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In this paper, we consider the problem of accurate force control using actuated winches, intended for use in real-time hybrid hydrodynamic model testing. The paper is also relevant to other cable-driven parallel robot applications that use force control in an inner control loop. For this problem, conventional strategies typically use actuated winches with torque-controlled servomotors directly connected to the cabled drum. In contrast, we propose using actuated winches with position-controlled servomotors that connect to the cabled drum via a clockspring. The servomotors are position controlled at drive-level and are rapid, accurate, robust, and simple to install. We show how this, combined with an accurate estimate of the clockspring deflection and stiffness, can yield fast and precise force tracking on moving objects. This includes proposing associated feedforward force-controllers that compensates for damping, angle-dependent force variations, delays, and non-constant clockspring characteristics. Extensive experimental testing on a 1 degree of freedom actuated mass-spring system supports the work. This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/

This paper addresses the problem of station-keeping of a surface vessel by means of the consecutive compensator approach. The horizontal motion of the vessel is described by a dynamic model. The model is set up in vessel parallel coordinates, with three degrees of freedom: longitudinal, transverse and rotational motion. It is assumed that the vessel is fully actuated, i.e. there is a sufficient number and type of actuators and a thrust allocation system to ensure full manoeuvrability. Thus, the control can be designed with the assumption of three independent inputs and three output signals. The longitudinal motion can be considered separately, but a cross-coupling exists between the transverse and rotational kinetics. There is uncertainty both in parameters and signals, due to the vessel mass, inertia, and damping, as well as the unmeasured derivatives. The proposed control ensures station-keeping when the vessel is subjected to external disturbances. The consecutive compensator, which is based on high-gain feedback, provides robustness. Stability analysis is presented considering the cross-terms as limited disturbances. This allows proof of exponential stability. Experimental results are included from the Marine Cybernetics Laboratory (MC lab) at the Centre for Autonomous Marine Operations and Systems (A MOS) at the Norwegian University of Science and Technology (Norges teknisk-naturvitenskapelige universitet, NTNU ). Two scenarios are investigated: the scaled vessel is subjected to external disturbance, and the vessel executes the " four corner test". The experiments illustrate the applicability of the method.

In this article, combinations of linear and nonlinear feedback terms are investigated for 3 degrees-of-freedom pose and velocity control of ships. Nonlinear control algorithms that are found in the literature often have linear feedback terms, which result in nice globally exponential stability properties when assuming no actuator constraints. However, considering that all actuators have saturation constraints, such stability properties are not feasible in practice. Applying nonlinear feedback terms can be a step to handle such constraints. As a result, this article explores nonlinear feedback terms for both the kinematic and kinetic control loops. Specifically, three controllers based on a cascaded backstepping control design are implemented and compared through simulations and model-scale experiments in an ocean basin. Stability properties and tuning rules for all the controllers are also provided. Interestingly, the use of nonlinear feedback terms gives the ability to constrain the feedback control inputs globally while simultaneously being able to change the convergence rates locally. The price to be paid is the introduction of additional tuning parameters. The three controller types are compared using performance metrics which consider both control accuracy and energy use.

This paper investigates some fundamental aspects of control system design for dynamic positioning of marine surface vessels. Different control design models are compared. The impact of applying linear and nonlinear models to describe the hydrodynamic damping loads are investigated, and common observer algorithms for DP are compared. The comparison includes a linear time-varying Kalman filter, an extended Kalman filter, and an unscented Kalman filter. To make the comparisons rigorous, optimization is employed to find optimal observer gains, and high-fidelity simulation data and full-scale experimental data are used to test the performance under operations/conditions with significant transient effects.

In real-time hybrid model testing, complex ocean structures are emulated by fusing numerical modelling with traditional hydrodynamic model testing. This is done by partitioning the ocean structure under consideration into a numerical and a physical substructure, coupled in real time via a measurement and control interface. The numerically computed load vector is applied to the physical substructure by means of multiple actuated winches so that the resulting experimental platform becomes a type of cable-driven parallel robot. In this context, the placement of the actuated winches is important to ensure that the loads can be accurately and robustly transferred to the physical substructure. This paper addresses this problem by proposing a performance measure and an associated actuator placement procedure that enables accurate force tracking and ensures that the numerically calculated loads can be actuated throughout the testing campaign. To clarify the application of the proposed procedure, it is applied to the design of a test setup for a moored barge. Overall, the paper represents a guideline for robust and beneficial actuator placement for real-time hybrid model testing using cable-driven parallel robots for load-actuation. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Control barrier functions (CBFs) ensure safety of controlled dynamical systems, by restricting the control inputs to render desired sets forward invariant. In this paper we propose a dynamic guidance scheme for autonomous vehicles, using CBFs to reactively generate an obstacle-free trajectory. By implementing the safety constraints on a kinematic guidance level, rather than on a lower-level control layer, we do not need to account for uncertainty in the ship dynamics explicitly. Moreover, for ships with well-proven and resilient control systems, this is an appropriate interface level, since it does not require modification of lower-level feedback control. The guidance scheme is applied to maneuvering of underactuated ships, using a virtual vessel with unicycle dynamics to trace out a feasible trajectory.

Control barrier functions (CBFs) have recently emerged as a means to ensure safety of controlled dynamical systems. CBFs are suitable for obstacle avoidance, where the CBF is often constructed from the distance and relative velocity between the vehicle and the obstacle. For vehicles required to maintain non-zero forward speed, ordinary (non-hybrid) CBFs cannot ensure safety due to vanishing control authority when the vehicle is oriented directly towards the obstacle. In this paper, synergistic CBFs are proposed, which is an intuitive extension of CBFs using ideas from synergistic Lyapunov functions. A synergistic CBF for obstacle avoidance for nonholonomic vehicles is constructed by shifting the orientations with vanishing control authority. This induces a penalty for traversing the obstacle in the counterclockwise or clockwise direction, where a logic variable is used to determine the preferred direction. The performance of the CBF is illustrated by a case study.

For the stable operation of the shipboard hybrid DC power systems, the DC bus voltage should be controlled within the recommended range by the regulations. However, the challenge comes from the load variations, and it makes the main bus voltage fluctuate. This paper proposes a control approach based on hybrid dynamical modeling of the DC-DC converter for the battery interface. In this method, the switching signal of the DC-DC converter is calculated to satisfy the Lyapunov stability criteria, and the reference power of the battery is generated for the peak-shaving of the load changes to stabilize the voltage. The effectiveness of the controller is evaluated with real ship load data which has a transient profile. The performance of the proposed control strategy is presented, and the results show that the proposed method can provide significant advantages in terms of fast and stable control performance, as well as the peak-shaving function by the battery to operate the diesel generators at the best efficiency point. © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

A reactive collision avoidance method for autonomous surface vehicles based on control barrier functions (CBFs) is proposed. An encounter between the ownship (the vessel that we control) and a target ship is classified, in accordance with the International Regulations for Preventing Collisions at Sea (COLREGs), to be either a head-on, overtake, give-way, stand-on or a safe situation with respect to the ownship. Subsequently, a spatial region is assigned to the target ship based on the classification, and this region is used to define a collision-free set. Based on this, a CBF is formulated to ensure forward invariance of the collision-free set. This CBF can then be applied as an inequality constraint to any guidance, navigation and control system with an optimization-based trajectory tracking or thrust allocation system. The method is verified through simulations and is seen to handle head-on, overtaking and crossing situations with both give-way and stand-on duty in compliance with COLREGs rules 13-15 and 17.

The next generation marine control systems will, as a step towards increased autonomy, have more automatic functionality in order to cope with a set of complex operations in unknown, challenging and varying environments while maintaining safety and keeping operational costs low. In this paper a hybrid control strategy for stationkeeping and maneuvering of marine vessels is proposed. The hybrid concept allows a structured way to develop a control system with a bank of controllers and observers improving dynamic positioning (DP) performance in stationary dynamics, changing dynamics including enhancing transient performance, and giving robustness to measurement errors. DP systems are used on marine vessels for automatic stationkeeping and tracking operations solely by use of the thrusters . In this paper a novel method improving the transient response of a vessel in DP is developed. The performance of the hybrid control system, including two observer candidates and one controller candidate, is demonstrated in model-scale experiments and on full-scale field data. The hybrid system has global stability properties.

Shipboard power systems consist of components such as gensets and loads that have continuous dynamic behaviour, and these can be represented by differential equations. Other components, such as breakers, exhibit discrete dynamic behaviour and can be represented by discrete equations. In this paper, a hybrid dynamical system framework is used to develop a mathematical model of a shipboard electrical power system. The continuous nonlinear swing dynamics of diesel generators are considered as continuous flow dynamics and logical connection in/out of generators is considered as discrete jump dynamics to model shipboard electrical power system as a hybrid dynamical system. A load-dependent start/stop table defines the logical rules to connect/disconnect next genset in sequence, extending the plant with additional discrete functions.

This paper presents a hybrid feedback controller suitable for orientation control of ships. A hybrid kinematic controller on the unit circle is constructed from the gradient of a synergistic potential function, which globally asymptotically stabilizes a desired orientation, with yaw rate viewed as control input. While this idea is not new, the potential function is novel and possesses some desired properties. The kinematic controller generates smooth reference signals for the desired velocity and acceleration, except at instances when the controller switches. Continuity of velocity and acceleration is achieved by controlling the yaw rate through a double integrator. Moreover, the velocity and acceleration converge to their desired values exponentially. The resulting closed-loop system is stable, provided the controller gains satisfy mild constraints. This is shown using a hybrid Lyapunov function. © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

Autonomous surface vehicles (ASVs) are safety-critical systems that must provide strict safety guarantees such as collision avoidance to enable fully autonomous operations. This paper presents a unified framework for safety-critical control of ASVs for maneuvering, dynamic positioning, and control allocation with safety guarantees in the presence of unknown ocean currents. The framework utilizes control Lyapunov function (CLF)- and control barrier function (CBF)-based quadratic programs (QPs), and is applicable to a general class of nonlinear affine control systems. The stabilization objective is formulated as a maneuvering problem and integral action is introduced in the CLFs to counteract the effect of unknown irrotational ocean currents. Furthermore, ocean current estimates are constructed for robust CBF design, and analytic conditions under which the estimates guarantee safety are derived. Subsequently, robust CBFs are designed to achieve collision avoidance of static obstacles. The paper concludes by verifying the framework in simulation for a double-ended passenger ferry. © 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

Resilience is an important feature of autonomous systems. To be resilient, a control system must be stable, robust, and safe. This paper explores the use of hybrid feedback controllers to ensure robustness towards uncertainties and disturbances in motion control systems for autonomous ships. Motivated by recent developments in control barrier functions (CBFs) for safe maneuvering of autonomous ships, a CBF-based hybrid kinematic controller for obstacle avoidance is proposed. The controller uses course angle as control input, making it suitable for ships with a limited speed envelope. The performance of the controller is illustrated by simulations, using an underactuated ship as a case study. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI: 10.1088/1757-899X/929/1/012022. Published under licence by IOP Publishing Ltd

This review presents a systematic summary of the state-of-the-art development of technological solutions, modeling, and control strategies of thruster-assisted position mooring (TAPM) systems. The survey serves as a starting point for exploring automatic control and real-time monitoring solutions proposed for TAPM systems. A brief historical background of the mooring systems is given. The kinematics and a simplified kinetic control-design model of a TAPM system are derived in accordance with established control methods, including a quasistatic linearized model for the restoring and damping forces based on low-frequency horizontal motions of the vessel. In addition, another two mooring line models, i.e., the catenary equation and the finite element method model, are presented for the purpose of higher-fidelity simulations. The basic TAPM control strategies are reviewed, including heading control, surge-sway damping, roll–pitch damping (for semisubmersibles), and line break detection and compensation. Details on the concepts of setpoint chasing for optimal positioning of a vessel at the equilibrium position are discussed based on balancing the mooring forces with the environmental loads and avoiding mooring line failure modes. One method for setpoint chasing is the use of a structural reliability index, accounting for both mean mooring line tensions and dynamic effects. Another method is the use of a lowpass filter on the position of the vessel itself, to provide a reference position. The most advanced method seems to be the use of a fault-tolerant control framework that, in addition to direct fault detection and isolation in the mooring system, incorporates minimization of either the low-frequency tensions in the mooring lines or minimization of the reliability indices for the mooring lines to select the optimal directions for the setpoint to move. A hybrid (or supervisory switching) control method is also presented, where a best-fit control law and observer law are automatically selected among a bank of control and observer algorithms based on the supervision of the sea-state and automatic switching logic.

This paper investigates different methods for compensating the mean and slowly varying environmental loads, and unmodeled dynamics (bias loads) in dynamic positioning of marine vessels. Four different methods are compared; using the bias estimate from an observer tuned to estimate position and velocity well, using a wave-filtered version of this bias load, using the estimate from a separate observer tuned to work well for estimating the bias loads, and finally traditional integral action on the tracking errors. The results show that the bias from the bias observer is the best solution, both in transients and steady state. Standard integral action matches the steady state performance, but is slower in transients. The estimate from the observer used for position and velocity is fast in transients, but too oscillatory in the bias state. The wave-filtered version of this has less oscillations, but falls short compared to the other methods due to added phase lag from the extra wave filter. Using a bias estimate from an observer has benefits over typical integral action, such as the possibility of offline or open-loop tuning and avoiding integral windup issues. For the comparison study, a 6 DOF simulation model of a supply vessel is used.

Lifting is a frequently used offshore operation. In this paper, a nonlinear model predictive control (NMPC) scheme is proposed to overcome the sudden peak tension and snap loads in the lifting wires caused by lifting speed changes in a wind turbine blade lifting operation. The objectives are to improve installation efficiency and ensure operational safety. A simplified three-dimensional crane-wire-blade model is adopted to design the optimal control algorithm. A crane winch servo motor is controlled by the NMPC controller. The direct multiple shooting approach is applied to solve the nonlinear programming problem. High-fidelity simulations of the lifting operations are implemented based on a turbulent wind field with the MarIn and CaSADi toolkit in MATLAB. By well-tuned weighting matrices, the NMPC controller is capable of preventing snap loads and axial peak tension, while ensuring efficient lifting operation. The performance is verified through a sensitivity study, compared with a typical PD controller. (C) 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Offshore wind turbines (OWTs) have become increasingly popular for their ability to harvest clean offshore wind energy. Bottom-fixed foundations are the most used foundation type. Because of its large diameter, the foundation is sensitive to wave loads. For typical manually assisted blade-mating operations, the decision to perform the mating operation is based on the relative distance and velocity between the blade root center and the hub, and in accordance with the weather window. Hence, monitoring the hub real-time position and velocity is necessary, whether the blade installation is conducted manually or automatically. In this study, we design a hub motion estimation algorithm for the OWT with a bottom-fixed foundation using sensor fusion of a global navigation satellite system (GNSS) and an inertial measurement unit (IMU). Two schemes are proposed based on a moving horizon estimator, a multirate Kalman filter, an online smoother, and a predictor. The moving horizon estimator mitigates the slow GNSS sampling rate relative to the hub dynamics. The multirate Kalman filter estimates the position, velocity, and accelerometer bias with a constant GNSS measurement delay. The online smoothing algorithm filters the delayed estimated trajectory to remove sudden step changes. The predictor compensates the delayed estimate, resulting in real-time monitoring. HAWC2 and MATLAB are used to verify the performance of the estimation algorithms, showing that a sufficiently accurate real-time position and velocity estimate with a high sampling rate is achieved. A sensitivity study compares the accuracy of different algorithms applied in various conditions. By combining both proposed algorithms, a sufficiently accurate estimation can be achieved for a wider scope of practical applications.

Dynamic positioning (DP) systems are used on marine vessels for automatic station keeping and tracking operations solely by use of thrusters. Observers are key components of DP systems, and two main types are proposed in this paper. The model-based type is used in steady state conditions since it is especially good at filtering out first order wave induced motions and predicting states in the case of signal loss, and the signal-based type typically has superior performance during transients. In this paper a hybrid observer including a signal-based part and a model-based part with a performance monitoring function is proposed. The observer part that provides the best estimate of the vessel position and heading is used in closed-loop control, thereby allowing for improved transient response while maintaining good steady-state performance. The contributions of this paper include the design of a hybrid signal-based and model-based observer with performance monitoring, stability analysis of the vessel with hybrid estimates in output feedback control, and simulations of a platform supply vessel during a set point and heading change.

Thruster-assisted position mooring (TAPM) is an attractive stationkeeping solution for long-term operation. Due to the complex environmental loads and system structure, increasing attention has been paid to improve the redundancy and reliability. This paper summaries the key research results when introducing the simultaneous localization and mapping algorithm to moored structures, which can provide an additional position reference system with uncertain anchor positions. It is especially cost-efficient for some applications alleviating the need for special sensors, such as, hydroacoustic sensors. The line-of-sight range mapping from tension measurements is discussed. Fairleads, the turret dynamics, and loading effects are considered to provide a more realistic and robust solution. A sensor network scheme and a state-space model are proposed, and an extended Kalman filter (EKF) is employed to estimate the uncertain anchor position.

The paper presents model predictive control (MPC) based on an optimization model drawn from the structure preserving model (SPM) for marine vessel power systems. Three objective functions are proposed to control frequency, transient load and power flow. The objective functions, in addition to a benchmark controller, are applied to a simulated power system. © 2018, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd.

Single blade installation is one of the methods for installing large wind turbine blades at an offshore site. During this installation, each blade is lifted by the main crane from the deck of an installation vessel or a transportation barge with blade root approaching the hub. The blade root is then bolted to the hub. The final mating phase is critical and requires high precision. Tugger lines from the crane boom are connected to the suspended blade to reduce pendulum motions. The entire process is typically completed without active tugger line force control. Due to wind‐induced blade loads, strict requirement on installation precision, and the limitations imposed by the lifting equipment, the single blade installation operations are subject to weather constraints. Therefore, developing techniques to reduce the blade motions and consequently shorten the installation time is desired. In this paper, an active control scheme is proposed to control the tugger line forces acting on a blade during the final installation stage before mating. A simplified three degree‐of‐freedom blade installation model is developed for the control design. An extended Kalman filter is used to estimate the blade motions and wind velocities. Feedback linearization and pole placement techniques are applied for the design of the controller. Simulations under turbulent wind conditions are conducted to verify the active control scheme, which effectively reduces the blade root motions in the wind direction.

(c) 2015, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved. This is the author' accepted and refereed manuscript to the article. Locked until 2017-01-01.

This paper presents a novel idea on a tension-based localization approach as a redundancy measure to handle the situation when the position reference (posref) signals are not available or significant GNSS drifts occur, such as sudden ionospheric disturbances, for thruster-assisted moored vessels. The only information needed is the tension measurements from tension cells. This method can improve the redundancy and safety of offshore operation, by detecting and verifying posref failure modes. It can even take over the posref function if one no longer trust the main posref measurements. Based on a residual signal, a fault detection and estimation approach is introduced and verified through simulations. © 2016. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

This paper deals with the problem of transient events in model-based observers for dynamic positioning of marine surface vessels. Traditionally, model-based observers experience a deterioration of performance during transients, and there is a give or take relationship between transient and steady state performance. To remedy this problem, we propose to use time-varying gains for a model-based observer. The gains are aggressive during transients to improve transient performance, and relaxed in steady state to lower the oscillations of the estimates. The proposed observer is analyzed with regard to stability. Its performance is verified in both a high-fidelity simulation model, and on experimental data with the research vessel (R/V) Gunnerus. In addition, a partial closed-loop validation with R/V Gunnerus has been performed. © 2017 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission

This paper deals with a nonlinear model predictive control (NMPC) scheme for a winch servo motor to overcome the sudden peak tension in the lifting wire caused by a lumped-mass payload at the beginning of a lifting off or a lowering operation. The crane-wire-payload system is modeled in 3 degrees of freedom with the Newton-Euler approach. Direct multiple shooting and real-time iteration (RTI) scheme are employed to provide feedback control input to the winch servo. Simulations are implemented with MATLAB and CaSADi toolkit. By well tuning the weighting matrices, the NMPC controller can reduce the snatch loads in the lifting wire and the winch loads simultaneously. A comparative study with a PID controller is conducted to verify its performance. (c) 2017 by ASME

A thrust allocation algorithm with capability to assist the power management system on a vessel with diesel-electric propulsion has recently been proposed. In the work presented here, the results show that the use of this thrust allocation algorithm produces a significant improvement in the stability of the electric bus frequency in the presence of large power load variations, as long as those load variations are small compared to the kinetic energy of the vessel at low speed. Further reduction in the frequency variations are achieved through use of a feedforward signal to the diesel engine governors, which enables them to react to surges in power consumption before they affect the frequency on the electric bus.

In this paper a time-delayed acceleration measurement is used to linearize a second order nonlinear system. A theoretical foundation is developed, and a control strategy is presented which handles the challenges related to classical linearization. Utilizing the acceleration measurement to directly cancel the nonlinearities constitute a feedforward approach which increases robustness with respect to unmodeled dynamics and model uncertainties. Due to the time-delay, one or more perturbation terms dependent on a previous state of the system appears. This work outlines a stability criterion which these terms must satisfy in order for the linearization to be valid, where it is shown that the performance of the scheme is dependent on the measurement time-delay. The proposed method proves feasible when compared to two other nonlinear control strategies, including a conventional linearization control law and a sliding mode control law. In comparison, the feedforward linearization features robust performance without aggressive control input.

This paper considers an AUV guidance system for collision-free transit and complete area coverage. A topologically organized neural network model is used to represent the operating environment. The activity of each neuron, characterized by a shunting equation, is used to represent the local environmental information. Targeted areas have the highest values. The AUV moves from areas with low activity to areas with higher activity like in a potential field navigation. The kinetic constraints of the AUV are taken into account by using Dubins theory to generate feasible paths.

Control allocation may produce particular thruster configurations which cause poor maneuverability and temporary loss of controllability in certain directions. The paper presents a new approach for dealing with these singular configurations, which allows to formulate the control allocation problem into optimization problems easy to set up. The approach is validated with numerical simulations on experimental model test data.

This paper presents a fault tolerant navigation system for a remotely operated vehicle (ROV). The navigation system uses hydro-acoustic position reference (HPR) and Doppler velocity log (DVL) measurements to achieve an integrated navigation. The fault tolerant functionality is based on a modified particle filter. This particle filter is able to run in an asynchronous manner to accommodate the measurement drop out problem, and it overcomes the measurement outliers by switching observation models. Simulations with experimental data show that this fault tolerant navigation system can accurately estimate the ROV kinematic states, even when sensor failures appear frequently.

This paper addresses observer design of mechanical systems subject to unknown external disturbances and unmodeled dynamics. For fast-acting disturbance forces, realistic and accurate physical models do not always exist, such as for dynamic positioning of marine vessels in ice-infested waters. To accommodate such forces and provide disturbance rejection, a delayed acceleration measurement is used to form an acceleration feedforward which captures the perturbations and enhances the observer phase margins. Through the acceleration measurement, an inherent sensor bias is introduced, creating a pseudo-force in the system. This is tracked and compensates by a bias estimate. The theoretical foundation is exemplified by the derivation of a DP observer and a simulation study featuring an inverted pendulum.

This paper addresses environmental disturbance rejection in dynamic systems by means of acceleration feedforward. A feedforward structure calculates the environmental force and magnitude by comparing the filtered acceleration measurement with the acceleration due to actuation and known forces. Direct compensation of the environment by feedforward will alleviate the position and velocity feedback terms. The presented scheme compensates environmental disturbances affecting the system in addition to model uncertainties. First, the design is used for a mechanical system and illustrated for an inverted pendulum. Then, the main application, being a dynamic positioning system affected by ice forces in an Arctic operation, is controlled by acceleration feedforward to give enhanced performance.

Acceleration signals have a powerful disturbance rejection potential in rigid body motion control, as they carry a measure proportional to the resulting force. Yet, they are seldom used, since measuring, decoupling, and utilizing the dynamic acceleration in the control design is not trivial. This paper discusses these topics and presents a solution for marine vessels building on conventional methods together with a novel control law design, where the dynamic acceleration signals are used to form a dynamic referenceless disturbance feedforward compensation. This replaces conventional integral action and enables unmeasured external loads and unmodel dynamics to be counteracted with low time lag. A case study shows the feasibility of the proposed design using experimental data and closed-loop high fidelity simulations of dynamic positioning in a harsh cold climate environment with sea-ice. © 2016 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission.

The maneuvering problem involves two tasks. The first, called the geometric task, is to force the system output to converge to a desired path continuously parametrized by a scalar @q. The second task, called the dynamic task, is to satisfy a desired dynamic behavior along the path. In this paper, this dynamic behavior is further specified as a speed assignment for @q(t). While the main concern is to satisfy the geometric task, the dynamic task ensures that the system output follows the path with the desired speed. An adaptive recursive design technique is developed for a parametrically uncertain nonlinear plant describing the dynamics of a ship. First the geometric part of the problem is solved. Then an update law is constructed that bridges the geometric design with the dynamic task. The design procedure is performed and tested by several experiments for a model ship in a marine control laboratory.

Good nonlinear maneuvering models, including numerical values, for control of ships are hard to find. This paper presents a complete modeling, identification, and control design for maneuvering a ship along a desired path. A variety of references have been applied to describe the ship model, its difficulties, limitations, and possible simplifications for the purpose of automatic control design. The numerical values of the parameters in the model are identified in towing tests and adaptive maneuvering experiments for a scaled model ship in a marine control laboratory.

The output maneuvering problem involves two tasks. The first, called the geometric task, is to force the system output to converge to a desired path parametrized by a continuous scalar variable @q. The second task, called the dynamic task, is to satisfy a desired dynamic behavior along the path. This dynamic behavior is further specified via a time, speed, or acceleration assignment. While the main concern is to satisfy the geometric task, the dynamic task ensures that the system output follows the path with the desired speed. A robust recursive design technique is developed for uncertain nonlinear plants in vectorial strict feedback form. First the geometric part of the problem is solved. Then an update law is constructed that bridges the geometric design with the speed assignment. The design procedure is illustrated through several examples.

A 3 degrees of freedom (surge, sway, and yaw) nonlinear controller for path following of marine craft using only two controls is derived using nonlinear control theory. Path following is achieved by a geometric assignment based on a line-of-sight projection algorithm for minimization of the cross-track error to the path. The desired speed along the path can be specified independently. The control laws in surge and yaw are derived using backstepping. This results in a dynamic feedback controller where the dynamics of the uncontrolled sway mode enters the yaw control law. UGAS is proven for the tracking error dynamics in surge and yaw while the controller dynamics is bounded. A case study involving an experiment with a model ship is included to demonstrate the performance of the controller and guidance systems.

Recent developments in maneuvering control of marine surface vessels have shown promising results. In this paper maneuvering of a towed body is addressed. The body may be actuated by a connection to a towing vessel or by local control forces. Actuation by connection forces are performed by tracking the towing vessel along a desired position reference. The desired position for the towing vessel is calculated from a desired force acting on the towed body and utilization of the connection's characteristics. Bandwidth limitations are discussed, and a combined solution with local actuation is proposed.

This paper investigates formation control of a fleet of vessels with a small amount of intervessel communication. The control objective for each vessel is to maintain its position in the formation. This is obtained by constructing individual parametrized paths for each vessel so that when the parametrization variables are synchronized, the vessels are in formation. Then, vectorial backstepping is used to solve an individual maneuvering problem for each vessel with the extension of a synchronization term in the resulting dynamic controllers to ensure that the vessels keep assembled in the desired formation.

In this paper, we present a system simulator of a marine vessel and power plant which contains the mechanical system with diesel engines, propellers, steering gear, and thrusters; the electrical system with generators, switchboards, breakers, and motors; and the plant level controllers with dynamic positioning controller, thrust control, and power management system. Interconnections are possible to simulate by using a multi domain simulator. This is important when evaluating system performance and fault handling. The simulator is implemented in Simulink and is modular, configurable and scalable. It can be extended to run on National Instruments’ cRIO embedded control and acquisition system, for real-time simulation.Copyright © 2015 by ASME

The output maneuvering problem involves two tasks. The first, which is the geometric task, is to force the output to converge to a desired path parametrized by a continuous scalar variable θ. The second task is to satisfy a desired speed assignment along the path. The main concern is to satisfy the geometric task. However, the speed assignment will ensure that the output follows the path with sufficient speed. A recursive control design technique is developed for nonlinear plants in vectorial strict feedback form of any relative degree. First the geometric part of the problem is solved. Then an update law is constructed that bridges the geometric design with the speed assignment. An extra degree of freedom is provided for an operator to specify the speed θ. A computer simulation with a marine vessel is performed to illustrate the design.

The concept of energy based Lyapunov control is extended to marine vessels with a nonsymmetric system inertia matrix. Acceleration feedback is used as the main tool to symmetrize the nonsymmetric part of the system inertia matrix. The main reason for a nonsymmetric mass distribution is hydrodynamic added mass which depend on the forward speed of the vessel and the frequency of the incoming waves. This is a well known phenomenon for marine vessels moving at positive speed in waves while low-speed applications like dynamic positioning systems are fairly well described with a symmetric system inertia matrix. The main contribution of the paper is a new Lyapunov-based design technique incorporating acceleration feedback to shape the kinetic energy of the system. Acceleration feedback is implemented in conjuncture with a nonlinear PID-controller derived from vectorial backstepping. The result is a uniformly globally asymptotically stable (UGAS) closed-loop control system applicable to marine vessels with nonsymmetric system inertia matrices. Typical applications are ships in maneuvering situations, vessels in transit and high speed craft where nonsymmetric added mass effects must be compensated for.

A thrust allocation method with a functionality to assist power management systems by using the hull of the ship as a store of potential energy in the field of environmental forces has been recently proposed and demonstrated to work in simulation. This functionality allows the thrust allocation algorithm to decrease the power consumption in the thrusters when a sharp increase in power consumption is demanded elsewhere on the ship. This way, the high-frequency part of the load variations on the power plant can be reduced, at the expense of minor (typically less than 1 meter) variations in the position of the vessel. The advantages from reduced variations in load include reduced wear-and-tear of the power plant, more stable frequency on the electric grid, reduced risk of blackout due to underfrequency, and more reliable synchronization when connecting additional generators or connecting bus segments. In the present work, this functionality is improved further by continuously monitoring the environmental forces and modifying the setpoint of the dynamic positioning algorithm to place the vessel a short distance (e.g. 20 cm) in the direction of steepest increase of the environmental force potential, thus maximizing the available potential energy. The increased potential energy creates additional capacity for assisting the power plant, which is shown in simulation to be significant. Preprint version. © IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works.

In this paper, a governor principle for marine dieselelectric power plants with a normal and an emergency mode is proposed. In the normal mode, the governor is tuned such that the variations in the electric frequency are weighed against operational costs such as specific fuel consumption, pollutant emissions and wear-and-tear of the diesel engine due to thermal variations. In emergency mode, the governor disregards the operational costs and attempts to keep the frequency as steady as possible. This leads to larger margins to the under-frequency condition and therefore reduces the risk of blackout. It also allows for a more reliable synchronization of additional generating sets to the electric grid. Because the emergency mode is entered under abnormal conditions only, the overall increase in operational costs resulting from this addition is negligible. The governor is implemented as a receding horizon controller.

A thrust allocation method with capabilities to assist the power management system on dynamically positioned ships is proposed in this paper. Its main benefits are reduction in frequency and/or load variations on the electric network, and a formulation of thruster bias which can be released when required by the power management system. To reduce load variations without increasing overall power consumption it is necessary to deviate from the thrust command given by the dynamic positioning system or joystick. The resulting deviation in position and velocity of the vessel is tightly controlled, and results show that small deviations are sufficient to fulfill the objective. For simplicity, the study has been limited to thrusters with fixed direction, having in mind that generalizations are fairly straightforward.

This paper addresses the problem of getting fully-actuated marine surface vessels to establish a formation before executing its mission, which here is to traverse a predetermined path. Whereas existing designs typically solve the problem by establishing the formation on the path, the proposed design in this paper allows the vessels to coordinate at an arbitrary location prior to a collective movement to the path. Protocols for group agreement form the basis of the proposed solution, while ideas from maneuvering control theory are incorporated to yield the desired path-following behavior. To demonstrate the design, a simulation is shown, where the formation's capability of handling a severe single vessel failure is illustrated.

An output feedback design for maneuvering systems is proposed by using an observer backstepping approach which applies damping terms to counteract disturbances to the controller. By using available exponential observers, stability of the interconnected observer-controller system is obtained. The design is fairly general, and equally applicable for maneuvering systems where an exponentially stable observer is available. As a case study, an output-feedback control design is performed for maneuvering a vessel where only position measurements are available. The resulting design can be viewed as a guidance-navigation-control system for a marine vessel. Simulation of the case study verifies the theoretical results

The maneuvering problem involves two tasks. The first one, called the geometric task, is to force the system states to converge to a desired parametrized path. The second task, called the dynamic task, is to satisfy a desired dynamic behavior along the path. The desired geometric path /spl xi/ is viewed as a target set /spl Xi/ which is parametrized by a scalar variable /spl theta/. The proposed dynamic controller consists of a stabilization algorithm that drives the state x(t) to the point /spl xi/(/spl theta/(t)), and a smooth dynamic optimization algorithm that selects the point /spl xi/(/spl theta/) in the set /spl Xi/ that minimizes the weighted distance between x and /spl xi/. Choosing a gain /spl mu/ large in the optimization algorithm, induces a two-time scale behavior or a closed-loop plant. In the fast time-scale /spl theta/(t) rapidly converges to the minimizer, and in the slow time-scale x(t) converges to /spl Xi/. Two motivational examples illustrate the design and the achieved performance of the closed-loop.