Actuator with reduced lens tilt error

An optical actuator is disclosed for positioning a lens to focus a laser light on a surface of a member, such as an optical disk, with minimum lens tilt error. The optical actuator includes a lens mount for supporting the lens, a plurality of flexures fixed to the lens mount, and a based spaced from the lens mount. The optical actuator further includes a motor structure for operating the actuator in energized and de-energized states. In the energized state, a force is applied to the lens mount to cause the lens mount to move in a focus direction. In the de-energized state, the flexures relax, causing the lens mount to move substantially in the focus direction. The optical actuator further includes an actuator travel stop disposed relative to the base and the lens mount for supporting the lens mount in a downward direction when the actuator is in the de-energized state, to thereby prevent the flexures from creeping in such a way so as to result in lens tilt.

CROSS-REFERENCE TO RELATED APPLICATIONS 
Reference is made to commonly assigned U.S. application Ser. No. 08/660,568 
filed Jun. 7, 1996 and entitled "Position Sensing for an Optical Recording 
Actuator" by Phil Marino et al. The disclosure of this related application 
is incorporated herein by reference. 
FIELD OF THE INVENTION 
This invention relates to optical actuators, and more particularly to 
focusing a laser beam on a surface, such as an optical disk, so that lens 
tilt error is minimized. 
BACKGROUND OF THE INVENTION 
An optical disk recording and reading device includes a lens which focuses 
a laser light beam onto an optical disk while the disk is rotating. Focus 
and/or tracking actuators are used in optical disk recording and reading 
devices to control the position of the lens relative to the rotating disk. 
The actuators are necessary because of unavoidable warp of the disks, 
spindle errors, and other mechanical and optical imperfections. 
Both focus and tracking of optical disks must continuously be maintained 
while the disks are recording and/or reading. Focus is controlled by 
adjusting the distance between the lens and the disk surface. Tracking is 
controlled by maintaining the lens at the correct radial position such 
that a single data track is read or written at a time. The motions of the 
actuator in the focus and tracking directions are controlled by a servo 
system. 
Current actuators typically have a mechanical suspension system to control 
the motion of the actuator tangent to the disk, as well as three 
orthogonal rotations. This suspension system often includes a set of 
flexures. The flexures are typically designed to have high levels of 
mechanical damping in order to suppress actuator resonances, and thereby 
improve the overall functioning of the optical disk recording and reading 
device. Materials and configurations which have high mechanical damping 
often have low mechanical stability, i.e., the flexures creep and/or 
stress relax with time, particularly at elevated temperatures. 
Alignment of the optical axis of the lens to the disk (i.e., lens tilt) 
must be maintained accurately for the system to function properly. The 
problem with the presently known and utilized optical actuator is that 
during the de-energized state, with time, the weight of the lens and lens 
holder causes the flexures to creep until the lens holder is resting on 
the base of the actuator. This contact, typically at an uncontrolled 
location, causes a moment on the flexures. This moment causes a torsional 
creep of the flexures, which results in a tilt motion being introduced to 
the lens and the lens holder. The lens tilt degrades the performance of 
the optical system, which consequently degrades the writing and reading 
reliability of the system. 
FIG. 1 shows a prior art optical actuator 10 with a lens 12, a lens mount 
14, an actuator base 16, and flexures 18. The flexures 18 are made of a 
high damping material, for example, viscoelastic plastic such as HYTREL 
elastomer (trademark of DUPONT). When the actuator 10 is in the 
de-energized state, the gravity creep with time would cause the lens mount 
14 to contact the actuator base 16 at an uncontrolled position. This 
induces a moment about the x-axis, as shown in FIG. 1, which over time, 
induce a rotation of the lens mount 14 and the lens 12 about the x-axis. 
The rotation of the lens mount 14 and the lens 12 introduce an undesirable 
lens tilt error. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an optical 
actuator with minimized lens tilt error caused by flexure creep. 
It is another object of this invention to provide an optical actuator with 
improved reliability when the system is subject to extended storage in 
high temperature conditions. 
These objects are achieved by an optical actuator for positioning a lens to 
focus a laser light beam on a surface of a member, such as an optical 
disk, with minimum lens tilt error, comprising: 
a) a lens mount for supporting the lens; 
b) a base spaced from the lens mount; 
c) a plurality of flexures fixed to the lens mount and the base; 
d) motor means for operating the actuator that is operative in energized 
and de-energized states, such that in the energized state a force is 
applied to the lens mount to cause the lens mount to move in a focus 
direction, and in the de-energized state permits the flexures to relax; 
and 
e) an actuator travel stop disposed relative to the base and the lens mount 
for supporting the lens mount in a downward direction when the actuator is 
in the de-energized state, to thereby prevent the flexures from creeping 
in such a way so as to result in lens tilt. 
ADVANTAGES 
An optical actuator according to this invention provides an optical 
actuator with minimized lens tilt error by substantially reducing a 
torsional creep of the flexures when the actuator is subject to extended 
storage in high temperature conditions. 
An optical actuator according to this invention further provides an optical 
actuator with increased performance because it permits the use of less 
stable and more highly damped flexure materials.

DETAILED DESCRIPTION OF THE INVENTION 
In the following description of FIG. 2, many parts correspond to the parts 
shown in prior art FIG. 1. Where parts or elements correspond to FIG. 1, 
the same numerals will be used. No new matter has been added because it is 
obvious from FIGS. 1 and 2 that many of the parts are identical. Referring 
to FIG. 2, a perspective of an optical actuator 10 in accordance with this 
invention is shown. The optical actuator 10 includes the lens 12 which is 
well known in the art focuses a laser light beam on a surface of a disk 
member (not shown). Preferably, the disk member is an optical reading 
and/or recording disk. The lens 12 is fixed to a lens mount 14 by an 
adhesive or other fastening structure not shown. Preferably, the lens 
mount 14 is made of a molded plastic, such as VECTRA.RTM. (Hoechst 
Celanese). The optical actuator 10 further includes flexures 18 that are 
fixed to the lens mount 14 and the actuator base 16. The optical actuator 
includes at least two spaced-apart flexures 18, which are arranged so that 
they are substantially parallel when viewed from one side. For a more 
complete disclosure of flexures 18, see the above-referenced commonly 
assigned applications. 
A conventional motor arrangement is shown which can be used to apply focus 
force to the lens mount 14. The motor arrangement includes a pair of 
magnets 24, which are fixed to a pair of magnetic yokes 26. The magnetic 
yokes 26 are secured to opposite sides of the actuator base 16. The motor 
arrangement further includes a focus motor coil 28 and four tracking motor 
coils 30 (only two of which are shown). The focus motor coil 28 is wrapped 
around its corresponding magnetic yokes 26 and the lens mount 14. Each 
tracking motor coil 30 is secured to a corner of the focus motor coil 28. 
In operation, current is applied to the focus motor coil 28 to create a 
magnetic field. By energizing the focus motor coil 28, the lens mount 14 
moves in a direction along the z-axis. By energizing the tracking motor 
coils 30, the lens mount 14 moves in a direction along the y-axis. 
When both the focus motor coil 28 and the tracking motor coils 30 are 
de-energized, the gravity creep of a prior optical actuator 10 would cause 
the lens mount 14 to contact the actuator base 16 at an uncontrolled 
position. However, in accordance with the present invention, an actuator 
travel stop 22 is positioned so as to contact and centrally support the 
weight of the optical actuator 10 when the focus motor coil 28 and the 
tracking motor coils 30 are in the de-energized state. Preferably, the 
actuator travel stop 22 is positioned below the center of gravity of the 
lens mount 14 in the tracking direction. The travel stop 22 is shown as 
molded directly into the actuator base 16 and can be formed of the same 
material as the actuator base 16. Alternatively, it can be formed 
separately from the base. In any event, for purposes of this disclosure, 
it will be considered to be disposed relative to the actuator base 16. 
When the optical actuator 10 is subject to extended storage, especially in 
high temperature conditions, the actuator travel stop 22 will 
substantially eliminate the moment on the flexures 18 about the x-axis. 
Elimination of this moment will substantially eliminate the rotational 
creep of the flexures 18, thereby substantially reducing lens tilt error. 
An alternative configuration, with the actuator travel stop 22 molded into 
the lens mount 14, instead of the actuator base 16, could also be used in 
accordance with the present invention. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof. However, it will be understood that 
variations and modifications can be effected within the spirit and scope 
of the invention. 
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TS LIST 
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10 optical actuator 
12 lens 
14 lens mount 
16 actuator base 
18 flexures 
22 actuator travel stop 
24 magnet 
26 magnetic yoke 
28 focus motor coil 
30 tracking motor coil 
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