Auto Resonance Based Identification of Rotating Systems

Rotating structures exhibit speed dependent natural frequencies and mode shapes that play an important role in the overall dynamics. Accurate experimental identification of these phenomena is of great importance for validating uncertainties in numerical models and for detecting potentially dangerous asynchronous frequencies, often obscured by the imbalance synchronous vibrations. As the speed dependent natural frequencies cannot be assessed experimentally without actually rotating the structure at the vicinity of these speeds, the task of exciting and measuring asynchronous frequencies during rotation without risking the integrity of the machine, becomes a great challenge. The present paper proposes an automatic and efficient method to excite a rotating structure at a selected modeshape, while controlling the vibration amplitude, such that a non-destructive test takes place. Automatic excitation of marginally stable vibration occurs upon introducing a phase shifting filter and a nonlinear feedback element. A digital signal processor carries out the latter, therefore the system behavior and the vibration levels are fully controllable. Theoretical analysis, based on the describing function method and modal filtering, is carried out and verified by numerical simulations. Finally, some experimental results are described and analyzed. The experimental system exhibits different modes of vibration that are excited selectively, at any desired speed of rotation and at any desired magnitude. This approach effectively reconstructs the Campbell diagram with only basic knowledge of the system’s modal behavior. It is also shown that one can switch, in situ while rotating the system, between modes of vibration in the presence of large imbalance forces.