Patent Publication Number: US-2015059325-A1

Title: Hybrid Apparatus and Method for Hydraulic Systems

Description:
TECHNICAL FIELD 
     This patent disclosure relates generally to hydraulic systems and, more particularly, to a hybrid closed-loop system for selectively driving two or more hydraulic actuators. 
     BACKGROUND 
     Hydraulic systems are known for converting fluid energy, tier example, fluid pressure, into mechanical power. Fluid power may be transferred from one or more hydraulic pumps through fluid conduits to one or more hydraulic actuators. Hydraulic actuators may include hydraulic motors that convert fluid power into shaft rotational power, hydraulic cylinders that convert fluid power into translational motion, or other hydraulic actuators known in the art. 
     In an open-loop hydraulic system, fluid discharged from an actuator is directed to a low-pressure reservoir, from which the pump draws fluid. In a closed-loop hydraulic system, a pump is coupled to a hydraulic motor through a motor supply conduit and a pump return conduit, such that all of the hydraulic fluid is not returned to a low-pressure reservoir upon each pass through the closed-loop. Instead, fluid discharged from an actuator in a closed-loop system is directed back to the pump for immediate recirculation. 
     A hydraulic actuator may receive fluid power from more than one pump. For example, even in so-called closed-loop systems, fluid may be diverted out of the closed-loop to limit pressure, or be deliberately flushed from the closed-loop circuit to a reservoir, to control a hydraulic fluid property such as temperature, viscosity, cleanliness, or the like. Thus, an actuator in a closed-loop system may receive fluid power from an external boost pump in addition to the closed-loop circuit pump to compensate for fluid diverted out of the closed-loop. 
     Conversely, a pump may supply fluid power to more than one actuator throughout a duty cycle of a machine. For example, U.S. Pat. No. 8,191,290 (hereinafter “the &#39;290 patent), entitled “Displacement-Controlled Hydraulic System for Multi-Function Machines,” purports to describe a hydraulic system capable of switching outputs of individual pumps between actuators to sequentially control multiple different machine functions of a multi-function machine. In turn, the &#39;290 patent touts a machine using a number of pumps less than the number of multiple functions of the machine. 
     According to the &#39;290 patent, valves enable switching of one pump between control of a swing motor and control of a blade actuator, and switching of another pump between a bucket control function and an actuator that controls an offset function of an articulated arm. However, as a result, the swing function and the blade function described in the &#39;290 patent may not be performed simultaneously, and the bucket control function and the articulated arm offset function described in the &#39;290 patent may not be performed simultaneously, thereby posing limited operability of the multiple functions. 
     Accordingly, there is a need for an improved hydraulic system to address the problems described above and/or problems posed by other conventional approaches. 
     SUMMARY 
     In one aspect, the disclosure describes a hydraulic system. The hydraulic system includes a first actuator fluidly coupled to a first rotating group in a first closed-loop circuit, a flow control module fluidly coupled to the first closed-loop circuit via a first conduit, a second actuator fluidly coupled to the flow control module via a second conduit, a second rotating group in selective fluid communication with the first conduit and the second conduit via the flow control module, and a controller operatively coupled to the flow control module. The controller is configured to operate the flow control module in a first mode, such that the flow control module effects fluid communication between the second rotating group and the first closed-loop circuit via the first conduit, and blocks fluid communication between the second rotating group and the second actuator via the second conduit, and operate the flow control module in a second mode, such that the flow control module blocks fluid communication between the second rotating group and the first closed-loop circuit via the first conduit, and effects fluid communication between the second rotating group and the second actuator via the second conduit. 
     In another aspect, the disclosure describes a machine including a hydraulic system. The hydraulic system includes a first actuator fluidly coupled to a first rotating group in a first closed-loop circuit, a flow control module fluidly coupled to the first closed-loop circuit via a first conduit, a second actuator fluidly coupled to the flow control module via a second conduit, a second rotating group in selective fluid communication with the first conduit and the second conduit via the flow control module, and a controller operatively coupled to the flow control module. The controller is configured to operate the flow control module in a first mode, such that the flow control module effects fluid communication between the second rotating group and the first closed-loop circuit via the first conduit, and blocks fluid communication between the second rotating group and the second actuator via the second conduit, and operate the flow control module in a second mode, such that the flow control module blocks fluid communication between the second rotating group and the first closed-loop circuit via the first conduit, and effects fluid communication between the second rotating group and the second actuator via the second conduit. 
     In yet another aspect, the disclosure describes a method of controlling a hydraulic system. The hydraulic system includes a first actuator fluidly coupled to a first rotating group in a first closed-loop circuit, a flow control module fluidly coupled to the first closed-loop circuit via a first conduit, a second actuator fluidly coupled to the flow control module via a second conduit, and a second rotating group in selective fluid communication with the first conduit and the second conduit via the flow control module. The method includes operating the flow control module in a first mode and operating the flow control module in a second mode. Operating the flow control module in the first mode includes effecting fluid communication between the second rotating group and the first closed-loop circuit via the first conduit, and blocking fluid communication between the second rotating group and the second actuator via the second conduit. Operating the flow control module in the second mode includes blocking fluid communication between the second rotating group and the first closed-loop circuit via the first conduit, and effecting fluid communication between the second rotating group and the second actuator via the second conduit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exemplary machine, according to an aspect of the disclosure. 
         FIG. 2  shows a schematic view of a linear hydraulic cylinder, according to an aspect of the disclosure. 
         FIG. 3  shows a schematic view of a hydraulic system, according to an aspect of the disclosure. 
         FIG. 4  shows a schematic view of a hydraulic system, according to an aspect of the disclosure. 
         FIG. 5  shows a schematic view of a hydraulic system, according to an aspect of the disclosure. 
         FIG. 6  shows a schematic view of a hydraulic system, according to an aspect of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary machine  10  having various systems and components that cooperate to accomplish a task. The machine  10  may embody a fixed or mobile machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, or another industry known in the art. For example, the machine  10  may be an earth moving machine such as an excavator (shown in  FIG. 1 ), a dozer, a loader, a backhoe, a motor grader, a dump truck, or another earth moving machine. The machine  10  may include an implement system  12  configured to move a work tool  14 , a drive system  16  for propelling the machine  10 , a power source  18  or other prime mover that provides power to the implement system  12  and the drive system  16 , and an operator station  20  that may include control interfaces for manual control of the implement system  12 , the drive system  16 , and/or the power source  18 . 
     The implement system  12  may include a linkage structure coupled to hydraulic actuators, which may include linear or rotary actuators, to move the work tool  14 . For example, the implement system  12  may include a boom  22  that is pivotally coupled to a body  23  of the machine  10  about a first horizontal axis (not shown), with respect to the work surface  24 , and actuated by one or more double-acting, boom hydraulic cylinders  26  (only one shown in  FIG. 1 ). The implement system  12  may also include a stick  28  that is pivotally coupled to the boom  22  about a second horizontal axis  30 , with respect to the work surface  24 , and actuated by a double-acting, stick hydraulic cylinder  32 . 
     The implement system  12  may further include a double-acting, tool hydraulic cylinder  34  that is operatively coupled between the stick  28  and the work tool  14  to pivot the work tool  14  about a third horizontal axis  36 . In the non-limiting aspect illustrated in  FIG. 1 , a head-end  38  of the tool hydraulic cylinder  34  is connected to a portion of the stick  28 , and an opposing rod-end  40  of the tool hydraulic cylinder  34  is connected to the work tool  14  by way of a power link  42 . The body  23  may be connected to an undercarriage  44  to swing about a vertical axis  46  by a hydraulic swing motor  48 . 
     Numerous different work tools  14  may be attached to a single machine  10  and controlled by an operator. The work tool  14  may include any device used to perform a particular task such as, for example, a bucket (shown in  FIG. 1 ), a fork arrangement, a blade, a shovel, a ripper, a dump bed, a broom, a snow blower, a propelling device, a cutting device, a grasping device, or any other task-performing device known in the art. Although the aspect illustrated in  FIG. 1  shows the work tool  14  configured to pivot in the vertical direction relative to the body  23  and to swing in the horizontal direction about the pivot axis  46 , it will be appreciated that the work tool  14  may alternatively or additionally rotate relative to the stick  28 , slide, open and close, or move in any other manner known in the art. 
     The drive system  16  may include one or more traction devices powered to propel the machine  10 . As illustrated in  FIG. 1 , the drive system  16  may include a left track  50  located on one side of the machine  10 , and a right track  52  located on an opposing side of the machine  10 . The left track  50  may be driven by a left travel motor  54 , and the right track  52  may be driven by a right travel motor  56 . It is contemplated that the drive system  16  could alternatively include traction devices other than tracks, such as wheels, belts, or other known traction devices. The machine  10  may be steered by generating a speed and/or rotational direction difference between the left travel motor  54  and the right travel motor  56 , while straight travel may be effected by generating substantially equal output speeds and rotational directions of the left travel motor  54  and the right travel motor  56 . 
     The power source  18  may include a combustion engine such as, for example, a reciprocating compression ignition engine, a reciprocating spark ignition engine, a combustion turbine, or another type of combustion engine known in the art. It is contemplated that the power source  18  may alternatively include a non-combustion source of power such as a fuel cell, a power storage device, or another power source known in the art. The power source  18  may produce a mechanical or electrical power output that may then be converted to hydraulic power for moving the linear or rotary actuators of the implement system  12 . 
     The operator station  20  may include devices that receive input from an operator indicative of desired maneuvering. Specifically, the operator station  20  may include one or more operator interface devices  58 , for example a joystick (shown in  FIG. 1 ), a steering wheel, or a pedal, that are located near an operator seat (not shown). Operator interface devices may initiate movement of the machine  10 , for example travel and/or tool movement, by producing displacement signals that are indicative of desired machine  10  maneuvering. As an operator moves interface device  58 , the operator may affect a corresponding machine  10  movement in a desired direction, with a desired speed, and/or with a desired force. 
       FIG. 2  shows a schematic view of a linear hydraulic cylinder  70 , according to an aspect of the disclosure. The linear hydraulic cylinder  70  may include a tube  72  defining a cylinder bore  74  therein, and a piston assembly  76  disposed within the cylinder bore  74 . A rod  78  is coupled to the piston assembly  76  and extends through the tube  72  at a seal  80 . A rod-end chamber  82  is defined by a first face  84  of the piston, the cylinder bore  74 , and a surface  86  of the rod  78 . A head-end chamber  88  is defined by a second face  90  of the piston and the cylinder bore  74 . 
     The head-end chamber  88  and the rod-end chamber  82  of the linear hydraulic actuator  70  may be selectively supplied with pressurized fluid or drained of fluid via the head-end port  92  and the rod-end port  94 , respectively, to cause piston assembly  76  to translate within tube  72 , thereby changing the effective length of the actuator to move work tool  14 , for example. A flow rate of fluid into and out of the head-end chamber  88  and the rod-end chamber  82  may relate to a translational velocity of the actuator, while a pressure differential between the head-end chamber  88  and the rod-end chamber  82  may relate to a force imparted by the actuator on work tool  14 . It will be appreciated that any of the boom hydraulic cylinders  26 , the stick hydraulic cylinder  32 , or the tool hydraulic cylinder  34 , shown in  FIG. 1 , may embody structural features of the linear hydraulic actuator  70  illustrated in  FIG. 2 . 
     A hydraulic area of the second face  90  of the piston may be greater than a hydraulic area of the first face  84  of the piston, at least because the rod  78  blocks fluid from acting on a portion of the first face  84 . According to an aspect of the disclosure, a hydraulic area of the second face  90  is substantially equal to a hydraulic area of the first face  84  plus a radial cross sectional area of the rod  78 . Thus, a change in head-end chamber  88  fluid volume for a given translation of the piston assembly  76  may be substantially equal to the change in rod-end chamber  82  fluid volume plus the corresponding volume of the rod  78  displaced by the translation of the piston  76 . 
     Accordingly, it will be appreciated that a volume of fluid displaced out of the rod-end port  94  to increase an effective length of the linear hydraulic actuator  70  may be smaller than a corresponding volume of fluid added to the head-end port  92  to maintain the head-end chamber  88  full of fluid. Conversely, it will be appreciated that a volume of fluid displaced out of the head-end port  92  to decrease an effective length of the linear hydraulic actuator  70  may be larger than a corresponding volume of fluid delivered through the rod-end port  94 . This difference between rod-end chamber  82  fluid displacement and head-end chamber  88  fluid displacement may be referred to herein as the “head-end disparity” of a hydraulic cylinder. 
     A rotary actuator may include first and second chambers located to either side of a fluid work-extracting mechanism such as an impeller, plunger, or series of pistons. When the first chamber is filled with pressurized fluid and the second chamber is simultaneously drained of fluid, the fluid work-extracting mechanism may be urged to rotate in a first direction by a pressure differential across the pumping mechanism. Conversely, when the first chamber is drained of fluid and the second chamber is simultaneously filled with pressurized fluid, the fluid work-extracting mechanism may be urged to rotate in an opposite direction by the pressure differential. The flow rate of fluid into and out of the first and second chambers may determine a rotational velocity of the actuator, while a magnitude of the pressure differential across the pumping mechanism may determine an output torque. It will be appreciated that any of the hydraulic swing motor  48 , the left travel motor  54 , or the right travel motor  56 , illustrated in  FIG. 1 , may embody the rotary actuator structure described above. Further, it will be appreciated that rotary actuators may have a fixed displacement or a variable displacement, as desired. 
       FIG. 3  shows a hydraulic system  100 , according to an aspect of the disclosure. The hydraulic system  100  includes a first actuator  102  and a second actuator  104 . The first actuator  102  may embody the structure of the linear hydraulic actuator  70  illustrated in  FIG. 2 . Thus, the first actuator  102  may have a head-end chamber  88 , a rod-end chamber  82 , a head-end port  92 , and a rod-end port  94 . It will be appreciated that the first actuator  102  may be a boom hydraulic cylinder  26 , a stick hydraulic cylinder  32 , or a tool hydraulic cylinder  34  of the machine  10 , as shown in  FIG. 1 , or serve any other hydraulic cylinder function known in the art. 
     The second actuator  104  may be a rotary actuator, as described previously. Thus, the second actuator  104  may be the hydraulic swing motor  48 , the left travel motor  54 , or the right travel motor  56  of the machine  10 , as illustrated in  FIG. 1 , or serve any other hydraulic motor function known in the art. According to an aspect of the disclosure, the second actuator  104  is the left travel motor  54  of the machine  10 . According to another aspect of the disclosure, the first actuator  102  is a boom hydraulic cylinder  26  of the machine  10 . 
     The first actuator  102  is fluidly coupled to a first rotating group  106  in a first closed-loop circuit  108 . The first rotating group  106  may act as a pump to convert input shaft power into fluid power within the first closed-loop circuit  108 , or the first rotating group  106  may act as a motor to convert fluid power within the first closed-loop circuit  108  into output shaft power. Further, the first rotating group  106  may be coupled to the power source  18  of the machine  10  directly or indirectly through a shaft  110 . Indirect coupling between the shaft  110  of the first rotating group  106  and the power source  18  may include a torque converter, a gear box, an electrical circuit, or other coupling method known in the art. Thus, the first rotating group  106  may either accept shaft power from the power source  18  of the machine  10 , or may deliver shaft power to the power source  18  of the machine  10  through the shaft  110 . 
     The first rotating group  106  may have variable displacement, which is controlled via controller  112  to draw fluid from its associated actuators and discharge the fluid at a specified elevated pressure back to the actuators in two different directions (i.e., the first rotating group  106  may be an over-center pump). The first rotating group  106  may include a stroke-adjusting mechanism, for example a swashplate, a position of which is hydro-mechanically adjusted based on, among other things, a desired speed of the actuators, to thereby vary an output (e.g., a discharge rate) of the first rotating group  106 . It is contemplated that first rotating group  106  may be coupled to the power source  18  in tandem (e.g., via the same shaft) or in parallel (e.g., via a gear train) with other pumps (not shown) of the machine  10 , as desired. 
     Further, the displacement of the first rotating group  106  may be adjusted from a zero displacement position at which substantially no fluid is discharged from first rotating group  106 , to a maximum displacement position in a first direction at which fluid is discharged from first rotating group  106  at a maximum rate into the conduit  114  of the first closed-loop circuit  108 . Likewise, the displacement of first rotating group  106  may be adjusted from the zero displacement position to a maximum displacement position in a second direction at which fluid is discharged from first rotating group  106  at a maximum rate into the conduit  116  of the first closed-loop circuit  108 . 
     The first rotating group  106  may also operate selectively as a motor. More specifically, when an associated actuator is operating in an overrunning condition (i.e., a condition where the actuator fluid discharge pressure is greater than the actuator fluid inlet pressure), the fluid discharged from the actuator may have a pressure elevated above an output pressure of the first rotating group  106 . In this situation, the elevated pressure of the actuator fluid directed back through the first rotating group  106  may act to drive the first rotating group  106  to rotate without assistance from the power source  18 . Under some circumstances, the first rotating group  106  may even be capable of imparting energy to the power source  18 , thereby improving an efficiency and/or capacity of the power source  18 . 
     It will be appreciated by those of skill in the art that the respective rates of fluid flow into and out of the first actuator  102  (if embodied as a linear actuator) during extension and retraction may not be equal. As discussed previously with respect to  FIG. 2 , more fluid may be forced out of the head-end chamber  88  than may be received by the rod-end chamber  82  during retraction of the first actuator  102 , and conversely, during extension of the first actuator  102 , more hydraulic fluid may be consumed by the head-end chamber  88  than is discharged from the rod-end chamber  82 . Thus, in order to accommodate the excess fluid discharged during retraction, and the additional fluid required during extension, the first closed-loop circuit  108  may include a makeup circuit  118  in fluid communication with a boost system  120  through boost conduit  122 , and in fluid communication with the first closed-loop circuit  108  at nodes  124  and  126 , for example. 
     The makeup circuit  118  may be configured to deliver hydraulic fluid from the boost conduit  122  into the first closed-loop circuit  108  when a pressure in the first closed-loop circuit is less than a first threshold pressure, and may be configured to discharge fluid from the first closed-loop circuit  108  into the boost conduit  122  when a pressure in the first closed-loop circuit  108  is greater than a second threshold pressure. It will be appreciated that the first threshold pressure, the second threshold pressure, or both may be related to a pressure in the boost conduit  122 . 
     The boost system  120  includes a boost pump  128 , which draws fluid from a hydraulic reservoir  130  and discharges the fluid into the boost conduit  122 . The boost pump  128  may be driven directly or indirectly by the power source  18  of the machine  10 , or another power source. The boost system may further include a relief valve  132  that drains fluid from the boost conduit  122  when a pressure in the boost conduit  122  exceeds a third threshold value. The relief valve  132  may discharge fluid drained from the boost conduit  122  to the hydraulic reservoir  130  or any other point in the hydraulic system  100  with sufficiently low pressure. 
     The boost system  120  may also include an accumulator  134  in fluid communication with the boost conduit  122 . The accumulator  134  may store hydraulic energy as a displacement of a resilient member included therein. The resilient member of the accumulator  134  may be a volume of a gas, a resilient bladder, a coil spring, a leaf spring, combinations thereof, or other resilient member known to persons having skill in the art. It will be appreciated that a fluid capacitance of the accumulator  134  may act to filter pressure oscillations in the boost conduit  122 , and a fluid resistance imposed on hydraulic fluid entering and exiting the accumulator  134  may act to damp pressure oscillations in the boost conduit  122 . 
     Thus, the boost pump  128 , the accumulator  134 , or combinations thereof may deliver fluid into the first closed-loop circuit  108  via the makeup circuit  118 . Alternatively, the relief valve  132 , the accumulator  134 , or combinations thereof may receive fluid discharged from the first closed-loop circuit  108  via the makeup circuit  118 . 
     The hydraulic system  100  includes a second rotating group  136  that is fluidly coupled to a flow control module  138  via conduits  140 ,  142  extending to ports  144  and  146 , respectively. Further, the second rotating group  136  is operatively coupled to a source of shaft power, such as, for example, the power source  18  of the machine  10 , or another power source. Similar to the first rotating group  106 , the second rotating group  136  may function as a pump or a motor, may have a variable displacement controlled by the controller  112 , and may embody the operational characteristics of an over-center pump, as discussed previously. 
     As a pump, the second rotating group  136  may impart fluid energy across port  144  and port  146 , and may delivery fluid power to the flow control module  138  in either of two flow directions, namely toward port  144  or toward port  146 . As a motor, the second rotating group  136  may convert fluid energy across port  144  and port  146  into torque, and transmit shaft power out of the second rotating group  136  in either a first direction or a second direction. 
     The flow control module  138  may selectively effect various states of fluid communication between the components of the hydraulic system  100 . In a first mode of operation, the flow control module  138  effects fluid communication between the second rotating group  136  and the first closed-loop circuit  108  via a conduit  148  connected to the port  150  of the flow control module  138 . Thus, when the flow control module  138  is operated in the first mode, the second rotating group  136  may act as a pump to deliver fluid power to the first closed-loop circuit  108  via conduit  142 , or the second rotating group  136  may act as a motor to convert fluid power from the first closed-loop circuit  108  into shaft power. 
     The first mode of the flow control module  138  may block fluid communication between the second rotating group  136  and the second actuator  104 , which is fluidly coupled to the flow control module  138  at port  152  via conduit  154 , and at port  156  via conduit  158 . According to an aspect of the disclosure, the first mode of operation of the flow control module  138  blocks all fluid communication between either port  152  or port  156  and any other port of the flow control module  138 . 
     Alternatively, a second operating mode of the flow control module  138  may effect fluid communication between the second rotating group  136  and the second actuator  104 , and may block fluid communication between the second rotating group  136  and the first closed-loop circuit  108  via the flow control module  138 . Thus, in the second operating mode, the second rotating group  136  may deliver fluid power to the second actuator  104 , or convert fluid power received from the second actuator  104  into shaft power. 
     In the second operating mode, the second rotating group  136  may operate in either an open-loop circuit or a closed-loop circuit. In an open-loop configuration, the flow control module  138  may effect fluid communication between port  144  and the hydraulic fluid reservoir  130  via port  164  and conduit  166 , and effect fluid communication between port  146  and either port  152  or port  156 , depending on the direction the second actuator  104  is to be driven. In turn, whichever of port  152  and port  156  is not coupled to port  146  is placed in fluid communication with a return conduit  168  to the hydraulic reservoir via port  170 , according to the second mode. 
     In a closed-loop configuration of the second operating mode for the second rotating group  136 , the flow control module couples port  146  to port  156 , and couples port  152  to port  144 . Then, the direction of motion of the second actuator  104  is determined by the direction of fluid flow through the second rotating group  136 . In its closed-loop configuration, one or both of port  144  and port  146  may be in fluid communication with the boost system  120  via port  172  and conduit  174  to at least provide makeup flow to the closed-loop including the second rotating group  136 . 
       FIG. 4  shows a hydraulic system  200 , according to an aspect of the disclosure. Similar to the hydraulic system  100  shown in  FIG. 3 , the hydraulic system  200  includes a first rotating group  106  fluidly coupled to a first actuator  102  via a first closed-loop circuit  108 , a second actuator  104 , a second rotating group  136 , and a boost system  120 . The hydraulic system  200  further includes a flow control module  202  fluidly coupled to the first closed-loop circuit  108  via the conduit  148 , fluidly coupled to the second actuator  104  via the conduits  154  and  158 , and fluidly coupled to the second rotating group  136  via the conduits  140  and  142 . The flow control module  202  may operate in first mode or a second mode which effect the states of fluid communication between ports  144 ,  146 ,  150 ,  152 ,  156 ,  164 ,  170 , and  172  as described above with respect to the hydraulic system  100 , shown in  FIG. 3 . Further, the controller  112  may cause the flow control module  202  to switch between operational modes according to a control signal transmitted from the controller  112  to the flow control module  202 . 
     In addition, the hydraulic system  200  further includes a third rotating group  204  fluidly coupled to a third actuator  206  via a second closed-loop circuit  208 , a fourth actuator  210 , and a fourth rotating group  212 . The third actuator  206  may embody structural features of the linear hydraulic actuator  70  illustrated in  FIG. 2 . Thus, the third actuator  206  may have a head-end chamber  88 , a rod-end chamber  82 , a head-end port  92 , and a rod-end port  94 . It will be cylinder  32 , or a tool hydraulic cylinder  34  of the machine  10 , as shown in  FIG. 1 , or serve any other hydraulic cylinder function known in the art. 
     The fourth actuator  210  may be a rotary actuator, as described previously. Thus, the fourth actuator  210  may be the hydraulic swing motor  48 , the left travel motor  54 , or the right travel motor  56  of the machine  10 , as illustrated in  FIG. 1 , or serve any other hydraulic motor function known in the art. According to an aspect of the disclosure, the fourth actuator  210  is right travel motor  56  of the machine  10  (see  FIG. 1 ). According to another aspect of the disclosure, the third actuator  206  is the stick hydraulic cylinder  32  of the machine  10  (see  FIG. 1 ). 
     The third rotating group  204  may act as a pump to convert input shaft power into fluid power within the second closed-loop circuit  208 , or the third rotating group  204  may act as a motor to convert fluid power within the second closed-loop circuit  208  into output shaft power. Further, the third rotating group  204  may be coupled to the power source  18  of the machine  10  directly or indirectly through a shaft  214 . Indirect coupling between the shaft  214  of the third rotating group  204  and the power source  18  may include a torque converter, a gear box, an electrical circuit, or other coupling method known in the art. Thus, the third rotating group  204  may either accept shaft power from the power source  18  of the machine  10 , or may deliver shaft power to the power source  18  of the machine  10  through the shaft  214 . Similar to the first rotating group  106 , the third rotating group  204  may have a variable displacement and may have operational attributes of an over-center pump. 
     The second closed-loop circuit  208  may include a makeup circuit  216  having operation similar to or different from the makeup circuit  118 . The makeup circuit  216  may be in fluid communication with the boost system  120  through the boost conduit  122 , and may be in fluid communication with the second closed-loop circuit  208  at nodes  218  and  220 , for example. 
     The makeup circuit  216  may be configured to deliver hydraulic fluid from the boost conduit  122  into the second closed-loop circuit  208  when a pressure in the second closed-loop circuit  208  is less than a fourth threshold pressure, and may be configured to discharge fluid from the second closed-loop circuit  208  into the boost conduit  122  when a pressure in the second closed-loop circuit  208  is greater than a fifth threshold pressure. It will be appreciated that the fourth threshold pressure, the fifth threshold pressure, or both may be relate to a pressure in the boost conduit  122 . 
     The boost pump  128 , the accumulator  134 , or combinations thereof may deliver fluid into the second closed-loop circuit  208  via the makeup circuit  216 . Alternatively, the relief valve  132 , the accumulator  134 , or combinations thereof may receive fluid discharged from the second closed-loop circuit  208  via the makeup circuit  216 . 
     The fourth rotating group  212  is fluidly coupled to the flow control module  202  via conduits  222 ,  224  extending to port  226  and port  228 , respectively. Further, the fourth rotating group  212  is operatively coupled directly or indirectly to a source of shaft power, such as, for example, the power source  18  of the machine  10 , or another power source. Similar to the first rotating group  106 , the fourth rotating group  212  may function as a pump or a motor, may have a variable displacement, and may embody operational characteristics of an over-center pump, as discussed previously. 
     As a pump, the fourth rotating group  212  may impart fluid energy across port  226  and port  228 , and may delivery fluid power to the flow control module  202  in either of two flow directions, namely toward port  226  or toward port  228 . As a motor, the fourth rotating group  212  may convert fluid potential energy across port  226  and port  228  into torque, and may transmit shaft power out of the fourth rotating group  212  in either a first direction or a second direction. 
     Similar to the flow control module  138  of the hydraulic system  100  (see  FIG. 3 ), the flow control module  202  may selectively effect various states of fluid communication between the components of the hydraulic system  200 . In the first mode of operation, the flow control module  202  effects fluid communication between the fourth rotating group  212  and the second closed-loop circuit  208  via the conduit  230  connected to the port  232  of the flow control module  202 . Thus, when the flow control module  202  is operated in the first mode, the fourth rotating group  212  may act as a pump to deliver fluid power to the second closed-loop circuit  208  via conduit  230 , or the fourth rotating group  212  may act as a motor to convert fluid power from the second closed-loop circuit  208  into shaft power. 
     The first mode of the flow control module  202  may block fluid communication between the fourth rotating group  212  and the fourth actuator  210 , which is fluidly coupled to the flow control module  202  at port  232  via conduit  234 , and at port  236  via conduit  238 . According to an aspect of the disclosure, the first mode of operation of the flow control module  202  blocks all fluid communication between either port  232  or port  236  and any other port of the flow control module  202 . 
     Alternatively, a second operating mode of the flow control module  202  may effect fluid communication between the fourth rotating group  212  and the fourth actuator  210 , and may block fluid communication between the fourth rotating group  212  and the second closed-loop circuit  208  via the flow control module  202 . Thus, in the second operating mode, the fourth rotating group  212  may deliver fluid power to the fourth actuator  210 , or convert fluid power received from the fourth actuator  210  into shaft power. 
     In the second operating mode, the fourth rotating group  212  may operate in either an open-loop circuit or a closed-loop circuit. In an open-loop configuration, the flow control module  202  may effect fluid communication between port  226  and the hydraulic fluid reservoir  130  via port  164  and conduit  166 , and effect fluid communication between port  228  and either port  233  or port  236 , depending on the direction the fourth actuator  210  is to be driven. In turn, whichever of port  233  and port  236  is not coupled to port  228  is placed in fluid communication with a return conduit  168  to the hydraulic reservoir via port  170 , according to the second mode. 
     In a closed-loop configuration of the second operating mode for the fourth rotating group  212 , the flow control module  202  couples port  228  to port  236 , and couples port  226  to port  233 . Then, the direction of motion of the fourth actuator  210  is determined by the direction of fluid flow through the fourth rotating group  212 . In its closed-loop configuration, one or both of port  226  and port  228  may be in fluid communication with the boost system  120  via port  172  and conduit  174  to at least provide makeup flow to the closed-loop including the fourth rotating group  212 . 
       FIG. 5  shows a hydraulic system  300  according to an aspect of the disclosure. Similar to hydraulic system  200  in  FIG. 4 , hydraulic system  300  has a first rotating group  106 , a first actuator  102 , a second rotating group  136 , a second actuator  104 , a third rotating group  204 , a third actuator  206 , a fourth rotating group  212 , a fourth actuator  210 , and a boost system  120 . The hydraulic system  300  also includes a flow control module  302  having a travel divert valve  304 , a first travel direction valve  306 , and a second travel direction valve  308 . 
     The travel divert valve  304  may have six ports  310 ,  312 ,  314 ,  316 ,  318 , and  320 . The port  310  is fluidly coupled to the fourth rotating group  212  via conduit  224 , and the port  312  is fluidly coupled to the second rotating group  136  via conduit  142 . When the travel divert valve  304  is configured in a first position, the port  310  is fluidly coupled to the port  316  via valve passage  320 , and the port  312  is fluidly coupled to the port  320  via valve passage  322 . When the travel divert valve  304  is configured in a second position, the port  310  is fluidly coupled to the port  314  via valve passage  324 , and the port  312  is fluidly coupled to the port  318  via valve passage  326 . 
     The port  316  of the travel divert valve  304  is fluidly coupled to the second closed-loop circuit  208  via the conduit  230 , and the port  320  of the travel divert valve  304  is fluidly coupled the first closed-loop circuit  108  via the conduit  148 . Thus, when the travel divert valve  304  is in its first position, the second rotating group  136  is in fluid communication with the first closed-loop circuit  108  via conduit  148 , and the fourth rotating group  212  is in fluid communication with the second closed-loop circuit  208  via conduit  230 . 
     The travel divert valve  304  may include a resilient member  328  that biases the travel divert valve  304  toward its first position. The travel divert valve  304  may further include an actuator  330  that may act to urge the travel divert valve  304  toward its second position. The actuator  330  may be operatively coupled to the controller  112  such that a control signal from the controller  112  to the travel divert valve  304  may position the travel divert valve  304  proportionally between its first position and its second position. Alternatively, the actuator  330  may toggle the travel divert valve  304  between its first position and its second position in response to a control signal from the controller  112 . The actuator  330  may be a hydraulic actuator, a pneumatic actuator, a solenoid actuator, or any other actuator known to those having skill in the art. 
     According to an aspect of the disclosure, the first position of the travel divert valve  304  corresponds to a first operational mode of the flow control module  302 . According to another aspect of the disclosure, the second position of the travel divert valve  304  corresponds a second operational mode of the flow control module  302 . 
     The first travel direction valve  306  has four ports  332 ,  334 ,  336 , and  338 . The port  332  of the first travel direction valve  306  is fluidly coupled to the port  318  of the travel divert valve  304  via conduit  340 , and the port  334  of the first travel direction valve  306  is fluidly coupled to the reservoir  130  via conduit  168 . The ports  336  and  338  of the first travel direction valve  306  are fluidly coupled to the second actuator  104  via the conduits  154  and  158 , respectively. 
     When the first travel direction valve  306  is in a first position, the port  334  is in fluid communication with both of the ports  336  and  338  via a valve passage  342 , and the port  332  is blocked from fluid communication with another port of the first travel direction valve  306  through the first travel direction valve  306 . Thus, when the first travel direction valve  306  is in the first position a fluid energy potential across the second actuator  104  is substantially zero. Therefore, the second actuator  104  may not move when the first travel direction valve  306  is configured in the first position. 
     When the first travel direction valve  306  is in a second position, the port  332  is in fluid communication with the port  336  via the valve passage  343 , and the port  334  is in fluid. communication with the port  338  via the valve passage  344 . When the first travel direction valve  306  is in a third position, the port  332  is in fluid communication with the port  338  via the valve passage  346 , and the port  334  is in fluid communication with the port  336  via valve passage  348 . Therefore, it will be appreciated that when the travel divert valve  304  is configured in its second position and the first travel direction valve  306  is configured in its second position, the second actuator  104  may be operated in a first direction. Further, it will be appreciated that when the travel divert valve  304  is configured in its second position and the first travel direction valve  306  is configured in its third position, the second actuator  104  may be operated in a second direction. 
     The first travel direction valve  306  may include one or more resilient members  370 , which bias the first travel direction valve  306  toward its first position. The first travel direction valve  306  may also include an actuator  372 , which is configured to urge the first travel direction valve  306  selectively toward either its second position or its third position. The actuator  372  may be a hydraulic actuator, a pneumatic actuator, a solenoid actuator, or another actuator known to persons having skill in the art. Further, the actuator  372  may be operatively coupled to the controller  112  such that a control signal from the controller  112  to the first travel direction valve  306  may toggle the position of the first travel direction valve  306  between its first position, its second position, and its third position. 
     In hydraulic system  300 , the second actuator  104  is operated in an open-loop mode such that the fluid energy potential across the second actuator  104 , to drive motion thereof, is substantially the difference in fluid pressure between conduit  142  and a pressure of the hydraulic fluid reservoir  130 , assuming negligible pressure losses between the second rotating group  136  and the second actuator  104 . 
     The second travel direction valve  308  has four ports  350 ,  352 ,  354 , and  356 . The port  352  of the second travel direction valve  308  is fluidly coupled to the port  314  of the travel divert valve  304  via conduit  358 , and the port  350  of the second travel direction valve  308  is fluidly coupled to the reservoir  130  via conduit  168 . The ports  354  and  356  of the second travel direction valve  308  are fluidly coupled to the fourth actuator  210  via the conduits  234  and  238 , respectively. 
     When the second travel direction valve  308  is in a first position, the port  350  is in fluid communication with both of the ports  354  and  356  via a valve passage  360 , and the port  352  is blocked from fluid communication with another port of the second travel direction valve  308  through the second travel direction valve  308 . Thus, when the second travel direction valve  308  is in the first position, a fluid energy potential across the fourth actuator  210  is substantially zero. Therefore, the fourth actuator  210  may not move when the second travel direction valve  308  is configured in the first position. 
     When the second travel direction valve  308  is in a second position, the port  352  is in fluid communication with the port  356  via the valve passage  362 , and the port  350  is in fluid communication with the port  354  via the valve passage  364 . When the second travel direction valve  308  is in a third position, the port  352  is in fluid communication with the port  354  via the valve passage  366 , and the port  350  is in fluid communication with the port  356  via valve passage  368 . Therefore, it will be appreciated that when the travel divert valve  304  is configured in its second position and the second travel direction valve  308  is configured in its second position, the fourth actuator  210  may be operated in a first direction. Further, it will be appreciated that when the travel divert valve  304  is configured in its second position and the second travel direction valve  308  is configured in its third position, the fourth actuator  210  may be operated in a second direction. 
     The second travel direction valve  308  may include one or more resilient members  374 , which bias the second travel direction valve  308  toward its first position. The second travel direction valve  308  may also include an actuator  376 , which is configured to urge the second travel direction valve  308  toward either its second position or its third position. The actuator  376  may be a hydraulic actuator, a pneumatic actuator, a solenoid actuator, or another actuator known to persons having skill in the art. Further, the actuator  376  may be operatively coupled to the controller  112  such that a control signal from the controller  112  to the second travel direction valve  308  may toggle the position of the travel divert valve  304  between its first position, its second position, and its third position. 
     In hydraulic system  300 , the fourth actuator  210  is operated in an open-loop mode such that the fluid energy potential across the fourth actuator  210 , to drive motion thereof, is substantially the difference in fluid pressure between conduit  224  and a pressure of the hydraulic fluid reservoir  130 , assuming negligible pressure losses between the fourth rotating group  212  and the fourth actuator  210 . 
       FIG. 6  shows a hydraulic system  400  according to an aspect of the disclosure. Similar to the hydraulic system  200  in  FIG. 4 , hydraulic system  400  has a first rotating group  106 , a first actuator  102 , a second rotating group  136 , a second actuator  104 , a third rotating group  204 , a third actuator  206 , a fourth rotating group  212 , a fourth actuator  210 , and a boost system  120 . The hydraulic system  400  also includes u flow control module  402  having a first travel divert valve  404  and a second travel divert valve  406 . 
     The first travel divert valve  404  may have five ports  408 ,  410 ,  412 ,  414 , and  416 . Port  408  and port  410  of the first travel divert valve  404  are in fluid communication with the second rotating group  136  via the conduit  142  and the conduit  140 , respectively. Port  412  and port  416  of the first travel divert valve  404  are in fluid communication with the second actuator  104  via conduit  154  and conduit  158 , respectively. Port  414  of the first travel divert valve is in fluid communication with the first closed-loop circuit  108  via the conduit  148 . 
     When the first travel divert valve  404  is disposed in a first position, port  408  is fluid coupled to port  414  via valve passage  418 , and ports  410 ,  412  and  416  are blocked from fluid communication with any other ports of the first travel divert valve  404  through the first travel divert valve  404 . Thus, when the first travel divert valve  404  is disposed in the first position, the second rotating group  136  is in fluid communication with the first closed-loop circuit  108  via the first travel divert valve  404 , and the second actuator  104  is blocked from fluid communication with the second rotating group  136  through the first travel divert valve  404 . 
     When the first travel divert valve  404  is disposed in a second position, the port  408  is in fluid communication with the port  412  via the valve passage  432 , the port  410  is in fluid communication with the port  416  via the valve passage  434 , and the port  414  is blocked from fluid communication with any other ports of the first travel divert valve  404  via the first travel divert valve. Thus, when the first travel divert valve  404  is disposed in the second position, the second rotating group  136  is fluidly coupled with the second actuator  104  in a closed-loop circuit via the first travel divert valve  404 . The hydraulic system  400  may include makeup check valves  470  and  472  to provide makeup flow from the boost system  120  to the closed-loop circuit established by the second position of the first travel divert valve  404 . 
     The first travel divert valve  404  may include a resilient member  436  that biases the first travel divert valve toward its first position. Further, the first travel divert valve  404  may include an actuator  438  that urges the first travel divert valve  404  toward its second position. The actuator  438  may be a hydraulic actuator, a pneumatic actuator, a solenoid actuator, or any other actuator known to persons having skill in the art. The actuator  438  may be operatively coupled to the controller  112 , such that the controller  112  may vary the position of the first travel divert valve  404  via a control signal transmitted from the controller  112  to the first travel divert valve  404 . 
     According to an aspect of the disclosure, the first position of the first travel divert valve  404  corresponds to a first operational mode of the flow control module  402 . According to another aspect of the disclosure, the second position of the first travel divert valve  404  corresponds to a second operational mode of the flow control module  402 . 
     The hydraulic system  400  may include an accumulator  420  that is fluidly coupled to the second rotating group  136  via conduit  422  extending from a node  424  on conduit  144 . The accumulator  420  may store hydraulic energy as a displacement of a resilient member included therein. The resilient member of the accumulator  420  may be a volume of a gas, a resilient bladder, a coil spring, a leaf spring, combinations thereof, or other resilient member known to persons having skill in the art. It will be appreciated that a fluid capacitance of the accumulator  420  may act to filter pressure oscillations in the conduit  140 , and a fluid resistance imposed on hydraulic fluid entering and exiting the accumulator  420  may act to damp pressure oscillations in the conduit  140 . 
     An accumulator valve  426  may be disposed in the conduit  422  between the node  424  and the accumulator  420 , and be fluidly coupled thereto via port  428  and port  430 , respectively. When the accumulator valve is disposed in a first position, the port  428  and the port  430  are blocked from fluid communication with one another. When the accumulator valve is disposed in a second position, the port  428  may be in fluid communication with the port  430 . Thus, when the accumulator valve is disposed in the second position, the second rotating group  136  may be in fluid communication with the accumulator  420  via the accumulator valve  426 . 
     The accumulator valve  426  may include a resilient member  429  that biases a position of the accumulator valve  426  toward its first position. The accumulator valve  426  may include an actuator  431  that urges the accumulator valve  426  toward its second position. The actuator  431  may be a hydraulic actuator, a pneumatic actuator, a solenoid actuator, or any other actuator known to persons of skill in the art. The actuator  431  may be operatively coupled to the controller  112 , such that the controller  112  may vary a position of the accumulator valve  426 . It will be appreciated that the controller  112  may cause the accumulator valve  426  to toggle between its first position and its second position, or alternatively, a position of the accumulator valve  426  may vary proportionally to a signal from the controller  112 . According to an aspect of the disclosure, the second position of the accumulator valve corresponds to a first operational mode of the flow control module  402 . 
     The second travel divert valve  406  may have six ports  440 ,  442 ,  444 ,  446 ,  448 , and  450 . The port  440  and the port  442  of the second travel divert valve  406  are in fluid communication with the fourth rotating group  212  via the conduit  222  and the conduit  224 , respectively. Port  444  and port  448  of the second travel divert valve  406  are in fluid communication with the fourth actuator  210  via conduit  234  and conduit  238 , respectively. The port  450  of the second travel divert valve  406  is in fluid communication with the second closed-loop circuit  208  via the conduit  230 , and the port  446  of the second travel divert valve  406  may be in fluid communication with the boost conduit  122  via conduit  452 . 
     When the second travel divert valve  406  is disposed in a first position, the port  440  may be in fluid communication with the port  446  via the valve passage  454 , the port  442  may be in fluid communication with the port  450  via the valve passage  456 , and the ports  444  and  448  may be blocked from fluid communication with any other ports of the second travel divert valve  406  via the second travel divert valve  406 . Thus, when the second travel divert valve  406  is disposed in its first position, the fourth rotating group  212  may be in fluid communication with the boost conduit  122 , and the second closed-loop circuit  208  via the second travel divert valve  406 . Further, the first position of the second travel divert valve  406  may block fluid communication between the fourth actuator  210  and the fourth rotating group  212  via the second travel divert valve  406 . 
     When the second travel divert valve  406  is disposed in a second position, the port  440  may be in fluid communication with the port  444  via the valve passage  458 , the port  442  may be in fluid communication with the port  448  via the valve passage  460 , and the ports  446  and  450  may be blocked from fluid communication with any other ports of the second travel divert valve  406  via the second travel divert valve  406 . Thus, when the second travel divert valve  406  is disposed in its second position, the fourth rotating group  212  is fluidly coupled with the fourth actuator  210  in a closed-loop circuit, and the boost conduit  122  and the second closed-loop circuit  208  are blocked from fluid communication with the fourth rotating group  212  via the second travel divert valve  406 . The hydraulic system  400  may include makeup check valves  474  and  476  to provide makeup flow from the boost system  120  to the closed-loop circuit established by the second position of the second travel divert valve  406 . 
     The second travel divert valve  406  may include a resilient member  462  that biases the second travel divert valve  406  toward its first position. Further, the second travel divert valve  406  may include an actuator  464  that may urge the second travel divert valve  406  toward its second position. The actuator  464  may be a hydraulic actuator, a pneumatic actuator, a solenoid actuator, or any other actuator known to persons having skill in the art. The actuator  464  may be operatively coupled to the controller  112 , such that the controller  112  may vary the position of the second travel divert valve  406  via a control signal transmitted from the controller l| 2  to the second travel divert valve  406 . 
     According to an aspect of the disclosure, the first position of the second travel divert valve  406  corresponds to a first operational mode of the flow control module  402 . According to another aspect of the disclosure, the second position of the second travel divert valve  406  corresponds to a second operational mode of the flow control module  402 . 
     INDUSTRIAL APPLICABILITY 
     The present disclosure may be applicable to any machine including a hydraulic system containing two or more hydraulic actuators. Aspects of the disclosed hydraulic system and method may promote operational flexibility of multi-actuator systems while limiting the number of rotating groups required therein, and may promote operational smoothness and energy efficiency of a hydraulic system. 
     During operation of machine  10 , shown in  FIG. 1 , an operator located within station  20  may command a particular motion of the work tool  14  in a desired direction and at a desired velocity by way of the interface device  58 . One or more corresponding signals generated by the interface device  46  may be provided to the controller  112  indicative of the desired motion, along with machine performance information, for example sensor data such as pressure data, position data, speed data, pump or motor displacement data, and other data known in the art. In response to the signals from interface device  46  and based on the machine performance information, controller  112  may generate control signals directed to a stroke-adjusting mechanism of any of the first rotating group  106 , the second rotating group  136 , the third rotating group  204 , the fourth rotating group  212 , or combinations thereof. 
     For example, to drive the first hydraulic actuator  102 , depicted in  FIG. 3 , at an increasing speed in an extending direction, the controller  112  may generate a control signal that causes the first rotating group  106  of the first closed-loop circuit  108  to increase its displacement in a first direction that results in delivery of pressurized fluid into the head-end chamber  88  via the head-end port  92  at a greater rate. When fluid from the first rotating group  106  is directed into the head-end chamber  88 , return fluid from the rod-end chamber  82  of the first hydraulic actuator  102  may flow through the rod-end port  94  back toward the first rotating group  106  in a closed-loop manner. 
     As discussed previously, the flow rate of fluid entering the head-end port  92  may be greater than the flow rate of fluid exiting the rod-end port  94  during extension of the first hydraulic actuator  102  because of the head-end disparity. And while the makeup circuit  118  may help to provide the additional fluid to the first dosed-loop circuit  108  to fill the head-end chamber  88  while extending the first hydraulic actuator  102 , the second rotating group  136  may also be used to contribute additional fluid to the first closed-loop circuit  108 . 
     Thus, during extension of the first hydraulic actuator  102 , the flow control module  138  may be operated in u first mode that effects fluid communication between the second rotating group  136  and the first closed-loop circuit  108 . According to an aspect of the disclosure, the controller  112  may send a signal to the second rotating group  136  to adjust its stroke to deliver approximately the difference between the head-end fluid flow and the rod-end fluid flow to the first closed-loop circuit  108  during extension of the first hydraulic actuator  102 , and the first rotating group  106  and the second rotating group  136  operate simultaneously to complete the operation. In turn, the fluid demand on the boost system  120  is reduced, allowing lower capacity components to be used therein. 
     Conversely, when the first hydraulic actuator  102  is contracted, the flow rate of fluid out of the head-end chamber  88  may be greater than the flow rate of fluid into the rod-end chamber  82 , because of the head-end disparity. Accordingly, the difference between the head-end flow and the rod-end flow may be removed from the first closed-loop circuit  108  through the second rotating group  136 , in combined operation with the first rotating group  106 , by operating the flow control module  138  in the first operating mode. 
     Further, it will be appreciated that when the first hydraulic cylinder  102  is either extended or contracted in an overrun condition, for example, such that operation of the first hydraulic actuator imparts fluid energy to the first closed-loop circuit  108 , the first rotating group  106 , the second rotating group  136 , or both may be operated as motors to deliver the fluid energy extracted from the first closed-loop circuit  108  to the power source  18  or the like. Alternatively, fluid energy extracted from the first closed-loop circuit  108  may be stored in the accumulator  420  by selectively opening and closing the accumulator valve  426 , as shown in  FIG. 6 . 
     When the operator wishes to operate the second actuator  104 , a signal from the controller  112  may configure the flow control module  138  in a second operating mode such that the second actuator  104  is driven by the second rotating group  136 . The second actuator  104  may be fluidly coupled to the second rotating group  136  by operation of the travel divert valve  304  and the first travel direction valve  306 , as shown in  FIG. 5 , or by operation of the first travel divert valve  404 , as shown in  FIG. 6 . 
     When the second rotating group  136  is fluidly coupled to the second actuator  104 , the second rotating group may not be available for cooperation with the first rotating group  106  to drive the first hydraulic actuator  102 . However, unlike conventional approaches, the first hydraulic actuator  102  may still be operated by the first rotating group  106  in conjunction with the boost system  120  to compensate for any head-end disparity effects. Indeed, the hydraulic power demand for operating functions such as the boom hydraulic cylinder  26  and the stick hydraulic cylinder  32  may be greatly reduced when the machine  10  is moving, so much so that the boost system  120  may be sufficient to counter any head-end disparity effects from operation of the first hydraulic actuator  102  or the third hydraulic actuator  206  while the travel motors  54 ,  56  are operating. 
     It will be appreciated that the fourth rotating group  212  may be used to either compensate for head-end disparity effects while operating the third hydraulic actuator  206 , or be used to operate the fourth hydraulic actuator  210  (see, e.g.  FIG. 4 ) depending on the mode of the flow control module  202 , similar to operation of the second rotating group  136  with respect to the first hydraulic actuator  102  and the second hydraulic actuator  104 . 
     As shown in  FIG. 6 , when the flow control module  402  is operated in its first mode, the second rotating group  212  may be used to simultaneously exchange fluid with the second closed-loop circuit  208  and the boost system  120  via conduit  230  and conduit  452 , respectively, Accordingly, the energy storage accumulator  420 , shown in  FIG. 6 , may enable hydraulic system operation with a smaller boost accumulator  134 . 
     Further regarding  FIG. 6 , it will be appreciated that fluid energy stored in the accumulator  420  may be selectively released into the hydraulic system  400  by the accumulator valve  426  to increase the hydraulic power available to the first actuator  102  and the second actuator, or delivered to the power source  18  as shaft power by using the second rotating group  136  as a motor to convert the stored fluid energy into shaft power. 
     According to an aspect of the disclosure, the first hydraulic actuator  102  is a boom hydraulic cylinder  26  of the machine  10 , the third hydraulic actuator  206  is the stick hydraulic cylinder  32  of the machine  10 , and the second hydraulic actuator  104  and the fourth hydraulic actuator  204  are the right travel motor  56  and left travel motor  54 , respectively, of the machine  10  (see  FIG. 1 ). Thus, when the flow control module is configured in its first operating mode, the first rotating group  106  and the second rotating group  136  may act together to operate the boom hydraulic cylinder  26 , and the third rotating group  204  and the fourth rotating group  212  may act together to operate the stick hydraulic cylinder  32 . 
     When the operator wishes to move the machine  10  relative to the work surface  24 , the right travel motor  56  and the left travel motor  54  may be driven by the second rotating group  136  and the fourth rotating group  212 , respectively, by configuring the flow control module in the second mode. And as discussed above, the boom hydraulic cylinder  26  and the stick hydraulic cylinder  32  still may be driven by the first rotating group  106  and the second rotating group  204 , respectively, while the travel motors  54 ,  56  operate to move the machine relative to the work surface  24 . 
     Even if not expressly stated, it is contemplated that any of the hydraulic systems  100 ,  200 ,  300 , and  400  may embody structures or functions of the other hydraulic systems discussed herein, and it is contemplated that any of the flow control modules  138 ,  202 ,  302 , and  402  may embody structures or functions of the other flow control modules discussed herein. Further, any of the flow control modules  138 ,  202 ,  302 , and  402  may be enclosed within a single housing, or be distributed throughout their corresponding hydraulic systems in a plurality of discrete housings. 
     Like reference numbers refer to similar elements herein, unless otherwise specified. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof 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 disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure 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.