Control Evaluations of Semiactive Fluid-ElastomericHelicopter Lag Damper

Abstract

A semiactive magnetorheological fluid―elastomeric device has been assembled and evaluated in control experiments to demonstrate its utility to compensate for measured changes as a helicopter lag damper. The baseline used in this study was lag damper (snubber) for a hingeless helicopter rotor that employs both elastomers and fluids: that is, a fluid―elastomeric damper. This damper was disassembled and modified to include two internal magnetorheological control valves, and the existing hydraulic fluid was replaced with magnetorheological fluid. Hence, the resulting prototype damper is a retrofit design that can easily be installed in a test rotor in future evaluations. The magnetorheological fluid―elastomeric damper was subject to extensive performance characterization at displacement amplitudes ranging from 0.8 to 3.4 mm, temperatures ranging from 20 to 55°C, and control currents up to 2.0 A. Using this database and a hydromechanical model of the damper, control systems were designed to track a reference damping profile, which is a function of amplitude only, in the presence of measured amplitude and temperature variations at the rotor lag frequency (3.8 Hz). Both open-loop (interpolating lookup table) and closed-loop (proportional-integral and gain scheduling) techniques were employed in simulations and experiments. The results demonstrated that all of the control schemes were effective, although a minimum tracking error was achieved with the closed-loop systems. Comparing the two closed-loop systems, the gain-scheduling controller provided a faster response overall, but the improvement was marginal.