Modelling of flexible slender systems for real-time simulation and control applications

Abstract

The main contributions in the present thesis are the development of two mathematical models describing the motion and forces in slender mechanical systems like cables. The main advantage by the models proposed is the possible real-time simulation performance which often is lacking in traditional model formulations. The main problem is often twofold; inversion of a full matrix for each time step and high mechanical and numerical stiffness. High material stiffness and high wave velocity in the longitudinal direction is by far the most common reason for the stiffness problem, and this requires very short time steps. For many applications the axial dynamics is of minor interest. Thus, the solution may be to separate the transversal and longitudinal dynamics. Both models proposed enables separation of dynamics, and one does not require matrix inversion.

Real-time models for cable systems are useful for both controller design and observer design. In the present study a new control concept for interconnection maneuvering is proposed. A towed body is controlled via the forces in the interconnection. These forces are adjusted by active positioning of the towing vessel. Long and non-stiff connections may require very high bandwidth in the towing vessel’s motion, and this is not always possible to fulfill. The proposed solution to this may be to introduce local actuators on the towed body to adjust the control forces.

Trawling is an application where the cable models and control concepts proposed may be used in future development. This application was the basis for starting this study, and the results may enable further industrial development of this technology.