JP6639532B2 - Image forming device - Google Patents

Description

  The present invention relates to an image forming apparatus including an optical scanning device, and more particularly, to a method for correcting a laser light amount in an optical scanning device.

  2. Description of the Related Art In recent years, image forming apparatuses such as copying machines and printers have been required to form images with high image quality and high accuracy. Generally, an optical scanning device provided in such an image forming apparatus includes a laser beam that scans a surface to be scanned of a photoconductor, a rotary polygon mirror for deflecting the laser beam, and a rotary polygon mirror. A scanning optical system using a drive motor is provided. The scanning optical system determines the speed and the irradiation position of the laser light when the laser light scans the surface to be scanned of the photoconductor by optical components such as an fθ lens and a reflection mirror. Since the light transmittance of the fθ lens and the reflectivity of the reflection mirror are not uniform in the scanning direction of the laser light, the amount of laser light at each irradiation position on the surface to be scanned is not uniform. As a result, there is a problem that density unevenness occurs in an image to be formed. Therefore, conventionally, correction for uniformizing the laser light amount on the photoconductor (hereinafter, referred to as shading correction) has been performed to suppress the occurrence of density unevenness. Among the shading correction techniques, as a technique for performing shading correction with a simpler configuration, a PWM signal (pulse width modulation signal) corresponding to a correction light amount is generated, and shading is performed based on a voltage obtained by smoothing the PWM signal. There is a method of controlling the correction amount. In this method, the light amount can be controlled by controlling the duty ratio (Duty) of the PWM signal, and the shading correction control can be performed with an inexpensive and simple configuration.

  For example, Patent Literature 1 discloses a shading circuit including an active filter that removes a frequency component of a PWM signal and a voltage conversion unit that converts a voltage of the PWM signal input to the active filter into a predetermined voltage range. I have. This shading circuit stabilizes the smoothing operation of the PWM signal corresponding to the exposure position, and as a result, it is possible to prevent the occurrence of a light amount step or the like due to the shading operation. Further, in the shading circuit of Patent Document 1, laser power control (Auto Power Control: APC) is performed in a non-image area in order to correct the laser power. Further, a BD (Beam Detector) for detecting a laser beam emitted from the non-image area is provided. When the BD detects the laser beam, the BD outputs a scanning timing signal serving as a reference of a scanning timing of the photoconductor by the laser beam.

JP 2011-25502 A

  In order for an image forming apparatus to be compatible with a wide range of productivity, it is required that the light amount correction range of a laser diode that emits laser light in an optical scanning device can be controlled in a wide range. The drive current of the laser diode is automatically adjusted by the laser light amount control (APC) operation so that the light emission amount of the laser light from the laser diode becomes a desired value in the image area of the photoconductor on which an image is formed. You. In the case of a circuit configuration that emits light in the image area with the light amount adjusted at the time of the laser light amount control (APC) operation, the light amount of the laser light incident on the BD that outputs the scanning timing signal in the non-image area is also equal to the laser diode. It is necessary to be able to detect in the range corresponding to the light amount correction range.

  However, when the inside of the image forming apparatus is in a high-temperature and high-humidity environment, the amount of laser light required in the image area may be lower than the light amount range in which the BD can be detected. In such a case, since the BD cannot detect the laser light emitted from the laser diode, no scanning timing signal is output. Therefore, in order to output the scanning timing signal from the BD, there arises a problem that the light amount range of the laser light in the image area must be limited. For example, this problem can be solved if the amount of laser light incident on a BD that outputs a scanning timing signal in a non-image area and the amount of laser light irradiated on a photosensitive drum in an image area can be individually controlled. . However, for that purpose, a control IC capable of controlling the drive current of the laser diode differently in the non-image area and the image area is required, which causes a problem of cost increase.

  The present invention has been made under such circumstances, and has as its object to emit a laser beam of a light amount detectable by a BD and a laser beam of a light amount necessary for image formation without increasing the cost. .

  In order to solve the above-described problems, the present invention includes the following configurations.

(1) A photoreceptor, a light source for emitting a light beam, a first light receiving element for receiving the light beam emitted from the light source, and a light beam emitted from the light source scans on the photoreceptor. Deflecting means for deflecting the light beam, developing means for developing an electrostatic latent image formed on the photoreceptor by scanning with the light beam deflected by the deflecting means using toner, Transfer means for transferring the toner image developed by the developing means to the transfer member, detection means for detecting the toner image for density detection formed on the transfer member, and reference for scanning timing of the photosensitive member by the light beam A second light receiving element for receiving the light beam deflected by the deflecting means to generate a timing signal, and a control IC for supplying a drive current to the light source in accordance with a drive signal A light beam received by the first light receiving element for a value of a driving current supplied to the light source during a scanning cycle of the light beam so that the light beam scanning the second light receiving element has a target light amount. a control IC that is set based on the light intensity, storage means for storing compensation data for correcting the amount of light corresponding to the scanning position of the light beam on the photosensitive member, provided outside of the control IC, The density detection toner detected by the detection unit such that a value of the drive current supplied to the light source is adjusted based on a value set by the control IC according to a scanning position on the photoconductor. An adjusting circuit that adjusts a voltage applied to the control IC based on an image density detection result and the correction data, wherein the adjustment circuit adjusts a voltage applied to the control IC based on the density detection result of the density detection toner image and the correction data. Comprising a regulating circuit including a pulse signal generating means for generating a first pulse signal having a pulse width corresponding to the scanning position on the body, the said adjustment circuit adjusts the value of drive current supplied to the light source Adjusting voltage output means for outputting a reference voltage for current adjustment for generating a second pulse signal based on the reference voltage by turning on / off in response to the first pulse signal; And a smoothing circuit for smoothing a second pulse signal to generate the voltage applied to the control IC .

  ADVANTAGE OF THE INVENTION According to this invention, the laser beam of the amount of laser beams which can detect BD, and the amount of laser beams required for image formation can be emitted, without increasing cost.

FIG. 1 is a diagram illustrating a configuration of an image forming apparatus according to an embodiment. FIG. 2 is a diagram illustrating a configuration of an optical scanning device according to an embodiment. FIG. 2 is a diagram illustrating a configuration of a semiconductor laser device according to an embodiment and APC. FIG. 2 is a diagram illustrating a light amount adjustment circuit according to the embodiment. FIG. 4 is a diagram illustrating correction of the amount of laser light by the shading circuit according to the embodiment. 7 is a graph showing the density of the toner pattern for density detection and the duty of the PWM signal according to the embodiment, and a diagram showing the relationship between the duty of the PWM signal and the integrated light amount; FIG. 4 is a diagram for explaining a problem of a laser light amount of a conventional example for comparison of the embodiments. FIG. 7 is a diagram illustrating the laser light amount in the non-image area and the image area according to the embodiment. 4 is a flowchart illustrating a control sequence for calculating a laser light amount having a target density according to the embodiment. FIG. 6 is a diagram illustrating an example of a table for calculating a laser light amount during image formation and a toner check pattern image according to the embodiment. 5 is a flowchart illustrating a shading control sequence according to the embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[Overall Configuration of Image Forming Apparatus]
FIG. 1 is a cross-sectional view illustrating an overall configuration of an image forming apparatus 100 according to the present embodiment, and illustrates a schematic configuration of an electrophotographic full-color printer. In the image forming apparatus 100 shown in FIG. 1, photosensitive drums 101a, 101b, 101c, and 101d, which are photosensitive members corresponding to each color, are charged by chargers 102a, 102b, 102c, and 102d. Here, the suffixes a, b, c, and d of the symbols represent colors such as yellow (Y), magenta (M), cyan (C), and black (K). Except for the case where a member of a specific color is described, the description is omitted. An electrostatic latent image (latent image) is formed on the charged photosensitive drum 101 by laser light (light beam) emitted from one optical scanning device 200 using a light emitting element as a light source. The amount of laser light is appropriately adjusted depending on the environment where the image forming apparatus 100 is installed, the age of the photosensitive drum 101, and the like, and a latent image having an appropriate potential is formed on the photosensitive drum 101. . The electrostatic latent image formed on the photosensitive drum 101 is developed using toner by a developing device 103 as each developing unit. The toner image of each color developed on each photosensitive drum 101 is transferred by being superimposed and transferred to an intermediate transfer belt 105 as a carrier by a transfer voltage applied to a transfer roller 104 as a transfer unit, and a color toner image is formed. Is formed on the intermediate transfer belt 105 (on the belt). The four colors of the toner image transferred onto the intermediate transfer belt 105 are collectively transferred onto a sheet S (recording material) as a recording material by a secondary transfer roller 106. After that, the sheet S carrying the unfixed toner image passes through a fixing device 107 serving as a fixing unit, and is subjected to a fixing process.

  The paper S is fed from the paper feed cassette 109 or the manual feed tray 110 or the like, and the conveyance timing is adjusted by a registration roller (hereinafter, referred to as a registration roller) 111. The sheet S whose timing has been adjusted by the registration roller 111 is conveyed to the nip portion between the secondary transfer roller 106 and the secondary transfer inner roller 21. During double-sided printing, the sheet S that has passed through the fixing device 107 is guided in the direction of the double-sided inversion path 112, the transport direction is reversed, transported in the opposite direction, and transported to the double-sided path 113. The sheet S conveyed to the double-sided path 113 is conveyed again to the registration roller 111 by the vertical path roller 114, where the image on the second side is formed, transferred, fixed, and discharged similarly to the first side.

  The environment sensor 141 is a sensor for measuring the temperature and humidity inside the image forming apparatus 100, which is the internal environment. The density sensor 142 detects the density of a toner check pattern formed on the intermediate transfer belt 105 described later.

[Image reading device]
An image reading device 115 is provided above the image forming apparatus 100. The image reading device 115 includes a lamp 1130 that irradiates light to the document surface, and mirrors 134, 135, and 136 that guide reflected light from the document 131 to the lens 132 and the CCD line sensor 133. The lamp 1130 and the mirror 134 are attached to the first mirror base 137. The mirrors 135 and 136 are attached to the second mirror base 138. The first mirror base 137 and the second mirror base 138 are connected to a drive motor (not shown) by a wire (not shown), and move in parallel with the original table glass 139 by the rotation drive of the drive motor. In addition, a reference white plate 140 serving as a reference of reading luminance is attached to an end of the document table glass 139. Light reflected from the document 131 is guided to a lens 132 via mirrors 134, 135, and 136, and is imaged on a light receiving portion of the CCD line sensor 133 by the lens 132. The CCD line sensor 133 photoelectrically converts the formed reflected light with a light receiving element, and outputs an electric signal corresponding to the amount of incident light. When the image reading device 115 reads an image on the sheet S, which is a transfer body on which the toner pattern for density detection is formed by the image forming apparatus 100, the toner pattern for density detection is formed on the platen glass 139. It is set so that the side of the platen facing the original platen glass 139 side. The density detection toner pattern (density detection toner image) is composed of a plurality of toner images having different densities.

[Configuration of Optical Scanning Device]
FIG. 2 is a diagram schematically illustrating a configuration of an optical scanning device 200 used in the image forming apparatus 100 according to the present embodiment. FIG. 2A is a diagram of the optical scanning device 200 as the exposure unit as viewed from above the rotary polygon mirror 205 as the deflecting unit. FIG. 2B is a diagram of the optical scanning device 200 as viewed from the side. FIG. 2C is a diagram illustrating a configuration of a laser diode (hereinafter, referred to as an LD) 201 that is a light emitting element. The optical scanning device 200 has four LDs 201. For example, the optical scanning device 200 includes an LD 201a for yellow, an LD 201b for magenta, an LD 201c for cyan, and an LD 201d for black. The optical scanning device 200 includes an LD 201, a collimator lens 202, an aperture stop 203, and a cylindrical lens 204. The optical scanning device 200 includes a rotating polygon mirror 205, a scanner motor 206, toric lenses 207e and 207f, diffractive optical elements 208e and 208f, and folding mirrors 209e, 209f, 130e, 130f, 131e, and 131f. The collimator lens 202 converts the light beam emitted from the LD 201 into a parallel light beam. The aperture stop 203 limits the light flux of the light beam passing therethrough. The cylindrical lens 204 has a predetermined refractive power (degree of refraction) only in the sub-scanning direction, and forms a light beam on the reflecting surface of the rotary polygon mirror 205 as an elliptical image long in the main scanning direction. In the optical path from the LD 201 to the rotary polygon mirror 205, YK is parallel and MC is parallel. Further, the YM and CK laser beams are incident on the surface of the rotary polygon mirror 205 so as to be symmetrical.

  The rotary polygon mirror 205 is rotated at a constant speed in a direction indicated by an arrow in the figure by a scanner motor 206, and deflects and scans the laser light imaged on the reflection surface. Here, in the rotating polygon mirror 205, the light beam for YM is scanned to the left and the light beam for CK is scanned to the right in the figure. At this time, the position where the light beam is reflected at 90 ° when viewed from the LD 201 is the center of the photosensitive drum 101, and the center of the image (FIG. 2A). That is, when the light beam is irradiated on the center of the photosensitive drum 101, the following is performed. The light beam 220 emitted from the LD 201 enters the rotating polygon mirror 205 at an angle of 45 °, and is reflected by the rotating polygon mirror 205 in a direction of 90 ° with respect to the optical path of the light beam 220 at the time of incidence.

  The light beams 221a and 221b reflected by the rotating polygon mirror 205 pass through the toric lens 207e and the diffractive optical element 208e. Thereafter, the light beam 221a is reflected by the turning mirror 209e, and is finally irradiated on the photosensitive drum 101a in the main scanning direction. On the other hand, the light beam 221b is reflected by the return mirrors 130e and 131e, and is finally irradiated on the photosensitive drum 101b in the main scanning direction. The light beam 221a and the light beam 221b are applied to the same position in the main scanning direction on the photosensitive drum 101a and the photosensitive drum 101b. The light beams 221c and 221d reflected by the rotating polygon mirror 205 pass through the toric lens 207f and the diffractive optical element 208f. Thereafter, the light beam 221d is reflected by the turning mirror 209f, and is finally irradiated on the photosensitive drum 101d in the main scanning direction. On the other hand, the light beam 221c is reflected by the return mirrors 130f and 131f, and is finally irradiated on the photosensitive drum 101c in the main scanning direction. The light beam 221c and the light beam 221d are applied to the same position in the main scanning direction on the photosensitive drum 101c and the photosensitive drum 101d.

  The toric lens 207 is an optical element having fθ characteristics, and is a refraction part having different refractive indexes in the main scanning direction and the sub-scanning direction. Both lens surfaces on the front and back sides of the toric lens 207 in the main scanning direction have an aspherical shape. The diffractive optical element 208 is an optical element having fθ characteristics, and is a long diffractive portion having different magnifications in the main scanning direction and the sub-scanning direction. The black light beam 222d scanned by the rotating polygon mirror 205 is incident on a BD (Beam Detector) 214, which is a detecting means, and is used for detecting the timing of the scanning light. When the light beam 222d is incident on the BD 214, the BD 214 generates a scanning timing signal (hereinafter, referred to as a BD signal) serving as a reference for scanning timing of the photoconductor with the laser beam. Image formation on the CK side is performed based on the timing at which the light beam 222d is detected by the BD 214, and drawing of an image is started after a certain time from the BD signal. In the image formation on the YM side, drawing of the image is started based on the BD signal generated by the BD 214. When writing the image on the YM side, if the BD 215 is provided, the CK is determined based on the timing when the virtual BD signal is generated from the BD 215 on which the light beam 222a is incident. An image is drawn by scanning in the direction opposite to the side. The surface of the rotary polygon mirror 205 when a BD signal is generated by the BD 214 is different from the surface of the rotary polygon mirror 205 that scans the YM light beam at that moment. For this reason, the position of the image of each color is adjusted by adding or subtracting a predetermined time from the actual BD signal generated by the BD 214 to the writing start time of the virtual BD signal generated by the virtual BD 215.

  FIG. 2B is a diagram viewed from the front of the main body. On the photosensitive drum 101, the spot of the light beam emitted from the LD 201 moves linearly in parallel with the rotation axis of the photosensitive drum 101 by scanning in the main scanning direction by the rotary polygon mirror 205. By rotating the photosensitive drum 101, the main scanning direction is turned back, and an image is written in the sub-scanning direction. After the surface of the photosensitive drum 101 is charged by the charger 102, the charged surface of the photosensitive drum 101 is scanned by a light beam. The potential of the surface of the photosensitive drum 101 changes depending on the intensity of the irradiated light beam.

[Configuration of light emitting element]
FIG. 3A is a configuration diagram of the semiconductor laser element 1101, as viewed from the LD 201 side in FIG. 3B. The LD 201 includes a laser chip having four light sources, LD 201a, 201b, 201c, and 201d. The LD 201 also emits light from the side opposite to the side from which the light beam emitted to the photosensitive drum 101 is emitted, and the light beam enters a photodiode (hereinafter referred to as PD) 403.

[Automatic light intensity adjustment (APC)]
The optical scanning device 200 irradiates a predetermined amount of light beam onto the surface of the photosensitive drum 101 to perform image formation before image formation and during image formation during execution of an image formation job (for details, refer to APC between lines. (To be described later), the light amount is adjusted. The light amount of the laser beam at the time of light amount adjustment is adjusted to the maximum light amount required at the time of image formation. First, a method for adjusting the light amount of the light beam will be described. The LD 201 has a structure as shown in FIGS. The light beam is called front light used for forming an image. In FIG. 3A, the light beam emitted to the right and the rear light whose emission direction is opposite to that of the front light are referred to. In FIG. 3A, there is a light beam emitted on the left side. The rear light is emitted with a fixed ratio of the amount of the front light. The PD 403 receives the rear light emitted from the LD 201. By monitoring the intensity of the light received by the PD 403, the amount of current supplied to the LD 201 is feedback-controlled. That is, the current flowing through the LD 201 is adjusted so that the output of the PD 403, which is the light amount detection means attached to the LD 201, becomes a predetermined value. Such control is referred to as automatic light intensity adjustment (hereinafter, referred to as APC: Auto Power Control). An operation mode for performing APC is called an APC mode. When an image is being drawn (hereinafter, referred to as an image mode), the light beam repeatedly blinks, so that the APC operation cannot be performed. Therefore, the APC operation is performed at a predetermined timing in the non-image area where the image is not drawn, that is, in the non-image area. Therefore, the laser control unit 304 described later turns on the switch 307 in the non-image area (see FIG. 4).

[Control circuit]
A method for switching between the APC mode and the image mode will be described with reference to a block diagram of a control circuit in FIG. The CPU 303 as a control unit sets the laser control unit 304 to output a laser control signal 318 for controlling the APC control circuit 301 as a driving unit. The laser control unit 304 is, for example, an ASIC (Application Specific Integrated Circuit). When a set value is set in the register 3041 from the CPU 303, the laser control unit 304 generates a laser control signal 318 based on the set value. Note that the CPU 303 and the laser control unit 304 may be configured as one IC.

  The laser control signal 318 is composed of a parallel signal of several bits called a CTL signal, and the control mode can be changed according to the pattern of the parallel signal. The control mode is changed for each laser scan. The information of the table shown in FIG. 3C is stored in the register 3041 of the laser control unit 304. The laser control unit 304 outputs a laser control signal 318 (CTL1, CTL0) shown in FIG. 3C based on the table shown in FIG.

  Here, the table shown in FIG. 3C will be described. The disable mode is a mode in which the power of the image forming apparatus 100 is turned off (OFF) or the image forming apparatus 100 transitions to an image forming job standby state. In the disable mode, the status of the laser control signal 318 is such that CTL1 is low level (L) and CTL0 is low level (L). In response to the laser control signals 318 of CTL1: L and CTL0: L being input to the APC control circuit 301, the APC control circuit 301 discharges a hold capacitor (hereinafter, referred to as a capacitor) 310 and outputs a voltage Vch of the capacitor 310. To 0V. Since the voltage Vch of the capacitor 310 is 0 V, the potential at both ends of the current limiting resistor 311 becomes 0 V. Therefore, in the current mirror circuit 309, the driving current 312 does not flow, and the driving current 327 does not flow.

  In the APC mode, the status of the laser control signal 318 is such that CTL1 is high level (H) and CTL0 is low level (L). In a state where the laser control signals 318 of CTL1: H and CTL0: L are input to the APC control circuit 301, the APC control circuit 301 turns on the transistor 328 regardless of the output of the LVDS buffer 326.

  In the image mode, the status of the laser control signal 318 is such that CTL1 is low level (L) and CTL0 is high level (H). In a state where the laser control signals 318 of CTL1: L and CTL0: H are input to the APC control circuit 301, the APC control circuit 301 turns on or off the transistor 328 according to the output of the LVDS buffer 326.

  In the OFF mode, the status of the laser control signal 318 is such that CTL1 is high level (H) and CTL0 is high level (H). In a state where the laser control signals 318 of CTL1: H and CTL0: H are input to the APC control circuit 301, the APC control circuit 301 turns off the transistor 328 regardless of the output of the LVDS buffer 326. Hereinafter, the mode change to the APC mode performed in each scanning cycle of the laser light is referred to as inter-line APC. The inter-line APC will be described with reference to FIG.

[APC between lines]
FIGS. 3C and 3I show BD signals generated by the BD 214. The BD signal generated by the BD 214 is at a high level when the BD 214 is not receiving a light beam, and is low when the BD 214 is receiving a light beam. Level. The time from the fall timing of the BD signal to the fall timing of the next BD signal is called a BD cycle, and is a time required for one scan of the light beam. The BD cycle includes an area where the light beam corresponding to the image data is irradiated on the photosensitive drum 101 (on the photoconductor), that is, an image area corresponding to an area where a toner image is formed. Note that a section from the falling timing of the BD signal to the start of the image area is defined as an image writing section. FIG. 3C and (ii) show operation modes (APC mode, image mode, OFF mode, etc.). 3 (c) and (iii) show the signal CTL1 of the second bit when the laser control signal 318 is constituted by, for example, a 2-bit parallel signal. FIGS. 3C and iv show the signal CTL0 of the first bit when the laser control signal 318 is constituted by, for example, a 2-bit parallel signal. The horizontal axis indicates time.

  When the operation mode is the APC mode, the laser control signal 318 has a pattern in which CTL0 has a low level (L) and CTL1 has a high level (H). When the operation mode is the APC mode (CTL0: L, CTL1: H), the BD 214 is irradiated with laser light to generate a BD signal (FIG. 3 (c) (i)). When a certain period of time elapses from the timing when the BD signal is generated from the BD 214, the operation mode becomes the OFF mode. When the operation mode is the OFF mode, the laser control signal 318 has a pattern in which CTL0 is at a high level and CTL1 is at a high level. Then, when the time to reach the image area elapses from the timing at which the BD signal is generated, the laser control signal 318 changes the CTL0 to the high level and the CTL1 to the low level, and switches the operation mode to the image mode. When the image area ends, the operation mode becomes the OFF mode (CTL0: H, CTL1: H), and before the BD cycle elapses, the operation mode becomes the APC mode (CTL0: L, CTL1: H) again.

(APC mode)
When the laser control signal 318 is CTL0: L and CTL1: H, the operation mode is the APC mode. At this time, when the laser light enters the PD 403, a current PD flows according to the amount of the incident light. The reference voltage Vref determined by the reference voltage generator 302 is input to the + terminal of the comparator 306. The voltage RM when the current PD flows through the predetermined semi-fixed resistor 305 is input to the minus terminal of the comparator 306. The comparator 306 compares the reference voltage Vref with the voltage RM, and controls the current supplied to the LD 201 such that the voltage RM becomes equal to the reference voltage Vref generated by the reference voltage generator 302. The capacitor 310 is charged so as to have a voltage Vch that determines a current when APC control is performed so that the light amount of the LD 201 becomes the target light amount. The voltage Vch is input to the + terminal of the operational amplifier 308. Further, a voltage Rs obtained by converting a current flowing through the current limiting resistor 311 is input to a negative terminal of the operational amplifier 308. The operational amplifier 308 and the current limiting resistor 311 convert the voltage Vch into a drive current 312 according to the voltage Vch. The current mirror circuit 309 generates a current equivalent to the drive current 312 from the drive current 312 and supplies the LD 201 as a drive current (Ild) 327 when the transistor 328 is on.

(OFF mode)
When the laser control signals 318 are CTL0: H and CTL1: H, the operation mode is the OFF mode. At this time, the LD 201 is turned off (OFF). In the OFF mode, the capacitor 310 maintains a charged state.

(Image mode)
When the laser control signals 318 are CTL0: H and CTL1: L, the operation mode is the image mode. At this time, the current is supplied to the LD 201 according to the image signal while the voltage Vch is maintained by the charging voltage Ch of the capacitor 310. In the image mode, image data 324 is output from the CPU 303 to the conversion unit 325. Here, the image data 324 is, for example, 4-bit bitmap data input to the CPU 303 from a controller (not shown) or the like, and includes gradation data. The conversion unit 325 converts the input image data 324 into a PWM signal having a pulse width (ON width: time during which the LD 201 emits light) according to the characteristics of the LD 201. For example, the conversion unit 325 converts the image data 324 into a PWM signal 323 corresponding to the gradation of the pixel, and outputs the PWM signal 323 to the APC control circuit 301.

  In the image mode, the APC control circuit 301 receives the PWM signal 323 corresponding to the density of the pixel, and turns on / off the laser drive current (Ild327) according to the PWM signal 323. Thereby, the LD 201 emits light or goes out. The CPU 303 outputs the image data 324 subjected to the screen processing and the γ correction to the conversion unit 325. The conversion unit 325 generates a PWM signal 323 corresponding to the image data 324 from a predetermined table (ILUT) in which the image data 324 and the PWM pattern are associated with each other, which will be described later. The PWM signal 323 is a signal that defines the time (on width) for causing the LD 201 to emit light in one pixel, and is input to the APC control circuit 301 described later as differential signals 323a and 323b that are in an inverse relationship to each other. You. The PWM signal 323 is generally transmitted by a low-voltage differential signaling (LVDS) and output to the APC control circuit 301. The APC control circuit 301 has an LVDS buffer 326. The LVDS buffer 326 returns the signal to a single-ended signal, and the LD 201 is turned on (light-emitting) / off (light-out) (switching) with a drive current having an adjusted light amount. Is performed.

(Disable mode)
The state in which the laser control signal 318 is CTL0: L and CTL1: L is a state in which the light amount value of the LD 201 during APC control is returned to 0, and such an operation mode is called a disable mode. In the disable mode, the charge of the capacitor 310 is discharged to set the voltage Vch to 0, and the LD 201 is turned off. When the printing on the paper S is completed and the output of the LD 201 is stopped, the disable mode is generally set.

  APC control needs to be performed outside the image area. Therefore, the CPU 303 detects the scanning timing based on the BD signal generated by the BD 214 via the laser control unit 304, and controls the laser control unit 304. The laser control unit 304 outputs the above-described pattern of the laser control signal 318 to the APC control circuit 301 at a predetermined timing according to the BD signal input by the BD 214.

[Drive current of LD]
The drive current 312 in FIG. 4 will be described. The drive current 312 includes a voltage Vch for controlling the voltage of the capacitor 310, a shading voltage (output voltage) Vshd output from the smoothing circuit 352, a resistance value Rs of the current limiting resistor 311, and a resistance value Rt (Rtｔ) of the resistor 317. Rs). Here, the voltage charged in the capacitor 310 during the APC control and held is defined as Vapc. The smoothing circuit 352 is a low-pass filter included in the shading circuit 350. The smoothing circuit 352 has an RC circuit including a resistor 3521 and a capacitor 3522. The drive current 312 is represented by the following equation.
Drive current 312 = Vapc / (Rs + Rt) -Vshd / Rt
Here, the current Ishd flowing from the shading circuit 350 via the resistor 317 is a current Ishd = Vshd / Rt.

  APC is performed in a non-image area during one scanning cycle. The switch 307 is off in an image area during one scanning cycle. Therefore, the capacitor 310 outputs the voltage Vch sampled in the APC mode. Therefore, the voltage Vch is constant in the image area during one scanning cycle except for the effect of spontaneous discharge, and the current value of the drive current 312 is constant at Vapc / (Rs + Rt).

  On the other hand, a shading circuit 350 described later controls the shading voltage Vshd according to the exposure position (also referred to as a scanning position) of the light beam in the main scanning direction. Therefore, in the image area in one scanning cycle, the current Ishd changes according to the exposure position of the light beam in the main scanning direction.

  In the image area, the current value (Vapc / (Rs + Rt)) is constant, and the current value (= Vshd / Rt) of the current Ishd changes according to the exposure position of the light beam in the main scanning direction. Therefore, by controlling the shading voltage Vshd according to the light beam exposure position in the main scanning direction, the drive current 312 can be controlled to a current value corresponding to the light beam exposure position in the main scanning direction.

[Shading circuit]
Here, the operation of the shading circuit 350 as the light amount correction means will be described. The laser control unit 304 reads a correction value of the light amount corresponding to each exposure position, that is, the position of the photosensitive drum 101 in the main scanning direction, from the ROM 3031 via the CPU 303. The light amount correction value corresponding to each exposure position is light amount correction data, and is hereinafter referred to as shading data. The laser control unit 304 outputs a PWM signal (SHDPWM signal) including a pulse having a pulse width (duty ratio) based on the shading data. Here, the laser control unit 304 switches the shading data used to generate the SHDPWM signal for each block during scanning of the light beam. Then, the laser control unit 304 outputs an SPWM PWM signal having a pulse width corresponding to the shading block.

  The laser control unit 304 includes a reference clock signal generation unit (hereinafter, referred to as a clock) that generates a reference clock signal having a constant frequency, and a counter that counts the reference clock signal. The reference clock signal generation unit generates a clock signal having a higher frequency than the BD signal (periodic signal). The laser control unit 304 counts the reference clock signal by an internal counter based on the BD signal, and switches the shading data at a count value corresponding to the boundary of the shading block.

  The voltage switch 354 is turned on / off according to the SHDPWM signal output from the laser control unit 304. As shown in FIG. 4, a bias application circuit 313 is provided between the voltage switch 354 and the smoothing circuit 352. The bias application circuit 313 applies a constant bias voltage Vbias to the output (Vref2) of the voltage switch 354. When the voltage switch 354 is on, the input voltage to the smoothing circuit 352 becomes the voltage (Vref2 + Vbias). The value of the bias voltage Vbias is a value much smaller than Vref2, and is a very small voltage having a value of 0V or more and very close to 0V. When the voltage switch 354 is off, the input voltage to the smoothing circuit 352 becomes the voltage Vbias. Therefore, when the voltage switch 354 is turned on / off by the SHDPWM signal, the input voltage to the smoothing circuit 352 oscillates between the voltage (Vref2 + Vbias) and the voltage Vbias. The smoothing circuit 352 smoothes an input and outputs a shading voltage Vshd. The laser control unit 304 controls the shading voltage Vshd output from the smoothing circuit 352 by setting the duty ratio of the SHPWM signal for each shading block. The shading voltage Vshd is a voltage based on the shading reference voltage Vref2, the bias voltage Vbias, and the duty ratio of the SHDPWM signal. As a result, the current value of the drive current 312 is adjusted according to each position in the main scanning direction, and the shading correction is performed.

(Shading correction control)
Next, the operation of the shading correction control will be described in detail. FIG. 5 is a timing chart for explaining the effect when the shading circuit 350 corrects the amount of laser light. (I) shows an area on the photosensitive drum 101 such as a non-image area (APC area) and an image area. Note that the image area is divided into a plurality of blocks when performing shading processing in the main scanning direction. For example, in the present embodiment, the image area is divided into six blocks. Hereinafter, these blocks are referred to as Block1, Block2, and the like. (Ii) shows the SHD PWM signal output from the laser control unit 304. (Iii) shows a current Ishd corresponding to the shading voltage Vshd. (Iv) shows the drive current 312. The horizontal axis is time. FIG. 5 shows a shading operation sequence during one scan. In this sequence, the image area is divided into a plurality of blocks, and the duty ratio of the SPWM PWM signal is set based on the shading data in each block.

  As described above, the drive current 312 is controlled by the shading voltage Vshd. For example, the shading voltage Vshd output from the smoothing circuit 352 increases as the pulse width of the SDPWM signal increases. Therefore, the current Ishd corresponding to the shading voltage Vshd also increases, and as a result, the drive current 312 decreases and the light amount of the light beam decreases. For example, in Block 1 shown in FIG. 5, the duty ratio of the SDPWM signal output by the laser control unit 304 is 0%. The light amount at this time is set to 100%. Since Block 2 controls the light amount to 95% of the light amount in Block 1, the duty ratio of the SHDPWM signal output from the laser control unit 304 is 5%. When the laser control unit 304 outputs a PWM signal having a duty ratio of 5%, the drive current 312 during the period of scanning Block 2 is controlled, and the light amount of the light beam is controlled to 95%. Similarly, in Blocks 3 to 6, the laser controller 304 can output an SHDPWM signal having a duty ratio corresponding to each block, so that the light amount of the light beam can be controlled to a light amount corresponding to each block. Although the SHDPWM signal in each block is shown by one pulse in FIG. 5, the laser control unit 304 generates a plurality of pulse signals in each block, and the smoothing circuit 352 smoothes the plurality of pulse signals in each block. To process.

  The smoothing circuit 352 outputs the shading voltage Vshd by smoothing the input signal, and smoothly changes the light amount between the shading blocks in the above-described sequence. The smoothing circuit 352 is a filter circuit including a capacitor and a choke coil or a resistor, and having an active filter using an operational amplifier. The cutoff frequency of the active filter is set so as to cut the frequency of the SHD PWM signal and pass the period of the shading block. At the switching timing of the pulse width of the SHDPWM signal (the switching timing of the shading block), the operation of the smoothing circuit 352 causes the shading voltage Vshd to change in a curve without a step. That is, by using the smoothing circuit 352, it is possible to prevent the amount of light from extremely changing at the switching timing of the pulse width of the SPWM PWM signal, thereby preventing the occurrence of streaks and unevenness on an image.

[Light quantity control by light quantity PWM signal]
A method of controlling the light amount of the LD 201 using the light amount PWM signal 320 will be described with reference to FIG. The laser control unit 304 outputs the light quantity PWM signal 320 to a field effect transistor (hereinafter, referred to as FET) 321 included in the reference voltage generation unit 302. The light quantity PWM signal 320 is a pulse signal and is a signal for determining the reference voltage Vref. By changing the duty of the light quantity PWM signal 320, the reference voltage Vref can be changed. The reference voltage generator 302 converts the internally generated voltage VR319 into an on / off signal by turning on / off the FET 321 according to the input light quantity PWM signal 320. Voltage VR319 is smoothed by a filter formed by resistor 1323 and capacitor 314 to generate reference voltage Vref. The reference voltage Vref is compared with the current from the PD 403 by the comparator 306 included in the APC control circuit 301 in the APC control. Thereby, light quantity control can be performed. Note that a resistor 1324 is provided to make the voltage VR319 low when the FET 321 is turned on. The switch 307 receives a sample hold signal and is used to switch the operation mode of the APC control circuit 301.

  The light amount PWM signal 320 is a signal for making the light amount of the LD 201 variable. The light amount of the LD 201 is adjusted by adjusting the resistance value of the semi-fixed resistor 305 while performing APC control in a state where the light amount PWM signal 320 is set to 100% in advance at the factory. Then, at the time of printing, a light amount suitable for an appropriate density is set by controlling the pulse width of the light amount PWM signal 320.

[Dispersion of target light intensity due to environment]
In the image forming apparatus, when the image forming speed changes or the surrounding environment changes, or when the power of the image forming apparatus is turned on or upon a request from the user, the amount of laser light to obtain an appropriate density is adjusted. By setting, the density of the image is set. At that time, a toner pattern for density detection is formed on the photosensitive drum 101 and is transferred onto the intermediate transfer belt 105. Then, the density of the toner pattern transferred onto the intermediate transfer belt 105 is detected by the density sensor 142, and the toner pattern printed on the sheet S is read by the image reading device 115 to obtain density information. Is performed. In forming the density detection toner pattern, the duty of the light amount PWM signal 320 output from the CPU 303 to the reference voltage generation unit 302 in the circuit of FIG. 4 is set to 100%. Further, the duty of the SHDPWM signal is set to 0% (fixed duty).

  For example, FIG. 6A shows the result of measuring the density of the toner pattern for density detection by reading the toner pattern for density detection printed on the paper S by the image reading device 115. is there. In FIG. 6A, the horizontal axis represents the duty of the PWM signal 323, the vertical axis represents the density of the toner pattern for density detection, and the black circle represents the measured density. In FIG. 6A, the duty of the PWM signal 323 at the time of forming the toner pattern is 100%, 90%, 80%, 70%, 60%, and 50%. For example, when the duty of the PWM signal 323 is 100%, the LD 201 is controlled to emit light throughout one pixel. Further, for example, when the duty of the PWM signal 323 is 50%, the LD 201 is controlled so that half of one pixel emits light and half of the pixel 201 is turned off. Here, the duty of the PWM signal 323 is an arbitrary duty different from the duty at the time of image formation.

  Also, in FIG. 6A, there are two plotted graphs, and the toner patterns formed in environments A (lower graph in the figure) and B (upper graph in the figure) where the temperature and the humidity are different. 3 shows the measurement results. Here, for example, the target density for the darkest toner pattern (hereinafter, referred to as target density) is set to 1.5. At this time, the duty of the PWM signal 323 for realizing the target density 1.5 in the environment A is between 70% and 80% from the measurement result of the lower graph in the figure. . Therefore, when the duty of the PWM signal 323 when the target density becomes 1.5 is obtained by performing linear interpolation, the duty of the PWM signal 323 corresponding to the target density 1.5 is calculated as 76%. On the other hand, the duty of the PWM signal 323 for realizing the target density 1.5 in the environment B is between 60% and 70% from the measurement result of the upper graph in the figure, and the target density 1. The duty of the PWM signal 323 corresponding to 5 is calculated as 67%. The environment B has a higher temperature and a higher humidity than the environment A. It can be seen that the duty of the PWM signal 323 decreases as the temperature and the humidity increase with respect to the same target density. Further, since the sensitivity is high when the photosensitive drum 101 is new, the duty of the PWM signal 323 for the same target density is lower than that of the photosensitive drum 101 having a long usage time, as in the case of the high-temperature and high-humidity environment B. Become smaller.

  FIG. 6B is a graph illustrating a relationship between the duty of the PWM signal 323 and the light amount of the LD 201. In FIG. 6B, the horizontal axis indicates the duty (%) of the PWM signal 323, and the vertical axis indicates the output light amount (%) of the LD 201 when the allowable output light amount of the LD 201 is 100%. The allowable light amount is a value determined in design from the specifications of the LD 201 and the specifications of the image forming apparatus (for example, the sensitivity of the photosensitive drum 101 and the maximum output density). Note that the ILUT (relation data) indicating the relationship between the duty of the PWM signal 323 and the light amount of the LD 201 shown in FIG. 6B is predetermined using the measurement result at the time of shipment from the factory, and is stored in the ROM 3031. Shall be By referring to the ILUT, the CPU 303 determines that the target light amount is 71% when the duty of the PWM signal 323 is 76%. The CPU 303 stores the target light amount (for example, 71%) in the RAM 3032 of the CPU 303. The CPU 303 functions as setting means for setting the amount of laser light. The CPU 303 performs printing with the target light amount (71%) stored in the RAM 3032 during printing. Here, the calculated target light amount of 71% becomes the duty of the light amount PWM signal 320 during image formation. As described above, the CPU 303 also functions as a correction unit that corrects the density of an image.

  Also, here, with respect to the PWM signal 323, the light amount of the LD 201 does not increase at a low duty, and starts increasing at a point where the duty becomes a certain level. Then, regarding the PWM signal 323, it is assumed that the light amount sharply increases to 100% at a high duty, and the light amount of the laser diode 201 reaches 100% before the duty of the PWM signal 323 reaches 100%. Such a characteristic of the light quantity of the LD 201 with respect to the duty of the PWM signal differs for each LD 201.

  In FIG. 6B, the light quantity of the LD 201 when the duty of the PWM signal 323 is 76% to correspond to the target density 1.5 in the environment A is the light quantity when the allowable output light quantity of the LD 201 is 100%. It can be seen that this corresponds to 71%. Similarly, the light amount of the LD 201 when the duty of the PWM signal 323 is 67% to correspond to the target density 1.5 in the environment B corresponds to 62% when the allowable output light amount of the LD 201 is 100%. You can see that Thus, in the environment A, the target light amount of the APC is set to 71% of the maximum light amount of the LD 201, and in the environment B, the target light amount of the APC is set to 62% of the maximum light amount of the LD 201.

  Subsequently, the upper graph shown in FIG. 6C is a graph showing the measurement result of the toner pattern in the environment C where the temperature and the humidity are higher than the environment B. In addition, in the figure, the lower graph is a graph showing the measurement result in the environment A of FIG. In FIG. 6C, the duty of the PWM signal 323 for realizing the target density 1.5 in the environment C is between 50% and 60% from the measurement result of the graph in FIG. The duty of the PWM signal 323 corresponding to the density 1.5 is calculated as 55%. FIG. 6D is a graph showing the relationship between the duty of the PWM signal 323 and the light amount of the LD 201 as in FIG. 6B described above. Here, the duty of the PWM signal 323 in the environment A and the environment C is shown. The relationship between the light amounts of the LD 201 is shown. From FIG. 6D, the light amount of the LD 201 when the duty of the PWM signal 323 is 55% to correspond to the target density 1.5 in the environment C is the light amount when the allowable output light amount of the LD 201 is 100%. It turns out that it corresponds to 48%.

  Here, the larger the amplitude when the BD receives the laser beam from the LD 201, the higher the accuracy of the writing position. In order to increase the amplitude, it is better that the amount of light incident on the BD is large. Therefore, conventionally, when the laser beam is incident on the BD, the driving current 327 having a value corresponding to the charging voltage ch of the capacitor 310 is supplied to the laser light source without operating the shading circuit 350. However, as described above, when image formation is performed immediately after a new photosensitive drum 101 having good sensitivity is mounted and in a high-temperature and high-humidity environment such as the environment C described above, the target light amount is low. Therefore, the drive current 327 flowing through the LD 201 is set low. Then, the light amount of the laser beam emitted from the LD 201 by the driving current 327 does not reach the lower limit of the light amount range in which the BD can be detected, and the BD signal is not generated.

[Issues in Conventional APC Control]
FIG. 7 is a diagram illustrating a target light amount in a non-image area (APC area) and a laser light amount in an image area in the above-described environments A, B, and C. FIG. 7A is a diagram illustrating a target light amount in a non-image area (APC area) and a laser light amount in an image area in the environment A and the environment B. The vertical axis indicates the laser light amount, and the horizontal axis indicates the laser light amount. Indicates time. FIG. 7B shows the target light quantity in the non-image area (APC area) and the laser light quantity in the image area in the environment A and the environment C, the vertical axis shows the laser light quantity, and the horizontal axis shows the time. .

  In FIG. 7A, the solid light indicates the target light quantity (laser light quantity) in the non-image area (APC area) and the laser light quantity corresponding to the image data in the image area in the case of the environment A. On the other hand, the target light quantity (laser light quantity) in the non-image area (APC area) and the laser light quantity corresponding to the image data in the image area in the case of environment B are indicated by broken lines. The double arrow α indicates the light amount range of the laser light that can be detected by the BD 214, and the double arrow β indicates the light amount range of the laser light that the BD 214 cannot detect. Here, the threshold value of the laser light amount in a range where the BD 214 can detect the laser light emitted from the LD 201 is set to 50% of the maximum light amount that the LD 201 can emit. As shown in FIG. 7A, the target light amounts in the APC area in the environment A and the environment B are 76% and 67%, respectively, so that the BD 214 can detect the laser light emitted from the LD 201. For this reason, the amount of laser light emitted from the LD 201 is low, the BD 214 cannot detect the laser light, and the BD signal is not output.

  In FIG. 7B, the solid light indicates the target light quantity (laser light quantity) in the non-image area (APC area) and the laser light quantity corresponding to the image data in the image area in the case of environment A. On the other hand, the target light quantity (laser light quantity) in the non-image area (APC area) and the laser light quantity corresponding to the image data in the image area in the case of the environment C are indicated by dashed lines. The double arrow α indicates the light amount range of the laser light that can be detected by the BD 214, and the double arrow β indicates the light amount range of the laser light that the BD 214 cannot detect. Note that the threshold value of the laser light amount in a range where the BD 214 can detect the laser light emitted from the LD 201 is set to 50% of the maximum light amount that the LD 201 can emit. As shown in FIG. 7B, the target light amount (76%) in the APC region in the environment A is such that the BD 214 can detect the laser light emitted from the LD 201. On the other hand, the target light amount (48%) in the APC region in the environment C is such that the BD 214 cannot detect the laser light emitted from the LD 201. Therefore, in the case of the environment C, the amount of laser light emitted from the LD 201 is low, the BD 214 cannot detect the laser light, and no BD signal is output.

[Setting of laser light amount during APC in this embodiment]
Next, a method of setting the laser light amount in the APC area in the present embodiment will be described with reference to FIG. FIG. 8 is a diagram illustrating a method for setting the laser light amount in the APC area according to the present embodiment. FIG. 8 is a diagram for explaining the setting of the target light amount in the APC region and the setting of the laser light amount in the image region, similarly to FIG. 7 described above. The vertical axis indicates the laser light amount, and the horizontal axis indicates the time. In addition, a on the vertical axis indicates a corrected target light amount (laser light amount) set as the light amount in the APC area in the present embodiment, and b on the vertical axis indicates the laser light amount at which the BD 214 can detect the laser light. The threshold of the light amount is shown. Further, c on the vertical axis indicates a target light amount (laser light amount) before correction. In this embodiment, when setting the target light amount in the APC area, it is determined whether the set light amount is a light amount detectable by the BD 214, and if the target light amount is not detectable (c in the figure), The light amount (a in the figure) increased to a predetermined light amount is set as the light amount to be set. Thereby, the BD 214 can detect the laser beam with the target light amount at the time of the APC, and the BD 214 outputs a BD signal. On the other hand, in the image area, the target light amount at the time of APC is increased from the light amount c to the light amount a. Therefore, in the image area, shading data corrected by data obtained by adding the shading offset amount (= light amount a−light amount c), which is the increase (difference light amount), to the shading data is set. As a result, the laser control unit 304 can form an image of a predetermined density by outputting the SHDPWM signal corresponding to the corrected shading data to the shading circuit 350.

[Calculation of target light amount]
Next, a method of calculating the target light amount according to the environment (temperature, humidity) will be described. FIG. 9 is a flowchart illustrating a control sequence for calculating a target light amount according to the environment (temperature and humidity). The processing shown in FIG. 9 is performed when the image forming speed changes or the surrounding environment changes, or when the power of the image forming apparatus is turned on or when the number of prints reaches a predetermined number. It is started when there is, and executed by the CPU 303.

  When the process illustrated in FIG. 9 is started, in step (hereinafter, referred to as S) 901, the CPU 303 acquires temperature information and humidity information inside the image forming apparatus 100 detected by the environment sensor 141. In step S <b> 902, the CPU 303 determines an environment category inside the image forming apparatus 100 based on the temperature information and the humidity information acquired in step S <b> 901. The table shown in FIG. 10A is an environment classification table in which the environment classification is divided into five stages according to the temperature and humidity inside the image forming apparatus 100. The vertical axis of the table indicates temperature [° C.], and the horizontal axis indicates the table. Indicates humidity [%]. The environmental category is divided into environmental category 1 to environmental category 5 from the lower temperature to the higher temperature and from the lower humidity to the higher temperature. The CPU 303 determines an environment category of the image forming apparatus 100 based on the temperature information and the humidity information acquired in S901 and the environment category table. For example, if the temperature detected by the environment sensor 141 in S901 is 25 ° C. and the humidity is 45%, the CPU 303 determines that the environment category is the environment category 3 from the environment category table.

  In step S <b> 903, the CPU 303 determines the charging voltage of the charger 102 that charges the photosensitive drum 101, which is one of the image forming conditions, based on the environment category determined in step S <b> 902. The table shown in FIG. 10B is a charging voltage setting table showing the correspondence between the environmental classification and the charging voltage of the charger 102 that charges the photosensitive drum 101. In the charging voltage setting table of FIG. 10B, the numbers 1 to 5 shown in the column of the environmental category correspond to the environmental category 1 to the environmental category 5, respectively. In the drawing, Y, M, C, and K correspond to the charger 102 that charges the photosensitive drums 101 of yellow, magenta, cyan, and black, respectively. Numerals 50, 100, 200, 300, and 400 in the table indicate the charging voltage (unit: volt) applied to the photosensitive drum 101 by the charger 102. The charging voltage applied to the photosensitive drum 101 by the charger 102 is a negative potential. Therefore, 50 means -50 volts, and 400 means -400 volts. The CPU 303 determines the charging voltage of the charger 102 based on the environment category determined in S902 and the charging voltage setting table. For example, if the temperature detected by the environment sensor 141 in S901 is 25 ° C. and the humidity is 45%, the environment category is environment category 3, and the charging voltage applied from the charger 102 to the photosensitive drum 101 is −200 volts. Become.

  In S904, the CPU 303 calculates the maximum laser light amount that can be irradiated from the LD 201 of the optical scanning device 200 to the photosensitive drum 101 based on the charging voltage determined in S903. The table shown in FIG. 10C is a maximum laser light amount value table showing the correspondence between the charging voltage applied from the charger 102 to the photosensitive drum 101 and the laser light amount set value (maximum light amount) that can be applied to the photosensitive drum 101. is there. 50 to 400 shown in the charging voltage column indicate charging voltages of -50 volts to -400 volts applied to the photosensitive drum 101 by the charger 102, respectively. The laser light amount setting value is the value of the light amount PWM signal 320 output from the laser control unit 304 to the reference voltage generation unit 302. The light quantity PWM signal 320 is represented by 8-bit data, and its value is represented by 0 to 255. The CPU 303 calculates the laser light maximum value based on the charging voltage of the charger 102 determined in S903 and the maximum laser light value table. For example, if the temperature detected by the environment sensor 141 in S901 is 25 ° C. and the humidity is 45%, the high voltage of the charger 102 is −200 volts, and the maximum laser light amount value in this case is 180.

  In S905, the CPU 303 forms an image of a toner check pattern as shown in FIG. 10D on the photosensitive drum 101 to which the charging voltage determined from the charger 102 has been applied. FIG. 10D is an example of a patch image for density detection in which the laser light amount is reduced stepwise with the maximum laser light amount value 180 being 100%. In the figure, (Y), (M), (C), and (K) indicate yellow, magenta, cyan, and black pattern data, respectively. The numerals 20, 74, 127, and 180 in the drawing indicate the laser light amount setting values corresponding to the toner check pattern, with the maximum laser light amount value 180 calculated in S904 being 100%. Here, in order to form a patch in which the laser light amount is changed in four steps up to the minimum value 20 of the laser light amount setting value, the setting value of the light amount PWM signal 320 of (180−20) ÷ 3 = 53 is changed by 53. Toner check pattern (density patch) is formed.

  In step S <b> 906, the CPU 303 detects the density of the toner check pattern formed on each of the photosensitive drums 101 and transferred onto the intermediate transfer belt 105 by the density sensor 142, and acquires the detection result. In step S907, the CPU 303 calculates a laser light amount corresponding to the target density by the above-described method based on the duty of the PWM signal and the density detection result of the density sensor 142. The CPU 303 stores the calculated laser light amount value in the RAM 3032, and ends the processing.

  In the processing illustrated in FIG. 9, an example in which the density of the toner check pattern transferred onto the intermediate transfer belt 105 is detected by the density sensor 142 has been described. However, the present embodiment is not limited to this. For example, a method may be used in which the toner check pattern is printed on paper S, the paper S on which the toner check pattern is printed is read by the image reading device 115, and the density of the toner check pattern is detected.

[Image formation control sequence]
FIG. 11 is a flowchart illustrating a control sequence of image formation according to the present embodiment. 11 is started at the time of image formation, and is executed by the CPU 303. When the process illustrated in FIG. 11 is started, in S1101, the CPU 303 acquires a laser light amount (target light amount) emitted from the LD 201 of the optical scanning device 200 from the RAM 3032 according to the target density at the time of image formation. The laser light amount value in this case is a laser light amount value calculated by the processing of FIG. 10 described above and stored in the RAM 3032. In step S1102, the CPU 303 determines whether the target light amount value is equal to or smaller than the laser light amount that can be detected by the BD 214 based on the laser light amount value acquired in step S1101, that is, the target light amount value and the threshold γ that is the lower limit of the laser light amount that can be detected by the BD 214. It is determined whether the value is a light amount value. If the CPU 303 determines that the target light amount value is equal to or larger than the threshold γ (more than the detectable light amount) and the BD 214 is a detectable laser light amount value, the process proceeds to S1103. On the other hand, if the CPU 303 determines that the target light amount value is less than the threshold value γ and the BD 214 is an undetectable laser light amount value, the process proceeds to S1104.

  In step S1103, the CPU 303 notifies the laser control unit 304 of a target light amount value which is a laser light amount value set in a non-image area (APC area). The laser control unit 304 sets the reference voltage corresponding to the target light amount value notified from the CPU 303 in the non-image area (APC area) to the reference voltage generation unit 302, and uses the light amount PWM signal 320 for determining the reference voltage Vref as a reference. Output to the voltage generator 302. Further, the CPU 303 reads a correction value (shading data) of the light amount corresponding to each exposure position of the photosensitive drum 101 from the ROM 3031 in order to respond to a request from the laser control unit 304 when controlling the laser light amount in the image area. Then, the CPU 303 stores the read shading data in the RAM 3032 in order to transmit the shading data in response to a request from the laser control unit 304, and advances the processing to S1106.

  In step S1104, the CPU 303 sets the laser light amount to be set in the non-image area (AP area) to a predetermined light amount value (corrected laser light amount value) larger than the threshold value γ, so that the difference between the predetermined light amount value and the target light amount value. The amount of shading offset, which is the amount of light, is calculated. Then, the CPU 303 notifies the laser control unit 304 of a predetermined light amount value (corrected laser light amount value) as a laser light amount value set in the non-image area (APC area). Note that the predetermined light amount value is a light amount value larger than the threshold value γ. The laser control unit 304 sends the light amount PWM signal 320 to the reference voltage generation unit 302 to set the reference voltage corresponding to the predetermined light amount value notified from the CPU 303 to the non-image area (AP area). Output. For example, in FIG. 7, the threshold value of the laser light amount in a range in which the BD 214 can detect the laser light emitted from the LD 201 is set to 50% of the maximum light amount that the LD 201 can emit, but the predetermined light amount value here is, for example, It is assumed that the maximum light quantity that can be emitted by the LD 201 is 53%. In step S1105, the CPU 303 reads from the ROM 3031 a light amount correction value (shading data) corresponding to each exposure position of the photosensitive drum 101. Then, the CPU 303 stores the corrected shading data in the RAM 3032 by adding the calculated shading offset amount to the read shading data.

  In step S1106, the CPU 303 executes a print operation to form an image on the sheet S, and when printing is completed, ends the processing in FIG. During the printing operation, a request to obtain shading data corresponding to the exposure position of the photosensitive drum 101 is transmitted from the laser control unit 304 to the CPU 303. The CPU 303 reads the shading data corresponding to the exposure position stored in the RAM 3032 in response to the acquisition request from the laser control unit 304, and transmits the data to the laser control unit 304.

  In the processing of FIG. 11, the CPU 303 transmits the corrected shading data in response to a request from the laser control unit 304. However, for example, the laser control unit 304 may correct the shading data. That is, the processing content of S1104 may be changed so that the CPU 303 notifies the laser control unit 304 of the calculated shading offset amount. Then, the CPU 303 reads the shading data corresponding to each exposure position of the photosensitive drum 101 from the ROM 3031 and stores the read shading data in the RAM 3032. The CPU 303 transmits the shading data stored in the RAM 3032 and not corrected by the shading offset amount in response to the shading data acquisition request from the laser control unit 304. On the other hand, the laser control unit 304 generates shading data corrected by adding the shading offset amount notified from the CPU 303 to the transmitted shading data. Then, the laser control unit 304 controls the light amount of the LD 201 based on the corrected shading data.

  As described above, when the target light amount at the time of image formation falls below the light amount range in which BD can be detected, the light amount in the non-image area (APC area) is set to a light amount larger than the target light amount in which BD can be detected. Thus, the correction is performed so that the BD can output the BD signal. On the other hand, in the image area, the amount of light to be emitted is switched to the amount of light corresponding to the target amount of light. Therefore, an offset amount, which is a difference between the amount of light in the APC region and the amount of target light, is added to the normal shading data. An image can be formed with a corresponding light amount.

  As described above, according to the present embodiment, it is possible to emit a laser beam of a light amount capable of detecting a BD and a laser beam of a light amount necessary for image formation without increasing the cost.

Reference Signs List 100 Image forming apparatus 101 Photosensitive drum 201 Laser diode 214 BD
301 APC control circuit 303 CPU
350 Shading circuit 3031 ROM

Claims (5)

A photoreceptor,
A light source for emitting a light beam;
A first light receiving element for receiving a light beam emitted from the light source;
Deflection means for deflecting the light beam so that the light beam emitted from the light source scans on the photoconductor,
Developing means for developing an electrostatic latent image formed on the photoconductor by using a toner by being scanned by the light beam deflected by the deflecting means,
Transfer means for transferring the toner image developed by the developing means to a transfer member,
Detecting means for detecting a toner image for density detection formed on the transfer body,
A second light receiving element that receives the light beam deflected by the deflecting unit to generate a timing signal serving as a reference of the scanning timing of the photoconductor by the light beam;
A control IC for supplying a drive current to the light source in response to a drive signal, the control IC supplying the drive current to the light source during a scan cycle of the light beam so that the light beam for scanning the second light receiving element has a target light amount A control IC for setting a value of the drive current based on a light amount of the light beam received by the first light receiving element;
Storage means for storing compensation data for correcting the amount of light corresponding to the scanning position of the light beam on the photosensitive member,
The detection means provided outside the control IC, and the value of the drive current supplied to the light source is adjusted based on a value set by the control IC in accordance with a scanning position on the photoconductor. An adjustment circuit that adjusts a voltage applied to the control IC based on the density detection result of the density detection toner image and the correction data, wherein the density detection result of the density detection toner image is An adjustment circuit including pulse signal generation means for generating a first pulse signal having a pulse width corresponding to a scanning position on the photoconductor based on the correction data;
With
The adjustment circuit is configured to output a reference voltage for current adjustment for adjusting a value of a drive current supplied to the light source, and to turn on / off in response to the first pulse signal. A generation unit configured to generate a second pulse signal based on the reference voltage; and a smoothing circuit configured to generate the voltage applied to the control IC by smoothing the second pulse signal. Image forming device. The transfer body is a belt to which a toner image is transferred from the photoconductor,
Image forming means including the developing means and the transfer means,
And environmental sensor for detecting the temperature Do及beauty humidity inside of the image forming apparatus,
Control means for controlling the image forming means;
With
Wherein, the concentration of the density detecting toner image on the belt which is formed based on an image forming condition according to the temperature and the humidity detected by the environment sensor, is detected by the detecting means, The image forming apparatus according to claim 1 , wherein a target light amount of a light beam that scans the photoconductor is calculated according to the detected density. Image forming means including the developing means and the transfer means,
And environmental sensor for detecting the temperature Do及beauty humidity inside of the image forming apparatus,
Fixing means for fixing the toner image transferred by the transfer means to a recording material,
Reading means for reading a toner image on the recording material,
Control means for controlling the image forming means;
With
The image forming unit further includes the fixing unit,
The control unit is configured to determine the density of the density detection toner image formed based on image forming conditions corresponding to the temperature and the humidity detected by the environment sensor, transferred to the recording material, and fixed. 2. The image forming apparatus according to claim 1 , wherein a target light amount of a light beam that scans the photosensitive member is calculated according to the density of the toner image read by a reading unit. A charging unit for charging the photoconductor to a predetermined potential,
Wherein the image forming conditions, the image forming apparatus according to claim 2 or claim 3 wherein the charging means is characterized in that the charging voltage applied to the photoreceptor. A resistor connected to the first light receiving element, for converting a current output from the first light receiving element into a light receiving voltage in accordance with the amount of light received;
A reference voltage generation unit configured to generate a reference voltage for current control for setting the value of the drive current by the control IC;
A comparator that is built into the control IC and compares the reference voltage generated by the reference voltage generation unit with the received light voltage;
A capacitor charged according to the comparison result of the comparator;
With
5. The control IC according to claim 1, wherein when the light beam scans the second light receiving element, a drive current having a value based on a potential of the capacitor is supplied to the light source. An image forming apparatus according to claim 1.

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