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In a method of operating a fluid-working machine, the volume of working fluid displaced during each cycle of working chamber volume is selected taking into account the availability of other working chambers. The status of each working chamber is monitored and a working chamber treated as unavailable

In a method of operating a fluid-working machine, the volume of working fluid displaced during each cycle of working chamber volume is selected taking into account the availability of other working chambers. The status of each working chamber is monitored and a working chamber treated as unavailable if it is found to be malfunctioning. A working chamber may be treated as unavailable if it is allocated to an alternative working function. A fault may be detected in a working chamber by determining whether a measured output parameter of the fluid working machine fulfills at least one acceptable function criterion taking into account the previously selected net displacement of working fluid by a working chamber during a cycle of working chamber volume to carry out the working function.

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1. A method of operating a fluid-working machine comprising a plurality of working chambers of cyclically varying volume, each said working chamber operable to displace a volume of working fluid which is selectable for each cycle of working chamber volume, the method comprising: selecting the volume

1. A method of operating a fluid-working machine comprising a plurality of working chambers of cyclically varying volume, each said working chamber operable to displace a volume of working fluid which is selectable for each cycle of working chamber volume, the method comprising: selecting the volume of working fluid displaced by one or more said working chambers during each cycle of working chamber volume to carry out a working function responsive to a received demand signal,selecting the volume of working fluid displaced by a working chamber of said plurality of working chambers during a cycle of working chamber volume taking into account an availability of the other said working chambers to displace fluid to carry out the working function, anddetermining that a second of said working chambers is unavailable to carry out the working function and responsively causing a first of said working chambers to execute an active cycle instead of an idle cycle. 2. A method of operating a fluid-working machine according to claim 1, the fluid-working machine comprising a controller, and at least one valve associated with each said working chamber operable to regulate the connection of the respective working chamber to a low pressure manifold or a high pressure manifold, said at least one valve associated with each said working chamber being electronically controllable under the active control of the controller to select the volume of working fluid displaced during said cycle of working chamber volume, wherein the controller receives the demand signal and actively controls the said electronically controllable valves, in phased relationship to cycles of working chamber volume, to select the displacement of fluid by one or more of the working chambers on each cycle of working chamber volume, responsive to the received demand signal. 3. A method of operating a fluid-working machine according to claim 1, comprising measuring the status of each said working chamber and determining the availability of each working chamber responsive thereto. 4. A method of operating a fluid-working machine according to claim 1, wherein a working chamber of said plurality of working chambers is treated as unavailable responsive to a detection that there is a fault associated with the working chamber. 5. A method of operating a fluid-working machine according to claim 1, wherein said working function is a first working function, andwherein a working chamber of said plurality of working chambers is treated as unavailable when the working chamber is allocated to a second working function different from the first working function. 6. A method of operating a fluid-working machine according to claim 5, wherein the fluid-working machine comprises one or more ports, one or more of which are associated with the first working function, andwherein the fluid-working machine is configurable to direct working fluid along a fluid path selectable from amongst a group of different fluid paths to carry out the first working function, each fluid path in the group of different fluid paths extending between one or more said ports and one or more said working chambers,wherein a working chamber of said plurality of working chambers is allocated to the first working function if the selected fluid path extends between the one or more ports associated with the first working function and the working chamber. 7. A method of operating a fluid-working machine according to claim 1, wherein the amount of fluid displaced by a first working chamber of said working chambers during an individual cycle of the working chamber volume of said first working chamber is greater than would be the case if a second working chamber of said working chambers was available to carry out the working function. 8. A method of operating a fluid-working machine according to claim 7, wherein each said working chamber is operable on each cycle of working chamber volume to carry out an active cycle in which the working chamber makes a net displacement of the volume of working fluid or an idle cycle in which the working chamber makes substantially no net displacement of the volume of working fluid,wherein, on at least some occasions, the first working chamber carries out an active cycle instead of an idle cycle as a result of the non-availability of the second working chamber. 9. A method of operating a fluid-working machine according to claim 7, wherein volume cycles of the first said working chamber are phased earlier or later than volume cycles of the second said working chamber. 10. A method of operating a fluid-working machine according to claim 1, wherein, when the demand indicated by the received demand signal is below a threshold, the selected volume of fluid displaced by at least one of the working chambers which are available to carry out the working function is substantially zero for at least some cycles of said working chamber volume. 11. A fluid-working machine comprising: a controller and a plurality of working chambers of cyclically varying volume, each said working chamber operable to displace a volume of working fluid which is selectable by the controller on each cycle of the working chamber volume,the controller operable to select the volume of working fluid displaced by one or more said working chambers on each cycle of the working chamber volume to carry out a working function responsive to a received demand signal,the controller operable to select the volume of working fluid displaced by the working chamber of said plurality of working chambers on a cycle of the working chamber volume taking into account an availability of other said working chambers to displace fluid to carry out the working function, the controller being configured to determine that a second of said working chambers is unavailable to carry out the working function and to responsively cause a first of said working chambers to execute an active cycle instead of an idle cycle. 12. A fluid-working machine according to claim 11, further comprising a working chamber status detection module. 13. A fluid working machine controller comprising: a working chamber database specifying the relative phase of a plurality of working chambers of a fluid working machine,a demand input for receiving a demand signal,a phase input for receiving a phase signal indicative of the phase of volume cycles of the working chambers of the fluid working machine, working chamber availability data specifying which of the plurality of working chambers are available, anda displacement control module operable to select a volume of working fluid to be displaced by each of a plurality of working chambers specified by the working chamber database on each cycle of the working chamber volume taking into account the received phase signal, the received demand signal and the working chamber availability data,wherein the fluid working machine controller is configured to determine from said working chamber availability data that a second of said working chambers is unavailable to carry out the working function and to responsively cause a first of said working chambers to execute an active cycle instead of an idle cycle. 14. A fluid working machine controller according to claim 13, operable to periodically determine the status of each said working chamber and to treat a working chamber of said plurality of working chambers as unavailable if the working chamber is determined to be functioning incorrectly. 15. A fluid working machine controller according to claim 13, operable to amend the availability data concerning a working chamber responsive to a change in a working function allocated to the working chamber. 16. A fluid working machine controller according to claim 13, wherein the displacement control module is operable to select the volume of the working fluid to be displaced by each of the plurality of working chambers by determining the timing of valve control signals. 17. A non-transitory computer readable recording medium comprising computer program code configured to, when executed on the fluid working machine controller of claim 13, operate the displacement control module of the fluid working machine controller. 18. A method of operating a fluid-working machine comprising a plurality of working chambers of cyclically varying volume, each said working chamber operable to displace a volume of working fluid which is selectable for each cycle of working chamber volume, the method comprising: selecting the volume of working fluid displaced by one or more said working chambers during each cycle of working chamber volume to carry out a working function responsive to a received demand signal, andselecting the volume of working fluid displaced by a working chamber of said plurality of working chambers during a cycle of working chamber volume taking into account an availability of the other said working chambers to displace fluid to carry out the working function, measuring the status of each said working chamber and determining the availability of each said working chamber responsive thereto. 19. A method of operating a fluid-working machine comprising a plurality of working chambers of cyclically varying volume, each said working chamber operable to displace a volume of working fluid which is selectable for each cycle of working chamber volume, the method comprising: selecting the volume of working fluid displaced by one or more said working chambers during each cycle of working chamber volume to carry out a first working function responsive to a received demand signal, andselecting the volume of working fluid displaced by a working chamber of said plurality of working chambers during a cycle of working chamber volume taking into account an availability of the other said working chambers to displace fluid to carry out the said first working function,wherein a working chamber of said plurality of working chambers is treated as unavailable when said working chamber is allocated to a second working function different from said first working function or when said working chamber is not allocated to a or any working function. 20. A method of operating a fluid-working machine according to claim 1, wherein a working chamber is treated as unavailable responsive to a detection that there is a fault associated with the working chamber, and wherein active cycles of the faulty working chamber which would otherwise have been selected are instead substituted with idle cycles.