Forghieri A., Dozio L., Ghiringhelli G.

The paper deals with the experimental evaluation of the feasibility and effectiveness of active control of longitudinal vibration on a helicopter gearbox support strut. The aim is to test practical solutions for helicopter interior noise reduction using active control techniques. Very annoying components of cabin noise are associated with gearbox meshing tones. A preliminary transmissibility analysis of the vibration path between the gearbox and the helicopter fuselage revealed the predominant contribution of the longitudinal vibration. Thus the experiments were arranged and performed on a strut under longitudinal disturbance conditions. The strut was connected with two end masses to roughly reproduce terminal impedances and equipped with a pair of surface-bonded piezoelectric patches. The control was the feed-forward FXLMS. The primary excitation was generated on one end-mass by an electro-mechanical shaker aligned with the beam longitudinal axis. The disturbance input power was regulated to achieve the strut vibration levels of the helicopter mock-up and to reproduce the frequency spectrum of interest. The reference signal was supplied directly by an accelerometer close to the excitation point. The error signal was derived by an accelerometer on the opposite end of the beam. An extensive set of experiments was performed in order to tune the optimal FXLMS control parameters. The analysis of the high voltage requirements of the control devices led to the implementation of a modified FXLMS algorithm, by including a set of digital band-pass filters designed to concentrate the control action on a priori selected disturbing tones. A careful study has been conducted on the type of pass-band filters to ensure low delay response, high cutting properties, no ripple phenomenon and limited computational requirements. Experimental results revealed the effectiveness of the proposed approach. Complete rejection of large amplitude narrow band disturbances was achieved with limited control effort.