Search

This article proposes a performance measure to evaluate the detection performance of a control chart with a given sampling strategy for finite or small samples sequence and prove that the CUSUM control chart with dynamic non-random control limit and a given sampling strategy can be optimal under the measure. Numerical simulations and real data for an earthquake are provided to illustrate that for different sampling strategies, the CUSUM chart will have different monitoring performance in change-point detection. Among the six sampling strategies that take only a part of samples, the numerical comparing results illustrate that the uniform sampling strategy (uniformly dispersed sampling strategy) has the best monitoring effect. [ABSTRACT FROM AUTHOR]

The gas production of offshore gas well testing is generally large, and sand production is inevitable, which will cause erosion risk of testing pipeline. In this research, surface process structure in deepwater gas well testing has been analyzed and the potential risk points of the process have been pointed out. According to the analysis results of elbow structure characteristics in potential risk points, material properties, natural gas production, solid phase change law, and material sampling of field elbow in testing surface process, combined with modified erosion testing system, the erosion of samples with different erosion times, erosion angles, fluid velocities and particle concentrations were studied. The results showed that the erosion number of particles on pipe samples was larger in the first 2 h. The erosion number of samples was rapidly increased with the increase in fluid velocity and sand concentration. When the angle between particle movement direction and sample wall plane was about 30°, erosion amount reached its maximum value. Impact forces between particles and solid materials change in vertical and horizontal components, affecting the morphology of erosion pits. Based on erosion experiment results and considering fluid flow theory, sand movement theory and OKA erosion model, numerical simulations of elbow erosion under different fluid velocities, particle concentrations and particle sizes were carried out and the erosion law of elbow was further explained. [ABSTRACT FROM AUTHOR]

In this paper, we study the nonlinear dynamics of a multiplex system consisting of neuronal networks each with an arbitrary number of FitzHugh-Nagumo neurons and intra-connections and delayed couplings. The network contains an autaptic connection formed by the axon of a neuron on its own soma or dendrites. The stability and instability of the network are determined and the existence of bifurcation is discussed. Then, the study turns to validate the theoretical analysis through numerical simulations. Abundant dynamical phenomena of the network are explored, such as coexisting multi-period oscillations and chaotic responses. [ABSTRACT FROM AUTHOR]

A one-equation Reynolds-averaged Navier–Stokes closure model is established for bypass transition in this paper. A new local indicator is proposed to describe the variation of turbulence intensities and pressure gradients. Based on this new indicator, a novel and efficient transition criterion is formulated. For laminar separation bubble induced transition, a reasonable modified intermittency factor is developed to complete the reattachment process and control the size of separation bubbles. Incorporated with Menter's k − ω shear stress transport turbulence model, the new transition-turbulence model is built for a high turbulence intensity environment. Several classical flow cases, including the ERCOFTAC (European Research Community on Flow, Turbulence and Combustion) series flat plates with various pressure gradients, the Pratt and Whitney low pressure turbine cascade, and a highly loaded linear compressor cascade, are all employed for the model verifications. Decent agreement with the experimental data and direct numerical simulation data can be obtained in a wide range of incoming flow conditions. [ABSTRACT FROM AUTHOR]

This paper focuses on the trajectory tracking control of unmanned underwater vehicles (UUVs) in the presence of dynamic uncertainties and time-varying external disturbances. Two adaptive integral terminal sliding mode control schemes, namely, adaptive integral terminal sliding mode control (AITSMC) scheme and adaptive fast integral terminal sliding mode control (AFITSMC) scheme are proposed for UUVs based on integral terminal sliding mode (ITSM) and fast ITSM (FITSM), respectively. Each control scheme is double-looped: composed of a kinematic controller and a dynamic controller. First, a kinematic controller is designed for each of the two control schemes. The two kinematic controllers are based on ITSM and FITSM, respectively. These kinematic controllers yield local finite-time convergence of the position tracking errors to zero meanwhile avoid the singularity problem in the conventional terminal sliding mode control (TSMC). Then, using the output of the kinematic controller as a reference velocity command, a dynamic controller is developed for each of the two control schemes. The two dynamic controllers are also based on ITSM and FITSM, respectively. An adaptive mechanism is introduced to estimate the unknown parameters of the upper bound of the lumped system uncertainty which consists of dynamic uncertainties and time-varying external disturbances so that the prior knowledge of the upper bound of the lumped system uncertainty is not required. The estimated parameters are then used as controller parameters to eliminate the effects of the lumped system uncertainty. The convergence rate of the integral terminal sliding variable vector is investigated and the local finite-time convergence of the velocity tracking errors to zero in the ITSM or FITSM is obtained. Finally, based on the designed kinematic and dynamic controllers, the finite-time stability of the full closed-loop cascaded system is shown. The two proposed control schemes improve the tracking accuracy over the existing globally finite-time stable tracking control (GFTSTC) and adaptive nonsingular TSMC schemes, and enhance the robustness against parameter uncertainties and external disturbances over the GFTSTC scheme. Compared with the conventional adaptive integral sliding mode control (AISMC) scheme, the two proposed control schemes offer faster convergence rate and stronger robustness against dynamic uncertainties and time-varying external disturbances for the trajectory tracking control of UUVs due to involving the fractional integrator. Comparative numerical simulations are performed on the dynamic model of the Omni Directional Intelligent Navigator UUV for two trajectory tracking cases. The convergence rate and robustness to uncertainties and disturbances are quantified as the convergent time and bounds of the steady-state position and velocity tracking errors, respectively. The results show that the two proposed control schemes improve at least 20s in convergence rate and enhance about 2% robustness in position tracking and 20% robustness in velocity tracking over the AISMC scheme. [ABSTRACT FROM AUTHOR]

To enhance the control system robustness of an underactuated surface vessel with model uncertainties and environmental disturbances, a robust adaptive trajectory tracking algorithm based on proportional integral (PI) sliding mode control and the backstepping technique is proposed. In this algorithm, a continuous adaptive term is constructed to reduce the chattering magenta phenomenon of the system due to the sliding mode surface, and the backstepping technique is employed to force the ship position and the orientation on the desired values. In addition, we have proved the Lyapunov stability of the closed-loop system under the discontinuous environment disturbances or the thruster discontinuity. Finally, numerical simulations are performed to demonstrate the effectiveness of this novel methodology. [ABSTRACT FROM AUTHOR]

This paper deals with the trajectory tracking problem of autonomous underwater vehicles (AUVs) in the presence of dynamic uncertainties and time-varying external disturbances. Three exponentially convergent robust controllers, namely, the min-max type controller, the saturation type controller and the smooth transition type controller are proposed to drive an AUV to track a predefined trajectory. It is shown that the filtered tracking errors, position tracking errors and velocity tracking errors for the three proposed controllers are exponentially convergent. Moreover, all the above tracking errors for the three proposed controllers can be shaped by specific analytic expressions and such expressions illustrate how the transient responses of the above tracking errors can be modified by adjusting the control parameters. The characteristics of the three proposed controllers are summarized and demonstrated with numerical simulations. Theoretical comparison analysis and comparative simulations with the existing RISE-based controller of AUV are presented to show the effectiveness of the three proposed exponentially convergent robust controllers. [ABSTRACT FROM AUTHOR]

The paper aims to develop a novel path following control law for nonlinear vessel models in two-time scales. For the 4-DOF path following model, we explore that the guidance dynamics are much slower than the ship motion dynamics. Based on such characteristic, a singular perturbation method is used to decompose the full system into two-time-scale subsystems. The two-time-scale structure allows independent analysis of dynamics in each time scale. Separate control strategies for the quasi-steady-state subsystem and the boundary layer subsystem are designed to stabilize the full system, yielding a rudder angle control law which is compact and uncomplicated to implement in practice. Singular perturbation methods are utilized to provide mathematical expressions for the upper bound of the singularly perturbed parameter and establish the exponential stability of the full system. The paper proves that the control law is robust in the presence of bounded perturbations and unmodeled dynamics, resulting states converge into an invariant set arbitrarily closed to the origin. Simulation results show the effectiveness and robustness of the proposed method for path following. The primary benefit of the proposed method is the simplicity of implementation. [ABSTRACT FROM AUTHOR]

Abstract: Similar to macroboats, microboats can be potentially employed to transport a target from one place to another in a small area for the purposes of delivery and detection. In previous work, we developed a type of self-propelled microboat which is capable of having straight motion at speeds of order 0.1m/s. In this work, using a combination of theoretical, numerical and experimental investigations, we explored another type of self-propelled microboat that could have radial motions. This type of microboat has side rudders. The side rudder induced an additional drag on one side, creating a moment to set the boat into an approximately circular motion. The radii of this motion could be controlled by varying the length of the side rudder. In this work, we designed, fabricated and tested eight microboats, whose rudders ranged from 0 to 10mm in length. The speeds of these microboats were also in the order of 0.1m/s. Depending on the rudder lengths, their moving paths had radii varying from infinity to 20mm. According to moving trajectories, the radius of motion and the travel distance both decreased with the increase in the rudder length. We further built a theoretical model to predict the radius of the first loop of the moving path. By comparing experimental results with numerical solutions to the theoretical model, we found: (i) the numerical results had reasonably good agreement with the experimental results, and (ii) the mismatch between numerical and experimental results increased with an increase in the rudder length due to the simplified consideration in the simulation. [Copyright &y& Elsevier]

In recent years, high energy consumption has gradually become a prominent problem in a data center. With the advent of cloud computing, computing and storage resources are bringing greater challenges to energy consumption. Virtual machine (VM) initial placement plays an important role in affecting the size of energy consumption. In this paper, we use binary particle swarm optimization (BPSO) algorithm to design a VM placement strategy for low energy consumption measured by proposed energy efficiency fitness, and this strategy needs multiple iterations and updates for VM placement. Finally, the strategy proposed in this paper is compared with other four strategies through simulation experiments. The results show that our strategy for VM placement has better performance in reducing energy consumption than the other four strategies, and it can use less active hosts than others. [ABSTRACT FROM AUTHOR]