<p>A unified controller for trajectory tracking and set-point regulation/stabilization of nonholonomic wheeled mobile robots is designed by using the hierarchical idea popular in unmanned aerial vehicles. As a preliminary, a smooth function for solving the argument of a rotating vector is obtained by switching between discontinuous functions, which motivates the definition of a novel function atan3. The hierarchical control is introduced to wheeled mobile robots, which includes an attitude planner based on atan3, nominal full-actuated position control, and planner-based attitude control, such that the exponential stability of the closed-loop system is achieved. It is easy to extend the control of nonholonomic wheeled mobile robots (WMRs) with unknown parameters and disturbances. The simulation results demonstrate the effectiveness of the control scheme and the necessity of introducing the function atan3.</p>