ISEP – DEE – Comunicações em eventos científicos
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- Adaptive Controller for Systems of Fractional Dynamics Based on Robust Fixed Point TransformationsPublication . Tar, J. K.; Rudas, I. J.; Bitó, J. F.; Tenreiro Machado, J. A.; Kozlowski, K.In this paper a discrete time approximation of Caputo’s fractional order derivatives is used for modeling the dynamic behavior of hypothetical fractional order systems the appropriate responses of which that can directly be manipulated by some physical agents are some fractional order time-derivatives of their state variables. A possible generalization of the concept of “initial conditions” of the integer order systems is proposed as “preceding history” for fractional order ones. It is shown that the number of the independent data characteristic to the “preceding history” can be made independent of the order of derivation. It is shown that the discrete time approximation proposed makes it possible to interpret the order of derivation in a higher range than in the case of the original integral form of Caputo’s definition. By providing a simple analysis of the so obtained time-sequences it is shown that by manipulating the order of differentiation in this model both dissipative and unstable behavior can be modeled. The dissipative casecorresponds to the presence of unmodeled internal degrees of freedom that are dynamically coupled to the directly controlled ones but cannot directly be controlled. The unstable case seems to be appropriate for modeling the behavior of systems coupled to some directly unmodeled exciting environment. For this purpose very simple mathematical estimations can be applied. The paradigm controlled is a fractional order Φ6 type Van der Pol oscillator that already obtained certain attention in the literature. It is shown that the simple fixed point transformations based adaptive control elaborated for integer order systems can be applied without any modification for fractional order ones.
- Adaptive nonlinear vibration control based on causal time-invariant green functions and on a novel branch of soft computingPublication . Tar, József K.; Rudas, Imre J.; Bitó, János F.; Tenreiro Machado, J. A.In this paper a simple nonlinear, adaptive approach inspired by the fractional derivatives based CRONE control is presented for vibration damping. Its key idea is replacement of the fractional derivatives with the mathematically less restricted concept of time-invariant Green functions. Instead of the traditional PID feedback terms it applies positive definite weighted moving average of the square of the error plus a nonlinear term making the error converge to zero. In this way simple kinematic design of the desired damping becomes possible. The adaptive part of the controller guarantees the realization of this kinematic design without making it necessary to the designer to have an accurate and complete dynamic model of the system to be controlled or to design sophisticated linear controller. The applicability of the approach is illustrated via simulations for a paradigm consisting of a pair of coupled damper linear oscillators under external excitation. One of the oscillators is not modeled by the controller. The adaptive loop successively maps the observed system behavior to the desired one without exerting any effort to identify the reasons of the differences. The approach was found be useful for solving vibration damping problems with unmodeled and uncontrolled internal degrees of freedom.
- Adaptive nonlinear vibration damping inspired by the concept of fractional derivativesPublication . Tar, József K.; Rudas, Imre J.; Bitó, János F.; Tenreiro Machado, J. A.In this paper a simple nonlinear, adaptive approach inspired by the CRONE method is presented for vibration control. It replaces the fractional derivatives with time-invariant Green functions. Being completed by a nonlinear feedback term it makes the positive definite weighted moving average of the square of the error converge to zero in the kinematic design of the desired damping the realization of which is guaranteed by the controller's adaptive nature. The burden of designing a sophisticated linear controller is evaded. The applicability of the approach is illustrated via simulations for a damped linear oscillator under external excitation at its resonance frequency. The adaptive loop simply successively maps the observed system behavior to the desired one without exerting any effort to identify the reasons of the differences. It is expected to be useful for solving even more complicated vibration damping problems with unmodeled and uncontrolled internal degrees of freedom.
- Adaptive reduction of the order of derivation in the control of a hydraulic differential cylinderPublication . Tar, József K.; Bitó, János F.; Rudas, Imre J.; Tenreiro Machado, J. A.Servo valve controlled hydraulic differential cylinders are non-linear, strongly coupled multivariable electromechanical tools applicable for driving e.g. manipulators. When the piston has finite but considerable velocity with respect to the cylinder the system’s behavior can be “linearized” because the viscous friction i.e. the main source of disturbance is smooth function of this velocity and causes linear damping. When this velocity is in the vicinity of zero the effect of adhesion is the dominating disturbance force that abruptly changes direction depending on the sign of this velocity. Furthermore, at zero relative velocity adhesion can compensate arbitrary forces within certain limits keeping the piston fixed. In the paper a concise application of the Stribeck model of friction and adhesion is reported in an adaptive control in which varying fractional order derivatives are used to reduce the hectic behavior of friction in the case of “critical” trajectories that asymptotically converge to a fixed position and zero velocity. Simulation results made by INRIA’s Scilab are presented. It is concluded that the combined application of the two adaptive techniques reported accurate control can be achieved without knowing the accurate model of the piston-cylinder system.
- Adaptive vibration damping based on casual time-invariant green-functions and fractional order derivatesPublication . Tar, József K.; Rudas, Imre J.; Pátkai, Béla; Tenreiro Machado, J. A.In this paper a simple nonlinear, adaptive control using causal time-invariant Green-functions and fractional order derivatives is applied for damping the vibration of a car forced during passing along a bumpy road. Its key idea is the replacement of the integer order derivates in a Green-functions-based nonlinear controller with a time-shift invariant, causal approximation of the Riemann-Liouville fractional derivative that also behaves like a Green-function. Since its physical essence is rather frequency filtering than providing inter order derivatives in limit cases, the approximation applied numerically is quite convinent. In this way simple kinematic design of the desired damping becomes possible. The adaptive part of the controller guarantees the realization of this kinematic design without making it necessary for the designer to have accurate and complete dynamic model of the system to be controlled or to design a sophisticated linear "CRONE" controller that has to take the responsability for the unknown dynamics of the system. The applicability of the approach is illustrated via simulations for a paradigm that is a rough model of a car. It was found that both adaptivity and the use of fractional order derivatives in the control are essential parts of the success of the method.
- Adaptive VS/SM Controller based on Robust Fixed Point TransformationsPublication . Tar, J. K.; Rudas, I. J.; Bitó, J. F.; Tenreiro Machado, J. A.; Kozlowski, K.The great advantage of the Variable Structure Sliding Mode controllers is their simplicity, and that they can efficiently work in the possession of a very rough model of the system to be controlled. Instead investing any effort into identifying the precise analytical model of this system they apply well timed bang-bang type control signals in order to drive the “error metrics” near zero during finite time. The effects of modeling errors and unknown external perturbations are compensated by the application of great amplitude for this fluctuation often induce chattering that can be obviated by smoothing these signals normally at the cost of degraded precision of trajectory tracking. The recently proposed adaptive control based on robust fixed point transformations drives the system nearby the kinematically arbitrarily prescribed trajectories by the use local basins of attraction in an iterative learning control also abandoning the identification of the precise model of the system. By the combination of these different approaches efficient adaptive VS/SM controller can be developed that more precisely can trace the desired trajectory kinematically prescribed by the usual relaxation of the error metrics. Chattering can be evaded by decreasing the amplitude of the fluctuating control signal without degrading the tracking precision. This statement is substantiated by simulation results obtained for a simple paradigm.
- Advanced control of robot in tecnological operationPublication . Tar, József K.; Rudas, Imre J.; Tenreiro Machado, J. A.; Horváth, LászlóA novel adaptive robot control tackling the problem of the approximately known robot dynamics and the unknown external dynamic interactions is presented in this paper. By applying uniform structures derived from the Euler-Lagrange equations in the most general and formal level it differs from and overcomes the limitations of classical feedfoward neural network-based approaches as far as the a priori unknown number of the necessary nodes and the scaling ranges of free parameters are concerned. Using a relatively simple structure of reduced number of parameters real time tuning can be carried out in the control. From the point of view of the possible local optimums resulting in improper control the structure here used seems to have the possible least complexity and coupling for a given degree of freedom robot. Several task- independent ancillary procedures also support the control. The method is illustrated via simulation in the case of a 3 active and one passive DOF SCARA arm used for polishing the surface of a bell-shaped work-piece.
- Algorithmic music composition: a surveyPublication . Ferreira, Isabel M.; Tenreiro Machado, J. A.This paper surveys some of the methods used for algorithmic composition and their evolution during the last decades. Algorithmic composition was motivated by the natural need to assist and to develop the process of music creation. Techniques and applications of algorithmic composition are broad spectrum, ranging from methods that produce entire works with no human intervention, up to methods were both composer and computer work closely together in real-time. Common algorithms used for music composition are based in stochastic, deterministic, chaotic and artificial intelligence methods.
- An adaptive robot control for thecnological operations based on uniform structures and reduced number of free parametersPublication . Tar, József K.; Tenreiro Machado, J. A.; Rudas, Imre J.; Bitó, János F.Grinding and polishing are typical application paradigms in which efficient control is needed for approximately and partially known multivariable, nonlinear, strongly coupled mechanical systems (robots) under strong dynamic interaction with an unmodeled environment. A novel adaptive approach to this problem using uniform structures and procedures as well as a passive compliant component as an essential part of the control was recently invented. The method seems to overcome the limitations of the classic approaches as limited speed of motion and supposed separability in the operational space supposing free directions for force/torque components and for free components of translation in their orthogonal sub-spaces. Like Soft Computing, instead developing the formally exact analytical model of the robot, its environment and the dynamic interaction between them the proposed method uses uniform structures but these are derived from the Euler-Lagrange equations considered in a general and formal level of abstraction. In contrast to the general approach fit to a quite wide class of problems, these structures are rather fit to a far narrower task of modeling and control of mechanical devices. This results in a drastic reduction in the number of tunable parameters, fast tuning for those parameters for which no a priori linguistic rules are available and uses simple fuzzy rules for tuning other parameters for which at least qualitative a priori known tuning are known. The proposed technique also is free from "scaling problems" so characteristic to the classic ones. The method is proved and illustrated via simulation in the case of a 3 DOF SCARA arm used for polishing a convex surface as an application paradigm.
- An educational remote laboratory for controlling a signal conditioning circuit with an LDR sensorPublication . Costa, Ricardo; Bastos, Paulo; Alves, Gustavo R.; Felgueiras, Carlos; Fidalgo, AndréIn the past few years, the technologic evolution of communication and information systems had a major impact in our lives. In education, this evolution broke some geographical barriers, facilitating students’ access to real laboratories using a simple device connected to the Internet, contributing therefore to improve teaching and learning methods. To support this evolution, this paper describes a remote laboratory that provides students’ access to a typical electronic instrumentation system. Through a set of webpages, users can remotely control a light intensity sensed through a Light Dependent Resistor (LDR) sensor connected in an arm of a configurable Wheatstone Bridge (WB), whose output differential voltage are amplified / attenuated by an Instrumentation Amplifier (IA) circuit. Through this laboratory, users are able to control several components of a typical instrumentation system, visualize changes and analyze some measurements, in the same way as they would do in a traditional hands-on laboratory. An overview of the developed remote laboratory and a contextualization within other available solutions are presented in this paper. At the end, some considerations for its adoption in education are described.