Patent Publication Number: US-8531497-B2

Title: Image forming apparatus and control method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus and a control method thereof. 
     2. Description of the Related Art 
     Recently, in the field of image forming apparatuses using electrophotographic technology, there are constant demands for reductions in size and cost. To realize such reductions in size and cost, a method has been discussed (see Japanese Patent Application Laid-Open No. 07-175005) which uses a galvanomirror fabricated by a semiconductor fabrication technique instead of a conventionally-used polygonal mirror. In this method, an image is formed by making the mirror resonate at a specific resonant frequency which is based on the mechanical dimensions of the galvanomirror, and by scanning a light beam in the main scanning direction. 
     Further, for a nested mirror (Japanese Patent Application Laid-Open No. 2005-208578), there are the qualities that the available scanning area is considered a constant angular velocity, and that the scanning angle can be made larger. As a result, a correction optical system can be made to have a compact and simple structure, which is suitable for a scanning apparatus in a compact, low-cost image forming apparatus. 
     If a light beam is deflected using a technique such as that described above to make a vibrating mirror resonate, wobbles occur in the resonance due to turbulence or the like caused by air resistance during the resonance operation. The wobbles can produce non-periodic jitter. 
     This jitter becomes apparent as angular velocity jitter of the vibrating mirror and image forming position jitter in the main scanning direction such as that illustrated in  FIG. 2A , which causes a difference in the width of the main scanning direction. This results in shake in the straight lines of the sub-scanning direction at the center and at the edges on the transfer medium, so that image quality deteriorates. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an image forming apparatus which predicts the non-periodic jitter of each scan, corrects according to the prediction, and satisfactorily holds an image forming position of the sub-scanning direction at the center and at the edges on the transfer medium during image formation. 
     According to an aspect of the present invention, an image forming apparatus includes a light beam output unit configured to output a light beam, a deflection unit for deflection scanning in a main scanning direction of a photosensitive member by reflecting the light beam from the light beam output unit, a timing information detection unit configured to detect timing information of the deflection scanning by the deflection unit, a calculation unit configured to calculate a correction amount of the main scanning direction for a next scan based on the timing information, a light beam modulation control unit configured to generate a light beam modulation signal based on image data and the correction amount, and a drive unit configured to drive the light beam output unit based on the light beam modulation signal. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  is a diagram illustrating a configuration example of an image forming apparatus according to an exemplary embodiment of the present invention. 
         FIGS. 2A and 2B  are diagrams respectively illustrating the degradation in image quality due to the jitter of a vibrating mirror, and the effects from an exemplary embodiment of the present invention. 
         FIG. 3  is a diagram for describing the relationship between change in scanning position over time, and timing information and scanning line length according to a first exemplary embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating one example of the processing according to the first exemplary embodiment of the present invention. 
         FIG. 5  is a diagram illustrating a correction example through the insertion of a pixel according to the first exemplary embodiment of the present invention. 
         FIG. 6  is a diagram illustrating a correction example through the deletion of a pixel according to the first exemplary embodiment of the present invention. 
         FIG. 7  is a diagram illustrating the relationship between phase difference φ and angular velocity θ′ according to a third exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention are described in detail below with reference to the drawings. 
     First Exemplary Embodiment 
       FIG. 1  is a diagram illustrating a configuration example of an image forming apparatus  20  according to an exemplary embodiment of the present invention. The image forming apparatus  20  includes a controller  21  and an image forming unit  22 . 
     The controller  21  controls the whole apparatus by a not-illustrated central processing unit (CPU), and generates image data which can be output by the image forming unit  22  from print data received from an external personal computer (PC)  10 . The image forming unit  22  develops an electrostatic latent image which was exposed on a photosensitive drum  226 , transfers the developed image to a transfer medium, and performs transportation processing to output the formed image. 
     First, an image generation unit  211  in the controller  21  analyzes print data that the controller  21  received from a PC  10 , performs image processing, and generates image data. The generated image data is output to a light beam drive unit  212  from the image generation unit  211  based on a requested timing of a vertical synchronizing signal output from the image forming unit  22 . 
     A target value storage unit  213  stores a target value to be utilized in calculating a correction amount which is output by a correction amount prediction unit  215 . While in the present exemplary embodiment the scanning time (timing information) of the main scanning direction is stored as the target value, the target value may be other information which can be utilized in predicting a correction amount, such as the resonant frequency of the mirror, main scanning line interval, and correction amount per scan. 
     A timing information detection unit  214  outputs timing information to the correction amount prediction unit  215  using a horizontal synchronizing signal output from the image forming unit  22 . 
     The correction amount prediction unit  215  converts the timing information output by the timing information detection unit  214  into a parameter representing an operation of a vibrating mirror  224 , and calculates a correction amount prediction value from the converted parameter and a target value which is stored in the target value storage unit  213 . Based on the calculated correction amount prediction value, a modulation correction amount of the light beam is output to the light beam modulation control unit  216 . 
     The light beam modulation control unit  216  outputs a light beam modulation signal for modulating the light beam to a light beam drive unit  212  based on the image data output from the image generation unit  211  and the modulation correction amount output from the correction amount prediction unit  215 . By inserting or deleting a small pixel piece according to the correction amount, the light beam modulation control unit  216  partially or fully expands or contracts the scanning line length of the main scanning direction to adjust the drawing time. 
     Further, the adjustment method of the scanning line length of the main scanning direction is not limited to the above-described example. The adjustment method may also be performed by changing the clock frequency, which becomes a standard when drawing the image data, for all or part of the main scanning. However, if the clock frequency is changed using a technique such as a programmable phase-locked loop (PLL), the PLL is locked after the frequency change control has been performed. As a result, there is a time delay until the frequency is changed, or the time until being locked is indefinite. Thus, an adjustment method which inserts/deletes the above-described small pixel piece according to the pixel location is more suitable. 
     The light beam drive unit  212  drives a light beam output unit  225  of the image forming unit  22  according to the light beam modulation signal designated by the light beam modulation control unit  216 . A vertical synchronizing signal generation unit  222  of the image forming unit  22  outputs a vertical synchronizing signal for synchronizing the writing-start position in the sub-scanning direction of the photosensitive drum  226  to the image generation unit  211 . A horizontal synchronizing signal generation unit  221  outputs a horizontal synchronizing signal based on light beam detection information from a writing-start-side light beam timing detection unit  227  and a writing-end-side light beam timing detection unit  228  that are located proximal to the photosensitive drum  226 . The horizontal synchronizing signal is input to the image generation unit  211 , timing information detection unit  214 , and vibrating mirror drive unit  223 . 
     The vibrating mirror drive unit  223  drives the vibrating mirror  224 . The vibrating mirror  224  reflects alight beam irradiated from the light beam output unit  225  and deflection-scans the light beam in a main scanning direction. The drive method of the vibrating mirror  224  may be electrostatic, electromagnetic, bimetal, piezoelectric, a combination of these, or other drive method. 
     The light beam output unit  225  makes the light beam blink using a light beam drive signal received from the light beam drive unit  212 . The blinking light beam is reflected by the vibrating mirror  224  and passes through a constant linear velocity conversion optical system  229 . The photosensitive drum  226  is scanned with the blinking light beam so that the photosensitive drum  226  is exposed. 
     The writing-start-side light beam timing detection unit  227  is a detection unit which detects the start of light beam scanning on the photosensitive drum  226 , and outputs a light beam detection signal to the horizontal synchronizing signal generation unit  221 . Further, the writing-end-side light beam timing detection unit  228  is a detection unit which detects the end of light beam scanning on the photosensitive drum  226 , and outputs a light beam detection signal to the horizontal synchronizing signal generation unit  221 . 
     Next, the predicted correction processing of the non-periodic jitter of each scan in a main scanning direction according to the exemplary embodiment of the present invention is described with reference to  FIGS. 3 and 4 .  FIG. 3  is a diagram describing the relationship between the change in scanning position over time made by the vibrating mirror  224 , and the timing information and scanning line length detected by the timing detection units  227  and  228 .  FIG. 4  is a flowchart of the processing according to the present exemplary embodiment. The processing corresponding to  FIG. 4  is executed based on a processing program corresponding to the respective processing units of  FIG. 1 . 
     In  FIG. 3 , the horizontal axis represents time, and the vertical axis represents scanning position. This scanning position corresponds to the angle θ formed between the vibrating mirror  224  and the photosensitive drum  226 . In  FIG. 3 , the light beam at times ta and tb is detected by the writing-start-side light beam timing detection unit  227 . Further, the light beam at times tc and td is detected by the writing-end-side light beam timing detection unit  228 . 
     In step S 401  of  FIG. 4 , the timing information detection unit  214  detects timing information based on the detected light beam for an n-th scan. The timing information detection unit  214  outputs the timing information to the correction amount prediction unit  215 . 
     The timing information t 1 n, t 2 n, and t 3 n for an n-th scan (n being a natural number) is determined by t 1 n =tb−ta, t 2 n=tc−ta, and t 3 n=td−ta. Further, the elapsed time from the second detection of the light beam at time tb by the writing-start-side light beam timing detection unit  227  shall be denoted as tαn. 
     The timing information is generated as follows. First, light beam detection information is output to the horizontal synchronizing signal generation unit  221  from the writing-start-side light beam timing detection unit  227  and writing-end-side light beam timing detection unit  228  of an n-th scan, which is the current scan. The horizontal synchronizing signal generation unit  221  outputs a horizontal synchronizing signal to the timing information detection unit  214  based on the light beam detection information. The timing information detection unit  214  generates timing information using the horizontal synchronizing signal, and outputs the generated timing information to the correction amount prediction unit  215 . 
     In step S 402 , the correction amount prediction unit  215  calculates a correction amount in the following manner based on the timing information and a target value for calculating the correction amount stored in the target value storage unit  213 . 
     In the present exemplary embodiment, letting the main scanning drawing period be ω, when driving the vibrating mirror  224  by a composite wave of a sine wave of angular velocity ωand a sine wave of angular velocity 2ω, the angle θ formed between the vibrating mirror  224  and the photosensitive drum  226  can be expressed using ω as follows.
 
θ=− A 1 sin(ω t )− A 2 sin(2ω t )  (1)
 
     Here, the coefficient A1 is the maximum wave amplitude of the sine wave of angular velocity ω, and the coefficient A 2  is the maximum wave amplitude of the sine wave of angular velocity 2ω. The curve  301  in  FIG. 3  corresponds to equation (1). In the present exemplary embodiment, beam control is performed utilizing the linear change over the section between time tb and time tc. 
     However, in actual control, jitter occurs in the angle of the vibrating mirror  224  due to air resistance and other factors, so that differences ΔA 1  and ΔA 2  between the respective target values and the respective actual values occur, and also so that a phase difference φ occurs. ΔA 1  is the difference between a target value A 1  and the actual value A 1 ′, ΔA 2  is the difference between a target value A 2  and the actual value A 2 ′, and φ is the phase difference between the sine wave of angular velocity ω and the sine wave of angular velocity 2ω. Differences ΔA 1  and ΔA 2  and phase difference φ are determined by a calculation performed by the correction amount prediction unit  215  based on the above-described timing information. 
     At the correction amount prediction unit  215 , the timing information differences Δt 1 n, Δt 2 n, and Δt 3 n are determined based on the drawing timing information t 1 n, t 2 n, and t 3 n of an n-th scan and the target timing information t 1 , t 2 , and t 3  when controlled by the targeted maximum amplitudes A 1  and A 2 .
 
 Δt 1 n=t 1 n−t 1
 
 Δt 2 n=t 2 n−t 2
 
 Δt 3 n=t 3 n−t 3  (2)
 
     Using the obtained differences Δt 1 n, Δt 2 n, and Δt 3 n, the correction amount prediction unit  215  determines errors ΔA 1 n, ΔA 2 n, and φn with the target value of an n-th scan from the following matrix calculation. 
     
       
         
           
             
               
                 
                   
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     The matrix M is a matrix representing the change in the time taken for a light beam to pass through the light beam timing detection units  227  and  228  when a control parameter including any of maximum amplitudes A 1 , A 2  or phase difference φ is slightly changed from the target value. The matrix M can be expressed as follows in terms of the time ta at which θ=θ0 and the target timing information t 1 n, t 2 n, and t 3   n.    
     
       
         
           
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     From the thus-determined errors ΔA 1 n, ΔA 2 n, and φn, the angle θ (t) can be expressed based on equation (1) as follows.
 
θ( t )=−( A 1+Δ A 1 n )sin(ω t )−( A 2+Δ A 2 n )sin(2ω t+φn)   (4)
 
     From equation (4), the angular velocity θ′ (t) of the vibrating mirror  224  at time t can be determined as follows.
 
θ′( t )=−( A 1+Δ A 1 n )ω cos(ω t )−2( A 2+Δ A 2 n )ω cos(2ω t+φn )  (5)
 
     Next, letting the angle formed between the vibrating mirror  224  and the photosensitive drum  226  when the writing-start-side light beam timing detection unit  227  detects the scanning start timing be θ0, the time t 0 (n+1) from t=0 until the light beam is detected by the writing-start-side light beam timing detection unit  227  (for the waveform of the composite wave drawing the n+1-th scan) can be determined from the following equation.
 
θ0=−( A 1+ ΔA 1 n )sin(ω t 0( n+ 1))−( A 2+Δ A 2 n )sin(2ω t 0( n+ 1)+φ n )  (6)
 
     Using the t 0 (n+1) determined by equation (6), the t 1 (n+1) of n+1-th scan, and the tα(n+1), an arbitrary time t can be expressed as follows.
 
 t=t 0( n+ 1)+ t 1( n+ 1)+ tα ( n+ 1)  (7)
 
     Based on equations (5) and (7), the respective angular velocities θ′(tα(n+1)) can be determined as follows.
 
θ′( tα ( n+ 1))=( A 1+Δ A 1 n )ω cos(ω( t 0( n+ 1)+ t 1( n+ 1)+ tα ( n+ 1)))+2( A 2+Δ A 2 n )ω cos(2ω( t 0( n+ 1)+ t 1( n+ 1)+ t α( n+ 1))+φ n )  (8)
 
     Here, letting an ideal angular velocity (target angular velocity) when no error occurs in the angular velocity be θ′ ideal, and the drawing time per pixel at such time be tpix_ideal (first drawing time), if there is an error in the angular velocity, then to align the drawing area of one pixel with the ideal case, the drawing time tpix_α (second drawing time) per pixel in the time tα(n+1) has to satisfy the following equation.
 
θ′( tα ( n+ 1))· tpix _α=θ′ideal· tpix _ideal  (9)
 
     Further, based on the difference between tpix_α for resolving the error and the actual drawing time tpix_ideal per pixel, the interval into which a pixel piece is inserted/deleted can be decided. Here, this difference can be expressed as in the following equation (10).
 
 tpix   —   α−tpix _ideal=(θ′ideal· tpix _ideal)/θ′( tα ( n+ 1)− tpix _ideal= tpix _ideal(θ′ideal/θ′( tα ( n+ 1))−1)  (10)
 
     In the present exemplary embodiment, an interval Pi into which a pixel piece is inserted/deleted based on equation (10) can be determined as a function of tα(n+1) as follows.
 
 Pi=θ′ ( tα ( n+ 1))/(θ′ideal−θ′( tα ( n+ 1)))  (11)
 
     In this manner, the correction amount prediction unit  215  can calculate the interval into which a pixel piece is deleted or inserted as a correction amount. 
     Next, in step S 403 , based on the calculated correction amount and the image data provided from the image generation unit  211 , the light beam modulation control unit  216  generates a light beam modulation signal. The light beam modulation control unit  216  partially or fully expands or contracts the scanning line length of the main scanning direction to adjust the drawing time by inserting or deleting a pixel according to the correction amount. A specific example of the processing in the light beam modulation control unit  216  is described next with reference to  FIGS. 5 and 6 . The remainder of  FIG. 4  is also described below. 
     In  FIG. 5 , as one example, a case where tpix_α-tpix_ideal=tpix_ideal/15 for a given drawing area is illustrated. Specifically, the actual drawing time tpix_ideal of one pixel is only tpix_ideal/15 shorter than the tpix_α for resolving an error in the case where an error has occurred. Therefore, the correction magnification of the main scanning direction is 16/15=1.07. Assuming the minimum pixel piece is a size of 1/8 of a pixel (tpix_ideal/8), since the magnification of the drawing area can be adjusted by inserting 8 pixel pieces (one pixel amount) per 15 pixels, the pixel insertion interval is every 15 pixels. 
     Further, in  FIG. 6 , as one example, a case where tpix_α-tpix_ideal=−tpix_ideal/16 for a given drawing area is illustrated. Specifically, the actual drawing time tpix_ideal of a pixel is only tpix_ideal/16 longer than the tpix_α for resolving the error in the case where an error has occurred. Therefore, the correction magnification of the main scanning direction is 15/16=0.94. Assuming the minimum pixel piece is a size of 1/8 of a pixel (tpix_ideal/8), since the drawing area magnification can be adjusted by deleting 8 pixel pieces (one pixel amount) per 16 pixels, the pixel deletion interval becomes every 16 pixels. 
     Although in  FIGS. 5 and 6  cases where a pixel piece was inserted/deleted as a whole one pixel amount were described, the insertion/deletion may also be carried out by dividing up into units of pixel pieces. 
     In step S 404  of  FIG. 4 , based on the thus-generated light beam modulation signal, the light beam drive unit  212  generates a light beam drive signal, and outputs the signal to the light beam output unit  225  to drive the light beam output unit  225 . In step S 405 , the light beam output unit  225  outputs the light beam to the vibrating mirror  224  according to the fed light beam drive signal, and performs exposure processing of the photosensitive drum  226  via the vibrating mirror  224 . 
     The adjustment of the magnification may also be realized by adjusting through increasing/decreasing the video clock frequency rather than inserting/deleting a pixel piece. 
     In this manner, interpolation/deletion intervals of the pixel vicinity for the next n+1-th scan can be decided based on the drawing timing information of the current n-th scan and the target value. By adjusting the magnification in this manner, image distortion due to the jitter of a vibrating mirror like that illustrated in  FIG. 2A  can be corrected, so that a good image like that illustrated in  FIG. 2B  can be obtained. 
     Second Exemplary Embodiment 
     While in the above-described first exemplary embodiment the magnification was determined by an equation, the magnification may also be determined by a configuration in which the properties of the vibrating mirror are measured in advance, the relationship between the measurement results and φ is retained as data, and the drive is corrected based on this data. 
     For example, dividing the main scanning direction into s-pieces, and letting the magnification at each area be a1 to as, the magnification coefficient at an area can be expressed as follows using k 1  to ks and a constant.
 
 ai=a 0+ ki·φ ( i= 1 to  s )
 
     Using the determined partial magnifications a 1  to as, a good image can be obtained by correcting the pixel width of the scanning area through pixel piece insertion/deletion or adjustment of the video clock. 
     Third Exemplary Embodiment 
     In the above-described second exemplary embodiment, although a partial magnification coefficient is retained for each area, the partial magnification coefficient may also be considered as a proportion of the main scanning direction.  FIG. 7  illustrates the relationship between the angular velocity θ′ and the phase difference φ in an available scanning area. Assuming that the angular velocity θ′ in the available scanning area can be approximated by a straight line, the relationship between a position x of a main scanning direction and a correction magnification a can be expressed as in the following equation using a proportional coefficient k of the phase difference φ.
 
 a ( x )= a 0+ k·φ·x  
 
     Using this a(x), a good image can be obtained by correcting the total magnification/partial magnification through pixel piece insertion/deletion of a pixel width of the scanning area, or adjustment of the video clock. 
     Other Exemplary Embodiments 
     The present invention may be applied to a system configured from a plurality of devices (such as, for example, a host computer, an interface device, a reader, another reader, and other computer devices and/or peripherals), as well as a system configured from one device (such as, a computer, a copying machine, a facsimile machine, or other processing device). 
     The present invention can also be achieved by feeding a storage medium storing a computer program code of a software program for realizing the above-described functions to a system, and having this system read and execute the program code. In this case, the storage medium storing this program code, wherein the program code itself read from the storage medium executes the functions of the above-described exemplary embodiments, constitutes an embodiment of the present invention. Further, based on an instruction of that program code, an operating system (OS) or other supporting program running on the computer may perform part or all of the actual processing, and the above-described functions may thus be executed by such processing. 
     In addition, the program code read from the storage medium may be written in a memory provided on a function expansion card inserted into the computer or a function expansion unit connected to the computer. Based on an instruction of that program code, a CPU or other processor provided on the function expansion card or function expansion unit can perform part or all of the actual processing, and the above-described functions may thus be executed. 
     A storage medium that stores a program code corresponding to an embodiment of the present invention, which embodiment may be that disclosed in the flowchart described above for example, serves as an embodiment of the present invention. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures, and functions. 
     This application claims priority from Japanese Patent Application No. 2007-168085filed Jun. 26, 2007, which is here by incorporated by reference herein in its entirety.