Tether Management System Research Articles

Continuous Vector Fields for Precise Cable-Guided Landing of Tethered UAVs

This letter presents a method for the precise cable-guided landing of tethered drones. The proposed approach is based on a continuous velocity vector field that relies on the tether variables only. We assume that the vehicle is attached to a landing platform through a tensioned and free-of-collision cable whose length and angles are measured by a tether management system. We do not assume that the cable is taut. The method is suitable for highly degraded environments, where traditional landing strategies based on GNSS and vision are not feasible, such as dark underground mines and caves. This letter shows that, besides being easy to implement and adjust, the proposed vector field is proven to correctly guide the vehicle to its desired landing position. Experiments with a quad-rotor vehicle landing on static and moving platforms illustrate the precision, robustness, and efficacy of the method.

Oxpecker: A Tethered UAV for Inspection of Stone-Mine Pillars

This paper presents a state-of-the-art tethered unmanned aerial vehicle (TUAV) for structural integrity assessment of underground stone mine pillars. The TUAV, powered by its tether, works in tandem with an unmanned ground vehicle (UGV) that hosts the TUAV batteries, a self-leveled landing platform, and the tether management system. The UGV and the TUAV were named Rhino and Oxpecker, respectively, given that the TUAV stays landed on the UGV while the ensemble moves inside a mine. The mission of Oxpecker is to create, using a LiDAR sensor, 3D maps of the mine pillars to support time-lapse hazard mapping and time-dependent pillar degradation analysis. Given the height of the pillars (7–12 m), this task cannot be executed by Rhino alone. This paper describes the drone’s hardware and software. The hardware includes the tether management system, designed to control the tension of the tether, and the tether perception system, which provides information that can be used for localization and landing in global navigation satellite systems (GNSS)-denied environments. The vehicle’s software is based on a state machine that controls the several phases of a mission (i.e., takeoff, inspection, and landing) by coordinating drone motion with the tethering system. The paper also describes and evaluates our approach for tether-based landing and autonomous 3D mapping of pillars. We show experiments that illustrate and validate our system in laboratories and underground mines.

On the theory and application of absolute coordinates-based multibody modelling of the rigid–flexible coupled dynamics of a deep-sea ROV-TMS (tether management system) integrated model

This paper presents a novel method to investigate the dynamics between the interconnected rigid and very flexible bodies of a deep-sea ROV-TMS with highly coupled dynamics. The aim is to develop an accurate yet effective system model, which is applicable to a wide range of deep-sea working-class ROV applications. The use of different sets of generalized coordinates in rigid–flexible multibody system usually leads to complex forms of constraints Jacobian and quadratic velocity vector when the interconnected rigid and flexible bodies share highly coupled dynamics. Thus, the compatibility of the formulations used in modelling is pivotal to a correct representation of the dynamics of an underwater multibody system with highly coupled dynamics. To this end, a mixed formulation, which is fully-based on absolute coordinates, is presented in modelling the dynamics of underwater tethered vehicle system. The development of system model of a basic ROV-TMS and theoretical implementations are presented in this paper. The applicability of the presented model is assessed through numerical simulations for the realistic scenarios during the deep-sea ROV operations. The numerical studies show that the proposed model is able to capture the nonlinear dynamics of cable and cable drum, and the nonlinear coupling dynamics between the tether cable and the ROV.

Tether Management System for Autonomous Inspection Missions in Mariculture Using an ASV and an ROV

ooperation of a remotely operated vehicle (ROV) and an autonomous surface vehicle (ASV) in prolonged robotic inspections missions in mariculture requires a tether management system (TMS) with a docking system mounted onto the ASV. The autonomous surface developed in the scope of HEKTOR (Heterogeneous Autonomous Robotic System in Viticulture and Mariculture) project is shortly presented. Furthermore, mechanical and electrical design of the TMS, the control scheme and communication protocol of the TMS with the ASV, and TMS-ROS interface for future high-level mission control are described. Laboratory pool and at-sea field trials of the TMS show that the chosen concept works in various settings.

Vision based autonomous docking for work class ROVs

This paper presents autonomous docking of an industry standard work-class ROV to both static and dynamic docking station (Tether Management System — TMS) using visual based pose estimation techniques. This is the first time autonomous docking to a dynamic docking station has been presented. Furthermore, the presented system does not require a specially designed docking station but uses a conventional cage type TMS. The paper presents and discusses real-world environmental tests successfully completed during January 2019 in the North Atlantic Ocean. To validate the performance of the system, a commercial state of the art underwater navigation system has been used. The results demonstrate a significant advancement in resident ROV automation and capabilities, and represents a system which can be retrofitted to the current ROV fleet.

Application of Thruster Fault-Tolerant Control System based on the Hybrid Method for Control Allocation in Real-World Environment

Fault-tolerant control system for marine robots, developed by researchers at Centre for Robotics & Intelligent Systems (CRIS), University of Limerick, utilises the hybrid method for control allocation, based on the integration of the pseudoinverse and the fixed-point iteration method. The algorithm is implemented as a two-step process. In the first step, the pseudoinverse solution is found, and the feasibility of the solution is examined analysing its components. If violation of actuator constraint(s) is detected, the fixed-point iteration method is activated in the second step. In this way, the hybrid method for control allocation can allocate the exact solution, optimal in the l2 sense, inside the entire attainable command set. This solution minimises a control energy cost function, the most suitable criteria for underwater applications. The performance of the thruster fault-tolerant control system has been validated during the research cruise CE19001, where the work-class ROV Étaín, deployed from support vessel Celtic Explorer via Tether Management System (TMS), successfully performed several complex subsea tasks with a faulty horizontal thruster. This paper describes the algorithm, provides geometric interpretation and presents the results from field trials.

Failure Analysis of Fasteners in a Remotely Operated Vehicle (ROV) System

A failure analysis study was carried out on AISI 204 Cu stainless steel full-threaded fasteners and end-threaded SS 316L fasteners used in a deep water work class ROV system. The AISI 204 Cu stainless steel full-threaded fasteners were used in a Ti–6Al–4V alloy pressure case. The end-threaded SS 316L fasteners were used for attaching the bottom fender in the tether management system (TMS) made of Al 6061-T6 alloy. The mechanical property of both the fasteners was found to be within standard specification. The SEM study of the full-threaded fasteners revealed that the corrosion pit formation in the unfractured interface area is the cause of crack initiation. Subsequently, the crack propagated by stress corrosion cracking and finally, failed by fast overload cleavage fracture. It can be reasoned that a lower wt.% of molybdenum in AISI 204 Cu stainless steel reduced the pitting corrosion resistance of the full-threaded fasteners. The end-threaded fastener failed by stress corrosion cracking, but corrosion pit was not found in the fracture surface, due to the lack of its interface area with the unfractured region. It can be inferred that the chlorine ions of seawater break the oxide film of stainless steel bolts to form corrosion pits. These corrosion pits act as a stress concentration point to initiate a crack, and subsequently the bolts fail by stress corrosion cracking, showing a brittle appearance.

Automated tether management system for extravehicular activities

AbstractSafe extravehicular activity (EVA) requires astronauts to employ tethers to ensure proximity to their spacecraft. This research strives to improve EVA efficiency by allowing crew members to remotely release and retract their safety tether from extended distances using a tether management system consisting of a remotely releasable robotic gripper and a retractor that controls the length of tether. This circumvents the current need to travel back to the tether anchor and manually release the tether, which requires the crew member to travel three times further than necessary. The automated tether management system is essentially a multifunctional teleoperated robotic system that has the potential to improve EVA efficiency, reduce crew‐member fatigue, and improve efficiency of existing robots such as Robonaut. Design and performance requirements are specified to comply with current safety standards. The multifunctional gripper is designed to provide fail‐safe self‐locking positive engagement on a variety of spacecraft anchors. The retractor employs motorized and passive retraction to minimize power consumption, allow the retractor to mimic existing retractable safety tethers functionality, and facilitate control over gripper flight. A nonconductive fiber‐optic core structural tether is designed to transmit commands to the gripper while reducing the risk of developing electrical charges at orbital velocities. Experiments in a simulated microgravity environment evaluate behavior during gripper release and retraction and provide guidelines for improved system operation. Based on these guidelines, the system is then further optimized via simulation for improved retraction considering orbital dynamics. These results provide a recommended operating envelope to allow safe automated gripper retraction. © 2007 Wiley Periodicals, Inc.

Marine tether dynamics: retrieval and deployment from a heaving platform

A tether management system of a winch module in a marine environment is proposed. For the purpose of this study the subsea body is considered as a mass attached at the free end of a long tether which is wrapped around a circular drum controlled by an external torque. The winch is considered mounted on a heaving platform. The effect of the heaving platform on the motions of the drum, tether and attached mass are studied with respect to the longitudinal vibrations of the tether in one dimension. The hydrodynamic effects are considered on the deployed portion of the tether and the tethered mass, both of which are assumed submerged in otherwise still water. The resulting nonlinear system of equations of motion is developed and solved numerically for an example of a marine tether system. The effect of the tether extensibility on the operation threshold of the system is identified. Comparison with an inextensible tether case is provided. The numerical results and their analysis for the retrieval/deployment of the system are presented.

Retrieval and deployment of a marine tether from a heaving platform

A tether management system of a winch module in a marine environment is proposed. For the purpose of this study the subsea body is considered a mass attached at the free end of a long tether which is wrapped around a circular drum controlled by an external torque. The winch is considered mounted on a heaving platform. The effect of the heaving platform on the motions of the drum, tether and attached mass are studied with respect to the longitudinal vibrations of the tether in one dimension. The resulting nonlinear system of equations of motion are developed and solved numerically for an example of a marine tether system.