Controller design and motion compensation for marine towed bodies

At sea, a vessel is subjected to waves. If the ship is towing a submerged body containing sensory equipment, then any ship motion at the surface can impart unwanted disturbances on the towed body via the tow line. To help compensate for these disturbances, a winch on-board the host vessel can be operated in response to the surface ship's motion by reeling in or reeling out the tow line. A corresponding method, however, is needed to determine how much cable the winch controller should pay in or out to effectively attenuate unwanted towed-body motion. This paper, therefore, proposes and explores four different approaches that, using various combinations of sensor measurements such as ship inertial measurement unit data and measured tow-line angle, can be used to try to establish appropriate winch control actions for motion compensation of marine towed bodies. Small-scale towed-body experiments using a spherical tow body as well as computer simulations are carried out to test the control approaches and, by analyzing the corresponding motion reduction achieved by each of these winch control strategies, the most effective method is identified.