Formation Control of Multiple Autonomous Underwater Vehicles Under Communication Delays

Time-delay owing to the communication delay in acoustic modem greatly influences the path following performances when a group of Autonomous Underwater Vehicles (AUVs) are deployed in formation. When the AUVs are employed in oceanic environment, the distance between the AUVs keeps on changing due to wave disturbances. Hence, time-delay which depends on the distance between AUVs also varies. In this brief, a time-delay estimator based on gradient-descent method has been designed to estimate the communication delays. Taking the estimated delay into account, the delayed error vectors are computed to design a suitable control input. As the reduction of Lyapunov function incorporating communication delay occurs on-line at each sampling interval, the path following control performance of follower AUV improves significantly. The simulation and experimental results show that the delay estimator accurately estimates the delay. Moreover, during experiment, the actual delay is significantly reduced as the time-stamping of packets of leader AUV states is avoided during formulation of the estimator. The follower AUV even under the effects of variable communication delay, maintains a desired distance from the leader AUV. The results also show that by imposing constraints on the control input, actuator saturation is avoided.