Marine Thrust Allocation Using Predictive Force Control
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Solution Overview
Problem
Traditional motion control systems for marine vessels face challenges in accurately distributing desired forces among thrusters, especially when using rotatable or azimuthing thrusters, leading to mismatches between desired and achieved forces and torques, particularly during aggressive maneuvers.
Innovation Solution
A motion control system comprising a higher-level motion controller and a lower-level thrust allocation controller, where the higher-level controller determines desired generalized forces and inputs them to the lower-level controller, which generates control signals for individual thrusters, taking into account both current and predicted future forces, and optionally considering propeller speed and pitch control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If traditional decoupled motion control systems are used, then the system is modular and easier to develop, but the control accuracy deteriorates due to limited capability to handle thruster constraints
Solution Approach 1:
The control system is divided into two hierarchical levels: a high-level motion controller that generates desired generalized forces and torques, and a low-level thrust allocation controller that distributes these forces to individual thrusters. This segmentation allows each level to focus on specific tasks while maintaining overall system modularity.
Solution Approach 2:
The high-level controller provides not only current but also predicted future desired generalized forces to the low-level controller. This preliminary action allows the thrust allocation controller to anticipate future constraints and plan thruster orientations in advance, improving control accuracy without sacrificing modularity.
2Reliability
If the bandwidth of the high-level controller is limited to avoid thruster orientation issues, then thruster reliability is improved, but the control performance deteriorates
Solution Approach 1:
The low-level thrust allocation controller uses predicted future desired generalized forces to determine optimal current thruster orientations in advance. This preliminary planning ensures thrusters are properly positioned before aggressive maneuvers, maintaining reliability while enabling high-performance control responses.
Solution Approach 2:
The system continuously monitors actual thruster states and orientations, using this feedback to adjust the thrust allocation strategy. This closed-loop control ensures thruster reliability is maintained while optimizing control performance based on real-time system conditions.
3Productivity
If combined motion control approaches are used, then control performance is improved, but the system complexity increases and modularity is lost
Solution Approach 1:
The control system maintains clear segmentation between high-level motion control and low-level thrust allocation, with well-defined interfaces. This modular architecture reduces system complexity while still achieving improved control performance through coordinated control strategies.
Solution Approach 2:
The interface between the high-level and low-level controllers acts as an intermediary, exchanging desired generalized forces and thruster state information. This intermediary layer enables coordinated control while maintaining the modularity and independence of each control level.
4Productivity
If aggressive maneuvers are commanded, then productivity is improved, but control accuracy deteriorates due to mismatch between desired and achieved forces
Solution Approach 1:
The low-level controller uses predicted future desired generalized forces to plan thruster orientations and force distributions in advance of aggressive maneuvers. This preliminary action ensures that thrusters are optimally positioned and forces are accurately distributed even during rapid maneuvers, maintaining both productivity and force accuracy.
Data Source
Figure 1~2A
Figure 2B~3
Figure 4
AI summary
A high-level motion controller (200) provides a lower-level thrust allocation (TA) (202) with generalized forces demanded on the ship (10) for the current instant of time as well as a prediction of future generalized forces τ^Dt=0T demanded over a predicted future time, and optionally one or more other reference predictions, such as a time-varying velocity and position of the ship η^D,ν^Dt=0TWhen predictions of the future desired forces and torque are made available to the lower-level thrust allocation, the lower-level thrust allocation will be aware of what is needed in the future and use the predictions to make efficient changes towards the desired future references, when it generates control signals (u) to thrusters available in the thruster system (100) to distribute the desired generalized forces into individual forces of the individual thrusters.