Programmable detector configuration for Fraunhofer diffraction particle sizing instruments

• US 5,007,737 A
• Filed: 11/01/1988
• Issued: 04/16/1991
• Est. Priority Date: 11/01/1988
• Status: Expired due to Fees

• Current Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Original Assignee: United States Of America As Represented By The Secretary Of The Air Force
• Inventors: Hirleman, Edwin D. Jr.
• Primary Examiner(s): McGraw, Vincent P.
• Assistant Examiner(s): Pham, Hoa
• Application Number:
  US07/266,952
• Time in Patent Office: 896 Days
• Field of Search: 356/335-343, 356/561, 356/356, 356/432, 250/574, 250/575
• US Class Current: 356/336
• CPC Class Codes: G01N 15/0211 Investigating a scatter or diffraction pattern
• CPC Class Codes: G01N 15/0227 using imaging; using holography
• CPC Class Codes: G01N 2021/4716 Using a ring of sensors, or a combination of diaphragm and sensors; Annular sensor

Abstract

An intelligent laser diffraction particle sizing system is disclosed. A portion of the primary laser beam is directed by beam splitter into a calibration leg, modulated, and passed through a diffraction reticle. The known diffraction signature of the reticle is then recombined with the primary beam to pass through the particle field. Both the modulated calibration diffraction signature and the portion of the primary beam scattered by the particles are collected by the transform lens and detected at the back focal plane using an X-Y photodiode array. The detector array elements are grouped into variable geometry annular rings. The ring detectors are continuously centered around the beam using feedback from the X-Y position detector which monitors beam deflection.

Claims

1. A particle sizing system for determining particle size distributions in liquids and gases in a particle field using Fraunhofer diffraction patterns, said particle sizing system comprising:

a means for illuminating said particle field with a Fraunhofer diffraction pattern, said Fraunhofer diffraction pattern having a center axis;

a transform lens which receives and focuses said Fraunhofer diffraction pattern from said particle field at its focal point to produce an output;

a transform plane which is fixed at the focal point of said transform lens, said transform plane comprising a spatial light modulator which physically separates different bands of light of said Fraunhofer diffraction pattern in order to produce an output by passing only light through concentric rings which are adjustable in radius and width about the diffraction pattern'"'"'s center axis, said spatial light modulator thereby allowing adjustable detection of selective particle size distributions since light scattered by very large is concentrated about comparatively small angles from said center axis, while light scattered by comparatively small particles in the particle field goes into comparatively large angles about said center axis of said diffraction pattern from said transform lens;

a detector which receives and measures said Fraunhofer diffraction pattern from said transform plane at increments that are adjusted by said spatial light modulator to indicate thereby said particle size distribution by detecting magnitudes of discrete angular distributions of scattered light in said Fraunhofer diffraction pattern from said transform plane;

an on-line test means which tests said particle sizing system using a particle field sample with a known particle size distribution which is compared to detected particle size distributions measured by said detector, said particle field sample comprising a diffraction reticle which includes a transparent sheet with a particle array of circular apertures with said known particle size distribution;

a means for detecting the center axis of said Fraunhofer diffraction pattern from said transform plane, said detecting means outputting a detection signal; and

a means for controlling said spatial light modulator and said detector, said controlling means receiving said detection signal from said detecting means, and physically moving said detector so that it is aligned with the center axis of said Fraunhofer diffraction pattern from said transform lens, said controlling means also adjusting said spatial light modulator to pass only light through concentric rings which are incrementally adjusted in radius and width about said center axis of said Fraunhofer diffraction pattern, said controlling means also receiving and displaying measurements made by said detector to indicate thereby said particle size distribution in said particle field.

2. A particle sizing system, as defined in claim 1, wherein said controlling means comprises:

a servomechanism which contains at least three pizoelectric transducers which are physically in contact with said detector to physically tilt it to be aligned with said center axis of said Fraunhofer diffraction pattern in response to a set of electrical adjustment signals; and

a computer which is electrically connected with said servomechanism, said detector, said spatial light modulator and said detecting means, said computer receiving said detection signal from said detecting means and determining a location of the center axis of said Fraunhofer diffraction pattern therefrom, said computer then sending said set of electrical adjustment signals to said servomechanism to cause it to tilt said detector so that it is aligned with said center axis of said Fraunhofer diffraction pattern.

3. A particle sizing system, as defined in claim 2, wherein said illuminating means comprises:

a laser which produces an illuminating laser beam; and

a first beam expander spatial filter which receives and expands said illuminating laser beam from said laser through a first slit to produce said Fraunhofer diffraction pattern for said particle field.

4. A particle sizing system, as defined in claim 3, wherein said on-line test means comprises:

a first beam splitter which receives and splits said illuminating beam from said laser and outputs thereby said illuminating beam to said first beam expander as well as a sample of said illuminating beam;

an acousto-optic modulator which receives and modulates said sample of said illuminating beam from said first beam splitter to produce thereby a modulated sample of said illuminating beam;

a second beam expander spatial filter which receives and expands said modulated sample of said illuminating beam through a second slit to produce an output which includes a Fraunhofer diffraction pattern of said modulated sample of said laser beam;

a diffraction reticle which intercepts said output of said second beam expander with said modulated sample with said known particle size distribution which is compared with the measurement made by said detector, said diffraction reticle producing an output thereby; and

a beam combiner which receives and combines the output of the diffraction reticle with the Fraunhofer diffraction pattern of said first beam expander spatial filter to produce a combined beam which is directed towards said transform lens.

5. A particle sizing system, as defined in claim 4, wherein said detecting means comprises:

a second beam splitter which receives and splits said output of said transform lens to forward it to said transform plane while extracting a sample; and

a photodiode array which receives and detects said sample from said second beam splitter to output thereby a detection signal which indicates an X and Y position of said output of said transform lens which, in turn, indicates to said computer the center axis of said Fraunhofer diffraction pattern.

6. A particle sizing system for determining particle size distributions in liquids and gases in a particle field using Fraunhofer diffraction patterns, said particle sizing system comprising:

a laser which produces an illuminating laser beam;

a first beam expander spatial filter which receives and expands said illuminating laser beam from said laser through a first slit to produce said Fraunhofer diffraction pattern for said particle field, said Fraunhofer diffraction pattern having a center axis;

a transform lens which receives and focuses said Fraunhofer diffraction pattern from said particle at its focal point to produce an output;

a transform plane which is fixed at the focal point of said transform lens, said transform plane comprising a spatial light modulator which physically separates different bands of light of said Fraunhofer diffraction pattern in order to produce an output by passing only light through concentric rings which are adjustable in radius and width about the diffraction pattern'"'"'s center axis, said spatial light modulator thereby allowing adjustable detection of selective particle size distributions since light scattered by very large particles is concentrated about comparatively small angles from said center axis, while light scattered by comparatively small particles in the particle field goes into comparatively large angles about said center axis of said diffraction pattern from said transform lens;

a detector which receives and measures said Fraunhofer diffraction pattern from said transform plane at increments that are adjusted by said spatial light modulator to indicate thereby said particle size distributions by detecting magnitudes of discrete angular distributions of scattered light in said Fraunhofer diffraction pattern from said transform plane;

a first beam splitter which receives and splits said illuminating beam from said laser and outputs thereby said illuminating beam to said first beam expander as well as a sample of said illuminating beam;

an acousto-optic modulator which receives and modulates said sample of said illuminating beam from said first beam splitter to produce thereby a modulated sample of said illuminating beam;

a second beam expander spatial filter which receives and expands said modulated sample of said illuminating beam through a second slit to produce an output which includes a Fraunhofer diffraction pattern of said modulated sample of said laser beam;

a diffraction reticle which intercepts said output of said second beam expander spatial filter with said modulated, sample with said known particle size distribution which is compared with the measurement made by said detector, said diffraction reticle producing an output thereby;

a beam combiner which receives and combines the output of the diffraction reticle with the Fraunhofer diffraction pattern of said first beam expander spatial filter to produce a combined beam which is directed towards said transforms lens;

a second beam splitter which receives and splits said output of said transform lens to forward it to said transform plane while extracting a sample;

a photodiode array which receives and detects said sample from said second beam splitter to output thereby a detection signal which indicates an X and Y position of said output of said transform lens which, in turn, indicates the center axis of said Fraunhofer diffraction pattern; and

a servomechanism which contains a least three piezoelectric transducers which are physically in contact with said detector to physically tilt the detector to be aligned with said center axis of said Fraunhofer diffraction pattern.

7. An on-line test system that tests an optical particle size distribution sensing system which illuminates a medium with an illuminating beam from a laser to produce a diffraction pattern of scattered light, said optical particle size distribution sensing system having a means for measuring said diffraction pattern of scattered light to determine therefrom a measured particle size distribution in said medium, said on-line test system comprising;

a first beam splitter which receives and splits said illuminating beam from said laser and outputs thereby a sample of said illuminating beam;

an acousto-optic modulator which receives and modulates said sample of said illuminating beam from said first beam splitter to produce thereby a modulated sample of said illuminating beam;

a beam expander spatial filter which receives and expands said modulated sample of said illuminating beam through a second slit to produce an output which includes a Fraunhofer diffraction pattern of said modulated sample of said laser beam;

a diffraction reticle which intercept said output of said beam expander spatial filter with said modulated sample with a known particle size distribution which is compared with the measurement made by said optical particle size distribution sensing system said diffraction reticle producing an output thereby; and

a beam combiner which receives and combines the output of the diffraction reticle with the Fraunhofer diffraction pattern of said beam expander spatial filter to produce a combined beam which is directed towards said measuring means of said optical particle size distribution sensing system so that said measured particle size distribution produced by said particle size distribution sensing is compared with said known particle size distribution of said diffraction reticle to yield thereform an indication of the optical particle size distribution sensing system'"'"'s accuracy.

8. An on-line test system, as defined in claim 7, wherein said diffraction reticle comprises a transparent sheet with a particle array of circular apertures with said known particle size distribution.