Modal control using real-time identification for time-varying structures

For a controlled structure, the changes of physical characteristics like natural frequencies, modal damping coefficients, and what's more on mode shapes will destabilize the controlled system. To keep the control performance and guarantee its robustness, a new modal control method using real-time identification is proposed to overcome the instability of controlled structure, especially induced by the inversion of mode shape. In this paper, a modal model of time-varying mechanical structure is supplied by an identifier. The choice of the kind of control permits to use this time-varying model for optimizing and self-adapting the controller. In order to supply the controller, a modal observer is used and also updated. For illustrative purpose, this proposed methodology is carried out on a simple but representative time-varying mechanical structure. On this example, an inertia modification leads not only to low modal frequency shifts but also to inversion of a mode shape. This mode shape inversion is shown to destabilize the controlled structure when the control system is not updated. The overall procedure will be described through simulations and performed for different operating conditions, which will prove that mode shapes have to be precisely determined and updated in the controller and observer to guarantee a robust modal control with high performances in spite of the changes of structure.