A compact closed-loop nanomanipulation system in scanning electron microscope
ABSTRACT This paper presents a nanomanipulation system for operation inside scanning electron microscopes (SEM). The system is small in size, capable of being mounted onto and demounted from an SEM through the specimen exchange chamber without breaking the high vacuum of the SEM. This advance eliminates frequent opening of the high-vacuum chamber, thus, incurs less contamination to the SEM, avoids lengthy pumping, and significantly eases the exchange of end-effectors (e.g., nano probes and grippers). The system consists of two independent 3-DOF Cartesian nanomanipulators based on piezo motors and piezo actuators. High-resolution optical encoders are integrated into the nanomanipulators to provide position feedback for closed-loop control. A look-then-move control system and a contact detection algorithm are implemented for horizontal and vertical nanopositioning. The system design, system characterization details, and system performance are described.
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ABSTRACT: ABSTRACT Undesired complex ,hysteretic nonlinearities are present to varying degree in virtually all smart material based sensors and actuators provided that they are driven with sufficient high amplitudes. This necessitates the development,of purely phenomenological models ,which characterize these nonlinearities in a way which is sufficiently accurate, robust, amenable,to control ,design for nonlinearity compensation and efficient enough ,for use in real-time applications. To fulfill these demanding ,requirements ,the present paper describes a new ,compensator ,design method ,for invertible complex,hysteretic nonlinearities which ,bases on the ,so- called Prandtl-Ishlinskii hysteresis operator. The parameter identification of this model can be formulated,as a quadratic optimization problem ,which ,produces ,the best L2 2 -norm approximation,for the measured,output-input data of the real hysteretic nonlinearity. Special linear inequality constraints for the parameters guarantee the unique solvability of the identification problem and the invertability of the identified model. This leads to a ,robustness ,of the ,identification procedure against unknown measurement errors, unknown model errors and unknown,model orders. The corresponding compensator,can be directly calculated and thus efficiently implemented,from the model ,by analytical ,transformation laws. Finally the compensator ,design method ,is used ,to generate,an inverse ,feedforward ,controller ,for ,a magnetostrictiveactuator. In comparision to the conventional controlled magnetostrictive actuator the nonlinearity error of the inverse controlled magnetostrictive actuator ,is lowered from about 30 % to about 3 %.
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ABSTRACT: We present a novel hysteresis compensation method for piezoelectric actuators. Instead of using any particular mathematical model of hysteresis, we consider the hysteresis nonlinearity as a disturbance over a linear system. A disturbance observer (DOB) is then utilized to estimate and compensate for the hysteresis nonlinearity. In contrast to the existing inverse model-based approaches, the DOB-based hysteresis compensation does not rely on any particular hysteresis model, and therefore provides a simple and effective compensation mechanism. Experimental validation of the proposed hysteresis compensation is performed on a PMN-PT cantilever piezoelectric actuator.American Control Conference, 2009. ACC '09.; 07/2009
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ABSTRACT: Piezoelectric, magnetostrictive, and shape memory alloy actuators are gaining importance in high-frequency precision applications constrained by space. Their intrinsic hysteretic behavior makes control difficult. The Prandtl-Ishlinskii (PI) operator can model hysteresis well, albeit a major inadequacy: the inverse operator does not exist when the hysteretic curve gradient is not positive definite, i.e., ill condition occurs when slope is negative. An inevitable tradeoff between modeling accuracy and inversion stability exists. The hysteretic modeling improves with increasing number of play operators. But as the piecewise continuous interval of each operator reduces, the model tends to be ill-conditioned, especially at the turning points. Similar ill-conditioned situation arises when these actuators move heavy loads or operate at high frequency. This paper proposes an extended PI operator to map hysteresis to a domain where inversion is well behaved. The inverse weights are then evaluated to determine the inverse hysteresis model for the feedforward controller. For illustration purpose, a piezoelectric actuator is used.IEEE/ASME Transactions on Mechatronics 11/2009; DOI:10.1109/TMECH.2008.2009936 · 3.65 Impact Factor