On optimal relaying strategies for VANETs over double Nakagami-m fading channels

Wireless Networks - Vehicular ad hoc networks (VANETs) are able to facilitate data exchange among vehicles and provide diverse data services. The benefits of cooperative communications are such as...     

Sum‐of‐Squares‐Based Finite‐Time Adaptive Sliding Mode Control of Uncertain Polynomial Systems With Input Nonlinearities

This paper proposes a novel adaptive sliding mode control (ASMC) for a class of polynomial systems comprising uncertain terms and input nonlinearities. In this approach, a new polynomial sliding surface is proposed and designed based on the sum‐of‐squares (SOS) decomposition. In the proposed method, an adaptive control law is derived such that the finite‐time reachability of the state trajectories in the presence of input nonlinearity and uncertainties is guaranteed. To do this, it is assumed that the uncertain terms are bounded and the input nonlinearities belong to sectors with positive slope parameters. However, the bound of the uncertain terms is unknown and adaptation law is proposed to effectively estimate the uncertainty bounds. Furthermore, based on a novel polynomial Lyapunov function, the finite‐time convergence of the sliding surface to a pre‐chosen small neighborhood of the origin is guaranteed. To eliminate the time derivatives of the polynomial terms in the stability analysis conditions, the SOS variables of the Lyapunov matrix are optimally selected. In order to show the merits and the robust performance of the proposed controller, chaotic Chen system is provided. Numerical simulation results demonstrate chattering reduction in the proposed approach and the high accuracy in estimating the unknown parameters.     

Feedback control of linear systems by multirate pulse-widthmodulation

In this note, sampled-data methods for modeling and control design of pulse-width modulated systems are examined. Both single-rate and multirate approaches are considered. Local controllability and observability analyses of the sampled-data models are performed, and feedback linearizability conditions for the sampled-data models are derived. Multirate nonlinear state and output feedback designs are presented that permit asymptotic regulation or tracking     

A novel antenna concept for future solar sails: application of Fresnel antennas

Recent studies have shown that interplanetary missions may be made possible using inflatable solar sails that employ solar-flux power, providing thrust for spacecraft while reducing onboard fuel requirements. These sails require a large surface area (i.e., 100 m in diameter) to capture enough solar flux for spacecraft navigation. In this paper, a novel communication antenna concept is proposed for future solar-sail missions, taking advantage of the large sail surface area via application of Fresnel-zone (FZ) antennas. This study focuses on utilizing a design/analysis methodology using physical optics (PO) and method of moments (MoM) for Fresnel-type antennas applicable to the solar-sail missions. Extensive parametric studies of Fresnel-zone antenna radiation characteristics have been performed, and the analytical methodologies were verified using a series of measurements. Fresnel-zone antenna gain is studied under different antenna configurations. Furthermore, a Fresnel-zone antenna under surface deformation is investigated to characterize Fresnel-zone antenna performance in the reflective mode. In addition, a new bandwidth-enhancement technique is introduced for Fresnel-zone antennas, to accommodate the dual-band operation ( X band uplink and downlink) of the antenna for the deep space network (DSN).     

Adaptive Friction Compensation: Modular Design with Passive Identifier

For a second‐order mechanical system incorporating Coulomb frictional effect, a nonlinear adaptive control that achieves a controller‐identifier separation is designed. This modularity is made possible by the strong input‐to‐state stability (ISS) property of the ISS controller with respect to the parameter estimation error as input. This input is independently guaranteed to be bounded by the passive identifier. We use two types of passive identifiers: z‐scheme passive identifier and x‐scheme passive identifier. These designs are more flexible than the Lyapunov‐based design and lead to lower control effort. In addition, the advantages and disadvantages of z‐scheme and x‐scheme are presented. Transient performance of the system is enhanced with a trajectory initialization technique. The validity and effectiveness of the proposed friction compensator is verified by simulation for position tracking control under the influence of Coulomb friction.     

A Reset‐Time Dependent Approach for Stability Analysis of Nonlinear Reset Control Systems

A reset mechanism in controller can affect the stability property of a closed loop control system. In a simple word, there are stable reset control systems with unstable base‐systems and also unstable reset systems with stable base‐systems. The Lyapunov stability theory is a strong tool to investigate the stability of a nonlinear system. In this paper, based on the well‐known Lyapunov stability concept, some stability conditions for nonlinear reset control systems are addressed. These conditions are dependent on the reset‐times and hence the reset‐time intervals are explicitly emerged in the stability conditions. Some applications of these results are used in numerical examples to show the effectiveness of the proposed approach.     

Robust delay dependent fault estimation for a class of interconnected nonlinear time delay systems

This paper focuses on the problem of fault estimation for a class of interconnected nonlinear systems with time varying delays. In contrast to the common assumption imposed on the problem in most literature, here, there is no need for the delay rate to be less than one. Both actuator and component faults are considered within the general fault model invoked as multiplicative faults in this study. Robust adaptive observers are used to detect and estimate simultaneously the states and the parameter faults in each subsystem. The designed observers ensure a prescribed H _∞ performance level for the fault estimation error, irrespective of the uncertainties which are assumed here to be the unknown interconnections between the subsystems. With the aid of H _∞ performance index, the common assumption regarding the observer matching condition is no longer required. Sufficient conditions for asymptotic stability of the observers are derived via a matrix inequality approach with the aid of LyapunovKrasovskii function. Finally, a simulation example is presented to show the validity and feasibility of the proposed method.

     

Transient performance improvement in indirect model reference adaptive control using perturbation‐based extremum seeking identifier

One of the main drawbacks of model reference adaptive control (MRAC) is the weakness of its transient performance. The key reason of this imperfection is parameter's estimation error convergence. For many cases in the closed‐loop control, the plant input signal cannot satisfy the persistence of excitation (PE) condition which yields poor parameters estimation error convergence. In this paper, we use a fast perturbation‐based extremum seeking (PES) scheme without steady‐state oscillation as the parameter identifier in indirect MRAC. The estimated parameters through the PES identifier contain the additive sinusoidal signals with distinct frequencies in the transient, which satisfy the PE condition of the plant input. Therefore, convergence of the parameters estimation error to zero will be guaranteed that results in improvement of transient performance for indirect MRAC. Also, the contrary effects on the steady‐state behaviour is eliminated since the sinusoidal excitation signals amplitude exponentially converge to zero and reinitiate with every change in the unknown parameters. Simulation results for a second order example have been presented to illustrate the effectiveness of the proposed scheme.     

Transient performance improvement of model reference adaptive control: LMI‐based resetting

In this paper, a resetting mechanism is proposed to enhance the transient performance of model reference adaptive control. While the suggested method has a simple structure, it is capable of taking into account both the desired steady‐state behavior and the transient response, simultaneously. Whenever the transient specification is not satisfying, there is a jump in the controller parameters. This jump is determined by designing an optimal reset law. At the reset times, the after‐reset values of parameters are calculated based on a minimization problem. The considered cost function is a mixed H₂/H_∞ criterion, which minimizes the tracking error. The optimization problem is converted to an LMI formulation, and the reset law is designed by solving this LMI at certain reset times. To verify the effectiveness of the proposed approach, simulation results are presented.     

Dead‐Zone Model Based Adaptive Backstepping Control for a Class of Uncertain Saturated Systems

In this paper, a dead‐zone based model of saturation phenomena is proposed. This method is capable of modelling diverse kinds of saturation, including both hard‐limited and soft‐limited. Due to use of a linear parameter approach, the proposed model is consistent with the available adaptive control techniques in the literature. In addition, based on the proposed model, an adaptive controller is designed for a class of nonlinear saturated systems, where, the shape of the saturation phenomenon is assumed to be unknown. The effectiveness of the proposed method and its robustness against initial condition variation and reference signal is evaluated via simulated examples for both spring–mass–damper and ship steering autopilot systems.