Robust output-feedback orbital stabilization for underactuated mechanical systems via high-order sliding modes

The manuscript deals with the robust orbital stabilization for a class of disturbed Euler–Lagrange systems with one degree of underactuation. The proposed strategy relies on the virtual holonomic constraints approach, using incomplete state measurements. First, a high-order sliding-mode extended observer estimates the state and the disturbances affecting the input channel. Then, proposing a new set of coordinates, the so-called virtual holonomic constraints, a robust output partial-feedback linearization approach takes the system into a double integrator with a particular zero dynamics. Thus, considering the general integral of motion of the zero dynamics, the orbital stabilization is reduced to stabilize a linear time-varying system. The resulting control law is a continuous signal. Therefore, the robustness to disturbances is addressed without the tarnishing effects of chattering. The closed-loop stability analysis is done using the Lyapunov theory. The feasibility of the method is illustrated experimentally in a cart-pendulum system.