Ships – Ballasting – Antirolling
Patent
1994-01-18
1995-09-26
Sotelo, Jesus D.
Ships
Ballasting
Antirolling
B63B 3900
Patent
active
054526744
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a system for stabilising the movement of ships or floating vessels. In particular the invention relates to a fin stabiliser control system for use with ships.
A fin stabiliser control system is used to control stabilisers of a ship in order to reduce ship motions. For mono-hulls, because of the forces and moments required, roll stabilisation only is normally used. For semi-submersibles, catamarans, or SWATH (Small Water-Plane Area Twin Hull) vessels control of other motions such as pitch or heave may also be practical.
Usually fins and their associated actuation systems exhibit two major non-linearities; slew rate limiting and fin-angle limiting. The first non-linearity; slew rate limiting, occurs because the actuation systems are only capable of drive the fins up to some finite rate. This rate may vary with the fin angle. The second occurs if the fin is operated above a certain angle, when "stalling" of the fin occurs.
This behaviour adversely affects control law performance and this can be exacerbated if the non-linearities generate intermodulation (coupling) between multiple control laws.
Existing stabiliser controllers employ fixed gain. This means as sea disturbances increase, the fin demand increases until it is limited. Under severe overload the fin spends its time either at the nose-up or nose-down limit position or slewing at its maximum slew rate between the nose-up and the nose-down limit positions. Because the maximum fin slew rate is finite, the effect when slew rate limiting occurs is that the fin position is delayed with respect to the fin demand. The slower the slew rate, the more pronounced this effect. The delay has the effect of reducing the effectiveness of the roll stabiliser control loop by introducing a phase delay, and the effectiveness of control law reduces rapidly with increasing phase delay.
There are two possible ways of dealing with this problem. The first way is to ensure that the maximum slew rate of the fin is such that the phase delays introduced for all reasonable disturbances are low enough not to matter. This has the advantage of simplicity, but the disadvantage what the drive system must be powerful enough, and provide sufficient torque, to provide the requisite slew rates.
The second way is to reduce the controller loop gain so that the fin slew rate is never exceeded and the nose-up and nose-down limits are never reached. However, as the control gain is reduced, the non-saturated roll reduction reduces as well. In this situation the principal advantage is that the drive system, power and torque capability may be reduced considerably when compared with the first potential solution.
In designing a suitable fin stabiliser controller, the main points to consider are the relative fall off in performance in the two types of potential solution with increase in disturbances for a given drive system power and torque capabilities, and also the trade off between performance and cost. In this regard, it is desirable to reduce the cost of the drive system whilst not comprising the roll reduction achieved.
An object of the present invention is to provide an improved fin stabiliser control system which obviates or mitigates at least one of the aforementioned disadvantages.
This is achieved by providing an automatic gain control system which uses the difference between the control laws demand signals and the actual fin angles to calculate a gain factor to reduce the control laws demand until the non-linearities almost disappear.
If more than one control law is used to generate a demand, the control laws can be prioritised and the available actuator power thus rationed. In particular, the automatic gain control (AGC) sub-system of the control law monitors the motions of the fins and detects when each fin is not following the control law demands it adjusts the gain of the control law so as to reduce the slew rate limit and the fin angle limit effects. A control law is a physical realisation of a mathematical equation which continuously generates actuat
REFERENCES:
patent: 3727572 (1973-04-01), Nelson et al.
patent: 4380206 (1983-04-01), Baitis et al.
McMeel George
Melville Robert G. A.
Sotelo Jesus D.
Vickers PLC.
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