901 research outputs found
Design of a stiffness adjustable magnetic fluid shock absorber based on optimal stiffness coefficient
With the rapid development of aerospace technology, the vibration problem of
the spacecraft flexible structure urgently needs to be solved. Magnetic fluids
are a type of multi-functional smart materials, which can be employed in shock
absorbers to eliminate these vibrations. Referring to the calculation methods
of stiffness coefficients of other passive dampers, the stiffness coefficient
formula of magnetic fluid shock absorbers (MFSAs) was derived. Meanwhile, a
novel stiffness adjustable magnetic fluid shock absorber (SA-MFSA) was
proposed. On the basis of the second-order buoyancy principle, a series of
SA-MFSAs were fabricated. The range of stiffness coefficients covered by these
SA-MFSAs contains the optimal stiffness coefficient estimated by formulas. The
repulsive force measurement and vibration attenuation experiments were
conducted on these SA-MFSAs. In the case of small amplitude, the relationship
between the repulsive force and the offset distance was linear. The simulation
and experiment curves of repulsive forces were in good agreement. The results
of vibration attenuation experiments demonstrated that the rod length and the
magnetic fluid mass influence the damping efficiency of SA-MFSAs. In addition,
these results verified that the SA-MFSA with the optimal stiffness coefficient
performed best. Therefore, the stiffness coefficient formula can guide the
design of MFSAs.Comment: 18 pages, 12 figure
Convergence Rate Analysis for Optimal Computing Budget Allocation Algorithms
Ordinal optimization (OO) is a widely-studied technique for optimizing
discrete-event dynamic systems (DEDS). It evaluates the performance of the
system designs in a finite set by sampling and aims to correctly make ordinal
comparison of the designs. A well-known method in OO is the optimal computing
budget allocation (OCBA). It builds the optimality conditions for the number of
samples allocated to each design, and the sample allocation that satisfies the
optimality conditions is shown to asymptotically maximize the probability of
correct selection for the best design. In this paper, we investigate two
popular OCBA algorithms. With known variances for samples of each design, we
characterize their convergence rates with respect to different performance
measures. We first demonstrate that the two OCBA algorithms achieve the optimal
convergence rate under measures of probability of correct selection and
expected opportunity cost. It fills the void of convergence analysis for OCBA
algorithms. Next, we extend our analysis to the measure of cumulative regret, a
main measure studied in the field of machine learning. We show that with minor
modification, the two OCBA algorithms can reach the optimal convergence rate
under cumulative regret. It indicates the potential of broader use of
algorithms designed based on the OCBA optimality conditions
An Image Segmentation Algorithm for Gradient Target Based on Mean-Shift and Dictionary Learning
In electromagnetic imaging, because of the diffraction limited system, the pixel values could change slowly near the edge of the image targets and they also change with the location in the same target. Using traditional digital image segmentation methods to segment electromagnetic gradient images could result in lots of errors because of this change in pixel values. To address this issue, this paper proposes a novel image segmentation and extraction algorithm based on Mean-Shift and dictionary learning. Firstly, the preliminary segmentation results from adaptive bandwidth Mean-Shift algorithm are expanded, merged and extracted. Then the overlap rate of the extracted image block is detected before determining a segmentation region with a single complete target. Last, the gradient edge of the extracted targets is recovered and reconstructed by using a dictionary-learning algorithm, while the final segmentation results are obtained which are very close to the gradient target in the original image. Both the experimental results and the simulated results show that the segmentation results are very accurate. The Dice coefficients are improved by 70% to 80% compared with the Mean-Shift only method
Asymptotic Optimality of Myopic Ranking and Selection Procedures
Ranking and selection (R&S) is a popular model for studying discrete-event
dynamic systems. It aims to select the best design (the design with the largest
mean performance) from a finite set, where the mean of each design is unknown
and has to be learned by samples. Great research efforts have been devoted to
this problem in the literature for developing procedures with superior
empirical performance and showing their optimality. In these efforts, myopic
procedures were popular. They select the best design using a 'naive' mechanism
of iteratively and myopically improving an approximation of the objective
measure. Although they are based on simple heuristics and lack theoretical
support, they turned out highly effective, and often achieved competitive
empirical performance compared to procedures that were proposed later and shown
to be asymptotically optimal. In this paper, we theoretically analyze these
myopic procedures and prove that they also satisfy the optimality conditions of
R&S, just like some other popular R&S methods. It explains the good performance
of myopic procedures in various numerical tests, and provides good insight into
the structure and theoretical development of efficient R&S procedures
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