Electrophotography – Control of electrophotography process – Control of fixing
Reexamination Certificate
2002-01-18
2003-08-19
Tran, Hoan (Department: 2852)
Electrophotography
Control of electrophotography process
Control of fixing
C219S216000
Reexamination Certificate
active
06608977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to temperature control apparatuses for controlling the temperature of a heating unit.
2. Description of the Related Art
In a xerographic apparatus such as a copy machine, a toner image formed on a photo-sensitive drum is transferred to a sheet of transfer paper by a transfer device. The toner image on the transfer paper is thermally fused to the transfer paper by a heat fuser. A fusing roller incorporated in the heat fuser includes a built-in heater for heating the fusing roller so that the temperature of the surface of the fusing roller is maintained at a constant temperature.
A temperature control apparatus for such a fuser includes, for example, the temperature control apparatuses shown in
FIG. 11
or FIG.
13
.
The temperature control apparatus shown in
FIG. 11
will now be described. An AC power supply
103
is connected through a switching circuit
104
to a heater
102
of a fusing roller
101
. A thermistor
105
converts the temperature of the surface of the fusing roller
101
into an output signal a which has a voltage value in accordance with the temperature. The output signal a is input to an input port P
0
of a microcomputer
106
and is subjected to A/D conversion. When a detected temperature is lower than a target temperature, the microcomputer
106
outputs H (ON) from an output port P
1
to the switching circuit
104
(signal b). When the detected temperature is higher than the target temperature, the microcomputer
106
outputs L (OFF) from the output port P
1
to the switching circuit
104
.
A thermoswitch
107
is connected between the AC power supply
103
and the heater
102
. When the microcomputer
106
continuously turns ON the output port P
1
that controls ON/OFF of the switching circuit
104
as a result of the microcomputer
106
being out of control due to electric noise, a software bug, etc, the switching circuit
104
is continuously ON. As a result, the temperature of the fusing roller
101
increases excessively. In response to that condition, the thermoswitch
107
is operated so as to cut off the power supply to the fusing roller
101
.
The temperature control apparatus shown in
FIG. 12
will now be described. The AC power supply
103
is connected through the switching circuit
104
to the heater
102
of the fusing roller
101
. The thermistor
105
converts the temperature of the surface of the fusing roller
101
into the output signal a including the voltage value in accordance with the temperature. The output signal a is input to an input port P
0
of a microcomputer
116
and is subjected to A/D conversion. Bit
3
of a register
1167
that controls the output port P
1
of the microcomputer
116
is assigned to switch ON/OFF the heater
102
. When a detected temperature obtained by converting the signal a input to the input port P
0
into a temperature is lower than a target temperature, the microcomputer
116
writes, for example,
1
, instructing “heater-ON” to bit
3
of the register
1167
. Accordingly, H (ON) is output from the output port P
1
to the switching circuit
104
(signal b). In contrast, when the detected temperature is higher than the target temperature, the microcomputer
116
writes
0
instructing “heater-OFF” to bit
3
of the register
1167
. Accordingly, L (OFF) is output from the port P
1
(signal b). Bits other than bit
3
of the register
1167
are assigned to control other input/output ports. The thermoswitch
107
, which cuts off the power supply to the heater
102
in case of excessive temperature rise of the fusing roller
101
, is connected between the AC power supply
103
and the heater
102
.
The temperature control apparatus shown in
FIG. 13
will now be described. Unlike the temperature control apparatus shown in
FIG. 12
, a register
1207
for writing
1
and
0
instructing heater ON/OFF is provided in an integrated circuit (IC)
120
outside a microcomputer
126
. An address bus, a data bus, and a control signal of the microcomputer
126
are connected to the IC
120
.
In the temperature control apparatuses shown in
FIGS. 11
to
13
, the thermoswitch
107
, which is supposed to operate in case of excess temperature rise, may not operate immediately when the temperature of the fusing roller
101
excessively increases. For example, when the temperature of the fusing roller
101
excessively increases from room temperature, the fusing roller
101
and a bus of the fusing roller
101
may break before the thermoswitch
107
is operated since it takes time before the temperature of the thermoswitch
107
increases.
In order to solve this problem, for example, a method is described in Japanese Laid-Open Patent No. 4-136881. According to the method, electricity to a heater is forced to be periodically turned OFF for a predetermined period of time. When a heater ON/OFF detection unit detects that the heater has been in the ON state for a predetermined period of time or longer, electricity to the heater is cut off.
According to the method, when electricity to the heater is cut off in response to a failure detected, it is impossible to determine whether the failure has occurred in a switching circuit such as a solid-state relay (SSR) or in a microprocessor.
Even when the temperature of a fusing roller is low, the power supply to the heater is periodically turned ON/OFF. As a result, the AC power supply voltage varies in accordance with interruption of current flowing to the heater during power feeding and cut-off periods.
In the temperature control apparatus shown in
FIG. 12
, the heater
102
is turned ON by simply writing
1
to bit
3
of the register
1167
. A failure due to a simple bug in the program of the microcomputer
116
or noise may turn ON the heater
102
.
In particular, because bits other than bit
3
of the register
1167
are assigned to other input/output ports, the register
1167
is frequently accessed for purposes other than turning ON/OFF the heater
102
. Accordingly, bit inversion may occur as a result of electric noise generated when the register
1167
is accessed for purposes other than heater ON/OFF, thus unnecessarily turning ON the heater
102
.
In the temperature control apparatus shown in
FIG. 13
, the IC
120
is provided outside the microcomputer
126
; the address bus, the data bus, and the control signal of the microcomputer
126
are connected to the IC
120
; and the microcomputer
126
writes to the register
1207
in the IC
120
. When controlling ON/OFF of the heater
102
, the buses and control signal may be influenced by electric noise.
When the microcomputer
126
tries to gain write access to another address, part of the address may be inverted by electric noise. The IC
120
may erroneously detect this as writing to the register
1207
.
In response to the false detection, the heater
102
may be turned ON. When the register
1207
is accessed to rewrite bits assigned to other functions, bit
3
for heater ON/OFF may be inverted by electric noise. As a result, the heater
102
may be turned ON unnecessarily.
SUMMARY OF THE INVENTION
Accordingly, it is a first object of the present invention to provide a temperature control apparatus for solving the foregoing problems and for stopping the power supply to a heater before the temperature of a fusing roller excessively increases.
A second object of the present invention is to provide a temperature control apparatus for solving the foregoing problems and for detecting a failure in the temperature control apparatus without unnecessarily turning ON/OFF a heater even when it is necessary to continuously supply electricity to the heater.
A third of object of the present invention is to provide a temperature control apparatus for solving the foregoing problems and for preventing a malfunction due to electric noise.
In accordance with these and other objects, there is provided a temperature control apparatus that includes a temperature detector for detecting the temperature of a heating unit; a switching circuit for turning
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