Radial piston pumps

Abstract

A radial piston pump 1 is disclosed. The pump 1 comprises a rotor (30 Fig 2) mounted for rotation on the pintle (40 Fig 2) of a manifold, the rotor comprising a plurality of piston chambers (32 Fig 2), a piston (34 Fig 2) being mounted in each chamber 32 for reciprocal movement. The pump 1 comprises a P-port 8 for connecting the pump to a hydraulic system. The P-port 8 is a port connected to the piston chambers such that, in use, fluid can flow between the P-port 8 and the piston chambers. The P-port 8 is connected to the piston chambers by a P flow-path 6. A shut-off valve 4 is positioned along the P flow-path 6 such that when the shut-off valve 4 is in an off-position flow between the P-port 8 and the piston chambers is prevented.

Description

Field of the Invention The present invention concerns radial piston pumps. The invention also concerns units comprising such pumps, and methods of operating such pumps. Background of the Invention Radial piston pumps are used in a wide variety of applications including automotive and aerospace applications. Typically, radial piston pumps comprise a plurality of pistons mounted in radially extending piston chambers formed in a piston housing. The piston housing may include a hollow centre with a shaft mounted eccentrically therein. Alternatively, the piston housing may be eccentrically mounted within a ring. Movement of the pistons may be produced by rotating the piston housing relative to the shaft and / or ring. An inside (or internally) impinged pump may be defined as a pump in which the fluid flows into the pistons via the interior of the pump housing. An outside (or externally) impinged pump may be defined as a pump in which the fluid flows to and from the pistons via structure located around the exterior of the piston housing. WO2017 / 098250 (Dornin Fluid Power Limited) and WO 2022 / 234284 (Dornin Fluid Power Limited) describe example radial piston pumps. It would be advantageous to provide other types of radial piston pumps to thereby broaden the range of applications in which such pumps can be used. In many systems the choice of pump is constrained by available space and / or power. Accordingly, it is generally desirable to increase the efficiency of a pump. Additionally or alternatively, it is generally desirable to reduce the size of pump required for a given flow rate. Additionally or alternatively, it is generally desirable to reduce the mechanical complexity of a pump. The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved radial piston pump and methods of operation thereof. Summary of the Invention In one aspect, the present invention may provide a radial piston pump comprising: a manifold comprising a pintle; a rotor mounted for rotation on the pintle, the rotor comprising a plurality of piston chambers, a piston being mounted in each chamber for reciprocal movement; and / or a P-port for connecting the pump to a hydraulic system. A P-port may be defined a port connected to the piston chambers such that, in use, fluid can flow between the P-port and the piston chambers. The flow-path connecting the P-port to the piston chambers may be referred to as a P flowpath. The pump may comprise a shut-off valve positioned along the P flow-path such that when the shut-off valve is in an off-position flow between the P-port and the piston chambers is prevented. Thus, radial piston pumps in accordance with the present invention may comprise a shut-off valve located on the flow-path between the piston chambers and the P-port such that the piston chambers can be isolated from the hydraulic system connected to the pump by said port. In this way, the pistons of the pump can be isolated from the hydraulic system thereby reducing or eliminating leakage from the hydraulic system via the pump (i.e., losses in the pump), and / or the need to provide power to the pump, thereby increasing efficiency. It may be that the pump is configured such that rotation of the rotor in a first direction causes fluid to flow from the piston chambers toward the P-port along the P flow-path, and rotation of the rotor in a second, opposite, direction causes fluid to flow from the P-port toward the piston chambers along the P flow-path. Thus, radial piston pumps in accordance with the present invention may be bidirectional as regards the flow of fluid through the piston chambers and the P flowpath may be a bidirectional flow path. In this way, the pump may both provide fluid to, and extract fluid from, a hydraulic system connected to the P-port. This may simplify the design of the hydraulic system and / or improve the control of pressure in the hydraulic system. Additionally, or alternatively, flow in both directions between the piston chambers and the hydraulic system may be prevented using a single shutoff valve, allowing for a more compact pump. It may be that the manifold is of a single-piece construction, for example produced using additive manufacturing. It may be that the manifold comprises (e.g. an internal surface of the manifold defines) a P flow-gallery defining at least part of the P flow-path. It may be that the manifold, e.g. a surface of the manifold, defines the pintle. It may be that a P-port permits flow in both directions (i.e. the P-port is a two-way or bidirectional port). It may be that the P flow-path is a bidirectional flow path. The or each P-port may be an orifice, for example an orifice in a surface of the manifold. The P-port may comprise and / or be configured to receive a connector. The pump may comprise a connector configured to connect the pump to a hydraulic system, the connector being received in the P-port and / or being integrally formed with the manifold defining the P-port. The connector may comprise a screw thread and / or a tapered region for interlocking with a corresponding connector on the hydraulic system. The connector may be a male or female connector. The connector may comprise a protuberance, for example extending around the longitudinal axis of the connector and being configured to form a seal with a corresponding connector. As used herein, and unless otherwise stated, the longitudinal axis of the pump refers to the axis of rotation of the rotor. Likewise, the longitudinal axis of the rotor refers to the axis of rotation of the rotor. The longitudinal axis of the pintle refers to the axis parallel to, for example coaxial with, the longitudinal axis of the rotor. A shut-off valve may be defined as a valve having an off-position in which there is substantially no flow through the valve. That is to say, the valve may be configured such that the flow of fluid through the valve is prevented when the valve is in the off-position. This is in contrast to e.g. a spool valve in which there is inevitably flow around the sides of the spool (a quiescent loss) even when the spool is in a position in which it closes all the intended flow paths through the spool. It may be that the pump does not comprise a servo valve positioned along the P flow-path. It may be that the pump does not comprise a servo valve. The shut-off valve may be configured for movement between an on-position in which fluid can flow through the valve and an off-position in which the flow of fluid is prevented. It may be that the shut-off valve is configured for movement between only two positions; the on-position and the off-position. Thus, it may be that the shut-off valve does not provide proportional control. The shut off valve may be located within, for example received within a cavity of and / or defined at least in part by, the manifold, for example as part of the singlepiece construction of the manifold. The shut off valve may comprise a valve member mounted for movement, for example linear movement, into and out of the off-position, for example between the off-position and the on-position. The valve member may be mounted for movement parallel to, for example along, the longitudinal axis of the pump. The shut-off valve may comprise a valve seat. It may be that the valve is configured such that in the off-position the valve member contacts the valve seat such that the flow of fluid through the valve, for example past the seat and / or member is prevented. The valve seat may define an orifice, the valve member being sized and shaped such that when received in the orifice the flow of fluid through the orifice is prevented. The valve member may be mounted for movement from a first position (the on-position) spaced apart from the valve seat to a second position (the off-position) in which the valve member contacts the valve seat. The valve seat may be located along the P flow-path. For example, the valve seat may comprise a constriction in a flow-gallery defining the P flow-path. It may be that the manifold comprises the valve seat. The manifold may define the valve seat, e.g. as part of the single-piece construction of the manifold. This may reduce the number of components, reduce the risk of leakage and / or allow for a more compact manifold for a given flow rate. Alternatively, the valve seat may be a separate component mounted on the manifold. It may be that the radial piston pump comprises a reservoir. It may be that the reservoir is connected to the piston chambers such that, in use, fluid can flow between the reservoir and the piston chambers. It may be that the reservoir is connected to the piston chambers via a T flow-path. The manifold may comprise a T flow-gallery defining at least part of the T flow-path. The T flow-gallery may be formed within (i.e. defined by) the manifold as part of its single-piece construction. In the case that the pump is bidirectional, it may be that pump is configured such that when the rotor rotates in a first direction, fluid flows from the T flow-path towards the P flow-path via the piston chambers, and when the rotor rotates in a second, opposite, direction, fluid flows from the P flow-path towards the T flow-path via the piston chambers. The pump may comprise a pressure-relief valve. A pressure relief valve (PRV) may be defined as a valve configured to open at a predetermined pressure. For example, a PRV may be biased to a closed position, for example using a spring, until the predetermined pressure is reached, at which point the biasing force is overcome and the valve is forced open. It will be appreciated that during normal operation, the PRV will typically only open when the shut-off valve is closed as otherwise rotation of the rotor can be controlled in order to supply fluid to and / or extract fluid from the hydraulic system and thereby control the pressure at the P-port. It may be that the P-port is connected to the reservoir via a R flow-path. It may be that the R flow-path branches from the P flow-path at a point between the shut-off valve and the P-port. The PRV may positioned along the R flow-path such that the PRV opens when the pressure at the P-port exceeds a predetermined pressure. Use of such a PRV may protect the hydraulic system to which the pump is connected from over-pressure. Including the PRV in the pump as opposed to the hydraulic system may allow for a plug-and-play design, in which the complex hydraulic elements are contained within the pump and the hydraulic system to which it is connected can be simplified. This may facilitate maintenance / repair of the hydraulic system. It may be that the P-flow path is defined wholly or in part by one or more P flow galleries comprised within (for example defined by) the manifold. Similarly, the T or R flow path may be defined wholly or in part by one or more T or R flow galleries respectively, said flow galleries comprised within (for example defined by) the manifold. That is to say, the manifold may define said galleries as part of its single-piece construction. It will be appreciated, given that the flow path is so defined by the flow galleries that where reference is made to the location and / or shape of the flow path within the manifold, this applies equally to the location and / or shape of the flow galleries within the manifold (and vice versa). It may be that the majority of the length of the P flow-path is defined by one or more P flow galleries formed in (i.e. defined by) the single-piece manifold. It may be that the majority of the length of the T flow-path and / or R flow path is defined by one or more T or R flow galleries formed in (i.e. defined by) the single-piece manifold. The length of the T flow-path may be defined as the length of the flow path between the reservoir and the piston chambers. The length of the P flow path may be defined as the length of the flow path between the piston chambers and the P port. The length of the R flow-path may be defined as the length of the flow path between PRV and the piston chambers. In the case that the length of the flow path varies depending on e.g. the route taken (when the flow path branches) and / or the piston chamber chosen as the start / end point, the minimum length shall be used. It may be that the P, T and / or R flow-paths extend along at least part of the length (the dimension along the longitudinal axis), for example the majority of the length of, the pintle. It may be that there is only a single P flow-gallery in the pintle for the majority of the length of the pintle. It may be that the P flow-gallery extends along the longitudinal axis of the pintle. It may be that the P flow-gallery branches at the axial position of the piston chambers along the longitudinal axis of the pintle. It may be that each of said branches extends from the centre-line of the pintle to the outer surface of the pintle. It may be that the P-flow gallery ends in an orifice in the outer surface of the pintle, for example at the axial position along the longitudinal axis of the or each layer of piston chambers along the pintle. It may be that one or more T flow galleries extend along the pintle. Said T flow galleries may extend parallel to the P flow gallery(ies) in the pintle. Said T flow galleries may be located radially outside the P flow gallery(ies) in the pintle. It may be that the or each T-flow gallery (or T flow path) ends in an orifice in the outer surface of the pintle, for example at the axial position along the pintle of the piston chamber. A flow gallery may be defined as a channel or passageway, for example formed in the manifold. A flow gallery may have a start, being an orifice or a branching point with another flow gallery. A flow gallery may have an end, being an orifice in or a branching point with another flow gallery. That is to say, the length of the flow gallery is the length of the uninterrupted passageway between orifices / branches. It may be that a portion of the P-flow path extends parallel to, for example along, the longitudinal axis of the pump and / or pintle. It may be that the P-flow path splits into at least two branches. For example, it may be that the P flow-path splits into at least two branches at a location along the flow-path between the shut-off valve and a P-port. In that case, it may be that said branches of the P-flow path then recombine before reaching the P-port. This branching may reduce pressure losses in the manifold and / or allow for a more compact manifold. Additionally or alternatively, the branching may more evenly distribute the pressure-force throughout the manifold and thereby reduce the amount of material required in the manifold. Again, where branching and / or the shape or form of the flow path is described, this applies equally to branching and / or the shape or form of the flow galleries (and vice versa). Thus, the manifold may comprise P flow-galleries that branch as described above. This branching and / or recombining may all take place within the manifold, i.e. the flow galleries involved may be defined by the manifold as part of its single piece construction. It may be that the pump comprises more than one P-port connected to the piston chambers. In the case that the unit has more than one P-port, then the unit will have more than one P flow-path, a P flow-path being the flow path from a P-port to the piston chambers. It may be that the pump comprises a shut-off valve positioned along each P flow-path such that when the shut-off valve(s) are in the off-position, flow between all the P-ports and the piston chambers is prevented. Thus, the pump may be configured such that the shut-off valve(s) can be used to isolate the piston chambers from any hydraulic systems to which it is connected. It may be that a single shut-off valve is located between the piston chambers and a point at which a common P flow-path branches into two or more P flow-paths, one for each P-port. Alternatively, a plurality of shut-off valve(s) may be used, one for each P flow-path. Alternatively, it may be that all the piston chambers of the pump are connected to the same P-port. For example, it may be that the pump comprises a single P-port, and a single shut-off valve, said single shut-off valve being positioned such that when the shut-off valve is in the off-position flow between the P-port and the piston chambers is prevented. Thus, the pump may comprise only one P-port and only one shut-off valve, while still allowing the pump to supply to, and extract fluid from, the hydraulic system(s) connected to said P-port. It may be that the manifold comprises the reservoir (either the whole of the reservoir or part thereof). The manifold may comprise the reservoir as part of its single piece construction. For example, the reservoir may comprise a void (for storing fluid) within (defined by) the manifold (hereafter a manifold-void). It may be that said manifold-void has a minimum dimension greater than, for example at least 50% greater than, the maximum diameter of any flow gallery. It may be that the manifoldvoid is annular, for example is a full or partial annulus. It may be that the manifoldvoid extends circumferentially around at least a portion of the P-flow path. For example, the P-flow path and the manifold void may be concentric and, optionally, coaxial. Additionally or alternatively, the pump unit may comprise a reservoir assembly, the reservoir assembly comprising the reservoir (either the whole of the reservoir or part thereof). The reservoir may comprise a void (for storing fluid) within the reservoir assembly (hereafter an assembly-void). The reservoir assembly may be connected to, for example mounted on, the manifold, for example such that the manifold-void and the assembly-void are connected so as to thereby define the reservoir void. Thus, the reservoir assembly may form a cap on the manifold void. The reservoir assembly may comprise an expandable structure, for example bellows or other structure known to the skilled person, moveable between a collapsed configuration and an expanded configuration. It may be that the expandable structure defines (in whole or in part) the assembly-void. The expandable structure may be biased towards the collapsed configuration such that fluid in the assembly-void must reach a predetermined pressure before the structure will expand. The reservoir assembly may comprise one or more springs arranged to bias the expandable structure towards the collapsed configuration. Other mechanisms for biasing the expandable structure towards the collapsed configuration will be apparent to the skilled person. The pump may comprise a pump motor configured to rotate the rotor relative to the pintle. The pump motor may comprise a plurality of magnets, a plurality of coils and a stator. It may be that the rotor comprising the plurality of piston chambers is mounted for rotation with respect to the stator. It may be that either the plurality of magnets or the plurality of coils is mounted on the stator and the other of the plurality of magnets and the plurality of coils is mounted on the rotor. Thus, radial piston pumps in accordance with the present invention may comprise a common rotor as between the pump and the pump motor. It may be advantageous to mount the magnets on the rotor, as the coils can thereby be located in a dry area of the pump unit. Use of such a common rotor may allow for a more compact pump for a given flow rate and / or reduce the mechanical complexity of the pump with respect to similar prior art pumps. It will be appreciated that the plurality of magnets or the plurality of coils are mounted on the rotor for rotation therewith, such that when a current is provided to the coils in the presence of the magnetic field of the magnets and electromotive force is generated thereby rotating the rotor. Electric motors per se are well known and will not be described further here. Further detail is provided in WO 2022 / 234284 (Domin Fluid Power Limited) the contents of which is incorporated herein by reference. The radial piston pump may comprise a shut-off valve motor. It may be that the shut-off valve motor is operable to move the shut-off valve into and out of the off-position. It may be that the pump is arranged such that rotation of the shut-off valve motor in a first direction causes the shut off valve to move from one of the on-position and the off-position to the other of the on-position and the off-position. The radial piston pump may comprise a valve cam. The valve cam may be connected to the shut-off valve motor for movement (for example rotation) therewith. The valve cam may be mounted for rotation about a longitudinal cam axis. The valve cam may have an outer surface, the radial distance of the outer surface from the longitudinal cam axis varying circumferentially (e.g. around the circumference of the cam). The pump may be arranged such that as the valve cam moves (for example rotates) between a first position and a second position, contact between the cam surface and the valve member causes the valve member to move from the on-position to the off-position (or vice versa). For example, the pump may be arranged such that the cam pushes the valve member from the off-position to the on-position (or vice versa), as the valve cam rotates from the first position to the second position. The pump may be configured such that the valve member is biased to one of the off-position and the on-position, such that the valve member occupies said position in the absence of a force from the valve cam. The pump may be configured such that gravity acts to maintain the valve member in one of the off-position and the on-position in the absence of a force from the valve cam. Additionally or alternatively, the pump may comprise a resilient member, for example a spring, arranged to maintain the valve member in one of the off-position and the on-position in the absence of a force from the valve cam. The valve cam may be located within a cavity defined, at least in part, by the manifold, for example as part of its single-piece construction. Say cavity may lie on the P-flow path, such that in use, fluid in the P flow-path flows around the valve cam. This may allow for a more compact pump. The valve motor may comprise a plurality of magnets, a plurality of coils and a stator. It may be that the valve cam is mounted for rotation with respect to the stator. It may be that either the plurality of magnets or the plurality of coils is mounted on the stator and the other of the plurality of magnets and the plurality of coils is mounted on the valve cam. Thus, radial piston pumps in accordance with the present invention may use the valve cam as the rotor of the valve motor. It may be advantageous to mount the magnets on the cam, as the coils can thereby be located in a dry area of the pump unit. Use of the valve cam as the valve motor rotor may allow for a more compact pump for a given flow rate and / or reduce the mechanical complexity of the pump with respect to similar prior art pumps. The pump may comprise a servo valve comprising a spool mounted for movement relative to one or more ports, for example ports formed in an internal surface (for example an internal surface of the manifold) defining a cavity in which the spool is received. Thus, movement of the spool relative to the ports may be used to control the flow of fluid through the manifold and / or to or from another hydraulic system. The spool may be connected to the shut-off valve motor and the valve cam for movement (for example rotation) therewith. In this way, the shut-off valve motor may be used to control both the operation of the shut-off valve, and the flow of fluid using the servo valve. As well as simplifying the construction of the pump, using an (optionally integrally formed) cam and spool valve connected to a single motor may ensure the operation of the valve or other element associated with the cam are correctly coordinated (indexed) without the need for complex control circuitry. It will be appreciated that the use of a single motor to rotate both a cam and the spool of a servo valve (for example wherein the cam and servo valve are held in a fixed position relative to each other, e.g. when the cam and spool are integrally formed) is not restricted to cams for use with shut-off valves and / or radial piston pumps, but may find application in other hydraulic units and / or systems. It may be that the axis of rotation of the shut-off valve motor and / or valve cam is perpendicular to the longitudinal axis of the pump, for example the axis of rotation of the rotor and / or the pump motor. This may allow for a more compact pump. Alternatively, it may be that the axis of rotation of the shut-off valve motor and / or valve cam is parallel to the longitudinal axis of the pump, for example the axis of rotation of the rotor and / or the pump motor. It may be that the shut-off valve motor and / or valve cam is concentric and / or coaxial with the longitudinal axis of the pump, with the rotor and / or the pintle. This may allow for a more compact pump. It may be that the direction of movement of the valve member is perpendicular to the longitudinal axis of the pump unit. It may be that the direction of movement of the valve member is perpendicular to, for example along, the longitudinal axis of the P flow-path and / or the T flow-path in the pintle. This may allow for a more compact pump. It may be that the direction of movement of the valve member is parallel to, for example along, the longitudinal axis of the pump unit and / or the pintle. It may be that the direction of movement of the valve member is parallel to, for example along, the longitudinal axis of the P flow-path and / or the T flow-path in the pintle. This may reduce pressure losses. It may be that the pintle and the rotor are concentric, for example coaxial. It may be that the radial piston pump is an internally impinged (inside impinged) piston pump. The radial piston pump, for example the rotor and / or pintle may comprise, consist essentially of, or consist of metal, for example steel, aluminium, bronze, titanium or other appropriate materials. The radial piston pump, for example the manifold and / or the rotor may be formed using an additive manufacturing process. It may be that the pintle is a protrusion extending outward in a first direction from the manifold. The side of the manifold on which the pintle is located may be referred to as a first side. If present, it may be that the reservoir void extends into the manifold from a second, opposite, side of the manifold. For example, the manifold may have a second surface, being the surface of the manifold on the second side. The reservoir void may be formed by a recess in the second surface. The reservoir assembly (if present) may be located (for example mounted) on the second side of the manifold. The valve member of the shut-off valve may extend into the manifold from the second side. The valve cam may be located closer to the first side of the manifold than the valve member. The valve cam and / or shut-off valve motor may be located in a recess formed in the manifold. The pump may comprise a pressure transducer arranged to provide indication of the pressure in the P flow-path and / or the P-port. For example, the pressure transducer may be connected to the P-flow path between the shut-off valve and the P-port. A flow-gallery, for example defined by the single-piece manifold, may connect the pressure transducer to the P-flow path. The pump may comprise a particle trap, for example as part of the reservoir. The particle trap may comprise a region of the reservoir shaped to slow the flow of the fluid through said region such that particles can drop out of the flow. The particle trap may comprise a grating configured to permit the passage of fluid but to retain particles exceeding a threshold size. It may be that all the piston chambers (and the pistons received therein) occupy substantially the same axial position along the length of the pintle. Thus, the piston chambers may be arranged in a single layer. The pump may comprise only a single layer of piston chambers. Alternatively, the pump may comprise two or more layers of piston chambers, the piston chambers (and pistons received therein) of each layer occupying substantially the same axial position along the length of the pintle, each layer being spaced apart along the length of the pintle from any other layer. It will be appreciated that each layer will require an associated set of flow paths (i.e., P and T flow paths to enable the flow of fluid to or from the pistons). The separate P flow paths associated with the layers may combine, for example into a single P flow path, such that only a single shut-off valve and / or P-port is required. The radial piston pump may comprise a pump cam surface, the rotor being mounted for rotation relative to the cam surface. It may be that the pump cam surface is arranged to control the radial movement of the pistons. The pintle, rotor and cam surface may be concentric, for example coaxial. The pump cam surface may comprise a surface facing towards the longitudinal axis of the rotor / pump. The cam surface may extend circumferentially around the rotor, for example around the whole of the circumference of the rotor. Each cam surface may extend for 360 degrees around the rotor. The cam surface may comprise one or more regions of decreasing radius (e.g. regions in which the radius is decreasing with distance in a first direction of rotation) and one or more regions of increasing radius (e.g. regions in which the radius is increasing with distance in the first direction of rotation). Each region of decreasing radius may be located between two regions of increasing radius, and vice versa. Each region of decreasing radius may be located opposite another region of decreasing radius. Each region of increasing radius may be located opposite another region of increasing radius. The profile of the cam surface may be defined as the variation of the radius of the cam surface around the circumference of the cam surface and / or rotor. The radius of the cam surface may be defined as the distance between the cam surface and the point about which the rotor rotates relative to the cam surface. It may be that the radial distance between the longitudinal axis of the rotor and the cam surface varies circumferentially (e.g. around the circumference of the rotor and / or cam surface). The profile of the cam surface may comprise one or more cycles, each cycle comprising a region of increasing radius and a region of decreasing radius. Thus, each cycle of the cam surface may correspond to a cycle (a complete forward and backwards movement) of the piston. The profile of the cam surface may comprise two, four, six or more cycles. The profile of the cam surface may comprise only two, four or six cycles. The radial piston pump may comprise a plurality of cam followers arranged to travel along pump cam surface as the rotor rotates relative to the cam surface, each piston being connected to a cam follower such that radial displacement of the cam follower results in radial displacement of the piston. The pump may comprise a control system configured to control the operation of the pump, for example the operation of the pump motor and / or the shut-off valve motor. The control system may be configured to control the shut-off valve motor (to move the shut-off valve) and / or the motor that rotates the rotor, in response to a user input and / or a signal received from a feedback system. The pump may comprise a feedback system, for example an electrical feedback system, configured to provide information on the position of the shut-off valve, state of the PRV valve (if present) and / or the speed of the rotor. The feedback system may comprise a pressure transducer arranged to provide information on the pressure at the P-port. The pressure transducer may be located along the P-flow gallery between the shut-off valve and the P-port. The control system may be configured to cause the pump motor to rotate in the first and / or second direction in dependence on the pressure at the P-port, for example as sensed by the pressure transducer, and thereby maintain the pressure at the P-port and / or in the hydraulic system at a target pressure. The control system may be configured to cause the valve motor to move the shut-off valve between the on- position and the off-position in dependence on the pressure at the P-port, for example as sensed by the pressure transducer. In another aspect of the invention, there is provided a radial piston pump unit comprising a radial piston pump according to any other aspect. The pump unit may comprise a housing. The elements of the pump described above may be located within and / or define a portion of the housing. For example, the P-port may be arranged such that fluid can flow into or out of the unit and the housing via the P-port. For example, the P-port may be formed in and / or extend through the housing. The manifold may define a part of the housing, or be located within the housing. The pintle, rotor, reservoir and reservoir assembly may be located (wholly) within the housing. It may be that the housing does not have a single-piece construction. The pump and / or pump unit may be connected to a hydraulic system to provided fluid to and / or extract fluid from the system, for example via the or each P port of the pump and / or pump unit. The hydraulic system may comprise an actuator arranged to move between a first position and a second position using energy provided to the system in the form of hydraulic pressure by the pump. The control system may provide fluid to and / or extract fluid from the system in order to achieve a target pressure at the P-port. It may be that the hydraulic system is an aerospace or automative hydraulic system. It may be that the hydraulic system is a brake system. For example, a brake system for a vehicle, for example an automotive vehicle or aircraft (including a fixed wing aircraft or a helicopter). The brake system may comprise a brake pad and an actuator configured to move the brake pad from a first position to a second position in order to effect braking of a wheel of the vehicle. It may be that the hydraulic system comprises and / or is arranged to control the movement of an aerodynamic system, for example as part of aircraft flight control system and / or automotive drag control system. It may be that the hydraulic system is an aircraft landing gear extension / retraction system. It may be that the hydraulic system is a lifting apparatus, for example a fork lift truck, cherry picker or other freight handling system. For example, a lifting apparatus on a submarine or naval vessel, for example a weapons handling system. It may be that the hydraulic system is a suspension system, for example an automotive suspension system. It may be that the hydraulic system is a automotive central hydraulic system arranged to power active aerodynamic surfaces and / or control heave. It may be that the hydraulic system is an injection moulding system. It may be that the hydraulic system is a hydraulic test bed or other test equipment. The control system may comprise a processor and an associated memory. It may be that the processor is configured to cause the pump to perform the method of the invention by executing instructions stored in the associated memory and / or controlling valve motor and / or rotor motor. According to another aspect of the invention, there is provided a method of operating a radial piston pump, for example the pump of any other aspect, connected to a hydraulic system via a P-port, the pump comprising a rotor comprising a plurality of piston chambers having pistons therein, a P flow-path connecting the piston chambers to the P-port, and a shut-off valve located along the P flow-path. It may be that the method comprises the shut-off valve being open for a first time period, wherein during said first time period the rotor rotates in a first direction so that fluid flows from the piston chambers towards the hydraulic system and the P-port; and the rotor rotates in a second direction to extract fluid from the hydraulic system towards the piston chambers via the P-port. It may be that the method then comprises the shut-off valve being closed for a second time period, such that fluid cannot flow in either direction between the hydraulic system and the piston chambers via the P-port. Thus, methods in accordance with the present invention may provide a radial piston pump that can switch between a first mode (which may be referred to as an active mode) in which fluid is supplied to or extract from the hydraulic system to which the pump is connected, and a second mode (which may be referred to a locked-off mode) in which the piston chambers of the pump are isolated from the hydraulic system, such that there are no losses in and / or power requirement for the pump. This may find particular application in systems which have occasional periods of changing demand, and long periods of steady state demand. For example, if the hydraulic system is a brake system, then the first mode can be thought of as an active braking mode (e.g. while a vehicle is moving) and the second mode can be thought of as a parking brake mode (e.g. while a vehicle is stationary, and the brake is simply engaged). It may be that during said first time period the rotor rotates in a first direction to supply pressurised fluid from the piston chambers to the hydraulic system via the P-port; and the rotor rotates in a second direction to extract fluid from the hydraulic system to the piston chambers via the P-port. It may be that during said second time period fluid cannot flow between the hydraulic system and the piston chambers via the P-port. The method may comprise during the first time period, rotation of the rotor switching between the first and second directions, for example by the control system, in order to supply to and / or extract fluid from the hydraulic system and thereby provide a target pressure at the P-port (and consequently in the hydraulic system). The pressure at the P-port may be determined using the pressure transducer located on the P flow path between the P-port and the shut-off valve. The target pressure (which may vary during the first time period) may be determined by a user, or by the control system in response to a user input for an action (e.g. braking). The method may comprise, during the second time period, the pressure-relief valve (PRV) opening when the pressure in the P-flow gallery exceeds a first predetermined threshold. This may prevent overpressure in the hydraulic system during the second mode of operation. For example, if a brake system has been put into park while the ambient environment is cold, but the environment then heats up such that pressure increases. The method may comprise, during the second time period, a user and / or the control system monitoring the pressure at the P-port, for example using a pressure transducer located on the P flow-path between the shut-off valve and the P-port. The method may comprise, in dependence on the pressure so measured, causing the shutoff valve to open and the rotor to rotate in the first and / or second direction in the event the pressure deviates by a predetermined amount from a target pressure. The target pressure may remain constant throughout the second time period. The target pressure in the second time period may be different from the target pressure in the first time period. The control system may automatically cause the shut-off valve to open and / or the rotor to rotate in the event the pressure so measured deviates by a predetermined amount from the target pressure, for example in the event the pressure drops below a predetermined threshold. It will be appreciated that opening of the shut-off valve ends the second time period. The method may comprise fluid flowing along the P flow-path in a first direction (from the piston chamber to the P-port) and a second direction, opposite to the first direction (from the P-port to the piston chambers). It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Fig. 1 shows a radial piston pump assembly in accordance with a first example embodiment; Fig. 2 shows the pump 2 of the assembly of Fig. 1 in more detail; Fig. 3 shows a perspective view of a radial piston pump unit in accordance with a second example embodiment; Fig. 4 shows a plan view of the radial piston pump unit of Fig. 3; Fig. 5 shows a cross-sectional view through the rotor of the radial piston pump unit of Fig. 3, as indicated by the line labelled pump in Fig. 6; Fig. 6 shows a cross-sectional view through the pump unit, as indicated by the line labelled Inlet in Fig. 4; Fig. 7 shows a cross-sectional view through the pump unit, as indicated by the line labelled Outlet-SOV (Shut-Off Valve) in Fig. 4; Fig. 8 shows a cross-sectional view through the pump unit, as indicated by the line labelled PRV in Fig. 4; and Fig. 9 shows a flow chart of an example method in accordance with the present invention. Detailed Description Fig. 1 shows a diagram of a radial piston pump assembly 1 in accordance with a first example embodiment of the invention. The pump assembly 1 comprises a rotor assembly 2 (see Fig. 2) connected to a reservoir 14 via a T flow-path 18, and to a P port 8 via a P flow-path 6. Fluid flows into and out of the pump assembly 1 via the P port 8. A shut-off valve 4 is located on the P flow-path 6. A pressure relief valve 10 is connected to the P flow-path 6, between the shut-off valve 4 and the P port 8. The pressure relief valve 10 is connected to the reservoir 14 via a R flow-path 16. A pressure transducer 20 is located on the P flow-path 6, between the shut-off valve 4 and the P port 8. The pressure transducer 20 is electrically connected to a control system 22. The control system 22 is electrically connected to a pump motor 24. The pump motor 24 is arranged to drive the rotor assembly 2. The control system 22 is electrically connected to a valve motor 26. The valve motor 26 is arranged to move the shut-off valve 4 between its on position and its off position. In Fig. 1, a single P-port 8 is fluidly connected to the shut-off valve 4, but it will be appreciated that in other embodiments, the P flow-path 6 could branch, such that a single shut-off valve 4 is connected to multiple P-ports. In the same or yet further embodiments, the P flowpath 6 could branch with each branch having a shut-off valve, each shut-off valve controlling the flow between the rotor assembly 2 and one or more P-ports. With reference to Fig. 2, rotor assembly 2 is internally impinged and comprises a rotor 30 including a plurality of radially extending piston chambers 32 formed in the body of the rotor 30. Only one piston chamber 32 is shown in Fig. 2 but it will be appreciated that the piston chambers are spaced equidistantly around the rotor 32. A piston 34 is located in each piston chamber 32. An inward facing cam surface 36 extends around the outside of the rotor 30. The radius of the cam surface 36 varies periodically with circumferential distance around the rotor 30. Each piston 34 comprises a cam follower 38 that contacts the cam surface 36. The rotor 30 is concentrically mounted on a pintle 40 forming part of a manifold. A P-flow gallery 42 extends along the longitudinal axis of the pintle 40, and defines a portion of the P flow-path 6. The P-flow gallery 42 splits into two branches extending radially in opposite directions to an openings 42a on the surface of the pintle 40 at the axial location of the piston chambers 32. Two T-flow galleries 44 extend parallel to the longitudinal axis of the pintle 40 and the P-flow gallery 42 and define a portion of the T flow path 18. The two T-flow galleries 44 are on opposite sides of the pintle 40 and each open onto the outer surface of the pintle at an opening at the axial location of the piston chambers 32. It will be appreciated that other numbers and arrangements of T-flow galleries and P-flow galleries within the pintle are possible. In use, the rotor 30 rotates relative to the cam surface 36. In some embodiments a spring (not shown) urges the piston 34 radially outward from the piston chamber 32 and accordingly maintains the cam follower 38 in contact with the cam surface 36. In other embodiments a spring is not required and centrifugal force, or the pressure of liquid flowing into the piston chamber 32 may be sufficient to maintain the follower in contact with the cam surface. Along portions of the cam surface 36 where the radius of the cam surface reduces with the relative rotation of the rotor 30 and cam surface 36 the piston 34 is pushed into the piston chamber 32 as a result of the contact between the cam follower 38 and the cam surface 36. Consequently, any liquid located in the piston chamber 32 is expelled from the piston chamber 32. Conversely, when the profile of the cam surface 36 is such that the radius of the cam surface 36 increases with rotation the piston 34 moves radially outward and fluid can enter the piston chamber 32. Thus, rotation of the rotor 30 relative to the varying profile of the cam surface 36 causes the pistons 34 to reciprocate in the piston chambers 32 thereby moving fluid through the rotor assembly 2. When the rotor 30 rotates in a first direction (clockwise in Fig. 3), the piston 34 is moving inward as the piston chamber 32 moves past the opening 42a of the P flow-gallery 42, and the piston 32 is moving outward as the piston chamber 32 moves past the opening 44a of the T flow-gallery 44. Thus, when the rotor 30 rotates in the first direction, fluid flows from the T flow-gallery 44 / T flow-path 18 to the piston chamber 32, and then from the piston chamber 32 to the P flow-gallery 42 / P flow-path 6. When the rotor 30 rotates in a second direction (anticlockwise in Fig. 3), the piston 34 is moving inward as the piston chamber 32 moves past the opening of the T flow-gallery 44, and the piston 32 is moving outward as the piston chamber 32 moves past the opening 42a of the P flow-gallery 44. Thus, when the rotor 30 rotates in the second direction, fluid flows from the P flow-gallery 42 / P flow-path 6 to the piston chamber 32, and then from the piston chamber 32 to the T flow-gallery 44 / T flow-path 18. Thus, the pump assembly 1 of Figures 1 and 2 is bidirectional. Having fluid flow to and from the rotor assembly 2 via the same port may facilitate the integration of the pump assembly 1 with a hydraulic system and / or allow for a more compact and / or simplified pump assembly in comparison to prior art designs. It will be appreciated that in order to be bidirectional, the rotor and pistons must have sufficiently low inertia so as to facilitate switching, and the pump (including the various flow passages, piston chambers, openings, rotor etc), must have a geometry that allows flow in both directions. In use, when the rotor assembly 2 is rotating in the first direction and shut-off valve 4 is open (i.e. on) pressurised fluid is output from the rotor assembly 2, and flows via P-flow path 6 to P-port 8 and from there out of the pump assembly 1 for use in a hydraulic system to which the pump assembly 1 is connected. When the rotor assembly 2 is rotating in the second direction and shut-off valve 4 is open (i.e. on) pressurised fluid is pulled from the hydraulic system to the rotor assembly 2 via P-port 8 and P flow-path 6. Thus, the pump assembly 1 can achieve a target pressure in the hydraulic system by running the rotor assembly 2 in the first or second direction in order to increase or lower pressure in the hydraulic system. If the hydraulic system is not doing any work or having any work done to it, such that there is no need to supply or extract fluid (e.g. because the hydraulic system is a brake that is to be engaged while a vehicle is parked) then the shut-off valve 4 can be moved to a closed (i.e. off) position. The rotor assembly 2 is then isolated from the hydraulic system, meaning there is no pressure loss from the pump and / or work required to drive the pump. Thus, pumps in accordance with the present example may allow for operation in two modes; an active mode in which fluid flows into and out of the pump assembly, and a second mode in which the pump is isolated from the hydraulic system. This may increase efficiency, compared to e.g. pumps using servo valves where there is ongoing fluid flow. Additionally, pumps in accordance with the present example may achieve this pressure control in a more compact and / or mechanically simpler way. In the event that the pressure in the hydraulic system increases while the shutoff valve 4 is closed, that pressure is transmitted to the fluid in the P-flow path 6 via P-port 8 and pressure-relief valve (PRV) 10 will open in the case that the pressure exceeds a predetermined threshold. Fluid will then flow via the PRV 10 and R flowpath 16 to the reservoir 14. This may occur when environmental changes such as heating cause an increase in pressure in the hydraulic system. In the event that the pressure in the hydraulic system reduces while the shut-off valve 4 is closed, this can be detected by pressure transducer 20 and, if necessary, control system 22 opens shutoff valve 26 by driving valve motor 26 and operates rotor assembly 2 by driving pump motor 24 to provide fluid to the hydraulic system. This may occur when environmental changes such as cooling, or a slow leave from the system, causes a decrease in pressure in the hydraulic system. Thus, pump assemblies in accordance with the present example embodiment may control the pressure in the hydraulic system to which they are connected, and thereby simplify the design of that hydraulic system. The pump assembly 1 may be in the form of a pump unit, being a modular self-contained unit that can be attached to a hydraulic system via the P-port 8. Fig. 3 shows an exterior view of a radial piston pump unit 1 in accordance with a second example embodiment of the invention. Figs 5 to 8 show various cross-sectional views of the unit 1 of Fig. 3, the cross-sections being taken in the planes indicated in Figs. 4 and 6. Like elements are shown with like reference numerals as between the first and second example embodiments. For example, the P-port is indicated with reference numeral 8 in Fig. 1 and Figs 3 to 8. Only those aspects of the second example embodiment which differ with respect to the first example embodiment and / or which have not already been described in connection with the first embodiment are discussed here. With reference to Fig. 3 to 8, the unit 1 comprises a housing 50 encasing the elements of the pump unit described in Fig. 1 and 2. Fluid flows into and out of the unit 1 only via P-port 8 in housing 50. The housing 50 also includes electrical connectors 52 for providing power and control to the pump unit 1, and attachment points 54 for mounting the unit 1. As is shown in, for example, Fig. 6, the pintle 40 of the unit of Fig. 1 is integrally formed as part of a single-piece manifold 31. The rotor 30 is mounted on the pintle 40. The pump motor 24 comprises coils 56 and magnets 58, and the magnets are mounted on the rotor 30 for rotation therewith, such that the rotor is common as between the pump motor 24 and the rotor assembly 2. A casing 74 divides the interior of the housing 50 into a dry side, in which electronics and the coils 56 are located, and a wet side in which the hydraulic components are located. Shutoff valve 4 comprises a valve member 60 mounted for linear movement with respect to a valve seat 62 defined by the manifold 31. Valve member 60 is raised by rotating valve cam 66 between a first and second position and thereby pushing valve member 60 upwards. Shut-off valve 4 is located along the P flow-path 6 which is defined by P flow-galleries 42 formed in manifold 31. As shown in Fig. 6 and Fig. 6, valve cam 66 is located along the P-flow 6 in a cavity defined by the manifold 31. The P flowpath 6 / P flow-galleries 42 extend along the longitudinal axis of the pintle 40, and immediately after (i.e. on the P-port side of) the shut-off valve 4 two spurs 43 extend radially outward in opposition directions from the P flow-path 42, each spur 43 ending at a pressure transducer 20. The reservoir 14 is formed by two elements in the pump unit 1 of the second example: a reservoir cavity 14a in the manifold 1 and a reservoir assembly 14b mounted to one side of the manifold. Reservoir cavity 14a is an annular void in the manifold. Reservoir assembly 14b comprises expandable bellows 68 which define a void in the housing 50, and which are biased against expansion by springs 70, such that fluid in the reservoir is maintained at a predetermined pressure (0.3 bar gauge in some example embodiments). Reservoir cavity 14a and reservoir assembly 14b are aligned so that together they define a single reservoir volume. At the lower end of the reservoir cavity 14a there is a flow trap in which dust and / or other particles may be collected, as fluid flows from the PRV 10 to the reservoir 14. In some embodiments, this is simply an area of the reservoir wherein the geometry of the reservoir slows the flow of fluid, allowing particulates to drop out. In other embodiments, a filter, for example a metal grating, is located in the reservoir and is configured to allow the passage of fluid while preventing the passage of said particles. The arrangement of the unit 1 means that gravity assists the valve member 60 in maintaining contact with the cam 66, and that fluid flows from the reservoir 14 under the action of gravity to the rotor assembly 2. Fig. 7 shows the valve motor 26, attached to valve cam 66, the axis of rotation of the valve motor 26 and valve cam 66 being perpendicular to the axis of rotation of the rotor assembly 2. Fig. 8 shows the coils 72 of the valve motor 26. The magnets (not shown) are integrally formed with the valve cam 66. The pump unit 1 comprises a control system (not shown) connected (for example electrically) to the electrical connectors 52, pressure transducers 20, valve motor 26 and pump motor 24. The control system comprises a processor and associated memory, the processor being configured to carry out the above and below-described functions of the apparatus by executing instructions stored in the associated memory. Fig. 9 shows an example method 100 of operating a radial piston pump, for example a pump unit 1 as described above, in accordance with the present invention. During a first time period 102, the rotor rotates in a first direction 104 so that fluid flows away from the piston chambers towards the P-port and the rotor rotates in a second direction 106 so that fluid flows towards the piston chambers from the P-port. Then the shut-off valve is closed 108. During a second time period 110 throughout which the shut-off valve is closed, fluid cannot flow in either direction between the piston chambers and the P-port. Optionally, the rotor is switched between rotation in the first direction 104 and second direction 106 during the first time period 102 in dependence on a signal received from the pressure transducer. In some embodiments this done automatically by the control system of the pump. Optionally, during the second time period the control system monitors 112 pressure at the P-port. In the event the pressure at the P-port deviates from a target pressure, the control system automatically opens 116 the shut-off valve and / or rotates 104, 106 the rotor in order to obtain the target pressure. Optionally, during the second time period, the PRV valve opens 114 in the event the pressure at the P-port exceeds a predetermined threshold. Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

1. A radial piston pump comprising:a manifold comprising a pintle;a rotor mounted for rotation on the pintle, the rotor comprising a plurality of piston chambers, a piston being mounted in each chamber for reciprocal movement;a P-port for connecting the pump to a hydraulic system, the P-port being a port connected to the piston chambers such that, in use, fluid can flow between the P-port and the piston chambers, the P-port being connected to the piston chambers by a P flow-path; anda shut-off valve positioned along the P flow-path such that when the shut-off valve is in an off-position flow between the P-port and the piston chambers is prevented.

2. A radial piston pump according to claim 1, wherein the pump is configured such that rotation of the rotor in a first direction causes fluid to flow from the piston chambers toward the P-port along the P flow-path, and rotation of the rotor in a second, opposite, direction causes fluid to flow from the P-port toward the piston chambers along the P flow-path.

3. A radial piston pump according to claim 2, wherein the P flow-path is a bidirectional flow path and / or the P-port is a bidirectional port.

4. A radial piston pump according to any previous claim, wherein the manifold is of a single-piece construction and the manifold comprises a P flow-gallery defining at least part of the P flow-path.

5. A radial piston pump according to any previous claim, wherein the shut-off valve comprises a valve member and a valve seat, the valve member being mounted for movement into and out of an off-position in which the valve member contacts the valve seat such that the flow of fluid through the shut-off valve is prevented, the valve seat being defined by the manifold as part of its single-piece construction.

6. A radial piston pump according to any previous claim, the pump comprising a shut-off valve motor, and a valve cam connected to the shut-off valve motor for movement thereby, the pump being arranged such that as the valve cam moves between a first position and a second position, contact between a cam surface of the valve cam and the valve member causes the valve member to move from one of an on-position and the off-position to the other of the on-position and the off-position.

7. A radial piston pump according to claim 6, wherein the axis of rotation of the shut-off valve motor and / or valve cam is perpendicular to the axis of rotation of the rotor.

8. A radial piston pump according to any previous claim, comprising a reservoir, the reservoir being connected to the piston chambers such that, in use, fluid can flow between the reservoir and the piston chambers, the reservoir being connected to the piston chambers via a T flow-path; and wherein the manifold comprises a T flow-gallery defining at least part of the T flow-path.

9. A radial piston pump according to claim 8, comprising a pressure-relief valve, and wherein the P-port is connected to the reservoir via a R flow-path such that, in use, fluid can flow from the P-port to the reservoir, and wherein the R flow-path is connected to the P flow-path between the shut-off valve and the P-port and the pressure relief valve is positioned along the R flow-path such that the pressure relief valve opens when the pressure at the P-port exceeds a predetermined pressure thereby permitting fluid to flow from the P-port towards the reservoir.

10. A radial piston pump according to claim 8 or claim 9, wherein the pintle comprises at least part of the P flow-path and / or the T flow-path, extending parallel to, for example along, the longitudinal axis of the pintle.

11. A radial piston pump according to any previous claim, wherein a portion of the P-flow path extends along the longitudinal axis of the pump, and splits into at least two branches between the shut-off valve and the P-port, and optionally, wherein said at least two branches recombine before reaching the P-port.

12. A radial piston pump according to any previous claim, the pump comprises only one P-port, and only one shut-off valve.

13. A radial piston pump according to any previous claim, wherein the pump is connected to the hydraulic system, and the hydraulic system is a brake system or a lifting apparatus.

14. A radial piston pump unit comprising a radial piston pump according to any previous claim.

15. A method of operating a radial piston pump connected to a hydraulic system via a P-port, the pump comprising a rotor comprising a plurality of piston chambers having pistons therein, a P flow-path connecting the piston chambers to the P-port, and a shut-off valve located along the P flow-path, the method comprising;the shut-off valve being open for a first time period, wherein during said first time period the rotor rotates in a first direction so that fluid flows from the piston chambers towards the hydraulic system and the P-port; and the rotor rotates in a second direction to extract fluid from the hydraulic system towards the piston chambers via the P-port; and thenthe shut-off valve being closed for a second time period, such that fluid cannot flow in either direction between the hydraulic system and the piston chambers via the P-port.

16. A method according to claim 15 comprising, during the first time period, rotation of the rotor being switched between the first and second directions in order to supply to and / or extract fluid from the hydraulic system and thereby provide a target pressure at the P-port, wherein said switching is carried out in dependence on a signal received from a pressure transducer arranged on the P flow-path between the shut-off valve and the P-port.

17. A method according to claim 16 comprising, a control system of the radial piston pump automatically causing the shut-off valve to open and / or the rotor to rotate in the event the signal received from the pressure transducer indicates the pressure at the P-port has deviated from a target pressure.

18. A method according to any of claims 15 to 17, wherein the pump comprises a pressure-relief valve and during the second time period, the pressure-relief valve opens when the pressure in the P-flow gallery exceeds a first predetermined5 threshold.

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