1,720,989 research outputs found
An approximation-based design optimization approach to eigenfrequency assignment for flexible multibody systems
Full text linkThe use of flexible multibody simulation has increased significantly over recent years due to the increasingly lightweight nature of mechanical systems. The prominence of lightweight engineering design in mechanical systems is driven by the desire to require less energy in operation and to reach higher speeds. However, flexible lightweight systems are prone to vibration, which can affect reliability and overall system performance. Whether such issues are critical depends largely on the system eigenfrequencies, which should be correctly assigned by the proper choice of the inertial and elastic properties of the system. In this paper, an eigenfrequency assignment method for flexible multibody systems is proposed. This relies on a parametric modal model which is a Taylor expansion approximation of the eigenfrequencies in the neighborhood of a configuration of choice. Eigenfrequency assignment is recast as a quadratic programming problem which can be solved with low computational effort. The method is validated by assigning the lowest eigenfrequency of a two-bar linkage by properly adding point masses. The obtained results indicate that the proposed method can effectively assign the desired eigenfrequency
Multi-domain optimization of the eigenstructure of controlled underactuated vibrating systems
Full text linkThe paper proposes a multi-domain approach to the optimization of the dynamic response of an underactuated vibrating linear system through eigenstructure assignment, by exploiting the concurrent design of the mechanical properties, the regulator and state observers. The approach relies on handling simultaneously mechanical design and controller synthesis in order to enlarge the set of the achievable performances. The underlying novel idea is that structural properties of controlled mechanical systems should be designed considering the presence of the controller through a concurrent approach: this can considerably improve the optimization possibilities. The method is, first, developed theoretically. Starting from the definition of the set of feasible system responses, defined through the feasible mode shapes, an original formulation of the optimality criterion is proposed to properly shape the allowable subspace through the optimal modification of the design variables. A proper choice of the modifications of the elastic and inertial parameters, indeed, changes the space of the allowable eigenvectors that can be achieved through active control and allows obtaining the desired performances. The problem is then solved through a rank-minimization with constraints on the design variables: a convex optimization problem is formulated through the “semidefinite embedding lemma” and the “trace heuristics”. Finally, experimental validation is provided through the assignment of a mode shape and of the related eigenfrequency to a cantilever beam controlled by a piezoelectric actuator, in order to obtain a region of the beam with negligible oscillations and the other one with large oscillations. The results prove the effectiveness of the proposed approach that outperforms active control and mechanical design when used alone
A novel approach for antiresonance assignment in undamped vibrating systems
No full textThis paper proposes a novel design method for antiresonance assignment in linear undamped vibrating systems through modifications of the inertial and elastic properties of the existing degrees of freedom. The method can handle arbitrary topologies of modifications and therefore provides a general formulation. Constraints on the feasible modifications can be included to represent technical constraints and design choices. Assignment is formulated as an inverse eigenvalue problem for the adjunct system, whose eigenvectors are treated as unknowns, and is solved through numerical optimization. Validation is proposed through a five-mass, simply-connected vibrating system
Genetic and environmental contributions to social anxiety across different ages: a meta-analytic approach to twin data.
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A Genetically Informed Study of the Covariation Between Childhood Anxiety Dimensions and Social Competence
No full textWe investigated the nature of latent shared etiological elements in 398 Italian twin pairs aged 8–17, explaining covariation between high levels of anxiety symptoms and low social competence. We found significant negative correlations between Child Behaviour Checklist/6–18 Social Competence Scale and three (Panic Anxiety, Separation Anxiety, Social Anxiety) out of five Screen for Child Anxiety Related Disorders anxiety subscales. Results from causal analysis seem to exclude the hypothesis that co-occurrence between Anxiety Symptoms and Social Competence could be due to a direct phenotypic effect of one trait upon the other. Multivariate analysis suggested that both genetic and shared environmental components contribute to the phenotypic correlation between Social Competence and Anxiety Subscales, whereas unique environmental factors have a negligible influence. This means that both common genetic and shared environmental causal factors contribute simultaneously to increase risk of having low Social Competence and high Anxiety scores. In particular, covariation with Social Competence seems to be influenced by both genetic and shared environmental causal components in Separation Anxiety and Social Anxiety, whereas environmental factors have an irrelevant influence for covariation with Panic/Somatic Anxiety Subscale. Our results support the adoption of a broader view of the relationships between psychopathology and diminished social competences in childhood for both clinicians and educators
Pole assignment for active vibration control of linear vibrating systems through Linear Matrix Inequalities
Full text linkThis paper proposes a novel method for pole placement in linear vibrating systems through state feedback and rank-one control. Rather than assigning all the poles to the desired locations of the complex plane, the proposed method exactly assigns just the dominant poles, while the remaining ones are free to assume arbitrary positions within a pre-specified region in the complex plane. Therefore, the method can be referred to as "regional pole placement". A two-stage approach is proposed to accomplish both the tasks. In the first stage, the subset of dominant poles is assigned to exact locations by exploiting the receptance method, formulated for either symmetric or asymmetric systems. Then, in the second stage, a first-order model formulated with a reduced state, together with the theory of Linear Matrix Inequalities, are exploited to cluster the subset of the unassigned poles into some stable regions of the complex plane while keeping unchanged the poles assigned in the first stage. The additional degrees of freedom in the choice of the gains, i.e., the non-uniqueness of the solution, is exploited through a semidefinite programming problem to reduce the control gains. The method is validated by means of four meaningful and challenging test-cases, also borrowed from the literature. The results are also compared with those of classic partial pole placement, to show the benefits and the effectiveness of the proposed approach
A Comprehensive Meta-analysis of Cognitive-Behavioral Interventions for Social Anxiety Disorder in Children and Adolescents
No full textValutazione microbiologica del rischio cardiogeno in pazienti pediatrici HIV+: una spia di infezione da miceti
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