Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
2001-01-29
2002-09-17
Huber, Paul W. (Department: 2653)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044320
Reexamination Certificate
active
06452879
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical storage apparatus for positioning an objective lens for irradiating a medium with a light beam from a light source to a target position on a medium by a tracking control and a focusing control and performing reproduction, recording, or erasure. More particularly, the invention relates to an optical storage apparatus for measuring and correcting an offset in a servo error signal caused by a change in an amount of reflection light from a medium.
2. Description of the Related Arts
Attention is paid to an optical disk as a removable storage medium as a core of multimedia which has rapidly been developing in recent years, and a magnetooptic disk (MO), a phase change optical disk (PD), and the like exist. An optical disk drive using such an optical disk as a storage medium has an objective lens for irradiating a medium with a light beam from a laser light source, which is mounted on an actuator movable in the radial direction of the medium, and performs a servo control of positioning the light beam to a target track position on the medium and positioning the objective lens so as to achieve focus on the medium by moving the objective lens in an optical axis direction. Such a servo control is performed by generating a servo error signal (a tracking error signal and a focusing error signal) indicative of a deviation from the target position of the objective lens on the basis of reflection light from the medium and positioning the objective lens to the target position on the basis of the servo error signal (tracking control and focusing control). Since the servo error signal used for the servo control is generated by receiving the reflection light from the medium by a photodetector, an amount of light received by the photodetector changes according to an amount of the reflection light from the medium and a light emitting power itself of the laser light source. Consequently, an offset component is generated in the servo error signal, and the servo stability deteriorates. The phenomenon of occurrence of an offset in the servo error signal due to a change in the amount of reflection light received by the photodetector will be described as follows. Although the phenomenon will be described with respect to a track system here, the phenomenon with respect to the focusing system is similar.
FIG. 1
shows a construction of a tracking error signal generating circuit. Specifically, the tracking error signal generating circuit includes: a photodetecting unit
300
having a pair of photodetectors
302
and
304
for receiving reflection light from a medium and performing a photoelectric conversion; current-to-voltage converters
306
and
308
for converting currents ia and ib from the photodetectors
302
and
304
to voltages Va and Vb, respectively; a subtractor
310
for obtaining a difference (Va−Vb) of the two voltage signals Va and Vb derived by the conversion by the current-to-voltage converters
306
and
308
; an adder
312
for obtaining a sum (Va+Vb); and a divider
314
for performing division between outputs of the subtractor
310
and the adder
312
and outputting the result as a tracking error signal E
10
. Although there is a case that the output of the subtractor
310
is used as a tracking error signal, generally, in order to suppress variations in amplitude according to the reflection light amount, the output of the subtractor
310
is divided by the total amount of reflection light calculated by the adder
312
, thereby making the amplitude of the tracking error signal constant. Consequently, an ideal output of the tracking error signal E
10
is obtained as follows.
E
10
=(Va−Vb)/(Va+Vb)
When individual differences in the current-to-voltage converters
306
and
308
are considered and it is assumed that a small offset Vofs exists on the output Va side, the track error signal E
10
is obtained as follows.
E
10
={(Va+Vofs)−Vb}/{(Va+Vofs)+Vb}
The offset Vofs is a small offset voltage which is always constant irrespective of currents supplied to the current-to-voltage converters
306
and
308
. When the outputs Va and Vb of the current-to-voltage converters
306
and
308
have sufficiently large values, an influence of the offset Vofs is small and an influence on the tracking error signal E
10
is also a little. When the difference between each of the outputs Va and Vb and the offset Vofs is too small to ignore the relation between the outputs Va and Vb and the offset Vofs, a change amount of the offset with respect to the amplitude of the tracking error signal E
10
becomes too large. Conventionally, to deal with the offset change in the tracking error signal due to variations in the amount of the reflection light from the medium, an offset correction is performed in such a manner that a tracking error signal is fetched by an A/D converter in a DSP (Digital Signal Processor), when a change in the reflection light amount is detected, an offset amount is measured, a correction amount is calculated from the offset amount and, after that, the correction amount is added to the tracking error signal in the DSP (JP11328696a and U.S. patent application Ser. No. 09/196,098).
FIG. 2
shows a conventional tracking servo control unit. The tracking error signal E
10
generated in
FIG. 1
is supplied to an adding circuit
200
for correcting an offset, and an arbitrary correction amount from a DSP
205
is added to the tracking error signal E
10
to thereby correct the offset. Unnecessary frequency band components in an offset-corrected tracking error signal E
11
are eliminated by a notching circuit
202
and a low pass filter
204
and, after that, the resultant signal is fetched as a tracking error signal E
12
by an A/D converter
206
in the DSP
205
. The DSP
205
supplies the tracking error signal fetched by the A/D converter
206
to a correction amount detecting unit
224
. An offset amount caused by a change in the amount of the reflection light in a sector ID area at the time of reproduction or in a data area at the time of recording or reproduction is measured, a correction amount to cancel out the offset amount is calculated and added to the output of the A/D converter
206
at an addition point
222
, and an offset-corrected tracking error signal E
13
is outputted. The correction amount detecting unit
224
uses an MOXID signal E
14
and a write gate signal E
15
supplied as signals for detecting a change in the reflection light to an edge port
232
. As shown in
FIG. 3A
, the MOXID signal is a logical signal which becomes at the H level in the data area in a medium sector and becomes at the L level in an ID area between sectors. Since the amount of reflection light decreases in the ID area, by using the MOXID signal, an offset is measured in the ID area and corrected.
FIG. 3B
shows the tracking error signal E
10
inputted to the adding circuit
200
, and an offset occurs in the ID area where the MOXID signal E
14
becomes at the L level.
FIG. 3C
shows sampling timings of the A/D converter
206
, the tracking error signal E
12
in
FIG. 3D
which has passed the notching filter
202
and the low pass filter
204
is sampled at timings of arrows to be converted to digital data, and the digital data is fetched. The correction amount detecting unit
224
measures an offset amount from a difference between sample values before and after the detection start timing in the ID area, calculates a correction amount having an amplitude of
FIG. 3E
from the measured offset amount, outputs the correction amount for a predetermined time, and adds the correction amount at the addition point
222
, thereby obtaining the offset-corrected tracking error signal E
13
as shown in FIG.
3
F. The track error signal E
13
in which the offset caused by a change in the reflection light amount has been corrected passes through an input gain multiplying unit
208
, a PID computing unit
210
, an output gain multiplying uni
Ikeda Toru
Nishida Masatsugu
Tsukahara Wataru
Yanagi Shigenori
Fujitsu Limited
Greer Burns & Crain Ltd.
Huber Paul W.
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