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Description  |
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TECHNICAL FIELD
This invention relates to temperature sensing and, more particularly, to a
method and apparatus for optically measuring temperature by
spectroscopically detecting and analyzing temperature-induced changes in
the wavelength spectrum of light interacting with semiconducting sensing
element.
BACKGROUND OF THE INVENTION
Conventional temperature probes, which utilize thermocouples, thermistors,
and other electrically conducting components, are often unusable in the
presence of electromagnetic fields because of electrical interference
problems and field perturbation effects. Metallic components such as lead
wires and connectors can cause erroneous temperature readings in the
presence of electromagnetic fields, can pick up electrical interference in
electrically noisy environments, and can transmit hazardous electrical
shocks in high voltage applications.
Optical temperature probes differ from conventional probes in that they
contain essentially no metallic or electrically conducting components.
Non-metallic temperature probes have applications in and near regions
having electromagnetic fields, such as in microwave ovens, motors,
transformers, and electrical generators. In addition, nonmetallic probes
preclude the possibility of potentially fatal electrical shocks when used
to measure temperature inside the human body.
Because of the advantages of nonmetallic temperature probes, several
techniques for optically measuring temperature have been proposed and
tested. Among these are methods described in the following articles: C. J.
Johnson, et al., "A Prototype Liquid Crystal Fiberoptic Probe for
Temperature and Power in R. F. Fields", Microwave Journal, Volume 18, No.
8, pp. 55-59, August, 1975; T. Cetas "A Birefringent Crystal Optical
Thermometer for Measurements for Electromagnetically Induced Heating,
USNC/URSI 1985 Annual Meeting, Boulder, Colo., Oct. 20-23, 1975; D.
Christensen, "Temperature Measurement Using Optical Etalons", 1975 Annual
Meeting of the Optical Society of America, Houston, Tex., Oct. 15-18,
1975; and "Novel Method for Measuring Transient Surface Temperature with
High Spatial and Temporal Resolution", Journal of Applied Physics, Vol.
43, No. 7, p. 3213, July, 1972. Other methods which are currently
commercially available include the characterization of fluorescent
emission from a fluorescent sensor as described in U.S. Pat. Nos.
4,448,547 and 4,459,044; the measurement of discrete wavelength emissions
from an excited semiconductor sensor as described in U.S. Pat. Nos.
4,376,890, 4,539,473; the use of a two-wavelength semiconductor sensor as
employed by Mitsubishi Corporation; and the use of a narrow band
wavelength source whose optical power is variably absorbed by a
semiconductor sensor as disclosed by the present inventor in U.S. Pat. No.
4,136,566.
Except for the fluorescent sensor technique, which measures the time
history of the emitted optical power, and the etalon technique, which
detects a discrete pass band frequency of a reflecting cavity, the prior
methods can be classified generally as "amplitude" techniques. In such
"amplitude" techniques, the intensity of the return signal is directly
proportional to the temperature. Furthermore, all of the prior methods
generally utilize only a small portion of the wavelength spectrum,
normally measuring the intensity of no more than two wavelengths of the
signal or emission.
A major disadvantage of the prior methods for optically measuring the
temperature is that the amplitude techniques are susceptible to inaccuracy
caused by drift in the source of intensity, variable optical losses in the
transmitting fibers, and other intensity variations unrelated to the
sensor temperature. These variations can be minimized by taking a ratio of
amplitudes of two wavelengths which interact with the sensor, but a
unilateral amplitude drift in either component of the ratio still results
in temperature measurement errors. Thus, a need has arisen for a
nonmetallic temperature measurement device having a higher degree of
accuracy and greater stability with respect to time.
SUMMARY OF THE INVENTION
The present invention optically measures the temperature of a semiconductor
sensor by spectroscopically determining the wavelength spectrum
characteristics of the spectrum of light interacting with a semiconductor
sensor. The invention includes a radiant energy source, typically having a
broad wave spectrum, transmission by a waveguiding means, such as optical
fibers, to an optical temperature sensor, interaction with the temperature
sensor, transmission of the temperature-modified spectrum back to a
receiver by wave guiding means, and detection of the received spectrum by
a spectrometer. Characteristics of the received spectrum are defined by
its interaction with the sensor, either by reflection from one or more
surfaces of the sensor or after transmission through the sensor. By
electrically processing the digital signal information of the received
spectrum, the value of the sensor temperature may be obtained.
The invention utilizes the entire wavelength spectrum of the radiant energy
which interacts with the semiconductor sensor. The invention can be
described as a "spectroscopic" or "wavelength spectrum" technique in that
it measures changes in the wavelength spectrum characteristics of the
sensor rather than changes in the intensity of the sensor's intersection
at one or more discrete wavelengths. The present invention is insensitive
to variations and drifts in radiant energy intensity. The wavelength
spectrum characteristics of radiant energy transmitted or reflected by the
sensor can be determined accurately, for example, by means of a
diffraction grating spectrometer coupled with a fastscanning photodiode
array. Furthermore, wavelength calibration apparatus is very stable over
time, thus eliminating the requirement of frequent recalibration of the
system.
The present invention measures temperature by spectroscopically measuring
changes in the wavelength spectrum due to the absorption edge of a
semiconductor. The rapid increase in the optical absorption of a
semiconductor as a function of decreasing wavelength is due to the
excitation of the valence band electrons into the conduction band by
incident photons which have sufficient energy hf (where "h" is Planck's
constant and "f" is the frequency) to bridge the energy gap between the
two bands. The energy gap varies monotonically with temperature at a rate
characteristic of the semiconductor used as the sensor. Therefore, the
wavelength spectrum which describes the absorption edge characteristics
(i.e., the non-absorbing region, the initial rise in absorption, the
slope, and the plateau at high absorption) will predictably move in
wavelength location as the temperature of the sensor changes. The
wavelength spectrum of the radiant energy transmitted or reflected by the
sensor reveals the form and wavelength location of the absorption edge of
the semiconductor, which is indicative of the temperature of the sensor.
In one embodiment of the invention, the source of radiant energy is a
broadband quartz-halogen lamp whose total wavelength spectrum of radiant
energy is focused onto one or more optical fiber lightguides. The radiant
energy passes through the optical fibers to the semiconductor sensor, is
transmitted through or reflected by the sensor and collected by one or
more receiving optical fibers, and is returned to a spectrometer which
determines the wavelength spectrum characteristics of the emitted radiant
energy. The spectrometer comprises a diffraction grating, collecting and
focusing lenses, and a photodetector array to receive the spectrum from
the grating. The diffraction grating and the focusing lenses spread the
wavelength of the return signal into a spatial pattern which is detected
by the photodetector array. Alternatively, a dispersive optical prism may
be used to separate the spectrum instead of a diffraction grating. Also a
moving mirror followed by a slit and photodetector may replace the
photodetector array. An analog-to-digital converter converts the analog
signal to digital signal information relating to the receiving spectrum
for processing by a digital computer. The computer utilizes a stored
algorithm to process the digitized spectral information returned from the
sensor, converting the spectral characteristics of the returned light
signal into the temperature at the sensor. The sensor temperature may be
output in a number of ways, such as a visual display, recorded on a
printer/plotter, or utilized by the computer to activate an audio/visual
alarm system.
In order to accomplish fast sequential temperature readings from a
plurality of probes and sensors with a common optical source and receiving
spectrometer, one embodiment of the invention transmits the light from the
optical source through an optical multiplexer to the temperature sensor
probes. The optical multiplexer may comprise an array of lenses placed
around the light source and a rotating shutter with an aperture sufficient
for the light to reach one lens at a time. Each lens has optical fibers
placed at the focal point to receive light from the source and transmit it
to one of the temperature sensors. The optical energy from the source is
thus directed to and collected from only one probe at a time, the
sequencing of the probes being achieved by the timing of the rotation of
the shutter. In another embodiment of the optical multiplexer, light from
the source is focused by a lens onto an oscillating or rotating mirror,
which directs the focused beams sequentially to an array of optical fiber
ends, each fiber end being associated with one of the temperature sensor
probes. Thus, the optical multiplexing systems allows a plurality of
temperature sensing probes to be timed-shared with one light source and
one spectrometer.
In yet another embodiment to obtain temperature readings from a plurality
of temperature sensing probes having an common optical source and
receiving spectrometer, the spectrum of light returned from each of the
sensors is processed electronically. In this embodiment the need for an
optical multiplexer is eliminated. A rectangular array of photodetectors
is provided in the spectrometer and each of the ends of the lightguides
from the temperature sensors is positioned so that a spectrum of received
light is directed or assigned to a separate linear array of photodiodes in
the rectangular array. As an example, the X-coordinate of the rectangular
array of photodiodes could represent the spectral characteristics of a
returned light signal from a sensor and the Y-coordinate could represent
an unique address corresponding to each of the individual temperature
sensors. This method of using a rectangular array of photodetectors in a
fast scan spectrometer and assigning an address to each lightguide
provides a means for electronically separating the received spectrum of
light from a plurality of temperature sensors in contrast to the
mechanical method of separating the light transmitted to each of the
temperature sensors with an optical multiplexer.
In one embodiment of the present invention, the digital computer utilizes
an algorithm which detects the absorption peak by detecting the wavelength
at which the intensity has dropped to one-half the peak intensity anywhere
else in the spectrum. Having determined the spectrum position
corresponding to one-half the maximum intensity, the computer utilizes a
calibration table in memory to convert this information to a temperature
reading. The temperature may be output to a visual display, a
printer/plotter for recording the temperature, or compared to a
predetermined value for activating an alarm.
In another embodiment of the present invention, the computer determines the
temperature from the received spectrum by fitting a curve to the spectral
edge, such as by utilizing the least squares method. The computer may then
convert the curve fitting parameters to the temperature by utilizing the
calibration table stored in memory. In addition, the computer may utilize
an algorithm to normalize out the reference spectrum, leaving only the
spectral change due to the sensor before fitting a curve to the spectral
edge of the received spectrum.
A BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and for further
advantages thereof, reference is now made to the following Description of
the Preferred Embodiments taken in conjunction with the accompanying
Drawings, in which:
FIG. 1 is a functional block diagram of one embodiment of the present
invention, wherein light is transmitted and received unidirectionally
along fiber optic lightguides to the sensor;
FIG. 2 is a functional block diagram of another embodiment of the present
invention, wherein light is transmitted and received bidirectionally along
fiber optic lightguides to the sensor;
FIG. 3 illustrates one embodiment of an optical multiplexer for use in the
present invention;
FIG. 4 illustrates a second embodiment of an optical multiplexer for use in
the present invention;
FIG. 5 illustrates one embodiment of a fast scan spectrometer system for
use in a system incorporating the present invention such as shown in FIGS.
1 and 2;
FIG. 6 illustrates an embodiment of a fast scan spectrometer system for use
in one embodiment of a system incorporating the present invention, wherein
the received spectrum of light for a plurality of temperature sensor
probes is separated electronically;
FIG. 7 is a flow chart of an algorithm for determining the probe
temperature utilizing the present invention;
FIG. 8 is a flow chart of a second algorithm for determining the probe
temperature utilizing the present invention;
FIGS. 9a-c illustrate variations of one embodiment of a semiconductor
sensor of the present invention;
FIGS. 10a-c illustrate variations of one embodiment of a semiconductor
sensor of the present invention;
FIGS. 11a-c illustrate variations of one embodiment of a semiconductor
sensor of the present invention;
FIGS. 12a-c illustrate variations of one embodiment of a semiconductor
sensor of the present invention;
FIGS. 13a-b illustrate variations of one embodiment of a semiconductor
sensor of the present invention;
FIGS. 14a-d illustrate variations of one embodiment of a semiconductor
sensor of the present invention; and
FIG. 15 is a graph illustrating the temperature-induced wavelength spectrum
change of the absorption edge of a semiconductor sensor of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 is a block diagram of a system 10 illustrating one embodiment of the
present invention for optically measuring temperature. A broadband radiant
energy source 12, such as a quartz-halogen lamp 112 (see FIGS. 3 and 4),
provides illumination for the temperature measuring system 10. The radiant
energy source 12 will provide illumination for a single semiconductor
temperature sensor, or the source 12 may be time-shared among a plurality
of semiconductor temperature sensors by utilizing an optical multiplexer
14 of the type described below in FIGS. 3 and 4. In another embodiment of
the invention shown in FIG. 6, means are provided for electronically
separating the illumination received from temperature sensor probes which
are continuously illuminated by the radiant energy source 12, eliminating
the need for an optical multiplexor.
A number of "n" optical fiber light guide(s) 16 (l-n) transmit radiant
energy from radiant energy source 12 to a number of n' temperature sensor
probe(s) 18 (l-n').
Lightguides 16 may be coupled to the source 12 through multiplexer 14 when
more than one sensor probe 18 is used.
A fast scan spectrometer system 20 detects the wavelength spectrum
characteristics of the light transmitted from temperature sensor probes 18
through a number of "m" lightguides 22 (l-m). One example of a
commercially available spectrometer that may be utilized as spectrometer
20 is manufactured by Spectron Instrument and identified as Model CE395.
The number of lightguides 22 from the temperature sensor probes 18 may
differ from the number of lightguides 16 to the probes 18, as illustrated
in FIGS. 9-14.
The spectrometer system 20 may also have a lightguide 24 connecting it to
the radiant energy source 12 through multiplexer 14 to provide a reference
signal of the wavelength spectrum of the source 12. This reference signal
may be used to monitor the condition of the light source, the light source
spectrum and to calibrate the photodetector array on a continuous basis.
However, the reference signal transmitted from the energy signal 12
through the lightguide 24 is not necessary to the operation of the
invention.
An analog signal from spectrometer 20 is transmitted through a line 26 to
an analog-to-digital (A to D) converter 28 for converting it to a digital
signal representing the received light spectrum. The digitized signal is
transmitted over a line 30 to a computer 32 for electronic signal
processing. Computer 32 is controlled through input means 34. The computer
32 also is connected to the radiant energy source 12 and optical
multiplexer 14 for operating and controlling their functions. Computer 32
includes a memory means for storing parameters for use in accordance with
one of the algorithms shown in FIGS. 7 and 8 for calculating the sensor
temperature. In addition, memory means of computer may be utilized to
store predetermined temperature limits set through input means 34 to allow
the computer 32 to determine when the temperature of probe 18 exceeds the
predetermined value. The computer 32 of the present invention may be
implemented with any one of a number of microprocessors and associated
memory means commercially available as plug in modules, such as plug in
modules built around the Motorola 68000 series microprocessor or the Intel
8086 microprocessor.
The computer 32 may output the calculated sensor temperature to a visual
display 36 or a printer/plotter 38 for recording temperature data. An
audio/visual alarm of 40 may also be provided to enable the computer to
determine when the sensor temperature exceeds a predetermined value and
providing an alarm to alert an operator that the temperature has exceeded
some predetermined range of temperatures.
A clock pulse is transmitted from computer 32 over control/clock signal
line 42 to A to D converter 28 and line 44 to clock the spectrometer 20.
The spectrometer 20 also transmits a control signal back to the computer
over lines 42 and 44 and through converter 28 to signal when the
spectrometer 20 is through scanning the received signals from the probe
18.
FIG. 2 illustrates an alternate embodiment of a system 50 of the present
invention for optically measuring temperature. The elements of the system
50 which are identical to the system 10 described in FIG. 1 are designated
with the same reference numerals having a "'" designation added. These
elements in system 50 common to those in System 10 will not be described
in connection with FIG. 2, as is understood that they function in a
similar manner described above in the description of system 10.
The temperature measuring system 50 includes a fiber directional coupler 52
for processing light transmitted bidirectionally to a plurality of "n"
lightguides 54 for temperature sensor probes 18'. In addition, the fiber
directional coupler 52 enables light to be transmitted from the
temperature sensor probes 18' unidirectionally to a plurality of "m"
lightguides 22'. Instead of a fiber direction coupler 52, it is to be
understood that a beam splitter and collimating lenses may be used.
FIG. 3 illustrates one embodiment of an optical multiplexer 14a for use in
the present invention. A rotating shutter 60, rotating as shown by
directional arrow 61, has an aperture 62 for allowing the time-sequenced
illumination of a plurality of probes from the same light source, a
quartz-halogen lamp 112. As described in connection with FIG. 1, the
computer 32 may be programmed to control both the lamp 112, including its
intensity, and the optical multiplexer 14a, including timing the
illumination from lamp 112. Light from lamp 112 is allowed to pass on a
time-sequenced basis through aperture 62 to an array of lenses 64 focusing
the light to a plurality of focal points 66 located at the ends of
lightguides 16. In addition, the optical multiplexer 14 allows light from
lamp 112 to be transmitted to the reference lightguide 24 for providing a
reference signal to the spectrometer 20 (FIG. 1).
FIG. 4 is an alternate embodiment of an optical multiplexer 14b. Light from
the quartz halogen lamp 112 is focused by a lens 70 onto a mirror 72 which
reflects the light to an array of focal points 74. The mirror 72 rotates
about an axis 76, as shown by directional arrow 77, so as to direct the
light from lamp 112 on a time sequenced basis to the plurality of "n"
lightguides 16. In addition, light is transmitted on a time-sequenced
basis through a lightguide 24 as a reference signal to spectrometer 20
(FIG. 1).
FIG. 5 illustrates one embodiment of the spectrometer system 20 shown in
FIGS. 1 and 2. The plurality of lightguides 22 from the temperature
sensors 18 enter the spectrometer system 20 through optical connectors 80,
or it is understood that the lightguides may be connected directly to the
spectrometer system 20. Reference lightguide 24 also may be connected with
the spectrometer system 20 through optical connectors 80. The referenced
lightguide 24 provides an optical reference spectrum. The optical
connectors 80 direct the light from lightguides 22 and 24 to an optical
focusing means 82, such as a system of lenses, to direct one of the
time-sequenced illuminated lightguides 22, 24 to an optical spectral
separator, such as diffraction grating 84, for separating the light into
its wavelength spectrum. The optical spectral separator may be implemented
by means of a diffraction grating 84 or a dispersive optical prism (not
shown) to spread the spectrum of the returned light signal into
corresponding angles. The received wavelength spectrum from diffraction
grating 84 is transmitted to a photodetector array 86 having a linear
array of photodiodes for measuring the wavelength characteristics of the
received light signal. The photodetector array 86 of spectrometer system
20 senses the temperature-induced wavelength spectrum shift of the
absorption edge of the semiconductor sensor and the temperature sensor
probe 18. The analog signal representative of the temperature-induced
wavelength spectrum shift is then converted to a digital signal through A
to D converter 28 and processed by the algorithm of computer 32 to
determine the temperature.
In another embodiment of the spectrometer 20, the photodetector array 86
may be replaced by a rotating mirror which projects the spectrum through a
pin hole or slit onto a single, stationary photodetector. While such a
spectrometer would have slower processing speed, it would have the
advantage of lower noise or a higher signal to noise ratio.
FIG. 6 illustrates an alternate embodiment of a spectrometer system 90 for
use in a system similar to that illustrated in FIGS. 1 and 2 but in which
the optical multiplexer 14 may be eliminated. The spectrometer system 90
provides means for electronically reading each of the returned light
signals from the plurality of temperature sensors 18 that are receiving a
continuous transmission of light from the broadband energy source 12. The
spectrometer system 90 includes optical connectors 92 for providing
connections to the lightguides 22 from probes 18 and the reference
lightguide 24 from the radiant energy source 12. Optical fiber coordinator
94 provides a vertical or lateral displacement of the lightguides 22, 24
wherein each return fiber or reference fiber is assigned a different
address along one axis (x or y) in a coordinate system. For purposes of
this description, the lightguides 22, 24 are assigned different positions
along the y-axis.
An optical spectral separator, such as diffraction grating 96, separates
the light from each of the lightguides 22, 24 into its wavelength spectrum
along the x-axis of a rectangular photodetector array 98. Each lightguide
22 from the temperature sensor probes, as well as the referenced light
guide 24, have a different vertical address along the y-axis assigned by
the optical fiber coordinator 94. A wavelength spectrum associated with
that lightguide is spread out along a horizontal row, or x-axis, of the
rectangular array (x,y) of photodetectors in array 98. The received
spectrum from each single lightguide appears as a horizontal line of
intensity upon the horizontal x-axis of the photodetector array 98. The
wavelength spectrum information for each of the probes 18 is transmitted
from the rectangular array photodetector 98 for all of the illuminated
lightguides 22, 24 to be processed by computer 32.
FIG. 7 illustrates a flow chart 110 for the computer 32 to process
information received from spectrometer 20 or spectrometer 90. If the
spectrometer 90 produces probe by probe information in a parallel fashion,
then a buffer would be used by computer 32 in order to serially process
this information. The spectrometer 90 may utilize a rectangular array 98
that produces probe information in a serial fashion. The flow chart 110
describes a method of detecting the absorption peak of the semiconductor
sensor by detecting the wavelength at which the intensity of light has
dropped to one-half of the peak intensity anywhere else in the received
spectrum of light. Following the process steps for the flow chart 110, the
computer 32 first causes the background (dark) spectrum to be read and
stored. The return spectrum from probe "n" is read and stored, and the
background spectrum is subtracted from the returned spectrum. The position
on the spectrum corresponding to one-half the maximum value is found, and
the computer 32 converts this position to a temperature reading using a
calibration table in memory. The temperature of the probe is then recorded
or displayed through visual display 36 or printer/plotter 38 (FIG. 1). The
computer next increments a probe counter by the equation n=n+1. A
comparison is made to determine if the probe number (n) in the counter is
now greater than the preset number for the maximum number of probes
(n.sub.max). If all probes "n" in the system have not processed, the
program returns to read and store the spectrum from the next probe. If all
probes have been processed, or "n" is greater than "n.sub.max ", the
program returns to the initial step and the probe counter is reset by
making n=1. The background (dark) of spectrum is read at the beginning of
each cycle as it may change with time.
FIG. 8 illustrates a flow chart 130 as an alternate method for computer 32
to calculate the temperature serially from a plurality of "n" probes 18.
The method of flow chart 130 differs from that in flow chart 110 in two
respects. First, the reference spectrum of the radiant energy source 12 is
normalized out. Second, a curve fit is performed to the spectral edge so
that all of the data points are used, not just the position of the
spectrum.
The method shown in the flow chart 130 begins by reading and storing the
background (dark) spectrum and reading and storing the source reference
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