An open loop hydraulic drive system

Abstract

Two open-loop hydraulic circuits 102,103 each having a pump 2, a hydraulic actuator 32, a valve 40 controlling flow direction and flow rate through the actuator, two hydraulic compensators 62,64 betwe

Description

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. Summary Against this background, there is provided a hydraulic system comprising a first open-loop hydraulic circuit; and a second open-loop hydraulic circuit. Each of the first and second open-loop hydraulic circuits comprises: a hydraulic pump; and a hydraulic actuator configured to receive hydraulic fluid from the hydraulic pump and return the hydraulic fluid to a hydraulic fluid outlet. Each of the first and second open-loop hydraulic circuits further comprises: a first valve assembly between the hydraulic pump and the hydraulic actuator configured to control direction of flow and flow rate of hydraulic fluid through the hydraulic actuator. Each of the first and second open-loop hydraulic circuits further comprises: a first hydraulic compensator between a first outlet of the first valve assembly and a first side of the hydraulic actuator. Each of the first and second open-loop hydraulic circuits further comprises: a second hydraulic compensator between a second outlet of the first valve assembly and a second side of the hydraulic actuator. In this way, the risk of an overrunning load in an open-loop hydraulic system comprising first and second hydraulic circuits is avoided. Unlike in the case of the single pump arrangement, where the higher pressure is metered down for supply to the hydraulic circuit with the lower pressure requirement, the pressure supplied by the pump of each of the first and second hydraulic circuits can be regulated to meet the pressure requirement of the respective one of the first and second hydraulic circuits. Consequently, while there is a need for two pumps in place of one, there is a reduced power consumption. The hydraulic system also avoids the need for additional compensation hardware in each of the first and second hydraulic circuits since the compensation is provided by each pump. 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 hydraulic system in accordance with the prior art and comprising a pair of open-loop hydraulic circuits 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 a hydraulic system in accordance with a first embodiment of the disclosure, the hydraulic system comprising a pair of open-loop hydraulic circuits; Figure 4 shows a hydraulic system in accordance with a second embodiment of the disclosure; Figure 5 shows a hydraulic system in accordance with a third embodiment of the disclosure; Figure 6 shows a hydraulic system in accordance with a fourth embodiment of the disclosure; Figure 7 shows a hydraulic system in accordance with a fifth embodiment of the disclosure; Figure 8 shows a hydraulic system in accordance with a sixth embodiment of the disclosure; and Figure 9 shows a hydraulic system in accordance with a seventh embodiment of the disclosure. 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 may simply consist of a directional control valve 40. 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. Ina 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. Ina third subset of valve positions, the first outlet 42 and the second outlet 44 may be blocked such that no hydraulic fluid flows through either the first outlet 42 and the second outlet 44. Figure 1 shows the first valve assembly 40 in the third subset of valve positions. The first valve assembly further comprises an intermediate outlet 46 and an intermediate inlet 48. The intermediate outlet 46 is connected via an intermediate conduit 47 to the intermediate inlet 48. The intermediate inlet 48 is selectively capable of receiving hydraulic fluid from the hydraulic fluid inlet 10. The intermediate outlet 46 is selectively capable of supplying hydraulic fluid to either the first outlet 42 or the second outlet 44. 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 40 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 40 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 30 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. The open-loop hydraulic circuit 100 further comprises a load sensing pressure compensating circuit 80. The load sensing pressure compensating circuit comprises a compensation valve 81 and a compensation circuit conduit 82 that connected between the intermediate conduit 47 of the first valve assembly 40 and the compensation valve 81. The flow to the actuator 30 can thereby be controlled by utilizing the load sensing pressure compensating circuit 80 to maintain a set pressure drop over the meter in orifice area of the first valve assembly 40. In doing this, the flow is also held constant regardless of load pressure. The meter in flow provided to the actuator 30 can then be precisely controlled by varying meter in orifice area of the first valve assembly 40. This enables precise control of flow metered into the actuator 30 regardless of the pressure or torque of the actuator 30. 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 share a pump 2 and a reservoir 4. Differential steering may be achieved by running one actuator 30 faster than the other actuator 30. 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 Ipm of hydraulic fluid may pass through the left actuator and a flow rate of 0.9X Imp 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. Note that all the number values (e.g. numbers of litres per minute) shown in Figure 3 and in all subsequent Figures are given for the purposes of illustration only and provide no limiting effect whatsoever. The Figure 3 arrangement is similar to the Figure 2 arrangement. Distinct from the Figure 2 arrangement, the Figure 3 arrangement comprises separate first and second pumps, one for each of the pair of open-loop hydraulic circuits 102, 103, and separate load sensing pressure compensating circuit 80, one for each of the pair of open-loop hydraulic circuits 102, 103. In addition, distinct from the Figure 2 arrangement, the hydraulic system of Figure 3 comprises a back pressure check valve 21 in the hydraulic fluid outlet 20 by which hydraulic fluid may return to the reservoir 4. In the example, the back pressure check valve is given an example backpressure value of 3 bar. The value may be chosen in order to be confident that cavitation will always be avoided in expected use cases. However, that pressure is not otherwise usefully deployed and can never be deployed for useful work. Depending on the direction of flow of hydraulic fluid through the actuator 30, one side (either the first side 32 or the second side 34) will be a meter in side of the actuator 30 and the other side (either the second side 34 or the first side 32) will be a meter out side of the actuator 30. When an overrunning load is encountered, the meter in side of the actuator 30 will be low in pressure, while the meter out side of the actuator 30 will become high pressure. Without any countermeasures, the pressure drop over the first valve assembly 40 meter out orifice area will increase, thereby increasing flow, resulting in a significant increase in speed of the actuator 30. This is prevented in the arrangement of Figure 3 by using the first hydraulic compensator 62 and the second hydraulic compensator 64 to maintain a constant pressure drop over the meter out orifice area of the actuator 30. More specifically, the first hydraulic compensator 62 and the second hydraulic compensator 64 are controlled to vary their open area so as to vary the pressure drop across them in order to maintain the constant pressure drop over the meter out orifice area in the actuators 30. As the pressure drop over the meter out orifice area is held constant, the flow out of the actuator 30 will also be held constant with an overrunning load, so preventing the motor speed from increasing. The speed of the actuator 30 can then be controlled by varying meter out orifice area of the first valve assembly 40. This is the same principle used to meter flow into the actuator 30 from the directional valve when pushing a load (for example, if the actuator 30 is a motor providing traction to move a machine up a hill). Thus, the speed of the actuator 30 (motor) can be controlled independent of pressure and actuator (motor) torque in all conditions. The resulting control over the actuator 30 is superior to that of an open loop circuit with counterbalance valves. To improve the performance of an open loop drive system beyond the benefit of the meter out compensation, dedicated pumps are used for each actuator (drive motor) 30 to improve power consumption and system efficiency. When steering or turning the machine using the arrangement of Figure 3, the pump paired with the inside track or wheel will be at a low pressure as the actuator 30 is driven by the outside track or wheel turning the machine, while the pump paired with the outside track or wheel will be at high pressure. As a result, the pump paired with the inside track or wheel will be at a low pressure, thereby requiring a fraction of the power compared to the previously described single pump open loop system. As a result, a two-pump open loop system will have improved turning performance over a single pump open loop system while allowing energy savings to be made and potentially allocated to other machine services. The Figure 4 arrangement is similar to the Figure 3 arrangement. Distinct from the Figure 3 arrangement, the Figure 4 arrangement deploys a variable displacement pump control circuit 82 on each of the two pumps 2 in place of a load sensing pressure compensating circuit 80 on each of the two pumps 2. An electronically variable displacement pump circuit 82 does not require margin pressure to control its displacement, and therefore the efficiency loss due to margin is saved during operation. As in the Figure 3 arrangement, the arrangement of Figure 4 comprises a back pressure check valve 21 in the hydraulic fluid outlet 20 by which hydraulic fluid may return to the reservoir 4. In the Figure 4 arrangement the back pressure check valve is given an example backpressure value of 7 bar. The value may be chosen in order to be confident that cavitation will always be avoided in expected use cases. However, that pressure is not otherwise usefully deployed and can never be deployed for useful work. The Figure 5 arrangement avoids the need for that permanent backpressure introduced by the backpressure check valve 21 of the Figures 3 and 4 arrangements. In the Figure 5 arrangement, a separate (third) pump 6 is used in combination with a pressure reducing valve 7 (set to 10 bar using the example values) to prevent the low side of the actuator 30 from dropping below 10 bar. The control pressure of the pressure reducing valve is communicated to the pump’s load sense compensator such that the discharge pressure of the pump is elevated proportionally. By elevating the pump discharge pressure, oil will be pumped into the low pressure side of the actuator at an elevated pressure, thereby avoiding cavitation of the actuator. As a result, elevated system pressure is only present as needed to prevent cavitation, and not all of the time during operation as would be the case with a backpressure device. As stated previously, the value may be chosen in order to be confident that cavitation will always be avoided in expected use cases. The third pump may be another pump which is already available on the machine and with a primary use other than its use shown in Figure 5. Thus the need for additional pump hardware can be avoided. Referring to the Figure 6 arrangement, in an alternative arrangement to Figure 5, the need for a third pump 6 may be avoided by providing a dedicated pressure reducing valve 11 in a separate line driven by one of the two pumps 2. With reference to the Figure 7 arrangement, instead of one pressure reducing valve 11 driven by one of the two pumps, there is a separate line for each of the first and second hydraulic circuits, each having its own pressure reducing valve 11. The pressure reducing valve 11 of the first hydraulic circuit is driven by the pump 2 of the first hydraulic circuit while the pressure reducing valve 11 of the second hydraulic circuit is driven by the pump 2 of the second hydraulic circuit. This results in improved operating efficiency. The arrangement of Figure 8 provides electronically controllable variable load sense relief valves 12 (one for each of the first hydraulic circuit and the second hydraulic circuit) which provide the ability to control both flow and pressure. In this way, flow control is achieved with a variable torque limit. The torque limit for each of the first hydraulic circuit and the second hydraulic circuit may be varied independently of the other. The arrangement of Figure 9 is similar to that of Figure 8 except that it uses a single electronically controllable variable load sense relief valve 12 shared between the first hydraulic circuit and the second hydraulic circuit. This reduces hardware requirements but enables one torque limit (which is variable) but at any one time the same torque limit applies to both the first hydraulic circuit and the second hydraulic circuit. It may be that on a particular machine a high torque is required when steering but a significantly lower torque is required when moving forwards without steering. Thus, the arrangements of Figure 8 and 9 may be particularly useful for imposing a higher torque limit during steering than when moving forwards without steering. Different torque requirement scenarios will depend upon the type of machine. For example, an excavator, a loader and a crane may all have different torque requirements in different modes of operation. Although all of the illustrated embodiments show the pumps of both the first hydraulic circuit and the second hydraulic circuit being driven by the same prime mover, they may be driven by separate prime movers. In such a scenario, the pump may be a fixed displacement pump and the speed of the prime mover may be varied in order to vary the flow. Each prime mover might be an electric motor. Each prime mover might be an internal combustion engine. Such an arrangement may, for example, be deployed with the configuration shown in Figure 4, with the pump displacement electronically controlled to achieve the required flow. The displacement would be fixed but the speed may be varied on each prime mover to achieve the desired flow, independently, from each pump. 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 such a manner that reduces power consumption by avoiding the need to meter down high pressure hydraulic fluid to whichever of the first and second hydraulic circuits has the lower pressure demand. Instead, each of the first and second hydraulic circuits includes a pump which is controlled to meet the pressure demand required by its hydraulic circuit. Consequently, while there is a need for two pumps in place of one, there is a reduced power consumption. Furthermore, the hydraulic system in accordance with the claims avoids the need for additional compensation hardware in each of the first and second hydraulic circuits since the compensation is provided by each pump. Each of the two pumps may share a power source, namely the prime mover, but is otherwise independent. Alternatively, each pump may be driven by its own separate prime mover. Furthermore, by comparison with a solution involving hydraulic fluid compensators in each of the first and second hydraulic circuits, control software may be simpler. 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 pumps of the first and second hydraulic circuits are controlled independently to supply only the required flow at the required pressure for each of the first and second hydraulic circuits. Thus, hydraulic fluid pressure is not metered down for no benefit, meaning improved efficiency. In short, the arrangement of the claimed hydraulic system provides improved turning performance and improved efficiency. When steering a dual path machine, very high torque may be required. However, when using a machine to push a pile without steering may require significantly lower torque. The arrangement of the present disclosure enables better matching of torque requirement to torque output, and thereby improved efficiency. Which of the embodiments set out above may be most appropriate in a particular situation may depend upon, among other things, the type of machine. For example, a loader, a crane and an excavator and their respective use cases may make one embodiment more favourable than another.

Claims

1. A hydraulic system comprising:a first open-loop hydraulic circuit; anda second open-loop hydraulic circuit;wherein each of the first and second open-loop hydraulic circuits comprises:a hydraulic pump;a hydraulic actuator configured to receive hydraulic fluid from the hydraulic pump and return the hydraulic fluid to a hydraulic fluid outlet;a first valve assembly between the hydraulic pump 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; anda second hydraulic compensator between a second outlet of the first valve assembly and a second side of the hydraulic actuator.

2. The hydraulic system of claim 1 further comprising a hydraulic pump drive input configured to drive both the hydraulic pump of the first open-loop hydraulic circuit and the hydraulic pump of the second open-loop hydraulic circuit.

3. The hydraulic system of claim 1 further comprising a first hydraulic pump drive input configured to drive the hydraulic pump of the first open-loop hydraulic circuit and a second hydraulic pump drive input configured to drive the hydraulic pump the hydraulic pump of the second open-loop hydraulic circuit.

4. The hydraulic system of any preceding claim wherein each of the first open-loop hydraulic circuit and second the open-loop hydraulic circuit are configured to maintain flow independent of pressure.

5. The hydraulic system of claim 4 wherein the load sensing circuit configured to maintain a constant pressure drop over first valve assembly meter in orifice area.

6. The hydraulic system of any preceding claim wherein the hydraulic pump of either or both of the first open-loop hydraulic circuit comprises a first variable displacement pumpand the hydraulic pump of the second open-loop hydraulic circuit comprises a second variable displacement pump.

7. The hydraulic system of any of claims 1 to 6 wherein the hydraulic pump of the first open-loop hydraulic circuit comprises a first electronically displacement-controlled pump and / or the hydraulic pump of the second open-loop hydraulic circuit comprises a second electronically displacement-controlled pump.

8. The hydraulic system of any preceding claim further comprising:a first hydraulic fluid supply line in fluid communication with the first open-loop hydraulic circuit between the hydraulic pump and the hydraulic actuator for selective supply of hydraulic fluid in order to maintain pressure in the hydraulic actuator and / or the hydraulic pump above a threshold pressure; and / ora second hydraulic fluid supply line in fluid communication with the second openloop hydraulic circuit between the hydraulic pump and the hydraulic actuator for selective supply of hydraulic fluid in order to maintain pressure in the hydraulic actuator and / or the hydraulic pump above a threshold pressure.

9. The hydraulic system of claim 8 further comprising one or more pressure reducing valves for reducing pressure in the first hydraulic fluid supply line and in the second hydraulic fluid supply line.

10. The hydraulic system of claim 8 or 9 wherein:the first hydraulic fluid supply line is in selective fluid communication with the hydraulic pump of the second open-loop hydraulic circuit; and / orthe second hydraulic fluid supply line is in selective fluid communication with the hydraulic pump of the first open-loop hydraulic circuit.

11. The hydraulic system of claim 8, claim 9 or claim 10 further comprising a third pump configured to supply hydraulic fluid to the first and second hydraulic fluid supply lines.

12. The hydraulic system of claim 8 wherein:the first hydraulic fluid supply line is supplied by the hydraulic pump of the first open-loop hydraulic circuit; and / orthe second hydraulic fluid supply line is supplied by the hydraulic pump of the second open-loop hydraulic circuit.

13. The hydraulic system of any preceding claim further comprising one or more variable load sense relief valves for reducing pressure in the first hydraulic fluid supply line and / or in the second hydraulic fluid supply line.

14. The hydraulic system of claim 13 wherein:the first load sense relief valve is in selective fluid communication between the first hydraulic actuator or the first valve assembly and the hydraulic pump of the first open-loop hydraulic circuit; andthe second load sense relief valve is in selective fluid communication between the second hydraulic actuator or the first valve assembly and the hydraulic pump of the second open-loop hydraulic circuit.

15. The hydraulic system of any preceding claim comprising one or more reservoirs configured to receive one or both of:the hydraulic fluid from the hydraulic fluid outlet of the first of the pair of open-loop hydraulic circuits; andthe hydraulic fluid from the hydraulic fluid outlet of the second of the pair of openloop hydraulic circuits.

16. The hydraulic system of claim 2 or any claim dependent on claim 2 wherein the hydraulic pump drive input comprises a coupling for connection to a prime mover.

17. The hydraulic system of claim 3 or any claim dependent on claim 3 wherein: the hydraulic pump drive input configured to drive the hydraulic pump of the first open-loop hydraulic circuit comprises a first coupling for connection to a first prime mover; andthe hydraulic pump drive input configured to drive the hydraulic pump of the second open-loop hydraulic circuit comprises a second coupling for connection to a second prime mover.

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