Electro-static contaminant removal from light modulating fluid
United Sta Towlson Inventor: Howard E. Towlson, Baldwinsville,  
 Apr. 1, 1975 [75 l N Y 57 ABSTRACT v [n the space between the opposed output window and [73] Asslgnee: General Elecmc Company rotating disk of a light valve, loose particulate material surmise in the fluid which becomes charged and held onthe 23 Filed; Sept 10 1973 nonconductive surface of the disk or the inside surface of the output window is repelled from one surface to [Zn Appl&#39; N05 395514 the opposing surface by applying a high voltage between the disk and exterior conductive shell. By peri- 1521 us. or .1 350/101, 204/299, 204/302, Odiwlly reversing P y of the pp voltage, with 3 5 an interval of zero voltage interposed between oppo- 51 Int. Cl. G02f 1/32, BOld 13/02 Site Polarity intervals, the Particles are moved back 1581 Field of Search, 350/101; 204/299, 302; and forth between the disk and Output windowwhen 3 3 5 the particles adhere to the disk, they are transported toward the fluid reservoir where they are repelled 5 References Cited from the disk into the bulk of the fluid and are subse- UNITED STATES PATENTS quently pulled through the fluid filter. 3.4219940 1/1970 Towlson 350/161 11 Claims, 6 Drawing Figures FIG!  22 r s&#34; Q 1 19* 26 25 3o-- l 14 g ELECTRON eu- 5a f :3 52- f [.TEIIIEU APR I I mean FIG.4  
 TIME (MINUTES) FIGZ) j l I I l I I SYNCHRONOUS MOTOR ELECTRO-STATIC CONTAMINANT REMOVAL FROM LIGHT MODULATING FLUID INTRODUCTION This invention relates to light valves for optically projecting images generated electronically on a fluid layer, and more particularly to a method and apparatus for electrostatically removing particulate material adhering to the nonconductive surface of the disk or the inside surface of the output window and directing it through a filter for the fluid.  
  One form of light valve suitable for optical projection of electronically generated images onto a remote display surface comprises an evacuated envelope containing an electron gun in alignment with a transparent disk. The disk is rotated through a reservoir of lightmodulating fluid to deposit a continuously replenished layer of fluid on the disk surface. An electron beam, generated by the electron gun, is directed through electrostatic beam deflecting and focusing means and is scanned across a portion of the light-modulating fluid layer so as to selectively deform the layer. The fluid deformations thus formed constitute diffraction gratings which, in conjunction with a Schlieren optical system, selectively control passage of light from a light source through the disk and through an output window in the light valve envelope in order to create visible images at a remote display surface on which the light impinges.  
  Light valves of the type described have hitherto operated satisfactorily for varying periods of time. Although the light-modulating fluid is free from particulate contamination at the outset of light valve operation, a buildup of contamination usually occurs as operation of the light valve progresses. This contamination is due mainly to wear of moving parts within the light valve, as well as presence of impurities. In addition, some particles are formed as a result of massive damage to fluid molecules caused by heavy electron or ion bombardment. In accordance with the invention shown and described in US. Pat. No. 3,489,940, issued Jan. 13, 1970 to H. E. Towlson and assigned to the instant assignee, the contaminants may be isolated by use of a baffle plate spaced in close proximity to the disk so that fresh filtered fluid dispensed on the side of the plate facing the disk maintains a fluid buffer region around the area of the disk whereon the fluid layer is deformed by the electron beam. The buffer region contained between the plate and the disk prevents contaminated, particulate-carrying sump fluid from reaching the disk surface facing the buffer plate.  
  Nevertheless, some contaminants do find their way onto the surface of the disk facing the output window and the inside surface of the output window itself. As the particulate matter builds up on these opposing glass surfaces, performance of the light-valve can deteriorate to the point where, ultimately, the disk and window must be cleaned in order for satisfactory operation to take place. This involves a costly procedure inasmuch as the evacuated envelope must be opened, and hence the light valve is essentially rebuilt.  
  The present invention concerns a light valve in which the nonconductive, front surface of the disk and the nonconductive, rear surface of the output window may be electrostatically cleaned at any time when the light valve is not being used to display images. The electrostatic cleaning does not require the envelope of the light valve to be opened. Instead, an electric field of predetermined configuration is established across a space encompassing the entire width of both the disk and output window. The field, being of predetermined amplitude and polarity, and being cycled in a predetermined fashion, causes particles adhering, by electrostatic attraction, to the front surface of the disk and to the rear or inside surface of the output window, to be repelled from the surface to which they are adherent, to the opposing glass surface. The next reversal of polarity causes these particles to transfer back to the surface from which they originated, for example, particles from the disk that had been repelled to the output window surface stick to the output window until polarity is reversed, and are then transferred back to the disk surface where they are carried along with the rotating disk. In this fashion, the particles are kept in motion in the fluid between the disk and output window surfaces. By having a discontinuity in the lower portion of the output window, particles repelled from the front surface of the disk are forced deep into the particulatefree fluid in the sump and are subsequently pulled through the fluid filter.  
  Accordingly, one object of the invention is to provide a system for electrostatically cleaning the interior surface of the output window and the front surface of the rotating disk in a light valve. I  
  Another object is to provide a light valve wherein any deterioration in performance due to particulate accumulation on opposing glass surfaces of the output window and rotating disk is reversible.  
  Another object is to provide electrostatic apparatus for preventing any significant quantities of contaminants in the fluid between the output window and rotating disk from becoming electrostatically adhered to either of these surfaces.  
  Another object is to provide a method of removing particulate matter from a portion of relatively stationary fluid situated within a narrow confinement in communication with a larger quantity of such fluid.  
  Briefly, in accordance with a preferred embodiment of the invention, a light valve contains a rotatable disk on which a layer of light-modulating fluid is carried, and an output window. A portion of the disk and output window are each submerged in a reservoir of the fluid, with a portion of the fluid being contained between the disk and output window. Contaminants are removed from the fluid by applying a dc voltage of cyclically reversing polarity across the fluid between the disk and output window while the disk is rotating, the voltage being of sufficiently high amplitude electrostatically to move particulate matter through the fluid substantially normal to opposed disk and output window surfaces. Means are provided for withdrawing fluid containing particulate matter from the reservoir and replenishing the reservoir with fluid in which the content of particulate matter has been minimized.  
 . In accordance with another preferred embodiment of the invention, a method of removing particulate matter from a portion of fluid situated between spaced surfaces located within a reservoir of the fluid, one of the surfaces being the surface of a rotating disk, comprises moving the particulate matter back and forth between the surfaces to allow different portions of the particulate matter to become attracted to the rotating disk and be transported to a fluid outlet within the reservoir. Fluid containing the particulate matter in suspension is drawn off from the reservoir, and the drawn-off fluid is replaced in the reservoir with fluid in which the content of particulate matter is at a minimum.  
 BRIEF DESCRIPTION OF THE DRAWINGS The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:  
  FIG. 1 is a partially cutaway side view of a portion of a light valve employing the instant invention;  
  FIG. 2 is a sectional view of the light valve shown in FIG. 1, taken along line 2-2;  
  FIG. 3 is a schematic diagram showing means for applying an electrostatic potential to a light valve for removing contaminants from the fluid,  
  FIG. 4 is an illustration of waveforms employed in electrostatically removing contaminants from the fluid of a light valve;  
 FIG. 5 is a schematic diagram of apparatus for electromechanically producing the waveforms shown in FIG. 4; and  
  FIG. 6 is a schematic and partially cutaway side view of apparatus for producing the waveforms shown in FIG. 4.  
 DESCRIPTION OF TYPICAL EMBODIMENTS FIG. 1 illustrates a light valve including the electrostatic contaminant removal means of the instant invention. The light valve comprises an envelope 10, typically of glass, containing a light output window portion 11 and a sump region 12 acting as a reservoir of lightmodulating fluid 13. The interior of enevelope is evacuated to a low gas pressure.  
  The light-modulating fluid is typically of the polybenzyltoluene type having a fluid viscosity of 1000 centistokes at 60C, with a vapor pressure in the range of l0 -l0 torr. The fluid contained in sump region 12 is that which has drained off of an optically transparent disk 14 which is continuously rotated on metallic bearings 15 about a metallic shaft 16, typically at a speed in the order of 3 revolutions per hour. A metallic spring is maintained in compression by having its metallic cap 21 affixed to a rigid, electrically conductive support member 22 which, in turn, is affixed to envelope 10 of the light valve by any suitable means (not shown). The opposite end of spring 20 bears against the body of a shaft 16. Consequently, a shoulder 23 on shaft 16 urges bearing 15 to force disk 14 against a plurality of protuberances 17, which may advantageously be formed of fritted glass droplets. These protuberances are affixed to output window 11.  
  Protuberances 17 maintain disk 14 at a distance of about 3 mils from light output window 11 so as to permit fluid 18 from reservoir 12 to rise by capillary action and fill the region between the disk and the output window, as described in greater detail in H. E. Towlson U.S. Pat. No. 3,385,991, issued May 28, 1968 and assigned to the instant assignee.  
  While the front surface of rotating disk 14 is uncoated, a thin film 24, which comprises a transparent conductive coating such as indium oxide, is carried on the rear surface of the disk. Coating 24 may be maintained at any desired potential since a conductive path is formed through bearing 15, shaft 16, spring 20, cap  
 21 and member 22, permitting a continuous electrical connection to coating 24 through a stationary connection (not shown) which may be made to member 22. An aperture 19 in member 22 permits passage of an electron beam 25, originating at an electron gun 26, to be directed towards conductive coating 24 on disk 14. Disk 14 itself is nonconductive, and preferably is comprised of glass.  
  A thin film of light-modulating fluid 27 is coated on conductive coating 14 and thus is situated in the direct path of electrons in electron beam 25. Beam 25 is focused and deflected by electron optical means (not shown) within the light valve and hence is swept, in raster fashion, over the surface of light-modulating fluid layer 27. The pattern of charges on layer 27 produced by electron beam 25 causes corresponding deformations in the thickness of of layer 27, resulting in formation of diffraction gratings 30. These gratings correspond to the image to be projected onto a remote display surface. Light from a light source (not shown) positioned behind electron gun 26 impinges upon a lenticular lens system 28 formed on the rear wall of envelope 10 and is directed by the lenticular lens system through aperture 19 onto diffraction gratings 30.  
 By modulation of electron beam 25 through application of suitable potentials to the electrostatic focus and&#39; deflection means, diffraction gratings 30 in fluid layer 27 are selectively controlled. Consequently, light passing through transparent rotatable disk 14 and output window 11 is selectively controlled and, in conjunction with externally located lenses of a Schlieren optical system (not shown) is projected on a remote display surface (not shown) to fonn an image representative of the intelligence modulating the electron beam.  
  A baffle plate 40, situated within sump 12, is spaced in close proximity to, and parallel to, disk 14. The spacing and orientation of baffle plate 40 with respect to disk 14 are maintained by spacer bars 41. By employing a metallic baffle plate, bars 41 may be welded to the baffle plate and mated with cavities 42 in output window 11. The baffle plate is then rigidly held in place by two spring clips 43.  
  The manner by which spring clips 43 hold baffle plate 40 to output window 11 is further illustrated in FIG. 2, which is a sectional view taken along line 2-2 of FIG. 1. In FIG. 2, the width of baffle plate 40 can be seen to be larger than that of disk 14, while output window 11 can be seen to have protruding regions 47 which position bars 41 and clips 43 at locations to avoid interference with rotation of disk 14. Baffle plate 40 may alternatively be constructed of glass, in which case spacer bars 41 are likewise constructed of glass and joined by fritting to output window 11 so as to render spring clips 43 unnecessary. The separation maintained between baffle plate 40 and disk 14 is typically in the order of 10-15 mils.  
  A raster area 48 on oil film 27, which is the region upon which electron beam 25 may impinge to form diffraction gratings 30, is illustrated in its relative position on disk 14 in FIG. 2. Although raster area 48 is shown as a discrete area, and is fixed with respect to output window 11, those skilled in the art will recognize that the raster area on disk 14 constitutes a circular band, due to the continuous rotation of the disk.  
  As shown in FIG. 1, the entire front end of the light valve is enclosed within a protective, metallic housing containing an opening 49 to allow egress of light.  
 The housing, typically comprised of aluminum, may conveniently be formed of two parts, a so-called clamshell 51 and support plate 52, secured together by bolts 53. The housing is supported upon glass protuberances formed on the light valve, clamshell 51 being supported upon three protuberances 54 substantially equally spaced about the light valve so that only one is illustrated, and support plate 52 being situated upon three protuberances 55 also substantially equally spaced about the light valve so that only one is illustrated. To protect protuberances 54 and 55 from damage due to forces exerted by housing 50, each of protuberances 54 is mated with a spring 56 fitted within a flanged cup 57 recessed in clamshell 51, while each of protuberances 55 is mated with a fiberglass pad 58 recessed in support plate 52.  
  Fluid conveying means 44 is disposed in predetermined configuration on the side of baffle plate 40 facing disk 14. This configuration permits flow of fresh filtered fluid outward in all directions parallel to the plane of disk 14, so as to maintain the entire region between disk 14 and baffle plate 40 filled with fresh filtered fluid only. By continually supplying fresh filtered fluid from a pump and filter 45, shown in FIG. 1, through a closed tube 46 to fluid conveying means 44, fluid pressure in the region between baffle plate 40 and disk 14 is maintained sufficiently high to prevent contaminated fluid from sump 12 from entering this region. Accordingly, as disk 14 rotates, transparent coating 24 is continually covered with fresh filtered fluid only, since the surface of coating 24 does not contact any of the contaminated fluid in sump 12.  
  Despite the fact that fresh filtered fluid is coated upon the disk, contaminants in the fluid can still produce blemishes in the displayed image if they arepresent in the region between light output window 11 and disk 14 in an area aligned with electron gun 26 and any location in raster area 48 shown in FIG. 2. In this region, precautions are necessary to insure cleanliness during assembly to minimize presence of any objectionable contaminants in the light valve. Nevertheless, after a finite total time of operation, some contaminant particles may appear in the region shown in FIG. 1 between light output window 1 1 and disk 14, and produce visible blemishes, typically doughnut-shaped spots, in the displayed image. Because of high intensity electrical fields that occur during normal operation of the light valve, these contaminants become charged and consequently adhere, by electrostatic attraction, to the uncoated, nonconductive surface of disk 14 or the inside surface of nonconductive output window 11. When the high intensity electrical fields are collapsed, however, as when the light valve is in a standby condition as defined, infra, the contaminant particles slowly discharge. Nevertheless, these particles never move very far from the surfaces to which they have adhered, since relative velocity of the fluid normal to the surfaces of disk 14 and output window 11 within the region defined by these surfaces is essentially zero. Only those contaminant particles that adhere to the disk and are carried into reservoir 12 encounter fluid that is in motion and hence, if the light valve is switched into a standby condition at the time such particles are being transported into reservoir 12 by the disk, they may drift away from the disk and ultimately be removed by filtration through pump and filter 45. At best, this but slightly alleviates the problem created by such contaminant particles since contaminant particles adhered to the output window and to other locations on the disk surface that are released into the region between the disk and output window above the reservoir encounter essentially no moving fluid capable of transporting them to pump and filter 45. When the light valve is next returned to an operating condition, the unremoved par ticles are once again attracted to the nonconductive surfaces of disk 14 and output window 11 through paths normal to the surfaces thereof.  
  By subjecting particles between disk 14 and output window 11 to electrical fields that cause them to move back and forth in that region while disk 14 is in rotation, disk 14 may be utilized as a means to transport contaminant particles from output window 11 as well as from the disk, into sump 12 for removal. As illustrated in FIG. 3, this is accomplished by applying a high voltage dc source across a pair of electrodes shown in fragmentary form, one electrode comprising conductive coating 24 on disk 14 and the other electrode comprising housing 50, so as to subject window 11, disk 14 and fluid 13 between the window and disk, to a high intensity electrical field. To avoid disruption of normal light valve operation, contaminant removal is performed when the light valve is in standby operation; that is, the electron beam is turned off, as are its focus, deflection and acceleration fields, but disk 14 is rotated and pump and filter 45, shown in FIG. 1, are in operation. Depending upon relative polarities of the electrical fields and each contaminant particle, each particle is influenced to move toward either disk 14 or output window 11. During intervening periods of zero electrical field, the particles are free to drift away from the surfaces to which they had been attracted. Accordingly, by judicous selection of duty cycle for the electrical field, the particles may all eventually be transported, by rotation of disk 14, into sump 12 for removal.  
  A typical duty cycle for voltage supplied by dc source of FIG. 3 is illustrated in FIG. 4. Thus, output voltage of the dc source is on for two and one-half minute intervals, switching from +8000 volts to -8000 volts and back again to +8000 volts, with intervening zero voltage intervals of one-half minute each. During each positive voltage interval, positive voltage is applied to conductive layer 24 on disk 14 and negative voltage to housing 50, shown in FIG. 3, causing positivelycharged contaminant particles to migrate through fluid 13 toward output window 11. At the same time, negatively-charged particles are caused to migrate through fluid 13 toward disk 14, and the motion of the disk carries these particles along until the voltage drops to zero, after which time the particles move into fluid 13. Those negatively-charged contaminant particles released from the disk in sump 12, shown in FIG. 1, especially those released below the bottom of output window 11, are caught up in the motion of fluid in the sump, and are filtered by pump and filter 45 so as to be removed from the fluid. Those contaminant particles released into fluid 13 between disk 14 and output window 11 above sump 12, however, remain essentially suspended at that location until the negative voltage interval, at which time the negatively-charged contaminant particles are caused to migrate through fluid 13 toward output window 1 1. At the same time, positivelycharged contaminant particles are caused to migrate toward disk 14 and be transported by motion of the disk until the voltage again drops to zero, at which time the contaminant particles start to reenter fluid 13. Those positively-charged contaminant particles released from the disk in sump 12, especially those released below the bottom of output window 11, are caught up in the motion of the fluid in the sump, and are removed by filtration.  
  FIG. illustrates apparatus which may comprise dc source 60 of FIG. 3. A pair of brushes 61 and 62, rotating at substantially constant speed in the direction indicated by the arrows, are energized with positive and negative potentials, respectively, through circular contacts 64 and 65, respectively, from a high voltage dc power supply 63. A pair of commutator surfaces 66 and 67 provide output voltages to conductive coating 24 on disk 14 and to housing 50, respectively, shown in FIG. 3.  
  From FIG. 5 it can be seen that, as brushes 61 and 62 rotate, brush 61 connects contact 64 to commutator 66 to provide a positive potential at the terminal and brush 62 connects contact 65 to commutator 67 to provide a negative potential at the I terminal. After a 2% minute interval of such potentials, brush 61 terminates contact with commutator 66 and brush 62 terminates contact with commutator 67, so that, during a one-half minute interval, both the i and I terminals produce zero volts. After the one-half minute interval, brush 61 contacts commutator 67 and brush 62 contacts commutator 66, so that the i terminal produces a negative potential and the T- produces a positive potential. After another 2% minute interval, both brushes again terminate contact with both commutators, so that both the i and 1 output terminals again produce zero voltage for one-half minute. The cycle is then repeated.  
  FIG. 6 represents another view of apparatus which may comprise dc source 60 of FIG. 3, wherein llike numbers designate like elements. As illustrated, a synchronous motor 68 drives a shaft 70 which rotates brushes 61 and 62 thereabout on different radii. As described in conjunction with FIG. 5, the output voltage produced is illustrated by the waveform of FIG. 4.  
  The foregoing describes a method and apparatus for electrostatically cleaning the interior surface of the output window and the front surface of the rotating disk in a light valve, rendering reversible any degradation in performance due to particulate accumulation on opposing glass surfaces of the output window and rotating disk. The invention prevents any significant quantities of contaminants in the fluid between the output window and rotating disk from becoming electrostatically adhered to either of these surfaces, and permits removal of contaminant particles from a portion of the fluid situated within a narrow confinement in communication with a larger quantity of the fluid.  
  While only certain preferred features of the invention have been shown by way of illustration, many modiflcations and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.  
 I claim:  
  1. In a light valve containing a rotatable disk on which a layer of light-modulating fluid is carried, a portion of said disk being submerged in a reservoir of said fluid, said light valve including an output window partially submerged in said reservoir such that a portion of said fluid is contained between said disk and said output window, apparatus for removing contaminants from said fluid comprising:  
 means for applying a dc voltage of cyclically reversing polarity across said portion of said fluid, said voltage being of sufflciently high amplitude electrostatically to move particulate matter away from said disk and said output window; and means for withdrawing fluid containing said particulate matter from said reservoir and replenishing said reservoir with fluid in which the content of particulate matter has been minimized. 2. The apparatus of claim 1 wherein said last-named means comprises:  
  fluid filtering means; and means circulating fluid through said reservoir and said filtering means so as to remove said particulate matter from fluid withdrawn from said reservoir. 3. In a light valve containing a rotatable disk having one surface covered with a conductive, transparent coating on which a layer of light-modulating fluid is carried, a portion of said disk being submerged in a reservoir of said fluid, said light valve including an output window partially submerged in said reservoir such that a portion of said fluid is contained between said disk and said output window, apparatus for removing contaminants from said fluid comprising:  
 a conductive surface situated outside said reservoir; means coupling a dc voltage of cyclically reversing polarity from said conductive transparent coating to said conductive surface such that a dc voltage appears across said portion of said fluid with sufficiently high amplitude electrostatically to move particulate matter away from said disk and said output window; and means for withdrawing fluid containing said particulate matter from said reservoir and replenishing said reservoir with fluid in which the content of particulate matter has been minimized. 4. The apparatus of claim 3 wherein said last-named means comprises:  
  fluid filtering means; and means circulating fluid through said reservoir and said filtering means so as to remove said particulate matter from fluid withdrawn from said reservoir. 5. In a light valve containing a rotatable disk having one surface covered with a conductive coating on which a layer of light-modulating fluid is carried, a portion of said disk being submerged in a reservoir of said fluid, said light valve including an output window partially submerged in said reservoir such that a portion of said fluid is contained between said disk and said output window, apparatus for removing contaminants from said fluid comprising:  
 a conductive surface situated outside said reservoir; a voltage source applying an electrical potential of one polarity to said conductive coating and an electrical potential of opposite polarity to said conductive surface, said electrical potentials electrostatically moving particulate matter through said fluid between said disk and said conductive surface; means coupled to said voltage source for cyclically reversing polarity of said electrical potentials according to a predetermined duty cycle so as to reverse the directions of movement of said particulate matter through said fluid; and means for withdrawing fluid containing said particulate matter from said reservoir and replenishing said reservoir with fluid in which the content of particulate matter has been minimized.  
  6. The apparatus of claim wherein said means for cyclically reversing polarity of said electrical potentials comprises first and second motor-driven brushes energized with positive and negative electrical potentials, respectively, said brushes contacting first and second commutator surfaces, respectively, during a first interval and contacting said second and first commutator surfaces, respectively, during a subsequent interval, said first commutator surface being coupled to said conductive coating and said second commutator surface being coupled to said conductive surface.  
  7. The apparatus of claim 6 wherein said first and second commutator surfaces are spaced apart from each other so that, for a time between said first interval and said subsequent intervals said first and second brushes apply no electrical potentials to said conductive coating and to said conductive surface by failure to contact either of said first and second commutator surfaces.  
  8. In a light valve containing a movable surface on which a layer of light-modulating fluid is carried, apparatus for removing contaminants from said fluid comprising:  
 means for applying a dc voltage of cyclically reversing polarity across a portion of said fluid, said voltage being of sufficiently high amplitude electrostatically to move particulate matter away from surfaces in contact with said fluid; and  
 means at a fixed location communicating with said layer of light-modulating fluid for replacing fluid containing said particulate matter with fluid in which the content of particulate matter has been minimized. 9. The apparatus of claim 8 wherein said last-named means comprises:  
  fluid filtering means; and means circulating said fluid containing said particulate matter through said filtering means so as to remove said particulate matter therefrom. 10. In a light valve containing a surface coated with a layer of light-modulating fluid supplied from a reservoir of said fluid, apparatus for removing contaminants from said fluid comprising:  
 means for applying a dc voltage of cyclically reversing polarity across a portion of said fluid, said voltage being of sufficiently high amplitude electrostatically to move particulate matter away from surfaces in contact with said fluid so as to allow said particulate matter to exist in suspension in said fluid; and means for withdrawing fluid containing said particulate matter in suspension from said reservoir and replenishing said reservoir with fluid in which the content of suspended particulate matter has been minimized. 11. The apparatus of claim 10 wherein said lastnamed means comprises:  
 fluid filtering means; and means circulating fluid through said reservoir and said filtering means so as to remove said particulate matter from fluid withdrawn from said reservoir.