Robust Aircraft Braking Control Laws Design and Validation
Creators: name:  A.W. NDIAYE 
name:   M. CASSARO 
name:   J. BIANNIC 
name:   C. COMBIER 
name:   C. ROOS 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: ADVANCED PERFORMANCE ENGINEERING
MECHANICAL

description: Despite the recent considerable enhancement of flight control and automation, runway excursions represent a growing percentage of the overall aviation accidents records [1, 2]. In this context, the control of the aircraft on-ground remains today a major issue with some unexplored room for improvement [3]. Specifically, the anti-skid control systems are still mostly logic-based heuristic algorithms monitoring the wheel deceleration. Such strategies are sub-optimal and lack generality. Recent advances for automotive applications [4] introduced novel strategies based on wheel slip control achieving better performance, robustness as well as reduced tuning effort [5, 6]. However, those ABS solutions cannot be directly transferred to aircraft for multiple reasons. A major one is the unavailability of the aircraft velocity measure to the braking control unit (BCU) for the sake of safety and modularity [7]. Besides limited measurements access, an aircraft antiskid control system has also more stringent requirements in a wider operating range. Mass variations, runway conditions and braking gain fluctuations make the control system design a challenging problem. In that respect, to unlock all the slip control benefits in aeronautics, a real challenge consists in designing robust, high-performing and generic feedback slip control loops that rely only on the landing gear common set of measurements, namely the braking pressure and the wheel speed [4, 5, 6, 7]. The proposed research aims to develop a robust wheel slip control method able to adapt to the current conditions via efficient wheel slip estimation and road friction curve identification. Preliminary analysis on a linearized single wheel model demonstrates a varying bandwidth transfer between the applied braking torque and the resulting wheel slip. The corresponding time constant is proportional to the inverse of the aircraft speed and to the friction curve slope. Further studies confirm that a simple self-scheduling modal proportional integral controller is able to regulate the varying system when the dynamics evolves on the stable region of the slip-friction curve. However, due to the actuator limitations, the so conceived closed-loop loses convergence properties as soon as the wheel slip reference exceeds the optimal value. Thus, this paper proposes a novel structured H? synthesis coupled with a Linear Parameter Varying (LPV) problem formulation to derive low order controllers with optimal robustness properties. The proposed approach ensures that the wheel slip setpoint is stable and the controller allows the system to reach this reference value from any initial condition including points in the unstable region. Satisfying tracking performances of the wheel slip setpoint are obtained under a certain admissible set of uncertainties regarding the current and optimal slip values as well as the brake gain variations. Closed-loop stability will be analyzed and guaranteed through LPV formulation. In the effort of making the proposed control law implementable in real-world applications, the global architecture is finally supplemented with robust sliding mode observers [7] providing accurate estimations of necessary signals feeding the designed control system. An extensive validation campaign is performed in simulation environment on a high-fidelity/medium-complexity aircraft on-ground model [8], able to capture a wide range of nonlinear and cross-coupled dynamics involved during braking. Performances and robustness are demonstrated, which lays the foundations for slip control implementation in aeronautic applications. Pertinent results will be presented and discussed in terms of accuracy, tuning effort and architecture complexity.