Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2002-08-30
2003-10-14
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S198100, C359S212100, C359S871000, C359S223100, C359S226200
Reexamination Certificate
active
06633422
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a rapidly rotatable optical mirror, and an optical scanner and a laser machining apparatus employing the improved mirror.
BACKGROUND OF THE INVENTION
The prior art optical mirror will be described hereinafter with reference to the accompanying drawings.
FIGS. 12A and 12B
show the structure of conventional lightweight optical mirror
120
.
Optical mirror
120
, as shown in
FIG. 12A
, comprises reflecting surface
121
and holder
128
which a motor shaft (not shown) is attached thereto. Holder
128
further comprises semi-circular cross section groove
122
for the motor shaft, and screw holes
123
.
FIG. 12B
shows the structure of the mirror seen from the rear surface. As shown in
FIG. 2B
, reflecting surface
121
has on its rear surface:
(1) mirror support beam
124
extending from holder
128
;
(2) plural strengthening ribs
125
extending from the both sides of beam
124
toward the rim of the rear surface of reflecting surface
121
; and
(3) peripheral ribs
126
that are disposed close to holder
128
and extended along the rim of the rear surface of reflecting surface
121
.
The optical mirror structured above operates in a manner, which will be described hereinafter. Optical mirror
120
(
FIGS. 12A
,
12
B) is attached directly to the rotary shaft of the motor (not shown), and used for a galvanometer scanner in which the rotation angle of the motor defines a reflecting direction of light.
Laser beam and illumination light are reflected by mirror surface
121
. The shape and area of the reflected light depend on the shape of incident light and the rotation angle of the optical mirror.
To attach optical mirror
120
to the motor shaft, the motor shaft is fitted in semi-circular groove
122
and held with a retaining ring (not shown) having also a semi-circular groove, then secured by screws at screw holes
123
. Diameters both of groove
122
and the groove of the retaining ring are generally sized to be a few micrometers bigger than that of the motor shaft. However, the perimeter of the motor shaft measures bigger than the perimeter of roughly semi-cylindrical shape formed from facing each semi-circular portion of groove
122
and the retaining ring's groove. Therefore, fastening the screws to secure the optical mirror to the motor shaft inconveniently applies a stress to screw holes
123
vertically with respect to the reflecting surface
121
.
Optical mirror
120
is required to keep enough rigidity against a distortion occurred between reflecting surface
121
and holder
128
while the motor is rotating. For keeping enough rigidity, mirror support beam
124
, plural ribs
125
, and peripheral ribs
126
close to holder
128
are formed on the rear surface of reflecting surface
121
. In addition, as shown in
FIGS. 12A and 12B
, reflecting surface
121
of optical mirror
120
and holder
128
are formed in one piece.
Mirror support beam
124
functions as an absorber of the vibrations created in the axial direction of the motor shaft while the motor is rotating. Further, ribs
125
and
126
make a large contribution to minimize the fluttering of the mirror when rotating.
With the structure described above, however, a distortion occurs locally in the mirror surface when fastening the screws. When an optical scanner with such optical mirror is used for controlling the traveling path of laser beam in laser machining, flaws have often been detected outside the machined main hole in a workpiece.
FIG. 13
shows a distortion in the mirror surface when the motor shaft is attached and secured by screws to the conventional optical mirror, indicating distorted area by the curves.
It is apparent from
FIG. 13
that the distortion which occurs at the screw holes disposed on the both sides of groove
122
is, through the peripheral ribs disposed on the rim of the mirror surface, carried to the mirror surface near the holder.
According to an amount of distortion measured by an interferometer, in the optical mirror made of a material containing beryllium for weight reduction, the Peak-Valley (P-V) value of the precision of the mirror surface measures no less than 4 &mgr;m. This amount of distortion is compatible to the optical path difference of approximately one-half of the wavelength (approx. 10 &mgr;m) of a carbon dioxide laser having relatively long wavelength. Generally, {fraction (1/20)}th of the wavelength of laser is defined to be optically aberration-free value (that is, approx. 0.5 &mgr;m for a carbon dioxide laser.) The P-V value in
FIG. 13
, however, shows as much as about 10 times the aberration-free value for the carbon dioxide laser.
Referring to
FIG. 14
, now will be described a two-dimensional optical scanner using the conventional optical mirror.
The conventional two-dimensional scanner, as shown in
FIG. 14
, comprises two sets of galvano-mirrors
140
A,
140
B and position control unit
148
. In
FIG. 14
, galvano-mirror
140
A further comprises motor
143
A having motor shaft
142
A, and optical mirror
141
A attached to motor shaft
142
A. Motor
143
A contains a position sensor (not shown) for position control. An output signal from the position sensor is fed into position control unit
148
for adjusting the position of the optical mirror. The explanation for galvano-mirror
140
B will be omitted because the mirror has the same structure as mirror
140
A described above. Hereinafter, depending on the parts constituting mirror
140
A or
140
B, either letter “A” or “B” is appended to the corresponding parts number.
Optical mirror
141
A of galvano-mirror
140
A, as shown in
FIG. 14
, horizontally rotates about motor shaft
142
A, while mirror
141
B of galvano-mirror
140
B vertically rotates about motor shaft
142
B.
The optical scanner structured above operates in a manner, which will be described hereinafter. Optical mirror
141
A reflects laser beam
145
shown in
FIG. 14
to direct an intended position on optical mirror
141
B. In response to the reflection, the position sensor, which is built in motor
143
A of galvano-mirror
140
A, detects the orientation of mirror
141
A. Getting the signal back from the position sensor, position control unit
148
adjusts the reflecting direction.
Similarly, in response to the light incident on mirror
140
B, the position sensor, which is built in motor
143
B, detects the orientation of mirror
141
B. Getting the signal back from the position sensor, position control unit
148
adjusts the reflecting direction.
However, with the two-dimensional scanner employing mirrors
141
A and
141
B that have the conventional structure, the aimed surface cannot be radiated with the laser beam reflected from mirrors
141
A and
141
B due to a bad distortion.
FIG. 15
shows an optical system of the laser machining apparatus equipped with the optical scanner illustrated in FIG.
14
. In
FIG. 15
, the conventional laser machining apparatus comprises:
a) laser oscillator
151
that produces a laser beam;
b) collimator
152
collimating the output laser beam from laser oscillator
151
;
c) mask changer
153
masking the collimated laser beam;
d) reflecting mirror
154
reflecting the laser beam passed through mask changer
153
;
e) two-dimensional optical scanner
155
scanning the incident laser beam through reflecting mirror
154
;
f) scanning lens
156
projecting the incident laser beam through optical scanner
155
; and
g) two-dimensional machining table
158
for mounting workpiece
157
to be machined with the projected laser beam. (Workpiece
157
is an object to be machined on machining table
158
.)
The laser machining apparatus structured above operates in a manner, which will be described hereinafter. Laser oscillator
151
produces laser beam. After changed the beam diameter by Collimator
152
, the laser beam is irradiated over the mask placed on mask changer
153
. A portion of the laser beam, which passes through the mask, is launched into optical scanner
155
for controlling the scanning direction. Then scanning lens
156
projects the shape
Karasaki Hidehiko
Kawazoe Kimiko
Phan James
Wenderoth , Lind & Ponack, L.L.P.
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