JP2020021010A - Image formation device - Google Patents

Abstract

To improve image quality even in an image formation device that performs micro-exposure.SOLUTION: An image formation device includes: a photosensitive drum 5; an exposure device 9 that includes a light source for radiating an optical beam and forms an electrostatic latent image by scanning the optical beam in a main scanning direction and irradiating the photosensitive drum 5; an optical element provided between the light source and the photosensitive drum 5; and a developing device 8 that develops the electrostatic latent image formed on the photosensitive drum 5 and forms a toner image. The image formation device performs usual exposure on an image portion on the photosensitive drum 5 in the main scanning direction and performs micro-exposure on a non-image portion different from the image portion with a light amount smaller than the normal exposure, and includes an engine controller 122 that corrects an amount of emission light according to image height when micro-exposure is performed.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a color image forming apparatus using an electrophotographic recording system, such as a laser printer, a copying machine, and a facsimile.

  2. Description of the Related Art Conventionally, an image forming apparatus such as a copying machine or a laser printer using an electrophotographic recording method has been known. In an electrophotographic recording type image forming apparatus, for example, the following electrophotographic process is executed. First, the surface of the photosensitive drum is uniformly charged to, for example, -600 V by a charging device. Thereafter, laser emission is performed by a laser exposure device, and an electrostatic latent image is formed on the photosensitive drum. Then, the toner image adheres to the electrostatic latent image by the developing device, and the toner image is transferred to the transfer target by the transfer device. Here, the laser light emitted from the laser exposure device is deflected at a constant angular velocity by the rotary polygon mirror, but the scanning speed on the scanned surface is not constant because the scanned surface is flat. For this reason, an fθ lens, an fθ mirror, and the like are arranged between the rotary polygon mirror and the photosensitive drum, and these are used to make the scanning speed on the surface to be scanned constant. However, the intensity of the laser beam on the photosensitive drum varies depending on the image height, even if the scanning speed of the laser beam via the fθ lens, the fθ mirror, or the like is constant on the surface to be scanned. Since the intensity of the laser beam due to the image height affects the density of the formed image, the image forming apparatus needs to correct the density fluctuation caused by the intensity of the laser beam due to the image height. Is done.

  The lens having the above-mentioned fθ characteristic is large in size and expensive. Therefore, for the purpose of reducing the size and cost of the image forming apparatus, it has been considered to use no lens itself or use a lens having no fθ characteristic. For example, in Patent Document 1, even when a spot of a laser beam on the surface of the photosensitive drum does not move at a constant speed on the surface of the photosensitive drum, the following is performed so that dots formed on the surface of the photosensitive drum have a constant width. It is disclosed that such correction is performed. That is, it is disclosed that electrical correction is performed so as to change the frequency of the image clock during one scan.

  On the other hand, in recent years, color printers have become widespread and become mainstream. In this color printer, in order to reduce the size of the apparatus, a method has been devised in which a high-voltage power supply is shared by performing weak exposure (micro exposure) on a portion (background portion) of the photosensitive drum on which toner does not adhere. (For example, see Patent Document 2).

JP-A-58-125064 JP 2013-007989 A

  In the above-described electrophotographic image forming apparatus, it is required that the intensity of the laser beam in the main scanning direction according to the image height be corrected with high accuracy. Further, in an image forming apparatus that performs minute exposure, there is a possibility that a phenomenon such as a normal fog or a reverse fog may occur depending on the intensity of the laser beam depending on the image height. The phenomenon of fogging not only lowers the quality of an image, but also consumes toner more than necessary. For this reason, even in an image forming apparatus that performs minute exposure, it is required to provide a means for correcting the intensity of laser light based on the image height to improve image quality.

  The present invention has been made under such a situation, and an object of the present invention is to improve image quality even in an image forming apparatus that performs minute exposure.

  In order to solve the above-described problem, the present invention has the following configuration.

  (1) an exposure unit that has a photoconductor, a light source that irradiates a light beam, and that scans the light beam in a main scanning direction and irradiates the photoconductor to form an electrostatic latent image; An optical element provided between the photoconductor and developing means for developing the electrostatic latent image formed on the photoconductor to form a toner image; An image forming apparatus that performs normal exposure on an image portion in a scanning direction and performs fine exposure with a smaller amount of light than the normal exposure on a non-image portion different from the image portion, the light emission amount when performing the fine exposure, An image forming apparatus comprising: a correction unit configured to perform correction in accordance with an image height that is a distance from an optical axis of the optical element in the main scanning direction.

  According to the present invention, the image quality can be improved even in an image forming apparatus that performs minute exposure.

FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment. FIG. 2 is a diagram illustrating a main part of an exposure apparatus according to an embodiment. FIG. 3 is a diagram illustrating a drive circuit having a three-level light intensity adjustment function according to the embodiment. FIG. 4 is a diagram illustrating an image height and a laser beam intensity on the surface of the photosensitive drum according to the embodiment; FIG. FIG. 7 is a diagram illustrating the waveform of each signal when an all-white image is printed according to the embodiment, and is a diagram illustrating the waveform of each signal during a sheet interval. FIG. 11 is a diagram illustrating a drive circuit having a three-level light intensity adjustment function according to another embodiment, and is a diagram illustrating waveforms of respective signals when an all-white image is printed.

  Hereinafter, preferred embodiments of the present invention will be illustratively described in detail with reference to the drawings. However, the components described in this embodiment are merely examples, and are not intended to limit the scope of the present invention.

<Schematic diagram of cross section of image forming apparatus>
FIG. 1 is a schematic sectional view of the color image forming apparatus. In the following description, a description will be given using a color image forming apparatus, but the present invention is not limited to this. In the following description, a portion to which toner is applied on the photosensitive drum is referred to as an image portion, and a portion to which no toner is applied is referred to as a non-image portion. The fine exposure of a non-image portion described later can be applied to, for example, a monochrome image forming apparatus. In the following, an in-line type color image forming apparatus will be described as an example, but a rotary type color image forming apparatus may be used, for example. In the following, an image forming apparatus having an intermediate transfer belt will be described. However, the present invention can be applied to an image forming apparatus employing a method of directly transferring a toner image developed on a photosensitive drum to a transfer material. Hereinafter, a color image forming apparatus employing an in-line system and an intermediate transfer belt system will be described in detail as an example.

  FIG. 1 is a diagram showing a configuration of a color laser printer (hereinafter, referred to as a printer) 50. The printer 50 has a plurality of photosensitive drums 5 (5Y, 5M, 5C, 5K). Here, Y is a code representing yellow, M is magenta, C is cyan, and K is a code representing black, and will not be described below unless a specific color is described. The printer 50 is a printer in which toner images formed on the respective photosensitive drums 5 are successively multiplex-transferred onto the intermediate transfer belt 3 as an intermediate transfer body to obtain a full-color print image. This method is called an in-line method or a four-drum method.

  The intermediate transfer belt 3 is an endless endless belt, and is suspended around a drive roller 12, a tension roller 13, an idler roller 17, and a secondary transfer opposing roller 18. The intermediate transfer belt 3 rotates at a process speed of 115 mm / sec in the direction of the arrow (clockwise) in the figure. The drive roller 12, the tension roller 13, and the secondary transfer opposing roller 18 are support rollers that support the intermediate transfer belt 3. The drive roller 12, the secondary transfer opposing roller 18 has a configuration of φ24 (mm), and the tension roller 13 has a configuration of φ16 (mm).

  Four photosensitive drums 5 are arranged in series in the moving direction of the intermediate transfer belt 3. The developing device 8 is a developing unit. The photosensitive drum 5Y having the yellow developing device 8Y is uniformly charged to a predetermined polarity and potential by the charging roller 7Y during the rotation process, and then receives the image exposure 4Y by the exposure device 9Y. That is, by causing a laser diode 107 (see FIG. 2), which will be described later, to emit light in accordance with the yellow image data, a laser beam is irradiated onto the photosensitive drum 5Y with a light amount and time corresponding to the yellow image data. As a result, an electrostatic latent image corresponding to the first color (yellow) component image of the target color image is formed on the photosensitive drum 5Y. Next, the yellow toner of the first color adheres to the electrostatic latent image and is developed by the yellow developing device 8Y as the first developing device. Thereby, visualization of the image is performed. Such a method in which the toner is developed on the portion where the electrostatic latent image is formed by the image exposure is referred to as a “reversal development method”.

  The yellow image formed on the photosensitive drum 5Y enters a primary transfer nip with the intermediate transfer belt 3. In the primary transfer nip portion, a primary transfer roller 10 </ b> Y as a voltage applying member is brought into contact with the back side of the intermediate transfer belt 3. A primary transfer voltage power supply (not shown) for applying a voltage is connected to the primary transfer roller 10Y. The intermediate transfer belt 3 first transfers a yellow image at a first color station, and then sequentially transfers multiple colors of magenta, cyan, and black from the photosensitive drums 5M, 5C, and 5K corresponding to the respective colors after the above-described process. . The four color toner images transferred onto the intermediate transfer belt 3 are rotated in the direction of the arrow (clockwise) in FIG.

  On the other hand, the recording material P stacked and stored in the paper feed cassette 1 is fed by the paper feed roller 2, conveyed to a nip portion of a registration roller pair (hereinafter, referred to as a registration roller pair) 6, and temporarily stopped. You. The temporarily stopped recording material P is supplied to the secondary transfer nip by the registration roller pair 6 in synchronization with the timing at which the four color toner images formed on the intermediate transfer belt 3 reach the secondary transfer nip. You. The toner image on the intermediate transfer belt 3 is transferred onto the recording material P by applying a voltage (about +1.5 kV) between the secondary transfer roller 11 and the secondary transfer opposing roller 18.

  The recording material P on which the unfixed toner image has been transferred is separated from the intermediate transfer belt 3 and sent to the fixing device 14 via the conveyance guide 19, where it is heated and heated by the fixing roller 15 and the pressure roller 16. Under the pressure, the toner image is melted and fixed on the surface. Thereby, a four-color full-color image is obtained. Thereafter, the recording material P is discharged from the discharge roller pair 20 to the outside of the apparatus, and one cycle of printing is completed. On the other hand, the toner remaining on the intermediate transfer belt 3 without being transferred to the recording material P in the secondary transfer portion is removed by a cleaning unit 21 disposed downstream of the secondary transfer portion.

  The above is the description of the schematic cross-sectional view of the printer 50. Next, in the following, in relation to the laser drive system, first, a diagram showing a main part of the exposure apparatus 9 will be described, and then, a circuit configuration of the laser drive system will be described in detail.

<Main parts of exposure equipment>
FIG. 2 shows a main part of the exposure apparatus 9. A drive current flows through a laser diode 107 (hereinafter, referred to as an LD 107) that is a light source and a light emitting element by the operation of a drive circuit 130 that is a drive unit. The LD 107 emits a laser beam (light beam) at an intensity level according to the drive current. The drive circuit 130 is a circuit for driving the LD 107 electrically connected to the engine controller 122 and the video controller 123, which are control means described later (see FIG. 3).

  The laser beam emitted by the LD 107 is shaped into a parallel beam by the collimator lens 134, is converted into a parallel beam, and is scanned in the horizontal direction of the photosensitive drum 5 by the rotating polygon mirror 133. The scanning direction of the laser beam is called a main scanning direction. A direction substantially perpendicular to the main scanning direction is called a sub-scanning direction. The scanned laser beam forms an image on the surface of the rotating photosensitive drum 5 by the fθ lens 132 and is exposed in a dot form.

  On the other hand, a reflection mirror 131 is provided corresponding to a scanning position on one end side of the photosensitive drum 5, and reflects a laser beam projected on a scanning start position toward a sensor (hereinafter, referred to as a BD) 121. The BD 121 outputs a signal (hereinafter, referred to as a BD signal) to the drive circuit 130 when the laser beam is incident. The drive circuit 130 determines the scanning start timing of the laser beam based on the BD signal output from the BD 121. Here, at the time of the forced light emission in the detection of the laser light, automatic light amount control (hereinafter also referred to as APC (Auto Power Control)) of the laser light amount is performed, and the light emission amount (light emission level) of the laser is adjusted.

<Configuration of drive circuit>
FIG. 3 shows the light amount level of the LD 107 in performing the micro-exposure so as to prevent the toner from adhering on the photosensitive drum 5 (on the photoreceptor) in the non-image portion and not to generate the positive fog or the reverse fog. This is a drive circuit that adjusts automatically.

  3, the driving circuit 130 shown in FIG. 2 corresponds to a portion surrounded by a dotted frame. The drive circuit 130 has comparator circuits 101, 111, 141, sample / hold circuits 102, 112, 142, and hold capacitors 103, 113, 143. The drive circuit 130 includes current amplifier circuits 104, 114, 144, reference current sources (constant current circuits) 105, 115, 145, and switching circuits 106, 116, 146. The drive circuit 130 has a current-voltage conversion circuit 109.

  The LD 107 has a cathode terminal connected to the switching circuits 106, 116, and 146. The anode terminal of the LD 107 is connected to the power supply Vcc. The photodiode (hereinafter referred to as PD) 108 has an anode terminal connected to the current-voltage conversion circuit 109. The cathode terminal of the PD 108 is connected to the power supply Vcc. The engine controller 122 receives the BD signal (synchronization detection signal) output from the BD 121. Further, the comparator circuit 101, the sample / hold circuit 102, the hold capacitor 103, the current amplifying circuit 104, the reference current source (constant current circuit) 105, and the switching circuit 106 correspond to a first light amount adjusting unit. The comparator circuit 111, the sample / hold circuit 112, the hold capacitor 113, the current amplifying circuit 114, the reference current source (constant current circuit) 115, and the switching circuit 116 correspond to a plurality of second light amount adjustment units, and the third light amount It corresponds to an adjustment unit. The comparator circuit 141, the sample / hold circuit 142, the hold capacitor 143, the current amplifying circuit 144, the reference current source (constant current circuit) 145, and the switching circuit 146 correspond to a plurality of second light amount adjustment units, and the fourth light amount It corresponds to an adjustment unit. Note that the first, third, and fourth are described to distinguish one from another, and which one is described as first is not particularly limited.

  The engine controller 122 includes an ASIC, a CPU, a RAM, and an EEPROM. The engine controller 122 performs not only control of the printer engine but also communication control with the video controller 123 and the like.

  The drive circuit 130 has an OR circuit 124, and an Ldrv signal of the engine controller 122 and a VIDEO signal as a first signal from the video controller 123 are connected to inputs. The OR circuit 124 outputs a Data signal, and the Data signal is connected to the switching circuit 106. The VIDEO signal is basically a signal based on print data (image data) sent from an external device such as a reader scanner connected to the outside or a host computer. However, as described above, the engine controller 122 also performs communication control with the video controller 123. Therefore, the engine controller 122 and the video controller 123 can share information of the EEPROM on the engine controller 122 and information of the image memory of the video controller 123 with each other.

  A VIDEO signal output from the video controller 123 as an output unit is input to a buffer 125 having an enable terminal (hereinafter, referred to as a buffer). The output of the buffer 125 is connected to the above-described OR circuit 124. At this time, the enable terminal is connected to a line from which the Venb signal from the engine controller 122 is output.

  Further, the engine controller 122 is connected to output an SH1, SH2, SH3, Base2, Base3, Ldrv, and Venb signal, which will be described later. The Base2 signal and the Base3 signal correspond to a plurality of signals. Further, the Base2 signal corresponds to a second signal, and the Base3 signal corresponds to a third signal. The Venb signal is for performing a mask process on the Data signal based on the VIDEO signal. The area where the Data signal is subjected to the mask processing is referred to as an image mask area, and the period during which the Data signal is subjected to the mask processing, in other words, the timing at which the Data signal is set as the image mask area is referred to as the image mask period. By setting the Venb signal to a disabled state (OFF state), the timing of the image mask area (image mask period) can be created.

  The first reference voltage Vref11, the second reference voltage Vref21, and the third reference voltage Vref31 are input to the positive terminals of the comparator circuits 101, 111, and 141, respectively. Outputs of the comparator circuits 101, 111, and 141 are input to sample / hold circuits 102, 112, and 142, respectively. The reference voltage Vref11 is set as a target voltage for causing the LD 107 to emit light at a normal print light emission level (first light emission level or first light amount). In addition, the reference voltage Vref21 is set as a target voltage of a light emission level (second light emission level or second light amount) for minute exposure. Further, the reference voltage Vref31 is set as a target voltage of a light emission level (third light emission level or third light amount) for minute exposure. The second light amount and the third light amount correspond to a plurality of light amounts. Note that the second light amount and the third light amount are lower than the first light amount.

  The hold capacitors 103, 113, and 143 are connected to the sample / hold circuits 102, 112, and 142, respectively. The outputs of the hold capacitors 103, 113, and 143 are input to the positive terminals of the current amplifier circuits 104, 114, and 144 via the sample / hold circuits 102, 112, and 142, respectively. The reference voltages Vref11, Vref21, and Vref31 mean settings in the drive circuit 130 for realizing a light emission level for normal printing and a light emission level for minute exposure.

  Reference current sources 105, 115, and 145 are connected to the current amplifying circuits 104, 114, and 144, respectively, and their outputs are input to switching circuits 106, 116, and 146, respectively. On the other hand, the fourth reference voltage Vref12, the fifth reference voltage Vref22, and the sixth reference voltage Vref32 are input to the negative terminals of the current amplifier circuits 104, 114, and 144, respectively. Here, the current Io1 (first drive current) is determined according to the difference between the output voltage of the sample / hold circuit 102 described above and the reference voltage Vref12. The current Io2 (second drive current) is determined according to the difference between the output voltage of the sample / hold circuit 112 and the reference voltage Vref22. The current Io3 (third drive current) is determined according to the difference between the output voltage of the sample / hold circuit 142 and the reference voltage Vref32. That is, the fourth to sixth reference voltages Vref12, Vref22, and Vref32 are voltage settings for determining the current.

  The switching circuit 106 is turned on / off by a Data signal which is a pulse modulation data signal. The switching circuit 116 is turned on / off by an input signal Base2, and the switching circuit 146 is turned on / off by an input signal Base3. Output terminals of the switching circuits 106, 116, and 146 are connected to the cathode terminal of the LD 107, and supply drive currents Idrv, Ib1, and Ib2. The anode terminal of the LD 107 is connected to the power supply Vcc. The cathode terminal of the PD 108 for monitoring the light amount of the LD 107 is connected to the power supply Vcc. The anode terminal of the PD 108 is connected to the current-voltage conversion circuit 109 and allows the monitor current Im to flow through the current-voltage conversion circuit 109. Thereby, the current-voltage conversion circuit 109 converts the monitor current Im to the monitor voltage Vm. The monitor voltage Vm is input to the negative terminals of the comparator circuits 101, 111, and 141 in negative feedback.

  Although FIG. 3 shows the engine controller 122 and the video controller 123 separately, the present invention is not limited to this. For example, a part or all of the engine controller 122 and the video controller 123 may be configured by the same controller. Further, the drive circuit 130 enclosed by a dotted frame in the drawing may be partially or entirely incorporated in the engine controller 122, for example.

<Laser intensity depending on image height on photosensitive drum surface>
In the case of the present embodiment, the characteristics such as the reflectance and the transmittance of the optical elements provided between the LD 107 and the photosensitive drum 5, such as the collimator lens 134, the rotating polygon mirror 133, and the fθ lens 132, through which the laser light passes, have the laser light Depends on the angle of incidence on the optical element. Therefore, the intensity of the laser light varies depending on the image height. The image height is a value that represents the position of an image on the surface of the photosensitive drum 5 as a distance from the optical axis of the optical element, and the value of the image height at the optical axis of the optical element is 0 [mm]. The value on the upstream side in the scanning direction is minus (-), and the value on the downstream side in the main scanning direction is plus (+).

  FIG. 4A is a diagram illustrating the unevenness of the intensity of the laser beam due to the image height on the surface of the photosensitive drum 5 when the LD 107 emits the same amount of light over the entire image height in the present embodiment. FIG. 4A shows the photosensitive drum 5Y and the charging roller 7Y, and the horizontal direction in FIG. 4A is the main scanning direction. In (ii), the horizontal axis indicates the image height [mm] on the surface of the photosensitive drum 5, and the vertical axis indicates the intensity [%] of the laser beam. In this embodiment, the length (hereinafter, referred to as width) of the photosensitive drum 5 in the longitudinal direction (main scanning direction) is 250 mm, and the width of the charging roller 7 in the longitudinal direction is 230 mm. 4A, an area where an image is formed on the photosensitive drum 5 is equal to or less than the width of the charging roller 7 in the longitudinal direction.

  In this embodiment, for example, the range from the image height of -120 mm to the image height of +120 mm is the effective area of the minute exposure. As shown in FIG. 4A, even when the LD 107 emits light with the same light amount over the entire image height, when the laser light intensity at the image height of -120 mm is 100%, the laser light intensity is 80% at the image height of +120 mm. It drops to near.

  FIG. 4B is a diagram for explaining a correction amount in a case where the laser light intensity changes due to the image height described in FIG. 4A is corrected. FIG. 4B (i) is a view similar to FIG. 4 (a) (i). In (ii), the horizontal axis indicates the image height [mm] on the surface of the photosensitive drum 5, and the vertical axis indicates the correction amount [%] of the laser beam. A correction amount (hereinafter, referred to as a correction amount) having the opposite characteristic as shown in FIG. 4B (ii) with respect to the intensity unevenness of the laser beam due to the image height on the surface of the photosensitive drum 5 shown in FIG. The light amount correction data is also stored in an EEPROM serving as a storage unit of the engine controller 122. As described above, the correction amount based on the change in the light amount of the laser light generated according to the image height is stored in the EEPROM in advance. The engine controller 122, which is also a correction unit, corrects the light amount of the LD 107 based on the light amount correction data stored in the EEPROM. Thereby, the unevenness of the laser beam intensity due to the image height on the surface of the photosensitive drum 5 becomes constant. The light amount correction data indicating the relationship between the image height and the correction amount of the laser light is also called a laser light intensity correction profile.

<Description of APC operation>
Next, the APC operation of this embodiment will be described. As shown in FIG. 4B, in the minute exposure, the point A has the minimum value of the correction amount of the laser light, and the point B has the maximum value of the correction amount of the laser light. The reference voltage Vref21 and the reference voltage Vref31 are set such that the point A is set to the third light emission level (third light amount) and the point B is set to the second light emission level (second light amount). There is a need. That is, the third light amount is the minimum light amount that can be set as the minute exposure, and the second light amount is the maximum light amount that can be set as the minute exposure.

  First, the engine controller 122 sets the reference voltage Vref31 and sets the sample / hold circuit 142 to a sampling state. In this state, when the LD 107 enters the light emitting state, the PD 108 monitors the light emission intensity (light emission amount) of the LD 107, and a monitor current Im3 proportional to the light emission intensity flows. Then, the monitor current Im3 flows to the current-voltage conversion circuit 109. Thereby, the current-voltage conversion circuit 109 converts the monitor current Im3 to the monitor voltage Vm3. Further, the current amplifier circuit 144 controls the drive current Ib2 based on the current Io3 flowing to the reference current source 145 so that the monitor voltage Vm3 matches the third reference voltage Vref31 which is the target value. As described above, the third light amount adjustment unit controls the drive current Ib2 such that the light emission intensity of the LD 107 at the point A becomes the third light emission level.

  Next, the engine controller 122 sets the reference voltage Vref21 and sets the sample / hold circuit 112 to the sampling state. At this time, the sample / hold circuit 142 is set to a hold state. When the LD 107 enters the light emitting state, the PD 108 monitors the light emission intensity (light emission amount) of the LD 107, and a monitor current Im2 proportional to the light emission intensity flows. Then, the monitor current Im2 flows to the current-voltage conversion circuit 109. Thereby, the current-voltage conversion circuit 109 converts the monitor current Im2 to the monitor voltage Vm2. Further, the current amplifying circuit 114 controls the drive current Ib1 based on the current Io2 flowing to the reference current source 115 so that the monitor voltage Vm2 matches the second reference voltage Vref21 which is the target value. As described above, the second light amount adjustment unit controls the drive current Ib1 so that the light emission intensity of the LD 107 at the point B becomes the second light emission level. As described above, the drive circuit 130 can drive the LD 107 so that the LD 107 has the third light amount at the point A and the LD 107 has the second light amount at the point B in the minute exposure.

  In normal exposure (exposure at the time of image formation), point C has the minimum value of the laser light correction amount, and point D has the maximum value of the laser light correction amount. It is necessary to set the reference voltage Vref11 so that the point D is set to the above-described first light emission level. The first light amount is the maximum light amount among the light amounts that can be set in the LD 107.

  The engine controller 122 sets the reference voltage Vref11 and sets the sample / hold circuit 102 to a sampling state. At this time, the sample / hold circuit 142 is set to a hold state. When the LD 107 enters the light emitting state, the PD 108 monitors the light emission intensity (light emission amount) of the LD 107, and a monitor current Im1 proportional to the light emission intensity flows. Then, the monitor current Im1 flows to the current-voltage conversion circuit 109. Thus, the current-voltage conversion circuit 109 converts the monitor current Im1 to the monitor voltage Vm1. Further, the current amplifying circuit 104 controls the drive current Idrv based on the current Io1 flowing to the reference current source 105 so that the monitor voltage Vm1 matches the first reference voltage Vref11 which is the target value. As described above, the first light amount adjustment unit controls the drive current Idrv such that the light emission intensity of the LD 107 at the point D becomes the first light emission level. As described above, the drive circuit 130 can drive the LD 107 so that the LD 107 has the first light amount at the point D in the normal exposure.

  When it is desired to set the third light amount as the minute exposure during the non-APC operation, the minute exposure state is maintained at the third light amount by setting the sample / hold circuit 142 to the hold state. Similarly, when it is desired to set the second light amount as the minute exposure, the sample / hold circuits 112 and 142 are set to the hold state, and the minute exposure state is maintained at the second light amount. When it is desired to set the first light amount as the normal exposure, the sample / hold circuits 102 and 142 are put into a hold state, and the Venb signal and the VIDEO signal are turned on (for example, high level), so that the first light amount is obtained. The normal light emitting state is maintained.

<About laser beam intensity correction>
Next, a method for correcting the intensity of the laser light according to the present embodiment will be described. FIG. 5A is a timing chart showing signals output from the engine controller 122 and the video controller 123 when an all-white image is printed. FIG. 5A shows a change in each signal in a laser light intensity correction profile, a laser light intensity, and a non-image area and an image area for one cycle in the main scanning direction. One cycle refers to a cycle from the output of a BD signal to the output of the next BD signal. VIDEO, Base2 and Base3 signals are shown as signals. Specifically, (i) of FIG. 5A shows the image height and the correction amount [%] of the intensity of the laser beam in FIG. 4B and (ii). (Ii) indicates an area on the photosensitive drum 5, where an area where image formation is performed is an image area, and other areas are non-image areas. (Iii) shows a BD signal output from the BD 121. For example, a change from the high level to the low level of the BD signal is expressed as the output of the BD signal. (Iv) indicates a VIDEO signal, which is a signal corresponding to image data. (V) shows a Base3 signal input from the engine controller 122 to the switching circuit 146. (Vi) indicates a Base2 signal input from the engine controller 122 to the switching circuit 116. (Vii) indicates the intensity of the laser beam.

  As shown in FIG. 5A, the engine controller 122 changes the duty of the Base2 signal in the non-image area based on the laser light intensity correction profile. Here, changing the duty of the signal means changing the width (time) of the high level or the low level of the signal. In this embodiment, the high-level width of the signal is changed. That is, the engine controller 122 reduces the duty of the Base2 signal in the non-image area on the side of the point A where the correction amount of the laser beam is minimum. On the other hand, the engine controller 122 sets the duty of the Base2 signal larger than the duty of the Base2 signal in the non-image area on the point A side in the non-image area on the point B where the correction amount of the laser beam is the maximum. . As shown in (v) of FIG. 5A, the engine controller 122 sets the Base3 signal to a high level in any of the non-image areas on the point A side and the point B side. The Base3 signal is turned on so that the LD 107 emits light with the third light quantity at a duty of 100%, in other words, the LD 107 always emits light.

  On the other hand, the engine controller 122 transmits the laser light intensity correction profile stored in the EEPROM on the engine controller 122 to the video controller 123 in the image area. The video controller 123 changes the duty of the VIDEO signal based on the laser light intensity correction profile received from the engine controller 122. That is, the engine controller 122 sends the VIDEO signal of the VIDEO signal from the point C where the laser light correction amount is minimum to the point D side where the laser light correction amount is maximum via the video controller 123. Control is performed to increase the duty. Note that, as shown in (v) of FIG. 5A, the engine controller 122 sets the Base3 signal to a high level in the entire image area. Thereby, even in the non-image portion in the image area, the minute exposure with the third light amount is performed.

  By performing such correction of the intensity of the laser light, the intensity of the laser light based on the image height on the surface of the photosensitive drum 5 can be adjusted. As shown in (vii) of FIG. 5A, by changing the duty of the Base2 signal or the VIDEO signal based on the laser light intensity correction profile at the time of minute light emission or at the time of normal printing, the laser based on the image height is changed. The change in light intensity is corrected. As shown in FIGS. 5A and 5B, the laser light intensity is such that the toner does not adhere to the photosensitive drum 5 in the non-image area, and is smaller than the laser light intensity in the image area. Has become. Here, an all-white image has been described, but when the image pattern changes, a VIDEO signal may be added according to the image pattern.

(Correction between sheets)
FIG. 5B is a timing chart showing signals output from the engine controller 122 and the video controller 123 during a sheet interval. The space between sheets refers to a rear end of a predetermined recording material P and a subsequent recording material P on which an image is formed following the predetermined recording material P when printing is performed continuously (hereinafter, referred to as continuous printing). Between the tip of (I) to (vii) in FIG. 5B are the same as (i) to (vii) in FIG.

  As shown in FIG. 5B, since no VIDEO signal is output between sheets, the entire area of the photosensitive drum 5 becomes a non-image area. Then, in the non-image area, the engine controller 122 changes the duty of the Base2 signal based on the laser light intensity correction profile. Specifically, the engine controller 122 increases the duty of the Base2 signal from the point A where the correction amount of the laser light intensity is minimum to the point B side where the correction amount of the laser light intensity is maximum. Control. Note that, as shown in (v) of FIG. 5B, the engine controller 122 sets the Base3 signal to a high level in the entire non-image area. By performing the laser light intensity correction, the laser light intensity based on the image height on the surface of the photosensitive drum 5 can be adjusted even between sheets. As shown in FIGS. 5 (b) and (vii), the laser light intensity is such that the toner does not adhere to the photosensitive drum 5 in the non-image area, and the image shown in FIGS. 5 (a) and (vii) It is smaller than the laser light intensity in the region.

<Regarding Modification of Laser Light Intensity Correction>
In the above description, the case where the signals output from the engine controller 122 and the video controller 123 are digitally switched based on the laser light intensity correction profile has been described. A description will be given of a modification in which the operation speed of the engine controller 122 cannot be increased or radiation noise is reduced. FIG. 6A is a diagram illustrating a driving circuit 130a that automatically adjusts the light amount level of the LD 107 according to the modification. In the modified example, the points different from the drive circuit 130 described in FIG. 3 will be described, and the same components as those described in FIG. 3 will be denoted by the same reference numerals and description thereof will be omitted.

  The drive circuit 130a according to the modified example includes a filter circuit 139 as a filter unit. For example, the filter circuit 139 is a low-pass filter (hereinafter, LPF). The filter circuit 139 is connected to the output of the current amplifier circuit 114, and the current amplifier circuit 114 is connected to the switching circuit 116 via the filter circuit 139. By changing the output of the current amplifier circuit 114 through the filter circuit 139, the change of the laser beam intensity becomes smooth.

  Next, a laser light intensity correction method according to this modification will be described. FIG. 6B is a timing chart showing signals output from the engine controller 122 and the video controller 123 when an all-white image is printed. (I) to (vii) in FIG. 5B are the same as (i) to (vii) in FIG.

  As shown in FIG. 6B, in the non-image area, the engine controller 122 changes the duty of the Base2 signal based on the laser light intensity correction profile. On the other hand, the engine controller 122 changes the duty of the VIDEO signal via the video controller 123 in the image area based on the laser light intensity correction profile. As a result, as shown in (vii) of FIG. 5B, the intensity of the laser light smoothly changes in the non-image area. For this reason, the number of times of switching of the Base2 signal in the non-image area can be reduced. Therefore, even if the operation speed of the engine controller 122 is not increased, the laser beam intensity unevenness due to the image height on the surface of the photosensitive drum 5 can be made constant by correcting the light amount of the LD 107, so that it is inexpensive and the emission noise is reduced. The effect can be reduced.

<Explanation of action and effect>
In this embodiment, a plurality of APC adjustments are performed in the minute exposure. Then, by correcting the light amount of the LD 107 using the results of the plurality of APC adjustments, the laser light intensity unevenness due to the image height on the surface of the photosensitive drum 5 caused by the characteristics such as the reflectance and the transmittance of the optical element is kept constant. can do. Therefore, it is possible to provide a high-quality image in which the phenomenon of normal fog or reverse fog does not occur. Further, only by changing the laser light intensity correction profile, it is possible to correct other factors such as uneven sensitivity of the photosensitive drum, and it is also possible to correct a combination of a plurality of factors. Further, by performing the correction of the present embodiment, it is possible to use a lens having no fθ characteristic, and it is possible to reduce the size and cost.

  As described above, according to the embodiment, the image quality can be improved even in an image forming apparatus that performs minute exposure.

5 Photosensitive drum 8 Developing device 9 Exposure device 122 Engine controller 133 Rotating polygon mirror

Claims (12)

A photoreceptor,
Exposure means having a light source for irradiating a light beam, forming an electrostatic latent image by irradiating the photosensitive member by scanning the light beam in the main scanning direction,
An optical element provided between the light source and the photoconductor,
Developing means for developing the electrostatic latent image formed on the photoconductor to form a toner image;
An image forming apparatus that performs normal exposure on an image portion in the main scanning direction on the photoconductor, and performs fine exposure with a light amount smaller than the normal exposure on a non-image portion different from the image portion,
An image forming apparatus, comprising: a correction unit configured to correct a light emission amount when performing the minute exposure according to an image height that is a distance from an optical axis of the optical element in the main scanning direction.   The image forming apparatus according to claim 1, wherein the correction unit corrects the light amount of the light source based on a correction amount based on a change in the light amount generated according to the image height.   The image forming apparatus according to claim 2, further comprising a storage unit that stores the correction amount. An output unit that outputs a first signal for causing the light source to emit light in accordance with image data input in an image area where image formation is performed on the photoconductor;
Control means for outputting a plurality of signals for performing the fine exposure in a non-image area on the photoconductor except for the image area,
A first light amount adjustment unit that adjusts the light amount of the light source in the image area to a first light amount, and a plurality of second light amount adjustment units that adjust the light amount of the light source in the minute exposure to a plurality of light amounts. Driving means for causing the light amount to emit at the first light amount based on the first signal in the image area, and emitting the light amount at the plurality of light amounts based on the plurality of signals in the non-image area; ,
Prepared,
The correcting unit corrects the light amount of the light source by changing a duty of one of the plurality of signals in the non-image area according to the correction amount, and in the image area, 4. The image forming apparatus according to claim 2, wherein the light amount of the light source is corrected by changing a duty of the signal according to the correction amount. The plurality of second light amount adjusting units adjust a light amount of the light source in the minute exposure to a second light amount, and a third light amount adjusting unit adjusts the light amount of the light source in the minute exposure to the second light amount. A fourth light amount adjustment unit that adjusts to a small third light amount,
The control unit outputs to the driving unit a second signal for causing the light source to emit light at the second light amount and a third signal for causing the light source to emit light at the third light amount. ,
The correction unit sets the second signal to a duty based on the correction amount while turning on the third signal in the non-image area, and turns on the third signal in the image area. 5. The image forming apparatus according to claim 4, wherein the light amount is corrected by setting the first signal to a duty based on the correction amount while keeping the state.   In a case where printing is performed continuously, the third signal is turned on during a sheet interval between the rear end of a predetermined recording material and the front end of a succeeding recording material on which printing is performed subsequent to the predetermined recording material. 6. The image forming apparatus according to claim 5, wherein the light amount is corrected by setting the second signal to a duty based on the correction amount while keeping the state.   The image forming apparatus according to claim 5, wherein the second light amount is a maximum light amount among light amounts that can be set as the minute exposure.   The image forming apparatus according to claim 5, wherein the third light amount is a minimum light amount among light amounts that can be set as the minute exposure.   The image forming apparatus according to claim 5, wherein the first light amount is a maximum light amount among light amounts that can be set for the light source. The second light amount adjustment unit includes:
A current amplifier circuit for amplifying a current corresponding to a current flowing through the light source,
A switching circuit that turns on and off based on the second signal;
A filter circuit connected between the current amplification circuit and the switching circuit,
The image forming apparatus according to claim 4, further comprising:   The image forming apparatus according to any one of claims 2 to 10, wherein the correction amount increases as the image height increases.   The light exposure according to any one of claims 1 to 11, wherein the minute exposure is a light amount such that toner is not applied to the photoconductor by the developing unit when the light source emits light. Image forming apparatus.

Images