Abstract:
A patch mark-forming unit forms a first patch mark at a first density on a surface. A light emitting unit emits an incident light onto the surface moving. The incident light reflected by the surface is divided into a mirror-reflected light and a diffusion-reflected light on the surface. A first detecting unit detects an amount of the diffusion-reflected light. The patch mark forming unit reforms a second patch mark at a second density weaker than the first density it the amount detected by the first detecting unit is larger than a threshold. A second detecting unit detects an amount of the mirror-reflected light reflected by the surface on which the second patch mark has been reformed. A position calculating unit calculates, based on the amount detected by the second detecting unit, a position on the surface at which an image should be formed. An image-forming unit forms an image at the position.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from Japanese Patent Application No. 2008-050262 filed Feb. 29, 2008 and No. 2009-32605 filed Feb. 16, 2009. The entire content of each of these priority applications is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an image-forming device capable of correcting displacements of images. 
       BACKGROUND 
       [0003]    Conventional color image-forming device forms correction patch marks of various colors on a rotating member such as a conveying belt, and detects the positions of the correction patch marks to correct the density of each color image and the displacement of different-color images. In such conventional image-forming device, the positions of the correction patch marks are detected by detecting the infrared beam reflected by the rotating member. Further, Japanese Patent Application Publication No. H09-152796 discloses an image-forming device that changes transfer voltage between when forming the correction patch marks and when forming images on a recording medium such as a paper sheet, so that developer is transferred to each object at highest possible efficiency. 
       SUMMARY 
       [0004]    However, the image-forming device disclosed in Japanese Patent Application Publication No. H09-152796 does not set the appropriate transfer voltage of the correction patch marks in view of the influence of the diffusion-reflected light. As the density of the developer other than black developer increases, the diffusion-reflected light can increase. Due to the increased diffusion-reflected light, the image-forming device disclosed in Japanese Patent Application Publication No. H09-152796 cannot detect the densities of the correction patch marks at high accuracy. Thus, the displacements of images cannot be appropriately corrected. 
         [0005]    In view of the above-described drawbacks, it is an objective of the present invention to provide an image-forming device that can suppress the influence of diffusion-reflected light that occurs when detecting the correction patch marks, in order to appropriately correct the displacements of images. 
         [0006]    In order to attain the above and other objects, the present invention provides an image-forming device including a moving member having a surface movable, a patch mark-forming unit, a light emitting unit, a first detecting unit, a density controlling unit, a second detecting unit, a position calculating unit, and an image-forming unit. The patch mark-forming unit forms a first patch mark at a first density on the surface. The light emitting unit emits an incident light onto the surface moving, at an incident angle for the surface. The incident light reflected by the surface is divided into a mirror-reflected light and a diffusion-reflected light on the surface. The mirror-reflected light is reflected by the surface at a reflected angle equal to the incident angle. The first detecting unit detects an amount of the diffusion-reflected light. The density controlling unit controls the patch mark forming unit to reform a second patch mark at a second density weaker than the first density if the amount detected by the first detecting unit is larger than a threshold. The second detecting unit detects an amount of the mirror-reflected light reflected by the surface on which the second patch mark has been reformed. The position calculating unit calculates, based on the amount detected by the second detecting unit, a position on the surface at which an image should be formed. The image-forming unit forms an image at the position. 
         [0007]    Another aspect of the present invention provides an image displacement correcting method. The method includes: forming a first patch mark at a first density on a surface; emitting an incident light onto the surface moving, at an incident angle for the surface, the incident light reflected by the surface being divided into a mirror-reflected light and a diffusion-reflected light on the surface, the mirror-reflected light being reflected by the surface at a reflected angle equal to the incident angle; detecting an amount of the diffusion-reflected light; reforming a second patch mark at a second density weaker than the first density if the detected amount of the diffusion-reflected light is larger than a threshold; detecting an amount of the mirror-reflected light reflected by the surface on which the second patch mark has been reformed; calculating, based on the detected amount of the mirror-reflected light, a position on the surface at which an image should be formed; and forming an image at the position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
           [0009]      FIG. 1  is a sectional side view schematically showing the configuration of a laser printer according to the present invention; 
           [0010]      FIG. 2  is a diagram schematically illustrating the configuration of a print density sensor incorporated in the laser printer; 
           [0011]      FIG. 3  is a circuit diagram showing the electrical configuration of the print density sensor; 
           [0012]      FIG. 4  is a block diagram showing the configuration of a control system of the laser printer; 
           [0013]      FIG. 5  is a flowchart explaining an automatic registration that the control system performs; 
           [0014]      FIG. 6  is a flowchart explaining, in detail, the process of setting threshold values in first and first sensors to perform the automatic registration; 
           [0015]      FIG. 7A  is a diagram showing a conveying belt that is rough with scratches; 
           [0016]      FIG. 7B  is a diagram showing changes of potentials of the second sensor with respect to the infrared beam reflected by the conveying belt shown in  FIG. 7A ; 
           [0017]      FIG. 7C  is a diagram showing changes of potentials of the first sensor with respect to the infrared beam reflected by the conveying belt shown in  FIG. 7A ; 
           [0018]      FIG. 8A  is a diagram showing the conveying belt that is rough with scratches and formed with correction patch marks; 
           [0019]      FIG. 8B  is a diagram showing changes of potentials of the second sensor with respect to the infrared beam reflected by the conveying belt shown in  FIG. 8A ; 
           [0020]      FIG. 8C  is a diagram showing changes of potentials of the first sensor with respect to the infrared beam reflected by the conveying belt shown in  FIG. 8A ; 
           [0021]      FIG. 9A  is a graph explaining a relation between a transmission density of a toner that forms the correction patch marks and an amount of the infrared beam detected by the first sensor; 
           [0022]      FIG. 9B  is a graph explaining a relation between a transmission density of a toner that forms the correction patch marks and an amount of the infrared beam detected by the second sensor; 
           [0023]      FIG. 10  is a flowchart explaining an interruption process that is performed when a cover is opened; 
           [0024]      FIG. 11  is a flowchart explaining an interruption process that is performed when a belt is replaced; and 
           [0025]      FIG. 12  is a flowchart explaining an interruption process that is performed when a print job is generated. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    An embodiment of the present invention will be described with reference to the accompanying drawings. In the embodiment described below, the present invention is applied to a laser printer connected to a computer for use. 
       1. OUTER APPEARANCE OF LASER PRINTER 
       [0027]      FIG. 1  is a sectional side view schematically showing the configuration of the laser printer  1 . The laser printer  1  is installed with the top turned upward in the direction of gravity, as is illustrated in  FIG. 1 . In most cases, the laser printer  1  is positioned, with the right side in  FIG. 1  set toward the user. The laser printer  1  has a housing  3 , which is shaped like a box (cube). On the top of the housing  3 , a discharge tray  5  is provided to hold recording sheets (recording media), such as paper sheets or OHP sheets that have been discharged from the housing  3  after data has been printed an them. 
         [0028]    In the present embodiment, a frame member (not shown) made of metal or resin is provided in the inside of housing  3 . A process cartridge  70  described later and a fixing unit  80  are detachably mounted on the frame member. 
         [0029]    The discharge tray  5  has an inclining surface  5   a  that is inclined downwardly from the front toward the rear of the upper surface of the housing  3 . At the rear end of the inclining surface Sa, a discharge unit  7  is provided to discharge any recording sheet on which data has been printed. 
       2. INTERNAL MECHANICAL CONFIGURATION OF LASER PRINTER 
       [0030]    The laser printer  1  has an image-forming unit  10  for forming images on recording sheets, a feeding unit  20  for feeding recording sheets to the image-forming unit  10 , and a conveying mechanism  30  for conveying a recording sheet. 
         [0031]    The laser printer  1  has a print density sensor  90  for detecting correction patch marks formed on a conveying belt  33  described later. A recording sheet on which an image is formed by the image-forming unit  10  is turned upward in an discharging chute (not shown), and then discharged from the discharge unit  7  onto the discharge tray  5 . 
       2.1. Configuration of Feeder 
       [0032]    The feeding unit  20  includes a feeding tray  21 , a feeding roller  22 , and a separation pad  23 . The feeding tray  21  is provided in the lowermost part of the housing  3 . The feeding roller  22  is located above the front edge of the feeding tray  21  to feed a recording sheet from the feeding tray  21  to the image-forming unit  10 . The separation pad  23  is positioned on a part opposing to the feeding roller  22  to apply a prescribed feeding resistance to a topmost recording sheet, thereby separating the topmost sheet from any other recording sheet. 
         [0033]    On the feeding tray  21 , the recording sheet is U-turned in the front part of the housing  3  and conveyed to the image-forming unit  10  arranged in a middle part of the housing  3 . A sheet-conveying path extends from the feeding tray  21  to the discharge tray  5 . A conveying roller  24  is arranged at one part of the sheet-conveying path, where the sheet is U-turned. The conveying roller  24  feeds the sheet toward the image-forming unit  10 . 
         [0034]    A pressing roller  25  is arranged at a part opposing to the conveying roller  24  across the recording sheet to press the recording sheet onto the conveying roller  24 . Specifically, an elastic member such as a coil spring  25   a  biases the pressing roller  25  toward the conveying roller  24 . 
       2.2. Configuration of Conveying Mechanism 
       [0035]    The conveying mechanism  30  includes a driving roller  31 , a driven roller  32 , and a conveying belt  33 . The driving roller  31  rotates as the image-forming unit  10  operates. The driven roller  32  is rotatably provided spaced apart from the driving roller  31 . The conveying belt  33  is wrapped around the driving roller  31  and the driven roller  32 . The recording sheet conveyed from the feeding tray  21  to the conveying belt  33  is conveyed to the four process cartridges  70 K,  70 Y,  70 M and  70 C, from each cartridge to the next one. The conveying mechanism  30 , that is, the driving roller  31 , the driven roller  32 , and the conveying belt  33  are detachable integrally by opening the upper cover of the housing  3 . Below the conveying belt  33 , a belt cleaner  34  described later is arranged to clean the correction patch marks from the surface of the conveying belt  33 . 
       2.3. Configuration of Image-Forming Unit 
       [0036]    The image-forming unit  10  includes a scanner unit  60 , a process cartridge  70 , and a fixing unit  80 . The image-forming unit  10  of this embodiment is a direct tandem type that can accomplish color printing. The process cartridge  70  has the process cartridges  70 K,  70 Y,  70 M and  70 C containing black toner, yellow toner, magenta toner and cyan toner, respectively. The process cartridges  70 K,  70 Y,  70 M and  70 C are arranged in the mentioned order from upstream side in a conveying direction of the sheets. The process cartridges  70 K,  70 Y,  70 M and  70 C have same structures with each other, except for colors of the toners (developers) Hereinafter, the four process cartridges  70 K,  70 Y,  70 M and  70 C will be generally referred to as the process cartridge  70 . 
         [0037]    The scanner unit  60  includes a laser beam source, a polygon mirror, an fθ lens, and a reflector to form an electrostatic latent image on each photosensitive drum  71  of the respective process cartridges  70 K,  70 Y,  70 M and  70 C. 
         [0038]    The process cartridge  70  is detachably mounted on the housing  3  below the scanner unit  60 . The process cartridge  70  has the photosensitive drum  71 , a charger  72 , a transfer roller  73 , and a developer cartridge  74  having a developing roller  74   a.    
         [0039]    The fixing unit  80  is arranged downstream of the photosensitive drum  71  in the conveying direction. The fixing unit  80  includes a heating roller  81  and a pressing roller  82  opposing to the heating roller  81  across the recording sheet. The heating roller  81  feeds a recording sheet forward, while heating the toner applied to the sheet. The pressing roller  82  presses the sheet onto the heating roller  81 . Thus, an image formed on the recording sheet is fixed. 
         [0040]    As the photosensitive drum  71  rotates, the surface thereof is positively and uniformly charged by the charger  72 . The surface is then scanned at high speed with the laser beam emitted from the scanner unit  60 . The part of the surface exposed to the laser beam therefore has a lower potential than the part not exposed. An electrostatic latent image that corresponds to an image to be formed on the recording sheet is therefore formed on the surface of the photosensitive drum  71 . 
         [0041]    Next, a development bias is applied to the developing roller  74   a , while rotating the developing roller  74   a  provided in the process cartridge  70 . The toner positively charged is supplied from the developing roller  74   a  to the surface of the photosensitive drum  71  positively and uniformly charged, which is exposed to the laser beam and has a lower potential. The electrostatic latent image on the photosensitive drum  71  is thereby changed to a visible image. That is, inverse development is achieved, forming a toner image on the surface of the photosensitive drum  71 . 
         [0042]    Thereafter, the toner image is transferred from the surface of the photosensitive drum  71  to a recording sheet, because of the transfer bias applied to the transfer roller  73 . The recording sheet on which the toner image is formed is conveyed to the fixing unit  80 . The fixing unit  80  heats the recording sheet, fixing the toner to the recording sheet. The image is thereby formed (printed) on the recording sheet. 
       2.4. Configuration of Print Density Sensor 
       [0043]      FIG. 2  is a diagram schematically illustrating the configuration of the print density sensor  90 . As shown in  FIG. 2 , the print density sensor  90  includes an infrared light-emitting diode  93 , a first sensor  91 , and a second sensor  92 . The infrared light-emitting diode  93  emits an infrared beam to the conveying belt  33  at an incidence angle θ 1 . The second sensor  92  detects the amount (intensity) of the infrared beam reflected by the conveying belt  33  at a reflection angle θ 2  equal to the incidence angle θ 1 . The first sensor  91  detects the amount (intensity) of the infrared beam reflected by the conveying belt  33  at a reflection angle different from the incidence angle θ 1 . 
         [0044]    The conveying belt  33  is made from a film in which carbon is dispersed. Therefore, the conveying belt  33  has electrical property for transferring toner, and the surface of the conveying belt  33  appears as black and is highly glossy. Since the surface of the conveying belt  33  is highly glossy, the conveying belt  33  causes mush mirror-reflected light. Since the surface of the conveying belt  33  appears as black, the conveying belt  33  can absorb infrared light and scarcely cause the diffusion-reflected light. When the diffusion-reflected light does not occur, the infrared beam is reflected only at the reflection angle θ 2  as the mirror-reflected light. Hence, when correction patch marks are not formed on the conveying belt  33 , the second sensor  92  detects strong reflected light, whereas the first sensor  91  scarcely detects reflected light. 
         [0045]    On the other hand, when correction patch marks are formed on the conveying belt  33 , the infrared beam reflected by the correction patch marks is divided into the mirror-reflected light and the diffusion-reflected light. Therefore, the first sensor  91  detects the reflected light reflected at the reflection angle different from the incidence angle θ 1 , whereas the second sensor  92  detects decreased reflected light. 
         [0046]    In this embodiment, the correction patch marks are monochrome images, and black toner, cyan toner, magenta toner and yellow toner are transferred to the conveying belt  33 , forming black, cyan, magenta and yellow correction patch marks, each shaped like a strip. 
         [0047]      FIG. 3  is a circuit diagram showing the electrical configuration of the print density sensor  90 . Note that the second sensor  92  and the first sensor  91  are identical in electrical configuration. Therefore, one of the electrical configurations thereof is shown in  FIG. 3 . 
         [0048]    As shown in  FIG. 3 , transistors Tr 1  and Tr 2  that compose an amplifier are turned on or off in response to a signal sen_led_on inputtted from a control unit  100  described later. When 3.3 v is applied from a DC power supply Vcc to the infrared light-emitting diode  93  through the amplifier circuit, the infrared light-emitting diode  93  emits the infrared beam. The second sensor  92  and the first sensor  91  are phototransistors, which are connected to a DC power supply Vcc of 3.3V via a variable resistor VR and a resistor R 1 , so that the electric current corresponding to the amount of received light passes through the variable resistor VR and the resistor R 1 . Therefore, as the amount of received light increases, voltage drops at the resistor R 1  and variable resistor VR, and the potential of the point A in  FIG. 3  decreases. This potential difference is input to a comparator  95 , and the comparator  95  compares the potential difference with a signal reg_mark_pwm input from the control unit  100 . 
         [0049]    Signal reg_mark_pwm is a PWM signal. The signal is smoothed by a smoothing circuit composed of a resistor R 3  and a capacitor C 1 . The signal thus smoothed is inputted into the comparator  95  via a resistor R 5 . Therefore, if the signal reg_mark_pwm corresponding to a prescribed threshold value is inputted into the comparator  95 , the comparator  95  can output a detection signal reg_mark_sen that rises to H level when the amount of received light the second sensor  92  (first sensor  91 ) has received exceeds the threshold value. 
       3. CONTROL SYSTEM OF LASER PRINTER 
       [0050]      FIG. 4  is a block diagram showing the configuration of the control system of the laser printer  1 . As shown in  FIG. 4 , the second sensor  92 , the first sensor  91  and the infrared light-emitting diode  93 , which constitute the print density sensor  90 , are connected to the control unit  100 , along with the above-mentioned image-forming unit  10  and a high-voltage power supply  99 . The high-voltage power supply  99  applies a development bias to the developing roller  74   a . The control unit  100  is composed mainly of a microprocessor that has a CPU  101 , a ROM  102  and a RAM  103 . The control unit  100  controls the image-forming unit  10 , the high-voltage power supply  99 , etc., as will be described later, in accordance with programs stored in the ROM  102 . A cover sensor  110 , a belt sensor  120 , and a display unit  130 , all being of known types, are connected to the control unit  100 . The cover sensor  110  detects the open of the upper cover of the housing  3 . The belt sensor  120  detects that the conveying belt  33  is mounted. The display unit  130  is provided on the surface of the housing  3 . 
       4. CONTROL PERFORMED BY CONTROL SYSTEM 
       [0051]    An automatic registration performed by the control unit  100  will be explained.  FIG. 5  is a flowchart explaining the automatic registration. In the automatic registration, the correction patch marks are formed on the conveying belt  33 , the positions of the correction patch marks are detected, and then the displacement of different-color images are corrected based on the detected positions of the correction patch marks. The automatic registration is started when, for example, the power switch of the laser printer  1  is turned on, as known in the art. 
         [0052]    As shown in  FIG. 5 , in Step S 1 , the control unit  100  increments a variable RN by one. The variable RN indicates the number of times the automatic registration has been performed since threshold values have been set in Step S 6  in the latest time. In other words, the variable RN is not reset even if the automatic registration is ended, unless the threshold values are set. 
         [0053]    In Step S 2 , the control unit  100  determines whether or not the variable RN has exceeded a predetermined value RN_S. If RN≧RN_S (Yes in S 2 ), in Step S 3  the control unit  100  sets flag SS to 1, and then, the operation goes to Step S 4 . When the flag SS is 1, the threshold values are set in Step S 6  described later. On the other hand, if RN&lt;RN_S (No in S 2 ), the operation goes to Step S 4 . That is, if RN&lt;RN_S (No in S 2 ), the threshold values are not set in Step S 6  since the flag SS is not set to 1 in Step S 3 . 
         [0054]    In Step S 4 , the control unit  100  corrects the sensitivity of the print density sensor  90  based on the surface condition of the conveying belt  33 . Specifically, the control unit  100  controls the infrared light-emitting diode  93  to emit the infrared light onto the conveying belt  33  on which the correction patch mark is not formed, and sets a resistance value of the variable resistor VR so that the potentials inputted from the first and first sensors  92  and  91  to the comparator  95  are saturated. Hereinafter, these potentials will be referred to as a potential of the sensor  91  and a potential of the sensor  92 . 
         [0055]    In Step S 5 , the control unit  100  determines the development bias DbB for the correction patch mark by using the equation of DbE=DbP×P 1 . In this equation, DbP is development bias applied when forming an image on the recording sheet, and P 1  is a correction coefficient. P 1  is set to prescribed initial value P 0  at first. 
         [0056]    If the surface of the conveying belt  33  is rough with scratches, the amount of the infrared light detected by the first sensor  91  and the second sensor  92  are not accurate. Therefore, in Step S 6 , the control unit  100  sets the threshold values of the first and second sensors  91  and  92  in view of scratches of the conveying belt  33 .  FIG. 6  is a flowchart explaining, in detail, this process of setting threshold value R 1  and threshold value R 2  in Step S 6 . 
         [0057]    In Step S 61 , the control unit  100  determines whether or not the flag SS is set to 1. If the flag SS≠1 (No in S 61 ), the process goes to Step S 7  of  FIG. 5 . That is, the threshold values are not set, since the flag SS is set to 0. 
         [0058]    On the other hand, if the flag SS=1 (Yes in S 61 ), in Step S 62 , the control unit  100  sets the flag SS and the variable RN to 0, and then, in Step S 63 , the control unit  100  controls the conveying belt  33  to rotate one turn, controlling the infrared light-emitting diode  93  to emit the infrared beam onto the conveying belt  33 , without forming the correction patch marks, in order to acquire waveforms signals indicating changes of the potentials of the first sensor  91  and the second sensors  92 . The potential of the first sensor  91  is identical to the potential between the first sensor  91  and the variable resistor VR in  FIG. 3 . The potential of the second sensor  92  is identical to the potential between the second sensor  92  and the variable resistor VR in  FIG. 3 . 
         [0059]    In Step S 64 , the control unit  100  calculates the threshold value R 1  of the first sensor  91  and the threshold value R 2  of the second sensor  92 , using the following equations, and the process goes to Step S 7  in  FIG. 5   
         [0000]        R 1= RB 1_min− RB 1 
         [0000]        R 2= RB 2_max+ RB 2 
         [0060]    where RB 1 _min is the minimum potential acquired by the first sensor  91  in Step S 63 , RB 1  is a preset adjustment parameter, RB 2 _max is the maximum potential acquired by the second sensor  92  in Step S 63 , and RB 2  is a preset adjustment parameter. 
         [0061]      FIG. 7A  is a diagram showing the conveying belt  33  that is rough with scratches.  FIG. 7B  is a diagram showing changes of the potentials of the second sensor  92  with respect to the infrared beam reflected by the conveying belt  33  shown in  FIG. 7A .  FIG. 7C  is a diagram showing changes of the potentials of the first sensor  91  with respect to the infrared beam reflected by the conveying belt shown in  FIG. 7A . 
         [0062]    If the surface of the conveying belt  33  is not rough with scratches and dust, and the like, most part of the infrared beam emitted from the infrared light-emitting diode  93  is mirror-reflected on the conveying belt  33  and detected by the second sensor  92 . However, if the surface of the conveying belt  33  is rough with scratches and dust, the infrared beam emitted from the infrared light-emitting diode  93  is also diffusion-reflected on scratches. Therefore, the amount of the infrared beam detected by the second sensor  92  is decreased in comparison with when the surface of the conveying belt  33  is not rough with scratches, causing the potential of the second sensor  92  increased as shown in  FIG. 7B . Further, if the surface of the conveying belt  33  is rough with scratches, the amount of the infrared beam detected by the first sensor  91  is increased in comparison with when the surface of the conveying belt  33  is not rough with scratches, causing the potential of the first sensor  91  decreased as shown in  FIG. 7C . 
         [0063]    As described above, if the surface of the conveying belt  33  is rough with scratches, the diffusion-reflected light can occur even if the correction patch mark is not formed on the conveying belt  33 . 
         [0064]    Therefore, in Step S 6 , the control unit  100  sets the threshold values R 1  and R 2  in view of the changes of the potentials of the first sensor  91  and the second sensor  92  that occur due to the scratches. Specifically, the control unit  100  sets the threshold R 1  to a value lower than the minimum potential corresponding to the maximum amount of the infrared beam detected by the first sensor  91 , and sets the threshold value R 2  to a value higher than the maximum potential corresponding to the minimum amount of the infrared beam detected by the second sensor  92 . 
         [0065]    In Step S 7  of  FIG. 5 , the control unit  100  controls the high-voltage power supply  99  to apply the development bias DbB determined in Step S 5  to the developing roller  74 A. Thus, the image-forming unit  10  forms the correction patch marks on the conveying belt  33 . In Step S 8 , the control unit  100  acquires the positions of the correction patch marks based on the potentials of the second sensor  92 . 
         [0066]      FIG. 8A  is a diagram showing the conveying belt  33  that is rough with scratches and formed with correction patch marks  300 Y and  300 M.  FIG. 8B  is a diagram showing changes of the potentials of the second sensor  92  with respect to the infrared beam reflected by the conveying belt  33  shown in  FIG. 8A  when the density of the correction patch marks is lower than the region indicated by two-dot dashed lines in  FIG. 9B  described later.  FIG. 8C  is a diagram showing changes of the potentials of the first sensor  91  with respect to the infrared beam reflected by the conveying belt  33  shown in  FIG. 8A  when the density of the correction patch marks is lower than the region indicated by two-dot dashed lines in  FIG. 9B  described later. 
         [0067]    The infrared beam reflected by the correction patch mark  300 Y is divided into the mirror-reflected light and the diffusion-reflected light. Therefore, the amount of the infrared beam reflected by the correction patch  300 Y and detected by the second sensor  92  is smaller than the amount of the infrared beam reflected by the conveying belt  33  that is not rough with scratches and detected by the second sensor  92 . Thus, as shown in  FIG. 8B , the potential of the second sensor  92  with respect to the infrared beam reflected by the correction patch mark  300 Y is higher than the potential of the second sensor  92  with respect to the infrared beam reflected by the conveying belt  33 . Above described result is also adapted to magenta and cyan patch marks. 
         [0068]    On the other hand, the amount of the infrared beam reflected by the correction patch  300 Y and detected by the first sensor  91 , is greater than the amount of the infrared beam reflected by the conveying belt  33  that is not rough with scratches and detected by the first sensor  91 . Therefore, as shown in  FIG. 8C , the potential of the first sensor  91  with respect to the infrared beam reflected by the correction patch mark  300 Y is lower than the potential of the first sensor  91  with respect to the infrared beam reflected by the conveying belt  33 . Above described result is also adapted to magenta and cyan patch marks. 
         [0069]    In Step S 9 , the control unit  100  determines whether or not the number of times the potentials of the second sensor  92  have exceeded the threshold value R 2  set in Step S 6  is identical to a preset value (i.e., the number of correction patch marks). 
         [0070]    When the conveying belt  33  is rough with scratches, the mirror-reflected light is decreased and the potential of the second sensor  92  is increased as shown in  FIG. 8B . If the scratch is fairly large, the potential of the second sensor  92  with respect to the infrared beam reflected by the scratches may be higher than the potential of the second sensor  92  with respect to the infrared beam reflected by the correction patch mark  300 Y. In such case, the potential of the second sensor  92  with respect to the infrared beam reflected by the correction patch mark  300 Y cannot exceeds the threshold value R 2 . Further, when the threshold RB 2  is extremely great, the potential of the second sensor  92  with respect to the infrared beam reflected by the correction patch mark  300 Y cannot also exceed the threshold R 2 . 
         [0071]    Therefore, if the number of times is not identical to the present value (No in S 9 ), in Step S 10 , the control unit  100  controls the display unit  130  to display an error message. 
         [0072]    On the other hand, if the number of times is identical to the preset value (Yes in S 9 ), in Step S 11 , the control unit  100  determines whether or not the potentials of the first sensor  91  has exceeded the threshold value R 1 . If the potential has not exceeded the threshold value R 1  (No in SIT), in Step S 12 , the control unit  100  corrects the displacement of different-color images based on the positions of the correction patch marks detected by the second sensor  92  in Step S 8 , and the process is then terminated. 
         [0073]    If the potentials of the first sensor  91  has exceeded the threshold value R 1  (Yes in S 31 ), in Step S 13 , the control unit  100  determines whether or not the correction coefficient P 1  used in Step S 5  is smaller than prescribed Pmin that is a minimum value of the correction coefficient P 1 . If P 1 ≧Pmin (No in Step  513 ), in Step S 14 , the control unit  100  subtracts a prescribed adjustment coefficient P 2  from the correction coefficient P 1  used in Step S 5 , and the process returns to Step S 5 . In Step S 5 , the control unit  100  determines the new development bias DbB by applying the new correction coefficient P 1  to the equation of DbB=DbP×P 1 ). Thus, the development bias DbB is reduced. 
         [0074]      FIG. 9A  is a graph explaining a relation between a transmission density of a toner that forms the correction patch marks and an amount of the infrared beam detected by the first sensor  91 .  FIG. 9B  is a graph explaining a relation between a transmission density of a toner that forms the correction patch marks and an amount of the infrared beam detected by the second sensor  92 . 
         [0075]    As shown in  FIG. 9A , if the color of the patch marks is black (K), no diffusion-reflected light does not occur, irrespective of the density (transmission density) of the correction patch marks. If the color of the patch marks is other than black, such as cyan (C), the diffusion-reflected light is increased in proportion to the density of the correction patch marks. Since the diffusion-reelected light is applied to the second sensor  92 , together with the mirror-reflected light, the amount of the infrared beam received by the second sensor  92  changes as shown in  FIG. 9B  That is, if the color of the patch marks is other than black and the density of the correction patch marks is too high, the infrared beam received by the second sensor  92  will be increased. In such case, the difference between the amount of the infrared beam received by the second sensor  92  when the correction patch marks are formed and the amount of infrared beam received by the second sensor  92  when the correction patch marks are not formed (transmission density=0) will be decreased. As the result, when the transmission density is high, the CPU  100  may determine wrongly that the patch marks are not formed, even if the patch marks are formed actually. 
         [0076]    In the present embodiment, if the potential of the first sensor  91  has exceeded the threshold value R 1 , the development bias DbB is reduced in order to increase the above difference. Specially, it is preferable that the amount of the infrared beam reflected by the correction patch marks formed with a toner other than the black toner and detected by the second sensor  92  falls in a region indicated by two-dot dashed lines in  FIG. 9B . The region includes a density corresponding to the lowest amount of the infrared beam reflected by the correction patch marks formed with a toner other than the black toner and detected by the second sensor  92 . If the development bias DbB is reduced as described above, the above difference becomes large. Therefore, accuracy of detecting the positions of the correction patch marks in Step SB can be increased by setting the adjustment parameter RB 2 , that is, the threshold value R 2  to a large value. Thus, in this embodiment, the influence of the diffusion-reflected light to the detection of the correction patch marks can be suppressed, to achieve appropriate correction of the displacement of different-color images. Then, Step S 6  and the subsequent steps are performed. 
         [0077]    If the correction coefficient P 1  is smaller than Pmin (No in S 13 ) the correction coefficient P 1  is set to initial value P 0  in Step S 14 , terminating the process (Step  310 ) of displaying an error message. In other words, the displacement of different-color images is not corrected if the amount of the diffusion-reflected light is excessively large. 
         [0078]    Further, in the present embodiment, in Step S 6 , the threshold value R 1  is set to potentials corresponding to the light amount higher than the amount of diffusion-reflected light that occurs due to the unevenness at the surface of the conveying belt  33 . The threshold value R 2  is set to potentials corresponding to the light amount smaller than the amount of mirror-reflected light that occurs due to the unevenness at the surface of the conveying belt  33 . Hence, the influence of the diffusion-reflected light can more be suppressed, thereby to correct the displacement of different-color images more appropriately. Moreover, the threshold values R 1  and R 2  are set (Yes in S 61 ) when the variable RN indicating the number of the automatic registration has been performed since the threshold values have been set in Step S 6  in the latest time has exceeded the predetermined value RN_S (Yes in S 2 ) Thus, the threshold values R 1  and R 2  can be set again at the time when the surface state of the conveying belt  33  may change. The more appropriate threshold values R 1  and R 2 , the more reliably can the influence of the diffusion reflected light be suppressed. 
       5. OTHER EMBODIMENTS OF THE INVENTION 
       [0079]    Although the present invention has been described with respect to specific embodiments, it will be appreciated by one skilled in the art that a variety of changes may be made without departing from the scope of the invention. 
         [0080]    For example, the Flag SS is set to 1 not only if variable RN has reached RN_S (Yes in S 2 ), but also in such a case as will be described.  FIG. 10  is a flowchart explaining the interruption process that is performed when the cover sensor  110  detects that the upper cover of the housing  3  has been opened. The interruption process is terminated when flag SS is set to 1 in Step S 31 . 
         [0081]      FIG. 11  is a flowchart explaining the interruption process that is performed when the belt sensor  120  detects the replacement of the conveying belt  33 . This interruption process is terminated, too, when flag SS is set to 1 in Step S 33 . 
         [0082]      FIG. 12  is a flowchart explaining the interruption process that is performed every time a print job is generated and the image-forming unit  10  therefore forms an image on a recording sheet. As shown in  FIG. 10 , variable PN indicating the number of sheets printed and reset to 0 at the time of setting the threshold value (refer to Step S 6 ) is incremented by one in Step S 35 . In the next step, i.e., Step S 36 , whether variable PN has reached or exceeded a preset value PN S. If PN&lt;PN_S (if No in S 36 ), the interruption process is terminated. If PN≧PN S (if yes in S 36 ), the operation goes to Step S 37 . In Step S 37 , flag SS is set to 1 and the process is terminated. Note that if the flag SS=1 (Yes in S 61 ), in Step S 62 , the variables PN are reset to 0. 
         [0083]    Thus, the more appropriate threshold values R 1  and R 2 , the more reliably can the influence of the diffusion reflected light be suppressed. 
         [0084]    Further, various parameters can be used as parameter for adjusting the image density in the process of forming correction patch marks. The correction patch marks may be adjusted in terms of density, by changing the transfer bias (transfer voltage) the intensity of the light applied to the photosensitive drums  71 , or the exposure time In this case, too, images can be formed in the same way as in the above-described embodiment, only if the transfer bias, the intensity of exposure light or the exposure time is set to an appropriate value. If the transfer bias is corrected, however, the toner not transferred may remain on the photosensitive drums  71  and may eventually be degraded. Such a problem would not arise in this invention, because the development bias is corrected as in the embodiment described above. Hence, the toners can be used over a long period of time. 
         [0085]    In the embodiment described above, development bias DbB set in Step S 5  for automatic registration is used for all toners of different colors. Instead, a bias of the same value for forming images on recording sheets may be used to form a black-correction patch mark on the conveying belt  33 , and development bias DbB may be used to form yellow-, magenta- and cyan-correction patch marks. 
         [0086]    The embodiment described above is a laser printer of the direct tandem type. The invention is not limited to laser printers of this type, nevertheless. The invention may be applied to an electro-photographic image-forming device of, for example, a four-cycle type. Further, the invention is not limited to an device in which correction patch marks are formed on the transfer belt  33 . Rather, correction patch marks may be formed on members (e.g., intermediate transfer members or photosensitive drums) that rotate as the image-forming unit  10  operates.