Method of tracking servo for optical pickup device

Details Method of tracking servo for optical pickup device Method of tracking servo for optical pickup device

2001-04-19

2003-11-25

Edun, Muhammad (Department: 2655)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C369S044270, C369S044370

Reexamination Certificate

active

06654323

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a novel method of tracking servo for an optical pickup device, and more specifically, to a technique which permits highly accurate tracking servo particularly in recording in an optical pickup device for use in a disk drive of a recordable optical disc as well.
2. Description of the Related Art
DPP (Differential Push Pull) method is well known as a method of tracking servo in a disk drive of an optical recording medium such as CD (Compact Disc), for instance.
The DPP method is a method of generating a tracking error signal through an operation of signals respectively outputted from photo detectors on the basis of a main beam MB and two side beams SB, SB.
Specifically, the DPP is by forming three spots respectively based on three beams, i.e., 0th-order diffraction light (a main beam) and ±1-st order diffraction light (side beams) on an optical disc with a diffraction means (a grating) arranged in a going path of a beam emitted from a laser light source, and then receiving returned light of the above three beams with photo detectors a, b, b respectively for applying a main spot MS based on the main beam to writing or reading of signals, while applying side spots SS, SS based on the side beams to tracking error detection.
As shown in
FIG. 1
, for instance, the main photo detector “a” for detecting the main spot MS is split into four sections crosswise, while each of the side photo detectors “b”, “b” for detecting the side spots SS, SS is split into two sections left and right. Incidentally, output signals from the respective split elements are denoted as A, B, C, D, E, F, G and H. Then, a tracking error signal is generated on the basis of operational output among the output signals from these photo detectors “a”, “b”, “b”.
That is, a DPP (Differential Push Pull) signal is obtained according to the following operation expression on the basis of an MPP (Main Push Pull) signal generated from the output signal of the main photo detector “a” and SPP
1
and SPP
2
(Side Push Pull) signals generated from the output signals of the side photo detectors “b”, “b”.
MPP
=(
B+C
)−(
A+D
)
SPP
1
=
E−F
SPP
2
=
G−H
DPP=MPP−K
·(
SPP
1
+
SPP
2
)
∴DPP=
((
B+C
)−(
A+D
))−
K
·((
E−F
)+(
G−H
))
K: Coefficient
FIG. 1
shows schematically the relation between the photo detectors “a”, “b”, “b” and the spots MS, SS, SS when optical components or the like are free from variations, and
FIG. 2
shows waveforms of the SPP
1
signal, the SPP
2
signal, the MPP signal and the DPP signal in FIG.
1
.
As is apparent from
FIG. 2
, the SPP
1
signal and the SPP
2
signal are outputted in the same phase, the MPP signal is outputted in the reverse phase to the SPP
1
signal and the SPP
2
signal, and the DPP signal is outputted in the same phase as the MPP signal.
FIG. 3
schematically shows the positional relation between the photo detectors “a”, “b”, “b” and the spots MS, SS, SS when spaces among the spots MS, SS, SS are different due to variations or the like in optical components (the widened spaces among the spots are shown in FIG.
3
).
FIG. 4
shows the waveforms of the SPP
1
signal, the SPP
2
signal, the MPP signal and the DPP signal in FIG.
3
. Incidentally, the waveform shown by a solid line in
FIG. 4
is of each PP (Push Pull) signal in signal reading, while the waveform shown by a broken line is of the SPP signal and the DPP signal in signal writing which will be described later.
In signal reading, although positive DC offset and negative DC offset are respectively produced in the SPP
1
signal and the SPP
2
signal, both the amounts of DC offset are of the same absolute value. Thus, these amounts of DC offset may be canceled according to the above operation expression to permit the DPP signal to be outputted in a DC offset free state, resulting in maintenance of an on-track state (Refer to a solid line in FIG.
4
).
FIG. 5
schematically shows the positional relation between the photo detectors “a”, “b”, “b” and the spots MS, SS, SS when the spots MS, SS, SS are one-sidedly (rightward in
FIG. 5
) shifted on the photo detectors “a”, “b”, “b” due to the positional difference of the objective and the photo detector or the like.
FIG. 6
shows the waveform of the SPP
1
signal, the SPP
2
signal, the MPP signal and the DPP signal in FIG.
5
. Incidentally, the waveform shown by a solid line in
FIG. 6
is of each PP (Push Pull) signal in signal reading, while the waveform shown by a broken line is of each PP (Push Pull) signal in signal writing which will be described later.
In signal reading, although DC offset is produced in any of the SPP
1
signal, the SPP
2
signal and the MPP signal, these amounts of DC offset may be canceled according to the above operation expression to permit the DPP signal to be outputted in the DC offset free state, resulting in maintenance of the on-track state
As described the above, the DPP method is considered to be an extremely effective method of tracking servo for an optical pickup of a signal reading system, for the reason that the amount of DC offset of the tracking error signal (the DPP signal) may be canceled according to the above operation expression, even in any of the case where the spaces among the spots MS, SS, SS are different due to the variations or the like in optical components as shown in
FIG. 3
, and the case where the spots MS, SS, SS are one-sidedly shifted on the photo detectors “a”, “b”, “b” due to the positional difference of the objective lens and the photo detector or the like as shown in FIG.
5
.
However, it is to be understood that only the case of reading the signals of the optical disc as described above permits the amounts of offset produced to be canceled, when the spaces among the spots MS, SS, SS are different due to the variations or the like in optical components as shown in
FIG. 3
or when the spots MS, SS, SS are one-sided on the photo detectors a, b, b due to the positional difference of the objective and the photo detector or the like as shown in FIG.
5
.
FIG. 7
schematically shows the positional relation between the main beam MB and the side beams SB
1
, SB
2
and recording tracks T in writing the signals onto an optical disc C.
In writing signals onto the optical disc C, one side beam SB
1
(the side beam preceding the main beam MB) is reflected from a disc surface placed in a non-written state that pits “d”, “d”, are not formed yet at all, while the other side beam SB
2
(the side beam following the main beam MB) is reflected from the disc surface between tracks T, T placed in the written state that the pits “d”, “d”, . . . are already formed. Thus, there is a problem in that a method similar to that of canceling the offset produced in reading the signals is not good enough to cancel the offset produced in writing the signals onto the optical disc.
That is, in writing the signals when the spaces among the spots MS, SS, SS are different due to the variations or the like in optical components to produce the offset, a disk drive of an optical recording medium, for example, an optical disc such as CD-R (Compact Disc-Recordable) and CD-RW (Compact Disc-Rewritable) or like optical disc causes the amount of DC offset
1
of the SPP
1
signal to be different from the amount of DC offset
2
of the SPP
2
signal, as shown by the broken line in the waveform diagram of FIG.
4
. Specifically, the SPP
2
signal produces the amount of offset
2
less in absolute value than the amount of offset
1
produced in the SPP
1
signal, and as a result, the amount of DC offset
3
, which may not be canceled by the above operation expression, is produced in the DPP signal provided as a final output signal.
On the other hand, in writing the signals when the spots MS, SS, SS are one-sidedly shifted on the photo detectors “a”, “b”, “b” due Lo the positional difference of the objective lens and the photo detector or the like to produce the offs

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