Frequency Scaling of Limit-Cycle Oscillation in a Launch Vehicle Electrohydraulic Actuation System

The flight acceptance integrated trials of an electrohydraulic actuation system which steers the flex nozzle showed the excitation of system resonance mode for a typical flight command profile. The resonance in flex nozzle control system where the actuator moves the nozzle inertia is determined by the combination of net mounting bracket stiffness at both ends, hydraulic actuator stiffness, and the nozzle inertia. The resonance frequency was excited internally due to the limit-cycle oscillation happened in the hydraulic valve harmonized with the resonance mode. The valve consists of linear force motor, hydraulic amplifier valve, and direct drive valve. The nonlinearities present in the system like magnetic hysteresis, diaphragm hysteresis, radial clearances, and spool lap conditions produce limit-cycle oscillation in the valve. A novel scheme of shifting the limit-cycle frequency to a higher value sufficiently away from the resonance mode for all operating condition is developed and discussed in this paper. Adaptively varying servo gain according to the operating zone of the valve is derived from the estimated and observed valve dynamics for shifting the limit-cycle frequency to a larger value without compromising the stability.