Abstract:
A digital hydraulic system including a fluid having three portions each at a pressure and a digital hydraulic transformer having a first and second element. The first element has a first end and a second end. The first and second elements operating along a common axis, and together defining at least four variable volume working chambers. The at least two of the at least four variable volume working chambers containing fluid directing force on the second element in a direction toward the first end. Another two of the at least four variable volume working chambers containing fluid directing force on the second element toward the second end. A control system individually selectively fluidically connects and disconnects each of the four variable volume working chambers to and from the three portions of the fluid at the three pressures.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a divisional of U.S. patent application Ser. No. 12/352,398, entitled “DIGITAL HYDRAULIC SYSTEM”, filed Jan. 12, 2009, which is incorporated herein by reference, which was a continuation-in-part of U.S. patent application Ser. No. 11/564,065, entitled “DIGITAL HYDRAULIC SYSTEM”, filed Nov. 28, 2006, which is incorporated herein by reference. U.S. patent application Ser. No. 11/564,065 was a non-provisional application based upon U.S. provisional patent application Ser. No. 60/740,345, entitled “DIGITAL HYDRAULIC SYSTEM”, filed Nov. 29, 2005. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a control system for a hydraulic work machine, and more particularly the present invention relates to the monitoring of potential and kinetic energies in movable elements of a hydraulic work machine, and to the control of hydraulic energy added to the hydraulic system. 
         [0004]    2. Description of the Related Art 
         [0005]    Hydraulics has a history practically as old as civilization itself. Hydraulics, more generally, fluid power, has evolved continuously and been refined countless times into the present day state in which it provides a power and finesse required by the most demanding industrial and mobile applications. Implementations of hydraulic systems are driven by the need for high power density, dynamic performance and maximum flexibility in system architecture. The touch of an operator can control hundreds of horsepower that can be delivered to any location where a pipe can be routed. The positioning tolerances can be held within thousandths of an inch and output force can be continuously varied in real time with a hydraulic system. Hydraulics today is a controlled, flexible muscle that provides power smoothly and precisely to accomplish useful work in millions of unique applications throughout the world. 
         [0006]    Work machines are commonly used to move heavy loads, such as earth, construction material, and/or debris. These work machines, which may be, for example, excavators, wheel loaders, bulldozers, backhoes, telehandlers and track loaders, typically include different types of work implements that are designed to perform various moving tasks. Work implements may be, for example, a loader, shovel, bucket, blade, or fork. For the purposes of the present disclosure, the term “work implement” may also include the individual components of the work implement, such as a boom or stick. The work implements of these work machines are commonly moved by hydraulic actuators powered by hydraulic systems, which use pressurized fluid to move the work implements. 
         [0007]    In many situations, the work implement of the work machine is raised to an elevated position. As the work implement may be relatively heavy, the work implement gains significant potential energy when raised to the elevated position. When the work implement is released from the elevated position the potential energy is usually converted to heat when the pressurized fluid is throttled across a valve and returned to the tank. Some of the potential energy of a work implement in an elevated position may be captured by redistributing that energy into an accumulator as a volume of pressurized hydraulic fluid. The stored energy can be used to perform useful work at a later time. 
         [0008]    In addition to potential energies associated with elevated implements of work machines, there may be substantial kinetic energy in implements moving linearly or rotatively at points in a work cycle. Examples of such points in work cycles include: a rapid decent of a work implement from an elevated position to a lower position, and the rotation of a work machine superstructure commonly referred to as the swing function. Upon deceleration of the moving work implement, some of the kinetic energy of a work implement in motion may be captured by redistributing that energy into an accumulator as a volume of pressurized hydraulic fluid. The stored energy can be used to perform useful work at a later time. 
         [0009]    Hydraulic transformers known in the art are designed to be used in conjunction with constant or semi-constant supply pressure as the energy source. The energy source may be driven by any of a variety of prime movers such as a diesel engine, gasoline engine, or an electric motor, and the energy supplied by the energy source may be supplemented by energy delivered by a hydraulic accumulator. Typically, however, there are no means by which a prime mover is governed to add energy only up to a pressure level less than a preset supply pressure. 
         [0010]    In order to take full advantage of the benefits allowed by the digital hydraulic system, it is necessary to control the energy input into the hydraulic system. 
         [0011]    In the event that a work implement has substantial potential and/or kinetic energy, it is advantageous in terms of energy efficiency to maintain a capacity for energy storage in the hydraulic accumulator approximately equal to the cumulative potential and kinetic energies of the work machine such that a maximum amount of potential and kinetic energy may be redistributed to the accumulator. 
         [0012]    What is needed in the art is a control system that controls hydraulic energy input by the prime mover based on potential and kinetic energies of the work machine. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention provides a digital hydraulic system including a hydraulic actuator, a digital hydraulic transformer and/or a digital hydraulic pump utilized in a system to controllably provide power. 
         [0014]    The invention in one form is directed to a digital hydraulic system including a fluid having three portions each at a pressure and a digital hydraulic transformer having a first and second element. The first element has a first end and a second end. The first and second elements operating along a common axis, and together defining at least four variable volume working chambers. The at least two of the at least four variable volume working chambers containing fluid directing force on the second element in a direction toward the first end. Another two of the at least four variable volume working chambers containing fluid directing force on the second element toward the second end. A control system individually selectively fluidically connects and disconnects each of the four variable volume working chambers to and from the three portions of the fluid at the three pressures. 
         [0015]    An advantage of the present invention is that energy utilization in a work machine may be optimized for maximum efficiency. 
         [0016]    Another advantage of the present invention is that no energy will be intentionally wasted upon redistribution of potential and kinetic energies in work implements. 
         [0017]    Yet another advantage of the present invention is that it can be utilized in four quadrant operation. 
         [0018]    Yet another advantage of the present invention is that it requires less cooling of the hydraulic fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
           [0020]      FIG. 1  illustrates a backhoe utilizing an embodiment of a digital hydraulic system of the present invention; 
           [0021]      FIG. 2  is a schematical illustration of an embodiment of digital hydraulic system of the present invention; 
           [0022]      FIG. 3  is another schematical illustration of the digital hydraulic system of  FIGS. 1 and 2 ; 
           [0023]      FIG. 4  is an illustrative table showing multiple operation modes of the digital hydraulic system of  FIGS. 1-3 ; 
           [0024]      FIG. 5  is a schematical illustration of an actuator/pump used by the digital hydraulic system of  FIGS. 1-3 ; 
           [0025]      FIG. 6  is a schematical illustration of a double acting actuator/pump usable by the hydraulic system of  FIGS. 1-3 ; 
           [0026]      FIG. 7  is a schematical cross-sectional view of single acting pump/actuator of  FIG. 5 ; 
           [0027]      FIG. 8  is a cross-sectional schematical illustration of a double acting pump/actuator of  FIG. 6 ; 
           [0028]      FIG. 9  is a schematical flow diagram of a control method utilizing the digital hydraulic system of  FIGS. 1-8 ; 
           [0029]      FIG. 10  is another embodiment of a digital hydraulic system of the present invention; 
           [0030]      FIG. 11 . illustrates a side view of a hydraulic excavator utilizing another embodiment of the energy management system of the present invention; 
           [0031]      FIG. 12  illustrates a top view of the hydraulic excavator of  FIG. 11 ; 
           [0032]      FIG. 13  is a schematical illustration of the energy management system of  FIGS. 11 and 12 ; and 
           [0033]      FIG. 14  is another schematical illustration of the energy management system of  FIGS. 11 and 12 . 
       
    
    
       [0034]    Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate embodiments of the invention and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0035]    Referring now to the drawings, and more particularly to  FIGS. 1-3 , there is shown a digital hydraulic system  10  being used in conjunction with a backhoe assembly. Digital hydraulic system  10  includes a power source  12 , a pump  14 , a human interface  16 , a control system  18 , an actuator  20 , a buffering device  22 , an accumulator  24 , a digital hydraulic transformer  26 , sense and control lines  28  and hydraulic lines  30  and  32 . Power source  12  provides mechanical power to actuate pump  14  to serve as a hydraulic source to provide pressurized fluid/flow to digital hydraulic system  10 . Pump  14  can be a typical hydraulic pump or may be a digital hydraulic pump  14  as described herein. Buffering device  22  serves an anti-cavitation function to absorb any impulses that may occur as the hydraulic fluid is switched by control system  18 . Additionally, buffering device  22  may serve an accumulation function. Although not illustrated, pump  14  and actuator  20  may have buffering devices associated with each. 
         [0036]    Human interface  16  can include a series of levers, to direct the operation of a piece of machinery, such as a backhoe. Human interface  16  is interactively connected with control system  18  to provide desired movement information from the operator to control system  18 . 
         [0037]    Control system  18  communicates with human interface  16  as well as to pump  14 , transformer  26  and actuator  20 . Transformer  26  includes a transtatic bridge  62  that schematically appears as a stepped cylinder in  FIG. 2  inside of a housing. Transtatic bridge  62  is not mechanically linked to anything outside of the housing and serves to transform a force against selected areas on one side to the fluid in other selected areas on the other side of transtatic bridge  62 . Unlike transtatic bridge  62  of hydraulic transformer  26 , the transtatic bridges that may be in pump  14  and/or actuator  20  may have a mechanical linkage that are respectively linked to a power source and a working piece. 
         [0038]    Control system  18  can also receive information from power source  12  and send instructions to power source  12  to alter the function of power source  12 . Control system  18  monitors pressure in accumulator  24 . Control system  18  can alter the pressure/fluid flow from pump  14  based upon a need to move actuator  20 . Further, control system  18  controls transformer  26  to adjust pressure in hydraulic line  32 . Control system  18  also reacts to loads encountered by actuator  20  such that when movement by actuator  20  is in a direction that lowers the potential energy of a raised mass, such as a bucket full of dirt, then the lowering of the mass along with the weight of the mechanism can be used to increase the pressure in accumulator  24 . In a like manner, control system  18  can utilize pressure on one side of transtatic bridge  62  to alter the pressure on another side of transtatic bridge  62 . For example, if accumulator  24  has reached a maximum pressure and hydraulic line  32  has a less than a desired pressure, transtatic bridge  62  can translate pressure from accumulator  24  to provide energy to hydraulic line  32 . 
         [0039]    When human interface  16  indicates the movement of actuator  20  as desired, control system  18  actuates control valves based upon a calculated required pressure to be applied to actuator  20  in order to obtain the desired movement thereof. For example, if human interface  16  directs a work piece  27 , which may be a tool  27 , connected to actuator  20  to encounter an object that is to be pushed by movement of actuator  20 , the position and movement of actuator  20  is monitored by control system  18  and appropriate pressure is supplied to hydraulic lines  32  by way of transtatic bridge  62 , which draws energy from hydraulic line  30 . So when tool  27  connected to actuator  20  encounters the object and human interface  16  indicates that tool  27  is to continue pushing, control system  18  detects either a slowed or stopped movement of tool  27  connected to actuator  20  and increases the pressure applied to actuator  20 . Alternatively, actuator  20  is reconfigured by valves attached thereto to alter the pressurized cross-sectional area of actuator  20  to cause the tool to continue pressing against the encountered object. Control system  18  can balance the required pressure to be delivered from transtatic bridge, with that of cross-sectional area of actuator  20  so as to efficiently apply only the needed pressurized fluid in the required flow volume and pressure to cause the desired movement of actuator  20 , based upon instructions from human interface  16 . 
         [0040]    For the sake of simplicity, a single pump and actuator control has been illustrated. However, the use of digital hydraulic components such as multiple actuators, transtatic bridges and/or pumps is also contemplated. Further, interaction of multiple control systems associated with selected sets of digital hydraulic components is also contemplated. 
         [0041]    Now, additionally referring to  FIG. 4 , there is shown a schematic illustration of the operating of a transtatic bridge embodied here as a step cylinder having four separate cross-sectional areas, which illustratively yield sixteen combinations of operation available from the selection of portions of the active areas under pressure in transformer  26 , actuator  20  and/or pump  14 . For example, mode 1 illustrates that none of the area has been selected by control system  18 . In mode 2, the smallest area is selected which is illustrated as the most central portion, which can indicate the pressures applied to the specified area. In mode three the area selected is twice area A and each stepped area is double the previous stepped area resulting in a binary digital hydraulic system. The selection of a desired cumulative area thereby directs the amount of pressure against a sealed piston to result in mechanical movement. 
         [0042]    The following table illustrates how the mode of operation relates to the binary selection of areas of a digital cylinder/piston arrangement of the present invention. The cumulative area relates to the ratio of the pressure of the high pressure line that is transferred. In transtatic bridge  62  of hydraulic transformer  26  the ratios are selectable on both sides so as to allow 143 unique overall ratios of pressure conversion. This is assuming that the areas on each side of transtatic bridge  62  are substantially the same. It is possible to have the two sides of transtatic bridge  62  to not be minor images of each other, but for the ease of illustration such is illustrated and described herein. The transtatic bridge of actuator  20  may have a different total area than transtatic bridge  62  and if it has four selectively pressurized sections as discussed herein, then the overall possibilities of unique power selections exceed 2,000. Differing numbers of pressurized sections and working area sizes are contemplated as a part of the present invention. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                 CUMU- 
                   
                   
               
               
                 MODE OF 
                   
                   
                   
                   
                 LATIVE 
                   
                 TRANSFOM 
               
               
                 OPERATION 
                 8A 
                 4A 
                 2A 
                 A 
                 AREA 
                 RATIO 
                 PRESSURE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0:15 
                 0 
               
               
                 2 
                 0 
                 0 
                 0 
                 1 
                 A 
                 1:15 
                 Ph/15 
               
               
                 3 
                 0 
                 0 
                 1 
                 0 
                  2A 
                 2:15 
                  2 * Ph/15 
               
               
                 4 
                 0 
                 0 
                 1 
                 1 
                  3A 
                 3:15 
                  3 * Ph/15 
               
               
                 5 
                 0 
                 1 
                 0 
                 0 
                  4A 
                 4:15 
                  4 * Ph/15 
               
               
                 6 
                 0 
                 1 
                 0 
                 1 
                  5A 
                 5:15 
                  5 * Ph/15 
               
               
                 7 
                 0 
                 1 
                 1 
                 0 
                  6A 
                 6:15 
                  6 * Ph/15 
               
               
                 8 
                 0 
                 1 
                 1 
                 1 
                  7A 
                 7:15 
                  7 * Ph/15 
               
               
                 9 
                 1 
                 0 
                 0 
                 0 
                  8A 
                 8:15 
                  8 * Ph/15 
               
               
                 10 
                 1 
                 0 
                 0 
                 1 
                  9A 
                 9:15 
                  9 * Ph/15 
               
               
                 11 
                 1 
                 0 
                 1 
                 0 
                 10A 
                 10:15  
                 10 * Ph/15 
               
               
                 12 
                 1 
                 0 
                 1 
                 1 
                 11A 
                 11:15  
                 11 * Ph/15 
               
               
                 13 
                 1 
                 1 
                 0 
                 0 
                 12A 
                 12:15  
                 12 * Ph/15 
               
               
                 14 
                 1 
                 1 
                 0 
                 1 
                 13A 
                 13:15  
                 13 * Ph/15 
               
               
                 15 
                 1 
                 1 
                 1 
                 0 
                 14A 
                 14:15  
                 14 * Ph/15 
               
               
                 16 
                 1 
                 1 
                 1 
                 1 
                 15A 
                 15:15  
                 15 * Ph/15 
               
               
                   
               
             
          
         
       
     
         [0043]    As can be seen in  FIG. 2 , transtatic bridge  62  is located within stepped cavities having hydraulic flow lines connected by way of valves. For the sake of illustration, position sensors  34  and  36  are associated with transtatic bridge  62  and position sensor  38  is associated with actuator  20 , herein illustrated as a simple dual acting cylinder. Valves  40 ,  42 ,  44  and  46  are associated with one side of transtatic bridge  62  and valves  48 ,  50 ,  52  and  54  are associated with an opposite side of transtatic bridge  62 . Valves  56  and  58  allow for the switching of the high pressure line to opposite sides of transtatic bridge  62 . Valve  60  allows for the reversed application of pressure to reach the actuator cylinder. Additionally valve  60  may be kept in a closed position until pressure, as measured by pressure sensor  70  is at the proper level to be applied to actuator  20 . 
         [0044]    As illustrated in  FIG. 2 , transtatic bridge  62  may be utilized to step the pressure up from the pressure contained in the high pressure line or step it down. For example, if the actuator is commanded to extend by the user in operation of human interface  16 , control  18  would sense the command and cause valve  60  to shift to the right thereby connecting the low pressure line to the right side of the working cylinder and the left side of the working cylinder being connected to an output of transtatic bridge  62 . For the lowest level of pressure, valve  40  is shifted to the left and valves  48 ,  50 ,  52  and  54  are likewise shifted to the left and valve  56  is shifted to the left thereby completing the fluid circuit to cause fluid flow from the high pressure line through valve  56  and valve  40 , which would represent a mode 2 operation on the left side of transtatic bridge  62 . The mode on the right side of transtatic bridge  62  would be in a mode 16 thereby causing the pressure of the fluid flowing to the left side of the actuator to be 1/15 th  of the pressure in the high pressure line. As can be understood, the selective positioning of valves  40 ,  42 ,  44  and  46  alter the amount of pressure driving transtatic bridge  62  and the selective use of valves  48 ,  50 ,  52  and  54  on the opposite side of transtatic bridge  62  selects the desired output pressure to be applied to the actuator when valves  56  and  58  are so positioned. Numerous combinations then of output pressure are available by the selective use of valves  40 - 54 . When transtatic bridge  62  approaches either position sensor  34  or  36 , valves  56  and  58  can be simultaneously reversed from their position along with an appropriate reversal of valves  40 - 54  so that when transtatic bridge  62  travels in an opposite direction it still supplies the desired pressure of hydraulic fluid to the actuator. Pressure sensors  64 ,  66 ,  68  and  70  provide information to control system  18  to optimally control the function of transtatic bridge  62 . 
         [0045]    Understanding of the control of transtatic bridge  62  allows for an easy understanding of transtatic bridge  118  of single acting actuator  100  having valves  102 ,  104 ,  106  and  108  that are hydraulically connected with pressure cylinders  110 ,  112 ,  114  and  116 , respectively. Pressure cylinders  110 - 116  are illustrated in schematic form and have stepped progressions, which for purposes of illustration can be understood to equate to the binarily oriented sixteen modes of  FIG. 4  although different increments are also contemplated. Actuator  100  is connected to high and low hydraulic lines, which can come directly from the pump, an accumulator or from the pressure created by transtatic bridge  62 . For ease of illustration the actual source of the pressure is not shown. The position of actuator  100  is detected by a position sensor, not shown, and when a new position is desired control system  18  selectively activates one or more of valves  102 ,  104 ,  106  and  108 . For example, for the least amount of force from actuator  100 , only valve  108  is activated causing the high pressure line to be directed to pressure cylinder  116 . In a like manner, as described above, combinations of the activation of valves  102 - 108  apply hydraulic fluid to a selected cross sectional area of actuator  100 . This tailoring of fluid connections allows the selected pressure cylinders to efficiently move shaft  120  of actuator  100  without relying upon a throttling method or dropping pressure through a flow rate reducer, which is common in the industry. The more efficient use of a pressurized hydraulic source by the present invention reduces the amount of energy required from power source  12  to operate hydraulic system  10  as compared to current hydraulic systems. 
         [0046]    Now, additionally referring to  FIG. 6 , there is shown a double acting actuator  200  having valves  202 ,  204 ,  206  and  208  operatively connected to opposing pressure cylinder pairs  210 ,  212 ,  214  and  216  of transtatic bridge  218 . The selective actuation of valves  202 - 208  cause a powered movement in both directions for reasons similar to those explained relative to  FIG. 5 . A shaft  220  may be attached to transtatic bridge  218  to convey force into/out of actuator  200 . 
         [0047]    Two cross-sectional examples are provided in  FIGS. 7 and 8  to show how different pressurized cavities can be utilized to produce an actuator/pump in accordance with the present invention. The pressurized cavities of  FIG. 7  correspond nicely with the end view presented in  FIG. 4  and the schematical presentation in  FIG. 5 , showing four separate pressurized areas. These areas can be separately pressurized to cause the movement of shaft  120  within housing  122 . In  FIG. 8 , another embodiment of an actuator  20  or  200  having a geometry that again has working areas that are selectively pressurized and which are annular in nature. For example, working area  72  is opposite matched working area  74  on the opposite end thereof. In a like manner area  76  is opposite  78 , area  80  is opposite area  82  and area  84  is opposite area  86 . The selective pressurization of different sides of working areas  72 - 86  modify the direction and force applied to the shaft extending from actuator  20 . The annular geometry of  FIG. 8  is again binarily related with the working areas being associated by a factor of two. 
         [0048]    Now, additionally referring to  FIG. 9  is an illustrative method  300  that utilizes the digital features of hydraulic system  10 . A user input is detected at step  302  and the direction is selected at step  304  as to whether actuator  20  should extend or retract. If the command from the user is to extend actuator  20 , then the method proceeds to step  306 . If the command from the user is to retract actuator  20 , then the method proceeds to step  308 . Steps  306  and  308  are similar in that a determination is made as to which side of the working cylinder has the largest pressure. If at step  306  the largest pressure is detected at transducer Pb then actuator/pump  20  functions as a pump to increase the pressure in an accumulator  24 . If at step  306  if pressure is greater at transducer Pa then actuator/pump  20  functions as an actuator. Continuing along the flow of Pa being greater than Pb then a transform ratio is selected for the valves to be actuated at step  310 . At step  312  the valves are engaged causing the operation to begin. If the piston velocity is within a predetermined tolerance then no action is taken at step  314 . However, if the piston velocity is not within a predetermined tolerance then an indication of the position as it changes with time is determined at step  316  to determine if the piston velocity is too slow or too fast as compared to the required user input detected at step  302 . If the movement is too fast then the transform ratio is decreased at step  318 . If it is determined that movement of the actuator is too slow then the transform ratio is increased at step  320  by selectively engaging valves similar to step  312 . 
         [0049]    In a like manner if the pressure detected by the Pb transducer is greater than Pa then actuator  20  functions as a pump thereby recovering energy from the movement of the load held by actuator/pump  20 . In a manner somewhat similar to the functioning of an actuator the transform ratio is selected just below unity at step  322 , which means that the actuator will then retract. Valves are shifted to begin the operation at step  324  and the movement is monitored at step  326  to determine if the piston velocity is within a predetermined tolerance. If the piston velocity is not within tolerance then a determination is made at step  328  as to whether the piston velocity is too slow or too fast as compared to the input required by the user at step  302 . If the movement is too slow then the transform ratio is reduced at step  330  and valves are reoriented similar to step  324  to alter the velocity of the piston. If at step  328  it is determined that piston velocity is too fast then the transform ratio is increased, thereby causing increased resistance to movement of the actuator, thereby increasing pressure in accumulator  24 . 
         [0050]    Now, additionally referring to  FIG. 10 , there is shown another embodiment of the present invention including digital hydraulic system  410  including a power source  412 , a pump  414 , an accumulator  416  and a transtatic bridge  418  operatively connected to a work piece  420 . The prime mover that provides mechanical work to the system is power source  412 , which is mechanically linked by linkage  422  to pump  414 . Pump  414  is a hydraulic source of pressure and flow, and may be a digital pump  14  as described herein being under the control of a system that selects portions of a transtatic bridge within pump  14  to control the flow and pressure delivered to hydraulic line  424 . Accumulator  416  stores and releases pressurized fluid by way of hydraulic line  424 . Transtatic bridge  418  is a transtatic bridge as described above and may be single or double acting. A linkage  426  may be a mechanical linkage  426  such as a shaft  426  that is connected to work piece  420  for the controllable movement thereof. Alternatively, linkage  426  may be a fluidic linkage that provides fluid pressure/flow to work piece  420 . For the sake of simplicity the valves and control system associated with system  410  have not been shown but would include the control and valve elements described herein to direct force to/from work piece  420 . 
         [0051]    Pump  14  again can be identical or substantially identical with an actuator  20  in its construct and control by control system  18 . Pump  14  can be also known as a variable displacement linear pump (VDLP)  14 , which can displace a variable amount of fluid per unit length of stroke or allow variable stroke per unit of volume displaced. Its function depends upon how it is plumbed and controlled, that is, whether a constant force on the piston or a constant fluid pressure is required from the VDLP. Considering that virtually any low frequency random oscillating motion could be harnessed as a usable energy source, many applications are possible for the VDLP beyond the energy supplied by way of a typical power source, such as an internal combustion engine. One potential application of the VDLP of the present invention could be a shock absorber on a vehicle, such as an automobile or bus. The device, when utilized in such an application, would displace a progressively larger amount of fluid per unit length of stroke as the velocity of the piston increases. This would function to cause greater resistance to motion and a greater fluid displacement as the piston velocity increases. Whenever a powerful random motion has to be damped or the need for an extreme hydraulic efficiency is present, the VDLP can be utilized to transform motion to a usable pressurized hydraulic flow. Digital hydraulic systems of the present invention allow a new flexibility of design applications. 
         [0052]    In a like manner a variable displacement linear actuator (VDLA)  20  may deliver a variable force output throughout its stroke with near instantaneous control response and near perfect efficiency as compared to conventional hydraulic systems. The double acting variable displacement linear actuator permits four quadrant operation, in which operational transition is seamless throughout the entire range of motoring and pumping. For example, a four quadrant linear actuator can produce a variable force in either direction while moving in either direction at nearly any velocity. If a control signal is sent by way of control system  18  to actuator  20  to produce some specific force in a particular direction and the opposing force of the load against it is less, the opposition force is overpowered, and the mechanism, along with the load, accelerate in the direction of the actuator force. If however, the opposing force of the load is greater than the force output of the VDLA, the mechanism and load travel in an opposite direction thereby causing the VDLA to operate as a VDLP. 
         [0053]    The digital hydraulic transformer (DHT), converts hydraulic energy by way of transtatic bridge  62 . An input flow at a given pressure can be converted to an output flow at another pressure level with minimal loss. The conversion is also reversible, as the product of the input pressure and flow is equal to the product of output pressure and flow. The transtatic bridge in pump  14  is connected to power source  12  to mechanically move the transtatic bridge so that the selectable flow and pressure of the working hydraulic fluid from pump  14  is produced. In a like manner, particularly since actuator  20  and pump  14  can be substantially similar, the transtatic bridge of actuator  20  can be connected to a work piece or load, so that the selected flow and pressure of the hydraulic fluid directed to the transtatic bridge determines the force applied to the work piece. Transtatic bridge  62  of hydraulic transformer  26  is not mechanically linked to a motive force or to a load. Rather transtatic bridge  62  serves to transfer one force-flow product to another force-flow product. 
         [0054]    In operation the digital hydraulic system of the present invention may present discrete pressures and flows, which may be altered by an interpolation method to provide a pressure and/or flow that is between the discrete selections. The interpolation methods include frequency modulation by the control system to vary the selection of adjacent discrete pressures/flows to provide a selection between the discrete outputs. Similarly a pulse width modulation technique can be used to interpolate the pressure/flow. Additionally, a servo valve, a throttling technique and/or a modulation of a poppet valve is contemplated to slightly alter a discrete output. 
         [0055]    Now additionally referring to  FIGS. 11-14 , there is shown a control system  590  being used in conjunction with a hydraulic excavator assembly. Control system  590  receives input from sensors  608  and estimating device  610 . It is contemplated that control system  590  also receives information from digital hydraulic transformer  588 . Control system  590  controls hydraulic energy source  580 . Sensors  608  include sensors  592 ,  594 ,  596 ,  598  and  600 . Hydraulic energy source  580  includes a prime mover  582  and a hydraulic pump  584 . Alternatively, hydraulic energy source  580  can include a prime mover-pump combination such as a free piston engine-pump, not shown. 
         [0056]    Prime mover  582  drives hydraulic pump  584 . Prime mover  582  can be an internal combustion engine, an electric motor or some other type of power providing apparatus. Hydraulic pump  584  can be a fixed displacement hydraulic pump or a variable displacement hydraulic pump. Prime mover  582  drives hydraulic pump  584  adding pressurized hydraulic fluid to accumulator  586  up to a fill level determined by control system  590 . Control system  590  determines a fill level of accumulator  586  based on input from sensors  608 . Digital hydraulic transformer  588  is fluidly connected to hydraulic energy source  580  and hydraulic accumulator  586 . Digital hydraulic transformer  588  is also connected to hydraulic cylinder  540 . Hydraulic cylinder  540  is operatively connected to load  602 . Load  602  can act on cylinder  540  in the direction of direction of arrow  604  or arrow  606  depending upon the position of load  602  in a gravitational field. As load  602  is raised to an elevated position in a gravitational field it gains potential energy. As load  602  is lowered to a lower position in the gravitational field it loses potential energy. If load  602  is moving in a direction and has mass it has kinetic energy. Digital hydraulic transformer  588  transfers energy between hydraulic accumulator  586  and hydraulic cylinder  540 . In the event that load  602  is moving in the opposite direction as load  602  is acting on cylinder  540 , energy is transferred from accumulator  586  to load  602 . In the event that load  602  is moving in the same direction as load  602  is acting on cylinder  540 , then energy is transferred from load  602  to accumulator  586 . In the event that load  602  is in motion and is caused to stop, the kinetic energy is transferred from load  602  through digital hydraulic transformer  588  into accumulator  586 . Estimating device  610  receives input from sensors  608 . Estimating device  610  estimates the amount of potential energy and kinetic energy in load  602  based on input from sensors  608 . Control system  590  controls hydraulic energy source  580  to allow sufficient capacity for additional hydraulic fluid in hydraulic accumulator  586  such that an amount of hydraulic energy approximately equal to the sum of potential energy and kinetic energy in load  602 , in the form of a volume of pressurized hydraulic fluid, is able to be added to accumulator  586 . 
         [0057]    Work machine  520  is comprised of stationary structure  524  and rotatable structure  522 . Stationary structure  524  is engaged with ground  510 , and rotatable structure  522  is rotatable with respect to stationary structure  524  by swing drive  546 . Onto rotatable structure  522  implement  530  is operatively mounted, which illustratively includes boom  532 , stick  534  and bucket  538 . Implement  530  is movable by hydraulic cylinder  540  with respect to rotatable structure  522 , and is shown engaging load  512 . Two positions of implement  530  are shown in  FIG. 11 : position  550  and position  552 . Two positions of rotating structure  522  are shown in  FIG. 12 : position  564  and position  566 . 
         [0058]    When work machine  520  raises implement  530  from position  552  to position  550  in the direction of arrow  560 , implement  530  and the engaged load  512  gain potential energy. 
         [0059]    When work machine  520  lowers implement  530  from position  550  to position  552  in the direction of arrow  562 , implement  530  and the engaged load  512  loses potential energy. Also while implement  530  is in motion in the direction of arrow  560  or arrow  562 , implement  530  and the engaged load  512  possesses kinetic energy. Control system  590  receives input from sensors  608  to estimate the potential energy in implement  530  and load  512  acting together on cylinder  540  as load  602 . Based on the estimate of potential energy and kinetic energy in load  602 , control system  590  lowers the target fill level of hydraulic fluid in accumulator  586  to leave enough capacity for the redistribution of the potential energy and kinetic energy in load  602  in the event that load  602  is lowered and/or brought to a stop. 
         [0060]    Similarly, rotating structure  522 , while rotating from position  564  to position  566 , possesses kinetic energy. Swing drive  546  applies a force to rotating structure  522  in the direction of direction arrow  572  to accelerate rotating structure  522  in the direction of direction arrow  572 . To bring rotating structure  522  to a stop at position  566 , swing drive  546  applies a force to rotating structure  522  in the direction of arrow  570 , and thus acts as a pump transferring kinetic energy to the accumulator. 
         [0061]    Control system  590  receives input from sensors  608  to estimate the kinetic energy in rotating structure  522  and lowers the target fill level of hydraulic fluid in accumulator  586  to leave enough capacity for the redistribution of the kinetic energy in rotating structure  522  in the event that rotating structure  522  is brought to a stop. 
         [0062]    For the sake of clarity, a single hydraulic energy source, digital hydraulic transformer and actuator control has been illustrated. It is to be understood that the use of multiple hydraulic energy sources, digital hydraulic transformers and/or hydraulic actuators, such as illustrated by cylinders  542  and  544 , along with swing drive  546 , is also contemplated. Further, interaction of multiple control systems associated with the control of individual digital hydraulic transformers and energy management systems are additionally contemplated. 
         [0063]    While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.