Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-11-30
2004-03-09
Pyo, Kevin (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S236000, C359S223100, C359S226200
Reexamination Certificate
active
06703603
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a controller for an optical scanner including an angle detecting unit and adapted to position a mirror supported by an axis of rotation.
In an optical scanner used in the laser marking and the laser drilling through a printed wiring board using a laser beam, a mirror mounted on the rotation shaft is rotated by an electric motor having the mirror therewith to change the angle of the mirror, whereby a laser beam emitted from a laser oscillator is irradiated on a predetermined position of an object to be fabricated.
2. Description of the Related Art
FIG. 12
is a schematic view showing the construction of a movable part in the optical scanner. A mirror
11
is mounted on an end of a rotation shaft
12
. The rotation shaft
12
is supported by a bearing
14
and a bearing
15
and is rotated by suffering the driving torque by a moving coil
13
incorporated integrally with the rotation shaft
12
to be positioned to a predetermined angle. Hereinafter, the mirror
11
, the rotation shaft
12
and the moving coil
13
which are rotated integrally are collectively referred to as an optical scanner
1
″ for short when applicable.
The optical scanner
1
has an angle detecting sensor (not shown) for detecting the rotation angle of the rotation shaft
12
, e,g., a variable capacitance type sensor. The variable capacitance type sensor is configured in such a way that a dielectric plate mounted on a rotation shaft is rotated together with the rotation shaft between a set of two fixed electrode plates. Then, the angle of the rotation shaft is detected in the form of an electrical signal as the change in the electrostatic capacity between the electrode plates. The technique of such a variable capacitance type sensor, for example, is disclosed in U.S. Pat. No. 3,517,282, U.S. Pat. No. 4,864,295 and JP-A-7-55500.
In addition, JP-A-4-127981 discloses a technique that a laser beam for measuring an angle is irradiated on a mirror and the reflected light beam is detected by a linear sensor to control the mirror angle in the feedback control. Furthermore, JP-A-63-147138 discloses a technique that a reflecting surface is formed on a component (mirror mount) to be fixed a mirror on a rotation shaft and the light beam emitted from a light emitting device reflected from the reflecting surface is detected by a light receiving device, thereby positioning the mirror.
FIG. 13
is a servo block diagram showing a configuration of the optical scanner. In order that an angle detection signal
22
is negatively fed back to an angle desired value signal
21
supplied from a host controller to make zero a value of a tracking error signal
23
, the tracking error signal
23
is integrated by an integral compensation circuit
24
. In addition, in order to maintain the stability of this servo mechanism, the angle detection signal
22
is inputted to both a proportional compensation circuit
25
and a derivative compensation circuit
26
, and the sum of the output signals from these circuits is subtracted from an output signal of the integral compensation circuit
24
to make the resultant signal a control input signal
27
. A motor driving circuit
28
supplies a motor driving current
29
proportional to the control input signal
27
to the optical scanner
1
. The motor driving current
29
flows through a moving coil
13
, thus, the driving torque proportional to the current value acts on the moving coil.
In the case where holes are formed through a printed wiring board by the laser beam, in order to form the holes through the fine circuit pattern with high accuracy, it is necessary to make the positioning error which should be equal to or smaller than about 10 &mgr;m when forming the holes. In addition, in order to shorten the laser drilling time, the enhancement of the drilling speed is required for the movement up to the drilling position of a next hole after completion of the one hole. For example, in the case where 1,000 holes are formed per second, the time required for the movement between the holes needs to be made shorter than 1 ms in average.
Now, the laser beam has the energy distribution and is made incident to the mirror
11
with the area extend held. For this reason, in order to form the high quality holes, it is desirable that the size of mirror
11
is large (i.e., the area thereof is large).
However, if the size of mirror
11
increases, then the torsional vibration and the flexural vibration of the rotation shaft
12
are both increased, which results in the impediment of the high response of the mirror positioning.
First of all, the description will be given with respect to the influence of the torsional vibration. If the size of mirror
11
increases, the moment of inertia around the rotation shaft
12
also increases, therefore, the natural frequency of the torsional vibration acting by the rotation shaft
12
decreases. The natural frequency of the first mode is minimum. One node of the torsion is then present in the longitudinal direction of the rotation shaft
12
and the both sides sandwiching the node are subjected to the angular displacement in an opposite phase. In addition, the natural frequency of the second mode is low next to that of the first mode. Two nodes are therefore present in the longitudinal direction of the rotation shaft
12
, and the both sides of the central part sandwiched between the two nodes are vibrated in the opposite phase with respect to that central part.
For example, when the angle detecting sensor is close to the mirror, the node of the torsional vibration is located between the angle detecting sensor and the moving coil
13
, whereby the angle detecting sensor and the moving coil
13
may be subjected to the angular displacement in the opposite phase in some cases. In such cases, in the above-mentioned servo mechanism, the torsional vibration frequency component of the angle detection signal
22
is positively fed back, so that the control becomes unstable. Though the control bandwidth is desirably wider in terms of the characteristics of the high response of the mirror positioning and the suppression of the low frequency disturbance, the control bandwidth is limited due to the torsional vibration.
In addition, when the node of torsional vibration overlaps the position of sensor or is located close to the sensor, since the vibration mode thereof can not be observed by the angle detecting sensor, the stabilization can not be obtained in terms of the control and hence the accuracy of positioning the laser beam is reduced.
Next, the description will hereinbelow be given with respect to the influence of the flexural vibration. It is desirable that the movable part of the optical scanner shown in
FIG. 12
is balanced around the rotation shaft
12
. However, if the mass of mirror with respect to the two sides of the moving coil
13
in the longitudinal direction which is subjected to the driving torque or the mass of mirror with respect to the axis line of rotation shaft
12
is different between the left and right sides thereof, the mass difference becomes the unbalance weight. The flexural vibration is then generated on the rotation shaft
12
with the rotation shaft
12
supported by the bearing
14
and the bearing
15
due to the force of inertia caused by the unbalance weight resulting from the operation of the optical scanner
1
. As a result, the mirror
11
vibrates in the parallel direction or in the perpendicular direction with respect to the mirror surface. In general, the optical scanner
1
does not have a sensor for detecting the flexural vibration of the rotation shaft
12
and an actuator for applying the force in the direction of the flexural vibration to the rotation shaft
12
. In addition, in the feedback control made by the above-mentioned servo mechanism, it is impossible to attenuate the flexural vibration which has been generated once. For this reason, it is impossible to enhance the accuracy of positioning the laser beam.
SUMMARY OF THE INVEN
Hirai Hiromu
Nakajima Kichio
Oishi Masahiro
Okubo Yaichi
Sakamoto Atsushi
Crowell & Moring LLP
Hitachi Via Mechanics Ltd.
Pyo Kevin
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