"GYRO-STABILISER" Field of the Invention

The present invention relates to an improved method and system of stabilising oscillating motion of a structure using a gyro-stabiliser and relates particularly, though not exclusively, to a gyro-stabiliser for stabilising floating platform motion induced by wave action such as the rolling motion of a marine vessel. A rotary damper system is also provided.

Australian provisional application 2007905651 entitled "Gyro-Stabiliser" filed 16 October 2007 is hereby incorporated by reference in its entirety for all purposes.

Background to the Invention

The use of gyroscopic stabilisers (gyro-stabilisers) in marine vessels for stabilising the rolling motion caused by wave action has been known for more than a century. The fundamental principal of gyro-stabilisers is that when a spinning flywheel is forced to rotate about an axis in the plane of the spinning axis, a torque is generated about an axis in the plane that includes the spinning axis and is perpendicular to the plane of forced rotation. There are two basic configurations for gyro-stabilisers that have been described in the prior art: gyros that have a vertical spinning axis (at mid-stroke of its precession); and, gyros that have a horizontal spinning axis. Horizontal axis gyros precess in a horizontal plane. Vertical axis gyros precess in a vertical plane. The gyroscopic (restoring) torque generated by the device due to precession, can be used to oppose a disturbance torque and hence attenuate possible disturbance motion. Some previously described gyro-stabilisers function by allowing the flywheel to precess within it's bearings due to the disturbance torque. The precession rate is typically damped using a fixed setting damper applied to the precession axis to ensure smooth precession rates. The damper setting is selected such that in the anticipated largest disturbance torque the device does not precess through angles exceeding a preferred angle. The restoring

torque (defined as the torque generated by the precession of the gyro) is used to attenuate the oscillating motion of the platform on which the gyro- stabiliser is mounted. End stops are usually fitted in order to ensure that the precession angle does not exceed the preferred angle. Disadvantages of this type of prior art arrangement are that (a) when the disturbance torque is less than the anticipated largest disturbance torque, the precession of the gyro is over-damped, resulting in lower precession rates than would otherwise be possible (and a resulting lower restoring torque than would otherwise be possible), and (b) when the disturbance torque is greater than the anticipated largest disturbance torque, the gyro precesses through an excessive range and crashes into the end-stops provided for this purpose. This situation is potentially dangerous as excessive accelerations may be induced leading to discomfort, loss of balance or fear, and possible structural damage to either the gyro or the supporting structure. The recommended procedure under these circumstances is to switch the gyro-stabiliser off (at exactly the time when it is most needed).

An example of this type of prior art arrangement may be found in European Patent Specification No. 0650890 by Mitsubishi Jukogyo K K. EP0650890 describes a passive oscillation suppression device which is based on damping the precession axis using a special rotary damper with a fixed flow restriction orifice. The rotary damper is connected to the gimbal (precession) shaft of the gimbal cage in which a spinning flywheel is supported, and acts to damp angular precession velocity. The precession rate of a passively damped gyro-stabiliser is reduced as compared to the free precession rate that would occur if there was no damping present. A passively damped gyro is workshop configured such that the level of damping causes the gyro to precess through a target maximum angular range in response to the target or 'design' disturbance torque. As a result, when the disturbance torque is less than the 'design' disturbance torque, the precession rate will be less than the free precession rate, and the resulting restoring torque will be less than optimum. In this circumstance, the passively damped gyro of EP0650890 provides less restoring torque than it could if the damping was reduced or removed, and so is inefficient. In addition to this, when the disturbance torque

is greater than the maximum target or 'design' disturbance torque, the damping level will be lower than is required to limit the precession range to within the target angular range. To prevent excessive precession of the flywheel, fixed hard-stops are fitted to the EP0650890 gyro. The result is that when the disturbance moment exceeds the maximum target or 'design' disturbance torque, the EP0650890 gyro flywheel cage will crash into the hard-stops. This is a potential cause of structural damage to the gyro unit or the vessel structure upon which it is mounted, and is considered to be an undesirable safety risk. The mitigating action for the vessel operator is to switch the gyro off, at a time when effective motion attenuation is presumably highly desirable.

Sea Gyro Pty Ltd has filed International Patent Application No. PCT/AU2004/001271 for a vessel stabilisation apparatus and method based on actively controlling the precession to ensure that the restoring torque generated forces the vessel to "follow the wave slope". The motion of the vessel caused by a wave having a wave slope is sensed, and on the basis of the sensed motion a restoring force is applied to or about the trunnion axis of a gimbal in which a gyroscope is mounted, so that the gyroscope produces a reactive torque which is applied to the vessel in a manner that forces the vessel to follow the wave slope. According to the Sea Gyro patent specification, since the gyroscope applies a torque to cause the vessel to follow the wave slope, the vessel can never resonate with the wave no matter what frequency the vessel or wave oscillates at. The Sea Gyro device requires that a sensor or a plurality of sensors is available to sense what the wave slope actually is. This is considered a disadvantage of the Sea Gyro system.

The present invention was developed with a view to providing a gyro- stabiliser system and method in which the precession of the spinning mass is actively controlled to produce a restoring torque that provides improved motion attenuation and increased safety. Although the invention will be described with particular reference to stabilising floating platform motion induced by wave action, it will be understood that it may also have application in stabilising oscillating motion on land based vehicles.

References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

Summary of the Invention According to an aspect of preferred arrangements herein described there is provided a method of stabilising an oscillating motion of a structure, by opposing a disturbance torque producing the oscillating motion using a gyro- stabiliser mounted on the structure, the method comprising: sensing the induced precession of the gyro-stabiliser, about a precession axis, as the structure oscillates; and actively controlling the precession of the gyro- stabiliser by applying a braking torque, said active control being solely responsive to the sensed precession.

The inventors have realised that it is possible to actively control the precession of a gyrostabiliser by applying a braking torque where the active control is solely responsive to the sensed precession. This is considered advantageous for a number of reasons including improved safety.

By relying solely on the sensed precession the system is not subject to the failure or incorrect calibration of external accelerometers, inclination devices and the like. Incorrect readings and failure (in extreme situations) can cause undesirable effects. This may cause the captain of a ship to order that a gyroscope be turned off, right when the gyroscope is required most.

In preferred arrangements of the present invention, actively controlling the precession comprises using solely the sensed precession as a variable in determining whether to increase or decrease the braking torque applied about the precession axis.

In 'smaller' disturbance conditions, the prior art systems known to the Applicant will combine to produce less undamped gyro precession. In these arrangements the flywheel precesses through less than its maximum range. In comparison preferred arrangements of the present invention, for a given size, mass and power consumption, will produce a greater reduction in vessel rolling motions.

Most gyro-stabilisers utilise some form of physical or software programme to provide 'hard stops' at the end of the maximum design precession angular range. It is well described in prior art that precession beyond 90 degrees causes instabilities in the restoring torque and that increasing precession angle causes undesirable 'cross-torques'. In the case where the passively damped gyro operates in 'larger' operating conditions, the flywheel cage will endeavour to precess beyond its design angular range. If hard stops are fitted (as they are on the MHI patent device) this will result in high physical accelerations and may result on structural damage to the gyro-stabiliser or the vessel structure to which it is mounted, both of which are highly undesirable unsafe outcomes. It may also in less violent situations cause vessel accelerations which may cause discomfort or fear in passengers. Again a highly undesirable outcome.

In 'larger' disturbance conditions, the flywheel of preferred arrangements will not precess through more than its maximum designed range. Comparatively the precession axis will not be under-damped and the rotary damper will produce relatively optimal damping without the necessity of hard stops.

Preferably when the precession rate or angle is within a specified angular range, the braking torque applied will be optimally reduced to ensure that the flywheel precesses as much as possible. When the rate or angle is greater than a threshold the braking torque applied is preferably be optimally increased to reduce the precession of the flywheel as much as possible. Furthermore, if the control system fails it is preferred that a hydraulic system providing the braking reduces to a passive braking system. According to another aspect of preferred arrangements herein described there is provided a gyro-stabiliser system for stabilising an oscillating motion of a structure by opposing a disturbance torque producing the oscillating motion, the system comprising: a gimbal-mounted spinning mass; a precession angle sensing means for sensing the induced precession of the spinning mass, about a precession axis thereof, as the structure oscillates;

a damping means for applying a braking torque to the spinning mass about the precession axis; and a control means responsive solely to the precession sensed by the precession angle sensing means, the control means being connected to the damping means for actively controlling the precession of the spinning mass by varying the braking torque applied; wherein the gyro-stabiliser is caused to precess at a rate calculated to produce an optimum restoring torque that attenuates the oscillating motion of the structure whilst inhibiting excessive precession angles. Preferably the control means is adapted to use solely the sensed precession as a variable, in determining whether the disturbance torque is lower than a restoring torque that the gyro-stabiliser is able to generate, and is adapted to remove the braking torque to allow free precession, if the disturbance torque is determined as being greater than the restoring torque. . According to another aspect of preferred arrangements herein described there is provided a rotary damper for dampening the precession of a spinning mass, the rotary damper comprising: a housing having an interior for holding a hydraulic fluid; and a rotary sweeper within the housing, the rotary sweeper being mechanically coupled to a precession axis shaft of the spinning mass and being pivotally mounted within the housing to divide the interior into a first chamber and a second chamber; wherein the housing defines one or more flow paths between the first chamber and the second chamber for allowing the hydraulic fluid to flow therebetween, the one or more flow paths being configured for actively controlling the flow of the hydraulic fluid between the first and second chambers to control the dampening of the precession of the spinning mass.

Preferably the one or more flow paths comprises a first active control flow path and a second flow path that is normally open. It is considered that the ability to have one normally open flow path and one variable flow path allows for the advantageous control of the rotary damper in both smaller disturbance and large disturbance conditions.

According to another aspect of preferred arrangements herein described there is provided a rotary damper for dampening the precession of a spinning mass, the rotary damper comprising: a housing having an interior for holding a hydraulic fluid; a rotary sweeper within the housing, the rotary sweeper being mechanically coupled to a precession axis shaft of the spinning mass and being pivotally mounted within the housing so as to divide the interior of the housing into a first chamber and a second chamber; and a conduit defining a first flow path and a second flow path from the first chamber and the second chamber, respectively, to allow for the flow of hydraulic fluid into and out of the first and second chambers, to facilitate control of the precession of the spinning mass, each of the first and second flow paths having a separate header tank for the hydraulic fluid and a separate flow restriction device for restricting the flow of hydraulic fluid through each flow path.

Preferably the rotary damper includes a control means adapted to send flow restriction commands to a first flow restriction device, forming part of the first flow path, and a second flow restriction device, forming part of the second flow path. In these preferred arrangements the separation of the header tank from the first and second chambers is advantageous as it allows for the ready and accessible use of variable control valves.

According to yet another aspect of preferred arrangements herein described there is provided a gyroscope damping system comprising: a mass adapted to spin about a precession axis and provide a torque that opposes a disturbance; one of more dampers for applying a braking torque to the mass when spinning about the precession axis; a conduit system for allowing the flow of fluid between the one or more dampers and a remote header tank spaced therefrom, the conduit system including at least one electronically controlled variable flow restrictor unit for controlling the braking torque applied by the one or more dampers by controlling the flow of fluid between the one or more dampers and the header tank.

According to yet another aspect of preferred arrangements herein described there is provided a method of stabilising an oscillating motion of a structure, by opposing a disturbance torque producing the oscillating motion, using a gyro-stabiliser mounted on the structure, the method comprising: sensing an induced precession of the gyro-stabiliser as the structure oscillates; and using fluid within one or more dampers to apply a braking torque to the mass when spinning about the precession axis, the fluid serving to resist the induced precession; and delivering the fluid through a conduit system in fluid communication between the one or more dampers and a remote header tank spaced therefrom, the conduit system using the at least one electronically controlled variable flow restrictor unit to restrict movement of the fluid by controlling the flow of fluid between the one or more dampers and the header tank. In preferred arrangements of the present invention the at least one electronically controlled variable flow restrictor unit comprises a single flow restrictor. These arrangements are advantageous for the reasons that they utilise a relatively small number of parts in a beneficial manner whereby greater safety is performed. Preferably the electronically controlled variable flow restrictor unit has an outlet that is connected to opposed ports of the one or more dampers by an arrangement of one way valves.

According to one aspect of the present invention there is provided a method of stabilising an oscillating motion of a structure, by opposing a disturbance torque producing the oscillating motion using a gyro-stabiliser mounted on the structure, the method comprising the steps of: sensing the induced precession angle of the gyro-stabiliser as the structure oscillates; applying a braking torque about the precession axis responsive to the sensed precession angle of the gyro-stabiliser; and, actively controlling the rate and/or angle of precession of the gyro-stabiliser by varying the applied braking torque, wherein the gyro-stabiliser is caused to

precess at a rate calculated to produce an optimum restoring torque that attenuates the oscillating motion of the structure whilst inhibiting excessive precession angles.

The structure may be a floating platform (ship, boat, vessel, offshore structure or production facility, barge, bridge, jetty, pontoon, buoy) having its motion induced by wave action.

When the disturbance torque (such as roll-inducing wave forces acting on a vessel's hull) is lower than the restoring torque that the gyro-stabiliser is able to generate, (based on its angular momentum and range of precession), the braking torque is preferably removed from about the precession axis to allow free precession. In this circumstance the gyro-stabiliser will produce a restoring torque equal to but opposing the disturbance torque resulting in zero or very low motion of the structure as the gyro-stabiliser precesses back and forth through its designed precession range. When the disturbance torque is greater than the maximum torque that the gyro-stabiliser can generate under free precession, whilst operating within its designed precession angle range, the damping or braking torque applied about the precession axis is increased to limit the precession angle to within the designed precession range. Active control of the rate and/or angle of precession is achieved by varying the applied braking torque, wherein the gyro-stabiliser is caused to precess at a rate calculated to produce an optimum restoring torque

According to another aspect of the present invention there is provided a gyro- stabiliser system for stabilising an oscillating motion of a structure by opposing a disturbance torque producing the oscillating motion, the system comprising:

a gimbal-mounted spinning mass; a precession angle sensing means for sensing the induced precession angle of the spinning mass about its precession axis as the structure oscillates; a damping means for applying a braking torque about the precession axis; and,

a control means responsive to the sensed precession angle for actively controlling the rate and/or angle of precession of the spinning mass by varying the braking torque applied by the damping means, wherein the gyro- stabiliser is caused to precess at a rate calculated to produce an optimum restoring torque that attenuates the oscillating motion of the structure whilst inhibiting excessive precession angles.

The braking torque is preferably varied by a control means that calculates the gyro-stabiliser precession angle (and can calculate derivatives of this as required), based on a sensing signal from the precession angle sensing means, and that sends appropriate control commands to the damping means to thereby generate a suitable braking torque about the precession axis.

In one embodiment the damping means is a rotary damper comprising a rotary sweeper that is mechanically coupled to a precession axis shaft of the spinning mass, the rotary sweeper being pivotally mounted within a housing so as to divide an interior of the housing into first and second chambers filled with hydraulic fluid, one or more variable flow paths for the hydraulic fluid being provided to facilitate control of the braking torque generated by the rotary damper. In one embodiment the one or more variable flow paths comprise first and second flow paths in fluid communication with the first and second chambers respectively, each flow path having a separate header tank for cooling the hydraulic fluid and a separate flow restriction device for restricting the flow of hydraulic fluid through the flow path. Preferably the control system sends appropriate flow restriction commands to the flow restriction devices to control the rate and/or angle of precession of the spinning mass by varying the braking torque applied by the rotary damper.

In another embodiment the one or more variable flow paths comprise a plurality of orifices that extend between the first and second chambers of the rotary damper, each orifice having a different diameter and therefore providing a different restriction on the flow of hydraulic fluid there through. Preferably each one of said plurality of orifices has a flow control valve which enables the flow of hydraulic fluid through that orifice to be regulated from the first to the second chamber or vice versa. Preferably the control system sends

appropriate flow restriction commands to sequentially select combinations of the various orifices to be open while the others remain closed. In this way the overall rotary damping of the sweeper caused by flow between the two chambers can be varied. By varying the rotary damping of the sweeper, the rate and/or angle of precession of the spinning mass maybe varied.

In an alternative embodiment the damping means is a linear damper comprising one of more hydraulic cylinders for applying a braking torque to the gimbal-mounted spinning mass about the precession axis. Preferably said one or more hydraulic cylinders comprise a first elongate hydraulic cylinder having a plurality of fluid ports provided at respective ends of the cylinder. Preferably a header tank is provided in fluid communication with the fliud ports via electronically controlled variable flow restrictor valves respectively. Preferably the first elongate hydraulic cylinder is one of a pair of elongate hydraulic cylinders mechanically coupled to a trunnion of the gimbal-mounted spinning mass via a torque arm.

The present invention typically does not require external measurement of the motion of the structure that is being stabilised. Rather, the gyro-stabiliser is itself used to sense the disturbance torque produced by the oscillating motion of the structure (as it induces precession of the gyro-stabiliser), by sensing the rate and/or angle of precession of the spinning mass. However external sensing of the motion of the structure may be employed in certain situations.

According to another aspect of the present invention there is provided a gyro- stabiliser for stabilising an oscillating motion of a structure by opposing a disturbance torque producing the oscillating motion, the gyro-stabiliser comprising: a gimbal-mounted spinning mass having a drive shaft extending along its axis of rotation; an electric spin motor for driving the spinning mass, the spin motor comprising a donut-shaped stator mounted concentrically about the axis of rotation of the spinning mass, and a rotor assembly mounted concentrically

within the stator and mechanically coupled to the drive shaft of the spinning mass.

Preferably the electric spin motor is a brushless AC servo motor. Typically the spinning mass is rotatably mounted within a gyro cage. In one embodiment the electric motor is a separate bolt-on unit that is externally mounted on the gyro cage. In another embodiment the rotor assembly is mounted directly onto a protruding portion of the drive shaft of the spinning mass and the stator may be mounted on an external surface of the gyro cage. Alternatively the rotor assembly may be mounted directly onto an internal portion of the drive shaft and the stator may be mounted on an internal surface of the gyro cage.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word "preferably" or variations such as "preferred", will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention.

Brief Description of the Drawings The nature of the invention will be better understood from the following detailed description of several specific embodiments of the gyro-stabiliser system and method, given by way of example only, with reference to the accompanying drawings, in which:

Figures 1(a), 1 (b) and 1 (c) illustrate a first embodiment of a gyro- stabiliser according to the present invention;

Figures 2(a) and 2(b) illustrate a second embodiment of a gyro- stabiliser according to the present invention;

Figure 3 is a functional block diagram of a preferred embodiment of a gyro-stabiliser system according to the present invention;

Figure 4 is a schematic section view of a first embodiment of the spinning mass and an electric spin motor employed in the gyro- stabiliser of Figure 1 ;

Figure 5 is a schematic section view of a second embodiment of the spinning mass and an electric spin motor that may be incorporated in the gyro-stabiliser of Figure 1 ;

Figure 6 is a schematic section view of a third embodiment of the spinning mass and an electric spin motor that may be employed in the gyro-stabiliser of Figure 1 ; Figure 7 is a schematic diagram of a preferred embodiment of a rotary damper according to the present invention that may be employed in a gyro-stabiliser system;

Figure 8 is a schematic diagram of a second embodiment of a rotary damper that may be employed in a gyro-stabiliser system; Figure 9 is a schematic diagram of a first embodiment of a linear damper that may be used in the gyro-stabiliser system of Figure 1 ;

Figure 10 illustrates a first embodiment of an arrangement showing how the linear damper of Figure 9 may be used to apply a braking torque about the trunnion axis of a gyro-stabiliser; Figure 11 illustrates a second embodiment of an arrangement showing how the linear damper of Figure 9 may be used to apply a braking torque about the trunnion axis of a gyro-stabiliser;

Figure 12 is a functional block diagram of a control method according to another preferred embodiment of the present invention; Figure 13 comprises four schematic diagrams of rotary dampers according to several embodiments of the present invention;

Figure 14 is a schematic diagram of a preferred embodiment of a rotary damper system according to the present invention;

Figure 15 is a schematic diagram of a preferred embodiment of a linear damper system according to the present invention;

Figure 16 s a schematic diagram of a further preferred embodiment of a linear damper system according to the present invention; and

Figure 17 and 18 comprise schematic diagrams of linear and rotary damper systems according to further embodiment of the present invention.

Detailed Description of Preferred Embodiments A preferred embodiment of a gyro-stabiliser 10 according to the present invention for a marine vessel, as illustrated in Figure 1 , comprises a gimbal- mounted spinning mass, which in this embodiment takes the form of a flywheel 12 driven by a spin motor 14. In this embodiment the spinning mass or flywheel 12 is rotatably mounted horizontally, so as to spin about a vertical axis of rotation S, within a gyro cage 16. The gyro cage 16 is, in turn, pivotally mounted on trunnions 18, which support the spinning mass 12 about a horizontal trunnion or precession axis P and allow it to precess naturally about the trunnion axis P. Trunnions 18 are held in trunnion bearings 20 mounted on each side of the gyro cage 16. Trunnion bearings 20 may be deep groove ball bearings with or without oil, tapered roller bearings with or without oil, plain bearings or hydro-dynamically lubricated plain bearings. The gyro cage 16 may be made from fabricated steel plate or cast from a suitable metallic material or manufactured using fibre-reinforced plastics material.

Figures 1 (a) to (c) illustrate the gyro-stabiliser 10 at various angles of precession about its precession axis P. Figure 1 (a) shows the gyro cage 16 at one extreme of its angular range of precession, whereas Figure 1(c) shows it at its other extreme. Figure 1(b) shows the gyro cage 16 at its midpoint or neutral position in which the spinning axis S is substantially vertical.

The spinning mass or flywheel 12 is rotatably mounted in the gyro cage 16 on main bearings 22. Like the trunnion bearings 20, main bearings 22 may be

deep groove ball bearings with or without oil, tapered roller bearings with or without oil, plain bearings or hydro-dynamically lubricated plain bearings. The spinning mass 12 is driven by a spin motor 14, which in this embodiment is mounted on top of the gyro cage 16 and is directly coupled to the spinning axis shaft 21 of the spinning mass 12. Alternatively the spin motor 14 may drive the spinning mass 12 indirectly via a drive belt mechanism or via a gearbox. The spin motor 14 may be a hydraulic or electric motor.

In this embodiment the spin motor 14 is an externally mounted electric brushless AC servo motor 14, as illustrated schematically in Figure 4. The electric motor 14 has a donut-shaped stator 24 about which a suitable winding (not visible) is provided to carry an electric drive current. An annular rotor assembly 26 is concentrically mounted within the stator 24 and is directly coupled via a mechanical rotor coupling 28 to the end of the drive shaft 21 of the spinning mass 12. The annular rotor assembly 26 comprises a laminated, cylindrical iron core with slots for receiving conductors (not visible), in which a secondary current is induced by the primary alternating current in the stator winding. The donut configuration of the electric motor 14 is compact in design, and produces sufficient torque to drive the spinning mass 12 at high speed. In Figure 4, the electric motor 14 is a separate bolt-on unit that is externally mounted on top of the gyro cage 16. The provision of the rotor coupling 28 enables the whole motor unit to be conveniently fitted, and/or removed for servicing as required, without having to dismantle the gyro cage 16. However the electric motor may also be integrated into the structure of the gyroscope if preferred. Figures 5 and 6 illustrate two alternative embodiments in which the rotor assembly 26 is mounted directly onto the main drive shaft 21 of the spinning mass 12.

In Figure 5, the drive shaft 21 extends beyond the confines of the gyro cage 16 and the rotor assembly 26 is mounted on the protruding portion of the drive shaft 21. The stator 24 is arranged concentrically about the rotor assembly 26 and is mounted on the external surface of the gyro cage 16. In Figure 6 the rotor assembly 26 is mounted on an internal portion of the drive

shaft 21, within the confines of the gyro cage 16. The stator 24 is again arranged concentrically about the rotor assembly 26 and is mounted on an internal surface of the gyro cage 16. This provides a very compact design, but makes it much more difficult to access the motor 14 should it require servicing, as it would require disassembly of the gyro cage 16.

A main base structure 30, which is typically secured to the deck of the marine vessel, supports the trunnion bearings 20 in which the trunnions 18, that support the flywheel 12, pivot. The base structure 30 is a rigid structure fabricated or cast from a suitable metallic material or manufactured using fibre-reinforced plastics.

Preferably an accelerometer 34 and temperature sensors 36 are fitted to the main bearings 22 of spinning mass 12 (Figure 1(c)) for condition monitoring. The accelerometer 34 is designed to detect vibrations within a defined bandwidth, which indicate an imbalance in the flywheel 12. Such vibrations could be due to an internal or external crack that may have opened up in the material of the spinning mass, which could lead to catastrophic failure if not detected early enough. Likewise, the temperature sensors 36 are designed to detect variations in the temperature of the bearings above a defined range that may indicate wear or excessive friction in the bearings. In the case of a temperature rise the speed of rotation may be eased back a bit. In the case of vibrations, the spin motor is switched off immediately.

The preferred embodiment of a gyro-stabiliser 10 further comprises a damping means for applying a braking torque about the trunnion or precession axis P. The braking torque may be applied by a variety of damping means including friction brakes, a hydraulic clutch, hydraulic linear actuators with flow controls across the 'A' and 'B' ports. In this embodiment the damping means is a linear damper 32 comprising a plurality of hydraulic cylinders 88 mounted on the base structure 30. A preferred arrangement of the linear damper 32 will be described in more detail below with reference to Figures 9, 10 and 11. In this embodiment a linear damper 32 is provided on each side of the gyro cage 16.

Figure 2 illustrates a second embodiment of a gyro-stabiliser 38 according the present invention. The gyro-stabiliser 38 is similar to the first embodiment and therefore the same reference numerals will be used for the like parts, which will not be described again. The spinning mass 12 is driven by an electric spin motor 14 in a similar manner to that of the previously described embodiment (see also Figures 4 to 6). The main difference between gyro-stabiliser 38 and gyro-stabiliser 10 is that a different damping means is employed. In this embodiment a rotary hydraulic damper 40 is employed, mechanically coupled to one of the trunnions 18 on the gyro cage 16. A rotary damper is advantageous because the damping force (braking torque) generated is directly proprtional to the rotational velocity.

The rotary damper 40 comprises a rotary sweeper 42 pivotally mounted within a cylindrical housing 44 and directly coupled to one of the trunnions 18. In Figure 2 the rotary damper is shown schematically with a cover plate removed to reveal the position of the rotary sweeper 42 as the gyro cage 16 precesses about the precession axis P. In Figure 2(a) the gyro cage 16 has precessed to its maximum precession angle in one direction (about 80°). The sweeper 42 is close to its maximum sweep within the housing 44 and the high pressure chamber is almost fully closed. In Figure 2(b) the gyro cage 16 is in its neutral position with the spin axis S in a vertical orientation. The sweeper 42 is as its midpoint within the housing 44 and the pressure in the chambers on either side is virtually equalised. The operation of the rotary damper 40 will be described in more detail below with reference to Figure 7.

Figure 3 is a functional block diagram illustrating a preferred embodiment of the gyro-stabiliser system according to the invention and how it may be integrated into a marine vessel. The gyro-stabiliser system comprises a gyro- stabiliser 10 or 38 in combination with a control means 60, which typically includes a precession valve controller 58 and a spin motor speed controller 64. The control means 60 is preferably housed in a cabinet that is separate from the gyro-stabiliser 10 or 38 and is located in a more convenient location within the marine vessel. In addition to the spin motor 14 and damper 32, the gyro-stabiliser 10 or 38 also typically includes a precesion angle sensor 62 for

sensing the precession angle of the spinning mass 12 about its precession axis P ₁ as the vessel is subject to rolling motion induced by wave action.

Control means 60 is responsive to the sensed precession angle for actively controlling the rate and/or angle of precession of the spinning mass 12 by varying the braking torque applied by the damper 32 or 40. In this way the gyro-stabiliser 10 or 38 is caused to precess at a rate calculated to produce an optimum restoring torque that attenuates the disturbance torque producing the rolling motion of the marine vessel, whilst inhibiting excessive precession angles. Control means 60 typically also includes a spin motor speed controller for controlling the speed of the spin motor 14, (and hence of the spinning mass 12) responsive to the rotary velocity of the spin motor 14 as sensed by a spin RPM encoder 66.

The spin motor 14 is powered from the ships AC power generator 122, whereas the precession valve controller 58 is powered from the ships DC power board 124. Preferably the control means 60 is provided with a control interface 68, which typically includes a simple keypad and VDU (visual display unit) (not illustrated). The control interface 68 provides ON/OFF control, mode selection and an indication of the status of various operational conditions. Figure 7 illustrates schemaically a preferred embodiment of the rotatary damper 40 employed in the gyro-stabiliser 38 of Figure 2. The rotary damper 40 comprises a rotary sweeper 42 pivotally mounted within a cylindrical housing 44 and directly coupled to one of the trunnions 18. Ports 46a and 46b provide a flow path for hydraulic fluid to/from first and second chambers 48a and 48b on the respective sides of the rotary sweeper 42.

Separate header tanks 50a and 50b (one for each of the two chambers 48) are in fluid communication with the ports 46a and 46b, respectively via electronically controlled (or fixed as the case demands) variable flow restrictor valves 52a and 52b respectively. Each port 46 of the rotary damper 40 is provided with two fluid paths to/from a respective header tank 50. Each fluid path is provided with a respective non-return valve 54 or 56. The placement and orientationof the non-return valves 54 and 56 forces the flow to proceed

along the appropriate path, such that when fluid is expelled from a chamber 48, it passes through the non-return valve 54 and the flow restrictor valve 52, whilst when fluid is drawn into a chamber 48 it flows directly from the header tank through the non-return valve 56. Depending on which way the rotary sweeper 42 is moving, one of the chambers 48 will the high pressure chamber and the other will be the low pressure chamber. Hydraulic fluid expelled from the high pressure chamber 48 of the rotary damper 40 flows through a non-return valve 54, then a variable flow restrictor valve 52, and into a header tank 50 where the fluid cools prior to the next cycle. On the low pressure side, hydraulic fluid flows from a separate header tank 50 through a non-return valve 56 and into the low pressure chamber 48 of the rotary damper 40. During the next oscillation, the low pressure side becomes the high pressure side. By varying the flow restriction, on the high pressure chamber port, the rate of rotary damping can be controlled. Actively controlling the flow restriction in the rotary damper 40 enables the rate and/or angle of precession to be controlled.

The electronic valve control unit 58 actively controls the variable restrictor valve 52 to vary the braking torque applied by the rotary damper 40. The control unit 58 is typically incorporated in a control means 60 (see Figure 3), which is responsive to a sensed precession angle for actively controlling the rate and/or angle of precession of the spinning mass 12 by varying the braking torque applied by the rotary damper 40. In this manner the gyro- stabiliser 38 is caused to precess at a rate calculated to produce a restoring torque that attenuates the rolling motion of the marine vessel. A precession angle sensing means 62 (Figures 1 and 3) is provided for sensing the precession angle of the spinning mass 12 about its precession axis as the structure oscillates. The precession angle sensing means 62 may take the form of an inductive linear variable displacement transducer (LVDT). In an inductive LVDT the linear displacement of, for example, an hydraulic cylinder caused by the precession of the spinning mass 12, is converted inductively into an electronic signal which can be processed in the control means 60 to calculate the rate and/or angle of precession. Alternatively a

rotary sensor or encoder may be coupled directly or indirectly via belt drive or gear mechanism to one of the trunnions 18 for sensing the precession rate and/or angle of the spinning mass.

Figure 8 illustrates another embodiment of a rotary damper 70 which may be employed in the gyro-stabiliser 38 according to the invention. Like the previous embodiment, the rotary damper 70 comprises a rotary sweeper 72 that is mechanically coupled to a main drive shaft of the spinning mass 12. The rotary sweeper 12 is pivotally mounted within a housing 74 so as to divide an interior of the housing into first and second chambers 76a and 76b filled with hydraulic fluid. One or more variable flow paths for the hydraulic fluid are provided to facilitate control of the braking torque generated by the rotary damper 70. In this embodiment the one or more variable flow paths are a plurality of orifices 78a, 78b, 78c, and 78d that extend between the first and second chambers 76a and 76b, each orifice 78 having a different diameter and therefore providing a different restriction on the flow of hydraulic fluid there through. Each one of the plurality of orifices 78 has a flow control valve 80 for controlling the flow of hydraulic fluid through that orifice from the first chamber 76a to the second chamber 76b or vice versa. The control means 60 sends appropriate flow restriction commands to the flow control valves 80 to control the rate and/or angle of precession of the spinning mass 12 by varying the braking torque applied by the rotary damper 70.

Figure 9 illustrates an embodiment of the linear damper 32 that may employed in the gyro-stabiliser 10 as a damping means for applying a braking torque about the precession axis P (see Figure 1). The linear damper 32 comprises an elongate hydraulic cylinder 88, having a piston 90 slidably mounted therein. The hydraulic cylinder 88 has a first pair of first and second fluid ports 92a and 92b at one end, and a second pair of first and second fluid ports 94a and 94b at the other end. A header tank 96 is in fluid communication with the ports 92b and 94b respectively via electronically controlled (or fixed as the case requires) variable flow restrictor valves 98a and 98b respectively.

Each pair of fluid ports 92 and 94 of the hydraulic cylinder 88 is provided with two fluid paths to/from respective sides of the header tank 96. Each fluid path is provided with a respective non-return valve 100 or 102. The placement and orientationof the non-return valves 100 and 102 forces the flow to proceed along the appropriate path, such that when fluid is expelled from the chamber on one side of the piston 90, it passes through the non-return valve 100 and the flow restrictor valve 98, whilst when fluid is drawn into the same chamber it flows directly from the header tank 96 through the non-return valve 102. As with the previous embodiments, an electronic valve control unit 104 actively controls the variable restrictor valves 98 to vary the braking torque applied by the linear damper 32a. Suitable processing in the control unit 104 ensures that flow restriction commands sent to the variable restrictor valves 98 result in a braking torque that is proportional to the rate and/or angle of precession.

The braking torque generated by a linear damper may be applied to the trunnion axis P of the gyro-stabiliser in a variety of ways. Figures 10 and 11 illustrate two possible embodiments of a linear damper 32 comprising a pair of hydraulic cylinders 88 for applying a braking torque to the trunnion axis. In the linear damper 32a of Figure 10 a torque arm 110 is fixed to one of the trunnions of the gyro cage 16 of the gyro-stabiliser 10. The torque arm 110 of this embodiment is provided with two spaced apart pivot joints 112a and 112b to which the ends of the pistons of respective hydraulic cylinders 88a and 88b are coupled. The other ends of the respective hydraulic cylinders 88a and 88b are pivotally anchored to the main base structure 30 of the gyro-stabiliser or the structure of the floating platform. This arrangement ensures that the linear damper 32a can provide effective damping through a 160° oscillation of the gyro cage 16, without exceeding the stroke length of the hydraulic cylinders

88.

Figure 11 illustrates a similar linear damper 32b in which a torque arm 116 is provided with only one pivot joint 118 to which the ends of the pistons of respective hydraulic cylinders 88a and 88b are coupled. In other respects the arrangement is similar to that of Figure 10. This arrangement likewise ensures that the linear damper 32b can provide effective damping through a 160° oscillation of the gyro cage, without exceeding the stroke length of the

hydraulic cylinders 88, although the available stroke length will need to be longer in this case.

Referring to Figure 12 there is shown a method 100 according to further preferred embodiment of the present invention. The method 100 comprises stabilising an oscillating motion of a structure 102, by opposing a disturbance torque 104 producing the oscillating motion. In order to achieve this, the method 100 uses a gyro-stabiliser 106 mounted on the structure 102.

At block 108 the method 100 includes sensing the induced precession of the gyro-stabiliser 106 about its precession axis as the structure 102 oscillates. At block 110 the method 100 includes actively controlling the precession of the gyro-stabiliser 106 by applying a braking torque.

In the method 100 the active control at block 110 is solely responsive to the sensed precession. In other words, the active control does not depend on any external measurements relating to the condition of the structure 102. Unlike the prior art the present embodiment does not require, for example, any accelerometers attached to the structure 102.

Furthermore, actively controlling the precession comprises using solely the sensed precession as a variable in determining whether the disturbance torque is lower than a restoring torque that the gyro-stabiliser is able to generate. If the disturbance torque is determined as being greater than the restoring torque, the method 100 at block 111 removes the braking torque to allow free precession.

No other variable such as the inclination of the structure 102 is used in determining whether the disturbance torque is lower than the restoring torque. Thus the precession angle, or derivatives of it, is the sole parameter of the system providing for optimum functionality in combination with considerations of safety. The method 100 is considered to be particularly useful in stabilising vessels includes relatively large boats.

It is to be appreciated that the method 100 is distinguished from most gyro- stabiliser methods for boats. In these prior art methods some form of physical arrangement is usually provided or software is usually programmed with 'hard

stops ¹ at the end of a maximum design precession angular range. Whilst it is known that having precession angles beyond 90 degrees causes instabilities in the restoring torque, and that increasing precession angle causes undesirable 'cross-torques', the present arrangement provides dampening solely on the basis of precession angle and allows free precession when the disturbance torque is less than a restoring torque. This is considered advantageous.

In the present case the method 100 is applied to a vessel where the gyrostabiliser 106 is passively damped in 'larger' operating conditions. In this case where disturbance torque is seen as being higher than the restoring torque the flywheel cage of the gyrostabiliser is preferably damped to precess within a predetermined angular range. As discussed, if the disturbance torque is seen as being lower than the restoring torque the braking torque is of course removed to allow free precession. By obviating the need for hard stops, as are fitted on prior art devices, the problems of high physical accelerations and structural damage to the gyrostabiliser or the vessel structure are addressed. The method 100 also advantageously serves to address less violent situations that can still cause vessel accelerations and discomfort or fear in passengers. Referring to Figure 13 there is shown a rotary damper 150 according to another preferred embodiment of the present invention. The rotary damper 150 is provided to dampen the precession of a spinning mass (not shown). The damper comprises a housing 152 having an interior 154 for holding a hydraulic fluid 156. The housing includes a rotary sweeper 158 within the housing 152. The rotary sweeper 158 is mechanically coupled to an axis of a shaft at a point 160. The shaft is connected to the spinning mass with the spinning mass being arranged to precess about the axis of a shaft 160.

The rotary sweeper 158 is pivotally mounted within the housing and divides the interior 154 into a first chamber 162 and a second chamber 164. The housing 152 defines two flow paths 166 between the first chamber 162 and

the second chamber 164. The two flow paths 166 allow the hydraulic fluid 156 to flow therebetween.

In the embodiment the two flow paths 166 are each configured for actively controlling the flow of the hydraulic fluid therethrough. As would be apparent, the rotary sweeper 158 operates to force hydraulic fluid 156 along the flow paths 166 to dampen and thereby control the precession of the spinning mass about the point 160.

Referring to Figure 13 there is shown a variation 168 of the rotary damper 150. The variation 168 forms another preferred embodiment of the present invention.

In comparison to the rotary damper 150, the one or more flow paths comprises a first active control flow path 170 and a second flow path 172. The first active control path 170 allows for active control while the second path that is a normally open flow path. The first and second flow paths 170, 172 together allow for variable control of the flow of hydraulic fluid.

In the arrangement, the first active control flow path 170 includes a single variable flow restrictor 174. The second control flow path 170 includes an orifice provided by an isolation valve 175. An electronic control means (not shown) is adapted to send flow restriction commands to the flow restrictor 174 and isolation valve 175, to control the precession of the spinning mass.

A third variation 176 is shown in Figure 13. The variation 176 forms a further preferred embodiment of the present invention. In the variation 176 there is provided a single first active control flow path 178 having a variable flow restrictor. A fourth variation 182 is shown in Figure 13. The variation 182 forms yet a further preferred embodiment of the present invention. As shown in Figure 13 there are provided a plurality of flow paths 184 each having an orifice 186 of a different restrictive coefficient. The flow paths 184 are provided in parallel so that different amounts of hydraulic fluid flow through respective flow paths 184 on movement of a sweeper 188. Each of the flow paths 184 has electrically operated isolation valve 190. A controller is provided so that the valves 190

can be operated to select different ones of the flow paths 184 to actively control the flow of fluid between chambers. That is, the controller is adapted to send flow restriction commands to select combinations of the various orifices to be open while the others remain closed. The fourth variation is considered to provide a multi-orifice arrangement in which a plurality of orifices connect the two fluid chambers. In the multi-orifice arrangement there is more than one orifice, each of which has an isolation valve that can be electronically or otherwise actuated to open or close the corresponding fluid path through the corresponding orifice. By opening a combination of one or more of the various orifices to allow flow from one chamber to the next, the resulting damping characteristic of the damper device can be varied. In the embodiment described, this advantageously requires relatively little power and can be achieved using a 'low powered ¹ controller devices. By increasing the number or orifices and using a number or variations or combinations of the sizes of orifice at any one time, an advantageously large number of possible damping settings for the rotary damper can be readily achieved. This allows the precession axis damping to be advantageously and actively adapted to changing operating conditions. In another embodiment of the invention, one or more of the orifices are replaced with one or more variable damping valves such as proportional flow control valves. As would be apparent a combination of variable damping valves and fixed orifices is possible, including the installation of a single variable damping valve. Referring to Figure 14 there is shown a rotary damper system 192 for dampening the precession of a spinning mass. The damper system 192 includes a conduit 194 defining first flow path 196 and a second flow path 198. The first flow path 196 extends between a first tank 200 to a first chamber 202. The second flow path 198 extends between a second separate tank 204 to a second chamber 206. In the manner shown the first and second flow paths 196 and 198 allow for the flow of hydraulic fluid into and out of the first chamber 202 and the second chamber 206. This is used to

facilitate the control of the precession of the spinning mass. The spinning mass is connected to a sweeper 207.

In the system 192 there is provided control means (not shown) adapted to send flow restriction commands to a first flow restriction device 208, forming part of the first flow path 196, as well as flow restriction commands to a second flow restriction device 210, forming part of the second flow path 198.

In the embodiment the first restriction device 208 and the second restriction device 210 are variable control restriction devices. The damping system 192 is advantageously used in a gyroscope mounted to a vessel and is adapted to provide relatively optimum damping in different operating conditions that depend on the wave environment, vessel heading, vessel speed, and the vessels motion response to wave characteristics. Only the precession is monitored by the system.

Referring to Figure 15 there is shown a further embodiment comprising a linear gyroscope damping system 212. The gyroscope damping system 212 is connected to a precession axis 214 about which a spinning mass is arranged to precess and provide a torque that opposes a disturbance.

The system 212 includes two dampers 216 arranged for applying a braking torque to the mass. The damping system 212 includes a conduit system 218 that allows the flow of fluid between the dampers 216 and two remote header tanks 220 spaced therefrom.

In the arrangement the conduit system 218 is divided into two halves, each of which includes two electronically controlled variable flow restrictor units 222. In total there are four restrictor units 222 that are arranged to control the braking torque applied by the dampers 216. This is achieved by controlling the flow of fluid between the dampers 216 the header tanks 220.

Referring now to Figure 16 there is shown another linear gyroscope damping system 224 according to a further preferred embodiment of the present invention. The damping system 224 includes a conduit system 226 that includes two electronically controlled variable flow restrictor units 228 respectively connected to header tanks 230. The number of restrictor units

228 is considered to be advantageous for reasons including maintenance and reliability. .

Referring to Figure 17 there is shown a liner damping system 232 according to another preferred embodiment of the present invention. The damping system 232 comprises a conduit system 234 having a single electronically controlled variable flow restrictor unit 236. This is considered to be particularly advantageously for reasons of maintenance, dependability and construction. The system 232 includes a first damper 240 and a second damper 242. An outlet 238 of the restrictor unit 236 is connected to a first port 244 of the first damper 240 as well as a second port 246. As shown in the Figure 17, the first port 244 and the second port 246 are opposed in the sense of the induced flow direction. The outlet 238 is also connected to a first port 248 of the second damper 242 and an opposed port 250. The outlet 238 is connected to the damper 240 and the damper 242 by an arrangement 251 of one way valves 252. The one way valves 252 total four in number. This is considered advantageous for the reasons of reliability, maintenance and manufacture.

Each of the four ports 244, 246, 248 and 250 are connected to two of the check valves. The upper ports 244 and 248 are connected to the same to check valves. The lower ports 246 and 248 are connected to the other two check valves. Furthermore each of the valves is connected in parallel to another of the check valves between the electronically controlled variable flow restrictor unit 236 and the first and second dampers 240, 242. The arrangement is clearly illustrated in Figure 17.

The check valves 252 deliver fluid to and from a header tank 256 with the flow restrictor unit controlling delivery of fluid into the damper 240 and the damper 242. This occurs in an alternating manner.

Referring to Figure 18 there is shown a rotary damper system 254 according to another preferred embodiment of the present invention. Whereas the linear damping system 232 included a normal header tank 256, the rotary

damper system 254 includes a header tank in the form of an accumulator 258. In both systems the variable flow restrictor unit comprises a single normally open fixed restrictor 260 and a single electronically controlled variable restrictor 262. The flow through the normally open restrictor is varied using a valve 264.

With reference to Figure 18 it is to be appreciated that by arranging the rotary damper such that fluid from each of the chambers flows to and from a remote tank, the rotary damper arrangement is a new and advantageous. As would be apparent, the chambers of the housing of a rotary damper may be ported to a separate fluid chamber which may be provided in the form of a normal tank, reservoir, accumulator or other fluid storage device. The fluid chamber may be at ambient or elevated or reduced pressure. A proportional flow control valve is preferably placed in-line between the chambers and the tank to damp the flow between the chambers and the tanks, The damping characteristics of the valve are then preferably altered in order to optimise the rotary dampers performance in changing operating conditions. Alternately a fixed orifice or other form of flow control valve could be placed in line in order to damp or otherwise control the flow of fluid between the chambers and the tank(s). This is not presently preferred. Now that preferred embodiments of the gyro-stabiliser system and method have been described in detail, it will be apparent that the described embodiments provide a number of advantages over the prior art, including the following:

(i) It enables the rate and/or angle of precession to be controlled to ensure, that in all conditions, the angular range of precession is maximised but does not exceed a preferred maximum angular range.

(ii) It provides increased motion attenuation and improved safety by actively controlling the rate and/or angle of precession. (iϋ) It does not require external measurement of the motion of the platform that is being stabilised. Rather, the gyro-stabiliser is itself

used to sense the disturbance torque (as it causes precession of the gyro-stabiliser).

(iv) Arrangements are provided having a plurality of dampers that require only a single flow restrictor unit. The arrangement may use a pair of rotary dampers and advantageously have a conduit system using only four check valve groups.

(v) Systems and methods that provide precession axis damping that actively respond to changing operating conditions, where undesirable outcomes could be avoided. Preferred arrangements of the present invention are considered to be advantageously efficient and safe in comparison to prior art arrangements.

It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. For example, the gyro-stabiliser may employ two vertical or horizontal axis spinning masses, spinning in opposite directions in the same plane and mounted in the same gimbal mount. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described.