Power-off damping in MR damper

[0002] This invention relates generally to a damper for use between corresponding parts as in a vehicle suspension application. In particular the invention deals with magneto-rheological fluid dampers and their ability to provide dampening characteristics in various power states.

[0003] Traditionally, a damper such as a shock absorber or a strut, is installed between corresponding parts where motion in a first part is to be damped in reference to a second part. A traditional damper consists of a body and a piston attached to a rod. The body has a smooth cylindrical interior surface and has an attachment feature at a closed end. The piston slides on the interior surface of the body, and the rod projects out an open end of the body. The open end where the rod projects is sealed around the rod, allowing only the rod to slide in and out of the body. An end of the rod outside the body has an attachment feature. Sealed inside the body is a liquid used as a damping fluid. Also, inside the damper body is a sliding piston that contains a volume of pressurized gas used to allow for fluid volume changes. In use, the attachment features of the rod and the closed end of the body are attached between parts to be damped such as between an automotive frame and a movable part of an automotive suspension. When the parts move, the damper strokes or changes length by the rod and piston moving within the body of the damper. When a force from the parts is applied to the damper, the force is transmitted to the damping fluid forcing the fluid through a restriction mechanism, thereby damping the parts. In magneto-rheological (MR) dampers, MR fluid (fluid containing iron particles) is used and the damping restriction mechanism is a magnetic flux established in the MR fluid. The magnetic flux is generated by electric coils with the level of damping generally proportional to the amount of electric current supplied to the coil. If no current is supplied to the damper coil, damping is at a minimum level. This presents obvious problems in that a current must be constantly supplied to the damper in order to provide a greater than minimum dampening effect by the MR damper.

[0004] Thus, what is desired is a MR damper that will provide a greater than minimum level of dampening when the power source is off.

[0005] The present invention is a MR damper that provides a higher than minimum level of damping when a power source to the MR damper is not supplying a control current. The damper of the present invention includes permanent magnets positioned to direct a magnetic flux across a MR fluid path causing a resistance to MR fluid flow. An electric coil cancels an affect of the permanent magnets when the control current is available to allow a control circuit more operating range. The permanent magnets allow for damping when no control current is available.

[0006] The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the presently preferred embodiment. The drawings that accompany the detailed description are described as follows:

[0007] FIG. 1 is a perspective top view of a damper assembly of the present invention;

[0008] FIG. 2 is a cross-sectional view along line A—A of FIG. 1; and

[0009] FIG. 3 is a cross-sectional view of a control valve assembly of the present invention.

[0010] In FIG. 1 a magneto-rheological (MR) damper assembly of the present invention is shown generally at 10. The MR damper assembly comprises a cylindrical housing 12, a rod 14 exiting a first end 16 of the housing 12, and an attachment assembly 18 exiting a second end 20 of the housing 12.

[0011] In FIG. 2, a cross section of the MR damper assembly 10 is shown. The MR damper assembly 10 is comprised of a rod 14 attached to a piston 24 and a gas cup 26. The interior chamber 15 is filled with MR fluid. An area 27 partitioned by the gas cup 26 contains a pressurized gas. The gas cup 26 and pressurized gas allow for fluid volume changes due to thermal expansion and displacement due to rod 14 movement. As will be appreciated by those skilled in the art, the piston 24 has no control valving. The damper assembly 10 is configured so that a stroking of the rod 14 and the piston 24 forces fluid through a side mounted control valve 36. When the rod 14 is moved or stroked out of the assembly 10 (a rebound stroke), MR fluid above the piston 24 flows through an orifice 28 to a passage 17 between a first tube 32 and a second tube 30, and through an orifice 34 to the control valve 36. Fluid is routed through the control valve 36 and returns to a lower damper chamber 38 through an orifice 40. The fluid continues through a passage 41 in a piston stop 42 to an area 44 under the piston 24. Flow is reverse to this process during a compression stroke.

[0012] In the control valve 36, fluid is routed past a magnetic core 46. Normal damping control is provided by a plurality of coils 48 located on the core 46. The coils 48 setup magnetic fields in the core 46 and in a baffle 50 that also controls the fluid flow path in the control valve 36. The magnetic fields restrain the flow of MR fluid flowing along the core 46 and thereby provide damping control.

[0013] In FIG. 3 a section of the control valve 36 is shown in closer detail. A power off control assembly, shown generally at 52, is mounted on the core 46. The power off control assembly 52 comprises a coil 54, two ring shaped permanent magnets 56, 58, and two magnetic shields 60, 62. Alternatively, the shields 60, 62 may be absent from the assembly. The permanent magnets 56, 58 are magnetized radially so that a flux path is setup as shown by the phantom arrow at 64. The permanent magnets 56, 58 and associated flux provides flow restriction or resistance without electrical power being supplied.

[0014] During powered operation of the damper assembly 10 as shown in FIG. 1, current is supplied to the coil 54 so that a field is created to reduce the effects of the field 64 setup by the permanent magnets 56, 58 and allow the other coils 48, as shown in FIG. 2, along the core 46 to control damping. The magnetic shields 60, 62 are constructed of a magnetically inert material such as aluminum, plastic or stainless steel, and reduce the flux shorting effects of having permanent magnets 56, 58 installed against an core 46 that is constructed of iron and is magnetically soft. Electrical connections (not shown) for the coils 48 and 54 are contained in the core 46.

[0015] The coil 54 that reduces the damping of the damper assembly 10 is dependent on power supplied by a circuit that provides a controlled damping current. Whereby, when there is no power being supplied to the control valve 36, the power off control assembly 52 will be active.

[0016] This invention has been described in an illustrative manner, and it is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.