JP5965858B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that performs double-sided printing of a color image using an electrophotographic system.

  Electrophotographic image formation includes a process of charging a photoconductive drum as an image carrier, a process of drawing an electrostatic latent image of an image indicated by image data on the charged photoconductive drum, The method includes a step of supplying toner to the latent image to form a toner image, a step of transferring an image composed of the toner image to a sheet, and a step of fixing the image transferred to the sheet to the sheet.

  In the charging step, there are a method using a charging bias composed of a DC bias and a method using a charging bias in which a DC bias and an AC bias are superimposed as a method for charging the photosensitive drum.

  Regarding a method using a charging bias in which a DC bias and an AC bias are superimposed, a problem that the image quality is deteriorated due to the AC bias is pointed out, and a technique that can solve this problem has been proposed (for example, see Patent Document 1). .

  On the other hand, in the fixing step, the image transferred to the paper is heated and fixed on the paper. For this reason, it is known that the water content of the paper evaporates in the fixing step, and as a result, the paper size is slightly reduced. In double-sided printing, an image is fixed on the front side of the paper, and then an image is formed on the back side of the paper. Therefore, the magnification is different between the image formed on the front side and the image formed on the back side.

  Therefore, it is necessary to adjust the magnification in the main scanning direction and the magnification in the sub-scanning direction between the image formed on the front side of the sheet and the image formed on the back side. In order to adjust the magnification in the sub-scanning direction, when drawing an electrostatic latent image corresponding to the image formed on the front surface and when drawing an electrostatic latent image corresponding to the image formed on the back surface, A technique for changing the rotational speed of a polygon mirror, which is an optical deflector, is known (see, for example, Patent Document 2).

  This technique will be described. In double-sided color printing, a color image formed on the front surface of a paper is a front image, and a color image formed on the back surface of the paper is a back image. A mode for drawing an electrostatic latent image used for forming a front image is a first drawing mode, and a mode for drawing an electrostatic latent image used for forming a back image is a second drawing mode.

  The electrostatic latent image is drawn by repeatedly irradiating the peripheral surface of the photosensitive drum with the light beam deflected by the polygon mirror and drawing the main scanning line. According to Patent Document 2, the rotation speed of the polygon mirror in the second drawing mode is set higher than the rotation speed of the polygon mirror in the first drawing mode. Thereby, in one drawing of the main scanning line, the movement amount in the sub-scanning direction of the peripheral surface of the photosensitive drum is smaller in the second drawing mode than in the first drawing mode. Therefore, the dimension of the electrostatic latent image corresponding to the back image in the sub scanning direction is smaller than the dimension of the electrostatic latent image corresponding to the front image in the sub scanning direction. Therefore, when the back image is formed on the back surface of the paper reduced by forming the front image, the magnification of the front image in the sub-scanning direction and the magnification of the back image in the sub-scanning direction can be adjusted to be the same.

JP 2010-164866 A JP 2004-25841 A

  The developing roller supplies toner used for developing the electrostatic latent image to the photosensitive drum. In the above-mentioned Patent Document 1, in the case of using a bias in which a DC bias and an AC bias are superimposed as the developing bias applied to the developing roller, the cycle of the AC component of the developing bias and the cycle of the AC component of the charging bias are as follows. If close, the problem that development unevenness occurs periodically is disclosed.

  In double-sided printing, as described above, in addition to the problem of adjusting the magnification in the sub-scanning direction between the front image and the back image, when a color image is formed by superimposing a plurality of color toner images, There is a problem of preventing the toner image from being shifted in the scanning direction in the front and back images.

  According to the present invention, when a color image is formed by superimposing a plurality of color toner images, the toner image is shifted in the sub-scanning direction. Image forming apparatus that can prevent color image (back image) from being generated, can adjust the magnification in the sub-scanning direction between the front image and the back image, and can prevent deterioration in image quality caused by AC bias The purpose is to do.

  An image forming apparatus according to the present invention that achieves the above object is an image forming apparatus that prints color images formed by superimposing toner images of a plurality of colors on both sides of a sheet, each including a mirror unit, A plurality of optical deflectors that drive a mirror unit; and a plurality of image forming units that respectively form assigned color toner images among the plurality of color toner images. A charging unit including a charging bias generation unit that generates a charging bias including an AC bias, an image carrier charged by the charging bias applied by the charging unit, and the plurality of optical deflectors A light beam is emitted in a state where the mirror portion of the optical deflector is driven, and the image carrier is irradiated with the light beam deflected by the mirror portion so as to irradiate the main scanning line. And an exposure unit that draws an electrostatic latent image and a light receiving element that receives the light beam deflected by the mirror unit of the corresponding optical deflector, and is output from the light receiving element. Using this signal, a toner is supplied to the electrostatic latent image drawn on the image carrier, and a light beam detection signal generation unit that generates a light beam detection signal that serves as a reference for drawing the main scanning line. By doing so, the developing unit that forms the toner image, and the front surface image that is the color image formed on the front surface of the paper and the back image that is the color image formed on the back surface of the paper. In order to adjust the magnification in the scanning direction, a first drawing mode for drawing the electrostatic latent image used for forming the front surface image and a second drawing for drawing the electrostatic latent image used for forming the back surface image Mode and And a drive control unit that varies the speed of driving the mirror unit of the corresponding optical deflector, and the image forming apparatus is further formed on the image carrier of the plurality of image forming units. A transfer unit that transfers the color image on which the toner image is superimposed to the paper, a fixing unit that heats and fixes the color image transferred to the paper, and the color image on the surface of the paper A fixing unit that reverses the sheet and conveys the sheet to the transfer unit and the fixing unit in order to form the color image on the back surface of the sheet, and a preset first cycle. A first reference pulse generator for generating a first reference pulse, and a second reference pulse generator for generating a second reference pulse having a preset second period shorter than the first period. And the compound The phase of the light beam detection signal generated by the light beam detection signal generation unit provided in the image forming unit in the first drawing mode among the number of image forming units matches the phase of the first reference pulse. And adjusting the phase of the light beam detection signal generated by the light beam detection signal generation unit provided in the image forming unit of the second drawing mode among the plurality of image forming units. A first adjustment unit that adjusts the phase of the second reference pulse to match, and an image forming unit in the first drawing mode among the plurality of image forming units, and a predetermined first of the first reference pulse. The charging bias value at a timing synchronized with a reference phase is adjusted to be a predetermined value, and the second drawing of the plurality of image forming units is performed. In the second image forming unit, a second adjustment is performed so that the value of the charging bias at the timing synchronized with a predetermined second reference phase of the second reference pulse becomes the predetermined value. A section.

  In the image forming apparatus according to the present invention, the driving speed of the mirror unit of the optical deflector is different between the first drawing mode and the second drawing mode. Thereby, the magnification in the sub-scanning direction can be adjusted between the front image and the back image.

  Further, in the image forming apparatus according to the present invention, the first drawing mode is used to match the phases of the light beam detection signals generated by the light beam detection signal generation unit provided in each of the plurality of image forming units. In the case of (1), the first reference pulse is used, and in the case of the second drawing mode, the second reference pulse is used. The first reference pulse is a pulse having a preset first period. The second reference pulse is a pulse having a preset second period shorter than the first period.

  Therefore, in the image forming apparatus according to the present invention, in each image forming unit, in the first drawing mode, the main scanning line is drawn on the basis of the light beam detection signal whose phase is matched with the first reference pulse. This is repeated to draw an electrostatic latent image. Therefore, it is possible to prevent the toner image from being shifted in the sub-scanning direction when the surface image is formed. In each image forming unit, in the second drawing mode, an electrostatic latent image is formed by repeatedly drawing a main scanning line based on a light beam detection signal whose phase is matched with that of the second reference pulse. draw. Therefore, it is possible to prevent the toner image from shifting in the sub-scanning direction when the back image is formed.

  Therefore, according to the image forming apparatus of the present invention, when a color image is formed by superimposing a plurality of color toner images, the toner image is prevented from shifting in the sub-scanning direction in either the front image or the back image. it can.

  In the image forming apparatus according to the present invention, in the first drawing mode, when the phase of the light beam detection signal matches the phase of the first reference pulse, the phase of the first reference pulse is generated by the light beam detection signal. It will be the timing. In the second drawing mode, when the phase of the light beam detection signal is matched with the phase of the second reference pulse, the phase of the second reference pulse is the timing at which the light beam detection signal is generated.

  According to the image forming apparatus of the present invention, the second adjustment unit synchronizes with the first reference phase of the first reference pulse (the timing at which the light beam detection signal is generated in the first drawing mode). And the charging bias value at the timing synchronized with the second reference phase of the second reference pulse (the timing at which the light beam detection signal is generated in the second drawing mode). Are set to the same predetermined value.

  Therefore, according to the image forming apparatus of the present invention, it is possible to prevent the image quality from being deteriorated due to the AC bias in the method of charging the image carrier using the charging bias including the AC bias.

  Furthermore, according to the image forming apparatus of the present invention, the first light beam detection signals generated by the light beam detection signal generation unit provided in each of the plurality of image forming units are matched with each other in phase. A reference pulse and a second reference pulse are used. Further, the first reference pulse and the second reference pulse are used to set the charging bias to a predetermined value. Therefore, these two objects can be achieved without increasing the number of reference pulses.

  In the above configuration, as the first adjustment unit, each of the plurality of image forming units has a phase of the light beam detection signal that matches a phase of the first reference pulse in the first drawing mode. In the second drawing mode, the corresponding optical deflector is controlled so that the phase of the light beam detection signal coincides with the phase of the second reference pulse. A light beam detection signal adjusting unit for adjusting driving is provided.

  This configuration embodies the first adjustment unit.

  In the above configuration, each of the plurality of image forming units includes a clock generation unit that generates a clock having a predetermined cycle that is the cycle of the AC bias, and the charging bias generation unit uses the clock. In the first drawing mode, each of the plurality of image forming units includes the first bias of the first reference pulse as the second adjustment unit. In the second drawing mode, the value of the charging bias at the timing synchronized with the reference phase is the predetermined value, and in synchronization with the second reference phase of the second reference pulse. The timing at which the clock generation unit generates the clock is set so that the value of the charging bias at the determined timing becomes the predetermined value. It comprises a clock adjustment portion for integer.

  This configuration embodies the second adjustment unit.

  According to the present invention, when a color image is formed by superimposing a plurality of color toner images, it is possible to prevent the toner image from being shifted in the sub-scanning direction in the front image and the back image, and the front image and the back image. The magnification in the sub-scanning direction can be adjusted between and the image quality can be prevented from being deteriorated due to the AC bias.

2 is an explanatory diagram for explaining an outline of an internal structure of the image forming apparatus according to the embodiment. FIG. FIG. 2 is a block diagram illustrating a configuration of the image forming apparatus illustrated in FIG. 1. 1 is a block diagram of an electrostatic latent image forming system provided in an image forming apparatus according to an embodiment. FIG. 6 is an explanatory diagram illustrating transitions of respective drawing modes of a yellow image forming unit, a magenta image forming unit, a cyan image forming unit, and a black image forming unit in the first embodiment. FIG. 7 is a waveform diagram showing a light beam detection signal, a first reference pulse, and a second reference pulse in the process of transition from the first drawing mode to the second drawing mode in the yellow control unit. FIG. 6 is a waveform diagram showing a light beam detection signal, a first reference pulse, and a second reference pulse in the process of transition from the second drawing mode to the first drawing mode in the yellow control unit. It is a wave form diagram which shows the 1st example of the relationship between a light beam detection signal and a charging bias. It is a wave form diagram which shows the 2nd example of the relationship between a light beam detection signal and a charging bias. It is a wave form diagram which shows the 3rd example of the relationship between a light beam detection signal and a charging bias. FIG. 6 is a waveform diagram showing a relationship among a light beam detection signal, a first reference pulse, a clock, and a charging bias in Embodiment 2. FIG. 10 is a waveform diagram showing a relationship among a light beam detection signal, a first reference pulse, a third reference pulse, a clock, and a charging bias in Modification 1 of Embodiment 2. FIG. 10 is a waveform diagram illustrating a relationship among a light beam detection signal, a first reference pulse, a second reference pulse, a clock, and a charging bias in Embodiment 3.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is an explanatory diagram for explaining the outline of the internal structure of an image forming apparatus 1 according to an embodiment of the present invention. The image forming apparatus 1 can be applied to, for example, a digital multifunction machine having functions of a copy, a printer, a scanner, and a facsimile. The image forming apparatus 1 includes an apparatus main body 100, a document reading unit 200, and a document feeding unit 300.

  A document reading unit 200 is disposed on the apparatus main body 100, and a document feeding unit 300 is disposed on the document reading unit 200.

  The document feeder 300 functions as an automatic document feeder, and can continuously send a plurality of documents placed on the document placement unit 301 to the document reading unit 200.

  The document reading unit 200 includes a carriage on which an exposure lamp or the like is mounted, a document table made of a transparent member such as glass, a CCD (Charge Coupled Device) sensor, and a document reading slit (all not shown). The CCD sensor outputs the read original as image data.

  The apparatus main body 100 includes a sheet storage unit 101, an image forming unit 103, and a fixing unit 105. The sheet storage unit 101 is disposed at the lowermost part of the apparatus main body 100 and includes two sheet cassettes 101a and 101b that can store a bundle of sheets.

  Among the paper cassettes 101a and 101b, in the bundle of paper stored in the selected cassette, the uppermost paper is sent out toward the paper conveyance path 107 of the apparatus main body 100 by driving a pickup roller (not shown). . The sheet is conveyed to the image forming unit 103 through the sheet conveyance path 107.

  The sheet conveying path 107 extends in a substantially vertical direction from the lower side to the upper side along one side surface (the right side surface in FIG. 1) of the apparatus main body 100, and the other side surface (the left side surface in FIG. 1). ) And extends substantially horizontally along the lower side of the document reading unit 200. A discharge tray 131 is provided at the end.

  The image forming apparatus 1 prints color images formed by superimposing toner images of four colors (a plurality of colors) of yellow, magenta, cyan, and black on both sides of a sheet. The image forming unit 103 forms toner images of four colors. The image forming unit 103 includes a yellow image forming unit 111Y, a magenta image forming unit 111M, a cyan image forming unit 111C, and a black image forming unit 111BK, which are arranged according to the order in which the toner images are transferred to the transfer belt 113. These units are a plurality of image forming units that respectively form toner images of assigned colors among toner images of four colors (a plurality of colors). These units have the same configuration and will be described by taking the yellow image forming unit 111Y as an example.

  The yellow image forming unit 111Y includes a photosensitive drum 115 and an exposure unit 117. Around the photosensitive drum 115, a charging unit 119, a developing unit 121, and a cleaning unit 123 are arranged. The charging unit 119 charges the peripheral surface of the photosensitive drum 115. The exposure unit 117 generates a light beam modulated in accordance with image data (image data output from the document reading unit 200, image data transmitted from a personal computer, image data received by facsimile, etc.) and is charged. Irradiate the circumferential surface of the photosensitive drum 115. As a result, an electrostatic latent image corresponding to yellow image data is formed on the peripheral surface of the photosensitive drum 115. In this state, by supplying yellow toner from the developing unit 121 to the peripheral surface of the photosensitive drum 115 as an image carrier, a toner image corresponding to yellow image data is formed on the peripheral surface.

  The transfer belt 113 can move counterclockwise while being sandwiched between the photosensitive drum 115 and the primary transfer roller 125. The yellow toner image is transferred from the photosensitive drum 115 to the transfer belt 113. The yellow toner remaining on the peripheral surface of the photosensitive drum 115 is removed by the cleaning unit 123. The above is the description of the yellow image forming unit 111Y.

  Above the yellow image forming unit 111Y, magenta image forming unit 111M, cyan image forming unit 111C, and black image forming unit 111BK, containers containing corresponding color toners, that is, a yellow toner container 127Y, a magenta toner container 127M, A cyan toner container 127C and a black toner container 127BK are arranged. Toner for each color is supplied with toner from the corresponding container.

  As described above, a yellow toner image is transferred to the transfer belt 113, and a magenta toner image is transferred onto the toner image. Similarly, a cyan toner image and a black toner image are transferred onto the transfer belt 113. . As a result, a color toner image is formed on the transfer belt 113. In this way, a color toner image (color image) is formed on the transfer belt 113 by transferring the toner image of each color pattern superimposed on the transfer belt 113. The color toner image is transferred by the secondary transfer roller 129 onto the sheet conveyed from the sheet storage unit 101 described above. The transfer belt 113 and the secondary transfer roller 129 constitute a transfer portion. The transfer unit transfers the color image on which the toner images formed on the peripheral surfaces of the photosensitive drums 115 of the four (plural) image forming units 111Y, 111M, 111C, and 111BK are superimposed onto the sheet.

  The sheet on which the color toner image (color image) is transferred is sent to the fixing unit 105. The fixing unit 105 includes a heating roller and a fixing roller. The paper on which the color toner image is transferred is sandwiched between these rollers. As a result, heat and pressure are applied to the color toner image and the paper, and the color toner image is fixed to the paper. The sheet is discharged to a discharge tray 131.

  In the case of duplex printing, the paper that has passed through the fixing unit 105 is sent to the switchback unit 106. The paper is sent to the paper transport path 108 by the switchback unit 106. The sheet conveyance path 108 extends in a substantially vertical direction from the upper side to the lower side along one side surface (the right side surface in FIG. 1) of the apparatus main body 100. The sheet conveyance path 108 joins the sheet conveyance path 107 at a location 109 upstream of the secondary transfer roller 129. The paper sent to the paper transport path 108 is sent to the paper transport path 107 in a state where the paper is reversed, and is sent to the secondary transfer roller 129 and the fixing unit 105 through the paper transport path 107. As a result, a color image can be transferred and fixed on the back side of the paper.

  The switchback unit 106, the sheet conveyance path 107, and the sheet conveyance path 108 constitute a conveyance unit. After fixing the color image on the front surface of the paper, the transport unit reverses the paper and transports it to the secondary transfer roller 129 (transfer unit) and the fixing unit 105 in order to form a color image on the back surface of the paper.

  FIG. 2 is a block diagram showing a configuration of the image forming apparatus 1 shown in FIG. The image forming apparatus 1 has a configuration in which an apparatus main body 100, a document reading unit 200, a document feeding unit 300, an operation unit 400, a control unit 500, and a communication unit 600 are connected to each other by a bus. Since the apparatus main body 100, the document reading unit 200, and the document feeding unit 300 have already been described, description thereof will be omitted.

  The operation unit 400 includes an operation key unit 401 and a display unit 403. The display unit 403 has a touch panel function, and displays a screen including soft keys. The user operates the soft key while viewing the screen to make settings necessary for executing functions such as copying.

  The operation key unit 401 is provided with operation keys including hard keys. Specifically, a function switch key for switching between a start key, a numeric keypad, a stop key, a reset key, a copy, a printer, a scanner, and a facsimile is provided.

  The start key is a key for starting operations such as copying and facsimile transmission. The numeric keypad is a key for inputting numbers such as the number of copies and a facsimile number. The stop key is a key for stopping a copy operation or the like halfway. The reset key is a key for returning the set contents to the initial setting state.

  The function switching key includes a copy key, a transmission key, and the like, and is a key for switching between a copy function, a transmission function, and the like. When the copy key is operated, an initial copy screen is displayed on the display unit 403. When the transmission key is operated, an initial screen for facsimile transmission and mail transmission is displayed on the display unit 403.

  The control unit 500 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), an image memory, and the like. The CPU executes control necessary for operating the image forming apparatus 1 for the above-described components of the image forming apparatus 1 such as the apparatus main body 100. The ROM stores software necessary for controlling the operation of the image forming apparatus 1. The RAM is used for temporary storage of data generated during execution of software, storage of application software, and the like. The image memory temporarily stores image data (image data output from the document reading unit 200, image data transmitted from a personal computer, image data received by facsimile, etc.).

  The control unit 500 includes, as functional blocks, a switching control unit 501, a first reference pulse generation unit 503, a second reference pulse generation unit 505, a yellow control unit 507, a magenta control unit 509, a cyan control unit 511, And a black control unit 513. These functional blocks will be described later.

  The communication unit 600 includes a facsimile communication unit 601 and a network I / F unit 603. The facsimile communication unit 601 includes an NCU (Network Control Unit) that controls connection of a telephone line with a destination facsimile, and a modulation / demodulation circuit that modulates / demodulates a signal for facsimile communication. The facsimile communication unit 601 is connected to the telephone line 605.

  The network I / F unit 603 is connected to a LAN (Local Area Network) 607. A network I / F unit 603 is a communication interface circuit for executing communication with a terminal device such as a personal computer connected to the LAN 607.

  FIG. 3 is a block diagram of the electrostatic latent image forming system 3 provided in the image forming apparatus 1 according to the present embodiment. Each of the yellow image forming unit 111Y, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit 111BK shown in FIG. Since the electrostatic latent image forming system 3 provided in these image forming units 111Y, 111M, 111C, and 111BK has the same configuration, the electrostatic latent image forming system 3 provided in the yellow image forming unit 111Y is replaced with the electrostatic latent image forming system 3 provided in the yellow image forming unit 111Y. An example will be described.

  The electrostatic latent image forming system 3 includes a photosensitive drum 115, a charging roller 11, a charging bias generation unit 13, an optical deflector 20, an exposure unit 117, and a yellow control unit 507.

  The photosensitive drum 115 is a specific example of an image carrier.

  A charging unit 119 shown in FIG. 1 is configured by the charging roller 11 and the charging bias generation unit 13. The charging roller 11 is substantially in point contact with the peripheral surface of the photosensitive drum 115. A charging bias V is supplied to the charging roller 11 from the charging bias generator 13.

  The charging bias generator 13 includes an AC bias generator 15 and a DC bias generator 17. The charging bias generator 13 generates a charging bias V in which the AC bias generated by the AC bias generator 15 and the DC bias generated by the DC bias generator 17 are superimposed. By applying a charging bias V to the peripheral surface of the photosensitive drum 115 through the charging roller 11, the peripheral surface of the photosensitive drum 115 is charged.

  The charging unit 119 charges the peripheral surface of the photosensitive drum 115 by a contact charging method. The method of charging the peripheral surface of the photosensitive drum 115 is not limited to the contact charging method, and may be a corona charging method.

  The optical deflector 20 includes a polygon mirror 21 and a motor 23. The polygon mirror 21 functions as a mirror unit and is rotated by a motor 23. The light beam LB emitted from the laser diode 29 is reflected by the polygon mirror 21 and draws a main scanning line on the peripheral surface of the photosensitive drum 115.

  The optical deflector 20 is not limited to the one constituted by the polygon mirror 21 and the motor 23. The optical deflector 20 only needs to include a mirror unit and drive the mirror unit. For example, a MEMS (Micro Electro Mechanical Systems) mirror can be used as the optical deflector 20.

  The exposure unit 117 includes a light beam detection signal generation unit 25, a laser diode 29, and an LD driver 31. The exposure unit 117 further includes a lens (not shown) that guides the light beam LB to the polygon mirror 21 and the photosensitive drum 115, but the description of the lens is omitted.

  The light beam detection signal generation unit 25 includes a photodiode 27 that is a light receiving element. The light beam LB deflected by the polygon mirror 21 is received by the photodiode 27. Based on the signal output from the photodiode 27, the light beam detection signal generation unit 25 generates a light beam detection signal BD that serves as a reference for timing for starting drawing of the main scanning line. The light beam detection signal BD is also referred to as a BD (Beam Detect) signal.

  The LD driver 31 executes various controls of the laser diode 29. More specifically, the LD driver 31 generates a drive current for the laser diode 29. The LD driver 31 receives the light beam detection signal BD and an image data signal indicating an image to be printed on the paper.

  In the period for drawing the main scanning line, the LD driver 31 lights the laser diode 29. After the light beam LB emitted from the laser diode 29 is deflected by the polygon mirror 21 and received by the photodiode 27, when the light beam LB reaches the effective scanning range R of the photosensitive drum 115, an image data signal is generated. Then, lighting control of the laser diode 29 is performed. As a result, the main scanning line is drawn on the photosensitive drum 115. The timing for starting drawing of the main scanning line is based on the light beam detection signal BD. When the light beam LB exceeds the effective scanning range R, the LD driver 31 forcibly turns on the laser diode 29 at a predetermined timing. Thus, the light beam LB deflected by the polygon mirror 21 can be received by the photodiode 27 when the next main scanning line is drawn.

  As described above, the exposure unit 117 emits the light beam LB while the polygon mirror 21 is rotated, and irradiates the peripheral surface of the photosensitive drum 115 with the light beam LB deflected by the polygon mirror 21. By repeating the drawing of the scanning line, an electrostatic latent image is drawn on the peripheral surface of the photosensitive drum 115.

  In this embodiment, each of the exposure units 117 of the yellow image forming unit 111Y, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit 111BK includes an optical deflector 20. However, the optical deflector 20 may be shared.

  For example, when the yellow image forming unit 111Y, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit 111BK are arranged in order, the exposure unit 117 and the magenta image forming unit of the yellow image forming unit 111Y. The 111M exposure unit 117 shares one optical deflector 20, and the cyan image forming unit 111C exposure unit 117 and the black image forming unit 111BK exposure unit 117 share one optical deflector 20.

  As described above, this embodiment can be applied to an aspect in which one image forming unit includes one optical deflector 20 and an aspect in which two image forming units share one optical deflector 20. In the former embodiment, there are four optical deflectors 20, and in the latter embodiment, there are two optical deflectors 20. Therefore, the exposure unit 117 of each of the image forming units 111Y, 111M, 111C, and 111BK emits the light beam LB while the polygon mirror 21 of the corresponding light deflector 20 among the plurality of light deflectors 20 is driven. The electrostatic latent image is drawn by repeatedly irradiating the circumferential surface of the photosensitive drum 115 with the light beam LB deflected by the polygon mirror 21 and drawing the main scanning line.

  The light beam detection signal generation unit 25 of each of the image forming units 111Y, 111M, 111C, and 111BK includes a photodiode 27 that receives the light beam LB deflected by the polygon mirror 21 of the corresponding optical deflector 20, and includes a photo diode. Using the signal output from the diode 27, a light beam detection signal BD serving as a reference for timing for drawing the main scanning line is generated.

  Before describing the yellow controller 507, the switching controller 501, the first reference pulse generator 503, and the second reference pulse generator 505 shown in FIG. 2 will be described.

  In double-sided color printing, a color image formed on the front surface of a paper is a front image, and a color image formed on the back surface of the paper is a back image. A mode for drawing an electrostatic latent image used for forming a front image is a first drawing mode, and a mode for drawing an electrostatic latent image used for forming a back image is a second drawing mode.

  The rotation speed of the polygon mirror 21 (FIG. 3) is different between the first drawing mode and the second drawing mode. The reason for this is to adjust the magnification in the sub-scanning direction between the front image and the back image as described in the background art. In the first drawing mode, the polygon mirror 21 is rotated at a first speed, and in the second drawing mode, the polygon mirror 21 is rotated at a second speed higher than the first speed. Since the rotation speed of the polygon mirror 21 in the second drawing mode is higher than the rotation speed of the polygon mirror 21 in the first drawing mode, the cycle of the light beam detection signal BD in the second drawing mode is the first Becomes shorter than the period of the light beam detection signal BD in the drawing mode.

  The switching control unit 501 controls the yellow image forming unit 111Y, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit 111BK to switch from the first drawing mode to the second drawing mode, And control which switches from 2nd drawing mode to 1st drawing mode is performed. More specifically, when the switching control unit 501 outputs an L-level switching signal SS (Y) to the yellow control unit 507, the yellow image forming unit 111Y enters the first drawing mode. When the switching control unit 501 outputs an H level switching signal SS (Y) to the yellow control unit 507, the yellow image forming unit 111Y enters the second drawing mode.

  When the switching control unit 501 outputs an L level switching signal SS (M) to the magenta control unit 509, the magenta image forming unit 111M enters the first drawing mode. When the switching control unit 501 outputs an H level switching signal SS (M) to the magenta control unit 509, the magenta image forming unit 111M enters the second drawing mode.

  When the switching control unit 501 outputs an L level switching signal SS (C) to the cyan control unit 511, the cyan image forming unit 111C enters the first drawing mode. When the switching control unit 501 outputs an H level switching signal SS (C) to the cyan control unit 511, the cyan image forming unit 111C enters the second drawing mode.

  When the switching control unit 501 outputs an L level switching signal SS (BK) to the black control unit 513, the black image forming unit 111BK enters the first drawing mode. When the switching control unit 501 outputs an H level switching signal SS (BK) to the black control unit 513, the black image forming unit 111BK enters the second drawing mode.

  In the first drawing mode, the switching signals SS (Y), SS (M), SS (C), and SS (BK) output from the switching control unit 501 are set to L level, and switching control is performed in the second drawing mode. Although the switching signals SS (Y), SS (M), SS (C), and SS (BK) output from the unit 501 are at the H level, the reverse may be possible.

  The drive control unit is provided by the switching control unit 501, and the motor driver 55 (FIG. 3) provided in each of the yellow control unit 507, the magenta control unit 509, the cyan control unit 511, and the black control unit 513. Realized. As will be described later with reference to FIG. 3, the motor driver 55 controls the rotation of the polygon mirror 21 by controlling the rotation of the motor 23.

  In the first drawing mode, the drive control unit adjusts the magnification in the sub-scanning direction between the front image and the back image, in the first drawing mode, the yellow image forming unit 111Y, the magenta image forming unit 111M, the cyan image forming unit 111C, And the rotation speed of the polygon mirror 21 of the optical deflector 20 corresponding to each of the black image forming units 111BK and the rotation of the polygon mirror 21 of the optical deflector 20 corresponding to each of these units in the second drawing mode. Different speed.

  In the present embodiment, each of the image forming units 111Y, 111M, 111C, and 111BK includes the optical deflector 20. Accordingly, in the first drawing mode, the rotation speed of the polygon mirror 21 of the optical deflector 20 provided in each of the image forming units 111Y, 111M, 111C, and 111BK, and the light provided in these units in the second drawing mode. The rotational speed of the polygon mirror 21 of the deflector 20 is made different.

  That is, in the first drawing mode for drawing the electrostatic latent image used for forming the surface image, the rotation speed of the polygon mirror 21 of the optical deflector 20 provided in each of the image forming units 111Y, 111M, 111C, and 111BK is set to the first. In the second drawing mode for drawing the electrostatic latent image used for forming the rear surface image, the rotation speed of the polygon mirror 21 of the optical deflector 20 provided in each of the image forming units 111Y, 111M, 111C, and 111BK is The second speed is different from the first speed. The drive control unit matches the magnification in the sub-scanning direction between the front surface image and the back surface image by making the second speed larger than the first speed.

  The first reference pulse generator 503 generates a first reference pulse P1 having a preset first period. The first reference pulse P1 is generated by the light beam detection signal BD generated in the first drawing mode by the light beam detection signal generator 25 (FIG. 3) provided in each of the image forming units 111Y, 111M, 111C, and 111BK. These pulses have the same period as

  The second reference pulse generator 505 generates a second reference pulse P2 having a preset second period. The second reference pulse P2 has the same period as the light beam detection signal BD generated by the light beam detection signal generation unit 25 provided in each of the image forming units 111Y, 111M, 111C, and 111BK in the second drawing mode. It is a pulse having a period.

  The first reference pulse generation unit 503 and the second reference pulse generation unit 505 are pulse generation circuits. Since the rotation speed of the polygon mirror 21 is higher in the second drawing mode than in the first drawing mode, the period of the light beam detection signal BD is higher in the second drawing mode than in the first drawing mode. short. Therefore, the cycle of the second reference pulse P2 is shorter than the cycle of the first reference pulse P1.

  The yellow control unit 507 controls the timing at which the light beam detection signal BD is generated in the yellow image forming unit 111Y. The magenta controller 509 controls the timing at which the light beam detection signal BD is generated in the magenta image forming unit 111M. The cyan control unit 511 controls the timing at which the light beam detection signal BD is generated in the cyan image forming unit 111C. The black control unit 513 controls the timing at which the light beam detection signal BD is generated in the black image forming unit 111BK. These control units have the same configuration, and the yellow control unit 507 will be described as an example.

  As shown in FIG. 3, the yellow control unit 507 includes an ASIC (Application Specific) including a light beam detection signal input unit 41, a light beam detection signal adjustment unit 43, a clock adjustment unit 47, a clock generation unit 51, and a multiplexer 45. Integrated Circuit).

  The multiplexer 45 receives the first reference pulse P1 and the second reference pulse P2. When the switching signal SS (Y) is at the L level, that is, when the yellow image forming unit 111Y is in the first drawing mode, the multiplexer 45 outputs the first reference pulse P1. When the switching signal SS (Y) is at the H level, that is, when the yellow image forming unit 111Y is in the second drawing mode, the multiplexer 45 outputs the second reference pulse P2.

  The light beam detection signal BD generated by the light beam detection signal generation unit 25 is input to the light beam detection signal input unit 41, and the light beam detection signal input unit 41 converts the input light beam detection signal BD into the light beam detection signal. This is sent to the adjustment unit 43.

  The light beam detection signal adjustment unit 43 uses feedback control to match the phase of the light beam detection signal BD with the phase of the first reference pulse P1 in the first drawing mode, and in the second drawing mode, The phase of the beam detection signal BD is matched with the phase of the second reference pulse P2. In the present embodiment, the timing at which the light beam detection signal BD falls is the timing at which the light beam detection signal BD is generated. Since the pulse width of the light beam detection signal BD is narrow, the phase of the light beam detection signal BD can be regarded as the timing at which the light beam detection signal BD is generated.

  The light beam detection signal adjustment unit 43 will be described in detail. The light beam detection signal adjustment unit 43 includes a phase comparison unit 53 and a motor driver 55. The motor driver 55 controls the rotation of the polygon mirror 21 by controlling the rotation of the motor 23.

  The light beam detection signal BD is input to the phase comparison unit 53. Further, when the multiplexer 45 outputs the first reference pulse P1 to the phase comparison unit 53, the first reference pulse P1 is input, and the multiplexer 45 outputs the second reference pulse P2. The second reference pulse P2 is input.

  When the first reference pulse P1 is input, that is, when the yellow image forming unit 111Y is in the first drawing mode, the phase comparison unit 53 and the phase of the first reference pulse P1 and the light beam detection signal BD The data indicating the phase difference is sent to the motor driver 55. The motor driver 55 adjusts the rotation of the polygon mirror 21 by adjusting the rotation of the motor 23 based on the data. The phase comparison unit 53 and the motor driver 55 repeat this to match the phase of the light beam detection signal BD with the phase of the first reference pulse P1.

  The phase comparison unit 53 detects the phase of the second reference pulse P2 and the light beam detection when the second reference pulse P2 is input, that is, when the yellow image forming unit 111Y is in the second drawing mode. The phase of the signal BD is compared, and data indicating the phase difference is sent to the motor driver 55. The motor driver 55 adjusts the rotation of the polygon mirror 21 by adjusting the rotation of the motor 23 based on the data. The phase comparison unit 53 and the motor driver 55 repeat this to match the phase of the light beam detection signal BD with the phase of the second reference pulse P2.

  The clock generation unit 51 generates a clock CK having a predetermined period that is an AC bias period. The AC bias generator 15 generates an AC bias using the clock CK generated by the clock generator 51. Specifically, the AC bias generator 15 generates an AC bias by changing the clock CK to a sine wave and increasing the voltage of the sine wave.

  The clock generation unit 51 generates a clock CK having the same cycle as that of the light beam detection signal BD. Since the period of the light beam detection signal BD is shorter in the second drawing mode than in the first drawing mode, the period of the clock CK is shorter in the second drawing mode than in the first drawing mode. .

  When the multiplexer 45 outputs the first reference pulse P1, the clock adjustment unit 47 receives the first reference pulse P1, and when the multiplexer 45 outputs the second reference pulse P2, Two reference pulses P2 are input.

  When the first reference pulse P1 is input, that is, when the yellow image forming unit 111Y is in the first drawing mode, the clock adjusting unit 47 charges the charging bias at the timing when the first reference pulse P1 falls. The timing at which the clock generation unit 51 generates the clock CK is adjusted so that V becomes a predetermined value. In other words, the clock adjustment unit 47 determines the value of the charging bias V at the timing synchronized with the predetermined reference phase of the first reference pulse P1 (the value of the charging bias V at the same phase as the phase of the first reference pulse P1). ) Adjusts the timing at which the clock generation unit 51 generates the clock CK so that it becomes a predetermined value V3.

  When the second reference pulse P2 is input, that is, when the yellow image forming unit 111Y is in the second drawing mode, the clock adjusting unit 47 charges the charging bias at the timing when the second reference pulse P2 falls. The timing at which the clock generation unit 51 generates the clock CK is adjusted so that V becomes a predetermined value. In other words, the clock adjustment unit 47 determines the value of the charging bias V at the timing synchronized with the predetermined reference phase of the second reference pulse P2 (the value of the charging bias V at the same phase as the phase of the second reference pulse P2). ) Adjusts the timing at which the clock generation unit 51 generates the clock CK so that it becomes a predetermined value V3. The predetermined value of the charging bias V is the same in the first drawing mode and in the second drawing mode.

  The light beam detection signal adjustment unit provided in the yellow control unit 507 is the same as the light beam detection signal adjustment unit 43 and the clock adjustment unit 47 provided in the magenta control unit 509, cyan control unit 511, and black control unit 513. 43 and the clock adjustment unit 47 are adjusted in the same manner.

  This embodiment includes Embodiment 1, Embodiment 2, and Embodiment 3. The first embodiment will be described.

(Embodiment 1)
According to the first embodiment, the magnification in the sub-scanning direction can be adjusted between the front surface image (color image formed on the front surface of the paper) and the back image (color image formed on the back surface of the paper). In addition, when the color images are formed by superimposing toner images of four colors of yellow, magenta, cyan, and black, it is possible to prevent the toner images from being shifted in the sub-scanning direction in the front image and the back image.

  As described above, the magnification in the sub-scanning direction can be adjusted between the front image and the back image. That is, the drive control unit includes a motor driver provided in each of the switching control unit 501 (FIG. 2), the yellow control unit 507, the magenta control unit 509, the cyan control unit 511, and the black control unit 513. 55 (FIG. 3). In the first drawing mode (mode for drawing an electrostatic latent image used for forming a surface image), the rotational speed of the polygon mirror 21 of the optical deflector 20 provided in each of the image forming units 111Y, 111M, 111C, and 111BK is set to the first. In the first speed and the second drawing mode (mode for drawing an electrostatic latent image used for forming the back side image), the polygon mirror 21 of the optical deflector 20 provided in each of the image forming units 111Y, 111M, 111C, and 111BK. The rotation speed is a second speed different from the first speed. The drive control unit matches the magnification in the sub-scanning direction between the front surface image and the back surface image by making the second speed larger than the first speed.

  Next, it will be described that the toner image can be prevented from being shifted in the sub-scanning direction when the color image is formed by superimposing the four color toner images in the front image and the back image.

  When forming a color image, a plurality of light beam detection signals BD are used. A color image is formed by superimposing a plurality of color toner images. When the plurality of colors are yellow, magenta, cyan, and black, a color image is formed by superimposing a yellow toner image, a magenta toner image, a cyan toner image, and a black toner image. In this case, there are four light beam detection signals BD. That is, a light beam detection signal BD used for drawing an electrostatic latent image corresponding to a yellow toner image, a light beam detection signal BD used for drawing an electrostatic latent image corresponding to a magenta toner image, and a cyan toner image. A light beam detection signal BD used for drawing the electrostatic latent image, and a light beam detection signal BD used for drawing the electrostatic latent image corresponding to the black toner image.

  If the phases of the four light beam detection signals BD do not coincide with each other, when toner images of four colors are superimposed, a maximum deviation occurs in the sub-scanning direction corresponding to the width of one main scanning line. Therefore, it is necessary to draw the electrostatic latent images corresponding to the toner images of the respective colors by matching the phases of the four light beam detection signals BD. For example, there is a technique (Technique 1) for matching the phase of the remaining light beam detection signal BD with the phase of one light beam detection signal BD (light beam detection signal BD for black).

  In the technique (Technology 2) for adjusting the magnification in the sub-scanning direction between the front image and the back image by adjusting the rotation speed of the polygon mirror 21 described above, the rotation speed of the polygon mirror 21 in the second drawing mode. Is made larger than the rotational speed of the polygon mirror 21 in the first drawing mode. Therefore, the period of the light beam detection signal BD is different between the first drawing mode and the second drawing mode. For this reason, when the techniques 2 and 1 are combined, the phases of the remaining light beam detection signals BD coincide with the phases of one light beam detection signal BD because the period of the light beam detection signal BD is different. I can't let you. This will be described in detail.

  FIG. 4 is an explanatory diagram for explaining the transition of the drawing modes of the image forming units 111Y, 111M, 111C, and 111BK in the first embodiment. In a print job in which double-sided printing of color images is performed on a plurality of sheets, (1) an electrostatic latent image corresponding to the color image formed on the surface of the first sheet is drawn, and (2) two sheets Draw an electrostatic latent image corresponding to the color image formed on the surface of the eye paper, (3) Draw an electrostatic latent image corresponding to the color image formed on the back surface of the first paper, 4) Draw an electrostatic latent image corresponding to the color image formed on the surface of the third sheet, and (5) Create an electrostatic latent image corresponding to the color image formed on the back surface of the second sheet. (6) Draw an electrostatic latent image corresponding to the color image formed on the surface of the fourth sheet, and (7) Static corresponding to the color image formed on the back side of the third sheet. An electric latent image is drawn.

  There is no second drawing mode between (1) and (2) when the electrostatic latent image corresponding to the color image formed on the surface of the first sheet is drawn. This is because there is no paper on which the color image is formed.

  In the tandem image forming apparatus, the image forming units for each color are arranged in tandem. In this embodiment, as shown in FIG. 1, a yellow image forming unit 111Y, a magenta image forming unit 111M, a cyan image forming unit 111C, and a black image forming unit 111BK are arranged along the longitudinal direction of the transfer belt 113 in order from the upstream. Arranged in tandem.

  Drawing of the electrostatic latent image used for forming one color image is completed in the order of the yellow image forming unit 111Y, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit 111BK. Therefore, as shown in (2), (4), and (6), in the yellow image forming unit 111Y, when the first drawing mode is completed, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit are formed. Without waiting for the end of the first drawing mode in the unit 111BK, the unit 111BK immediately switches to the second drawing mode. When the first drawing mode is completed, the magenta image forming unit 111M immediately enters the second drawing mode without waiting for the cyan image forming unit 111C and the black image forming unit 111BK to end the first drawing mode. Switch. In the cyan image forming unit 111C, when the first drawing mode ends, the cyan image forming unit 111C immediately switches to the second drawing mode without waiting for the black image forming unit 111BK to end the first drawing mode.

  Similarly, as shown in (3) and (5), in the yellow image forming unit 111Y, when the second drawing mode ends, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit 111BK. The first drawing mode is immediately switched without waiting for the end of the second drawing mode. When the second drawing mode is completed, the magenta image forming unit 111M immediately enters the first drawing mode without waiting for the cyan image forming unit 111C and the black image forming unit 111BK to end the second drawing mode. Switch. In the cyan image forming unit 111C, when the second drawing mode ends, the cyan image forming unit 111C immediately switches to the first drawing mode without waiting for the end of the second drawing mode in the black image forming unit 111BK.

  The above sequential switching control is executed by the switching control unit 501 (FIG. 2). In other words, the switching control unit 501 sequentially proceeds from the image forming unit in which the first drawing mode is completed to the second among the yellow image forming unit 111Y, the magenta image forming unit 111M, the cyan image forming unit 111C, and the black image forming unit 111BK. Are switched to the first drawing mode sequentially from the image forming unit in which the second drawing mode is completed.

  In the tandem image forming apparatus 1, the image forming units are controlled to be in the first drawing mode until all of the four image forming units 111Y, 111M, 111C, and 111BK end the first drawing mode. In addition, it is possible to control the image forming units to be in the second drawing mode until all of the four image forming units 111Y, 111M, 111C, and 111BK end the second drawing mode. However, with this method, it is not possible to improve the printing speed when performing duplex printing on a plurality of sheets. Therefore, sequential switching control is used.

  According to the sequential switching control, the image forming unit in the first drawing mode and the image forming unit in the second drawing mode simultaneously exist. Therefore, when the black image forming unit 111BK is in the second drawing mode, the image forming units of other colors may be in the first drawing mode or vice versa. The period of the light beam detection signal BD is different between the first drawing mode and the second drawing mode. For this reason, in order to match the phases of the four light beam detection signals BD, the phases of the remaining light beam detection signals BD cannot be matched with the phases of one light beam detection signal BD.

  Therefore, in the first embodiment, in the first drawing mode, the phases of the four light beam detection signals BD are matched with the phases of the first reference pulse P1. In the case of the second drawing mode, the phases of the four light beam detection signals BD are matched with the phase of the second reference pulse P2. This will be described using the yellow control unit 507 as an example.

  FIG. 5 shows waveforms of the light beam detection signal BD, the first reference pulse P1, and the second reference pulse P2 in the process of transition from the first drawing mode to the second drawing mode in the yellow control unit 507. FIG. In the first drawing mode, the cycle of the light beam detection signal BD is the same as the cycle of the first reference pulse P1, and in the second drawing mode, the cycle of the light beam detection signal BD is the second reference pulse. It is the same as the period of P2.

  In the first drawing mode, the light beam detection signal adjustment unit 43 (FIG. 3) controls the optical deflector 20 so that the phase of the light beam detection signal BD matches the phase of the first reference pulse P1. To adjust the rotation of the polygon mirror 21. When the drawing mode of the yellow image forming unit 111Y is switched from the first drawing mode to the second drawing mode, the light beam detection signal adjusting unit 43 sets the phase of the light beam detection signal BD to the second reference pulse P2. The rotation of the polygon mirror 21 is adjusted by controlling the optical deflector 20 so as to coincide with the phase of.

  FIG. 6 shows waveforms of the light beam detection signal BD, the first reference pulse P1, and the second reference pulse P2 in the process of transition from the second drawing mode to the first drawing mode in the yellow control unit 507. FIG. In the second drawing mode, the light beam detection signal adjustment unit 43 controls the optical deflector 20 to control the polygon mirror 21 so that the phase of the light beam detection signal BD matches the phase of the second reference pulse P2. Adjust the drive. When the drawing mode of the yellow image forming unit 111Y is switched from the second drawing mode to the first drawing mode, the light beam detection signal adjusting unit 43 sets the phase of the light beam detection signal BD to the first reference pulse P1. The rotation of the polygon mirror 21 is adjusted by controlling the optical deflector 20 so as to coincide with the phase of.

  The same applies to the control unit for magenta 509, the control unit for cyan 511, and the control unit for black 513, and the light beam detection signal adjustment unit 43 has the first phase of the light beam detection signal BD in the first drawing mode. The phase of the light beam detection signal BD is adjusted by adjusting the rotation of the polygon mirror 21 so as to coincide with the phase of the reference pulse P1, and in the case of the second drawing mode, the phase of the light beam detection signal BD is the first. The phase of the light beam detection signal BD is adjusted by adjusting the rotation of the polygon mirror 21 so as to coincide with the phase of the second reference pulse P2.

  The yellow control unit 507, the magenta control unit 509, the cyan control unit 511, and the light beam detection signal adjustment unit 43 provided in the black control unit 513 constitute a phase adjustment unit. As described with reference to FIG. 4, the image forming unit in the first drawing mode and the image forming unit in the second drawing mode coexist at the same time. The phase adjusting unit is generated by the light beam detection signal generating unit 25 provided in the image forming unit in the first drawing mode among the image forming units 111Y, 111M, 111C, and 111BK by adjusting the rotation of the polygon mirror 21. The phase of the light beam detection signal BD to be matched with the phase of the first reference pulse P1, and the image forming unit in the second drawing mode among the image forming units 111Y, 111M, 111C, 111BK. Is adjusted so that the phase of the light beam detection signal BD generated by the light beam detection signal generation unit 25 included in is matched with the phase of the second reference pulse P2.

  As described above, according to the first embodiment, in each of the image forming units 111Y, 111M, 111C, and 111BK, in the first drawing mode, the light beam detection signal BD whose phase is matched with the first reference pulse P1. The electrostatic latent image is drawn by repeatedly drawing the main scanning line with reference. Therefore, it is possible to prevent the toner image from being shifted in the sub-scanning direction when the surface image is formed. Further, according to the first embodiment, in each of the image forming units 111Y, 111M, 111C, and 111BK, in the second drawing mode, the light beam detection signal BD whose phase is matched with the second reference pulse P2 is used as a reference. Then, the electrostatic latent image is drawn by repeatedly drawing the main scanning line. Therefore, it is possible to prevent the toner image from shifting in the sub-scanning direction when the back image is formed.

  Therefore, according to the first embodiment, it is possible to prevent the toner image from being shifted in the sub-scanning direction when the four color toner images are superimposed to form the color image in the front image and the back image.

(Embodiment 2)
According to the second embodiment, in the method of charging the peripheral surface of the photosensitive drum 115 using the charging bias V including the AC bias, it is possible to prevent the image quality from being deteriorated due to the AC bias. First, a phenomenon in which image quality deteriorates due to an AC bias will be described.

  FIG. 7 is a waveform diagram showing a first example of the relationship between the light beam detection signal BD and the charging bias V. FIG. 8 is a waveform diagram showing a second example thereof, and FIG. 9 is a waveform diagram showing a third example thereof. A symbol Vdc indicates the magnitude of the DC bias included in the charging bias V. The periodic change of the charging bias V is caused by an AC bias, and the cycle of the AC bias becomes the cycle of the charging bias V.

  Since the charging bias V periodically changes, the amount of charge periodically changes according to the position of the circumferential surface of the photosensitive drum 115. In the present embodiment, the timing at which the light beam detection signal BD falls is the timing at which the light beam detection signal BD is generated. This is because the light beam LB deflected by the polygon mirror 21 shown in FIG. 27 is the timing detected by H.27. The light beam detection signal BD is a reference for drawing the main scanning line. The timing at which one light beam detection signal BD falls serves as a reference for drawing one main scanning line.

  In the first example shown in FIG. 7, since the cycle of the light beam detection signal BD and the cycle of the charging bias V are different, the value of the charging bias V is detected at the timing when the light beam detection signal BD falls. It differs for each timing when the signal BD falls. Therefore, on the peripheral surface of the photoconductive drum 115, the change state of the charge amount at the location where the main scanning line is drawn differs for each main scanning line. In other words, the variation pattern of the image density differs for each main scanning line in the main scanning direction. This is because when the period of the light beam detection signal BD and the period of the charging bias V (the period of the AC bias) are the same or close to each other, it appears to the human eye as a periodic change in image density, and the image quality deteriorates. It becomes.

  In the second example shown in FIG. 8 and the third example shown in FIG. 9, the cycle of the light beam detection signal BD and the cycle of the charging bias V are the same. For this reason, at the timing when the light beam detection signal BD falls, the value of the charging bias V does not differ for each timing when the light beam detection signal BD falls, and is the same. Therefore, in any case, a periodic change in image density does not occur.

  However, at the timing when the light beam detection signal BD falls, the value of the charging bias V is V1 in the second example shown in FIG. 8 and V2 in the third example shown in FIG. For this reason, the second example shown in FIG. 8 and the third example shown in FIG. 9 have different image density fluctuation patterns in the main scanning direction, and even when the same image is compared, the image density appears to fluctuate. . The reason why the value of the charging bias V is different at the timing when the light beam detection signal BD falls even when the cycle of the light beam detection signal BD and the cycle of the charging bias V are the same will be described.

  In the image forming apparatus 1, if the print job is completed and there is no next print job, the control unit 500 (FIG. 2) rotates the polygon mirror 21 and generates the charging bias V until the next print job is generated. Control to stop. When the next print job is generated, the rotation of the polygon mirror 21 and the generation of the charging bias V are resumed. At this time, if there is no adjustment between them, even if the period of the light beam detection signal BD and the period of the charging bias V are the same, the timing at which the light beam detection signal BD falls (the light beam detection signal BD is At the timing of generation), the value of the charging bias V cannot be the same as before the generation of the charging bias V is stopped.

  Therefore, for example, before the rotation of the polygon mirror 21 and the generation of the charging bias V are stopped, the value of the charging bias V is V1 (FIG. 8) at the timing when the light beam detection signal BD falls. When rotation of the polygon mirror 21 and generation of the charging bias V are restarted, the value of the charging bias V becomes V2 (FIG. 9) at the timing when the light beam detection signal BD falls.

  According to the second embodiment, even if the rotation of the polygon mirror 21 and the generation of the charging bias V are stopped, the value of the charging bias V at the timing when the light beam detection signal BD falls is calculated before the generation of the charging bias V is stopped. Can be set to a predetermined value V3 which is the same value as. Although the reference pulse will be described by taking the first reference pulse P1 as an example, the same can be said for the second reference pulse P2.

  The first reference pulse generator 503 (FIG. 2), the second reference pulse generator 505 (FIG. 2), the light beam detection signal adjuster 43 (FIG. 3), and the clock adjuster 47 (FIG. 3) An adjustment unit is configured. As will be described below, the adjustment unit rotates the polygon mirror 21 by the optical polarizer 20 and the charging bias so that the light beam detection signal BD is generated when the charging bias V is a predetermined value V3. At least one of V is adjusted. In other words, adjustment is made so that the light beam detection signal BD falls when the charging bias V is a predetermined value V3.

  FIG. 10 is a waveform diagram showing the relationship among the light beam detection signal BD, the first reference pulse P1, the clock CK, and the charging bias V in the second embodiment. When the rotation of the polygon mirror 21 is resumed after the rotation of the polygon mirror 21 is stopped, the light beam detection signal adjustment unit 43 in FIG. 3 determines that the phase of the light beam detection signal BD is the first reference pulse P1. The rotation of the motor 23 is adjusted so as to coincide with the phase. The state in which the phase of the light beam detection signal BD matches the phase of the first reference pulse P1 is the timing at which the phase of the first reference pulse P1 is generated.

  When the generation of the charging bias V is resumed after the generation of the charging bias V is stopped, the clock adjusting unit 47 in FIG. 3 charges the charging bias V at a timing synchronized with a predetermined reference phase of the first reference pulse. (In other words, the value of the charging bias V at the same phase as the phase of the first reference pulse P1) is set to a predetermined value V3 that is the same value as before the generation of the charging bias V is stopped. The clock generation unit 51 adjusts the timing for generating the clock CK.

  As described above, even when the rotation of the polygon mirror 21 and the generation of the charging bias V are stopped, the value of the charging bias V at the timing when the light beam detection signal BD falls (that is, the timing when the light beam detection signal is generated). Can be set to the same predetermined value V3 as before the generation of the charging bias V is stopped. Therefore, according to the second embodiment, as shown in FIGS. 8 and 9, the value of the charging bias V at the timing when the light beam detection signal BD falls is not different, and is always a predetermined value V3. Can be. Therefore, according to the second embodiment, in the method of charging the peripheral surface of the photosensitive drum 115 using the charging bias V including the AC bias, it is possible to prevent the deterioration of the image quality caused by the AC bias.

  The second embodiment can be applied not only to printing a color image but also to printing a monochrome image. The second embodiment can be applied not only to double-sided printing but also to single-sided printing.

  In the second embodiment, as shown in FIG. 10, the ratio between the period of the light beam detection signal and the period of the charging bias is 1: 1. Therefore, in the first drawing mode, the clock generation unit 51 generates the clock CK having a ratio of 1: 1 between the period of the first reference pulse P1 and the period of the clock CK. Although not shown, in the second drawing mode, the clock generator 51 generates a clock CK in which the ratio of the period of the second reference pulse P2 to the period of the clock CK is 1: 1. The clock adjusting unit 47 (FIG. 3) sets the charging bias V at a timing synchronized with a predetermined reference phase of the first reference pulse P1 for each cycle of the first reference pulse P1 and for each cycle of the clock CK. The timing at which the clock generator 51 generates the clock CK is adjusted so that the value (the value of the charging bias V at the timing when the first reference pulse P1 falls) becomes a predetermined value V3.

  As described above, in the second embodiment, the ratio between the period of the light beam detection signal BD and the period of the charging bias V is 1: 1. However, the ratio between the period of the light beam detection signal BD and the period of the charging bias V is not necessarily 1: 1. The present invention can be applied even when the ratio between the period of the light beam detection signal BD and the period of the charging bias V is m: n (m and n are integers different from each other). This will be described as a first modification.

  FIG. 11 is a waveform diagram showing the relationship among the light beam detection signal BD, the first reference pulse P1, the third reference pulse, the clock CK, and the charging bias V in Modification 1. The ratio between the period of the light beam detection signal BD and the period of the charging bias V is 3: 4.

  In the first drawing mode, the clock generator 51 (FIG. 3) generates the third reference pulse using the first reference pulse P1. The ratio between the period of the first reference pulse P1 and the period of the third reference pulse is 3: 4. The clock adjustment unit 47 adjusts the timing at which the clock generation unit 51 generates the clock CK so that the value of the charging bias V becomes a predetermined value V3 at the timing when the third reference pulse falls. As a result, the value of the charging bias V at a timing synchronized with a predetermined reference phase of the first reference pulse P1 every three cycles of the first reference pulse P1 and every four cycles of the clock CK is a predetermined value. V3.

  Although not shown, the same processing is performed in the second drawing mode. That is, the clock generation unit 51 generates the third reference pulse using the second reference pulse P2. The ratio between the period of the second reference pulse P2 and the period of the third reference pulse is 3: 4. The clock adjustment unit 47 adjusts the timing at which the clock generation unit 51 generates the clock CK so that the value of the charging bias V becomes a predetermined value V3 at the timing when the third reference pulse falls. As a result, the value of the charging bias V at a timing synchronized with a predetermined reference phase of the second reference pulse P2 every three cycles of the second reference pulse P2 and every four cycles of the clock CK is a predetermined value. V3.

  In the first modification, at the timing when the light beam detection signal BD falls every time, the value of the charging bias V does not become the predetermined value V3, but at the timing when the light beam detection signal BD falls once every three times. However, the value of the charging bias V cannot be set to a predetermined value V3. Therefore, as in the first example shown in FIG. 7, a periodic change in image density appears.

  If the value of the charging bias V can be set to a predetermined value V3 only at the timing when the light beam detection signal BD falls once every several tens of times, as in the first example shown in FIG. Periodic changes in concentration can be recognized by the human eye. However, as in the first modification, if the value of the charging bias V can be set to a predetermined value V3 at the timing when the light beam detection signal BD falls once every three times (several times), the image density periodically changes. Changes cannot be recognized by the human eye.

  Modification 2 will be described. In the second embodiment, as shown in FIGS. 3 and 10, in the first drawing mode, when the charging bias V is a predetermined value V3, the light beam detection signal BD is generated. The timing at which the light beam detection signal BD is generated is adjusted using the first reference pulse P1. Although not shown, the second reference pulse is used to generate the light beam detection signal BD when the charging bias V is a predetermined value V3 in the second drawing mode. The timing at which the light beam detection signal BD is generated is adjusted using P2.

  In the second modification, the light beam detection signal BD is input to the clock adjustment unit 47 (FIG. 3) instead of the first reference pulse P1 and the second reference pulse P2. Thus, the clock adjustment unit 47 generates the clock so that the charging bias V becomes a predetermined value V3 when the light beam detection signal BD is generated (when the light beam detection signal BD falls). The unit 51 adjusts the timing for generating the clock CK.

  The modification 2 is superior to the second embodiment in that the first reference pulse P1 and the second reference pulse P2 can be omitted. On the other hand, Embodiment 2 is superior to Modification 2 in the following points.

  As shown in FIG. 3, the AC bias included in the charging bias V is generated using a clock CK. The clock adjustment unit 47 adjusts the phase of the charging bias V by adjusting the timing at which the clock generation unit 51 generates the clock CK. By adjusting the timing at which the clock generation unit 51 generates the clock CK with the phase of the light beam detection signal BD (that is, the timing at which the light beam detection signal BD is generated) as a direct reference, the light beam detection signal BD is adjusted. The value of the charging bias V at the timing at which is generated can be set to a predetermined value V3 (Modification 2).

  However, in the second modification, after the rotation of the polygon mirror 21 is stabilized (that is, after the cycle of the light beam detection signal BD is stabilized), the light beam detection signal is adjusted by adjusting the timing for generating the clock CK. The value of the charging bias V at the timing when the BD is generated is set to a predetermined value V3. When the rotation of the polygon mirror 21 is resumed after the rotation of the polygon mirror 21 is stopped, it takes a relatively long time until the rotation of the polygon mirror 21 is stabilized. In the second modification, the light beam detection signal BD is generated. The value of the charging bias V at this timing cannot be quickly set to the predetermined value V3.

  On the other hand, according to the second embodiment, when the rotation of the polygon mirror 21 is resumed after the rotation of the polygon mirror 21 is stopped, the first reference pulse P1 having the same period as the period of the light beam detection signal BD. , The value of the charging bias V at the timing when the light beam detection signal BD is generated is set to a predetermined value V3. That is, as shown in FIG. 10, the rotation of the polygon mirror 21 is adjusted so that the phase of the light beam detection signal BD (the timing at which the light beam detection signal BD is generated) matches the phase of the first reference pulse P1. In addition, the timing for generating the clock CK is adjusted so that the value of the charging bias V at the same phase as the phase of the first reference pulse P1 becomes a predetermined value V3. For this reason, the timing for generating the clock CK can be adjusted before the rotation of the polygon mirror 21 is stabilized (that is, before the cycle of the light beam detection signal BD is stabilized).

  Therefore, according to the second embodiment, when the rotation of the polygon mirror 21 is resumed after the rotation of the polygon mirror 21 is stopped, the value of the charging bias V at the timing at which the light beam detection signal BD is generated is quickly determined. It can be set to a predetermined value V3.

(Embodiment 3)
The third embodiment is a combination of the first embodiment and the second embodiment. In the third embodiment, as described with reference to FIGS. 5 and 6 in the first embodiment, in the first drawing mode, the phase of the light beam detection signal BD matches the phase of the first reference pulse P1. In the second drawing mode, the phase of the light beam detection signal BD matches the phase of the second reference pulse P2. This is executed by the light beam detection signal adjustment unit 43 (FIG. 3) provided in each of the image forming units 111Y, 111M, 111C, and 111BK.

  In the third embodiment, as described in the second embodiment, in the first drawing mode, as shown in FIG. 10, the phase of the light beam detection signal BD matches the phase of the first reference pulse P1. As described above, the light beam detection signal adjustment unit 43 adjusts the timing at which the light beam detection signal BD is generated by adjusting the rotation of the polygon mirror 21. In the first drawing mode, the clock is set so that the charging bias V becomes a predetermined value V3 at the timing when the first reference pulse P1 falls (the timing when the light beam detection signal BD is generated). The adjustment unit 47 (FIG. 3) adjusts the timing at which the clock generation unit 51 generates the clock CK. In other words, in the case of the first drawing mode, the clock adjustment unit 47 so that the charging bias V at the timing synchronized with the predetermined first reference phase of the first reference pulse P1 becomes the predetermined value V3. Adjusts the timing at which the clock generator 51 generates the clock CK.

  Although not shown, in the case of the second drawing mode, the light beam detection signal adjustment unit 43 is connected to the polygon mirror 21 so that the phase of the light beam detection signal BD matches the phase of the second reference pulse P2. The timing at which the light beam detection signal BD is generated is adjusted by adjusting the rotation. In the second drawing mode, the clock is set so that the charging bias V becomes a predetermined value V3 at the timing when the second reference pulse P2 falls (the timing when the light beam detection signal BD is generated). The adjustment unit 47 adjusts the timing at which the clock generation unit 51 generates the clock CK. In other words, in the case of the second drawing mode, the clock adjustment unit 47 so that the charging bias V at the timing synchronized with the predetermined second reference phase of the second reference pulse P2 becomes the predetermined value V3. Adjusts the timing at which the clock generator 51 generates the clock CK.

  Such adjustment by the light beam detection signal adjustment unit 43 and the clock adjustment unit 47 is executed in each of the image forming units 111Y, 111M, 111C, and 111BK.

  Therefore, according to the third embodiment, for example, when transitioning from the second drawing mode to the first drawing mode, the adjustment shown in FIG. 12 is performed in each of the image forming units 111Y, 111M, 111C, and 111BK. The light beam detection signal adjustment unit 43 starts from the state where the phase of the light beam detection signal BD matches the phase of the second reference pulse P2, so that the phase of the light beam detection signal BD matches the phase of the first reference pulse P1. Switch to the matching state. The clock adjustment unit 47 starts from the state where the value of the charging bias V becomes a predetermined value V3 at the timing when the second reference pulse P2 falls (the timing when the light beam detection signal BD is generated). At a timing when the pulse P1 falls (a timing when the light beam detection signal BD is generated), the state is switched to a state where the value of the charging bias V becomes a predetermined value V3.

  According to the third embodiment, for the same reason as in the first embodiment, in double-sided printing of a color image, the magnification in the sub-scanning direction can be adjusted between the front image and the back image, and yellow, magenta, When the toner images of four colors of cyan and black are superimposed, it is possible to prevent the toner images from being shifted in the sub-scanning direction in the front and back images. Further, according to the third embodiment, for the same reason as in the second embodiment, in the method of charging the peripheral surface of the photosensitive drum 115 using the charging bias V including the AC bias, the image quality is deteriorated due to the AC bias. Can be prevented.

  The first reference pulse P1 and the second reference pulse P2 in the third embodiment have the same role as the first reference pulse P1 and the second reference pulse P2 in the first embodiment, and in the second embodiment It has the same role as the first reference pulse P1 and the second reference pulse P2. Therefore, according to the third embodiment, the effects of the first embodiment and the second embodiment can be achieved without increasing the number of reference pulses.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 3 Electrostatic latent image forming system 13 Charging bias generation part 20 Optical deflector 21 Polygon mirror 23 Motor 25 Light beam detection signal generation part 27 Photodiode (light receiving element)
43 Light Beam Detection Signal Adjustment Unit 47 Clock Adjustment Unit 51 Clock Generation Unit 55 Motor Driver (Component of Drive Control Unit)
105 Fixing unit 106 Switchback unit (component of conveyance unit)
107 Paper transport path (components of transport unit)
108 Paper transport path (component of transport section)
113 Transfer belt (component of transfer section)
115 Photosensitive drum (image carrier)
117 Exposure unit 119 Charging unit 129 Secondary transfer roller (component of transfer unit)
501 Switching control unit (component of drive control unit)
503 First reference pulse generator (reference pulse generator)
505 Second reference pulse generator (reference pulse generator)

Claims (3)

An image forming apparatus that prints color images formed by superimposing toner images of a plurality of colors on both sides of a sheet,
A plurality of optical deflectors each including a mirror unit and driving the mirror unit;
A plurality of image forming units that respectively form assigned color toner images among the plurality of color toner images;
Each of the plurality of image forming units is
A charging unit including a charging bias generation unit that generates a charging bias including an AC bias;
An image carrier charged by the charging bias applied by the charging unit;
A light beam is emitted in a state where the mirror portion of the corresponding light deflector among the plurality of light deflectors is driven, and the image carrier is irradiated with the light beam deflected by the mirror portion to be mainly used. An exposure unit for drawing an electrostatic latent image by repeatedly drawing a scanning line;
A light receiving element that receives the light beam deflected by the mirror unit of the corresponding optical deflector, and that serves as a reference for timing of drawing the main scanning line using a signal output from the light receiving element A light beam detection signal generator for generating a beam detection signal;
A developing unit that forms the toner image by supplying toner to the electrostatic latent image drawn on the image carrier;
In order to adjust the magnification in the sub-scanning direction between the front image that is the color image formed on the front surface of the paper and the back image that is the color image formed on the back surface of the paper, In the first drawing mode for drawing the electrostatic latent image used for forming and the second drawing mode for drawing the electrostatic latent image used for forming the back surface image, the mirror section of the corresponding optical deflector A drive control unit that varies the speed of driving
The image forming apparatus further includes:
A transfer unit that transfers the color image on which the toner image formed on the image carrier of the plurality of image forming units is superimposed onto the paper;
A fixing unit that heats and fixes the color image transferred to the paper to the paper;
After fixing the color image on the front surface of the paper, in order to form the color image on the back surface of the paper, a transport unit that reverses the paper and transports it to the transfer unit and the fixing unit;
A first reference pulse generator that generates a first reference pulse having a preset first period;
A second reference pulse generator that generates a second reference pulse having a preset second period shorter than the first period;
Of the plurality of image forming units, the phase of the light beam detection signal generated by the light beam detection signal generation unit provided in the image forming unit in the first drawing mode is the phase of the first reference pulse. The phase of the light beam detection signal generated by the light beam detection signal generation unit provided in the image forming unit in the second drawing mode among the plurality of image forming units is adjusted. A first adjustment unit for adjusting the phase to match the phase of the second reference pulse;
In the image forming unit in the first drawing mode among the plurality of image forming units, a value of the charging bias at a timing synchronized with a predetermined first reference phase of the first reference pulse is a predetermined value. And the charging at the timing synchronized with a predetermined second reference phase of the second reference pulse in the image forming unit in the second drawing mode among the plurality of image forming units. A second adjustment unit that adjusts the bias value to be the predetermined value;
Among the plurality of image forming units, the image forming unit that has finished the first drawing mode is sequentially switched to the second drawing mode, and the image forming unit that has finished the second drawing mode is sequentially A switching control unit for switching to the first drawing mode,
The transfer unit includes a transfer belt on which the toner images formed on the image carrier of the plurality of image forming units are transferred in a superimposed manner, and is formed by the toner images transferred on the transfer belt in a superimposed manner. Transferring the color image to the paper,
The plurality of image forming units are arranged in tandem along a longitudinal direction of the transfer belt .   As the first adjustment unit, each of the plurality of image forming units is configured so that, in the first drawing mode, the phase of the light beam detection signal matches the phase of the first reference pulse, and In the second drawing mode, the driving of the mirror unit is adjusted by controlling the corresponding optical deflector so that the phase of the light beam detection signal matches the phase of the second reference pulse. The image forming apparatus according to claim 1, further comprising a light beam detection signal adjustment unit. Each of the plurality of image forming units includes a clock generation unit that generates a clock having a predetermined cycle that is a cycle of the AC bias.
The charging bias generation unit includes an AC bias generation unit that generates the AC bias using the clock,
As the second adjustment unit, each of the plurality of image forming units has a value of the charging bias at a timing synchronized with the first reference phase of the first reference pulse in the first drawing mode. In the second drawing mode, the value of the charging bias at the timing synchronized with the second reference phase of the second reference pulse is set to the predetermined value. 3. The image forming apparatus according to claim 1, further comprising a clock adjustment unit that adjusts a timing at which the clock generation unit generates the clock so that the obtained value is obtained.

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