Decentralized control of sound radiation from periodically stiffened panels
Active structural acoustic control has previously been used to reduce low-frequency sound radiation from relatively simple laboratory structures. However, significant implementation issues have to be addressed before active control can be used on large, complex structures such as an aircraft fuselage. The purpose of this project is to extend decentralized structural control systems from individual bays to more realistic airframe structures. In addition, to make this investigation more applicable to industry, potential control strategies are evaluated using a realistic aft-cabin disturbance identified from flight test data.
This work focuses on decentralized control, which implies that each control unit is designed and implemented independently. While decentralized control systems are relatively scalable, performance can be limited due to the destabilizing interaction between neighboring controllers. An in-depth study of this problem demonstrates that the modeling error introduced by neighboring controllers can be expressed as the product of the complementary sensitivity function of the neighboring control unit multiplied by a term that quantifies the diagonal dominance of the plant. This understanding can be used to improve existing control strategies. For instance, decentralized performance can often be improved by penalizing control effort at the zeros of the local control model. This stabilizes each control unit and reduces the modeling error induced on neighboring controllers. Additional analyses show that the performance of decentralized model-based control systems can be improved by augmenting the structural damping using robust, low-authority control strategies such as direct velocity feedback and positive position feedback. Increasing the structural damping can supplement the performance of the model-based control strategy and reduce the destabilizing interaction between neighboring control units. Instead of using low-authority controllers to stabilize the decentralized control system, another option is to modify the model-based design. Specifically, an iterative approach is developed and validated using real-time control experiments performed on a structural-acoustic system with poles close to the stability boundary, non-minimum phase zeros, and unmodeled dynamics. Experiments demonstrate that the iterative control strategy, which combines frequency-shaped linear quadratic Gaussian (LQG) control with loop transfer recovery (LTR), is capable of achieving 12dB peak reductions and a 3.6dB integrated reduction in radiated sound power from a rib-stiffened aluminum panel.