Patent Publication Number: US-7210292-B2

Title: Hydraulic system having variable back pressure control

Description:
TECHNICAL FIELD 
   The present disclosure relates generally to a hydraulic system, and more particularly, to a hydraulic system having variable back pressure control. 
   BACKGROUND 
   Work machines such as, for example, excavators, loaders, dozers, and other types of heavy machinery use multiple hydraulic actuators in conjunction with a linkage system to accomplish a variety of tasks. The hydraulic actuators may include a tube having a head-end pressure chamber and a rod-end pressure chamber separated by a piston assembly. The tube may be connected to one portion of the linkage assembly, while the piston assembly may be connected to a different portion. The head and rod-end pressure chambers may be selectively filled with or drained of pressurized fluid to move the piston assembly relative to the tube, which affects movement of the linkage system. During movement of the linkage system, it is possible for gravity acting on the linkage system to cause the piston assembly to force draining of fluid from one of the rod or head-end chambers faster than fluid can fill the other of the rod or head-end chambers. In this situation, a void or vacuum may be created by the expansion of the filling chamber (voiding). Voiding can result in undesired and/or unpredictable movement of the work machine and could damage the hydraulic actuators. 
   One method of minimizing voiding within a hydraulic actuator is described in U.S. Pat. No. 5,868,059 (the &#39;059 patent) issued to Smith on Feb. 9, 1999. The &#39;059 patent describes an electrohydraulic valve arrangement in combination with an implement pump, a tank, and a hydraulic cylinder having a rod-end chamber and a head-end chamber. The valve arrangement includes a plurality of electrohydraulic displacement control independent metering valve modules and a return check valve disposed in an outlet between the valve arrangement and the tank to generate a back pressure for the valve arrangement. This generated back pressure may limit the rate that fluid drains from the head-end or rod-end chambers. If the drain rate is limited to the same as or less than the fill rate of the other of the head-end or rod-end chambers, voiding may be minimized. The level of the back pressure is established by a spring. 
   Although the electrohydraulic valve arrangement of the &#39;059 patent may minimize voiding, it may do so inefficiently. In particular, because the back pressure restriction is always active, regardless of the likelihood of voiding, the pump supplying pressurized fluid to the electrohydraulic valve arrangement must be continuously operated at a high power usage level to overcome the continuous back pressure restriction. In addition, because the back pressure restriction is constant, velocity control of the hydraulic actuators may be limited. There may be situations when it is desirable to reduce or increase the back pressure restriction to allow for increased or decreased velocity of the associated linkage system. 
   The disclosed hydraulic system is directed to overcoming one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present disclosure is directed to a hydraulic system for a work machine having a linkage system. The hydraulic system includes a tank configured to hold a supply of fluid and at least one hydraulic actuator associated with the linkage system to affect movement of the linkage system. The at least one hydraulic actuator has a first pressure chamber and a second pressure chamber. The hydraulic system also includes an independent metering valve associated with the first pressure chamber. The independent metering valve has a valve element movable between a first position at which fluid communication between the first pressure chamber and the tank is blocked, and a second position at which fluid is allowed to drain from the first pressure chamber to the tank. The hydraulic system further includes at least one sensor configured to sense a parameter indicative of a pressure in the second pressure chamber, and a controller in communication with the independent metering valve and the sensor. The controller is configured to move the valve element of the independent metering valve in response to the pressure. 
   In another aspect, the present disclosure is directed to a method of operating a hydraulic system associated with a linkage system. The method includes moving an independent metering valve element between a first position and a second position to selectively block fluid from or drain fluid from a first chamber of a hydraulic actuator to a tank. The method also includes sensing a parameter indicative of a pressure within a second pressure chamber of the hydraulic actuator. The method further includes moving the independent metering valve element between the first and second positions in response to the pressure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side-view diagrammatic illustration of an exemplary disclosed work machine; and 
       FIG. 2  is a schematic illustration of an exemplary disclosed hydraulic system for the work machine of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates an exemplary work machine  10 . Work machine  10  may be a fixed or mobile machine that performs some type of operation associated with an industry such as mining, construction, farming, transportation, or any other industry known in the art. For example, work machine  10  may be an earth moving machine such as an excavator, a dozer, a loader, a backhoe, a motor grader, or any other earth moving machine. Work machine  10  may include a linkage system  12 , a work tool  14  attachable to linkage system  12 , one or more hydraulic actuators  30   a–c  interconnecting linkage system  12 , an operator interface  16 , a power source  18 , and at least one traction device  20 . 
   Linkage system  12  may include any structural unit that supports movement of work machine  10  and/or work tool  14 . Linkage system  12  may include, for example, a stationary base frame (not shown), a boom  13 , and a stick  15 . Boom  13  may be pivotally connected to the frame, while stick  15  may be pivotally connected to boom  13  at a join  17 . Work tool  14  may pivotally connect to stick  15  at a joint  19 . It is contemplated that linkage system  12  may alternatively include a different configuration and/or number of linkage members than what is depicted in  FIG. 1 . 
   Numerous different work tools  14  may be attachable to stick  15  and controllable via operator interface  16 . Work tool  14  may include any device used to perform a particular task such as, for example, a bucket, a fork arrangement, a blade, a shovel, a ripper, a dump bed, a broom, a snow blower, a propelling device, a cutting device, a grasping device, or any other task-performing device known in the art. Work tool  14  may be configured to pivot, rotate, slide, swing, lift, or move relative to work machine  10  in any manner known in the art. 
   Operator interface  16  may be configured to receive input from a work machine operator indicative of a desired work tool movement. Specifically, operator interface  16  may include an operator interface device  22  such as, for example, a multi-axis joystick located to one side of an operator station. Operator interface device  22  may be a proportional-type controller configured to position and/or orient work tool  14  and to produce an interface device position signal indicative of a desired movement of work tool  14 . It is contemplated that additional and/or different operator interface devices may be included within operator interface  16  such as, for example, wheels, knobs, push-pull devices, switches, pedals, and other operator interface devices known in the art. 
   Power source  18  may be an engine such as, for example, a diesel engine, a gasoline engine, a gaseous fuel-power engine such as a natural gas engine, or any other engine known in the art. It is contemplated that power source  18  may alternatively embody another source of power such as a fuel cell, a power storage device, an electric or hydraulic motor, or another source of power known in the art. 
   Traction device  20  may include tracks located on each side of work machine  10  (only one side shown). Alternatively, traction device  20  may include wheels, belts, or other traction devices. Traction device  20  may or may not be steerable. It is contemplated that if work machine  10  embodies a stationary machine, traction device  20  may be omitted. 
   As illustrated in  FIG. 2 , work machine  10  may include a hydraulic system  24  having a plurality of fluid components that cooperate together to move work tool  14 . Specifically, hydraulic system  24  may include a tank  26  holding a supply of fluid, and a source  28  configured to pressurize the fluid and to direct the pressurized fluid to hydraulic actuators  30   a–c . While  FIG. 1  depicts three actuators, identified as  30   a ,  30   b , and  30   c , for the purposes of simplicity, the hydraulic schematic of  FIG. 2  depicts only one hydraulic actuator. Hydraulic system  24  may include four independent metering valves, including a head-end supply valve  32 , a head-end drain valve  34 , a rod-end supply valve  36 , and a rod-end drain valve  38 . Thus, two independent metering valves may be associated with each end of a hydraulic actuator  30   a–c . The hydraulic system  24  also may include a head-end pressure sensor  40  and a rod-end pressure sensor  42  associated with each hydraulic actuator  30   a–c . Hydraulic system  24  may further include a linkage sensor  46  and a controller  48  in communication with the fluid components of hydraulic system  24  and operator interface device  22 . It is contemplated that hydraulic system  24  may include additional and/or different components such as, for example, accumulators, restrictive orifices, check valves, pressure relief valves, makeup valves, pressure-balancing passageways, temperature sensors, tool recognition devices, and other components known in the art. 
   Tank  26  may constitute a reservoir configured to hold a supply of fluid. The fluid may include, for example, a dedicated hydraulic oil, an engine lubrication oil, a transmission lubrication oil, or any other fluid known in the art. One or more hydraulic systems within work machine  10  may draw fluid from and return fluid to tank  26 . It is also contemplated that hydraulic system  24  may be connected to multiple separate fluid tanks. 
   Source  28  may be configured to produce a flow of pressurized fluid and may include a pump such as, for example, a variable displacement pump, a fixed displacement pump, or any other source of pressurized fluid known in the art. Source  28  may be drivably connected to power source  18  of work machine  10  by, for example, a countershaft  50 , a belt (not shown), an electrical circuit (not shown), or in any other suitable manner. Alternatively, source  28  may be indirectly connected to power source  18  via a torque converter (not shown), a gear box (not shown), or in any other manner known in the art. It is contemplated that multiple sources of pressurized fluid may be interconnected to supply pressurized fluid to hydraulic system  24 . 
   Hydraulic actuators  30   a–c  may include fluid cylinders that interconnect work tool  14  and linkage system  12 . It is contemplated that hydraulic actuators other than fluid cylinders may alternatively be implemented within hydraulic system  24  such as, for example, hydraulic motors or any other type of hydraulic actuator known in the art. As illustrated in  FIG. 2 , each of hydraulic actuators  30   a–c  may include a tube  52  and a piston assembly  54  disposed within tube  52 . One of tube  52  and piston assembly  54  may be pivotally connected between members of linkage system  12  and/or work tool  14 . Each of hydraulic actuators  30   a–c  may include a first chamber  56  and a second chamber  58  separated by a piston  60 . First and second chambers  56 ,  58  may be selectively supplied with pressurized fluid from source  28  and selectively drained of the fluid to cause piston assembly  54  to displace within tube  52 , thereby changing the effective length of hydraulic actuators  30   a–c . The expansion and retraction of hydraulic actuators  30   a–c  may function to assist in moving work tool  14  and linkage system  12 . 
   Piston assembly  54  may include piston  60  axially aligned with and disposed within tube  52 , and a piston rod  62  connectable to the frame of work machine  10 , boom  13 , stick  15 , or work tool  14  (referring to  FIG. 1 ). Piston  60  may include a first hydraulic surface  64  and a second hydraulic surface  66  opposite first hydraulic surface  64 . An imbalance of force caused by fluid pressure on first and second hydraulic surfaces  64 ,  66  may result in movement of piston assembly  54  within tube  52 . For example, a force on first hydraulic surface  64  being greater than a force on second hydraulic surface  66  may cause piston assembly  54  to displace to increase the effective length of hydraulic actuators  30   a–c . Similarly, when a force on second hydraulic surface  66  is greater than a force on first hydraulic surface  64 , piston assembly  54  will retract within tube  52  to decrease the effective length of hydraulic actuators  30   a–c . A flow rate of fluid into and out of first and second chambers  56  and  58  may determine a velocity of hydraulic actuators  30   a–c , while a pressure of the fluid in contact with first and second hydraulic surfaces  64  and  66  may determine an actuation force of hydraulic actuators  30   a–c . A sealing member (not shown), such as an o-ring, may be connected to piston  60  to restrict a flow of fluid between an internal wall of tube  52  and an outer cylindrical surface of piston  60 . 
   Head-end supply valve  32  may be disposed between source  28  and first chamber  56  and configured to regulate a flow of pressurized fluid to first chamber  56  in response to a command velocity from controller  48 . Specifically, head-end supply valve  32  may include a proportional spring biased valve mechanism that is solenoid actuated and configured to move between a first position at which fluid is allowed to flow into first chamber  56 , and a second position at which fluid flow is blocked from first chamber  56 . Head-end supply valve  32  may be movable to any position between the first and second positions to vary the rate of flow into first chamber  56 , thereby affecting the velocity of hydraulic actuators  30   a–c . It is contemplated that head-end supply valve  32  may alternatively be hydraulically actuated, mechanically actuated, pneumatically actuated, or actuated in any other suitable manner. It is further contemplated that head-end supply valve  32  may be configured to allow fluid from first chamber  56  to flow through head-end supply valve  32  during a regeneration event when a pressure within first chamber  56  exceeds a pressure directed to head-end supply valve  32  from source  28 . 
   Head-end drain valve  34  may be disposed between first chamber  56  and tank  26 , and configured to regulate a flow of fluid from first chamber  56  to tank  26  in response to the command velocity from controller  48 . Specifically, head-end drain valve  34  may include a proportional spring biased valve mechanism that is solenoid actuated and configured to move between a first position at which fluid is allowed to flow from first chamber  56  and a second position at which fluid is blocked from flowing from first chamber  56 . Head-end drain valve  34  may be movable to any position between the first and second positions to vary the rate of flow from first chamber  56 , thereby affecting the velocity of hydraulic actuators  30   a–c . It is contemplated that head-end drain valve  34  may alternatively be hydraulically actuated, mechanically actuated, pneumatically actuated, or actuated in any other suitable manner. 
   Rod-end supply valve  36  may be disposed between source  28  and second chamber  58  and configured to regulate a flow of pressurized fluid to second chamber  58  in response to the command velocity from controller  48 . Specifically, rod-end supply valve  36  may include a proportional spring biased valve mechanism that is solenoid actuated and configured to move between a first position at which fluid is allowed to flow into second chamber  58  and a second position at which fluid is blocked from second chamber  58 . Rod-end supply valve  36  may be movable to any position between the first and second positions to vary the rate of flow into second chamber  58 , thereby affecting the velocity of hydraulic actuators  30   a–c . It is contemplated that rod-end supply valve  36  may alternatively be hydraulically actuated, mechanically actuated, pneumatically actuated, or actuated in any other suitable manner. It is further contemplated that rod-end supply valve  36  may be configured to allow fluid from second chamber  58  to flow through rod-end supply valve  36  during a regeneration event when a pressure within second chamber  58  exceeds a pressure directed to rod-end supply valve  36  from source  28 . 
   Rod-end drain valve  38  may be disposed between second chamber  58  and tank  26  and configured to regulate a flow of fluid from second chamber  58  to tank  26  in response to a command velocity from controller  48 . Specifically, rod-end drain valve  38  may include a proportional spring biased valve mechanism that is solenoid actuated and configured to move between a first position at which fluid is allowed to flow from second chamber  58  and a second position at which fluid is blocked from flowing from second chamber  58 . Rod-end drain valve  38  may be movable to any position between the first and second positions to vary the rate of flow from second chamber  58 , thereby affecting the velocity of hydraulic actuators  30   a–c . It is contemplated that rod-end drain valve  38  may alternatively be hydraulically actuated, mechanically actuated, pneumatically actuated, or actuated in any other suitable manner. 
   Head and rod-end supply and drain valves  32 – 38  may be fluidly interconnected. In particular, head and rod-end supply valves  32 ,  36  may be connected in parallel to a common supply passageway  68  extending from source  28 . Head and rod-end drain valves  34 ,  38  may be connected in parallel to a common drain passageway  70  leading to tank  26 . Head-end supply and drain valves  32 ,  34  may be connected in parallel to a first chamber passageway  72  for selectively supplying and draining first chamber  56  in response to the command velocity from controller  48 . Rod-end supply and drain valves  36 ,  38  may be connected in parallel to a common second chamber passageway  74  for selectively supplying and draining second chamber  58  in response to the command velocity from controller  48 . For the purposes of this disclosure, the pressure of the fluid within first and second chamber passageways  72  and  74  during draining of the associated first or second chamber is defined as back pressure that results from piston  60  pushing fluid through an orifice (not shown) within the associated drain valve. This back pressure may oppose the motion of piston  60 . 
   Head and rod-end pressure sensors  40 ,  42  may be in fluid communication with first and second chambers  56 ,  58 , respectively and configured to sense the pressure of the fluid within first and second chambers  56 ,  58 . Head and rod-end pressure sensors  40 ,  42  may be further configured to generate a hydraulic actuator load signal indicative of the pressures within first and second chambers  56 ,  58 . 
   Linkage sensor  46  may be operably connected to linkage system  12  and configured to monitor an operating parameter of linkage system  12 . In one example, linkage sensor  46  may include a gravitational position sensor attached to a side of boom  13  or stick  15 . In this example, linkage sensor  46  may be configured to determine a position and/or an orientation of the linkage member to which it is attached. It is also contemplated that linkage sensor  46  may alternatively embody an angle sensor attached to a pivot joint of work machine  10  to determine an orientation of a linkage member of linkage system  12 . It is further contemplated that linkage sensor  46  may embody an internal or external position sensor associated with one or more of hydraulic actuators  30   a–c  to determine an extension/retraction position of the respective cylinder. This extension/retraction information may be utilized to calculate the position and/or orientation of the associated linkage members. The position and orientation information monitored and/or determined by linkage sensor  46  may be used to derive additional operating parameters for linkage system  12  such as, for example, velocity, acceleration, jerk, and other parameters known in the art. It is still further contemplated that linkage sensor  46  may embody additional or different types of sensors as are known in the art that can be used to monitor or determine the position, orientation, velocity, and other similar operating parameters of linkage system  12 . 
   Controller  48  may embody a single microprocessor or multiple microprocessors that include a means for controlling an operation of hydraulic system  24 . Numerous commercially available microprocessors can be configured to perform the functions of controller  48 . It should be appreciated that controller  48  could readily be embodied in a general work machine microprocessor capable of controlling numerous work machine functions. Controller  48  may include a memory, a secondary storage device, a processor, and any other components for running an application. Various other circuits may be associated with controller  48  such as power supply circuitry, signal conditioning circuitry, solenoid driver circuitry, and other types of circuitry. 
   One or more maps relating operational parameters of linkage system  12  to pressure information for hydraulic actuators  30   a–c  may be stored in the memory of controller  48 . Each of these maps may be in the form of a 2-D or 3-D table. Controller  48  may be configured to reference these tables during actuation of head and rod-end supply and drain valves to determine appropriate minimum and/or desired pressure values for the one of the first and second chambers currently being filled with pressurized fluid. It is also contemplated that instead of relating operational parameters of linkage system  12  directly to pressure information for head and rod-end drain valves  34  and  38 , the maps may alternatively relate the operational parameters to valve element positions that result in the minimum or desired pressure values. The relationship between valve element position and minimum or desired pressure values may be determined during lab and/or field testing of work machine  10 , and may be periodically recalibrated and updated. 
   Controller  48  may be configured to receive input from operator interface device  22 , head and rod-end pressure sensors  40 ,  42 , and linkage sensor  46 , and to actuate hydraulic actuators  30   a–c  in response to the input and the relationship map. Specifically, controller  48  may be in communication with head and rod-end supply and drain valves  32 – 38  of hydraulic actuators  30   a–c  via communication lines  80 – 86  respectively, with operator interface device  22  via a communication line  88 , with head and rod-end pressure sensors  40 ,  42  via communication lines  90  and  92 , and with linkage sensor  46  via a communication line  93 , respectively. Controller  48  may receive the interface device position signal from operator interface device  22 , the linkage parameter signal from linkage sensor  46 , the pressure signals from head and rod-end pressure sensors  40 ,  42 , and reference the relationship map stored in the memory of controller  48  to determine appropriate pressure values or valve element settings for the one of the first and second chambers that controller  48  is currently filling. Controller  48  may then command movement of the valve elements that result in the minimum or desired pressure values. 
   INDUSTRIAL APPLICABILITY 
   The disclosed hydraulic system may be applicable to any work machine that includes a hydraulic actuator where it is desirable to minimize voiding within the hydraulic actuator while improving efficiency of the work machine. The disclosed hydraulic system may minimize voiding by providing back pressure within the hydraulic actuator at a level and at times appropriate for the current operating conditions of the work machine. The operation of hydraulic system  24  will now be explained. 
   As illustrated in  FIG. 2 , hydraulic cylinders  30   a–c  may be movable by fluid pressure in response to an operator input. Fluid may be pressurized by source  28  and selectively directed to head-end and rod-end supply valves  32  and  36 . In response to an operator input to either extend or retract piston assembly  54  relative to tube  52 , controller  48  may direct the pressurized fluid to the appropriate one of first and second chambers  56 ,  58  by causing one of head-end and rod-end supply valves  32  and  36  to move to the flow-passing position. Substantially simultaneously, controller  48  may actuate the appropriate one of head-end and rod-end drain valves  34 ,  38  to drain fluid from the appropriate one of the first and second chambers  56 ,  58  to tank  26 , thereby creating a force imbalance on piston  60  that causes piston assembly  54  to move. For example, if an extension of hydraulic cylinders  30   a–c  is requested, head-end supply valve  32  may be moved to the open position to direct pressurized fluid from source  28  to first chamber  56 . Substantially simultaneous to the directing of pressurized fluid to first chamber  56 , rod-end drain valve  38  may be moved to the open position to allow fluid from second chamber  58  to drain to tank  26 . If a retraction of hydraulic cylinders  30   a–c  is requested, rod-end supply valve  36  may be moved to the open position to direct pressurized fluid from source  28  to second chamber  58 . Substantially simultaneous to the directing of pressurized fluid to second chamber  58 , head-end drain valve  34  may be moved to the open position to allow fluid from first chamber  56  to drain to tank  26 . 
   During movement of linkage system  12 , it is possible for gravity acting on one or more members of linkage system  12  to move piston  60  in a direction causing expansion in one of first and second chambers  56 ,  58  faster than pressurized fluid can be introduced into the chamber. For example, during downward and/or inward movement of stick  15 , a heavy load within work tool  14  may drive stick  15  in such a way that fluid is forced from second chamber  58  of hydraulic actuator  30   b  faster than fluid can fill first chamber  56 . Without intervention, the pressure within first chamber  56  may drop to a point where movement of the linkage system may be unpredictable or undesirable (voiding). In order to prevent this voiding situation, it may be necessary to increase the back pressure in second chamber passageway  74  to opposes motion of piston assembly  54 . 
   Back pressure may be increased by moving the valve element of the draining valve toward the closed direction. In the example described above, back pressure within second chamber passageway  74  may be increased by moving the valve element of rod-end drain valve  38  to increase flow restriction from second chamber  58 . The increasing restriction results in increased back pressure. 
   Controller  48  may be configured to increase the back pressure of the draining valve in response to various inputs. In the example above, controller  48  may receive a signal from head-end pressure sensor  40  indicating a low pressure level within first chamber  56  that is filling, signifying that voiding is already occurring or may be about to occur. Controller  48  may then determine an operating condition (position, orientation, velocity, load, etc.) of linkage system  12  via linkage sensor  46  and determine either a desired pressure value or a minimum pressure value from the relationship map stored in the controller&#39;s memory that corresponds to that operating condition. Controller  48  may then compare the pressure signal from head-end pressure sensor  40  with the desired or minimum pressure value and move the valve element of rod-end drain valve  38  to either increase the flow restriction through that valve. Alternatively, controller  48  may reference only the operating condition of linkage system  12  with the map stored in the memory of controller  48  to determine an appropriate position of the valve element of the draining valve that results in the desired or minimum back pressure value. 
   Although the example described above references a low pressure situation within first chamber  56 , controller  48  would respond similarly to a low pressure situation within second chamber  58 . Likewise, controller  48  may react to a high pressure situation in either of first or second chambers  56  and  58  by moving the appropriate valve elements to decrease flow restriction, thereby lowering pressure within the associated chamber. 
   Because controller  48  selectively increases back pressure to oppose piston movement, hydraulic system  24  is efficient. Specifically, because controller  48  only increases flow restriction when a potential for voiding exists, the output of source  28  is only increased during those situations rather than constantly operating at a higher energy consumption rate. Further, because the amount of restriction is proportional to the potential for voiding, source  28  may be operated at a lower average energy consumption rate, as compared to constantly operating at the maximum restriction. 
   In addition, because controller  48  can control back pressure in response to an operating condition of linkage system  12 , velocity control of linkage system  12  may be improved. Specifically, if the potential for voiding is minimal, flow restriction from either first or second chambers  56 ,  58  may be reduced to increase velocity of the associated linkage members. In contrast, if more precise control over positioning of the linkage member of linkage system  12  is desired, controller  48  may increase the flow restrictions. These increase or decreased flow restrictions may be related to angular orientations and/or positions of the linkage members of linkage system  12 . For example, when work tool  14  is extended to an upper angle or position where sufficient ground clearance is available, increased velocity may be desired to improve cycle time. When work tool  14  is at a lower angle or position for loading or unloading, slower velocities may be desired for improved accuracy in the placement of work tool  14 . 
   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.