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Extended State Observers (ESO) provide the appealing capability of estimating states and disturbances (like modeling errors) simultaneously. In this paper we present a modified fullstate linear ESO as an augmentation of a baseline attitude tracking controller for a hexacopter. The baseline controller is designed using Nonlinear Dynamic Inversion with relative degree 2. By feeding back the estimated modeling error from the ESO, the performance as well as the robustness of the overall control system is significantly improved. These capabilities are demonstrated in outdoor flight tests with a hexacopter, where controlled flight is achieved even under severe actuator degradation. Due to the linear structure of the presented ESO, this might be an alternative to adaptive controllers in view of certification issues.

This paper presents a fault tolerant position tracking controller for a hexarotor system. The proposed controller has a cascaded structure composed of a position and an attitude control loop. The nominal controller is augmented by an adaptive control allocation which compensates for faults and failures within the propulsion system without reconfiguration of the controller. Simultaneously, it is able to implement a degraded control strategy which prioritizes specific control directions in the case of extreme degradation. The main contribution is a controller that is a step closer to application scenarios by including outdoor GPS-based flight tests, onboard computation and the handling of unknown degradation and failure of any rotor.

A position tracking controller for a hexacopter is presented which is robust against disturbances like modeling errors or propulsion efficiency degradation. The presented controller stands out because of its simple design which does not resort to reconfiguration and avoids the necessity of a Failure Detection and Isolation (FDI) filter. The baseline controller has a cascaded structure with two loops. The outer loop corresponds to the position and velocity control and is designed using linear control. The inner loop corresponds to the attitude control which is a Nonlinear Dynamic Inversion (NDI). The baseline controller is augmented by an Extended State Observer (ESO) which provides an estimate of the disturbances and modeling errors which is then fed back to the control law. The performance as well as the robustness of the control system is significantly improved as demonstrated in simulation, where controlled flight is achieved even under severe actuator degradation.