Abstract:
Directional control valves are arranged with restrictable center passageways connected in series to a fixed displacement pump and with restrictable power and exhaust passageways straddling loads connected in parallel to the same fixed displacement pump. Pressure responsive valves located between the loads and the restrictable exhaust passageways reduce interactions between the loads. Another pressure responsive valve located between the fixed displacement pump and the restrictable center passageways maintains an appropriate division of flow between the restrictable center passageways and the restrictable power and exhaust passageways.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/070,509, filed on Jan. 6, 1998, which provisional application is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The invention relates to open center hydraulic systems, which are generally noted for ruggedness, simplicity, low cost, tolerance of dirt, and ease of service. 
     BACKGROUND 
     Open center hydraulic systems generally comprise a fluid power source, such as a fixed displacement pump, having a low-pressure side and a high-pressure side. A reservoir is connected to the low-pressure side to supply the fixed displacement pump with fluid. One or more loads, such as hydraulic cylinders or motors, regulated by open center directional control valves are connected to the high-pressure side to utilize the fluid power generated by the fixed displacement pump. The fluid flow path is from the fixed displacement pump, which draws fluid from the reservoir through any one or more of the open center directional control valves and their associated loads before returning to the reservoir. 
     The open center directional control valves are typically spool valves having one normally open orifice (NO 1 ) in parallel with a pair of normally closed orifices (NC 2  and NC 3 ) that straddle a load. Two directions of flow control through the load (e.g., forward and reverse) require a second pair of normally closed orifices (NC 4  and NC 5 ). A common spool regulates flow through all of the orifices. One direction of spool movement gradually closes the normally open orifice NO 1  of a center core passage and gradually opens the normally closed orifice NC 2  of a power core passage as well as the normally closed orifice NC 3  of an exhaust core passage. An opposite direction of spool movement also gradually closes the normally open orifice NO 1  of the center core passage while gradually opening the normally closed orifice NC 4  of another power core passage and the normally closed orifice NC 5  of another exhaust core passage. Additional open center spool valves for controlling other loads can be arranged as individual valves or multiple valve sections that are stacked or sandwiched together as a single unit. 
     The valve spool usually contains longitudinal machined or pressed slots called metering notches which provide for a more gradual opening or closing of the spool orifices. Many different styles of metering notches are incorporated by manufacturers in order to achieve gradual flow control, especially at low flow rates. Some valve spools have metering notches in communication with the normally open orifice NO 1  as well as both normally closed orifices NC 2  and NC 3 , and others have metering notches only in communication with the normally open orifice NO 1  of the center core passage and the normally closed orifices NC 3  of the exhaust core orifice. The degree of spool valve overlap also varies from manufacturer to manufacturer. 
     Each load is connected to a separate branch line controlled by one of the spool valves. With all of the spool valves in the neutral or off position, fluid flows virtually unrestricted from the fixed displacement pump through the normally open orifices NO 1  of the center core passages and back to the reservoir. Shifting the directional valves from neutral toward one direction or the other gradually restricts flows through the normally open orifices NO 1  of the central core passages and pressurizes the power core passages. Further spool movement gradually opens the normally closed orifices NC 2  or NC 4  of the power core passages permitting flows to the loads (e.g., cylinders or motors). Return flows from the loads encounter the normally closed orifices NC 3  and NC 5  of the exhaust core passages, which are also gradually opened by yet further movement of the spool to allow return flows to the reservoir. Since the normally closed orifices NC 3  and NC 5  of the exhaust core passages provide the final restriction of flows returning from the loads to the reservoir, these orifices (NC 3  and NC 5 ) are primarily responsible for regulating the operating speeds of the loads. 
     The central core passages of the multiple spool valves are connected in series to the fixed displacement pump, while each of the power core passages of the same spool valves are connected in parallel to the fixed displacement pump. Thus, closing any one of the normally open orifices NO 1  in series creates the potential for pressure in all of the power core passages, including the power core passage whose normally closed orifice NC 2  or NC 4  is gradually opened by further movement of the same spool valve. 
     At any given setting combination of the spool valves, the pressure in the power core passages, which is substantially equivalent to the output pressure of the fixed displacement pump (i.e., the system pressure), varies as a function of the total flow resistance in the load branches. For example, a maximum system pressure occurs at a given setting combination of the spool valves when the load flow resistance is high enough to force all of the fixed amount of flow from the pump through the normally open orifices NO 1  of the center core passages to the reservoir. Any flow through the load branches (i.e., through either pair of normally closed orifices NC 2 , NC 3  or NC 4 , NC 5  of the power and exhaust core passages) to the reservoir reduces the potential system pressure from this maximum for the given valve setting combination. A minimum system pressure occurs at the same valve setting combination when the load flow resistance in the load branches is also at a minimum, permitting a maximum flow through the load branches to the reservoir. Predetermined flow rates cannot be established for any given setting of the individual spool valves; because variations in the load flow resistance alters system pressure, which affects the total amount of flow through the branch lines. System pressure and the resulting flow through the branch lines are also affected by variations in the valve settings. 
     In addition, when two or more spool valves are operated simultaneously, the flow rate through any one load branch can be affected by variations in the load flow resistance of another load branch. At a given system pressure, the load branch exhibiting the relatively decreased load resistance will receive more flow unless its spool valve is returned toward a more neutral position (i.e., “throttled back”). These instabilities make open center hydraulic systems difficult to control. Operators of open center systems often complain of a lack of fine metering control and high forces at valve control levers that further interfere with operator control and contribute to operator fatigue. 
     Attempts have been made to improve the flow control characteristics of open center valves by reducing variations in the bypass flow rate. See, for example, U.S. Pat. No. 4,139,021 to Ailshie et al. and U.S. Pat. No. 4,178,962 to Tennis. However, neither of these patents address the problem of flow instability caused by concurrently operating loads. 
     SUMMARY OF INVENTION 
     My invention reduces flow instabilities caused by interactions among concurrent loads in different load branches as well as load variations within individual branches of open center systems. A mechanism, which can be referred to as “meter-out pressure compensation” is used to relate directional control valve positions to more stable flow rates through the load branches. Another mechanism referred to as “bypass pressure compensation” can be used to supplement and further enhance the meter-out pressure compensation. 
     According to one embodiment, the open center system includes the usual features of a reservoir, a fixed displacement pump, and a plurality of directional control valves that direct flow between a plurality of load branches and a bypass line to the reservoir. Each load branch is also connected to the reservoir. The directional control valves are preferably spool valves each having a normally open orifice NO 1  along a center core passage, at least one pair of normally closed orifices NC 2  and NC 3  along respective power core and exhaust core passages that straddle a load, and a common spool that is movable for adjusting the sizes of the three orifices NO 1 , NC 2 , and NC 3 . The center core passages of the spool valves are arranged in series along the bypass line between the fixed displacement pump and the reservoir, and the power core and exhaust core passages of each spool valve are together arranged in parallel with the power core and exhaust core passages of the other spool valves between the fixed displacement pump and the reservoir. 
     To reduce flow instabilities in accordance with my invention, each of the load branches is fitted with a branch pressure reducing valve between the load and the normally closed orifice NC 3  of the exhaust core passage. Sensing lines of the branch pressure reducing valve straddle the normally closed orifice NC 3  and work in connection with an adjustable bias to progressively close the branch pressure reducing valve above a setpoint differential pressure. 
     The objective is to maintain a constant pressure across the normally closed orifice NC 3  of the exhaust core passage so that any one position of the spool commands a constant flow rate from the load to the reservoir regardless of variations in system pressure or load flow resistance variation. An increasing pressure differential across the normally closed orifice NC 3  above the setpoint closes the branch pressure reducing valve to prevent an unwanted increase in flow through the normally closed orifice NC 3 . A decreasing pressure differential across the normally closed orifice NC 3  below the setpoint opens the branch pressure reducing valve to prevent an unwanted decrease in flow through the normally closed orifice NC 3 . 
     The constant pressure differential across the normally closed orifice NC 3  is preferably maintained throughout most of the range of spool valve positions to hold the upstream load at a constant speed regardless of its flow resistance, the flow resistance of loads in other branches, or other effects on the output pressure of the fixed displacement pump. However, at spool valve positions approaching “full throttle” (i.e., wide open settings of the normally closed valves NC 2  and NC 3 ), the branch pressure reducing valve preferably has little or no effect on the load speed. A control orifice of the branch pressure reducing valve is sized so that at its wide open setting, little or no restriction to flow is exhibited. Thus, the branch pressure reducing valve preferably supports fine speed control at slow to moderate load speeds, where such control is most needed, yet permits full flow at higher load speeds to maintain a full range of possible load speeds. 
     A proper setting of the setpoint differential pressure is important to achieving the desired operation of the branch pressure reducing valve. Too high a setpoint differential pressure renders the branch pressure reducing valve ineffective throughout most, if not all, of the range of fluid flow rates through the affected load branch. Too low a setpoint differential pressure can limit the range of fluid flow rates (i.e., limit the maximum load speed) and can produce excessive back pressure in the system, which reduces efficiency. Thus, the branch pressure reducing valve is preferably limited to restricting flow to only when the normally closed orifice NC 3  of the exhaust core passage is also restricting flow to regulate load speed within a range less than its maximum speed at full flow. 
     The branch pressure reducing valves most effectively compensate for momentary load flow resistance decreases, because the branch pressure reducing valves are designed to restrict excess flows through the normally closed orifices NC 3 . Momentary increases in load flow resistance can temporarily reduce exhaust flows from the loads, resulting in insufficient flows through the normally closed orifices NC 3 . Accordingly, another embodiment of my invention provides an additional bypass pressure reducing valve located along the bypass line just upstream of the normally open orifices NO 1 . Sensing lines of the bypass pressure reducing valve straddle the series of normally open orifices NO 1  and work in connection with an adjustable bias to progressively close the bypass pressure reducing valve above a setpoint differential pressure. 
     The objective of the bypass pressure reducing valve is to prevent variations in the total flow resistance of the load branches from affecting the division of flow between the bypass line and the multiple load branches. The constant pressure drop across the series of normally open orifices NO 1  equates individual positions of the spool valves to fixed amounts of flow through the bypass line to the reservoir. Since the output flow of the pump is fixed, a fixed amount of remaining flow is also forced through the load branches. 
     The setpoint differential pressure of the bypass pressure reducing valve is also preferably set in relation to the characteristic pressure profile of the system to cover a range of normal operations. Set too low, the bypass pressure reducing valve wastes energy. Set too high, the bypass pressure reducing valve has too little effect on flows through the bypass line. 
     The bypass pressure reducing valve enhances the performance of the branch pressure reducing valves in two main respects. First, a momentary increase in the total flow resistance of the load branches, which would normally force a larger percentage of the flow through the bypass line and reduce the combined flow through the load branches, is balanced by an additional restriction in the bypass line to maintain the same level of flow through the load branches. This assures adequate flow through the normally closed orifices NC 3  so that the branch pressure reducing valves can continue to carry out their meter-out pressure compensating function. 
     Second, a momentary decrease in the total flow resistance to the load branches, which would normally force a smaller percentage of the flow through the bypass line and increase the combined flow through the load branches, is balanced by a reduced restriction in the bypass line to maintain the same level of flow through the branches. This reduces the amount of restriction and resulting back pressure against the loads required of the branch pressure reducing valves to maintain the desired flow rates through the normally closed orifices NC 3 . With the addition of the bypass pressure reducing valve, the main remaining tasks of the branch pressure reducing valves involve compensating for changes in the pattern of load flow resistance among the loads and compensating for changes in system pressure accompanying the operation of the spool valves. 
     While operation of the branch and bypass pressure reducing valves are desirable under many circumstances to manage flow instabilities, both activation and deactivation of these valves can be controlled by the addition of control valves that can be operated to interfere with the sensing of setpoint conditions. For example, shut-off valves can be located in the sensing lines approaching the reservoir for developing back pressures that prevent the setpoint conditions of the pressure reducing valves from being achieved. 
    
    
     DRAWINGS 
     FIG. 1 is a circuit diagram of an open center hydraulic system containing branch pressure reducing valves to reduce flow instabilities in load branch lines. Directional control valves are depicted as three separately controllable orifices linked by a mechanical arm to more clearly represent their separate functions. 
     FIG. 2 is a similar diagram in which a bypass pressure reducing valve has been added to a bypass line for controlling a division of flow between the bypass line and the branch lines. 
     FIG. 3 is a circuit diagram of an alternative open center hydraulic system containing both bypass and branch pressure reducing valves. Conventional symbols are used to represent the directional control valves, which control forward and reverse directions of flow through the loads. 
    
    
     DETAILED DESCRIPTION 
     The open center hydraulic system  10  of FIG. 1 includes a fixed displacement pump  12  driven by a motor  14  for drawing fluid from a reservoir  16  and for pumping the fluid at a fixed rate along a common supply line  18  that splits into three load branches  20   a ,  20   b , and  20   c , as well as a common bypass line  22 . Three normally open control orifices NO 1a , NO 1b , and NO 1c  interrupt the common bypass line  22  that returns fluid to the reservoir  16 . 
     The normally open control orifices NO 1a , NO 1b , and NO 1c  are mechanically linked by control arms Ma, Mb, and Mc to respective pairs of normally closed control orifices NC 2a  and NC 3a , NC 2b  and NC 3b , and NC 2c , and NC 3c  that straddle respective loads La, Lb, and Lc. The loads La and Lc are depicted as hydraulic cylinders, and the load Lb is depicted as a hydraulic motor. Ordinarily, the one normally open control orifice (e.g., NO 1a ) and the two normally closed orifices (e.g., NC 2a  and NC 3a ) associated with each branch  20   a ,  20   b , and  20   c  are incorporated into respective directional control valves, such as spool valves, but FIG. 1 depicts these control orifices as discrete components to better illustrate their individual functions. 
     Initially, all of the fixed rate flow from the pump  12  is returned to the reservoir along the bypass line  22 . Little system pressure is developed to oppose the flow. However, adjusting any of the control arms Ma, Mb, and Mc to progressively close one of the normally open control orifices NO 1a , NO 1b , or NO 1c  resists the flow of fluid along the bypass line  22  and develops a system pressure reaching into the three branch lines  20   a ,  20   b , and  20   c . Further movement of the control arms Ma, Mb, or Mc progressively opens the normally closed control orifices NC 2a , NC 2b , or NC 2c  for releasing a portion of the flow to the loads La, Lb, or Lc. Movement of the loads La, Lb, or Lc enables fluid to reach the normally closed control orifices NC 3a , NC 3b , or NC 3c , which are progressively opened by yet further movement of the control arms Ma, Mb, or Mc for returning the fluid to the reservoir  16  along a common return line  24 . 
     The normally closed control orifices NC 3a , NC 3b , and NC 3c  provide so-called “meter-out” functions for controlling the load speed. In prior designs, any one position of the control arms Ma, Mb, or Mc could result in a range of load speeds depending on the system pressure and the load resistance in the load branches  20   a ,  20   b , and  20   c . This flow instability can be corrected by positioning branch pressure reducing valves  26   a ,  26   b , and  26   c  just upstream of the normally closed control orifices NC 3a , NC 3b , and NC 3c . Pairs of pressure sensing lines  28   a  and  30   a ,  28   b  and  30   b , and  28   c  and  30   c  straddle the normally closed control orifices NC 3a , NC 3b , and NC 3c  to provide feedback pressures to the branch pressure reducing valves  26   a ,  26   b , and  26   c.    
     The branch pressure reducing valves  26   a ,  26   b , and  26   c  are biased at setpoint differential pressures to maintain constant pressure differences across the normally closed control orifices NC 3a , NC 3b , and NC 3c . By eliminating variability in differential pressure across the normally closed control orifices NC 3a , NC 3b , and NC 3c , each different size opening of the normally closed control orifices NC 3a , NC 3b , and NC 3c  commands a specific flow rate through the normally closed control orifices NC 3a , NC 3b , and NC 3c  regardless of the system pressure upstream of the branch pressure reducing valves  26   a ,  26   b , and  26   c.    
     The proper setpoint for the differential pressure can be determined in comparison to its effect on the overall system pressure at the fixed displacement pump  12 . In the no load condition (i.e., no load flow resistance), each load branch  20   a ,  20   b , and  20   c  exhibits a characteristic system pressure profile throughout its range of operation (i.e., range of spool travel). Starting at neutral in a typical open center hydraulic system, the system pressure tends to increase with spool travel to a level pressure before decreasing to a minimum pressure approaching the end of spool travel. The setpoint differential pressure of the branch pressure reducing valves  26   a ,  26   b , and  26   c  can be adjusted to only slightly raise the level or peak system pressure during a first portion of the spool travel, while having no affect on the minimum system pressure near the end of spool travel. 
     Alternatively, the setpoint differential pressure can be determined with a similar effect in comparison to the characteristic pressure drops that occur across the normally closed control orifices NC 3a , NC 3b , and NC 3c  throughout the range of spool travel. Typically, the pressure drop parallels the change in system pressure by rising to a level with increasing spool travel before falling off toward the end of spool travel. In this instance, the setpoint differential pressure is set at a differential pressure that is less than the maximum pressure drop within the range of spool travel but more than the minimum pressure drop associated with the end of spool travel. As a result, the branch pressure reducing valves  26   a ,  26   b , and  26   c  permit the normally closed control orifices NC 3a , NC 3b , and NC 3c  to exhibit fine metering-out control over load speeds independent of system pressure fluctuations or load flow resistance throughout a range of load speeds without interfering with the maximum load speeds attainable by the system. The characteristic pressure profiles of the load branches can also be changed to take better advantage of the setpoint differential pressure controls, such as by modifying the opening and closing relationships among the normally open control orifice NO 1  and the two normally closed orifices NC 2  and NC 3  in each branch. 
     FIG. 2 depicts a similar open center hydraulic system  40 . Components in common with the open center hydraulic system  10  are labeled with like reference numerals and will not be described further. The hydraulic system  40  differs by the addition of a bypass pressure reducing valve  42  that can be connected to the bypass line  22  upstream of the three normally open control orifices NO 1a , NO 1b , and NO 1c . Pressure sensing lines  44  and  46  communicate a differential pressure across all three normally open control orifices NO 1a , NO 1b  and NO 1c  to the bypass pressure reducing valve  42 . Any differences between the sensed differential pressure and a setpoint differential pressure adjust the opening and closing of the bypass pressure reducing valve  42  to maintain a constant pressure drop across the three normally open control orifices NO 1a , NO 1b , and NO 1c . 
     At any one combination of spool position settings for the three normally open control orifices NO 1a , NO 1b , and NO 1c , the constant pressure drop commands a fixed amount of flow through the bypass line  22  to the reservoir  16 . Since the output flow of the pump  12  is fixed, a fixed amount of remaining flow is also forced through the load branches  20   a ,  20   b , and  20   c . For example, an increase in the total flow resistance of the load branches  20   a ,  20   b , and  20   c , which would normally force a larger percentage of the flow through the bypass line  22  and reduce the combined flow through the load branches  20   a ,  20   b , and  20   c , is balanced by an additional restriction in the bypass line  22  to maintain the same distribution of flow between the bypass line  22  and the load branches  20   a ,  20   b , and  20   c.    
     The setpoint differential pressure of the bypass pressure reducing valve  42  is preferably set in relation to the characteristic pressure profile of the system to cover a range of normal operations. Set too low, the bypass pressure reducing valve  42  wastes energy. Set too high, the bypass pressure reducing valve  42  has too little effect on flows through the bypass line  22 . 
     Overall system performance can be enhanced by using the bypass pressure reducing valve  42  in combination with the branch pressure reducing valves  26   a ,  26   b , and  26   c . The bypass pressure reducing valve  42  provides a steady flow of fluid to the load branches  20   a ,  20   b , and  20   c  despite variations in the total flow resistance of the load branches  20   a ,  20   b , and  20   c . This assures that the branch pressure reducing valves  26   a ,  26   b , and  26   c  receive sufficient flow for carrying out their intended functions during momentary increases in the total load flow resistance. Though to a lesser extent, the bypass pressure reducing valve  42  can also reduce excess flow to the load branches  20   a ,  20   b , and  20   c  caused by momentary decreases in the total load flow resistance. This reduces the work required of the branch pressure reducing valves  26   a ,  26   b , and  26   c , which are more suited for restricting the excess flow. 
     Another open center hydraulic system  50  is depicted by FIG. 3 in a more conventional format. Directional control valves  52   a ,  52   b , and  52   c , which are preferably spool valves, replace the combination of one normally open control orifice NO 1  and two pairs of normally closed control orifices NC 2 , NC 3  and NC 4 , NC 5 . In addition, as implied by the two pairs of normally closed orifices, the hydraulic system  50  supports opposite directions of load control. 
     Flow proceeds from a fixed displacement pump  54  along a common supply line  56  that splits into three branch supply lines  58   a ,  58   b , and  58   c  and a bypass line  60  that returns flow to a reservoir  55 . The bank of directional control valves  52   a ,  52   b , and  52   c  are supplied in series along the bypass line  60  and are supplied in parallel by the three branch supply lines  58   a ,  58   b , and  58   c . Two working/exhaust lines  62   a  and  64   a ,  62   b  and  64   b , and  62   c  and  64   c  are connected to different ports of the directional control valves  52   a ,  52   b , and  52   c  to carry fluid in opposite directions to and from loads La, Lb, and Lc. Return lines  66   a ,  66   b , and  66   c  from the directional control valves  52   a ,  52   b , and  52   c  are combined to provide an alternative path to the reservoir  55 . 
     Movement of directional control valve actuators (e.g., valve handles)  68   a ,  68   b , or  68   c  in one direction from a neutral starting point closes off normally open flow along the bypass line  60  and produces a working pressure in the working/exhaust lines  62   a ,  62   b , or  62   c  for moving the loads La, Lb, or Lc. Exhaust flow from the loads La, Lb, or Lc is returned to the directional control valves  52   a ,  52   b , and  52   c  along the working/exhaust lines  64   a ,  64   b , or  64   c . After metering by the instant position of the directional control valves  52   a ,  52   b , and  52   c , the exhaust flow is returned to the reservoir  55  along the return lines  66   a ,  66   b , or  66   c . Movement of directional control valve actuators (e.g., valve handles)  68   a ,  68   b , or  68   c  in the opposite direction from the neutral starting point generates a similar flow pattern except that the working/exhaust lines  64   a ,  64   b , or  64   c  convey flows to the loads La, Lb, or Lc and the working/exhaust lines  62   a ,  62   b , or  62   c  return flows to the directional control valves  52   a ,  52   b , and  52   c.    
     Both the working/exhaust lines  62   a ,  62   b , or  62   c  and the working/exhaust lines  64   a ,  64   b , or  64   c  are interrupted by branch pressure reducing valves  70   a  and  72   a ,  70   b  and  72   b , and  70   c  and  72   c . However, each of the branch pressure reducing valves  70   a  and  72   a ,  70   b  and  72   b , and  70   c  and  72   c  is associated with a check valve bypass  74   a  and  76   a ,  74   b  and  76   b , and  74   c  and  76   c  to bypass flows from the directional control valves  52   a ,  52   b , and  52   c  through the otherwise impeding branch pressure reducing valves  70   a  or  72   a ,  70   b  or  72   b , and  70   c  or  72   c . As a result, the branch pressure reducing valves  70   a  and  72   a ,  70   b  and  72   b , and  70   c  and  72   c  only restrict exhaust flows from the loads La, Lb, and Lc to the directional control valves  52   a ,  52   b , and  52   c.    
     Differential pressure across the meter-out function of the directional control valves  52   a ,  52   b , and  52   c  can be monitored by each of the branch pressure reducing valves  70   a  and  72   a ,  70   b  and  72   b , and  70   c  and  72   c  through exhaust flow sensing lines  78   a  or  80   a ,  78   b  or  80   b , and  78   c  or  80   c  in combination with return flow sensing lines  82   a  or  84   a ,  82   b  or  84   b , and  82   c  or  84   c . The setpoint differential pressures for the branch pressure reducing valves  70   a  and  72   a ,  70   b  and  72   b , and  70   c  and  72   c  are preferably set as described above to provide fine metering-out control over load speeds independent of system pressure fluctuations or load flow resistance throughout an initial range of load speeds without interfering with the maximum load speeds attainable by the system. 
     A bypass pressure reducing valve  86  is positioned along the bypass line  60  upstream of the three directional control valves  52   a ,  52   b , and  52   c . Sensing lines  88  and  90  monitor the differential pressure across the three directional control valves  52   a ,  52   b , and  52   c  and, in combination with a predetermined bias, control operation of the bypass pressure reducing valve  86  to restrict excess flow through the bypass line  60 . The bypass pressure reducing valve  86  maintains a setpoint differential pressure across the three directional control valves  52   a ,  52   b , and  52   c  to preserve a fixed flow distribution between the bypass line  60  and the three branch supply lines  58   a ,  58   b , and  58   c  despite load flow resistance variations. Each different position combination of the control valve actuators  68   a ,  68   b , and  68   c  within the working range of the bypass pressure reducing valve  86  supports a different total flow rate through the three branch supply lines  58   a ,  58   b , and  58   c  independent of variations in the total load flow resistance of the branch lines. 
     The setpoint differential pressure of the bypass pressure reducing valve  86  is preferably set to balance tradeoffs between flow stability and efficiency in accordance with the characteristic pressure profile of the hydraulic system  50  and its expected range of use. However, some systems, which are modified to include the meter-out pressure compensation provided by the branch pressure reducing valves  70   a-c  and  72   a-c , may not require the bypass pressure reducing valve  86  to achieve sufficient flow control. 
     Either or both the branch pressure reducing valves  70   a-c  and  72   a-c  and the bypass pressure reducing valve  86  can be deactivated to save energy when improved control over load speed is not needed. A shut-off valve  92  is located along a common portion  94  of return flow sensing lines  82   a-c  and  84   a-c  and can be closed to develop a back pressure in the return flow sensing lines that prevents the differential setpoint conditions from being achieved to close any of the branch pressure reducing valves  70   a-c  and  72   a-c . The back pressure is developed because of small leakages from the branch pressure reducing valves  70   a-c  and  72   a-c  through the return flow sensing lines  82   a-c  and  84   a-c . Reopening the shut-off valve  92  releases the accumulated leakage to the reservoir  55  and permits the branch pressure reducing valves  70   a-c  and  72   a-c  to operate normally. 
     A shut-off valve  96  interrupts the sensing line  90  from the bypass pressure reducing valve  86 . Closing this valve  96  has a similar effect of preventing the setpoint conditions for operation of the bypass pressure reducing valve  86  from being achieved regardless of the actual differential pressure across the three directional control valves  52   a ,  52   b , and  52   c.    
     Alternatively, separate shut-off valves could be associated with the two operating directions of each of the directional control valves  52   a ,  52   b , and  52   c . For example, separate shut-off valves could be located in each of the return flow sensing lines  82   a-c  and  84   a-c  for separately deactivating any one of the branch pressure reducing valves  70   a-c  and  72   a-c.    
     A control system could also be used to statically or dynamically adjust the setpoint differential pressures of the branch pressure reducing valves  70   a-c  and  72   a-c  to vary the meter-out control between load branches or between different operating demands. For example, the setpoint differential pressures can be temporarily reduced at a cost of efficiency and overall speed to provide more control over a limited range of load speeds. The ratio of actuator movement to speed variation can be enlarged by reducing the setpoint differential pressure. On the other hand, the control system could also be used to reduce or eliminate the effects of one or more of the branch pressure reducing valves  70   a-c  and  72   a-c  (such as by controlling the shut-off valve  92 ). The control system could also be used to adjust the setpoint differential pressure of the bypass pressure reducing valve to better match either ongoing or anticipated operating conditions. 
     Applicability 
     My invention is particularly intended as an improvement to backhoes and other excavators that include open center hydraulic systems, but also has wide applicability throughout the field of mobile hydraulics as well as to stationary open center hydraulic systems requiring improved flow stability between load branches.