This paper addresses the trajectory tracking problem for a wheeled mobile robot (WMR), considering the presence of sliding effects that violate the nonholonomic constraints. Using the singular perturbation approach, the unicycle robot is modelled as a dynamic system which is referenced to the trajectory to be tracked. Indeed, due to the transgression of the nonholonomic constraints, the dynamic model becomes time varying and highly non linear which impose the use of a robust stabilizing control. For this purpose, we propose a solution based on a second order sliding mode control, ensuring the asymptotic convergence of the unicycle about the reference trajectory. Simulation results are reported showing the efficiency of the proposed control with respect to the non ideal constraints and to the nonlinearities.