Controlling transient deviations from adaptation lines in turbojet engine compressor fields via sliding mode

Abstract

One possible approach to control turbofan engines is the use of high pressure and low pressure compressor field parameters. The adaptation line represents the locus of intermediate steady states between initial and final states. This is modeled to generate the smooth time varying compressor rates, mass flow rate and rotation rates of high pressure and low pressure compressors. Outer loop controls track the compression rates using the mass flow rates' differences of operating points with adaptation conditions. High pressure and low pressure inner loops track prescribed mass flow rates using respectively the fuel mixture and the afterburner lid variable section area as controls. This allows the shaping of the transient trajectories, departing from and returning to the adaptation line. It allows safe operation, close to the stall and surging limits, thereby authorizing greater compressors' accelerations during the transient. In order to overcome the complexity of the nonlinear plant representation and its uncertainty, output tracking in sliding mode is used. It is approximated by a smoothing sliding mode controller. A model of the adaptation line generates the time varying profiles of the high pressure and the fan compression rates associated with the adaptation line. This allows to shape the transient trajectories departing from the adaptation lines and returning to the adaptation lines at a steady state in the compressor fields. Thrust rate changes of a PW-F100 engine type are simulated. They show that both outer and inner loops of high pressure and low pressure spools are perfectly tracked.