Optics: measuring and testing – Velocity or velocity/height measuring – With light detector
Reexamination Certificate
2002-01-29
2003-08-26
Buczinski, Stephen C. (Department: 3662)
Optics: measuring and testing
Velocity or velocity/height measuring
With light detector
C073S861060, C708S422000
Reexamination Certificate
active
06611319
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical flow sensor which cross correlates digitized signals from a plurality of photodetectors that are separated in a direction parallel to a predetermined direction of gas flow and to a method of determining gas flow velocity in a predetermined direction.
2. Description of the Prior Art
Accurate flow rate measurement represents one of the biggest problems in industrial and environmental monitoring for contaminants in gaseous discharges. The problem of airborne pollution has become so acute that virtually all industries of any size are now monitored for compliance with applicable regulations or laws limiting gaseous discharges. While the determination of the concentration of pollutants, contaminants, noxious gases, and toxic gases in a gas flow has reached a satisfactory level of reliability, monitoring of the rate, and hence the volume of gas flow, has heretofore been rather inaccurate. As a consequence. The determination of the total extent of pollutants emitted from any particular facility has heretofore been uncertain.
Current technologies that estimate flow rate often rely upon a point sensor that intrudes into the path of a flowing gas, for example in the chimney or smoke stack of an industrial processing plant. The estimates for flow rate achieved with such instruments are often inadequate for industrial and environmental applications. One problem is that the intrusion of a sensor head into the flow medium, alters the resulting flow measurements. Furthermore, the flow medium being monitored is often very dirty and corrosive. Direct contact of the sensing head with the flow medium often quickly fouls the sensing head so that maintenance of the sensor is a major problem.
Attempts have been made to devise ultrasonic gaseous flow measuring devices. However, ultrasonic gaseous flow measuring instruments are very expensive and are also rather inaccurate.
Since pollutants and contaminants often provide a distinctive color of conglomerated dust particles to a gaseous discharge, it is sometimes possible to visually monitor chimneys and smokestacks from a distance in order to ascertain a general estimate of rate of gaseous discharge. However, this requires a direct line of sight to the facility involved. Also, some noxious and toxic gases, such as carbon monoxide, are colorless. Moreover, visual observation is not an option at night or in inclement weather. Unfortunately, a monitored industrial facility may limit the volume of its gaseous discharges during the daytime only to increase them at night when the volume of gaseous discharge cannot be observed.
SUMMARY OF THE INVENTION
The present invention adopts an entirely new approach to measurement of the velocity of a gaseous flow. The invention utilizes an optical transmitter and an optical receiver located on opposite sides of a gas flow passageway, such as a chimney or smokestack. A beam of light is transmitted across the gas flow passageway. The receiver includes a plurality of photodetectors longitudinally separated from each other in a direction parallel to the predetermined direction of gas flow through the passageway. Particulate matter and eddies in the flowing gas produce scintillations in the beam of light transmitted across the passageway.
These scintillations are detected by all of the photodetectors on the opposite side of the passageway, but detection does not occur at the same time in each photodetector. Rather, since the particle or eddy that creates the scintillation is traveling downstream in the gas flow, the photodetector located furthest upstream will detect the scintillation earlier in time than a photodetector located further downstream. This physical phenomenon is similar to the moving shadow cast by a bird or a piece of debris being carried along on the wind. Temporal cross correlation analysis is then performed on the digitized signals in a digital signal processor utilizing a fast correlation algorithm in which the total number of calculations is proportional to 2N, as contrasted with conventional systems in which the number of correlations is proportional to N
2
.
Fast Correlation Algorithm (FCA)
One major challenge of calculating cross correlation of two data strings with N time delays is that conventional cross-correlation techniques need a number of calculations proportional to N
2
. When N is a large number, these calculations are extremely time consuming. To speed up the calculation of the cross-correlation function, a Fast Correlation Algorithm (FCA) according to the present invention has been developed as follows.
1. For two data strings with N (=2
n
) data points, i.e., A
1
, A
2
, . . . A
N
, and B
1
, B
2
, . . . B
N
, the correlation C
1
can be calculated as the ensemble average of the product of A
i
and B
i+1
, where i=1 . . . N−1. This calculation needs a total of N multiplications.
2. By averaging the two adjacent points of both strings to form two new strings each with length N/2. The correlation C
2
can be calculated with N/2 multiplications.
3. Averaging the two adjacent points of the previous two strings once again to form another two strings each with length N/4. The correlation C
4
can be calculated with N/4 multiplications.
4. Iteratively doing this process n (=log
2
N) times. A total of n strings formed each with the data length of N, N/2, N/4 . . . 4, 2, respectively.
From these calculations, the cross correlations C
1
, C
2
, C
4
. . . C
N/2
can be obtained to retrieve the crosswind. The major advantage of the Fast Correlation Algorithm (FCA) is that the total number of calculations is proportional to 2N instead of N
2
. This represents a reduction by a factor of N/2 in the number of calculations required for cross correlation. This crucial reduction, especially when N is a large number, allows the presently commercially available two-channel digital-signal-processors (DSP) to be utilized since they are fast enough to process the crosswind retrieval algorithm due to the reduced number of calculations that are required.
In one broad aspect, the present invention may be considered to be an optical flow sensor for measuring gas flow velocity in a predetermined direction of gas flow. The optical flow sensor of the invention is comprised of an optical transmitter, an optical receiver, signal-controlled gain amplifier circuits, analog-to-digital converters, and a digital signal processor. The optical transmitter generates a collimated optical beam across the predetermined direction of gas flow. The optical receiver includes a plurality of receiving lenses all located in optical communication with the optical transmitter and in the path of the optical beam. The receiving lenses are separated from each other in a direction parallel to the predetermined direction of gas flow. A separate optical photodetector is provided for each of the receiving lenses. Each photodetector produces an electronic output that varies in response to scintillations occurring in the optical beam.
A separate signal controlled gain amplifier circuit is coupled to each of the optical photodetectors. A separate analog-to-digital converter is coupled to each of the gain amplifier circuits for separately digitizing outputs from each of the gain amplifier circuits.
A digital signal processor is coupled to receive inputs from all of the gain amplifier circuits. The digital signal processor determines the temporal cross variance of the digitized outputs for scintillation events detected separately by all of the photodetectors. Cross correlation is performed using a logarithmic time delay function based upon an original data string with N=2
n
data points. The fast correlation algorithm averages successive data points in pairs to form a new data string having a length half that of the original data string. The fast correlation algorithm averages each two adjacent points of the new data string to form another data string having a length half that of the data string immediately prior the
Buczinski Stephen C.
Optical Scientific, Inc.
Thomas Charles H.
LandOfFree
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