An orbital-free molecular dynamics study of melting in K_20, K_55, K_92, K_142, Rb_55 and Cs_55 clusters

The melting-like transition in potasium clusters K_N, with N=20, 55, 92 and 142, is studied by using an orbital-free density-functional constant-energy molecular dynamics simulation method, and compared to previous theoretical results on the melting-like transition in sodium clusters of the same sizes. Melting in potasium and sodium clusters proceeds in a similar way: a surface melting stage develops upon heating before the homogeneous melting temperature is reached. Premelting effects are nevertheless more important and more easily established in potasium clusters, and the transition regions spread over temperature intervals which are wider than in the case of sodium. For all the sizes considered, the percentage melting temperature reduction when passing from Na to K clusters is substantially larger than in the bulk. Once those two materials have been compared for a number of different cluster sizes, we study the melting-like transition in Rb_55 and Cs_55 clusters and make a comparison with the melting behavior of Na_55 and K_55. As the atomic number increases, the height of the specific heat peaks decreases, their width increases, and the melting temperature decreases as in bulk melting, but in a more pronounced way.Comment: LaTeX file. 6 pages with 17 pictures. Final version with minor change

Robust vehicle steering control design based on the disturbance observer

A robust steering controller is introduced for improving the yaw dynamics of a passenger car. A specific two degree of freedom control architecture known as the disturbance observer is adapted to the vehicle yaw dynamics control problem and shown to robustly improve performance. The relevant design specifications are formulated in terms of eigenvalues (T-stability) and in frequency domain as bounds on weighted sensitivity and complementary sensitivity functions (B-stability). The parameter space method is used to map the specifications for controller design. A Popov criterion based nonlinear stability analysis is also carried out to prove absolute stability in the presence of actuator rate limitation. Simulations are used to demonstrate the effectiveness of the final design

Robust two degree of freedom vehicle steering control satisfying mixed sensitivity constraint

Robust steering control is used here for improving the yaw dynamics of a passenger car. A specific two degree of freedom control structure is adapted to the vehicle yaw dynamics problem and shown to robustly improve performance. The design study is based on six operating conditions for vehicle speed and the coefficient of friction between the tires and the road representing the operating domain of the vehicle. The relevant design specifications are formulated as attaining Hurwitz stability and a mixed sensitivity frequency domain bound. Simple, and therefore, easily implementable controller transfer functions with two design parameters are chosen for the two steering controller degrees of freedom. Using the parameter space method, the design specifications are mapped into the plane of controller parameters. The effectiveness of the final design is demonstrated using simulations

Robust Two Degree-of-Freedom Vehicle Steering Controller Design

Robust steering control based on a specific two degree of freedom control structure is used here for improving the yaw dynamics of a passenger car. The usage of an auxiliary steering actuation system for imparting the corrective action of the steering controller is assumed. The design study is based on six operating conditions for vehicle speed and the coefficient of friction between the tires and the road representing the boundary of the operating domain of the vehicle. The design is carried out by finding the region in controller parameter plane where Hurwitz stability and a mixed sensitivity frequency domain constraint are simultaneously satisfied. A velocity based gain scheduling type implementation is used. Moreover, the steering controller has a fading effect that leaves the low frequency driving task to the driver, intervening only when necessary. The effectiveness of the final design is demonstrated using linear and nonlinear simulations

Large envelope flight control satisfying H-infinity robustness and performance specifications

A method of designing large envelope flight controllers for high performance aircraft is presented. The approach combines the simple two-degree of freedom architecture often referred to as a disturbance observer and recent advances in parameter-space control design techniques to form a new approach for large envelope flight control design. The procedure enables the designer to explicitly define the desired closed-loop dynamics and ensures that the closed-loop is stable and satisfies weighted frequency response magnitude (H-infinity) specifications. The result is a straightforward procedure that enables the design of a robust flight controller that "forces" the closed-loop dynamics to behave like the specified 'desired dynamics' despite disturbances, modeling uncertainties, and variations in aircraft dynamics due to changing flight conditions. The procedure is presented by designing a pitch-rate controller for the F-16 Variable Stability In-Flight Simulator Test Aircraft (VISTA). The controller provides predicted 'Level 1' flying qualities throughout the large design flight envelope, which is demonstrated using both linear simulations and a high fidelity, nonlinear simulation. Finally, the controller is compared to a 7th order and a 10th order linear parameter-varying (i.e. gain-scheduled) controller and found to provide better performance and robustness

Robust Vehicle Steering Control Design Based on the Disturbance Observer

A robust steering controller is introduced for improving the yaw dynamics of a passenger car. A specific two degree of freedom control architecture known as the disturbance observer is adapted to the vehicle yaw dynamics control problem and shown to robustly improve performance. The relevant design specifications are formulated in terms of eigenvalues (Gamma-stability) and in frequency domain as bounds on weighted sensitivity and complementary sensitivity functions (B-stability). The parameter space method is used to map the specifications for controller design. A Popov criterion based nonlinear stability analysis is also carried out to prove robust absolute stability (Theta-stability) in the presence of actuator rate limitation. Simulations are used to demonstrate the effectiveness of the final design

Robust two degree of freedom vehicle steering controller design

Robust steering control based on a specific two degree of freedom control structure is used here for improving the yaw dynamics of a passenger car. The usage of an auxiliary steering actuation system for imparting the corrective action of the steering controller is assumed. The design study is based on six operating conditions for vehicle speed and the coefficient of friction between the tires and the road representing the boundary of the operating domain of the vehicle. The design is carried out by finding the region in controller parameter plane where Hurwitz stability and a mixed sensitivity frequency domain constraint are simultaneously satisfied. A velocity based gain scheduling type implementation is used. Moreover, the steering controller has a fading effect that leaves the low frequency driving task to the driver, intervening only when necessary. The effectiveness of the final design is demonstrated using linear and nonlinear simulations