An optimal actuator placement methodology for active control of helicopter airframe vibrations

Active control of helicopter vibrations, with actuators in the airframe structure, is appealing in terms of simplicity, air-worthiness, and effectiveness. The actuator locations of current active control systems are centrally located in the cabin or near the main rotor. A new actuator placement methodology is proposed and explored in this research. The approach uses an optimization process to distribute a set of actuators at optimal locations throughout the airframe. A reduced-order airframe dynamic model is utilized for the design methodology synthesis and system analysis. Oscillatory loads at the main rotor hub and at the horizontal tail surface excite the airframe model. Actuation loads are integrated with the airframe dynamic model. A hybrid optimization methodology is formulated to simultaneously determine optimal actuator placement and control actions. The optimization process couples an optimal control formulation with a Simulated Annealing optimization routine. An analytical study is performed comparing optimally distributed actuator configurations to a representative state-of-the-art centralized actuator configuration. It is shown that actuators in the airframe can more effectively control some of the dominant airframe modes, rather than actuators centralized near the main rotor support assembly. When compared to a representative state-of-the-art centralized control configuration, the distributed actuator configuration can achieve significantly greater vibration reduction with less control effort. One of the case studies shows that the centralized actuator configuration reduces vibration by 49% (below 0.07 g); however, the optimally distributed actuator configuration reduces vibration by 90% (below 0.03 g) and requires 50% less control effort. The proposed design methodology that simultaneously determines actuator placement and the associated control action is extremely beneficial to reduce vibration and control effort.