Compact, rugged sensor for optical measurement of the size of particles suspended in a fluid

COMPACT, RUGGED SENSOR FOR OPTICAL MEASUREMENT OF THE SIZE OF PARTICLES SUSPENDED IN A FLUID

ABSTRACT
A compact, rugged sensor for use in the measurement of particle sizes in a fluid stream is disclosed. The sensor permits the use of a light emitting diode as a light source by mounting a high grade optical system with its focal point at the point light source provided by the light emitting diode to collimate the generated light. This collimated light passes through an aperture formed in one dimension by the width of the fluid flow stream and formed in a second perpendicular dimension by a narrow transparent slit in an otherwise opaque shield adjacent the fluid flow stream. The collimated light source, together with the use of the flow stream as a light path boundary, substantially increases the accuracy of the sensor. All of this is accomplished in conjunction with a solid state light emitting diode source which permits the sensor to be used in shock and vibration environments which would have prohibited particle measurement using prior art sensors.
COMPACT, RUGGED SENSOR FOR OPTICAL MEASUREMENT OF THE SIZE OF PARTICLES SUSPENDED IN A FLUID
BACKGROUND OF THE INVENTION
The present invention relates to particle size measurement devices, and devices used for counting particles in fluid streams wherein the particles are discriminated on the basis of their size.
More specifically, the present invention pertains to a sensing element used in such systems, the sensing element comprising a light source and light sensitive element positioned on opposite sides of a fluid flow channel. Particles carried by a fluid stream in the flow channel cast a shadow on the light sensitive element, which shadow causes a change in the output signal level from the light sensitive device. By utilizing sensitive, well known prior art signal detectors, the change in this signal can be correlated with a given particle size, and thus particles of various sizes

can be measured and counted, the detector signal being supplied to a discriminator circuit which divides the particles into size groups.
While the state of the particle size detecting art is quite advanced, numerous problems have continued unsolved in prior art devices. These problems both limit the permissible environment in which the sensors may be utilized and, at the same time, limit the sensor accuracy. It will be understood by those skilled in the art that particle size discriminating devices, such as those of the present invention, are used in a wide variety of applications. For example, these devices are used for monitoring the quantity and size of impurities in liquid streams to determine when liquids meet cleanliness specifications. They are also used in the production of powders and granulated material to monitor grain size and size distribution. Other uses for the equipment are well known and are limited only by the requirement that the particles to be measured must be carried past the light source and sensing elememt in a fluid or air stream and must be small enough t6 pass through the flow channel provided between these elements.
In the past there has been a requirement for compromise between the smallest particle sizes which could be measured by such devices and the accuracy and reliability of the devices. As smaller and smaller particle sizes are measured, the light flow path is commonly also reduced in cross-sectional area.