Widanalage Dhammika Widanage

Technical communique: Initial estimates for the dynamics of a Hammerstein system

This paper studies the generation of initial estimates for the dynamic part of a Hammerstein model. It will be shown that ARMAX or Box-Jenkins models result in better initial estimates than ARX or output-error (OE) models even in the absence of disturbing noise. This will be proven by noticing that a static nonlinear system can be replaced by a static gain plus a nonlinear noise source that acts in a completely similar way to disturbing noise for the study of the second-order properties of the estimators in the prediction error framework.

Effects of Overlapping and Windowing on Frequency Response Function Estimates of Systems With Random Inputs

The use of block overlapping and time windowing in the estimation of the frequency response functions (FRFs) of linear systems is considered. The systems are subjected to stationary white random excitation, and there is no external noise. The sources of errors for the bias and variance of the frequency response are identified to be due to the Fourier transforms of the transient terms or end effects that arise due to nonperiodic signals. Using a pure time delay as an example for the system, it is shown that there is a limit to the improvement achievable in reducing the variance as the block overlap is increased.

A study of the influence of measurement timescale on internal resistance characterisation methodologies for lithium-ion cells

The power capability of a lithium ion battery is governed by its resistance, which changes with battery state such as temperature, state of charge, and state of health. Characterizing resistance, therefore, is integral in defining battery operational boundaries, estimating its performance and tracking its state of health. There are many techniques that have been employed for estimating the resistance of a battery, these include: using DC pulse current signals such as pulse power tests or Hybrid Pulse Power Characterization (HPPC) tests; using AC current signals, i.e., electrochemical impedance spectroscopy (EIS) and using pulse-multisine measurements. From existing literature, these techniques are perceived to yield differing values of resistance. In this work, we apply these techniques to 20 Ah LiFePO4/C6 pouch cells and use the results to compare the techniques. The results indicate that the computed resistance is strongly dependent on the timescales of the technique employed and that when timescales match, the resistances derived via different techniques align. Furthermore, given that EIS is a perturbative characterisation technique, employing a spectrum of perturbation frequencies, we show that the resistance estimated from any technique can be identified – to a high level of confidence – from EIS by matching their timescales.

Comparison of several data-driven non-linear system identification methods on a simplified glucoregulatory system example

In this paper, several advanced data-driven nonlinear identification techniques are compared on a specific problem: a simplified glucoregulatory system modeling example. This problem represents a challenge in the development of an artificial pancreas for T1DM treatment, since for this application good nonlinear models are needed to design accurate closed-loop controllers to regulate the glucose level in the blood. Block-oriented as well as state-space models are used to describe both the dynamics and the nonlinear behavior of the insulin-glucose system, and the advantages and drawbacks of each method are pointed out. The obtained nonlinear models are accurate in simulating the patients behavior, and some of them are also sufficiently simple to be considered in the implementation of a model-based controller to develop the artificial pancreas.

Design and Application of Signals for Nonlinear System Identification

Abstract This paper discusses the design, implementation and the advantages of three types of signals for nonlinear system analysis and identification. They belong to the class of multisine signals and are the random phase, positively skewed and crest factor optimised multisine signals. A straightforward routine to combine such a signal with the systems typical input signal is discussed. The advantages of using such signals is illustrated through the results obtained from identifying the dynamics of a mechanical wet-clutch system.

Effects of Overlapping and Windowing on the Estimates of Frequency Response Functions of Linear Systems with Random Input Signals

The use of block overlapping and time windowing in the estimation of frequency response functions of linear systems is considered. The systems are subjected to stationary white random excitation and there is no external noise. The sources of errors for the bias and variance of the frequency response are identified to be due to the Fourier transforms of the transient terms or end effects that arise due to non-periodic signals. Using a pure time delay as an example for the system, it is shown that there is a limit to the improvement achievable in reducing the variance as the block overlap is increased.

Assigning the Nonlinear Distortions of a Two-input Single-output System

In a multiple-input multiple-output (MIMO) nonlinear system the nonlinear contributions from each input to each output may vary significantly. The nonlinear distortions are composed of contributions that are either purely due to each individual input or a combination with some of the inputs. A three stage experimental design method valid for a wide range of nonlinear systems is presented that detects and classifies, in the frequency domain, the level of these nonlinear contributions. For systems that require nonzero operating points, the contribution from each input and their combinations is conditioned by the operating levels. Periodic broadband excitation signals with several harmonics suppressed are used as the inputs to reduce the noise contributions and evaluate the nonlinear distortion levels present at the suppressed harmonics. Alternatively, each input signal can be designed with a harmonic specification such that the harmonics of the output signal indicate the presence of these nonlinear contributions. As a single experiment technique it requires less time for measurements, however the input harmonics become very sparse as the order of the nonlinearity increase that the signals becomes impractical for experimental use. Experimental results for a two-input single-output system are presented demonstrating the effectiveness of the techniques.

Testing of Commercial Electric Vehicle Battery Modules for Circular Economy Applications

Increasingly international academic and industrial communities desire to better understand, implement and improve the sustainability of vehicles that contain embedded electrochemical energy storage. Underpinning a number of studies that evaluate different circular economy strategies for the electric vehicle (EV) battery system are implicit assumptions about the retained capacity or State-of-Health (SoH) of the battery. International standards and best-practice guides exist that address the performance evaluation of both EV and HEV battery systems. However, a common theme in performance testing is that the test duration can be excessive and last for a number of hours. The aim of this research is to assess whether energy capacity and internal resistance measurements of Li-ion based modules can be optimized, reducing the test duration to a value that may facilitate further End-of-Life (EoL) options. Experimental results for a Porsche Panamera Hybrid module and a Tesla Model S P85 module that highlight a reduction of the duration of a commercial battery module characterization test by circa 70%. This reduction is accompanied by levels of measurement accuracy for retained energy capacity in the order of 1% for module test temperatures equal to 25°C. Improvement of 85% is achieved for resistance testing while still retaining levels of measurement accuracy in the order of 2% for module temperatures equal to 25°C. Based on these experimental results, a quick characterization test sequence is proposed and within a robust system test framework would allow different organizations to prioritize the relative importance of test accuracy versus experimental test time when grading used Li-ion modules.