State estimation of nonlinear systems

University dissertation from Luleå tekniska universitet

Abstract: Observer design for nonlinear systems is a popular problem in control theory that has been studied from many angles. Since the system state variables, in general, are not available, state estimation is essential in many control applications, which is why the problem is so attractive for researchers. One example of a process that has nonlinear dynamics is wood drying. In the wood drying process, there are some unmeasurable variables such as the moisture content at the surface and inside the wood, which are important for controlling the drying process for the purpose of minimizing the energy consumption of the wood drying kiln. However, to the best of our knowledge, there is no straightforward observer design for the wood-drying process in the current literature. In the first two research papers that compose this thesis, after introducing a state space realization of the wood drying process, a novel method for estimating the moisture content of the wood during drying is proposed. Compared to typical systems, observer design for nonlinear uncertain systems with time delays, is significantly more complicated and thus attractive for research. In this thesis, the problem of the observer design for a class of uncertain discrete-time nonlinear systems with unknown time delay has also been investigated. The study shows that by using upper and lower bounds of the time delay, the time delay can be excluded in the observer structure. The third and fourth papers mostly focus on this topic based on an optimization approach.

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