Optics: measuring and testing – By particle light scattering – With photocell detection
Reexamination Certificate
1999-09-28
2001-08-28
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
By particle light scattering
With photocell detection
C356S337000
Reexamination Certificate
active
06281973
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
REFERENCE TO A “MICROFICHE APPENDIX”
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of determining particle size distribution and more specifically to an optical detection system and method for the measurement of particle size distribution in a flow field that is oscillated at various frequencies.
2. Discussion of the Related Art
The measurement of particle size distribution finds use in the process industries in the manufacture of pharmaceuticals, chemicals, abrasives, ceramics, pigments and the like, where the particle size affects the quality of the manufactured product.
A number of methods presently exist for determining the size distribution of particulate material for particles in the approximate size range of 0.1 to 1000 microns in diameter. The conventional method of measurement at high concentration is dynamic light scattering, as taught by U.S. Pat. No. 5,094,532 to Trainer et al., patented Mar. 10, 1992. This patent discloses a fiber optic Doppler anemometer and method that directs a beam of light into a scattering medium that contains particles in Brownian motion. The frequency of the scattered light is compared to non-scattered light emitted from the scattering medium and results in the generation of a first signal having a magnitude that is indicative of the difference in frequency between the scattered light and the non-scattered light. A second signal is generated having a magnitude that varies with frequency on a linear scale. The frequency scale of the second signal is then translated into a logarithmic scale and deconvolved to determine the size and distribution of moving particles within the scattering medium. The translation and deconvolving requires translation of analog signals to digital signals and subsequent processing by a central processor and a vector signal processor using fast Fourier transform techniques (FFT). In order to solve for a known particle size distribution of over 80 particle diameters the method just described must sample over 80 frequencies. Even though this method provides an accurate measurement of particle size distribution, it does require a long time period (usually greater than two minutes) to process all of the sample frequencies, due primarily to the stochastic nature of Brownian motion. This technique is best suited for use in a laboratory with samples that have been extracted from a process and properly prepared for measurement analysis. Additionally, this method is strongly dependent upon dispersant viscosity and temperature and the use of non-flowing sample delivery systems. Although this technique provides accurate results for particles having diameters less than 1 micron, it exhibits poor size and volume accuracy for particles above 1 micron.
Another recognized technique and method for measuring the size distribution of very small particles is static light scattering, or angular light scattering. In this method, a collimated monochromatic light beam irradiates an ensemble of particles that flow perpendicularly through the collimated light beam. Light scattered from the particles emerges from the interaction over a range of angles from the axis of the collimated beam. The scattered light is collected by a lens placed in the path of the scattered light. The scattered light patterns focused in the focal plane of the lens are typically measured by an array of photodetectors placed in the focal plane. The angular extent of the scatter pattern is determined by the size of the particles. The smaller the particle, the wider the angular extent of the scatter; the larger the particle, the narrower the angular extent of the scatter.
One such method is taught by U.S. Pat. No. 5,416,580 to Trainer, patented on May 16, 1998, which uses multiple light beams to irradiate the particles. This method has demonstrated excellent measurement results for particles in the 0.1 to 3000 micron range in flowing sample systems, without temperature or viscosity dependency. Unlike the dynamic scattering techniques, measurements can be made in less than five seconds with repeatability superior to that of the dynamic light scattering. However, in order to produce good measurement accuracy for a process sample at a high concentration, for example 10% by volume, the process sample must be properly diluted with a dispersant medium to minimize the particle concentration.
Each of the above described techniques is limited to a certain range of particle size, concentration and shape. Particles of many shapes are encountered in the aforementioned industrial processes. In certain applications hydrodynamic particle size measurement techniques present a better correlation to the product quality than the optical particle size measuring techniques for irregularly shaped particles. A particularly difficult region is between 0.5 to 1 microns, where both static and dynamic scattering can present somewhat of a less-than-accurate measurement of particle size distribution. Hydrodynamic particle size measurement techniques include a basic concept of detecting a particle's motion or oscillations in a fluid dispersant caused by a vibrating surface or an ultrasonic wave. Depending on the oscillating frequency applied to the dispersant fluid, the particles will closely follow the oscillation of the dispersant fluid.
BRIEF SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide an optical detection system and method that employs an oscillating flow field used in the accurate measurement of particles suspended within a dispersant medium.
In accordance to the object of the present invention there is provided an optical detection system and method comprising a light source for producing light energy and a first light guide for conveying the light energy from the light source to a first light guide face end that is immersed in the dispersant medium. A first portion of the light energy exits the face end to irradiate the particles contained in the dispersant medium and a second portion of the light energy is reflected from the face end back into the first light guide.
The optical system of the present invention further includes a frequency oscillator that produces at least one signal representing a specific frequency within a frequency range. A frequency transducer mounted to the first light guide receives the frequency oscillator signal and oscillates the first light guide face end at the applied specific frequency. The transducer oscillations are further coupled into the dispersant medium, causing the dispersant medium and the particles contained therein to oscillate at the oscillation frequency of the first light guide face end. The light energy scattered by the oscillating particles is captured by the first light guide face end and is mixed within the first light guide with the reflected light energy, producing an optical Doppler beat signal for the applied specific frequency. A second light guide, optically connected to the first light guide, conveys the optical Doppler beat signal to a light detection device. The light detection device produces an output signal representative of the optical Doppler beat signal.
A mixer circuit receives the specific frequency signal from the frequency oscillator and the output signal from the detection device and is arranged to produce and track the center of a plurality of derived harmonics for the input-specific frequency signal. The mixer circuit further generates frequency components for the input Doppler beat signal for each derived harmonic that is used for producing a total power value signal for each derived harmonic of the frequency.
The total power value signals are applied to a signal processing system that calculates a particle motion amplitude signal for each applied specific frequency. The particle motion amplitude signal is used to determine the percentage of the total particles wh
Microtrac, Inc.
Nguyen Sang H.
Pham Hoa Q.
Wood Herron & Evans LLP
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