U.S. patent application number 12/532954 was filed with the patent office on 2012-05-24 for linear machine having a primary part and a secondary part.
Invention is credited to Carsten Buhrer, Jakob Fallkowski, Ingolf Hahn, Jan Wiezoreck.
Application Number | 20120129700 12/532954 |
Document ID | / |
Family ID | 39627583 |
Filed Date | 2012-05-24 |
United States Patent
Application |
20120129700 |
Kind Code |
A1 |
Wiezoreck; Jan ; et
al. |
May 24, 2012 |
LINEAR MACHINE HAVING A PRIMARY PART AND A SECONDARY PART
Abstract
A linear machine comprises a primary part and a secondary part.
The primary part forms a receptacle around an axis and has a
plurality of annular primary coils, which are arranged
concentrically with respect to the axis, can have alternating
current applied to them, and are separated by intermediate
elements, in order to produce a magnetic field in the receptacle.
The secondary part, which can be moved relative to the primary part
by the magnetic field in the receptacle along the axis, is provided
with secondary coils having superconductor windings. In order to
produce a linear motor which allows high force densities, the
intermediate elements are produced from non-magnetizable material
and the primary coils and the secondary coils are arranged with an
air-gap winding, wherein the secondary coils are manufactured from
a high-temperature superconductor and direct current can be applied
or is applied to them. Force densities of more than 18 N/cm.sup.2
can be achieved in the receptacle by the linear motors.
Inventors: |
Wiezoreck; Jan; (Bonn,
DE) ; Hahn; Ingolf; (Bonn, DE) ; Buhrer;
Carsten; (Bonn, DE) ; Fallkowski; Jakob;
(Haruestof, DE) |
Family ID: |
39627583 |
Appl. No.: |
12/532954 |
Filed: |
March 25, 2008 |
PCT Filed: |
March 25, 2008 |
PCT NO: |
PCT/EP08/02333 |
371 Date: |
February 2, 2012 |
Current U.S.
Class: |
505/166 ;
310/12.21 |
Current CPC
Class: |
H02K 35/00 20130101;
H02K 55/04 20130101; Y02E 10/30 20130101; H02K 41/03 20130101; Y02E
10/38 20130101; Y02E 40/625 20130101; H02K 3/47 20130101; Y02E
40/60 20130101 |
Class at
Publication: |
505/166 ;
310/12.21 |
International
Class: |
H02K 41/02 20060101
H02K041/02; H01L 39/08 20060101 H01L039/08 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2007 |
DE |
10 2007 015 168.5 |
Claims
1. A linear machine comprising: a primary part having a plurality
of annular primary coils, arranged concentrically with respect to
an axis (A) and separated from one another by intermediate
elements, and a secondary part having a plurality of secondary
coils aplliable with direct current and arranged axially alongside
one another with alternating polarity and having superconductor
windings, with one part being movable relative to the other part
parallel to the axis, wherein the arrangement of the primary coils
in the primary part is in the form of an air-gap winding with the
intermediate elements composed of non-magnetizable material, and
the secondary coils are composed of windings of a high-temperature
superconductor, as a result of which force densities of more than
18 N/cm.sup.2 can be achieved, with the secondary coils being
annular and being arranged concentrically with respect to one
another around a supporting body, and with spacing elements being
arranged between the secondary coils, on which spacing elements the
secondary coils are supported in the axial direction.
2. The linear machine as claimed in claim 1, wherein the
arrangement of the secondary coils in the secondary part is in the
form of an air-gap winding.
3. The linear machine as claimed in claim 1, wherein no
magnetizable material for concentration of the magnetic flux is
arranged between the primary coils of the primary part and between
the secondary coils of the secondary part.
4. The linear machine as claimed in claim 1, wherein a volume ratio
of the primary coils to the intermediate elements is more than
70%.
5. The linear machine as claimed in claim 1, wherein the primary
coils are composed of windings of a normal conductor composed of
one of aluminum and copper.
6. The linear machine as claimed in claim 1, wherein the primary
coils are manufactured from windings of a superconductor.
7. The linear machine as claimed in claim 1, further including a
yoke which sheaths the primary coils and the intermediate elements
and is composed of one of non-magnetic material and
non-magnetizable material.
8. The linear machine as claimed in claim 7, wherein the yoke has
slots on its internal circumference, on which slots the
intermediate elements are anchored.
9. The linear machine as claimed in claim 1, wherein one of the
primary coils (21) and the secondary coils are encapsulated in a
sheath, with the intermediate elements at least partially
comprising the sheath.
10. The linear machine as claimed in claim 1, wherein a current
density of more than 50 A/mm.sup.2 is applied to the secondary
coils, and the magnetic field of the secondary coils is aligned
parallel to the axis.
11. The linear machine as claimed in claim 1, wherein the
supporting body is cylindrical and the secondary coils are arranged
about an outer surface of the supporting body.
12. The linear machine as claimed in claim 11, wherein the
supporting body is one of non-magnetizable and composed of
non-magnetizable material.
13. The linear machine as claimed in claim 1, wherein the spacing
elements are one of non-magnetizable and composed of
non-magnetizable material.
14. The linear machine as claimed in claim 13, wherein the
secondary coils have a width, and the distance between adjacent
secondary coils corresponds at least to twice the width of the
secondary coils.
15. The linear machine as claimed in claim 1, wherein alternating
current is applied to the primary coils, the primary part and
secondary part being moved relative to one another by applying
current to the primary and secondary coils, and the linear machine
forms a linear motor.
16. The linear machine as claimed in claim 15, wherein the primary
coils are manufactured from windings of a superconductor, with the
alternating current being applied oscillating at a frequency of
less than 100 Hz.
17. The linear machine as claimed in claim 1, wherein one of the
primary part and the secondary part can be moved parallel to the
axis, on an externally-operating basis, wherein current which is
induced in the primary coils by the axial movement between the
primary part and the secondary part can be tapped off, and the
linear machine forms a generator.
18. A linear machine including a primary part and a secondary part,
the primary part of the linear machine comprising: a plurality of
annular primary coils which are arranged concentrically with
respect to an axis and are separated by intermediate elements, the
arrangement of the primary coils in the primary part is in the form
of an air-gap winding with intermediate elements composed of
non-magnetizable material, and alternating current applied with a
phase shift to the primary coils which are located alongside one
another.
19. A linear machine including a primary part and a secondary part,
the secondary part of the linear machine comprising: a plurality of
secondary coils which are arranged axially alongside one another
and have superconductor windings, the arrangement of the secondary
coils is in the form of an air-gap winding, wherein the secondary
coils are manufactured from a high-temperature superconductor and
direct current of opposite polarity is applied to secondary coils
which are located alongside one another.
Description
[0001] This application claims priority to and the benefit of the
filing date of International Application No. PCT/EP2008/002333,
filed 25 Mar. 2008, which application claims priority to and the
benefit of the filing date of German Application No. 10 2007
015168.5, filed 27 Mar. 2007, both of which are hereby incorporated
by reference into the specification of this application.
[0002] The invention relates to a linear machine having a primary
part, which has a plurality of annular primary coils, which are
arranged concentrically with respect to an axis and are separated
from one another by intermediate elements, and having a secondary
part which has a plurality of secondary coils, to which direct
current can be applied and which are arranged axially alongside one
another with alternating polarity and have superconductor windings,
with one part being movable backward and forward relative to the
other part parallel to the axis.
BACKGROUND
[0003] DE 195 42 551 A1 discloses a linear motor having a
hollow-cylindrical primary part, which has annular primary coils
which are arranged concentrically with respect to a movement axis
of a secondary part and can be operated with polyphase current.
Annular laminates composed of soft-magnetic material are arranged
between the primary coils, are used as intermediate elements to
separate adjacent primary coils, and form magnetizable teeth, in
order to amplify the magnetic flux and to pass this to the
receptacle in which the secondary part is arranged. The primary
coils and the annular laminates are accommodated in a
hollow-cylindrical yoke composed of magnetizable material, which
forms a magnetic return path. The secondary part is arranged such
that it can move axially within the receptacle that is formed by
the primary part. The secondary part has a plurality of field
magnets composed of superconductor windings, which are arranged one
behind the other with alternating polarity in the axial direction.
In DE 195 42 551, the magnetic fields of the secondary windings are
at right angles to the axis of the secondary part. In order to
produce this field direction using wound coils, the axis of each
individual coil through which current flows must be at right angles
to the movement axis of the linear motor. Only if permanent magnets
or superconducting solid-body magnets are used can these magnets
rest with their inner circumferential surface on a cylindrical yoke
composed of magnetizable material. Although these then have an
annular shape, they are magnetized radially, however. In the case
of wound secondary coils, in contrast, an arrangement must be
chosen in which the wound coils are offset alongside one another in
the circumferential direction and in the axial direction on the
casing surface of the supporting body. The magnetic forces which
are produced when current is applied to the primary and secondary
coils produce a relative movement between the primary part and the
secondary part.
[0004] EP 1 465 328 A1 discloses a linear motor in which the
primary part and secondary part are arranged reversed, such that
the secondary part is on the outside, and surrounds the primary
part.
[0005] The capability to magnetize the soft-magnetic teeth is
restricted because magnetic saturation occurs in the soft-magnetic
material. In order to achieve higher force densities between the
primary part and secondary part with high current densities in the
coils of the primary part, it has been proposed that the number of
turns in the primary coils be increased or that the amount of
magnetizable material be increased. These measures have allowed
force densities of about 8 N/cm.sup.2 to be achieved in the trial
stage for round and polysolenoid linear motors. However, the
physical size and the weight of the linear motors have to be
significantly increased to do this.
[0006] A concept for a linear motor, in which the stator has
primary coils composed of a superconductor material which comprise
high-temperature superconducting double-pancake coils, is known
from Superconductor Science and Technology, 17 (2004), page 445 to
449. In order to achieve a force density in the order of magnitude
of 14 N/cm.sup.2 with the linear motor, an actuator is proposed
which is fitted with NdFeB magnets. As an alternative concept, an
actuator is proposed which comprises solid-body superconductors,
which are formed by means of a combination of iron laminate wafers
and YBCO.
[0007] EP 0 425 314 A1 discloses a linear motor in which both the
coils in the primary part and the coils in the secondary part
comprise saddle-type coils which are curved in the form of an arc
and are arranged axially offset with respect to one another. The
magnetic field of the saddle-type coils in the primary part and in
the secondary part is at right angles to the movement axis.
SUMMARY OF INVENTION
[0008] In accordance with the present invention, provided is a
linear machine in which considerably higher force densities are
made possible by design measures on the primary part and/or
secondary part, even for linear machines of small physical
size.
[0009] According to one aspect of the invention, the arrangement of
the primary coils in the primary part is in the form of an air-gap
winding with intermediate elements composed of non-magnetizable
material, and the secondary coils are composed of windings of a
high-temperature superconductor, as a result of which force
densities of more than 18 N/cm.sup.2 can be achieved. The linear
machine is in the form of a linear motor, in which a relative
movement is produced between the primary part and secondary part
parallel to the axis by applying current to the primary and
secondary coils, via the magnetic fields that are produced in this
way, and the invention will be described in the following text
primarily with reference to this. However, the linear machine may
also be in the form of a generator, in which a current which is
induced in the primary coils by the relative movement between the
primary part and secondary part is converted in order to obtain
energy. The high force densities when the linear machine is in the
form of a linear motor can be achieved by applying alternating
current to the primary coils and direct current to the secondary
coils. Since the arrangement of the primary coils and also the
arrangement of the secondary coils are in the form of an air-gap
winding, that is to say no magnetizable material for flux guidance
is arranged either between the primary coils or between the
secondary coils, the force density in the case of the linear
machine according to the invention is not limited by saturation
magnetization.
[0010] According to another aspect, current level in the primary
part, that is to say the current in the circumferential direction
per axial length of the primary part, can be increased in
comparison to known linear motors without enlarging the physical
size of the linear motor, as a result of which the force density,
which is proportional to the current level, rises without
saturation effects. No iron or magnetizable material for
concentration of the magnetic flux is arranged between the primary
coils. The use of secondary coils composed of high-temperature
superconducting material, which has a critical temperature which is
higher than 77 K, in the secondary part allows large direct
currents to be applied to the secondary coils, in order to make it
possible to produce extremely strong magnetic fields in the
receptacle. A further advantage with the linear motor according to
the invention is that a force profile which is virtually smooth in
the axial direction is achieved since the air-gap winding means
that there are largely no reluctance forces in practice, and in
consequence scarcely any cogging forces occur. Furthermore, since
there are no permanent magnets and magnetizable material in the
primary part and secondary part, and no magnetic forces therefore
occur when the current that is supplied is switched off, the linear
motor can be serviced and cleaned relatively easily.
[0011] According to yet another aspect, a high current level in the
primary part can be achieved in particular by choosing a high
filling factor for the primary part. The filling factor is defined
as the volume ratio of the volume of the primary coils through
which current flows to the volume of the intermediate elements and
any intermediate spaces that there may be between the primary
coils. The filling factor of the primary part is greater than 70%,
and in particular greater than 85%. Primary coils which are
adjacent in the axial direction preferably have an alternating
current with a phase shift of 120.degree. applied to them, as a
result of which the linear motor forms a three-phase motor. In the
case of a two-phase motor or a polyphase motor with more than three
phases, the phase shift may be adapted or chosen differently.
[0012] According to one exemplary embodiment, the primary coils may
have windings composed of a normal conductor, in particular such as
a conductor composed of aluminum or copper, as a result of which
the primary coils may, for example, be liquid-cooled or gas-cooled
in a cost-effective manner. Cooling with water or oil, for example,
is particularly advantageous. In particular, the normal conductor
may also be formed from a hollow conductor, whose internal tube is
used for cooling. Alternatively, according to another exemplary
embodiment, the windings of the primary coils could be composed of
or be manufactured from a superconducting conductor, in particular
a high-temperature superconducting conductor. The current that is
applied should then be applied using alternating current at a
frequency of less than 100 Hz, in particular of less than 50 Hz, in
order to keep alternating-current losses in the superconducting
primary coils low, which would otherwise have to be compensated for
by additional coolant. In the linear motor according to the
invention, force densities of more than 18 N/cm.sup.2 can be
achieved, and when using superconductors both in the secondary
coils and in the primary coils, it is even possible to achieve
force densities of more than 25 N/cm.sup.2. Cooling lines through
which a coolant can flow may also be formed between the coils, or
gaps may be left open between the primary coils and if appropriate
the intermediate elements, in order to cool the primary coils. The
intermediate elements may be in the form of annular segments thus
allowing a coolant to be passed to the end faces, which are not
covered by the annular segments, of the primary coils. The
intermediate elements may extend over the entire area, partially or
with intermediate spaces over the radial height of the primary
coils. The intermediate elements may also comprise grid structures,
hollow bodies or grid bodies, which are sufficiently mechanically
robust and at the same time allow a coolant to flow through
them.
[0013] Furthermore, according to yet another aspect, the primary
coils and the intermediate elements are sheathed by a yoke, which
is composed of non-magnetizable material, in particular a
lightweight material without any iron. Alternatively, the yoke may
be composed of material which does contain iron and/or which can be
magnetized, for magnetic field shielding. In particular, the yoke
and the intermediate elements may form a mechanical holding
structure for the primary coils. In order to anchor the
intermediate elements in the axial direction as well, the yoke may
have slots on its internal circumference, in which slots the
intermediate elements engage in an interlocking manner. Anchoring
the intermediate elements on the yoke allows the primary coils to
be supported in the axial direction on the intermediate elements,
which means that the yoke can absorb the magnetic field forces
which act on the primary coils, in the axial direction. According
to one exemplary embodiment, forming the primary part without any
iron can achieve a particularly lightweight design for the primary
part and therefore for the linear machine, while avoiding
saturation effects at the same time. Alternatively, the yoke may
have a magnetizable material in order to form a return path for the
magnetic flux.
[0014] According to still yet another aspect, the primary coils may
be encapsulated in plastic, for example in synthetic resin, in
particular in epoxy resin. The intermediate elements in one
advantageous refinement of the invention are likewise manufactured
from plastic, for example synthetic resin, in particular epoxy
resin, and can be reinforced with fiber reinforcement, for example
by insertion of glass fiber material.
[0015] According to still yet another aspect, the superconducting
secondary coils can carry high current densities, for example
current densities of more than 50 A/mm.sup.2, furthermore of more
than 70 A/mm.sup.2 and in particular more than 100 A/mm.sup.2, thus
making it possible to produce an extremely strong magnetic field by
means of the secondary coils. The flux densities which can be
produced by the secondary part in the air-gap may reach more than
0.5 Tesla, more than 1 Tesla, and possibly up to 2 Tesla. The
secondary part has a cylindrical supporting body adjacent to or on
whose casing surface the secondary coils are arranged. The
supporting body of the secondary part is produced from a
non-magnetic material, for example from fiber-reinforced plastic.
The supporting body could also be produced from or be composed of a
magnetic material, in particular iron. In one refinement, the
secondary coils have an annular shape and are arranged
concentrically with respect to one another with respect to the
axis, mounted on the associated supporting body of the secondary
part. Secondary coils which are adjacent in the axial direction
have direct current applied to them in antiphase, by opposite
polarity connection, during operation. Once again, in order to
create the air-gap winding, non-magnetizable, annular spacing
elements can be arranged between the secondary coils, on which
spacing elements the secondary coils are supported in the axial
direction. In this exemplary embodiment, adjacent secondary coils
are at a distance from one another which is at least twice as
great, and preferably more than this, as the width of the
respective secondary coils in the axial direction. A plurality of
coils can also be combined to form a pack, all having the same
current flow direction (connected in series or in parallel). A
reverse current direction is then in each case applied only to
adjacent coil packs.
[0016] Further advantages and features of the invention will be
described with reference to exemplary embodiments, which are
illustrated schematically in the drawing, of a linear motor as a
linear machine.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a linear motor according to the invention with
a primary part and a secondary part, according to a first exemplary
embodiment and in the form of a longitudinal section; and
[0018] FIG. 2 shows a perspective view of the secondary part from
FIG. 1.
DETAILED DESCRIPTION
[0019] Referring now to the drawings wherein the showings are for
the purpose of illustrating preferred and alternative embodiments
of the invention only and not for the purpose of limiting same,
FIG. 1 illustrates a linear motor, which is annotated 10 in its
totality, with a primary part 20 and a secondary part 30. The
primary part 20 bounds a cylindrical receptacle 11 in which the
secondary part 30 can move backward and forward along a central
axis A. In the illustrated exemplary embodiment, the primary part
20 has five primary coils 21 which are arranged concentrically with
respect to the axis A. The drawing shows only one motor section
from an entire motor since, for example, the number of coils or
coil packs must be divisible by three, for example, for three-phase
operation. The primary coils 21 comprise annular disk coils to
which phase-shifted alternating current or three-phase current can
be applied, phase-shifted through 120.degree., for example, on the
external circumference via contacts which are not illustrated, in
order to produce a magnetic traveling field by means of the primary
coils 21 in the receptacle 11. The windings, which are composed of
a copper conductor, of the primary coils 21 are encapsulated in
epoxy resin, to provide mechanical robustness. Annular intermediate
elements 22 are likewise arranged between the primary coils 21, on
which intermediate elements 22 the end faces of the primary coils
21 are supported in the axial direction. The intermediate elements
22 extend in the axial direction from the internal circumference of
the primary coils 21 to the external circumference of the primary
coils 21. A hollow-cylindrical yoke 23, on which the intermediate
elements 22 are anchored (not illustrated), rests on the external
circumference of the intermediate elements 22 and of the primary
coils 21. The yoke 23 and the intermediate elements 22 thus form a
mechanical holding structure for the primary coils 21 that are
accommodated therein.
[0020] The yoke 23 around the primary part 20 may be composed of
non-magnetizable material or, for shielding purposes, also of
magnetizable material. In the latter case, it is even possible for
the force density to be increased. If the yoke 23 is composed of
electrically conductive material, then it can preferably be formed
by means of laminated or slotted materials, in order to reduce
alternating-current losses.
[0021] By way of example, the intermediate elements 22 may be
composed of glass-fiber-reinforced plastic and, according to the
invention, therefore cannot be magnetized, as a result of which the
magnetic field which is produced in the receptacle 11 when current
is applied to the primary coils 21 is not limited by saturation
magnetization of the intermediate elements 22. There is essentially
no magnetizable material for flux guidance located between the
primary coils 21. The arrangement of the primary coils 21 located
alongside one another in the axial direction is therefore in the
form of a so-called air-gap winding. These "air-gaps" between the
primary coils 21 are filled with the intermediate elements 22,
which are possibly partially hollow and/or are used exclusively for
insulation. Very broad primary coils 21 with a large number of
turns per unit axial length can therefore be used in the primary
part 20. Since the volume of the intermediate elements 22 occupies
only a fraction of the volume of the primary coils 21, the filling
factor of the primary part with turns which carry current and also
produce a magnetic field (traveling field) is considerably more
than 50%. A higher current can therefore be introduced into the
primary coils 21 of the primary part 20.
[0022] The secondary part 30, which is illustrated in FIG. 1 and
FIG. 2, has annular secondary coils 31, which are arranged
concentrically with respect to the axis A and are composed of a
high-temperature superconductor. These secondary coils 31, which
are superconductive at cryogenic temperatures of more than 20 K,
have direct current applied to them, with secondary coils 31 which
are adjacent in the axial direction being connected in antiphase.
The high-temperature superconductor windings and secondary coils 31
in the secondary part 30 may be in the form of pancake coils,
double-pancake coils, packs composed of these pancake coils or
short solenoid coils. Annular spacing elements 32 are likewise
arranged between the secondary coils 31, and are arranged
concentrically with respect to the axis A. The spacing elements 32
are composed of glass-fiber-reinforced epoxy resin and are arranged
together with the secondary coils 31 on a hollow-cylindrical
supporting tube 33. The hollow-cylindrical supporting tube 33 may
be manufactured from soft-magnetic magnetizable material such as
soft-magnetic iron, or may likewise be composed, for example, of
glass-fiber-reinforced plastic. In order to allow the secondary
coils 31 to be cooled, for example using liquid nitrogen, the
cryostat 34 is provided with a double-walled tube 36. The
intermediate space, which is not illustrated, between the "warm"
outer tube wall and the "cold" inner tube wall of the tube 36 is
evacuated, in order to prevent heat from being introduced from the
outside into the cryostat 34, or to constrain it. If required, an
insulation layer composed of commercially available super
insulation sheet can also be fitted around the cold tube wall.
Force is transmitted from the secondary part 30 to the cryostat 34
by means of schematically indicated transmission elements 35a and
35b. The transmission elements 35a, 35b are composed of a material
of low thermal conductivity and high mechanical strength, for
example of glass-fiber-reinforced plastics. The secondary coils 31
can be operated with current densities of up to 100 A/mm.sup.2. The
linear motor 10 with a primary part 20 designed according to the
invention and with an air-gap winding of the primary coils and a
secondary part 30 designed according to the invention allows force
densities of more than 18 N/cm.sup.2 to be achieved in the
receptacle 11 between the primary part and the secondary part, in
order to move the secondary part 30 parallel to the axis A.
[0023] Numerous modifications will be evident to a person skilled
in the art from the above description and the dependent claims. The
number of primary and secondary coils in the axial direction is
only an example and, in particular, may vary with the width of the
coils and the overall length of the linear motor. The secondary
coils may also be arranged in a spiral shape. The yoke and the
supporting tube for the secondary part may also be composed of
material containing iron. The supporting tube for the secondary
part may also be omitted, if the secondary coils have been firmly
connected to one another together with the spacers, for example by
vacuum impregnation. Alternatively, the supporting tube for the
secondary part may be composed of laminated and slotted
magnetizable material, or likewise, for example, from
glass-fiber-reinforced plastic. Hard-magnetic materials may also be
used as a supporting tube in the secondary part through which
direct current flows. Particularly when using normally conductive
primary coils, they can be cooled indirectly or preferably
directly, for example by water, oil, gas or nitrogen (N.sub.2).
Alternatively, it is also possible to use suitable gas or dry
cooling, which allows an operating temperature of below 77 K, for
example 20 K or 30 K. In order to further reduce eddy current
losses in the primary part, the primary coils may be provided with
braided-wire windings. If required, a second primary part could
also be arranged within the secondary part, in order to further
increase the force density. Instead of the secondary part, the
primary part could also be moved parallel to the axis by the
magnetic field that is produced when current is applied. The
primary part could be arranged internally, and the secondary part
could be arranged externally. In one refinement of the linear
machine as a generator, the secondary part to which direct current
is applied could be moved mechanically, for example by a rising and
falling buoy of a wave-driven power station. The current which is
induced in the primary windings of the primary part by means of
this movement of the secondary part could be used to obtain energy,
with the linear machine then acting as a generator. Instead of the
secondary part, the primary part could also carry out the backward
and forward movement parallel to the axis with the secondary part
being stationary, without departing from the scope of protection of
the attached claims.
[0024] It will be appreciated that various of the above-disclosed
and other features and functions, or alternatives thereof, may be
desirably combined into many other different systems or
applications. Also that various presently unforeseen or
unanticipated alternatives, modifications, variations or
improvements therein may be subsequently made by those skilled in
the art which are also intended to be encompassed by the following
claims.
* * * * *