  In this paper the stability of a closed-loop cascade control system in the
trajectory tracking task is addressed. The considered plant consists of
underlying second-order fully actuated perturbed dynamics and the first order
system which describes dynamics of the input. The main theoretical result
presented in the paper concerns stability conditions formulated based on the
Lyapunov analysis for the cascade control structure taking advantage of the
active rejection disturbance approach. In particular, limitations imposed on a
feasible set of an observer bandwidth are discussed. In order to illustrate
characteristics of the closed-loop control system simulation results are
presented. Furthermore, the controller is verified experimentally using a
two-axis telescope mount. The obtained results confirm that the considered
control strategy can be efficiently applied for mechanical systems when a high
tracking precision is required.
