Mobile wheeled inverted pendulum mechanical models are benchmark underactuated dynamical systems with inherently unstable dynamics and pose a difficult challenge to control with desired accuracy. This paper seeks to achieve an efficient control scheme based on the notion of sliding modes to achieve the desired response under perturbations. In this work, a mathematical model of a two wheeled inverted pendulum mobile robot is considered and controller design is carried out without linearizing the model to ensure efficiency in large operating region. The development of the robust controller is carried out by selecting the crucial sliding parameters by minimizing the quadratic index and regular form design approach. The undesirable phenomenon of chattering in the control law has been eliminated by making smooth approximations using sigmoid function.