Brakes – Internal-resistance motion retarder – Magnetic fluid or material
Reexamination Certificate
2002-05-07
2004-04-20
Graham, Matthew C. (Department: 3683)
Brakes
Internal-resistance motion retarder
Magnetic fluid or material
Reexamination Certificate
active
06722480
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a magnetically actuated motion control device. In particular the present invention relates to magnetically actuated motion control devices that vary the contact force between a first member and a second member in accordance with a generated magnetic field.
BACKGROUND OF THE INVENTION
Magnetically actuated motion control devices such as magnetically controlled dampers or struts provide motion control, e.g., damping that is controlled by the magnitude of an applied magnetic field. Much of the work in the area of magnetically controlled dampers has focused on either electrorheological (ER) or magnetorheological (MR) dampers. The principle underlying both of these types of damping devices is that particular fluids change viscosity in proportion to an applied electric or magnetic field. Thus, the damping force achievable with the fluid can be controlled by controlling the applied field. Examples of ER and MR dampers are discussed in U.S. Pat. Nos. 5,018,606 and 5,284,330, respectively, assigned to Lord Corporation of Cary, N.C.
Generally, MR fluids have high yield strengths and viscosities, and therefore are capable of generating greater damping forces than ER fluids. In addition, the viscosities of MR fluids are precisely controlled by easily produced magnetic fields that are generated by energizing simple low voltage electromagnetic coils. As a result, dampers employing MR fluids have become preferred over ER dampers.
Because ER and MR fluid dampers involve fluid damping, the dampers must be manufactured with precise valving and seals. In particular, such dampers typically require a dynamic seal and a compliant containment member and as a result, prior art MR and ER dampers are not easy to manufacture or assemble. Further, the ER and MR fluid dampers can have significant “off-state” forces when the devices are operated at high speeds and the off-state forces can further complicate their manufacture and assembly. Off-state forces refer to those forces at work in the damper when the damper is not energized.
As a result of the shortcomings associated with prior art MR and ER fluid devices, magnetically actuated alternatives to traditional MR fluid motion control devices have been developed. Such magnetically actuated prior art devices are disclosed in pending U.S. Pat. No. 6,378,671 for “Magnetically Actuated Motion Control Device” and in pending divisional application of the allowed '365 application having Ser. No. 10/080,293, filed Feb. 20, 2002 for a “System Comprising Magnetically actuated Motion Control Device”. Both of the issued patent and pending application are assigned to Lord Corporation of Cary, N.C. The prior art magnetically actuated devices disclosed in the applications contain no MR or ER fluid, yet provide a variable level of coulombic or friction damping that is controlled by the magnitude of the applied magnetic or electric field. Prior art magnetically actuated motion control devices overcome a number of the shortcomings associated with MR and ER fluid devices. For example, prior art magnetically actuated motion control devices: may be manufactured and assembled relatively simply and at a relatively low cost; allow for very loose mechanical tolerances and fit between components; do not require a dynamic seal or a compliant containment member; have particularly low off-state forces and provide for a wide dynamic range between the off-state and a maximum damping force. The wide dynamic range is particularly evident when the devices are operated at high speeds.
An exemplary prior art magnetically actuated motion control device disclosed in the pending applications referred to in paragraph [0005] hereinabove is illustrated generally in
FIGS. 1
,
2
and
3
. The prior art motion control device or damper is identified generally at
101
in FIG.
1
and includes a tubular housing
103
defining a cavity
105
in which a piston
107
is located and moveable linearly therein along axis
123
. Each end of the damper preferably includes a conventional, well known structure which facilitates attaching damper
101
to other structures, such as clevis eye
121
for attaching the end to a portion of a damped component. The housing
103
includes a least one axially aligned slot
109
. The slot may also be referred to as a longitudinally extending slot. The prior art device
101
of
FIG. 1
comprises eight slots. All eight of the slots are illustrated in FIG.
2
and five of the slots are illustrated in FIG.
1
. The slots pass through the housing wall to define flexible bands, tabs, or fingers
111
. The slots
109
extend through the wall of the housing
103
and extend axially nearly the entire length of the housing.
Piston
107
includes a shaft
112
having a magnetically active portion
113
made up of at least one, and preferably two electromagnetic coils
115
set in a magnetically permeable core
117
. The portion
113
may also be referred to as a piston head hereinafter. Although here the magnetically permeable core
117
is hollow, the core can alternatively be a solid bobbin. A hollow core allows space for locating connecting wires
119
therein. As shown in
FIG. 3
, the piston head
113
also defines a plurality of annular poles
114
A,
114
B,
114
C and
114
D located adjacent the axially directed portions of the coils
115
. The poles
114
A-
114
D have substantially the same dimensions. The poles comprise substantially the same overall axial dimension identified as P in
FIG. 3 and a
constant lateral dimension equal to approximately one quarter of the diameter D and such lateral dimension is identified as D/4 in FIG.
3
. The axial pole dimension P remains substantially constant as the pole extends laterally along the dimension D/4. The poles have substantially rectangular cross sections and hold a constant radial clearance
127
between the outer periphery of the poles and the housing wall when the coils are not energized. In the prior art device
101
, the magnetic flux produced when the electromagnets are energized is substantially constant through the poles
114
A-D, the inner portion of piston head
113
and housing wall
104
, and the constant flux is illustrated by the equally spaced flux lines
125
in FIG.
3
. The constant flux is primarily a result of the substantially constant dimensions of the poles, active portion
113
and wall
104
.
A current source
118
supplies current to the coils
115
through wires
119
. Current flowing through the coils
115
creates a magnetic field that draws the housing
103
in toward the piston head
113
. As indicated above, the created magnetic field is illustrated in
FIG. 3
by field lines
125
. Also shown in
FIG. 3
the field surrounds the coil
115
and passes through the poles
114
, inner portion of head
113
and housing wall
104
. Like head
113
, the housing
103
is also made from a magnetically permeable material that will be attracted by the magnetic field including, but not limited to, steels and other iron alloys. The amount of current flowing through the coils
115
is generally directly proportional to the magnitude of the magnetic field generated. Thus, control of the electric current flowing through the coils
115
can be used to control the normal or pressing force between the inner surface of the housing
103
and the outer surface of the piston
107
, thereby controlling the damping effect of the damper
101
.
The slotted housing
103
and the head
113
of the piston
107
are preferably made from low carbon, high permeability steel, although other magnetically permeable materials can be used. The slots
109
are preferably evenly spaced around the circumference of the housing
103
so that axial-periodic symmetry is maintained. The pair of coils
115
is preferably wired such that they produce magnetic fields in opposite directions as indicated by the directional arrows associated with the field lines
125
illustrated in FIG.
3
. This configuration allows the magnetic field produced by each coil
115
to add r
Gnibus Michael M.
Graham Matthew C.
Lord Corporation
Murphy III Edward F.
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