Abstract:
A fuser control system and method are provided for improving the image control of printing systems, including digital front-end processors, color printers and post-finishing system. This automatic image control system, including measurement and calibration, by registering the thermal load measurements.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to the field of printing, and more particularly to processes and apparatus for maintaining quality in digital reproduction systems by controlling the fuser used electrostatographic printing process, and in particular to the control of temperature of fusing members. 
     BACKGROUND OF THE INVENTION 
     In electrostatographic imaging and recording processes such as electrophotographic reproduction, an electrostatic latent image is formed on a primary image-forming member such as a photoconductive surface and is developed with a thermoplastic toner powder to form a toner image. The toner image is thereafter transferred to a receiver, e.g., a sheet of paper or plastic, and the toner image is subsequently fused to the receiver in a fusing station using heat or pressure, or both heat and pressure. The fuser station can include a roller, belt, or any surface having a suitable shape for fixing thermoplastic toner powder to the receiver. 
     The fusing step in a roller fuser commonly consists of passing the toned receiver between a pair of engaged rollers that produce an area of pressure contact known as a fusing nip. In order to form the fusing nip, at least one of the rollers typically has a compliant or conformable layer on its surface. Heat is transferred from at least one of the rollers to the toner in the fusing nip, causing the toner to partially melt and attach to the receiver. In the case where the fuser member is a heated roller, a resilient compliant layer having a smooth surface is typically used which is bonded either directly or indirectly to the core of the roller. Where the fuser member is in the form of a belt, e.g., a flexible endless belt that passes around the heated roller, it typically has a smooth, hardened outer surface. 
     Most roller fusers, known as simplex fusers, attach toner to only one side of the receiver at a time. In this type of fuser, the roller that contacts the unfused toner is commonly known as the fuser roller and is usually the heated roller. The roller that contacts the other side of the receiver is known as the pressure roller and is usually unheated. Either or both rollers can have a compliant layer on or near the surface. In most fusing stations having a fuser roller and an engaged pressure roller, it is common for only one of the two rollers to be driven rotatably by an external source. The other roller is then driven rotatably by frictional contact. 
     In a duplex fusing station, which is less common, two toner images are simultaneously attached, one to each side of a receiver passing through a fusing nip. In such a duplex fusing station there is no real distinction between fuser roller and pressure roller, both rollers performing similar functions, i.e., providing heat and pressure. 
     Two basic types of simplex heated roller fusers have evolved. One uses a conformable or compliant pressure roller to form the fusing nip against a hard fuser roller, such as in a DocuTech 135 machine made by the Xerox Corporation. The other uses a compliant fuser roller to form the nip against a hard or relatively non-conformable pressure roller, such as in a Digimaster 9110 machine made by Heidelberg Digital L.L.C. A fuser roller designated herein as compliant typically includes a conformable layer having a thickness greater than about 2 mm and in some cases exceeding 25 mm. A fuser roller designated herein as hard includes a rigid cylinder, which may have a relatively thin polymeric or conformable elastomeric coating, typically less than about 1.25 mm thick. A compliant fuser roller used in conjunction with a hard pressure roller tends to provide easier release of a receiver from the heated fuser roller, because the distorted shape of the compliant surface in the nip tends to bend the receiver towards the relatively non-conformable pressure roller and away from the much more conformable fuser roller. 
     A conventional toner fuser roller includes a cylindrical core member, often metallic such as aluminum, coated with one or more synthetic layers, which typically include polymeric materials made from elastomers. 
     One common type of fuser roller is internally heated, i.e., a source of heat for fusing is provided within the roller for fusing. Such a fuser roller normally has a hollow core, inside of which is located a heating source, usually a lamp. Surrounding the core is an elastomeric layer through which heat is conducted from the core to the surface, and the elastomeric layer typically contains fillers for enhanced thermal conductivity. A different kind of fuser roller that is internally heated near its surface is disclosed by Lee et al. in U.S. Pat. No. 4,791,275, which describes a fuser roller including two polyimide Kapton® sheets (sold by DuPont® and Nemours) having a flexible ohmic heating element disposed between the sheets. The polyimide sheets surround a conformable polyimide foam layer attached to a core member. According to J. H. DuBois and F. W. John, Eds., in Plastics, 5th Edition, Van Nostrand and Rheinhold, 1974, polyimide at room temperature is fairly stiff with a Young&#39;s modulus of about 3.5 GPa-5.5 GPa (1 GPa=1 GigaPascal=10.sup.9 Newton/m.sup.2), but the Young&#39;s modulus of the polyimide sheets can be expected to be considerably lower at the stated high operational fusing temperature of the roller of at least 450 degrees F. 
     An externally heated fuser roller is used, for example, in an Image Source  120  copier, and is heated by surface contact between the fuser roller and one or more external heating rollers. Externally heated fuser rollers are also disclosed by O&#39;Leary, U.S. Pat. No. 5,450,183, and by Derimiggio et al., U.S. Pat. No. 4,984,027. 
     A compliant fuser roller may include a conformable layer of any useful material, such as for example a substantially incompressible elastomer, i.e., having a Poisson&#39;s ratio approaching 0.5. A substantially incompressible conformable layer including a poly(dimethyl siloxane) elastomer has been disclosed by Chen et al., in the commonly assigned U.S. patent application Ser. No. 08/879,896, now U.S. Pat. No. 6,224,978, which is hereby incorporated by reference. Alternatively, the conformable layer may include a relatively compressible foam having a value of Poisson&#39;s ratio much lower than 0.5. A conformable polyimide foam layer is disclosed by Lee in U.S. Pat. No. 4,791,275 and a lithographic printing blanket are disclosed by Goosen et al. in U.S. Pat. No. 3,983,287, including a conformable layer containing a vast number of frangible rigid-walled tiny bubbles that are mechanically ruptured to produce a closed cell foam having a smooth surface. 
     Receivers remove the majority of heat during fusing. Since receivers may have a narrower length measured parallel to the fuser roller axis than the fuser roller length, heat may be removed differentially, causing areas of higher temperature or lower temperature along the fuser roller surface parallel to the roller axis. Higher or lower temperatures can cause excessive toner offset (i.e., toner powder transfer to the fuser roller) in roller fusers. However, if differential heat can be transferred axially along the fuser roller by layers within the fuser roller having high thermal conductivity, the effect of differential heating can be reduced. 
     Improved heat transfer from the core to the surface of an internally heated roller fuser will reduce the temperature of the core as well as that of mounting hardware and bearings that are attached to the core. Similarly, improved heat transfer to the surface of an externally heated fuser roller from external heating rollers will reduce the temperature of the external heating rollers as well as the mounting hardware and bearings attached to the external heating rollers. 
     In the fusing of the toner image to the receiver, the area of contact of a conformable fuser roller with the toner-bearing surface of a receiver sheet as it passes through the fusing nip is determined by the amount pressure exerted by the pressure roller and by the characteristics of the resilient conformable layer. The extent of the contact area helps establish the length of time that any given portion of the toner image will be in contact with, and heated by, the fuser roller. 
     A fuser module is disclosed by M. E. Beard et al., in U.S. Pat. No. 6,016,409, which includes an electronically-readable memory permanently associated with the module, whereby the control system of the printing apparatus reads out codes from the electronically readable memory at install to obtain parameters for operating the module, such as maximum web use, voltage and temperature requirements, and thermistor calibration parameters. 
     In a roller fusing system, the fusing parameters, namely the temperature, nip-width, and speed of the fusing member, are fixed and controlled within certain specifications for a given range of receivers. Generally the system changes the temperature or/and speed according to the receiver weights or types. The changing of temperature in an internally heated fuser roller takes time to stabilize. If the receivers are presented at a too-rapid rate, the fuser roller may not have returned to its working temperature when the next receiver arrives. Consequently, the receivers must be stopped or slowed until the temperature of the fuser roller has come within acceptable range and such stopping or slowing results in degradation of receiver throughput rate. The same is true for speed changes. Regardless of whether the speed of presentation or the fuser roller temperature itself is being adjusted by the system, the temperature stabilization time required by a fusing member can constrain the speed of presentation of receivers. 
     The fixing quality of toned images of an electrophotographic printer depends on the temperature, nip-width, process speed, and thermal properties of the fusing member, toner chemistry, toner coverage, and receiver type. To simplify the engineering and control of a roller fusing system, as many as possible of the above parameters are considered and then fixed during the system&#39;s design. The fusing parameters such as temperature, nip-width, process speed, and thermal properties of the fusing member are optimized for the most critical case. 
     Complicating the system&#39;s design is the fact that the toner coverage and the receiver type (weight, coated/uncoated) can vary from image to image in a digital printer. Therefore, some of the above listed parameters need to be adjusted according to the image contents and the receiver types to assure adequate image fixing. Typically, the fuser temperature is adjusted and kept constant for a dedicated run with a particular receiver. The temperature is adjusted higher from the nominal, for heavier receivers and lower for lighter receivers. For some heavy receivers, the speed must also be reduced. 
     The change of fuser temperature and/or reduction of speed results in reduced productivity. Furthermore, if different receiver types are required in a single document, extra time is needed to collate images on different receivers into the document. 
     A digital printer with multiple paper supplies allows running RIPPED information that varies from image to image onto multiple receivers in a single document run. Since the RIPPED image may vary from one occurrence to the next, both in image color and image density, the workload on the fuser may vary significantly. U.S. Pat. No. 5,956,543, issued to Aslam et al. optimizes the image fixing of toned images on a specified receiver by optimally selecting the fuser temperature, nip-width and speed. However, it does not address the image fixing quality issues when multiple types and weights of receivers are mixed during a document mode operation of an electrophotographic printer. 
     The present invention overcomes this shortcoming by making fuser temperature control more efficient and accurate and allowing it to occur automatically during the printing run. The following invention solves the current problems with fuser temperature control in a wide variety of situations. 
     SUMMARY OF THE INVENTION 
     In accordance with an object of the invention, both a system and a method are provided for improving the temperature control of a fuser in a printing system, and specifically the efficiency and accuracy of the fuser control. More specifically, the invention relates to the use of a system to control the transfer of heat rapidly to a fuser roller in an electrophotographic printer and the resulting fuser temperature based on a number of factors including receiver type and fuser temperature. The invention uses stored media process set points, input image content, and input media type data to regulate the heat transfer rate by varying the nip width between the fuser roller and the receiver. The adjustments are sufficiently rapid and accurate that the invention allows for the printing of many different media weights and types in a print run without restrictions on media run lengths, without collation requirements per run, and without productivity losses due to slowing of feed rate for heavier receivers. One embodiment of this method includes the automatic adjustment of thermal load measurements for use in a receiver catalogue. 
     While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, it is believed the invention will be better understood from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the characteristics of this invention the invention will now be described in detail with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic illustration of a printer system according to the present invention for use in conjunction with a image control system and method. 
         FIG. 2  is a schematic diagram of the fuser assembly according to this invention; 
         FIG. 3  is a schematic diagram showing the heating rollers and the fuser roller, and the nips between them, for the fuser assembly of  FIG. 2 ; 
         FIG. 4  is a schematic diagram showing the fuser roller and the pressure roller, and the nip between them, for the fuser assembly of  FIG. 2 ; 
         FIG. 5  is a schematic diagram showing a fuser roller with a single backup roller; 
         FIG. 6  is a graph showing the relationship between the applied load and nipwidth, according to this invention, giving the power transferred at different levels of load; and 
         FIG. 7  is a block diagram of the fuser control mechanism according to this invention, illustrating the process steps according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus and methods in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     Various aspects of the invention are presented in  FIGS. 1-7  which are not drawn to scale and in which like components are numbered alike. According to one aspect of the invention, the thermal response of the fuser with sheets being fed through the fuser is simulated in the fuser prior to feeding sheets through the fuser. The thermal response may be simulated in a manner that minimizes thermal droop, or it may be simulated in a manner that maintains a tentering force, or it may be simulated in a manner that accomplishes both. According to a further aspect of the invention, the thermal response of the fuser with sheets being fed through the fuser is controlled to maintain a desired tentering force. The desired tentering force may be varied based on sheet width, or sheet heat absorbing capacity, or sheet stiffness, or combinations of these (all combinations thereof being included within the purview of the invention). 
       FIG. 1  shows a fuser  10  including a fuser roller  12  and a pressure roller  14 . The fuser  10  further has a fuser roller heater  16 , and a fuser temperature sensor  18 , which inputs to a logic and control system  20 , also referred to as a Logic Control Unit (LCU), that controls the heating of the fuser roller heater  16 . The fuser  10  has a run condition, and an idle condition. The fuser roller  12  and the pressure roller  14  form a nip  22 . A receiving sheet, also referred to as a receiver,  24  is considered to have entered the fuser  10  when it has entered the nip  22 . The heater  16  may be electrothermal, radiative, convective, or other heat source suitable for fusing images, internal or external to the fuser roller, the particular type of heat source not being critical in the practice of the invention. 
     According to an aspect of the invention, an improved method of operation of a fuser  10  for fixing toner images to a receiver  24  includes controlling the thermal load in conjunction with the fuser roller  12  prior to the receiver entering the fuser  10 , by using the logic and control system  20  to activate the fuser roller heater  16  in response to information contained in the LCU  20 . By performing a run (virtual or real) including fusing the toner to the sheet of receiver media by a fusing member during an interframe interval before the arrival of each receiver, wherein the nip width between the external heating rollers and the internally heated roller are adjusted to vary the amount of heat and the resulting temperature related information used to set a new thermal set point and stored for future runs. 
     This helps prevent thermal droop because it eliminates the lag time between the arrival of the receiver  24  and the activation of the fuser roller heater  16 . 
     A schematic diagram of one fuser assembly disclosed in this invention is shown in  FIG. 2 . The fuser assembly includes a fuser roller  12  and a pressure roller  14 . Fuser roller  12  is heated with an internal heat source  16  (lamp) and external heating rollers  30  and  32  that make up a nip  26  with fuser roller  12 . The number and sizes of external heating rollers and the sizes of the fuser roller  12  and pressure roller  14  depend on the printer process speed and the heat requirements for proper image fixing. Any toner or paper dust contamination on the heating members  30  and  32  is cleaned with a cleaning web  56  trained around takeup and supply rollers  40  and  42  respectively and corresponding back up rollers  36  and  38 . In alternative embodiments, the cleaning is accomplished by other mechanisms well known in the art, such as blade cleaning or tacky rollers for example. 
     The receiver (sheet) release from the fuser roller  12  and pressure roller  14 , is accomplished by a pair of air knives  58 . In alternative embodiments of the invention, mechanical pawls or skive fingers for example, are utilized for receiver stripping, replacing the air knives. Further, toner offset prevention is accomplished by application of a release fluid to the fusing member rollers. The release fluid applicator is not shown in the diagram, but either a donor roller type or a web type applicator may be employed. 
     The fuser roller  12  includes an aluminum core  44 , an elastomeric base-cushion  46  (relatively more compliant than the pressure roller), a conductive elastomeric intermediate layer  48  (5 to 10 mm (mils)) thick depending on the process speed), and finally a thin (1-2 mm (mils)) top release coating  50 . The external heating rollers  30 ,  32  are conductive metallic (steel, aluminum, etc.) cores with a finished metalized hard surface such as chrome, nickel, anodized aluminum, etc. Other embodiments of the external heating rollers use conductive Teflon® based coatings on the respective conductive cores. 
     The external heating rollers  30 ,  32  are heated with internal lamps  34 . A predetermined desired temperature of fuser roller  12  is maintained by an internal heat lamp  16  during the standby mode when external heating rollers  30 ,  32  are not engaged. The heat input for fusing of toner comes mainly from external heating rollers  30 ,  32  to the fuser roller  12  during the print mode. A limited amount of additional heat comes from the fusing roller&#39;s internal heat source  16  as a thermal ballast during the print mode to keep the core of the fuser roller  12  within the desired predetermined temperature range. 
     A sheet S n  bears a toner image I n . As indicated in  FIG. 7 , the toner content of the image and the type of media that receives the image are provided to the digital front end  205  (hereafter referred to as DFE) associated with the printer. The digital front end  205  and media catalog  212  provide the printer machine control  210  with signals representing respectively image content, and type of media and parameters of such media type being used. For quality control purposes, the apparatus has a media sensor  201  that senses the type and weight of the sheet S n  and an image content sensor  202  senses the amount of toner that forms the image, I n . The heating roller controller  220 , associated with the machine control  210 , controls the nip between heating rollers  30 ,  32  and  12  as well as the temperature of each heating roller  30 ,  32 . The fuser roller nip width controller  230 , associated with the machine control  210 , controls the temperature of roller  12  and the nip between rollers  12  and  14 . 
     The fuser assembly according to this invention adjusts the fuser member roller  12 , temperature to various set points by changing the nip width  62  (see  FIG. 3 ) or contact time between the heating rollers  30 ,  32  and the fuser member roller. The temperature of the heating rollers  30  and  32  is maintained constant, but the heat input to the fuser roller  12  is controlled by the heating roller nip width (dwell time)  62  between the heating rollers and the fuser member roller. The graph of  FIG. 6  shows an example of the relationship between the applied load and nipwidth and corresponding power that can be transferred to the fuser roller for every 10.degree. C. temperature difference between the heating rollers and the fuser roller. 
     The fuser assembly according to this invention also applies print engine intelligence as referred to above. The fuser process set points (fuser nipwidth, fuser member temperature, and energy requirements) for various types of media are stored as lookup tables in a media catalog  212  for the machine control unit  210  (see  FIG. 7 ). The media can include heavy stock cover material, interior page print material, insert material, transparency material, or any other desired media to carry text or image information. A typical machine control unit  210  includes a microprocessor and memory or microcomputer. It stores and operates a program that controls operation of the machine in accordance with programmed steps and machine inputs, such as temperature of the fusing rollers. Temperature data is supplied, for example, by a thermocouple (not shown) or any other suitable thermal sensor in a manner well known to those skilled in the art. As a sheet of a specific media type is requested, the DFE  205  provides a data signal to the machine control unit  210  (or alternatively, directly to an independent control for the fuser assembly) that is representative of the image contents and the type of media sheet coming to be fixed. The machine control unit  210  sets the fuser conditions (temperature; dwell time) from the media catalog  212  as a function of the data provided by the DFE  205 . Machine control unit  210  directs the heating roller nip width control  220  for heating rollers  30 ,  32  to adjust the nipwidth  62  according to the power requirements of the fuser roller  12  per the information provided from media catalog  212 . Machine control unit  210  also directs the fuser roller nip width controller  230  for fusing roller  12  and pressure roller  14  to adjust the fuser nip  64  per the information provided from media catalog  212 . 
     The energy in the fuser roller  12  is stored only in its top coating and the conductive intermediate layer (5-10 mm (mils)). See  FIG. 4  and  FIG. 7 . Therefore, after the passage of each sheet through the fuser nip  22 , the fuser surface temperature drops significantly and heat energy needs to be restored back in the fuser roller  12  by the heating rollers  30 ,  32  during their contact time. Since the heating rollers  30 ,  32  are made of thermally conductive materials; the heat transfer rate to the fuser roller  12  is fast. As one media type is followed by a different media type, the machine control unit  210  is informed of the different types and it loads the corresponding fuser setup conditions from the media catalog  212 . Consequently the fuser nip  22 , as well as fuser roller temperature (driven by the nipwidth  62 ) is adjusted to the correct value during the inter-frame between two sheets. Both controllers  220  and  230  change the respective nips  26  and  22  dynamically, in any well-known manner, during the inter-frame between two sheets. 
     Each nip control may include a cam and a stepper motor for a fixed displacement nip, a set of air regulated cylinders for constant load nip, a combination of both, or any combination of these and other electromechanical mechanisms well-known in the art. Since the temperature of the fusing roller (as driven by the heating rollers nip) and the nipwidth between the fusing and pressure members can be manipulated and adjusted for each sheet, such a fusing assembly system allows mixing of many different media weights and types seamlessly without any restriction on the run length of each media. 
     In distinct embodiments of the invention, the fusing member may be in the form of a roller, a belt or a sleeve, or variations thereof as are well known in the art. 
     In a further embodiment of the invention (see  FIG. 5 ), the cleaning web  56  may be placed in contact with the external heating rollers  30 ,  32  using only a single back up roller  36 . 
     The invention confers the advantage of enabling the printer to run jobs in document mode while mixing a variety of receivers, without loss of productivity or fusing quality. The invention also facilitates seamless printing on the widest possible ranges of media types and weights. 
     When the system detects a thermal load difference, that is a difference between a stored or expected, target thermal load and a detected thermal load that is different then expected then additional steps can be taken. The detected thermal load is first compared to the thermal load values, or thermal load related values, stored in the LCU, for that receiver, as discussed above. If the thermal difference exceeds a threshold amount (for that receiver and print job type) then the system will alert, through a reply function, the operator or system and suggest a new value that can be utilized in subsequent runs. This function would be implemented in conjunction to the embodiments described above or as a stand-alone method and a signal can be generated if the thermal load difference is above the threshold amount Although it could be partially automated it is important that the operator is able to override any such alert since the alert could be due to a desired change in the receiver and is preferably an alert system and method. 
     Those skilled in the art understand that the functional elements of the sensors  201 ,  202  and the controllers  220 ,  230  may be implemented in different ways. In lieu of actual sensors, the machine may be pre-set for specific media types, weights and toner content. Likewise, the controllers  220 ,  230  may use electric stepper motors, hydraulics or pneumatic operators and other equivalent means to move the rollers and set the nips. 
     The invention has been described in detail with particular reference to certain preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     PARTS LIST Fuser  10 ) fuser roller  12 ) pressure roller  14 ) fuser roller heater  16 ) fuser temperature sensor  18 ) LCU  20 ) Nip ( 12 &amp;  14 )  22 ) receiver  24 ) Nip ( 30 ,  32  and  12 )  26 ) external heating roller  30 &amp; 32 ) back up rollers  36 ,  38 ) supply rollers  40 ,  42 ) aluminum core  44 ) elastomeric base cushion layer  46 ) elastomeric intermediate layer  48 ) top release coating  54 ) cleaning web  56 ) pair air knives  58 ) external heating roller nip width  62 ) fuser nip  64 ) media sensor  201 ) image content sensor  202 ) digital front end (DFE)  205 ) machine control unit  210 ) media, which could be the difference between the target and detected thermal load catalog  212 ) heating roller controller  220 ) fuser roller nip width controller  230 ).