These authors contributed equally to this work.
This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/).
Technological obstacles to the use of rotarytype swing arm actuators to actuate optical pickup modules in smallformfactor (SFF) disk drives stem from a hinge’s skewed actuation, subsequently inducing offaxis aberrations and deteriorating optical quality. This work describes a dualstage seesawswivel actuator for optical pickup actuation. A triplelayered bimorph bender made of piezoelectric materials (PZTs) is connected to the suspension of the pickup head, while the tunable vibration absorber (TVA) unit is mounted on the seesaw swing arm to offer a balanced force to reduce vibrations in a focusing direction. Both PZT and TVA are designed to satisfy stable focusing operation operational requirements and compensate for the tilt angle or deformation of a disc. Finally, simulation results verify the performance of the dualstage seesawswivel actuator, along with experimental procedures and parametric design optimization confirming the effectiveness of the proposed system.
Swing arm actuators have been applied to small form factor (SFF) optical pickup heads or near field hybrid recording systems. Trends in optical disk drives include developing smallformfactor (SFF) for holographic optics and extending the applications of nearfield optics. In such a high speed positioning servo system, the head/disk spacing variation must be maintained as small as possible during disk operations. While attempting to reduce the size of optical heads, several works have scaled down all components of a conventional unit [
Two possible actuated mechanisms are available for active and passive headpositioning control. For active actuation, YeackScranton devised a secondary stage PZT bender with an active slider for contact recording [
This work describes a novel a dualstage seesawswivel actuator based on optical sensing by using a PZT bimorph component to compensate for the tilt angle variation in focusing stroke. A piezoelectric actuated suspension for a secondarystage actuator is also developed to implement fine head positioning to assist the primary VCM actuator and maintain a constant flying height during the focusing operation, as shown in
The optical axis shift during the primary VCM actuation is compensated for by using the active bimorph micro actuator. A TVA can passively absorb the residual tilt variations while actively generating a reacting force for the moment conservation. The proposed mechanism is a seesaw type swivel actuator using piezoelectric active control collaboratively with passive TVA design. The following sections describe the proposed mechanism.
In actuated slider suspension, the tilt motion of the optical actuator causes the variation of focal spot position on the photodetector, as shown in
A common method of piezoelectric actuated suspension is driven along the opposite data (track) tangential direction in an allowable stroke. The proposed seesaw swiveldrive actuator uses a dualstage leverage mechanism (
The primary structure with arbitrary distributions of mass, stiffness and damping is subjected to vibration suppression by an absorber subsystem. Sensing variation capabilities in the opticaxis direction are investigated to ensure the suppression of robust vibration. Consisting of two flexible cantilever beams extended from a balanced mass, a cantilever TVA subsystem is illustrated from the perspective of both improving bandwidth and increasing shock protection.
The vibration suppression of structures is based on independent design of dynamic absorbers, while taking into account the selected tuning mass of the subsystem structure. Effective structural stiffness is thus adjusted by adjusting the beam dimensions after a preliminary primitive design.
DeVoe and Pisano [
The elongation force
Therefore, integrating
Additionally, the rotary angle of PZT is:
The motion of a cantilever beam can be represented as [
By using the separation of variables method, the general solution can be expressed as
Replacing
The general solution of the mode shapes in
The boundary condition of
By substituting
The natural frequency is solved from the following equation:
For
From the previous derivations of the equations, two objective functions of the optimization are selected: lateral displacement error
Several attempts have been made to seek optimum absorber parameters when the main system has significant damping. Den Hartog first tackled the optimum solution of a tuned mass damper that is connected to a primary system with the intention of reducing its dynamic response [
The equation of the damped model in
The amplitudes of the system displacement,
Taking the absolute of
Substituting the natural frequency,
According to Den Hartog’s theorem [
Design of the PZT loop compensator in the dualstage actuator expands the bandwidth of the overall system. The primary VCM continuously provides coarse positioning, while the secondary actuator gives fine positioning, vibration and disturbances rejection.
The open loop bandwidth of the dualstage system is approximately 3.8 kHz, with a phase margin of 30 degrees and a gain margin of 5 dB. This represents a significant enhancement over the singlestage VCM system, which has a 0.46 kHz bandwidth for the same stability margins. The PZT actuator can reject high frequency disturbances. Notably, the resonant peak of the PZT actuator at 0.9 kHz is lower than the open loop bandwidth and does not pose stability problems.
The proposed astigmatic detection method combined with differential phase detection (DPD) is verified through simulation and experimental results. By using the quadrant photo detector array, the astigmatic signal is generated (based on the phase difference between the sum of the diagonal elements, as shown in
Focus error signal (FES),
The DPD signal is defined
DPD signal,
DPD signal,
Feasibility of the proposed secondary stage PZTactuator to compensate for the tilt actuation induced by the primary VCM actuator is demonstrated by conducting an experiment in which the dual stage actuator is kept in the infocus condition.
Both results closely correspond to the numerical calculations. The measurement accuracy of the photodetector is validated.
This work presents a miniaturized seesaw swivel actuator and suspension assembly with a piezoelectricbased microactuator and TVA absorber. Among the unique features of the proposed dual stage actuator include a rotary actuator for trackfollowing and a combined piezoVCM actuated suspension nutation for laser focusing. By using the optimization procedure, the original design is improved in terms of bandwidth and stability. The dynamic response of the dualstage actuator is also shown, in which the PZT actuator rejects a higher frequency disturbance. The difference between the experimental frequency response of PZT bender and the numerical results is within 5.6%. Lower than the open loop bandwidth, the resonant peak of the PZTactuator at 0.9 kHz does not pose stability problems. Simulation and experimental results indicate that the passive TVA reduces the system vibration by 80% with respect to the peak amplitudes. This work also investigates the relationship between the tilt sensor sensitivity and working distance of the dual stage actuator based on laser autocollimation. The tilt variance of optical axis is reduced by 60% from a singlestage drive with the implementation of PZT compensation. Performance optimization of the combined piezoVCM actuator is also evaluated to demonstrate the effectiveness of the proposed optical sensing applications.
Effective length of PZT (mm)
Effective Length of arm (mm)
Distance from disk center(mm)
Rotary angle of PZT bender (rad)
Rotary angle of arm (rad)
Rotary angle of disk (rad)
Width of PZT bender (mm)
Thickness of PZT (mm)
Thickness of carbon (mm)
Density of PZT (kg/m^{3})
Density of carbon (kg/m^{3})
Density of TVA (kg/m^{3})
Young’s modulus of PZT (GPa)
Young’s modulus of carbon (GPa)
Moment of inertia, PZT (m^{4})
Moment of inertia, carbon (m^{4})
Charging constant of PZT (C/N)
PZT input Voltage (V)
The frequency ratio between system and input
The frequency ratio between system and TVA
Mass ratio between system and TVA
Mass of dualstage system (kg)
Mass of TVA (kg)
Damping constant of PZT
Damping constant of arm
Damping constant of TVA
Stiffness of system (N/m)
Stiffness of PZT (Nm/rad)
Equivalent stiffness of PZT (N/m)
Stiffness of TVA (N/m)
Damping ratio of dualstage system
Damping ratio of TVA
This work was supported by TDPA project 98EC17A07S1011, MOEA, and NSC project NSC 962221E009149, Taiwan. The authors would like to thank the anonymous reviewers for useful comments and suggestions.
Schematic illustration of the bimorph dualstage actuator with the TVA unit indicated.
Misalignment in the incline angle of the optical axis.
Conceptual diagram of the combined bimorph piezoVCM actuator based active tilt compensation.
Developed tunable vibration absorber with system dynamics behavior: principles and practices.
Structure variables of triplelayer bimorph actuator:
Geometry and coordinates for the dual stage actuator based on electromagnetism and mechanism (
Flowchart of the PZT optimization procedure.
Effective length
Deflection curve of hysteresis loop in a bimorph bender with the applied voltage swing from ±125 V.
Measured frequency response of optimum PZT bender.
A suspended mass
Normalized amplitudes of vibrations:
Normalized amplitudes of vibrations when
Prototype of the dualstage seesaw actuator.
Experimental setup of the dual stage actuator utilized in the active vibration control scheme and measurement point of the laser vibrometer.
Frequency responses of actuator assemblies with (solid line) and without (dashed line) TVA unit.
Single and dualstage actuated suspension frequency response: VCM input to head displacement (dash line); VCM and PZT combined actuation input to head displacement (solid line).
Intensity profiles of the reflected beam on the photodiode and the output astigmatism signals (Scurve),
Geometrical model to detect the optical angle: optical layout for the differential detection method and focal spot shifts along the photodiode (
Comparison of two actuation directions with the DPD measurement results from the tilt error signal (black) and lateral deviation (red).
Experimental verification of dual stage compensation: with (red line) and without (black line) piezoactuated stage actuator.
Parametric optimization investigates the design parameters.
Objective function 


Optimization parameter  
upper/lower bound  10 ≤ 
10 ≤ 
10 ≤ 
0.1 ≤  
Constrain  2  
Algorithm  
Iterations  64  59 
Optimization value  
FuncCount  485  603 
Stepsize  0.3014  0.31 
Firstorderopt  80.1485  79.3485 
Constrviolation  15.0831  21.2495 
result 
Verification of numerical optimization.
 

967  985  1.86%  
6,065  5,874  3.14% 