Patent Publication Number: US-8526873-B2

Title: Image forming apparatus and threshold setting method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2010-043616 filed in Japan on Feb. 27, 2010. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an image forming apparatus, and a threshold setting method. 
     2. Description of the Related Art 
     As a printing system for forming images on both sides of a web without a feed hole as typified by a continuous long belt-like sheet, for example, as described in Japanese Patent No. 3680989, a printing system capable of printing an image on the second side of a web correctly so as to align with an image on the first side even if the web discharged from a first printing device is shrunk or expanded with environment has been proposed and put to practical use. 
       FIG. 1  shows a total configuration of an electrophotographic device applied to a conventional printing system. In  FIG. 1 , “W” denotes a web; usually, it is a paper web in most cases. The web W delivered from a feeding device (not shown) is fed toward a web buffer mechanism  2  by being guided by a guide roller  1  arranged on a feed path to thread through a printing device P. Then, the web W passes through a guide member  3 , a foreign-body removing mechanism  4 , a tension applying mechanism  5 , a guide shaft  6 , and a guide plate  7 , and is fed into an imaging unit  10  by feed rollers  8  and  9 . In the imaging unit  10 , through charging, exposure, and developing processes, a toner image is formed on a photosensitive drum  101 , and after that, the toner image is transferred onto the web W by the action of a transfer unit  105 . 
     The web W gets off a feed belt  11 , and is fed to a fixing unit  13  via a buffer plate  12 . When reaching the fixing unit  13 , the web W is preheated by a preheater  13   a . After that, the web W is fed while being sandwiched in a nip between a pair of fixing rollers composed of a heat roller  13   b  and a pressure roller  13   c , and applied with heat and pressure by the heat roller  13   b  and the pressure roller  13   c  thereby fusing and fixing the toner image on the web W. 
     Furthermore, a reference numeral  16  denotes a mark detecting unit (a mark sensor) for detecting a positioning mark  17  formed on the web W as shown in  FIG. 2 . The mark sensor  16  is an optical sensor composed of a light-emitting element and a light-sensitive element. The positioning mark  17  is formed on near the top of each page, and is used as a reference position for aligning positions of images that are formed on both sides of the web W by first and second printing devices which are connected to each other. Specifically, the second printing device detects the positioning mark  17  formed by the first printing device, and prints out an image on the second side of the web W correctly so as to align with an image on the first side by controlling, i.e., changing the rotating speeds of the feed rollers  8  and  9  and the photosensitive drum  101  (for example, see Japanese Patent No. 3680989 and Japanese Patent Application Laid-open No. 2003-266825). 
     Conventionally, in such a printing system, when an image is printed to fit in a business form of a preprinted sheet, the first printing device needs to form the positioning mark  17  on the preprinted sheet. However, to bother to form the positioning mark  17  on the preprinted sheet in addition to the business form causes an extra cost; therefore, recently, a method to use a portion of the business form, such as a company name or logo printed at the specified position on each page, as a positioning mark has been implemented. 
     A company name or logo on a preprinted sheet differs in layout or a color from one business form to another, so it is necessary to provide a way to detect a plurality of colors at an arbitrary position; therefore, by providing a mechanism capable of moving the mark detecting unit to an arbitrary position in a main-scanning direction or by making a sensitivity adjustment in accordance with a color of the company name or logo using an optical sensor with sensitivity to the plurality of colors, the company name or logo can be used as a positioning mark. 
     On the other hand, as a method for the sensitivity adjustment, there is a method to cause the mark sensor  16  to recognize a color of the positioning mark  17  and a ground color of the web W and set an intermediate color of the two colors as a threshold. At this time, it is preferable to arrange the positioning mark  17  in the center of a detectable area of the mark sensor  16 . As a method to detect the center of the positioning mark  17 , for example, as proposed in Japanese Patent Application Laid-open No. 2002-207338 and Japanese Patent Application Laid-open No. 2002-174936, there is a detection method in which using a sensor that outputs 0 V when the mark sensor  16  detects the positioning mark  17  and 5 V when the mark sensor  16  does not detect the positioning mark  17 , the position of the center of a line connecting the center voltage in a fall region of an analog signal output from the sensor and the center voltage in a subsequent rise region is set as the center of the positioning mark  17 . 
     Furthermore, Japanese Patent Application Laid-open No. 2000-318221 has proposed a detection method in which using the mark sensor  16  that outputs an analog voltage according to a color of an object to be detected, a voltage between a voltage value±α that is output when detecting the ground color of the web W, and a peak voltage value that is output when detecting the positioning mark  17  is set as a threshold voltage. Two points at which an output voltage of the mark sensor  16  intersects with the threshold voltage near the peak voltage are obtained, and then the midpoint of the two points is set as the center of the positioning mark  17 . 
     Moreover, as a simplified method, there is known a method that a scale  18  as shown in  FIG. 8  is provided, and an operator visually sets the positioning mark  17  so that the positioning mark  17  is positioned roughly in the center of a detectable area φ of the mark sensor  16 . 
     The mark sensor  16  is a sensor that the light-sensitive element detects that a light emitted from the light-emitting element is reflected or absorbed on the basis of a color of a target object, and recognizes a color level of the target object according to an amount of light received. Therefore, when the ground color of the web W and the positioning mark  17  are both contained within the detectable area φ, an amount of light received, i.e., a color level varies according to the proportion of the positioning mark  17  in the detectable area φ. 
     Consequently, when the visual position adjustment is made, even though an operator believes that the positioning mark  17  is positioned in the center of the detectable area φ of the mark sensor  16 , the positioning mark  17  may deviate from the detectable area φ, or the position of the positioning mark  17  varies among operators, and therefore, a result of the sensitivity adjustment varies. 
       FIG. 9A  is a diagram illustrating a case in which the positioning mark  17  is positioned in the center of the detectable area φ of the mark sensor  16 ;  FIG. 9B  is a diagram illustrating a case in which the positioning mark  17  is not positioned in the center of the detectable area φ of the mark sensor  16 .  FIG. 10  is a diagram showing color levels of the ground color of the web W and the positioning mark  17  recognized by the mark sensor  16  and a threshold level in the cases shown in  FIGS. 9A and 9B . 
     With respect to an object of which the size is larger than the detectable area φ like the web W, it is easy to make a position adjustment, so a variation in color recognized by the mark sensor  16  is small. Namely, L 0 ≈L 0 ′ holds. On the other hand, when an object is small in size like the positioning mark  17 , it is difficult to arrange the object to fit perfectly in the detectable area φ of the mark sensor  16 . When a sensitivity adjustment is made in a state where the positioning mark  17  deviates from the detectable area φ as shown in  FIG. 9B , a color level L 1 ′ of the positioning mark  17  is in a relation of L 1 ′&gt;L 1 . Furthermore, when an exactly intermediate level of the color levels of the ground color of the web W and the positioning mark  17  is set as a threshold, a threshold level TH′ is in a relation of TH′&gt;TH. 
     In this manner, when a sensitivity adjustment is made in a state where the positioning mark  17  deviates from the detectable area φ, the mark sensor  16  recognizes the color of the positioning mark  17  as a color close to the ground color of the web W, so a contrast with the color of the web W is insufficient; therefore, if a disturbance, such as flapping of the web, is produced while the web is fed, there arises a problem in that the positioning mark  17  is incorrectly detected even though there is no positioning mark  17 . 
     Furthermore, to arrange the positioning mark  17  in the center of the detectable area φ of the mark sensor  16  using any of the above-described methods disclosed in Japanese Patent Application Laid-open No. 2002-207338, Japanese Patent Application Laid-open No. 2002-174936, and Japanese Patent Application Laid-open No. 2000-318221, data processing, such as an analog-to-digital (A/D) conversion of an output signal of the mark sensor  16 , is required and a circuit configuration becomes complex. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     An image forming apparatus according to an aspect of the present invention that forms an image on a web with reference to a position of a positioning mark on the web, includes: a moving unit that causes a relative movement of the web and a mark detecting unit in a predetermined direction, the mark detecting unit detecting a ground color of the web and a mark color which is a color of the positioning mark; a measuring unit that measures a measurement value corresponding to a moving amount of the mark detecting unit with respect to the web in a time from when a color detected by the mark detecting unit is switched from the ground color to the mark color till when a color detected by the mark detecting unit is back to the ground color; and a setting unit that sets a threshold used for determining whether the color detected is the ground color or the mark color on the basis of the mark color, which is detected by the mark detecting unit at the time when the mark detecting unit is moved in a reverse direction for a distance of half the measurement value since the color detected is back to the ground color. 
     A threshold setting method according to another aspect of the present invention executed in an image forming apparatus for forming an image on a web with reference to a position of a positioning mark on the web, includes: causing, by a moving unit, a relative movement of the web and a mark detecting unit, which detects a ground color of the web and a mark color that is a color of the positioning mark, in a predetermined direction; measuring, by a measuring unit, a measurement value corresponding to a moving amount of the mark detecting unit with respect to the web in a time from when a color detected by the mark detecting unit is switched from the ground color to the mark color till when a color detected by the mark detecting unit is back to the ground color; and setting, by a setting unit, a threshold used for determining whether the color detected is the ground color or the mark color on the basis of the mark color, which is detected by the mark detecting unit at the time when the mark detecting unit is moved in a reverse direction for a distance of half the measurement value since the color detected is back to the ground color. 
     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 longitudinal sectional view illustrating an outline of a structure of a printing device embodying one aspect of the present invention; 
         FIG. 2  is a plan view of a web shown in  FIG. 1 , and illustrates an example of positioning marks formed on the web; 
         FIG. 3  is a block diagram illustrating a configuration and a function for detecting the positioning mark, which are included in the printing device shown in  FIG. 1 ; 
         FIG. 4  is a graph showing a change in a color detection level of a mark sensor shown in  FIG. 1  when the positioning mark passes through a mark detectable field of view of the mark sensor; 
         FIG. 5  is a flowchart illustrating contents of a sensitivity adjustment mode and a check mode according to a first embodiment of the present invention using the configuration and the function for detecting the positioning mark shown in  FIG. 3 ; 
         FIG. 6  is a flowchart illustrating contents of a sensitivity adjustment mode according to a third embodiment of the present invention; 
         FIGS. 7A and 7B  are plan views illustrating two examples of relative sizes of a detectable area of a mark detecting unit and the positioning mark; 
         FIG. 8  is a plan view illustrating a scale used to set a mark position as a preliminary preparation at the time of sensitivity adjustment; 
         FIGS. 9A and 9B  are plan views illustrating two examples of the relative position of the positioning mark to the detectable area of the mark detecting unit; and 
         FIG. 10  is a plan view showing a level of a detection signal of the mark sensor and a set threshold level. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present embodiments, when the size of a selected positioning mark is too small or too large with respect to a detectable field of view of a mark detecting unit, a warning is issued. Therefore, when a portion of a business form selected as a positioning mark, such as a company name or logo, is inappropriate as a positioning mark, an operator can recognize this from the warning. 
     Other objects and features of the present invention will be revealed in the following description of the embodiments with reference to the accompanying drawings. 
     First Embodiment 
     An image forming apparatus according to a first embodiment has the same device configuration as that of the conventional image forming apparatus described above with reference to  FIGS. 1 and 2 .  FIG. 3  shows an outline of a web control device  21  included in the image forming apparatus. The web control device  21  embodies one aspect of the present invention. In  FIG. 3 , a mark sensor  16  as a mark detecting unit is a sensitivity-adjustable optical sensor composed of a light-emitting element and a light-sensitive element. 
     In a sensitivity adjustment, the mark sensor  16  detects a color of a positioning mark  17  that one wants to detect and a ground color of a web W, and an intermediate level of color levels of the two colors is set as a threshold. In the present embodiment, a sensor output after the sensitivity adjustment is an L level when the positioning mark  17  is detected and an H level when the ground color of the web W is detected, and an L-level sensor output is a mark detection signal. In image formation, this sensor output is subjected to waveform shaping and then given as a mark detection signal (a page timing signal) to an image formation control system. 
     To make the sensitivity adjustment, the web control device  21  includes a ground-color hold circuit  30 , a mark-color hold circuit  31 , an averaging circuit  32 , and a comparison circuit  33 . The ground-color hold circuit  30  holds a detection signal of the mark sensor  16  at the time when a ground-color hold switch  19  is pressed. The mark-color hold circuit  31  holds a detection signal of the mark sensor  16  at the time when a mark-color hold switch  29  is pressed. The averaging circuit  32  outputs a threshold level which is an intermediate-level (average-level) signal of hold signal levels of the hold circuits  30  and  31 . The comparison circuit  33  generates an H-level (ground color) binary signal (mark detection pulse) from a detection signal of the mark sensor  16  if a level of the detection signal of the mark sensor  16  is equal to or higher than the threshold level; the comparison circuit  33  generates an L-level (mark) binary signal from a detection signal of the mark sensor  16  if a level of the detection signal of the mark sensor  16  is lower than the threshold level. Incidentally, as another form, the hold circuits  30  and  31  can be replaced by an A/D converter and a memory (a register), and the averaging circuit  32  can be replaced by an average-value calculating circuit and a D/A converter. Furthermore, as still another form, not the web control device  21  but the mark sensor  16  can include functions of the hold circuits  30  and  31 , the averaging circuit  32 , and the comparison circuit  33 . 
     A sensitivity-adjustment-mode switch  20  is a switch for selectively specifying either a sensitivity adjustment mode in which the positioning mark  17  is arranged in the center of a detectable area φ of the mark sensor  16 , or a check mode in which a warning to an operator to change the positioning mark  17  is issued if the positioning mark  17  is small. A timer  22  measures a mark detection time of a mark detection signal output from the mark sensor  16 . A memory  24  is a memory for storing therein a measurement result of the mark detection time. A web-feed-speed switching unit  23  switches the web feed speed to a speed V 1  when the sensitivity adjustment mode or the check mode is selected. When an imaging unit  10  forms an image on a web W, the web feed speed is switched to a speed according to a web feed command from the image formation control system (not shown). A web-feed-drive control unit  25  controls forward rotation, stoppage, and reverse rotation of a feed-roller driving motor  27  for driving web feed rollers  8  and  9  shown in  FIG. 1  to rotate and also controls the driving speed of the feed-roller driving motor  27 . An error detecting unit  26  detects an error if a mark detection time is shorter than a prescribed value. An error display unit  28  warns an operator to change the positioning mark  17  in response to detection of the error. 
       FIG. 5  shows outlines of a sensitivity adjustment and a sensitivity check made by the web control device  21 . The sensitivity adjustment mode is a mode in which the positioning mark  17  is arranged in the center of the detectable area φ of the mark sensor  16  on the basis of an output signal of the mark sensor  16 ; as described above, to cause the mark sensor  16  to recognize the positioning mark  17 , it is necessary to set an appropriate threshold in advance. Therefore, as a preliminary preparation for the transition to the sensitivity adjustment mode, a coarse adjustment of the sensitivity using a scale  18  as shown in  FIG. 8  is made. This is for roughly adjusting the sensitivity of the mark sensor  16  in accordance with the colors of the positioning mark  17  and the web W. In the coarse adjustment, the web W is moved so that a whole field of view of the mark sensor  16  is made up of only the ground color portion, and the ground-color hold switch  19  is pressed thereby causing the ground-color hold circuit  30  to hold (memorize) a ground-color detection signal (voltage level) of the mark sensor  16 ; then, the web W is moved so that the positioning mark  17  is positioned roughly in the center of the field of view of the mark sensor  16 , and the mark-color hold switch  29  is pressed thereby causing the mark-color hold circuit  31  to hold a mark-color detection signal (voltage level) of the mark sensor  16 . This brings a threshold, an output of the averaging circuit  32 , to an intermediate level TH″ of the ground color level and the mark color level. Then, the web W is moved so that the positioning mark  17  is positioned on the downstream side of the mark sensor  16 . The preliminary preparation for the sensitivity adjustment mode is completed. 
     When the sensitivity-adjustment-mode switch  20  is pressed (turned on) and released from being held down within three seconds, an image forming system makes the transition to the sensitivity adjustment mode. Then, the web-feed-speed switching unit  23  sets the feed speed of the web W to the speed V 1 , and the feed of the web W is started, and then the mark sensor  16  starts detecting the positioning mark  17  (Steps S 1  and S 2 ). After that, when the positioning mark  17  formed on the web W reaches the detectable area φ of the mark sensor  16  and a color level detected by the mark sensor  16  falls below the threshold TH″ set in the coarse adjustment, a mark detection signal changes from the H level to the L level, and the timer  22  starts measuring a mark detection time (Steps S 3  and S 4 ). When the web W is further fed, and the positioning mark  17  passes through the detectable area φ of the mark sensor  16 , and then a color level detected by the mark sensor  16  exceeds the threshold TH″, a mark detection signal changes from the L level to the H level. At this timing, the measurement of the mark detection time is stopped (Steps S 5  and S 6 ), and a measurement result T 1  is stored in the memory  24 , and then the feed of the web W is stopped (Step S 7 ). After the feed of the web W is stopped, the web-feed-drive control unit  25  initiates the feed of the web W at the speed V 1  in a reverse direction this time, and at a timing when a mark detection signal changes from the H level to the L level, the timer  22  starts measuring a mark detection time (Step S 8 ). Then, when a measurement time of the timer  22  reaches a time T 1 / 2 , a mark detection signal of the mark sensor  16  is held in the mark-color hold circuit  31 , i.e., the mark-color hold circuit  31  updates the mark detection signal to a mark detection signal of the mark sensor  16  at the time, and the web-feed-drive control unit  25  stops the feed of the web W (Steps S 9  and S 10 ). When the mark detection signal held in the mark-color hold circuit  31  is updated to a mark detection signal of the mark sensor  16  at the time, the averaging circuit  32  gives a threshold corresponding to the updated mark detection signal to the comparison circuit  33 . At this time, the positioning mark  17  is positioned in the center of the sensor detectable area φ as shown in  FIG. 4 , so a contrast difference of the mark color with respect to the ground color of the web W is maximized. 
     That is the sensitivity adjustment mode. Incidentally, if the feed speed V 1  of the web W is high speed, it may be difficult to position the positioning mark  17  in the center of the sensor detectable area φ. This is due to failing to correctly measure the measurement time T 1  because the positioning mark  17  passes through the sensor detectable area φ while the feed speed of the web W is accelerated or due to a braking distance when the feed of the web W is stopped. Therefore, if the web feed speed at the time of image formation is high speed, it is preferable to set the speed V 1  to a slower rate than that is at the time of image printing. 
     Subsequently, the check mode in which a warning to an operator to change the positioning mark  17  is issued is explained with reference to a flowchart shown in  FIG. 5 . When the sensitivity-adjustment-mode switch  20  is pressed and held down for a prescribed time or longer, for example, for more than three seconds, the check mode is started. In the check mode, in the same manner as in the sensitivity adjustment mode, the timer  22  measures a detection time of the positioning mark  17 , and a measurement time t 1  is stored in the memory  24  (Steps S 11  to S 16 ). Then, the error detecting unit  26  compares the measurement time t 1  stored in the memory  24  with a preset criterion value (a set value) (Step S 17 ). If the measurement time t 1  is shorter than the criterion value, the error display unit  28  warns an operator to change the positioning mark  17  (Step S 18 ). 
     Here, the criterion value for determination of an error is explained with reference to  FIGS. 7A and 7B . Here, a detectable area of the mark sensor  16  in the web feed direction is denoted by φ 1 , the feed speed of the web W is denoted by V 1 , and a measurement time of the positioning mark  17  is denoted by t 1 . If a length of the positioning mark  17  in the web feed direction is smaller than φ 1  as shown in  FIG. 7A , a relation of “t 1 &lt;φ 1 /V 1 ” holds true; if a length of the positioning mark  17  in the web feed direction is larger than φ 1  as shown in  FIG. 7B , a relation of “t 1 &gt;φ 1 /V 1 ” holds true. Therefore, for example, when the mark detection time t 1  is in the relation of “t 1 &lt;φ 1 /V 1 ”, a warning to an operator to change the positioning mark  17  is issued. Incidentally, if the present check mode is implemented after implementation of the sensitivity adjustment mode, a more highly accurate warning can be issued. 
     Second Embodiment 
     In a second embodiment, the image forming apparatus includes a main-scanning drive mechanism for moving the mark sensor  16  in a main-scanning direction perpendicular to a sub-scanning direction, i.e., a feed direction of the web W at the time of image formation. In the sensitivity adjustment mode, the positioning mark  17  is arranged in the center of the detectable area φ in the main-scanning direction on the basis of an output signal of the mark sensor  16 . Namely, the mark sensor  16  is driven to move at a speed V 2  in the main-scanning direction perpendicular to the feed direction of the web W at the time of image formation (the sub-scanning direction); when a color detected by the mark sensor  16  is switched from the color of the positioning mark  17  to the ground color in accordance with the movement of the mark sensor  16 , the movement of the mark sensor  16  is stopped, and the mark sensor  16  is driven to move in the reverse direction at the speed V 2 ; a time T 2 , which is from when a color detected by the mark detecting unit is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the mark sensor  16  till when a color detected by the mark detecting unit is back to the ground color, is measured; when a color detected by the mark sensor  16  is back to the ground color, the mark sensor  16  is driven to move at the speed V 2  in the reverse direction; when a time T 2 / 2  has passed since a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the mark sensor  16 , the movement of the mark sensor  16  is stopped, and a color detected by the mark sensor  16  at the time, i.e., the mark color is used for setting of a threshold. 
     Also in the second embodiment, as a preliminary preparation for the transition to the sensitivity adjustment mode, a coarse adjustment of the sensitivity using the scale  18  as shown in  FIG. 8  is made. This is for roughly adjusting the sensitivity of the mark sensor  16  in accordance with the colors of the positioning mark  17  and the web W. In the coarse adjustment, the web W is moved in the sub-scanning direction so that a whole field of view of the mark sensor  16  is made up of only the ground color portion, and the ground-color hold switch  19  is pressed thereby causing the ground-color hold circuit  30  to hold (memorize) a ground-color detection signal (voltage level) of the mark sensor  16 ; then, the web W is moved in the sub-scanning direction and the mark sensor  16  is moved in the main-scanning direction so that the positioning mark  17  is positioned roughly in the center of the field of view of the mark sensor  16 , and the mark-color hold switch  29  is pressed thereby causing the mark-color hold circuit  31  to hold a mark-color detection signal (voltage level) of the mark sensor  16 . This brings a threshold, an output of the averaging circuit  32 , to an intermediate level TH″ of the ground color level and the mark color level. Then, the mark sensor  16  is moved in the main-scanning direction so that the mark sensor  16  is positioned on the upstream side of the positioning mark  17 . The preliminary preparation for the sensitivity adjustment mode is completed. 
     When the sensitivity-adjustment-mode switch  20  is pressed (turned on) and released from being held down within three seconds, the image forming system makes the transition to the sensitivity adjustment mode. Then, a sensor drive circuit starts driving the mark sensor  16  to move at the speed V 2  in a forth direction toward the positioning mark  17 , and the mark sensor  16  starts detecting the positioning mark  17 . After that, when the detectable area φ of the mark sensor  16  reaches the positioning mark  17  on the web W and a color level detected by the mark sensor  16  falls below the threshold TH″ set in the coarse adjustment, a mark detection signal changes from the H level to the L level and the timer  22  starts measuring a mark detection time. When the sensor drive circuit further drives the mark sensor  16  to move in the main-scanning direction, and the detectable area φ of the mark sensor  16  passes through the positioning mark  17 , and then a color level detected by the mark sensor  16  exceeds the threshold TH″, a mark detection signal changes from the L level to the H level. At this timing, the measurement of the mark detection time T 2  is stopped, and a measurement result T 2  is stored in the memory  24 , and then the movement of the mark sensor  16  in the main-scanning direction is stopped. After the movement of the mark sensor  16  is stopped, the mark sensor  16  is driven to move in a back direction of the main-scanning direction this time, and at a timing when a mark detection signal changes from the H level to the L level, the timer  22  starts measuring a mark detection time. Then, when a measurement time of the timer  22  reaches a time T 2 / 2 , a mark detection signal of the mark sensor  16  is held in the mark-color hold circuit  31 , i.e., the mark-color hold circuit  31  updates the mark detection signal to a mark detection signal of the time, and the movement of the mark sensor  16  in the main-scanning direction is stopped. When the mark detection signal held in the mark-color hold circuit  31  is updated to a mark detection signal of the mark sensor  16  at the time, the averaging circuit  32  gives a threshold corresponding to the updated mark detection signal to the comparison circuit  33 . At this time, the positioning mark  17  is positioned in the center of the sensor detectable area φ in the main-scanning direction as shown in  FIG. 4 , so a contrast difference of the mark color with respect to the ground color of the web W is maximized. 
     That is the sensitivity adjustment mode. The check mode, in which a warning to an operator to change the positioning mark  17  is issued, is started when the sensitivity-adjustment-mode switch  20  is pressed and held down for a prescribed time or longer, for example, for more than three seconds. In the check mode, in the same manner as in the sensitivity adjustment mode, the timer  22  measures a detection time of the positioning mark  17 , and a measurement time t 2  is stored in the memory  24 . Then, the error detecting unit  26  compares the measurement time t 2  stored in the memory  24  with a preset criterion value (a set value). If the measurement time t 2  is shorter than the criterion value, the error display unit  28  warns an operator to change the positioning mark  17 . Here, a detectable area of the mark sensor  16  in the web feed direction is denoted by φ 1 , the main-scanning-direction moving speed of the mark sensor  16  is denoted by V 2 , and a measurement time of the positioning mark  17  is denoted by t 2 . If a width of the positioning mark  17  in the main-scanning direction is smaller than φ 1 , a relation of “t 2 &lt;φ 1 /V 2 ” holds true; if a width of the positioning mark  17  in the main-scanning direction is larger than φ 1 , a relation of “t 2 &gt;φ 1 /V 2 ” holds true. Therefore, for example, when the mark detection time t 2  is in the relation of “t 2 &lt;φ 1 /V 2 ”, a warning to an operator to change the positioning mark  17  is issued. Incidentally, if the present check mode is implemented after implementation of the sensitivity adjustment mode, a more highly accurate warning can be issued. The other configurations and functions in the second embodiment are identical to those in the first embodiment. 
     Third Embodiment 
     In a third embodiment, the positioning mark  17  is positioned in the center of a viewing field area φ of the mark sensor  16  in the sub-scanning direction in the same manner as in the first embodiment, and then the mark sensor  16  is positioned in the center of the viewing field area φ in the main-scanning direction in the same manner as in the second embodiment, and then a detected color signal of the mark sensor  16  at the time is held in the mark-color hold circuit  31 , and the mark sensor  16  is kept at the position (a main-scanning-direction optimum position). Namely, in the third embodiment, the web W is driven to move at the speed V 1 ; a time T 1 , which is from when a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the web W till when a color detected by the mark sensor  16  is back to the ground color, is measured; when a color detected by the mark sensor  16  is back to the ground color, the web W is driven to move in the reverse direction at the speed V 1 ; when a time T 1 / 2  has passed since a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the web W, the movement of the web W is stopped, and the mark sensor  16  is driven to move in the main-scanning direction perpendicular to the feed direction of the web W (the sub-scanning direction); when a color detected by the mark sensor  16  is switched from the color of the positioning mark  17  to the ground color in accordance with the movement of the mark sensor  16 , the movement of the mark sensor  16  is stopped, and the mark sensor  16  is driven to move in the reverse direction at the speed V 2 ; a time T 2 , which is from when a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the mark sensor  16  till when a color detected by the mark sensor  16  is back to the ground color, is measured; when a color detected by the mark sensor  16  is back to the ground color, the mark sensor  16  is driven to move in the reverse direction at the speed V 2 ; when a time T 2 / 2  has passed since a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the mark sensor  16 , the movement of the mark sensor  16  is stopped, and a color detected by the mark sensor  16  at the time, i.e., the mark color is used for setting of a threshold; the mark sensor  16  is kept at the position, and used for detection of the subsequent positioning mark  17 . 
       FIG. 6  shows contents of SAMa, the “sensitivity adjustment mode” according to the third embodiment. In this “sensitivity adjustment mode” SAMa, first, the sensitivity adjustment mode SAM according to the first embodiment (Steps S 1  to S 10 ) is executed, and the positioning mark  17  on the web W is positioned in the center of the viewing field area φ of the mark sensor  16  in the sub-scanning direction; then, in the same manner as the sensitivity adjustment mode according to the second embodiment, the mark sensor  16  is driven to move in the main-scanning direction perpendicular to the feed direction of the web W (the sub-scanning direction); when a color detected by the mark sensor  16  is switched from the color of the positioning mark  17  to the ground color, the movement of the mark sensor  16  is stopped, and the mark sensor  16  is driven to move at the speed V 2  in the reverse direction; a time T 2 , which is from when a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  till when a color detected by the mark sensor  16  is back to the ground color, is measured; when a color detected by the mark sensor  16  is back to the ground color, the mark sensor  16  is driven to move at the speed V 2  in the reverse direction; when a time T 2 / 2  has passed since a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17 , the movement of the mark sensor  16  is stopped, and a color detected by the mark sensor  16  at the time, i.e., the mark color is used for setting of a threshold (Steps S 21  to S 32 ); the mark sensor  16  is kept at the position, and used for detection of the subsequent positioning mark  17 . 
     In the check mode according to the third embodiment in which a warning to an operator to change the positioning mark  17  is issued, whether the measurement times T 1  and T 2  are within respective setting ranges is determined, and an operator is informed of an error if the measurement times T 1  and T 2  are out of the setting ranges. If the check mode is implemented after implementation of the sensitivity adjustment mode, a more highly accurate warning can be issued. The other configurations and functions in the third embodiment are identical to those in the first embodiment. 
     Fourth Embodiment 
     In a fourth embodiment, moving amounts P 1  (corresponding to a moving amount in the time T 1 ) and p 1  (corresponding to a moving amount in the time t 1 ) of the web W are measured instead of measurements of the times T 1  and t 1  made in the first embodiment. To transfer a toner image formed on the photosensitive drum  101  of the imaging unit  10  onto a predetermined position of the web W based on the positioning mark  17  formed on the web W, the image forming apparatus ( FIG. 1 ) produces one feed synchronization pulse (timing pulse) with each movement of a predetermined short distance of the web W in synchronization with the feed of the web W. Then, the image forming apparatus starts counting the number of the feed synchronization pulses (measuring a moving amount of the web W) each time the mark sensor  16  detects the positioning mark  17 , and the count value is referenced in the image formation control. In the fourth embodiment, the moving amounts P 1  and p 1  are measured by counting the number of the feed synchronization pulses. Then, in the fourth embodiment, a moving amount P 1  of the web W in a time from when a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the web W till when a color detected by the mark sensor  16  is back to the ground color is measured; when a color detected by the mark sensor  16  is back to the ground color, the web W is driven to move in the reverse direction; a color detected by the mark sensor  16  at the time when a moving amount of the web W since a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  is P 1 / 2 , i.e., the mark color is used for setting of a threshold. The other configurations and functions are identical to those in the first embodiment. 
     Fifth Embodiment 
     In a fifth embodiment, moving amounts P 2  (corresponding to a moving amount in the time T 2 ) and p 2  (corresponding to a moving amount in the time t 2 ) of the mark sensor  16  in the main-scanning direction are measured instead of measurements of the times T 2  and t 2  made in the second embodiment. Namely, in the fifth embodiment, an encoder is installed on a drive shaft of the main-scanning drive mechanism described in the second embodiment; the mark sensor  16  is driven to move in the main-scanning direction, and moving amounts P 2  and p 2  of the mark sensor  16  in the main-scanning direction are measured by counting the number of pulse signals generated by the encoder. A count value, i.e., a moving amount P 2  of the mark sensor  16  in a time from when a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the mark sensor  16  till when a color detected by the mark sensor  16  is back to the ground color is measured; when a color detected by the mark sensor  16  is back to the ground color, the mark sensor  16  is driven to move in the reverse direction; a color detected by the mark sensor  16  at the time when a moving amount of the mark sensor  16  since a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  is P 2 / 2 , i.e., the mark color is used for setting of a threshold. The other configurations and functions are identical to those in the second embodiment. 
     Sixth Embodiment 
     In a sixth embodiment, the positioning mark  17  is positioned in the center of the viewing field area φ of the mark sensor  16  in the sub-scanning direction in the same manner as in the fourth embodiment, and then the mark sensor  16  is positioned in the center of the viewing field area φ in the main-scanning direction in the same manner as in the fifth embodiment, and a detected color signal of the mark sensor  16  at the time is held in the mark-color hold circuit  31 , and the mark sensor  16  is kept at the position (the main-scanning-direction optimum position). Namely, in the sixth embodiment, the web W is driven to move, and a moving amount P 1  of the web W in a time from when a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the web W till when a color detected by the mark sensor  16  is back to the ground color is measured; when a color detected by the mark sensor  16  is back to the ground color, the web W is driven to move in the reverse direction; when a moving amount of the web W since a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  is P 1 / 2 , the movement of the web W is stopped, and the mark sensor  16  is driven to move in the main-scanning direction perpendicular to the feed direction of the web W (the sub-scanning direction); when a color detected by the mark sensor  16  is switched from the color of the positioning mark  17  to the ground color in accordance with the movement of the mark sensor  16 , the movement of the mark sensor  16  is stopped, and the mark sensor  16  is driven to move in the reverse direction; a moving amount P 2  of the mark sensor  16  in the main-scanning direction in a time from when a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17  in accordance with the movement of the mark sensor  16  till when a color detected by the mark sensor  16  is back to the ground color is measured; when a color detected by the mark sensor  16  is back to the ground color, the mark sensor  16  is driven to move in the reverse direction; when the mark sensor  16  is moved for a distance P 2 / 2  since a color detected by the mark sensor  16  is switched from the ground color to the color of the positioning mark  17 , the movement of the mark sensor  16  is stopped, and a color detected by the mark sensor  16  at the time, i.e., the mark color is used for setting of a threshold; the mark sensor  16  is kept at the position, and used for detection of the subsequent positioning mark  17 . 
     According to the present invention, a positioning mark is set in the center of a detectable area of a mark detecting unit, a color of the positioning mark is detected, and the detected color is used for setting of a threshold; therefore, a contrast difference between the color of the positioning mark and a ground color of a web is maximized. Consequently, even if a disturbance, such as flapping of the web, is produced while the web is fed, there is no false detection of the positioning mark. 
     Although the invention has been described with respect to specific embodiments 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.