A magnetic bias apparatus for a magnetooptical disk drive apparatus

Abstract: A magnetic bias apparatus for generating a magnetic field for recording or erasing information in a magnetooptical disk drive apparatus is disclosed. The magnetic bias apparatus has a permanent magnet for generating a bias magnetic field which is rotatable about a rotating axis, a coil for rotating the permanent magnet, and a circuit for driving the coil. The central axis of the coil is directed toward the rotating axis and perpendicular to the line connecting both the two poles of the permanent magnet. The apparatus can be constructed in a reduced height while the permanent can be rapidly rotated.

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: This invention relates to a magnetic bias apparatus for a magnetooptical disk drive apparatus in which information is recorded on and erased from a recording medium using the magnetooptical effect. 2. Description of the prior art: The technique of magnetooptically recording information on a recording medium such as a disk has drawn great attention. A magnetooptical recording apparatus using this technique is provided with a magnetic bias apparatus which is disposed in a position opposite to an optical head. Figure 11 diagrammatically illustrates a prior art magnetooptical recording apparatus. The apparatus of Figure 11 comprises a motor 1a for rotating a magnetooptical disk 1 , an optical head 2 , and a magnetic bias apparatus A which faces the optical head 2 through the disk 1 . The optical head 2 includes an object lens 2a, and a semiconductor laser device (not shown). The magnetic bias apparatus A has a permanent magnet 3 , and a coil 4 which is arranged above the magnet 3 (i.e., above the N-pole of the magnet). The magnet 3 is rotatable about an axis 3a . When data erasing is performed, the S-pole of the magnet 3 is directed as shown in Figure 11 to face the disk 1 so as to produce a magnetic field of a predetermined direction, and a light beam emitted from the optical head 2 is converged onto the disk 1 , thereby erasing data stored on the disk 1 . When data is to be recorded on the disk 1 , the coil 4 is driven to rotate the magnet 3 about the axis 3a by a half turn, so that the N-pole faces the disk 1 and a magnetic field of a direction opposite to the predetermined direction is applied to the disk 1 . Then, a light beam modulated in accordance with the data to be recorded is converged onto the disk 1 to record the data on the disk. Alternatively, the relationships of the directions of the magnetic fields may be inverted, in other words, the magnet 3 may be positioned so that when data is to be erased the N-pole faces the disk 1 and when data is to be recorded the S-pole faces the disk 1 . It is desirable that a magnetooptical recording apparatus should be as small and as thin as possible. For this reason, it is necessary for a magnetic bias apparatus also to be made as thin as possible. In an apparatus having the structure shown in Figure 11, the coil 4 for reversing the direction of the magnetic field is positioned above the permanent magnet 3 , and therefore the magnetic bias apparatus cannot be thinly constructed. Figure 12 illustrates another magnetic bias apparatus. In the apparatus shown in Figure 12, the coil 4 and the permanent magnet 3 are juxtaposed in parallel so that the overall height of the apparatus can be decreased. Although the apparatus shown in Figure 12 can solve the above-mentioned problem, it has another problem in that the coil 4 generates the magnetic flux as shown in Figure 12, and therefore the rotating force f which is caused by the magnetic field of the coil 4 and acts on the magnet 3 is extremely small, resulting in that a significant amount of time is required for rotating the magnet 3 or the response characteristic of the magnetic bias apparatus is poor. SUMMARY OF THE INVENTION The magnetic bias apparatus for a magnetooptical disk drive apparatus of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises: a permanent magnet for generating a bias magnetic field, said permanent magnet being rotatable about a rotating axis; a coil for rotating said permanent magnet, said coil being located beside the permanent magnet; and drive means for driving said coil; characterised in that the central axis of said coil is directed toward said rotating axis and perpendicular to the line connecting both the two poles of said permanent magnet. In preferred embodiments, a body of a ferromagnetic material is disposed at the center of said coil. In preferred embodiments, the length of said body is smaller than the length of said coil along said central axis. In preferred embodiments, said body is a substantially rectangular body. In preferred embodiments, one of the surfaces of said body faces said permanent magnet. In preferred embodiments, said one surface is a curved one which is concaved in the direction along said central axis and departing from said rotating axis of said permanent magnet. Thus, the invention described herein makes possible the objectives of: (1) providing a magnetic bias apparatus in which the magnetic flux for rotating the permanent magnet can be effectively applied to the permanent magnet; (2) providing a magnetic bias apparatus in which the permanent magnet can be rapidly rotated; (3) providing a magnetic bias apparatus which can be constructed in a reduced height or thickness; and (4) providing a magnetic bias apparatus which is low in height or small in thickness while the permanent magnet can be rapidly rotated. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a perspective view diagrammatically illustrating a magnetic bias apparatus according to the invention. Figure 2 is a diagram illustrating the operation of the apparatus of Figure 1. Figure 3 is a diagram illustrating another apparatus according to the invention. Figure 4 is a plan view illustrating the arrangement of the apparatus of Figure 3. Figure 5 is a perspective view illustrating the arrangement of the apparatus of Figure 3. Figure 6 illustrates the relationship between the length of the ferromagnetic body and the strength of the magnetic field produced by the coil in the apparatus of Figure 3. Figure 7 illustrates the change of the strength of the magnetic field produced by the permanent magnet with the increase of the distance from the permanent magnet in the apparatus of Figure 3. Figure 8 illustrates the relationship between the length of the ferromagnetic body and the strength of the force in the apparatus of Figure 3. Figure 9 is a plan view illustrating the arrangement of a further apparatus according to the invention. Figure 10 is a diagram illustrating the embodiment shown in Figure 9. Figure 11 is a diagram illustrating a conventional apparatus. Figure 12 is a diagram illustrating another conventional apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows an embodiment of this invention. The magnetic bias apparatus A of this embodiment comprises a permanent magnet 3 for generating a magnetic field, a coil 4 for rotating the permanent magnet 3 . The permanent magnet 3 is rotatable about an axis 7 which is supported by an adequate supporting means (not shown). The apparatus A further comprises a driving circuit 8 for driving the coil 4 , a magnetic detector 5 which detects the direction and strength of the magnetic field, and a changeover switch 9 by which the polarity of the voltage applied to the coil 4 is inverted in accordance with the output of the magnetic detector 5 . The magnetic detector 5 detects the position or rotating angle of the permanent magnet 3 , and in accordance with the degree of the detected rotating angle outputs a signal to the driving circuit 8 , so that the amount and direction of the current flowing the coil 4 are controlled, thereby controlling the position or rotating angle of the permanent magnet 3 . More specifically, the magnetic detector 5 outputs a voltage which corresponds to the strength of the magnetic field generated by the permanent magnet 3 , and the output voltage is coupled to the driving circuit 8 . The driving circuit 8 supplies a voltage for driving the coil 4 to rotate the permanent magnet 3 so that the poles of the magnet 3 are positioned at a desired positional relationship. In this embodiment, the coil 4 is a cylindrical coil. The axis Y of the coil 4 is directed toward the axis 7 of the permanent magnet 3 , and is perpendicular to the line ( Z in Figure 5) connecting the N- and S-poles of the permanent magnet 3 when the magnet 3 is at either of the recording position or the erasing position. When the permanent magnet 3 is to be inverted or rotated, the switch 9 is set to the position for supplying a current of the direction by which a magnetic field of the direction as indicated by an arrow M in Figure 2 is generated, and the coil 4 is driven by the current. In this situation, since the coil 4 is positioned in the manner described above, the direction of the force F caused by the magnetic field M of the coil 4 and applied to the magnet 3 coincides with the rotating direction of the magnet 3 at the initial stage of the rotation, whereby the force F is effectively applied to the magnet 3 . According to this embodiment, therefore, the rapid inversion of the permanent magnet 3 can be achieved, and an efficient magnetic bias apparatus is obtained. Figure 3 diagrammatically illustrates another embodiment of the invention. This embodiment is constructed in the same manner as that shown in Figure 1 except that a body 10 having a rectangular section and made of a ferromagnetic material such as iron is fixedly positioned at the vicinity of the center of the coil 4 . Figure 4 shows the positional relationship between the coil 4 and the ferromagnetic body 10 . As shown in Figure 4, the length L₁₀ of the ferromagnetic body 10 along the axis Y is smaller than the length L₄ of the coil 4 (in this embodiment, L₁₀ = (3/4) x L₄ ). In the apparatus of this embodiment, when the magnetic field M (Figure 2) is generated by the coil 4 , the ferromagnetic body 10 is magnetized by the magnetic field M . The existence of the magnetized ferromagnetic body 10 contributes to the further increase of the strength of the force F . Figure 6 illustrates the relationship between the length L₁₀ of the ferromagnetic body 10 and the strength of the magnetic field produced by the coil 4 . Figure 7 illustrates the change of the strength of the magnetic field produced by the permanent magnet 3 with the increase of the distance from the magnet. Figure 8 illustrates the relationship between the length L₁₀ of the ferromagnetic body 10 and the strength of the force F . As seen from Figure 6, the larger the length L₁₀ of the ferromagnetic body 10 , the greater the strength of the magnetic field produced by the coil 4 becomes. As Figure 7 shows, however, the closer to the permanent magnet 3 , the greater the strength of the magnetic field produced by the permanent magnet 3 becomes. When the ferromagnetic body 10 is excess in size, therefore, it may be drawn to the permanent magnet 3 . Preferably, therefore, the length L₁₀ of the ferromagnetic body 10 is set to be from about 1/5 to about 4/5 of the length L₄ of the coil 4 . Figure 9 shows a ferromagnetic body 11 used in another embodiment of the invention. As shown in Figure 10, the ferromagnetic body 11 is fixedly positioned at the vicinity of the center of the coil 4 , in the same manner as the embodiment of Figure 3. The surface 11a of the ferromagnetic body 11 which faces the permanent magnet 3 is curved or concaved in the direction opposite to the permanent magnet 3 . In other words, the length L₁₁ is gradually decreased, and has the minimum value at the center in the direction X (Figure 5). Since the strength of the magnetic field generated by the permanent magnet 3 reaches the maximum in the vicinity of the center in the direction X and is gradually decreased toward both ends, the influence on the ferromagnetic body 11 from the magnetic field generated by the permanent magnet 3 is made uniform, and thus the number of the magnetic flux can be increased or the force F (Figure 2) can be multiplied, as compared with the embodiment shown in Figure 3. As mentioned above, the magnetic bias apparatus according to the invention is able to achieve high speed reversing of the magnetic field with a simple construction, and is suitable for a small and thin magnetooptical disk drive apparatus. Furthermore, because the magnetic flux is used efficiently in the magnetic bias apparatus according to the invention, the amount of the current supplied to the coil can be reduced, with the result of the reduced electric power consumption. Hence, the present apparatus is very useful also in the view point of the electric power saving. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of this invention. Accordingly, it is not intended that the scope of the claims appended hereto be limited to the description as set forth herein, but rather that the claims be construed as encompassing all the features of patentable novelty that reside in the present invention, including all features that would be treated as equivalents thereof by those skilled in the art to which this invention pertains.

Claims: A magnetic bias apparatus for a magnetooptical disk drive apparatus, comprising: a permanent magnet for generating a bias magnetic field, said permanent magnet being rotatable about a rotating axis; a coil for rotating said permanent magnet, said coil being located beside the permanent magnet; and drive means for driving said coil; characterised in that the central axis of said coil is directed toward said rotating axis and perpendicular to the line connecting both the two poles of said permanent magnet. An apparatus according to claim 1, wherein a body of a ferromagnetic material is disposed at the center of said coil. An apparatus according to claim 2, wherein the length of said body is smaller than the length of said coil along said central axis. An apparatus according to claim 2, wherein said body is a substantially rectangular body. An apparatus according to claim 4, wherein one of the surfaces of said body faces said permanent magnet. An apparatus according to claim 5, wherein said one surface is a curved one which is concaved in the direction along said central axis and departing from said rotating axis of said permanent magnet.

Inventors: KURODA MOTOKI, MATSUDA NOBUHIDE, KURODA, MOTOKI, MATSUDA, NOBUHIDE
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: G11B11
IPC codes: G11B11