Systematic experimental exploration of nonlinear structures with control-based continuation

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In numerous research areas across engineering and the applied sciences there are nonlinear structures or systems for which there are inadequate or multiple competing mathematical models. This is often caused by a poor understanding of the physics at the appropriate scale. Two examples of this are the study of shimmy oscillations in aircraft landing gear, and the onset of chatter during high-speed machining. In these areas experimental investigations are of fundamental importance in order to resolve the details that the models cannot.However, systematically investigating nonlinear dynamics in an experiment is fraught with difficulties due to the potential for sudden changes in the dynamics as system parameters are varied; moreover, the changes may be qualitative as well as quantitative. For example, for particular choice of parameters a system may have a single stable steady-state whereas for another choice it may become bistable. Of particular interest are the boundaries between different types of qualitative behaviour (so-called bifurcations). This proposal seeks to make the determination of these boundaries in a physical experiment a normal and routine task by leveraging ideas from control theory and dynamical systems, and so reveal previously unseen dynamical phenomena.Control-based continuation is a new method, developed in Bristol in 2008, for systematically characterising the qualitative behaviour of a physical nonlinear experiment. Its potential has been recognised by experimenters in other institutions, who are now applying it to their own systems. The key idea is that dynamical features of the system, such as stability changes or the onset of mixed-mode oscillations, can be found and tracked directly in a physical experiment using a combination of feedback control and numerical path following techniques. Thus a `map' or bifurcation diagram showing different regions of qualitative behaviour can be traced out.To enable the widespread uptake of control-based continuation three key objectives must be satisfied. (1) It must be possible to determine the local linearisation of a steady-state, thus providing additional dynamical information and a means to perform on-line controller design/adaptation. (2) The underpinning numerical methods must be made fast and robust. (3) The scalability of control-based continuation to multi-degree-of-freedom systems must be demonstrated. This proposal seeks to address all three points.