Walter Caharija

Path following of underactuated autonomous underwater vehicles in the presence of ocean currents

A control strategy allowing underactuated underwater vehicles to accomplish 3D straight line path following tasks in the presence of ocean currents is developed. The ocean current is considered constant, irrotational and acting in any direction of the inertial frame. The control technique is based on a modified three-dimensional Line-of-Sight (LOS) guidance law with two integrators and three feedback controllers. The integral effect is introduced to successfully counteract the drifting caused by the unknown current. The integration laws are chosen to reduce the risk of wind-up effects. Furthermore, due to the irrotational nature of the ocean current, it is seen that the vehicle dynamics can be defined in terms of relative velocities only. This significantly simplifies the control system compared to approaches based on absolute velocities, and as a result no adaption laws are required. Uniform global asymptotic stability and uniform local exponential stability for the closed loop system are proven and explicit bounds on the guidance law parameters are given.

Integral Line-of-Sight Guidance and Control of Underactuated Marine Vehicles: Theory, Simulations, and Experiments

This paper presents an extensive analysis of the integral line-of-sight (ILOS) guidance method for path-following tasks of underactuated marine vehicles, operating on and below the sea surface. It is shown that due to the embedded integral action, the guidance law makes the vessels follow straight lines by compensating for the drift effect of environmental disturbances, such as currents, wind, and waves. The ILOS guidance is first applied to a 2-D model of surface vessels that includes the underactauted sway dynamics of the vehicle as well as disturbances in the form of constant irrotational ocean currents and constant dynamic, attitude dependent, and forces. The actuated dynamics are not considered at this point. A Lyapunov closed-loop analysis yields explicit bounds on the guidance law gains to guarantee uniform global asymptotic stability (UGAS) and uniform local exponential stability (ULES). The complete kinematic and dynamic closed-loop system of the 3-D ILOS guidance law is analyzed in the following and hence extending the analysis to underactuated autonomous underwater vehicles (AUVs) for the 3-D straight-line path-following applications in the presence of constant irrotational ocean currents. The actuated surge, pitch, and yaw dynamics are included in the analysis where the closed-loop system forms a cascade, and the properties of UGAS and ULES are shown. The 3-D ILOS control system is a generalization of the 2-D ILOS guidance. Finally, results from simulations and experiments are presented to validate and illustrate the theoretical results, where the 2-D ILOS guidance is applied to the cooperative autonomous robotics towing system vehicle and light AUV.

Relative Velocity Control and Integral LOS for Path Following of Underactuated Surface Vessels

Abstract A control technique for path following applications of underactuated surface vessels in the presence of constant irrotational ocean currents is revisited and improved. It is shown that in steady state the presented guidance law paired with measurements of the absolute speed and the relative speed of the vessel yields to an estimation of the ocean current. The control strategy is based on a modified two-dimensional Line-of-Sight (LOS) guidance law with integral action and two feedback controllers. The integral effect in the LOS guidance is introduced to counteract slowly varying disturbances like wave drift, wind load and current. In particular the effect of sea current is studied and the chosen integration law is defined to reduce the risk of wind-up effects. Moreover, due to the irrotational nature of the ocean current, only relative velocities are used in the feedback loop. Compared to the approach based on absolute velocities, redefining the vessel model with relative velocities significantly simplifies the control system. Closed loop uniform local exponential stability is achieved for path following of straight-line paths. Finally, simulations are presented to support the theoretical results.

Integral LOS guidance for horizontal path following of underactuated autonomous underwater vehicles in the presence of vertical ocean currents

A control strategy allowing underactuated underwater vehicles to perform horizontal path following tasks in the presence of vertical irrotational ocean currents is developed. It is based on a modified three-dimensional Line-of-Sight (LOS) guidance law with integral action and three adaptive feedback controllers. The traditional LOS guidance technique is often used in the marine field for path following purposes. It is however highly susceptible to environmental disturbances such as unknown currents. The integral effect is hence successfully introduced to counteract the current and the chosen integration law is defined to reduce the risk of wind-up effects. Furthermore, a simple translation of the equations of motion is used to simplify the underactuated design problem. The closed loop stability is addressed and path following of horizontal straight-line paths is proved. Finally, simulation results are presented.

Path following of underactuated marine surface vessels in the presence of unknown ocean currents

Unmanned marine crafts constitute a priority area within several fields of study, and there are still many challenges related to making such vessels autonomous. A basic task of an autonomous marine craft is to follow a general path in the presence of unknown ocean currents. This paper presents a method to achieve this for surface vessels. The results are an extension of the results in [1] regarding path following of space curves when no ocean currents are present, and introduce a virtual Serret-Frenet reference frame that is anchored in and propagates along the desired path. The closed-loop system consists of an ocean current observer, a guidance law, a controller and an update law to drive the Serret-Frenet frame along the path, and is shown to be UGAS. Simulation results are presented to verify the theoretical results.

Relative velocity control and integral line of sight for path following of autonomous surface vessels: Merging intuition with theory

The integral line-of-sight guidance law for path following applications of autonomous surface vessels is presented in a unified manner, merging intuitive and theoretical aspects of this valuable control technique. Straight line path following scenarios of underactuated surface vessels in the presence of unknown constant irrotational ocean currents are considered. The integral line-of-sight guidance and two feedback controllers are combined into a cascaded configuration where the integral effect in the line-of-sight guidance is introduced to counteract the disturbance. The chosen integration law is defined to reduce the risk of wind-up effects, and it is shown that the integral action in the line-of-sight guidance law performs a vectorial sum between the vessel relative velocity and the unknown current velocity to compensate for the drift. Moreover, only relative velocities are used in the feedback loop since the ocean current is assumed constant and irrotational. Redefining the vessel model with relative velocities significantly simplifies the control system compared to the approach based on absolute velocities. Closed-loop uniform local exponential stability is achieved for path following of straight line paths. Furthermore, in steady state, the presented guidance law paired with measurements of the absolute speed and the relative speed of the vessel yields to an estimation of the ocean current. Simulations are presented to support the theoretical results.

ILOS Guidance - Experiments and Tuning

Abstract A recently proposed Integral Line-of-Sight (ILOS) guidance law is applied to an underactuated Unmanned Semi-Submersible Vehicle (USSV) for path following of straight lines. Derived from the popular Line-of-Sight guidance, the ILOS methods adds integral action to increase robustness with respect to environmental disturbances such as sea currents, wind and waves that unavoidably affect maritime operations. Integral action makes the vehicle sideslip and hence compensate for the disturbances acting in the underactuated sway direction. Furthermore, the integrator of the ILOS implemented in this paper has embodied, analytically derived, anti-windup properties. It is shown that even if an accurate model of the vessel dynamics is not available, a simple kinematic model and a few test runs give enough information to correctly choose the guidance law parameters. Data from sea trials are presented to verify the ILOS theory and give an experimentally based understanding of the behavior of the USSV when different look-ahead distances and integral gains are used.

Path following of marine surface vessels with saturated transverse actuators

This paper presents a control technique for low speed path following applications of fully actuated marine surface vehicles with saturated transverse actuators. Path following of straight lines is considered. The control system is designed to counteract disturbances caused by ocean currents and combines the Integral Line-of-Sight guidance law (ILOS) with a nonlinear saturated sway controller. The ocean current is considered constant, irrotational and acting in any direction of the inertial frame. The control configuration is derived from controllers designed for underactuated marine vehicles and exploits both sway actuators and side-slipping to compensate for the drift and guarantee successful path following. The additional use of transverse actuators for disturbance compensation increases mission flexibility when trade-offs between path following speed, power usage and energy consumption arise. This represents an extension of the underactuated case. The vehicle dynamics are defined in terms of relative velocities only, since the ocean current is assumed irrotational. Closed loop uniform global asymptotic stability and uniform local exponential stability are achieved and explicit bounds on the guidance law parameters are given. The theoretical results are supported by simulations.

Topics on current compensation for path following applications of underactuated underwater vehicles

Abstract Ocean currents compensation techniques for underactuated underwater vehicles based on a three-dimensional Line-of-Sight (LOS) guidance law with integral action are discussed and applied in order to achieve precise path following. The current is considered as constant, unknown and acting in all directions, including directions in which there are no actuator forces. An integral term introduced in the guidance law counteracts the current by allowing the vehicle to side-slip and pitch while keeping the desired course. It is shown that the integral action in the LOS guidance law performs a vectorial sum between the vehicles relative velocity and the unknown current velocity to compensate for the drift. Path following of straight lines is considered and the problem is addressed at the kinematic level only since the LOS guidance law can be seen as a decoupling tool between the actuated surge dynamics and the unactuated sway and heave dynamics of the vehicle. Stability of the closed loop kinematic system is analyzed and simulation results show path convergence.

Path Following of Underactuated Marine Underwater Vehicles in the Presence of Unknown Ocean Currents

The use of autonomous marine vehicles, and especially autonomous underwater vehicles, is rapidly increasing within several fields of study. In particular, such vehicles can be applied for sea floor mapping, oceanography, environmental monitoring, inspection and maintenance of underwater structures (for instance within the oil and gas industry) and military purposes. They are also highly suitable for operations below ice-covered areas in the Arctic. However, there are still many challenges related to making such underwater vehicles autonomous. A fundamental task of an autonomous underwater vehicle vessel is to follow a general path in the presence of unknown ocean currents. There exist several results for underwater vehicles to follow a general path when no ocean currents are present [1] and to follow a geometrically simple path such as a straight line when ocean currents affect the vehicle [2, 3], but the problem of general path following in the presence of unknown ocean currents has not been solved yet. This paper presents a method to achieve this. The results are an extension of the results in [1], and introduce a virtual Serret-Frenet reference frame that is anchored in and propagates along the desired path. The closed-loop system consists of an ocean current observer, a guidance law, a controller and an update law to drive the Serret-Frenet frame along the path, and is shown to be asymptotically stable given that certain assumptions are fulfilled. This guarantees that the autonomous underwater vehicle will converge to the desired path and move along it with the desired velocity. Simulation results are presented to verify and illustrate the theoretical results.Copyright