Scielo RSS <![CDATA[Latin American applied research]]> vol. 33 num. 4 lang. es <![CDATA[SciELO Logo]]> <![CDATA[A new approach to Wiener-like modeling]]> In this paper we propose a Wiener-like approximation scheme that uses Rational Wavelets for the linear dynamical structure and Orthonormal High Level Canonical Piecewise Linear functions for approximating the nonlinear static part. This structure allows to approximate anynonlinear, time-invariant, causal dynamic systems with fading memory and has the following advantages: capability of time-frequency location, design of the linear dynamic part taking into account the a priori knowledge of the system, and minimum number of parameters of Orthonormal High Level Canonical Piecewise Linear functions determined straightforwardly. <![CDATA[On-line estimation of communication time delay in a robotic teleoperation system]]> An estimation technique to compute on-line the communication time-delay in a robotic teleoperation system is presented. The proposed algorithm is based on the concept of correlation function and time-delay estimation. The estimation is obtained with an additional signal injected through the communication channel, that is correlated to another signal (of the same kind) generated at the remote station. This way, the correlation function is computed along with its maximum which, multiplied by the sampling time, will result in the desired time delay. Simulation results show the improvement of the teleoperation system performance when using both the communication time delay compensation structure and the time delay estimator. Finally, a comparison between the proposed algorithm and Matlab's xcorr function to compute time delay is performed. <![CDATA[An experience on stable control of mobile robots]]> This paper is based on a previous work (Carelli et al., 1999). In this paper, mobile robot control laws, including obstacle avoidance based on distance sensorial information are developed. The mobile robot is assumed to evolve in a semi-structured environment. The control systems are based on the use of the extended impedance concept, in which the relationship between fictitious forces and motion error is regulated. The fictitious forces are generated with the information provided by sensors on the distance from the obstacle to the robot. The control algorithms also avoid the potential problem of control command saturation. The paper includes the stability analysis of the developed control systems, using positive definite potential functions. <![CDATA[Identification of partially known models of the Susqueda hidroelectric power plant]]> This paper presents the identification of a hydroelectric power turbine dynamics. Knowledge of power plant behaviour is fundamental to obtain reliable and efficient operation of power systems. Starting from models already proposed, some modifications are suggested in order to adjust real plant response, recorded from different conditions and situations, to model behaviour. <![CDATA[Robot control with inverse dynamics and non-linear gains]]> A motion control strategy for robot manipulators, with inverse dynamics and non-linear proportional-derivative gains is presented. On account of a possible interaction of the robot with the environment, impedance is incorporated to modify the robot's motion references according to the interaction force. The gains, that are non-linear state functions, allow to improve robot performance and to prevent actuator saturation. It is proved that an asymptotically stable closed-loop system is obtained with the proposed controller. Simulation results on a 3-dof robot show a good performance of the controller with variable gains, as opposed to that of a constant gain PD controller. <![CDATA[Quantized-state control: a method for discrete event control of continuous systems]]> This paper introduces a new method for the digital implementation of controllers designed in continuous time. Through the quantization of its state and input variables the original continuous controller is mapped into a discrete event model within the DEVS formalism framework that can be implemented in a digital device. Under certain conditions on the original continuous control system (CCS), this implementation guarantees regional convergence in finite time of system trajectories to arbitrarily small regions around the equilibrium points even in the presence of A/D and D/A quantization effects. The convergence of the new scheme to the CCS is demonstrated when the quantization width goes to zero. Further, a design algorithm for the digital controller is given, which fulfills specifications of admissible final error and convergence speed. Also discussed is the computational efficiency of the scheme, along practical implementation issues. Two numerical examples are provided illustrating some benefits of the new method. <![CDATA[An output feedback algorithm for trajectory tracking in control affine nonlinear systems]]> In this work we present an output feedback algorithm that solves the trajectory tracking problem in control affine nonlinear systems. This algorithm, is an improvement, for this class of systems, of that of (Mancilla Aguilar et al. 2000a), since it reduces the chattering effect on the control while keeping the original performance. In addition, and via a high gain observer, it deals with discrete output measurements instead of the states, as the original algorithm does. <![CDATA[Stability analysis of degenerate Hopf bifurcations for discrete-time systems]]> A methodology for the stability analysis of invariant cycles emerging from Hopf bifurcations in discrete-time nonlinear systems is presented. The technique is formulated in the so-called frequency-domain and it is based on the Nyquist stability criterion and a higher-order harmonic balance method. The study of a planar cubic map is included for illustration. <![CDATA[On the design of robust orthogonal adaptative decision feedback equalizers for uncertain dispersive channels]]> This paper presents the theoretical aspects of a methodology for the design of robust orthogonal adaptive decision feedback equalizers. The dispersive transmission channel is assumed to have a transfer function type of description with small uncertainties in the parameters. A decision feedback equalizer is designed minimizing a mean square error objective function that takes into account the uncertain description of the channel. The equalizer is conceived with an orthogonal basis structure, so that the basis parameters inherit the robustness properties of the design to parameter perturbations. Adaptation of the coefficients that linearly combine the basis elements is also considered and the development of the adaptation algorithm is included. The resulting equalizer has a very flexible, modular and easy to implement structure. An example with comparisons of performance with FIR designs is included. <![CDATA[Groebner bases for designing dynamical systems]]> The design or synthesis of systems exhibiting a prescribed trajectory is presented in this paper. The design process is based on algebraic concepts, and it relies heavily on the use of Groebner bases. It is assumed that both the trajectory and its dynamics can be represented as algebraic relationships between the variables of the system and their first derivatives. The method yields a dynamical systems with the desired behavior as one of its many solutions. <![CDATA[Robust identification of PWL-Wiener Models: use in model predictive control]]> In this paper a robust identification strategy for Wiener like models constituted by a linear dynamic block in series with a Piecewise Linear (PWL) function as the nonlinear static gain is presented. The proposed realization allows straightforward characterization of the static gain uncertainty. A robust Model Predictive Control (MPC) algorithm, using the presented modeling strategy, is developed to guarantee that no constraints in the feedback loop are violated. <![CDATA[Acoustic emission signal analysis in machining processes using wavelet packets]]> The acoustic emission (AE) phenomenon is useful for monitoring machining processes. AE is directly related to the tool condition, since it is generated by plastic deformation, abrasion, debris fracture, and crack propagation. In the research reported in this paper several AE parameters were measured during the wearing process of a tool insert in turning tests. While some of these parameters are indicative of the wear degree, their applicability in industrial activities is limited. This occurs because the establishment of a control threshold value strongly depends on the process variables. In this paper parameters obtained from the Wavelet Packet (WP) transform coefficients, were employed. They were the Power and the Entropy of the WP coefficients both in the 300 kHz - 600 kHz frequency range. These two parameters were adequate and specific for the determination of the tool condition. <![CDATA[Hammerstein an Wiener model identification using rational orthonormal bases]]> In this paper, non iterative algorithms for the identification of (multivariable) Hammerstein and Wiener systems are presented. The proposed algorithms are numerically robust, since they are based only on least squares estimation and singular value decomposition. For the Hammerstein model, the algorithm provides consistent estimates even in the presence of coloured output noise, under weak assumptions on the persistency of excitation of the inputs. For the Wiener model, consistency of the estimates can only be guaranteed in the noise free case. Key in the derivation of the results is the use of rational orthonormal bases for the representation of the linear part of the systems. <![CDATA[On the existence of norm-estimators for switched systems]]> In this paper, we prove the existence of norm-estimators for switched nonlinear systems. The proof is based on an existing converse Lyapunov theorem for IOSS nonlinear systems, and on the association of the switched system with a nonlinear system with inputs and disturbances that take values in a compact set. <![CDATA[Stability analysis of a certain class of time-varying hybrid dynamical systems]]> In this work we study the exponential stability of a class of hybrid dynamical systems that comprises the sampled-data systems consisting of the interconnection of a time-varying nonlinear continuous-time plant and a time-varying nonlinear discrete-time controller, assuming that the sampling periods are not necessarily constant. For this purpose we develop an Indirect Lyapunov Method of analysis, and show that under adequate hypotheses the exponential stability of the hybrid dynamical system is equivalent to the exponential stability of its linearization. <![CDATA[Control of the reaching mode in variable structure systems]]> This paper focuses on the behaviour of variable structure systems with dynamic control, particularly during the reaching mode of operation. It is shown that stability problems may arise during this reaching phase. The causes of these problems are closely related with the problems of windup commonly found in conventional control systems with actuator constraints. Methods for stabilization of the reaching mode are proposed which are based on the concepts of 'realizable reference' and observers. Well-known algorithms that have been previously proposed from empiric ideas, can now be rigorously derived using these concepts. The theoretical framework developed by Kothare and co-workers in the context of windup is generalized to study and design control algorithms for the reaching mode. <![CDATA[Solving the electroencephalography forward problem with a meshless method]]> We describe a numerical method to solve the quasistatic Maxwell equations to obtain the electric potential distribution generated by a point source of current density inside a body of arbitrary shape and constant conductivity. The method needs only a set of nodes on the surface and inside the body, but it does not need a mesh connecting the nodes. The proposed meshless method is compared against the boundary elements method evaluating its performance when solving the electroencephalography forward problem. <![CDATA[A method fo continuous-time identification of moored systems]]> In this paper a method for continuous-time identification for the class of moored semisubmersible marine systems based on totally measured states is presented. The exponential convergence of parameter trajectories is analyzed in the context of conditions for persistency of excitation (PE). A regression for the estimator is constructed containing generically 312 parameters to be identified. The presented analysis has revealed that the regressor must expand a space of only 24 dimensions instead of 312 for unbiased estimates. Under monochromatic excitation, PE conditions are expected to be satisfied only in chaotic behaviors. A case study of a real moored crane-platform is modelled and simulated to verify such conditions.