Innovative Gain Reconfiguration for Active Fault-Tolerant Flight Control: Balance of Stability and Agility

TL;DR

The paper addresses loss of control due to actuator faults in agile aircraft by introducing a reconfigurable active fault-tolerant flight control framework that uses Incremental Attainable Acceleration Set (IAAS) to constrain inner-loop commands and an analytical outer-loop gain update law to preserve stability while maintaining agility. The method updates inner-loop gains online via IAAS with a sigmoid-like tuning function to balance robustness and responsiveness, and it derives an outer-loop gain relation from inner-loop dynamics and time constants to meet a prescribed bandwidth ratio. Key contributions include the IAAS-based fault accommodation, the tuning-function-driven inner-loop reconfiguration, the analytical outer-loop update law, and in-flight validation showing superior stability-agility performance compared to fixed-gain controllers on a severe actuator fault scenario. The approach enables safer, real-time fault-tolerant operation for over-actuated aircraft, demonstrated on a high-fidelity F-16 model and applicable to demanding mission profiles. Overall, the work advances practical active fault-tolerant flight control by tightly integrating controllability constraints with dynamic reconfiguration to sustain performance under severe faults.

Abstract

In this study, a distinct reconfigurable fault-tolerant flight control strategy is addressed for mitigating one of the persistent safety-critical issue, i.e. loss of control triggered by actuator faults. The attainable acceleration set notion is taken a step further towards incremental attainable acceleration set through a slight modification that enables instantaneous controllability checks. The inner-loop gains are updated in case of a fault using incremental attainable acceleration set and a tuning function, which is in charge as a compensator of agility and robustness. Additionally, the outer-loop gains are also such reconfigured that holding the bandwidth ratio of the successive loops at a prudent level to ensure the closed-loop stability; for this reason, an analytical outer-loop gain update law is derived based on the inner-loop gains and bandwidth, actuator and command filter time constants. Subsequently, the proposed architecture is assessed under a severe fault scenario with a demanding maneuver mission. Noticeably, the proposed method fulfills the expectations of stability and agility sufficiently, and surpasses the fixed-gain approach.

Figures (9)

• Figure 1: Entire flight control architecture: nonlinear dynamic inversion based control law derivation and weighted pseudo-inverse based incremental nonlinear control allocation application to the nonlinear flight dynamics.
• Figure 2: Background for the reconfiguration strategy: A sample IAAS at an arbitrary time step including commanded acceleration vector, $\dot{\omega}_c$, which violates the boundary. Also, $\dot{\omega}_{c,r}$ is the reconfigured acceleration command by reconfiguring $K_\omega$ to $K_{\omega,r}$.
• Figure 3: Tuning function behaviour with the change of hyper-parameters.
• Figure 4: The representation of the nonlinear closed-loop system in the linearized form due to the nonlinear dynamic inversion: $\Dot{\Omega}_\nu$ denotes the commanded virtual input, i.e. $\omega_c$, while $\Dot{\omega}_\nu$ also denotes the virtual input.
• Figure 5: State history of the test maneuver: reconfigured gain and fixed gain.

Theorems & Definitions (1)

• Corollary IV.1