Thrust-coefficient-coupled extended state observer for robust control of a coaxial omnidirectional tiltrotor aircraft

A coaxial omnidirectional tiltrotor aircraft (COTA) exhibits strong coupling among thrust generation, attitude dynamics, and external disturbances, particularly during aggressive maneuvers and physical interaction tasks. A conventional extended state observer (ESO) typically treats the commanded control wrench as its known input. However, this assumption becomes problematic for COTAs because the actual control wrench depends on actuator states and thrust coefficients and may deviate significantly from the commanded input due to actuator dynamics and thrust-coefficient variations. To address this input mismatch, this paper proposes a thrust-coefficient-coupled extended state observer (TCESO). The TCESO reconstructs the observer-side input in real time from measured actuator states together with an online estimate of the thrust coefficient. This design ensures that the observer is driven by a term that more closely approximates the actual physical input, thereby yielding a more accurate internal model. To improve robustness and avoid dual disturbance compensation, a decoupled strategy is adopted: the instantaneous thrust-coefficient estimate is used exclusively within the observer, whereas a filtered and subsequently frozen value identified during takeoff is used for control allocation. Real-world flight experiments, including aerodynamic-disturbance rejection, human–robot interaction, and physical interaction, demonstrate that the proposed TCESO framework provides better or more balanced performance than both a baseline SE(3) controller and a conventional ESO-based controller, with the clearest advantages appearing in attitude-critical and interaction-rich scenarios.