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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a linear motor, and more
specifically to a permanent magnet type linear motor for use in a machine
tool or the like.
2. Description of Relate Art
Prior to turning to the present invention, it would be preferable to
describe, with reference to FIGS. 6(A) to 6(C) and FIGS. 7 and 8, a
conventional linear motor which is pertinent to the present invention.
FIG. 6(A) is a side elevation of a linear motor (generally denoted by
reference number 8), and FIG. 6(8) is a cross section taken along a
section line A-B of FIG. 6(A).
As shown FIG. 6(A), the linear motor 8 generally comprises a stator 10 and
a movable body 12. The movable body 12 is positioned apart from the stator
10 leaving an air gap therebetween so as to be movable with respect
thereto. The stator 10 is comprised of a plurality of permanent magnets
14, each of which is rectangular in this case and has a lengthwise axis in
the direction normal to the drawing of FIG. 6(A). The magnets 14 are arms
in an equally spaced manner on a plate-like iron base (support) 16. The
polarities of the magnets 14 are such as to change alternately in the
moving direction of the movable body 12 (viz., in the horizontal direction
in the drawing). Each of the arrows attached to the permanent magnets 14
indicates the magnetized direction of the corresponding magnet. On the
other hand, the moving body 12 comprises an armature core 18 of magnetic
material and a plurality of armature coils 20 respectively wound around
the teeth (or leg-like projections) 22 of the core 18.
The armature coils 20 are concentratedly wound around the teeth 22 of the
core 18, respectively, and coupled so as to take the form of balanced
three-phase circuit, wherein the three phases are respectively denoted by
U, V, and W. The balanced three-phase circuit or connection per se is well
known in the art, and thus the further description will be omitted for
simplifying the instant disclosure.
In the linear motor 8 shown in FIG. 6(A), the eight permanent magnets 14
are arranged such as to oppose nine teeth 22 of the armature core 18.
Arrows shown in FIG. 6(B) schematically indicate the winding directions of
the armature coils 20 provided around the teeth 22. The nine coils 20
produce eight magnetic fields between the adjacent teeth. As is known in
the art, when the phases of the currents flowing through the coils 20 are
controlled, the moving body 12, which is held by a suitable support (not
shot), moves linearly above the stator 10.
The number of the teeth 22 and the number of the permanent magnets 14
opposing them, are not limited to the above-mentioned ones. By way of
example, FIG. 6(C) shown one example wherein the nine teeth 22 of the
armature core 18 are arranged such as to face the six permanent magnets
14, in the case of which the coils 20 carried by the nine teeth 22 are
supplied with the U-, V- and W-phase currents so as to generate six
magnetic fields at the side of the magnets 14.
FIGS. 7(A) and 7(B) schematically illustrate the manner wherein the linear
motor 8 of FIG. 6(A) is installed in a machine tool 30 (only part thereof
is illustrated). FIG. 7(A) is a front elevation of the linear motor 8
together with the part of the machine tool 30 as viewed in the direction
of the movement of the movable body 12. FIG. 7(B) is a schematic side
elevation of the linear motor 8 and part of the stator 10 of FIG. 7(A) as
viewed in the direction perpendicular to the movement of the movable body
12.
As shown in FIG. 7(A), the movable body 12 is fixed to the lower side of a
table 32 which is provided with linear guides 34a and 34b extending in the
direction of the movement of the movable body 12. The stator 10 has been
mounted on a plate-like bottom member 36 of the machine tool 30. The
machine tool 30 is further equipped with two side members 38a and 38b
which stand vertically at the opposite ends of the bottom member 36. The
machine tool 30 is still further equipped with two linear guides 40a and
40b on the tops of the side members 38a and 38b, respectively. The
above-mentioned guides 34a and 34b, which are fired to the lower side of
the table 32, are slidably mounted on the liner guides 40a and 40b when
the movable body 12 has been assembled with the stator 10.
At the final stage of assembly of the linear motor 8 as shown in FIGS. 7(A)
and 7(B), the table 32 accompanying the movable body 12 is lowered as
shown by arrows 42a and 42b. Thus, the armature core 22 of magnetic
material approaches the permanent magnets 14 with the result in occurrence
of an extremely large amount of magnetic attraction force imparted on the
magnetic core 18 as indicated by a broad open arrow 44. Accordingly, in
order to precisely position the movable body 12 on the stator 10 against
such a large magnetic force, it is inevitable to prepare a jig (typically
rigid and bulk which is dedicated to the assembly itself. However, it is
practically difficult to settle such jig on or in the vicinity of the
machine tool 30 due to a limited working space. In the case where a
suitable jig is unable to prepare for the assembly of the linear motor 8
FIG. 6(A)), the above-mention prior art has encountered the problem that
it needs a fairly long time until completing the assembly of the linear
motor 8, in addition to which it is not typically expected to precisely
mount the movable body 12 onto the stator 10, leading to the fact that the
designed performances or characteristics of the machine tool 30 is not
expected.
FIGS. 8(A) and 8(B), which respectively correspond to FIGS. 7(A) and 7(B),
schematically illustrate that the linear motor a has been assembled on the
machine tool 30. When the linear motor 8 has been assembled, the armature
core 18 is in close proximity to the permanent magnets 14, and as such,
the magnetic attraction force imparted on the core 18 is very large, which
is approximately several times the nominal (rated) driving force of the
linear motor 8 (for example). As a result, the frictions between the guide
rails 34a and 40a and also between the guide rails 34b and 40b are large
to a considerable extent, and thus, such a large friction may result in
decrease in the life time of the guides 34a-34b and 40a-40b. In order to
overcome this problem, it is conceivable to increase the contact area
between the linear guides, which, however, may arise another difficulties
that the movable portion undesirably increases in weight and thus the
acceleration of the movable body 12 is lowered.
One approach to overcoming the above-mentioned problems is disclosed in
Japanese Laid-open Patent Application No. 10-257750, according to which a
two opposing stators are provided between which a movable body is arranged
to linearly move. In this case, the two magnetic attraction forces exerted
on the two stators are cancelled, and thus, it is possible to reduce the
friction between the liner guides of the stators and the movable body.
However, the prior art disclosed in Japanese laid-open Patent Application
is still encountered the problem that it is not easy to assemble the bear
mortar. That is to say, according to this prior art, the two stators are
firstly provided on the lower (bottom) frame of a machine tool as in the
first prior art (FIG. 2(A)), after which the movable body, which has been
fixed on the lower surface of a table, is lowered toward the space between
the two stators. As mentioned above, the magnetic attraction force is very
large and thus it is absolutely necessary to prepare a mechanism dedicated
to the linear motor assembly as in the first prior art, which results in
the same difficulty with the first prior art.
Therefore, in case the linear motor is assembled without use of such an
assembly-assisting mechanism, it needs a quite long time until finishing
the assembly on the machine tool due to strong magnetic attractions, and
in this instance, it is usually not expected to precisely assemble the
linear motor. Accordingly, it is often the case that the movable body is
liable to be misaligned, resulting in the fact that the air gap between
the stator and the opposing magnets is unable to be uniformly maintained.
As a result, it is high liable to under induce cogging forces which lead
to uneven driving forces.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a linear
motor which can be accurately assembled onto a machine tool in a short
time period.
Another object of the present invention is to provide a linear motor which,
once installed on a machine tool is able to considerably reduce the
frictions between the engaged linear guides and also able to reduce
cogging forces to a considerable extent.
One aspect of the present invention resides in a linear motor comprising; a
stator having a magnet support which extends in parallel with the
lengthwise direction of the linear motor and whose cross section
perpendicular to the lengthwise direction of the linear motor is polygon
or circular, the stator further having a plurality of permanent magnet
arrays mounted on the outer surface of the magnet support in parallel with
the lengthwise direction of the linear motor, the suitor being held at
both ends thereof by stator supports; and a movable body having a hollow
member extending in parallel with the length direction of the linear motor
and surrounding part of the stator, and having a plurality of armature
modules each of which comprises an armature core and armature coils
mounted thereon, the plurality of armature modules being mounted on the
inner surfaces of the hollow member such that the lengthwise axes thereof
are in parallel with the lengthwise direction of the linear motor and such
that the armature modules respectively face the permanent magnet arrays
with an air gap therebetween.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will become more
clearly appreciated from the following description taken in conjunction
with the accompanying drawing in which like elements or portions are
denoted by like reference numerals and in which:
FIG. 1(A) is a sectional view of a linear motor according to an embodiment
of the present invention, which linear motor is installed in a machine
tool;
FIG. 1(B) is a sectional view t along section line C-D of FIG. 1(A);
FIG. 2 is a diagram schematically showing the linear motor of FIGS. 1(A)
and 1(B) installed in a machine tool;
FIG. 3 is a dial schematically showing displacements of magnetic cores and
magnetic array of FIGS. 1(A) and 1(B);
FIG. 4 is a graph showing the waves of cogging forces (torques) of four
armature modules together with the wave of a combined cogging force when
displacing the magnetic cores and the magnetic arrays as shown in FIG. 3;
FIG. 5 illustrates schematically a variant of the embodiment shown in FIGS.
1(A) to 4;
FIGS. 6(A) to 6(C) are each being a sectional view schematically showing an
armature core and a magnetic array for describing prior art which is
pertinent to the present invention;
FIG. 7(A) is a sectional view schematically showing a conventional linear
motor which is lowered so as to be installed in a machine tool;
FIG. 7(B) is a section view of the linear motor shown in FIG. 7(A);
FIG. 8(A) is a sectional view schematically showing the conventional linear
motor of FIG. 7(A) which has been installed in the machine tool; and
FIG. 8(B) is a sectional view of the linear motor shown in FIG. 8(A).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be described with
reference to FIGS. 1(A) to 5. In the following, the portions or members,
which are identical to or correspond to those of the prior art referred to
in the opening are denoted by like reference numerals. Further, all like
elements or portions may not be denoted by reference numerals for the sake
of simplifying the draw.
FIG. 1(A) is a sectional view schematically showing a linear motor
(generally denoted by 48) according to a preferred embodiment of the
present invention, together with part of the machine tool 30 referred to
in FIG. 7(A), as viewed from the direction along which a movable body 50
is linearly driven (viz., as viewed from the lengthwise direction of the
linear motor 48). The linear motor 48 shown in FIG. 1(A) has been already
installed in the machine tool 30. FIG. 1(B) is a sectional view of the
linear motor 48 and part of the machine tool 30 taken along section line
C-D of FIG. 1(A).
As shown in FIGS. 1(A) and 1(B), as stator 52 is comprised of a pipe 54
whose cross section perpendicular to the lengthwise direction of the
linear motor 48 is substantially square in this particular case. The
stator 52 further comprises a plurality of permanent magnet arrays 56
mounted on the four outer surfaces of the pipe (viz., hollow square
member) 54. Each magnet array 56 is comprised of a plurality of permanent
magnets 57 (see FIG. 1(B) which are equally spaced with each other, and
the magnetized directions of the magnets 57 change alternately in the
direction of driving the movable body 50 as in the prior art. Still
further, the stator 52 is provided with two pairs of linear guides 58 on
the upper and lower outer surfaces of the square pipe 54. The stator 52 is
firmly held at the both ends thereof by means of suitable supports 60 (see
FIG. 1(B)).
The movable body 50 comprises a square pipe (viz., hollow square member) 62
which is fit attached to the lower surface of the table 32. The square
pipe 62 is equipped with four armature cores 64, each of which is made of
magnetic material and fixed to the inner walls of the tube 62 and which
respectively face the corresponding magnet arrays 56 leaving an air gap
therebetween. Coils 65 are respectively wound around the corresponding
teeth 67 of each of the four armature cores 64 as in the prior art. In
addition, the movable body 50 comprises two pars of linear guides 66,
which at respectively fixed to the upper and lower inner walls of the
square pipe 62 and which are slidably with the corresponding linear guides
58, as schematically illustrated in FIG. 1(A).
A combination of one armature core 64 and the coils 64 provided thereon
will be referred to as an armature module (or simply "module") for the
sake of convenience of description, and the four modules in total are
respectively denoted by No. 1 to No. 4. The modules No. 1 to No. 4 are
identical in configuration with each other.
The directions of the magnetic attraction forces, which are imparted on the
modules No. 1 to No. 4, are indicated by four arrows 70 in FIG. 1(A). It
is understood that the magnetic attraction forces in the vertical
direction (in the figure) are respectively canceled, and in the same
manner, those in the horizontal direction are also canceled. Accordingly,
the forces exerted on the linear guides 58 provided on the upper and lower
surfaces of the square pipe 54 can be reduced to a considerable extent,
which implies that it is not necessary to strongly build the guides 58. In
other words, especially, the upper linear guides 54 are sufficient in
terms of rigidity if they can support the weights of the table 32, the
movable body 50, the guides 34a and 34b, etc. That is to say, it is no
longer necessary to consider the influences of the permanent magnetic
forces as in the prior art. In addition, the same discussion is applicable
to the guides 66, 34a-34b, and 40a-40b.
Reference is made to FIG. 2, which corresponds to FIG. 1(A) and
schematically illustrates a manner of assembling the liner motor 48 onto
the machine tool 30.
Before the linear motor 48 is installed onto the mine tool 30 as shown in
FIG. 2, the modules No. 1 to No. 4 are mounted on the inner walls of the
square pipe 62 of the movable body 50. The table 32 is attached to the
spare pipe 62 after or before the above-mentioned mounting of the modules
No. 1 to No. 4. It is to be noted that the mounting of the modules No. 1
to No. 4 on the inner walls of the tube 62 can be done without any
difficulty in that no magnetic fields exit at this working step.
Thereafter, the stator 52 is seared into the space which is defined by the
modules No. 1 to No. 4. In this case, as mentioned above, the magnetic
attraction forces imparted on the opposite magnetic cores 64 of the
modules No. 1 to No. 4 are effectively cancelled, and accordingly it is
not difficult to insert the stator 52 into the predetermined space within
the square tube 62. Subsequently, the supports 60 are fixed to the both
ends of the stator 52. The above-mentioned assembly of the linear motor 48
can be implemented at an ample space away from the machine tool 30 or an
appropriate workshop, and hence, it is possible to address the aforesaid
problem of preparing a rigid assemble gear dedicated to the assembly and
settling the same on a limited space on the machine tool 30.
As shown in FIG. 2, the linear motor 48, which is attached to the lower
surface of the table 32, is lowered toward the machine tool 30 using an
appropriate instrument such as a crane or the like. In this instance, the
magnetic forces may exist between the linear motor 48 and each of the
vertical frames 38a and 38b of the machine tool 30. However, the magnetic
attraction forces exerted on the vertical frames 38a and 38b are opposite
in direction and am small relative to the those indicated by arrows 70,
and accordingly, the linear motor 48 can precisely be assembled to the
machine tool 30 without difficulty. When the linear motor 48 is settled on
the machine tool 30 by engine the linear guides 34a and 34b with the
counterparts 40a and 40b, the supports 60 we then fixed to the bottom
frame 36 of the machine tool 30 as shown in FIG. 1(B).
The embodiment will further be described so as to discuss, with reference
to FIGS. 3 and 4, another feature thereof via which the cogging torques
(forces) can effectively be reduced with the structure mentioned above.
FIG. 3 is a drawing schematically showing relative positions of the four
armature cores 64 and also relative positions of the permanent magnet
arrays 56 so as to reduce the cogging forces of the linear motor as
mentioned later. As shown in FIG. 3, it is assumed that the linear motor
is confided such that the eight permanent magnets face the nine teeth 67
of the armature core 64 as in the case of FIG. 6(A). Designating the pitch
of the permanent magnets 57 by .tau., each of the armature cores 64 is
displaced by .tau./4 with respect to the adjacent ones in the direction of
driving the movable body 50. In a similar manner, each of the magnet
arrays 56 is displaced by .tau./4 with respect to the adjacent magnet
arrays in the direction of driving the movable body 50.
The displacement of the armature cores 64 and the magnetic arrays 56 is not
limited to .tau./4, and in general, it is sufficient if the displacement
is a natural number multiple of .tau./4.
In the above, it can be said that each of the modules No. 1 to No. 4
instead of the armature cores is displaced with each other.
FIG. 4 is a diagram which schematically shows the waves of four cogging
forces generated at the four modules No. 1 to No. 4 together with the wave
of the combined cogging force in the case where the armature cores 64 and
the magnetic arrays 56 are displaced as shown in FIG. 3. In this case, it
is assumed that a pitch .tau. of the permanent magnets of each magnet
arrays is 24 mm. As shown in FIG. 4, although each of the cogging forces
is as large as about 32N, the cogging force are canceled with each other,
resulting in the fact that the combined cogging force can be lowered as
low as approximately 2N.
In the above-mentioned embodiment, the stator 52 is provided with the
square tube 54 on which the four magnet arrays 56 are mounted such as to
face the four modules No. 1 to No. 4 of the movable body 50. That is, the
linear motor 48 is provided with the four pairs or combinations of the
magnet arrays and the modules. However, it is within the scope of the
present invention to provide more than four pairs of the magnet arrays and
the modules. By way of example, FIG. 5 illustrates one variation of the
aforesaid embodiment wherein six pairs of the magnet arrays and the
modules (denoted by No. 1 to No. 6) are provided. In FIG. 5, each of the
elements or portions corresponding to those of the aforesaid embodiment is
denoted by like reference numeral plus a prime. As shown in FIG. 5, a
stator 52' comprises a frame member 54' which corresponds to the square
tube 54. The member 54' has eight surfaces among which the six surfaces
are used to mount thereon the permanent magnets 56' and the two surfaces
are utilized to provide two linear guides 58'. On the other hand, a
movable body 50' is comprised of a frame 62' whose inner surface is
configured so as to mount thereon the six modules No. 1 to No. 6 and two
linear guides 66'. It is understood that if more than four pairs of magnet
arrays and modules should be provided in a linear motor, the cross section
of the frame member 54' for mounting the magnet arrays thereon may be
polygon. As an alterative, the frame member 54' is able to take the form
of the hollow circular member on the condition that the magnetic arrays
are shaped so as to be mounted on the cylindrical surface.
The foregoing descriptions show one preferred embodiment and one
modification thereof. However, other various modifications are apparent to
those skilled in the art without departing from the scope of the present
invention which is only limited by the appended claims. Therefore, the
embodiment and modification shown and described are only illustrated, not
restrictive.
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