Robust disturbance compensation for a duocopter by gain-scheduled nonlinear control and observer design

A cascaded control strategy for an innovative Duo-copter test rig - a helicopter with two rotors combined with a guiding mechanism - is presented in this paper. The guiding mechanism consists of a rocker arm with a sliding carriage that enforces a planar workspace. The Duocopter is attached to the carriage by a rotary joint and offers 3 degrees of freedom. The derived system model has similarities with a planar model of a quadrotor but involves additional terms due to the guiding mechanism. A gain-scheduled cascaded control strategy using extended linearisation is proposed: an outer MIMO control loop is responsible for the nonlinear control of both the horizontal and the vertical Duocopter positions, whereas the rotation angle of the Duocopter is controlled in a linear inner control loop. An additional feedforward control takes into account known parts of the coupling forces between the carriage and the rocker. The control structure is extended by a sliding mode observer (SMO) that provides estimates for the state vector and, moreover, estimates for remaining errors concerning the feedforward coupling forces. The sum of the feedforward part and the estimated part can be used to robustly and accurately compensate for the impact of the guiding mechanism on the motion of the Duocopter frame. Thereby, an excellent tracking performance in vertical and horizontal directions can be achieved. The efficiency of the proposed control strategy is demonstrated by experiments.