Active control of radiated noise from a cylindrical shell using external piezoelectric panels

Abstract

Control architectures and methodologies are developed for the reduction of radiated noise from a thick-walled cylindrical shell using external piezoelectric panels. The proposed approach is to cover the shell's outer surface with curved active composite panels, and to reduce the radiated noise by controlling the motion of each panel's outer surface (i.e., the radiating surface), instead of the shell's outer surface. The use of external piezoelectric panels proposed in this thesis has significant advantages over the conventional approach of directly controlling the structure in reducing radiated noise from stiff structures. The reason is that the proposed approach needs much less control authority, and allows the control system to be significantly less dependent on the dynamic characteristics of the structure, than the conventional approach. The control architecture is composed of local controllers, which are implemented for each panel to reduce its vibration level, and a global controller, which makes the shell a weak radiator by coordinating all of the panels simultaneously. For each local control, two different feedback controllers are implemented simultaneously. The first feedback controller takes the acceleration of the outer surface of each panel and uses high gain to minimize its motion. The other feedback loop, which is denoted as the feedforward controller in this thesis, takes acceleration on the inside surface of the panel and aims at canceling the motion of radiating surface. Several controller configurations were designed, implemented and compared, in order to find the one that is the simplest to implement, while achieving the required closed-loop performance and stability margins.

(cont.) After covering the surface of the cylindrical shell with active composite panels, the panel-level tonal controllers were designed and implemented on the shell vibrating in water. The controllers yielded more than 20 dB of attenuation at the target frequency in the acceleration over the radiating surface, although the actual noise level was increased under closed-loop control due to the flaws in the internal accelerometers in the panels. For global control, a new wavenumber domain sensing method has been developed and applied to feedback controller design for active structural acoustic control. The approach is to minimize the total acoustic power radiated from vibrating structures in the wavenumber domain. We found that the method greatly simplifies the design of MIMO LQG controllers for active structural acoustic control, by reducing the effort to model the acoustic radiation from the structure, and by reducing significantly the number of transfer functions that should be identified to get a plant model. The new sensing method was numerically validated on a beam structure and a cylindrical shell with active composite panels mounted.