Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-03-26
2001-11-06
Hindi, Nabil (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044110, C369S053230
Reexamination Certificate
active
06314065
ABSTRACT:
The invention relates to an optical scanning apparatus for scanning an optical record carrier having at least a first and second superjacent information layers, comprising
an optical system including means for focusing a radiation beam to a focal point,
a focus actuator for controlling the focusing means to shift the focal point in a direction perpendicular to the layers,
a focus error detector for deriving a focus error (FE) signal indicative of the direction and distance between the focal point and the layer to be scanned,
a focus control circuit for deriving a focus actuator drive signal in response to the FE signal and supplying said drive signal to the focus actuator to cause the actuator to maintain the focal point substantially on the layer to be scanned,
a first controller for generating a binary loop-control signal which assumes a first logical value controlling a modification of the characteristics of the focus control circuit after receiving a layer-jump command for changing the scanning from the first to the second layer and a second logical value controlling the restoring of the characteristics after receiving a transition of a binary close signal.
An optical record carrier may comprise several information layers to increase its storage capacity. These layers may be scanned by a single optical beam which is brought to a focal point by means of an objective lens. A focus control circuit keeps the focal point on an information layer, the circuit controlling the position of the objective lens in response to a focus error signal, which represents the deviation of the focal point from the information layer. The focus error signal is derived from the radiation reflected or transmitted from the record carrier and intercepted by a photo-detection system. The radiation from the record carrier is also used to generate a so-called central aperture (CA) signal, which is a measure for the total amount of radiation reflected by the record carrier. The CA signal may be used to form an information signal representing the information read from the record carrier.
When a scanning apparatus starts a scanning session, it must move the objective lens towards the record carrier. During this movement, the focal point crosses the so-called entrance plane of the record carrier, a low-reflection boundary surface of a substrate of the record carrier through which the information layers are scanned. In order to avoid the focal point to lock on the entrance plane, the focal control circuit checks the amplitude of the CA signal. When the amplitude due to the entrance plane is below a certain level, the circuit will not lock the focal point on the entrance plane, and continue moving the objective lens towards the record carrier. When the focal point approaches the information layer of the record carrier, the high reflection of this layer will cause the CA signal to cross said level, and the focus control circuit will lock the focal point on the information layer.
The European patent application nr. 0 020 199 discloses a scanning apparatus comprising a focus control circuit for changing the focal point between two information layers of an optical record carrier. When changing to another information layer, the feedback loop of the focus control circuit is opened, the actuator is driven in the appropriate direction, and the level of the high-frequency content of the CA signal is monitored. When the amplitude of the high-frequency content crosses a certain level, the focal point is close to the information layer, and the feedback loop is closed. The focus control circuit then takes care that the focal point follows the information layer.
A disadvantage of the know scanning apparatus is, that the changing the focal point between layers does not operate anymore if one or both layers have not (yet) been provided with information. Since in that case the CA signal does not comprise the high-frequency content, said level will not be crossed and the feedback loop will not be closed.
It is an object of the invention to provide an optical scanning apparatus that can reliably focus on all information layers of an optical record carrier, whether the layers contain any information or not.
The scanning apparatus according to the invention is thereto characterized in that the optical scanning apparatus comprises
a second controller for generating the close signal in response to the FE signal and comprising an inhibitor for inhibiting the transition of the close signal in an interval around a zero-crossing of the FE signal in between subsequent zero-crossings corresponding to the information layers. It has turned out that a layer jump can be realized reliably by only monitoring the FE signal, which is not affected by the presence or absence of information contained in the information layers. This is in contrast with the known focus control circuits, which use both the FE signal and CA signal. When using only the FE signal, a special precaution must then be taken when moving the focal point from one layer to the other. If the feedback loop of the focus control circuit were to be kept closed when changing to another information layer, the actuator would acquire too much speed around the zero-crossing of the FE signal situated in between two information layers. As a consequence, the actuator might shoot over the designated information layer. The apparatus according to the invention solves the problem by opening the feedback loop or part of it in an interval around said zero-crossing, thereby avoiding the undue speed increase.
An important contribution to the speed increase comes from the differentiating branch in the feedback loop due to the slope of the FE signal around said zero-crossing. The part of the feedback loop to be opened is therefore preferentially the differentiating branch. The other two branches of the feedback loop, i.e. the proportional and integrating branch, can, to a certain extent, control the speed of the actuator during the jump in response to the motion of the record carrier.
In another embodiment of the apparatus according to the invention, all branches of the feedback loop are opened during a jump. The motion of the actuator during the jump can then be controlled independently from the motion of the record carrier.
A preferred embodiment of the scanning apparatus according to the invention is characterized in that the inhibitor comprises a comparator for detecting upward crossings of a preset level by the absolute value of the FE signal, and the second controller generates the transition of the close signal after the 2N
th
upward crossing after the layer-jump command. When subsequent information layers have subsequent ordinal numbers, then N is the absolute difference in ordinal number between the first and second information layer. The FE signal has generally the shape of the known S-curve, having a zero value when the focal point is far removed from an information layer and when the focal point is located on the information layer, and with a different sign on both sides of the information layer. When the focal point is moved through the information layers, each layer will give rise to an S-curve. The closeness of the neighbouring S-curves causes the emergence of a clear zero-crossing of the FE signal in between two information layers. The comparator detects when the absolute value of the FE signal crosses the preset level from a value below to a value above the preset below. Such a crossing is called an upward crossing. The first upward level crossing after start of the layer jump belongs to the S-curve of the information layer the focal point has just left. If the feedback loop were to be closed on the first upward crossing, as is common in the known scanning apparatuses, the focus control unit might drive the focal point back to the information layer it has just left. For a jump to a neighbouring layer the inhibitor according to the invention will allow that the feedback loop will only be closed after the 2
nd
upward level crossing. The 2
nd
upward level crossing occurs in the S-cur
Belk Michael E.
Hindi Nabil
U.S. Philips Corporation
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