Linear motor

Details Linear motor Linear motor

2003-03-12

2004-09-07

Mullins, Burton (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Linear

C310S154010, C310S266000

Reexamination Certificate

active

06787945

ABSTRACT:

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
34
a
and
34
b
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
38
a
and
38
b
which stand vertically at the opposite ends of the bottom member
36
. The machine tool
30
is still further equipped with two linear guides
40
a
and
40
b
on the tops of the side members
38
a
and
38
b
, respectively. The above-mentioned guides
34
a
and
34
b
, which are fired to the lower side of the table
32
, are slidably mounted on the liner guides
40
a
and
40
b
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
42
a
and
42
b
. 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
34
a
and
40
a
and also between the guide rails
34
b
and
40
b
are large to a considerable extent, and thus, such a large friction may result in decrease in the life time of the guides
34
a
-
34
b
and
40
a
-
40
b
. 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 tim

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