Patent Publication Number: US-9836001-B2

Title: Image forming apparatus having measuring unit

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to an image forming apparatus. 
     Description of the Related Art 
     In an image forming apparatus such as electrophotographic type image forming apparatus, due to an environmental condition such as temperature and humidity and charged amount of toner of developer, density of an output image and a relative position of an image with respect to a sheet change. Thereby, the image forming apparatus forms a measurement image and corrects the density of the output image based on a measurement result of the measurement image by a sensor. Also, the image forming apparatus adjusts an image forming condition for adjusting the relative position of the image with respect to the sheet. 
     Conventionally, to prevent adhesion of scattering paper dust or toner on a detecting surface of a sensor, an image forming apparatus comprising a sensor with a shutter and that controls opening/closing operation of the shutter through an opening/closing mechanism is known (U.S. Pat. No. 6,321,044). 
     However, in the image forming apparatus of this type, if the toner or stain is adhered on the opening/closing mechanism of the shutter, sometimes, the shutter cannot normally be opened/closed. In this case, the sensor may receive reflection light from a rear side of the shutter at a timing when the density of the measurement image is detected. Due to this, when a sensor output value for the measurement image is wrongly detected, it is not possible to control, with high accuracy, an image forming condition of the image forming apparatus. 
     Thus, the present invention is directed to restrict the image forming condition from being adjusted by the measurement result of the sensor measured when the shutter is not normally opened. 
     SUMMARY OF THE INVENTION 
     The image forming apparatus of the present disclosure comprises an image forming apparatus comprising an image bearing member; an image forming unit configured to form an image on the image bearing member; a measuring unit having a first sensor, a second sensor, and a shutter, and configured to measure a measurement image formed on the image bearing member, wherein the shutter protects the first sensor and the second sensor; a driving unit configured to be driven to move the shutter, wherein, in a state in which the shutter is at a first position, the shutter prohibits measurement of the measurement image by the first sensor and the second sensor and wherein, in a state in which the shutter is at a second position, the measurement image is measured by the first sensor and the second sensor; and a controller configured to: control the image forming unit to form the measurement image on the image bearing member, drive the driving unit to move the shutter to the second position, control the measurement unit to measure the measurement image, control whether or not to change an image forming condition of the image forming unit based on a first output value corresponding to a measurement result of the first sensor and a second output value corresponding to a measurement result of the second sensor. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic longitudinal sectional view illustrating an example of a configuration of an image forming apparatus according to a first embodiment. 
         FIG. 2  is a partial enlarged view of a display screen of an operation panel. 
         FIG. 3  is a block diagram for explaining an example of functional configuration of the image forming apparatus. 
         FIGS. 4A and 4B  are diagrams for explaining stabilization control in the image forming apparatus. 
         FIG. 5  is a diagram schematically representing a state in which density of the measurement image formed between sheets is detected. 
         FIGS. 6A and 6B  are schematic diagrams of a unit comprising a shutter and a density detection sensor which is viewed from an intermediate transfer body side. 
         FIG. 7  is a graph for explaining gradation characteristic. 
         FIG. 8  is a diagram for explaining forming timing of the measurement image formed by the image forming apparatus between sheets. 
         FIG. 9  is a diagram for explaining a relation of opening/closing state of the shutter and detection/non-detection of the density detection sensor with respect to the measurement image. 
         FIG. 10  is a flowchart illustrating an example of processing procedure of a gradation correction of the image forming apparatus. 
         FIG. 11  is a diagram for explaining forming timing of the measurement image formed by the image forming apparatus according to a second embodiment between sheets. 
         FIG. 12  is a diagram for explaining a relation of opening/closing state of the shutter and detection/non-detection of the density detection sensor with respect to the measurement image. 
         FIG. 13  is a diagram for explaining a relation of opening/closing state of the shutter and detection/non-detection of the density detection sensor with respect to the measurement image according to a third embodiment. 
         FIG. 14  is a graph for explaining detection of half-opened state of the shutter by the density detection sensor according to a fourth embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     In the following, embodiments are described with reference to the accompanying drawings. In the present embodiment, a description is provided with an example of an electrophotographic laser beam printer, which is an example of the image forming apparatus. Further, as the image forming apparatus, other image forming apparatus, such as inkjet printer, sublimation type printer, etc., can be used. 
     Further, according to the present embodiment, it is possible to prevent the image forming condition of the image forming apparatus from being adjusted based on the measurement result of the sensor measured when the shutter is not normally opened. 
     First Embodiment 
       FIG. 1  is a schematic longitudinal sectional view illustrating an example of a configuration of an image forming apparatus according to the present embodiment. An image forming apparatus  100  comprises a housing body  101  and an operation panel  180 . In the housing body  101 , various mechanisms which constitute an image forming unit for forming an image are arranged. In the image forming unit, by scanning laser light on an image bearing member of a photosensitive drum  105 , an electrostatic latent image is formed. Then, the electrostatic latent image is developed. To perform multiple transfer of the developed image to an intermediate transfer body  106  to further transfer a color image having performed multiple transfer to a recording medium such as a sheet (hereinafter referred to as transfer member)  110 , a transfer processing mechanism is arranged. Also, a fixing processing mechanism for fixing the toner image having transferred to the transfer member  110 , a sheet feeding processing mechanism for the transfer member  110 , a conveyance processing mechanism for the transfer member  110  etc. are arranged. 
     The image forming apparatus  100  also comprises respective laser scanner units  107  which correspond to each color of yellow (Y), magenta (M), cyan (C), and black (K). The laser scanner unit  107  comprises a laser driver which turns ON/OFF the laser light emitted from a semiconductor laser emitting device (not shown). For example, when the laser scanner unit  107  receives an image signal (image data) with its image resolution being, for example, 2400 [dpi] from a controller  300  (described later), the laser scanner unit  107  accordingly turns ON/OFF the laser light as mentioned. The laser light emitted from the semiconductor laser emitting device is distributed in a scanning direction through a rotating polygon mirror (not shown). The laser light distributed in a main scanning direction is guided to the photosensitive drum  105  through a reflection mirror  109  to expose a surface of the photosensitive drum  105 . 
     On the other hand, by charging the photosensitive drum  105  by a primary charger  111  and by scanning exposure of the laser light, the electrostatic latent image is formed on the photosensitive drum  105 . The electrostatic latent image then is developed into a toner image by toner supplied from a developing device  112  (described later). Then, the toner image developed on the photosensitive drum  105  is transferred on the intermediate transfer body  106  to which a voltage of reverse characteristic to the toner image is applied (primary transfer). As mentioned, the photosensitive drum  105  carries and conveys the image formed by the image forming unit. It is noted that, when forming the color image, respective colors are formed in order on the intermediate transfer body  106  from a yellow (Y) station  120 , a magenta (M) station  121 , a cyan (C) station  122 , and a black (K) station  123 . Thereby, a full color visible image is formed on the intermediate transfer body  106 . 
     The full color visible image formed on the intermediate transfer body  106  is transferred to the transfer member  110  fed from a storage  113  of the transfer member. In particular, the transfer member  110  is brought into pressure contact with the intermediate transfer body  106  by the transfer roller  114 . Then, a voltage of reverse characteristic to the toner is applied to the transfer roller  114 . In this manner, the visible image is transferred to the transfer member  110  which is synchronized and fed in a sub-scanning direction by a sheet feeding processing mechanism (secondary transfer). It is noted that the photosensitive drum  105  and the developing device  112  are attachable/detachable to/from the image forming apparatus  100 . 
     Also, a start position detection sensor  115  and a sheet feeding timing sensor  116  are arranged around the intermediate transfer body  106 . The start position detection sensor  115  determines a print start position when performing image formation. The sheet feeding timing sensor  116  is used to control sheet feeding timing of transfer member  110 . A density detection sensor  117  is also arranged. The density detection sensor  117  is used to measure the density of the patch image (measurement image) for a density correction when controlling the density. It is noted that, based on the detection result of the density detection sensor  117  (measured data), stabilization control (described later) is executed. Also, the detail of the density detection sensor  117  is described later. 
     The fixing processing mechanism comprises a first fixing unit  150  and a second fixing unit  160  for fixing the toner image transferred to the transfer member  110  by heat and pressure. The first fixing unit  150  comprises a fixing roller  151  for applying heat to the transfer member  110 , a pressurizing belt  152  for bringing the transfer member  110  into pressure contact with the fixing roller  151 , and a post-fixing sensor  153  for detecting completion of fixation. Each roller is a hollow roller, respectively having a heater inside, and is configured to convey the transfer member  110  at the same time each roller is rotationally driven. The second fixing unit  160  is positioned downstream of the conveyance path of the transfer member  110  as compared to the first fixing unit  150 . The second fixing unit  160  adds gloss and secures fixability to the toner image fixed on the transfer member  110  by the first fixing unit  150 . Similar to the first fixing unit  150 , the second fixing unit  160  also comprises a fixing roller  161 , a pressurizing roller  162 , and a post-fixing sensor  163 . 
     It is noted that some sheets are not required to go through the second fixing unit  160  depending on a type of the transfer member  110 . In this case, to reduce energy consumption, the transfer member  110  is guided to a conveyance path  130  through a conveyance path switching flapper  153  to discharge the transfer member  110  not via the second fixing unit  160 . 
     The transfer member  110  is guided to a conveyance path  135  through a conveyance path switching flapper  132 . Then, after the position of the transfer member  110  is detected by a reverse sensor  137 , switchback operation is performed to the transfer member  110  at a reverse section  136 . Then, a preceding edge of the transfer member  110  is changed. A color sensor  200  is a color sensor for detecting the measurement image formed on the transfer member  110 . In a case where a color detection operation is instructed through the operation panel  180 , density adjustment, gradation adjustment, multi-color adjustment, etc., are performed based on the detection result of the color sensor  200 . It is noted that controls concerning the image forming processing performed by each mechanism (for example, sheet feeding processing) are performed through an image forming control unit  102  (described later). 
     [Operation Panel] 
       FIG. 2  is a partial enlarged view of a display screen of the operation panel  180 . A soft switch  500  displayed on the display screen is a button for turning ON/OFF a power source of the image forming apparatus  100  main body. A copy start key  501  is a button for instructing to start copying. A reset key  502  is a button used to back an image forming mode of the image forming apparatus  100  to a normal mode. Here, in the normal mode, “full color: single side” image is set to be formed. A numeric keypad  503  is a keypad used to input a numeric value such as the number of image forming sheets. A clear key  504  is a button used to clear the numeric value input. A stop key  505  is a button used to stop copying during continuous copying. 
     A touch panel  506  displays the setting of various modes and a state of a printer. Also, it receives an input through touch operation. An interruption key  507  is a button used to interrupt during the continuous copying or while using the image forming apparatus  100  as a facsimile machine or a printer to execute other operations. A password key  508  is a button used to manage the number of copies individually or sectorally. A guidance key  509  is a button pressed down when using a guidance function. 
     A function key  510  is a key used when changing a function of the image forming apparatus  100 . A user mode key  511  is a button used to switch to a mode that is managed and set by a user. In particular, the user mode key is used when the user adjusts sensitivity of the sensor, performs a calibration mode of density and color, registers sheets, and changes setting time until the image forming apparatus  100  enters an energy saving mode. A color measuring mode  414  is a button used to switch the image forming apparatus  100  to a color measuring mode. 
     Also, a full color mode key  512  is a button selected when forming the full color image. A monochrome mode key  513  is a button selected when forming a monochrome image (or single color image). In the following description, selection and execution of pseudo halftone processing pattern (hereinafter referred to as pseudo halftone processing) etc. are instructed through, for example, the operation panel  180 . 
     [Image Processing Unit] 
       FIG. 3  is a block diagram for explaining an example of functional configuration of the image forming apparatus  100 . The image forming apparatus shown in  FIG. 3  is connected to a host computer  301  via a communication line such as a network (for example, in compliance with 10base-T, IEEE 802.3). 
     The controller  300  controls an operation of the image forming apparatus  100 . Also, the controller  300  comprises a host I/F unit  302 , an input/output buffer  303 , a read only memory (ROM)  304 , and an image information generation unit  305 . The controller  300  also comprises a maximum density condition determination unit (Vcont: development contrast potential)  306 , a gradation correction table generation unit (gamma LUT: gamma look up table)  307 , and a multi-dimensional table generation unit (ICC profile)  308 . The controller  300  also comprises a random access memory (RAM)  309 , a central processing unit (CPU)  313 , a raster image processor (RIP) unit  314 , a color processing unit  315 , a gradation correction unit  316 , a pseudo halftone processing unit  317 , an image forming I/F unit  318 , and an opening/closing determination unit  319  which determines opening/closing of a shutter  407 , which is described later. Each functional configuration is connected to allow transmission and reception of various information through a system bus. 
     The host I/F unit  302  is an interface for transmission and reception of information to and from the host computer  301 . The input/output buffer  303  transmits and receives a control code from the host I/F unit  302  and data from each communication means. The CPU  313  controls the entire operation of the controller  300 . Control programs executed by the CPU  313  and various control data are stored in the ROM  304 . The RAM  309  is used as a work memory for performing calculation required to translate the control code and data or processing of print data. The image information generation unit  305  generates various image objects based on the data received from the host computer  301 . 
     The RIP unit  314  develops the image object into a bit map image. The color processing unit  315  performs multi-color color conversion processing (described later). The gradation correction unit  316  executes single color gradation correction. The pseudo halftone processing unit  317  executes the pseudo halftone processing which is referred to as dither matrixes, error diffusion method etc. The image forming I/F unit  318  transfers the converted image to the image forming unit. In this manner, the image is formed. Also, in the image forming apparatus  100 , for example, two types of the pseudo halftone processing pattern can be executed. The image forming apparatus  100  separately forms the measurement image for density correction in one or more pseudo halftone processing. Also, the image forming apparatus  100  can form the measurement image of one or more colors separately. Then, based on the detection result, through which the density of the measurement image is detected, image adjustment is performed, which is to optimize the maximum density condition and the gradation correction table. 
     The maximum density determination unit  306  determines the maximum density correction condition to adjust the maximum density. The gradation correction table generation unit  307  determines the gradation correction coefficient based on the maximum density correction condition determined. To correct the variation of the multi-color, the multi-color table generation unit  308  generates ICC profile, which is multi-dimension LUT. It is noted that each adjustment result in the maximum density condition determination unit  306 , the gradation correction table generation unit  307 , and the multi-color table generation unit  308  is primarily stored in the table storage unit  310  in the RAM  309 . 
     A panel I/F unit  311  mediates transmission and reception of information between the controller  300  and the operation panel  180 . A memory I/F unit  312  mediates transmission and reception of information between the controller  300  and an external memory unit  181  which is used to store print data, information of various print devices etc. It is noted that each of the image information generation unit  305 , the maximum density condition determination unit  306 , the gradation correction table generation unit  307 , and the multi-color table generation unit  308  in which the correction result of the multi-color is reflected are stored in the ROM  304  as a function module. 
     Also, information of the ICC profile, the gamma LUT, and the Vcont used at the time of forming the image is appropriately managed and updated. It is noted that the feature of the present disclosure, i.e., change of the exposure condition between the sheets (an interval between a preceding sheet and the following sheet) is determined by the maximum density condition determination unit  306  as mentioned. Then, the determination result is notified to the image forming control unit  102 . The exposure condition is changed (reflected) before printing the measurement image in the first pseudo halftone processing (described later). 
     Based on the detection result (measured data) of the density detection sensor  117 , the opening/closing determination unit  319  determines whether the shutter  407  is in an opened state or in a closed state. The determination result is notified to the CPU  313 . The CPU  313  determines whether to perform the correction or not in accordance with the determination result received and notifies the gradation correction table generation unit  307  of the determination result. If it is determined that the shutter  407  is in the closed state, the CPU  313  controls not to update the gradation correction table. In this manner, the controller  300  functions as an image adjustment unit which changes the image forming condition. 
     [Outline of Density Detection Sensor] 
       FIGS. 4A and 4B  are diagrams for explaining stabilization control in the image forming apparatus  100 .  FIG. 4A  represents a shutter open state.  FIG. 4B  represents a shutter closed state. Detail of the shutter open state and the shutter closed state is described later with  FIG. 6 . 
     The density detection sensor  117  comprises a light emitting unit  400  and a light receiving unit  401 . Light Io emitted from the light emitting unit  400  is reflected on the surface of the intermediate transfer body. Then, the reflected light Ir is measured at the light receiving unit  401 . The reflected light measured at the light receiving unit  401  is monitored at an LED light amount control unit  403 . The monitored result is then sent to the image forming control unit  102 . The image forming control unit  102  calculates the density based on the light source light Io and the measured value of the reflected light Ir. For example, the measured value of the output value, 0 to 5 [V], is standardized into a digital signal value of 0 to 1023 levels. Then, the density is calculated. In this manner, the density detection sensor  117  can detect the density of an object to be detected on an optical path including the measurement image. 
     As shown in  FIG. 4A , a state in which the density detection sensor  117  can measure the measurement image on the intermediate transfer body  106  is the shutter open state. Further, as shown in  FIG. 4B , a state in which the density detection sensor  117  can measure a surface of the shutter  407  is the shutter closed state. It is noted that the shutter  407 , lying between the density detection sensor  117  and the intermediate transfer body  106 , is provided on the optical path between the density detection sensor  117  and the measurement image. 
       FIG. 5  is a diagram schematically representing a state in which the density detection sensor  117  detects the measurement image formed between the sheets. As shown in  FIG. 5 , three measurement images corresponding to each color of cyan, magenta, and yellow are formed between the sheets of normal images page  1  and page  2  formed on the intermediate transfer body  106 . The density detection sensor  117  can also measure the density of one or more colors separately. As shown in  FIG. 5 , the density detection sensor  117  comprises a density detection sensor  117 F for detecting the measurement image of cyan, a density detection sensor  117 C for detecting the measurement image of magenta, and a density detection sensor  117 R for detecting the measurement image of yellow. For example, in the shutter closed state, the detecting surfaces of the density detection sensor  117 F and the density detection sensor  117 C are covered. Each sensor is arranged in a direction orthogonal to a conveying direction and at different positions. Further, through an adjustment of the light amount of the density detection sensor of each color, intensity of the emitted light is adjusted such that, when the light is emitted to a rear side of the shutter  407  when the power is ON before forming the image, an output value reaches a target value. The target value corresponds to the output value of a “shutter rear side” described in Tables 1 to 3 described later. Further, the respective target values of the density detection sensors  117 F,  117 C, and  117 R are previously determined by an experiment. Due to an attachment error etc. of the shutter  407 , the target value of each density detection sensor may differ. A motor  408  is a driving source used to open/close the shutter  407  and functions as an opening/closing mechanism of the shutter  407 . Note that, as to Bk (black), the correction timing is separately provided for correction. 
     [Configuration of Shutter] 
       FIGS. 6A and 6B  are schematic diagrams when a unit comprising the shutter  407  and the density detection sensor  117  shown in  FIGS. 4A and 4B  are viewed from the intermediate transfer body side.  FIG. 6A  shows the shutter closed state in which the shutter  407  is moved to a first position.  FIG. 6B  shows the shutter open state in which the shutter  407  is moved to a second position. For example, the shutter  407  is comprised of one metal plate. Three openings are provided on a surface of the metal plate. Further, the shutter  407  is provided to be relatively slidable with respect to the density detection sensor  117  and is integrally movable to the first position and to the second position by a driving force of the motor  408 . Further, before one pair of the measurement images passes a measurement position, the shutter  407  is moved to the first position from the second position. As shown in  FIG. 6A , in the shutter closed state, the density detection sensor  117  detects positions around the dotted line in  FIG. 6A . Further, as shown in  FIG. 6B , in the shutter open state, the position of the detecting surface of the density detection sensor  117  matches with the position of the shutter opening so that the detecting surface is opened. As a result, detection of one or more measurement images formed on the intermediate transfer body becomes possible. To prevent stains of the detecting surface of the density detection sensor  117  due to the scattering toner, etc., the shutter  407  is closed when the measurement image is not detected. 
     [Outline of Stabilization Control] 
     The density detection sensor  117  is used for the stabilization control for obtaining correct color tone in a recorded image. It means that the measurement image experimentally formed (printed) on the intermediate transfer body is detected through the density detection sensor  117 . It is noted that the stabilization control includes, for example, “Dmax control” and “halftone control”. 
     In the Dmax control, exposure amount is made variable and an image formed using developer (hereinafter referred to as “developer image”) is experimentally formed. Then, the density of the developer image is measured to calculate the exposure amount corresponding to the target density of each color. Further, in the halftone control, developing images of different stages, formed with the exposure amount calculated by the Dmax control and having experienced half-toning such as screening are experimentally formed. The developing image is measured and a table (a gamma LUT), in which an input/output relation is corrected such that an output result for an input signal becomes target density characteristic, is formed. The gamma LUT is stored in the gradation correction unit  316  to wait for the next image formation. The image forming apparatus  100  prints and detects the measurement image to which one or more pseudo halftone processing have been applied between the sheets which are in continuous output. Then, to change the maximum density condition based on the detection result, the image forming apparatus  100  changes the exposure amount and the value of the gamma LUT. This is described in detail in the following. 
       FIG. 7  is a graph for explaining gradation characteristic. The solid line in  FIG. 7  represents the gradation characteristic. Further, the solid line TGT represents one example of an ideal gradation characteristic. To have a density corresponding to the value of the input signal included in the image data, the value of the input signal is converted. The conversion is performed using conversion condition (gradation correction condition) represented by the dotted line in  FIG. 7 . The gamma LUT is a table for correcting the input/output relation such that the output result for the input signal becomes the target density characteristic. The gamma LUT is stored in the gradation correction unit  316  to wait for the next image formation. 
       FIG. 8  is a diagram for explaining forming timing of the measurement image formed between the sheets by the image forming apparatus. In  FIG. 8 , numbers ( 1  to  15 ) enclosed in a rectangular shape respectively represent the normal images page  1  to page  15 . Further, as shown in  FIG. 8 , the measurement image of the first pseudo halftone processing (170 [lpi]) and the measurement image of a second pseudo halftone processing (230 [lpi]) are formed between the sheets of the normal images page  4  and page  5  which are in continuous output. 
       FIG. 9  is a diagram for explaining the relation of opening/closing state of the shutter  407  and detection/non-detection of the density detection sensor  117  with respect to the measurement image. For example, the density detection sensor  117  performs detection in the normal image page  4  which is before detecting the measurement image formed on the intermediate transfer body. In this case, the shutter is in the closed state so that, as shown in  FIG. 9 , the density detection sensor  117  detects the surface of the shutter  407 . In  FIG. 9 , for simplification, “detection” is represented as “D”, and “non-detection” is represented as “N”. It is noted that the surface of the shutter  407  detected by the density detection sensor  117  is expressed as a shutter rear side. The detection result of the shutter rear side detected by the density detection sensor  117  is expressed as ref (reference measured data). Further, the shutter  407  is opened between the sheets of the normal images page  4  and page  5 . The measurement images of one or more density levels are detected therebetween. After the detection, the shutter  407  is in the closed state again. Next, a description is provided with regard to a method to determine whether there is opening/closing abnormality of the shutter  407  or not. 
     [Determination of Opening/Closing Abnormality of Shutter] 
     Table 1 below shows one example of what is detected at the detection timing as described in  FIG. 9 . In Table 1, the detection result of the shutter rear side by the density detection sensor  117  is expressed as Ref. Each of the letters C, M, Y represents color. In particular, C represents cyan, M represents magenta, and Y represents yellow. Each of the numbers placed after the letters represents the pseudo halftone processing. Further, in Table 1, D1 represents the detection in a low density area of the measurement image, D2 represents the detection in a medium density area of the measurement image, and D3 represents the detection in a high density area of the measurement image. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 SENSOR 
                 SENSOR 
                 SENSOR 
               
               
                   
                 117F 
                 117C 
                 117R 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Correction Color 
                 Cyan 
                 Magenta 
                 Yellow 
               
               
                 Shutter Rear Side 
                 C-ref 
                 M-ref 
                 Y-ref 
               
               
                 170 lpi Low density 
                 C1-D1 
                 M1-D1 
                 Y1-D1 
               
               
                 170 lpi Medium density 
                 C1-D2 
                 M1-D2 
                 Y1-D2 
               
               
                 170 lpi High density 
                 C1-D3 
                 M1-D3 
                 Y1-D3 
               
               
                 230 lpi Low density 
                 C2-D1 
                 M2-D1 
                 Y2-D1 
               
               
                 230 lpi Medium density 
                 C2-D2 
                 M2-D2 
                 Y2-D2 
               
               
                 230 lpi High density 
                 C2-D3 
                 M2-D3 
                 Y2-D3 
               
               
                   
               
            
           
         
       
     
     The detection characteristic shown in Table 2 as below is obtained if the shutter  407  is normally operated in the regulated opening/closing operation at the timing shown in  FIG. 9 . 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 SENSOR 
                 SENSOR 
                 SENSOR 
               
               
                   
                 117F 
                 117C 
                 117R 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Correction Color 
                 Cyan 
                 Magenta 
                 Yellow 
               
               
                 Shutter Rear Side 
                 540 
                 520 
                 505 
               
               
                 170 lpi Low density 
                 101 
                 90 
                 96 
               
               
                 170 lpi Medium density 
                 389 
                 420 
                 442 
               
               
                 170 lpi High density 
                 700 
                 705 
                 698 
               
               
                 230 lpi Low density 
                 105 
                 93 
                 97 
               
               
                 230 lpi Medium density 
                 400 
                 433 
                 462 
               
               
                 230 lpi High density 
                 702 
                 708 
                 701 
               
               
                   
               
            
           
         
       
     
     On the other hand, the detection characteristic shown in Table 3 as below is obtained as the detection result of the density detection sensor if abnormality occurs and the shutter  407  is not normally operated, including the case, for example, where the shutter  407  is not turned into the open state. In this case, the value of the detection result of the density detection sensor is almost equal to the value of the detection result of the shutter rear side. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 SENSOR 
                 SENSOR 
                 SENSOR 
               
               
                   
                 117F 
                 117C 
                 117R 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Correction Color 
                 Cyan 
                 Magenta 
                 Yellow 
               
               
                 Shutter Rear Side 
                 540 
                 520 
                 505 
               
               
                 170 lpi Low density 
                 541 
                 521 
                 503 
               
               
                 170 lpi Medium density 
                 540 
                 520 
                 504 
               
               
                 170 lpi High density 
                 542 
                 520 
                 505 
               
               
                 230 lpi Low density 
                 541 
                 522 
                 506 
               
               
                 230 lpi Medium density 
                 539 
                 521 
                 505 
               
               
                 230 lpi High density 
                 540 
                 519 
                 504 
               
               
                   
               
            
           
         
       
     
     Based on these detection characteristics, the CPU  313  performs arithmetic operations  1  and  2  as shown below through the opening/closing determination unit  319  to determine whether the shutter  407  is in the open state or in the closed state. The CPU  313  compares the detection result of each measurement image with Ref and calculates the difference therebetween. Then, the CPU  313  determines whether the difference which is at a predetermined reference value or below (for example, Δ±2 level or below) is included in the comparison result or not. If the difference which is Δ±2 level or below is included in the comparison result, the CPU  313  also determines whether the differences of the rest of the colors obtained by the comparison are Δ±2 level or below or not. If it is determined that all the differences of the rest of the colors obtained by the comparison are Δ±2 level or below, the CPU  313  determines that the shutter is in the closed state (arithmetic operation  2 ). 
     In this manner, if all the detection results of one or more density detection sensors are almost the same value as the value of the detection result of the shutter rear side, the CPU  313  determines that the shutter is in the closed state. This is because, as shown in Table 3, the detection result of the rear side color is almost equal to the detection results in the low density to high density areas, which causes difficulty in distinguishing from the normal state. It means that when determining whether the shutter  407  is in the open state or in the closed state based only on the detection result of the density detection sensor of one color, in many cases, the density adjustment is unnecessarily stopped. Thereby, in the image forming apparatus  100  according to the present embodiment, using the detection result which detected the measurement image formed at almost the same timing in a sub-scanning direction, the open/closed state of the shutter  407  is determined. Then, if it is determined that the shutter  407  is in the closed state based on the calculation result, the CPU  313  controls not to update the gradation correction table to stop to change the image forming condition. 
     [Processing Procedure of Gradation Correction] 
       FIG. 10  is a flowchart illustrating one example of a processing procedure of the gradation correction of the image forming apparatus  100 . Each processing shown in  FIG. 10  is mainly performed by the CPU  313 . When receiving a print instruction from a user, the CPU  313  starts to form an image (S 001 ). The CPU  313  confirms that the shutter  407  is in the closed state (S 002 ). This restricts contamination of the detecting surface of the density detection sensor  117  due to scattering toner. 
     The CPU  313  determines whether it is time to start density detection previously set or not (S 003 ). If it is determined that it is time to start the density detection (S 003 : Y), before starting to form the measurement image, the CPU  313  performs the density detection in the shutter closed state (S 004 ). Then, the CPU  313  temporarily stores the detection result Ref in the shutter closed state in an external memory unit  181  (S 005 ). If not (S 003 : N), the CPU  313  goes back to the processing of Step S 002 . 
     The CPU  313  forms one or more measurement images between the sheets (S 006 ) and turns the shutter  407  into the open state (S 007 ). The CPU  313  instructs each density detection sensor ( 117 F,  117 C,  117 R) to start detection. Then, the CPU  313  obtains the detection result of each color separately (S 008 ). The CPU  313  compares the detection result Ref obtained in the processing of Step S 004  with the detection result obtained in the processing of Step S 008 . Then, based on the comparison result, the CPU  313  calculates the difference therebetween (S 009 ). 
     The CPU  313  determines whether a color having the difference which is Δ±2 or below is included in the comparison result or not (S 010 ). If it is determined that no color having the difference which is Δ±2 or below is included in the comparison result (S 010 : N), the CPU  313  performs the gradation correction processing (S 013 ). If not (S 010 : Y), the CPU  313  also determines whether the difference of the rest of the colors obtained by the comparison is Δ±2 or below or not (S 011 ). If it is determined that no color having the difference which is Δ±2 or below is included in the comparison result of the rest of the colors (S 011 : N), the CPU  313  performs the gradation correction processing (S 013 ). 
     If not (S 011 : Y), the CPU  313  determines that the shutter  407  is in the closed state when detecting the measurement image by each density detection sensor. Then, the CPU  313  stops the gradation correction processing (S 012 ). The CPU  313  turns the shutter  407  into the closed state (S 014 ). Thereafter, the CPU  313  determines whether a series of the image formation is finished or not (S 015 ). If the image formation is not finished, the CPU  313  goes back to the processing of the step S 002 . 
     In this manner, the image forming apparatus  100  of the present embodiment can detect the opening/closing abnormality of the shutter  407  using the density detection sensor  117  used for the gradation correction. It means that, without a dedicated shutter opening/closing sensor, the gradation characteristic (gamma LUT) of the apparatus can be controlled with high accuracy. This allows restriction of occurrence of a defect (tone jump) when performing the gradation correction. In addition, an image forming apparatus with low cost and high stability can be provided. It is noted that, in the above, the description has been provided for the case where the reference value of the difference based on the comparison result is ±2. A maximum output voltage of the sensor, 5[V], is converted to a digital signal value of 1023 level. The digital signal value is adjusted such that maximum density region becomes 700, 0.3[%] of which is the reference value of ±2. 
     Further, a sensor detection value, 0.3[%] in the specified maximum density is a value which should be changed depending on noise component of sensor or main body and bit number used when converting analog signals to digital signals, so it is not intended to limit the number to 0.3[%]. It is noted that, when determining whether the shutter  407  is closed or not, considering the fact that there are many cases where the gradation correction processing is stopped so that stability is impaired, it is desired that the value is set within about 1[%]. 
     Further, in the above, the description of detecting a shutter member itself has been provided. As the shutter member, those with various materials and surface properties, from mold to plate, can be employed. In particular, by employing the shutter member with high uniformity of the shutter rear side or the shutter member to which toner is hardly adhered, occurrence of variation in the detection result when the shutter is in the closed state can be prevented. It is noted that, to prevent the occurrence of variation in the detection result detected when the shutter is in the closed state, a reference member, managed to satisfy predetermined standard such as uniformity, may be arranged at detecting portion of the shutter  407  and the density detection sensor may read the reference member. 
     Second Embodiment 
     In the present embodiment, a description is provided with regard to an image forming apparatus capable of separately forming one measurement image between the sheets and performing the gradation correction by integrating three measurement images. Note that the same symbols are used for the functional components which are identical to those as described in the first embodiment and the description thereof will be omitted. 
       FIG. 11  is a diagram for explaining forming timing of the measurement image formed by the image forming apparatus according to the present embodiment between the sheets. Numbers ( 1  to  15 ) enclosed in a rectangular shape respectively represent the normal images page  1  to page  15 . As shown in  FIG. 11 , the image forming apparatus according to the present embodiment forms, for example, the measurement image of low density of the first pseudo halftone processing between the sheets of the normal images page  1  and page  2 . Further, the image forming apparatus forms the measurement image of medium density of the first pseudo halftone processing between the sheets of the normal images page  2  and page  3 . Further, the image forming apparatus forms the measurement image of high density of the first pseudo halftone processing between the sheets of the normal images page  3  and page  4 . Then, the density detection sensor  117  detects the density of the three measurement images separately. Similarly, the image forming apparatus forms the measurement image of low density of the second pseudo halftone processing between the sheets of the normal images page  6  and page  7 . Further, the image forming apparatus forms the measurement image of medium density of the second pseudo halftone processing between the sheets of the normal images page  7  and page  8 . Further, the image forming apparatus forms the measurement image of high density of the second pseudo halftone processing between the sheets of the normal images page  8  and page  9 . In this manner, the image forming apparatus repeatedly forms the images corresponding to each density separately from the end of the image formation to the start of the next image formation (between the sheets) to form the measurement image. 
     Then, the image forming apparatus performs the gradation correction of the first pseudo halftone processing (170 [lpi]) based on the detection results of the three measurement images formed between the normal images page  1  and page  2 , page  2  and page  3 , and page  3  and page  4 . The image forming apparatus also performs the gradation correction of the second pseudo halftone processing (230 [lpi]) based on the detection results of the three measurement images formed between the normal images page  6  and page  7 , page  7  and page  8 , and page  8  and page  9 . 
       FIG. 12  is a diagram for explaining a relation of opening/closing state of the shutter  407  and detection/non-detection of the density detection sensor with respect to the measurement image. The density detection sensor  117  detects, for example, the shutter rear side in the normal image page  1 , which is before detecting the measurement image formed on the intermediate transfer body. Then, the density detection sensor  117  detects the measurement image formed between the sheets of the normal images page  1  and page  2 . Thereafter, the density detection sensor  117  again detects the shutter rear side in the normal image page  2 . Then, the image forming apparatus according to the present embodiment determines the opening/closing state of the shutter  407  based on the difference between the detection result of each measurement image and Ref detected immediately before detecting each measurement image. 
     Here, there may be a case where, due to interference caused by adhesion of scattering objects such as toner on the surface of the shutter, driving member etc., malfunction is temporarily caused to the opening/closing operation of the shutter  407 . For example, when abnormality occurs in the opening/closing operation of the shutter and none of the measurement images can be detected, while performing the opening/closing operation of the shutter, the abnormal state sometimes returns to the normal state due to vibration of the opening/closing operation of the shutter. The image forming apparatus of the present embodiment determines the opening/closing state of the shutter  407  for each measurement image formed between the sheets separately. Then, if it is determined that the shutter is in the closed state, the image forming apparatus controls to stop the gradation correction of the pseudo halftone processing corresponding to this. Further, in the pseudo halftone processing to be performed next, the opening/closing state of the shutter  407  is again determined. 
     In Table 4 as below, the difference between the detection result of the measurement image of low density and the detection result Ref of the shutter rear side is ±2 level or below. Also, the difference between the detection result of the measurement image of medium density and the detection result Ref of the shutter rear side is ±2 level or below. So, it is determined that the shutter  407  is in the closed state at the timing of detecting the measurement image. On the other hand, the shutter  407  is in the open state at the timing of detecting the measurement image of high density. So, it is determined that the operation state is returned to the normal state when the measurement image of high density is detected. In this case, among the three densities, low, medium, and high, two of them exceed the difference of ±2 level. Thereby, it is determined that the state is abnormal (NG). Thus, it is controlled not to perform the gradation correction in the pseudo halftone processing at this time. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 SENSOR 
                 SENSOR 
                 SENSOR 
               
               
                   
                 117F 
                 117C 
                 117R 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Correction Color 
                 Cyan 
                 Magenta 
                 Yellow 
               
               
                 170 lpi Shutter Rear Side 
                 540 
                 520 
                 505 
               
               
                 Just before low density 
               
               
                 170 lpi Low density 
                 541 
                 521 
                 503 
               
               
                 Low density determination 
                 NG 
                 NG 
                 NG 
               
               
                 170 lpi Shutter Rear Side 
                 540 
                 520 
                 504 
               
               
                 Just before medium density 
               
               
                 170 lpi Medium density 
                 542 
                 520 
                 505 
               
               
                 Medium density determination 
                 NG 
                 NG 
                 NG 
               
               
                 170 lpi Shutter Rear Side 
                 541 
                 521 
                 505 
               
               
                 Just before high density 
               
               
                 170 lpi High density 
                 700 
                 702 
                 698 
               
               
                 High density determination 
                 OK 
                 OK 
                 OK 
               
            
           
           
               
               
            
               
                 Overall Determination 
                 NG (when detecting low density and 
               
               
                   
                 medium density, shutter was closed.) 
               
               
                   
               
            
           
         
       
     
     As mentioned, in the image forming apparatus according to the present embodiment, it is controlled not to perform the gradation correction processing when it is at least once determined that the shutter is in the closed state in the same pseudo halftone processing. For example, if, among the low density, medium density, and high density, the gradation correction emphasizing only the specific density is performed, it is considered to perform the gradation correction based on the detection result of the measurement image corresponding to the specific density. In this case, however, the detection result of the measurement image of the specific density and the detection result of the measurement image of other density largely vary, which may cause a correction level difference etc. Thereby, by performing the gradation correction processing like the image forming apparatus according to the present embodiment, occurrence of the correction level difference etc. can be restricted. 
     If it is continuously determined that the shutter is in the closed state as above, some abnormality may be caused to the apparatus. For example, a message like “density detection sensor shutter is in abnormal state. Please call a serviceman.” may be displayed on the display screen of the touch panel  506  of the apparatus to prompt a user for requesting service. Further, the message can automatically be notified to a service station via network. 
     Third Embodiment 
     The descriptions have been provided in the first embodiment and the second embodiment for the case where the measurement images having different densities are formed in the sub-scanning direction. In this case, in the main scanning direction, the measurement images corresponding to each color having the same density are separately formed. When the detection result of the measurement image of medium density is close to the detection result Ref of the shutter rear side, sometimes, the opening/closing operation of the shutter  407  is wrongly determined. In the present embodiment, a description is provided with regard to the image forming apparatus which forms the measurement image having different densities in the main scanning direction. Note that the same symbols are used for the functional components which are identical to those as described in the first embodiment and the second embodiment and the description thereof will be omitted. 
       FIG. 13  is a diagram for explaining a relation of opening/closing state of the shutter  407  and detection/non-detection of the density detection sensor  117  with respect to the measurement image according to the present embodiment. As shown in  FIG. 13 , the measurement images having different densities are formed in the main scanning direction. For example, as shown in Table 5 as below, the measurement images having different densities are separately formed for each color between the same sheets. Further, the measurement images are formed such that the densities of the neighboring measurement images in the sub-scanning direction are different from each other. In this case, in one detection, the measurement images having different densities are to be detected. Thereby, there causes large difference between the detection result Ref of the shutter rear side and the detection result of the measurement images of low density, medium density, or high density. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 SENSOR 
                 SENSOR 
                 SENSOR 
               
               
                   
                 117F 
                 117C 
                 117R 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Correction 
                 Cyan 
                 Magenta 
                 Yellow 
               
               
                 Color 
               
               
                 First sheet 
                 Low density 
                 High density 
                 Medium density 
               
               
                 interval 
               
               
                 Second sheet 
                 Medium density 
                 Low density 
                 High density 
               
               
                 interval 
               
               
                 Third sheet 
                 High density 
                 Medium density 
                 Low density 
               
               
                 interval 
               
               
                   
               
            
           
         
       
     
     As mentioned, in the image forming apparatus according to the present embodiment, the measurement images having different densities are separately formed for each color between the same sheets in the main scanning direction. This certainly prevents a situation where the shutter  407  is wrongly determined that it is in the closed state whereas actually it is in the open state. 
     Fourth Embodiment 
     For example, there is sometimes a case where causes problem in slidability of each part due to adhesion of scattering objects such as toner on the shutter  407 . In this case, sometimes, the shutter  407  is half-opened (a half-opened state). In the present embodiment, a description is provided with regard to an image forming apparatus capable of coping with a case where so called half-opened state is caused. Note that, in the present embodiment, similar to the case of the third embodiment, the measurement images having different densities are formed in the main scanning direction. Further, the same symbols are used for the functional components which are identical to those as described in the first, second, and third embodiments and the description thereof will be omitted. 
       FIG. 14  is a graph for explaining detection of the half-opened state of the shutter  407  by the density detection sensor. In  FIG. 14 , detection result of the density detection sensor is represented for each color separately. A vertical axis shows the detection result of the density detection sensor. A lateral axis shows measurement image density area (low density, medium density, high density). Further, in the following, a description is provided for a case where the shutter  407  is in the half-opened state only between the normal images page  1  and page  2  shown in  FIG. 13 . 
     In  FIG. 14 , a black round mark in each graph shows that the shutter  407  was half-opened. Further, a white round mark represented by a solid line shows that the shutter  407  was normally opened. Further, a white round mark represented by a dotted line shows that previous opening/closing of the shutter was normal. Further, a dotted line in each graph shows the detection result of the shutter rear side. It is noted that even if the density of the measurement image to be detected is changed, the density of the shutter rear side is not changed. Thereby, if the shutter  407  is in the closed state, the density value is equal to the density value of the shutter rear side. If the shutter  407  is in the half-opened state, it means that, comparing the latest detection result with the detection result this time, the detection result this time is closer to the density value of the shutter rear side than the latest detection result. 
     The image forming apparatus according to the present embodiment determines whether the shutter  407  is in the half-opened state or not by comparing vector approximating to the detection result of the shutter rear side with the detection result detected when operated normally. One example of the processing procedure of this determination is shown below as STEP 1 to STEP 6. 
     STEP 1: Detect density of the shutter rear side (Ref). STEP 2: Obtain latest detection result through which it is determined that the shutter is in the open state (n−1). STEP 3: Obtain detection result this time (n). STEP 4: Compare the normal and the latest detection result with the detection result this time. Then, determine whether ratio of the difference is more than predetermined ratio or not (for example, 10% or more) (((n)−(n−1)/(n)*100%). STEP 5: If it is determined, in STEP 4, the ratio of the difference is 10% or more, further determine whether (n) is a value closer to (Ref) than (n−1). STEP 6: If (n) is a value closer to (Ref) than (n−1), perform STEP 1 to 5 for the detection results of the rest of the colors. If it is found that (n) is a value closer to (Ref) than (n−1) for the rest of the colors (C, M, Y), it is determined that the shutter  407  is in the half-opened state and the measurement image is not normally detected. Then, the gradation correction is stopped. 
     As mentioned, in the image forming apparatus according to the present embodiment, it is possible to determine whether the shutter  407  is in the half-opened state or not. This enables to more surely restrict the occurrence of tone jump when performing the gradation correction. 
     Further, the descriptions have been provided in the first to the fourth embodiments with regard to the example where the detection sensor  117  is provided with the shutter  407  and the density of the image is adjusted based on the measurement result of the measurement image by the density detection sensor  117 . However, relative misregistration of the yellow image, magenta image, cyan image, and black image may be corrected based on the measurement result of the measurement image by the density detection sensor  117 . In this case, the stations  120 ,  121 ,  122 , and  123  form a measurement image for separately measuring relative position of the images of each color component on the intermediate transfer body  106 . Then, based on the measurement result of the measurement image by the density detection sensor  117 , the image forming position in the respective stations  120 ,  121 ,  122 , and  123  is adjusted. In the image forming apparatus, if the difference between a sensor output value corresponding to the measurement image for separately measuring the relative position of the images of each color component and a sensor output value corresponding to the reference member is a predetermined reference value or below, the image forming position needs to be adjusted based on the measurement result. In this case, it is possible to restrict a case where the relative position of the images of each color component is wrongly changed by the sensor output value output when the shutter  407  is not opened. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2015-153699, filed Aug. 3, 2015, which is hereby incorporated by reference herein in its entirety.