Abstract:
A load sense open center hydraulic system provides efficiency and operator feedback and includes one or more constant flow-open center valves ( 218 ); respective one or more parallel power cores ( 238 ) operatively coupled to the one or more constant flow-open center valves; a variable capacity pump ( 246 ) directly fluidly connected to the one or more parallel power cores; and a pressure compensated flow control fluidly connected between the variable capacity pump and the one or more constant flow-open center valves. The one or more constant flow-open center valves are not fluidly coupled to a fixed capacity pump.

Description:
RELATED APPLICATIONS 
     This application is a national phase of International Application No. PCT/US2014/016902 filed on Feb. 18, 2014 and published in the English language, which claims the benefit of U.S. Provisional Application No. 61/765,231 filed Feb. 15, 2013, the disclosures of each are hereby incorporated herein by reference. 
    
    
     FIELD OF INVENTION 
     The present invention relates to hydraulic valve systems used, for example, in off-road earth moving, construction, and forestry equipment, such as backhoes, log loaders, feller bunchers, wheel loaders, and the like. Hydraulic valve systems are utilized, for example, to cause cylinders to move a boom or bucket loader in a backhoe. The present invention relates to an improved design for such hydraulic valve systems, and more particularly to an efficient open center hydraulic system with feedback. 
     BACKGROUND 
     Open center hydraulic circuits use pumps which supply a continuous flow. The flow is returned to tank through a control valve&#39;s open center; that is, when the control valve spool is centered, it provides an open return path to tank and the fluid is not pumped to a high pressure. Otherwise, if the control valve is actuated, it routes fluid to and from an actuator and tank. The fluid&#39;s pressure will rise to meet any resistance, because the pump has a constant output. If the pressure rises too high, fluid returns to tank through a pressure relief valve. Multiple control valves may be stacked in series. This type of circuit typically uses inexpensive, constant displacement pumps. 
     SUMMARY OF INVENTION 
     The major downside of a constant-flow, open-center system is energy efficiency. Because the pump is fixed, the whole pump flow is always pressurized at the highest function pressure. Throttling losses in the valve, especially at low flow demand (i.e. during slow movements), can be substantial. Competing load sense technology uses variable pumps to adjust the pump flow to function demand, but loses the pressure dependency of the speed of the movement, and therefore the operator does not get any feedback on the forces at the implement being commanded. 
     The present invention takes the advantages of both prior art systems and combines them. 
     According to one aspect of the invention an open center hydraulic system includes one or more constant-flow, open center valves; a parallel power core operatively coupled to the one or more constant-flow, open center valves; a variable capacity pump fluidly connected to and configured to provide pressurized fluid to the one or more constant-flow, open center valves, and fluidly connected to the parallel power core; and a flow restrictor fluidly connected between the variable capacity pump and the one or more constant-flow, open center valves. 
     Optionally, the variable capacity pump is directly fluidly connected to the parallel power core. 
     Optionally, the flow restrictor is a pressure compensated flow control fluidly connected between the variable capacity pump and the one or more constant-flow, open center valves. 
     Optionally, the one or more constant-flow, open center valves include a plurality of serially connected constant-flow, open center valves. 
     Optionally, the one or more constant-flow, open center valves are not fluidly coupled to a fixed capacity pump. 
     Optionally, the pump is a load-sense pump. 
     Optionally, the pump is set up as a remote pressure control pump. 
     Optionally, the flow restrictor is a fixed metering device. 
     Optionally, the flow restrictor is a variable metering device. 
     Optionally, at least one of the one or more constant-flow, open center valves includes a valve spool having notches configured to create a flow restriction thereat such that the pressure in the open center passageway increases with spool stroke. 
     Optionally, at least one of the one or more constant-flow, open center valves includes a valve spool which has associated therewith: (A) a first hydraulic port and a second hydraulic port; (B) a first spool passage between the parallel power core and a first hydraulic port associated with the spool, that is capable of being opened or closed depending upon the position of the spool; (C) a second spool passage between the parallel power core and a second hydraulic port associated with the spool, that is capable of being opened or closed depending upon the position of the spool; (D) a third spool passage between a tank galley and the first hydraulic port associated with the spool, that is capable of being opened or closed depending upon the position of the spool; (E) a fourth spool passage between the tank galley and the second hydraulic port associated with the spool, that is capable of being opened or closed depending upon the position of the spool; (F) a fifth spool passage, wherein an open center core passes through the fifth spool passage, and wherein, depending upon the position of the spool, the spool may permit hydraulic fluid to flow through the fifth spool passage and the open center core in an unrestricted manner, or the spool may partially restrict the hydraulic fluid flowing through the fifth spool passage and the open center core. 
     Optionally, the spool has at least a neutral position, a first non-neutral position, and a second non-neutral position, wherein: (A) in the neutral position, the spool permits hydraulic fluid to flow through the fifth spool passage and the open center core passing therethrough in an unrestricted manner, and the spool blocks the flow of hydraulic fluid through the first spool passage, the second spool passage, the third spool passage, and the fourth spool passage, (B) in the first non-neutral position, the spool partially restricts the flow of hydraulic fluid through the fifth spool passage and the open center core passing therethrough, the spool opens the first spool passage between the power core and the first hydraulic port associated with the spool allowing hydraulic fluid to flow from the power core to the first hydraulic port, the spool opens the fourth spool passage between the tank galley and the second hydraulic port associated with the spool allowing hydraulic fluid to flow from the second hydraulic port to the tank galley, the spool closes the second spool passage between the power core and the second hydraulic port associated with the spool, and the spool closes the third spool passage between the tank galley and the first hydraulic port associated with the spool; and (C) in the second non-neutral position, the spool partially restricts the flow of hydraulic fluid through the fifth spool passage and the open center core passing therethrough, the spool opens the second spool passage between the power core and the second hydraulic port associated with the spool allowing hydraulic fluid to flow from the power core to the second hydraulic port, the spool opens the third spool passage between the tank galley and the first hydraulic port associated with the spool allowing hydraulic fluid to flow from the first hydraulic port to the tank galley, the spool closes the first spool passage between the power core and the first hydraulic port associated with the spool, and the spool closes the fourth spool passage between the tank galley and the second hydraulic port associated with the spool. 
     Optionally, each spool in each of the one or more valves has associated therewith a spool activator, wherein each of said spool activators is capable of causing movement of the spool associated therewith to either a neutral position, a first non-neutral position, or a second non-neutral position. 
     Optionally, the hydraulic system, further includes a signal port associated with the variable displacement pump, wherein an increase in hydraulic fluid pressure received by the signal port cause the variable displacement pump to pump hydraulic fluid at an increased pressure rate, and wherein a decrease in hydraulic fluid pressure received by the signal port causes the variable displacement pump to pump hydraulic fluid at a decreased pressure rate; a sense signal passage hydraulically connecting the open center core and the signal port, wherein the hydraulic connection between the sense signal passage and the open center core is located between the pump and the spool of the first one of the one or more valves downstream of the pump in the open center core; wherein when activation of one or more of the spools of one or more of the valves occurs in a manner causing one or more of the activated spools to be in either a first non-neutral position, or a second non-neutral position, increased hydraulic fluid pressure in the open center core is hydraulically communicated through the sense signal passage to the signal port. 
     According to another aspect of the invention, a valve assembly including a one or more constant-flow, open center valves; an open center valve input port; a parallel power core operatively coupled to the one or more constant-flow, open center valves; and a power core input port separate from the open center valve input port, wherein the parallel power core is not fluidly connected to an open center core of the open center valves downstream of the input ports. 
     Optionally, the valve assembly includes a variable capacity pump fluidly connected to the parallel power core input port and the open center valve input port. 
     Optionally, the variable capacity pump is directly fluidly connected to the parallel power core input port. 
     Optionally, the valve assembly includes a flow restrictor fluidly connected between the variable capacity pump and the one or more constant-flow, open center valves 
     Optionally, the flow restrictor is a pressure compensated flow control fluidly connected between the variable capacity pump and the one or more constant-flow, open center valves. 
     Optionally, the one or more constant-flow, open center valves include a plurality of serially connected constant-flow, open center valves. 
     Optionally, the one or more constant-flow, open center valves are not fluidly coupled to a fixed capacity pump. 
     Optionally, the pump is a load-sense pump. 
     Optionally, the pump is set up as a remote pressure control pump. 
     Optionally, the flow restrictor is a fixed metering device. 
     Optionally, the flow restrictor is a variable metering device. 
     Optionally, at least one of the one or more constant-flow, open center valves includes a valve spool having notches configured to create a flow restriction thereat such that the pressure in the open center passageway increases with spool stroke. 
     Optionally, at least one of the one or more constant-flow, open center valves includes a valve spool having open center notches substantially smaller than power core notches. 
     Optionally, at least one of the one or more constant-flow, open center valves includes a valve spool which has associated therewith: (A) a first hydraulic port and a second hydraulic port; (B) a first spool passage between the parallel power core and a first hydraulic port associated with the spool, that is capable of being opened or closed depending upon the position of the spool; (C) a second spool passage between the parallel power core and a second hydraulic port associated with the spool, that is capable of being opened or closed depending upon the position of the spool; (D) a third spool passage between a tank galley and the first hydraulic port associated with the spool, that is capable of being opened or closed depending upon the position of the spool; (E) a fourth spool passage between the tank galley and the second hydraulic port associated with the spool, that is capable of being opened or closed depending upon the position of the spool; (F) a fifth spool passage, wherein an open center core passes through the fifth spool passage, and wherein, depending upon the position of the spool, the spool may permit hydraulic fluid to flow through the fifth spool passage and the open center core in an unrestricted manner, or the spool may partially restrict the hydraulic fluid flowing through the fifth spool passage and the open center core. 
     Optionally, the spool has at least a neutral position, a first non-neutral position, and a second non-neutral position, wherein: (A) in the neutral position, the spool permits hydraulic fluid to flow through the fifth spool passage and the open center core passing therethrough in an unrestricted manner, and the spool blocks the flow of hydraulic fluid through the first spool passage, the second spool passage, the third spool passage, and the fourth spool passage, (B) in the first non-neutral position, the spool partially restricts the flow of hydraulic fluid through the fifth spool passage and the open center core passing therethrough, the spool opens the first spool passage between the power core and the first hydraulic port associated with the spool allowing hydraulic fluid to flow from the power core to the first hydraulic port, the spool opens the fourth spool passage between the tank galley and the second hydraulic port associated with the spool allowing hydraulic fluid to flow from the second hydraulic port to the tank galley, the spool closes the second spool passage between the power core and the second hydraulic port associated with the spool, and the spool closes the third spool passage between the tank galley and the first hydraulic port associated with the spool; and (C) in the second non-neutral position, the spool partially restricts the flow of hydraulic fluid through the fifth spool passage and the open center core passing therethrough, the spool opens the second spool passage between the power core and the second hydraulic port associated with the spool allowing hydraulic fluid to flow from the power core to the second hydraulic port, the spool opens the third spool passage between the tank galley and the first hydraulic port associated with the spool allowing hydraulic fluid to flow from the first hydraulic port to the tank galley, the spool closes the first spool passage between the power core and the first hydraulic port associated with the spool, and the spool closes the fourth spool passage between the tank galley and the second hydraulic port associated with the spool. 
     Optionally, each spool in each of the one or more valves has associated therewith a spool activator, wherein each of said spool activators is capable of causing movement of the spool associated therewith to either a neutral position, a first non-neutral position, or a second non-neutral position. 
     Optionally, the valve assembly includes a signal port associated with the variable displacement pump, wherein an increase in hydraulic fluid pressure received by the signal port cause the variable displacement pump to pump hydraulic fluid at an increased pressure rate, and wherein a decrease in hydraulic fluid pressure received by the signal port causes the variable displacement pump to pump hydraulic fluid at a decreased pressure rate; a sense signal passage hydraulically connecting the open center core and the signal port, wherein the hydraulic connection between the sense signal passage and the open center core is located between the pump and the spool of the first one of the one or more valves downstream of the pump in the open center core; wherein when activation of one or more of the spools of one or more of the valves occurs in a manner causing one or more of the activated spools to be in either a first non-neutral position, or a second non-neutral position, increased hydraulic fluid pressure in the open center core is hydraulically communicated through the sense signal passage to the signal port. 
     The foregoing and other features of the invention are hereinafter described in greater detail with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic drawing of an embodiment of a conventional open center hydraulic valve system having two valves, eight spools, and eight functions corresponding to the spools. 
         FIG. 2A  is a cross-sectional view illustrating the operation of a spool of a conventional simple CFO valve in the neutral position. 
         FIG. 2B  is a cross-sectional view illustrating the operation of a spool of a conventional simple CFO valve activated in a non-neutral first position to lift a load. 
         FIG. 2C  is a cross-sectional view illustrating the operation of a spool of a conventional simple CFO valve activated in a non-neutral second position to lower a load. 
         FIG. 3  shows a schematic drawing of an exemplary load sense open center hydraulic system having a fixed metering device; and 
         FIG. 4  shows a detail view of an exemplary load sense open center hydraulic system having a pressure compensated flow control metering device. 
         FIG. 5  shows a schematic drawing of an exemplary open center hydraulic system having a remote pressure controlled pump. 
         FIG. 6  shows a detail schematic view of a remote pressure controlled pump. 
     
    
    
     DETAILED DESCRIPTION 
     Conventional open center hydraulic systems  110  include a fixed displacement pump connected to one or more constant-flow, open center (CFO) valve banks as depicted in  FIG. 1  (see also U.S. Patent Application Publication 2009/0025380, the entire contents of which are hereby incorporated herein by reference). The open center hydraulic valve system  110  of  FIG. 1  presently is in common use, for example, in off-road earth moving, construction, and forestry equipment. Major benefits of these systems are the low complexity valve technology, short response times to operator input and pressure dependent movement of functions, allowing the operator to get a better ‘feel’ for the forces at the implement. 
     While variations in the basic design of such an open center hydraulic valve system  110  exist, the fundamental components and operation of such a system are briefly described below. 
     The open center hydraulic valve system  110  of  FIG. 1  typically includes a hydraulic fluid tank  112 , one or more constant flow open center hydraulic valve banks (“valves”)  114 , and a fixed displacement pump  116 . Each valve  114 , in turn, may include one or more spools  118 , with each spool  118  being activated by a spool actuator  120 . The spool actuators  120  may be activated by an equipment operator using a number of known means (not illustrated), such as mechanically (for example, using a lever), electrically (for example, using a solenoid receiving an electrical signal from a switch, a joystick, a computer, or other means), hydraulically, pneumatically, or otherwise. 
     In order to illustrate the operation of a spool  118  to selectively interconnect hydraulic pathways within a valve, a simplified set of drawings illustrating how a spool  118  of a simple constant flow open center (“CFO”) valve  136  is capable of redirecting the constant flow of hydraulic fluid is provided in  FIGS. 2A, 2B, and 2C . There, spool  118  is capable of providing selective hydraulic communication with either of a pair of hydraulic ports  122  and  124 , depending upon the position of spool  118 . The hydraulic ports  122  and  124  are hydraulically connected to a cylinder  126  on either side of a piston  128 . The simple CFO valve  136  has a number of internal hydraulic pathways which permit the spool  118 , depending on its position, to direct hydraulic fluid flow to or from hydraulic ports  122  and  124 . 
     For example, in  FIG. 2A , the spool  118  is in the neutral position. In that position, fixed displacement pump  116  pumps hydraulic fluid at a constant rate through open center core  130 . The spool  118  does not obstruct or restrict the hydraulic fluid flow through the open center core  130 , which proceeds to the tank galley  132 , and then through tank galley  132  to hydraulic fluid tank  112 . The spool  118  in the neutral position blocks the flow of hydraulic fluid to or from hydraulic ports  122  and  124 , on the one hand, and either the open center core  130  or the tank galley  132 , on the other hand. The result is that no net hydraulic fluid flows into or out of cylinder  126  either above or below piston  128 . The piston  128  and associated load  134  do not raise or lower. 
     In  FIG. 2B , on the other hand, spool  118  is caused to move to a first non-neutral position (upward) where spool  118  partially restricts the hydraulic fluid flow provided by fixed displacement pump  116  through open center core  130 , raising the hydraulic pressure of the hydraulic fluid upstream of the spool  118  (i.e., between the spool  118  and the fixed displacement pump  116 ). The spool  118  also opens a hydraulic pathway within the simple CFO valve  136  for net hydraulic fluid to flow from the open center core  130  through hydraulic port  122  into the cylinder  126  below the piston  128 . At the same time, spool  118  opens a hydraulic pathway in simple CFO valve  136  between hydraulic port  124  and the tank galley  132  allowing net hydraulic fluid to flow out of the cylinder  126  above the piston  128  to the tank galley  132  and to hydraulic fluid tank  132 . The result is that there is net hydraulic fluid flow into the cylinder  126  below the piston  128  and out of the cylinder  126  above the piston  128 ; thus, the piston  128  and its associated load  134  is caused to rise. 
     Further, in  FIG. 2C , spool  118  is caused to move to a second non-neutral position (downward), causing spool  118  to partially restrict the hydraulic fluid flow provided by fixed displacement pump  116  through open center core  130 , raising the hydraulic pressure upstream of the spool  118 . The spool  118  opens a hydraulic pathway within the simple CFO valve  136  permitting net hydraulic fluid flow from the open center core  130  through hydraulic port  124  into the cylinder  126  above the piston  128 , while at the same time opening a hydraulic pathway between hydraulic port  122  and tank galley  132  allowing net hydraulic fluid to flow out of the cylinder  126  below the piston  128 . The result is that the piston  128  and its associated load  134  are lowered. 
     The operation of the spool  118  in the system  110  is similar to the operation of the spool  118  in the simple CFO valve  136  described above; however, as illustrated and disclosed in the schematic diagram of  FIG. 1 , the fluid pathways within open center hydraulic valve system  110  that are selectively interconnected by spool  118  differ to a certain extent. 
     Referring once again to the open center hydraulic valve system  110  illustrated in  FIG. 1 , each spool  118  is capable of selective hydraulic communication with a pair of associated hydraulic ports  122  and  124 . Each pair of hydraulic ports  122  and  124 , in turn, may communicate hydraulically with equipment applications (such as a boom on a backhoe) in which the open center hydraulic valve system  110  is used to operate, typically utilizing a cylinder and a piston. The hydraulic ports selectively provide pressurized hydraulic flow to or from the cylinder on either side of the piston. 
     Referring again to  FIG. 1 , each spool  118  of each valve  114 , and, hence, each pair of hydraulic ports  122  and  124  associated with each spool  118 , is associated with a function of the application on the equipment within which the open center hydraulic valve system  110  is utilized. In the example illustrated in  FIG. 1 , one of the spools  118  (and the associated pair of ports  122  and  124 ) is associated with the each of the following functions, which can be found, for example, in a backhoe: boom, bucket, stick, swing, stabilizer, boom loader, bucket loader, and auxiliary. Those functions are chosen for purposes of illustration, and, as would be recognized by skilled practitioners, those functions can vary, depending on the equipment and applications to which the open center hydraulic valve system  110  is assigned. 
     The valves  114  include several hydraulic fluid pathways that may be selectively interconnected by activation of the spool  118 , including an open center core  130 , a power core  138 , and a tank galley  132 . The fixed displacement pump  116  pumps hydraulic fluid (at a constant flow rate for a given engine speed) from the hydraulic fluid tank  112  into the open center core  130 . The tank galley  132  returns hydraulic fluid to the hydraulic fluid tank  112 , where it is available to be re-pumped. The valves  114  also include a hydraulic connection between the open center core  130  and the power core  138 , namely, an open center/power core passage  140  within the valve. Typically, the valves  114  may also include smaller internal valves utilized to prevent, for example, overpressure or incorrect flow direction in the system, such as relief valves  142 , or load drop check valves  144 , which are understood by those having skill in the art, and are not discussed fully for the sake of brevity. 
     The open center hydraulic valve system  110  is typically housed in a standard manifold (not illustrated) attached to the equipment (e.g., construction, earth moving, or forestry equipment, such as a backhoe) in which the open center hydraulic valve system  110  is being used. The fixed displacement pump  116  is typically driven by a power take-off (not illustrated), which, in turn, is directly mounted to a transmission (not illustrated), which is connected to the prime mover (for example, an internal combustion engine) of the equipment in which the open center hydraulic valve system  110  is being used. 
     The operation of the spools  118  in each of the valves  114  to direct hydraulic fluid flow to and to permit fluid flow from associated hydraulic ports  122  and  124  to cause, for example, a piston to move within a cylinder and thereby cause movement of a functional aspect of the equipment on which the open center hydraulic valve  110  is mounted, is well-known to skilled practitioners, and can be ascertained by skilled practitioners by reference solely to the schematic diagram found in  FIG. 1 . For purposes of the following explanation, hydraulic ports  122  and  124  will be assumed to be hydraulically connected to a cylinder  126  above and below a piston  128 , respectively, in a manner similar to that illustrated in  FIGS. 2A, 2B, and 2C . 
     As can be seen in  FIG. 1 , and will be described further below, when a spool  118  is caused by spool actuator  120  to be in the neutral position (with the open center core  130  unrestricted by the spool  118 , and the fluid passageways between either the open center core  130  or the tank galley  132 , on the one hand, and the pair of hydraulic ports  122  and  124  associated with the spool  118 , on the other hand, being obstructed by the spool  118 , no net hydraulic fluid flows to or from the hydraulic ports  122  and  124  to the cylinder  126  on either side of the piston  128 , and thus, the piston  128  does not move. Instead, the hydraulic fluid delivered at a constant flow rate (for a given engine speed) by the fixed displacement pump  116  flows unrestricted through the open center core  130  and through the open center of the spools  118  to the tank galley  132  and to the hydraulic fluid tank  112  where it is re-pumped. Hence, the function to which the piston  128  and cylinder  126  is associated (e.g., the position of the boom) does not change, because there is no net change in hydraulic fluid in the cylinder  126  either above or below the piston  128 . The piston  128  therefore does not move. 
     If, as shown in  FIG. 1 , the spool actuator  120  is activated by an operator to cause the spool  118  to move from the neutral position to a first non-neutral position, the constant flow of hydraulic fluid delivered by the fixed displacement pump  116  is caused by the partial restriction by the spool  118  of the open center core  130  to increase in pressure. Referring to  FIG. 1 , the increase in fluid pressure in the open center core  130  is communicated to the power core  138  through the open center/power core passage  140 . As shown in  FIG. 1 , the activated spool  118  allows pressurized hydraulic fluid to flow from the power core  138  to the first hydraulic port  122  associated with the activated spool  118  into the cylinder  126  under the piston  128 . The activated spool  118  simultaneously allows fluid to flow out of the cylinder  126  through the second hydraulic port  124  associated with the activated spool  118  which is connected above the piston  128 . That fluid flows through the tank galley  132  to the hydraulic fluid tank  112  (where it is re-pumped). Thus, the net effect is that hydraulic fluid under pressure flows into the cylinder  126  below the piston  128 , and hydraulic fluid flows out of the cylinder  126  above the piston  128 . This causes the piston  128  and associated load  134  to rise and the function to change (e.g., it causes the boom and any associated load to rise). 
     On the other hand, if, as shown in  FIG. 1 , the spool operator manipulates the actuator  120  to cause the spool  118  to move from the neutral position to a second non-neutral position, that once again causes partial restriction of the open center  130 , and causes the fluid flowing through the open center core  130  to increase in pressure. That increase in hydraulic pressure is once again communicated from the open center core  130  to the power core  138  through open center/power core passage  140 . At the same time, hydraulic fluid is allowed by the activated spool  118  to flow out of the cylinder  126  under the piston  128  through the connected hydraulic port  122  associated with activated spool  118  and through the tank galley  132  to the hydraulic fluid tank  112 . Also at the same time, the spool directs pressurized fluid (under pressure from the fixed displacement pump  116  due to partial restriction of the opening in the open center core  130  by the spool  118 ) to flow from the power core  138  through the associated hydraulic port  124  into the cylinder  126  above the piston  128 . Thus, hydraulic fluid under pressure is introduced to the cylinder  126  above the piston  128 , and hydraulic fluid is drained from the cylinder  126  below the piston  128 . This causes the piston  128  to lower and the equipment function to change (e.g., the boom and any associated load is caused to lower). A skilled artisan would recognize, of course, that this activation of spools  118  in the valves  114  can be utilized to operate a number of different equipment functions having moving components, and would not be limited to booms (or to backhoes). 
     Further details of the operation of the open center hydraulic valve system  110  illustrated in  FIG. 1  are described below. The explanation herein concerning the operation of a single spool  118  (and its associated pair of hydraulic ports  122  and  124 ) within a single valve  114  associated with a particular single function is illustrative, and is not limited to that particular single spool  118  or valve  114 , and applies to other spools  118  and valves  114  within the open center hydraulic valve system  110  as well. 
     Because the pump for the open center hydraulic system  110  is a fixed displacement pump  116 , the flow of the hydraulic fluid supplied by the fixed displacement pump  116  is constant for a given engine speed for the equipment in which the system  110  is mounted. 
     When the spool actuators  120  in the valves  114  in the open center hydraulic system  110  are in the neutral position, all of the associated spools  118  are likewise in the neutral position. As illustrated in  FIG. 1 , the centers of the valve spools  118  are open, the net flow paths to the associated hydraulic ports  122  and  124  (from the open center core  130  or the power core  138 ), or from the hydraulic ports  122  and  124  (to the tank galley  132 ), are blocked by the spools  118 , and all net hydraulic fluid flow pumped by the fixed displacement pump  116  from the hydraulic fluid tank  112  at a constant flow rate flows unrestricted through the open center core  130  through the spools  118  to the tank galley  132  and then back to the hydraulic fluid tank  112  where it is again available to be pumped. 
     When one of the functions associated with the open center hydraulic system  110  is desired to be activated, the spool actuator  120  associated with that function is activated by an equipment operator in order to move the associated spool  118  (left or right, as shown in the schematic in  FIG. 1 ) in order to partially restrict or “pinch” the opening through the open center core  130  to the tank galley  132 . This partial restriction of hydraulic fluid flow by the spool  118  in the open center core  130  partially restricts flow to the tank galley  132 , and, in turn, increases the pressure of the hydraulic fluid in the open center core  130  being provided at constant flow by the fixed displacement pump  116 . The resulting increased hydraulic fluid pressure in the open center core  130  is transmitted hydraulically through the open center/power core passage  140  to the power core  138 . 
     If the chosen spool actuator  120  is activated with the intention of causing the piston  128  to move to a first non-neutral position as illustrated in  FIG. 1  (and to thereby, for example, lift a boom and associated load), then not only is the open center core  130  partially restricted to cause an increase in pressure to occur in the open center core  130  and be transmitted to the power core  138 , but the spool  118  at the same time opens a hydraulic passage in the valve  114  between associated hydraulic port  122  (hydraulically connected to a cylinder  126  below the piston  128 , in the manner illustrated in  FIG. 2B ) and the power core  138 . The hydraulic fluid, having increased hydraulic pressure in the power core  138 , is transmitted through associated hydraulic port  122 . Simultaneously, activated spool  118  opens a hydraulic passage in the valve  114  between associated hydraulic port  124  (hydraulically connected to a cylinder  126  above the piston  128 , in the manner illustrated in  FIG. 2B ) and the tank galley  132 . The result is that hydraulic fluid under pressure from the power core  138  flows through associated hydraulic port  122  and begins filling the cylinder  126  below the piston  128 , and hydraulic fluid is permitted to leave the cylinder  126  above the piston  128  by flowing through associated hydraulic port  124  into the tank galley  132  to return to the hydraulic fluid tank  112 , where it is available to be re-pumped. By adding pressurized hydraulic fluid to the cylinder  126  below the piston  128 , and by reducing hydraulic fluid in the cylinder  126  above the piston  128 , the piston  128  and its associated load  134  is lifted. 
     Conversely, if the chosen spool actuator  120  is activated with the intention of causing the piston to move to a second non-neutral position as illustrated in  FIG. 1 , (and to, for example, cause a boom to lower), then not only does the activated spool  118  cause the open center core  130  to be partially restricted to cause an increase in fluid pressure in the open center core  130  to be hydraulically transmitted to the power core  138  via open center/power core passage  140 , but also the activated spool  118  opens a hydraulic passage in the valve  114  between the associated hydraulic port  124  (hydraulically connected to cylinder  126  above the piston  128 ) and the power core  138  (with pressurized hydraulic fluid). Simultaneously, the activated spool  118  opens a passage in valve  114  between associated hydraulic port  122  (hydraulically connected to cylinder  126  below the piston  128 , in the manner illustrated in  FIG. 2C ) and the tank galley  132 , allowing hydraulic fluid to flow out of the cylinder  126  below the piston  128  to the tank galley  132  and the hydraulic fluid tank  112 . The result is that hydraulic fluid under pressure from the power core  138  begins filling the cylinder  126  above the piston  128 , and hydraulic fluid begins leaving the cylinder  126  below the piston  128 . The piston  128  and its associated load  134  lowers (in this example, the boom and load is lowered). 
     Because the open center hydraulic valve system  110  illustrated in  FIG. 1  utilizes a fixed displacement pump  116  operating at a constant flow for a given engine speed for the equipment on which it is mounted, all power used to generate unused hydraulic fluid flow (such as hydraulic fluid constantly flowing through the open center core  130  when the spools  118  are in the neutral position) is a loss. Nevertheless, the size and power of the fixed displacement pump  116  in such a system must accommodate not only sufficient hydraulic flow and system pressure to operate the multiple functions operated by the valves  114  at rated load conditions, but also must sustain the constant hydraulic flow through the open center core  130  (as well as overcome line losses) in order for the system to operate properly. A relatively large and powerful fixed displacement pump  116  running constantly is therefore required for the open center hydraulic valve system  110 . And, as noted above, a considerable portion of the power of the fixed displacement pump  116  in such a system is required to deliver hydraulic fluid flow that is frequently unused by the functions of the system, for example, the unused flow that constantly passes through the open center core  130  to the hydraulic fluid tank  112 , only to be re-pumped (when one or more, often all, spools  118  are not activated and the functions are idle). Hence, significant inefficiencies are inherent in the open center hydraulic valve system  110 . 
     A number of factors have spurred equipment manufacturers and hydraulic systems designers to attempt to overcome the inefficiencies and shortcomings of the hydraulic valve systems, including open center hydraulic valve system  110 . New emissions standards and a desire for fuel savings have caused designers and manufacturers to attempt to design equipment and hydraulic systems that are more fuel efficient, and more power efficient, by achieving greater horsepower management. Manufacturers and designers likewise desire to avoid significant increases in the size, weight, and expense of providing alternatives to the prior art systems. Since the pump of the system  110  described above is fixed, the whole pump flow is always pressurized at the highest function pressure. Throttling losses in the valve, especially at low flow demand (i.e. during slow movements), can be substantial. 
     One potential alternative is to replace the fixed displacement pump  116  of the open center hydraulic valve system  110  illustrated in  FIG. 1  with a variable displacement piston pump. In such a potential alternative, however, the existing valves  114  in the open center hydraulic valve system  110  would be required to be replaced by considerably larger, considerably heavier, and considerably more expensive valves in order to permit the higher hydraulic fluid flow required by such a replacement. Such a potential alternative would be cost prohibitive, and the installation of such a large, heavy system would be highly undesirable because, in many if not most applications, there is limited room available on equipment for the hydraulic system to be mounted. 
     Competing load sense technology uses variable pumps to adjust the pump flow to function demand, but loses the pressure dependency of the speed of the movement; therefore the operator no longer get any feedback on the forces at the implement. 
     The invention combines the advantages of conventional CFO systems with the advantages of load sense systems. 
     An exemplary hydraulic system  210  is illustrated schematically in  FIGS. 3 and 4  in a manner using schematic symbols that would be understood by persons skilled in the art. The hydraulic system  210  shares similarities with the above-referenced system  110 , and consequently the same reference numerals but indexed by 100 are used to denote structures corresponding to similar structures in the system. In addition, the foregoing description of the system  110  is equally applicable to the system  210  except as noted below. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the systems may be substituted for one another or used in conjunction with one another where applicable. 
     The exemplary hydraulic system  210  includes a hydraulic fluid tank  212 , and one or more standard open center hydraulic valve banks  214  modified in the manner described and illustrated herein (“fixed/variable valves”). Each fixed/variable valve  214  may include one or more spools  218 , each activated by an associated spool actuator  220 . As previously discussed, the spool actuator  220  may be activated by an operator using a variety of known means, including mechanically, electrically, hydraulically, pneumatically, or otherwise. 
     In the exemplary hydraulic system  210 , the fixed displacement pump of the system  110  is replaced by a variable capacity (e.g., variable displacement piston type) load sense type pump  246 . 
     The system  210  of the present invention may be housed in a standard manifold (not illustrated) attached to the equipment (e.g., off-road construction, earth moving, or forestry equipment—not illustrated) in which the system  210  is being used. The variable displacement pump  246  may be driven by a power take-off (not illustrated), which, in turn, is mounted to a transmission (not illustrated) connected to the prime mover of the equipment. 
     Each spool  218  of the system  210  operates in the same manner as described above for spools  118  in the conventional open center hydraulic valve system  110  to provide selective hydraulic communication with hydraulic ports  222  and  224  associated with each spool  218 . In a typical application of the invention, each pair of hydraulic ports  222  and  224  communicate hydraulically with a cylinder on opposite sides of a piston to cause piston movement, in a manner similar to that described above for the open center hydraulic valve system  110 . In order to prevent undue repetition, to serve the function of brevity, and to avoid belaboring what is known to skilled practitioners in the art, the operation of the hydraulic ports  222  and  224  hydraulically connected to a cylinder on either side of a load-supporting piston in the fixed/variable hybrid system  210  is the same as explained and illustrated for hydraulic ports  122  and  124  hydraulically connected to the cylinder  126  on either side of piston  128 , and load  134  in the prior art open center hydraulic valve system  110  previously described and illustrated (see, e.g.,  FIGS. 1, 2A, 2B, and 2C ) except as noted herein. 
     The variable capacity pump is tied directly only to the CFO valve&#39;s parallel power core. The CFO valve is configured without any connection or passageway between the power core and open center core. The CFO valves open center passageway is also supplied with flow from the load sense pump, but indirectly through a fixed or variable (to easily adjust the control gains) metering device  250 . This may be, for example, a pressure compensated flow control. As a load sense pump works to maintain a set pressure differential between its outlet port and its load sense signal port, a constant, yet small, amount of flow is allowed to pass through the CFO open center passageway. This amount is a signal flow and is small compared to the amount passing through the parallel power core, for example, less than about 30% of main flow. Preferably, the signal flow is about 1-10 liters per minute versus. 
     The metering device, for example the pressure compensated flow control  250 ′ as shown in  FIG. 4 , is set to allow a constant, yet small, amount of flow to pass through the CFO open center passageway. The notches of the valve spools restricting the flow through the open center passageway are cut specifically to create a restriction in the open center passageway so that the pressure in the open center passageway increases with spool stroke. The notches in the open center are substantially smaller than the parallel power core notches to provide the reduced signal flow, and are preferably less than 10%, and more preferably about 1-4% the size of the parallel power core notches. 
     As the load sense line of the load sense pump is connected to the open center passageway as well, any spool stroke increases the pressure command to the load sense pump. The load sense pump&#39;s control will change its displacement to match the pressure in the load sense line plus a certain margin pressure at its outlet port. (That is, unless the load sense pump is saturated, i.e. required to provide more flow than possible at maximum displacement.) 
     The flow for the individual functions is then determined by the design of the spools in the parallel power core, the load pressure for the individual functions and the spool position of all sections providing an operator experience very similar to the open center ‘feel’. However since the variable pump adjusts its displacement automatically to provide just the required pressure created by the restriction in the open center passageway, no excess flow is wasted. 
     Turning now to  FIG. 5 , an exemplary embodiment of the hydraulic system is shown at  310 . The system  310  is substantially the same as the above-referenced system  210 , and consequently the same reference numerals but indexed by 100 are used to denote structures corresponding to similar structures in the system. In addition, the foregoing description of the system  210  is equally applicable to the system  310  except as noted below. Moreover, it will be appreciated upon reading and understanding the specification that aspects of the systems may be substituted for one another or used in conjunction with one another where applicable. 
     The same or similar functionality as described above with reference to the system  210  can also be achieved by using a pump set up as a remote pressure control pump as shown in  FIG. 5 . 
     In particular,  FIG. 6  shows an example of a schematic of this control option operable with most variable pumps. As shown, a small orifice  350  is integrated into the pump supplying flow to the control port (V), which may be connected to the open center passageway of the CFO valve. In this case, no external metering device needs to be plumbed into the circuit. 
     As the signal line of the remote pressure controlled pump  346  is connected to the open center passageway as well, any spool stroke increases the pressure command to the remote pressure controlled pump. The remote pressure controlled pump&#39;s control will change its displacement to match the pressure in the signal line at its outlet port. (That is, unless the pump is saturated, i.e. required to provide more flow than possible at maximum displacement.) 
     As with the load sense system  210 , since the remote pressure controlled pump adjusts its displacement automatically to provide just the required pressure created by the restriction in the open center passageway, no excess flow is wasted. 
     Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.