Study on Attitude Control of a Cabin-Suspended Catamaran by Using a Double-Loop Control System

Abstract

A series of cabin-suspended ships, named Wave Harmonizer, has been developed since 2008. The model ship consists of cabin part, hull part and conjunction part which is mounted in-between the cabin and the hull. The possibility and feasibility of introducing suspensions into small vessels are investigated. Effectiveness evaluations are made in two aspects: motion reduction of the cabin and wave energy harvesting through the oscillating cabin. According to the research results obtained in 2015 and 2016, it was found that the deck of the cabin may bear obvious inclination while weights were loading or unloading from it. Moreover, in relative long waves, the effectiveness of the motion reduction of the skyhook controller was insufficient. To solve those problems, an attitude control strategy is proposed. It is adopted as an outer-loop of the existing controller. Inclination sensors are employed to collect the rotational angle of the cabin in terms of pitch and roll. After finishing the design and construction of the double-loop control system, open loop tests are carried out in dry and wet conditions. The chain mechanism of the ship is investigated. Then a bench test is operated to validate the control concept and performance of the double-loop control system. Finally, tank tests are implemented to examine the inclination reduction of the cabin at regular head waves. In this paper, the development of the double-loop control system is described, experimental results are demonstrated with respect to the heave and pitch motion reduction of the cabin at the condition of with/without the outer control loop. It shows that the inclination reduction at the loading/unloading condition and the walking-on-deck condition are significant. However, in waves the effectiveness is not clearly verified. It suggests that the influence of the apparent gravity on the inclination measurement in waves should be investigated and solutions to accurately detect the inclination of the cabin should be sought.