JP2013043432A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a density from changing to an extent that it can be visually recognized by a man in an image formed by scanning with laser light corrected in light quantity using an analog signal output from a DA converter.SOLUTION: An image forming apparatus includes a photosensitive element, a light source part scanning a surface of the photosensitive element with the laser light, an image forming part forming the image on a recording medium using a latent image formed on the photosensitive element, a correction value output part outputting a correction value of the light quantity of the laser light, a DA converter having a PDM (Product Data Management) signal output part outputting a pulse density modulation signal including pulses of the number according to the correction value per part cycle which is a cycle shorter than a cycle corresponding to a maximum spatial frequency that a man is able to visually recognize and a low pass filter outputting an analog signal that a high frequency component of the pulse density modulation signal is cut, and a laser control part radiating the laser light according to image data and adjusting the light quantity of the laser light according to the analog signal.

Description

  The present invention relates to an image forming apparatus, and more particularly to a technique for correcting a light amount at the time of exposure in the image forming apparatus.

  Conventionally, in an exposure apparatus provided in an image forming apparatus, a latent image is formed on the peripheral surface of the photoconductor by scanning the peripheral surface of the photoconductor with a laser beam through a condenser lens or a mirror at a constant speed. The forming technique is known. At this time, due to the characteristics of the optical element, such as the incident angle of the laser beam to the condenser lens that passes through until reaching the peripheral surface of the photosensitive member, and the thickness of the condenser lens, the light amount of the laser beam is It is known that it varies depending on the scanning position on the surface.

  Therefore, in order to uniformly correct the light amount of the laser beam at such a scanning position, for example, in Patent Document 1 below, a light source unit that generates a light beam, a deflection unit that deflects and scans the light beam from the light source unit, In the beam scanning type image forming apparatus having the scanning imaging optical means for condensing the light beam from this deflecting means as a light spot on the surface to be scanned, the scanning position of the light spot is detected and detected. A technique for controlling the amount of laser light corresponding to the scanning position of the light spot based on light amount correction data given in advance corresponding to the scanning position is described.

  Specifically, since the laser light source changes the light amount of the laser light according to the input analog signal level, the light amount correction data is converted into an analog signal by a DA (digital analog) converter and output to the light source. It has become.

JP 2000-71510 A

  However, when control is performed to correct the light amount of the light source based on the correction value (light amount correction data) using the above technique, the ripple caused by the operating frequency of the DA converter is superimposed on the analog signal. Therefore, a density change corresponding to the fluctuation indicated by the ripple appears in the formed latent image, and there is a possibility that the density change is visible to the user when viewing the image formed using the latent image. It was.

  The present invention has been made in view of such circumstances, and humans are present in an image formed by scanning a laser beam whose light amount is corrected using an analog signal indicating a correction value output from a DA converter. An object of the present invention is to provide an image forming apparatus capable of avoiding a visible change in density.

  An image forming apparatus according to the present invention is formed on a photosensitive member having a surface on which a latent image is formed by scanning with a laser beam, a light source unit that scans the surface of the photosensitive member with the laser beam, and the photosensitive member. An image forming unit that forms an image on a recording medium using a latent image, a correction value output unit that outputs a correction value of the amount of laser light, and a period that corresponds to the maximum spatial frequency that can be visually recognized by a person A PDM signal output unit that outputs a pulse density modulation signal including a pulse train including a number of pulses corresponding to the correction value per unit cycle, which is a cycle shorter than the cycle, and the pulse density output from the PDM signal output unit A DA converter including a low-pass filter that outputs an analog signal obtained by cutting off a high-frequency component of the modulation signal; and a light source unit that is connected to the light source unit according to image data corresponding to the latent image. Together emit a laser beam, and a laser control unit for adjusting in accordance with light amount of the laser light to the analog signal.

  According to this configuration, the PDM signal output unit outputs a pulse density modulation signal whose unit period is a period shorter than the visual period, which is a period corresponding to the maximum spatial frequency that can be visually recognized by a person. And the analog signal which cut the high frequency component of the said pulse density modulation signal is output from a low pass filter. Therefore, the ripple of the analog signal output from the DA converter fluctuates at a period shorter than the visual recognition period, that is, a ripple that fluctuates at a frequency high enough to be invisible to humans.

  Then, the laser control unit causes the light source unit to emit laser light according to the image data corresponding to the latent image, and according to an analog signal including a ripple that fluctuates at a high frequency such that the person cannot visually recognize the laser beam. The amount of laser light is adjusted. For this reason, it is possible to avoid a change in density that can be visually recognized by a human being in an image formed on a recording medium using a latent image formed by scanning with the adjusted amount of laser light.

  Moreover, it is preferable that the said visual recognition period is time for the said light source part to scan the said photoreceptor surface 1/16 mm.

  The human eye cannot recognize a spatial frequency of 70 cpd (cycles per degree) or more, which is equivalent to 16 lines per mm when viewed from a distance of 250 mm. It has been. That is, when a latent image of 16 lines or more is formed per 1 mm on the surface of the photosensitive member and then an image is formed on the paper surface using the latent image, a person cannot distinguish and visually recognize each line. .

  According to this configuration, since the time for scanning the photoconductor surface by 1/16 mm is set as the visual recognition period, the ripple of the analog signal output from the DA converter is shorter than the time for scanning the photoconductor surface by 1/16 mm. In the latent image formed by the scanning of the laser light whose light amount is adjusted according to the analog signal, the density change occurs at 16 or more locations per 1 mm. For this reason, it is possible to avoid a change in density that can be visually recognized by a person in an image formed on the recording medium using the latent image.

  According to the present invention, an image formed by scanning a laser beam whose amount of light is corrected using an analog signal indicating a correction value output from a DA converter can be seen to have a density change that can be visually recognized by a person. It is possible to provide an image forming apparatus that can be avoided.

1 is a schematic structural diagram of a copying machine as an example of an image forming apparatus according to the present invention. 1 is a block diagram illustrating an example of an electrical configuration of a copying machine. An explanatory view for explaining an example of a configuration of an optical scanning device. Explanatory drawing for demonstrating an example of a structure of DA converter. (A) is explanatory drawing for demonstrating an example of the pulse signal output from the PDM counter of 4-bit structure. (B) is explanatory drawing for demonstrating that an example of a pulse density modulation signal is output from a PDM signal output part using the pulse signal shown to (a). Explanatory drawing for demonstrating that the pulse signal according to the correction value output from a correction value output part by a selector is selected. 6 is a flowchart showing an example of control for correcting the amount of laser light for exposing a photosensitive drum.

  Embodiments according to the present invention will be described below with reference to the drawings. FIG. 1 is a schematic structural diagram of a copying machine as an example of an image forming apparatus according to the present invention. FIG. 2 is a block diagram showing an example of the electrical configuration of the copying machine 1 shown in FIG.

  As shown in FIG. 1, the copying machine 1 includes a main unit 8, a stack tray 3 disposed on the left side of the main unit 8, a document reading unit 5 disposed on the top of the main unit 8, and a document A document feeding unit 6 disposed above the reading unit 5 and a control unit 10 disposed inside the main body unit 8 are provided.

  An operation panel unit 47 is provided at the front part of the copying machine 1. The operation panel unit 47 includes a display unit 473 and an operation key unit 476. The display unit 473 is configured by, for example, a liquid crystal display having a touch panel function. The operation key unit 476 includes various key switches such as a start key for a user to input a print execution instruction and a numeric keypad for inputting the number of copies to be printed.

  The document reading unit 5 includes a scanner unit 51 including an exposure lamp 511 and a CCD (Charge Coupled Device) 512 (FIG. 2), a document table 52 and a document reading slit 53 made of a transparent member such as glass.

  The scanner unit 51 is configured to be movable by a drive unit (not shown). When reading a document placed on the document table 52, the scanner unit 51 is moved along the document surface at a position facing the document table 52, and the document image is scanned. The image data acquired while scanning is output to the control unit 10. Further, when reading a document fed by the document feeding unit 6, the document is moved to a position facing the document reading slit 53 and synchronized with the document feeding operation by the document feeding unit 6 via the document reading slit 53. The image of the original is acquired and the image data is output to the control unit 10.

  The document feeding unit 6 includes a document placing unit 61 for placing a document, a document discharge unit 62 for discharging a document whose image has been read, and a document placed on the document placing unit 61. A document transport mechanism 63 is provided that feeds the sheets one by one to transport them to a position facing the document reading slit 53 and discharges them to the document discharge section 62.

  The main body 8 includes a plurality of paper feed cassettes 461, a paper feed roller 412 that feeds the paper from the paper feed cassette 461 one by one and transports it to the image forming unit 40, and an image on the paper carried out of the paper feed cassette 461. An image forming unit 40, a discharge tray 48 on which a sheet on which an image is formed is discharged, and a control unit 10 that controls operation of the entire apparatus.

  The image forming unit 40 includes a paper transport unit 41, an optical scanning device 42, a photosensitive drum (photosensitive member) 43, a developing unit 44, a transfer unit 45, and a fixing unit 46.

  The paper transport unit 41 is provided in the paper transport path in the image forming unit 40, and transports the paper transported by the paper feed roller 412 to the photosensitive drum 43 and transports the paper to the stack tray 3. A transport roller 414 for transporting the paper, a transport roller 415 for transporting the paper to the discharge tray 48, and the like.

  The optical scanning device 42 outputs a laser beam based on the image data output from the control unit 10 and exposes the photosensitive drum 43 with the laser beam, thereby forming an electrostatic latent image on the photosensitive drum 43. To do.

  The developing unit 44 forms a toner image by attaching toner to the electrostatic latent image on the photosensitive drum 43. The transfer unit 45 transfers the toner image on the photosensitive drum 43 to a sheet. The fixing unit 46 heats the sheet on which the toner image is transferred to fix the toner image on the sheet.

  The control unit 10 includes, for example, a CPU (Central Processing Unit) that executes predetermined arithmetic processing, a ROM (Read Only Memory) that stores a predetermined control program, a RAM (Random Access Memory) that temporarily stores data, ASIC (Application Specific Integrated Circuits), which is dedicated hardware configured to be capable of high-speed processing of predetermined processing such as image processing, and peripheral circuits thereof are provided.

  As shown in FIG. 2, the document reading unit 5, the image forming unit 40, the operation panel unit 47, and the reference oscillator 70 are connected to the control unit 10. The control unit 10 executes the control program stored in the ROM or the like by the CPU, thereby controlling the operation of each unit in the apparatus and executing copying of the document image. The control unit 10 takes the operation timing of each unit using the reference clock signal oscillated from the reference oscillator 70.

  Specifically, the control unit 10 causes the optical scanning device 42 to output laser light according to the image data read from the document by the document reading unit 5, thereby forming a latent image on the photosensitive drum 43. Then, the developing unit 44, the transfer unit 45, the fixing unit 46, and the sheet conveying unit 41 are used to form an image on the sheet.

  Below, control which correct | amends the light quantity of the laser beam which exposes the photosensitive drum 43 among the control performed by the control part 10 is demonstrated. In connection with the control for correcting the amount of laser light, the control unit 10 particularly functions as a correction clock generation unit 11, a correction value output unit 12, and an image signal output unit 13. Details of the correction clock generation unit 11, the correction value output unit 12, and the image signal output unit 13 will be described later.

  FIG. 3 is an explanatory diagram for explaining an example of the configuration of the optical scanning device 42. As shown in FIG. 3, the optical scanning device 42 includes a light source unit 29, an optical sensor 21, a DA (digital analog) converter 22, and a laser control unit 23.

  The light source unit 29 includes a laser light source 91, a collimator lens 92, a prism 93, a polygon mirror 94, and an f-θ lens 95.

  The laser light source 91 outputs a laser beam having a light amount corresponding to the power supplied from the laser control unit 23 described later.

  The collimator lens 92 condenses the laser light output from the laser light source 91. The prism 93 converts the light transmitted through the collimator lens 92 into parallel light and emits it toward the polygon mirror 94.

  The polygon mirror 94 has a plurality of reflecting surfaces that reflect incident light toward the photosensitive drum 43, and rotates at a constant speed in the direction of an arrow in FIG. 3, for example, by a driving force of a driving motor (not shown).

  The f-θ lens 95 forms an image of the laser beam reflected by the polygon mirror 94 in a spot shape having a predetermined diameter on the peripheral surface of the photosensitive drum 43, and the peripheral surface of the photosensitive drum 43 is formed on the photosensitive drum. 43 is scanned at a constant speed in the main scanning direction, which is the direction in which the support shaft 43a that rotatably supports the shaft 43 extends.

  The optical sensor 21 is provided in the vicinity of the end of the photosensitive drum 43 on the start side of the image scanning line. When receiving the laser beam, the optical sensor 21 outputs a detection signal BD indicating that the laser beam has been received to the control unit 10.

  The DA converter 22 converts a digital signal indicating a correction value CVD for correcting the amount of laser light input from the correction value output unit 12 described later into an analog signal CVA, and converts the analog signal into a laser control unit 23 described later. Output to.

  When the laser control unit 23 receives an image signal VSA having a signal level corresponding to image data corresponding to a latent image formed on the peripheral surface of the photosensitive drum 43 output from the image signal output unit 13 described later, Electric power corresponding to the signal level of the image signal VSA is supplied to the laser light source 91. As a result, the laser light source 91 outputs a laser beam having a light amount corresponding to the image data. Further, the laser control unit 23 corrects the amount of laser light by adjusting the power supplied to the laser light source 91 in accordance with the analog signal CVA indicating the correction value CVD output from the DA converter 22.

  Hereinafter, the control for correcting the light amount of the laser light that exposes the photosensitive drum 43 by the correction clock generation unit 11, the correction value output unit 12, the image signal output unit 13, the DA converter 22, and the laser control unit 23 will be described in detail. To do.

  The correction clock generation unit 11 divides the reference clock signal CLK oscillated from the reference oscillator 70 to generate a correction clock signal CVCLK and outputs it to the DA converter 22.

  The correction value output unit 12 outputs a correction value CVD of the amount of laser light toward the DA converter 22.

  Specifically, the amount of laser light when scanning on the peripheral surface of the photosensitive drum 43 depends on optical characteristics such as the incident angle with respect to the f-θ lens 95 and the thickness of the f-θ lens 95. It is known that the amount of light varies depending on the position.

  Therefore, for example, a correction value CVD indicating the degree of correction of the amount of laser light is determined in advance in association with the scanning position of the laser light on the peripheral surface of the photosensitive drum 43 based on experimental values such as test operation. It is stored in a memory such as a ROM. The correction value output unit 12 grasps the scanning position of the laser beam based on the elapsed time after the detection signal BD is input, acquires the correction value CVD associated with the grasped scanning position from the memory, Output to the DA converter 22.

  The image signal output unit 13 outputs to the laser control unit 23 an image signal VSA having a signal level corresponding to the image data of the latent image formed on the peripheral surface of the photosensitive drum 43 among the image data output from the document reading unit 5. Output toward.

  For example, the DA converter 22 includes a PDM signal output unit 31 and a low-pass filter 35 as shown in FIG.

  The PDM signal output unit 31 is a unit having a period shorter than a visual recognition period, which is a period corresponding to the maximum spatial frequency that can be visually recognized by a person while synchronizing with the correction clock signal CVCLK output from the correction clock generation unit 11 For each period T, a pulse density modulation signal PDMS including a pulse train including a number of pulses corresponding to the correction value CVD output from the correction value output unit 12 is output. That is, the PDM signal output unit 31 outputs the pulse density modulation signal PDMS used as a carrier wave for sending information indicating the correction value CVD.

  The low pass filter 35 generates an analog signal CVA by smoothing the pulse density modulation signal PDMS output from the PDM signal output unit 31, and outputs the generated analog signal CVA toward the laser control unit 23.

  Specifically, the PDM signal output unit 31 includes a PDM counter 32, a selector 33, and an OR operation circuit 34.

  The PDM counter 32 includes, for example, a register R of 4 bits b0 to b3, counts the number of pulses of the correction clock signal CVCLK output from the correction clock generation unit 11, and stores it in the register R.

  The register R is a 4-bit binary counter. For example, the PDM counter 32 outputs the bit b0 as a pulse signal x1 as it is, as shown by the solid line part in FIG. 5A, and synchronizes with the rising timing of the bit b1 for one period of the correction clock signal CVCLK. A pulse signal that is only at a high level is output as a pulse signal x2, and in synchronization with the rising timing of bit b2, a pulse signal that is at a high level only for a period of one cycle of the correction clock signal CVCLK is output as a pulse signal x3. In synchronization with the rising timing of b3, a pulse signal that is at a high level only for one period of the correction clock signal CVCLK is output as a pulse signal x4. The period of the pulse signal x4 having the longest period is a unit period T described later.

  In FIG. 5 (a), the dotted line portion described overlaid on the pulse signal x2 indicates a period in which the bit b1 of the second digit of the register R is 1, and the dotted line portion described overlaid on the pulse signal x3 is The period in which the bit b2 in the third digit of the register R is 1 is shown, and the period in which the bit b3 in the fourth digit of the register R is 1 is shown.

  The selector 33 selects a pulse signal corresponding to the correction value CVD output from the correction value output unit 12 from the pulse signals x1 to x4 output from the PDM counter 32, and performs an OR operation on the selected pulse signal. Output toward the circuit 34.

  Specifically, since the resolution of the DA converter 22 is determined according to the number of bits of the register R provided in the PDM counter 32 and the number of pulse signals corresponding to each bit, the correction value CVD is adjusted accordingly. The range is divided in advance into a number of levels corresponding to the resolution of the DA converter 22. For example, as shown in FIG. 6, the correspondence between the correction value CVD and the pulse signals x1 to x4 is determined in advance and stored in a memory such as a ROM. In the example shown in FIG. 6, the signal selected according to the level of the correction value CVD is indicated by “1”.

  The logical sum operation circuit 34 outputs a signal indicating the logical sum of the pulse signals output from the selector 33 as the pulse density modulation signal PDMS.

  For example, when the register R is composed of 4 bits b0 to b3 as shown in FIG. 4 and four pulse signals x1 to x4 corresponding to the bits b0 to b3 are output, as shown in FIG. The range of the correction value CVD is divided into 16 levels in advance, and the correspondence between each level and the pulse signals x1 to x4 is determined in advance and stored in a memory such as a ROM.

  For example, when the correction value CVD at the first level (1/16) of the 16 levels is input, the selector 33 selects the pulse signal x4 and outputs it to the OR circuit 34. Then, the logical sum operation circuit 34 outputs a pulse signal indicating the pulse signal x4 as the pulse density modulation signal PDMS. That is, in this case, as shown in FIG. 5B, the PDM signal output unit 31 outputs a pulse density modulation signal PDMS composed of a pulse train including one pulse during the unit period T.

  In the same manner, the selector 33 selects the second stage (2/16) level in the 16th stage, the fourth stage (4/16) level in the 16th stage, or the eighth stage (8/8) in the 16th stage. When the correction value CVD at the level of 16) is input, the pulse signals x3, x2, and x1 are selected and output to the OR operation circuit 34, respectively. In this case, as shown in FIG. 5B, the PDM signal output unit 31 outputs a pulse density modulation signal PDMS composed of a pulse train including two, four, and eight pulses during the unit period T, respectively. Output.

  On the other hand, the selector 33 may select two pulse signals. For example, when the correction value CVD of the third stage (3/16) level among the 16 stages is input, the selector 33 selects the two pulse signals x4 and x3, and the two pulse signals x4 and x4 are selected. x3 is output to the logical sum operation circuit 34. Then, the logical sum operation circuit 34 outputs a pulse signal indicating the logical sum of the two pulse signals x4 and x3 as the pulse density modulation signal PDMS. That is, in this case, the PDM signal output unit 31 outputs a pulse density modulation signal PDMS composed of a pulse train including three pulses during the unit period T as shown in FIG.

  Similarly, when the correction value CVD at the 12th stage (12/16) of the 16 stages is input, the selector 33 selects the two pulse signals x2 and x1, and the two pulse signals x2 , X1 are output to the logical sum operation circuit 34. Then, the logical sum operation circuit 34 outputs a pulse signal indicating the logical sum of the two pulse signals x2 and x1 as a pulse density modulation signal PDMS. That is, in this case, the PDM signal output unit 31 outputs a pulse density modulation signal PDMS composed of a pulse train including 12 pulses during a unit period T as shown in FIG.

  Similarly, the selector 33 sets the level of the 5th stage (5/16) in the 16th stage, the level of the 6th stage (6/16) in the 16th stage, and the 9th stage (9/16) in the 16th stage. Or the correction value CVD of the 10th stage (10/16) of the 16 stages is also input, the two pulse signals x4, x2, the two pulse signals x3, x2, 2 The two pulse signals x4 and x1 and the two pulse signals x3 and x1 are selected and output to the OR operation circuit 34. In this case, although not shown in the figure, the PDM signal output unit 31 includes a pulse density modulation signal PDMS composed of pulse trains including 5, 6, 9, and 10 pulses during the unit period T, respectively. Is output.

  The selector 33 may select three pulse signals. For example, the selector 33 selects the three pulse signals x4, x3, and x2 when the correction value CVD at the seventh stage (7/16) of the 16 stages is input, and the three pulse signals are selected. x4, x3, and x2 are output to the logical sum operation circuit 34. Then, the logical sum operation circuit 34 outputs a pulse signal indicating the logical sum of the three pulse signals x4, x3, and x2 as a pulse density modulation signal PDMS. That is, in this case, the PDM signal output unit 31 outputs a pulse density modulation signal PDMS composed of a pulse train including seven pulses during the unit period T as shown in FIG.

  In the same manner, the selector 33 can change the level of the 11th stage (11/16) in the 16th stage, the level of the 13th stage (13/16) in the 16th stage, or the 14th stage (14/14 in the 16th stage). Also when the correction value CVD of level 16) is input, three pulse signals x4, x3, x1, three pulse signals x4, x2, x1 and three pulse signals x3, x2, x1 are selected, respectively. To the logical sum operation circuit 34. In this case, although not shown, the PDM signal output unit 31 outputs a pulse density modulation signal PDMS composed of a pulse train including 11 pulses, 13 pulses, and 14 pulses, respectively, during the unit period T. .

  In addition, the selector 33 may select four pulse signals. For example, the selector 33 selects four pulse signals x4, x3, x2, and x1 when the correction value CVD at the 15th (15/16) level among the 16 levels is input. Then, the logical sum operation circuit 34 outputs a pulse signal indicating the logical sum of the four pulse signals x4, x3, x2, and x1 as a pulse density modulation signal PDMS. That is, in this case, as shown in FIG. 5B, the PDM signal output unit 31 outputs a pulse density modulation signal PDMS composed of a pulse train including 15 pulses during the unit period T.

  The selector 33 may not select any pulse signal. For example, when the correction value CVD at the 0th stage (0/16) level among the 16 stages is input, or when the correction value CVD is not input, the selector 33 sets the correction value CVD to the laser beam. None of the pulse signals x1 to x4 is selected to indicate that the light amount is not corrected. Since the pulse signal is not input from the selector 33, the logical sum operation circuit 34 outputs a low level signal as the pulse density modulation signal PDMS. That is, in this case, as shown in FIG. 5B, the PDM signal output unit 31 outputs a pulse density modulation signal PDMS that does not include any pulses during the unit period T.

  The unit period T is set to be shorter than the visual period, which is a period corresponding to the maximum spatial frequency that can be visually recognized by a person. As described above, the unit period T corresponds to the rising period of the most significant bit b3 of the register R provided in the PDM counter 32, that is, the pulse period when the pulse period of the pulse density modulation signal PDMS is the longest. ing. Further, the resolution of the DA converter 22 is determined according to the number of bits of the register R provided in the PDM counter 32 and the number of pulse signals corresponding to each bit. Accordingly, the period of the correction clock signal CVCLK, the number of bits of the register R provided in the PDM counter 32 determined by the resolution of the output of the analog signal required for the DA converter 22, and the number of pulse signals corresponding to each bit are obtained. The unit period T is adjusted by appropriately setting.

  Specifically, the human eye cannot recognize a spatial frequency of 70 cpd (cycle per degree) or more, which corresponds to 16 lines per mm when viewed from a distance of 250 mm. It is known to do. In other words, a person has an image of 16 lines or more per mm from a position separated by a distance of 250 mm (25.4 × 16 = 406.4 lines per inch, that is, an image with a resolution of 406.4 dpi). It is known that each line cannot be distinguished and viewed when looking at.

  Therefore, in the correction clock generation unit 11, for example, the time required for the light source unit 29 to scan the peripheral surface of the photoconductive drum 43 by 1/16 mm is defined as a visual recognition cycle, and the light source unit 29 has the photoconductive drum 43. The period of the correction clock signal CVCLK is adjusted and set so as to be shorter than the time required for scanning the peripheral surface of 1/16 mm.

  For example, as shown in FIG. 4, the output resolution of the analog signal required by the DA converter 22 is 16, and the PDM counter 32 includes a register R of 4 bits b0 to b3 according to this, and each bit When the four pulse signals x1 to x4 are output corresponding to b0 to b3, the scanning speed is 100 m per second, that is, the time required to scan 1/16 mm is 10 Suppose that it is / 16 microseconds. In this case, the correction clock generator 11 divides the reference clock signal CLK so that the period of the correction clock signal CVCLK is set to 10/256 microseconds (the frequency of the correction clock signal CVCLK is 25.6 MHz).

  Hereinafter, the flow of control for correcting the amount of laser light for exposing the photosensitive drum 43 will be described with reference to FIG.

  When the user inputs a copy function execution instruction by operating the operation panel unit 47 or the like, and the control unit 10 starts the image forming operation by the image forming unit 40 in accordance with the image data output from the document reading unit 5 ( S1) The laser controller 23 causes the laser light source 91 to output laser light having a predetermined light amount without adjusting the light amount of the laser light (S2).

  When the detection signal BD output from the optical sensor 21 is received by the control unit 10 (S3; YES), the correction clock generation unit 11 divides and generates the reference clock signal output from the reference oscillator 70. The corrected clock signal CVCLK is output to the DA converter 22 (S4). In step S4, the cycle of the correction clock signal CVCLK output from the correction clock generation unit 11 is set in advance so that the unit cycle T is shorter than the viewing cycle.

  Next, the correction value output unit 12 acquires the correction value CVD corresponding to the scanning position of the laser beam from the memory and outputs it to the DA converter 22 (S5).

  In the DA converter 22, the PDM signal output unit 31 uses the correction clock signal CVCLK input from the correction clock generation unit 11 and the correction value CVD input from the correction value output unit 12. In addition, a pulse density modulation signal PDMS composed of a pulse train including a number of pulses corresponding to the correction value CVD is output to the low-pass filter 35 (S6).

  The low-pass filter 35 smoothes the pulse density modulation signal PDMS output from the PDM signal output unit 31 to generate an analog signal CVA, and outputs the generated analog signal CVA toward the laser control unit 23 ( S7).

  Then, the laser control unit 23 of the image data output from the image signal generation unit 13 and output from the document reading unit 5 is an image of a latent image formed at the scanning position of the laser beam on the peripheral surface of the photosensitive drum 43. The image signal VSA having a signal level corresponding to the data is corrected in accordance with the signal level of the analog signal CVA output from the DA converter 22, and a laser beam having a light amount corresponding to the corrected signal level is output from the laser light source 91. (S8).

  Then, the control unit 10 determines whether or not the exposure for one line is completed (S9), and when it is determined that the exposure for one line is not completed (S9; NO), the laser beam is used. In order to advance scanning in the main scanning direction, the process proceeds to step S4.

  On the other hand, when it is determined in step S9 that the exposure for one line is completed (S9; YES), the control unit 10 determines whether the exposure for all lines is completed (S10).

  When it is determined in step S10 that the exposure for all lines has not been completed (S10; NO), the control unit 10 rotates the photosensitive drum 43 at a predetermined rotation angle so as to perform exposure for the next line. Only the rotation is performed, and the process proceeds to step S2.

  On the other hand, when it is determined in step S10 that the exposure for all the lines has been completed (S10; YES), the control unit 10 ends the exposure of the photosensitive drum 43 by the exposure unit 29 and at the same time, the photosensitive drum 43. This completes the control of the correction of the light amount of the laser light for exposing the light.

  As described above, in step S6, the PDM signal output unit 31 outputs the pulse density modulation signal PDMS whose unit period T is a period shorter than the viewing period, which is a period corresponding to the maximum spatial frequency that can be visually recognized by humans, In step S7, an analog signal CVA from which the high frequency component of the pulse density modulation signal PDMS is cut is output from the low pass filter. Therefore, the ripple of the analog signal CVA output from the DA converter 22 fluctuates at a cycle shorter than the visual recognition cycle, that is, a ripple that fluctuates at a frequency high enough to be invisible to humans.

  In step S8, the laser control unit 23 causes the light source unit 29 to emit laser light according to the image data corresponding to the latent image, and the ripple fluctuates at a high frequency so that the person cannot visually recognize the ripple. The amount of laser light is adjusted in accordance with an analog signal CVA including For this reason, it is possible to avoid a change in density that can be visually recognized by a human being in an image formed on a recording medium using a latent image formed by scanning with the adjusted amount of laser light.

  The present invention is not limited to the configuration of the above embodiment, and various modifications can be made. For example, in the above-described embodiment, the example in which the image forming apparatus according to the present invention is applied to the copying machine 1 has been described. The present invention may be applied to a scanner device, a facsimile device, a printer device, and a copy device.

  In the above embodiment, the configurations and settings shown in FIGS. 1 to 6 are merely examples, and the present invention is not limited to the embodiment. For example, the register R provided in the PDM counter 32 is not limited to a 4-bit register, but may be an 8-bit register.

1 Copying machine (image forming device)
DESCRIPTION OF SYMBOLS 10 Control part 11 Correction clock generation part 12 Correction value output part 13 Laser control part 23 DA converter 29 Light source part 31 PDM signal output part 32 PDM counter 33 Selector 34 OR operation circuit 35 Low pass filter 40 Image formation part 42 Optical scanning device 43 Photoconductor drum (photoconductor)
70 Reference oscillator 91 Laser light source x1 to x4 Pulse signal b1 to b4 Bit CVD correction value PDMS Pulse density modulation signal T Unit period

Claims (2)

A photoreceptor having a surface on which a latent image is formed by scanning with laser light;
A light source unit that scans the surface of the photoreceptor with the laser beam;
An image forming unit that forms an image on a recording medium using a latent image formed on the photoreceptor;
A correction value output unit that outputs a correction value of the amount of laser light;
A PDM signal that outputs a pulse density modulation signal composed of a pulse train including a number of pulses corresponding to the correction value per unit period, which is a period shorter than a visual recognition period that is a period corresponding to the maximum spatial frequency visible to a human A DA converter comprising: an output unit; and a low-pass filter that outputs an analog signal obtained by cutting a high-frequency component of the pulse density modulation signal output from the PDM signal output unit;
A laser control unit that emits the laser light to the light source unit according to image data corresponding to the latent image, and adjusts the amount of the laser light according to the analog signal;
An image forming apparatus comprising:   The image forming apparatus according to claim 1, wherein the visual recognition period is a time for the light source unit to scan the surface of the photosensitive member by 1/16 mm.

Images