Coherent light generators – Particular active media – Gas
Reexamination Certificate
2001-09-19
2004-11-02
Gagliardi, Albert J. (Department: 2878)
Coherent light generators
Particular active media
Gas
C372S057000, C372S037000
Reexamination Certificate
active
06813301
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a structure of a reflux fan for an excimer laser apparatus. More specifically, the present invention relates to a structure that supports and rotates a rotary shaft of a reflux fan for an excimer laser apparatus.
2. Description of the Background Art
A reflux fan for laser gas circulation in an excimer laser apparatus must have low vibration characteristic and durability. To meet such a demand, a magnetic bearing that realizes maintenance-free, non-contact support has been proposed for a bearing to be used in the reflux fan.
For example, U.S. Pat. No. 5,848,089 and Japanese Patent Laying-Open No. 11-303793 disclose examples of use of the magnetic bearing. In these references, a structure is disclosed in which a rotary shaft is supported in a non-contact manner by two radial magnetic bearings and an axial magnetic bearing consisting of an axial electromagnet, and a motor rotor fixed on the rotary shaft is driven to rotate by a motor stator on a stator side.
FIG. 41
is a cross sectional view showing a basic structure of a fan circulating excimer gas and peripheral portion thereof. In
FIG. 41
, a fan
203
is arranged in a chamber
201
, and a laser gas is sealed inside the chamber
201
. Fan
203
fixed on a rotary shaft
202
rotates in chamber
201
. Magnetic bearings supporting rotary shaft
202
are arranged on opposite sides of chamber
201
. On the left side, arranged are: a radial magnetic bearing
206
including a radial electromagnet
204
and a position sensor
205
; an axial magnetic bearing
210
including axial electromagnets
207
and
208
and a position sensor
209
; a motor
211
including a motor rotor
218
and a motor stator
217
; and a protective bearing
212
as a touch down bearing that can support both the radial and axial directions to protect rotary shaft
202
.
On the right side of
FIG. 41
, provided are: a radial magnetic bearing
215
including a radial electromagnet
213
and a position sensor
214
; and a protective bearing
216
that can support only the radial direction.
Here, inner diameters of protective bearing
212
, motor stator
217
and radial electromagnet
204
are adjusted to be approximately co-axial. A gap between the inner diameter of protective bearing
212
and rotary shaft
202
opposing thereto is set slightly smaller than a minimum dimension of the gap between the inner diameters of motor stator
217
and radial electromagnet
204
and the rotary shaft
202
opposing thereto, so as to prevent contact between rotary shaft
202
and radial electromagnet
204
or motor stator
214
.
Axial magnetic bearing
210
and radial magnetic bearings
206
and
215
detect the position of rotary shaft
202
by means of position sensors
209
,
205
and
214
, respectively, provides signals by comparing operation between respective position sensor outputs and an instruction value, which signals are phase-compensated by a control circuit, not shown, and current-amplified by a power amplifier, so that a current is caused to flow in a coil of a corresponding electromagnet.
In the reflux fan for circulating laser gas in the excimer laser apparatus shown in
FIG. 41
, the pressure of reflux gas is as high as up to 5000 hPa. In order to rotate fan
202
under such a high output, it is necessary to increase output of motor
211
driving the rotary shaft
202
. Because of high motor output, however, attraction between motor rotor
218
and motor stator
217
constituting motor
211
becomes undesirably strong. This not only increases disturbance on rotary shaft
202
containing motor rotor
218
, but also affects control stability of radial magnetic bearing
206
supporting rotary shaft
202
.
Support by the radial magnetic bearing must be controlled stably both in a state of non-rotation in which motor
211
does not have any influence and in a state of maximum rotation in which motor has significant influence. Further, rotation in every pressure range lower than the maximum value of 5000 hPa of the reflux gas pressure is necessary. Therefore, it has been difficult to ensure stability of control of the magnetic bearing.
FIG. 42
shows a structure near the radial magnetic bearing portion, and
FIG. 43
is a block diagram of the magnetic bearing control system illustrating an influence of the motor on the radial magnetic bearing of FIG.
41
.
Referring to
FIG. 42
, a desired distance between electromagnet
204
of the radial magnetic bearing and rotary shaft
202
is represented by X
0
, and displacement x from the distance X
0
is measured. Based on the measurement, attraction force of electromagnetic
204
of the radial magnetic bearing is adjusted, so as to control floating position of rotary shaft
202
. Here, motor
211
is positioned close to radial magnetic bearing
206
, and control of radial magnetic bearing
206
is influenced by motor
211
.
Referring to
FIG. 43
, P(s) represents an object of control of magnetic bearing itself not considering motor
211
, G(s) represents transfer function of magnetic bearing control circuit, and x represents displacement from the position of prescribed floating distance X
0
of the rotary shaft. After comparing operation between an output x′ of the position sensor detecting the displacement x of the rotary shaft with an instruction value r, an electromagnetic force Fa calculated by magnetic bearing control circuit G(s) consisting of a control circuit including proportional, integral and differential elements acts on rotary shaft
202
, so that rotary shaft
202
is supported at a prescribed position. Here, Km represents a negative spring constant of motor
211
.
Referring to
FIG. 42
, when rotary shaft
202
moves downward, attraction force Fm from motor stator
217
below increases, and spring constant attains seemingly negative. The value Km increases as the output of motor
211
increases, that is, as the attraction force between motor rotor
218
and motor stator
217
increases. In other words, control of the magnetic bearing must be designed in consideration of the value Km, while the value Km varies widely (when rotary drive is stopped, Km attains 0). Therefore, it is difficult to ensure stability of supporting the magnetic bearing in every state.
FIG. 44
represents a gain curve of Bode diagram of the object of control (in
FIG. 43
, transfer function from Fa to x), of the magnetic bearing when the motor is driven and not driven. In
FIG. 44
, solid line a represents gain curve when the motor is not rotating, while solid line b represents gain curve when the motor is being driven. It is understood that the gain curve lowers in a low frequency range, when the motor is driven. Because of this decrease in gain in the low frequency range and because of the characteristic that the gain curve is flat (gain frequency gradient is approximately 0) over a wide range in the low frequency range, controllability of the magnetic bearing degrades.
FIGS. 45A and 45B
represent open loop transfer function when a magnetic bearing control circuit ensuring stability in both states (when motor is driven and not driven) is designed, based on the object of control of the magnetic bearing, in which
FIG. 45A
represents gain characteristic, and
FIG. 45B
represents phase characteristic.
Referring to
FIG. 45A
, solid line c represents the open loop transfer function when motor is not driven, while solid line d is when the motor is driven. When motor
211
is driven, gain margin shown in
FIG. 45A
decreases from A to A′, and it is understood that the margin in stability of control decreases significantly. As a countermeasure, it may be possible to set a cross over frequency to higher frequency side to suppress the influence of motor. In the excimer laser apparatus, however, discharge at a high voltage is utilized for laser oscillation, to excite laser gas. Therefore, in order to prevent influence of high frequency noise, in the magnetic bearing used for the excimer laser apparatus, it is necessary to lower as much as p
Ozaki Takayoshi
Tokunaga Hironori
Yada Yuuji
Yamada Koji
Gagliardi Albert J.
Monbleau Davienne
NTN Corporation
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