U.S. patent application number 09/232423 was filed with the patent office on 2002-01-03 for system and method for determining combustion temperature using infrared emissions.
Invention is credited to DEMILO, CHARLES, GLASHEEN, WILLIAM M., STEGLICH, HELMAR R..
Application Number | 20020001333 09/232423 |
Document ID | / |
Family ID | 22873038 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020001333 |
Kind Code |
A1 |
GLASHEEN, WILLIAM M. ; et
al. |
January 3, 2002 |
SYSTEM AND METHOD FOR DETERMINING COMBUSTION TEMPERATURE USING
INFRARED EMISSIONS
Abstract
The present invention relates to a combustion temperature
sensor, and, more particularly, to a combustion temperature sensor
that measures infrared energy emitted at several preselected
wavelengths from a flame and/or a flame's hot gas at a turbine
inlet location and applies the energy signals to a calculation
model to yield temperature.
Inventors: |
GLASHEEN, WILLIAM M.;
(WILMINGTON, MA) ; DEMILO, CHARLES; (WILMINGTON,
MA) ; STEGLICH, HELMAR R.; (WILMINGTON, MA) |
Correspondence
Address: |
HAYES SOLOWAY HENNESSEY
GROSSMAN & HAGE
175 CANAL STREET
MANCHESTER
NH
031012335
|
Family ID: |
22873038 |
Appl. No.: |
09/232423 |
Filed: |
January 15, 1999 |
Current U.S.
Class: |
374/131 ;
374/120; 374/130 |
Current CPC
Class: |
G01J 5/041 20130101;
H05H 1/0043 20130101; G01J 5/602 20130101; G01J 5/0014
20130101 |
Class at
Publication: |
374/131 ;
374/130; 374/120 |
International
Class: |
G01J 005/00; G01J
005/02 |
Claims
1. In a system for measuring IR energies of a flame in a gas
turbine having a combustion chamber and a turbine section, the
improvement comprising a lens for collecting infrared energy from
that portion of the flame nearest to the inlet to the turbine
section, said lens focusing said infrared energy on one end of an
optical fiber, said lens and optical fiber comprising sapphire, a
mounting means for supporting said lens and the one end of the
optical fiber in fixed relation to the flame, means for supplying
compressed air to said mounting means to shield said mounting means
and said lens from combustion gases in said flame, means for
positioning the other end of the optical fiber in position to
direct a beam of infrared energy transmitted by the fiber onto a
plurality of infrared detectors positioned in a second mounting
means spaced from the turbine, spectral separation means for
separating said infrared radiation into a plurality of different
ranges of infrared frequency and directing each of said ranges of
infrared radiation onto an associated detector, and optical
chopping means for interrupting the infrared beam at a
predetermined frequency before it reaches said beam splitter
means.
2. The system of claim 1 wherein said lens and optical fiber
mounting means are sealed, and the air is directed around the
sealed mounting means.
3. The system of claim 2 wherein said sealed mounting means is
carried in a separate chamber which communicates with said
combustion chamber through an opening in a wall of the combustion
chamber and the compressed air enters said combustion chamber
through said wall opening to prevent combustion gases from entering
said separate chamber.
4. The system of claim 1, wherein said spectral separation means
comprises a plurality of narrow range infrared filters, each filter
being positioned adjacent a separate and passing only a narrow
range of infrared energy to its associated filter.
5. The system of claim 1, wherein at least one of said ranges of
infrared energy corresponds to a peak CO.sub.2 emission range and
another corresponds to a peak H.sub.2O range and a third
corresponds to a peak background temperature range of infrared
energy emitted from the combustion chamber walls.
6. In a system for measuring discrete IR energies of a flame in a
turbine having a combustion chamber and a gas turbine section, the
improvement comprising a lens for collecting energy from that
portion of the flame nearest the inlet to the turbine section, said
lens focusing said IR energy on one end of an optical fiber, said
lens and optical fiber comprising sapphire, a mounting means for
supporting said lens and the one end of the optical fiber in fixed
relation to the flame, means for supplying compressed air to said
mounting means to shield said mounting means from combustion gases,
means for positioning the other end of the optical fiber in
position to direct a beam of IR energy radiation transmitted by the
fiber onto a plurality of infrared detectors positioned in a second
mounting means spaced from the turbine, each detector having a
filter means for passing a predetermined wavelength of infrared
radiation to an associated detector, and optical chopping means for
interrupting the infrared beam at a predetermined frequency before
it impinges on said filter means, and means for controlling the
temperature of said detectors.
7. The system of claim 6, wherein a portion of the fiber optic
cable furthest removed from said lens is formed of chalcogenide
(As.sub.2S.sub.3).
8. The system of claim 2, wherein the air is fed circumferentially
around the selected mounting means.
9. The system of claim 1, wherein the optical chopper operates at a
frequency on the order of 65 Hz.
10. The system of claim 1, wherein said end of said sapphire
optical fiber is positioned in the focal plane of said lens.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a combustion temperature
sensor, and, more particularly, to a combustion temperature sensor
that measures infrared energy emitted at several preselected
wavelengths from a flame and/or a flame's hot gas at a turbine
inlet location and applies the energy signals to a calculation
model to yield temperature. Particular utility for the present
invention is in the field of gas turbine engines; although other
utilities are contemplated herein.
[0003] 2. Description of Related Art
[0004] Combustion gas turbine designers and users can benefit from
knowledge of flame temperature to determine, for example, NOx and
CO emission concentrations, flame control, and flame-off
conditions. Knowledge of these parameters can be used for increased
turbine efficiency, and increased turbine blade life and
reliability, as well as decreased pollution. While much effort has
been devoted in the past to the problem of flame temperature
determination, previously developed systems have been lacking in
the ability to come up quickly and reliably with accurate and
useful flame temperature measurements.
[0005] One example of such a temperature measurement system is
shown in Cashdollar et al., U.S. Pat. No. 4,142,417. This patent
discloses an IR measuring pyrometer used to calculate both particle
and gas temperature from an explosion or fire. In this system three
or four IR wavelength measurements (1.57, 2.30, 3.46, and 5.19 um)
are obtained to compute temperature of the particles and gas.
Significantly, these wavelengths are chosen to avoid discrete
emission bands of gases in the hot flame, e.g., those emission
bands which correspond to the quantized energies of the vibrational
and rotational states of molecules. The wavelength measurements, as
provided by Cashdollar et al., are restricted to dust cloud flames
which are optically "thick" (i.e., gas cloud flame is optically
opaque at the chosen wavelength), to eliminate the need to
compensate for background radiation. Thus, this system would be
incapable of operating in an optically thin environment, such a
turbine, since background radiation from the wall on the other side
of the flame would be detected and would destroy any measurement
obtained.
[0006] In still other prior art examples, temperature measurement
is determined by detecting UV radical (e.g., OH, CO, CH, CHO, C,
etc.) emission bands in the combustion chamber. For example, German
Laid Open Patent Application DE 4028922/A1 and published PCT
Application WO 98/07013, each disclose methodologies for
temperature determination in a combustion chamber using UV spectral
emissions from a variety of gaseous radicals. Radicals, by their
very nature, are short-lived as compared to molecular gas
constituents, and thus, determination of temperature from molecular
gas components is more stable. While UV-spectral combustion
temperature determination may be adequate for some purposes, such a
system cannot be used for temperature control at the turbine inlet
location. In addition, UV combustion temperature determination
cannot provide information that can be used to improve turbine
blade life and stability.
[0007] It has been proposed (e.g., En Urga Paper 1997) to determine
temperature by observing the entire IR spectrum and directly
correlating certain radiation intensities of molecular CO.sub.2 and
H.sub.2O. However, the harsh operating environment inside the
turbine prohibit such a direct measurement. In addition the cost of
producing a fiber optic fiber that is capable of both transmitting
the entire spectrum without degrading in the harsh operating
environment is too prohibitive. Thus, engineering trade-offs must
be reached between the ability to effectively observe and transmit
optical energy signals within a turbine environment, and to obtain
appropriate IR wavelength intensities for accurate temperature
measurement.
[0008] The present invention solves the aforementioned shortcomings
of the prior art by selecting an optical fiber and detector that
can withstand the operating environment of a turbine and transmit
certain, meaningful wavelengths of optical energy to determine
temperature. More specifically, the present invention includes
improvements in the relationship of the various elements of the
optical system to each other and to the flame. A lens is positioned
so that it collects infrared (IR) radiation from that portion of
the flame nearest the inlet to the turbine section. The lens
focuses the IR energy on one end of an optical fiber, with a
mounting structure supporting one end of the optical fiber in fixed
relation to the flame. Compressed air is supplied to the mounting
structure to shield the lens from combustion gases in the flame.
The other end of the fiber is positioned to direct a beam of IR
energy onto a plurality of detectors positioned in a second
mounting structure spaced from the turbine. A spectral separation
mechanism is provided before the detectors to separate the incident
IR radiation into a plurality of narrow-range IR frequencies. An
optical chopping mechanism is provided for interrupting the IR beam
(at a predetermined frequency) before the beam reaches the spectral
separating mechanism. In this way, the detector receives a chopped,
narrow-range IR signal. The signals are converted to appropriate
electrical signals, processed to determine optical energy, and
preferably compared to a predefined look-up table to determine a
temperature value for a given set of detected optical energy
signals.
[0009] It should be emphasized that the disclosure in this
application includes "best mode" descriptions of preferred related
technologies (e.g., temperature calculation via a look-up table)
which are not part of the instant claimed invention. This
disclosure is amplified for purposes of completeness.
SUMMARY OF THE INVENTION
[0010] Accordingly, the present invention provides a system and
method for determining combustion temperature using infrared
emissions. The present invention includes a sensor, a signal
conditioning stage and a temperature determining stage to provide
temperature measurement at a turbine inlet location.
[0011] In the present invention, an optical system is focused on
the flame as the temperature to be measured. As mentioned above, it
is desired to measure the flame temperature when the combustion
process is essentially complete, i.e., the gaseous products of
combustion contain stable compounds of H.sub.2O and CO.sub.2. For
this purpose, the IR radiation for that portion of the flame
closest to the turbine inlet is measured. The resultant optical
signal is focused on one end of a fiber optic cable and the other
end of the fiber optic cable emits light into an optical detection
system. This optical detection system includes an optical chopper,
after which the optically chopped signal impinges on a number of
separate detectors which convert the optical signal into an
electric current. Each of the optical sensors is preferably
provided with a selective filter which passes only a very limited,
discrete range (i.e., narrow band filter) of infrared wavelength.
In the preferred embodiment, four wavelength filters are used: one
to pass wavelengths of radiation specifically emitted by CO.sub.2,
one to pass wavelengths specifically emitted by H.sub.2O, one to
pass a correlated wavelength of CO.sub.2 and H.sub.2O, and one to
pass a background radiation wavelength. The resultant composite
signal is then processed to obtain a stable optical energy signal
at each of the selected wavelengths. Preferably, the signal
processing includes programmable gain amplifiers and digital to
analog circuits for preparation of the signals for computer
calculations.
[0012] The IR signals must be fed to an optical detector which is
subject to careful temperature control so that temperature effects
of the detector can be eliminated in so far as is technically
feasible. The optical chopper causes a zero optical signal to be
available at a given chopping rate (such as 65 Hz) as well as the
regular optical signal. Since only the difference between the two
signals is used, any DC slow drift is eliminated. In this case,
each channels' programmable gain amplifier is controlled by the
computer and the signal processing system so that the signal
remains in the middle A/D range where accuracy is best. The
hardware also includes the use of the digital to analog converters
to generate an offset to the signal to assist in further keeping
the A/D conversion accurate.
[0013] A calibration is also performed. The purpose of calibrating
the instrument is to account for component variations from sensor
to sensor. Calibration consists of converting an electrical
(voltage) signal from each detector element, to an optical
(radiation) signal which is used in the software program to
determine temperature. A blackbody radiation source is used for
this purpose. Since the amount of radiation exiting a blackbody
source is well known, there is a direct relation to the detector
response. A standard instrument blackbody with emissivity
.gtoreq.0.99 has a very well defined spectral emission as a
function of its temperature. Optical radiation, at different
blackbody temperatures, transmits through the entire optical
system, and the voltage response from each of the four detector
elements is measured. The detector output as a function of
uwatts/steradian-cm is then calculated for each blackbody
temperature. These data yield a graph to convert detector reading
to the radiation intensity valued that are necessary for flame
temperature back calculation.
[0014] The temperature calculation is performed by using a
multidimensional look-up-table (LUT). The LUT is created by the
following four steps. (1) A stochastic simulation is carried out to
mimic the CO.sub.2 and H.sub.2O concentrations and temperatures
over a broad range of values, and over the path length present in
the turbine. The CO.sub.2 concentrations vary from 0.005 to 0.08
mole fraction and H.sub.2O varies from 0.005 to 0.16 mole fraction.
The temperature is varied over the range of interest from
500.degree. C. to 1400.degree. C. (2) The radiation intensities
leaving these simulated paths are calculated using a narrow band
model such as RADCAL. (3) Preferably, the resultant intensities are
first sorted into a four dimensional table, with the radiation at
each of the three wavelengths arranged in three columns. The
temperature corresponding to the three intensity values are stored
in a fourth column, in the four-dimensional LUT. (4) The sorted
values are then averaged to provide a convenient number of
intensities (typically 8 to 50) along the three dimensions, with
temperature forming the fourth dimension. This table forms the
LUT.
[0015] After the LUT is obtained, it is stored into memory. During
operation, one of the intensity values is chosen as a background
radiation channel and is used to correct the intensities of the
other 3 wavelengths, and the corrected intensities at these three
wavelengths are used to find the temperature using a sequential
search routine. This search is very fast, since the LUT has been
sorted in an ascending (or descending) order. To improve speed,
equi-spaced intensities (or the logarithm of the intensities) with
indexing can also be used.
[0016] The details of the various aspects of the system are
described below in more detail hereinafter, and are specifically
claimed in the copending application of ______,______ filed on even
date herewith.
[0017] It will be appreciated by those skilled in the art that
although the following Detailed Description will proceed with
reference being made to preferred embodiments and methods of use,
the present invention is not intended to be limited to these
preferred embodiments and methods of use. Rather, the present
invention is of broad scope and is intended to be limited as only
set forth in the accompanying claims.
[0018] Other features and advantages of the present invention will
become apparent as the following Detailed Description proceeds, and
upon reference to the Drawings, wherein like numerals depict like
parts, and wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a simplified block diagram of the major elements
of the system of the present invention;
[0020] FIG. 2 is a detailed view of a preferred lens assembly (the
preferred dimensions of the lens are shown in FIG. 2A);
[0021] FIG. 3 is a detailed view of lens mounting and fiber optic
cable;
[0022] FIG. 4 is a detailed view of the relationship between the
fiber optic cable, the optical chopper and the IR detector of the
present invention;
[0023] FIGS. 5A and 5B is a side view and front view, respectively,
of the preferred IR detector assembly of the present invention;
[0024] FIG. 6 is a block diagram of the preferred embodiment of the
signal conditioning system;
[0025] FIGS. 7A and 7B depict the operational flow of the signal
conditioning system of FIG. 6; and
[0026] FIG. 8 is a simplified block diagram of the temperature
calculation of the present invention; and
[0027] FIG. 9 is a plot of intensity vs. wavelength for several
emission spectra.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0028] As an overview, FIG. 1, depicts a block diagram
representation of the major elements which make up the various
aspects of the invention. A lens 10 focuses an IR image of a flame
whose temperature is to be measured onto one end of an optical
fiber 12. The other end of the fiber emits the IR radiation through
an optical chopper 14 towards a group of optical filters 18 and IR
detectors 20. The frequency of the optical chopper 14 is controlled
by electronic 16 and the temperature of the IR detectors is
controlled by a temperature controller 22 within a narrow,
predetermined range (e.g., 0.degree. C., +/-0.050.degree. C.) to
prevent DC voltage drifts and thermal voltage effects. Temperature
controller preferably includes a Peltier cooler, although other
closed-loop cooling systems known in the art are deemed within the
scope of the present invention. The electrical output of the IR
detectors is fed to a signal conditioning stage 24 to provide a
stable optical energy signal for a wide range of operating
temperatures. The output from the signal conditioning stage is fed
into temperature stage 26 to calculate a flame temperature 30. Each
of these components of the present invention are discussed in
detail below.
[0029] FIGS. 2 and 2A depict the preferred form of the lens
assembly of the present invention. The details and preferred
dimensions are tabulated in FIG. 2A. Preferably, the lens 10 is
composed of Al.sub.2O.sub.3. To withstand the sever operating
environment inside the turbine near the combustion flame, the mount
50 is preferably formed of Kovar, although other materials capable
of withstanding the temperatures and pressures within a turbine are
contemplated. Kovar is preferred when AgCu brazing alloy is used to
secure the lens 10 in the mount 50, since Kovar permits direct
brazing. The preferred lens having the dimensions illustrated will
give a focal length of about 21.27 mm for a wavelength in the IR
range of 2.275-2.885 .mu.m.
[0030] In order to mount the lens 10 in a secure position with
respect to the IR input end of a fiber optic cable, the preferred
design of mounting is constructed as schematically illustrated in
FIG. 3. As shown the Kovar lens mount 50 of FIG. 3 is supported on
a mount holder 52 which, at its rear 54, is connected to an outer
lens housing 56 and an optical fiber mounting fixture 66, thus
permitting a predetermined fixed relationship between lens 10 and
the input end 58 of the optical fiber 12. This permits the input of
the fiber to be positioned in the approximate focal plane of lens
10 at the IR wavelengths of interest.
[0031] As can be seen, there is a space 55 between the lens mount
holder 52 and outer mount holder 56. Into this space 55 a supply of
high pressure purge air 62 is introduced through air inlet 60. This
air is introduced in a tangential fashion to provided radial and
circumferential flow through space 55. The purge air exits through
fitting 64 which serves also as the mount for the optical system on
the side of the turbine combustion chamber, a portion of which is
shown at 65. This portion is adjacent to the inlet to the turbine
(not shown). The IR signal enters the fitting 64 and impinges on
the lens 10 where it is focussed on the end 50 of the optical
fiber. Since there is a constant flow of air through fitting 64
into the combustion chamber, no products of combustion can flow
into space 55 (where such products might otherwise deposit on the
lens 10 resulting in a decrease in IR signal strength and faulty
temperature measurement). For convenience, the high pressure air
from the turbine compressor stage (not shown) can be used as the
purge air source and fed into the fitting, although a dedicated air
source is deemed equivalent. This purge air has the dual function
of cooling the lens mount and preventing combustion products from
approaching the lens.
[0032] The fiber optic cable is preferably formed of
Al.sub.2O.sub.3 for the first meter (or some predetermined distance
away from the intense heat and mechanical stress around the
combustion flame), starting at the IR input end and then continues
as As.sub.2 S.sub.3 for another meter. These two fibers have a
diameter of about 0.4 mm and are optically aligned at their
junction and are protected by a fiberglass buffer layer within an
outer cable sheath 68 of stainless steel.
[0033] At the output end of cable 12, there is provided a cable
mounting fixture 72 which suitably secures the cable to the
detector housing 74 illustrated schematically in FIG. 4. This
detector housing supports the output end of the cable 12 in
position to direct the output IR past an optical chopper 78 towards
the detector assembly 79. As shown, the optical chopper is placed
in the path of the incident radiation between the end of the fiber
12 and the detector 79, and is preferably a tuning fork-type and is
driven at 65 Hz by suitable electronics 76. Alternatively, the
chopper can be formed with a spinning wheel having a plurality of
openings and synchronized (via synchronizing drive electronics,
described below) as a function of rotational velocity.
[0034] The IR detector assembly 79 preferably comprises four
separate detector elements 82 mounted behind four IR filters 80. A
more detailed, enlarged plain view of the assembly 79 are shown in
FIGS. 5A and 5B. The detectors 82 are preferably mounted on Peltier
coolers 86 for closed-loop temperature control of the detectors.
Peltier coolers are known in the art and commercially available. In
a preferred embodiment of the invention the detectors 82 are lead
sulfide and the four filters having the following peak IR
transmissions: 2.28 .mu.m (+/-0.005 .mu.m) chosen as a background
emission selected to be away from the emission radiation of
CO.sub.2 and H.sub.2O; 2.6 .mu.m (+/-0.015 .mu.m) selected as the
H.sub.2O emission wavelength; 2.70 .mu.m (+/-0.015 .mu.m) selected
as the CO.sub.2 emission wavelength; and 2.8 .mu.m (+/-0.005 .mu.m)
selected as a combined CO.sub.2 and H.sub.2O emission wavelength.
The spectral plot (wavelength vs. intensity) is shown in FIG. 9,
and each of the four emission peaks are indicated in this figure.
Each of these wavelength are depicted The raw electrical signal
from the IR detectors 82 exits through wiring 84.
[0035] Referring now to FIG. 6, a detailed block-diagram
representation of the preferred embodiment of the signal
conditioning stage 24 of the present invention is depicted.
Essentially, signal conditioning stage 24 consists of a feedback
loop utilizing a programmable gain amplifier (PGA) 102, and A/D
converter 104 and a digital processor/controller 106. It should be
noted at the outset that the circuit shown in FIG. 6 can be
duplicated as necessary for each sensor (discussed above), although
the A/D converter 104 and the processor/controller 106 are
preferably comprised of multiple input devices (via multiplexing,
not shown) that can accommodate multiple sensor signal inputs.
[0036] As disclosed above, the sensor 108 (which includes the lens,
lens housing, fiber optic cable, detector, etc.) preferably
includes an electromechanical signal chopper (e.g. an
electronically controlled tuning fork, a wheel, etc.) that causes
the detector to obtain 2 signals: a "dark" signal when the chopper
is closed (i.e., no optical signal is obtained by the sensor), and
a "light" signal when the chopper is open. Preferably, the chopper
frequency is set at 65 Hz., although other frequencies are
envisioned, provided that the associated circuitry shown in FIG. 6
has time to settle between light and dark signals.
[0037] The output signals from the sensor are fed into a
programmable gain amplifier (PGA) 102. The PGA preferably includes
a difference amplifier 110 and a programmable gain amplifier 112,
and is utilized to adjust signal level as a function of intensity,
and to obtain a signal output that is in the middle of the
operating range of the A/D converter 104 to increase overall
dynamic range of the system 100. In the preferred embodiment, the
A/D converter 104 has inputs from the PGA 102 and the chopper
signal synchronizing signal from the signal chopper drive 14. The
output is fed into processor/controller 106. Processor/controller
106 obtains the values of the "light" and "dark" signals and
calculates and determines appropriate OFFSET 118 and GAIN 120
values, as described below.
[0038] In block diagram form, the preferred operational flow of the
signal conditioning stage 24 is shown in FIG. 7A and 7B. For
clarity, corresponding reference numbers of the components of FIG.
6 are omitted. Before the instrument, which includes the sensor 108
and signal conditioning stage 24, is placed in an operating
environment, the sensor and associated electronics embodied in FIG.
6 are calibrated 130 (FIG. 7A). System calibration 130 is provided
to obtain a calibration constant (K) for each system. Those skilled
in the art will recognize that variations will exist between each
component in the instrument (and between instruments), and thus, it
is desirable to calibrate each system by determining each system's
input/output transfer constant. Thus, each system is measured to
determine its operational curve. Optical energy (OE) is related to
observed optical voltage (V). Accordingly, an input optical signal
of a known temperature is input into the system 132. A blackbody
radiation source is preferably used for this purpose, since the
amount of radiation exiting a blackbody source is well known, there
can obtained a direct relationship with the response of the sensor
and associated electronics. Preferably, a standard blackbody with
emissivity >0.99 is used having a well-defined spectral emission
as a function of temperature. Optical radiation, at different
blackbody temperatures, transmits through the sensor 108, and the
voltage response from each of the sensors is measured. The detector
output (uwatts/steradian-cm) is then calculated for each blackbody
temperature. A constant K is determined 134 by observing the output
signal as a function of the input signal (optical energy (OE)-K*
input voltage). It should be noted that the input/output
relationship is not necessarily linear, and thus, K may reflect a
nonlinear curve relating input to output. Preferably, several test
input values are used to determining a constant for each input,
thus, a calibration curve, not shown, is obtained. The
processor/controller stores K 136, to be used in calculating
optical energy and temperature, discussed below.
[0039] The operational flow of conditioning an optical input signal
from each sensor (as described above) at a given wavelength, is
shown in FIG. 7B, with reference to the system 24 of FIG. 6. From
the signal chopper and PGA, the processor/controller obtains the
raw values of the "light" signal 142 and the "dark" signal 144,
each alternating according to the chopper frequency (e.g. 65 Hz.)
synchronized via the A/D converter. Preferably, light and dark
signals are obtained at the leading edge and falling edge of the
chopper reference signal, respectively. Each value is stored by
processor/controller for a predetermined time (e.g., 5 second
buffer memory) to compare current values with previous values. The
value of the "dark" signal (background radiation) and the "light"
signal are used to calculate an OFFSET value 148, which is
differentially compared to the chopper signal from the sensor in
the PGA 152. Preferably, OFFSET value is determined so that a
positive value is always obtained from the PGA. Knowing the
amplitude of the signals, processor/controller calculates an
appropriate GAIN value 150, and inputs this value to the PGA 154.
Preferably, the GAIN dynamically adjusts the raw signal value to
keep the amplified signal in the "middle" of the A/D converter
input voltage range, thereby increasing dynamic range and avoiding
saturation. For example, if the operating voltage range of the A/D
is 0 to 5 V., the gain applied will maintain the signal value at
approximately 2.5 V. The output of the PGA is fed into the A/D
converter and supplied to processor/controller 156. As with the raw
signal values, the gain values can be maintained in buffer memory
for a predetermined time. Optical energy (OE) is then calculated
158 as a function of input optical intensity (in millivolts) and
the calibration constant K (described above), for each of the four
wavelengths from the sensor. In the preferred embodiment, four
optical energy output signals 122 are obtained from which
temperature is determined, as described below.
[0040] To determine temperature, the four optical energy signals
are provided as inputs to a predetermined lock-up table (described
below) in which temperature is back-calculated using known optical
energy vs. temperature model calculations. As shown in FIG. 8,
three optical energy signals corresponding to CO.sub.2 and H.sub.2O
emission spectra and one background radiation signal (corresponding
to the background radiation from combustion) 122 are determined,
from the above-described process. The background radiation signal
is subtracted from each of the other three emission signals 160,
using a correction factor that corrects for filter widths and
relative emissivity at each of the three wavelengths. This
correction factor can be assumed constant (i.e., the relationship
between the given wavelengths for any given instrument is fixed,
and exhibits little, if any, change as a function of optical
energy, temperature, etc.). As shown in the figure,
I.sub.i'=I.sub.l-I.sub.b (.alpha.); where .alpha.=the correction
factor, I.sub.l=, the intensity value at a preselected wavelength
(in the preferred embodiment, three wavelengths are chose, as
described above), and I.sub.b=the background intensity value. The
calculation is performed at preferably 3 wavelengths: 2.6 .mu.m
(+/-0.015 .mu.m), 2.70 .mu.m (+/-0.015 .mu.m) and 2.8 .mu.m
(+/-0.005 .mu.m); with wavelength 2.28 .mu.m (+/-0.005 .mu.m) being
used as the background radiation signal. The three adjusted signals
are used as inputs in determining temperature from the look-up
table.
[0041] The preferred construction of the LUT is described below.
Using a narrow-band radiation model, such as provided by
commercially available spectroscopy simulation models (e.g.,
RADCAL: A Narrow-Band Model for Radiation Calculations in a
Combustion Environment, Gossander, William L., NIST Technical Note
1402, April 1993), a simulated calculation of radiation intensity
(at one or more wavelengths) as a function of (1) path length, (2)
temperature, and (3) molar concentration of CO.sub.2 and H.sub.2O
is obtained. For the present invention and the intended operating
environment, the path length (i.e., the distance between the sensor
and the turbine wall can be assumed to be homogeneous. In addition,
it is known that the temperature range can vary from 900 to 1500
degrees C., and, for hydrocarbon flames, the molar concentration of
CO.sub.2 and H.sub.2O can vary between 0 and 0.075, and 0 and 0.15,
respectively. Thus, a temperature range of 800 to 1600 degrees C.,
and molar concentration values of 0.01 to 0.08 and 0.01 to 0.16
(CO.sub.2 and H.sub.2O, respectively) are used. To improve speed,
equi-spaced intensities values (or the logarithm of the
intensitites) can be used. Thus, for example, the present invention
can be adapted to run the calculation using 48 equi-spaced variable
values within the above-noted temperature and concentration range.
Of course, the present invention can realize a larger number
calculations, which would increase the accuracy (by decreasing the
error associated with interpolation) at the expense of expanding
the size of the LUT.
[0042] Ultimately, the only unknown quantity of concern is
temperature. Thus, in the preferred embodiment, the LUT comprises
three columns of intensity values and one column of corresponding
temperature values. To that end, preferably a 4-dimensional look-up
table (LUT) 162 is used to determine temperature 164 from the three
radiation signals. Of course, the LUT can be constructed in
n-dimensions, corresponding to the number of sensor signals used.
The resultant table (LUT) is stored as a database (not shown) and
is essentially formatted as plurality of columns, one column for
each wavelength chosen, and a final column of corresponding
temperature values for each row of wavelengths, and is stored in
memory (not shown). Referring again to FIG. 8, the three signals
122 received from the signal conditioning stage are compared (e.g.,
using a search algorithm performed by, e.g., processor/controller
106) to the corresponding intensity values created in the LUT, and
a corresponding temperature value is obtained. The processor that
receives the radiation intensity values and compares these values
to the LUT may be a separate processor without departing from the
present invention.
[0043] Thus, it is evident that there has been provided a
combustion temperature sensor system and method for operating same
that fully satisfy both the aims and objectives hereinbefore set
forth. It will be appreciated that although specific embodiments
and methods of use have been presented, many modifications,
alternatives and equivalents are possible. For example,
processor/controller 106, A/D converter 104, PGA 102, D/A 116, and
LUT (memory) 162 can be any custom made or off-the-shelf components
known in the art, and may be provided in one unified system or be
part of a modular, interchangeable system, provided that the stated
functionality is obtained. Although not shown, system 10 can be
adapted with appropriate I/O ports to permit, for example, the
signals to be displayed (e.g., using appropriately modified LCD
and/or LED display modules) and to provide user-control of the
various operational parameters herein described. To that end, the
system 10 can include an RS485 digital bus to output the obtained
temperature calculation to a display and/or mass storage (not
shown). Further modifications are possible. For example, instead of
the signal conditioning stage (FIG. 6) as provided herein, the
signals received from the sensor 108 can be first fed into an A/D
converter and supplied to processor/controller 106 for
conditioning. To that end, the signals can be appropriately
digitized at a sufficient sampling rate and bit-depth to achieve a
desired resolution and dynamic range.
[0044] Still other modifications are possible. As described above
in reference to the LUT, since each column of the table can be
quite large (for example, 48.times.48.times.48), the LUT can be
appropriately condensed, thereby saving memory space, and modified
to optimize a sequential look-up operation. While not wishing to be
bound by example, the intensity-temperature table stored in the LUT
may be modified as follows. The first column of radiation intensity
values can be arranged in ascending (or descending) order, while
keeping the corresponding values in the remaining columns in the
proper row. The first k values of the first column are averaged to
obtain 1 value; where k=n.times.n; and n is the number of
calculations made within the above-noted temperature and molar
concentration range. This is repeated n times. The first k values
of column 2 are then arranged in ascending (or descending) order,
and the first n values are averaged, to obtain 1 value. The first n
values in column 3 are then arranged in ascending order, and
repeated. Of course, the above process assumes three wavelengths
and 1 temperature value, in accordance with the preferred
embodiment. However, this process can extend to any given number of
wavelengths calculated. It will also be apparent to those skilled
in the art that the intensity values obtained by the sensor and
those calculated in the LUT may not precisely coincide. Thus, the
present invention can be appropriately modified with standard
interpolation techniques (e.g., linear, cubic spline, etc.).
Alternately, the value of n may be increased so that the distance
between any two intensity values for a given wavelength is within
error tolerances. In yet another modification, the LUT can be
modified to include a curve-fit polynomial expression of
intensity-temperature values, which can be approximated for each
intensity value obtained by the sensor.
* * * * *