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Adaptive Ship Steering Characteristic

IP.com Disclosure Number: IPCOM000086612D
Original Publication Date: 1976-Oct-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 23K

Publishing Venue

IBM

Related People

Gaffney, JE: AUTHOR [+2]

Abstract

A problem with ship steering is the representation of the ship's characteristics in the "control model" (which is implemented by the autopilot) in such a manner that the "control model" adequately represents affects of both the environment (sea conditions, wind, etc.) and ship hydrodynamics (loading, speed, etc.) to enable accurate heading control to be maintained in a dynamic environment. Described is a ship autopilot algorithm whose parameters change in response to the variations in operational conditions.

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Adaptive Ship Steering Characteristic

A problem with ship steering is the representation of the ship's characteristics in the "control model" (which is implemented by the autopilot) in such a manner that the "control model" adequately represents affects of both the environment (sea conditions, wind, etc.) and ship hydrodynamics (loading, speed, etc.) to enable accurate heading control to be maintained in a dynamic environment. Described is a ship autopilot algorithm whose parameters change in response to the variations in operational conditions.

An existing autopilot algorithm calculates rudder commands (delta c) to cause the heading (psi) to adhere to a desired heading (psi d) selected by the operator. The control equation implemented is: (I) Delta(c) - delta(o) = A(1) (psi - psi(d)) A(2)psi where "delta(o)" is the value of "null rudder". That value of rudder (angle) required so that the yaw producing forces are approximately zero is psi - psi(d) = psi = o.

Earlier pilot functions assume that the controlled ship's dynamics in the yaw dimension are represented by: (II) psi = -C(delta(c) - delta(o)) = -C x R.

Equations I and II may be combined into an over "control model" given by:
(III) Psi CA(2)psi CA(1)psi = CA(1)psi(D).

In earlier work, it has been assumed that "C" is a constant (known as the ship's steering characteristic) and it is so represented in the control model. However, both theory and experimental evidence demonstrate that "C" varies as the square of the ship's speed. The theoretical value of "C" is: C = rho.l.s/2/ over J. where rho = specific weight of water acceleration of gravity

l = distance between ship's center

of gravity and its rudder

A = effective area of rudder

J = moment of inertia of ship about

vertical through center of gravity.

It should be noted that in the physical environment, the moment of inertia is affected by a coupling of the rotation about the horizontal axis (roll). It is dependent upon loading of the vessel and can be directly rela...