1,721,001 research outputs found
Bifurcation analysis of attitude dynamics in rigid spacecraft with switching control logics
No full textA numerical continuation method is used for bifurcation analysis of the complex behavior that appears in the attitude dynamics of rigid spacecraft as a result of switching logics in control system elements. Because the stability analysis of limit cycles is carried out in most circumstances, the application of the continuation method raises a number of issues when such aspects as nonlinearities that can not be approximated by differentiable functions and noninvertible input-output relations are taken into consideration. An approach based on the continuation of equilibrium points of the Poincare map for the analysis of dynamic systems with motion-dependent discontinuities is presented. Spacecraft configurations featuring single-axis control loops are investigated, where momentum wheels or reaction thrusters are used. In the latter case, the characterization of the dynamic behavior of a pulse-modulated control system is used as an example of the proposed methodology. The results show that the considered configurations can be successfully analyzed by the continuation technique, without performing extensive numerical simulation
Accurate positioning of a payload suspended to a quadrotor unmanned aircraft
No full textThis paper presents a trajectory control strategy for a quadrotor carrying a suspended
load. Following the derivation of a set of simplified equations of motion for the coupled
vehicle slung-load systems, the inverse simulation method based on an integration algorithm
allows for the determination of the control action to track an assigned trajectory, with the pendular
payload acting as a disturbance on vehicle dynamics. Results concerning two situations
relevant from the perspective of practical applications, namely payload in the air and towed in
water, are presented and discussed. The approach appears suitable and effective for the selection
of feedforward control laws that account of hanging load loads, either as a preliminary
step in control system design or to assess vehicle performance limits in specified missions
Single-Axis Pointing of Underactuated Spacecraft in the Presence of Path Constraints
No full textA 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
Acquisition of a Desired Pure-Spin Condition for a Magnetically Actuated Spacecraft
No full textA 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
Assessment of Helicopter Model Fidelity through Inverse Simulation
No full textThe paper presents a technique for assessing the reliability of a set of helicopter models in predicting the required control action when executing a given (set of) manoeuvre task(s). An inverse simulation algorithm based on the integration method is used in order to derive the time-history of control commands necessary for following a prescribed ight path. A quantitative comparison between the control laws thus obtained is performed in order to assess the reliability of lower order models with respect to the baseline, most complete one, adopted as a reference for the analysis. Two metrics are developed, one for evaluating a global error level in the definition of the required control law, and a second one for the identification of the uncertainty in the control action when adopting a lower order model. A total of 9 main rotor dynamic models, 3 main rotor inow models and 3 fuselage aerodynamic databases are combined in order to obtain as many as 13 difierent helicopter simulation models, analyzed in 3 manoeuvres: a hurdle-hop, a slalom and a lateral repositioning. The evaluation of the uncertainty associated with the command law identified by means of simpler models is thus performed in terms of the considered metrics, the validity of which is then tested on two more manoeuvres: a pop-up-pop-down manoeuvre and a 180 deg fast turn. The results show that most of the times uncertainty intervals are correctly identified, although with some degree of conservativeness, when less demanding manoeuvres are dealt with. © 2011 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved
Full-envelope robust control of a shrouded-fan unmanned vehicle
No full textThe development is described of a rate-command system for the control of a novel unmanned vehicle, the baseline model of which is highly nonlinear and presents fast and unstable open-loop modes. Structured singular-value design methodology is used to achieve the desired command response characteristics under specified uncertainties taking into consideration typical problems of small-size helicopters and ducted-fan vehicles such as rate-limited servos and significant time delays. Two robust linear controllers are designed for the low- and high-speed subsets of the operating envelope, and full-envelope flight control is achieved by switching between controllers as the threshold airspeed is traversed. Following an analysis of scaling effects, controller performance is evaluated against rotorcraft handling-qualities specifications. Flight control system development is assessed by piloted, hardware-in-the-loop simulation in the full range of operating conditions, with the controller implemented in the flight computer and pilot commands transmitted to the vehicle via radio link. Simulation testing also shows that the control system has good turbulence gust rejection performance and is robust to significant variations of c.g. position
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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