Design, modeling, and control of a long stroke compliant tip-tilt-piston micropositioning stage driven by voice coil motors.

In this paper, a long-stroke parallel compliant tip-tilt-piston micropositioning stage driven by voice coil motors (VCMs) is proposed. The stage is equipped with three sets of driving arms, which include a spherical hinge, VCM, and parallelogram guide mechanism, evenly spaced at 120° intervals. The spherical hinge is composed of orthogonal leaf-spring beams, and the VCM is embedded into the parallelogram mechanism to form a compact design. The compliance matrix method and the geometric method facilitated the determination of compliance in all six degree-of-freedom directions of the spherical hinge and the derivation of kinematic equations for decoupling the motion of the stage. In addition, finite element analysis was utilized to determine the maximum stroke and stress of the stage. To validate the proposed design, a stage prototype was constructed and subjected to experimental testing. Furthermore, a feedback controller was designed, integrating proportional integral controller, notch filter, and sliding mode controller feedforward. The experimental results indicate that the stage can achieve a long stroke of ±50.75 mrad × ±44.2 mrad × ±4.425 mm, with the natural frequencies in the three-axis direction of 22.3 × 25.5 × 25.5 Hz3. In addition, the maximum relative tracking error was maintained below 5.25%, highlighting the effectiveness of the control technique in achieving a high tracking performance.

Structural design and experimental evaluation of a coarse-fine parallel dual-actuation XY flexure micropositioner with low interference behavior.

This paper proposes a novel two degree of freedom large range coarse-fine parallel dual-actuation flexure micropositioner (CFPDFM) with low interference behavior. First, the structure and working principle of the CFPDFM are introduced. Then, based on the stiffness matrix method, the analytical models of the motion range, input stiffness, and amplification ratio of the mechanism are established. Subsequently, the accuracy of the analytical model is verified by finite element analysis and experiments. Moreover, the dual-servo cooperative drive control strategy is designed to improve the closed-loop positioning ability of the CFPDFM. Finally, the CFPDFM experimental system is built to verify the plane trajectory tracking performance. The results reveal that the micropositioner can achieve a total range of 3.02 × 3.11 mm2, a resolution of ≤40 nm, low interference performance of coarse and fine actuators, good cross-axis decoupling, and planar complex trajectory tracking performance, while possessing a compact structure. It can be used in many plane positioning situations requiring large range and high precision.

Push-pull large stroke flexure-based micropositioning stage driven by electromagnetic actuators with complementary double configuration.

This paper aims at presenting a solution to overcome the problems of small driving force and the evident nonlinear characteristics of the large stroke flexure-based micropositioning stage driven by a voice coil motor (VCM). The push-pull mode of complementary configurations of VCMs on both sides is adopted to improve the magnitude and uniformity of the driving force, and model-free adaptive control (MFAC) is combined to achieve accurate control of the positioning stage. First, the micropositioning stage based on the compound double parallelogram flexure mechanism driven by double VCMs in the push-pull mode is proposed, and its most prominent features are introduced. Then, a comparison of the driving force characteristics of a single VCM and dual VCMs is conducted, and the results are empirically discussed. Subsequently, the static and dynamic modeling of the flexure mechanism was carried out and verified by finite element analysis and experimental tests. After that, the controller for the positioning stage based on MFAC is designed. Finally, three different combinations of different controllers and VCM configuration modes are used to track the triangle wave signals. The experimental results show that compared with the other two combinations, the maximum tracking error and root mean square error of the combination of MFAC and push-pull mode are significantly reduced, which fully proves the effectiveness and feasibility of the method proposed in this paper. At the same time, the reduction of current in the coil confirms the advantages of the push-pull mode.

Data-driven fractional order feedback and model-less feedforward control of a XY reluctance-actuated micropositioning stage.

This paper proposes a compound data-driven control method to solve the problems of low damping resonance, different dynamic properties, and hysteresis in the large-range compliant micropositioning stage driven by a Maxwell reluctance actuator. First, in order to verify the proposed control algorithm, a reluctance-actuated, XY compliant micropositioning stage is constructed according to the principle of operation of a reluctance actuator. Second, in order to eliminate the influence of complex dynamics on the controller design, a fractional order proportional-integral feedback controller is designed using a data iterative feedback turning algorithm. Third, the finite impulse response feedforward filter is optimized using experimental data, and the on-line inverse estimation of the system frequency response function and its iterative feedforward compensation are carried out to further eliminate the influence of light damping resonance. Finally, the proposed control method is used for tracking the experiment and compared with other methods. The experimental results show that the proposed control method can better meet the requirements of high precision, fast speed, and strong anti-interference ability for large stroke micro/nanopositioning and tracking.

Analytical model and experimental verification of an elliptical bridge-type compliant displacement amplification mechanism.

In this paper, an analytical model of elliptical bridge-type compliant mechanism is established. Compared with the traditional bridge mechanism, the elliptical bridge-type compliant mechanism has the advantages of light weight, high natural frequency, and more uniform stress distribution. Based on the strain energy and Castigliano's displacement theorem, a static model for calculating the displacement amplification ratio and input stiffness is established. Then, the Lagrange equation is used to establish the dynamic model to calculate the natural frequency. After that, finite element simulation and experimental test are both used to verify the proposed analytical model. The results show that the maximum error between the analytical model and finite element simulation is within 8.25% and that of the experimental results is within 6.25%. The conclusion of this paper provides an accurate prediction analytical method for the mechanical performance index design of the elliptical compliant mechanism, which has important theoretical significance.

Paired double parallelogram flexure mechanism clamped by corrugated beam for underconstraint elimination.

This Note presents a kind of paired double parallelogram (DP) flexure mechanism clamped by a sinusoidal corrugated beam to solve the problem of underconstraint of its secondary stages. The results show that the proposed mechanism effectively constrains the undesired motions of the secondary stages without changing the stiffness and the first-order natural frequency in the working direction. The second- and third-order natural frequencies of the mechanism are increased by 9.2% and 30.8%, respectively, which can effectively improve the dynamic characteristics of the DP mechanism.

Development of a novel line structured light measurement instrument for complex manufactured parts.

This paper studies a novel structured light measurement instrument for complex manufactured parts. In this research, a quick and accurate light plane calibration method and a novel light strip center extraction method were used to establish the measurement instrument. First, the intrinsic and external parameters of the camera are calibrated by Zhang's method. Second, a novel light plane calibration technique is proposed by using the original parameters and images in the process of camera calibration, which can be used to improve the accuracy of the measurement system because both the camera calibration and light plane calibration are based on the same target. Third, a subpixel extraction method is presented to extract the light strip center on the basis of the geometric center method. Finally, the constructed instrument is used to detect the geometric information of the complex manufactured parts. The experimental results show that the maximum measurement error is 0.1 mm under a measurement range of 50 mm and the accuracy is 0.2%, which indicates that the measurement instrument is fast, robust, and accurate enough to be applied to many kinds of measurements, such as railroad track deformation, the inner wall of a pipe, and so on.

Design and analysis of a displacement amplifier with high load capacity by combining bridge-type and Scott-Russell mechanisms.

To improve the load capacity of flexure-based amplification mechanisms, a flexure-based displacement amplification mechanism is developed in this study by combining the bridge-type and the Scott-Russell mechanisms. The four arms of the traditional bridge mechanism are replaced by four Scott-Russell mechanisms that provide the advantages of good symmetry, compact structure, and higher load capacity. An analytical model is formulated using a stiffness matrix to analyze the natural frequency, input and output stiffnesses, displacement amplification ratio, and load capacity of the proposed mechanism. The design model was subjected to both experimental methods and finite element analysis for verification, and the two sets of results were in good agreement. This proves the effectiveness and applicability of the analytical model and the proposed amplification mechanism. The output stiffnesses of the modified bridge-type amplifier and the traditional bridge-type amplifier are 0.158 N/µm and 0.041 N/µm, respectively, indicating excellent load performance of the modified bridge-type amplifier.

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.

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.

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.