Time-Frequency Analysis of the Chirp Response for Rapid Electrochemical Impedance Estimation

Fuel cell fault diagnostics has been the focus of numerous research efforts. These efforts seek to address the durability concerns that continue to hinder the penetration of such an attractive energy source. A tool that has gained popularity for diagnostics and characterisation, is the Electrochemical Impedance Spectroscopy (EIS). Information derived from the EIS provides relevant details on the electrochemical characteristics of the system which aid diagnostics. However, classical EIS, implemented using sinusoidal perturbations takes a long time making it unsuitable for real-time measurement. This work seeks to address the challenge by investigating the use of a frequency-rich signal - a chirp. Furthermore, the use of the wavelet transform as an analysis tool of choice is presented. The characteristic nature of the chirp signal makes for a broadband frequency sweep over time possible, hence enabling a faster impedance estimation. Useful information is gained by the localisation of the signal vectors in both the time and frequency space employing the continuous wavelet transform for analysis and the resulting decomposition is then harnessed for impedance calculation. This approach is first tested in simulation and results for equivalent circuit are shown. It is shown that the generated impedance well approximates the theoretical values. Various condition to be observed regarding the proposed techniques are also discussed. The technique is then further tested on a developed rig to demonstrate its practical use and results show good performance relative to commercial frequency response analyser (FRA) implementing the single sine technique while enabling a short-time measurement.