Abstract:
Printers that correct for polygon phasing errors. Such printers include a raster output scanner, a moving photoreceptor, a page sensor for sensing the position of an image area, a start-of-scan sensor for sensing the start of scan, a light valve array having a plurality of electrically controlled light valves for selectively passing light, and a system controller that controls the light valve(s). The system controller initially selects one of the light valves. When the page sensor senses the beginning of a page the system controller starts selecting sequential light valves at a controlled rate. After a start-of-scan occurs the system controller stops sequencing the light valves. The light valve that passed light when the start-of-scan occurred continues to pass light. Beneficially, the system controller monitors the photoreceptor motion. If the photoreceptor motion changes the system controller then selects a light valve that moves the scan line toward the proper position.

Description:
FIELD OF THE INVENTION 
     This invention relates to electrophotographic marking machines. More particularly, it relates to aerially correcting the process direction spot position using an electronically addressable liquid crystal plate. 
     BACKGROUND OF THE INVENTION 
     Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a substantially uniformly charged photoreceptor with a light image representation of a desired document. In response to that light image the photoreceptor discharges so as to create an electrostatic latent image of the desired document on the photoreceptor&#39;s surface. Toner particles are then deposited onto that latent image to form a toner image. That toner image is then transferred from the photoreceptor onto a copy substrate, such as a sheet of paper. The transferred toner image is then fused to the copy substrate, usually using heat and/or pressure. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image. 
     The foregoing broadly describes a black and white electrophotographic marking machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, called the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner to produce a black toner image. The recharge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various latent images and consequently the color toners are placed in a superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate. 
     The foregoing color printing process can be performed in a various ways. For example, in a single pass printer wherein a composite image is produced in a single pass of the photoreceptor through the machine. This requires a charging, an exposing, and a developing station for each color of toner. Single pass printers are advantageous in that they are relatively fast since a composite color image can be produced in one cycle of the photoreceptor. 
     One method of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser light source (or sources), a rotating polygon having a plurality of mirrored facets, and pre-polygon and post-polygon optical systems. The light source radiates a laser beam into the pre-polygon optical system. The optical system collimates the laser beam and directs the collimated beam onto the rotating polygon facets. Those facets reflect the incoming beam into a sweeping beam that is directed into the post-polygon optical system. The post-polygon optical system corrects for various defects (such as wobble correction and scan line non-linearities) and focuses the sweeping beam onto a photoreceptor, thereby producing a light spot. As the polygon rotates the spot traces lines, referred to as scan lines, on the photoreceptor. By moving the photoreceptor in a process direction (also referred to as the slow scan direction) as the spot traces scan lines in the fast scan direction, the surface of the photoreceptor is raster scanned by the spot. During scanning, the laser beam is modulated by image data synchronized with the movement of the spot across the photoreceptor. Thus, individual picture elements (“pixels”) of the image are sequentially created on the photoreceptor. 
     While raster output scanners are beneficial, they have problems. One set of problems relates to scan line position errors in the slow scan direction. Scan line position errors of greater than 10% of the nominal line spacing can be noticeable in a half tone or continuous tone image. Because of the sensitivity of the human eye to color variations, color images are even more susceptible to scan line position errors. 
     Scan line position errors arise from many sources, such as polygon and/or photoreceptor motion flaws, facet and/or photoreceptor surface defects, photoreceptor stretching, and phasing errors between photoreceptor motion and facet position. Phasing errors arise because when the photoreceptor is in the proper position to receive an image a facet may not be in position to produce a scan line. As the printer delays writing a scan line until a facet is properly positioned the photoreceptor continues advancing. When a facet is properly positioned the photoreceptor has advanced, producing a scan line error. While phasing errors are generally small, in high quality systems, particularly color, the errors can be noticeable. 
     Scan line position errors can be corrected using closely spaced light valves (such as liquid crystal modulators, reflecting Fabry-Perot modulators, total internal reflective modulators, or a waveguide modulator/amplifier) that selectively block portions of a light beam from reaching the photoreceptor. Reference U.S. Pat. No. 5,049,897 issued on Sep. 17, 1991 to Ng entitled “Method and Apparatus for Beam Displacement in a Light Beam Scanner,” and U.S. Pat. No. 5,764,273, issued on Jun. 9, 1998 to Paoli entitled, “Spot Position Control Using a Linear Array of Light Valves.” 
     The use of closely spaced light valves to selectively block portions of a light beam is a useful technique since the position of a scan line on a photoreceptor is directly controlled by selecting which light valve(s) should pass light. That technique is particularly beneficial for correcting for phasing errors. Unfortunately, prior art techniques of selecting which light value(s) to turn on require the determination of the existence and the extent of scan line position errors. Only then can the proper light valve(s) be selected. U.S. Pat. No. 5,764,273 teaches using a feedback control system comprised of marks on the photoreceptor, a synchronization strobe and sensor, a signal processing circuit, a control apparatus, and a switching circuit that selects the proper light valves. Alternatively, U.S. Pat. No. 5,764,273 teaches using stored data and a switching circuit. U.S. Pat. No. 5,049,897 teaches using an encoder that monitors the web (photoreceptor) speed, phase-locking the raster output polygon motor to the web (photoreceptor), logic circuitry that compares the web (photoreceptor) speed with a predetermined constant, a logic and control unit (LCU) that calculates a potential scan line spacing error and that generates a control signal, and a driver that uses the control signal to select the proper light valve(s) to pass light. 
     While the prior art techniques of selecting the proper light valve(s) to correct for slow scan spot position errors are beneficial, they are rather complex, costly and/or difficult to implement. This is particularly true when correcting for phasing errors. Thus, a need exists for an improved method of determining which light valve(s) should be selected so as to correct for slow-scan direction spot position errors. Even more beneficial would be a simple, easily implemented method of selecting the proper light valve(s) to pass light when correcting for phasing errors. 
     SUMMARY OF INVENTION 
     The principles of the present invention provide for raster output scanner based printers that correct for polygon phasing errors. A printer in accordance with the principles of the present invention includes a laser-based raster output scanner, a moving photoreceptor, a page sensor for sensing the position of an image area on the photoreceptor, a start-of-scan sensor for sensing the start of scan, a light valve array having a plurality of electrically controlled light valves that selectively pass light, and a system controller that selects which light valve(s) that passes light. The system controller initially selects one of the light valves to pass light. When the page sensor senses the beginning of a page the system controller starts selecting sequential light valves, beneficially at a rate that depends upon the motion of the photoreceptor. When the start-of-scan sensor detects a start-of-scan, the system control stops sequencing the light valves. When the sequencing stops the light valve that passed light when the start-of-scan occurred continues to pass light. Beneficially, the system controller continues to monitor the photoreceptor motion. If the photoreceptor motion changes the system controller then selects a light valve such that the scan line moves toward the proper position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features of the present invention will become apparent as the following description proceeds and upon reference to the drawings, in which: 
     FIG. 1 is a schematic illustration of a printing apparatus according to the principles of the present invention; 
     FIG. 2 is a schematic illustration of selected printer elements producing a scan line; and 
     FIG. 3 is schematically illustrates a system controller selecting a light valve that 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present invention will be described in connection with a preferred embodiment, it should be understood that the present invention is not limited to that embodiment. On the contrary, the scope of the present invention covers all alternatives, modifications, and equivalents as may be included within the spirit and scope of the appended claims. 
     FIG. 1 illustrates an electrophotographic printing machine  8  that is suitable for use with the principles of the present invention. The printing machine  8  is a single pass, Recharge-Expose-and-Develop, Image-on-Image (Read IOI) printer. However, it is to be understood that the present invention is applicable to many other types of systems. Therefore, it is to be understood that the following description of the printing machine  8  is only to assist the understanding of the principles of the present invention. 
     The printing machine  8  includes an Active Matrix (AMAT) photoreceptor belt  10  which travels in the direction indicated by the arrow  12 . Belt travel is brought about by mounting the photoreceptor belt about a driven roller  14  and about tension rollers  16  and  18 , and then driving the driven roller  14  with a motor  20 . 
     As the photoreceptor belt travels each part of it passes through each of the subsequently described process stations. For convenience, a single section of the photoreceptor belt, referred to as the image area, is identified. The image area is that part of the photoreceptor belt which is to receive the various actions and toner layers that produce the final composite color image. While the photoreceptor belt may have numerous image areas, since each image area is processed in the same way a description of the processing of one image area suffices to explain the operation of the printing machine  8 . 
     The imaging process begins with the image area passing a “precharge” erase lamp  21  that illuminates the image area so as to cause any residual charge which might exist on the image area to be discharged. Such erase lamps are common in high quality systems and their use for initial erasure is well known. 
     As the photoreceptor belt continues its travel the image area passes a charging station comprised of a DC corotron  22 . The DC corotron charges the image area in preparation for exposure to create a latent image for black toner. For example, the DC corotron might charge the image area to a substantially uniform potential of about −500 volts. It should be understood that the actual charge placed on the photoreceptor will depend upon many variables, such as the black toner mass that is to be developed and the settings of the black development station (see below). 
     After passing the charging station the image area advances to an exposure station  24 A. At the exposure station the charged image area is exposed to a modulated laser beam  26 A from a raster output scanner  27 A that raster scans the image area such that an electrostatic latent representation of a black image is produced. Significantly, the position of the laser beam  26 A on the photoreceptor is determined for each facet of a rotating, multi-faceted polygon that is within the exposure station. Using the determined position the scan line position is corrected and the laser beam modulation is controlled such that the black latent image is imaged at a known position on the photoreceptor. A more detailed description of the raster output scanner  27 A (as well as the raster output scanners  27 B- 27 D that are discussed below) and the determining and control of the laser beam&#39;s position is given subsequently. 
     Still referring to FIG. 1, after passing the exposure station  24 A the exposed image area with the black latent image passes a black development station  32  that advances black toner  34  onto the image area so as to develop a black toner image. Biasing is such as to effect discharged area development (DAD) of the lower (less negative) of the two voltage levels on the image area. The charged black toner  34  adheres to the exposed areas of the image area, thereby causing the voltage of the illuminated parts of the image area to be about −200 volts. The non-illuminated parts of the image area remain at about −500 volts. 
     After passing the black development station  32  the image area advances to a recharging station  36  comprised of a DC corotron  38  and an AC scorotron  40 . The recharging station  36  recharges the image area and its black toner layer using a technique known as split recharging. Split recharging is described in U.S. Pat. No. 5,600,430, which issued on Feb. 4, 1997, and which is entitled, “Split Recharge Method and Apparatus for Color Image Formation.” Briefly, the DC corotron  38  overcharges the image area to a voltage level greater than that desired when the image area is recharged, while the AC scorotron  40  reduces that voltage level to that which is desired. Split recharging serves to substantially eliminate voltage differences between toned areas and untoned areas and to reduce the level of residual charge remaining on the previously toned areas. This benefits subsequent development by different toners. 
     The recharged image area with its black toner layer then advances to an exposure station  24 B. There, a laser beam  26 B from a raster output scanner  27 B exposes the image area to produce an electrostatic latent representation of a yellow image. In a manner similar to that of the laser beam  26 A, the position of the laser beam  26 B on the photoreceptor is determined and controlled, and the laser beam modulated is controlled such that the yellow latent image is in superimposed registration with the black latent image. Again, a more detailed description of the raster output scanners ( 27 A- 27 D) and the determining and control of the laser beam&#39;s position are given subsequently. 
     The now re-exposed image area then advances to a yellow development station  46  that deposits yellow toner  48  onto the image area. After passing the yellow development station the image area advances to a recharging station  50  where a DC scorotron  52  and an AC scorotron  54  split recharge the image area. 
     An exposure station  24 C then exposes the recharged image area. A modulated laser beam  26 C from a raster output scanner  27 C then exposes the image area to produce an electrostatic latent representation of a magenta image. In a manner similar to that of the laser beams  26 A and  26 B, the position of the laser beam  26 C on the photoreceptor is determined and controlled, and the laser beam  26 C is modulated such that the magenta latent image is in superimposed registration with the black and yellow latent images. Again, a more detailed description of the raster output scanners ( 27 A- 27 D) and the determining and control of the laser beam&#39;s position are given subsequently. 
     After passing the magenta exposure station the now re-exposed image area advances to a magenta development station  56  that deposits magenta toner  58  onto the image area. After passing the magenta development station the image area advances another recharging station  60  where a DC corotron  62  and an AC scorotron  64  split recharge the image area. 
     The recharged image area with its toner layers then advances to an exposure station  24 D. There, a laser beam  26 D from a raster output scanner  27 D exposes the image area to produce an electrostatic latent representation of a cyan image. A more detailed description of the raster output scanners ( 27 A- 27 D) and the determining and control of the laser beam&#39;s position are given subsequently. 
     After passing the exposure station  24 D the re-exposed image area advances past a cyan development station  66  that deposits cyan toner  68  onto the image area. At this time four colors of toner are on the image area, resulting in a composite color image. However, the composite color toner image is comprised of individual toner particles that have charge potentials that vary widely. Directly transferring such a composite toner image onto a substrate would result in a degraded final image. Therefore it is beneficial to prepare the composite color toner image for transfer. 
     To prepare the composite toner image for transfer a pretransfer erase lamp  72  discharges the image area to produce a relatively low charge level on the image area. The image area then passes a pretransfer DC scorotron  80  that performs a pre-transfer charging function. The image area continues to advance in the direction  12  past the driven roller  14 . A substrate  82  is then placed over the image area using a sheet feeder (which is not shown). As the image area and substrate continue their travel they pass a transfer corotron  84  that applies positive ions onto the back of the substrate  82 . Those ions attract the negatively charged toner particles onto the substrate. 
     As the substrate continues its travel is passes a detack corotron  86 . That corotron neutralizes some of the charge on the substrate to assist separation of the substrate from the photoreceptor  10 . As the lip of the substrate  82  moves around the tension roller  18  the lip separates from the photoreceptor. The substrate is then directed into a fuser  90  where a heated fuser roller  92  and a pressure roller  94  create a nip through which the substrate  82  passes. The combination of pressure and heat at the nip causes the composite color toner image to fuse into the substrate. After fusing, a chute, not shown, guides the substrate to a catch tray, also not shown, for removal by an operator. 
     After the substrate  82  is separated from the photoreceptor belt  10  the image area continues its travel and passes a preclean erase lamp  98 . That lamp neutralizes most of the charge remaining on the photoreceptor belt. After passing the preclean erase lamp the residual toner and/or debris on the photoreceptor is removed at a cleaning station  99 . The image area then passes once again to the precharge erase lamp  21  and the start of another printing cycle. 
     The printer  8  also includes a system controller  101  that controls the overall operation of the printer. The system controller preferably comprises one or more programmable microprocessors that operate in accordance with a software program stored in a suitable memory. Of importance to understanding the principles of the present invention is that the system controller synchronizes the overall operation of the printer  8  and provides video information to the laser beams  26 A- 26 D. 
     The system controller also drives the motor  20  such that the photoreceptor  10  advances at a nominal rate. However, because of various factors discussed in the “Background of the Invention,” the absolute position of the image area is not accurately known. In particular, since the polygon rotation is not synchronized with the photoreceptor motion a facet is not necessarily in position to write a scan line when the image area is in position (hence phasing errors). The printer  8  addresses phasing error problems using printer elements shown in FIG.  2 . 
     As shown in FIG. 2, a generic raster output scanner  27  includes a laser diode  206  that produces a laser beam  26 . As emitted the laser beam  26  is diverging. A spherical lens  214  collimates the divergent beam and while a polarizing filter  215  polarizes the collimated beam. The polarized and collimated laser beam illuminates a liquid crystal array  216  that is comprised of a plurality of closely space, individually selectable, liquid crystal elements. The liquid crystal array includes a common (shared) back electrode and a plurality of front electrodes, one for each liquid crystal element. In a manner that is subsequently explained, the system controller  101  selects one (other systems may select more than one) liquid crystal element to pass the laser beam. The system controller then applies an excitation voltage to the selected liquid crystal element. That excitation voltage causes the liquid crystals of the selected liquid crystal element to align themselves orthogonal to the electrode. The result in that the portion of the polarized laser beam  26  that illuminates the selected liquid crystal element passes through the selected element without rotation. However, the potions of the polarized laser beam  26  that illuminates the unselected liquid crystal elements are rotated 90°. For example, FIG. 2 shows an upper laser beam  26 A and a lower laser beam  26 B. If the system controller applies an excitation voltage to an “upper” front electrode the laser beam  26 A passes through the selected upper element without a polarization shift. Alternatively, if a “lower” liquid crystal element is selected the laser beam  26 B passes through the selected lower liquid crystal element without a polarization shift. 
     After passing through the liquid crystal array  216  the laser beam illuminates a polarizer plate  218 . The polarizer plate is aligned such that it passes the portion of the polarized laser beam that passed through the selected liquid crystal element. The other portions of the laser beam  26  are blocked by the polarizer plate. Thus, by selecting various liquid crystal elements the system controller  101  controls where the laser beam emerges from the polarizer plate  218 . FIG. 2 shows the laser beam  26 A emerging. 
     Light passed by the polarizer plate  218  passes through a cylindrical lens  220  that focuses the beam in the slow scan (process) direction onto a polygon  222  having a plurality of mirrored facets  224 . The polygon  222  rotates in the direction  226 . This rotation causes the laser beam  26  to sweep in a scan plane. The sweeping laser beam passes through a post-scan optics system  228  that reconfigures the beam into a circular (or elliptical) cross-section and that refocuses that laser beam  26  onto the surface of the photoreceptor  10 . The post-scan optics also corrects for various problems such as scan non-linearity (f-theta correction) and wobble (scanner motion or facet errors). The laser beam produces a light spot that sweeps across the photoreceptor in the direction  103 , thereby tracing a scan line  230 . 
     The liquid crystal element selected by the system controller  101  influences the slow scan (process) direction position of the scan line  230 . For example, if the system controller selected a different liquid crystal element the relative position of the scan line  230  on the photoreceptor would change to that of scan line  230 ′. That change would depend upon the separation of the individual liquid crystal elements and on the system&#39;s magnification. For example, in one embodiment, if the system controller switches between liquid crystal elements that are separated by 100 microns, the scan line moves 60 microns on the photoreceptor. 
     The principles of the present invention relate to selecting the individual liquid crystal element or elements that pass the laser beam without a polarization rotation. For example, the printing machine  8  corrects for phasing errors by selecting among the individual liquid crystal elements. Phasing error correction corrects for the spatial difference caused by photoreceptor motion (in the direction  12 ) during the time between when the image area is in position and when a facet is in position. The mechanics of that correction is described below. 
     To sense when an image area is in position the printer  8  includes a page sensor  304 . The page sensor senses light  306 , from a light source  308  that passes through a slot  310  in the photoreceptor. When light is sensed the page sensor signals the system controller  101 . The system controller uses page sensor signals to know when to expose and image. The image area is exposed to produce a black image after one page sensor signal, then the image area is exposed to produce a yellow image at the next page sensor signal, and so on. Thus, the page sensor signals are used to register the individual exposures. 
     The printer  8  also includes a start-of-scan sensor  312 . The start-of-scan sensor signals the system controller  101  when the laser beam  26  begins to sweep across the sensor. From the start-of-scan signals the system controller  101  knows the exact position of the polygon at an instant in time. 
     Still referring to FIG. 2, the printer  8  further includes a motion sensor  432  that senses a plurality of evenly spaced marks  430  on the photoreceptor. The motion sensor outputs motion signals at a rate that depends upon the motion of the photoreceptor. If the photoreceptor motion increases, so does the rate of the motion signals. 
     Turning now to FIG. 3, the system controller  101  uses the page sensor signals, the start-of-scan sensor signals, and the motion signals to select which of the individual liquid crystal element(s) pass light. The system controller  101  includes N liquid crystal element control lines, the lines  402 A through  402 N. When an excitation voltage is applied to a particular control line an associated liquid crystal element passes light. 
     In operation, a Liquid Crystal Element Select network  422  within the system controller  101  initially “parks” the laser beam  26  at a predetermined position on the photoreceptor  10  by applying an excitation voltage to a control line  402 A. The system controller then determines how many motion signals occur between start-of-scan signals. That number is then divided by N (the number of liquid crystal elements), resulting in a number W. when a page signal is received a clock  420  begins counting the motion signals. After W motion signals the clock applies a step signal to the Liquid Crystal Element Select network  422 . The Liquid Crystal Element Select network  422  then moves the excitation voltage from control line  402 A to control line  402 B. After W more motion signals, the excitation voltage moves to  402 C. This process continues until a start-of-scan signal occurs. The Liquid Crystal Element Select network then holds the excitation voltage on the control line that was excited when a start-of-scan signal occurred. After the image is fully exposed the Liquid Crystal Element Select network once again parks the laser beam at a predetermined position until another page sensor signal occurs. 
     Tracking photoreceptor motion using motion signals and “locking” the excitation on a particular control line when a start-of-scan signal occurs is beneficial for correcting for phasing errors. However, the printer  8  also corrects for motion errors after the phasing errors are corrected. The system controller accomplishes this by tracking the motion signals. If the motion signals occur at a constant rate the system controller  101  knows that the photoreceptor motion is constant. However, if the motion signal rate changes the system controller knows that the motion of the photoreceptor has changed. In that case, the Liquid Crystal Element Select network steps the excitation voltage onto the control line that would bring the scan line back toward the proper position. That is, the position that would be proper if the photoreceptor motion was constant. 
     While the principles of the present invention have been described in conjunction with a specific embodiment, many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the present invention is intended to embrace all alternatives, modifications and variations that fall within the spirit and scope of the claims.