Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2001-02-23
2003-10-14
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044350, C369S053230
Reexamination Certificate
active
06633522
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for controlling a light spot on a recording layer of an optical disk.
2. Description of the Background Art
FIG. 10
denotes an example of a circuit configuration for realizing focus pull-in to a recording layer of optical disk such as a CD, DVD or the like. In the figure, reference numeral
0
denotes a light emitting optical system containing a semiconductor laser diode (LD), reference numeral
1
denotes an optical disk,
2
an objective lens,
3
a focus actuator which is rigid-body-like connected to the objective lens
2
and disposed in a magnetic circuit,
4
a half-mirror,
5
a photo-electric conversion element,
6
a focus error detector,
7
a phase compensator,
8
a change-over switch,
9
a driver amplifier,
10
a search waveform generator,
11
a zero-crossing detector for a focus error signal,
12
an AND circuit taking a logic product of a second control signal E from an external host device such as a microcomputer or the like and an output of the zero-crossing detector
11
and
13
an addition amplifier producing a whole addition signal.
FIG. 11
denotes a view showing an operation of each signal at a time of focus pull-in of a CD or DVD. In the figure, a symbol A denotes a focus drive signal which is output from the driver amplifier
9
to be applied to the focus actuator
3
, B a focus error signal output from the focus error detector
6
(a pointer which directs a positional relationship between the optical
1
and the objective lens
2
), C a total additional signal which is output from the addition amplifiers
13
and sums up totally the photo-electric conversion signals obtained from each area of the photo-electric conversion elements
5
, D a first control signal which a total addition signal C is over a predetermined threshold value S and becomes “H” at a zero-crossing point of the focus error signal, E a second control signal from the host device which is “H” when the focus control loop is closed and is “L” when it is open, F a third control signal which the change-over switch
8
selects the focus control signal in a case of “H” and selects the focus search signal in a case of “L”, T
1
the zero-crossing point of the focus error signal and Sr
2
a threshold value of the total addition signal for detecting a valid focus error signal.
A description will be given to a general focus pull-in operation hereinafter. As usual, a pull-in range of a focus servo lies in a relatively narrow range of a several to twenty &mgr;m. In order to pull-in a focus servo, it is necessary to shift the focus actuator
3
up to the pull-in range of the focus servo. For example, one Hz of saw tooth search waveform is output from the search waveform generator
10
. When a system is not a mode which closes the focus control loop, as a second control signal E from a host device is “L”, a third control signal F becomes “L” and a focus search signal is selected at the change-over switch
8
. In this case, a search waveform is input to the driver amplifier
9
to drive a focus actuator
3
. Based on the search waveform, a focus search operation that an objective lens
2
comes close to an optical disk
1
or goes away therefrom is carried out.
A focus error signal is a signal based on a reflection light from the optical disk
1
which changes together with the focus search operation and a laser light which is projected from a semiconductor laser diode (LD)
0
and reflected at a recording surface of the optical disk
1
, is received by the photo-electric conversion element
5
and detected by the focus error detector
6
, for example, by using a well known astigmatism method. The focus error signal detected herein is output to a phase compensator
7
, which is formed by a phase-lead filter which leads a phase of the band width near 1 KHz and a suitable servo gain is given here. After the focus error signal obtains an adequate phase and a gain characteristic, the signal will be output to the change-over switch
8
as a focus control signal.
The focus error signal is also output to the zero-crossing detector
11
. The zero-crossing detector
11
outputs a first control signal D that, when a signal level of the whole addition signal is over a predetermined threshold value Sr
2
and the focus error signal detects a zero-crossing point T
1
, the zero-crossing detector
11
becomes “H”, while when a signal level of the whole addition signal is less than a predetermined threshold value Sr
2
, the zero-crossing detector
11
becomes “L”.
In this case, if the system is in a mode which closes the focus control loop, as a second control signal E from the host device becomes “H” and a third control signal F from the zero-crossing detector
11
is valid, a change-over switch
8
selects a focus control signal at a zero-crossing point T
1
of the focus error signal and the focus actuator
3
is activated. Thus, a pull-in operation for the focus control is effected.
When a rotational drive of a disk is generally activated, a face deflection occurs. An absolute quantity of the face deflection is decided by a standard of each disk. As for DVD, ±300 &mgr;m is a standard limit. For example, when a DVD of the standard limit is driven by one double velocity, a rotational speed at the most internal circumference part of a disk is about 23 Hz, and suppose that an amount of the face deflection quantity is proportional to a radius of the disk, the face deflection is about ±125 &mgr;m at the most internal circumference, and the maximum face deflection velocity is about 18 mm/sec according to the following expression.
V=A·&pgr;·f·
cos(2&pgr;·
f·t
)
where V is the face deflection velocity, A is an amount of face deflection and f is a rotational frequency.
On the other hand, since a speed of focus search is generally 10 mm/sec, the relative velocity between an objective lens and an optical disk is 30 mm/sec at the most. (This corresponds to a sum of the largest face deflection and the focus search speed.) As a rule, a pull-in capability of focus control depends on a phase compensation filter, a setting amount of gain, a sensitivity of the focus actuator or the like, and is 30 mm/sec, more or less. Since the pull-in capability exceeds the largest relative velocity of the objective lens and optical disk in all the region, it is possible to close a focus control loop.
Recently, a reproduction speed of the optical disk device became higher year by year and accordingly, the rotational speed of the disk has been made higher. For example, at the most internal circumference of CD, it is 16 times, and at that of DVD, the rotational speed is about four times. Considering a case of four times of DVD reproduction speed, and if the face deflection amount of the disk is ±300 &mgr;m in the standard limit, the largest face deflection velocity at the most internal circumference part is about ±70 mm/sec, and the maximum relative velocity is 80 mm/sec, where the speed for focus search is 10 mm/sec. As described above, since a possible velocity for pulling-in of the focus control system is usually 30 mm/sec, as shown in
FIG. 12
, there is very little region that the relative velocity of the objective lens and the optical disk is less than 30 mm/sec and it is seen that it is quite difficult to pull-in the focus control.
FIG. 12
shows a timely variation of the relative velocity between the optical disk and the objective lens and a horizontal axis shows a time(sec) and a vertical axis a relative velocity (mm/sec). A thick line at 10 mm/sec of the relative velocity shows the focus search velocity. An area between 10 mm/sec to ±30 mm/sec shows an area which the focus servo system can pull-in.
As described above, according to the prior art focus servo pull-in method, when the face deflection of the disk is large, the reproduction speed is fast, it often fails in the pull-in operation of servo. Therefore, it is a serious problem that as things stand, scarcely any servo can pull-in.
SUMMARY OF THE
Huber Paul W.
Mitsubishi Denki & Kabushiki Kaisha
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