Full Envelope Nonlinear Controller Design for a Novel Electric VTOL(eVTOL) Air-taxi via INDI Approach Combined with CA

Suiçmez, Emre Can
On-demand urban air mobility (UAM) has become very popular in recent years with the introduction of the electric vertical take-off and landing (eVTOL) aircraft concept. Thanks to the key advantages of electric propulsion (e.g., very low noise and zero carbon-emission), short/medium range eVTOL "air-taxi" concept emerged as a feasible solution considering the requirements of the on-demand UAM. With this motivation, flight control problems of a novel eVTOL air-taxi are discussed and a unified flight controller is designed considering the full flight envelope. The air-taxi has a fixed-wing surface to have aerodynamically efficient forward flight, and uses only tilting electric propulsion units (i.e. the pure thrust vector control) to achieve full envelope flight control. The aircraft does not have any conventional control and stability surfaces such as aileron, elevator, rudder, horizontal/vertical tail. Therefore, the unified controller design becomes more challenging compared to the conventional aircraft configuration. The flight dynamics model of the air-taxi does not exist in literature since the air-taxi has a novel configuration. First, a preliminary flight dynamics model is generated using the component build-up approach for hover and high speed forward flight. Then, the hover and forward flight models are merged to simulate the transition dynamics. Two main challenges regarding the flight control are the severe nonlinearities in the flight dynamics during the transition flight and deterioration of the controller's performance in specific flight conditions due to the limited control authority (i.e., the actuator saturation). The first challenge is resolved via designing a sensor-based nonlinear controller for the entire flight envelope using the Incremental Nonlinear Dynamic Inversion (INDI) method. The INDI approach has improved robustness to modeling errors compared to the classical nonlinear dynamic inversion (NDI) methods. Therefore, the INDI based controller design fits very well to the problem considering the severe nonlinearities in the flight dynamics model. The INDI controller is formulated specifically considering the highly coupled pure thrust vector control approach. For the second problem, an online optimization-based Control Allocation (CA) algorithm is designed and integrated into the INDI controller. Resolving the actuator saturation related problems requires special attention due to the thrust vector control's coupled nature. The CA prioritizes the rotational channels over the translational channels to adequately allocate the limited control authority in case of actuator saturation. Various nonlinear simulation tests are performed considering the full envelope flight control, disturbance rejection characteristics at limited control authority and criticality of the CA design, robustness to model parameters, etc. Simulation results show that the controller has satisfactory performance, disturbance rejection characteristics, and significant robustness to the modeling errors. Moreover, it is observed that the CA plays a vital role in guaranteeing stable flight in case of severe actuator saturation.


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Citation Formats
E. C. Suiçmez, “Full Envelope Nonlinear Controller Design for a Novel Electric VTOL(eVTOL) Air-taxi via INDI Approach Combined with CA,” Ph.D. - Doctoral Program, Middle East Technical University, 2021.