Virtual Instrumentation Based Active Fin Control for Roll Stabilization

Journal of Naval Architecture and Marine Engineering December, 2005 http://jname.8m.net VIRTUAL INSTRUMENTATION BASED ACTIVE FIN CONTROL FOR ROLL STABILIZATION V. Anantha Subramanian1, G. Asokumar1 and Thaju Mohamed2 1 Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai- 60036, India. Ph. 0091-4422574812, e-mail: , 2 Department of Electrical Engineering, Indian Institute of Technology Madras, Chennai-60036, India Abstract The objective of this work is to develop a non-traditional strategy for providing control of ship roll motion in a seaway using active fins. This is based on a multidisciplinary approach. It consists of the assessment of the hydrodynamic coefficients of a ship using a numerical package [SEDOS], representation of the hydrodynamic coefficients by suitable polynomial functions, identification of the time dependent fundamental frequency in the roll disturbance signal sample, generation of control signal for an appropriate fin angle based on the fin lift characteristics of the chosen fin and employment of feedback control to maintain the fin at the appropriate angle for maximum control. Once the ship based hydrodynamic coefficients are generated, the entire control algorithm is developed in the virtual instrumentation mode using LabVIEW environment. One of the disadvantages of the traditional stabilizer system is the enormous hardware involved in the controllers and instrumentation panels, which makes maintenance and troubleshooting difficult. With the help of virtual instrumentation, design of the stabilization controller and instrumentation panels can be carried out much more effectively, in comparison with the traditional hardware approach. Key words: Roll stabilization, Active fin control, Virtual instrumentation NOMENCLATURE: αt θ C.G A B C CL CLα V Sf ρ ωe Angle of attack , deg Angle encountered by fin with hull vertical plane, deg Centre of Gravity, m Roll virtual mass moment of inertia, t-m2 Roll damping coefficient, t-m2/sec Roll restoring moment coefficient, kN-m Coefficient of lift Lift slope coefficient, rad-1 Ship speed, m/s Fin profile area, m2 Density of seawater, t/m3 Encounter frequency, rad/s 1813-8535 © 2005 ANAME Publication. All rights reserved. V. Anantha Subramanian, G. Asokumar & Thaju Mohamed / Journal of Naval Architecture and Marine Engineering 2(2005) 11-24 1. Introduction Motion stabilization devices are often adopted in vessels in order to reduce their motions in a seaway. The main oscillations of heave, pitch and roll are the principal reasons for limiting the operability of a vessel, and also for causing adverse influence on human comfort and habitability. Devices used for stabilization may be passive or active. Any successful device requires combined consideration of the hydrodynamic forces as well as control engineering. With the help of stabilizers, the amplitude, rate and acceleration of the motion as well as some dynamic effects i.e., deck wetness and slamming can be reduced considerably. There are three well known ways to reduce forced motions. They are damping stabilization, tuning stabilization and equilibrium stabilization. The present work is based on the principle of equilibrium stabilization with active stabilizer fins using virtual instrumentation based control. The term “virtual instrumentation” is used to represent a PC based control system which is used to acquire data from physical transducers and then manipulate them in specific ways using a very high level graphical environment. In the graphical environment, symbolic icons are used that operate in the same way as real instruments do. A non-traditional strategy is presented to provide motion control for ship roll motion using fins. The basic ship motion characteristics are obtained by means of a prediction program (SEDOS). Using the fin performance characteristics, the countering forces required and the consequent fin movement are achieved in a control loop. One of the disadvantages of the traditional stabilizer system is the enormous hardware involved in the controllers and the instrumentation panels, which makes maintenance and troubleshooting difficult. With the help of virtual instrumentation, the design of the stabilization controller with virtual panels can be performed with a great deal of flexibility and effectiveness. In this work, the control has been designed and implemented using virtual instrumentation in the LabVIEW environment. 2. Motivation Fin stabilizers have been used for motion stabilization typically in passenger ships and warships. Conolly (1968) developed a linear theory to predict rolling motions under the action of active stabilizers. Lloyd (1974) made comparisons of Conolly’s prediction with laboratory model measurements and quantified the interaction coupling effects between roll, sway and yaw motions. Gunsteren (1974) developed a method for the design of stabilizer including the dynamic effects of the fins and the ship. Tsuyoshi et al. (1994) investigated the influence of fin area and the control method on the reduction of roll with the fin stabilizer. Sgobbo and Parsons (1999) studied the effects of the rudder and fins on the rolling motion of the ship using three degree of freedom (3-DOF) model. They presented a set of equations which give the effect of fins and rudder on the damping matrix and the roll added mass moment of inertia matrix. Significant roll reduction can be achieved using the MIMO rudder/fin controller. Samoilescu and Radu (2002) presented the control engineering aspects of a fin system which takes into account roll velocity, roll acceleration, roll angle, natural list and ship speed in a feedback control system. Virtual instrument based control and measurements are being increasingly used as versatile, flexible technique [Rahman and Pichlik, 1999] in many control engineering applications. In the light of the above developments and based on the hydrodynamic coefficients of the ship and the fins, a control algorithm is developed. Based on the input from a roll angle sensor and knowledge of ship hydrodynamic coefficients as well as fin lift characteristics, programming has been undertaken for virtual instrumentation in the LabVIEW environment. Therefore in place of involved hardware instrumentation and circuits for achieving feedback control, a compact software based control is presented. The virtual instrumentation integrates the hydrodynamic characteristics of the ship with correct choice of fin angle in a feed back control loop. 3. Analytical Motion Prediction and Motion Control Equation A motion analysis package namely SEDOS [Soeding, 1988] has been used for obtaining the ship hydrodynamic coefficients for motion prediction. The method is based on strip theory. The program performs calculation for the motion analysis of ship in regular waves and in natural, stationary seas. Most strip theory methods do this by solving the boundary value problems for the velocity (...truncated)