Electromagnetic actuator

An electromagnetic actuator includes a magnetic flux conductive material case, an electrical current conductive coil, a magnetic flux conductive material core, and a pair of radially polarized magnetic flux developing elements. The coil is disposed in the case coextensively adjacent to its interior wall. The core is movably received within the chamber with motion of the core occurring between the first end and the second end of the case such that a first region of the core traverses the coil between its first end and its midpoint, and a second region of the core traverses the coil between its second end and its midpoint. A first one of the magnetic elements is carried by the first region and a second one of the magnetic elements is carried by the second region so that magnetic flux across the coil between the first region and the case is in a first direction, and magnetic flux across the coil between the second region and the case is in a second direction. The coil is arranged so that an electrical current in the coil between the first coil and the midpoint flows in an opposite direction with respect to the direction of the current in the coil between the second coil and the midpoint. Therefore, the flux current cross product of the flux in the first direction with the coil current and the flux current cross product of the flux in the second direction with the coil current are additive.

FIELD OF THE INVENTION 
The present invention relates generally to electromagnetic actuators, and 
more particularly to an improved electromagnetic actuator capable of 
providing relatively large output forces in response to relatively 
low-level electrical input signals when compared to actuators of similar 
design. 
BACKGROUND OF THE INVENTION 
Electromagnetic actuators are well known. In many applications, the output 
force of the actuator is controlled by and is a function of an electrical 
control or command signal, and as such can be used in a variety of 
applications. One type of electromagnetic actuator is the "linear" 
actuator wherein output force is linearly proportional to the input 
electrical current. For example, as described in U.S. Pat. No. 4,892,328, 
issued Jan. 9, 1990 and assigned to the assignee of the invention 
disclosed in the present application (hereinafter the "'328 patent"), one 
type of a linear electromagnetic actuator is employed as part of an 
electromagnetic strut assembly in a vehicle suspension system for 
controlling the level and orientation of the vehicle sprung mass relative 
to the vehicle unsprung mass. More particularly, a radially polarized 
permanent magnet is carried by the sprung mass and is disposed coaxially 
within a coil carried by the unsprung mass. A current is applied to the 
coil in an amplitude and polarity selected to develop an axial force which 
maintains the orientation of the sprung and unsprung mass in a 
predetermined orientation. 
In the '328 patent, the radially polarized magnet is mounted on an 
elongated rod made of magnetically "soft" iron to provide a flux return 
path to the radially inner pole of the magnet. Accordingly, the magnetic 
flux which is developed from the radially outward pole and which radially 
passes through the coil must be directed by some means to either end of 
the elongated rod so that there is a complete magnetic circuit. However, 
the flux path external of the coil for return to the elongated rod cannot 
again pass through the coil. Otherwise, the coil current interacting with 
the return flux will develop a force in opposition to the force developed 
by the coil current interacting with the flux at the radially outer pole 
of the permanent magnet. Therefore, to keep the flux path between the ends 
of the elongated rod external of the coil, the coil is housed in a 
non-magnetic material housing with the flux path external of the coil 
being through the vehicle spring and upper and lower spring seats between 
which the elongated rod carrying the radially polarized magnet is 
connected. However, the non-magnetic material housing provides a high 
reluctance flux path which decreases the flux density across the coil from 
the outer radial pole of the permanent magnet, thereby decreasing the 
overall efficiency of the device. 
An improved linear electromagnetic actuator, as described in the '343 
patent, overcomes the above described limitation. The actuator of the '343 
patent is capable of providing relatively large output forces in response 
to relatively small level command signal. As set forth in the '343 patent, 
the actuator comprises first and second cylindrical assemblies coaxially 
mounted and movable relative to one another along a common axis of 
relative movement. 
The first assembly includes three coils, each of different radius, disposed 
coaxially about the axis of relative movement. The second assembly 
includes at least a pair of axially spaced apart cylindrical magnets, each 
radially polarized so that flux is directed in a radial direction from 
each magnet between the inner and intermediate coils. At least a second 
pair of similar magnets are positioned between the intermediate and outer 
coils. The magnets are polarized so that one of the magnets of each set 
provides flux in a radially inward direction while the other magnet of 
each set provides flux in a radially outward direction. The magnets 
providing the inwardly directed flux are axially aligned, as are the 
magnets providing the outwardly directed flux so that the magnets of each 
polarity orientation provide all of the radial flux through the same 
sections of the coil assembly. 
The second assembly also includes a center core member positioned inside of 
the inner coil and a cylindrical tube positioned around the outer coil, 
both coaxial with the axis of relative movement to provide an axial return 
path for the radial flux developed by the magnets. Brushes are provided 
between the sets of magnets and at the opposite ends of the magnets for 
applying control current in the coils in one direction through the 
inwardly directed flux and in the other direction to the outwardly 
directed flux so that the current/flux force created in accordance with 
Lorenz' Law will be additive. In the embodiment shown in the '343 patent, 
the magnets, core element and outer cylinder all move relative to the 
coils in response to the force provided. The magnets are preferably made 
of a high magnetic energy product material producing relatively high flux 
density, such as for example, neodymium-iron-boron or samarium cobalt. 
The actuator shown in the '343 patent provides relatively high output 
forces in response to relatively low command signals as compared to the 
electromagnetic device disclosed in the '328 patent. Since the first and 
second assemblies move relative to each other, either assembly may be used 
to actuate an external device. However, the coil assemblies alone, whether 
structurally stationary with respect to the actuator device or coupled to 
the actuator device, may not be sufficiently structurally rigid to 
withstand the forces applied between the two assemblies. Furthermore, if 
the core, outer cylinder and intermediate cylindrical sections which carry 
the permanent magnets are coupled to the actuated device, additional 
weight is added to the actuated mass, requiring higher currents or 
reducing bandwidth. 
To impart sufficient structural integrity to the coil, the coil may be 
carried by the core, and the radially polarized magnets may then move in a 
separate assembly external of the coil as shown in either FIG. 4 or FIG. 6 
of the '158 patent. In FIG. 4 of the '158 patent, electrical connection is 
made to the coil through brushes which are carried with the moving magnet 
assembly. In FIG. 6 of the '158 patent, brushes are eliminated by 
providing for a first half of the coil to be counterwound with respect to 
the second half of the coil. The magnets with the flux radial in the first 
direction will move along the first coil half and the magnets with the 
opposite radial polarization will move along the counterwound coil half 
such that a current through the coil will interact with the flux to 
develop an additive force so that the first and second assemblies move 
relative to each other. 
Yet another type of actuator providing support for the coil is shown in 
Prior Art FIG. 1. More particularly, FIG. 1 shows shows a prior art 
actuator 10 which is commercially available from Northern Magnetics of Van 
Nuys, Calif. The prior art actuator 10 includes magnetic flux conductive 
cylindrical case 12 having an inner wall 14 extending between a first end 
16 and second end 18 of the case 12. An electrical current conductive coil 
20 is wound on a nonmagnetic material coil carrier 21. The carrier 21 is 
coaxially secured within the case 12 with the windings of the coil 20 
being intermediate the carrier 21 and the inner wall 14. The windings are 
made from thin copper wire. 
A core assembly 19 includes an axially polarized cylindrical magnet 22 
having a first magnetic pole at its first end 24 and a second opposite 
magnetic pole at its second end 26. A first disc shaped magnetic flux 
conductive material pole piece 28 is attached to the first end 24 of the 
permanent magnet 22. A second magnetic flux conductive material pole piece 
30 is connected to the second end 26 of the permanent magnet 22. 
The permanent magnet 22 and the first and second pole pieces 28, 30 are 
coaxially mounted to a cylindrical rod 32 which, in turn, is coaxially 
received by end caps 34, 36 in axial slidable engagement. Each end cap 34, 
36 is attached to the cylindrical case 12. The rod 32 is received in 
slidable engagement in coaxial bores 38, 40 in each respective end cap 34, 
36. It is to be noted that the cylindrical rod 32 and end caps 34, 36 are 
of nonmagnetic material. The cylindrical bores 38, 40 may include bearings 
(not shown) to reduce frictional losses. The actuator 10 is one type of 
moving core actuator. 
Accordingly, the first pole piece 28 provides flux in a radial first 
direction across the coil 20 and the second pole piece 30 provides flux in 
the opposite radial direction across the coil 2. Ideally, the flux is 
confined to the case 12 in the axial section between the present position 
of the first pole piece 28 and the second pole piece 30. Thus, if current 
is put into the coil 20 at its midpoint 42, with the current return being 
at a first end 44 and a second end 46, with each end 44, 46 connected in 
common, then the current flux cross product with each pole piece 28, 30 
will be additive. Alternatively, the coil 20 of the prior art actuator 10 
may also be counterwound at either side of the midpoint 42 as set forth in 
the '158 patent. 
However, the ideal flux confinement does not exist. Since the magnet 22 is 
axially polarized, there will be leakage of the flux from the first pole 
piece 28 to the second pole piece 30 at the point they are attached to the 
rod 32 through the center bore of the rod 32. Furthermore, a flux path 
will emanate from the tops of the first pole piece 28 and the second pole 
piece 30 external of the case 12 since the tops of the pole pieces merely 
extend the axial polarization of the magnet 22. Accordingly, not all the 
available flux from the magnet 22 is being utilized to provide radial flux 
in confined axial regions of the coil 20. This flux loss reduces the total 
output power available from the actuator 10. 
Furthermore, to obtain a radially polarized high flux density which remains 
constant in the axial direction, each pole piece 28, 30 must be relatively 
thin in their axial dimension. Otherwise, the flux density will be at a 
maximum where each pole piece 28, 30 is adjacent to cylindrical magnet 22 
and decrease in the axial direction away from the cylindrical magnet 22. 
Enlarging the axial dimension of the pole pieces 28, 30 will also not 
change the total flux across the coil since the total available flux is 
determined by the permanent magnet 22. Therefore, only a small portion of 
the total current within the coil 20 is available to interact with a high 
flux density for producing an axial force since the high density radial 
flux is confined to a very narrow axial region. Therefore, much higher 
currents and power consumption are required for the prior art actuator 10 
to achieve the same types of output forces available through the actuators 
disclosed in the above-referenced patents. 
Another limitation on the total output force available from the prior art 
actuator 10 is due to the coil carrier 21 being disposed between the pole 
pieces 28, 30 and the coil 20. The nonmagnetic material carrier 21 
enlarges the gap in which the flux is confined, thereby reducing field 
strength. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to overcome 
one or more disadvantages and limitations of the actuator identified in 
prior art FIG. 1. A significant object of the present invention is to 
provide a moving core linear actuator which obtains high flux densities in 
a radial direction across larger axial lengths of the coil than in the 
prior art. 
According to a broad aspect of the present invention, an electromagnetic 
actuator includes a magnetic flux conductive material case, an electrical 
current conductive coil, a magnetic flux conductive material core, and a 
pair of magnetic flux developing elements. The case has a first case end, 
a second case end, and an interior wall extending between the first case 
end and the second case end. The interior wall defines a chamber within 
the case. The coil is disposed in the chamber coextensively adjacent to 
the interior wall. The coil has a first coil end disposed proximate to the 
first case end, a second coil end disposed proximate to the second case 
end, and a midpoint. The core has a first core end, a second core end, and 
an exterior wall extending between the first core end and the second core 
end. The exterior wall of the core has a first region adjacent to the 
first core end and a second region adjacent to the second core end. The 
core is movably received within the chamber with motion of the core 
occurring between the first case end and the second case end such that the 
first region traverses the coil between the first coil end and the 
midpoint, and the second region traverses the coil between the second coil 
end and the midpoint. The coil and the exterior wall are in a facing 
relationship with respect to each other. Each of the magnetic flux 
developing elements has a first pole face of a first magnetic polarity and 
a second pole face of a second, opposite magnetic polarity. The first one 
of the magnetic elements is carried by the first region with its first 
pole face being adjacent to the first region and its second pole face 
being distal from the first region in a spaced apart relationship to the 
coil. A second one of the magnetic elements is carried by the second 
region with its first pole face being distal from the second region in a 
spaced apart relationship to the coil and its second pole face being 
adjacent to the second region. Magnetic flux across the coil between the 
first region and the interior wall of the case is in a first direction, 
and magnetic flux across the coil between the second region and the 
interior wall of the case is in a second direction in opposition to the 
first direction. The coil is arranged so that an electrical current in the 
coil between the first coil end and the midpoint flows in an opposite 
direction with respect to the direction of the current in the coil between 
the second coil end and the midpoint. Therefore, the flux current cross 
product of the flux in the first direction with the coil current and the 
flux current cross product of the flux in the second direction with the 
coil current are additive. 
A feature of the present invention is that the use of the magnetic elements 
on the exterior wall of the core puts a magnetic pole face adjacent to the 
coil such that the flux density from such magnetic pole face is constant 
across the coil. Therefore, the pole face can be made axially larger than 
the pole piece of the prior art so that coil current interacts with larger 
flux to produce greater output force for a given current. An advantage of 
the present invention is that by being able to use lower current for a 
given output force, resistive losses in the coil are minimized. 
Another feature of the present invention is that the coil may be formed 
from a relatively thin, but wide, insulated rectangular stock and edge 
wound to produce a self supporting coil structure. This self supporting 
structure eliminates the need for a coil carrier, as in the prior art, and 
allows the gap between the coil and pole face of the magnetic element to 
be minimized to increase, advantageously, the magnetic field strength 
through the coil. 
Another feature of the present invention is that for any axially directed 
flux return path within the core, such flux path is confined within the 
magnetic flux conductive material, even if the core is hollow. This 
advantageously eliminates flux leakage which is present in the hollow 
center of the rod of the prior art actuator. This advantage arises from 
the feature of the pole faces of the magnetic elements being adjacent to 
the core confine. This feature also eliminates the external flux leakage 
present in the prior art actuator. 
These and other objects, advantages and features of the present invention 
will become readily apparent to those skilled in the art from a study of 
the following description of an exemplary preferred embodiment when read 
in conjunction with the attached drawing and appended claims.

DESCRIPTION OF AN EXEMPLARY PREFERRED EMBODIMENT 
Referring now to FIG. 2, there is shown a linear electromagnetic actuator 
50 constructed according to the principles of the present invention. The 
actuator 50 includes a case 52, an electrical current conductive coil 54, 
a core 56, and a pair of magnetic flux developing elements 58, 60. 
Although each of the above-mentioned elements of the actuator 50 will be 
described hereinbelow as being cylindrical in construction and coaxially 
disposed with respect to each other, it is to be understood that other 
geometries which satisfy the cooperation between the elements are within 
the scope of the present invention. 
The case 52 is an elongated cylinder fabricated from magnetic flux 
conductive material. The case 52 has a first case end 62, a second case 
end 64, and an interior wall 66 extending axially between the first case 
end 62 and the second case end 64. The interior wall 66 defines a chamber 
within the cylindrical case 52. 
The coil 54 is disposed in the chamber coextensively adjacent to the 
interior wall 66. The coil 54 has a first coil end 68 disposed proximate 
the first case end 62 and a second coil end 70 disposed proximate the 
second case end 64. The coil 54 further has a midpoint 72. As will be 
described in greater detail hereinbelow, the first coil end 68, the second 
coil end 70 and the midpoint 72 are provided so that electrical connection 
may be made to the coil 54. 
The core 56 is a cylinder of magnetic flux conductive material. The core 56 
has a first core end 74, a second core end 76, and a cylindrical exterior 
wall 78 extending between the first core end 74 and the second core end 
76. The exterior wall 78 has a first region 80 adjacent the first core end 
74 and a second region 82 adjacent the second core end 76. 
The core 56 is coaxially received in the chamber of the case 52 and mounted 
therein in axially slidable engagement. Accordingly, the cylindrical 
exterior wall 78 of the core 56 is radially spaced from the coil 54. 
Motion of the core 56 occurs between the first case end 62 and the second 
case end 64 such that the first region 80 traverses the coil 54 in the 
axial direction between the first coil end 68 and the midpoint 72, and the 
second region 82 axially traverses the coil 54 between the second coil end 
70 and the midpoint 72. 
The magnetic elements 58, 60 are radially polarized and each have a first 
pole face 58S, 60S of a first magnetic polarity and a second pole face 
58N, 60N of a second magnetic polarity opposite the first polarity. The 
first magnetic element 58 is carried by the first region 80, with its 
first pole face 58S being adjacent the first region 80 and its second pole 
face 58N being radially distal from the first region in a spaced 
relationship to the coil 54. Similarly, the second magnetic element 60 is 
carried by the second region 82. The first pole face 60S of the second 
magnetic element 60 is radially distal from the second region 82 in a 
spaced relationship to the coil 54 and its second pole face 60N is 
adjacent the second region 82. 
Accordingly, the magnetic flux developed by the magnetic flux developing 
elements 58, 60 is radially confined between the first region 80 and the 
axial section of the interior wall 66 facing the first region 80 and 
further confined between the second region 82 and the axial section of the 
interior wall 66 facing the second region 82. Furthermore, since the first 
magnetic element 58 is of reverse polarity to the second magnetic element 
60, the radial flux between the first region 80 and the interior wall 66 
will be in the first direction and the radial flux between the second 
region 82 and the interior wall 66 will be in the second opposite radial 
direction. Since magnetic flux will follow the path of lowest reluctance, 
the axially directed flux will occur in the core 56 between the first 
region 80 and the second region 82 and in the case 52 in an axial portion 
where the core 56 is present. For similar reasons, the flux emanating 
radially from the pole face 58N or the pole face 60S will not tend to 
fringe in an axial direction within the chamber of the case 52. 
The coil is arranged so that an electrical current in the coil between the 
first coil end 68 and the midpoint 72 flows in an opposite direction with 
respect to the direction of the current in the coil between the second 
coil end 70 and the midpoint 72. Accordingly, the flux current cross 
product of the flux in the first radial direction between the pole face 
58N and the current in the coil 54 and the flux current cross product of 
the flux in the second radial direction from pole face 60S and the current 
in the coil 54 are additive. 
As best seen in FIG. 2, the coil current is flown in opposite direction, as 
hereinabove described, by applying the current to the midpoint 72 of a 
coil which is continuously wound along its axial length. The first coil 
end 68 and the second coil end 70 are connected in common to provide a 
current return path to the source of current. 
With reference to FIG. 3 there shows a linear actuator 50', which is an 
alternative embodiment of the linear actuator 50 described with reference 
to FIG. 2. In the linear actuator 50' prime, the coil 54' is wound in a 
first direction between its first coil end 68 and the midpoint 72, and 
counterwound in a second direction between the midpoint 72 and the second 
coil end 70. Such counterwound coil is described in conjunction with FIG. 
6 in the '158 patent. Accordingly, a current applied to the first coil end 
68 with the current return taken from the second coil end 70 will provide 
for a reverse of a direction of current at the midpoint 72 so that the 
flux current cross product in the actuator is additive, as described 
hereinabove. 
Each of the magnetic elements 58, 60 are in the above-described embodiment 
of the present invention radially polarized cylindrical magnets. It is 
also possible to construct such polarized magnetic elements 58, 60 from a 
series of rectangular bar magnets disposed edge to edge about either the 
first region 80 or second region 82 to approximate a radially polarized 
cylindrical magnet. Such construction of bar magnets is described in 
conjunction with FIG. 3 of the '158 patent. 
In either the linear actuator 50 of FIG. 2, or the linear actuator 50' of 
FIG. 3, the construction may further include a first end cap 84 and a 
second end cap 86. Each end cap 84, 86 is fabricated from nonmagnetic 
material. The first end cap 84 attaches to the first end 62 of the case 52 
and the second end cap 86 attaches to the second end 64 of the case 52, as 
best seen in either FIG. 2 or FIG. 3. The first end cap 84 has a coxial 
bore 88. A rod 90, fabricated from nonmagnetic material, is received in 
axial slidable engagement within the bore 88 and attaches to the first end 
74 of the core 56. The rod provides for external connection of an actuated 
device (not shown) to the moving core 56. The bore 88 may include linear 
bearings or bushings 89 as is well known in the art. 
If double ended action of the actuator 50 or actuator 50' is desired, the 
second end cap 86 may include a coaxial bore 92. A second rod 94 is 
received within the bore 92 in axially slidable engagement and attaches to 
the second end 76 of the core 56. Similarly, the bore 92 may also include 
linear bearings or bushings 93. 
The relative movement of the flux developing elements 58, 60 with respect 
to the case 52 will cause eddy currents to be developed on the interior 
wall 66. These eddy currents will produce an electromotive force opposing 
the motion of the core 56 with respect to the case 52. In some 
applications, this natural damping effect is desirable. However, to 
eliminate such eddy currents, a current suppression laminate may be 
disposed in the interior wall 66 and adjacent to coil 54 or the coil 54'. 
The use of such laminates is well known in the art. 
With reference to FIG. 4, there is shown a fragmentary view of the coil 54 
(or 54'). The coil 54 is preferably constructed from wire of rectangular 
cross section and edge wound so that the length L of the wire is radially 
directed and its width W is axially directed. Construction of such edge 
wound coils is fully set forth in the '158 patent. 
With reference to FIG. 5, there is shown a core 56' which is an alternative 
embodiment of the core 56 described in reference to FIG. 2 or FIG. 3. The 
core 56' is of hollow cylindrical construction and fabricated from 
magnetic flux conductive material. The core 56' is mounted on a 
cylindrical rod 94 fabricated from nonmagnetic material. A first end 96 of 
the rod 94 is received within the bore 88 of the first end cap 84 and a 
second end 98 of the rod 94 is received within the bore 92 of the second 
end cap 86. Bearings or bushings 89, 93 may also be printed. 
In core 56', the magnetic elements 58, 60 are mounted to the respective one 
of the first or second regions 80, 82 as described hereinabove. In either 
embodiment of core 56 or core 56', the axial flux will be confined to the 
core material because a pole face of the magnetic elements 58, 60 is 
adjacent to the core material. Any flux emanating from one pole face will 
seek the path of lowest reluctance to the next opposite polarity pole 
face. With either core 56 or core 56' this path is always within magnetic 
flux conductive material with respect to the pole faces adjacent the core 
56 or 56'. For those pole faces distal from the core 56 or 56', the lowest 
reluctance flux path is radial across the gap to the inner wall 66 of the 
case 52. Axial fringing is obviated by the fact that the flux path for 
such axial fringing is longer and, hence, of higher reluctance than the 
smallest radial path from the pole face. 
Therefore, in actuator 50 or actuator 50', with either core 56 and 56', the 
flux path between pole faces of the magnetic elements is either confined 
to magnetic material between opposite polarity pole faces or cross a gap 
with a pole face facing such gap. In either instance, flux leakage is 
obviated for the above-stated reasons. In the prior art device of FIG. 1, 
a gap occurs across magnetic material only without a pole face of a magnet 
adjacent such gap. Therefore, the high reluctance discontinuity of such 
gap in the prior art will cause the flux to fringe about such gap causing 
leakage described in conjunction with the prior art device. 
With reference to FIG. 6, there is shown core 56" as carried, in axial 
slidable engagement, to a second end cap 86'. The second end cap 86 
includes a post 100 extending coaxially toward the core 56". The core 56" 
includes a bore 102 dimensioned to receive the post 100 in slidable 
engagement. Bearings or bushings 104 may also be mounted to the post 100. 
The first rod 90 is received through the bore 88 of the first end cap 84 
as hereinabove described. 
There has been described hereinabove an exemplary preferred embodiment of 
the linear actuator according to the principles of the present invention. 
Those skilled in the art may now make numerous uses of, and departures 
from, the above-described embodiments without departing from the inventive 
concepts disclosed herein. Accordingly, the present invention is to be 
defined solely by the scope of the following claims.