ROS assisted toner patch generation for use in tri-level imaging

Abstract: Toner patch generation for use in tri-level imaging is effected using a laser ROS (48). Two toner patches are formed using a single toner patch generator (52) of the type commonly used in the prior art. The patch generator (52), used by itself serves to form one toner patch latent image and together with the ROS exposure device (48) of the imaging apparatus is used to form the other toner patch latent image.

Description: This invention relates generally to highlight color imaging and more particularly to the formation of tri-level highlight color images in a single pass. The invention can be utilized in the art of xerography or in the printing arts. In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a photoreceptor. The photoreceptor comprises a charge retentive surface. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation. This charge pattern is made visible by developing it with toner. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques. The concept of tri-level, highlight color xerography is described in US-A 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness. In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided into three levels, rather than two levels as is the case in conventional xerography. The photoreceptor is charged, typically to -900 volts. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V cad or V ddp ). V ddp is the voltage on the photoreceptor due to the loss of voltage while the P/R remains charged in the absence of light, otherwise known as dark decay. The other image is exposed to discharge the photoreceptor to its residual potential, i.e.V dad or V c (typically -100 volts) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background area is exposed such as to reduce the photoreceptor potential to halfway between the V cad and V dad potentials, (typically -500 volts) and is referred to as V white or V w . The CAD developer is typically biased about 100 volts closer to V cad than V white (about -600 volts), and the DAD developer system is biased about -100 volts closer to V dad than V white (about 400 volts). As will be appreciated, the highlight color need not be a different color but may have other distinguishing characteristics. For, example, one toner may be magnetic and the other nonmagnetic. The present invention addresses the problem of providing, in a tri-level xerographic system, two separate toner test patches, each for use in controlling a respective color development system, while providing sufficient exposure latitude to allow for dirt buildup on the exposure lens and for high charge levels required as a photoreceptor ages and permitting small increments in exposure. The present invention relates to toner patch generation in a tri-level imaging apparatus. A charge retentive surface is uniformly charged in areas including the interdocument zone. A patch generator is utilized to form a first test patch by discharging a predetermined area of the interdocument zone to a level V tb intermediate the CAD image voltage level and the background voltage level, V Mod . Using an image exposure ROS, another predetermined area of the interdocument zone is discharged to the background voltage level, V Mod . Using the toner patch generator, the predetermined area discharged to the background level is then discharged to a voltage level V tc intermediate the DAD image voltage level and the background level. The present invention provides a method of creating tri-level images on a charge retentive surface, including the steps of: uniformly charging said charge retentive surface; using a test patch generator, forming a first test patch, having a first test voltage level, on said charge retentive surface; using an image exposure structure, utilized for forming tri-level images, and said test patch generator for making superposed exposures, thereby forming a second test patch, having a second test voltage level, different to said first test voltage level, on said charge retentive surface. According to another aspect of the invention there is provided an apparatus for creating tri-level images on a charge retentive surface, said apparatus comprising: means for uniformly charging said charge retentive surface; a test patch generator for forming a first test patch, having a first test voltage level, on said charge retentive surface; and exposure structure for forming tri-level images; said test patch generator and said exposure structure cooperating to form a second test patch, having a second test voltage level, different to said first test voltage level, on said charge retentive surface. Preferably, said means for uniformly charging said retentive surface comprises means for uniformly charging image and interdocument areas of charge retentive surface. Preferably, said test patch generator comprises means for discharging a predetermined section of one of said areas to a voltage level intermediate a first image voltage level and a background voltage level. Preferably, said exposure structure comprises means for discharging said another predetermined section of said areas to approximately said background voltage level and said test patch generator comprises means for discharging said another predetermined section from approximately said background voltage level to a voltage level intermediate said background voltage level and a second image voltage level. Preferably, said exposure structure comprises a laser ROS. Preferably, said means for discharging a predetermined section of one of said areas comprises means for discharging a predetermined section in said interdocument area. Preferably, the apparatus further comprises means for developing said predetermined section with toner and developing said another predetermined section with toner having physical properties different from the physical properties of the toner used to develop said predetermined area. Preferably, the apparatus further comprises means for developing said predetermined section and said another predetermined area section comprises means for developing said sections with toners of different colors. Figure 1a is a plot of photoreceptor potential versus exposure illustrating a tri-level electrostatic latent image; Figure 1b is a plot of photoreceptor potential illustrating single-pass, highlight color latent image characteristics; Figure 2 is schematic illustration of a printing apparatus incorporating the inventive features of the invention; and Figure 3 a schematic of the xerographic process stations including the active members for image formation as well as the control members operatively associated therewith of the printing apparatus illustrated in Figure 2. Figure 4 is a block diagram illustrating the interconnection among active components of the xerographic process module and the control devices utilized to control them. For a better understanding of the concept of tri-level, highlight color imaging, a description thereof will now be made with reference to Figures 1a and 1b. Figure 1a shows a Photolnduced Discharge Curve (PIDC) for a tri-level electrostatic latent image according to the present invention. Here V 0 is the initial charge level, V ddp (V CAD ) the dark discharge potential (unexposed), V w (V Mod ) the wvhite or background discharge level and V c (V DAD ) the photoreceptor residual potential (full exposure using a three level Raster Output Scanner, ROS). Nominal voltage values for V CAD , V Mod and V DAD are, for example, 788, 423 and 123, respectively. Color discrimination in the development of the electrostatic latent image is achieved when passing the photoreceptor through two developer housings in tandem or in a single pass by electrically biasing the housings to voltages which are offset from the background voltage V Mod , the direction of offset depending on the polarity or sign of toner in the housing. One housing (for the sake of illustration, the second) contains developer with black toner having triboelectric properties (positively charged) such that the toner is driven to the most highly charged (V ddp ) areas of the latent image by the electrostatic field between the photoreceptor and the development rolls biased at V black bias (V bb ) as shown in Figure 1b. Conversely, the triboelectric charge (negative charge) on the colored toner in the first housing is chosen so that the toner is urged towards parts of the latent image at residual potential, V DAD by the electrostatic field existing between the photoreceptor and the development rolls in the first housing which are biased to V color bias (V cb ). Nominal voltage levels for V bb and V cb are 641 and 294, respectively. As shown in Figures 2 and 3, a highlight color printing apparatus 2 in which the invention may be utilized comprises a xerographic processor module 4, an electronics module 6, a paper handling module 8 and a user interface (IC) 9. A charge retentive member in the form of an Active Matrix (AMAT) photoreceptor belt 10 is mounted for movement in an endless path past a charging station A, an exposure station B, a test patch generator station C, a first Electrostatic Voltmeter (ESV) station D, a developer station E, a second ESV station F within the developer station E, a pretransfer station G, a toner patch reading station H where developed toner patches are sensed, a transfer station J, a preclean station K, cleaning station L and a fusing station M. Belt 10 moves in the direction of arrow 16 to advance successive portions thereof sequentially through the various processing stations disposed about the path of movement thereof. Belt 10 is entrained about a plurality of rollers 18, 20, 22, 24 and 25, the former of which can be used as a drive roller and the latter of which can be used to provide suitable tensioning of the photoreceptor belt 10. Motor 26 rotates roller 18 to advance belt 10 in the direction of arrow 16. Roller 18 is coupled to motor 26 by suitable means such as a belt drive, not shown. The photoreceptor belt may comprise a flexible belt photoreceptor. Typical belt photoreceptors are disclosed in US-A 4,588,667, US-A 4,654,284 and US-A 4,780,385. As can be seen by further reference to Figures 2 and 3, initially successive portions of belt 10 pass through charging station A. At charging station A, a primary corona discharge device in the form of dicorotron indicated generally by the reference numeral 28, charges the belt 10 to a selectively high uniform negative potential, V 0 . As noted above, the initial charge decays to a dark decay discharge voltage, V ddp (V CAD ). The dicorotron is a corona discharge device including a corona discharge electrode 30 and a conductive shield 32 located adjacent the electrode. The electrode is coated with relatively thick dielectric material. An AC voltage is applied to the dielectrically coated electrode via power source 34 and a DC voltage is applied to the shield 32 via a DC power supply 36. The delivery of charge to the photoconductive surface is accomplished by means of a displacement current or capacitative coupling through the dielectric material. The flow of charge to the P/R 10 is regulated by means of the DC bias applied to the dicorotron shield. In other words, the P/R will be charged to the voltage applied to the shield 32. For further details of the dicorotron construction and operation, reference may be had to US-A 4,086,650 granted to Davis et al on April 25, 1978. A feedback dicorotron 38 comprising a dielectrically coated electrode 40 and a conductive shield 42 operatively interacts with the dicorotron 28 to form an integrated charging device (ICD). An AC power supply 44 is operatively connected to the electrode 40 and a DC power supply 46 is operatively connected to the conductive shield 42. Next, the charged portions of the photoreceptor surface are advanced through exposure station B. At exposure station B, the uniformly charged photoreceptor or charge retentive surface 10 is exposed to a laser based input and/or output scanning device 48 which causes the charge retentive surface to be discharged in accordance with the output from the scanning device. Preferably the scanning device is a three level laser Raster Output Scanner (ROS). Alternatively, the ROS could be replaced by a conventional xerographic exposure device. The ROS comprises optics, sensors, laser tube and resident control or pixel board. The photoreceptor, which is initially charged to a voltage V 0 , undergoes dark decay to a level V ddp or V CAD equal to about -900 volts to form CAD images. When exposed at the exposure station B it is discharged to V c or V DAD equal to about -100 volts to form a DAD image which is near zero or ground potential in the highlight color (i.e. color other than black) parts of the image. See Figure 1a. The photoreceptor is also discharged to V w or V mod equal to approximately minus 500 volts in the background (white) areas. A patch generator 52 (Figures 3 and 4) in the form of a conventional exposure device utilized for such purpose is positioned at the patch generation station C. It serves to create toner test patches in the interdocument zone which are used both in a developed and undeveloped condition for controlling various process functions. An Infra-Red densitometer (IRD) 54 is utilized to sense or measure the voltage level of test patches after they have been developed. After patch generation, the P/R is moved through a first ESV station D where an ESV (ESV 1 ) 55 is positioned for sensing or reading certain electrostatic charge levels (i. e. V DAD , V CAD , V Mod and V tc ) on the P/R prior to movement of these areas of the P/R moving through the development station E. At development station E, a magnetic brush development system, indicated generally by the reference numeral 56 advances developer materials into contact with the electrostatic latent images on the P/R. The development system 56 comprises first and second developer housing structures 58 and 60. Preferably, each magnetic brush development housing includes a pair of magnetic brush developer rollers. Thus, the housing 58 contains a pair of rollers 62, 64 while the housing 60 contains a pair of magnetic brush rollers 66, 68. Each pair of rollers advances its respective developer material into contact with the latent image. Appropriate developer biasing is accomplished via power supplies 70 and 71 electrically connected to respective developer housings 58 and 60. A pair of toner replenishment devices 72 and 73 (Figure 2) are provided for replacing the toner as it is depleted from the developer housing structures 58 and 60. Color discrimination in the development of the electrostatic latent image is achieved by passing the photoreceptor past the two developer housings 58 and 60 in a single pass with the magnetic brush rolls 62, 64, 66 and 68 electrically biased to voltages which are offset from the background voltage V Mod , the direction of offset depending on the polarity of toner in the housing. One housing e.g. 58 (for the sake of illustration, the first) contains red conductive magnetic brush (CMB) developer 74 having triboelectric properties (i. e. negative charge) such that it is driven to the least highly charged areas at the potential V DAD of the latent images by the electrostatic development field (V DAD - V color bias ) between the photoreceptor and the development rolls 62, 64. These rolls are biased using a chopped DC bias via power supply 70. The triboelectric charge on conductive black magnetic brush developer 76 in the second housing is chosen so that the black toner is urged towards the parts of the latent images at the most highly charged potential V CAD by the electrostatic development field (V CAD - V black bias ) existing between the photoreceptor and the development rolls 66, 68. These rolls, like the rolls 62, 64, are also biased using a chopped DC bias via power supply 71. By chopped DC (CDC) bias is meant that the housing bias applied to the developer housing is alternated between two potentials, one that represents roughly the normal bias for the DAD developer, and the other that represents a bias that is considerably more negative than the normal bias, the former being identified as V Bias Low and the latter as V Bias High . This alternation of the bias takes place in a periodic fashion at a given frequency, with the period of each cycle divided up between the two bias levels at a duty cycle of from 5-10 % (Percent of cycle at V Bias High ) and 90-95% at V Bias Low . In the case of the CAD image, the amplitude of both V Bias Low and V Bias High are about the same as for the DAD housing case, but the waveform is inverted in the sense that the the bias on the CAD housing is at V Bias High for a duty cycle of 90-95%. Developer bias switching between V Bias High and V Bias Low is effected automatically via the power supplies 70 and 74. For further details regarding CDC biasing, reference may be had to EP-A-0429309, published 29 May 1991, corresponding to U. S. Patent Application Serial No. 440,913 filed November 22, 1989 in the name of Germain et al. In contrast, in conventional tri-level imaging as noted above, the CAD and DAD developer housing biases are set at a single value which is offset from the background voltage by approximately -100 volts. During image development, a single developer bias voltage is continuously applied to each of the developer structures. Expressed differently, the bias for each developer structure has a duty cycle of 100%. Because the composite image developed on the photoreceptor consists of both positive and negative toner, a negative pretransfer dicorotron member 100 at the pretransfer station G is provided to condition the toner for effective transfer to a substrate using positive corona discharge. Subsequent to image development a sheet of support material 102 (Figure 3) is moved into contact with the toner image at transfer station J. The sheet of support material is advanced to transfer station J by conventional sheet feeding apparatus comprising a part of the paper handling module 8. Preferably, the sheet feeding apparatus includes a feed roll contacting the uppermost sheet of a stack copy sheets. The feed rolls rotate so as to advance the uppermost sheet from stack into a chute which directs the advancing sheet of support material into contact with photoconductive surface of belt 10 in a timed sequence so that the toner powder image developed thereon contacts the advancing sheet of support material at transfer station J. Transfer station J includes a transfer dicorotron 104 which sprays positive ions onto the backside of sheet 102. This attracts the negatively charged toner powder images from the belt 10 to sheet 102. A detack dicorotron 106 is also provided for facilitating stripping of the sheets from the belt 10. After transfer, the sheet continues to move, in the direction of arrow 108, onto a conveyor (not shown) which advances the sheet to fusing station M. Fusing station M includes a fuser assembly, indicated generally by the reference numeral 120, which permanently affixes the transferred powder image to sheet 102. Preferably, fuser assembly 120 comprises a heated fuser roller 122 and a backup roller 124. Sheet 102 passes between fuser roller 122 and backup roller 124 with the toner powder image contacting fuser roller 122. In this manner, the toner powder image is permanently affixed to sheet 102 after it is allowed to cool. After fusing, a chute, not shown, guides the advancing sheets 102 to a catch trays 126 and 128 (Figure 2), for subsequent removal from the printing machine by the operator. After the sheet of support material is separated from photoconductive surface of belt 10, the residual toner particles carried by the non-image areas on the photoconductive surface are removed therefrom. These particles are removed at cleaning station L. A cleaning housing 100 supports therewithin two cleaning brushes 132, 134 supported for counter-rotation with respect to the other and each supported in cleaning relationship with photoreceptor belt 10. Each brush 132, 134 is generally cylindrical in shape, with a long axis arranged generally parallel to photoreceptor belt 10, and transverse to photoreceptor movement direction 16. Brushes 132, 134 each have a large number of insulative fibers mounted on base, each base respectively journaled for rotation (driving elements not shown). The brushes are typically detoned using a flicker bar and the toner so removed is transported with air moved by a vacuum source (not shown) through the gap between the housing and photoreceptor belt 10, through the insulative fibers and exhausted through a channel, not shown. A typical brush rotation speed is 1300 rpm (136 rads -1 ), and the brush/photoreceptor interference is usually about 2 mm. Brushes 132, 134 beat against flicker bars (not shown) for the release of toner carried by the brushes and for effecting suitable tribo charging of the brush fibers. Subsequent to cleaning, a discharge lamp 140 floods the photoconductive surface 10 with light to dissipate any residual negative electrostatic charges remaining prior to the charging thereof for the successive imaging cycles. To this end, a light pipe 142 is provided. Another light pipe 144 serves to illuminate the backside of the P/R downstream of the pretransfer dicorotron 100. The P/R is also subjected to flood illumination from the lamp 140 via a light channel 146. Figure 4 depicts the the interconnection among active components of the xerographic process module 4 and the sensing or measuring devices utilized to control them. As illustrated therein, ESV 1 , ESV 2 and IRD 54 are operatively connected to a control board 150 through an analog to digital (A/D) converter 152. ESV 1 and ESV 2 produce analog readings in the range of 0 to 10 volts which are converted by Analog to Digital (A/D) converter 152 to digital values in the range 0-255. Each bit corresponds to 0.040 volts (10/255) which is equivalent to photoreceptor voltages in the range 0-1500 where one bit equals 5.88 volts (1500/255). The digital value corresponding to the analog measurements are processed in conjunction with a Non-Volatile Memory (NVM) 156 by firmware forming a part of the control board 150. The digital values arrived at are converted by a digital to analog (D/A) converter 158 for use in controlling the ROS 48, dicorotrons 28, 90, 104 and 106. Toner dispensers 160 and 162 are controlled by the digital values. Target values for use in setting and adjusting the operation of the active machine components are stored in NVM. In the tri-level xerographic system of the present invention, two separate toner patches must be generated, one for controlling a color development system and the other for controlling a black development system. As will be appreciated, it would be desirable to use a single patch generator for generating both patches. This, however, can be quite difficult for a single device because of the requirements imposed by factors inherent to tri-level xerography and the necessary precision of the exposure device. These requirements include: 1. sufficient exposure latitude needed to allow for nominal dirt (toner) build-up on the exposure lens. 2. sufficient exposure latitude needed to allow for high charge levels required as the p/r ages. 3. small increments in exposure needed over the 255 available control steps to allow for precise voltage control (less than 6 volts per step). 4. items 1-3 must be maintained for both low exposure needed at the high end (black patch) of the PhotoInduced Discharge Curve (PIDC) and for high exposure (color patch) at the low end of the PIDC. Item 4 is the most constraining. A single device that can expose both the black and color toner patches from a fully charged P/R has insufficient latitude to maintain items 1 through 3. In order to effect the generation of two toner patches using the patch generator 52, the ROS 48 is utilized to expose a predetermined interdocument area to the background voltage, V Mod . The patch generator is used to complete the exposure of the predetermined interdocument area to reduce the voltage level in that area to the desired toner patch voltage, V tc for the color test patch. The foregoing is facilitated by the fact that the color toner patch voltage is always lower than the intermediate background voltage, V Mod . The black test patch voltage, V tb is achieved using only the exposure provided by the patch generator 52. The magnitude of black patch voltage, V tb is between the CAD voltage, V CAD and the black developer bias voltage, V Black Bias while the magnitude of the color patch, V tc is between the DAD voltage level and the color developer bias, V color bias .

Claims: A method of creating tri-level images on a charge retentive surface (10), including the steps of: uniformly charging said charge retentive surface (10); using a test patch generator (52), forming a first test patch, having a first test voltage level (V tb ), on said charge retentive surface (10); using an image exposure structure (48), utilized for forming tri-level images, and said test patch generator (52) for making superposed exposures, thereby forming a second test patch, having a second test voltage level (V tc ), different to said first test voltage level (V tb ), on said charge retentive surface (10). The method according to claim 1 wherein said step of uniformly charging said charge retentive surface (10) comprises charging image and interdocument areas of said charge retentive surface (10) . The method according to claim 1 or 2 wherein said step of forming a first test patch comprises using a test patch generator (52) to discharge a predetermined section of one of said areas to a voltage level (V tb ) intermediate a first image voltage level (V CAD ) and a background voltage level (V MOD ). The method according to claims 1, 2 or 3 wherein said step of forming a second test patch comprises, using said image exposure structure (48) to discharge another predetermined section of said areas to approximately said background voltage level (V MOD ) and using said test patch generator (52) for discharging said another predetermined area from approximately said background voltage level (V MOD ) to a voltage level (V tc ) intermediate said background voltage level (V MOD ) and a second image voltage level (V DAD ). The method according to any of claims 1 to 4 wherein said step of using said image exposure structure (48) comprises using a laser ROS (48). The method according to any of the preceding claims wherein the step of discharging a predetermined section of one of said areas comprises discharging a predetermined section in said interdocument area. The method according to claim 4 including the steps of developing said predetermined section with toner and developing said another predetermined section with toner having physical properties different from the physical properties of the toner used to develop said predetermined section. The method according to claim 7 wherein the steps of developing comprises developing said predetermined area and said another predetermined area with toners of of different colors. Apparatus for creating tri-level images on a charge retentive surface (10), said apparatus comprising: means (28,38) for uniformly charging said charge retentive surface (10); a test patch generator (52) for forming a first test patch, having a first test voltage level (V tb ), on said charge retentive surface (10); and exposure structure (48) for forming tri-level images; said test patch generator (52) and said exposure structure (48), in use, making superposed exposures to form a second test patch, having a second test voltage level (V tc ), different to said first test voltage level (V tb ), on said charge retentive surface (10). Apparatus according to claim 9 wherein said means for uniformly charging said charge retentive surface comprises means for uniformly charging image and interdocument areas of said charge retentive surface.

Inventors: MACDONALD DANIEL W, SCHEUER MARK A, MACDONALD, DANIEL W., SCHEUER, MARK A.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1992
Main classification: G03G15
IPC codes: G03G15, G01M11, H04N1