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Description  |
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TECHNICAL FIELD
The present invention relates to the field of temperature sensors, and,
more particularly, to optical fiber temperature sensors.
BACKGROUND ART
Electrically-based temperature sensors, such as thermocouples and resistive
temperature devices (RTD), are commonly used to measure temperature. There
are numerous applications in which the response time, accuracy or immunity
from electromagnetic interference (EMI) of these electrically-based
temperature sensors is inadequate. Fiber optic temperature sensors have
been developed with improved response time and accuracy, and the optical
fiber is inherently immune to EMI.
It is known to measure temperature with optical means sensitive to
thermally-generated optical radiation. The spectral radiance, L, of a
thermal radiation source is expressed in optical power (watts) per unit
area (square meters), per unit wavelength (meter) and per unit solid angle
(steradian). The relationship between spectral radiance, L, and
temperature, T, (on the Kelvin scale) of the emitting source is given by
the Planck equation:
##EQU1##
.epsilon.=emissivity .lambda.=wavelength in units of meters
C.sub.1 =3.741.times.10.sup.-16 watt meter.sup.2
C.sub.2 =0.01439 meter Kelvin
The emissivity, .epsilon., is the ratio of the emission efficiency of a
source to that of a perfectly-emitting source of the same shape.
Emissivity may vary with wavelength. To determine the optical power, P,
(in units of watts) collected by an optical fiber, it is necessary to
integrate the spectral radiance over the appropriate wavelength interval,
.DELTA..lambda.; the effective area, A; and acceptance angle, .omega., of
the optical fiber:
##EQU2##
where .theta. is the angle between the normals of the emitting and
receiving surfaces.
For ease of installation, it is known to use a light guide containing one
or more optical fibers to transmit the thermal radiation from the sensed
region to the optical detector, as shown, for example, in U.S. Pat. No.
1,894,109. In such an arrangement, it is necessary to know the emissivity
of the radiation source in order to determine the temperature. For most
materials, the emissivity varies with wavelength and also in time due to
such effects as oxidation, contamination and mechanical wear. Undetected
changes in emissivity introduce errors in optical temperature measurement
techniques.
A way to minimize the effects of emissivity changes is to locate an optical
cavity in the region for which the temperature is to be sensed. By
suitably choosing the shape of the cavity, a change in the emissivity of
the interior surface of the cavity will result in a
proportionately-smaller change in the effective emissivity of the cavity
aperture. When the emissivity of the cavity approaches unity, it is called
a blackbody cavity.
The use of blackbody cavities for optical temperature measurement is
described in U.S. Pat. Nos. 3,626,758 and 4,576,486. In the latter patent,
the blackbody cavity is formed by coating the end of single-crystal
optical fiber for a length 20 times the diameter of the fiber. The
variation is the effective emissivity of this cavity was less than 1% over
a range of test conditions. Accurate temperature measurements can be made
as long as the coating is not perturbed. In practice, the coating is
eroded in adverse environments. The coating thickness can be increased or
additional layers added for protection, but only with a corresponding
increase in the response time of the probe. A further limitation of the
coating technique is that it can be applied only to the core of the
optical fiber, whereas many optical fibers have an annular cladding
surrounding the core as an integral part of the fiber. A further
limitation of U.S. Pat. No. 4,576,486 is that the temperature range does
not extend below 500.degree. C.
Another example of an optical temperature sensor is disclosed in U.S. Pat.
No. 4,362,057. The thermal radiation from an optical fiber itself is
employed to make a temperature sensor. Even transparent materials such as
glass have a finite emissivity in the near-infrared, producing thermal
radiation above 100.degree. C. This self-generated radiation can be
detected at the remote end of the fiber and suitable electronics used to
convert an optical signal level to a temperature. This approach is
applicable to the measurement of temperature over a region having
dimensions of inches or feet, but is not suitable for smaller regions.
Improvements to the ruggedness, response time, usable type of optical
fibers, temperature range, and spatial resolution of optical temperature
sensors, achieved by the present invention, renders them capable of
exploitation for temperature sensing and measurement in such industrial
applications as combustion systems, petroleum and chemical processing, and
locations with high electromagnetic fields.
DISCLOSURE OF INVENTION
According to the present invention, the temperature sensor apparatus
comprises an optical temperature probe and a control module. The optical
temperature probe contains an optical fiber with a tip end located in the
temperature environment to be sensed and a remote end connected to a
control module. The optical fiber in the vicinity of said tip end is made
to have a recess containing a thermally-emissive surface. The emissive
surface generates optical radiation in proportion to its temperature and
the control module has the means to convert the measured optical power to
a signal proportional to temperature.
An important feature of the present invention is that the emissive source
is located within the optical fiber. Thus, the fiber itself protects the
emissive surface and thereby maintains the emissivity of the surface at a
fixed value. By using the probe configuration of the present inventive
apparatus to keep the emissivity constant over time, it is not necessary
to form a blackbody cavity.
The thermally-emissive surface is positioned near the tip end of the
optical fiber by drilling or otherwise creating a recess near the end of
the fiber. The thermal-emissivity characteristic of this surface must be
greater than the thermal-emissivity characteristic of the optical fiber.
The emissive surface can be prepared by increasing the emissivity of the
optical fiber material, for example, by roughening. Alternatively, the
emissivity of the recess region can be increased by bringing a different
material in contact with the surface of the recess. For example, the
recess can be coated with a thin film, or the recess filled with an
insert. The materials used in the recess are selected to be compatible
with the temperature range and environment in which the probe is to be
used.
Glass is usable as an optical fiber at temperatures up to 600.degree. C.
and is resistive to many chemicals. Quartz optical fibers are similar to
glass in chemical resistance and can be used up to 1000.degree. C.
Sapphire is an optical material with an operating temperature range up to
2000.degree. C., and is well known for its hardness and resistance to
chemical attack. For an optical fiber having a core-and-cladding
construction, the recess, if axial, should be smaller than the central
core of the fiber. The thermal emission generated at the surface of the
recess and within the acceptance angle of the core will be propagated down
the fiber.
The control module employs techniques similar to those described in the
literature such as, for example, U.S. Pat. Nos. 4,362,057 and 4,576,486.
Optical power detectors made from materials such as silicon, germanium,
aluminum-gallium arsenide or lead sulfide can be selected, based on their
sensitivity to the spectral distribution of optical power at the desired
temperature range. The electric voltage produced by the optical power
detector is measured and the control module includes the means to convert
the optical signal level to an output proportional to temperature.
Accordingly, the general object of the present invention is to provide an
improved optical fiber temperature sensor which has fast response time and
simultaneously is rugged enough to withstand hostile environments.
It is a further object to configure the thermal emission source for use
with either optical fiber made from a single-crystal, such as sapphire, or
an optical fiber having a core-and-cladding construction made from glass
or quartz.
It is a further object to localize to a small region the volume in which
the temperature is sensed.
It is a further object to allow the probe to be made of all dielectric
materials so that the probe may be used near high voltage equipment and in
areas containing strong electromagnetic fields.
It is a further object to make the sensor cost effective, easy to
manufacture, and capable of measuring temperature in the range of from
175.degree. C. to 2000.degree. C.
Other objects and advantages of the present invention will be apparent from
the foregoing and ensuing detailed description, the drawings, and the
appended claims.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic representation of an optical fiber temperature sensor
apparatus embodying the present invention associated with a temperature
environment to be sensed.
FIG. 2 is an enlarged fragmentary perspective view of the
temperature-sensing tip end portion of the optical fiber shown in FIG. 1,
partly in section, and showing a thermally-emissive surface provided by an
axial recess at such tip end.
FIG. 3 is a view similar to FIG. 2, showing another form of
thermally-emissive surface provided by a radial recess at the tip end of
the optical fiber.
FIG. 4 is a view similar to FIG. 2, showing the wall of the recess
roughened.
FIG. 5 is a view similar to FIG. 2, showing the recess coated with a thin
film.
FIG. 6 is a view similar to FIG. 2, showing the recess filled with an
insert.
FIG. 7 is one species of sensor apparatus depicted in FIG. 1, partly in
section, and showing the optical fiber as a single filament.
FIG. 8 is another species of sensor apparatus depicted in FIG. 1, partly in
section, and showing the optical fiber as having a core-and-cladding.
FIG. 9 is still another species of sensor apparatus depicted in FIG. 1,
partly in section, and showing the optical fiber element as including a
high-temperature section optically coupled to a low-temperature section.
FIG. 10 is a view similar to FIG. 7, and showing details, schematically, of
the control module.
FIG. 11 is a depiction of a sapphire optical fiber connected to a flexible
optical fiber by a commercial form of connector, illustrated partly in
section.
FIG. 12 is a side elevational view, partly in section, of the sapphire
optical fiber shown in FIG. 11, in which the tip end is provided with an
axial recess having the dimensions indicated, and filled with a zirconia
powder insert.
FIG. 13 is a graph plotting temperature sensed by the sapphire optical
fiber shown in FIG. 12, versus volts produced by a germanium detector.
MODE(S) FOR CARRYING OUT THE INVENTION
Embodiments of the optical fiber temperature sensor apparatus, with its
species variations, are illustrated in the drawings.
Referring to FIG. 1, the apparatus is shown as comprising a probe 20 and a
control module 21. The probe includes an optical fiber element 22 having a
tip end portion 23 for insertion into a region where temperature is to be
detected, such region being indicated in FIG. 1 by broken lines and
represented generally by the numeral 24. End portion 23 has an end face or
tip 25. The opposite or remote end 26 of optical fiber element 22 is
operatively connected to control module 21, as will be explained later
herein. Between fiber ends 25 and 26, the optical fiber element 22
provides a radiant energy transmission path.
Within region 24, the tip end portion 23 of the optical fiber element 22
includes means providing a thermally-emissive surface, represented
generally by the numeral 28 in FIG. 1, which is internal to the
aforementioned radiant energy path. This emissive surface 28 has a
thermal-emissivity characteristic higher than that of the optical fiber
element 22, and generates thermal radiation in proportion to the
temperature of such emitting surface. This radiation is transmitted along
a path within optical fiber element 22 from tip end 23 to the control
module 21 located at remote end 26. Control module 21 includes the means
to measure the optical power and produce an output proportional to the
temperature of the emissive surface 28. This output may be an analog
electrical signal, a digital electrical signal or a visual temperature
display, any of which is well understood by those skilled in the art.
Illustrative configurations of the temperature probe tip end portion 23 are
shown In FIGS. 2 and 3. In FIG. 2, emissive surface 28 is shown as
provided by the peripheral wall of an axially-disposed, central,
cylindrical recess 28a opening to the tip or end face 25 of the optical
fiber element. Alternatively, surface 28 may be provided by the peripheral
wall of a radially-disposed cylindrical recess 28b, as shown in FIG. 3. In
either case, the surface forming the boundary defining the wall of the
recess 28a or 28b is made to have a higher thermal emissivity than the
optical fiber element 22. It is to be understood that the recess providing
the emissive surface 28 may be other than cylindrical, such as conical.
The emissive surface 28 can be provided by roughening the surface of the
recess so as to have a higher emissivity than the optical fiber, or
coating the recess surface with a material having a higher emissivity than
the optical fiber, or filling the recess with a refractory element or
compound having a greater emissivity than the optical fiber. Examples of a
suitable refractory material include oxides of aluminum, silicon,
zirconium and yttrium.
These alternative arrangements are illustrated in FIGS. 4-6 for the axial
recess 28a shown in FIG. 2. In FIG. 4, this axial recess is shown as
having a roughened surface 29. In FIG. 5, the axial recess 28a is shown as
lined with a coating 30. In FIG. 6, the axial recess 28a is shown as
filled with an insert 31. In these variations, the emissive surface 28 is
provided by the roughened surface 29 in FIG. 4; by the outer peripheral
surface of the coating 30 in FIG. 5; and by the outer peripheral surface
of the insert 31 in FIG. 6.
The radial recess 28b shown in FIG. 3 may be similarly provided with a
roughened surface, or a coating, or an insert, as illustrated in FIGS.
4-6. In order to avoid a redundancy of illustrations, such variations for
radial recess 28b have not been depicted.
The optical fiber element 22 (FIG. 1) can be a light guide 32 made of a
single material, as shown in FIG. 7. In FIG. 8, the light guide 33 is
shown as constructed with a core 34 surrounded by a cladding 35. Where
high temperature operation is desired, say, in the range of from
1000.degree. C. to 2000.degree. C., it is necessary to use a crystalline
material with a high melting point, such as sapphire. A length of rigid
optical-quality sapphire is shown at 36 in FIG. 9, and for ease of
installation it can be joined by a suitable optical connector 38 to a
flexible section of optical fiber 39 outside the high temperature region.
The control module 21 includes means to measure the optical power at the
remote end 26 of the optical fiber element 22, and convert the measured
value to an output signal proportional to the temperature of the
thermally-emissive surface 28. Referring to FIG. 10, the control module 21
is shown as being operatively joined to the optical temperature probe 20
by a suitable connector 40. While the control module 21 may be variously
constructed, it is shown as including means, represented by the arrow 41,
to convey the optical radiation to an optical filter 42 which has a
spectral bandpass centered at wavelength .lambda..sub.1. This filter is
used to control the wavelength range of the radiant energy transmitted via
the path, represented by the arrow 43, to the optical power detector 44.
This detector produces a voltage, V.sub.1, proportional to the optical
power striking its photosensitive surface. This voltage is transferred to
a signal processor 45 via suitable means, indicated generally at 46. This
signal processor converts voltage V.sub.1 to an output signal, represented
at 48, which is proportional to the temperature of emissive surface 28.
For some applications, greater accuracy can be achieved by measuring the
optical power in two wavelength intervals, as also illustrated in FIG. 10.
Within the control module 21, the optical radiation is split into two
beams, the second of which is represented by the arrow 49. This second
beam passes through a second filter 50 with a spectral bandpass centered
at wavelength .lambda..sub.2, and is transmitted via path 51 to a second
optical power detector 52, thus producing a second voltage, V.sub.2, which
is conveyed via suitable means indicated at 53 to signal processor 45.
Techniques for using optical measurements at two wavelengths are known by
those skilled in the art, and are described in U.S. Pat. No. 4,362,057,
for example.
A probe geometry suitable for general use is illustrated in FIG. 11, and
represented by the numeral 201. There the probe 201 is shown as including
a high-temperature optical fiber section 54 and a low-temperature optical
fiber section 55, connected together by an optical fiber connector 56 of
any suitable construction.
Referring to FIG. 12, the high temperature optical fiber section 54 is
represented by a sapphire rod having a diameter of 0.050 inch [1.27
millimeters (mm)] and a length of 3 inches (76.2 mm). Located in the tip
end of this sapphire optical fiber 54 is a cylindrical recess 58 in the
form of a centrally-disposed axial recess, opening to the tip end face 59.
Recess 58 has a 0.020 inch (0.508 mm) diameter and 0.060 (1.524 mm) inch
depth. These small dimensions of the optical fiber 54 and recess 58 allow
the probe to rapidly equilibrate with its temperature environment. Recess
58 is preferably filled with a refractory powder 60, such as Zircar ZYBF-2
powder (92% zirconium oxide and 8% yttrium oxide), manufactured by ZIRCAR
Products Inc. The outer periphery of insert 60 provides thermally-emissive
surface 28.
The low-temperature optical fiber section 55 may be made of glass, provided
with a core having a diameter of 200 microns (0.0079 inch) and a cladding
having an outer diameter of 240 microns (0.0094 inch).
A connector 56 suitable for joining the two optical fiber sections 54 and
55 is described in U.S. Pat. No. 4,607,911, owned by the assignee of the
present application. The description of the connector in said U.S. Pat.
No. 4,607,911, is incorporated herein by reference.
The optical power transmitted by the probe 201 shown in FIG. 11 is
preferably measured by a germanium photodetector. The electrical signal in
volts produced by such a photodetector is shown in FIG. 13 as a function
of the temperature surrounding the tip end portion of the sapphire probe
54.
While embodiments of the inventive improvement have been shown and
described, persons skilled in the art will readily appreciate that various
modifications besides those mentioned may be made without departing from
the spirit of the invention, as defined and differentiated by the
following claims.
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