Controlling the power dissipation of a fixing device

Information

  • Patent Grant
  • 6229120
  • Patent Number
    6,229,120
  • Date Filed
    Thursday, November 12, 1998
    26 years ago
  • Date Issued
    Tuesday, May 8, 2001
    23 years ago
  • Inventors
  • Original Assignees
  • Examiners
    • Walberg; Teresa
    • Fastovsky; Leonid
    Agents
    • Wisdom; Gregg W.
Abstract
A fixing device, such as a halogen bulb fuser or an instant on fuser, includes a first heating element supplied with substantially constant average power by a power control circuit in preparation for and during fusing. Additionally, the instant on fuser includes a second heating element supplied by the power control circuit. The power control circuit measures the average voltage across and the average current through the first heating element to detect changes in the thermal load applied to the first heating element. In preparation for fusing, the power control circuit applies sufficient power to the second heating element for fusing the expected average thermal load. If the power control circuit detects a level of thermal loading on the instant on fuser different than the expected average thermal load, the power control circuit adjusts the power supplied to the second heating element to compensate for the level of thermal loading different than the average expected thermal load. The power control circuit includes a pulse width modulator that adjusts the duty cycle of a drive signal applied to the gate terminal of a first triac coupled in series with the first heating element to maintain substantially constant power dissipation in the first heating element. To compensate for thermal loading different than the average expected thermal load, the pulse width modulator adjusts the duty cycle of a drive signal applied to the gate terminal of a second triac coupled in series with the second heating element.
Description




FIELD OF THE INVENTION




This invention relates to the fixing of toner to print media in an electrophotographic printing system. More particularly, this invention relates to a fixing device used in an electrophotographic printing system.




BACKGROUND OF THE INVENTION




Heating elements have been used to fix toner to print media in electrophotographic printing Prior art technology employs one or more resistive heating elements enclosed in a glass bulb which is inserted into a cylinder formed of a thermally conductive material such as aluminum. The exterior surface of the cylinder has a release layer formed from a low adhesion material, such as TEFLON, to reduce toner adhesion to the surface. This embodiment of a fixing device uses a kind of fuser typically referred to as a halogen bulb fuser. The heat generated by the resistive heating element is transferred to the exterior surface of the halogen bulb fuser through radiation, convection and thermal conduction through the wall of the cylinder. Frequently, the glass bulb is filled with a halogen gas to allow the heating element to be operated at a higher temperature.




Another prior art fixing device implementation, using a type of fuser known as an instant on fuser, includes a strip of material forming a resistive heating element. The resistive heating element can be formed on the ceramic substrate through a thick film deposition process. The resistive heating element is covered by a coating of glass. The coating of glass permits low friction rotation of a film sleeve over the glass as well as providing electrical insulation. Typically, in an instant on fuser, the resistive heating element is fabricated on the ceramic substrate with the electrical connections at one end of the long axis of the fuser. Multiple resistive heating elements may be used in the instant on fuser.




A significant technical problem encountered in the use of fixing devices is obtaining an accurate measurement of the temperature on the surface of the fixing device contacting the print media. Generally, in a halogen bulb fuser a single temperature sensor, such as a thermistor, is located near one end and in sliding contact with the halogen bulb fuser outside the path the print media follows as it passes over the halogen bulb fuser. During fixing, print media must pass between the halogen bulb fuser and a pressure roller. In order to permit the print media to pass between the halogen bulb fuser and the pressure roller without the risk of a paper jam, the temperature sensor is typically located on the side of the halogen bulb fuser opposite the region through which the print media passes during fixing. Additionally, in an instant on fuser, the temperature sensor is typically located on the side of the instant on fuser opposite the resistive heating elements in order to eliminate the risk of paper jams. The temperature sensor is part of a circuit which regulates the flow of power to the one or more heating elements within the fixing device in an attempt to establish a uniform temperature profile across the surface of the fixing device. Because the temperature sensor is located relatively remote from the region in which fixing occurs, the difficulty in precisely controlling the fixing temperature is increased.




Contact between the print media and the surface of the fixing device results in a decrease in the surface temperature of the fixing device in those locations on the surface contacting the print media. The temperature sensor provides a measure of the temperature on the surface of the fixing device outside of the print media path in an area which is not thermally loaded. Because of this, an assumption about the surface temperature offset between areas outside of the print media path and areas within the print media path must be made to provide effective control of the fixing device surface temperature in the region contacting the print media. As the width of the print media varies, the value of this temperature offset can change substantially as a result of differences in the thermal loading. This variation in temperature offset increases the difficulty in providing fixing device surface temperatures optimal for fixing toner.




An additional consideration is that the amount of thermal loading from the print media is variable depending upon such print media characteristics as the thermal mass of the print media, the thermal conductivity of the material used for the print media, the surface finish of the print media, and the moisture content of the print media. For example, different thermal loading of the fixing device results from papers having different weights and different surface finishes. Or, between paper of the same weight having different moisture contents, typically as a result of exposure to different humidities, the fixing device is thermally loaded to different degrees. Another factor contributing to the uncertainty of the temperature offset value is the thermal time constant associated with the fixing device and the variable thermal load of the print media. This thermal time constant results in a time varying temperature offset between locations on the surface of the fixing device inside and outside the print media path as the fixing device reaches steady state. These factors create a wide range of variability in the temperature offset between areas of the surface of the fixing device in the path of the print media and the location of the temperature sensor outside of the path of the print media. These types of problems make the use of a temperature sensor outside of the print media path unsuitable for tight control of the fixing device surface temperature.




In past attempts at improving the accuracy of the measurement of the fixing device surface temperature, temperature sensors have been placed in the path of the print media in an attempt to obtain a more accurate measurement of the surface temperature of the fixing device. However, a difficulty encountered in using conventional temperature sensors, such as thermistors, is that, fibers from the print media accumulate on the surface of the sensor and prevent it from obtaining an accurate measure of the fuser surface temperature in the print media path. This difficult has limited the usefulness of locating certain types of temperature sensors in the print media path.




Other attempts at improving the accuracy of the measurement of the fixing device surface temperature have included using temperature sensors that do not contact the surface of the fixing device and are located near the paper path. Although these implementations are less susceptible to paper jams, they are more expensive to implement and they are also susceptible to coating by airborne paper fibers which reduce their ability to reliably measure the surface temperature of the fixing device.




A need exist for an implementation of a fixing device that has the capability to provide a more accurate measurement of the temperature on the surface of the fixing device in the vicinity of the region that contacts the print media for improving control of the fixing device surface temperature while having less sensitivity to the accumulation of the paper fibers and not increasing the likelihood of paper jams.




SUMMARY OF THE INVENTION




Accordingly, a fixing device for fusing toner to print media includes a first heating element and a second heating element. The fixing device further includes a power control circuit configured for selectively supplying a substantially constant average power to the first heating element and configured for adjusting the power supplied to the second heating element in response to thermal loading of the first heating element by the print media.




In a fixing device for fixing toner to print media, with the fixing device including a power control circuit, a first heating element coupled to the power control circuit, and a second heating element coupled to the power control circuit, a method for controlling the power applied to the second heating element to compensate for thermal loading by the print media different than a predetermined level of thermal loading has been developed. The method includes the steps of applying a substantially constant average power to the first heating element and applying power to the second heating element corresponding to the predetermined level of thermal loading. The method further includes the step of adjusting the power supplied to the second heating element in response to thermal loading of the first heating element different than the predetermined level of thermal loading.




An electrophotographic printing system for printing print data on print media using toner includes a formatter arranged for receiving the print data to generate formatted print data and a scanner configured for receiving the formatted print data to generate a laser beam modulated based upon the formatted print data. The electrophotographic printing system further includes a photoconductor onto which the laser beam impinges to generate a latent electrostatic image and a developer for developing the toner onto the latent electrostatic image. Additionally, the electrophotographic printing system includes a transfer device for transferring the toner on the photoconductor onto the print media. The electrophotographic printing system further includes a fixing device including a first heating element, a second heating element, and a power control circuit configured for selectively supplying a substantially constant average power to the first heating element and configured for adjusting the power supplied to the second heating element in response to thermal loading of the first heating element by the print media.











DESCRIPTION OF THE DRAWINGS




A more thorough understanding of the invention may be had from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which:





FIG. 1

is a simplified cross section of an electrophotographic printer including an embodiment of the fixing device.





FIG. 2

shows an implementation of the fixing device using an instant on fuser.





FIG. 3

shows a simplified schematic of a power control circuit used in the fixing device





FIG. 4

shows an implementation of the fixing device using a halogen bulb fuser.





FIG. 5

shows a high level flow diagram of a method of the using the fixing device to compensate for changes in thermal loading.











DETAILED DESCRIPTION OF THE DRAWINGS




The present invention is not limited to the specific exemplary embodiments illustrated herein. Although the embodiments of the fixing device will be discussed in the context of a monochrome electrophotographic printer, one of ordinary skill in the art will recognize by understanding this specification that the fixing device has applicability in both color and monochrome electrophotographic image forming systems. Furthermore, although the embodiments of the fixing device will be discussed in the context of a monochrome electrophotographic printer, one of ordinary skill in the art will recognize by understanding this specification that other types of electrophotographic printing systems such as electrophotographic copiers could use the fixing device.




Referring to

FIG. 1

, shown is a simplified cross sectional view of an electrophotographic printer


1


containing an embodiment of the fixing device including an instant on fuser


2


. It should be recognized that although the disclosed embodiment of the fixing device is discussed in the context of an electrophotographic printer


1


using instant on type fuser


2


, it could also be applied to other types fusers, such as a halogen bulb type fuser.




Charge roller


3


is used to charge the surface of photoconductor drum to a predetermined voltage. A laser diode (not shown) inside laser scanner


5


emits a laser beam


6


which is pulsed on and off as it is swept across the surface of photoconductor drum


4


to selectively discharge the surface of the photoconductor drum


4


. Photoconductor drum


4


rotates in the clockwise direction as shown by the arrow


7


. Developer roller


8


is used to develop the latent electrostatic image residing on the surface of photoconductor drum


4


after the surface voltage of the photoconductor drum


4


has been selectively discharged. Toner


9


which is stored in the toner reservoir


10


of electrophotographic print cartridge


11


moves from locations within the toner reservoir


10


to the developer roller


8


. The magnet located within the developer roller


8


magnetically attracts the toner to the surface of the developer roller


8


. As the developer roller


8


rotates in the counterclockwise direction, the toner on the surface of the developer roller


8


, located opposite the areas on the surface of photoconductor drum


4


which are discharged, is moved across the gap between the surface of the photoconductor drum


4


and the surface of the developer roller


8


to develop the latent electrostatic image. A high voltage bias applied to developer roller


8


creates the electric field necessary to project toner from developer roller


8


onto photoconductor drum


4


.




Print media


12


is loaded from paper tray


13


by pickup roller


14


into the paper path of the electrophotographic printer


1


. Print media


12


moves through the drive rollers


1


so that the arrival of the leading edge of print media


12


below photoconductor drum


4


is synchronized with the rotation of the region on the surface of photoconductor drum


4


having a latent electrostatic image corresponding to the leading edge of print media


12


. As the photoconductor drum


4


continues to rotate in the clockwise direction, the surface of the photoconductor drum


4


, having toner adhered to it in the discharged areas, contacts the print media


12


which has been charged by transfer roller


16


so that it attracts the toner particles away from the surface of the photoconductor drum


4


and onto the surface of the print media


12


. The transfer of toner particles from the surface of photoconductor drum


4


to the surface of the print media


12


does not occur with one hundred percent efficiency and therefore some toner particles remain on the surface of photoconductor drum


4


. As photoconductor drum


4


continues to rotate, toner particles which remain adhered to its surface are removed by cleaning blade


17


and deposited in toner waste hopper


18


.




As the print media


12


moves in the paper path past photoconductor drum


4


, conveyer belt


19


delivers the print media


12


to fuser


2


. Print media


12


passes between pressure roller


20


and the sleeve


21


surrounding instant on fuser


2


. Pressure roller


20


forces print media


12


against sleeve


21


deforming sleeve


21


. Pressure roller


20


provides the drive force to rotate sleeve


21


around instant on fuser


2


as pressure roller


20


rotates. At the instant on fuser


2


, heat is applied to print media


12


through the sleeve


21


so that the toner particles are fused to the surface of print media


12


. Output rollers


22


push the print media


12


into the output tray


23


after it exits instant on fuser


2


.




Power control circuit


24


applies power to instant on fuser


2


to control the temperature of instant on fuser


2


. The embodiment of instant on fuser


2


shown in

FIG. 1

uses multiple heating elements to provide the energy necessary to fuse toner to print media


12


. An electrical parameter is monitored on one of the heating elements located on instant on fuser


2


in order to determine the thermal load applied to instant on fuser


2


. In response to the monitoring of the electrical parameter, power control circuit


24


adjusts the power supplied to the remaining heating elements in order to compensate for the increased thermal load.




Formatter


25


receives print data, such as a display list, vector graphics, or raster print data, from the print driver operating in conjunction with an application program in host computer


26


. Formatter


25


converts this relatively high level print data into a stream of binary print data. Formatter


25


sends the stream of binary print data to controller


27


. In addition, formatter


25


and controller


27


exchange data necessary for controlling the electrophotographic printing process. Controller


27


supplies the stream of binary print data to laser scanner


5


. The binary print data stream sent to the laser diode in laser scanner


5


pulses the laser diode to create the latent electrostatic image on photoconductor drum


4


.




As part of initiating the electrophotographic printing process, controller


27


commands high voltage power supply


28


to apply the necessary bias voltages to charge roller


3


, developer roller


8


, and transfer roller


16


in order to develop an image onto print media


12


. In addition, controller


27


controls a drive motor (not shown in

FIG. 1

) that supplies mechanical drive power to a printer gear train that moves various components in the electrophotographic system (such as photoconductor drum


4


, pickup roller


14


, and driver rollers


15


). Controller


27


also initiates the application of power by power control circuit


24


to instant on fuser


2


so that instant on fuser


2


is ready to begin fusing toner to print media


12


when the print media moves between instant on fuser


2


and pressure roller


20


. Power control circuit


24


initially supplies sufficient power to maintain the surface temperature of instant on fuser


2


at the fusing temperature for the average thermal load. As print media


12


moves between instant on fuser


2


and pressure roller


20


, power control circuit


24


modulates the power supplied to instant on fuser


2


to compensate for a level of thermal loading different than the average thermal load. Further details on electrophotographic processes can be found in the text “The Physics and Technology of Xerographic Processes”, by Edgar M. Williams, 1984, a Wiley-Interscience Publication of John Wiley & Sons, the disclosure of which is incorporated by reference herein.




Shown in

FIG. 2

is a view of instant on fuser


2


coupled to power control circuit


24


. Instant on fuser


2


includes a first heating element


100


and a second heating element


102


. First heating element


102


is used for monitoring the thermal load applied to instant on fuser


2


. First heating element


100


is positioned relative to second heating element


102


so that the leading edge of print media


12


passes over first heating element


100


before passing over second heating element


102


.




It should be recognized that although the first heating element


100


shown in

FIG. 2

has approximately the same width as second heating element


102


, first heating element


100


could be narrow relative to the width of print media


12


. Furthermore, although the first heating element


100


supplies sufficient power to print media


12


to assist in fixing toner, the level of power supplied to first heating element


100


may be less than that needed for fusing with the second heating element


102


supplying the majority of the heat required for fusing. Additionally, although instant on fuser


2


shown in

FIG. 2

use two heating elements for fixing toner


9


to print media


12


, one of ordinary skill could extend the concepts disclosed in this specification to implementations of an instant on fuser or a halogen bulb fuser using more than two heating elements.




Shown in

FIG. 3

is a simplified schematic of power control circuit


24


and a line voltage source. Power control circuit


24


is configured to provide a constant power to first heating element


100


(represented as a resistive element). First buffer amplifier


200


is connected across current sense resistor


202


. Current sense resistor


202


is located in series with first triac


204


to measure the current flowing through heating element


100


. First low pass filter


206


is coupled to the output of first buffer amplifier


200


. The output of first low pass filter


206


is coupled to pulse width modulator


208


. Second buffer amplifier


210


is connected across first heating element


100


. The output of second buffer amplifier


210


is coupled to second low pass filter


212


. The output of second low pass filter


212


is coupled to pulse width modulator


208


. A first output of pulse width modulator


208


is coupled the gate terminal of first triac


204


. The power for first heating element


100


is supplied by line voltage source


214


which may be either a 110/120 V source or a 220/240 V source. Second heating element


102


is connected in series with second triac


216


and line voltage source


214


. A second output of pulse width modulator


208


is connected to the gate terminal of second triac


216


.




Pulse width modulator


208


is designed to regulate the value of power supplied to first heating element


100


to maintain the power dissipation in first heating element


100


substantially constant during the time in which first heating element


100


is operated. Substantially constant as it is used in the context of the power supplied to first heating element means maintaining the power dissipation across first heating element


100


within a range that is typical for the precision that can be achieved in electronic power control devices. First low pass filter


206


attenuates the ripple present in the voltage across current sense resistor


202


. The output of first low pass filter


206


represents the average value of the current flowing through first heating element


100


. Second low pass filter


212


attenuates the ripple present in the voltage across first heating element


100


. The output of second low pass filter


212


represents the average value of the voltage across first heating element


100


. Pulse width modulator


208


controls the pulse width of the drive signal supplied to the gate terminal of first triac


204


so that the product of the average value of the voltage across first heating element


100


and the average value of the current through first heating element


100


remains substantially constant. The product of the average voltage across and the average current through first heating element


100


is equal to the average power dissipation in first heating element


100


. By maintaining the product of the average voltage and the average current substantially constant, the average power dissipation in first heating element


100


is substantially constant.




Pulse width modulator


208


is designed to set the power dissipation in first heating element


100


and second heating element


102


at a value corresponding to the average expected thermal load. The expected thermal load is determined by measuring the average characteristics (such as, basis weight, density, caliper, specific heat, moisture content) of the print media type anticipated to be used most frequently. Additionally, because toner coverage on the print media affects the thermal loading, the expected level of toner coverage would need to be measured to determine the expected thermal load.




Pulse width modulator


208


controls the power dissipated by first heating element


100


and second heating element


102


by setting the duty cycle of the drive signal applied to the gate terminal of first triac


204


and to the gate terminal of second triac


216


. As previously mentioned, the duty cycle of the signals applied to the gate terminals of first triac


204


are set so that the power dissipated by first heating element


100


and second heating element


102


generates the temperature on the surface of instant on fuser


2


needed for properly fusing toner on print media


12


having average characteristics. However, proper fusing of toner is needed for print media presenting a wide range of thermal loads to instant on fuser


2


.




Power control circuit


24


adjusts the power dissipation of second heating element


102


to compensate for variations in the thermal load applied to instant on fuser


2


. When power control circuit


24


is commanded by controller


27


to begin applying power to first heating element


100


and second heating element


102


, pulse width modulator


208


sets the duty cycle to the gate terminal of first triac


204


and second triac


216


so that the temperature on the surface of instant on fuser


2


is at the proper level for fusing toner onto print media


12


having average characteristics.




When the leading edge of the print media


12


to be fused moves between instant on fuser


2


and backup roller


20


, first heating element


100


is encountered first. The thermal loading of first heating element


100


by print media


12


affects its resistance. Power control circuit


24


responds to this change in resistance by changing the duty cycle of the drive signal applied to the gate terminal of first triac


204


in order to maintain the product of the average voltage across and average current through first heating element


100


at a substantially constant value. Pulse width modulator uses the signals from first low pass filter


206


and second low pass filter


212


to make the necessary adjustments to the duty cycle of the drive signal applied to the gate terminal of first triac


204


. If thermal loading by print media


12


decreases the resistance of first heating element


100


, pulse width modulator


208


responds by changing the duty cycle to increase the current flow through first heating element


100


to maintain the power dissipated at a substantially constant value. If thermal loading by print media


12


increases the resistance of first heating element


100


, pulse width modulator


208


responds by changing the duty cycle to decrease the current flow through first heating element


100


to maintain the power dissipated at a substantially constant value.




Pulse width modulator


208


includes a circuit to control the duty cycle of the signal used to drive the gate terminal of first triac


204


and a circuit to control the duty cycle of the signal used to drive the gate terminal of second triac


216


. The signals from first low pass filter


206


and second low pass filter


212


are used by pulse width modulator


208


to adjust the duty cycle of the drive signal applied to the gate terminal of second triac


216


. Based upon the knowledge of the relationship between the thermal load applied to first heating element


100


and the change in resistance that results (as indicated by the change in current flow), pulse width modulator


208


makes the necessary adjustment in the duty cycle of the drive signal applied to the gate terminal of second triac


216


.




Consider the case in which the material used to construct first heating element


100


has a positive temperature coefficient of resistivity over the operating temperature range. If a constant voltage source were applied to first heating element


100


, the application of a thermal load to first heating element


100


would tend to lower its temperature, resulting in a resistance decrease. The resistance decrease would, in turn, result in increased current flow through first heating element


100


. For example, if the resistance changes by a factor of x (0<x<1), then the current flowing through first heating element


100


would increase by a factor of 1/x. Correspondingly, the power dissipated by first heating element


100


goes up by a factor of 1/x. The increase in the power dissipation by first heating element


100


resulting from the increase in the thermal load at least partially offsets the temperature decrease (and corresponding current increase) resulting from the increase in the thermal load. Therefore, accurately detecting a change in the thermal load is made more difficult by the effect of offsetting the temperature decrease.




Consider the case in which a constant current source is used to supply power to first heating element


100


, with first heating element


100


constructed from a material having a negative temperature coefficient of resistivity. The application of a thermal load to first heating element


100


would tend to lower its temperature, resulting in a resistance increase. The resistance increase would, in turn, result in increased power dissipation in first heating element


100


. For example if the resistance changes by a factor of x (x>1), then the voltage across first heating element


100


would increase by a factor of x. The increase in power dissipation by first heating element


100


resulting from the increase in the thermal load at least partially offsets the temperature decrease (and corresponding voltage increase) resulting from the increase in the thermal load. Therefore, accurately detecting a change in the thermal load is made more difficult by the effect of offsetting the temperature decrease.




The power control circuit


24


applies substantially constant power to first heating element


100


. Because substantially constant power is applied, detecting changes in the thermal loading of instant on fuser


2


is more easily done than it would be if a constant current source or a constant voltage source were applied. Consider the case in which first heating element


100


is formed from a material having a positive temperature coefficient of resistivity over its operating temperature range. Applying a thermal load to first heating element


100


would tend to lower its temperature, resulting in a resistance decrease. In response, pulse width modulator


208


adjusts the duty cycle of the drive signal applied to first triac


204


in order to reduce the current flowing through first heating element


100


to maintain substantially constant power dissipation. Therefore, the change in the temperature of first heating element


100


brought about by an increase in the thermal load is not offset by an increase in the power dissipated in it, allowing for a more accurate detection of a change in the thermal load.




Pulse width modulator


208


uses the measurement of the average current through and average voltage across the first heating element


100


to control the power dissipation in second heating element


102


. By knowing the relationship between the change in current through first heating element


100


or the change in voltage across first heating element


100


and the change in the thermal load, the power supplied to second heating element


102


can be adjusted to compensate for the change in thermal load applied to instant on fuser


2


.




The functions of pulse width modulator


208


may be implemented in a variety of ways. For example, pulse width modulator


208


could be implemented using an analog multiplier to multiply the output of first low pass filter


212


and second low pass filter


206


to generate a feedback signal. The feedback signal could be applied to an error amplifier in the type of switch mode control circuits commonly used in commercially available integrated circuits. An example of the general class of switch mode control circuits that might be used is Motorola part number MC34063 discussed in Motorola application note AN920, the disclosure of which is incorporated into this patent application by reference. By properly selecting the reference voltage to which to compare the feedback signal, the duty cycle of the drive signal supplied to the gate of first triac can be set to provide the desired power to first heating element


100


.




A second switch mode control circuit could use either the average current value from the output of first low pass filter


206


or the average voltage value from the output of second low pass filter


212


as a feedback signal to apply to the error amplifier in the switch mode control circuit. By properly selecting the reference voltage to which to compare the feedback signal, the duty cycle of the drive signal supplied to the gate of second triac


216


can be set to provide, to second heating element


102


, the power necessary for fusing a large variety of print media types. An increasing thermal load on first heating element


100


, results in a drop in the average voltage and average current outputs from first


206


and second low pass filter


212


. The second switch mode control circuit would respond by increasing the duty cycle of the drive signal applied to the gate terminal of second triac


216


, thereby boosting power supplied to second heating element


102


.




Pulse width modulator


208


could also be implemented using analog to digital converters to digitize the output of first low pass filter


206


and second low pass filter


212


. A processor in pulse width modulator


208


could perform a multiplication of these digitized outputs and use the result to generate a digital signal used to drive the gate terminal of first triac


204


in order to maintain constant power dissipation in first heating element


100


. The processor could also generate a digital signal from one of the digitized outputs of first low pass filter


206


and second low pass filter


212


to drive the gate terminal of second triac


216


to compensate for a change in the thermal load detected by first heating element


100


. Both of the digital signals used to drive the gate terminals of first triac


204


and second triac


216


may need to be level shifted in order to meet the drive requirements of the triac.




Alternatively, the processor could utilize a look-up table to select a duty cycle of the drive signals to the gate terminals of first triac


204


and second triac


216


. In this alternative implementation, the processor computes an index based upon the values of the average current and average voltage associated with first heating element


100


. The processor than uses the computed index to access the duty cycle values stored in the look-up table. With the duty cycle values loaded from the look-up table, the processor then applies the drive signals to the gate terminals of first triac


204


and second triac


216


in order to control the power dissipation in first heating element


100


and second heating element


102


.




Shown in

FIG. 4

is an implementation of a halogen bulb fuser


300


that could be used with the power control circuit


24


to adjust for changes in the thermal load applied to the halogen bulb fuser


300


. In this implementation, sensing element


302


, is used to detect the thermal load applied to halogen bulb fuser


300


. Sensing element


302


may be constructed as a thin resistive heating element formed onto the surface of halogen bulb fuser


300


. Sensing element


302


would be located on halogen bulb fuser


300


between the release layer covering the surface of halogen bulb fuser


300


and the wall of the cylinder


304


surrounding halogen bulb


306


. The release layer is not shown in FIG.


4


. Because cylinder


304


is typically formed from aluminum, sensing element


302


must be electrically insulated from the surface of cylinder


304


. Halogen bulb


306


provides the primary source of energy for fixing toner


9


to print media


12


. Power control circuit


24


supplies substantially constant power to sensing element


302


. In a manner similar to adjustment of the power supplied to the instant on fuser


2


, power control circuit


24


adjusts the power supplied to halogen bulb fuser


300


in response to changes in the thermal load.




Shown in

FIG. 5

is a flow chart of a method for using the instant on fuser


2


with power control circuit


24


. First, in step


400


, a level of power sufficient for fixing toner to the nominal thermal load is applied to first heating element


100


and second heating element


102


by power control circuit


24


. Next, in step


402


, power control circuit


24


monitors the voltage across first heating element


100


and the current through first heating element


100


to detect changes in the thermal load applied to first heating element


100


. Then, in step


404


power control circuit


24


maintains the power dissipation in first heating element


100


at a substantially constant level. Next, in step


406


, power control circuit


24


adjusts the power applied to second heating element


102


to compensate for thermal loading different from the nominal thermal load.




Although several embodiments of the invention have been illustrated, and their forms described, it is readily apparent to those of ordinary skill in the art that various modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.



Claims
  • 1. A fixing device for fusing toner to print media, comprising:a first heating element; a second heating element; and a power control circuit configured for selectively supplying a substantially constant average power to the first heating element and configured for adjusting the power supplied to the second heating element in response to thermal loading of the first heating element by the print media.
  • 2. The fixing device as recited in claim 1, wherein:the capability of the power control circuit for adjusting the power supplied to the second heating element in response to thermal loading of the first heating element by the print media includes increasing the power supplied to the second heating element for thermal loading greater than a predetermined level and decreasing the power supplied to the second heating element for thermal loading less than the predetermined level.
  • 3. Using a power supply for supplying power to the first heating element and the second heating element, the fixing device as recited in claim 2, wherein:the power control circuit includes a first switch coupled in series with the first heating element and for coupling in series with the power supply; the power control circuit includes a second switch coupled in series with the second heating element and for coupling in series with the power supply; and the power control circuit includes a pulse width modulator coupled to the first switch and the second switch, with the pulse width modulator configured for actuating the first switch to deliver substantially constant average power to the first heating element and configured for actuating the second switch to adjust the power supplied to the second heating element in response to thermal loading of the first heating element by the print media.
  • 4. The fixing device as recited in claim 3, wherein:the pulse width modulator supplies a first signal to the first switch and a second signal to the second switch; the first signal switches between a first state, for placing the first switch in an open state, and a second state, for placing the first switch in a closed state; and the second signal switches between a third state, for placing the second switch in an open state, and a fourth state, for placing the second switch in a closed state.
  • 5. The fixing device as recited in claim 4, wherein:the pulse width modulator includes the capability to adjust a duration of the second state to deliver substantially constant average power from the power supply to the first heating element and the pulse width modulator includes the capability to adjust a duration of the fourth state to adjust the power supplied to the second heating element for thermal loading different than the predetermined level.
  • 6. The fixing device as recited in claim 5, wherein:the power control circuit includes a first sensor coupled to the pulse width modulator and configured for measuring a first parameter related to power dissipation in the first heating element; the power control circuit includes a second sensor coupled to the pulse width modulator and configured for measuring a second parameter related to power dissipation in the first heating element; and the pulse width modulator includes the capability to adjust the duration of the second state responsive to at least one of the first parameter and the second parameter and includes the capability to adjust the duration of the fourth state responsive to at least one of the first parameter and the second parameter for thermal loading different than the predetermined level.
  • 7. The fixing device as recited in claim 6, wherein:the first parameter includes the current through the first heating element; and the second parameter includes the voltage across the first heating element.
  • 8. The fixing device as recited in claim 7, wherein:the first switch and the second switch each include a triac.
  • 9. In a fixing device for fixing toner to print media, with the fixing device including a power control circuit, a first heating element coupled to the power control circuit, and a second heating element coupled to the power control circuit, a method for controlling the power applied to the second heating element to compensate for thermal loading by the print media different than a predetermined level of thermal loading, the method comprising the steps of:applying a substantially constant average power to the first heating element; applying power to the second heating element corresponding to the predetermined level of thermal loading; and adjusting the power supplied to the second heating element in response to thermal loading of the first heating element different than the predetermined level of thermal loading.
  • 10. The method as recited in claim 9, wherein:the step of adjusting includes increasing the power the supplied to the second heating element for thermal loading greater than the predetermined level and decreasing the power supplied to the second heating element for thermal loading less than the predetermined level.
  • 11. With the power control circuit including a first switch coupled in series with the first heating element and for coupling to a power supply, a second switch coupled in series with the second heating element and for coupling to the power supply, and a pulse width modulator coupled to the first switch and the second switch, the method as recited in claim 10, wherein:the step of applying the substantially constant average power to the first heating element includes actuating the first switch; and the step of applying power to the second heating element corresponding to the predetermined level of thermal loading includes actuating the second switch.
  • 12. With the pulse width modulator supplying a first signal to the first switch and a second signal to the second switch, with the first signal switching between a first state, for placing the first switch in an open state, and a second state, for placing the first switch in a closed state, and with the second signal switching between a third state, for placing the second switch in an open state, and a fourth state, for placing the second switch in a closed state, the method as recited in claim 11, wherein:the step of applying the substantially constant average power to the first heating element includes adjusting a duration of the second state; and the step of adjusting includes adjusting a duration of the fourth state for thermal loading of the first heating element different than the predetermined level.
  • 13. With the power control circuit including a first sensor coupled to the first heating element and the pulse width modulator and with the power control circuit including a second sensor coupled to the first heating element and the pulse width modulator, the method as recited in claim 12, wherein:the step of applying the substantially constant average power to the first heating element includes measuring a first parameter related to power dissipation in the first heating element and measuring a second parameter related to power dissipation in the first heating element; and the step of adjusting the duration of the second state includes adjusting the duration of the second state in response to at least one of the first parameter and the second parameter.
  • 14. An electrophotographic printing system for printing print data on print media using toner, comprising:a formatter arranged for receiving the print data to generate formatted print data; a scanner configured for receiving the formatted print data to generate a laser beam modulated based upon the formatted print data; a photoconductor onto which the laser beam impinges to generate a latent electrostatic image; a developer for developing the toner onto the latent electrostatic image; a transfer device for transferring the toner on the photoconductor onto the print media; and a fixing device including a first heating element, a second heating element, and a power control circuit configured for selectively supplying a substantially constant average power to the first heating element and configured for adjusting the power supplied to the second heating element in response to thermal loading of the first heating element by the print media.
  • 15. The electrophotographic printing system as recited in claim 14, wherein:the capability of the power control circuit for adjusting the power supplied to the second heating element in response to thermal loading of the first heating element by the print media includes increasing the power supplied to the second heating element for thermal loading greater than a predetermined level and decreasing the power supplied to the second heating element for thermal loading less than the predetermined level.
  • 16. Using a power supply for supplying power, the electrophotographic printing system as recited in claim 15, wherein:the power control circuit includes a first switch coupled in series with the first heating element and for coupling in series with the power supply; the power control circuit includes a second switch coupled in series with the second heating element and for coupling in series with the power supply; and the power control circuit includes a pulse width modulator coupled to the first switch and the second switch, with the pulse width modulator configured for actuating the first switch to deliver substantially constant average power to the first heating element and configured for actuating the second switch to adjust the power supplied to the second heating element in response to thermal loading of the first heating element by the print media.
  • 17. The electrophotographic printing system as recited in claim 16, wherein:the pulse width modulator supplies a first signal to the first switch and a second signal to the second switch; the first signal switches between a first state, for placing the first switch in an open state, and a second state, for placing the first switch in a closed state; and the second signal switches between a third state, for placing the second switch in an open state, and a fourth state, for placing the second switch in a closed state.
  • 18. The electrophotographic printing system as recited in claim 17, wherein:the pulse width modulator includes the capability to adjust a duration of the second state to deliver substantially constant average power from the power supply to the first heating element and the pulse width modulator includes the capability to adjust a duration of the fourth state to adjust the power supplied to the second heating element for thermal loading different than the predetermined level.
  • 19. The electrophotographic printing system as recited in claim 18, wherein:the power control circuit includes a first sensor coupled to the pulse width modulator and configured for measuring a first parameter related to power dissipation in the first heating element; the power control circuit includes a second sensor coupled to the pulse width modulator and configured for measuring a second parameter related to power dissipation in the first heating element; and the pulse width modulator includes the capability to adjust the duration of the second state responsive to at least one of the first parameter and the second parameter and to adjust the duration of the fourth state responsive to at least one of the first parameter and the second parameter for thermal loading different than the predetermined level.
  • 20. The electrophotographic printing system as recited in claim 19, wherein:the first parameter includes the current through the first heating element; and the second parameter includes the voltage across the first heating element.
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