Sliding mode control of a quad rotor helicopter using nonlinear sliding surface

In this paper, a sliding mode controller (SMC) based on a nonlinear sliding surface (NSS) is designed for controlling a quad rotor helicopter (quadrocopter). It is a well-known that a low overshoot can be achieved with a cost of longer settling time, although a shorter settling time is needed for quick response in a quadrocopter system. In the conventional SMC, the sliding surface is designed as a linear surface that provides a constant damping ratio. The value of damping ratio should be adjusted in order to obtain an optimal performance by making a tradeoff between the two criteria; overshoot and settling time. In this paper, an NSS is designed so that the damping ratio of the control system can be varied from its initial low value to a final high value in a finite time. A low value of damping ratio will cause a quick response, and the later high damping ratio will avoid overshoot, and therefore the control performance can be optimized. First, a dynamics model of a quadrocopter is presented. Next, an SMC with an NSS is designed for tracking control of a quadrocopter. The stability of the proposed control system is proved based on the Lyapunov stability theory. The effectiveness of the proposed design is verified by simulation in which comparative results with the conventional linear sliding surface (LSS) is shown. The NSS is more effective compared to the conventional LSS when the disturbances exist.