Ships – Steering mechanism – Remote control steering excluding manual operation
Reexamination Certificate
1999-04-02
2002-09-17
Morano, S. Joseph (Department: 3617)
Ships
Steering mechanism
Remote control steering excluding manual operation
C114S14400A, C114S1440RE, C700S028000, C701S021000
Reexamination Certificate
active
06450112
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for the automatic positioning of a vessel, by which the vessel can be moved, by propulsion means, in the direction opposite of the main external disturbance, or position deviation, forces. And more particularly, the present invention relates to an improved thruster allocation logic for automatic positioning of the vessel, and specifically, to applying numerical optimization methods to the problem of force and moment allocation in vessel position and rate control.
BACKGROUND OF THE INVENTION
In order to perform the dynamic positioning of the vessel there must be provided propulsion means, which act to hold the vessel accurately at a working position. Proper commands to the propulsion means must be determined with a combination of control laws and thruster allocation logic. Examples of the types of vessels involved are aircraft, spacecraft, submarines, surface ships, and other such vessels. The propulsion means include jet propulsion, rocket propulsion, propellers, adjustable propellers, screws and rudders. It is also possible to combine transverse thrust systems with active propellers.
When a vessel is being dynamically positioned, the vessel is frequently placed with its bow facing in the direction of the resultant of the disturbance forces since in this position it will have the lowest wind, water and/or wave resistance, i.e. the external influences will apply the lowest force levels to the vessel. Another method for setting the required vessel heading is described in U.S. Pat. No. 4,089,287 to Kranert, et al., which discloses an automatic method and apparatus for setting the required vessel heading to minimize the influence of external disturbance forces. Position or rate control of such vessels as aircraft, surface ships, underwater vehicles and rockets has been achieved using a variety of control laws including Proportional/Integral/Derivative, H-infinity, and nonlinear sliding mode controls. All control laws have common inputs and outputs.
FIG. 1
illustrates the basic components and relationships involved in current state of the art vessel position control. Desired Position Command Source
101
, generates the required vessel positions. The required position inputs can come from an operator, an automatic path planner, or other external sources. Inputs to the Control Laws Logic
102
include required position or rate for the degrees of freedom to be controlled. The other required input to the control laws is feedback on the state of the vessel from Position/Rate Measurement
108
, such as Differential Global Positioning Systems, hydro-acoustic positioning systems, and Kalman filters.
Outputs of the Control Law
102
logic are the force or moment in the controlled degree of freedom required achieving the desired position or rate. These forces and moments are usually generated without regard to the vessel's capability to generate them. Output of External Forces Detector
103
, for determining disturbing forces and moments, may supplement the required control forces and moments in a Feed Forward
104
. Examples of disturbing forces include, but are not limited to wind, wave, current, gravity and manual thruster commands, and may be detected with a variety of sensors and operator inputs. The Feed Forward
104
may be simply accomplished by addition of forces and moments that compensate for the disturbing forces and moments to the required Control Laws
102
forces and moments via electrical circuit or numerically in a computer. These disturbing forces are detected by sensors and/or inputted to the computer by an operator. The combination of Control Law
102
, External Forces
103
, and Feed Forward
104
comprise force setting means and moment setting means. The total required forces and moments are then allocated to the available effectors in Thrust Allocation Logic
105
before commands pass to the Force Generating Effectors
106
, which generate forces and turning moments that act on Vessel
107
.
Until now, the allocation of the forces and moments required to control vessel position to the available vessel Force Generating Effector
106
, such as thrusters and rudders, has been synthesized with complicated, highly structured logic. Each new configuration of effectors has required customization of computer software to calculate the best set of commands to the effectors that achieve the required forces and moments on the vessel while observing constraints on the effectors and power availability. Often, there is an infinite number of possible effector command sets that can achieve the required net forces and moments on the vessel. Conventional Thrust Allocation Logic methods do not always select the optimal solution. In some cases, no command set is found that achieves the required forces and moments and the vessel position control is compromised. This may be due to physical limitations of the available effectors or due to failure of the Thrust Allocation Logic to find an existing feasible set.
The set of required forces and moments determined by the Control Laws Logic
102
and Feed Forward
104
can usually be achieved in more than one way. Consider a relatively simple system of a surface ship with two main longitudinal propellers each with its own rudder. There are four independent commands to be determined, one for each effector. Suppose that only three degrees of freedom are to be controlled: fore/aft, port/starboard, and heading. The sum of forces from the two main props must equal the required fore/aft force (ignoring drag forces on the rudders). Rudders acting in the flow of the main propellers must provide the required port/starboard force. The moment required to control heading must be met by the differential forces of the main propellers acting across their lateral (port/starboard) separation plus the moment due to the rudders and their longitudinal (fore/aft) separation from the vessel center of rotation. The four unknowns (commands) are therefore under-specified by the three governing equations.
In some vessel configurations, a fourth equation can be specified, such as a requirement to minimize thruster power, and the four equations are solved simultaneously to determine the necessary effector commands. This is satisfactory only if the fourth equation is the appropriate one for the application and the resulting set of equations can be solved, but that is often not the case. In general, the goals for the allocation of control forces do not lend themselves to simple mathematical solution.
In addition to meeting the required set of control forces, there are frequently other allocation goals to be achieved. These goals include: minimum change from current set of commands; minimum power usage; minimize the maximum effector command; minimize the sum of the squares of the effector commands; minimize the difference between the minimum and the maximum effector commands; and establish preferences for use of one set of effectors over another set.
If the required set of control forces can not be achieved, then it may be desirable to come as close as possible to such control forces, sacrificing control in some degrees of freedom in favor of others. While maximizing performance goals, or minimizing penalties, there may be allocation constraints on the solution. They can be equality constraints or inequality constraints, and they can be linear or nonlinear in the control variables. The most obvious constraints are simple bounds on the allowable commands. Other constraints on an individual effector might include: minimum levels, due to clutching, stiction, or other mechanism; unallowable command regions, such as thruster wash angles spoiling hydrophone sensors or other thrusters, critical shaft speed avoidance, etc.; and minimum thruster level to reduce azimuth control chattering.
Other constraints affect multiple effectors simultaneously, such as maximum total power levels and minimum total power levels required, for example, to meet minimum generator power loading. The requirement to meet the required cont
Erickson Don E.
Morano S. Joseph
Nautronix, Inc.
Vasudeva Ajay
LandOfFree
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