Fusing station having release-oil level detector

A heat and pressure fusing station in an electrostatographic copier or printer includes a level detecting apparatus for detecting a low oil level condition in a release oil reservoir of such a fusing station. The level detecting apparatus includes a temperature responsive thermistor connected to a voltage divider and a voltage comparator circuit, and supported within the reservoir so as to be exposed and above the level of release oil in the reservoir when there is a low oil condition in the reservoir.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to fuser and pressure roller-type fusing stations 
for fusing toner images in an electrostatographic apparatus such as 
copiers and printers. More particularly, this invention relates to such a 
fusing station that includes a release-oil level detector. 
2. Description Relative to the Prior Art 
In electrostatographic apparatus such as copiers and printers, fuser and 
pressure roller-type fusing stations as disclosed for example in U.S. Pat. 
Nos. 4,870,445 and 4,870,446 issued Sep. 26, 1989 in the names of Collier 
et al. and Bickerstaff et al., respectively, are well known for fusing 
toner images on suitable receivers. Usually, the fuser roller of such a 
station is heated, and rotatably forms a fusing nip with an unheated 
pressure roller. A suitable receiver sheet with an unfused toner image 
thereon is fed through the fusing nip such that the heated fuser roller 
directly contacts and heats the toner image on the receiver sheet. 
A common problem associated with such fusing stations is that the toner 
particles which form the toner image, partially offset undesirably from 
the receiver sheet onto the surface of the fuser roller. As disclosed, for 
example, in the patents cited above, and in commonly assigned U.S. Pat. 
No. 4,994,862, issued Feb. 19, 1991 in the name of the present inventor 
Linn C. Hoover, such an offsetting problem can be prevented by the 
application of release oil to the surface of the fuser roller. 
Typically, a system for supplying and applying such release oil is mounted 
in a remote and blind location within a copier or printer, and includes a 
reservoir that can hold a substantial quantity of such oil. The oil is 
withdrawn from the reservoir by suitable means, and is used up by being 
applied to, and released from, the surface of the fuser roller. 
Eventually, however, the quantity of oil in the reservoir can all run out 
or be all used up, and if not replenished, can undesirably result in 
reoccurrence of the toner offsetting problem, and hence in poorly fused 
images from the fusing station. There is, therefore, a need for a simple 
and reliable release oil level detector for use in association with such a 
reservoir in order to prevent such a run out and reoccurrence of the toner 
offsetting problem. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a simple and reliable 
apparatus for detecting a low oil level condition in the release oil 
reservoir of an electrostatographic fusing station. 
It is another object of the present invention to provide such a level 
detecting apparatus that has relatively few parts. 
In accordance with the present invention, a release oil level detecting 
apparatus is provided for detecting a low oil level condition in a release 
oil reservoir of the release oil supply system of an electrostatographic 
fusing station. The level detecting apparatus includes first means which 
constitute a voltage divider circuit that comprises a power source with an 
input voltage R.sub.IN, an output voltage V.sub.OUT, a reference 
resistance R.sub.r, and a temperature responsive variable resistance means 
R.sub.T. The level detecting apparatus also includes a voltage comparator 
circuit means, and means for supporting the variable resistance means 
R.sub.T within the reservoir such that such means R.sub.T is above the 
level of release oil in the reservoir when a low oil level condition 
exists in the reservoir. The voltage comparator circuit means is connected 
to the voltage divider circuit means for sensing a sudden change in the 
output voltage V.sub.OUT of the voltage divider circuit means. The level 
detecting apparatus further includes alerting means which are connected to 
the voltage comparator circuit means for signaling the existence of such a 
low oil level condition in response to such a sensed change in the output 
voltage V.sub.OUT of the voltage divider circuit means.

DETAILED DESCRIPTION OF THE INVENTION 
Because fusing stations or apparatus are well known for use in 
electrostatographic process machines such as copiers and printers, the 
description of the present invention will be directed in particular to 
elements of such a fusing apparatus forming part of, or cooperating more 
directly with, the present invention. Elements of such a fusing station 
not specifically shown or described herein are ordinarily selectable from 
those known in the prior art. 
Referring now to the drawing, a fuser and pressure roller-type fusing 
apparatus or station, for use in an electrostatographic process machine 
such as a copier or printer, is shown generally as 10. Further details 
regarding such a copier or printer may be found, for example, in U.S. Pat. 
No. 4,791,450, the contents of which are incorporated herein with this 
reference. As shown, the fusing station or apparatus 10 includes a 
rotatable heated fuser roller 12, means 14, such as a quartz lamp for 
heating the fuser roller 12, and a rotatable pressure roller 16 which 
forms a fusing nip 18 with the fuser roller 12. As is well known, the 
fuser and pressure rollers 12 and 16, respectively, should have 
constructions suitable for producing clean, desirable fused images on a 
suitable receiver such as a copy sheet of paper 20. As is well known, the 
fuser roller 12 may also be heated externally by suitable means. The 
fusing apparatus 10, as such, can be used in an electrostatographic copier 
or printer to fuse unfused toner images 22 on the receiver 20 which is 
passed, for example, in the direction of arrow 24, through the fusing nip 
18. The receiver 20 is passed therethrough such that the heated fuser 
roller 12 directly contacts and heats the unfused toner images 22 
resulting in fused images 26. 
The quality of the resulting fused images 26 exiting the nip 18 depends in 
significant part on prevention of the toner particles, which form the 
images 22, from offsetting from the sheet 20 to the surface of the fuser 
roller 12 during such fusing. To prevent such offsetting of the toner 
particles, the fusing apparatus 10 includes a release oil supply and 
application apparatus or system 32. The system 32 contains a substantial 
supply of release oil, such as silicone oil, and applies such oil to the 
surface of the heated fuser roller 12. 
As shown, the system 32 includes a reservoir 38 for holding the release 
oil, a porous release oil application roll 36, and feed means 40 for 
automatically feeding the release oil from the reservoir 38 to the 
application roll 36. The reservoir 38, as shown, is connected by an oil 
feed tube 42 to the application roll 36. The feed tube 42, as such, is 
part of the feed means 40 and is made of a flexible or resilient material 
to allow it to be pinched. The release oil application roll 36, for 
example, is mounted fixedly on, and for rotation with, a rotatable shaft S 
such that the roll 36 frictionally contacts the surface of the fuser 
roller 12. Mounted as such, rotation of the fuser roller 12, for example 
by the copier's drive (not shown), frictionally drives the application 
roll 36, thereby causing the roll 36 to continuously apply release oil 
thereto. Additionally, such frictional rotation of the application roll 36 
also in turn rotates the shaft S which is fixedly connected to the roll 
36. 
As shown, the feed means 40 may include the feed tube 42, a one-way check 
valve 44, a cam driven pressure bar assembly 46, a relief valve 48, and a 
cam 50 cooperating with the pressure bar assembly 46. The cam driven 
pressure bar assembly 46 is mounted for contacting the feed tube 42 below 
the check valve 44. The assembly 46 includes a pressure bar 52 that has a 
tail or first end, and a large arcuate head 54 for directly contacting the 
feed tube 42. It also includes a stationary pressure plate 56, and a 
return spring 58 that is connected to the moving head 54 of the pressure 
bar 52. The pressure bar 52 is movable in the direction of the arrows 
shown and has a feed tube compressing position and a feed tube release 
position relative to the tube 42. The pressure plate 56 is mounted on the 
side of the feed tube 42 that is directly across from, and opposite the 
point of contact between the feed tube 42 and the head 54 of bar 52. The 
spring 58 functions to return the pressure bar 52 to the right in FIG. 1 
from its tube compressing position to its tube release position when the 
bar is released by cam 50. 
The cam 50, like the application roller 36, is also mounted fixedly on the 
shaft S for rotation therewith. As mounted, the cam 50 is in continuous 
rotating contact with the first or tail end of the pressure bar 52 which 
is biased against the cam 50 for intermittently driving the pressure bar 
52 from its tube release position into compressing contact with the feed 
tube 42. Such compressing and squeezing contact with the tube 42 pumps and 
feeds release oil within the tube to the application roll 36. Feeding of 
the oil from the reservoir 38, as such, or by other suitable means, causes 
the level of the oil therein to drop. 
In order to prevent the reoccurrence of the problem of toner particles 
offsetting from the sheet 20 to the fuser roller 12 when the system 32 
runs out of oil, the present invention provides a level detecting system 
or apparatus designated generally as 60. The apparatus 60 is useful for 
detecting a low oil level or (LOL) condition in the reservoir 38. As 
shown, the apparatus 60 includes first means shown as 62 which constitutes 
a voltage divider circuit. The means 62 includes a constant current power 
source, for example a 100 milliamp service, that has an input voltage 
V.sub.IN of 24 VDC for example, an output voltage V.sub.OUT, a fixed 
reference resistance R.sub.r for example of 155 ohms, and a temperature 
responsive variable resistance means R.sub.T such as a PTC thermistor 
having a shelf resistance of 50-150 ohms at 25.degree. C. (77.degree. F.). 
The level detecting apparatus 60 also includes a voltage comparator circuit 
means 63 and means 72 for supporting the variable resistance means R.sub.T 
within the reservoir 38 such that the thermistor R.sub.T is immersed in 
release oil 64 when there is sufficient oil in the reservoir but is above 
the level LOL of oil in the reservoir and exposed when a low oil level 
condition exists in the reservoir 38. The voltage comparator circuit means 
63 is connected to the voltage divider circuit means 62 for sensing a 
sudden change, for example a sudden drop, in the output voltage V.sub.OUT 
of the voltage divider circuit 62. Alerting means shown as 64 may be 
connected through an LCU (logic and control unit) to the voltage 
comparator circuit 63 for generating a signal signaling the existence of 
such a low oil level condition in response to the sudden drop in voltage 
output of the divider circuit. The alerting means 64 can be a sound alarm 
or a warning light on an operator control panel. In addition, besides 
actuating the alerting means, the LCU can be programmed to output a signal 
to shut down the entire electrostatographic reproduction apparatus for 
example after a predetermined number of copies, should the alerted low oil 
level condition remain uncorrected. 
As shown, the means 72 for supporting the thermistor R.sub.T within the 
reservoir 38 consists of a siphon tube and includes a cap 70 for capping 
an aperture, for example at the top thereof, into the inside of the 
reservoir. The siphon tube 72 is fitted through the cap 70 as shown, and 
has an output end 74 that lies above the cap 70 and is associated with the 
oil feed tube 42. The siphon tube 72 also has an intake end 76 that has an 
intake port 80 for taking up oil from the reservoir. The intake end 76, as 
shown, lies below the cap 70 for positioning the intake port 80 as near as 
possible to the bottom 82 of the inside of the reservoir 38. The length of 
the siphon tube 72 is such that when the cap 70 is properly assembled to 
the reservoir, the intake port 80 of the tube 72 will lie Just below the 
predetermined low oil level line LOL inside the reservoir. 
As further shown, the level detecting apparatus 60 includes electrically 
conductive traces 84, 86 which are formed into the walls of the tube 72, 
and of the cap 70. The traces 84, 86 have a first set of connecting means 
or terminals on the cap 70 and a second such set of connecting means or 
terminals at or near the intake end 74 of the tube 72. The conductive 
traces 84, 86 can be formed integrally with the siphon tube 72 and cap 70 
for example by the "Mold-n-Plate" (trademark of Kollmorgen Corporation) 
process used by the Pathtek Company of Rochester, N.Y. It is a 
3-dimensional process in which the traces are molded directly into the 
walls of the tube and cap 72, 70, respectively. 
In the apparatus 60, the first set of terminals to the traces 84, 86 are 
appropriately connected as by soldering to electrical wiring means 88A, 
88B, from the voltage divider and comparator circuits 62, 64, 
respectively. The second set of terminals of the traces 84, 86 near the 
intake port 80 are similarly connected appropriately as by soldering to 
the leads of the thermistor R.sub.T to complete the electrical connections 
of the apparatus 60. As supported and connected, the thermistor R.sub.T 
will be completely immersed in release oil when there is sufficient oil in 
the reservoir, but will be above the oil level and hence exposed to 
ambient temperature air above the oil level when the reservoir is in the 
predetermined low oil level (LOL) condition. 
The thermistor R.sub.T should be selected such that when a predetermined 
amount of electrical current is passed through such thermistor, such 
current will cause the thermistor to heat up, and such that if the heat is 
not dissipated, the heat will suddenly cause a significant increase in the 
resistance value of the thermistor. As selected, the thermistor R.sub.T 
should be such that when in the ambient air above the release oil level 
within the reservoir 38, the current therethrough will cause such 
undissipated heating thereof, and hence a sudden increase in its 
resistance value. The amount of current through the thermistor should 
additionally be determined such that when such a thermistor is completely 
immersed in the release oil, the release oil in contact therewith, will 
quickly dissipate any heat therefrom caused by the current, thus 
preventing any significant increase in its resistance value. 
For example, a successful level detecting apparatus 60 was built using a 
100 milliamp current source at 24 volt DC, a fixed reference resistance 
R.sub.r of 155 ohms, and a PTC thermistor R.sub.T having a shelf 
resistance of 85 ohms at 25.degree. C. (77.degree. F.). The maximum 
temperature of the thermistor R.sub.T was about 120.degree. F. when 
immersed in oil, and about 240.degree. F. when then exposed in air above 
the oil level. The circuit resistance of the thermistor R.sub.T at about 
240.degree. F. was estimated to be about five times its shelf resistance 
value. 
While the invention has been described with regard to a temperature 
responsive thermistor, those skilled in the art will recognize that other 
temperature responsive elements may be substituted for the thermistor, 
including transistors, diodes, etc. Detectors may detect changes in 
voltage, in current, or even the amount of light or color, for example of 
a light-emitting diode. 
As can be seen, the present invention provides a simple and reliable level 
detecting apparatus 60 for detecting a low oil level condition in the 
release oil reservoir 38. The apparatus 60 has relatively few parts and 
yet is effective in preventing reoccurrence of a toner offsetting problem 
in the fusing station of the present invention. 
The invention has been described in detail with particular reference to a 
presently preferred embodiment, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.