Dynamic information storage or retrieval – Information location or remote operator actuated control – Selective addressing of storage medium
Patent
1985-12-09
1989-04-18
Corbin, John K.
Dynamic information storage or retrieval
Information location or remote operator actuated control
Selective addressing of storage medium
350255, 369 45, G02B 702, G11B 700
Patent
active
048221392
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to an optical head capable of picking up or erasing recorded information by converging a laser beam on a rotating disc type recording medium by means of a condenser lens to record the information, and accessing an arbitrary position on the disc for recording or reproducing, that is, accessing randomly at high speed.
BACKGROUND OF THE INVENTION
In the field of optical disc player, recently an optical head capable of accessing randomly an arbitrary position on a disc at high speed is attracting a wide attention.
The conditions of high speed access may be summarized below.
That is, there are two requirements for high speed access of the optical head. The first is that the speed of moving the optical head in the radial direction of the disc should be high, and the second is that the tracking servo should be stable.
To access at high speed, generally two processes--that is, rough access and fine access--are combined.
The rough access is a process of moving the optical head approximately to the radical position at a desired address position on the disc. The approximate radial position is known, for example, by attaching a linear potentiometer to the optical head and reading its voltage. Supposing the distance between the radial position before moving and that of the desired address track to be x, the fastest rough access is achieved by the optical head up to x/2 position at maximum acceleration and slowing down from x/2 to x position at maximum deceleration. It is ideal if the desired track is reached by the rough access alone, and in a magnetic disc or the like, the access is completed by this rough access alone. However, in the magnetic disc, the track width is about tens of .mu.m to hundreds of .mu.m, while the track width of an optical disc is as narrow as 1.6 .mu.m, and it is extremely difficult to reach the desired address track by the rough access alone. Accordingly, by reading the address of the track reached by rouch access, the light spot is caused to jump from this address track to the desired address track by a jumping pulse. This is called fine access.
Therefore, to access in the shortest time, it is necessary to quicken the moving speed of the optical head and bring it as closely to the desired track as possible in the rough access. In the fine access, it is required to jump over may tracks accurately in a short time, and for this purpose, the wide drawing range of the tracking servo and high servo gain are indispensable.
Various optical heads have been known conventionally, but, for example, to access within 0.1 second, sufficient characteristics are not obtained by either rough access or fine access. The main reason is that the moving part is heavy. Conventionally, the usual practice is to move the entire optical head by means of a linear motor, but since the optical head comprises a laser, a focusing actuator and its magnetic circuit, tracking actuator and its magnetic circuit, mirrors and a polarizing prism, lenses, a photo detector, a preamplifier,and other parts, its weight is at least several hundred grams. The rough access time T is expressed, assuming the moving part weight to be m, drive force to be F and moving distance to be x, as follows. for a moving distance of x/2 and decelerate from x/2 to x, the equation of motion example, the rough access time is T=0.15 sec. Therefore, in the prior art, it takes as long as 0.15 sec for the rough access alone.
In the fine access, conventionally, the drawing range of tracking was too narrow. In a disc for recording and reproducing, the method of picking up an error signal of the tracking servo from the far-field pattern of diffracted light from the guide groove for tracking is called the far-field method, and in the conventional tracking method of swinging the objective lens in the radial direction of the disc or using a tracking mirror, the drawing range of the tracking servo is narrow.
For instance, in the method of swinging the lens for tracking purposes, as shown in FIG. 1(a), the reflected ligh
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Ben Loha
Corbin John K.
Matsushita Electric - Industrial Co., Ltd.
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