There are numerous examples for remote inspection on industrial plant and civil structures where a rapid preliminary inspection with either a wheeled remote sensing agent (RSA) or a small unmanned aerial vehicle (UAV) removes the need to physically send a person into a potentially dangerous environment (expensive and hazardous), and removes the need for expensive supporting hardware (scaffolding, safety equipment etc). Many such examples are to be found in nuclear, oil and gas and civil structures where a fundamental requirement for monitoring exists. Since 2003, The University of Strathclyde has been involved with research as part of the UK Research Centre for Non Destructive Evaluation (RCNDE) to develop robotic deployment of NDE measurement probes. The use of multiple inspection vehicles (coupled with different sensing modalities), allows for a flexible, reconfigurable, and adaptive approach to remote health monitoring. However to be effective in real inspection applications, effective co-ordination between multiple agents must be integrated into the system, along with accurate positioning. In this paper we discuss using artificial potential fields for formation structuring for NDE robots. A theoretical framework is developed and supported by experimental measurements of simple formation structuring in the laboratory environment using five independent NDE robots.
The advantages of decentralised multi-spacecraft architectures for many space applications are well understood. Distributed antennas represent popularly envisaged applications of such an architecture; these are composed of, typically, receiving elements carried on-board multiple spacecraft in precise formation. In this paper decentralised control, based on artificial potential functions, together with a fractal-like connection network, is used to produce autonomous and verifiable deployment and formation control of a swarm of spacecraft into a fractal-like pattern. The effect of using fractal-like routing of control data within the spacecraft generates complex formation shape patterns, while simultaneously reducing the amount of control information required to form such complex formation shapes. Furthermore, the techniques used ensures against swarm fragmentation, which can otherwise be a consequence of the non-uniform connectivity of the communication graph. In particular, the superposition of potential functions operating at multiple levels (single agents, subgroups of agents, groups of agents) according to a self-similar adjacency matrix produces a fractal-like final deployment with the same stability property on each scale. Results from the investigations carried out indicate the approach is feasible, whilst outlining its robustness characteristics, and versatility in formation deployment and control. Considering future high-precision formation flying and control capabilities, this paper considers, for the first time and as an example of a fractally fractionated spacecraft, a decentralised multi-spacecraft fractal shaped antenna. Furthermore, multi-spacecraft architecture exploiting fractal-like formations can be considered to investigate multi-scale phenomena in areas such as cosmic radiation and space plasma physics. Both numerical simulations and analytic treatment are presented, demonstrating the feasibility of deploying and controlling a fractionated fractal antenna in space through autonomous decentralised means. This work frames the problem of architecture and tackles the one of control, whilst not neglecting actuation.
This paper describes the design of a three-dimensional formation flying guidance and control algorithm for a swarm of autonomous Unmanned Aerial Vehicles (UAVs), using the new approach of bifurcating artificial potential fields.We consider a decentralized control methodology that can create verifiable swarming patterns, which guarantee obstacle and vehicle collision avoidance.Based on a steering and repulsive potential field the algorithm supports flight that can transition between different formation patterns by way of a simple parameter change.The algorithm is applied to linear longitudinal and lateral models of a UAV.An experimental system to demonstrate formation flying is also developed to verify the validity of the proposed control system.Nomenclature
This paper investigates the development of a new guidance algorithm for a formation of unmanned aerial vehicles.Using the new approach of bifurcating potential fields, it is shown that a formation of unmanned aerial vehicles can be successfully controlled such that verifiable autonomous patterns are achieved, with a simple parameter switch allowing for transitions between patterns.The key contribution that this paper presents is in the development of a new bounded bifurcating potential field that avoids saturating the vehicle actuators, which is essential for real or safety-critical applications.To demonstrate this, a guidance and control method is developed, based on a six-degreeof-freedom linearized aircraft model, showing that, in simulation, three-dimensional formation flight for a swarm of unmanned aerial vehicles can be achieved.
A number of ship roll stabilizing devices are reviewed, namely passive tanks, controlled passive tanks, activated tanks and activated fins. For each device, the scheme of operation is explained and the advantages, disadvantages and limitations are reviewed. The use of hybrid stabilizing systems with several different types of devices is mentioned.
Abstract The capability to predict fatigue damage continues to be critical for determining the operational life and inspection intervals of connectors and components used in offshore applications. Subsea well intervention systems are subjected to wave induced cyclic bending moments and understanding the fatigue performance of equipment is essential for determining safe operating envelopes. In this paper, a validated fatigue analysis methodology is presented for non-preloaded connectors that are used within subsea well intervention systems. The fatigue analysis methodology addresses limitations in current standards when calculating the fatigue capacities of non-preloaded connectors with different interacting component materials (i.e. low alloy steel and nickel based alloys). The methodology considers the effect on the fatigue life of both non-axisymmetric geometry/loading, FAT loading, as well as the interaction of different connector materials, capturing any potential change in hot spot locations. Three different non-preloaded connections (i.e. consisting of threaded and load shoulder connectors) were analysed using 3-D finite element analysis models, where ΔM-N curves and the associated crack initiation locations were calculated for each connector. Full-scale resonance fatigue tests were successfully performed on the three different connector types, validating the ΔM-N curves calculated using the fatigue analysis methodology. Fatigue failure (i.e. through-wall crack) was achieved in all tests between 100,000 and 5,000,000 cycles matching the predicted crack initiation location from the analysis for each connection. The validated methodology provides accurate calculation of the fatigue life and correct identification of hot spot locations. Using the validated approach described in this paper within the design process can lead to significant improvements in future designs of connectors, enabling safer operational limits and extending the service life of subsea systems.
The distributed control of spacecraft flying in formation has been shown to have advantages over conventional single spacecraft systems. These include scalability, flexibility and robustness to failures. This paper considers the real problem of actuator saturation and shows how bound control laws can be developed that allow pattern formation and reconfigurability in a formation of spacecraft using bifurcating potential fields. In addition the stability of the system is ensured mathematically through dynamical systems theory.
Fragmentation of particle swarms into isolated subgroups occurs when interaction forces are weak or restricted. In the restricted case, the swarm experiences the onset of bottlenecks in the graph of interactions that can lead to the fragmentation of the system into subgroups. This work investigates the characteristics of such bottlenecks when the number of particles in the swarm increases. It is shown that certain characteristics of the bottleneck can be captured by considering only the number of particles in the swarm. Considering the case of a connected communication graph constructed in the hypothesis that each particle is influenced by a fixed number of neighboring particles, a limit case is determined for which a lower limit to the Cheeger constant can be derived analytically without the need for extensive algebraic calculations. Results show that as the number of particles increases, the Cheeger constant decreases. Although ensuring a minimum number of interactions per particle is sufficient, in theory, to ensure cohesion, the swarm may face fragmentation as more particles are added to the swarm.
This paper attempts to design a guidance law using bifurcating potential fields and velocity field for a swarm of autonomous Unmanned Aerial Vehicles (UAVs). We consider an autonomous flight system that can create different three-dimensional swarming patterns so as to guarantee obstacle and vehicle collision avoidance. The guidance law, which is derived from a steering and repulsive potential field, can express variable geometric patterns for a formation flight of UAVs. The system can transition between different formation patterns by way of a simple parameter change. We also describe the design method for potential field that is flexible enough to respond to a variety of UAV performance and mission. Numerical simulation is performed to verify the validity of the proposed guidance law.
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