Note: Descriptions are shown in the official language in which they were submitted.
1075033
Background of the Invention
In Nelson, United States Patent No. 3,870,467, it was
: disclosed that the reduction efficiency of a chemical recovery
furnace could be monitored by measuring the amount of sodium
vapor produced in the process. The Nelson disclosure proposed
to do this by measuring the intensity of radiation in the
sodium band taround 5890 angstroms) characteristically produced
by the sodium vapor that burns as it rises from the char bed.
Though burning sodium is a source of light in the
sodium band, other sources of such light typically exist in a
recovery furnace. The glow of the char bed and the intermittent
flaring that occurs there both give off some light in the same
waveband as the light produced by the burning sodium, so the
total light in the sodium band is composed of light from several
sources other than burning sodium. In more general terms, any
, - time a process is monitored by measuring the amount of light
occuring in a characteristic waveband, light from sources other
than the process are likely to be included in the measurement.
A As a result, the monitoring of a process by merely measuring the
- 20 amount of light emitted in a characteristic waveband can lack
precision.
Summary of the Invention
Accordingly, the present invention is a method and
apparatus for producing a measurement of the characteristic
light produced by a process. According to the invention, the
general level of background light intensity is determined by
measuring the light intensityin at least one waveband that is not
identical to the characteristic waveband. This background-light
intensity is substracted from the total light intensity in the
characteristic waveband. The resultant quantity is the
~ -2-
i: '
~075033
intensity of only that part of a characteristic light that is
produced by the process.
: In one broad aspect, the invention may be summarized
as residing in a method of monitoring a process by measuring
the intensity of light occurring in a waveband having wave-
lengths characteristic of the process, the light including back-
ground light not produced by the process, the improvement com-
prising: measuring the intensity of light in at least one of a
. plurality of reference wavebands, the intensities in combination
10 being representative of the intensity of the background light
in the characteristic waveband; and subtracting a quantity
proportional to a weighted average of the intensities of light
measured in the reference wavebands from the intensity of light
measured in the characteristic waveband, thereby producing an
indication of the intensity of that portion of the light in the
characteristic waveband that is produced by the process.
In another broad aspect, the invention may be
summarized as residing in an apparatus for monitoring a process,
which apparatus comprises means for producing an output indic-
ative of the intensity of light occurring in a waveband havingwavelengths characteristic of the process, the light including
background light not produced by the process, the improvement
comprising: means for measuring the intensity of light in each
of a plurality of reference wavebands so as to define a curve of
.: intensity-wavelength ordered pairs, the intensities in combin-
- ation being representative of the intensity of the background
light of the characteristic waveband; and means, receiving as
its inputs the outputs of the reference-waveband means and the
characteristic-waveband means, for producing a signal indicative
of the difference between the intensity indicated by the
-2a-
`` 1075033
characteristic-waveband means and a quantity proportional to an
intensity value occurring in the characteristic waveband on
the curve defined by the intensity-wavelength ordered pairs
derived from the outputs of the reference-waveband means. - -
Brief Description of the Drawings
These and further features and advantages of the
invention become evident in the description of the embodiment -
shown in the drawings attached, wherein:
-2b-
.~
, . . .
; ` 1075033
Figure 1 is the side elevation view of a light-receiving
apparatus used in the preferred embodiment of the present invention;
Figure 2 is a front elevation view of the same apparatus;
Figure 3 is a front elevation view of a rotating filter used
with the preferred embodiment of the present invention;
Figure ~ is a side sectional view of a rotating-filter assembly;
Figure 5 is a graph of a typical output signal from the rotating-
: - filter assembly; and
Figure 6 is a block diagram of the signal processing circuitry
for use with the preferred embodiment.
Detailed Descri~tion of the Preferred Embodiment
Figure 1 is a side elevation view of the light-receiving apparatus
used in Nelson. The apparatus has been slightly modified in order to adapt
it to use in the preferred embodiment of the present invention. In Nelson,
it was the intent to monitor the rate of the reduction reaction in a char
bed by measuring the intensity of light characteristic of a related process,
namely the burning of sodium. The light-receiving apparatus is one of a
plurality aimed at the furnace in which the burning of sodium occurs. A
fiber-optic bundle 32, possibl~ capped with a lens 30 for concentrating
20 light on the bundle, is housed within compartment 34, ~hich is attached
- to plate 26. Plate 26 supports slide 28, which holdstwo filters 36, as
shown in Figure 2. Plate member 26 includes aperture 38 through which
light shines on fiber-optic bundle 32. Slide 28 is arranged so tha~ one
window 36 covers aperture 38 while the other window 36 is exposed for
25 cleaning. As can be seen in Figure 1, a tubular section 22 is attached
to the front surface of plate 26, and light from the furnace passes
through tubular section 22. Connection 24 on tubular section 22 admits
purge gas into tubular section 22 in order to prevent blowback from the
furnace, which could cause black liquor and ash to deposit on window 36.
30 Air, steam, or another relatively inert gas mignt typically be used for
purging. Fiber-optic bundles 32 are led from the viewing assemblies to
- a rotating filter assembly, a front vie~ of which is shown in Figure 3.
--3--
5751650
~ .
.
1075033
Three filters, 44, 46 and 48 having different pass bands are unevenly
spaced about rotating disc 50. One of the filters passes light in a
waveband that includes the sodium-light wavelengths. The other two
filters pass light in wavebands equally spaced on either side of the
sodium band. For example, filter 44 might be a sodium-light filter
with half-power wavelengths of 5840 angstroms and 5940 angstroms, while
filters 46 and 48 might be filters that admit light in bands equally
spaced on either side of the sodium band. Typical pass bands for these
filters might be 5540 to 5640 angstroms and 5240 to 5340 angstroms. As
can be seen in Figure 4, disc 50 is rotated by shaft 56 within frame 52.
Fiber-optic bundles 32, one from each viewing apparatus, are mounted so
that each one is in registration with one of the detectors 54, typically
a photo-diode, photo-resistor, or other light-sensitive circuit element.
Rotating disc 50 brings filters 44, 46 and 48 into registration with each
of the bundle-detector pairs in succession, and signals indicative of the
intensity of light within the chosen waveband are transmitted to associated
circuitry. The result of this arrangement is that each detector 54
receives light within each waveband in sequence as the filters pass between
it and the fiber-optic bundle from the furnace quadr~nt associated with it.
The signal sent by each detector 54 resembles the example shown in
Figure 5. As can be seen in Figure 5, the detector 54 gives a reading of
zero intensity during a substantial portion of each revolution. During
other periods, B filter is situated between the detector and its fiber-optic
bundle, permitting light of wavelengths within the waveband passed by the
filter to strike the detector, ~hich sends a signal indicative o~ the
intensity of light within the passed waveband to associated circuitry. For
example, the peak 58 could be a signal sent when filter 44, the sodium-light
filter, passes between a detector 54 and its associated fiber-optic bundle
32. The signal rises as the filter comes more directly in front of detector
54. A narrow plateau occurs during the time when detector 54 recei~-es light
from the entire face of the fiber-optic bundle 32, and the signal falls off
-4-
C751650
' 1075033
as the filter passes on to the next detector. Assuming that di3c 50
rotates in a clockwise direction, peak 60 is indicative of the intensity of
light passed by ~ilter 46, and peak 62 is indicative of the intensity of
light passed by filter 48. Tnus, the combination with the apparatus of
Figures 1 and 2 of one of the filters 44 and any of the detectors 54
constitutes means for producing an output indicative of the indensity of
light occurring in a waveband having wavelengths characteristic of the
process of burning sodium. Similarly, the combination with the apparatus
of Figures 1 and 2 of filters 46 and 48 and any of the detectors 54
constitutes means for producing an output indicative of the intensity of
light in at least one reference waveband, which intensity is representative of
the intensity of background light in the characteristic waveband that was
not produced by the process of burning sodium.
The uneven spacing of the filters would be desirable if signal-
processing circuitry were to be operated in a synchronous manner, that is,
with no direct connection between rotating disc 50 and the synchronizing
circuitry.
Peaks 60 and 62 are of importance because they give an indication
of the general level of background light. The light hitting fiber-optic
bundle 32 and being transmitted by it to the filter assembly is, of
course, composed of light occurring in a whole spectrum of wavelengths,
the light of some wavelengths being more intense than the light of other
wavelengths. Even though the intensities vary, however, most sources of
light have fairly continuous intensity-versus- wavelength characteristics
through most regions of their spectra. In other words, the light intensity
at one wavelength will typically be close to the light intensity at nearby
wa~elengths. At certain character1stic wavelengths, though, the light
intensity greatly exceeds the intensity of light at nearby wavelengths.
The present invention takes advantage of this phenomenon. Since
background sources have relatively smooth intensity-versus-wavelength
C751650
1075033
characteristics at and near the sodium-light wavelengths, an indication
of the background intensity at the sodium-light wavelengths can be
produced by interpolating between the intensities o~ light occurring
at nearby reference wavelengths. Accordingly, the signal-processing
circuitry shown in block-diagram form in Figure 6 averages the values of
peaks 60 and 62 and uses the resultant quantity as the intensity of the
- background light at the sodium-light wavelengths. This background
intensity is subtracted from peak 58 to determine what portion of the
sodium-light radiation intensity is caused by the sodium burning above
the char bed. me resultant intensity has been found to be a measure
of the sodium concentration above the char bed. Since the amount of sodium
produced by the process in the char bed is an indication of reduction
efffciency, the output of the Figure 6 circuitry (described below) is a ~-
more exact indication of reduction efffciency than is the total intensity
of all radiation within the sodium band.
e preferred embodiment employs an averaging circuit to generate
a quantity that represents the background-light intensity, but it is
apparent that many modifications of the preferred embodiment could be
designed that wo-~;d use the basic principal of the present invention
without using a circuit for producing an unweighted average. The averaging
circuit is merely one example of any number of embodiments that could be used
in the present invention. A simpler embodiment, for instance, might
generate the background-intensity indication by producing a signal
proportional to the intensity of light in only one waveband. A slightly
more complicated apparatus might use a weighted average of two wavelengths
to make up for different reference-waveband wiaths or for the fact that
i one reference waveband is closer than the other to the characteristic
wavelength. A weighted-average embodiment, of course, is only a more general
case of the ordinary average used in the preferred embodiment described
below. Accordingly, the term wei~hted average is used in the claims to
refer to the addition of the outputs of any number of reference-waveband
filters, each of which output may be multiplied by an appropriate
C751650
--6--
~075033
coefficient before the addition. Wei~hted svera~e therefore includes the
use of only one reference intensity, the use of the ordinary average of two
or more rePerences, and the use of differing coefficients in the computation
of the average of two or more reference intensities.
An even more elaborate setup might employ one of the curve-fitting
methods known to mathemstics. Such sn embodiment would perform a function
having the effect of finding a curve in the intensity-wavelength two-space
that fits the dats taken at the reference wavelengths, and it would use a value
of thst curve occurring in the charscteristic wsveband ss the background-light
intensity. This would differ from the weighted-sverage embodiment in that the
formuls for producing the background-intensity v~lue would not in general be a
linesr function of the reference intensities. Whichever of these embodiments
is used, the bssic principle is the same--using nearby wavelengths to give an
indication of what the background-radiation intensity is at the characteristic
wavelengths.
Block 64 of Figure 6 represents the rotating-filter assembly of
Figures 3 and 4, and signal line 65 represents the signal from one of the
detectors 54 of Figure 4. Lines 81 represent the signals from the other thr~e
detectors 54. Since signal line 65 contains indications of the intensities of
radistion in all these wsvebands, it is fed to synchronizer 66, which is a cir-
cuit for separsting those intensities from each other. Synchronizer 66 typicallyconsists of three sample-and-hold circuits, one for each filter, and the sample-and-hold circuits are triggered either through a connection to the di~c, therebyproviding synchronization, or by an internally generated trigger signal that
employs the uneven spacing of the filters to determine the orientation of the
disc. Of course, if it is desired to synchronize the sample-and-hold circuits
by means of a direct connection to disc 50, uneven spacing of the filters is
- not necessary.
The intensity indication from the sodium-light filter is fed to
block 58, which represents an amplifier that may or may not be used
depending on the buffering and amplification requirements of the
C751650 ~7~
~ 1075033
particular design. Signals indicative of the intensities of the light passed
by the two reference filters 46 and 48 are fed to block 70, which represents
an averaging circuit, typically an amplifier, that produces a signal indicative
of the average of the intensities indicated by the reference-filter signals.
; 5 A signal is thereby produced that represents the background-light level at the
sodium wavelengths. The resulting signal is fed, along with the output of block
68, to block 72. Block 72 is typically a difference amplifier, and it subtracts
the reference signal from the char&cteristic-wavelength signal. From the above
definition of weighted averaOEe, it can be appreciated that blocks 70 and 72 in
combination constitute means for producing a signal indicative of the differencebetween the intensity indicated by the characteristic-waveband means and a quan-tity proportional to a weighted average of the intensities indicated by the
reference-waveband means. Block 74 is a linear or logarithmic amplifier whose
purpose is to drive a monitor, represented by block 78, in case the output of
the subtraction circuit is not appropriate for that function. Block 78 would
be a DC meter in the typical case.
Block 82 represents parallel signal-processing paths that perform
the same function as blocks 56, 58, 70, 72, 74 and 76 on the other three sigr~lsproduced by block 64. The result is that the light produced by the burning
sodium above the char bed is monitored by, typically, four DC meters, one of
which is represented by block 78, and the other three of which are part of
block 84. In addition, the signal produced by the circuits represented by
block 74 and their corresponding parts of block 82 are averaged to give an
indication of the reduction efficiency of the entire furnace. This averaging
function is included in block 84. Finally, it may be desired to monitor the
level of background light in order, for instance, to inform the operator of
unwanted flames or flaring around the viewing ports or in the char bed. This
background-light level would be indicated by block 80 and corresponding parts
of block 84, typically DC meters.
~0 The circuit just described could easily be operated at disc speeds
of around 1000 RPM. Sampling would then occur approximately once every 60
C751650 -ô-
~075033
milliseconds. Thls would reduce to insignificance any inaccuracies produced
by the fact that different reference-light levels are not measured simultan-
eously, since light-intensity variations would occur at a rate that is much
slower than the sampling rate.
While the invention has been described in con~unction with a specific
embodiment, it is evident that many alternatives, modifications, and variations
will be apparent to those skilled in the art in light of the foregoing descrip-
tion. Accordingly, it is intended to cover all such alternatives as fall withi~
the spirit and broad scope of the appended claims.
What is claimed is:
C751650 -9-
