Nonlinear Control of an Aeroelastic System with a Non-smooth Structural Nonlinearity

Non-smooth nonlinearities such as freeplay, bilinear/piece-wise linear stiffness are among the various types of nonlinearity that have been encountered in aeroelastic systems. Freeplay, for example, may begin to appear as a result of ageing of components such as bolted joints and control surfaces, and has been known to be the cause of flutter-induced limit cycle oscillation (LCO). Therefore, it is evident that effectively controlling these nonlinearities is essential in avoiding the onset of LCO, or indeed any other type of nonlinear response. The present paper addresses the control of systems with non-smooth structural nonlinearities, through application of the feedback linearisation method. In systems with smooth nonlinearities, the required nonlinear feedback is also smooth, and therefore does not give rise to complexities associated with the feedback linearisation method. On the other hand, when controlling systems with non-smooth nonlinearities, the necessary control inputs are also non-smooth, and the applicability of feedback linearisation to such systems is of interest. This task is undertaken in the present work, through the use of numerical simulations on a 3 degree of freedom aeroservoelastic model. An example of a case where the parameterisation of the nonlinearity is uncertain is also addressed.