Patent Publication Number: US-11385562-B1

Title: Image forming apparatus and position control method

Description:
FIELD 
     Embodiments described herein relate generally to an image forming apparatus and a position control method. 
     BACKGROUND 
     When forming an image on a sheet, the image forming apparatus scans the scanning area of a photoreceptor drum with a laser beam along the main scanning direction. The laser beam contains a signal in units of a pixel. Thus, the image forming apparatus forms a plurality of dots along the main scanning direction by scanning the laser beam through the scanning area from a dot start position to a dot end position. There may be cases where the positions of dots formed along the main scanning direction deviate from the reference or intended positions, thus there is a possibility that the position of the image on the sheet deviates from an expected or intended location. In order to prevent the position of the image from deviating, it is necessary to correct the positions of dots along the main scanning direction to be at appropriate positions. 
     In the related art, there is an image forming apparatus that detects a scanning laser beam with a sensor in order to prevent a deviation in the positions of dots along the main scanning direction. The image forming apparatus determines the timing for starting formation of the dots (dotting) along the main scanning direction based on a horizontal synchronization signal output from the sensor in response to the detection of the laser beam. 
     However, the reaction time of the sensor from the exposure by the laser beam to the actual output of the horizontal synchronization signal by the sensor may differ depending on the intensity of the laser beam. For example, different light intensities may be output according to the type of sheet being printed, the state of the photoreceptor drum deterioration, and the like. Thus, there may be a problem with deviation of the positions of dots along the main scanning direction from the reference positions if the laser beam output varies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an image forming apparatus according to an embodiment. 
         FIG. 2  is a schematic configuration of an image forming apparatus according to an embodiment. 
         FIG. 3  is a schematic configuration of an exposing unit. 
         FIG. 4  is a schematic configuration of a printing unit. 
         FIG. 5  is a block diagram of an image forming apparatus according to an embodiment. 
         FIG. 6  is a block diagram of a control unit, 
         FIGS. 7-9  are diagrams illustrating examples of a calculation process of a delay amount difference value by an image forming apparatus according to an embodiment. 
         FIG. 10  is a flowchart illustrating a calculation process of the delay amount difference value by an image forming apparatus according to an embodiment. 
         FIG. 11  is a flowchart illustrating a correction process of dot timing along a main scanning direction by an image forming apparatus according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An image forming apparatus and a position control method are described. The apparatus and control method are capable of reducing deviations from intended dotting positions along a main scanning direction that might otherwise be caused by changes in the characteristics of a laser beam. 
     According to one embodiment, an image reading apparatus includes an exposure unit, a light sensor, and a controller. The exposure unit is configured to selectively emit light to positions along a first scan direction to write pixels in accordance with a main scan line of image data. The light sensor is at a position to receive light from the exposure unit and configured to output a synchronization signal according to the detection of the light emitted for each main scan line of image data. The controller is configured to adjust a starting pixel position along the first scan direction for a main scan line of image data based on a change in an output interval of the synchronization signal resulting from a change in an intensity of light emitted by the exposure unit. 
     An image forming apparatus and a position control method according to certain, non-limiting example embodiments will be described with reference to the drawings. 
       FIG. 1  is a diagram illustrating an example of an internal configuration of an image forming apparatus  1  according to the embodiment. The image forming apparatus  1  is an electrophotographic image forming apparatus. For example, the image forming apparatus  1  is a multi-function peripheral (MFP). The image forming apparatus  1  may be referred to as a printer or a copier in some instances. In the present embodiment, the case where the image forming apparatus  1  is a double-tandem type image forming apparatus will be described as an example, but the present disclosure is not limited to this type of image forming apparatus. 
     The image forming apparatus  1  can generate image data. The image data is digital data generated by reading an image from recording medium such as sheet of paper or the like or the image data may be provided to the image forming apparatus  1  from an external device. The image forming apparatus  1  forms an image corresponding to the image data on a sheet by using toner. For example, the sheet is paper, film, or the like. In general, the sheet may be made of any material as long as the image forming apparatus  1  can form an image on the sheet. 
     The image forming apparatus  1  includes an operation display unit  2 , a scanner unit  3 , a printing unit  4 , a paper feeding unit  5 , a conveying unit  6 , and a paper ejecting unit  7 . 
     The operation display unit  2  includes a display unit  11  and an operation unit  12 . 
     The display unit  11  operates as an output interface and displays characters, text and images to a user. For example, the display unit  11  is a display device such as a liquid crystal display (LCD) or an organic Electro Luminescence (EL) display. The display unit  11  displays various information related to operations and functions of the image forming apparatus  1 . 
     The operation unit  12  operates as an input interface for a user and receives inputs reflecting the instructions from the user. For example, the operation unit  12  includes a plurality of buttons, keys, switches, or the like. The operation unit  12  receives user input operations via the buttons and the like. 
     The display unit  11  and the operation unit  12  may be integrated with each other as a touch panel display or the like. For example, the operation display unit  2  may be a touch panel type liquid crystal display. That is, the operation display unit  2  may operate as both an output device and an input device. 
     In a case where the operation mode of the image forming apparatus  1  is a scan (scanner) mode, the scanner unit  3  reads image information from an object to be scanned. The scanner unit  3  comprises, for example, a contact image sensor (CIS), a charge coupled device (CCD), or the like. The scanner unit  3  reads an image from a sheet, document, or other object by using a sensor to generate image data. 
     In a case where the operation mode of the image forming apparatus  1  is a copy (copier) mode, the printing unit  4  prints images on sheets based on the image data generated by the scanner unit  3 . In other operation modes, the printing unit  4  may print images based on image data acquired from another information processing apparatus (an external device) via a network or the like. The printing unit  4  forms an image on a sheet with toner. A sheet with an image formed thereon may be referred to as a hard copy, a printout, or the like. 
     As non-limiting examples of toner, a decolorable toner, a non-decolorable toner (“normal toner”), or a decorative toner (“specialty toner”) are described. The decolorable toner can be decolorized by an external stimulus, such as heat, light of a specific wavelength, or pressure. In this context “decolorized” or “decolorable” refers to an image or material that changes from an initial, printed color to either become a different color matching a base color of the paper on which the image was printed or otherwise substantially visually undetectable with an unaided eye. External stimuli which might be utilized for decolorizing decolorable images are temperature changes, exposure to light of a specific wavelength, and pressure changes, or combinations of such stimuli. 
     As a decolorable toner, any toner may be used as long as the toner has the above-described characteristics. For example, a colorant of the decolorable toner may be a leuco dye. The decolorable toner may be appropriately combined with a color developing agent, a decolorizing agent, a discoloration temperature adjusting agent, and the like. 
     The paper feeding unit  5  supplies the sheets to the printing unit  4 . The paper feeding unit  5  supplies sheets one by one to the printing unit  4  in accordance with a timing at which the printing unit  4  forms the toner image. The paper feeding unit  5  includes paper feeding cassettes  15 ,  16 , and  17 . Each of the paper feeding cassettes  15 ,  16 , and  17  stores sheets of a predetermined size and type. In this context, a sheet type may be based, for example, on sheet thickness, such that plain paper is one type and thick paper is another type. 
     The paper feeding cassettes  15 ,  16 , and  17  include pickup rollers  15 - 1 ,  16 - 1 , and  17 - 1 , respectively. The pickup rollers  15 - 1 ,  16 - 1 , and  17 - 1  respectively take out a sheet from the paper feeding cassettes  15 ,  16 , and  17 . The pickup rollers  15 - 1 ,  16 - 1 , and  17 - 1  supply the taken-out sheets to the conveying unit  6 . 
     Sheets specifically for forming decolorable images may be stored in anyone of the plurality of paper feeding cassettes  15 ,  16 , and  17 . In some examples, since an image of a sheet formed with the decolorable toner may be later decolorized, the sheet may be reused after the decolorization processing of the previously printed sheet in a decolorizing mode. Thus, it is possible to reuse such a sheet a plurality of times. 
     The conveying unit  6  transports the sheet between various portions of the image forming apparatus  1 . In the following description, since the sheets are conveyed from the paper feeding unit  5  to the paper ejecting unit  7 , points along the sheet conveyance path that are closer to the paper feeding unit  5  (may be referred to as “on the paper feeding unit  5  side” are referred to as being on an upstream side along a sheet conveyance direction Vs of the sheet conveyance path, and points along the sheet conveyance path that are closer to the paper ejecting unit  7  (may be referred to as “on the paper ejecting unit  7  side”) are referred to as being on a downstream side along the sheet conveyance direction Vs of the sheet conveyance path. 
     The conveying unit  6  includes a pair of conveying rollers  20  and a pair of registration rollers  21 . 
     The conveying roller  20  conveys the sheets supplied from the pickup rollers  15 - 1 ,  16 - 1 , and  17 - 1  to the registration roller  21 . The conveying roller  20  abuts the downstream end of the sheet (“leading edge”) against the nip  21 - 1  of the registration rollers  21 . The conveying roller  20  thus adjusts the position of the downstream end of the sheet. 
     The registration roller  21  temporarily stops the sheet being conveyed by the conveying roller  20 . The registration roller  21  then sends the sheet toward a secondary transfer unit  37  in accordance with the timing at which the toner image is formed on an intermediate transfer body  32 . The toner image on the intermediate transfer body  32  is transferred to the sheet by the secondary transfer unit  37 . The registration rollers  21  face each other along a conveying path between the conveying roller  20  and the secondary transfer unit  37 . A nip  21 - 1  is formed between the pair of registration rollers  21 . 
     The registration roller  21  aligns the downstream ends of each sheet sent from the conveying roller  20  at the nip  21 - 1 , and after that the alignment, conveys the sheet along to the secondary transfer unit  37  side. 
     A reversing unit  25  reverses the sheets after fixing unit  40  by a switchback operation. The reversing unit  25  conveys the reversed sheet back to the front of the registration roller  21  again. The reversing unit  25  reverses the sheet so a toner image can be formed on the back surface of a sheet. Thus, double-sided printing can be performed on sheets. 
     The printing unit  4  is illustrated in more detail in  FIG. 2 .  FIG. 2  is a diagram illustrating one example of the schematic configuration of image forming apparatus  1  according to the embodiment. 
     The printing unit  4  includes a transfer unit  30  and a fixing unit  40 . 
     The transfer unit  30  includes an exposing unit  31 , the intermediate transfer body  32 , a cleaning blade  33 , image generating units  34  and  35 , primary transfer rollers  36 - 1  and  36 - 2 , a secondary transfer unit  37 , a temperature detection unit  38 , and a temperature adjustment unit  39 . 
     The temperature detection unit  38  detects the temperature around the secondary transfer unit  37 . For example, the temperature detection unit  38  is a temperature sensor. 
     The temperature adjustment unit  39  functions to adjust the temperature around the secondary transfer unit  37  based on the detection result of the temperature detection unit  38 . For example, the temperature adjustment unit  39  is a fan. The temperature adjustment unit  39  may have more functions and purpose beyond just adjusting the temperature around the secondary transfer unit  37 . For example, the temperature adjustment unit  39  may function for the purpose of ejecting or venting ozone from the image forming apparatus. 
     The image transfer process in the image forming apparatus  1  includes a first transfer step and a second transfer step. 
     In the first transfer step, the primary transfer rollers  36 - 1  and  36 - 2  can each transfer a toner image from respective photoreceptor drums  34 - 1  and  35 - 1  to the intermediate transfer body  32 . 
     In the second transfer step, the secondary transfer unit  37  transfers the toner images formed on the intermediate transfer body  32  to the sheet. In general, the toner images from each of the generating units  34  and  35  are stacked one upon the other on the intermediate transfer body  32  before being transferred together to the sheet at the secondary transfer unit  37 . 
     The scanner unit  3  reads an image from a sheet, document, or object to be scanned. For example, the scanner unit  3  reads a color image on a sheet and the generates image data corresponding to scanned color image. The scanner unit  3  outputs the generated image data to an image processing unit  8 . 
     The image processing unit  8  controls the exposing unit  31  based on a color signals corresponding to the image data from the scanner unit  3 . 
     The exposing unit  31  irradiates the photoreceptor drums  34 - 1  and  35 - 1  of the image generating units  34  and  35  with light (exposure) according to respective color signals. 
     The intermediate transfer body  32  rotates in the direction of an arrow A of  FIG. 2 . The intermediate transfer body  32  may be a belt. A toner image is formed on the surface of the intermediate transfer body  32 . 
     The cleaning blade  33  removes the toner still attached to the intermediate transfer body  32  after the secondary transfer process. The cleaning blade  33  is, for example, a plate-shaped member. In some examples, cleaning blade  33  is made of a resin such as urethane resin. The tip of the cleaning blade  33  presses against the intermediate transfer body  32  to scrape off toner from the intermediate transfer body  32 . Instead of, or in addition to, a cleaning blade  33 , a charged brush may be brought into contact with the intermediate transfer body  32 . 
     The image generating units  34  and  35  form images using toner of different respective colors or types. The image generating units  34  and  35  are provided along the intermediate transfer body  32  in series. Image generating unit  34  is before the image generating unit  35  in the present example. 
     In the present example, image generating unit  34  utilizes a decolorable toner. The image generating unit  34  (referred to in this context as “decolorized image generating unit  34 ”) contains the decolorable toner and thus transfers a decolorable toner image to the intermediate transfer body  32 . In the present example, the decolorable toner has a blue color when initially fixed to a sheet. 
     The image generating unit  35  is downstream of the decolorized image generating unit  34  along the rotation direction A of the intermediate transfer body  32 . The image generating unit  35  utilizes a non-decolorable toner  32 . In the embodiment, the image generating unit  35  (referred to in this context as “non-decolorized image generating unit  35 ”) contains a non-decolorable black toner. 
     The image forming apparatus  1  of this example executes printing in the following two modes:
         Monochrome toner mode (An image is formed with non-decolorable toner)   Decolorable toner mode (An image is formed with decolorable toner).       

     The mode can be selected by the user of the image forming apparatus  1 . 
     In the monochrome toner mode, an image is formed by operation of the non-decolorized image generating unit  35  using of black non-decolorable toner. The monochrome toner mode can be selected if the user desires to print a general monochrome image. For example, this mode is used if important materials are to be printed without reusing paper. 
     In the decolorable toner mode, an image is formed by operating just the decolorized image generating unit  34  using the decolorable toner. The decolorable toner mode can be selected if the paper is to be reused at a later time. 
     The decolorized image generating unit  34  and the non-decolorized image generating unit  35  have generally the same configuration although different toners are contained therein. Therefore, the decolorized image generating unit  34  will be particularly described as a representative example of both the image generating units  34  and  35 . 
     The decolorized image generating unit  34  includes a photoreceptor drum  34 - 1 , a developer  34 - 2 , a charger  34 - 3 , and a cleaning blade  34 - 4 . 
     The photoreceptor drum  34 - 1  is one type of image carrier. The photoreceptor drum  34 - 1  has a photoreceptor on the outer peripheral surface. For example, the photoreceptor is an organic photoconductor (OPC). 
     The developer  34 - 2  contains a developing agent. The developer includes toner. The developer  34 - 2  supplies toner to the photoreceptor drum  34 - 1 . For example, the toner is used as a one-component developing agent or as a two-component developing agent in combination with a carrier. For example, as a carrier, iron powder or polymer ferrite particles having a particle size of several tens of microns are used. In the present embodiment, a two-component developing agent containing a non-magnetic toner is used. 
     The charger  34 - 3  uniformly charges the surface of the photoreceptor drum  34 - 1 . 
     The cleaning blade  34 - 4  removes toner attached to the photoreceptor drum  34 - 1 . 
     The outline of the operation of the decolorized image generating unit  34  will be described. 
     The photoreceptor drum  34 - 1  is charged to a predetermined potential by the charger  34 - 3 . Next, the exposing unit  31  selectively irradiates the photoreceptor drum  34 - 1  with light (e.g., a laser beam). The electrostatic potential of the area irradiated with light in the photoreceptor drum  34 - 1  changes. An electrostatic latent image is formed on the surface of the photoreceptor drum  34 - 1  as a result of the selective irradiation. The electrostatic latent image on the surface of the photoreceptor drum  34 - 1  is then developed with the developing agent of the developer  34 - 2 . That is, a developed image formed toner is on the surface of the photoreceptor drum  34 - 1 . 
     The developed image formed on the surface of the photoreceptor drum  34 - 1  is then transferred to the intermediate transfer body  32  by the primary transfer roller  36 - 1  facing the photoreceptor drum  34 - 1 . 
     An image is formed by using only the decolorable toner in this example. That is, by the operation of the decolorized image generating unit  34 , a decolorable toner image is formed on the intermediate transfer body  32 . 
     If just the non-decolorized image generating unit  35  operates, a non-decolorable toner image is formed on the intermediate transfer body  32 . 
     The non-decolorized image generating unit  35  includes a photoreceptor drum  35 - 1 , a developer  35 - 2 , a charger  35 - 3 , and a cleaning blade  35 - 4 . 
     The respective one of the primary transfer rollers  36 - 1  or  36 - 2  is used for transferring the toner image formed by the image generating units  34  and  35  to the intermediate transfer body  32 . 
     The second transfer step will be described. 
     The secondary transfer unit  37  includes a secondary transfer roller  37 - 1  and a facing secondary transfer roller  37 - 2 . In the secondary transfer unit  37 , the intermediate transfer body  32  and the secondary transfer roller  37 - 1  are in contact with each other. For rectifying paper jams and the like, the intermediate transfer body  32  and the secondary transfer roller  37 - 1  may be configured to be separable from each other. 
     A bias voltage is applied to the secondary transfer roller  37 - 2 . Thus, an electric field is generated between the secondary transfer roller  37 - 2  and the secondary transfer roller  37 - 1 . With this electric field, the secondary transfer unit  37  transfers the toner image formed on the intermediate transfer body  32  to the sheet. The sheet is then guided to the fixing unit  40 . 
     The fixing unit  40  can be controlled to be in a fixing mode or a decolorizing mode. In the fixing mode, the toner image is fixed to the sheet. In the decolorizing mode, the toner image is decolorized. The fixing unit  40  fixes the toner image to the sheet by heat and pressure. For example, the fixing unit  40  includes a heated roller (heating unit) and a pressuring unit. After the fixing unit  40  the sheet can be ejected from the paper ejecting unit  7  to the outside of the image forming apparatus  1 . 
     The exposing unit  31  of the printing unit  4  will be described.  FIG. 3  is a plan view illustrating an example of a schematic configuration of the exposing unit  31  according to the present embodiment. 
     The exposing unit  31  includes a light source  50 , a light control circuit  51 , a light deflection unit  52 , a first imaging lens  53 , a second imaging lens  54 , an optical path changing unit  55 , an optical path changing unit  56 , a mirror  57 , and a sensor  58 . 
     The light source  50  in this example is a Laser Diode (LD) that emits a laser beam (light). In other examples, the light source  50  may be a Light Emitting Diode (LED). 
     The light control circuit  51  includes a laser driver that causes the light source  50  to emit light. The light source  50  outputs an optically-modulated laser beam based on an optical modulation signal by the control of the light control circuit  51 . For example, the optical modulation signal is generated based on an image data signal and a horizontal synchronization signal. The laser beam emitted from the light source  50  can be supplied to a collimator lens then be incident on the reflecting surfaces of the polygon mirror  52 - 1 . 
     The light deflection unit  52  in this example includes a regular polyhedral polygon mirror  52 - 1  having a reflecting surface formed on each side of the regular polyhedron. The polygon mirror  52 - 1  is rotated around the rotation axis  52 - 2  by the motor  52 - 3 . The polygon mirror  52 - 1  rotates in the direction of an arrow Ra at a steady angular velocity and deflects the laser beam at each reflecting surface generally increasing the direction of arrow Rb as the polygon mirror  52 - 1  rotates. It is generally preferable that the polygon mirror  52 - 1  rotates at constant angular velocity. 
     The polygon mirror  52 - 1  continuously reflects the laser beam output from the light source  50  at positions along the axial directions of the photoreceptor drums  34 - 1  and  35 - 1 . The polygon mirror  52 - 1  continuously reflects the laser beams from the light source  50  in a direction paralleling the scanning lines during reading the image by the scanner unit  3 . The laser beams reflected by the polygon mirror  52 - 1  pass through the first imaging lens  53  and the second imaging lens  54 , and at the exposure position on the photoreceptor drums  34 - 1  and  35 - 1 , the photoreceptor drums  34 - 1  and  35 - 1  are sequentially irradiated at positions along the axial directions. 
     The first imaging lens  53  and the second imaging lens  54  provide predetermined optical characteristics to the laser beam reflected by the polygon mirror  52 - 1 . The first imaging lens  53  and the second imaging lens  54  extend in the axial directions of the photoreceptor drums  34 - 1  and  35 - 1 . The first imaging lens  53  and the second imaging lens  54  help form an image on the photoreceptor drums  34 - 1  and  35 - 1  so that the relationship between the rotation angle of the polygon mirror  52 - 1  and the focal length satisfies an image height requirement. The first imaging lens  53  and the second imaging lens  54  cooperate with a cylindrical lens or the like to provide convergence to the laser beam reflected by the polygon mirror  52 - 1 . 
     The rotational axis of the photoreceptor drums  34 - 1  and  35 - 1  is parallel to the main scanning direction in the image formation process. The surfaces of the photoreceptor drums  34 - 1  and  35 - 1  are scanned along the main scanning direction with the laser beam reflected by the polygon mirror  52 - 1 . Reference numeral “Sc” in  FIG. 3  denotes the main scanning direction. The main scanning direction Sc is a direction parallel to the rotational axis. The sub-scanning direction is a direction perpendicular to the main scanning direction and thus corresponds to a circumferential direction of the photoreceptor drums  34 - 1  and  35 - 1 . 
     As depicted in  FIG. 4 , the optical path changing units  55  and  56  are arranged between the second imaging lens  54  and the photoreceptor drums  34 - 1  and  35 - 1 . 
     The optical path changing unit  55  directs a decolorizing image laser beam BD that passes through the second imaging lens  54  toward the photoreceptor drum  34 - 1 . The optical path changing unit  55  includes a plurality of mirrors  55 - 1 ,  55 - 2 , and  55 - 3 . The laser beam BD that passes through the second imaging lens  54  is reflected by the mirrors  55 - 1 ,  55 - 2 , and  55 - 3  in this order so as to be incident on the photoreceptor drum  34 - 1 . 
     The optical path changing unit  56  directs a non-decolorizing image laser beam BK that passes through the second imaging lens  54  toward the photoreceptor drum  35 - 1 . The optical path changing unit  56  includes a plurality of mirrors  56 - 1 ,  56 - 2 , and  56 - 3 . The laser beam BK that passes through the second imaging lens  54  is reflected by the mirrors  56 - 1 ,  56 - 2 , and  56 - 3  in this order so as to be incident on the photoreceptor drum  35 - 1 . 
     As depicted in  FIG. 3 , mirror  57  reflects light that passes through the first imaging lens  53  toward the sensor  58 . 
     The sensor  58  is arranged in an area away from the photoreceptor drums  34 - 1  and  35 - 1 . For example, the sensor  58  is positioned to receive light from near the starting edge side in the main scanning direction Sc. In other words, the sensor  58  receives light from a laser beam near the beginning of a scan in the main scanning direction Sc. The sensor  58  detects the laser beam with which scanning area J 1  will be scanned in order to synchronize the start timing of dot formation along the main scanning direction Sc. 
     In this context, the scanning area J 1  denotes the area on the surface of the photoreceptor drums  34 - 1  and  35 - 1 , which is scanned with the laser beam during the latent image formation process involving selective exposure of particular dot locations (image pixels) along the main scanning direction Sc according to the image data. The scanning start position in this context refers to the starting edge position of the scanning area J 1  along the main scanning direction Sc. The scanning start position is located at the starting edge of a scanning line. The scanning end position in this context refers to the ending edge position opposite to the scanning start position in the main scanning direction Sc. The scanning end position is located at the ending edge of a main scanning line formed in the scanning area J 1 . 
     The sensor  58  is a horizontal synchronization sensor in this example. The sensor  58  supplies a horizontal synchronization signal to the light control circuit  51 . The horizontal synchronization signal is used for a switching timing of each line formed in the scanning area J 1 . The horizontal synchronization signal can be used as a signal that indicates the end of a scanning of one line and the start of a scanning of another line (the next line). 
     The light control circuit  51  determines the time to start outputting the image data for each line based on the detection result of the sensor  58 . Specifically, the time is the dotting start time for dots along the main scanning direction Sc. 
       FIG. 5  is a block diagram illustrating an example of the functional configuration of the image forming apparatus  1  according to an embodiment. Each functional unit of the image forming apparatus  1  is connected via a system bus  100 . 
     The control unit  101  controls the operation of each functional unit of the image forming apparatus  1 . The control unit  101  executes various processes by executing a software program or the like. The program can be recorded in advance in, for example, the storage device  103 . The program may be recorded in advance in the memory  104  or an external recording medium or the like. The control unit  101  acquires the instructions input by the user via the operation display unit  2 . The control unit  101  executes a control process based on the acquired instruction(s). 
     For example, the control unit  101  can control the rotation speed of the secondary transfer roller  37 - 1 . 
     The control unit  101  may increase the temperature of the fixing unit  40  to switch to decolorizing mode from the fixing mode. That is, the control unit  101  operates the fixing unit  40  to be at two or more target temperatures. Specifically, the memory  104  stores two target temperatures (set points) for the fixing unit  40 . The control unit  101  loads the target temperature from the memory  104  according to the selected mode and operates the fixing unit  40  according to the target temperature. A first temperature is a temperature target in the decolorizing mode. A second temperature is a temperature target in the fixing mode. The second temperature is lower than the first temperature. 
       FIG. 6  is a block diagram illustrating an example of the functional configuration of the control unit  101  according to an embodiment. 
     The control unit  101  includes a time measuring unit  110 , a light amount switching unit  111 , a delay amount difference calculation unit  112 , a sheet information acquisition unit  113 , a deteriorated state acquisition unit  114 , and a timing correction unit  115 . 
     The time measuring unit  110  measures the interval at which the horizontal synchronization signal output from the sensor  58  is sent to the light control circuit  51 . The time measuring unit  110  measures the interval with a counter that counts, for example, the number of internal clock cycles. 
     The light amount switching unit  111  switches the power output of the laser beam from the light source  50 . In the present embodiment, the light amount switching unit  111  switches between a first light amount setting and a second light amount setting. The second light amount setting results in a laser beam that has less power (intensity) than the laser beam at the first light amount setting. The light supplied at first light amount setting is referred to as a “normal light amount,” and the light supplied at the second light amount setting is referred to as a “dimmed light amount”. In the present embodiment, the light amount switching unit  111  is configured to switch to between two different light amount settings, but in other examples the light amount switching unit  111  may be configured to switch between three or more light amount settings. 
     The delay amount difference calculation unit  112  calculates the difference (hereinafter, referred to as a “delay amount difference value”) between the delay amount when the laser beam is a normal light amount and the delay amount when the laser beam is a dimmed light amount. The delay amount referred to herein is the time from when the laser beam is first incident on the sensor  58  to when the sensor  58  subsequently outputs the horizontal synchronization signal to the light control circuit  51 . The delay amount difference calculation unit  112  records the calculated delay amount difference value in the storage device  103 . 
     The sheet information acquisition unit  113  acquires information (“sheet information”) indicating the type of sheet on which image formation will be performed. The type of sheet relevant in this context is, for example, whether the sheet is plain paper or thick paper. The sheet information can be generated, for example, when the user performs an operation with the operation unit  12  for selecting the type of sheet. 
     The deteriorated state acquisition unit  114  acquires information indicating the deteriorated state of the photoreceptor drums  34 - 1  and  35 - 1 . The deterioration referred to here is, for example, increased difficulty in changing the photoreceptor potential when exposed to the same light amount. The information indicating the deteriorated state of the photoreceptor drums  34 - 1  and  35 - 1  may be generated by any method. For example, the information may be a deterioration metric calculated or estimated based on the number of times the photoreceptor drums  34 - 1  and  35 - 1  have been used in an image formation process, the total exposure time to which each photoreceptor drum  34 - 1  and  35 - 1  has been subjected, and/or an age (e.g., the time since installation) of the photoreceptor drums  34 - 1  and  35 - 1 . 
     The light amount switching unit  111  acquires sheet information from the sheet information acquisition unit  113 . The light amount switching unit  111  also acquires the sheet type/light amount correspondence information recorded in advance in the storage device  103 . The sheet type/light amount correspondence information indicates a light amount to be used with different sheet types. For example, the normal light amount is to be used with plain paper, but the dimmed light amount is to be used with thick paper. 
     The light amount switching unit  111  specifies the light amount setting according to type of sheet based on the sheet type/light amount correspondence information. The light amount switching unit  111  switches the light amount setting for the laser beam output from the light source  50  to the specified light amount setting as necessary. 
     The light amount switching unit  111  may be configured to switch the light amount setting based on the information indicating the deteriorated state of the photoreceptor drums  34 - 1  and  35 - 1  as acquired from the deteriorated state acquisition unit  114 . In this case, the light amount switching unit  111  acquires the deteriorated state/light amount correspondence information from the storage device  103 . The deteriorated state/light amount correspondence information indicates the light amount to be used according to the deterioration state of the photoreceptor drums  34 - 1  or  35 - 1 . For example, the normal light amount is to be used with a more deteriorated photoreceptor drum, and the dimmed light amount is to be with a less deteriorated photoreceptor drum. The light amount switching unit  111  specifies the light amount setting according to the deteriorated state of the photoreceptor drums  34 - 1  and  35 - 1  based on the acquired deteriorated state/light amount correspondence information. The light amount switching unit  111  switches the light amount setting for the laser beam output from the light source  50  to the specified light amount as necessary. 
     The timing correction unit  115  acquires the delay amount difference value recorded in the storage device  103  by the delay amount difference calculation unit  112 . The timing correction unit  115  performs the control to correct the dotting start timing of dots in the main scanning direction Sc based on the acquired delay amount difference value. 
     For example, when the dimmed light amount is being used, the timing correction unit  115  corrects the delay in the dotting start timing of dots by the delay amount difference value. The delay amount difference value is, for example, a value such as a number of dots, a number of clock cycles, or a time. On the other hand, when the normal light amount is being used, the timing correction unit  115  may leave the dotting start timing of dots along the main scanning direction Sc unadjusted. 
     The network interface  102  transmits/receives data to/from another apparatus. The network interface  102  operates as an input interface and receives data transmitted from another apparatus. The network interface  102  also operates as an output interface and transmits data to another apparatus. 
     The storage device  103  stores various data. For example, the storage device  103  is a hard disk or a solid-state drive (SSD). For example, in this context various data may include digital data, screen data of a setting screen, setting information, a print job, a print job log, or the like. The various data may also include the above-described delay amount difference value, sheet type/light amount correspondence information, deteriorated state/light amount correspondence information, and the like. 
     The memory  104  temporarily stores the data being used by each functional unit. For example, the memory  104  is a random-access memory (RAM). For example, the memory  104  temporarily stores digital data, print jobs, print job logs, and the like. 
     The correction control of the deviation in the dotting positions of dots in the main scanning direction Sc caused by the change in the light amount of the laser beam will be described. 
     The image forming apparatus  100  according to the present embodiment changes the speed of scanning of the laser beam according to the type of sheet on which the image is being formed. For example, when a thick paper is to be used, the speed of scanning by the laser beam is controlled to be slower than that when plain paper is to be used. 
     In general, the faster the speed of scanning, the shorter the time that the laser beam dwells at the same nominal position, and therefore, it can be necessary to increase the intensity of the laser beam accordingly. Similarly, the slower the speed of scanning, the lower the intensity of the laser beam that can be used. However, delay times for output of the signal from the sensor  58  can also change with laser beam power. Generally, the higher the intensity of the laser beam, the shorter the delay time between when the laser beam enters the sensor  58  to when the sensor  58  outputs the horizontal synchronization signal to the light control circuit  51 . On the other hand, the lower the intensity of the laser beam, the longer the delay time between when the laser beam enters the sensor  58  to when the sensor  58  outputs the horizontal synchronization signal to the light control circuit  51 . 
     When delay time is long, the dotting start timing of dots in the main scanning direction Sc is delayed. Furthermore, since the light amount setting of the laser beam can be switched according to changes in the speed of scanning, the dotting positions of dots in the main scanning direction Sc can be further deviated from intended or expected position. 
     The image forming apparatus  1  according to the embodiment corrects the dotting start timing of dots in the main scanning direction Sc based on the delay amount difference value. The image forming apparatus  1  thus suppresses the occurrence of deviations in the dotting positions of dots in the main scanning direction Sc. 
     The image forming apparatus  1  according to the present embodiment calculates the delay amount difference value as in the following examples. 
       FIGS. 7, 8, and 9  are diagrams illustrating examples of the delay amount difference value calculation process by the image forming apparatus  1 . In  FIGS. 7 to 9 , the horizontal axis represents the time axis. 
     In  FIGS. 7 to 9 , the upper horizontal axes indicate the timings at which the laser beam emitted from the light source  50  enters the sensor  58 , respectively. As illustrated in each figure, the interval at which the laser beam enters the sensor is an equal interval, which is an interval Ta. The horizontal axis in the lower portions of  FIGS. 7 to 9  indicate the timing at which the sensor  58  outputs the horizontal synchronization signal to the light control circuit  51  according to the detection of the laser beam. 
       FIG. 7  illustrates the case where the light amount of the laser beam is a normal light amount.  FIG. 8  illustrates the case where the light amount of the laser beam is a dimmed light amount. As illustrated in  FIG. 7 , the delay amount (delay interval) from the time when the laser beam enters the sensor to the time when the sensor  58  outputs a horizontal synchronization signal to the light control circuit  51  is a fixed interval Tb. As illustrated by the horizontal axis in the lower portion, since the same delay amount (interval Tb) occurs in each period, the interval at which the horizontal synchronization signal is output from the sensor  58  is still equal to the interval Ta. That is, the interval at which the laser beam enters the sensor  58  and the interval at which the horizontal synchronization signal is output from the sensor  58  are the same, though offset from each other in time. 
     As illustrated in  FIG. 8 , the delay amount (delay interval) from the time when the laser beam enters the sensor to the time when the sensor  58  outputs a horizontal synchronization signal to the light control circuit  51  is the fixed interval Tc. The interval Tc is longer than the interval Tb. As illustrated by the horizontal axis in the lower portion of  FIG. 8 , since the same delay amount (interval Tc) occurs in each period, the timing at which the horizontal synchronization signal is output from the sensor  58  is still equal to the interval Ta, though offset in time. 
     The image forming apparatus  1  according to the present embodiment calculates the delay amount difference value by switching the light amount from the time when the laser beam enters the sensor  58  to the time when the laser beam enters the sensor  58  again in the next period (next main scanning). In the example illustrated in  FIGS. 7 to 9 , the delay amount difference value is the value of the interval (Tc−Tb). 
       FIG. 9  illustrates the case where the normal light amount is switched to the dimmed light from one period to the next (the next main scanning). The interval at which the horizontal synchronization signal is output from the sensor  58 , which is illustrated on the horizontal axis in the lower portion, is the interval Ta in a normal light amount period. 
     However, as illustrated in  FIG. 9 , the interval at which the horizontal synchronization signal is output from the sensor  58  changes for the next period when the light amount is switched. In this example, the normal light amount is switched to the dimmed light amount after the second period, thus the interval at which the horizontal synchronization signal will be output from the sensor  58  becomes the interval (Ta+Tc−Tb). 
     The image forming apparatus  1  according to the embodiment can calculate the delay amount difference value (interval (Tc−Tb)) from an interval (for example, the interval Ta) of the output of the horizontal synchronization signal in the normal period and an interval (for example, the interval (Ta+Tc−Tb)) of the output of the horizontal synchronization signal in a period in which the light amount is switched. 
     As expressed by the Equation (1), when the interval Ta is 200 μsec (microseconds), the interval (Tc−Tb) is 10 nsec (nanoseconds), and the length of the scanning line is 250 mm, the deviation in the dotting positions of dots in the main scanning direction Sc between the case of normal light amount and the case of dimmed light amount is 12.5 μm (microns).
 
[(250×1000)/(200×1000)]×10=12.5  Equation (1):
 
     In such a case, for example, when switching from the normal light amount to the dimmed light amount, the image forming apparatus  1  according to the embodiment corrects the dotting start position for dots along the main scanning direction Sc by 12.5 μm in the main scanning direction Sc. 
     Hereinafter, an example of the calculation process of the delay amount difference value by the image forming apparatus  1  will be described. 
       FIG. 10  is a flowchart illustrating the calculation process of the delay amount difference value by the image forming apparatus  1 . The time at which the calculation process of the delay amount difference value is executed is, for example, an initial start-up time, each start-up time, or a time between printing jobs of the image forming apparatus  1 . 
     The control unit  101  starts the rotation of the light deflection unit  52  of the exposing unit  31  (ACT 001 ). Next, the light control circuit  51  sets the light amount of the laser beam emitted from the light source  50  to the normal light amount based on the instruction from the control unit  101  (ACT 002 ). Next, the light control circuit  51  controls the light source  50  and starts emitting the laser beam (ACT 003 ). 
     Next, the control unit  101  detects whether or not the rotation of the light deflection unit  52  is stable (ACT 004 ). In a case where the control unit  101  detects that the rotation of the light deflection unit  52  is not stable (NO in ACT 004 ), the control unit  101  waits until the rotation becomes stable. When the control unit  101  detects that the rotation of the light deflection unit  52  is stable (YES in ACT 004 ), the control unit  101  controls the light control circuit  51  and starts detection of the horizontal synchronization signal output from the sensor  58 . The light control circuit  51  waits for the arrival of the horizontal synchronization signal (ACT 005 ). 
     When the light control circuit  51  detects the horizontal synchronization signal output from the sensor  58  (YES in ACT 005 ), the control unit  101  resets the counter of the time measuring unit  110  (ACT 006 ). As described above, the counter referred to herein is a counter for the time measuring unit  110  to measure the interval at which the horizontal synchronization signal is input to the light control circuit  51 . The light control circuit  51  again waits for the arrival of the horizontal synchronization signal (ACT 007 ). 
     When light control circuit  51  detects the horizontal synchronization signal output from the sensor  58  (YES in ACT 007 ), the control unit  101  acquires the count value of the counter of the time measuring unit  110  and resets the counter again (ACT 006 ). The count value acquired herein is, for example, a value indicating the interval (interval Ta) of the first period on the horizontal axis in the lower portion of  FIG. 9 . The time measuring unit  110  outputs the acquired count value to the delay amount difference calculation unit  112 . 
     Next, the light control circuit  51  switches the light amount of the laser beam emitted from the light source  50  to the dimmed light amount based on the instruction from the control unit  101  before the next horizontal synchronization signal is detected (ACT 009 ). Again, the light control circuit  51  waits for the arrival of the horizontal synchronization signal (ACT 010 ). 
     When the light control circuit  51  detects the horizontal synchronization signal output from the sensor  58  (YES in ACT 010 ), the control unit  101  acquires the count value of the counter of the time measuring unit  110  (ACT 011 ). The count value acquired herein is, for example, a value indicating the interval (interval (Ta+Tc−Tb)) of the second period on the horizontal axis in the lower portion of  FIG. 9 . The time measuring unit  110  outputs the acquired count value to the delay amount difference calculation unit  112 . 
     Next, the delay amount difference calculation unit  112  calculates the delay amount difference value. For example, the interval (Tc−Tb)) is calculated from the interval in the first period (a period with a length of interval Ta in this example) and the interval in the second period (a period with a length of interval (Ta+Tc−Tb) in this example) (ACT 012 ). Next, the delay amount difference calculation unit  112  records the calculated delay amount difference value in the storage device  103  (ACT 013 ). 
     The calculation process of the delay amount difference value by the image forming apparatus  1  illustrated in the flowchart of  FIG. 10  is thus completed. 
     In the process illustrated in the flowchart of  FIG. 10 , the image forming apparatus  1  first sets the normal light amount and then calculates the delay amount difference value by switching to the dimmed light amount. However, the present disclosure is not limited thereto, and the image forming apparatus  1  may first set the dimmed light amount and then calculate the delay amount difference value by switching to the normal light amount. 
     By the above-described processes, the delay amount difference value is calculated in advance and recorded in the storage device  103  before the printing job (image formation on the sheet) by the image forming apparatus  1 . When each printing job is executed, the image forming apparatus  1  reads the delay amount difference value from the storage device  103  according to the light amount setting of the laser beam and corrects the dotting positions of dots in the main scanning direction Sc by the laser beam. 
     Hereinafter, an example of the correction process of the dotting timing of dots in the main scanning direction Sc by the image forming apparatus  1  will be described. The processes described below are processes performed, for example, when each printing job is executed. 
       FIG. 11  is a flowchart illustrating the correction process of the dotting timing of dots in the main scanning direction Sc by the image forming apparatus  1 . 
     The sheet information acquisition unit  113  of the control unit  101  acquires the sheet information (ACT 101 ). As described above, the sheet information is information indicating the type of sheet (such as plain paper or thick paper) to be used in the printing. The sheet information acquisition unit  113  acquires the sheet information generated by, for example, the user performing an operation of selecting the type of sheet by the operation unit  12 . 
     Next, the light amount switching unit  111  acquires sheet information from the sheet information acquisition unit  113 . The light amount switching unit  111  also acquires the sheet type/light amount correspondence information recorded in advance in the storage device  103 . As described above, the sheet type/light amount correspondence information is information in which the type of sheet and the light amount are stored in correspondence with each other. 
     The light amount switching unit  111  specifies the light amount setting for the type of sheet indicated by the acquired sheet information by referring to the sheet type/light amount correspondence information. The light amount switching unit  111  sets the light amount for the laser beam output from the light source  50  to the specified light amount setting (ACT 102 ). 
     In this process, the light amount switching unit  111  is configured to determine the light amount to be used based on the sheet information acquired from the sheet information acquisition unit  113 , but the present disclosure is not limited thereto. For example, the light amount switching unit  111  may be configured to determine the light amount to be used based on the information indicating the present deterioration state of the photoreceptor drums  34 - 1  and  35 - 1  as acquired from the deteriorated state acquisition unit  114 . 
     In such a case, the light amount switching unit  111  identifies the light amount to be used based on the state of the photoreceptor drums  34 - 1  and  35 - 1  as indicated by the acquired information by referring to the deteriorated state/light amount correspondence information recorded in advance in the storage device  103 . Then, the light amount switching unit  111  switches the light amount setting to the specified light amount. 
     Next, the control unit  101  starts the rotation of the light deflection unit  52  of the exposing unit  31  (ACT 103 ). Next, the light control circuit  51  controls the light source  50  and starts emitting the laser beam (ACT 104 ). Next, the timing correction unit  115  of the control unit  101  acquires a delay amount correction value recorded in advance in the storage device  103  (ACT  105 ). The delay amount correction value acquired herein is, for example, a value calculated by the process illustrated by the flowchart of  FIG. 10 . 
     Next, the control unit  101  detects whether or not the rotation of the light deflection unit  52  is stable (ACT 106 ). When the control unit  101  detects that the rotation of the light deflection unit  52  is not stable (NO in ACT 106 ), the control unit  101  waits until the rotation becomes stable. When the control unit  101  detects that the rotation of the light deflection unit  52  is stable (YES in ACT 106 ), the control unit  101  controls the light control circuit  51  and starts detection of the horizontal synchronization signal output from the sensor  58 . The light control circuit  51  waits for the arrival of the horizontal synchronization signal (ACT 107 ). 
     When the light control circuit  51  detects the horizontal synchronization signal output from the sensor  58  (YES in ACT 107 ), the timing correction unit  115  performs correction control of the dotting timing of dots in the main scanning direction Sc based on the light amount setting determined by the light amount switching unit  111  in the process of the ACT  102 . 
     When the light amount setting determined by the light amount switching unit  111  is the normal light amount (YES in ACT 108 ), the timing correction unit  115  waits for a time corresponding to the delay amount difference value (ACT 109 ), and after that, starts the dotting of dots in the main scanning direction Sc by the laser beam (ACT 110 ). If the light amount setting determined by the light amount switching unit  111  is the dimmed light amount (NO in ACT 108 ), the timing correction unit  115  starts the dotting of dots in the main scanning direction Sc by the laser beam without performing the above-described waiting (ACT 110 ). 
     The control unit  101  repeatedly executes the processes of ACT  107  to ACT  110  until all the dotting of dots by the laser beam is completed (ACT  111 ). By the above description, the correction process for the dotting timing along the main scanning direction Sc is completed. 
     As described above, the image forming apparatus  100  according to the present embodiment includes a light source  50 , a sensor  58 , and a control unit  101 . The light source  50  emits a laser beam (light) that is used to scan the scanning area J. The sensor  58  is arranged at a position which is also irradiated with the laser beam. The sensor  58  detects the laser beam emitted at each main scanning. The sensor  58  outputs a horizontal synchronization signal (synchronization signal) that synchronizes the writing positions of dots (pixels) in the main scanning direction Sc according to the detection of the laser beam. The control unit  101  controls the dotting positions (writing positions) of dots in the main scanning direction Sc based on the change in the output interval of the horizontal synchronization signal that can be caused by a change in the light output amount of the laser beam. 
     With such a configuration, the image forming apparatus  1  can correct the deviation in the writing positions of dots (pixels) in the main scanning direction Sc, which may otherwise be caused due to the change in the light amount setting of the laser beam, based on a delay amount difference value calculated in advance. 
     According to the image forming apparatus  1  of the present embodiment, even when the light amount of the laser beam is changed according to, for example, the type of sheet and/or the deteriorated state of the photoreceptor drum(s), it is possible to suppress the deviation in the dotting positions of dots in the main scanning direction that might otherwise be caused by the change. Accordingly, the image forming apparatus  1  can prevent the occurrence of deviation in the transfer position of the image on the sheet. 
     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.