Abstract:
In order to avoid cavitation in a boom cylinder head end at the beginning of a dig cycle, fluid from an alternate source is supplied to the head end before or in addition to fluid supplied by the main boom-up hydraulic circuit. In one embodiment, an electronic hydraulic valve, related sensors, and control system determines the beginning of a dig operation and uses fluid at an intermediate pressure to rapidly provide fluid to a boom head end cylinder to prevent voiding or cavitation before fluid under high pressure from the main pump can be brought to the cylinder. An on/off fluid switch is activated early in a dig operation to address low pressure at the boom cylinder head end and provide an alternate path for fluid into the cylinder in reaction to the boom being lifted by a motion of the stick and bucket in contact with the work surface.

Description:
RELATED APPLICATION 
       [0001]    The present application is a continuation-in-part of and claims priority to U.S. patent application Ser. No. 13/721,719 entitled, “Hydraulic System For Controlling a Work Implement,” which is hereby incorporated by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to hydraulic implements and more particularly to improving performance and fuel economy in machines with boom, stick and bucket linkages which include excavators and backhoe loaders. 
       BACKGROUND 
       [0003]    When operating hydraulic equipment conditions may arise when a sudden change in configuration causes voiding in hydraulic boom cylinders. For example, when an excavating bucket contacts the ground at the beginning of a dig, a reaction force against the bucket including support for the weight of the implement, may be transmitted through the stick and cause the boom to be pushed up faster than the boom cylinder can respond. This upward force can draw the rod and piston from the boom cylinder and cause a low pressure situation at the head end of the boom cylinders. 
         [0004]    EP1416096A1 discloses a system that monitors for a number of conditions including low boom cylinder head end pressure to draw oil from the return line to the boom cylinder head end. The &#39;096 reference fails to disclose a hydraulic circuit, components, and control system that meters fluid to a boom cylinder head end based on a defined point in the dig operation to reduce or eliminate voiding in the boom cylinder. 
       SUMMARY 
       [0005]    According to one aspect of the disclosure, a method of providing fluid to a cylinder in an implement when the cylinder experiences low pressure includes delivering fluid to a head end of the cylinder from both a first fluid source and a second fluid source, the first fluid source providing fluid at a first pressure higher than a second pressure from the second fluid source. The method may also include identifying a condition that occurs while delivering fluid to the head end of the cylinder from both the first and second fluid sources and responsive to identifying the condition, sending a signal to a valve causing the second fluid source to be disconnected from the head end of the cylinder. 
         [0006]    According to another aspect of the disclosure, a method of reducing voiding in a head end of a cylinder of a boom of an excavator may include connecting the head end of the cylinder to a first fluid source at a first pressure to initiate a transfer of fluid from the first fluid source to the head end of the cylinder, determining that a dig operation is underway, and responsive to determining that the dig operation is underway, connecting the head end of the cylinder to a second fluid source at a second pressure to initiate a transfer of fluid from the second fluid source to the head end of the cylinder. The second pressure is lower than the first pressure. After connecting the head end of the cylinder to the second fluid source, identifying a condition and disconnecting the second fluid source from the head end of the cylinder responsive to identifying the condition. 
         [0007]    In yet another aspect of the disclosure, an apparatus for providing fluid to a cylinder in an implement may include a first fluid source that provides fluid at a high pressure, the cylinder having a head end, the head end controllably coupled to the first fluid source via a spool valve, a head end pressure sensor and a control stick position sensor. The a second fluid source has a lower pressure than the first fluid source. The apparatus may also include a control valve that operates responsive to an electrical signal to selectively connect the second fluid source to the head end, and a controller coupled to the head end pressure sensor, the control stick position sensor, and the control valve, wherein the controller generates the electrical signal to close the control valve to disconnect the second fluid source responsive to identification of a condition. 
         [0008]    These and other benefits will become apparent from the specification, the drawings and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a view of an implement at a work site; 
           [0010]      FIG. 2  is a block diagram of an electrohydraulic circuit for use in the excavator of  FIG. 1 ; 
           [0011]      FIG. 3  is a block diagram of another electrohydraulic circuit for use in the implement of  FIG. 1 ; 
           [0012]      FIG. 4  is a block diagram of a hydraulic circuit for use in the implement of  FIG. 1 ; 
           [0013]      FIG. 5  is a block diagram of a controller suitable for use with the electrohydraulic circuits of  FIG. 2  and  FIG. 3 ; 
           [0014]      FIG. 6  is a flowchart of a method of reducing dig force in a hydraulic implement; 
           [0015]      FIG. 7  is a flowchart amplifying the method illustrated in  FIG. 6 ; and 
           [0016]      FIG. 8  is a graph of spool valve displacement vs. spool valve opening for nominal and modified valves. 
       
    
    
     DESCRIPTION 
       [0017]      FIG. 1  illustrates an exemplary excavator  102  at a work site  100 . While an excavator is discussed and described, the techniques and apparatus disclosed below are applicable to and can be implemented with any application or configuration which utilizes a boom, stick and work implement and/or any number of other boom/stick/bucket machines, including, but not limited to, shovels and backhoes, and may include machines that may have a single or multiple cylinders operating the boom. The excavator  102  is shown with its bucket in contact with a work surface  104 . The excavator  102  is shown in this simplified drawing with an implement  120  having a boom  106  and a boom cylinder  108  that raises and lowers the boom  106 . The implement  120  also has a stick  110  and its corresponding stick cylinder  112  as well as work implement, shown and hereinafter referred to as bucket  114  for the purposes of illustration, and a bucket cylinder  116 . 
         [0018]    The various arrows illustrate gravity, cylinder forces, and reaction forces which may be present during a dig operation of the implement  120 . The weight of implement  120 , including, but not limited to, the boom  106 , the stick  110 , and the bucket  114  (and their associated cylinders, hydraulic lines, pivots, etc.) can be supported at a boom pivot  118 , by the boom cylinders  108 , and by the work surface  104  at the contact point with the bucket. Ideally, at least at the beginning of the dig operation most of the weight of the implement  120  can be borne by the boom cylinders  108  so that the ground engaging elements (not depicted) of the bucket  114  can enter the work surface  104  cleanly with minimal fiction force. 
         [0019]    However, as the dig operation progresses and the bucket  114  is inserted into and drawn through the work surface  104 , by curling the bucket  114 , by drawing the stick  110  inwardly towards the boom  106  and boom pivot  118 , or both, there can be an upward reaction force that lifts the bucket  114  and stick  110  up, causing, in the view shown in  FIG. 1 , the boom  106  to rotate counterclockwise about the boom pivot  118 . 
         [0020]    This rotation or lifting can cause the boom cylinder rods (e.g.,  160  of  FIG. 2 ) to be forcibly drawn out of the boom cylinder  108 . As will be discussed more below, this action of the boom cylinder rods  160  can cause a temporary void  166  of the fluid in the head end of the boom cylinders  108 . While this condition exists, the temporary void  166  or disparity can result in an insufficient amount of pressurized fluid within the head end  152  of the boom cylinders  108  available such that the boom cylinders are temporarily no longer able to provide lift and/or support the weight of the implement  120 . As a result, at least a portion of the unsupported implement weight can be transferred to the bucket/work-surface interface, and can substantially increase the frictional or drag force opposing the movement of the bucket  114  into and through the work surface  104 . An operator generally issues a boom up command while digging but the response of the system may not be fast enough to power the boom cylinders in this short-lived initial state, generally no longer than 2-3 seconds, which may at least be partially due to the lack of on-demand pressurized fluid in order to make up for the temporary void  166 . Studies have shown that this additional frictional force during that 2-3 second interval can cause a significant increase in fuel consumption in the overall operation of the excavator  102 . 
         [0021]    Existing boom cylinder head-end check valves, e.g., check valve  168  of  FIG. 2 , may be installed to provide supplemental fluid to the boom cylinders, but these are generally too small to provide a meaningful response in a timely manner. Further, because these check valve  168  is connected to the rod end cylinder-to-tank line  162 , the pressure supplying the fluid may be inconsistent or too low to overcome the small size of the check valve  168  with a sufficient volume of fluid 
         [0022]    To address this situation, a controller and/or specialized hydraulic circuit (not depicted in  FIG. 1 ) may be used in the excavator  102  to rapidly respond to the conditions associated with cavitation in the head end of the boom cylinder  108  and prevent the undue frictional forces at the beginning of a dig, resulting in an overall fuel savings of 5% or more in some machines. 
         [0023]      FIG. 2  is a block diagram of an electrohydraulic circuit  130  for use in the excavator of  FIG. 1 . The circuit  130  includes one or more main hydraulic pumps  132 . 
         [0024]    In a conventional manner, the pump  132  may supply high pressure fluid via a fluid line  134  to a stick spool valve  136  with individual valves  138  and  140  that connect, respectively, the pump  132  to the head end  144  of the stick cylinder  112  and the rod end  146  to the tank line  148 . 
         [0025]    The pump  132  may also be connected to a head end  152  of the boom cylinder  108  via a first boom cylinder spool  150  using valve  154  and line  156 . The rod end  158  of the boom cylinder  108  may be connected to the tank line  148  via line  162  and valve  164 . A check valve  168  may operate in a conventional manner to allow fluid flow between the tank line  148  and the boom cylinder line  156 . As discussed above, these check valves are generally either too small to be effective during the transient of the initial dig operation or cause feel and handling problems if increased in size. 
         [0026]    As illustrated, when the rod  160  is drawn out of the boom cylinder  108  during the beginning of a dig operation, the supply of fluid in the head end  152  of the boom cylinder  108  cannot be replenished quickly enough via valve  154  and a void area  166  may be created. As discussed above, this void  166  may exist for several seconds, during which time the boom cylinder  108  provides virtually no lift to support the implement  120 . 
         [0027]    In the embodiment of  FIG. 2 , the void  166  may be eliminated using a secondary boom cylinder spool  170  to provide fluid to the head end  152  of the boom cylinder. As shown, valve  172  may connect the tank line  148  to the boom cylinder line  156  via line  174 . The valve  176  that would typically connect to line  178  and the rod end line  162  to the pump  132  is not connected. 
         [0028]    When a boom up pilot command is received via line  182 , that is, a control signal used to open the secondary boom cylinder spool  170  via line  180 , and a determination may be made that a dig operation is underway the controller  190  issues a command to electrohydraulic valve  184  via control  186  to connect pilot pressure source  188  to the valve control line  180  and override the boom up pilot command. During this override period, the valve  172  connects the tank line  148  to the head end  152  of the boom cylinder  158  as illustrated. This provides a temporary, high-volume flow path for fluid under pressure from the rod end  158  back into the head end  152 . While the pressure supplied from the tank line  148  may be insufficient to actually lift the implement  120 , enough pressure is provided to significantly reduce the implement weight causing frictional force at the bucket  114 . After certain conditions are reached the controller  190  may turn off the valve  184  and allow the normal pilot command signal via line  182  to again control the secondary boom cylinder spool  170 . 
         [0029]      FIG. 3  is a block diagram of another electrohydraulic circuit  200  for use in the excavator of  FIG. 1 .  FIG. 3  repeats a substantial portion of the elements of  FIG. 2  with respect to the stick cylinder  112 , stick spool valve  136 , pump  132 , boom cylinder  108 , and boom cylinder spool valve  150 . In this illustrated embodiment, the void  166  may be eliminated using a hydraulic circuit  202  with an electrohydraulic valve  204  under the control of the controller  190 . In this embodiment, the controller  190  may evaluate a number of conditions to conclude that a dig operation has begun and turn on the electrohydraulic valve  204  to couple a source of pilot pressure source  188  to the head end  152  of the boom cylinder  108 . These conditions are discussed in more detail below. 
         [0030]    The controller  190  or an engine control module (ECM) managing that function will signal the electrohydraulic valve  204  to close after certain other conditions have been identified, which are also discussed in more detail below. An orifice  206  restricts flow to help ensure that the pilot pressure source  188  is not reduced below a working level while the fluid is injected into the boom cylinder head in  152 . In this embodiment, using the pilot pressure source  188  as the source of pressurized fluid provides a more uniform pressure compared to the rod end cylinder to tank line  148 . Additionally, because the pilot pressure source is generally well below that of the main pump  132  and also well below that required to physically lift the boom  106 , the goal of reducing or preventing cavitation is met without introducing so much pressure that the boom  106  may be moved unintentionally. As long as the boom cylinder can support some portion of the implement weight, a significant reduction in friction force at the bucket may be realized. 
         [0031]      FIG. 4  is a block diagram  400  of a hydraulic circuit for use in the implement of  FIG. 1 . Unlike the electrohydraulic circuits of  FIGS. 2 and 3 , the hydraulic circuit of  FIG. 4  does not use an electrically-controlled valve to supply fluid to the cylinder head end during the initial dig operation to eliminate the void  166 . 
         [0032]    As discussed above, an operator, or an autonomous function, may desire to dig earth or other material at work site  100  with the depicted excavator  102 , and then dump the material into a haul truck (not shown) or other holding vehicle. As the work implement control system  108  responds to dig commands, for example, “stick in” and “bucket close,” the stick cylinder  112  may extend so that the stick  110  is urged in toward the cab, and the bucket cylinder  116  may extend so that the bucket  114  may begin to close, moving downwards and curling inward towards the stick  110  and cab, digging material and then holding it as is well known by ordinary persons skilled in the art. While the bucket  114  is digging, interaction between the bucket  114  and the material  104  the bucket  114  is digging may cause a resistive load to be applied to the bucket  114 . This resistive load may create a moment on the implement  120 , which may cause an extension of the boom cylinder  108  even though the operator is not inputting a “boom up” command. This unintended extension of the boom cylinder  108  may create a void  166  in the boom cylinder  108  as well as increase pressure at a rod-end  158  of the boom cylinder  108 . 
         [0033]    The combination line relief with check or a reconfigured makeup valve  169  and, in some embodiments, a second makeup valve  404 , may be configured to provide additional fluid flow to the head end  152  of the boom cylinder  108  to fill the void. Thus, the boom cylinder  108  is filled with fluid before a subsequent “boom up” command by the operator and the boom cylinder  108  can move in response to the “boom up” command without delay. Further, even though the fluid supplied via the makeup valve(s)  169  and  404  do not provide sufficient pressure to actually lift the implement  120 , the fluid does have sufficient pressure to help support the implement  120  thereby reducing the friction force caused at the bucket  114 -work surface  104  interface by reducing the normal force at the point of contact. 
         [0034]    Because a boom up command at the beginning of the dig cycle connects high pressure line  134  to the low, potentially zero, pressure of the boom cylinder via the control valve  402 , there is a potential to drop the pressure in the fluid line  134  enough to affect performance in other areas of the implement  120  or excavator  102  in general. To address this, the spool valve may be modified to limit the flow of fluid over an initial range of operation by the operator. 
         [0035]    Referring briefly to  FIG. 8 , a graph  420  illustrates an exemplary opening area versus spool displacement for the valve opening of the metering control valve  150  in a rod extension position. Although units are not illustrated in  FIG. 8 , the x-axis  424  of the graph  420  may represent spool displacement in mm, while the y-axis  422  of the graph  420  may represent the valve opening area in mm 2 . The graph  420  includes a first curve  426  showing a conventional opening versus displacement for a metering control valve and a second curve  428  showing an exemplary opening versus displacement for metering control valve  402  in accordance with the disclosure. 
         [0036]    The area of the valve opening varies as the spool valve  402  is displaced in the metering control valve  150 . In one embodiment of the illustrated exemplary graph  420 , the area of the valve opening may vary from 0 mm 2  at 0 mm spool displacement (i.e., closed) to a maximum valve opening area of about 185 mm 2  at 11 mm spool displacement (i.e., maximum spool displacement). One embodiment of the second curve  428  may represent a reduced initial opening area up to about 10 mm spool displacement. For example, over about the first 5.5 mm spool displacement (or about 50% of total spool displacement), the valve opening area may be less than 5 mm 2  or less than 3% of maximum valve opening area). Over about the first 6.5 mm of spool displacement, the valve opening area may be less than about 10 mm 2  (or less than 5.5% of maximum valve opening area), which is about one-half the area of the valve opening of the conventional valve at 6.5 mm displacement, as represented by curve  426 . 
         [0037]      FIG. 5  is a block diagram of a controller  190  suitable for use with the electrohydraulic circuits of  FIG. 2  and  FIG. 3 . The controller  190  may be a standalone unit or may be part of another electronic control module of the excavator  102 . The controller  190  may include a processor  262  that is coupled to a memory  264  by a data bus  266 . The data bus  266  may also provide connectivity to input controls  268 , a communication port  270  that supports communication with an external bus  272 , and sensor inputs  274 . The sensor inputs  274  may collect data from a variety of sensors such as pressure sensors at the pump  132 , head end  152  and rod end  158  of the boom cylinder  108 , the tank line  148 , and the pilot pressure source  188 . The input controls may also include control stick positions or control pressure values so that the controller  190  can determine operator actions with respect to the implement  120 . 
         [0038]    The memory  264  may include modules such as an operating system  276 , utilities  278  for performing various functions such as diagnostics and communication, strategy code  284  supporting execution of the disclosed system and method, and various modules  282 ,  284  that may provide, among other things, timers, comparison functions, lookup tables, etc. 
       INDUSTRIAL APPLICABILITY 
       [0039]      FIG. 6  is a flowchart of a method  300  of reducing dig force in a hydraulic implement  120 . At a block  302  a head end  152  of a boom cylinder  108  may be connected to a first fluid source, such as a pump  132 , via a valve  154 . At block  304 , a check may be made to determine if the hydraulic implement  120  is commencing a dig operation. More details about determining when a dig operation is beginning is discussed below with respect to  FIG. 7 . If a dig operation is beginning, the “yes” branch may be taken to block  306  where the head end  152  of the boom cylinder  108  may be connected to a second fluid source so that fluid is transferred from the second fluid source to the head end  152  of the boom cylinder  108 . In one embodiment, the second fluid source may be a tank line  148  pressurized by a rod end  158  of the boom cylinder  108 . In another embodiment, the second fluid source may be a pilot pressure source  188 . In either case, a pressure of the second fluid source will be less than the pressure at the main pump because the main pump is active by definition during a dig operation. 
         [0040]    After the second fluid source is connected to the head end  152  of the boom cylinder  108 , at block  308 , a controller  190  may monitor for one or more conditions. For example, a timer may be started after connecting the second fluid source that, in one embodiment, expires in a range of from 2 to 3 seconds. In another example, pressure at the head end  152  of the boom cylinder  108  may be monitored and the condition set when the head end pressure exceeds a threshold value, such as a pressure of the pilot pressure source  188 . In other embodiments, another selected pressure below that of the main pump  132  may be designated. When the condition at block  308  is met, the “yes” branch from block  308  may be taken to block  310  where the second fluid source is disconnected from the head end  152  of the boom cylinder  108 . 
         [0041]    Returning to block  304 , if no dig operation is detected execution may return to block  302  and the process repeated. In an embodiment, the loop repeats in a range of about every 8-12 ms. Other loop times may be supported based on a number of factors such as available processing capacity in the controller  190 . 
         [0042]    Returning to block  308 , if none of the conditions are identified, execution may loop at block  308  until at least the timer has expired. 
         [0043]    In the exemplary embodiments, the condition that ends the secondary fluid flow to the cylinder head end  158  may occur either at the expiration of a time period, such as two seconds, or when pressure at the head end  152  of the cylinder  108  reaches a level indicative of fluid from the main pump  132  arriving in sufficient volume to overcome any voiding. 
         [0044]      FIG. 7  is a flowchart amplifying the method  300  illustrated in  FIG. 6 . A method  320  may be used to determine when a dig operation is beginning. At block  322 , execution may begin from block  302  of  FIG. 6 . At block  324  and  326  an evaluation been may be made to determine if either the stick  110  or the bucket  114  is being drawn in, that is, toward the excavator  102 , indicative of a dig operation. 
         [0045]    If either or both of these conditions exists, execution may continue at block  328  and a determination may be made if the pressure at the main pump  132 , that is, a first fluid source, is above a first threshold pressure. This indicates that an operation is underway and the main pump  132  is active. In an embodiment the first threshold pressure may be a range of 8000-12,000 Kpa and typically may be in a range of 9000-11,000 Kpa. 
         [0046]    If so, execution may continue at block  330 , and a determination may be made if pressure at the head end  152  of the boom cylinder  108  is below a second threshold, indicating that the boom cylinder rod  160  is being drawn out, causing low pressure at the head end  152 . In an embodiment, the second threshold may be in a range of 800-1200 Kpa and any pressure less than the second threshold may meet the criteria. In an embodiment, the pressure may be zero. 
         [0047]    If the condition at block  330  is met the “yes” branch may be taken to block  332  where, for example, a flag may be set indicating a dig operation is commencing and execution returned to block  304  of  FIG. 6 . If at block  326 ,  328 , or  330  the tested-for condition does not exist, execution may immediately fall to block  334 , the flag indicating a dig operation may be cleared if needed and operation may be returned to block  304  of  FIG. 6 . The method  300  disclosed in  FIG. 6  and  FIG. 7  is but one example of how such a routine may be implemented but other embodiments are possible given this disclosure of what conditions are relevant to the operation. 
         [0048]    The system and method disclosed above, in its various embodiments, is particularly applicable to excavators, such as excavator  102 , but may also be used in other applications where hydraulic fluid voiding or cavitation occurs due to stresses on a hydraulic cylinder. The embodiments discussed above benefit operators of heavy hydraulic equipment, such as excavators, by offering a significant, measurable, fuel savings over prior art systems through the reduction of friction during the critical initial moments of a dig operation. Because no changes are required to the original boom cylinder spool valves  150  these savings can be realized in existing equipment with minimal new gear and/or modifications to hydraulic lines and existing controller strategies. 
         [0049]    In accordance with the provisions of the patent statutes and jurisprudence, exemplary configurations described above are considered to represent a preferred embodiment of the invention. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.