297 research outputs found
Electrochemical impedance spectroscopy beyond linearity and stationarity - a critical review
Electrochemical impedance spectroscopy (EIS) is a widely used experimental
technique for characterising materials and electrode reactions by observing
their frequency-dependent impedance. Classical EIS measurements require the
electrochemical process to behave as a linear time-invariant system. However,
electrochemical processes do not naturally satisfy this assumption: the
relation between voltage and current is inherently nonlinear and evolves over
time. Examples include the corrosion of metal substrates and the cycling of
Li-ion batteries. As such, classical EIS only offers models linearised at
specific operating points. During the last decade, solutions were developed for
estimating nonlinear and time-varying impedances, contributing to more general
models. In this paper, we review the concept of impedance beyond linearity and
stationarity, and detail different methods to estimate this from measured
current and voltage data, with emphasis on frequency domain approaches using
multisine excitation. In addition to a mathematical discussion, we measure and
provide examples demonstrating impedance estimation for a Li-ion battery,
beyond linearity and stationarity, both while resting and while charging
A nonlinear model of vortex-induced forces on an oscillating cylinder in a fluid flow
A nonlinear model relating the imposed motion of a circular cylinder,
submerged in a fluid flow, to the transverse force coefficient is presented.
The nonlinear fluid system, featuring vortex shedding patterns, limit cycle
oscillations and synchronisation, is studied both for swept sine and multisine
excitation. A nonparametric nonlinear distortion analysis (FAST) is used to
distinguish odd from even nonlinear behaviour. The information which is
obtained from the nonlinear analysis is explicitly used in constructing a
nonlinear model of the polynomial nonlinear state-space (PNLSS) type. The
latter results in a reduction of the number of parameters and an increased
accuracy compared to the generic modelling approach where typically no such
information of the nonlinearity is used. The obtained model is able to
accurately simulate time series of the transverse force coefficient over a wide
range of the frequency-amplitude plane of imposed cylinder motion.Comment: Accepted for publication in Journal of Fluids and Structure
Optimal control of wave energy systems considering nonlinear Froude–Krylov effects: control-oriented modelling and moment-based control
Motivated by the relevance of so-called nonlinear Froude–Krylov (FK) hydrodynamic effects in the accurate dynamical description of wave energy converters (WECs) under controlled conditions, and the apparent lack of a suitable control framework effectively capable of optimally harvesting ocean wave energy in such circumstances, we present, in this paper, an integrated framework to achieve such a control objective, by means of two main contributions. We first propose a data-based, control-oriented, modelling procedure, able to compute a suitable mathematical representation for nonlinear FK effects, fully compatible with state-of-the-art control procedures. Secondly, we propose a moment-based optimal control solution, capable of transcribing the energy-maximising optimal control problem for WECs subject to nonlinear FK effects, by incorporating the corresponding data-based FK model via moment-based theory, with real-time capabilities. We illustrate the application of the proposed framework, including energy absorption performance, by means of a comprehensive case study, comprising both the data-based modelling, and the optimal moment-based control of a heaving point absorber WEC subject to nonlinear FK force
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