3.5 Selection of most relevant MRCs
Although a complete system will need to include enough MRCs to handle a lot of different situations, it is quite a lot of work. Therefore, to be able to detail each MRC and be able to incorporate these into a control system for testing, 2 MRCs are selected for further development. These are the following:
1. Crash stop
2. Evasive maneuver:
2.1. Evasive maneuver starboard 2.2. Evasive maneuver port
In this section, each MRC is detailed into functional demands for guidance and control. Relevant causes that trigger them are also given.
3.5.1 Crash stop
This MRC is activated when the situation demands that the vessel comes to a full stop as fast as possible.
This can for example be activated if the vessel is on a collision course with another vessel or obstacle, and the guidance function cannot find a feasible path to avoid it. It should also be activated if the vessel is on course to enter a dangerous area. If the system encounters a navigational conflict such that no safe path to the destination can be planned, this should also be activated. This can for example happen if the traffic picture suddenly gets very busy. Lastly, a crash stop should be performed if the vessel deviates from the desired path by a distance larger than a set safety margin.
For a ship with azimuth propellers, a crash stop can be done by turning the propellers 180°, or by reversing the rotation of the propellers. A third method is to point the azimuths30° and reversing the propellers. This utilizes the drag of the azimuth unit in addition to the thrust produced by the propellers.
The effectiveness of these procedures is investigated by Nowicki (2014) and Park et al. (2020). They find that turning the azimuths around and turning the azimuths outward is the most effective method, respectively. However, these experiments were done for large ships (277 mand70 m) with two azimuths placed at the stern. For the smallest ship, reversing the propellers is almost as good as turning them outward (6% longer stopping length). Ultimately, the best solution is dependent on the specific vessel.
Therefore, the method where the propeller revolutions are reversed is assumed to be sufficient here. This is relatively simple to implement and seems to be quite effective. Then it must be noted that for a vessel that will be put into operation, tests should be performed to find the best method.
For this mode, the guidance layer does not need to provide a desired position. The output of the controller block in this mode should then be full throttle in the opposite direction of the velocity as long as the vessel has a forward speed. After the vessel has come to a stop, the system can transition to station-keeping until the situation is cleared.
3.5.2 Evasive maneuver
An evasive maneuver should be performed when the nominal COLAV system has failed or an obstacle has not been detected in time to be avoided, such that a collision is imminent. The result should then be
a hard turn to either port or starboard. The new heading after the turn should be sufficient to avoid the obstacle. To relate this maneuver to classical ships, an evasive maneuver would entail that the captain turns the rudder hard to either port or starboard. milliAmpere does not have a rudder, so this behavior must be mimicked for the fully-actuated case. During the maneuver, the speed should be ramped to zero.
The maneuver is complete when the vessel has obtained the new heading and stopped. Then the vessel can transition into stationkeeping until the situation is resolved.
To complete this task, the guidance function must provide a heading reference that takes the vessel into a turning circle maneuver, and keeps the heading constant when the heading has changed enough.
Additionally, a reference speed must be provided, that gradually ramps to zero at the end of the maneuver.
A controller must be in place to control the heading and speed to the desired references.
Chapter 4
Problem formulation
The overall goal of this thesis is to develop a control system capable of mode switching between nominal operation modes and emergency modes. In this chapter, the problem is formulated as functional demands for the subsystems that are treated in this thesis. Two subsystems are in focus; the mode supervisor and the motion control system. The proposed system architecture including these two systems can be seen in Figure 4.1. The details of the red blocks are not considered in this thesis, while the green boxes are in focus. Therefore, three subproblems are specified: the supervisor problem, the guidance problem and the control problem. At the end of the chapter, the limitations and assumptions used during the thesis work are given.
Figure 4.1:Proposed system architecture.
4.1 Mode supervisor problem
To switch between the modes described in Chapter 3, a mode supervisor is to be implemented. This will be integrated with the rest of a conventional GNC system as shown in Figure 4.1. As seen, the mode supervisor will interact with the rest of the GNC system. It will take input from the following modules:
• Situational awareness: from the situational awareness/sensor fusion module, information about the surroundings in terms of the traffic and weather situation.
• Motion control system: from the motion control system, the mode supervisor receives informa-tion on whether a collision-free path can be constructed or not.
Based on the information received, the mode supervisor should determine which mode that should be used. The alternatives are three NCMs (undocking, crossing, docking) or three MRCs (crash stop, evasive maneuver port, evasive maneuver starboard). The supervisor shall run continuously, but measures have to be taken to avoidchattering, which is the problem when a signal is switching back and forth rapidly.
Therefore, when an MRC is entered and nominal operation is interrupted, the MRC should be finished before another switching is considered.
The output from the mode supervisor should be which mode the system should operate in. This signal is fed to the motion control system, which in turn will implement the corresponding changes. The working method of the affected subsystems in each mode will be formulated in the following sections.