Manual Ship Motion Control: Course Keeping and Roll Stabilisation Using Rudder and Fins

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Perez, Tristan () Ship Motion Control: Course Keeping and Roll Stabilisation Using Rudder and Fins. Advances in Industrial Control.
Table of contents

The short range information may be used to drive minor course changes along the long range path to avoid high energy waves which are not resolved with the long range data. Referring now to FIG. These include heave , roll , surge , yaw , pitch , and sway , or any combination thereof.

Ship motion control : course keeping and roll stabilisation using rudder and fins

Each of these movements can have dangerous effects on a ship The encounter quadrant may vary based on the sea state approach and ship speed, although not limited thereto. The encounter quadrant may comprise any area of concern for the ship. For example, it may comprise a 90 degree angle of view, degrees, or even a full degree area around the ship , although not limited thereto. In vessels having difficulty entering port at lower speeds in a large following sea, a yaw autopilot may be improved by knowledge of approaching sea conditions from, for example, the astern quadrant rather than the forward quadrant.

The orientation of the quadrant may be important for sensing the future encounter wave profile.

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In some pure beam cases a wider azimuth range of encounter prediction is of use. If a vessel is rolling with a natural period of 6 seconds, a wave length in beam seas of 9. In this case if the wave length is For this example, the wave speed will be 9. Many vessels are very soft in yaw and have a long period if they are stable, and thus a hull proceeding at 12 knots being overtaken slowly by the wave can present directional control challenges. This is the case when the ship is operating down sea following and stern quarter where the velocity of wave propagation is greater than hull speed and the wave is catching up to the hull.

The forward quarter may be used for higher speeds.

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For instance, the encounter of a bow quarter sea at a heading of degrees between the heading of the ship and the direction of wave propagation, a wave is encountered at a local angle of 45 degrees and the distance to travel from one crest to the next is 1. This distance during the encounter period is the sum of the ship's forward advance and the wave celerity advance along the line of encounter.

Control input may use measurement of far field sea conditions along the vessel's designated course This may represent sea conditions over an area greater than a minimum distance representing the time necessary for computation of statistical measures of the conditions, which may be up to several kilometers or more. Long range sea state characterization of the far field may provide a statistical view of the larger sea state around the vessel. The output may include significant wave height and direction of the seas. The vessel may steer toward the destination, but through the statistical areas of lower wave energy to reduce vertical accelerations.

The course may vary as a moving section toward the destination, updated as the sea state map is updated. The data to compile the moving average of the far field sea state may be collected through back scatter of an existing ship board radar system. Back scatter is typically discarded as noise so the operator can focus on other vessels or objects of concern. The back scatter can be examined to provide indication of the frequency of waves, the intensity or energy of the waves, as well as the predominant wave train direction. The collected data may be compiled into moving average of wave energy, length and direction over a period of time.

Further, predictive wave slope computation may be performed on the near field The size of the near field may extend from a fraction of a second of forward advance distance e. Regardless of the sea encounter direction e.


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As seas become larger and steeper, the resulting ship motions that occur when the ship transits the wave become more severe. Near field mapping may utilize input from one or more sensors or sensor arrays, etc. In one embodiment, short range sea state prediction of the near field may use short range sensors to provide input for deterministic sea wave prediction DSWP.

This may use short range radar to provide data on the immediate waves approaching the vessel from any direction. This may be limited by the mast height of the sensor mounting, but preferably is approximately wave lengths, depending on the wave period, or 30 seconds, depending on the encounter frequency.

Use of this output as a determination of the immediate wave front a few seconds ahead of the vessel and the distribution across the wave front is helpful in course correction. This input may be used to provide a course heading to direct the vessel away from the area of high wave energy towards lower energy and reduced vertical acceleration. There is a desire to balance between the size of the sensor area in the near field and the size of wave that may result in extreme motion of the ship and detection and characterization of sets of waves and prediction of their propagation characteristics.

This may be satisfied with a multiple range, time correlated, scanning technique.

Ship Motion and Wave Simulation

In one embodiment, a Fourier representation of the sea surface may be used to predict future sea shapes e. As such, LiDAR or a similar technology may provide a direct measure and input to the control system. Prediction of sea state characteristics may break down into data input and analysis to inform system design, and output in the shorter term and longer term actions. A ship's immediate history of motions may inform the system of probable future motions. Knowledge of the short term database of responses may suggest future events. For example, on a vessel having an 8-second natural roll period, roll rate may be the maximum in worst-case rolling sets e.

That may be too small of a lead and effectiveness may be reduced by the phase shift lag in the action of motion control devices e. When the roll rate is at its maximum, the hull may have acquired the maximum angular momentum that must be countered to damp the roll motion.


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  5. Therefore, it may be desirable to lead the rate signal as much as possible. If the gain is sufficiently high in the stable range, the system may reach saturation maximum command before the rate signal reaches maximum. Too much gain may lead to instability. Other than immediate announcements, such as fault lights and sounds to indicate adverse conditions such as temperature or pressure limits, these events take place very quickly and modifications of control strategies may be part of the designed system response.

    For example, these may be used to recommend changes in system configuration such as gains, equipment parameters e. Such changes may be useful both for immediate and future implementation. Operational parameters e. For example, if a vessel is not underway but an oncoming wave has been detected, it may be preferable to position a fin to provide for maximum movement e. Far future analysis may also inform the operators of trends whether favorable or unfavorable. Visual displays may show operators projected and recommended courses of action.

    A number of benefits are realized by statistical mapping and motion characterization. For example, it permits evaluation of present and recent past conditions. The system may comprise one or more computers having hardware configured with software executing on computer readable media. The one or more computers may comprise multiple computers in electronic communication with each other over a network.

    The computer s may comprise an analyzer that analyzes conditions e. Discussed further below, the analyzer may also analyze stored ship data and condition data , although not limited thereto. One or more algorithms may be used to analyze input data. One or more sensor s may be used to anticipate sea conditions to provide an input that optimizes algorithms used by the motion control system.

    This provides future knowledge of the sea as it is developing to improve optimization of the gains and the weighting of the various control inputs to optimize the overall performance of the motion control algorithms. This may include the history of past motions and the expected motions based on measurements of the approaching sea conditions. Measuring approaching sea conditions may be performed using a sensor that can be either active or passive depending on operational requirements. For military applications, passive sensors may be optical, ultrasonic with limited range , etc.

    Radar, laser or other sensors that range out to a kilometer or more may reveal themselves to others in the operational area and may not be preferable for military applications.

    Citations en double

    The present teachings may be utilized with current motion sensor packs MSP , which may include:. An MSP may be configured for a range of direct and indirect measurements. These can consist of direct measurements of roll, pitch and yaw angles and rates as well as tri-axial measurements of acceleration of the rigid body ship's structure.

    A leading term such as roll pitch or yaw acceleration may be used to improve control.

    Tristan Perez - Citations Google Scholar

    For example, at points per second from a roll rate gyro a smoothed roll rate first derivative estimate of roll acceleration can predict increases or reductions in roll rate which will occur 2 seconds later for an 8 second roll period. This may be implemented using a low pass digital filter. A smoothed projection through 0. The phase shift may be equivalent to the projected estimate of the acceleration based on the least squares coefficients and the leading period of the calculation.

    This may lead to a different result than the low pass filter approach.

    Rigid body roll accelerations may also be measured by mounting Z-axis accelerometers. Inertia-torsion devices may also be used to measure angular acceleration. These devices are analogous to the mass-spring type of linear acceleration measurement. It is also possible to incorporate heave rate feedback.

    It is a particularly useful feedback signal when the system includes T-foil s forward and trim tabs or interceptors aft. The combination, in phase command, both forward and aft, damps the heave motion and reduces acceleration over the entire length of the hull, both port and starboard.