The present invention relates generally to piston dampers, and more particularly to a magnetorheological (MR) piston. 
Conventional piston dampers include MR dampers having a cylinder containing an MR fluid and having an MR piston which slideably engages the cylinder.  The MR fluid passes through an orifice of the MR piston.  Exposing the MR fluid in the orifice to a varying magnetic field, generated by providing a varying electric current to an electric coil of the MR piston, varies the damping effect of the MR fluid in the orifice providing variably-controlled damping of relative motion between the MR piston and the cylinder.  The electric current is varied to accommodate varying operating conditions, as is known to those skilled in the art.  A rod has a first end attached to the upper end of the MR piston and a second end extending outside the cylinder.  The cylinder and the rod are attached to separate structures to dampen relative motion of the two structures along the direction of piston travel. 
A known design includes an MR piston having a substantially annular, magnetically energizable passageway (the above-described orifice) and a magnetically non-energizable passageway (also called a bypass) positioned radially inward from the magnetically energizable passageway.  The MR piston includes a one-piece piston core machined from a steel bar, an electric coil (also called an MR piston coil) disposed in a circumferential recess of the piston core, upper and lower piston plates longitudinally bounding the MR piston core, and a piston ring circumferentially surrounding the piston core and the coil.  The coil is a wound length of electrically-insulated wire having one end electrically attached to an electrically-insulated electrode which passes through the piston core and having the other end grounded to the piston core.  A plastic lining is molded over the piston core and to the wall of the electrode/coil wire passageway of the piston core to further insulate the piston core from the coil and to locate the electrode through the center of the passageway of the piston core.  Another layer of plastic is molded over the coil to protect the wire from damage from the velocity of the MR fluid flow in the magnetically energizable passageway formed from a longitudinal space between the piston ring and the piston core/coil assembly. 
What is needed is an improved magnetorheological piston. 
In a first expression of an embodiment of the invention, a magnetorheological piston includes a magnetorheological-piston coil and a magnetorheological-piston core.  The magnetorheological-piston coil has a longitudinal axis and has an inner-diameter portion and an outer-diameter portion.  The magnetorheological-piston core includes separate upper core, center core, and lower core pieces.  The center core piece is located longitudinally between the upper core and lower core pieces and has a circumferential surface positioned radially inward from the inner-diameter portion of the coil.  The coil is longitudinally positioned between the upper core and lower core pieces. 
In a second expression of an embodiment of the invention, a magnetorheological piston includes a magnetorheological-piston coil and a magnetorheological-piston core.  The magnetorheological-piston coil has a longitudinal axis and has an inner-diameter portion and an outer-diameter portion.  The magnetorheological-piston core includes separate upper core, center core, and lower core pieces.  The center core piece is located longitudinally between the upper core and lower core pieces and has a circumferential surface positioned radially inward from the inner-diameter portion of the coil.  The coil is longitudinally positioned between the upper core and lower core pieces.  Each of the upper core, center core, and lower core pieces is a powder-metal core piece. 
In a third expression of an embodiment of the invention, a magnetorheological piston includes a magnetorheological-piston coil, a magnetorheological-piston core, a nonmagnetic sleeve, and an electrically-insulating coil end piece.  The magnetorheological-piston coil has a longitudinal axis and has an inner-diameter portion and an outer-diameter portion.  The magnetorheological-piston core includes separate upper core, center core, and lower core pieces.  The center core piece is located longitudinally between the upper core and lower core pieces and has a circumferential surface positioned radially inward from the inner-diameter portion of the coil.  The coil is longitudinally positioned between the upper core and lower core pieces.  Each of the upper core, center core, and lower core pieces is a powder-metal core piece.  The sleeve is longitudinally bounded by the upper core and lower core pieces and circumferentially surrounds the coil.  The sleeve has an inner surface facing the coil and has an outer surface which defines an inner wall portion of a magnetically energizable passageway.  The electrically-insulating coil end piece is longitudinally disposed between and in contact with the lower core piece and the coil and is longitudinally disposed between and in contact with the lower core and center core pieces. 
In a fourth expression of an embodiment of the invention, a magnetorheological piston includes a magnetorheological-piston coil and a magnetorheological-piston core.  The magnetorheological-piston coil has a longitudinal axis.  The magnetorheological-piston core includes at least two separate core pieces, wherein each of the core pieces is a powder-metal core piece, and wherein the coil is positioned to circumferentially surround at least a portion of at least one of the core pieces. 
Several benefits and advantages are derived from one or more of the expressions of an embodiment of the invention.  Having at least two separate pieces which make up the magnetorheological-piston core allows a more-expensive-to-make machined piston core to be replaced with a less-expensive-to-make piston core having at least two piston core pieces such as two core pieces having less-machined or non-machined shapes.  Having upper and lower core pieces longitudinally surround the piston coil and having the center core piece with a circumferential surface located radially inward from the inner-diameter of the coil, or having at least two separate core pieces, provides a construction allowing for each core piece to be a powder-metal core piece resulting in overall cost savings for the piston core.  Having a nonmagnetic sleeve provide protection for the coil wire from the velocity of the MR fluid eliminates the prior art step of over-molding the coil.  Having the electrically-insulating coil end piece and an electrically-insulating coating on those portions of the upper core and center core pieces eliminates the plastic lining of the prior art.