An H-Infinity Technique for Improved Higher Harmonic Control

This paper presents two methods for designing higher harmonic controllers to reduce harmonic vibration in helicopters. These design methods produce controllers with improved performance, as measured by peak sensitivity (a measure of control system robustness) and control system bandwidth, as compared to traditional HHC controllers. In our framework, we translate the problem of determining whether the control system meets the bandwidth and peak sensitivity requirements into the problem of determining whether the sensitivity transfer function of the control system, weighted by a filter that embodies the requirements, satisfies an H∞ norm bound. Using H∞ theory allows a controller to be synthesized that meets the given requirements, if such a controller exists. The resulting controller is of high order, because a high-order weighting filter is needed to describe the requirements. To address this problem, we present a second design methodology for designing suboptimal controllers of fixed order, which nevertheless have performance significantly better than traditional HHC controllers. The methodologies are demonstrated at two flight conditions on models derived from wind tunnel data for the SMART rotor.