U.S. patent application number 12/831765 was filed with the patent office on 2010-10-28 for 3d actuator for otpical disc system.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to Jadranko Dovic, Hendrik Josephus Goossens, Bart Hendriks, Bernardus Johannes Stinesen, Jacobus Cornelis Gerardus Van Der Sanden, Johannes Antonius Van Rooij.
Application Number | 20100271912 12/831765 |
Document ID | / |
Family ID | 29595040 |
Filed Date | 2010-10-28 |
United States Patent
Application |
20100271912 |
Kind Code |
A1 |
Van Rooij; Johannes Antonius ;
et al. |
October 28, 2010 |
3D ACTUATOR FOR OTPICAL DISC SYSTEM
Abstract
An optical pick-up actuator (1) has a lens holder (2). The lens
holder has tracking (5r) and focusing (5f) coils which
substantially extend in two parallel planes at a side of the lens
holder (2). A magnet system (7) is arranged separately from the
lens holder and extends beyond said planes, said magnet system
cooperating with the tracking and focusing coils, the coil systems
being arranged for effecting tilt through cooperation with the
magnet system. Preferably, a coil system is provided at each of two
opposite sides of the lens holder.
Inventors: |
Van Rooij; Johannes Antonius;
(Eindhoven, NL) ; Hendriks; Bart; (Lommel, BE)
; Dovic; Jadranko; (Eindhoven, NL) ; Stinesen;
Bernardus Johannes; (Eindhoven, NL) ; Van Der Sanden;
Jacobus Cornelis Gerardus; (Eindhoven, NL) ;
Goossens; Hendrik Josephus; (Shanghai, CN) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS
N.V.
Eindhoven
NL
|
Family ID: |
29595040 |
Appl. No.: |
12/831765 |
Filed: |
July 7, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12030533 |
Feb 13, 2008 |
7779433 |
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12831765 |
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10516154 |
Nov 30, 2004 |
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12030533 |
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Current U.S.
Class: |
369/44.15 ;
G9B/7 |
Current CPC
Class: |
G11B 7/0956 20130101;
G11B 7/0933 20130101; G11B 7/0935 20130101; G11B 7/0932
20130101 |
Class at
Publication: |
369/44.15 ;
G9B/7 |
International
Class: |
G11B 7/00 20060101
G11B007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 4, 2002 |
EP |
02077194.5 |
Claims
1. Optical pick-up actuator comprising a lens holder suspended by
suspension means and having tracking and focusing coils an
objective lens, means for tilting the optical lens holder, a coil
system at a side of the lens holder, said coil system comprising a
focusing coil system substantially extending in a first plane and a
tracking coil system substantially extending in a second plane
parallel to the first plane, the actuator having a magnet system
separate from the lens holder and extending substantially beyond
the first and the second plane, seen from the lens holder, said
magnet system cooperating with the tracking and focusing coils, the
focusing and/or tracking coil systems being arranged for effecting
tilt through cooperation with the magnet system.
2. Optical pick-up actuator as claimed in claim 1, characterized in
that the lens holder comprises a magnet system separate from the
lens holder beyond the first and the second plane at each of two
mutually opposed sides of the lens holder comprising a focusing
coil system substantially extending in a first plane and a tracking
coil system substantially extending in a second plane, so as to be
parallel to the first plane and associated with each of said coil
systems.
3. Optical pick-up actuator as claimed in claim 1, characterized in
that the first and the second plane substantially coincide.
4. Optical pick-up actuator as claimed in claim 1, characterized in
that the magnet system 7 comprises an arrangement of sub-magnets,
the magnetic axes of at least some of said sub-magnets having a
non-perpendicular orientation relative to the first and/or the
second plane of the coils.
5. Optical pick-up actuator as claimed in claim 4, characterized in
that at least some of the sub-magnets have a diagonal orientation
relative to the first and/or the second plane.
6. Optical pick-up actuator as claimed in claim 1, characterized in
that at least one of said coil focusing and/or tracking coil
systems comprises one or more pairs of coils at each opposite side,
wherein said pair or pairs of coils forms or form a means for
tilting the lens holder.
7. Optical pick-up actuator as claimed in claim 6, characterized in
that the pair of the coils forming a means for tilting the lens
holder is arranged substantially mirror-symmetrically with respect
to a mirror plane, said mirror plane extending through and parallel
to an optical axis of the lens holder and substantially
perpendicular to the planes of the planar coil arrangements.
8. Optical pick-up actuator as claimed in claim 6, characterized in
that both focus and tracking coil systems are mirror-symmetrically
arranged with respect to said mirror plane, wherein the pair of
coils forming the means for tilting is arranged at opposite sides
of said plane at some distance from the mirror plane, and the other
coil system is arranged adjacent the mirror plane.
9. Optical pick-up actuator as claimed in claim 6, characterized in
that the center of gravity of the coil system not comprising the
means for tilting the lens holder is arranged closer to the center
of gravity than the coils comprised in the means for tilting the
lens holder.
10. Optical pick-up actuator as claimed in claim 7, characterized
in that the center of gravity of the coil system not comprising the
means for tilting the lens holder is arranged substantially
adjacent the center of gravity of the lens holder.
11. Optical pick-up actuator as claimed in claim 1, characterized
in that the pair of the coils forming a means for tilting the lens
holder forms part of the focus coil system.
12. Optical pick-up actuator as claimed in claim 1, characterized
in that the lens holder comprises a magnetic yoke.
13. Optical pick-up actuator as claimed in claim 1, characterized
in that the tracking and focus coil systems share a common
electrical line.
14. Optical read and/or write system comprising an optical pick-up
actuator according to claim 1.
Description
FIELD OF THE INVENTION
[0001] The invention relates to an optical pick-up actuator
comprising a lens holder suspended by suspension means and having
tracking and focusing coils, magnets for cooperation with the
tracking and focusing coils, an objective lens, and means for
tilting the optical lens holder.
[0002] The invention also relates to an optical read and/or write
system comprising an optical pick-up actuator.
BACKGROUND OF THE INVENTION
[0003] Optical pick-up actuators as well as optical read and/or
write systems comprising an optical pick-up actuator are known. The
actuator comprises a lens holder suspended by suspension means.
Tracking and focusing coils on the lens holder in co-operation with
magnets on a fixed part allow the lens holder to be moved in a
radial direction (tracking) and a vertical direction (focusing).
Compared with pick-up actuators that have coils on a fixed part and
magnets on the coil holder, this type of actuators is lighter and
better able to track and focus. Such an actuator and system is
disclosed in US 2001/0030815. In this actuator a means for tilting
the lens holder is also provided. A shaft is provided around which
the lens holder can be tilted. Tilting, i.e. rotating about an
axis, allows an improved control over the movements of the lens
holder and consequently an improved correspondence between the
optical axis of the lens in the lens holder and the optical medium
to be read or to be written. The means for tilting the lens holder
known from US 2001/0030815 are, however, rather complicated and
thus costly.
SUMMARY OF THE INVENTION
[0004] It is an object of the invention to provide an actuator of
the type described in the opening paragraph having a relatively
simple and effective means for tilting the lens holder.
[0005] To this end the actuator is characterized in that the
actuator comprises a coil system at a side of the lens holder, said
coil system comprising a focusing coil system substantially
extending in a first plane and a tracking coil system substantially
extending in a second plane parallel to the first plane, the
actuator having a magnet system separate from the lens holder
extending substantially beyond the first and second plane, seen
from the lens holder, said magnet system cooperating with the
tracking and focusing coils, the focusing and/or tracking coil
systems being arranged for effecting tilt through cooperation with
the magnet system.
[0006] Present and future designs will make a good focal and/or
radial positioning accuracy ever more important, but it is
especially the power dissipation that is of great importance. The
more dense the information is stored, the more important an active
tilt control will become especially in write systems. Too large a
power dissipation leads to structural damage or the need for spin
down to prevent damage. This problem becomes ever more important as
the read/write-speeds increases, and the density of information on
the disks increases. With increasing speeds, power dissipation in
the actuator becomes ever more important. Too much power
dissipation can lead to overheated coils and degradation of the
quality of the lens (structural damage). Furthermore, too much
power dissipation will increase the system temperature, which can
limit the lifetime of the laser(s). To minimize power dissipation,
the actuator efficiency should be optimized. The present invention
offers a solution to one or more of the above given problems.
Minimizing the total mass of the lens holder will increase the
efficiency. This is achieved by minimizing the coil mass by means
of combining focus, tracking, and tilt functionality in the coil
system. A separate tilt coil system is thereby prevented, reducing
the mass of the lens holder and the complexity of the design. Tilt
functionality may be achieved by combining focusing and tilt, or
tracking and tilt, or a combination of focusing, tracking, and tilt
functionality. The magnet system is positioned separately from
(i.e. not attached to) the lens holder extending substantially,
seen from the lens holder, beyond the first and second plane and
cooperates with the tracking and focusing coils combined for
effecting tracking, tilt, and focusing. Thus the mass of the lens
holder is further reduced as compared with lens holders comprising
(part of the) magnet system. In general, the advantage of having
the coil system combine a focus, radial, and tilt actuator in one
plane (or in planes close to each other without iron or magnets in
between coils) is that the following can be combined: [0007] 1.
constant air gap between magnets and coils independent of focus,
radial, and tilt stroke; with: [0008] 2. very compact design of the
lens holder (small in space and mass and consequently with high
efficiencies and high resonance frequencies); with: [0009] 3. a
combined magnet system, which can be of relatively simple design,
for instance one single multi-pole magnet or a limited array, for
instance one array of 2 pole magnets, for each set of focus,
radial, tilt coils. A small magnet volume helps to keep the
actuator dimensions small and to minimize the cost. Not having a
(part of) the magnet system within the lens holder saves weight of
the lens holder, thus increasing the efficiency
[0010] The lens holder may have a single coil system at one side of
the lens holder, in simple and relatively low-cost embodiments, but
preferably the lens holder comprises a magnet system separate from
the lens holder at opposite sides of the lens holder comprising a
focusing coil system (5f, 5f1, 5f2) substantially extending in a
first plane (Pcoilf) and a tracking coil system (5r) substantially
extending in a second plane (Pcoilr), parallel to the first plane
and associated with each of said coil systems, beyond the first and
second plane. Such a symmetrical arrangement allows for better
accuracy, and on average the current through each coil system will
be less, so that the power dissipation is more evenly distributed
over the lens holder in comparison with asymmetrical embodiments
(coil system at one side of the lens holder only).
[0011] Preferably, the first and second plane substantially
coincide, i.e. the coil system extends substantially in a planar
arrangement. A planar arrangement of the coil system, i.e. an
arrangement in which the focus and tracking coils extend
substantially in a plane, offers the possibility of a relatively
simple design of the lens holder and a high efficiency as compared
with designs in which the coils are arranged one behind the
other.
[0012] Preferably, the combined magnet system comprises an
arrangement of sub-magnets, and the magnetic axes of at least some
of said sub-magnets have a non-perpendicular orientation in respect
of the first and/or second plane of the coils. Such an oriented
magnet system, wherein the magnetic axes of at least some of the
generated magnetic fields are oriented non-perpendicularly to the
planes of the coils, makes it possible to shape the magnetic fields
associated with the magnet system so that a higher efficiency is
obtained. In such a preferred embodiment, the power dissipation is
further reduced and enables a more efficient use of the coils,
which could also serve to reduce the number of turns of the coils
and/or the extension of the coils, thus reducing the weight of the
lens holder. Preferably, the orientation of some of the sub-magnets
is in a diagonal direction with respect to the planes of the coils.
This makes for a simple design. Use of sub-magnets with
non-perpendicular orientation also provides a reduction in residual
tilt during tracking and focusing. Residual tilt is unwanted tilt
during a tracking or focus movement.
[0013] Preferably, at least one of said coil focusing and/or
tracking coil systems comprises one or more pairs of coils at each
opposite side, wherein said pair(s) of coils forms or form a means
for tilting the lens holder. Splitting up at least one of the focus
and/or tracking coils into one or more pairs of coils provides a
simple design for tilting the lens holder (by offering the coils of
the pair a slightly different current). Yet, the planar arrangement
of the coil system offers the possibility of a good focus as well
as radial efficiency, while the power dissipation needed for a
maximum tilt is small, as will be illustrated below.
[0014] Preferably, the pair of the coils forming a means for
tilting the lens holder is arranged substantially
mirror-symmetrically with respect to a mirror plane, through and
parallel to an optical axis of the lens holder and substantially
perpendicular to the planes of the planar coil arrangements. This
allows for a simple and accurate control of the tilt. Preferably,
in such arrangements, both focus and tracking coil systems are
mirror-symmetrically arranged with respect to said mirror plane,
wherein the system forming the means for tilting is arranged at
opposite sides of said plane at some distance from the mirror
plane, and the other coil system is arranged near the mirror plane.
The cross-talk between tilt and tracking and/or focusing is made
small thereby.
[0015] In preferred embodiments, the focus and the tracking coil
systems share a common electrical line. This reduces the number of
electrical lines to the coil systems and thus reduces the
complexity of the design
[0016] The coil systems may be composed of wound coils, but in
embodiments may also be made by coils printed on a foil. The latter
will result in a further reduction of the weight of the lens
holder.
[0017] It is a further object of the invention to provide an
optical read/write system comprising an optical pick-up actuator.
To this end an optical read system comprises an optical pick-up
actuator in accordance with the invention. The advantages of the
optical pick-up improve the functioning of the optical pick-up per
se, however, this provides an improvement in the functioning of the
optical read system in toto. The possibility of a reduction in
power dissipation has a positive effect on the optical read system
in toto, since the power dissipated in or near the optical pick-up
actuator is dissipated towards other parts of the optical read
system, effecting also the proper functioning of said parts.
[0018] These and other aspects of the invention are apparent from
and will be elucidated with reference to the embodiments described
hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] In the drawings:
[0020] FIG. 1 schematically shows in perspective a pick-up actuator
in accordance with the invention.
[0021] FIG. 2 shows in perspective a detail of a pick-up actuator
in accordance with the invention.
[0022] FIG. 3 shows in perspective a lens holder for a pick-up
actuator in accordance with the invention.
[0023] FIGS. 4a and 4b illustrate electrical circuits for an
optical read system comprising a pick-up actuator in accordance
with the invention.
[0024] FIGS. 5a, 5b and 5d shows pick-up actuators in accordance
with embodiments of the invention in side elevation.
[0025] FIG. 5c is a side elevation of a magnet system for an
actuator.
[0026] FIGS. 6a and 6b show an embodiment (the H-variant) of the
invention.
[0027] FIG. 7 shows an embodiment of the invention in which the
focus and tracking coils extend one behind the other, i.e. in
parallel separate planes.
[0028] FIG. 8 illustrates in a graphical form the efficiency of
actuators in accordance with several embodiments of the
invention.
[0029] FIG. 9 illustrates embodiments of the invention having
yokes.
[0030] FIGS. 10a to 10c illustrate a specific arrangement for the
magnet system in accordance with a preferred embodiment of the
invention.
[0031] FIG. 11 illustrates an actuator in which use is made of the
magnet systems illustrated in FIG. 10.
[0032] FIGS. 12 to 14 illustrate a further example of a device in
accordance with the invention.
[0033] The Figures are not drawn to scale. Generally, identical
components are denoted by the same reference numerals in the
Figures.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0034] FIG. 1 shows schematically a pick-up actuator in accordance
with the invention. The actuator 1 comprises a lens holder 2 with a
lens 3, suspended by suspension means 4. The lens holder 2
comprises a lens system, part of the radial tracking coil 5r of
which is shown in FIG. 1. Furthermore, it comprises a fixed part 6
comprising a magnet system 7 for cooperation with the coil system
on the lens holder. The coil system and the magnet system which
face each other across the gap between the lens holder and the
fixed part cooperate so as to effect radial, focus, as well as tilt
control. This limits the power consumption in comparison with
designs in which a separate tilt control system is used, or lens
holders in which magnets in-between coils systems (and thus on the
coil holder) are used.
[0035] FIG. 2 shows in perspective view a detail of the pick-up
actuator of FIG. 1. At the opposite sides 2a and 2b of coil holder,
a coils system 5 comprising a radial coil 5r (i.e. a coil for
correction of the position of the lens holder along the radial
direction) is provided, as well as a focus coil system 5f which
comprises a pair of coils 5f1 and 5f2. The coil system extends
substantially in the plane P.sub.coils. Naturally the coils 5r, 5f1
and 5f2 will have a width, so the fact that the coils extend
substantially in said plane P.sub.coils is to be understood in a
practical sense, and not to be unduly restricted to a mathematical
interpretation of this condition. In this preferred embodiment the
tracking and focusing coils extend in a single plane. Within the
framework of the invention in the broadest sense, the coils systems
may extend in two parallel planes, one system behind the other.
Such systems could be used if a relatively large extent of the
coils is needed.
[0036] The advantage of having the coil system combine a focus,
radial, and tilt actuator, in one plane (or in planes close to each
other without iron or magnets in between coils) is that the
following can be combined: [0037] 1. constant air gap between
magnets and coils independent of focus, radial, and tilt stroke;
with: [0038] 2. very compact design of the lens holder (small in
space and mass and consequently with high efficiencies and high
resonance frequencies); with: [0039] 3. a relatively simple
combined magnet system, for instance one single multi-pole magnet
or a limited array, for instance one array of 2 pole magnets, for
each set of focus, radial, tilt coils. A small magnet volume helps
to keep the actuator dimensions small and to minimize the cost. Not
having a (part of) the magnet system within the lens holder saves
weight of the lens holder, thus increasing the efficiency
[0040] FIG. 3 shows schematically a lens holder for an actuator in
accordance with the invention. This Figure shows a number of
features of preferred embodiments. It shows, for example, a plane
defined by an optical axis of the lens (an axis through a center of
the lens, substantially perpendicular to the lens) and a direction
substantially perpendicular to the sides 2a, 2b of the lens holder.
The radial coil system (5r) and the focus coil system (5f1, 5f2)
are arranged substantially mirror-symmetrically with respect to
this plane. The center of gravity of the lens holder is
schematically indicated by the star and lies a small distance below
the lens. The focus coil system (5f1, 5f2) lies below this center
of gravity in this embodiment, seen in a direction along the focus
direction, whereas the radial coil system (or tracking coil system,
as it is sometimes called) lies substantially at the same level as
the center of gravity. In preferred embodiments, the center of
gravity coincides with 2D (focus-track plane) suspension stiffness
center. Both the focus coils (5f1, 5f2) and the radial coils (5r)
are positioned such that the resulting radial and focus forces are
acting through (close to) the center of gravity and 2D stiffness
center.
[0041] FIG. 4a illustrates an electrical circuit for an optical
read system comprising a pick-up actuator in accordance with the
invention. The read/write system comprises a tilt control 41, a
focusing control 42, and a radial displacement control 45, which
are familiar elements of read/write systems. These detectors
generate control signals Utilt, Ufoc and Urad, respectively. Ufoc
and Utilt are sent jointly via adders 43 and 44 to the coils 5f1
and 5f2 generating Ifoc1 and Ifoc2. The difference between Ifoc1
and Ifoc2 equals the (effective) tilt current that causes the lens
holder to tilt. The sum of Ifoc1 and Ifoc2 equals the (total
effective) focus current Ifoc for focus control. Urad is imposed on
the radial coil generating Irad for radial control. In this
preferred arrangement, the focus and tracking coils system share a
common electrical line 46. The total number of electrical lines is
then 4, the common line 46, a line for the radial coil, and two
lines for the focus coils 5f1, 5f2. Having only four lines needed
for correction of radial position, focus, and tilt renders it
possible to use the hinges of a simple, commonly used 4-hinge
suspension, and the electrical connectors also allow for a simple
electrical arrangement.
[0042] FIG. 4b shows an electrical circuit requiring 6 connectors,
which makes it possible to separate the coils (5f1, 5f2, 5r)
electrically, preventing electrical cross-talk. Furthermore, this
arrangement makes it possible to use simple, straightforward
electronic drivers. However, this arrangement will be mechanically
more complex and more expensive.
[0043] FIG. 5a is a side elevation of a lens holder with the radial
coil 5r and the focus coils 5f1 and 5f2. Also shown are the magnets
7. Sending a current through the coil 5r will shift the lens holder
in the radial direction rad, whereas a sum of the currents through
the coils 5f1 and 5f2 will shift the lens holder along the focus
direction foc, a difference in currents between the coils 5f1 and
5f2 causing the lens holder to tilt, as indicated by the arrow
`tilt` in FIG. 5a.
[0044] FIG. 5b is a side elevation of a lens holder with radial
coils 5r1 and 5r2 and focus coils 5f1 and 5f2. Also shown are the
magnets 7. Sending a current through the coils 5r1 and 5r2 will
shift the lens holder in the radial direction rad, whereas a sum of
the currents through the coils 5f1 and 5f2 will shift the lens
holder along the focus direction foc, a difference in currents
between the coils 5f1 and 5f2 causing the lens holder to tilt along
a vertical axis, as indicated by the arrow `tilt` in FIG. 5a. With
this arrangement it is also possible to tilt (by sending slightly
different currents through coils 5r1 and 5r2) to effect a `tilt`
along a horizontal axis.
[0045] FIG. 5c shows a magnet system 7 with a yoke 5j at the rear
side of the magnet system in front elevation (F) and side elevation
(S). Such yokes enable the magnetic field to be enhanced and
modified (directed in certain directions).
[0046] FIG. 5d shows an arrangement which is in effect a variation
of the arrangement shown in FIG. 5a. The coils 5f1 and 5f2 are
slightly elongated below the coil 5r. Such an arrangement allows
coils 5f1 and 5f2 to cover a larger area, thus improving efficiency
(because they are slightly larger) and reducing the current need
and power consumption, or with the same power consumption allowing
for a larger maximum shift or tilt.
[0047] FIGS. 6a and 6b also show an embodiment of the actuator in
accordance with the invention. In this case the radial coils (5r1,
5r2, 5r3, 5r4) are split up, taking care of both radial and tilt
control. The single focus coil (5f) is just for focus control. In
this example the radial coils 5r1 and 5r3 are put in series, as are
the coils 5r2 and 5r4.
[0048] In the embodiments shown in FIGS. 1 to 5a and 5d, the
arrangement of the coil system can be seen as constituting a `T`
arrangement where coils 5f1 and 5f2 form one part of the `T` and
the coil 5r the other part. The arrangement of FIGS. 6a and 6b
looks more like an H, wherein coils 5r1 to 5r4 constitute the two
legs of the H, and coil 5f the crossbar. In this arrangement it is
the focus coil which is not split, whereas the tracking or radial
coil 5r is split up into two pairs of coils. In this example the
coils 5r1 and 5r3 are put in series, as are the coils 5r2 and 5r4.
A current common through all coils 5r1 to 5r4 will effect a radial
shift, a difference in current between 5r1-5r3 and 5r2-5r4 will
cause a difference in radial shift between the top part and the
bottom part of the lens holder, thus resulting in a tilt.
[0049] Yet another embodiment is formed by translating coil 5f
downwards, in which case the coils form a U (the coils 5r1 to 5r2
forming two legs of the `U`, the coil 5f the bottom of the
`U`).
[0050] In the above embodiments, all coils (system 5r and 50 are
arranged in a planar arrangement, i.e. the coils extends in a plane
P.sub.coils. Within the framework of the invention in the broadest
sense, the coils systems may extend in two parallel planes, one
system behind the other. Such systems could be used if a relatively
large extent of the coils is needed. To investigate the effect of
the planar arrangement feature, the inventors have made designs in
which one coil system was positioned behind the other system (seen
from the lens holder), and in which either the focus coils or the
rad coils were split to effect tilting. FIG. 7 shows such a system
(for the U-form) in which the focus coils are nearer to the holder
than the radial coils, or in other words, the radial coils are
nearest to the magnets. The radial (tracking) coil system extends
in a first plane Pcoilr and the focusing coil system extends in a
second, parallel plane Pcoilf. In the embodiments of FIGS. 1 to 6,
the parallel planes Pcoilr and Pcoilf substantially coincide.
Putting a set of coils farther away from the magnets will results
in a lower K-factor (linear motor constant [N/A]), but this
embodiment makes it possible to build the lens holder more compact
(and maybe also reduce its weight).
[0051] The efficiencies of various embodiments which are all
designed within the same, specific space boundaries are given in
FIG. 8. The vertical axis denotes focus efficiency, the horizontal
axis the radial efficiency, and the data in mW next to the points
give the power dissipation in milliwatts for 8 mrad tilt. The
points are respectively: [0052] 71: U form, focus coil split, focus
coil nearest to magnet [0053] 72: U form, focus coil split, radial
coil nearest to magnet (as in FIG. 7) [0054] 73: U form, radial
coil split, radial coil nearest to magnet [0055] 74: U form, radial
coil split, focus coil nearest to magnet [0056] 75: U form, radial
coil split, all coils in planar arrangement [0057] 76: H form,
radial coil nearest to magnet [0058] 77: H form, all coils in
planar arrangement (as in FIGS. 6a, 6b) [0059] 78: as 77, but with
inner yoke (will be further explained below) [0060] 79: T form, all
coils in planar arrangement (as in FIGS. 1-5)
[0061] What is preferred is a low value for the tilt dissipation
(preferably below 100, more preferably below 200 mW), combined with
high values for both the focus efficiency and the radial
efficiency. The designs that give such values are 75, 76, and 77 to
79, especially 77 to 79, with the best result for design 79. Thus
it follows that: [0062] 1. designs in which all coils are
positioned in a planar arrangement (Pcoilf substantially coinciding
with Pcoilr) generally give better results than designs in which
the coil are positioned one behind the other in terms of
efficiency, provided the rest of the design remains unchanged,
[0063] 2. The H- and the T-form are preferred to the U-form, [0064]
3. The T-form gives the best results.
[0065] While the invention has been described in connection with
preferred embodiments, it will be understood that modifications
thereof within the principles outlined above will be evident to
those skilled in the art, and thus the invention is not limited to
a single or any one of the preferred embodiments but is intended to
encompass such modifications.
[0066] One such modification is shown, for example, in FIG. 9. The
actuator shown is similar to the actuator having the coil system in
H-form as shown in FIGS. 6a and 6b, except for the fact that a yoke
(a piece of metal with a high magnetic permeability .mu.) is placed
behind the coils extending in a slit in the holder, the yoke itself
being attached to a fixed part, i.e. not to the lens holder. This
increases and directs the magnetic fields generated by the focus
and tracking coils, which as a consequence reduces the needed
currents and thus the power dissipation, which becomes apparent
when the power dissipations of points 77 (H-form without yoke) and
78 (H-form with yoke) are compared. The yoke will require a larger
and more complex lens holder owing to, for example, the provision
of the slit and may thus increase the weight of the lens holder,
but on the other hand it will also increase the efficiency and make
it possible to reduce the weight and/or extent of the coils.
[0067] In a preferred embodiment, the combined magnet system 7
comprises an arrangement of sub-magnets, the magnetic axes of at
least some of said sub-magnets having a non-perpendicular
orientation to the first and/or second plane. Such an oriented
magnet system, wherein the magnetic axis of the magnetic fields is
non-perpendicularly oriented to the planes of the coils, makes it
possible to shape the magnetic fields associated with the magnet
system such that a higher efficiency is obtained. This reduces the
power dissipation and renders possible a more efficient use of the
coils, which could also be employed to reduce the number of turns
of the coils and or the extension of the coils, thus reducing the
weight of the lens holder.
[0068] FIGS. 10a to 10c illustrate such a magnet arrangement.
[0069] Small electromagnetic actuators conventionally comprise, as
is schematically shown in FIG. 10a, two permanent magnets 7
magnetized oppositely and perpendicularly to the upper surface
facing the coil 5r 5f, together with an optional yoke 92. The
magnets and the yoke together form a stator. The rotor is formed by
the coil 5r, 5f (denoted by the dotted line) through which a
current flows that interacts with the magnetic field in the
y-direction (B.sub.y) to generate a Lorentz force in the
x-direction. The field generated by such a pair of magnets is,
however, not optimal. There is a need to increase the efficiency of
the actuator. In order to increase the efficiency in embodiments of
the invention, the magnet system comprises sub-magnets with an
orientation that is non-perpendicular.
[0070] FIG. 10b gives one possible arrangement. In this arrangement
two sub-magnets 7b and 7d with perpendicular magnetic field
orientation are separated by a magnet 7c with a parallel
orientation and flanked at both sides by magnets 7a and 7e. The
magnetic field is perfectly symmetrical and better optimized than
the field generated by the arrangement shown in FIG. 10a. The
concept, although within the scope of the preferred embodiments,
does require 5 magnets and thus a relatively large space.
[0071] FIG. 10c gives a more preferred arrangement, using only 4
magnets. This is a simpler design requiring less space, and
calculations have shown that the magnetic field is even better than
that of the arrangement shown in FIG. 10b. [0072] The arrangement
of FIG. 10c may be used in several embodiments, [0073] The magnet
array 7a to 7d may be double-sided, [0074] The magnet array may be
combined with a yoke, analogous to the yoke shown in FIG. 10a,
[0075] The magnets 7a to 7d may be made in (slightly) different
sizes. Smaller inner magnets 7b and 7d lead to the possibility of
using a smaller coil and therefore a smaller moving mass, leading
to increased efficiency.
[0076] FIG. 11 shows the concept of the arrangement schematically
shown in FIG. 10c to improve the efficiency of the actuator. The
upper half of the magnet array comprises four magnets diagonally
magnetized as shown in the plan view. The two bottom magnets are
perpendicularly magnetized. Comparison of such a design with a
design in which there are two perpendicularly magnetized upper
magnets (instead of the four diagonally magnetized magnets as in
FIG. 11) show a remarkable increase in efficiency:the motor
constant (ratio of force to current) has increased by 50% while the
acceleration efficiency (ratio of acceleration to square root of
electrical power) has increased by 25%.
[0077] Furthermore, residual tilt (i.e. a small residual tilt
during tracking and/or focusing movement) is reduced, and the
linearity of the movements (i.e. the relation between the current
and the movement) is improved. Both of these effects make control
of the movements easier.
[0078] The invention is embodied in each new characteristic and
each combination of characteristics. Any reference signs do not
limit the scope of the claims. The word "comprising" does not
exclude the presence of other elements than those listed in a
claim. Use of the word "a" or "an" preceding an element does not
exclude the presence of a plurality of such elements. The Figures
and embodiments are shown by way of example and do not restrict the
scope of the claims. For example, where in the Figures the coils
are wound on the holder, in embodiments the coil may be printed or
otherwise provided on a foil.
[0079] Further advantageous embodiments are, for example, those in
which the magnet system as shown in FIGS. 10a to 10c (the
arrangement of sub-magnets) is used in combination with an
asymmetric coil arrangement (i.e. a coil system at one side of the
lens holder only). The increased efficiency offered by the
sub-magnet arrangement gives the possibility of using smaller coils
and/or less current, thereby reducing power dissipation and any
unevenly distributed weight.
[0080] It is also advantageous to use the magnet system as shown in
FIGS. 10a to 10c in combination with embodiments in which the first
and the second plane do not coincide (i.e. the focusing and
tracking coils are positioned one (partly) behind the other). The
magnetic fields generated by the sub-magnet arrangement are more
directed towards the coils and have on average a greater extent in
the direction of the coils. This is of advantage if one of the
coils is further away from the magnet system than the other.
[0081] FIGS. 12 to 14 illustrate a further example in which the
holder 2 with lens 3 has coils 5r and 5f at both sides. Neither of
these coils is split, but the configuration of the coils (more or
less forming an L, with one of the coils being the I of the L and
the other one being the _ of the L), forming an L at one side of
the holder and an .right brkt-bot. at the other side, enables the
5r and 5f coils to effect tilt in cooperation. In this example,
therefore, it is the focusing and the tracking coil systems
together which are arranged for effecting tilt through cooperation
with the magnet system.
[0082] The invention may be briefly described as follows:
An optical pick-up actuator has a lens holder. The lens holder has
tracking and focusing coils which substantially extend in two
parallel planes at a side of the lens holder. A magnet system is
arranged separately from the lens holder and extends beyond said
planes, said magnet system cooperating with the tracking and
focusing coils, the coil systems being arranged for effecting tilt
through cooperation with the magnet system.
[0083] Preferably, a coil system is provided at each of two
mutually opposed sides of the lens holder.
* * * * *