Abstract:
A hydraulic circuit controls a doubling acting cylinder of a vehicle suspension to provide load leveling and shock absorption functions. A set of solenoid valves control the application of pressurized hydraulic fluid from a supply line to the cylinder and from the cylinder to a tank return line to raise and lower the vehicle for load leveling. The chambers of the cylinder are interconnected by a parallel arrangement of a check valve, orifice and a relief valve. Another parallel arrangement of a check valve, orifice and a relief valve couples the cylinder to an accumulator. These parallel arranged components enable the doubling acting cylinder to function as a passive shock absorber. A lock-out valve is provided in the preferred embodiment a to defeat the shock absorber operation and provide a very stiff suspension.

Description:
Related Applications  
       [0001]    This application claims benefit of U.S. Provisional Patent Application No. 60/207,068 filed May 25, 2000. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to suspension systems for off-road equipment, such as agricultural tractors, and more particularly to such suspension systems that provide hydraulic load leveling.  
         BACKGROUND OF THE INVENTION  
         [0003]    Off-road equipment, such as construction and agricultural vehicles, can carry widely varying loads. When a relatively heavy load is applied to the equipment, the vehicle body is forced downward with respect to the axles supporting the wheels on which the vehicle rides. This results in compression of the suspension which can adversely affect the maneuverability of the vehicle. On the other hand, if the suspension is configured for very heavy loads, the vehicle may have an undesirable ride under light load conditions.  
           [0004]    As a result, many vehicles have automatic load leveling systems which employ one or more hydraulic cylinders between the axle and the frame of the vehicle to ensure that the frame is maintained at the proper height above the axle. When a heavy load is applied to the frame, the drop of the frame is sensed and additional hydraulic fluid is applied to the cylinder to raise the frame the desired distance from the axle. Thereafter, when the load is removed from the vehicle the frame will rise significantly above the axle. When this occurs hydraulic fluid is applied to the opposing chamber of the cylinder to lower the frame with respect to the axle. This type of automatic hydraulic load leveling system ensures that the frame and axle will be at the desired separation regardless of the size of the load applied to the vehicle.  
           [0005]    One of the drawbacks of this load leveling system is that the opposite chambers of the double acting cylinder have separate pressure control circuits and require high pump pressure to move the cylinder in both directions. Thus the consumption of fluid from the pump for load leveling may adversely affect the availability of fluid pressure for other functions powered by the tractor. In order to compensate for that power consumption, the pump capacity would have to be increased thus raising the cost of the hydraulic system.  
           [0006]    Although the piston within the load leveling hydraulic cylinders moves under heavy loads, the piston does not move in response to the relatively small forces due to driving the vehicle over rough terrain. Therefore, the cylinders provide a very stiff the suspension system with negligible shock absorption. This results in a very rough ride, which can be uncomfortably for the operator.  
         SUMMARY OF THE INVENTION  
         [0007]    The present system provides a hydraulic load leveling system that has a passive mode that provides shock absorption.  
           [0008]    A hydraulic circuit controls a suspension of a vehicle having a cylinder and piston for load leveling functionality. The hydraulic circuit has a first node and a second node that is connected to a piston chamber of the cylinder. A first control valve has an inlet, for connection to a supply line for pressurized hydraulic fluid in the vehicle, and has a outlet which is coupled to the first node. A control valve assembly connects the first node to a tank return line of the vehicle. In the preferred embodiment, the control valve assembly comprises a second control valve connected to operate a pilot valve. The second control valve has an inlet for connection to the pump supply line and has an outlet. The pilot operated valve has a control port connected to the outlet of the second control valve, a first port coupled to the first node, and a second port for connection to the tank return line. This group of components provides the load leveling function where the control valves are electrically operated to raise and lower the vehicle.  
           [0009]    The shock absorption is implemented by an accumulator coupled to the first node and two valve subcircuits. The first subcircuit includes a first check valve coupling the first node to the second node and permits fluid to flow through the first check valve only in a direction from the first node to the second node. A first subcircuit orifice is connected in parallel with the first check valve, and a first relief valve preferably is connected in parallel with the first check valve and opening when pressure at the second node is a predefined amount greater than pressure at the first node. The second subcircuit includes a second check valve coupling the second node to a port of the rod chamber wherein fluid can flow through the second check valve only in a direction from the second node to the rod chamber. A second subcircuit orifice is connected in parallel with the second check valve, and preferably a second relief valve is connected in parallel with the second check valve and opening when pressure in the rod chamber is a predefined amount greater than pressure at the piston chamber.  
           [0010]    The second subcircuit meters the flow of hydraulic fluid between the chambers of the cylinder thereby enabling the cylinder to act as a shock absorber. Because a rod is attached to one side of the piston, one of the cylinder chambers has less volume that the other. The extra fluid required for the larger chamber is sent into and out of the accumulator as needed in response to operation of the first subcircuit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a front view of an off-road vehicle that incorporates a regenerative suspension system according to the present invention;  
         [0012]    [0012]FIG. 2 is a schematic diagram of a hydraulic circuit of the regenerative suspension system;  
         [0013]    [0013]FIG. 3 is a cross sectional view through a valve assembly employed in the present hydraulic circuit; and  
         [0014]    [0014]FIG. 4 illustrates a disk used in the valve assembly of FIG. 3; and  
         [0015]    [0015]FIG. 5 is a schematic diagram of an alternative hydraulic circuit for the regenerative suspension system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    With initial reference to FIG. 1, an off-road vehicle  10 , such as an agricultural tractor, has a body  12  with a frame that is linked to axles to which the wheels of the vehicle are attached. For example, the front axle  14  is coupled to the body  12  by a pair of hydraulic cylinders  18  and has a pair of wheels  16  attached to it. As will be described, pressurized hydraulic fluid is applied to and drained from the cylinders  18  to control the distance that the body  12  of the tractor is above the front axle  14 . This hydraulic system ensures that a relatively constant separation distance exists regardless of the load applied to the tractor  10 .  
         [0017]    As shown in FIG. 2, the cylinder  18  has an internal bore in which a piston  20  is slidably received thereby forming a rod chamber  21  and a piston chamber  22  within the cylinder on opposite sides of the piston. The rod and piston chambers  21  and  22  vary in volume as the piston moves within the cylinder. The cylinder  18  is attached to the frame of the tractor body  12  while the remote end of the piston rod  24  is attached to the front axle  14 .  
         [0018]    The cylinder chambers  21  and  22  are connected to a regenerative hydraulic circuit  30  that controls the flow of fluid from a pump supply line  32  and back to a tank return line  34 . Specifically, the pump supply line  32  is connected to an inlet of a first control valve  36  that has a spool which is driven by a solenoid. Depending upon the position of that spool, an outlet  37  of the first control valve  36  is connected either to the pump supply line  32  or to the tank return line  34 . That latter connection occurs when the solenoid is de-energized.  
         [0019]    The outlet  37  of the first control valve  36  is connected to the load sense circuit (LS)  38  to provide a control signal to a variable displacement pump on the tractor  10  which supplies hydraulic fluid to the pump supply line  32 . A supply check valve  40  couples this outlet  37  to a first node  42  in the hydraulic circuit  30  and prevents the flow of hydraulic fluid from that first node back to the first control valve  36 .  
         [0020]    The first node  42  is coupled to the tank return line  34  by a control valve assembly  45  comprising a pilot valve  46  operated by a second solenoid control valve  50 . Specifically the first node  42  is connected through a drain orifice  44  to an inlet port of a zero-leakage, pilot operated valve  46 . An outlet port of the pilot operated valve  46  is connected to the tank return line  34 . The position of the pilot operated valve  46  is determined by pressure in a control line  48  which is coupled by a second control valve  50  to the pump supply line  32 . Both the first and second control valves  36  and  50  have solenoid operators which drive their respective spools in response to an electrical signal from a controller  52 , as will be described. Although the preferred embodiment of the hydraulic circuit  30  employs two valves  46  and  50  in the control valve assembly  45 , a single valve could be utilized. A relief orifice  54  couples the control line  48  to the tank return line  34  and acts as a bleed path for the pressure within the control line  48  when the second control valve  50  is in the closed state.  
         [0021]    The first node  42  is connected to an accumulator  56 . A valve subcircuit  58  comprises a first relief valve  60 , a first orifice  62  and a first check valve  64  connected in parallel between the first node  42  and an intermediate node  66 . The first relief valve  60  opens when the pressure at the intermediate node  66  exceeds a predefined pressure level. Fluid flows through the second check valve  64  only in the direction from the first node  42  to the intermediate node  66 .  
         [0022]    The intermediate node  66  is coupled to a second node  70  by a solenoid operated, lock-out valve  68  which also is operated by the controller  52 . The lock-out valve  68  has a fully open state when the solenoid is energized and a de-energized state in which an orifice connects the intermediate and second nodes  66  and  70 . An alternative embodiment of the lock-out valve  68  completely closes the connected between those nodes  66  and  70  in the de-energized state.  
         [0023]    The second node  70  is connected directly to the piston chamber  22  of the cylinder  18 , and by a second valve subcircuit  72  to the rod chamber of cylinder  18 . The second valve subcircuit  72  comprises a second relief valve  74 , a second orifice  76  and a second check valve  78  connected in parallel between the second node  70  and the piston chamber  21 . The second relief valve  74  opens when the pressure in the rod chamber  21  exceeds a predetermined level. Fluid is able to flow through the second check valve  78  only in a direction from the second node  70  to the rod chamber  21 .  
         [0024]    A safety pressure relief valve  79  couples the second node  70  to the tank return line  34  to relieve any dangerously high pressure occurring in the cylinder chambers  21  or  22 .  
         [0025]    Although separate elements can be utilized for each of the first and second valve subcircuits  58  and  72 , the three elements of each subcircuit can be efficiently integrated into a single assembly shown in FIG. 3. To simplify the description, this assembly will be explained with respect to the first subcircuit  58  which controls the flow of hydraulic fluid between first and second nodes  42  and  70 . However it should be understood that the second valve subcircuit  72  has an identical structure.  
         [0026]    The first valve subcircuit  58  is mounted within a bore  82  in valve housing  80  where the circular bore extends between the two nodes  42  and  70 . The valve subcircuit  58  comprises a body  84  with first and second ends  81  and  83  with an intermediate section there between. The intermediate section has a circular first flange  85  with a threaded outer circumferential surface that enables the body  84  to be threaded into the bore  82  until securely engaging a shoulder  86 . A plurality of apertures  87  extend through the periphery of the first flange  85  so that fluid is able to flow between the first and intermediate nodes  42  and  66 , as will be described.  
         [0027]    The body  84  has a first cylindrical section  88  that projects from the first flange  85  toward the intermediate node  66  and defines the second end  83 . The first check valve  64  of subcircuit  58  is formed by an annular member, or disk,  90  that has a central aperture through which the first cylindrical section  88  extends. A slip ring  92  retains the check valve disk  90  on to the first cylindrical section  88  while allowing the disk to slide longitudinally along the cylindrical section to control.  
         [0028]    A second flange  93  extends outwardly from the body  84  between the first flange  85  and the first end  81 . The second flange  93  has an annular lip  94  extending therefrom toward the first end  81  thereby forming a cavity, or recess,  95  on one side of the second circular projection  93  and opening toward the first node  42 . The second flange  93  and lip  94  have outer diameters that are less that the diameter of the bore  82  which creates a passage  99  around those elements. A central aperture  96  extends into the body from the second end  83  thereby opening into the intermediate node  66 . A plurality of angled passages  97  extend between the central aperture  96  and the recess cavity  95 . The central aperture  96  and angled passages  97  for passage by which pressure at the second node is communicated to that cavity  95 .  
         [0029]    The first end  81  of the body  84  has a second cylindrical section  98  projecting coaxial from the second flange  93  toward the first node  42 . A disk pack  100  comprises a plurality of annular disks  101  that are mounted on the second cylindrical section  98  and held in place by a washer  103  and nut  102  which is threaded onto the end of the second projection. By tightening the nut  102  to a defined torque (e.g. 6.8 Nm), the disks are forced against the edges of the flanges  94  and act as a spring having a bias force determined by the torque on the nut  102 . The innermost disk  104  that abuts the edge of the flange  94  has a serrated edge  105  with notches  106  shown in FIG. 4, which collectively form the orifice  62  along that edge as seen in FIG. 3.  
         [0030]    With reference to FIGS. 2 and 3, the subcircuit&#39;s first check valve  64  is formed by the disk-shaped member  90  and the surfaces of the body  84 . Specifically, fluid is able to flow from the first node  42  through passage  99  around the second flange  93  into a chamber  108  and then into the apertures  87  in the first flange  85  where the fluid abuts the disk-shaped member  90 . If the pressure in at the first node  42  is greater than the pressure at the intermediate node  66 , the fluid pushes the disk-shaped member  90  along the first cylindrical section  88  and away from the first flange  85 . That action opens a passageway between the disk-shaped member and the body  85  so that fluid can flow to the intermediate node  66 . Conversely, when the pressure at the intermediate node  66  is greater than the pressure at the first node  42 , the fluid pushes disk-shaped member  90  against the first flange  85 , thereby closing the passageway and preventing the fluid flow to the first node.  
         [0031]    The orifice  62  of the subcircuit is formed by the plurality of notches  106  in the inner disk  104  which allow fluid to flow in either direction between the first and intermediate nodes  42  and  66 . Specifically, the fluid flowing through the orifice from the first node  42  goes through chamber  95 , angled passages  97  and aperture  96  in the body  84  to the intermediate node  66  and is able to flow in the opposite direction through those passages.  
         [0032]    The first relief valve  60  is formed by the disk pack  100 . The pressure at the first node  42  acts on one side of the disk pack  100  while pressure at the intermediate node  66  is communicated via aperture  96  and angled passages  97  into the cavity  95  where it acts on the other side of the disk pack. When the pressure at the first node  42  is greater than pressure at the intermediate node  66 , the plurality of disks  101  in the disk pack  100  are pressed against the flange  94 , thereby restricting fluid flow to that which occurs through the orifice notches  62 . However, when pressure at the intermediate node  66  is greater than that at the first node  42  by an amount that exceeds the force applied by nut  102 , the edges of the disks are pushed away from the lip  94 . This action opens a larger area fluid passage between the cavity  95  and the first node  42 .  
         [0033]    Referring again to the operation of the hydraulic circuit shown in FIG. 2, when the load on the tractor  10  increases significantly causing its body to drop with respect to the axle, the piston  20  moves upward in the cylinder  18 . In order to raise the body of the tractor, additional pressurized hydraulic fluid has to be added to the piston chamber  22  of the cylinder. This is accomplished by the controller  52  opening the first solenoid operated control valve  36  so that the hydraulic fluid in the pump supply line  32  flows through the supply check valve  40  to the first node  42 . From the first node  42  the fluid continues through the first check valve  64  in the first subcircuit  58  and an opened lock-out valve  68  to the piston chambers  21  and  22 . In response, the tractor body  12  rises because the area of the piston exposed in the upper cylinder chamber  22  is greater than the piston area in the lower chamber  21  due to the area occupied by the rod  24 . As a consequence, the greater pressure in the upper chamber  22  will exert a greater force on the piston  20  forcing it downward.  
         [0034]    A sensor (not shown) on the truck undercarriage indicates when the tractor body  12  has raised to the proper distance from the axle  14 . At that time, controller  52  de-energizes the first control valve  36  to disconnect the hydraulic circuit  30  from the pump supply line  32 . Pressure at the outlet  37  of the first control valve is relieved through the valve to the tank return line  34  so that the pressure does not affect the load sense line  38  when the first control valve is de-energized. The supply check valve  40  prevents the fluid that has been applied to the cylinder  18  from flowing backward through this connection to the tank return line  34 .  
         [0035]    Similarly, when a heavy load is removed from the tractor  10 , the relatively high pressure in piston chamber  22  tends to force the piston  22  downward, raising the tractor body away from the axle  14 . The automatic load leveling system senses this movement and the controller  52  responds by opening the second control valve  50  while maintaining the lock-out valve  68  in the open position. This solenoid operated second control valve  50  acts as a pilot valve controlling the operation of the pilot operated valve  46 . Specifically, opening the second control valve  50  applies pressurized fluid from the pump supply line  32  through the control passage  48  to the pilot chamber of valve  46  causing the latter valve to open. This relieves pressure in the cylinder  18  by allowing the fluid therein to drain to the system tank through the tank return line  34  until the tractor body  12  is at the proper height above the axle  14 . Specifically, fluid from the piston chamber  22  flows through the open lock-out valve  68  to the first subcircuit  58  causing the first relief valve  60  to open. Because of the orifice formed by the notches  106  in disk  104  the pressure on both sides of the disk pack  100  usually is equal. Thus the relief valve opens when that pressure exceeds the force exerted by the nut  102 . The fluid continues to flow through the first node  42  and orifice  44  to the pilot operated valve  46  and into the tank return line  34 .  
         [0036]    Some of the fluid from the piston chamber  21  flows through the second node  70  and the second check valve  78  of the second subcircuit  72  into the expanding rod chamber  21 . Thus, the rod chamber  21  does not require fluid from the pump supply line  32  during this phase of load leveling. As a consequence, the present hydraulic circuit  30  enables the body  12  to be lowered by employing its own weight and without the use of pressurized fluid from the pump supply line  32 .  
         [0037]    When the tractor body  11  lowers to the proper height, the controller  52  closes the second control valve  50 . At that point the pressure within the control passage  42  bleeds to the tank return line  34  through orifice  54  resulting in closure of the pilot operated valve  46 .  
         [0038]    When load leveling is not active, the present hydraulic circuit  30  acts as a shock absorber, as long as the controller  52  maintains the lock-out valve  68  in the open position, i.e. opposite to that illustrated in FIG. 2. As the vehicle encounters rough terrain, the front wheels  16  move up and down with respect to the body  12 . When the vehicle encounters a bump, the axle  14  pushes the rod  24  and piston  20  upward in the cylinder  18  forcing fluid to flow from the upper piston chamber  22  through the second node  70  and the second valve subcircuit  72  into the rod chamber  21 . The movement of the piston  20  is dampened by restriction of that fluid flow due to the size of the tubing interconnecting the cylinder chambers  21  and  22 . It also will be appreciated that the volume of the rod chamber  21  is less than that of the piston chamber  22  because of the rod  24 . The excess fluid flows through the open lock-out valve  68  and the first orifice  62  of first subcircuit  58  into the accumulator  56 . If the bump is sever, a relatively high pressure created in the piston chamber  22  may cause the relief valve  60  in the first subcircuit  58  to open, thus aiding the transfer of fluid into the accumulator  56 . This fluid is stored under pressure in the accumulator. Note that the supply check valve  40  and the closed pilot operated valve  46  prevent the flow of this hydraulic fluid further backward through the circuit  30 .  
         [0039]    Thereafter, when the body  12  of the vehicle  10  tends to rise away from the axle  14 , the rod  24  connected to the axle pulls the piston  20  downward within the cylinder  18  in the orientation shown in FIG. 2. This motion of the piston  20  forces fluid from the rod chamber  21  back through the hydraulic circuit to the piston chamber  22 . Specifically, the fluid will flow from the rod chamber  21  through the second orifice  76  of the second subcircuit  72 , then through the second node  70 , and into the piston chamber  22 .  
         [0040]    Should pressure in the rod chamber  21  be significantly greater than that in the piston chamber  22 , the second relief valve  74  in second subcircuit  72  will open providing a bypass path for the fluid to flow around the second orifice  76  and rapidly into the piston chamber  22 . When the pressure differential decreases the second relief valve  74  closes, so that second orifice  76  restricts the flow of fluid between the two chambers  21  and  22 .  
         [0041]    Because the piston chamber  22  is larger than the rod chamber  21 , the fluid previously stored under pressure in the accumulator  56  is drawn through the first node  42  and the first check valve  64  of the first subcircuit  58 , then through the fully opened lock-out valve  68  and into the piston chamber  22 . The fluid from the accumulator makes up for the difference in volume between the two chambers  21  and  22 .  
         [0042]    Under some operating conditions, it is desirable that off-road equipment have a very stiff suspension which is achieved by disabling, or locking-out, the shock absorption function of the present hydraulic circuit  30 . In this case, the controller  52  de-energizes the lock-out valve  68  placing it in the position illustrated in FIG. 2 in which a relatively small orifice connects the intermediate and second nodes  66  and  70  of the hydraulic circuit  30 . This restricts the flow of excess fluid from the piston chamber  22  of cylinder  18  to the rod chamber  21  because of the size differential of those two chambers. That is, as the piston  20  moves upward, a greater amount of fluid has to be pushed out of the piston chamber  22  than can be accommodated by the expansion of the rod chamber  21 . Thus when the lock-out valve  68  is closed, movement of the piston is restricted, because the excess fluid cannot freely flow into the accumulator  56  due to the relatively small orifice of the closed lock-out valve. An alternative embodiment of the lock-out valve  68  eliminates that orifice so that the connection between nodes  66  and  70  is closed completely in the valve&#39;s de-energized state. Both embodiments provide a very stiff acting suspension for the vehicle  10  when the lock-out valve  68  is de-energized.  
         [0043]    [0043]FIG. 5 illustrates an alternative embodiment of a regenerative hydraulic circuit  200  that performs these functions in which the second subcircuit is incorporated into the cylinder piston. The components of the alternative hydraulic circuit  200  that correspond each components of the first circuit  30  in FIG. 2 have identical reference numerals. Specifically, the components between the first node  42  and the pump supply line  32  and the tank return line  34  are the same as in the previous embodiment. Similarly, an accumulator  56  is connected to the first node  42  which in turn is coupled to a second node  202  by the lock-out valve  68 . A pressure relief valve  79  connects the second node  202  to the tank return line  34 .  
         [0044]    The second node  202  in the alternative hydraulic circuit  200  is connected to the piston chamber  204  of the cylinder  18  by a subcircuit  208 . The rod chamber  206  of the cylinder  18  is not connected directly to any external components. The subcircuit  208  comprises a pressure relief valve  210 , an orifice  212  and a check valve  214 . The relief valve  210  opens when the pressure in the piston chamber  204  is a predetermined amount greater than the pressure at the second node  202 . The orifice  212  connects the piston chamber  204  to the second node  202  and the check valve  214  permits fluid to flow there through only from the second node  202  to the piston chamber  204 .  
         [0045]    The piston  216  in cylinder  18  has a rod  215  connected to it and incorporates the structure of the second subcircuit  217 . Specifically, piston  216  has an orifice  218  extending there through between the piston and rod chambers  204  and  206 . An internal check valve  220 , within the piston, allows the free flow of fluid only in a direction from the piston chamber  204  to the rod chamber  206 . Flow in the opposite direction from the rod chamber  206  into the piston chamber  204  is permitted by a pressure relief valve  222  when the pressure in the rod chamber is a predetermined amount greater than that in the piston chamber. Thus, elements  218 ,  220 , and  222  correspond respectively to components  76 ,  78  and  74  in the circuit embodiment in FIG. 2.  
         [0046]    The alternative hydraulic circuit  200  functions in the same manner as that described previously with respect to the first hydraulic circuit  30 . However, this circuit has the advantage of fewer connections to other components.