Linear transducer

A linear transducer includes a pair of electrical conductors and first and second transducer members. The electrical conductors are spaced apart along a reference axis. The first and second transducer members are positioned transversely adjacent to each other for relative axial movement. They form a magnetic path of high permeance about each of the electrical conductors. The first transducer member includes a magnetic flux source having a transversely oriented internal magnetic field vector. It extends axially between the electrical conductors to complete two variable magnetic flux circuits. One such flux circuit links each of the conductors. The second transducer member has a magnetic flux dividing separator to variably apportion magnetic flux from the flux source between the resulting magnetic flux circuits as the transducer members move axially relative to each other.

TECHNICAL FIELD 
This invention relates to linear transducers for effecting conversions 
between electrical energy and kinetic energy. 
BACKGROUND OF THE INVENTION 
Linear-motion electro-mechanical transducers, referred to herein as linear 
transducers, are useful in a number of applications, and are particularly 
useful in conjunction with reciprocating motion devices such as Stirling 
cycle machines. 
Optimum efficiency is a primary consideration in the design of a linear 
transducer, with regard to utilization of power, heat, materials, size, 
and weight. The ratio of energy output to transducer weight is one factor 
which is particularly important. Minimizing plunger mass is another 
important consideration, in order to minimize the momentum which must be 
periodically reversed during machine reciprocation. When a transducer is 
to be used with a free-piston Stirling engine, axial symmetry around the 
longitudinal axis of reciprocation may be important to allow the plunger 
to rotate or spin. Optimum utilization of magnetic fields is another 
important design consideration, as is preservation of flux source 
magnetization. 
FIG. 1 is a cross-sectional view of a prior art linear transducer 10 which 
has been found to have an efficient and desirable design. This linear 
transducer is described by Robert W. Redlich in his U.S. Pat. No. 
4,602,174 (FIGS. 4-7 of the Redlich patent). Transducer 10 comprises a 
high-permeability stator, including an inner stator block 12 and an outer 
stator block 13. As described in the referenced patent, blocks 12 and 13 
can be diametrically opposite one another, duplicated in quadrature, or 
continuously revolved in a circular, axially-symmetrical embodiment. In 
each instance, inner stator block 12 has an annular peripheral channel 14 
which contains a toroidal electrical coil or winding 16. 
Channel 14 is surrounded by a pair of poles 19 and 21 which extend radially 
outward from inner stator block 12 on either side of electrical winding 
16. Poles 19 and 21 have generally cylindrical outwardly-facing surfaces 
which form a pair of inner pole faces 18 and 20, respectively. 
Outer stator block 13 spans inner pole faces 18 and 20. It has an inwardly 
facing surface which forms an outer pole face 22 opposite to and facing 
inner pole faces 18 and 20. The inner and outer pole faces have generally 
complementary diameters, with the diameter of inner pole faces 18 and 20 
being smaller than the corresponding diameter of outer pole face 22 so 
that an annular air gap 23 extends axially through the stator. Inner and 
outer stator blocks 12 and 13 form a magnetic path around electrical 
winding 16 as shown by the arrows in FIG. 1. 
An annular magnet 26 is positioned to reciprocate longitudinally within 
annular air gap 23, between the inner and outer pole faces. Magnet 26 has 
a radially-oriented magnetic polar axis. It has an axial length 
approximately equal to the axial length of a single inner pole face, and 
does not axially span the two inner pole faces 18 and 20. As magnet 26 
reciprocates axially from one inner pole face to the other, it alternately 
completes magnetic flux circuits of opposite polarity around coil 16, 
producing an electric current within coil 16. 
While the linear transducer shown in FIG. 1 performs well in many 
applications, it has several disadvantages. One disadvantage is that 
magnet 26 must repeatedly travel through an intermediate position, in the 
"gap" between inner pole faces 18 and 20, in which there is no adjacent 
high permeability path for magnetic flux. Repeated transitions between 
high and low permeability conditions eventually degrade magnet 
performance. Also, the flux source is exposed to opposing fields from 
other flux sources or other parts of the same flux source. 
Another disadvantage of transducer 10 is that any resulting magnetic flux 
circuit around coil 16 always includes a relatively large air gap between 
the outer pole face and one inner pole face. This large gap tends to 
reduce the magnetic flux intensity in the flux circuit, therefore reducing 
efficiency and limiting peak power output. In addition, the induction in 
the stator block of transducer 10 is heteropolar, resulting in relatively 
high specific core losses. This reduces the efficiency of the device. 
The invention described below is a linear transducer which addresses the 
efficiency factors mentioned above and eliminates the noted disadvantages 
of the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This disclosure of the invention is submitted in furtherance of the 
constitutional purposes of the U.S. Patent Laws "to promote the progress 
of science and useful arts." U.S. Constitution, Article 1, Section 8. 
FIGS. 2-4 show, in schematic form, many of the features incorporated in the 
preferred physical embodiments described below. FIG. 2 shows a linear 
transducer 50 which converts electrical energy to bi-directional 
mechanical force or velocity, or visa versa. More specifically, transducer 
50 may be used either as a linear generator or motor. In addition, 
transducer 50 may be used as a displacement or motion sensor in 
applications where it is desired to track the linear position or velocity 
of a moving or reciprocating element. The term "linear" is used in a 
general sense and is intended to encompass the term "curvilinear." 
Similarly, the terms "axis," "axial," and "axially," as used below and in 
the claims, are not intended to foreclose transducers having curvilinear 
or other translational movements. 
Transducer 50 comprises a pair of electrical conductors 51 and 52 separated 
from one another along a reference axis X--X. Each conductor optionally 
comprises a plurality of windings or wraps, depending upon the desired 
output characteristics of the transducer. Conductors 51 and 52 can be 
externally connected if desired. 
Transducer 50 includes first and second magnetically permeable transducer 
members 53 and 55 which are positioned transversely adjacent to each other 
for relative axial movement. Transducer members 53 and 55 form magnetic 
paths of high permeability. Each such magnetic path links a single one of 
electrical conductors 51 and 52. 
More specifically, first transducer member 53 includes an axially-extending 
magnetic flux source 54 positioned generally between the two conductors 51 
and 52. Magnetic flux source 54 is an elongated permanent magnet having 
its north-to-south magnetization vector oriented along its transverse 
width, perpendicular to reference axis X--X. Flux source 54 extends 
partially between the two conductors 51 and 52 along reference axis X--X. 
Flux source 54 could alternatively comprise an electro-magnet configured 
to have a similar magnetic field as the permanent magnet of FIG. 2. 
Second transducer member 55 comprises a yoke positioned around conductors 
51 and 52 and flux source 54. The yoke is formed of a material having high 
magnetic permeability, such as iron or laminated iron. It surrounds or 
links conductors 51 and 52, having first and second U-shaped yoke sections 
58 and 60. Each of yoke sections 58 and 60 has a pair of transversely 
opposed poles or limbs 62 and 64 which extend axially inward along the 
transverse sides of flux source 54. Second transducer member 55 thus forms 
two independent magnetic flux paths of high permeability: one through yoke 
section 58 to link conductor 51, and one through yoke section 60 to link 
conductor 52. Flux source 54, positioned between limbs 62 and 64 of yoke 
sections 58 and 60, completes an independent magnetic flux circuit through 
each of yoke sections 58 and 60 to independently link each of electrical 
conductors 51 and 52. 
A flux dividing means, comprising a low permeability separator or gap 66, 
separates yoke sections 58 and 60 and divides the two magnetic flux paths 
from each other. Flux dividing separator 66 is axially positioned between 
electrical conductors 51 and 52, transversely adjacent flux source 54 and 
between the magnetic flux circuits. It essentially bisects second 
transducer member 55 to form opposed yoke sections 58 and 60. It also 
largely isolates the magnetic paths formed by yoke sections 58 and 60 from 
each other. Separator 66 can be positioned adjacent on one side of flux 
source 54, as shown. Alternatively, a separator could be positioned 
adjacent both sides of flux source 54. 
Flux source 54 and flux dividing separator 66 are axially movable with 
respect to one another. As such movement occurs, flux dividing separator 
66 apportions magnetic flux from flux source 54 between the two magnetic 
flux circuits formed by second transducer member 55. 
For instance, as shown in FIG. 3, moving flux source 54 along the reference 
axis results in proportionally more of the flux produced by flux source 54 
being apportioned to one of yoke sections 58 and 60, increasing the flux 
density linking one conductor while decreasing the flux density linking 
the other conductor. The changing flux density induces an electric field 
in conductors 51 and 52. If the conductors are connected to an external 
circuit, a varying electric current will be induced through the circuit. 
Similarly, providing a current through conductors 51 and 52 causes 
corresponding force or movement between flux source 54 and flux dividing 
separator 66. In most cases, flux dividing separator 66 has a narrow axial 
width so that substantially all of the magnetic flux from the magnetic 
flux source remains within one or the other of the two magnetic circuits. 
FIG. 4 shows the result of moving second transducer member 55 and flux 
dividing separator 66 relative to a stationary flux source 54 and 
stationary conductors 51 and 52. As illustrated, the result is the same as 
shown in FIG. 3. In both cases, separator 66 effectively splits the flux 
lines emanating from flux source 54, directing them through separate flux 
paths. As the axial position of separator 66 varies, the magnetic field 
linking each of the conductors varies as well. 
Flux dividing separator 66 can be formed from any type of material having a 
low magnetic permeability, such as air or a vacuum. In the preferred 
embodiments described below, the flux dividing separator comprises either 
an air gap or an integral section of solid material within an iron flux 
conductor. For example, the flux dividing separator may be formed of a 
plexiglass, Lexan, or phenolic. 
Most embodiments constructed in accordance with the principles of this 
invention will necessarily include at least one low permeability flux 
circuit gap in each of the magnetic circuits. For instance, transducer 50 
has a small transverse circuit gap between flux source 54 and the opposed 
yoke poles 62 and 64. Such a gap may be necessary to allow axial movement 
of flux source 54 relative to flux dividing separator 66. However, a 
significant advantage of the invention is that such gaps can be reduced to 
a minimum, in contrast to prior art devices such as the Redlich transducer 
described above. Specifically, the unique configuration enabled by the 
present invention allows all such circuit gaps to have transverse widths 
significantly less than the transverse width of the flux source. 
The transducer described above provides a number of advantages over prior 
art devices. For instance, the use of a single flux source simplifies both 
design and construction. The fact that each conductor is lumped, rather 
than distributed axially as in some transducer designs, allows the device 
to be modelled by a filament at a single axial coordinate rather than 
requiring a model of an axially-extended surface. 
Furthermore, all parts of the flux source are exposed to a high and 
essentially constant permeance external flux circuit throughout all 
extremes of operation. This is in sharp contrast to many existing 
transducers, which base their operation on a magnet being passed between 
high and low permeance external conditions. Here, each axial segment of 
the flux source is simply passed from one high permeance flux circuit to 
another, rather than being passed in and out of a single high permeance 
flux circuit. A related advantage is found in the fact that no part of the 
flux source is forced to withstand opposing magnetic fields from another 
flux source or from another part of the same flux source (except for 
reaction fields from the two conductors). These factors allow a high 
induction field operating point, resulting in high voltage characteristics 
and high resistance to demagnetization. 
A small region of the magnetic flux source (the portion adjacent the low 
permeability flux dividing separator) can be subjected to external 
conditions of relatively lower permeance. However, the flux from this 
region that links each of the conductors remains constant throughout all 
positions of the flux source. The result is that the transducer has a 
highly linear transfer function. The structure also greatly reduces axial 
end effects, further improving the linear response of the device. 
In addition, a narrow low permeability flux dividing separator will insure 
that all or nearly all flux emanating from the magnetic flux source, even 
that which emanates from adjacent the low permeability region, will enter 
one or the other of the yoke limb faces, thereby reducing losses and 
practically eliminating any need for shielding. These features prevent 
much of the eddy-braking and AC resistance power losses typical of prior 
devices. 
Another advantage of the invention is that induction fields in the magnetic 
circuits are essentially homopolar, varying from zero to a maximum 
magnitude without changing sense or direction. This is in contrast to 
prior art devices in which the induction fields vary about zero. The 
invention can therefore be made to have a very low specific core loss. 
In addition to the advantages described above, a device can be constructed 
in accordance with this invention to have a terminal inductance which is 
independent of relative positions between the operative parts of the 
machine. 
FIG. 5 shows a simple yet practical physical embodiment of a linear 
transducer in accordance with the invention, generally designated by the 
reference numeral 100. Transducer 100 incorporates the general features of 
transducer 50, described above, comprising first and second transducer 
members 120 and 121 positioned transversely adjacent one another for 
relative axial movement. The general structure of the device is dictated 
by the shape of second transducer member 121, formed by a yoke which is 
generally in the shape of an elongated toroid or cylinder. 
Second transducer member 121 is symmetrical about reference axis X--X. 
Although axial symmetry is not essential, it is preferred in many 
applications, particularly where the transducer is to be used with a 
rotating hydrodynamic bearing. Nevertheless, similar results can be 
obtained with differently shaped yoke cross-sections, such as polygonal, 
or with one or more discrete stacks. In addition, transducers can be 
coupled for increased output. 
A pair of annular electrical conductors 112 and 114 are spaced from one 
another along reference axis X--X. The transducer members form two 
independent paths of high magnetic permeance to independently surround or 
link each of electrical conductors 112 and 114. Each magnetic path links a 
single one of the electrical conductors. Conductors 112 and 114 preferably 
comprise coils or windings which are electrically isolated from each 
other, at least with regard to internal connections. If desired, the 
windings or conductors can be externally connected. 
Second transducer member 121 comprises a pair of axially opposed toroidal 
yoke sections 116 and 118 which form a stationary stator in the preferred 
embodiment. Yoke sections 116 and 118 are fabricated from a material of 
high magnetic permeability such as iron. They are sized, shaped, and 
positioned to form two paths of high magnetic permeance: one which links 
each of electrical conductors 112 and 114. To improve performance, yoke 
sections 116 and 118 are divided into radial or transverse laminations. In 
most applications electrical conductors 112 and 114 are fixed within yoke 
sections 116 and 118. However, a configuration in which electrical 
conductors 112 and 114 move relative to yoke sections 116 and 118 is also 
feasible. 
First transducer member 120 forms a plunger which moves or reciprocates 
along reference axis X--X, and which in actual application is usually 
connected to an external mechanical device (not shown). First transducer 
member 120 comprises an annular magnetic flux source 122 which is 
symmetrical and coaxially centered about reference axis X--X. Magnetic 
flux source 122 has a transversely or radially oriented internal magnetic 
field vector. It extends axially between yoke sections 116 and 118 to 
complete two variable magnetic flux circuits: one through each of the 
magnetic paths linking electrical conductors 112 and 114. Magnetic flux 
source 122 is axially movable relative to yoke sections 116 and 118, along 
with first transducer member 120. The relative amount of flux linking each 
of conductors 112 and 114 depends on the axial position of flux source 
122. 
Each toroidal yoke section 116 and 118 has a generally U-shaped 
cross-section comprising transversely-opposed inner and outer yoke poles 
or limbs 124 and 126 which extend axially on opposite transverse sides of 
electrical conductors 112 and 114 and flux source 122. Limbs 124 and 126 
of each yoke section 116 and 118 form transversely-opposed limb faces 128 
and 130 which face each other. Inner limbs 124 extend toward each other, 
leaving a gap therebetween which forms a low permeability flux dividing 
separator 132. Outer limbs 126 are axially joined or adjacent to one 
another to form a solid outer cylindrical wall around the transducer. 
Preferably, no air gap is left between outer limbs 126. 
The limb faces from each yoke section 116 and 118 align to form a 
continuous and elongated axially-extending air gap between electrical 
conductor 112 and electrical conductor 114. The U-shaped cross-section of 
each yoke section forms an independent magnetic path of high permeance 
between its corresponding limb faces to link a single one of electrical 
conductors 112 and 114. 
Limb faces 128 and 130 are spaced from each other by a transverse or radial 
width which is approximately complementary to the transverse or radial 
width of magnetic flux source 122. Magnetic flux source 122 is positioned 
relative to yoke sections 116 and 118 for axial movement within the 
axially extending air gaps of yoke sections 116 and 118 to complete the 
two variable magnetic flux circuits which link electrical conductors 112 
and 114. 
As noted, second transducer member 121 has a low permeability flux dividing 
separator 132 between its inner limbs 124 to axially separate yoke 
sections 116 and 118. Flux dividing separator 132 is formed by an air gap 
which is positioned axially between electrical conductors 112 and 114 to 
variably apportion the magnetic flux from the magnetic flux source between 
the two magnetic circuits depending on the relative axial position of 
magnetic flux source 122. Flux dividing separator 132 has a narrow axial 
width so that substantially all of the magnetic flux from magnetic flux 
source 122 remains in one or the other of the magnetic circuits, thereby 
preventing flux leakage. Magnetic flux source 122 has an axial length 
sufficient to span low permeability flux dividing separator 132. 
Operation of transducer 100 is best observed with reference to FIGS. 6 and 
7, in which simplified cross-sectional views are shown at two different 
transducer member positions. Transducer 100 usually operates in 
conjunction with relative oscillatory motion of first transducer member 
120 and magnetic flux source 122. In FIG. 6, magnetic flux source 122 is 
shown in an intermediate position, midway between electrical conductor 112 
and electrical conductor 114. Flux from flux source 122 is evenly divided 
between the circuits which link the two conductors. 
In FIG. 7, flux source 122 is shown in an extreme axial position, toward 
yoke section 116 and electrical conductor 112. As a result of movement 
toward yoke section 116, most of the flux lines emanating from flux source 
122 are directed through the flux circuit linking electrical conductor 
112. The flux which links electrical conductor 114 is reduced in 
comparison to the intermediate position. Flux source 122 can be moved 
toward another extreme position, toward yoke section 118 and electrical 
conductor 114, causing the major portion of flux to link electrical 
conductor 114 through yoke section 118. By oscillating flux source 122 
between its two extreme positions, the flux linking each of electrical 
conductors 112 and 114 varies from a minimum to a maximum, causing induced 
electro-motive forces in conductors 112 and 114. 
Flux dividing separator 132 between the two magnetic paths insures that the 
flux entering a particular limb face will affect only the associated yoke 
section and conductor, and will not leak to the other yoke section. Such 
leakage would decrease flux changes from relative motion between the flux 
dividing separator and the flux source. 
Limb faces 128 and 130 preferably have uniform widths and shapes along 
their axial length, forming an air gap of uniform transverse or radial 
width. This provides for linear transducer response, which is desired in 
most instances. However, in some cases it is desirable to provide limb 
faces and air gaps which vary along their axial length, thus creating 
non-linear (or corrected linear) transducer responses. The limb faces 
could also be varied to cause a magnetic spring force between components. 
The embodiment described above has members 120 and 121 which can rotate 
relative to each other during operation. This feature is useful in certain 
applications, particularly where the invention must be connected to a 
Stirling cycle machine having hydrodynamic or "spin" bearings. 
Further advantages result from extending the outer poles to meet each 
other. For instance, the solid cylindrical outer wall reduces leakage by 
presenting a constant, high permeability path at all points along the 
outer surface of the magnetic flux source. The solid outer wall also 
reduces the effect of external magnetic fields on the transducer. In 
addition, the embodiment of FIG. 5 is more resistant to demagnetization 
from temperature and current effects than previous devices. 
FIGS. 8 and 9 show a second preferred embodiment of a linear transducer in 
accordance with the invention, generally designated by the reference 
numeral 150. Transducer 150 is similar to transducer 100, shown in FIG. 5. 
Accordingly, similar components have been referenced by identical 
reference numerals, with the addition of the suffix "a" to the common 
reference numerals in FIGS. 8 and 9. 
Transducer 150 comprises first and second transducer members 120a and 121a, 
both of which are preferably symmetrical about reference axis X--X, and 
which are configured for relative axial movement or reciprocation. A pair 
of annular electrical conductors 112a and 114a are spaced from one another 
along reference axis X--X. 
Second transducer member 121a comprises a pair of axially opposed toroidal 
stator yoke sections 116a and 118a. Each of yoke sections 116a and 118a 
forms a path of high magnetic permeability which links a single one of 
conductors 112a and 114a. Conductors 112a and 114a are preferably fixed 
within yoke sections 116a and 118a. 
First transducer member 120a moves or reciprocates along reference axis 
X--X. It comprises an annular magnetic flux source 122a coaxially centered 
about reference axis X--X. Magnetic flux source 122a has a transversely or 
radially oriented internal magnetic Yield vector. It extends axially 
between yoke sections 116a and 118a to complete two variable magnetic flux 
circuits: one through each of the magnetic paths linking electrical 
conductors 112a and 114a. 
Each toroidal yoke section 116a and 118a has a generally U-shaped 
cross-section comprising transversely-opposed inner and outer yoke limbs 
124a and 126a. Electrical conductors 112a and 114a are positioned between 
the yoke limbs. Limbs 124a and 126a of each yoke 116a and 118a extend 
axially inward, forming transversely-opposed pole faces 128a and 130a 
which face each other. Outer limbs 126a extend toward each other, leaving 
a low permeability flux dividing separator 132a therebetween. Inner limbs 
124a, however, are axially joined to one another to form an inner 
cylindrical wall 144 within the transducer. No axial air gap need be left 
between inner poles 124a. 
Inner cylindrical wall 144, formed by inner limbs 124a, has a plurality of 
axially extending slots 154. A plurality of spokes 156 extend generally 
radially inward from magnetic flux source 122a to a central plunger or 
member 158 along reference axis X--X. 
Limb faces 128a and 130a define axially extending air gaps therebetween. 
The axially-extending air gaps from each yoke section 116a and 118a align 
to form a continuous and elongated annular gap extending axially between 
the electrical conductors and between the yoke sections. The U-shaped 
cross-section of each yoke section 116a and 118a forms a path of high 
magnetic permeance between its corresponding pole faces to link one of 
electrical conductors 112a and 114a. 
The axially extending air gaps have transverse widths which are 
approximately complementary to the transverse width of magnetic flux 
source 122a. Magnetic flux source 122a is positioned relative to yoke 
sections 116a and 118a for axial movement within the axially extending air 
gaps of yoke sections 116a and 118a to complete the magnetic flux circuits 
linking electrical conductors 112a and 114a. 
Transducer 150 has the advantage that its low permeability flux dividing 
separator 132a can be made much narrower than is the case with the 
previously described embodiment 100. In the previous embodiment, the low 
permeability flux dividing air gap was required to accommodate axial 
movement of the members, and was thereby required to be at least as long 
as the overall range of relative member movement. In transducer 150, 
however, axial plunger movement is accommodated by slots 154. Low 
permeability flux dividing separator 132a, being in the outer wall of the 
transducer, can be made as small as desired, as long as it is large enough 
to sufficiently isolate the two magnetic circuits from each other. 
Because of the small flux dividing separator in transducer 150, essentially 
all of the magnetic flux source is subjected to high permeance external 
conditions. Therefore demagnetization tendencies are greatly reduced. 
Considerations such as this are critical in applications requiring a high 
degree of reliability, such as in systems which are to be used in outer 
space. The characteristics describe, immediately above also enable 
transducer 150 to have less length and total mass than the previously 
described embodiments, factors which are also important in many 
applications. 
FIG. 10 shows a cross-section of a third preferred embodiment of a linear 
transducer in accordance with the invention, generally designated by the 
reference numeral 200. Transducer 200 incorporates the unique features of 
transducer 50 (FIG. 2), comprising first and second transducer members 202 
and 204, positioned transversely adjacent one another for relative axial 
motion. The general structure of the device is dictated by the shape of 
second transducer member 204, which is preferably a cylindrical block of 
iron alloy or iron alloy laminate. 
First and second transducer members 202 and 204 are preferably symmetrical 
about a reference axis X--X, to allow free rotation of first transducer 
member 202 within second transducer member 204. Second transducer member 
204 has an inner surface 210 coaxially entered about reference axis X--X. 
Inner surface 210 is preferably cylindrical. First transducer member 202 
has an outer surface 212 which is sized and shaped to fit within and 
transversely adjacent to inner surface 210 of second transducer member 204 
for axial movement or reciprocation relative thereto. Outer surface 212 is 
therefore also preferably cylindrical, having a diameter only slightly 
less than that of inner surface 210. In some cases it may be possible to 
provide a sliding or contacting fit between first and second transducer 
members 202 and 204. 
Second transducer member 204 has an axially spaced pair of annular 
peripheral channels 216 which extend into second transducer member 204 
from inner surface 210. A pair of electrical conductors 220 and 222 are 
fixedly received within annular channels 216. Electrical conductors 220 
and 222 are thus spaced from one another along reference axis X--X. Each 
electrical conductor 220 and 222 preferably comprises an annular coil or 
winding. The number of wraps or turns in each winding is determined by the 
desired response characteristics of the transducer. 
First and second transducer members 202 and 204 are made of a high 
permeability material such as iron, and are preferably laminated in the 
transverse or radial direction to improve performance. They are positioned 
and configured to form separate magnetic paths of high permeance linking 
each of electrical conductors 220 and 222. 
A magnetic flux source 224 is positioned between second transducer member 
204 and first transducer member 202. Magnetic flux source 224 is 
preferably annular in shape to correspond to the cylindrical shapes of the 
transducer members. Magnetic flux source 224 is preferably a permanent 
magnet having a transversely or radially oriented internal magnetic field 
vector. An electro-magnet can also be used, provided it is designed to 
produce a similar magnetic field. Magnetic flux source 224 is preferably 
fixed inside of second transducer member 204 between electrical conductors 
220 and 222 to complete two variable magnetic flux circuits, one through 
each of the magnetic paths linking electrical conductors 220 and 222, 
respectively. Magnetic flux source 224 therefore forms an integral part of 
second transducer member 204, forming a portion of inner surface between 
210 between conductors 220 and 222. Inner and outer surfaces 210 and 212 
are preferably spaced from one another by a width less than the transverse 
width of magnetic flux source 224. 
First transducer member 202 preferably comprises an elongated bar which is 
bisected by a low permeability flux dividing separator 230. Flux dividing 
separator 230 comprises a region of low permeability material. For 
example, flux dividing separator 230 may be formed of a plastic such as 
plexiglass, Lexan, or a phenolic. Low permeability flux dividing separator 
230 extends within and throughout a transverse cross-section of first 
transducer member 202 to magnetically isolate the ends of first transducer 
member 202 from each other. However, the axial width of low permeability 
flux dividing separator 230 is as small as possible to provide minimal 
interference to external systems from flux emanating from magnetic flux 
source 224, so that substantially all of the magnetic flux from magnetic 
flux source 224 forms part of one or the other of the two magnetic 
circuits. 
Magnetic flux source 224 and low permeability flux dividing separator 230 
are positioned for relative axial movement. More specifically, first 
transducer member 202, including low permeability flux dividing separator 
230, can move or reciprocate axially within second transducer member 204. 
Low permeability flux dividing separator 230 is axially positioned between 
electrical conductors 220 and 222 to variably apportion magnetic flux from 
magnetic flux source 224 between the two magnetic circuits depending on 
the relative axial position of the transducer elements. Low permeability 
flux dividing separator 230 therefore separates the magnetic flux circuits 
from each other and varies the flux density within each of the magnetic 
flux circuits. 
FIGS. 11 and 12 illustrate operation of transducer 200. In FIG. 11, first 
transducer member 202 is shown in an intermediate position, with low 
permeability flux dividing separator 230 positioned midway between 
electrical conductors 220 and 222. Flux from flux source 224 is evenly 
divided between the magnetic circuit linking electrical conductor 220 and 
the magnetic circuit linking conductor 222. In FIG. 12, however, first 
transducer member 202 is shown in an extreme axial position, with low 
permeability flux dividing separator 230 positioned toward electrical 
conductor 220. In this position, very little flux flows in the circuit 
linking electrical conductor 220, while most of the flux from flux source 
224 is directed by low permeability flux dividing separator 230 through 
the magnetic circuit linking electrical conductor 222. In the other 
extreme axial position (not shown), with low permeability flux dividing 
separator 230 positioned toward electrical conductor 222, most flux is 
directed through the magnetic circuit linking electrical conductor 220. 
Moving or oscillating first transducer member 202 within second transducer 
member 204 thereby varies the flux density in each of the two flux 
circuits, producing an electro-motive force and a corresponding current in 
each of electrical conductors 220 and 222. 
Transducer 200 is particularly well suited to high temperature 
environments. This embodiment fixes the flux source and the conductors on 
the same body, allowing cooling of both components by cooling only second 
transducer member 204. Accordingly, transducer 200 can operate in a very 
hot environment while the magnet is protected from thermal demagnetization 
through cooling. This embodiment also permits a stationary magnet to be 
used in a mechanically robust structure. 
FIG. 13 shows a cross-section of a fourth preferred embodiment of a linear 
transducer in accordance with the invention, generally designated by the 
reference numeral 300. Transducer 300 is similar in concept and operation 
to transducer 200 of FIG. 10, with the relative radial positions of the 
two transducer members essentially being reversed. Transducer 300 
comprises first and second transducer members 302 and 304, positioned 
transversely adjacent one another for relative axial movement. 
First and second transducer members 302 and 304 are preferably symmetrical 
about a reference axis X--X, to allow free rotation of first transducer 
member 302 within second transducer member 304. Second transducer member 
304 is preferably a cylindrical sleeve or elongated cylinder having a 
cylindrical inner surface 310 coaxially centered about reference axis 
X--X. First transducer member 302 preferably comprises an elongated bar or 
hollow tube having an outer surface 312 which is sized and shaped to fit 
within and transversely adjacent to inner surface 310 of second transducer 
member 304 for axial movement or reciprocation relative thereto. Outer 
surface 312 is therefore also preferably cylindrical, having a diameter 
only slightly less than that of inner surface 310. In some cases it may be 
possible to provide a sliding or contacting fit between first and second 
transducer members 302 and 304. 
First transducer member 302 has an axially spaced pair of annular 
peripheral channels 316 which extend into first transducer member 302 from 
outer surface 312. A pair of electrical conductors 320 and 322 are fixedly 
received within annular channels 316. Electrical conductors 320 and 322 
are thus spaced from one another along reference axis X--X. Each 
electrical conductor 320 and 322 preferably comprises an annular coil or 
winding. The number of wraps or turns in each winding is determined by the 
desired response characteristics of the transducer. First and second 
transducer members 302 and 304 form two independent paths of high magnetic 
permeance, each linking a single one of electrical conductors 320 and 322. 
First and second transducer members 302 and 304 are made of a high 
permeability material such as iron, and are preferably laminated in the 
transverse or radial direction to improve performance. 
A magnetic flux source 324 is positioned between second transducer member 
304 and first transducer member 302. Magnetic flux source 324 is 
preferably annular in shape to correspond to the cylindrical shapes the 
transducer members. Magnetic flux source 324 is illustrated as an 
electro-magnet having externally-powered conductors 325. Conductors 325 
must be sized and placed to create a transversely or radially oriented 
internal magnetic field vector. A permanent magnet could also be used as 
illustrated in the previous embodiments. Magnetic flux source 324 is fixed 
inside transducer member 302 between electrical conductors 320 and 322 to 
complete two variable magnetic flux circuits, one through each of the 
magnetic paths linking electrical conductors 320 and 322. Magnetic flux 
source 324 therefore forms an integral part of first transducer member 
302, and also forms a portion of outer surface 312. Inner and outer 
surfaces 310 and 312 are preferably spaced from one another by a width 
less than the transverse width of magnetic flux source 324. 
Second transducer member 304 is bisected by a low permeability flux 
dividing separator 330, comprising a region of low permeability material. 
For example, flux dividing separator 230 may be formed of material such as 
plexiglass, Lexan, or a phenolic. Low permeability flux dividing separator 
330 extends completely around second transducer member 304 to 
substantially magnetically isolate the ends of second transducer member 
304 from each other. However, the axial width of low permeability flux 
dividing separator 330 is as small as possible to provide minimal 
interference to external systems from flux emanating from magnetic flux 
source 324, so that substantially all of the magnetic flux from magnetic 
flux source 324 remains in one or the other of the magnetic circuits. 
Magnetic flux source 324 and low permeability flux dividing separator 330 
are positioned for relative axial movement. Low permeability flux dividing 
separator 330 is axially positioned between electrical conductors 320 and 
322 to variably apportion magnetic flux from magnetic flux source 324 
between the two magnetic circuits depending on the relative axial 
positions of the transducer elements. Low permeability flux dividing 
separator 330 therefore separates the magnetic flux circuits from each 
other and varies the flux density within each of the magnetic flux 
circuits. 
Using an electro-magnet as the magnetic flux source allows creation of a 
variable magnetic transfer function which can be controlled over time or 
made to depend on the axial position of the transducer elements. This can 
be accomplished by controlling the electro-magnet currents as a function 
of a control variable such as relative axial member position, or by the 
selection of fixed electromagnet currents and locations. When used as a 
linear alternator, this feature can be used to control power and voltage 
output while maintaining constant stroke amplitude, to compensate for 
changing load conditions. Use of an electro-magnet in the place of a 
permanent magnet is not limited to transducer 300. Electro-magnets could 
be advantageously used in each of the embodiments described herein. 
Transducer 300 has a very high operating temperature capability, in part 
due to the absence of permanent magnets. Permanent magnets have operating 
temperature limits due to Curie point or other permanent magnet 
demagnetization effects. Transducer 300 can be operated up to the 
temperature limits of the conductor and yoke materials. This feature is a 
tremendous advantage in high temperature power conversion systems such as 
space power Stirling systems. 
In compliance with the statute, the invention has been described in 
language more or less specific as to methodical features. It is to be 
understood, however, that the invention is not limited to the specific 
features described, since the means herein disclosed comprise preferred 
forms of putting the invention into effect. The invention is, therefore, 
claimed in any of its forms or modifications within the proper scope of 
the appended claims appropriately interpreted in accordance with the 
doctrine of equivalents.