1,721,207 research outputs found
Adaptive control of launch vehicles in atmospheric flight
Full text linkThe research focuses on the development of innovative control techniques for attitude control of a launch vehicle (LV) during the atmospheric flight, and to investigate their possible benefits in terms - for instance - of improved disturbance rejection capability, as well as, a means for reducing the burden of recurrent activities of mission integration and flight program software finalization.
In this respect, a complete nonlinear mathematical model of the launch vehicle dynamics, comprehensive of all relevant aspects for the attitude control problem is first developed. Next, linearized equations of motion are derived under the assumption of small deviations of the vehicle motion from a reference trajectory. The time-invariant linear model is used to synthesize a baseline controller (BC) that features two proportional-derivative (PD) components for attitude and translational motion control, plus filters to phase-stabilize and notch the bending modes by using classical, frequency-based, analysis, and control design techniques.
Among several advanced algorithms referenced in the literature, the Adaptive Augmenting Control (AAC) has been selected and implemented in the LV flight control system in order to retain the functionality and proven record of success of classically designed linear control systems, while consistently and predictably improving their performance and robustness in expanded flight and/or uncertainty parameter envelopes. AAC adjusts the action of a baseline PID-type controller by means of a forward loop gain multiplicative adaptive law that, basically, on-line modulates BC output either to minimize the error with respect to a reference model or to limit undesirable high-frequency response in the control path.
In order to fully exploit the AAC potentiality, an effective and reliable tuning procedure for AAC gains is developed, where a robust design optimization (RDO) problem is formulated, and the goal is to maximize a statistical metric that describes FCS performance measured over a set of LV simulations.
Finally, an analysis of the effects of uncertainties on bending mode characteristics is carried out. Variations of bending mode parameters have a significant and negative impact on AAC performance and, consequently, on LV stability. In this respect, the use of adaptive filters is investigated in order to further improve flight control system robustness. An adaptive notch filter is designed, the parameters of which are updated continuously by an adaptation algorithm that uses the pitch rate sensor output so as to estimate the unknown parameters of the filter and precisely match the actual bending mode frequency
Risk assessment in mission planning of uninhabited aerial vehicles
Full text linkA procedure for evaluating the risk related to the use of unmanned aerial systems over populated areas is proposed. A nominal trajectory, planned for performing a given mission, is represented by means of motion primitives, that is segments and arcs flown in a steady-state condition. The risk of hitting a person on the ground after catastrophic failure is evaluated as a function of vehicle reliability and population density (assumed known), and position of the impact point (which depends on initial conditions at the time of failure and trajectory flown afterwards). In the deterministic case, a lethal area is introduced and the risk at each point on the ground is proportional to the amount of time spent by the point inside the lethal area. Under the assumptions of a ballistic fall, the position of the lethal area with respect to the nominal trajectory depends only on altitude and velocity at the time of failure. When the effect of navigation errors is introduced, impact points are described by a statistical impact footprint, assuming that position and velocity errors at time of failure are normally distributed with known standard deviations. The two approaches are compared for a fictitious, yet realistic, mission scenario
The concept of idiopathic epilepsy. In:
No full textUpdate on genes that cause epilepsy and the concept of genetic epilepsie
Mialaret G. — Introduction à la pédagogie. P.U.F. «L'éducateur », p. 1977, 5e édition, augmentée et mise à jour Mialaret G. — Les sciences de l’éducation. P.U.F. «Que sais-je » 1976 AVANZINI G. — Introduction aux sciences de l’éducation. Privat. Toulouse, 1976, Coll. «Mésopé»
No full textVexliard Alexandre. Mialaret G. — Introduction à la pédagogie. P.U.F. «L'éducateur », p. 1977, 5e édition, augmentée et mise à jour Mialaret G. — Les sciences de l’éducation. P.U.F. «Que sais-je » 1976 AVANZINI G. — Introduction aux sciences de l’éducation. Privat. Toulouse, 1976, Coll. «Mésopé». In: Bulletin de psychologie, tome 31 n°337, 1978. pp. 968-970
Two-timescale magnetic attitude control of Low-Earth-Orbit spacecraft
Full text linkSpacecraft 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
Optimal cruise performance of a conventional helicopter
Full text linkThis 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
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