Design and analysis of a mini-linear actuator for optical disk drive

Abstract

Describes a mini-linear optical pickup actuator and virtual path method for analysis of an open magnetic circuit. The magnetic circuit of the proposed actuator becomes a closed circuit when the virtual path model is applied to the outer air magnetic flux. The validity of the virtual path method is proved using a finite-element method and force experiment. A frequency response experiment shows the availability of the proposed actuator as a radial direction actuator for optical disk drives.

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 20033337
Design and Analysis of a Mini-Linear Actuator
for Optical Disk Drive
Joon Hyuk Park, Yoon Su Baek, and Young-Pil Park
Abstract—This paper describes a mini-linear optical pickup
actuator and virtual path method for analysis of open magnetic
circuit. The magnetic circuit of the proposed actuator becomes a
closed circuit when virtual path model is applied to the outer air
magnetic flux. The validity of the virtual path method is proved
using a finite-element method and force experiment. A frequency
response experiment shows the availability of the proposed
actuator as a radial direction actuator for optical disk drives.
Index Terms—Linear actuator, magnetic circuit, optical disk
drive, virtual path method, voice coil motor (VCM).
I. INTRODUCTION
A
production cost, removability, and portability. However, the
conventional mechanism of the optical pickup actuator is
shown to have the limits to enhance performance [1].
In this paper, we propose a voice coil motor (VCM)-type
mini-linear actuator and discuss its possibility of application to
an ODD.
The magnetic circuit method, namely the permeance method,
is one of the widely used tools for analysis and design since
it can express the magnetic system in simple mathematical
forms. However, it has been applied to the closed circuit in
most studies. One of the major reasons is that it is very difficult
to model the reluctance of the outer air path in an open circuit.
Therefore, the open magnetic circuit could not be shown as a
simple mathematical form.
Thus, in this work, we also propose the virtual path method
to model the outer air reluctance and apply this method for the
analysis of the proposed actuator. Using this method, an open
magnetic circuit can be expressed as a simple mathematical
form by approximation.
N OPTICAL disk drive (ODD) is one of the major
information storage devices since it has advantages in
II. VCM-TYPE MINI-LINEAR ACTUATOR
Fig. 1 shows the proposed linear actuator. To increase the
thrust force, two coils and permanent magnets are positioned at
bothsidesoftheactuator.Thisactuatorisdesignedsothatsingle
magnetic flux passes through the air gaps and yokes to reduce
therotationmomentcausedbythedifferenceofthethrustforces
at both sides. To reduce the size, the inner yokes are designed
ManuscriptreceivedJanuary17,2002.ThisworkwassupportedbytheKorea
Science and Engineering Foundation (KOSEF), CISD, Yonsei University under
Grant R11-1997-042-090001-0.
The authors are with the Mechanical Engineering Department, Yonsei Uni-
versity, Seoul, Korea (e-mail: watchcon@yonsei.ac.kr; ysbaek@yonsei.ac.kr;
park2814@yonsei.ac.kr).
Digital Object Identifier 10.1109/TMAG.2003.816249
Fig. 1.Prototype of mini-linear actuator.
to guide the mover and the outer yokes of permanent magnets
are removed. Therefore, the magnetic system of the actuator is
an open circuit.
III. MAGNETIC ANALYSIS OF OPEN CIRCUIT
Magnetic circuit design including permanent magnets has
been studied widely [2], [3]. But in most studies, it has been
applied to analyze and/or design the closed circuit because
it is very difficult to express the open circuit in a simple
mathematical formula.
Therefore, we suggest the virtual path method that can be
used to model the outer magnetic flux of open magnetic circuit
by approximation. An open magnetic circuit can be expressed
in a simple mathematical form using a virtual path method.
The rudimentary concept of the virtual path method is that
most of the magnetic flux passes where the reluctance is at a
minimum. Thus, the proper shape of the magnetic flux path for
the outer air is determined roughly and is modified to minimize
the outer air reluctance.
To make the analysis simple, let us consider the magnetic
circuit composed of only two right-hexahedron-type permanent
magnets as shown in Fig. 2. Each magnetic flux that passes be-
tween faces 1 and face 2 is neglected because the property of
the permanent magnets is considered to be orthotropic. Faces A
are divided into 16 sections. Each section of face A has an inde-
pendent virtual path and all virtual paths are directed toward the
other face A because theoretically all the magnetic flux except
one from the center of the permanent magnet should be curved.
To reduce the error, the virtual path shape between the two
faces A needs to be selected properly. In this paper, we select a
half-annulus and a couple of quarter-elliptical shapes.
Reluctances for various geometries have been studied widely
[4]. The entire reluctance of the virtual path equals the sum of
reluctancesofparallelyconnectedeightpairsofthevirtualpath.
0018-9464/03$17.00 © 2003 IEEE

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3338IEEE TRANSACTIONS ON MAGNETICS, VOL. 39, NO. 5, SEPTEMBER 2003
(a)
(b)(c)
Fig. 2.
right-hexahedron permanent magnets. (b) Flux path model by half-annulus and
quarter-elliptical shapes. (c) Elliptical shape of the virtual path model.
Virtual path model for outer magnetic flux. (a) Geometry of the
From the geometry as shown in Fig. 2 and the parallel law of
electric resistances, the entire reluctance of the virtual path can
be expressed as
(1)
where
When
(2)
where
Here,
the virtual path is the function of
constants and
is determined to minimize the reluctance of the
virtual path. From the rudimentary concept
is the permeability of free space. Total reluctance of
because , , , and are
(3)
In the case where
(4)
(a)
(b)
Fig.3.
(b) Magnetic circuit diagram.
Magneticcircuitoftheproposedactuator.(a)Magneticcircuitelement.
When we use the isotropic materials like iron in an magnetic
circuit, a virtual path model needs to be modified accordingly
because magnetic flux flows in all directions in the isotropic
materials. If one of right-hexahedron of the Fig. 2(a) is iron and
the other is a permanent magnet, the magnetic flux of the faces
1and2of thepermanentmagnet canstillbe neglected,butthere
exists the magnetic flux that passes from faces 1 and 2 of iron
to face A of the permanent magnet. The sum of reluctances of
the parallely connected path from faces 1 and 2 of iron to face
A of permanent magnet is determined as
(5)
where
Here, the entire reluctance of the virtual path model of the
outer air is equal to the sum of the reluctances of the parallely
connected path from face A of iron to face A of the permanent
magnet and the path from face 1 and 2 of iron to face A of the
permanent magnet.
IV. EXPERIMENT
Fig. 3 shows the magnetic circuit of the proposed linear actu-
ator.
indicates the reluctance of the outer air of the proposed
actuator, determined using the virtual path method.
The validity of the virtual path method can be estimated from
Figs. 4 and 5.

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PARK AND BAEK: DESIGN AND ANALYSIS OF MINI-LINEAR ACTUATOR FOR OPTICAL DISK DRIVE3339
Fig. 4.
permanent magnets.
Magnetic flux density of the air gap between two right prim-type
Fig.5.
magnet and iron.
Magneticfluxdensityoftheairgapbetween7?10?5mmpermanent
Fig. 6. Force experiment between two permanent magnets.
There is a good agreement between the analytical results
using the magnetic circuit with virtual path model and finite-el-
ement method (FEM) results. As the air gap becomes smaller
the error increases. This may be caused by the edge effect of
the permanent magnet, which is neglected in the magnetic
circuit analysis.
Fig. 6 shows the experimental results of the force between
the two permanent magnets. The edge effect appears intensely
morethanthatinFEMresults.But,wecanstillconcludethatthe
virtual path method has the validity from the force experimental
results.
Using the magnetic circuit analysis with virtual path model,
themagneticfluxdensityintheprimaryairgapsoftheproposed
actuator is 0.318 T and the average values of FEM results are
0.304 T. The errors are about 4.5% compared to the magnetic
circuit analysis results.
Fig. 7.Frequency response of the prototype actuator.
TABLE I
CHARACTERISTICS OF MINI-LINEAR ACTUATOR
Fig. 7 shows the frequency response of the proposed actuator
and the characteristics of the prototype actuator are described in
the Table I.
V. CONCLUSION
Inthispaper,wesuggestaVCM-typemini-linearactuatorfor
ODD. The virtual path method is also proposed to analyze the
outer air magnetic flux of open magnetic circuit.
As a result, the magnetic system of theproposed actuator was
expressed in a simple mathematical formula. It provides us an
analytical tool to optimize the design of the open magnetic cir-
cuit of the proposed actuator for higher thrust force while main-
taining a small size.
This paper has significance in that it offers the analytical tool
to analyze the open magnetic circuit and to optimize the design
ofanopenmagneticcircuitinsimplemathematicalexpressions.
Using the FEM and force experiments, validity of the virtual
path method was proved and we applied the virtual path method
to analyze the proposed linear actuator.
Experimental frequency response results show that the pro-
posed linear actuator can be used as a radial direction actuator
for ODD and that it has a large thrust force margin enough to
load the focusing actuator.
For further studies, we will improve the design by optimiza-
tion and develop the focusing actuator for the VCM-type mini-
linear actuator. Then, we will investigate the availability of the
entire system as an optical pickup actuator.
REFERENCES
[1] K. H. Park, C. H. Choi, and J. Ryu, “Hybrid actuator for high speed and
high precision optical disk drives,” Mechatronics, vol. 11, pp. 527–543,
2001.
[2] H. A.Leupold, “Approachesto permanentmagnetcircuit design,”IEEE
Trans. Magn., vol. 29, pp. 2341–2346, Nov. 1993.
[3] W.-B. Tsai and T.-Y. Chang, “Analysis of flux leakage in a brushless
permanent-magnetmotorwithembeddedmagnets,”IEEETrans.Magn.,
vol. 35, pp. 543–547, Jan. 1999.
[4] H.C.Roters,ElectromagneticDevices,1sted.
pp. 116–150.
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