Hydraulic fluid power transmission

Abstract

The invention relates to a hydraulic fluid power transmission comprising: actuator apparatus having a plurality of chambers, each of the chambers having a respective fluid driving surface configured to drive or be driven by hydraulic fluid therein; a discretised pressure control system configured to selectively connect one or more first chambers of the said plurality of chambers to one of a plurality of hydraulic fluid sources or sinks, at least two of the plurality of hydraulic fluid sources or sinks having different fluid pressures; a continuous pressure control system configured to control the pressure of hydraulic fluid, or a flow rate of hydraulic fluid, input to or output from one or more second chambers of the said plurality of chambers, the pressure or flow rate of the hydraulic fluid input to or output from the second chambers being thereby variable within a range of pressures or flow rates respectively; and a controller configured to control the discretised and continuous pressure control systems to thereby regulate a property of the actuator affected by the hydraulic fluid pressure in the said chambers.

Claims

1. A hydraulic fluid power transmission comprising: actuator apparatus having a plurality of chambers, each of the chambers having a respective fluid driving surface configured to drive or be driven by hydraulic fluid therein; a discretised pressure control system including a plurality of bodies configured to contain hydraulic fluid therein, wherein each of the plurality of bodies is configured to function as a hydraulic fluid source or a hydraulic fluid sink, wherein at least two of the plurality of bodies have different fluid pressures, and wherein the discretised pressure control system is configured to selectively connect each of a plurality of first chambers of the plurality of chambers, independently, to respective ones of the plurality of bodies to produce a stepped load profile; a continuous pressure control system configured to control the pressure of hydraulic fluid, or a flow rate of hydraulic fluid, input to or output from one or more second chambers of the said plurality of chambers; and a controller configured to receive a demand signal indicative of a demand value of a property of the actuator apparatus, the property being affected by the hydraulic fluid pressure in the chambers, and to control the discretised and continuous pressure control systems in response to the received demand signal to thereby regulate the property of the actuator apparatus.

2. The hydraulic fluid power transmission according to claim 1 wherein the controller is configured to control the discretised and continuous pressure control systems such that a contribution of the second chambers to the said property of the actuator apparatus at least partially compensates for a difference between a contribution of the said first chambers to the said property of the actuator apparatus and the demand value of the said property of the actuator apparatus indicated by the received demand signal.

3. The hydraulic fluid power transmission according to claim 2 wherein the controller is configured to control the discretised and continuous pressure control systems to thereby regulate the property of the actuator apparatus the received demand signal for the said property of the actuator apparatus.

4. The hydraulic fluid power transmission according to claim 3 wherein the controller is configured to generate a discretised demand and a continuous demand from the said received demand signal for the said property of the actuator apparatus, the controller being configured to control the discretised pressure control system to follow the discretised demand and to control the continuous pressure control system to follow the continuous demand.

5. The hydraulic fluid power transmission according to claim 4 wherein the discretised demand is determined by offsetting the received demand for the said property of the actuator apparatus.

6. The hydraulic fluid power transmission according to claim 5 wherein the discretised demand is determined by offsetting the received demand by half the load range available from an individual second chamber of the said second chambers.

7. The hydraulic fluid power transmission according to claim 1 wherein the continuous pressure control system comprises one or more variable displacement pump/motors operable to operate as a pump or as a motor in different operating modes.

8. The hydraulic fluid power transmission according to claim 7 wherein the continuous pressure control system comprises a variable displacement pump or a variable displacement motor in a common shaft arrangement with another unit exchanging hydraulic fluid with a high pressure one of the plurality of bodies.

9. The hydraulic fluid power transmission according to claim 8 wherein the common shaft is coupled to a prime mover or energy sink, and wherein the controller is configured to control the net shaft torque to provide a smoothed net power transfer between the common shaft and the prime mover or energy sink.

10. The hydraulic fluid power transmission according to claim 1 wherein the continuous pressure control system is configured to control the pressure or flow rate of hydraulic fluid into or out of one of the one or more second chambers responsive to a change of volume of that chamber.

11. The hydraulic fluid power transmission according to claim 1 wherein the continuous pressure control system is configured to control the flow rate or pressure of hydraulic fluid into or out of a said second chamber to cause a desired change in pressure taking into account the volume of the said second chamber and an effective bulk modulus of the fluid volume.

12. The hydraulic fluid power transmission according to claim 1 wherein the controller is configured to synchronise a change in pressure of at least one of the one or more second chambers with a corresponding change in pressure in one or more of the plurality of first chambers.

13. The hydraulic fluid power transmission according to claim 1 wherein the fluid driving surfaces of the said chambers are coupled to each other such that the forces exerted by hydraulic fluid on the respective fluid driving surfaces of the chambers are additive or subtractive.

14. A method of transmitting hydraulic fluid power, the method comprising: providing an actuator apparatus having a plurality of chambers, each of the chambers having a respective fluid driving surface configured to drive or be driven by hydraulic fluid therein; providing a plurality of bodies configures to contain hydraulic fluid therein, each of the plurality of bodies being configured to function as a hydraulic fluid source or a hydraulic fluid sink, wherein at least two of the plurality of hydraulic fluid sources or sinks have different fluid pressures; receiving a demand signal indicative of a demand value of a property of the actuator apparatus, the property being affected by the hydraulic fluid pressure in the chambers; and in response to the received demand signal: selectively connecting each of a plurality of first chambers of the plurality of chambers to one of the plurality of bodies, independently, to produce a stepped load profile; and controlling the pressure or flow rate of hydraulic fluid input to or output from said one or more second chambers of the said plurality of chambers by varying the pressure or flow rate of hydraulic fluid flowing into or out of the said second chambers within a range of pressures or flow rates respectively to thereby regulate the property of the actuator apparatus.

Description

DESCRIPTION OF THE DRAWINGS

(1) An example embodiment of the present invention will now be illustrated with reference to the following Figures in which:

(2) FIG. 1 is a schematic hydraulic circuit diagram of a hydraulic transmission which uses continuous pressure control enabled by a pair of variable displacement pump-motors on a common shaft;

(3) FIG. 2 is a schematic hydraulic circuit diagram of a hydraulic transmission which uses quantised pressure control enabled by two pairs of electronically controlled valves configured to selectively connect each of a pair of actuator chambers to a high pressure energy store and a low pressure reservoir;

(4) FIG. 3 is a schematic hydraulic circuit diagram of a hybrid hydraulic transmission having a pair of actuators each with four hydraulic fluid chambers acting about a pivot to allow torque and power to be transmitted about it, the pressure of all four chambers of one actuator and the pressure of three of the other being controlled by a discretised pressure control system and the pressure within the remaining actuator of the said other actuator being controlled by a continuous pressure control system;

(5) FIG. 4 is a block diagram of an algorithm for calculating a demand to be met by the continuous pressure control system of FIG. 3;

(6) FIG. 5 is a block diagram of an algorithm implemented by the continuous pressure control system to meet the demand calculated by the algorithm of FIG. 4;

(7) FIG. 6 is a graph showing the various demand and output signals from example quantized and continuous pressure control systems;

(8) FIG. 7 is a block diagram illustrating the hydraulic transmission of FIG. 3 used in a heave compensation winch; and

(9) FIG. 8 is a flow chart showing a method for controlling the discretised pressure control system and the continuous pressure control system.

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT

(10) FIG. 3 is a schematic hydraulic circuit diagram of a hybrid hydraulic fluid transmission comprising first and second linear hydraulic actuators 100, 102 acting about a pivot to allow torque and power to be transmitted about it, the first and second actuators 100, 102 each having four actuator chambers 104, 106, 108, 110 and 141, 142, 143, 144 respectively. The hydraulic actuators 100, 102 are hydraulic rams of apparatus for extracting energy from water waves, the actuators 100, 102 being driven by the relative rotation of buoyant body members (not shown) provided in a body of water (e.g. an ocean), the relative rotation being caused by waves in the body of water, the buoyant body members being coupled to each other by a coupling which permits relative rotation between the body members and which causes movement within each of the actuators 100, 102 of a shaft into and out of a cylinder 114, thereby causing first and second pistons 116, 118 provided on the shaft 112 to reciprocate in the cylinder 114 to perform work. The pistons 116, 118 of the actuators 100, 102 each have opposing respective fluid driving surfaces configured to drive or be driven by hydraulic fluid in the respective chambers 104-110 (in respect of the first actuator 100), 141-144 (in respect of the second actuator 102). A divider 125 is provided between the pistons 116, 118 of each actuator 100, 102 so that the pistons 116, 118 of each actuator 100, 102 operate in series with each other. Chambers 104, 106 are provided on either side of the first piston 116 of the first actuator 100, while chambers 108, 110 are provided on either side of the second piston 118 of the first actuator 100. Chambers 141, 142 are provided on either side of the first piston 116 of the second actuator 102, while chambers 143, 144 are provided on either side of the second piston 118 of the second actuator 102. The pressures in the chambers 104-110, 141-144 are controlled in order to control the resistance of the movement of the pistons 116, 118 with the cylinder 114 of each actuator 100, 102, in order to maximise the energy extracted by the hydraulic rams.

(11) Three chambers 104, 106 and 108 of the first actuator 100 and all four chambers 141-144 of the second actuator 102 (hereinafter referred to as first chambers) are selectively connected to a high pressure hydraulic fluid accumulator 120 and a low pressure hydraulic fluid reservoir 122 by way of respective hydraulic fluid lines and respective banks 124 of electronically controlled valves coupled between the respective chambers 104-108, 141-144 and the accumulator 120 and the reservoir 122. The banks 124 thus operate as a discretised pressure control system. Each of the banks 124 of electronically controlled valves are identical, so only the bank 124 coupled between the chamber 104 of the first actuator 100 and the accumulator 120 and reservoir 122 is described below for brevity.

(12) The bank 124 comprises first and second electronically controlled high pressure valves 126, 128 arranged in parallel and configured to selectively connect the chamber 104 to the accumulator 120. Each of the high pressure valves 126, 128 have open states in which hydraulic fluid can flow through them between the chamber 104 and the accumulator 120 and closed states in which hydraulic fluid cannot flow through them between the chamber 104 and the accumulator 120. The first high pressure valve 126 has a greater fluid flow area therethrough when it is in its open state than the second high pressure valve 128. This is illustrated in FIG. 3 by way of a hydraulic impedance in series with the second high pressure valve 128. The second high pressure valve 128 (e.g. by virtue of having a valve member with a smaller cross sectional area than that of the first high pressure valve 126) is capable of transitioning between its open and closed states more quickly than the first high pressure valve 126, but the maximum fluid flow rate through the second high pressure valve 128 (when open) is less than the maximum fluid flow rate through the first high pressure valve 126 (when open).

(13) The bank 124 further comprises first and second electronically controlled low pressure valves 130, 132 arranged in parallel and configured to selectively connect the chamber 104 to the reservoir 122. The first low pressure valve 130 has a greater fluid flow area therethrough when it is in its open state than the second low pressure valve 132. This is illustrated in FIG. 3 by way of a hydraulic impedance in series with the second high pressure valve 132. The second low pressure valve 132 (e.g. by virtue of having a valve member with a smaller cross sectional area than that of the first low pressure valve 130) is capable of transitioning between its open and closed states more quickly than the first low pressure valve 130, but the maximum fluid flow rate through the second low pressure valve 132 (when open) is less than the maximum fluid flow rate through the first low pressure valve 130 (when open).

(14) The bank 124 further comprises a large cross sectional flow area low pressure check valve 134 operable to selectively connect the chamber 104 to the low pressure reservoir 122 in order to maintain the pressure in the chamber 104 at least at the pressure in the low pressure reservoir (which may be charged above atmospheric pressure), for example when the chamber 104 is increasing in volume (by movement of the shaft 112 and the piston 116), to prevent cavitation. The check valve 134 is configured to open when the pressure in the chamber 104 falls below the pressure of the reservoir 122.

(15) The valves of the bank 124 control the timing and the transition profiles (i.e. the way in which the pressure changes over time) of the chamber 104 between a high pressure state, when it is connected to the accumulator 120, and a low pressure state when it is connected to the reservoir 122. More specifically, the second low or high pressure valves 128, 132 open first, to begin the pressure transition gradually. When the pressure transition is underway, the first low or high pressure valves 128, 132 are opened to allow the full rate of hydraulic fluid flow through the combination of first and second low or high pressure valves, to thereby complete the pressure transition. This prevents extremely quick large pressure transitions from occurring, which could cause a shock to the system. In addition, the timing profiles of the transitions are typically controlled to permit a pressure change in chamber 110 to synchronise with a pressure change in one or more of the first chambers 104-108, 141-144 and to optimise the performance and accuracy of the overall system. Typically in practice the timing profiles of the transitions of the chamber 104 between the low and high pressure states ensures that the control latency of the chamber 104 is greater than the control latency of the chamber 110.

(16) The valves of the banks 124 are typically electronically controlled solenoid valves which are controlled by a controller 140 provided in electronic communication with the valves. The controller 140 acts to selectively open and close the valves by turning a solenoid on and off, the valves being biased to the open or closed positions by a (typically passive) biasing mechanism such as a spring. The banks 124 of low and high pressure valves are thus part of a discretised control system under the control of the controller 140 which is configured to selectively connect the first chambers 104-108, 141-144 to the accumulator 120 and the reservoir 122. The controller typically comprises one or more computer processors configured to run computer program code. It may be that the controller 140 is distributed across a plurality of processors.

(17) As explained in the Background of invention, use of this discretised control method alone leads to an undesirable stepped output.

(18) Chamber 110 (which will be referred to as a second chamber hereinafter) is connected to a continuous pressure control system comprising a first variable displacement hydraulic pump-motor 150 coupled to a second variable displacement hydraulic pump-motor 152 by a common shaft 151, the variable displacement hydraulic pump-motors 150, 152 each being operable to function as a pump or a motor in different operating modes. The variable displacement hydraulic pump-motors 150, 152 are also coupled to an electricity generator 154 by way of the common shaft 151 (which is operable to drive the generator 154). Connection between the chamber 110 and the continuous pressure control system is by way of a hydraulic line 160, and the chamber 110 is also selectively connected to the low pressure reservoir 122 by an electronically controlled valve 162 which is controlled by, and in electronic communication with, the controller 140. Valve 162 will either open passively in the event that the pressure in the chamber 110 falls below the pressure in the reservoir 122 (to prevent cavitation) or actively on the command of the controller 140 to disable flow from the pump-motors 150,152 into or out of the chamber 110.

(19) Depending on the pressure requirements in the second chamber 110, one of the first and second variable displacement hydraulic pump-motors 150, 152 operates in a motoring mode to convert high pressure fluid (received from the chamber 110 or from the accumulator 120) to torque on the shaft 151 and output low pressure fluid to the reservoir 122. The other of the first and second variable displacement hydraulic pump-motors 150, 152 operates in pumping mode to convert torque of the shaft to pressurised fluid which is either provided to the accumulator 120 or to the chamber 110. The displacements of the hydraulic pump-motors 150, 152 can be varied in each case to consume or provide hydraulic fluid from or to the second chamber 110 having a pressure which is variable across a (typically continuous) range.

(20) It will be understood that if the continuous pressure control system is required to consume high pressure fluid from the chamber 110, the first variable displacement hydraulic pump-motor 150 functions in the motoring mode, converting high pressure fluid from the chamber 110 to a torque on the shaft 151. The second variable displacement hydraulic pump-motor operates in pumping mode, converting torque from the shaft 151 to high pressure fluid provided to the accumulator. In addition, some of the torque on the shaft 151 is converted to electricity by the generator 154. This helps to reduce the number of rotating machines required to generate power, thereby improving efficiency of power generation. The pump-motors 150, 152 also provide an efficient means for starting and speed control of the shaft. The quantity of high pressure fluid consumed can be controlled by adjusting the displacements of the hydraulic pump-motors 150, 152.

(21) If the continuous pressure control system is required to provide high pressure fluid to the chamber 110, the second variable displacement hydraulic pump-motor 152 functions in motoring mode, converting high pressure hydraulic fluid from the accumulator 120 to torque on the shaft 151. In this case, the first variable displacement hydraulic pump-motor 150 functions in pumping mode, converting torque on the shaft 151 to pressurised hydraulic fluid provided to the chamber 110. The quantity of high pressure fluid provided can be controlled by adjusting the displacements of the hydraulic pump-motors 150, 152.

(22) In practice, the pressure in the second chamber 110 is controlled by the pump-motors 150, 152 demanding a little less or more than the flow required to match the ‘geometric flow’ that the chamber 110 is providing or absorbing as a result of the motion of the actuator shaft 112. The ‘geometric flow’ is the flow required to maintain a constant pressure in the chamber 110, taking into account increases or decreases in the volume of the chamber 110 as a result of motion of the actuator shaft 112. If the pump-motors 150, 152 are absorbing flow from the chamber 110, and if they absorb a little less flow than the ‘geometric flow’ being generated, the pressure in the chamber 110 will rise. Conversely if the pump-motors 150, 152 absorb a little more than the geometric flow, the pressure in the second chamber 110 will fall. In the inverse case where the actuator 100 is absorbing flow from the pump-motors 150, 152, if the pump-motors 150, 152 provide greater flow to the chamber 110 than the ‘geometric flow’ required, the pressure in the chamber 110 will rise. If the pump-motors 150, 152 provide less than the geometric flow, the pressure in the chamber 110 will fall. If the actuator is stationary, there is no ‘geometric flow’ of course and the pump-motors 150, 152 only need to deliver a small flow to pressurise the chamber 110 accordingly. It is through this mechanism that the pump-motors 150, 152 can deliver continuous pressure control over the chamber 110, and thereby continuous load control.

(23) In one example, the variable displacement pump-motors 150, 152 comprise (synthetically commutated) variable displacement pump/motor(s), each comprising at least one working chamber of cyclically varying volume (typically a plurality of working chambers of cyclically varying volume), a high pressure manifold, a low pressure manifold and a plurality of valves which regulate the flow of fluid between the at least one working chamber and the low and high pressure manifolds, at least one valve associated with the or each working chamber being an electronically controlled valve operable in phased relationship to cycles of working chamber volume to select the net volume of working fluid displaced by the respective working chamber during each successive cycle of working chamber volume. The quantity of pressurised fluid consumed or provided by the continuous pressure control system can be varied by the controller 140 by activating and deactivating working chambers of the pump-motors 150, 152 on each cycle of working chamber volume (by way of controlling opening and closing of respective valves of the pump-motors).

(24) FIG. 8 is a flow chart showing a method for controlling the discretised pressure control system and the continuous pressure control system. The desired resistance of the actuator 100 to movement of the shaft 112 caused by relative rotation of the buoyant body members is provided to the controller 140 in the form of a computer derived load demand signal (functioning as the received demand) to be received 802 at the controller 140. The first chambers 104-108, 141-144 and the second chamber 110 work to meet the load demand. As will be explained below, the contribution of the second chamber 110 (and thus the continuous pressure control system) to meet the load demand can be selected to at least partially (preferably fully) compensate for a difference between the received demand and a contribution of the first chambers 104-108, 141-144 (and thus the discretised pressure control system) to meet the load demand, thereby significantly reducing (preferably removing) the steps in the resistance profile caused by the contribution of the first chambers 104-108, 141-144 (and thus the discretised pressure control system).

(25) As shown in FIG. 6, the demand 180 received 802 by the controller is a smooth curve. The controller 140 is configured to generate 804 from the demand signal 180 a discretised load demand 220 (also shown in FIG. 6), the discretised demand 220 being the portion of the demand to which the first chambers 104-108, 141-144 are required to meet. The controller 140 is configured to cause 808 the first chambers 104, 141-144 to meet the discretised demand 220 as best they can such that the first chambers 104-108, 141-144 provide the main load and power flow. The difference or error between the load delivered by the first chambers 104-108, 141-144 and the received demand 180 forms 806 a continuous demand 222 for the second chamber 110 to deliver 808.

(26) In this example the (single) second chamber 110 is operable to provide a force in only one direction (to the left in the view of FIG. 3). A single second chamber 110 acting in one direction only can be made to correct the steps in the discrete system by generating the discretised demand 220 by: offsetting 804 (i.e. applying an offset to) the received demand 180 by (typically) half of the load range of the second chamber 110 (the load range of the second chamber being the range from the minimum contribution of the second chamber 110 to meet the demand to the maximum contribution of the second chamber 110 to meet the demand) in a direction opposite to that in which the second chamber 110 is configured to provide an active force to generate an offset demand 181; and then quantising the offset demand 181 to generate the discretised demand 220. This biases the correction signal to always remain in the right sense for the second chamber 110 to correct it. To further clarify, this approach and effect is illustrated clearly in FIG. 6. The received demand signal 180 is offset down by half of the range of the second chamber which can be seen from the maximum and minimum range of the continuous demand 222. This means that (in this example) the continuous demand 222 inherently remains positive and within the load range that can be applied by the second chamber 110.

(27) For alternative embodiments in which the actuator 100 is a rotary actuator, the direction of fluid flow into and out of the actuator chambers could be reversed as required as an alternative to the above approach.

(28) It will be understood that if at least one second chamber 110 (i.e. at least one chamber in respect of which hydraulic fluid provided thereto or consumed therefrom having a continuously controlled pressure) is provided which is operable to provide a force in one direction and at least one second chamber 110 is provided which is operable to provide a force in the opposite direction, it would not be necessary to offset the discretised demand from the received demand as the continuous pressure controlled chambers could act to compensate for errors between the discrete pressure controlled chambers 104-108, 141-144 in either direction.

(29) If there is relative clockwise motion of the buoyant body members around the pivot axis shown in FIG. 3 (between the ends of the double headed arrow between actuators 100, 102), the first actuator 100 will retract (the shaft 112 of the first actuator 100 moves to the right in the view of FIG. 3) and the second actuator will extend (the shaft 112 of the second actuator 102 moves to the left in the view of FIG. 3). In this case, the volumes of the first chambers 104, 108 of the first actuator 100 will increase and the volumes of the first chamber 106 and the second chamber 110 of the first actuator 100 will decrease. The volumes of the first chambers 141, 143 of the second actuator 102 will increase and the volumes of the first chambers 142, 144 of the second actuator 102 will decrease. The opposite is of course true if there is relative anticlockwise motion of the buoyant body members. In either case, as the shafts 112 of the first and second actuators 100, 102 move, the controller 140 controls the states of the high and low pressure valves in the valve blocks 124 associated with each first chamber 104-108, 141-144 to select the pressure of each first chamber 104-108, 141-144 as low or high (by selectively connecting them to the reservoir 122 or accumulator 120 respectively) to meet the discretised demand 220. Pressure within each of the first chambers 104-108, 141-144 can be controlled to either resist or assist movement of the shafts 112 of the actuators 100, 102 caused by the relative rotation of the buoyant body members, although it will be understood that the overall aim is that the relative rotation causes a net flow to the accumulator 120 (and to enable power to be extracted from the system).

(30) The pressure within the second chamber 110 is also controlled by controlling the pressure and/or volume of working fluid flowing into the chamber 110 from the pump-motors 150, 152. However, as explained above, while the pressure within the first chambers 104-108, 141-144 is restricted to either the pressure of the accumulator 120 or the pressure of the reservoir 122, the pressure within the second chamber 110 is controllable within a continuous range of values by the pump-motors 150, 152. This property can be used to smooth the steps in the output provided by the first chambers 104-108, 141-144, to thereby improve the quality of the output as a whole, while retaining much of the benefits (e.g. improved efficiency) of the discrete pressure control system of the first chambers 104-108, 141-144.

(31) In order to calculate a continuous demand 222 to smooth the steps in the output provided by the first chambers 104-108, 141-144, the controller 140 is configured to receive pressure measurement signals from pressure sensors within the first chambers 104-108, 141-144, identify which of the chambers 104-108, 141-144 contributes to the demand and derive the continuous demand 222 from the received pressure measurements and the said identification. As illustrated in FIG. 4, the continuous demand 222 is typically derived by subtracting the measured (stepped) contributions of the chambers 104-108, 141-144 from the received load demand, taking into account the area of the relevant fluid driving surface of piston 118 of the first actuator 100.

(32) FIG. 5 is a block diagram of an example algorithm implemented by the controller 140 to control the continuous pressure control system (and thus the pressure in the second chamber 110) to meet the continuous demand 222. The algorithm receives the continuous pressure demand 222 as an input to a difference operator 192 which also receives as another input the measured pressure 194 in the second chamber 110 (typically measured by a pressure sensor configured to measure the pressure in the second chamber 110, the pressure sensor 110 being in electronic communication with the controller 140), the difference operator 192 being configured to subtract the measured pressure 194 from the continuous pressure demand 222 to provide a difference signal 196. This feedback loop provides fine control to correct minor errors in the pressure of the second chamber 110 to meet the continuous pressure demand 222.

(33) The difference signal 196 is input to (in this example) a proportional integral (PI) control block 198 which in turn provides a control signal 199 to a summing operator 200. Also provided to the summing operator 200 is an input 202 which enables the algorithm to take into account measurements (e.g. measurements of speed, acceleration) of the motion of the actuator shaft 112 (e.g. rate of change of volume 204 of the second chamber 110) and knowledge of the compliance of the volume of the second chamber 110 (i.e. volume 206 of chamber 110 and the stiffness 208 of the hydraulic working fluid) to determine the required flow to or from the second chamber 110. For example, measurements of the motion of the actuator shaft 112 can be used by the algorithm to determine the flow rate required to maintain constant pressure in the chamber 110 taking into account motion of the actuator shaft 112. Knowledge of the compliance of the volume of the second chamber 110 can be used by the algorithm to determine change in fluid volume at the working pressure required to achieve a given pressure change. As shown in FIG. 5, knowledge of the compliance of the volume of the second chamber 110 can be taken into account by differentiating the continuous pressure demand with respect to time and multiplying the differentiated continuous pressure demand by the volume of chamber 110 divided by the fluid stiffness. This is then taken into account, together with the rate of change of actuator volume, to provide the input 202 to the summing operator 200.

(34) The output of the summing operator 200 is fed as a flow demand signal to the pump-motors 150, 152 which delivers a flow rate of hydraulic fluid to or from the second chamber 110 as close as possible to the flow demand. It will be understood that the flow rate of hydraulic fluid flowing into or out of the second chamber 110 typically refers to the volume of hydraulic fluid flowing into or out of the second chamber per unit time, but that this may be delivered by controlling a volume of hydraulic fluid flow to or from the pump-motors 150, 152 per cycle of rotation of the rotatable shaft 151. The delivered flow rate, together with the volume 230 of the chamber 110 (which is determined by the externally induced motion 232 of the actuator shaft 112 of the first actuator 100), determine the pressure in the second chamber 110.

(35) It will be understood that, in an alternative embodiment, the pressure within the chamber 110 can be controlled by providing the pump-motors 150, 152 with a continuous pressure demand signal, in which case the pump-motors 150, 152 are operable to determine the required flow rate of hydraulic fluid into or out of the chamber 110 to meet with pressure demand signal and to control the displacement (e.g. by activating and deactivating working chambers) accordingly.

(36) This combination of open and closed loop control enables the second chamber 110 to accurately correct for the difference between the contribution to the overall demand by the first chambers 104-108, 141-144 and the overall demand itself, reducing (preferably removing) the effect of the steps in the contribution from the first chambers 104-108, 141-144.

(37) As explained above, FIG. 6 shows the discretised load demand 220 and the continuous demand 222 (being the error between the load delivered by the first chambers 104-108, 141-144 and the overall demand 180). Also shown in FIG. 6 is the overall output 224 provided by the chambers 104-110, 141-144 which faithfully follows (and is therefore overlaid on) the overall demand 180, other than for a portion 224a where the demand cannot be met due to saturation (i.e. the demand exceeds the capability of the actuator 100).

(38) Referring back to FIG. 3, a system pressure relief valve 169 is connected between the pump-motors 150, 152. A further system pressure relief valve 171 is provided between the reservoir 122 and the accumulator 120. A further electrical generator 172 is connected to the accumulator 120 by way of a variable displacement motor 173 configured to receive pressurised fluid from the accumulator 120 and to convert received pressurised fluid from the accumulator 120 into torque on a shaft 174 which drives the generator 172 to generate electrical energy.

(39) Typically the valves in the banks 124 are selected and tuned to provide the best overall response in combination with the second chamber 110. For example, the valves in the banks 124 may be selected such that the peak rate of change of the pressure acting in the first chambers 104-108, 141-144 can be matched by the second chamber 110. This allows the contribution of the second chamber 110 to compensate for a step change contributed by a first chamber 104-108, 141-144 to the load.

(40) The controller 140 may of course include constraints to ensure that the overall output remains smooth and controlled, and to prevent overload or reaching endstops (for example). In some applications, constraints may be applied by the controller 140 to avoid the overall demand signal 180 from exceeding the capabilities of the hydraulic transmission. The controller 140 may restrict the rate of change of the total load to lie within the load range of the second chamber 110, which is in turn a function of its flow range limit and the compliance of its volume. The controller 140 may similarly be configured to constrain the peak rate of change of pressure acting in the first chambers 104-108, 141-144 to lie within the load range of the second chamber 110.

(41) The fact that most of the flow and power is handled by the first chambers 104-108, 141-144 (and thus the discrete pressure control system), and the fact that the high instantaneous power through the second chamber 110 is exchanged directly between the two pump-motors 150, 152 (rather than through a prime mover or generator) makes the hydraulic transmission disclosed herein particularly suitable for handling large instantaneous power flows while maintaining a very high system efficiency.

(42) Further variations and modifications may be made within the scope of the invention herein described.

(43) For example, although the apparatus for extracting energy from water waves aims to provide a net flow to the accumulator 120 (and to extract energy from the system by way of generators 154, 172), the hydraulic transmission described herein is equally applicable to an actuator system which causes a net flow from the accumulator 120 (that is, an actuator system which is driven by hydraulic fluid from the hydraulic transmission to perform work on a load). In this case, the electricity generator 154 is typically replaced by a prime mover, such as a diesel engine or electric motor, so as to add power to the system. Additionally or alternatively, the electrical generator 172 and variable displacement hydraulic motor 173 may be replaced by a prime mover (such as a diesel engine or electric motor) and a variable displacement hydraulic pump, again so as to add power to the system. Indeed, the hydraulic transmission described herein is generally applicable to any system where high power flows across all four quadrants are required (‘four quadrant’ means a system that can apply a load in either direction during motion in either direction).

(44) FIG. 7 is a block diagram of a heave compensation winch comprising an actuator 300 comprising a plurality of chambers (not shown) and a hydraulic transmission 302 configured to selectively control the connection of each of a plurality of the said plurality of first chambers of the actuator to a high pressure fluid store 304 and a low pressure hydraulic fluid reservoir 306 in accordance with a load demand. The hydraulic transmission 302 is also configured to control the pressure in a second chamber of the said plurality of chambers within a continuous range of pressures. In accordance with the principles discussed above, the hydraulic transmission is configured to control the pressure in the second chamber to at least partially compensate for a difference between the contribution of the first chambers to a received demand. In this case, the demand is typically a varying load on a cable which varies the speed of the winding in and out of the cable by the winch. It will be understood that the winch is an example of an application where there is a net flow of power to the actuator. Accordingly, the electricity generator 154 is typically replaced by a prime mover, such as a diesel engine or electric motor, so as to add power to the system and/or the electrical generator 172 and variable displacement hydraulic motor 173 are replaced by a prime mover (such as a diesel engine or electric motor) and a variable displacement hydraulic pump, again so as to add power to the system.

(45) Although seven first chambers 104-108, 141-144 and a single second chamber 110 are illustrated in FIG. 3, it will be understood that any number of first chambers and second chambers could be provided. The best choice of design will depend on the specific application requirements. Typically a minimum of two first chambers and one second chamber will be provided.

(46) Although the chambers 141-144 are illustrated in FIG. 3 as being in parallel with the chambers 104-108, it will be understood that they could alternatively be connected in series.

(47) In some applications, it is possible to use a single pump-motor 150 in place of the pair of pump-motors 150, 152 of the continuous pressure control system of FIG. 3.

(48) In some embodiments, the variable displacement pump-motors 150, 152 can be replaced by variable flow control valves, each of which is configured to regulate the flow of hydraulic fluid between the second chamber 110 and the accumulator 120 or the reservoir 122.

(49) Although a linear actuator 100 is described above, the hydraulic transmission may additionally or alternatively comprise one or a number of rotary actuators comprising one or more chambers whose pressures are controlled discretely and one or more chambers whose pressures are controlled over a continuous range of pressures in a similar way to the linear actuator 100 to create a rotary output directly or through a gear system, or indeed a linear output using, for example, a rack and pinion system.