Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1998-10-06
2001-07-17
Tran, Thang V. (Department: 2651)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044140, C369S044350
Reexamination Certificate
active
06262953
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of actuator coil positioning systems such as those used to position a lens of an optical disc drives.
BACKGROUND OF THE INVENTION
The invention relates to an apparatus for reading and/or writing information from/on a recording medium, comprising a scan element for scanning the medium and a servo loop for positioning said scan element, said servo loop comprising
an actuator for driving the scan element, said actuator comprising an actuator coil and
a control means conceived to feed the actuator coil with a drive current dependent on a position signal which is indicative of the actual position of the scan element and a reference signal which is indicative of the desired position of the scan element.
The invention also relates to an actuator for an apparatus for reading and/or writing information from/on a recording medium, which actuator comprises an actuator coil and a magnetic member.
The invention further relates to a control means for driving an actuator coil in order to position a scan element in an apparatus for reading and/or writing information from/on a recording medium, the control means being conceived to feed the actuator coil with a drive current dependent on a position signal which is indicative of the actual position of the scan element and a reference signal which is indicative of the desired position of the scan element.
Such an apparatus is known from JP-A-08212579. The known apparatus is an optical drive for reading and or writing information on an optical disk. The apparatus comprises an optical head with an objective lens and a so called Solid Immersion Lens (SIL). The objective lens is positioned relative to the disc by means of a first actuator. The SIL is positioned relative to the objective lens by means of a second actuator which is operative between the objective lens and the SIL. In order to derive a position signal indicative of the position of the SIL relative to the objective lens, the capacitance of a capacitor formed by a holder of the SIL and a holder of the objective lens is measured.
The above citations are hereby incorporated in whole by reference.
SUMMARY OF THE INVENTION
The inventors recognize that a disadvantage of this measurement is that it imposes additional requirements on the construction of the optical head, which are difficult to meet because of the small dimensions of such a SIL.
It is an object of the invention to provide an apparatus, an actuator and a circuit for driving an actuator coil as defined above which overcome said disadvantage. To this end the apparatus according to the invention is characterized in that
the apparatus comprises an inductance influencing means to vary the inductance of the actuator coil as a function of the position of the scan element and
the control means are conceived to derive the position signal from the actual inductance of the actuator coil.
A first advantage of these measures is that no additional wires are required. A second advantage is that the measurement of the inductance of the actuator coil is less influenced by other parameters such as the actual position and length of wires than a measurement of the capacitance between two lens holders. As a result, there is more freedom of design and the quality of the position signal is much better so that the scan element can be positioned more accurately with the servo loop.
The measure as defined in dependent claim
2
has the advantage that the bandwidth of the servo loop can be large because the influence of an electrically conductive and non-magnetic material on the inductance of the actuator coil is not affected by the magnetic field resulting from the drive current fed through the actuator coil. Such a material is for example copper, aluminum or silver. When an inductance influencing means comprising a electrically and non-magnetic material approaches the actuator coil, the magnetic field from the actuator coil causes eddy currents in this material. These eddy currents result in a magnetic field opposing the magnetic field generated by the actuator coil and in a reduction of the actuator coil inductance.
Usually an inductance influencing means comprising a magnetic material such as ferrite or iron is used in order to influence the inductance of a coil. Use of such a material results in an increase of the inductance of a coil when the inductance influencing means and the actuator coil approach each other. It has been found that the influence of such a material on the inductance of the actuator coil is affected by the drive current through the actuator coil. This results in so called magnetic cross-talk from the drive current via the magnetic material to the position signal. The mechanism causing this magnetic cross-talk is based on the fact that magnetic materials have a relative magnetic permeability which depends on the magnetic field strength imposed on them. This magnetic cross-talk limits the bandwidth in which the servo loop for positioning the scan element can be used.
Another advantage of the measure as defined in dependent claim
2
is that an electrically conductive and non-magnetic element can easily be implemented in an actuator and can have a second function for example as a mounting ring.
The measure as defined in dependent claim
3
has the advantage that the measurement current results in a cyclic voltage variation across the actuator coil, the amplitude and the phase of this variation being indicative of the inductance of the actuator coil and hence being suitable as a position signal. Such a position signal in the form of a cyclic measurement voltage can easily be separated from voltage variations due to variations of the drive current, for example with a synchronous detector or a bandpass filter. By this separation so called electric cross-talk from the drive current to the position signal can be counteracted. Such electric cross-talk also limits the bandwidth of the servo loop for positioning the scan element and therefore should preferably be prevented.
The measure as defined in dependent claim
4
has the advantage that voltage variations at the measurement frequency due to a change of the actuator coil inductance can be separated from voltage variations resulting from variations of the drive current with simple means such as a high-pass filter.
An actuator usually comprises a magnetic member such as a permanent magnet and in many cases also a ferromagnetic flux guide. The measure as defined in dependent claim
5
has the advantage that, at the measurement frequency, the interaction between such a magnetic element and the actuator coil is reduced so that magnetic cross-talk from the drive current to the position signal is counteracted. As a result, the servo loop can be used in a much larger frequency range and the scan element can be positioned with a far better accuracy. Preferably, the actuator does not comprise magnetic flux guides so that only permanent magnetic members have to be shielded.
The measure as defined in dependent claim
6
has the advantage that the penetration depth of the electromagnetic waves resulting from the measurement current in an electrically conductive material is so small that a metal foil suffices to shield the magnetic member or members. Such a foil is attractive as it hardly influences the dimensions and the weight of the actuator or the scan element. In addition, such a foil can also act as the inductance influencing means. Hence, the addition of such a foil to an existing actuator makes the actuator very suitable for use in an appraratus according to the invention.
The measure as defined in dependent claim
7
has the advantage that the measurement current through the actuator coil is always amplified by resonance so that the measurement voltage indicative of the inductance of the actuator coil is also amplified. This amplification results in an improved signal to noise ratio of the position signal so that the scan element can be positioned more accurately. Another advantage of the measure of dependent claim
7
is that the actua
Kahlman Josephus A. H. M.
van Rosmalen Gerard E.
Belk Michael E.
Tran Thang V.
U.S. Philips Corporation
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