Abstract

Vibration testing of nonlinear structures presents various challenges that are not only related to the nonlinear structure itself but also to the exciter. As the exciter and the structure are coupled, the structure's nonlinear behaviour distorts the exciter force amplitude and frequency content, making the test results more difficult to compare with numerical predictions or even less accurate when isolating and identifying nonlinear normal modes (NNMs).

This paper uses an adaptive feedforward cancellation (AFC) real-time control algorithm to track multi-harmonic force reference signals accurately. The controller estimates the disturbance generated by the structure and adjusts the exciter's input to compensate for it, recovering the desired force signal. The advantage of this approach is that it does not rely on the knowledge of the exciter and structure models. It only requires knowledge of the phase at the output of the exciter, which is straightforward to estimate experimentally from the force measurements.

The effectiveness of the AFC for exciter force control is demonstrated numerically and experimentally on a cantilever beam structure excited by an electromagnetic shaker. The beam's free tip is attached to a spring mechanism, giving rise to geometric nonlinearities and mode interactions resulting in strong harmonic distortions in the excitation force. The method is further demonstrated on a compressor blade from Roll-Royce's aero-engine.

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