1,124 research outputs found
De Giorgi’s approach to hyperbolic Cauchy problems: the case of nonhomogeneous equations
No full textIn this paper we discuss an extension of some results obtained by Serra and Tilli, in 2012 and 2016, concerning an original conjecture by De Giorgi on a purely minimization approach to the Cauchy problem for the defocusing nonlinear wave equation. Precisely, we show how to extend the techniques developed by Serra and Tilli for homogeneous hyperbolic nonlinear PDEs to the nonhomogeneous case, thus proving that the idea of De Giorgi yields in fact an effective approach to investigate general hyperbolic equations
Decreto Poletti, jobs act e esoneri contributivi. Cosa è cambiato nel mercato del lavoro italiano?
No full textFabrizio Patriarca e Riccardo Tilli valutano se il Jobs Act, in particolare con l’introduzione del contratto a tutele crescenti, abbia ridotto il ricorso al lavoro a termine da parte delle imprese. Sulla base dei dati disponibili, Tilli e Patriarca mostrano come l’obiettivo della riduzione del lavoro a tempo determinato sia stato disatteso a causa degli effetti di un’altra riforma, il cosiddetto “Decreto Poletti”, che a marzo 2014 ha liberalizzato ulteriormente l’uso dei contratti a termine, in contrasto con gli obiettivi del Jobs Act
Discussion on "Power Flow Control of a Doubly Fed Induction Machine Coupled to a Flywheel" (by C. Batlle, A. Dòria-Cerezo and R. Ortega)
No full textA Hybrid Sensorless Observer for the Robust Global Asymptotic Flux Reconstruction of Permanent Magnet Synchronous Machines
No full textWe propose a hybrid sensorless observer for permanent magnet synchronous machines with global asymptotic stability guarantees. Exploiting the constraint of the rotor flux on a circle of unknown radius, we design an integrator system with periodic jumps triggered by a clock to generate a linear regression containing the flux estimation error. Then, a normalized projected gradient descent identifier provides the observer estimates. For the closed-loop system, it is shown that there exists a robustly globally asymptotically stable compact attractor, which, additionally, ensures zero estimation error if appropriate Persistency of Excitation (PE) conditions are satisfied. In this respect, sufficient conditions ensuring PE are provided for the angular speed and the clock period
A modular approach to dynamic modelling of heat exchangers in vapor compression cycles
No full textThis paper describes a modular approach to the dynamic modelling of heat exchangers (condenser and evaporator) in vapor compression cycles. The model of the heat exchangers is obtained by properly connecting a small number of generic modules where a fluid flows, exchanging heat and, possibly, changing state (fromliquid to vapor and vice-versa). The dynamicmodel of the generic element is phase- independent and is valid in every working condition. Unlike other approaches described in the literature, the mass exchange rate from vapor to liquid is explicitly computed in order to comply with state-trajectories constraints which are defined according to the current phase condition and highlighting when a phase-transition is possible. This leads to an effective representation of constrained evolutions and smooth jumps among different phases. The proposed dynamic elements are characterized by dynamic-orientation of some interface variables; this crucial feature for correct modelling is treated carefully in composing them and a suitable standardized interface is defined
Robust control of a throttle body for drive by wire operation of automotive engines
No full textIn recent years, ever more stringent requirements in terms of emissions control, driveability, and safety of automobiles have led to the development of the drive by wire (DBW) concept, a new architecture for engine control systems, with the purpose of managing air, fuel and ignition in an integrated way. The throttle control plays an important role in the development of DBW systems. Despite its apparent simplicity, the position control of the throttle valve is quite a complex problem, due to application constraints and system characteristics. Very high robustness must be linked with limited cost, as required by a mass production device. A cascaded control structure including a nonlinear trajectory generator filter is adopted, allowing each different control problem to be solved with the most suitable control algorithm and implementation technology. In this regard, the use of variable structure control techniques is the key element to reaching the solution. Extensive simulation tests are reported to show the performance of the proposed control algorithm. A throttle step from 0.5° to 89.5° indicates good position tracking under realistic operating conditions, with a position error smaller than 1°. The same simulation is performed at a battery voltage of 9 V to check the controller robustness. A prototype controller is presented. The experimental implementation of the controller for a step from 2.5° to 85.5° indicates a very smooth position trajectory with a maximum dynamic position error of 7°. A small throttle step from 1° to 7° (which contains the nonlinearity of the limp home mode spring) was also tested and resulted in very good position response with the maximum position error of 2°. Application specifications are fully satisfied both in terms of control performance and controller cost
A UGAS Sensorless Observer for Permanent Magnets Synchronous Machines including Estimation and Compensation of Dead-Times Effects
No full textIn this work, a novel sensorless observer is proposed for Permanent Magnet Synchronous Machines, formally dealing with stator voltage actuation non-idealities. Rotor speed, position and stator fluxes, as well as the unknown parameters of the voltage perturbations are reconstructed considering a fixed reference frame for both the machine and the voltage actuator non-linear effects. Stator currents and commands for the voltage actuator are assumed to be the only known signals. The estimation scheme is proven to be Uniformly Globally Asymptotically Stable by means of rigorous results from adaptive systems theory. The effectiveness of this solution is validated by realistic simulation tests, including a detailed model of the power converter. Discretization of the presented solution is addressed accurately. A comparison is provided to show the advantages of the proposed observer against a solution which does not adopt any mechanism to compensate for the mismatch between ideal and actuated stator voltages
A novel control solution for improved trajectory tracking and LVRT performance in DFIG-based wind turbines
No full textThis paper presents a new control strategy for the rotor side converter of Doubly-Fed Induction Generator based Wind Turbine systems, under severe voltage dips. The main goal is to fulfill the Low Voltage Ride Through performance, required by modern grid codes. In this respect, the key point is to limit oscillations (particularly on rotor currents) triggered by line faults, so that the system keeps operating with graceful behavior. To this aim, a suitable feedforward-feedback control solution is proposed for the DFIG rotor side. The feedforward part exploits oscillation-free reference trajectories, analytically derived for the system internal dynamics. State feedback, designed accounting for control voltage limits, endows the system with robustness and further tame oscillations during faults. Moreover, improved torque and stator reactive power tracking during faults is achieved, proposing an exact mapping between such quantities and rotor-side currents, which are conventionally used as controlled outputs. Numerical simulations are provided to validate the capability of the proposed approach to effectively cope with harsh faults
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