JP2004098595A - Image forming apparatus and laser scanning control method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of correcting the scanning length of a laser beam by adding or eliminating minute pixels and capable of eliminating an image generated when the minute pixels are eliminated. <P>SOLUTION: When the minute pixels are eliminated (step S501), it is judged whether the density of pixels for eliminating the minute pixels is high or low (step S502). This judgment of the density of the pixels is performed corresponding to whether the density of the pixels is lower than a preset density standard. When the density of the pixels is low, the minute pixels of "O" are eliminated (step S503). When density of the pixels is high, the minute pixels of "1" are eliminated (step S504). <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus for exposing and scanning an image carrier with a laser beam, and a laser scanning control method thereof.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an image forming apparatus that performs image exposure using laser light, a rotating polygonal mirror (polygon mirror) is irradiated with laser light, and the reflected light is exposed to a photoconductor to obtain an electrostatic latent image on the photoconductor. It is configured.
[0003]
In recent years, in order to realize high-speed image formation, a technique of arranging a plurality of lasers in the sub-scanning direction and simultaneously exposing two lines, for example, has been used. As a color printer, there is a so-called tandem type printer in which four photoconductors are arranged in a line, and each photoconductor is exposed by an independent laser. When a plurality of lasers are arranged in the sub-scanning direction to simultaneously expose a plurality of lines, the scanning lengths cannot be equal unless the optical path length from the laser light source to the surface of the photosensitive member is equal. Similarly, in a tandem-type printer, the scanning lengths are not equal unless the optical path lengths of the four lasers are equal.
[0004]
Here, a laser exposure apparatus for exposing two lines simultaneously will be described with reference to FIG. FIG. 11 is a diagram schematically showing a main part configuration of a conventional laser exposure apparatus for simultaneously exposing two lines.
[0005]
In a laser exposure apparatus for exposing two lines at the same time, as shown in FIG. 11A, two laser beams are deflected by a polygon mirror 833 rotating at a constant speed and irradiated onto a photosensitive drum 811. That is, by scanning the photosensitive drum 811 in the main scanning direction with two laser beams, an electrostatic latent image is formed on the photosensitive drum 811. Here, the two laser beams are applied to the photosensitive drum 811 at a predetermined interval in the sub-scanning direction. With such a configuration, image formation can be performed with half the number of scans as compared with the case where the photosensitive drum 811 is scanned with one laser. As shown in FIG. 11C, an f-θ lens 834 is provided between the polygon mirror 833 and the photosensitive drum 811, and the f-θ lens 234 reflects a deflection beam. At the same time as condensing the laser beam thus obtained, distortion is corrected so as to guarantee the temporal linearity of scanning. Therefore, the laser light that has passed through the f-θ lens 834 is coupled and scanned on the photosensitive drum 811 at a constant speed.
[0006]
[Problems to be solved by the invention]
However, since the two laser beams are irradiated on the photosensitive drum 811 at a predetermined interval in the sub-scanning direction, as shown in FIG. Are respectively La and Lb, the optical path lengths La and Lb are different. Here, it is assumed that a relational expression of La <Lb holds, and the difference is ΔL.
[0007]
The fact that the optical path lengths La and Lb of the respective laser beams from the polygon mirror 833 to the photosensitive drum 811 are different from each other means that the effective pixel range of one line in the main scanning direction on the photosensitive drum 811 as shown in FIG. Is different for each laser beam. Here, assuming that the scanning length of the laser beam in the case of the optical path length La is Xa and the scanning length of the laser beam in the case of the optical path length Lb is Xb, the relational expression of Xa <Xb is established.
[0008]
When an image is formed in such a state, an image having jagged edges at every other line is obtained. In today's situation where the demand for high resolution for images is increasing, the jagged edges do not meet the high resolution requirements.
[0009]
As described above, if the scanning lengths are different, image defects such as color misregistration will occur. Conventionally, the optical path length for each laser beam, that is, the scanning length of each laser beam is made equal by increasing the optical and mechanical accuracy. Measures have been taken. However, there is a limit in improving optical and mechanical accuracy, and it is becoming impossible to satisfy the demand for higher resolution.
[0010]
SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus capable of correcting a scanning length of a laser beam by adding or deleting a minute pixel and eliminating loss of an image caused when a minute pixel is deleted, and a laser scanning control thereof. It is to provide a method.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the present invention converts an input pixel-based image signal into frequency-modulated data decomposed into a predetermined number of small pixels synchronized with a high-frequency clock having an integral multiple of the period of a basic clock. A modulating unit, and a scanning unit that modulates and drives a laser light source based on the frequency modulation data output from the modulating unit, and scans the latent image carrier with laser light emitted from the modulated and driven laser light source. In the image forming apparatus, the modulating unit corrects a scanning length by the laser light by adding or deleting minute pixels from frequency modulation data of an arbitrary pixel on a scanning line of the laser light. A correction unit, wherein the correction unit is configured to delete minute pixels from the frequency modulation data in accordance with a pixel density indicated by the frequency modulation data. And determining a minute pixel to be removed from the serial frequency modulation data.
[0012]
Further, in the image forming apparatus, when the pixel density indicated by the frequency modulation data is high, the deletion unit determines a micro pixel corresponding to the laser light output among the micro pixels of the frequency modulation data as a pixel to be deleted. When the pixel density indicated by the frequency modulation data is low, the minute pixels corresponding to the no output of the laser beam among the minute pixels of the frequency modulation data are determined as pixels to be deleted.
[0013]
According to the present invention, in order to achieve the above object, an input pixel-based image signal is converted into frequency-modulated data that is decomposed into a predetermined number of minute pixels synchronized with a high-frequency clock having a cycle that is an integral multiple of the basic clock. A modulating means for converting, a scanning means for modulating and driving a laser light source based on the frequency modulation data output from the modulating means, and scanning over the latent image carrier with laser light emitted from the modulated and driven laser light source; A laser scanning control method for an image forming apparatus, comprising: adding or deleting minute pixels to or from a frequency modulation data of an arbitrary pixel on a scanning line of the laser light to reduce a scanning length by the laser light. A correcting step of correcting, and in the correcting step, when deleting a minute pixel from the frequency modulation data, a pixel indicated by the frequency modulation data is deleted. And determining a minute pixel to be removed from the frequency modulation data according to time.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0015]
FIG. 1 is a vertical cross-sectional view illustrating a main configuration of an image forming apparatus according to an embodiment of the present invention.
[0016]
As shown in FIG. 1, the image forming apparatus 1 is a tandem-type color image forming apparatus employing an electrophotographic system, and includes an image forming unit, a sheet feeding unit, an intermediate transfer unit, a transport unit, a fixing unit, and an operation unit. , And a control unit (not shown).
[0017]
The image forming section has four stations a, b, c, d arranged side by side, and their basic configuration is the same. Specifically, in the image forming section, photosensitive drums 11a, 11b, 11c, and 11d as image carriers are pivotally supported at their centers, and are rotationally driven by a drive motor (not shown) in the direction of the arrow. Around the photosensitive drums 11a to 11d, roller chargers 12a, 12b, 12c, 12d, scanners 13a, 13b, 13c, 13d, and developing devices 14a, 14b, 14c, 14d are arranged in order in the rotation direction. . The roller chargers 12a to 12d apply a uniform amount of charge to the surfaces of the photosensitive drums 11a to 11d. The scanners 13a to 13d expose and scan the photosensitive drums 11a to 11d with a laser beam modulated according to a recording image signal. Thus, an electrostatic latent image is formed on the photosensitive drums 11a to 11d. The developing devices 14a to 14d store developers (toners) of four colors of yellow, cyan, magenta, and black, respectively, and supply the developers to the photosensitive drums 11a to 11d, thereby forming electrostatic latent images on the photosensitive drums 11a to 11d. The latent image is visualized as a toner image. The visualized toner image is transferred to the intermediate transfer belt 30. By the above process, image formation by each toner is sequentially performed.
[0018]
The paper feeding unit includes a portion for storing the transfer material P, a roller for transporting the transfer material P, a sensor for detecting passage of the transfer material P, a sensor for detecting the presence or absence of the transfer material P, and a transfer material. It comprises a guide (not shown) for transporting P along the transport path. The transfer material P can be stored in each of the cassettes 21a, 21b, 21c, 21d, the manual tray 27, or the deck 28, and the transfer material P in the cassettes 21a to 21d is collected by the pickup rollers 22a, 22b, 22c, 22d. It is sent out one by one. Although a plurality of transfer materials P may be sent out by the pickup rollers 22a to 22d, only one sheet is reliably separated by the BC rollers 23a, 23b, 23c, and 23d.
[0019]
The transfer material P separated only by one of the BC rollers 23a to 23d is further transported by pull-out rollers 24a to 24d and a pre-registration roller 26, and further transported to the registration roller 25. Further, the transfer material P stored in the manual feed tray 27 is separated by a BC roller 29, and is conveyed to a registration roller 25 by a pre-registration roller 26. The transfer material P stored in the deck 28 is transported by the pickup roller 60 to the paper feed roller 61, and is separated by the paper feed roller 61, and is transported to the pull-out roller 62. Further, the transfer material P is transported to the registration roller 25 by the pre-registration roller 26.
[0020]
The intermediate transfer unit will be described in detail. The intermediate transfer belt 30 is a belt made of a material such as PET (polyethylene terephthalate) or PVdF (polyvinylidene fluoride). The intermediate transfer belt 30 is wound around a driving roller 32, a tension roller 33, and a driven roller 34 for transmitting a driving force thereto. The drive roller 32 is formed of a metal roller having a surface coated with rubber (urethane or chloroprene) having a thickness of several millimeters, and the coating prevents slippage with the intermediate transfer belt. The driving roller 32 is driven to rotate by a stepping motor (not shown). Primary transfer rollers 35a to 35d to which a high voltage for transferring a toner image to the intermediate transfer belt 30 is applied are disposed on the back of the intermediate transfer belt 30 at positions where the respective photosensitive drums 11a to 11d and the intermediate transfer belt 30 face each other. Have been.
[0021]
The tension roller 33 is urged by a spring (not shown) and applies an appropriate tension to the intermediate transfer belt 30. The driven roller 34 is arranged to face the secondary transfer roller 36 so as to form a secondary transfer area with the intermediate transfer belt 30 interposed therebetween. The secondary transfer area is formed by a nip between the secondary transfer roller 36 and the intermediate transfer belt 30. The secondary transfer roller 36 is pressed against the intermediate transfer belt 30 with an appropriate pressure. On the intermediate transfer belt 30, a cleaning device 50 for cleaning the image forming surface of the intermediate transfer belt 30 is disposed downstream of the secondary transfer area. The cleaning device 50 includes a cleaner blade 51 (material). And a waste toner box 52 for storing waste toner.
[0022]
The fixing unit 40 includes a fixing roller 41a having a heat source such as a halogen heater therein, a pressing roller 41b pressed against the fixing roller 41a (the roller may also have a heat source), and a fixing roller 41a. And an inner discharge roller 44 for transporting the transfer material P discharged from the pressure roller 41b.
[0023]
On the other hand, the transfer material P conveyed to the registration roller 25 is temporarily stopped by stopping the rotation of the roller 26 located upstream of the registration roller 25, and then is transferred to the registration roller 25 in synchronization with the image forming timing of the image forming unit. By restarting the rotation of the upstream roller 26 including the above, the sheet is sent to the secondary transfer area. In the secondary transfer area, an image is transferred to the transfer material P, and the image transferred to the transfer material P is fixed to the transfer material P in the fixing unit 40. After the transfer material P after fixing has passed through the inner discharge roller 44, the transport destination is switched by the switching flapper 73. When the switching flapper 73 is on the face-up paper discharge side, the transfer material P is discharged to the face-up paper discharge tray 2 by the external paper discharge roller 45. On the other hand, when the switching flapper 73 is on the face-down discharge side, the transfer material P is transported in the direction of the reversing rollers 72a, 72b, 72c and discharged to the face-down discharge tray 3.
[0024]
Note that a plurality of sensors are arranged on the transfer path of the transfer material P in order to detect the passage of the transfer material P. These sensors include a paper feed retry sensor 64a, 64b, 64c, 64d, a deck paper feed sensor 65, a deck pull-out sensor 66, a registration sensor 67, an internal paper discharge sensor 68, a face-down paper discharge sensor 69, a double-sided pre-registration sensor 70, There is a double-side refeed sensor 71 and the like. Cassette paper absence sensors 63a, 63b, 63c, and 63d for detecting the presence or absence of the transfer material P are disposed in the cassettes 21a to 21d that store the transfer material P. A manual paper tray presence / absence sensor 74 for detecting the presence / absence of the material P is disposed, and a deck paper presence / absence sensor 75 for detecting the presence / absence of the transfer material P in the deck 28 is disposed on the deck 28.
[0025]
The control unit includes a control board (not shown) for controlling the operation of the mechanism in each unit, a motor drive board (not shown), and the like.
[0026]
The operation unit 4 is disposed on the upper surface of the image forming apparatus 1, selects a paper supply unit (the paper supply cassettes 21 a to 21 d, the manual tray 27, the deck 28) in which the transfer material P is stored, and a paper discharge tray (face). It is possible to select the up tray 2 and the face down tray 3), specify a tab sheet bundle, and the like.
[0027]
Next, the operation of the apparatus will be described. Here, a case where the transfer material P is transported from the cassette 21a will be described as an example.
[0028]
After a lapse of a predetermined time from the issuance of the image forming operation start signal, first, the transfer material P is sent out one by one from the cassette 21a by the pickup roller 22a. Then, the transfer material P is transported by the paper feed roller 23 to the registration roller 25 via the pull-out roller 24a and the pre-registration roller 26. At this time, the registration roller 25 is stopped, and the leading end of the paper strikes the nip. Thereafter, the registration roller 25 starts rotating at the timing when the image forming section starts forming an image. The rotation start timing is set to a timing at which the transfer material P and the toner image primarily transferred from the image forming unit onto the intermediate transfer belt 30 exactly coincide with each other in the secondary transfer area.
[0029]
On the other hand, in the image forming section, when an image forming operation start signal is issued, a high voltage is applied to the toner image formed on the photosensitive drum 11d at the most upstream side in the rotation direction of the intermediate transfer belt 30 by the above-described process. The primary transfer is performed on the intermediate transfer belt 30 in the primary transfer area by the transferred transfer roller 35d. The primary-transferred toner image is transported to the next primary transfer area. In this case, the image formation is performed with a delay of the time when the toner image is conveyed between the stations of the image forming unit, and the next toner image is transferred by aligning the leading edge of the image with the previous image. Become. Thereafter, the same steps are repeated, and finally, the four color toner images are primarily transferred on the intermediate transfer belt 30.
[0030]
Thereafter, when the transfer material P enters the secondary transfer area and comes into contact with the intermediate transfer belt 30, a high voltage is applied to the secondary transfer roller 36 in synchronization with the passage timing of the transfer material P. Then, the four color toner images formed on the intermediate transfer belt 30 by the above-described process are transferred onto the surface of the transfer material P. Thereafter, the transfer material P is guided to a nip between the fixing roller 41a and the pressure roller 41b, and receives heat and pressure when passing through the nip. Thus, the toner image is fixed on the surface of the transfer material P. Then, the transfer material P is discharged to the face-up discharge tray 2 or the face-down tray 3 according to the switching direction of the switching flapper 73. The image forming apparatus 1 is connected to a document reading device (not shown) that reads an image of a document and converts the image into image data.
[0031]
Next, the configuration of the scanner units 13a to 13d will be described with reference to FIG. FIG. 2 is a plan view schematically showing a configuration of the scanner units 13a to 13d of the image forming apparatus of FIG.
[0032]
Each of the scanner units 13a to 13d has a laser driving device 231 that drives a semiconductor laser 243, as shown in FIG. A PD sensor (not shown) for detecting a part of the laser light is provided inside the semiconductor laser 243, and the laser driving device 231 uses a detection signal of a photodiode sensor (PD sensor; not shown) using a detection signal. APC (Auto Power Control) control of the semiconductor laser 243 is performed. The laser light emitted from the semiconductor laser 243 becomes almost parallel light by the collimator lens 235 and the stop 232 and enters the polygon mirror (rotating polygon mirror) 233 with a predetermined beam diameter. The polygon mirror 233 rotates at a constant angular velocity in the direction indicated by the arrow in the figure, and with this rotation, the laser light incident on the polygon mirror 233 is reflected as a deflection beam whose angle continuously changes. . The laser beam reflected as a deflected beam is condensed by the f-θ lens 234. At the same time, the f-θ lens 234 corrects distortion so as to guarantee the temporal linearity of scanning, so that the laser beam passing through the f-θ lens 234 is applied to the photosensitive drum 11 (the photosensitive drums 11a to 11d). (Equivalent) and scanning is performed at a constant speed in the direction of the arrow in the figure. In the vicinity of one end of the photosensitive drum 11, a beam detect (hereinafter, referred to as BD) sensor 236 for detecting a laser beam reflected from the polygon mirror 233 is provided, and a detection signal of the BD sensor 236 is It is used as a synchronization signal for synchronizing the rotation of the data and the writing of data.
[0033]
In such a laser driving device 231, in order to keep the amount of laser light during one scan constant, the output of the laser light is detected in the light detection section during one scan and the drive current of the semiconductor laser 243 is reduced by one. A driving method of holding the data during scanning is adopted.
[0034]
Next, a specific control method for the semiconductor laser 243 by the laser driving device 231 will be described with reference to FIG. FIG. 3 is a circuit diagram showing a specific control configuration for the semiconductor laser 243 of the laser driving device 231 of FIG.
[0035]
In the laser driving device 231, as shown in FIG. 3, a laser chip including one laser 343 </ b> A and one photodiode (hereinafter, referred to as PD) sensor 343 </ b> B is used as the semiconductor laser 243. Two current sources, a bias current source 341 and a pulse current source 342, are used as driving power sources for the semiconductor laser 243, thereby improving the light emission characteristics of the laser 343A. In addition, in order to stabilize the light emission of the laser 343A, feedback control is performed on the bias current source 341 using an output signal from the PD sensor 343B, thereby automatically controlling the amount of bias current. That is, when the logic element 340 outputs an ON signal to the switch 349 in response to the full lighting signal FULL from the sequence controller 347, the sum of the currents from the bias current source 341 and the pulse current source 342 flows to the semiconductor laser 243. The output signal from the PD sensor 343B is input to a current / voltage converter (I / V) 344 and converted into a voltage signal. This voltage signal is amplified by an amplifier (Gain) 345 and then input to the APC circuit 346 as a voltage signal VPD. The APC circuit 346 supplies a control signal VAPC corresponding to the input voltage signal VPD to the bias current source 341. This circuit system is called an APC (Auto Power Control) circuit system, and is currently generally used as a circuit system for driving a laser. The laser 343A has a temperature characteristic. As the temperature increases, the amount of current for obtaining a constant light amount increases. In addition, since the laser 343A generates heat, a constant amount of light cannot be obtained only by supplying a constant current, and these have a significant effect on image formation. As a method for solving this problem, a method of controlling the amount of current flowing for each scan using the above-described APC circuit method for each scan so that the light emission characteristics of each scan is constant is adopted.
[0036]
The laser light controlled to a constant light amount in this manner is turned on / off by turning on / off the switch 349 according to the data modulated by the modulation unit 348, and a latent image corresponding to the laser light is formed on the photosensitive drum 11 ( It is formed on the photosensitive drums 11a to 11d).
[0037]
Next, the configuration of the modulation section 348 will be described with reference to FIGS. 4 is a block diagram showing the configuration of the modulator 348 of FIG. 3, FIG. 5 is a timing chart of input / output signals of the frequency divider 461 of the modulator 348 of FIG. 4, and FIG. 6 is a modulator of the modulator 348 of FIG. FIG. 7 is a timing chart of the input / output signals of the input / output signals of the modulation circuit 348 of FIG.
[0038]
As shown in FIG. 4, the modulation section 348 includes a PLL circuit 460, a frequency dividing circuit 461, a modulation circuit 462, an output circuit 463, and a counter circuit 464.
[0039]
The PLL circuit 460 receives a basic clock (basic CLK) as an input, and outputs a high-frequency clock n times the basic clock. This high frequency clock is input to the frequency dividing circuit 461 and the output circuit 463, respectively. The frequency dividing circuit 461 counts the input high frequency clock once every x times and outputs a clock (main clock) obtained by dividing the input high frequency clock by 1 / x (see FIG. 5). x may be any positive integer. Here, for convenience of description, it is assumed that a main clock having the same cycle as the basic clock input to PLL circuit 460 is output by dividing the frequency by 1 / n. The clock output from the frequency dividing circuit 461 is input to the counter circuit 464.
[0040]
The modulation circuit 462 modulates input data in synchronization with a clock signal described later. Usually, in order to represent the gradation of the laser, the lighting time within a unit time is controlled by PWM modulation. Therefore, in the present embodiment, it is assumed that PWM modulation (particularly digital PWM modulation) is performed. explain. For example, when the A-bit input data is PWM-modulated, the input data is 2 bits. ^A Is converted to pulse width data. here,
2 ^A = N
The constant is determined so that The modulation circuit 462 generates pulse width data from the input data, and outputs the pulse width data to the output circuit 463 (see FIG. 6). The output circuit 463 outputs a PWM signal synchronized with the high-frequency clock output from the PLL circuit 460 and a clock signal synchronized with the high-frequency clock according to the pulse width data output from the modulation circuit 462, and the PWM signal is a logic element. At 340 (FIG. 3), the clock signal is output to an image processing unit (not shown) and modulation circuit 462, respectively (see FIGS. 7 (a), (e) and (f)).
[0041]
The counter circuit 464 counts the clock (clock obtained by dividing the high frequency clock by 1 / n) output from the frequency dividing circuit 461 (see FIG. 7B). When the count value reaches a predetermined value, the counter circuit 464 outputs a count A signal to the output circuit 63 (see FIG. 7C). When the count value reaches a predetermined value, the counter circuit 464 outputs a count B signal to the modulation circuit 462 (see FIG. 7D). Here, the count B signal and the count A signal are output with a difference of one clock.
[0042]
When the counter circuit 464 outputs the count A signal to the output circuit 463, the output circuit 463 performs an operation different from a normal operation. In the normal operation, one cycle (of a PWM signal and a clock signal) is generated by n high-frequency clocks. On the other hand, when the count A signal is input, a PWM signal having a cycle different from the above cycle is generated. It outputs data and clock signals (see FIG. 7).
[0043]
In the present embodiment, in order to realize high-speed image formation, a technique is used in which the photosensitive drum 11 is exposed with laser light generated from two semiconductor lasers 43 provided at an interval in the sub-scanning direction. . This technique is well known.
[0044]
Next, the operation when the modulation circuit 462 receives the count B signal from the counter circuit 464 will be described with reference to FIGS. FIG. 8 is a flowchart showing the operation when the modulation circuit 462 inputs the count B signal from the counter circuit 464, and FIG. 9 is a diagram showing the output waveform of the modulation circuit 462.
[0045]
Upon receiving the count B signal, the modulation circuit 462 converts the received input data according to the flowchart shown in FIG. 8, and outputs the converted data to the output circuit 463 at the next clock. In the modulation circuit 462, the input data is converted into pulse width data in synchronization with the high frequency clock. In this embodiment, one pixel of the input image data is converted into pulse width data having a resolution of 8 bits. For example, the input data of density 3 can be represented as shown in FIG. The input data of the density 6 can be represented as shown in FIG. That is, each pixel can be decomposed into minute pixels synchronized with the high-frequency clock. Therefore, it is possible to reduce the scanning length by deleting a predetermined number of minute pixels from an arbitrary pixel.
[0046]
The flowchart in FIG. 8 shows a procedure for deleting a minute pixel from an arbitrary pixel. First, in step S501, in order to adjust the scanning length (to shorten the scanning length), it is determined whether to delete a small pixel. Here, when deleting a minute pixel, in step S502, it is determined whether the density of the pixel from which the minute pixel is to be deleted is high or low. The determination of the density of the pixel is made according to whether the density is higher or lower than a preset density reference. If the density of the pixel is low, the small pixel of “0” is deleted in step S503. If the density is high, in step S504, the minute pixel “1” is deleted.
[0047]
For example, when a minute pixel is deleted from the pixel shown in FIG. 9A (the configuration of eight clocks), the result shown in FIG. 9B is obtained. In this case, since the density of the pixel from which the minute pixel is deleted is low, the minute pixel of "0" is deleted, and the number of high-frequency clocks constituting this pixel becomes seven. In the case where minute pixels are deleted from the pixel (eight clocks) shown in FIG. 9C, the result shown in FIG. 9D is obtained. In this case, since the density of the pixel from which the minute pixel is deleted is high, the minute pixel of "1" is deleted, and the number of high-frequency clocks constituting this pixel becomes seven.
[0048]
Next, a specific operation when the output circuit 463 receives the count A signal from the counter circuit 464 will be described with reference to FIG. FIG. 10 is a block diagram showing a configuration of the output circuit 463 in the modulation section 348 of FIG.
[0049]
As shown in FIG. 10, the output circuit 463 includes a modulation control unit 770, nine D-type flip-flops 771a to 771i, nine two-input AND circuits 772a to 772i, a three-input selector circuit 773, and two It includes a two-input selector circuit 774, 775, a nine-input OR circuit 776, and a two-input OR circuit 777.
[0050]
The modulation circuit 462 modulates the input image data into 8-bit pulse width data. Each bit of the pulse width data is input to one of the inputs of two-input AND circuits 772a to 772i. Here, the same data is input to two-input AND circuits 772h and 772i.
[0051]
The flip-flops 771a to 771i output the input of the D terminal to the Q terminal at the rise of the high-frequency clock from the PLL circuit 460. The output of the Q terminal is connected to the other of the inputs of each of the two-input AND circuits 772a to 772i. At the same time, the flip-flops 771a to 771i are cascaded such that the output of the flip-flop 771a is connected to the input of the flip-flop 771b and the output of the flip-flop 771b is connected to the input of the flip-flop 771c. The output of the flip-flop 771g is also connected to a three-input selector circuit 773 and a two-input selector circuit 774. The output of the two-input AND circuit 772h is also connected to the three-input selector circuit 773 and the two-input selector circuit 775. The output of the flip-flop 771i is also connected to a three-input selector circuit 773.
[0052]
Outputs of the two-input AND circuits 772a to 772i are input to a nine-input OR circuit 776, and the nine-input OR circuit 776 outputs its output as a PWM signal.
[0053]
The three-input selector circuit 773 selects the output of the flip-flops 771g, 771h, and 771i according to the output of the modulation control unit 770, and the selected output is input to one of the inputs of the two-input OR circuit 777. The other inputs of the two-input selector circuits 774 and 774 are connected to GND.
[0054]
The modulation control unit 770 switches the switching operation of the selector circuits 773 to 775 based on the output of the counter circuit 464. That is, the output of the modulation control unit 770 is whether the output of the flip-flop 771g is input to the flip-flop 771h in the case of the two-input selector circuit 774, or the output of the flip-flop 771g in the case of the two-input selector circuit 775. Is controlled whether or not to input the data.
[0055]
A timing signal having a width of one high-frequency clock is input to the other input of the two-input OR circuit 777, and the output is input to the flip-flop 771a.
[0056]
Next, the operation of the output circuit 463 will be described.
[0057]
In the output circuit 463, a signal having a width of one high-frequency clock is input to the two-input OR circuit 777 as a timing signal in synchronization with the high-frequency clock input to the flip-flops 771a to 771i. Thus, one of the outputs of the ring-shaped shift register including the flip-flops 771a to 771i is always "1". The modulation control unit 770 receives the output of the counter circuit 463 and controls the switching operation of each of the selector circuits 773 to 775 so as to control the size of the ring-shaped shift register. When one pixel is composed of seven high-frequency clocks (clocks output from the PLL circuit 460), the output of the flip-flop 771g is selected by the three-input selector circuit 773, and GND is output by the two-input selector circuits 774 and 775. Select Thus, a ring-shaped shift register including the seven flip-flops 771a to 771g is formed. When one pixel is constituted by eight high-frequency clocks, the output of the flip-flop 771h is selected by the three-input selector circuit 773, the output of the flip-flop 771g is selected by the two-input selector circuit 774, and the two-input selector circuit is selected. At 775, GND is selected. Thus, a ring-shaped shift register including the eight flip-flops 771a to 771h is formed. When one pixel is constituted by nine high-frequency clocks, the output of the flip-flop 771i is selected by the three-input selector circuit 773, the output of the flip-flop 771g is selected by the two-input selector circuit 774, and the two-input selector circuit is selected. At 775, the output of flip-flop 771h is selected. Thus, a ring-shaped shift register including the nine flip-flops 771a to 771i is formed. By these switching, the outputs of the flip-flops 771a to 771i output "1" once for the 7/8/9 clock signal.
[0058]
The pulse width data is set in the two-input AND circuits 772a to 772i. The two-input AND circuits 772a to 772i change the data for each pixel (= 7/8 / 9CLK), and set the set data. AND operation of "1" once during a period of 7/8/9 high frequency clock is performed, and each AND output is OR-operated by a 9-input OR circuit 776. Thereby, a PWM signal composed of 7/8/9 high frequency clocks can be output.
[0059]
Although not shown, the same configuration is used to input an image clock pattern at a position corresponding to image data, or to a specific position (for example, flip-flops 771a and 771e) of flip-flops 771a to 771g. By inputting the output to the JK flip-flop circuit, a clock signal composed of 7/8/9 high-frequency clocks can be output in the same manner as the PWM signal. Assuming that the default clock signal is composed of eight high-frequency clocks, a predetermined signal (see FIG. 7C) is output from the counter circuit 464 to a desired pixel. For a pixel composed of normal eight high-frequency clocks, a three-input selector circuit 773 selects the output of flip-flop 771h, and a two-input selector circuit 774 outputs the output of flip-flop 771g and the two-input selector circuit 775. Then, control is performed so as to select GND. As a result, a PWM signal having a width corresponding to eight high-frequency clocks is output.
[0060]
When the main scanning length of the laser beam to be corrected is longer than the main scanning length of the reference laser beam, that is, when the main scanning length of the laser beam to be corrected is shortened, 1 The correction for shortening the pixel section is set. That is, in this case, control is performed so that a PWM signal having a width of seven high-frequency clocks is output to a plurality of pixels in a scanning section of one line.
[0061]
Further, when the main scanning length of the laser to be corrected is shorter than the main scanning length of the reference laser, a correction for lengthening one pixel section is set. That is, in this case, control is performed so that a PWM signal having a width corresponding to nine high-frequency clocks is output to a plurality of pixels in a scanning section of one line.
[0062]
As described above, by removing or adding a minute pixel from an arbitrary pixel, it is possible to freely change the scanning length, and when deleting a minute pixel, the position to be deleted is determined according to the density of the pixel. By making the change, the loss of the image and the loss of the portion where the image is not formed can be eliminated.
[0063]
【The invention's effect】
As described above, according to the present invention, the scanning length by laser light is corrected by adding or deleting minute pixels to or from the frequency modulation data of any pixel on the scanning line of laser light, When deleting a minute pixel from the modulation data, the minute pixel to be deleted is determined from the frequency modulation data according to the pixel density indicated by the frequency modulation data. The length can be corrected, and the loss of an image that occurs when minute pixels are deleted can be eliminated. As a result, a high-resolution image without image degradation can be easily obtained.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view illustrating a configuration of a main part of an image forming apparatus according to an embodiment of the invention.
FIG. 2 is a plan view schematically illustrating a configuration of scanner units 13a to 13d of the image forming apparatus in FIG.
FIG. 3 is a circuit diagram showing a specific control configuration for a semiconductor laser 243 of the laser driving device 231 of FIG.
FIG. 4 is a block diagram illustrating a configuration of a modulation unit 348 of FIG.
FIG. 5 is a timing chart of input / output signals of a frequency dividing circuit 461 of the modulation unit 348 of FIG.
FIG. 6 is a timing chart of input / output signals of a modulation circuit 462 of the modulation unit 348 of FIG.
FIG. 7 is a timing chart of input / output signals of a counter circuit 464 and an output circuit 463 of the modulation unit 348 of FIG.
FIG. 8 is a flowchart showing an operation when the modulation circuit 462 receives the count B signal from the counter circuit 464.
FIG. 9 is a diagram showing an output waveform of a modulation circuit 462.
FIG. 10 is a block diagram showing a configuration of an output circuit 463 in the modulation section 348 of FIG.
FIG. 11 is a diagram schematically showing a main part configuration of a conventional laser exposure apparatus for exposing two lines simultaneously.
[Explanation of symbols]
11a, 11b, 11c, 11d Photosensitive drum
13a, 13b, 13c, 13d Scanner unit
231 Laser drive
233 Polygon mirror
243 Semiconductor Laser
348 modulation section
460 PLL circuit
461 frequency divider
462 Modulation circuit
463 output circuit
464 counter circuit
770 Modulation control unit
771a-771i flip-flop
772a-772i 2-input AND circuit
776 9-input OR circuit

Claims (4)

A modulating means for converting an input pixel-based image signal into frequency-modulated data decomposed into a predetermined number of minute pixels synchronized with a high-frequency clock having a cycle of an integral multiple of the basic clock; and A laser light source that is modulated based on the frequency modulation data, and a scanning unit that scans the latent image carrier with laser light emitted from the modulated laser light source.
The modulating unit has a correcting unit that corrects a scanning length by the laser light by adding or deleting a fine pixel to frequency modulation data of an arbitrary pixel on a scanning line of the laser light,
When the fine pixel is deleted from the frequency modulation data, the correction unit determines a small pixel to be deleted from the frequency modulation data according to a pixel density indicated by the frequency modulation data. Image forming device. When the pixel density indicated by the frequency modulation data is high, the deletion unit determines, as the pixel to be deleted, the micro pixel corresponding to the laser light output among the micro pixels of the frequency modulation data, and determines the pixel indicated by the frequency modulation data. 2. The image forming apparatus according to claim 1, wherein when the density is low, among the minute pixels of the frequency modulation data, the minute pixels corresponding to the no output of the laser beam are determined as pixels to be deleted. A modulating means for converting an input pixel-based image signal into frequency-modulated data decomposed into a predetermined number of minute pixels synchronized with a high-frequency clock having a cycle of an integral multiple of the basic clock; and A laser light source that is modulated based on frequency modulation data, and a scanning unit that scans the latent image carrier with laser light emitted from the modulated laser light source. ,
By adding or deleting small pixels to the frequency modulation data of any pixel on the scanning line of the laser light, a correction step of correcting the scanning length by the laser light,
In the correcting step, when a minute pixel is deleted from the frequency modulation data, a minute pixel to be deleted from the frequency modulation data is determined according to a pixel density indicated by the frequency modulation data. A laser scanning control method for an image forming apparatus. When the pixel density indicated by the frequency modulation data is high, among the fine pixels of the frequency modulation data, the fine pixel corresponding to the laser light output is determined as a pixel to be deleted, and when the pixel density indicated by the frequency modulation data is low, 4. The laser scanning control method for an image forming apparatus according to claim 3, wherein, among the minute pixels of the frequency modulation data, the minute pixel corresponding to the no laser beam output is determined as a pixel to be deleted.

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