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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a magnetic drive unit including a voice
coil and a magnetic circuit for moving a driver member such as an
objective lens in an optical pickup in small intervals, and more
particularly to a magnetic drive unit capable of moving the voice coil
linearly in small intervals.
Recently available digital audio disc (DAD) players have an optical pickup
for reading information recorded on a disc. The optical pickup has a
magnetic drive unit for keeping an objective lens a constant distance from
the disc at all times to focus a detecting beam spot on the
information-recorded surface in the disc. FIG. 1 of the accompanying
drawings is a side elevational view, partly in cross section, of a
conventional magnetic drive unit 11'. The magnetic drive unit 11' operates
on the same principle as that of an audie loudspeaker. The magnetic drive
unit 11' includes a magnet 11b disposed centrally in a yoke 11c and a pole
piece 11d bonded to an upper surface of the magnet 11b. The pole piece 11d
and an upper flange 11e of the yoke 11c which confronts the pole piece 11d
jointly define therebetween a magnetic gap 14 in which a voice coil 11'a
is disposed. The voice coil 11'a supports thereon a driver member
comprising an objective lens 8 held in confronting relation to the disc,
which is denoted at 1. The objective lens 8 is moved vertically through
small intervals by varying a current flowing through the voice coil 11'a.
Another magnetic drive unit 12' with the yoke 11c serving as a drive
member has a voice coil 12'a. The objective lens 8 can be moved
horizontally in small intervals by varying a current flowing through the
voice coil 12'a. A light beam emitted from an optical detector 15
comprising such as a laser diode is reflected by a prism 7 and focused by
the objective lens 8 onto the information-recorded surface in the disc 1.
The beam spot on the information-recorded surface in the disc 1 can be
focused thereon through small movements of the objective lens 8 driven by
the magnetic drive units 11', 12', and adjusted thereby to follow the
tracks on the disc 1 accurately. In the field of the optical pickup, the
magnetic drive unit 11' is called a "focusing servomechanism" and the
magnetic drive unit 12' is called a "tracking servomechansim".
The voice coil 11'a is normally constructed of windings of a copper wire.
Since the copper wire is of a round cross section, the coil windings have
a poor space factor. The space factor is particularly low where the coil
has a plurality of winding layers. To cope with this, there has been
proposed to construct the voice coil 11'a of windings of a wire having a
rectangularly cross section which gives a better space factor. However,
when such a wire is wound, the radially outward portion of the coil is
subjected to a tensile stress, while the radially inward portion undergoes
a compressive stress, with the result that the coil will be distorted
during the manufacturing process. Another proposal is a printed coil which
however is required to be produced in a complex manufacturing process and
hence is costly to fabricate.
The voice coil 11'a has a uniform winding density. The magnetic field
generated in the magnetic gap 14 between the flange 11e of the yoke 11c
and the pole piece 11d has a high flux density in its central area, the
flux density becoming progressively lower in a direction away from the
central area. Accordingly, the force acting on the voice coil 11'a tends
to vary as the voice coil 11'a is moved in the magnetic gap 14.
SUMMARY OF THE INVENTION
With the conventional shortcomings in view, it is an object of the present
invention to provide a magnetic drive unit including a voice coil which
has a high space factor and which can be linearly moved in small
intervals.
According to the present invention, there is provided a magnetic drive unit
including a magnetic circuit composed of a yoke and a magnet which define
a magnetic gap therebetween, and a voice coil movably disposed in the
magnetic gap for moving a driven member, the voice coil comprising turns
formed by helically cutting a tubular conductive body, the turns being
spaced at pitches which are larger within the magnetic gap and become
progressively smaller in directions away from the magnetic gap. The turns
are of a rectangular cross section and insulated by a mass of resin coated
on their surfaces.
The above and other objects, features and advantages of the present
invention will become more apparent from the following description when
taken in conjunction with the accompanying drawings in which preferred
embodiments of the present invention are shown by way of illustrative
example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view, partly in cross section, of a
conventional magnetic drive unit;
FIG. 2 is a schematic view of an optical pickup having magnetic drive units
according to the present invention;
FIG. 3 is an enlarged cross-sectional view of portions encircled at A in
FIG. 2;
FIG. 4 is a perspective view of a means for forming a voice coil;
FIG. 5 is an enlarged cross-sectional view of a magnetic drive unit
according to another embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As shown in FIG. 2, an optical pickup 16 for use in a DAD player comprises
an actutator 13 for driving an objective lens 8 under servo control, and
an optical detector 15. The optical detector 15 is composed of a laser
diode 2, a beam splitter 3, a collimating lens 4, a 1/4-wave plate 6, a
cylindrical lens 9, and a photodiode 10.
The actuator 13 includes magnetic drive units comprising a focusing
servomechanism 11 and a tracking servomechanism 12, which are of the same
construction. More specifically, magnets 11b, 12b are disposed centrally
in yokes 11c, 12c, respectively, and voice coils 11a, 12a are disposed in
magnetic gaps 14 in magnetic circuits composed of the yokes 11c, 12c and
the magnets 11b, 12b. An objective lens 8 serving as a driven member is
mounted on the voice coil 11a, and the other voice coil 12a is connected
to the yoke 11c which also serves as a driven member. The focusing
servomechanism 11 moves in small intervals for focusing a beam spot
emitted from the objective lens 8 on the information-recorded surface 1b
in the disc 1. The tracking servomechanism 12 moves in small intervals for
enabling the beam spot to follow tracks of pits p on the
information-recorded surface 1b in the disc 1.
FIG. 3 illustrates at an enlarged scale each of encircled portions A in
FIG. 2. Since the focusing servomechanism 11 and the tracking
servomechanism 12 are of the same construction, only the focusing
servomechanism 11 will hereinafter be described.
A magnetic gap 14 having a width C is defined between an upper flange 11e
of the yoke 11c and a pole piece 11d bonded on an upper surface of the
magnet 11b in confronting relation to the flange 11e. The voice coil 11a
disposed in the magnetic gap 14 is formed by cutting a tubular conductive
body 17 along a helical path as shown in FIG. 4. The tubular conductive
body 17 may be cut by a lathe or laser. As shown in FIG. 3, a pitch P at
which the cutter or laser beam is fed along during machining operation is
progressively greater (from P to .DELTA.P) while the cutter or laser beam
goes from the center of the conductive body 17 to its opposite ends.
Accordingly the number of turns per unit length of the voice coil 11a is
progressively greater from the center thereof toward the opposite ends
thereof. The cut wire is of a rectangular cross section. After the voice
coil has been machined, a coating of resin, for example, is deposited on
the cut surfaces and the inner and outer peripheral surfaces of the coil
to insulate the cut turns. The voice coil 11a thus fabricated is
vertically movably supported, together with the objective lens 8 for
example, by a leaf spring. Under normal conditions, a portion of the voice
coil 11a which has the pitch .DELTA.P is positioned in the magnetic gap
14.
Information reading operation of the optical pickup will be described.
The disc 1 is clamped on a turntable (not shown) and rotated thereby. A
beam emitted from the laser diode 2 in the optical detectgor 15 passes
through the beam splitter 3, the collimating lens 4, and the 1/4-wave
plate 6, and is then reflected by the prism 7 toward the objective lens 8.
A beam spot is radiated from the objective lens 8 onto the
information-recorded surface 1b in the disc 1. The beam reflected from the
disc surface 1b goes through the objective lens 3 and changes its
direction through 90.degree. in the beam splitter 3 to travel through the
cylindrical lens 9. The beam is then detected by the photodiode 10 for
reading information therefrom.
Operation of the magnetic drive units is as follows: The focusing
servomechanism 11 is actuated to focus the beam spot from the objective
lens 8 on the disc surface 1b. More specifically, when a current flowing
through the voice coil 11a is varied, the voice coil 11a is slightly moved
vertically in FIG. 3 to drive the objective lens 8. Since the number of
turns of the voice coil 11a per unit length is progressively increased
from the center of the voice coil toward the ends thereof, the flux
density distribution (which is progressively coarser in directions away
from the center of the magnetic gap 14) in the magnetic gap 14 can
effectively utilized for moving the voice coil 11a linearly. Specifically,
in a position where the flux density is small away from the magnetic gap
14, an effective electromagnetic force can be generated as the number of
turns of the voice coil 11a is large in such a position close to the ends
of the voice coil 11a.
Due to the influence of a flux leakage from a side of the magnet 11b within
the yoke 11c, the flux densities at positions 14a, 14b away from the
magnetic gap 14 are asymmetrical with respect to the magnetic gap 14. With
the number of turns of the voice coil 11a being increased and selected
appropriately in such positions remote from the magnetic gap 14, the
influence of the asymmetrical flux densities can be reduced, and any
variations in the electromagnetic force which would be caused by movement
of the voice coil 11a into the yoke 11c can also be reduced, thereby
providing a linear driving force.
In the tracking servomechanism 12 having the other magnetic drive unit, a
current is passed through the voice coil 12a to enable the beam spot to
follow the tracks on the disc 1. When such a current is varied, the voice
coil 12a moves the focusing servomechanism 11 as a whole in horizontal
directions. At this time, the voice coil 12a is also linearly moved in the
manner described above since the voice coil 12a is of the same
construction as that of the voice coil 11a.
FIG. 5 shows at an enlarged scale a magnetic drive unit according to
another embodiment of the present invention. The magnetic drive unit,
generally designated at 18, is composed of a yoke 11c having an upper
flange 11e, a magnet 11b disposed in the yoke 11c, a pole piece 11d
mounted on an upper surface of the magnet 11b, and a double-layer voice
coil 19 including an insulating material 20 and disposed in a magnetic gap
14 defined between the flange 11e and the pole piece 11d. The layers of
the voice coil 19 are formed in the same manner as that in which the voice
coil 11a of FIG. 3 is formed. Because of the double-layer construction,
the voice coil 19 has a better response when moved in small intervals.
In the above embodiments, the voice coils 11a, 12a, 19 have different
numbers of turns per unit length which are achieved by varying the pitch
at which the cutter or laser beam is fed along during machining operation.
Howver, the tubular conductive body as shown in FIG. 4 may be cut at equal
pitches, and thereafter the voice coil may be elongated in its central
portion to increase the pitches therein and the increased pitches may be
fixed by a mass of resin placed between the adjacent turns. The magnetic
drive unit may be incorporated in an audio loudspeaker.
The prevent invention is advantageous for the following reasons:
(1) Since a voice coil is formed by helically cutting a tubular conductive
body, the efficiency of making the voice coil is much higher than possible
with a conventional process in which a wire is wound to form a voice coil.
(2) The turns of the voice coil have a distortion-free rectangular cross
section, and hence the voice coil has an increased space factor.
Therefore, the magnet can be smaller in size, and the magnetic drive unit
can also be smaller in size.
(3) The pitches of the helical turns of the voice coil are relatively large
in the magnetic gap between the yoke and the magnet, and become
progressively smaller in direction away from the magnetic gap. As a
consequence, the influence due to a varying flux distribution of a
magnetic field generated in the magnetic gap is reduced to allow the voice
coil to move linearly.
(4) As the voice coil is formed by helically cutting a tubular conductive
body, as described above, it has a uniform diameter and a uniform
thickness throughout its axial length. This can reduce the width C (FIG.
3) of the magnetic gap 14 to as small an extent as possible. Accordingly,
the voice coil can efficiently utilize the magnetic field produced in the
magnetic gap, and the magnetic drive unit can be reduced in size.
Although certain preferred embodiments have been shown and described, it
should be understood that many changes and modifications may be made
therein without departing from the scope of the appended claims.
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