The control point concept for nonlinear trajectory-tracking control of autonomous helicopters with fly-bar

An alternative approach for automatic trajectory-tracking control of small unmanned helicopters with fly-bar is proposed. This approach uses the spatial three-dimensional coordinates of a point on the helicopter's local coordinate frame other than its centre of gravity, called the control point, and the helicopter's yaw angle as four control outputs. The helicopter is assumed to have four independent control inputs. With this choice of control outputs, the helicopter's input–output model becomes a square control system, which opens the possibility of implementation of many robust nonlinear control methods that are suitable for such systems. The helicopter, which has six rigid body degrees of freedom (DOFs), has two underactuated DOFs (UA-DOFs). It is proved that the zero-dynamics of the UA-DOFs are inherently stable, leading to a stable control system. A sliding mode controller is designed for trajectory-tracking of the outputs. It is verified via simulations that the response of the control outputs and UA-DOFs are in fact stable.