Patent Publication Number: US-6219085-B1

Title: Method and system for improved performance of adjustable printer clocks in an electrophotographic device

Description:
RELATED PATENT APPLICATION 
     The present patent application is related to a copending application U.S. Ser. No. 09/024,476 filed on Feb. 17, 1998, entitled “METHOD AND SYSTEM FOR THE MODIFICATION OF THE TIMING OF A PLURALITY OF PEL CLOCKS IN AN ELECTROPHOTOGRAPHIC DEVICE”. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to data processing systems and, in particular, to electrophotographic reproduction devices. Still more particularly, the present invention relates to improving performance of adjustable printer clocks in an electrophotographic reproduction device. 
     2. Description of the Related Art 
     Electrophotographic reproduction devices can generally be divided into copiers or printers. The present invention relates primarily to printers. 
     In an electrophotographic printer, a charged photoconductor is selectively discharged by the operation of a print or imaging station, to provide an electrostatic latent image on the photoconductor&#39;s surface. This latent image corresponds to the visual image that is to be printed, first by applying toner to the photoconductor, and then by transferring the toner image to the surface of substrate material such as a sheet of plain paper. 
     Electrophotographic reproduction devices may be constructed to apply toner to either the photoconductor&#39;s discharged area, or to the photoconductor&#39;s charged area. The former type of device is called a discharged area developing device, whereas the latter device is called a charged area developing device. 
     The broad spirit and scope of the invention are not to be limited to a scanning light beam since, as will be appreciated by those of skill in the art, such a scanning beam generally comprises a moving point or spot of light, that can be modulated in intensity—for example, on and off, to form an electrostatic latent image on the photoconductor. 
     Thus, the term scanning laser beam, as used herein, is intended to mean any moving point of light to which the photoconductor is sensitive, and which operates to sequentially print the small picture elements, or PELs, of one or more image rows, as the point of light sequentially scans the photoconductor, image row after image row. 
     In printers of this type, the image to be printed comprises an electronic image signal that may, for example, reside in the page memory of a data processing system. In this page memory, each photoconductor PEL area that is to be discharged may, again by way of example, be represented by a binary “1”, in which case each PEL this is to be left in its charged state would be represented by a binary “0”. As the spot of light moves across a photoconductor PEL row, the row content of the page memory signal is gated utilizing a PEL clock to control or modulate the spot of light in accurate synchronism with the position of the moving spot of light. 
     The art has recognized the need to synchronize the gating of the print data signal to laser beam modulator means as a function of the beam&#39;s position prior to each photoconductor scan. For example, it is known to use a photosensor that is located on an image plane that is established by the physical location of the photoconductor. As a result of the detection of the laser beam at this start of scan position, a timing pulse, or beam detection indicator, is generated to start the flow of print data to a beam modulator. 
     Imaging stations used in electrophotographic printers often utilize more than one scanning laser beam. In systems utilizing two beams, for example, each beam scans a different PEL row approximately concurrently so that two PEL rows are simultaneously scanned. Therefore, two beam detection indicators are generated, one for each beam. A fixed time offset between a first pel of a first beam and a first pel of a second beam may be equivalent to an offset of several tens of pels on order to avoid interference between the adjacent beams. 
     Print quality is at least partially determined by the sensitivity of the printer to adjustments and/or modifications to the print start location of each beam, as well as to the size and length of a PEL. For duplex printers, adjustments and/or modifications may need to be made to the print start location of each beam, and size and length of a PEL to compensate for paper shrinkage which occurs after printing the front size of the duplex-printed page. 
     Therefore a need exists for a method and system in an electrophotographic reproduction devices for improving performance of adjustable printer clocks. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the present invention to provide an improved data processing system. 
     It is another object of the present invention to provide an improved electrophotographic reproduction device. 
     It is yet another object of the present invention to provide a method and system in an electrophotographic reproduction device for improving performance of adjustable printer clocks. 
     The foregoing objects are achieved as is now described. A method and system are disclosed for improving performance of adjustable printer clock signals in an electrophotographic device. A source clock signal is output having a source frequency. A first clock signal is output having a first frequency. The first frequency is substantially related to the source frequency by a ratio of n 1 :m 1 . An alignment signal is generated for adjusting a first printer clock signal with respect to a second printer clock signal. In response to the first clock signal and the alignment signal, the first printer clock signal is output having a first printer frequency. The first printer frequency is substantially related to the source clock frequency by a ratio of n 1 n 2 :m 1 m 2 . 
     The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features are set forth in the appended claims. The present invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of a preferred embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts a pictorial representation of an electrophotographic printer embodying the present invention; 
     FIG. 2 illustrates a pictorial representation of a scanning laser imaging station of the printer of FIG. 1 in accordance with the method and system of the present invention; 
     FIGS. 3A and 3B illustrate a pictorial representation of a print receiving material, such as paper, two “printed” PEL rows of an image, and a printed measurement device in accordance with the method and system of the present invention; 
     FIG. 4 is a block diagram of a circuit for generating a plurality of PEL reduced-jitter clocks where the timing of each of the clocks is finely adjustable in accordance with the method and system of the present invention; 
     FIG. 5 is a timing diagram of selected signals generated by the device of FIG. 4 in accordance with the method and system of the present invention; and 
     FIG. 6 is a high level flow chart illustrating the printing of PELs utilizing finely adjustable printer clock signals in accordance with the method and system of the present invention. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention and its advantages are better understood by referring to FIGS. 1-6 of the drawings, like numerals being used for like and corresponding parts of the accompanying drawings. 
     This invention will be illustrated within the framework of electrophotographic devices wherein prints are produced by sequentially creating an image of the print subject on a photoconductor surface, developing the image with toner, transferring the toner image to print receiving material, and then fusing the toner image to the print receiving material. In most electrophotographic machines, the electrophotographic process is of the transfer type, wherein photoconductor material is placed around a rotating drum, or is arranged as a belt to be driven by a system of rollers. In a typical transfer process, photoconductor material is passed under a stationary charge generating station, to thereby place a relatively uniform electrostatic charge, usually several hundred volts, across the entirety of the photoconductor surface. Next, the photoconductor is moved to an imaging station where the photoconductor receives light rays from a light-generating source. These light rays discharge the photoconductor to relatively low levels when the light source is fully powered, while the photoconductor continues to carry high voltage levels when the light source is turned off, or when it is powered at intermediate levels, or for a relatively short time duration. In that manner, the photoconductor is caused to bear an electrostatic latent charge pattern which corresponds to the printing, shading, etc., which is desired to be printed on the receiving material. 
     Light-generating sources in an electrophotographic printer frequently comprise lasing means in which a laser beam is modulated by a character generator and a modulator to control the power or length of time that the beam exposes the photoconductor in each particular picture element (PEL) area. In a multiple beam lasing system, multiple modulating means modulates more than one beam at a time so that multiple lines or rows of PELs are written at a time. 
     The present invention is described in a two-beam system. However, those skilled in the art will recognize that any greater number of beams may be utilized. For example, in a four-beam system, the method and system described below for adjusting one beam with respect to another is expanded so that any of the four beams may be adjusted with respect to any of the other beams. 
     After producing a latent image on the photoconductor, the latent image is moved to a developing station where developing material called toner is placed on the image. Toner is usually in the form of a colored powder which carries a charge designed to cause the powder to deposit on selected areas of the photoconductor. 
     The developed image is then moved from the developing station to a transfer station where copy receiving material, usually paper, is juxtaposed to the developed toner image, as a charge is placed on the backside of the paper, so that when the paper is thereafter stripped from the photoconductor, a toner image is held on the paper&#39;s surface as toner is concomitantly removed from the photoconductor. 
     The remaining electrophotographic process steps provide permanent bonding or fusing of the toner to the copy paper, and cleaning of residual toner from the photoconductor so that the photoconductor can be reused. 
     FIG. 1 depicts a electrophotographic printer incorporating this invention. Photoconductor material  10  is placed on the surface of a drum  11  which is driven by motive means, not shown, to rotate at a substantially constant speed in the direction A. A charge generator  12  places a uniform charge of several hundred volts across the surface of photoconductor  10  at the location of charging station  13 . The charged photoconductor is mounted in a dark enclosure, not shown, and thereafter rotates to the location of a printhead  14  which is comprised of a light-generating source, such as the laser beam generator shown in FIG.  2 . 
     Printhead  14  selectively exposes the charged photoconductor to light as the photoconductor passes through imaging station  15 . As a result, imaging station  15  operates to discharge photoconductor  10  in areas which are desired to be developed with toner (Discharged Area Development, DAD process), or to discharge the photoconductor in areas which are to remain free of toner (Charged Area Development, CAD process). 
     For a DAD process, the discharged areas of photoconductor  10  are developed at developing station  16  by developer apparatus  17  which operates to apply toner so that the photoconductor thereafter carries a visually perceptible image of the print data that previously operated to control printhead  14 . In a CAD process, the charged areas are developed. In either case, the developed image now rotates to transfer station  18  whereat print paper, moving in the direction B, is juxtaposed with the surface of photoconductor  10 . A charge opposite in polarity to the charge on the toner is placed on the backside of the print paper by transfer charge generator  19 , such that when the paper is stripped from the surface of the photoconductor, toner is attracted to the paper and leaves the surface of photoconductor  10 . Any remaining residual toner is cleaned from the photoconductor at cleaning station  20  by operation of cleaning apparatus  21 . 
     The selective application of light rays to photoconductor  10  at imaging station  15  is accomplished through operation of printhead modulator means  22 . For a semiconductor laser diode, printhead modulator  22  may be comprised of a power supply, which will either turn the light source on for longer or shorter periods of time to accomplish varying degrees of photoconductor discharge, in accordance with image pattern data, or modulator  22  will turn the light-generating source on to a greater or lesser illumination intensity in accordance with that data. In any event, modulation will occur in accordance with the data contained in buffer memory  23 . Print data is first sent to a raster buffer  24 , and then to printhead modulator  22 , as is well known by those skilled in the art. 
     FIG. 2 illustrates an optical scanning system which can be used as printhead  14  of FIG.  1 . In FIG. 2, a laser beam  25  is shown emanating from laser module  26 . Beam  25  passes through beam modulator  22  and then through cylindrical lens  27  for focusing the beam onto the facets  28  of a rotating polygon mirror  29 . The beam is reflected from the moving mirror facets, and then through a negative spherical lens group  30 , an anamorphic lens group  31 , and a positive spherical lens group  32 , to the surface of photoconductor  10 . FIG. 2 also depicts a fold mirror  33 , and exit window  34 , and the length of the scan  35  across photoconductor  10 . 
     A reflective surface  36  is provided to reflect light from the laser beam to a photo-detector means  37 . Means  37  provides a start of scan, beam detection indicator. 
     As the laser beam traverses photoconductor scan line  35  while moving at a substantially constant speed during the formation of one PEL row of a print data image, the binary print image signal present on conductor  38  may operate to control modulator  22  to an on or off state for each image PEL that is to be printed. The phase at which modulator  22  is controlled during each scan line  35  must be such that the PELs of one scan row will align with the corresponding PELs of adjacent scan rows. 
     FIGS. 3A and 3B illustrate a pictorial representation of a print receiving material, such as paper, two “printed” PEL rows of an image printed on a print receiving material  41 , such as paper, and a printed measurement device in accordance with the method and system of the present invention. A first PEL row  40  is generated in response to a first beam detect indicator during a first scan where a first beam is synchronized by a first PEL clock. A second PEL row  42  is generated in response to a second beam detect indicator during a second scan where a second beam is synchronized by a second PEL clock. As shown in FIG. 3A, prior to adjusting the timing of the PEL clocks by adjusting the timing of the second beam with respect to the timing of the first beam, the positions of PELs  44  and  46  are not aligned. 
     In addition, a printed measurement device  43 , such as a ruler, is also printed on paper  41 . Ruler  43  may be utilized to determine a start print location for a first PEL, as well as PEL size and length. In a preferred embodiment, the length of the ruler is the length of multiple PELs. A user may modify or adjust PEL size by inputting parameters into the printer which causes ruler  43  to be lengthened or shortened. The user may also cause ruler  43  to be moved on the paper in order to position the start print position at a new location. 
     This is useful particularly in duplex printers. In duplex printers, one ruler  43  is printed on a front of paper  41  and a second ruler  43  is printed on a back of paper  41 . A user may compare the position and length of the two rulers to align them so that printing on the back of the paper will be aligned with printing on the front. 
     When the position of ruler  43  is moved, an offset from its original position is created. When ruler  43  is lengthened or shortened, an offset from its original length is created. These offsets are utilized as described below to modify the printer clocks in order to print ruler  43  in its modified position with its modified length. 
     In addition to permitting an adjustment to the length and position of ruler  43 , and thus to a first printed PEL, the placement of multiple PEL lines may be adjusted with respect to each other. In order to adjust the placement of the PELs of concurrent scans at the desired linear positional relationship with respect to each other, the timing of the second beam is adjusted with respect to the timing of the first beam. In this manner, the second scan can be adjusted, and therefore, the physical position of the PEL row may be adjusted with either a positive or negative delay, with respect to the first scan. As shown in FIG. 3B after adjusting the timing of the second beam with respect to the timing of the first beam utilizing an offset  48  of the position of one PEL from a second PEL, the positions of PELs  44  and  46  are aligned. 
     FIG. 4 is a block diagram of a circuit for generating a plurality of PEL reduced jitter clocks where the timing of each of the clocks is finely adjustable in accordance with the method and system of the present invention. Printhead  14  generates a plurality of beam detect indicators, one for each beam utilized by the printer. These beam detect indicators may be included either as two separate timing pulses in a single signal, or each as a timing pulse in a separate signal. In the preferred embodiment, the two pulses are included in a single signal. A beam detect signal including the two beam detect pulses is received by a fixed delay line  50  which generates signals  51 ,  52 ,  90 , and  91 . Signal  51  is the beam detect signal which includes no delay. Signal  52  is the beam detect signal including preferably a 125 nsec delay. Those skilled in the art will recognize that by a selection of different circuitry components, a different delay may be utilized in order to produce the same result. Other signals having a different amount of delay may be generated from delay line  50  as needed. 
     The first beam detect signal  51  is input directly into logic means  53 . The second beam detect signal  52  is delayed by programmable delay line  54 . The amount of the programmable delay is determined utilizing an offset adjustment value entered by a user. Prior to initiating a print operation, a user selects an adjustment, or offset, value. The offset value is an amount by which a physical position for a first PEL in a second PEL row is offset from a physical position for a first PEL in a first PEL row as shown in FIG.  3 A. The offset is the number of PELs by which the second row is offset from the first. The offset may include both an integer number of PELs as well as a fractional amount of a PEL. The programmable delay line  54  adds a delay equivalent to the fractional amount of the offset to second signal  52 . A beam detect adjustment signal  55  is generated from programmable delay line  54  and is equal to signal  52  delayed a fractional PEL amount. In this manner, signal  55  is adjusted with respect to signal  51  and includes the fractional PEL offset. 
     Logic means  53  receives as input signals  51  and  52 . Logic means  53  and counting means  56  generate PEL clock enable signals  57  and  58 , and signals  80  and  81 . Signals  57  and  58  are beam alignment adjustment signals. A between beam detect signal  59  is also generated by logic means  53 . PEL clock enable signal  57  is associated with first PEL row  40 . PEL row  40  is generated by a first scan which is synchronized by a first PEL clock  70 . The first scan is initiated in response to PEL clock enable signal  57 . PEL clock enable signal  58  is associated with second PEL row  42 . PEL row  42  is generated by a second scan which is synchronized by a second PEL clock  72 . The second scan is initiated in response to PEL clock enable signal  58 . 
     PEL clock enable signal  57  is generated in accordance with the following logical equation: Signal  57 =Signal  51 +/Signal  52 +/Signal  59 . Signal  59  is generated utilizing signal  55 . Signal  59  becomes a logical “1” at the falling edge of the first pulse of signal  55 , and becomes a logical “0” at the falling edge of the second pulse of signal  55 . PEL clock enable signal  58  is generated in accordance with the following logical equation: Signal  58 =Signal  51 +/Signal  55 +Signal  59 . In this manner, signal  58 , which is associated with the second scan, is offset with respect to signal  57 , which is associated with a first scan. The offset includes the fractional PEL amount. 
     Signals  70  and  72  are used to run two separate counters,  56 A and  56 B, that implement the full pel offset adjustment. In general, all scans are delayed by some integer number pels. If scan 2 is delayed less than scan 1, this amounts to a negative adjustment to scan 2. In the preferred embodiment, scan 1 is delayed a fixed amount of 8 pels and scan 2 delayed by 0 to 15 pels for a −8 to +7 pel adjustment window. This saves register space and logic in a two beam implementation, but a more general implementation would make all delays adjustable. As each counter reaches its target count, it outputs a START signal  80 ,  81  that is synchronous with the adjusted pel clock The START signals are reset during the first 25 nsec of signal  51 . 
     Signals  57  and  58  are received by phase-locked-loop means  60  and  62 , respectively. Frequency oscillator  64  outputs a source clock signal  65  having a source frequency and provides it as an input to a PLL  67 . Phase-locked-loop  67  outputs a first clock signal  69  having a first frequency and provides it as an input to both PLLs  60  and  62 . Those skilled in the art will recognize that an additional PLL is added for each additional beam in system  10  which receives as an input a beam alignment signal and clock signal  69 . 
     Phase-locked-loop  67  receives source clock signal  65  and generates clock signal  69 . The frequency of signal  69  is substantially related to the frequency of signal  65  by a ratio of n 1 :m 1 . Phase-locked-loop  60  receives clock signal  69  and generates printer clock signal  70  having a first printer frequency. The frequency of signal  70  is substantially related to the frequency of signal  69  by a ratio of n 2 :m 2 . Therefore, the frequency of signal  70  is substantially related to the frequency of signal  65  by a ratio of n 1 n 2 :m 1 m 2 . Similarly, phase-locked-loop  62  receives clock signal  69  and generates printer clock signal  72  having a second printer frequency. The frequency of signal  72  is substantially related to the frequency of signal  69  by a ratio of n 2 :m 2 . Therefore, the frequency of signal  72  is substantially related to the frequency of signal  65  by a ratio of n 1 n 2 :m 1 m 2 . By utilizing PLL  67 , jitter in PLL&#39;s  60  and  62  may be reduced. 
     Phase-locked-loops  60  and  62 , act as programmable oscillators. PLL  60 , generates PEL clock  70  in response to PEL clock enable signal  57 . PLL  62  generates PEL clock  72  in response to PEL clock enable signal  58 . PEL clock  72 , therefore, is delayed with respect to PEL clock  70  in order to align the positions of the first PELs in PEL rows  40  and  42  as desired in accordance with the user input offset. 
     Logic means  71  receives PEL clock  70  as well as signals  59 ,  90 , and  91 . Logic means  71  generates signal  84 . Logic means  73  receives PEL clock  72  as well as signals  59 ,  90 , and  91 . Logic means  73  generates signal  85 . 
     FIG. 5 is a timing diagram of selected signals generated by the device of FIG. 4 in accordance with the method and system of the present invention. As depicted in FIG. 5, signals  57  and  58  include the partial PEL offset value. 
     START signal  80  occurs on the eighth rising edge of signal  70 . START signal  81  occurs on the 0 to 15th rising edge of signal  72 . As depicted in FIG. 5, signal  80  occurs on the fourth edge relative to signal  70 , thus causing a negative adjustment relative to signal  70 . Signal  82  is added to signal  70  to do early set up of drivers. Signal  82 =(Pulse Beam Detect Signal+7 nsec)*/(Pulse Beam Detect Signal+100 nsec)*/Signal  59 . Signal  83 =(Pulse Beam Detect Signal+7 nsec)*(Pulse Beam Detect Signal+100 nsec) * Signal  59 . Signal  90 =pulse beam detect signal delayed 75 nsec. Signal  91 =pulse beam detect signal delayed 100 nsec. 
     Signals  84  and  85  are then the resultant PEL clocks where signal  85  is than delayed by either a positive or negative adjustable amount from signal  84 . 
     FIG. 6 is a high level flow chart illustrating the printing of PELs utilizing finely adjustable printer clock signals in accordance with the method and system of the present invention. The process starts as depicted at block  100  and thereafter passes to block  102  which illustrates the printing of a reference page which has a ruler printed on both the front and back sides of the page. Multiple beam alignment PEL lines are also printed on the page. Next, block  104  depicts the comparing of the position and length of the ruler printed on the front of the page against the position of the ruler printed on the back of the page. The process then passes to block  106  which illustrates the comparing of one of the beam alignment PEL lines against another beam alignment PEL line. Next, block  108  depicts the permitting of a modification of the length of a PEL by receiving a modification to the length of the reference ruler. Thereafter, block  110  illustrates the permitting of a modification of a start print position by receiving a modification to a print position of the reference ruler. 
     The process then passes to block  112  which depicts a determination of an offset of a first beam alignment PEL line from a second beam alignment PEL line. Thereafter, block  114  illustrates a generation of a beam alignment signal utilizing the offset as described above. Next, block  116  depicts the delaying of the second printer clock signal as described above utilizing the alignment signal to compensate for the beam alignment offset. The process then passes to block  118  which illustrates a determination of a target printer clock frequency. Block  120 , then, depicts a determination of a ratio, n 1 n 2 :m 1 m 2 , to utilize in order to modify oscillator frequency to be equal to the desired target printer clock frequency. Next, block  122  illustrates a determination of n 1 , n 2 , m 1 , and m 2  to generate the determined ratio. The values for n 1 , n 2 , m 1 , and m 2  may be selected to further customize the device  10 . For example, the values could be chosen to maintain a low frequency difference between the PLL&#39;s. In addition, the values could be chosen to prohibit one or more frequencies from falling into a particular band of frequencies. The process then passes to block  124  which depicts the printing of another reference page with rulers and PEL lines in adjusted positions utilizing the ratio and alignment signal. 
     While a preferred embodiment has been particularly shown and described, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention.