Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2002-07-12
2004-08-10
Nguyen, Thong (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S223100, C359S224200
Reexamination Certificate
active
06775039
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a driving circuit for an optical scanner in which light from a light source is reflected and an optical scanner performing a one- or two-dimensional scan with this reflected light is driven.
2. Description of the Related Art
Some of conventional optical scanners are disclosed in Japanese Patent Kokai Nos. Hie 7-175005 and Hie 10-123449. Each of these optical scanners is fabricated by semiconductor manufacturing technology and has the features of compactness and small thickness.
FIG. 1
shows a diagram for illustrating an operating principle of the optical scanner. As shown in this figure, the optical scanner includes a mirror portion
101
in which a coil pattern (a driving coil
102
) is provided parallel with a mirror face
101
a
; spring portions
104
a
and
104
b
for oscillating the mirror portion
101
; and permanent magnets
105
a
and
105
b
arranged close to the mirror portion
101
, for producing a magnetic field nearly parallel with the mirror face
101
a
where the mirror portion
101
is in a stationary state. The spring portions
104
a
and
104
b
are connected to a support, not shown, to be fixed to an arbitrary member. By supplying an alternating current (of a frequency t) to the driving coil
102
, a force obeying the left-hand rule is generated in a direction normal to the mirror face
101
a
to oscillate the mirror portion
101
at the frequency f.
When the alternating current is represented by I (=I
0
sin (2&pgr;ft)), the strength of the magnetic field by H (a magnetic flux density B), the number of turns of the coil by N, the area of the coil by S, and a vacuum magnetic constant by &mgr;
0
, an oscillating angle &thgr; and a generating force F in this case have the relation expressed by the following equation:
F=&mgr;
0
NHSI
0
sin(2&pgr;ft)·cos &thgr; (1)
The oscillating angle &thgr; can be found by solving the following equation of motion:
θ
¨
=
-
k
θ
-
D
θ
.
+
F
J
(
2
)
Here, k is a torsion spring constant of the spring portion and has the relation of k=(2&pgr;f
c
)
2
, where f
c
is a mechanical resonant frequency of the optical scanner, D is an attenuation coefficient, and J is the moment of inertia of the optical scanner.
The relation between the oscillating angle &thgr; and the frequency f of the alternating current, in which the oscillating angle &thgr; is thought of as small, can be expressed from Eqs. (1) and (2) by the following equation:
θ
(
f
)
=
μ
0
NHSI
0
J
1
{
k
-
(
2
π
f
)
2
}
2
+
D
2
(
2
π
f
)
2
(
3
)
FIG. 2A
shows a plot of Eq. (3). As shown in
FIG. 2A
, the maximum oscillating angle (oscillating amplitude) is obtained when the driving frequency f of the alternating current is caused to coincide with the mechanical resonant frequency f
c
.
From this reason, it is a common practice for the drive of the optical scanner to cause the frequency of a driving signal to coincide with the mechanical resonant frequency of the optical scanner.
In order to stabilize the drive of the optical scanner mentioned above, it is necessary to provide a sensor for detecting the oscillating condition of the optical scanner. In the optical scanner using such a sensor, as disclosed, for example, in Japanese Patent Kokai No. Hie 11-242180, it is known that, in addition to the structure of
FIG. 1
, a coil pattern (hereinafter referred to as a sensor coil
103
), different from the driving coil
102
, is provided nearly concentrically on the same plane as the driving coil
102
in the mirror portion
101
(see FIG.
3
), and when the mirror portion
101
is oscillated, an electromotive force generated by the interlinkage of the sensor coil
103
with the magnetic field is detected and thereby the oscillating condition is detected.
Here, in the technique of detecting the oscillating condition of the optical scanner of the structure mentioned above, an electromotive force V
r
generated in the sensor coil
103
is given by the following equation:
V
r
=N
s
BS
s
{dot over (&thgr;)}·cos &thgr; (4)
where N
s
is the number of turns of the sensor coil, B is the magnetic flux density, and S
s
is the area of the sensor coil.
Now, consider the case where the optical scanner is driven with the mechanical resonant frequency f
c
. When the driving signal is expressed as I=I
0
sin (2&pgr;f
c
t), the oscillation of the optical scanner is retarded in phase by 90° with respect to the driving signal, thus giving
&thgr;=−&thgr;
0
·cos(2&pgr;
f
c
t
) (5)
Therefore, the electromotive force V
r
expressed by Eq. (4), in which the oscillating angle &thgr; (&thgr;
0
) is thought of as small, can be approximated by the following equation:
V
r
=
N
s
B
S
s
θ
0
2
π
f
c
·
sin
(
2
π
f
c
t
)
·
cos
{
-
θ
0
·
cos
(
2
π
f
c
t
)
}
≈
N
s
B
S
s
θ
0
2
π
f
c
·
sin
(
2
π
f
c
t
)
(
6
)
Whereby, it is found that the electromotive force generated in the sensor coil is 90° ahead in phase with respect to the oscillation of the optical scanner. (Also, if the connections of both ends of the coil are replaced, the sign of the electromotive force will reverse and the phase will be retarded by 90°, and the following description is given on the basis of this practice.) Thus, in the resonant frequency drive, the phase relations of the driving signal, the drive of the optical scanner, and the electromotive force of the sensor coil (a sensor signal) are as shown in
FIGS. 4A
,
4
B, and
4
C, respectively, and the driving signal (
FIG. 4A
) coincides in phase with the sensor signal (FIG.
4
C).
Here, where the optical scanner is driven at an arbitrary frequency which is much lower than the resonant frequency, the oscillation of the optical scanner, when the driving signal is expressed as I=I
0
sin(2&pgr;ft), coincides in phase with the driving signal, thus giving
&thgr;=−&thgr;
0
·sin(2&pgr;
ft
) (7)
Therefore, the electromotive force V
r
expressed by Eq. (4), in which the oscillating angle &thgr; (&thgr;
0
) is thought of as small, can be approximated by the following equation:
V
r
=
N
s
B
S
s
θ
0
2
π
f
·
cos
(
2
π
f
t
)
·
cos
{
θ
0
·
sin
(
2
π
f
t
)
}
≈
N
s
B
S
s
θ
0
2
π
f
·
cos
(
2
π
f
t
)
(
8
)
A common control driving circuit for operating the optical scanner with stability is disclosed in Japanese Patent Kokai No. Hie 09-101474. This control driving circuit has a frequency follow-up control function (a positive feedback control function) for always driving the optical scanner with the resonant frequency and an amplitude control function (a negative feedback control function) for operating the optical scanner with stability at a desired oscillating amplitude.
However, when the control drive of the optical scanner with the sensor is made, the following problem {circle around (1)} arises.
Specifically, the sensor signal (the electromotive force produced in the sensor coil), as shown in Eq. (6) or (8), is proportional to the driving frequency. Consequently, when resonant frequency follow-up control such as that described in Kokai No. Hie 09-101474 is made, the mechanical resonant frequency of the optical scanner fluctuates due to changes of ambience and with
Allen Denise S.
Nguyen Thong
Olympus Corporation
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