Abstract:
A pressure compensated load sense hydraulic system and method is disclosed where a first pressure compensated valve controls flow between a pump and a first function based on a highest function load; and a second pressure compensated valve controls flow between the pump and a second function based on the highest function load. First and second operator controls activate the first and second functions, respectively. When the first function is stalled and the second function is activated, the controller closes the first valve to remove the first function load from the load sense circuit and prevent flow to or from the first function. The controller can determine the first function is stalled when a timer exceeds an initialization period. When the controller closes the first valve, it can cycle the first valve between a shutoff period where the valve is closed, and a refresh period where the valve is opened.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    The present disclosure relates to hydraulic systems, and more particularly to a system and method that improves the efficiency of pressure compensated load sense hydraulic systems. 
       BACKGROUND 
       [0002]    Pressure compensated load sense hydraulic systems are used in many industries, including for example mobile construction and forestry equipment. Pressure compensated load sense systems allow for better multi-functioning performance due to improved flow sharing with pressure compensation. Without pressure compensation, the flow in a hydraulic system will go to the flow path of least resistance. For example, when commanding two functions (F1 and F2) in a hydraulic system with more flow than the flow source can provide, uneven flow sharing will occur when the two functions have differing loads. In a situation where the max pump flow is 10 GPM (gallons per minute) and a flow of 10 GPM is being commanded to each of the two functions F1 and F2, where F1&#39;s load requires 3000 psi (pounds per square inch) to move and F2&#39;s load only requires 1000 psi to move, then F2 will move first and F1 will not move until F2 stops being commanded or reaches a physical limit such as the end of stroke of a cylinder. Pressure compensators can be added to hydraulic systems to improve this situation. 
         [0003]    If pressure compensation is added to the hydraulic system in the same example as above, each function F1 and F2 will get a portion of the max flow based on each function&#39;s percentage of the sum of the requested flow. This is accomplished through a pressure balance using a pressure compensator for each of the functions F1 and F2. The highest function load (3000 psi) is applied on one side of each pressure compensator trying to force that compensator closed, and the individual function&#39;s load (3000 psi for F1, and 1000 psi for F2) on the opposite side of the compensator trying to force the compensator open. The compensator of the lower loaded function F2 will be forced closed since the highest function load will be higher than the lower loaded function&#39;s load. This added restriction to the lower loaded function will make both flow paths to F1 and F2 have equal resistance allowing flow sharing between two unequally loaded functions. In this situation, F1 flow request is 10 GPM and the F2 flow request is 10 GPM for a total requested flow of 20 GPM. Since the F1 flow request is 50% of the sum of the requested flow, F1 will get 50% of the available pump flow which is 5 GPM. And since the F2 flow request is also 50% of the sum of the requested flow, F2 will get 50% of the available pump flow, 5 GPM, as well. 
         [0004]    Although pressure compensation has the benefit of flow sharing, the pressure compensators introduce added restriction and loss in power that results in heat production. When multifunctioning, the larger the difference between the smallest and greatest load, the larger the amount of heat that is produced. The power lost to heat is represented by the following fluid power equation: 
         [0000]      Power= P*Q/ 1714 
         [0005]    where:
       P=the pressure drop across the compensator in PSI,   Q=the flow rate through the compensator in GPM, and   Power=the power loss in horsepower (HP).       
 
         [0009]    It would be desirable to reduce the pressure difference between functions requesting flow in a pressure compensated hydraulic circuit to reduce the power loss and heat production of the pressure compensation. 
       SUMMARY 
       [0010]    A pressure compensated load sense hydraulic system for a machine is disclosed, where the pressure compensated load sense hydraulic system includes first and second hydraulic functions, a hydraulic pump that provides flow to the first and second hydraulic functions, first and second pressure compensated hydraulic valves, first and second operator controls, and a controller. The first pressure compensated hydraulic valve controls flow between the hydraulic pump and the first hydraulic function and restricts flow based on a highest function load. The first hydraulic function has a first function load, the second hydraulic function has a second function load, and the highest function load is the greater of the first and second function loads. The second pressure compensated hydraulic valve controls flow between the hydraulic pump and the second hydraulic function and restricts flow based on the highest function load. The first operator control generates first control signals to activate the first hydraulic function, and the second operator control generates second control signals to activate the second hydraulic function. The controller receives the first and second control signals. While the first control signals indicate the first hydraulic function is stalled, the controller closes the first hydraulic valve to prevent flow to or from the first hydraulic function and block the first function load from the pump when the second hydraulic function is activated, and the controller reopens the first hydraulic valve to allow flow to or from the first hydraulic function and unblock the first function load from the pump when the second hydraulic function is deactivated. 
         [0011]    The pressure compensated load sense hydraulic system can also include an initialization timer that tracks time since the first hydraulic function was activated each time the first operator control begins to generate the first control signals to activate the first hydraulic function; and the controller can determine the first control signals indicate the first hydraulic function is stalled when the initialization timer exceeds an initialization period. The initialization period can be, for example, 10 seconds. 
         [0012]    When the controller closes the first hydraulic valve while the first control signals indicate the first hydraulic function is stalled and the second hydraulic function is activated, the controller can cycle the first hydraulic valve between a shutoff period during which the first hydraulic valve is closed to prevent flow to or from the first hydraulic function; and a refresh period during which the first hydraulic valve is opened to allow flow to or from the first hydraulic function. The shutoff period can be, for example, 5 seconds, and the refresh period can be, for example, 1 second. 
         [0013]    The machine can be, for example, a skidder where the first hydraulic function can be a hydraulic grapple, and the second hydraulic function can be a hydraulic boom or a hydraulic arch. 
         [0014]    The pressure compensated load sense hydraulic system can also include a load sense circuit that hydraulically couples a load sense relief valve, a hydraulic control of the hydraulic pump, a control port of the first pressure compensated hydraulic valve, and a control port of the second pressure compensated hydraulic valve. The load sense circuit can sense the highest function load of hydraulic functions receiving flow from the hydraulic pump. The first pressure compensated hydraulic valve can include a first spool valve that controls flow between the hydraulic pump and the first hydraulic function, and a first pressure compensator that restricts flow between the hydraulic pump and the first hydraulic function based on the highest function load. The second pressure compensated hydraulic valve can include a second spool valve that controls flow between the hydraulic pump and the second hydraulic function, and a second pressure compensator that restricts flow between the hydraulic pump and the second hydraulic function based on the highest function load. The first pressure compensator can restrict flow between the hydraulic pump and the first hydraulic function based on a pressure balance between the highest function load sensed by the load sense circuit and the first function load. The second pressure compensator can restrict flow between the hydraulic pump and the second hydraulic function based on a pressure balance between the highest function load sensed by the load sense circuit and the second function load. 
         [0015]    A pressure compensated load sense hydraulic method is disclosed for a machine that includes first and second hydraulic functions, and a hydraulic pump that provides flow to the first and second hydraulic functions. The method includes controlling flow between the hydraulic pump and the first hydraulic function using a first pressure compensated hydraulic valve based on a highest function load; controlling flow between the hydraulic pump and the second hydraulic function using a second pressure compensated hydraulic valve based on the highest function load; monitoring first control signals that activate the first hydraulic function; monitoring second control signals that activate the second hydraulic function; determining when the first hydraulic function is stalled; and while the first hydraulic function is stalled, closing the first hydraulic valve to prevent flow to or from the first hydraulic function and block the first function load from the pump when the second hydraulic function is activated, and reopening the first hydraulic valve to allow flow to or from the first hydraulic function and unblock the first function load from the pump when the second hydraulic function is deactivated. The first hydraulic function has a first function load, the second hydraulic function has a second function load, and the highest function load is the greater of the first and second function loads. 
         [0016]    Determining when the first hydraulic function is stalled can include resetting an initialization timer each time the first control signals start or stop activating the first hydraulic function; running the initialization timer while the first control signals continue activating the first hydraulic function; and determining the first hydraulic function is stalled when the initialization timer exceeds an initialization period. The initialization period can be, for example, 10 seconds. Closing the first hydraulic valve while the first hydraulic function is stalled and the second hydraulic function is activated can include closing the first hydraulic valve for a shutoff period to prevent flow to or from the first hydraulic function; opening the first hydraulic valve for a refresh period to allow flow to or from the first hydraulic function; and repeating the closing the first hydraulic valve for a shutoff period and opening the first hydraulic valve for a refresh period steps while the first hydraulic function is stalled and the second hydraulic function is activated. The shutoff period can be, for example, 5 seconds, and the refresh period can be, for example, 1 second. 
         [0017]    The machine can be, for example, a skidder where the first hydraulic function is a hydraulic grapple, and the second hydraulic function is a hydraulic boom or a hydraulic arch. 
         [0018]    The pressure compensated load sense hydraulic method can also include sensing the highest function load of hydraulic functions receiving flow from the hydraulic pump using a load sense circuit that hydraulically couples a load sense relief valve, a hydraulic control of the hydraulic pump, a control port of the first pressure compensated hydraulic valve, and a control port of the second pressure compensated hydraulic valve. The first pressure compensated hydraulic valve can include a first spool valve and a first pressure compensator, and the second pressure compensated hydraulic valve can include a second spool valve and a second pressure compensator. Controlling flow between the hydraulic pump and the first hydraulic function can include controlling flow between the hydraulic pump and the first hydraulic function using the first spool valve, and restricting flow between the hydraulic pump and the first hydraulic function based on the highest function load using the first pressure compensator. Controlling flow between the hydraulic pump and the second hydraulic function can include controlling flow between the hydraulic pump and the second hydraulic function using the second spool valve, and restricting flow between the hydraulic pump and the second hydraulic function based on the highest function load using the second pressure compensator. Restricting flow between the hydraulic pump and the first hydraulic function can include pressure balancing by the first pressure compensator between the highest function load sensed by the load sense circuit and the first function load. Restricting flow between the hydraulic pump and the second hydraulic function can include pressure balancing by the second pressure compensator between the highest function load sensed by the load sense circuit and the second function load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The above-mentioned aspects of the present disclosure and the manner of obtaining them will become more apparent and the disclosure itself will be better understood by reference to the following description of the embodiments of the disclosure, taken in conjunction with the accompanying drawings, wherein: 
           [0020]      FIG. 1  illustrates an exemplary machine having a pressure compensated load sense hydraulic system; 
           [0021]      FIG. 2  illustrates an exemplary embodiment of a grapple; 
           [0022]      FIG. 3  illustrates some exemplary operator controls for a machine having a pressure compensated load sense hydraulic system; 
           [0023]      FIG. 4  is an exemplary schematic for a pressure compensated load sense hydraulic system; and 
           [0024]      FIG. 5  is an exemplary control flow for an improved pressure compensated load sense hydraulic system. 
       
    
    
       [0025]    Corresponding reference numerals are used to indicate corresponding parts throughout the several views. 
       DETAILED DESCRIPTION 
       [0026]    The embodiments of the present disclosure described below are not intended to be exhaustive or to limit the disclosure to the precise forms in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of the present disclosure. 
         [0027]      FIG. 1  illustrates an example of a work machine, a skidder  100 , having a pressure compensated load sense hydraulic system. The present disclosure is not limited, however, to skidders but also is intended to include other work machines used in construction, forestry, agriculture and other industries having a pressure compensated load sense hydraulic system. As such, while the figures and forthcoming description may relate to a wheeled skidder, it is to be understood that the scope of the present disclosure extends beyond a wheeled skidder, and the term “machine” or “work machine” will also be used. The term “machine” or “work machine” is intended to be broader and encompass other vehicles besides a skidder. 
         [0028]    The machine  100  includes a front vehicle frame  110  coupled to a rear vehicle frame  120 . Front wheels  112  support the front vehicle frame  110 , and the front vehicle frame  110  supports an engine compartment  124  and an operator cab  126 . Rear wheels  122  support the rear vehicle frame  120 , and the rear vehicle frame  120  supports a boom assembly  114 . The engine compartment  124  houses a vehicle engine or motor, such as a diesel engine which provides the motive power for driving the front and rear wheels  112 ,  122 , and for operating the other components associated with the machine  100 . The operator cab  126 , where an operator sits when operating the machine  100 , includes a plurality of controls (e.g., joysticks, pedals, buttons, etc.) for controlling the machine  100  during operation thereof. 
         [0029]    As shown in  FIG. 1 , the boom assembly  114  includes an arch section  130 , a boom section  140  and a grapple  150 . A proximal end of the arch section  130  is pivotably coupled to the rear vehicle frame  120  and a distal end of the arch section  130  is pivotably coupled to the boom section  140 . One or more arch hydraulic cylinders  132  are controllable by the operator to move the arch  130 . A proximal end of the boom section  140  is pivotably coupled to the arch section  130  and a distal end of the boom section  140  is pivotably coupled to the grapple  150 . One or more boom hydraulic cylinders  142  are coupled to the proximal end of the boom section  140  and are controllable by the operator to move the boom  140 . A proximal end of the grapple  150  is coupled to the distal end of the boom section  140 . 
         [0030]    An exemplary embodiment of a grapple  150  is shown in  FIG. 2 . The grapple  150  includes a base  154 , left and right tongs  156 ,  158 , and left and right hydraulic cylinders  166 ,  168 . The base  154  is coupled to the distal end of the boom section  140 . The proximal ends of the left and right tongs  156 ,  158  are coupled to the base  154 , and the distal ends of the left and right tongs  156 ,  158  are controllable by the left and right hydraulic cylinders  166 ,  168  to open and close the grapple  150 . The left hydraulic cylinder  166  has a head end coupled to the base  154 , and a piston end coupled to the proximal end of the left tong  156 . The right hydraulic cylinder  168  has a head end coupled to the base  154 , and a piston end coupled to the proximal end of the right tong  158 . The operator can control extension and retraction of the left and right hydraulic cylinders  166 ,  168  to open and close the grapple  150 . When the left and right hydraulic cylinders  166 ,  168  are retracted, the proximal ends of the left and right tongs  156 ,  158  are brought closer together, which pulls apart the distal ends of the left and right tongs  156 ,  158 , which opens the grapple  150 . When the left and right hydraulic cylinders  166 ,  168  are extended, the proximal ends of the left and right tongs  156 ,  158  are pushed apart, which brings together the distal ends of the left and right tongs  156 ,  158 , which closes the grapple  150 . Thus in this embodiment, the operator can retract the left and right tong cylinders  166 ,  168  to open the grapple  150  to surround a payload (e.g. trees or other woody vegetation), and then extend the left and right tong cylinders  166 ,  168  to close the grapple  150  to grab, hold and lift the payload so the machine  100  can move it to another desired location. 
         [0031]      FIG. 3  illustrates an example of operator controls  300  for the arch, boom and tong hydraulic cylinders  132 ,  142 ,  166 ,  168 . The operator controls  300  include a boom control  302 , an arch control  304  and a grapple control  306 . The boom control  302  controls extension and retraction of the boom hydraulic cylinders  142  to move the boom  140 . The arch control  304  controls extension and retraction of the arch hydraulic cylinders  132  to lower and raise the arch  130 . The grapple control  306  controls extension and retraction of the tong hydraulic cylinders  166 ,  168  to open and close the grapple  150 . The boom, arch and grapple controls  302 ,  304 ,  306  send electrical signals to an electrical controller  310  and the controller  310  sends command signals to control the boom, arch and tong hydraulic cylinders  142 ,  132 ,  166 ,  168  over control lines  320 . 
         [0032]      FIG. 4  is a schematic of an exemplary pressure compensated load sense hydraulic system  400  for the arch, boom and tong hydraulic cylinders  132 ,  142 ,  166 ,  168 . The hydraulic system  400  includes a hydraulic pump  402 , a load sense relief valve  404 , a fluid reservoir  406 , a load sense drain orifice  408 , a boom spool valve  412 , an arch spool valve  414 , a grapple spool valve  416 , a boom pressure compensator  422 , an arch pressure compensator  424 , a grapple pressure compensator  426 , a boom load check valve  432 , an arch load check valve  434 , and a grapple load check valve  436 . The command signals sent by the controller  310  control the positions of the boom, arch and grapple spool valves  412 ,  414 ,  416 . The pump  402  hydraulically powers the arch  130 , boom  140  and grapple  150 . 
         [0033]    The boom, arch and grapple spool valves  412 ,  414 ,  416  are shown as electrically controlled, closed center, proportional control valves. Electrically controlled solenoids can be used to shift the spool valves  412 ,  414 ,  416 , where the solenoids are proportional and therefore can be varied to change the stroke of the spool valves  412 ,  414 ,  416 . The more electrical current commanded to one of the solenoids, the more each respective spool valve  412 ,  414 ,  416  will stroke. The boom, arch and grapple pressure compensators  422 ,  424 ,  426  are shown as proportional and therefore can vary the flow. The boom, arch and grapple pressure compensators  422 ,  424 ,  426  are shown downstream of the boom, arch and grapple spool valves  412 ,  414 ,  416  to represent a post-compensated hydraulic load sense system. The improvement to the pressure compensated load sense hydraulic system can be used in both post-compensated and pre-compensated load sense systems. When one of the solenoids is electrically commanded on, and the associated spool valve  412 ,  414 ,  416  shifts to allow flow from the pump  402  to its respective hydraulic function cylinder, the passage through the spool valve  412 ,  414 ,  416  from the pump inlet side to the associated pressure compensator  422 ,  424 ,  426  is effectively a variable metering orifice. The variable metering orifice is commonly referred to as the inlet metering orifice. As the spool valve  412 ,  414 ,  416  strokes more, the open area of this inlet metering orifice becomes larger. The larger the open area of the metering orifice, the larger the flow rate through the spool valve  412 ,  414 ,  416  at a given pressure drop. 
         [0034]    A pump outlet line  452  hydraulically couples the pump  402  to the boom, arch and grapple spool valves  412 ,  414 ,  416 . A reservoir line  456  hydraulically couples the boom, arch and grapple spool valves  412 ,  414 ,  416  to the reservoir  406 . The boom spool valve  412  controls flow between the pump  402  and the boom hydraulic cylinder  142 , and between the boom hydraulic cylinder  142  and the reservoir  406  to control raising and lowering of the boom  140 . The arch spool valve  414  controls flow between the pump  402  and the arch hydraulic cylinder  132 , and between the arch hydraulic cylinder  132  and the reservoir  406  to control raising and lowering of the arch  130 . The grapple spool valve  416  controls flow between the pump  402  and the grapple hydraulic cylinders  166 ,  168 , and between the grapple hydraulic cylinders  166 ,  168  and the reservoir  406  to control opening and closing of the grapple  150 . 
         [0035]    A load sense circuit  454  hydraulically couples a hydraulic control of the pump  402 , the load sense relief valve  404 , the load sense drain orifice  408 , the lower control ports of the boom, arch and grapple pressure compensators  422 ,  424 ,  426 , and the block flow sides of the boom, arch and grapple load check valves  432 ,  434 ,  436 . The load sense circuit  454  indicates the highest function load of the arch, boom and tong hydraulic cylinders  132 ,  142 ,  166 ,  168  that are currently receiving flow from the pump  402  through their associated spool valve  412 ,  414 ,  416 . 
         [0036]    In a scenario when the operator is sending commands using the boom, arch and grapple controls  302 ,  304 ,  306  to raise the arch and boom  130 ,  140 , and close the grapple  150 , the controller  310  can send command signals to the solenoids of the boom, arch and grapple spool valves  412 ,  414 ,  416  to shift the spool valves  412 ,  414 ,  416  to the bottom positions. In the bottom position, the boom spool valve  412  controls variable flow from the hydraulic pump  402  through the lower flow path to the boom pressure compensator  422 ; and the boom pressure compensator  422  controls variable flow through the middle flow path of the boom spool valve  412  to retract the boom hydraulic cylinder  142 . In the bottom position, the arch spool valve  414  controls variable flow from the hydraulic pump  402  through the lower flow path to the arch pressure compensator  424 ; and the arch pressure compensator  424  controls variable flow through the middle flow path of the arch spool valve  414  to retract the arch hydraulic cylinder  132 . In the bottom position, the grapple spool valve  416  controls variable flow from the hydraulic pump  402  through the lower flow path to the grapple pressure compensator  426 , and the grapple pressure compensator  426  controls variable flow through the middle flow path of the grapple spool valve  416  to extend the tong hydraulic cylinders  166 ,  168 . The load sensed in moving the boom, arch and tong hydraulic cylinders  142 ,  132 ,  166 ,  168  is communicated back through the middle flow paths of the boom, arch and grapple spool valves  412 ,  414 ,  416  to the allow flow sides of the boom, arch and grapple load check valves  432 ,  434 ,  436 , respectively. The load sense circuit  454  senses the highest load pressure through the boom, arch and grapple load check valves  432 ,  434 ,  436 . Those of skill in the art will realize that if one or more of the hydraulic functions (boom, arch or grapple  140 ,  130 ,  150 ) is inactive (no flow being commanded), then the respective spool valve  412 ,  414 ,  416  will be in the closed position and the load sense circuit  454  will not sense the load pressure from the inactive hydraulic function regardless of its pressure relative to the other active hydraulic functions. 
         [0037]    In this scenario, a pressure balance is performed by the pressure compensators  422 ,  424 ,  426  for each of the active hydraulic functions. The load sense circuit  454  applies the highest function load on the bottom side of each of the boom, arch and grapple pressure compensators  422 ,  424 ,  426  trying to force the respective compensator closed, and the individual function&#39;s load is applied on the top side of each of the boom, arch and grapple pressure compensators  422 ,  424 ,  426  trying to force the respective compensator open. The compensators of the lower loaded function(s) will be forced closed since the highest function load will be higher than the load of the respective function. This added restriction to the lower loaded function(s) will make all of the flow paths to the active hydraulic functions have equal resistance allowing flow sharing between unequally loaded functions. As explained above, although pressure compensation has the benefit of flow sharing, the pressure compensators introduce added restriction and loss in power that results in heat production. 
         [0038]    On some machines (e.g. the skidder  100 ) with a pressure compensated load sense system, the situation of having a large difference between the heaviest and lightest load can occur frequently when commanding the grapple  150  closed with constant tong squeeze to hold and move a payload. Constant tong squeeze is when the tong close function is constantly being activated to provide constant pressure through the tong hydraulic cylinders  166 ,  168  on the tongs  156 ,  158  in the closing direction. This constant pressure closing the tongs  156 ,  158  is necessary to keep a hold of a payload in the grapple  150  when skidding. Without this constant force on the tongs  156 ,  158  to hold the payload, the payload may fall out of the grapple  150 . When constant tong squeeze is active, the tong hydraulic cylinders  166 ,  168  are stalled out against the payload which causes the pressure in the load sense circuit  454  to increase until the load sense relief setting of the load sense relief valve  404  is reached. This brings the hydraulic pump  402  to maximum system pressure. Therefore when another function is cycled, e.g. the arch  130  or boom  140 , a large pressure difference can exist between the highest load sense produced by the grapple  150  and the load of the arch  130  or boom  140 . This is particularly the case when the arch  130  or boom  140  is being lowered. Lowering the arch  130  or boom  140  is a gravity-aided function and therefore does not take much pressure to move. The arch  130  or boom  140  are also functions that usually require a large amount of flow to obtain adequate cycle times. The result of this large pressure difference and the flow rate results in a significant power loss generating heat. 
         [0039]    Since the boom  140 , arch  130 , and grapple  150  hydraulic functions are electro-hydraulically controlled using the controls  302 ,  304 ,  306 , the controller  310  knows when each of the boom, arch and tong controls  302 ,  304 ,  306  are being used to activate the boom, arch and tong hydraulic functions  140 ,  130 ,  150 . Thus, when the tong control  306  is being used to constantly squeeze or close the tongs  156 ,  158  of the grapple  150 , and the boom or arch controls  302 ,  304  are activated to lower the boom or arch  140 ,  130 , the controller  310  can suspend the tong close command. When the tong close command is suspended, the grapple spool valve  416  is released to the neutral state (middle position) blocking flow between the hydraulic pump  402  and the tong hydraulic cylinders  166 ,  168 , and trapping hydraulic fluid between the tong spool valve  416  and the tong hydraulic cylinders  166 ,  168 . Blocking flow between the hydraulic pump  402  and the tong hydraulic cylinders  166 ,  168  also blocks the load of the tong hydraulic cylinders  166 ,  168  from the load sense circuit  454 . Since the hydraulic fluid is trapped between the tong spool valve  416  and tong hydraulic cylinders  166 ,  168 , the only way to lose squeeze force on the logs is from hydraulic system leakage or if the tongs  156 ,  158  are forced open by some external force causing a tong work port relief to open on the tong hydraulic cylinders  166 ,  168 . An external force causing the cylinder pressure to reach the work port relief setting on the tong hydraulic cylinders  166 ,  168  is possible with or without the improvement to the pressure compensated load sense hydraulic system. To address hydraulic system leakage, since it typically only takes a few seconds to lower the arch  130  or boom  140 , this is usually not enough time for the cylinder pressure of the tong hydraulic cylinders  166 ,  168  to leak down. To help ensure leakage does not cause the grapple  150  to lose hold of the payload, a suspend timer can be implemented to only allow the tong close command to be suspended a certain suspend time period, e.g. 5 seconds. After the suspend time period, the tong close command can be reactivated for a reactivation time period, e.g. 1 second. As long as the tong close command is still active and the arch  130  or boom  140  lower command is still active, this suspend and reactivate process can be repeated by the controller  310  to suspend the tong close command for the suspend time period and reactivate the tong close command for the reactivation time period. 
         [0040]    A grapple close or tong squeeze initialization time period can also be implemented to help ensure that the grapple  150  is securely closed on the payload held by the tongs  156 ,  158  before suspending the tong squeeze command. The tong squeeze initialization time period can be set to any desired time, e.g. 10 seconds. The controller  310  can be configured to not suspend a tong squeeze command during the initial tong squeeze initialization time period that the tong squeeze command is active so that the tongs have an opportunity to fully close and grab against the payload. Otherwise, without a proper clamp force on the payload, the payload may slip out of the grapple  150  when the machine starts to travel. So if an arch  130  or boom  140  lower is commanded during the initial tong squeeze initialization time period of a tong squeeze command, the controller  310  will not suspend the tong squeeze command. After the tong squeeze initialization time period has passed, then it is assumed that the tongs  156 ,  158  have had a chance to fully close against the payload and the tong squeeze command can be suspended when an arch  130  or boom  140  lower command is given by the operator. 
         [0041]    The following example helps illustrate the potential power savings of the improvement to the pressure compensated load sense hydraulic system illustrated in  FIG. 4 . Assume that the load sense relief valve  404  has a load sense relief setting of 2700 psi (pounds per square inch) which means that the load sense circuit  454  will be limited to 2700 psi. Assume also that the load sense margin of the pump  402  is set to 300 psi. The outlet pressure produced by the pump  402  will equal the load sense pressure plus the pump load sense margin. Therefore in this example, the maximum system pressure will be 3000 psi. Also assume that the bias springs for the pressure compensators  422 ,  424 ,  426  are negligible in force and that plumbing losses are negligible as well. Lastly, assume that the pressure required to lower the arch  130  is 500 psi, the flow requirement to lower the arch  130  is 50 GPM (gallons per minute), and the pump  402  is adequately sized to provide a flow of 50 GPM. 
         [0042]    The above assumptions will be used for the load sensing hydraulic system  400  without the improvement in the situation where the operator is commanding a constant tong squeeze for the grapple  150 , and the operator is commanding a lowering of the arch  130 . With constant tong squeeze for the grapple  150  active, the controller  310  will send control signals to the solenoid of the grapple spool valve  416 , shifting the grapple spool valve  416  open to allow the pump  402  to flow hydraulic fluid to the head end of the tong cylinders  166 ,  168 , closing the tongs  156 ,  158  around a payload, e.g. logs or other material. Once the tongs  156 ,  158  are fully closed against the payload, the tong cylinders  166 ,  168  will be stalled which will cause the load sense circuit  454  to reach the load sense relief limit of 2700 psi. The outlet pressure of the pump  402  will be equal to the highest load sense signal (2700 psi from the tong cylinders  166 ,  168 ) plus the load sense margin (300 psi) which is 3000 psi. The load sense pressure on the bottom side of the arch compensator  424  will be the highest load sense pressure of 2700 psi. This load sense pressure of 2700 psi will force the arch compensator  424  to the closed position. When the operator activates lowering of the arch  130  using the arch control  304  to a full (100%) lowering command, the controller  310  will send control signals to the solenoid of the arch spool valve  414 , shifting the arch spool valve  414  to full stroke. With the arch spool valve  414  shifted, the outlet pressure (3000 psi) of the pump  402  will initially be seen at the top side of the arch compensator  424 . With pump outlet pressure (3000 psi) on the top side and load sense pressure (2700 psi) on the bottom side of the arch compensator  424 , the arch compensator  424  will open allowing flow to the arch cylinder  132 . Once the arch compensator  424  is open, the pressure upstream of the arch compensator  424  will initially want to drop to the pressure it takes to lower the arch  130  which is 500 psi. But once the pressure upstream of the arch compensator  424  is lowered to the highest load sense pressure of 2700 psi, the arch compensator  424  will start to close. The arch compensator  424  will then restrict flow to the arch cylinder  132  such that the pressure upstream of the arch compensator  424  will be equal to the highest load sense pressure creating a pressure balance on each side of the arch compensator  424 . This means that the pressure upstream of the arch compensator  424  will be equal to 2700 psi resulting in a 300 psi drop across the inlet metering orifice in the arch spool valve  414  (3000 psi−2700 psi=300 psi). The inlet metering orifice in the arch spool valve  414  can be designed such that there will be 300 psi drop at 50 GPM at full stroke. The arch compensator  424  will then have the highest load sense pressure of 2700 psi upstream and the arch lower load of 500 psi downstream which results in a 2200 psi pressure drop across the arch compensator  424  at 50 GPM. Using the above fluid power equation, Power=P*Q/1714, calculates a power loss of approximately 64 HP going straight to heat in this particular example. 
         [0043]    The same assumptions and situation will now be used for the load sensing hydraulic system  400  with the improvement, which will suspend the constant tong squeeze command when the arch  130  or boom  140  is being lowered. With constant tong squeeze for the grapple  150  active, the controller  310  sends control signals to the solenoid of the grapple spool valve  416 , shifting the grapple spool valve  416  open to continuously supply flow of hydraulic fluid towards the head end of the tong cylinders  166 ,  168 , closing the tongs  156 ,  158  around a payload. After a tong squeeze initialization time period (e.g. 10 seconds) to ensure that the tongs  156 ,  158  are fully closed around the payload, the controller  310  determines the tong cylinders  166 ,  168  are stalled and suspends the tong squeeze command when lowering of the arch  130  or boom  140  is active. When lowering of the arch  130  is commanded using the arch control  304 , the controller  310  will send control signals to the solenoid of the arch spool valve  414 , shifting the arch spool valve  414  to lower the arch  140 . When the controller  310  sends the control signals to the solenoid of the arch spool valve  414 , the controller  310  will stop sending the control signals to the solenoid of the grapple spool valve  416  which allows the grapple spool valve  416  to return to its neutral position blocking flow between the hydraulic pump  402  and the tong cylinders  166 ,  168  through the grapple spool valve  416 , which also blocks the load of the tong cylinders  166 ,  168  from the load sense circuit  454 . With the grapple spool valve  416  closed, the maximum load sense signal will be the 500 psi load coming from the arch cylinder  132 . Thus, the pressure upstream of the arch compensator  424  will equal the highest load sense signal. So the pressures upstream and downstream of the arch compensator  424  will both be 500 psi, and therefore there will essentially be no pressure drop or heat production across the arch compensator  424 . So in this example, 64 HP of heat generation will be saved. When the operator stops commanding lowering of the arch  130 , the controller  310  will no longer see the arch lowering command from the arch control  304  and will restart sending control signals to the solenoid of the grapple spool valve  416  to shift the grapple spool valve  416  open to direct flow of hydraulic fluid towards the head end of the tong cylinders  166 ,  168 . This opens pump flow to the tong cylinders  166 ,  168 , and also opens load pressure from the tong cylinders  166 ,  168  into the load sense circuit  454 , allowing the outlet pressure of the pump  402  to reach the highest load sense signal plus pump margin and a constant squeeze will again be applied on the payload in the grapple  150 . 
         [0044]      FIG. 5  shows an example of a more general control flow  500  for an improved pressure compensated load sense hydraulic system with at least two hydraulic functions. At block  502  the system determines whether a first hydraulic function (HF 1 ) is stalled. This can be done by waiting a predetermined initialization time period or by other methods. The system waits at block  502  until it determines that a first hydraulic function (HF 1 ) is stalled. When the system determines that HF 1  is stalled, control proceeds to block  504 . 
         [0045]    At block  504  the system checks if an operator command is being issued by an operator control to activate another hydraulic function (HF 2 ). If no operator commands are being issued to activate another hydraulic function, control goes back to block  502 . If operator commands are being issued to activate another hydraulic function (HF 2 ), control proceeds to block  506 . 
         [0046]    At block  506 , the system sends command signals to activate HF 2  and to suspend HF 1 , which blocks flow to and from HF 1  and also removes the load of HF 1  from the pump. At block  508  the system starts a suspend timer to track a suspend time period for HF 1 . Control then proceeds to block  510 . 
         [0047]    At block  510  the system checks if operator commands are still being issued by an operator control for HF 2 . If operator commands are no longer being issued to activate HF 2 , the system reactivates HF 1  at block  512 , and control returns to block  502 . If operator commands are still being issued to activate HF 2 , control proceeds to block  514 . 
         [0048]    At block  514  the system checks if operator commands are still being issued by an operator control for HF 1 . If operator commands are no longer being issued to activate HF 1 , control returns to block  502 . If operator commands are still being issued to activate HF 1 , control proceeds to block  516 . 
         [0049]    At block  516  the system checks if the suspend time period for HF 1  has passed. If the suspend time period for HF 1  has passed, the system reactivates HF 1  and starts a reactivation timer at block  518  to track a reactivation time period for HF 1 , and then control proceeds to block  520 . If the suspend time period for HF 1  has not passed, control returns to block  510 . 
         [0050]    At block  520  the system checks if operator commands are still being issued by an operator control for HF 2 . If operator commands are no longer being issued to activate HF 2 , the control returns to block  502 . If operator commands are still being issued to activate HF 2 , control proceeds to block  522 . 
         [0051]    At block  522  the system checks if operator commands are still being issued by an operator control for HF 1 . If operator commands are no longer being issued to activate HF 1 , control returns to block  502 . If operator commands are still being issued to activate HF 1 , control proceeds to block  524 . 
         [0052]    At block  524  the system checks if the reactivation time period for HF 1  has passed. If the reactivation time period for HF 1  has passed, control passes to block  526  where the system suspends HF 1 , and then returns to block  508  to restart the suspend timer. If the reactivation time period for HF 1  has not passed, control returns to block  520 . 
         [0053]    While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered as exemplary and not restrictive in character, it being understood that illustrative embodiment(s) have been shown and described and that all changes and modifications that come within the spirit of the disclosure are desired to be protected. It will be noted that alternative embodiments of the present disclosure may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations that incorporate one or more of the features of the present disclosure and fall within the spirit and scope of the present invention as defined by the appended claims.