Optimising the performance of optical data storage drives based on a novel
seesaw – swivel actuator for a holographic module
Yu-Cheng Lin
1, Po-Chien Chou
1, Kuan-Chou Hou
2, Stone Cheng
2, Jin-Chern Chiou
3, Hsi-Fu Shih
4 1Department of Mechanical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
2
Institution of Electrical and Control Engineering, National Chiao Tung University, Hsinch 30010, Taiwan
3
School of Medicine, China Medical University, Taichung 40402, Taiwan
4
Mechanical Engineering Department, National Chung-Hsing University, Taichung 402, Taiwan
E-mail: [email protected]
Published in Micro & Nano Letters; Received on 14th March 2011; Revised on 11th July 2011
This Letter presents an optical module with a holographic optical element (HOE) for the small-form-factor pickup head. The proposed module is mounted on a novel seesaw swivel-drive mechanism with unique features, including a rotary actuator for track-following; in addition, a seesaw arm nutates along a pivot for laser focusing. Additionally, the actuating force is increased within the limit of actuator size constraints by using a Halbach-magnet-array in biaxial voice coil motors. Experimental assessment of the holographic module and actuator reveals that the HOE module ensures the optical performance and stability of the system, together with a performance superior to a classic hinge-type swing arm actuator, as attributed to enhanced durability and higher robustness.
1. Introduction: This work describes an innovative optical con-figuration with a holographic optical element (HOE) for small-form-factor (SFF) pickup design, as developed to comply the requirements of new generation of optical devices [1 – 3]. The overall module of the optical system is then mounted on the seesaw – swivel actuator to perform the laser focusing and track-fol-lowing operations. Next, the dynamic characteristics of electromag-netic and structural analyses are studied by deriving this biaxial actuator design. Additionally, an attempt is made to reduce the thick-ness of driving mechanism without sacrificing the actuating force by utilising the proposed ring yoke with Halbach-magnet-array (HMA) to the increase magnetic flux of one side without the cover yoke. All structural and electromagnetic circuit parts are optimised in numerical simulations to evaluate the biaxial motion performance. Furthermore, the optimised design that satisfies all specifications is explained [4, 5]. Finally, the stability and performance of the proposed holographic module and actuator are evaluated[6, 7]. 2. Device design: SFF optical configuration design by using an HOE is based on the optical specifications inFig. 1. The proposed devices are fabricated and tested to demonstrate the uniqueness of their performances and advantages over conventional ones. After the 458 turning mirror (not shown) reflects the horizontal laser
beam upwards, microprisms 2 and 1 redirect the beam upwards towards HOE. After the reflected beam enters HOE perpendicu-larly, the zero-order laser beam passes through the objective lens and focuses on the optical disk. In this work, HOE is developed as a beam-splitting element to simplify the optical path and optical components. In the optical return path, the returning beam is diffracted by the HOE patterns and reflected by microprism 1 to microprism 2. The microprisms provide support substrates as well as calibration reference planes based on the virtual image method. Finally, the returning beam projected on the quadrant photodetector (PD) generates the focus error signal (FES), tracking error signal (TES) and radio frequency signal (RFS) from the HOE diffraction.
The assembly process of this HOE module optical head are illus-trated inFig. 2. This device is fabricated and tested to achieve the microminiaturisation of optical sensor. The 635 nm laser assembly technology is utilised to play an important role in stabilising new holographic pickup structures. InFig. 2a, 635 nm edge-emitting laser diode chip, a quadrant photodetector and a 458 turning mirror are mounted on a substrate.Fig. 2bshowed two microprisms are stuck on two sides of the substrate. InFig. 2c, an HOE module is bound on these two prisms. In the final step,Fig. 2d, an objective lens module (objective lens and lens holder) is adjusted on HOE via astigmatic focus detection method.
Many previous works use the swing arm type to realise the microstorage system [8, 9]. An effective solution increases the second torsion mode to limit the servo bandwidth of the actuating range, whereas the other one strengthens the hinge connection with embossments to optimise the suspension shape; the excessive load between the suspension and swing arm also diminishes the mechanical durability. However, the hinge springs are susceptible to permanent fracturing under long-term operating conditions. Consequently, the durability of the actuator is improved by design-ing a seesaw-type rotary actuator without a hdesign-inge sprdesign-ing to provide the miniaturisation and enable the nutation to focus on the motion.
Fig. 3shows the overall structure of the seesaw actuator. The com-posite alloy seesaw arm is connected to two steel balls by the fixed cylinder pins. The cylinder pin restricts the ball, which makes contact with the side of arm and provides one degree of freedom in vertical axis (with the direction of the pin clamped). Between the seesaw arm and the pins is a rotary junction, which consists of a lateral bracket. The function of a limited working distance (+0.2 mm) that is less than the effective focal length (0.525 mm) can avoid abrupt impacts on the head to damage the disk. Figure 1 Schematic diagram of the optical head, where the laser is bounded
on a submount and emitting light from the direction into the paper
Micro & Nano Letters, 2011, Vol. 6, Iss. 7, pp. 571 – 574 571 doi: 10.1049/mnl.2011.0135
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The Institution of Engineering and Technology 2011Moreover, this study uses the HMA array to achieve vertical focus-ing motion, despite its potential for trackfocus-ing actuator usage[10]. The ring yoke with HMA has a thin actuation structure without sacrificing flux density. Owing to its unique magnet array feature that increases the magnetic flux on one side without using a cover yoke, each dimension of the array is obtained to achieve higher actuation performance, optical path and optical components. In
the optical return path, the returning beam is diffracted by the HOE patterns and reflected by microprism 1 to microprism 2. The microprisms provide support substrates as well as calibration refer-ence planes based on the virtual image method. Finally, the return-ing beam projected on the quadrant PD generates the FES, TES and RFS from the HOE diffraction.
3. Performance evaluation and optimisation: The design process of a seesaw arm is optimised for the seesaw-type actuator to comply with actuation performance requirements. The dynamic character-istics, that is modal shapes, of the seesaw arm-type actuator are analysed using finite-element methods. Fig. 4 illustrates the finite-element meshes and mode analysis. For the simulation,
Table 1lists the resonance frequencies of various vibrational modes. Driving mechanism is optimised to determine the size constraints without hindering the performance by the voice coil motor (VCM) actuation force. Fig. 5 shows this optimisation, which starts by defining six varied dimensions of the structural parts as design vari-able candidates and also, determining their corresponding con-straints that arise from the limited space. The optimal sizing
Figure 4 Finite-element model and vibrational modes
Table 1 Tolerance and optima value of six design parameters
Paremeters A B C D E F
Initial values, X0, mm 11 12 13 2.8 0.8 7.5
Upper bound (210%) 9.9 10.8 11.7 2.52 0.72 6.25 Lower bound (+10%) 12.1 13.2 14.3 3.08 0.88 8.35 Optimal values 11.2 13.0 14.1 3.01 0.85 8.12
Figure 5 Candidate six design parameters for operating optimisation Figure 2 Fabrication processes of the HOE module sensor
a Laser diode, photodiode and turning mirror b 458 prisms
c HOE glass substrate d Objective lens modulus
Figure 3 Structure of the seesaw – swivel actuator for SFF pickup head
572 Micro & Nano Letters, 2011, Vol. 6, Iss. 7, pp. 571 – 574
involves assigning the actuator sensitivity to maximise the objective function, with the intention of increasing the frequencies of flexible modes. Equation (1) shows the objective function. Notably, the weighting factor is not applied
max [(Ffocusing/Fi.focusing)2+ (Ftracking/Fi.tracking)2] (1)
where Ffocusingdenotes the mode I in the focusing direction, and
Ftracking refers to the mode II in the tracking direction.
Additionally, Fi.focusingand Fi.trackingrepresent the initial resonance
frequency, respectively. InTable 2, the optimal results are calcu-lated within the variation of the model parameters A – F with respect to the initial model parameters A0– F0, in the range of,
210 to 10%, respectively.
Fig. 6 summarises the FEM analysis results for the optimised magnetic array dimensions. Based on the electromagnetic analysis, flux density and actuation force are compared with those of the cover yoke with a conventional magnet array. The important position of the magnetic flux is 0.5 mm above the magnet array surface where coils are located.Fig. 7shows a cross-sectional mag-netic flux along the A – A′working path of the magnet array. The maximum flux density is0.45 T.Figs. 6and7also present the flux density along the radius direction (R) with various track-follow-ing angles (u). The flux densities are uniformly distributed on both focusing and tracking coils actuation range. A ring yoke with HMA is the optimum candidate for the seesaw – swivel actuator, capable of providing an additional 4.5% actuation force and superior thick-ness and mass properties over that of the ring yoke combined with the cover yoke.
4. Experimental results:Fig. 3shows a prototype of the fabricated seesaw – swivel actuator with SFF optical head. In the experimental implementation, the relative displacement of the fabricated actuator is assessed using a laser doppler vibrometer. In addition to record-ing both the input and output signals of time-domain responses, a dynamic signal analyser obtains the frequency responses of the VCM actuator.Fig. 8shows the measured frequency response of the prototype. The small resonance peak at 3.85 kHz is the first Table 2 Characteristics of resonance frequency modes
Mode Resonance frequency, Hz Response
I 281.59 cantilever
II 2375.0 1st bending
III 5113.4 2nd bending
Figure 6 3D electromagnetic analysis and air gap flux density optimisation
Figure 7 FEM results from various magnetic flux variations along A – A′ polyline
Figure 8 Frequency responses of the fabricated seesaw – swivel device a Focusing direction
b Tracking direction
Micro & Nano Letters, 2011, Vol. 6, Iss. 7, pp. 571 – 574 573 doi: 10.1049/mnl.2011.0135
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The Institution of Engineering and Technology 2011bending mode of the actuator. Simulation results (4 kHz) and measurement results (3.85 kHz) slightly differ in actuation perform-ance, possibily owing to structural differences during manufactur-ing and assembly. Additionally, mechanical properties identified during the experiment may also distort the results.
The experimental setup to evaluate the optical performance of the integrated SFF optical device includes an integrated SFF pickup unit, a seesaw – swivel device, an optical signal conversion amplifier and an optical disc. The quadrant detector adopts the astig-matic method for focusing detection and push – pull method for tracking detection. Additionally, an attempt is made to verify the focusing performance of this HOE optical module. To do so, the focusing actuator is driven by a 5 Hz triangular signal with a mag-nitude of +140 mV to force the optical module to pass its focal point to the disc. Fig. 8shows the detected results, whereas the simulated symmetric S-curve is used for comparison. The simu-lation and experimental results are satisfactory and demonstrate the feasibility of the seesaw – swivel actuator with the SFF optical head (seeFig. 9).
5. Conclusions: This work has developed a novel holographic module and seesaw – swivel device design for an integrated micro pickup head for small-form-factor applications. In addition to mag-nifying the focusing force, this accurate level mechanism strength-ens the stiffness of the load suspstrength-ension. Moreover, exactly how the driving force affects this actuator is studied to elucidate the biaxial performance. Furthermore, the mechanical and electromagnetic properties are optimised to determine the size constraints without inhibiting the performance by the VCM actuation force. Also, the fabricated device is evaluated based on dynamic experiments and optical measurements. The actuator performance and the focus per-formance for SFF optical head are verified as well. Importantly, this work demonstrates the feasibility of this novel SFF pickup head with a seesaw – swivel device for next-generation optical storage-systems.
6. Acknowledgments: Yu-Cheng Li and Po-Chieu Chou contribu-ted equally to this work.
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Figure 9 Focusing error signal (S-curve) a Simulation
b Experimental results
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