STRUCTURE IDENTIFICATION OF NONLINEAR MODELS FOR QMB POLYMER-COATED SENSORS

This article deals with the representation of quartz-microbalance (QMB) polymer-coated sensors by block-structured models, their structural identification and their functional identification. The block-structured modelling approach is one of the techniques available for obtaining a complete description of the dynamic behaviour of a non-linear sensing device without having detailed information about its inner structure. A QMB sensor, employed for the detection of mixtures of n-octane and toluene, is studied at the high concentration range (up to 10 000 ppm), where non-linear behaviour is expected. The sensor is exposed to time-varying concentrations of toluene and n-octane having quasi-white power spectral density. It is recognized that the sensor is very likely to admit a special model structure, called the 'Wiener model', which is a simple non-linear cascade consisting of sequential dynamic linear, static non-linear and dynamic linear processes. The model is completely identified and gives a good approximation to the response of the sensor to any time-varying concentration. Our approach has not yet been widely applied because of the complication of the structure testing procedure, but it is demonstrated that the method is well suited for the study of most chemical transducers provided certain considerations and preliminary analyses are made, as described in this paper.