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Description  |
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This invention relates generally to fusing devices used in an
electrophotographic printing machine, and more particularly concerns a
control system employed therein for anticipating the temperature
deviations of the fuser device and correcting for these deviations
automatically.
Generally, an electrophotographic printing machine employs a
photoconductive member which is charged to a substantial uniform potential
to sensitize the surface thereof. The charged portion of the
photoconductive member is exposed to a light image of an original document
being reproduced. Exposure of the charged photoconductive member
selectively dissipates the charge thereon in the irradiated areas. This
records an electrostatic latent image on the photoconductive member
corresponding to the informational areas contained within the original
document. After the electrostatic latent image is recorded on the
photoconductive member, the latent image is developed by bringing a
developer material into contact therewith. Generally, the developer
material comprises toner particles adhering triboelectrically to carrier
granules. The toner particles are attracted from the carrier granules to
the latent image forming a latent powder image on the photoconductive
member. The toner powder image is then transferred from the
photoconductive member to a copy sheet. The toner particles are heated to
permanently affix the powder image to the copy sheet.
In a commercial printing machine of the foregoing type, the fusing device
employs a heated roller to heat the toner particles and permanently affix
them to the copy sheet. However, it is necessary to insure that the toner
particles do not adhere to the fuser roller. In the event that the toner
particles adhere to the fuser roller, they may be subsequently transferred
to successive copy sheets degradating the quality thereof. Thus, the fuser
roller must operate within a temperature latitude dictated by the
properties of the toner particles. At one extreme, the fuser roller must
be heated sufficiently to permanently affix the toner particles to the
copy sheet. While at the other extreme, the fuser roller temperature must
not exceed the maximum limit wherein toner particles are offset from the
copy sheet and remain adhering to the fuser controller.
Generally, the fuser roller is heated by a suitable heat source to a
pre-determined temperature. At the surface of the roller, a temperature
detecting device continuously measures the surface temperature of the
roller. A control circuit associated with the temperature detector
regulates the amount of power furnished to the heating element of the
fuser roller so as to control the surface temperature thereof. It has been
found that as the fuser roller contacts the copy sheet, the fuser roller
surface temperature drops below the intended steady state operating
temperature. This temperature drop is caused by both the lag in the
temperature detector and the thermal mass of the fuser roll core. As the
control systems responds to these lags, the fuser roll temperature
increases to its operating temperature. However, after completion of a
copy run, due to the thermal energy stored in the fuser roll and the lag
in the temperature sensor, the surface temperature overshoots the designed
steady state stand-by temperature. At some later time, the temperature
returns to the steady state stand-by condition. Operation of the fusing
system in this manner is inefficient and may produce copy quality defects.
For example, the copies going through the fusing device initially may not
be heated sufficiently to permanently affix the toner powder image to the
sheet. Alternatively, temperature overshoots at the end of a copy run
increase the temperatures at which the first few copies of the following
job experience. This may lead to offsetting of the toner particles from
the copy sheets to the fuser roll. Various approaches have been devised to
control the temperature of fusing devices. The following disclosures
appear to be relevant:
U.S. Pat. No. 4,046,990 Patentee: White Issued: Sept. 6, 1977
U.S. Pat. No. 4,145,599 Patentee: Sakurai Issued: Mar. 20, 1979
U.S. Pat. No. 4,318,612, Patentee: Brannan, et al. Issued: Mar. 9, 1982
U.S. Pat. No. 4,415,800 Patentee: Dodge, et al. Issued: Nov. 15, 1983
The pertinent portions of the foregoing disclosures may be briefly
summarized as follows:
White discloses a heater roll disposed internally of a fuser roll with a
temperature sensor contacting the core thereof. Upon energizing the
printing machine, a controller detects the need to raise the core
temperature, as measured by the temperature sensor, to an idle
temperature. During a copy run, a selectively insertable resistor is added
to the control circuit. With the addition of the resistor, the controller
regulates at a pre-determined higher controlled setting. At this higher
control setting, the heater is actuated until the core reaches the
pre-determined temperature appropriate for fusing in the run state. After
the copy run is completed the resistor is removed from the circuit
returning to the idle temperature.
Sakurai, et al., describes a thermister contacting the surface of a fuser
roll and being also connected to a heat source. The thermister set-point
temperature is variable. When the detected fuser temperature is less than
the set-point temperature, the fuser is energized. The set-point
temperature during copying is greater than the set-point temperature
during stand-by. The stand-by set-point is greater than the set-point
temperature after a copy run. This latter set-point temperature, in turn,
is equal or greater than the set-point temperature during the waiting
time. In this way, the temperature of the fuser roller is limited to a
narrow range.
Brannan, et al., discloses a fuser roller temperature controller that
adjusts the set-point temperatures so that at a cold start the set-point
temperature is higher than for a relatively hot start. During copying, the
fusing temperature set-point varies as a function of the area of the sheet
to be fused. Larger sheets have a higher fuser temperature set-point with
the set-point being reduced at specified intervals during the copy run.
Dodge, et al., describes a fuser roller temperature control system which
monitors the fuser roll temperature during warm up. The fuser roll
temperature is sampled for decreasing threshold intervals. Sampling
terminates when the measured fuser roll temperature exceeds the threshold
temperature. The copier is then enabled for normal copying.
In accordance with one aspect of the features of the present invention,
there is provided an apparatus for fusing images to a sheet during a copy
run. Means are provided for applying heat to at least the images on
successive sheets advanced, in seriatum, thereto for substantially
permanently affixing the images to the sheets. Means detect the
temperature of the heat applying means and transmit a signal indicative
thereof. Means control the heat applying means. The controlling means
compares the time derivative of the signal received from the detecting
means at initialization of the copy run to a first constant and energizes
the heat applying means when the first constant is less than the time
derivative of the signal. After the copy run, the controlling means
compares the time derivative of the signal received from the detecting
means to a second constant and de-energizes the heat applying means when
the second constant is less than the time derivative of the signal. During
the copy run, after the time derivative of the signal is less than the
first constant and a specified time period has elapsed, the controlling
means compares the signal from the detecting means to a third constant and
generates an error signal indicative of the difference therebetween to
control the heat applying means.
Pursuant to another aspect of the features of the present invention, there
is provided an electrophotographic printing machine of the type having a
fusing apparatus for fusing toner powder images transferred to copy sheets
during a copy run of the printing machine. The improved fusing apparatus
includes means for applying heat to at least toner images on successive
sheets advanced, in seriatum, thereto for substantially permanently
affixing the toner powder image to the copy sheets. Means detect the
temperature of the heat applying means and transmits a signal indicative
thereof. Means control the heat applying means. The controlling means
compares the time derivative of the signal received from the detecting
means at initialization of the copy run to a first constant and energizes
the heat applying means when the first constant is less than the time
derivative at the signal. After the copy run, the controlling means
compares the time derivative of the signal received from the detecting
means to a second constant and de-energizes the heat applying means when
the second constant is less than the time derivative of the signal. During
the copy run, after the time derivative of the signal is less than the
first constant and a specified time period has elapsed, the controlling
means compares the signal received from the detecting means to a third
constant and generates an error signal indicative of the difference
therebetween to control the heat applying means.
Other aspects of the invention will become apparent as the following
description proceeds and upon reference to the drawings, in which:
FIG. 1 is an elevational view depicting an electrophotographic printing
machine incorporating with features of the present invention therein;
FIG. 2 is a fragmentary, elevational view depicting the fusing device used
in the FIG. 1 printing machine;
FIG. 3 is a graph showing the temperature variation of the center surface
of the fuser roller used in the FIG. 2 fusing device without the control
scheme of the present invention being employed;
FIG. 4 is a block diagram illustrating the control system regulating the
temperature of the FIG. 2 fusing device; and
FIG. 5 is a flow diagram showing the control scheme employed by the control
logic of FIG. 4.
While the present invention will hereafter be described in connection with
a preferred embodiment thereof, it will be understood that it is not
intended to limit the invention to that embodiment. On the contrary, it is
intended to cover all alternatives, modification, and equivalents that may
be included within the spirit and scope of the invention as defined by the
appended claims.
For a general understanding of the features of the present invention,
references is made to the drawings. In the drawings, like reference
numerals have been used throughout to designate identical elements. FIG. 1
schematically depicts the various components of an illustrative
electrophotographic printing machine incorporating the fusing system of
the present invention therein. It will become evident from the following
discussion that the fusing system of the present invention is equally well
suited for use in a wide variety of electrostatographic printing machines,
and is not necessarily limited in its application to the particular
printing machine shown herein.
Inasmuch as the art of electrophotographic printing is well known, the
various processing stations employed in the FIG. 1 printing machine will
be shown hereandafter schematically and their operation described briefly
with reference thereto.
As shown in FIG. 1, the electrophotographic printing machine employes a
belt 10 having a photoconductive surface 12 deposited on a conductive
substrate 14.
Preferably, photoconductive surface 12 is made from a selenium alloy with
conductive substrate 14 being made from an aluminum alloy. Other suitable
photoconductive materials and conductives substrates may also be employed.
Belt 10 moves in the direction of arrow 16 to advance successive portions
of photoconductive surface 12 sequentially through the various processing
stations disposed about the path of movement thereof. Belt 10 is entrained
about stripping roller 18, tensioning roller 20 and drive roller 22.
Stripping roller 18 is mounted rotatably so as to rotate with the movement
belt 10. Tensioning roller 20 is resiliently urged against belt 10 to
maintain belt 10 under the desired tension. Drive roller 22 is rotated by
motor 24 coupled thereby suitable means, such as a drive belt. As roller
22 rotates, belt 10 advances in the direction of arrow 16.
Initially, a portion of photoconductive surface 12 passes through charging
station A. At charging station A, a corona generating device, indicated
generally by the reference numeral 26, charges photoconductive surface 12
to a relatively high, substantially uniform potential.
Next, the charged portion of photoconductive surface 12 is advanced through
imaging station B. At imaging station B, a document handling unit,
indicated generally by the reference numeral 28, is positioned over platen
30 of the printing machine. Document handling unit 28 sequentially feeds
documents from a stack of document placed by the operator face down in a
normal forward collating order in a document stacking and holding tray. A
document feeder located, below the tray, forwards the bottom document of
the stack to a pair of takeaway rollers. The bottommost sheet is then
sent, by rollers through a document guide to a feed roll and conveyor
belt. The conveyor belt advances the document onto platen 30. After
imaging, the original document is fed from platen 30 by the conveyor belt
into a guide and feed roll pairs which advance the document into an
inverter mechanism, or back to the document stack through feed roll pairs.
A decision gate is provided to divert the document either to the inverter
or to the feeder roll pairs. Imaging of a document on platen 30 is
achieved by lamps 32 which illuminate the document positioned thereon.
Light rays reflected from the document are transmitted through lens 64.
Lens 64 focuses the light image of the original document onto the charged
portion of photoconductive surface 12 to selectively dissipate the charge
thereon. This records an electrostatic latent image on photoconductive
surface 12 which corresponds to the informational areas contained within
the original document. Thereafter, belt 10 advances the electrostatic
latent image recorded on photoconductive surface 12 to development station
C.
With continued reference to FIG. 1, at development station C, a pair of
magnetic developer rollers, indicated generally by the reference numerals
36 and 38, advance developer material into contact with the electrostatic
latent image. The latent image attracts toner particles from the carrier
granules of the developer material to form a toner powder image on
photoconductive surface 12 of belt 10.
Belt 10 then advances the toner image powder to transfer station D. At
transfer station D, a copy sheet is moved into contact with the toner
powder image. Transfer station D includes a corona generating device 40
which sprays ions onto the backside of the copy sheet. This attracts the
toner powder image from photoconductive surface 12 of belt 10 to the
sheet. After transfer, conveyor 42 advances the copy sheet to a fusing
station E.
The copy sheets are selected from one of the trays 44 or 46 and advanced to
transfer station D by conveyor belts 70 and feed rolls 72. After transfer
of the toner image powder to the first side of the copy sheet, the sheet
is advanced by conveyor 42 to fusing station E.
Fusing station E includes a fusing system indicated generally by the
reference numeral 48. Preferably, the fusing system includes a heated
fuser roller 50 and a back-roller 52 with the toner image on the sheet
contacting fuser roller 50. In this manner, the powder image is
permanently affixed to the copy sheet. The detailed structure of fusing
system 48 and the control scheme thereof will be described hereinafter
with reference to FIGS. 2 through 5 inclusive.
After fusing, the copy sheets are fed to decision gate 54 which functions
as an inverter selector. Depending upon the position of gate 54, the
sheets will be deflected into a sheet inverter 56 or bypass inverter 56
and be fed directly to a second decision gate 58. The sheets which bypass
inverter 56 turn a 90.degree. corner in the sheet path before reacting
gate 58. This inverts the sheets into a face up orientation so that the
image side, which has been transferred and fused, is face up. If inverter
path 56 is selected, the opposite is true, i.e., the last printed side is
face down. The second decision gate 58 either deflects the sheet directly
into an output tray 60 or deflects the sheets into a transport path which
carries them on without inversion to a third decision gate 62. Gate 62
either passes the sheets directly on without inversion into the output
path of the copier or deflects the sheets onto a duplex inverter roller
64. Roller 64 inverts and stacks the sheets to be duplexed in duplex tray
66 when gate 62 so directs. Duplex tray 66 provides intermediate buffer
storage for those sheets which have been printed on one side in which an
image will be subsequently printed on the side opposed thereto, i.e., the
sheets being duplexed. Due to the sheets being inverted by roller 64, the
sheets are stacked in tray 66 face down. The sheets are stacked in duplex
tray 66 on top of one another in the order in which they are copied.
In order to complete duplex copying, the simplex sheets in duplex tray 66
are fed, in series, by bottom feeder 68 from tray 66 back to transfer
station d for transfer of the toner powder image to the opposed side of
the copy sheet. Conveyors 70 and rollers 72 advance the sheet along the
path which produces an inversion thereof. However, inasmuch as the
bottommost sheet is fed from duplex tray 66, the proper or clean side of
the copy sheet is in contact with belt 10 at transfer station D so that
the toner powder image on photoconductive surface 12 is transferred
thereto. The duplex sheets are then fed through the same path as the
simplex sheets to be stacked in tray 60 for subsequent removal by the
machine operator.
Invariably, after the copy sheet is separated from photoconductive surface
12 of belt 10 some residual particles remain adhering thereto. These
residual particles are removed from photoconductive surface 12 at cleaning
station F. Cleaning station F includes a rotatably mounted fiberous brush
74 in contact with photoconductive surface 12 of belt 10. These particles
are cleaned from photoconductive surface 12 of belt 10 by the rotation of
brush 74 in contact therewith. Subsequent to cleaning, a discharge lamp
(not shown) floods photoconductive surface 12 with light to dissipate any
residual photostatic charge remaining thereon for prior to the charging
thereof the next successive imaging cycle.
Controller 76 is preferrably a programmable microprocessor which controls
all the machine functions. The controller provides the storage and
comparison of counts of the copy sheets, the number of documents being
recirculated in the document sets, the number of copy sheet selected by
the operator, time delays, jam correction control, fuser temperature
control, etc. The control of all of the systems in the printing machine
may be accomplished by conventional control switch input from the printing
machine console selected by the operator. Conventional sheet path sensors
or switches may be employed for tracking or keeping track of the position
of the documents and copy sheets. Controller 76 contains the necessary
logic for regulating the temperature of fuser 48.
It is believed that the foregoing description is sufficient for purposes of
the present invention to illustrate the general operation of an
electrophotographic printing machine incorporating the features of the
present invention therein.
Referring now to the specific subject matter of the present time.
Invention, fuser 48 will be described with reference to FIG. 2 through 5
inclusive.
As shown in FIG. 2, fuser 48 includes a fuser roller, indicated generally
by the reference numeral 50, and a back-up roller, indicated generally by
the reference numeral 52. A temperature sensor 78 contacts the exterior
circumstantial surface of fuser roller 50.
Preferably, temperature sensor 78 is thermistor wherein the resistance
thereof varies as a function of the detected temperature. The output
signal from temperature sensor 78 is a voltage. Fuser roller 50 is
composed of a hollow tube 80 having a thin covering 82 thereon. A heat
source 84 is disposed interiorly of tube 80. Tube 80 is made from a metal
material having the desired heat conductivity characteristics. By way of
example, aluminum, copper and other metals having a high thermal
conductivity are suitable for use as a tube. Preferrably, covering layer
82 is made from silicone rubber. Heating element 84 is preferrably a
halogen lamp. Lamp 84 is connected to sensor 78 through controller 76.
Back-up roller 52 has a relatively thick layer of silicone rubber 86 on
metal tube 88. Back-up roller 52 is mounted rotatably on bracket 90.
Bracket 90 is actuated by controller 76 to pivot so as to press back-up
roller 52 into contact with fuser roller 50 to define a nip therebetween
through which the copy sheet passes. Switch 92 detects the presence or
absence of the copy sheet in fusing system 48 and indicates the status
thereof to controller 76. Rollers 50 and 52 remain spaced from each other
whenever fusing is not occurring. When fusing is occurring, roller 52
pivots so as to press against fuser roller 50. Back-up roller 52 and fuser
roller 50 are adapted to rotate during the fusing operation so as to
advance the copy sheet therethrough. Heat source 84, which may be a
halogen lamp, or infrared lamp, amongst others, is located internally of
fuser roller 80. During the operation of an internally heated fuser
roller, the surface of the fuser roller experiences temperature variations
which are due to the changes in the thermal load thereon. Temperature
control is achieved through a proportional, resistor thermistor 78 coupled
to control 76 which, in turn, regulates the heat output from heat source
84. Temperature variations occur as a result of the system going from a
stand-by mode, wherein fuser roller 50 is at its stand-by temperature and
back-up roller 52 is significantly cooler, to an operating or fusing mode,
in which the copy sheet passes between the fuser roller and back-up roller
at elevated temperatures. Large amounts of heat are transferred to the
copy sheet and back up roller 52 during the fusing process. This
drastically lowers the surface temperature of fuser roller 50.
Turning now to FIG. 3, there is shown a typical temperature cycle for fuser
roller 50 when only being controlled in direct proportion to the voltage
output from temperature sensor 78. A copy run is initiated at time,
t.sub.p, when the fuser roller surface is at its stand-by temperature
t.sub.s. As the fuser roller engages the back-up roller and a copy sheet
passes through the nip therebetween, the surface temperature of the fuser
roller drops below the intended steady state run temperature T.sub.d. This
droop is due both to the lags in the temperature controlling device and
the thermal mass of the fuser roller core. As the system responds to these
lags, the surface temperature of the roller increases to its operating or
run temperature, T.sub.r, at time t.sub.ssr, and remains very close to
this temperature until the end of the copy run, at t.sub.sp. After
completion of a copy run, due to the thermal energy stored in the fuser
roller and the time lag in the temperature sensor, the surface temperature
of the fuser roller overshoots the designed steady state stand-by
temperature to T.sub.o, at time t.sub.max. At some later time, the
temperature returns to the stand-by temperature, T.sub.s. Operation of the
fusing system in this manner is inefficient and may cause copy quality
problems. If the operating latitude of the fusing system is limited, the
copy sheets going through the fusing system at the start of the copy run
may not be adequately fixed. On the other hand, temperature overshoots, at
the end of the copy run, may increase the temperatures which the first few
copies of the next successive copy run experience. This may lead to
offsetting of the toner particles from the copy sheet to the fuser roller.
These offset toner particles adhering to the fuser roller may be
transferred to the next copy degradating the quality thereof. Thus, it is
highly desirable to minimize the droop and overshoot temperature
excursions during operation of the fusing system. This may be achieved by
employing a control system which anticipates these excursion and adjusts
the system to minimize their effects.
In order to minimize the droop and overshoot during a copy run, the fusing
system controller must be able to anticipate the fuser roller surface
temperature as a function of several parameters, i.e., the length of the
copy run, the size of the copy sheet being employed, and the mode that the
copy sheet is being operated in, i.e., simplex, duplex or computer forms
feeding, etc. In addition to the foregoing, the control system must be
able to anticipate the surface temperature variations during a particular
run. In order to achieve this, the control logic must determine the
magnitude of the first derivative of the temperature sensor voltage output
with respect to time and compare this value with pre-determined boundary
values throughout the copy run. Based on these values, the control system
determines the heat output from the fuser roller heat source.
Referring now to FIG. 4, temperature sensor 78 develops a voltage output
signal which is indicative of the measured surface temperature of the
fuser roller. The voltage signal from the temperature sensor 78 is
transmitted to controller 76. Controller 76 determines the time derivative
of the voltage signal transmitted thereto. The time derivative of the
voltage signal is compared to pre-determined boundary values. The boundary
values are chosen through emperical means to correspond to the actual
measured values of the rate of change of the fuser roller surface
temperature. One of the boundary values is a pre-selected constant which
is compared to the time derivative of the voltage from temperature sensor
78 at the beginning, or initialization, of the copy run. The other
boundary value is a constant which is compared to the time derivative of
the voltage from temperature sensor 78 at the end of the copy run. The
decision of whether or not to energize fuser lamps 84 is made by comparing
the stored constants with the time derivative of the voltage from
temperature sensor 78. At the initialization of a copy run, the time
derivative of the voltage from temperature sensor 78 is compared with the
first constant or boundary value. If this constant is greater than the
absolute value of the time derivative of the voltage, fuser lamp 84
remains off otherwise, fuser lamp 84 is energized. The constant is
selected so that nominally, fuser lamp 84 will be energized at the
beginning of a copy run, and, as such, is selected to be slightly less
than the desired time derivative of voltage from temperature sensor 78.
After a period of time, with fuser lamp 84 being energized, the surface
temperature of the fuser roller increases and the time derivative of the
voltage output from temperature regulator 78 decreases such that the time
derivative is less than the constant. At this point, controller logic 76
defaults to the normal proportional control mode. At the end of the copy
run, the system again calculates the time derivative of the voltage output
from temperature sensor 78 and compares this with the second constant. If
the second constant is greater than the absolute value of the time
derivative, the fuser lamp is energized, otherwise the fuser lamp remains
off. It is desirable to have the fuser lamp remain off immediately after
completion of the copy run so that the surface temperature of the fuser
roller does not excessively overshoot the stand-by condition. The second
constant is chosen to achieve the foregoing. Controller 76 defaults to the
normal proportional control mode once the surface temperature of the fuser
roller is at the stand-by condition. Controller 76 determines the time
derivative of the voltage from temperature sensor 78 by subtracting
successive voltage measurements from temperature sensor 78 and dividing by
the elapsed time therebetween. The output from controller 76 regulates the
power output from high voltage power supply 94. High voltage power supply
94 is coupled to fuser lamp 84 and, dependent upon the input thereto,
regulates the heat output therefrom.
Turning now to FIG. 5, there is shown a flow diagram describing the
operation of the control scheme. As shown thereat, the temperature sensor
voltage output V.sub.t, time t, is compared to the temperature sensor
voltage, V.sub.t-a, at time t-a. The difference in the voltage outputs is
then divided by a seconds, i.e. the time difference between the two
voltage readings. This determines the time derivative of the voltage,
.chi.. The time derivative of the voltage, .chi., is then compared with
constant, b.sub.1, at the start of the copy run. If .chi., i.e. the time
derivative of the voltage is greater than the constant, b.sub.1, fuser
lamp 84 is energized. Conversely, if .chi., i.e. the time derivative of
the voltage output, is less than b.sub.1, the fuser lamp is de-energized.
At the end of the copy run, .chi., the time derivative of the voltage
output, is compared to a second constant, b.sub.2. If .chi., the time
derivative of the voltage output, is greater than the second constant
b.sub.2, fuser lamp 84 is de-energized. Conversely, if .chi., the time
derivative of the voltage output, is less than the second constant,
b.sub.2, fuser lamp 84 is energized. At all other times, the control
system defaults to the normal proportional control.
In recapitulation, it is evident that the control system for the fusing
devise of the present device minimizes temperature droops and overshoots
at the surface of the fuser roller. The surface temperature of the fuser
roller is regulated within specified temperature latitudes to insure that
toner particles are not offset from the copy sheet to the fuser roller and
to provide adequate heat for permanently affixing the toner particles to
the copy sheet. This type of fusing control produces excellent, high
quality copies.
It is, therefore, evident that there has been provided in accordance with
the present invention a fusing system that fully satifies the aims and
advantages hereinbefore set forth. While this invention has been described
in conjunction with a specific embodiment thereof, it is evident that many
alternatives, modifications, and variations will be apparent to those
skilled in the art. Accordingly, it is intended to embrace all such
alternatives, modifications, and variations as fall within the spirit and
broadscope of the appended claims.
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