In this paper, we propose to improve the robustness of the well-known PD controller through an adaptable disturbance observer. The proposed disturbance observer is based on the Super-Twisting Algorithm, and is designed with adaptive gains. The stability analysis of the resulting closed-loop controller/observer is achieved using Lyapunov arguments. Real-time experiments in different operating conditions are conducted to demonstrate the effectiveness and robustness of the proposed methodology, compared to the nominal PD control.