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- W1934206655 abstract "System identification is the art of modelling of a process (physical, biological,etc.) or to predict its behaviour or output when the environment conditionor parameter changes. One is modelling the input-output relationship of a system,for example, linking temperature of a greenhouse (output) to the sunlight intensity(input), power of a car engine (output) with fuel injection rate (input). In linearsystems, changing an input parameter will result in a proportional increase in thesystem output. This is not the case in a nonlinear system. Linear system identificationhas been extensively studied, more so than nonlinear system identification.Since most systems are nonlinear to some extent, there is significant interest in thistopic as industrial processes become more and more complex.In a linear dynamical system, knowing the impulse response function of asystem will allow one to predict the output given any input. For nonlinear systemsthis is not the case. If advanced theory is not available, it is possible to approximatea nonlinear system by a linear one. One tool is the Best Linear Approximation(Bla), which is an impulse response function of a linear system that minimises theoutput differences between its nonlinear counterparts for a given class of input. TheBla is often the starting point for modelling a nonlinear system. There is extensiveliterature on the Bla obtained from input signals with a Gaussian probabilitydensity function (p.d.f.), but there has been very little for other kinds of inputs.A Bla estimated from Gaussian inputs is useful in decoupling the linear dynamicsfrom the nonlinearity, and in initialisation of parameterised models. As Gaussianinputs are not always practical to be introduced as excitations, it is important toinvestigate the dependence of the Bla on the amplitude distribution in more detail.This thesis studies the behaviour of the Bla with regards to other types of signals,and in particular, binary sequences where a signal takes only two levels. Such aninput is valuable in many practical situations, for example where the input actuatoris a switch or a valve and hence can only be turned either on or off.While it is known in the literature that the Bla depends on the amplitudedistribution of the input, as far as the author is aware, there is a lack of comprehensivetheoretical study on this topic. In this thesis, the Blas of discrete-timetime-invariant nonlinear systems are studied theoretically for white inputs with an arbitrary amplitude distribution, including Gaussian and binary sequences. In doingso, the thesis offers answers to fundamental questions of interest to system engineers,for example: 1) How the amplitude distribution of the input and the systemdynamics affect the Bla? 2) How does one quantify the difference between theBla obtained from a Gaussian input and that obtained from an arbitrary input?3) Is the difference (if any) negligible? 4) What can be done in terms of experimentdesign to minimise such difference?To answer these questions, the theoretical expressions for the Bla have beendeveloped for both Wiener-Hammerstein (Wh) systems and the more general Volterrasystems. The theory for the Wh case has been verified by simulation and physicalexperiments in Chapter 3 and Chapter 6 respectively. It is shown in Chapter 3that the difference between the Gaussian and non-Gaussian Bla’s depends on thesystem memory as well as the higher order moments of the non-Gaussian input.To quantify this difference, a measure called the Discrepancy Factor—a measure ofrelative error, was developed. It has been shown that when the system memory isshort, the discrepancy can be as high as 44.4%, which is not negligible. This justifiesthe need for a method to decrease such discrepancy. One method is to design a randommultilevel sequence for Gaussianity with respect to its higher order moments,and this is discussed in Chapter 5.When estimating the Bla even in the absence of environment and measurementnoise, the nonlinearity inevitably introduces nonlinear distortions—deviationsfrom the Bla specific to the realisation of input used. This also explains why morethan one realisation of input and averaging is required to obtain a good estimate ofthe Bla. It is observed that with a specific class of pseudorandom binary sequence(Prbs), called the maximum length binary sequence (Mlbs or the m-sequence), thenonlinear distortions appear structured in the time domain. Chapter 4 illustratesa simple and computationally inexpensive method to take advantage this structureto obtain better estimates of the Bla—by replacing mean averaging by medianaveraging.Lastly, Chapters 7 and 8 document two independent benchmark studies separatefrom the main theoretical work of the thesis. The benchmark in Chapter 7 isconcerned with the modelling of an electrical Wh system proposed in a special sessionof the 15th International Federation of Automatic Control (Ifac) Symposium onSystem Identification (Sysid) 2009 (Schoukens, Suykens & Ljung, 2009). Chapter 8is concerned with the modelling of a ‘hyperfast’ Peltier cooling system first proposedin the U.K. Automatic Control Council (Ukacc) International Conferenceon Control, 2010 (Control 2010)." @default.
- W1934206655 created "2016-06-24" @default.
- W1934206655 creator A5037508947 @default.
- W1934206655 date "2013-01-01" @default.
- W1934206655 modified "2023-09-26" @default.
- W1934206655 title "Study of the best linear approximation of nonlinear systems with arbitrary inputs" @default.
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