Signe Moe

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.

Set-Based Tasks within the Singularity-Robust Multiple Task-Priority Inverse Kinematics Framework: General Formulation, Stability Analysis, and Experimental Results

Inverse kinematics algorithms are commonly used in robotic systems to transform tasks to joint references, and several methods exist to ensure the achievement of several tasks simultaneously. The multiple task-priority inverse kinematics framework allows tasks to be considered in a prioritized order by projecting task velocities through the nullspaces of higher priority tasks. This paper extends this framework to handle setbased tasks, i.e. tasks with a range of valid values, in addition to equality tasks, which have a specific desired value. Examples of set-based tasks are joint limit and obstacle avoidance. The proposed method is proven to ensure asymptotic convergence of the equality task errors and the satisfaction of all high-priority set-based tasks. The practical implementation of the proposed algorithm is discussed, and experimental results are presented where a number of both set-based and equality tasks have been implemented on a 6 degree of freedom UR5 which is an industrial robotic arm from Universal Robots. The experiments validate the theoretical results and confirm the effectiveness of the proposed approach.

Stability analysis for set-based control within the singularity-robust multiple task-priority inverse kinematics framework

Inverse kinematics algorithms are commonly used in robotic systems to accomplish desired behavior, and several methods exist to ensure the achievement of several tasks simultaneously. The multiple task-priority inverse kinematics framework allows tasks to be considered in a prioritized order by projecting task velocities through the nullspaces of higher priority tasks. This paper extends this framework to handle set-based tasks, i.e. tasks with a range of valid values, in addition to equality tasks, which have a specific desired value. Examples of such tasks are joint limit and obstacle avoidance. The proposed method is proven to ensure asymptotic convergence of the equality task errors and the satisfaction of all high-priority set-based tasks. Simulations results confirm the effectiveness of the proposed approach.

Incorporating set-based control within the singularity-robust multiple task-priority inverse kinematics

Inverse kinematics is an active research domain in robotics since several years due to its importance in multiple robotics application. Among the various approaches, differential inverse kinematics is widely used due to the possibility to real-time implementation. Redundant robotic systems exhibit more degrees of freedom than those strictly required to execute a given end-effector task, in such a case, multiple tasks can be handled simultaneously in, e.g, a task-priority architecture. This paper addresses the systematic extension of the multiple tasks singularity robust solution, also known as Null-space Based Behavioral control, to the case of set-based control tasks, i.e, tasks for which a range, rather than a specific value, is assigned. This is the case for several variables such as, for example, mechanical joint limits of robotic arms as well as obstacle avoidance for any kind of robots. Numerical validation are provided to support the solution proposed.

Real-time hand guiding of industrial manipulator in 5 DOF using Microsoft Kinect and accelerometer

Human computer interaction is currently developing toward more intuitive and natural ways of communication. The main goal of research presented in this paper is the development of a system for controlling a robot manipulator in 5 DOF using human motions. The developed system reads and interprets sensor data from a Microsoft Kinect and an accelerometer embedded in a smartphone and use this data and the robot kinematics to generate a position and orientation reference to the robot controller allowing real-time interaction with the robot. The system has been implemented and tested on a Universal Robot manipulator arm with 6 joints. This robot has an embedded controller able to receive a reference in both Cartesian and joint space and calculate and track trajectories that fulfills this reference. The main result of this research is the architecture of an industrially oriented hand guiding system.

Line-of-sight curved path following for underactuated USVs and AUVs in the horizontal plane under the influence of ocean currents

An essential ability of autonomous unmanned surface vessels (USVs) and autonomous underwater vehicles (AUVs) moving in a horizontal plane is to follow a general two-dimensional path in the presence of unknown ocean currents. This paper presents a method to achieve this. The proposed guidance and control system only requires absolute velocity measurements for feedback, thereby foregoing the need for expensive sensors to measure relative velocities. The closed-loop system consists of a guidance law and an adaptive feedback linearizing controller combined with sliding mode, and is shown to render the path cross-track error dynamics UGAS and USGES. Simulation results are presented to verify the theoretical results.

Experimental results for set-based control within the singularity-robust multiple task-priority inverse kinematics framework

Inverse kinematics algorithms are commonly used in robotic systems to achieve desired behavior, and several methods exist to ensure the achievement of numerous tasks simultaneously. The multiple task-priority inverse kinematics framework allows a consideration of tasks in a prioritized order by projecting task velocities through the null-spaces of higher priority tasks. Recent results have extended this framework from equality tasks to also handling set-based tasks, i.e. tasks that have an interval of valid values. The purpose of this paper is to further investigate and experimentally validate this algorithm and its properties. In particular, this paper presents experimental results where a number of both set-based and equality tasks have been implemented on the 6 Degree of Freedom UR5 which is an industrial robotic arm from Universal Robots. The experiments validate the theoretical results.

Set-based Line-of-Sight (LOS) path following with collision avoidance for underactuated unmanned surface vessel

A cornerstone ability of an autonomous unmanned surface vessel (USV) is to avoid collisions with stationary obstacles and other moving vehicles while following a predefined path. USVs are typically underactuated, and this paper extends recent results in set-based guidance theory to an underactuated surface vessel, resulting in a switched guidance system with a path following mode and a collision avoidance mode. This system can be used with any combination of path following and collision avoidance guidance laws. Furthermore, a specific guidance law for collision avoidance is suggested that ensures tracking of a safe radius about a moving obstacle. The guidance law is specifically designed to assure collision avoidance while abiding by the International Regulations for Preventing Collisions at Sea (COLREGs). It is proven that the USV successfully circumvents the obstacles in a COLREGs compliant manner and that path following is achieved in path following mode. Simulations results confirm the effectiveness of the proposed approach.

Null-space-based behavior guidance of planar dual-arm UVMS

Dual-arm Underwater Vehicle-Manipulator Systems are able perform a variety of interventions tasks, and there are still many challenges related to making such vehicles autonomous. This paper presents a guidance method for generating reference trajectories for such a system by considering the main manipulator and the vehicle base as a leader unit and the secondary manipulator as a follower unit. The desired behavior of the system is expressed as different tasks, and the reference trajectories are calculated using pseudo-inverse Jacobian task matrices and the null-space-based method. The proposed method has been implemented for a particular planar UVMS and simulated for a defined set of tasks. Simulation results confirm the correctness of the proposed method.

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