Simplified trawl system modeling and design of a depth control system using fuzzy logic

Abstract

This paper describes a simplified mathematical model of a mid-water trawl system developed to help and understand the dynamic behavior of trawl, and to design the depth control system of the mid-water trawl operation. The equations of the motion for the vertically moved trawl system were obtained using Lagrange’s equation with respect to the variables of the displacement of the trawler, the length of the trawl warp, the angle of the warp to the horizon, and the angle of the hand rope to the horizon. These equations were modified as simplified types. A fuzzy logic controller was designed to control the depth of a mid-water trawl net. The fuzzy controller calculates the tension of the trawl warp to be changed, based on the depth error between desired depth and actual depth of the trawl net and the ratio of changes in the depth error. The error and error change are calculated every second of sampling time. The control input, i.e., the desirable tension of the trawl warp is determined by inference from the linguistic control rules, which is usually carried out by crew experienced in controlling the depth of the trawl net during manual operation. Computer simulations indicated that the calculated motion of the trawl system using the simplified mathematical model agreed reasonably with those of the original model and actual trawl system obtained from field experiments, and the proposed fuzzy controller rapidly followed the desired depth without steady-state error.