Patent Publication Number: US-7218875-B2

Title: Apparatus and process for fuser control

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a 111A application of Provisional Application Ser. No. 60/556,091, filed Mar. 24, 2004, entitled APPARATUS AND PROCESS FOR FUSER CONTROL by Susan C. Baruch, et al. 

   BACKGROUND OF THE INVENTION 
   The invention is in the field of fusing and fusing apparatus for print media, particularly for fusing toner to print media and other variations. 
   Fusers are commonly implemented in electrographic print systems to fix toner, for example, to a print media such as a sheet of paper or plastic. Fuser temperature may be maintained by a feedback control loop that senses fuser roller surface temperature and turns heater lamps on and off in a pulse-width-modulated duty cycle to maintain roller temperature at a setpoint. At the beginning of a run, if the system has been in standby mode, fuser roller temperature is at, or very near, the desired setpoint. During the run, fuser roller temperature will undergo a transient decline, reaching a minimum and then begin to recover, eventually coming back up to the setpoint. During the transient, fuser roller temperature can fall to a level where fusing quality is compromised with reduced adhesion of the toner and increased crack-width in the fused toner. The amount of this transient “droop” depends on the heat capacity of the receiver, which in turn depends on the specific heat and mass of the receiver sheet. 
   Heavy coated papers represent a worst case due to greater mass and specific heat. One control scheme uses proportional-integral control with added feed-forward compensation to try to anticipate the transient droop and compensate by adding additional heat. The feed-forward is open loop since there is no sensor to measure heat removed by the receiver. An improved apparatus and control system is desired. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic representation (end view) of a fuser assembly according to an aspect of the invention. 
       FIG. 2  is a plot of temperature versus time according to an aspect of the invention. 
       FIG. 3  is a schematic representation (end view) of a fuser assembly according to a further aspect of the invention. 
       FIG. 4  is a schematic representation (side view) of a fuser assembly according to a further aspect of the invention. 
       FIG. 5  is a schematic representation (end view) of a fuser assembly according to a further aspect of the invention. 
       FIG. 6  is a plot of heat power versus print media width according to further aspect of the invention. 
       FIG. 7  is a bottom view of the  FIG. 5  fuser assembly showing the heater rollers. 
       FIG. 8  is a plot of heat power versus print media width according to further aspect of the invention. 
       FIG. 9  is a schematic representation of an embodiment having a distributed control system. 
       FIG. 10  is a schematic representation (end view) of a fuser assembly according to a further aspect of the invention. 
       FIG. 11  presents a process according to an aspect of the invention. 
       FIGS. 12 and 13  present schematic diagrams of an electrographic marking or reproduction system in accordance with the present invention. 
       FIG. 14  presents a plot of torque versus time for a fuser roller. 
       FIG. 15  presents a plot of fusing force versus time. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Various aspects of the invention are presented in  FIGS. 1–15 , which are not drawn to any particular scale, and wherein like components in the numerous views are numbered alike. Referring now to  FIGS. 1 and 2 , a fusing apparatus  100  and process for an electrographic printer comprising a fusing nip  102  comprising two rollers  104  and  106 , and at least one heating nip  108 . A print media is fed through the fusing nip  102 , as is well known in the art. The heating nip  108  comprises a heater roller  110  and a first of the rollers  106 , and the heater roller  110  comprises a heat source  112 . A first temperature sensor  114  operative to sense a roller temperature  120  of the first of the rollers  106  is provided, a second temperature sensor  116  operative to sense a heater roller temperature  122  of the heater roller  110  is provided. A controller  118  is provided, the controller  118  being operative to regulate the roller temperature  120  while limiting a maximum heater roller temperature through interaction with the heat source  112  and with input from the first temperature sensor  114  and the second temperature sensor  116 . 
   In  FIG. 2 , there is a standby  124  prior to print media being fed to the fusing nip  102  for fusing wherein the heater roller temperature  122  is in a steady-state. The standby  124  may correspond to the heater roller temperature  122  being a temperature setpoint for fusing. A run  126  follows the standby  124  wherein a stream of print media is fed to the fusing nip  102 . Generally, at the end of run  126 , the fuser assembly  100  returns to standby  124 . During run  126 , the stream of print media draws a substantial quantity of heat energy out of the first of the rollers  106  causing an effect known as “temperature droop”, represented as the substantial dip in roller temperature  120 . The controller  118  responds by switching power on to the heat source  112  and the heater roller temperature  122  may increase (depending on the amount of heat transferred to the print media) until a time is reached wherein the controller  118  regulates the heater roller temperature  122 . Since, the controller  118  is operative to limit the heater roller temperature  122  while controlling the roller temperature  120 , the roller temperature  120  may not rise as quickly, indicated in  FIG. 2  by temperature plot  128 , since the controller  118  effectively caps the quantity of heat energy that heater roller  110  can deliver to the first of the rollers  106 . However, a quick rise in roller temperature  120  is still desired in order to minimize droop. If the fuser is still unable to transfer sufficient heat, as determined by the roller temperature  120 , skip frames can be added to the printing process (“skip frames” are a temporary reduction in printing rate, on a page-by-page basis, where no paper is passed through the fuser). 
   An advantage with this control scheme lies in regulating the heater roller temperature  122 , and indirectly the heat source  112 . Preferably, the controller  118  is operative to prevent the heater roller temperature  122  from exceeding a predetermined maximum heater roller temperature, which may prevent damage to the heater roller or burn-out of the heat source  112 , which may be a heat lamp, for example (of course other suitable heaters may be implemented, particularly electrothermal heaters). The effect of the controller  118  capping the quantity of heat energy that the heater roller  110  can deliver to the first of the rollers  106  may be offset by configuring the fuser assembly  100  to supply sufficient heat energy for a range of expected print media stocks. Thus, faster recovery from droop may be provided while also providing better control of the heat source  112 . 
   According to one embodiment, although not so limited, the controller  118  switches power on to the heat source  112  until the heater roller temperature  122  reaches a maximum heater roller temperature, and then the controller  118  switches power off to the heat source  112 . In response, the roller temperature  120  continues to increase but at a slower rate, the heater roller temperature  122  decreases and the controller switches power on and off to the heat source  112  cyclically until the roller temperature  120  reaches a controlled temperature at the temperature setpoint for fusing. 
   Still referring to  FIGS. 1 and 2 , and according to a further embodiment, another heating nip  109  may be provided comprising another heater roller  111  and the first of the rollers  106 . The another heater roller  111  comprises another heat source  113 . A third temperature sensor  117  is provided operative to sense another heater roller temperature of the another heater roller  111 . The controller  118  is operative to control the roller temperature while limiting the another heater roller temperature  123  of the another heater roller  111  through interaction with the another heat source  113 . The controller  118  is in communication with the third temperature sensor  117 . The controller  118  may be operative to regulate the roller temperature  120  while limiting another maximum heater roller temperature through interaction with the heat source  112  and with input from the first temperature sensor  114  and the third temperature sensor  117 , analogous to the control of the heater roller temperature  122  as previously described herein, and as shown by temperature plot  132 . The controller  118  may be operative to prevent the another heater roller temperature  123  from exceeding another predetermined maximum heater roller temperature. 
   The controller  118  may be operative to establish a heating power ratio between the heat source  112  and the another heat source  113 . Temperature plot  134  represents the another heater roller temperature  123  for a desired heating power ratio. The desired heating power ratio may not be achieved, as indicated by temperature plots  130  and  132 , since regulating the temperature of the roller  106  and the temperatures of the heat sources  112  and  113  may be a greater priority. Temperature plot  130  is an example where not as much heat power is needed to fuse the print media. Temperature plot  132  is an example where more heat power is needed to fuse the print media. Overall, the system is more responsive and flexible compared to prior art systems. Of course, there are many possible variations in the temperature plots and these examples are representative only to assist in understanding. 
   According to one embodiment, the two rollers  104  and  106  comprise a pressure roller and a fuser roller, respectively, the first of the rollers  106  being the fuser roller. The roller temperature is a surface temperature of the first of the rollers  106 , the heater roller temperature  122  is a surface temperature of the heater roller  110 , and the another heater roller temperature  123  is a surface temperature of the another heater roller  111 . 
   Referring now to  FIG. 3 , an apparatus  200  and process is presented according to a further aspect of the invention. Apparatus  200  comprises a fuser assembly  202  operative to fuse print media  210 , a thickness sensor  204  operative to sense a print media thickness  206 , and a controller  218  operative to control a fusing control parameter based at least in part upon the print media thickness  206 . The fuser assembly  202  may comprise a heat source  208 , and the at least one fusing control parameter may comprise heat power applied to the heat source  208 . The at least one fusing control parameter may also comprise a temperature setpoint for a temperature related to fusing in the fuser assembly  202 , for example the surface temperature of the roller  106 . As shown in  FIG. 4 , the fuser assembly  202  may comprise a loading mechanism operative to establish a fusing force in the fusing nip  102  (forcing the rollers  104  and  106  toward each other), and the at least one fusing control parameter may comprise the fusing force. The loading mechanism  210  may comprise any suitable mechanism for generating a fusing force, for example screws, cams, levers, pneumatics, hydraulics, and electromechanical devices (including motors and stepper motors). 
   Changing the fusing force may influence the temperature of certain components in the fuser assembly. For example, referring again to  FIG. 2 , reducing the fusing force during standby  124  tends to reduce wear on the rollers  106  and  104  and also tends to increase the heater roller temperature  122  and/or  123 . The fusing force may be increased just prior to the run  126 . 
   The thickness sensor  204  may be a multi-feed sensor located upstream from the fuser assembly (as shown in  FIG. 3 ). A multi-feed sensor may also be used to detect multiple feeds of print media from a media supply, and also includes the ability to sense the thickness of a single print medium. Multi-feed sensors are well known in the art. 
   Referring again to  FIG. 3 , the controller  218  may be operative to vary heating of the at least one heated roller  106  based at least in part upon the print media thickness  206  in advance of print media  210  reaching the fusing nip. According to one embodiment, the controller  218  is operative to regulate a fusing temperature of the at least one heated roller  106  according to a fusing setpoint temperature; the controller being operative to increase the fusing setpoint temperature in response to an increase in the print media thickness  206 . According to another embodiment, the controller  218  is operative to regulate a fusing temperature of the at least one heated roller  106  according to a fusing setpoint temperature; the controller  218  being operative to decrease the fusing setpoint temperature in response to a decrease in the print media thickness  206 . The controller  218  may be operative to do both. The fusing setpoint temperature may be a function of the print media thickness  206 . 
   The controller  218  may be operative to increase heating power in response to an increase in the print media thickness  206 . According to another embodiment, the controller  218  is operative to decrease heating power in response to a decrease in the print media thickness  206 . The controller  218  may be operative to do both. The heating power may be a function of the print media thickness  206 . 
   The fusing nip  102  may comprise a heated roller  106 , the controller being operative to increase heating power to the heated roller  106  in response to an increase in the print media thickness  206 . According to another embodiment, the fusing nip  102  comprises a heated roller  106 , the controller  218  being operative to decrease heating power to the heated roller  106  in response to a decrease in the print media thickness  206 . The controller  218  may be operative to do both. The heating power may be a function of the print media thickness  206 . 
   Referring now to  FIGS. 5 and 6 , a fusing apparatus  300  and a process for an electrographic printer according to a further aspect of the invention is presented. The fusing apparatus  300  is similar to apparatus  100  and comprises a heater roller  310  with a heat source  312 , and a controller  318  operative to establish a heating power for the heat source  312  dependent upon a print media width. At least three heating powers  320 ,  322 ,  324 , corresponding to at least three print media widths  330 ,  332 ,  334 , are provided. According to one embodiment, the controller  318  is operative to increase the heating power with an increase in print media width. According to another embodiment, the controller  318  is operative to decrease the heating power with a decrease in print media width. The controller  318  may be operative to do both. 
   Referring now to  FIG. 8 , the controller  318  may be operative to linearly increase the heating power from a first heating power  346  to a second heating power  348  with an increase in print media width from a first print media width  336  to a greater second print media width  338 . The controller may be operative to linearly decrease the heating power with a decrease in print media width from a second print media width to a lesser first print media width. The controller  318  may be operative to do both. 
   Referring now  FIGS. 5 and 7 , the heat source  312  within the heater roller  310  has an operable width  342  (over which the heat source  312  generates heat). Another heater roller  311  may be provided comprising another heat source  313 , the another heat source  313  having another operable width  340  (over which the heat source  313  generates heat), the operable width  342  being greater than the another operable width  340 . Referring again to  FIG. 8 , the controller  318  may be operative to establish another heating power  344  for the another heat source  313  not dependent upon the print media width. The another heating power  344  may be constant, for example. 
   Referring now to  FIG. 9 , an embodiment is presented comprising a fusing apparatus  400  and a process comprising a distributed controller  418 . Output from the temperature sensors  114 ,  116  and  117  is multiplexed by a multiplexer  402  to a thermistor amplifier board  404 . Output from the thermistor amplifier board is communicated to an analog to digital converter  406  and then to a feedback controller  408  that processes the information and communicates with a feed forward controller  410 . Output from the thickness sensor  204  is communicated to an analog to digital converter  412  and then to the feed forward controller  410 . Output from the feed forward controller is communicated to a first solid state relay  414  and a second solid state relay  416  that switch power to the heat source  312  and the another heat source  313  through a multiplexer  420 . 
   Referring now to  FIG. 10  an embodiment comprising a fusing apparatus  500  and a process is presented comprising moving a stream of print media  504  through a fuser assembly  502 , and changing at least one fusing control parameter while the stream of print media  504 , all of a same type, is moving through the fuser assembly  502 . A controller  506  may be provided that is operative to change the at least one fusing control parameter in accordance with this process. In the example presented in  FIG. 10 , the fuser assembly  502  comprises the two rollers  104  and  106 . The stream of print media  504  be a single stream, or one of a plurality of streams of print media. For example, a stream of print media  504 , all of a same type, may precede or follow a stream of print media  504 , all of a same another type. One or more print media of another type may be intermingled between streams, and/or placed at the beginning and/or end of a stream. As used herein, the term “stream” means at least two sheets, and may comprise at least three sheets, at least four sheets, or a multitude of sheets. 
   The process may comprise changing the fusing control parameter between ends (a leading edge and a trailing edge) of a single print media  505 . This may be implemented by the controller being operative to change the fusing control parameter in the manner just described. 
   The process may also comprise changing the fusing control parameter while the stream of print media  504 , all of the same type, is moving through the fuser assembly, based at least in part on a thickness of the print media, the size of the print media, and/or the bending stiffness of the print media. Again, this may be implemented by the controller  506  being operative to change the fusing control parameter in the manner just described. 
   The fusing assembly  502  may comprise the fusing nip  102  having a fusing force, the at least one fusing parameter being the fusing force. For example, the fusing force may be decreased from a beginning of the stream of print media  504  to an end of the stream of print media  504  (e.g. 90 to 100% of max load at the beginning, 75 to 85% of max load at the end). This may be implemented by the controller  506  being operative to decrease the fusing force concurrently with the stream of print media  504 . This process may compensate for heating and thermal expansion of the fuser roller over the length of a run, and minimize wrinkling of prints at the beginning of a run, maintain adequate nip load for good fusing quality during thermal droop, and then minimize image defects (“slapdown” or “lakes”) due to excessive differential overdrive at the end of the run. The process may also comprise monotonically decreasing the fusing force concurrently with the stream of print media  504 . 
   Alternatively or in addition, the process may comprise increasing fusing force upon a fusing temperature decreasing to a predetermined temperature. This may at least partially compensate for the decreased fusing temperature and provide suitable fusing, especially during thermal droop. Again, this may be implemented by the controller  506  being operative to increase the fusing force upon the fusing temperature decreasing to the predetermined temperature. 
   Still referring to  FIG. 10 , an example of a fusing nip loading mechanism  508  is presented comprising a lever  510  that rotates about a fixed pivot  512 . The roller  104  is mounted to the lever at a pivot  514 . An actuator  516  applies a variable load to the lever under the control of the controller  506 . The actuator  516  may comprise any suitable mechanism for generating a fusing force, for example screws, cams, levers, pneumatics, hydraulics, and electromechanical devices (including motors and stepper motors). 
   The roller  106  may be a fuser roller, and the roller  104  may be a pressure roller, the fuser roller having a cross-sectional diameter that is constant along a length of the fuser roller. In some prior fusing systems, it has been advantageous to vary the pressure exerted by the pressure member against the receiver sheet and fuser member. This variation in pressure can be provided, for example in a fusing system having a pressure roller and a fuser roller, by slightly modifying the shape of the fuser roller and/or pressure roller. The variance of pressure, in the form of a gradient of pressure that changes along the direction through the nip that is parallel to the axes of the rollers, can be established, for example, by continuously varying the overall diameter of the fuser roller and/or pressure roller along the direction of its axis such that the diameter is smallest at the midpoint of the axis and largest at the ends of the axis, in order to give the fuser roller and/or pressure roller a subtle “bow tie” or “hourglass” shape. This causes the pair of rollers to exert more pressure on the receiver sheet in the nip in the areas near the ends of the rollers than in the area about the midpoint of the rollers. This gradient of pressure helps to prevent wrinkles and cockle in the receiver sheet as it passes through the nip. A fuser roller is disclosed in United patent application Publication US 2004/0023144 A1, filed Aug. 4, 2003, in the names of Jerry A. Pickering and Alan R. Priebe, the contents of which are incorporated by reference as if fully set forth herein. Changing the fusing force over the stream of print media may eliminate the need for changing the diameter of the fuser roller and/or pressure roller along the direction of its axis. 
   Still referring to  FIG. 10 , an embodiment is presented comprising moving a print medium  505  through the fusing nip  102  comprising the fusing force, and changing the fusing force while the print medium  505  is moving through the fusing nip  102 . The process may comprise decreasing the fusing force before, during, or after the print medium  505  enters the fusing nip, and subsequently increasing the fusing nip load force while the print medium  505  is within the fusing nip  102 . The process may comprise decreasing the fusing force before, during, or after the print medium  505  leaves the fusing nip. More specifically, and as shown in  FIG. 11 , the process may comprise decreasing the fusing force in an interframe gap  518  within the fusing nip  102  immediately before the print medium  505 , and subsequently increasing the fusing force while the print medium  505  is within the fusing nip  102 , and decreasing the fusing force as the print medium leaves the fusing nip  102 . Referring again to  FIG. 10 , the fusing force may be gradually increased to a predetermined fusing force for the balance of the print medium  505  while the print medium  505  is within the fusing nip  102 . Very thick print media engenders a rapid change in fuser drive torque, as presented  FIG. 14  (LE indicates the leading edge of the print medium and TE indicates the-trailing edge of the print medium), in order to maintain the match between print media velocity and imaging member velocity. Under some conditions, the fuser drive servo may not have sufficient bandwidth to maintain the speed match during this nip entry transient. If the print medium velocity falls behind imaging member velocity, relative motion at the sheet/member interface will cause image smear in the transfer nip. The processes just described enable a gradual change in drive torque preferably within the bandwidth of the drive servo thus minimizing transfer smear of prints when printing on thick sheets. 
   An example of a fusing force profile is presented in  FIG. 15 . The fusing force before and after TE and LE is of a magnitude that permits the print medium velocity to match the imaging member velocity as the print medium enters the fusing nip. The fusing force applied over the bulk of the print medium between TE and LE is sufficient for adequate fusing, 400 pounds in one example. 
   The fusing force between ends (a leading edge and a trailing edge) of the print medium  505  may be changed while the print medium  505  is moving through the fusing nip  102  based at least in part on a thickness of the print medium  505 , a size of the print medium  505 , or a bending stiffness of the print medium  505 . The thickness sensor  204  ( FIG. 3 ) may be implemented to sense a thickness of the print medium  505 , and the fusing force between ends (a leading edge and a trailing edge) of the print medium  505  may be changed while the print medium  505  is moving through the fusing nip  102  based at least in part on the thickness of the print medium sensed by the thickness sensor  204 . The thickness sensor  204  may be a multi-feed sensor located upstream from the fusing nip  102 . 
   As previously described, these processes may be implemented by the controller  506  being operable to perform one or more steps. 
   Referring now to  FIGS. 12 and 13 , a printer machine  10  that implements the fusing apparatus and processes of the invention includes a moving electrographic imaging member  18  such as a photoconductive belt which is entrained about a plurality of rollers or other supports  21   a  through  21   g , one or more of which is driven by a motor to advance the belt. By way of example, roller  21   a  is illustrated as being driven by motor  20 . Motor  20  preferably advances the belt at a high speed, such as 20 inches per second or higher, in the direction indicated by arrow P, past a series of workstations of the printer machine  10 . Alternatively, belt  18  may be wrapped and secured about only a single drum, or may be a drum. 
   Printer machine  10  includes a controller or logic and control unit (LCU)  24 , preferably a digital computer or microprocessor operating according to a stored program for sequentially actuating the workstations within printer machine  10 , effecting overall control of printer machine  10  and its various subsystems. LCU  24  also is programmed to provide closed-loop control of printer machine  10  in response to signals from various sensors and encoders (e.g.  57 ,  76 ) Aspects of process control are described in U.S. Pat. No. 6,121,986 incorporated herein by this reference. 
   A primary charging station  28  in printer machine  10  sensitizes belt  18  by applying a uniform electrostatic corona charge, from high-voltage charging wires at a predetermined primary voltage, to a surface  18   a  of belt  18 . The output of charging station  28  is regulated by a programmable voltage controller  30 , which is in turn controlled by LCU  24  to adjust this primary voltage, for example by controlling the electrical potential of a grid and thus controlling movement of the corona charge. Other forms of chargers, including brush or roller chargers, may also be used. 
   An exposure station  34  in printer machine  10  projects light from a writer  34   a  to belt  18 . This light selectively dissipates the electrostatic charge on photoconductive belt  18  to form a latent electrostatic image of the document to be copied or printed. Writer  34   a  is preferably constructed as an array of light emitting diodes (LEDs), or alternatively as another light source such as a laser or spatial light modulator. Writer  34   a  exposes individual picture elements (pixels) of belt  18  with light at a regulated intensity and exposure, in the manner described below. The exposing light discharges selected pixel locations of the photoconductor, so that the pattern of localized voltages across the photoconductor corresponds to the image to be printed. An image is a pattern of physical light which may include characters, words, text, and other features such as graphics, photos, etc. An image may be included in a set of one or more images, such as in images of the pages of a document. An image may be divided into segments, objects, or structures each of which is itself an image. A segment, object or structure of an image may be of any size up to and including the whole image. 
   Image data to be printed is provided by an image data source  36 , which is a device that can provide digital data defining a version of the image. Such types of devices are numerous and include computer or microcontroller, computer workstation, scanner, digital camera, etc. These data represent the location and intensity of each pixel that is exposed by the printer. Signals from data source  36 , in combination with control signals from LCU  24  are provided to a raster image processor (RIP)  37 . The Digital images (including styled text) are converted by the RIP  37  from their form in a page description language (PDL) to a sequence of serial instructions for the electrographic printer in a process commonly known as “ripping” and which provides a ripped image to a image storage and retrieval system known as a Marking Image Processor (MIP)  38 . 
   In general, the major roles of the RIP  37  are to: receive job information from the server; parse the header from the print job and determine the printing and finishing requirements of the job; analyze the PDL (Page Description Language) to reflect any job or page requirements that were not stated in the header; resolve any conflicts between the requirements of the job and the Marking Engine configuration (i.e., RIP time mismatch resolution); keep accounting record and error logs and provide this information to any subsystem, upon request; communicate image transfer requirements to the Marking Engine; translate the data from PDL (Page Description Language) to Raster for printing; and support diagnostics communication between User Applications The RIP accepts a print job in the form of a Page Description Language (PDL) such as PostScript, PDF or PCL and converts it into Raster, a form that the marking engine can accept. The PDL file received at the RIP describes the layout of the document as it was created on the host computer used by the customer. This conversion process is called rasterization. The RIP makes the decision on how to process the document based on what PDL the document is described in. It reaches this decision by looking at the first 2K of the document. A job manager sends the job information to a MSS (Marking Subsystem Services) via Ethernet and the rest of the document further into the RIP to get rasterized. For clarification, the document header contains printer-specific information such as whether to staple or duplex the job. Once the document has been converted to raster by one of the interpreters, the Raster data goes to the MIP  38  via RTS (Raster Transfer Services); this transfers the data over a IDB (Image Data Bus). 
   The MIP functionally replaces recirculating feeders on optical copiers. This means that images are not mechanically rescanned within jobs that require rescanning, but rather, images are electronically retrieved from the MIP to replace the rescan process. The MIP accepts digital image input and stores it for a limited time so it can be retrieved and printed to complete the job as needed. The MIP consists of memory for storing digital image input received from the RIP. Once the images are in MIP memory, they can be repeatedly read from memory and output to the Render Circuit. The amount of memory required to store a given number of images can be reduced by compressing the images; therefore, the images are compressed prior to MIP memory storage, then decompressed while being read from MIP memory. 
   The output of the MIP is provided to an image render circuit  39 , which alters the image and provides the altered image to the writer interface  32  (otherwise known as a write head, print head, etc.) which applies exposure parameters to the exposure medium, such as a photoconductor  18 . 
   After exposure, the portion of exposure medium belt  18  bearing the latent charge images travels to a development station  35 . Development station  35  includes a magnetic brush in juxtaposition to the belt  18 . Magnetic brush development stations are well known in the art, and are preferred in many applications; alternatively, other known types of development stations or devices may be used. Plural development stations  35  may be provided for developing images in plural colors, or from toners of different physical characteristics. Full process color electrographic printing is accomplished by utilizing this process for each of four toner colors (e.g., black, cyan, magenta, yellow). 
   Upon the imaged portion of belt  18  reaching development station  35 , LCU  24  selectively activates development station  35  to apply toner to belt  18  by moving backup roller or bar  35 a against belt  18 , into engagement with or close proximity to the magnetic brush. Alternatively, the magnetic brush may be moved toward belt  18  to selectively engage belt  18 . In either case, charged toner particles on the magnetic brush are selectively attracted to the latent image patterns present on belt  18 , developing those image patterns. As the exposed photoconductor passes the developing station, toner is attracted to pixel locations of the photoconductor and as a result, a pattern of toner corresponding to the image to be printed appears on the photoconductor, thereby forming a developed image on the electrostatic image. As known in the art, conductor portions of development station  35 , such as conductive applicator cylinders, are biased to act as electrodes. The electrodes are connected to a variable supply voltage, which is regulated by programmable controller  40  in response to LCU  24 , by way of which the development process is controlled. 
   Development station  35  may contain a two component developer mix which comprises a dry mixture of toner and carrier particles. Typically the carrier preferably comprises high coercivity (hard magnetic) ferrite particles. As an example, the carrier particles have a volume-weighted diameter of approximately 30μ. The dry toner particles are substantially smaller, on the order of 6μ to 15μ in volume-weighted diameter. Development station  35  may include an applicator having a rotatable magnetic core within a shell, which also may be rotatably driven by a motor or other suitable driving means. Relative rotation of the core and shell moves the developer through a development zone in the presence of an electrical field. In the course of development, the toner selectively electrostatically adheres to photoconductive belt  18  to develop the electrostatic images thereon and the carrier material remains at development station  35 . As toner is depleted from the development station due to the development of the electrostatic image, additional toner is periodically introduced by toner auger  42  into development station  35  to be mixed with the carrier particles to maintain a uniform amount of development mixture. Toner auger  42  is driven by a replenisher motor  41  controlled by a replenisher motor control  43 . This development mixture is controlled in accordance with various development control processes. Single component developer stations, as well as conventional liquid toner development stations, may also be used. 
   A transfer station  46  in printing machine  10  moves a receiver sheet S into engagement with photoconductive belt  18 , in registration with a developed image to transfer the developed image to receiver sheet S. Receiver sheets S may be plain or coated paper, plastic, or another medium capable of being handled by printer machine  10 . Typically, transfer station  46  includes a charging device for electrostatically biasing movement of the toner particles from belt  18  to receiver sheet S. In this example, the biasing device is roller  46   b , which engages the back of sheet S and which is connected to programmable voltage controller  46   a  that operates in a constant current mode during transfer. Alternatively, an intermediate member may have the image transferred to it and the image may then be transferred to receiver sheet S. After transfer of the toner image to receiver sheet S, sheet S is detacked from belt  18  and transported to fuser station  49  where the image is fixed onto sheet S, typically by the application of heat. Alternatively, the image may be fixed to sheet S at the time of transfer. The fuser station  49  implements the one or more of apparatus and processes previously described in relation  FIGS. 1–12 . A fuser entry guide may be implemented between the transfer station  46  and the fuser station, for example, as described in U.S. patent application Ser. No. 10/668,416 filed Sep. 23, 2003, in the names of John Giannetti, Giovanni B. Caiazza, and Jerome F. Sleve, the contents of which are incorporated by reference as if fully set forth herein. 
   A cleaning station  48 , such as a brush, blade, or web is also located behind transfer station  46 , and removes residual toner from belt  18 . A pre-clean charger (not shown) may be located before or at cleaning station  48  to assist in this cleaning. After cleaning, this portion of belt  18  is then ready for recharging and re-exposure. Of course, other portions of belt  18  are simultaneously located at the various workstations of printing machine  10 , so that the printing process is carried out in a substantially continuous manner. 
   LCU  24  provides overall control of the apparatus and its various subsystems as is well known. LCU  24  will typically include temporary data storage memory, a central processing unit, timing and cycle control unit, and stored program control. Data input and output is performed sequentially through or under program control. Input data can be applied through input signal buffers to an input data processor, or through an interrupt signal processor, and include input signals from various switches, sensors, and analog-to-digital converters internal to printing machine  10 , or received from sources external to printing machine  10 , such from as a human user or a network control. The output data and control signals from LCU  24  are applied directly or through storage latches to suitable output drivers and in turn to the appropriate subsystems within printing machine  10 . 
   Process control strategies generally utilize various sensors to provide real-time closed-loop control of the electrostatographic process so that printing machine  10  generates “constant” image quality output, from the user&#39;s perspective. Real-time process control is necessary in electrographic printing, to account for changes in the environmental ambient of the photographic printer, and for changes in the operating conditions of the printer that occur over time during operation (rest/run effects). An important environmental condition parameter requiring process control is relative humidity, because changes in relative humidity affect the charge-to-mass ratio Q/m of toner particles. The ratio Q/m directly determines the density of toner that adheres to the photoconductor during development, and thus directly affects the density of the resulting image. System changes that can occur over time include changes due to aging of the printhead (exposure station), changes in the concentration of magnetic carrier particles in the toner as the toner is depleted through use, changes in the mechanical position of primary charger elements, aging of the photoconductor, variability in the manufacture of electrical components and of the photoconductor, change in conditions as the printer warms up after power-on, triboelectric charging of the toner, and other changes in electrographic process conditions. Because of these effects and the high resolution of modern electrographic printing, the process control techniques have become quite complex. 
   Process control sensor may be a densitometer  76 , which monitors test patches that are exposed and developed in non-image areas of photoconductive belt  18  under the control of LCU  24 . Densitometer  76  may include a infrared or visible light LED, which either shines through the belt or is reflected by the belt onto a photodiode in densitometer  76 . These toned test patches are exposed to varying toner density levels, including full density and various intermediate densities, so that the actual density of toner in the patch can be compared with the desired density of toner as indicated by the various control voltages and signals. These densitometer measurements are used to control primary charging voltage V O , maximum exposure light intensity E O , and development station electrode bias V B . In addition, the process control of a toner replenishment control signal value or a toner concentration setpoint value to maintain the charge-to-mass ratio Q/m at a level that avoids dusting or hollow character formation due to low toner charge, and also avoids breakdown and transfer mottle due to high toner charge for improved accuracy in the process control of printing machine  10 . The toned test patches are formed in the interframe area of belt  18  so that the process control can be carried out in real time without reducing the printed output throughput. Another sensor useful for monitoring process parameters in printer machine  10  is electrometer probe  50 , mounted downstream of the corona charging station  28  relative to direction P of the movement of belt  18 . An example of an electrometer is described in U.S. Pat. No. 5,956,544 incorporated herein by this reference. 
   Other approaches to electrographic printing process control may be utilized, such as those described in International Publication Number WO 02/10860 A1, and International Publication Number WO 02/14957 A1, both commonly assigned herewith and incorporated herein by this reference. 
   Raster image processing begins with a page description generated by the computer application used to produce the desired image. The Raster Image Processor interprets this page description into a display list of objects. This display list contains a descriptor for each text and non-text object to be printed; in the case of text, the descriptor specifies each text character, its font, and its location on the page. For example, the contents of a word processing document with styled text is translated by the RIP into serial printer instructions that include, for the example of a binary black printer, a bit for each pixel location indicating whether that pixel is to be black or white. Binary print means an image is converted to a digital array of pixels, each pixel having a value assigned to it, and wherein the digital value of every pixel is represented by only two possible numbers, either a one or a zero. The digital image in such a case is known as a binary image. Multi-bit images, alternatively, are represented by a digital array of pixels, wherein the pixels have assigned values of more than two number possibilities. The RIP renders the display list into a “contone” (continuous tone) byte map for the page to be printed. This contone byte map represents each pixel location on the page to be printed by a density level (typically eight bits, or one byte, for a byte map rendering) for each color to be printed. Black text is generally represented by a full density value (255, for an eight bit rendering) for each pixel within the character. The byte map typically contains more information than can be used by the printer. Finally, the RIP rasterizes the byte map into a bit map for use by the printer. Half-tone densities are formed by the application of a halftone “screen” to the byte map, especially in the case of image objects to be printed. Pre-press adjustments can include the selection of the particular halftone screens to be applied, for example to adjust the contrast of the resulting image. 
   Electrographic printers with gray scale printheads are also known, as described in International Publication Number WO 01/89194 A2, incorporated herein by this reference. As described in this publication, the rendering algorithm groups adjacent pixels into sets of adjacent cells, each cell corresponding to a halftone dot of the image to be printed. The gray tones are printed by increasing the level of exposure of each pixel in the cell, by increasing the duration by way of which a corresponding LED in the printhead is kept on, and by “growing” the exposure into adjacent pixels within the cell. 
   Ripping is printer-specific, in that the writing characteristics of the printer to be used are taken into account in producing the printer bit map. For example, the resolution of the printer both in pixel size (dpi) and contrast resolution (bit depth at the contone byte map) will determine the contone byte map. As noted above, the contrast performance of the printer can be used in pre-press to select the appropriate halftone screen. RIP rendering therefore incorporates the attributes of the printer itself with the image data to be printed. 
   The printer specificity in the RIP output may cause problems if the RIP output is forwarded to a different electrographic printer. One such problem is that the printed image will turn out to be either darker or lighter than that which would be printed on the printer for which the original RIP was performed. In some cases the original image data is not available for re-processing by another RIP in which tonal adjustments for the new printer may be made. 
   Processes for developing electrostatic images using dry toner are well known in the art. The term “electrographic printer,” is intended to encompass electrophotographic printers and copiers that employ a photoconductor element, as well as ionographic printers and copiers that do not rely upon a photoconductor. 
   Electrographic printers typically employ a developer having two or more components, consisting of resinous, pigmented toner particles, magnetic carrier particles and other components. The developer is moved into proximity with an electrostatic image carried on an electrographic imaging member, whereupon the toner component of the developer is transferred to the imaging member, prior to being transferred to a sheet of paper to create the final image. Developer is moved into proximity with the imaging member by an electrically-biased, conductive toning shell, often a roller that may be rotated co-currently with the imaging member, such that the opposing surfaces of the imaging member and toning shell travel in the same direction. Located adjacent the toning shell is a multipole magnetic core, having a plurality of magnets, that may be fixed relative to the toning shell or that may rotate, usually in the opposite direction of the toning shell. The developer is deposited on the toning shell and the toning shell rotates the developer into proximity with the imaging member, at a location where the imaging member and the toning shell are in closest proximity, referred to as the “toning nip.” 
   According to a further aspect of the invention a process is provided, comprising forming an electrostatic image on an imaging member, forming a developed image on the electrostatic image, moving a print medium past the imaging member, transferring the developed image to the print medium, moving the print medium through a fusing nip comprising a fusing force, and changing the fusing force while the print medium is moving through the fusing nip. This process may be carried out while the print medium is contacting the imaging member during transfer of the developed image to the print medium and while the print medium is moving through the fusing nip. As previously described, smearing of the image proximate the trailing edge of the print medium may be avoided. 
   Although certain aspects of the invention have been described with external heat sources, such as heater rollers  110  and  111 , internal heat sources may be implemented as well, for example inside rollers  104  and/or  106  instead of or in addition to one or more external heat sources. 
   It should be understood that the programs, processes, methods and apparatus described herein are not related or limited to any particular type of computer or network apparatus (hardware or software), unless indicated otherwise. Various types of general purpose or specialized computer apparatus may be used with or perform operations in accordance with the teachings described herein. While various elements have been described as being implemented by software, in other embodiments hardware or firmware implementations may alternatively be used, and vice-versa. Similarly, the controllers may implement software, hardware, and/or firmware. In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the present invention. 
   The claims should not be read as limited to the described order or elements unless stated to that effect. In addition, use of the term “means” in any claim is intended to invoke 35 U.S.C. §112, paragraph 6, and any claim without the word “means” is not so intended. 
   Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the true scope and spirit of the invention as defined by the claims that follow. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof. 
   PARTS LIST 
   
       
       IDB image data bus 
       LE leading edge of the print medium 
       LED light of emitting diodes 
       MIP marking image processor 
       MSS marking subsystem services 
       P arrow 
       PDL page description language 
       S receiver sheet 
       TE trailing edge of the print medium 
         10  printer machine 
         18  belt or photoconductive belt 
         18   a  surface 
         20  motor 
         21   a  through  21   g  plurality of roller or other supports 
         24  logic and control (LCU) 
         28  charging station 
         30  programmable voltage controller 
         32  writer interface 
         34  exposure station  34   
         34   a  writer 
         35  development station 
         35   a  moving backup roller or bar 
         36  image data source 
         37  raster image processor (RIP)  37   
         38  marking image processor (MIP) 
         39  render 
         40  programmable controller 
         41  replenisher motor 
         42  toner auger 
         43  replenisher motor control 
         46  transfer station 
         46   a  programmable voltage controller 
         46   b  roller 
         48  cleaning station 
         49  fuser station 
         50  electrometer probe 
         57  sensor 
         76  densitometer 
         100  fusing apparatus 
         102  fusing nip 
         104  roller 
         106  roller 
         108  heating nip 
         109  heating nip 
         110  heater roller 
         111  another heater roller 
         112  heat source 
         113  another heat source 
         114  first temperature sensor 
         116  second temperature sensor 
         117  third temperature sensor 
         118  controller 
         120  roller temperature 
         122  heater roller temperature 
         123  another heater roller temperature 
         124  standby 
         126  run 
         128  temperature plot 
         130  temperature plot 
         132  temperature plot 
         134  temperature plot 
         200  fusing apparatus 
         202  fuser assembly 
         204  thickness sensor 
         206  print media thickness 
         208  heat source 
         210  print media 
         218  controller 
         300  fusing apparatus 
         310  heater roller 
         311  another heater roller 
         312  heat source 
         313  another heat source 
         318  controller 
         320 ,  322 ,  324  three heating powers 
         330 ,  332 ,  334  three print media widths 
         336  first print media width 
         338  second print media width 
         340  heater roller width 
         342  another heater roller width 
         344  another heating power 
         346  first heating power 
         348  second heating power 
         400  apparatus 
         402  multiplexer 
         404  thermistor amplifier board 
         406  digital converter 
         408  feedback controller 
         410  feed forward controller 
         412  analog to digital converter 
         414  a first solid state relay 
         416  second solid state relay 
         418  distributed cotnroller 
         420  multiplexer 
         500  fusing apparatus 
         502  fuser assembly 
         504  print media 
         505  print medium 
         506  controller 
         508  fusing nip loading mechanism 
         510  lever 
         512  fixed pivot 
         514  pivot 
         516  actuator 
         518  interframe gap