Abstract:
An active suspension system isolates a first member from vibration in a second member. A hydraulic actuator is connected between the first and second members and includes a cylinder having a second chamber and a first chamber defined on opposite sides of a piston in the cylinder. A rod is attached to the piston with a first end extending out of the cylinder and a second end within the third chamber. A valve arrangement controlling flow of fluid between the first and second chambers and a source of pressurized hydraulic fluid and a tank which control applies force to the piston that counteracts the transmission of vibration between the first and second members. A load leveling valve assembly connects the third cylinder chamber selectively to the source or the tank to maintain the piston centered in the cylinder under static load conditions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not Applicable  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     The present invention relates to active and semi-active hydraulic suspension systems for isolating a component, such as an operator cab or a seat, from vibrations in other sections of a vehicle while traveling over rough terrain; and more particularly to such hydraulic suspension systems which incorporate automatic load leveling.  
         [0005]     2. Description of the Related Art  
         [0006]     Vibration has an adverse affect on the productivity of work vehicles in which an operator cab is supported on a chassis. Such vehicles include agricultural tractors, construction equipment, and over the road trucks. The vibrations experienced by such vehicles reduce their reliability, increase mechanical fatigue of components, and most importantly produce human fatigue due to motion of the operator&#39;s body. Therefore, it is desirable to minimize vibration of the vehicle cab or the seat in which the operator sits and of other components of the vehicle.  
         [0007]     Traditional approaches to vibration mitigation employed either a passive or an active suspension system to isolate the vehicle cab or seat along one or more axes to reduce bounce, pitch, and roll of the vehicle. Passive systems typically placed a series of struts between the vehicle chassis and the components to be isolated. Each strut included a parallel arrangement of a spring and a shock absorber to dampen movement. This resulted in good vibration isolation at higher frequencies produced by bumps, potholes and the like. However, performance a lower frequencies, such as encountered by a farm tractor while plowing a field, was relatively poor. The lower frequency vibrations can be in the same range as the natural frequency of the passive suspension system, thereby actually amplifying the vibration. Therefore, such previous vehicle suspension systems often performed poorly in the range of vibration frequencies to which the human body is most sensitive, i.e. one to ten Hertz.  
         [0008]     Active and semi-active suspension systems place a cylinder and piston arrangement between the chassis and the cab or seat of the vehicle to isolate that latter component. The piston divides the cylinder into two internal chambers and an electronic circuit operates valves which control the flow of hydraulic fluid between the chambers.  
         [0009]     U.S. Pat. No. 4,887,699 discloses an semi-active vibration damper in which the valve is adjusted to control the flow of fluid from one cylinder chamber into the other chamber. The valve is operated in response to one or more motion sensors, so that the fluid flow is proportionally controlled in response to the motion.  
         [0010]     U.S. Pat. No. 3,701,499 describes a type of active isolation system in which a servo valve selectively controls the flow of pressurized hydraulic fluid from a source to one of the cylinder chambers and controls exhaustion of oil from the other chamber back to a tank supplying the source. A displacement sensor and an accelerometer are connected to the mass which is being isolated from vibration and provide input signals to a control circuit. In response, the control circuit operates the servo valve to determine into which cylinder chamber fluid should be supplied, from which cylinder chamber fluid should be drained and the rate of those respective flows. This application of pressurized fluid to the cylinder produces movement of the piston which counters the vibration.  
         [0011]     For optimum vibration damping, the piston should be centered between the cylinder ends under static conditions. However, the piston may drift toward one end of the cylinder due to changes in the load on the vehicle. A similar drift occurs during prolonged vibrating conditions, such as when an agricultural tractor is plowing a field. Other effects, such as leakage of hydraulic fluid and friction between the piston and the cylinder, also affect the position of the piston under static conditions. To compensate for that piston drift, prior suspension systems included a sensor that indicated the distance between the vehicle components to which the cylinder/piston rod combination was connected and thus provide an indication of piston drift within the cylinder. In response to that signal, main control valve was opened to apply more fluid into one of the two cylinder chambers and exhaust fluid from the other chamber under static conditions to re-center the piston.  
         [0012]     However this type of load leveling increased the power requirements of the active suspension system because the dynamic response has to overcome the weight of the supported mass with each activation. This requires that the pump of the vehicle&#39;s hydraulic system operate above the normal standby pressure that occurred otherwise when other hydraulic devices were not being operated, such as when the vehicle was being driven along the ground.  
       SUMMARY OF THE INVENTION  
       [0013]     An active suspension system is provided to isolate a first member from vibrations in a second member. That system is hydraulically operated and includes a source of pressurized hydraulic fluid and a tank connected to furnish fluid to the source. A first hydraulic actuator is connected between the first and second members and comprises a cylinder with a piston therein that defines a first chamber and a second chamber in the cylinder. The cylinder further includes a third chamber that is sealed from the first and second chambers. A rod is connected to the piston and has a first end extending out of the cylinder and a second end of the rod extending into the third chamber.  
         [0014]     An electrically operated, valve arrangement controls the flow of fluid between the source and the tank and each of the first and second cylinder chambers. In one state, the valve arrangement applies the pressurized fluid to the first chamber and exhausts fluid from the second chamber to the tank. In another state, the valve arrangement applies the pressurized fluid to the second chamber and exhausts fluid from the first chamber to the tank. At other times, the valve arrangement disconnects the first and second cylinder chambers from both the source and the tank. A controller operates the valve arrangement to control the flow of fluid to and from the first and second chambers to apply force to the piston in a manner that which attenuates transmission of vibration from the second member to the first member.  
         [0015]     A load leveling valve assembly connects the third chamber of the cylinder selectively to the source and the tank to adjust a static position of the piston within the cylinder. Such adjustment substantially centers the piston between the extreme ends of its travel.  
         [0016]     In a preferred embodiment, a displacement sensor detects the position of the piston within the cylinder and provides a position signal to the controller. The controller responds, when the position signal indicates significant derivation of the piston from the center position under a static load condition, by activating the load leveling valve assembly to add or exhaust fluid to or from the third chamber. That action centers the piston.  
         [0017]     Different configurations of the valve arrangement can be employed. One configuration utilizes a three-position, four-way spool valve, while another configuration uses separate three-way valves for each of the first and second chambers. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIGS. 1 and 2  are rear and side views, respectively, of an agricultural tractor incorporating a suspension system according to the present invention;  
         [0019]      FIG. 3  is a representation of the active suspension system for the agricultural tractor;  
         [0020]      FIG. 4  is a diagram of the hydraulic circuit for one of the vibration isolators in the active suspension system;  
         [0021]      FIG. 5  is a longitudinal cross sectional view through a cylinder in the vibration isolator in which the cylinder incorporates a displacement sensor;  
         [0022]      FIG. 6  is a longitudinal cross sectional view through a cylinder which incorporates a second version of a displacement sensor;  
         [0023]      FIG. 7  is a longitudinal cross sectional view through a cylinder which incorporates a third version of a displacement sensor; and  
         [0024]      FIG. 8  is a diagram of an alternative hydraulic circuit for one of the vibration isolators in the active suspension system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     With reference to  FIGS. 1 and 2 , a vehicle  10 , such as an agricultural tractor, has a cab  12  within which an operator sits on seat  15 . The cab  12  is supported on the chassis  14  of the vehicle by three vibration isolators  16 ,  17  and  18 . The first and second vibration isolators  16  and  17  are attached to the vehicle cab at the rear of the chassis  14 . The third vibration isolator  18  is located at the center of the front of the cab  12 . The three vibration isolators  16 ,  17  and  18  can be located at other positions underneath the cab and other numbers of isolators can be employed. Although the present invention is being described in the context of an isolation system which supports the cab  12  of the vehicle  10 , this system also could be employed to isolate only the operator seat  15  from the floor of the cab  12 . Similar vibration isolators also could be incorporated into the suspension for each wheel of an automobile and used in vibration mitigating systems for other types of equipment.  
         [0026]     The vehicle cab  12  is susceptible to motion in several degrees of freedom. Movement in a vertical direction Z is commonly referred to as “bounce”, whereas “roll” is rotation about the X axis of the vehicle  10 , while rotation about the Y axis is referred to as “pitch.” The illustrated three-point active suspension, provided by the three vibration isolators  16 - 18 , addresses motion in these three degrees of freedom. However, one and two point suspension systems which address fewer degrees of freedom can also utilize the present invention.  
         [0027]      FIG. 3  depicts the system  20  for operating the three vibration isolators  16 - 18 . A pump  22 , that is driven by the engine of the vehicle  10 , draws fluid from a tank  24  and forces the fluid under pressure through a supply conduit  26  connected to the vibration isolators  16 - 18 . The fluid returns from the vibration isolators  16 - 18  through a return conduit  28  back to the tank  24 .  
         [0028]     The vibration isolators  16 - 18  are operated by control signals received from a microcomputer based electronic controller  30 , however a separate controller could be provided for each vibration isolator. The conventional controller  30  includes a memory which stores a software program for execution by the microcomputer. The memory also stores data used and produced by execution of that software program. Additional circuits are provided for interfacing the microcomputer to sensors and solenoid operated control valve for each vibration isolator  16 - 18  as will be described.  
         [0029]      FIG. 4  illustrates the hydraulic circuit  32  for the first vibration isolator  16 , with the understanding that the other two vibration isolators  17  have identical hydraulic circuits. The first vibration isolator  16  has a hydraulic actuator  34  which comprises a hydraulic cylinder  36 , pivotally connected to the chassis  14  of the vehicle, and a piston  37  with a rod  38  that is pivotally attached to the vehicle cab  12 . However, the connections can be reversed in other installations of the vibration isolator  16 . The piston  37  divides the interior of the cylinder  36  into a first chamber  41  and a second chamber  42 , and an internal wall  40  in the cylinder defines a third chamber  43  into which a surface  39  of the piston rod  38  faces. The third chamber  43  is connected to an accumulator  45  which under normal operation of the vibration isolator  16  receives fluid therefrom when the piston rod  38  is forced farther into the third chamber  43  as the piston  37  moves and sends fluid back into the third chamber  43  when the piston rod  38  is partially withdrawn from the third chamber.  
         [0030]     Although a cylinder could be constructed as depicted schematically in  FIG. 4 , in which the three chambers  41 ,  42  and  43  are located longitudinally along the cylinder, doing so creates a relatively lengthy cylinder as the third chamber  43  has to be as long as the combined lengths of the first and second chambers  41  and  42  in order to accommodate the full travel of the piston  37 . Such a relatively large hydraulic actuator  34  severely limits the places at which the vibration isolator  16  can be used. As a consequence, a novel hydraulic actuator as shown in  FIG. 5  has been developed which reduces the overall length of the device. This is accomplished by incorporating the third hydraulic chamber  43  inside a tubular piston rod.  
         [0031]     The novel hydraulic actuator has first, second and third ports  56 ,  60  and  62  for connection to hydraulic fluid conduits. The cylinder of the hydraulic actuator  34  has a tubular housing  52  with first and second ends  53  and  54  and a bore  51  there between. An end cap  55 , with an aperture  57  there through, is sealed to the housing  52  to close the first end  53 . The second port  60  is adjacent to the first end  53 . The second end  54  is closed by a fitting  58  sealed thereto and through which the first and third ports  56  and  62  lead to the bore  51  of the tubular housing  52 . The third port  62  opens into a first cavity  66  in the middle of the an interior surface  65  of the fitting  58 . The first port  56  communicates with an annular recess  67  extending around the first cavity  66  on the fitting&#39;s interior surface  65 . The annular recess  67  defines a portion of the first chamber  41  of the hydraulic actuator. The fitting  58  also has a first coupling  64  for pivotally attaching the hydraulic actuator  34  to the chassis  14  of the motor vehicle  10 .  
         [0032]     An interior tube  68  is pressed into the first cavity  66  of the fitting  58  and extends at one end into the tubular housing  52  terminating a small distance before the end cap  55 . The interior tube  68  has a central passage  69  extending from the one end to and opposite end. The opposite end has a resilient ring  70  attached thereto that acts as a stop against which the piston rod abuts in the fully retracted position and the piston abuts in the fully extended position.  
         [0033]     The piston rod  38  comprises a tubular rod body  74  that extends into the cylinder&#39;s tubular housing  52  through the aperture  57  in the end cap  55  and around the interior tube  68 . Thus rod body  74  has a central aperture  75  within which a portion of the interior tube  68  is located. O-rings in the aperture  57  provide a fluid tight seal around the rod body  74 . The piston  37  is affixed to the interior end of the tubular rod body  74  in a fluid tight manner and has an aperture  77  through which the interior tube  68  extends with O-ring seals there between that allow the piston to slide within the cylinder bore  51 . The outer circumferential surface of the piston  37  engages the inner circumferential surface of the cylinder housing  52  and has external O-rings there between to provide a fluid tight seal. The piston  37  is able to slide longitudinally within the cylinder  36  along both the cylinder housing  52  and the interior tube  68 . The first chamber is located between the piston  37  and the fitting  58  and the second chamber  42  is formed between the exterior of the rod body  74  and the interior of the cylinder housing  52 .  
         [0034]     The piston rod  38  has a plug  78  sealed into the end of the rod body  74  that projects outward from the cylinder  36 . This plug  78  has a second coupling  80  for attaching the hydraulic actuator  34  to the vehicle cab  12 . The third chamber  43  of the hydraulic actuator  34  is formed within the tubular rod body  74  between the plug  78  and the free end of the cylinder interior tube  68  and around the circumferential outer surface of the interior tube to the piston  37 . The plug  78  of the piston rod  38  has the surface  39  that faces into the third chamber  43 .  
         [0035]     A displacement sensor  48  is integrated into the hydraulic actuator  34  to provide an electrical signal indicating the amount that the piston rod  38  extends from the cylinder and thus the distance between the vehicle cab  12  and the chassis  14 . Specifically, a rod-like sensor member  82  of an electrically non-conductive material is secured in an interior end of the plug  78  so as to extend along the passage  69  of the interior tube  68 . As seen in  FIG. 5 , a gap exists between the outer surface of the sensor member  82  and the inner surface of the passage  69  allowing fluid to flow between the third port  62  in the cylinder fitting  58  and the third chamber  43  at the opposite end of the interior tube. Two stripes  83  of electrically resistive material commonly used in potentiometers are deposited separated from each other along the length of the sensor member  82 . As used herein, the term “electrically resistive” means a material having a significant resistivity that the material would not be used as an electrical conductor where resistance is an undesired characteristic. Alternatively, only one of the stripes  83  may be formed of electrically resistive material while the other stripe is an electrical conductor, such as copper or aluminum. The two stripes  83  are connected by electrical wires to a pair of contacts  84  in a connector  85  on the outer surface of the plug  78 , so that the displacement sensor  48  can be connected by an electrical cable to the controller  30 . A wiper  86  of electrically conductive material is located at the interior end of the interior tube  68  and contacts both of the resistive stripes  83  on the sensor member  82  to provide an electrical bridge between those stripes. As the piston rod  38  slides into and out of the cylinder  36 , the wiper  86  bridges the two resistive stripes  83  at different locations along the length of the sensor member  82  thereby varying the resistance appearing across the two contacts  84  of connector  85 . The magnitude of that resistance changes with variation of the distance that the piston rod  38  extends from the cylinder  36  and thus the linear displacement between the vehicle cab  12  and the chassis  14 . The wiper  86  has small apertures there through to allow fluid flow through the interior tube passage  69  between the third chamber  43  and the third port  62 .  
         [0036]     Alternatively as shown in  FIG. 6 , the displacement sensor  48  comprises stripes  100  and  102  of electrically resistive material deposited along the wall of the central aperture  75  in the rod body  74  with a wiper  104  located on the outer surface at the interior end of the interior tube  68 . The wiper  104  has notches in the outer circumferential surface to allow fluid flow there through. In another version of the displacement sensor illustrated in  FIG. 7 , the electrically resistive stripes  110  and  112  are be deposited along the wall of the passage  69  in the interior tube  68  with wires leading to a connector  114  mounted on the fitting  58 . In this alternative, a wiper  116  is positioned on the rod-like sensor member  82 .  
         [0037]     Returning to hydraulic circuit of the first vibration isolator  16  in  FIG. 4 , the cylinder  36  is connected to the supply and return conduits  26  and  28  by a three-position, four-way proportional control valve  44  which may be a conventional spool type valve, for example. The control valve  44  is moved from one position to another by solenoids which are activated by output signals from the electronic controller  30 . In the illustrated center-off position, the first and second chambers  41  and  42  of the cylinder  36  are disconnected from the supply and tank return conduits  26  and  28 . In one activated position, the control valve  44  connects the supply conduit  26  to the second chamber  42  and the tank return conduit  28  to the first chamber  41 . This applies pressurized fluid to the second chamber  42  which tends to drive the piston  37  so that the rod  38  is retracted into the cylinder  36 , thereby decreasing the distance between the vehicle cab  12  and the chassis  14 . In the other activated position of the control valve  44 , the supply conduit  26  is connected to the first chamber  41  of the cylinder  36  and the second chamber  42  is connected to the tank return conduit  28 . Here, pressurized fluid applied to the first chamber  41  drives the piston  37  to extend the rod from the cylinder, thereby increasing the distance between the vehicle cab  12  and chassis  14 .  
         [0038]     The controller  30  operates the control valve  44  in response to input signals received from sensors on the vehicle  10 . One such sensor is an accelerometer  46  that is attached to the vehicle chassis  14  and produces an electrical signal indicating vibrations that affect the vehicle cab. Other types of vibration sensors, such as a velocity sensor can be utilized to provide this vibration indicating input signal. The accelerometer  46  or other type of vibration sensor also can be mounted on the vehicle cab  12  instead of the chassis  14 . The displacement sensor  48  also is connected to the controller  30  which measures the resistance of that sensor to determine the relative displacement (Z rel ) between the vehicle cab  12  and chassis  14 .  
         [0039]     The controller  30  receives the signals from displacement sensor  48  and the accelerometer  46  which indicate instantaneous motion of the vehicle chassis  14  and determines movement of the piston  37  which is required to cancel that instantaneous motion from affecting the cab  12 . Next the controller  30  ascertains the direction and amount of fluid flow required to produce that desired vibration canceling movement of the piston  37  and then derives the magnitude of electric current to apply to the control valve  44  to produce that fluid flow. That electric current magnitude is a function of the desired fluid flow and the characteristics of the particular control valve  44 . The position and degree to which the control valve  44  is opened are respectively based on the direction and magnitude of the vibrational motion.  
         [0040]     Referring to  FIGS. 4 and 5 , when the control valve  44  is activated to retract the piston rod  38 , pressurized fluid from the pump  22  enters the second port  60  of the hydraulic actuator  34  and then flows into the second chamber  42  between the cylinder housing  52  and the tubular rod body  74 . The pressure within the second chamber  42  exerts a force on an annular first surface  88  around the piston  37 . At the same time, the second port  60  is coupled by the control valve  44  to the tank  24 , thereby permitting fluid within the first chamber  41  on the opposite side of the piston  37  to be exhausted from the hydraulic actuator. As a result of a greater force being applied to the annular first surface  88  than to the piston&#39;s second surface  89  in the first chamber  41 , the piston  37  is forced to the right in the orientation in  FIG. 5  retracting the piston rod  38  farther into the cylinder  36  which draws the chassis and vehicle cab closer together.  
         [0041]     Inversely, when the control valve  44  is placed in a position that couples the output of the pump  22  to the first port  56  of the hydraulic actuator, pressurized fluid is applied to the first chamber  41 . In this state of the control valve  44 , the second port  60  and thus the first cylinder chamber  41  are connected to the tank  24 . Now, a greater pressure exists in the first chamber  41  than in the second chamber  42  thereby applying more force against the second surface  89  of the piston  37  than against the opposite annular first surface  88 , which tends to extend the piston rod  38  from the cylinder  36 .  
         [0042]     The piston  37  should be approximately centered between the extreme ends of its travel within the cylinder, when only static external forces act on the hydraulic actuator  34 , i.e. vibration is not occurring. This centered position optimizes the ability of the vibration isolator to accommodate motion of the vehicle cab in either direction. However, leakage of hydraulic fluid, friction between the piston and the cylinder, and changes in the load of the vehicle affect the position of the piston under static conditions. If the static position of the piston too close to one end of the cylinder, the piston may be prevented from moving enough toward that end to adequately counteract subsequently occurring vibrations. The centered position is indicated by the resistance of the displacement sensor  48  produced by the position of the wiper  86  along the sensor member  82  which resistance is measured at the controller  30 . If during the static state, the displacement sensor  48  indicates a significant deviation of the piston from the center position, either due to drift of the hydraulic actuator  34  or to a significant change in the load acting on the vehicle, the controller  30  commences a load leveling operation.  
         [0043]     With reference to  FIGS. 4 and 5 , that operation employs a load leveling circuit  90  and involves opening a directional valve  92  to couple a load leveling conduit  94  to either the output of the pump  22 , in order to raise the vehicle cab with respect to the chassis, or to the tank  24  to lower the vehicle cab. The load leveling conduit  94  is attached to all three vibration isolators  16 - 18  in which the conduit is connected to a load leveling valve  96 . The load leveling valve  96  is a solenoid operated, bidirectional proportional valve the controls the amount of fluid being supplied to or exhausted from the respective hydraulic actuator  34  when the controller  30  determines that the static position of that hydraulic actuator requires adjustment. When the load leveling valve  96  is open, fluid can flow to or from the third chamber  43  of the hydraulic actuator depending upon the position of the directional valve  92 . To raise the piston  37  within the cylinder  36 , the directional valve  92  is placed into the position in which the pump output is applied to the load leveling conduit  94  and the load leveling valve  96  is opened. The action adds fluid into the third chamber  43  which applies more force to the surface  39  of the piston rod  38  thereby extending the piston rod from the cylinder  38 . Similarly, to lower the piston  37  the load leveling valve  96  is opened while the directional valve  92  is positioned to couple the load leveling conduit  94  to the tank  24 . This latter action decreases the amount of fluid in the third chamber and retracts the piston rod into the cylinder  36 . Therefore the position of the directional valve  92  determines whether raising of lowering is to occur and the state of the load leveling valve  96  of a given vibration isolator determines whether its associated hydraulic actuator is to be adjusted. While the load leveling valve  96  is opened, the four-way proportional control valve  44  may also have to be activated to alter the amounts of fluid within the first and second chambers  41  and  42  to allow motion of the piston  37 , however force does not have to be applied to the piston to accomplish the load leveling. In fact, the center “closed” position of the control valve  44  may have a orifice that connected between the first and second cylinder chambers to enable fluid to flow there between to allow piston motion.  
         [0044]      FIG. 8  discloses an alternative hydraulic circuit  200  for a vibration isolator  16 - 18 . The hydraulic actuator  34  and other components of the circuit that are identical to those in the embodiment of  FIG. 4  have been assigned identical reference numerals. The primary distinction between the circuits in  FIGS. 4 and 6  is that the single control valve  44  has been replaced by a pair of three-way control valves  201  and  202  in  FIG. 6 . The first of these proportional control valves  201  connects the first chamber  41  of the hydraulic actuator  34  selectively to the pump supply conduit  26  or the return conduit  28  and has a center position in which the first chamber  41  is disconnected from both of those conduits. The second control valve  202  provides the identical function with respect to the second chamber  42  of the hydraulic actuator  34 . Both the control valves  201  and  202  have solenoid operators which are activated by the controller  30  in similar manner to that described previously with respect to the single control valve  44 . However, by providing separate proportional control valves, the flow into each cylinder chamber  41  and  42  can be independently controlled.  
         [0045]     The alternative hydraulic circuit  200  also has a different version of the load leveling circuit  204  to manage the pressure within the third chamber  43  and thus the static position of the piston  37 . Instead of the load leveling circuit having a directional valve  92  in common with all the vibration isolators  16 - 18 , this alternative provides a proportional load leveling valve  206  in each isolator to couple the third chamber  43  of the respective cylinder  36  selectively to either the supply or return conduit  26  or  28 . The load leveling valve  206  is a three-position, three-way type valve which when activated by the controller  30  determines the whether fluid from the supply conduit  26  flows into the third chamber or fluid from that chamber flows into the return conduit  28  and the rate of such flow.  
         [0046]     While the load leveling valve  206  is opened, the three-way control valves  201  and  202  may also have to be activated to connect both of the first and second chambers  41  and  42  to the return conduit  28  allow motion of the piston  37 . That connection enables fluid for fluid from the cylinder chamber that is collapsing to the chamber that is expanding.  
         [0047]     This latter version of the load leveling circuit  204  can be used with the four-way, three-position proportional control valve  44  in  FIG. 4 , and conversely the load leveling circuit  90  in  FIG. 4  can be used with the pair of three-way control valves  201  and  202  in  FIG. 6 .  
         [0048]     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.