Robust H-infinity Controller Design For INDI-Controlled Quadcopters Using Online Parameter Identification

Abstract

This work presents the design of a robust gain-scheduled controller for attitude control of quadcopters, using an Incremental Nonlinear Dynamic Inversion (INDI)-based inner loop with online identification of its system parameters. A linearized model including uncertainty in the identified parameters and unmodeled dynamics is presented for robustness analysis, followed by a set of design requirements. With this, a cascaded feedback attitude controller with a feedforward filter was synthesized for a symmetric quadcopter using signal-based H-infinity closed-loop shaping. Subsequent linear analysis demonstrated good robustness margins and performance characteristics, which were further validated through nonlinear simulations and experimental flights, showing good performance under uncertainty.
This methodology was then extended using co-design to develop a gain-schedule for varying actuator time constants. The approach achieved the requirements over the entire range. The resulting gain-scheduled controller exhibited good stability margins, with nonlinear simulations confirming effective tracking performance under uncertainty. Experimental evaluation of the gain-scheduled controller was conducted through flight tests with full online parameter identification. Even though the identified parameters during these tests were far outside the defined uncertainty range, acceptable flight performance comparable to simulation results was maintained for actuator time constants below 40 ms. For slower actuators, performance was degraded but this may have been due to the extreme uncertainties rather than the controller itself.