Torque Control

Abstract

A power take-off arrangement for a wave energy converter is described. The arrangement has a gearbox having a first shaft (20) connected to the wave energy converter (1) which is adapted to receive, in use, rotational motion from the wave energy converter. The gearbox has a second shaft connecting the annulus gear (31) to the generator shaft (36). A third shaft (38) is connected to one or more means (39) of applying torque to it. Torque applied by the one or more means (39) may be varied in operation so as to alter the relative rotational velocities and accelerations of the second and third shafts in response to any given rotational velocity or acceleration of the first shaft.

Claims

1. A power take-off arrangement for a wave energy converter, the power take-off arrangement including: a gearbox comprising: a first shaft connected to the wave energy converter and adapted to receive, in use, rotational motion from the wave energy converter; a second shaft connected to a generator; and a third shaft connected to one or more means of applying torque thereto; characterised in that the torque applied by the one or more means may be varied in operation so as to alter the relative rotational velocities and accelerations of the second and third shafts in response to any given rotational velocity or acceleration of the first shaft.

2. The power take-off arrangement according to claim 1, wherein the gear box is a three-way planetary gear arrangement.

3. The power take-off arrangement according to claim 2, wherein said third shaft of the gearbox is connected to a central sun gear of the planetary gear arrangement.

4. The power take-off arrangement according to claim 2, wherein the three-way gearing arrangement includes bevel gearing.

5. The power take-off arrangement according to claim 1, wherein the first shaft is connected to the wave energy converter by direct connection or via another portion of the power take-off arrangement.

6. The power take-off arrangement according to claim 1, wherein the first shaft is adapted to receive, in use, unidirectional rotational motion.

7. The power take-off arrangement according to claim 1, wherein the means of applying torque to the third shaft comprises a disc brake.

8. The power take-off arrangement according to claim 6, wherein the means of applying torque to the third shaft comprises a hydraulic pump.

9. The power take-off arrangement according to claim 6, wherein the means of applying torque to third shaft comprises an electrical generator.

10. The power take-off arrangement according to claim 6, wherein the means of applying torque to third shaft comprises a motor generator.

11. A wave energy converter comprising a power take-off arrangement as described in claim 1.

12. A method of operating a power take-off arrangement according to claim 1 comprising reducing torque applied to the third shaft when the instantaneous loading on the transmission system exceeds a load limit for the wave energy converter.

13. A method of operating a power take-off arrangement according to claim 1 comprising reducing torque applied to said the shaft to achieve a ‘survival’ state

14. A method of operating a power take-off arrangement according to claim 1 comprising modulating the torque to maintain a constant rotation speed at a main generator.

15. A method of operating a power take-off arrangement according claim 1 comprising applying torque to the third shaft and the electrical generator or motor generator generating electricity which may be exported to the grid or used to power on-board systems.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0038] FIG. 1 shows an example of an attenuator-type wave energy converter of the prior art;

[0039] FIG. 2 shows one embodiment of a power take-off of the present invention suitable for mounting in the hinge of the wave energy converter of FIG. 1;

[0040] FIG. 3 shows an embodiment of the invention in which the means of applying torque to the third shaft comprises a disc brake mounted on the shaft and a number of brake calipers mounted to the housing;

[0041] FIG. 4 shows an embodiment of the invention in which the means of applying torque to the third shaft comprises both a disc brake and a hydraulic pump;

[0042] FIG. 5 shows a schematic view of a system by which the flow of hydraulic fluid produced by the hydraulic pump of FIG. 4 may be used to generate electrical power; and

[0043] FIG. 6 shows an embodiment of the invention in which the means of applying torque to shaft comprises both a disc brake and an electrical generator or optionally motor/generator.

DETAILED DESCRIPTION OF THE INVENTION

[0044] Referring now to FIG. 1, which shows an example of an attenuator-type wave energy converter (1) of the prior art comprising a first buoyant pontoon assembly (2) and a second buoyant pontoon assembly (3), connected by a hinge (4) which permits relative rotation of the pontoon assemblies around the hinge axis. The first pontoon assembly is restrained by a mooring (5) to the sea bed. In response to an incoming wave stream in direction (6) the pontoon assemblies (2, 3) will tend to follow the water surface (7), causing a reciprocating rotational motion at the hinge (4). This is an example of the type of device to which the present invention may be fitted.

[0045] Thus, FIG. 2 shows one embodiment of a power take-off of the present invention suitable for mounting in the hinge (4) of the wave energy converter (1) of FIG. 1. The mounting arrangement is such that the motion of the wave energy converter produces a relative rotation between the input shaft (20) and the housing (21) about the axis (22).

[0046] In this arrangement, there is a primary planetary gearing stage (23), comprising an annulus gear (24) which is rigidly mounted in the housing (21). A number of planet gears (25) are rotatably mounted on planet pins (26), the pins in turn being supported by a planet carrier (27). The planet gears (25) are arranged in mesh with the annulus gear (24). A central sun gear (28) is mounted on a sun shaft (29) and arranged in mesh with the planet gears (25).

[0047] There is also a second planetary gearing stage (30), again comprising an annulus gear (31), planet gears (32), planet pins (33), a planet carrier (34) and a sun gear (35). The planet carrier (34) is torsionally connected to the sun shaft (29) of the first planetary gearing stage (23). The annulus gear (31) is torsionally connected to the rotor shaft (36) of a generator. The sun gear (35) is mounted on a sun shaft (38) which extends axially through the bore of the generator rotor shaft (36) and is torsionally connected to a means (39) of controlling the torque applied to the shaft (40). The generator additionally comprises stator windings (41) mounted in the housing (21) such that rotation of the generator rotor (37) produces a current in the windings (41).

[0048] In operation, it will be apparent that if the shaft (38) is constrained such that it cannot rotate with respect to the housing (21), rotational of the input shaft (20) will cause rotation of the generator rotor shaft (36) with a fixed speed ratio determined only by the numbers of teeth in the planetary gear arrangements (23) and (30). If, however, the sun shaft (38) is permitted to rotate, the second planetary gearing arrangement (30) acts as a differential or three-way gearing arrangement in which the ratio of speeds between any two of the input shaft (20), generator rotor shaft (36) and sun shaft (38) is dependent on the speed of the third. Thus by controlling the magnitude and direction of torque applied to the shaft (38), the acceleration of the generator rotor shaft (36) in response to a given acceleration of the input shaft (20) may be varied across a wide range.

[0049] Thus the gearbox has a first shaft (20) connected to the wave energy converter (1) which is adapted to receive, in use, rotational motion from the wave energy converter. The gearbox has a second shaft connecting the annulus gear (31) to the generator shaft (36). A third shaft (38) is connected to one or more means (39) of applying torque to it. Torque applied by the one or more means (39) may be varied in operation so as to alter the relative rotational velocities and accelerations of the second and third shafts in response to any given rotational velocity or acceleration of the first shaft.

[0050] FIG. 3 shows an embodiment of the invention in which the means (39) of applying torque to shaft (38) comprises a disc brake (42), mounted on the shaft (38), and a number of brake calipers (43) mounted to the housing (21).

[0051] In a first mode of operation, the brake calipers are used to prevent the disc (42), shaft (38), and sun gear (35) from rotating relative to the housing (21). Since the sun shaft (38) is the lowest torque part of the geared system, the torque capacity of the brake disc (42) and calipers (43) is minimised by this layout. In this mode, all power entering the transmission from the input shaft (20) is extracted from the generator (37, 41).

[0052] In a second mode of operation, the brake calipers are released, allowing rotation of disc (42), shaft (38) and sun gear (35) relative to the housing (21). In response to a given acceleration of the input shaft, both the generator rotor shaft (36) and the sun shaft (38) will accelerate, the relative accelerations being determined by the relative inertias of these two shafts and the components mounted to them, and the torque acting on the generator. Since the inertia of the sun shaft (38) and associated components will be considerably lower than that of the generator rotor shaft (36) and associated components, the acceleration of the sun shaft (38) will be higher.

[0053] The system may transition from the second mode of operation to the first mode of operation by reapplying pressure to the brake calipers (43). This may be done while the shaft (38) is moving, in which case power will be dissipated as heat until the speed of the shaft (38) relative to the housing (21) is reduced to zero. Alternatively, the application of the brakes may be timed to coincide with the speed reducing to zero as the WEC reverses direction. By applying the brakes at this point the wear on the brake pads is effectively zero. This is accomplished by means of a controller and means for sensing rotational velocities and torques (not shown).

[0054] FIG. 4 shows an embodiment of the invention in which the means (39) of applying torque to shaft (38) comprises both a disc brake (42) and a hydraulic pump (50).

[0055] FIG. 5 shows a schematic view of a system by which the flow of hydraulic fluid produced by the hydraulic pump (50) of FIG. 4 may be used to generate electrical power.

[0056] In a first mode of operation of such a combined system, as shown in FIG. 3, the brake calipers (43) are used to prevent the disc (42), shaft (38) and sun gear (35) from rotating. All power entering the transmission from the input shaft (20) is extracted from the generator (37, 41).

[0057] In a second mode of operation, the brake calipers (43) release the disc (42), permitting rotation of the shaft (38) and attached hydraulic pump (50). Rotation of the hydraulic pump (50) causes a movement of hydraulic fluid. A connected hydraulic system such as that shown in FIG. 5 may be controlled by a supervisory system to vary the resistance to this fluid flow. The resistance is created by extracting energy through the hydraulic motor (61) and attached electrical generator (62). At low resistance, the behaviour of the system will be similar to the second mode of operation of the system of FIG. 3. Increasing the resistance of the hydraulic system will increase the proportion of any given acceleration of the input shaft (20) which is transferred to the generator rotor shaft (36), and reduce the proportion which is transferred to the sun shaft (38). By further increase in resistance, the majority of the braking energy dissipation required to transition from the second mode of operation to the first mode of operation may be achieved by the hydraulic system (and thus converted to useful electrical energy) with only a smaller, final portion achieved by the disc brake (42) to bring the speed of the sun shaft relative to the housing to zero and hold it there.

[0058] In a third mode of operation, the brake is released whenever the main generator rotor (37) approaches the maximum rotational speed and electrical output frequency which can be accommodated by the power converters. Further acceleration of the prime mover will largely result in acceleration of the shaft (38) and hydraulic pump (50). The main generator (37, 41) is able to remain connected to the grid while the additional power is extracted by the hydraulic system and secondary generator (62).

[0059] FIG. 6 shows an embodiment of the invention in which the means (39) of applying torque to shaft (38) comprises both a disc brake (42, 43) and an electrical generator or optionally motor/generator (70). This system is able to operate in the same modes of operation as the system of FIG. 4, but additionally is able to feed energy back into the system—the energy being drawn either from storage capacity on board the WEC or from the electricity grid. This capability may provide additional flexibility in ensuring the rotation speed of the main generator (37, 41) spends the maximum amount of time operating within the frequency envelope of the power converters.

[0060] It will be apparent that other embodiments of the present invention are possible which are not described here. For example, the three-way gearing arrangement described may be achieved by other arrangements of planetary gearing, or by arrangements of bevel gearing, as is common in automotive differentials. The primary gearing stage (23) may be replaced by an alternative gearing arrangement, by multiple gearing stages, or omitted entirely such that the input shaft (20) was directly connected to the planet carrier (34), depending on the desired range of overall transmission ratios to be achieved by the system.

[0061] The present invention could be arranged to form part of a power take-off system also comprising the transmission system described in Patent EP2425123. In operation, the torque control capabilities of the present invention could be used to mitigate the dynamic shock-loading associated with the engagement of the one-way transmission elements, thus allowing the advantages of uni-directional generator rotation described in Patent EP2425123 to be realised.