Agile and Precise Attitude Control of Tiltrotor Aircraft in Transition Flight

A tiltrotor aircraft (TRA) can both hover in the air (hover flight state) and cruise quickly (level flight state). Transition flight is a transitional state between these two flight states, during which tiltable rotors rotate, forming a unique state for TRA. Revolving of the tiltrotor shaft leads to strong nonlinearity and time-varying uncertainty, which weakens the stability of attitude control with linearization in transition flight. The safety of TRA is critical, but it can be easily harmed due to the weak control stability. This study is to regulate the TRA attitudes agilely and precisely simultaneously, by ensuring fast convergence speed and small convergence error, i.e., a prescribed transient and steady state performance (PTSSP). A constraint-based adaptive robust prescribed performance control (CARPPC) is proposed for TRA to follow the desired attitude while guaranteeing PTSSP, with no approximations or linearizations invoked. The desired attitudes and PTSSP are respectively described as equality and inequality servo constraints. A constraint-based control with state transformation is introduced to render both equality and inequality constraint-following. An adaptive law is established for online estimation of unknown uncertainty bounds to compensate time-varying uncertainty. Both theoretical proofs and simulation results demonstrated that CARPPC can realize agile and precise attitude control robustly in transition flight.