Hydraulic system having area controlled bypass

1. A hydraulic system comprising:

a first source of pressurized fluid;

at least one fluid actuator;

a first valve disposed between the first source and the at least one fluid actuator being configured to selectively communicate pressurized fluid from the first source to a tank in response to a first command; and

a controller configured to determine the first command by determining a plurality of commands for the first valve and selecting one of the plurality of commands as the first command.

2. The hydraulic system of claim 1 wherein the at least one fluid actuator is a first plurality of fluid actuators, the hydraulic system further including a first passageway fluidly communicating the first source, the first valve, and the first plurality of fluid actuators.

3. The hydraulic system of claim 1 , wherein the controller is further configured to receive an operator input and communicate the first command to the first valve and communicate a second command to the first source.

4. The hydraulic system of claim 3 , wherein the second command is at least partially based on an estimated amount of flow of pressurized fluid through the first valve and a predetermined amount of pressurized fluid flow through the first source.

5. The hydraulic system of claim 3 , wherein:

the first command is determined via a look-up table relating operator inputs and displacement values of the first valve; and

the second command is determined by:

estimating a first valve flow via a look-up table relating the displacement values of the first valve and first valve flows,

determining a first source flow via a look-up table relating operator inputs and first source flows, and

adding the estimated first valve flow and the determined first source flow.

6. The hydraulic system of claim 1 , further including:

a second source of pressurized fluid;

a second plurality of fluid actuators; and

a second valve disposed between the second source and the second plurality of fluid actuators, the second valve being movable in response to a third command, the third command being at least partially based on a predetermined flow area of the second valve.

7. The hydraulic system of claim 6 , wherein the at least one actuator is a first plurality of actuators, the hydraulic system further including:

a first passageway fluidly communicating the first source, the first valve, and the first plurality of fluid actuators; and

a second passageway fluidly communicating the second source, the second valve, and the second plurality of fluid actuators.

8. The hydraulic system of claim 7 , further including:

a third valve disposed downstream of the first and second valves and being configured to selectively fluidly communicate pressurized fluid from the second source to at least one of the first plurality of actuators.

9. A method of operating a hydraulic system comprising:

pressurizing a fluid;

directing pressurized fluid toward a first valve, the first valve having a first flow passageway and a first valve stem;

selectively directing an amount of the pressurized fluid through the first flow passageway to a tank; and

estimating the flow of pressurized fluid directed through the first flow passageway.

10. The method of claim 9 , further including selectively moving the first valve stem to vary the area of the first flow passageway.

11. The method of claim 9 , further including:

selectively directing pressurized fluid to a first fluid actuator and moving the first valve stem to a first position; and

selectively communicating pressurized fluid to a second fluid actuator and moving the first valve stem to a second position;

wherein an area of the first flow passageway in the first position is different than an area of the first flow passageway in the second position.

12. The method of claim 9 , wherein pressurizing a fluid includes pressurizing a first fluid to a first pressure and directing the first fluid at a first flow rate, and pressurizing a second fluid to a second pressure and directing the second fluid at a second flow rate, the method further including:

directing the fluid having the first flow rate toward the first valve;

directing the fluid having the second flow rate toward a second valve;

selectively permitting at least a portion of the first fluid to flow to the tank through the first valve; and

selectively permitting at least a portion of the second fluid to flow to the tank through the second valve.

13. The method of claim 9 , wherein pressurizing a fluid includes pressurizing a fluid with a first source, the method further including:

determining a first command at least partially based on a look-up table relating operator inputs and displacement values of the first valve at least partially based on the predetermined flow areas;

determining a second command at least partially based on a look-up table relating the displacement values of the first valve and estimated valve flow rates;

determining a third command at least partially based on a look-up table relating operator inputs and first source flow rates;

determining a fourth command at least partially based on the sum of the second and third commands;

selectively communicating the first command to the first valve; and

selectively communicating the fourth command to a first source of pressurized fluid.

14. The method of claim 9 , further including:

selectively directing pressurized fluid to a first chamber of a first fluid actuator;

selectively directing pressurized fluid from a second chamber of the first actuator to the tank.

15. The method of claim 14 , further including:

directing pressurized fluid via a first fluid passageway toward the first valve and toward the actuator; and

directing a portion of pressurized fluid from the first fluid passageway to the tank via the first valve.

16. A machine comprising:

an implement;

a frame;

a first hydraulic actuator configured to affect movement of the implement;

a second hydraulic actuator configured to affect movement of at least a part of the frame;

a hydraulic system including:

a tank,

first and second sources of pressurized fluid,

a first valve configured to selectively permit a pressurized fluid flow to the tank in response to a first area command, and

a second valve configured to selectively permit a pressurized fluid flow to the tank in response to a second area command; and

a controller configured to determine first and second source commands at least partially based on an estimated flow of pressurized fluid directed through the first and second valves, respectively.

17. The machine of claim 16 ,

wherein the controller is further configured to selectively communicate the first and second area commands to the first and second valves, respectively.

18. The machine of claim 17 , wherein the controller is configured to determine the first and second area commands by determining first and second valve displacements at least partially based on first and second look-up tables, each relating operator inputs with predetermined first and second valve displacements to determine first and second valve flow areas.

19. The machine of claim 17 wherein the controller is further configured to determine the first and second source commands by:

estimating first and second fluid flows through the first and second valves at least partially based on third and fourth look-up tables, each relating valve displacements and fluid flows;

determining first and second source flows at least partially based on fifth and sixth look-up tables, each relating operator inputs and source flows;

adding the estimated fluid flow through the first valve to the determined fluid flow through the first source; and

adding the estimated fluid flow through the second valve to the determined fluid flow through the second source.

20. The machine of claim 17 , wherein:

the first valve includes a flow area configured to permit pressurized fluid through the first valve to the tank; and

the controller is further configured to:

determine a plurality of first area commands based in part on a plurality of operator inputs, and

communicate the one of the plurality of first area commands resulting in the largest flow area of the flow passageway to the first valve.

21. The machine of claim 17 , further including an operator interface configured to selectively communicate operator inputs to the controller;

wherein the controller selectively affects movement of the first and second hydraulic actuators via the hydraulic circuit.