Helicopter Vibration Reduction Using Robust Control

This dissertation presents a control law for helicopters to reduce vibration and to increase damping using individual blade control. control synthesis is used to develop a robust controller usable in different operating conditions with different helicopter flight speeds. The control design is applied in simulation to the four-blade BO 105 helicopter rotor, which is equipped with an individual blade control system, where the pitch rod links are replaced by hydraulic actuators, allowing blade pitch control to be superimposed to the swashplate commands. Either oscillatory hub loads can be reduced or fuselage vibration can be targeted directly. As concerns hub loads, vibration can be cancelled ( ) in three outputs simultaneously, e.g. in all three hub forces or in the vertical hub force and in the roll and pitch moments. A number of more than three outputs exceeds the number of three degrees of freedom available for vibration reduction of the four-blade rotor. Vibration can then only be reduced moderately, e.g. by , for all three hub forces and for the two moments about the roll and pitch axis. Reducing hub vibration, however, does not necessarily lead to reduced vibration in the cabin. When individual blade control inputs, aimed at minimizing hub loads, are introduced, fuselage accelerations increase by a factor of up to three. Therefore, a finite-element model of the flexible fuselage is coupled with the aeromechanical rotor model. The resulting coupled rotor-fuselage model allows vibration to be calculated and controlled at locations in the cabin, such as at the pilot and copilot seats and in the load compartment. In simulation, a simultaneous vibration reduction of is achieved at the pilot and copilot seats. The control law is developed with the constraint of no sensors and, consequently, no measurements in the rotating blades. However, to increase lag damping, the lag rates must be fed back. The use of a model-based control strategy enables lag damping to be enhanced from 0.5% to >3% critical damping by feeding back the observed lag rates, only requiring measurements of the hub loads. In order to consider the periodicity of the plant in the controller design, a time-periodic gain-scheduled controller is developed. The results of the simulation confirm the common viewpoint that incorporating more knowledge about the plant into the controller, instead of designing a more robust and thus conservative controller, improves performance or robustness against other influences. H