Abstract:
A hydraulic system is disclosed having at least two hydraulic circuits. The disclosed system compares pressures between the hydraulic circuits and alters valve commands of the circuit associated with higher pressures in order to reduce overall system pressure.

Description:
RELATED APPLICATIONS 
       [0001]    This patent application is a continuation-in-part of U.S. patent applicaion Ser. No. 11/238,962, filed Sep. 30, 2005, which is hereby incorporated by reference. 
     
    
     TECHNICAL FIELD  
       [0002]    The present disclosure relates generally to a hydraulic system, and more particularly, to a hydraulic system having multiple circuits. 
       BACKGROUND  
       [0003]    Hydraulic systems are often used to control the operation of hydraulic actuators of machines. These hydraulic systems typically include valves, arranged within hydraulic circuits, fluidly connected between the actuators and pumps. These valves may each be configured to control a flow rate and direction of pressurized fluid to or from respective chambers within the actuators. 
         [0004]    In some instances, multiple actuators may be connected to a common pump. During actuation of multiple actuators one actuator may require a significantly higher pressure from the pump than other actuators. Actuation of one such actuator may also create undesirable pressure or flow conditions in other parts of the system. The pressure and flow of the fluid provided to each actuator can be controlled, in part, by valves between the pump and the actuator. It is generally desirable to control the valves in a way that improves the efficiency of the system. 
         [0005]    One method of reducing pressure fluctuations in hydraulic systems is described in U.S. Pat. No. 5,878,647 (“the &#39;647 patent”) issued to Wilke et al. While the hydraulic circuit described in the &#39;647 patent may reduce pressure fluctuations, it may also result in unnecessarily high system pressure. 
       SUMMARY OF THE INVENTION 
       [0006]    A hydraulic system is disclosed having a source of pressurized fluid, and a first hydraulic circuit configured to receive pressurized fluid from the source. The first hydraulic circuit is provided with a first valve and a first actuator, the first valve being disposed between the source and the first actuator, and the first actuator operating at a first pressure. A second hydraulic circuit is also provided and configured to receive pressurized fluid from the source. The second hydraulic circuit includes a second valve and a second actuator, the second valve being disposed between the source and the second actuator, and the second actuator operating at a second pressure. The hydraulic system also includes a controller configured to receive a command input, and based on the command input, determine a first initial flow passing command for the first valve and a second initial flow passing command for the second valve. The controller further determines which of the first pressure and the second pressure is a higher pressure and alters the initial flow passing command for a high pressure valve, the high pressure valve being the one of the first valve and the second valve corresponding to the higher pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a diagrammatic illustration of a disclosed machine; and 
           [0008]      FIG. 2  is a schematic illustration of a disclosed hydraulic system. 
       
    
    
     DETAILED DESCRIPTION  
       [0009]      FIG. 1  illustrates an exemplary machine  10 . Machine  10  may be a fixed or mobile machine that performs some type of operation 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 include a generator set, a pump, a marine vessel, or any other suitable operation-performing machine. Machine  10  may include a frame  12 , an implement  14 , and hydraulic actuators  20   a ,  20   b  connected between implement  14  and frame  12 . Alternatively, hydraulic actuator  20   a  may be connected between implement  14  and frame  12  while hydraulic actuator  20   b  may be connected between a separate implement (not shown) and frame. Machine  10  may also include more than the two actuators  20   a ,  20   b  specifically discussed herein. 
         [0010]    As illustrated in  FIG. 2 , machine  10  may further include a hydraulic system  25  configured to affect movement of hydraulic actuators  20   a ,  20   b  so as to move, for example implement  14 . Hydraulic system  25  may further include two hydraulic circuits  50   a ,  50   b  configured to control the operation of hydraulic actuators  20   a ,  20   b , respectively. 
         [0011]    Hydraulic system  25  may further include a source  26  of pressurized fluid and a tank  28 . Hydraulic circuits  50   a ,  50   b , may each include a pressure compensating valve  30   a ,  30   b . Each hydraulic circuit  50   a ,  50   b  may further include two supply valves  31   a ,  31   b : a head-end supply valve  32   a ,  32   b  and a rod-end supply valve  34   a ,  34   b ; as well as two drain valves  33   a ,  33   b : a head-end drain valve  36   a ,  36   b , and a rod-end drain valve  38   a ,  38   b . Each hydraulic circuit may also include a head-end make-up valve  40   a ,  40   b , a head-end relief valve  42   a ,  42   b , a rod-end make-up valve  44   a ,  44   b , and a rod-end relief valve  46   a ,  46   b . It is contemplated that hydraulic system  25  may include additional and/or different components such as, for example, a temperature sensor, a position sensor, an accumulator, and/or other components known in the art. 
         [0012]    Hydraulic actuators  20   a ,  20   b  may include a piston-cylinder arrangement, a hydraulic motor, and/or any other known hydraulic actuator having one or more fluid chambers therein. According to an embodiment of this disclosure, hydraulic actuators  20   a ,  20   b  may include a tube  51   a ,  51   b  and a piston assembly  52   a ,  52   b . Hydraulic actuators  20   a ,  20   b  may also include a head-end chamber  54   a ,  54   b  and a rod-end chamber  56   a ,  56   b  separated by piston assembly  52   a ,  52   b.    
         [0013]    Source  26  may be configured to produce a flow of pressurized fluid and may include a variable displacement pump such as, for example, a swashplate pump, a variable pitch propeller pump, and/or other sources of pressurized fluid known in the art. Source  26  may be controlled by a control system  100  and may be drivably connected to a power source (not shown) of machine  10  by, for example, a countershaft (not shown), a belt (not shown), an electrical circuit (not shown), and/or in any other suitable manner. Source  26  may be disposed between tank  28  and hydraulic actuators  20   a ,  20   b  and may be configured to be controlled by control system  100 . 
         [0014]    Pressure compensating valves  30   a ,  30   b  may be proportional control valves disposed between source  26  and an upstream supply passageway  60   a ,  60   b , respectively, and may be configured to control a pressure of the fluid supplied to upstream supply passageway  60   a ,  60   b , respectively. Pressure compensating valves  30   a ,  30   b  may include a proportional valve element that may be spring and hydraulically biased toward a flow passing position and hydraulically biased toward a flow blocking position. 
         [0015]    Pressure compensating valves  30   a ,  30   b  may be movable toward the flow blocking position by a fluid directed via a fluid passageway  78   a ,  78   b  from a point between pressure compensating valve  30   a ,  30   b  and upstream supply passageway  60   a ,  60   b . A restrictive orifice  80   a ,  80   b  may be disposed within fluid passageway  78   a ,  78   b  to minimize pressure and/or flow oscillations within fluid passageway  78   a ,  78   b . Pressure compensating valve  30   a ,  30   b  may be movable toward the flow passing position by the combined forces of a spring and a fluid directed via a fluid passageway  82   a ,  82   b  from a shuttle valve  74   a ,  74   b . A restrictive orifice  84   a ,  84   b  may be disposed within fluid passageway  82   a ,  82   b  to minimize pressure and/or flow oscillations within fluid passageway  82   a ,  82   b . It is contemplated that the proportional valve element of pressure compensating valve  30   a ,  30   b  may alternately be spring biased toward a flow blocking position, that the fluid from fluid passageway  82   a ,  82   b  may alternately bias the valve element of pressure compensating valve  30   a ,  30   b  toward the flow blocking position, and/or that the fluid from passageway  78   a ,  78   b  may alternately move the proportional valve element of pressure compensating valve  30   a ,  30   b  toward the flow passing position. It is also contemplated that pressure compensating valve  30   a ,  30   b  may alternately be located downstream of supply valves  31   a ,  31   b , or in any other suitable location. It is further contemplated that restrictive orifices  80   a ,  80   b , and  84   a ,  84   b  may be omitted, if desired. 
         [0016]    Supply valves  31   a ,  31   b  may be disposed between source  26  and hydraulic actuator  20   a ,  20   b , respectively, and may be configured to regulate a flow of pressurized fluid to actuators  20   a ,  20   b . Specifically, head-end supply valves  32   a ,  32   b  may be disposed between source  26  and head-end chamber  54   a ,  54   b , and rod-end supply valves  34   a ,  34   b  may be disposed between source and rod-end chambers  56   a ,  56   b , respectively. Depending on the direction of actuation of the actuator  20   a ,  20   b , one of head-end supply valve  32   a ,  32   b  or rod-end supply valve  34   a ,  34   b  will provide the supply of pressurized fluid to the actuator  20   a ,  20   b  for its respective circuit  50   a ,  50   b . For example, if pressurized fluid is provided to the head end  54   a  of actuator  20   a  in circuit  50   a , head-end supply valve  32   a  would be the acting supply valve  31   a  in circuit  50   a.    
         [0017]    Supply valves  31   a ,  31   b  may each include a proportional valve element that may be spring biased and solenoid actuated to move the valve element to any of a plurality of positions from a first position in which fluid flow may be substantially blocked from flowing toward actuator  20   a ,  20   b  to a second position in which a maximum fluid flow may be allowed toward actuator  20   a ,  20   b . Additionally, the proportional valve elements of supply valves  31   a ,  31   b  may be controlled by control system  100  to vary the size of a flow area through which the pressurized fluid may flow. 
         [0018]    Drain valves  33   a ,  33   b  may be disposed between hydraulic actuator  20   a ,  20   b  and tank  28  and may be configured to regulate a flow of pressurized fluid from head-end chamber  54   a ,  54   b , or rod-end chamber  56   a ,  56   b , depending on the direction of actuation. Specifically, head-end drain valves  36   a ,  36   b  and rod-end drain valves  38   a ,  38   b  may each include a two-position valve element that may be spring biased and solenoid actuated between a first position at which fluid may be allowed to flow from head-end chamber  54   a ,  54   b  or rod-end chamber  56   a ,  56   b , depending on the direction of actuation, and a second position at which fluid may be substantially blocked from flowing from head-end chamber  54   a ,  54   b  or rod-end chamber  56   a ,  56   b . Supply valves  31   a ,  31   b  and drain valves  33   a ,  33   b  may be fluidly interconnected as illustrated in  FIG. 2 . 
         [0019]    Shuttle valve  74   a ,  74   b  may be disposed within downstream system signal passageway  62   a ,  62   b . Shuttle valve  74   a ,  74   b  may be configured to fluidly connect the one of head-end supply valve  32   a ,  32   b  and rod-end supply valve  34   a ,  34   b  having a lower fluid pressure to pressure compensating valve  30   a ,  30   b . In this manner, shuttle valve  74   a ,  74   b  may resolve pressure signals from head-end supply valve  32   a ,  32   b  and rod-end supply valve  34   a ,  34   b  to allow the lower outlet pressure of the two valves to affect movement of pressure compensating valve  30   a ,  30   b  via fluid passageway  82   a ,  82   b.    
         [0020]    Hydraulic system  24  may include additional components to control fluid pressures and/or flows within hydraulic system  24 . Specifically, hydraulic system  24  may include pressure balancing passageways  66   a ,  66   b  configured to control fluid pressures and/or flows within hydraulic system  24 . Pressure balancing passageways  66   a ,  66   b  may fluidly connect upstream supply passageway  60   a ,  60   b  and downstream system signal passageway  62   a ,  62   b . Pressure balancing passageways  66   a ,  66   b  may include restrictive orifices  70   a ,  70   b , to minimize pressure and/or flow oscillations within fluid passageways  66   a ,  66   b . Hydraulic system  24  may also include a check valve  76   a ,  76   b  disposed between pressure compensating valve  30   a ,  30   b  and upstream supply passageway  60   a ,  60   b  and may be configured to block pressurized fluid from flowing from upstream supply passageway  60   a ,  60   b  to pressure compensating valve  30   a ,  30   b.    
         [0021]    Control system  100  may be configured to control the operation of head-end supply valves  31   a,    31   b  and drain valves  33   a ,  33   b  source  26 . Control system  100  may include a controller  102  configured to receive pressure signals from pressure sensors  108   a ,  108   b ,  108   c  via communication lines  112   a ,  112   b . Controller  100  may also be configured to deliver control signals to supply valves  31   a ,  31   b , drain valves  33   a ,  33   b , and source  26  via communication lines  112   a ,  112   b . It is contemplated that the pressure and control signals may each be any conventional signal, such as, for example, a pulse, a voltage level, a magnetic field, a sound or light wave, and/or another signal format. 
         [0022]    Controller  102  may be configured to control hydraulic system  24  in response to the pressure signals received from pressure sensors  108   a ,  108   b ,  108   c . Controller  102  may be configured to perform one or more algorithms to determine appropriate output signals to control the movement of the valve elements of, and thus the amount of flow directed through, supply valves  31   a,    31   b  and drain valves  33   a ,  33   b  and to control the output, e.g., displacement and/or input speed, of source  26 . Controller  102  may determine the appropriate control signals by, for example, predetermined equations, look-up tables, and/or maps. It is further contemplated that controller  102  may control the operation of other components within hydraulic system  24 . 
         [0023]    In operation, source  26  provides pressurized fluid to either head-end chamber  54   a ,  54   b  or rod-end chamber  56   a ,  56   b  of one or more actuators  20   a ,  20   b , depending on the direction of actuation. Flow of fluid to the actuator  20   a ,  20   b  may be controlled in part by control of source  26 . For example, source  26  may be a variable displacement axial piston pump, in which case the rate of flow from source  26  may be controlled by the angle of the swashplate and/or the speed of the pump. 
         [0024]    Flow of pressurized fluid from the source  26  to actuator  20   a ,  20   b  may also be controlled in part by the respective supply valve  31   a ,  31   b . By altering the flow passing area of supply valve  31   a ,  31   b , the flow of fluid to the respective actuator  20   a ,  20   b , and the pressure drop over supply valve  31   a ,  31   b  may be controlled. 
         [0025]    When multiple circuits  50   a ,  50   b  simultaneously request flow to actuate multiple actuators  20   a ,  20   b , one circuit, e.g. circuit  50   a , may require fluid at a higher pressure than the other circuit, e.g. circuit  50   b . In this situation, controller  102  may determine which circuit  50   a ,  50   b  is at a higher pressure. Controller  102  may then determine the available flow from the source  26 . The flow available from source  26  may be limited, for example, by a maximum flow rate of source  26 , in which case available flow could depend on, among other things, a maximum speed and displacement of source  26 . Alternatively, the flow available from source could be limited by available power, in which case available flow could depend on, among other things, an output pressure from source  26  and the power available to source  26 . 
         [0026]    During multi-function operations, controller  102  may apportion available flow from source  26  between each circuit  50   a ,  50   b . For example, controller  102  may control multiple supply valves  31   a ,  31   b , to be actuated to flow passing positions to direct pressurized fluid to respective chambers, e.g., head-end chambers  54   a ,  54   b  or rod-end chambers  56   a ,  56   b , of the multiple hydraulic actuators  20   a ,  20   b . For example, controller  102  may include logic that relates a set of inputs, such as an operator input or inputs, to a initial flow passing position of supply valves  31   a ,  31   b , and/or drain valves  33   a ,  33   b . The logic may include a look-up table, an algorithm, priority schemes or other methods for relating inputs to desired flow passing positions of supply valves  31   a ,  31   b  as may be known in the art. 
         [0027]    Controller  102  may receive multiple pressure signals from pressure sensors  108   a ,  108   b  associated with the multiple circuits  50   a ,  50   b  and pressure sensor  108   c  associated with source  26 . Controller  102  may then compare pressure signals between the hydraulic circuits  50   a ,  50   b  to determine which circuit  50   a ,  50   b , or more specifically which actuator  20   a ,  20   b , is operating at a higher pressure. For example, if pressurized fluid is provided to head-end chamber  54   a  and head-end chamber  54   b , controller  102  may compare a pressure downstream of head-end supply valve  32   a  with a pressure downstream of head-end supply valve  32   b . If the pressure downstream of head-end supply valve  32   a  is greater than the pressure downstream of head-end supply valve  32   b , controller  102  may provide a high-pressure altered command, such that the flow passing position of supply valve  32   a  is larger, i.e. passes fluid with less restriction, than the initial command would have caused. The alteration of the initial command provided to such high-pressure supply valve  31   a,    31   b,  e.g. head-end supply valve  32   a , may cause the flow passing area of the respective valve to be increased by a percentage, by a fixed displacement, or by any other method of causing an increase in flow passing area. 
         [0028]    If the pressure difference between the high pressure supply valve  31   a,    31   b , e.g. head-end supply valve  32   a , and the low pressure supply valve  31   a ,  31   b , e.g. head-end supply valve  32   b , is below a predetermined value, controller  102  may provide a high-pressure altered command to both the high pressure supply valve  31   a ,  31   b , and the low pressure supply valve  31   a ,  31   b , such that the flow passing area of the low pressure supply valve  31   a,    31   b  is increased in a manner similar to the increase in the flow passing area of the high pressure supply-valve  31   a,    31   b.  In doing so, a smooth transition may be facilitated when a low pressure supply-valve  31   a ,  31   b  becomes the high-pressure supply valve  31   a ,  31   b,  and vice versa. 
         [0029]    A high-pressure altered command may be provided to both supply valves  31   a,    31   b  during a period of time in which the pressure differential between the supply valves  31   a ,  31   b  is below a predetermined pressure value. Alternatively, a high-pressure altered command may be provided to both supply valves  31   a,    31   b  for a predetermined period of time after the pressure differential drops below a predetermined value. In yet another alternative, the high-pressure altered command may be provided to both supply valves  31   a,    31   b  for the greater of a period of time in which the pressure differential between the supply valves  31   a,    31   b  is below a predetermined value and a predetermined period of time after the pressure differential drops below a predetermined value. 
       INDUSTRIAL APPLICABILITY 
       [0030]    The disclosed hydraulic system may be applicable to increase the efficiency of a machine  10 . By altering the command to the high-pressure supply valve  31   a ,  31   b,  the overall pressure demand on source  26  may be reduced. For example, considering that head-end supply valve  32   a  may be, for a desired operation, the high-pressure supply valve, pressure compensating valve  30   a  may maintain a constant pressure drop between source  26  and first hydraulic actuator  18 . By altering head-end supply valve  32   a , the pressure differential between upstream supply passageway  60   a  and first chamber passageway  61   a  may be reduced. Consequently, this lower pressure differential may then affect the balance of the proportional valve element of pressure compensating valve  30   a  to a more open position. As such, both the pressure drop over the compensating valve  30   a  and the supply valve  31   a  may be reduced, and less pressure may be required from source  26 . As such, a reduction in the required output of the power source drivably connected to source  26  may be realized or the displacement of source  26  may be increased to realize an increased flow of pressurized fluid. 
         [0031]    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. 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.