  In this paper, a novel robust tracking control law is proposed for
constrained robots under unknown stiffness environment. The stability and the
robustness of the controller are proved using a Lyapunov-based approach where
the relationship between the error dynamics of the robotic system and its
energy is investigated. Finally, a 3DOF constrained robotic arm is used to
prove the stability, the robustness and the safety of the proposed approach.
