Abstract:
Variably controllable motion-damping devices are disclosed that utilize a magneto-rheological fluid (MRF). One configuration of such a device includes a first valving region and a second valving region connected to the first valving region. Between the first and second valving regions is a plate having a first surface adjacent the first valving region and a second surface adjacent the second valving region. At least one of the first and second surfaces is treated to impart a significantly increased shear yield stress to columnized MRF particles passing over the surface, compared to an otherwise similar untreated surface. An MRF is contained in the first and second valving regions such that the MRF can flow through the first and second valving regions. Motion of a first object relative to a second object is damped by causing flow of MRF through the first and second valving regions. A magnet produces a magnetic field within at least one of the valving regions. MRF flow is constrained by the magnetic field and the treated surface sufficiently to damp the relative motion.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of prior pending U.S. Provisional Patent Application No. 60/559,387, filed Apr. 2, 2004, which is herein incorporated by reference. 

   FIELD 
   The present application is directed to, inter alia, vibration- and motion-dampers, and more particularly, to controllable magneto-rheological fluid dampers. 
   BACKGROUND 
   During the past decade there has been increasing interest in the development of controllable shock absorbers, vibration-dampers, and the like that utilize electro-rheological fluid (ERF) or magneto-rheological fluid (MRF). The possibility of using ERF- or MRF-based damping devices in various applications has made these controllable devices attractive to the vibration-control field. Controllable energy-dissipation devices can potentially be used in a variety of mechanical and structural systems such as bicycles, motorcycles, automobiles, trucks, ships, trains, airplanes, bridges, buildings, sports equipment, and any of various other systems or structures requiring vibration control. 
   In an MRF, micron-sized, magnetically polarized particles are suspended in a carrier fluid such as silicone oil or mineral oil. MRF is capable of responding to an applied magnetic field in a few milliseconds. The material properties of an MRF can change rapidly by increasing or decreasing the intensity of the applied magnetic field. The material property can be viewed as a controllable change in the apparent viscosity of the fluid by varying the current supplied to, for example, an adjacent electromagnet. A higher fluid apparent viscosity can be exploited to provide a higher damping force or pressure-drop across an MRF valve. This is the phenomenon behind the controllability of MRF dampers and related devices. 
   Certain types of MRF dampers, such as described in U.S. Pat. No. 5,277,281 to Carlson et al. and U.S. Pat. No. 6,510,929 to Gordaninejad et al., are known in the art. However, these dampers exhibit various drawbacks and/or application limitations. 
   Different parameters and variables within an MRF valve can affect MRF valve performance. For example, the shear yield stress of different magnetic colloidal suspensions has been measured by Lemaire and Bossis utilizing different rheometer plate materials. Lemaire and Bossis, “Yield Stress and Wall Effects in Magnetic Colloidal Suspensions,”  J. Phys. D, Appl. Phys . 24:1473-1477, 1991. The roughness and material of the rheometer plates were varied to produce wall effects. Two paramagnetic plate surfaces, stainless steel and glass, and one ferromagnetic plate surface, iron, were investigated. The smooth surfaces of glass plates produced almost zero yield stress. Stainless steel plates produced higher yield stress, basically due to the roughness of the surface. The roughness of the stainless steel and iron plates were identical; however, due to wall interactions of the fluid with the iron plates, the highest yield stress was observed with iron plates. 
   Increased torque performance of an ERF clutch has been achieved by attaching porous fabric materials to the clutch surfaces. Monkman, “Addition of Solid Structures to Electrorheological Fluids,”  J. Rheol . 35(7):1385-1392, 1991. The increase was attributed to increased electroviscosity due to fabric polarization. Efforts to increase the performance of MRF and ERF by structured modifications in torsional flow modes were reported by Gorodkin et al., “Surface Shear Stress Enhancement under MR Fluid Deformation,”  Proc . 8 th    Intl. Conf. on Electrorheol. Fluids and Magnetorheol. Susp . pp. 847-852, Nice, France, 2001, and by Abu-Jdayil et al., “Effects of Electrode Morphology on the Behavior of Electrorheological Fluids in Torsional Flow,”  J. Intell. Matl. Syst . &amp;  Strmt . 13:3-11, 2002, respectively. Gorodkin et al. studied the effects of radial and circumferential grooves on a magnetorheometer, and showed that a circumferential groove did not increase measured torque because the grooves were parallel to the flow direction. However, radial grooves that were perpendicular to the torsional flow created an increased shear stress. 
   SUMMARY 
   Disclosed below are representative embodiments that are not intended to be limiting in any way. Instead, the present disclosure is directed toward novel and nonobvious features, aspects, and equivalents of the disclosed embodiments of the controllable magneto-rheological fluid devices as described below. The disclosed features and aspects of the embodiments can be used alone or in various novel and nonobvious combinations and sub-combinations with one another. 
   Embodiments of a controllable magneto-rheological fluid (MRF) device for variably controlling the flow of MRF in passages within the device are herein described. The characteristics of an MRF flowing through the passages can be controlled by subjecting the MRF to a magnetic field of a selectively variable strength. The magnetic field causes ferrous particles within the MRF to form chain-like structures which can adhere to or otherwise interact with one or more surfaces of a fluid passage to create or change a shear yield stress in the passage that resists the flow of the MRF. Increasing the surface area of a passage can increase the shear yield stress. Accordingly, in some aspects of embodiments of the device, two passages are provided in series to optimize the controllability of the MRF flow through the device. Furthermore, in some aspects, one or more surfaces of a passage through which the MRF flows can be surface-treated to increase, decrease, or otherwise alter the controllability of the MRF flow. 
   Specific embodiments of a motion damping device for damping motion of a first object relative to a second object can comprise a first valving region and a second valving region that is coupled to the first valving region. The device can also comprise a plate positioned between the first and second valving regions. The plate can have a first surface adjacent the first valving region and a second surface adjacent the second valving region. At least one of the first and second surfaces can comprise a surface that has been treated so as to impart a significantly increased shear yield stress to columnized MRF particles passing over the surface, compared to an otherwise similar untreated surface. (A “significant” increase is one that is measurable and that represents at least an incremental increase that is favorable from an application point of view. The magnitude of a “significant” increase depends upon various factors such as the size of the MRF device and the magnitude and direction of forces associated with the movement to be damped, etc.) The device can also comprise a magneto-rheological fluid (MRF) contained in the first and second valving regions in a manner allowing the MRF to flow through the first and second valving regions. Motion of the first object relative to the second object causes a flow of MRF through the first and second valving regions. Also, the device can include a magnet configured to produce a magnetic field within at least one of the first and second valving regions. The flow of MRF is constrained by the magnetic field and the treated surface sufficiently to damp the motion of the first object relative to the second object. As used herein, “motion” can include motion of a nature that is termed “vibration” in the art. 
   By way of example, the treated surface can comprise a first set of parallel grooves that extend in a first direction and a second set of parallel grooves that extend in a second direction perpendicular to the first direction. 
   In certain embodiments the first object can be a piston and the second object can be a cylindrical housing in which the piston is movably disposed. In some implementations, at least the first valving region, the second valving region, and the plate can be contained within the piston. In this configuration the magnet can be associated with at least one of the valving regions. The magnet can be contained within the piston. In other implementations, at least the first valving region, the second valving region, and the plate are external to the piston, wherein the magnet is associated with at least one of the valving regions. In specific embodiments, the external components are contained within an external housing mounted to the piston housing. 
   In certain embodiments the first valving region can comprise a first generally disk-shaped space, and the second valving region can comprise a second generally disk-shaped space. The first valving region and the second valving region can be, but need not be, parallel to each other. Also, the valve plate can be, but need not be, generally disk-shaped. 
   The magnet can be an electromagnet that includes an electromagnetic coil. Alternatively, for example, the magnet can be a permanent magnet that is either stationary or movable relative to the at least one valving region. In some embodiments, the magnet can be an electromagnet, a permanent magnet, or combination of both. 
   Certain embodiments of a damping device for damping movement of a first object relative to a second object comprises a valving region and a valve plate that is adjacent the valving region. The valve plate has a treated surface that defines a boundary of the valving region. The treated surface of the valve plate is configured to impart a significantly increased shear yield stress to columnized MRF particles passing over the surface, compared to an otherwise similar untreated surface. The device also includes a magneto-rheological fluid (MRF) contained in the valving region in a manner allowing the MRF to flow through the valving region and across the treated surface. A magnet configured to produce a magnetic field within at least the valving region is also included. Movement of the first object relative to the second object causes a flow of MRF through the first valving region, in which the flow is constrained by the magnetic field and the treated surface sufficiently to damp the motion of the first object relative to the second object. 
   In certain embodiments the valving region includes a first valving region and a second valving region, wherein the valve plate is positioned between the first and second valving regions. The first valving region can comprise a first generally disk-shaped space, and the second valving region can comprise a second generally disk-shaped space. In some implementations, the first valving region and the second valving region are parallel to each other. The treated surface can be a first-valve plate surface, wherein the valve plate further comprises a second valve-plate surface defining a boundary of the second valving region. In some implementations, the valve-plate second surface comprises a treated surface. 
   The treated surface can comprise a first set of parallel grooves that extend in a first direction and a second set of parallel grooves extending in a second direction perpendicular to the first direction. Other surface-treatment schemes alternatively can be employed to increase the surface roughness compared to an untreated surface. 
   The first object can be a piston and the second object can be a cylindrical housing in which the piston is movably disposed. In other embodiments, the first object can be a mass to which the piston is attached, and the second object can be a mass to which the cylinder is attached. In some implementations, the first valving region, the second valving region, and the plate can be contained within the piston. In other implementations, the first valving region, the second valving region, and the plate are external to the cylindrical housing. 
   Certain embodiments of a motion-damping device have a movable piston that comprises a first valving stage comprising a first primary valving region that is coupled to a first secondary valving region. A first valve plate is positioned between the first primary valving region and the first secondary valving region. The device also comprises a second valving stage in series with the first valving stage, the second valving stage comprising a second primary valving region that is coupled to a second secondary valving region. A second valve plate is positioned between the second primary valving region and the second secondary valving region. The device also comprises a magneto-rheological fluid (MRF) contained in the first and second primary valving regions and the first and second secondary valving regions in a manner allowing the MRF to flow through these valving regions. The device includes a first magnet configured to produce a magnetic field within at least one of the first primary valving region and the first secondary valving region, and a second magnet configured to produce a magnetic field within at least one of the second primary valving region and the second secondary valving region. Motion of the piston can be damped by the flow of MRF through the first primary and secondary valving regions and the second primary and secondary valving regions as the first and second magnets produce their respective magnetic fields. The magnets need not produce the respective magnetic fields simultaneously. 
   Furthermore, the first valve plate can comprise a first surface defining a boundary of the first primary valving region and a second surface defining a boundary of the first secondary valving region. The second valve plate can comprise a first surface defining a boundary of the second primary valving region and a second surface defining a boundary of the second secondary valving region. In some implementations, at least one of the first and second surfaces of the first and second valve plates comprises a treated surface. The treated surface is configured to impart a significantly increased shear yield stress to columnized MRF particles passing over the surface, compared to an otherwise similar untreated surface. 
   In certain embodiments the piston is movably disposed within a cylindrical housing. In such configurations the first and second primary valving regions, the first and second secondary valving regions, and the first and second valve plates can be contained within the piston or located externally to the cylindrical housing. The first and second magnets can each include electromagnetic coils. 
   The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a partial cross-sectional view of a motion-damper according to one embodiment of the present application. 
       FIG. 2  is a partial cross-sectional view, along the line  2 - 2  in  FIG. 1 , showing an internal magneto-rheological fluid (MRF) damper valve. 
       FIG. 3  is a side elevational view of a motion-damper according to one embodiment having an external MRF by-pass valve and a satellite accumulator. 
       FIG. 4  is a cross-sectional view of the external MRF by-pass valve of the motion-damper of  FIG. 3 . 
       FIG. 5  is a perspective view of a valve plate of the MRF by-pass valve of  FIG. 4 , in which the valve-plate surface has been treated. 
       FIG. 6A  is a graph illustrating the pressure drop across the MRF by-pass valve of  FIG. 5  as a function of time for various activation electrical currents and a constant volumetric flow rate of 22.8 cm 3 /s. 
       FIG. 6B  is a cross-sectional view of an embodiment of an MRF damper valve. 
       FIG. 7  is a graph illustrating the shear yield stress of activated MRF particles against different surfaces of the MRF by-pass valve of  FIG. 4  as a function of activation electrical current for surfaces of varying roughness. 
       FIG. 8  is a cross-sectional view of a damper according to one embodiment having two internal MRF damper valves in series. 
   

   DETAILED DESCRIPTION 
   Described below are embodiments of controllable magneto-rheological fluid (MRF) devices in which the flow of MRF in respective passages within the devices are variably controlled. Generally, an MRF as known in the art contains ferrous particles that form chain-like columns when the fluid is subjected to a magnetic field. The columns are generally oriented parallel to the direction of the magnetic field flux lines. For optimal effect, the flux lines desirably are at an approximately 90-degree angle to surfaces of a valving region, or approximately perpendicular to the direction of MRF flow through the valving region. The chains of ferrous particles thus oriented effectively increase the viscosity of the MRF as the MRF flows through the valving region. The increased apparent viscosity generally results in an increased pressure drop between an entrance and an exit of a valving region, which increases the damping force of the MRF device. 
   Referring to  FIG. 1 , an embodiment of a controllable MRF device  1  includes a work cylinder  3  and an internal work piston  6  positioned within the work cylinder. The work cylinder  3  defines an internal compression chamber  7  on one side of the work piston  6  and an internal rebound chamber  5  on the other side  4  of the work piston  6  as shown. The compression chamber  7  and the rebound chamber  5  are at least substantially filled with an MRF, such as one generally known in the art. The work piston  6  includes outer (circumferential) seals configured to create a sealed barrier between the rebound chamber  5  and the compression chamber  7  and allow the work piston to move axially within the work cylinder  3  while maintaining the sealed barrier. 
   The MRF device  1  further includes a piston rod  11  with an internal end coupled to the piston  6  within the work cylinder  3 , and an external end extending to outside the work cylinder  3 . The work cylinder  3  includes an end cap  37  threadably coupled to the cylinder. The end cap  37  defines an opening through which the piston rod  11  extends. A seal (not detailed, but well understood in the art of, e.g., hydraulic pistons) is provided around the opening to seal the opening against the piston rod  11  to prevent MRF from leaking out of the work cylinder  3 . 
   In some embodiments, a mounting element, such as rod ring  2  or rod clevis, is attached to or integrated into the external end of the piston rod  11 . The mounting element, or rod ring  2 , is generally configured to be coupled to a structure or mass that moves in the axial direction of the MRF devices relative to the work cylinder  3 . Furthermore, a mounting element, such as fixed support ring  9  or fixed clevis, can be attached to or integrated into the work cylinder  3  and configured to be coupled to a fixed structure or mass. The MRF device  1  is thus configured such that movement of the movable structure is relative to the fixed structure in the axial direction correspondingly moves the work piston  6  within the work cylinder  3  in the axial direction. Axial movement of the work piston  6  results in the displacement of MRF contained within the device  1 . 
   The MRF device  1  includes an accumulator that comprises an accumulator piston  8  and an accumulator chamber  13  that are situated “below” the work piston  6  and compression chamber  7  in the figure. As the work piston  6  compresses, i.e., moves axially in a direction toward the support ring  9 , the accumulator accommodates for the added volume of the piston rod  11 . Specifically, the accumulator piston  8  moves in the same axial direction as the work piston  6  and compresses a gas in the accumulator chamber  13 . The gas can be, for example, air or nitrogen. The accumulator chamber  13  contains a predetermined amount of gas sufficient to accommodate the volume of the piston rod  11  over the full stroke of the piston rod. The accumulator piston  8  includes a circumferential seal  12  that prevents the gas from leaking into the compression chamber  7  and prevents the MRF from leaking into the accumulator chamber  13 . Alternatively to the  FIG. 1  embodiment, the accumulator can be external as shown in  FIG. 3  and described in more detail below. 
   The MRF device  1  includes a magnetic element configured to produce a magnetic field having a flux oriented through the device in a direction generally parallel to the axis of the device. In the  FIG. 1  embodiment (with a portion of the piston removed for illustrative purposes), the magnetic element is a wound magnetic coil  10  positioned within the work piston  6 . As detailed in  FIG. 2 , the coil  10  is wound around a coil-receiving portion  42  formed in a piston-support member  40 . Alternatively, in certain embodiments, the magnetic element can be a wound magnetic coil  10  positioned coaxially around the exterior of at least a portion of work cylinder  3 . Although the magnetic element shown in  FIGS. 1 and 2  is a wound magnetic coil, it is recognized that any element capable of producing a magnetic field through the MRF device  1  can be used. For example, the use of one or more permanent magnets in place of, or in conjunction with, the electromagnet may be used. Additionally, the use of an electromagnet to counteract the constant magnetic field of one or more permanent magnets (or a converse configuration) can be used to produce a reverse-controlled mode. If one or more permanent magnets are used, they can be mounted so as to be displaceable in position relative to the MRF fluid. 
   A magnetic field produced by the coil  10  is activated and variably controlled by the application thereto of varying levels of electric current. In other words, the coil  10  in the depicted embodiment is an electromagnetic coil that produces a magnetic field having a magnitude that is proportional to the amplitude of the electric current applied to the coil. The electric current can be applied to the coil  10  via external electrical wire leads  4  ( FIG. 1 ) that are connected to a selectively variable external electrical power source (not shown). In the depicted embodiment, the wire leads  4  extend longitudinally through the piston rod  11  to the coil  10 . 
   As shown in  FIG. 2 , the work piston  6  includes an internal MRF damper valve indicated generally at  39 . The damper valve  39  includes a primary valving region  16  and a secondary valving region  17 . The valving regions  16 ,  17  generally define passageways through which MRF can flow. The valving regions  16 ,  17  are fluidly coupled to each other by an MRF flow passageway  41  oriented generally in the vertical direction as shown. Generally, the valving regions  16 ,  17  are each partially bounded by a valve plate  44  affixed within a cavity  45  formed in a piston-support member  40 . More specifically, the primary valving region  16  is the space defined between a first major surface  46  of the valve plate  44  and an end surface  53  of the valve plate receiving cavity. The secondary valving region  17  is the space defined between a second major surface  48  of the valve plate  44  and an interior surface  49  of an inlet manifold plate  15 . 
   The valve plate  44  in the depicted embodiment is generally disk-shaped with the first major surface  46  and the second major surface  48  extending radially substantially parallel to each other. The cavity  45  in the piston-support member  40  is generally cylindrically-shaped defined by a vertically extending sidewall surface  51 , the end surface  53 , and an open end opposite the end surface  53 . The diameter of the cavity  45  is larger than the diameter of the valve plate  44  such that, when the valve plate  44  is positioned co-axially within the cavity  45 , the peripheral surface of the valve plate  44  and the sidewall surface  51  define the MRF flow passageway  41 . It can be appreciated from the foregoing that the primary valving region  16  and the secondary valving region  17  are generally disk-shaped spaces, and the MRF flow passageway  41  is generally a ring-shaped space. This configuration improves the contact area to which MRF ferrous particles formed into chains can adhere. In other words, because the MRF flow is distributed over a larger area around the valve plate  44 , more MRF chains can be formed and adhered to the walls of the valving regions  16 ,  17  for increasing the viscosity of the MRF and improving the damping effects exhibited by the device  1 . 
   Although the primary valving regions and second valving regions shown in the illustrated embodiments define respective generally disk-shaped spaces, it is recognized that one or more of the regions can define respective spaces other than disk-shaped. For example, the regions could have converging, diverging or circular surfaces and define cone-shaped, spherical-shaped, or semi-spherical shaped spaces. 
   The inlet manifold plate  15  includes an inlet port  14  through which MRF contained in the compression chamber  7  can flow. The inlet port  14  is positioned generally at the center of the manifold plate  15 , co-axially with the work cylinder  3  and work piston  6 . Although the illustrated embodiment has one inlet port  14 , it is recognized that one or more inlet ports can be formed in the inlet manifold plate  15 . The piston-support member  40  defines an inner orifice  32   a  coupled to a piston-rod inner orifice  55  extending through the center of the piston rod  11 . The piston-rod inner orifice  55  is connected to one or more exit ports  57  through which MRF can flow into the rebound chamber  5 . It will be appreciated that this embodiment permits multiple inlet ports  14  and exit ports  57 , thereby allowing for separate compression and rebound flow paths. Thus, the MRF device  1  can achieve different compression and rebound force characteristics if desired. Also, multiple inlet ports and exit ports give greater flexibility in the design of the damping forces by varying the number of inlet and outlet ports. 
   In operation, during compression of the work piston  6 , MRF from the compression chamber  7  enters the internal MRF damper valve  39  by flowing through the inlet port  14  into the secondary valving region  17 . The MRF diverges and flows through the secondary valving region  17  from the axis A of the valve plate  44  (coincident with the axis of the piston), radially outward from the axis A toward the MRF flow passageway  41 . The MRF then flows through the MRF flow passageway  41 , from which the fluid enters the primary valving region  16 . Flowing radially inward through the primary valving region  16  toward the axis A, the MRF fluid converges at the axis and flows through the inner orifice  32   a  and the piston-rod inner orifice  55 . The MRF then flows through the exit ports  57  into the rebound chamber  5 . In this way, the rebound chamber  5  and the compression chamber  7  are fluidly coupled via the internal MRF damper valve  39 . 
   As MRF flows through the secondary valving region  17  and the primary valving region  16 , the electromagnetic coil  10  can be activated to produce a magnetic field in the valving regions. The magnetic field can be represented by magnetic-flux lines extending generally perpendicularly to the direction of the MRF flow as the fluid flows through the valving regions, i.e., perpendicularly to the first and second major surfaces  46 ,  48  of the valve plate  44 . Subjected to the magnetic field, the ferrous particles in the MRF (especially in a stationary fluid) form chain-like columns oriented along or substantially parallel to the magnetic flux lines. The ends of the chain-like columns contact the surfaces of the primary and secondary valving regions  16 ,  17 . The resulting shear yield stress between the surfaces and the chain-like columns acts to resist flow of the MRF through the valving regions  16 ,  17 . The shear yield stress can be increased by increasing the current supplied to the electromagnetic coil  10  and decreased by decreasing the current supplied to the coil. With increased magnetic field, the MRF becomes more viscous, which results in increased damping forces. 
   The same concepts can be applied to the MRF fluid as it moves in the reverse direction, i.e., from the rebound chamber  5  to the compression chamber  7 . 
   Turning now to the embodiment illustrated in  FIG. 3 , a controllable MRF device  18  includes a work cylinder  3  and work piston (not shown) positioned within the work cylinder. The MRF device  18 , like the embodiment of  FIGS. 1 and 2 , includes a piston rod  11  coupled to the piston. An external end of the rod includes a rod clevis  21  that can be secured to a movable structure that moves relative to a fixed structure and a fixed clevis  22  that is secured to the fixed structure. The piston of the device  18  functions much like the piston of the MRF device  1  except that MRF in the MRF device  18  is not displaced between a rebound chamber and a compression chamber via the piston, but rather via an external (by-pass) MRF damper valve  25 . The work cylinder  3  includes an external accumulator  24  to compensate for potential volume changes within the compression chamber caused by various conditions such as piston rod displacement and temperature changes. 
   The external MRF damper valve  25  shown in  FIG. 3  (and detailed in  FIG. 4 ) includes an outer housing  29  with a compression line  23   a  that couples the compression chamber of the work cylinder  3  with the housing  29  and a rebound line  23   b  that couples the rebound chamber of the work cylinder  3  with the housing  29 . The housing  29  can be mounted to the work cylinder  3  by mounting brackets  20 . 
   The external MRF damper valve  25  comprises components similar to the components of the internal MRF damper valve  39  shown in  FIG. 2 . Similar to the internal damper valve  39 , the external MRF damper valve  25  includes an inlet-manifold plate  27  having an inlet orifice  34 ; a valve plate  35  with a first surface  36   a  substantially parallel to an opposing second surface  36   b , a support member  28  with a valve-plate receiving cavity  61 , a magnet-receiving cavity  63 , and an inner orifice  32   b ; and an electromagnetic coil  10  wrapped coaxially within the magnet-receiving cavity  63 . The internal damper valve  25  also includes an outlet-manifold plate  26  that defines an outlet orifice  33 . 
   The external MRF damper valve  25  operates in a manner similar to the internal MRF damper valve  39  except that MRF from the compression chamber of the work cylinder  3  enters the damper valve  25  through the inlet orifice  34  via the compression line  23   a . The MRF then flows through a secondary valving region  31  defined between the second surface  36   b  of the valve plate  35  and an interior surface of the inlet-manifold plate  27 . The MRF then flows through an MRF passageway  65  defined between the peripheral surface of the valve plate  35  and a sidewall surface of the valve-plate receiving cavity  61 . The MRF then flows through the primary valving region  30  and enters the inner orifice  32   b . The MRF exits the external damper valve  25  through the outlet orifice  33  and flows into the rebound chamber of the work cylinder  3  via the rebound line  23   b.    
   In this embodiment, employing a primary valving region  30  and a secondary valving region  31  provides greater MRF control, and thus provides potentially enhanced performance, compared to only a single primary valving region. As illustrated in the graph of  FIG. 6A , a measured pressure drop across the external valve  25  is shown as a function of time for various current amplitudes applied to the magnetic coil  10  and a constant MRF volumetric flow rate. Generally, the pressure drop (ΔP MR ) across an MRF valve  90  having a primary valving region  92  and a secondary valving region  94  as shown in  FIG. 6B , i.e., symmetrically-shaped valving regions where the primary valving region is substantially the same shape as the secondary valving region, can be calculated by use of the following equation, 
             Δ   ⁢           ⁢     P   MR       =       2   *         r   1     -       d   1     2         h   1       ⁢       τ   1     ⁡     (     B   1     )         +     2   *         r   2     -       d   2     2         h   2       ⁢       τ   2     ⁡     (     B   2     )                 
where τ 1  and τ 2  are the shear yield stress of the MRF in the primary valving region and the secondary valving region, respectively. As indicated, τ 1  and τ 2  are functions of the applied magnetic field density B 1  and B 2 , respectively. The remaining parameters of the equation are set out in  FIG. 6B . As shown by the equation, the contributions of the valving regions  92 ,  94  on the pressure drop across the MRF valve  90  are dependent on the shear yield stress τ 1  and τ 2  of the MRF in the respective regions.
 
   Although the primary and secondary valving regions of the illustrated embodiments each define a generally symmetrically shaped space and have substantially the same shape as the other, it is recognized that valving regions of other embodiments can be non-symmetrical in shape and be shaped differently, while providing at least the same benefits and advantages of the illustrated embodiments. For example, depending on the application, half of a valving region could be of a disk-shaped space and the opposite half could be half of a cone-shaped space. Also, an MRF valve could have a primary valving region that defines a generally disk-shaped space and a secondary valving region that defines a generally cone-shaped or hemispherical-shaped space. In embodiments where the valving regions are non-symmetrical and/or do not define the same shape, the equation described above would not apply. 
   For added control, additional MRF cavities can be formed in the MRF valve. For example, multiple MRF damper valves as detailed in  FIG. 2  or  FIG. 4  can be connected together in tandem such that MRF fluid exiting a first MRF damper valve passes through a second MRF damper valve, etc. Alternatively, one or more “passive” valving cavities, such as valving cavity  67  shown in  FIG. 4 , can be employed. In the valving cavity  67 , MRF fluid flow is at most only partially affected by the applied magnetic field, but nevertheless is forced to assume some radial circulation as the fluid passes from the inner orifice  32   b  thru the outlet orifice  33 . Although not shown, multiple outlet orifices can be positioned at radially outward positions around the valving cavity  67  to improve radial flow of the MRF through the cavity. 
   As in the internal MRF damper valve  39  of  FIG. 2 , the magnetic coil  10  of the external MRF damper valve  25  includes wire leads  4  that can be coupled to a power source to energize the magnetic coil  10  for producing a magnetic field through the valve  25 . The MRF in the external MRF damper valve  25  behaves in the same manner as the MRF in the internal MRF damper valve  39  as the fluid passes through the magnetic field while flowing through the primary and secondary valving regions  30 ,  31 . Accordingly, the external MRF damper valve  39  is operable to increase the shear yield stress of the MRF particles against the surfaces of the valving regions  30 ,  31  to increase the viscosity of the MRF, which results in increased damping forces. 
   In some embodiments of an internal valve and/or external valve, the first surface and/or second surface of the valve plate can have a surface treatment configured to increase the shear yield stress for a given current supplied to the coil  10 . For example,  FIG. 5  depicts a valve plate  35 , being exemplary of the valve plates of the internal and external valves described herein having a surface treatment  38 . The surface treatment  38  shown in  FIG. 5  includes a first set of parallel grooves  38   a  extending in a first direction and a second set of parallel grooves  38   b  extending in a second direction that is perpendicular to the first direction. The grooves  38   a ,  38   b  are thus oriented such that most of the MRF flowing across the surface treatment  38  is generally not flowing parallel to the grooves. Accordingly, the shear yield stress of the MRF flowing through a valving region where the valve plate surface within the valving region has a surface treatment, as shown in  FIG. 5 , will be greater than if the valve plate surface lacked surface treatment. 
   Altering the width and depth of the grooves  38   a ,  38   b  can affect the characteristics of the MRF as it flows through the valving regions. As shown in  FIG. 7 , experimental results show that increasing the groove depth while maintaining the width of the grooves  38   a ,  38   b  tends to increase the pressure drop through the valving regions. In other words, deeper grooves tend to result in increased shear yield stress and increased damping forces. 
   Although the surface treatment  38  shown in  FIG. 5  includes two set of grooves  38   a ,  38   b  extending perpendicular to each other, it is recognized that other configurations of grooves or alternative surface treatment can be implemented. For example, the surface treatment could include a set of co-axial circular grooves of incrementally greater diameters extending away from the center axis. In some embodiments, the surface treatment could be arrays of geometrically shaped topographic features such as hemispheres, pyramids, and/or rectangular bodies; features formed by machining, etching, or the like; chemical treatments; coatings; arrays of other materials, etc. 
   In some embodiments in which the first surface  36   a  and the second surface  36   b  have a surface treatment, the surface treatments can be of the same type. In other embodiments, the first surface  36   a  has a surface treatment of a type different than the surface treatment of the second surface  36   b . For example, the first and second surfaces  36   a ,  36   b  could have respective surface treatments each being two set of grooves extending perpendicular to each other, or the first surface  36   a  could have a surface treatment with two set of grooves extending perpendicular to each other and the second surface  36   b  could have a surface treatment that is a set of co-axial circular grooves of incrementally greater diameters. 
   In certain embodiments of internal and external MRF damper valves, the portion of the surface of the inlet-manifold plate located within the secondary valving region  31  and/or the portion of the surface of the valve-plate receiving cavity located within the primary valving region  30  can include a surface treatment for increasing the shear yield stress of columnized MRF particles passing by the surface. In this way, each of the opposing surfaces of the first and second valving regions  30 ,  31  can include a surface treatment. Although less preferable, it is recognized that the first and second surfaces of the valve plate  35  can be configured without a surface treatment while the inner surface of the inlet-manifold plate  27  and/or the inner surface of the valve-plate receiving cavity  61  can have a surface treatment. 
   A third exemplary embodiment is shown in  FIG. 8 , in which an MRF damper valve  47  includes a first valving stage having a first primary valving region  50  and a first secondary valving region  52 , and a second valving stage having a second primary valving region  60  and a second secondary valving region  62 . The first valving stage includes a corresponding first magnetic coil  54  and first valve plate  56 . The second valving stage includes a corresponding second magnetic coil  64  and a second valve plate  66 . As illustrated in  FIG. 8 , the first valving stage and the second valving stage are configured with each other, in series, with an inner orifice  72  of a valve support member  70  fluidly coupling the stages together. The MRF damper valve  47  also includes an inlet manifold plate  80  having an inlet orifice  74  and an outlet-manifold plate  82  having an outlet orifice  76 . 
   In operation, MRF flows into the MRF damper valve  49  through the inlet orifice  74  and diverges to flow through the first secondary valving region  52 . The MRF continues to flow through a first passageway defined between the circumferential outer surface of the first valve plate  56  and the inner surface of the valve-support member  70  and into the first primary valving region  50 . The MRF converges to flow through the inner orifice  72  and converges into the second secondary valving region  62  as the MRF flows out of the inner orifice. Similarly, the MRF continues to flow through a second passageway defined between the circumferential outer surface of the second valve plate  66  and the support valve-support member  70  and into the second primary valving region  60 . The MRF then flows through the outlet orifice  76  prior to exiting the MRF damper valve  47 . 
   Similar to the first and second embodiments previously described, the first magnetic coil  54  can be energized to produce a magnetic field that alters the characteristics of the MRF as the fluid flows through the first valving regions  50 ,  52  and the second magnetic coil  64  can be energized to produce a magnetic field that alters the characteristics of the MRF as the fluid flows through the second valving regions  60 ,  62 . Accordingly, positioning first and second valving stages, each comprising a respective primary valving region and a respective secondary valving region, in series advantageously provides increased control of the MRF characteristics as the MRF flows through the valve. 
   Although the embodiment illustrated in  FIG. 8  comprises two valving stages connected together in series, it is recognized that more stages can be configured in series to provide even greater control over the MRF characteristics. Alternatively, or in addition, two or more stages can be connected together in parallel. Furthermore, one or more of the surfaces of the valve plates, support members, or manifold plates within the valving regions can include a respective surface treatment to increase the shear yield stress of the MRF as described above. 
   The MRF damper valve  47  of this embodiment can be internally contained within a piston or be housed externally to the piston. With some embodiments in which the MRF damper valve  47  is internal, although the two valving stages in series shown in  FIG. 8  would be contained in a single piston, it is recognized that multiple pistons containing one or more valving stages can be positioned in series. Regarding some embodiments in which the damper valve  49  is external, the two or more valving stages in series can comprise a first external housing containing one or more valving stages connected in series with a second external housing containing one or more valving stages. In other embodiments, more than two external housings having one or more respective valving stages can be coupled in series. 
   It is appreciated that, in some embodiments of a controllable MRF damper device, while in an off-state, i.e., no electrical activation current applied to the electromagnet(s), the device can still provide significant viscous damper forces or pressure drops across the device. This allows for a fail-safe mode of operation in the event of an electrical system failure. For example, multiple electromagnetic configurations are possible to facilitate pressure drops in the device when in the off-state. Although not required, these configurations can include permanent magnets. Also exemplary is the use of multiple inlet and or outlet ports to provide additional passageways that can act to increase the pressure drop in the off-state. 
   In some embodiments, the damping device can include at least one spring or spring-like element in series or parallel to provide spring-dashpot effects. A spring can provide, for example, a bias to the damper such that the piston preferably assumes a first position (e.g., at one end of a stroke) rather than a second position (e.g., at the other end of the stroke). Spring-dashpot effects are characterized by a controlled rate of extension or retraction of the rod from the cylinder under load. 
   Although axial motion of a movable piston or other object has been described above, other motions, such as rotary motion or combinations of linear and rotary motions, of movable pistons or other objects can be applied in the damper devices of the present application. 
   Generally, the components of controllable MRF devices of the present application can be made from a ferrous material such as mild steel, a non-ferrous material such as stainless steel, aluminum alloy, other metal, polymeric material, or other composite material, or combination thereof. 
   Having illustrated and described the principles of the disclosed embodiments, it will be apparent to those skilled in the art that the embodiments can be modified in arrangement and detail without departing from such principles. In view of the many possible embodiments, it will be recognized that the described embodiments include only examples and should not be taken as a limitation on the scope of the invention. Rather, the invention is defined by the following claims. We therefore claim as the invention all possible embodiments and their equivalents that come within the scope of these claims.