Robust nonlinear control of a 7 DOF model - scale helicopter under wind gusts using disturbance observers

Nowadays, high levels of agility, maneuverability and capability of operating in reduced visual environments and adverse weather conditions are the new trends of helicopter design. Helicopter flight control systems should make these performance requirements achievable by improving tracking performance and disturbance rejection capability. Robustness is one of the critical issues which must be considered in the control system design for such highperformance autonomous helicopter, since any mathematical helicopter model, especially those covering large flight domains, will unavoidably have uncertainty due to the empirical representation of aerodynamic forces and moments. Recently the control problems of unmanned scale helicopter have been attracted extensively attention of control researchers. As the helicopter can hover, it is used to implement many important flight missions such as rescue, surveillance, security operation, traffic monitoring, etc. However, helicopter, which is difficult to hover, is more complicated than other familiar control objects. Helicopter is dynamic unstable when it flights in hover mode at nearly zero forward speed. Moreover, the helicopter is open-loop unstable and most mathematical model contain a moderate-high degree of uncertainty models associated with neglected dynamics and poorly understood aeromechanical couplings. Therefore, it is very important to design a stable controller for unmanned helicopter. Many previous works focus on (linear and nonlinear, robust, ...) control (Beji and Abichou, 2005) (Frazzoli et al., 2000) (Koo and Sastry, 1998), including a particular attention on the analysis of the stability (Mahony and Hamel, 2004), but very few works have been made on the influence of wind gusts acting on the flying system, whereas it is a crucial problem for out-door applications, especially in urban environment : as a matter of fact, if the autonomous flying system (especially when this system is relatively slight) crosses a crossroads, it can be disturbed by wind gusts and leave its trajectory, which could be critical in a highly dense urban context. In (Martini et al., 2008), thw controllers (an approximate feedback control AFLC and approximate disturbance observer AADRC) are designed for a nonlinear model of a 7 DOF helicopter using in its approximate minimum phase model. In (Pflimlin et al., 2004), a 3