Note: Descriptions are shown in the official language in which they were submitted.
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TEMPERATURE SENSING DEVICE
BACKGROUND OF THE INVENTION
The present invention relates to a temperature
sensing device which has a sensing element located in
its interior, the outward surface of which is shaped
in accordance with the shape of the surface whose
temperature is to be measured.
In various technical fields it is necessary
to measure the surface temperature of rolls, rollers
or of other curved or flat surfaces.
Temperature sensing devices are, e.g., known from
the fixing stations of electrophotographic copiers, and
are, e.g., described in U.S. Patent No. 3,888,622.
Temperature sensing devices of that type are constituent
parts of temperature measuring circuits for determining
and controlling the surface temperatures of the fixing
rolls. Among other elements, such a measuring circuit
comprises a probe shoe containing at least one tem-
perature sensing element, the probe being arranged
close to the moving surface of the heated fixing roll,
but without touching it. Within the air gap between
the heated fixing roll and the probe shoe, an induction
field is produced and a magnetic, preferably fluid
medium is introduced into this gap. This medium pos-
sesses a relatively high coefficient of thermal con-
ductivity, whereby a rapid and effective heat flow
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from the surface to be measured to the temperature
sensing element is obtained.
The temperature sensing elements are arranged
in the interior of the cylindrical probe shoe made of a
non-magnetic material. The probe shoe is fastened to a
free end of a support arm which is also made of a non-
magnetic material, such as, e~g., aluminum, plastic
or the like. The other end of the support arm is
fastened to the frame of the apparatus. In the back
side of the support arm, i~e., opposed to the fixing
roll, a cut out is provided in which a circular magnet
is installed which is divided into two halv~s along a
vertical centerline. Each half of the magnet comprises
a number ~f north and south pole pieces, with opposite
pole pieces belng located adjacent to each other.
There is no direct contact between the sur-
face to be measured of the fixing roll and the probe
shoe containing the temperature sensing elements.
The toner fixing device known from German
Offenlegungsschrift No 2,420,161 comprises a temperature
measurin~ device for measuring the surface temperature
of a roll and for controlling the heating of the roll.
This temperature measuring device is composed of a
photosensitive cell with fiber optics which is arranged
close to the fixing roll, and of a light source illumi-
nating a fluorescent strip on the circumferential surface
of the fixing roll. The intensity of the light emitted
by the fluorescent strip is in indirect proportional
ratio to the temperature of the strip, i.e., to the
temperature of the surface of the fixing roll. The
light is led to the photosensitive cell via the fiber
optics. The output signal of the cell is amplified and
led to a control unit controlling the current supply of
a quartz lamp whichproduces the fusing of the toner on
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a fusing roll of the fixing device.
In this temperature measuring unit, too, there is no
direct contact between the temperature sensing element and
the surface to be measured. This device is quite complicated,
however, since a photosensitive cell with fiber optics and a
light source for illuminating the fluorescent strip on the surface
of the fixing roll to be measured are required, and it is rather
accident-sensitive, because the fiber optics is easily contam-
inated by toner which then causes failure of the temperature
measuring device.
SUMMAR~ OF THE INVENTION
It is therefore an object of the present invention to
provide an improved temperature sensing device.
Another object of the invention resides in providing
an improved temperature sensing device which over a longer period
of time is maintenance-free and makes possible a rapid measurement
of the temperature of a moving surface.
It is also an object of the invention to provide such
a temperature sensing device which is technically uncomplicated,
can be inexpensively built and is relatively free of wear.
In accomplishing the foregoing objects, there has been
provided in accordance with the present invention a temperature
sensing element for sensing the temperature of a moving external
surface, comprising: a contact body of a self-lubricating
material having an exterior contact surface shaped to conform
to the surface whose temperature is to be sensed, said contact
body having an internal recess therein; a temperature sensing
element positioned in the internal recess of said contact body;
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a cast body of a high-temperature resistant casting material,
said cast body being firmly bonded to said contact body of self-
lubricating material; a contact area between said contact body
and said cast body having at least one blind bore into which
a reinforcing metal sleeve is inserted; a supportin~ bracket
having at least one end inserted into said metal sleeve; a
supporting member running parallel to the surface to be measured,
said bracket being attached to said supporting member; and
biasing means cooperating with a pin and connected to said
supporting member for exerting a torque on said supporti.ng
member for pressing the contact surface against the surface to
be measured. In one preferred embodiment, the contact body is
comprised of graphite, the temperature sensing element is sealed
in a glass pearl which is inserted into the internal recess of
the contact body, and the internal recess is lined with a
heat-conductive paste to ensure a good thermal contact between
the contact body and the glass pearl containing the temperature
sensing element,
Further objects, features and advantages of the
present invention will become apparent from the detailed
description of preferred embodiments which follows.
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'BRIEF DESCRIPTION OF THE;D~A~XNGS
In the drawings:
Figure 1 is a sectional view of the temperatu,re
sensing device according to the present invention;
Figure 2 is a side view of the temperature
sensing device, seen in the direction of the arrow A
in Figure l;
Figure 3 is a schematic view, partly in section,-
illustrating the mounting of the temperature sensing de-
vice of the invention by means o~ a bracket and a support
pipe; and
Figure 4 is a schematic view of the arrangement
of the temperature sensing device contacting the surface
whose temperature is to be measured.
DETAILED DESCRIPTION OF PREFE~RED EMBODIMENTS
The invention pertains to a temperature sensing
device of the type described in the introduction of the
application, which is composed of a body of a self-
lubricati,ng material having an outward surface which
serves as the contact surface, and of a cast body of
a high-temperature resistant casting material, which
after casting and setting is firmly bonded to the body
of self-lubricating material. The body of self-lubri-
cating material serves as the fixture for the sensing
element.
In one embodiment of the invention, the sensing
element is sealed in a glass pearl which has been in-
serted into a bore of the body of a self-lubricating
material. This bore contains a heat-conductive paste
ensuring a good thermal contact between the body of
self-lubricating material and the glass pearl in which
the sensing element is sealed~ Thus a rapid temperature
measurement is made possible, since the temperature
present at the contact surface of the body of self-
lubricating material is led, via the heat-conductive
paste, through the body of self-lubricating material and
to the sensing element in the glass pearl with practically
no delay. It is advisable to connect the sensing ele-
ment to connecting wires which are embedded in the cast
body. Via these connecting wires, the ends of which
project from the cast body, the sensing element is
connected to an electrical circuit containing, inter
alia, an amplifier for amplifying the output signal of
the sensing element to the strength required.
In a further embodiment of this invention,
wired parallel to the sensing elements are circuit ele-
ments which are connected to the connecting wires and
embedded in the cast body~ In the area of the contact
surface between the cast body and the body of self-
lubricating material, bores are provided in the body
of self-lubricating material which are reinforced by
means of metal sleeves. Into these bores, the ends
of a bracket are introduced which is supported in a
pipe, parallel to the surface whose temperature is to
be measured. This embodiment makes it possible to
pivot the temperature sensing device around the bracket,
in order to bring the contact surface of the temperature
sensing device into the correct position with respect
to the surface to be measured. The sensing element
preferably is a negative-temperature coefficient re-
sistor. The casting material used may, e.g., be sili-
cone rubber.
The present invention will be explained in
more detail by way of an exemplary embodiment in the
attached figures of drawing.
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The temperature sensing device 1 which is
schematically shown in Figure 1 is composed o~ a body 2
of a self-lubricating material and of a cast body 5.
Graphite is preferably used as the self-lubricating
material for the body 2, but it is also possible to
use a sintered material containing finely divided
graphite particles, intercalated polytetrafluoro-
ethylene or an alloy of soft metals, such as, e.g.,
bronze. In the body 2 of self-lubricating material,
a bore 9 is provided into which a glass pearl 7 is
inserted having its upper end extending into the
cast body 5~ A sensing element 6, e.g., a negative-
temperature coefficient resistor, is sealed in the
glass pearl 7. Any sensing element causing either a
current change or a voltage change as a reaction to
temperature variations detected by the temperature
sensing device 1 can be used as the sensing element
in the present invention.
The outward surface of the body 2 forms a
contact surface 10, the shape of which is adapted
to the surface whose te~perature is to be measured,
as shown in Figures 2 and 4.
The body 2 of a self-lubricating material
serves as the fixture for the sensing element 6. In
the bore 9, a heat-conductive paste 4 is contained
which ensures a good thermal contact between the
body 2 and the glass pearl 7 in which the sensing
element 6 i5 included. Connecting wires 11 and 12,
which are embedded in the cast body 5, lead from the
sensing element 6 to the outside. It is advisable to
form the ends of these connecting wires 11 and 12 as
plug wires. Thus a simple plug-and-socket connection
can be created between the temperature sensing device 1
and an electronic circuit for the further processing
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of the electric output signals of the sensing element 6.
This electronîc circuit is conventional and is not
shown in the figures of drawing.
T~e cast body 5 is made of a high-temperature
resistant casting mass, such as, e.g., a silicone rubber
or a similar material having a thermal conductivity
of less than a.5 W m l K l.
The thermal conductivity of the casting mass
for the body 5 is low compared with the thermal con-
ductivity of the materials used for producing the body 2,the thermal conductivity of the latter varying between
about 4 and 250 W m l K l
Due to the fact that the body 2 possesses a
large contact surface lO and a small massl an excellent
thermal contact is made possible between the temperature
sensing device l and the moving surface 17 (Figure 4)
whose temperature is to be measured, i.e., the tempera-
ture chan~es of the moving surface 17 are registered
with practically no delay. Since the thermal conducti-
vity of the cast body 5, or respectively of the castingmass used for its production, is low, the cast body S
hardly contributes to the undesired, detrimental total
thermal capacitance of the temperature sensing device l,
and thus its influence on the temperature measurement
is kept very small.
Due to the self-lubricating properties of the
body 2, and, in the preferred embodiment, especially
to the anti-friction proQerties of graphite, it is not
necessary to apply a special anti-friction layer onto
this body 2, which would be necessary if a conventional
metal body was used as the fixture for the sensing ele-
ment 6. Another disadvantage which can be avoided by
using a self-lubricating material is the fact that,
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becau~e of the high temperatures of about 180C to
200C to be measured, only very few plastic materials
can be used for the anti-friction layer~ Such layers
are subjected to strong wear, which leads to their
rapid destruction, for they must be very thin in order
not to impede the heat transfer too much.
In Figure 1, horizontal kores 13 in the kody 2 are shown which
in part are located in the border area between the body
2 of self-lubricating material and the cast body 5.
In those cases where the self-lubricating material
chosen for the body 2 is graphite, reinforcing sleeves 3
are inserted into these bores 13. As shown in Figure 3,
the metal sleeves 3 receive the ends of a bracket 14
which is installed in a support pipe 15. Bores 18, 18'
and 19, 19' are provided in the support pipe 15l
through which the arms of the bracket 14 are led.
The ends of the support pipe 15 are fastened to the
frame of the apparatus where the temperature device 1
is installed. This apparatus,such as a photocopy
machine, i8 not shown in any further detail.
As can be seen from Figure 4, the support
pipe 15 runs parallel to the surface 17 whose tempera-
ture is to be measured, which may be a metal, a
sintered material or a plastic-coated surface.
The support pipe 15 is connected with an
angular spring 16 which near its free end is pressed
against a pin 20. The spring 16 exerts a torque onto
the support pipe 15, whereby it is ensured that the
bracket 14 and thus the temperature sensing device 1
fastened to it is pressed against the surface 17.
By lifting the spring 16 off the pin 20, the tem-
perature sensing device 1 can easily be removed from
the surface 17, so that the temperature sensing device 1
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can be pivoted a~ound the support pipe 15, as indicated
by the double arrow B in Figure 4~
Embedded in the cast body 5 are circuit ele-
ments 8(Figure l~, which are wired parallel to the
sensing element 6 and connected to the connecting
wires ll and 12. These circuit elements may be com-
posed of one or of several resistors ensuring that,
even when there is a high sensor resistance due to
an insufficient temperature of the surface 17, the
circuit (not shown) processing the signals coming
from the temperature sensing device 1 is not switched off.
On the other hand, this resistor or these resistors also ensure
that surface 17 is not overheated because, independent
from the surface temperature to be measured, at least
some current flow takes place through this resistor
or these resistors, as long as the line is not inter-
rupted, ~ithout this resistor or these resistors,
the circuit could not differentiate between a high
sensor resistance and a line interruption, i,e., an
infinite aensor re8istance, so that, in case of a line
interruption, there would be the danger of surface 17
overheating, and thus of a fire caused due to this
overheating.
Compared with known, non-contiguous methods
of temperature measurement by means of temperature
sensing devices requiring a high technical expense,
the contact temperature measuring device according to
the present invention shows little wear and delay.
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