Information
-
Patent Grant
-
6597879
-
Patent Number
6,597,879
-
Date Filed
Tuesday, May 21, 200222 years ago
-
Date Issued
Tuesday, July 22, 200321 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Chen; Sophia S.
- Campbell; Candice C.
Agents
- Akin Gump Strauss Hauer & Feld, L.L.P.
-
CPC
-
US Classifications
Field of Search
US
- 399 69
- 399 67
- 399 33
- 399 320
- 399 328
- 399 335
- 219 216
- 219 619
- 219 469
- 219 470
-
International Classifications
-
Abstract
A fixing unit fixes a developing material deposited on a recording medium by heating and pressing the recording medium. The fixing unit includes a first number of heating members that apply heat to the recording medium and a second number of switches that supply electric power to the first number of heating members. Each of the heating members receives electric power through a series circuit of the switches. Each of the switches responds to a surface temperature of a corresponding one of the heating members so that when the surface temperature exceeds a predetermined value, the electric power is shut off.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fixing unit incorporated in an electrophotographic recording apparatus, and more particularly to a fixing unit where the developer material deposited on a print medium is pressed and heated to fuse.
2. Description of the Related Art
A conventional fixing unit for use in an electrophotographic printer includes a rotating heat roller and a rotating backup roller. The surfaces of the heat roller and backup roller have a rubber material or a resin material wrapping around them. The heat roller is cylindrical and has a built-in heater in the form of, for example, a halogen lamp. Electric power is supplied to the heater, which in turn generates heat to heat the heat roller to a desired temperature.
The heat roller has a temperature sensor in the form of a thermistor. The temperature sensor detects the temperature of the surface of the heat roller. The detection signal causes a control circuit to turn on and off the electric power supplied to the halogen lamp, thereby maintaining the surface temperature of the heat roller to a substantially constant value. For safety of the system, there is provided a thermostat that shuts off electric power when the feedback control operates abnormally to overheat the heat roller. The thermostat shuts off the electric power before the temperature of the heat roller exceeds a maximum allowable value, thereby preventing an abnormal increase in temperature.
FIG. 6
illustrates another conventional fixing unit. Referring to
FIG. 6
, instead of a heat roller and a backup roller, a fixing unit
1
uses two heat rollers
2
and
3
that heat a print medium both from the front side and from the back side simultaneously. This type of fixing unit is advantageous when the printing speed of the electrophotographic printer is to be increased. The recording medium passes through the fixing unit at a high speed and therefore heat rollers must apply a sufficient amount of heat to the printing medium in a short time during which the printing medium passes through the fixing unit.
There are provided thermistor sensors
4
and
5
on the heat rollers
2
and
3
, respectively. The thermistor sensors
4
and
5
are connected to control circuits
18
and
19
through connectors
6
and
7
, respectively. The control circuits
8
and
9
are connected to an a-c main line
12
through thyristors
10
and
11
.
Thermostats
13
and
14
are disposed on the surfaces of the heat rollers
2
and
3
, respectively. The thermostat
13
has one cord connected to a halogen lamp
15
in the heat roller
2
and the other cord connected to the a-c main line
12
through a fuse
16
and the connector
6
. Likewise, the thermostat
14
has one cord connected to a halogen lamp
17
in the heat rollers
3
and the other cord connected to the AC main line
12
through the fuse
16
.
FIG. 7
is a graph that illustrates changes in the surface temperature of the heat rollers in the conventional art when temperature control fails. Curve A indicates the surface temperature of the heat roller
22
when temperature control fails and Curve B shows the surface temperature of the heat roller
23
when temperature control operates normally.
The operation of the conventional fixing unit of the aforementioned construction will be described. If the temperature control involving one of the thermistor sensors
4
and
5
should fail and a heat roller in a failed control system is overheated, a corresponding thermostat operates to shut off the circuit before the temperature reaches a tolerable value. The temperature of the heat roller
23
under abnormal temperature control will start to decrease. The heat roller
22
under normal temperature control maintains its surface temperature substantially at the target value. Therefore, as shown by Curve A, the temperature of the heat roller
23
will not decrease rapidly but slowly reach the target value at time t
3
.
With the aforementioned conventional fixing unit that employs two heat rollers, the thermostats are connected to separate circuits. If one of the feedback control systems fails, a corresponding thermostat in the failed system is shut off. A thermostat in the normally operating system is not shut off but performs its on and off operation under the control of the output of a corresponding thermistor. In other words, the surface of a normally operating heat roller is maintained at a desired temperature. The temperature of an abnormally operating heat roller will not decrease and the abnormal condition will remain for a long time.
SUMMERY OF THE INVENTION
An object of the invention is to provide a fixing unit in which when a heat roller is overheated due to an abnormal condition, the abnormal condition is prevented from lasting for a long time.
A fixing unit fixes a developing material deposited on a recording medium by heating and pressing the recording medium. The fixing unit includes a first number of heating members that apply heat to the recording medium and a second number of switches that supply electric power to the first number of heating members. Each of the heating members receives electric power through a series circuit of the switches. Each of the switches responds to a surface temperature of a corresponding one of the heating members so that when the surface temperature exceeds a predetermined value, the electric power is shut off.
The first number of heating elements may be in parallel with each other.
The first number is equal to the second number.
The first number of heating members may include two heating members and the second number of switches may include two switches. The two switches form a series circuit with each other and the two heating members form a parallel circuit with each other. The series circuit is connected in series with the parallel circuit.
The first number of heating members may include three heating members and the second number of switches may include three switches. The three switches form a series circuit with each other and the three heating members form a parallel circuit with each other. The series circuit is connected in series with the parallel circuit.
The first number of heating members may include two heating members and the second number of switches may include four switches. A first one of the two heating members forms a first series circuit with first two of the four switches. A second one of the two heating members forms a second series circuit with second two of the four switches. The first series circuit and the second series circuit are connected in parallel with a power source.
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
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 limitative of the present invention, and wherein:
FIG. 1
is a schematic diagram, illustrating a fixing unit according to a first embodiment of the invention;
FIG. 2
is a graph that illustrates changes in the surface temperature of the heat roller in the first embodiment;
FIG. 3
is a schematic diagram, illustrating a fixing unit according to a second embodiment;
FIG. 4
is a cross-sectional view of the fixing unit of
Fig. 3
;
FIG. 5
illustrates a fixing unit according to a third embodiment;
FIG. 6
illustrates a conventional fixing unit; and
FIG. 7
is a graph that illustrates changes in the surface temperature of the heat roller in the conventional fixing unit when temperature control fails.
DETAILED DESCRIPTION OF THE INVENTION
First Embodiment
FIG. 1
is a schematic diagram, illustrating a fixing unit according to a first embodiment of the invention.
Referring to
FIG. 1
, a fixing unit
21
includes two heat rollers
22
and
23
. The surfaces
24
and
25
of the heat rollers
22
and
23
are covered with a layer of a rubber material or a resin material. Halogen lamps
26
and
27
are disposed in the heat rollers
22
and
23
, respectively, and serve as a heater. The halogen lamp
26
has one end
26
a
connected to a cord
28
and the other end
26
b
connected to a cord
29
. The halogen lamp
27
has one end
27
a
connected to a cord
30
and the other end
27
b
connected to the cord
29
.
Thermistor sensors
4
and
5
are disposed on the heat rollers
22
and
23
, respectively, and connected to control circuits
8
and
9
through connectors
6
and
7
, respectively.
Thermostats
13
and
14
are disposed on the surfaces of the heat rollers
22
and
23
. The thermostats
13
and
14
take the form of an overtemperature thermostat.
The cord
28
connects the halogen lamp
26
and a cathode of a thyristor
10
through a connector
6
. An anode of the thyristor
10
is connected to one of the terminals of an a-c main line
12
. The cord
30
connects the halogen lamp
27
and the cathode of a thyristor
11
through a connector
7
. The anode of the thyristor
11
is connected to one of the terminals of the a-c main line
12
. The cord
32
connects the thermostat
13
to the main line
12
through the connector
6
and fuse
16
. The aforementioned circuit connection completes a series connection between the thermostat
13
and thermostat
14
.
Temperature-controlling circuits
8
and
9
are connected to a temperature-setting circuit
34
and to gates of the thyristors
10
and
11
. The temperature-setting circuit
34
sends a command to the temperature-controlling circuits
8
and
9
to control the temperatures of the heat rollers
22
and
23
, respectively.
The operation of the first embodiment will be described. When the printer is powered on and a printing operation is initiated, the temperature-setting circuit
34
sends a temperature-setting command to the temperature-controlling circuits
8
and
9
to set the surfaces of the heat rollers
22
and
23
to a target temperature. The temperature-controlling circuit
8
compares a detection signal from the thermistor sensor
4
with a target temperature. If the detection signal is lower than the target temperature, then the temperature-controlling circuit
8
provides a signal to the gate of the thyristor
10
to turn on the thyristor
10
. Then,the thyristor
10
allows a-c current to flow therethrough, the a-c current flowing through the cord
28
into the halogen lamp
26
to heat the heat roller
22
.
The thermistor
4
monitors the surface temperature of the heat roller
22
. If the temperature monitored by the thermistor
4
exceeds a threshold value, then the temperature-controlling circuit
8
provides a signal to the gate of the thyristor
10
, thereby turning off the thyristor
10
. In response to the signal, the thyristor
10
shuts off the a-c current flowing through it, so that no current flows through the halogen lamp
26
and therefore the heat roller
22
begins to cool down. If the surface temperature of the heat roller
22
decreases below the threshold value, the aforementioned operation is performed so that current flows through the halogen lamp
26
again. By repeating the aforementioned operation, the surface temperature of the heat roller
22
is maintained substantially to a target temperature.
A similar temperature control is performed for the heat roller
23
. That is, the temperature-controlling circuit
9
compares a detection signal from the thermistor sensor
5
with a target temperature received from the temperature-setting circuit
34
. The comparison result is used to drive the thyristor
11
to control the current flowing through the halogen lamp
27
, so that the surface temperature of the heat roller
23
is maintained substantially to the target-temperature.
During normal operation, the a-c currents flowing out of the halogen lamps
26
and
27
are added together at the terminal of the thermostat
14
and then further flows through the thermostat
13
, connected in series with the thermostat
14
, to the fuse
16
.
If the feedback control through the thermistor sensors
4
and
5
should fail so that the halogen lamps
26
and
27
are overheated, the thermostat in the circuit having an overheated halogen lamp operates to shut off the electric power supplied thereto. For example, if a foreign matter is caught between the thermistor sensor
4
and the heat roller
22
, the foreign matter prevents the thermistor sensor
4
from detecting the surface temperature of the heat roller
22
properly. As a result, a large current flows through the halogen lamp
26
and may cause the surface temperature of the heat roller
22
to exceed a target value.
When the surface of the heat roller
22
exceeds the upper limit temperature, the thermostat
13
operates to shut off the current flowing through the halogen lamp
26
. Therefore, the current flowing through the halogen lamp
27
is also shut off. Shutting off the current that flows through the halogen lamps
26
and
27
causes the surface temperature of the heat rollers
22
and
23
to rapidly decrease.
FIG. 2
is a graph that illustrates changes in the surface temperature of the heat roller in the first embodiment.
Referring to
FIG. 2
, Curve A indicates the surface temperature of the heat roller
22
and Curve B shows the surface temperature of the heat roller
23
. Upper limit temperature is a temperature beyond which the thermostat
13
operates to shut off the current through the circuit and target temperature is a temperature value toward which the surface temperatures of the heat rollers
22
and
23
are controlled. As shown in
FIG. 2
, when the surface temperature of the heat roller
22
increases to the upper limit temperature at time t
1
, the thermostat
13
operates to shut off the current through it, as well as the current flowing through the halogen lamp
27
in the heat roller
23
. Thus, the surface temperature of the heat roller
23
also decreases. The decrease in the surface temperature of the heat roller
23
allows the surface temperature of the heat roller
22
to decrease promptly. In fact, the time required for the heat roller
22
to cool down to the target temperature is t
2
in
FIG. 2
, shorter than t
3
in FIG.
7
. As described previously, the thermostats
13
and
14
take the form of an overtemperature thermostat. That is, the thermostat opens at, for example, 150° C. and closes at, for example, below 0° C. Thus, once the thermostat opens at a high abnormal temperature, the circuit will remain open after the heat rollers cool down to room apparatus. The use of an overtemperature thermostat enhances safety of the apparatus. A thermal fuse may be used in place of the overtemperature thermostat.
As described above, the thermostats
13
and
14
are connected in series with a parallel circuit of the halogen lamps
26
and
27
. Therefore, when a failure of the temperature control for one of the heat rollers
22
and
23
causes a corresponding heat roller to be overheated, a corresponding thermostat operates to shut off the current flowing through the halogen lamps
26
and
27
. This makes an abnormal condition to quickly terminate, thereby improving safety of the fixing unit
21
.
Second Embodiment
While the first embodiment has been described with respect to a fixing unit having two heat rollers, more heat rollers may be employed. A second embodiment differs from the first embodiment in that the fixing unit uses three heat rollers.
FIG. 3
is a schematic diagram, illustrating a fixing unit according to the second embodiment.
FIG. 4
is a cross-sectional view of the fixing unit of FIG.
3
.
Referring to
FIG. 3
, a heat roller
41
is in contact with the heat roller
22
. A thermistor sensor
42
is disposed on a surface
82
of the heat roller
41
. The thermistor sensor
42
is electrically connected through a connector
43
to a temperature-controlling circuit
44
. The temperature-controlling circuit
44
is connected to the gate of a thyristor
45
and the temperature-setting circuit
34
.
A thermostat
46
is disposed on the surface
82
of the heat roller
41
. A cord
47
connects the thermostat
46
to the thermostat
13
, and a cord
48
connects the thermostat
46
to the a-c main line
12
through the connector
43
and the fuse
16
.
A halogen lamp
49
has one end
49
b thereof connected to one terminal of the thermostat
14
through a cord
29
, and the other end
49
a
thereof connected to the cathode of the thyristor
45
through a cord
50
and the connector
43
. The anode of the thyristor
45
is connected to the another terminal of the a-c main line
12
. The rest of the construction is the same as the first embodiment.
Referring to
FIG. 4
, the heat rollers
22
,
23
, and
41
rotate in directions shown by arrows A, B, and C and the recording medium travels in a direction shown by arrow D. The positional relation between rollers
23
and
41
and the roller
22
may be interchanged so that the recording medium travels upward after it passes through between the roller
23
and the roller
22
. An angle θ and the diameters of the rollers can be selected by considering the flexibility of the recording medium. This type of fixing unit is advantageous when high-speed printing is performed. When the recording medium passes through the fixing unit at a high speed, the recording medium cannot receive a sufficient amount of heat. The configuration of
FIG. 4
effectively increases the time during which the recording medium is subjected to heating. The recording medium is guided by a guide member, not shown, that extends substantially through the angle θ in such a way that the recording medium passes between the heat roller
22
and heat roller
41
and then between the heat roller
22
and heat roller
23
.
When an abnormal condition occurs in any one of the three heat rollers
41
,
22
, and
23
, the current flowing through the halogen lamps
49
,
113
, and
27
in all other heat rollers is shut off, thereby preventing the abnormal condition from lasting for a long time. The second embodiment has been described with respect to three rollers
22
,
23
, and
41
rotate in contact with the recording medium. An additional heat roller may be combined to these heat rollers
22
,
23
, and
41
so that the additional heat roller rotates in contact with one of the three rollers
22
,
23
, and
41
, and heats the recording medium indirectly.
Third Embodiment
FIG. 5
illustrates a fixing unit according to a third embodiment.
With the first and second embodiments, the thermostats are simply connected in series regardless of the number of heat rollers. The third embodiment differs from the first and second embodiments in that there are as many series-connections of thermostats as there are heat rollers.
Referring to
FIG. 5
, a fixing unit according to the third embodiment includes two heat rollers
52
and
53
. The heat rollers have surfaces
54
and
55
, respectively, which are covered with a rubber material or a resin material. The heat rollers
52
and
53
have halogen lamps
56
and
57
built therein, respectively. The halogen lamps
56
and
57
have one ends thereof connected to cords
58
and
59
and the other ends thereof connected to cords
60
and
61
.
There are provided the thermistor sensors
4
and
5
on the surface of the heat rollers
52
and
53
, respectively. The thermistor sensors
4
and
5
are connected to temperature-controlling circuits
64
and
65
through connectors
62
and
63
, respectively. Thermostats
66
and
67
are disposed on the surface of the heat roller
52
, and thermostats
68
and
69
are disposed on the heat roller
53
.
A first series circuit is formed as follows: A cord
58
connects the halogen lamp
56
in the heat roller
52
to the thermostat
69
on the heat roller
53
. A cord
70
connects the thermostat
69
to the thermostat
66
on the heat roller
52
. A cord
7
l connects the thermostat
66
to an a-c mail line
73
through the connector
62
and a fuse
72
. The cord
59
connects a cathode of a thyristor
74
through the connector
62
to the halogen lamp
56
. The thyristor
74
has an anode connected to the a-c main line
73
.
Another series circuit is formed as follows: A cord
60
connects the halogen lamp
57
in the heat roller
53
to the thermostat
67
on the heat roller
52
. A cord
75
connects the thermostat
67
to the thermostat
68
on the heat roller
53
. A cord
76
connects the thermostat
68
to the a-c main line
73
through the connector
63
and the fuse
72
. A thyristor
77
has a cathode connected through the cord
61
and the connector
63
to the halogen lamp
57
, and an anode connected to the a-c supply
73
.
The current that flows through the thermostats
66
and
69
is equal to the current that flows through the halogen lamp
56
. The current that flows through the thermostats
67
and
68
is equal to the current that flows through the halogen lamp
57
.
The thermostats
66
and
67
disposed on the heat roller
52
are designed to operate at substantially the same temperature. Likewise, the thermostats
68
and
69
disposed on the heat roller
53
are designed to operate at substantially the same temperature.
The temperature controlling circuits
64
and
65
are connected to a temperature-setting circuit
78
and gates of the thyristors
74
an
77
. The temperature-setting circuit
78
provides a command signal to the temperature-controlling circuits
64
and
65
to perform the temperature control for the heat rollers
52
and
53
.
The operation of the third embodiment will now be described. In response to the command signal from the temperature-setting circuit
78
, the temperature-controlling circuit
64
sends an ON signal to the thyristor
74
. The thyristor
74
then operates to allow an a-c current to flow through the halogen lamp
56
in the heat roller
52
. The current that flows through the halogen lamp
56
also flows through the thermostat
69
on the heat roller
53
, then through the thermostat
66
on the heat roller
52
, and finally returns to the a-c main line
73
through the connector
62
and fuse
72
.
In response to the command signal from the temperature-setting circuit
78
, the temperature-controlling circuit
65
sends an ON signal to the thyristor
77
. The thyristor
77
then operates to allow an a-c current to flow through the halogen lamp
57
in the heat roller
53
. The current that flows through the halogen lamp
57
also flows through the thermostat
67
on the heat roller
52
, then the thermostat
68
on the heat roller
52
, and finally returns to the a-c main line
73
through the connector
63
and fuse
72
.
If the feedback control through one of the thermistor sensors should fail to properly operate and a corresponding heat roller is overheated, the thermostat on the overheated heat roller operates to shut off electric power through it. For example, when the heat roller
52
is overheated, the thermostats
66
and
67
operate simultaneously or substantially simultaneously to shut off the current flowing through the halogen lamps
56
and
57
. Likewise, when the heat roller
53
is overheated, the thermostats
68
and
69
operate simultaneously or substantially simultaneously to shut off the current flowing through the halogen lamps
56
and
57
.
For example, if a foreign material is trapped between the thermistor
4
and the heat roller
52
, the foreign matter prevents the thermistor sensor
4
from detecting the surface temperature of the heat roller
52
properly. As a result, a large current flows through the halogen lamp
56
and may cause the surface temperature of the heat roller
52
to exceed the upper limit temperature.
If the surface temperature of the heat roller
52
exceeds the upper limit temperature, the thermostats
66
and
67
operate. In other words, the thermostat
66
shuts off the current flowing through the halogen lamp
56
while the thermostat
67
shuts off the current flowing through the halogen lamp
57
. Shutting off the currents that flow through the halogen lamps
56
and
57
allows the heat rollers
52
and
53
to cool down rapidly.
According to the third embodiment, when the temperature control for one of a plurality of heat rollers fails and causes the associated heat roller to be overheated, the system operates to shut off not only the current flowing through the halogen lamp for the heat roller under failed temperature control but also the current flowing through the other halogen lamps for the heat rollers under normal temperature control. This way of operation prevents the abnormal condition from lasting a long time and improves safety of the system.
The circuit is configured in such a way that the current that flows through the respective thermostats is equal to the current for one halogen lamp. This allows employing inexpensive thermostats having a low current rating.
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 are intended to be included within the scope of the following claims.
Claims
- 1. A fixing unit that fixes a developing material deposited on a recording medium by heating and pressing the recording medium, comprising:a first number of heating members that apply heat to the recording medium; a second number of temperature-sensitive shut-off devices in a first series connection; and wherein each of said first number of heating members and the first series connection form a second series connection in such a way that each of said first number of heating members receives electric power through the first series connection of said second number of temperature-sensitive shut-off devices, each of said second number of temperature-sensitive shut-off devices receiving heat from a surface of a corresponding one of said first number of heating members to turn off the electric power in response to a temperature of the surface, the electric power supplied to the heating members being shut off simultaneously when the surface temperature exceeds a predetermined value.
- 2. The fixing unit according to claim 1, wherein said first number of heating elements are in parallel with each other.
- 3. The fixing unit according to claim 2, wherein said temperature-sensitive shut-off devices are overtemperature thermostats.
- 4. The fixing unit according to claim 2, wherein said temperature-sensitive shut-off devices are thermal fuses.
- 5. The fixing unit according to claim 1, wherein said first number is equal to said second number.
- 6. The fixing unit according to claim 5, wherein said temperature-sensitive shut-off devices are overtemperature thermostats.
- 7. The fixing unit according to claim 5, wherein said temperature-sensitive shut-off devices are thermal fuses.
- 8. The fixing unit according to claim 1, wherein said first number of heating members include two heating members and said second number of temperature-sensitive shut-off devices includes two temperature-sensitive shut-off devices,wherein the two temperature-sensitive shut-off devices form a series circuit with each other and the two heating members form a parallel circuit with each other, the series circuit being connected in series with the parallel circuit.
- 9. The fixing unit according to claim 1, wherein said first number of heating members includes three heating members and said second number of temperature-sensitive shut-off devices includes three temperature-sensitive shut-off devices,wherein the three temperature-sensitive shut-off devices form a series circuit with each other and the three heating members form a parallel circuit with each other, the series circuit being connected in series with the parallel circuit.
- 10. The fixing unit according to claim 1, wherein said first number of heating members includes two heating members and said second number of temperature-sensitive shut-off devices includes four temperature-sensitive shut-off devices,wherein a first one of the two heating members forms a first series circuit with the first two of the four temperature-sensitive shut-off devices and a second one of the two heating members forms a second series circuit with the second two of the four temperature-sensitive shut-off devices, the first series circuit and second series circuit being connected in parallel with a power source.
- 11. The fixing unit according to claim 1, wherein said temperature-sensitive shut-off devices are overtemperature thermostats.
- 12. The fixing unit according to claim 1, wherein said temperature-sensitive shut-off devices are thermal fuses.
Priority Claims (1)
Number |
Date |
Country |
Kind |
2001-204726 |
Jul 2001 |
JP |
|
US Referenced Citations (7)
Foreign Referenced Citations (2)
Number |
Date |
Country |
408110732 |
Apr 1996 |
JP |
411316517 |
Nov 1999 |
JP |