Lei-Jie Lai

Design and control of a decoupled two degree of freedom translational parallel micro-positioning stage

This paper presents a novel decoupled two degrees of freedom (2-DOF) translational parallel micro-positioning stage. The stage consists of a monolithic compliant mechanism driven by two piezoelectric actuators. The end-effector of the stage is connected to the base by four independent kinematic limbs. Two types of compound flexure module are serially connected to provide 2-DOF for each limb. The compound flexure modules and mirror symmetric distribution of the four limbs significantly reduce the input and output cross couplings and the parasitic motions. Based on the stiffness matrix method, static and dynamic models are constructed and optimal design is performed under certain constraints. The finite element analysis results are then given to validate the design model and a prototype of the XY stage is fabricated for performance tests. Open-loop tests show that maximum static and dynamic cross couplings between the two linear motions are below 0.5% and -45 dB, which are low enough to utilize the single-input-single-out control strategies. Finally, according to the identified dynamic model, an inversion-based feedforward controller in conjunction with a proportional-integral-derivative controller is applied to compensate for the nonlinearities and uncertainties. The experimental results show that good positioning and tracking performances are achieved, which verifies the effectiveness of the proposed mechanism and controller design. The resonant frequencies of the loaded stage at 2 kg and 5 kg are 105 Hz and 68 Hz, respectively. Therefore, the performance of the stage is reasonably good in term of a 200 N load capacity.

Design of a decoupled 2-DOF translational parallel micro-positioning stage

In this paper, a new type of decoupled 2-DOF translational parallel micro-positioning stage is designed to realize the 2-DOF ultra-precision linear motion. The stage consists of two piezoelectric actuators (PZTs) and a monolithic compliant mechanism. The monolithic compliant mechanism adopts two types of compound double parallel four-leaf flexures and a mirror symmetric structure to reduce the input and output cross coupling and parasitic motion. Based on the stiffness matrix method and screw theory, a mathematical model is constructed to analyze the compliant mechanism. The optimal design is performed in view of performance constraints. The design results show good static and dynamic performances of the stage, which are well validated by the simulation of finite-element-analysis (FEA) and experimental results. The experimental results show that the proposed stage has a full range of 40µm × 40µm when the full voltage(100V) is applied on the two PZTs. Besides, the stage only has the maximum cross coupling of -50dB between the two axes, low enough to utilize single-input-single-out(SISO) control strategies for positioning and tracking.

Hysteresis modeling of piezoelectric actuators using the fuzzy system

An approach of hysteresis modeling in piezoelectric actuators is presented based on the multi-input single-output (MISO) fuzzy system. The proposed model adopts first-order Takagi-Sugeno (T-S) fuzzy system and transforms the multi-valued hysteresis into a one-to-one mapping with the extended input space vector. The generated fuzzy subspaces (multi-dimensional fuzzy sets) assign the maximum membership degree to the input data vectors. Fewer fuzzy subspaces are obtained by introducing the nearest neighbor and super radius concepts. The consequent parameter optimization is implemented after training the fuzzy system. Experimental results demonstrate that this methodology is algorithmically easy and can achieve high modeling accuracy.

Development of An Automatic Approaching System for Electrochemical Nanofabrication Using Visual and Force-Displacement Sensing

In this paper, a fast automatic precision approaching system is developed for electrochemical nanofabrication using visual and force-displacement sensing. Before the substrate is fabricated, the template should approach the substrate accurately to establish the initial gap between the template and substrate. During the approaching process, the template is first quickly moved towards the substrate by the stepping motor until a specified gap is detected by the visual feedback. Then, the successive approach using the switch of macro-micro motion with a force-displacement sensing module is triggered to make the template contact with the substrate to nanometre accuracy. The contact force is measured by the force-displacement sensing module which employs the high-resolution capacitive displacement sensor and flexure compliant mechanism. The high sensitivity of this capacitive displacement sensor ensures high accuracy of the template-substrate contact. The experimental results show that the template can reach the substrate accurately and smoothly, which verifies the effectiveness of the proposed approaching system with the visual and the force-displacement sensing modules.

A digital lock-in amplifier based contact detection technique for electrochemical nanolithography

This paper presents a digital lock-in amplifier (DLIA) based technique to detect the template-substrate contact in electrochemical nanolithography. This technique is applied to a specially designed electrochemical nanolithography system for verification. The system adopts a macro-micro positioning setup consisting of a fine stepping motor to drive the macropositioning stage and a PZT(lead zirconate titanate, Pb[ZrxTi1−x]O3) actuator to drive the micropositioning stage. The template is mounted on a force-displacement sensing module which is attached to the PZT actuated micropositioning stage and the substrate is mounted on a holder which is merged in the solution. When the template approaches the substrate, it is controlled to oscillate at a certain frequency. Two capacitive displacement sensors are used to measure the template oscillation. Afterwards, a digital lock-in amplifier is adopted to separate the oscillation information from the raw signal. The contact is determined by monitoring the separated oscillation information. Finally, experiment tests are conducted to verify the effectiveness of the digital lock-in amplifier. Experimental results demonstrate that the developed DLIA technique makes the template-substrate contact to nanometer accuracy.

Development of an electrochemical micromachining instrument for the confined etching techniques

This study proposes an electrochemical micromachining instrument for two confined etching techniques, namely, confined etchant layer technique (CELT) and electrochemical wet stamping (E-WETS). The proposed instrument consists of a granite bridge base, a Z-axis coarse/fine dual stage, and a force sensor. The Z-axis coarse/fine dual stage controls the vertical movement of the substrate with nanometer accuracy. The force sensor measures the contact force between the mold and the substrate. A contact detection method based on a digital lock-in amplifier is developed to make the mold-substrate contact within a five-nanometer range in CELT, and a force feedback controller is implemented to keep the contact force in E-WETS at a constant value with a noise of less than 0.2 mN. With the use of the confined etching techniques, a microlens array and a curvilinear ridge microstructure are successfully fabricated with high accuracy, thus demonstrating the promising performance of the proposed micromachining instrument.

Design and Analysis of a Spatial Remote Center of Compliance Mechanism

This paper presents a novel monolithic spatial remote center of compliant orientation-adjusting mechanism to resolve the parallelism alignment problem in the application of micro/nanofabrication. The mechanism is combined by two leaf-type isosceles-trapezoidal flexure pivots in a parallel manner to enable the spatial rotations around a fixed remote center. Based on the stiffness matrix method, the static model of the compliant mechanism is constructed to directly give the compliance factors that completely define the elastic response of the mechanism. The locations of remote center of compliance are also analyzed for the compliant mechanism in different loading cases. The finite element analysis results are then given to validate the analytical model and the remote center locations. The deviation of the analytical approach is less than 7% with respect to the finite element analysis method. Using the analytical model, the influences of the geometry parameters on the compliance factors and the remote center locations are graphically evaluated to provide theoretical guidelines for the practical design. The spatial remote center of compliant mechanism has the advantage of simple structure, balance, compactness, and can achieve high precision of rotation during the orientation motions.

Improving Tracking Precision of Piezoceramic Actuators Using Feedforward-Feedback Control

This paper presents a feedforward-feedback controller to improve tracking precision of piezoceramic actuators with hysteresis and creep nonlinearities. Rather than the commonly used approach to construct an inverse of the hysteresis model in the feedforward path, a direct inverse hysteresis compensation method is used to linearize the asymmetric hysteresis nonlinearity with a modified Prandtl-Ishlinskii model. Considering the limitation of the robustness of the feedforward controller, a proportional integral derivative controller is integrated in the feedback loop to mitigate the modeling uncertainty and creep nonlinearity. To demonstrate the performance improvement of the feedforward-feedback control strategy, a piezoceramic actuated platform is built, and comparative tests are conducted on the experimental platform. In comparison with the open-loop operation, the maximum tracking error of the feedforward-feedback controller is reduced from 6.47 μm to 30 nm, and the maximum hysteresis caused error is reduced from 13.19% to less than 0.1% with respect to the desired displacement range. The experimental results clearly demonstrate the feasibility and effectiveness of the developed feedforward-feedback controller using the modified Prandtl-Ishlinskii model.