Abstract:
Systems and methods for controlling and actuating actuators that perform multiple functions in a machine. Such systems and methods encompass a hydraulic system adapted to control and actuate the actuators of the machine. The hydraulic system includes variable displacement pump/motors connected to the engine in parallel. A first of the pump/motors controls a first of the actuators, and a second of the pump/motors is adapted to draw power from and deliver power to the engine and the first actuator, as well as control at least a second of the actuators. An energy storage device is connected in series with the second pump/motor and the second actuator, and accumulates a fluid pumped thereto by the second pump/motor, as well as delivers the fluid to the second pump/motor, depending on whether the second pump/motor delivers is delivering or drawing power from the engine or first actuators.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/453,368, filed Mar. 16, 2011, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to systems for operating hydraulic circuits. More particularly, this invention relates to hydraulic systems containing multiple displacement-controlled (DC) actuators, for example, of a multi-function machine, and an energy storage system therefor. 
         [0003]    Compact excavators, wheel loaders and skid-steer loaders are examples of multi-function machines whose operations involve controlling movements of various implements of the machines.  FIG. 1  illustrates a compact excavator  100  as having a cab  101  mounted on an undercarriage  102  via a swing bearing (not shown) or other suitable device. The undercarriage  102  includes tracks  103  and associated drive components, such as drive sprockets, rollers, idlers, etc. The excavator  100  is further equipped with a blade  104  and an articulating mechanical arm  105  comprising a boom  106 , a stick  107 , and an attachment  108  represented as a bucket, though it should be understood that a variety of different attachments could be mounted to the arm  105 . The functions of the excavator  100  include the motions of the boom  106 , stick  107  and bucket  108 , the offset of the arm  105  during excavation operations with the bucket  108 , the motion of the blade  104  during grading operations, the swing motion for rotating the cab  101 , and the left and right travel motions of the tracks  103  during movement of the excavator  100 . In the case of a compact excavator  100  of the type represented in  FIG. 1 , the blade  104 , boom  106 , stick  107 , bucket  108  and offset functions are typically powered with linear actuators  109 - 114  (represented as hydraulic cylinders in  FIG. 1 ), while the travel and swing functions are typically powered with rotary hydraulic motors (not shown in  FIG. 1 ). 
         [0004]    On conventional excavators, the control of these functions is accomplished by means of directional control valves. However, throttling flow through control valves is known to waste energy. In some current machines, the rotary functions (rotary hydraulic drive motors for the tracks  103  and rotary hydraulic swing motor for the cabin  101 ) are realized using displacement control (DC) systems, which notably exhibit lower power losses and allow energy recovery. In contrast, the position and velocity of the linear actuators  109 - 114  for the blade  104 , boom  106 , stick  107 , bucket  108 , and offset functions typically remain controlled with directional control valves. It is also possible to control linear hydraulic actuators directly with hydraulic pumps. Several pump-controlled configurations are known, using both constant and variable displacement pumps. Displacement control of linear actuators with single rod cylinders has been described in U.S. Pat. No. 5,329,767 and German Patents DE000010303360A1, EP000001588057A1 and WO002004067969. Other aspects of using displacement control systems can be better appreciated from further reference to Zimmerman et al., “The Effect of System Pressure Level on the Energy Consumption of Displacement Controlled Actuator Systems,” Proc. of the 5th FPNI PhD Symposium, Cracow, Poland, 77-92 (2008), and Williamson et al., “Efficiency Study of an Excavator Hydraulic System Based on Displacement-Controlled Actuators,” Bath ASME Symposium on Fluid Power and Motion Control (FPMC2008), 291-307 (2008), whose contents are incorporated herein by reference. An example of the capability of achieving reductions in energy requirements using displacement control systems is taught in U.S. Published Patent Application No. 2010/0162593, whose contents are incorporated herein by reference. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0005]    The present invention provides a system and method for controlling and actuating multiple actuators that perform multiple functions in a machine using variable displacement pumps. 
         [0006]    According to a first aspect of the invention, a hydraulic system is provided that is adapted to be installed on a machine that includes an engine and multiple actuators that perform multiple functions of the machine, and the hydraulic system is adapted to control and actuate the multiple actuators of the machine. The hydraulic system includes a plurality of first variable displacement pump/motors adapted to be powered in parallel by the engine and a second variable displacement pump/motor adapted to be connected to the engine in parallel with the first variable displacement pump/motors. The first variable displacement pump/motors are operable to control flow of a first fluid to control first actuators of the multiple actuators and the corresponding functions performed thereby. The second variable displacement pump/motor is adapted to draw power from and deliver power to the engine, draw power from and deliver power to the first actuators through the first variable displacement pump/motors, and control flow of a second fluid to control at least a second actuator of the multiple actuators and the corresponding function performed thereby. The hydraulic system further includes an energy storage device connected in series with the second variable displacement pump/motor and the second actuator. The energy storage device is adapted to accumulate the second fluid pumped thereto by the second variable displacement pump/motor when the second variable displacement pump/motor draws power from the engine or from the first actuators through the first variable displacement pump/motors, and is adapted to deliver the second fluid to the second variable displacement pump/motor when the second variable displacement pump/motor delivers power to the engine or to the first actuators through the first variable displacement pump/motors. 
         [0007]    According to a second aspect of the invention, a machine is provided that includes an engine, multiple linear actuators and at least a first rotary actuator that perform functions of the machine, and a hydraulic system that controls and actuates the linear actuators and the first rotary actuator. The hydraulic system includes a plurality of first variable displacement pump/motors powered in parallel by the engine, and a second variable displacement pump/motor connected to the engine in parallel with the first variable displacement pump/motors. The first variable displacement pump/motors are operable to control flow of a first fluid to control at least some of the linear actuators and the corresponding functions performed thereby. The second variable displacement pump/motor is capable of drawing power from and delivering power to the engine, drawing power from and delivering power to at least some of the linear actuators through the first variable displacement pump/motors, and controlling flow of a second fluid to control the first rotary actuator and the corresponding function performed thereby. The hydraulic system further includes an energy storage device connected in series with the second variable displacement pump/motor and the first rotary actuator. The energy storage device is adapted to accumulate the second fluid pumped thereto by the second variable displacement pump/motor when the second variable displacement pump/motor draws power from the engine or from at least some of the linear actuators through the first variable displacement pump/motors, and is adapted to deliver the second fluid to the second variable displacement pump/motor when the second variable displacement pump/motor delivers power to the engine or to at least some of the linear actuators through the first variable displacement pump/motors. 
         [0008]    Another aspect of the invention is an excavator machine that includes an engine, multiple linear actuators that control a first set of multiple implements of the machine, at least a first rotary actuator that controls at least a second implement of the machine, and a hydraulic system that controls and actuates the linear actuators and the first rotary actuator. The hydraulic system includes a plurality of first variable displacement pump/motors powered in parallel by the engine, and a second variable displacement pump/motor connected to the engine in parallel with the first variable displacement pump/motors. Each of the first variable displacement pump/motors controls flow of a first fluid to control a corresponding one of the linear actuators and a corresponding one of the first set of multiple implements. The second variable displacement pump/motor is capable of drawing power from and delivering power to the engine, drawing power from and delivering power to the linear actuators through the first variable displacement pump/motors, and controlling flow of a second fluid to control the first rotary actuator and the second implement. The hydraulic system further includes a hydraulic accumulator connected in series with the second variable displacement pump/motor and the first rotary actuator. The hydraulic accumulator is adapted to accumulate the second fluid pumped thereto by the second variable displacement pump/motor when the second variable displacement pump/motor draws power from the engine or from the linear actuators through the first variable displacement pump/motors, and is adapted to deliver the second fluid to the second variable displacement pump/motor when the second variable displacement pump/motor delivers power to the engine or to the linear actuators through first variable displacement pump/motors. 
         [0009]    Other aspects of the invention include methods of operating hydraulic systems and machines of the types described above. A particular method includes powering a plurality of first variable displacement pump/motors and a second variable displacement pump/motor that are connected in parallel to an engine of a machine. The first variable displacement pump/motors control flow of a first fluid to control the linear actuators and the corresponding functions performed thereby, and the second variable displacement pump/motor is operated to draw power from or deliver power to the engine, and/or draw power from or deliver power to the linear actuators through the first variable displacement pump/motors, and/or control flow of a second fluid to control the first rotary actuator and the corresponding function performed thereby. The second fluid is accumulated in an energy storage device connected in series with the second variable displacement pump/motor and the first rotary actuator. The energy storage device accumulates the second fluid pumped thereto by the second variable displacement pump/motor when the second variable displacement pump/motor draws power from the engine or from the linear actuators through the first variable displacement pump/motors. Furthermore, the energy storage device delivers the second fluid to the second variable displacement pump/motor when the second variable displacement pump/motor delivers power to the engine or to the linear actuators through the first variable displacement pump/motors. 
         [0010]    A technical effect of the invention is the ability of a hydraulic system to capture energy from actuators or an engine of a machine, store the captured energy in an energy storage device, and then deliver the captured energy to the engine/actuators in a controlled manner and time frame. The invention is particularly adapted for use with architectural arrangements of both rotary and linear actuators that are used to control implements of a machine, and offers the possibility of significantly reducing the engine power, energy, and fuel consumption requirements of such machines. 
         [0011]    Other aspects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  schematically represents a compact excavator of a type known in the prior art. 
           [0013]      FIG. 2  schematically represents a hybrid (series-parallel) displacement-controlled hydraulic system in accordance with an embodiment of the invention and suitable for controlling linear and rotary actuators of a machine, such as the excavator of  FIG. 1 . 
           [0014]      FIGS. 3 and 4  schematically represent alternative embodiments of hydraulic systems of the invention. 
           [0015]      FIG. 5  schematically represents a hydraulic control suitable for use in the hydraulic system of  FIG. 4 . 
           [0016]      FIGS. 6 through 10  schematically represent additional embodiments of hydraulic systems of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    As noted above, the present invention relates to architectural arrangements of hydraulic actuators for machine systems having both rotary and linear actuators, a nonlimiting example of which is the excavator  100  represented in  FIG. 1 . The invention is a hybrid system adapted to capture energy from the actuators or an engine of the machine, store the captured energy in an energy storage device, and then deliver the stored energy back to the engine and/or actuators in a controlled manner and time frame. According to a particular aspect of the invention, the hybrid system is a purely hydraulic hybrid system in which one or more hydraulic accumulators serve as the energy storage device. As will be appreciated from the following discussion, the linear actuators may be single rod cylinders each controlled by a variable displacement pump using a method referred to as displacement control. The hybrid system is capable of recovering power from the linear actuators, for example, energy from negative loads such as gravity-assisted lowering or actuator braking, and is also capable of transferring the recovered energy back to a variable displacement pump for actuating the linear actuators. According to a preferred aspect of the invention, at least some of the rotary actuators of the system are controlled by a method referred to herein as secondary control, in which a hydraulic pump is connected in series to the accumulator and supplies power to one or more rotary actuators using one or more variable displacement hydraulic motors. Because the excavator  100  of  FIG. 1  is a useful example of a machine that utilizes both rotary and linear actuators, the following discussion will make reference to the excavator  100  of  FIG. 1 , though it should be understood that the invention is not so limited. 
         [0018]      FIG. 2  represents an embodiment of the invention as a system  10  that includes three displacement-controlled linear actuators (hydraulic cylinders)  12 ,  14  and  16  and three secondary-controlled rotary actuators (motors)  18 ,  20  and  22 , though it should be understood that the invention pertains to any number of linear and rotary actuators as long as there is at least one of each. As a point of reference, the linear actuators  12 ,  14  and  16  may correspond to any of the linear actuators  109 - 114  of the excavator  100  of  FIG. 1 , and the rotary actuators  18 ,  20  and  22  may correspond to any of the rotary actuators (not shown) of the excavator  100  of  FIG. 1 . As such, it should be appreciated that the system  10  and its actuators  12  through  22  are suitable for installation on a machine to operate various implements of the machine. Again referring to  FIG. 1 , the linear actuators  12 ,  14  and  16  can be adapted to power the blade  104 , boom  106 , stick  107 , bucket  108  and offset functions of the excavator  100 , and the rotary actuators  18 ,  20  and  22  can be variable displacement motors adapted to power the travel (tracks  103 ) and swing functions of the excavator  100 . It is not necessary that the linear actuators  12 ,  14  and  16  be controlled using displacement control, in that they could be controlled using a single pump and control valves, or by any other means. However, the greatest benefits come when they are controlled using a means that allows energy recovery. 
         [0019]      FIG. 2  further depicts the system  10  as what may be referred to as a hybrid displacement-controlled hydraulic system or a series-parallel displacement-controlled hydraulic system for controlling the linear and rotary actuators  12  through  22 , and represents an embodiment of the system  10  as comprising four variable displacement pump/motors  24 ,  26 ,  28  and  30  connected to an engine  32  by any suitable connection  34 , for example, shafts, gear boxes, belt drives, etc. Three of the variable displacement pump/motors  24 ,  26  and  28  are individually fluidically coupled to the linear actuators  12 ,  14  and  16 , such that the system  10  utilizes a single pump/motor  24 ,  26  and  28  for each linear actuator  12 ,  14  and  16 , respectively. In contrast, the fourth variable displacement pump/motor  30  is fluidically coupled to all of the rotary actuators  18 ,  20  and  22 . Power can be transferred between these pump/motors  24  through  30  through their connections  34  to the engine  32 . Flow of any suitable hydraulic fluid from the pump/motors  24 ,  26  and  28  to their respective actuators  12 ,  14  and  16  is represented as being through a hydraulic circuit that includes lines  38  and  40  that supply the hydraulic fluid to either of two chambers of the linear actuators  12 ,  14  and  16 . Each pair of lines  38  and  40  for each actuator  12 ,  14  and  16  is interconnected with check valves, through which the hydraulic circuit for each pump/motor  24 ,  26  and  28  is fluidically connected to a low pressure accumulator  42 . The engine  32  is represented as driving a charge pump  44  that is fluidically connected to the accumulator  42 , which serves as a low pressure flow source for the pump/motors  24 ,  26  and  28 , and not as an energy storage device. 
         [0020]    The fourth variable displacement pump/motor  30  can be referred to as an energy storage pump  30 , in that the pump  30  is adapted to be responsible for storing excess energy recovered from the linear actuators  12 ,  14  and  16  and/or delivered by the engine  32  into a high pressure accumulator  36 , and then distributing that energy back to the engine  32  and/or the linear actuators  12 ,  14  and  16  at a later time as needed. The energy storage pump  30  is also responsible for providing the necessary flow for the rotary actuators  18 ,  20  and  22 . As such, an energy storage device in the form of the accumulator  36  is directly linked in series to the energy storage pump  30  and to each of the rotary actuators  18 ,  20  and  22 . In the embodiment of  FIG. 2 , the secondary-controlled rotary actuators  18 ,  20  and  22  are in an open circuit, and each rotary actuator  18 ,  20  and  22  is connected to a reservoir to ensure that a continuous supply of hydraulic fluid is available to each rotary actuator  18 ,  20  and  22 .  FIG. 2  further shows the optional inclusion of a valve  46  for locking the hydraulic fluid within the high pressure accumulator  36 , and a valve  48  for limiting the pressure within the hydraulic circuit containing the rotary actuators  18 ,  20  and  22 . 
         [0021]    As should be apparent from  FIG. 2 , the linear actuators  12 ,  14  and  16  and the rotary actuators  18 ,  20  and  22  may be interconnected solely through their mechanical connections  34  to the engine  32 , such that the hydraulic circuit containing the linear actuators  12 ,  14  and  16  and the hydraulic circuit containing the rotary actuators  18 ,  20  and  22  may be fluidically isolated from each other and contain two separate fluids. However, it should also be apparent that these hydraulic circuits can be fluidically interconnected as a result of sharing a common reservoir. 
         [0022]    The system  10  represented in  FIG. 2  (as well as  FIGS. 3-10 ) can be referred to as a series-parallel hybrid displacement-controlled system in the sense of the following. The circuit containing the rotary actuators  18 ,  20  and  22  is a series hybrid because power is transferred in series from the engine  32  to the accumulator  36  (operating as a secondary power supply) and the implement(s) controlled by the rotary actuators  18 ,  20  and  22  (for example, the tracks  103  and/or swing functions of the excavator  100 ). Furthermore, energy recovered from these same implement(s), for example, energy from negative loads such as actuator braking, can be returned through the same path. This series hybrid circuit operates in parallel with each of the displacement-controlled linear actuators  12 ,  14  and  16  and the implement(s) controlled by the linear actuators  12 ,  14  and  16  (for example, the boom  106 , stick  107  and bucket  108  of the excavator  100 ), which can still receive power from the engine  32  and/or the accumulator  36  in parallel. In addition, any power recovered by the linear actuators  12 ,  14  and  16 , for example, energy from negative loads such as gravity-assisted lowering of the implements, can be transferred through their respective connections  34  to the energy storage pump  30 , which then stores the recovered energy in the high pressure accumulator  36 . 
         [0023]      FIG. 3  represents a second embodiment of a hybrid displacement-controlled hydraulic system  10  of the invention that is similar to  FIG. 2 , but differs from the embodiment of  FIG. 2  as a result of the secondary control of the rotary actuators  18 ,  20  and  22  being within a closed circuit, as opposed to each rotary actuator  18 ,  20  and  22  being connected to a reservoir as represented in  FIG. 2 .  FIG. 3  shows the closed circuit as also being connected to the low pressure accumulator  42 , which ensures that a continuous supply of hydraulic fluid is available to each rotary actuator  18 ,  20  and  22 . As such, the linear actuators  12 ,  14  and  16  and the rotary actuators  18 ,  20  and  22  are both mechanically and fluidically interconnected through their mechanical connections  34  to the engine  32  and through the fluid lines to the low pressure accumulator  42 , such that the hydraulic circuits containing the linear actuators  12 ,  14  and  16  and rotary actuators  18 ,  20  and  22  contain the same hydraulic fluid. However, it should also be apparent that these hydraulic circuits can be fluidically interconnected as a result of sharing a common reservoir. 
         [0024]      FIGS. 4 and 5  represent a third embodiment of a hybrid displacement-controlled hydraulic system  10  of the invention that is similar to  FIG. 2 , but differs by showing that the high pressure available from the high pressure accumulator  36  can be used as inputs to controls  50  for each of the variable displacement pump/motors  24 ,  26  and  28  for the linear actuators  12 ,  14  and  16  and for the energy storage pump  30  and each rotary actuator  18 ,  20  and  22 . In particular,  FIG. 5  represents an example of one of the controls  50  for the pump/motors  24 ,  26  and  28 . The control  50  includes a line-in  52  from the high pressure accumulator  36  and an electronically-controlled hydraulic valve  54  that controls the flow of hydraulic fluid from the line-in  52  to a hydraulic cylinder  56 , whose output (position) is used to control the pump/motor  24 / 26 / 28  as schematically represented in  FIG. 5 . This additional capability is beneficial because the high pressure of the accumulator  36  is available within the system  10  at no extra energy cost, and the relative high hydraulic pressure available from the accumulator  36  allows for a reduction in size of the means (valve  54  and cylinder  56 ) that would typically be used to control the operations of the variable displacement pump/motors  24 ,  26  and  28 . Alternatively, the high pressure available from the accumulator  36  can be employed with valves  54  and cylinders  56  of a more conventional size to more rapidly operate the valves  54  and cylinders  56 , resulting in faster control capabilities for the controls  50  and the pump/motors  24 ,  26  and  28  they control. 
         [0025]      FIG. 6  represents a fourth embodiment of a hybrid displacement-controlled hydraulic system  10  of the invention that is similar to  FIG. 2 , but differs from the embodiment of  FIG. 2  as a result of the inclusion of an auxiliary attachment  58  that is fluidically connected to the energy storage pump  30 , rotary actuators  18 ,  20  and  22 , and high pressure accumulator  36  through an electronically-controlled hydraulic valve  59 .  FIG. 6  is notable for illustrating an additional benefit of the system  10 , particularly in relation to conventional displacement control systems, for example, of the type conventional used to control the rotary functions (rotary hydraulic drive motors for the tracks and rotary hydraulic swing motor for the cabin) of excavators of the type represented in  FIG. 1 . Perhaps the largest disadvantage of conventional displacement control systems is their requirement for one pump for each actuator. For an excavator of the type shown in  FIG. 1 , such a requirement conventionally necessitates the use of six pump/motors for the six primary functions of the excavator  100  (swing, boom, stick, bucket, left travel track, and right travel track). These machines often have options for high flow auxiliary attachments in addition to the primary working functions of the machine. For excavators utilizing a conventional displacement control system, this would require an additional high flow pump or would require disablement of one of the standard functions to allow one of the pumps to power the auxiliary attachment. In contrast, the system  10  in  FIG. 6  is represented as incorporating the auxiliary attachment  58  without the further addition of another pump and without the need to disable a standard function of the machine. It should be appreciated that  FIG. 6  represents only one of a variety of possible approaches for powering one or more auxiliary attachments (functions)  58  of the excavator (or other machine) using the energy storage pump  30  in the system  10 , evidencing the ability of the system  10  to be more versatile by allowing miscellaneous additional functions to easily be integrated into the system  10  without requiring additional pumps. 
         [0026]      FIGS. 7 through 10  represent further embodiments of hybrid displacement-controlled hydraulic systems  10  of the invention that incorporate various aspects of the embodiments of  FIGS. 2 through 6 , as well as additional features within the scope of the invention. 
         [0027]      FIGS. 7 and 8  represent open circuit hydraulic systems  10  similar to the open circuit hydraulic systems of  FIGS. 2 ,  4  and  6 , and  FIGS. 9 and 10  represent closed circuit hydraulic systems  10  similar to the closed circuit hydraulic system of  FIG. 3 . Each of  FIGS. 7 through 10  further represents its respective system  10  as including an anti-cavitation valve  60 , which can be of a type known used in conventional hydraulic systems. In addition, the systems  10  of  FIGS. 7 through 10  are further represented as including additional valves  48  for limiting pressures within the individual hydraulic circuits associated with the variable displacement pump/motors  24 ,  26  and  28  that control the linear actuators  12 ,  14  and  16 . The open circuit systems  10  of  FIGS. 7 and 8  differ from each other and the closed circuit systems  10  of  FIGS. 9 and 10  differ from each other by the manner in which their respective locking valves  46  are positioned. Finally, each of  FIGS. 7 through 10  represents the inclusion of multiple auxiliary attachments (functions)  58 , similar to  FIG. 6 , and each represents line breaks that represent that any number of linear actuators  12 ,  14  and  16  and rotary actuators  18 ,  20  and  22  can be incorporated in the systems  10 . 
         [0028]    From the above, it can be seen that the present invention and hybrid displacement-controlled systems  10  thereof can achieve significant energy savings as compared to conventional control systems in which control of the functions of a multi-function machine is accomplished by means of directional control valves, and in which throttling flow through the control valves results in wasted energy. In addition, the invention offers further energy savings by providing a means to recover and store energy from the machine actuators. Because energy can be stored and power can be transferred between the linear actuators  12 ,  14  and  16  and the rotary actuators  18 ,  20  and  22 , the invention also makes it possible to reduce the peak power requirement and improve the operating efficiency of the engine  32  (or other power supply) by controlling the load using the energy stored in the high pressure accumulator  36 , while still being capable of providing peak power demands to the actuators  12  through  22 . Compared to alternate hybrid system designs, the hybrid systems  10  of this invention are capable of reducing the number of components needed to control the actuators  12  through  22  because a single pump  30  can be used for all rotary actuators  18 ,  20  and  22  in the system  10 . The invention can also be beneficial to systems, equipment and machines that use displacement-controlled linear actuators because the high pressure accumulator  36  is capable of providing a high pressure source that can improve the response of the displacement-controlled actuators and allow the actuators to be more compact. 
         [0029]    Results from a simulation study using mathematical models that compared a conventional non-hybrid displacement-controlled hydraulic system with hybrid displacement-controlled systems  10  of this invention demonstrated that the rated engine power of an engine of an excavator (for example,  FIG. 1 ) could be reduced by approximately half with the hybrid systems  10  without losing any performance from the digging functions of the excavator. Furthermore, the simulation predicted that excavators equipped with the hybrid systems  10  may consume about 20% less fuel than the simulated non-hybrid displacement-controlled systems based on a high power cycle when the excavator was operated by an expert operator, and even greater fuel savings were predicted if the excavator were operated by a novice operator on a low power cycle. Simulations also quantified benefits of hybrid hydraulic systems  10  of the invention, particularly in terms of engine power, energy, and fuel consumption. 
         [0030]    In addition to excavators, the invention can be implemented on a variety of heavy mobile hydraulic machines, such as wheel loaders and other similar material-handling machines having both linear and rotary actuators. Suitable pumping capacities of the pump/motors  24 ,  26  and  28  and the energy storage pump  30  and suitable operating pressures and capacities for the high pressure accumulator  36  and low pressure accumulator  42  will depend on the particular application. 
         [0031]    While the invention has been described in terms of specific embodiments, it is apparent that other forms could be adopted by one skilled in the art. For example, the functions of certain components of the systems could be performed by components of different construction but capable of a similar (though not necessarily equivalent) function. Furthermore, other methods of control could be used for control of the linear actuators  12 ,  14  and  16  than described. For example a single pump could be used to provide flow and pressure and control valves could be used to control the motion of the linear actuators  12 ,  14  and  16 . Though power could still be transferred from the accumulator  36  to the linear actuators  12 ,  14  and  16 , using such a method would reduce the efficiency of the invention as a result of preventing power from being recovered from the linear actuators  12 ,  14  and  16  to be stored in the accumulator  36 . Accordingly, it should be understood that the invention is not limited to the specific embodiment illustrated in the drawings, and the scope of the invention is to be limited only by the following claims.