1. Field of the Invention
The present invention relates generally to an apparatus for generating magnetic field varying in response to an information signal, and more particularly, is directed to improvements in an apparatus operative to generate a magnetic field varying in response to an information signal, which is used for recording information signals on a magnetooptic disc with magnetic field modulation.
2. Description of the Prior Art
There has been proposed a so-called optical writable disc as one type of record disc which is available for optically writing an information signal in a record track provided thereon and optically reading an information signal written in the record tracks provided thereon. A magnetooptic disc is one of such optical writable discs and distinguished in that erasure of information signals and writing of the information signals are able to be conducted repeatedly thereon.
The magnetooptic disc comprises a disc-shaped transparent base, a vertically magnetized layer provided for forming a recording layer on a surface of the disc-shaped transparent base, a reflecting layer formed with a thin metallic layer provided on the vertically magnetized layer and a protective layer provided to cover the reflecting layer. Recording of information on the magnetooptic disc is effected by erasing an information signal written therein and writing a new information signal therein, and reproduction of information from the magnetooptic disc is effected by reading an information signal written therein.
In the event of writing of an information signal on the magnetooptic disc, the magnetooptic disc is rotated on an axis of rotation at a central portion thereof and a laser light beam is caused to impinge upon the vertically magnetized layer on a record track portion of the magnetooptic disc, to which a predetermined external magnetic field acts, so that a part of the vertically magnetized layer on the record track portion upon which the laser light beam impinges is magnetized in a direction depending upon the external magnetic field with temperature rise due to impingement of the laser light beam. The information signal is written in the vertically magnetized layer on the record track portion with inversions in the direction of magnetization provided in the vertically magnetized layer by means of a combination of the laser light beam controlled to be constant in intensity and the external magnetic field modulated in direction in response to the information signal or a combination of the laser light beam modulated in intensity in response to the information signal and the external magnetic field produced to be constant in direction.
For writing the information signal in the vertically magnetized layer of the magnetooptic disc with the combination of the laser light beam controlled to be constant in intensity and the external magnetic field modulated in direction in response to the information signal, a magnetic field generating apparatus which is operative to generate a magnetic field varying in response to the information signal and to cause the magnetic field thus generated to act to the vertically magnetized layer of the magnetooptic disc is employed. The magnetic field generating apparatus includes as a main component a magnetic head comprising a magnetic core and a coil wound on the magnetic core to be supplied with a current varying in response to the information signal. It is preferable that the magnetic field generated by the magnetic field generating apparatus to be applied to the vertically magnetized layer of the magnetooptic disc be provided with sufficiently large intensity for acting on the vertically magnetized layer of the magnetooptic disc, and therefore the magnetic head included in the magnetic field generating apparatus is required to be disposed at a position as close as possible to the surface of the magnetooptic disc.
However, it is usual that the magnetooptic disc is rotated with a portion surrounding a central portion thereof and being subjected to periodic deviations in a direction of an axis of rotation during the rotation of the magnetooptic disc, so that it is very difficult to maintain the magnetic head included in the magnetic field generating apparatus to be close to but not to contact the surface of the magnetooptic disc without any particular control for the position of the same. Accordingly, there has been proposed a servocontrol system for the magnetic head included in the magnetic field generating apparatus wherein the magnetic head is mounted on an actuator and variations in a space between the magnetic head mounted on the actuator and the surface of the magnetooptic disc are detected to produce a detection output which is used for driving the actuator to keep the space between the magnetic head mounted on the actuator and the surface of the magnetooptic disc at a predetermined small value regardless of the periodic deviations of the portion of the magnetooptic disc during the rotation of the magnetooptic disc.
In the operation of such a servocontrol system, detection of the variations in the space between the magnetic head and the surface of the magnetooptic disc is achieved, for example, by detecting variations in capacitance between the magnetic head and the reflecting layer provided in the magnetooptic disc.
For detecting the variations in capacitance between the magnetic head and the reflecting layer provided in the magnetooptic disc in order to carry out the detection of the variations in the space between the magnetic head and the surface of the magnetooptic disc, which is required in the servocontrol system for keeping the space between the magnetic head and the surface of the magnetooptic disc at the predetermined small value regardless of the periodic deviations of the portion of the magnetooptic disc during the rotation of the magnetooptic disc, it is necessary to provide a capacitance detecting electrode to be opposite to the magnetooptic disc and to move together with the magnetic head. For meeting this requirement, the capacitance detecting electrode is attached to the magnetic head or the actuator on which the magnetic head is mounted so as to be driven by the actuator together with the magnetic head. However, this results in a disadvantage that the magnetic field generating apparatus including the magnetic head, the actuator and the capacitance detecting electrode results in a complicated configuration and is large in size, and also in another disadvantage that the actuator is burdened with driving a heavy load. Further, in the magnetic field generating apparatus including the magnetic head, the actuator and the capacitance detecting electrode, it would be feared that an eddy current is induced in the capacitance detecting electrode due to the magnetic field generated by the magnetic head and that the capacitance detecting electrode would be undesirably heated by means of eddy current loss.