Patent Publication Number: US-8966892-B2

Title: Meterless hydraulic system having restricted primary makeup

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a hydraulic system and, more particularly, to a meterless hydraulic system having restricted primary makeup functionality. 
     BACKGROUND 
     A conventional hydraulic system includes a pump that draws low-pressure fluid from a tank, pressurizes the fluid, and makes the pressurized fluid available to multiple different actuators for use in moving the actuators. In this arrangement, a speed of each actuator can be independently controlled by selectively throttling (i.e., restricting) a flow of the pressurized fluid from the pump into each actuator. For example, to move a particular actuator at a high speed, the flow of fluid from the pump into the actuator is restricted by only a small amount. In contrast, to move the same or another actuator at a low speed, the restriction placed on the flow of fluid is increased. Although adequate for many applications, the use of fluid restriction to control actuator speed can result in flow losses that reduce an overall efficiency of a hydraulic system. 
     An alternative type of hydraulic system is known as a meterless hydraulic system. A meterless hydraulic system generally includes a pump connected in closed-loop fashion to a single actuator or to a pair of actuators operating in tandem. During operation, the pump draws fluid from one chamber of the actuator(s) and discharges pressurized fluid to an opposing chamber of the same actuator(s). To move the actuator(s) at a higher speed, the pump discharges fluid at a faster rate. To move the actuator with a lower speed, the pump discharges the fluid at a slower rate. A meterless hydraulic system is generally more efficient than a conventional hydraulic system because the speed of the actuator(s) is controlled through pump operation as opposed to fluid restriction. That is, the pump is controlled to only discharge as much fluid as is necessary to move the actuator(s) at a desired speed, and no throttling of a fluid flow is required. An exemplary meterless hydraulic system is disclosed in U.S. Patent Publication 2009/0165450 of Cherney et al. that published on Jul. 2, 2009 (“the &#39;450 publication). 
     Although an improvement over conventional hydraulic systems, the meterless hydraulic system of the &#39;450 publication may still be less than optimal. In particular, the hydraulic system of the &#39;450 publication may suffer from instabilities during transitional operations (i.e., during operations that transition between resistive and overrunning modes), pump overspeeding during operation in the overrunning mode, and/or damaging pressure spikes. 
     The hydraulic system of the present disclosure is directed toward solving one or more of the problems set forth above and/or other problems of the prior art. 
     SUMMARY 
     In one aspect, the present disclosure is directed to a hydraulic system. The hydraulic system may include a primary pump, a hydraulic actuator, and first and second passages fluidly connecting the primary pump to the hydraulic actuator in a closed-loop manner. The hydraulic system may also include a charge circuit, a makeup valve movable to selectively allow charge fluid from the charge circuit to enter the first or second passages, and at least one restricted pilot passage configured to direct pilot fluid to the makeup valve to move the makeup valve and allow the charge fluid into the first and second passages. 
     In another aspect, the present disclosure is directed to a method of operating a hydraulic system. The method may include pressurizing fluid with a pump, directing pressurized fluid from the pump through a hydraulic actuator to move the actuator, and returning fluid from the hydraulic actuator back to the pump in a closed-loop manner. The method may also include directing at least one restricted flow of pilot fluid to move a makeup valve and selectively allow charge fluid to join with pressurized fluid from the pump or with the fluid returning to the pump. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a pictorial illustration of an exemplary disclosed machine; 
         FIG. 2  is a schematic illustration of an exemplary disclosed hydraulic system that may be used in conjunction with the machine of  FIG. 1 ; 
         FIGS. 3-5  are cross-sectional and schematic illustrations of an exemplary disclosed load-holding valve that forms a portion of the hydraulic system of  FIG. 2 ; 
         FIG. 6  is an enlarged schematic illustration of a portion of the hydraulic system of  FIG. 2 ; 
         FIG. 7  is a cross-sectional illustration of an exemplary disclosed displacement control valve that forms a portion of the hydraulic system of  FIG. 2 ; and 
         FIG. 8  is a schematic illustration of another exemplary disclosed hydraulic system that may be used in conjunction with the machine of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary machine  10 . Machine  10  may be 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, machine  10  may be an earth moving machine such as an excavator (shown in  FIG. 1 ), a backhoe, a loader, or a motor grader. Machine  10  may include a power source  12 , a tool system  14  driven by power source  12 , and an operator station  16  situated for manual control of tool system  14  and/or power source  12 . 
     Tool system  14  may include linkage acted on by hydraulic actuators to move a work tool  18 . For example, tool system  14  may include a boom  20  that is vertically pivotal about a horizontal boom axis (not shown) by a pair of adjacent, double-acting, hydraulic cylinders  22  (only one shown in  FIG. 1 ), and a stick  24  that is vertically pivotal about a stick axis  26  by a single, double-acting, hydraulic cylinder  28 . Tool system  14  may further include a single, double-acting, hydraulic cylinder  30  that is connected to vertically pivot work tool  18  about a tool axis  32 . In one embodiment, hydraulic cylinder  30  may be connected at a head-end  30 A to a portion of stick  24  and at an opposing rod-end  30 B to work tool  18  by way of a power link  34 . Boom  20  may be pivotally connected to a frame  36  of machine  10 , while stick  24  may pivotally connect tool  18  to boom  20 . It should be noted that other types and configurations of linkages and actuators may be associated with machine  10 , as desired. 
     Operator station  16  may include devices that receive input from a machine operator indicative of desired machine maneuvering. Specifically, operator station  16  may include one or more operator interface devices  37 , for example a joystick, a steering wheel, or a pedal, that are located proximate an operator seat (not shown). Operator interface devices  37  may initiate movement of machine  10 , for example travel and/or tool movement, by producing displacement signals that are indicative of desired machine maneuvering. As an operator moves interface device  37 , the operator may affect a corresponding machine movement in a desired direction, with a desired speed, and/or with a desired force. 
     For purposes of simplicity,  FIG. 2  illustrates the composition and connections of only hydraulic cylinder  22 . It should be noted, however, that hydraulic cylinders  28 ,  30 , and/or any other hydraulic actuator of machine  10 , may have a similar composition and be hydraulically connected in a similar manner, if desired. 
     As shown in  FIG. 2 , hydraulic cylinder  22  may include a tube  38  and a piston assembly  40  arranged within tube  38  to form a first chamber  42  and an opposing second chamber  44 . In one example, a rod portion  40 A of piston assembly  40  may extend through an end of second chamber  44 . As such, second chamber  44  may be considered the rod-end chamber of hydraulic cylinder  22 , while first chamber  42  may be considered the head-end chamber. 
     First and second chambers  42 ,  44  may each be selectively supplied with pressurized fluid and drained of the pressurized fluid to cause piston assembly  40  to displace within tube  38 , thereby changing an effective length of hydraulic cylinder  22  and moving (i.e., lifting and lowering) boom  20  (referring to  FIG. 1 ). A flow rate of fluid into and out of first and second chambers  42 ,  44  may relate to a translational velocity of hydraulic cylinder  22  and a rotational velocity of boom  20 , while a pressure differential between first and second chambers  42 ,  44  may relate to a force imparted by hydraulic cylinder  22  on boom  20  and by boom  20  on stick  24 . An expansion and a retraction of hydraulic cylinder  22  may function to assist in moving boom  20  in different manners (e.g., lifting and lowering boom  20 , respectively). 
     To help regulate filling and draining of first and second chambers  42 ,  44 , machine  10  may include a hydraulic system  46  having a plurality of interconnecting and cooperating fluid components. Hydraulic system  46  may include, among other things, a primary circuit  48  configured to connect a primary pump  50  to hydraulic cylinder  22  in a generally closed-loop manner, a charge circuit  52  configured to selectively accumulate excess fluid from and discharge makeup fluid to primary circuit  48 , and a controller  54  configured to control operations of primary and charge circuits  48 ,  52  in response to input from an operator received via interface device  37 . 
     Primary circuit  48  may include a head-end passage  56  and a rod-end passage  58  forming the generally closed loop between primary pump  50  and hydraulic cylinder  22 . During an extending operation, head-end passage  56  may be filled with fluid pressurized by primary pump  50 , while rod-end passage  58  may be filled with fluid returned from hydraulic cylinder  22 . In contrast, during a retracting operation, rod-end passage  58  may be filled with fluid pressurized by primary pump  50 , while head-end passage  56  may be filled with fluid returned from hydraulic cylinder  22 . 
     Primary pump  50  may have variable displacement and be controlled to draw fluid from hydraulic cylinder  22  and discharge the fluid at a specified elevated pressure back to hydraulic cylinder  22  in two different directions. That is, primary pump  50  may include a stroke-adjusting mechanism  60 , for example a swashplate, a position of which is hydro-mechanically adjusted based on, among other things, a desired speed of hydraulic cylinder  22  to thereby vary an output (e.g., a discharge rate) of primary pump  50 . The displacement of pump  50  may be adjusted from a zero displacement position at which substantially no fluid is discharged from primary pump  50 , to a maximum displacement position in a first direction at which fluid is discharged from primary pump  50  at a maximum rate into head-end passage  56 . Likewise, the displacement of pump  50  may be adjusted from the zero displacement position to a maximum displacement position in a second direction at which fluid is discharged from primary pump  50  at a maximum rate into rod-end passage  58 . Primary pump  50  may be drivably connected to power source  12  of machine  10  by, for example, a countershaft, a belt, or in another suitable manner. Alternatively, primary pump  50  may be indirectly connected to power source  12  via a torque converter, a gear box, an electrical circuit, or in any other manner known in the art. 
     Primary pump  50  may also selectively be operated as a motor. More specifically, when an extension or a retraction of hydraulic cylinder  22  is in the same direction as a force acting on boom  20 , the fluid discharged from hydraulic cylinder  22  may be elevated and function to drive primary pump  50  to rotate with or without assistance from power source  12 . Under some circumstances, primary pump  50  may even be capable of imparting energy to power source  12 , thereby improving an efficiency and/or capacity of power source  12 . 
     It will be appreciated by those of skill in the art that the respective rates of hydraulic fluid flow into and out of first and second chambers  42 ,  44  during extension and retraction of hydraulic cylinder  22  may not be equal. That is, because of the location of rod portion  40 A within second chamber  44 , piston assembly  40  may have a reduced pressure area within second chamber  44 , as compared with a pressure area within first chamber  42 . Accordingly, during retraction of hydraulic cylinder  22 , more hydraulic fluid may flow out of first chamber  42  than can be consumed by second chamber  44  and, during extension of hydraulic cylinder  22 , more hydraulic fluid may be required to flow into first chamber  42  than flows out of second chamber  44 . In order to accommodate the excess fluid during retraction and the need for additional fluid during extension, primary circuit  48  may be provided with a primary makeup valve (PMV)  62 , two secondary makeup valves (SMV)  64 , and two relief valves  66 , each connected to charge circuit  52  via a passage  67 . 
     PMV  62  may be a pilot-operated, spring-centered, three-position valve movable based on a pressure differential between head- and rod-end passages  56 ,  58 . In particular, PMV  62  may be movable from a first position (shown in  FIG. 2 ) at which fluid flow through PMV  62  may be inhibited, to a second position at which fluid flow from passage  67  through PMV  62  into head-end passage  56  is allowed via a makeup passage  68 , and to a third position at which fluid flow from passage  67  through PMV  62  into rod-end passage  56  is allowed via a makeup passage  70 . A first pilot passage  72  may connect a pilot pressure signal from makeup passage  68  to an end of PMV  62  to urge PMV  62  toward the second position, while a second pilot passage  74  may connect a pilot pressure signal from makeup passage  70  to an opposing end of PMV  62  to urge PMV  62  toward the third position. When the pressure signal within first pilot passage  72  sufficiently exceeds the pressure signal within second pilot passage  74  (i.e., exceeds by an amount about equal to or greater than a centering spring bias of PMV  62 ), PMV  62  may move toward the second position, and when the pressure signal within second pilot passage  74  sufficiently exceeds the pressure signal within first pilot passage  72 , PMV  62  may move toward the third position. First and second pilot passages  72 ,  74  may each include a fixed restrictive orifice  76  that helps to reduce pressure oscillations having a potential to cause instabilities in movement of PMV  62 . PMV  62  may be spring-centered toward the first position. 
     It should be noted that when PMV  62  is in the first position, flow through PMV  62  may either be completely blocked or only restricted to inhibit flow by a desired amount. That is, PMV  62  could include restrictive orifices (not shown) that block some or all fluid flow when PMV  62  is in the first position, if desired. The use of restrictive orifices may be helpful during situations where primary pump  50  does not return to a perfect zero displacement when commanded to neutral. Accordingly, any reference to the first position of PMV  62  as being a flow-inhibiting position is intended to include both a completely blocked condition and a condition wherein flow through PMV  62  is limited but still possible. 
     Although restrictive orifices  76  within first and second pilot passages  72 ,  74  may help reduce instabilities associated with PMV  62 , they may also slow a reaction of PMV  62 . Accordingly, SMVs  64  may be provided within a passage  77  connecting passage  67  with head- and rod-end passages  56 ,  58  to enhance responsiveness of primary circuit  48 . In the disclosed embodiment, SMVs  64  may be check type valves that are operative at set pressure differentials between passage  67  and head- and rod-end passages  56 ,  58 , respectively. It will be appreciated that the SMVs  64  may unseat to permit flow only into primary circuit  48  when the pressure of fluid within passage  67  is greater than the pressures in head- and rod-end passages  56 ,  58 , respectively. 
     Relief valves  66  may be provided to permit flow between head- and rod-end passages  56 ,  58  and passage  67 , allowing fluid to be relieved from primary circuit  48  into charge circuit  52  when a pressure of the fluid exceeds a set threshold of relief valves  66 . Relief valves  66  may be set to operate at relatively high pressure levels in order to prevent damage to hydraulic system  46 , for example at levels that may only be reached when piston assembly  40  reaches an end-of-stroke position and the flow from primary pump  50  is nonzero, or during a failure condition of hydraulic system  46 . Relief valves  66  may connect via relief passages  69  to head- and rod-end passages  56 ,  58  at or near ports of first and second chambers  42 ,  44 , for example at locations between any load-holding check valves and hydraulic cylinder  22 . 
     In order to help reduce a likelihood of primary pump  50  overspeeding during a motoring retraction of hydraulic cylinder  22 , primary circuit  48  may be provided with at least one regeneration valve  78 . Regeneration valve  78  may be disposed within a regeneration passage  80  that extends between head- and rod-end passages  56 ,  58 , and be movable between a first or flow-blocking position (shown in  FIG. 2 ) and a second or flow-passing position. When regeneration valve  78  is in the flow-passing position, some or all of the fluid discharged from first chamber  42  may be directly routed into second chamber  44 , without the fluid first passing through primary pump  50 . Regeneration valve  78  may only be moved to the flow-passing position during a motoring retraction, and movement of regeneration valve  78  may be accomplished hydraulically via pressure control of fluid within a regeneration control passage  82 . That is, any time a force generated by fluid within regeneration control passage  82  acting on a first end of regeneration valve  78  exceeds a combined spring force and force from fluid within a pilot passage  84  (i.e., a force of fluid from rod-end passage  58 ) acting on an opposing end of regeneration valve  78 , regeneration valve  78  may move toward the flow-passing position. Control of the pressure within regeneration control passage  82  will be described in more detail below in connection with displacement control of primary pump  50 . 
     First circuit  48  may be provided with load-holding valves  86  and  88  to inhibit unintended motion of tool system  14  (referring to  FIG. 1 ). Load-holding valves  86 ,  88  may be associated with head- and rod-end passages  56 ,  58 , respectively, and configured to inhibit fluid flow to and from the associated chambers of hydraulic cylinder  22 , thereby locking the movement of hydraulic cylinder  22  when movement of hydraulic cylinder  22  has not been requested by the operator of machine  10 . Each of load-holding valves  86 ,  88  may include a first or default position (shown in  FIG. 2 ) at which substantially no fluid flow from hydraulic cylinder  22  through load-holding valves  86 ,  88  is allowed, and a second or active position at which flow through load-holding valves  86 ,  88  and movement of hydraulic cylinder  22  is substantially unrestricted. Load-holding valves  86 ,  88  may be urged toward their default positions when movement of hydraulic cylinder  22  is not requested, and moved toward their active positions when movement is requested. 
     Each load-holding valve  86 ,  88  may be hydraulically operated to move between the flow-passing and flow-blocking positions. In particular, each load-holding valve  86 ,  88  may include a pump-side pilot passage (PSPP)  90 , a first actuator-side pilot passage (FASPP)  92 , a second actuator-side pilot passage (SASPP)  94 , and a control pilot passage (CPP)  96 . A restrictive orifice  98  may be disposed within each SASPP  94  that provides for a restriction in fluid flow through SASPP  94 . Pressurized fluid from within PSPP  90  and FASPP  92  may act separately on a first end of each load-holding valve  86 ,  88  to urge the corresponding valve toward its flow-passing position, while pressurized fluid from within SASPP  94  and CPP  96  may act together with a spring-bias on an opposing second end of each load-holding valve  86 ,  88  to urge the valve towards its flow-blocking position. In order to facilitate movement of load-holding valves  86 ,  88  from their flow-blocking positions toward their flow-passing positions, CPP  96  may be selectively reduced in pressure, for example by way of connection to a low-pressure tank  99  of charge circuit  52 . When CPP  96  is connected to tank  99 , fluid from within PSPP  90  and/or FASPP  92  may generate a combined force during movement of hydraulic cylinder  22  that is sufficient to overcome the spring bias of load-holding valves  86 ,  88  and move load-holding valves  86 ,  88  to the flow-passing positions. To move load-holding valves  86 ,  88  to their default or flow-blocking position, CPP  96  may be pressurized with fluid (or at least blocked and allowed to be pressurized with fluid from hydraulic cylinder  22 ), the resulting force combined with the spring bias acting at the second end of load-holding valves  86 ,  88  being sufficient to overcome any force generated at the opposing end of load-holding valves  86 ,  88 . With this configuration, even if tool system  14  is loaded and generating force on hydraulic cylinder  22 , any pressure buildup between load-holding valves  86 ,  88  and hydraulic cylinder  22  caused by the loading may be communicated with both the first and second ends of load-holding valves  86 ,  88  via FASPP  92  and SASPP  94 , thereby counteracting each other and allowing the pressure within CPP  96  to control motion of load-holding valves  86 ,  88 . In fact, in some embodiments, a pressure area of load-holding valves  86 ,  88  exposed to SASPP  94  may be greater than a pressure area exposed to FASPP  92  such that any buildup of pressure caused by the loading of tool system  14  may actually result in a greater valve-closing force (i.e., a greater force urging load-holding valves  86 ,  88  toward their flow-blocking positions) for a given pressure buildup. Details of the selective connection of CPP  96  to tank  99  will be discussed in greater detail below. 
     An exemplary load-holding valve  86  is illustrated in  FIGS. 3-5 . While  FIGS. 3-5  illustrate only load-holding valve  86 , it should be noted that the same configuration may likewise be associated with load-holding valve  88 , if desired. In the illustrated embodiment, load-holding valve  86  may be a poppet-type valve having a poppet element  100  moveable within a valve block  102  between the flow-blocking position (shown in  FIG. 3 ) at which a nose portion  104  of poppet element  100  engages a seat  106  of valve block  102 , and the flow-passing position (shown in  FIG. 4 ) at which nose portion  104  is away from seat  106 . 
       FIG. 3  illustrates load-holding valve  86  in the flow-blocking position during a time when movement of hydraulic cylinder  22  is not being requested by the operator of machine  10  via interface device  37 . At this point in time, because no request is being made by the operator, primary pump  50  may be destroked to about a zero displacement position such that a pressure of fluid within PSPP  90  is low and generating little force, if any, urging poppet element  100  toward the flow-passing position. At this same time, a load acting through tool system  14  on hydraulic cylinder  22  may generate a relatively high pressure within first chamber  42  that is transmitted to FASPP  92 . This high-pressure fluid may be communicated to nose portion  104 , as well as to a base portion  107  of poppet element  100  via SASPP  94 . Because CPP  96  may be pressurized at this time (i.e., not connected to tank  99 ) and because base portion  107  may have a larger pressure area when compared with nose portion  104 , a valve-closing force generated at base portion  107  by the pressurized fluid may be greater than a valve-opening force generated at nose portion  104  by the same fluid. Accordingly, poppet element  100  may be moved to and/or maintained in the flow-blocking position shown in  FIG. 3 . 
       FIG. 4  illustrates load-holding valve  86  in the flow-passing position during a time when movement of hydraulic cylinder  22  is being requested by the operator via interface device  37 . At this point in time, primary pump  50  may be pressurizing fluid directed into hydraulic cylinder  22 , and CPP  96  may be connected to tank  99 . The high-pressure fluid acting on a shoulder portion  108  and on nose portion  104  of poppet element  100 , combined with the low-pressure connection to base portion  107 , may generate a force imbalance that causes poppet element  100  to move toward and/or be maintained in the flow-passing position shown in  FIG. 4 . It should be noted that, even though the high-pressure fluid from primary pump  50  may be communicated with base portion  107  via SASPP  94 , restrictive orifice  98  may restrict flow through SASPP  94  such that pressure does not significantly build at base portion  107  and affect (i.e., inhibit) movement of poppet element  100  to the flow-passing position at this time. 
       FIG. 5  illustrates load-holding valve  86  in a position associated with a malfunction of hydraulic system  46 . That is, CPP  96  should normally be connected with tank  99  any time PSPP  90  is pressurized. However, there may be some situations when this does not occur. For example, when pump  50  is commanded to zero displacement but, for one reason or another, pump  50  does not achieve zero displacement (e.g., when displacement actuator  134  becomes stuck), or when CPP  96  somehow becomes inadvertently pinched closed, PSPP  90  may be pressurized at the same time that CPP  96  is pressurized. During this condition, after valve element  100  is driven to the closed or flow-blocking position, pressurized fluid from pump  50  (i.e., from PSPP  90 ) may act on nose  104  and shoulder  108  to urge valve element  100  toward the flow passing position, while fluid from CPP  96  may simultaneously be forced by the movement of valve element  100  from CPP  96  into FASPP  92  via SASPP  94  and restrictive orifice  98 . Because of the restriction of orifice  98 , however, this flow of fluid from CPP  96  into FASPP  92  may be too slow, resulting in excessive pressure spikes within CPP  96  and/or PSPP  90 . In order to help reduce these excessive pressure spikes during a malfunction condition, fluid from within CPP  96  may also be allowed to escape into FASPP  92  via a bypass passage  109  and check valve  110 . 
     Returning to  FIG. 2 , charge circuit  52  may include at least one hydraulic source fluidly connected to passage  67  described above. For example, charge circuit  52  may include a charge pump  112  and/or an accumulator  114 , both of which may be fluidly connected to passage  67  via a common passage  116  to provide makeup fluid to primary circuit  48 . Charge pump  112  may embody, for example, an engine-driven, fixed displacement pump configured to draw fluid from tank  99 , pressurize the fluid, and discharge the fluid into passage  67  via common passage  116 . Accumulator  114  may embody, for example, a compressed gas, membrane/spring, or bladder type of accumulator configured to accumulate pressurized fluid from and discharge pressurized fluid into common passage  116 . Excess hydraulic fluid, either from charge pump  112  or from primary circuit  48  (i.e., from operation of primary pump  50  and/or hydraulic cylinder  22 ) may be directed into either accumulator  114  or into tank  99  by way of a charge pilot valve  118  disposed in a return passage  120 . Charge pilot valve  118  may be movable from a flow-blocking position toward a flow-passing position as a result of fluid pressures within common passage  116  and passage  67 . 
     As shown in  FIGS. 2 and 6 , a pressure relief valve  122  may be disposed within a drain passage  124  that extends between common passage  116  and return passage  120  to regulate fluid flow from charge circuit  52  into tank  99 , and a restrictive orifice  123  may be disposed within common passage  116  between passage  67  and drain passage  124 . Pressure relief valve  122  may be pilot-operated and spring-biased to move between a first position at which fluid flow into tank  99  is inhibited, and a second position at which fluid is allowed to flow from common passage  116  into return passage  120 . Pressure relief valve  122  may be spring-biased toward the first position, and movable toward the second position when a pressure acting on pressure relief valve  122  generates a force exceeding the spring bias of pressure relief valve  122 . A resolver  126  may be disposed to selectively communicate a pilot signal via pilot passages  128 ,  130  from the higher-pressure one of head- and rod-end passages  56 ,  58  with pressure relief valve  122  to allow the signal to act on pressure relief valve  122  and urge pressure relief valve  122  toward the second position. Restrictive orifice  123  may help to dampen pressure oscillations within common passage  116  and somewhat isolate fluid makeup operations from displacement control operations associated with primary pump  50 . When pressure relief valve  122  is moved to its second or flow-passing position, the pressure of fluid within passage  116  downstream of restrictive orifice  123  may drop to bring displacement actuator  134  to a lesser displacement value (possibly to zero). This will happen, for example, when hydraulic actuator  22  reaches its end of stroke position or is acting against a sufficiently high load. It should be noted that the form of override described above can also be implemented as a power-override, if desired, during which circuit pressures are not resolved but instead act simultaneously to bring the displacement of actuator  134  to a zero value. 
       FIG. 6  illustrates a portion of charge circuit  52  that is configured to affect displacement control of primary pump  50  and operation of load-holding valves  86 ,  88 . In particular,  FIG. 6  shows a displacement control valve  132  configured to control motion of a displacement actuator  134  that is mechanically connected to stroke-adjusting mechanism  60  of primary pump  50 . In the illustrated embodiment, displacement control valve  132  is a solenoid-actuated, three-position valve that is movable by pilot pressure in response to control signals from controller  54  (referring to  FIG. 2 ). It should be noted, however, that although displacement actuator  134  is shown and described as being electro-hydraulically controlled, it is contemplated that displacement actuator  134  may alternatively be purely mechanically or hydro-mechanically controlled, if desired. 
     When displacement control valve  132  is in the first position (shown in  FIG. 6 ), the pressures within first and second chambers  136 ,  140  may be substantially balanced (i.e., first and second chambers  136 ,  140  may be exposed to substantially similar pressures) such that displacement actuator  134  is spring-biased toward a neutral position that returns the displacement of primary pump  50  to zero displacement. In particular, when displacement control valve  132  is in the first position, first and second chambers  136 ,  140  may be fluidly communicated with common passage  116  leading to charge pump  112  and accumulator  114  and simultaneously communicated with return passage  120  leading to tank  99 . The simultaneous connection of both first and second chambers  136 ,  140  to common passage  116  and return passage  120  may allow for an equal amount of pressure buildup within first and second chambers  136 ,  140  that is less than a full pressure of common passage  116 . This equal and slightly elevated, yet limited, pressure (e.g., about 2-3 MPa) within first and second chambers  136 ,  140  may facilitate movement of displacement control valve  132  to the neutral position while also providing for a quick displacement response of primary pump  50  during subsequent movement of displacement control valve  132  to the second or third positions. When displacement control valve  132  is moved to the first position, regeneration control passage  82  may also be connected to common passage  116  and return passage  120 . Because regeneration control passage  82  may be drained of fluid (or at least exposed to a lower pressure) when displacement control valve  132  is in the first position, regeneration valve  78  may be spring-biased to its flow-blocking position, thereby inhibiting fluid flow from rod-end passage  58  to head-end passage  56  via regeneration passage  80 . CPP  96  may be blocked at this time by displacement control valve  132 , to facilitate movement of load-holding valves  86 ,  88  to their flow-blocking positions. 
     When displacement control valve  132  is in the second position (i.e., the position associated with downward movement of displacement control valve  132  in  FIG. 6  away from the first position), fluid may be allowed to flow from charge pump  112  and/or accumulator  114  into second chamber  140  of displacement actuator  134  via common passage  116  and a pilot passage  139  to urge displacement actuator  134  to move in a first direction indicated by an arrow  142 . At this same time, fluid may be allowed to drain from first chamber  136  of displacement actuator  134 , from regeneration control passage  82  associated with regeneration valve  78 , and from CPP  96  associated with load-holding valves  86  into tank  99  via pilot passage  137  and return passage  120 . Because regeneration control passage  82  may be drained of fluid when displacement control valve  132  is in the second position, regeneration valve  78  may be spring-biased to its flow-blocking position, thereby inhibiting fluid flow from rod-end passage  58  to head-end passage  56  via passage  80 . CPP  96  may be unblocked at this time, to facilitate movement of load-holding valves  86 ,  88  to their flow-passing positions. 
     When displacement control valve  132  is in the third position (i.e., the position associated with upward movement of displacement control valve  132  in  FIG. 6  away from the first position), fluid may be allowed to flow from charge pump  112  and/or accumulator  114  into first chamber  136  of displacement actuator  134  via common passage  116  and pilot passage  137  to urge displacement actuator  134  to move in a second direction indicated by an arrow  138  and into regeneration control passage  82 . At this same time, fluid may be allowed to drain from second chamber  140  of displacement actuator  134  via pilot passage  139  and from load-holding valves  86 ,  88  into tank  99  via return passage  120 . Because regeneration control passage  82  may be pressurized with fluid when displacement control valve  132  is in the third position, regeneration valve  78  may be moved to its flow-passing position, thereby allowing fluid flow from rod-end passage  58  to head-end passage  56  via regeneration passage  80 . CPP  96  may be unblocked at this time, to facilitate movement of load-holding valves  86 ,  88  to their flow-passing positions. 
     Displacement control valve  132  may be spring-biased toward the first position and selectively moved by pressurized fluid from common passage  116  acting on ends of displacement control valve  132  via a pilot passage  144  into the second and third positions based on signals from controller  54 . Flows of pressurized fluid into first and second chambers  136 ,  140  of displacement actuator  134  that are achieved when displacement control valve  132  is in the first and second positions, respectively, may affect the motion of displacement actuator  134 . Those of skill in the art will appreciate that the motion of displacement actuator  134  may control the position of stroke-adjusting mechanism  60 , and, hence, the displacement of primary pump  50  and associated flow rates and directions of fluid flow through head- and rod-end passages  56 ,  58 . When displacement control valve  132  is in the first position, stroke-adjusting mechanism  60  may be centered or “zeroed” by biasing forces, such that primary pump  50  may have substantially zero displacement (i.e., such that primary pump  50  may be displacing little, if any, fluid into either of head- or rod-end passages  56 ,  58 ). When displacement control valve  132  is in the second position, stroke-adjusting mechanism may be shifted upward (relative to the embodiment of  FIG. 6 ) to provide a positive displacement of primary pump  50  (a displacement of fluid into head-end passage  56 ), the resulting angle or position of stroke-adjusting mechanism  60  determining a volume of fluid displaced. When displacement control valve  132  is in the third position, stroke-adjusting mechanism may be shifted downward (relative to the embodiment of  FIG. 6 ) to provide a negative displacement of primary pump  50  (a displacement of fluid into rod-end passage  58 ), the resulting angle or position of stroke-adjusting mechanism  60  determining a volume of fluid displaced. 
     During operation, the operator of machine  10  may utilize interface device  37  (referring to  FIG. 2 ) to provide a signal that identifies the desired movement of hydraulic cylinder  22  to controller  54 . Based upon one or more signals, including the signal from interface device  37 , and, for example, a current position of hydraulic cylinder  22 , controller  54  may command displacement control valve  132  to advance to a particular one of the first-third positions. 
       FIG. 7  illustrates a physical embodiment of displacement control valve  132 . In this embodiment, displacement control valve  132  may include a valve element, for example a spool  146 , that is slidably disposed within a stationary cage portion  148 . Stationary cage portion  148  may be located within a valve block  149  and at least partially define passages  82 ,  96 ,  116 ,  120 ,  137 ,  139 , and  144 , such that, as spool  146  slides lengthwise up and down (relative to  FIG. 6 ) within stationary cage portion  148 , different combinations of the passages may be interconnected. For example,  FIG. 6  illustrates the third position of displacement control valve  132 , wherein spool  146  is shifted downward to connect pressurized fluid from common passage  116  with passages  82  and  139  and to connect passages  137  and  96  with the low pressure of return passage  120 . 
     In some embodiments, displacement actuator  134  may be provided with a mechanical feedback device  150  that is configured to adjust an operating state of displacement control valve  132  as displacement actuator  134  is actuated. Mechanical feedback device  150  may include a link  152  that is pivotally restrained at a midpoint  154 , and a movable cage portion  156  that is connected to a first end of link  152  and disposed proximate stationary cage portion  148  at passages  137 ,  139 . In some embodiments, movable cage portion  156  may actually form a portion of passages  137 ,  139 . Link  152  may also be connected at a second end to displacement actuator  134 , such that as displacement actuator  134  translates between the positive and negative displacement positions, link  152  may pivot about midpoint  154  and cause movable cage portion  156  to slide along an outer surface of stationary cage portion  148 . As movable cage portion  156  slides relative to stationary cage portion  148  in response to movement of displacement actuator  134  toward a greater displacement position, passages  137  and  139  may be increasingly restricted and eventually become blocked. In this manner, mechanical feedback device  150  may facilitate incremental movement of displacement actuator  134  in response to movement of displacement control valve  132 . 
     Controller  54  may embody a single microprocessor or multiple microprocessors that include components for controlling operations of hydraulic system  46  based on input from an operator of machine  10  and based on sensed or other known operational parameters. Numerous commercially available microprocessors can be configured to perform the functions of controller  54 . It should be appreciated that controller  54  could readily be embodied in a general machine microprocessor capable of controlling numerous machine functions. Controller  54  may include a memory, a secondary storage device, a processor, and any other components for running an application. Various other circuits may be associated with controller  54  such as power supply circuitry, signal conditioning circuitry, solenoid driver circuitry, and other types of circuitry. 
       FIG. 8  illustrates an alternative embodiment of hydraulic system  46 . Similar to the embodiment of  FIG. 2 , hydraulic system  46  of  FIG. 8  includes primary circuit  48  and charge circuit  52 . In contrast to the embodiment of  FIG. 2 , however, primary circuit  48  of  FIG. 8  may include an additional resolver  158  associated with each pressure relief valve  66 . In this configuration, resolvers  158  may selectively connect head- and rod-end passages  56 ,  58  at the higher-pressure side of load-holding valves  86 ,  88 , respectively, to the corresponding pressure relief valve  66 . It is contemplated that passages  109  and/or check valves  110  may be omitted from the configuration of  FIG. 8 , if desired. With this configuration, additional protection from pressure spikes may be provided. 
     INDUSTRIAL APPLICABILITY 
     The disclosed hydraulic system may be applicable to any machine where improved hydraulic efficiency and performance is desired. The disclosed hydraulic system may provide for improved efficiency through the use of meterless technology. The disclosed hydraulic system may provide for enhanced performance through the selective use of novel primary and charge circuits. Operation of hydraulic system  46  will now be described. 
     During operation of machine  10 , an operator located within station  16  may command a particular motion of work tool  18  in a desired direction and at a desired velocity by way of interface device  37 . One or more corresponding signals generated by interface device  37  may be provided to controller  54  indicative of the desired motion, along with machine performance information, for example sensor data such a pressure data, position data, speed data, pump displacement data, and other data known in the art. 
     In response to the signals from interface device  37  and based on the machine performance information, controller  54  may generate control signals directed to displacement control valve  132  to move displacement control valve  132  to one of the first-third positions described above. For example, to extend hydraulic cylinder  22  at an increasing speed, controller  54  may generate a control signal that causes displacement control valve  132  to move a greater extent toward the second position, at which a greater amount of pressurized fluid from charge circuit  52  (i.e., from common passage  116 ) may be directed through displacement control valve  132  and into first chamber  136 . The increasing amount of pressurized fluid directed into first chamber  136  may cause movement of displacement actuator  134  that increases a positive displacement of primary pump  50 , such that fluid is discharged from primary pump  50  at a greater rate into head-end passage  56 . At this same time, CPP  96  may be communicated with tank  99  via displacement control valve  132 , such that load-holding valves  86 ,  88  are moved to and/or maintained in their flow-passing positions, thereby allowing the pressurized fluid within head-end passage  56  to enter first chamber  42  and the fluid within second chamber  44  to be drawn back to primary pump  50  via rod-end passage  58 . 
     To retract hydraulic cylinder  22  at an increasing speed, controller  54  may generate a control signal that causes displacement control valve  132  to move a greater extent toward the third position, at which a greater amount of pressurized fluid from charge circuit  52  (i.e., from common passage  116 ) may be directed through displacement control valve  132  and into second chamber  140 . The increasing amount of pressurized fluid directed into second chamber  140  may cause movement of displacement actuator  134  that increases a negative displacement of primary pump  50 , such that fluid is discharged at a greater rate from primary pump  50  into rod-end passage  58 . At this same time, CPP  96  may be communicated with tank  99  via displacement control valve  132 , such that load-holding valves  86 ,  88  are moved to and/or maintained in their flow-passing positions, thereby allowing the pressurized fluid within rod-end passage  58  to enter second chamber  44  and the fluid within first chamber  42  to be drawn back to primary pump  50  via head-end passage  56 . 
     Regeneration of fluid may be possible during retraction operations of hydraulic cylinder  22 , when the pressure of fluid exiting first chamber  42  of hydraulic cylinder  22  is elevated (e.g., during motoring retraction operations). Specifically, during the retracting operation described above, when displacement control valve  132  is in the third position, the fluid of common passage  116  may be connected with regeneration valve  78 . When the charge pressure in communication with regeneration valve  78  creates a force acting on regeneration valve  78  greater than a valve-closing spring-bias, regeneration valve  78  may open and allow pressurized fluid from first chamber  42  to bypass primary pump  50  and flow directly into second chamber  44 . This operation may reduce a load on primary pump  50 , while still satisfying operator demands, thereby increasing an efficiency of machine  10 . 
     When an operator stops requesting movement of hydraulic cylinder  22  (e.g., when the operator releases interface device  37 ), controller  54  may correspondingly signal displacement control valve  132  to move to its first or neutral position. When displacement control valve  132  is in its first position, first and second chambers  136 ,  140  may both be simultaneously exposed to substantially similar pressures (e.g., simultaneously connected to both common and return passages  116 ,  120 ), thereby allowing displacement actuator  134  to center itself and destroke primary pump  50 . At this same time, CPP  96  associated with load-holding valves  86 ,  88  may be blocked from tank  99  via displacement control valve, thereby allowing pressure to build within CPP  96 . As the pressure builds within CPP  96 , load-holding valves  86 ,  88  may eventually be caused to move toward their flow-blocking positions, thereby effectively holding hydraulic cylinder  22  in its current position and hydraulically locking hydraulic cylinder  22  from movement. Operation may be similar when machine  10  is turned off and/or the operator activates a hydraulic lock-out switch (not shown). 
     In the disclosed embodiments of hydraulic system  46 , flow provided by primary pump  50  may be substantially unrestricted such that significant energy is not unnecessarily wasted in the actuation process. Thus, embodiments of the disclosure may provide improved energy usage and conservation. In addition, the meterless operation of hydraulic system  46  may allow for a reduction or even complete elimination of metering valves for controlling fluid flow associated with hydraulic cylinder  22 . This reduction may result in a less complicated and/or less expensive system. 
     The disclosed hydraulic system may provide for stable operation of hydraulic cylinder  22 . Specifically, the disclosed hydraulic system may improve stability of cylinder operation through the use of a restricted primary makeup valve. That is, the restrictions associated with PMV  62  may help to reduce pressure oscillations that occur during makeup operations. These reductions in pressure oscillations may help to stabilize movement of hydraulic cylinder  22 , particularly during transitional operations when hydraulic cylinder  22  is transitioning between resistive and overrunning loads. 
     The disclosed hydraulic system may also provide for enhanced pump overspeed protection. In particular, during overrunning retracting operations of hydraulic cylinder  22 , when fluid exiting first chamber  42  of hydraulic cylinder  22  has elevated pressures, the highly-pressurized fluid may be rerouted back into second chamber  44  of hydraulic cylinder  22  via regeneration valve  78 , without the fluid ever passing through primary pump  50 . Not only does the rerouting help improve machine efficiencies, but the bypassing of primary pump  50  may also reduce a likelihood of primary pump  50  overspeeding. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed hydraulic system. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed hydraulic system. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.