Abstract:
An image forming apparatus includes a light source, a light beam controlling mechanism, a sensor, and a signal controller. The light source emits parallel light beams with an angle smaller than 90° relative to a sub-scanning direction. The light beam controlling mechanism controls a scanning of the light beams. The sensor detects one of the light beams and generates a line synchronous signal. The signal controller delays the data streams and generates PLL clock signals divided into different clock signals having a same frequency and having phases sequentially varied. The signal controller selects one of the different clock signals and drives the light source with the selected clock signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The disclosed invention is directed to a method and apparatus for image forming, and more particularly to a method and apparatus for image forming that uses multiple laser beams. 
     2. Discussion of the Background 
     A high speed printing and a high resolution are increasingly demanded for electrophotographic technology and a multiple laser beam technique has recently been developed as one solution. The multiple laser beam technique typically uses a laser diode array that includes a plurality of laser diodes generally arranged in line in a direction relative to a sub-scanning direction of the laser beams. In practice, the laser diode array is mounted in an optical system in such a way that the laser diode array has an angle smaller than 90° relative to a direction corresponding to a main scanning direction of the laser beams. This is to justify a pitch of lines drawn down in a sub-scanning direction on a surface of a writing member (e.g., a photoconductive drum). 
     In connection with the above-mentioned multiple laser beam technology, several attempts are described in published Japanese unexamined patent applications No. 2000-118038, No. 06-227037, No. 06-300980, and No. 09-174924, for example. 
     However, the above-mentioned laser diode array has a drawback. That is, when the laser diode array including laser diodes is mounted with a slanting angle, the lines drawn down by the laser beams have an undesired pitch or a displacement in a main scanning direction. To eliminate such an undesired pitch in the main scanning direction, a beam detection for synchronization is required for each of the laser diodes. However, since a distance between any adjacent two of the laser diodes is relatively small, the beam detection for synchronization for each of the laser diodes cannot easily be performed. Therefore, an issue arises as to how to eliminate such an undesired pitch in the main scanning direction. 
     In addition, this laser diode array having laser diodes with a slanting angle, as described above, involves another drawback. That is, some optical systems may require a reversed slanting direction of laser diodes due to a structural reason, for example. In this case, if a beam detection on a specific laser beam (e.g., typically a laser beam of a channel 1) for synchronization is conducted, this optical system is required to perform a relatively complex control of a synchronization because the channel-1 laser beam of the laser diode array in the reversed slanting direction draws a line at an innermost position in the main scanning direction. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of the present invention is to describe a novel image forming apparatus with improvements. 
     In one example, this novel image forming apparatus includes a light source, a light beam controlling mechanism, a sensor, and a signal controller. 
     The light source includes a plurality of light emitting elements arranged in line for simultaneously emitting a plurality of parallel light beams. The light source is arranged with an angle smaller than 90° relative to a sub-scanning direction of the plurality of parallel light beams. The light beam controlling mechanism is configured to control the plurality of parallel light beams to simultaneously scan a plurality of lines on a surface of a photoconductive member in a main scanning direction. The sensor is configured to detect one of the plurality of parallel light beams and to generate a line synchronous signal upon detecting the one of the plurality of parallel light beams. The signal controller is configured to synchronize and to modulate a plurality of parallel image data streams that respectively drive the plurality of light emitting elements of the light source to emit the plurality of parallel light beams in accordance with the plurality of parallel image data streams. 
     In this novel image forming apparatus, the controller may include a plurality of FIFOs, a PLL circuit, a frequency divider, a synchronous clock generator, and a plurality of drivers. The plurality of FIFOs are configured to delay the plurality of parallel data streams, respectively. The PLL circuit is configured to generate a PLL clock signal having an integral multiple frequency of a pixel clock signal. The frequency divider is configured to divide the PLL clock signal into a plurality of clock signals having a same frequency and having phases sequentially varied. The synchronous clock generator is configured to select one of the plurality of clock signals having the same frequency and having the phases sequentially varied. The plurality of drivers are arranged and configured to drive the plurality of light emitting elements, respectively, with the one of the plurality of clock signals selected by the synchronous clock generator. 
     The light source may be a laser diode array including a plurality of laser diodes. 
     The sensor may arbitrarily be set to detect a predetermined one of the plurality of parallel light beams. 
     The predetermined one of the plurality of parallel light beams may be a light beam emitted by a light emitting element for scanning a line on the surface of the photoconductive member ahead of other light beams in the main scanning direction. 
     Another object of the present invention is to describe a novel method of image forming with improvements. 
     In one example, this novel method of image forming includes the steps of arranging, causing, detecting, inputting, providing, dividing, selecting, separating, delaying, synchronizing, modulating, and driving. 
     The arranging step arranges a light source with an angle smaller than 90° relative to a sub-scanning direction of a photoconductive member. The light source includes a plurality of light emitting elements in line. The causing step causes the plurality of light emitting elements to simultaneously emit a plurality of parallel light beams. The detecting step detects one of the plurality of parallel light beams and generates a line synchronous signal upon detecting the one of the plurality of parallel light beams. The inputting step inputs image data. The providing step provides a PLL clock signal having an integral multiple frequency of a pixel clock signal. The dividing step divides the PLL clock signal into a plurality of pixel clock signals having a same frequency and having phases sequentially varied. The selecting step selects one of the plurality of pixel clock signals having the same frequency and having the phases sequentially varied. The separating step separates the image data into a plurality of parallel data streams. The delaying step delays the plurality of parallel data streams, respectively, with the one of the plurality of pixel clock signals selected in the selecting step. The synchronizing step synchronizes the plurality of parallel image data streams with the one of the plurality of pixel clock signals selected in the selecting step. The modulating step modulates a plurality of driving signals with the plurality of parallel image data streams. The driving step drives the plurality of light emitting elements with the plurality of parallel image data streams, respectively, synchronized in the synchronizing step. 
     The arranging step may arrange a laser diode array including a plurality of laser diodes. 
     The detecting step may arbitrarily detect a predetermined one of the plurality of parallel light beams. 
     The predetermined one of the plurality of light beams may be a light beam emitted by a light emitting element for scanning a line on the surface of the photoconductive member ahead of other light beams in the main scanning direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a block diagram of an image forming apparatus according to a preferred embodiment of the present invention; 
     FIG. 2 is a schematic diagram of an optical writing system included in the image forming apparatus of FIG. 1; 
     FIG. 3 is an illustration for explaining a laser diode array including a plurality of laser diodes; 
     FIG. 4 is a circuit diagram of the laser diode array of FIG. 3; 
     FIG. 5 is a block diagram of an optical writing controller included in the image forming apparatus of FIG. 1; 
     FIG. 6 is an illustration for explaining positional displacements of laser diodes; 
     FIG. 7 is a timing diagram showing a relationship between a PLL clock signal PLLCLK and pixel clock signals WCLK 1 -WCLK 7 ; 
     FIG. 8 is a block diagram of a PLL circuit and a video signal processor of FIG. 5; 
     FIG. 9 is a timing diagram for explaining an allowable delay for synchronizing image data between writing and reading; 
     FIG. 10 is a block diagram of a PLL controller included in the PLL circuit of FIG. 8; 
     FIG. 11 is an illustration for explaining different positional displacements of the laser diodes; and 
     FIG. 12 is timing diagrams for the cases of FIG.  6  and FIG. 11 in comparison. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In describing preferred embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, an electrophotographic digital copying apparatus  1  according to a preferred embodiment of the present invention is explained. As shown in FIG. 1, the digital copying apparatus  1  includes a photoconductive drum  2 , a development unit  3 , an optical writing unit  4 , a sheet cassette  5 , a sheet feed roller  6 , a sheet guide  7 , a transfer roller  8 , a fixing unit  9 , an ejection roller  10 , and an ejection tray  11 . The digital copying apparatus  1  further includes an image sensor  12 , a document plate  13 , a document transporting mechanism  14 , a document tray  15 , and an optical writing controller  16 . 
     The development unit  3  is rotatably mounted to the digital copying apparatus  1 , and performs an image development process according to electro photography. The optical writing unit  4  generates laser light and scans the surface of the rotating photoconductive drum  2  with the laser light modulated in accordance with image data under the control of the optical writing controller  16 . The sheet cassette  5  contains recording sheets. The sheet feed roller  6  feeds a recording sheet to the sheet guide  7  that guides the recording sheet to an image transferring position between the photoconductive drum  2  and the transfer roller  8 . The transfer roller  8  transfers a toner image formed on the photoconductive drum  2  onto the recording sheet and transports the recording sheet carrying the toner image thereon to the fixing unit  9 . The fixing unit  9  fixes the toner image and sends the recording sheet having the fixed toner image thereon to the ejection roller  10 . The ejection roller  10  ejects the recording sheet onto the ejection tray  11 . 
     The image sensor  12  may be a close-contact type sensor mounted to a midpoint position of a sheet passage formed by the document transporting mechanism  14  to read an image of a document. The document plate  13  is a plate on which at least one sheet of a document to be copied is placed. The document transporting mechanism  14  transports the document from the document plate  13  to the document tray  15 . 
     When copying of a document is started, a document placed on the document plate  13  is automatically inserted into the document transporting mechanism  14 . The document is transported to the original tray  15  by the document transporting mechanism  14 . During the time the document is passing by the image sensor  12 , the image sensor  12  optically reads an image of the document and generates image data in response to the read image. The image data generated by the image sensor  12  is sent to the optical writing unit  4  that generates laser light modulated with the image data and emits the laser light onto the charged surface of the photoconductive drum  2  under the control of the optical writing controller  16 . As a result, an electrostatic latent image is formed on the photoconductive drum  2 . 
     The electrostatic latent image thus formed on the photoconductive drum  2  is developed with toner by the development unit  3  into a toner image, which is a visualized image. The toner image is then transferred onto a recording sheet by the transfer roller  8 . The recording sheet carrying the toner image thereon is then transported to the fixing unit  9  that applies pressure and heat to the toner image carried on the recording sheet so that the toner image is fixed on the recording sheet. After the fixing process, the recording sheet is ejected by the ejection roller  10  to the ejection tray  11 . Thereby, one operational cycle of the electrophotographic copying procedure is completed. In this procedure, image data may be input from an external data source (not shown), e.g. a personal computer, through a data cable (not shown). 
     FIG. 2 illustrates an exemplary structure of the optical writing unit  4 . As illustrated in FIG. 2, the optical writing unit  4  includes a laser diode array device  20 , a collimate lens  21 , an aperture  22 , a cylindrical lens  23 , a polygon mirror  24 , a pair of fθ lenses  25 , a correction lens  26 , a mirror  27 , a synchronous mirror  28 , and a synchronous detection sensor  29 . In FIG. 2, a direction parallel to an axis of the photoconductive drum  2  is referred to as a main scanning direction and a direction perpendicular to the axis of the photoconductive drum  2  is referred to as a sub-scanning direction. 
     As illustrated in FIG. 3, the laser diode array device  20  includes a plurality of light emitting elements, e.g., four-channel LDs (laser diodes)  30 , arranged in line. In this example, the LDs  30  are independently controlled to emit laser beams B 1 -B 4  that are modulated according to image data. As illustrated in FIG. 3, the laser diode array device  20  has a surface  20   a  in which the LDs  30  are arranged in line with a predetermined distance between any adjacent two of the LDs  30 . FIG. 4 shows an electric circuit diagram of the laser diode array device  20 . As shown in FIG. 4, the laser diode array device  20  further includes a photo receiving element  31  (hereinafter referred to as a PD  31 ). 
     The laser beams B 1 -B 4  modulated according to image data are formed into predetermined beam shapes through the collimate lens  21 , the aperture  22 , and the cylindrical lens  23 , and are transmitted to the polygon mirror  24 . More specifically, the collimate lens  21  makes the laser beams B 1 -B 4  parallel, and the aperture  22  that has a slit formed in accordance with a writing density shapes the parallel laser beams B 1 -B 4  by cutting out undesired beam portions. The cylindrical lens  23  brings each of the laser beams B 1 -B 4  into focus so that each of the laser beams B 1 -B 4  will have a predetermined beam diameter to form a spot of a predetermined size on the surface of the photoconductive drum  2 . The cylindrical lens  23  transmits the thus focused laser beams B 1 -B 4  to the polygon mirror  24 . 
     The laser beams B 1 -B 4  impinge on the polygon mirror  24 , which is rotated at a predetermined speed, and are transmitted to the mirror  27  so that the laser beams B 1 -B 4  scan the mirror  27 . The mirror  27  is fixed with an angle relative to the propagation direction of the laser beams B 1 -B 4  so as to reflect the laser beams B 1 -B 4  towards the surface of the photoconductive drum  2 . Thereby, the laser beams B 1 -B 4  scan the surface of the photoconductive drum  2  in the main scanning direction X. During this process, after being reflected by the polygon mirror  24 , the laser beams B 1 -B 4  propagating at a constant angular speed are converted into beams propagating at a constant speed by the pair of the fθ lenses  25  and are corrected by the correction lens  26 . After that, the laser beams B 1 -B 4  have their directions changed towards the photoconductive drum  2  by the mirror  27 . 
     When the laser beams B 1 -B 4  are caused to scan the surface of the photoconductive drum  2 , they lay down four trails, having a predetermined pitch between any adjacent two laser beams, on the surface of the photoconductive drum  2  in the sub-scanning direction Y. 
     The synchronous mirror  28  is, as shown in FIG. 2, mounted in a laser beam passage at a starting edge of a main scanning delta out of an image writing zone. The position of the synchronous mirror  28  is adjustable to selectively receive one of the laser beams B 1 -B 4 . By this arrangement, one of the laser beams B 1 -B 4  is caused to impinge on the synchronous mirror  28  and the reflected of that one laser beam is detected by the synchronous detection sensor  29  each time before the scanning of the surface of the photoconductive drum  2 . Upon detecting the one of the laser beams B 1 -B 4 , the synchronous detection sensor  29  generates a synchronous detection signal SYNC that is used to determine a start time of image writing in the main scanning direction X. 
     FIG. 5 shows an exemplary configuration of the optical writing controller  16 . As shown in FIG. 5, the optical writing controller  16  includes a video signal processor  40 , pulse width modulators (PWMs)  41   a - 41   d,  laser diode drivers (LDDs)  42   a - 42   d,  an APC (automatic power control)  44 , a PLL (phase-locked loop) circuit  45 , and digital-to-analog converters  47   a - 47   d.  In FIG. 5, the laser diode array device  20  is referred to as an LDA (laser diode array) and the synchronous detection sensor  29  is referred to as an SDS (synchronous detection sensor). 
     Based on the synchronous detection signal SYNC output by the SDS  29 , the video signal processor  40  of the optical writing controller  16  starts to receive image data for four lines with an associated clock signal CLK-ID from an image processing section (not shown). The video signal processor  40  has internal line memories (not shown) and stores the received image data for four lines into the internal line memories, and outputs the image data for four lines at the same time to the PWMs  41   a - 41   d  in synchronism with the rotation of the polygon mirror  24 . 
     The PWMs  41   a - 41   d  generate signals having pulse widths modulated in accordance with the image data for the respective four lines and output the pulse modulated signals to the LDDs  42   a - 42   d,  respectively. The LDDs  42   a - 42   d  then drive the four-channel LDs  20   a - 20   d,  respectively, of the LDA  20  in accordance with the pulse modulated signals. The APC  44  determines driving voltages for driving the LDDs  42   a - 42   d  based on a signal from the PD  31  of the LDA  20  and a control signal CONT 1  sent from the video signal processor  40 , and applies the determined driving voltages to the LDDs  42   a - 42   d.    
     When the video signal processor  40  transmits the image data to the PWMs  41   a - 41   d  based on the synchronous detection signal SYNC sent from the SDS  29 , as described above, the video signal processor  40  also generates a basic timing signal CONT 2  used by the image processing section (not shown) for its transmission of the image data for four lines to the video signal processor  40 . The DACs  47   a - 47   d  control the LDDs  42   a - 42   d,  respectively, in accordance with the signals sent from the video signal processor  40  to control light amounts of the LDs  20   a - 20   d,  respectively. The PLL circuit  45  receives a reference PLL clock signal REFCLK from a clock signal generator (not shown) and the synchronous detection signal SYNC generated by the SDS  29 . Based on these signals, the PLL circuit  45  generates pixel clock signals WCLKa-WCLKd (see FIG. 8) for synchronizing the image data input to the PWMs  41   a - 41   d.    
     FIG. 6 shows positional displacements on the surface of the photoconductive drum  2  in connection with data channels CH 1 -CH 4  of the laser beams B 1 -B 4 . In FIG. 6, data channels CH 1 -CH 4  represent data lines in the main scanning direction X scanned by the LDs  20   a - 20   d,  respectively. As shown in FIG. 6, the channels CH 1  and CH 2  have a displacement X 1 , the channels CH 1  and CH 3  have a displacement X 2 , and the channels CH 1  and CH 4  have a displacement X 3 . 
     FIG. 7 shows a timing chart of a PLL basic clock signal PLLCLK and the pixel clock signals WCLK 1 -WCLK 7 . In this case, the PLL circuit  45  sequentially generates pixel clock signals WCLK 1 -WCLK 7  with a one-clock delay relative to PLLCLK to divide a frequency of the PLL basic clock signal into one-eighth the original frequency. One of the pixel clock signals WCLK 1 -WCLK 7  is arbitrarily selected according to an amount of the above-mentioned positional displacements X 1 -X 3 . 
     FIG. 8 shows an exemplary configuration of the PLL circuit  45  and a PWM (pulse width modulation) control portion of the video signal processor  40 . As shown in FIG. 8, the PLL circuit  45  includes a PLL (phase-locked loop) controller  201 , a frequency divider  202 , and a synchronous clock generator  203 . The PWM control portion of the video signal processor  40  includes an image data controller  204  and a FIFO (first-in and first-out) circuit  205  that includes FIFOs  205   a - 205   d . In the PLL circuit  45 , the PLL controller  201  receives the reference PLL clock signal REFCLK and generates a clock signal PLLCLK that is a frequency-multiplied signal with a VCO (voltage controlled oscillator)  2016  (see FIG. 10) included in the PLL controller  201 . The frequency divider  202  divides the clock signal PLLCLK into 1/n, in which n is an integer, and generates a clock signal CLKA that has a 1/n frequency of the clock signal PLLCLK. The synchronous clock generator  203  receives the clock signal PLLCLK from the PLL controller  201 , the clock signal CLKA from the frequency divider  202 , and the synchronous detection signal SYNC from the SDS  29 , and generates the pixel clock signals WCLK 1 -WCLK 7  that sequentially have a phase delay by a clock of the clock signal PLCLK. That is, the pixel clock signals WCLK 1 -WCLK 7  and the clock signal CLKA have the same frequency but have different phases. 
     One of the pixel clock signals WCLK 1 -WCLK 7  is selected based on the synchronous detection signal SYNC and is output as write clock signals WCLKa-WCLKd from the synchronous clock generator  203  to drive the PWMs  41   a - 41   d  to correct the positional displacements X 1 -X 3 . In the video signal processor  40 , the image data and the associated clock signals sent from the image processing section (not shown) are separated into write data WDATA 1 -WDATA 4  for the channels CH 1 -CH 4 , respectively, gated with the clock signal CLKA from the frequency divider  202 . The write data WDATA 1 -WDATA 4  of the channels CH 1 -CH 4  are then input to the FIFOs  205   a - 205   d  gated with the clock signal CLKA from the frequency divider  202  and write enable signals WE 1 -WE 4  sent from the image data controller  204 . 
     The image data are read out from the FIFOs  205   a - 205   d  with the write clock signals WCLKa-WCLKd and read enable signals RE 1 -RE 4  sent from the FIFOs  205   a - 205   d , respectively. 
     With the above-described structure, as shown in FIG. 9, a delay in unit of pixels can be set within a time period T 1 , which is a difference of the rise times between the write enable signal WE and the read enable signal RE. In addition, a phase delay in unit of ⅛ of a dot can be set within a time period T 2  by a selection of the write clock signals WCLKa-WCLKd. A total delay to correct an amount of the positional displacement can be set within a total delay T 3  composed of the time periods T 1  and T 2 . 
     FIG. 10 shows an exemplary structure of the PLL controller  201  of the PLL circuit  45 . As shown in FIG. 10, the PLL controller  201  includes frequency dividers  2011  and  2012 , a phase frequency detector (PFD)  2013 , a charge pump (CP)  2014 , a loop filter (LF)  2015 , and the above-mentioned VCO  2016 . The reference PLL clock signal REFCLK is divided into a 1/p clock signal by the frequency divider  2011 , in which p is an interger. The 1/p clock signal is input to the PFD  2013  and is compared with a 1/q clock signal of the PLLCLK from the frequency divider  2012 , in which q is an integer. The PFD  2013  outputs a signal representing a phase difference between the two clock signals and is converted into an analog signal by the CP  2014 . The analog signal output by the CP  2014  is input to the VCO  2016 , which oscillates in accordance with the input analog voltage and generates the clock signal PLLCLK. 
     FIG. 11 shows a case in which the laser beam B 4  of the channel CH 4  is ahead in the main scanning direction. In this case, the laser beam detection performed by the synchronous detection sensor  29  is made on the laser beam B 4  of the channel CH 4 . 
     FIG. 12 shows two timing charts A 1  and A 2  in comparison. The timing chart A 1  represents the case of the operation in which the laser beam B 1  of the channel CH 1  is ahead in the main scanning direction, as shown in FIG. 6, and the time chart A 2  represents the case of the operation in which the laser beam B 4  of the channel CH 4  is ahead in the main scanning direction, as shown in FIG.  11 . Thus, the synchronous detection sensor  29  is arbitrary set to detect a previously determined one of the laser diodes  30  for the channels CH 1 -CH 4 . 
     Numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. 
     This patent specification is based on Japanese patent application No. JPAP2001-063696 filed on Mar. 7, 2001, in the Japanese Patent Office, the entire contents of which are hereby incorporated by reference herein.