Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2002-05-14
2003-06-24
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S012060, C318S135000, C361S031000
Reexamination Certificate
active
06583527
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a linear motor having a structure in which permanent magnets and a coil are subjected to relative movement, and an apparatus and a method for protecting such a linear motor.
BACKGROUND OF THE INVENTION
Movable coil-type linear motors have conventionally been widely used as driving means for positioning articles in stroke ranges of about 10-100 cm (for instance, Japanese Patent Publication No. 58-49100 and Japanese Utility Model Laid-Open No. 63-93783). The movable coil-type linear motor comprises a plurality of permanent magnets magnetized in their thickness directions and arranged such that their different magnetic poles are opposing each other, and a movable coil assembly moving in a magnetic gap defined between the opposing permanent magnets (or between permanent magnets and a yoke) in perpendicular to a magnetic flux.
Such a linear motor is free from a center yoke in a magnetic circuit portion, and comprises a plurality of closed-loop magnetic fluxes in the magnetic gap, so that a magnetic flux is not concentrated in part of the magnetic path. Accordingly, this linear motor can generate a uniform magnetic flux density in an overall range of a long stroke.
FIG. 5
schematically shows one example of a mechanical portion of the linear motor, in which permanent magnets and a coil are subjected to relative movement. This linear motor comprises a pair of flat plate-shaped yokes
1
,
1
made of ferromagnetic materials such as soft iron, a pair of permanent magnet rows
102
,
102
constituted by a plurality of permanent magnets
2
magnetized in their thickness directions, and attached to the inner surface of the flat plate-shaped yokes
1
,
1
respectively, such that they are opposing via a magnetic gap
3
, and supports
4
,
4
attached to both ends of a pair of yokes
1
,
1
to provide the magnetic gap
3
. In each permanent magnet row
102
, the permanent magnets
2
are arranged on each yoke
1
,
1
in a longitudinal direction, such that N poles and S poles alternately appear on the surfaces of the permanent magnets
2
, and that different magnetic poles appear on the surfaces of the permanent magnets
2
opposing via the magnetic gap
3
. A plurality of closed-loop magnetic circuits are constituted by a pair of yokes
1
,
1
and a pair of opposing permanent magnets
2
,
2
(refer to FIG.
5
). Incidentally, the supports
4
are preferably formed by the same ferromagnetic materials as those of the yokes
1
.
FIG. 6
schematically shows another example of the linear motor. This linear motor comprises a plurality of permanent magnets
2
arranged on one yoke
1
to constitute magnetic circuits, instead of having a structure that the different poles of a plurality of permanent magnets
2
are opposing via the magnetic gap
3
.
FIGS. 5 and 6
respectively show movable coil-type, linear motors, both of which are essentially the same except for difference in that the number of the combination of a permanent magnet row
102
and a yoke
1
is two or one. Because the linear motor of
FIG. 5
has a larger magnetic flux density at the same current level, it provides a larger thrust.
The coil
5
is constituted by flat multi-phase coils with the winding direction of the coil
5
in perpendicular to a magnetic flux direction in a magnetic gap
3
. A plurality of coils
5
(only one coil is shown for simplicity) are arranged longitudinally along the permanent magnet row
102
, and the directions of their magnetic poles are detected to switch a coil to which current is supplied and the direction of current by a means such as a magnetic field detecting element, etc.
In the movable coil-type linear motor, a plurality of coils
5
are arranged in a stroke direction to generate a large thrust. A plurality of coils
5
are integrally fixed to a non-magnetic holder (not shown) to constitute a mover. The mover is movably supported by a sliding member (not shown) in a longitudinal direction of a permanent magnet row
102
, and the holder is integrally fixed to a table (carriage) on which an article is placed. The holder is made of non-magnetic materials such as resins, aluminum, ceramics, etc. so as not to provide magnetic influence on a closed-loop magnetic circuit. To make the magnetic gap
3
as small as possible, the holder is preferably as thin as possible.
In order that the coil
5
receives thrust to move in a constant direction, the direction of current should be changed successively according to the polarity of the opposing permanent magnets
2
. When the coil
5
faces a boundary of the adjacent permanent magnets
2
,
2
, there is no thrust of movement. Current supply is thus stopped to the coil
5
. The holder is equipped with one element (usually Hall element) for detecting the polarity of the permanent magnets
2
per one phase of the coil. Accordingly, in the case of the three-phase coil, the holder is equipped with three detecting elements.
An arranging pitch is not the same for the coil
5
and the permanent magnets
2
. If both were the same, there would appear a moment in which the combined force of the thrust of each coil
5
is zero, resulting in a large thrust ripple, which leads to the cogging of the coil
5
. With the arranging pitch of the coil
5
and the permanent magnets
2
deviated form each other, the coil
5
undergoes smooth movement with a reduced thrust ripple. In that case, electric current should be supplied successively to part of a plurality of coils
5
. The timing of ON/OFF to supply electric current to each coil
5
and the direction of electric current are determined depending on the output of the hole element.
When current is supplied to the coil
5
, it receives thrust in the longitudinal direction of the yoke
1
by the Fleming's rule, a mover (not shown) integrally provided with the coil
5
moves in the longitudinal direction of the yoke
1
. When current is supplied to the coil
5
in an opposite direction, thrust in an opposite direction acts on coil
5
, resulting in moving the mover in an opposite direction. Accordingly, current supply to the coil
5
and the direction of that current can be chosen to move the mover to the predetermined position. The strength of this thrust is proportional to current flowing through the coil
5
.
Contrary to the above embodiment, the coil may be a stator, and the permanent magnets may be movers to achieve the same function. Though the coil
5
is disposed in the magnetic gap
3
in the above embodiment, as shown in
FIG. 6
, a linear motor may be free from a magnetic gap
3
. In such a linear motor, the coil
5
is movable on permanent magnets
2
disposed on a yoke
1
.
When current is caused to flow through the coil
5
, Joule heat is generated, and when the temperature of the coil
5
is elevated to a level exceeding its heat resistance limit, the coil
5
is burned down. As a method preventing this problem, for instance, Japanese Patent Laid-Open No. 4-67763 proposes the mounting of a temperature-detecting element such as a thermistor, etc. to a coil. However, though a surface portion of the coil is well cooled by air-flowing effect, etc., heat is likely to be accumulated inside the coil, resulting in the tendency that the temperature of the coil is higher in an inner portion than in a surface portion. Because a temperature-detecting element such as a thermistor, etc. is mounted onto the coil surface, it is impossible to detect the temperature inside the coil. Accordingly, it has been found that the method disclosed in Japanese Patent Laid-Open No. 4-67763 fails to accurately detect the temperature at which the coil
5
is burned down. In addition, it suffers from the problem that the mounting of a thermistor, etc. onto the coil surface makes wiring, etc. more difficult.
To avoid such problems, there is a known method for generating a coil overheat signal when the product of I×T, in which I is a level of current flowing through a coil, and T is time in which the current flows, exceeds a predetermined value. As one example thereof, Jap
Hashimoto Hiroshi
Takedomi Seiki
Hitachi Metals Ltd.
Morgan & Lewis & Bockius, LLP
Mullins Burton S.
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