Abstract:
An apparatus is provided having an active material forming a surface thereof in contact with a rheological fluid. The active material is controllable to vary a surface geometry thereof between a rough or nonsmooth surface geometry to increase drag, and achieve an increase in stress transmitted through the fluid, when a field is applied, while allowing a smooth surface geometry and an associated reduction in stress transmitted through the fluid when the field is removed. A method of enhancing performance of an apparatus that includes a rheological fluid is also provided.

Description:
TECHNICAL FIELD 
     The invention relates to an apparatus with a rheological fluid and a mechanical assembly having an active material surface in contact with the fluid. 
     BACKGROUND OF THE INVENTION 
     Rheological fluid devices, such as magnetorheological (MR) and electrorheological (ER) fluid clutches, polishing devices, and damping mechanisms, such as shock absorbers, utilize the increased yield stress exhibited by a rheological fluid when exposed to a field (e.g., a magnetic or electrical field) in order to control the interface of two mechanical members, such as the coupling of a stator and a rotor that are relatively rotatable. The transmission of torsional or axial forces between the members is controlled by turning the field on and off. Rheological fluids include magnetorheological and electrorheological fluids, and are sometimes referral to as controllable rheological fluids or field-responsive fluids. 
     For example, an MR fluid is a suspension of magnetizable particles in a carrier fluid. When exposed to a sufficiently high magnetic field, and when the MR fluid is under shear flow, the magnetizable particles will line up in columns of higher concentration, aligned with the direction of applied magnetic field. It has been observed that forming a grooved surface on one or both mechanical members of a torque-transmitting device, with the grooved surface in contact with an MR fluid in an annular space between the members, enhances the stress transmitted through the MR fluid relative to that of a surface without grooves. The grooved surface creates drag in the fluid, inhibiting slip at the boundary of the fluid and the member or members having the grooved surface. The drag is thus beneficial for intended operation in “clutch on” (i.e., magnetic field applied) conditions. 
     SUMMARY OF THE INVENTION 
     An apparatus is provided that includes a rheological fluid characterized by an increase in yield stress under an applied field. The apparatus also includes a mechanical assembly, such as a clutch assembly, that is configured to provide the field. The mechanical assembly includes an active material component forming a surface in contact with the rheological fluid. The active material component has a variable surface geometry: it is characterized by a first surface geometry when the active material component is not activated and by a second surface geometry when the active material component is activated. One of the surface geometries is rougher than the other. (As used herein to describe an active material surface, “uneven”, “unsmooth” or “rough” are intended to be equivalent in meaning.) Activation of the active material component is controlled so that the rougher surface geometry is on contact with the rheological fluid when the field is applied (e.g., when increased torque is desired in the case of a torque-transmitting device) and the other, relatively smooth surface geometry is in contact with the rheological fluid when the field is removed (e.g., when drag is not desirable). Preferably, the second surface geometry is the rougher surface geometry so that the active material component must be activated to provide the drag that promotes increased transfer of stress through the fluid. A surface roughness in the range of 1-500 microns, and preferably between 20 and 30 microns, may deliver optimum stress transfer enhancement. As is known to those skilled in the art, active materials, such as shape memory alloys, may be prestressed to exhibit a shape change when activated. Thus, the shape change of the active material may cause the rougher surface, which may be characterized by protrusions and/or depressions in the surface of the active material. 
     In a torque-transmitting clutch assembly with such an active material surface, enhanced torque capacity is achieved without the expense of machining grooves into the stator and/or rotor of the clutch to create a rough surface geometry. Furthermore, overheating of the stator and/or rotor is avoided. Overheating can occur in the absence of surface roughness due to magnetized particles dragging along the surface of the stator and/or rotor. 
     During “clutch off” (i.e., magnetic field removed) operating conditions of an MR clutch, the objective is to decouple the clutch members and minimize rotational resistance of the clutch (i.e., to allow the rotor to rotate relative to the stator with minimal resistance). Thus, the drag associated with a grooved clutch surface is not desirable in a “clutch off” state. According to at least one aspect of the invention, a torque-transmitting device is provided having an active material component on a surface thereof in contact with an MR fluid in order to meet the differing objectives of the “clutch on” and “clutch off” operating conditions. Activation of the active material component is controllable to vary a surface geometry thereof between a rough or nonsmooth surface geometry to increase drag, and thereby achieve the increased torque and clutch capacity associated therewith, when a magnetic field is applied, while allowing smooth surface geometry and associated reduction in drag when the magnetic field is removed (and torque transmission is not desired). 
     Active materials include those compositions that can exhibit a change in stiffness properties, shape and/or dimensions in response to an activation signal, which can be an electrical, magnetic, thermal or a like signal depending on the different types of active materials. Preferred active materials include but are not limited to the class of shape memory materials, and combinations thereof. Shape memory materials, also sometimes referred to as smart materials, refer to materials or compositions that have the ability to “remember” their original shape, which can subsequently be “recalled” by applying an external stimulus (i.e., an activation signal). As such, deformation of the shape memory material from the original shape can be a temporary condition. Shape memory materials such as shape memory alloys and polymers represent a class of thermally-activated smart materials that undergo a reversible phase transformation responsible for dramatic stress-induced and temperature-induced recoverable deformation behavior. 
     In one embodiment, a clutch assembly includes a cylindrical stator concentrically arranged with a cylindrical rotor to form an annular cavity therebetween that is filled with an MR fluid. The MR fluid couples the rotor and stator to transmit torque therebetween when the magnetic field is applied. The stator is configured to provide the magnetic field that causes magnetization of the MR fluid. It should be appreciated that the stator is not stationary. The stator is driven by a power source. The active material component forming the surface in contact with the MR fluid is on at least one of the stator and the rotor. Alternatively, multiple active material components may be used, such as on both the stator and the rotor. The active material component may be a coating on or a portion of the rotor and/or stator. 
     “Activation” of an active material component means that a signal or trigger is provided to begin actuation (contraction, expansion, bending or other shape change) of the active material component. The active material component used herein may be activated in any of a number of different ways. For example, the active material component may be configured to be activated by thermal activation, electrical activation, magnetic activation or chemical activation. In the case of thermal activation, viscous heating of the MR fluid which occurs due to shearing of the MR fluid when a magnetic field is applied may supply the temperature change necessary to activate the active material component. Because the MR fluid cools when the magnetic field is removed, deactivation of the active material component is thus also linked to the magnetic field (i.e., the absence thereof). In such an embodiment, the active material component may be considered “sell-activating”, as it responds to the temperature of the MR fluid, and no separate activation signal is required. 
     Exemplary shape memory materials include shape memory alloys (SMAs), electroactive polymers (EAPs) such as dielectric elastomers, ionic polymer metal composites (IPMC), piezoelectric polymers and shape memory polymers (SMPs), magnetic shape memory alloys (MSMA), shape memory ceramics (SMCs), baroplastics, piezoelectric ceramics, magnetorheological (MR) elastomers, composites of the foregoing shape memory materials with non-shape memory materials, and combinations comprising at least one of the foregoing shape memory materials. For convenience and by way of example, reference herein will be made to shape memory alloys. The shape memory ceramics, baroplastics, and the like can be employed in a similar manner as will be appreciated by those skilled in the art in view of this disclosure. For example, with baroplastic materials, a pressure induced mixing of nanophase domains of high and low glass transition temperature (Tg) components effects the shape change. Baroplastics can be processed at relatively low temperatures repeatedly without degradation. SMCs are similar to SMAs but can tolerate much higher operating temperatures than can other shape-memory materials. An example of an SMC is a piezoelectric material. In the preferred embodiments of the invention, the active material component is an SMA material. 
     Shape memory alloys are alloy compositions with at least two different temperature-dependent phases. The most commonly utilized of these phases are the so-called martensite and austenite phases. In the following discussion, the martensite phase generally refers to the more deformable, lower temperature phase whereas the austenite phase generally refers to the more rigid, higher temperature phase. When the shape memory alloy is in the martensite phase and is heated, it begins to change into the austenite phase. The temperature at which this phenomenon starts is often referred to as austenite start temperature (A s ). The temperature at which this phenomenon is complete is often called the austenite finish temperature (A f ). When the shape memory alloy is in the austenite phase and is cooled, it begins to change into the martensite phase, and the temperature at which this phenomenon starts is often referred to as the martensite start temperature (M s ). The temperature at which austenite finishes transforming to martensite is often called the martensite finish temperature (M f ). The range between A s  and A f  is often referred to as the martensite-to-austenite transformation temperature range while that between M s  and M f  is often called the austenite-to-martensite transformation temperature range. It should be noted that the above-mentioned transition temperatures are functions of the stress experienced by the shape memory alloy sample. Generally, these temperatures increase with increasing stress. In view of the foregoing properties, deformation of the shape memory alloy is applied preferably at or below the austenite start temperature (at or below A s ). Subsequent heating above the austenite start temperature causes the deformed shape memory material sample to begin to revert back to its original (nonstressed) permanent shape until completion at the austenite finish temperature. Thus, a suitable activation input or signal for use with shape memory alloys is a thermal activation signal having a magnitude that is sufficient to cause transformations between the martensite and austenite phases. 
     The temperature at which the shape memory alloy remembers its high temperature form (i.e., its original, nonstressed shape) when heated can be adjusted by slight changes in the composition of the alloy and through thermo-mechanical processing. In nickel-titanium shape memory alloys, for example, it can be changed from above about 100 degrees Celsius to below about −100 degrees Celsius. The shape recovery process can occur over a range of just a few degrees or exhibit a more gradual recovery over a wider temperature range. The start or finish of the transformation can be controlled to within several degrees depending on the desired application and alloy composition. A torque-transmitting device within the scope of the invention has been shown to function as intended with activation temperatures ranging from −40 degrees Celsius to 400 degrees Celsius. 
     Suitable shape memory alloy materials include, but are not intended to be limited to, nickel-titanium based alloys, indium-titanium based alloys, nickel-aluminum based alloys, nickel-gallium based alloys, copper based alloys (e.g., copper-zinc alloys, copper-aluminum alloys, copper-gold, and copper-tin alloys), gold-cadmium based alloys, silver-cadmium based alloys, indium-cadmium based alloys, manganese-copper based alloys, iron-platinum based alloys, iron-palladium based alloys, and the like. The alloys can be binary, ternary, or any higher order so long as the alloy composition exhibits a shape memory effect. 
     A method of enhancing performance of an apparatus that includes the mechanical assembly and rheological fluid as described above includes activating the active material component in contact with the fluid to thereby cause one of an increase or a decrease in the surface roughness of the active material component. After activating, the method further includes deactivating the active material component to cause the other of an increase or a decrease on the surface roughness of the active material component. 
     As disclosed, an MR clutch with a variable surface geometry may be capable of delivering a higher torque transfer (e.g., 100-400 percent increase), or the same torque transfer at a lower electrical output, or the same torque transfer but having a smaller size (e.g., having a 100 to 400 percent smaller working area), as compared to an MR clutch with smooth surfaces in contact with the MR fluid having the same size and electrical power input, and may also have less drag when torque-transfer is not desired than does an MR clutch having a permanently rough or grooved surface geometry. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary cross-sectional schematic illustration of a magnetorheological torque-transmitting device within the scope of the invention; 
         FIG. 2  is a schematic perspective illustration of a portion of an active material component in the form of a coating included on the torque-transmitting device of  FIG. 1 , in an activated state and having a relatively rough surface geometry; and 
         FIG. 3  is a schematic perspective illustration of the active material coating of  FIG. 2  in an inactivated or deactivated state and having a relatively smooth surface geometry. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers refer to like components,  FIG. 1  illustrates an apparatus or torque-transmitting device  10 , also referred to as a MR fluid coupling or an MR fluid clutch, including a mechanical assembly, which in this embodiment is a clutch assembly  12 . The clutch assembly  12  includes a cylindrical rotor  14  concentrically rotatable within a cylindrical stator  16  having an outer portion  18  and a magnetic inner portion  20 . A drive pulley (not shown) may be attached around the outer periphery of the stator  16  to cause it to rotate. The pulley may be driven, in turn, by a serpentine belt such as those used to power accessory items on an engine such as the air conditioner compressor, water pump, power steering pump or alternator. The stator  16  includes a magnetic field generator  22 . The rotor  14 , having a rotational degree of freedom with respect to the stator  16 , is in direct mechanical communication with a rotatable shaft  24  that has a rotational axis  26 . The shaft  24  may be a driven member of a fan, a transmission member, or any other rotatable mechanical component. The stator  16  and rotor  14  define an annular space  28 , and are coupled via an MR fluid  30  disposed within the annular space  28 . As discussed further with respect to  FIGS. 2 and 3 , two active material components  29  and  31  are formed on the stator  16  and rotor  14 , respectively, in contact with the MR fluid  30  to increase the torque capacity of the torque-transmitting device  10  when in an “on” (magnetized) stare, while reducing parasitic drag of the torque-transmitting device  10  when in an “off” (unmagnetized) state. The MR fluid  30  includes a carrier fluid, depicted as the white space filling the annular space  28 , and magnetizable particles suspended in the base fluid, depicted as dots within the annular space  28 . 
     An exemplary MR fluid composition generally comprises magnetizable particles, a carrier fluid and additives. The magnetizable particles of the MR fluid composition are comprised of, for example, paramagnetic, superparamagnetic, or ferromagnetic compounds or a combination comprising at least one of the foregoing compounds. The number of magnetizable particles in the MR fluid composition depends upon the desired magnetic activity and viscosity of the fluid, but may be from about 0.01 to about 60 volume percent, based on the total volume of the MR fluid composition. In one embodiment, the number of magnetizable particles in the MR fluid composition may be from about 1.5 to about 50 volume percent, based on the total volume of the MR fluid composition. As is known, the static yield stress of MR fluids increases nonlinearly with increasing volume fraction of magnetizable particles in the MR fluid. The carrier fluid forms the continuous phase of the MR fluid composition. Examples of suitable carrier fluids are natural fatty oils, mineral oils, polyalpha-olefins, polyphenylethers, polyesters (such as perfluorinated polyesters, dibasic acid esters and neopentylpolyol esters), phosphate esters, synthetic cycloparaffin oils and synthetic paraffin oils, unsaturated hydrocarhon oils, monobasic acid esters, glycol esters and ethers (such as polyalkylene glycol), synthetic hydrocarbon oils, perfluorinated polyethers, halogenated hydrocarbons, or the like, or a combination comprising at least one of the foregoing carrier fluids. Exemplary carrier fluids are those which are non-volatile, non-polar and do not contain amounts of water greater than or equal to about 5 wt %, based upon the total weight of the carrier fluid. In general, it is desirable for the MR fluid composition to have a viscosity of about 50 to about 500 centipoise at 40 degrees Celsius in the off-state. On-state yield stresses for MR fluid compositions are about 10 to about 100 kilopascals (about 1.5-to about 15 pound per square inch). These yield stresses would be measured at magnetic flux densities on the order of about 1 to about 2 tesla (that is, when the particles are magnetically saturated). 
     The magnetic field generator  22  is in field communication with the MR fluid  30  in the annular space  28 , which is illustrated generally by flux lines  32 . Stator  16  and rotor  14  include magnetic portions  34 ,  36 , respectively, and non-magnetic portions  38 ,  40 , respectively, which serve to guide the magnetic field in a manner suitable for the purposes disclosed herein. Magnetic portion  36  is also herein referred to as a stator portion of the magnetic field generator  22 . Suitable magnetizable materials include but are not limited to iron, steel, carbonyl iron, or the like, or a combination comprising at least one of the foregoing magnetizable materials. Suitable non-magnetic materials include but are not limited to stainless steel, aluminum, brass, plastics, or the like, or a combination comprising at least one of the foregoing non-magnetic materials. Alternatively, an air gap may be employed in place of or in addition to the use of non-magnetic portions. 
     Magnetic field generator  22  includes a stationary magnetic core  42 , and a field coil  44  that is energized via external leads and power source (not shown but well known in the art). Exemplary oil seals  46  serve to prevent leakage of the MR fluid  30  from annular space  28 . A stator housing  48  retains the outer portion  18  and the magnetic inner portion  20  of the stator  16  to one another. The rotor  14  is driven by rotation of the stator  16  by torque transfer via the fluid  30  when the fluid  30  is exposed to magnetic flux generated by energizing the field coil  44 . A controller, switch, or other mechanism, not shown, selectively energizes the field coil  44  when operating conditions warrant the transfer of torque. 
     It has been observed that, when the MR fluid is in shear flow and exposed to a sufficiently high magnetic field, the columns of magnetic particles within the MR fluid will coalesce into “stripes” or bands of particles with a high concentration. This stripe formation results in an increase in the apparent viscosity of the MR fluid, which can be as much as two to ten times larger than that of the same MR fluid without stripes. This increase in apparent yield stress or viscosity can occur because of the non-linear relationship between the static yield stress and volume fraction of magnetizable particles in the MR fluid. A corresponding torque increase in a clutch, for example, on the order of 2-4 times would be reasonable for a clutch having structure promoting the formation of stripes as compared to a clutch without such structure. The active material components described here may also promote the formation of such stripes, as described below. 
     In an exemplary embodiment, each of the an active material components  29 ,  31  form a surface in contact with the MR fluid  30  that is configured so as to promote drag at appreciable magnetic flux densities while at the same time permitting the reduction of drag of the MR fluid when it is not desired to transmit torque via the torque-transmitting device  10 . The active material component  29  is a layer on the inner cylindrical surface of the stator outer portion  18 . The active material component  31  is a layer on the outer cylindrical surface of the rotor  14 . It should be appreciated that only one of these components may be provided in alternative embodiments, or active material component(s) could instead or in addition be provided on one or both of the inner cylindrical surface  54  of the rotor  14  and the outer cylindrical surface  56  of the inner magnetic portion  20  of the stator  16 . 
     The active material components  29 ,  31  are activatable and deactivatable to effect a change in surface geometry that promotes torque transfer through the MR fluid  30  within the annular space  28  in response to rotation of the rotor  14 , as well as to reduce drag when torque transmission is not desired, as described below. Referring to  FIG. 2 , a perspective view of a portion of the active material component  31  formed as a layer on rotor  14  in contact with the MR fluid  30  in  FIG. 1  reveals an active material surface  60  having a rough surface geometry characterized by protrusions  62  and depressions  64 . Preferably, the active material component  31  is formed such that the protrusions  62  and depressions  64  are relatively aligned as grooves and valleys, as shown, promoting the formation of stationary flow channels to enhance striping. The surface roughness from the peak of the protrusion  62  to the base of the depression  64 , represented as the peak to base height H 1  of the typical protrusion  62  and the base to peak height H 2  of the typical depression  64 , may be in the range of 1 to 500 microns, with a preferred range of 20-30 microns, to provide the necessary surface roughness. Preferably, the surface  60  has this rough surface geometry when the active material component  31  is activated. Preferably, the active material component  31  is activated by the viscous heating of the MR fluid  30  caused by magnetization of the MR fluid  30 . Alternatively, electrical wires, not shown, may be embedded in the active material component  31  such that an electrical signal, causing resistive heating, may activate the active material component  31 . 
     Referring to  FIG. 3 , the active material component is shown in an inactivated state, in which it is referenced as  31 A, and in which the surface in contact with the MR fluid  30  is the same surface as that of  FIG. 2  but has a relatively smooth surface geometry absent the protrusions  62  and depressions  64  of the surface geometry of  FIG. 2 , and therefore is referred to as  60 A. The inactivated state may result from removal of the magnetic field from the torque-transmitting device  10  (i.e., the cessation of current in field coil  44 ), and the subsequent cooling of the MR fluid  30  in response to removal of the field. It should be appreciated that the surface  60 A in contact with the MR fluid  30  may alternatively represent the activated state of the active material component  31 A, while activation may cause a return to the rough surface geometry  60 . In that instance, activation would be controlled to occur generally concurrently with cessation of current in field coil  44 . 
     It should be appreciated that, although a clutch assembly for torque transmission is described herein, within the scope of the invention, an active material component forming a surface in contact with the rheological fluid in other torque transmitting applications, such as in a polishing apparatus, or in applications utilizing pressure-driven flow, such as a damping mechanism or shock absorber, may also enhance the transmission of stress through the fluid. 
     Thus, a method of enhancing performance of an apparatus that includes a rheological fluid, discussed with respect to the torque-transmitting device  10  of  FIGS. 1-3 , includes activating an active material component  31  in contact with a rheological fluid  30  to thereby cause an increase or a decrease in the surface roughness of the active material component  31  and increase or decrease, respectively, stress transmission (e.g., torque transfer) through the rheological fluid  30  in contact therewith. The method then includes deactivating the active material component  31  to thereby cause the other of an increase or decrease of the surface roughness of the active material component  31 . The torque-transmitting device  10  having the active material component(s)  29 ,  31  described herein when operated according to the method described herein permits efficient torque transfer when in a “clutch on” state and the minimization of undesirable drag when in a “clutch off” state. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.