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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a linear DC brushless motor, and more
particularly, to a linear DC brushless motor incorporating an armature
core which can remarkably reduce detent thrust or drive force, in other
words, can reduce such detent thrust caused by an end effect of its
armature core attributable to its peculiar nature that the armature core
of a linear motor must inevitably be composed of a linear core having a
finite or limited length, namely having both ends in the direction of
travel.
2. Prior Art
As linear motors have increasingly been applied to the office automation
(OA) devices or to precision measuring instruments, various demands have
been raised with respect to their servo control performance or precise
positioning characteristics and as a consequence, linear DC brushless
motors have attracted user's interest as one which is particularly
suitable for such usage.
Generally, a linear DC brushless motor (hereinafter merely referred to LDM)
is constructed based on such a manner that an ordinary DC brushless motor
(or AC servomotor) is reformed into a linear shape. As its general
construction of an LDM is shown in FIG. 7, movable element, namely, an
armature core 1 is reciprocally movable right and left along its travel of
motion shown by an arrow line in FIG. 7. The armature core 1 has a large
number of teeth 1A or "comb teeth".
In the slots defined between successively adjacent teeth of the core,
plurality of coils are fitted to make up a set of armature core in the
movable element. In more detail, one of the legs of a preformed toroidal
coil, say a first coil is inserted into one slot of the core between two
teeth adjacent with each other expecting those at both ends in the
direction of travel of the motor, the next leg, in a similar manner, to
the next slot, and further leg(s) of the coil or a subsequent coil is
inserted in the subsequent slot, one after another to make up a set of an
armature core.
With regard to the stator, a large number of permanent magnets constituting
a linear stator are arrayed on a plate of magnetic material to form an
alternate magnetic pole arrangement as shown also in the drawing.
The relative position of the armature core with respect to the stator is
detected by a sensor (not shown). Incorporated further in the LDM system
are a control circuit 5 and a driver 6, the former controls the current to
be supplied to each coil 2, responsive to the relative position of these
two elements detected by the sensor, while the latter supplies the thus
controlled current to the coils through a power feed line 7. There have
been found undesirable phenomena in such type LDM, referred to "detent
thrust" or "rippled thrust" attributable to the inherent magnetic pole
construction of the stator, and one of the measures taken heretofore to
reduce the detent thrust is such one as shown by FIG. 8, namely, permanent
magnets 4 of the stator are arrayed parallel in skewed relation with
respect to the length-wise axis of the stator 3.
It becomes possible to reduce the detent thrust caused by both the gaps or
boundaries between adjacent magnetic poles and the teeth slots, by relying
on the aforesaid skewed arraying of the stator permanent magnets, however,
other problems of detent thrust due to the end effect of the armature
core, that is, abrupt change in permeance from full value to almost zero,
or viceversa, between the end touch and the exterior of the armature core,
have not yet been solved.
These unsolved problems of the latter type are attributable to such
inherent nature of the linear motor that the armature core which moves in
itself or is movable relative to the stator has a finite length, namely,
having both ends and this is the cause of the detent thrust as mentioned
above which cannot be solved only by arraying the stator permanent magnets
in a skewed manner as explained above.
SUMMARY OF THE INVENTION
In view of the drawbacks mentioned above, the present invention aims to
provide a linear DC brushless motor which can solve the drawbacks in the
prior art linear DC brushless motor as mentioned above and further
remarkably reduce the detent thrust caused by the end effect of the
armature core and thereby able to perform smooth running even at a low
speed.
Explanation will now be made in the following paragraphs as to by what
technical concept the present invention has solved the pending problem of
detent thrust and to what extent the detent thrust could be reduced by
gradually reducing the unit volume of the core, for example, by partly
cutting out at least one tooth at each end of the armature core in the
direction of the travel of the linear motor, thereby avoiding abrupt chage
of the permeance of the armature core.
The detent thrust when no measure is taken to reduce the end effect of the
armature core is represented, as shown in FIG. 3 by a sinusoidal curve A
having a length of period equal to the pitch of the poles, that is, the
length of one permanent magnet pole (N pole or S pole).
If each of a pair of end teeth 1B of the armature core 1 is partly cut away
by transversely cutting a half of the length l as shown in FIG. 1, the
detent thrust can be reduced to such one having a half amplitude but the
same phase, as shown by the wave form B in FIG. 3, as compared with the
case where no such measures has been taken, shown by curve A of FIG. 3.
Next, the detent thrust attributable to the second tooth 1C from each end
of the armature core will be considered.
The wave form under this condition can be represented by the curve C in
FIG. 3 having a half amplitude and a shifted phase as compared with those
of curve A. Assuming the teeth pitch of the armature core is set such that
three teeth of the armature core are disposed per each pole length of the
permanent magnetic pole (N pole or S pole), the shift of the phase will be
120.degree. as shown by the dash and dot line C in FIG. 3.
First embodiment of the present invention adopted such an armature having a
pair of outermost end teeth at each end in the direction of travel
(hereafter merely referred to "end") being partly cut away, and the total
detent thrust effected by this measure is represented by the curve D in
FIG. 3, which is obtained by synthesizing two sinusoidal curves B and C of
the same period and a half amplitude of curve A but its phase being
shifted.
Consequently, if the extent of skewing at the two axial ends of the
armature core is set S equal to the length of one pole of the permanent
magnet of the stator, the detent thrust due to the end effect can be made
cancelled with each other within a polar pitch (0 to 2.pi.), so it would
become possible to reduce the detent thrust under problem.
According to the second embodiment of the present invention, outermost
tooth at each end of the armature core 1 is attached integrally with at
least one end member of right-angled triangular pillar.
In the third embodiment of the present invention, outermost tooth and
plural teeth disposed inside next to the outermost tooth at each end of
the armature core 1 are slantly cut away as if they were cut by a vertical
plane passing through the outer ridge of the outermost pole and through
the outer ridge of the transversely oppsite side of the armature core.
Similarly, in this case too, the detent thrust can be made such one as
represented by a gentle curve without having any appreciable abrupt change
in the permeance, thus ripples in the thrust can be remarkably reduced.
Reduction in the detent thrust, namely, the ripples in the thrust brought
about by reducing the volume of the axially end portion of the armature by
means of the above mentioned embodiments is generally referred to a skew
effect obtained by the modified armature configuration of the tooth or
teeth at both axial end.
As to the coils used in the present invention, coils of any particular size
or shape corresponding to the modified teeth end construction are not
required, but prefabricated ordinary toroidal coils for specific model
linear motor can be used by inserting each leg of them in the slot between
the adjacent teeth as a usual manner, this is because the volume of the
coils at the end portion of the armature core exceeding the reduced volume
of the end tooth or teeth merelely acts as idle portions which impart no
substantial disadvantage to the permeance of this type of linear motor. It
goes without saying that the coils, if it is preferred, may be made to
have a shape corresponding to the modified shape of slot or sloots between
the teeth at the end portiopn of the core.
Explanation will now be made in the following part of the specification by
referring to accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of an armature core showing an example of the
first embodiment of the present invention;
FIG. 2 is a perspective view of an armature core showing another example of
the first embodiment;
FIG. 3 is a graph showing a relationship between the pole pitch and the
detent thrust when a part of the tooth or teeth on each axial end of the
armature core have been cut away or attached with triangular pillar or
pillars;
FIG. 4 is a perspective view of an armature core showing an example of the
second embodiment of the present invention;
FIG. 5 is a perspective view of an armature core showing another example of
the second embodiment;
FIG. 6 is a perspective view of an armature core showing an example of the
third embodiment of the present invention;
FIG. 7 is a front view showing a general construction and arrangement of
the conventional linear DC brushless motor; and
FIG. 8 is a plan view showing the skewed arrangement of the permanent
magnets in the stator of the conventional linear DC brushless motor shown
in FIG. 7.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, the structural parts and portions constitute the
present invention being the same as or equal to that referred to in the
prior art shown by FIGS. 7 and 8 are shown by using the same reference
numeral(s), in order to help reader's clear understanding by way of
comparison and further the detailed explanation thereof are omitted. FIG.
1 is a perspective view showing an entire part of the armature core 1, in
which the tooth 1B at each axial end of the core 1 is formed to have its
length one half the transverse length l of the core 1 as can be clearly
seen from the drawing. The armature core 1 can be made by stacking a large
number of thin sheets of steel for electric use such as silicon steel, one
after another, and thus forming the core 1 as an integral member in a
effective manner as explained below.
This can be done in the following way, first by making steel sheets each
having n minus one (n-1) teeth (where n is the specified teeth number of
the core 1), in other words, one end of each sheet having teeth of
specified number is cut away so as to remove one tooth, then stacking the
thus prepared sheets until the stacked sheets amount to a half thickness
of the specified transverse length l of the core 1 and then by stacking
another half thickness by reversing and stacking the sheets in such a way
that the other ends of the sheets having no end tooth are placed at
opposite end.
Alternatively, the armature core of this configuration can be made by
compacting powdered iron.
FIG. 2 shows the construction according to another example of the first
embodiment of the present invention, in this case the outermost end tooth
1B at each axial end of the armature core is formed to have one third of
the transverse length l of the armature core, while the second tooth from
each axial end of the armature core is formed to have two thirds of the
transverse length l of the armature core. If other tooth or teeth adjacent
to the thus cut teeth at each end is cut away, the detent thrust of the
armature core can be further reduced.
By virtue of using the armature core 1 having a shape as shown by FIG. 1,
the detent thrust at one core end effected by the cut away tooth 1B can be
reduced to such an extent as having one half amplitude and the same phase
but having no such cut away tooth (curve B of FIG. 3). The detent thrust
at each axial end effected by the second tooth 1C from the end can be
represented by the curve C (dash and dot line in FIG. 3), that is, the
detent thrust has a wave form in FIG. 3 showing a half amplitude and
shifted phase, thus the synthesized detent thrust effected by the armature
core 1 can be shown by curve D having reduced amplitude of the prior art
core. The extent of such phase shift varies depending on the number of
teeth per one pole, for example, if three teeth are set for each pole, the
phase shift would be 120.degree..
The wave form of the detent thrust caused by the end effect will be
represented by sinusoidal curve shown in FIG. 3 having their one period
equal to the length of polar pitch.
Accordingly, if the skew at the end is set to be a polar pitch, the detent
thrust cancel with each other between 0 and 2.pi. and thereby large extent
of reduction is obtainable.
Explanation will now be made on the second embodiment of the present
invention. In FIG. 4, the member or parts the same as or equal to those
used for the conventional device are also shown by the same numeral used
in FIGS. 7 and 8 and the explanation thereof is also omitted. FIG. 4 is a
perspective view showing an entire part of the armature core 1 according
to the second embodiment of the present invention. In FIG. 4, each of the
end pillar members A or B has right angled triangular top and bottom faces
each having its width and height the same as that, respectively, of the
armature core 1 and is made of material for magnetic use such as
structural carbon steel of JIS Standards SS 41 and it is attached to the
end teeth 1B at right or left end of the armature core 1 by means of a
bonding agent or the like.
The top ridges of the thus attached end pillar members are directed
opposite with respect to its lengthwise axis, that is the direction of the
top ridge of the pillar A at the front lower part is directed rightward,
while that of the top ridge of the pillar B at the rear upper part is
directed leftward.
As a way of making, the core can be readily produced by stacking a large
number of preformed thin sheets of material such as silicon steel one on
another into a laminated structure and by attaching an end member A or B
on the outer face of the thus prepared respective end tooth 1B.
Instead of using laminated thin silicon steel sheets, the armature core of
this type also can be formed by compaction.
By virtue of the armature core 1 formed into such a configuration as
mentioned above, the core 1 as a whole can impose a skew effect to reduce
the detent thrust otherwise generates in the armature core itself.
Detent thrust reduced by the core end construction in accordance with this
embodiment can be represented by sinusoidal curves having its period equal
to a polar pitch as shown in the FIG. 3.
Accordingly, if the extent of skew in the direction of the travel of the
armature core is selected as a polar pitch, the detent thrust due to the
end effect can be cancelled within the polar pitch of 0 to 2.pi. thereby
large extent of meritorious effect for reducing the detent thrust can be
attained.
FIG. 5 shows another example of the second embodiment of the present
invention, wherein the end face of the end tooth 1B at each axial end is
attached with two right-angled triangular pillars of equal shape and size.
The top and bottom faces of right-angled triangular shape have two sides
forming the right-angle, having a length of 1/2 of the transverse width l
of the armature core 1, thereby two rectangular side faces of equal size
are formed in one pillar, and one side face out of the two rectangular
side faces are selected to be placed on the end face of the end tooth and
one rectangular side face of the other triangular pillar is similarly
placed on the end face of the end tooth such that the two pillars are
placed side by side on the end face of the end tooth 1B in a manner that
their top ridges are placed being opposed head to head with each other.
FIG. 6 is a perspective view showing a preferred form of the entire part of
the armature core 1 according to the third embodiment of the present
invention. Outermost end tooth 1B, the next tooth 1C and the further next
tooth 1D at each axial end portion of the armature core 1 are shown,
having their respective cut end as if they have been slantly cut by a
plane passing through the transversely outermost ridge G of the end tooth
1B and the ridge F of the fourth tooth 1E, in more detail, the outermost
ridge G of the fourth tooth for forming the cut teeth at the lower front
side is shown at right side of FIG. 6, but the ridge G forming the cut
teeth at the rear upper side is shown at the left upper part of FIG. 6.
In a similar manner as explained in the preceding embodiment, the armature
core 1 including the slantly cut teeth at the ends can be made by stacking
a large number of thin sheets of silicon steel into a laminated state or
by compacting powdered iron instead of silicon steel sheets. By virtue of
the armature core 1 constructed to have such a configuration as explained
above, the entire core member can impose a skew effect on the set of
permanent magnets 4, thereby if the extent of skew is set, for example,
equal to the pitch of the poles, the detent thrusts caused by the end
effect can be largely reduced since they are cancelled each other within a
range of pole pitch (0 to 2.pi.).
As explained above, the detent thrust encountered in the armature core of
the linear DC brushless motor can be greatly reduced by applying very
simple structural modification to the end portion of the core in the
following manner,
(1) either cutting out only the outermost end tooth at each axial end, or
cutting out other teeth such as second, third or further teeth in the end
part of the core,
(2) one or more rightangled triangular pillars are attached on the end
tooth on each axial end of the core by bonding or,
(3) the outermost end tooth and a plurality of teeth adjacent to the end
tooth at both axial end are slantly cut by a plane slantly extending from
one lengthwise side of the core to the other side of the core.
Thus the present invention can provide remarkable meritorious effects to
the field of linear DC brushless motor.
While the invention has been particularly shown and described in reference
to preferred embodiments thereof, it will be understood by those skilled
in the art that changes in form and details may be made therein without
departing from the spirit and scope of the invention.
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