This paper describes the application of Active Fiber Composite (AFC) actuators, a hybrid piezoelectric device, to the reduction of acoustic radiation from a cylindrical shell by active control methods. AFCs were developed to provide a mechanically robust method for large-area, orthotropic actuation and sensing in active structures. The actuation layer is formed by small diameter piezoelectric fibers that are unidirectionally aligned and imbedded in a resin matrix system. By the nature of its structure, an AFC actuator allows use of the primary piezoelectric effect in the plane of the composite. A cylindrical shell testbed is used for this experiment due to the predominance of this structure, and the resulting general interest, within the field of underwater acoustics. To control acoustic radiation from the cylindrical shell, the AFC actuators, placed at optimal locations determined using numerical models, are used to generate a strain field that counteracts the strain associated with acoustically efficient shell motions. Using an end-mounted accelerometer as the error measurement, an adaptive LMS algorithm is used to minimize the error signal in real-time. Experimental are supplied to validate both the device and the methodology in a complex, real-world environment.
Active control architectures for broadband acoustic radiation reduction are compared both analytically and experimentally on a representative structure to quantify the capabilities and limitations of the proposed control methodologies. Specifically, three broad categories of control are compared: classical feedback (rate feedback), optimal feedback (Linear Quadratic Gaussian), and adaptive feedforward control (x-filtered Least Mean Square). A simple radiation model of the light fluid, structural-acoustic behavior of the composite panel is utilized to design the compensators and predict closed-loop performance. For each type of control, a single input, single output design is implemented on a composite panel with embedded active fiber composite piezoelectric actuators. Both the vibration and acoustic performance is recorded for each experiment under equivalent conditions to facilitate a generalizable comparison. Experimental results lead to generalized conclusions pertaining to the application of active structural-based control to improve the acoustic performance of complex structures.
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