Abstract:
A magnetorheological fluid delivery system includes a mixing and tempering vessel. Fluid is admitted to the vessel via a plurality of tangential ports, creating a mixing of the fluid in the vessel and promoting homogeneity. Fluid may be reconstituted in the vessel by metered addition of carrier fluid. A fixed-speed centrifugal pump disposed in the vessel pressurizes the system. Fluid is pumped through a magnetic-induction flowmeter and a magnetic flow control valve having solenoid windings whereby MR fluid is magnetically stiffened to restrict flow. A closed-loop feedback control system connects the output of the flowmeter to performance of the valve. A nozzle having a slot-shaped bore dispenses MR fluid for re-use in the work zone. A planar-diaphragm flush-mounted pressure transducer at the entrance to the nozzle and flowmeter inferentially measure relaxed viscosity and provide signals to a computer for dispensing metered amounts of carrier fluid into the mixing vessel to assure correct composition of the reconstituted fluid as it is dispensed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to methods and apparatus for circulating and dispensing fluids; more particularly, to methods and apparatus for circulating and dispensing fluids having magnetorheological properties; and most particularly, to methods and apparatus for managing and metering magnetorheological fluids being used in a magnetorheological finishing apparatus. 
     2. Discussion of the Related Art 
     It is well known in the art of finishing and polishing surfaces to use, as a finishing agent, particulate fluid suspensions having magnetorheological properties. Such fluids, known as magnetorheological fluids (MR fluids), comprise magnetically soft particles which can become oriented and magnetically linked into fibrils in the presence of a superimposed magnetic field, thereby increasing the apparent viscosity of the fluid by many orders of magnitude. Such increase is known as magnetic “stiffening” of the MR fluid. It is further known to incorporate finely-divided abrasives into MR fluids used in finishing and polishing to increase the rate of removal of material. Non-stiffened, or magnetically relaxed, MR fluid can be stored and pumped as a low-viscosity fluid, having a viscosity typically of about 50 cp or less, then stiffened to a semi-rigid paste of 10 5  cp or more in a magnetic work zone for finishing or polishing, then relaxed again outside the work zone for collection, reconditioning, and reuse. Apparatus and methods for magnetorheological finishing and for delivery of MR fluids are disclosed in, for example, U.S. Pat. No. 5,951,369 issued Sep. 14, 1999 and U.S. Pat. No. 5,971,835 issued Oct. 26, 1999, both to Kordonski et al., the relevant disclosures of which are herein incorporated by reference. 
     MR fluid finishing apparatus typically includes a fluid delivery system (FDS) for dispensing MR fluid onto a rotating carrier surface, whereon the fluid is carried into and out of the work zone. MR fluid is a relatively unstable suspension because the magnetic particles tend readily to agglomerate and to settle out of suspension and thereby stagnate. Thus, a primary concern in configuring an FDS for MR fluid is keeping the fluid relatively homogeneous in the system, and very highly homogeneous at the point of dispensing into the work zone. An FDS must receive spent fluid from the work zone, recondition the fluid for reuse as by adjusting the temperature and viscosity, homogenize the adjusted fluid, and redispense the fluid into the work zone at a controlled flow rate. A suitable prior art FDS is disclosed in U.S. Pat. No. 5,951,369 incorporated above. 
     Because of these various requirements, the prior art FDS is relatively complex and includes a first peristaltic pump for removing spent fluid from a scraper at the work zone and returning the fluid to a reservoir; a mixer in the reservoir for rehomogenizing the fluid; a tempering subsystem at the reservoir for cooling the fluid, which tends to become heated in the work zone; a second peristaltic pump and cylindrical nozzle having a fixed restriction for redispensing the fluid; a pulse-dampener for removing pulses generated by the pumps; and a viscosity measuring and correcting subsystem. Flow may be controlled by manually setting the speed of the second pump, and preferably is monitored via a magnetic induction flowmeter. 
     Several problems are presented by the prior art FDS. 
     First, the system is cumbersome, as it is essentially an assemblage of discrete components, each intended to perform a single task. Thus, the system is wasteful of space. 
     Second, the flow control system requires a positive-displacement (PD) pump. Some known PD pumps such as gear pumps are unsuited to the task of pumping MR fluids. A peristaltic pump can meet the positive-displacement need over a short period of time; however, the pulsating output mandates the pulse-dampening apparatus already noted, and the delivery lines within the pump are subject to fatigue and must be replaced frequently. 
     Third, correct composition of the MR fluid being redispensed is inferred from an inline viscometer which incorporates a cylindrical nozzle that, for flow reasons, must be relatively long and thus is cumbersome. In the flow and composition control strategy employed, a constant input pressure at the entrance to the nozzle and a constant flowrate at the flowmeter indicate a constant viscosity and hence constant composition of the fluid being dispensed. 
     What is needed is an improved fluid delivery system for managing MR fluid in an MR finishing apparatus wherein flow is inherently smooth, pulsations are not generated, and pulsation dampening is unnecessary; wherein the dispensing flow is maintained at a desired flowrate by a closed-loop flow control subsystem; wherein the composition of the MR fluid is automatically corrected to aim during a reconditioning step; wherein the sizes of components such as a dispensing nozzle are minimized; and wherein mixing, tempering, and pressurizing of MR fluid is performed in a single vessel. 
     It is a primary objective of the invention to provide a simple, compact fluid delivery system for managing and dispensing magnetorheological fluid for use by a magnetorheological finishing apparatus. 
     SUMMARY OF THE INVENTION 
     Briefly described, a magnetorheological fluid delivery system in accordance with the invention comprises various elements connected by conduit means, including a mixing and tempering vessel. Fluid being returned from use in a work zone is admitted to the vessel via a plurality of tangential ports near the bottom of the vessel, creating a mixing of the fluid in the vessel and thus promoting homogeneity. Fluid may be reconstituted in the vessel by metered addition of carrier fluid to compensate for carrier fluid lost in the work zone. A centrifugal pump, preferably operating at a fixed speed, collects the fluid from the vessel and pressurizes the system. Preferably, the pump is disposed in the vessel. Fluid is fed through a magnetic-induction flowmeter and a magnetic valve having solenoid windings whereby fluid may be controllably stiffened and thus flow restricted by the associated viscous drag created in the bore of the valve. A closed-loop feedback control system connects the output of the flowmeter to performance of the valve. A nozzle having a slot-shaped bore dispenses MR fluid for re-use in the work zone. A flush diaphragm pressure transducer at the entrance to the nozzle inferentially measures relaxed viscosity and provides signals to a computer for dispensing metered amounts of carrier fluid into the mixing vessel to assure correct composition of the reconstituted fluid as it is dispensed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features, and advantages of the invention, as well as presently preferred embodiments thereof, will become more apparent from a reading of the following description in connection with the accompanying drawings in which; 
         FIG. 1  is a schematic view of a prior art fluid delivery system for magnetorheological fluids, substantially as disclosed as FIG. 10 in U.S. Pat. No. 5,951,369; 
         FIG. 1   a  is a cross-sectional view of a prior art nozzle useful in the delivery system shown in  FIG. 1 ; 
         FIG. 2  is a schematic view of an improved fluid delivery system in accordance with the invention; 
         FIG. 3  is an isometric view, partially in cutaway, of a mixing/tempering vessel and a pressurizing pump; 
         FIG. 4  is a plan view of a magnetic valve; 
         FIG. 5  is a cross-sectional view taken along line  5 — 5  in  FIG. 4 ; 
         FIG. 6  is a schematic cross-sectional view of the valve shown in  FIGS. 4 and 5 , showing the direction and intensity of magnetic flux within the valve; and 
         FIG. 7  is an isometric view, partially in cutaway, of an improved viscometric nozzle in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The benefits and advantages of a magnetorheological fluid delivery system in accordance with the invention may be better appreciated by first considering a prior art system. 
     Referring to  FIG. 1 , a prior art fluid delivery system  10  (FDS) is shown for providing MR fluid  11  to a carrier surface  12  of a magnetorheological finishing apparatus (not otherwise shown) at a constant aim flow rate and viscosity; for recovering MR fluid from the carrier surface; and for conditioning recovered MR fluid for re-use. MR fluid  11  is scraped from the carrier surface  12  by scraper  14  and returned via line  16  to an inline mixing and tempering vessel  18  wherein agglomerates are broken up, carrier fluid is replenished as described below, and the reconstituted MR fluid is re-tempered to an aim temperature. A prior art system typically includes a supplementary peristaltic pump  20  to acquire the spent MR fluid from scraper  14  and deliver it to vessel  18 . Retempered MR fluid is withdrawn from vessel  18  by a primary peristaltic delivery pump  22  and is delivered through an inline magnetic-induction flowmeter  24 . The output of peristaltic pumps is cyclic and therefore a pulse dampener  26  is required in the fluid delivery system downstream of pump  22 . Flowmeter  24  and the drive for pump  22  are connected to a computer  28  which sets a flow aim and the rotational speed of the pump. From the flowmeter, MRF passes through nozzle  30  and is discharged for work onto carrier surface  12 . 
     Referring to  FIG. 1   a , prior art nozzle  30  is an inline capillary rheometer or viscometer at the discharge end of the fluid delivery system and comprises a capillary tube  32  formed of a non-magnetic material, for example, stainless steel or ceramic, having a length to diameter ratio preferably greater than about 100:1. Tube  32  is surrounded by a magnetic shield  34  formed preferably of a magnetically soft material, for example, low-carbon cold rolled steel. Tube  32  and shield  34  are spaced apart by one or more non-magnetic centering spacers  36  and by a non-magnetic transition piece  38  for smoothly narrowing the MR fluid flow from the diameter of the supply line  40  to the diameter of tube  32 . Disposed between supply line  40  and transition piece  38  is a pressure transducer  42  having a diaphragm  44  for sensing line pressure at the entrance to the capillary tube and sending a signal thereof to computer  28 . Since nozzle  30  is disposed at the end of the delivery line, the pressure drop through the nozzle may be measured relative to ambient pressure, and thus only one pressure sensor is required. Computer  28  is programmed with an algorithm for calculating MR fluid viscosity as a function of pressure and flowrate through nozzle  30 . When a predetermined upper viscosity control limit is exceeded, computer  28  signals metering pump  46  to inject a computer-calculated replenishing amount of carrier fluid into mixing/tempering chamber  18  where the fluid is mixed into the recirculating MRF. 
     Referring to  FIGS. 2 through 7 , an improved and compact fluid delivery system  50  (FDS) in accordance with the invention is shown for providing MR fluid to a carrier surface  12  of a magnetorheological finishing apparatus (not otherwise shown) at a constant aim flow rate and viscosity; for recovering MR fluid from the carrier surface; and for conditioning recovered MR fluid for re-use. System  50  includes significant improvements in mixing, pumping, metering, and dispensing over prior art system  10 , and is significantly less complex and more compact. Several components of system  10  are eliminated, including supplementary pump  20  and pulsation dampener  26 . 
     As shown in  FIG. 2 , MR fluid  11  is scraped from the carrier surface  12  by scraper  14  and returned via line  16  to an improved inline mixing and tempering vessel  52  wherein agglomerates are broken up, carrier fluid is replenished, and the reconstituted MR fluid is re-tempered to an aim temperature and prepared to be re-dispensed onto carrier surface  12 . 
     As shown in  FIG. 3 , improved vessel  52  includes an insulating jacket  54  surrounding a mixing chamber  56 . Spent MR fluid being returned from scraper  14  is drawn into chamber  56  via at least one passage  58 , and preferably a plurality of such passages, extending from a splitting block  60  on the underside of vessel  52  and entering into chamber  56  through jacket  54  at ports  55  near the bottom  62  of the chamber and substantially tangential to the inner wall  64  of the chamber. This configuration causes a high level of swirling agitation of MR fluid within chamber  56  without resort to a separate mechanical mixer as is required in prior art mixing chambers. MR fluid is drawn into splitting block  60  from return line  16 , wherein the fluid flow is split into a plurality of streams following passages  58 . As in the prior art system, replenishing carrier fluid is injected from a source (not shown) via replenishment pump  46  and line  66  in response to commands from computer  28 , either into return line  16  as shown in  FIG. 2  or directly into vessel  52 . 
     Disposed within chamber  56  is a centrifugal pump  66  having a vertical drive shaft  68  supporting a conventional vaned impeller  70  near the bottom  62  of the chamber. Preferably, impeller  70  is vaned on both the upper and lower surfaces thereof to balance the pumping load and to increase the output volume. Pump housing  72  surrounds the shaft and impeller and is closed at its lower end by an end plate  74  having a central aperture  76  for receiving the outer end of shaft  68  and impeller  70  and for admitting MR fluid from the lower part of chamber  56  to impeller  70 . Housing  72  is provided with an inlet passage  78  for admitting MR fluid from the upper part of chamber  56  to impeller  70 . An outlet passage  80  extends within housing  72  from the periphery of impeller  70  through jacket  54  to the exterior of vessel  52 . Housing  72  is further surrounded by tempering coils  82  of a conventional liquid heat exchanger tempering system (not shown) for adjusting the temperature of MR fluid within chamber  56  to a predetermined aim in known fashion. 
     Pump drive  84  is disposed outside and above vessel  52  and is coupled to shaft  68  via a central bore in housing  72 , which housing also functions as the closing cover for vessel  52 . Drive  84  is operationally connected via conventional interface conversion elements to control computer  28  which may, via connection  85 , set and maintain the rotational speed of pump  66 , preferably at a predetermined fixed speed selected to optimize the output of the pump, for example, 3200 rpm. Alternatively, the speed of the pump may be set manually by conventional electromechanical means. 
     Referring to  FIGS. 3 through 6 , a novel magnetic flow control valve  86  and a conventional magnetic induction flowmeter  24  are disposed inline downstream of pump  66 . Flowmeter  24  senses the flow volume of material passing therethrough and communicates with computer  28  which then sends a controlling signal to valve  86  to adjust the flow sensed by flowmeter  24  to some predetermined aim. The flowmeter, valve, and computer thus form a conventional closed-loop feedback control system. Because pump  66  is a centrifugal pump and therefore non-positive-displacement, unlike prior art peristaltic pump  22 , hydraulic slip can occur within the pump, permitting valve  86  simply to throttle the pump output. 
     Magnetic flow control valve  86  comprises a solenoid without an armature, the MR fluid replacing the armature, and having first and second end caps  87 , 89  having first and second nipples  91 , 93 , respectively for connection of the valve into the FDS. The end caps are magnetically linked by a cylindrical housing  95  which also functions as a magnetic shunt. Hollow first and second magnet polepieces  88 , 90 , respectively extend axially towards each other from end caps  87 , 89 , respectively, within windings  92  which may be, for example, 1000 ampere-turns. Polepieces  88 , 90  are separated by a non-magnetic spacer  94  also within the windings and preferably having an axial bore of the same diameter as the bores in the polepieces, such that the axial passageway  96  extending through valve  86  is of a single non-restricted diameter. Spacer  94  forms and fills a magnetic gap between the polepieces. Each of polepieces  88 , 90  is tapered toward the other, preferably conically, on an outer surface thereof as shown in  FIGS. 5 and 6 , such that magnetic flux is directed and concentrated towards the gap, creating a magnetic field  98  within passageway  96  in which the flux lines are substantially parallel to the axis of the passageway, as shown in FIG.  6 . In operation, when the windings are de-energized, passageway  96  exerts low viscous drag on MR fluid flowing through the valve. Flow through the valve is limited only by the diameter of passageway  96 , the output pressure of pump  66 , and the mechanical restrictions in the FDS downstream of the valve. When the windings are controllably energized in response to signals from computer  28 , MR fluid in the magnetic field is magnetically stiffened within the valve to a higher apparent viscosity, thus creating increased flow resistance due to viscous drag on the walls of passageway  96 . Flow is thus controllably decreased from the non-energized level. The MR fluid becomes again relaxed, of course, upon passing out of the valve. By controllably varying the intensity of the magnetic field by varying the current through windings  92 , computer  28  is able to control the flow through the FDS in response to a predetermined flow aim and to the actual flow as measured by flowmeter  24 . 
     Referring to  FIG. 7 , an improved dispensing nozzle  30   a  is an inline capillary rheometer or viscometer at the discharge end of fluid delivery system  50  and comprises a barrel  32   a  formed of a non-magnetic material, for example, stainless steel or ceramic. Barrel  32   a  is surrounded by a magnetic shield  34   a  formed preferably of a magnetically soft material, for example, low-carbon cold rolled steel. A non-magnetic transition piece  38   a  smoothly narrows the MR fluid flow from the diameter of the supply line  40  into barrel  32   a . Extending through shield  34   a  and barrel  32   a  and exposed to the material flowpath is a pressure transducer  42   a  for sensing line pressure at the entrance to the capillary tube and sending a signal thereof to computer  28 . Since nozzle  30   a  is disposed at the end of the delivery line, the pressure drop through the nozzle may be measured relative to ambient pressure, and thus only one pressure transducer is required. Computer  28  is programmed with an algorithm for calculating MR fluid viscosity as a function of pressure and flowrate through nozzle  30   a . When a predetermined upper viscosity control limit is exceeded, computer  28  signals replenishment pump  46  to inject a computer-calculated replenishing amount of carrier fluid into either return line  16  or mixing/tempering vessel  52  wherein the fluid is mixed into the recirculating MR fluid. 
     A particular feature and advantage of nozzle  30   a  over prior art nozzle  30  is the incorporation of a non-cylindrical slot-shaped flow passage  100  through barrel  32   a  rather than the conventional cylindrical flow passage in nozzle  30 . Passage  100  has first and second opposed parallel planar walls  102  having a longer transverse length than third and fourth opposed walls  104 . A first advantage is that passage  100  dispenses MR fluid onto carrier surface  12  as a pre-formed ribbon. A second advantage is that pressure transducer  42   a  may be mounted in a planar wall  102  of passage  100 , permitting the use of an inexpensive flush diaphragm  44   a  in replacement of the prior art diaphragm  44 . A third advantage is that a slot-shaped passage exhibits increased viscous drag of the MR fluid because of greater surface area per unit length; therefore, a significantly shorter nozzle can yield a back pressure at transducer  42   a  equal to the back pressure present at prior art transducer  42 . 
     Pressure drop along a slot-like channel and a round pipe are presented as follows: 
               Δ   ⁢           ⁢     P   slot       =       2   ⁢           ⁢   μ   ⁢           ⁢     L   slot     ⁢   Q         b   3     ⁢   w               (     Eq   .           ⁢   1     )                 Δ   ⁢           ⁢     P   pipe       =       8   ⁢           ⁢   μ   ⁢           ⁢     L   pipe     ⁢   Q       π   ⁢           ⁢     R   pipe   4                 (     Eq   .           ⁢   2     )             
 
where μ is fluid viscosity, L slot  is slot length, b is slot half-height, w is the width of the channel, L pipe  is pipe length, R is pipe radius and Q is flow rate. When the pressure drop, flow rate, and viscosity in both channels are the same, then 
                   2   ⁢     L   slot           b   3     ⁢   w       =       8   ⁢     L   pipe         π   ⁢           ⁢     R   4           ⁢     
     ⁢   or           (     Eq   .           ⁢   3     )                 L   slot     =       L   pipe     ⁢           ⁢       4   ⁢     b   3     ⁢   w       π   ⁢           ⁢     R   4                   (     Eq   .           ⁢   4     )             
 
     To provide the same fluid velocity in both channels, the channels&#39; cross sectional areas must be the same
 
2 b w= 3.14 R   2  and L slot   =L   pipe (2 b   2   /R   2 )  (Eq. 5)
 
     The cross-sectional area of a cylindrical tube having a radius of 1.5 mm is about the same as the cross-sectional area of a slot-shaped passage having a slot height of 1.5 mm and slot width of 5 mm. Thus, for example, a prior art cylindrical nozzle  30  having a tube length of 200 mm can be replaced with an improved nozzle  30   a  having a barrel  32   a  with a slot length of about 100 mm. Such a shortening of the nozzle greatly enhances the desirable compactness of an MR fluid delivery system. 
     From the foregoing description it will be apparent that there has been provided an improved delivery system for magnetorheological fluid. Variations and modifications of the herein described fluid delivery system will undoubtedly suggest themselves to those skilled in this art. Accordingly, the foregoing description should be taken as illustrative and not in a limiting sense.