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.
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.
MicroSPECT/CT image acquisition and analysis for thyroidal RAI uptake is greatly improved by the cradle and the CTViewer software, respectively. Furthermore, the approach of superimposing thyroid VOIs from t24 images to select thyroid VOIs on corresponding aligned t1 images can be applied to studies in which the target tissue has differential radiotracer retention from surrounding tissues.
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