Abstract:
An electro-hydraulic control system for pump control is disclosed. The hydraulic actuator is configured to control the inclination of a swashplate. The position of the hydraulic actuator is controlled by controlling the flow of pressurized fluid into and out of two pressure chambers, one on either side of the actuator. A fluid passageway is provided that selectively connects the passageway to tank. The passageway has an orifice for each pressure chamber, and the actuator is configured to selectively block all or a portion of one or more of the orifices, depending on the position of the actuator. The components of the control system are configured such that the actuator will return to a neutral or near-neutral position upon loss of electric power.

Description:
RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from U.S. Provisional Application No. 61/254,773 by Hongliu Du, filed Oct. 26, 2009, the contents of which are expressly incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to a hydraulic actuator, and more particularly, to a fail neutral electro-hydraulic control system for controlling a variable displacement pump. 
       BACKGROUND 
       [0003]    Variable displacement hydraulic pumps are widely used in hydraulic systems to provide pressurized hydraulic fluid for various applications. Many types of machines such as dozers, loaders, and the like, rely heavily on hydraulic systems to operate, and utilize variable displacement pumps to provide a greater degree of control over fixed displacement pumps. 
         [0004]    Various control schemes have been utilized to control the swashplate angle of such variable displacement hydraulic pumps. One such control scheme is disclosed in U.S. Pat. No. 6,553,891, filed Jul. 9, 2001, to Carsten Fiebing, which is hereby incorporated by reference. However, it may be beneficial to provide a control scheme that fails to a neutral position upon loss of power. 
       SUMMARY OF THE INVENTION 
       [0005]    A hydraulic system is disclosed having a source of pressurized fluid, a tank, an actuator disposed between a first pressure chamber and a second pressure chamber, a fluid passageway having a first orifice in selective communication with the first pressure chamber and a second orifice in selective communication with the second pressure chamber, and a drain valve disposed in the fluid passageway having an open position and a closed position. According to this disclosure fluid is passable from both the first orifice and the second orifice to the tank when the drain valve is in the open position, and fluid is restricted from passing from both the first orifice and the second orifice to the tank when the drain valve is in the closed position. 
         [0006]    A method for controlling an inclination of a swashplate is further disclosed. This method includes the steps of changing the inclination of a swashplate by energizing a first electrical device associated with a first control valve, de-energizing a second electrical device associated with a second control valve, and energizing a third electrical device associated with a drain valve; and returning the swashplate to a neutral position or a near-neutral position by de-energizing the first electrical device, de-energizing the second electrical device, and de-energizing the third electrical device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a side-view diagrammatic illustration of an exemplary machine; 
           [0008]      FIG. 2  is a schematic illustration of an exemplary transmission; and 
           [0009]      FIG. 3  is a schematic illustration of exemplary pump control hardware in a first condition; 
           [0010]      FIG. 4  is a schematic illustration of the exemplary pump control hardware of  FIG. 3  in a second condition; and 
           [0011]      FIG. 5  is a schematic illustration of another embodiment exemplary pump control hardware. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]      FIG. 1  illustrates an exemplary machine  10 . Machine  10  may be a fixed or mobile machine that performs operations associated with an industry such as mining, construction, farming, or any other industry known in the art. For example, machine  10  may be an earth moving machine such as a dozer, a loader, a backhoe, an excavator, a motor grader, a dump truck, or any other earth moving machine. Machine  10  may also embody a generator set, a pump, a marine vessel, or any other suitable machine. Referring to  FIGS. 1 and 2 , machine  10  may include a frame  12 , an implement  14 , traction devices  18  such as wheels or tracks, and a transmission  20  ( FIG. 2 ) to transfer power from an engine  16  ( FIG. 2 ) to the traction devices  18 . 
         [0013]    As illustrated in  FIG. 2 , the transmission  20  may be a hydrostatic transmission and may include a source of pressurized fluid, for example a primary pump  22  driven by the engine  16 , a motor  24  and a bypass relief valve  26 . In practice, transmission may be a continuously variable transmission (CVT), parallel path variable transmission (PPV), or other transmission known in the art. According to the present disclosure, the primary pump  22  may be a variable displacement pump such as a variable displacement axial piston pump, the displacement of which may be varied by changing the angle of inclination of a swashplate (not shown). The motor  24  may be a fixed displacement hydraulic motor. However, the motor  24  may alternatively be a variable displacement motor. The transmission  20  may further include another source of pressurized fluid, for example a charge pump  28  providing pressurized fluid to swashplate control hardware  30 , which is illustrated in greater detail in  FIG. 3 . 
         [0014]      FIG. 3  illustrates a portion of the control hardware  30 . Control hardware  30  includes an actuator  50  having a connection portion  52  configured to accept a swashplate control arm (not shown), such that translation of the actuator  50  effects a change in an angular orientation of the primary pump&#39;s  22  swashplate (not shown). The position of actuator  50  is controlled by a first pressure chamber  54  and a second pressure chamber  56 . First pressure chamber  54  is selectively placed in communication with charge pump  28  and tank  40  by a first three-position three-way control valve  58 , which is actuated by an electrical device, such as a solenoid  61 , acting against a mechanical device, such as a spring  63 . Similarly, second pressure chamber  56  is selectively placed in communication with charge pump  28  and tank  40  by a second three-position three-way control valve  60 , which is actuated by an electrical device, such as a solenoid  65 , acting against a mechanical device, such as a spring  67 . 
         [0015]    With respect to  FIG. 3 , movement of actuator  50  to the right is effected by de-energizing the solenoid  61  associated with the first control valve  58  to place the first pressure chamber  54  in communication with charge pump  28  and energizing the solenoid  65  associated with the second control valve  60  to place the second pressure chamber  56  in communication with tank  40 . Similarly, movement of actuator  50  to the left is effected by energizing the solenoid  61  associated with the first control valve  58  to place the first pressure chamber  54  in communication with tank  40  and de-energizing the solenoid  65  associated with the second control valve  60  to place the second pressure chamber  56  in communication with charge pump  28 . 
         [0016]    A fluid passageway  62  is provided between the first control chamber  54  and the second control chamber  56 . The passageway  62  has a first orifice  68  connecting the passageway  62  with the first pressure chamber  54  and a second orifice  70  connecting the passageway  62  with the second pressure chamber  56 . In the embodiment illustrated in  FIG. 3 , the first and second orifices  68 ,  70  are blocked by the actuator  50  when the actuator  50  is in a neutral position, as illustrated. The neutral position of the actuator may be characterized by the actuator being substantially centered with respect to the first and second orifices  68 ,  70 . It is contemplated that a neutral and near-neutral position of the actuator will correspond to a substantially neutral orientation of the swashplate, and a null or minimal displacement of the primary pump  22 . 
         [0017]    In the embodiment illustrated in  FIG. 3  a relatively small movement of the actuator  50  to the right will open the first orifice  68  to the first pressure chamber  54 , and a relatively small movement of the actuator  50  to the left will open the first orifice  70  to the second pressure chamber  54 . A drain valve  64  is disposed within the passageway  62  having an open and a closed position. A mechanical device, such as a spring  72 , biases drain valve  64  toward the open position and an electrical device, such as a solenoid  74 , biases the drain valve  64  toward the closed position. When drain valve  64  is in the open position, fluid is capable of passing through passageway  62  to tank  40 . When drain valve  64  is in the closed position, fluid is restricted from flowing through passageway  62  to tank  40 , and from flowing to either of the first or second orifices  68 ,  70  from the other of the first or second orifices  68 ,  70 . 
       INDUSTRIAL APPLICABILITY 
       [0018]    During normal operation of the primary pump  22 , solenoid  74  is energized, moving drain valve  64  to the closed position. In this manner pressurized fluid may be provided to and from the first and second chambers  54 ,  56  to move actuator  50  and change the angle of the swashplate and, thus the displacement of the primary pump  22 . 
         [0019]    Upon loss of electrical power, the control hardware  30  may assume the configuration illustrated in  FIG. 4 . In this condition the first and second control valves  58 ,  60  are actuated by their respective springs to a flow passing position, such that both first and second pressure chambers  54 ,  56  are in communication with charge pump  28 . Furthermore, drain valve  64  is also biased by spring  72  to the open position. With further reference to  FIG. 4  actuator  50  is left of a neutral position, thereby communicating second pressure chamber  56  with tank  40  by way of passageway  62  through an exposed area, A p2 , of the second orifice  70 . The flow of fluid from second pressure chamber  56  to tank  40  will result in the second pressure chamber  56  being at a lower pressure than the first pressure chamber  54 . This pressure imbalance will bias the actuator  50  towards a neutral position. However, due to forces acting on the swashplate, the swashplate arm may exert a force, F s , on the actuator  50  as well. Thus, the actuator  50  will move to an equilibrium position, which will generally be close to a neutral position. Neglecting the effects of friction, this equilibrium area of A p1  can be approximated by Eq. 1, in which A c  is the metering area of the second control valve  60 , A act  is surface area of the right side of the actuator  50  being acted upon by the pressure in the second pressure chamber  56 , and P charge  is the pressure of the fluid being discharged from the charge pump  28 . 
         [0000]    
       
         
           
             
               
                 
                   
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         [0020]    Accordingly, by using Eq. 1, the steady state position of the actuator  50  can be approximated by using a map comparing actuator  50  position to the exposed area, A p2 , of the second orifice  70 . 
         [0021]    In another embodiment illustrated in  FIG. 5 , actuator  50  is sized such that it is underlapping in a neutral position, which is to say that in a neutral position both the first and second orifices  68 ,  70  are in communication with their respective pressure chambers  54 ,  56 . In such an underlapping condition, an equilibrium position in terms of A p1  and A p2  can be approximated by Eq. 2, where A p1  is the exposed area of the first orifice  68 . 
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         [0022]    Accordingly, in an underlapping condition, by using Eq. 2, the steady state position of the actuator  50  can be approximated by using map comparing actuator  50  position to the difference of the square of the exposed areas, i.e. A p1   2 −A p2   2 . 
         [0023]    It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed hydraulic system. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed hydraulic system. In particular, it will be apparent to those skilled in the art that the control system describe herein for use on a variable displacement pump, may also be utilized on a variable displacement motor. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.