 |
Description  |
|
|
The following invention relates to permanent magnet linear electric motors,
in particular those used for applications requiring extended travel.
Numerous types of linear motor exist for effecting powered travel over
extended distances. Examples are a) the AC linear induction motor, b) the
`sandwiched coil` construction permanent magnet linear motor, and c) dc
brushless motors of various configurations. Each of these examples suffers
from various deficiencies. In the case of the AC induction motor, there
are substantial electromagnetic and resistive losses inherent in both its
construction and mode of operation, and the maximum velocity of movement
is limited by factors such as the frequency of the AC supply used, and the
practical limitations on the effective pole pitch of the armature coils.
In addition, for systems in which the stator is used as the moving part,
armature coils are required over the full length of travel, which is both
expensive, and wasteful of energy. In the case of sandwich type
constructions, in which the motor's armature coils are located between two
facing rows of magnets, it is self evident that heat cannot escape
conveniently from the said coils. The ability to dissipate heat is a key
measure of the effectiveness of any linear motor design. In the case of
flat dc brushless motors, these necessitate the use of a copious volume of
permanent magnet material along their entire length, and are therefore
expensive. Furthermore, the ways must be flat, and in many designs a very
tight air gap must be maintained.
An ideal design is one in which a) a relatively limited volume of permanent
magnet material is needed per unit length of travel, b) there is no need
for a critical airgap between the stator and armature, and c) heat
generated by the travelling armature coils can easily be dissipated. Such
a motor, now in widespread use, is described in UK granted patent no.
2079068. Although a successful commercial design, an inherent disadvantage
of this tubular type of construction, is however, the limited travel that
may be realised. This is due to the tendency of the stator to bow under
the effect of gravity, and thereby come into contact with the moving coils
of the armature, which coaxially surround it. Travel is therefore limited,
in practice, to no more than two metres. An ideal motor is one which
combines the inherent advantages of the aforementioned invention, with the
ability to travel extended distances.
According to the present invention, there is provided a linear motor
comprising an armature and a stator coaxial with one another, the stator
having a plurality of magnetic flux generators extending along the
longitudinal axis of the motor over the required travel of the armature
relative to the stator and providing a repeating sequence of magnetic
poles along said axis, the armature having a plurality of phases of drive
coils coaxial with the stator for providing, when appropriately
commutated, thrust, the coils being wound such that they substantially
surround the stator but leave a single gap extending transversely of the
longitudinal axis of the motor to allow the presence of means, extending
radially through the gap, for the mechanical support of the stator.
This gap means that unlike prior linear motors using coaxial drive coils,
the stator and its flux generators are not restricted to being supported
only at the opposite ends of the stator and can be supported at locations
intermediate its ends and, indeed, over its entire length.
In the present motor, due to the coaxial arrangement of the drive coils,
the thrust is produced by currents passing through the drive coils
interacting with the lines of magneto motive force produced by the stator
flux generators which are principally directed radially relative to the
motor longitudinal axis. The parts of the coil conductors in which this
thrust is generated are those which extend circumferentially around the
motor longitudinal axis.
The requirement of the provision of the stator support gap is, on the face
of it, in conflict with the winding of the armature coils from continuous
electrical conductors (wire or tape) for this reason: the coil conductor
cannot, because of the gap, extend 360.degree. around the motor axis and
must therefore after one part-turn (i.e. 360.degree. minus a gap angle)
turn back on itself around the motor axis. However, this would result in
the current flow being in the opposite turning direction, cancelling out
the thrust generated by the part-turn in question. In an embodiment of the
invention, this problem is overcome as follows:
each coil is configured as two sub-coils which are spaced from one another
longitudinally of the motor and have winding portions which extend
clockwise and anti-clockwise respectively round the axis of the motor;
these sub-coils are longitudinally spaced apart by a distance, chosen in
relation to the spacings of the stator poles, such that they are subject
to radial lines of force from the stator poles which are of opposite
polarities. Thus, the current flows in the sub-coils, being of opposite
clock senses, produce thrust forces in the same longitudinal direction.
In order to assist visualisation of the way in which the coil is wound,
each "turn" of the coil winding is made up of the following contiguous
portions:
starting at one side of the gap, a portion which extends in a circular arc,
in a plane perpendicular to the motor axis, around the motor axis to the
other side of the gap (this circular arc subtends less than 360.degree.
around the motor axis to leave the mechanical support gap);
from there, a portion which turns perpendicular to that plane, i.e. now
parallel to the motor axis, along the length of the armature to the other
sub-coil of the coil;
from there a portion corresponding to the first portion, extending
circumferentially around the motor axis through an arc corresponding to
the first-mentioned one but counterclockwise to the starting side of the
gap;
finally, a portion extending parallel to the motor axis, clear of the gap,
returning to the first sub-coil.
In the illustrated embodiment of the invention, the coils of the respective
phases of the motor overlap one another longitudinally of the motor and,
to allow for this, the portions of the conductors which extend between the
two sub-coils are not exactly parallel to the motor axis but are shaped to
allow the overlapping of the coils. In a preferred embodiment of the
invention, the stator flux generators are arranged such that successive
flux generators have their magnetic poles facing one another i.e. in a NS
. . . SN . . . NS . . . SN . . . sequence. Further, the armature sub-coils
are contiguously overlapped so that there are no longitudinal spaces left
between sub-coils of respective phases of the motor. The stator flux
generators are conveniently constituted by a stacked sequence of permanent
magnets and intermediate spacers so as to provide the required NS . . . SN
. . . NS . . . SN . . . sequence.
Thus, in this arrangement, because the armature coils do not circumscribe
the stator of the motor, the stator may be mechanically supported along
its full length, so enabling the provision of a motor of whatever length
is required to meet a particular application. At the same time, the
maximum possible flux linkage between the armature coils and the stator
magnetic stack is achieved, by virtue of their coaxial alignment and
operation. A tubular linear motor results, with a number of salient
advantages, as follows.
Firstly, and most significantly, the electromagnetic efficiency of the
motor arising from the manner in which flux squeezing occurs due to the
disposition of the permanent magnets. (By way of explanation, because like
poles are facing, virtually all of the magnetic energy stored within the
permanent magnets is caused to emanate radially, for direct linking with
the coils of the armature.) Secondly, because the coils are arranged
contiguously, all of the length of the armature is occupied by copper, so
optimising the number of turns working against the flux emanating from the
stator. Thirdly, again arising from the favourable permanent magnet field
pattern, there is no need for the use of iron laminations between
successive coils, to enhance performance. The mass of the armature
consequently is substantially only that of the windings and their housing,
rather than that of windings and iron, and the resulting light armature
provides a highly improved dynamic response for eg servo positioning.
Fourthly, and importantly, because the coils surround the stator, heat can
readily escape through the walls of the housing in which they are located.
Fifthly, because the coaxial alignment of the coils relative to the stator
may be determined by external precision guidance means (eg a precision
recirculating ball linear bearing slide), a close air gap may be achieved
between stator and armature, so further enhancing performance. Sixthly, as
already mentioned above, there is no effective limit to the travel that
may provided by a motor of this construction.
As will be appreciated, the essence of the invention herein disclosed is to
provide a tubular linear motor, but in such a format that its stator can
be supported along its full length. An inherent limitation arises from
this construction, inasmuch that the individual turns of each sub-coil of
any given phase, clearly cannot be completed circumferentially around the
stator. Thus, each part turn, having been formed during winding to
surround the stator as far as possible, (in order to maximise flux
linkage), must then traverse along the length of one pole pitch of the
armature, in order to form a part turn of the next sub-coil, and then
traverse again back to the original coil to form a further part turn., and
so on. It will be appreciated that the `traversing portion` connecting
each sub coil to the next must occupy space with that of its neighbour's
interconnections, and this sharing of space must be optimised during
production to provide a feasible solution.
According to an embodiment of the invention, the interconnections between
sub coils of any given phase of the armature are so formed that the
traversing portions interconnecting the sub coils are interleaved with the
traversing portions interconnecting the sub coils of the other phase(s),
the arrangement being such that the traversing portions of wire
interconnecting the various subcoils share, substantially, the same cross
sectional area. Thus, by this means, rather than there being bulges where
the traversing portions cross one another, as they leave to form the turns
of each sub coil, the individual turns may rise naturally from the
traversing portions. This new arrangement both minimises I2R losses in the
non-effective traversing portion of the coil, and also ensures as much
`turn` as possible coaxially surrounds the stator. The combination of
these two factors helps realise as high efficiency as is possible from
this design.
In a feature of this embodiment of the invention, the traversing portion of
wire interconnecting the sub coils is arranged during manufacture of the
armature to lie directly against the inside surface of the housing walls
in which the coils are situated, so as to ensure the maximum dissipation
of resistive heat losses through the housing walls.
The invention will now be described with reference to the accompanying
drawings in which:
FIGS. 1a, b, c & d are schematic representations of the stator of the
motor.
FIGS. 2a,b,c and d are schematic representations of the armature of the
motor.
FIG. 3 shows a perspective view of the armature and stator together.
FIG. 4 shows the armature coils embedded within a heat-sinking housing,
with linear bearings for guiding the passage of the housing relative to
the stator.
FIG. 5 shows in detail a winding arrangement of the coils of the motor.
Referring now to FIG. 1, views of the stator of a linear motor as herein
described are shown at a)-d). Referring now to FIG. 1a, a
non-ferromagnetic tube 10 containing a sequence of permanent magnets, is
mounted onto a proprietary type support stand, 12. The sequence of
permanent magnets is as disclosed in UK granted patent 2,079,068, and is
shown schematically in FIGS. 1, b & c. Each magnet, 13, is disc shaped in
the particular design illustrated in FIGS. 1a-c, and is housed and
maintained within the tube 10, as shown in FIG. 1b. (To suit mechanical
mounting requirements, magnets and spacers of other cross sections may be
employed, such as in the arrangement shown in FIG 1d, provided the
magnetic sequence is maintained.) The magnets are each separated one from
the other by spacers, 14. These may be of a suitable ferromagnetic
material, to provide the maximum radial field. Like poles of each magnet
face one another, in other words, they are stacked in a spaced NS . . . SN
. . . NS . . . sequence. The diameter and width of the disc magnets is
chosen to optimise the required performance/cost ratio for any given
design. The stand 12 used to support the tube may be of any suitable
commercial design, but preferably is not fabricated from ferromagnetic
material which would otherwise provide a degree of magnetic short circuit
to the magnetic flux radiated from the stator, and so limit the motor's
efficiency. The upper portion of the stand extends through a cap G which
extends the entire length of the motor armature so as to provide
mechanical support for the stator magnets and their tubular housing.
Referring now to FIG. 2, a three phase coil 15 of the armature is shown
pictorially at a) & b). Each coil comprises two circular portions, as
shown at 16 & 17 in 2c. It is these circular portions which cut the lines
of flux emanated from the stator, and thus produce linear force
longitudinally of the motor. Each turn of each of the circular portions is
connected, as shown, to the corresponding turn of its facing portion by
the traversing portions of conductor, 18. Each set of two sub coils is
interleaved with neighbouring coils to form a complete set, as shown at
(b) and (a). It will be noted that the circumferential direction of
current in phase A will be opposite to that of its mating coil, A, simply
due to the current flow direction. This is as necessary to effect the
correct magnetic polarity of each sub coil in correspondence to that which
is created by the stator permanent magnet fields. Thus, when portion A
overlies an area of the stator coinciding with the field emanating
from--say--facing North poles of the stator, portion A overlies an area
with facing South poles. This reversal of current direction, resulting
from the method of winding and interconnecting the coils, thereby ensures
a consistent direction of thrust is obtained from a set of energised sub
coils, in accordance with Fleming's left hand rule, regardless of the
position of the coils over the armature. Note, the coils do not
necessarily have to be wound so as to form just two facing sub coils.
Instead, where a particularly long armature is envisaged, a series of sub
coils can be formed, in one continuous sequence, as shown in FIG. 2d. It
is a feature of this arrangement, however, that because of the winding
pattern, the very end sub coils do have one half of the number of turns of
the intermediate sub coils.
FIG. 3 shows a double set of armature coils, 19 and 20, positioned
coaxially around a stator of the motor 21. It is readily apparent that
many sets can be added to increase the thrust available for any specific
application, or by the use of an extended series of sub coils as explained
above.
A key advantage of this invention is the manner in which heat can be
dissipated from the armature coils, owing to the fact that they are
cylindrical, and surround the stator. FIG. 4 indicates how this is
realised in practice. The armature coils 22 are sealed within a protective
housing, 23, with external fins, 24. As well as providing a highly
effective heat sink, the housing protects the coils physically, as well as
ensuring that they are held in coaxial alignment. The load to be
positioned by the linear motor may be clamped to the housing by means of
the T slots, 25. To ensure a precise orientation of the armature housing
relative to the stator, and accurate guidance as it traverses to and fro,
a linear guidance system may be employed. This is shown at 26 and 27.
Linear re-circulating ball bearing blocks are affixed to the base of the
housing , and these are in turn guided along by the linear slides 28 and
29. In practice, such guides afford outstanding precision. A fine air gap
may therefore be achieved between the inner diameter of the coils 24, and
the outer diameter of the stator tube (not shown), so further enhancing
the performance of the motor. It will be appreciated that this arrangement
is shown by way of example only, there being many orientations and
location possibilities for the linear guidance system.
It will be apparent from a close inspection of the coil configurations of
FIGS. 2a and 2b, that the longitudinally extending portions of traversing
wire 18 interconnecting the various sub coils, must all share the same
physical space. To optimise this procedure, and minimise the length of
interconnecting wire, (and therefore ameliorate the parasitic I.sup.2 R
losses associated therewith), the coils of the motor are formed together
during construction such that all of the traversing portions 18 of the
various sub coils are interleaved or interlaced within the same effective
cross-sectional area along the armature. Thus, referring to FIG. 5, it
will be seen that the vertical portions of each coil (representing the
circumferential portion of each coil), are--of course--separate, but that
the traversing portions 18 are all interleaved. This provides a simple and
elegant solution to the practical matter of winding the armature coil for
as efficient operation as possible. Note, the third phase, phase C, is
omitted for clarity.
Thus the illustrated arrangement provides a motor in which the winding of
the armature coils and the dispersion of the longitudinally extending
traversing portions 18 is such that the single gap G is left in the
circumference of the armature, allowing the passage of the upper part of
the support stand 12 into the interior of the coils where it can support
the stator magnet assembly. At the same time, the arcuate portions of the
armature drive coils substantially surround the stator permanent magnets
apart from in the position of the gap G. The arc around the longitudinal
axis of the motor occupied by the gap G is preferably not more than
approximately 30.degree..
While the above discloses several possible realisations of the linear motor
of the invention, numerous variations will be apparent to those skilled in
the art.
* * * * *
|
|
 |
|
 |
Description |
|
