Quantized Feedback Sliding Mode Control for AUV Based on Nonlinear Disturbance Observer

In a practical engineering application of autonomous undersea vehicles(AUVs), state variables must be quantified and transmitted to a controller because of a digital loop. Consequently, the stability of the system is significantly affected. To achieve AUV trajectory-tracking control, a quantized feedback sliding mode control(SMC) method based on a nonlinear disturbance observer(NDO) is proposed by considering the influence of state and control input quantization. First, NDO is used to estimate and compensate for external and unknown interference to suppress buffeting, which is a common step in traditional SMC. Then, an SMC scheme is proposed, in which the state variables and the upper bound of the quantization error of the input are added to the switching term of the SMC to overcome the influence of quantization on the stability of the system. In the first step, the trajectory is proved to be driven to a small strip area regardless of the initial value. In the second step, the sliding surface, s, converges to zero by dynamically adjusting the quantization parameter. Finally, the stability of the system is proved by applying the Lyapunov function. The simulation results show that the designed quantized sliding mode tracking controller allows the AUV to successfully track the expected trajectory