Compact, low cost particle sensor

A compact, low cost particle sensor utilizing a photodetector (31) which directly collects light scattered by particles (33) entrained in a fluid traversing a beam of light (32). The beam of light (32) is aligned such that it is in close proximity to the photo detector (31). The beam of light (32) is typically provided by a laser and associated focusing/collimating optics. The beam of light (32) intersects a portion of the fluid flow permitting a low pressure drop system and fluid flow generated by a low cost, low pressure device such as an axial fan (50).

FEDERALLY SPONSORED RESEARCH

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SEQUENCE LISTING OR PROGRAM

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BACKGROUND

1. Field of Invention

This invention relates generally to systems that use light scattering techniques for the detection of particles in a fluid (e.g. liquid or gas), which systems are generally referred to in the art as particle sensors or particle counters.

2. Prior Art

Typically, a particle counter works by drawing a sample of air through a beam of light and detecting the light scattered off the particles entrained in the air flow. These particles scatter light in proportion to their size, composition, shape and other physical properties. Lenses, mirrors, or other light collection techniques are used to increase the portion of the scattered light which is focused onto a photoelectric device (hereinafter referred to as a photodetector). The photodetector converts this scattered light into an electrical signal. This electrical signal is typically a pulse whose amplitude is related to the amount of scattered light reaching the photodetector and whose duration is typically related to the transit time of the particle through the beam of light. Thus, from the photodetector output and associated circuitry information about the number and size of particles in a sampled volume of air can be determined.

At the present time particle counters typically cost several thousand dollars or more. Particle counters typically contain a number of expensive components or assemblies. Typically, a blower or pump is used to generate the necessary vacuum to draw the fluid flow through a sensor assembly/chamber. The sensor assembly is typically sealed except for an inlet and exhaust opening. The inlet typically has a “nozzle” or “inlet jet” which may be a machined or formed component through which the air to be sampled passes before entering the beam of light. As particle counters typically assume the total flow of air through the instrument is being sampled for particles, care must be taken in the alignment of the nozzle over the beam of light so that all air leaving the nozzle passes through the beam. The sensor will also typically contain collection optics to gather a large percentage of the light scattered off particles passing through the beam. These can consist of expensive components such as mirrors or lenses. In addition, particle counters typically use pressure sensors and/or mass flow sensors to determine the volumetric flow through the beam of light. The above components add significant cost to a particle counter.

There are many applications in which monitoring the concentration of airborne particles would be useful, such as testing indoor air quality, but a cost of several thousand dollars is a deterrent. Therefore, a need exists for a light scattering device which eliminates many of the above expensive components to provide low cost particle monitoring.

SUMMARY

The invention is an improvement in a light scattering particle sensor or optical particle counter. In accordance with one embodiment, the cross-sectional area of the flow passage through the sensor is larger than the area of the beam of light which it intersects. Thus, only a portion of the air flow is illuminated by the beam of light and only a portion of the air flow is sampled for particles. Also in the improvement, the beam of light passes in close proximity to a photodetector such that a sufficiently large percentage of the light scattered off the particles will directly strike the photodetectors as to enable particle detection without the need for a light collection system utilizing mirrors, lenses, or other light collection techniques.

The large flow passage through the sensor allows the sensor to operate at very low vacuums of less than 0.2 inches of H2O (1 inch of H2O, or water, is defined as a differential pressure of 248.84 pascals at 60 degrees Fahrenheit and a vacuum of 1 inch of H2O is a differential pressure of 248.84 pascals from ambient pressure at 60 degrees Fahrenheit). It further allows loose tolerances on the sealing of the sensor because minor leaks will not appreciably affect the flow rate through the sensor. In contrast, current particle sensors typically contain a block with an inlet and exit, but otherwise tightly sealed, referred to as a “flow cell”, “sensor chamber”, “detector housing”, “sensor assembly”, or other such name. The approach of this invention allows this block to be eliminated and the entire enclosure for the particle counter to be made of two plastic pieces injection molded to standard tolerances. Operation at low vacuum also permits the use of a low cost axial fan or blower to generate the air flow. The large flow passage also permits the elimination of a nozzle or inlet jet which typically require precise alignment to the beam of light.

In another embodiment of the invention the need to measure the flow via a pressure and/or flow sensor is eliminated by measuring the pulse width of the photo detector output to determine the transit time of the particles through the beam of light and calculate the flow rate.

In another embodiment of the invention a light baffle is placed between the beam of light and the photodetector to improve the particle size resolution.

In another embodiment of the invention a lens is placed between the beam of light and the photodetector to improve the particle size resolution.

Other details of the invention are set forth in the following detailed description and in the drawings.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of the invention is illustrated inFIG. 1which shows a perspective view of particle sensor10.FIG. 2illustrates an exploded view of particle sensor10showing that the enclosure is made up of two pieces, an enclosure front11and an enclosure back12which are held together with screws. Also, a circuit board20is attached to the enclosure front11. Circuit board20has attached to it photodetector31and light source30. In this embodiment, the light source30is a collimated/focused laser beam. In this embodiment, the photodetector31is a Silicon PIN photodiode. In this embodiment, circuit board20contains circuitry familiar to those skilled in the art which will power the light source30, convert the photodetector31output to electrical pulses, process those pulses to obtain size and count information, control the fan50(shown inFIG. 4), output data for display on the LCD14(shown inFIG. 1), monitor switch13inputs, and perform all other control and input/output functions for the particle sensor.

FIG. 3is a section view illustrating the positioning of the light source30and photodetector31relative to the baffles15,16, and17. These baffles are molded into the rear enclosure12and serve to control stray light and direct air flow within the particle sensor (arrows show the direction of flow). The control of stray light, as is known to those skilled in the art, is important to reduce unwanted output from the photodetector31. The baffles15,16, and17function to reduce stray light reaching the photodetector31from outside the particle sensor. In addition, baffles16and17reduce stray light from the light source30by forming what is known to those skilled in the art as a “light trap”, “light stop”, “beam dump”, “beam stop”, etc.FIG. 3also illustrates the relationship between air flow passage34. the beam of light32, the photodetector31, and the air flow. The air flow passage34is located upstream (“upsteam” is defined as the direction from which the air flow is coming) of the beam of light32. Thus, the air first flows through the air flow passage34and then through the beam of light32.

FIG. 4is a section view which further illustrates the air flow (shown by arrows) within the particle sensor. The air is drawn in through openings18at the top of the enclosure back12and exhausted out through openings19at the bottom of the enclosure back12. In this embodiment, the air flow is created by axial fan50.FIG. 4also illustrates how the baffles15,16(not shown), and17are part of the enclosure back12and contact the circuit board20.

FIG. 5is a diagram (not to scale) which shows the positioning of the light source30, the light beam32, the photodetector31, and the air flow passage34. Also shown are particles33entrained in the air flow. In the diagram, the direction of air flow is into the page through air flow passage34. The area of air flow passage34is greater than the area of the light beam32under the air flow passage34such that only a portion of the particles33passing through the particle sensor will traverse the light beam32. By way of example, the cross sectional area of the flow passage34could be 75 square millimeters, the width of the beam of light32could be 0.5 millimeters, and the height of the beam above the photodetector could be 1.0 millimeters, although other geometries are possible. Again, by way of example, with the above geometry, many typical low cost axial fans (60 mm×60 mm) will produce less than 0.1 inches of pressure drop across the flow passage. As can be seen from the example dimensions and the small size of the axial fan, this embodiment permits a compact particle sensor to be constructed.

Continuing withFIG. 5, that portion of particles33which traverse the light beam32will scatter light as they pass through the beam. A portion of this scattered light is illustrated inFIG. 5by arrows. As can be seen inFIG. 5, particles near the center of the photodetector31will scatter more light onto the photodetector31than particles near the edge of the photodetector. The pulse output of the photodetector for a given size particle will tend to be relatively uniform for particles near the center and will drop off rapidly for particles near or beyond the edge of the photodetector31.

The rate at which air passing through the particle sensor is sampled for particles is the “effective flow rate” and is less than the actual flow rate of air through the air flow passage34. To a first approximation, the effective flow rate is the flow of air through the light beam32directly over the photodetector31. A more accurate calculation of the effective flow rate can be made by those skilled in the art by using Mie scattering theory, the light beam width, the geometry of the photodetector31relative to the light beam32, the velocity of the air passing through the light beam, and the sensitivity of the photodetector as a function of the angle of incidence of the scattered light. Alternatively, those skilled in the art may determine the effective flow rate by 1) calibrating the count threshold of the photodetector output to its median response to uniform sized calibration particles, 2) measuring the count rate of the calibration particles, 3) determining the true concentration per unit volume of air of the calibration particles using a reference particle counter such as a Condensation Nucleus Counter, 4) calculating the effective flow rate by dividing the count rate by the true concentration and multiplying by 2. In this embodiment, the circuit board20contains a microprocessor and associated circuitry which, using techniques known to those skilled in the art, determines the count rate by monitoring the output of photodetector31. This can be done using either analog, digital, or a mix of methods. The microprocessor then calculates the concentration of particles per unit volume by using the count rate and the effective flow rate. If the speed of the particles through the light beam32changes then the effective flow rate will change accordingly. The microprocessor can compensate for any change in flow rate by monitoring the pulse width of the photodetector response pulse and adjusting the value used for the effective flow rate when calculating the particle concentration.

In another embodiment, the particle sensor can control the axial fan or other flow generating device, using techniques known to those skilled in the art, to maintain a nominal pulse width and thus maintain a nominal effective flow rate.

Another embodiment is shown inFIG. 6which has a light baffle35between the light beam32and the photodetector31. As known by those skilled in the art, the light baffle35improves the ability of the particle sensor to resolve particle size by blocking light from the more distant particles.

In another embodiment, a lens (not shown), with or without a light baffle, can be added between the light beam and the photodetector to further improve the particle size resolution.

Although the air flow passage is shown as rectangular inFIG. 5andFIG. 6and in a particular size relationship to the light beam32and the photodetector31, other arrangements are possible including a non-rectangular shape for the air flow passage34, an air flow passage34narrower than the photodetector31, and other geometric configurations.

In an alternate embodiment, which is not described in the prior art, the photodetector output is digitally processed in a manner distinct from that described in U.S. Pat. No. 5,870,190. In this new method the pulses will be digitized in a manner similar to that described in U.S. Pat. No. 5,870,190 but peak detection will not be used to size the particles. Rather, the digitized pulses will essentially be integrated by summing the digital values obtained for each distinct pulse. This summation will be related to the total amount of light scattered by the particle and will be used to determine the particle size.

In an alternate embodiment, the photodetector output is digitally processed in a manner distinct from that described in U.S. Pat. No. 5,870,190. In this new method the pulses will be digitized in a manner similar to that described in U.S. Pat. No. 5,870,190 but peak detection will not be used to size the particles. Rather, the digitized output of the photodetector will be continuously monitored to check for a transition through the count threshold and if so a particle will be counted for the size corresponding to that threshold.

Although the description above contains many specificities, these should not be construed as limiting the scope of the embodiment but as merely providing illustrations of some of the presently preferred embodiments. Thus the scope of the embodiments should be determined by the appended claims and their legal equivalents, rather than by the examples given.