FUSER CONTROL CIRCUIT FOR COPYING APPARATUS
United States Patent 3735092
An arrangement for controlling the fuser heat source in an electrostatic type reproduction machine to maintain fuser temperatures within a desired operating range and protect the reproduction machine in the event fuser temperatures become too high. The arrangement includes a thermistor to sense temperature conditions within the fuser, an amplifier to amplify the thermistor signal output to a level permitting relatively inexpensive switching devices to be used directly as fuser and reproduction machine controls without the need for relatively sensitive and correspondingly expensive intermediate comparator circuitry, and switching devices for operating the fuser heat source and controlling the reproduction machine in response to predetermined fuser temperature conditions.
US Patent References:
Firing angle control circuit for silicon controlled rectifiers
Engelson - August 1967 - 3337792
ELECTRICAL HEATING SYSTEMS
Portas et al. - February 1970 - 3496339
REPROGRAPHIC APPARATUS HEATER UNIT CONTROL
Lawes et al. - April 1970 - 3505497
Firing angle control circuit for silicon controlled rectifiers
Engelson - August 1967 - 3337792
ELECTRICAL HEATING SYSTEMS
Portas et al. - February 1970 - 3496339
REPROGRAPHIC APPARATUS HEATER UNIT CONTROL
Lawes et al. - April 1970 - 3505497
Application Number:
05/189859
Publication Date:
05/22/1973
Filing Date:
10/18/1971
View Patent Images:
Export Citation:
Assignee:
Xerox Corporation (Stamford, CT)
Primary Class:
Other Classes:
219/505, 219/216
International Classes:
G03G15/20; G05D23/24; G05D23/20; H05B1/02
Field of Search:
219/494,501,505,216
Primary Examiner:
Gilheany, Bernard A.
Assistant Examiner:
Bell F. E.
Claims:
What is claimed is
1. In a copying apparatus having a fuser for fixing toner developed images on the copy output of said apparatus, the combination comprising
2. The copying apparatus according to claim 1 in which said copying apparatus control means includes second switching means operative to shut down said copying apparatus in response to a second preset signal from said amplifier means reflecting a predetermined overtemperature in said fuser.
3. The copying apparatus according to claim 1 including means for monitoring operation of said temperature variable resistor operative on failure of said resistor to abort said amplifying means to generate said second preset signal whereby said second switching means shuts down said copying apparatus.
4. In a copying apparatus having a fuser for fixing toner developed images on the copy output of said apparatus, the combination comprising
5. In a copying apparatus having a fuser for fixing toner developed images on the copy output of said apparatus, the combination comprising:
6. The copying apparatus according to claim 5 including means to delay response of said resistor monitoring means whereby to avoid premature response of said resistor monitoring means and shut down of said fuser heating means.
1. In a copying apparatus having a fuser for fixing toner developed images on the copy output of said apparatus, the combination comprising
2. The copying apparatus according to claim 1 in which said copying apparatus control means includes second switching means operative to shut down said copying apparatus in response to a second preset signal from said amplifier means reflecting a predetermined overtemperature in said fuser.
3. The copying apparatus according to claim 1 including means for monitoring operation of said temperature variable resistor operative on failure of said resistor to abort said amplifying means to generate said second preset signal whereby said second switching means shuts down said copying apparatus.
4. In a copying apparatus having a fuser for fixing toner developed images on the copy output of said apparatus, the combination comprising
5. In a copying apparatus having a fuser for fixing toner developed images on the copy output of said apparatus, the combination comprising:
6. The copying apparatus according to claim 5 including means to delay response of said resistor monitoring means whereby to avoid premature response of said resistor monitoring means and shut down of said fuser heating means.
Description:
This invention relates to a fuser for electrostatic type reproduction machines, and more particularly, to a fuser for electrostatic type reproduction machines incorporating improved fuser temperature controlling means.
In electrostatic type reproduction machines, a toner delineated image of the original document being copied is electrostatically formed on the copy material. In order to render the image permanent, the image is fused by passing copy material through a heated oven, conventionally called a fuser. There, a combination of heat and pressure melts, i.e. fuses, the toner onto the copy material to form a permanent image.
In machines of this type, correct fuser temperatures are critical. If the fuser temperature is too low, fusing may be incomplete. This is most often evidenced by smearing or loss of image, particularly when the copy material is handled. If the fuser temperature is too high, there is danger that the copy material may burn or char.
In controlling fuser temperatures, a resistive type transducer, normally a thermistor, is usually used. The thermistor, which is disposed in heat exchange relationship with the fuser, is circuited to provide a signal change in response to changes in fuser temperatures, this signal being relied upon to start and stop the fuser heat source as required to maintain desired fuser temperatures. However, thermistor type devices suffer from the fact that their response or sensitivity to temperature change ordinarily decreases with increasing temperature. In the relatively high operating temperature range of modern fusers, thermistor sensitivity is very low. Hence, the thermistor signal output band over the fuser operating temperature range is quite small. This, in turn, has required that relatively sensitive comparator circuits be provided if accurate control over fuser temperatures are to be obtained, particularly where other machine control functions such as overtemperature shutdown protection are provided.
It is a principle object of the present invention to provide a new and improved fuser for an electrostatic type reproduction machine.
It is an object of the present invention to provide a control for copy machine fusers having means to amplify the output of the fuser temperature responsive transducer across the fuser operating temperature range.
It is a further object of the present invention to provide an improved control for maintaining operating temperatures in the fuser of an electrostatic copying machine.
It is an object of the present invention to provide an improved temperature controller for the fuser of a copy machine employing a resistive type transducer to sense fuser temperature conditions, with means to amplify the transducer signal output over at least the fuser operating temperature range to enhance control accuracy and avoid the need for relatively complex and expensive comparison circuitry.
It is an object of the present invention to provide a fuser control for accurately maintaining critical fuser operating temperatures employing relatively simple and inexpensive switching devices.
It is a further object of the present invention to provide, in an electrostatic type copying apparatus having a fuser temperature sensor with means to amplify the signal output thereof, control means effective on failure of the temperature sensor to prevent signal amplification and shut down the copying apparatus.
It is an object of the present invention to provide a fuser control designed to hold fuser temperatures in a copying machine within a prescribed operating limit while preventing operation of the copy machine whenever fuser temperatures are too low.
It is an object of the present invention to provide a copy machine control effective to shut down the machine on a fuser over-temperature condition.
This invention relates to a copying apparatus having a fuser for fixing the toner developed images of the copying apparatus, comprising, in combination: circuit means adapted to generate a control signal proportional to temperature conditions in the apparatus fuser, the circuit means including a temperature variable resistor in heat exchange relation with the fuser, the resistive change of the resistor in response to changes in temperature in the fuser normally being relatively small within the operating temperature range of the fuser with the result that the sensitivity of the circuit means is relatively low in the fuser operating temperature range; means for amplifying the signal output of the circuit means at least in the fuser operating temperature range whereby to avoid the need for relatively sensitive output comparator circuitry and instead permit use of relatively simple switching means; means for heating the fuser; and switching means for controlling the fuser heating means, the switching means being adapted to render the fuser heating means inoperative in response to a first amplifier means signal reflecting attainment of normal operating temperature in the fuser.
Other objects and advantages will be apparent from the following description and drawings in which:
FIG. 1 is a schematic sectional view of an electrostatic type reproduction machine embodying the principles of the present invention;
FIG. 2 is an enlarged cross sectional view showing the fusing apparatus of the reproduction machine shown in FIG. 1;
FIG. 3 is a plot of the temperature versus resistance relationship for a typical thermistor;
FIG. 4 is a plot of the temperature versus output signal relationship for the thermistor of FIG. 3 with unamplified thermistor signal output shown in dotted lines and amplified thermistor signal output shown in solid lines; and
FIG. 5 is a schematic circuit representation of the fuser control arrangement of the present invention.
Referring particularly to FIGS. 1 and 2 of the drawings, an exemplary copier/reproduction machine, designated generally by the numeral 10 and incorporating the fuser control arrangement of the present invention, is there shown. As in all electrostatic systems such as the xerographic type machine illustrated, a light image of a document to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material to form a xerographic powder or toner image, corresponding to the latent image on the plate surface. The toner image is then electrostatically transferred to a support surface where it is fused by a fusing device so that the toner image is permanently adhered to the support surface.
In machine 10, an original document 12 to be copied is placed upon a transparent support platen 14 fixedly arranged in an illumination assembly generally indicated by the reference numeral 15 and disposed at the left end of the machine. While upon the platen, the document 12 is illuminated, thereby producing image rays corresponding to the informational areas on the original. The image rays are projected by means of an optical system onto the photosensitive surface of a xerographic plate. In the exemplary copier/reproduction machine 10, the xerographic plate is in the form of a flexible photoconductive belt 17 supported in a belt assembly 18.
The support assembly 18 for photoconductive belt 17 includes three rollers 20, 21 and 22 located with parallel axes at approximately the apices of a triangle. The upper roller 22 is rotatably supported on shaft 23 which in turn is rotatably driven by a suitable motor and drive means (not shown) to drive belt 17 in the direction shown by the arrow in FIG. 1. During this movement of the belt, the reflected light image of the original document 12 on platen 14 is flashed upon the photoreceptor surface of belt 17 at an exposure station 25 to produce an electrostatic latent image thereon.
The continued movement of photoconductive belt 17 carries the electrostatic image through a developing station 26 in which there is positioned a developer assembly generally indicated by the reference numeral 28. There the latent electrostatic image is developed by means of toner through the use of a multiple magnetic brush system 29.
The developed electrostatic image is carried by belt 17 to the transfer station 30 where the developed image is transferred to a support surface, normally a sheet of copy paper 31, brought forward between transfer roller 32 and belt 17. In order to accomplish transfer of the developed image solely by means of the electrical bias on transfer roller 32, the copy sheet 31 is moved at substantially the same speed as belt 17. A sheet transport mechanism generally indicated at 34 is provided to advance copy sheets 31 from a paper handling mechanism generally indicated by the reference numeral 35 to transfer station 30.
Following transfer, the copy sheet 31 is stripped from belt 17 and conveyed through fuser 38 wherein the toner image is permanently fused or affixed thereto. Following fusing, the finished copy is discharged into output tray 39.
Photoconductive belt 17 comprises a photoconductive layer of selenium, which is the light receiving surface and imaging medium for the apparatus, on a conductive backing. Further details regarding the structure of the belt assembly 12 and its relationship with the machine and support therefor may be found in the copending application Ser. No. 102,312, filed Dec. 29, 1970, and assigned to the same assignee.
Referring now to FIG. 2, fuser 38 includes a suitable housing 40 within which is disposed a lower heated fuser roll 41 and an upper pressure roll 42, rolls 41, 42 cooperating to form a nip through which the copy sheets 31 pass. Rolls 41, 42 are suitably supported for rotation and driven in unison by a suitable drive means (not shown). Pressure roll 42 is comprised of a relatively soft material such as Teflon, Neoprene, and the like with the result that pressure contact between the rolls 41, 42 deforms the surface of pressure roll 42. In this way, an increased contact arc between the copy sheet and the heated fuser roll 41 is obtained.
In the exemplary arrangement illustrated, fuser roll 41 is hollow, the roll 41 being formed from a suitable heat conductive material. A source of heat such as lamp 44 is disposed therewithin. A suitable temperature variable resistor, i.e. thermistor 45 is supported on a fuser housing 40 in heat exchange relation therewith to sense temperature conditions within the fuser housing 40.
Referring to FIGS. 3 and 4, where resistance vs. temperature and output voltage vs. temperature relationships for a typical thermistor are plotted, the latter for both unamplified and amplified voltage outputs of thermistor 45, it will be seen that the change in thermistor resistance becomes progressively less and less with increasing temperatures. In the relatively high operating temperature range of fuser 38, which typically extends from a minimum operating temperature of 350° F (T 1 ) to a normal operating temperature of 430° F (T 2 ), the change in thermistor resistance, R 1 to R 2 is relatively small. The unamplified signal output of thermistor 45 in this temperature range, v 1 to v 2 , is correspondingly small, a signal output of approximately two volts being typical. This relatively small change in signal output, which covers the operating temperature range of fuser 38, renders accurate control of fuser temperatures difficult to obtain. Where, as in the case of electrostatic copying machines, fuser temperatures are critical if legible and permanent copy is to be obtained, accurate control of fuser temperatures is essential, and relatively expensive and sophisticated comparator circuits would normally be required to obtain the necessary control accuracy.
To avoid the need for expensive and complex comparator type circuits, the signal output of thermistor 45 across the operating temperature range of fuser 38 is amplified. This in turn enables relatively simple and inexpensive switches 55, 56, 57 to be used as control devices instead.
Referring to the circuit diagram of FIG. 5, the signal output of thermistor 45 is fed to a suitable operational amplifier 50. Amplifier 50 is gated to amplify thermistor signals equal to or less than v 1 by means of a suitable source 51 of d.c. voltage. The signal amplification accordingly takes place at fuser temperatures equal to or above minimum fuser operating temperature T 1 . The amplified signal from amplifier 50 is impressed on the control gate of switching amplifiers 55, 56, 57. As will appear, switching amplifiers 55, 56, 57 control fuser heating and operation of copying machine 10. The switching amplifiers 55, 56, 57 are each biased to a suitable control level by voltage source 51, various resistor combinations 58, 59 being provided to obtain the different individual switching responses from each of the switching amplifiers 55, 56, 57 as required to provide the requisite copying machine control.
Switching amplifier 55 controls operation of fuser heating lamp 44. For purposes of illustration, the fuser lamp control consists of switch 60 in the power line to lamp 44 with the switch operating coil 60' in series with the output gate of amplifier 55. On triggering of switching amplifier 55 to a conducting state, coil 60 is energized to close switch 60 and complete the energizing circuit to fuser heat lamp 44.
Switching amplifiers 56, 57 control operation of copying machine 10, amplifier 56 serving to prevent operation of copying machine 10 on start-up until temperatures of fuser 38 reach a predetermined level. Schematically, a switch 63 is provided in the copying machine operating circuit, the operating coil 63' therefor being in series relationship with the output gate of amplifier 56. Switch 63 prevents or inhibits operation of the copying machine 10 until closed by coil 63' upon energization thereof through triggering of amplifier 56 to a conducting state.
Switching amplifier 57 serves to shut down copy machine 10 should fuser temperatures become too high as represented by temperature T 3 of FIGS. 3 and 4. Schematically, the output gate of amplifier 57 is series connected with the operating coil 67' for normally closed switch 67 in the operating circuit of copying machine 10. On triggering of amplifier 57 to a conducting state, coil 67' opens switch 67 to shut down copying machine 10.
On start-up of copying machine 10, which is effected by operator closure of a suitable start or PRINT control, schematically represented by switch 69, the temperature of fuser 38 may be below minimum fuser operating temperature T 1 . This normally occurs when copying machine 10 is unused for a relatively long period or on first start-up following an overnight shutdown. In this circumstance, resistance of thermistor 45 is relatively high so that the signal output of thermistor 45 to amplifier 50 is relatively high. Since the thermistor signal output is greater than v 1 , the voltage for which amplifier 50 is gated to conduct, amplifier 50 is held in a non-conducting condition. Since switching amplifiers 55, 56, 57 are set to respond to predetermined signal inputs from amplifier 50, amplifiers 55, 56, 57 remain in their quiescent state. In this situation, switching amplifier 55 is in a conducting state while amplifiers 56, 57 are in a blocking state. The signal from amplifier 55 energizes coil 60' to close switch 60 and energize fuser heating lamp 44. Lamp 44, in turn, heats fuser roll 41.
Switch 63, which is closed only on triggering of switching amplifier 56, is open thereby preventing operation of copying machine 10. With amplifier 57 in a blocking condition, switch 67 is closed.
With operation of heating lamp 44, temperatures within fuser 38 rise. This results in a decrease in resistance of thermistor 45. On fuser 38 reaching the temperature range of interest, which is normally the minimum operating temperature T 1 , the unamplified signal output of thermistor 45 equals v 1 and amplifier 50 is switched to a conducting state. The signal V 1 , which appears at the output gate of amplifier 50 and which represents an amplified version of the output signal generated by thermistor 45 in response to temperatures of fuser 38, is impressed on the control gates of switching amplifiers 55, 56, 57. The bias level on amplifiers 55, 57 as determined by the values of their individual resistor combinations 58, 59 hold amplifiers 55, 57 in their quiescent state. However, the resistor combination 58, 59 for switching amplifier 56 is chosen so that the amplified signal V 1 from amplifier 50, representing attainment of the minimum operating temperature T 1 in fuser 38, switches amplifier 56 to a conducting state. With switching of amplifier to a conducting state, relay coil 63' is energized to close switch 63 and enable operation of copying machine 10.
Fuser lamp 44 continues to heat fuser 38, the rise in fuser temperature being accompanied by a continued decrease in resistance of thermistor 45 and hence a decrease in the amplified signal output from amplifier 50. On fuser 38 reaching normal fuser operating temperature T 2 , the signal output V 2 from amplifier 50 switches switching amplifier 55 to a non-conducting state. The resultant deenergization of relay coil 60' opens switch 60 to interrupt power to fuser lamp 44 and terminate heating of fuser 38.
It is understood that with fuser lamp 44 off, temperatures in fuser 38 decrease. Following a small temperature decrease, as sensed by thermistor 45, the signal from amplifier 50 switches switching amplifier 55 to a conducting state to actuate fuser lamp 44 and heat fuser 38. Thus, while copying machine 10 is operational, amplifier switches between a conducting and non-conducting state in response to changes in fuser temperatures as sensed by thermistor 45 to maintain fuser 38 at approximately temperature T 2 . It is understood that if fuser temperatures fall below the minimum operative temperature T 1 , switching amplifier 56 switches to a non-conducting state to open switch 63 and prevent further operation of copying machine 10.
Should temperatures of fuser 38 rise above the normal fuser operating temperature, T 2 , to the maximum safe temperature T 3 , the amplified signal output V 3 of thermistor 45 switches amplifier 57, to a conducting state. The signal from amplifier 57 energizes coil 67' to open switch 67 and shut down copying machine 10.
In order to protect fuser 38 from over-heating in the event thermistor 45 fails, in which event resistance thereof would become extremely high and switching amplifier 55 would normally be held in a conducting state with fuser heating lamp 44 energized, a second control gate circuit to amplifier 50 incorporating transistor 70 is provided. The base of transistor 70 is connected through adjustable resistor 71 to the output side of thermistor 45 while the emitter of transistor 70 is connected with a control gate of amplifier 70.
During normal operation of thermistor 45, transistor 70 is conductive to provide a gate signal to amplifier 70. In the event that the resistance of thermistor 45 exceeds a predetermined value indicating a failure or malfunction of thermistor 45, transistor 70 is switched to a non-conducting state. This in turn causes the output voltage of amplifier 50 to drop to a low level. The low level output from amplifier 50 on such failure or malfunction of thermistor 45, which is below the triggering signal V 3 of switching amplifier 57, triggers amplifier 57 to a conducting state thereby energizing relay coil 67' and opening switch 67. Opening of switch 67 shuts down the copying machine 10 as described heretofore. A suitable delay circuit 72 is provided to prevent premature triggering of switching amplifier 57 when fuser 38 is first warmed up.
If desired, switch 67 may comprise a manually resettable type switch requiring, once opened, manual reclosing before operation of copying machine 10 can be resumed. In addition, triggering of switching amplifier 57 may also serve to actuate an alarm or signal to alert the operator to the fuser over-temperature condition.
While the invention has been described with reference to the structure disclosed, it is not confined to the details set forth, but is intended to cover such modifications or changes as may come within the scope of the following claims.
In electrostatic type reproduction machines, a toner delineated image of the original document being copied is electrostatically formed on the copy material. In order to render the image permanent, the image is fused by passing copy material through a heated oven, conventionally called a fuser. There, a combination of heat and pressure melts, i.e. fuses, the toner onto the copy material to form a permanent image.
In machines of this type, correct fuser temperatures are critical. If the fuser temperature is too low, fusing may be incomplete. This is most often evidenced by smearing or loss of image, particularly when the copy material is handled. If the fuser temperature is too high, there is danger that the copy material may burn or char.
In controlling fuser temperatures, a resistive type transducer, normally a thermistor, is usually used. The thermistor, which is disposed in heat exchange relationship with the fuser, is circuited to provide a signal change in response to changes in fuser temperatures, this signal being relied upon to start and stop the fuser heat source as required to maintain desired fuser temperatures. However, thermistor type devices suffer from the fact that their response or sensitivity to temperature change ordinarily decreases with increasing temperature. In the relatively high operating temperature range of modern fusers, thermistor sensitivity is very low. Hence, the thermistor signal output band over the fuser operating temperature range is quite small. This, in turn, has required that relatively sensitive comparator circuits be provided if accurate control over fuser temperatures are to be obtained, particularly where other machine control functions such as overtemperature shutdown protection are provided.
It is a principle object of the present invention to provide a new and improved fuser for an electrostatic type reproduction machine.
It is an object of the present invention to provide a control for copy machine fusers having means to amplify the output of the fuser temperature responsive transducer across the fuser operating temperature range.
It is a further object of the present invention to provide an improved control for maintaining operating temperatures in the fuser of an electrostatic copying machine.
It is an object of the present invention to provide an improved temperature controller for the fuser of a copy machine employing a resistive type transducer to sense fuser temperature conditions, with means to amplify the transducer signal output over at least the fuser operating temperature range to enhance control accuracy and avoid the need for relatively complex and expensive comparison circuitry.
It is an object of the present invention to provide a fuser control for accurately maintaining critical fuser operating temperatures employing relatively simple and inexpensive switching devices.
It is a further object of the present invention to provide, in an electrostatic type copying apparatus having a fuser temperature sensor with means to amplify the signal output thereof, control means effective on failure of the temperature sensor to prevent signal amplification and shut down the copying apparatus.
It is an object of the present invention to provide a fuser control designed to hold fuser temperatures in a copying machine within a prescribed operating limit while preventing operation of the copy machine whenever fuser temperatures are too low.
It is an object of the present invention to provide a copy machine control effective to shut down the machine on a fuser over-temperature condition.
This invention relates to a copying apparatus having a fuser for fixing the toner developed images of the copying apparatus, comprising, in combination: circuit means adapted to generate a control signal proportional to temperature conditions in the apparatus fuser, the circuit means including a temperature variable resistor in heat exchange relation with the fuser, the resistive change of the resistor in response to changes in temperature in the fuser normally being relatively small within the operating temperature range of the fuser with the result that the sensitivity of the circuit means is relatively low in the fuser operating temperature range; means for amplifying the signal output of the circuit means at least in the fuser operating temperature range whereby to avoid the need for relatively sensitive output comparator circuitry and instead permit use of relatively simple switching means; means for heating the fuser; and switching means for controlling the fuser heating means, the switching means being adapted to render the fuser heating means inoperative in response to a first amplifier means signal reflecting attainment of normal operating temperature in the fuser.
Other objects and advantages will be apparent from the following description and drawings in which:
FIG. 1 is a schematic sectional view of an electrostatic type reproduction machine embodying the principles of the present invention;
FIG. 2 is an enlarged cross sectional view showing the fusing apparatus of the reproduction machine shown in FIG. 1;
FIG. 3 is a plot of the temperature versus resistance relationship for a typical thermistor;
FIG. 4 is a plot of the temperature versus output signal relationship for the thermistor of FIG. 3 with unamplified thermistor signal output shown in dotted lines and amplified thermistor signal output shown in solid lines; and
FIG. 5 is a schematic circuit representation of the fuser control arrangement of the present invention.
Referring particularly to FIGS. 1 and 2 of the drawings, an exemplary copier/reproduction machine, designated generally by the numeral 10 and incorporating the fuser control arrangement of the present invention, is there shown. As in all electrostatic systems such as the xerographic type machine illustrated, a light image of a document to be reproduced is projected onto the sensitized surface of a xerographic plate to form an electrostatic latent image thereon. Thereafter, the latent image is developed with an oppositely charged developing material to form a xerographic powder or toner image, corresponding to the latent image on the plate surface. The toner image is then electrostatically transferred to a support surface where it is fused by a fusing device so that the toner image is permanently adhered to the support surface.
In machine 10, an original document 12 to be copied is placed upon a transparent support platen 14 fixedly arranged in an illumination assembly generally indicated by the reference numeral 15 and disposed at the left end of the machine. While upon the platen, the document 12 is illuminated, thereby producing image rays corresponding to the informational areas on the original. The image rays are projected by means of an optical system onto the photosensitive surface of a xerographic plate. In the exemplary copier/reproduction machine 10, the xerographic plate is in the form of a flexible photoconductive belt 17 supported in a belt assembly 18.
The support assembly 18 for photoconductive belt 17 includes three rollers 20, 21 and 22 located with parallel axes at approximately the apices of a triangle. The upper roller 22 is rotatably supported on shaft 23 which in turn is rotatably driven by a suitable motor and drive means (not shown) to drive belt 17 in the direction shown by the arrow in FIG. 1. During this movement of the belt, the reflected light image of the original document 12 on platen 14 is flashed upon the photoreceptor surface of belt 17 at an exposure station 25 to produce an electrostatic latent image thereon.
The continued movement of photoconductive belt 17 carries the electrostatic image through a developing station 26 in which there is positioned a developer assembly generally indicated by the reference numeral 28. There the latent electrostatic image is developed by means of toner through the use of a multiple magnetic brush system 29.
The developed electrostatic image is carried by belt 17 to the transfer station 30 where the developed image is transferred to a support surface, normally a sheet of copy paper 31, brought forward between transfer roller 32 and belt 17. In order to accomplish transfer of the developed image solely by means of the electrical bias on transfer roller 32, the copy sheet 31 is moved at substantially the same speed as belt 17. A sheet transport mechanism generally indicated at 34 is provided to advance copy sheets 31 from a paper handling mechanism generally indicated by the reference numeral 35 to transfer station 30.
Following transfer, the copy sheet 31 is stripped from belt 17 and conveyed through fuser 38 wherein the toner image is permanently fused or affixed thereto. Following fusing, the finished copy is discharged into output tray 39.
Photoconductive belt 17 comprises a photoconductive layer of selenium, which is the light receiving surface and imaging medium for the apparatus, on a conductive backing. Further details regarding the structure of the belt assembly 12 and its relationship with the machine and support therefor may be found in the copending application Ser. No. 102,312, filed Dec. 29, 1970, and assigned to the same assignee.
Referring now to FIG. 2, fuser 38 includes a suitable housing 40 within which is disposed a lower heated fuser roll 41 and an upper pressure roll 42, rolls 41, 42 cooperating to form a nip through which the copy sheets 31 pass. Rolls 41, 42 are suitably supported for rotation and driven in unison by a suitable drive means (not shown). Pressure roll 42 is comprised of a relatively soft material such as Teflon, Neoprene, and the like with the result that pressure contact between the rolls 41, 42 deforms the surface of pressure roll 42. In this way, an increased contact arc between the copy sheet and the heated fuser roll 41 is obtained.
In the exemplary arrangement illustrated, fuser roll 41 is hollow, the roll 41 being formed from a suitable heat conductive material. A source of heat such as lamp 44 is disposed therewithin. A suitable temperature variable resistor, i.e. thermistor 45 is supported on a fuser housing 40 in heat exchange relation therewith to sense temperature conditions within the fuser housing 40.
Referring to FIGS. 3 and 4, where resistance vs. temperature and output voltage vs. temperature relationships for a typical thermistor are plotted, the latter for both unamplified and amplified voltage outputs of thermistor 45, it will be seen that the change in thermistor resistance becomes progressively less and less with increasing temperatures. In the relatively high operating temperature range of fuser 38, which typically extends from a minimum operating temperature of 350° F (T 1 ) to a normal operating temperature of 430° F (T 2 ), the change in thermistor resistance, R 1 to R 2 is relatively small. The unamplified signal output of thermistor 45 in this temperature range, v 1 to v 2 , is correspondingly small, a signal output of approximately two volts being typical. This relatively small change in signal output, which covers the operating temperature range of fuser 38, renders accurate control of fuser temperatures difficult to obtain. Where, as in the case of electrostatic copying machines, fuser temperatures are critical if legible and permanent copy is to be obtained, accurate control of fuser temperatures is essential, and relatively expensive and sophisticated comparator circuits would normally be required to obtain the necessary control accuracy.
To avoid the need for expensive and complex comparator type circuits, the signal output of thermistor 45 across the operating temperature range of fuser 38 is amplified. This in turn enables relatively simple and inexpensive switches 55, 56, 57 to be used as control devices instead.
Referring to the circuit diagram of FIG. 5, the signal output of thermistor 45 is fed to a suitable operational amplifier 50. Amplifier 50 is gated to amplify thermistor signals equal to or less than v 1 by means of a suitable source 51 of d.c. voltage. The signal amplification accordingly takes place at fuser temperatures equal to or above minimum fuser operating temperature T 1 . The amplified signal from amplifier 50 is impressed on the control gate of switching amplifiers 55, 56, 57. As will appear, switching amplifiers 55, 56, 57 control fuser heating and operation of copying machine 10. The switching amplifiers 55, 56, 57 are each biased to a suitable control level by voltage source 51, various resistor combinations 58, 59 being provided to obtain the different individual switching responses from each of the switching amplifiers 55, 56, 57 as required to provide the requisite copying machine control.
Switching amplifier 55 controls operation of fuser heating lamp 44. For purposes of illustration, the fuser lamp control consists of switch 60 in the power line to lamp 44 with the switch operating coil 60' in series with the output gate of amplifier 55. On triggering of switching amplifier 55 to a conducting state, coil 60 is energized to close switch 60 and complete the energizing circuit to fuser heat lamp 44.
Switching amplifiers 56, 57 control operation of copying machine 10, amplifier 56 serving to prevent operation of copying machine 10 on start-up until temperatures of fuser 38 reach a predetermined level. Schematically, a switch 63 is provided in the copying machine operating circuit, the operating coil 63' therefor being in series relationship with the output gate of amplifier 56. Switch 63 prevents or inhibits operation of the copying machine 10 until closed by coil 63' upon energization thereof through triggering of amplifier 56 to a conducting state.
Switching amplifier 57 serves to shut down copy machine 10 should fuser temperatures become too high as represented by temperature T 3 of FIGS. 3 and 4. Schematically, the output gate of amplifier 57 is series connected with the operating coil 67' for normally closed switch 67 in the operating circuit of copying machine 10. On triggering of amplifier 57 to a conducting state, coil 67' opens switch 67 to shut down copying machine 10.
On start-up of copying machine 10, which is effected by operator closure of a suitable start or PRINT control, schematically represented by switch 69, the temperature of fuser 38 may be below minimum fuser operating temperature T 1 . This normally occurs when copying machine 10 is unused for a relatively long period or on first start-up following an overnight shutdown. In this circumstance, resistance of thermistor 45 is relatively high so that the signal output of thermistor 45 to amplifier 50 is relatively high. Since the thermistor signal output is greater than v 1 , the voltage for which amplifier 50 is gated to conduct, amplifier 50 is held in a non-conducting condition. Since switching amplifiers 55, 56, 57 are set to respond to predetermined signal inputs from amplifier 50, amplifiers 55, 56, 57 remain in their quiescent state. In this situation, switching amplifier 55 is in a conducting state while amplifiers 56, 57 are in a blocking state. The signal from amplifier 55 energizes coil 60' to close switch 60 and energize fuser heating lamp 44. Lamp 44, in turn, heats fuser roll 41.
Switch 63, which is closed only on triggering of switching amplifier 56, is open thereby preventing operation of copying machine 10. With amplifier 57 in a blocking condition, switch 67 is closed.
With operation of heating lamp 44, temperatures within fuser 38 rise. This results in a decrease in resistance of thermistor 45. On fuser 38 reaching the temperature range of interest, which is normally the minimum operating temperature T 1 , the unamplified signal output of thermistor 45 equals v 1 and amplifier 50 is switched to a conducting state. The signal V 1 , which appears at the output gate of amplifier 50 and which represents an amplified version of the output signal generated by thermistor 45 in response to temperatures of fuser 38, is impressed on the control gates of switching amplifiers 55, 56, 57. The bias level on amplifiers 55, 57 as determined by the values of their individual resistor combinations 58, 59 hold amplifiers 55, 57 in their quiescent state. However, the resistor combination 58, 59 for switching amplifier 56 is chosen so that the amplified signal V 1 from amplifier 50, representing attainment of the minimum operating temperature T 1 in fuser 38, switches amplifier 56 to a conducting state. With switching of amplifier to a conducting state, relay coil 63' is energized to close switch 63 and enable operation of copying machine 10.
Fuser lamp 44 continues to heat fuser 38, the rise in fuser temperature being accompanied by a continued decrease in resistance of thermistor 45 and hence a decrease in the amplified signal output from amplifier 50. On fuser 38 reaching normal fuser operating temperature T 2 , the signal output V 2 from amplifier 50 switches switching amplifier 55 to a non-conducting state. The resultant deenergization of relay coil 60' opens switch 60 to interrupt power to fuser lamp 44 and terminate heating of fuser 38.
It is understood that with fuser lamp 44 off, temperatures in fuser 38 decrease. Following a small temperature decrease, as sensed by thermistor 45, the signal from amplifier 50 switches switching amplifier 55 to a conducting state to actuate fuser lamp 44 and heat fuser 38. Thus, while copying machine 10 is operational, amplifier switches between a conducting and non-conducting state in response to changes in fuser temperatures as sensed by thermistor 45 to maintain fuser 38 at approximately temperature T 2 . It is understood that if fuser temperatures fall below the minimum operative temperature T 1 , switching amplifier 56 switches to a non-conducting state to open switch 63 and prevent further operation of copying machine 10.
Should temperatures of fuser 38 rise above the normal fuser operating temperature, T 2 , to the maximum safe temperature T 3 , the amplified signal output V 3 of thermistor 45 switches amplifier 57, to a conducting state. The signal from amplifier 57 energizes coil 67' to open switch 67 and shut down copying machine 10.
In order to protect fuser 38 from over-heating in the event thermistor 45 fails, in which event resistance thereof would become extremely high and switching amplifier 55 would normally be held in a conducting state with fuser heating lamp 44 energized, a second control gate circuit to amplifier 50 incorporating transistor 70 is provided. The base of transistor 70 is connected through adjustable resistor 71 to the output side of thermistor 45 while the emitter of transistor 70 is connected with a control gate of amplifier 70.
During normal operation of thermistor 45, transistor 70 is conductive to provide a gate signal to amplifier 70. In the event that the resistance of thermistor 45 exceeds a predetermined value indicating a failure or malfunction of thermistor 45, transistor 70 is switched to a non-conducting state. This in turn causes the output voltage of amplifier 50 to drop to a low level. The low level output from amplifier 50 on such failure or malfunction of thermistor 45, which is below the triggering signal V 3 of switching amplifier 57, triggers amplifier 57 to a conducting state thereby energizing relay coil 67' and opening switch 67. Opening of switch 67 shuts down the copying machine 10 as described heretofore. A suitable delay circuit 72 is provided to prevent premature triggering of switching amplifier 57 when fuser 38 is first warmed up.
If desired, switch 67 may comprise a manually resettable type switch requiring, once opened, manual reclosing before operation of copying machine 10 can be resumed. In addition, triggering of switching amplifier 57 may also serve to actuate an alarm or signal to alert the operator to the fuser over-temperature condition.
While the invention has been described with reference to the structure disclosed, it is not confined to the details set forth, but is intended to cover such modifications or changes as may come within the scope of the following claims.