Search

This article presents an analytical framework for investigating the cruise performance of conventional helicopter configurations. Starting from the analysis of power required in straight-and-level flight, endurance and range performance of turbine- and battery-powered rotorcraft are considered, for which it is assumed that fuel consumption and constant-power battery discharge models are, respectively, made available. The original contributions of the article are represented by (a) a closed-form formulation for expected endurance and range for both classes of vehicles, where electrical helicopters have not been dealt with in previous studies and (b) the analytical derivation of an accurate estimate for best endurance and best range airspeeds as a function of relevant system parameters. The approach is validated by analyzing two reference helicopters, showing good physical insight and better accuracy with respect to other techniques available in the literature, for the identification of an energy-efficient cruise flight strategy.

The paper describes an analytical and experimental framework which investigates performance of electrically-driven ground vehicles powered by battery packs, with the objective of providing simple indications for preliminary sizing of rovers designed for mission on the surface of extraterrestrial bodies. For this class of missions, power is at a premium, and energy-efficient locomotion is clearly a critical issue. An analytical model for the estimate of cruise distance is derived and mobility performance is thus analyzed as a function of cruise speed, relevant vehicle parameters, soil mechanical properties, and atmospheric data. Results are then validated by numerical simulations and an experimental campaign for an Earth-based rover.

The problem of aiming a generic body-fixed axis along an inertially fixed direction is dealt with for an underactuated spacecraft in the presence of a non-zero residual angular momentum, when only two reaction wheels can exchange angular momentum with the spacecraft platform. An analytical condition for the feasibility of the desired pointing is derived first, together with a closed-form solution for the corresponding attitude with zero platform angular rate. A nonlinear controller is then developed in the framework of singular perturbation theory, enforcing a two-timescale response to the system. Convergence to the desired attitude, when the pointing direction falls within admissible limits, is then proved for rest-to-rest maneuvers and randomly generated initial tumbling conditions for a configuration representative of a small-size satellite.

Spacecraft attitude stabilization based on active magnetic actuators represents a challenging problem, since the available control torque is constrained on a plane orthogonal to the direction of the local geomagnetic field, making the system instantaneously underactuated. A novel magnetic controller is proposed, driving a satellite flying on a Low-Earth-Orbit to three-axis stabilization on a prescribed attitude in the Nadir-pointing orbit frame. A proof of stability is provided for an idealized configuration (axisymmetric spacecraft with no disturbances). Robustness of the control technique against environmental disturbances, parameter uncertainties, corrupted measurements, and other control implementation issues is then demonstrated by numerical simulations, where the effect of magnetic residual dipoles is mitigated by online estimation.

This paper presents an analytical framework for addressing the hovering performance of a battery–powered multirotor. The estimation of power required for flight is investigated and an analytical model is proposed to describe the rotor figure of merit as a function of few relevant blade parameters, without the need for ad hoc experiments. The model is derived after a discussion about the aerodynamics of rotating blades. The formulation in terms of Reynolds number is supported by an experimental campaign, performed on a set of commercial–of–the–shelf propellers optimized for small–scale multirotor applications. By imposing the balance between required and available power, the hovering time is predicted by an integral formulation developed for a constant–power battery discharge process. The best endurance condition is obtained in terms of optimum battery capacity and flight time. The methodology, applicable to the design phase of novel multirotor configurations, is finally validated by flight tests.

This paper presents a trajectory control strategy for a multirotor aircraft carrying a suspended load. The load is modeled as a pendulum connected by a rigid link to the center of gravity of the vehicle. Starting from the dynamic equations describing the motion of the coupled vehicle slung-load systems, a nonlinear controller is proposed that simultaneously performs trajectory tracking and payload swing damping. Controller gains are chosen so that the system exhibits a two-time-scale behavior, with fast dynamics for the pendulum and slow dynamics for the positioning task. Under these conditions, the basic results of singular perturbation theory are evoked for both the proof of stability and the preliminary design of control gains. Results of numerical simulations are provided in order to assess the stability and performance of the considered approach.

This paper presents an analytical and experimental framework for investigating the performance of fixed-wing electrically-driven propeller aircraft. After the experimental derivation of a novel constant power discharge model for lithium-polymer (Li-Po) batteries, closed-form expressions for flight endurance and range are derived by equating power required in steady level flight with power supplied by the battery pack. Best endurance and best range airspeed are obtained. A methodology is also described to optimally size the battery capacity for a given set of battery and aircraft characteristics. The proposed approach is validated by application to a test case relative to a small-size unmanned aerial vehicle.

This paper describes the aerosol measurement setup and results obtained during the BEXUS18 (Balloon-borne Experiments for University Students) stratospheric balloon within the A5-Unibo (Advanced Atmospheric Aerosol Acquisition and Analysis) experiment performed on 10 October 2014 in northern Sweden (Kiruna). The experimental setup was designed and developed by the University of Bologna with the aim of collecting and analyzing vertical profiles of atmospheric ions and particles together with atmospheric parameters (temperature, relative humidity, and pressure) all along the stratospheric ascent of the BEXUS18 stratospheric balloon. Particle size distributions were measured with the MeteoModem Light Optical Aerosol Counter (LOAC) and air ion density was measured with a set of two commercial and portable ion counters. Though the experimental setup was based upon relatively low-cost and light-weight sensors, vertical profiles of all the parameters up to an altitude of about 27 km were successfully collected. The results obtained are useful for elucidating the relationships between aerosols and charged particles between ground level and the stratosphere, with great potential in collecting and adding useful information in this field, also in the stratosphere where such measurements are rare. In particular, the equipment detected coherent vertical profiles for particles and ions, with a particularly strong correlation between negative ions and fine particles, possibly resulting from proposed associations between cosmic rays and ions as previously suggested. In addition, the detection of charged aerosols in the stratosphere is in agreement with the results obtained by a previous flight and with simulations conducted with a stratospheric ion–aerosol model. However, further measurements under stratospheric balloon flights equipped with a similar setup are needed to reach general conclusions about such important issues.

The purpose of this paper is the design and the performance analysis of a control technique that allows simultaneous attitude control and angular momentum management for a Low Earth Orbit satellite equipped with a set of three magnetic actuators and three reaction wheels. A Proof of global asymptotic stability is derived for a control law driving a rigid spacecraft towards a prescribed, yet generic attitude fixed in the orbit frame. An analytical technique allowing for control gain selection is also proposed, which provides optimal convergence performance for the proposed controller, as demonstrated by means of Monte Carlo simulations. Robustness to external disturbances is finally evaluated within a realistic implementation scenario.

This paper addresses a methodology for autonomous motion planning of multirotor aircraft in obstructed environments. The control strategy allows the vehicle to online generate quasi-optimal trajectories with limited computational load while performing collision avoidance tasks. The problem is formulated in a model-predictive control architecture in which motion planning and trajectory tracking processes are solved separately. The first process is based on a spline path planning approach to generate smooth and safe trajectories. The second process elaborates trajectory inputs in terms of commanded thrust magnitude and desired attitude rates in order to steer the vehicle during the mission task. Results of both numerical simulations and, for the first time, an experimental validation are provided in order to assess the performance of the approach in the presence of external disturbances and unmodeled dynamics, provided adequate time horizon and update frequency are selected for the numerical optimization algorithm.

The aim of this paper is the analysis of simultaneous attitude control and momentum-wheel management of a spacecraft by means of magnetic actuators only. A proof of almost global asymptotic stability is derived for control laws that drive a rigid satellite toward attitude stabilization in the orbit frame when the momentum wheel is aligned with one of the principal axes of inertia. Performance of the proposed control laws is demonstrated by numerical simulations under actuator saturation. Robustness to external disturbances and model uncertainties is also evaluated.

A rigorous proof of global exponential stability is derived for a magnetic control law that drives a rigid satellite toward a pure-spin condition around a prescribed principal axis of inertia with a desired angular rate. The proof represents an extension and a generalization of a method proposed by two of the authors of the present note for demonstrating global asymptotic stability for a B-dot-like control law that detumbles a spacecraft to rest by means of magnetic actuators only. The proof of stability in the case of acquisition of a non-zero desired angular rate pure spin state is derived in terms of robustness of the global exponential stability of a nominal system by means of generalized exponential asymptotic stability in variations (GEASV) tools. To this aim, the error dynamics equation is first derived in the classical form of a nominal system perturbed by a vanishing perturbation term. Then, after proving the generalized exponential stability for the nominal system, such result is extended to the perturbed system. As a further contribution, an approach for the choice of the control law gain is proposed to the present application, thus allowing to perform the acquisition of the desired pure-spin condition in quasi-minimum time from arbitrary initial tumbling conditions. Stability and performance of the approach are extensively tested by means of numerical simulation.

The paper presents an analytical framework for addressing the hovering time prediction of rotary-wing aircraft, with a particular focus on multi-rotor platforms. By imposing the balance between required and available power, the endurance expression is derived as a function of airframe features, rotor parameters, and battery capacity. The best endurance condition is also obtained in terms of optimum capacity and hovering time, by means of two approximate closed-form solutions. The proposed methodology was validated by means of numerical simulation and flight testing. Results show the effectiveness of the proposed approach.

In underactuated conditions, satellite attitude actuators can deliver a control torque with two components only. The paper develops a general solution for the problem of attitude maneuver planning for underactuated spacecraft by means of a sequence of N admissible rotations, where a rotation is considered to be admissible if it takes place around an axis which lies on the plane where the actuator system can deliver a control torque. An exact solution for the sequence of two feasible rotations that provides a desired reorientation is derived, where the minimum angular- path sequence can also be identified analytically. The optimal sequence for a higher number of steps is obtained by means of a numerical optimization procedure. The features of the optimal sequences for increasing values of N are investigated in order to obtain a better understanding of their characteristics.

Volcanic activity has often affected human life both at large and at small scale. For example, the 2010 Eyjafjallajokull eruption caused severe economic damage at continental scale due to its strong effect on air traffic. At a local scale, ash fall and lava flow emission can cause harm and disruption. Understanding precursory signals to volcanic eruptions is still an open and tricky challenge: seismic tremor and gas emissions, for example, are related to upcoming eruptive activity but the mechanisms are not yet completely understood. Furthermore, information related to gases emission mostly comes from the summit crater area of a volcano, which is usually hard to investigate with required accuracy. Although many regulation problems are still on the discussion table, an increasing interest in the application of cutting-edge technology like unmanned flying systems is growing up. In this sense, INGV (Istituto Nazionale di Geofisica e Vulcanologia) started to investigate the possibility to use unmanned air vehicles for volcanic environment application already in 2004. A flight both in visual- and radio-controlled mode was carried out on Stromboli volcano as feasibility test. In this work we present the preliminary results of a test performed by INGV in collaboration with the University of Bologna (aerospace division) by using a multi-rotor aircraft in a hexacopter configuration. Thermal camera observations and flying tests have been realised over a mud volcano located on its SW flank of Mt. Etna and whose activity proved to be related to early stages of magma accumulation within the volcano.

An analytical and experimental framework addressing formation control of multirotor aircraft with collision avoidance capability is developed and presented. The proposed strategy applies to a formation of cooperating vehicles under the assumption of a broadcast transmission of position data. Starting from the point-mass dynamic model of the single aircraft in the formation, a set of constraints on the vehicles position is introduced and a nonlinear controller based on potential field method is proposed to simultaneously perform trajectory tracking, formation geometry keeping, and collision avoidance. Stability of the closed-loop control system is then proven by means of the Lyapunov method. Results of both numerical simulations and an experimental validation are provided in order to assess the proposed approach in the presence of external disturbances and unmodeled dynamics.

A sizing tool for the definition of the configuration of electrically powered multirotor platforms is developed, which accounts for a realistic battery discharge model. The tool is developed to provide the community with the possibility of deriving the best configuration for performing a given task, while accounting for specific constraints and performance requirements. An evolutionary algorithm is used for searching the design space and to identify feasible designs with optimal performance in terms of maximum hovering time on the target and payload weight fraction.

This paper shows an ambitious methodology of autonomous navigation for multirotor UAVs in obstructed environments. The following strategy was formulated to ensure to ultirotor vehicles the capability to produce autonomously quasi-optimal and safe trajectories, although generally they have at their disposal limited computational resources on board. The problem is formuled in a model predictive control (MPC) architecture in which motion planning and trajectory tracking processes are solved separately like they were stored in two different devices. The first one uses a spline-based motion planning approach to generate smooth and safe trajectories. At this step also a multicopter’s simplified dynamic model and environment information are taken in account. The second one uses trajectory inputs (total thrust and attitude angle rates) to steer the multicopter during the flight. Both adequate time horizon and update frequency are chosen in order to take in account disturbances and dynamics model mismatch. The following methodology will be validated with simulations and future work include experimental tests in outdoor environement. ispartof: pages:19-24 ispartof: Proceedings of the 4th International Workshop on Research, Education and Development on Unmanned Aerial Systems pages:19-24 ispartof: International Workshop on Research, Education and Development on Unmanned Aerial Systems location:Linköping, Sweden date:3 Oct - 5 Oct 2017 status: published

A strategy for attitude path planning that performs single-axis pointing in the presence of obstacles along the angular path and constraints on admissible rotation axes is presented. The proposed kinematic solution is based on the identification of the crossing condition for a baseline single-step maneuver with an undesired set of pointing, seen as a conical obstacle along the attitude path. If crossing with the forbidden region occurs, an alternative two-step maneuver is selected, where both rotation axes are admissible and the cones spanned by the sensor boresight are tangent to the prohibited cone. Given the small number of admissible two-step maneuvers (eight at most), it is possible to select the shortest overall angular path with limited computational effort. The test case discussed demonstrates the effectiveness of the approach and provides further insight into the geometry of the cones.

The paper proposes a novel approach for single-axis pointing of an underactuated spacecraft using only two reaction wheels (RWs), which is based on a simple yet effective wheel rate command. The control law can be used for aiming the line of sight of a sensor, a nozzle, or an antenna toward a target direction, or solar panels toward the sun, after failure of one wheel for a nonredundant control system hardware or, in the case of multiple failures, for redundant systems. The proposed control methodology represents the practical, dynamic implementation of a kinematic planning scheme proposed by the same authors, developed under the assumptions of zero overall angular momentum and triaxial inertia tensor. Under the zero angular momentum hypothesis, the control law also provides three-axis stabilization at zero angular speed. Only rest-to-rest maneuvers are thus dealt with, as a steady residual rotation rate around an arbitrary body-fixed axis cannot be attained in general unless the axis is the principal of inertia and aligned with the spin axis of one of the two active reaction wheels. The limited computational demand of the control law also makes it a practical solution in the case of small-size satellites, where the computational budget is severely limited by the available CPU processing capabilities. The effects of a nonzero residual angular momentum and control axes not aligned with the principal axes of inertia is also investigated, highlighting limitations on pointing precision and convergence performance.

Extreme environments like active volcanoes exhibit many difficulties in being studied by in situ techniques. For example, during eruptions, summit areas are very hard to be accessed because of logistics problems and/or volcanic hazards. The use of remote sensing techniques in the last 20 years by satellite or airborne platforms has proven their capabilities in mapping and monitoring the evolution of volcanic activity. This approach has become increasingly important, as much interest is actually focused on understanding precursory signals to volcanic eruptions. In this work we verify the use of cutting-edge technology like unmanned flying system thermally equipped for volcanic applications. We present the results of a flight test performed by INGV in collaboration with the University of Bologna (Aerospace Division) by using a multi-rotor aircraft in a hexacopter configuration. The experiment was realized in radio controlled mode to overcome many regulation problems which, especially in Italy, limit the use of this system in autonomous mode. The overall goal was not only qualitative but also quantitative oriented. The system flew above an Italian mud volcano, named Le Salinelle, located on the lower South West flank of Mt. Etna volcano, which was chosen as representative site, providing not only a discrimination between hot and cold areas, but also the corresponding temperature values. The in-flight measurements have been cross-validated with contemporaneous in-situ acquisition of thermal data and from independent measurements of mud/water temperature.

In a recent work by the authors the concept of Fading Gaussian Deterministic filter was investigated. The algorithm is based on a set of equations derived from the minimization of a cost function where earlier data are progressively de-weighted by a fading factor. In such a way, the estimation was proved to be less prone to problem unknowns. A tuning procedure was proposed that allows the resulting globally best estimator to evaluate the covariance of an effective measurement noise and the true estimation error, without any a-priori assumption. In the present paper, a general formulation is derived where the observed system is influenced by a control input. Also, a proof is derived for the proposed tuning criterion, which is shown to provide, under certain assumptions, the fading factor that best dampens the modeling errors with respect to measurement noise. The validity of the proposed approach is investigated by means of both numerical simulations and an experimental campaign, where height estimation is performed by fusing information from MEMS accelerometers and a barometric altimeter.

The paper investigates the dynamic behavior of a spacecraft when a single magnetic torque-rod is used for achieving a pure spin condition by means of the so-called Y-dot control law. Global asymptotic convergence to a pure spin condition is proven on analytical grounds when the dipole moment is proportional to the rate of variation of the component of the magnetic field along the desired spin axis. Convergence of the spin axis towards the orbit normal is then explained by estimating the average magnetic control torque over one orbit. The validity of the analytical results, based on some simplifying assumptions and approximations, is finally investigated by means of numerical simulation for a fully non-linear attitude dynamic model, featuring a tilted dipole model for Earth׳s magnetic field. The analysis aims to support, in the framework of a sound mathematical basis, the development of effective control laws in realistic mission scenarios. Results are presented and discussed for relevant test cases.

Electrified aerosols have been previously detected in the troposphere and mesosphere, not only during disturbed weather but also in fair weather atmosphere. The detection of electric charges and discharges of volcanic ashes (Gilbert et al., 1991; Mather and Harrison, 2006; James et al., 2008; Harrison et al., 2010) and of Saharan dust layers (Ulanowski et al., 2007; Nicoll et al., 2011) are only a few examples of charged aerosols in the troposphere. On the contrary, charged particles have been seldom detected in the stratosphere (Renard et al., 2013). The “A5-Unibo” experiment by the University of Bologna has been developed to the aim of studying vertical profiles of atmospheric ions and particles all along the flight of the BEXUS18 stratospheric flight. The experiment was flown from the Esrange Space Center from Kiruna (Sweden) on 10th October 2014 within the REXUS/BEXUS programme. The experiment (Figure 1) collected vertical profiles of: 1) atmospheric parameters such as temperature, relative humidity, and pressure; 2) particles size distribution with an innovative aerosol counter (LOAC “Light Optical Particle Counter”); 3) ion (both positive and negative) densities (Air Ion Counter, AlphaLab Inc.). The preliminary results obtained (Figure 2) indicate good correlations between particle concentrations and ions above the tropopause level and in the stratosphere, and suggest in particular the presence of layers of electrified aerosols in the middle stratosphere similar to Renard et al. (2013). Even though model calculations are now being used to quantify the electrification of the aerosols with a stratospheric aerosol-ion model (Rawal et al., 2013; Renard et al., 2013), the need for further measurements to be conducted on stratospheric balloons clearly emerges. In fact, the presence of layers of electrified aerosols could have significant implications for sprite formation.

This paper introduces a novel concept of visual-haptic display for situational awareness improvement for crowded and low altitude airspace situations. The visual augmentation display that constitutes of Virtual Fences delimiting no-fly zones, and a specific tri-dimensional highlight graphics that enhances visibility of other remotely piloted or autonomous agents, as well as conventional manned aircraft operating in the area is presented first. Then the Shared Control paradigm and the Haptic Force generation mechanism, based on a Proportional-Derivative-like controller applied to repulsive forces generated by the Virtual Fences and other UAVs are introduced and discussed. Simulations with 26 pilots were performed in a photo-realistic synthetic environment showing that the combined use of Visual-haptic feedback outperforms the Visual Display only in helping the pilot keeping a safe distance from no-fly zones and other vehicles.

The use of magnetic torquers on satellites flying inclined Low Earth Orbits (LEO) arises challenging problems when dealing with control strategies: since the avail- able torque is perpendicular to the local magnetic field, the Attitude Control Sys- tem (ACS) is inherently underactuated if no other attitude effector is available. In this paper a rigorous proof of global asymptotic stability is derived for a control law that leads the satellite to a desired pure spin condition around a prin- cipal axis of inertia. A heuristic contribution is then added, inspired by quaternion feedback control and a previous geometric result obtained by the authors, which allows for pointing the spin axis along the normal to the orbital plane.

DOI: 10.2514/1.53074 In this paper, a control law that detumbles a spacecraft with magnetic actuators only is developed. A rigorous mathematical proof of global asymptotic convergence from arbitrary initial tumbling conditions to zero angular velocity in a time-varying magnetic field is derived. Furthermore, a simple criterion for determining a reasonable value of the control gain is developed. The selected gain results in a quasi-minimum time detumbling for different initialconditions,inthepresenceofmagneticcoilsaturation.Performanceoftheproposeddetumblingcontrollawis demonstrated by numerical simulations on a large set of test cases using a Monte Carlo approach.

In this paper, kinematics equations of attitude parameters are derived for cases where the Euler rotation theorem cannot be applied and the single rotation that takes an initial reference frame to a target reference frame cannot be performed. It is the case when the nominal rotation is not allowed along a prescribed direction in space, namely an “underactuation” direction. As a matter of fact, a non-nominal maneuver planning scheme, expressed in terms of Euler axis/angle parameters, is admitted for the minimization of the alignment error between the target and the attainable attitude. The derivation of kinematic equations, describing the time evolution of non-nominal rotation parameters, is performed by means of rigorous algebraic manipulations within a unified framework, where the underactuation direction is prescribed in either the moving frame or the target frame.

A tuning algorithm is presented allowing the optimal estimation of an unknown deterministic state vector observed through noisy measurements. The estimation technique named the fading Gaussian deterministic filter is based on a KF-like set of recursive equations and is considered to be optimal because the gain matrix is an algebraic consequence of a formal minimization of a cost function, with no other assumption. This fading behavior allows the algorithm to work despite other unknowns in the problem that are not measurement noise. A numerical example was provided where angular rate estimation is performed from magnetometer readings of a tumbling satellite. Results were obtained by searching offline for the best configuration over a filter bank, sharing the same structure and simulated measurements, but using different fading factors. The tuning procedure proved to work satisfactorily under large. modeling errors, with remarkable performance in the estimation of the angular rates from noisy readings. In fact, in the tuned condition only, the algorithm was able to estimate both the actual covariance of the measurement noise and the true error covariance related to the state vector estimate with satisfactory accuracy.

Attitude regulation proves to be a challenging problem, when magnetic actuators alone are used as attitude effectors, since they do not provide three independent control torque components at each time instant. In this paper a rigorous proof of global exponential stability is derived for a magnetic control law that leads the satellite to a desired spin condition around a principal axis of inertia, pointing the spin axis toward a prescribed direction in the inertial frame. The technique is demonstrated by means of numerical simulation of a few example maneuvers. An extensive Monte Carlo simulation is performed for random initial conditions, in order to investigate the effect of changes in control law gains.

Unmanned platforms with a conventional fixed wing configuration can be designed according to well assessed aircraft sizing techniques. Conversely, the design of multi-rotor vehicles is still based mainly on scale-model enthusiastic experience. At present there has been a limited effort by the community in developing a systematic approach for sizing this unconventional class of flying vehicles. This paper proposes a revised version of a classical aircraft sizing methodology, based on statistical data available in the literature, with the objective of preliminary sizing an electric multi-rotor configuration, taking into account mission profile and a few performance requirements.

T HE interest in electrically-driven propeller airplanes has been steadily increasing over the last 15 years with applications ranging from small electric remotely piloted vehicles [1] up to large high-altitude/long-endurance aircraft [2], passing through general aviation size airplanes powered by means of hydrogen fuel cells [3]. The art and science of preliminary sizing of conventional aircraft has been the subject of many textbooks over the years [4,5] and, in this framework, reasonably accurate range and endurance prediction against design performance requirements plays quite obviously a crucial role. Unfortunately, the extension of the results valid for conventional configurations to electrically powered aircraft is not always straightforward. In a recent paper, [6] the equations for the evaluation of range and endurance of battery operated electrical aircraft are derived, keeping into account the effects of Peukert’s law on the battery discharge process [7]. Wilhelm Peukert performed tests on lead–acid batteries and discharging them with a constant current. His analysis proved that the discharge time Δt and the discharge current i satisfy the law [7]