Command Limiting in Reconfigurable Flight Control

Limits on the motion and on the rate of motion of the actuators driving the control surfaces of aircrafts significantly affect the performance of flight control systems. After a failure, the constraints become even more restrictive, because of the loss of control power and because of the increase in cross-couplin gs between the axes. In the paper, model reference adaptive control algorithms are considered for flight control reconfiguration and four methods are proposed to handle the problems arising due to actuator saturation. The methods are based on a scaling of the control inputs, a relaxation of the control requirements, a scaling of the reference inputs, and a least-squares approximation of the commanded accelerations. Simulations demonstrate the effectiveness of the methods for the control of a twin-engine fighter aircraft. It is found that even the simplest method considerably improves the responses compared to those obtained without modification. Further, despite the widely different concepts and complexity levels of the four methods of command limiting, the performance is found to be similar in most cases.