An open loop hydraulic drive system

Abstract

An open-loop hydraulic circuit comprising: a hydraulic fluid inlet, a hydraulic fluid outlet, and a hydraulic actuator 30 configured to receive hydraulic fluid from the hydraulic fluid inlet and return the hydraulic fluid to the hydraulic fluid outlet. The circuit further comprises a first valve assembly 40 between the hydraulic fluid inlet and the hydraulic actuator configured to control direction of flow and flow rate of hydraulic fluid through the hydraulic actuator. The circuit further comprises a first hydraulic compensator 62 between a first outlet of the first valve assembly and a first side of the hydraulic actuator, and a second hydraulic compensator 64 between a second outlet of the first valve assembly and a second side of the hydraulic actuator. The circuit further comprises: a first conduit 52 between the first outlet of the first valve assembly and the first hydraulic compensator, and a second conduit 54 between the second outlet of the first valve assembly and the second hydraulic compensator. The circuit further comprises a third conduit 56 providing a connection between the first conduit and the second conduit, the third conduit comprising a second valve assembly 70 configured, in a first mode, to enable passage of hydraulic fluid between the first conduit and a hydraulic fluid outlet and, in a second mode, to enable passage of hydraulic fluid between the second conduit and the hydraulic fluid outlet.

Description

[0001]AN OPEN LOOP HYDRAULIC DRIVE SYSTEM Technical Field The disclosure relates to hydraulic circuits for controlling flow of hydraulic fluid to wheels, tracks, tools and the like. Background Hydraulic circuits may be either open loop or closed loop. In a closed loop hydraulic circuit, hydraulic fluid runs almost continuously between a hydraulic pump and a hydraulic actuator, such as a piston or a motor. In an open loop hydraulic circuit, hydraulic fluid starts at a reservoir, runs though a hydraulic pump and a hydraulic actuator, and returns to the reservoir where it loses heat before being recycled through the circuit. Closed loop circuits enable finer control of the speed and direction of the hydraulic actuator than is possible in open loop circuits. Open loop circuits are generally less expensive, less complex and easier to maintain than closed loop circuits. One issue with open loop hydraulic circuits is the risk of an overrunning load, which can be power intensive to control. Hydraulic fluid from the hydraulic actuator is generally metered to drop its pressure before returning to the reservoir. At the same time, hydraulic fluid at a low pressure is required to prevent cavitation on the incoming side of the hydraulic actuator and to prevent runaway. Where hydraulic fluid is in use elsewhere at a higher pressure, the pump is required to meet that high pressure requirement which is then metered down to provide the lower pressure hydraulic fluid to the incoming side of the hydraulic actuator. This can result in considerable wasted energy. Open loop systems generally have high parasitic loads. This means that open loop systems are often considered inappropriate for low power machines where there is a desire for a well performing drive system. A low power machine may be considered to be a machine in which the maximum power consumption of a hydraulic drive pump of the machine exceeds the power available to the hydraulic drive pump from the power supply of the machine (e.g. the power available from an engine). Summary Against this background, there is provided an open-loop hydraulic circuit comprising: a hydraulic fluid inlet; a hydraulic fluid outlet; and a hydraulic actuator configured to receive hydraulic fluid from the hydraulic fluid inlet and return the hydraulic fluid to the hydraulic fluid outlet. The open-loop hydraulic circuit further comprises: a first valve assembly between the hydraulic fluid inlet and the hydraulic actuator configured to control direction of flow and flow rate of hydraulic fluid through the hydraulic actuator. The open-loop hydraulic circuit further comprises: a first hydraulic compensator between a first outlet of the first valve assembly and a first side of the hydraulic actuator; and a second hydraulic compensator between a second outlet f the first valve assembly and a second side of the hydraulic actuator. The open-loop hydraulic circuit further comprises: a first conduit between the first outlet of the first valve assembly and the first hydraulic compensator; and second conduit between the second outlet of the first valve assembly and the second hydraulic compensator. The open-loop hydraulic circuit further comprises: a third conduit providing a connection between the first conduit and the second conduit, the third conduit comprising a second valve assembly configured, in a first mode, to enable passage of hydraulic fluid between the first conduit and a hydraulic fluid outlet and, in a second mode, to enable passage of hydraulic fluid between the second conduit and the hydraulic fluid outlet. In this way, the arrangement of the present disclosure deploys open-loop hydraulics but removes the need for metered down high-pressure hydraulic fluid to be supplied to the incoming side of the hydraulic actuator. Furthermore, in an open-loop hydraulic system comprising first and second hydraulic circuits, the risk of an overrunning load is avoided. Drawings Embodiments of the disclosure are illustrated in the following drawings in which: Figure 1 shows an open-loop hydraulic circuit in accordance with the prior art; Figure 2 shows a pair of open-loop hydraulic circuits in accordance with the prior art configured for use in propulsion of a machine that uses differential propulsion for left and right sides in order to facilitate steering; Figure 3 shows an open-loop hydraulic circuit in accordance with a first embodiment of the disclosure; Figure 4 shows an enlarged view of some of the features of the first embodiment; Figure 5 shows a pair of open-loop hydraulic circuits of the first embodiment with the secondary valve assembly being closed; Figure 6 shows the pair of open-loop hydraulic circuits of Figure 5 with the secondary valve assembly being partially open; Figure 7 shows an open-loop hydraulic circuit in accordance with a second embodiment of the disclosure; Figure 8 shows an enlarged view of some of the features of the second embodiment; Figure 9 shows a pair of open-loop hydraulic circuits of the second embodiment with the secondary valve assembly being closed; and Figure 10 shows the pair of open-loop hydraulic circuits of Figure 9 with the secondary valve assembly being partially open. Detailed Description Figure 1 shows an open-loop hydraulic circuit in accordance with the prior art. An open-loop hydraulic circuit 100 comprises a hydraulic fluid inlet 10, a hydraulic fluid outlet 20 and a hydraulic actuator 30 configured to receive hydraulic fluid from the hydraulic fluid inlet 10 and return the hydraulic fluid to the hydraulic fluid outlet 20. The hydraulic fluid inlet 10 may be supplied by a hydraulic fluid pump 2 which pumps hydraulic fluid from a reservoir 4. The hydraulic fluid pump 2 may be driven by a prime mover 3. The hydraulic fluid outlet 20 may return the hydraulic fluid to the reservoir 4. Although the reservoir 4 is shown in Figure 1 as being distributed between a plurality of locations, the reservoir 4 may be one single reservoir 4. The open-loop hydraulic circuit 100 further comprises a first valve assembly 40 between the hydraulic fluid inlet 10 and the hydraulic actuator 30 configured to control direction of flow and flow rate of hydraulic fluid through the hydraulic actuator 30. The first valve assembly 40 has a first outlet 42 and a second outlet 44. In a first subset of valve positions, the first valve assembly 40 may direct hydraulic fluid out of the first outlet 42 and receive hydraulic fluid back through the second outlet 44. In a second subset of valve positions, the first valve assembly 40 may direct hydraulic fluid out of the second outlet 44 and back through the first outlet 42. In a third subset of valve positions, the first outlet 42 and the second outlet 44 may be blocked such that no hydraulic fluid flows. Figure 1 shows the first valve assembly 40 in the third subset of valve positions. If the first valve assembly 40 were to be moved to the right relative to what is shown in Figure 1, it would switch from operating in the third subset of valve positions to operating in the first subset of valve positions. If the first valve assembly 40 were to be moved to the left relative to what is shown in Figure 1, it would switch from operating in the third subset of valve positions to operating in the second subset of valve positions. The open-loop hydraulic circuit 100 further comprises a first conduit 52 between the first outlet 42 and a first side 32 of the hydraulic actuator 30, and a second conduit 54 between the second outlet 44 and a second side 34 of the hydraulic actuator 30. The first conduit 52 comprises a first hydraulic compensator 62 and a first check valve 63.The second conduit 54 comprises a second hydraulic compensator 64 and a second check valve 65. In order to drive the actuator in a first direction, the first valve assembly is operated in the first subset of valve positions such that hydraulic fluid travels from the first outlet 42, through the first check valve 63, into the first side 32 of the hydraulic actuator 30, through the hydraulic actuator 30, out of the second side 34 of the hydraulic actuator 30, through the second compensator 64 and into the second outlet 44 of the first valve assembly 40. In order to drive the actuator in a second direction (which is opposite to the first direction), the first valve assembly is operated in the second subset of valve positions such that hydraulic fluid travels from the second outlet 44, through the second check valve 65, into the second side 34 of the hydraulic actuator 30, through the hydraulic actuator 30, out of the first side 32 of the hydraulic actuator 30, through the first compensator 62 and into the first outlet 42 of the first valve assembly 40. In order to stop driving the actuator in either direction, the first valve assembly 40 is moved into the third subset of valve positions whereby no hydraulic fluid flows between the first valve assembly 40 and the actuator 30 in either direction. Other components of the open-loop hydraulic circuit 100 are shown but not described since their function is conventional in the field of hydraulic circuits. Figure 2 shows a pair of open-loop hydraulic circuits 102, 103 in accordance with the prior art open-loop hydraulic circuits 101 shown in Figure 1. The pair of open-loop hydraulic circuits is configured for use in propulsion of a machine that uses differential propulsion for left and right sides in order to facilitate steering. The pair of open-loop hydraulic circuits may share a pump 2 and may share a reservoir 4. Differential steering may be achieved by running one actuator faster than the other actuator. This is achieved by supplying hydraulic fluid at different flow rates to the left and right open-loop hydraulic circuits. For example, the valves may be controlled such that a flow rate of X lpm of hydraulic fluid may pass through the left actuator and a flow rate of 0.9X lmp may pass through the right actuator. In this way, the differential speeds between the left and right actuators will cause the machine to move towards the right. Where a particularly tight turn is sought, it may be possible to flow hydraulic fluid through one actuator in a first direction and through the other actuator in a second direction, opposite to the first direction. In this way, it may be possible for the machine to rotate without moving forwards or backwards. One issue with the arrangements of Figures 1 and 2 is the risk of an overrunning load, which can be power intensive to control. Hydraulic fluid from the hydraulic actuator is generally metered to drop its pressure before returning to the reservoir. At the same time, hydraulic fluid at a low pressure is required to prevent cavitation on the incoming side of the hydraulic actuator and to prevent runaway. Where hydraulic fluid is in use elsewhere at a higher pressure, the pump is required to meet that high pressure requirement which is then metered down to provide the lower pressure hydraulic fluid to the incoming side of the hydraulic actuator. This can result in considerable wasted energy. Figure 3 shows an open-loop hydraulic circuit in accordance with a first embodiment of the disclosure. The Figure 3 arrangement corresponds to the Figure 1 arrangement of the prior art. The Figure 3 arrangement is similar to the Figure 1 arrangement. Distinct from the Figure 1 arrangement, the Figure 3 arrangement further comprises a third conduit 56 providing a connection between the first conduit 52 and the second conduit 54. The third conduit 56 comprises a second valve assembly 70 that comprises a bidirectional valve 72. The second valve assembly 70 is configured, in a first mode, to enable passage of hydraulic fluid via the bidirectional valve 72 between the first conduit 52 and the hydraulic fluid outlet 20. The second valve assembly 70 is configured, in a second mode, to enable passage of hydraulic fluid via the bidirectional valve 72 between the second conduit 54 and the hydraulic fluid outlet 20. The second valve assembly 70 is configured, in a third mode, to prevent passage of hydraulic fluid in either direction. The bidirectional valve 72 may comprise a solenoid acting against a biasing means in order to control flow through the bidirectional valve 72. An enlarged view of the third conduit 56 and surrounding features of the Figure 3 arrangement is shown in Figure 4. Figure 5 shows an arrangement 201 comprising a pair of open-loop hydraulic circuits 202, 203, each of the type of open-loop hydraulic circuit 200 shown in Figure 3. Figure 5 also shows flow rates of hydraulic fluid for a particular use scenario at various positions in the pair of hydraulic circuits 202, 203. In the Figure 5 scenario, there is a call for 75 litres per minute of hydraulic fluid through the actuator of the left side open-loop hydraulic circuit 202, and a call for 100 litres per minute of hydraulic fluid through the actuator of the right side open-loop hydraulic circuit 203. The bidirectional valve 72 of the secondary valve assembly 70 of both the left side hydraulic circuit 202 and the right side hydraulic circuit 203 is closed. Therefore, in order to be able to supply 75 litres per minute to the actuator 30 of the right side hydraulic circuit 303 and 100 litres per minute to the actuator of the left side hydraulic circuit 302, the pump 2 is called to provide 175 litres per minute (75 + 100 = 175). Figure 6 shows the same hardware as in Figure 5. The bidirectional valve 72 of the secondary valve assembly 70 of the right side hydraulic circuit 203 remains closed as in Figure 5. However, by contrast with Figure 5, the bidirectional valve 72 of the secondary valve assembly 70 of the left side hydraulic circuit 202 is open. This allows 50 litres per minute of hydraulic fuel to pass between the second conduit 54 of the left side open-loop hydraulic circuit 202 and the first conduit 52 of the left side open-loop hydraulic circuit 202. In this way 50 litres per minute of hydraulic fuel is directly recirculated to the actuator 30 of the left side hydraulic circuit 202. Consequently, only 25 litres per minute of hydraulic fluid needs to be supplied from the first valve assembly in order to provide 75 litres per minute to the actuator 30 of the left side hydraulic circuit 202. Since only 25 litres per minute of hydraulic fluid is required to be supplied via the first valve assembly 40 of the left side open-loop hydraulic circuit 202, the left side open-loop hydraulic circuit 202 requires the pump to supply to it only 25 litres per minute in place of the previous 75 litres per minute (per the Figure 5 scenario). Therefore, the total pump requirement across both left and right side open-loop hydraulic circuits 202, 203 is 125 litres per minute in place of the previous 175 litres per minute (per the Figure 5 scenario). This means that the total power consumption of the pump 2 is significantly reduced in the Figure 6 scenario relative to the Figure 5 scenario. The hardware arrangement of Figure 5 and Figure 6 does not allow for the second valve assembly 70 (on either the left side open-loop hydraulic circuit 202 or the right side open- loop hydraulic circuit 203) to influence the direction of flow of hydraulic fluid between the second conduit 54 and the first conduit 52. There may be circumstances where the absence of such functionality would render the third conduit 56 unsuitable for use since it would result in flow of hydraulic fluid in a direction opposite to that desired. This issue does not arise in the arrangement shown in Figures 7 to 10. Referring to Figure 7, it will be noted that the arrangement is similar to that of Figure 3 except for the architecture around the second valve assembly 70. The second valve assembly 70 of the Figure 7 arrangement comprises a first one-way valve 76 facilitating flow only between the first conduit 52 and second conduit 54 and a second one-way valve 74 facilitating flow only between the second conduit 54 and the first conduit 52. In this way, the arrangement of Figure 7 allows the approach of the present disclosure to be deployed in a broader range of scenarios since it facilitates prevention of flow of hydraulic fluid in the opposite direction from that which is intended. The first one-way valve 76 comprises a first solenoid acting against a first biasing means in order to control flow through the second valve assembly 70 in the first direction. The second one-way valve 74 comprises a second solenoid acting against a second biasing means in order to control flow through the second valve assembly 70 in the second direction. An enlarged view of the third conduit 56 and surrounding features of the Figure 7 arrangement is shown in Figure 8. Figure 9 shows an arrangement 301 comprising a pair of open-loop hydraulic circuits 302, 303, each of the type of open-loop hydraulic circuit 300 shown in Figure 7. Figure 9 also shows flow rates of hydraulic fluid for a particular use scenario at various positions in the pair of open-loop hydraulic circuits 302, 303. In the Figure 9 scenario, there is a call for 75 litres per minute of hydraulic fluid through the actuator of the left side open-loop hydraulic circuit 302, and a call for 100 litres per minute of hydraulic fluid through the actuator of the right side open-loop hydraulic circuit 303. In both the left side open-loop hydraulic circuit 302 and the right side open-loop hydraulic circuit 303, both the first one- way valve 76 and the second one-way valve 74 are closed such that no hydraulic fluid flows in the third conduit 56 in either direction. Therefore, in order to be able to supply 75 litres per minute to the actuator 30 of the right side hydraulic circuit 303 and 100 litres per minute to the actuator of the left side hydraulic circuit 302, the pump 2 is called to provide 175 litres per minute (75 + 100 = 175). Figure 10 shows the same hardware as in Figure 9. In the right side hydraulic circuit 303, both the first one-way valve 76 and the second one-way valve 74 are closed such that in the right side hydraulic circuit no hydraulic fluid flows in the third conduit 56 in either direction. In the left side hydraulic circuit 302, the first one-way valve 76 is closed while the second one-way valve 74 is open. In this way, hydraulic fluid is able to flow from the second conduit 54 of the left side open-loop hydraulic circuit 302 to the first conduit 52 of the left side open-loop hydraulic circuit 302 via the second one-way valve 74 and the third conduit 56. However, hydraulic fluid is prevented from flowing from the first conduit 52 of the left side open-loop hydraulic circuit 302 to the second conduit 54 of the left side open-loop hydraulic circuit 303 since the second one-way valve 76 is closed. This allows 50 litres per minute of hydraulic fuel to pass between the second conduit 54 of the left side open-loop hydraulic circuit 302 and the first conduit 52 of the left side open- loop hydraulic circuit 302. In this way 50 litres per minute of hydraulic fuel is directly recirculated to the actuator 30 of the left side hydraulic circuit 302. Consequently, only 25 litres per minute of hydraulic fluid needs to be supplied from the first valve assembly in order to provide 75 litres per minute to the actuator 30 of the left side hydraulic circuit 302. Since only 25 litres per minute of hydraulic fluid is required via the first valve assembly 40, the left side open-loop hydraulic circuit 302 requires the pump to supply to it only 25 litres per minute in place of the previous 75 litres per minute (per the Figure 9 scenario). Therefore, the total pump requirement across both left and right side open-loop hydraulic circuits 302, 303 is 125 litres per minute in place of the previous 175 litres per minute (per the Figure 9 scenario). This means that the total power consumption of the pump 2 is significantly reduced in the Figure 10 scenario relative to the Figure 9 scenario. Furthermore, relative to the arrangement of Figures 3 to 6, it is possible to deploy the strategy facilitated by arrangement 301 of Figures 7 to 10 in circumstances where hydraulic fuel might be at risk of travelling in a direction opposite to that intended. While the disclosure refers to left side and right side hydraulic circuits, the skilled person will recognise that the invention is not limited to requiring literal deployment on left and right sides of any particular application. Indeed, although a specific example application of differential steering has been mentioned, the invention may be equally applicable to other applications of hydraulic circuits. In the claims, the terms left and right side are substituted by primary and secondary. The open-loop hydraulic circuit may comprise a controller configured to control the positions of the first and second valve assemblies of the open-loop hydraulic circuit. The controller may receive an actuator flow requirement R1 litres per minute indicative of flow rate of hydraulic fluid required through the hydraulic actuator. The controller may control the pump in order to deliver sufficient flow rate of hydraulic fluid. The controller may facilitate a flow rate of P1litres per minute through the first valve assembly 40. The controller may control the second valve assembly 70 to facilitate a flow rate S1litres per minute through the second valve assembly 70. In this way, the controller may control the open-loop hydraulic circuit such that the hydraulic actuator 30 receives a flow rate of R1= P1+ S1. In a hydraulic system comprising left and right open-loop hydraulic circuits, the controller may be further configured to receive an actuator flow requirement R1litres per minute indicative of flow rate of hydraulic fluid required through the hydraulic actuator of the left (primary) open-loop hydraulic circuit. The controller may be configured to facilitate a flow rate of P1litres per minute through the first valve assembly of the left (primary) open-loop hydraulic circuit. The controller may be configured to control the second valve assembly of the left (primary) open-loop hydraulic circuit to facilitate a flow rate S1litres per minute through the second valve assembly of the left (primary) open-loop hydraulic circuit. In this way, the controller may control the hydraulic system such that the hydraulic actuator of the left (primary) open-loop hydraulic circuit receives a flow rate of R1 = P1 + S1. The controller may further control the pump assembly so as to supply a flow rate of S1 + S2 wherein S2 is a flow rate in litres per minute requested through the first valve assembly of the right (secondary) open-loop hydraulic circuit. Industrial Applicability A hydraulic system in accordance with the claims deploys an open-loop hydraulic system comprising first and second hydraulic circuits with protection against overrunning loads. In one scenario, the first and second hydraulic circuits are deployed in a dual path machine whose speed and direction are controlled independently (such as in an excavator or loader with dual path tracks or wheels). In this way, turning the machine may be achieved by running the left and right paths at different speeds. When turning, the inside path (driven by, say, the first hydraulic circuit) requires a lower pressure than the outside path (driven by the second hydraulic circuit). In the claimed solution, the first hydraulic compensator and the second hydraulic compensator prevent an overrunning load.

Claims

Claims 1. An open-loop hydraulic circuit comprising: a hydraulic fluid inlet; a hydraulic fluid outlet; a hydraulic actuator configured to receive hydraulic fluid from the hydraulic fluid inlet and return the hydraulic fluid to the hydraulic fluid outlet; a first valve assembly between the hydraulic fluid inlet and the hydraulic actuator configured to control direction of flow and flow rate of hydraulic fluid through the hydraulic actuator; a first hydraulic compensator between a first outlet of the first valve assembly and a first side of the hydraulic actuator; a second hydraulic compensator between a second outlet of the first valve assembly and a second side of the hydraulic actuator; a first conduit between the first outlet of the first valve assembly and the first hydraulic compensator; a second conduit between the second outlet of the first valve assembly and the second hydraulic compensator; a third conduit providing a connection between the first conduit and the second conduit, the third conduit comprising a second valve assembly configured, in a first mode, to enable passage of hydraulic fluid between the first conduit and a hydraulic fluid outlet and, in a second mode, to enable passage of hydraulic fluid between the second conduit and the hydraulic fluid outlet.

2. The open-loop hydraulic circuit of claim 1 wherein the second valve assembly comprises a bidirectional valve having a first solenoid acting against a first biasing means in order to control flow through the second valve assembly.

3. The open-loop hydraulic circuit of claim 2 wherein the first mode and the second mode are selectable by the pressure of hydraulic fluid in the first conduit relative to the pressure of hydraulic fluid in the second conduit.

4. The open-loop hydraulic circuit of claim 1 wherein the second valve assembly comprises: a first one-way valve facilitating flow between the first conduit and the hydraulic fluid outlet; anda second one-way valve facilitating flow between the second conduit and the hydraulic fluid outlet.

5. The open-loop hydraulic circuit of claim 4 wherein: the first one-way valve comprises a first solenoid acting against a first biasing means in order to control flow through the second valve assembly in the first direction; and the second one-way valve comprises a second solenoid acting against a second biasing means in order to control flow through the second valve assembly in the second direction.

6. A hydraulic system comprising the open-loop hydraulic circuit of any preceding claim and further comprising a controller configured to: receive an actuator flow requirement R1litres per minute indicative of flow rate of hydraulic fluid required through the hydraulic actuator; facilitate a flow rate of P1litres per minute through the first valve assembly; control the second valve assembly to facilitate a flow rate S1litres per minute through the second valve assembly; such that the hydraulic actuator receives a flow rate of R1= P1+ S1.

7. The hydraulic system of claim 6 comprising a primary open-loop hydraulic circuit and a secondary open-loop hydraulic circuit; wherein each of the primary open-loop hydraulic circuit and the secondary open- loop hydraulic circuit comprises the open-loop hydraulic circuit of any preceding claim.

8. The hydraulic system of claim 7 further comprising a pump assembly and a reservoir; wherein the pump assembly is configured to supply both: the hydraulic fluid inlet of the first of the pair of open-loop hydraulic circuits; and the hydraulic fluid inlet of the second of the pair of open-loop hydraulic circuits; and wherein the reservoir is configured to receive both: the hydraulic fluid from the hydraulic fluid outlet of the first of the pair of open-loop hydraulic circuits; and the hydraulic fluid from the hydraulic fluid outlet of the second of the pair of open-loop hydraulic circuits.

9. The hydraulic system of claim 8 wherein the assembly comprises a pair of hydraulic pumps, one for each of the pair of open-loop hydraulic circuits.

10. The hydraulic system of claim 8 or claim 9 wherein the controller is further configured to: receive an actuator flow requirement R1 litres per minute indicative of flow rate of hydraulic fluid required through the hydraulic actuator of the primary open-loop hydraulic circuit; facilitate a flow rate of P1 litres per minute through the first valve assembly of the primary open-loop hydraulic circuit; control the second valve assembly of the primary open-loop hydraulic circuit to facilitate a flow rate S1litres per minute through the second valve assembly of the primary open-loop hydraulic circuit; such that the hydraulic actuator of the primary open-loop hydraulic circuit receives a flow rate of R1= P1+ S1; and control the pump assembly so as to supply a flow rate of S1+ S2wherein S2is a flow rate in litres per minute requested through the first valve assembly of the secondary open-loop hydraulic circuit.

11. A machine comprising the hydraulic system of claim 8, claim 9 or claim 10, and an engine configured to supply power to the pump assembly, wherein a maximum power consumption of the pump assembly exceeds a power available to the pump assembly from the engine.

12. A method of controlling the open-loop hydraulic circuit in accordance with any one of claims 1 to 5, the method comprising: matching flow rate through the second valve assembly with an imposed flow restriction through the first valve.

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