Browsing by Issue Date, starting with "2006-07"
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- Comparison of different orders padé fractional order PD0.5 control algorithm implementationsPublication . Silva, Manuel; Tenreiro Machado, J. A.; Barbosa, RamiroThis paper studies the performance of different order Padé Fractional Order (FO) PD0.5 controllers applied to the leg joint control of a hexapod robot with two dof legs and joint actuators with saturation. For simulation purposes the robot prescribed motion is characterized through several locomotion variables and for the walking performance evaluation are used two indices, one based on the mean absolute density of energy per travelled distance and the other on the hip trajectory errors. A set of simulation experiments reveals the influence of the different order Padé PD0.5 controllers tuning upon the proposed indices.
- Fractional dynamics in genetic algorithmsPublication . Pires, E. J. Solteiro; Tenreiro Machado, J. A.; Oliveira, P. B. de MouraThis paper investigate the fractional-order dynamics during the evolution of a Genetic Algorithm (GA). In order to study the phenomena involved in the GA population evolution, themutation is exposed to excitation perturbations during some generations and the corresponding fitness variations are evaluated. Three similar functions are tested to measure its influence in GA dynamics. The input and output signals are studied revealing a fractional-order dynamic evolution.
- Fractional-order harmonics in the trajectory control of redundant manipulatorsPublication . Duarte, Fernando B. M.; Marcos, Maria da Graça; Tenreiro Machado, J. A.Redundant manipulators are superior to classical arms because they allow the trajectory optimization, the obstacle avoidance, and the resolution of singularities. For this type of manipulators, the kinematic control algorithms adopt generalized inverse matrices that may lead to unpredictable responses. Motivated by these problems this paper studies the complexity revealed by the trajectory planning scheme when controlling redundant manipulators. The result reveals a chaotic phenomenon and the existence of fractional order sub-harmonics in the robot signals.
- Analysis of fractional - order robot axis dynamicsPublication . Rosario, J. M.; Dumur, D.; Tenreiro Machado, J. A.Robots are complex mechatronics systems where several electric drives are employed to control the movement of articulated structures. In industrial environments they must perform tasks with rapidity and accuracy in order to produce goods and services with minimal production time. These procedures require the use of flexible robots which can act in a large workspace, thus subjected to important parameters variations and nonlinear dynamics effects. This paper investigates the fractional order dynamics during the evolution of trajectories of three robotic joints, considering the complete system dynamics
- Fractional control of two arms working in cooperationPublication . Ferreira, N. M. Fonseca; Tenreiro Machado, J. A.; Galhano, Alexandra; Cunha, J. BoaventuraThis paper analyzes the performance of two cooperative robot manipulators. It is studied the implementation of fractional-order algorithms in the position/force control of two robots holding an object. The experiments reveal that fractional algorithms lead to performances superior to classical integer-order controllers.
- Fractional dynamics in the describing function analysis of nonlinear 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 three components namely, the viscous, the Coulomb and the static forces. 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 content and the limit cycle prediction.
- Evolutionary techniques in circuit design and optimizationPublication . Reis, Cecília; Tenreiro Machado, J. A.; Cunha, J. BoaventuraSeveral Evolutionary Algorithms (EAs) are applied in the design and optimization of digital circuits, namely Genetic Algorithms (GAs), Memetic Algorithms (MAs) and swarm intelligence with Particle Swarm Optimization (PSO). GAs are optimization and search techniques based on the principles of genetics and natural selection. MAs are evolutionary algorithms that include a stage of individual optimization as part of its search strategy, being the individual optimization in the form of a local search. The combination of a global and a local search is a strategy used by many successful hybrid optimization approaches. PSO is a population-based search algorithm that starts with a population of random solutions called particles. In a PSO scheme each particle flies through the search space with a velocity that is adjusted dynamically according with its historical behavior. Therefore, the particles have a tendency to fly towards the best search area along the search process. In this line of thought, this paper presents the results for digital circuits design using the three above EAs.
- Fractional-order evolutionary design of digital circuitsPublication . Reis, Cecília; Tenreiro Machado, J. A.; Cunha, J. Boaventura; Figueiredo, LinoThis paper analyses the performance of a Genetic Algorithm (GA) in the synthesis of digital circuits using a new approach. The novel concept extends the classical fitness function by introducing a fractional-order dynamical evaluation. The dynamic fitness function results from an analogy with control systems where it is possible to benefit the proportional algorithm by including a differential component. For this purpose the non integer derivative is approximated through Pad´e fractions. The experiments reveal superior results when comparing with the classical fitness method.
- Fractional order fourier spectra in robotic manipulators with vibrationsPublication . Lima, Miguel F. M.; Tenreiro Machado, J. A.; Crisóstomo, ManuelThis paper presents a fractional system perspective in the study of signals captured during impacts and vibrations of mechanical manipulators. In order to acquire and study the signals an experimental setup was developed. The system acquires data from the sensors, in real time, and, in a second phase, processes it through an analysis package. The experimental study provides useful information that can assist in the design of a control system to be used in eliminating or reducing the effect of vibrations.
- Fractional electrical dynamics in fruits and vegetablesPublication . Jesus, Isabel S.; Tenreiro Machado, J. A.; Cunha, J. BoaventuraFractional calculus (FC) was originally developed in a pure mathematical viewpoint. However, nowadays FC is applied in many emerging fields of physics and engineering. This paper studies the fractional electrical impedance of vegetables and fruits having FC modelling in mind. In this line of thought, are developed several experiments for measuring the impedance of botanical elements, based in the Bode and polar diagrams. An electrical circuit that models these systems is presented and conclusions are drawn.