Patent Publication Number: US-6662705-B2

Title: Electro-hydraulic valve control system and method

Description:
TECHNICAL FIELD 
     The present invention is directed to a system and method for controlling an electro-hydraulic valve arrangement. In particular, the present invention is directed to a system and method for controlling an electro-hydraulic valve arrangement to perform a pump check function. 
     BACKGROUND 
     Hydraulic actuators, such as piston/cylinder arrangements or fluid motors, are commonly used to move work implements, such as, for example, buckets, shovels, loaders, backhoes, rakes, trenchers, forklifts, etc., that are carried on work machines. The hydraulic actuators provide the power necessary to move the work implement to accomplish an operation. Depending on the type of work implement and the requirements of the work machine, one or more hydraulic actuator may be connected to the work implement. 
     Each hydraulic actuator typically includes at least two fluid chambers that are disposed on opposite sides of a moveable element. The moveable element of each hydraulic actuator is, in turn, connected to the work implement that is to be moved. The work machine usually carries a pump that is connected to the hydraulic actuator and provides pressurized fluid to one or the other of the fluid chambers of the hydraulic actuator. Typically, an electro-hydraulic valve arrangement is placed in fluid connection between the pump and the hydraulic actuator to control a flow rate and direction of pressurized fluid to and from the fluid chambers. 
     When it is desirable to move the work implement in a certain direction, the electro-hydraulic valve arrangement is moved to place the pump in fluid connection with one chamber of the hydraulic actuator at the same time that fluid is allowed to flow out of the other chamber. This creates a pressure differential over the moveable element of the hydraulic actuator. Provided that the force exerted on the moveable element by the pressurized fluid is great enough to overcome the resistant force of the work implement, the moveable element will move towards the area of lower fluid pressure existing in the opposite chamber of the hydraulic actuator, thereby moving the work implement. 
     If however, the pressure of the fluid leaving the pump is less than the pressure of the fluid in the hydraulic actuator, the fluid will tend to flow from the actuator towards the pump, i.e. in a reverse direction. If the fluid were allowed to flow unchecked, the moveable element of the hydraulic actuator would move in an undesirable manner. 
     Typically, as shown in U.S. Pat. No. 4,967,557, a mechanical check valve is disposed in the fluid connection between the pump and the electro-hydraulic valve arrangement. The mechanical check valve is a spring loaded valve that only allows fluid to flow in one direction, e.g., from the pump to the electro-hydraulic valve arrangement. When the pressure differential over the check valve is positive, i.e. the pressure of the fluid on a first side of the valve is greater that the pressure of the fluid on the opposite side of the valve, the force of the fluid will overcome the spring force and open the check valve. If, however, the pressure of the fluid on the first side of the valve is less than the pressure on the opposite side of the valve, the valve will close and prevent fluid from flowing through the valve. 
     The use of mechanical check valves to perform the pump and load check functions may be disadvantageous to the overall system. For instance, each mechanical check valve may add cost to the overall system. In addition, the inclusion of a mechanical check valve may increase the size of the overall system. 
     The present invention provides a system and method for controlling an electro-hydraulic valve arrangement that solves all or some of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     To attain the advantages in accordance with the purposes of the invention, as embodied and broadly described herein, the invention is directed to a method of controlling an electro-hydraulic valve arrangement that is disposed in fluid connection between a source of pressurized fluid and an actuator. According to the method, a signal is received to open the electro-hydraulic valve arrangement to provide a flow of fluid from the source of pressurized fluid to the actuator. A source pressure that is representative of the pressure of fluid between the source of pressurized fluid and the electro-hydraulic valve arrangement is determined. An actuator pressure that is representative of the pressure of the fluid between the electro-hydraulic valve arrangement and the actuator is also determined. The generated signal is modified to prevent the electro-hydraulic valve arrangement from opening when the source pressure is less than the actuator pressure to prevent a reverse flow of fluid from the actuator to the source of pressurized fluid. 
     In another aspect, the invention is directed to a system for controlling a hydraulic actuator that includes a hydraulic actuator and a source of pressurized fluid. An electro-hydraulic valve arrangement is positioned in fluid connection with the source of pressurized fluid and the hydraulic actuator and is operable to control a flow rate of fluid from the source of pressurized fluid to the hydraulic actuator. A first pressure sensor senses a source pressure that is representative of the pressure of the fluid between the source of pressurized fluid and the electro-hydraulic valve arrangement. A second pressure sensor senses an actuator pressure that is representative of the pressure of the fluid between the electro-hydraulic valve arrangement and the hydraulic actuator. A control device receives a signal to open the electro-hydraulic valve arrangement and prevents the electro-hydraulic valve arrangement from opening when the source pressure is less than the actuator pressure. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is a schematic and diagrammatic illustration of a control system in accordance with one embodiment of the present invention; 
     FIG. 2 is a first embodiment of a flowchart illustrating a process for controlling the electro-hydraulic valve arrangement of FIG. 1; and 
     FIG. 3 is a second embodiment of a flowchart illustrating a process for controlling the electro-hydraulic valve arrangement of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     A system and method for controlling an electro-hydraulic valve arrangement is provided. The electro-hydraulic valve arrangement is used to control a flow of pressurized fluid to a hydraulic actuator. In the currently contemplated embodiment and as illustrated in the figures, the hydraulic actuator is a piston cylinder combination. However, the hydraulic actuator may be another type of actuator, such as, for example, a fluid motor. An exemplary embodiment of a control system for an electro-hydraulic valve arrangement is illustrated in FIG.  1  and is generally designated by the reference number  10 . 
     In the accompanying figures, a single electro-hydraulic valve arrangement and actuator combination is illustrated. However, the system and method described herein are equally applicable to hydraulic circuits that include multiple electro-hydraulic valve arrangement and actuator combinations. 
     As shown in FIG. 1, control system  10  is connected to a hydraulic actuator  12 , which includes a housing  64  containing a piston  60 . Piston  60  is slidably received in housing  64  for movement in a first direction (as indicated by arrow  66 ) and in a second direction (as indicated by arrow  68 ). Piston  60  is connected to a piston rod  62 , which extends through housing  64  and is connected to a load  14 . 
     It is contemplated that load  14  may be an implement of a work machine, such as, for example, a bucket, fork, or other earth or material moving implement. These types of work machines may include, for example, wheel loaders, track type loaders, and hydraulic excavators. 
     Also shown in FIG. 1 is housing  64  that defines a first chamber  56  on one side of piston  60  and a second chamber  58  on the opposite side of piston  60 . Both the first chamber  56  and the second chamber  58  are configured to receive and hold a pressurized fluid. Piston rod  62  extends through second chamber  58  and housing  64 . 
     A source of pressurized fluid is provided to supply pressurized fluid to the hydraulic actuator. It is contemplated that the source of pressurized fluid may be a pump  18  of any variety readily apparent to one skilled in the art, such as, for example, a piston pump, gear pump, vane pump, or gerotor pump. In the currently contemplated embodiment, the pump is a variable capacity pump, although it is contemplated that the pump may be a fixed capacity pump with a bypass valve. 
     As illustrated in FIG. 1, pump  18  is placed in fluid connection with a tank  20  through fluid line  46 . Tank  20  contains a supply of fluid at an ambient pressure. Pump  18  is also connected to fluid line  48 , which leads to an electro-hydraulic valve arrangement  16 . 
     Electro-hydraulic valve arrangement  16  is placed in fluid connection between pump  18  and hydraulic actuator  12 . Electro-hydraulic valve arrangement  16  is selectively operable to fluidly connect one of the first and second chambers  56 ,  58  of hydraulic actuator  12  with pump  18  while fluidly connecting the other of the first and second chambers with the tank. Electro-hydraulic valve arrangement  16  may also be closed to prevent fluid from flowing into or out of either the first chamber or the second chamber. 
     As illustrated in FIG. 1, electro-hydraulic valve arrangement  16  is connected to pump  18  through fluid line  48  and to tank  20  through a fluid line  50 . Electro-hydraulic valve arrangement  16  includes four independent metering valves  22 ,  24 ,  26 , and  28 . Other types of electro-hydraulic valve arrangements, such as, for example, split spool valves and three-position electro-hydraulic valves may also be used. 
     As also shown in FIG. 1, electro-hydraulic valve arrangement  16  is placed in fluid connection with hydraulic actuator  12  through fluid lines  52  and  54 . Specifically, first metering valve  22  and second metering valve  24  are connected to first chamber  56  of hydraulic actuator  12  through fluid line  52 . Third metering valve  26  and fourth metering valve  28  are connected to second chamber  58  of hydraulic actuator  12  through fluid line  54 . In the currently contemplated embodiment, each independent metering valve is a proportional valve, i.e. is operable to allow a variable flow rate of fluid to flow therethrough. The fluid flow rate that is allowed to flow through a particular valve depends upon system and load requirements. 
     As further illustrated in FIG. 1, first independent metering valve  22  controls the rate at which pressurized fluid flows from pump  18  to first chamber  56 . Second independent metering valve  24  controls the rate at which fluid flows from first chamber  56  to tank  20 . Third independent metering valve  26  controls the rate at which fluid flows from pump  18  to second chamber  58 . Fourth independent metering valve  28  controls the rate at which fluid flows from second chamber  58  to tank  20 . 
     First metering valve  22  includes a first solenoid  30 . In the disclosed embodiment, energizing first solenoid  30  acts on first metering valve  22  to move the valve towards an open position to place first chamber  56  in controlled fluid connection with pump  18 . A first spring  32  also acts on first metering valve  22  to return first metering valve  22  to a closed position when first solenoid  30  is de-energized. 
     Second metering valve  24  includes a second solenoid  34 . In the disclosed embodiment, energizing second solenoid  34  acts on second metering valve  24  to move the valve towards an open position to place first chamber  56  in controlled fluid connection with tank  20 . A second spring  36  also acts on second metering valve  24  to return the valve to a closed position when second solenoid  34  is de-energized. 
     Third metering valve  26  includes a third solenoid  38 . In the disclosed embodiment, energizing third solenoid  38  acts on third metering valve  26  to move the valve towards an open position to place second chamber  58  in controlled fluid connection with pump  18 . A third spring  40  also acts on third metering valve  26  to return the valve to a closed position when third solenoid  38  is de-energized. 
     Fourth metering valve  28  includes a fourth solenoid  42 . In the disclosed embodiment, energizing fourth solenoid  42  acts on fourth metering valve  28  to move the valve towards an open position to place second chamber  58  in controlled fluid connection with tank  20 . A fourth spring  44  also acts on fourth metering valve  28  to return the valve to a closed position when fourth solenoid  42  is de-energized. 
     In this embodiment, the motion of hydraulic actuator  12  is controlled by selectively and controllably opening and closing independent metering valves  22 ,  24 ,  26 , and  28 . In standard operation, to move hydraulic actuator  12  in a first direction (as illustrated by arrow  66 ), first metering valve  22  and fourth metering valve  28  are controllably opened at the same time by energizing first solenoid  30  and fourth solenoid  42 . This places first chamber  56  in connection with pump  18  and second chamber  58  in connection with tank  20 . This configuration allows pressurized fluid to flow to first chamber  56  and also allows displaced fluid to flow from second chamber  58  to tank  20 . The pressurized fluid entering first chamber  56  exerts a force on piston  60  to move load  14  in the first direction (as indicated by arrow  66 ). When the operation is complete, first solenoid  30  and fourth solenoid  42  are de-energized, thereby allowing first spring  32  and fourth spring  44  to return first metering valve  22  and fourth metering valve  28  to their closed positions. 
     Similarly, to move hydraulic actuator  12  in a second direction (as illustrated by arrow  66 ) second metering valve  24  and third metering valve  26  are controllably opened at the same time by energizing second solenoid  34  and third solenoid  38 . This places second chamber  58  in connection with pump  18  and first chamber  56  in connection with tank  20 . This configuration allows pressurized fluid to flow to second chamber  58  and also allows displaced fluid to flow from first chamber  56  to tank  20 . The pressurized fluid entering second chamber  58  exerts a force on piston  60  to move load  14  in the second direction (as indicated by arrow  68 ). When the operation is complete, second solenoid  34  and third solenoid  38  are de-energized, thereby allowing second spring  36  and third spring  40  to return second metering valve  24  and third metering valve  26  to their closed positions. 
     A first pressure sensor  70  is provided to sense a source, or pump, pressure that is representative of the pressure of the fluid between pump  18  and electro-hydraulic valve arrangement  16 . First pressure sensor  70  may be disposed at any point in system  10  that will allow first pressure sensor  70  to sense a fluid pressure that is representative of the pressure of the fluid between pump  18  and electro-hydraulic valve arrangement  16 . 
     As illustrated in FIG. 1, the first pressure sensor  70  is connected to fluid line  48 . First pressure sensor  70  senses the pressure of the fluid in fluid line  48 , which is representative of the fluid pressure between pump  18  and electro-hydraulic valve arrangement  16 . First pressure sensor may be disposed at any point along fluid line  48 , including the fluid exit of pump  18  and the fluid inlet of electro-hydraulic valve arrangement  16 . 
     A second pressure sensor  72  or  74  is provided to sense an actuator pressure that is representative of the pressure of the fluid between electro-hydraulic valve arrangement  16  and hydraulic actuator  12 . Second pressure sensor  72  or  74  may include one or more pressure sensors disposed in the system to sense the pressure of the fluid between electro-hydraulic valve arrangement  16  and at least one of the first and second chambers  56 ,  58  of hydraulic actuator  12 . Second pressure sensor  72  or  74  may be disposed at any point within system  10  that will allow the pressure sensor to sense a pressure representative of the fluid pressure between electro-hydraulic valve arrangement  16  and at least one chamber  56 ,  58  of hydraulic actuator  12 . 
     As will be described in greater detail below, the pressures sensed by first pressure sensor  70  and second pressure sensor  72  or  74  are used to determine the pressure difference between the pump pressure and the actuator pressure. As an alternative, a pressure differential sensor may be used to determine the pressure difference between the pump pressure and the actuator pressure. The output of the pressure differential sensor would indicate whether the pump pressure was greater or less than the actuator pressure. The output of the pressure differential sensor may also indicate, in appropriate units, the magnitude of the pressure difference. 
     As illustrated in FIG. 1, first chamber pressure sensor  72  is connected to fluid line  52  and second chamber pressure sensor  74  is connected to fluid line  54 . First chamber pressure sensor  72  senses the pressure of the fluid in fluid line  52 , which is representative of the fluid pressure within first chamber  56  and of the fluid pressure between electro-hydraulic valve arrangement  16  and hydraulic actuator  12 . Second chamber pressure sensor  74  senses the pressure of the fluid in fluid line  54 , which is representative of the fluid pressure within second chamber  58  and of the fluid pressure between electro-hydraulic valve arrangement  16  and hydraulic actuator  12 . 
     First chamber pressure sensor  72  and second chamber pressure sensor  74  may be disposed at any point along fluid lines  52  and  54  or may be connected directly to first chamber  56  and second chamber  58 , provided that the sensed pressures are representative of the fluid pressure between electro-hydraulic valve arrangement  16  and the respective chamber  56 ,  58  of hydraulic actuator  12 . First chamber pressure sensor  72  and second chamber pressure sensor  74  may also be disposed at the outlet of electro-hydraulic valve arrangement  16 , such as at the outlets of first independent metering valve  22  and third independent metering valve  26 . 
     A control device  88  is provided to govern the position of electro-hydraulic valve arrangement  16  and thereby control the rate and direction of fluid flow to hydraulic actuator  12 . In response to a received signal to open electro-hydraulic valve arrangement  16  to provide a requested flow rate of fluid to hydraulic actuator  12 , control device  88  will prevent electro-hydraulic valve arrangement  16  from opening when the pump pressure is less than the actuator pressure. In addition, control device  88  may compute a scaling factor based on the difference between the pump pressure and the actuator pressure. Control device  88  applies the scaling factor to the requested flow rate to determine an actual flow rate of fluid to provide to hydraulic actuator  12  and adjusts the position of electro-hydraulic valve arrangement  16  accordingly. The flowcharts of FIGS. 2 and 3 describe illustrative methods of controlling electro-hydraulic valve arrangement  16 . 
     As illustrated in FIG. 1, control device  88  is connected between a control lever  84  and system  10 . Control device  88  preferably includes a computer, which has all components required to run an application, such as, for example, a memory, a secondary storage device, a processor, such as a central processing unit, and an input device. One skilled in the art will appreciate that this computer can contain additional or different components. Furthermore, although aspects of the control system are described as being stored in memory, one skilled in the art will appreciate that these aspects can also be stored on or read from other types of computer program products or computer-readable media, such as computer chips and secondary storage devices, including hard disks, floppy disks, CD-ROM, or other forms of RAM or ROM. 
     Control device  88  governs the position of electro-hydraulic valve arrangement  16  and thereby controls the rate and direction of fluid flow into and out of hydraulic actuator  12 . Control device  88  is connected to first solenoid  30 , second solenoid  34 , third solenoid  38 , and fourth solenoid  42  through control lines  82 . By selectively energizing and de-energizing first, second, third, and fourth solenoids  30 ,  34 ,  38 , and  42 , control device  88  controls the rate and direction of fluid flow into and out of first and second chambers  56  and  58  of hydraulic actuator  12 . 
     As shown in FIG. 1, a spool position sensor  45  may be operatively engaged with each of first, second, third, and fourth metering valves  22 ,  24 ,  26 ,  28 . Each spool position sensor  45  detects the actual position of the spool within the respective metering valve. The measured position of each spool may be transmitted to control device  88 . Control device  88  may use this feedback to more accurately control the flow rate of fluid though each of first, second, third, and fourth metering valves  22 ,  24 ,  26 ,  28 . 
     Control device  88  is connected to control lever  84 . Control device  88  may be connected to control lever  84  through control line  86  or through another connection such as for example, a remote control  85  or and automatic control. An operator manipulates control lever  84  to control the motion of load  14 . The operator may move control lever  84  to a first operative position to move load  14  in the first direction (as indicated by arrow  66 ). In response, control device  88  energizes the appropriate solenoid, or solenoids, to connect first chamber  56  with pump  18  and second chamber  58  with tank  20 . This configuration results in the movement of load  14  in the first direction. 
     The operator may also move control lever  84  to a second operative position to move load  14  in the second direction (as indicated by arrow  68 ). In response, control device  88  energizes the appropriate solenoid, or solenoids, to connect second chamber  58  with pump  18  and first chamber  56  with tank  20 . This configuration results in the movement of load  14  in the second direction. 
     In addition, the operator may move control lever  84  to a neutral position to stop the motion of load  14  or to prevent load  14  from moving. In response, control device  88  de-energizes all solenoids so that electro-hydraulic valve arrangement  16  returns to a closed position to prevent fluid from flowing into or out of hydraulic actuator  12 . 
     As illustrated in FIG. 1, control device  88  is also connected to first pressure sensor  70  through control line  76 , first chamber pressure sensor  72  through control line  78 , and second chamber pressure sensor  74  through control line  80 . Each pressure sensor provides control device  88  with a sensed pressure. In the currently contemplated embodiment, each pressure sensor provides a sensed pressure to control device  88  on a periodic basis, such as every 5 ms. 
     INDUSTRIAL APPLICABILITY 
     The operation of the aforementioned system will now be described with reference to the attached drawings. An exemplary method  110  for controlling electro-hydraulic valve arrangement  12  is presented in the flowchart of FIG.  2 . Method  110  may be implemented in the system, for example, by an application stored in the memory of the computer of control device  88 . 
     With reference to FIG. 1, when an operator moves control lever  84  to either a first operative position or a second operative position to move hydraulic actuator  12  in either the first direction (as indicated by arrow  66 ) or the second direction (as indicated by arrow  68 ), a signal is generated to open electro-hydraulic valve arrangement  16  (step  112  of FIG.  2 ). The generated signal may be electronic or mechanical. 
     Control device  88  determines the pump pressure (P p ) (step  114 ). The pump pressure (P p ) may be determined by sensing the pressure of the fluid between pump  18  and electro-hydraulic valve arrangement  16  through a sensor, such as first pressure sensor  70 . The pump pressure (P p ) may be sensed on a periodic basis, such as every 5 ms. Alternatively, the pump pressure (P p ) may be sensed only upon receipt of a signal to open electro-hydraulic valve arrangement  16 . The pump pressure (P p ) may also be determined by reference to a representative pump pressure, such as, for example, the standard operating pressure or stand-by pressure of the pump, that is stored in the memory of control device  88 . 
     Control device  88  also reads the actuator pressure (P a ) as sensed by either the first chamber pressure sensor  72  or second chamber pressure sensor  74  (step  116 ). The actuator pressure (P a ) may be sensed on a periodic basis, such as every 5 ms. Alternatively, the actuator pressure (P a ) may be sensed only upon receipt of a signal to open electro-hydraulic valve arrangement  16 . 
     Control device  88  compares the pump pressure (P p ) to the actuator pressure (P a ) for the chamber to which pump  18  is to be connected, i.e. the pressure of first chamber  56  if hydraulic actuator  12  is to be moved in the first direction (as indicated by arrow  66 ) or the pressure of second chamber  58  if hydraulic actuator  12  is to be moved in the second direction (as indicated by arrow  68 ). If the pump pressure (P p ) is less than the actuator pressure (P a ) for the respective chamber, control device  88  will modify the signal provided by the control lever (i.e. the generated signal) to prevent electro-hydraulic valve arrangement  16  from opening (step  122 ). 
     If the pump pressure (P p ) is greater than the actuator pressure (P a ) for the respective chamber, control device  88  will open electro-hydraulic valve arrangement  16  (step  120 ). Opening electro-hydraulic valve arrangement  16  places pump  18  in fluid connection with the respective chamber of hydraulic actuator  12  to move actuator  12  in the desired direction. 
     After electro-hydraulic valve arrangement  16  is opened, control device  88  may continue to monitor both the pump pressure (P p ) and the actuator pressure (P a ). If the pump pressure (P p ) drops below the actuator pressure (P a ), control device  88  will immediately close electro-hydraulic valve arrangement  16  to prevent an undesirable reverse flow of fluid. 
     It is also contemplated that control device  88  may account for inaccuracies in the pressure sensors. Because pressure sensors do not always provide an accurate pressure reading, a variable, such as pressure offset (P o ), may be included to compensate for any possible error in the pressure readings. A pressure drop calculation including the pressure offset is as follows: 
     
       
         Pressure Difference= P   p   −P   a   P   o    
       
     
     As will be understood from this equation, the inclusion of the pressure offset (P o ) provides a safety margin. In the currently contemplated embodiment, the value of the pressure offset (P o ) is based on the specified margin of error for the pressure sensors. The value of the pressure offset should be approximately equal to the sum of the margin of error for the pump pressure sensor and one of the first and second chamber pressure sensors. By subtracting the pressures offset (P o ) in the pressure difference calculation, control device  88  ensures that the pump pressure (P p ) exceeds the actuator pressure (P a ) by at least the margin of error for the pressure sensors providing the values of the pump pressure and the actuator pressure. 
     By preventing electro-hydraulic valve arrangement  16  from opening and closing electro-hydraulic valve arrangement  16  when the pump pressure (P p ) is less than the actuator pressure (P a ), a reverse flow of fluid, where fluid flows from either first chamber  56  or second chamber  58  through electro-hydraulic valve arrangement  16  towards pump  18 , is prevented. If reverse flow was allowed, an undesirable movement of load  14  may occur. Thus, by controlling the position of electro-hydraulic valve arrangement  16  based on the pump pressure (P p ) and the actuator pressure (P a ), control device  88  performs a pump check function. This eliminates the need to include a separate mechanical check valve between pump  18  and electro-hydraulic valve arrangement  16 . 
     Another exemplary process  130  for controlling electro-hydraulic valve arrangement  16  is illustrated in the flowchart of FIG.  3 . When the operator moves control lever  84  to generate movement of hydraulic actuator  12 , control device  88  determines a requested flow rate of fluid into and out of first and second chambers  56  and  58  of hydraulic actuator  12  (step  132 ). As will be appreciated by one skilled in the art, the flow rate determination will be based on system parameters and requirements, such as, for example, chamber size, pump specifications, and actuator speed. 
     Control device  88  receives the sensed pump pressure (P p ) (step  134 ) and the sensed actuator pressure (P a ) (step  136 ) as described previously. Control device  88  then computes a scaling factor (step  138 ). The scaling factor calculation is based on the difference between the pump pressure (P p ) and the actuator pressure (P a ). The scaling factor is a value between 0 and 1 that represents the percentage of the requested flow rate that should be provided to the actuator given the current state of the hydraulic system. 
     A scaling factor of 0 indicates that the electro-hydraulic valve should be closed, i.e. the pump pressure (P p ) is less than the actuator pressure (P a ). A scaling factor of 1 indicates that the pump pressure (P p ) is sufficient to fully meet the system needs and the electro-hydraulic valve arrangement should be opened to provide an actual flow rate that is equal to the requested flow rate. A scaling factor of between 0 and 1 indicates that the pump pressure is marginally greater than the actuator pressure and some, but not all, of the system requirements may be met. Accordingly, the electro-hydraulic valve arrangement should be opened to provide an actual flow rate that is less than the requested flow rate. In this way, the computed scaling factor provides for limited flow under some operating conditions in a manner analogous to a mechanical check valve being partially opened. 
     The following formula may be used to determine the scaling factor (F s ): 
     
       
           F   S   =K   p *( P   p   −P   a )  
       
     
     where, K p  is a constant that represents the minimum pressure difference between the pump pressure and the actuator pressure that is necessary to meet all of the requirements of the system. K p  is dependent upon the particular system requirements and on the type of electro-hydraulic valve arrangement being controlled. In the currently contemplated embodiment, K p  is the reciprocal of this minimum pressure difference. For example, if the specifications of a particular system indicate that the pressure difference between the pump pressure and the actuator pressure be at least 100 kPa (14.5 psi) before the electro-hydraulic valve arrangement can meet all of the needs of the system, K p  will be equal to {fraction (1/100)} or 0.01. 
     As will be apparent from the calculation and description above, the computed value of F s  may be greater than 1 in the situation where the pump pressure (P p ) is much greater than the actuator pressure (P a ). In addition, the above calculation may yield a result that is less than 0 when the pump pressure (P p ) is less than the actuator pressure (P a ). Because the scaling factor must be limited to a value between 0 and 1, a computed value of F s  that is less than 0 means that a scaling factor of 0 should be applied to the requested flow rate and a computed value of F s  that is greater than 1 means that a scaling factor of 1 should be applied to the requested flow rate. 
     The computation of F s  may include a feedback component that accounts for the response time of the electro-hydraulic valve arrangement. The following formula may be used to account for the responsiveness of the electro-hydraulic valve. 
     
       
           F   s   =K   p *( P   p   −P   a )+ K   d [( P   p   −P   a )−( P   p   −P   a ) (−1) ] 
       
     
     where, K d  is a constant that indicates the responsiveness of the particular electro-hydraulic valve arrangement being controlled and (P p −P a ) (−1)  is the previous sample of the pressure difference between the pump pressure and the actuator pressure. By including this component, the computation of F s  will take into account the rate of change of the pressure difference between the pump pressure (P p ) and the actuator pressure (P a ). 
     After the scaling factor has been computed, control device  88  applies the scaling factor to the requested flow rate to determine an actual flow rate that the system is capable of providing to the actuator (step  140 ). This is accomplished by multiplying the requested flow rate by the scaling factor. If the scaling is 0, the actual flow rate will be 0. If the scaling factor is 1, the actual flow rate will be equal to the requested flow rate. Control device  88  then adjusts the position of electro-hydraulic valve arrangement  16  to provide the actual flow rate to hydraulic actuator  12  (step  142 ). 
     Thus, the present invention has wide applications in a variety of machines incorporating hydraulic actuators. The present invention may provide advantages in that it provides a cost effective and highly efficient system and method for controlling an electro-hydraulic valve arrangement to perform the pump check function. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the method and system for controlling an electro-hydraulic valve arrangement without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and their equivalents.