Coherent light generators – Particular active media – Gas
Reexamination Certificate
2001-02-20
2003-02-11
Paladini, Albert W. (Department: 2827)
Coherent light generators
Particular active media
Gas
C074S574300, C361S144000, C414S749200
Reexamination Certificate
active
06519273
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a radial magnetic bearing for freely rotatably levitating a rotation shaft having a circulation fan disposed inside a container having a corrosive gas sealed-in, such as excimer laser apparatus, and a circulation fan apparatus provided with the radial magnetic bearing.
2. Description of the Related Art
Magnetic bearing, being different from contact type bearing such as sliding bearing or ball bearing, supports a rotor non-contactingly, thereby offering advantages such as: 1) mechanical loss is low; 2) friction and wear are non-existent; 3) lubricating oil is not required; 4) low vibration and noise; and 5) maintenance free. Some examples of application of magnetic bearing having such features include turbomolecular pumps used for generating a vacuum environment that contains little impurities and spindles for highspeed machining tools.
If the magnetic bearing is to be used in an environment that is extremely averse to impurities or corrosive environment, problems arise from emission of gaseous substances from materials of the magnetic bearing such as magnetic strips, copper coils and organic materials, for example, and from corrosion of these materials. For this reason, a protective coating is applied to the magnetic bearing so as to protect the materials of the magnetic bearing from the corrosive environment. An example of using magnetic bearings for freely rotatably levitating the rotation shaft of a circulation fan is an excimer laser apparatus.
FIG. 6
shows a cross sectional view of a conventional excimer laser apparatus, and
FIG. 7
is an enlarged view of a key section of FIG.
6
. In the conventional excimer laser apparatus, as shown in
FIG. 6
, a laser vessel
10
that seals in a laser gas such as a halogen group gas, is provided with: a pre-ionizing electrode (not shown) for pre-ionizing the laser gas; and at least a pair of main discharge electrodes
12
,
12
to obtain an electric discharge to enable oscillation of laser light. Further, inside the laser vessel
10
is provided a circulation fan
14
for producing a highspeed flow of the laser gas between the main discharge electrodes
12
,
12
.
The circulation fan
14
has a rotation shaft
16
passing through the laser vessel
10
and extending between both end sections of the laser vessel
10
. The rotation shaft
16
is freely rotatably supported by magnetic bearings
20
,
22
and an axial magnetic bearing
24
non-contactingly, which are placed at each end portions of the laser vessel
10
. Also, a motor
26
is provided on the axial-end side of the radial magnetic bearing
20
at one-end.
A displacement sensor
20
a
and an electromagnet
20
b
of one-end radial magnetic bearing
20
and the stator
26
a
of the motor
26
are housed in the motor housing
28
, and their inside surfaces are protected by a thin walled cylindrical isolation wall
30
made of a material that is resistant to corrosion against halogen group gases contained in the laser gas, for example, austenite type stainless steels such as SUS316L and the like. Accordingly, the displacement sensor
20
a
, electromagnet
20
b
and the stator
26
a
of the motor
26
are prevented from coming into contact with the laser gas. A displacement sensor
22
a
of the radial magnetic bearing
22
and the electromagnet
22
b
at the opposing-end are similarly constructed, and are housed inside the bearing housing
32
, and their inner surfaces are protected by an isolation wall
34
.
Displacement sensor targets
20
c
,
22
c
and electromagnet targets
20
d
,
22
d
of the radial magnetic bearings
20
,
22
, and the rotor
26
b
of the motor
26
are affixed to the rotation shaft
16
, and are disposed so as to oppose the respective displacement sensors
20
a
,
22
a
and electromagnets
20
b
,
22
b
of the radial magnetic bearings
20
,
22
, and the stator
26
a
of the motor
26
. The displacement sensor targets
20
c
,
22
c
, and electromagnet targets
20
d
,
22
d
for the radial magnetic bearings
20
,
22
, and the rotor
26
b
of the motor
26
affixed to the rotation shaft
16
are installed inside the sealed container communicating with the laser vessel
10
. Therefore, they are required to be resistant to corrosion by the laser gas and not contaminate the laser gas.
Therefore, the displacement sensor targets
20
c
,
22
c
and electromagnet targets
20
d
,
22
d
are generally made by applying a Ni plating on the surface of a laminated steel plate or cladding the surface with stainless steel, or using a single piece ferromagnetic material resistant to corrosion by the laser gas, for example, Permalloy (Fe—Ni alloy containing 35-80% Ni). Also, because the rotor
26
b
of the motor
26
is made of a composite of laminated steel plate and aluminum alloys or a permanent magnet, Ni-plating does not adhere tightly and uniformly to the surface, and for this reason, contact with the laser gas is prevented by creating a sealed space on its surface produced by installing the isolation wall
36
made of a stainless steel.
However, in the conventional radial magnetic bearings, if the electromagnet target is made of a structure produced by surface treatment such as Ni plating on laminated steel sheets, Ni plating does not adhere tightly to the laminated steel sheets, so that there is a possibility that the plating can peel off to expose the laminated steel to corrosion. Furthermore, because of the lamination structure, the surface area is large and gases can be trapped on the surface to cause potential contamination of the laser gas.
Also, when a structure made of stainless steel cladding is used, because the distance between the electromagnet and the electromagnet target of the radial magnetic bearing must be increased by an amount equal to the sheet thickness of the isolation wall, the size of the electromagnets tends to increase.
Further, when a structure made of a single piece ferromagnetic material resistant to corrosion is used for the radial magnetic bearing
20
, as shown in
FIG. 7
, eddy current E is generated in the interior of the electromagnet target
20
d
due to variations in the magnetic fields introduced by the rotation of the rotation shaft
16
, and the magnetic flux M generated by the electromagnet
20
b
is reduced by the eddy current E in the electromagnet target
20
d
so that the magnetic strength is lowered. Especially, the eddy current E increases in proportion to the square of the speed of magnetic field change so that as the rotational speed of the rotation shaft
16
increases, drop in the magnetic strength becomes noticeable. The same phenomenon occurs at the opposing-end radial magnetic bearing
22
.
SUMMARY OF THE INVENTION
The present invention is performed in view of the background presented above, and it is an object of the present invention to provide a magnetic bearing that does not generate gas contamination and has good corrosion resistance, and enables to rotatably support a levitated body without contact while generating a magnetic force of appropriate strength, and a circulation fan apparatus equipped with the magnetic bearing.
According to an aspect of the present invention, there is provided a magnetic bearing having an electromagnet for supporting a levitated body, a displacement sensor for detecting a levitated position of the levitated body, and a controller for supplying signals and excitation currents to the displacement sensor and the electromagnet through cables; wherein an electromagnet target of the magnetic bearing that generates variations in magnetic field due to rotation of the levitated body, is comprised of a single piece of ferromagnetic material, and is provided with an electrical insulation structure oriented parallel to magnetic fluxes generated by the electromagnet.
According to the above magnetic bearing, because the electromagnet target is comprised of a single piece ferromagnetic material, the surface area of the electromagnet target is less compared with a similar electromagnet target made by laminat
Aiyoshizawa Shun-ichi
Barada Toshimitsu
Ooyama Atsushi
Sekiguchi Shinichi
Shinozaki Hiroyuki
Armstrong Westerman & Hattori, LLP
Ebara Corporation
Paladini Albert W.
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