1,720,974 research outputs found
Direct identification of nonlinear damping: application to a magnetic damped system
No full textIn the identification of mechanical systems through inverse receptance methods, a linear model is usually assumed, at least on a first approximation. However, most of the systems have a degree of nonlinearity in their elastic or dissipative properties. Usually, stiffness nonlinearities are identified, while damping nonlinearities are neglected or even not detected. Indeed, an equivalent linear damping model, correctly representing the nonlinear dissipation of the system under testing condition, can always be found, but it is test-specific and not generally valid. This paper addresses the identification of dissipative models for multi degrees of freedom mechanical systems with a single damping nonlinearity. Based on the direct damping matrix identification through inverse receptance methods, this research proposes an extension of the Stabilised Layers Method, valid for linear systems, to damping nonlinearities, resulting in the identification of the test-independent linear and nonlinear damping matrices of the system. The proposed method is theoretically derived for the identification of nonlinear damping forces depending on powers of displacement and velocity. The sinusoidal input describing function approximation of the nonlinear damping is exploited to identify the coefficients of the nonlinear damping force from the system receptances, measured under sinusoidal sweeps at constant amplitudes of oscillation across the nonlinearity. The presented method is applied to identify the linear and nonlinear damping matrices of a multi degrees of freedom system with a localised nonlinear magnetic damper. The coefficients of the nonlinear magnetic damping force are identified for two configurations of the magnetic damper. The proposed approach is a parametric method to identify the nonlinear damping models of mechanical systems using standard experimental techniques usually adopted for linear systems. The identified model is valid under general excitation forces, predicting the system behaviour in a broader range of operation than the test-specific linear equivalent model
Damping identification and localisation via Layer Method: Experimental application to a vehicle chassis focused on shock absorbers effects
No full textThis paper investigates the effects of the shock absorbers of a passenger body in white car on identified modal properties and damping matrices. The authors propose an experimental approach to evaluate the changes induced by localised dissipations on damping matrices and modal behaviour. The shock absorbers behave as localised viscous dampers; therefore, the global system results non-proportionally damped. The chassis dynamics is studied through experimental tests, e.g. roving hammer modal analysis. The chassis is tested in three different configurations to include or exclude the shock absorbers effects on the dynamic behaviour. A new approach to damping matrix identification is presented and applied to the chassis experimental data. The system viscous and hysteretic damping matrices are identified from the experimental data based on the inverse frequency response functions method combined with original physical and topological constraints able to describe the system mechanical properties. The effects of the suspension system on the first flexible modes are evaluated comparing the modal properties of the three experimental sets of mode shapes. A numerical method is developed to attenuate noise and to reduce the incompleteness of experimental data. Finally, the influence of localised dampers, their identification and spatial distribution are discussed
Parametric Analysis and Voltage Generation Performance of a Multi-directional MDOF Piezoelastic Vibration Energy Harvester
No full textPiezoelectric vibration energy harvesting has been extensively investigated in recent years and the majority of results focus on using the cantilever beam model under base driven motion. The main focus of this paper is to perform a parametric analysis of multi degrees of freedom piezo-elastic energy harvester to optimize the capability curve exploiting crossing/veering between modes. The structure under test consists of a combination of slender beams with one or more orthogonal beam segments placed on it. The resulting combined structure exhibits bending vibration modes in orthogonal planes. The cross and veering phenomena are studied in deep, attempting to improve the resulting mechanical-electrical energy conversion of the combined structure. A numerical model of the system under investigation is developed considering also non-classical damping. A parametric analysis of the system’ s performance due to geometrical and electrical properties variations are investigated to design a broadband harvester. An experimental analysis is performed on a test rig specially built to investigate the crossing and veering phenomena effects on the resulting output voltage from the energy harvester. Numerically simulated and experimental data are compared to provide information for updating the model as well as to address the efficiency of the harvester in terms of voltage generation
Stabilised Layer Method for linear and nonlinear spatial non classical damping identification
No full textIn the Layer Method the damping matrix is written as a sum of several layers characterised by a semidefinite positive elementary matrix. Each layer is modulated by an unknown damping coefficient and finally expanded to the system total dimensions, using a localisation matrix based on the system topology. The linear and nonlinear spatial damping distribution most close to the real dissipations can be identified directly from experimental FRFs combining the Stabilised Layer Method approach with the inverse receptance method. In this paper the Stabilised Layer Method is experimentally applied to a simple nonclassically viscous damping system and to a quite complex industrial example as a body in white chassis. Finally a nonlinear system with a localised magnetic eddy-current damping is numerically investigated. The nonlinear damping coefficients are identified from numerical nonlinear frequency response functions with additional random noise
Experimental partial feedback linearisation: comparison between two active control strategies on a non-smooth nonlinear system
No full textDynamic balance of the head in a flexible legged robot for efficient biped locomotion
No full textIn the biped robotics domain, head oscillations may be extremely harmful, especially if the robot is teleoperated, since vibrations strongly reduce the operator’s spatial awareness. In particular, undesired head oscillations occur in under-actuated robots, where springs and passive mechanisms are used to achieve a human-like motion. This paper proposes an approach to reduce the vibrations of a biped robot’s head; the proposed solution does not affect the dynamic locomotion properties, on which specific control logic could have been already tuned. The approach is tested on Rollo, a flexible-biped-wheeled robot, whose head vibrates throughout the robot locomotion. The two requirements, i.e., head vibration reduction and unchanged Rollo locomotion properties, are traduced in constraints to the robot possible modifications. Based on a 1D finite element model of the robot, tuned on experimental modal analysis, the undesired vibration causes are detected, and a solution for their reduction is proposed. Rollo’s head vibration amplitude is attenuated using a tuned vibration absorber, which achieves impressive performance in the robot. An archetype of the proposed vibration absorber is tailored designed on Rollo, without invasive changes to the robot structure. The proposed approach solves a significant problem in the biped robotic research community. The approach used to reduce the Rollo head oscillations may be utilized in other biped robot machines with or without flexible legs
Strain proportional damping in Bernoulli-Euler beam theory
No full textIn structural dynamics, different damping models are used; however, due to modal decomposition, those models typically result in the use of the damping ratio as the modal damping parameter. If proportional viscous damping is used, the damping ratio can be related to the mass and stiffness parameters of a particular dynamic system, i.e. the damping is structure-specific. Lord Rayleigh introduced the idea of proportional damping based on the global kinetic and potential energies of a dynamic system. This global or system-wide approach becomes questionable at the local scale, i.e., at a particular location of the researched system: for a particular mode, the potential energy is related to the strain mode shape and the kinetic energy is related to the displacement mode shape. As the strain and displacement mode shapes have different spatial distributions, also the spatial distributions of the potential and kinetic energies differ. Based on the Bernoulli-Euler beam theory, this research proposes an extension to the proportional damping approach, which results in a material-specific damping parameter. It is shown that using this material damping parameter and the assumption of damping energy proportionality to the local modal strain energy, the modal damping ratio of each mode can be obtained theoretically. This finding was confirmed against several experimental test-cases. The proposed material-specific damping parameter opens up the possibility to obtain the structure-specific damping parameters using the theoretical/numerical mode shapes
A proposal of dynamic behaviour design based on mode shape tracing: numerical application to a motorbike frame
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