Note: Descriptions are shown in the official language in which they were submitted.
:` - L~ .
,
.... . .
3~7
`
!
This invention relates to an improved heated pressure
-~ fusing system for use with an electrostatographic reproduction
0~
system and in particular to a closed loop control circuit for
maintaining constant temperature at the fusing nip under
varying operating conditions.
It has been recognized that one of the preferred
ways for fusing a powder image to a substrate is to bring the
- powder into direct contact with a hot surface, such as a
heated roller. The roller surface may be dry, i.e. no appli-
cation of a liquid release agent to the surace of that roller
- as described for example, in U. S. Patents 3,498,596 and
3,666,247. Alternatively, the fuser roll surface may be
coated with a release agent such as silicone oil as described
in U. S. Patents 3,268,351 and 3,256,002. Heated pressure
fusing systems are particularly well suited for high speed
and volume copier/duplicator systems. It has been found,
however, that higher temperatures and energy losses occur
under varying operating conditions which include changes in
ambient conditions, different copy volumes and/or speeds,
varying sheet weights, etc. As a result of these higher tem-
; peratures the sheets tend to curl and stick to the roll sur-
faces which is undesirable from the standpoint of sheet
handling and stripper finger wear on the roll surface due
to excessive paper forces.
In the prior art devices only a single sensing
device has been used to detect temperature at or near the nip
as described for example, in U. S. Patents 3,688,082 and
3,313,913 and British Patent 1,316,616. While these devices
are satisfactory in a sense, they have certain disadvantages
as already mentioned above.
- ~-
- ~
3~7
The present invention is an improved heated pressure ;
fusing system which enables a constant temperature to be
maintained at the fusing nip under varying load conditions.
Thus, in accordance with the present teachings, a
fusing system is provided with a first and second fuser -
roller formung a nip and an apparatus for maintaining a
substantially constant temperature at the nip between the
rollers. A first sensor is provided for detecting the
temperature of the first roller-surface and a second sensor -
. . .
being provided to detect the temperature of the second roller
surface. The first and second sensors producing first and
second output resistances respectfully and responsive to
the temperatures. A first circuit is provided connected
to the first and second output resistances to produce a
combined output resistance indicative of the temperature -~
at the nip. A second circuit is provided coupled with
the first circuit and receiving input voltage signals
representative of the combined output resistance and input
voltage signals representative of a reference set point
indicative of the desired nip temperature to produce electrical
output signals to the heating means. The electrical ouput
signal~vary with the difference of the input voltage signals
in response to the combined surface temperature of the rollers
to maintain a substantially constant temperature at the nip.
''
Figure 1 illustrates schematically a xerographic
reprod~lcing apparatus incorporating a heated pressure fuser
roll apparatus constructed in accordance with the present
q.~,~ -
L
3'~7
.,, .
invention;
Figure 2 is a graphic illustration illustrating
fusing temperatures and their effect on offset and fusing
-- quality;
Figure 3 is a block diagram illustrating details
of the present invention; and
Figure 4 is a circuit diagram according to the
present invention.
Referring now to the drawings there is shown in
Figure 1 an embodiment of the fusing system of the subject
invention in a suitable environment such as an automatic
xerographic reproducing machine. The automatic xerographic
reproducing machine includes a xerographic plate or surface
- 10 formed in the shape of a drum. The plate has a photoconduc-
tive layer or light receiving surface on a conductive backing,
journaled in a frame to rotate in the direction indicated
.; . ~. ~ .. .
by the arrow. The rotation will casue the plate surface to
sequentially pass a series of xerographic processing stations.
For the purpose of the present disclosure the several xero-
? graphic processing stations in the path of movement of the
plate surface may be described functionally as follows:
A charging station A, at whi~h a uniform electro-
static chaxge is deposited on the photoconductive platel
An exposure station B, at which light or a radiation
pattern of copies to be reproduced is p~ojected onto the plate
surface to dissipate the charge in the ~xposed areas thereof
to thereby form a latent electrostatic image of the copy to
be reproduced;
A developing station C, at wh~ch xerographic develop-
ing material, including toner particles ~aving an e~ectrostatic
--4--
. ~L r ~ 7
charge opposite that of the latent electrostatic image, is
cascaded over the latent electrostatic image to form a toner
powder image in configuration of the copy being reproduced;
A transfer station D at which the toner powder image
is electrostatically transferred from the plate surface to a
trans~er material or a support surface;
A drum cleaning and discharge station E at which
the plate surface is wiped to remove residual toner particles
remaining thereon after image transfer and at which the plate
is exposed to a relatively bright light source to effect
substantially complete discharge of any residual electrostatic
charge remaining thereon; and
- A fusing station F at which the powder image is
permanently affixed to the support material to produce a high
- 15 quality print with a minimum of offset in accordance with the
invention as will be explained more fully hereinafter.
_ Referring now to Figure 2 there is shown a graphic
display illustrating fuser roll temperature on a horizontal
axis and pressure or backup roll temperature on a vertical
axis. It will be appreciated that when the temperature is
in the range of over 370F for the fuser roll and over 250F
for the backup or pressure roll there is difficulty in avoiding
offset on the rolls. Also where temperatures are low, for
- example, the fuser temperature is below 370F and temperature
of pressure or backup roll is below 150F that there is
difficulty in obtaining high quality fixes. The rectangular
block area or zone H depicts the range for maintaining the
temperature of the fuser roll ~f offset is to be minimal and
fusing quality is to be high during the operation of a present
or single temperature sensing device. As copies are run on
--5--,
I :
.. - ..
~':J4~3U7
the machine the pressure roll temperature will continue to
rise until offsetting occurs. Block area or zone I shows
the parameters for operation with the present invention. As
copies are produced on the machine the pressure roll temperature
will rise. This will cause the fuser roll temperature to drop.
The dropping fuser roll temperature will stablize the pressure
roll temperature. The operation will insure that the tempera-
ture in the nip is always within the fusing window, i.e.,
greater than minimum and less than offset temperature regard-
less of varying ambient conditions, copying volume and/or
speed, and sheet stock.
In accordance with the present invention a constant
temeprature at the nip or contact arc is maintained to effect
a minimum of offset while maintaining high quallty image
fusing. As best shown in Figure 3 the fusing roll surface
is thermally sens~ by a sensor 201 and the pressure or backup
roll is thermally sensed by a sensor 205. An input 210 from
sensor 201 and an input 211 from sensor 205 is combined in a
sensing combining network 215. Desirably the sensing combining
network 215 is a parallel circuit such that an output signal
- 220 is a result of the combined effect of the temperatures
of the fusing rolls and pressure roll surfaces. This output
220 normally is the sum of the reciprocals of the two sensors
201 and 205. The signal 220 is then supplied to a control
circuit 222 which becomes output signal 230 in the form of
discrete on-off electrical signals which are supplied to one
- or more heating elements 235 which supplies heat to the nip
of the fuser roll. These heat pulses may be supplied to any
suitable electrical heating element such as a lamp or the like.
It has been found that the temperature of the pressure or
--6--
.. -. ~ . ..
~r~ 3~7
. . .
backup roll is sufficiently high that this will serve as
feedback to the control for the fusing nip enabling a greater
- latitude of temperature to be used for the heating element
- associated with the fuser rolls as best shown by zone I in
Figure 2. Typically, it has been found that the fuser roll
may range in temperature from 380 F to 310F when the pressure
roll temperature ranges from 150F to 290F, respectively.
The operation of the invention may be best understood
in connection with the circuit of Figure 4. This circuit is
designed to give a high output of operational amplifier 250,
when the combined resistance of sensors 201 and 205 is greater
than a predetermined set point. When the combined resistance
drops below this set point the operational amplifier 250
gives a low output which will de-energize heating elements
235. The system is designed to control the temperature about
the set point adding heat when the resitance goes above and
_ _ allowing the system to cbol when the resistance goes below.
In the machine standby mode when both sensors 201 and 205 are
cold or the combined sensor re stance is greater than the set
point, the output of amplifier 250 will go high causing the
heating elements to turn on. Since the pressure roll is still
cold its sensor 205 has a very high resistance and has a
negligible effect on the set point. The heating elements
will continue to oscillate as the fuser roll sensor 201
o~cillates about the set point.
When copies are being produced the pressure roll
comes into contact with the fuser roll to provide a nip for
fusing. The pressure roll will start heating up causing the
pressure roll sensor 205 resistance t~ decrease. Since the
combined resistance is controlled at ~e same set point,
~43~7
wllen the pressure roll sensor 205 resistance decreases it
forces the fuser roll sensor 201 resistance to increase in
order to maintain the combined resistance constant. An
increase in fuser roll sensor 201 resistance will cause the
fuser roll temperature to drop. Since the heating elements
are located inside the fuser roll, equilibrium will be reached
when both the fuser roll and pressure roll temperatures
stablize resulting in a constant temperature in the nip.
' By this system it has been found that a constant
temperature maintained at the fuser nip insures high quality
fixes without undesirable offset on the surface of the fuser
roll. Moreover, it will be appreciated that with a closed
loop system there is a conservation of energy for producing
heat at the fusing nip. It will be further appreciated that
with the constant temperature at the fusing nip that the
pa,per will not curl and that high quality fixes will be
maintained at varying copying volumes and operating conditions.
While there have been shown and described and pointed
out the fundamental novel features of the invention as applied
to a preferred embodiment, it will be understood that various
~ .
omission and substitutions and changes in the form and details
- of the device illustrated and in its operation may be made
by those skilled in the art without departing from the spirit
of the invention.
., .
