Hammerstein-Wiener system estimator initialization

In nonlinear system identification, the system is often represented as a series of blocks linked together. Such block-oriented models are built with static nonlinear subsystems and linear dynamic systems. This paper deals with the identification of the Hammerstein-Wiener model, which is a block-oriented model where a linear dynamic system is surrounded by two static nonlinearities at its input and output. The proposed identification scheme is iterative and will be demonstrated on measurements. It will be proven that on noiseless data and in absence of modeling errors, the optimization procedure converges to the true system locally.     

The lubricity of graphite flake inclusions in sintered polyimides affected by chemical reactions at high temperatures

The internal lubrication of 15 vol% graphite flakes added to polyimide was investigated during sliding at 50-200 N normal loads, 0.3 m/s and 23-260 °C, showing that chemical reactions in the polyimide bulk have an important effect on the sliding efficiency of the graphite. The latter are illustrated by Raman spectroscopy, thermo-analytical measurements and mass spectroscopy. At low temperatures (23-100 °C), transfer consists of pure graphite and mechanical shear prevails. At intermediate temperatures (120-180 °C), transfer consists of coexistent polyimide and graphite. A regime with decreasing friction is related to hydrolysis reactions in the bulk polyimide, lowering the effective water content in the sliding interface. At higher temperatures (180-260 °C), transfer consists of mixed polyimide-graphite. A low friction regime is related to imidisation of the bulk polymer and interfacial water supply, favouring the graphite lubricating properties. The graphite debris morphology occurs as flakes (low temperature) or powders (intermediate temperatures), likely resulting from cleavage between the basal planes or embrittlement across the basal planes.     

Design of multisine excitations to characterize the nonlineardistortions during FRF-measurements

Multisine excitation signals are designed which will be used to detect and qualify the nonlinear distortions on frequency response function (FRF) measurements, and this without making additional experiments. The main idea in this method is to apply well chosen periodic excitations where not all harmonics are excited. The nonexcited frequency lines are used to detect, qualify and quantify the nonlinear distortions, while the FRF is measured at the same time at the excited harmonics. The selection of the multisine that will be used depends on the characteristics of the system that are studied. For that reason, not every multisine will produce in every case accurate, robust, and reliable results for every system. This paper presents some signals by which the following aims are accomplished: simplicity, robustness, and a reliable nonlinear characterization     

Grey-box state-space identification of nonlinear mechanical vibrations

The present paper deals with the identification of nonlinear mechanical vibrations. A grey-box, or semi-physical, nonlinear state-space representation is introduced, expressing the nonlinear basis functions using a limited number of measured output variables. This representation assumes that the observed nonlinearities are localised in physical space, which is a generic case in mechanics. A two-step identification procedure is derived for the grey-box model parameters, integrating nonlinear subspace initialisation and weighted least-squares optimisation. The complete procedure is applied to an electrical circuit mimicking the behaviour of a single–input, single–output (SISO) nonlinear mechanical system and to a single–input, multiple–output (SIMO) geometrically nonlinear beam structure.     

Reduction of the Gibbs phenomenon applied on nonharmonic time base distortions

A sine wave fitting procedure for characterizing measurements of a periodic signal in the presence of additive noise and an unknown time base distortion is presented. If the time base distortion is modeled by a Fourier series, it suffers from the Gibbs phenomenon (ringing) at the borders of the data record. Usually, this is solved by ignoring data samples at the borders. The latter is unacceptable for very short data records where measuring a sample is (very) expensive and/or (very) time consuming. This paper presents a solution that suppresses the ringing in the estimated time base distortion without ignoring data samples at the borders. The theory is illustrated on simulations and on real vessel density in the wood of a mangrove tree from Kenya (Rhizophora mucronata).     

Why are nonlinear microwave systems measurements so involved?

Performing nonlinear measurements on microwave devices is a complex task. This paper introduces, step by step, the key concepts that make the difference between linear S-parameter measurements and nonlinear measurements. The main goal here is to make nonlinear measurements more accessible to the practicing microwave engineer.     

Identification of a Wiener–Hammerstein system using the polynomial nonlinear state space approach

In this paper, the Polynomial NonLinear State Space (PNLSS) approach is applied to model a nonlinear system with a Wiener–Hammerstein structure. To obtain good initial estimates, the best linear approximation of the system under test is first identified. Next, this linear model is extended to a polynomial nonlinear state space model to capture also the system's nonlinear behavior. The identification procedure is applied to measurement data.     

Insights in the molecular structure of low- and high-molecular weight poly(styrene-maleic anhydride) from vibrational and resonance spectroscopy

Poly(styrene-maleic anhydride) or SMA are versatile copolymers available in different grades of molecular weight (M_w = 5500–170,000 g/mol) and molar percentage of maleic anhydride (22–50 mol-%). As specific grades can be used for the synthesis of nanoscale structures in paper industry, a better understanding of the molecular structure was needed. Therefore, a broad range of SMA grades was analyzed by means of size exclusion chromatography, infrared spectroscopy, Raman spectroscopy and nuclear magnetic resonance spectroscopy. While the nominal composition could be determined by ¹H NMR and Raman spectroscopy, the combination with ¹³C NMR and FTIR allowed a more detailed characterization of the molecular structure. The high-molecular weight SMA grades show a heterogeneous molecular structure with segments including high styrene and maleic anhydride moieties, respectively. The formation of film patterns was evaluated by deposition from an acetone solution, resulting in homogeneous films, fibrous structures, or spherical shapes with decreasing diameter as a function of maleic anhydride content including primary particle sizes of 100 nm. This study illustrates intrinsic molecular features of SMA grades that are capable for self-assembly, and can provide a selection tool for future nanoparticle formulation.     

Can errors-in-variables systems be identified from closed-loop experiments?

Errors-in-variables (EIV) systems are known to be identifiable not generally, but under some specific conditions. These conditions are normally formulated for open-loop systems. This paper examines to what extent an EIV system can be identifiable from closed-loop experiments.     

Preparation of di‐butyryl‐chitin scaffolds by using salt leaching method for tissue engineering and their characteristics

Scaffolds are used as support material in treatment of damaged tissues such as cartilage and bone. With the help of scaffolds, damaged tissues can be cured in shorter period with less pain. Chitin is one of the most important scaffold materials curing the damaged tissues while providing a support for related part of the body during healing period. It is biocompatible and biodegradable; however it can not be solved by common solvents leading to the major drawback for this kind of applications. Therefore di‐butyril‐chitin (DBC), which is a chitin derivative and can be solved easily in solvents like acetone, ethanol, and methanol, is preferred for scaffold production instead of chitin. In this study, DBC scaffolds were produced for orthopedic applications and their structural and mechanical properties such as porosity, elasticity, compressibility, and strength were tested to confirm their suitability for such end‐uses. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008