Patent Publication Number: US-2012031087-A1

Title: Hydraulic circuit with multiple pumps

Description:
FIELD OF INVENTION 
     This invention is related to a hydraulic circuit and particularly, to a hydraulic circuit having multiple pumps for supplying fluid to an actuator. 
     BACKGROUND OF THE INVENTION 
     Some known hydraulic circuits, such as those commonly used in mobile machinery, for example, excavators, include two pumps. Since an excavator includes a minimum of four separate functions (boom, arm, bucket, and swing), each pump acts as a primary source for two of the functions. For example, in most excavator circuits, a first pump acts as the primary hydraulic fluid source for the swing and bucket functions and acts as a secondary hydraulic fluid source for the boom function during raising operation; while a second pump acts as the primary hydraulic fluid source for the boom and arm functions and acts as a secondary hydraulic fluid source for the bucket function. As a result of this design, during operation of the excavator, both the first and second pumps often operate at relatively low displacements. For example, during actuation of only the swing and boom function, the first pump may be operating at a 50% displacement for operating the swing, while the second pump may be operating at a 30% displacement for operating the boom. Generally, hydraulic pumps are quite inefficient at partial displacements. As a result of these inefficiencies, hydraulic circuits of the type described above can be costly to operate. 
     SUMMARY OF THE INVENTION 
     According to the invention, a hydraulic circuit is provided that includes at least one actuator that may be powered for performing a function. A plurality of valves are associated with the at least one actuator for controlling a flow of fluid into and out of the at least one actuator. The hydraulic circuit also includes multiple pumps for supplying fluid to the at least one actuator. The multiple pumps includes a first pump for primarily powering the at least one actuator for movement in a first direction and a second pump for primarily powering the at least one actuator for movement in a second direction, opposite the first direction. 
     According to one embodiment, an electronic controller controls the valves. The controller is responsive to signals from an input device for controlling the valves. 
     According to an embodiment, the first pump provides fluid into a first supply conduit and, the second pump provides fluid into a second supply conduit. A mixing valve is connected between the first and second supply conduits. The mixing valve is responsive to the controller for fluidly connecting the first and second supply conduits. 
     According to another embodiment, the hydraulic circuit includes a fluid power storage sub-system having an accumulator and a valve for controlling a flow of fluid out of the accumulator. The controller controls the valve of the fluid power storage sub-system for powering the at least one actuator using fluid from the accumulator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a hydraulic circuit constructed in accordance with a first embodiment of the invention; 
         FIG. 2  illustrates a hydraulic circuit constructed in accordance with another embodiment of the invention; and 
         FIG. 3  illustrates a hydraulic circuit constructed in accordance with yet another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a hydraulic circuit  10  constructed in accordance with a first embodiment of the present invention. The hydraulic circuit  10  includes an actuator  12  having a head side chamber  14  and a rod side chamber  16 . The head side chamber  14  and the rod side chamber  16  are separated by a piston  13  of a piston/rod assembly  15 . The actuator  12  may be powered for operating a function, shown generally by reference numeral  18 . The hydraulic circuit  10  also includes two hydraulic pumps  24  and  26 . In the embodiment illustrated in  FIG. 1 , the pumps  24  and  26  are variable displacement pumps that may be actuated overcenter so as to act like motors. The pumps  24  and  26  are controlled for maintaining a substantially constant outlet pressure. In one embodiment, the pumps  24  and  26  are axial piston pumps having a movable swashplate, however, any type of hydraulic pumps capable of varied displacement may be used. A power source  28  is connected to the pumps  24  and  26  and is operable for driving the pumps. The power source  28  may include a combustion engine, an electric motor, or any other known source of motive power. During operation for pumping fluid, pump  24  pulls fluid from a tank  30  and provides the fluid into supply conduit  34 . Likewise, during operation for pumping fluid, pump  26  pulls fluid from the tank  30  and provides the fluid into supply conduit  36 . 
     The hydraulic circuit  10  of  FIG. 1  also includes a plurality of valves associated with the actuator  12  for controlling the flow of fluid into and out of the actuator. The valves include two supply side valves  40  and  42 , and two return side valves  44  and  46 . In an alternative embodiment, the two return side valves may be combined into a single three-position valve. The hydraulic circuit  10  may optionally include a mixing valve  48 . As the hydraulic circuit  10  of  FIG. 1  includes only a single actuator  12 , a single mixing valve  48  is included in the circuit. When a hydraulic circuit includes more than one actuator, one or more mixing valves may be used. Supply side valve  40  is connected between and controls the flow of fluid between supply conduit  34  and a conduit  54  leading to the head side chamber  14  of the actuator  12 . Supply side valve  42  is connected between and controls the flow of fluid between supply conduit  36  and a conduit  56  leading to the rod side chamber  18  of the actuator  12 . Return valve  44  is connected between and controls the flow of fluid between conduit  54  and a return conduit  58 . Similarly, return valve  46  is connected between and controls the flow of fluid between conduit  56  and the return conduit  58 . The mixing valve  48  connects and controls the flow between supply conduits  34  and  36 . 
       FIG. 1  illustrates each valve  40 ,  42 ,  44 ,  46 , and  48  as a bi-directional pressure compensating valve. The valves, however, can be any known type of valve including uni-directional valves. The use of bi-directional valves for at least the supply valves  40  and  42  and the mixing valve  48 , however, enables additional control modes for the hydraulic circuit  10 , as is discussed below. 
       FIG. 1  also illustrates an optional fluid power storage sub-system  70 . The fluid power storage sub-system  70  includes an accumulator  72 , an associated valve  74  and, optionally, a charge pump  76 . The charge pump  76  is operatively connected to the pumps  24  and  26  and the power source  28 .  FIG. 1  illustrates a common shaft driving the pumps  24  and  26  and the charge pump  76 . The charge pump  76  is operable for pulling fluid from the tank  30  and providing the fluid to the accumulator  72  via charge conduit  78  for filling the accumulator. A check valve  80  located in charge conduit  78  prevents fluid from the accumulator  72  from flowing back through the charge conduit  78  toward charge pump  76 . The valve  74  connects the accumulator  72  to conduit  54  and controls a flow of fluid out of the accumulator. The valve  74  is a bi-directional valve for enabling the accumulator  72  to provide fluid to the conduit  54  and for enabling the conduit  54  to provide fluid to the accumulator  72 . 
     The hydraulic circuit  10  also includes an electronic controller  64 . The controller  64  is operatively connected to and controls the operation of the valves  40 ,  42 ,  44 ,  46 ,  48 , and  74 . The controller  64  is response to input signals provided from an operator input device  66  for controlling the valves  40 ,  42 ,  44 ,  48 , and  74  in a manner for operating the actuator as desired by an operator. Each of the valves  40 ,  42 ,  44 ,  46 , and  48  is responsive to the control signals for opening and closing to control the flow of fluid through the valve. The controller  64  also may control the power source  28  or, alternatively, may communicate with another controller that controls the power source  28 . The pumps  24  and  26  also may be responsive to the control signals from the controller  64  for changing their displacement, such as by changing an angle of their associated swashplates. Alternatively, the pumps  24  and  26  may be self-controlled to maintain a substantially constant pressure at their outputs. 
     With reference again to the pumps  24  and  26 , pump  24  is the primary pump for supplying fluid for powering the actuator  12  for movement in a first direction, while pump  26  is the primary pump for supplying fluid for powering the actuator  12  for movement in a second direction, opposite the first direction. FIG.  1  illustrates pump  24  as the primary pump for providing fluid to the head side chamber  14  of the actuator  12  and, illustrates pump  26  as the primary pump for providing fluid to the rod side chamber  16  of the actuator  12 . If the demand of the actuator  12  is such that the primary pump is insufficient for powering the actuator, the mixing valve  48  may be opened and the other pump an this operation, the secondary pump) may be used to supplement the flow of fluid provided by the primary pump. 
     The hydraulic circuit  10  of  FIG. 1  has a variety of control modes. The controller  64  controls at least the valves  40 ,  42 ,  44 ,  46 ,  48 , and  74  for controlling the flow of fluid through the hydraulic circuit  10 . The controller  64  controls the valves  40 ,  42 ,  44 ,  46 ,  48 , and  74  and optionally, controls the pumps  24  and  26 , in a manner to provide the highest efficiency for the hydraulic circuit  10  while performing as commanded by the input signals received from operator input device  66 . 
     To extend the actuator  12  of  FIG. 1 , fluid is provided to the head side chamber  14  of the actuator  12 . In response to a pressure differential between the head side chamber  14  and the rod side chamber  16  of the actuator  12 , the piston/rod assembly  15  moves and fluid exits the rod side chamber  16  of the actuator. Below are various control modes for extending the actuator  12  in the hydraulic circuit  10  of  FIG. 1 .
         Operate the power source  28  to drive pump  24  while opening valve  40  to allow fluid to flow from pump  24  through conduit  34 , valve  40 , and conduit  54  to the head side chamber  14  of the actuator  12 . Valve  46  is opened to allow fluid exiting the rod side chamber  16  to flow to tank  30  via conduit  56 , valve  46 , and conduit  58 .   Open valve  74  to allow fluid to flow from the accumulator  72  through valve  74  and a portion of conduit  54  to the head side chamber  14  of the actuator  12 . Valve  46  is opened to allow fluid exiting the rod side chamber  16  to flow to tank  30  via conduit  56 , valve  46 , and conduit  58 .   Open both valves  40  and  74  and operate to the pump  24  so that the pump  24  and the accumulator  72  both provide fluid to the head side chamber  14  of the actuator  12 . Valve  46  is opened to allow fluid exiting the rod side chamber  16  to flow to tank  30  via conduit  56 , valve  46 , and conduit  58 . This control mode is used when pump  24  is insufficient to operate the actuator  12  as commanded by the operator input device  66  and the accumulator  72  is used to supplement the fluid flow from pump  24 .   In the event that the flow from pump  24  and the accumulator  72  is insufficient for powering the actuator  12  as commanded, valve  74  associated with the accumulator  72  may be closed and the mixing valve  48  may be opened so that pump  26  may be used to supplement (or augment) flow to the head side chamber  14  of the actuator  12 . Valve  46  is opened to allow fluid exiting the rod side chamber  16  to flow to tank  30  via conduit  56 , valve  46 , and conduit  58 . In this control mode, pump  24  is the primary pump and pump  26  is a secondary pump that supplements the flow of pump  24 . Instead of both pumps  24  and  26  operating at partial displacement, pump  24  (the primary pump) is operated at full displacement and additional flow is supplemented by pump  26  (the secondary pump). The accumulator  72  may be used, as necessary, for further supplementing the flow provided from pumps  24  and  26 .   To utilize the energy of the fluid exiting the rod side chamber  16  of the actuator  12 , valve  46  may be controlled to remain closed and valve  42  may be opened to direct the flow to pump  26 , which is controlled (or actuated) overcenter so as to act as a motor. Pump  26 , acting as a motor, drives pump  24  (or aids the power source  28  in driving pump  24 ) for providing fluid to the head side chamber  14 . The accumulator  72  may be used, as necessary, for further supplementing the flow from pump  24 . Additionally, charge pump  76  is driven by pump  26  acting as a motor so that the accumulator  72  may be charged during this control mode.   In another control mode, the flow of fluid exiting the rod side chamber  16 , after passing through valve  42 , may be directed through the mixing valve  48  to supply conduit  34  to supplement (or augment) the flow from pump  24  as possible given the pressures in the supply conduits  34  and  36 .       

     To retract the actuator  12 , fluid is provided to the rod side chamber  16  of the actuator  12 . In response to a pressure differential between the rod side chamber  16  and the head side chamber  14  of the actuator  12 , the piston/rod assembly  15  moves and fluid exits the head side chamber  14  of the actuator  12 . Below are various control modes for retracting the actuator  12  in the hydraulic circuit of  FIG. 1 .
         Operate the power source  28  to drive pump  26  while opening valve  42  to allow fluid to flow from pump  26  through conduit  36 , valve  42 , and conduit  56  to the rod side chamber  16  of the actuator  12 . Valve  44  is opened to allow fluid exiting the head side chamber  14  via conduit  54  to flow to one or both of the tank  30  and, if valve  74  is opened, the accumulator  72  to at least partially fill the accumulator.   In the event that the flow from pump  26  is not sufficient for powering the actuator  12  as commanded, the mixing valve  48  may be opened and pump  24  may be used to supplement (or augment) flow to the rod side chamber  16  of the actuator  12 . Valve  44  is opened to allow fluid exiting the head side chamber  14  via conduit  54  to flow to one or both of the tank  30  and, if valve  74  is opened, the accumulator  72 . In this control mode, pump  26  is the primary pump and pump  24  is a secondary pump that supplements the flow of pump  26 . Instead of both pumps  24  and  26  operating at partial displacement, pump  26  (the primary pump) is operated at full displacement and additional flow is supplemented by pump  24  (the secondary pump).   To utilize the energy of the fluid exiting the head side chamber  14  of the actuator  12 , valve  44  remains closed and valve  40  is opened to direct the flow to pump  24 , which is controlled overcenter to act as a motor. Pump,  24  acting as a motor, drives pump  26  (or aids in driving pump  26 ) for providing fluid to the rod side chamber  16 .   In another mode, some of the flow of fluid exiting the head side chamber  14 , after passing through valve  40 , may be directed through the mixing valve  48  to supply conduit  36  for regeneration back to the rod side chamber  16 . The remainder of the fluid exiting the head side chamber  14  is directed to one of the accumulator  72  or the tank  30 .       

       FIG. 2  illustrates a hydraulic circuit  100  constructed in accordance with another embodiment of the invention. The hydraulic circuit  100  includes multiple actuators. The actuators illustrated in  FIG. 2  include three linear actuators  102 ,  104 , and  106  and one rotary actuator  108 ; however, any type or combination of types or actuators may be included in the hydraulic circuit  100 . Actuator  102  includes a piston/rod assembly  110  that is movable for actuating its associated function, shown generally by reference numeral  112 . The piston/rod assembly  110  separates a head side chamber  114  and a rod side chamber  116  of the actuator  102 . Actuator  104  includes a piston/rod assembly  120  that is movable for actuating its associated function, shown generally by reference numeral  122 . The piston/rod assembly  120  separates a head side chamber  124  and a rod side chamber  126  of the actuator  104 . Similarly, actuator  106  includes a piston/rod assembly  130  that is movable for actuating its associated function, shown generally by reference numeral  132 . The piston/rod assembly  130  separates a head side chamber  134  and a rod side chamber  136  of the actuator  106 . Actuator  108  includes first and second ports  140  and  142 , respectively. Fluid entering the first port  140  tends to cause clockwise rotation (or movement in a first direction) of a rotating portion of the actuator  108 . Fluid entering the second port  142  tends to cause counter-clockwise rotation (or movement in a second direction) of a rotating portion of the actuator  108 . 
     The hydraulic circuit  100  also includes two hydraulic pumps  150  and  152 . The pumps  150  and  152  are variable displacement pumps that may be actuated overcenter so as to act like motors. The pumps  150  and  152  are controlled for maintaining a substantially constant outlet pressure. In one embodiment, the pumps  150  and  152  are axial piston pumps having a movable swashplate, however, any type of hydraulic pumps capable of varied displacement may be used. A power source  154  is connected to the pumps  150  and  152  and is operable for driving the pumps. During operation for pumping fluid, pump  150  pulls fluid from a tank  158  and provides fluid into supply conduit  160 . Likewise, during operation for pumping fluid, pump  152  pulls fluid from the tank  158  and provides fluid into supply conduit  162 . 
     As can be seen with reference to  FIG. 2 , pump  150  is connected via conduit  160  to one side of each actuator.  FIG. 2  illustrates pump  150  connected to the head side chambers  114 ,  124 , and  134  of each of actuators  102 ,  104 , and  106 , respectively, and to the first port  140  of actuator  108 . Thus, in the example illustrated in  FIG. 2 , pump  150  acts as a primary pump for supplying fluid for powering actuators  102 ,  104 , and  106  for movement in an extending direction and for powering actuator  108  for clockwise rotation. In  FIG. 2 , pump  152  is connected via conduit  162  to the rod side chamber  116 ,  126 , and  136  of each of actuators  102 ,  104 , and  106  and to the second port  42  of actuator  108 . Thus, in the example illustrated in  FIG. 2 , pump  152  acts as a primary pump for supplying fluid for powering actuators  102 ,  104 , and  106  for movement in a retracting direction and for powering actuator  108  for counter-clockwise rotation. 
       FIG. 2  also illustrates an optional mixing valve  170  for fluidly connecting supply conduits  160  and  162 . The mixing valve  170  illustrated in  FIG. 2  is a three-position valve that is biased into a neutral (closed) position. The mixing valve  170  may be actuated to a first position for connecting flow from supply conduit  160  to supply conduit  162  or, may be actuated to a second position for connecting flow from supply conduit  162  to supply conduit  160 . Flow between the supply conduits  160  and  162  enables the pumps  150  and  152  to combine flows, if necessary, so that one pump may supplement the flow of the other pump as described with reference to  FIG. 1 . 
     The hydraulic circuit  100  of  FIG. 2  also includes a plurality of valves for controlling the flow of fluid into and out of each of the actuators  102 ,  104 ,  106 , and  108 . In  FIG. 2 , each actuator  102 ,  104 ,  106 , and  108  includes four valves. The four valves include two supply side valves  180  and  182  and two return side valves  184  and  186 . In the illustrated embodiment, at least the supply side valves  180  and  182  are bi-directional valves, such as, for example, bi-directional pressure compensating valves similar to those illustrated in  FIG. 1 . The return side valves  184  and  186  may be similar to the supply side valves  180  and  182  or simply may be two-position uni-directional valves for either blocking flow to tank  158  or enabling flow to tank  158 . Alternatively, the return side valves may be combined into a single three-position valve. 
       FIG. 2  also illustrates two pressure sensors  190  and  192 . Pressure sensor  190  is adapted for sensing the pressure within supply conduit  160  and for outputting a pressure signal indicative of the sensed pressure. Similarly, pressure sensor  192  is adapted for sensing the pressure within supply conduit  162  and for outputting a pressure signal indicative of the sensed pressure. 
     The hydraulic circuit  100  of  FIG. 2  also includes a controller  200 . The controller  200  receives signals from the pressure sensors  190  and  192  and also receives signals from an input device  202 . The input device  202  may be, for example, a joystick for receiving commands from an operator, in which case the signals from the input device  202  are indicative of the operator commanded actuation of the actuators  102 ,  104 ,  106 , and  108 . The controller  200  is responsive to the input signals from the input device  202  and the pressure signals from the pressure sensors  190  and  192  for controlling the pumps  150  and  152  and the valves  170 ,  180 ,  182 ,  184 , and  186  of the hydraulic circuit  100  in a manner to provide the highest efficiency while performing as commanded. The controller  200  also may prioritize actuation of the various actuators  102 ,  104 ,  106 , and  108  and control the valves  170 ,  180 ,  182 ,  184 , and  186  in a manner for providing priority to one or more actuators. Various control modes for the hydraulic circuit  100  of  FIG. 2  are described below. These described control modes do not provide priority to any of the actuators. From the description provided, those skilled in the art should recognize how to control the valves  170 ,  180 ,  182 ,  184 , and  186  in a manner for providing priority to one or more actuators. 
     To extend one or more of the actuators  102 ,  104 , and  106  and/or cause clockwise rotation of actuator  108 , the hydraulic circuit  100  of  FIG. 2  is controlled in one of the following control modes:
         Operate the power source  154  to drive pump  150  while opening the supply side valves  180  of the actuators  102 ,  104 ,  106 , and  108  to allow fluid to flow from conduit  160  to the appropriate head side chamber  114 ,  124 ,  134 , respectively, of the actuators  102 ,  104 , and  106  to be extended and/or to the first port  40  of rotary actuator  108 . Appropriate return side valves  186  of the actuators  102 ,  104 ,  106 , and  108  are opened to allow fluid exiting the actuators to flow to tank  158 .   In the event that the flow from pump  150  is not sufficient for powering the actuators  102 ,  104 ,  106 , and  108  as commanded, the mixing valve  170  is opened and pump  152  is used as a secondary source to supplement (or augment) fluid flow to the head side chambers of the actuators  102 ,  104 , and  106  to be extended and/or to the first port  40  of the rotary actuator  108 . The controller  200  may make a determination that pump  150  is not sufficient for powering actuators  102 ,  104 ,  106 , and  108  by monitoring pressure sensor  190 . Alternatively, if supply side valve  180  is a pressure compensating valve, the controller  200  may monitor a position of the compensator for determining whether pump  150  is sufficient for powering actuators  102 ,  104 ,  106 , and  108 . As the compensator has a moving spool (or poppet) that moves in response to changes in pressure, the position of the spool (or poppet) is indicative of pressure. Thus, the compensator acts as the pressure sensor. Appropriate return side valves  186  of the actuators  102 ,  104 ,  106 , and  108  are opened to allow fluid exiting the actuators to flow to tank  158 .   To utilize the energy of the fluid exiting the actuators  102 ,  104 ,  106 , and  108 , fluid is supplied to the actuators  102 ,  104 ,  106 , and  108  as set forth above and the return side valves  186  are controlled to the closed position. The supply side valves  182  are opened to direct the fluid flow exiting the actuators to pump  152 , which is controlled overcenter to act as a motor. Pump  152 , acting as a motor, drives pump  150  (or aids in driving pump  150 ) for providing fluid.   In another mode, the flow of fluid exiting the rod side chamber of the one or more actuators being extended, for example, chamber  126  of actuator  104 , may be directed through the supply side valve  182  into conduit  162 . The fluid may pass from conduit  162  through the mixing valve  170  (when appropriately positioned) and into conduit  160  to be directed into chamber  124  of actuator  104 , via supply side valve  180  as possible given pressures in the conduits  160  and  162 .       

     To retract one or more of the actuators  102 ,  104 , and  106  and/or cause counter-clockwise rotation of actuator  108 , the hydraulic circuit  100  is controlled in one of the following control modes:
         Operate the power source  154  to drive pump  152  while opening the appropriate supply side valves  182  to actuators  102 ,  104 ,  106 , and  108  to allow fluid to flow from conduit  162  to the appropriate rod side chamber  116 ,  126 ,  136 , respectively, of the actuators  102 ,  104 , and  106  to be retracted and/or to the second port  42  of the rotary actuator  108 . Appropriate return side valves  184  of the actuators  102 ,  104 ,  106 , and  108  are opened to allow fluid exiting the actuators to flow to tank  158 .   In the event that the flow from pump  152  is not sufficient for powering the actuators  102 ,  104 ,  106 , and  108  as commanded, the mixing valve  170  is opened and pump  150  is used as a secondary source to supplement (or augment) fluid flow to the rod side chambers of the actuators  102 ,  104 , and  106  to be retracted and/or the second port  42  of the rotary actuator  108 . The controller  200  may make a determination that pump  152  is not sufficient for powering actuators  102 ,  104 ,  106 , and  108  by monitoring pressure sensor  192 . Alternatively, if supply side valve  182  is a pressure compensating valve, the controller  200  may monitor a position of the compensator for determining whether pump  152  is sufficient for powering actuators  102 ,  104 ,  106 , and  108 . Appropriate return side valves  184  of the actuators  102 ,  104 ,  106 , and  108  are opened to allow fluid exiting the actuators to flow to tank.   To utilize the energy of the fluid exiting the actuators  102 ,  104 ,  106 , and  108 , fluid is supplied to the actuators  102 ,  104 ,  106 , and  108  as set forth above and the return side valves  184  are controlled to the closed position. The supply side valves  180  are opened to direct the fluid flow exiting the actuators to pump  150 , which is controlled overcenter to act as a motor. Pump  150 , acting as a motor, drives pump  152  (or aids in driving pump  152 ) for providing fluid.   In another mode, the flow of fluid exiting the head side chamber of one or more actuators being retracted, for example, chamber  124  of actuator  104 , may be directed through the supply side valve  180  into conduit  160 . The fluid may pass from conduit  160  through the mixing valve  170  (when appropriately positioned) and into conduit  162  to be directed into chamber  126  of actuator  104 , via supply side valve  182  as possible given pressures in conduits  160  and  162 .       

     At times, it may be desirable to actuate a majority of the actuators  102 ,  104 ,  106 , and  108  in one direction and a minority of the actuators in an opposite direction. For example, assume that actuators  102  and  104  are commanded to extend, actuator  108  is commanded to rotate clockwise, and actuator  106  is commanded to retract. In such a scenario, pump  150 , which based upon the commanded actuation acts as the primary fluid source for the majority of the actuators  102 ,  104 , and  108 , may be used for powering all of the actuators, including actuator  106 , if capable. To power actuator  106  with fluid from pump  150 , the controller  200  opens mixing valve  170  to enable fluid flow from supply conduit  160  into supply conduit  162  and valves  182  and  184  associated with actuator  106  are opened for enabling fluid flow into chamber  136  and out of the chamber  134 . In the event that pump  150  is incapable of supplying sufficient fluid for actuating the actuators  102 ,  104 ,  106 , and  108  as desired, the controller  200  will close the mixing valve  170  and supply fluid for actuator  106  from pump  152 . 
       FIG. 3  illustrates a hydraulic circuit  100 A constructed in accordance with yet another embodiment of the invention. Portions of  FIG. 3  that are similar to those described above with reference to  FIG. 2  use the same reference number as used in  FIG. 2  with the addition of the suffix “A” and are not described in detail with reference to  FIG. 3 . The hydraulic circuit  100 A of  FIG. 3  includes a fluid power storage sub-system  210  associated with actuator  102 A. Those skilled in the art should recognize that the other actuators  104 A,  106 A, and  108 A may include a similar fluid power storage sub-system or multiple actuators may share a common fluid power storage sub-system. The fluid power storage sub-system  210  includes an accumulator  212 , an associated valve  214  and a charge pump  216  that is coupled to and driven by the power source  154 A. When a hydraulic circuit includes multiple fluid power storage sub-systems a common charge pump may be used. The charge pump  216  is operatively connected to the pumps  150 A and  152 A and the power source  154 A. The charge pump  216  is operable for pulling fluid from the tank  158 A and providing the fluid to the accumulator  212  via conduit  220  for filling the accumulator. A check valve  222  located in conduit  220  prevents fluid from the accumulator  212  from flowing back through conduit  220  toward charge pump  216 . The valve  214  connects the accumulator  212  to supply conduit  160 A. The valve  214  is a bi-directional valve for enabling the accumulator  212  to provide fluid to the supply conduit  160 A and for enabling the supply conduit  160 A to provide fluid to the accumulator  212 . Fluid from the accumulator  212  may be used alone or in combination with fluid from pump  150 A (and supplemental pump  152 ) for extending actuator  102 A. The accumulator  212  may be charged by fluid provided by the charge pump  216 , by fluid exiting the head side chamber  114 A of the actuator  102 A, by fluid provided by pump  150 A, or by a combination of the these devices. 
       FIG. 3  also illustrates two actuators  104 A and  106 A having regeneration valves  230  that enable the supply side valves  180 A and  182 A to be fluidly connected. The regeneration valve  230  illustrated in  FIG. 3  is merely representative and may be formed by structures integral with the supply side valves  180 A and  182 A. Those skilled in the art should recognize that any number of the actuators may include regeneration valves  230 . The regeneration valves  230  direct fluid flowing out of a chamber with a volume that is being reduced and into a chamber with a volume that is being expanded. The control modes of the hydraulic circuit  100 A in  FIG. 3  are similar to those described with reference to  FIG. 2  with the addition of the use of the fluid power storage sub-system  210  for actuator  102 A, which is similar to that described with reference to fluid power storage sub-system  70  in  FIG. 1 , and the use of the regeneration valves  230  for actuators  104 A and  106 A. 
     Although the principles, embodiments and operation of the present invention have been described in detail herein, this is not to be construed as being limited to the particular illustrative forms disclosed. They will thus become apparent to those skilled in the art that various modifications of the embodiments herein can be made without departing from the spirit or scope of the invention.