Behavior Flexibility for Autonomous Unmanned Aerial Systems

Autonomous unmanned aerial systems (UAS) could supplement and eventually subsume a substantial portion of the mission set currently executed by remote pilots, making UAS more robust, responsive, and numerous than permitted by teleoperation alone. Unfortunately, the development of robust autonomous systems is difficult, costly, and time-consuming. Furthermore, the resulting systems often make little reuse of proven software components and offer limited adaptability for new tasks. This work presents a development platform for UAS which promotes behavioral flexibility. The platform incorporates the Unified Behavior Framework (a modular, extensible autonomy framework), the Robotic Operating System (a RSF), and PX4 (an open- source flight controller). Simulation of UBF agents identify a combination of reactive robotic control strategies effective for small-scale navigation tasks by a UAS in the presence of obstacles. Finally, flight tests provide a partial validation of the simulated results. The development platform presented in this work offers robust and responsive behavioral flexibility for UAS agents in simulation and reality. This work lays the foundation for further development of a unified autonomous UAS platform supporting advanced planning algorithms and inter-agent communication by providing a behavior-flexible framework in which to implement, execute, extend, and reuse behaviors

Secondary emission by positrons

In Chapter I the existing literature on secondary emission is reviewed from the point of view of possible effects which might be observed with positrons. Secondary emission by electrons of energy ~ 2 KeV has been extensively studied, and work on positive ions has been reported. No work at all on positrons has been published, and in the energy region covered by the present research ( ~ 5 - 500 KeV) there has been no theoretical work and practically no experimental work on electrons. Some simple theoretical ideas concerning secondary emission by positrons are put forward in Chapter II. Secondary emission by ''Potential Ejection", which has been observed for positive ions but is impossible for electrons, is considered as a possible process for positrons; if such a process can occur for positrons it would only be predominent for particles with an energy ~ 1eV. Some known differences in the behaviour of positrons and electrons are then discussed, from the point of view of any effect these might have on the secondary emission of the particles. It is concluded that no large differences are to be expected in the energy range which can be investigated by experiments which are feasible at present. The second part of Chapter II outlines the basic principle of the experiment, which was to compare the secondary emission by positrons and electrons of the same energy under identical conditions of geometry and target surface, A ?-spectrometer and a copper 64 source, which emitted positrons and electrons, provided focused beams of particles. The secondary electrons were detected with an Allen type electron multiplier, and the number of primary particles was counted with a thin windowed Geiger counter. Chapter III describes the electron multiplier and the associated electronics, and discusses briefly some measurements on its performance. In Chapter IV a preliminary experiment on secondary emission without using the spectrometer is described, which confirmed that there were no large differences in secondary emission by electrons and positrons at high energies. Chapter V describes the ?-spectrometer and the rotating coil method used to measure the magnetic field. Chapter VI describes the main experiments to determine, the relative secondary emission of electrons to that of positrons. Some absolute measurements were also made. It was found that above ~ 50 KeV was about 1.04; as the energy was reduced began to rise rapidly, exceeding 2 below 10 KeV. As such large values of were not expected on any existing theory, a very thorough investigation was carried out to establish that the results were not due to any instrumental errors. The final results for platinum, after all the corrections, none of which was very large, had been applied, were as follows;- Energy KeV 6.5 10 20 50-500 m 3.25 + 1.15 1.7 + 0.3 1.2 + 0.1 1.04. + 0.025 A copper-beryllium target gave similar results. In the final Chapter some tentative explanations for the results are put forward, For energies greater than 100 KeV a semi- quantitative theory is given. It was assumed that the secondary yield was proportional to the energy loss of the particles, and that a primary could produce a secondary as it entered the target, or as it left the target, if it did so as a result of scattering within the target. Using recent data on the energy loss of positrons and electrons, and the results of Seliger, who found that electrons were backscattered by ~30% more than positrons, values of m of the right order of magnitude are predicted. Below ~100 KeV the simple theory breaks down, but other factors which become important at lower energies enable this theory to be extended, so that it can possibly account for the results down to 20 KeV. This extended theory does not seem adequate to explain the large values of m observed below 20 KeV. Some very tentative ideas are put forward concerning processes by which positrons and electrons might liberate secondary electrons, which suggest qualitatively that electrons may be favoured. It is concluded that more experimental and theoretical work is required before the results at low energies can be understood

Genetic relationship between prepuberal plasma FSH levels and reproductive performance in Lacaune ewe lambs

International audienc

A study of subsonic transport aircraft configurations using hydrogen (H2) and methane (CH4) as fuel

The acceptability of alternate fuels for future commercial transport aircraft are discussed. Using both liquid hydrogen and methane, several aircraft configurations are developed and energy consumption, aircraft weights, range and payload are determined and compared to a conventional Boeing 747-100 aircraft. The results show that liquid hydrogen can be used to reduce aircraft energy consumption and that methane offers no advantage over JP or hydrogen fuel

Cracking in asphalt materials

This chapter provides a comprehensive review of both laboratory characterization and modelling of bulk material fracture in asphalt mixtures. For the purpose of organization, this chapter is divided into a section on laboratory tests and a section on models. The laboratory characterization section is further subdivided on the basis of predominant loading conditions (monotonic vs. cyclic). The section on constitutive models is subdivided into two sections, the first one containing fracture mechanics based models for crack initiation and propagation that do not include material degradation due to cyclic loading conditions. The second section discusses phenomenological models that have been developed for crack growth through the use of dissipated energy and damage accumulation concepts. These latter models have the capability to simulate degradation of material capacity upon exceeding a threshold number of loading cycles.Peer ReviewedPostprint (author's final draft