Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-06-03
2001-07-24
Psitos, Aristotelis M. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S047360, C369S280000, C369S053130, C369S300000
Reexamination Certificate
active
06266305
ABSTRACT:
The present invention relates to a device for reading from and/or writing to optical recording media, which device has variable delay elements, according to the preamble of Claim
1
, and also to a method for setting the delay times of the delay elements.
A device of this type is disclosed in U.S. Pat. No. 4,785,441. In this known device, errors in the tracking signal which are caused by tilting of the optical recording medium or by different pit depths in the optical recording medium are compensated for by altering the delay times of the variable delay elements on the basis of a phase comparison carried out during operation.
The known device may be regarded as having the disadvantage that although errors caused by tilting of the optical recording medium or by different pit depths of the optical recording medium can be compensated for relatively well, the nature of the detection of an error caused by lens displacement is not optimal. This is due to the fact that error portions from other error sources, for example changes in the component properties caused by ageing or changing environmental conditions, also have an influence. This leads to the compensation actually not being optimal.
The object of the present invention is to propose a device which has the best possible compensation of the error in the track error signal and thus in the tracking signal, which error is caused on account of the lens movement.
This object is achieved according to the invention by means of the features specified below. In this case, an artificial interference signal is fed to the tracking unit, the track error signal caused thereby is compared with the interference signal and the result is fed to the control unit. This has the advantage that errors in the track error signal which are caused by lens deflection are optimally compensated for. The interference signal generating unit effects a deflection of the lens and thus an error in the track error signal, the output signal of the phase comparator. This error is not compensated for as long as the tracking control loop is not closed. An error caused by the lens deflection is thus initially manifested without any compensation in the track error signal. The interference signal and the track error signal are compared by means of the control unit and the delay times of the variable delay elements are set optimally using the result of this comparison. This means that after setting no or only a very weak residual influence of the interference signal remains in the track error signal.
In accordance with a first variant according to the invention, two further delay elements having a fixed delay time are provided, with the result that each detector element is assigned a delay element. By shortening or prolonging the delay times of the variable delay elements in comparison with those of the fixedly set delay elements, it is possible to delay the output signals of one as well as the other pair of detector elements relative to the respective other pair. This has the advantage that great variability can nevertheless be obtained by means of a small number of delay elements.
According to the invention, a further variant provides for a variable delay element to be assigned to each of the detector elements of the four-quadrant detector, and also for a switching unit to be provided which serves for connecting a respective detector element pair to a control output of the control unit. This has the advantage that the setting of the delay times which are suitable for optimal compensation is simplified since the output signals in each case of the more suitable pair of detector elements are temporally delayed by means of the associated delay elements. Delay elements which are constructed more simply and are thus more cost-effective can be used here, in accordance with a first embodiment, since all the delay elements can be designed to be adjustable only in one direction. That is to say their variable delay time is increased proceeding from a start value, preferably zero. In doing this, it is not necessary to proceed from an average start value of the delay from which the delay time can then be increased or decreased. Equipping each detector element with a variable delay element makes it possible, moreover, not only to compensate for an error caused by lens deflection but also to compensate for an offset in the track error signal or other adaptations, for example to undesirable delays caused by component tolerances, or similar interfering influences.
Another possibility according to the invention, which is to manage only with two variable delay elements, consists in respectively connecting these elements by means of a switching unit to the detector elements whose signals are to be delayed. This has the advantage that the delay times of the delay elements need to be altered only in one direction, for example exclusively prolonged.
The control unit advantageously has a comparison unit, to whose inputs the output signal of the phase comparator and the output signal of the interference signal generating unit are applied, the output signal of the comparison unit serving for setting the variable delay elements. This has the advantage that the comparison unit provides both a direction signal, which indicates whether the delay to be set must be positive or negative, or which pair of detector elements is to be delayed, and an absolute value signal, which indicates the magnitude of the required delay. The comparison unit advantageously has a synchronous demodulator.
According to a further advantageous refinement of the device, the control unit has an offset determining unit, to whose input the output signal of the phase comparator is applied, and whose output signal serves for setting the variable delay elements. This has the advantage that an offset possibly present in the track error signal is likewise compensated for by the setting of the delay times of the delay elements. For this purpose, the track error signal is advantageously integrated and the pair of detector elements to be delayed as well as the magnitude of the required delay are determined from the sign and absolute value of the output signal of the integrator.
The two error compensation units for offset and for errors caused by lens deflection are advantageously combined in order to be able to generate a track error signal which is as error-free as possible.
The invention provides for at least one of the delay elements to be arranged between a summation point for output signals of the detector elements of the four-quadrant detector and the phase comparator. This has the advantage that the offset compensation is effected with delay elements which are independent of the compensation of the lens movement.
In this case, the invention provides both solutions with two variable delay elements and a simple switching unit, and solutions with a single delay element and a somewhat more complicated switching unit. This has the advantage that, depending on the given boundary conditions, it is possible to realize the version which is the most favourable in each case in cost terms or as seen from the production outlay. In general, solutions with a small number of delay elements are preferable here since they take up a small chip area when realized as an integrated circuit.
The invention furthermore provides for an output signal of the control unit to be split up into absolute value and sign by means of a circuit block. This has the advantage that, for example, the sign signal can be used directly for driving a switching unit which, consequently, receives a quasi-digital driving signal of defined amplitude. Furthermore, this makes it unnecessary to effect sign determination e.g. in the case of the delay elements.
A particularly advantageous refinement in particular of the detection arrangement of the device according to the invention is specified below. This refinement has the advantage that detection arrangements and control units for setting the delay elements and thus for compensating for systematic errors in the track error signa
Deutsche Thomson-Brandt GmbH
Kiel Paul P.
Psitos Aristotelis M.
Tripoli Joseph S.
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