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Claims  |
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What is claimed:
1. A linear motor comprising:
a displacer arranged for movement parallel to an axis, and carrying a
plurality of axially spaced magnetizable elements having surfaces parallel
to said axis, and
a stator extending parallel to said axis, comprising a plurality of
magnetic-pole-defining members having pole-defining surfaces parallel to
said displacer surfaces, successive magnetic-pole-defining surfaces being
axially and angularly spaced with respect to each other; and means for
selectively increasing magnetic flux through angularly selected surfaces
of said stator to attract said displacer to selected axial positions with
respect to said stator.
2. A motor as claimed in claim 1, characterized in that said displacer
magnetizable element surfaces are circumferential surfaces coaxial with
said axis, and
said displacer comprises an axially extending body formed of a magnetizable
material interconnecting said magnetizable elements.
3. A motor as claimed in claim 2, characterized in that said displacer
magnetizable surfaces are formed on said body.
4. A motor as claimed in claim 1, characterized in that at least a first
and a second of said angularly spaced stator surfaces are axially aligned
and are formed on respective first and second stator
magnetic-pole-defining members, said first and second members being
separated by at least a third stator member having third pole-defining
surfaces angularly indexed to an angular position spaced from that of said
first and second surfaces, said third member being free from pole-defining
surfaces in axial alignment with the pole-defining surfaces on said first
and second members, and
said stator comprises coil means for simultaneously increasing magnetic
flux through said first and second surfaces, and alternatively increasing
magnetic flux through said third surfaces.
5. A motor as claimed in claim 4, characterized in that each stator member
has a same given length, and has n pole-defining surfaces arranged
angularly about said axis as n/2 circumferentially aligned pairs of
angularly adjoining surfaces, said angularly adjoining surfaces being
spaced 360.degree./n relative to each other on centers,
successive stator members have pole-defining surfaces indexed 360.degree./n
angularly in a same given direction, said third stator member having
pole-defining surfaces axially aligned with spaces between pole-defining
surfaces of said first member; and said first and third members being
separated axially by a fourth stator member,
said coil means simultaneously increases magnetic flux through pairs of
axially aligned magnetic-pole-defining surfaces on axially adjoining
members, and
said displacer magnetizable surfaces have an axial length approximately
equal to the axial length of two stator members,
whereby one condition of excitation of said coil means increases magnetic
flux through a first axially aligned pair of stator surfaces on said first
and fourth stator members, and attracts one displacer magnetizable surface
into axial alignment with said first and fourth stator members; and
another condition of excitation increases flux through a second axially
aligned pair of stator surfaces on said fourth and third members, indexed
360.degree./n from said first pair of axially aligned surfaces, and
attracts said one displacer surface into alignment with said fourth and
third stator members.
6. A motor as claimed in claim 5, characterized in that said displacer
magnetizable surfaces are formed on said body.
7. A motor as claimed in claim 2, characterized in that said displacer
magnetizable surfaces are formed on said body.
8. A motor as claimed in claim 1, characterized in that said displacer
magnetizable surfaces are formed on said body.
9. A linear motor comprising:
an elongated stator having a first gap-defining surface extending in a
relative movement direction parallel to an axis, said stator comprising a
plurality of axially spaced magnetic-pole-defining members having pole
faces lying at least substantially in said surface, and coil means for
selectively magnetizing said members,
a displacer having a second gap-defining surface facing said first surface,
parallel to and coaxial with said axis, and
means for mounting said displacer with respect to said stator to permit
displacer movement in said direction,
characterized in that each magnetic-pole-defining member of said stator is
an angular-pole-shaping member having a plurality of teeth at
angularly-spaced locations around the periphery of said first gap-defining
surface, said teeth being separated by spaces and constituting poles
having pole faces providing a low reluctance magnetic path to said first
surface,
said angular-pole-shaping members are arranged as a coaxial stator stack,
said teeth being arranged in a repeating sequence of relative angular
positions of said teeth, and said stack having a plurality of aligned
winding slots formed by said spaces and defined by said teeth,
said coil means includes at least first and second coils, each coil having
a portion disposed in a respective slot,
said displacer comprises a displacer stack having a plurality of
substantially identical magnetic members aligned perpendicularly to said
direction, said displacer magnetic members having surfaces lying
substantially along said second gap-defining surface, a space between said
first and second gap defining surfaces being a first air gap, and
said members of one of said stacks are spaced at a given axial pitch and
the members of the other of said stacks are spaced axially at a pitch at
least twice said given pitch; said stator and displacer are so arranged
that, responsive to a given excitation of one of said coils only, said
displacer is attracted to a first of a repeating series of stable
positions with respect to said stator, a first of said displacer magnetic
members being aligned with teeth of one of said magnetic-pole-forming
members at a first angular location; and responsive to a second excitation
of another of said coils only, said displacer is attracted to a second
position, adjoining said first position, at which second position one of
said displacer members is attracted to teeth at an angular location spaced
from said first angular location.
10. A motor as claimed in claim 9, characterized in that said stator
angular-pole-shaping members are identical members, arranged with each
successive member rotated a given angular distance with respect to the
preceding member, said distance being an integral multiple of the angular
spacing between said slots.
11. A motor as claimed in claim 10, characterized in that said motor is a
hybrid motor, comprising:
first and second stator sections, axially spaced, and
field magnet means for providing a field flux which crosses said air gap
from said first stator section to said displacer in a given flux
direction, and crosses said air gap from said second stator section to
said displacer in a direction opposite said given flux direction.
12. A motor as claimed in claim 11, characterized in that each stator
angular-pole-shaping member has four said teeth having pole faces, two
said slots being formed between adjoining said teeth having pole faces
along said gap-defining surface, said given angular distance being equal
to said angular spacing between slots.
13. A linear motor comprising:
an elongated stator having a first gap-defining surface extending in a
relative movement direction parallel to an axis, said stator comprising a
plurality of axially spaced magnetic-pole-defining members having pole
faces lying at least substantially in said surface, and coil means for
selectively magnetizing said members,
a displacer having a second gap-defining surface facing said first surface,
parallel to and coaxial with said axis, and
means for mounting said displacer with respect to said stator to permit
displacer movement in said direction,
characterized in that said stator comprises an inner stator portion and an
outer stator portion, and means for mounting said stator portions fixedly
with respect to each other,
one of said stator portions comprises said magnetic-pole-defining stator
members, each said magnetic-pole-defining member being an
angular-pole-shaping member having a given axial length generally equal to
the axial length of the displacer magnetic members; and around the
periphery of said first gap-defining surface, said angular-pole-shaping
member has a plurality of equiangularly spaced teeth, said teeth being
separated by spaces and constituting poles having pole faces having a low
reluctance magnetic path to said first surface,
said angular-pole-shaping members are arranged as a coaxial stack having a
plurality of generally axially extending winding slots, the successive
pole-shaping members of the stack being arranged in a repeating angular
sequence of relative angular positions of the low reluctance path teeth,
said coil means includes at least first and second coils, each coil having
a portion disposed in a respective slot,
said other stator portion comprises a corresponding plurality of
angular-pole-shaping members having a corresponding plurality of teeth at
angularly-spaced locations around the periphery of a third gap-defining
surface, said teeth of the members of said other portion being separated
by spaces and constituting poles having pole faces providing a low
reluctance magnetic path to said third surface,
said members of said second stator portion being arranged as a coaxial
stator stack with the teeth thereof constituting poles having pole faces
being arranged in axial and angular alignment with, and facing,
corresponding teeth of the members of said first portion,
said displacer comprises a displacer stack having a plurality of
substantially identical rotation-symmetrical magnetic members alternating
with substantially identical nonmagnetic members, coaxially with said
axis, at least a plurality of said magnetic members being disposed in
axial positions between said inner and outer stator portions, and
said angular-pole-shaping members are so arranged that, responsive to a
given excitation of one of said coils, said displacer is attracted to a
first of a repeating series of stable positions with respect to said
stator; and responsive to a second excitation of another of said coils,
said displacer is attracted to a second position, adjoining said first
position and spaced therefrom a distance equal to said axial length.
14. A motor as claimed in claim 13, characterized in that said stator
angular-pole-shaping members are identical members, arranged with
successive members rotated a given angular distance with respect to the
preceding member, said distance being an integral multiple of the angular
spacing between said slots.
15. A motor as claimed in claim 14, characterized in that said motor is a
hybrid motor, comprising;
first and second stator sections, axially spaced, and
field magnet means for providing a field flux which crosses said air gap
from said first stator section to said displacer in a given flux
direction, and crosses said air gap from said second stator section to
said displacer in a direction opposite said given flux direction.
16. A motor as claimed in claim 15, characterized in that each stator
angular-pole-shaping member has four said teeth having pole faces, two
said slots being formed between adjoining said teeth having pole faces
along said gap-defining surface, said given angular distance being equal
to said angular spacing between slots. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
Engines or machines which produce mechanical output power, involving
relative movement of two elements against a load drag or force, fall
generally into two classes: translational, or linear, motors, and rotation
motors. Where precise control or extremely long life are desired, electric
motors are frequently preferred as the prime power source. However,
because of many factors relating to the need for magnetic return paths and
the cost of electromagnetic structures per unit volume, wherever a
relatively long linear motion or stroke is desired starting with an
electrical power source, the most common solution is to provide a rotary
electric motor connected to a gear train driving a rack and pinion
combination, or a rotating nut and threaded shaft combination. While
multi-pole linear induction motors have been proposed as traction motors
for electric railroads, these have not yet proved commercially
practicable.
Many linear power sources are required to operate only over a finite
distance, such as one-third to three times the principal dimension of a
conventional electric motor producing the necessary amount of power. This
is too long a stroke for efficient coupling of a single pole device such
as a solenoid, so that in practice these applications frequently use a
rotating motor which rotates the nut on a threaded shaft. However, where
long life or sinusoidal linear movement of the output shaft are desired,
this structure is not a very satisfactory solution.
2. Description of the Prior Art
It was recognized long ago that the basic design of a polyphase synchronous
motor could be followed, to provide a linear motor having a linearly
extended series of transverse pole structures. A 3-phase structure of this
type is described in the paper "Commutation and Control of Step Motors" by
Ish-Shalom and Manzer, Proceedings of the 14th Annual Symposium,
Incremental Motion Control Systems and Devices, published by the
Incremental Motion Control Systems Society in June, 1985. While published
after the conception of the invention embodied in FIGS. 1-3 herein, this
paper provides a useful background description of the electronic drive,
and the principles underlying such a linear motor.
To achieve the high-power density associated with the use of a permanent
magnet as a field magnetism source, the so-called "hybrid stepping motor"
was developed and is described, for example, in a published thesis,
"Static Performance of a Hybrid Stepping Motor with Ring Coils", by Ben.
H. A. Goddijn, Sept. 9, 1980, published in Waalre, The Netherlands. In
such a hybrid motor a field flux passes through a total motor magnetic
circuit, arranged with teeth extending transversely to the direction of
linear motor, the teeth on the stator having the same pitch as those on
the rotor, but with certain teeth offset by 90.degree. of one full cycle
of electrical excitation in what is, effectively, a two phase electric
motor. Each of the two driving coils affects half of the teeth on the
armature, adding to the flux in one group of teeth to raise the flux level
nearly to saturation, and bucking the field flux in the other teeth down
to nearly zero. Relative movement occurs so that a tooth on the field
structure is brought into alignment with the tooth which has been driven
nearly to saturation.
Such a motor, as a linear structure, is clearly the linear equivalent of
the rotary synchronous or stepping motor disclosed in U.S. Pat. No.
4,206,374. While it is possible to design such a structure for linear
operation, the mass of the displacer or linearly moving element will be
relatively high, and the manufacturing cost will also be quite high
because of the large numbers of transversely extending teeth which are
required. If the displacer is made shorter than the stator, to reduce the
moving mass, and the coils are placed on the stator for the same reason,
then a great number of coils are required to produce the necessary
alternation of polarities along the length of the stator. This makes such
a motor extremely expensive, and also requires that the poles be
relatively far apart in the longitudinal direction. If such a motor is
stepped by applying full current alternatively to one coil or the other,
relative coarse stepping motion is obtained.
SUMMARY OF THE INVENTION
An object of the invention is to provide a multiple pole linear motor
having all coils on the stator, with a very simple coil arrangement.
Another object of the invention is to provide a linear motor which, without
resort to incremental stepping by partial excitation of each winding,
provides a large number of very fine steps.
Yet another object of the invention is to simplify manufacturing, by
assembling the soft magnetic portions of the motor from laminations which
can be stacked on a mandrel, with coils being inserted in longitudinally
extending slots.
A further object of the invention is to minimize the moving mass of a
linear motor by making the displacer in the shape of a thin-walled tube.
A still further object of the invention is to provide a hybrid linear motor
whose displacer is made of alternating members in a pattern which is
continuous to each side of the field magnet. Then the stroke can be made
so long that certain displacer members interact with stator members to one
side of the magnet at one end of the stroke, and with stator members to
the other side of the magnet at the other end of the stroke.
In a motor according to the invention, a displacer is arranged for linear
movement parallel to an axis, and carries a plurality of axially spaced
magnetizable elements having surfaces parallel to the axis, the
magnetizable element surfaces being separated by spaces having an equal
axial length. The stator extends parallel to the same axis, and is formed
by a plurality of magnetic-pole-defining members having pole-defining
surfaces parallel to the surfaces of the displacer magnetizable elements.
Magnetic-pole-defining surfaces on successive stator members are axially
and angularly spaced with respect to each other. An electromagnetic means
selectively increases magnetic flux through the pole-defining surfaces of
the stator at selected angular positions to attract the displacer to axial
positions in which the magnetizable elements are aligned with
pole-defining surfaces having increased flux.
In a preferred embodiment, at least a first and a second of the stator
members are axially aligned, and separated by at least a third stator
member, the first and second members having stator pole-defining surfaces
which are angularly spaced, and the third member has pole-defining
surfaces angularly indexed to a position spaced from that of the first and
second surfaces, the third member being free from any pole-defining
surfaces aligned with those of the first and second members. A coil means
in the stator simultaneously increases magnetic flux through first and
second pole-defining surfaces of the first and second members, or
alternatively increases the magnetic flux through pole-defining surfaces
on the third stator member.
Still more preferably, each stator member has a same given length, and has
n pole-defining surfaces arranged angularly about the axis as n/2
circumferential aligned pairs of adjoining surfaces, these adjoining
surfaces being spaced 360.degree./n relative to each other on centers.
Successive stator members in a section have their pole-defining surfaces
indexed 360.degree./n angularly in a same direction. The first and third
stator members, described above, are axially separated by a fourth stator
member, such that one pole defining surface of a circumferential pair on
the fourth stator member is axially aligned with one pole-defining surface
on the adjoining first member; while the other pole-defining surface of
that pair on the fourth member is aligned axially with a pole-defining
surface on the third member to the other side of the fourth member. The
stator coils are arranged so that they will simultaneously increase the
magnetic flux through two axially adjoining magnetic-pole-defining
surfaces having the same angular alignment; and the displacer magnetizable
members have surfaces having an axial length approximately equal to the
length of two successive stator members, so that for a given excitation of
one coil the displacer magnetizable surface will align opposite the first
and fourth stator members. Excitation of a selected different coil
increases the flux through the pole-defining surfaces indexed
360.degree./n in one direction, the surfaces being on the fourth and third
members so that the displacer is attracted to step an axial distance equal
to a stator member length.
Still more preferably, in this embodiment the motor is formed as a hybrid
motor having two stator sections separated by an axially poled permanent
magnet. The displacer is formed as a thin hollow tube having equal length
rings made of magnetizable and nonmagnetizable material alternating along
the length of the displacer. An inner stator, coaxial with the outer
stator has inner stator members having magnetic-pole-defining surfaces
axially and angularly aligned opposite those of the outer stator. The
inner stator is formed as two sections connected by a magnetic return
path; or alternatively interconnected by a permanent magnet poled in the
opposite axial direction, so as to increase the field flux passing
radially inward from the outer to inner stators of one section, and
radially outward from the inner to outer stators of the other section.
In a different embodiment according to the invention, the displacer is
readily manufactured from an axially extending cylindrical body formed of
a magnetizable material, the displacer magnetizable elements being annular
rings about that body. In a further preferred embodiment, the displacer
elements and magnetizable surfaces are formed on that body.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a longitudinal section of a hybrid motor according to the
invention, having a tubular displacer and an inner stator,
FIG. 2 is a perspective view of the displacer of FIG. 1, only the
alternating magnetic and nonmagnetic rings being shown,
FIGS. 3a-3f are diagrammatic views of axially aligned outer and inner
stator members, showing the affects of different coil excitation, and
FIG. 4 is a longitudinal section of a hybrid motor according to the
invention having a solid displacer with integral return path.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The embodiments of FIGS. 1-3 have a unique, simple lightweight displacer
which has complete circular symmetry. This displacer is translated in the
axial direction in the space lying between an inner gap-defining surface
of an outer stator, and an outer gap-defining surface of an inner stator.
The outer stator and inner stator each comprise pole-defining members
formed by a plurality of axially spaced angular-pole-shaping members,
successive members being indexed angularly and spaced axially at a given
pitch distance, equal to the axial thickness of an individual member which
is preferably a single lamination.
Referring to FIG. 1, a linear motor 1 has three principal parts: an outer
stator 2 containing a stack of members or laminations to be described
below, and windings (for clarity, only the end turns are shown in this
view); an inner stator 3 having a correspondingly indexed set of members
or laminations to be described below; and a displacer 4 which is free to
move axially between the inner and outer stators.
The outer stator 2 is assembled within front and rear housing parts 5 and 6
which are separated by a field magnet 7, which is an axially magnetized
ring magnet. Front and rear outer lamination stacks fit snugly within the
housing parts 5 and 6, and abut the respective pole faces of the magnet 7.
The inner stator 3 is made up of two stacks of laminations mounted on an
inner stator mandrel 9, and spaced by a center return path ring 10 which
is aligned axially with the field magnet 7. The inner stator 3 is held in
position with respect to the outer stator 2 by a stabilized, filled resin
block 12, through which electrical connections may be made to the outer
stator coils.
As shown more clearly in FIG. 2, the displacer is formed primarily of an
alternating series of magnetic rings 14 and nonmagnetic rings 15 each of
which is quite thin radially and extends axially a length equal to the
thickness of two successive stator members or laminations. A shaft
extension 16 extends through the front of the motor, to enable connection
to a load which is to be driven by the motor.
To minimize weight and electrical losses, the nonmagnetic rings 15 may be
made of a stable, filled synthetic resin material; or they may be made
from a nonmagnetic, moderately high resistance metal or a ceramic material
which can be machined to shape, and bonded readily to the magnetic rings
14.
The lamination arrangement for the motor 1 is shown more clearly by
comparing FIG. 1 with FIGS. 3a-3f. FIGS. 3a, 3b, 3c and 3f show four
successive outer stator angular-pole-shaping members, which are indexed
45.degree. counterclockwise in sequence. Each of these pole-shaping
members is formed preferably as a single lamination, typically punched
from a thin strip of a high permeability magnetic metal such as 2V
Permendur, an alloy of 49% iron, 49% cobalt and 2% vanadium. Each member
or lamination has four inwardly extending long teeth 25, each terminating
in a pole face 26 subtending an angle somewhat less than 40.degree. about
the axis 18. The long teeth 25 are arranged as two diametrically opposed
pairs of adjacent teeth which are spaced 45.degree. apart on centers. Four
short teeth 27, having a tooth width the same as that of the long teeth,
are arranged in pairs spaced 45.degree. on centers between the long teeth
25, thus forming a symmetrical structure of alternating two long teeth,
two short teeth, two long teeth, and two short teeth around the periphery
of a circular cylindrical gap-forming surface 28 shown, for clarity, in
FIG. 3c as dashed lines. Because each of the teeth is less than 45.degree.
in width, eight equally spaced slot spaces 29 are formed between the
respective adjacent teeth as diametrically opposed slot pairs 29a, 29b,
29d and 29e.
The inner stator 3 is made similarly of a series of laminations 31-34,
sequential laminations being shown in Figs. 3a, 3b, 3c, and 3f. Each inner
angular-pole-shaping member or lamination has two long teeth 35 which
subtend an angle of somewhat less than 90.degree. and are arranged
directly opposite the respective pairs of pole faces 26 formed on the
adjoining long teeth 25 of the outer stator member 21-24. The long teeth
35 of each inner stator member lie along a circular cylindrical
gap-forming surface 38, shown in dashed lines in FIG. 3c. Between the
diametrically opposed teeth 35 on the inner stator there are symmetrical
recessed portions 37. The recessed portions 37 are spaced inwardly from
the gap forming surface 38, and the short teeth 27 have their inner ends
similarly spaced from the gap forming surface 28, a sufficient distance
that they form high reluctance paths to the gap-forming surface, in
comparison to the paths through the long teeth 25 and the inner stator
teeth 35.
In the position shown in FIG. 1, one of the displacer magnetic rings 14 is
aligned axially with adjoining outer members 21, 22 and adjoining inner
members 31, 32. Thus, by comparison with FIGS. 3a and 3b, it will be clear
that the magnetic ring lies in the stator air gap formed between the two
pole faces 26 and the face of an inner stator long tooth 35. The magnetic
ring is preferably made to have a radial thickness only slightly smaller
than the radial gap between the teeth 35 and 25, so that the total air
gap, from a long tooth 25 to the axially aligned magnetic ring 14, and
from the ring to the inner stator tooth 35 is very small in comparison
with the gap between the short teeth 27 and the magnetic ring 14, or the
recessed portion 37 and the ring 14.
The coils 13 are preferably arranged as two coil sets, one of which
provides a current distribution shown conventionally as the point and tail
of an arrow in the slots 29a and 29b of FIGS. 3a-3c, to attract the
displacer toward one of a first set of positions; and with reverse
polarity (the opposite direction of current) to attract the displacer
toward one of another set of positions. The second coil set produces the
current distribution shown similarly in slots 29d and 29e in FIGS. 3d-3f,
or the opposite current direction, to attract the displacer towards
respective other sets of positions.
From FIG. 1 it will be seen that the field magnet 7 tends to produce field
flux flowing from the front outer stator members, inwardly across the air
gap to the front inner stator members, rearwardly through the center
return path ring and mandrel of the inner stator, and then outwardly from
the rear inner stator members, across the rear air gap, to the rear outer
stator members. The member shown in FIG. 3a is a front member, so that
will be clear that the current pattern shown will produce a flux through
the teeth 25a which is additive to that caused by the field magnet 7,
while the same current distribution tends to reduce or buck the field flux
through the short teeth 27a that are 90.degree. away. Similarly, as shown
in FIG. 3b, this same current distribution will provide additive flux in
long teeth 25b and reduced flux in short teeth 27b. Thus, each of the
facing members 21, 31 and 22, 32 exerts a strong magnetic pull to hold the
magnetic ring 14 centered axially with respect to these two adjoining
stator members. At the same time, this current pattern will produce very
little flux through the short teeth 27c of outer stator member 23, shown
in FIG. 3c because of the long gap caused by the recessing of the face of
the tooth 27c from the outer gap-defining surface 28.
FIGS. 3d, 3e and 3f show the members of FIGS. 3b and 3c, and the fourth
member pair respectively, with the current distribution 90.degree. later
in time for one electrical cycle, if the motor is being operated as a
linear synchronous motor, or one displacer step to the left, as viewed in
FIG. 1, if the motor is being operated as a stepping motor. Clearly the
current distribution has a pattern just like that of FIGS. 3a-3c, except
rotated 45.degree. counterclockwise. As a result, flux is additive in the
long teeth 25d of outer stator member 22, and 25e in stator member 23; and
is reduced or bucked in the short teeth 27d, 27e of members 22 and 23, and
the long teeth 25f of outer stator member 24. This causes the displacer to
move to the left one axial pitch distance, equal to the thickness of one
stator member, so that the magnetic member 14 is now aligned axially
between the members 22, 23 and 32, 33.
In the hybrid motor shown in FIG. 1, it will be clear that flux passes
radially inward from the front outer stator section to the front inner
stator section, and radially outward from the rear stator inner section to
the rear outer section. Because a given stator coil tends to increase flux
inwardly in all those teeth which it surrounds, a coil current
distribution shown in FIGS. 3a-3c increases the flux through the long
teeth shown in section in the front stator section of FIG. 1; while it
bucks the field flux through those stator teeth having the same angular
position in the rear stator section. Thus this same current distribution
increases the flux flowing outwardly through the long teeth of the stator
members 23 and 24 of the rear section, which are aligned axially with the
magnetizable elements 14 then present between the rear inner and outer
stator sections.
The displacer can also be positioned or stepped distances less than one
axial pitch by energizing both coil sets simultaneously, either with the
same current in each coil or with differing magnitudes.
Single Stator Embodiments
The embodiment shown in section in FIG. 4 differs from that described
above, in that the return path for magnetic flux forms part of the
displacer, or armature, so that no inner stator is required; and in that
the displacer can be formed as one unitary body because nonmagnetic
spacers are not required.
The stator 2 shown in FIG. 4 may be identical to that of FIG. 1, with only
the resin block 12 of FIG. 1 being replaced by a disc 42 through which
electrical connections may be made to the stator coils and a flux-blocking
non-magnetic spacer 5b fitted between the housing part 5a and a mounting
and bearing part 5c.
The displacer 40 is made of a single block of magnetic iron, having
alternating cylindrical pole surfaces 41 and recesses 45, equally spaced
with a pole and a recess length each being equal to the length of two
successive stator pole-shaping members. Structurally, this displacer is
thus like that shown in FIG. 2 except that the entire interior of the
displacer is filled with magnetic material. Thus the stepping sequence of
the embodiment of FIG. 4 is identical to that of FIG. 1.
Compared with the embodiment of FIG. 1, the single-stator embodiment has
the disadvantage that the displacer has a far greater mass. The developed
magnetic axial force may also be less than that of the FIG. 1 embodiment,
if the stator 2 dimensions are the same, except that it may be feasible to
operate with a smaller radial clearance in the gap between displacer and
stator because of simplified fabrication and greater rigidity of the
displacer.
For some applications, the motor of FIG. 4 may be made shorter than that of
FIG. 1, while producing the same stroke, because the rear of the motor can
conveniently be made essentially as a mirror image of the front, with a
bore through which the displacer may travel axially.
Where structural length is extremely critical, the recesses 45 of the
displacer may be filled with a nonmagnetic material, preferably one which
is a poor electrical conductor, so that the displacer has a smooth
cylindrical surface. With this variation special bearings at the two ends
of the motor may be eliminated, and the displacer may be guided radially,
in whole or in part, by the cylindrical faces formed in the stator on the
pole-shaping members.
Other Embodiments
The angular-pole-defining members may each comprise a number of thin
laminations aligned axially, the laminations forming one member having
identical angular alignment and those forming the next member all being
indexed the same amount with respect to the first member. Where relatively
high stepping frequencies, or operating frequencies for synchronous
operation, are desired such a structure greatly reduces eddy current
losses.
Those of ordinary skill in the art will recognize that it is not necessary
to have similar structures to each side of the magnet 7. In a variation of
the embodiment of FIG. 1, which reduces the axial length of the motor for
a given stroke, the stator-pole-defining members may all be located to one
side axially of the magnet, while a minimum-length return path connects
the inner and outer stators to the other side. It is so clear that it is
not necessary that the magnet be configured as shown, or located solely in
the outer stator. Further, the field magnet need not be a permanent
magnet. The field flux can be provided by a field coil cooperating with
the magnetic structure, such as a ring coil surrounding a magnetic
material, or inside a magnetic sleeve such as the housing 6. Either a
permanent magnet or an electromagnet can be part of the inner stator, for
example replacing return path ring 10.
Other variations also include placing windings in slots in the inner
stator, in addition to or in place of slotted laminations for the outer
stator. To avoid imperfect radial alignment of the inner stator with
respect to the outer stator, it is possible to provide a series of axial
slots in the shaft extension 16 of the FIG. 1 displacer, so that the legs
of a spider may pass through these slots to center the front end of the
inner stator very precisely with respect to the housing part 5 of the
outer stator 2.
It will be clear that many other winding and/or magnetic pole
configurations can be utilized in accordance with the basic concept of the
invention, as described in the appended claims. For example, with the same
lamination shape, different coil distributions can involve conductors from
different coils sharing the same slot. Indexing of successive
pole-defining members need not advance continually in one direction, nor
is it necessary that two successive pole-defining members have long
pole-defining portions simultaneously experiencing additive magnetization;
in that circumstance the length of the displacer magnetic ring will be
appropriately selected. Of course, it is not necessary that the magnetic
and nonmagnetic rings, or the alternating cylindrical pole structures 41
and recesses 45, each have the same axial length. Where space constraints
are especially severe respecting motor length, it would also be possible
to have two concentric displacer sections moving in gaps between an inner,
an intermediate and an outer stator section. The field magnet might then
be radially poled and could be part of the intermediate stator or be in
the radial return path at the end of the motor.
* * * * *
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