Search

In this paper, the automatic control of a single unmanned surface vehicle (USV) pushing a floating load is developed and theoretically analyzed. This represents a challenging control problem, since the manipulated load is underactuated and its open-loop dynamics is inherently unstable. Thus, a stabilizing controller must be designed. To this end, a scheme that combines partial feedback linearization with local linearization of the remaining nonlinear terms is proposed. Such scheme simplifies the design of a variable structure controller, that has robustness characteristics to parametric uncertainties and matched disturbances. The proposed closed-loop control system has local stability properties. Small-scale experimental results in calm waters, and simulation results, illustrate the performance of the proposed control system.

The synthesis of externally positive filters that can be used to modulate control signals of output-feedback sliding mode controllers is presented. These filters are useful to estimate the norm of unmeasured signals in systems with parametric uncertainties. These norm observers are a robust alternative to state observers, that may be difficult to design for uncertain systems. The first step of the synthesis is the search of the envelope function, which is an upper bound for the norm of each admissible impulse response of the uncertain system. After this step, the approximation of a positive filter with an impulse response that is an upper bound for the envelope function is carried out. The search of the envelope function and the synthesis of the approximation filters are formulated as non-convex optimisation problems that can be solved by genetic algorithms. It is important to design norm observers that are not too conservative, because the modulation signals should be kept small in order to reduce the amplitude of the control signals, power losses, and undesirable chattering effects that may occur in sliding-mode control. The proposed methods are illustrated by numerical examples.

Embarcações não tripuladas têm inúmeras aplicações dentre as quais o transporte de cargas, no qual a capacidade de uma embarcação seguir outra em movimento é interessante com vistas à otimização e ao transporte cooperativo. Este trabalho apresenta o desenvolvimento de um sistema de controle para que uma embarcação não tripulada siga um alvo. O sistema de controle desenvolvido é avaliado experimentalmente em uma embarcação pequena utilizando-se um sistema baseado em câmeras para medir as posições do alvo e da embarcação. O bom desempenho obtido indica que esse sistema de controle servirá como ponto de partida para o desenvolvimento de estratégias para o controle de frotas.

In this paper, sliding mode control (SMC) is developed to regulate the water content in the soil. The movement of water in the soil is modeled by the Richards equation, which is a parabolic partial differential equation (PDE). The non-collocated SMC of PDEs is considered, that is, where the sensor and the actuator are at different positions. Two control objectives are considered: the mean water content control, and the water content control at a specified depth. For these purposes, non-standard sliding manifolds are proposed. The existence of the sliding mode in this control system is proved using a Lyapunov function, and the stability properties are analyzed using a frequency stability criterion. Simulation results indicate the excellent transient performance and the complete rejection of external disturbances caused by water evaporation, outperforming on–off and PI controllers.

Sliding mode control (SMC) is applied to soil irrigation. The control objective is to regulate the water content in the soil. The movement of water in the soil is modeled by the Richards equation, which is a parabolic partial differential equation (PDE). The non-collocated SMC of PDEs is considered, that is, where the sensor and the actuator are at different positions, which was not found in the literature. The existence of sliding mode in this control system is proved using a Lyapunov function, and the stability properties are analyzed using a frequency criterion of stability. Simulation results indicate the excellent transient performance and the complete rejection of external disturbances caused by water evaporation.

In this paper, an adaptive sliding mode approach based on monitoring functions to deal with disturbances of unknown bounds is proposed. An uncertain linear plant is considered as well as a quite general class of (non) smooth disturbances. Global tracking is demonstrated using only output feedback. The proposed adaptation method is able to make the control gain large when the disturbance grows and decrease it if the latter vanishes, allowing for a reduced chattering occurrence. Simulations are presented to show the potential of the new adaptation scheme in this adverse scenario of possibly growing or temporarily large disturbances.

This paper proposes a sliding mode control scheme for a class of nonlinear systems with multiple time delays, in the state variables and in the output signal. The unmeasured state of the system is estimated by an asymptotic observer for the zero dynamics and by cascaded high-gain observers for a chain of integrators with a nonlinear input disturbance which compose the complete state. Global asymptotic stability of the closed-loop system is obtained using only output feedback. The use of observers prevents undesirable chattering phenomena. Simulation results show effective performance in different scenarios, including application to missile guidance in the presence of seeker delays.

The synchronization of chaotic systems is achieved using output-feedback sliding mode control even in the presence of disturbances. Norm observers connected in cascade estimate upper bounds of the state variables required for the realization of the control law. This approach allows the proof of global stability and exponential convergence of the state errors based on Lyapunov's stability theory and input-to-state-stability properties of a broad class of chaotic systems. Thus, the global synchronization is guaranteed such that the initial conditions of the master (transmitter) and slave (receiver) systems can be arbitrary. The unified formulation of the chaotic systems with time-varying parameters allows periodic switching between Lorenz and Chen attractors. Simulation results illustrate the fast synchronization of master–slave chaotic oscillators and their application to a secure communication system using a time-varying cryptographic key and nonlinear encryption functions, which improve the privacy of the proposed scheme.

Commercial and research use of unmanned aerial vehicles (UAVs) have been drawing greater attention in the last years mainly due to its low cost implementation. Applications range from transportation and construction to rescue and military missions. In such context, this work presents an unmanned air vehicle control algorithm based on visual servoing using a motion capture system, including the main routines used to implement it. Using an AR.Drone quadrotor and a Vicon motion capture system, an algorithm that integrates both platforms to generate control actions for solving problems in robotics and visual servoing control is developed. Relying on a target tracking problem, on which the goal is to make the vehicle follow a moving target, a discrete time Proportional-Derivative control (PD) algorithm is proposed in which the error signal is the difference between target and vehicle positions obtained by the Vicon system. Experimental results which validate the proposal are shown and illustrate the performance of the control system. Some implementation details are also discussed.

A variable structure model-reference adaptive control VS-MRAC of impedances and admittances driving-point DP functions is proposed. Only voltage and current measurements are required to implement the controllers. The inclusion of a prefilter in the reference model allows the synthesis of quite general DP functions, even with nonminimum phase zeros and unstable poles. It is shown that the stability of the closed-loop system depends only on the source DP function and the chosen reference model. The VS-MRAC guarantees global exponential stability properties, robustness to parameter uncertainties, disturbances, and unmodelled dynamics. Simulation results illustrate the performance and robustness of the VS-MRAC. Copyright © 2016 John Wiley & Sons, Ltd.

This study presents an output-feedback control algorithm based on unit vector sliding mode for a class of multivariable systems. The control objective is to force each output signal to track a desired reference trajectory, while retaining good performance despite parameter uncertainties, unmatched disturbances and actuators faults that eventually may occur in the plant. Owing to the new approach proposed to tackle this problem, which involves a linear matrix inequality to be satisfied by the control distribution matrix, no upper bound on this matrix is required. Moreover, a remarkable result is that the proposed fault-tolerant controller can even handle variations in the order of the system dynamics. The analysis shows that the resulting closed-loop system is globally exponentially stable. A simulation example with a chain of trailers driven by redundant actuators illustrates the design and effectiveness of the proposed strategy.

In this chapter, we propose an adaptive sliding mode control approach based on monitoring functions, to deal with disturbances of unknown bounds. An uncertain linear system is considered as well as a quite general class of non-smooth disturbances. Global tracking is demonstrated using only output feedback. The proposed adaptation method is able to make the control gain less conservative, but large enough when the disturbance grows and allows it to decrease if the latter vanishes, leading to reduced chattering effects. Simulations are presented to show the potential of the new adaptation scheme in this adverse scenario of possibly growing, temporarily large, or vanishing disturbances.

In this chapter, we propose an adaptive sliding mode approach based on the extended equivalent control concept to cope with disturbances of unknown bounds in nonlinear systems. Some advantages with respect to previous proposed methods are its simplicity and capability of rejecting non smooth disturbances. Unlike other adaptive approaches, overestimation of the controller gain and the loss of the sliding motion can be avoided. The developed method guarantees ideal sliding modes and alleviates the chattering phenomena. Theoretically, the sliding variable becomes identically null after some finite time in spite the disturbances. Global stability is proved. Simulation results illustrate the potential and limitations of this novel adaptation strategy.

This paper addresses the dynamic modeling and the energy storage analysis of a wave energy hyperbaric converter, which consists of a set of oscillating bodies (named as pumping modules) linked to hydropneumatic accumulators and an electric generating unit. A mathematical model of the accumulator is presented and a model for the generating unit is proposed, including a nonlinear model of a Pelton turbine. Then, the hydrodynamic, mechanical, and electrical characteristics of the subsystems that compose the converter are discussed. With the proposed model, it is possible to evaluate the dynamic behavior of the entire system. That is, for a given incident ocean wave, it is possible to evaluate all the system state variables and the generated electric power, including the quality (fluctuation, for example) of the generated voltage and frequency for islanded or power-grid-connected operation. Simulation results considering the proposed wave-to-wire model under the action of regular and irregular incident waves are presented to illustrate the performance of the system.

This paper proposes a sliding-mode control scheme for a class of nonlinear systems with multiple time delays, in the state variables and in the output signal. The unmeasured state of the system is estimated by an asymptotic observer for the zero dynamics and high-gain observers connected in cascade for a chain of integrators with a nonlinear input disturbance which compose the complete state. The proposed control strategy guarantees global asymptotic stability of the closed-loop system using only output feedback. The use of observers prevents undesirable chattering phenomena. Simulation results illustrate the effectiveness of this scheme.

An output-feedback sliding-mode controller is proposed for multivariable uncertain systems with unknown time-varying state delays and unmatched non-linear terms. The non-linearities are allowed to depend not only on the system output, but also on its unmeasurable state, and can be polynomial. This scheme is based on model-reference unit vector control under a mild Hurwitz condition for the high-frequency gain matrix. To achieve global stability properties, the amplitude of the control signal is given by a norm observer for the unmeasured state. For the first time, the design of norm observers is applied to a class of time-delay systems. This approach guarantees finite-time convergence of the output-tracking error to zero. Simulation results illustrate the application of the developed control scheme.

In this paper, we propose an adaptive sliding mode approach based on average control to deal with disturbances of unknown bounds in nonlinear plants. Some advantages with respect to earlier results in the literature are its simplicity and the possibility of including non smooth disturbances. Differently from other adaptive algorithms, no overestimation for the controller gain neither the loss of the sliding motion are observed. The developed method allows for ideal sliding modes and reduces the chattering phenomena. In theory, the sliding variable which drives the relay function becomes identically zero after some finite time. Global stabilization is demonstrated and numerical simulations illustrate the potential of the new adaptation scheme.

The focus of this paper is on the synthesis of first-order filters which generate approximations for upper bounds of some signals needed in sliding mode control laws. The approximation is based on optimization methods which are applied to reduce the amplitude of the control signal. This paper also considers the application of these filters in output-feedback sliding mode controllers of linear systems in regular form with uncertain parameters and uncertain order. An experimental setup where the filters are applied to the model-reference control of an electromechanical system is presented to illustrate the design method.

This paper proposes an output-feedback sliding mode control design for a class of uncertain multivariable plants with nonlinear disturbances. The approach used here is based on the control parameterization employed in model-reference adaptive control. The disturbances are allowed to be unmatched and to depend not only on the plant output but also on its unmeas-urable state. A less restrictive condition on the uncertainty of the high frequency gain matrix is also obtained.

A unit vector based output-feedback model-reference adaptive controller (UV-MRAC) for a class of uncertain multivariable nonlinear systems is proposed. This paper generalizes a previous controller which can be applied to plants with uniform relative degree one. Here, the relative degree can be arbitrary. The resulting sliding mode controller is applicable to plants with nonlinear state dependent disturbances which are possibly unmatched with respect to the plant input. The closed loop system has global or semi-global exponential stability with respect to some small residual set and the controller is free of peaking phenomena which could appear in high gain observer based schemes.

This note considers the robust output tracking problem using a model-reference sliding mode controller for linear multivariable systems of relative degree one. It is shown that the closed loop system is globally exponentially stable and the performance is insensitive to bounded input disturbances and parameter uncertainties. The strategy is based on output-feedback unit vector control to generate sliding mode. The only required a priori information about the plant high frequency gain matrix K/sub p/ is the knowledge of a matrix S/sub p/ such that -K/sub p/S/sub p/ is Hurwitz which relaxes the positive definiteness requirement usually needed by other methods.

This paper presents a unit vector based output-feedback model-reference adaptive control (UV-MRAC) for uncertain multi-input-multi-output (MIMO) linear systems. Some features of this new algorithm are: (i) Less restrictive a priori knowledge about the high frequency gain matrix of the plant is required. (ii) It applies to plants of any uniform relative degree. (iii) The closed loop system is exponentially stable with respect to some small residual set. (iv) The controller is free of peaking.

This chapter briefly describes the main results developed by the authors in the area of output feedback sliding mode control. For the sake of simplicity, the focus is maintained on uncertain single-input-single-output (SISO) nonlinear systems, although several results have been extended to the control of multi-input-multi-output (MIMO) systems. For the considered class of nonlinear systems, linear growth restriction on the unmeasured states is assumed, while less restrictive conditions are imposed to the growth of nonlinearities depending on the measured output. We present different tracking controllers for plants with arbitrary relative degree. We consider several approaches to overcome the relative degree obstacle: linear or variable structure lead filters, high-gain observers with constant or dynamic gain, global hybrid estimation schemes combining lead filters or observers with locally exact differentiators based on high-order sliding mode. Global or semi-global stability properties can be proved either for asymptotic exact tracking or for tracking within a small residual error. Some experimental results are presented to illustrate the applicability of the control schemes in real systems.

This paper considers the problem of optimizing the efficiency of the primary conversion on an ocean wave energy hyperbaric converter. This wave energy converter (WEC) is an oscillating-body linked to a hyperbaric chamber. By solving the equation of motion of the power take-off mechanism, it is shown that optimal conditions for improving the absorbed power relate the buoy parameters and the chamber pressure with the incident waves. This work also proposes an algorithm for finding the best time interval that the body remains locked when latching control strategy is used to enhance the efficiency of the WEC. Numerical simulations are presented in order to evaluate the optimum conditions of the hyperbaric WEC and the performance of the proposed algorithm.

This chapter addresses the problem of model-reference sliding mode control of uncertain linear multi-input-multi-output systems by output-feedback. After a brief revision of the main approaches and results available in the literature, a new method to solve the problem is presented using unit vectors as switching functions to induce sliding modes. It is named Unit Vector Model-Reference Adaptive Control (UV-MRAC).

The focus of this paper is on the synthesis of first order filters which generate approximations for upper bounds of the norm of signals needed in the modulation function of some sliding mode control laws. The design of output-feedback sliding mode controllers for uncertain multivariable linear systems in regular form is considered. Optimization methods are applied to reduce the amplitude of the control signals. The model-reference control of a mechanical system with uncertain parameters and uncertain order illustrates the design method.

Neste trabalho, propõe-se um controlador por modelo de referência e modo deslizante baseado em observador de alto ganho. Para plantas monovariáveis com grau relativo maior do que um, a lei de chaveamento é gerada utilizando-se o estado observado, enquanto que a função de modulação da lei de controle utiliza sinais gerados por filtros de estado. Mostra-se que este esquema possui estabilidade global e exponencial em relação a um conjunto residual arbitrariamente pequeno. Além disso, o sistema em malha fechada não apresenta o fenômeno de peaking que pode prejudicar o desempenho. Resultados experimentais ilustram o bom desempenho obtido pelo algoritmo em condições realistas. A model-reference variable structure controller based on a high gain observer (HGO) is proposed and analyzed. For single-input-single-output plants with relative degree larger than one, the switching law is generated using the observed state while the modulation function in the control law is generated using signals from state variable filters. With this scheme, global exponential stability with respect to an arbitrarily small residual set is achieved. Moreover, the closed loop system shows no peaking phenomena which could degrade the performance. Experimental results illustrate the good performance obtained with the algorithm in actual conditions.