Fins stabilisers are devices which are fitted to the hull of a ship and utilised to ameliorate its
rolling motions. They apply a regulated moment about the ship's axis of roll in order to
oppose the sea induced disturbances. Recognising their unsurpassed performance, the Royal
Navy, since the 1950's, equips all its vessels with fin stabilisers. It can be shown that the
rudders, in vessels of appropriate size, also have the potential to be harnessed as roll
stabilisers Rudder Roll Stabilisation (RRS) without degrading the ship's course-keeping.
Thus creating a more stable platform for the human operators and equipment.
The reported success of RRS imparted an impetus to the Royal Navy to initiate this study.
The objectives are to ascertain whether RRS is possible without rudder modifications and to
establish whether enhanced levels of stabilisation would accrue if the fins and RRS were
operated in congress. The advantages in this novel approach being: avoidance of redesign
and refit of rudders, three modes of operation (fins alone, RRS alone and combined RRS
and fins), reduced fin activity and by implication self-generated noise, and amenability to be
retrofitted by simple alteration of any existing ship's autopilot software.
The study initially examined the mathematical models of the ship dynamics, defining
deficiencies and evaluating sources of uncertainty. It was postulated that the dual purpose
of the rudder can be separated into non-interacting frequency channels for controller design
purposes. An integrated design methodology is adopted to the roll stabilisation problem.
Investigating the capabilities of the rudder servomechanism, a new scheme, the
Anti-Saturation Algorithm (ASA) was proposed which can eliminate slew rate saturation.
Application of the ASA is generic to any servomechanism.
The effects of lateral accelerations of the ship on human operators was examined. This
resulted in an unique contribution to the Lateral Force Estimator problem in terms of
generating time domain models and defining the limitations of the applicability of a control
design strategy.
Linear Quadratic Guassian and two types of classical controllers were constructed for the
RRS and fins. A novel application of linear robust control theory to the ship roll
stabilisation problem resulted in H . controllers whose performance was superior to the
other design methods. This required the development of weight functions and the
identification and quantification of possible sources of uncertainty. The structured singular
value utilised this information to give comparable measures of robustness.
The sea trials conducted represent the first experience of the integrated ship roll stabilisation
approach. Experimental results are detailed. These afforded an invaluable opportunity to
validate the software employed to predict ship motion. The data generated from the sea
trials concurs with the simulations data in predicting that enhanced levels of roll stabilisation
are possible without any modification to the rudder system. They also confirm that when the
RRS is acting in congress with the fin stabilisers the activity of both actuators diminishes.
Date of Award | 1995 |
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Original language | English |
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Awarding Institution | |
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CONTROL TECHNIQUES APPLIED TO INTEGRATED SHIP MOTION CONTROL
SHARIF, M. T. (Author). 1995
Student thesis: PhD