U.S. patent application number 09/797030 was filed with the patent office on 2001-08-30 for image forming apparatus.
This patent application is currently assigned to Oki Data Corporation. Invention is credited to Hayashi, Kuniharu, Murakami, Tatsuya, Nagamine, Masamitsu, Yajima, Hiroyuki, Yamane, Tsutomu.
Application Number | 20010017992 09/797030 |
Document ID | / |
Family ID | 27342513 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017992 |
Kind Code |
A1 |
Hayashi, Kuniharu ; et
al. |
August 30, 2001 |
Image forming apparatus
Abstract
An image forming apparatus has a fixing unit. A heat roller
heats a toner image on a print medium to fuse the toner image. A
first temperature detector is in contact with a longitudinal end
portion of the heat roller, and generates a first temperature data.
A second temperature detector opposes the heat roller and is in
proximity to a substantially middle portion of the heat roller. The
second temperature detector generates a second temperature data.
Depending on the first temperature data and the second temperature
data, a controller performs temperature control either in a first
mode where the heater is energized such that the first temperature
data is equal to a first target value, or in a second mode where
the heater is energized such that second temperature data is equal
to a second target value.
Inventors: |
Hayashi, Kuniharu; (Ube-shi,
JP) ; Yamane, Tsutomu; (Tokyo, JP) ; Nagamine,
Masamitsu; (Tokyo, JP) ; Yajima, Hiroyuki;
(Tokyo, JP) ; Murakami, Tatsuya; (Tokyo,
JP) |
Correspondence
Address: |
AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P.
ONE COMMERCE SQUARE
2005 MARKET STREET, SUITE 2200
PHILADELPHIA
PA
19103
US
|
Assignee: |
Oki Data Corporation
|
Family ID: |
27342513 |
Appl. No.: |
09/797030 |
Filed: |
February 28, 2001 |
Current U.S.
Class: |
399/68 ; 219/216;
399/69; 399/70; 432/60 |
Current CPC
Class: |
G03G 15/205
20130101 |
Class at
Publication: |
399/68 ; 399/69;
399/70; 219/216; 432/60 |
International
Class: |
G03G 015/20 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 29, 2000 |
JP |
053322/00 |
Feb 29, 2000 |
JP |
053332/00 |
Mar 15, 2000 |
JP |
072137/00 |
Claims
What is claimed is:
1. A fixing unit for use in an image forming apparatus, comprising:
a heat roller, heating a toner image on a print medium; a heater
disposed in said heat roller and supplying heat to said heat
roller; a first temperature detector disposed in contact with a
first surface area of an end portion of said heat roller outside of
a second surface area of said heat roller with which the print
medium passes in contact, said first temperature detector
generating a first temperature data; a second temperature detector
disposed to oppose said heat roller, said second temperature
detector being in proximity to a substantially middle portion of
the second surface, said second temperature detector generating a
second temperature data; and a controller, controlling supply of
electric power to said heater, said controller switching based on
the first temperature data and the second temperature data between
a first temperature control mode where said controller controls
supply of electric power to said heater such that the first
temperature data is equal to a first target value, and a second
temperature control mode where said controller controls supply of
electric power to said heater such that the second temperature data
is equal to a second target value.
2. The image forming apparatus according to claim 1, wherein when a
fixing operation of the print medium has been completed, said
controller switches from the second temperature control mode to the
first temperature control mode.
3. The image forming apparatus according to claim 1, wherein if the
second temperature data decreases below a certain value during the
first temperature control mode, said controller switches from the
first temperature control mode to the second temperature control
mode.
4. The image forming apparatus according to claim 11, wherein when
continuous printing is being performed in the first temperature
control mode and a number of printed pages reaches a certain value,
said controller switches from the first temperature control mode to
the second temperature control mode.
5. The image forming apparatus according to claim 1, wherein said
controller controls supply of electric power to said heater in the
first temperature control mode during a standby state where the
image forming apparatus waits for a print command; wherein when the
print medium has a width smaller than a reference width and the
image forming apparatus receives a print command of continuous
printing, said controller controls supply of electric power to said
heater in the second temperature control mode.
6. The image forming apparatus according to claim 1, wherein said
heater includes a first heater element having a first length and a
second heater element having a second length shorter than the first
length; wherein when printing is performed on the print medium
having a smaller width than a reference width, said controller
supplies electric energy to the second heater element, said
controller supplies electric energy to the second heater element
and the first heater element if the first temperature data
decreases below a lower limit.
7. The image forming apparatus according to claim 1, wherein when
the first temperature data becomes equal to the first target value,
said controller sets the second temperature data as the second
target value.
8. The image forming apparatus according to claim 1, wherein said
controller performs a correction operation in which the second
temperature data is corrected to determine an approximate actual
surface temperature of said heat roller; wherein said controller
stores a first correction value and a second correction value, the
first correction value being a difference between the first
temperature data and a first item of data that describes the actual
surface temperature for a proper fixing operation, and the second
correction value being a difference between the first temperature
data and the second temperature data, wherein when said controller
is performing a printing operation, said controller adds the first
correction value and second correction value to the second
temperature data, thereby determining the approximate actual
surface temperature of said heat roller.
9. The image forming apparatus according to claim 8, wherein said
controller performs a warm-up operation where when the print medium
is not being passing through the fixing unit, said heat roller is
rotated and said controller supplies electric power to said heater
such that the actual surface temperature becomes the first item of
data; wherein the second correction value is determined immediately
before the warm-up operation is halted.
10. The image forming apparatus according to claim 9, wherein said
controller detects an initial value of a surface temperature of the
heat roller after power up and before said controller supplies
electric power to said heater; wherein said controller performs the
warm-up operation for a period of time in accordance with the
initial value, the period of time being increased stepwise as the
initial value decreases.
11. The image forming apparatus according to claim 9, wherein the
initial value of the surface temperature is the second temperature
data after power up and before said controller supplies electric
power to said heater.
12. The image forming apparatus according to claim 1, wherein said
second temperature detector opposes said heat roller with a gap
therebetween, wherein said controller incorporates a control
program in which a size of the gap is determined based on the first
temperature data, the second temperature data, and the first items
of data, and then the fixing temperature is controlled in
accordance with the size of the gap.
13. The image forming apparatus according to claim 1, wherein when
the first temperature data exceeds a certain value during the
second temperature control mode, the controller halts printing;
wherein when the first temperature data decreases to a third target
value after the first temperature has exceeded the certain value,
the controller resumes printing.
14. The image forming apparatus according to claim 1, wherein when
the first temperature data exceeds a first value during the second
temperature control mode; said controller causes the print medium
to be advanced at a low speed, and wherein when the first
temperature data decreases to a second value after said controller
has caused the print medium to be advanced at a low speed, said
controller causes the print medium to be advanced at a high
speed.
15. The image forming apparatus according to claim 1, wherein said
controller incorporates a temperature control program in which an
amplitude of ripple in the second temperature data is determined
and the fixing temperature of the heat roller is corrected in
accordance with the amplitude of ripple.
Description
BACKGROUND OF THE INVENTION
[0001] Field of the Invention
[0002] The present invention relates to an image-forming apparatus
such as a laser printer in which electrophotography is used to form
an image on print paper, and more particularly to a
temperature-controlling apparatus that controls the temperature of
a fixing unit that fixes an image transferred onto the print
paper.
[0003] With an electrophotographic image-forming apparatus, laser
light illuminates a photoconductive drum to form an electrostatic
latent image thereon. The electrostatic latent image is then
developed with toner into a toner image. Then, the toner image is
transferred from the photoconductive drum to print paper. Finally,
the toner image on the print paper is fixed at the fixing unit and
then the paper is discharged from the fixing unit.
[0004] FIG. 30 illustrates a fixing unit and the associated control
sections of a conventional image-forming apparatus.
[0005] The conventional image-forming apparatus will be described
with reference to FIG. 30.
[0006] A fixing unit 100 includes a heat roller 1, a pressure
roller 20, a heater 2, two springs 21, and a temperature detector
30. The heat roller 1 heats a toner image, transferred to a print
medium such as A5 size paper 70 or A4 size paper 80, to fix the
toner image. The heater 2 generates heat and supplies the heat to
the heat roller 1. The pressure roller 20 is rotatably supported by
a shaft 22 and rotates in pressure contact with the heat roller 1.
The springs 21 are disposed at longitudinal ends of the shaft 22
and urge the shaft 22 against the heat roller 1 such that the
pressure roller 20 is in pressure contact with the heat roller 1.
The temperature detector 30 is in contact with the outer
circumferential surface of the heat roller 1 to detect the
temperature of the heat roller 1.
[0007] In order to uniformly heat the toner image on the print
medium 70 or 80, the heat roller 1 takes the shape of a long,
hollow cylinder made of a highly heat conductive metal material.
The heat roller 1 has a rubber material formed on an outer
circumferential surface of the cylinder with a resin coating on the
rubber. The coating is highly heat resistant and has good
mold-releasing characteristic. The heat roller 1 and the pressure
roller 20 are rotated in directions shown by arrows 90 and 91 of
FIG. 30, respectively.
[0008] The heater 2 takes the form of, for example, a halogen lamp
that extends in the longitudinal direction of the heat roller 1 but
is not in contact with the heat roller 1.
[0009] The pressure roller 20 is a rubber roller in pressure
contact with the heat roller 1. The pressure roller 20 is rotated
such that the print medium 70 or 80 is pulled in between the heat
roller 1 and the pressure roller 20 and is advanced in a direction
shown by arrow 92.
[0010] The temperature detector 30 takes the form of, for example,
a temperature measuring thermistor. The temperature detector 30 is
disposed on the circumferential surface of the heat roller 1
outside of a paper path in which the A4 size paper (medium 80)
passes. In this specification, A4 size paper in portrait
orientation is a maximum size of print medium that the fixing unit
can accept. Specifically, the temperature detector 30 is a distance
L2 (e.g. 0-10 mm) away from the paper path.
[0011] A control block 200 shown in FIG. 30 will be described.
[0012] A main controller 11 controls the supply of electric power
to the heater 2, transport of the print medium 70 or 80, and drive
of the pressure roller 20. An analog/digital (A/D) converter 12
that converts temperature data in analog form detected by the
temperature detector 30 into a digital form that can be processed
in the main controller 11. A power switch 13 that switches on and
off the electric power supplied to the heater 2 from a power supply
14 under the control of the main controller 11. In accordance with
the instructions from the main controller 11, a drive controller 15
drives the drive source 16 in the form of, for example, a motor
that drives the pressure roller 20 in rotation. A memory 17 stores
a control program for the main controller 11 and control data. The
operating panel 18 includes an input section 18b and a display
section 18a. The input section 18b allows the user to set the size
of print paper and print density prior to printing. The display
section 18a displays current settings and messages to the
operator.
[0013] The warm-up operation of the aforementioned conventional
image-forming apparatus is performed as follows:
[0014] For example, upon power up, the warm-up operation starts.
The main controller 11 causes the power switch 13 to shift from an
OFF position to an ON position, so that the power supply 14
supplies electric power to the heater 2. At the same time, the main
controller 11 causes the drive controller 15 to drive the motor 16
into rotation. Thus, the motor 16 drives the pressure roller 20 and
heat roller 1 in rotation in the directions shown by arrows 90 and
91. In the warm-up operation, when the temperature detected by the
detector 30 reaches a target temperature (e.g., 170.degree. C.) at
which the fixing operation can be adequately carried out, the
supply of electric power from the power supply to the heater 2 is
interrupted. Thereafter, feedback control is performed through a
loop (the detector 30.fwdarw.A/D converter 12.fwdarw.main
controller 11.fwdarw.power switch 13.fwdarw.heater 2.fwdarw.heat
roller 1), thereby maintaining the detected temperature to the
target temperature.
[0015] The operation when the image-forming apparatus receives
print data from a host apparatus will be described.
[0016] Upon receiving the print data from the host apparatus, the
main controller 11 provides a command to the power switch 13 so
that the power switch 13 directs electric power to the heater 2 as
required. The main controller 11 also provides a command to the
drive controller 15, thereby causing the drive controller 15 to
drive the motor 16 to drive the pressure roller 20 and heat roller
1 in rotation. Then, the main controller 11 causes the print medium
70 or 80 to be fed to the image-forming section from a paper
cassette, not shown. A toner image is transferred onto the print
medium 70 or 80 and the print medium is advanced by a registry
roller, not shown, in a direction shown by white arrow 92 and abuts
the pressure roller 20. The print medium is pulled in between the
rotating heat roller 1 and the pressure roller 20. When the print
medium passes between the heat roller 1 and pressure roller 20, the
toner on the print medium 70 or 80 is melted and permanently
adheres to the print medium due to the pressure applied by the
pressure roller 20. The print medium 70 or 80 is then discharged
with a transport device, not shown, to the outside of the
printer.
[0017] As described above, the temperature detector 30 is disposed
in contact with an area outside of the surface of the heat roller 1
that contacts the print medium. The electric power supplied to the
heater 2 is switched on and off such that a temperature detected by
the temperature detector 30 is maintained at a predetermined
value.
[0018] When the print medium 70 or 80 passes between the heat
roller 1 and pressure roller 20, the print medium absorbs heat from
the heat roller 1 to fuse the toner deposited thereon, thereby
causing the temperature of the heat roller 1 to decrease. This
creates a temperature difference between an area on the heat roller
1 that contacts the print medium and an area on the heat roller 1
that does not contact the print medium.
[0019] For example, when A4 size paper (print medium 80) passes
through an area Lb, the heat roller 1 loses heat to the A4 size
paper, creating a temperature difference between the area Lb and a
point E. However, the distance L2 between the area Lb and the point
E is very short, e.g., 0-10 mm. Thus, the temperature difference
between the area Lb and the point E is not large, so that the
temperature in the area Lb can be maintained at a predetermined
target temperature by feedback control using a temperature detected
by the temperature detector 30 in contact with the point E.
[0020] Also, when, for example, A5 size paper (print medium 70)
smaller than A4 passes through the area 1a, the heat roller 1 loses
heat to the A5 size paper, creating a temperature difference
between the area 1a and the point E. However, the distance L1
between the area 1a and the point E is much longer than L2. Thus,
the heat does not transfer so fast through the heat roller 1, so
that a temperature difference between the area 1a and the point E
is large. Despite the fact that the temperature at the point E can
be controlled to a predetermined target value, the temperature in
the area 1a is not sufficient for proper fixing. In other words,
when a plurality of pages are printed continuously, the temperature
difference increases. Thus, the aforementioned conventional
controlling method cannot maintain the proper temperature of the
heat roller 1 in direct contact with a narrow-width print
medium.
SUMMARY OF THE INVENTION
[0021] The present invention was made in view of the aforementioned
drawbacks of the conventional apparatus.
[0022] An object of the invention is to provide an image-forming
apparatus for printing on print media having a variety of widths
wherein when pages of a small-width print medium are printed
continuously, the temperature of the heat roller in contact with
the print medium is maintained at a constant temperature.
[0023] A fixing unit is used in an image forming apparatus. A heat
roller heats a toner image on a print medium. A heater is disposed
in the heat roller and supplies heat to the heat roller. A first
temperature detector is in the form of a thermistor. The first
temperature detector is disposed in contact with a first surface
area of an end portion of the heat roller outside of a second
surface area of the heat roller with which the print medium passes
in contact. The first temperature detector generates a first
temperature data. A second temperature detector is disposed to
oppose the heat roller in proximity to a substantially middle
portion of the second surface, and generates a second temperature
data. A controller controls supply of electric power to the heater.
Based on the first temperature data and the second temperature
data, the controller switches between a first temperature control
mode and a second temperature control mode. The first temperature
control mode is a mode where the controller controls supply of
electric power to the heater such that the first temperature data
is equal to a first target value. The second temperature control
mode is a mode where the controller controls supply of electric
power to the heater such that the second temperature data is equal
to a second target value.
[0024] When a fixing operation of the print medium has been
completed, the controller switches from the second temperature
control mode to the first temperature control mode.
[0025] If the second temperature data becomes below a certain value
during the first temperature control mode, the controller switches
from the first temperature control mode to the second temperature
control mode.
[0026] When continuous printing is being performed in the first
temperature control mode and a number of printed pages reaches a
certain value (e.g., second target value minus 15.degree. C.),the
controller switches from the first temperature control mode to the
second temperature control.
[0027] The controller controls supply of electric power to the
heater in the first temperature control mode during a standby state
where the image-forming apparatus waits for a print command. When
the print medium has a width smaller than a reference width and the
image forming apparatus receives a print command of continuous
printing, the controller controls supply of electric power to the
heater in the second temperature control mode.
[0028] The heater includes a first heater element having a first
length and a second heater element having a second length shorter
than the first length. When printing is performed on the print
medium having a smaller width than a reference width, the
controller supplies electric energy to the second heater element.
If the first temperature data decreases below a lower limit, then
the controller supplies electric energy to the second heater
element and the first heater element.
[0029] When the first temperature data becomes equal to the first
target value, the controller sets the second temperature data as
the second target value.
[0030] The controller performs a correction operation in which the
second temperature data is corrected to determine an approximate
actual surface temperature of the heat roller. The controller
stores a first correction value and a second correction value, the
first correction value being a difference between the first
temperature data and a first item of data that describes the actual
surface temperature for a proper fixing operation, and the second
correction value being a difference between the first temperature
data and the second temperature data. When the controller is
performing a printing operation, the controller adds the first
correction value and second correction value to the second
temperature data, thereby determining the approximate actual
surface temperature of the heat roller. The controller performs a
warm-up operation where when the print medium is not being passing
through the fixing unit, the heat roller is rotated and the
controller supplies electric power to the heater such that the
actual surface temperature becomes the first item of data. The
second correction value is determined immediately before the
warm-up operation is halted. The controller detects an initial
value (e.g., second temperature data) of a surface temperature of
the heat roller after power up and before the controller supplies
electric power to the heater. The controller performs the warm-up
operation for a period of time in accordance with the initial
value, the period of time being increased stepwise as the initial
value decreases.
[0031] The second temperature detector opposes the heat roller with
a gap therebetween. The controller incorporates a control program
in which a size of the gap is determined based on the first
temperature data, the second temperature data, and the first items
of data, and then the fixing temperature is controlled in
accordance with the size of the gap.
[0032] When the first temperature data exceeds a certain value
during the second temperature control mode, the controller halts
printing; wherein when the first temperature data decreases to a
third target value after the first temperature has exceeded the
certain value, the controller resumes printing.
[0033] When the first temperature data exceeds a first value during
the second temperature control mode; the controller causes the
print medium to be advanced at a low speed. When the first
temperature data decreases to a second value after the controller
has caused the print medium to be advanced at a low speed, the
controller causes the print medium to be advanced at a high
speed.
[0034] The controller incorporates a temperature control program in
which an amplitude of ripple in the second temperature data is
determined and the fixing temperature of the heat roller is
corrected in accordance with the amplitude of ripple.
[0035] Further scope of applicability of the present invention will
become apparent from the detailed description given hereinafter.
However, it should be understood that the detailed description and
specific examples, while indicating preferred embodiments of the
invention, are given by way of illustration only, since various
changes and modifications within the spirit and scope of the
invention will become apparent to those skilled in the art from
this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The present invention will become more fully understood from
the detailed description given hereinbelow and the accompanying
drawings which are given by way of illustration only, and thus are
not limiting the present invention, and wherein:
[0037] FIG. 1A is a perspective view of a fixing unit and a control
block diagram of the fixing unit according to a first
embodiment;
[0038] FIG. 1B is a cross-sectional view taken along line II-II of
FIG. 1A;
[0039] FIG. 2 is a flowchart illustrating a warm-up operation in
the first embodiment;
[0040] FIG. 3 is a flowchart illustrating a temperature control of
the fixing unit according to the first embodiment;
[0041] FIG. 4 shows the relationship between the number of
continuously printed pages and the temperature at the middle
portion of the heat roller 1 when printing is performed on A4 size
paper;
[0042] FIG. 5 is a graph similar to FIG. 4 and shows the
relationship between the number of continuously printed pages and
the temperature at the middle portion of the heat roller 1 when
printing is performed on A5 size paper;
[0043] FIG. 6 illustrates the relationship between the adhesion of
toner and the surface temperature of heat roller 1 when the print
medium is transported at a paper speed of 2 in./sec and the target
value of surface temperature is 170.degree. C.;
[0044] FIG. 7 is a flowchart illustrating a temperature control of
the heat roller 1 according to the second embodiment;
[0045] FIG. 8 shows the relationship between the temperature
T.sub.M of the longitudinal middle portion of the heat roller and
the number of printed pages when continuous printing is performed
on the A5 size paper;
[0046] FIG. 9 is a flowchart illustrating the temperature control
operation according to the third embodiment;
[0047] FIG. 10 shows the relationship between the temperatures of
various parts of the heat roller 1 and the number of pages when
continuous printing is performed on a plurality of pages of A5 size
paper;
[0048] FIG. 11 is a flowchart illustrating the temperature control
of the fixing unit according to the fourth embodiment;
[0049] FIG. 12 is a block diagram illustrating a method of
correcting the surface temperature of the heat roller and a control
system that uses the method;
[0050] FIG. 13 is a flowchart illustrating a correction method
according to the fifth embodiment, the correction method being
carried out by the controller of the control system 1 of FIG.
12;
[0051] FIG. 14 is a timing chart illustrating the operations of
various parts of the control system 1 when the control system 1
operates according to the flowchart of FIG. 13;
[0052] FIG. 15 illustrates temperatures at various points on the
heat roller 1 along the length of the heat roller 1 when the
correction of temperature begins at time t4 during the warm-up
operation;
[0053] FIG. 16 illustrates the relationship between difference
T.sub.S-T.sub.M at time t4 of FIG. 14 and the surface temperature
T.sub.M detected by the non-contact type temperature detector 40
immediately before time t0;
[0054] FIG. 17 is a flowchart illustrating a procedure of the sixth
embodiment of a correction method, the procedure being carried out
by the controller of the control system 1 of FIG. 12;
[0055] FIG. 18 is a timing chart illustrating the operations of
various parts of the control system 1 when the control system 1
performs a warm-up operation according to the flowchart of FIG.
17;
[0056] FIGS. 19-21 are flowcharts illustrating a procedure of the
seventh embodiment;
[0057] FIG. 22 is a timing chart illustrating the operation of
various parts of the control system 1 when the control system
operates according to the flowchart of FIGS. 19-21;
[0058] FIG. 23 is a partially cross-sectional view illustrating a
heat roller and heater and 2B assembled therein;
[0059] FIG. 24 is a block diagram illustrating a control system for
controlling the fixing temperature;
[0060] FIG. 25 is a timing chart that illustrates the control
operation for controlling the surface temperature of the heat
roller;
[0061] FIG. 26 illustrates the temperature profile across the
length of the heat roller;
[0062] FIG. 27 illustrates the relationship between the actual
surface temperatures of the heat roller and the gaps G between the
heat roller and the non-contact type temperature detector;
[0063] FIG. 28 illustrates the relationship between the actual
surface temperatures of the middle portion of the heat roller and
the temperatures of the left end portion of the heat roller when
the temperature control is performed based on the contact type
temperature detector;
[0064] FIGS. 29A-29B illustrate the outline of the correction
procedure of temperature ripple; and
[0065] FIG. 30 illustrates a fixing unit and the associated control
sections of a conventional image-forming apparatus.
DETAILED DESCRIPTION OF THE INVENTION
[0066] Embodiments of the invention will be described in detail
with reference to the accompanying drawings.
[0067] First Embodiment
[0068] {Construction}
[0069] FIG. 1A is a perspective view of a fixing unit and a control
block diagram of the fixing unit according to a first
embodiment.
[0070] FIG. 1B is a cross-sectional view taken along line II-II of
FIG. 1A.
[0071] A fixing unit 300 of FIG. 1A differs from the conventional
fixing unit of FIG. 30 in that a non-contact type temperature
detector 40 is disposed substantially in the longitudinal middle of
a heat roller 1 in which a heater 2 extends and is very close to
the heat roller but not in contact therewith. The heater 2 takes
the form of, for example, a halogen lamp. The temperature detector
40 is about 1 mm spaced apart from the circumferential surface of
the heat roller 1 that opposes an A5 size print medium, so that the
toner on the heat roller 1 is prevented from migrating to the
temperature detector. The temperature detector 40 is, for example,
a temperature-detecting thermistor.
[0072] The temperature detector 40 need not be in the longitudinal
middle of the heat roller 1, providing that the temperature
detector 40 is substantially at the middle of a surface area of the
heat roller 1 in contact with the A5 size print medium. A contact
type temperature detector 30 is disposed on the circumferential
surface of the heat roller 1 outside of a paper path in which A4
size paper (medium 80) passes. The temperature detector 30 is in
contact with the surface of the heat roller 1.
[0073] The non-contact type temperature detector 40 should be as
close to the surface of the heat roller 1 as possible. However, if
the temperature detector 40 is too close to the heat roller 1, the
temperature detector 40 may move into contact engagement with the
heat roller 1 when the heat roller 1 vibrates during rotation. If
the temperature detector 40 contacts the surface of the heat roller
1, toner will migrate from the heat roller 1 to the temperature
detector 40 with the result that the temperature detector 40
becomes unable to accurately detect the surface temperature of the
heat roller. The distance between the temperature detector 40 and
the surface of the heat roller 1 is of about 1 mm to ensure that
the temperature detector 40 will not contact the heat roller 1.
[0074] Paper-size detectors 50 and 60 take the form of, for
example, a photo coupler and are disposed in the transport path of
the print medium 70 or 80. The main controller 11 receives the
outputs of the paper-size detectors 50 and 60 and the temperature
detectors 30 and 40. Based on the outputs of the paper size
detectors 50 and 60, the main controller 11 determines whether the
print paper being transported is A4 size paper or A5 size paper. If
the paper-size detector 50 generates an output but the paper-size
detector 60 does not, then the main controller 11 determines that
the paper being transported is A5 size paper. If both the
paper-size detectors 50 and 60 generate their outputs, the main
controller 11 determines that the paper being transported is A4
size paper.
[0075] The control block 400 of FIG. 1A differs from the
conventional control block of FIG. 30 in that the control block 400
operates under another temperature control program and has a
storage area 17b and a storage area 17c. The storage area 17b
stores target temperatures TTE1 and TTE2 for the temperature
detector 30, and the storage area 17c stores target temperatures
TTM1 and TTM2 for the contact temperature detector 40. The control
block 400 is so configured because the image-forming apparatus uses
two temperature detectors 30 and 40 instead of one temperature
detector.
[0076] {Warm-up Operation}
[0077] FIG. 2 is a flowchart illustrating a warm-up operation in
the first embodiment.
[0078] At step S1, when the operator turns on the image-forming
apparatus, the heater 2 is energized and the temperature detectors
30 and 40 begin to detect the temperatures of the heat roller 1.
Thus, the temperature of the heat roller 1 starts to increase from
room temperature. At step S2, a decision is made to determine
whether temperature T.sub.E detected by the temperature detector
30, is higher than the target temperature TTE1 for the temperature
detector 30. For example, the target temperature TTE1 is
170.degree. C. for a paper speed of 2 in./sec and 200.degree. C.
for a paper speed of 4 in./sec. If T.sub.E<TTE1 at step S2, then
the program repeats step S2 until T.sub.E.gtoreq.TTE1. If
T.sub.E.gtoreq.TTE1 at step S2, then the program proceeds to step
S3 where the main controller 11 stores a temperature T.sub.M
detected by the temperature detector 40 into the storage area 17c,
the detected temperature T.sub.M being used as a target temperature
TTM1 for the temperature detector 40 during the operation
illustrated in FIG. 3. The temperature T.sub.E of the heat roller 1
goes up and reaches TTE1 and then exceeds TTE1. The step S2 is
actually carried out in a very short time and repeated many times.
Therefore, when T.sub.E is substantially equal to TTE1, the answer
at step S2 is YES and the target temperature TTM1 stored in the
storage area 17c can be considered to be a value of T.sub.M when
the T.sub.E=TTE1. For example, the target temperature TTM1 stored
in the area 17c is 155.degree. C. for the paper speed of 2 in./sec
and 170.degree. C. for the paper speed of 4 in./sec. At step S4,
the heater 1 is de-energized. At step S5, a decision is made to
determine whether T.sub.E<TTE1. If the answer is NO at step S5,
the program repeats step S5 until T.sub.E<TTE1. If the answer is
YES at step S5, the program proceeds to step S6 where since
T.sub.E<TTE1, the heater 2 is again turned on and the program
jumps back to step S2.
[0079] With the warm-up operation in FIG. 2, a feedback control is
carried out such that the temperature T.sub.E becomes equal to the
target temperature TTE1. Further, the target temperature TTM1 for
the temperature detector 40 is determined based on the temperature
T.sub.M 40 when the temperature T.sub.E reaches the target
temperature TTE1.
[0080] {Temperature Control of the First Embodiment}
[0081] FIG. 3 is a flowchart illustrating a temperature control of
the fixing unit according to the first embodiment.
[0082] At step S11, a decision is made to determine whether a
command for printing has been input by the operator through the
input section 18b of the operating panel 18. If the answer is NO at
step S11, then the program returns to step S11. If the answer is
YES at step S11, then the program proceeds to step S12 where a
decision is made to determine whether the print paper transported
to the fixing unit is A4 size paper (print paper having a maximum
width in portrait orientation). If the paper is A4 size paper, then
the program proceeds to step S13, and if the paper size is not A4,
then the program proceeds to step S23. In other words, the main
controller 11 switches between a temperature control (S13-S17)
based on the output of the contact type temperature detector 30 and
a temperature control (S23-S27) based on the output of the
non-contact type temperature detector 40. The temperature control
performed at steps S13-S17 is based on the temperature T.sub.E
detected by the contact type temperature detector 30 and is
referred to as a contact detector mode or CD mode hereinafter. The
temperature control performed at steps S23-S27 is based on the
temperature T.sub.M detected by the contact type temperature
detector 40 and referred to as a non-contact detector mode or NCD
mode hereinafter.
[0083] In the CD mode, electric power to the heater 2 is turned on
and off to control the temperature of the heat roller 1 such that
T.sub.E becomes equal to TTE1.
[0084] In the NCD mode, electric power to the heater 2 is turned on
and off to control the temperature of the heat roller 1 such that
T.sub.M becomes equal to TTM1. The two modes of temperature control
are selectively performed in accordance with the size of the print
medium; A4 size or A5 size.
[0085] {Temperature Control in the CD Mode}
[0086] At step S13, a decision is made to determine whether
T.sub.E.gtoreq.TTE1. If T.sub.E.gtoreq.TTE1, the program proceeds
to step S14, and if T.sub.E<TTE1, the program repeats step S13.
At step S14, the heater is de-energized. At step S15, a decision is
made to determine whether T.sub.E<TTE1. If the answer is YES at
step S15, the program proceeds to step S16 where the heater 2 is
again energized, and then proceeds to step S17. If the answer is NO
at step S15, the program repeats step S15. At step S17, a decision
is made to determine whether the fixing operation has been
completed. If the answer is YES at step 17, then the program
returns to the warm-up operation shown in FIG. 2. If the answer is
NO at step S17, then the program loops back to step S13.
[0087] {Temperature Control in the NCD Mode}
[0088] At step S23, a decision is made to determine whether
T.sub.M.gtoreq.TTM1. TTM1 is the target temperature stored in the
area 17c at step S3. Other experimental value may be used in place
of TTM1. If the answer is YES at step S23, the program proceeds to
step S24 where the heater 2 is de-energized. If the answer is NO a
step S23, the program repeats step S23. At step S25, a decision is
made to determine whether T.sub.M<TTM1. If the answer is NO at
step S25, the program repeats step S25. If the answer is YES at
step S25, the program proceeds to step S26 where the heater 2 is
energized again, and then proceeds to step S27 where a decision is
made to determine whether the fixing operation has been completed.
If the answer is YES at step S27, then the program jumps back to
the warm-up operation of FIG. 2. If the answer is NO at step S27,
then the program loops back to step S23 where a decision is made to
determine whether T.sub.M.gtoreq.TTM1.
[0089] FIG. 4 shows the relationship between the number of
continuously printed pages and the temperature at the middle
portion of the heat roller 1 when printing is performed on A4 size
paper 80. FIG. 4 compares the results obtained in the
aforementioned two temperature control modes: the CD mode and the
NCD mode.
[0090] FIG. 5 is a graph similar to FIG. 4 and shows the
relationship between the number of continuously printed pages and
the temperature at the middle portion of the heat roller 1 when
printing is performed on A5 size paper 70.
[0091] The temperature at the middle of the heat roller 1 was
measured with a contact type temperature measuring thermistor. The
print medium was transported at a paper speed of 2 in./sec, the
target value of the surface temperature was 170.degree. C., and the
maximum acceptable width of print medium was 8.5 inches (letter
size). [0044]1
[0092] As is clear from FIG. 4, when continuous printing of 100
pages is performed on A4 size print medium 80 in the CD mode, there
is no significant difference between the target value of the
surface temperature and the actual surface temperature. However,
when the temperature is controlled in the NCD mode, the actual
surface temperature was about 30.degree. C. below the target value
of 170.degree. C. as shown in FIG. 5. This is because a surface
area 1a of the heat roller 1 in contact with the A5 size print
medium 70 loses heat to the print medium 70 and therefore the
temperature of the surface area 1a decreases, while a surface area
lb of the heat roller 1 not in contact with the print medium 70
does not lose a significant amount of heat to the print medium 70
and therefore the temperature does not decrease.
[0093] Thus, it is advantageous that when continuous printing is
performed using the A5 size print medium 70, the surface
temperature can be controlled more accurately in the NCD mode than
in the CD mode. Further, a temperature decrease of about 7.degree.
C. in the NCD mode dose not cause significant deterioration in
fixing performance.
[0094] As described above, the first embodiment prevents decreases
in the surface temperature of the heat roller 1 when continuous
printing is performed on a plurality of pages of a print medium
whose width is smaller than that of the print medium of the maximum
acceptable size.
[0095] The fixing results of a heat roller can be evaluated in
terms of adhesion of toner to the print medium. The adhesion is
evaluated as follows:
[0096] 1. Measure the print density (D1) of a solidly printed
medium.
[0097] 2. Place an adhesive tape on an area of the solidly printed
medium such that the weight of the adhesive tape presses down the
toner. Then, place a 100 g/cm.sup.2 cylindrical weight on the
adhesive tape.
[0098] 3. Remove the weight and then peel off the adhesive
tape.
[0099] 4. Measure the print density (D2) of the area from where the
adhesive tape was peeled off and compare the print density D2 with
D1.
[0100] 5. Calculate (D2/D1).times.100%, which is the adhesion of
the toner.
[0101] The adhesion of toner is determined by an amount of heat
supplied from the heat roller 1 to the print medium 70 or 80. When
the print medium 70 or 80 passes through the fixing unit 300 at a
paper constant speed, the adhesion can be determined only in terms
of the surface temperature of the heat roller 1. Adhesion of toner
higher than 90% is sufficient.
[0102] FIG. 6 illustrates the relationship between the adhesion of
toner and the surface temperature of heat roller 1 when the print
medium 70 or 80 is transported at a paper speed of 2 in./sec and
the target value of surface temperature is 170.degree. C. It is to
be noted that surface temperatures of the heat roller 1 below
155.degree. C. causes the adhesion of toner to become less than
90%, leading to poor fixing results. Such poor fixing results can
be overcome by controlling the surface temperature of the heat
roller 1 in the NCD mode.
[0103] Second Embodiment
[0104] In order to switch between the CD mode and the NCD mode, the
first embodiment requires the paper-size detectors 50 and 60 to
detect the width of the supplied print medium, or the user to input
information on the width of the print medium through the input
section 18b. Providing the paper-size detector increases the
overall manufacturing cost. Inputting information on paper size
makes the overall key operation more complex and creates a chance
of inputting erroneous information. A second embodiment is to
overcome this drawback and is characterized in that temperature
T.sub.M, is used to automatically switch between the CD mode and
the NCD mode.
[0105] The construction of the second embodiment is substantially
the same as that shown in FIG. 1. The second embodiment differs
from the first embodiment in that the paper size detectors 50 and
60 are not provided and a different control program is used for
switching between the CD mode and the NCD mode. The warm-up
operation in the second embodiment is the same as that of the first
embodiment.
[0106] FIG. 7 is a flowchart illustrating a temperature control of
the heat roller 1 according to the second embodiment. This
operation is performed instead of the operation described with
reference to FIG. 3.
[0107] Step S31 is the same as step S11 of FIG. 3. The operation
performed at steps S32-S35 is an operation in the NCD mode and the
same as that performed at steps S13-S16. The operation performed at
steps S42-S45 is an operation in the NCD mode and the same as that
performed at steps S23-S26. Thus, their description is omitted for
simplicity.
[0108] The second embodiment differs from the first embodiment in
that step S12 is replaced by step S36. At step S12, the size of
print medium is checked to determine whether the temperature
control should be performed in the CD mode or in the NCD mode. At
step S36, the temperature T.sub.M is checked to determine whether
the temperature controlled should be performed in the CD mode or in
the NCD mode.
[0109] A decision is made at step S36 to determine whether if
T.sub.M<TTM1-15.degree. C. If the answer is NO at step S36, then
the program jumps back to step S32. If the answer is YES at step
S36, then the program proceeds to step S37 where a decision is made
to determine whether the fixing operation has been completed. If
the answer is YES at step S37, then the program returns to the
warm-up operation of FIG. 2. If the answer is NO at step S37, then
the program proceeds to step S42 where the temperature control mode
is switched from the CD mode to the NCD mode.
[0110] At step S46, a decision is made to determine whether the
fixing operation has been completed. If the answer is YES at step
S46, then the program jumps back to the warm-up operation of FIG.
2. If the answer is NO at step S46, the program loops back to step
S42 for repeating the temperature control in the NCD mode.
[0111] It is to be noted that at step S36, T.sub.M is used to
switch between the CD mode and the NCD mode. In other words, paper
size is determined whether it is A5 or A4, for example. If
T.sub.M<TTM1-15.degree. C., then it is determined that the print
medium is A5 size paper and the temperature control is switched
from the CD mode to the NCD mode. The operation at step S36 uses
the phenomenon that in the CD mode, the surface temperature of the
heat roller 1 becomes lower at the middle potion of the roller than
at the end portion if printing is performed on a narrow-width print
medium such as A5 size paper.
[0112] FIG. 8 shows the relationship between the temperature
T.sub.M of the longitudinal middle portion of the heat roller and
the number of printed pages when continuous printing is performed
on the A5 size paper.
[0113] Referring to FIG. 8, for the reasons mentioned above, the
temperature T.sub.M in the longitudinally middle portion of the
heat roller 1 gradually decreases when the temperature is being
controlled in the CD mode after the warm-up operation. When T.sub.M
reaches a lower limit, i.e., 140.degree. C., the temperature
control is switched from the CD mode to the NCD mode where the
heater 2 is energized such that the surface temperature stays at an
equilibrium temperature of about 170.degree. C. Thus, in the second
embodiment, too, good fixing results are obtained when continuous
printing is performed on a plurality of pages of A5 size paper.
[0114] Third Embodiment
[0115] Referring to FIG. 8, when the temperature in the
longitudinally middle portion of the heat roller 1 reaches an
equilibrium at 170.degree. C., the temperature T.sub.E at the point
E of the heat roller 1 has increased to 190.degree. C. In the
second embodiment, when the temperature of the point E exceeds
200.degree. C., a phenomenon referred to as "hot offset" occurs.
"Hot offset" is a phenomenon in which melted toner loses its
viscosity to be deposited on the heat roller and adheres to an
unwanted area of the print medium. As is clear from FIG. 8, T.sub.M
increases only up to 190.degree. C., which is not high enough for
"hot offset" to occur. However, recent demands for high speed
printing requires high speed transportation of the print medium in
the image forming apparatus. This in turn requires higher values of
various target temperatures.
[0116] For example, in the first and second embodiments, if the
transportation speed of the print medium is increased from 2
in./sec to 4 in./sec, then the target temperature of the contact
type temperature detector 30 should be increased to 200.degree. C.
This high temperature (i.e., 200.degree. C.) causes "hot offset".
Thus, toner should be improved in order to avoid "hot offset".
However, improvement to the toner may not be a sufficient solution
to "hot offset" because high temperatures cause deformation of
peripheral parts, evaporation of oil at the bearings of the heat
roller 1, and the wearing away of bearings. Therefore, the
temperature T.sub.E should not exceed a highest temperature of
230.degree. C. even if improved toner is used.
[0117] A third embodiment is characterized in that the T.sub.E is
used to prevent "hot offset" from occurring when the transportation
speed of print medium is increased to 4 in./sec.
[0118] In the third embodiment, the print medium is transported at
a speed of 4 in./sec, the target temperature of the heat roller 1
is 200.degree. C., and a maximum acceptable width of the print
medium in portrait orientation is 8.5 in. (letter size). The
control block of the third embodiment is similar to that of FIG. 1
except that the paper-size detectors 50 and 60 are not provided and
a different control program is used to switch between the CD mode
and the NCD mode. The warm-up operation of the third embodiment is
the same as that of FIG. 2.
[0119] FIG. 9 is a flowchart illustrating the temperature control
operation according to the third embodiment. This operation is
performed in stead of the operation described with reference to
FIG. 3 or the operation described with reference to FIG. 7.
[0120] The control performed by steps S51-S65 is the same as that
performed by steps S31-S45 in the second embodiment, and therefore
the description thereof is omitted.
[0121] The third embodiment differs from the second embodiment in
that an operation performed by steps S66-S74 is added to the second
embodiment. In other words, when T.sub.E.gtoreq.TTE2+30.degree. C.
(TTE2 is, for example, 200.degree. C., and TTE2+30.degree. C. is a
highest allowable temperature), the supply of electric power to the
fixing unit 300 and paper transporting devices is interrupted and
the display section 18a shows a warning to indicate to the user
that image-forming operation cannot be carried out. When
T.sub.E<TTE2+30.degree. C., the supply of electric power is
resumed and the warning is canceled.
[0122] At step S66, a decision is made to determine whether the
fixing operation has been completed. If the answer is YES at step
S66, then the program jumps back to the warm-up operation of FIG.
2. If the answer is NO at step S66, then the program proceeds to
step S67 where a decision is made to determine whether
T.sub.E.gtoreq.TTE2+30.degree. C. If the answer is YES at step S67,
the program proceeds to step S71. If the answer is NO at step S67,
the program loops back to step S62 for controlling the temperature
of the heat roller 1 in the NCD mode.
[0123] At step S71, the main controller 11 stops supplying electric
power to the fixing unit 300 and the drive source of the medium
transporting mechanism, and causes the display section 18a to
display a warning indicating to the user that image-forming
operation cannot be carried out.
[0124] At step S72, a decision is made to determine whether
T.sub.E<TTE2. If the answer is YES at step S72, then the program
proceeds to step S73 where the main controller 11 resumes to supply
electric power to the fixing unit 300 and the drive source of the
medium transporting mechanism and causes the display section 18a
not to display the warning. If the answer is NO at step S72, the
program repeats step S72. At step S74, a decision is made to
determine whether the fixing operation has been completed. If the
answer is YES at step S74, then the program jumps to the warm-up
operation of FIG. 2. If the answer is NO at step S74, then the
program loops back to step S62.
[0125] FIG. 10 shows the relationship between the temperatures of
various parts of the heat roller 1 and the number of pages when
continuous printing is performed on a plurality of pages of A5 size
paper. It is to be noted that the temperature control in the NCD
mode begins shortly after the warm-up operation.
[0126] Referring to FIG. 10, the surface temperature (dot-dash line
curve) in the longitudinal middle of the heat roller 1 is
maintained to 200.degree. C. when the temperature is controlled in
the NCD mode after the warm-up operation, but the temperature
T.sub.E increases gradually as the number of continuously printed
pages increases. When T.sub.E (solid line curve) reaches a maximum
value (highest allowable temperature of about 230.degree. C.), the
main controller 11 stops supplying electric power to the fixing
unit 300 and the drive source of the medium transporting mechanism.
Thus, thereafter if printing is not performed, T.sub.E gradually
decreases because the heat roller 1 loses heat to the surroundings.
When T.sub.E has decreased to the target temperature 200.degree.
C., the main controller 11 resumes to supply electric power to the
fixing unit 300 and the drive source of the medium transporting
mechanism, so that T.sub.E gradually increases as printing is
performed continuously on a plurality of pages thereafter. The
aforementioned operation is repeated in the third embodiment so
that good fixing results can be obtained reliably when continuous
printing is performed on a plurality of pages of A5 size print
medium 70.
[0127] Fourth Embodiment
[0128] In the third embodiment, when the main controller 11 stops
supplying electric power to the fixing unit 300 and the
surroundings and waits until the temperature decreases due to
natural heat dissipation to the surroundings. Thus, if a large
number of pages are to be printed, the apparatus halts printing
many times so that an overall printing time increases.
[0129] A fourth embodiment is characterized in that when a print
medium is transported at a speed of 4 in./sec, T.sub.E is used to
prevent "hot offset" while still maintaining the same printing
time. The control block diagram of the fourth embodiment differs
from that of FIG. 1 in that the paper-size detectors 50 and 60 are
not provided and a different control program is used to switch
between the CD mode and the NCD mode. The warm-up operation of the
fourth embodiment is the same as that of FIG. 2.
[0130] FIG. 11 is a flowchart illustrating the temperature control
of the fixing unit according to the fourth embodiment.
[0131] This temperature control is performed in stead of the
temperature control in the second embodiment described with
reference to FIG. 7 or the temperature control in the third
embodiment described with reference to FIG. 9.
[0132] The control performed by steps S81-S97 is the same as that
performed by steps S51-S67 in the third embodiment, and the control
performed by steps S102-S105 is the same as that performed by steps
S23-S26 in the first embodiment. Therefore, the description thereof
is omitted.
[0133] The fourth embodiment differs from the third embodiment in
that the temperature control performed by steps S101-S108 is added.
In other words, when T.sub.E exceeds a maximum allowable
temperature TTE2+30.degree. C., the print medium is transported at
a lower speed and the main controller 11 causes the display section
18a to display a warning that image-forming speed will be
decreased, and when T.sub.E<TTE1, the print medium is
transported at a higher speed and the main controller 11 causes the
display section 18a not to display the warning.
[0134] If T.sub.E.gtoreq.TTE2+30.degree. C. at step S97 (e.g.,
TTE2=200.degree. C.), then the program proceeds to step S101.
[0135] At step S101 the following operations are performed. The
transportation speed of the print medium is switched from 4 in./sec
to 2 in./sec. The target temperature of the contact type
temperature detector 30 is switched from TTE2 to TTE1 (i.e.,
170.degree. C.). The target temperature of the non-contact type
temperature detector 40 is switched from TTM2 (e.g., 180.degree.
C.) to TTM1 (i.e., 155.degree. C.). The main controller causes the
display section 18a to display a warning that the image forming
speed will be decreased.
[0136] At step S106, a decision is made to determine whether
T.sub.M<TTE1. If the answer is NO at step S106, then the program
returns to step S102. If the answer is YES at step S106, then the
program proceeds to step S107.
[0137] At step S107, the following operations are performed. The
transportation speed is increased from 2 in./sec to 4 in./sec. The
target temperature of the contact type temperature detector 30 is
switched from TTE1 (e.g., 170.degree. C.) to TTE2 (e.g.,
200.degree. C.). The target temperature of the non-contact type
temperature detector 40 is switched from TTM1 (e.g., 155.degree.
C.) to TTM2 (e.g., 180.degree. C.). The main controller 11 causes
the display section 18a not to display a warning that the image
forming speed will be decreased.
[0138] At step S108, a decision is made to determine whether the
fixing operation has been completed. If the answer is YES at step
S108, the program jumps to the warm-up operation of FIG. 2. If the
answer is NO at step S108, then the program loops back to step S92
where temperature control is performed in the NCD mode for a paper
speed of 4 in./sec.
[0139] A printing operation according to the fourth embodiment was
carried out continuously on 100 pages of A5 size print medium 70,
and the printing operation was completed in seven minutes. The same
printing operation was completed in ten minutes in the third
embodiment. Thus, the fourth embodiment improves the printing time
of the third embodiment by three minutes.
[0140] In the fourth embodiment, T.sub.E is used to control the
temperature of the heat roller 1 in such a way that when the
transportation speed of the A5 size print medium is increased to 4
in./sec, the temperature T.sub.E at the point E will not increase
to cause "hot offset". Moreover, the fourth embodiment provides as
short a printing time as possible when a large number of pages are
printed.
[0141] In the fourth embodiment, two different transportation
speeds of the print medium are required, so that the size of the
control program may be large and a large memory area may be
required. Therefore, either the third embodiment or the fourth
embodiment may be selectively employed depending on whether cost is
of prime importance or speed is of prime importance.
[0142] The present embodiment has been described with respect to a
case where a maximum size of print medium is A4 size in portrait
orientation and a smaller size of print medium is A5 size in
portrait orientation. The size of print medium is not limited to
these particular sizes and a print medium having an arbitrary size
such as a letter size instead of A4 size may be employed. The
target temperatures and limit temperatures are not limited to
TTE1=155.degree. C., TTE1=170.degree. C., TTM2=180.degree. C.,
TTE2=200.degree. C., TT.sub.E+30=230.degree. C., and the lower
limit=140.degree. C., but other temperatures may be employed.
[0143] Fifth Embodiment
[0144] A fifth embodiment is directed to a correction of the
surface temperature of the heat roller 1 detected by the
non-contact type temperature detector 40, and a temperature control
based on the corrected surface temperature.
[0145] In the fifth embodiment, a temperature T.sub.M disposed in
the longitudinally middle portion (i.e., point M) of the heat
roller 1, is corrected to obtain a corrected temperature T.sub.C,
which is accurately close to an actual surface temperature T.sub.S
at the middle portion of a longitudinally extending heat roller.
The correction value is determined at the end of the warm-up
operation carried out immediately after power up of the fixing
unit, so that the corrected temperature T.sub.C can be obtained
based on the temperature T.sub.M during a printing operation.
[0146] FIG. 12 is a block diagram illustrating a method of
correcting the surface temperature of the heat roller and a control
system that uses the method.
[0147] Referring to FIG. 12, a control system 1 calculates an
actual surface temperature of a heat roller 1 of a fixing unit 101
of FIG. 1. The control system 1 also controls the on-off operation
of a heater 2 in the form of a halogen lamp.
[0148] The controller 2 receives a temperature T.sub.M at a point M
(FIG. 1) detected by a non-contact type temperature detector 40 and
a temperature T.sub.E at the point E detected by a contact type
temperature detector 30. The controller 2 then calculates the
corrected temperature T.sub.C using a later described procedure.
The controller 2 outputs a switching signal Vc that causes the
heater 2 to turn on and off and a drive signal Dm that causes a
fixing motor 117 to drive the heat roller 1 in rotation.
[0149] FIG. 13 is a flowchart illustrating a correction method
according to the fifth embodiment, the correction method being
carried out by the controller 400 of the control system 1 of FIG.
12.
[0150] FIG. 14 is a timing chart illustrating the operations of
various parts of the control system 1 when the control system 1
operates according to the flowchart of FIG. 13
[0151] {Warm-up Operation}
[0152] The flowchart of FIG. 13 will be described with reference to
FIG. 14.
[0153] At step S1, the fixing unit 101 is turned on and the motor
117 is turned on. At step S2, the contact type temperature detector
30 detects a surface temperature T.sub.E of the heat roller 1 (FIG.
1). At step S3, a decision is made to determine whether the
temperature T.sub.E.ltoreq.h1. If the answer is YES at step S3,
then the program proceeds to step S4 where the heater 2 is turned
on, and then jumps back to step S2. The heater 2 is turned on at
time t0 of FIG. 14. Steps S2-S4 are repeated until the answer is NO
at step S103.
[0154] If the answer is NO at step S3, it is assumed that the
temperature T.sub.E exceeds h1 at time t1 of FIG. 14 and the
program proceeds to steps S5-S6 where the fixing motor 117 is
turned on and the controller 2 starts to count an elapsed time.
Then, at step S7, the controller 2 reads the temperature T.sub.E at
the point E. At step S8, a decision is made to determine whether
T.sub.E.gtoreq.h2. If the answer is No at step S8, then the program
returns to step S7 to repeat steps S7-S8 until
T.sub.E.gtoreq.h2.
[0155] If the answer is YES at step S8, the program proceeds to
step S9 where the heater 2 is turned off. At step S10, the program
waits until elapsed time reaches or exceeds Ta. The temperature of
the heat roller 1 continues to increase for a certain period of
time after the halogen lamp 2 is turned off. The temperature
T.sub.E passes h2 at time t2, then h3 and finally decreases to h3
at time t3. The temperature h3 is a fixing temperature. The time
period Ta is a period from time t2 to time t3 and is a time elapse
from turnoff of the heater 2 at step S9 until the temperature of
the heat roller 1 falls to the temperature h3. The length of Ta is
determined by experiment.
[0156] At step S1, the controller 2 reads the temperature T.sub.E
again. At step S12, a decision is made to determine whether
T.sub.E.ltoreq.h3. If the answer is YES at step S12, the heater 2
is turned on at step S13; if NO, the heater 2 is turned off at step
14. At step S15, a decision is made to determine whether elapsed
time reached or exceeds Tb. If YES at step S15, then the later
described correction of detected temperature T.sub.M is performed
at step S16. Then, at step S17, the fixing motor 117 is turned off,
thereby completing the warm-up operation.
[0157] Steps S11-S15 correspond to time t3 of FIG. 14 and steps S16
corresponds to time t4 to time t5. The correction of temperature
begins at time t4 and the control loop S11-S15 maintains the
temperature T.sub.E at the fixing temperature h3.
[0158] {Correction of Temperature}
[0159] The correction of temperature performed at step S16 will now
be described with reference to FIGS. 14 and 15.
[0160] FIG. 15 illustrates temperatures at various points on the
heat roller 1 along the length of the heat roller 1 when the
correction of temperature begins at time t4 during the warm-up
operation.
[0161] When the warm-up operation starts from a cold start, i.e.,
the printer is turned on when the temperature inside the printer is
the same as room temperature, the surface temperatures of the heat
roller 1 reaches equilibrium at time t4.
[0162] The rod-like heater 2 generates more heat at the middle
thereof than at end portions thereof and the heat roller 1
dissipates heat at end portions thereof than at the middle portion
thereof. Thus, at time t4 at which the heat roller has not lost
heat to the print medium yet, the temperature at the longitudinal
middle portion of the heat roller 1 is stable and several degrees
to several tens degrees Celsius higher than that at the
longitudinal end portions.
[0163] Da is a temperature difference between an actual surface
temperature T.sub.S at the middle of the heat roller 106 at time t4
and a temperature T.sub.E at portion E of the heat roller 1 at time
t4. Therefore, Da=T.sub.S-T.sub.E. Da is experimentally
determined.
[0164] Db is a temperature difference between a temperature T.sub.E
at an end portion (i.e., point E) of the heat roller and a
temperature T.sub.M at the middle portion (i.e., point M) of the
heat roller 1 detected by the non-contact type temperature detector
40. Therefore, Db=T.sub.E-T.sub.M. A difference between an actual
surface temperature T.sub.S at the middle of the heat roller 1 and
a temperature T.sub.M, i.e., a temperature correction value
T.sub.S-T.sub.M can be obtained by using Da and Db. Thus, the
correction T.sub.S-T.sub.M is equal to Da+Db. The value of Da+Db is
substantially constant not only at time t4 but also during the
printing operation where the heat roller 1 is maintained at high
temperature. Thus, reading temperature T.sub.M with the non-contact
type temperature detector 40 allows calculation of corrected
surface temperature T.sub.S=T.sub.M+(Da+Db) which is an
approximated actual temperature at the middle of the longitudinal
heat roller 1.
[0165] Db is measured and set when the warm-up operation is
performed. Thus, the correction operation accommodates the gap G
(FIG. 1) between the non-contact type temperature detector 40 and
the surface of the heat roller 1, which varies due to manufacturing
errors. Thus, the correction operation is very advantageous.
[0166] When continuous fixing is performed on a plurality of pages,
the temperature in the surface area 1a (i.e., point M) in contact
with the print medium will decrease to a value lower than the
temperature at the point as the number of printed pages increases.
In such a case, a corrected temperature or actual surface
temperature Tc can be obtained by equation
T.sub.C=T.sub.M+(Da+Db).
[0167] Sixth Embodiment
[0168] The method described in the fifth embodiment suffer from the
problem that .DELTA.Da varies depending on the interior temperature
of the printer before the printer is turned on. Therefore, a
corrected temperature T.sub.C may not be close to an actual
temperature. This problem occurs, for example, when the printer is
turned on again relatively short time after the printer was turned
off.
[0169] FIG. 16 illustrates the relationship between difference
T.sub.S-T.sub.M at time t4 of FIG. 14 and the surface temperature
T.sub.M immediately before time t0.
[0170] As is clear form FIG. 16, if the warm-up operation is
performed from a cold start where room temperature is 28.degree.
C., i.e., T.sub.M=28.degree. C., then T.sub.S-T.sub.M=33.degree. C.
at time t4.
[0171] If the warm-up operation is performed when
T.sub.M=100.degree. C., then T.sub.S-T.sub.M=27.degree. C. at time
t4. In other words, the value of T.sub.S-T.sub.M is 6.degree. C.
lower when T.sub.M=100.degree. C. than when T.sub.M=28>C. This
case occurs, for example, if the printer is turned off after a
printing operation and then the printer is turned on again a little
later.
[0172] The temperature profile along the length of the heater 2 is
responsible for variations of T.sub.S-T.sub.M at time t4 depending
on initial conditions. Usually, the heater 2 generates more heat at
a longitudinally middle portion thereof than at longitudinal end
portions thereof. If the heater 2 is energized until the
temperature T.sub.E at the point E reaches a predetermined value,
then temperature profile at the point M overshoots. The overshoot
of temperature profile at the point M is larger when the initial
surface temperature of the heat roller 1 is close to room
temperature than when the initial surface temperature of heat
roller 1 is relatively higher than room temperature. Thus, the
temperature at the point M is high when the initial surface
temperature of the heat roller 1 is close to room temperature.
[0173] In order to minimize the overshoot, the heat roller 1 and
the pressure roller 107 are rotated so that the heat corresponding
to the overshoot is transferred to the pressure roller 107.
[0174] As shown in FIG. 16, the lower the temperature of the heat
roller 1 prior to the warm-up operation is, the larger the
overshoot is. Thus, in the sixth embodiment, the heat roller 1 is
energized for different lengths of time depending on the surface
temperature T.sub.M prior to the warm-up operation, thereby
effectively minimizing the overshoot. Then, the correction
described in the fifth embodiment is performed.
[0175] In the sixth embodiment, thereby reducing adverse effects of
the overshoot in temperature of the surface area 1a of the heat
roller 1 in contact with the print medium.
[0176] FIG. 17 is a flowchart illustrating a procedure of the sixth
embodiment of a correction method, the procedure being carried out
by the controller 2 of the control system 1 of FIG. 12.
[0177] FIG. 18 is a timing chart illustrating the operations of
various parts of the control system 1 when the control system 1
performs a warm-up operation according to the flowchart of FIG.
17.
[0178] The flowchart of FIG. 17 will be described with reference to
FIG. 18. Many of the steps in the flowchart of FIG. 17 are the same
as those of FIG. 13. Thus, the common steps are indicated but the
description thereof is omitted for simplicity.
[0179] At step S1, the fixing unit 101 is turned on at time tp. At
step S2, the non-contact type temperature detector 40 (FIG. 1)
detects a surface temperature T.sub.M of the heat roller 1. This
surface temperature is T.sub.M at time tp in FIG. 18. At step S3, a
decision is made to determine whether a temperature
T.sub.M.ltoreq.70.degree. C. or T.sub.M>70.degree. C. At step
S4, a decision is made to determine whether
T.sub.M.ltoreq.110.degree. C. or T.sub.M>110.degree. C.
[0180] If T.sub.M.ltoreq.70.degree. C. at step S3, then the main
controller sets Tb to Tb=.tau..times.3 (step S5) where r is a
predetermined value. If NO at step S3 and YES at step S4, (i.e.,
70.degree. C.<T.sub.M<110.degree. C.), then the main
controller sets Tb to Tb=.tau..times.2 at step S6. If NO at step S4
(i.e., T.sub.M>110.degree. C.), then the main controller sets Tb
to Tb=.tau. at step S7.
[0181] After steps S5-S7, the program continues to step S2 of the
flowchart of FIG. 13.
[0182] Specifically, steps S2-S7 of FIG. 17 are carried out for a
time period from time tp to time t0 of FIG. 18, thereby setting the
value of Tb, which is a time period from time t1 to time t4 in FIG.
18. Tb is determined in accordance with the value of T.sub.M
detected by the non-contact type temperature detector 30 at time
tp.
[0183] During Tb, the heat roller 1 is rotated to minimize the
overshoot in temperature at the longitudinal middle portion (point
M) of the heat roller 1, so that the variation in T.sub.S-T.sub.M
due to the overshoot can be minimized before the temperature
correction is carried out at time t4.
[0184] Since Tb is determined in accordance with the temperature of
the longitudinal middle portion of the heat roller 1 before the
heater 2 is turned on, the warm-up operation can be performed in a
minimum time.
[0185] Seventh Embodiment
[0186] {Control of Temperature}
[0187] A method of controlling temperature in the continuous
printing mode will be described. When the surface temperature in
the area 1a becomes lower than the area 1b, the method is used to
maintain the surface temperature of the area 1a (i.e., point M) of
the heat roller 1 through which the print medium passes to a
predetermined fixing temperature.
[0188] FIGS. 19-21 are flowcharts illustrating a procedure of the
seventh embodiment.
[0189] FIG. 22 is a timing chart illustrating the operation of
various parts of the control system 1 when the control system
operates according to the flowchart of FIGS. 19-21.
[0190] FIG. 22 shows the relationship between a temperature T.sub.M
and a temperature T.sub.E when continuous printing is performed on
more than N pages of a print medium.
[0191] The temperature T.sub.M is a temperature at a longitudinally
middle M of the heat roller 1 detected by the non-contact type
temperature detector 40. The temperature T.sub.E is a temperature
at an area of the heat roller not in contact with the print medium
detected by the contact type temperature detector 30.
[0192] The controller 2 of the control system 1 of FIG. 12 performs
specific steps.
[0193] The flowcharts of FIGS. 19-21 will be described with
reference to FIG. 22.
[0194] Upon receiving a print command at step S1 of FIG. 19, a
decision is made at step S2 to determine whether the printing
operation is continuous printing. Continuous printing is a printing
mode during which a plurality of pages are printed and the heat
roller 1 and the pressure roller 107 are continuously rotating. In
the following description, the printing operation assumes to be
activated immediately after the warm-up operation that is carried
out upon power up.
[0195] If it is determined at step S2 that the printing operation
is not continuous printing, the program proceeds to step S3. At
step S3, the heater 2 is turned on and off to perform temperature
control such that the temperature T.sub.E is maintained to a target
temperature H in the CD mode. The target temperature H is initially
set to h3 at step S4. The temperature control is continued until
step S6 is completed.
[0196] If it is determined at step S2 that the printing operation
is continuous printing, then the program proceeds to step S7 where
a decision is made to determine whether n>N. The n is the number
of pages to be printed and is loaded into a counter, not shown. If
n>N at step S7, then the program proceeds to steps S8-S15 where
temperature control is performed in the CD mode. In other words,
the halogen lamp is cycled on and off such that the temperature
T.sub.E is maintained to the target temperature H. The temperature
control using the contact type temperature detector 30 continues
until the temperature control is switched at later described step
S31 (FIG. 21) to the temperature control using the non-contact type
temperature detector 40.
[0197] FIG. 22 is a graph showing the relationship between the
actual surface temperature T.sub.S at the point M and the
temperature T.sub.E at the point E when the temperature control is
performed through steps S8-S15. The temperature control starts when
the target temperature H is set to H=h3 and the number n is set to
n=0 (zero) at step S10. Steps S9-S10 are performed at time t1 the
control FIG. 22 and steps S8-S15 are repeated until time t3.
[0198] When the print medium passes between the heat roller and the
pressure roller, the heat of an area of the heat roller in contact
with the print medium is lost to the print medium. As a result, the
temperature of the area decreases with increasing number of pages
of print medium to be printed and then reaches equilibrium in which
the difference in temperature between the area in contact with the
print medium and an area outside the area in contact with the print
medium is a and remains constant for some time after the print
medium has passed.
[0199] Steps S11-S15 are the same as steps S23-S28 of FIG. 20 and
are performed during the period between times t1-t3. In other
words, the target temperature H is incremented by a predetermined
value .beta. every time one page is printed. As the number of
printed pages increases, the target temperature H is increased
until H=h3+a at step S14 at time t2. Thereafter, H=h3+.alpha. is
maintained after t2. Once the target temperature reaches
H=h3+.alpha., then steps S11-S14 are iterated, i.e., step S15 is
not performed. Steps S11-S15 are iterated until n>N at step
S13.
[0200] When n>N at step S13, then the program proceeds to step
S30 of FIG. 21 where the temperature T.sub.M is loaded into a
register. The temperature T.sub.M stored in the register is used as
a target temperature Y. Then, the heater 2 is controlled at step
S31 to turn on and off such that the temperature T.sub.M is the
same as the target temperature Y (i.e., NCD mode). The temperature
control in the NCD mode continues until the printing is completed
at later described step S35 or step S39.
[0201] If n<N at step S7, then the program jumps to step S20 of
FIG. 20. At steps S20-S26, the heater 2 is turned on and off to
perform temperature control in the CD mode such that the
temperature T.sub.E is maintained to a target temperature H=h3.
This temperature control continues until step S26 has been
completed. The target temperature H is set to h3 at step S21 and
the counter is set to n=0 (zero) at step S22.
[0202] Steps S23-S25 and S27-28 are the same as steps S11-S12 and
S14-S15 of FIG. 19. One page is printed at step S23 and the counter
is incremented by one, i.e., n=n+1 at step S24. At step S25, a
decision is made to determine whether there is any remaining data.
If NO at step S25, then the program proceeds to step S26 where
printing is completed. If YES at step S25, then the program
proceeds to step S27.
[0203] At step S28, the target temperature H is increased by .beta.
to offset the loss of heat. In other words, the target temperature
H is incremented by b.degree. C. every time one page is printed.
Steps S23-S25 and S27-S28 are iterated to print on a plurality of
pages. As the number of printed pages increases, the target
temperature H is increased until H=h3+.alpha. at step S27.
Thereafter, H=h3+.alpha. is maintained. When all the print data has
been printed, the printing operation is completed at step S26.
[0204] The procedure of FIG. 21 begins at time t3 of FIG. 22 and is
repeated for the period between time t3 and time t4. At step S32, a
decision is made to determine whether T.sub.E.ltoreq.MAX. MAX is a
highest tolerable value of temperature T.sub.E over which
temperature should not be increased, and is determined by taking
the temperature resistance of the associated components into
account. If NO at step S32, then steps S32, S33, S37, and S38 are
performed to continue to print from time t3 to time t4 (FIG.
22).
[0205] Thus, the surface temperature T.sub.M of the area 1a of the
heat roller 1 in contact with the print medium can be maintained to
h3, even if the temperatures of the areas 1a and lb differ by more
than a during the printing operation due to changes in external
temperature. The value a is the difference in temperature between
an area in contact with the print medium and an area outside the
area when the temperature of the area decreases to reach
equilibrium with increasing number of printed pages. However,
temperature T.sub.E increases as a result of the temperature
control.
[0206] When T.sub.E>MAX at step S32, then the program proceeds
to step S34 where the heater 2 is turned off, and then proceeds to
step S35 where the printing is stopped. It is to be noted that the
printing is halted but the heat roller 1 and pressure roller 107
continue to rotate. The program waits a certain period of time Tk,
and then loops back to step S32. When T.sub.E.ltoreq.MAX at step
S32, the temperature control is resumed at step S33 to turn on and
off the heater 2 in the NCD mode such that temperature T.sub.M is
maintained to the target temperature Y, and the printing is
restarted at step S37. Step S33 is performed at time t5 of FIG. 22.
The printing operation is continued until it is determined at step
S38 that there is remaining print data and the printing operation
is completed at step S39.
[0207] In the seventh embodiment, the non-contact type temperature
detector 40 is used to detect the temperature of the heat roller
before the halogen lamp is turned on. Instead, the contact type
temperature detector 30 may be used to detect the temperature of
the heat roller before the halogen lamp is turned on.
[0208] In the seventh embodiment, when the number of printed pages
reaches a certain value N, the temperature control is switched from
the CD mode to the NCD mode. However, the temperature control is
not limited to this and the temperature control may be performed
based on, for example, an elapsed time from when continuous
printing is begun.
[0209] Eighth Embodiment
[0210] FIG. 23 is a partially cross-sectional view illustrating a
heat roller 1 and heater 2A and 2B assembled therein. The heaters
2A and 2B take the form of a halogen lamp.
[0211] A contact type temperature detector 30 is disposed at the
point E, which is outside of an area in contact with a print medium
having a maximum width that the fixing unit can accept. The contact
type temperature detector 30 is directly in contact with the coated
surface of the heat roller 1. A non-contact type temperature
detector 40 is disposed substantially in the longitudinally middle
portion of the heat roller 1 and faces the surface of the heat
roller 1 with a gap G of about 1 mm therebetween. The heat roller 1
has heaters 2A and 2B that serve as a heat source. The heater 2A
has an effective length that generates heat for fixing a print
medium having A3 size (297.times.420 mm). The heater 2B has an
effective length that generates heat for fixing A4 size print
medium having a width of about 210 mm. The rest of the construction
is the same as the fixing unit of the first embodiment.
[0212] FIG. 24 is a block diagram illustrating a control system for
controlling the fixing temperature.
[0213] The control system includes paper-size detectors (switches)
50, 60, an operating panel 18, a CPU 11, a ROM 17, a
temperature-controlling circuit 15, a heater-driving circuit 16,
and a fixing unit 300. The temperature-controlling circuit 15
converts information read from the ROM 14 into a controlling signal
and provides the control signal to the heater-driving circuit 16,
which in turn drives the two heaters 2A and 2B to emit light and
heat. The temperature-controlling circuit 15 also monitors the
temperatures of the heat roller 1 detected by the contact type
temperature detector 30 and the non-contact temperature detector
40. The temperature-controlling circuit 15 controls the
heater-driving circuit 16 in accordance with the detected
temperatures to switch on and off an a-c voltage supplied to the
heaters 2A and 2B.
[0214] FIG. 25 is a timing chart that illustrates the control
operation for controlling the surface temperature of the heat
roller 1. FIG. 25 shows temperatures of various parts of the heat
roller 1 at a setup state, a standby state, and a continuous
printing state when print data is printed on a narrow-width print
medium.
[0215] Referring to FIG. 25, a solid line curve indicates
temperatures detected by the non-contact type temperature detector
40. A dot-dash line curve represents actual temperatures of a
surface area on the heat roller that opposes the non-contact type
temperature detector 40. A dotted line curve shows temperatures
detected by the contact type temperature detector 30. It is to be
noted that the temperatures represented by the dot-dash line are
higher than temperatures indicated by the solid line. This is
because there is a small gap G between the surface of the heat
roller and the non-contact type temperature detector 40.
[0216] T0 is a target temperature of temperature of a left end
portion E of the heat roller 1, detected by the contact type
temperature detector 30 during the standby state. T1 is an average
value of actual temperatures of the longitudinal middle of the heat
roller 1 during the standby state. T2 is an average value of
detection temperatures of the longitudinal middle of the heat
roller 1, detected by the non-contact type temperature detector 40,
during the standby state. T3 is a target value of temperatures
detected by the non-contact type temperature detector 40 during the
continuous printing state of a narrow-width print medium. T4 is a
target value of actual temperature of the longitudinal middle of
the heat roller 1 during the continuous printing state of the
narrow-width print medium. T5 is a lower limit value of the
temperature of the left end portion of the heat roller 1. The
temperature of the left end portion of the heat roller 15 detected
by the contact type temperature detector should not decrease below
T5.
[0217] Upon power-up of the printer, the heaters 2A and 2B are
energized, the temperature control enters setup state where the
temperature of the end portion of the heat roller 1 increases to
T0. During the setup state, the temperature control is carried out
in the CD mode. Both the heaters 2A and 2B are turned on only for
an early period of the setup state. The heater 2B is then turned
off, thereby preventing overshoot in temperature rise as the
temperature of the heat roller 1 increases. In order to reduce
overshoot so that the warm-up time is shorter in the setup state,
the two heaters 2A and 2B should be turned on concurrently only for
an optimum length of time. The power consumption of the lamps and
the heat capacity of the heat roller 1 determine the time period
during which the two heaters 2A and 2B are turned on. The optimum
length of such time period is experimentally determined.
[0218] Then, the temperature control shifts from the setup state to
the standby state. In the standby state, the temperature is
controlled in the CD mode such that actual surface temperature of
the heat roller 1 at the middle portion thereof is maintained to T1
higher than T0, and the left end portion is maintained to T0. T1 is
determined by the temperature control based on the temperature T0
detected by the contact type temperature detector 30. The
difference T1-T0 is a difference in actual temperature between the
left end portion and the longitudinal middle portion of the heat
roller 1.
[0219] FIG. 26 illustrates the temperature profile across the
length of the heat roller.
[0220] Referring to FIG. 26, the heat roller 1 usually has a curved
temperature profile across its length with a low temperature T11 at
both longitudinal ends and a high temperature in the middle.
However, because there is a gap G of about 1 mm between the heat
roller 1 and the non-contact type temperature detector 40, the
temperature T2, detected by the non-contact type temperature
detector 40, is lower than T1.
[0221] Upon a command at time tp for continuous printing of a
narrow-width print medium, the temperature control is carried out
in the NCD mode. In other words, when the operator specifies a
print medium having a smaller width than A4 size paper in portrait
orientation, or the paper size detectors 50, 60 (FIG. 24) detect
such a print medium, the CPU 13 performs the temperature control
based on the non-contact temperature detector 40.
[0222] Referring to FIG. 25, when the CPU 13 receives a print
command at time tp, the CPU 13 forcefully terminates the standby
state and begins a printing operation. The CPU 13 switches
temperature control from the CD mode to the NCD mode. Then, the CPU
13 reads a fixing temperature controlling program corresponding to
the size of the print medium from the ROM 14 and sends the program
to the temperature-controlling circuit 15. Under the control of the
program, the temperature control is carried out such that a
temperature T.sub.M is equal to T3, which is the target value of
temperatures detected by the non-contact temperature detector 40.
Therefore, the actual temperature T.sub.S of the middle portion of
the heat roller 1 varies back and forth about T4 within a
predetermined range of error. Since the print medium has a narrower
width than A4 size paper, the heater 2A is turned off and the
heater 2B is turned on and the actual temperature T.sub.S at the
middle portion of the heat roller 1 increases from T1 to T4.
[0223] Printing is activated at time tp and the actual temperature
T.sub.S of the middle portion of the heat roller 1 reaches T4 at
time tq. In order to ensure good fixing results, it is desirable
that the actual temperature T.sub.S of the middle portion of the
heat roller 1 reaches T4 before the leading end of the print medium
reaches the heat roller 1. Thus, a time length tp-tq should be
equal to or shorter than the time required for the print medium to
reach the heat roller 1 after the printing is activated. In the
eighth embodiment, the actual temperature T.sub.S of the middle
portion of the heat roller 1 is increased from T1 to T4. The actual
temperature T.sub.S of the middle portion of the heat roller 1
during the standby state may be selected to be T4, in which case
temperature control is performed such that the temperature T.sub.M
is equal to T3. This is advantageous when the electrophotographic
printer operates at high printing speed and therefor the leading
end of the print medium reaches the heat roller 1 in a shorter
time.
[0224] During continuous printing, the heater 2B is energized and
the temperature control is performed in the NCD mode where the
non-contact type temperature detector 40 is used to perform the
temperature control of the surface area of the heat roller 1 in
contact with the print medium. This ensures that the actual surface
temperature T.sub.S of the middle portion of the heat roller 1 is
maintained at T4, which is a proper fixing temperature. However,
when only the heater 2B is used to heat the heat roller 1, the
temperature T.sub.E of the longitudinal end portions of the heat
roller 1 will gradually decrease during the continuous printing
operation. The decreases in the temperature T.sub.E of the
longitudinal end portions of the heat roller 1 should be maintained
within a certain range because there may be a case in which
printing is performed on a wide-width print paper shortly after
printing on a narrow-width print paper. In order to address such a
situation, when the temperature detected by the contact type
temperature detector 30 falls below T5, the heater 2A is also
energized to increase the temperature of the longitudinal end
portions of the heat roller 1 above T5. As described above, in the
continuous printing, the temperature control can be advantageously
performed using both the contact type temperature detector 30 and
non-contact type temperature detectors 3.
[0225] At time tr at which the continuous printing is completed,
the temperature control is switched from the NCD mode and the CD
mode to the CD mode. At this moment, if the temperature detected by
the contact type temperature detector 30 has fallen below T0, the
heater 2A is energized to increase the temperature to T0.
Energizing the heater 2A may cause the actual temperature T.sub.S
of the middle portion of the to increase to a value higher than T4
and may also cause the temperature T.sub.M to increase to a value
higher than T3. In order to minimize the temperature rise at the
middle portion of the heat roller 1, the value of T5 should be
selected to such that T5 is not too low compared to T0.
[0226] The T5 is preferably set to a temperature in the range from
T0 to T0-10.degree. C. Experiment showed that the range of
(T0-10)<T5<T0 allows the temperature of the middle portion of
the heat roller 1 to be maintained within a predetermined range
over which fixing result is good.
[0227] Thus, the fixing unit according to the invention is capable
of preventing not only temperature fall of the surface area of the
heat roller through which the print medium passes but also excess
temperature rise of the longitudinal end portions, which is outside
of the surface area.
[0228] The non-contact type temperature detector 40 disposed at the
middle portion of the heat roller is free from unwanted toner
deposition, preventing contamination of the print paper and
unstable temperature control of the heat roller. The non-contact
type temperature detector 40 eliminates a cleaning member such as a
cleaning felt and maintenance associated with oil such as silicone
oil, ensuring good print results at all times.
[0229] The non-contact type temperature detector 40 does not
frictionally slide on the heat roller 1 by nature, therefore the
coating layer on the surface of the heat roller 1 will not wear out
quickly, prolonging the lives of components as well as ensuring
good print results.
[0230] Ninth Embodiment
[0231] The gap G between the non-contact type temperature detector
40 and the heat roller 1 may vary over time. Changes in gap G will
cause the temperature difference between actual temperature of
longitudinal middle of the heat roller 1 and the temperature
detected by the non-conduct type temperature detector 40. In
addition, the rotating surface of the heat roller 1 creates
convection of air surrounding the heat roller 1, which in turn
causes changes in the temperature detected by the non-contact type
temperature detector 40.
[0232] Some measure should be taken in order to solve the problem
of changes in gap G over time. However, maintaining an accurate gap
is difficult. Instead, the temperature control can be performed
most efficiently through the use of the following correction method
in which the temperature is controlled in accordance with the size
of gap G.
[0233] The method of correcting the temperature control by the use
of the non-contact type temperature detector 40 will described in
detail.
[0234] FIG. 27 illustrates the relationship between the actual
surface temperatures of the heat roller 1 and the gaps G between
the heat roller 1 and the non-contact type temperature detector 40.
Curves A-C show actual surface temperatures versus the gaps for
different detection temperatures, detected by the non-contact
temperature detector 40. Curve A shows a case where the detection
temperature is 170.degree. C., Curve B shows a case where the
detection temperature is 180.degree. C., and Curve C shows a case
where the detection temperature is 190.degree. C. In the ninth
embodiment, the heat roller 1 has an outer diameter of 36 mm and a
thickness of 1.2 mm, and the heater 2B has a power rating of 600 W
and a length sufficient for fixing A4 size paper (about 210 mm in
width).
[0235] As is clear from FIG. 27, there is a linear relation between
the gap and the surface temperature in each case. For example, when
the detection temperature of the middle portion of the heat roller
is 180.degree. C., the actual surface temperatures are about
196.degree. C. for gap=1 mm, 190.degree. C. for gap=0.7 mm, and
202.5.degree. C. for gap=1.3 mm, respectively. In other words, the
difference between the actual temperature and detection temperature
is large for a large gap, and small for a small gap. Moreover,
regardless of the detection temperature, the actual surface
temperature is 9 to 10.degree. C. higher than the detection
temperature for gap=0.7 mm and 15 to 16.degree. C. higher than the
detection temperature for gap=1 mm. The ROM 14 (FIG. 24) stores a
fixing temperature-controlling program and "data A" that describes
the relation between the gaps and actual surface temperatures for
different detection temperatures shown in FIG. 27.
[0236] FIG. 28 illustrates the relationship between the actual
surface temperatures of the middle portion of the heat roller 1 and
the temperatures of the left end portion of the heat roller 1 when
the temperature control is performed based on the contact type
temperature detector 30. In other words, the temperature control is
performed in the CD mode. It is to be noted that actual surface
temperatures of the middle portion of the heat roller 1 are
linearly related to temperatures of the left end portion of the
heat roller 1. The heat roller 1 used to measure the relationships
of FIG. 28 is the same in material, shape, and size as that used to
measure the relationships of FIG. 27. In the standby sate, as shown
in FIG. 26, the heat roller 1 has a curved temperature profile
across its length with a low temperature T11 at both longitudinal
ends and a high temperature in the middle.
[0237] The temperature profile across the heat roller 1 depends on
the heat capacity of the heat roller 1, the amount of heat
generated by the heaters 2A and 2B, and light radiation pattern of
the heaters 2A and 2B. The measured data in FIG. 28 indicates that
detection temperature is generally 14 to 16.degree. C. higher than
the temperature of the left end portion of the heat roller 1. Thus,
the ROM 14 (FIG. 24) stores a fixing temperature-controlling
program and "data B" that describes the relation between the actual
temperatures of the middle portion of the heat roller and the
temperatures of the left end portion of the heat roller 1 shown in
FIG. 28.
[0238] Thus, the ROM 14 stores both data A and data B and
associated control programs. The contact type temperature detector
30 detects the temperature at the left end portion of the heat
roller 1 and the non-contact type temperature detector 40 detects
the temperature of the middle portion of the heat roller. The size
of gap can be determined fairly accurately by using the correlation
between the temperature detected by the contact type temperature
detector 30 and the surface temperature detected by the non-contact
type temperature detector 40.
[0239] {Method of Correcting Control Temperature}
[0240] The method of correcting control temperature in the NCD mode
will be described. It is assumed that data A and data B have
default values for gap=1 mm and target value T3=180.degree. C.,
detected by the non-contact temperature detector 40.
[0241] Here, it is assumed that the temperature correction is
performed when the fixing unit is in the standby state. From data B
(FIG. 28) read from the ROM 14, T4 is around 196.degree. C. for
T3=180.degree. C. At this moment, if the temperature detected by
the non-contact temperature detector is 170.degree. C., then data A
(FIG. 27) is read from the ROM 14. From the values of T4 and T3, it
is determined that the gap=1.6 mm. Thus, the gap between the
surface of the heat roller 1 and the non-contact type temperature
detector 40 deviates 0.6 mm from the default value of 1 mm.
[0242] Upon activation of continuous printing of a narrow-width
print medium, the temperature control is switched from the CD mode
to the NCD mode. Here, it is assumed that the temperature control
is to be performed such that T4=205.degree. C. with, for example,
T3=190.degree. C., i.e., the temperature control is to be carried
out by using Curve C of FIG. 27. However, since it has been
determined that the gap=1.6 mm, the temperature control using Curve
C will result T4=215.degree. C. as shown in FIG. 27, which is about
10.degree. C. higher than T4=205.degree. C. Thus, in order to
perform a temperature control equivalent to that based on Curve C
of FIG. 27, Curve B should be used so that T3=180.degree. C. gives
T4=207.degree. C. for gap=1.6 mm. Thus, the temperature control can
be performed such that T4=207.degree. C., which is very close to
T4=205.degree. C.
[0243] When data A for a default value of the gap G cannot be used
due to changes in gap, the use of data A and data B can control the
surface temperature of the middle portion of the heat roller 1 to a
desired value or very close to it. This enables temperature control
regardless of changes in gap over time.
[0244] FIG. 27 illustrates only three cases of 170.degree. C.,
180.degree. C., and 190.degree. C. of the detection temperature
detected by the non-contact type temperature detector 40. The
non-contact type temperature detector 40 in the form of a
thermistor usually has a resolution of 2 to 3.degree. C. and
therefore closer detection temperatures can be employed for a
temperature control having higher resolution.
[0245] The aforementioned correction of the target value of
detection temperatures, detected by the non-contact type
temperature detector 40, can be automatically carried out. For
example, a default value of the gap G may be set to 1 mm at the
factory. The printer may be programmed such that every time the
printer is turned on, automatic correction is performed immediately
before the heater 2A for wide-width medium is turned off at the end
of the standby state. This automatic correction ensures that the
temperature control is properly carried out at all times.
[0246] Tenth Embodiment
[0247] When the temperature control is performed based on the
temperature detected by a non-contact type temperature detector,
detected temperatures may be affected by convection of air
surrounding the temperature detector and/or flow of air created by
a built-in fan. Therefore, a non-contact type temperature detector
does not respond as quickly to heat as a contact type temperature
detector. Recent non-contact type heat-sensitive elements are in
the form of a thermistor of a miniature bead type and a thin-film
type having a small heat capacity, and they are as sensitive to
heat as contact-type thermistor elements. However, the non-contact
type thermistor becomes less responsive to changes in heat with
increasing gap between the heat source and the thermistor. As a
result, the controlled temperature has a larger ripple or
fluctuation in the non-contact type thermistor than in the contact
type thermistor.
[0248] A tenth embodiment is characterized in that the target value
of the temperature T.sub.M is increased in accordance with the
amplitude of ripple or fluctuation of T.sub.S.
[0249] FIGS. 29A-29B illustrate the outline of the correction
procedure of temperature ripple.
[0250] FIG. 29A illustrates the ripple in the surface temperature
of the left end portion of the heat roller 1 when the temperature
control is performed with the contact type temperature detector 30
is used.
[0251] Describing a ripple-like curve, the surface temperature
varies within or slightly beyond a temperature range having a lower
limit temperature at which the halogen lamp should be turned on and
an upper limit temperature at which the halogen lamp should be
turned off.
[0252] FIG. 29B illustrates the ripple in the surface temperature
of the middle portion of the heat roller 1 when the temperature
control is performed using the non-contact type temperature
detector 40.
[0253] The heat responsiveness of the non-contact type temperature
detector 40 often causes a ripple in the surface temperature of the
middle portion of the heat roller. This ripple in the surface
temperature has an amplitude of about five times the ripple in the
responsiveness of the non-contact type temperature detector. If,
for example, the amplitude of ripple is greater than 10.degree. C.,
the fixing results may not be good enough at temperatures near the
lower limit temperature. Large amplitudes of the ripple in the
surface temperature seriously affect the fixing results of thick
print medium such as a post card and an envelope.
[0254] The following is the temperature control according to the
tenth embodiment performed by the control system of FIG. 24.
[0255] The temperature-controlling circuit 15 detects the amplitude
of ripple in the temperature detected by the non-contact type
temperature detector 40 during continuous printing. The ROM 14
stores a ripple-correcting program and "data C" that describes the
relation between the amplitude of ripple in the heat responsiveness
of the non-contact type temperature detector 40 and the target
value of the surface temperature of the heat roller.
[0256] In the tenth embodiment, it is assumed that the non-contact
type temperature detector 30 has an amplitude of ripple in the
responsiveness of, for example, 10.degree. C. If the resolution of
the non-contact type temperature detector 40 is 2.degree. C., then
the amplitude of 10.degree. C. is equal to 10.degree.
C..div.2.degree. C.=5. Then, it is defined that the amplitude of
ripple is 5 units. Then, an actual target value of the surface
temperature of the heat roller 1 is set 2 units higher than a
desired target value. Likewise, if the amplitude of ripple in the
responsiveness is equal to 6 units, then the target value is set 3
units higher than the desired value. For larger amplitudes, the
target value is increased by one unit for every one unit of the
amplitude of ripple. The temperature-controlling circuit 15
determines the setting of target value according to the data C that
is read out of the ROM 14. The ROM 14 stores the data C and a
ripple correcting program, not shown.
[0257] The temperature control performed using the non-contact type
temperature detector 40 causes the actual surface temperature of
the heat roller to gradually decrease by a small amount. In order
to overcome such a drawback, the tenth embodiment is applicable to
increase the target temperature stepwise.
[0258] The invention being thus described, it will be obvious that
the same may be varied in many ways. Such variations are not to be
regarded as a departure from the spirit and scope of the invention,
and all such modifications as would be obvious to one skilled in
the art intended to be included within the scope of the following
claims.
* * * * *