Abstract:
A hydraulic actuator includes an energy recuperation device which harvests the energy generated from the stroking of a shock absorber. The energy recuperation device can function in a passive energy recovery mode for the shock absorber to store recovered energy as fluid pressure or it can be converted to another form of energy such as electrical energy.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present disclosure is a continuation of U.S. application Ser. No. 13/286,457, filed Nov. 1, 2011 and presently allowed, the entire disclosure of which is hereby incorporated by reference into the present application. 
    
    
     FIELD 
     The present disclosure is directed to passive and active suspension systems. More particularly, the present disclosure is directed to passive and active suspension systems that harvest the energy generated during the damping of the suspension system. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     Suspension systems are provided to filter or isolate the vehicle&#39;s body (sprung portion) from the vehicle&#39;s wheels and axles (unsprung portion) when the vehicle travels over vertical road surface irregularities as well as to control body and wheel motion. In addition, suspension systems are also used to maintain an average vehicle attitude to promote improved stability of the vehicle during maneuvering. The typical passive suspension system includes a spring and a damping device in parallel with the spring which are located between the sprung portion and the unsprung portion of the vehicle. 
     Hydraulic actuators, such as shock absorbers and/or struts, are used in conjunction with conventional passive suspension systems to absorb unwanted vibration which occurs during driving. To absorb this unwanted vibration, hydraulic actuators include a piston located within a pressure cylinder of the hydraulic actuator. The piston is connected to the sprung portion or body of the vehicle through a piston rod. Because the piston is able to restrict the flow of damping fluid within the working chamber of the hydraulic actuator when the piston is displaced within the pressure cylinder, the hydraulic actuator is able to produce a damping force which counteracts the vibration of the suspension. The greater the degree to which the damping fluid within the working chamber is restricted by the piston, the greater the damping forces which are generated by the hydraulic actuator. 
     In recent years, substantial interest has grown in automotive vehicle suspension systems which can offer improved comfort and road handling over the conventional passive suspension systems. In general, such improvements are achieved by utilization of an “intelligent” suspension system capable of electronically controlling the suspension forces generated by hydraulic actuators. 
     Different levels in achieving the ideal “intelligent” suspension system called a semi-active or a fully active suspension system are possible. Some systems control and generate damping forces based upon the dynamic forces acting against the movement of the piston. Other systems control and generate damping forces based on the static or slowly changing dynamic forces, acting on the piston independent of the velocity of the piston in the pressure tube. Other, more elaborate systems, can generate variable damping forces during rebound and compression movements of the hydraulic actuator regardless of the position and movement of the piston in the pressure tube. 
     The movement produced in the hydraulic actuators in both the passive and active suspension systems converts mechanical energy and this mechanical energy is changed into heat of the hydraulic actuator&#39;s fluid and the components of the actuator. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     The present disclosure provides the art with a system which captures the energy generated in a passive suspension system. The energy is captured in a way that the energy can be reused later, or the energy can be converted into another form of energy such as electrical energy. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG. 1  is a diagrammatic illustration of a vehicle incorporating the active energy harvesting suspension system in accordance with the present disclosure; 
         FIG. 2  is a schematic view of one of the active energy harvesting devices illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic view of the active energy harvesting device illustrated in  FIG. 2  illustrating the components of the active energy harvesting device; 
         FIG. 4  is a schematic view of the active energy harvesting device illustrated in  FIG. 3  showing fluid flow during a passive rebound mode of the active energy harvesting device; 
         FIG. 5  is a schematic view of the active energy harvesting device illustrated in  FIG. 3  showing fluid flow during an active rebound operation mode; 
         FIG. 6  is a schematic view of the active energy harvesting device illustrated in  FIG. 3  showing fluid flow during a passive compression mode of the active energy harvesting device; 
         FIG. 7  is a schematic view of the active energy harvesting device illustrated in  FIG. 3  showing fluid flow during an active compression operation mode; 
         FIG. 8  is a diagrammatic illustration of an active energy harvesting suspension system in accordance with another embodiment of the present disclosure; 
         FIG. 9  is a schematic view of the active energy harvesting device illustrated in  FIG. 8  showing fluid flow during a passive rebound mode of the active energy harvesting device; 
         FIG. 10  is a schematic view of the active energy harvesting device illustrated in  FIG. 8  showing fluid flow during an active rebound operation mode of the active energy harvesting device; 
         FIG. 11  is a schematic view of the active energy harvesting device illustrated in  FIG. 8  showing fluid flow during a passive compression mode of the active energy harvesting device; 
         FIG. 12  is a schematic view of the active energy harvesting device illustrated in  FIG. 8  showing fluid flow during an active compression operation mode of the active energy harvesting device. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. There is shown in  FIG. 1 , a vehicle incorporating an active energy harvesting suspension system in accordance with the present disclosure and which is designated generally by the reference numeral  10 . Vehicle  10  includes a rear suspension  12 , a front suspension  14  and a body  16 . Rear suspension  12  has a transversely extending rear axle assembly (not shown) adapted to operatively support a pair of rear wheels  18 . The rear axle is attached to body  16  by means of a pair of active energy harvesting devices  20  and by a pair of springs  22 . Similarly, front suspension  14  includes a transversely extending front axle assembly (not shown) to operatively support a pair of front wheels  24 . The front axle assembly is attached to body  16  by means of a pair of active energy harvesting devices  26  and by a pair of springs  28 . Active energy harvesting devices  20  and  26  serve to dampen the relative motion of the unsprung portion (i.e., front and rear suspensions  12 ,  14 ) with respect to the sprung portion (i.e., body  16 ) of vehicle  10 . Sensors (not shown), at each wheel  18  and each wheel  24 , sense the position and/or the velocity and/or the acceleration of body  16  in relation to rear suspension  12  and front suspension  14 . While vehicle  10  has been depicted as a passenger car having front and rear axle assemblies, active energy harvesting devices  20  and  26  may be used with other types of vehicles or in other types of applications including, but not limited to, vehicles incorporating non-independent front and/or non-independent rear suspensions, vehicles incorporating independent front and/or independent rear suspensions or other suspension systems known in the art. Further, the term “hydraulic actuator” as used herein is meant to refer to shock absorbers and hydraulic dampers in general and thus will include McPherson struts and other hydraulic damper designs known in the art. 
     Referring to  FIG. 2 , one of active energy harvesting devices  20  is illustrated schematically. While  FIG. 2  only illustrates active energy harvesting device  20 , active energy harvesting devices  26  include the same components discussed below for active energy harvesting device  20 . The only difference between active energy harvesting devices  20  and  26  may be the way in which the active energy harvesting device is attached to the sprung and/or unsprung portion of the vehicle. 
     Active energy harvesting device  20  comprises a hydraulic actuator  30 , a four quadrant convertor assembly  32 , a pump/turbine  34  and a motor/generator  36 . Four quadrant convertor assembly  32 , pump/turbine  34  and motor/generator  36  define means for recuperating energy. Hydraulic actuator  30  comprises a pressure tube  40  having a fluid chamber  42  that is divided into an upper working chamber  44  and a lower working chamber  46  by a piston assembly  48 . Piston assembly  48  is slidingly received within pressure tube  40  and piston assembly  48  includes a piston rod  50  that extends through upper working chamber  44  and is attached to the sprung portion of vehicle  10 . Pressure tube  40  is attached to the unsprung portion of vehicle  10 . 
     Referring now to  FIG. 3 , four quadrant convertor assembly  32  comprises a pair of check valves  60 ,  62 , a pair of hydraulic inductance units  64 ,  66  and a four quadrant convertor  68 . Four quadrant convertor  68  comprises four check valves  70 ,  72 ,  74  and  76  and four two state valves  78 ,  80 ,  82  and  84 . 
     Check valves  60  and  62  are disposed in a fluid line  86  which extends between upper working chamber  44  and lower working chamber  46 . A fluid line  88  extends from fluid line  86  at a position between check valve  60  and  62  to four quadrant convertor  68 . Check valve  60  prohibits fluid flow from upper working chamber  44  to fluid line  88  but allows fluid flow from fluid line  88  to upper working chamber  44 . Check valve  62  prohibits fluid flow from lower working chamber  46  to fluid line  88  but allows fluid flow from fluid line  88  to lower working chamber  46 . 
     Hydraulic inductance unit  64  is disposed within a fluid line  90  which extends between fluid line  86  where it is in communication with upper working chamber  44  and four quadrant convertor  68 . Hydraulic inductance unit  66  is disposed within a fluid line  92  which extends between fluid line  86  where it is in communication with lower working chamber  46  and four quadrant convertor  68 . A fluid line  94  extends between four quadrant convertor  68  and pump/turbine  34 . 
     Four quadrant convertor  68  includes a fluid line  96  within which check valves  70 ,  72 ,  74  and  76  are disposed. Fluid line  88  connects to fluid line  96  at a position between check valves  72  and  76 . Fluid line  90  connects to fluid line  96  at a position between check valves  70  and  72 . Fluid line  92  connects to fluid line  96  at a position between check valves  74  and  76 . Fluid line  94  connects to fluid line  96  at a position between check valves  70  and  74 . Check valve  70  allows fluid flow from fluid line  90  to fluid line  94  but prohibits fluid flow from fluid line  94  to fluid line  90 . Check valve  72  allows fluid flow from fluid line  88  to fluid line  90  but prohibits fluid flow from fluid line  90  to fluid line  88 . Check valve  74  allows fluid flow from fluid line  92  to fluid line  94  but prohibits fluid flow from fluid line  94  to fluid line  92 . Check valve  76  allows fluid flow from fluid line  88  to fluid line  92  but prohibits fluid flow from fluid line  92  to fluid line  88 . Both the combination of check valves  70  and  72  and the combination of check valves  74  and  76  allow fluid flow from fluid line  88  to fluid line  94  but prohibit fluid flow from fluid line  94  to fluid line  88 . 
     Two state valves  78  and  80  are disposed in a fluid line  98  which extends from fluid line  96  at a position between check valves  70  and  74  to fluid line  96  at a position between check valves  72  and  76 . A fluid line  100  extends from fluid line  98  at a position between the two state valves  78  and  80  to fluid line  96  at a position between check valves  70  and  72  where fluid line  100  is also in communication with fluid line  90 . Two state valves  82  and  84  are disposed in a fluid line  102  which extends from fluid line  96  at a position between check valves  70  and  74  to fluid line  96  at a position between check valves  72  and  76 . A fluid line  104  extends from fluid line  102  at a position between the two state valves  82  and  84  to fluid line  96  at a position between check valves  74  and  76  where fluid line  104  is also in communication with fluid line  92 . 
     Fluid line  94  is connected to one side of pump/turbine  34  and to one side of a two state valve  110 . A fluid line  112  connects an accumulator  114  to fluid line  94 . The opposite ends of pump/turbine  34  and two state valve  110  are connected to a fluid line  116  which extends from a fluid reservoir  118  to fluid line  86  at a position between check valves  60  and  62  where fluid line  116  is also in communication with fluid line  88 . 
     Motor/generator  36  is mechanically connected to pump/turbine  34 . When motor/generator  36  is used as a motor, motor/generator  36  will operate pump/turbine  34  to pump fluid in active energy harvesting device  20 . When motor/generator  36  is used as a generator, fluid within active energy harvesting device  20  will drive pump/turbine  34  which will in turn drive motor/generator  36  to generate electrical energy. The accumulator  114  can also be used to store hydraulic energy. 
     As illustrated in  FIG. 2 , active energy harvesting device  20  provides for the capturing of incoming energy in a way that the energy can be reused later or in a way that the energy can be converted into another form of energy. Active energy harvesting device  20  can also control the forces in hydraulic actuator  30  in both an active and a passive mode.  FIG. 2  illustrates a layout of coupling through a hydraulic medium. Forces on and motion of wheel  18  of vehicle  10  are converted into pressures and flows of the hydraulic fluid which in turn are converted into torque and speed at pump/turbine  34 . Motor/generator  36  converts this energy into electrical energy. An additional advantage of the present disclosure is that energy flow in the opposite direction is also possible. Motor/generator  36  can be driven by electrical energy to drive the motion of hydraulic actuator  30 . 
     Typically, the motion energy provided to wheel  18  from road contact is high frequency. This poses inertia limitations on pump/turbine  34  and motor/generator  36 . These limitations affect the ability of pump/turbine  34  and motor/generator  36  to handle the hydraulic power needed. This issue can be resolved by separating the high bandwidth side from the low bandwidth side by the use of four quadrant convertor  68 . 
     Four quadrant convertor  68  separates a semi-fixed pressure level at accumulator  114  to the high frequency side of hydraulic actuator  30 . Valves  78 ,  80 ,  82  and  84  are two state valves, on or off, in order to prevent large amounts of hydraulic losses. The hydraulic bursts caused by the switching of valves  78 ,  80 ,  82  and  84  are smoothened in accumulator  114  and hydraulic inductance units  64  and  66 . Accumulator  114  smoothens the pressure drops caused by the switching of valves  78 ,  80 ,  82  and  84  and accumulator  114  provides enough flow to drive hydraulic actuator  30 . Hydraulic inductance units  64  and  66  smoothen the flow of fluid to hydraulic actuator  30  and decouple the pressure in accumulator  114  from the pressures in the upper and lower working chambers  44  and  46  of hydraulic actuator  30 . 
     Energy can be delivered to or retracted from accumulator  114  by means of motor/generator  36 . Two state valve  110  is a pressure control valve that secures the various hydraulic fluid storage components of active energy harvesting device  20  for peak fluid pressures. 
     During a rebound stroke in the passive mode as illustrated in  FIG. 4 , check valve  62  allows fluid to flow into lower working chamber  46 . On the upper side of piston assembly  48 , fluid pressure is created in upper working chamber  44  by the upward movement of piston assembly  48 . Depending on the damping characteristic, fluid flow flows through hydraulic inductance unit  64 . When two state valve  80  is open, the flow rate increases as determined by the applied external force and the inductance constant of hydraulic inductance unit  64 . In order to reach a specific pressure in upper working chamber  44 , a specific duty cycle is applied to two state valve  80 . When two state valve  80  is closed, hydraulic inductance unit  64  continues the existing flow through check valve  70  at a decreasing rate. The flow through check valve  70  is directed to accumulator  114 . The flow through two state valve  80  is directed through check valve  62  and into lower working chamber  46 . Additional fluid required in lower working chamber  46  is provided by fluid reservoir  118  through fluid line  116 , through check valve  62  and into lower working chamber  46 . These various flows are illustrated by the arrows in  FIG. 4 . Thus, the pressure in upper working chamber  44  can be regulated and excess energy is recuperated. 
     During a rebound stroke in the active mode as illustrated in  FIG. 5 , check valve  76  and hydraulic inductance unit  66  allow fluid to flow into lower working chamber  46 . On the lower side of piston assembly  48 , fluid pressure is applied to lower working chamber  46  causing upward movement of piston assembly  48 . Depending on the damping characteristics, fluid flows through hydraulic inductance unit  64 . Two state valve  80  is continuously open to allow fluid flow out of upper working chamber  44  through hydraulic inductance unit  64 , through two state valve  80  and either through check valve  76 , through hydraulic inductance unit  66  and into lower working chamber  46  or through fluid line  88 , through fluid line  116  and into fluid reservoir  118  depending on the amount of force required. In order to reach a specific pressure within lower working chamber  46 , a specific duty cycle is applied to two state valve  82 . When two state valve  82  is closed, hydraulic inductance unit  64  continues the existing flow through two state valve  80 , through check valve  76 , through hydraulic inductance unit  66  and into lower working chamber  46 . When two state valve  82  is open, fluid flow is allowed from accumulator  114 , through two state valve  82 , through hydraulic inductance unit  66  and into lower working chamber  46 . Additional fluid required for lower working chamber  46  is provided by fluid reservoir  118  through fluid line  116 , through check valve  76 , through hydraulic inductance unit  66  and into lower working chamber  46 . These various flows are illustrated by the arrows in  FIG. 5 . Thus, the pressure in lower working chamber  46  can be regulated using the excess energy stored in accumulator  114 . 
     During a compression stroke in the passive mode as illustrated in  FIG. 6 , check valve  60  allows fluid to flow into upper working chamber  44 . On the lower side of piston assembly  48 , fluid pressure is created in lower working chamber  46  by the downward movement of piston assembly  48 . Depending on the damping characteristic, fluid flow flows through hydraulic inductance unit  66 . When two state valve  84  is open, the flow rate increases as determined by the applied external force and the inductance constant of hydraulic inductance unit  66 . In order to reach a specific pressure in lower working chamber  46 , a specific duty cycle is applied to two state valve  84 . When two state valve  84  is closed, hydraulic inductance unit  66  continues the existing flow through check valve  74  at a decreasing rate. The flow through check valve  74  is directed to accumulator  114 . The flow through two state valve  84  is directed either through check valve  60  and into upper working chamber  44  or through fluid line  116  to fluid reservoir  118  depending on the amount of force required. Additional fluid required in upper working chamber  44  is provided by fluid reservoir  118  through fluid line  116 , through check valve  60  and into upper working chamber  44 . These various flows are illustrated by the arrows in  FIG. 6 . Thus, the pressure in lower working chamber  46  can be regulated and excess energy is recuperated. 
     During a compression stroke in the active mode as illustrated in  FIG. 7 , check valve  72  and hydraulic inductance unit  64  allow fluid to flow into upper working chamber  44 . On the lower side of piston assembly  48 , fluid pressure is created in lower working chamber  46  by the downward movement of piston assembly  48 . Depending on the damping characteristics, fluid flows through hydraulic inductance unit  66 . Two state valve  84  is continuously open to allow fluid flow out of lower working chamber  46  through hydraulic inductance unit  66 , through two state valve  84 , through check valve  72 , through hydraulic inductance unit  64  and into upper working chamber  44 . In order to reach a specific pressure within upper working chamber  44 , a specific duty cycle is applied to two state valve  78 . When two state valve  78  is closed, hydraulic inductance unit  66  continues the existing flow through two state valve  84 , through check valve  72 , through hydraulic inductance unit  64  and into upper working chamber  44 . When two state valve  78  is open, fluid flow is allowed from accumulator  114 , through two state valve  78 , through hydraulic inductance unit  64  and into upper working chamber  44 . Additional fluid from lower working chamber  46  is directed to fluid reservoir  118  through fluid line  88  and fluid line  116 . These various flows are illustrated by the arrows in  FIG. 7 . Thus, the pressure in upper working chamber  44  can be regulated using the excess energy stored in accumulator  114 . 
     While the above discussion illustrates the reuse of the energy stored in the passive mode during the active mode, the energy stored in accumulator  114  can be directed through pump/turbine  34  and into fluid reservoir  118 . The fluid flowing through pump/turbine  34  will drive pump/turbine  34  which will in turn drive motor/generator  36  which can be used as a generator to generate electrical power. Also, when the fluid pressure in accumulator  114  is below a specified pressure, motor/generator  36  can be driven by electrical power to operate pump/turbine  34  and pump hydraulic fluid from fluid reservoir  118  to accumulator  114 . 
     The above system allows for full four quadrant operation. The system can send and retrieve energy from and to hydraulic actuator  30  is both rebound and compression movements of hydraulic actuator  30 . In the above system, pump/turbine  34  only has to provide energy to the system when the pressure in accumulator  114  is below a specified pressure. In prior art active systems, a pump has to constantly provide pressure to the system. 
     Referring now to  FIG. 8 , an active energy harvesting device  220  in accordance with another embodiment of the present disclosure is illustrated. Active energy harvesting device  220  can replace active energy harvesting device  20  or active energy harvesting device  26 . Active energy harvesting device  220  comprises hydraulic actuator  30 , pump/turbine  34 , motor/generator  36 , two state valve  110 , accumulator  114 , fluid reservoir  118 , operational valve system  222 , pressure regulation system  224  and a check valve  226  disposed within piston assembly  48 . Pump/turbine  34 , motor/generator  36 , operational valve system  222  and pressure regulation system  224  define means for recuperating energy. 
     Operation valve system  222  comprises a pair of valves  230 ,  232  and a check valve  234 . Pressure regulation system  224  comprises a hydraulic inductance unit  240 , a pair of check valves  242  and  244  and a pair of two state valves  246  and  248 . Fluid lines as illustrated in  FIG. 8  fluidly connect the various components of active energy harvesting device  220 . 
     During a rebound stroke in the passive mode as illustrated in  FIG. 9 , check valve  234  allows fluid to flow into lower working chamber  46 . On the upper side of piston assembly  48 , fluid pressure is created in upper working chamber  44  by the upward movement of piston assembly  48 . Fluid flows from upper working chamber  44  through hydraulic inductance unit  240 . When two state valve  248  is open, the flow rate increases determined by the applied external forces and the inductance constant of hydraulic inductance unit  240 . In order to reach a specific pressure in upper working chamber  44 , a specific duty cycle is applied to two state valve  248 . When two state valve  248  is closed, hydraulic inductance unit  240  continues the existing flow through check valve  242  at a decreasing rate. The flow through check valve  242  is directed to accumulator  114 . Two state valve  246  is placed in a closed position. The flow through two state valve  248  is directed through check valve  234  and into lower working chamber  46 . Additional fluid flow required in lower working chamber  46  is provided by fluid reservoir  118  through check valve  234  and into lower working chamber  46 . Valves  230  and  232  remain open in this operating mode. Thus, the pressure in upper working chamber  44  can be regulated and excess energy is recuperated. 
     During a rebound stroke in the active mode as illustrated in  FIG. 10 , valve  230  is closed to allow fluid flow from upper working chamber  44  to lower working chamber  46 . On the upper side of piston assembly  48 , fluid pressure is created in upper working chamber  44  by the upward movement of piston assembly  48 . Depending on the damping characteristics, fluid flows from upper working chamber  44  through valve  230  and into lower working chamber  46 . In order to reach a specified pressure within lower working chamber  46 , a specific duty cycle is applied to two state valve  246 . When two state valve  246  is open, fluid flow is allowed from accumulator  114 , through two state valve  246 , through hydraulic inductance unit  240 , through valve  230  and into lower working chamber  46 . Additional fluid required for lower working chamber  46  is provided by fluid reservoir  118  through check valve  234  and/or through check valve  244  and hydraulic inductance unit  240 . Thus, the pressure in lower working chamber  46  can be regulated using the excess energy stored in accumulator  114 . Valve  232  is open in this mode and two state valve  248  is closed in this mode. 
     During a compression stroke in the passive mode as illustrated in  FIG. 11 , check valve  226  allows fluid to flow into upper working chamber  44  from lower working chamber  46 . On the lower side of piston assembly  48 , fluid pressure is created in lower working chamber  46  by the downward movement of piston assembly  48 . Fluid flows from lower working chamber  46 , through check valve  226 , through upper working chamber  44 , and through hydraulic inductance unit  240 . When two state valve  248  is open, the flow rate increases as determined by the applied external forces and the inductance constant of hydraulic inductance unit  240 . In order to reach a specific pressure in upper working chamber  44 , a specific duty cycle is applied to two state valve  248 . When two state valve  248  is closed, hydraulic inductance unit  240  continues the existing flow through check valve  242  at a decreasing rate. The flow through check valve  242  is directed to accumulator  114  through check valve  242 . Two state valve  246  is placed in a closed position. The flow through two state valve  248  is directed into fluid reservoir  118 . Valves  230  and  232  remain open in this operating mode. Thus, the pressure in upper working chamber  44  can be regulated and excess energy is recuperated. 
     During a compression stroke in the active mode as illustrated in  FIG. 12 , valve  232  is closed to allow fluid flow from lower working chamber  46  to fluid reservoir  118 . On the lower side of piston assembly  48 , fluid pressure is created in lower working chamber  46  by the downward movement of piston assembly  48 . Depending on the damping characteristics, fluid flows from lower working chamber  46  through valve  232 , through check valve  244 , through hydraulic inductance unit  240  and into upper working chamber  44 . In order to reach a specified pressure within upper working chamber  44 , a specific duty cycle is applied to two state valve  246 . When two state valve  246  is open, fluid flow is allowed from accumulator  114 , through two state valve  246 , through hydraulic inductance unit  240  and into upper working chamber  44 . Thus, the pressure in upper working chamber  44  can be regulated using the excess energy stored in accumulator  114 . Valve  230  and two state valve  248  are open in this mode. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.