Magnetic Levitation Stage Research Articles

A Long-Range 6-DOF Absolute Displacement Sensor for Magnetic Levitation Stage Based on the Coding Principle

A Long-Range 6-DOF Absolute Displacement Sensor for Magnetic Levitation Stage Based on the Coding Principle

Development of a Novel Dual Servo Magnetic Levitation Stage

The main objective of this paper is to propose, design, and control a novel dual servo magnetic levitation stage which is precise and vacuum compatible. The dual servo mechanism, comprising a coarse stage and a fine stage, was applied to a magnetic levitation stage system for the first time. The dual servo stage achieves high precision and a long stroke at the same time. The fine stage, which comprises voice coil motors, achieves high-precision motion by overcoming the limit of the coarse stage, the form of which is a planar motor. The planar motor was mathematically modeled and analyzed with respect to the main design parameters, after which the fine stage was optimally designed to be driven by high force. Both stages including a common heat exchanger were manufactured, and the heat exchanger cools down the heat given off from the planar motor and voice coil motors. The position measuring system consisted of laser interferometers and capacitive sensors, and the integrated dual servo stage was controlled with a master–slave control scheme. The experimental results showed a precision of 10 nm, thus confirming the suitability of the developed magnetic levitation stage for a high-precision fabrication process such as wafer lithography.

보이스코일모터를 이용한 초정밀 자기부상 스테이지

Voice coil motor is an actuator that employs a non-contact electromagnetic force between its coils and magnets, and is widely used in precision stages on account of its good linearity between current and force. Additionally, these advantages are beneficial for implementing the magnetic levitation mechanism. Unlike the case of general use of a voice coil motor where 1-DOF translational motion is applied, 6-DOF magnetic levitation causes variation in the relative position and orientation between the coil and the magnet. Accordingly, the force constant also changes. In this study, high-level precision positioning of 20 ㎚ was realized through the design and control of a magnetic levitation stage using voice coil motors. In the process, the change in the force of the voice coil motor was found to be approximately 1% within the range of motion. Therefore, the proposed design was verified to be suitable for high-precision stages.

Design Concept and Structural Configuration of Magnetic Levitation Stage with Z-Assist System

Magnetic levitation technology is expected to provide a solution for achieving nanometer-scale positioning accuracy. However, magnetic leakage limits the application of the magnetic levitation stage. To reduce magnetic density, motors should be installed at an appropriate distance from the table. This increases the axis interference between the horizontal thrust and the pitching, making it difficult to achieve stable levitation. In this study, a magnetic levitation stage system that has a unique motor structure fusing a gravity compensation function and pitching moment compensation is proposed. This compensation mechanism operates automatically using the passive magnetic circuit structure, ensuring that noises from the coil current and the timing gaps do not affect the driving characteristics and that neither wiring nor sensors are required. The basic characteristics were evaluated through the driving experiments, and the efficiency of the proposed gravity and pitching moment compensation system was demonstrated.

Compact maglev stage system for nanometer-scale positioning

Semiconductor manufacturing systems and ultra-precise machine tools now require nanometer-scale positioning accuracy. To improve positioning accuracy, it is efficient to support the top table with a noncontact guide system to prevent guide friction and heat transfer from the lower table or base. A magnetic levitation (maglev) stage can accomplish ultra-precise positioning accuracy with six-degrees-of-freedom (6DOF) control even in vacuum conditions. However, if the gravity of the levitated table is supported by the thrust of a linear motor, heat generation from the motor coil dramatically increases. In addition, a larger motor is required, which causes the moving mass to increase and the mechanical response to deteriorate. We aimed to develop a compact maglev stage for which the levitated mass is less than 1 kg and that is dramatically more lightweight than existing maglev stages. This compact feature was enabled by our newly proposed gravity compensation system with repellent force and a planar motor structure. The developed stage system also has long strokes, specifically 200 mm in the X and Y-directions on a horizontal plane. We designed a maglev stage with the following design concepts to create its compact structure: (1) Reduce top-table mass to minimize the motor dimensions and enable a light weight and high responsiveness. (2) Measure the top-table position from the base to eliminate positioning error and isolate vibrations of the coarse stage. (3) Install a motor in a symmetrical layout in view from the Z-axis to enable the same driving characteristics between the X and Y axes. The results of the performance evaluation showed that the developed maglev stage system with a compact structure with 0.81 kg levitated mass has ±10 nm positioning stability.

Magnetic levitation stages for planar and linear scan application with nanometer resolution1

This article describes design rules for the magnetic constrains, the sensors and the controller for planar and linear stages for a scanning application with a resolution at nanometer level. The main advantage of magnetic levitated systems in relation to “standard” air bearing systems or mechanical guided systems is that they provide perfect cleanroom conditions. The other advantage is that there is no influence by inaccurate guiding. Because of closed-loop control in all 6 DOFs, there is also stationary accuracy and the highest repetition, which only depends on the resolution of the hysteresis-free sensor system. Most of industrial applications require free access to the movable top plate, requiring that all types of sensors should be placed below the platform. The main challenge for pushing the technology into “industry nanometer level” is how to get it to work with low-price distance sensors at nanometer resolution and a motion range of millimeters. A key point is the design of the controller structure for fast access of the actuators and many sensors, in case of the new planar system there are 24 coil pairs and 23 sensors working in the servo loop at 20 kHz.

Design process of square column-shaped voice coil motor for magnetic levitation stage

Magnetic levitation is state-of-the-art technology to realize 6 degree of freedom motion stages. The major weakness of magnetic levitation technology is the small force of the actuators, since electromagnetic actuators usually have small force compared to other actuators. Moreover, they always have to be turned on in order to compensate for the weight of the stage. In this paper, voice coil motors, a kind of electromagnetic actuator, are designed for a magnetic levitation stage. Two different types of voice coil motors are mathematically modeled, analyzed and optimized to obtain high force. According to the optimized final designs, the voice coil motors are manufactured and their performances are evaluated. The optimal design results are compared with the finite element simulations and the test results of the manufactured products. Both types of voice coil motors show high force. The results are quantified and compared. Through the design process and the evaluation of the voice coil motors, a practical design guideline for a voice coil motor is suggested.

Analysis of coupled motion constraints and coupling errors for a six-axis magnetic levitation stage

This paper presents the coupled motion constraints and coupling errors analysis of a magnetic levitation stage, which are two aspects of the coupling characteristics of magnetic levitation stage caused by force coupling. Aiming at the motion constraint coupling problem, this paper proposes a motion constrain method based on the force distribution matrix of the magnetic levitation stage. By this method, the motion constraint intervals without input saturation for single-degree-of-freedom motion, in-plane motion, and out-of-plane motion of magnetic levitation stage are established. When used for motion control, these constraints can provide the motion constraint specifications for the magnetic levitation stage, avoid the input saturation problem caused by the actuation force coupling, and provide a theoretical basis for magnetic levitation stage trajectory planning. Aiming at reducing the coupling error, this paper proposes a strategy to evaluate the coupling degree of the magnetic levitation stage actuation force by the condition number of the actuation force transfer matrix and the singular value of the force and torque error matrix. On this basis, the force and torque coupling errors caused by the translational and rotational movements of the magnetic levitation stage are studied, and the mechanism and characteristics of the coupling error between the interaction of translational and rotational movements are revealed. Based on the obtained results, the decoupling algorithm of the magnetic levitation stage is designed. Experimental results of the normalized step response demonstrate that the linearity of the magnetic levitation stage will be destroyed by the current saturation, and coupling error can also be introduced. Therefore, it is necessary to study the motion constrain strategy to provide comprehensive criteria for the trajectory planning and controller design. Experimental results of six-axis coupling error analysis show that the coupling error of x- and y-axes translational movement is reduced by 69.34% and 69.60%, respectively. This method provides a theoretical basis for the decoupling of the magnetic levitation stage and reducing the coupling error.

Magnetically Levitated Parallel Actuated Dual-Stage (Maglev-PAD) System for Six-Axis Precision Positioning

This paper presents a magnetically levitated parallel actuated dual-stage (maglev-PAD) motion system for six-axis precision positioning. This dual-stage motion system consists of a six-degrees-of-freedom (6-DOF) maglev primary stage for coarse positioning and a 2-DOF planar motion flexure-based secondary stage for fine positioning. Its coarse-fine positioning is realized through a parallel actuation concept. Unlike the conventional designs, the secondary stage in this dual-stage system is serially connected to the primary stage via the flexure hinges while the actuation coils are laid underneath the primary and secondary stages. This design concept largely minimized the size and weight of the levitated platform, thus improving the dynamic characteristics. Furthermore, the primary and secondary stages are truly contactless and most importantly cableless since no power cable is attached to the secondary stage. Design, analysis, and modeling of the Maglev-PAD motion system are presented in this paper. A prototype was developed and demonstrated that the proposed concept largely improves the static positioning errors and significantly compensates the dynamic tracking errors beyond the performance of the primary maglev stage.

Flexure-Based Magnetically Levitated Dual-Stage System for High-Bandwidth Positioning

Bandwidth is a critical specification for motion positioning systems because fast response to reference and broad-band disturbance rejection is highly desirable in industrial applications, e.g., two-dimensional (2-D)/3-D scanning. This paper presents a parallel-actuation, dual-stage concept to enhance the bandwidth of magnetically levitated (maglev) positioning system, which is realized by utilizing compliant joints to construct a monolithic-cut flexure-based fine positioning stage within the primary maglev stage, hence turning such a dual-stage system into a fully cable-less maglev system. An integrated design approach is employed to design the flexure-based secondary stage by optimizing both the mechanical parameters and the controller parameters, where various specifications, e.g., stability, performance, and saturation, are considered under the proposed framework. Experimental results have shown that the prototype can achieve a root-mean-square error of 43 nm in the principal axis even though the accuracy of the primary maglev stage is limited in micron-level because of the noise of capacitive sensors. Results also show that the developed prototype can significantly improve the closed-loop bandwidth of the maglev system from 20 to around 200 Hz.

Active damping and disturbance rejection control of a six-axis magnetic levitation stage.

This paper presents the design and application of a novel active damping and disturbance rejection controller for a magnetic levitation stage. Feedback linearization, based on the rigid-body dynamics of the levitated stage, and force distribution, based on a time-varying force distribution matrix that takes six-axis motion of a floater into account, are adopted to establish a decoupled and linearized dynamics between the six inputs and the six-axis motion. By integrating an augmented state estimator that provides full state and disturbance estimation, a linear controller that provides active damping for each axis is designed, providing the whole controller with active damping and disturbance rejection capability. In addition, the parameters of the designed controller can be easily selected based on the desired damping and natural frequency, while the parameters of the augmented estimator can be determined according to the desired estimator bandwidth and first system resonance, which make the parameter tuning have a clear physical meaning. Finally, the designed controller was implemented in a field programmable gate array-based control system. Experimental results of the proposed controller and comparison with the previously designed controller are provided to illustrate the feasibility and effectiveness of the designed control algorithm.

Differential Geometry Soft Variable Structure Control of Magnetic Levitation Movement Stage

Background: It is easy to carry out a wide range of high-precision micro movement on the magnetic levitation stage which also turns to be suitable in precision manufacturing and ultra-precision measurement field. In view of the strong coupling and nonlinear characteristics of the dynamic model of the sandwich layer structure, the differential geometry soft variable structure control method is proposed. Keywords: Magnetic levitation movement stage, differential geometry, feedback linearization, soft variable structure controller, variable saturation, upside coil.

A numerical study of forces for sandwiched maglev stage

A novel sandwiched maglev positioning stage is proposed in this paper. Mathematical model and finite element models are made up to analyze levitation force and driving force on maglev stage in two different structures—ordinary and sandwiched, which can verify the better performance of sandwiched st ructure. Also, the analysis of the relation between current input and working forces help a lot to the control of the maglev stage.

Prototype of 6-DOF Magnetically Levitated Stage Based on Single Axis Lorentz force Actuator

We present a prototype of 6-DOF magnetically levitated stage based on single axis Lorentz force actuator is capable of positioning down to micron in several millimeter travel range with a simple and compact structure. The implementation of the Lorentz force actuators, instrument modeling, and motion controller of the maglev system are described. With the force-gap relationship of the actuator solved by Gaussian quadrature, 2-demsional (2D) lookup table is employed to store the result for the designing of control system. Based on the force-gap relationship, we choose the suitable stroke of each actuator to develop the stability of the maglev stage. Complete decoupling matrix is analyzed here to establish a decoupled dynamics between resulting six axis motion and eight outputs to the actuators. The design travel volume of the stage is 2mm×2mm×2mm in translation and 80mrad×80mrad×40mrad in rotation. With a constant gain and critical damping PID controller, the resolution of the translation is 2.8um and 4um root mean square in horizontal and vertical direction respectively. Experimental results are presented to illustrate the positioning fluctuations, step responds and multi axis motion. Some comparative tests are taken to highlight the advantage resulted from the accurate force-gap relationship, suitable stroke, and complete decoupling matrix.

Design and optimization of long stroke planar motion maglev stage using copper strip array

The main purpose of this proposed maglev stage is to transport a 450 mm wafer in a vacuum environment. To realize a long stroke and a large size mover, a stator was designed using a copper strip array. The mover contains a halbach magnet array and the fixture part has characteristics of low weight and high structural bandwidth for fast response. Since the relationship between the design variables and the system performance is complicated, an optimization procedure was used to obtain optimal design variables. Based on the optimization solution, detailed design was performed and the system’s stability was checked by several simulations. Finally, the proposed maglev system was manufactured.

Numerical Calculation of the Magnetic Field and Force in Cylindrical Single-Axis Actuator

This paper investigates a kind of cylindrical linear single-axis actuator that is used in the magnetic levitation stage to provide fine movement. The magnetic flux density distribution due to the permanent magnet is determined using the surface charge method, and the magnitude of force generated by the actuator is calculated using the Lorentz force equation. Both of them can be solved by numerical integration. The results are verified by measurements. The magnetic force components are fitted as an analytical style that can be used in the real-time controlling of this actuator for the magnetic levitation stage.

Design of a VCM Actuator Using Halbach Magnet Array for Active Vibration Isolation System

This paper proposes the concept development and design of a VCM actuator for Active Vibration Isolation System (AVIS). Active vibration isolating method was constructed passive isolator and active isolator. Spring was used for isolating passively and actuator like voice coil motor was used for active isolating. The proposed active vibration isolating system could isolate a six degree-of-freedom disturbance effectively. In case of AVIS, payload is variable according to the type of instruments. Therefore, actuator should maintain uniform performance with payload change. The proposed VCM actuator satisfied this performance through relative relation between magnet array and coils. In addition, for maximizing actuator performance, actuator should generate large force density. Therefore, we apply the Halbach magnet array to proposed actuator. The Halbach magnet array helps with reinforcing magnetic flux field. The reinforced magnetic field takes a role to generate more powerful Lorentz force to isolate disturbance. Finally, in this paper, we propose the actuator that novel type using voice coil motor with the Halbach magnet array and optimize each design variables to generate maximum output. These AVIS can use to eliminate disturbance in Maglev stage. Maglev stage need AVIS to increase its performance is necessary. In the future, AVIS and Maglev system will be integrated.

Levitation Control of a Planar Magnetic Levitation Stage Based on Improved Auto-Disturbance Rejection Controller

In the present study, we proposed a magnetic levitation positioning stage for the high-precision positioning and scanning tasks in micro-electronics manufacturing and precision machining. This stage consists of a thrust system and a levitation system, the latter is a strong-coupling and nonlinear MIMO system related closely to both levitation height of each terminal magnetic bearing and instantaneous plane location of the movers and is focused on in our study. The dynamic model of this levitation system is so complicated that it is very difficult to obtain an ideal control accuracy and robustness by linear control theory, therefore, an improved auto-disturbance rejection controller was designed to realize decoupling and disturbance rejection and overcome the rough characteristics of traditional auto-disturbance rejection controller. The simulation and experiment showed that the performance of improved auto-disturbance rejection controller is superior to the traditional auto-disturbance rejection controller and with a better dynamic，static and robust performance.

고정밀 회전 및 축방향 이송을 위한 신개념 원통형 자기부상 스테이지

In this paper, a conceptual design and a detailed design of novel cylindrical magnetic levitation stage is introduced. This is came from planar-typed magnetic levitation stage. The proposed stage is composed of cylinder-typed permanent magnet array and semi-cylinder-typed 3 phase winding module. When a proper current is induced at winding module, a magnetic levitation force between the permanent magnet array and winding module is generated. The proposed stage can precisely move the cylinder to rotations and translations as well as levitations with the magnetic levitation force. This advantage is useful to make a nano patterning on the surface of cylindrical specimen by using electron beam lithography under vacuum. Two methods are used to calculate required magnetic levitation forces. The one is 2D FEM analysis, the other is mathematical modeling. This paper shown that results of two methods are similar. An assistant plate is introduced to reduce required currents of winding module for levitations in vacuum. The mathematical model of cylindrical magnetic levitation stage is used for dynamic simulation of magnetic levitations. A lead-lag compensator is used for control of the model. Simulation results shown that the detail designed model of the cylindrical magnetic levitation stage with the assistant plate can be controlled very well.

Experiments of a Novel Magnetic Levitation Stage for Wide Area Movements

In this paper, a novel planar type magnetic levitation system without other assistant devices is proposed and it can move with 6 degree of freedom (X, Y, Z, <TEX>${\theta}_X$</TEX>, <TEX>${\theta}_Y$</TEX>, <TEX>${\theta}_Z$</TEX>) in wafer size as well as in nano scale positioning.The mover is composed with 2-D Halbach permanent magnet array and the stator is composed with <TEX>$10{\times}10$</TEX> coil arrays.It was composed in laboratory and tested with short stroke (4 [mm]) and long stroke (160 [mm])movements. The errors of short movement test is [X, Y, Z, <TEX>${\theta}_X$</TEX>, <TEX>${\theta}_Y$</TEX>, <TEX>${\theta}_Z$</TEX>]<TEX>${\leq}$</TEX> [<TEX>${\pm}200nm$</TEX>, <TEX>${\pm}200nm$</TEX>, <TEX>${\pm}250nm$</TEX>, <TEX>${\pm}3urad$</TEX>, <TEX>${\pm}2urad$</TEX>, <TEX>${\pm}1urad$</TEX>]The errors of long stroke movement test is [X, Y, Z, <TEX>${\theta}_X$</TEX>, <TEX>${\theta}_Y$</TEX>, <TEX>${\theta}_Z$</TEX>]<TEX>${\leq}$</TEX> [<TEX>${\pm}200nm$</TEX>, <TEX>${\pm}200nm$</TEX>, <TEX>${\pm}250nm$</TEX>, <TEX>${\pm}1.5urad$</TEX>, <TEX>${\pm}2urad$</TEX>, <TEX>${\pm}0.5urad$</TEX>].