Abstract:
An electrostatic transfer control method that avoids undesired retransfer effects. A printing device develops and transfers several control patches. The patches are transferred at different electrostatic set points and a control strategy is utilized involving one or more density sensors to measure the transferred toner patches whereby the obtained density information can be used to compute the optimal value of electrostatic transfer bias. Print operators can adjust the bias value based on preferences for predetermined standards.

Description:
BACKGROUND 
     The present disclosure relates to multi-color document processing systems such as printers, copiers, multi-function devices, etc., and to control techniques for operating the same. The disclosures of U.S. Patent Application Publication Nos. 2008/0152369 to DiRubio et al., 2008/0152371 to Burry et al., and 2010/0329702 to Dirubio et al., and the disclosure of U.S. patent application Ser. No. 12/612,121, filed Nov. 4, 2009 and entitled “Dynamic Field Transfer Control in First Transfer” to Lee are hereby incorporated by reference in their entireties. Multi-color toner-based xerographic printing systems typically employ two or more xerographic marking devices to individually transfer toner of a given color to an intermediate transfer structure, such as a drum or belt (referred to as first transfer operations), with the toner being subsequently transferred (in a second transfer operation) from the intermediate medium to a sheet or other final print medium, after which the twice transferred toner is fused to the final print. 
     Retransfer occurs when toner on the intermediate belt from previous, upstream marking devices is wholly or partially removed (scavenged) due to high transfer fields within the current transfer nip. High fields in the transfer nips in the downstream marking devices can adversely modify the charge state of the toner on the intermediate transfer belt (ITB) through air breakdown mechanisms, further exacerbating retransfer. When this happens, the desired amount of one or more toner colors is not transferred to the final printed sheet, and the retransfer problem worsens as the number of colors increases. Retransfer at a given marking device may be reduced by lowering the transfer field strength at that device, but this may lead to incomplete transfer during image building at that device. In other words, the transfer nip may be transferring toner to the ITB at one region in the cross-process direction (image building), which requires high fields, while simultaneously scavenging toner from the ITB in another region (retransfer). In addition, the quality requirements of multi-color document processing systems are constantly increasing, with customers demanding improved imaging capabilities without adverse effects of retransfer and incomplete transfer. 
     Current xerographic transfer controls are optimized against many noise factors such as relative humidity and age of the components. However, the controls may not be optimized for image content, which is ultimately important to end users and customers. While transfer is quite robust for image building, retransfer is still a problem since this defect reduces image quality and increases toner-to-waste (increases run cost). Retransfer is also magnified when products have more than four colors. 
     One proposed solution is U.S. Patent Application Publication No. 2010/0329702 to DiRubio et al., published Dec. 30, 2010, entitled “Multi-Color Printing System and Method for Reducing the Transfer Field Through Closed Loop Controls”, which minimizes retransfer by detecting the amount of toner transferred to the intermediate transfer belt and employing closed loop controls. However, even this solution leaves residual toner and thus is not a complete solution to the retransfer problem. 
     Another proposed solution is U.S. patent application Ser. No. 12/612,121, filed Nov. 4, 2009, to Lee, entitled “Dynamic Field Transfer Control In First Transfer”, which presents a multi-color document processing system and method to control color retransfer by allowing operators to override nominal electrostatic transfer control settings and set more optimum conditions for a variety of specific and particular print jobs. This, thus, disables marking devices which are not needed for a particular print job and operates devices required for printing at a reduced transfer field levels for the first transfer. This solution reduces transfer, but at the penalty of reducing the speed at which the print device operates. In addition, this approach may cause a reduction in the amount of toner that transfers from the photoreceptor (P/R) to the ITB. A phenomenon called “incomplete transfer.” 
     BRIEF DESCRIPTION 
     The present embodiments disclose an electrostatic transfer control method that optimizes transfer efficiency, color gamut, and image quality. More particularly compensating for undesired retransfer effects. A printing device develops and transfers several control patches. The patches are transferred at different electrostatic set points and a control strategy is utilized involving one or more density sensors to measure the transferred toner patches whereby the obtained density information can be used to compute the optimal value of electrostatic transfer bias. The subject control strategy can allow print operators to adjust the bias value based on preferences for predetermined standards. The embodiments provide a more robust first transfer system which can also be applied to more than four color IBT marking engines. 
     A first embodiment comprises a method of operating the document processing system having a plurality of marking devices of different colors individually operable to transfer marking material in a first transfer operation onto an intermediate transfer structure. The method comprises (a) importing a plurality of control patches of preselected colors wherein the colors are repetitively imported at a plurality of electrostatic transfer bias set points, (b) sensing a density of the control patches, (c) detecting a highest density color of the repetitively imported patches, and (d) determining an optimal first transfer bias based on the detected highest density, whereby subsequent operation of the document printing system selectively employs the optimal first transfer bias. 
     An additional embodiment comprises a system for controlling print transfer. The system comprises a printer including a plurality of one color print modules, each module comprising a print head and an adjoining nip, each module associated with one individual color. At least one sensor is associated with each individual color. An algorithm calculates the optimal transfer bias based on data gathered by the sensors. A graphical user interface facilitates user data entry and approval acknowledgment data. The processor further receives the data gathered by the sensor, receives user entered settings data and executes the algorithm using the received data. The printer prints at least one test patch printed out in response to a calculated optimal transfer bias. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present subject matter may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the subject matter, in which: 
         FIG. 1  is a flow diagram illustrating an exemplary method for operating a document processing system in accordance with one or more aspects of the disclosure; 
         FIG. 2  is a simplified schematic of a document printing system showing the printhead, rollers, nips and detecting sensors in relation to a printing device; 
         FIG. 3  shows a plurality of color transfer onto a shared intermediate transfer structure (ITB); 
         FIG. 4  is a detailed side elevation view illustrating an exemplary multi-color embodiment of the system of  FIG. 2  in accordance with the present disclosure; and 
         FIG. 5  shows a closed schematic view of a print nip and system controlling processor. 
     
    
    
     DETAILED DESCRIPTION 
     Several embodiments or implementations of the different aspects of the present disclosure are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features, structures, and graphical renderings are not necessarily drawn to scale. Certain embodiments are illustrated and described below in the context of exemplary multi-color document processing systems that employ multiple xerographic marking devices or stations in which toner marking material is first transferred to an intermediate structure and ultimately transferred to a final print medium to create images thereon in accordance with a print job. However, the techniques and systems of the present disclosure may be implemented in other forms of document processing or printing systems that employ any form of marking materials and techniques in which marking device fields are used for material transfer, such as ink-based printers, etc., wherein any such implementations and variations thereof are contemplated as falling within the scope of the present disclosure. 
     An exemplary method  100  is illustrated in  FIG. 1  and  FIGS. 2-5  illustrate various aspects of an exemplary tandem multi-colored document processing system  200  having a plurality of marking devices which may be operated according to the exemplary method  100 , wherein marking devices as used herein includes without limitation marking engines, marking stations, etc. The method  100  involves operating the marking devices in a normal mode to selectively transfer marking material onto an intermediate transfer medium in accordance with a print job with transfer field elements of the devices being operated at a first set of field levels (i.e. transfer biases), and in a second or enhanced mode wherein the transfer biases are adjusted to produce highest density colors, or are further adjusted in accordance with an operator selectively applied preference. 
     While the exemplary method  100  is illustrated and described in the form of a series of acts or events, the various methods of the disclosure are not limited by the illustrated ordering of such acts or events except as specifically noted, and some acts or events may occur in different order and/or concurrently with other acts or events apart from those illustrated and described herein, and not all illustrated steps may be required to implement a process or method in accordance with the present disclosure. The illustrated method  100 , moreover, may be implemented in hardware, processor-executed software, or combinations thereof, in one or more control elements operatively associated with a document processing system in order to provide the selective functionality set forth herein for a given print job, such as in a printing system as shown in  FIGS. 2-5 , wherein the disclosure is not limited to the specific applications and implementations illustrated and described herein. 
     Referring to  FIGS. 2-5 , the document processing system  200  comprises a multi-engine marking assembly including a system controller  122  and marking devices  102  which may be operated in accordance with the method  100  in a normal printing mode. The system  200  includes a plurality of xerographic marking devices  102  individually operable by the controller  122  to transfer toner marking material onto an intermediate transfer structure  103 , in this case, a shared intermediate transfer belt (ITB)  103  traveling in a counter clockwise direction in the figures past the xerographic marking devices  102 , also referred to as marking engines, marking elements, marking stations, etc. In other embodiments, a cylindrical drum may be employed as an intermediate transfer structure with marking devices  102  positioned around the periphery of the drum to selectively transfer marking material thereto in a first transfer operation. 
     As best shown in  FIG. 5 , each exemplary xerographic marking device  102  includes a photoreceptor drum  104 , a pre-transfer charging subsystem  106 , by which the toner image of a given color (e.g., cyan, magenta, yellow, black, or one or more spot toners or gamut extension colors such as orange or violet) is developed on the photoreceptor  104  and transferred electrostatically to the intermediate transfer structure  103  using a biased transfer roller (BTR)  105  located on the inside of the intermediate transfer belt  103 . The BTR  105  operates at a transfer field value provided by a field strength control device according to a first transfer field level signal or value provided by the controller  122  for setting the transfer field used by the device  102  to transfer marking material, in this case, toner, to the structure  103 . In operation of the device  102 , marking material (e.g., toner  114  for the first (Yellow) device  112  detailed in  FIG. 2 ) is supplied to the drum  104 . In a first transfer operation, a surface of the intermediate medium  103  is adjacent to and/or in contact with the drum  104  and the toner  114  is transferred to the medium  103  with the assistance of the biased transfer roller  105 , where the BTR  105  induces charge into the BTR and the intermediate structure surface  103  to attract oppositely charged toner  114  from the drum  104  to the belt surface  103  as it passes through a nip created between the drum  104  and the charged transfer roller  105 , where the transfer charging is controlled by a bias control  101  operated by the system controller  122 . The toner  114  ideally remains on the surface of the ITB  103  after it passes through the nip for subsequent transfer (along with any other toner transferred by downstream devices  102 ) and ultimately fusing to the final print media  108  via the secondary transfer device  107  and fuser  110 . 
     As also shown in  FIG. 2 , the individual marking devices  102  may include one or more sensors  160  for sensing toner adhesion, toner mass per unit area, or other marking material transfer characteristic associated with the drum  104  and/or the intermediate transfer structure  103 . The device-specific sensors  160  in  FIG. 2  provide input signals or values to the controller  122 , such as an optical (e.g. reflective) sensor  160  downstream of the BTR  105  for sensing the residual mass per unit area (RMA) of marking material (e.g., toner)  114  not transferred from the drum  104  to the belt  103 , and an optional sensor  160  upstream of the BTR  105  for sensing the developed toner mass per unit area (DMA) or an optional sensor (e.g. an optical reflectance sensor)  160  downstream of the BTR  105  for sensing the transferred mass per unit area on the ITB  103 . One or more sensors  160  may be provided for measuring a marking material transfer condition of the medium  114  separate from any of the marking devices  102 , such as the sensor  160  shown in  FIG. 2 . Any type of sensor or sensors  160  may be employed which measure or sense toner state characteristics from which the toner transfer state of the marking device  102  can be derived. Suitable types of sensors  160  are described in DiRubio et al., U.S. Pat. No. 7,190,913, filed Mar. 31, 2005, owned by the assignee of the present disclosure, the entirety of which is incorporated by reference. 
     In normal operation, the marking devices  102  (e.g.,  FIG. 4 ) may suffer from incomplete transfer in which case a small amount of toner  114  remains on the drum  104  downstream of the BTR  105 , particularly for low transfer field levels. The exemplary sensor  160  is operatively coupled with the controller  122  and located proximate the downstream side of the drum  104  to detect the amount of untransferred toner  114  remaining on the drum  104 , where the illustrated example provides the sensor  160  as a residual mass per unit area (RMA) sensor that measures or senses the mass of residual toner  114  per a given area on the drum surface remaining after the drum  104  passes the nip at the BTR  105 . The device  102  (or the system  200  generally) can optionally include additional sensors, such as a transferred mass/area (TMA) sensor for sensing the amount of toner  114  that is transferred to the intermediate medium  103 , and a developed mass/area (DMA) sensor that detects the amount of toner  114  supplied on the drum  104  upstream of the nip at the BTR. 
     As illustrated in  FIGS. 2 and 4 , any integer number N marking devices  102  may be included in the system  200  of  FIG. 1 , where N is two or more. In one exemplary implementation, the system  200  may include six such marking devices  102 , as in the example of  FIG. 4 , and typical systems  200  may include four devices  102 , one each for yellow (Y, toner  114 ), magenta (M, toner  124 ), cyan (C, toner  134 ) and black (K, toner  144 ). The marking devices  102  individually include at least one first transfer field component (e.g.,  106  in  FIG. 5 ) controlling a first transfer field used to transfer marking material onto the intermediate transfer structure  103  with a transfer field control input receiving a first transfer field level signal or value  101  from the controller  122 . Each of the xerographic marking devices  102  is operable under control of the controller  122  to transfer toner of a corresponding color to the intermediate transfer belt  103 , where the first device  102  encountered by the ITB  103  in one example provides yellow toner  114 , the next device provides magenta toner  124 , the next provides cyan toner  134 , and the last device  102  provides black toner  144 , although other organizations and configurations are possible in which two or more marking devices  102  are provided. 
     The system  200  in  FIG. 4  includes an embodiment of the document processing system  100  with six marking stations  102  along with a transfer station  106 , a supply of final print media  108 , and a fuser  110  as described in  FIG. 2  above. In normal operation, print jobs  118  are received at the controller  122  via an internal source such as a scanner and/or from an external source, such as one or more computers  116  connected to the system  200  via one or more networks  124  and associated cabling  120 , or from wireless sources. Moreover, user prompting and selections can be made using a user interface  123  associated with the system  200  and/or with the computers  116 . 
     As shown in  FIGS. 2 and 4 , the system  200  also includes a secondary transfer component  106  ( FIG. 2 ) disposed downstream of the marking devices  102  along a lower portion of the intermediate belt path to transfer marking material in a second transfer operation from the belt  103  to an upper side of a final print medium  108  (e.g., precut paper sheets in one embodiment) traveling along a path from a media supply ( FIG. 4 ). After the transfer of toner to the print medium  108  at the transfer station  107 , the final print medium  108  is provided to a fuser type affixing apparatus  110  on the path in which the transferred marking material is fused to the print medium  108 . The system  200  may also include a scanner or other suitable image sensing apparatus downstream of the secondary transfer component  106  for sensing the image created by the first and second transfer operations, and providing corresponding image signals or values to the controller  122 . 
     The controller  122  is operative to perform various control functions and may implement digital front end (DFE) functionality for the system  200 , where the controller  122  may be any suitable form of hardware, processing component(s) with processor-executed software, processor-executed firmware, programmable logic, or combinations thereof, whether unitary or implemented in distributed fashion in a plurality of components, wherein all such implementations are contemplated as falling within the scope of the present disclosure and the appended claims. In a normal printing mode, the controller  122  receives incoming print jobs  118  and operates the marking devices  102  to transfer marking material onto the intermediate medium  103  in accordance with the print job  118 , in particular, by providing first transfer field level signals or values  101  to control the transfer fields of the first transfer field components  105 . The controller  122 , moreover, operates the secondary transfer component  107 , the fuser  110 , and interfaces with the various sensors  160  and the network  124  in the illustrated embodiments. 
     With particular reference to  FIG. 2 , it can be seen that the transfer belt moves in a direction  190 , with caving tension adjustment  180  initiated by a sensor  160 . The developing color section  170  transfers color patches of yellow  110 , magenta  120 , cyan  130  and black  140  via preselected electrostatic fields imparted on the print drums  112 ,  122 ,  132 ,  142  respectively, for inks of yellow  114 , magenta  124 , cyan  134  and black  144 . Nip  109  is disposed at the first junction at which the print drum  112  intersects the belt  103  and support roll  105 . The nips are located where the yellow  116 , magenta  126 , cyan  136  and black  146  photoreceptors meet the intermediate transfer belt  103 . The color transfer is completed when ink is supplied for yellow  118 , magenta  128 , cyan  138  and black  148 . The lead patch  138  is made up of a yellow patch and a magenta patch in a two-step process which usually involves the subject retransfer. 
     More particularly, one retransfer step occurs at the cyan nip  136  and one occurs at the black nip  146 . During retransfer air breakdown occurs within the first transfer nip thus transferring wrong sign toner. The wrong sign toner retransfers to the photoreceptor drums, away from the intended intermediate transfer belt. The retransfer defect is spatially non-uniform, which can cause the final print to look mottled and non-uniform. Because of the amount of retransfer nips that a particular image may go through during the printing process, a process often referred to as a retransfer history, this defect is especially noticeable in blended color patches such as red (Y+M) and green (Y+C). The density of the retransferred ink is measured by sensor  160  and may vary by the adjustment of distance between ink drums. 
     The printer develops and transfers several control patches. These patches are transferred at different electrostatic set points. The proposed control strategy utilizes one or more density sensors to measure the transferred toner patches and uses the density information to compute the optimal value of electrostatic transfer bias. The control strategy can allow the print operators to adjust the optimal value of electrostatic transfer bias based on their preferences, which provides a more robust first transfer system. The control strategy can be applied to more than four colors xerographic intermediate transfer belt  103  marking engines. 
       FIG. 3  presents the present applications, proposed principles of operation  200 . The intermediate transfer belt  150  contains two sections  270 ,  280  containing color control and traveling in a forward  260  direction. During the setup process prior to a print job, the printer develops three or more yellow  210 , magenta  220 , red  230 , and green  240  control patches. The color of control patches are designed depending on the marking engine architecture. The patches are transferred using bias set points. A bias set point is a voltage level at which a first transfer is set to occur, and is measured in either microamps or volts. The initial value is a nominal value and subsequent patches are set to variance point at three or more electrostatic transfer bias set points such as, but not limited to, nominal, ±10%, ±20%. The density sensors  250 , located after the last first transfer nip, measure the density of these patches. The control algorithm then determines the highest density of each color patch from the sweep of transfer bias set points, and through transfer functions, computes the optimal first transfer bias. The control patches may or may not be transferred to a substrate at second transfer. If the decision is to skip second transfer they will be removed via the intermediate transfer belt (ITB)  103  cleaning process. 
     An optimal first transfer point is the best layer single and uses a function to allow a user to specify and enter a set of complex color weights in order to optimize performance. In the present case, the weighted colors would be cyan, magenta, yellow, and black. In alternative embodiments, the four colors could be different and there may be more than four colors or less than four colors. Responses from sensors are used to evaluate the final output color. 
     The following transfer function computes the optimal first transfer bias:
 
Optimal First Transfer Bias=(0.5*( R+G )−0.5*( M+Y ))* X+ 0.5*( M+Y )
 
Where R=transfer bias when red patch&#39;s density is the highest
 
     G=transfer bias when green patch&#39;s density is the highest 
     M=transfer bias when magenta patch&#39;s density is the highest 
     Y=transfer bias when yellow patch&#39;s density is the highest 
     and X=weight from 0 to 1, where 0: single separation is more desired and 1: blended color is more desired, X can be operator adjustable based on his/her preference. By default, X is set to 0.5. 
     When the optimal first transfer bias is applied, the operator can opt to print a sample for viewing and making any changes. If the operator is satisfied with the print quality, the operator then accepts the settings and runs his print job. If he does not accept the settings, then he can adjust the X level via a graphical user interface on the printer. The printer then readjusts according to the input value and re-prints the print sample for the operator to approve. In other configurations, the printer can automatically print the job without seeking the operator&#39;s input. This process may be repeated iteratively until the operator is satisfied with the settings and print quality, or the operator ceases to enter data or make choices, or indicates otherwise. 
     For example, during setup, the blended colors transferred optimally at 30 uA and single colors transferred optimally at 20 uA. The printer computes 25 uA as the optimal first transfer bias. The operator then selects to view the print sample. If the operator wishes to print a monochrome job, he can adjust the weight value of X to zero. The printer then readjusts the optimal first transfer bias to 20 uA and makes a print sample. If the operator accepts the new print, the operator starts his print job. If not, he repeats the process until he is satisfied with the print sample. 
     The proposed embodiments are significantly advantageous over current xerographic intermediate transfer belt  103  transfer control strategy because they take into account image content, which ultimately is important to end-users and customers. In addition, the proposed method is fairly easy and affordable to integrate since it utilizes hardware that exists in today&#39;s printing systems. 
       FIG. 5  presents a detailed view of the nip  109  at which the print head  104  impacts the print media  151 , which is supported by a roller  105  on the opposite side of the print media under the nip  109 . A sensor  160  measures the density of the ink placed on the print media  151  and transmits the data to a system controller  122 . This information is used to transmit signals  101  to a grounded controller unit  161 . 
       FIG. 1  presents an embodiment comprising the method  10  of inserting transfer bias values and measuring the results with a sensor  160  in order to employ the method in a manner that reduces transfer bias. First, three transfer biases are selected  11  and the transfer biases set points are selected  20 . Then the transfer biases and set points are used to print test patches  30 . A sensor  160  then measures the density of the test patches  40 . From this, a control algorithm determines the highest density of each color patch  50  and the data is used as an optimal first transfer bias is computed  60 . Highest density can generally be associated with lowest retransfer problems. After the settings have been set up, a sample is printed and evaluated  70 . If the operator evaluates  80  (such as through a user interface  123 ) and is satisfied  97  with the sample printing, then the settings are saved and used to print a print job. However, if the setting evaluated to be not satisfactory  93 , then the process is repeated  95  starting with selecting transfer bias points  20 . This method may employ a computer processor to perform the calculations necessary to incorporate input user data, to perform calculations on that or any other data, and to interpret and perform calculations on any of the sensor data. 
     It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.