Koichi Sakata

Design and control of 6-DOF high-precision scan stage with gravity canceller

High-precision scan stages are used for fabrication of integrated circuits, liquid crystal displays and so on. To fabricate such precise devices, not only stages position but also stages attitude needs to be controlled rapidly and precisely. In this paper, an experimental 6-degree-of-freedom (6-DOF) high-precision stage with a novel 6-DOF air bearing called “gravity canceller” is designed and fabricated. The 6-DOF stage consists of a fine stage and a coarse stage. The gravity canceller compensates for the fine stages gravity and supports the fine stage without friction. This structure enables us to reduce heat which is generated close to the fine stage. For a 6-DOF control problem, attitude control is as important as translational control. Rotational motion, however, has nonlinearity and coupling arising from dynamics and kinematics which could degrade the attitude control performance. Therefore, in our past paper, our research group proposed a multi-input multi-output nonlinear feedforward attitude controller to compensate such problems. Experiments were performed to verify the effectiveness of the attitude controller by using the new experimental 6-DOF stage.

Settling time shortening method using final state control for high-precision stage with decouplable structure of fine and coarse parts

High-precision stages require high-speed and high-precision control to improve their production throughput and quality. However, it is expected that their motion speed and accuracy will reach a limit in the near future if the structure of the conventional high-precision stage is used. Therefore, the authors designed and fabricated a stage called the catapult stage which has a decouplable structure consisting of a fine stage and a coarse stage. This stage is different from conventional dual stages in which the fine stage would be disturbed by the coarse stage since they contact with each other. This paper proposes a novel control system design for the catapult stage, and a settling time shortening control method using final-state control (FSC). So far, FSC is mainly applied to the applications such as hard disk drives whose initial states are the zero. However, it is important to consider the initial states for the catapult stage since the initial position, velocity and acceleration of the catapult stage are not equal to zero. Simulations and experimental results demonstrate the effectiveness of the proposed methods.

Application of mode switching control using initial state variables in constraint final-state control to high-precision dual stage

Precise positioning stages with high-speed and high-precision control performance are increasingly required for improving production efficiency and quality. In this paper, a final-state control method considering input limitation is applied to a novel high-precision dual stage during acceleration. In the method, the timing to activate the final-state control is automatically determined by a proposed criterion. The method can significantly reduce the calculation time so that real-time implementation becomes possible. Experimental results illustrate that the method can reduce the maximum thrust of the fine stage without degrading control performance.

Proposal of attitude control for high-precision stage by compensating nonlinearity and coupling of Euler's equation and rotational kinematics

High-precision stages are used for the fabrication of integrated circuits, liquid crystal displays and so on. Since higher integration density and product quality are continuously required for the development of industry and informatics, not only the stage position but also the stage attitude needs to be controlled rapidly and precisely. The attitude is determined by roll, pitch and yaw motions which are affected by nonlinearity and coupling caused by Eulers equation and rotational kinematics. These effects deteriorate the attitude control performance. This paper proposes a MIMO nonlinear feedforward attitude controller which compensates such effect. The effectiveness of the proposed approach is verified by simulations and experiments.

Integrated design of mechanism and control for high-precision stages by the interaction index in the Direct Nyquist Array method

High-precision stages are widely used in the semiconductor and flat panel industry. Because these stages have six degrees of freedom to control, coupling forces can deteriorate its control performance and stability. If heights of the center of gravity (CoG), the center of rotation (CoR), the actuation point, and the measurement point are not the same, couping between the x and θy motions occurs. In this paper, integrated design method of mechanism and control is proposed utilizing the Direct Nyquist Array (DNA) method and a changeable actuation height stage by means of multiple actuator arrangement in the x direction. Due to the model analysis and the optimal actuation height, the coupling can be reduced with a simple precompensator. By drawing the Generalized Gershgorin Band (GGB), the effectiveness of the proposed method is verified through experiments.

Frequency separation self resonance cancellation for vibration suppression control of a large-scale stage using multiple position sensors

High-precision stages are industrial equipment for micro-fabrications. Fast and precise positioning control is very important technology related to the improvement of the throughput and the product quality. Especially, a large-scale stage has a low resonance mode which disturbs fast and precise positioning because of the structures of the stage. In this paper, a novel feedback system is proposed in a SIMO system using multiple position sensors which are located at an actuator side and a load side. The proposed feedback system has two remarkable features. One is that the effect of the vibration suppression and the phase stabilization of the resonance mode can be tuned simultaneously only by two parameters. The other is that the tuning of the resonance mode does not affect the rigid mode at all. Finally, simulations and experiments with a XY -gantry-stage are performed to show the advantages of the proposed feedback control system.

Decoupling control by the center of rotation and gravity hybrid-driven method for high-precision scan stage with multiple actuators

In a multi-input multi-output control system, coupling force between multiple axes can deteriorate control performance and stability. In this paper, a decoupling method utilizing a high-precision stage with multiple actuators is proposed. According to a model considering the misalignment between the center of gravity (CoG), the center of rotation (CoR), the actuation point, and the measurement point, the coupling characteristics from the translational force to the angle can be changed by varying the height of the actuation point. The model indicates that a CoR-driven method can suppress the coupling in the low frequency range and a CoG-driven method can suppress the coupling in the high frequency range. This paper proposes a CoR and CoG hybrid-driven method using complementary filters to place the actuation point at the CoR and the CoG in low and high frequency ranges, respectively. The effectiveness of the proposed method is verified by experiments.

Reduction of impact force by model prediction and final-state control for a high precision catapult stage

The catapult stage is a high-precision dual stage with a new structure which, by allowing contact between fine and coarse parts, is compatible with increasing size and acceleration. Impact force occurs in acceleration and deceleration regions, and may degrade a stage. This paper thus proposes a control method to reduce the impact force that considers the thrust limitation by combining model prediction and final-state control. In the proposed method, the timing to activate the fine stage is automatically determined by solving an optimization problem. Simulation and experimental results demonstrate that the method can significantly reduce the impact force between the fine and coarse parts.

Trajectory tracking control for pneumatic actuated scan stage with time delay compensation

A pneumatic actuator has several advantages such as low heat generation, high weight power ratio, and low cost. However, it has several disadvantages such as time delays and nonlinearities. Because pressure and position feedback band-widths are limited by the time delay problem, it is difficult to implement a pneumatic actuator for a scan stage. Therefore, this paper proposes a modified Smith predictor for it and implements for an experimental scan stage. The effectiveness of the proposed control system is validated by frequency and time domain experiments.