Autonomous take-off and landing of a tethered aircraft: a simulation study

The problem of autonomous launch and landing of a tethered rigid aircraft for airborne wind energy generation is addressed. The system operates with ground-based power conversion and pumping cycles, where the tether is repeatedly reeled in and out of a winch installed on the ground and linked to an electric motor/generator. In order to accelerate the aircraft to take-off speed, the ground station is augmented with a linear motion system composed by a slide translating on rails and controlled by a second motor. An onboard propeller is used to sustain the forward velocity during the ascend of the aircraft. During landing, a slight tension on the line is kept, while the onboard control surfaces are used to align the aircraft with the rails and to land again on them. A model-based, decentralized control approach is proposed, capable to carry out a full cycle of launch, low-tension flight, and landing again on the rails. The derived controller is tested via numerical simulations with a realistic dynamical model of the system, in presence of different wind speeds and turbulence, and its performance in terms of landing accuracy is assessed. This study is part of a project aimed to experimentally verify the launch and landing approach on a small-scale prototype.Comment: This is the longer version of a paper submitted to the 2016 American Control Conference 2016, with more details on the simulation parameter

Laser-Assisted Bremsstrahlung for Circular and Linear Polarization

We numerically evaluate the cross sections for spontaneous bremsstrahlung emission in a laser field for both circular and linear laser polarization, in a regime where the classical ponderomotive energies for the considered laser intensities are considerably larger than the rest mass of the electron. A fully relativistic quantum-electrodynamic approach using the Volkov solutions of an electron in an external field and Dirac-Volkov propagators for the intermediate electrons is applied. We compare circular to linear polarization and point out several interesting features of the laser-dressed cross sections. Regularizations in both electron and photon propagators are required. Specifically, imaginary mass and energy shifts of the electron must be implemented near resonances which correspond to Doppler-shifted harmonics of the laser frequency. We also introduce a screening to the Coulomb potential in order to avoid long-range Coulomb infinities at zero momentum transfer

Spatial mapping and manipulation of two tunnel-coupled quantum dots

The metallic tip of a scanning force microscope operated at 300 mK is used to locally induce a potential in a fully controllable double quantum dot defined via local anodic oxidation in a GaAs/AlGaAs heterostructure. Using scanning gate techniques we record spatial images of the current through the sample for different numbers of electrons on the quantum dots (i.e., for different quantum states). Owing to the spatial resolution of current maps, we are able to determine the spatial position of the individual quantum dots, and investigate their apparent relative shifts due to the voltage applied to a single gate

Autonomous Takeoff and Flight of a Tethered Aircraft for Airborne Wind Energy

A control design approach to achieve fully autonomous takeoff and flight maneuvers with a tethered aircraft is presented and demonstrated in real-world flight tests with a small-scale prototype. A ground station equipped with a controlled winch and a linear motion system accelerates the aircraft to takeoff speed and controls the tether reeling in order to limit the pulling force. This setup corresponds to airborne wind energy (AWE) systems with ground-based energy generation and rigid aircrafts. A simple model of the aircraft's dynamics is introduced and its parameters are identified from experimental data. A model-based, hierarchical feedback controller is then designed, whose aim is to manipulate the elevator, aileron, and propeller inputs in order to stabilize the aircraft during the takeoff and to achieve figure-of-eight flight patterns parallel to the ground. The controller operates in a fully decoupled mode with respect to the ground station. Parameter tuning and stability/robustness aspect are discussed, too. The experimental results indicate that the controller is able to achieve satisfactory performance and robustness, notwithstanding its simplicity, and confirm that the considered takeoff approach is technically viable and solves the issue of launching this kind of AWE systems in a compact space and at low additional cost

Linear take-off and landing of a rigid aircraft for airborne wind energy extraction

An overview of recent results on the take-off and landing phases of airborne wind energy systems with a rigid aircraft is given. The considered take-off approach employs a linear motion system installed on the ground to accelerate the aircraft to take-off speed and on-board propellers to sustain the climb up to operational altitude. Theoretical analyses are employed to estimate the power, additional on-board mass and land occupation required to realize such a take-off strategy. A realistic dynamical model of the tethered aircraft is then employed, together with a decentralized control approach, to simulate the take-off maneuver, followed by a low-tension flight and a landing maneuver back on the linear motion system. The consequences of different wing loadings for this approach are discussed as well. The simulation results indicate that the take-off and landing can also be accomplished in turbulent wind conditions with good accuracy when the wing loading is relatively small. On the other hand, with larger wing loading values the performance is worse. Possible ways to improve the approach and further research directions are finally pointed out

The relevance of electrostatics for scanning-gate microscopy

Scanning-probe techniques have been developed to extract local information from a given physical system. In particular, conductance maps obtained by means of scanning-gate microscopy (SGM), where a conducting tip of an atomic-force microscope is used as a local and movable gate, seem to present an intuitive picture of the underlying physical processes. Here, we argue that the interpretation of such images is complex and not very intuitive under certain circumstances: scanning a graphene quantum dot (QD) in the Coulomb-blockaded regime, we observe an apparent shift of features in scanning-gate images as a function of gate voltages, which cannot be a real shift of the physical system. Furthermore, we demonstrate the appearance of more than one set of Coulomb rings arising from the graphene QD. We attribute these effects to screening between the metallic tip and the gates. Our results are relevant for SGM on any kind of nanostructure, but are of particular importance for nanostructures that are not covered with a dielectric, e.g. graphene or carbon nanotube structures.ISSN:1367-263

A Small-Scale Prototype to Study the Take-Off of Tethered Rigid Aircrafts for Airborne Wind Energy

The design of a prototype to carry out takeoff and flight tests with tethered aircrafts is presented. The system features a ground station equipped with a winch and a linear motion system. The motion of these two components is regulated by an automatic control system, whose goal is to accelerate a tethered aircraft to takeoff speed using the linear motion system, while reeling out the tether from the winch with low pulling force and avoiding entanglement. The mechanical, electrical, measurement, and control aspects of the prototype are described in detail. Experimental results with a manually piloted aircraft are presented, showing a good match with previous theoretical findings