Abstract:
A vehicle has an axle connected to a frame by at least one hydraulic cylinder with two chambers separated by a piston. A hydraulic circuit controls flow of fluid between two cylinder chambers and between the chambers and an accumulator to dampen motion of the frame relative to the axle. The hydraulic circuit includes a control valve and a pair of check valves arranged so that the single control valve is able to lock-out the cylinder to emulate a rigid connection of the frame to the axle, as needed.

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 suspension systems for off-road vehicles, such as those used in agricultural and construction businesses, and more particularly to adjustable hydraulically controlled suspension systems.  
         [0005]     2. Description of the Related Art  
         [0006]     Off-road vehicles, such as construction and agricultural equipment, carry a wide range of loads over different types of surfaces. Thus, the suspension systems between the wheels and the frame of these vehicles are important for optimum operator comfort, productivity and safety. The suspension system must be adjustable depending upon the weight of the load applied to the equipment. A very heavy load forces the vehicle body downward with respect to the axles, which can adversely affect the maneuverability of the vehicle. On the other hand, if the suspension is configured for relatively heavy loads, the vehicle may have an undesirable ride under lighter loads.  
         [0007]     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 to a desired distance from the axle. When that heavy load is removed from the vehicle, the frame rises significantly above the axle. When this occurs hydraulic fluid is drained from 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 are maintained at the desired separation regardless of the size of the load applied to the vehicle.  
         [0008]     The hydraulic cylinder also functions as a shock absorber by regulating the flow of fluid from a chamber on one side of the piston to the chamber on the other side as the vehicle bounces when traveling over the ground. Although this shock absorbing action is beneficial to creating a smoother ride and greater operator comfort, there are times when it is necessary to lock-out the front axle to produce a “solid axle”. For example, when the vehicle is carrying a heavy load there may not be a need for shock absorber action of the suspension as the tire perform that task. Also the shock absorber action is not required at relatively low speeds.  
         [0009]     The lock-out function in prior suspension systems employed a plurality of electrically operated hydraulic valves to block the flow of fluid between the cylinder chambers in additional to flow between each chamber and sources of fluid, such as a pump and an accumulator. This multiple valve arrangement added significantly to the expense of the suspension.  
         [0010]     Therefore, it is desirable to provide a less expensive lock-out mechanism which does not adversely affect the performance characteristics of the suspension system.  
       SUMMARY OF THE INVENTION  
       [0011]     A hydraulic circuit for controlling a suspension of an off-highway vehicle that has a cylinder with a piston which defines a first chamber and a second chamber within the cylinder. The hydraulic circuit has a first node to which the first chamber of the cylinder is connected and a second node to which the second chamber is connected. A first check valve, connected between the first node and the second node, allows fluid to flow only in a direction from the second node to the first node.  
         [0012]     A control valve connects the first node to an accumulator. A second check valve couples the second node to the accumulator and allows fluid to flow only in a direction from the accumulator to the second node.  
         [0013]     In the preferred embodiment of the hydraulic circuit, the control valve has a closed position in which fluid is allowed to flow through the valve only from the accumulator to the first node, and fluid is blocked from flowing in an opposite direction. The control valve has a open position in which fluid is able to flow in either direction between the accumulator and the first node. As a variation of this embodiment, a conventional load leveling system is connected to the hydraulic circuit to supply and drain fluid from the cylinder in order to maintain the piston generally centered in the cylinder as the load on the vehicle changes.  
         [0014]     When the control valve is open, forces exerted on the suspension system cause the cylinder to compress and extend freely as the fluid is able to flow in both directions between the cylinder and the accumulator. The degree to which the control valve opens can be varied to alter the amount of motion damping provided by the cylinder. Closing the control valve places the suspension hydraulic circuit  30  in the locked-out state which replicates a suspension in which the axle is rigidly connected to the vehicle frame. Now fluid attempting to exit the second chamber is blocked from reaching the accumulator by the second check valve and the control valve. In this state, the first check valve and the control valve block any fluid attempting to exit the second chamber of the cylinder. As a consequence, the cylinder piston is unable to extend or retract in either direction and the position of the suspension remains stationary. This hydraulic circuit configuration enables engagement and disengagement of the lock-out function to be performed by a single control valve.  
         [0015]     If desired, the present hydraulic circuit configuration also permits both the compression and extension of the cylinder in response to forces acting of the vehicle to be dampened with a single control valve. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a front view of an agricultural tractor the incorporates a suspension system according to the present invention;  
         [0017]      FIG. 2  is a schematic diagram of a first embodiment of the hydraulic circuit of the suspension system;  
         [0018]      FIG. 3  illustrates an alternative electrohydraulic valve assembly for the first embodiment of the hydraulic circuit; and  
         [0019]      FIG. 4  is a schematic diagram of a second embodiment of the hydraulic circuit of the suspension system. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     With initial reference to  FIG. 1 , an off-road vehicle  10 , such as an agricultural tractor, has a body  12  with a frame  14  that is linked to axles to which the wheels of the vehicle are attached. For example, the front axle  16  is coupled to the frame  14  by a pair of hydraulic cylinders  17  and  18  and has a pair of front wheels  20  attached to it. As will be described, pressurized hydraulic fluid is applied to and drained from the cylinders  17  and  18  to control the distance that the body  12  of the tractor is above the front axle  16 . This hydraulic system adjusts dynamically to ensure that a relatively constant separation distance exists regardless of the load applied to the vehicle  10 .  
         [0021]     The control of the tractor suspension shall be described with respect to cylinder  18  with the understanding that the other cylinder  17  is controlled in the identical manner. Referring to  FIG. 2 , the cylinder  18  has an internal bore in which a piston  22  with a rod  24  is slidably received; thereby forming a first, or rod, chamber  26  and a second, or piston, chamber  28  within the cylinder on opposite sides of the piston. The rod and piston chambers  26  and  28  vary in volume as the piston  22  moves within the cylinder  18 . In order to understand the subsequent description of the system operation, it is important to note that some of the volume of the rod chamber is taken up by the piston rod  24 . Therefore, a smaller quantity of fluid is required to retract the rod into the cylinder (i.e. compress the cylinder) than is required to extend the piston rod (i.e. extend the cylinder) the same distance. Either the cylinder  18  or piston rod  24  is attached to the tractor frame  14 , while the other one is attached to the front axle  16 .  
         [0022]     In a first embodiment of the present invention, the cylinder  18  is connected to a regenerative hydraulic circuit  30  that controls the flow of fluid into and out of each cylinder chamber  26  and  28 . The hydraulic circuit  30  has a first node  31  to which the rod chamber  26  is connected and a second node  33  to which the piston chamber  28  is connected. A first check valve  32  is connected directly between the first and second nodes  31  and  33 , and thus directly between the two cylinder chambers  26  and  28 . The first check valve  32  is oriented to allow fluid to flow only in a direction from the second node  33  to the first node  31 . The terms “directly connected” and “directly connecting” as used herein mean that the associated components are connected together by a conduit without any intervening element, such as a valve, an orifice or other device, which restricts or controls the flow of fluid beyond the inherent restriction of any conduit and its couplings.  
         [0023]     An accumulator  38 , of a type conventionally used in suspension systems, is coupled by a second check valve  36  to the second node  33 . The second check valve  36  permits fluid to flow only in the direction from the accumulator  38  to the second node  33 . Conventional devices, such as ball or poppet style check valves can be used as the first and second check valves  32  and  36 . A third node  35  is defined in the conduit between the second check valve  36  and the accumulator  38 .  
         [0024]     An electrohydraulic poppet control valve  34  is connected between the rod chamber  26  and the accumulator  38 . The control valve  34  unidirectional in that it only controls the flow of fluid from the first node  31  to the third node  35 . Specifically, in the closed position, show in the drawing, fluid flow can occur through the valve from the third node  35  to the first node  31 , but flow in the opposite direction is blocked. In the open position, fluid is able to flow in either direction through the control valve  34 . A solenoid or other type of electrical actuator operates the control valve  34  to open the valve to varying degrees and thus proportionally control the flow of fluid there through. Several types of poppet and spool valves may be used for the control valve  34 .  FIG. 3  shows an alternative configuration that employs an electrohydraulic pilot valve  45  having a fully closed position, which blocks flow in both directions, and a parallel connected external check valve  46 . Therefore, as used herein the term “control valve” includes the various valve assemblies, including a plurality of physically separate valves, which perform the function of the control valve  34  as described herein.  
         [0025]     By controlling the flow area of the control valve  34  on a real-time basis, the degree to which the suspension dampens motion of the vehicle body  12  can be changed dynamically. This is possible with a single valve because the first and second check valves  32  and  36  direct the fluid flow through the control valve  34  during both extension and compression of the cylinder  18 . In contrast, previous systems required a pair of valves that independently controlled the compression flow and extension flow to adjust the degree of motion damping.  
         [0026]     Referring again to  FIG. 2 , the motion damping function is implemented with a sensor  48  that is attached to the body  12  or frame  14  of the vehicle  10 . The sensor provides an electrical input signal to a controller  49  which operates the control valve  34 . The sensor  48  may be an accelerometer in which case the electrical input signal indicates acceleration of the vehicle body  12  and the controller  49  integrates that signal to derive the velocity of the body. Alternatively, the sensor  48  can detect the displacement or position of the frame  14  with respect to the axle  16  and the controller differentiates the sensor signal to obtain the vehicle body&#39;s velocity. The controller  49  responds to the velocity by determining the amount of flow that is required between the cylinder chambers to dampen that velocity and thus determine the degree to which the control valve should open.  
         [0027]     The hydraulic circuit  30  enables a single proportional control valve  34  to proportionally control extension and compression damping of the vehicle suspension and to provide a cylinder lock-out function. When the control valve  34  is open, forces exerted on the suspension system cause the cylinder  18  to compress and extend freely as the fluid is able to flow in either direction between the cylinder chambers  26  and  28  and the accumulator  38 .  
         [0028]     When the cylinder  18  compresses and the piston chamber  28  becomes smaller, fluid flow from the piston chamber directly to the accumulator  38  is blocked by the second check valve  36 . Instead that flow is routed through the first check valve  32  to the first node  31 . From the first node  31 , some of the fluid fills the smaller rod chamber  26 , while the rest of the fluid flows through the control valve  34  into the accumulator  38  where it is stored.  
         [0029]     When the cylinder extends, the fluid exiting the rod chamber  26  is blocked by the first check valve  32 . As a result this fluid is directed through the control valve  34  and the second check valve  36  into the piston chamber  28  of the cylinder  18 . Because the expanding piston chamber  28  requires more fluid than is exiting the rod chamber  26 , additional fluid is drawn from the accumulator  38  through the second check valve  36  into the piston chamber  28  to prevent cavitation.  
         [0030]     Closing the control valve  34  places the suspension hydraulic circuit  30  in the lock-out state. Now fluid attempting to exit the piston chamber  28  is blocked from reaching the accumulator by the second check valve  36  and the closed control valve  34 . Although the rod chamber  26  is tending to expand, its smaller size is insufficient to contain all the fluid attempting to exit the piston chamber  28 . Alternatively in this closed state of the control valve  34 , any fluid attempting to exit the cylinder rod chamber  26  is blocked by the first check valve  32  and the control valve. As a result, the piston  22  is unable to move significantly in either direction and the position of the vehicle suspension remains stationary.  
         [0031]     An optional conventional load leveling system  40  controls the flow of pressurized fluid from a pump  42  to the first node  31  between the rod chamber  26  and the control valve  34  in the hydraulic circuit  30 . This system  40  also controls the flow of fluid from that first node  31  to a tank  44  that supplies the pump  42 . Alternatively the load leveling system  40  can be connected to the second or third node  33  or  35 . The load leveling system  40  includes a sensor (not shown) which detects the distance between the tractor frame  14  and the axle  16 . The load leveling system  40  responds to relatively long duration changes in that distance which as noted previously are caused by variation of the load applied to the tractor. If that frame to axle distance is significantly small, as occurs under a heavy load, fluid from the pump  42  is fed into the first node  31 . With the control valve  34  open, that pressure is applied to the rod chamber  26  of the cylinder  18  and through the second check valve to the cylinder piston chamber. Because the surface area of the piston  22  is greater in the piston chamber  28 , the rod  24  will extend, raising the tractor frame  14  with respect to the axle  16 . Alternately, when that frame to axle distance is significantly large, as occurs when a heavy load is removed, the load leveling  40  drains fluid from the first node  31  into the tank  44  which retracts the piston rod  24  lowering the tractor frame  14  toward the axle  16 .  
         [0032]     The load leveling system  40  also operates to relieve excessive pressure which may occur in the hydraulic circuit  30 . In order for a pressure relief valve in the load leveling system  40  to respond to excessive pressure in all sections of the hydraulic circuit  30  that system must be connected at the first node  31 . Otherwise, a load leveling system, connected to the second or third node  33  or  35 , will be isolated from a section of the hydraulic circuit  30  by the first or second check valve  32  or  36  when the control valve  34  is closed.  
         [0033]      FIG. 4  shows a second regenerative hydraulic circuit  50  which utilizes a normally-open electrohydraulic poppet control valve  52  in place of the normally-closed valve  34  in  FIG. 4 . Otherwise, the two hydraulic circuits function in the same manner and identical components have been assigned the same reference numerals in both figures.  
         [0034]     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.