Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Patent
1996-06-17
1998-06-09
Tran, Thang V.
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
369 4435, 369 4436, G11B 709
Patent
active
057646058
DESCRIPTION:
BRIEF SUMMARY
The invention relates to methods and arrangements for G factor alignment or for setting a device, providing control signals or a track error signal in the case of deviations of the scanning beam from the information track of a recording substrate, in control loops for track guidance of optical scanning devices which for the purpose of track guidance of the scanning device make use, for example, of the known transverse push-pull or differential push-pull methods. Such scanning devices are fitted, for example, to CD players, videodisc players, draw disc players and magnetooptical recording and playback devices.
Push-pull refers in general to a method for generating a track error signal for the track control loop in the case of radial or lateral deviation of the scanning beam from the information track of the recording substrate, the push-pull signal being the differential signal of an at least bipartite photodetector which permits the scanning device or the scanning beam to be guided on the track of the recording substrate. The light reflected by the optical recording substrate and detected by the photodetector has an intensity distribution, which is a function of the nature of the incidence of the scanning beam on an information track or of the track positioning, is converted into electric signals and used as reference input variable or push-pull signal in the track control loop, which keeps the scanning beam on the information track. For this purpose, a track control amplifier or track controller controls a scanning device or a so-called pickup. The pickup contains a laser which generates the scanning beam, a movable holder, referred to as an actuator, of an objective lens for fine positioning and focusing of the scanning beam, which is controlled by a servo device, a beam splitter for splitting the transmitting and receiving directions of the light, and a photodetector for evaluating the light reflected by the recording substrate, cf. Electronic Components & Applications, Vol. 6, No. 4, 1984, pages 209 to 215. In order to be able to scan the relatively large track area of a recording substrate, the pickup is arranged as a rule on a coarse drive which can be moved perpendicularly to the information track and forms with the servo device the so-called radial drive for track guidance of the scanning device. The servo device, also referred to as the vernier drive, is provided in this case particularly in order to be able to follow rapid track displacements owing to disc runout or eccentricities of the recording substrate. Because of disc runout or eccentricity of the recording substrate, the objective lens is in perpetual movement via the pickup in order to follow the information track of the recording substrate. The optical axis of the scanning device, which is a function of the position of the objective lens varies its position and wanders over the photodetector, and the light reflected by the recording substrate reaches the photodetector as a function of the current position of the actuator. The result of this is that despite ideal positioning of the scanning beam on the information track of the recording substrate a track error signal or push-pull signal is generated owing to inclination and movement of the actuator and a deviation from the information track is simulated. Furthermore, the push-pull signal can simulate track deviations which originate from the mechanical stability or instability of the pickup and its adjustment. The photodetectors are fastened to the pickup by means of an adhesive, for example, and float in the adhesive as a function of temperature and air humidity, as a result of which movements are caused. It is therefore not possible to distinguish by means of the push-pull signal whether the track error signal is actually to be ascribed to a track deviation, an inclination of the actuator, instabilities in the pickup or else to properties of the recording substrate. The so-called transverse push-pull and the differential push-pull are already known for the purpose of avoiding this disad
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Patent Abstracts of Japan, vol. 14, No. 465, and Japan Pat. No. 2-185723.
Buchler Christian
Zucker Friedhelm
Burke Alexander J.
Deutsche Thomson-Brandt GmbH
Kurdyla Ronald H.
Tran Thang V.
Tripoli Joseph S.
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