Abstract:
A first correction pattern and a second correction pattern are formed on an endless conveyor belt. The first correction pattern is located ahead of the second correction pattern on the conveyor belt with respect to the direction of movement of the conveyor belt. A sensor detects positional information of the correction patterns. A timing changing unit (controlling unit) changes a detection timing of the second correction pattern from a time point at which the sensor detects the second correction pattern.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present document incorporates by reference the entire contents of Japanese priority document, 2006-190722 filed in Japan on Jul. 11, 2006. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a positional-deviation correction apparatus, a method of controlling a positional deviation, and an image forming apparatus to obtain an image visualized by superimposing a plurality of colors by an electrophotographic printer and the like. 
   2. Description of the Related Art 
   In image forming apparatuses, it is common to perform correction of positional deviation by checking how a test pattern is printed on a recording medium. In the technology disclosed in Japanese Patent No. 3506891, a test pattern is printed on the left and right sides of a recording medium in a conveying direction and includes a first pattern and a second pattern opposing to each other. The first pattern includes stripes formed at a specific interval, and each strip has a specific line width. The second pattern also includes strips formed at a specific interval, and each strip has a specific line width; however, the specific interval is different from the interval of the first pattern. 
   In color image forming apparatuses, it is common to perform positional alignment of each of the colors. Japanese Patent Application Laid-Open No. 2002-244387 discloses a technology that has a mode for executing each of a plurality of processes, enables a user to select a desired mode and performs positional alignment of the colors appropriate to the user selected mode. 
   In the related art, a positional-deviation correction apparatus creates images of a plurality of sets of positional deviation correction patterns and detects positional information of each set, in an unambiguous time from start of exposure. However, due to a tolerance in layout size of the positional-deviation correction apparatus, such as a stretching of an intermediate transfer belt, there is an occasion that this unambiguous time cannot be inserted between the respective sets of the correction patterns. This leads to a problem that not all the positional information of the sets of the correction patterns can be read. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to at least partially solve the problems in the conventional technology. 
   According to an aspect of the present invention, a positional-deviation correction apparatus includes a first-pattern forming unit that forms a plurality of sets of first correction patterns on an endless conveying unit; a second-pattern forming unit that forms a second correction pattern before the correction-pattern forming unit forms the correction patterns; and a detecting unit configured to detect positional information of each set of the first correction patterns and the second correction pattern; and a timing changing unit that changes a detection timing for detecting positional information of each set of the first correction patterns from a time point at which the detecting unit detects positional information of the second correction pattern. 
   According to another aspect of the present invention, a method of controlling positional-deviation includes forming a second correction pattern on an endless conveying unit; forming a plurality of sets of first correction patterns on the endless conveying unit after the second correction pattern; detecting positional information of each set of the first correction patterns and the second correction pattern; and changing a detection timing, at the detecting, for detecting positional information of each set of the first correction patterns from a time point at which positional information of the second correction pattern is detected at the detecting. 
   According to still another aspect of the present invention, an image forming apparatus includes a first-pattern forming unit that forms a plurality of sets of first correction patterns on an endless conveying unit; a second-pattern forming unit that forms a second correction pattern before the correction-pattern forming unit forms the correction patterns; a detecting unit configured to detect positional information of each set of the first correction patterns and the second correction pattern; and a timing changing unit that changes a detection timing for detecting positional information of each set of the first correction patterns from a time point at which the detecting unit detects positional information of the second correction pattern. 
   The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an image forming apparatus according to an embodiment of the present invention; 
       FIG. 2  is a perspective view for explaining an example of arrangements of an image creating unit, correction marks, and sensors shown in  FIG. 1 ; 
       FIG. 3  is a schematic diagram for explaining a configuration of the sensor; 
       FIG. 4  is a schematic diagram for explaining a structure of a slit shown in  FIG. 3 ; 
       FIG. 5  is a schematic diagram for explaining an example of arrangements of the sensors and the correction patterns; 
       FIG. 6  is a block diagram of a controlling system; 
       FIG. 7  is a functional block diagram of a positional-deviation correction apparatus; 
       FIG. 8A  is a schematic diagram for explaining an example of arrangements of the correction patterns and a second pattern (a type beginning with one line); 
       FIG. 8B  is a schematic diagram for explaining another example of arrangements of the correction patterns and the second pattern (a type beginning with a line for each set); 
       FIG. 9  is a flowchart of a positional deviation controlling operation; 
       FIG. 10A  is a schematic diagram for explaining an example of arrangements of the correction patterns and the second pattern (a type providing one line for each of the sets); 
       FIG. 10B  is a schematic diagram for explaining another example of arrangements of the correction patterns and the second pattern (a type providing lines for each of the sets, respectively); and 
       FIG. 11  is a flowchart of a positional deviation controlling operation. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Exemplary embodiments of the invention will be described in detail with reference to the accompanying drawings. 
   As shown in  FIG. 1 , in an image forming apparatus according to an embodiment of the present invention, image forming units of respective colors are arranged along a conveyor belt (endless transporting unit). This arrangement is a so-called tandem type arrangement. A plurality of image forming units (electrophotographic processing units)  6 Y,  6 M,  6 C, and  6 BK are arranged along the conveyor belt  5  sequentially from the upstream side in a conveying direction of the conveyor belt  5 . The image forming units  6 Y,  6 M,  6 C, and  6 BK are collectively referred to as an image creating unit  30 . 
   Paper (recording paper) are stacked in a paper feed tray  1 . A paper feed roller  2  picks-up one paper  4  from the paper feed tray  1 , and a pair of separation rollers  3  conveys the paper  4  to the conveyor belt  5 . The conveyor belt  5  conveys the paper  4  to the image creating unit  30 . 
   An internal configuration of each of the image forming units  6 Y,  6 M,  6 C, and  6 BK are the same, except that each of them corresponds to a different color. Specifically, the yellow image forming unit  6 Y forms a yellow image, the magenta image forming unit  6 M forms a magenta image, the cyan image forming unit  6 C forms a cyan image, and the black image forming unit  6 BK forms a black image. 
   The yellow image forming unit  6 Y will be described in detail below. Because the other image forming units  6 M,  6 C, and  6 BK have the same configuration as the yellow image forming unit  6 Y, their description is omitted. 
   The conveyor belt  5  is an endless belt. The conveyor belt  5  is rolled on a driving roller  7  and a driven roller  8 . A driving motor (not shown) rotates and drives the driving roller  7 . The conveyor belt  5  rotates with the rotation of the driving roller  7 . The driving motor, the driving roller  7 , and the driven roller  8  function as a driving unit for the conveyor belt  5 . 
   The paper  4  conveyed to the conveyor belt  5  sticks to the conveyor belt  5  because of an electrostatic adsorption force, and the paper  4  moves with the rotation of the conveyor belt  5 . First, the paper  4  is conveyed to a position below the yellow image forming unit  6 Y, and a yellow toner image on the yellow image forming unit  6 Y is transferred onto the paper  4 . 
   The yellow image forming unit  6 Y includes a photosensitive drum  9 Y that is a photosensitive body. A charger  10 Y, a developing unit  12 Y, a cleaning unit (not shown), and a de-charger  13 Y are arranged around the photosensitive drum  9 Y. An optical writing unit  11  emits laser lights  14 Y,  14 M,  14 C, and  14 BK, which are exposure light corresponding to colors of an image formed by the respective image forming units  6 Y,  6 M,  6 C, and  6 BK. 
   A surface of the photosensitive drum  9 Y is exposed to the laser light  14 Y corresponding to the yellow image, after uniformly charged by the charger  10 Y in the dark, thereby forming an electrostatic latent image. The developing unit  12 Y visualizes the electrostatic latent image with the yellow toner, thereby forming the yellow toner image on the photosensitive drum  9 Y. 
   The toner image is transferred on the paper  4  by a transferring unit  15 Y, at a position (transfer position) where the photosensitive drum  9 Y comes in contact with the paper  4  on the conveyor belt  5 . As a result, a yellow toner image is formed on the paper  4 . After removing unnecessary residual toner left on the surface of the photosensitive drum  9 Y with the cleaning unit, the photosensitive drum  9 Y that has finished the transferring of the toner image is de-charged with the de-charger  13 Y. Then, the photosensitive drum  9 Y waits for the formation of the next image. 
   The conveyor belt  5  then conveys the paper  4  with the yellow toner image to a position below the magenta image forming unit  6 M. A photosensitive drum  9 M of the magenta image forming unit  6 M carries a magenta toner. The magenta toner image is transferred on the paper  4  so that it superposes onto the yellow image already formed on the paper  4 . 
   Then, the conveyor belt  5  conveys the paper  4  to the cyan image forming unit  6 C and then to the black image forming unit  6 BK. As a result, a cyan toner image, and a black toner image are superimposed and transferred on the paper  4 . Accordingly, a full color image is formed on the paper  4 . The paper  4  with the full color image is then separated from the conveyor belt  5  and conveyed to a fixing unit  16  where the full color image is fixed to the paper  4 . 
   Sometimes a positional deviation occurs between the respective colors, because the toner images of the respective colors do not superimpose at a position where they should theoretically. Such a positional deviation can occur due displacement of the axes of the photosensitive drums  9 Y,  9 M,  9 C, and  9 BK, non-parallelism of the photosensitive drums  9 Y,  9 M,  9 C, and  9 BK, displacement of a deflecting mirror (not shown) that deflects laser light in the optical writing unit  11 , a error in writing timing of the electrostatic latent image to the photosensitive drums  9 Y,  9 M,  9 C,  9 BK, and the like. 
   Other known causes of the positional deviation of the respective colors are, a skew, a positional deviation of a resist in a sub-scanning direction, a magnification error in a main scanning direction, the positional deviation of the resist in the main scanning direction, and the like. 
   The positional deviation leads to degraded image quality, so that the positional deviation needs be corrected. As shown in  FIG. 1 , sensors  17 ,  18 , and  19  are arranged at the downstream side of the black image forming unit  6 BK along the main scanning direction, which is a direction perpendicular to the direction of rotation of the conveyor belt  5 . The sensors  17 ,  18 , and  19  function as an image detecting unit. 
     FIG. 2  is an enlarged perspective view of the image detecting unit and a periphery thereof.  FIG. 3  is a schematic diagram of the image detecting unit. The image detecting unit includes a light emitting unit  20 , a slit  21 , and a light receiving unit  22 . The image detecting unit detects positional deviation correction patterns  23  formed on the conveyor belt  5 . One image detecting unit is arranged at each of the both edges and the center in the main scanning direction of the conveyer belt  5 . A correction pattern  23  is formed on the conveyer belt  5 . The correction pattern  23  includes correction marks  23   a ,  23   b , and  23   c  each corresponding to each of the image detecting unit. 
     FIG. 4  is an enlarged view of the slit  21 . The slit  21  includes two openings: one for detecting a line parallel to the main scanning direction (hereinafter, “parallel line”) and another one for detecting a line inclined to the parallel line (hereinafter, “oblique line”). 
     FIG. 5  is an enlarged view of the correction pattern  23 . Each of the correction marks  23   a ,  23   b , and  23   c  includes two sets of black (K), magenta (M), yellow (Y), and cyan (C) lines. One set of the lines includes parallel lines and the other set includes oblique lines. Each line is separated by a predetermined target length d from an adjoining line. By doing so, detection signals are produced in a typical wave form with peaks and valleys, when the line reaches the opening of the slit, thereby enabling to find the center of the line accurately. 
     FIG. 6  is a block diagram of a controlling system  40  that processes the data detected by the image detecting unit. The controlling system  40  includes a central processing unit (CPU)  31  that executes a predetermined arithmetic processing based on the detected data. Specifically, the CPU  31  calculates respective amounts of skew, positional deviation of the resist in the sub-scanning direction, magnification error in the main scanning direction, and positional deviation of the resist in the main scanning direction. 
   The skew can be corrected, for example, by adjusting the inclination of the deflecting mirror in the optical writing unit  11 , or by adjusting the optical writing unit  11 . The positional deviation of the resist in the sub-scanning direction can be corrected, for example, by controlling a write start timing of the line and a surface phase of a polygon mirror. The magnification error in the main scanning direction can be corrected, for example, by changing image writing frequency. The positional deviation of the resist in the main scanning direction can be corrected by correcting write start timing in the main scanning direction. 
     FIG. 5  is an example of a minimum set of patterns required to calculate different color registration deviation amounts of respective colors. However, there are alternative ways to offset fluctuations caused by rotation of the photosensitive body, an intermediate transfer belt, the conveyor belt and the like. For example, a plurality of sets of marks may be formed with respect to one cycle of the photosensitive body, and their correction patterns may be detected by the sensors  17 ,  18 , and  19 , thereby taking a mean value of the detection result. Thus, more accurate detection can be performed. 
   The process to process the detected data is to be explained with reference to  FIG. 6 . The signals obtained from the light receiving unit  22  are amplified by an amplifier (AMP)  24 . Signal components only detected at the line are passed through by a filter  25 , and converted into digital data from analog data, by an analog/digital (A/D) converter  26 . Sampling of the data is controlled by a sample controlling unit  27 , and the sampled data is stored in a first-in-first-out (FIFO) memory  28 . When the detection of one set of correction marks is finished, the stored data is loaded in the CPU  31  and a random access memory (RAM)  32  by a data bus via an I/O port  29 . The CPU  31  executes a predetermined arithmetic processing, and calculates the different deviation amounts. 
   In a read only memory (ROM)  33 , not only the computer program to calculate the various deviation amounts, but also various computer programs to control the positional-deviation correction apparatus and the image forming apparatus according to the present embodiment of the present invention are stored. The CPU  31  monitors the detection signals from the light receiving unit  22  at an appropriate timing, and controls an amount of emitting light by a light emitting amount controlling unit  35 . This enables to carry out a reliable detection even if the conveyor belt and the light emitting unit  20  may deteriorate, for example. The level of light-receiving signals from the light receiving unit  22  is always kept constant. Accordingly, the CPU  31  and the ROM  33  function as controlling units to control an entire operation of the image forming apparatus. 
     FIG. 7  is a block diagram of a functional configuration of a positional deviation correction portion in the image forming apparatus according to the embodiment of the present invention. Here, respective configurations of  FIGS. 1 to 6  are shown as one functional block. In  FIG. 7 , the reference numeral  40  denotes the controlling system having a microcomputer such as the CPU  31 , the RAM  32 , the ROM  33 , and a timer (not shown). The reference numeral  50  denotes a second pattern hereinafter described. Their functions are to be described below with reference to the flowcharts of  FIGS. 9 and 11 . 
   A flowchart of a method of arithmetically controlling a positional deviation correction (first controlling example) is shown in  FIG. 9 . A process of exposing the second pattern  50  and a plurality (N sets) of the correction patterns  23  is started (step S 11 ). The second pattern  50  is preferably in yellow.  FIGS. 8A and 8B  are schematic diagrams of the second pattern  50  and N sets of the correction patterns  23 . Subsequently, it is decided whether the second pattern  50  is detected by the sensors  17 ,  18 , and  19  (step S 12 ). If the second pattern  50  is detected by the sensors  17 ,  18 , and  19 , the system control is given to step S 13 . At step S 13 , the timer starts reading positional information of the correction patterns  23  of the N-th set. The timer may have a timer value corresponding to a pattern interval between the second pattern  50  and the correction patterns  23  formed at step S 11 . 
   At step S 14 , it is decided whether the timers have reached a predetermined value. When the timer has reached the predetermined value, the positional information of one set of marks in the RAM  32  is loaded (step S 15 ). Then, it is judge whether the positional information of all the sets of marks are loaded in the RAM (step S 16 ). If the positional information of all the sets of marks are not loaded, the system control is returned to step S 13 . If the positional information of all the sets of marks are loaded, the system control is goes to step S 17 . 
   At step S 17 , different deviation amounts are calculated based on the loaded positional information. At step S 18 , the correction amounts corresponding to the deviation amounts are stored in the RAM, and the controlling of the positional deviation correction is completed. 
   A flowchart of a different method for arithmetically controlling the positional deviation (second controlling example) is shown in  FIG. 11 . The process shown in  FIG. 11  is executed by the controlling unit  40 . 
   At step S 21 , exposing is started of the second pattern  50  and a plurality (N sets) of the correction patterns  23 . The second pattern  50  is preferably in yellow.  FIGS. 10A and 10B  are schematic diagrams of the second pattern  50  and N sets of the correction patterns  23 . At step S 22 , it is decided whether the second pattern  50  is detected by the sensors  17 ,  18 , and  19 . If the second pattern  50  is detected by the sensors  17 ,  18 , and  19 , the system control goes to step S 23 . At step S 23 , the timer starts reading the positional information of the correction patterns  23  of the N-th set. The timer may have the timer value corresponding to the pattern interval between the second pattern  50  and the correction patterns  23  formed at step S 21 . 
   At step S 24 , it is decided whether the timers have reached a predetermined value. If the timers have reached the predetermined value, the positional information of one set of marks is loaded in the RAM  32  (step S 25 ). It is judge whether the positional information of all the sets of marks are loaded in the RAM  32  (step S 26 ). If the positional information of all the sets of marks are not loaded, the system control is returned to step S 22 . If the positional information of all the sets of marks are loaded, the system control is goes to step S 27 . 
   At step S 27 , different deviation amounts are calculated based on the loaded positional information. At step S 28 , the correction amounts corresponding to the deviation amounts are stored in the RAM  32 , and the controlling of the positional deviation correction is completed. 
   The respective embodiments are exemplary embodiments of the present invention, and various modifications are possible within the scope and spirit of the present invention. For example, while in the respective embodiments, the configuration of forming the correction patterns to the conveyor belt is described, the endless transporting unit where the image is formed may be an intermediate transfer belt. 
   While the slit is used as the image detecting unit, as long as the correction patterns  23  can be detected, it is not limited to this configuration, but may be the one without the slit. Also, the detection patterns are aligned vertically and horizontally, but as long as the positional deviation can be detected, it is not limited to this, and the detection patterns may be peak patterns and the like. 
   In this manner, the second pattern  50 , which is different from the correction patterns  23 , is formed ahead of the correction patterns  23  in the sub-scanning direction. The timing to start detecting the positional information of each set of the correction patterns  23  is decided, after detecting the positional information of the second pattern  50 . This prevents a problem in which the positional information of the correction patterns  23  cannot be detected due to tolerance in the layout size, and the accurate positional correction cannot be performed. 
   The second pattern  50  is formed before forming the correction patterns  23 . The timing to start detecting the positional information of a number of sets of the correction patterns is determined, after detecting the positional information of the second pattern  50 . The positional information of the second set of the second pattern  50  is detected, after detecting the final set of the correction patterns. Repeating these can prevent a problem in which the positional information of the correction patterns  23  cannot be detected due to the tolerance in the layout size resulting from environmental fluctuation and the like, and the accurate positional deviation correction cannot be performed. 
   The second pattern  50  is formed before forming the correction patterns  23 . The second pattern  50  is formed in an image forming color furthest from the positional detecting unit. The timing to start detecting the positional information of each set of the correction patterns  23  is determined, after detecting the positional information of the second pattern  50 . This prevents a problem in which the positional information of the correction patterns cannot be detected due to the tolerance in the layout size, and the accurate positional deviation correction cannot be performed. 
   A low-cost apparatus that is capable of detecting the second pattern  50  without fail, and reducing toner consumption required to form the second pattern  50  can be achieved. 
   According to an aspect of the present invention, it is possible to accurately detect the positional deviation and correct the positional deviation. 
   According to another aspect of the present invention, the positional-deviation correction apparatus forms the second pattern that is different from the correction patterns, before forming the correction patterns. The timing to start detecting the positional information of a number of set of the correction patterns is determined, after detecting the positional information of the second pattern. The positional information of the second set of the second pattern is detected, after detecting the final set of the patterns. As a result, by repeating these, it offers an advantage of preventing the problem in which the positional information of the correction patterns cannot be detected due to the tolerance in the layout size resulting from the environmental fluctuation and the like, and the accurate positional deviation correction cannot be performed. 
   According to still another aspect of the present invention, the positional-deviation correction apparatus according to the first or second aspect forms the second pattern that is different from the correction patterns, before forming the correction patterns. The second pattern is formed in the image forming color furthest from the positional detecting unit. The timing to start detecting the positional information of each set of the correction patterns is determined, after the positional information of the second pattern is detected. As a result, it offers an advantage of preventing the problem in which the positional information of the correction patterns cannot be detected due to the tolerance in the layout size, and the accurate positional deviation correction cannot be performed. 
   According to still another aspect of the present invention, the positional-deviation correction apparatus according to the first or second aspect forms the second pattern in one straight line in the main scanning direction. As a result, it offers an advantage of detecting the second pattern without fail. 
   According to still another aspect of the present invention, the positional-deviation correction apparatus according to the first or second aspect sets a number of the second pattern corresponding to the number of the positional detecting unit. As a result, it offers an advantage of detecting the second pattern without fail, and reducing toner consumption required to form the second pattern. 
   According to still another aspect of the present invention, the method of controlling positional deviation forms the second pattern that is different from the correction patterns, before forming the correction patterns. The timing to start detecting the positional information of each set of the correction patterns is determined, after detecting the positional information of the second pattern. As a result, it offers an advantage of solving the problem of the related art in which the positional information of the correction patterns cannot be detected due to the tolerance in the layout size, and the accurate positional deviation correction cannot be performed. 
   According to still another aspect of the present invention, the method of controlling positional deviation forms the second pattern that is different from the correction patterns, before forming the correction patterns. The timing to start detecting the positional information of a number of set of the correction patterns is determined, after detecting the positional information of the second pattern. The positional information of the second set of the second pattern is detected, after detecting the final set of the patterns. As a result, by repeating these, it offers an advantage of preventing the problem in which the positional information of the correction patterns cannot be detected due to the tolerance in the layout size resulting from environmental fluctuation and the like, and the accurate positional deviation correction cannot be performed. 
   According to still another aspect of the present invention, the method of controlling positional deviation according to the sixth or seventh aspect forms the second pattern that is different from the correction patterns, before forming the correction patterns. The second pattern is formed in the image forming color furthest from the positional detecting unit. The timing to start detecting the positional information of each set of the correction patterns is determined, after detecting the positional information of the second pattern. As a result, it offers an advantage of preventing the problem in which the positional information of the correction patterns cannot be detected due to the tolerance in the layout size, and the accurate positional deviation correction cannot be performed. 
   According to still another aspect of the present invention, the method of controlling positional deviation according to the sixth or seventh aspect forms the second pattern in one straight line in the main scanning direction. As a result, it offers an advantage of detecting the second pattern without fail. 
   According to still another aspect of the present invention, the method of controlling positional deviation according to the sixth or seventh aspect sets the number of the second pattern corresponding to the number of the positional detecting unit. As a result, it offers an advantage of detecting the second pattern without fail, and reducing toner consumption required to form the second pattern. 
   According to still another aspect of the present invention, the image forming apparatus forms the second pattern that is different from the correction patterns, before forming the correction patterns. The timing to start detecting the positional information of each set of the correction patterns is determined, after detecting the positional information of the second pattern. As a result, it offers an advantage of solving the problem of the related art in which the positional information of the correction patterns cannot be detected due to the tolerance in the layout size, and the accurate positional deviation correction cannot be performed. 
   According to still another aspect of the present invention, the image forming apparatus forms the second pattern that is different from the correction patterns, before forming the correction patterns. The timing to start detecting the positional information of a number of set of the correction patterns is determined, after detecting the positional information of the second pattern. The positional information of the second set of the second pattern is detected, after detecting the final set of the patterns. As a result, by repeating these, it offers an advantage of preventing the problem in which the positional information of the correction patterns cannot be detected due to the tolerance in the layout size resulting from environmental fluctuation and the like, and the accurate positional deviation correction cannot be performed. 
   According to still another aspect of the present invention, the image forming apparatus according to the eleventh or twelfth aspect forms the second pattern that is different from the correction patterns, before forming the correction patterns. The second pattern is formed in the image forming color furthest from the positional detecting unit. The timing to start detecting the positional information of each set of the correction patterns is determined, after detecting the positional information of the second pattern. As a result, it offers an advantage of preventing the problem in which the positional information of the correction patterns cannot be detected due to the tolerance in the layout size, and the accurate positional deviation correction cannot be performed. 
   According to still another aspect of the present invention, the image forming apparatus according to the eleventh or twelfth aspect forms the second pattern in one straight line in the main scanning direction. As a result, it offers an advantage of detecting the second pattern without fail. 
   According to still another aspect of the present invention, the image forming apparatus according to the eleventh or twelfth aspect sets the number of the second pattern corresponding to the number of the positional detecting unit. As a result, it offers an advantage of detecting the second pattern without fail, and reducing toner consumption required to form the second pattern. 
   Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.