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Claims  |
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We claim:
1. An image forming apparatus comprising:
a process unit for forming an unfixed toner image on a sheet;
a fixing unit for fixing the unfixed toner image onto said sheet, said
fixing unit comprising a heating member and a pressurizing member which
are in contact with each other under pressure to form a fixing nip;
temperature detection means for detecting a temperature of said heating
member of said fixing unit;
control means for controlling the temperature of said heating member based
on an output from said temperature detection means;
estimation means for estimating an amount of heat transferred from said
heating member to said pressurizing member; and
fixing condition changing means for changing a fixing condition of said
fixing unit based on the amount of transferred heat estimated by said
estimation means, wherein
said control means determines a control target temperature of said heating
member and proportionally controls an amount of electric power supplied to
said heating member in proportion to a difference between said detected
temperature of said heating member and said target temperature, and
wherein said estimation means estimates the amount of heat transferred to
said pressurizing member based on the control target temperature of said
control means and the temperature of said heating member which is detected
by said temperature detection means.
2. An image forming apparatus according to claim 1, wherein said fixing
condition changing means estimates a temperature of said pressurizing
member based on the amount of transferred heat estimated by said
estimation means, and changes the fixing condition based on the estimated
result.
3. An image forming apparatus according to claim 1, wherein said fixing
condition changing means changes the control target temperature of said
control means.
4. An image forming apparatus according to claim 1, wherein said
pressurizing member is a rotating body, and said fixing condition changing
means adjusts a rotation time of said rotating body which is elapsed
before the sheet reaches said fixing nip.
5. An image forming apparatus according to claim 1, wherein said fixing
condition changing means adjusts a time period during which electric power
supply to said heating member is cut off, irrespective of a control output
of said control means in the proportional control.
6. An image forming apparatus according to claim 1, wherein said control
means controls the temperature of said heating member to a control target
temperature by controlling an electric power supplied to said heating
member, and said apparatus further comprises sheet detection means for
detecting whether said sheet is present in said fixing nip, and wherein
said estimation means estimates the amount of heat transferred from said
heating member to said pressurizing member, by measuring the amount of
electric power supplied to said heating member by said control means
during a period when said sheet detecting means is not detecting the
presence of the sheet in said fixing nip, and
said fixing condition changing means changes the control target temperature
of said control means.
7. An image forming apparatus according to claim 6, wherein said apparatus
further comprises sheet transportation judging means for judging whether a
sheet is being transported toward said fixing unit, and said estimation
means estimates the amount of heat transferred to said pressurizing member
by measuring the amount of electric power supplied to said heating member
by said control means during a period, said period continuing from an
elapse of a predetermined time after a time when said transportation
judging means judges that a sheet is transported, to a time when said
sheet enters said fixing unit.
8. An image forming apparatus according to claim 6, wherein said estimation
means estimates the amount of heat transferred to said pressurizing member
based on the temperature of said heating member which is detected by said
temperature detection means.
9. An image forming apparatus according to claim 1, wherein:
said heating member is a heating roller having an electric heat generating
element disposed inside said roller,
said pressurizing member is a pressurizing roller,
said control means controls a temperature of said heating roller to a
control target temperature, by controlling an electric power supplied to
said heating roller, and wherein said apparatus further comprises:
sheet detection means for detecting whether a sheet is present in said
fixing nip;
and rotation detection means for detecting whether said pressurizing roller
is rotating,
said estimation means estimates an amount of heat transferred to said
pressurizing roller in a period during which no sheets are present in said
fixing unit and said pressurizing roller is rotating, and
said fixing condition changing means changes the control target temperature
of said control means.
10. An image forming apparatus according to claim 9, wherein said
estimation means estimates the amount of transferred heat by integrating
an electric power supplied to said heating roller by said control means.
11. An image forming apparatus according to claim 9, wherein said
estimation means estimates the amount of transferred heat by integrating a
time during which no sheets are present in said fixing unit and said
pressurizing roller is rotating.
12. An image forming apparatus according to claim 9, wherein said control
means generates a signal for driving one of said heating roller and said
pressurizing roller, and said rotation detection means detects the
rotation based on the driving signal from said control means.
13. An image forming apparatus according to claim 9, wherein said apparatus
further comprises a sheet-feeding means for feeding said sheet between
said heating roller and said pressurizing roller, and wherein said sheet
detection means includes a sheet-feeding sensor disposed in said
sheet-feeding unit for detecting whether said sheet is present.
14. An image forming apparatus according to claim 9, wherein said
estimation means stores the estimated value of the amount of transferred
heat at an end of temperature control, and corrects an estimated value of
the amount of transferred heat when the temperature control is started
again, by using the stored estimated value.
15. An image forming apparatus according to claim 14, wherein said
estimation means corrects the estimated value of the amount of transferred
heat based on an elapsed time after the end of temperature control.
16. An image forming apparatus according to claim 14, wherein said
estimation means corrects the estimated value of the amount of transferred
heat based on the temperature of said heating roller detected when the
temperature control is restarted.
17. An image forming apparatus according to claim 14, wherein said
estimation means stores the estimated value of the amount of transferred
heat to said pressurizing roller at the end of temperature control of said
heating roller, and corrects an estimated value of the amount of
transferred heat when the temperature control is restarted, by comparing a
predetermined temperature which is determined by the stored estimated
value of the amount of transferred heat, with the detected temperature of
said heating roller, and by, if the detected temperature is higher than
the predetermined temperature, using the predetermined temperature, and,
if the detected temperature is lower than the predetermined temperature,
using the detected temperature.
18. An image forming apparatus according to claim 9, wherein said
estimation means performs the estimation by gradually changing the
estimated value of the amount of transferred heat based on an output of
said temperature detection means of said heating roller in a period during
which the temperature control of said heating roller is not performed.
19. An image forming apparatus comprising:
a process unit for forming an unfixed toner image on a sheet;
a fixing unit for fixing the unfixed toner image onto said sheet, said
fixing unit comprising a heating member and a pressurizing member which
are in contact with each other under pressure to form a fixing nip;
temperature detection means for detecting a temperature of said heating
member of said fixing unit;
control means for controlling the temperature of said heating member based
on an output from said temperature detection means;
estimation means for estimating an amount of heat transferred from said
heating member to said pressurizing member; and
fixing condition changing means for changing a fixing condition of said
fixing unit based on the amount of transferred heat estimated by said
estimation means, wherein
said heating member is a heating roller having an electric heat generating
element disposed inside said roller,
said pressurizing member is a pressurizing roller,
said control means controls a temperature of said heating roller to a
control target temperature, by controlling an electric power supplied to
said heating roller, and wherein said apparatus further comprises:
sheet detection means for detecting whether a sheet is present in said
fixing nip; and
rotation detection means for detecting whether said pressurizing roller is
rotating,
said estimation means estimates an amount of heat transferred to said
pressurizing roller in a period during which no sheets are present in said
fixing unit and said pressurizing roller is rotating, and
said fixing condition changing means changes the control target temperature
of said control means, and wherein
said estimation means stores the estimated value of the amount of
transferred heat at an end of temperature control, and corrects an
estimated value of the amount of transferred heat when the temperature
control is started again, by using the stored estimated value.
20. An image forming apparatus according to claim 19, wherein said
estimation means corrects the estimated value of the amount of transferred
heat based on an elapsed time after the end of temperature control.
21. An image forming apparatus according to claim 19, wherein said
estimation means corrects the estimated value of the amount of transferred
heat based on the temperature of said heating roller detected when the
temperature control is restarted.
22. An image forming apparatus according to claim 19, wherein said
estimation means stores the estimated value of the amount of transferred
heat to said pressurizing roller at the end of temperature control of said
heating roller, and corrects an estimated value of the amount of
transferred heat when the temperature control is restarted, by comparing a
predetermined temperature which is determined by the stored estimated
value of the amount of transferred heat, with the detected temperature of
said heating roller, and by, if the detected temperature is higher than
the predetermined temperature, using the predetermined temperature, and,
if the detected temperature is lower than the predetermined temperature,
using the detected temperature.
23. An image forming apparatus comprising:
a process unit for forming an unfixed toner image on a sheet;
a fixing unit for fixing the unfixed toner image onto said sheet, said
fixing unit comprising a heating member and a pressurizing member which
are in contact with each other under pressure to form a fixing nip;
temperature detection means for detecting a temperature of said heating
member of said fixing unit;
control means for controlling the temperature of said heating member based
on an output from said temperature detection means;
estimation means for estimating an amount of heat transferred from said
heating member to said pressurizing member; and
fixing condition changing means for changing a fixing condition of said
fixing unit based on the amount of transferred heat estimated by said
estimation means, wherein
said control means controls the temperature of said heating member to a
control target temperature by controlling an electric power supplied to
said heating member, and said apparatus further comprises sheet detection
means for detecting whether said sheet is present in said fixing nip, and
wherein
said estimation means estimates the amount of heat transferred from said
heating member to said pressurizing member, by measuring an amount of
electric power supplied to said heating member by said control means
during a period when said sheet detecting means is not detecting the
presence of the sheet in said fixing nip, and
said fixing condition changing means changes the control target temperature
of said control means.
24. An image forming apparatus according to claim 23, wherein said
apparatus further comprises sheet transportation judging means for judging
whether a sheet is being transported toward said fixing unit, and said
estimation means estimates the amount of heat transferred to said
pressurizing member by measuring the amount of electric power supplied to
said heating member by said control means during a period, said period
continuing from an elapse of a predetermined time after a time when said
transportation judging means judges that a sheet is transported, to a time
when-said sheet enters said fixing unit.
25. An image forming apparatus according to claim 23, wherein said
estimation means estimates the amount of heat transferred to said
pressurizing member based on the temperature of said heating member which
is detected by said temperature detection means.
26. An image forming apparatus comprising:
a process unit for forming an unfixed toner image on a sheet;
a fixing unit for fixing the unfixed toner image onto said sheet, said
fixing unit comprising a heating member and a pressurizing member which
are in contact with each other under pressure to form a fixing nip;
temperature detection means for detecting a temperature of said heating
member of said fixing unit;
control means for controlling the temperature of said heating member based
on an output from said temperature detection means;
estimation means for estimating an amount of heat transferred from said
heating member to said pressurizing member; and
fixing condition changing means for changing a fixing condition of said
fixing unit based on the amount of transferred heat estimated by said
estimation means, wherein
said heating member is a heating roller having an electric heat generating
element disposed inside said roller,
said pressurizing member is a pressurizing roller,
said control means controls a temperature of said heating roller to a
control target temperature, by controlling an electric power supplied to
said heating roller, and wherein said apparatus further comprises:
sheet detection means for detecting whether a sheet is present in said
fixing nip; and
rotation detection means for detecting whether said pressurizing roller is
rotating, wherein
said estimation means estimates an amount of heat transferred to said
pressurizing roller in a period during which no sheets are present in said
fixing unit and said pressurizing roller is rotating, and
said fixing condition changing means changes the control target temperature
of said control means, and wherein
said estimation means estimates the amount of transferred heat by
integrating an electric power supplied to said heating roller by said
control means.
27. An image forming apparatus according to claim 26, wherein said control
means generates a signal for driving one of said heating roller and said
pressurizing roller, and said rotation detection means detects the
rotation based on the driving signal from said control means.
28. An image forming apparatus according to claim 26, wherein said
apparatus further comprises a sheet-feeding means for feeding said sheet
between said heating roller and said pressurizing roller, and wherein said
sheet detection means includes a sheet-feeding sensor disposed in said
sheet-feeding unit for detecting whether said sheet is present.
29. An image forming apparatus according to claim 26, wherein said
estimation means performs the estimation by gradually changing the
estimated value of the amount of transferred heat based on an output of
said temperature detection means of said heating roller in a period during
which the temperature control of said heating roller is not performed.
30. An image forming apparatus comprising:
a process unit for forming an unfixed toner image on a sheet;
a fixing unit for fixing the unfixed toner image onto said sheet, said
fixing unit comprising a heating member and a pressurizing member which
are in contact with each other under pressure to form a fixing nip;
temperature detection means for detecting a temperature of said heating
member of said fixing unit;
control means for controlling the temperature of said heating member based
on an output from said temperature detection means;
estimation means for estimating an amount of heat transferred from said
heating member to said pressurizing member; and
fixing condition changing means for changing a fixing condition of said
fixing unit based on the amount of transferred heat estimated by said
estimation means, wherein
said heating member is a heating roller having an electric heat generating
element disposed inside said roller,
said pressurizing member is a pressurizing roller,
said control means controls a temperature of said heating roller to a
control target temperature, by controlling an electric power supplied to
said heating roller, and wherein said apparatus further comprises:
sheet detection means for detecting whether a sheet is present in said
fixing nip; and
rotation detection means for detecting whether said pressurizing roller is
rotating, wherein
said estimation means estimates an amount of heat transferred to said
pressurizing roller in a period during which no sheets are present in said
fixing unit and said pressurizing roller is rotating, and
said fixing condition changing means changes the control target temperature
of said control means, and wherein
said estimation means estimates the amount of transferred heat by
integrating a time during which no sheets are present in said fixing unit
and said pressurizing roller is rotating.
31. An image forming apparatus according to claim 30, wherein said control
means generates a signal for driving one of said heating roller and said
pressurizing roller, and said rotation detection means detects the
rotation based on the driving signal from said control means.
32. An image forming apparatus according to claim 30, wherein said
apparatus further comprises a sheet-feeding means for feeding said sheet
between said heating roller and said pressurizing roller, and wherein said
sheet detection means includes a sheet-feeding sensor disposed in said
sheet-feeding unit for detecting whether said sheet is present.
33. An image forming apparatus according to claim 30, wherein said
estimation means performs the estimation by gradually changing the
estimated value of the amount of transferred heat based on an output of
said temperature detection means of said heating roller in a period during
which the temperature control of said heating roller is not performed. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates particularly to a thermal fixing apparatus which is
used in an image forming apparatus using the electrophotographic
technique. More particularly, the invention relates to a fixing apparatus
comprising a heating member and a pressurizing member which are in contact
with each other under pressure, and further comprising temperature sensing
means for sensing the temperature of the heating member or a heating
element. The fixing apparatus proportionally controls the electric power
supplied to a heat generating member in accordance with the output result
of the temperature sensing means, so as to control the heating member to a
target temperature. A material to be heated and carrying an unfixed toner
image is nipped by and transported through the pressurized contact portion
between the heating member and the pressurizing member, whereby the toner
image is fixed onto the heated material.
2. Related art
Conventionally, a thermal fixing apparatus using a heat roller, a heat
belt, or the like is generally used as a fixing apparatus. Such a fixing
apparatus involves the following drawbacks. An excessively low temperature
of a pressurizing member may cause faulty fixing. In contrast, an
excessively high temperature of the pressurizing member may cause a heated
material (a paper sheet) to be crinkled, or may cause the toner to be
stuck to the heating member (high-temperature offset).
To solve the problems, Published Unexamined Japanese Utility Model
Application No. Sho. 55-181258 discloses a method of controlling the
temperature of the heating roller in which the temperature of the heating
roller is set at a first set temperature in the printing operation from
power on till a predetermined number of prints is produced, and it is set
at a second set temperature in the subsequent printing operation. This
technique is based on the concept that the temperature of the heating
roller is reduced by the quantity of an increase of the temperature of the
pressurizing roller, which results from the progression of the printing
operation.
Published Unexamined Japanese Utility Model Application No. Sho. 60-169664
discloses another temperature control in which when during a period from
the end of supplying electric power to the fixing unit to a preset time
point, the electric power is supplied again to the fixing unit, the fixing
temperature is set at a first predetermined temperature. After the preset
time point, the fixing temperature is set at a second predetermined
temperature higher than the first one. This technique is based on the
concept that the fixing unit will be cooled after the predetermined time
elapses from the end of the fixing operation, and hence it must be set at
a temperature higher than the predetermined fixing temperature.
Published Unexamined Japanese Patent Application No. Hei. 5-273890
discloses a temperature control method in which a set temperature of the
heating roller is controlled at a temperature different from a
predetermined temperature for a fixed time interval, on the basis of the
roller temperature before the copying operation starts. This technique is
based on the concept that whether or not the fixing unit is cooled is
judged on the basis of the roller temperature.
The temperature control method in which the control temperature is switched
to another on the basis of the number of prints has the following
drawbacks.
The printer which receives print data from a host computer, for example,
and develops the print data into an image pattern before its printing, or
a facsimile which receives compressed image data through a telephone line
and expands the compressed image data to develop it into an image pattern
before its print, prints the print data of plural pages in a manner that
before it discharges a print of a page, it starts the printing operation
of the next page. In other words, the printer operates in a successive
print mode.
When the printer operates in the successive print mode, the time taken for
the transfer of the print data or the development of the print data varies
depending on the print contents and the ability of the computer which
produces the print data. Accordingly, the paper passage time varies and
the temperature rise of the pressurizing roller also varies depending on
those factors.
Under this condition, if the set temperature is switched to another
depending on the number of prints, the temperature of the pressurizing
roller rises when the paper passage time is long, thereby causing crinkle
of the paper sheet. When the paper passage time is short, the roller
temperature is low, thereby causing fixing faulty.
The temperature control method in which the set temperature is switched
depending on the sensed temperature of the heating roller immediately
before the print operation starts or the elapsing time after the print
operation, has the following drawbacks.
FIG. 26 is a graph showing how the temperature of the heating roller and
the pressurizing roller fall after the printing operation ends. As shown,
the temperature falling curve, indicated by a solid line, after one print
is completed, is greatly different in shape from the temperature falling
curves, indicated by broken lines, after a number of prints are completed.
If a cooled state of the fixing unit is judged depending on whether or not
the sensed temperature of the heating roller immediately before the print
operation starts is equal to or higher than 90.degree. C. or lower than
90.degree. C., and the heating roller is set to a temperature on the basis
of the judgement, the temperature of the heating roller is 65.degree. C.
(P2) after one print is completed, and 85.degree. C. (P1) after a number
of prints are completed. These values of temperature are greatly different
from each other. In this case, when the temperature of the heating roller
is relatively low, it is mistakenly judged that the fixing unit is
satisfactory heated. Conversely, when it is relatively high, it is
mistakenly judged that the fixing unit is cooled. This results in fixing
faulty, crinkle of paper sheet, and the like.
The same mistaken judgement also occurs also in a case where the cooled
state of the fixing unit is judged on the basis of the elapsing time from
the end of the printing operation.
In a case where the cooled state of the fixing unit is judged depending on
whether or not 10 minutes elapses after the printing operation ends, the
temperature of the heating roller is 65.degree. C. (P4) after one print is
completed, and 85.degree. C. (P3) after a number of prints are completed.
These values of temperature are also greatly different from each other.
One of the possible ways to avoid the mistaken Judgement is such that a
point P5, for example, in the figure is selected for checking whether or
not the fixing unit is cooled so that in any case, the temperature of the
pressurizing roller, and the elapsing time and the temperature of the
heating roller are related in one-to-one correspondence. As also seen from
FIG. 18, such a heating roller temperature is low in most cases, only the
judgement as to whether or not the fixing unit is extremely cooled is
allowed.
In an extremely limited case, it is used as information to switch the set
temperature of the heating roller. Such an elapsing time is long, several
tens minutes, so that it cannot be used for the information to switch the
set temperature of the heating roller till the elapsing time terminates.
In a case that the printing operation is intermittently repeated at the
intervals of print rest of several tens minutes or shorter, the
information on whether or not the fixing unit is cooled cannot be used for
the information to switch the set temperature of the heating roller. As a
result, when the printing operation is intermittently repeated, the paper
sheet will be curled or the gloss level will occur to greatly vary the
picture quality. Additionally, crinkle and fixing faulty will be caused.
On the other hand, an image forming apparatus such as a printer or a
facsimile apparatus which receives image data transmitted from an external
apparatus and performs internal processing of the image data has the
following problem. When images are to be formed successively, a time
interval from the previous image output (the n-th sheet) to the next image
output (the (n+1)th sheet), i.e., a time interval between sheets which are
successively fed to the fixing apparatus, is not constant depending on the
amount and the type of image data. In the fixing apparatus, therefore, the
temperature of the pressurizing member is varied depending on the time
interval between sheets, which results in faulty fixing, the formation of
paper crinkles or high-temperature offset. For example, in the case where
the amount of image data is so large that a long time is required to
transmit the image data and to develop the image data in the apparatus, or
in the case where the image data, such as graphic image data, are
complicated so that a long time is required for the processing thereof, it
takes a long time to output the subsequent image, and hence the time
interval between sheets is prolonged.
In most image forming apparatuses, in order to shorten the start-up time of
the fixing apparatus, the temperature of the heating member in the
interval between sheets is maintained equal to or slightly lower than the
temperature during the image forming operation. For the purpose of
realizing the above, the electric power supply to the heating member and
the rotation driving of the heating roller are performed irrespective of
whether a paper sheet is present in the fixing apparatus or not. As a
result, if the time interval between sheets is long, the temperature of
the pressurizing member is raised by the heat transferred from the heating
member, so that paper crinkles and high-temperature offset may occur. In
contrast, if the time interval between sheets is short, the amount of heat
transferred from the pressurizing member to a paper sheet during a period
in which the paper sheet is present in the fixing apparatus (a
sheet-feeding period) is larger than the amount of heat supplied from the
heating member to the pressurizing member during the time interval between
sheets. Thus, the temperature of the pressurizing member is lowered, and
faulty fixing may occur.
Thus, as described above, the temperature of the pressurizing member
seriously affects the characteristics of fixing the toner image onto the
heated material.
In view of the above-mentioned problems, it is desirable that the
temperature of the heating member is appropriately set in accordance with
the temperature of the pressurizing member. For example, Japanese Patent
Publication (Kokai) No. SHO 50-39554 discloses an arrangement in which a
temperature sensing means is disposed in the pressurizing member as well
as in the heating member, and the temperature control is performed as
follows. When the temperature of the pressurizing member is low, the
temperature of the heating member is raised, and, when the temperature of
the pressurizing member is high, the temperature of the heating member is
lowered.
Recently, at home or at a workplace, there arises a situation where one
person uses one or more image forming apparatuses such as a printer, a
facsimile apparatus, and a copy machine. In such a usage environment, it
is requested to shorten the waiting time which extends from the power-on
of an image forming apparatus to a time when the image formation is
actually enabled (quick start property), or to reduce the power
consumption of the image forming apparatus.
In order to realize the quick start, it is necessary to shorten the warm-up
time of a fixing apparatus (a time required for reaching a temperature at
which a fixing process can be done). In order to reduce the power
consumption, the power supply to the fixing apparatus must be stopped in a
time period during which the image forming operation is not actually
performed (waiting time), or the temperature of the heating member must be
controlled so as to be lower than that attained during the image forming
process. When the fixing apparatus is configured so that the power supply
to the fixing apparatus is stopped or reduced in level, however, the
operation of warming up the fixing apparatus must be performed each time
when the image forming process is to be started. Also in this case, the
quick start property is still essential for the image forming apparatus.
When the temperature of the pressurizing member is sensed and the heating
member is controlled in accordance with the sense result, it may be
possible to shorten the warm-up time of a fixing apparatus, and in turn
improve the quick start property.
The temperature of the pressurizing member depends on the use history of
the image forming apparatus (for example, the time period during which the
apparatus was operated in the previous image forming process, the time
period between the end of the previous operation and the start of the next
operation, the waiting time after the apparatus is powered on, and the
time during which the image forming operation is continuously performed as
a result of a successive sheet supply). In a system wherein only the
temperature of the heating member is used as the judgment criterion,
however, the set temperature of the heating member is determined in
consideration of the case where the temperature of the pressurizing member
is low, and hence the required warm-up time is almost constant in a manner
substantially irrespective of the temperature of the pressurizing member.
By contrast, in a system wherein the temperature of the heating member is
controlled in accordance with that of the pressurizing member, for
example, the fixing apparatus can perform the fixing operation even when
the temperature of the heating member is low, as far as that of the
pressurizing member is high. In this system, therefore, the operation of
warming up the fixing apparatus can be completed in a shorter time than
that in the system wherein only the temperature of the heating member is
used as the judgment criterion. In the case where the temperature of the
pressurizing member is low, furthermore, the temperature of the heating
member must be raised. In this case, the required warm-up time is
comparative to that in the system wherein only the temperature of the
heating member is used as the judgment criterion. In the case where the
temperature of the pressurizing member is low, however, the warm-up time
can be made shorter than that required in the system wherein only the
temperature of the heating member is used as the judgment criterion, by,
for example, performing a control such as that an electric power greater
in amount than the usual one is supplied to the heating member.
As the system of controlling the temperature of the fixing apparatus in
consideration of the use history of the image forming apparatus, various
methods have been proposed. In such methods, the temperature of a fixing
apparatus is controlled based on a certain judgment criterion, for
example, each time when the operating time of the apparatus exceeds a
predetermined value, or when the number of image forming operations is
greater than a predetermined value. However, use histories of image
forming apparatuses are different in a various manner. In personal uses
each conducted by one person, particularly, use histories are widely
varied by various reasons such as the difference in usage environment, for
example, indoor use or outdoor use, or in frequency of use, for example,
use at home or use at a workplace. Therefore, it is difficult to conduct
the control based on a single judgment criterion with respect to all use
histories of various image forming apparatuses.
In order to control the temperature of a fixing apparatus correctly in
accordance with the use history of an image forming apparatus,
consequently, it is indispensable to actually sense the temperatures of a
heating member and a pressurizing member and conduct the control based on
the sense result.
Japanese Patent Publication (Kokai) No. HEI5-289562 discloses a technique
in which, when a fixing apparatus (a pressurizing member, etc.) is cold,
an electric power must be supplied in a larger amount to a heating member
so that the temperature of the heating member is maintained at a given
level, and, when the fixing apparatus is warm, an electric power can be
supplied in a smaller amount to the heating member. The publication
discloses a fixing apparatus which uses a heating member consisting of a
film of a small heat capacity, and a heating element (film heating
method), and also a system in which the temperature of the fixing
apparatus must be sensed with the objective of preventing the heating
element from being over-heated. As the method of sensing the temperature
of a fixing apparatus, the publication discloses the following methods: a
fixed amount of electric power is supplied to a heating element when a
heating element is to start the operation, and the temperature rising rate
of the heating element at this time is sensed; the power supply to a
heating element is stopped between sheets which are successively supplied
to the fixing apparatus (between sheets), and the temperature lowering
rate when the heat of the heating element is dissipated is sensed; the
power supply to the heating element between sheets is controlled so as to
have either of two levels of HIGH/LOW, and the temperature variation
(temperature ripple) of the heating element is sensed; and an electric
power supplied to the heating element in a sheet passing period during
which a sheet passes through the fixing apparatus is sensed.
On the other hand, in view of the demand for electric power saving in
recent years, the electric power supply to the fixing apparatus is cut off
after the printing is finished. In this case, depending on the printing
history, the temperature of the pressurizing roller is inconsistently
varied, and hence the fixing characteristic is also varied. Accordingly,
in order to improve the technique, a technique for estimating the
temperature of the pressurizing roller is proposed.
Further, Japanese Patent Publication (Kokai) No. HEI. 5-289562 discloses a
technique in which the power supply to a heating element is terminated
between successively fed sheets and a temperature lowering rate is sensed
as the heat of the heating element dissipates.
As described above, it is essential for a fixing apparatus to appropriately
set the temperature of the heating member in a manner which is dependent
on the temperature of the pressurizing member.
However, if the temperature sensing means is provided for sensing the
temperature of the pressurizing member as disclosed in Japanese Patent
Publication (Kokai) No. SHO 50-39554, the construction of the fixing
apparatus becomes complicated. In general, the temperature sensing means
is of the contact type, and the pressurizing member is made of a soft
elastic material. For these reasons, in a portion of the pressurizing
member with which the temperature sensing means is in contact, a
pressurizing roller wears or is partly broken, and the pressurizing force
cannot be obtained in that portion. As a result, there arise problems of
the occurrence of faulty fixing and paper crinkles.
Moreover, Japanese Patent Publication (Kokai) No. HEI5-289562 discloses
several methods of sensing the temperature of a pressurizing member. In
the case where the temperature rising rate of a heating element is sensed
when a fixed amount of electric power is supplied to the heating element,
for example, the supply of the fixed amount of electric power is conducted
irrespective of the set temperature to which the heating member must
eventually reach. Therefore, a time period for sensing the temperature
rising rate of the heating element must always be elapsed before the
heating element is controlled to the set temperature. In such a method, if
the heating element heats the material to be heated via a film having a
small heat capacity as in the case of the film heating method, the
temperature of the heating member is easily raised. Accordingly, the
sensing of the temperature rising rate can easily be performed in a short
time. In a generally used thermal fixing apparatus such as a heating
roller, however, the heat capacity of the heating member is so large that
it is impossible to sense the rising of the temperature of the heating
member in a short time with high accuracy.
Similary, as in the case of Japanese Patent Publication (Kokai) No. HEI.
5-289562 in which the power supply to the heating element is stopped
between successively fed sheets, and the temperature lowering rate of the
heating element is sensed, if the heating element (a heat generating
element) supplies the heat to the material to be heated via a film having
a small heat capacity in the same way as in the film heating method, the
temperature of the heating member is easily lowered. Accordingly, the
sensing of the temperature lowering rate can easily be performed in a
short time, and the temperature of the heating member can easily be raised
again to the target temperature after the temperature is once lowered. In
a generally used thermal fixing apparatus such as a heat roller, however,
the heat capacity of the heating member is so large that it is impossible
to sense the lowering of the temperature of the heating member in a short
time period with a high degree of accuracy. Moreover, a control
arrangement in which the temperature of the heating member is once lowered
and then raised to the target temperature wastefully consumes electric
power and time, and hence such a control is not desirable.
Also in the case where the electric power supply to the heating element
between sheets is controlled so as to have either of two levels of
HIGH/LOW and the temperature variation (temperature ripple) of the heating
element is sensed, if the heating element heats the material to be heated
via a film having a small heat capacity in the same way as in the film
heating method, the temperature of the heating member is easily varied.
Accordingly, the sensing of the temperature variation can easily be
performed in a short time. In a generally used thermal fixing apparatus
such as a heating roller, ho | | |
