CROSS-REFERENCE TO RELATED CASES

[0001] This application claims the benefit of U.S. Provisional Application Ser. No. 61/454,180; filed March 18, 201 1 , the disclosure of which is expressly incorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention relates to hydraulic actuation systems for extending a hydraulic cylinder in a work machine, and particularly to to hydraulic systems requiring switching the cylinder extension mode from a powered mode to a regeneration mode.

BACKGROUND OF THE INVENTION

[0003] In a typical unbalanced (differential) hydraulic cylinder, the cross- sectional area of the chamber on the head side of the piston is greater than the cross-sectional area of the chamber on the rod side of the piston. When the cylinder is extended, more fluid is needed to fill the head-end or extend chamber of the cylinder than is being discharged from the rod-end or retract chamber. Conversely, less fluid is needed to fill the rod-end chamber than is being discharged from the head-end chamber when the cylinder is being retracted.

[0004] Hydraulic cylinders are subjected to varying loads during motion. The loads can be resistive loads or overrunning loads. The cylinder is subjected to overrunning loads when that load acts in the same direction of motion, such as lowering a wheel loader boom or lowering an excavator boom or its arm with gravity assistance. Hydraulic cylinders can be subjected to both types of loads in the same extend or retract stroke. For example, when a wheel loader bucket is fully curled in (cylinder fully extended) and is given a curl out (retract) command motion could start with a resistive load and then at some point in the stroke the load turns into a gravity assisted overrunning load. The bucket cylinder in this case is said to have gone over-center. This load transition from resistive to overrunning or vice versa should not affect the speed of the actuator and should be seamless to the machine operator. Controlling overrunning loads using traditional spool valves has not been energy efficient.

[0005] In existing hydraulic systems using these spool valves for control, pressurized hydraulic fluid is supplied from a pump to a cylinder (actuator) and hydraulic fluid flows out of the actuator to a tank. The flow to the actuator and out of the actuator is controlled by a spool. The position of the spool controls the flow of the hydraulic fluid. The construction of the four way spool valve is such that a given position of the spool determines the 'flow in' and the 'flow out' restriction sizes. Thus, metering in and metering- out are coupled. A certain restriction size on the inlet corresponds to a certain restriction size on the outlet. Therefore, it is a one degree of freedom system, and either the speed of the actuator or the pressure in just one chamber are controlled but not both.

[0006] Consequently, spool valve controlled systems can provide for good motion control but it cannot achieve energy saving potential at the same time. Spools are usually designed such that the outlet is restricted to limit the flow and prevent the load from falling at uncontrollable speeds in the case of cylinders subjected to overrunning loads. However, in other operating conditions, such as lifting a load, this restriction is not needed yet it is inherent in the design of the spool valve. This causes energy loss that is unnecessary if the design would allow for separate meter-in and separate meter-out i.e. independent metering.

[0007] To achieve independent metering, at least four valves are needed to operate one cylinder. One valve connects one of the work ports of the cylinder to the supply line that contains pressurized oil. A second valve connects the other work port to the supply line. A third valve connects the first port to the return line which leads to the tank. Finally a fourth valve is needed to connect the second port to the return line. Using this four valve configuration, while only two valves will be active for a given condition, allows for independent metering and also allows for energy regeneration, which has the potential for reducing the pump flow demand and thus achieve energy saving. Independent metering design provides for two degrees of freedom. In this case it is not necessary to design for a tight restriction on the return flow that is useful in overrunning load cases, but not needed otherwise. Regeneration mode also has the advantage of higher speeds than a standard (powered) mode albeit at smaller loads or negative (overrunning) loads. In other configurations, five valves are used to achieve regeneration when the individual valves are not bi-directional. The fifth valve connects both chambers of the cylinder to perform regeneration.

[0008] One problem with independent metering designs is mode switching. When the actuator goes over-center, e.g. load changes from a resistive load to an overrunning load, some valves have to close and others have to open. Since these valves are not infinitely fast, closing and opening them can potentially cause discontinuity in the motion of the cylinder. So, independent metering valves can be more efficient - but controlling the valves during mode switching can be challenging.

SUMMARY OF THE INVENTION

[0009] At least one embodiment of the invention provides a hydraulic circuit comprising: an hydraulic cylinder having a cylinder rod end and a cylinder base end, a hydraulic fluid inlet in the cylinder base end, and a hydraulic fluid outlet in the cylinder rod end; a hydraulic pump; a hydraulic reservoir; a first valve selectively connecting in fluid communication the hydraulic pump and the hydraulic fluid inlet in the cylinder base end during extension of the cylinder rod; a second control valve selectively opening a fluid passageway connected to the hydraulic fluid outlet in the cylinder rod end of the hydraulic cylinder during extension of the cylinder rod; a third valve selectively opening to connect in fluid communication an outlet of the second control valve to a hydraulic fluid reservoir when the rod is subjected to a resistive load and alternatively closing to force flow from the outlet of the second control valve to pass through a one-way check valve to flow to the hydraulic fluid inlet at the head end of the hydraulic cylinder when the rod is subjected to a resistive load.

[0010] At least one embodiment of the invention provides a hydraulic circuit comprising: an hydraulic cylinder having a cylinder rod end and a cylinder base end, a hydraulic fluid inlet in the cylinder base end, and a hydraulic fluid outlet in the cylinder rod end; a hydraulic pump; a hydraulic reservoir; a first control valve selectively connecting in fluid communication the hydraulic pump and the hydraulic fluid inlet in the cylinder base end during extension of the cylinder rod; a second control valve selectively opening a fluid passageway connected to the hydraulic fluid outlet in the cylinder rod end of the hydraulic cylinder during extension of the cylinder rod; a third valve selectively connecting in fluid communication an outlet of the second control valve to a hydraulic fluid reservoir when the rod is subjected to a resistive load and alternatively connecting the outlet of the second control valve to the hydraulic fluid inlet at the head end of the hydraulic cylinder through a check valve when the rod is subjected to a resistive load; a fourth control valve selectively connecting in fluid communication the hydraulic pump and a hydraulic fluid inlet in the cylinder rod end of the hydraulic cylinder during retraction of the cylinder rod; and a fifth control valve selectively connecting in fluid communication a hydraulic fluid outlet in the cylinder base end and the hydraulic reservoir during retraction of the cylinder rod.

[0011] At least one embodiment of the invention provides a method of extending a cylinder rod of a power cylinder comprising the steps of: activating a first control valve to fluidly connect an outlet of a hydraulic pump to an inlet port of a cylinder base end of the power cylinder allowing hydraulic fluid into the power cylinder; simultaneously activating a second control valve to connect an outlet port on a rod side of the power cylinder to allow flow through the second control valve; activating the third valve when the power cylinder is subjected to a resistive load allowing flow from the second control valve through the third valve to drain to a fluid reservoir; closing the third valve when the power cylinder is subjected to an overrunning load causing flow from the second control valve to flow through a one-way check valve to the inlet port of the cylinder base end of the power cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of this invention will now be described in further detail with reference to the accompanying drawings, in which:

[0013] FIG. 1 is a schematic of prior art cylinder system having five valves to control the cylinder motion;

[0014] FIG. 2 is a schematic of a portion of the prior art cylinder components for extension in powered mode;

[0015] Fig. 3 is a schematic of a portion of the prior art cylinder components for extension in regeneration mode;

[0016] Fig. 4 is a schematic of a portion of an embodiment of a cylinder system including a regeneration circuit of the present invention providing both power mode and regeneration mode; and

[0017] Fig. 5 is a schematic of an embodiment of a cylinder system including the extension components of the circuit shown in FIG. 4 as well as the circuit for retraction of the cylinder rod.

DETAILED DESCRIPTION OF THE INVENTION

[0018] With reference to Fig. 1 , the prior art hydraulic system 1 10 can be used on a mobile machine such as a backhoe, an excavator, or an industrial machine such as a press. Cylinder 1 1 can be replaced by a motor to perform a rotary function instead of a linear motion. The hydraulic system 1 10 includes a pump 12 that is driven by a motor or an engine (not shown) to draw hydraulic fluid from a tank 13 (connection not shown) to establish the hydraulic fluid under pressure to a supply line 14. Supply line 14 is connected to node 's' and return line 22 is connected to node 't'. Five electro-hydraulic proportional control valves 25, 26, 27, 28, and 29 control the flow of hydraulic fluid from the pump to the cylinder and from the cylinder to tank and thus control the motion of the cylinder. The control valve 25 is connected to node 's' through conduit 15 and is connected to head side 17 of cylinder 1 1 through conduit 16. Thus, control valve 25 controls fluid from pump 12 through supply line 14 and conduit 15 to the head side 17 of the cylinder 1 1 . The control valve 27 is connected to the rod side 18 of cylinder 1 1 through conduit 19 and is connected to node 't' through conduit 21 . Thus control valve 27 controls fluid drained from rod side 18 of cylinder 1 1 through conduit 19 to tank 13 through conduits 21 and 22. The control valve 29 is connected to the rod side 18 of cylinder 1 1 through conduit 20 and is connected to the head side 17 of cylinder 1 1 through conduit 23. Thus control valve 27 controls fluid flow from rod side 18 to head side 17 of cylinder 1 1 . Control valves 26 and 28 are similar in function to control valves 27 and 25 respectively, but they are used when the cylinder is retracting to connect fluid from pump 12 to rod side 18 and drain fluid from head side 17 to tank 13.

[0019] There are at least two modes of operation to extend the cylinder. With reference to Fig. 2 (which is the prior art system 10 of FIG. 1 with the retracting valves and conduits removed for clarity and focus on one extension mode of the cylinder rod), hydraulic fluid flows from pump 12 through conduits 14 and 15 and is modulated through control valve 25, then through conduit 16 to head side 17 of cylinder 1 1 . Flow is drained from rod side 18 through conduit 24 and is modulated through control valve 27 then through conduits 21 and 22 to tank 13. This is called standard or powered mode. [0020] With reference to Fig. 3 (which is the prior art system 10 of FIG. 1 with the retracting valves and conduits removed for clarity and focus on second extension mode of the cylinder rod), hydraulic fluid flows from pump 12 through conduits 14 and 15 and is modulated through control valve 25, then through conduit 16 to head side 17 of cylinder 1 1 . Flow is drained from rod side 18 through conduit 23 and is modulated through control valve 29 then through conduits 20 and 24 to head side 17 of cylinder 1 1 to augment flow from pump 12 that is modulated through valve 25. This is required because the volume of flow drained from the rod side 18 is less than the volume required at head side 17 for the cylinder to extend. This is called regeneration mode.

[0021] Powered mode can push larger loads than regeneration mode, but regeneration mode can extend the cylinder at higher speeds than powered mode. Therefore, it is often desired to switch between the two modes while the cylinder is extending. For example, consider a backhoe or an excavator that has its arm retracted all the way and its boom raised away from the ground. If an extend command is given for the arm, then the first portion of the stroke is performed in the air with gravity assistance, and thus a negative load. Then the load starts to increase until it goes through the over center position where the load turns positive and keeps increasing till the end of the stroke. In another scenario, the arm would hit the ground to start the digging process and in this case the resistive load that needs to be pushed by the cylinder would increase significantly. In these scenarios, it may be desirable to start the extension of the stroke in regeneration mode to go fast; since the load is negative or a small positive value, at a certain point in the stroke (e.g. start of digging) a mode switch to powered mode would be desirable to enable the cylinder to push high resistive loads. It is always desired that this transition or mode switch be smooth enough that it does not affect the motion of the cylinder and would not be perceived by the operator. However, since switching modes would entail closing valve 29 and starting to modulate valve 27 instead, and since these valves have a limited response time (are not infinitely fast), a perfectly smooth transition is difficult to achieve.

[0022] The extension portion of the circuit 10 of the invention is shown in FIG. 4 and the entire circuit 10 is shown in FIG. 5. Referring now to FIG. 4, to extend the cylinder 1 1 in either regeneration mode or standard mode, the same two valves 25 and 27 are used to control the cylinder. The valves can be independent metering valves, electro-hydraulic proportional control valves, or any other valve that is appropriate. In the present embodiment, there is no switching to another valve to change modes. For the cylinder to operate in powered mode, flow is supplied from pump 12 through line 14 to node 's' and then through line 15 through valve 25, line 16, and then to head side 17 of cylinder 1 1 . Flow is also drained from rod side 18 of cylinder 1 1 through hydraulic line 24 through valve 27, and line 21 . Valve 30 is a normally open valve and flow would go through valve 30 and line 22 to tank 13.

[0023] For cylinder 1 1 to operate in regeneration mode, flow is supplied from pump 12 through line 14, node 's', line 15 through valve 25, line 16, and then to head side 17 of cylinder 1 1 . Flow is also drained from rod side 18 of cylinder 1 1 through line 24 and through valve 27. Valve 30 is given a command to close and thus flow is blocked from going to tank and is instead directed through line 32, through check valve 31 and hydraulic line 33 to head side 17 of cylinder 1 1 .

[0024] In another embodiment, valve 30 is a proportional valve that would be given a command between 0 and 100 % to control the amount of flow that is directed to tank 13 through line 22 and the remaining amount of flow that would be directed through hydraulic line 32 through check valve 31 and hydraulic line 33 to head side 17 of cylinder 1 1 . This would be a mixed mode that is neither standard mode or regeneration mode. [0025] Referring to Fig. 5, the complete circuit 10 is shown with control valves 26 and 28 used when the cylinder is retracting to connect fluid from pump 12 to rod side 18 through conduits 14 and 15 and drain fluid from head side 17 to tank 13 through conduit 35.

[0026] Although the principles, embodiments and operation of the present invention have been described in detail herein, this is not to be construed as being limited to the particular illustrative forms disclosed. They will thus become apparent to those skilled in the art that various modifications of the embodiments herein can be made without departing from the spirit or scope of the invention.