Patent Publication Number: US-11644667-B2

Title: Image forming apparatus and control method of image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/941,290, filed on Mar. 30, 2018, the entire contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an image forming apparatus, and a control method of the image forming apparatus. 
     BACKGROUND 
     In the related art, in a laser exposure-type color multifunction peripheral (MFP), or a printer, a control method of performing an operation to shift a position of printing for each color (hereinafter referred to as “color position shift operation”), or the like, at a point of time in which a continuous printing output or an intermittent printing output passes a preset time, has been used. However, in the color position shift operation of the related art, there is a case in which a color position shift is too large as a result of temperature changes inside a laser scanning unit, caused by heat generation by a polygon mirror motor, or the like, especially when continuous printing is performed, or when the preset time is too long. In addition, in the color position shift operation of the related art, there is also a case in which, when the preset time is too short, color position shift operations are performed too frequently, and as a result, productivity of a printing output decreases, or the lifespan of a developer or a photoconductive drum decreases. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an external view of an information processing device according to an embodiment. 
         FIG.  2    is a diagram of a scanning system of a printing unit. 
         FIG.  3    is another diagram of the scanning system of the printing unit. 
         FIG.  4    is a block diagram of a control system of the information processing device. 
         FIG.  5    is a diagram which illustrates an orientation of a first sensor used for horizontal synchronization. 
         FIG.  6    is a diagram which illustrates an orientation of the first sensor and a second sensor which detects a change in scanning position. 
         FIG.  7    is a diagram which describes a method of detecting a change in scanning position. 
         FIG.  8    is a diagram which illustrates another example of an orientation of the first sensor and the second sensor. 
         FIG.  9    is a timing diagram of signals of the first sensor and the second sensor in scanning positions right after the time color position shift operation was performed. 
         FIG.  10    is a timing diagram of signals of the first sensor and the second sensor in scanning positions after some passage of time from the time color position shift operation was performed. 
         FIG.  11    is a flowchart which illustrates an example of power-on processing which checks for the need for color position shift operation according to an embodiment. 
         FIG.  12    is a conceptual diagram showing example patterns produced during the color position shift operation. 
         FIG.  13    is a flowchart which illustrates steps of the color position shift operation. 
         FIG.  14    is a flowchart which illustrates steps carried out to determine whether to perform color position shift operation or the correction processing. 
     
    
    
     DETAILED DESCRIPTION 
     An image forming apparatus includes first and second light sensors positioned in a laser scanning system of at least one color, such that scanned light is detected by the first light sensor and then by the second light sensor and light sensing surfaces of the first and second light sensors are not parallel, and a control unit connected to the first and second light sensors and configured to determine a time difference in the timing of light detection by the first and second light sensors and to execute a color position shift operation upon determining that the time difference is greater than a first threshold value. 
     Hereinafter, an information processing device, an information processing system, and a control method of the information processing device according to an embodiment will be described with reference to drawings. In the following embodiment, a multifunction peripheral is depicted as an example of the information processing device. 
       FIG.  1    is an external view of an information processing device  100  according to the embodiment. 
     As illustrated in  FIG.  1   , the information processing device  100  is a multifunction peripheral which can forma toner image on a sheet. The sheet is, for example, paper, or the like. The sheet may be any sheet on which the information processing device  100  can form an image. 
     Further, the information processing device  100  can read an image on a sheet. The sheet may be any sheet from which the information processing device  100  can read an image. The information processing device  100  generates digital data by reading an image illustrated on the sheet, and generates an image file. 
     The information processing device  100  is provided with a display  110 , a control panel  120 , a printing unit  130 , a sheet accommodating unit  140 , an image reading unit  150 , a plurality of sensor units  160 Y- 160 K (shown in  FIG.  3   ), sensors  210 , a control unit  170 , and a storage unit  180 . The printing unit  130  of the information processing device  100  may be a device which fixes a toner image onto a sheet. According to the embodiment, the printing unit  130  will be described as an example of a device which fixes a toner image. 
     The display  110  is an image display device such as a liquid crystal display, or an organic electroluminescence (EL) display. The display  110  displays various information related to the information processing device  100 . In addition, the display  110  outputs a signal corresponding to an operation input performed by a user to the information processing device  100 . The display  110  receives an operation of the user. 
     The control panel  120  includes a plurality of buttons. The control panel  120  receives an operation inputs by the user. The control panel  120  outputs a signal according to an operation performed by the user to the information processing device  100 . The display  110  and the control panel  120  may be configured as an integrated touch panel. 
     The printing unit  130  executes image forming processing. In the image forming processing, the printing unit  130  forms an image on a sheet based on image information generated by the image reading unit  150 , or image information received from outside of the device through a communicating path. In addition, as will be described later, the printing unit  130  includes a light source, a driving unit of the light source, a polygon mirror, a driving unit of the polygon mirror, or the like. In detail, the printing unit  130  includes, for example, four photoconductive drums (shown in  FIG.  2   ), intermediate transfer belt ITB having an endless belt shape, four primary transfer rollers PTRY-PTRK (shown in  FIG.  2   ). The primary transfer rollers PTRY-PTRK contact a surface of the intermediate transfer belt ITB. The four photoconductive drums (shown in  FIG.  2   ) correspond to yellow color, magenta color, cyan color and black color respectively. The printing unit  130  forms toner image on each of the photoconductive drums respectively, transfers the toner image formed on the photoconductive drums onto the intermediate transfer belt ITB (to execute a primary transfer) in collaboration with the primary transfer rollers PTRY-PTRK, and then further transfer the toner image on the intermediate transfer belt ITB onto a sheet (to execute a secondary transfer). 
     The sheet accommodating unit  140  accommodates sheets which are subjected to the image forming by the printing unit  130 . 
     The image reading unit  150  reads an image of a reading target as brightness and darkness of light. For example, the image reading unit  150  reads an image printed on a sheet set as a reading target. The image reading unit  150  records image data read from the reading target. The recorded image data may be transmitted to another information processing device through a network. The recorded image data may be formed as an image on a sheet using the printing unit  130 . 
     Each of the plurality of sensor units  160 Y- 160 K includes a pair of light detecting sensors, detects light radiated from the light source provided in the printing unit  130 , and outputs the detection result to the control unit  170 . 
     The sensors  210  acquire density values of a toner image formed on the surface of intermediate transfer belt ITB when carrying out the alignment processing. When visible images are formed on the intermediate transfer belt ITB, the sensors  210  acquire different density values. In one embodiment, the sensors  210  are arranged at front and rear sides of the intermediate transfer belt ITB, such that from the viewpoint of  FIG.  2   , the sensor  210  at the rear side is behind and obscured by the sensor  210  at the front side. The control unit  170  controls the information processing device  100  according to a control application program or setting stored in the storage unit  180 . The control unit  170  performs a color position shift operation based on the detection result output by the sensor units  160 Y- 160 K. Here, the color position shift operation is an operation to correct a shift in the printing position of each color which occurs due to a position shift, or the like, of scanning systems of optical systems of two or more colors, which are provided in the printing unit  130 . The color position shift operation will be described later. 
     The control unit  170  includes a processor and a memory. The processor performs the operation of functional units described herein by executing programs or the like stored in the memory or the storage unit  180 . The processor is, for example, a central processing unit (CPU). Alternatively, the functions of the control unit  170  can be realized by a control circuit, an ASIC, a programmed processor, and a combination thereof. The memory is, for example, volatile memory, non-volatile memory, or a combination thereof. 
     The storage unit  180  stores a control application program, setting, various threshold values, a scanning speed of the light source, and for each of the sensor units  160 Y- 160 K, a time measurement representing an amount of time that elapses between light detections by the pair of light detecting sensors therein, and an angle θ formed by the pair of light detecting sensors. The storage unit  180  is, for example, a flash memory, a hard disk drive (HDD), or a solid state drive (SSD). 
     An example structure of the scanning system of the printing unit  130  will be described below, using  FIGS.  2  and  3   .  FIG.  2    is a diagram which illustrates the structure of the scanning system of the printing unit  130  according to the embodiment.  FIG.  3    is another diagram which illustrates the structure of the scanning system of the printing unit  130  according to the embodiment which is viewed from above. 
     As illustrated in  FIG.  2   , the scanning system of the printing unit  130  includes a first light source (not illustrated), an fθ lens  1321 , a second light source (not illustrated), an fθ lens  1322 , a third light source (not illustrated), a polygon mirror  1301 , an fθ lens  1323 , a fourth light source (not illustrated), an fθ lens  1324 , reflection mirrors  1341 ,  1342 ,  1351 ,  1352 ,  1361 ,  1362 ,  1363 ,  1364 ,  1381 ,  1382 ,  1383  and  1384 , and sensor units  160 Y,  160 M,  160 C, and  160 K. The sensor units  160 Y,  160 M,  160 C, and  160 K are positioned adjacent to the end of reflection mirrors  1381 ,  1382 ,  1383  and  1384  in the scanning direction, respectively, as shown in  FIG.  3   . The configuration of the embodiment is an example of including four light sources, and it is not limited to this. The light sources may be two or more. In addition, in the example illustrated in  FIG.  3   , the sensor unit is provided for each color; however, the sensor unit may be provided for just one color, e.g., the black color. 
     The respective first light source, second light source, third light source, and fourth light source are semiconductor lasers, for example. The first light source is a light source corresponding to black color, for example. The second light source is a light source corresponding to cyan color, for example. The third light source is a light source corresponding to magenta color, for example. The fourth light source is a light source corresponding to yellow color, for example. 
     The polygon mirror  1301  polarizes light beams which are input by rotating under a control of the control unit  170 . 
     The reflection mirrors  1341  and  1351  reflect a light beam which is polarized by the polygon mirror  1301 . 
     A light beam reflected by the mirror  1351  is input to the reflection mirror  1361 , and then the reflection mirrors  1361  and  1381  guide the light beam toward a photoconductive drum for the black color. The sensor unit  160 K is used for horizontal synchronization and a change in scanning position in the sub-scanning direction with respect to the light beam reflected by the reflection mirror  1381 . 
     The light beam reflected by the reflection mirror  1351  is also input to the reflection mirror  1362 , and then the reflection mirrors  1362  and  1382  guide the light beam toward a photoconductive drum for the cyan color. The sensor unit  160 C is used for horizontal synchronization and a change in scanning position in the sub-scanning direction with respect to the light beam reflected by the reflection mirror  1382 . 
     The reflection mirrors  1342  and  1352  reflect the light beam which is polarized by the polygon mirror  1301 . 
     The light beam reflected by the reflection mirrors  1342  and  1352  is input to the reflection mirror  1363 , and then the reflection mirrors  1363  and  1383  guide the light beam toward a photoconductive drum for the magenta color. The sensor unit  160 M is used for horizontal synchronization and a change in scanning position in the sub-scanning direction with respect to the light beam reflected by the reflection mirror  1383 . 
     The light beam reflected by the reflection mirrors  1342  and  1352  is input to the reflection mirror  1364 , and then the reflection mirrors  1364  and  1384  guide the light beam toward a photoconductive drum for the yellow color. The sensor unit  160 Y is used for horizontal synchronization and a change in scanning position in the sub-scanning direction with respect to the light beam reflected by the reflection mirror  1384 . 
     Subsequently, a configuration example of a control system of the information processing device  100  according to the embodiment will be described. 
       FIG.  4    is a block diagram of hardware components of the control system of the information processing device  100  according to the embodiment. As illustrated in  FIG.  4   , the information processing device  100  is provided with the display  110 , the control panel  120 , the printing unit  130 , the sensor unit  160 Y- 160 K, the control unit  170 , and the storage unit  180 . 
     The printing unit  130  is provided with a driving unit  131 , a light source  132 , a driving unit  133 , and a polygon mirror  1301 . 
     The sensor unit  160 Y for yellow color is provided with a first sensor  161 Y (also referred to herein as a “first light detecting sensor”), and a second sensor  162 Y (also referred to herein as a “second light detecting sensor”). The sensor unit  160 M for magenta color is provided with a first sensor  161 M, and a second sensor  162 M. The sensor unit  160 C for cyan color is provided with a first sensor  161 C, and a second sensor  162 C. The sensor unit  160 K for black color is provided with a first sensor  161 K, and a second sensor  162 K. Hereinafter, reference numeral  160  is used for representing the sensor units  160 Y- 160 K, reference numeral  161  is used for representing the first sensors  161 Y- 161 K, and reference numeral  162  is used for representing the second sensors  162 Y- 162 K. 
     The control unit  170  is configured to function as a change amount detecting unit  171 , a counter  172 , and an image forming unit  173 . 
     In the example illustrated in  FIG.  4   , one light source in the plurality of light sources is illustrated by being extracted. The one light source is the first light source for black color ( FIG.  2   ), for example. 
     The driving unit  131  drives the light source  132  according to a control of the control unit  170 . 
     The driving unit  133  drives the polygon mirror  1301  according to a control of the control unit  170 . 
     Each of the first sensors  161 Y,  161 M,  161 C and  161 K is used for horizontal synchronization, and outputs the detection result to the change amount detecting unit  171  of the control unit  170 . 
     Each of the second sensors  161 Y,  161 M,  161 C and  161 K is configured to detect a change in scanning position in the sub-scanning direction with respect to the light beam, and outputs the detection result to the change amount detecting unit  171 . 
     For example, in a case of black color, the change amount detecting unit  171  obtains a time difference between the detection of the light beam by the first sensor  161 K and the detection of the light beam by the second sensor  162 K. For example, the change amount detecting unit  171  obtains the time difference by causing the counter  172  to start counting when the first sensor  161 K outputs the detection result, and counting a time until the second sensor  162 K outputs the detection result. The change amount detecting unit  171  determines whether or not the time difference is large enough to require color position shift operation by comparing the obtained time difference with a predetermined threshold value stored in the storage unit  180 . In addition, when information denoting a detected point of time (e.g., a time stamp) is included in the detection result output by the sensor  160 K, the change amount detecting unit  171  may obtain the time difference from the detected point of time. 
     The counter  172  performs a start of counting, and ending of counting according to a control of the change amount detecting unit  171 . 
     The image forming unit  173  performs image forming processing by controlling the printing unit  130 . The image forming unit  173  causes the printing unit  130  to form an image according to a received instruction. The received instruction is, for example, position shift processing with respect to each color. The received instruction may be printing, copying, faxing, or the like, received from a user or a maintenance person. 
     Subsequently, an example of a location and an orientation of the sensor unit  160 K will be described. The other sensor units  160 Y,  160 M and  160 C have the same configuration as that of the sensor unit  160 K. 
       FIG.  5    is a diagram which illustrates an example of a location and an orientation of the first sensor  161 K included in the sensor unit  160 K which is used for horizontal synchronization according to the embodiment. As illustrated in  FIG.  5   , the first sensor  161 K which is used for horizontal synchronization is disposed so that the longitudinal direction of the first sensor  161 K is orthogonal to the scanning direction on a substrate  1600 K. 
       FIG.  6    is a diagram which illustrates an example of a location and an orientation of the first sensor  161 K and the second sensor  162 K, which is configured to detect a change in scanning position according to the embodiment. As illustrated in  FIG.  6   , the second sensor  162 K is disposed on the substrate  1600 K at a predetermined angle which is different from the angle of the first sensor  161 K, with respect to the scanning direction on the substrate  1600 K. The predetermined angle is set to an angle θ with respect to the first sensor  161 . 
       FIG.  7    is a diagram which describes a method of detecting a change in scanning position according to the embodiment. 
     In  FIG.  7   , the reference numeral g 1  denotes a scanning position obtained from the most recent color position shift operation. In the case of the scanning position of the reference numeral g 1 , the position at which the light passes, as detected by the first sensor  161 K is a 1 , and the position at which the light passes, as detected by the second sensor  162 K is b 1 . A time difference between the detection of light at the position a 1  and the detection of light at the position b 1  is t 1 . 
     In addition, in  FIG.  7   , a reference numeral g 2  denotes a scanning position that is shifted after a time has elapsed since the most recent completion of color position shift operation. In the case of the scanning position of the reference numeral g 2 , the position at which the light passes, as detected by the first sensor  161 K is a 2 , and a position at which the light passes, as detected by the second sensor  162 K is b 2 . A time difference between the detection of light at the position a 2  and the detection of light at the position b 2  is t 2 , which is larger than t 1 . 
     In addition, the distance between the detected positions of the light in the case of the reference numeral g 1  and the detected positions of the light in the case of the reference numeral g 2  (representing a shift in scanning position in the sub-scanning direction) is set to Δy [mm]. In addition, an angle between the first sensor  161 K and the second sensor  162 K is set to θ. 
     If a scanning speed (i.e., moving speed of the scanned light on the surface of the photoconductive drum) is set as 
                     Δ   ⁢           ⁢   y     =       v   ·     (       t   ⁢           ⁢   2     -     t   ⁢           ⁢   1       )         tan   ⁢           ⁢   θ               (   1   )               
v [mm/s], the change in scanning position in the sub-scanning direction Δy [mm] is expressed by equation (1).
 
     By determining an allowable amount of the shift Δy [mm], that is, an allowable shift amount, it is possible to determine an allowable change in the time difference between the detection of light at a position on one sensor and the detection of light at a position on the other sensor, that is, it is possible to determine a threshold value for the time difference. In addition, in  FIG.  2   , a horizontal direction with respect to a paper denotes a direction of the change Δy [mm] (Δyy, Δym, Δyc, and Δyb). In addition, the allowable amount of the change Δy [mm] is less than 2 lines, for example. When a resolution is 600 dpi, one line is approximately 42 [μm]. 
     The configurations of the first sensor  161 K and the second sensor  162 K illustrated in  FIGS.  6  and  7    are examples, and they are not limited to these. 
       FIG.  8    is a diagram which illustrates another example of an orientation of the first sensor  161 K and the second sensor  162 K according to the embodiment. In  FIG.  8   , the direction of the arrow g 100  denotes a scanning direction of the scanning light. 
     A diagram in a region denoted by a reference numeral g 101  is an example in which the second sensor  162 K is slightly rotated to the counterclockwise direction with respect to the first sensor  161 K. 
     A diagram in a region denoted by a reference numeral g 102  is an example in which the first sensor  161 K is slightly rotated to the clockwise direction with respect to the second sensor  162 K. 
     A diagram in a region denoted by a reference numeral g 103  is an example in which the first sensor  161 K is slightly rotated to the counterclockwise direction with respect to the second sensor  162 K. 
     In the information processing device  100 , the first sensor and the second sensor are provided in respective optical systems corresponding to each of the colors. Alternatively, in the information processing device  100 , only the first sensor  161 K and the second sensor  162 K may be provided in the optical system fora color of which a use frequency is high, for example, black. Alternatively, in the information processing device  100 , the first sensor and the second sensor may be provided in an optical system corresponding to at least one color that is not black. Alternatively, in the information processing device  100 , one sensor unit may be disposed with respect to any one of optical systems of colors on both ends which are disposed in the sheet conveying direction. The colors on both ends in the sheet conveying direction are, for example, a yellow color or a black color in  FIG.  2   , in a case of four colors. 
     In the examples illustrated in  FIGS.  5  to  7   , the pair of sensors are disposed on one substrate, however, a sensor in which a light receiving unit corresponding to the pair of sensors is provided in one chip may be adopted. In addition, the pair of sensors may be provided on separate substrates. 
     In the examples illustrated in  FIGS.  4  to  8   , an example in which the sensor unit is provided as the pair of sensors is described, however, the number of the sensors for one sensor unit may be two or more. In this case, predetermined angles of at least two sensors with respect to the light scanning direction is different, that is, the angle formed by the two sensors is angle θ. 
     In addition, an angle θ formed by the first sensor  161  and the second sensor  162 , and the time difference t 1  in an initial position of a scanning line may be stored in the storage unit  180  at a time of shipment, for example. In a case where the first sensor  161  and the second sensor  162  are provided in each of the light sources  132 , the angle θ formed by the first sensor  161  and the second sensor  162 , and the time difference t 1  in an initial position of a scanning line may be stored for each of the light sources  132  in the storage unit  180  at a time of shipment, for example. 
     Subsequently, examples of signals of the first sensor  161  and the second sensor  162  at a scanning position after a time of the passage of light from recent execution of color position shift operation will be described. 
       FIG.  9    is a timing diagram of signals of the first sensor  161  and the second sensor  162  at the scanning position after a passage of time from the most recent execution of the color position shift operation according to the embodiment. In  FIG.  9   , a horizontal axis denotes a time, and a vertical axis denotes a level of the signals. 
     As illustrated in  FIG.  9   , a time difference obtained right after the most recent prior execution of color position shift operation, between the detection of light by the first sensor  161  and the detection of light by the second sensor  162  is t 1 . 
     Subsequently, examples of signals of the first sensor  161  and the second sensor  162  at the scanning position after some passage of time will be described. 
       FIG.  10    is a timing diagram of signals of the first sensor  161  and the second sensor  162  at the scanning position after some passage of time from a time the color position shift operation was most recently executed. In  FIG.  10   , a horizontal axis denotes a time, and a vertical axis denotes a level of the signals. 
     As illustrated in  FIG.  10   , a time difference between the timing of the detection of light by the first sensor  161  and the timing of the detection of light by the second sensor  162  is t 2 . 
     Subsequently, an example of procedure of processing which is performed by the control unit  170  will be described. 
       FIG.  11    is a flowchart which illustrates an example of power-on processing, which is carried out by the control unit  170  and checks for the need for color position shift operation according to the embodiment. In the example illustrated in  FIG.  11   , it is assumed that color position shift operation is performed when the information processing device  100  enters a power ON state from a power OFF state, and the measured time difference t 1  between the detection of the light by the first sensor  161  and the detection of the light by the second sensor  162  is stored in the storage unit  180 . In addition, it is assumed that the first sensor  161  is disposed so as to be perpendicular to the scanning direction, and the angle formed by the first sensor  161  and the second sensor  162  is θ. 
     (ACT S 1 ) The change amount detecting unit  171  starts counting at a falling timing at which a detection result is changed from an H (high) level to an L (low) level, when the first sensor  161  outputs the detection result. 
     (ACT S 2 ) The change amount detecting unit  171  obtains the time difference t 2  in the timing of detecting of light by the first sensor  161  and the second sensor  162  by ending counting at a falling timing at which the detection result is changed from the H level to the L level, when the second sensor  162  outputs the detection result. 
     (ACT S 3 ) The change amount detecting unit  171  reads the time difference t 1 , the scanning speed v, the angle θ formed by the first sensor  161  and the second sensor  162 . Subsequently, the change amount detecting unit  171  obtains the change Δy in the scanning position in the sub-scanning direction by substituting the time difference t 1 , the scanning speed v, the angle θ which is formed, and the obtained time difference t 2  which are read in the equation (1). 
     (ACT S 4 ) The control unit  170  compares the change Δy in the scanning position in the sub-scanning direction which is calculated by the change amount detecting unit  171  with the first threshold value stored in the storage unit  180 . Subsequently, the control unit  170  proceeds to processing in ACT S 5  when the change Δy in the scanning position in the sub-scanning direction is equal to or more than the first threshold value (Yes in ACT S 4 ). Alternatively, the control unit  170  ends the processing when the change Δy in the scanning position in the sub-scanning direction is less than the first threshold value (No in ACT S 4 ). 
     (ACT S 5 ) The control unit  170  performs the color position shift operation. The control unit  170  then ends the processing after the color position shift operation. 
     In addition, the control unit  170  performs processing from ACT S 1  to ACT S 5  after an elapse of a predetermined time (for example, one minute) and every printing ending time, after ending of printing of a predetermined number of sheets, at a predetermined point of time and at a time of ending printing, every time of printing one sheet, and the like. 
     Here, an example of color position shift operation will be described. 
     As illustrated in  FIG.  4   , a case in which the printing unit  130  includes the first light source to the fourth light source for four colors will be described as an example. When light sources of four colors are provided, the information processing device  100  is also provided with four optical systems such as mirrors. In addition, as illustrated in  FIG.  2   , mirrors are provided for each color. In printing, when printing of only a black color is frequently performed, a frequency of light emitting of the first light source for black color becomes high. Due to this, when a temperature of the optical system for black color becomes higher than the optical system of another color, a difference between the optical systems occurs in optical characteristics, and there is a case in which a shift occurs between the change Δy 1  in the scanning position in the sub-scanning direction of the black color and the change Δy 2  in the scanning position in the sub-scanning direction of another color. 
     Here, an example of the color position shift operation will be described. 
     A shift in printing position in each color is caused by any of the following: 
     I. A wavelength of light is shifted due to a temperature change. 
     II. A positioning component in the device is subjected to thermal expansion. 
     III. A distance between the optical element and the photoconductive drum changes, for example, as a result of an exchange of any part of the printing unit  130 , or the like, of the photoconductive drum. 
     In such cases, the control unit  170  instructs the image forming unit  173  to form an image so that a distance at respective predetermined positions on the upstream side and the downstream side in the transport direction becomes the same as each other, for example. This technique is described in Japanese Patent Application No. 2017-084732. 
     In the related art, a control for the color position shift operation, or the like, is performed at a point of time when the total cumulative time for continuous or intermittent printing exceeds a preset maximum. 
     In the color position shift operation according to this embodiment, the control of color position shift operation is described with reference to  FIG.  12   .  FIG.  13    is a flowchart which illustrates steps of the color position shift operation. 
     The control unit  170  causes the printing unit  130  to form a predetermined first pattern with a first toner (e.g., black toner) and a predetermined second pattern with a second toner different from the first toner (e.g., yellow toner), on the surface of the intermediate transfer belt ITB (ACT  901 ). The sensors  210  detect the patterns formed on the surface of the intermediate transfer belt ITB which passes through the sensors  210  (ACT  902 ). The control unit  170  calculates distances tr 1  and tr 2  on the basis of output of the sensor  210  at the rear side. Further, the control unit  170  calculates distances tf 1  and tf 2  on the basis of output of the sensor  210  at the front side (ACT  903 ). The printing unit  130  carries out the color position shift operation such that tr 1 , tr 2 , tf 1  and tf 2  become equal values by shifting an output timing of emission of the laser light from the light sources in the sub-scanning direction (ACT  904 ). 
     When the color position shift operation is executed at a point of time at which a preset time elapses, if the preset time is too long, there is a case in which a temperature change in the laser scanning unit becomes too large due to heat generation of the optical system such as a motor for rotating the polygon mirror, and a color position shift also becomes too large, especially when continuous printing is continued. In addition, in the related art, when the preset time is too short, there is a case in which a color position shift operation is too frequently executed, and productivity of a printing output decreases, or a lifespan of the developer or the photoconductive drum decreases. 
     According to the above described embodiment, two or more sensors of which angles are different with respect to the optical system of at least one color, and the change amount detecting unit  171  which detects a time difference in detecting timing of the sensors are provided. In this manner, according to the embodiment, since the color position shift operation is performed when a time difference in the timing of light detection by the two sensors has changed by a predetermined amount after the execution of the most recent color position shift operation, it is possible to correct for the color position shift at a more proper timing as compared to the related art. In addition, according to the embodiment, it is possible to suppress a decrease in productivity of a printing output, or a decrease in lifespan of the developer or the photoconductive drum. 
     In addition, in the example illustrated in  FIG.  11   , an example in which color position shift operation is performed when the change Δy in the scanning position in the sub-scanning direction is equal to or more than the threshold value is described; however, it is not limited to this. For example, when the change Δy in the scanning position in the sub-scanning direction is one line or more, and less than two lines, the control unit  170  performs a correction of color position shift rather than performing the color position shift operation. 
       FIG.  14    is a flowchart which illustrates steps carried out to determine whether to perform color position shift operation or the correction processing. In addition, description of the same processing given in  FIG.  11    will be omitted by using the same reference numerals. 
     (ACT S 1  to ACT S 3 ) The change amount detecting unit  171  performs ACT S 1  to ACT S 3 . The change amount detecting unit  171  proceeds to processing in ACT S 11  after the processing. 
     (ACT S 11 ) The control unit  170  compares the change Δy in the scanning position in the sub-scanning direction which is calculated by the change amount detecting unit  171  with the first threshold value stored in the storage unit  180 . The first threshold value is a value of two lines, for example. Subsequently, when the change Δy in the scanning position in the sub-scanning direction is equal to or more than the first threshold value (Yes in ACT S 11 ), the control unit  170  proceeds to processing in ACT S 5 . Alternatively, the control unit  170  proceeds to processing in ACT S 12 , when the change Δy in the scanning position in the sub-scanning direction is less than the first threshold value (No in ACT S 11 ). 
     (ACT S 12 ) The control unit  170  compares the change Δy in the scanning position in the sub-scanning direction which is calculated by the change amount detecting unit  171  with the second threshold value stored in the storage unit  180 . The second threshold value is, for example, a distance between adjacent two scanning lines. Subsequently, the control unit  170  proceeds to the correction processing in ACT S 13  when the change Δy in the scanning position in the sub-scanning direction is the second threshold value or more, and is less than the first threshold value (Yes in ACT S 12 ). Alternatively, the control unit  170  ends the processing when the change Δy in the scanning position in the sub-scanning direction is less than the second threshold value (No in ACT S 12 ). 
     (ACT S 13 ) The control unit  170  performs correction processing. The control unit  170  ends the processing after the correction processing. 
     Here, an example of the correction processing will be described. 
     For example, it is assumed that the change Δy in the scanning position in the sub-scanning direction of a black color is the second threshold value or more, and less than the first threshold value. 
     The control unit  170  controls the driving unit  131  so as to advance or delay a driving timing of the first light source ( FIG.  2   ) for black color, depending on whether a value of a shift of one line is a positive value or a negative value. The control unit  170  may advance or delay a driving timing, by performing a control of shifting a timing of a line buffer provided in the driving unit  131 . The control unit  170  may shift a frequency of a clock signal of phase locked loop (PLL) which is used when controlling the printing unit  130  between printing and printing, and may perform a correction of one line of a horizontal synchronous timing at the printing start position, similarly. 
     The control unit  170  performs the correction processing by advancing or delaying a light emitting timing of the light source  132  in this manner. 
     In addition, the control unit  170  similarly controls the driving unit  131  so as to advance or delay the driving timing of the light source  132  depending on whether the value of the shift of one scanning line is a positive value or a negative value, when the change Δy in the scanning position in the sub-scanning direction of the light source  132  of another color is also the second threshold value or more, and less than the first threshold value. In addition, when the value of the shift is a positive value, it is a case of being shifted to the right direction with respect to the paper in  FIG.  2   , for example. When the value of the shift is a negative value, it is a case of being shifted to the left direction with respect to the paper in  FIG.  2   , for example. 
     According to the above described embodiment, two or more sensors of which angles are different with respect to the optical system of at least one color, and the change amount detecting unit  171  which obtains a time difference in the timing of detecting of light by the sensors, are provided. In this manner, according to the embodiment, since the color position shift operation is performed when a time difference between the light detection by the two sensors is changed by a predetermined amount since the most recent execution of the color position shift operation, it is possible to only correct the color position shift when the time difference changed by less than the predetermined amount. 
     In the above described each embodiment, an example in which the control unit  170  is a hardware functional portion is described; however, the control unit may be a functional portion which is executed, using software. 
     According to the above described at least one embodiment, it is possible to correct a color position shift by using two or more sensors of which orientations are different with respect to the optical system of at least one color, and the change amount detecting unit  171  which measures a time difference in the timing of detecting light by the sensors. 
     While certain embodiments have been described these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms: furthermore various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.