Patent Publication Number: US-7908853-B2

Title: Hydraulic balancing for steering management

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a hydraulic system, and more particularly, to a hydraulic system having a controller for balancing fluid flow between one or more steering actuators and one or more other implements. 
     BACKGROUND 
     Many machines use multiple hydraulic actuators to accomplish a variety of tasks. Examples of such machines include without limitation dozers, loaders, excavators, motor graders, and other types of heavy machinery. The hydraulic actuators in such machines are linked via fluid flow lines to a pump associated with the machine to provide pressurized fluid to the hydraulic actuators. Chambers within the various actuators receive the pressurized fluid in controlled flow rates and/or pressures in response to operator demands or other signals. Although most such machines are deigned to allow multiple actuators to be used simultaneously, in certain circumstances the demanded fluid flow will exceed the output capabilities of the fluid pump, especially when a single such pump is used. In the event that a flow of fluid supplied to one of the actuators is less than what is demanded by the machine operator or control system, the affected actuator may respond too slowly, too gently, or otherwise behave in an unexpected manner. 
     Given this problem, various solutions have evolved in the art. One method of accommodating a demand for fluid flow that is greater than the capacity of an associated pump is described in U.S. Appl. 20060090459 by Devier et al. entitled “Hydraulic System Having Priority Based Flow Control” (“the &#39;459 application”). The &#39;459 application describes a hydraulic system controller that is configured to receive input indicative classifying a plurality of fluid actuators as being either of a first or a second type. When an input indicative of a desired flow rate for the plurality of fluid actuators is received, the controller determines a current flow rate of the source. If all demanded flow rates can be met, the controller demands this amount of flow. Otherwise, the controller demands the desired flow rate only for the first type of fluid actuator and scales down the desired flow rate for the second type of fluid actuator. When the desired flow rate just for the first type of fluid actuators alone exceeds the current flow rate of the source, the controller scales down the desired flow rate for all of the fluid actuators. Thus there are three regimes in which the controller of the &#39;459 application operates. 
     The disclosed hydraulic system is directed to overcoming one or more of the problems set forth above. It should be appreciated that the foregoing background discussion is intended solely to aid the reader. It is not intended to limit the disclosure or claims, and thus should not be taken to indicate that any particular element of a prior system is unsuitable for use, nor is it intended to indicate any element, including solving the motivating problem, to be essential in implementing the examples described herein or similar examples. 
     BRIEF SUMMARY 
     The disclosure describes, in one aspect, a method of allocating hydraulic fluid between actuators in a machine. A controller accepts a first command to provide a first requested fluid flow to a first actuator, wherein the first actuator is a steering actuator, and a second command to provide a second requested fluid flow to a second actuator, wherein the second actuator is not a steering actuator. The system adjusts the first and second commands to produce adjusted first and second commands corresponding to adjusted first and second fluid flows, such that the sum of the adjusted first and second fluid flows is less than or equal to a maximum available flow and the adjusted first fluid flow meets or exceeds the lesser of the first requested fluid flow and a threshold curve that is a function of engine speed. 
     Other aspects, features, and embodiments of the described system and method will be apparent from the following discussion, taken in conjunction with the attached drawing FIGS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side-view of an exemplary disclosed machine; 
         FIG. 2  is a schematic top-view of an exemplary disclosed machine; 
         FIG. 3  is a schematic system illustration of an exemplary disclosed hydraulic system for a machine such as illustrated in  FIG. 1 ; 
         FIG. 4  is a schematic diagram illustrating control circuits of a machine such as illustrated in  FIG. 1 ; 
         FIG. 5  is a flow allocation plot illustrating allocation of hydraulic flow between a steering actuator and a tool actuator; and 
         FIG. 6  is a flow chart illustrating an exemplary process usable by a controller for allocating fluid flow between a steering actuator and a tool actuator within a machine such as illustrated in  FIG. 1-2 . 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to a system and method for controlling a flow of hydraulic fluid in a plurality of parallel circuits in a machine. In particular, a controller applies one or more thresholds to control the flow priority among parallel circuits when the flow demanded for all circuits exceeds the available flow, e.g., from a hydraulic pump of the machine. Although the disclosure pertains to machines having more than one pump, the disclosed techniques are particularly advantageous in machines where only a single pump is available. The use of a single pump is often driven by machine size, engine power limitations, or cost requirements, and it is especially important to provide appropriately managed hydraulic fluid flows in such a machine to prevent inadequate machine performance. 
       FIG. 2  illustrates an example machine  70 . The mobile machine  70  is a wheel loader system that includes moveable components  71 , a power source  72  for providing power to move moveable components  71 , and controls  73  for controlling the motion of moveable components  71 . The mobile machine  70  includes a propulsion system  74 . Moveable components  71  include steering devices  75 ,  76  that transmit steering forces to steer mobile machine  70 . The steering devices  75 ,  76  are wheels in the illustrated example, but may additionally or alternatively comprise other types of devices. Moveable components  71  may include components that connect to steering devices  75 ,  76  and allow adjustment of a steering angle θ between steering devices  75  and steering devices  76 . For example, moveable components  71  may include a frame section  77  to which steering devices  75  mount, and a frame section  78  to which steering devices  76  mount. A pivot joint  79  between frame sections  77 ,  78  may allow adjustment of steering angle θ by allowing frame sections  77 ,  78  to pivot relative to one another about an axis  80 . 
     Power source  72  supplies pressurized hydraulic fluid to hydraulic cylinder with housing  81  and drive member  82 . Controls  73  will typically though not invariably include an operator-input device  83 , provisions for gathering information about the motion of moveable components  71  and/or actuator  84 , and provisions for controlling actuator  84 . Actuator  84  may be a linear actuator, a rotary actuator, or a type of actuator that generates motion other than purely rotational or linear motion. 
     Actuator  84  is drivingly connected to moveable components  71 . For example, as  FIG. 2  shows, actuator  84  may be directly drivingly connected to each frame section  77 ,  78  and, through each frame section  77 ,  78 , indirectly drivingly connected to steering devices  75 ,  76 . This allows actuator  84  to drive frame sections  77 ,  78  and steering devices  75 ,  76 . In some embodiments, actuator  84  is connected to frame sections  77 ,  78  in a manner that enables actuator  84  to adjust steering angle  0  by pivoting frame section  77  and steering devices  75  about axis  80  relative to frame section  78  and steering devices  76 . 
     Although the following discussion makes reference primarily to the machine  70  of  FIG. 1 and 2 , it will be appreciated that the same hydraulic and mechanical principles apply equally to other machines. As more generally illustrated in  FIG. 3 , the machine  70  includes a hydraulic system  26  having a plurality of fluid components that cooperate together to move a tool and/or propel machine  70 . Specifically, hydraulic system  26  includes a tank  28  for holding a supply of fluid and a source  30  configured to pressurize the fluid and to direct the pressurized fluid to one or more hydraulic cylinders  32   a - c , to one or more fluid motors  34 , and/or to any other additional fluid actuator known in the art. Hydraulic system  26  also includes a control system  36  in communication with some or all of the components of hydraulic system  26 . Although not shown, it is contemplated that hydraulic system  26  will generally include other components as well such as, for example, accumulators, restrictive orifices, check valves, pressure relief valves, makeup valves, pressure-balancing passageways, and other components known in the art. 
     The fluid in tank  28  comprises, for example, a specialized hydraulic oil, an engine lubrication oil, a transmission lubrication oil, or other suitable fluid known in the art. One or more hydraulic systems within machine  70  draw fluid from and return fluid to tank  28 . In an embodiment, hydraulic system  26  is connected to multiple separate fluid tanks. 
     Source  30 , also referred to herein as a fluid pump, produces a pressurized flow of fluid and may comprise a variable displacement pump, a fixed displacement pump, a variable delivery pump, or other source of pressurized fluid. Source  30  may be connected to power source  18  by, for example, a countershaft  38 , a belt (not shown), an electrical circuit (not shown), or in other suitable manner, or may be indirectly connected to power source  18  via a torque converter, a gear box, or in other appropriate system. As noted above, multiple sources of pressurized fluid may be interconnected to supply pressurized fluid to hydraulic system  26 . 
     In the disclosed technique, it is often useful to be able to measure the flow of fluid provided by source  30 . A flow rate available from source  30  may be determined, e.g., by sensing an angle of a swash plate within source  30 , by observing a command sent to source  30 , or by other suitable means. The flow rate may alternately be determined by a flow sensor such as coriolis sensor or otherwise, configured to determine an actual flow output from source  30 . It is also possible to estimate expected flow based on other inputs and/or parameters. The flow rate available from the source  30  can generally be reduced or increased for various reasons within practical limitations. For example, a source displacement may be lowered to ensure that demanded pump power does not exceed available power from power source  18  at high pump pressures, or to reduced or increase pressures within hydraulic system  26 . 
     Hydraulic cylinders  32   a - c  may for example connect a tool to frame  77  or  78  via a direct pivot, via a linkage system with each of hydraulic cylinders  32   a - c  forming one member in the linkage system (referring to  FIG. 1 ), or in any other appropriate manner. Each of hydraulic cylinders  32   a - c  includes a tube  40  and a piston assembly (not shown) disposed within tube  40 . One of tube  40  and the piston assembly may be pivotally connected to frame  77 ,  78 , while the other of tube  40  and the piston assembly is pivotally connected to a tool. Tube  40  and/or the piston assembly may alternately be fixedly connected to either frame  77 ,  78  or work implement or connected between two or more members of frame  77 ,  78 . For example, actuator  84  is connected between frame members  77 ,  78  to steer the machine  70  when actuated. 
     The piston may include two opposing hydraulic surfaces, one associated with each of the first and second chambers. An imbalance of fluid pressure on the two surfaces causes the piston assembly to axially move within tube  40 . For example, a fluid pressure within the first hydraulic chamber acting on a first hydraulic surface being greater than a fluid pressure within the second hydraulic chamber acting on a second opposing hydraulic surface may cause the piston assembly to displace to increase the effective length of hydraulic cylinders  32   a - c . Similarly, when a fluid pressure acting on the second hydraulic surface is greater than a fluid pressure acting on the first hydraulic surface, the piston assembly may retract within tube  40  to decrease the effective length of hydraulic cylinders  32   a - c . 
     A sealing member (not shown), such as an o-ring, may be connected to the piston to restrict a flow of fluid between an internal wall of tube  40  and an outer cylindrical surface of the piston. The expansion and retraction of hydraulic cylinders  32   a - c  may function to assist in moving a tool. 
     Each of hydraulic cylinders  32   a - c  includes at least one proportional control valve  44  that functions to meter pressurized fluid from source  30  to one of the first and second hydraulic chambers, and at least one drain valve (not shown) that functions to allow fluid from the other of the first and second chambers to drain to tank  28 . In an embodiment, proportional control valve  44  includes a spring biased proportional valve mechanism that is solenoid actuated and configured to move between a first position at which fluid is allowed to flow into one of the first and second chambers and a second position at which fluid flow is blocked from the first and second chambers. The location of the valve mechanism between the first and second positions determines a flow rate of the pressurized fluid directed into the associated first and second chambers. The valve mechanism may be movable between the first and second positions in response to a demanded flow rate that produces a desired movement of tool  14 . The drain valve typically includes a spring biased valve mechanism that is solenoid-actuated and configured to move between a first position at which fluid is allowed to flow from the first and second chambers and a second position at which fluid is blocked from flowing from the first and second chambers. Although the illustrated example employs solenoid valves, the proportional control valve  44  and the drain valve may alternately be hydraulically actuated, mechanically actuated, pneumatically actuated, or actuated in another suitable manner. 
     With respect to driving the machine  70 , motor  34  ( FIG. 3 ) may be a variable displacement motor or a fixed displacement motor and is configured to receive a flow of pressurized fluid from source  30 . The flow of pressurized fluid through motor  34  causes an output shaft  46  connected to a traction device, e.g., wheels  75 ,  76 , to rotate, thereby propelling and/or steering the machine  70 . The motor  34  may alternately be indirectly connected to a traction device via a gearbox or in any other manner known in the art. Motor  34  or other motor may be connected to a different mechanism on machine  70  other than the traction device. For example, motor  34  or other motor may be connected to a rotating work implement, a steering mechanism, or other machine mechanism known in the art. Motor  34  may include a proportional control valve  48  that controls a flow rate of the pressurized fluid supplied to motor  34 . Proportional control valve  48  may include a spring biased proportional valve mechanism that is solenoid actuated and configured to move between a first position at which fluid is allowed to flow through motor  34  and a second position at which fluid flow is blocked from motor  34 . The location of the valve mechanism between the first and second positions determines a flow rate of the pressurized fluid directed through the motor  34 . 
     Control system  36  includes a controller  50  embodied in a single microprocessor or multiple microprocessors and associated standard electronic systems such as buffers, memory, multiplexers, display drivers, power supply circuitry, signal conditioning circuitry, solenoid driver circuitry, etc. for running an application or program, to control the operation of hydraulic system  26 . Numerous commercially available microprocessors can be configured to perform the functions of controller  50 . It will be appreciated that controller  50  may be embodied in a general machine microprocessor capable of controlling numerous machine functions. 
     Controller  50  is configured to receive input from operator interface  16  and to control the flow rate of pressurized fluid to hydraulic cylinders  32   a - c  and motor  34  in response to the input. Specifically, controller  50  is in communication with proportional control valves  44  of hydraulic cylinders  32   a - c  via communication lines  52 ,  54 , and  56  respectively, with proportional control valve  48  of motor  34  via a communication line  58 , with first operator interface device  22  via a communication line  60 , and with second operator interface device  24  via a communication line  62 . In the illustrated embodiment, controller  50  receives proportional signals generated by the first operator interface device  22  and selectively actuates one or more of proportional control valves  44  to selectively fill the first or second actuating chambers associated with hydraulic cylinders  32   a - c  to produce the desired tool movement. Controller  50  also receives the proportional signal generated by the second operator interface device  24  and selectively actuates proportional control valve  48  of motor  34  to produce the desired rotational movement of the traction device(s). 
     Controller  50  is in communication with source  30  via a communication line  64  and is configured to change the operation of the source  30  in response to a demand for pressurized fluid. Specifically, controller  50  may be configured to determine a desired flow rate of pressurized fluid that is required to produce machine movements desired by a machine operator (total desired flow rate) and indicated via first and/or second operator interface devices  22 ,  24 . Controller  50  may be further configured to determine a current flow rate of source  30  and a maximum flow capacity of source  30 . Controller  50  may be configured to increase the current flow rate of source  30  if the total desired flow rate is greater than the current flow rate and the current flow rate is less than the maximum flow capacity of source  30 . 
     In an embodiment, the controller  50  is also configured to selectively reduce the desired flow rate of pressurized fluid to hydraulic cylinders  32   a - c  and/or motor  34  under certain circumstances as will be described in greater detail. In particular, if the total commanded flow rate exceeds the available flow rate, one or more of hydraulic cylinders  32   a - c  and/or motor  34  will not receive an adequate flow of pressurized fluid and the associated movements of machine  70  may be unpredictable. 
     In overview, when controller  50  determines that the total desired flow rate exceeds the available flow rate of source  30 , the demanded flow rate for one or more of hydraulic cylinders  32   a - c  and/or motor  34  is reduced by moving the associated proportional control valves  44 ,  48  towards the second position. This allows a predictable flow of pressurized fluid to be made available to each such entity in response to an input received via operator interface  16 , thereby providing predictable machine and tool movement. 
     From the foregoing, the manner in which the various system hydraulic components interact and are controllable will be appreciated. In the following, the electromechanical systems for controlling flow and movement will not be further detailed or referred to, but it will be appreciated that the steps carried out by the controller  50  are implemented using the systems and interrelationships described above. 
       FIG. 4  is a schematic diagram  100  illustrating the control circuits of the machine  10  at a conceptual level to aid in understanding the present disclosure. The operator controls  101  provide one or more signals  102  to a translation algorithm (translation module)  103  that outputs valve control commands  104  corresponding to the desired machine movements. It will be appreciated that the algorithm  102  operates in conjunction with input from a number of system sensors  105  as described above as well. The valve control commands  104  are processed via a hydraulic priority algorithm (balancing module)  106 , operating in conjunction with data reflecting the available fluid flow from flow estimator  107 , to produce adjusted valve commands  108 . 
     The adjusted valve commands  108  are further refined via a closed loop transformation (closed loop transformation module)  109  based on feedback from the system sensors  105 . This is necessitated because the valve control commands  104  and adjusted valve commands  108  are empirically based, and the actual operating environment and/or condition of the machine  10  may result in inaccuracies in these values. The closed loop transformation  109  outputs refined valve control signals  110 . The refined valve control signals  110  are provided to the appropriate valves  111  to effect movement of the associated actuators  112 , resulting qualitatively in the desired machine movement, although the magnitude and/or speed of the movement may be reduced from that commanded via the operator controls  101 . 
     The thresholds governing hydraulic flow priority are illustrated with respect to demanded flows and available fluid flow in the chart  300  of  FIG. 5 . The chart  300  assumes competition for fluid between two functions, one of which is a steering function. The flow to the steering function is bounded between a maximum allowable flow  301  and a minimum allowable flow  302 . The lower bound  302  on the priority threshold  304  in this embodiment is a minimum acceptable flow for the steering actuators, such as that set by ISO 5010. Thus, the actual flow to the steering actuators will not exceed the maximum acceptable flow, nor will it decrease below the mandated minimum set by ISO 5010. 
     The amount of fluid flow available for distribution is shown as maximum available flow  303  (MAPF). The maximum available flow  303  may be limited by a mechanical stop or by an electronic stop such as a torque limit, power limit, displacement limit, flow limit, and so on. This curve  303  is linear with engine speed in a middle portion but plateaus at higher engine speeds due to a flow limit. In the illustrated example, maximum available flow  303  also drops off at lower engine speeds due to limitations imposed by an electronic controller. 
     A priority threshold  304  sets a minimum level of flow to the steering actuator, such that the flow provided to the steering actuator will always equal or exceed the priority threshold  304 . Although the priority threshold  304  is a function of engine speed in the illustrated example, it may additionally or alternatively be a function of one or more other machine variables or parameters such as machine speed, linkage position, bucket and/or lift arm position, pump speed, pump pressures, etc. Finally, curve  305  illustrates the difference between maximum available flow  303  and a full demanded implement flow to a second actuator, i.e., for-tool movement. 
     In operation, the steering actuator is always guaranteed to receive an amount of flow corresponding to the lesser of the demanded flow and the amount of flow set by the priority threshold  304 . Thus, the chart  300  represents four regions of operation labeled Region  1 , Region  2 , Region  3 , and Region  4  within which fluid flow priority is adjusted differently. In Region  1 , the difference between maximum available flow  303  and the requested flow to the tool actuator falls within this region. In this case, there is no need to prioritize the fluid flows between the steering actuator and tool actuator, and each thus receives its requested flow. 
     In Region  2 , the system may be flow-limited in that the difference between maximum available flow  303  and the requested flow to the tool actuator falls below the maximum flow limit for the steering actuator. Thus, in this region, if the requested flow to the steering actuator exceeds the difference between maximum available flow  303  and the requested flow to the tool actuator, the flow to the steering actuator is reduced to the priority threshold  304 . 
     In Region  3 , the system may again be flow-limited in that the difference between maximum available flow  303  and the requested flow to the tool actuator falls below the maximum flow limit for the steering actuator. However, in this region, if the requested flow to the steering actuator exceeds the difference between maximum available flow  303  and the requested flow to the tool actuator, the flow to the steering actuator is increased to the priority threshold  304 . This increase to the steering actuator flow comes at the expense of the tool actuator, which now receives a flow that is somewhat less than that requested. 
     In Region  4 , the system is not flow-limited in that the difference between maximum available flow  303  and the requested flow to the tool actuator is greater than the flow requested for the steering actuator. In this region, each implement receives its requested flow. 
     In an embodiment, the controller  50  implements the priority system shown in chart  300  to control a steering actuator and at least one tool actuator. The resulting control instructions executed by the controller  50  are illustrated diagrammatically via the flow chart  400  of  FIG. 6 . At an initial state  401 , the controller determines whether the difference between the MAPF and the tool actuator flow request (Uimp_req) is less than 0, i.e. whether there is insufficient flow available to satisfy even the requested flow for the tool actuator. If this condition is met, the process flows to state  402  and the controller  50  sets a preliminary tool actuator flow (Uimp_prelim) equal to the maximum available flow and flows to state  403 . Otherwise, the process flows directly to state  403  and sets the preliminary tool actuator flow (Uimp_prelim) equal to the tool actuator flow request (Uimp_req). 
     At state  404 , the controller  50  determines whether the difference between the MAPF and the preliminary tool actuator flow (Uimp_prelim) is greater than or equal to a steering actuator flow request (Bimp_req). If this condition is met, the process  400  flows to state  405 , sets a flow limit flag (flow_limited_flag) equal to zero, sets an actual tool actuator flow (Uimp_actual) equal to the preliminary tool actuator flow (Uimp_prelim), sets an actual steering actuator flow (Bimp_actual) equal to the requested steering actuator flow (Bimp_req), and flows to state  412 . 
     If at state  404  the condition was not met, then the process  400  sets the flow limit flag (flow_limited_flag) equal to one and flows to state  406 . At state  406 , the controller  50  determines whether the difference between the MAPF and the preliminary tool actuator flow (Uimp_prelim) exceeds a priority threshold (priority_threshold). If this condition is met, the process  400  flows to state  407 . At state  407 , the process  400  sets actual tool actuator flow (Uimp_actual) equal to the preliminary tool actuator flow (Uimp_prelim), actual steering actuator flow (Bimp_actual) equal to the difference between the maximum available flow and the preliminary tool actuator flow (Uimp_prelim), and flows to state  411 . Otherwise, the process flows directly from state  406  to state  408 . 
     At state  408 , the process  400  determines whether the steering actuator flow requested (Bimp_req) is less than the priority threshold (priority_threshold). If this condition is met, the process  400  flows to state  409 . At state  409 , the process  400  sets the actual tool actuator flow (Uimp_actual) equal to the difference between the maximum available flow and the steering actuator flow requested (Bimp_req). In addition, the controller  50  sets the actual steering actuator flow (Bimp_actual) equal to the steering actuator flow requested (Bimp_req). From state  409 , the process  400  flows to state  410 . 
     If the condition at state  408  is not met, the process  400  sets the actual tool actuator flow (Uimp_actual) equal to the difference between the maximum available flow and the priority threshold (priority_threshold), sets the actual steering actuator flow (Bimp_actual) equal to the priority threshold (priority_threshold), and flows to state  410 . 
     Thus, it can be seen that the actual tool actuator flow (Uimp_actual) and actual steering actuator flow (Bimp_actual) will be set to one of four combinations depending upon the maximum available flow, the priority threshold  304 , and the operator-requested flow levels. In the first combination, there is adequate flow to meet all requests and the flow is not deemed to be limited. In the remaining three combinations, the flow is deemed to be limited, and the actual steering actuator flow (Bimp_actual) will be set to the priority threshold  304 , the requested flow, or another value that is a function of the maximum available flow and the tool actuator flow request (Uimp_req). In this manner, the flow provided to the steering actuator is never less than the lesser of the priority threshold and the actual flow requested for that implement. 
     In operation, this results in at least acceptable steering ability for safety and operator experience purposes without causing sluggish operation with respect to other implements while steering, and without causing undesirably slow steering while operating other implements simultaneously. Thus, for example, in the case of a steerable machine having a bucket being used for loading material into a truck or container, the machine may be freely and safely steered while in motion at the same time that the bucket is being raised, lowered, or tilted. 
     INDUSTRIAL APPLICABILITY 
     The industrial applicability of the hydraulic flow control system described herein will be readily appreciated from the foregoing discussion. A technique is described wherein the flow of hydraulic fluid to one or more steering actuators and to one or more non-steering actuators such as for a bucket tilt/lift/lower function are controlled to maintain the flow to the steering actuators within predefined bounds while setting the flow to the non-steering actuators to the remaining available flow or the requested flow for the unbounded flow implement. 
     The disclosed hydraulic system is applicable to any hydraulically actuated machine that includes a plurality of fluidly connected hydraulic actuators where flow sharing is desired to alleviate unpredictable and undesirable movements of the machine. Nonexhaustive examples of machines within which the disclosed principles may be used include landfill compactors, backhoe loaders, wheel loaders, motor graders, wheel dozers, articulated trucks and the like. 
     The disclosed hydraulic system apportions an available flow rate (for example, a maximum available flow) of a source of pressurized fluid among the plurality of fluidly connected hydraulic actuators dynamically according to the requested flow amounts as well as a speed-variable priority threshold  304  for the steering actuator. In this maimer, predictable operation of machine  70  and any implements in use is maintained, while keeping the fluid flow to the steering actuator from exceeding a maximum allowable flow or from falling below a predefined priority threshold curve  304 . 
     During operation of machine  70 , a machine operator manipulates first and/or second operator interface devices  22 ,  24  to create a desired movement of the machine  70 . Throughout this process, first and second operator interface devices  22 ,  24  generate signals indicative of desired flow rates of fluid supplied to hydraulic cylinders  32   a - c  and/or motor  34  to accomplish the desired movements. After receiving these signals, controller  50  executes the process of flow chart  400  in keeping with plot  300  to generate actual flow request commands to move the implements in question. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations may differ in detail from the foregoing examples. All references to specific examples herein are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the claims or disclosure more generally. All language of distinction and disparagement with respect to certain features of the described system or the art is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the claims entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Accordingly, the attached claims encompass all modifications and equivalents as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed unless otherwise indicated herein or otherwise clearly contradicted by context.