Ultra-precision Positioning Research Articles

Yaw error correction of ultra-precision stage for scanning beam interference lithography systems

The stage yaw error is a key factor affecting the phase distortion of gratings produced by scanning beam interference lithography system. In order to solve this problem, a coarse-fine dual-stage mechanism is proposed, in which an ultra-precision fine positioning stage with yaw error correction function is developed. To achieve nanoscale positioning and sub-microradian yaw motion accuracy, four Lorentz motors are used to drive the fine stage. The internal coupling factors and the mechanism of Lorentz motors motion control are analyzed. Besides, the Abbe error caused by the yaw error is investigated. Positioning and scanning experiments are conducted and the outcomes show that maximum yaw error is 0.33 μrad during constant velocity scanning, which completely meets the grating fabrication requirements.

An efficient method for nonlinear characteristic analysis of fixed contact surface and interface in bolted joint and its application

This paper proposes a modelling method for nonlinear characteristic analysis of bolted joint considering the combined effect of contact surface and the interface interaction. First, the discontinuous contact region of the bolted structure is regarded as a flat continuous medium based on the hypothesis of equivalent virtual material. Second, two user-defined elements are respectively developed to simulate the contact surface and flexible interface. Then, a typical example of bolted structure is simulated using the proposed method. The comparison of the simulation results and the measurements indicates that the method is valid for the nonlinear characteristic analysis of the joint. Moreover, the method also presents advantage in computational efficiency according to the comparison with the conventional FE model. Hence, the proposed method provides a feasible solution for bolted problem in practical engineering applications, such as the precision instrument design, ultra-precision position control and multi-body dynamics simulation of complex mechanical system.

Multi-Objective Topology Optimization of a Compliant Parallel Planar Mechanism under Combined Load Cases and Constraints

This paper focuses on a new type of configuration design of a compliant parallel mechanism (CPM) planar continuum structure and its characteristic analysis of vibration-inherent frequency for planar motion, which can suppress the impact of random vibration in ultra-precision positioning and manufacturing equipment and improve the inherent frequency response of the mechanism. Firstly, a vector-mapping isomorphism between the fully CPM and conventional isomorphic parallel mechanism was constructed with a kinematic differential Jacobian matrix. Then, the mathematical model of topology optimization was put forward considering the compromise programming on the static stiffness and mean vibration-inherent frequency of the mechanism as the design variable and the minimization of compliance as the objective function. A constraint of volume fraction was considered and multi-objective micro displacement mechanism topology optimization based on a prismatic-revolute-revolute (3-PRR) planar nano-positioning continuum structure was performed using the solid isotropic material with penalization (SIMP) technique, which combines the criteria of the optimization algorithm and the vector isomorphic mapping method. Multi-objective topology optimization of the continuum structure micro displacement mechanism was investigated and presented by optimizations with different initial rejection rates. The simulation results show that the stiffness and vibration suppression performance of the continuum structure were improved, whereas the positioning of differential kinematics characteristics of the 3-PRR micro displacement planar fully CPM and isomorphic prototype mechanism retain the same. The modal analysis also provides a rational configuration for the micro displacement mechanism dimensional design and its optimal modal parameters. The crossover oscillation in frequency response of the continuum structure was reduced and quickly converged in the optimization iterations. The performance of the optimized mechanism was verified by the experiments on a planar fully compliant micro displacement continuum structure based on Lead Zirconate Titanate (PZT) actuator.

Practical controller design for ultra-precision positioning of stages with a pneumatic artificial muscle actuator

This paper presents a practical controller design method for ultra-precision positioning of pneumatic artificial muscle actuator stages. Pneumatic artificial muscle (PAM) actuators are safe to use and have numerous advantages which have brought these actuators to wide applications. However, PAM exhibits strong non-linear characteristics, and these limitations lead to low controllability and limit its application. In practice, the non-linear characteristics of PAM mechanism are difficult to be precisely modeled, and time consuming to model them accurately. The purpose of the present study is to clarify a practical controller design method that emphasizes a simple design procedure that does not acquire plants parameters modeling, and yet is able to demonstrate ultra-precision positioning performance for a PAM driven stage. The practical control approach adopts continuous motion nominal characteristic trajectory following (CM NCTF) control as the feedback controller. The constructed PAM driven stage is in low damping characteristic and causes severe residual vibration that deteriorates motion accuracy of the system. Therefore, the idea to increase the damping characteristic by having an acceleration feedback compensation to the plant has been proposed. The effectiveness of the proposed controller was verified experimentally and compared with a classical PI controller in point-to-point motion. The experiment results proved that the CM NCTF controller demonstrates better positioning performance in smaller motion error than the PI controller. Overall, the CM NCTF controller has successfully to reduce motion error to 3µm, which is 88.7% smaller than the PI controller.

超精密位置決めステージ向け微小振動除去機構に関する研究

超精密位置決めステージ向け微小振動除去機構に関する研究

Analysis of an Ultra-precision Positioning System and Parametrization of Its Structural Model for Error Compensation

Conventional compensation of position errors of machine tools relies only on measured values. Due to this principle it is not always possible to compensate the errors in time, especially dynamic ones. Moreover, the relevant control variables cannot always be measured directly. Thus, this approach proves to be insufficient for high precision applications. In this context, a model-based error prediction allows for minimal position errors. However, ultra-precision applications set high demands for the models’ accuracy. This paper presents the design of an accurate and real time-capable structural model of an ultra-precision positioning system. The modeling method for the developed ultra-precision demonstrator is shown and the initial parameter identification is presented.

A novel approach for mitigating the effects of pre-rolling/pre-sliding friction on the settling time of rolling bearing nanopositioning stages using high frequency vibration

Nanopositioning stages are used for ultra-precise positioning in many different applications. Among the bearing options available for long-range nanopositioning stages, rolling bearings are the most cost effective. However, rolling bearing nanopositioning (RB-NP) stages are well known to take a long time to reach their target positions in point-to-point positioning tasks due to pre-rolling/pre-sliding friction. This paper presents vibration assisted nanopositioning (VAN) – a novel approach for reducing the settling time of RB-NP stages using high frequency vibration (aka dither). VAN synergistically combines the electromechanical design and control of RB-NP stages to mitigate pre-rolling/pre-sliding friction using dither, without jeopardizing positioning precision. It is shown in simulations to exhibit superior positioning performance and robustness compared to model-based friction compensation methods. Experiments carried out on a prototype stage incorporating the VAN approach show a 52% reduction of mean settling time, based on 50 trials, when compared to a case without VAN. Moreover, the heat and potential wear induced by dither are shown to be practically insignificant.

Design and experimental validation of an ultra-precision Abbe-compliant linear encoder-based position measurement system

The design and development of an Abbe-compliant linear encoder-based measurement system for position measurement with a targeted 20nm uncertainty (k=2) in machine tools and CMMs is presented. It consists of a linear scale and a capacitive sensor, mounted in line on an interface which is guided in the scale's measurement direction and driven by a linear motor based on the output signal of the capacitive sensor. The capacitive sensor measures the displacement of a target surface on the workpiece table. The functional point, which is the center of a tool or touch probe, is always aligned with the scale and capacitive sensor such that this configuration is compliant with the Abbe principle. Thermal stability is achieved by the application of a thermal center between the scale and capacitive sensor at the tip of the latter, which prevents both components to drift apart. Based on this concept, a prototype of a one-DOF measurement system was developed for a measurement range of 120mm, together with an experimental setup aimed at verifying the reproducibility of the system for changing ambient conditions of ±0.5°C and ±5%rh and the repeatability during tracking of a target surface over a short period of time. These experiments have shown that the measurement uncertainty of the one-DOF system is below 29nm with a 95% confidence level.

Dynamic modeling method for air bearings in ultra-precision positioning stages

Air bearings have been widely used in ultra-precision positioning stages due to the property of nearly zero friction or wear. Small vibration of the bearing reduces the overall moving and positioning precision of the stage and hampers its applications in fabrication facilities requiring nanometer moving and positioning precision. In order to improve system precision, knowledge of the dynamic characteristics of air bearings is the first and crucial step. However, it is still a challenge to set up an accurate dynamic model for air bearings due to the system complexity. In this article, a novel method for the dynamic modeling of air bearing is proposed, which takes into account the dynamics in both the moving direction and the supporting direction. An ultra-precision positioning dual stage is investigated using the proposed dynamic modeling method. This stage has two sets of air bearings and can be used in integrated circuit fabrication equipments. Moreover, dynamic behaviors of the ultra-precision positioning dual stage are studied and compared with experimental results to validate the effectiveness and accuracy of the proposed method.

Development of an ultrasonic linear motor with ultra-positioning capability and four driving feet

This paper presents a novel linear piezoelectric motor which is suitable for rapid ultra-precision positioning. The finite element analysis (FEA) was applied for optimal design and further analysis, then experiments were conducted to investigate its performance. By changing the input signal, the proposed motor was found capable of working in the fast driving mode as well as in the precision positioning mode. When working in the fast driving mode, the motor acts as an ultrasonic motor with maximum no-load speed up to 181.2mm/s and maximum thrust of 1.7N at 200Vp-p. Also, when working in precision positioning mode, the motor can be regarded as a flexible hinge piezoelectric actuator with arbitrary motion in the range of 8μm. The measurable minimum output displacement was found to be 0.08μm, but theoretically, can be even smaller. More importantly, the motor can be quickly and accurately positioned in a large stroke.

FEI Titan 80-300 TEM

The FEI Titan 80-300 TEM is a high-resolution transmission electron microscope equipped with a field emission gun and a corrector for the spherical aberration (CS) of the imaging lens system. The instrument is designed for the investigation of a wide range of solid state phenomena taking place on the atomic scale, which requires true atomic resolution capabilities. Under optimum optical settings of the image CS-corrector (CEOS CETCOR) the point-resolution is extended up to the information limit of well below 100 pm with 200 keV and 300 keV electrons. A special piezo-stage design allows ultra-precise positioning of the specimen in all 3 dimensions. Digital images are acquired with a Gatan 2k x 2k slow-scan charged coupled device camera.

Development of compact three-degrees-of-freedom compensation system for geometric errors of an ultra-precision linear axis

We report a compensation system for geometric errors of a linear axis to achieve the ultra-precision positioning. The system was miniaturized by integrating the drive and measuring components. A novel joint with three DOFs based on a flexure hinge was developed to obtain the ultra-precision positioning. The drive component, which was adapted to the parallel mechanism, had a high structural stiffness and rapid response characteristics. The rotational stiffness of the joint was calculated and the mode shapes were analyzed to verify the feasibility of three-DOF motion. The measurement component consisted of capacitive sensors and a reference mirror to measure the three geometric errors of the linear axis. In addition, the optimal position between the reference coordinate system and a sensor was determined to minimize the standard uncertainties of the measured errors. The compensation system was fabricated and evaluated experimentally. A compensation algorithm based on a recursive technique was developed to improve the positioning accuracy of the system. The geometric errors of the linear axis were measured and compensated for, and a marked improvement was observed compared to the case with no error compensation. The peak-to-valley error of the translational and rotational errors were reduced by 92% and 98%, respectively.

直動転がり案内を用いたリニアモータ駆動による超精密位置決め

High precision positioning can be achieved by incorporating positioning control, if accurate modeling of frictional behavior occurring in such linear motion ball bearings is conducted. Final goal in this research is an achievement of ultra-precision positioning by building the accurate frictional model on displacement range under nano-meter level. For that reason, it is necessary to clarify the frictional behavior in minute displacement range. This paper describes that nonlinear spring behavior is shown in range of amplitude of displacement under 100μm. In addition, it indicates difference characteristics when the amplitude of displacement is continuously changed. Moreover, the frictional behavior in minute displacement is influenced by changing very small velocity compared with experimental results of sinusoidal and triangle feed.

Advanced Materials for Demanding Applications

This was a special conference. It was small enough (60+ delegates) but covering a wide range of topics, under a broad end-use focussed heading. Most conferences today either have hundreds or thousands of delegates or are small and very focussed. The topics ranged over composite materials, the testing of durability aspects of materials, and an eclectic set of papers on radar screening using weak ionized plasmas, composites for microvascular applications, composites in space rockets, and materials for spallation neutron sources etc. There were several papers of new characterisation techniques and, very importantly, several papers that started with the end-user requirements leading back into materials selection. In my own area, there were three talks about the technology for the ultra-precise positioning of individual atoms, donors, and complete monolayers to take modern electronics and optoelectronics ideas closer to the market place.The President of the Institute opened with an experience-based talk on translating innovative technology into business. Everyone gave a generous introduction to bring all-comers up to speed with the burning contemporary issues. Indeed, I wish that a larger cohort of first-year engineering PhD students were present to see the full gamut of what takes a physics idea to a success in the market place.I would urge groups to learn from Prof Alison McMillan (a Vice President of the Institute of Physics) and Steven Schofield, to set up conferences of similar scale and breadth. I took in more than I do from mega-meetings, and in greater depth. Professor Michael KellyDepartment of EngineeringUniversity of Cambridge

Magnetic Fluid Seal for Linear Motion System with Gravity Compensator

Demands of ultra-precision machining systems have recently increased for machining three dimensional nano-geometries. Such systems require not only horizontal positioning systems but also vertical positioning systems with a nanometer positioning accuracy. However, the gravity load of moving mass negatively affects the positioning accuracy especially in a vertical positioning system. Therefore, it is necessary to compensate the influence of the gravity load for vertical ultra-precision positioning. Although the gravity compensation method using a non-contact vacuum cylinder has been proposed, it is difficult to improve the sealing performance of the non-contact seal. Thus, this study proposes the magnetic fluid seal for linear motion system with a gravity compensator. In general, magnetic fluid seals have been applied to rotary motion systems, however there are few studies on linear motion systems so far. When a magnetic fluid seal is applied to linear motion systems, the magnetic fluid leaks out of the seal clearance due to the movement, and it cannot keep the performance as a seal. In this study, it is possible to prevent leaking the magnetic fluid from the seal clearance and also to maintain the performance of the seal by holding a magnetic fluid by the two pole pieces shaped tapered tip and putting it near the piston made of non-magnetic material and coated with oil repellent layer. The proposed magnetic fluid seal performance has been evaluated. As a result, the result confirmed that the proposed magnetic fluid seal is useful as a linear motion seal, and the oil repellent layer on the piston surface can largely improve the sealing performance.

LFT Structured Uncertainty Modeling and Robust Loop-Shaping Controller Optimization for an Ultraprecision Positioning Stage

In this paper, a practical modeling and robust controller optimization strategy is presented for an ultraprecision positioning stage with position-dependent dynamics to achieve ultraprecision positioning accuracy. A linear-fractional-transformation structured uncertainty modeling procedure is proposed to describe the varying dynamics of the stage. The modeling process involves the global curve fitting of frequency response functions and dimensionality reduction for the uncertainty structure so that the uncertainty set could be minimized. Then, a robust loop-shaping controller optimization method is presented to improve the control performances. The optimization objective includes the control bandwidth and the disturbance rejection ability, and μ analysis is employed as a nonconservative robust condition with respect to the structured uncertainty. A genetic algorithm is then utilized to determine the optimal parameters of the controller. Comparative experiments on a developed ultraprecision positioning stage are finally conducted, and the results validate that significant improvements on rising time, settling time, and positioning accuracy have been achieved.

Modeling and analysis of an over-constrained flexure-based compliant mechanism

Bridge-type micro-displacement amplifier with flexure hinges is a classic displacement amplification mechanism. Existing theoretic models cannot predict its amplification ratio and input stiffness accurately and make it very difficult to confirm the amplifier’s performance and error compensation by means of these models, which is very significant in ultra-precision positioning. This paper focuses on the development of design equations that can accurately calculate the ideal displacement amplification ratio and input stiffness of the amplifier based on the thought of statically indeterminate structure. Force Method, Maxwell–Mohr Method, principle of superposition and deformation compatibility are used together to establish uncanonical linear homogeneous equations. The analytical results are verified by FEA simulations. The influence of the geometric parameters on the amplifier performance is investigated. It is noted that amplifier performance is more sensitive to the longitudinal distance of flexure hinges. Besides, two same-sized amplifiers with the opposite output directions can be clearly differentiated by these equations.

2D24 Investigation of pressure control in balancing cylinder for hybrid electric-pneumatic ultra-precision vertical positioning device(The 12th International Conference on Motion and Vibration Control)

Many types of positioning devices have been developed for machine tools. Of these, state-of-the-art ultra-precision vertical positioning devices most often employ pneumatic balancing cylinder(s) along with an electric linear motor to achieve energy-efficient ultra-precise positioning. However, the relationship between the accuracy of ultra-precision positioning and fluctuation of pressure in a balancing cylinder is currently unknown. In order to further clarify this relationship, we fabricated an electro-pneumatic hybrid ultra-precision vertical positioning stage and used it to investigate the relationship between the accuracy of ultra-precision positioning and pressure fluctuations in a balancing cylinder. Next, a high precision quick response pneumatic regulator (HPQR) that had been previously developed by the authors was applied to pressure control of the balancing cylinder using the newly developed positioning stage. Our experimental results indicate the superiority of the proposed method compared with conventional methods.

Tape drive track following using cascade control

Ultra-precise positioning of the recording head over the data tracks is required to achieve the aggressive track density scaling envisioned for future tape storage systems. The track-follow control system is responsible for reducing the misalignment between the tape and the head created primarily by lateral motion of the flexible medium. The challenge of improving the track-following accuracy becomes even more difficult in the presence of external vibration disturbances. Typically, position error signal (PES) information is measured at the tape head using pre-formatted servo information and is used in a feedback controller during track-following to maintain the position of the read/write elements at the desired position during tape transport. In this paper, a track-follow scheme is introduced in which a high-bandwidth low-noise position sensor is used in conjunction with the position information read back by the head from preformatted servo patterns on the tape. A cascade control configuration is proposed, in which first an inner loop is designed based on the actuator position information provided by the sensor. The high-bandwidth and low-noise characteristics of the sensor enable a high closed-loop bandwidth of the inner system that effectively compensates for external vibration disturbances. Subsequently, using a cascade structure, a PES-based controller is designed that compensates for the lateral tape motion disturbances. Simulation and experimental results are presented that illustrate the performance of the proposed scheme compared to a conventional PES-based track-follow control system.

【超精密位置決め専門委員会】超精密位置決め専門委員会の活動

【超精密位置決め専門委員会】超精密位置決め専門委員会の活動