The invention relates to a motion compensation system, in particular a motion compensation system for compensating motion of a floating body such as a boat floating on water.

Motion compensation systems are generally known and are for example used for transferring loads, in particular people, to and/or from a vessel. For example from the Ampelmann® system as disclosed in general in NL1027103, or systems disclosed in WO2012/138227 and W02013/10564.

NL 1027103 discloses a vessel with a Stewart platform, also known as a hexapod, for compensating motions of a ship. The platform comprises a surface, borne on six hydraulic cylinders, and motion sensors. During use, with the aid of the sensors, the motions of the vessel are measured. With the aid of these measurements, the orientation of the cylinders is driven continuously so that the surface remains approximately stationary relative to the fixed world. A luffing gangway is connected to the platform. In this manner, motions of the vessel are compensated and for instance people or loads can be transferred from the vessel onto a stationary offshore construction, or vice versa.

The hydraulic cyhnders and/or actuators used in known motion compensation systems are powered by means of a hydraulic system which includes a fluid reservoir, a pressure line, a return line and a pump. The pump can move fluid from the reservoir via the pressure line to the cylinder in order to extend or retract the cylinder. In particular, one can shift a control valve, to direct the fluid from the pump to one side of the piston. Fluid from the other side of the piston can return to the reservoir via the return line. A disadvantage of known motion-compensation systems is that a considerable amount of energy is wasted during operation of the system. Therefore, it is an object of the invention to provide a motion compensation system that is more energy-efficient than known motion compensation systems.

To this end, the invention provides a motion compensation system for compensating motion of a floating body such as a boat floating on water, comprising a carrier for bearing, moving and/or transferring a load on the floating body, and one or more actuators for moving the carrier, the one or more actuator including a movable element and a motor module for driving the movable element, wherein the motor module of at least one actuator includes an electric machine that is arranged for using a movement of the movable element for regenerating electric power.

By providing a motor module that includes an electric machine that is arranged for using a movement of the movable element to regenerate electric power a more energy-efficient system is obtained. Typically, a substantial part of the motion that is compensated, e.g. the motion induced by sea waves, averages out over time. As a consequence, in motion compensation applications the energy generated by braking the platform is roughly equal to the energy consumed by driving the platform. According to the invention one is able to regenerate the braking energy, which would otherwise have been wasted. One can, for example, use this energy instantaneously for driving other actuators or for powering other systems.

In this way a more energy efficient motion-compensation system is obtained.

The system can advantageously be used so as to benefit from a periodic movement that is carried out by the movable element. The periodic movement may be associated with periodic external conditions such as a periodic movement of a local water level induced by surface waves on the sea.

Advantageously, the electric machine may be arranged for alternatingly driving the movable element as a motor and using a movement of the movable element for regenerating electric power as a generator. However, in principle one actuator may be solely used as a generator, while a second actuator is solely used for driving the carrier.

The one or more actuators may be arranged for providing the carrier with a translational and/or rotational movement relative to the floating body. As an example, one, two, three, four or more translational and/or rotational movements relative to the floating body can be realized by the one or more actuators. As a further example, a first number of actuators may be arranged for driving a hexapod on which hexapod the carrier has been mounted and/or a second number of actuators may be arranged for driving at least one carrier such as a motion compensation platform, a gangway or a crane. Optionally, the second number of actuators may be arranged for driving movable structures of a gangway, such as for performing a telescoping, luffing and/or slewing movement of the gangway.

According to a further aspect, the system may comprise an electric power storage unit connected to the motor module of the actuator. By providing an electric power storage unit, one is able to momentarily store the regenerated energy, which can then be used at a later time for powering the same or other actuators. Furthermore, the power storage unit can act as an emergency power supply.

Preferably, the motor module further includes a DC/AC converter having an AC terminal that is connected to the electric machine, and a DC terminal that is connected to the electric power storage unit.

Advantageously, the electric power storage unit may include a capacitor rack. By providing a capacitor one is able to quickly store and release the regenerated energy. In addition, by providing the capacitors in a rack a compact system is obtained.

Optionally, the electric power storage unit also includes a power input unit for receiving electric power from an boat AC power supply. In addition or instead of the AC power input unit a DC power input unit can be provided. Preferably, the system comprises a common DC bus interconnecting DC terminals of respective DC/AC converters to the electric power storage unit.

Preferably, the one or more actuators are electro-hydraulic actuators. For example, this allows one to make optimal use of present hydraulic technology.

Advantageously, the system may further comprise a control unit for controlling an initial charging process of the one or more motor modules and/or the electric power storage unit.

Further, the system may comprise a current limiting element and/or a current interrupting element for limiting and/or interrupting an electric current flowing from the power input unit towards a motor module of the respective one or more actuators. By providing a current interrupting element and/or hmiting element one is able to limit the total current. For example, this allows one to connect the system to an external AC power supply with a DC/AC converter that has a relatively low maximal wattage, thereby reducing the overall system size.

Advantageously, the system may further be provided with a switching element controlled by the control unit for bypassing the current hmiting element and/or the current interrupting element. For example, this allows one to by-pass the current hmiting and/or the current interrupting element once the system is sufficiently charged. In this way, the energy efficiency of the system is further improved.

The motion compensation system may comprise an actuator control unit for controlling operation the electric machine of the at least one actuator. Preferably, such an actuator control unit is arranged for configuring the electric machine to act as a motor when the driving force direction and the moving direction of the movable element coincide, and for configuring the electric machine to act as a generator when the driving force direction and the moving direction of the movable element are opposite to each other. In this way, a maximum amount of energy may be regenerated from the external environment.

The motion compensation system may optionally comprise multiple carriers for bearing, moving, and/or transferring a load.

According to a further aspect, at least one carrier of the motion compensation system is a motion compensation platform, a gangway or a crane.

Also, the invention relates to a motion compensation method of a boat floating on water, comprising a step of providing one or more motor modules for driving a movable element of a respective actuator for moving a carrier for bearing, moving and/or transferring a load, further comprising a step of using a movement of the movable element of at least one actuator for regenerating electric power via an electric machine of the respective motor module.

Optionally, the motion compensation method comprises a step of controlling an initial charging process of the one or more motor modules and/or an electric power storage unit connected to the one or more motor modules.

Further, the invention relates to a computer program product, comprising computer readable instructions for causing a processor to perform a motion compensation method of a boat floating on water, the method comprising a step of providing one or more motor modules for driving a movable element of a respective actuator for moving a carrier for bearing, moving and/or transferring a load, and a step of using a movement of the movable element of at least one actuator for regenerating electric power via an electric machine of the respective motor module.

The invention will be further elucidated on the basis of exemplary embodiments which are represented in the drawings. The exemplary embodiments are given by way of non-limitative illustration of the invention. In the drawings: Fig. 1 shows a boat provided with a motion compensation system according to the invention;

Fig. 2 shows a diagrammatic view of a motion compensation system according to the invention;

Fig. 3 shows a further diagrammatic view of another motion compensation system according to the invention;

Fig. 4 shows a close-up of an actuator of the system of Fig. 2 in four different stages of motion-compensation;

Fig. 5a shows a first embodiment of the motion compensation system according to the invention provided with a current limiting, interrupting and/or switching element;

Fig. 5b shows a second embodiment of a motion compensation system according to the invention provided with a current limiting, interrupting and/or switching element, and

Fig. 6 shows a flow-chart for a motion compensation method.

In the figures identical or corresponding parts are represented with the same reference numerals. The drawings are only schematic representations of embodiments of the invention, which are given by manner of non-limited examples.

Fig. 1 shows a boat 2 provided with a motion-compensation system 1 according to the invention. The system 1 comprises a carrier 3, which is embodied as a platform mounted on a hexapod on the boat 2, the hexapod comprising six actuators 4. The carrier 3 further includes a gangway.

Fig. 2 shows a diagrammatic view of a motion compensation system 1 according to the invention. Said system 1 is arranged for compensating motion of a boat 2 or another floating body floating on water, such as a pontoon. In particular, said system 1 comprises a carrier 3 or arm for bearing, moving, transferring and/or lifting a load on the boat. For example, the carrier may be implemented as a gangway, a platform or a crane arm. Said system 1 may comprise multiple carriers 3 for bearing, moving, transferring and/or lifting load. The motion compensation system 1 comprises one or more actuators 4, for moving the carrier 3. For example, the one or more actuators 4 can move the carrier relative to the boat by translational and/or rotational movement. Translational movement may include horizontal movement in the plane parallel to the water surface, as well as vertical movement perpendicular to the water surface, and rotational movement may include pitch, yaw and/or roll motion. The system 1 shown in Fig. 2 has six actuators 4a-4f for driving a hexapod and three other actuators 4g-4i for driving a gangway. For example, a platform may be mounted on the hexapod having six actuators 4a-4f, which is able to compensate for linear/translational and rotational movement of the floating object. The gangway may be attached to the platform, and may be movable relative to the platform using the three actuators 4g-4i. The one or more actuators 4 include a movable element 4-1, see also Fig. 3, and a motor module 4-2 for driving the movable element 4-1. For example, by connecting an output of the motor module, e.g. an output shaft, with the movable element one is able to retract or extend the actuator. The output of the motor module may directly be connected to the movable element 4-1, or the movable element may be moved by an hydraulic system that is operated using a pump 4-5 that is driven by the output of the motor module. The motor module 4-2 of at least one actuator includes an electric machine 4-3, also referred to as M, that is arranged for using a movement of the movable element for regenerating electric power. Said electric machine 4-3 may, for example, be implemented as a single motor-generator unit. It is noted, however, that, alternatively, the generator and motor may be implemented as separate units. It is further noted that the at least one actuator which includes an electric machine 4-3 may solely be used for regenerating electric power, while a further actuator is used for driving the carrier.

In a preferred embodiment the electric machine 4-3 is arranged for alternatingly driving the movable element 4- 1 as a motor and using a movement of the movable element 4-1 for regenerating electric power as a generator. To the use of movement of the movable element for regenerating power is also referred as braking. If the motor module 4-2 is driving the movable element 4-1 of the actuator 4 then power flows from the motor module 4-2 towards the movable element 4-1 of the actuator 4. Contrarily, if the motor module 4-2 is braking the actuator, also referred herein as regenerating, power flows from the movable element 4-1 towards the motor module 4-2.

The motor module 4-2 for driving the movable element, preferably, includes a DC/AC converter 4-2a having an AC terminal 4-2a’ that is connected to the electric machine, and a DC terminal 4-2 a” that is connected to the electric power storage unit 5. In case a DC motor is driving the movable element of the actuator, the system may include a DC/DC converter instead. The electric machine 4-3 is a high power electric machine 4-3 designed for delivering relatively high power for supporting and moving the one or more carriers 3. However, alternatively, the electric machine may be designed so as to deliver less power, e.g. for controlling a relatively small size crane.

The system 1 shown in Fig. 2 has the optional feature of an electric power storage unit 5 that is connected to the motor module 4-2 of the actuator 4. Such power storage unit may for example include a battery pack or a capacitor 5-2. In particular, a capacitor 5-2 has the advantage that it can be quickly charged, and is able to provide a high power. In a preferred embodiment, the capacitor 5-2 may be a so-called ultra-capacitor. To obtain a compact system the ultra-capacitors can advantageously be arranged in a rack. It is also possible that the power storage unit 5 includes a battery pack as well as a capacitor 5-1. The provision of a power storage unit 5 may be beneficial for various reasons. For example, it enables one to store the regenerated energy, which can then be subsequently used in driving the actuators. Further, it may serve as back-up power supply if another power source fails.

Optionally, the electric power storage unit 5 includes a power input unit 5-1, as shown in Fig. 2, for receiving electric power, for example from an AC power supply on the boat or floating object. Said AC power may for example be generated using a generator. Alternatively, the electric power storage unit 5 includes a power input unit for receiving electric power from a DC power supply. Preferably the power input unit includes an AC/DC or DC/DC converter for converting the AC or DC power that is generated on the boat or floating object to DC. An AC/DC converter may for example be implemented as an active line module.

The system 1 preferably comprises a common DC bus 6, as shown in Fig. 2, interconnecting DC terminals of respective DC/AC and/or DC/DC converters to the electric power storage unit. Such a DC bus 6 may, for example, be formed by a DC busbar. In the network topology, shown in Fig. 1, the capacitor 5-2 of the power storage unit 5, the external power supply and the motor modules 4-2 are connected in parallel to the DC bus bar. In this way the external power supply can charge the capacitor 5-2 and power the motor modules 4-2. When charged, the capacitor 5-2 is able to power the actuators.

Optionally, the capacitor 5-2 is connected to the DC bus 6 via an intermediate DC-DC converter 5-2a, as shown in Fig.2. This enables one to store more electrostatic energy in the capacitor 5-2.

Optionally, the system 1 comprises a control unit for controlling an initial charging process of the one or more motor modules and/or the electric power storage unit. Said control unit for controlling an initial charging process, also referred to as charging control unit, may for example be integrated within a central control unit 8-1 or can be implemented as a separate unit. Advantageously, a smart-charging control unit is provided. Said smart-charging control unit may be integrated within the central control unit 8-1 or can be implemented as a separate unit. The smart-charging control unit is configured to control the charging level of the power storage unit, e.g. such that the power which is (re)generated during braking can be stored within the power storage unit, e.g. depending on sea conditions. For example, if the sea is very rough, the energy that is regenerated during braking can be very high. In order to be able to store all this energy it may be necessary to keep the charging level below a charging threshold at the beginning of the regenerating step. Thereby, it is also avoided that the batteries or capacitors are overcharged, which can have adverse effects. In case energy from multiple actuators is regenerated, then the beginning of a regenerating step may be defined as a time instant when the actuators together generate a net amount of energy. At the same time it may be advantageous to maximize the charging threshold such that in case of power failure a maximum amount of energy is available. Hence, if the sea is relatively calm, it may be beneficial to use a relatively high charging threshold, as the energy generated during the braking will be relatively low, and can thus be stored even if the charging level at the start of the regenerating step was already high.The smart-charging control unit can further be configured to control the charging level of the power storage unit to a level that is high enough to control operation of the motion compensation system after an abrupt or soft power breakdown and/or to a level that is low enough so as to minimize energy that is dissipated after terminating normal operation of the motion compensation system.

Preferably, the system 1 comprises an actuator control unit for controlling operation of the electric machine of the at least one actuator. For example the actuator control unit may be arranged for configuring the electric machine 4-3 to act as a motor when the driving force direction and the moving direction of the movable element 4-1 coincide, and for configuring the electric machine to act as a generator when the driving force direction and the moving direction of the movable element are opposite to each other. The actuator control unit may be integrated within the motor module 4-2, or can be part of the central control unit 8-1, or can be implemented as a separate unit.

The system 1 shown in Fig. 2 further includes the optional feature of one or multiple discharge resistors 5-2b connected to the DC bus 6 via a discharge control unit 5-2c.

Fig. 3 shows a further diagrammatic view of another motion compensation system according to the invention. The system 1 has one actuator 4 with a movable element 4-1 which is provided within a cylinder 4-4. The movable element 4-1 of the actuator can be driven by a motor module 4-2. Said motor module 4-2 is connected to a DC bus 6 via a DC-AC converter 4-2a. Further, the motor module 4-2 includes an electric machine 4-3. In the shown embodiment, an output of the electric machine is connected to a pump 4-5, which can operate or drive a movement of the movable element 4-1. The electric machine 4-3 is arranged for using a movement of the movable element 4-1 relative to the cylinder 4-4 for (Degenerating electric power. Put differently, the electric machine 4-3 can be used as an electric power generator, powered by a movement of the movable element 4-1. If the electric machine is used as an electric power generator, power flows from the actuator 4 towards the DC bus 6. Then it can power or charge another unit connected to the DC bus 6. In particular, it can charge the power storage unit 5 that is connected to the DC bus 6. The power storage unit 5 includes a capacitor 5-2 that is connected to the DC bus 6 via a DC-DC converter 5-3. In addition, the system 1 includes a power input unit, herein embodied as a AC-DC converter 5-1, for receiving electric power from a boat AC power supply.

Fig. 4 shows a close-up of the system of Fig. 2 in four different stages of motion-compensation. In particular, each of the four panels of Fig. 3 depicts an actuator 4, in this specific embodiment corresponding to an hydraulic cylinder 4-4, of which the movable element 4-1 can be extended or retracted via a pump 4-5 that is driven by an output of the motor module 4- 1, more specifically by the electric machine 4-3. Said motor module also comprises an electric machine that is arranged for using a movement of the movable element for regenerating electric power. Put differently, the actuator can also act as a generator.

The upper left panel, Fig. 4 (a) corresponds to the situation wherein the actuator is extending or moving upwardly with a velocity vcylinder and wherein the hydraulic pump generates a force Fcylinder acting in the same direction as the velocity vcylinder. Hence, the actuator is consuming power P= | Fcylinder | * | vcylinder | . Such a situation may for example arise when a wave is lowering a vessel with a motion compensated platform mounted thereon. Then, the actuator is expanding, thereby keeping the platform at a constant height, while the vessel is moving downwards. In order to compensate for the force of gravity acting on the platform, the actuator has to apply a force Fcylinder in the opposite direction. The actuator is then consuming power

P= I Fcylinder | * | vcylinder | . If the capacitor is charged, the power will flow from the capacitor towards the motor module and/or actuator.

The upper right panel, Fig. 4 (b) corresponds to the situation wherein the actuator is retracting or moving downwardly with a velocity vcylinder, while the hydrauhc pump generates a force Fcylinder acting against the retraction. Put differently, the force Fcylinder and the velocity of the actuator vcylinder are pointing in opposite directions. Such a situation may for example arise when a wave is lifting a vessel with a motion compensated platform mounted thereon. Then, the actuator is retracting, thereby keeping the platform at a constant height, whereas the vessel is rising. In order to compensate for the force of gravity acting on the platform, the actuator has to apply a force Fcyl in the opposite direction. So contrary to the situation shown in Fig. 4 (a),, the actuator is (re)generating power P= I Fcylinder | * | vcylinder | . This power can be used to (re)charge the power storage unit 5, e.g. the capacitor 5-2. To this end, the busbar or DC bus 6 connecting the motor module and the capacitor allows for a bidirectional power flow.

Both in Fig. 4(a) and Fig. 4(b) the force Fcylinder is pointing in the upward direction, corresponding to a pushing action. This may, for example, be the case when the motion-compensated platform is supported by the actuator from below.

The lower left panel, Fig. 4 (c), corresponds to the situation wherein the actuator is extending upwardly, while the hydraulic pump generates a force Fcylinder acting against the extension, i.e. in the direction opposite to the velocity vcylinder. As in Fig. 4 (b) the actuator is (re)generating power P= I Fcylinder | * | vcylinder | , which can be used to (re)charge the capacitor.

The lower-right panel, Fig. 4(d), corresponds to the situation wherein the actuator is retracting downwardly, and the hydraulic pump generates a force Fcylinder acting in the same direction as the velocity of the cylinder vcylinder. Both in Fig. 4(c) and Fig. 4(d) the force Fcylinder is pointing in the downward direction, corresponding to a pulling action. This may, for example, be the case if a motion-compensated gangway is suspended from above by an actuator.

Typically, a substantial part of the motion that is compensated, e.g. the motion induced by sea waves, averages out over time. As a consequence, in motion compensation applications the energy generated by braking and consumed by driving the actuators are roughly equal to each other. As the system 1 allows one to momentarily store the braking energy in the capacitors one obtains a system that is more energy efficient than existing motion-compensation systems. Since in the system 1, the power storage unit 5, and/or the capacitor 5-1 is able to provide the major part of the power that is delivered to the actuator 4, one is able to use an external power source with relatively low instantaneous power compared to the maximum instantaneous power consumed by the actuator. Further, one is able to use an AC-DC converter for connecting the system to an external power source with a reduced maximum instantaneous power. In this way, the system can be kept compact and/or relatively inexpensive. Furthermore, the power storage unit 5 can be used as an emergency power source in case the external power source fails.

It is noted instead of using electro-hydraulic actuators, the system 1 may use fully electronic actuators or a combination of electro-hydraulic and fully electronic actuators.

Fig. 5(a) and (b) depict a diagrammatic view of a motion compensation system 1 with a current limiting element and/or current interrupting element 7 for limiting and/or interrupting an electric current flowing from the power input unit towards a motor module of the respective one or more actuators. Preferably, the system 1 further comprises a switching element for by-passing the current limiting element and/or the current interrupting element. Such a current interrupting element and/or a switching element, for example a by-pass relay, may be controlled by a control unit, for example by the control unit for controlling an initial charging process of the one or more motor modules and/or the electric power storage unit. The control unit may be integrated with the central control unit 8-1. The system 1 depicted in Fig. 5(a) includes four motor modules of a first group FG and six motor modules of a second group SG. The first group of motor modules FG may for example drive a motion-compensated gangway, and the second group of motor modules SG may for example drive a hexapod. The individual motor modules are also referred to as MM. In the shown embodiment, a capacitor 5-2 and the motor modules 4-2 are all connected in parallel. In addition, Fig. 5(a) shows a pre-charging module 7 that is connected in series with the second group of motor modules SG and the capacitor 5-2. The combination of the second group of motor modules SG, the capacitor 5-2 and the pre-charging module 7 is connected in parallel with the first group of motor modules FG. The pre-charging module 7 comprises a current limiting element, here embodied as a pre-charging resistor 7-1, and a switching element, here embodied as a by-pass relay 7-2. When powering on the system 1 shown in Fig. 5(a) the pre-charging module 7, with the by-pass relay 7-2 open, limits the current that powers the second group of motor modules SG. Once the system 1 is charged, e.g. 99.3% charged, the by-pass relay 7-2 can be closed, and subsequently the system 1 can be operated normally, i.e. can be used to compensate motion. It is noted that the pre-charging module 7 in the system 1 shown in Fig. 5(a) may also be used in the absence of the first group of motor modules. Further, the number of motor modules in the first and/or second group can also be different.

Fig. 5(b) shows a diagrammatic view of a motion compensation system 1 with a pre-charging module 7. The system 1 in Fig. 5(b) includes a first group of motor modules FG and a second group of motor modules SG. The first group comprises four motor modules, e.g. for driving a gangway, and the second group comprises six motor modules, e.g. for driving a hexapod on which a platform is mounted. The motor modules of the first group are connected in parallel to the external power supply. Each of the motor modules of the second group is connected via the pre-charging module 7 to the external power supply. The pre-charging module 7 comprises a current limiting element 7-1, here embodied as a pre-charge resistor, connected in parallel with a switching element 7-2, here embodied as by-pass relay, which together are connected in series with a current interrupting element 7-3, here embodied as an isolator relay. By having the isolator relay open, the external power supply charges only the first group. When, the first group of motor modules FG is charged, e.g. up to 99.3%, the isolator relay may be closed, while keeping the by-pass relay open. Then the second group of motor modules SG is being charged, during which the pre- charging resistor limits the current. When both the first and second group of motor modules are charged, the by-pass relay can be closed, thereby enabhng the normal operation of the system 1. It is noted that the first and second groups of actuators may also include different numbers of actuators.

By providing a current limiting, a current interrupting and/or a switching element the current flowing from the external power supply may be limited, for example during charging of the system 1. In this way, the current limiting, current interrupting and switching element allows one to connect the system 1 to the external power supply using an AC-DC converter with a reduced maximal power.

Fig. 6 shows a flow chart for a motion compensation method 100 of a floating body such as a boat floating on water. The method includes a step 110 of providing one or more motor modules for driving a movable element of a respective actuator for moving a carrier for bearing, moving, lifting and/or transferring a load on the floating body. The method further comprises a step 120 of using a movement of the movable element of at least one actuator for regenerating electric power via an electric machine of the respective motor module. Optionally, the method includes a step 130 of controlling an initial charging process of the one or more motor modules and/or an electric power storage unit connected to the one or more modules.

The method for motion compensation 100 can be performed using a dedicated hardware structures, such as FPGA, PLC and/or ASIC components. Otherwise, the method can also at least partially be performed using a computer program product comprising instructions for causing a processor, such as a dedicated processor or a general purpose computer, to perform the above described steps of the method 100 according to the invention. All steps can in principle be performed on a single processor. However, it is noted that at least one step can be performed on a separate processor. A processor can be loaded with a specific software module. Dedicated software modules can be provided, e.g. from the Internet. It will be clear to the skilled person that the invention is not limited to the exemplary embodiment represented here. Many variations are possible.

Such variations shall be clear to the skilled person and are considered to fall within the scope of the invention as defined in the appended claims.