Abstract:
A pump displacement control arrangement uses the inherent swivel torques of a fluid translating device in cooperation with a proportional force feedback to more consistently and precisely control the displacement of the fluid translating device. The subject invention uses a variable displacement control arrangement having an actuator mechanism coupled to a swash plate of the fluid translating device and controlled by a proportional valve arrangement to control the displacement of the fluid translating device. A force feedback mechanism is disposed between the actuator mechanism and the proportional valve arrangement and provides a more precise and repeatable displacement control.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates generally to an electro-hydraulic pump control system for controlling displacement of a pump. More particularly, the invention is directed to a method and arrangement for a hydraulic pump control that utilizes pump characteristics determined from operation of a pump and a force feedback control.  
         BACKGROUND  
         [0002]    Variable displacement pumps are well known in the industry to drive an implement or a hydraulic motor or any combinations thereof. It is also well known that the speed of an actuator (i.e., hydraulic cylinder) and/or pressure of the fluid in the system may be controlled by varying the displacement of the hydraulic pump. Variable displacement pumps generally include a drive shaft, a rotatable cylinder barrel having multiple piston bores, and pistons held against a tiltable swash plate biased by a spring mechanism. When the swash plate is tilted relative to the longitudinal axis of the drive shaft, the pistons reciprocate within the piston bores to produce a pumping action. Each piston bore is subject to intake and discharge pressures during each revolution of the cylinder barrel. As the piston bores sweep pass the top and bottom center positions, a swivel force is generated on the swash plate as a result of the reciprocating pistons and pressure carryover within the piston bores. This swivel torque, depending on certain operating parameters of the pump, urges the swash plate to change its displacement position. In some variable displacement pump control systems, the swivel torque forces are utilized for controlling the displacement. For example, U.S. Pat. No. 5,564,905, which issued on Oct. 15, 1996 to Noah D. Manring, teaches using the forces generated by swivel torques to control the arcuate movement of the port plate within the pump thus controlling the forces being generated by the swivel torques which then are used to control the position of the swash plate. Additionally, U.S. Pat. No. 6,179,570, which issued on Jan. 30, 2001 to David P. Smith, teaches using the inherent forces generated by the swivel torques to aid in the control of the speed of a fluid motor. It is desirable to provide a control that not only uses the inherent swivel forces but to also provide a control that has a minimum number of moving parts, good controllability throughout the whole operating range, is precise and repeatable in positioning the swash plate.  
         SUMMARY OF THE INVENTION  
         [0003]    In one aspect of the subject invention, a variable displacement control arrangement is provided for controlling the displacement of a variable displacement fluid translating device having a pressure outlet port and an adjustable swash plate. The control arrangement includes an actuator mechanism connected to the adjustable swash plate and a source of pressurized pilot fluid connected through a proportional valve arrangement to the actuator mechanism. A force feedback mechanism is disposed between the actuator mechanism and the proportional valve arrangement.  
           [0004]    In another aspect of the subject invention, a method of controlling the displacement of a fluid translating device having an adjustable swash plate is provided and includes the steps of providing a source of pressurized pilot fluid, providing an actuator mechanism connected to the adjustable swash plate, providing a proportional valve arrangement between the source of pressurized pilot fluid and the actuator mechanism, and providing a force feedback mechanism between the actuator mechanism and the proportional valve arrangement.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a diagrammatic representation of a variable displacement axial piston pump illustrating a barrel having a plurality of bores, a port plate in contact with the barrel, a plurality of piston assemblies disposed in the bores and an adjustable swash plate in contact with the plurality of piston assemblies;  
         [0006]    [0006]FIG. 2 is a diagrammatic representation of a surface of the port plate of FIG. 1;  
         [0007]    [0007]FIG. 3 is a graph illustrating representative swivel forces being generated in one size of a pump; and  
         [0008]    [0008]FIG. 4 is a partial diagrammatic and a partial schematic representation of a variable displacement control arrangement incorporating the subject invention. 
     
    
     DETAILED DESCRIPTION  
       [0009]    Referring to FIGS. 1 and 2, a diagrammatic free-body representation of a fluid translating device  10  is illustrated. The fluid translating device  10  (hereinafter referred to as ‘the pump’) includes a barrel  12  rotatable about a pump axis  14 . The barrel has a plurality of equally-spaced, circumferentially arranged piston bores  16  provided therein. Each one of a plurality of pistons  18  is reciprocatably disposed in the respective piston bores  16 . A swash plate  20  is conventionally mounted adjacent one end of the barrel  12  for tilting movement about a swash plate axis  22  to adjust the stroke of the respective pistons. The swash plate  20  is continuously biased towards the maximum displacement position by a spring  24 . A stationary head  26  is disposed at the other end of the barrel  12  and has an intake passage  28  and a discharge passage  30 . A ball and socket joint  31  connects the base of each piston  18  to a slipper  32  that is maintained in sliding contact with the swash plate  20  in a known manner. The centers of the ball and socket joints  31  are coincident with the swash plate axis  22 .  
         [0010]    As best illustrated in FIG. 2, a flat timing port plate  34  is disposed between the barrel  12  and the stationary head  26 . The port plate  34  has an arcuate intake port  36  and an arcuate discharge port  38  extending therethrough for continuous communication with the respective intake and discharge passages  28 , 30  in the stationary head  26 . In a known manner, the barrel  12  is disposed in sliding contact with the port plate  34  so that the piston bores  16  sequentially open into the intake and discharge ports  36 ,  38  of the port plate  34  in a timed relationship as the barrel  12  rotates. As is well known, a swivel torque (naturally existing moment) tends to increase or decrease the angle of the swash plate  20  depending on the operating conditions of the fluid translating device  10 . With the barrel  12  rotating in the clockwise direction through each rotation, as viewed in FIG. 2, each piston bore  16  sequentially communicates with the intake port  36 , sweeps through a BDC position, communicates with the discharge port  38 , and after further rotation, sweeps through a TDC position to again communicate with the intake port  36 . During this rotation, some of the fluid from the intake port  36  is trapped in the respective piston bores  16  and carried through the BDC position and likewise, some of the pressurized fluid in the discharge port  38  is trapped in the respective piston bores  16  and carried through the TDC position. The accumulated effect of the forces generated by the individual pistons  18  during each revolution results in swivel torques acting on the swash plate  20 . As noted above, these swivel torques will either generate a force tending to increase the angle of the swash plate  20  or decrease the angle thereof depending on the operating conditions of the pump  10 .  
         [0011]    Referring to FIG. 3, even though swivel torque may be based on many different operating conditions, such as pressure, temperature, port plate architecture and timing to name a few, for example, the shown graph illustrates the relationship of two exemplary operating conditions of the pump  10 . A positive swivel torque urges the swash plate  20  towards a greater displacement position and a negative swivel torque urges the swash plate  20  towards a lesser displacement position.  
         [0012]    In an exemplary embodiment, the pump  10  may include a maximum displacement of 250 cubic centimeters (cc) having multiple operating speeds (RPM) and which produce system pressures up to 40,000 kilopascals (kPa), for example (FIG. 3). Dotted line  40  represents the swivel forces being generated within the exemplary pump  10  being operated at 800 RPM. Represented by the line  40 , the swivel forces are at a minimum value when the system pressure is below 10,000 kPa and, in contrast, are approximately −13 kilonewtons (kN) when the system pressure is approximately 35,000 kPa. Dashed line  42  represents the swivel forces being generated within the exemplary pump  10  while being operated at 1600 RPM. Represented by the line  42 , the swivel forces may be approximately +2 kN when the system pressure is below 10,000 kPa and, in contrast, are approximately −17 kN when the system pressure is approximately 35,00 kPa. Solid line  44  represents the swivel forces being generated within the pump  10  while being operated at 2250 RPM. Represented by the line  44 , the swivel forces are approximately +5 kN when the system pressure is below 10,000 kPa and, in contrast, are approximately —18 kilonewtons (kN) when the system pressure is approximately 35,000 kPa. It will be understood that pumps of different operating capacities, having different inherent swivel torques may also produce similar results, however, it should be recognized that when operating at higher system pressures, the swivel torques will normally be urging the swash plate  20  towards a smaller displacement position.  
         [0013]    Referring to FIG. 4, a fluid system  48  is illustrated and includes a variable displacement control arrangement  50  (hereinafter referred to as ‘the control arrangement’) disposed between a reservoir  52  and a known work system  54 . The control arrangement  50  includes the pump  10  having the adjustable swash plate  20  and the intake and discharge passages  36 , 38 . The intake passage  36  is connected to the reservoir  52  and the discharge passage  38  is connected to the work system  54  through an outlet port  56  thereof.  
         [0014]    The control arrangement  50  includes an actuator mechanism  58  that is operative to move the swash plate  20  between its minimum (MIN) and maximum (MAX) displacement positions. The actuator mechanism  58  is connected to the swash plate  20  by a mechanical link mechanism  60 . The actuator mechanism  58  includes an actuator member  62  disposed within the control arrangement  50  and is connected to the mechanical link mechanism  60 . The actuator member  62  has a first end portion  64  of a predetermined cross-sectional area disposed in a first pressure chamber  66  defined in the control arrangement  50 . The first pressure chamber  66  is in communication with the outlet port  56  of the pump  10  by a passage  68 . A spring member  69  is disposed in the first pressure chamber  66  and is operatively in contact with the first end portion  64  of the actuator member  62 . The spring member  69  functions to move the swash plate  20  away from its minimum displacement position during initial startup. The actuator member  62  also has a second end portion  70  of a predetermined cross-sectional area. The second end portion  70  is disposed in a second pressure chamber  72  of the control arrangement  50 . In an exemplary embodiment, the cross-sectional area of the first end portion  64  is smaller than the cross-sectional area of the second end portion  70 , however it is envisioned that other suitable cross-sectional areas of the first and second end portions  64 ,  70  may be used. The cross-sectional area of the first end portion  64  of the actuator member  62  is sized to provide a force that would offset the maximum swivel torque that would be acting to decrease the displacement of the pump  10 . That force is the cross-sectional area of the first end portion  64  times the pressure at the outlet port  56 . The larger, second end portion  70  is sized to produce a force that would offset or balance the maximum swivel torque that would be acting to increase the displacement of the pump  10 . That force is the cross-sectional area of the second end portion  70  times a lower control pressure hereinafter described.  
         [0015]    A source of pressurized pilot fluid  74  (hereinafter referred to as ‘the pilot pump’) is connected to the second pressure chamber  72  of the actuator mechanism  62  through a proportional valve arrangement  76  (hereinafter referred to as ‘the valve’) disposed within the control arrangement  50 . The pilot pump  74  is one example of the constant, low pressure source noted above. A force feedback mechanism  78 , such as a spring  80 , is disposed between the actuator member  62  and the valve  76  and is operative to bias the actuator member  62  towards its first operative position. The valve  76  is movable towards its second operative position in response to an electrical signal received through an electrical line  82  from a controller  84 . In the subject arrangement, the controller  84  is of a known electronic type. The degree of movement of the valve  76  is proportional to the magnitude of the electrical signal received from the controller  84 . In turn, the magnitude of the electrical signal being generated by the controller may be dependent on a control scheme in the form of a control algorithm, for example.  
         [0016]    At the first operative position of the valve  76 , pressurized fluid from the pilot pump  74  is in communication with the second pressure chamber  72  and in the second operative position thereof, the pilot pump  74  is blocked from the second pressure chamber  72  and the second pressure chamber  72  is in communication with the reservoir  52 .  
         [0017]    Industrial Applicability  
         [0018]    In use with no electrical signal being generated by the controller  84 , the actuator member  62  is in its leftmost position, as viewed in FIG. 4, since the pressure of the fluid from the pilot pump  74  acting on the cross-sectional area of the second end portion  70  is sufficient to move the actuator member  62  and thus move the swash plate  20  to its minimum displacement position.  
         [0019]    When pressurized fluid flow is required in the work system  54 , the controller  84  generates an electrical signal and directs the electrical signal through the electrical line  82  to the solenoid of the valve  76 . The valve  76  moves against the bias of the force feedback mechanism  78  an amount proportional to the magnitude of the electrical signal. As the valve  76  moves towards its second operative position, a portion of the pressurized fluid within the second pressure chamber  72  is vented to the reservoir  52  thus reducing the pressure within the second pressure chamber  72 . As a result of the lower pressure within the second pressure chamber  72 , the actuator member  62  moves in a rightward direction, as viewed in FIG. 4. As the actuator member  62  moves, the displacement of the swash plate  20  is increased through the action of the mechanical link mechanism  60 . As the actuator member  62  moves in the rightward direction, the force of the force feedback mechanism  78  is increased. Once the force of the force feedback mechanism  78  is increased to the point at which it overcomes the force established by the electrical signal, the valve  76  is maintained in a balanced position, thus maintaining a constant pressure in the second pressure chamber  72 . If additional pressurized fluid is needed in the work system  54 , the controller  84  increases the electrical signal and the force created by the solenoid moves the valve  76  further to the left, thus further decreasing the pressure in the second pressure chamber  72 . With a further decrease of pressure in the second pressure chamber  72 , the actuator member  62  moves further to the right resulting in the swash plate  20  moving to a greater angle of displacement. Again, as the force of the force feedback mechanism  78  increases, it reaches a point again at which the force therefrom balances the force established by the electrical signal and the pressure in the second pressure chamber  72  is maintained at a constant pressure level. As can be readily recognized from the above, any increase or decrease in the electrical signal from the controller  84  results in a proportional increase or decrease of the displacement of the pump  10 .  
         [0020]    In view of the foregoing, it is readily apparent that a variable displacement control arrangement  50  is provided that uses the favorable direction of the inherent swivel torques within the pump  10  to provide a simple control arrangement that has good controllability throughout the whole operating range, independent of the pump discharge pressure, and is very repeatable and precise in positioning the swash plate  20 . This repeatability comes from the inherent, internal closed loop of the force feedback/valve mechanism. This same control arrangement  50  could be used for other modes of operation, such as, flow control pressure cut-off, torque limiting control, etc. by merely using a different control software within the controller  84 .  
         [0021]    Other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.