MULTI-LOBE PUMP FOR PARTICLE COUNTERS

Abstract: A particle counter having a housing (13) having a view volume (11) where particles are counted by light scattering or obscuration of a beam (20) from a light source (19) intersecting the gas flow path. The light after interaction with the particles detected by a detector (21). There is a pump (25) connected to a gas outlet port (17) which establishes a gas flow through the view volume (11). The pump (25) has a housing; a pair of shafts having lobes is mounted in the pump housing. The lobes in the pump housing do not contact each other or the walls of the housing, thereby limiting particles to those suspended in the gas flow.

Description: Description MULTI-LOBE PUMP FOR PARTICLE COUNTERS TECHNICAL FIELD The invention relates to apparatus used in instruments for detecting, counting or measuring particles in a gas stream, particularly air. More specifically, the invention relates to an improved gas flow system in particle counters. BACKGROUND ART A typical prior art counter for particles suspended in a fluid is disclosed in U.S. Pat. No. 4,746,216 to K. Gross, assignor to Pacific Scientific Company. This patent describes a particle counter in which gas flows through a space within an enclosure, known as a view volume, monitored by a laser beam. The beam intersects the gas flow and illuminates particles flowing through the view volume. Light obscured by the particles, or scattered by the particles, is detected by an electronic detector sensitive to the obscuration or scattering of light. The resultant electrical signals are interpreted as particle counts. The need for improved particle counters is largely driven by the semiconductor manufacturing industry. Silicon wafers have become larger in size while, at the same time, line widths and features on chips laid out on the wafer have become smaller. Chip size has become larger with more complex functions on each chip. Defects and particles which previously caused little harm now can render a large portion of a wafer useless. Thus, the role of small particles becomes increasingly important in monitoring quality of chip production. Air quality in clean rooms is especially critical at many stages of chip production. Semiconductor companies routinely monitor air quality at all stages of chip production. While various improvements have been made to particle counters themselves, including the optics, nozzle characteristics, lasers and laser cavities, detector attributes and signal electronics, little attention has been paid to the pumps which are used to maintain the air flow through the air through the view volume. Typically, pumps used in prior art particle counters use pistons, diaphragms or vanes to move a fluid carrier medium, such as air. Such pumps move relatively large volumes of the carrier medium at low cost. However, most prior art pumps are noisy, generate heat, and are somewhat inefficient. An object of the invention was to provide an improved pump for a particle counter which provides for quiet operation, relatively cool running and is efficient at flow rates and pressures used in particle counters. SUMMARY OF THE INVENTION The above object has been achieved in a particle counter having a flow of sample gas through a flow cell propelled by a pump of the lobe type. The number of lobes may vary. Where twin rotors have more than one lobe, the lobes are interleaving. In the case where rotors have a single lobe, the lobes are cam- shaped. The interleaving lobes are typical of Roots pumps, frequently used in the automotive industry in superchargers. A motor drives the shafts carrying the rotors using a controller provided with feedback from a flowmeter so that a desired air volume may be pumped through the gas flow cell. The interleaving rotors are quiet, efficient and do not introduce particles because clearance is maintained between interleaving rotors and also with surrounding walls . BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram plan of the apparatus of the present invention. Fig. 2 is a cross section of a rotor chamber used in the apparatus of Fig. 1. Fig. 3 is a cutaway view of a rotor chamber and motor as illustrated in Fig. 1. Figs. 4-7 are alternate embodiments of lobe type rotors for use in the rotor chamber illustrated in Fig. 1. BEST MODE OF CARRYING OUT THE INVENTION With reference to Fig. 1, a particle view volume 11, within a gas flow cell, is defined within a housing 13, which may be a block of metal or other material which does not generate particles. A first of two intersecting bores accommodates a gas inlet port 15 which faces a gas outlet port 17. A second orthogonal bore seats laser 19 which directs a beam 20 toward detector 21, intersecting the gas flow stream between gas inlet port 15 and gas outlet port 17. The beam 20 illuminates individual particles flowing from the gas inlet port 15 to the gas outlet port 17 within the gas flow cell. The illuminated zone formed by the beam and the gas flow path, as seen by the detector, is the view volume. The beam 20 has a characteristic changed, either by obscuration of the beam in which case the detector 21 is in line with the beam, or by scattering, in which case the detector 21 is at an angle to the axis of the beam so that only light scattered by particles is detected and not light coming directly from the laser. Gas flow cells of the type described above are illustrated in Fig. 1 of U.S. Pat. No. 4,746,215 to K. Gross. The gas outlet port 17 is connected to an optional particle filter 26 which feeds gas to rotor chamber 25 which acts as a pump, pulling gas through the gas flow cell from the gas inlet. Rotors in the rotor chamber are described below and are driven by a motor 27 which may be a DC motor, or alternatively an AC motor or a battery-driven motor. Motor 27 operates under control of a motor controller 29, providing a command signal indicating the speed at which motor 27 is to operate. Gas pulled through the gas flow cell by the pump, comprised of the rotor chamber 25 and the motor 27, moves through exhaust pipe 31 and passes through flowmeter 33 before going through a particle filter 37 and into gas exhaust port 39. Flowmeter 33, operating under command of controller 35, measures output gas velocity and sends a signal back to motor controller 29 which uses the flowmeter signal for a comparison with a commanded motor speed signal delivered on line 40 from an external source. The comparison of the two signals results in an error signal which is used to continuously adjust the speed of motor 27 so that the motor will turn the rotors at a velocity so that the desired output flow rate is achieved. A signal from counter 23 may be used to influence motor controller 29. If particle counts are too fast or too great, exceeding the capacity of the system, the flow may be slowed to allow the counter to be within an accurate measuring range. For this purpose, an output signal from counter 23 is taken along line 32 to motor controller 29. A motor speed adjustment may be commanded on line 40 to compensate for increased loads before the motor 27, such as long or constricted outlet ports 17. Fig. 2 shows first rotor 41 with three lobes 43, 45 and 47. These three lobes interleave with the three lobes 53, 55 and 57 of rotor 51. Note that lobe 47 does not contact lobe 53 or lobe 55 but comes in close proximity. Similarly, lobes 43, 45 and 57 come close to wall 49 of the rotor housing 50, but do not contact the wall. Rotor 41 is mounted on a central shaft 42 and rotates about the shaft, while rotor 51 is mounted on central shaft 52 and rotates about that shaft. Each of the lobes has a circular, mass-reducing cutout 54 which lowers the moment of inertia of the rotor. A motor, not shown, directly drives one of the shafts while a gear drives the second shaft and maintains synchronism. In operation, gas would be drawn in to a first port 61, connected to the gas flow cell and then be pumped out through gas outlet port 63 toward a flowmeter. The placement of the flowmeter is not important . The flowmeter could be positioned either before or after filter 37. In Fig. 3, motor 65 may be seen to have a shaft 67 directly turning rotor 51, as well as a first gear 71. The first gear 71 meshes with a second gear 73 which turns rotor 41 in synchronism with rotor 51. Motor 65 is preferably a DC motor which can operate off of a battery or an AC supply having a rectifier converting local AC to DC at the appropriate voltage. Operation with DC allows the pump of the present invention to be portable or to use AC as a backup supply. Housing 50 is airtight except for a gas inlet port and a gas outlet port 63. The lobes 41 and 51 extend the full height of housing 50 which is typically only a few inches tall, with the first rotor 41 and second rotor 51 being mounted totally within the housing and supported by bearing support walls 75 and 77. The bearings are sealed units so that no particles enter the gas flow stream from bearing wear. Fig. 4 shows twin lobe rotors featuring first rotor 71 and second rotor 73 within housing 75. The rotors interleave in a non-contacting way as described above. Similarly, in Fig. 5 the rotors 77 and 79 have a single lobe for each rotor with a cam-like surface. The two rotors rotate in synchronism, without contact. Fig. 6 shows two three-lobe rotors 83 and 85, supported on two shafts in a manner similar to the two rotors illustrated in Fig. 2. In the case of Fig. 6, the rotors have solid cross-section and a slightly different shape than the rotors illustrated in Fig. 2. The lobes contact neither other lobes nor housing wall surfaces. In Fig. 7, two circumferential rotors 91 and 93 are shown. Although the two rotors do not technically employ lobes, the circumferential protrusions 92 and 94 associated with rotor 91 and the circumferential protrusions 96 and 98 associated with rotor 93 may be considered to be lobes for purposes of this invention. Rotating characteristics are similar, with no contact among the lobes nor the housing wall surface. The pumps of the present invention, resembling Roots pumps, are ideal movers of a gas flow stream in a particle counter, reducing noise, increasing efficiency and being operable by a wide variety of motors.

Claims: Claims 1. A system for counting particles in a gas stream comprising, a gas flow cell having a gas flow path defined by a gas inlet, a gas outlet, an optical beam impinging upon the flow path, and an optical detector in communication with light which has impinged upon the flow path, and a multi -rotor pump connected to the gas outlet side of the gas flow cell, each rotor having shafts with interleaving lobes, and an electric motor driving the shafts at selectable speeds. 2. The particle counting system of claim 1 having a flowmeter connected to the gas outlet of the flow cell velocity generating an electrical signal indicative of gas flow and a feedback path carrying the electrical signal from the flowmeter to the electric motor, thereby regulating the speed of the driven shafts. 3. A system for counting particles in a gas stream comprising, a gas flow cell defined within a first air tight housing supporting a plurality of interleaving lobe, non-contacting rotors spanning a dimension of the housing between spaced apart gas inlet and outlet ports, a motor outside of the housing communicating rotary power to the rotors inside of the housing in a direction pumping gas having suspended particles into the gas inlet port and expelling gas from the gas outlet port of the housing in a gas flow path through the first housing, a second housing having a light beam ingress port and a light egress port, the second housing being in gas flow communication with the first housing wherein the gas flow path established by the rotors extends into the second housing, a light beam projected into the light beam ingress port of the second housing in a direction traversing the gas flow path, interacting with particles in the gas flow path, and a light detector associated with the light egress port positioned to receive light which has interacted with said particles in the gas flow path. 4. The system of claim 3 wherein the number of rotors is three . 5. The system of claim 3 wherein the number of rotors is two. 6. The system of claim 3 wherein said motor has an electrically connected controller capable of varying the speed of the motor. 7. The system of claim 6 further defined by a flowmeter operatively associated with the gas flow path through the housing, the flowmeter electrically connected to the controller, providing feedback flow velocity control of the motor. 8. The system of claim 3 wherein the light egress port is at an angle to the light ingress port whereby light scattered from particles is detected. 9. The system of claim 8 wherein a laser is associated with the light ingress port. 10. The system of claim 3 wherein said light detector is an array detector. 11. In a particle counter of the type having a gas flow path through a volumetric space illuminated by a beam of light whose light is obscured or scattered by particles in a flowing stream of gas in the flow path wherein the obscured or scattered light is indicative of a particle count, the improvement comprising, a pump having a plurality of shafts, each shaft having at least one lobe interleaving with a lobe on another shaft in a non-contacting manner in gas flow communication with said volumetric space. 12. The apparatus of claim 11 further comprising, a flowmeter positioned to measure gas flow through the volumetric space, the flowmeter generating an electrical signal representing flow velocity. 13. The apparatus of claim 12 further comprising, a motor operating said lobe pump and having a motor controller adjusting flow velocity through the volumetric space to a desired velocity represented by an electrical signal. 14. The apparatus of claim 13 further comprising an electrical feedback circuit receiving the electrical signal representing flow velocity and an electrical signal representing the desired velocity, the feedback circuit generating an error signal by comparing the flow velocity and the desired velocity, the error signal transmitted to the motor controller thereby adjusting the flow velocity through the volumetric space. 15. The apparatus of claim 11 wherein said lobe pump comprises first and second shafts, each shaft having at least one lobe. 16. The apparatus of claim 15 wherein each shaft carries at least two lobes. 17. The apparatus of claim 15 wherein each shaft carries three lobes. 18. The apparatus of claim 15 wherein each shaft carries circumferential lobes . 19. The apparatus of claim 15 wherein each lobe rotates within a walled housing with a peripheral region of each lobe approaching the housing wall but not contacting the wall. 20. The apparatus of claim 11 wherein said pump is in a first housing being in air flow communication with a second housing defining said volumetric space. 21. The apparatus of claim 13 wherein the motor controller has an input signal from a particle counter, indicating whether particle counts are exceeding the capacity of the system. 22. The apparatus of claim 11 further comprising a filter disposed in the gas flow path between the pump and said volumetric space.

Inventors: ADAMS CRAIG D, ADAMS, CRAIG, D.
Applicants: MET ONE INC, MET ONE, INC.
Application year: 1998
Main classification: G01N21
IPC codes: F04C18, F04C28, G01N15, G01N21