Event-triggered ADRC-MFAC for intelligent tugs in escort operations with a thrust re-allocation strategy

Abstract

Compared to cargo ships, tugs have powerful propulsion, flexible maneuvering, high degree of automation and operational efficiency, therefore, intelligent tugs with high autonomy have received increasing attention worldwide. When a large cargo ship enters a port, one or more tugs are assigned to perform escort operations to follow the ship. To automate this process, this paper proposes an event-triggered control method to address the escort control problem of autonomous tugs. It combines model-free adaptive control (MFAC) and active disturbance rejection control (ADRC) with an event-triggered mechanism to optimize control frequency with stability guarantees. The coupled effects between surge and sway movements are treated as unknown disturbances, allowing decoupled control of surge speed and heading angle. Considering the deterioration of rudder efficacy at low propeller revolutions, this paper also proposes a model-free thrust re-allocation strategy, which enhances its applicability and adaptability to other tugs with varying specifications. Model-scaled simulations and experiments, as well as full-scale simulations on marine simulators have been conducted to validate and evaluate the proposed method.