Browsing by Issue Date, starting with "2008-06"
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- A new method for approximating fractional derivatives: application in non-linear controlPublication . Tenreiro Machado, J. A.; Galhano, AlexandraThe theory of fractional calculus goes back to the beginning of the theory of differential calculus, but its application received attention only recently. In the area of automatic control some work was developed but the proposed algorithms are still in a research stage. This paper discusses a novel method, with two degrees of freedom, for the design of fractional discrete-time derivatives. The performance of several approximations of fractional derivatives is investigated in the perspective of nonlinear system control.
- Describing function of a simple mechanical system with non-linear frictionPublication . Duarte, Fernando B. M.; Tenreiro Machado, J. A.This paper studies the describing function (DF) of systems constituted by a mass subjected to nonlinear friction. The friction force is decomposed in two components namely, the viscous and the Coulomb friction. The system dynamics is analyzed in the DF perspective revealing a fractional-order behaviour. The reliability of the DF method is evaluated through the signal harmonic contents.
- Numerical calculation of fractional derivatives of non-smooth dataPublication . Tenreiro Machado, J. A.; Galhano, AlexandraThis paper addresses the calculation of fractional derivatives of fractional order for non-smooth data. The noise is avoided by adopting an optimization formulation using genetic algorithms (GA). Given the flexibility of the evolutionary schemes it is established a hierarchical GA composed by a series of two GAs, each one with having a distinct fitness function.
- Down syndrome: otolaryngological effects of rapid maxillary expansionPublication . Moura, C. Pinto de; Andrade, D.; Cunha, L. M.; Tavares, M. J.; Gonçalves, Maria João; Vaz, P.; Barros, H.; Pueschel, S. M.; Clemente, M. PaisPhenotypical Down syndrome includes pharyngeal and maxillary hypoplasia and, frequently, constricted maxillary arch with nasal obstruction. This clinical trial assessed the effects of rapid maxillary expansion on ENT disorders in 24 children with Down syndrome randomly allocated to receive either rapid maxillary expansion or not. Each group received ENT and speech therapy assessments before expansion and after the device had been removed. In the rapid maxillary expansion group, the yearly ENT infection rate was reduced when assessed after device removal (p < 0.01). The parents of rapid maxillary expansion children reported a reduction in respiratory obstruction symptoms. Audiological assessment revealed improvements in the rapid maxillary expansion group (p < 0.01). Cephalometry showed increased maxillary width in the rapid maxillary expansion group. Rapid maxillary expansion resulted in a reduction in hearing loss, yearly rate of ENT infections and parentally assessed symptoms of upper airway obstruction, compared with no treatment. These findings are probably related to expanded oronasal space, due to rapid maxillary expansion.