Self-excited vibrations in drill-string systems are one of the main causes of failure and efficiency reduction in drilling operations.To suppress these vibrations, an active control strategy is proposed in this article based on a distributed drillstring model.Herein, the coupled axial-torsional dynamics of the drill string are taken into account.This coupling takes place through the bit-rock interaction, consisting of the cutting and the frictional components.The drill-string model is expressed as a neutral-type delay differential equation (NDDE) with constant and state-dependent state delays and constant input delays.As a first step in the novel controller design, a compensator is designed to mitigate the reflective waves at the top side of the string, which, in turn, results in the elimination of the neutral terms and some of the constant time delays in the delay system model.This supports a simplified next step of stabilizing controller design.Second, a new method is proposed to provide sufficient conditions for exponential stability with a prescribed minimal transient decay rate.Based on these conditions, a parametric feedback control law is designed.Finally, to make the controller causal, a predictor is designed which predicts the future state by only employing top-side measurements, available in practice.A simulation-based case study reflecting real-life scenarios is presented to illustrate the effectiveness of the proposed controller.It is also illustrated that the controller is robust against parametric uncertainties and measurement noise.

This paper investigates nonlinear forced vibrations of homogeneous Euler-Bernoulli microbeams with clamped-clamped boundary conditions. Here, the nonlocal strain gradient theory is incorporated to achieve the governing nonlinear partial differential equation of motion, including mid-plane stretching and damping effects. Using the Galerkin approach, a reduced equation of motion is derived under a central harmonic force. The perturbation technique is employed to examine the nonlinear forced vibration behavior of microbeam. Frequency responses of microbeam are presented for primary, super-harmonic, and sub-harmonic resonances. Simulation results indicate role of size effect on the vibration behavior of microbeam. Moreover, the effects of different physical parameters on the vibration behavior of microbeam are studied. Finally, the proposed approach is compared with a numerical solution to demonstrate the accuracy and validity of the presented analytical solution.

In this paper, the vortex-induced vibrations (VIV) of two bladeless wind energy converters (BWECs) are investigated through wind tunnel experiments, CFD-FEM simulations reduced-order model. BWECs consist of a blunt body attached to the tip of two flexible coaxial beams. In BWEC1, the blunt body is a truncated conic cylinder, whereas in BWEC2 it is a right cylinder. Due to periodic shedding vortices, the BWECs undergo vibrations that can be converted to electrical energy. An analytical reduced-order model is derived for the BWECs by incorporating a semiempirical model for the fluctuating aerodynamic lift coefficient into the Euler–Bernoulli theorem for the flexible support. The reduced-order model involves two principal assumptions: linear mode shapes for the aerodynamic lift force and a semiempirical model for the lift coefficient. The objective of the present research is to study and validate the accuracy of these two assumptions. To this end, wind tunnel experiments were accomplished to measure the tip displacement and CFD-FEM simulations were performed to obtain lift force distribution. Parameters of the reduced-order model are obtained using a genetic algorithm that minimizes the least squared error between the results of the model and the measurements of the experiments. To examine the assumptions of the reduced-order model, further CFD-FEM simulations are performed. The results of the CFD-FEM simulations confirmed the validity of the presumed lift force mode shapes. Moreover, it is justifiably inferred that the semiempirical lift model is the source of inconsistencies between the model and the wind tunnel experiments in high wind speeds of the post-lock-in region. In conclusion, the proposed reduced-order model is shown to be adequately accurate near the lock-in wind speed, which is the most significant working condition of the VIV energy harvesters.

This paper introduces a new type of control strategy for dynamical systems subjected to uncertainties. The suggested controller combines the capabilities of sliding mode control with fractional-order control and a specified adaption law. The stability of the closed-loop system is examined based on Lyapunov stability theory. This control scheme can be employed in many conventional systems; however, in this paper, we try to apply it to an under-actuated system as a novel extension to the variety of its applications. The idea originates from the practical features of fractional calculus including the facts that the fractional derivative has a memory of past values and it preserves a large stability region and has more free parameters to enhance the performance of the controller. Computer simulations are included to highlight the efficiency and applicability of the proposed controller in control of an under-actuated system even in the presence of parameter uncertainties.

Removing space debris of various sizes, configurations, and properties from Earth’s orbits is one of the main missions of world space agencies. The existence of deactivated bodies within the path of other spacecraft increases the risk of collision. Althgough towing a satellite through a tether and taking it out of orbit may be a definite solution for space debris removal, most deactivated satellites have some fuel remaining in their fuel tank. This remaining liquid slosh within the tank directly affects satellite motion. To improve the mathematical modeling, the existence of unburned fuel is considered. More specifically, this research focuses on dynamic modeling and control of a selected type of satellite by taking into account the sloshing effect of debris captured using a tug and a tether. In this work, we consider the postcapture phase and consider the combined set of debris as a tethered satellite system (TSS). In this regard, we use both the classic and modified forms of the Lagrange method to derive the governing equation, which entails the calculation of the total kinetic and potential energies of the system. This system is modeled using two completely different simulation methods to create confidence and ensure the performance of our modeling: the MATLAB simulator and NX Siemens software, which is a type of CAD software. The final results of these two programs show an acceptable correlation. Finally, to reveal the effects of different parameters on the system variables, we perform a parametric study.

Functional electrical stimulation (FES) is an effective method to induce muscle contraction and to improve movements in individuals with injured central nervous system. In order to develop the FES systems for an individual with gait impairment, an appropriate control strategy must be designed to accurate tracking performance. The goal of this study is to present a method for designing proportional-derivative (PD) and sliding mode controllers (SMC) for the FES applied to the musculoskeletal model of an ankle joint to track the desired movements obtained by experiments on two healthy individuals during the gait cycle. Simulation results of the developed controller on musculoskeletal model of the ankle joint illustrated that the SMC is able to track the desired movements more accurately than the PD controller and prevents oscillating patterns around the experimentally measured data. Therefore, the sliding mode as the nonlinear method is more robust in face to unmodeled dynamics and model errors and track the desired path smoothly. Also, the required control effort is smoother in SMC with respect to the PD controller because of the nonlinearity.

In this paper, a distributed model in terms of neutral-type time-delay equations is presented to investigate the global nonlinear axial-torsional dynamical behavior of a drilling string. A rate-independent bit-rock interaction law is employed for both cutting and frictional forces at the bit. A model is proposed for the estimation of the depth of cut which is valid in the case of bit bouncing and the bit reverse rotation. Illustrative simulation results are presented for a representative case study, which demonstrate the existence of the bit-bounce and reverse-rotation in some practical operating conditions, and indicates the need for taking the resulting multiple regenerative effects into account.

This article proposes an active control strategy to suppress self-excited coupled axial-torsional vibrations of a distributed drill-string system while the coupling takes place through the bit-rock interaction. The drill-string model is expressed as Neutral-type Delay Differential Equations (NDDEs) with constant and state-dependent state delays and constant input delays. As a first step in the novel controller design, an implementable input transformation is introduced, resulting in the elimination of the neutral terms from the equations of motion. This supports a simplified next step of stabilizing controller design. In the second step, a new analytic method named the “Eigenvector Contradiction Method” is proposed to provide sufficient conditions to ensure that all eigenvalues have real parts less than a prescribed value. Based on this criterion, an automated parametric feedback control law is designed. A case study simulation is presented to illustrate the effectiveness of the proposed control strategy.

SUMMARY In this article, a novel mechanism for planar one-legged hopping robots is proposed. The robot consists of a flat foot which is pinned to the leg and a reciprocating mass which is connected to the leg via a prismatic joint. The proposed mechanism performs the hopping by transferring linear momentum between the reciprocating mass and its main body. The nonlinear equations of the motion of the robot are derived using the Euler–Lagrange equations. To accomplish a stable jump, appropriate trajectories have been planned. To guarantee a stable response for this nonlinear system, a sliding-mode controller is implemented. The performance of the hopping robot is investigated through numerical simulations. The results confirm the stability of the hopping robot through the jump cycle on a flat surface and in climbing up and down ramp and stairs.

Hepatitis C is a viral infection that appears as a result of the Hepatitis C Virus (HCV), and it has been recognized as the main reason for liver diseases. HCV incidence is growing as an important issue in the epidemiology of infectious diseases. In the present study, a mathematical model is employed for simulating the dynamics of HCV outbreak in a population. The total population is divided into five compartments, including unaware and aware susceptible, acutely and chronically infected, and treated classes. Then, a Lyapunov-based nonlinear adaptive method is proposed for the first time to control the HCV epidemic considering modelling uncertainties. A positive definite Lyapunov candidate function is suggested, and adaptation and control laws are attained based on that. The main goal of the proposed control strategy is to decrease the population of unaware susceptible and chronically infected compartments by pursuing appropriate treatment scenarios. As a consequence of this decrease in the mentioned compartments, the population of aware susceptible individuals increases and the population of acutely infected and treated humans decreases. The Lyapunov stability theorem and Barbalat's lemma are employed in order to prove the tracking convergence to desired population reduction scenarios. Based on the acquired numerical results, the proposed nonlinear adaptive controller can achieve the above-mentioned objective by adjusting the inputs (rates of informing the susceptible people and treatment of chronically infected ones) and estimating uncertain parameter values based on the designed control and adaptation laws, respectively. Moreover, the proposed strategy is designed to be robust in the presence of different levels of parametric uncertainties.

In this study, semi-global practical asymptotic stability of a class of nonlinear cascade systems with upper triangular configuration has been investigated. In particular, using general results presented on stabilization of the discrete-time systems, a semi-global practical asymptotic stabilizing controller has been designed and the essential conditions for the semi-global asymptotic stability of this class of nonlinear cascade systems have been presented. The controller-design framework is based on the approximate discrete-time model of the system and the corresponding cross term constructed Lyapunov function. To illustrate the effectiveness of the proposed scheme, it has been applied to some examples and also been compared with other counterpart results in this context. (C) 2020 Elsevier Inc. All rights reserved.

In this article, dynamic pull-in instability of a suspended microchannel resonator (SMR) is studied by using homotopy analysis method which leads to a semi-analytic solution. The corresponding SMR is modeled using the modified strain gradient theory. Also, the micro-flow size effect corresponding to the profile of flow velocity and the fringing field effect associated with the electrostatic actuation will be considered in model development of the SMR. Moreover, the effects of flow velocity on dynamic pull-in instability will be investigated and a comparison with two other theories of continuum mechanics, namely modified couple stress theory and classical theory will be made. It is illustrated that the simulation results agree well with numerical previously published data.

In this article, modeling and control of a rotating hub-beam system are studied. The system consists of a solid rotating cylinder and an attached flexible arm with a payload at the end. The rotation is supposed to be in the presence of gravity and the flexible arm is assumed to be a Euler-Bernoulli beam. To derive the equations of motion of the system, Lagrange’s method is applied. Moreover, Galerkin’s technique is employed to discretize the equations of motion. Furthermore, designing an appropriate two-time (slow and fast) scale controller in the presence of uncertainties is considered in order to track the desired hub angular position and suppress vibrations of the arm simultaneously. For the so-called slow subsystem, a novel controller design is proposed as two different cases, with and without the presence of uncertainties in system dynamics are considered; and accordingly, a control law for tracking the desired path is introduced based on the idea of using the cross-term constructed Lyapunov function. For the fast subsystem, a pole placement technique is used to suppress vibration of the beam. The simulation results indicate notable effectiveness of the proposed controller.

The design process of a novel adaptive critic based neuro-fuzzy controller is described. The concept is based on separating state variables into groups, assigning the groups to a multi-layer structure, and employing individual neuro-fuzzy controllers for each layer. The correlation procedure between layers is then defined and a proportional–derivative critic is generalized to be used with this structure. The controller’s structure leads to a high reduction in the number of tunable parameters. For easier tuning, different gains are considered in the structure and an approach for the synthesis of the networks’ initial parameters is given. The effectiveness of the controller is then verified by investigating two case studies: 1. Ball and beam regulation 2. Stabilizing the chaotic spinning disk’s lateral vibrations. Simulations proved the feasibility and robustness of the proposed controller.

In this paper, the problems of control and stabilization of switched nonlinear cascade systems is investigated. The so called simultaneous domination limitation (SDL) is introduced in previous works to assure the existence of a common quadratic Lyapunov function (CQLF) for switched nonlinear cascade systems. According to this idea, if all subsystems of a switched system satisfy the SDL, a CQLF can be constructed by employing the back-stepping approach. The major shortcoming of the SDL is that this limitation cannot be satisfied for complicated switched nonlinear systems. Therefore, a CQLF cannot be constructed by employing the back-stepping approach. Moreover, if SDL is satisfied, only stabilization problem can be solved. In this paper, a new approach based on state conversion is introduced to solve the stabilization and control problems of switched nonlinear cascade systems without any limitation. Several simulation and experimental studies are provided to show the effectiveness of the proposed approach.

As a first endeavour, the out-of-plane vibration characteristics of laminated functionally graded graphene platelets reinforced composite (FG-GPLRC) curved beams bonded by piezoelectric layers are investigated. The displacement components are approximated through the beam thickness direction based on the first-order shear deformation theory (FSDT). Accordingly, the shear deformation and rotary inertia effects due to both the torsional and flexural deformations are considered. The effective mechanical properties of the nanocomposite layers are estimated using the modified Halpin-Tsai model. The governing equations are derived by employing Hamilton's principle, which are discretized in the spatial domain using the differential quadrature method (DQM). After validating the approach, some useful results are provided which can be used for future researches. In this regards, the effects of graphene platelets (GPLs) distribution patterns, GPLs weight fraction and dimensions, number of GPLs reinforced layers, piezoelectric layer thickness, the curved beam geometric parameters and boundary conditions on the vibrational characteristics of the laminated FG-GPLRC curved beams embedded in piezoelectric layers are studied.

This study is concerned with the design of a nonsingular decoupled terminal sliding mode controller for a class of fourth-order under-actuated uncertain nonlinear systems with unknown external disturbance. For the unmeasured disturbance, a disturbance observer with finite-time convergence of estimation error to zero is proposed. The nonsingular decoupled terminal sliding mode controller is designed by utilizing the output of the proposed disturbance observer. Also, an input saturation constraint and control singularity are considered in the controller design. The finite-time stability and convergence of the disturbance observer are proved for the closed-loop system. In addition, the control of an electrostatically actuated Timoshenko nanobeam subjected to Casimir force is simulated to demonstrate the effectiveness and performance of the proposed control scheme.

Background: Walker is one of the most important devices to help people who need assistance to maintain balance or stability while walking. A walker is often used by people who deal with reduced muscle strength of the lower limbs or reduced range of motion in hip, knee or ankle. One of the things that should be considered when using walker is its handle's optimal height. This height could be different according to the physical parameters of each person's height, weight, gender and age and can significantly affect the level of assistance and ergonomic aspects of the device. Methods: In this article, the basis for manufacturing an intelligent walker is presented by proposing an algorithm for determining the optimal height of the handle according to the users' characteristics. To achieve this, an experimental test was first performed on 47 subjects. After performing this experimental test, the information about each person, as well as the level of satisfaction declared by the user for the specific height was recorded. Then an intelligent algorithm was generated by an adaptive neuro-fuzzy classifier and trained accordingly. The input of the classifier is the user characteristics and the output is the optimal height. Results: The algorithm predicted the optimal height with 87% accuracy. The result of the test was to obtain an optimal height for the subjects tested. The results obtained from the classifier show that optimal height depends on factors such as gender, age, height and weight. Conclusion: In this method, in order to obtain the optimal height for each person, one of the main parameters was the degree of satisfaction of the individual, which was not taken into account in the previous studies in this field. In this intelligent algorithm, by entering information of an individual, his/her optimal height can be calculated. The proposed method can be utilized in improving intelligent walkers.

In this paper, a control system is proposed for the vibration suppression of a semi-submersible offshore wind turbine equipped with a tuned liquid multi-column damper (TLMCD). The TLMCD consists of three columns of liquid integrated into the superstructure of the semi-submersible platform. To improve the vibration suppression performance of the TLMCD, unlike previous works, the TLMCD is operated in a semi-active mode by introducing three flow control valves that allow liquid to transfer between columns. Since the rotor dynamics may greatly affect the platform vibrations, it has been included in the derived nonlinear dynamic model. In the proposed control loop, the closed-loop performance objectives of platform stabilization and rotor speed regulation are accomplished by two distinct controllers. For the platform stabilization purpose, three controller design methods of displacement-based ground-hook, velocity-based ground-hook, and bang–bang are investigated. Meanwhile, to achieve the rated rotor speed, two methods of the H ∞ and gain-scheduling control schemes are attempted. The closed-loop performance is investigated through numerical simulations. Realistic wind profiles along with a wave disturbance are implemented in the numerical simulations. The results show that the gain-scheduling control scheme for the rotor speed regulation and the velocity-based ground-hook method for the platform stabilization outperform other methods. Furthermore, to study the effect of the design parameters of the TLMCD on the closed-loop performance, several cases with different values of the design parameters are examined and compared.

This paper deals with vibration control of micro-scale structures; i.e. MEMS devices. For modeling of the structures, finite element method which is a distinguished and accurate technique will be used. This method, however, leads to a model with high number of degrees of freedom which may cause computational costs especially for control problems. Hence, we will apply the second order Krylov subspace method based on multi-moment matching to obtain a reduced order model which is in the form of a second order bilinear system. For vibration suppression of the corresponding micro-structure, a quadratic feedback controller and also a linear state feedback controller using linear matrix inequality (LMI) will be designed. Finally, a micro-cantilever beam will be considered as a practical case study and simulation results of applying the proposed method will be presented. (C) 2019 Elsevier Inc. All rights reserved.