagram of a control circuit of the color ink jet printer according to the first embodiment. 
     A controller  400  is a main control unit. It has a CPU  401  in the form of, for example, a microcomputer; ROM  403  which stores programs, required tables, and other fixed data; and RAM  405  with an area for use to load image data, work area, and the like. A host machine  410  is a source of image data. Specifically, it may be a computer which creates or processes images and other data related to printing, a reader which reads images, or the like. Image data, commands, status signals, and the like are transmitted and received to/from the controller  400  via an interface (I/F)  412 . 
     A console  420  is constituted of a switch group which accepts commands from a user. It includes, a power switch  422 , switch  424  used to start printing, and recovery switch  426  used to start suction recovery. It also includes a registration adjustment start switch  427  for manual registration adjustment and registration adjustment value setting input section  429  for use to enter adjustment values manually. 
     A sensor group  430  is used to detect condition of the apparatus. It consists of the reflective optical sensor  130 , the photocoupler  109  used to detect the home position, a temperature sensor  434  mounted at an appropriate location to detect ambient temperature, and the like. 
     A head driver  440  is used to drive a discharge heater  441  in the print head  201  according to print data and the like. The head driver  440  has a shift register which align print data along the discharge heater  441  and a latch circuit which latches with an appropriate timing. Also, the head driver  440  has a logic circuit element which operates the discharge heater  441  in sync with a drive timing signal, timing setting unit which appropriately sets a drive timing (discharge timing) for dot formation/alignment, and the like. 
     The print head  201  is equipped with a sub-heater  442 . The sub-heater  442  is used for temperature adjustment in order to stabilize ink discharge characteristics. It may be formed on the print head substrate simultaneously with the discharge heater  441  and/or mounted on the print head body or head cartridge. 
     A motor driver  450  drives a main scanning motor  452 . A sub-scanning motor  462  is used to convey (sub-scan) the printing medium  108  and driven by a motor driver  460 . 
     &lt;Variations in Amount of Conveyance of Printing Medium by Conveying Roller&gt; 
     Normally, a printing medium such as paper is conveyed through rotation of a conveying roller (hereinafter referred to as a “roller”). For example, if the roller is 47 mm in circumference, the printing medium is conveyed 47 mm by one rotation of the roller. Generally, however, there is slight deviation in the amount of conveyance when a printing medium is conveyed by a conveying roller. 
       FIG. 5  is a diagram schematically showing variations in a feed rate in one cycle of a roller. In the figure, the ordinate represents the amount of feed rate variation and abscissa represents the amount of paper conveyance. As can be seen from the figure, the feed rate of paper can be expressed roughly by two components. 
     One of the components is a fixed component (A in  FIG. 5 ) within one rotation of the roller. It is dependent on the paper type, individual apparatus, and environment. The other component is a variable component (B in  FIG. 5 ) which has a period equal to one rotation of the roller. It is dependent on accuracy of the roller, flexure of the roller, and mounting of a roller support member. That is, the amount of paper conveyance can be approximated by the sum of the two components. 
     Incidentally, since the fixed component (A in  FIG. 5 ) is dependent on operating environment, registration adjustment should be performed in the environment in which recording operation is actually performed. On the other hand, since the variable component (B in  FIG. 5 ) is dependent on the individual apparatus, adjustment needs to be made only once before shipment or the like. 
       FIGS. 6A and 6B  are schematic diagrams showing differences in the amount of paper conveyance according to roller cross-sectional shape. 
     If it is assumed that the rotation angle of a roller for paper conveyance is constant and if roller cross-sectional shape is a perfect circle, when the roller is rotated by an angle of R, the amount of conveyance is a constant L 0 , as shown in  FIG. 6A . However, if the roller has an odd-shaped cross section, the amount of conveyance when the roller is rotated by an angle of R varies with the rotational position of the roller. For example, if the roller has an elliptic cross section, as shown in  FIG. 6B , the paper is conveyed by L 1  at a certain position, and conveyed by L 2  at another position during the rotation of the roller. In that case, there is a relationship L 1 &gt;L 0 &gt;L 2 , and the variations in paper conveyance depend on the cycle of the roller. Moreover, these conveyance amounts L 0 , L 1  and L 2  are approximately equal to the length of the arc when the angle is “R”. 
     Such variations in the amount of paper conveyance dependent on the cycle of the roller affect actual images. Variations in the amount of paper conveyance dependent on the cycle of the roller mean deflection in the landing position of ink droplets. 
     In  FIGS. 6A and 6B  generation of a conveyance amount displacement component within roller rotation by using the difference in cross-sectional shape of a roller that is a perfect circle and one that is oval-shaped was explained. Incidentally, it can be thought that the cause of the displacement component is not the cross-sectional shape of the roller, but is some other cause. 
       FIGS. 22A-22D  show displacement of a conveyance amount originating from misalignment of the rotational axis of a conveyance roller. 
       FIG. 22A  shows that the center of the diameter (central axis) of the roller  116  and the rotational axis  118  when the printing apparatus supports the roller  116  have the same shape. Further,  FIG. 22B  shows the center of the diameter of the roller  116  and the rotational axis of the roller  118  in the misaligned state. Moreover, the center of the diameter of the roller  116  is the point at which the dotted line intersects the plus sign in  FIG. 22B . Further, frame format diagrams of the roller state when the roller  116  shown in  FIGS. 2A ,  2 B is rotated around the rotational axis  118  are shown in  FIGS. 22C ,  22 D, respectively.  FIG. 22C  is a frame format diagram of the roller  116  in  FIG. 22A  being rotated when the center of the diameter of roller  116  and the rotational axis are the same, and because they are the same, the cross-sectional view when looking at the roller from the side is the same as the outer shape of the roller even when the roller  116  is rotated. Further,  FIG. 22D  is a frame format diagram of the roller for which the center of the diameter and the rotational axis are not the same in  FIG. 22B  when it is rotated, and because they are not the same, the cross-sectional view when looking at the roller from the side changes as in  FIG. 22D , along with the roller  116 . As can be understood from  FIG. 22D , when the center of the diameter of the roller and the rotational axis are misaligned, the conveyance amount when the roller is rotated over a predetermined angle, that is, the length of the arc corresponding to the predetermined angle, differs. For this reason, the conveyance amount of the printing medium differs depending on the rotational starting position of the roller. 
     Further, bending of the conveyance roller can be given as another cause for generation of a displacement component.  FIG. 23A  shows a roller  177  with no bending, while  FIG. 23B  shows a roller  117  when bending occurs. As shown in  FIG. 23B , long rollers have a possibility of flexing in response to bending. As shown in  FIG. 23B , the conveyance amount of the printing medium can differ depending on the rotational starting position of the roller when bending occurs as well. 
     As explained above, several causes exist for differences in conveyance amount in response to the rotational starting position of the conveyance roller, that is, causes for differences in conveyance amount in a single revolution of the conveyance roller. Although there are several causes for the difference in conveyance amount, the problem occurring when an image is formed on a printing medium due to a differing conveyance amount is the same, and since it is possible to adapt to the shift in conveyance amount due to several causes in the present embodiment, the recording position misalignment in the conveyance direction can be reduced. Further, the application of the present invention is not limited to causes for the occurrence of the shift in conveyance amount explained above. 
       FIGS. 7A and 7B  are diagrams illustrating the effect of variations in the amount of paper conveyance on recording, where the variations are dependent on the cycle of the roller. 
     When the roller is located in L 1  in  FIG. 6B , the paper is conveyed a larger distance than usual, and thus actual recording position is lower than desired recording position as shown in  FIG. 7A . On the other hand, when the roller is located in L 2  in  FIG. 6B , the paper is conveyed a shorter distance than usual, and thus actual recording position is higher than ideal recording position as shown in  FIG. 7A . Consequently, there are density differences such as shown in  FIG. 7B  even when a uniform image is recorded. Such irregularities stand out clearly on a uniform scene such as a background of a landscape, impairing high quality prints. 
     &lt;Derivation of Fixed Component&gt; 
     Normally, adjustment of the amount of paper conveyance unit adjustment of a fixed component (A in  FIG. 5 ) dependent on the paper type, individual apparatus, and environment. In conventional techniques, deviation in the amount of conveyance is derived using adjustment patterns and used as a conveyance adjustment value. However, due to the effect of the variable component described above, the position at which the adjustment value for the fixed component is obtained can vary depending on the timing of registration adjustment operation. 
       FIG. 8  is a diagram schematically showing changes in feed rate according to the position (phase) of a conveying roller. If a registration adjustment is made at position ( 1 ) in  FIG. 8 , an adjustment value larger than the fixed component is obtained. If a registration adjustment is made at position ( 3 ), an adjustment value smaller than the fixed component is obtained. The adjustment value for the amount of conveyance can be derived almost properly corresponding to the fixed component if derived at position ( 2 ) in  FIG. 8 . However, it is generally difficult to find this position because the variable component depends on the accuracy of the roller, flexure of the roller, and mounting of a roller support member. 
     However, as described above, the variations in the amount of conveyance have a period equal to one rotation of the conveying roller. In particular, if the period of variation can be approximated by one period of a sine function, it can be seen from  FIG. 5  that the amounts of variations at two points corresponding to ½ rotation of the conveying roller are equal in absolute value but opposite in sign. That is, an average amount of variations at the two points corresponding to ½ rotation of the conveying roller is equal to an average amount of conveyance per rotation of the conveying roller. 
     Thus, it can be seen that by controlling the rotation of the conveying roller based on the average amount of conveyance determined in this way, it is possible to reduce the effect of the fixed component (A in  FIG. 5 ). 
     &lt;Detection of Displacement in Conveyance using Reference Patterns (Outline)&gt; 
     Next, description will be given of a method for detecting displacement in conveyance corresponding to positions of the conveying roller. 
       FIG. 9  is a diagram schematically showing the print head according to the first embodiment. Inks of six colors are used including black (Bk), light cyan (LC), cyan (C), light magenta (LM), magenta (M), and yellow (Y). Each of the six inks have an EVEN column and ODD column. That is, there are a total of 12 nozzle columns (=6 colors×2 columns) in the carriage driving direction. 
     Also, 640 nozzles are arranged in each nozzle column to provide a resolution of 600 dpi in the paper conveying direction. The EVEN and ODD nozzle columns of each color are placed being displaced 1/1200 inch in the paper conveying direction. Consequently, the resolution in the paper conveying direction is 1,200 dpi when both EVEN columns and ODD columns are used for recording. 
     In the following description, a print head with two nozzle columns per color as shown in  FIG. 9  will be taken as an example. However, a print head in which each color consists of a single nozzle column can also be treated in a similar manner if even-numbered nozzles and odd-numbered nozzles are regarded as EVEN nozzle columns and ODD nozzle columns, respectively. Incidentally, the EVEN and ODD nozzle columns of Bk will be taken as an example in the following description, but the same is true to the other colors. 
       FIGS. 10A and 10B  are diagrams illustrating procedures for printing reference patterns. Incidentally, the nozzle columns will be divided into two parts in the paper conveying direction and the upstream half of the nozzles will be referred to as “upstream nozzles” and the downstream half of the nozzles will be referred to as “downstream nozzles.” 
     First, reference patterns (first patterns) indicated by white dots in  FIG. 10A  are recorded using the upstream nozzles. Patterns recorded continuously in the direction orthogonal to the conveying direction are used as the reference patterns although details will be described later. Incidentally, any of the upstream nozzles are available for use, but it is assumed here for simplicity of explanation that all the upstream nozzles in the ODD column are used for recording. 
     Next, the paper is conveyed by a distance equal to half the length of the nozzle column. Feed resolution is a variable which depends on printer performance, but it is assumed here that the paper can be conveyed theoretically at a resolution of 9,600 dpi. That is, the paper is conveyed theoretically at 1/9600 inch per pulse. Under these conditions, to convey the paper by a distance equal to half the length of the nozzle column:
 
640*25.4/1200=13.55 [mm],
 
a theoretical instruction pulse value (count) to be used is:
 
(640*25.4/1200)/25.4*9600=5120 (pulses)
 
     After the paper conveyance, adjustment patterns (second patterns) indicated by black dots in  FIG. 10B  are recorded around the locations of the reference patterns (white dots) recorded earlier, using the downstream nozzles. It is assumed here for simplicity of explanation that all the downstream nozzles in the EVEN column are used for recording. 
       FIGS. 11A and 11B  are schematic diagrams of patterns printed one over another. The white dots represent dots in the reference patterns formed on the medium (paper) using the upstream nozzles in the ODD column and the black dots represent dots in the adjustment patterns formed using the downstream nozzles in the EVEN column. Incidentally, the white dots and black dots are used for simplicity of explanation, and actually they are formed by the ink discharged from nozzles of the same color ink (Bk). They do not represent density. 
     If the amount by which the paper is conveyed based on an instruction pulse value after the white dots are recorded is equal to half the length of the nozzle column, a region with an area factor of nearly 100% is formed by printing the black dots over the white dots as shown in  FIG. 11A . Hereinafter, the region formed by overprinting will be referred to as a “patch.” 
     On the other hand, depending on the accuracy of the individual apparatus or changes in the printing medium caused by the environment and the like, the amount by which the paper is conveyed based on the instruction pulse value may deviate from the value equal to half the length of the nozzle column. In that case, even if black dots are overprinted, a patch with an area factor lower than 100% (50% at the minimum) will be formed as shown in  FIG. 11B . 
     In forming a patch such as shown in FIG.  11 B, suppose the area factor becomes 100% when the instruction pulse value is set, for example, at 5122 rather than 5120. In that case, it can be seen that for the combination of the given printer and printing medium, the correct instruction pulse value needed to feed the printing medium by 13.55 mm is 5122. That is, it is possible to derive the correct instruction pulse value by checking the area factors of the patches produced by varying the instruction pulse value for conveyance after white dots are recorded. The difference (+2 in this case) between the correct instruction pulse value (5122 in this case) and theoretical instruction pulse value (5120 in this case) corresponds to displacement in conveyance. A concrete method for constructing a patch using the above-described principle will be described below. 
     &lt;Adjustment Patch Configuration Example 1&gt; 
       FIG. 12  is a diagram illustrating adjustment patches (configuration example 1). Incidentally, with the adjustment patterns (second patterns) constituting the patches illustrated here, an adjustment range of the instruction pulse count is set at +/−5 pulses. Further more, to make it easier to make a selection by checking visually, five columns each of patches and solid patterns are arranged alternately in the main scanning direction. 
     Enlarged view A in  FIG. 12  shows a patch whose pulse adjustment value is “0.” After the reference patterns indicated by white dots are recorded, the paper is conveyed by the amount corresponding to an instruction pulse value of 5120 and the adjustment patterns indicated by the black dots are recorded. The resulting patch theoretically has an area factor of approximately 100%. 
     Enlarged view B in  FIG. 12  shows a patch whose pulse adjustment value is “+3.” After the reference patterns indicated by white dots are recorded, the paper is conveyed by the amount corresponding to an instruction pulse value of 5123 and the adjustment patterns indicated by the black dots are recorded. The resulting patch theoretically has an area factor of approximately 75%. 
     Enlarged view C in  FIG. 12  shows a patch whose pulse adjustment value is “+5.” After the reference patterns indicated by white dots are recorded, the paper is conveyed by the amount corresponding to an instruction pulse value of 5125 and the adjustment patterns indicated by the black dots are recorded. The resulting patch theoretically has an area factor of approximately 50%. 
     As described above, when recording a patch whose theoretical pulse adjustment value is “0,” the area factor is approximately 100%. However, depending on the accuracy of the individual apparatus or changes in the printing medium caused by the environment and the like, the amount of paper conveyance corresponding to the instruction pulse value can differ from its theoretical value. That is, a patch area factor of approximately 100% may be produced by a pulse adjustment value other than “0.” 
     Incidentally, the adjustment pattern of “+5” and adjustment pattern of “−5” in  FIG. 12  causes displacement equivalent to one pixel dot. Thus, it can be seen that one out of 11 patterns always produces an area factor of approximately 100%. This makes it possible to determine a pulse adjustment value corresponding to an adjustment pattern with an area factor of approximately 100%. Incidentally, the pulse adjustment value corresponds to the amount of deviation in conveyance. 
     &lt;Adjustment Patch Configuration Example 2&gt; 
     With configuration example 1, it is necessary to change the instruction pulse value during recording of a pattern. This makes it necessary to arrange patches in the paper conveying direction. However, the arrangement in the paper conveying direction results in increased paper consumption. On the other hand, configuration example 2 allows paper feed adjustment to be made without changing the instruction pulse value. 
       FIGS. 13A and 13B  are diagrams illustrating adjustment patches (configuration example 2). In the figures, seven patches are recorded in the main scanning direction. 
     First, reference patterns (first patterns) indicated by white dots in  FIG. 13A  are recorded using the upstream nozzles. Incidentally, any of the upstream nozzles are available for use, but it is assumed here for simplicity of explanation that all the upstream nozzles in the ODD column are used at intervals of four nozzles for recording. That is, the spacing between the two dot columns in the reference patterns shown in  FIG. 13A  is approximately 1/150 inch. Incidentally, the reference patterns arranged in the main scanning direction are identical to each other. 
     Next, to convey the paper by a distance equal to half the length of the nozzle column, the conveying roller is rotated with a theoretical instruction pulse value of 5120. 
     After the paper conveyance, adjustment patterns (second patterns) indicated by black dots in  FIG. 13B  are recorded around the locations of the reference patterns (white dots) recorded earlier, using the downstream nozzles. It is assumed here that the downstream nozzles in both the EVEN and ODD columns are used for recording. Specifically, by using, as a reference position, the 320th ODD-column nozzle position in the downstream direction from the upstream nozzles in the ODD column used to record the reference patterns, the adjustment patterns are recorded using the nozzles at seven locations displaced in the conveying direction in steps of one dot. In  FIG. 13B , the nozzles used are located at −3, −2, −1, 0, +1, +2, and +3 positions displaced in steps of one dot from the reference position indicated by adjustment patterns of patches at position ( 3 ). That is, adjustment patterns displaced by −2, 0, or +2 are recorded using the nozzles in the ODD column and adjustment patterns displaced by −3, −1, +1, or +3 dots are recorded using the nozzles in the EVEN column. 
     Theoretically, the adjustment pattern at the reference position of ( 3 ) has the lowest patch area factor, which theoretically is approximately 12.5% (=100/8). However, depending on the accuracy of the individual apparatus or changes in the printing medium caused by the environment and the like, the amount of paper conveyance corresponding to the instruction pulse value can differ from its theoretical value. In that case, the patch area factor will exceed 12.5%. 
     Incidentally, the adjustment pattern of “−3” and adjustment pattern of “+3” in  FIG. 13B  cause displacement equivalent to seven pixel dots. Thus, it can be seen that one out of seven patches always produces an area factor of approximately 12.5%. There is almost a one-to-one correspondence between area factors and densities. Thus, it is possible to determine the amount of dot displacement by detecting the patch with the lowest density using the optical sensor  130 . The amount of dot displacement corresponds to the amount of displacement in conveyance. 
       FIG. 14  is a diagram showing an example of detection of the adjustment patches (configuration example 2) shown in  FIG. 13B , where the ordinate represents intensity of irregularly reflected light. It can be seen that the stronger the reflected light, the lower the density. Thus, in  FIG. 14 , by using an adjustment value of “0,” which is the adjustment value for the patch at position ( 3 ), it is possible to derive an adjustment value comparable to the nozzle resolution. 
     It is also preferable to use function approximation such as shown by the curve in  FIG. 14 . That is, a function is derived, for example, by applying the least-squares method to intensity values of reflected light for the seven patches. Then, by deriving and using paper feed adjustment position which corresponds to a maximum value of an approximated curve, it is possible to obtain an adjustment value with an accuracy higher than the nozzle resolution. 
     &lt;Adjustment Patch Configuration Example 3&gt; 
     Configuration example 3 is similar to configuration example 2, but the number of divisions of nozzles which record adjustment patterns is increased to further increase adjustment resolution. In the following example, the nozzles are divided into eight parts. 
       FIGS. 15A and 15B  are diagrams illustrating how nozzle columns are divided into two parts and eight parts, respectively. In the case of two-part split ( FIG. 15A ), reference patterns (first patterns) are recorded using the upstream ½ of the nozzles, the paper is conveyed by L×½, and then adjustment patterns (second patterns) are recorded using the downstream ½ of the nozzles. In the case of eight-part split ( FIG. 15B ), reference patterns (first patterns) are recorded using the upstream ⅛ of the nozzles, the paper is conveyed by L×⅞, and then adjustment patterns (second patterns) are recorded using the downstream ⅛ of the nozzles. That is, the amount of conveyance between the upstream pattern formation and downstream pattern formation is increased by 1.75 times. 
     Consequently, if the deviation in the amount of conveyance is constant on a per-sheet basis, white noise components are averaged and reduced, improving the S/N ratio. Thus, adjustment accuracy of the eight-part split patterns can be higher than that of the two-part split patterns. Suppose, there is a deviation equivalent to one pulse in relation to an instruction pulse value of 1280 (=5120/4). In the case of two-part split, the patches are subjected to a deviation equivalent to four pulses. In the case of eight-part split, the patches are subjected to a deviation equivalent to seven pulses. That is, the eight-part split has a larger impact on the patches. 
     Furthermore, in the case of eight-part split, the amount of conveyance per stroke is approximately 3.4 mm, making it possible to take 14 measurements per rotation of the roller. Thus, by using the average value of the 14 measurements as the amount of paper conveyance, it is possible to calculate the amount of paper conveyance more stably. 
     &lt;Flow of Deriving Average Amount of Conveyance and Instruction Pulse Value&gt; 
       FIG. 16  is a flowchart showing procedures for deriving an average amount of conveyance and instruction pulse value during one rotation of a conveying roller. Incidentally, although any of the three adjustment patch types described above may be selected freely, configuration example 2 will be cited here. 
     In Step S 1601 , an adjustment patch is formed at a first position (phase) of the conveying roller. That is, a reference pattern (first pattern) is formed using upstream nozzles and an adjustment pattern (second pattern) is formed using downstream nozzles. 
     In Step S 1602 , the amount of dot displacement at the first position (phase) is derived by measuring the adjustment patch formed in Step S 1601 . Details have been described in “Adjustment patch configuration example 2” and will be omitted here. 
     In Step S 1603 , the conveying roller is rotated by a half turn (180 degrees) from the position (phase) at which the reference pattern (first pattern) has been formed in Step S 1601 . Incidentally, the rotation angle of the conveying roller can be detected with much higher accuracy than the amount of dot displacement using an encoder (not shown) mounted on the conveying roller. 
     In Step S 1604 , an adjustment patch is formed at a second position (phase) of the conveying roller. That is, a reference pattern (first pattern) is formed using upstream nozzles and an adjustment pattern (second pattern) is formed using downstream nozzles. 
     In Step S 1605 , the amount of dot displacement at the second position (phase) is derived by measuring the adjustment patch formed in Step S 1604 . Details have been described in “Adjustment patch configuration example 2” and will be omitted here. 
     In Step S 1606 , an instruction pulse value corresponding to an average amount of conveyance is derived. That is, an average amount of displacement is calculated from the amount of dot displacement at the first position (phase) and amount of dot displacement at the second position (phase). Then, a correct instruction pulse value ( 5122  in this case) is calculated from a pulse adjustment value (e.g., +2) corresponding to the average amount of displacement and a theoretical instruction pulse value (e.g.,  5120 ). The calculated correct instruction pulse value as the rotational amount of the conveyance roller during conveyance of the printing medium executed after print scanning in image formation is set, and the conveyance roller is driven based on the set pulse value. By driving the conveyance roller in this manner, the conveyance shift amount of a fixed component resulting from a full rotation of the conveyance roller is absorbed, enabling image formation with low density unevenness. 
     As described above, the average amount of displacement is derived from the amount of displacement at two positions (phases) of the conveying roller. The use of the average amount of displacement makes it possible to derive an almost constant correction value regardless of the timing of adjustment operation. By driving the conveying roller using the correction value thus derived, it is possible to reduce misregistration in the conveying direction of the printing medium. 
     In the above description, assuming that the variable component (B in  FIG. 5 ) can be approximated by a sine curve with a period approximately equal to one rotation of the conveying roller, the average amount of conveyance has been derived from two positions 180 degrees out of phase with each other. When the variable component varies in a complicated manner, the average amount of conveyance can be derived with higher accuracy if it is derived from phases of more than two different points. 
     Also, the friction and amount of slippage between the conveyance roller and the printing medium differs depending on the type of printing medium. For this reason, by setting the amount of rotation of the roller for each type of printing medium, the average conveyance amount can be calculated with even higher accuracy. 
     (Second Embodiment) 
     A method for reducing the fixed component by deriving the average amount of conveyance has been described in the first embodiment. However, eccentricity of the conveying roller or the like can cause degradation of recorded images as shown in  FIG. 7 . Thus, in a second embodiment, description will be given of a method for controlling the conveying roller by detecting a variable component in each phase during one rotation of the conveying roller in addition to detection of the fixed component described in the first embodiment and deriving an adjustment value in each phase. Incidentally, equipment configuration and the like is the same as the first embodiment, and thus description thereof will be omitted. 
     &lt;Variable Component Due to Eccentricity&gt; 
     It is known, for example, that the variable component (B in  FIG. 5 ) affects a recorded image formed by 4 p 1  of ink droplets if its amplitude is larger than 30 μm. That part of the variable component which is attributable to roller deformation and roller flexure can be reduced to 30 μm or below with conventional machine accuracy. On the other hand, it is difficult to reduce that part of the variable component which is attributable to displacement in the mounting position of a roller support member to 30 μm or below. 
       FIGS. 17A and 17B  are diagrams illustrating structures of a conveying roller and roller support member.  FIG. 17A  is an external perspective view. There is no variable component when the central axis of the conveying roller coincides with the central axis of the roller support member. However, misalignment (eccentricity) can occur between the two axis depending on the tightening condition of mounting screws as shown in  FIG. 17B . This will produce the variable component of conveyance. 
     &lt;Adjustment Patch Configuration Example&gt; 
       FIG. 18  is a diagram showing measured values of feed rate for approximately 2.5 rotations of the conveying roller when there is eccentricity. In the figure, the ordinate represents the amount of feed rate variation and abscissa represents the position of the conveying roller. It can be seen that there are peculiar variations in the feed rate with a period equal to one rotation of the conveying roller. However, variations in the feed rate include variable components other than sine functions. Consequently, although it is possible to measure the amount of variation using the nozzles in intervals A and B, since the amount of variation can fluctuate due to paper slippage and the like, it is likely that the S/N ratio (signal component/noise component) is low, making it difficult to measure eccentricity with high accuracy. 
     Incidentally, variable components other than sine functions are mainly attributable to paper slippage and the like as described above. It is known that the paper slippage and the like can be regarded as white noise (random noise). Therefore, with increases in the amount of conveyance, the variable components other than sine functions are averaged and noise is reduced relatively. That is, the S/N ratio is increased. However, simply increasing the amount of conveyance increases the amount (length) of the printing medium needed for registration adjustment. Thus, description will be given below of a method for reducing the variable components other than sine functions while curbing increases in consumption of the printing medium. 
       FIG. 19  is a diagram illustrating nozzle positions when a nozzle column is divided into A to H intervals (eight parts). When the conveying roller can be moved in steps of approximately ⅛ the length of the nozzle length at a time, the amount of displacement in conveyance can be detected in the same manner as adjustment patch configuration example 2 according to the first embodiment. That is, adjustment patches can be formed by forming reference patterns (first patterns) using the nozzles in interval A and forming adjustment patterns (second patterns) using the nozzles in interval B. However, the amount of conveyance between interval A and interval B is very small (approximately 3.4 mm). Thus, as described above, because of fluctuations in the amount of variation due to paper slippage and the like, it is difficult to detect only the amount of displacement attributable to eccentricity with accuracy. 
       FIG. 20  is a diagram showing detected amounts of displacement when there is no fluctuation in the amount of variation due to paper slippage or the like. The figure shows measurement data of the amounts of displacement in an exemplary manner when adjustment patches are formed in A-B, A-H, and B-H intervals. It can be seen from the figure that the difference between measured values in the A-H interval and B-H interval should be equivalent in principle to measured values in the A-B interval. 
     Actually, the white noise component described above is superimposed on the measured values in the A-B, A-H, and B-H intervals. However, the amounts of conveyance in the A-B, A-H, and B-H intervals are approximately 3.4 mm, 23.7 mm, and 20.3 mm, respectively. Consequently, the noise level in the A-H interval is an average (integration) of seven measurements taken in the A-B interval while the noise level in the B-H interval is an average of six measurements taken in the A-B interval. Thus, it can be seen that the amount of displacement can be detected more accurately if the difference between the A-H interval and B-H interval is used as measurement data in the A-B interval instead of using data obtained by direct measurement in the A-B interval. This method makes it possible to derive accurate adjustment values for the instruction pulse value without increasing the amount of recording in the paper conveying direction. 
     &lt;Modeling of Variable Components&gt; 
     The method described above makes it possible obtain the amount of displacement for each stroke of paper conveyance (approximately 3.4 mm). Therefore, by repeating measurements  14  (=47/3.4) times, it is possible to obtain the amount of displacement in each phase during one rotation of the conveying roller, and thereby derive an adjustment value for the instruction pulse value. 
     Incidentally, as described above, it is known that the variable component attributable to displacement (eccentricity) in the mounting position of the roller support member is equal in period to one rotation of the roller and has the same effect in the positive and negative directions. Thus, the variable component can be modeled (approximated) using a sine function, making it possible to derive more accurate adjustment values for the instruction pulse value. Also, the sine function can be determined uniquely using four or more measurement points (amounts of displacement) during one rotation of the conveying roller, making it possible to speed up adjustment operation. 
     &lt;Flow of Deriving Instruction Pulse Value According to Position (Phase) of Conveying Roller&gt; 
       FIG. 21  is a flowchart showing procedures for deriving the amount of displacement and instruction pulse value in each phase during one rotation of a conveying roller. 
     In Step S 2101 , an adjustment patch is formed. That is, a reference pattern (first pattern) is formed using upstream nozzles and an adjustment pattern (second pattern) is formed using downstream nozzles. 
     In Step S 2102 , the amount of dot displacement at the position (phase) of formation of the adjustment patch is derived by measuring the adjustment patch formed in Step S 2101 . Details have been described in the first embodiment and will be omitted here. 
     In Step S 2103 , the conveying roller is rotated by a predetermined angle from the position (phase) at which the reference pattern (first pattern) has been formed in Step S 2101 . For example, it is rotated here by a ¼ turn (approximately 11.8 mm). Incidentally, the rotation angle of the conveying roller can be detected with much higher accuracy than the amount of dot displacement using an encoder (not shown) mounted on the conveying roller. 
     In Step S 2104 , it is checked whether amounts of displacement have been acquired at four or more positions (phases) during one rotation of the conveying roller. If yes, the flow goes to Step S 2105 . Otherwise flow returns to Step S 2101 . 
     In Step S 2105 , modeling (function approximation) is performed based on the amounts of displacement derived at the positions (phases) of the conveying roller in Steps S 2101  to S 2104 . In the presence of eccentricity and the like, it is preferable to express the amounts of displacement in terms of a sine function with a period equal to one rotation of the conveying roller. 
     In Step S 2106 , a correct instruction pulse value for each phase of the conveying roller is derived using the function modeled in Step S 2105 . That is, the correct instruction pulse values for the phases of the conveying roller detected by the encoder are derived from the function. 
     By controlling the conveying roller based on the instruction pulse values thus derived, it is possible to reduce misregistration in the conveying direction of the printing medium. The second embodiment, in particular, can reduce displacement within one rotation of the conveying roller. 
     (Third Embodiment) 
     As a method for acquiring the conveyance amount for each phase angle of a rotation of the conveyance roller other than the embodiments mentioned above, the following method can also be used. 
       FIG. 24A  and  FIG. 24B  explain a method for acquiring a conveyance amount of a printing medium. 
     Moreover, because the nozzle-space distance and accuracy of the print head necessary for the method to acquire the conveyance amount of the present embodiment are regulated by the print head creation process, known values are used. In particular, the present method uses a nozzle-space distance to acquire an amount of misalignment of the conveyance amount. 
     First, as shown in  FIG. 24A , by discharging ink from nozzle  1  and nozzle  9  of the nozzle array of the print head as the carriage scans, two straight lines are printed in the scanning direction. Moreover, the distance between the two straight lines formed on the printing medium is the same as the distance between nozzles  1  and  9 . 
     Next, the distance between the two straight lines formed on the printing medium is measured using an optical sensor attached to the carriage.  FIG. 24B  shows a frame format diagram of detection. First, in order to make detection of the two printed straight lines by the optical sensor possible, the carriage is moved to position the optical sensor. Then, without moving the carriage, a conveyance operation is executed on the printing medium. The conveyance operation on the printing medium conveys the printing medium by rotating the conveyance rollers, and writes the rotation amount of the conveyance roller to memory from the encoder. Specifically, first, the encoder value when the optical sensor detects the straight line formed by nozzle  1  is written to memory as an initial value. Next, upon detection of the straight line formed by nozzle  9  by the optical sensory using conveyance operation of the printing medium, the encoder value at the time of detection is written to memory. The difference between these encoder values is the rotation amount of the conveyance roller required to convey the printing medium the distance from nozzle  1  to nozzle  9 , and is the encoder pulse amount for driving the conveyance motor to rotate the conveyance roller. Because the nozzle-space distance of the print head is known, the distance between nozzle  1  and nozzle  9  can be determined. Also, the difference between the distance determined using the difference in encoder values and the distance between nozzle  1  and nozzle  9  is the shift in the conveyance amount. Here, by further taking the accuracy of the nozzle-space distance into consideration, the shift in conveyance amount can be determined with even higher accuracy. 
     By printing the two straight lines shown in  FIG. 24B , and by measuring the distance between the two lines for one revolution of the conveyance roller, the conveyance amount for each roller position (conveyance shift amount) can be acquired. When the roller is a perfect circle, the amounts of printing medium conveyed for all roller positions are the same. However, as explained earlier, when the conveyance roller is not a perfect circle, the amounts of printing medium conveyed for all roller positions are not the same. 
     Moreover, while nozzles  1  through  9  are used in the explanation above, the numbers are not restricted, and any nozzles can be used. When selecting the nozzles to be used to print the straight line in the present embodiment, it is preferable to select nozzles with a distance similar to the conveyance amount during actual printing. 
     Also, when the light-emission unit ( FIGS. 3 ,  31 ) of the optical sensor uses a visible-light LED, the output of the sensor decreases when the straight lines formed on the printing medium are detected by the optical sensor. When the output of the sensor in an area where nothing is printing on the printing medium is 100%, and the output in an area where something is printed is 0%, a change in output of approximately 25% is sufficient for determining the existence of a straight line. This is a situation in which the output is 75% that of an area where nothing is printed. 
     For this reason, the thickness of the printed line must be approximately ¼ the size of the sensor aperture unit. In other words, a sensor with an aperture unit that is approximately 4 times the printed straight line must be used. This means that when the line is formed with a width of 100 μm, the aperture unit must be 400 μm, and a sensor with even higher accuracy is necessary for length measurement. 
     In this way, by using the method described above, the shift in conveyance amount corresponding to a position of one revolution of a conveyance roller can be acquired. As in the first embodiment, by detecting the conveyance amount at two or more positions on the conveyance roller, the fixed components of the roller rotation such as type of printing medium and environment can be acquired, and a conveyance amount corresponding to the printing medium that decreases the effect of these fixed components can be acquired. 
     As shown above, the present invention has a characteristic not of depending on a method for acquiring a conveyance amount, but of realizing paper medium conveyance amount adjustment that reduces the effect of shift components due to a revolution of the conveyance roller. According to the present invention, an error component during acquisition of conveyance amount due to a conveyance roller shift is acquired by an operation of rotating a roller less than one revolution can be acquired, and execution of conveyance control which decreases an error component is possible. 
     Also, when rotating a conveyance roller over more than one revolution in acquiring a conveyance amount or a conveyance error, the amount of consumed printing medium and time for acquisition increase by the amount of the conveyance roller rotation. However, in the present invention, because a conveyance amount or conveyance error is acquired with a rotation amount of a rotation roller in less than one revolution, the consumed amount of printing medium and time for acquisition is reduced. Moreover, although a conveyance amount of the printing medium with respect to a predetermined rotation amount is acquired by a plurality of roller revolutions, even if rotation of a conveyance roller or a conveyance amount acquisition operation in less than one rotation is executed a plurality of times, the overall rotation amount of the conveyance roller is still less than one revolution. For this reason, the amount of printing medium required for the conveyance amount acquisition operation is shorter than the circumference of the conveyance roller, and can be reduced to significantly less than the amount of printing medium required for conventional conveyance amount correction. 
     Moreover, by rotating a conveyance roller by less than one revolution, regarding position determination of a plurality of sampling points when acquiring conveyance amount, it is desirable to divide the circumference of the conveyance roller by a constant number and acquire the positions. For example, if the conveyance amount is to be acquired with 8 sampling points, the conveyance amount is acquired at positions A-H as shown in  FIG. 25 . Here, for example, the conveyance amount of the medium in rotating the conveyance roller from point A to B (A-B space) is acquired. Similarly, the conveyance amount of the medium for B-C, C-D, . . . , H-A spaces are acquired. From the average of the plurality of acquired conveyance amounts, an approximation of the conveyance amount of the printing medium for ⅛ of a revolution of the conveyance roller can be calculated. Moreover, as a rotation amount of the conveyance roller during acquisition of the conveyance amount, rather than a rotation amount smaller than the rotation in A-B space, a predetermined rotation amount smaller than 45° from point A as a starting point can be used. In this case, each of the points A-H become rotational starting positions during conveyance amount acquisition, and the conveyance amount of the printing medium is detected by rotating the conveyance roller one predetermined rotation amount at a time from each rotation starting position. According to the present invention, even when executing a conveyance amount acquisition operation a plurality of times (plurality of points) in this way, the overall rotation of the conveyance roller is still less than one revolution. 
     Also, although a composition in which the conveyance amount is acquired using a plurality of points was described, the conveyance amount must be acquired using a minimum of 2 sampling points, and in this case it is preferable to acquire the conveyance amount using 2 points positioned 180° from each other on the roller. Normally, because the shape of the conveyance roller becomes close to an oval shape, the shift component can be reduced in most cases by acquiring the conveyance amount using 2 sampling points offset 180° from each other. 
     (Other Embodiments) 
     Embodiments of the present invention have been described in detail above, but the present invention may be applied either to a system consisting of two or more devices or to an apparatus consisting of a single device. For example, the present invention may take the form of an image output terminal of an information processing apparatus such as a computer installed either integrally or separately, a copying machine combined with a reader or the like, or a facsimile machine equipped with transmission and reception capabilities. 
     Incidentally, the present invention can also be achieved by a configuration in which a software program that implements the functions of the embodiments described above is supplied to a system or apparatus either directly or remotely and a computer in the system or apparatus reads out and executes the supplied program code. Thus, program code itself installed on the computer to implement functions and processes of the present invention on the computer also implements the present invention. 
     In that case, the program code may take any form including object code, programs executed by an interpreter, and script data supplied to an OS as long as it has program functions. 
     Recording media available for use to supply programs include, for example, floppy (registered trademark) disks, hard disks, optical disks (CD and DVD), magneto-optical disks, MO disks, magnetic tapes, and non-volatile memories. 
     The functions of the above embodiments may be implemented not only by the program read out and executed by the computer, but also by part or all of the actual processing executed, in accordance with instructions from the program, by an OS running on the computer. 
     Furthermore, the functions of the above embodiments may also be implemented by part or all of the actual processing executed by a CPU or the like contained in a function expansion board inserted in the computer or a function expansion unit connected to the computer if the processing is performed in accordance with instructions from the program code that has been read out of the storage medium and written into memory on the function expansion board or unit. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2006-056899, filed Mar. 2, 2006, and Japanese Patent Application No. 2007-047886, filed Feb. 27, 2007, which are hereby incorporated by reference herein in its entirety.