Fluid working machine and method of operating a fluid working machine

Abstract

A fluid working machine of the type comprising working chambers of cyclically varying volume and low and high pressure valves to regulate the flow of working fluid into and out of the working chamber, from low and high pressure manifolds, in which the valves are electronically controlled on each cycle of working chamber volume, by way of valve actuation signals, to determine the net displacement of working chambers. Additional valve actuator signals are generated in response to determination that a valve or working chamber has been inactive to adapt the valve to operate more reliably when subsequently actuated, but without significantly altering the net displacement of working chambers.

Claims

1. A method of operating a fluid working machine, the machine comprising a rotatable shaft and a plurality of working chambers having working volumes which vary cyclically with rotation of the rotatable shaft, each working chamber having a low pressure valve which regulates the flow of working fluid between the working chamber and a low pressure manifold and a high pressure valve which regulates the flow of working fluid between the working chamber and a high pressure manifold, each low pressure valve being an electronically controlled valve having a valve actuator, the method comprising transmitting primary actuation signals to the actuators, in phased relationship to cycles of working chamber volume, to thereby actively control the said electronically controlled valves and so determine the net displacement of each working chamber on each cycle of working chamber volume, and selecting the net displacement of a group of one or more of the working chambers to follow a target, and further determining whether a valve inactivity test is met in respect of a said low pressure valve of a said working chamber and, responsive to said determination, transmitting one or more additional actuator signals to the actuator of the said low pressure valve, which additional actuator signals do not significantly change the net displacement of working fluid by the respective working chamber.

2. A method according to claim 1, wherein the valve inactivity test comprises that the respective working chamber has not undergone an active cycle for a predetermined period of time, or for a predetermined number of cycles of working chamber volume.

3. A method according to claim 1, wherein the valve inactivity test comprises determining that a valve has not been actuated for a predetermined period of time, or for a predetermined number of cycles of working chamber volume.

4. A method according to claim 1, wherein the transmission of said additional actuator signals takes places within one cycle of working chamber volume, or within two cycles of working chamber volume or within another predetermined number of cycles of working chamber volume immediately preceding the transmission of actuation signals to the respective valve actuator to select the net displacement of working chamber so that the net displacement of the group of working chambers follows the target.

5. A method according to claim 1, wherein the additional actuator signals do not lead to opening of the high pressure valve.

6. A method according to claim 5, wherein the said low pressure valve is closed and then opened again during a single expansion stroke of the respective working chamber.

7. A method according to claim 6, wherein the said low pressure valve is closed and then opened again within a period of time which is less than 25% of the period of a cycle of working chamber volume and the said opening again takes place before bottom dead centre.

8. A method according to claim 6, wherein the said low pressure valve is closed and then opened again within 72 of phase of the cycles of working chamber volume.

9. A method according to claim 1, wherein, responsive to said determination, the one or more additional actuator signals are transmitted to the actuator of said the low pressure valve of the respective working chamber, to cause the respective low pressure valve to close and then to open again while the respective working chamber remains sealed from the high pressure manifold.

10. A method according to claim 9, wherein the said low pressure valve is closed and then opened again within a period of time which is less than 25% of the period of a cycle of working chamber volume and the said opening again takes place before bottom dead centre.

11. A method according to claim 1, wherein responsive to said determination, the one or more said additional actuator signals are transmitted to the respective valve actuator to urge the low pressure valve open wherein no actuation signal to cause the actuator to urge the low pressure valve open has been transmitted to the respective valve actuator for a period of time which is at least the period of a cycle of working chamber volume.

12. A method according to claim 1, wherein responsive to said determination, the one or more additional actuator signals are transmitted to the actuator of the low pressure valve of the respective working chamber, to cause the respective valve to open and then to close again while the respective working chamber remains sealed from the high pressure manifold.

13. A method according to claim 1, wherein at least one of the working chambers has made no net displacement of working fluid for one or more consecutive cycles, with the respective high pressure valve closed, and with the respective low pressure valve in either state (a) in which the low pressure valve remains open so that the respective working chamber remains in fluid communication with the low pressure manifold or state (b) in which the respective low pressure valve remains closed so that the respective working chamber remains sealed, and responsive to said determination, closing or opening the respective low pressure valve to swap from state (a) to state (b) or vice versa.

14. A method according to claim 1, wherein, responsive to said determination, while the said respective working chamber is contracting, the additional actuator signals are transmitted to the actuator of the low pressure valve of the respective working chamber, to cause the respective valve to move from an open position to a closed position to seal the working chamber and thereby cause the pressure in the respective working chamber to increase as the respective working chamber further contracts.

15. A method according to claim 13, wherein, once the respective high pressure valve opens, pressurized working fluid from the respective working chamber passes out of the respective working chamber into the high pressure manifold, through the high pressure valve, while the respective working chamber contracts and then substantially the same amount of pressurized working fluid from the high pressure manifold passes into the respective working chamber from the high pressure manifold as the respective working chamber then expands.

16. A method according to claim 14, wherein, once the respective high pressure valve opens, pressurized working fluid from the respective working chamber passes out of the respective working chamber into the high pressure manifold, through the high pressure valve, while the respective working chamber contracts and then substantially the same amount of pressurized working fluid from the high pressure manifold passes into the respective working chamber from the high pressure manifold as the respective working chamber then expands.

17. A method according to claim 1, wherein the additional actuator signals do not lead to the net displacement of working fluid because they do not cause the said valve to move but they temporarily adapt the actuator to function more quickly or more reliably in response to a later actuation signal.

18. A fluid working machine comprising a rotatable shaft and a plurality of working chambers having working volumes which vary cyclically with rotation of the rotatable shaft, each working chamber having a low pressure valve which regulates the flow of working fluid between the working chamber and a low pressure manifold and a high pressure valve which regulates the flow of working fluid between the working chamber and a high pressure manifold, each low pressure valve being an electronically controlled valve having a valve actuator, a controller configured to transmit primary actuation signals to the actuators, to thereby actively control the said electronically controlled valves in phased relationship to cycles of working chamber volume and so determine the net displacement of each working chamber, and selecting the net displacement to match the net displacement of the working chambers to follow a target, and also further determining whether a valve inactivity test is met in respect of a said low pressure valve of a said working chamber and, responsive to said determination to transmit one or more actuation signals to the actuator of the said low pressure valve, which additional actuator signals do not significantly change the net displacement of working fluid by the respective working chamber.

19. A fluid working machine according to claim 18, wherein the additional actuator signals do not lead to opening of the high pressure valve.

20. A fluid working machine according to claim 18, wherein the said low pressure valve is closed and then opened again during a single expansion stroke of the respective working chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0102] An example embodiment of the invention will now be illustrated with reference to the following Figures:

[0103] FIG. 1 is a schematic diagram of a prior art fluid working machine;

[0104] FIG. 2 is a cross section through an electronically controlled LPV;

[0105] FIG. 3 is a schematic diagram of LPV position, HPV position, working chamber pressure and solenoid actuation signals for both pumping (upper traces) and motoring (lower traces) in prior art fluid working machines;

[0106] FIG. 4 is a flow chart of the procedure carried out by the controller to generate actuation signals;

[0107] FIG. 5 is a schematic diagram of LPV position, HPV position, working chamber pressure and LPV solenoid current in a cycle in which the LPV moves but there is no net displacement of working fluid;

[0108] FIG. 6 is a schematic diagram of LPV position, HPV position, working chamber pressure and solenoid currents in an alternative cycle in which the LPV moves but no fluid is displaced to the high pressure manifold;

[0109] FIG. 7 is a schematic diagram of LPV position, HPV position, working chamber pressure and solenoid currents in a cycle in which the LPV and HPV moves but there is no net displacement of working fluid; and

[0110] FIG. 8 is a schematic diagram of LPV position, HPV position, working chamber pressure and solenoid currents in a cycle in which the working chamber swaps from one inactive mode to another inactive mode, with movement of the LPV.

DETAILED DESCRIPTION OF AN EXAMPLE EMBODIMENT

[0111] FIG. 1 is a schematic diagram of an individual working chamber 2 in a fluid-working machine 1, which typically comprises a plurality of corresponding working chambers. The fluid-working machine may be a pump, which carries out pumping cycles, or a motor which carries out motoring cycles, or a pump-motor which can operate as a pump or a motor in alternative operating modes and can thereby carry out pumping or motoring cycles. The net throughput of fluid is determined by the active control of electronically controllable valves, in phased relationship to cycles of working chamber volume, to regulate fluid communication between individual working chambers of the machine and fluid manifolds. Individual chambers are selectable by a controller, on a cycle by cycle basis, to either undergo an active cycle in which they displace a predetermined fixed volume of fluid or to undergo an inactive cycle with no net displacement of fluid, thereby enabling the net throughput of the machine to be matched dynamically to a demand.

[0112] An individual working chamber 2 has a volume defined by the interior surface of a cylinder 4 and a piston 6, which is driven from a crankshaft 8 by a crank mechanism 9 and which reciprocates within the cylinder to cyclically vary the volume of the working chamber. A shaft position and speed sensor 10 determines the instantaneous angular position and speed of rotation of the shaft, and transmits shaft position and speed signals to a controller 12, which enables the controller to determine the instantaneous phase of the cycles of each individual working chamber. The controller typically comprises a microprocessor or microcontroller which executes a stored program in use.

[0113] The working chamber comprises an actively controlled low pressure valve in the form of an electronically controllable face-sealing poppet valve 14, which faces inwards toward the working chamber and is operable to selectively seal off a channel extending from the working chamber to a low pressure manifold 16. The working chamber further comprises a high pressure valve 18. The high pressure valve faces outwards from the working chamber and is operable to seal off a channel extending from the working chamber to a high pressure manifold 20.

[0114] An example LPV 100 is shown in cross section in FIG. 2. The LPV has a valve body 101, a first port 102 in communication with a working chamber and a second port 104 which leads to the high pressure manifold through a plurality of radially extending apertures. If the machine is a pump, the first port is the inlet and the second port is the outlet and the net flow out of the working chamber is by path 118. In a motor, the first port is the outlet and the second port is the inlet and the flow of fluid is reversed. In a machine operable as either a pump or a motor the first and second ports can function as inlet or outlets depending on the direction of fluid flow.

[0115] The valve includes an armature 106 which is formed integrally with a valve stem 108 which connects the armature to a poppet valve head 110, functioning as the valve member. The armature and solenoid are part of a magnetic circuit conducted through the valve body. The poppet valve head is operable between the open position illustrated in FIG. 2 and a closed position in which is seals against a valve seat 112.

[0116] A solenoid 114 can be used to close the valve under the active control of the controller and a return spring 116 is provided to bias the armature away from the electromagnet and therefore bias the poppet valve head to the open position. The solenoid and armature together function as the actuator. A barrier 120 on the working chamber side of the valve head, away from the valve seat, fixed to the valve assembly by radial connecting arms 122 between which fluid can flow. The barrier defines a chamber 124 which communicates with a constricted flow region 126 around the periphery of the valve member. When fluid flows out through the valve assembly, along the flow path 118, the pressure drops in the constricted flow region and therefore also in the reduced pressure chamber providing an opening force which counteracts forces on the poppet valve arising from the flow of working fluid along path 118. The valve stem extends beyond the poppet valve head, through an aperture 130 in the barrier and includes a flange 132 which cooperates with the periphery of the aperture to limit movement of the poppet valve head away from the valve seat so that there is always at least some fluid in the chamber between the barrier and the poppet valve head. This reduces the formation of squeeze film at this location which would provide additional resistance to closing, increasing the power consumption of the valve assembly and reducing the operating speed.

[0117] The HPV may be an electronically controlled valve with a solenoid acting on an armature coupled to a valve member, generally corresponding to the LPV, although for a dedicated pump it may be a simply spring loaded check valve, for example.

[0118] In the LPV shown, the armature, valve stem and valve member function as a travelling member which moves backwards and forwards to open and close the valve. Oil films form between the travelling member and the body of the valve, for example at the valve sealing line, but also between the armature and the body. In some embodiments, the travelling member comprises two or more parts which do not always move together, for example, the armature may bear on the valve stem to close the valve but able to move away from the valve stem under the control of the actuator, with the valve stem and valve member biased towards the armature by a spring.

[0119] FIG. 3 shows the details of a full stroke active pumping cycle (top) and motor cycle (bottom).

[0120] The figure shows the variation within time in LPV position 200A, HPV position 202A, LPV solenoid current 204A and working chamber pressure 208A (which is illustrated relative to the low pressure manifold pressure) during a pumping cycle, as well as the variation in LPV position 200B, HPV position 202B, the LPV solenoid current 204B, HPV solenoid current 206 and working chamber pressure 208B during a motoring cycle. The timing of events is shown relative to cycles of working chamber volume 210 between the point of maximum volume, bottom dead centre (BDC) and point of minimum volume, top dead centre (TDC).

[0121] A pumping cycle begins with the LPV and HPV closed. Shortly before BDC a current is passed through the LPV solenoid, as shown in the upper part of FIG. 3. As a result, a closing force is applied to the LPV valve member. The force exerted on the armature exceeds the biasing force from the LPV spring and the LPV opens. Pressure in the working chamber rises as the working chamber contracts whilst sealed and the HPV opens passively once the pressure differential between the working chamber and the high pressure manifold is sufficiently low that the net force urging the high pressure valve open exceeds the forces urging the HPV closed arising from the pressure differential across the HPV valve member. Working fluid is then displaced from the working chamber into the high pressure manifold.

[0122] The HPV closes passively when the piston reaches TDC and the working chamber begins to expand again. The LPV then opens during the expansion stroke once the pressure within the working chamber is sufficiently close to the low pressure manifold that the spring biasing the low pressure valve can overcome the force due to the pressure differential across the LPV valve member. During the subsequent expansion stoke, the LPV remains open and hydraulic fluid is received from the low pressure manifold.

[0123] At or around BDC, the controller determines whether or not the LPV should be closed. If so, fluid within the working chamber is pressurized and pumped to the HPV during the subsequent contraction phase of working chamber volume, as before. However, if the LPV remains open, fluid within the working chamber is vented back to the low pressure manifold and an inactive cycle occurs, in which there is no net displacement of fluid to the high pressure manifold. In an inactive cycle, the low and high pressure valves will both remain inactive; the high pressure valve will remain closed and the low pressure valve will remain open (although it is also known to carry out in inactive cycle in which the low pressure valve remains closed).

[0124] In some embodiments, the LPV will be biased open and will need to be actively closed by the controller if a pumping cycle is selected. In other embodiments, the LPV will be biased closed and will need to be actively held open by the controller if an idle cycle is selected. The HPV may be actively controlled, for example an actuation signal may be used to provide additional force to urge it open or closed, although for the pumping cycle described above it is sufficient for the HPV to be a check valve.

[0125] With reference to the lower part of FIG. 3, in order to carry out a motoring cycle, both the LPV and HPV are actively controlled. During a contraction stroke, fluid is vented to the low pressure manifold through the low pressure valve. The low pressure valve is closed before top dead centre, causing pressure to build up within the working chamber as it continues to reduce in volume. Once sufficient pressure has been built up, a current is applied to the HPV solenoid so that the HPV opens, and fluid flows into the working chamber from the high pressure manifold. Once the HPV is open, the energy required to keep it open may be reduced and the mean current is reduced 212 using pulse width modulation. Shortly before bottom dead centre, the HPV is actively closed, whereupon pressure within the working chamber falls, enabling the low pressure valve to open around or shortly after bottom dead centre.

[0126] In some embodiments, the low pressure valve will be biased open and will need to be actively closed by the controller. In other embodiments, the low pressure valve will be biased closed and will need to be actively held open by the controller if an inactive cycle is selected. The low pressure valve typically opens passively, but it may open under active control to enable the timing of opening to be carefully controlled. Thus, the low pressure valve may be actively opened, or, if it has been actively held open this active holding open may be stopped. The high pressure valve may be actively or passively opened.

[0127] In some embodiments, instead of selecting only between idle cycles and full stroke pumping and/or motoring cycles, the fluid-working controller is also operable to vary the precise phasing of valve timings to create partial stroke pumping and/or partial stroke motoring cycles. In a partial stroke pumping cycle, the low pressure valve is closed later in the exhaust stroke so that only a part of the maximum stroke volume of the working chamber is displaced into the high pressure manifold. Typically, closure of the low pressure valve is delayed until just before top dead centre. In a partial stroke motoring cycle, the high pressure valve is closed and the low pressure valve opened part way through the expansion stroke so that the volume of fluid received from the high pressure manifold and thus the net displacement of fluid is less than would otherwise be possible.

[0128] FIG. 4 sets out the procedure carried out by the fluid working machine controller to generate actuation signals. The procedure starts 300 and a register storing an accumulator is set to zero 302. As the rotatable shaft turns, each time it reaches a position where a displacement decisions should be made for a working chamber, it queries a received target demand signal 304 and then adds the received demand, expressed in consistent units, to the accumulator. A higher positive value indicates a higher amount of as yet unmet demand for displacement. The value of the accumulator is then used to select the displacement of the working chamber selected 308. In a known implementation, the working chamber will be caused to carry out an active cycle in which it displaces the maximum possible displacement if the accumulator is greater than half (for example) of the displacement of an individual working chamber. Other decision making algorithms are known to the person skilled in the art, for example in WO 2015/040360 (Caldwell et al.) or WO 2011/104549 (Rampen and Laird). The decision are made so that the total displacement follows that indicated by the target displacement, although actual displacement and target displacement need not perfectly match, for example, to avoid broken cylinders or the generation of unwanted frequencies. The controller then generates actuation signals 310 to actively control the LPV of the respective working chamber (and the HPV if required) at the correct times within the cycle of working chamber volume to generate an active or inactive cycle as required, as shown in FIG. 3. The actuation signals are typically transmitted to a switching circuit which switches current to the respective solenoid on or off in response, for example using a FET. The accumulator is then updated 312 by subtracting the amount of displacement carried out by the working chamber. Accordingly, the accumulator continues to monitor the displacement which has been demand but not as yet met. The decision as to the displacement made by the working chamber is then recorded 314. The algorithm then repeats, considering the next working chamber in turn 316.

[0129] This procedure generates actuation signals to cause the net displacement of the working chambers to follow a target demand. That is the aim is to establish a fluid input or output from the machine that accurately matches the target demand. In parallel, the controller also repetitively tests 350 whether a valve inactivity test is met for each working chamber in turn. In one embodiment, this valve inactivity test is met if the working chamber has not been instructed to carry out an active cycle (with a net displacement of working fluid) for a number of cycles exceeding a threshold period of time, as determined from the recorded data concerning past displacement decisions. The data concerning past displacement decisions could be as simple as a register which is incremented on each cycle that a respective working chamber carried out an inactive cycle and is set to zero each time that the respective working chamber carries out an active cycle. In another embodiment, the valve inactivity test is met if the working chamber has not carried out an active cycle for a threshold period of time, again determined from the recorded data concerning past displacement decisions. In this case, it is necessary to store when a working chamber was last used or to store data concerning the time variation of the speed of rotation of the rotatable shaft, which determines the instantaneous frequency of cycles of working chamber volume.

[0130] If the controller determines 350 that the valve inactivity test is met for a working chamber, and if an active cycle is not already underway 352 it then generates additional actuator signals 254, timed relative to cycles of working chamber volume, as shown in FIG. 5. The additional actuator signals generate an increase in LPV solenoid current 250 just before bottom dead centre to close the LPV for a brief period of time 254 until the LPV is commanded to open again shortly thereafter. The increase in LPV current is a first additional actuator signal 250 and the decrease in LPV current 252 is a second additional actuator signal. The current changes required to open and close the LPV depend on the nature of the LPV. For example, if the LPV was biased closed instead of biased open, the first additional actuator signal would be current flow stopping and the second additional actuator signal would be current flow restarting.

[0131] This extra actuation of the LPV, closing and opening again, has no effect on net displacement by the working chamber. The flow of working fluid from the low pressure manifold into the working chamber is simply briefly interrupted when the LPV closes and once it opens again, additional fluid flows in to fill the volume of the working chamber. The pressure in the working chamber remains low and the HPV does not open.

[0132] The additional actuator signals 250, 252 and the closure and subsequent opening of the LPV 254 are best carried out at around, and ideally just before, bottom dead centre. The LPV should be reopened before the normal LPV closing time during an active pumping cycle, at least if there is any possibility that an active pumping cycle will immediately follow this extra actuation. It would be less energy efficient for the extra actuation of the LPV to take place close to a mid-stroke position, half way between TDC and BDC, when the rate of fluid flow is the highest, as this would put excessive forces on the LPV.

[0133] This extra transmission of actuator signals to the LPV has several beneficial effects. Firstly, it may disturb the working fluid film around the valve member, returning the fluid film to a thickness which is desirable and likely to remain consistent during ongoing frequent actuation of the valve. Hence, the valve will have more consistent and fast response during subsequent actuations. Accordingly, the fluid film has been adapted. Secondly, the coils of the LPV solenoid will be heated up by the current flow, and so achieve a temperature which will be more consistent with that maintained during a regular actuation. Accordingly, the solenoid has been adapted. The valve member, valve stem and armature may also be aligned more centrally and or axially. Accordingly, the configuration of the valve member, valve stem and armature has been adapted. Finally, there can be beneficial magnetic effects. The armature, and other magnetic materials which form the magnetic circuit between the solenoid and the armature in use, will result in build-up of remanence or remanent magnetization as well. Accordingly, the magnetic circuit is adapted temporarily to maintain characteristics which it will have during normal operation with frequent actuation of the LPV.

[0134] Accordingly, any additional actuation of the LPV arising from the additional actuator signals has adapted the LPV so as to cause it to respond to subsequent actuations in a manner consistent with its response during normal operation, with frequent actuation. This adaptation is temporary and will be lost over time if the valve again becomes inactive.

[0135] Note that some of these benefits will be obtained even if the magnitude of the LPV current is not sufficient to cause the LPV to fully open, perhaps sufficient only to temporarily unseat the LPV member, and perhaps not move the LPV member at all. However, in some embodiments it may be important for the LPV to fully open.

[0136] FIG. 6 illustrates an alternative timing for additional actuator signals to the LPV. In this example, the current to the LPV solenoid is increased to close the LPV and decreased to allow the LPV to open, just before top dead centre. Again, the increase in current to close the LPV and the subsequent decrease in current to allow the LPV to open are first and second additional actuator signals. The pressure in the working chamber increases during this time (because the working chamber is temporarily sealed from both the high pressure and low pressure manifolds, while it continues to contract) but the pressure does not reach the pressure required to open the HPV and vent fluid to the high pressure manifold. Hence, this extra actuation of the LPV again has the effect of moving the LPV valve member, obtaining the benefits set out above, but without leading to a net displacement of working fluid.

[0137] With reference to FIG. 7, if the LPV is closed for a sufficiently long period of time, sufficiently far before TDC, the pressure in the working chamber will increase to the pressure required to move the HPV valve member, leading to the HPV opening 256 briefly. The pressure required to cause movement will typically be around the high pressure manifold pressure but will depend on the direction and magnitude of spring biasing forces on the HPV valve member and whether there is a current in the HPV solenoid. It may be that the HPV valve member is moved only sufficiently to unseat it and not sufficiently to move it from the closed position to entirely open and it may be that the HPV is not moved sufficiently to allow fluid to pass. However, if the LPV closure timing is brought forward and/or the duration of LPV closure is increased, the HPV will open fully.

[0138] In order to ensure some movement of the HPV, additional actuator signals 258, 260 may be generated to cause current to be start being passed through the HPV solenoid and then to stop being passed through the HPV solenoid. However, in some embodiments this may be optional as the HPV may anyway open due to the pressure differential across the HPV valve member.

[0139] Opening of the HPV while the working chamber is contracting has the effect that working fluid will be pumped from the working chamber to the high pressure manifold. In order to avoid substantial net displacement, the HPV is kept open until after top dead centre so that as the working chamber expands again, working fluid is received back from the high pressure manifold into the working chamber. The amount of fluid received back from the high pressure manifold is balanced with the amount which was pumped aiming for no net displacement of working fluid.

[0140] This approach also has the advantage of adapting not only the LPV but also the HPV. It is therefore useful where the HPV has been inactive, to precondition the HPV for more reliable subsequent actuation.

[0141] In a further embodiment, additional actuator signals may be sent to the LPV actuator of a working chamber which has carried out an inactive cycle to switch the LPV from open to closed, or vice versa. If, as shown in FIG. 8, the LPV was previously open and the working chamber was carrying out an inactive cycle in which working fluid was received from the low pressure manifold and vented to the low pressure manifold with no net displacement, the LPV is closed and for at least the following cycle the working chamber undergoes an idle cavitation cycle in which it remains sealed from both the low pressure and high pressure manifold, with no net displacement. Alternatively, if an idle cavitation cycle has taken place, the LPV will be opened and for the following cycle the working chamber will carry out an inactive cycle by maintain the LPV in the open position so that the same amount of working fluid is received from and then output to the low pressure manifold. In this approach, the LPV is actuated but the working chamber is effectively switched from one type of inactive cycle to another type of inactive cycle with no displacement to or from the high pressure manifold and therefore no net displacement of working fluid.

[0142] In the above examples, the valve inactivity test considers whether a working chamber meets an inactivity test (from which it can be inferred that a valve of the working chamber is inactive).

[0143] There are a number of different inactivity tests which can be considered in the step 350 of determining whether to generate additional actuator signals.

[0144] 1. Additional actuator signals may be generated periodically, for example after a predetermined period of time (e.g. 5 seconds) of non-use of a valve or working chamber. This period of time may be increased after a sufficiently long period of time or number of inactive cycles, in order to conserve power.

[0145] 2. Additional actuator signals may be generated after a predetermined number of inactive cycles of working chamber volume. The predetermined number of inactive cycles may be increased after a sufficiently long period of time or number of inactive cycles.

[0146] 3. Additional actuator signals may be generated for the working chambers which have been inactive for the longest period of time or number of cycles of working chamber volume.

[0147] 4. It may be determined that a working chamber is going to be used for an active cycle which has not undergone an active for cycle for a predetermined period of time or number of cycles of working chamber volume and additional actuator signals may be generated just before the working chamber is used, to carry out an active cycle, typically within a period of 0.1-2.0 times the period of cycles of working chamber volume before valve actuation signals are sent to the LPV of the working chamber to start an active cycle.

[0148] 5. It may be determined that one or more specific working chambers have a valve which is not meeting a performance criteria (e.g. which is opening too slowly) and the additional actuator signals may be generated more frequently, e.g. on a number of consecutive cycles, for corresponding specific valves.

[0149] 6. Active actuation signals may in part be generated in response to a measurement by a sensor. For example a measurement of oil film thickness within a valve may be obtained by a thickness sensor (e.g. an ultrasonic thickness gauge). If the oil film thickness meets a criterion (e.g. is less than a predetermined thickness) additional actuator signals are sent to the respective valve.

[0150] Accordingly, the invention enables the fluid working machine to perform more reliably than would otherwise be the case.