Pergamon 0967-0661 (95)00003-8 ControlEng. Practice, Vol. 3, No. 3, pp. 321-328, 1995 Copyright © 1995 Elaevie~ ScicmceLtd Printed in Great Britain. All rights rmea'ved 0967-0661/95 $9.50 + 0.00 THE IMPLEMENTATION OF FIXED RULEBASE FUZZY LOGIC TO THE CONTROL OF SMALL SURFACE SHIPS M.N. Polkinghorne*, G.N. Roberts**, R.S. Burns* and D. Wlnwood*** *University of Plymouth, Drake Circus, Plymouth, UK **Gwent College of Higher Education, Newport, UK ** * M arinex Industries Ltd, Factory Road, Upton, Poole, UK (Received April 1994; in final form August 1994) ABSTRACT. For ship control, the Proportional plus Integral plus Derivative (PID) controllers remam common-place. However, increasingly new autopflotstrategies,promisinghiaher levels of robustaess and/or adaptive qualities, are being proposed as possible successors to the PlD. Fuzzy logic is a modem control technique which is ctmrently finding an increasingand diverse range of novel applications, both in its fixed.rulebase and intelligent forms. By means of full scale sea- trials, a newly developedfuzzy logic autopilot is evaluatedfor both course-keepingand course- changing, and a comparison made to its conventional equivalent. Key Words. Fuzzy Control;Marine Systems; Ship Control; Autepilot;Small Vessel. 1. INTRODUCTION Since marine vessels are non-linear, time-variant systems, any changes in speed, water depth or mass loading may cause a change in their dynamic characteristics. The severity of the weather will also alter the magnitude and direction of any disturbance effects caused by the wind, waves and current. The problem of antopilot control for such vessels is therefore inherently difficult. This is particularly so in the case of small craft whose sensitivity to incorrect control action is accentuated by their responsiveness to helm adjustments. Small vessels may be considered to be those of less than thirty meters in length and could be for commercial or leisure usage. Due to the small draft of the type of ship considered, when the external environmental disturbances are applied to the hull, the low inertia present creates little resistance to the induced heading change. The autopilot performance must therefore be swift and decisive to counter any such effects by employing an opposing rudder condition. For any successful autopilot design, it is a necessity that the obtainable level of performance must be either robust and relatively insensitive to the alterations in vessel dynamics and external disturbance factors, or alternatively, must be adjustable by the mariner on demand. In practice the latter has been proven to be unsuccessful due to a general inability of mariners to fully understand the consequences of their actions when presented with a range of tuneable parameters. The resulting performance levels in such cases are normally still inadequate. The result of poor controller performance may be an oscillatory down-track course which increases distance and therefore trip time and fuel consumption. Wild and undesirable rudder movements may be produced, which not only cause excessive wear to the rudder mechanism, but also use unnecessary power. The latter is of particular importance when considering sail vessels whose power is often limited to a battery supply. In the field of large ships, various modern control techniques have been applied to large ships in an attempt to improve antopilot performance over the entire operating envelope: I-I °° (Fairbairn and Grimble, 1990), Optimality (Burns, 1990), Self- Tuning (Mort, 1983), Model Reference (van Amerongen and Unink Ten Care, 1975), Neural Networks (Hndo at al., 1989) and Fuzzy Logic (van Amernngen et al., 1977). In the case of small vessels there has been little research of this kind. Previous studies by the authors (Polkinghorne et al. 1992, 1993) have demonstrated the scope for fuzzy logic control in this area. The accepted robust qualities of the fuzzy technique and its ability to be 321