Electrophotography – Diagnostics – Fixing
Reexamination Certificate
1997-12-18
2001-01-09
Royer, William J. (Department: 2852)
Electrophotography
Diagnostics
Fixing
C374S153000, C374S183000, C399S069000
Reexamination Certificate
active
06173131
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image heating apparatus used in an image forming apparatus such as a copying machine, a printer and the like.
2. Related Background Art
In the past, a thermal fixing means including a heat roller was mainly used to fix a toner image developed in an image forming portion and transferred to a recording sheet to the recording sheet as a permanent image. In general, a fixing roller including a heater therein is urged against a pressure roller to form a fixing nip therebetween. A toner image is thermally fixed to the recording sheet by heat from the heater while the recording sheet is being passed through a nip between the fixing roller and the pressure roller.
Control of temperature of the heat roller is effected by a temperature sensor such as a thermistor contacted with the rotating heat roller. In the present day, the temperature control of the heat roller has been effected by using a CPU (central processing unit) in control of almost of all copying machines.
In the temperature control effected by the CPU, a heat amount given to the heat roller by the CPU is controlled by converting voltage (corresponding to a temperature of the heat roller) from the temperature sensor into a digital amount by means of an A/D (analogue/digital) converter.
[Explanation of Construction of Conventional Fixing Device]
FIG. 4
shows a conventional fixing device.
Here, explanation is made regarding an example of a fixing device in which an upper heat fixing roller and a lower pressure roller are urged against each other to form a fixing nip where a toner image (developed at an image forming portion and transferred to a recording sheet) is fixed to the recording sheet as a permanent image.
In
FIG. 4
, a fixing roller
40
a
contacted with the toner image has an outer diameter of 60 mm and is constituted by an aluminium core cylinder
418
, an HVT (high temperature vulcanizing type) silicone rubber layer
417
having a thickness of 1 mm and coated on the core cylinder, and a special adding type silicone rubber layer
416
coated on the rubber layer
417
.
On the other hand, the pressure roller
40
b
has an outer diameter of 60 mm and is constituted by an aluminium core cylinder
412
, an HVT silicone rubber layer having a thickness of 1 mm and coated on the core cylinder, and a special adding type silicone rubber layer
415
having a thickness of 1 mm and coated on the silicone rubber layer.
In the fixing roller
40
a,
a convey roller heater (heat generating means)
409
is disposed within the core cylinder
418
, and, in the pressure roller
40
b,
a heater
413
is disposed within the core cylinder
412
, so that the recording sheet is heated from both sides. A temperature of the fixing roller
40
a
is detected by a thermistor
410
contacted with the fixing roller
40
a
and a temperature of the pressure roller
40
b
is detected by a thermistor
411
contacted with the pressure roller
40
b.
The halogen heaters
409
,
413
are controlled by a control device
414
on the basis of detected temperatures so that the temperature of the fixing roller
40
a
is maintained to 170° C. (constant) and the temperature of the pressure roller
40
b
is maintained to 165° C. (constant). The fixing roller
40
a
and the pressure roller
40
b
are urged against each other with total pressure of 80 kg by means of a pressurizing mechanism (not shown). In
FIG. 4
, symbol
0
denotes an oil applying device (mold releasing agent applying device); C denotes a cleaning device; and C
1
denotes a cleaning blade for removing oil and contamination from the pressure roller
40
b.
In the oil applying device
0
, dimethyl silicone oil
408
in an oil pan
407
is picked up by oil pick-up rollers
406
and
405
and is applied to the fixing roller
40
a
by an oil applying roller
404
while regulating an oil applying amount by means of an oil applying amount regulating blade
403
. In the cleaning device C, the surface of the fixing roller
40
a
is cleaned by a web
402
contacted with the fixing roller
40
a
by an abut roller
401
. In the above-mentioned fixing device, the recording sheet on which a non-fixed toner image was borne is conveyed to the fixing nip (between the fixing roller
40
a
and the pressure roller
40
b
), where the recording sheet is heated and pressurized from both sides, thereby fixing the toner image onto the recording sheet. In this case, toners adhered to the fixing roller
40
a
and the pressure roller
40
b
are removed by the cleaning device C and the cleaning blade C
1
, respectively.
However, when the temperature of the heat roller detected by a single thermistor with high accuracy is unreliable, due to temperature errors in a series of conversion circuits from the thermistor to the CPU and temperature properties of the thermistor, if a low temperature reading accuracy of a thermistor selected for high temperature detecting is worsened, the low temperature cannot be measured accurately by the thermistor selected for the high temperature. Thus, when the surface temperature of the roller is heated to a desired temperature, due to poor abutment of a thermistor for low temperature and/or breakage of a signal line of the thermistor, it is difficult to quickly detect an abnormality of increase in temperature of the heat roller.
[Explanation of Causes for Worsening Low Temperature Reading Accuracy of Thermistor for High Temperature]
FIG. 1
shows a typical temperature detection circuit using a thermistor. In
FIG. 1
, the reference numeral
107
denotes a thermistor;
103
,
104
and
106
denote fixed resistances;
105
denotes a circuit resistance component such as current limitter resistance;
102
denotes an OP amplifier; and
101
denotes detected voltage corresponding to detection temperature of the thermistor
107
.
When reference resistance value is R
0
and temperature is T
0
, a resistance value R
th
of the thermistor is represented by the following equation (1):
R
th
=R
0
×exp
B
{(1/
T
)−(1/
T
0
)} (1)
where, B is a thermistor B constant (K) and T is an absolute temperature.
Further, when the above equation (1) is converted with respect to T, the following equation can be obtained:
T=
(
T
0
×B
)/(
T
0
×{Ln
R−
Ln
R
0
}+B
) (2)
The above equation (2) can be rewritten as follows:
1/
T=
(1/
T
0
)+{(Ln
R
th
/(
B
×Ln
R
0
)} (3)
When T
0
, R
0
and B are regarded as constants, and &agr;=1/T
0
and &bgr;=B×Ln R
0
, the following equations are obtained:
1/
T=&agr;+
(Ln
R
th
/&bgr;) (4)
T=
1/{(&agr;×Ln
R
th
)+&bgr;} (5)
Thus, a reciprocal of natural logarithm of the resistance error of the thermistor in the circuit appears as an error of the reading temperature. Since the resistance value of the thermistor for high temperature is increased in an exponential function manner as the temperature is decreased, the error factor of the resistance value regarding the temperature is also increased. This is one error factor of the reading temperature.
Regarding the detected voltage
101
, voltage obtained by dividing power source voltage (Vcc) by a resistance value (R
1
) of the division resistance
106
and the resistance value (R
th
) of the thermistor
107
is outputted, and is represented by the following relation:
Detected voltage
101
(out)={
R
th
/(
R
1
+R
th
)}×
Vcc
(6)
FIG. 2A
is a graph showing a relation between the measured temperature of the thermistor and the detected voltage, which represents a relation between the detected voltage
101
in the circuit shown in FIG.
1
and the temperature detected by the thermistor. In
FIG. 2A
, the abscissa indicates the detection temperature value of the thermistor and the ordinate indicates the detected voltage value.
The fixing device in electrophotography is normally used u
Kitamura Shingo
Noguchi Junichi
Watanabe Nobuyuki
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Royer William J.
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