Patent Publication Number: US-7905088-B2

Title: Energy recovery and reuse techniques for a hydraulic system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 60/865,710 filed on Nov. 14, 2006 and U.S. Provisional Patent Application No. 60/913,457 filed on Apr. 23, 2007. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hydraulic systems that control fluid flow to a hydraulic actuator which moves a mechanical component on a machine, and in particular to recovering energy from the hydraulic actuator and utilizing the recovered energy subsequently to power the hydraulic actuator. 
     2. Description of the Related Art 
     Construction and agricultural equipment employ hydraulic systems to operate different mechanical elements. For example, an excavator is a common construction machine that has boom pivotally coupled at one end to a tractor and having a bucket at the other end for scooping dirt and other material. A cylinder assembly is used to raise and lower the boom and includes a cylinder with a piston therein which defines two chambers in the cylinder. A rod connected to the piston is typically attached to the boom and the cylinder is attached to the body of the excavator. The boom is raised and lowered by extending and retracting the rod out of and into the cylinder. 
     Other machines use different types of hydraulic actuators to produce motion of a mechanical element. The term “hydraulic actuator”, as used herein, generically refers to any device, such as a cylinder-piston arrangement or a rotational motor for example, that converts hydraulic fluid flow into mechanical motion. 
     During powered extension and retraction of the cylinder assembly, pressurized fluid from a pump is usually applied by a valve assembly to one cylinder chamber and all the fluid exhausting from the other cylinder chamber flows through the valve assembly into a return conduit that leads to the system tank. Under some conditions, an external load or other force acting on the machine enables extension or retraction of the cylinder assembly without significant fluid pressure from the pump. This is often referred to as an overrunning load. In an excavator for example, when the bucket is filled with heavy material, the boom can be lowered by the force of gravity alone. That external force drives fluid out of one chamber of the boom&#39;s hydraulic cylinder through the valve assembly and into the tank. At the same time, an amount of fluid is drawn from the pump through the valve assembly into the other cylinder chamber which is expanding, however because that incoming fluid is not driving the piston, it does not have to be maintained at a significant pressure for this boom motion to occur. In this situation, the fluid is exhausted from the cylinder under relatively high pressure, thereby containing energy that normally is lost when the pressure is metered through the valve assembly. 
     To optimize efficiency and economical operation of the machine, it is desirable to recover the energy of that exhausting fluid, instead of dissipating it in the valve assembly. Some prior hydraulic systems sent that exhausting fluid to an accumulator, where it was stored under pressure for later use in powering the machine. However, a challenge to efficient energy recovery and reuse is that the stored hydraulic fluid has to be at the proper pressure and volume to power an actuator. The relationship between the pressure and volume of the exhausting fluid and those parameters of the accumulator varies instantaneously and determines whether that fluid can be stored. For example, if the external force acting on the cylinder assembly is insufficient to pressurized the exhausting fluid above the level of pressure in the accumulator, then that fluid cannot be stored. 
     At another time when use of the fluid in the accumulator is desired, the instantaneous relationship between the pressure and volume of the accumulator and that required of the fluid to power the hydraulic actuator determines whether the accumulator fluid can be used. For example, if the load on the hydraulic actuator requires a greater pressure than the accumulator pressure, then the recovered fluid cannot be employed. Also if the hydraulic actuator needs to move so far as to require a greater volume of fluid than is stored in the accumulator, effective operation may be difficult to achieve. Another limiting factor is that as the hydraulic actuator consumes fluid from the accumulator, the accumulator pressure decreases reducing the ability of the remaining fluid to power the actuator. 
     Therefore, a need exists to provide an effective techniques for recovering and reusing energy in a hydraulic system. 
     SUMMARY OF THE INVENTION 
     A hydraulic system has first and second hydraulic cylinders that are mechanically connected in parallel to operate a component of a machine and each cylinder has first and second chambers. A control valve assembly, such as a Wheatstone bridge arrangement of four electrohydraulic proportional valves for example, has a first workport and a second workport. The first workport is connected to the first chamber of the first cylinder and is isolated from the first chamber of the second cylinder. The second workport is connected to the second chambers of both the first and second hydraulic cylinders. The control valve assembly is operated to connect each of first and second workports selectively to the supply conduit and the return conduit. 
     An energy recovery apparatus of the hydraulic system comprises a cylinder separation control valve controlling fluid flow between the first chamber of the first cylinder and the first chamber of the second cylinder. An accumulator is connected to a recovery control valve that controls fluid flow to and from the first chamber of the second cylinder. This enables fluid that is forced out of that first chamber by an external load to be routed into the accumulator where it is stored under pressure. Subsequently, the stored fluid is used to power one or both of the hydraulic cylinders. 
     In another aspect, the present invention provides a first pump connected to the supply conduit. A supply valve controls fluid flow from a second pump to the first chamber of the second cylinder. By closing the supply valve and opening the cylinder separation control valve, both the first and second hydraulic cylinders are controlled in unison by the control valve assembly. Alternatively, closing the cylinder separation control valve, the first hydraulic cylinder is controlled by the control valve assembly, while the second hydraulic cylinder is controlled by opening the supply valve. 
     In a preferred embodiment of the hydraulic system, a workport shunt control valve is connected to first and second workports to enable fluid to flow directly there between. 
     In another aspect of the invention, an energy recovery apparatus is provided including a hydraulic cylinder to operate a component of a machine. The energy recovery apparatus includes a first chamber and a second chamber. A control valve assembly including a first workport and a second workport is connected to the first and second chamber, such that the first workport is in fluid communication with the first chamber of the hydraulic cylinder and the second workport is in fluid communication with the second chamber of the hydraulic cylinder, and such that operation of the control valve assembly connects each of first and second workports selectively to the supply conduit and the return conduit. A workport shunt control valve is in fluid communication with both the first workport and the second workport to control fluid flow there between. The system includes an accumulator and a recovery control valve that controls fluid flow to the accumulator from the first chamber of the cylinder. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of an excavator that incorporates a hydraulic system according to the present invention; 
         FIG. 2  is a schematic diagram of the portion of the hydraulic system for operating actuators that raise and lower a boom of the excavator; 
         FIG. 3  is a schematic diagram of an alternative portion of the hydraulic system for the boom; 
         FIG. 4  is a schematic diagram of another alternative portion of the hydraulic system for the boom; 
         FIGS. 5-9  are abbreviated schematic diagrams of the alternative portion of the hydraulic system in  FIG. 3  in different modes of energy recovery; and 
         FIGS. 10-15  are abbreviated schematic diagrams of the alternative portion of the hydraulic system in  FIG. 3  in various modes of reusing the recovered energy. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the present invention is being described in the context of use on an excavator, it can be implemented on other types of hydraulically operated equipment. 
     With initial reference to  FIG. 1 , an excavator  10  is composed of a cab  11  that is supported on a crawler, and a boom assembly  12  attached to the cab for up and down motion. The boom assembly  12  is subdivided into a boom  13 , an arm  14 , and a bucket  15  pivotally attached to each other. The boom  13 , that is coupled to the cab  11 , is able to pivot up and down when driven by a pair of hydraulic cylinder assemblies  16  and  17  mechanically connected in parallel between the cab and the boom. On a typical excavator the cylinder of these assemblies  16  and  17  is attached to the cab  11  while the piston rod is attached to the boom  13 , thus the force of gravity acting on the boom tends to retract the piston rod into the cylinder. Nevertheless, the connection of the cylinder assemblies could be such that gravity tends to extend the piston rod from the cylinder, and many energy recovery techniques to be described also can be used with that configuration. The arm  14 , supported at the remote end of the boom  13 , is able to swing forward and backward, and the bucket  15  is pivotally coupled at the tip of the arm. Another pair of cylinder assemblies  18  and  19  independently operate the arm  14  and bucket  15 . The bucket  15  can be replaced with other work heads. 
     With reference to  FIG. 2 , the cylinder assemblies  16 ,  17 ,  18  and  19  on the excavator  10  are part of a first hydraulic system  20  that has a source  21  of hydraulic fluid, which comprises a first pump  22  and a tank  23 . The first pump  22  draws fluid from the tank  23  and forces the fluid under pressure through a backflow check valve and into a supply conduit  25  that furnishes pressurized fluid to all the hydraulic functions on the excavator. After being used to power a hydraulic function, such as function  30  for raising and lowering the boom  13 , the fluid flows back to the tank  23  via a return conduit  26  in which the fluid is pressurized by a spring loaded tank check valve  24 . Although the hydraulic system  10  powers several hydraulic functions on the excavator  10 , attention is being focused on the boom function  30  to simplify the explanation of the present energy recovery and reuse techniques. 
     The boom function  30  raises and lowers the boom  13  by controlling the flow of fluid to and from the boom cylinder assemblies  16  and  17 , each having a cylinder, a piston with a rod. The first boom cylinder assembly  16  has a first boom cylinder  31  with a first piston  27  slideably received therein which divides the cylinder interior into a rod chamber  33  and a head chamber  34  on opposite sides of the piston. The second boom cylinder assembly  17  has a second boom cylinder  32  with a second piston  29  slideably received therein which divides the cylinder interior into another rod chamber  36  and head chamber  38  on opposite sides of the piston. The volumes of the rod and head chambers change as the associated piston slides within the respective cylinder. In the exemplary excavator  10  of  FIG. 1 , each boom cylinder  31  or  32  is attached to the cab  11  and each piston  27  or  29  is attached to the boom  13  by a piston rod  35  or  37 , respectively. 
     The rod chambers  33  and  36  are directly connected together hydraulically. A bidirectional, EHP cylinder separation control valve  39  directly couples the head chambers  34  and  38 , and preferably is directly connected to each head chamber. Closing the cylinder separation control valve  39  isolates the head chambers from each other and opening the cylinder separation control valve  39  provides a direct path between the two head chambers. A “control valve” is defined herein to mean a valve that is manually operated by a person or electrically operated. The term “directly connected” as used herein means that the associated components are connected together by a conduit without any intervening element, such as a valve, an orifice or other device, which restricts or controls the flow of fluid beyond the inherent restriction of any conduit. As used herein, stating that a hydraulic component “directly couples” two other elements means that the hydraulic component provides a path for fluid to flow between those two other elements without flowing through a control valve assembly or through the supply or return conduits in which fluid flows to and from other hydraulic functions. A statement herein that a control valve provides a “direct path” between two components or elements of the hydraulic system means that path does not contain another control valve. 
     A control valve assembly  40  couples the boom cylinder assemblies  16  and  17  to the supply and return conduits  25  and  26  and controls the flow of fluid there between. When the control valve assembly  40  supplies pressurized fluid to the head chambers  34  and  38  in the boom cylinders  31  and  32  and drains fluid from the rod chambers  33  and  36 , each piston rod  35  and  37  is extended from its cylinder, thereby raising the boom  13 . Similarly, supplying pressurized hydraulic fluid from the supply conduit  25  to the rod chambers  33  and  36  and draining fluid from the head chambers  34  and  38 , retracts the piston rods  35  and  37  into the boom cylinders  31  and  32 , thereby lowering the boom  13 . At those times that are commonly referred to as powered extension and powered retraction, the cylinder separation control valve  39  is opened to operate both boom cylinder assemblies  16  and  17  in unison. 
     The control valve assembly  40  comprises four electrohydraulic proportional (EHP) control valves  41 ,  42 ,  43  and  44  that are connected in a Wheatstone bridge arrangement. Alternatively, a solenoid operated spool valve can be used in place of the four EHP control valves  41 - 44 . Preferably, each EHP control valve  41 - 44  is a pilot-operated, bidirectional control valve, such as the valve described in U.S. Pat. No. 6,745,992 for example, that if necessary incorporates a conventional anti-cavitation valve. The first EHP control valve  41  directs the flow of hydraulic fluid from the supply conduit  25  to a first workport  46 , which is connected by a first actuator conduit  47  to a node  51  between the head chamber  34  of the first cylinder  31  and the cylinder separation control valve  39 . The head chamber  38  of the second boom cylinder  32  is connected to the first actuator conduit  47 , and thus to the head chamber  34  of the first cylinder  31 , by the cylinder separation control valve  39 , which thereby isolates the first workport  46  from head chamber  38  and the two head chambers from each other. The second EHP control valve  42  governs the flow of fluid between the first workport  46  to the return conduit  26 . The third EHP control valve  43  controls a path for fluid to flow between the supply conduit  25  and both cylinder rod chambers  33  and  36  that are connected to a second workport  48  by a second actuator conduit  49 . The fourth EHP control valve  44  is connected between the rod chambers  33  and  36  and the return conduit  26 . 
     The four EHP control valves  41 - 44 , as well as the cylinder separation control valve  39 , are solenoid operated independently by electrical signals from a system controller  50 . By opening both the first and fourth EHP control valves  41  and  44 , along with the cylinder separation control valve  39 , pressurized fluid is applied to the head chambers  34  and  38  and fluid drains from the rod chambers  33  and  36  to extend the piston rods  35  and  37  and raise the boom  13 . Similarly, opening the second and third EHP control valves  42  and  43 , as well as the cylinder separation control valve  39 , sends pressurized fluid into the rod chambers  33  and  36  and drains fluid from the head chambers  34  and  38  to retract the piston rods  35  and  37 , thereby lowering the boom  13 . 
     The system controller  50  is a microcomputer based device that receives control signals from several joysticks  52  by which a human operator designates desired motion of the hydraulic actuators on the excavator. The system controller  50  also receives signals from a supply conduit pressure sensor  54  and a return conduit pressure sensor  55 . Separate pressure sensors  56  and  57  are provided for the cylinder head chambers  34  and  38 , respectively, while another pressure sensor  58  measures pressure in the rod chambers  33  and  36  of the boom cylinder assemblies  16  and  17 . To simplify electrical wiring, the rod chamber pressure sensor  58  preferably is located proximate to the second workport  48 , with the understanding that its pressure measurement may be affected by pressure losses in the second actuator conduit  49 . The pressure sensors  56 ,  57  and  58  for the cylinder chambers produce signals indicating the amount of force F acting on the boom  13 . The system controller  50  responds to the pressure measurements by operating the variable displacement first pump  22  to regulate pressure in the supply conduit  25  in order to satisfy the pressure demands of the different hydraulic actuators on the excavator. 
     The first hydraulic system  20  includes several additional valves and other components that form an apparatus which enable energy recovery and reuse for the boom function  30 . Specifically, an accumulator  60  is provided to store fluid recovered from the boom cylinder assemblies  16  and  17 . An additional pressure sensor  59  is located at the port  61  of the accumulator  60  and produces a signal to the system controller  50  indicating the pressure within the accumulator. The accumulator  60  is coupled to the head chamber  38  of the second boom cylinder assembly  17  by a bidirectional, EHP recovery control valve  62  and is isolated from the head chamber  34  of the first boom cylinder assembly  16 . An electrohydraulic accumulator charging and reuse control valve  66  provides a direct path between the supply conduit  25  and the port  61  of the accumulator  60 . An electrohydraulic pump return control valve  68  directly connects the port of the accumulator  60  to the inlet of the first pump  22 , and a relief control valve  70  directly connects a node  64  at the second cylinder&#39;s head chamber  38  to the tank return conduit  26 . The node  64  is isolated by the cylinder separation control valve  39  from the head chamber  34  of the first cylinder  31 . An EHP workport shunt control valve  65  provides a direct path between the first and second workports  46  and  48 , and preferably is directly connected to each workport. All these additional control valves  39 ,  62 ,  65 ,  66 ,  68  and  70  are operated by signals from the system controller  50 . 
     By selectively operating various combinations of these valves fluid is routed to and from boom cylinder assemblies  16  and  17  and the first pump  22 , the tank  23  and the accumulator  60 . Fluid exhausting from the boom cylinder assemblies, during gravitational lowering of the boom  13 , can be stored under pressure in the accumulator and then subsequently used instead of fluid from the first pump, thereby saving the energy that otherwise would be required to drive that pump. The different modes of energy recovery resulting from operating various combinations of valves will be described later. 
     The present recovery system also can charge the accumulator  60  with fluid directly from the first pump  22  when none of the hydraulic functions on the machine is being used or when the hydraulic functions that are operating require only a relatively small amount of pump fluid. At those times, the accumulator charging and reuse control valve  66  is opened to connect the supply conduit  25  directly to the port  61  of the accumulator  60 . The pressure sensors  54  and  59  indicate when the pressure of the supply conduit is greater than the existing pressure in the accumulator  60  so that charging will occur. 
     Another mode that reuses the stored energy involves opening the pump return control valve  68 , thereby routing stored pressurized fluid from the accumulator  60  to the inlet of the first pump  22 . This is particularly useful when the inlet of the pump has a high pressure inlet capability. This energy recovery unloads the torque on the engine which is driving the first pump  22  even though the accumulator pressure is less than the load pressure of the cylinder assemblies  16  and  17  and thus can not be used to power the cylinder assemblies directly. In this case, the first pump only has to use torque from the engine to fulfill the pressure difference between the accumulator  60  and the load pressure on the cylinder assemblies. 
     With continuing reference to  FIG. 2 , the first hydraulic system  20  also includes a swing function  80  that bidirectionally rotates the excavator cab  11  and the boom assembly  12  with respect to the crawler  9 . A variable displacement second pump  82  furnishes pressurized fluid via a second supply conduit  83  to the swing function  80 . A control valve assembly  84 , similar to control valve assembly  40 , controls the flow of hydraulic fluid from the second pump  82  to a motor  86  and from the motor to the tank  23 . The motor  86  has two ports and the valve assembly  84  selectively connects the second pump  82  to one port and connects the other port to the tank, thereby defining the direction that fluid flows through the motor and thus the direction that the cab  11  rotates about the crawler  9 . 
     The two ports of the motor  86  also are connected to the inputs of a shuttle valve  88  that has an outlet coupled by a pressure operated valve  90  to the port  61  of the accumulator  60 . The pressure operated valve  90  opens when pressure at the outlet of the shuttle valve  88  exceeds a given level that occurs when the rotation of the cab  11  is coming to a stop. At that time, the pressurized fluid is routed to the accumulator  60  instead of through the valve assembly  84  to the tank  23 . Therefore, the energy of the fluid exhausting from the motor  86  at these times is stored in the accumulator  60 . 
     The stored fluid may be used by the boom function  30 , as described previously, or may be used to power the swing function motor  86 . To accomplish the latter operation, a bidirectional, electrohydraulic supply control valve  92  is opened to convey fluid from the accumulator  60  to the inlet of the valve assembly  84 . This accumulator fluid is used in place of or as a supplement to fluid from the second pump  82 . 
     By tying the first and second boom cylinder assemblies  16  and  17  together, the loading on those cylinders is equalized on the production system, but a degree of control freedom is lost. Greater efficiency can be achieved by separating the head chambers  34  and  38  of the two boom cylinder assemblies  16  and  17  to minimize pressure compensation losses on the machine&#39;s hydraulic system. 
       FIG. 3  depicts an alternative second hydraulic system  96  that accomplishes this greater degree of freedom. This second hydraulic system  96  is similar to the first hydraulic system  20  in  FIG. 2  and like components have been assigned identical reference numerals. The difference being that the supply control valve  92  in the previously described system  20  has been replaced by bidirectional, electrohydraulic supply control valve  98  that provides a direct path between the second supply conduit  83  from the second pump  82  and the head chamber  38  of the second boom cylinder  32 . Preferably the supply control valve  98  is directly connected between the second supply conduit and the head chamber  38 . This enables the boom to be raised using the fluid from the first pump  22  to drive the first boom cylinder assembly  16  under the control of the control valve assembly  40 , while supply control valve  98  controls application of fluid from the second pump  82  to the second boom cylinder assembly  17 . 
     EXAMPLE 1 
     Assume that the first pump  22  supplies fluid to other hydraulic functions on the machine and is running at 300 bar pressure to satisfy the highest demand of those functions. In addition, assume that still other hydraulic functions are connected to the second pump  82 , which is running at 200 bar pressure to satisfy its highest fluid demand. Further assume that 250 bar pressure is required to lift the load on the boom  13 . 
     With a conventional system, the first pump  22  would stay at 300 bar and the extra 50 bar would be “burned” as pressure compensation losses. In that conventional system, the pressure of the second pump  82  would rise to 250 bar and its other hydraulic functions would produce pressure compensation losses, due to the pressure being greater than required at those functions. 
     With the system shown in  FIG. 3 , the first pump  22  continues operating at 300 bar and the second pump  82  continues to operate at 200 bar, thus a combined average of 250 bar. Each of those pumps supplies fluid to the boom cylinder assemblies  16  and  17 , the first pump through control valve assembly  40  and the second pump through the supply control valve  98 . As a result, each cylinder assembly moves with a different amount of pressure and thus different force. Nevertheless, the resultant net force on the boom  13  is the same as with the conventional system. 
     EXAMPLE 2 
     Assume that there is another hydraulic function connected to the first pump  22  that already has consumed all that pump&#39;s output flow. If raising the boom  13  is commanded, then the second pump  82  can furnish all the power to the boom through supply control valve  98  and the second cylinder assembly  17 , while fluid for the head chamber  34  of first cylinder  31  is drawn from the return conduit  26  through the anti-cavitation check valve in the second EHP control valve  42 . 
     The functionality of examples 1 and 2 can be provided by a third hydraulic system  100  that uses solenoid operated spool valves, such as depicted in  FIG. 4 . Hydraulic system  100  includes a boom function  102  in which the same components as in the previously described systems have been identified with identical reference numerals. The head chambers  34  and  38  of the first and second boom cylinders  31  and  32  are coupled hydraulically by a bidirectional, electrohydraulic cylinder separation control valve  39 . An electrohydraulic shunt control valve  65  is connected between the ports for the rod and head chambers of the first cylinder  31 . 
     The third hydraulic system  100  has a hydraulic fluid source  21  formed by first and second pumps  22  and  82  which draw fluid from a tank  23  and operates the boom function  102 , a swing function  80 , and other functions on the machine which are not illustrated. The output of the first pump  22  feeds a first supply conduit  25  that is connected to an inlet of a three-position, four-way, solenoid operated first spool valve  104  that constitutes a control valve assembly of the boom function. An outlet of the first spool valve  104  is connected to the return conduit  26  that leads to the tank  23 . The first spool valve  104  has two workports, one  48  connected directly to the rod chambers  33  and  36  of the two hydraulic cylinders and the other workport  46  connected directly to the head chamber  34  of the first hydraulic cylinder  31 . A first relief valve  106  is connected between the first workport  46  and the return conduit  26 . 
     The outlet of the second pump  82  feeds a second supply conduit  83  that is connected to the inlet of a three-position, four-way, solenoid operated second spool valve  108  that forms a supply control valve. The outlet of the second spool valve  108  is connected to the return conduit  26 . The second spool valve  108  has a pair of workports one of which is connected directly to the rod chambers  33  and  36  of the hydraulic cylinders and the other workport is directly connected to the head chamber  38  of the second hydraulic cylinder  32 . A second relief valve  110  is coupled between the head chamber  38  and the return conduit  26 . The two spool valves  104  and  108  can be operated independently to apply fluid from each of the two pumps  22  and  82  to the two first and second cylinders  31  and  32  in much the same way as control valves  41 - 44  and  98  functioned in the second hydraulic system  96  in  FIG. 3 . 
     The third hydraulic system  100  also has an accumulator  112  connected by a bi-directional, electrohydraulic valve  114  to the head chamber  38  of the second cylinder  32 . This accumulator  112  can be used to store and recycle energy with respect to the first and second hydraulic cylinders  31  and  32  in much the same manner as described with respect to the accumulators in the hydraulic systems in  FIGS. 2 and 3 . 
     Energy Recovery 
     The boom function can be operated in several modes, in some of which energy is recovered from an overrunning load. An overrunning load condition occurs on the exemplary excavator  10  when the load and weight of the boom assembly  12  exerts a force that tends to retract the piston rods  35  and  37  into the boom cylinders  31  and  32 , thereby forcing fluid out of the head chambers  34  and  38  without pressurizing the rod chambers  33  and  36 . At that time, instead of sending the exhausting fluid to the tank  23 , it is directed into the accumulator  60  where the fluid is stored under pressure. The present energy recovery and reuse techniques involve operating the hydraulic circuit in several of the different energy recovery modes as the excavator boom  13  is lowered. Selection of a particular energy recovery mode is based on the pressures within the head and rod chambers of the boom cylinders  31  and  32  and the existing pressure within the accumulator  60 . The pressure relationships must be such that the fluid will flow in the proper directions as described for each particular energy recovery mode as described hereinafter. The accumulator pressure is indicated by pressure sensor  59 , pressures in the head chambers  34  and  38  are measured by sensors  56  and  57 , respectively, and the pressure in both rod chambers  33  and  36  is measured by sensor  58 . 
     Several of the energy recovery modes are depicted in  FIGS. 5-9  which are abbreviated schematic diagrams of the second hydraulic system  96  in  FIG. 3 . In these depictions primary fluid flow paths are indicated by a wide solid line, and partial or optional flow paths, that occur depending on specific operating conditions, are indicated by heavy dashed lines. Thin solid lines indicate paths through which fluid does not flow in the depicted mode. This flow indicating convention also is utilized for energy reuse modes shown in  FIGS. 10-15 , which will be described subsequently. 
     Assume that the initial position of the boom assembly  12  is relatively high, thereby having a relatively large amount of potential energy. As a result, the boom exerts a force on each cylinder assembly  16  and  17  that produces sufficient pressure in their head chambers  34  and  38  to charge the accumulator  60  as shown in the dual cylinder energy recovery mode of  FIG. 5 . Here, the pressure at the accumulator is below the threshold provided by the following inequality:
 
 P   59 &lt;( P   56   +P   57 )/2− P   58   /R  
 
Here, P 59  is the pressure at the accumulator from sensor  59 , P 56  is the pressure at the head chamber  34  of the first cylinder assembly  16  from sensor  56 ; P 57  is the pressure at the head chamber  38  of the second cylinder assembly  17  from pressure at sensor  57 ; and P 58  is the pressure in the rod chambers  33  and  36  of the boom cylinder assemblies  16  and  17 , from sensor  58  (See  FIG. 3 ). R is the ratio of areas at the head chambers  34  and  38 , and the rod chambers  33  and  36 . The cylinder ratio is given by the equation:
 
 R=πr   A   2 /(πr A   2 −πr ROD   2 )
 
Here, r A  is the radius of the head chambers  34  and  38 , and r ROD  is the radius of the piston rods  35  and  37 . R is a constant for the selected cylinder assemblies  16  and  17  chosen for the hydraulic circuit. The term (P 56 +P 57 )/2−P 58 /R is referred to as the dual cylinder energy recovery mode differential pressure herein. In addition, it should be noted that the above inequality may be modified to include losses due to friction and other factors.
 
     In the dual cylinder energy recovery mode  121 , the fluid exhausting from the head chambers  34  and  38  is combined by an open cylinder separation control valve  39  and flows through an open recovery control valve  62  to charge the accumulator  60 . The recovery control valve  62  is modulated to proportionally control the velocity of the boom. Fluid required to fill the expanding rod chambers  33  and  36  as the boom descends is drawn through the control valve assembly  40 . Specifically, fluid from other functions of the machine is drawn from the return conduit  26  through the anti-cavitation check valve in the fourth EHP control valve  44 . Because the force of gravity is lowering the boom, the fluid drawn from the return conduit  26  does not have to be at a high pressure. If this anti-cavitation flow is insufficient, the third EHP control valve  43  can be opened to furnish fluid from the first pump  22  to the rod chambers  33  and  36 . The descent of the boom  13  reaches a position at which the force exerted on the two cylinder assemblies  16  and  17  no longer produces sufficient pressure in both head chambers to continue charging the accumulator  60 . When the pressure at the accumulator is below the threshold provided by the following inequality:
 
 P   59 &lt;(( P   56   +P   57 )/2− P   58   /R )*2
 
the energy recovery transitions into a split cylinder energy recovery mode  122  depicted in  FIG. 6 , that intensifies the pressure in one cylinder head chamber to charge the accumulator. The right side of this inequality is referred to as the split cylinder energy recovery mode differential pressure herein. It should be noted that the above inequality may be modified to include losses due to friction and other factors. While the recovery control valve  62  remains open to continue charging the accumulator  60 , the second EHP control valve  42  is gradually opened as the cylinder separation control valve  39  is closed. This sends pressurized fluid from the head chamber  34  of the first boom cylinder  31  through second EHP control valve  42  and the anti-cavitation valve in the fourth EHP control valve  44  to the rod chambers  33  and  36  of both boom cylinders. Closing the cylinder separation control valve  39 , isolates the two boom cylinders  31  and  32  from each other and shifts the two head chambers  34  and  38  from an initial equal pressure condition to states in which those chambers have different pressures and thus exert different forces. In the split cylinder energy recovery mode  122  the force from the boom is supported by only the second cylinder assembly  17  and thus the pressure in the head chamber  38  of the second cylinder  32  has higher pressure for charging the accumulator than when the boom force was supported by both cylinder assemblies  16  and  17  as in the dual cylinder energy recovery mode  121  shown in  FIG. 5 .
 
     The head chamber  38  of the second cylinder  32  produces a sufficiently high pressure therein to continue charging the accumulator  60 . Thus fluid from that head chamber  38  is directed through the recovery control valve  62  into the accumulator  60 . During this split cylinder energy recovery mode  122 , the recovery control valve  62  and the second EHP control valve  42  are modulated to control the rate at which the boom  13  continues to lower. 
     In the split cylinder energy recovery mode  122 , if the amount of the head chamber fluid is inadequate to fill both rod chambers  33  and  36 , the third EHP control valve  43  can be opened to furnish supplemental fluid from the first pump  22 . That supplemental fluid does not have to be at a particular pressure as it is not used to drive the cylinder assemblies  16  and  17 , but only to fill the expanding rod chambers. On the other hand, if the head chamber  34  of the first cylinder  31  contains more fluid than is needed to fill both rod chambers  33  and  36 , as occurs with a very large diameter piston rods, the excess fluid can be sent to the return conduit  26  by selectively opening the second EHP control valve  42 . 
     Because the flow of fluid from each head chamber  34  and  38  is controlled separately in the split cylinder energy recovery mode  122 , the forces on each side of the boom  13  may be unequal producing a twisting action thereon. To avoid that condition, a pseudo-split cylinder energy recovery mode  123  shown in  FIG. 7  can be employed. This mode can be entered directly from the dual cylinder energy recovery mode ( FIG. 5 ) when the pressure on the accumulator falls below the threshold provided by the following equation:
 
 P   59 &lt;( R/R− 1)*(( P   56   +P   57 )/2 −P   58   /R )
 
The right side of this inequality is referred to as the pseudo-split cylinder energy recovery mode differential pressure herein. It should be noted that the above inequality may be modified to include losses due to line losses, friction and other factors.
 
     In this mode, the cylinder separation control valve  39  remains open to communicate pressure between the two head chambers  34  and  38 . The EHP workport shunt control valve  65  opens to convey pressurized fluid from the head chamber  34  of the first boom cylinder  31  to both rod chambers  33  and  36 . 
     On a typical excavator, the boom cylinder assemblies  16  and  17  have large diameter piston rods  35  and  37 , so that as the piston moves the volume of each rod chamber  33  and  36  may change half the amount that the volume of each head chamber changes, for example. This means that in the pseudo-split cylinder energy recovery mode  123 , the fluid exhausting the first cylinder&#39;s head chamber  34  is sufficient to fill both of the expanding rod chambers  33  and  36 . Therefore, fluid does not flow through the open cylinder separation control valve  39 , however if that one to two volume relationship does not exist, any additional fluid needed to fill the rod chambers  33  and  36  can come through the cylinder separation control valve from the second cylinder&#39;s head chamber  38 . Nevertheless, most, if not all, of the fluid in head chamber  38  of the second cylinder  32  flows into the accumulator  60 . 
     When operation in a split cylinder energy recovery mode  122  or  123  reaches a point at which there no longer is sufficient pressure available from the head chamber  38  of the second cylinder  32  to charge the accumulator, but is greater than zero, as given by the following equation:
 
( P   56   +P   57 )/2− P   58   /R&gt; 0
 
the boom operation transitions into a cross chamber energy recovery mode  124  depicted in  FIG. 8 . The left side of this inequality is referred to as the cross chamber energy recovery mode differential pressure herein. It should be noted that the above inequality may be modified to include losses due to friction and other factors. In the cross chamber energy recovery mode  124  the recovery control valve  62  typically closes to preserve a relatively high pressure charge in the accumulator  60 . Nevertheless, there may be enough residual pressure in the head chamber  38  of the second boom cylinder  32  to continue charging the accumulator as indicted by pressure sensors  57  and  59  ( FIG. 3 ) and thus the recovery control valve  62  may be partially open in this mode. In either case, the cylinder separation control valve  39  opens along with the workport shunt control valve  65  so that some fluid from both head chambers  34  and  38  is conveyed into to fill the expanding rod chambers  33  and  36 . Because the aggregate amount of fluid exhausting from the head chambers is more than is needed to fill the rod chambers, the second EHP control valve  42  opens so to convey that excess fluid into the return conduit  26  and onward to the tank  23 .
 
     It should be noted that the energy recovery modes  121 ,  122 ,  123 , and  124  do not need to follow the sequence as described above. The selection of one of the energy recovery modes  121 ,  122 ,  123 , and  124  should be based on the recovery efficiency benefits that each mode would provide at a given time. Accordingly, any energy recovery mode may transition to any of the other energy recovery modes, and an appropriate selection can be made by the system controller  50  based on the equations provided herein. 
     In the cross chamber energy recovery mode  124 , the accumulator reaches peak storage capability. In addition, as the cylinder separation control valve  39  opens, pressure in the two cylinder head chambers  34  and  38  begins to equalize again. Although the preferred embodiment incorporates the workport shunt control valve  65 , that valve could be eliminated as a cost saving measure if the split cylinder energy recovery mode  123  is not used. In that case, at the times when the workport shunt control valve would be opened, the control valve assembly  40  is operated by opening the second and fourth EHP control valves  42  and  44  to convey fluid through one of those pairs between the two workports  46  and  48  along with opening the isolation valve  39 . 
     Eventually the boom  13  reaches such a low position that the forces due to gravity alone are insufficient to continue lowering the boom fast enough for efficient operation of the excavator. Pressure from a pump now is needed to further lower the boom. At this juncture, the operation transitions to a powered energy mode  125  shown in  FIG. 9 . Now the third EHP control valve  43  opens to apply pressurized fluid from the first pump  22  to the rod chambers  33  and  36  of both boom cylinders  31  and  32 . This pressurized fluid propels the pistons to further retract the piston rods thereby driving the boom  13  downward. The fluid exhausting from the head chambers  34  and  38  at this time is conveyed by the opened cylinder separation control valve  39  and the second EHP control valve  42  into the return conduit  26 . The second and third EHP control valves  42  and  43  are modulated to control the velocity of the boom. 
     The positions of the boom  13  and arm  14  of the excavator  10  affect the amount of force that the boom exerts on the cylinder assemblies  16  and  17  and thus the amount of energy that can be recovered. The amount of force corresponds to the cylinder chamber pressures as measured by the sensors  56 ,  57  and  58 . Therefore, the signals from those sensors along with the accumulator pressure sensor  59  enable the system controller  50  to determine which of the energy recovery modes are practical and which one will recover the most energy. 
     Energy Reuse 
     When it comes time to extend the piston rods from the boom cylinders  31  and  32  and raise the boom  13  against a load force F acting downward, fluid can be recycled from the accumulator  60  in place of or in addition to using pressurized fluid from the first pump  22 . In a first energy reuse mode  131  shown in  FIG. 10 , fluid stored in the accumulator  60  is fed via open recovery control valve  62  and cylinder separation control valve  39  to both cylinder head chambers  34  and  38 . Fluid that is exhausting from the rod chambers  33  and  36  flows via an opened fourth EHP control valve  44  into the return conduit  26 . 
     It should be understood that often the accumulator  60  is not charged to a pressure level that is sufficient to drive both cylinder assemblies  16  and  17 . In addition, the quantity of fluid stored in the accumulator also may not be sufficient to fill both head chambers  34  and  38 . In such instances, a second energy reuse mode  132  depicted in  FIG. 11  is implemented in which the recovery control valve  62  is opened while the cylinder separation control valve  39  is closed. This directs fluid from the accumulator  60  into only the head chamber  38  of the second cylinder  32 . The recovery control valve  62  typically is fully open to eliminate metering losses on the flow from the accumulator. The head chamber  34  of the first cylinder  31  receives pressurized fluid from the first pump  22  via the first EHP control valve  41 . Thus, the first cylinder  31  is driven by pump fluid and the second cylinder  32  by fluid from the accumulator. The first EHP control valve  41  and the recovery control valve  62  are modulated to control the rate at which the boom raises. While this is occurring, fluid exiting the two rod chambers  33  and  36  flows through an opened fourth EHP control valve  44  into the return conduit  26 . 
     The second pump  82  may be connected by a second supply valve  99  to the port of the head chamber  34  for the first boom cylinder  31 , in which case pressurized fluid from the second pump can be supplied to that head chamber to augment fluid from the first pump  22 . To accomplish this, the second supply valve  99  meters fluid to the head chamber  34  for the first boom cylinder  31 , while the first EHP control valve  41  is used to meter fluid flow. 
     Eventually, fluid from the accumulator  60  is depleted and can no longer be utilized to drive the second cylinder  32 . At that time, the hydraulic system operation may enter a third energy reuse mode  133  illustrated in  FIG. 12  in which fluid from the second pump  82  is used instead of or as a supplement to fluid from the accumulator  60 . This is accomplished by opening the supply control valve  98  to direct fluid from the second pump  82  to the head chamber  38  of the second cylinder  32 . The head chamber  34  of the first cylinder  31  continues to receive fluid from the first pump  22  via the control valve assembly  40  and fluid exhausting from the rod chambers  33  and  36  also is fed through the control valve assembly to the return conduit  26 . In third energy reuse mode  133 , the first EHP control valve  41  and the supply control valve  98  are modulated to control the rate at which the boom  13  raises. 
       FIG. 13  shows a fourth energy reuse mode  134  in which the outputs of the first and second pumps  22  and  82  are combined by the cylinder separation control valve  39  and applied to both head chambers  34  and  38 . In the fourth energy reuse mode  134 , fluid from the first pump  22  is conveyed by the first EHP control valve  41  to head chambers  34  and  38 , while the supply control valve  98  conveys fluid from the second pump  82  to those same chambers. Some fluid may flow from the accumulator  60  depending upon the pressure level therein. Fluid that is exhausting from the rod chambers  33  and  36  flows via an opened fourth EHP control valve  44  into the return conduit  26 . 
       FIG. 14  illustrates a fifth energy reuse mode  135  in which fluid from only the first pump  22  powers the head chambers  34  and  38  of both hydraulic cylinder assemblies  16  and  17 . The second pump  82  does not supply the boom function  30  in this mode. Now the first EHP control valve  41  controls the flow of fluid from the first pump  22  to the head chambers  34  and  38  and the rate at which the boom is raised. The fourth EHP control valve  44  controls the fluid flow from the rod chambers  33  and  36  to the return conduit  26 . 
     In the first through fifth energy reuse modes  131 - 135  the force acting on the boom  13  tended to lower the boom. In other operational states of the excavator  10 , an external force tends to raise the boom  13 . For example with reference to  FIG. 1 , assume that the boom assembly  12  is fully extended for its farthest reach from the excavator cab  11  and then the arm cylinder assembly  18  is powered to draw the bucket toward the cab to dig into the ground. Resistance to this digging action exerts an upward force which tends to raise the boom without applying pressurized fluid from either pump  22  or  82  to the boom cylinder assemblies  16  and  17 . 
     While this upward force is being exerted on the boom  13 , the portion of the hydraulic system for the boom cylinder assemblies  16  and  17  can be configured as depicted in  FIG. 15 . In this sixth reuse mode  136 , the forces acting on the boom  13  further extend the piston rods from the cylinders  31  and  32  which forces fluid from the rod chambers  33  and  36  to the second workport  48  of the control valve assembly  40 . The fourth EHP control valve  44  now is opened to a degree that controls the boom to a desired velocity and conveys the exhausting fluid into the return conduit  26 . However, the expanding head chambers  34  and  38  produce a low pressure at the first workport  46  which causes the anti-cavitation valve within the second EHP control valve  42  to open conveying the pressurized fluid from the return node to the first workport  46 . That fluid continues to flow from the first workport  46  to both head chambers  34  and  38  via a now opened cylinder separation control valve  39 . Because the combined volume of the head chambers  34  and  38  is greater than the combined volume of the two rod chambers  33  and  36  additional fluid is required to fill the head chambers. That additional fluid is drawn into the control valve assembly  40  either from the return conduit  26  or if sufficient pressure does not exist in that conduit as indicated by pressure sensor  55 , the first EHP control valve  41  is opened to furnish fluid from the first pump  22 . The fluid from the first pump does not have to be supplied at a particular pressure as it is not driving the cylinders, but merely filling the expanding chambers. 
     Although the hydraulic system is described above as including a cylinder separation control valve  39 , advantages of the invention related to recovery and reuse of energy in the accumulator as discussed above can also be achieved without this valve. Here, the head chamber  34  of the first cylinder assembly  16  and head chamber  38  of the second cylinder assembly  17  are tied together in fluid communication, rather than coupled to the cylinder separation control valve  39 . During a recovery operation, in which excess pressure is provided to the accumulator, a circuit constructed in this way would operate as described above with respect to  FIGS. 5 ,  7 ,  8  and  9 , moving through the modes of  FIGS. 5 ,  7 ,  8 , and  9  as described above. During reuse, referring to  FIGS. 2 and 3 , fluid flows from the accumulator  60  through port  61  to charging and reuse control valve  66  which is opened to supply conduit  25 . The first pump  22  may also provide additional fluid to the supply conduit  25  in this reuse mode. Although two cylinders  16  and  17  are shown, when the cylinder separation valve  39  is removed, a single cylinder can be used. Irrespective of whether one or two cylinders is used, a single pressure sensor  56  or  57  can be used. 
     The foregoing description was primarily directed to preferred embodiments of the present invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention.