Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-09-17
2001-02-27
Tran, Thang V. (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044290, C369S044340
Reexamination Certificate
active
06195319
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical pickup position control device to be used in, for example, an optical recording and reproducing device, and, more specifically, relates to an optical pickup position control device which automatically performs, with precision and in a short time, adjustment of offset, etc. of a deviation signal expressing misalignment of a light spot, formed by the optical pickup on the surface of a disk, with a track centerline or focusing point of the disk.
BACKGROUND OF THE INVENTION
Devices, such as compact disk devices and minidisk devices, which optically reproduce (or record) data are provided with an optical pickup, which projects laser light onto a disk, and receives light reflected therefrom. In such devices, the position of the optical pickup with respect to the disk is generally controlled by means of tracking control and focus control. As is commonly known, tracking control is control which causes the light projected by the optical pickup to correctly follow a target track on the disk, and focus control is control which adjusts the focus position of the laser light in order to form on the target track a light spot of a predetermined diameter (i.e., a focused light spot).
Conventional devices for performing this kind of control include, for example, those disclosed in Japanese Unexamined Patent Publication Nos. 5-217315/1993 (Tokukaihei 5-217315; hereinafter “Document 1”), 5-151592/1993 (Tokukaihei 5-151592; hereinafter “Document 2”), and 5-151590/1993 (Tokukaihei 5-151590/1993; hereinafter “Document 3”).
In Document 1, gain of a focus servo loop or of a tracking servo loop (servo computing section) are adjusted automatically with each servo loop in a closed state. In Document 1, a signal of a predetermined frequency, produced by an oscillator (VCO), is applied to the servo loops. Further, by means of a band-pass filter, a signal of a certain frequency is extracted from the output of the servo computing section, and this extracted signal is multiplied with the signal from the oscillator. From the resulting signal, unneeded frequency components are eliminated using a notch filter, and gain is adjusted according to the output value of the notch filter.
In order to determine the state of a servo loop quickly and simply, it is most effective to apply, as above, an external signal to the servo loop. Accordingly, almost all disk devices including an automatic adjustment structure which are now on the market adopt the method of applying an external signal. For this reason, there are many cases in which the external signal producer and the various filters function solely as members for automatic adjustment, and are not used during normal reproducing. Further, since adjustment of the servo loop can only be performed with the servo loop in a closed state, it becomes necessary to repeat the operations of closing and opening the servo loop for each adjustment.
In Document 2, with the tracking servo loop in an open state, the pickup is moved a predetermined time or a distance necessary to cross a predetermined number of tracks. At this time, track deviation signals, which express an amount of deviation from the centerline of the target track, are measured from each side of the track, and a tracking error signal, which is a difference between these two deviation signals, is outputted. Then, by adjusting the gain of one of a pair of variable gain amplifiers through which the two track deviation signals are sent, so that a mean value of the tracking error signals approaches zero, the gain of the other variable gain amplifier is controlled in accordance with this adjustment. When tracking balance has been attained by repeating this gain adjustment until the mean value is zero, the gain control value at this time is stored in storage means (a memory).
In adjusting the balance, the gain of one of the variable gain amplifiers is adjusted, and a comparator determines whether the mean value of the tracking error signals is a value within a predetermined range. If it is determined that the mean value is outside the predetermined range, gain is again adjusted, and gain adjustment must be repeated until the mean value falls within the predetermined range.
In a focus control device according to Document 3, when not receiving the light beam reflected from the memory medium, two signals from position detecting means are measured, and, using these measured values, an offset correction quantity is set. With this method, offset arising in the circuit is corrected in a state free of influence from reflected light and stray light. In order to avoid the influence of gain and balance adjustment, this offset adjustment is performed prior to gain and balance adjustment.
After offset has been corrected, the position detecting means are returned to a state in which they can receive the light beam reflected from the memory medium, and gain adjustment is performed. In gain and balance correction, after adjusting one of the foregoing two signals, fine tuning of a control target position is performed. In fine tuning, gain correction quantity is roughly set so that the maximum amplitudes of the two signals from the position detecting means are equal. Then, when rough adjustment of focus has substantially been completed, gain correction quantity is further varied within a certain range, and the intensity of reflected light from the light projected onto the track is detected. At this time, as shown in
FIG. 9
, gain and balance correction quantities are set so that the intensity of the reflected light is maximum.
Incidentally, the present invention is chiefly directed toward tracking adjustment, but it may also be directed toward focus adjustment. Accordingly, the focus adjustment method of Document 3, which may also be applied to tracking adjustment, has been discussed above as prior art.
In typical optical pickup position control devices, as seen in Document 1, in performing tracking gain adjustment, an external signal is generally applied to the servo loop. This adjustment is naturally only possible when the servo loop is in a closed state. For this reason, control of external signal application, servo opening and closing, etc. becomes complicated. Further, since an external signal unnecessary in normal servo loop operations is applied, the servo system naturally becomes unstable, and the reliability of adjustment is impaired.
Moreover, since it is necessary to provide means for producing the external signal to be applied to the servo loop during tracking gain adjustment, and to perform complex switching of filters, etc. in order to extract the external signal, circuit structure and circuit processing are complicated. Further, if the S/N ratio of the signal to be reproduced has not yet been adjusted, passing the output of the servo computing section through the filters, etc. does not generally improve the S/N ratio, and the gain may not be correctly adjusted. There is also the drawback that adjustment using an external signal takes time. Furthermore, since gain adjustment is performed with the external signal inputted, unlike a case without input of an external signal, gain is adjusted in a somewhat different state due to the influence of each external signal. This accordingly leads to the drawback that tracking gain cannot be adjusted with the optimum value.
With the tracking balance adjustment of Document 2, in order to easily obtain a tracking error signal, the optical head is forcibly moved. Further, by repeatedly increasing and decreasing the gain of a variable gain amplifier with the tracking error signal passing through a low-pass filter, gain is adjusted so that the average value of the tracking error signal is zero. With this adjustment method, since various steps are repeated, the time needed for adjustment is lengthened. Again, the time needed for adjustment is also lengthened by the time required to move the optical head to the predetermined position.
With Document 3 relating to focus control, in order to avoid influence from the optical system
Hino Terufumi
Ohshita Akihiro
Nixon & Vanderhye P.C.
Sharp Kabushiki Kaisha
Tran Thang V.
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