Apparatus for driving objective lens

An apparatus for driving an objective lens in focusing and tracking directions for projecting a light beam upon an information track on an optical record medium, including a lens holder for holding the objective lens, resilient wires for supporting the lens holder movably in the focusing and tracking directions, a focusing coil wound on the lens holder, two pairs of tracking coils secured to opposite side walls of the lens holder, said opposite side walls being aligned in a track direction, permanent magnets each arranged to be faced to respective side walls, and a yoke having upright portions arranged to face respective magnets. In order to suppress an undesired rolling resonance, forces generated at first portions of tracking coils in the tracking direction are selectively increased or forces generated at second portions of tracking coils in the focusing direction are selectively decreased.

BACKGROUND OF THE INVENTION 
Field of the Invention and Related Art Statement 
The present invention relates to an apparatus for driving an objective lens 
for use in reading and writing information out of and onto an optical 
record medium. 
There has been developed an optical information reading and writing 
apparatus for reproducing and recording information out of and onto an 
optical record medium by projecting a focused spot of a light beam upon 
the record medium. In such an optical information reading and writing 
apparatus, the information is read out and recorded on the record medium 
along an information track with the aid of an optical head which includes 
an objective lens for projecting the light beam spot upon the record 
medium, a mechanism for supporting the objective lens movably in a 
tracking direction as well as in a focusing direction, and an apparatus 
for driving the objective lens in the tracking and focusing directions in 
accordance with tracking error and focusing error, respectively. It should 
be noted that the tracking direction is perpendicular both to the optical 
axis of the objective lens and to the track direction in which the 
information track extends, and the focusing direction is in parallel with 
the optical axis of the objective lens. When use is made of an optical 
record disc, the information track is formed as a spiral track or 
concentric circular tracks. Then, the tracking direction is a tangential 
direction of the spiral or circular track. The above mentioned objective 
lend driving apparatus has been proposed in, for example, Japanese Patent 
Publications Kokai Sho Nos. 59-221,839, 62-149,044, 62-149,045 and 
62-149,047. 
FIGS. 1 and 2 show a known objective lens driving apparatus disclosed in 
the Japanese Patent Publication Kokai Sho No. 59-221,839. An objective 
lens 1 is supported by a lens holder 2, and a focusing coil 3 is wound 
around the outer side wall of the holder 2. On opposite sides of the 
holder which are aligned in the track direction are applied respective 
pairs of tracking coils 4. The holder 2 is connected to a base 5 by means 
of four resilient wires movably both in the focusing direction and in the 
tracking direction. To the base 5 is secured a yoke 7 having upright 
portions 7a, 7b, and permanent magnets 8 are secured to end plates of the 
yoke. The holder 2 and the yoke 7 are assembled such that portions of the 
focusing and tracking coils 3 and 4 are situated in magnetic fields formed 
between the upright portions 7a, 7b of yoke 7 and permanent magnets 8. By 
conducting electric currents corresponding to the focusing and tracking 
errors through the focusing and tracking coils 3 and 4, respectively, the 
holder 2 and thus the objective lens 1 are moved in the focusing and 
tracking directions, so that the correctly focused light spot is projected 
on the information track of the optical record medium. 
In the known objective lens driving apparatus described above, the movable 
portion including the objective lens 1, holder 2 and coils 3 and 4 might 
be rotated about an axis (Y-axis) which is in parallel with the track 
direction and is perpendicular both to the focusing direction (FO) and to 
the tracking direction (Tr). Hereinafter this rotating movement is called 
the rolling resonance. In order to suppress the rolling resonance, there 
has been proposed to make a center of tracking force acting upon the 
tracking coils 4 in the tracking direction coincident with the center of 
gravity (G) of the movable portion so that any moment about the Y-axis 
could not be generated. 
However, even if the center of tracking force is made coincident with the 
center of gravity of the movable portion, when the movable portion is 
moved or shifted in the focusing direction due to the focusing servo 
control, the equivalent center of tracking force is shifted from the 
center of gravity so that the rolling resonance is generated, because the 
magnetic flux density in the magnetic gap in which the tracking coils are 
moved has such a distribution that the magnetic flux density is decreased 
toward the upper and lower ends of the magnetic gap. 
Now the generation of the rolling resonance will be further explained in 
detail. FIGS. 3 and 4 are schematic views showing the objective lens 1, 
holder 2, tracking coils 4 and permanent magnet 8 viewed in the Y-axis. 
The magnetic gap is situated in front of the permanent magnet 8 and is 
extended in parallel with the plane of the drawing. G denotes the center 
of gravity of the movable portion comprising the objective lens 1, holder 
2, focusing and tracking coils 3 and 4. When an electric current I is 
conducted through the tracking coils 4 in directions shown by arrows in 
FIG. 3, in portions 4b of the tracking coils 4 which portions extend in 
the focusing direction Fo there are generated forces F in the tracking 
direction Tr, and at the same time in portions 4a and 4c which extend in 
the tracking direction Tr there are produced forces f.sub.1 and f.sub.2, 
respectively in the focusing direction Fo. As illustrated in FIG. 3, when 
a center of the forces F is coincided with a direction which passes 
through the center of gravity G and is in parallel with the tracking 
direction Tr and the forces f.sub.1 and f.sub.2 have the same magnitude, 
there is not produced any moment about the Y-axis, so that the movable 
portion is shifted only in the tracking direction Tr without causing the 
undesired rolling resonance. 
However, when the movable portion is shifted in the focusing direction Fo, 
the force f.sub.1 becomes decreased, but the force f.sub.2 becomes 
increased, so that f.sub.1 &lt;f.sub.2, because the distribution of the 
magnetic flux density in the magnetic gap is decreased abruptly toward the 
upper and lower ends of the permanent magnet 4 as depicted in FIG. 5. 
Further, the center of the force F is shifted downward by a distance 
.DELTA.Z as illustrated in FIG. 4 with respect to the center of gravity G. 
Since the forces f.sub.1 and f.sub.2 have the opposite directions, the two 
sets of tracking coils 4 are subjected to a moment M.sub.1 which is equal 
to 2(f.sub.2 -f.sub.1)l, wherein l is a distance between the center of 
gravity G and the center points of the forces f.sub.1 and f.sub.2 measured 
in the tracking direction Tr. Moreover, due to the shift .DELTA.Z of the 
movable portion, there is also produced a moment M.sub.2 amounting to 
2F.multidot..DELTA.Z. Since the above mentioned moments M.sub. 1 and 
M.sub.2 have the same direction, there is produced the very large rolling 
resonance. That is to say, the movable portion is subjected to the rolling 
resonance which is caused by a sum of the two moments M.sub.1 and M.sub.2. 
This influence can be expressed by an equivalent shift .DELTA.Z' of the 
point of the force F with respect to the center of gravity G, said 
equivalent shift .DELTA.Z' being calculated as follows: 
##EQU1## 
When the equivalent shift .DELTA.Z' becomes large, there is generated a 
large rolling resonance. It should be noted that the above explained 
rolling resonance is equally generated when the movable portion is shifted 
downward in FIG. 3, but the direction of the rolling resonance is 
inverted. 
In the known objective lens driving apparatus just explained above, it is 
difficult to suppress the generation of the undesired rolling resonance 
effectively, so that the phase in the frequency characteristic of the 
tracking servo control fluctuates and the tracking control could not be 
carried out correctly. Therefore, the yield of the objective lens driving 
apparatus becomes reduced. In order to solve the above problem, one may 
consider to increase the height of the magnetic gap such that the central 
portion of the distribution curve of magnetic flux density has a flat 
portion and the tracking coils are moved within this flat central portion. 
However, this solution might induce another problem that the height of the 
yoke 7 and permanent magnets 8 has to be increased, and thus the height of 
the whole apparatus could not be made small. 
SUMMARY OF THE INVENTION 
The present invention has for its object to provide a novel and useful 
apparatus for driving an objective lens without causing the undesired 
rolling resonance. 
According to the invention, an apparatus for driving an objective lens 
which is used to project a light beam onto an information track on an 
optical record medium including a lens holder for holding the objective 
lens, a supporting means for supporting the lens holder movably in first 
and second directions which are orthogonal to each other, at least one 
coil secured to the lens holder and a magnetic means for generating a 
magnetic flux passing through said coil to generate a force acting in the 
first direction, the improvement being characterized in that a first force 
generated at a first portion of the coil which first portion extends in 
said second direction is increased relative to a second force generated at 
a second portion of the coil, said second portion extending in the first 
direction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 6 to 8 show a first embodiment of the objective lens driving 
apparatus according to the invention. FIG. 6 is an exploded perspective 
view, FIG. 7 is a side view and FIG. 8 is a cross sectional view cut along 
a line A--A in FIG. 7. An objective lens 11 for projecting a light beam 
upon an optical record medium not shown is secured to a lens holder 12. 
Around the side wall 12a of the holder 12 is wound a focusing coil 13 (see 
FIGS. 7 and 8) and rectangular plates 15a and 15b made of magnetic 
material are clamped into and adhered to recesses 12b formed in opposite 
side walls of the holder 12, said opposite side walls being aligned in the 
track direction which is perpendicular both to the optical axis of the 
objective lens 11 and to the direction of information rack on the record 
medium. The focusing coil 13 is wound over the magnetic plates 15a, 15b. 
Two pairs of tracking coils 4 are secured to the opposite side walls of 
the holder 12 by means of an adhesive agent. In the present embodiment, 
the tracking coil pair is shaped in the form of glasses. The magnetic 
plates 15a, 15b are provided at portions 14b of the tracking coils 14 
which portions generate the force in the tracking direction, but are not 
existent at portions 14a, 14c which produce the force in the focusing 
direction. As illustrated in FIG. 8, to a yoke 17 made of magnetic 
material is secured a permanent magnet 18 with the aid of an adhesive 
agent. By supplying electric currents to the focusing and tracking coils 
13 and 14, the holder 12 and thus the objective lens 11 are moved in the 
focusing and tracking directions, respectively. 
FIG. 9 shows a graph of the distribution of magnetic flux density within 
the magnetic gap defined by the upper and lower ends of the permanent 
magnets 18. A solid curve A illustrates the magnetic flux density 
distribution when the movable portion including the objective lens 11, 
holder 12, focusing and tracking coils 13 and 14, and magnetic plates 15a, 
15b, is not shifted, and a broken curve B represents the distribution of 
magnetic flux density when the movable portion is shifted upward in the 
focusing direction. As shown in FIG. 8, a magnetic gap lg' at the magnetic 
plate 15a is shorter than a magnetic gap lg at the remaining portion, so 
that the magnetic flux density at the magnetic plate is locally high. When 
the movable portion is shifted upward, the magnetic flux density at the 
magnetic plates 15a, 15b is remained high. Therefore, the portions 14b of 
the tracking coils 14 are always subjected to the high magnetic flux 
density, and thus the force F generated at this portions 14b becomes large 
relative to that generated at the portions 14a, 14c. That is to say, by 
providing the magnetic plates 15a, 15b selectively at the portions 14b of 
the tracking coils 14, the forces f.sub.1, f.sub.2 generated in the 
focusing direction at the portions 14a, 14c are not changed, but the 
forces F produced in the tracking direction at the portions 14b are 
selectively increased. As explained above, the equivalent shift .DELTA.Z' 
can be expressed by .DELTA.Z'=(f.sub.2 -f.sub.1)l/F+.DELTA.Z, and when the 
force F is exclusively increased with respect to the forces f.sub.1 and 
f.sub.2, .DELTA.Z' becomes small, and thus the rolling resonance can be 
suppressed. 
In the embodiment so far explained, the magnetic flux density is locally 
increased at the portions 14b of the tracking coils 14 by securing the 
rectangular magnetic plates 15a, 15b to the holder 12 at the portions 14b. 
It should be noted that the same effect can be attained by other means. 
For instance, the shape of the magnetic plates may be any shapes other 
than rectangular. Further, the number of magnetic plates may be one, three 
or more than three. 
In the above embodiment, the force F applying in the tracking direction is 
exclusively increased in order to reduce the equivalent shift .DELTA.Z'. 
According to another aspect of the invention, the same effect may be 
attained by decreasing the forces f.sub.1, f.sub.2 acting in the focusing 
direction with respect to the force F. 
FIG. 10 is a schematic side view showing a second embodiment of the 
objective lens driving apparatus according to the invention. In the 
present embodiment, portions similar to those of the previous embodiment 
are illustrated by the same reference numbers used in FIGS. 6 to 8. In 
FIG. 10, the movable portion is shown to be shifted in the rightward. In 
the present embodiment, the portions 14a, 14b of the tracking coils 14 
which extend in the tracking direction Tr have a short length m.sub.2 as 
compared with the known apparatus illustrated in FIGS. 1 to 4. A length 
m.sub.1 of the portions 14b of the tracking coils 14 extending in the 
focusing direction Fo is the same as that of the known apparatus. Now the 
forces applied to the movable portion will be explained. The tracking 
direction is denoted by X-axis, the focusing direction is in the Z-axis. 
An origin (X, Z)=(0, 0) of the coordinate system is set to the center of 
gravity G when the movable portion is not shifted in the focusing 
direction. Now the magnetic flux density is represented by B (X, Z), then 
moments M.sub.1 and M.sub.2 acting about the center of gravity G due to 
the forces applied to the portions 14a, 14c of tracking coils 14 may be 
expressed as follows. 
##EQU2## 
Therefore, the total moment M.sub.1 generated about the center of gravity 
G due to the forces generated at the portions 14a, 14c of the two tracking 
coils 14 may be expressed in the following manner. 
##EQU3## 
The total length of portions of the tracking coil 14 which are situated in 
the effective magnetic gap may be represented by m.sub.1 +2m.sub.2. Now, 
there is defined a ratio .alpha. of the length m.sub.1 of the portion 14b 
of the tracking coil 14 which generates the force F acting in the tracking 
direction Tr to the total length m.sub.1 +2m.sub.2, i.e. 
##EQU4## 
FIG. 11 is a graph showing the variation of the moment M.sub.1 about the 
center of gravity G with respect to .alpha. which varies from 0.35 to 1.0. 
As can be seen from FIG. 11, the moment M.sub.1 is decreased in accordance 
with the increase in .alpha.. When .alpha. exceeds 0.45, the moment 
M.sub.1 
is reduced abruptly, and when .alpha.=1, the moment M.sub.1 becomes zero. 
When .alpha.=0.5, the moment M.sub.1 is decreased by about 30% of that at 
.alpha.=0.35. In order to suppress the rolling resonance sufficiently, 
.alpha. is preferably made larger than 0.5. That is to say, in order to 
suppress the undesired rolling resonance it is preferable to form the 
tracking coil 14 such that the length m.sub.1 of the portion 14b is longer 
than twice of the length m.sub.2 of the portions 14a, 14c. 
FIG. 12 is a perspective view showing a third embodiment of the objective 
lens driving apparatus according to the invention. In the present 
embodiment, tracking coils 16 are wound on corners of a cubic holder 12 
such that portions 16a, 16c which serve to generate forces in the focusing 
direction situate on upper and lower surfaces of the holder 12. Then 
effective length of the portions 16a, 16c generating the forces in the 
focusing direction can be reduced, and thus the value of .alpha. can be 
increased. Further, the tracking coil portions 16a, 16c situate in the 
magnetic gap at portions having lower magnetic flux density, so that the 
forces generated by these portions 16a, 16c are further reduced. In this 
manner, the rolling resonance can be suppressed to a large extent. 
FIG. 13 is a perspective view illustrating a fourth embodiment of the 
objective lens driving apparatus according to the invention. In this 
embodiment, two tracking coils 17 are wound on the holder 12 such that 
portions 17b are arranged on opposite side walls of the holder and 
portions 17a, 17c are provided on upper and lower surfaces of the holder. 
FIG. 14 is a perspective view depicting a fifth embodiment of the objective 
lens driving apparatus, in which rectangular tracking coils 18 are wound 
on the holder 12 such that portions 18a, 18c are bent over the upper and 
lower surfaces of the holder 12. Also in the fourth and fifth embodiments, 
the rolling resonance can be reduced efficiently. 
FIGS. 15 and 16 show a sixth embodiment of the objective lens driving 
apparatus according to the invention. In this embodiment, shielding plates 
19 made of magnetic material such as iron are provided on the holder 12 
and tracking coils 14 such that the plates cover the portions 14a, 14c of 
tracking coil 14. In this construction, the portions 14a, 14c of the 
tracking coils 14 are shielded by the plates 19 from the magnetic flux, so 
that the effective length of these portions 14a, 14c is reduced, and the 
rolling resonance can be suppressed materially. 
The present invention is not limited to the embodiments explained above, 
but many modifications and alternations may be conceived by those skilled 
in the art within the scope of the invention. In the above embodiments, 
the movable portion including the objective lens, lens holder and coils is 
supported movably in the focusing and tracking directions by means of four 
resilient wires, but the movable portion may be supported with the aid of 
any other supporting means. Further, the tracking coils may be formed in 
any desired shape. Moreover, in the above embodiments, the invention is 
applied to the apparatus for driving the objective lens in the tracking 
direction, but the invention may be equally applied to the apparatus for 
driving the objective lens in the focusing direction or tangential, i.e. 
track direction.