1,721,180 research outputs found
A stability analysis based on dissipativity of linear and nonlinear repetitive control
No full textThis paper deals with repetitive control (RC). More specifically, a parametrised version of the repetitive compensator, i.e. of the infinite-dimensional controller employed in RC schemes, modelled as a boundary control system (BCS) in port-Hamiltonian form is presented. Well-posedness and stability of such control scheme are rigorously addressed thanks to novel tools based on dissipativity theory and originally developed for the stabilisation of BCS. Here, the linear and the nonlinear cases are tackled, and in both the cases the classes of plants for which RC schemes are exponentially stable are determined. Moreover, and explicit motivation of perfect asymptotic tracking and disturbance rejection for exponentially stable RC systems without relying on the internal model theory is provided. To show the validity of the analysis, simulations are reported. (C) 2019, IFAC (International Federation of Automatic Control) Hosting by Elsevier Ltd. All rights reserved
Viscoplastic simple shear at finite strains
Full text linkThe equations governing the simple shear deformation of an incompressible inelastic material undergoing finite strain are derived in this paper. The constitutive assumptions are kept in their most general form to allow the incorporation of widely used viscoplastic or viscoelastic models from the literature. It is shown that, while for a hyperelastic material the simple shear problem is completely determined by a single parameter, the amount of shear, in the viscoplastic case, the elastic deformation is the superposition of a triaxial stretch and a simple shear, whose determination requires the solution of three coupled nonlinear evolution equations. We evaluate such a solution for different material models and compare it with three-dimensional finite element simulations to assess its accuracy. We further assess the performance of these models using experimental data from filled rubber, focusing on their ability to capture the observed behaviour, such as the well-known Payne effect. Additionally, we extend our simple shear solution to address torsion and the extension of thin-walled cylinders. These derivations and analyses offer valuable insights for experimentalists engaged in the mechanical characterization of soft materials
Drop Size Evolution during the Phase Separation of Liquid Mixtures
No full textAfter quenching a partially miscible, initially homogeneous, critical liquid mixture to a temperature T deeply below its critical point of miscibility, we observed the formation of rapidly coalescing droplets, whose size grows linearly with time. Eventually, when their size reaches a critical length which is roughly equal to one tenth of the capillary length, the nucleating drops start sedimenting and the two phases rapidly segregate by gravity. This behavior was observed for both density-segregated and quasi-isopycnic systems, showing that gravity cannot be the driving force responsible for the enhancement of the coalescence among the nucleating drops. This result is in line with previous theoretical works based on the diffuse interface model, predicting that the phase separation of low viscosity liquid mixtures is a convection-driven process, induced by a body force which is proportional to the chemical potential gradients. Finally, at later times, following the evolution of isolated drops of the secondary emulsion, we saw that their size grows in time like t^1/3
Martinetti piatti: una applicazione per la valutazione di interventi di consolidamento in murature
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Repetitive Control Meets Continuous Zero Phase Error Tracking Controller for Precise Tracking of B-Spline Trajectories
No full textIn this paper, a novel repetitive control scheme is presented and discussed, based on the so-called B-spline filters. These dynamic filters are able to generate a B-spline trajectory if they are fed with the sequence of control points defining the curve. Therefore, they are ideal tools for generating online reference signals with the prescribed level of smoothness for driving dynamic systems, possibly together with a feedforward compensator. In particular, a continuous zero phase error tracking controller (ZPETC) can be used for tracking control of nonminimum phase systems but because of its open-loop nature it cannot guarantee the robustness with respect to modeling errors and exogenous disturbances. For this reason, ZPETC and trajectory generator have been embedded in a repetitive control scheme that allows to nullify interpolation errors even in nonideal conditions, provided that the desired reference trajectory and the disturbances are periodic. This paper is based on the results presented in the conference paper [L. Biagiotti, F. Califano, and C. Melchiorri, 'Repetitive control of non-minimum phase systems along b-spline trajectories,' in Proc. IEEE 55th Conf. Decis. Control, 2016, pp. 5496-5501.], where asymptotic stability of the overall control scheme has been proved mathematically, but extends such results with an experimental validation based on a nonminimum phase system. Different models of the same physical system have been identified and used in the implementation of this model-based control scheme, allowing a real evaluation of the relationship between control system performance and model accuracy
Large Scale, Unidirectional Convection during Phase Separation of a Density Matched Liquid Mixture
No full textComplete phase segregation may occur on a 10 centimeter scale even in the absence of buoyancy, due to unidirectional, large scale rapid bulk flows driven by chemical potential gradients. Using a hexadecane-acetone nearly density-matched liquid mixture in a 20 cm-long condenser tube with a 1 cm diameter, we observed the rapid axial migration of the acetone-rich drops towards the warmer regions of the condenser. Conversely, the hexadecane-rich drops moved in the opposite direction, therefore ruling out thermocapillary effects as a possible explanation of the phenomenon. These flows lead to a complete phase segregation within 10 seconds, with the formation of a single interface perpendicular to the axial direction. Changing the temperature gradient along the tube from 0.25 C/cm to 1 C/cm no change was detected, with typical drop speeds up to 6 cm/s, irrespectively of the distance of the drop from the wall, showing that the phenomenon is not due to a flow instability
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