Access Type

Open Access Thesis

Date of Award

January 2025

Degree Type

Thesis

Degree Name

M.S.

Department

Mechanical Engineering

First Advisor

Nabil Chalhoub

Second Advisor

Valery Pylypchuk

Abstract

Emergency towing vessels are a vital component of maritime safety. This study works to improve on previous works in the area of automating the towing of disabled ships. In doing so control methods for transient three degree of freedom dynamics of a tug and tow system which is connected with a finite element cable are evaluated. A Heaviside unit step function is inserted to void both the elastic and viscous interaction between the massive elements whenever the distance between two neighboring masses appears to be below the corresponding undeformed distance. This follows the representation of the cable as a one-dimensional structure whose segments cannot hold compressive deformations. The system is influenced by environmental forces which include sea current and wind. The control strategies utilize two different error signals that pass through a PID controller. The first phase evaluates a tension control error which incorporates the tension in the segment attached to the tug and the desired tension signal which follows an underdamped second order linearly damped oscillator profile up to an initial desired speed. The second control phase evaluates two different methods of velocity control. The first method uses the speed of the tug in comparison to a smooth desired speed function that ramps up from the initial desired speed to a defined cruising speed while also evaluating the tension in the segment attached to the tug. In doing so the segment tension is passed through a smooth sigmoidal function that evaluates the tension with respect to the desired segment tension. The output of this function increases or decreases the desired velocity of the tug to either protect the cable or increase cruising speed. The second method utilizes a weighted control error which sways the control signal between tracking and tension control to prevent the cable from over tensioning. The tension controller reduces the tension spikes in the cable while maintaining optimal control of the ensemble. The tension zoning velocity controller is effective at mitigating rises in tension at the cost of speed during increased resistance time periods. The weighted control error is effective at maintaining desired speeds while incorporating a tension failsafe in the event of excessive tension. The results show the effectiveness of tension control and the incorporation of this in the tracking control phase as well as the ability to maintain controllability in the event of tension loss.

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