Robust coordinated motion control of an underwater vehicle-manipulator system with minimizing restoring moments

For autonomous manipulation in water, an underwater vehicle-manipulator system (UVMS) should be able to generate trajectori9es for the vehicle and manipulators and track the planned trajectories accurately. In this paper, for trajectory generation, we suggest a performance index for redundancy resolution. This index is designed to minimize the restoring moments of the UVMS during manipulation, and it is optimized without impeding the performance of a given task. As a result, the restoring moments of the UVMS are decreased, and control efforts are also reduced. For tracking control of the UVMS, a nonlinear H_∞ optimal control with disturbance observer is proposed. This control is robust against parameter uncertainties, external disturbances, and actuator nonlinearities. Numerical simulations are presented to demonstrate the performance of the proposed coordinated motion control of the UVMS. The results show that control inputs for tracking are reduced, and the UVMS can successfully track generated trajectories.     

State-dependent Riccati equation-based robust dive plane control of AUV with control constraints

The paper treats the question of suboptimal dive plane control of autonomous underwater vehicles (AUVs) using the state-dependent Riccati equation (SDRE) technique. The SDRE method provides an effective mean of designing nonlinear control systems for minimum as well as nonminimum phase AUV models. It is assumed that the hydrodynamic parameters of the nonlinear vehicle model are imprecisely known, and in order to obtain a practical design, a hard constraint on control fin deflection is imposed. The problem of depth control is treated as a robust nonlinear output (depth) regulation problem with constant disturbance and reference exogenous signals. As such an internal model of first-order fed by the tracking error is constructed. A quadratic performance index is chosen for optimization and the algebraic Riccati equation is solved to obtain a suboptimal control law for the model with unconstrained input. For the design of model with fin angle constraints, a slack variable is introduced to transform the constrained control input problem into an unconstrained problem, and a suboptimal control law is designed for the augmented system using a modified performance index. Using the center manifold theorem, it is shown that in the closed-loop system, the system trajectories are regulated to a manifold (called output zeroing manifold) on which the depth tracking error is zero and the equilibrium state is asymptotically stable. Simulation results are presented which show that effective depth control is accomplished in spite of the uncertainties in the system parameters and control fin deflection constraints.     

Reliability analysis of H-infinity control for a container ship in way-point tracking

Efficient control of ships in a designed trajectory is always a significant charge for ship maneuverings. The purpose of this paper is to design a robust H_∞ controller and a reliability analysis for a container ship in a way-point tracking. First, the H_∞ controller is designed for a container ship because of model parameters’ uncertainties and external disturbances such as waves, winds and ocean currents. Then, to evaluate the reliability of the designed controller, a well-known reliability analysis technique is employed to achieve the predefined heading angle overshoot (that is less than 20%) in way-point tracking. To do this, three random variables including wind speed, wind direction and wave direction are considered as the inputs due to their significant effect on overshoot, compared to other variables. The results demonstrate the capability of the designed H_∞ controller against modeling uncertainties and external disturbances in way point tracking control.     

NNFFC-adaptive output feedback trajectory tracking control for a surface ship at high speed

An adaptive output feedback controller based on neural network feedback-feedforward compensator (NNFFC) which drives a surface ship at high speed to track a desired trajectory is designed. The tracking problem of the surface ship at low speed has been widely investigated. However, the coupling interactions among the forces from each degree of freedom (DOF) have not been considered in general. Furthermore, the influence of the hydrodynamic damping is also simplified into a linear form or neglected. On the contrary, coupling interactions and the nonlinear characteristics of the hydrodynamic damping can never be neglected in high speed maneuvering situation. For these reasons, the influence of the nonlinear hydrodynamic damping on the tracking precision is considered in this paper. Since the hydrodynamic coefficients of the surface ship at high speed are very difficult to be accurately estimated as a prior, it will be compensated by NNFFC as an unknown part of the tracking dynamics system. The stability analysis will be given by the Lyapunov theory. It indicates that the proposed control scheme can guarantee that all the signals in the closed-loop system are uniformly ultimately bounded (UUB), and numerical simulations can illustrate the excellent tracking performance of the surface ship at high speed under the proposed control scheme.     

双船中层拖网网位控制的静力学解析

中层拖网是开发利用中上层鱼类资源的重要工具之一,能否有效实施对拖网作业水层的控制,是实现瞄准捕捞和提高捕捞生产效率的关键。本文提出了1种基于有限元原理的拖网曳纲形状与张力的理论计算方法,以期依据中层拖网拖曳过程中不同拖速、曳纲长度和重锤配备条件下曳纲响应的计算机模拟结果,预测拖网可能的作业水层,以实现中层拖网网位控制,为中层拖网曳纲设计和网位调整提供理论依据。     

A Stability Criterion for Time-Delay Tension Leg Platform Systems Subjected to External Force

Stability analysis plays a central role in nonlinear system theory and engineering application.Over the past few yeats,the stability analysis of fuzzy systems has been proposed and there are many successful applications in practical engineering.Therefore,in this paper firstly proposed is the stability analysis on oceanic structure by fuzzy models.In the present study,Takagi-Sugeno (T-S) fuzzy model is proposed for a time delay tension leg platform (TLP) system subjected to an external wave force.In terms of stability analysis,linear matrix inequality (LMI) conditions are derived via Lyapunov theory to guarantee the stability of the TLP system.     

Numerical study on the effect of submerged depth on the horizontal plate wave energy converter

The growing search for clean and renewable energy sources has given rise to the studies of exploring sea wave energy. This paper is concerned with the numerical evaluation of the main operational principle of a submerged plate employed for the conversion of wave energy into electrical one. The numerical model used to solve the conservation equations of mass, momentum and transport of volume fraction is based on the finite volume method （FVM）. In order to tackle with the flow of mixture of air-water and its interaction with the device, the multiphase model volume of fluid （VOF） is employed. The purpose of this study is the evaluation of a numerical model for improvement of the knowledge about the submerged plate wave energy converter, as well as the investigation of the effect of the distance from the plate to the bottom of the sea （HP） on the performance of the converter. The simulations for several distances of the plate from the seabed show that the optimal efficiency is 64%, which is obtained for HP=0.53 m （88% of the depth）. This efficiency is 17% larger than that found in the worst case （HP=0.46 m, 77% of the depth）.     

Depth inversion in shallow water based on nonlinear properties of shoaling periodic waves

Two depth inversion algorithms (DIA) applicable to coastal waters are developed, calibrated, and validated based on results of computations of periodic waves shoaling over mild slopes, in a two-dimensional numerical wave tank based on fully nonlinear potential flow (FNPF) theory. In actual field situations, these algorithms would be used to predict the cross-shore depth variation h based on sets of values of wave celerity c and length L, and either wave height H or left–right asymmetry s₂/s₁, simultaneously measured at a number of locations in the direction of wave propagation, e.g., using video or radar remote sensing techniques. In these DIAs, an empirical relationship, calibrated for a series of computations in the numerical wave tank, is used to express c as a function of relative depth k_oh and deep water steepness k_oH_o. To carry out depth inversion, wave period is first predicted as the mean of observed L/c values, and H_o is then predicted, either based on observed H or s₂/s₁ values. The celerity relationship is finally inverted to predict depth h. The algorithms are validated by applying them to results of computations for cases with more complex bottom topography and different incident waves than in the original calibration computations. In all cases, root-mean-square (rms)-errors for the depth predictions are found to be less than a few percent, whereas depth predictions based on the linear dispersion relationship—which is still the basis for many state-of-the-art DIAs—have rms-errors 5 to 10 times larger.     

Nonlinear adaptive control of an underwater towed vehicle

This paper addresses the problem of simultaneous depth tracking and attitude control of an underwater towed vehicle. The system proposed uses a two-stage towing arrangement that includes a long primary cable, a gravitic depressor, and a secondary cable. The towfish motion induced by wave driven disturbances in both the vertical and horizontal planes is described using an empirical model of the depressor motion and a spring-damper model of the secondary cable. A nonlinear, Lyapunov-based, adaptive output feedback control law is designed and shown to regulate pitch, yaw, and depth tracking errors to zero. The controller is designed to operate in the presence of plant parameter uncertainty. When subjected to bounded external disturbances, the tracking errors converge to a neighbourhood of the origin that can be made arbitrarily small. In the implementation proposed, a nonlinear observer is used to estimate the linear velocities used by the controller thus dispensing with the need for costly sensor suites. The results obtained with computer simulations show that the controlled system exhibits good performance about different operating conditions when subjected to sea-wave driven disturbances and in the presence of sensor noise. The system holds promise for application in oceanographic missions that require depth tracking or bottom-following combined with precise vehicle attitude control.     

Autopilot control synthesis for path tracking maneuvers of underwater vehicles

This paper is concerned with the robust control synthesis of autonomous underwater vehicle(AUV) for general path following maneuvers.First,we present maneuvering kinematics and vehicle dynamics in a unified framework.Based on H∞ loop-shaping procedure,the 2-DOF autopilot controller has been presented to enhance stability and path tracking.By use of model reduction,the high-order control system is reduced to one with reasonable order,and further the scaled low-order controller has been analyzed in both the frequency and the time domains.Finally,it is shown that the autopilot control system provides robust performance and stability against prescribed levels of uncertainty.     

An assessment of fuzzy logic vessel path control

A fuzzy logic controller for ship path control in restricted waters is developed and evaluated. The controller uses inputs of heading, yaw rate, and lateral offset from the nominal track to produce a commanded rudder angle. Input variable fuzzification, fuzzy associative memory rules, and output set defuzzification are described. Two maneuvering situations are evaluated: track keeping along a specified path where linearized regulator control is valid; and larger maneuvers onto a specified path where nonlinear modeling and control are required. For the track keeping assessment, the controller is benchmarked against a conventional linear quadratic Gaussian (LQG) optimal controller and Kalman filter control system. The Kalman filter is used to produce the input state variable estimates for the fuzzy controller as well. An initial startup transient and regulator control performance with an external hydrodynamic disturbance are evaluated using linear model simulations of a crude oil tanker. A fully nonlinear maneuvering model for a smaller product tanker is used to assess the larger maneuvers     

Midwater fishes collected in the vicinity of the Sub-Polar Front,Mid-North Atlantic Ocean,during ECOMAR pelagic sampling

The ECOMAR project was a multidisciplinary process study conducted in the mid-North Atlantic, coincident hydrodynamically with the Sub-Polar Front (SPF; 48–54°N) and topographically with Charlie-Gibbs Fracture Zone of the Mid-Atlantic Ridge, as part of the Census of Marine Life field project MAR-ECO. Midwater trawling was conducted during the 2007 and 2009 ECOMAR expeditions at 14 stations north and south of the SPF, day and night, in four discrete depth intervals from 0 to 1000 m. A total of 56 species of midwater fishes representing 44 genera and 18 families were collected, several of which are new records for the region and/or were not previously sampled during MAR-ECO sampling. An annotated species list with depth-of-capture data is provided. Three species of the genus Cyclothone (Cyclothone braueri, Cyclothone microdon and Cyclothone pallida) and the myctophid Benthosema glaciale combined to contribute ~88% of all specimens collected. This finding differs from results of previous net-based sampling in the same area, likely due to sampling scheme differences (diel sampling, upper 800 m concentration) and gear selectivity (mesh size, trawl speed). Quantitative data from ECOMAR midwater sampling and the previous 2004 G.O. Sars MAR-ECO expedition are compared. Despite differences in gear between the major MAR-ECO expeditions, abundance estimates of some dominant species were remarkably similar. Data showed that the SPF is an asymmetrical, taxon-specific biogeographic boundary for deep-pelagic fishes in the North Atlantic; the SPF is semi-permeable to some species in one direction, while a strong boundary for species in another direction. Deeper-living fish species did not appear as affected by the SPF as a boundary.     

Prediction of scour depth at breakwaters due to non-breaking waves using machine learning approaches

Coastal structures may cease to function properly due to seabed scouring. Hence, prediction of the maximum scour depth is of great importance for the protection of these structures. Since scour is the result of a complicated interaction between structure, sediment, and incoming waves, empirical equations are not as accurate as machine learning schemes, which are being widely employed for the coastal engineering modeling. In this paper, which can be regarded as an extension of Pourzangbar et al. (2016), two soft computing methods, a support vector regression (SVR), and a model tree algorithm (M5′), have been implemented to predict the maximum scour depth due to non-breaking waves. The models predict the relative scour depth (S_max/H₀) on the basis of the following variables: relative water depth at the toe of the breakwater (h_toe/L₀), Shields parameter (θ), non-breaking wave steepness (H₀/L₀), and reflection coefficient (Cr). 95 laboratory data points, extracted from dedicated experimental studies, have been used for developing the models, whose performances have been assessed on the basis of statistical parameters. The results suggest that all of the developed models predict the maximum scour depth with high precision, the M5′ model performed marginally better than the SVR model and also allowed to define a set of transparent and physically sound relationships. Such relationships, which are in good agreement with the existing empirical findings, show that the relative scour depth is mainly affected by wave reflection.     

Fishery trends, resource-use and management system in the Ungwana Bay fishery Kenya

The present study assessed trends in resource-use, partitioning and management in the Ungwana Bay fishery, Kenya, using surplus production models. The fishery is one of East Africa’s important marine fisheries sustaining a bottom trawl commercial fishery and a resident-migrant artisanal fishery. Two models: Schaefer (1954) and Gulland and Fox (1975) were applied to catch-effort data over a 21-year period to model maximum sustainable yield (MSY) and optimal effort (f_MSY) to examine the status of resource exploitation and provide reference points for sustainable management. In the artisanal fishery, model MSYs range from 392-446 t to 1283-1473 t for shrimps and fish respectively compared to mean annual landings of 60 t for shrimp and 758 t for fish. These landings represent <50% of the model MSYs suggesting under exploitation in the sub-sector. Moreover, current fishing effort applied stands at <0.5 f_MSY. On the other hand, mean annual landings in bottom trawl commercial fishery, at about 330 t for shrimps and 583 t and fish represent about 90% of the model MSYs of 352-391 t and 499-602 t for shrimps and fish respectively. Therefore, the bottom trawl commercial fishery is likely under full exploitation. Similarly, the current effort is estimated at >0.7 f_MSY. Resource management in the bay is faced with numerous problems including resource-use conflicts, poor economic conditions in artisanal fishery, poor legislation, and inadequate research augmented by poor reporting systems for catch-effort statistics. Thus, the fishery lacks clearly defined exploitation regimes. Fisheries research and assessment of the marine resources are important for sustainability of the fishery. Moreover, income diversification in the poverty ridden artisanal fishery would go a long way in addressing resource-use conflicts and use of deleterious fishing methods in the sub-sector. Borrowing from the successes of the Japanese community-based fisheries resource management (CBFRM) which has easily resolved numerous fisheries management issues in coastal small-scale commercial fisheries, and the beach management unit (BMU) system which has been applied to the artisanal fisheries of south coast Kenya with enormous benefits, it is envisaged that a hybrid CBFRM-BMU system presents the best approach to sustainable resource-use in the Ungwana Bay fishery.     

Analysis of bycatch in the South African midwater trawl fishery for horse mackerel Trachurus capensis based on observer data

The South African midwater trawl fishery targets adult horse mackerel Trachurus capensis. The bulk of the catch is taken by a single freezer-trawler, the biggest fishing vessel operating in South African waters. As fishing takes place off the south coast in ecologically sensitive areas, there are concerns about the potential impacts of this fishing operation on non-target species. Fishing behaviour and bycatch of this fishery from 2004 to 2014 were investigated by analysing observer records with regard to catch composition, volume and temporal and spatial patterns. The midwater trawl fishery was estimated to have caught 25 415 tonnes annually, with a bycatch of 6.9% of the total catch, by weight. There are species overlaps with various fisheries, namely the demersal trawl, small-pelagic, line, shark longline and squid fisheries, yet the total bycatch estimates from this fishery are generally small relative to catches taken in the target fisheries. Bycatch species with the highest average annual catches were chub mackerel Scomber japonicus, redeye roundherring Etrumeus whiteheadi, ribbonfish Lepidopus caudatus and hake Merluccius spp. Large-fauna bycatch species included sunfish Mola mola as well as a number of CITES II- and IUCN-listed species, such as Cape fur seal Arctocephalus pusillus, dusky shark Carcharhinus obscurus, smooth hammerhead shark Sphyrna zygaena and thresher sharks Alopias spp. The 97.9% observer coverage is high and the 6.9% bycatch rate low compared to other South African fisheries; however, due to the large size of the individual hauls (average of 46.3 t), the average sampling rate of 1.56% is low. Our analyses suggest that bycatch in the South African midwater trawl fishery has been lower than in other South African fisheries and similar fisheries elsewhere, but due to the combination of high catch volumes and low sampling rates, estimation errors for rare species are high and there is a substantial risk of incidental unmonitored bycatch of rare large fauna and aggregations of small fauna. This could be mitigated by spatio-temporal management of this fishery, to avoid fishing in high-risk areas, and the introduction of an electronic monitoring programme.     

An Adaptive Single Neural Control for Variable Step-Size P&O MPPT of Marine Current Turbine System

Marine current energy has been increasingly used because of its predictable higher power potential. Owing to the external disturbances of various flow velocity and the high nonlinear effects on the marine current turbine (MCT) system, the nonlinear controllers which rely on precise mathematical models show poor performance under a high level of parameters’ uncertainties. This paper proposes an adaptive single neural control (ASNC) strategy for variable step-size perturb and observe (P&O) maximum power point tracking (MPPT) control. Firstly, to automatically update the neuron weights of SNC for the nonlinear systems, an adaptive mechanism is proposed to adaptively adjust the weighting and learning coefficients. Secondly, aiming to generate the exact reference speed for ASNC to extract the maximum power, a variable step-size law based on speed increment is designed to strike a balance between tracking speed and accuracy of P&O MPPT. The robust stability of the MCT control system is guaranteed by the Lyapunov theorem. Comparative simulation results show that this strategy has favorable adaptive performance under variable velocity conditions, and the MCT system operates at maximum power point steadily.

     

Mathematical Modeling of the Hydrodynamic Forces on a Trawl Door

Precise control of trawl systems is assumed to be beneficial from both economic and environmental reasons. Using the trawl doors as actuators could increase the amount of available control forces. Adequate mathematical models of the hydrodynamic fores on the trawl doors are needed for control system design and verification. This paper presents a method for mathematical modeling of the hydrodynamic forces on the trawl doors. These forces are divided into steady-state forces and transient effects. The six degrees of freedom (six dof) steady-state hydrodynamic coefficients of a trawl door have been found as a function of its angles of attack and slip, based on wind-tunnel experiments. The coefficients are parameterized for smoothing and computational performance, and methods for extending the validity of the model in terms of orientation and trawl door shape are presented. The transient effects are described as functions of relative accelerations between the trawl door and the ambient water, angular velocities of the trawl door and circulation buildup. These effects are manifestations of variations in the flow around the trawl door and its wake, and a numerical method based on potential theory is employed to investigate them. A computational efficient, nonlinear, state–space model of the hydrodynamic forces is finally proposed. It accounts for steady-state and unsteady hydrodynamic forces and moments in six dof, suitable for trawl control system design and analysis.     

Wave evolution over submerged sills: tests of a high-order Boussinesq model

A Boussinesq model accurate to O(μ)⁴, μ=k₀h₀ in dispersion and retaining all nonlinear effects is derived for the case of variable water depth. A numerical implementation of the model in one horizontal direction is described. An algorithm for wave generation using a grid-interior source function is derived. The model is tested in its complete form, in a weakly nonlinear form corresponding to the approximation δ=O(μ²), δ=a/h₀, and in a fully nonlinear form accurate to O(μ²) in dispersion [Wei, G., Kirby, J.T., Grilli, S.T., Subramanya R. (1995). A fully nonlinear Boussinesq model for surface waves: Part 1. Highly nonlinear unsteady waves. J. Fluid Mech., 294, 71–92]. Test cases are taken from the experiments described by Dingemans [Dingemans, M.W. (1994). Comparison of computations with Boussinesq-like models and laboratory measurements. Report H-1684.12, Delft Hydraulics, 32 pp.] and Ohyama et al. [Ohyama, T., Kiota, W., Tada, A. (1994). Applicability of numerical models to nonlinear dispersive waves. Coastal Engineering, 24, 297–313.] and consider the shoaling and disintegration of monochromatic wave trains propagating over an elevated bar feature in an otherwise constant depth tank. Results clearly demonstrate the importance of the retention of fully-nonlinear effects in correct prediction of the evolved wave fields.     

Prediction of significant wave height using regressive support vector machines

Wave parameters prediction is an important issue in coastal and offshore engineering. In this literature, several models and methods are introduced. In the recent years, the well-known soft computing approaches, such as artificial neural networks, fuzzy and adaptive neuro-fuzzy inference systems and etc., have been known as novel methods to form intelligent systems, these approaches has also been used to predict wave parameters, as well. It is not a long time that support vector machine (SVM) is introduced as a strong machine learning and data mining tool. In this paper, it is used to predict significant wave height (H_s). The data set used in this study comprises wave wind data gathered from deep water locations in Lake Michigan. Current wind speed (u) and those belonging up to six previous hours are given as input variables, while the significant wave height is the output parameter. The SVM results are compared with those of artificial neural networks, multi-layer perceptron (MLP) and radial basis function (RBF) models. The results show that SVM can be successfully used for prediction of H_s. Furthermore, comparisons indicate that the error statistics of SVM model marginally outperforms ANN even with much less computational time required.     

Wave modelling in the vicinity of submerged breakwaters

This paper describes a simple method for modelling wave breaking over submerged structures, with the view of using such modelling approach in a coastal area morphodynamic modelling system.A dominant mechanism for dissipating wave energy over a submerged breakwater is depth-limited wave breaking. Available models for energy dissipation due to wave breaking are developed for beaches (gentle slopes) and require further modifications to model wave breaking over submerged breakwaters.In this paper, wave breaking is split into two parts, namely: 1) depth-limited breaking modelled using Battjes and Janssen's (1978) theory [Battjes, J.A. and Jannsen, J.P.F.M. (1978). Energy loss and setup due to breaking of random waves. Proceedings of the 16th Int. Conf. Coast. Eng., Hamburg, Germany, pp. 569-587.] and 2) steepness limited breaking modelled using an integrated form of the Hasselmann's whitecapping dissipation term, commonly used in fully spectral wind–wave models. The parameter γ₂, governing the maximum wave height at incipient breaking (H_max = γ₂d) is used as calibration factor to tune numerical model results to selected laboratory measurements. It is found that γ₂ varies mainly with the relative submergence depth (ratio of submergence depth at breakwater crest to significant wave height), and a simple relationship is proposed. It is shown that the transmission coefficients obtained using this approach compare favourably with those calculated using published empirical expressions.