Patent Publication Number: US-7210396-B2

Title: Valve having a hysteretic filtered actuation command

Description:
TECHNICAL FIELD 
   The present disclosure is directed to a valve and, more particularly, to a valve having a hysteretic filtered actuation command. 
   BACKGROUND 
   Hydraulic systems are often used to control the operation of hydraulic actuators of work machines. These hydraulic circuits typically include valves that are fluidly connected between the actuator and a pump and valves that are fluidly connected between the actuator and a reservoir. The valves control a flow rate and direction of pressurized fluid to and from chambers of the actuator to create pressure differentials within the actuator to affect movement thereof. Often, one or more of these valves are controlled in response to the pressure of the pressurized fluid within a portion of the hydraulic system and/or an associated chamber of the hydraulic actuator to reduce lag time between changing operational demands and valve actuation. Pressures within the hydraulic systems and, in particular, within chambers of the hydraulic actuators, however, may oscillate rapidly causing the valves to have overactive displacements which may lead to valve instability and/or premature wear. 
   A method of operating a hydraulic actuator is described in U.S. Pat. No. 6,467,264 B1 (“the &#39;264 patent”) issued to Stephenson et al. The &#39;264 patent discloses a pair of supply valves to direct fluid from a pump to respective head-end and rod-end chambers of a piston-cylinder arrangement. The &#39;264 patent also discloses a pair of drain valves to direct fluid from respective head-end and rod-end chambers of the piston-cylinder arrangement to a reservoir. Each of the head-end and rod-end valves are proportional valves actuated by solenoids to selectively allow fluid to and/or from the piston-cylinder arrangement. The &#39;264 patent further discloses a metering valve to control the pressure drop across the drain valves to improve the accuracy of the flow of fluid to the reservoir. 
   Although the metering valve of the &#39;264 patent may control the pressure drop across a drain valve directing fluid from the piston-cylinder arrangement to the reservoir, it may not increase stability of the drain valve by reducing overactive displacements. 
   The present disclosure is directed to overcoming one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   In a first aspect, the present disclosure is directed to a valve system including a controller and a valve including a valve element and a valve bore. The valve element is selectively movable relative to the valve bore at least partially in response to a signal communicated from a controller. The communicated signal is at least partially based on a load on the actuator and a determined pressured drop. The determined pressure drop is at least partially based on a hysteretic filter. 
   In another aspect, the present disclosure is directed to a method of actuating a valve having a valve element movable relative to a valve bore. The method includes determining a desired flow of pressurized fluid through the valve at least partially based on an operator input. The method also includes determining a load on an actuator fluidly connected upstream of the valve. The method further includes determining a desired pressure drop at least partially based on the determined load pressure and a hysteretic filter. The method still further includes determining a desired flow area of the valve at least partially based on the determined flow of pressurized flow and the determined pressure drop. The method still further includes moving the valve element to establish the determined flow area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of an exemplary disclosed hydraulic system; 
       FIG. 2  is a flow chart of an exemplary method to control the head-end and rod-end drain valves of  FIG. 1 ; and 
       FIG. 3  is a schematic illustration of an exemplary hysteretic filter logic for determining the pressure drop across the head-end and rod-end drain valves of  FIG. 1 . 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates a hydraulic system  10  that may include various components that cooperate to actuate hydraulic cylinder  12 . Hydraulic cylinder  12  may be connected to various work machine components, such as, for example, linkages (not shown), work implements (not shown), and/or frames (not shown). Hydraulic system  10  may include a source  14  of pressurized fluid, a tank  16 , a head-end supply valve  18 , a head-end drain valve  22 , a rod-end supply valve  20 , and a rod-end drain valve  24 . It is contemplated that hydraulic system  10  may include additional and/or different components such as, for example, a pressure sensor, a temperature sensor, a position sensor, a controller, an accumulator, and/or other components known in the art. 
   Hydraulic actuator  12  may include a piston-cylinder arrangement, a hydraulic motor, and/or any other known hydraulic actuator having one or more fluid chambers therein. For example, hydraulic actuator  12  may include a tube  50  and a piston assembly  52  disposed within tube  50 . One of tube  50  and piston assembly  52  may be pivotally connected to a frame, while the other of tube  50  and piston assembly  52  may be pivotally connected to a work implement. Hydraulic actuator  12  may include a first chamber  54  and a second chamber  56  separated by piston assembly  52 . The first and second chambers  54 ,  56  may be selectively supplied with pressurized fluid to cause piston assembly  52  to displace within tube  50 , thereby changing the effective length of hydraulic actuator  12 . The expansion and retraction of hydraulic actuator  12  may function to assist in moving one or both of the frame and the work implement. It is contemplated that hydraulic actuator  12  may be connected to and/or between any components of a work machine to affect relative movement therebetween. 
   Displacement of piston assembly  52  may be caused by an imbalance of force acting on opposite sides of piston assembly  52  as is conventional in the art. An imbalance of force may be caused by fluid pressure within one of first and second chambers  54 ,  56  being different than fluid pressure within the other one of first and second chambers  54 ,  56 . It is noted that a relatively large pressure differential may establish an overrunning operation of hydraulic actuator  12  and relatively small pressure differential may establish a restrictive operation of hydraulic actuator  12 . For example, an overrunning operation may be desired for quick movement of piston assembly  52 , e.g., when a load acts against the movement of hydraulic actuator  12 . For another example, a restrictive operation may be desired for a slow movement of hydraulic actuator  12 , e.g., when a load acts with the movement of hydraulic actuator  12 . 
   Source  14  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  14  may be drivably connected to a power source (not shown) of a work machine by, for example, a countershaft, a belt, an electrical circuit, and/or in any other suitable manner. Source  14  may be dedicated to supplying pressurized fluid only to hydraulic system  10 , or alternately may supply pressurized fluid to additional hydraulic systems (not shown) within a work machine. 
   Tank  16  may include a source of low pressure, such as, for example, 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 working fluid known in the art. One or more hydraulic systems may draw fluid from and return fluid to tank  16 . It is also contemplated that hydraulic system  10  may be connected to multiple, separate fluid tanks. It is contemplated that tank  16  may include any low pressure fluid source known in the art, such as, for example, a sump. 
   Head-end and rod-end supply valves  18 ,  20  may be disposed between source  14  and hydraulic actuator  12  and may be configured to regulate a flow of pressurized fluid to first and second chambers  54 ,  56 . Specifically, head-end and rod-end supply valves  18 ,  20  may each include a two-position spring biased valve mechanism that may be solenoid actuated and configured to move between a first position at which fluid is allowed to flow into first and second chambers  54 ,  56  and a second position at which fluid flow is blocked from flowing to first and second chambers  54 ,  56 . It is contemplated that head-end and rod-end supply valves  18 ,  20  may include additional and/or different valve mechanisms such as, for example, a proportional valve element and/or any other valve mechanisms known in the art. 
   Head-end and rod-end drain valves  22 ,  24  may be disposed between hydraulic actuator  12  and tank  16  and may be configured to regulate a flow of pressurized fluid from first and second chambers  54 ,  56  to tank  16 . Specifically, head-end and rod-end drain valves  22 ,  24  may each include a proportional spring biased valve mechanism that may be solenoid actuated and configured to move between a plurality of flow passing positions at which fluid is allowed to flow from first and second chambers  54 ,  56  and a flow blocking position at which fluid is blocked from flowing from first and second chambers  54 ,  56 . It is contemplated that head-end and rod-end drain valves  22 ,  24  may include additional and/or different valve mechanisms such as, for example, a two-position valve element and/or any other valve mechanism known in the art. 
   Head-end and rod-end supply and drain valves  18 ,  20 ,  22 ,  24  may be fluidly interconnected. In particular, head-end and rod-end supply valves  18 ,  20  may be connected in parallel to a common supply passageway  25  that may be configured to fluidly communicate pressurized fluid from source  14  to head-end and rod-end supply valves  18 ,  20 . Head-end and rod-end drain valves  22 ,  24  may be connected in parallel to a common drain passageway  27  that may be configured to fluidly communicate pressurized fluid from head-end and rod-end drain valves  22 ,  24  to tank  16 . Head-end supply and drain valves  18 ,  22  may be connected in parallel to a first chamber passageway  26  that may be configured to fluidly communicate pressurized fluid to and from first chamber  54 . Rod-end supply and drain valves  20 ,  24  may be connected in parallel to a second chamber passageway  28  that may be configured to fluidly communicate pressurized fluid to and from second chamber  56 . 
   A controller  30  may control the actuation of head-end and rod-end drain valves  22 ,  24 . Controller  30  may include one or more microprocessors, a memory, a data storage device, a communications hub, and/or other components known in the art. It is contemplated that controller  30  may be integrated within a general work machine control system capable of controlling additional various functions of a work machine. Controller  30  may be configured to receive input signals from first and second pressure sensors  32 ,  34  via first and second communication lines  36 ,  38 . Controller  30  may perform one or more algorithms to determine appropriate output signals to control head-end and rod-end drain valves  22 ,  24  and may deliver the output signals via third and fourth communication lines  40 ,  42 . It is contemplated that controller  30  may be further configured to receive additional inputs  44  indicative of various operating parameters of hydraulic system  10  and/or additional components of an associated work machine  10 , such as, for example, temperature sensors, position sensors, and/or any other parameter known in the art. It is also contemplated that controller  30  may be configured to control the operation of head-end and rod-end supply valves  18 ,  20  and/or additional components  46  of hydraulic system  10  and/or an associated work machine, such as, for example, visual displays and/or any other component known in the art. 
   First and second pressure sensors  32 ,  34  may include any known pressure sensor and may be configured to sense the pressure indicative of the pressurized fluid within first and second chambers  54 ,  56 . First and second pressure sensors  32 ,  34  may be disposed at any location relative to hydraulic system  10 , such as, for example, relative to first and second chamber supply passageways  26 ,  28 , relative to first and second chambers  54 ,  56 , and/or any other suitable location. 
     FIG. 2  illustrates an exemplary method  200  which controller  30  may perform to determine a desired flow area of head-end and rod-end drain valves  22 ,  24  to establish a desired actuation thereof. Method  200  will be described with reference to the actuation of head-end drain valve  22  for clarification purposes. It is understood that method  200  may be applicable to the actuation of rod-end drain valve  24 . Method  200  may include determining a desired flow of pressurized fluid (step  202 ), determining a circuit load (step  204 ), determining a desired pressure drop (step  206 ), determining a desired flow area (step  208 ), and may repeat (step  210 ) continuously, as desired. 
   Step  202  may include determining a flow of pressurized fluid desired to flow through head-end drain valve  22  and may be based at least in part on an operator input. Specifically, controller  30  may be configured to determine a desired flow of pressurized fluid through head-end drain valve  22  for a particular operation of hydraulic actuator  12  by, for example, look-up tables, equations, and/or maps. It is contemplated that the desired flow of pressurized fluid may also be based on, for example, the control of rod-end supply valve  20 , the valve dynamics of rod-end supply valve  20 , and/or be determined by other known methods. 
   Step  204  may include determining a circuit load which approximates the load on hydraulic actuator  12 . Specifically, controller  30  may approximate the load on hydraulic actuator  12  based on the forces acting on hydraulic actuator  12  by sensing pressures of the pressurized fluid within first and second chambers  54 ,  56 . For example, the circuit load may be determined by relating the sensed pressures to circuit loads via, for example, look-up tables, equations, and/or maps. It is contemplated that controller  30  may determine the circuit load by determining the imbalance of force across piston assembly  52  by proportionally relating the pressure of pressurized fluid within first chamber  54  and the area of the first chamber side of piston assembly  52  to the pressure of pressurized fluid within second chamber  56  and the area of second chamber side of piston assembly  52 . It is also contemplated that controller  30  may determine the circuit load as an approximation based on only the pressure of fluid in the one of first and second chambers  54 ,  56  fluidly connected to tank  16 . It is further contemplated that the circuit load may be determined by any other suitable method known in the art, such as, for example, through the use of a load cell suitably connected to actuator  12  as is known in the art. It is noted that circuit load, as used herein, approximates a load on an actuator as affected by internal system forces, e.g., hydraulic pressures acting on a piston within a cylinder, and/or external forces, e.g., loads acting to extend and/or retract the actuator, friction forces, and/or inertial forces. It is further noted that because hydraulic system  10  may have a plurality of actuators, hydraulic system  10  may have a plurality of circuit loads each representing the load on an associated actuator. 
   Step  206  may include determining a pressure drop across head-end drain valve  22  based in part on a functional relationship with the determined circuit load. Specifically, controller  30  may be configured to determine a desired pressure drop across head-end drain valve  22  via a hysteretic filter logic  300  and on the determined circuit load. Hysteric filter logic  300  will be described in more detail below with reference to  FIG. 3 . 
   Step  208  may include determining a flow area of head-end drain valve  22  based on a functional relationship with the desired flow and the desired pressure drop. Specifically, controller  30  may be configured to determine a desired flow area of the valve element of head-end drain valve  22  necessary to direct the desired flow of pressurized fluid through head-end valve  22  and provide the desired pressure drop across head-end drain valve  22 . The desired flow area may be determined by, for example, look-up tables, equations, and/or maps. It is noted that for a given desired flow of pressurized fluid, a substantially constant pressure drop may result in a substantially constant flow area, e.g., fluid flow may be a function of the pressure drop across a constant flow area, as is known in the art. It is contemplated that a change in desired flow of pressurized fluid may result in a corresponding change in flow area regardless of a change in pressure drop. It is also contemplated that controller  30  may control the displacement of the valve element of head-end drain valve  22  to establish the desired flow area therethrough. 
     FIG. 3  illustrates an exemplary hysteretic filter logic  300  which controller  30  may perform to determine the desired pressure drop for head-end drain valve  22  (step  206 ). Hysteretic filter logic  300  may be configured to determine a desired pressure drop that may be different than a previous pressure drop only when a determined circuit load exceeds maximum or minimum thresholds. Hysteretic filter logic  300  may further be configured to relate increasing circuit loads with desired pressure drops based on a first functional relationship y k =f 1 (x k ), wherein y k  represents the desired pressure drop and x k  represents the determined circuit load. Hysteretic filter logic  300  may further be configured to relate decreasing circuit loads with desired pressure drops based on a second functional relationship y k =f 2 (x k ). It is contemplated that the functional relationships for increasing and decreasing circuit loads may represent any mathematical relationship such as, for example, linear, parabolic, and/or other powered relationships relating determined circuit load and desired pressure drop. It is also contemplated that the functional relationship for decreasing circuit loads would establish a greater desired pressure drop than the functional relationship for increasing circuit loads. As such, hysteretic filter logic  300  may include a bias toward establishing restrictive operation of hydraulic actuator  12  rather than an overrunning operation of hydraulic actuator  12 . 
   Hysteretic filter logic  300  may start (step  302 ) when a desired actuation of hydraulic actuator  12  is performed and, more particularly, may start (step  302 ) when an actuation of head-end drain valve  22  is desired. Hysteretic filter logic  300  may receive an input y k−1  indicative of the last determined desired pressure drop (step  304 ) and may calculate the maximum x k max  and minimum x k min  load pressure threshold values (step  306 ) based on the last determined pressure drop. Hysteretic filter logic  300  may also receive an input x k  indicative of the present circuit load (step  308 ). Hysteretic filter logic  300  may compare the present circuit load x k  with the maximum x k max  and minimum x k min  load pressure threshold values (steps  310 ,  314 ) to select and perform an appropriate functional relationship y k =y k−1 , y k =f 1 (x k ), y k =f 2 (x k ) to determine the desired pressure drop y k  based on the present circuit load x k  (steps  312 ,  316 ,  318 ). Hysteretic filter logic  300  may output the determined desired pressure drop y k  (step  320 ) and may repeat (step  322 ) to continuously determine desired pressure drops as controller  30  actuates head-end drain valve  22 , as desired. Hysteretic filter logic  300  may end (step  324 ) when actuation of head-end drain valve  22  is no longer desired. 
   Step  304  may include establishing the last determined desired pressure drop. It is contemplated that for the first sequence performed by hysteretic filter logic  300 , the last determined pressure drop may be initially set to any constant, such as, for example, zero. 
   Step  306  may include determining the maximum x k max  and minimum x k min  load pressure threshold values based on the algebraic inverse of the functional relationships for increasing and decreasing circuit loads. Specifically, the maximum threshold value may be determined by algebraically inverting the functional relationship for increasing circuit loads. For example, if the functional relationship for increasing circuit loads is a linear relationship, such as, for example, y=f 1 (x)=x+C, where y represents a desired pressure drop, f 1 (x) represents the increasing functional relationship, x represents the circuit load, and C represents a constant, the maximum threshold value may be determined as x=f 1   −1 (y)=y−C. The minimum threshold value may be similarly determined. 
   Step  310  may include determining whether or not the determined circuit load is greater than or equal to the minimum threshold value and less than or equal to the maximum threshold value. If so, hysteretic filter logic  300  may progress to step  312  which may include determining the desired pressure drop to be substantially equal to the previous determined pressure drop. As such, hysteretic filter logic  300  may not establish a new pressure drop because the determined circuit load may not be sufficiently different than the previous circuit load. If not so, hysteretic filter logic  300  may progress to step  314 . 
   Step  314  may include determining whether or not the determined circuit load is greater than the maximum threshold value. If so, hysteretic filter logic  300  may progress to step  316  which may include determining the desired pressure drop based on the increasing functional relationship. As such, hysteretic filter logic  300  may establish a new pressure drop because the determined circuit load may have sufficiently increased over that of the previous circuit load, e.g., the circuit load may have sufficiently changed to indicate increasing loads are acting on hydraulic actuator  12 . If not so, hysteretic filter logic  300  may progress to step  318 . 
   Step  318  may include determining the desired pressure drop based on the decreasing functional relationship. If hysteretic filter logic  300  progresses to step  318 , the determined circuit load may be recognized to be less than the minimum threshold value because the determined circuit load is not greater than or equal to the minimum threshold value (step  310 ) and the determined circuit load is not greater than the maximum threshold value (step  314 ). As such, hysteretic filter logic  300  may establish a new pressure drop because the determined circuit load may have sufficiently decreased over that of the previous circuit load, e.g., the circuit load may have sufficiently changed to indicate decreasing loads are acting on hydraulic actuator  12 . 
   Step  320  may include outputting the appropriately determined desired pressure drop which may then be functionally related with the desired flow of pressurized fluid to determine the desired flow area of head-end drain valve  22  in step  208  of method  200  ( FIG. 2 ). As noted above, for a given flow area of head-end drain valve  22 , the amount of desired flow therethrough may be a function of the pressure drop across head-end drain valve  22 . 
   INDUSTRIAL APPLICABILITY 
   The disclosed valve may be applicable to any hydraulic system that includes a fluid actuator where fluid is directed from the actuator to a tank. The disclosed valve may reduce overactive valve actuation due to pressure oscillations, reduce energy necessary to operate the hydraulic actuator by establishing overrunning operations when appropriate, improve valve response to changing system pressures, and/or improve operation of the hydraulic system. The operation of hydraulic system  10  and, in particular, head-end drain valve  22  will be explained below. 
   Referring to  FIG. 1 , hydraulic cylinder  12  may be movable by fluid pressure in response to an operator input. Fluid may be pressurized by source  14  and directed to head-end and rod-end supply valves  18  and  20 . In response to an operator input to either extend or retract piston assembly  52  relative to tube  50 , one of the valve elements of one of head-end and rod-end supply valves  18 ,  20  may move to the open position to direct the pressurized fluid to the appropriate one of first and second chambers  54 ,  56 . Controller  30  may, in response to operator input, determine a desired flow area for the appropriate one of head-end and rod-end drain valves  22 ,  24  desired to be moved into a flow passing position to direct pressurized fluid to tank  16 . 
   Referring to  FIG. 2 , controller  30  may determine a desired flow of pressurized fluid through the flow passing drain valve, e.g., the one of head-end and rod-end drain valves  22 ,  24  desired to be moved into a flow passing position based in part on the operator input. Specifically, controller  30  may, for a given operator input, determine (step  202 ) a corresponding flow of pressurized fluid that may be desired through one of head-end and rod-end drain valves  22 ,  24  to establish an appropriate pressure differential across piston assembly  52  ( FIG. 1 ) to cause a desired movement of hydraulic actuator  12 . 
   For example, for an extension of hydraulic actuator  12  and a given flow of pressurized fluid through head-end supply valve  18 , a relatively large flow of pressurized fluid through rod-end drain valve  22  (overrunning operation) may provide a greater pressure differential across piston assembly  52  than a relatively small flow of pressurized fluid through rod-end drain valve  22  (restrictive operation). A similar relationship may be appropriate for a retraction of hydraulic actuator  12 . It is contemplated that overrunning and resistive movement of hydraulic actuator  12  may be adjusted and/or controlled for any number of various operator inputs to extend and/or retract hydraulic actuator  12 , as desired. 
   The following explanation of a restrictive retraction of hydraulic actuator  12  is provided for clarification purposes only. It is noted that the operation of hydraulic system  10  and, in particular, the operation of hysteretic filter logic  300  explained below is applicable to control hydraulic actuator  12  in any number of various operations. 
   Referring to  FIG. 1 , to retract hydraulic actuator  12 , rod-end supply valve  20  may move to a flow passing position to direct a flow of pressurized fluid to second chamber  56  in response to an operator input. Controller  30  may receive pressure signals from first and second pressure sensors  32 ,  34 . 
   Referring to  FIG. 2 , controller  30  may determine a desired flow of pressurized fluid through head-end drain valve  22  required to affect the appropriate retraction of hydraulic actuator  12  for the desired operator input (step  202 ). Controller  30  may also resolve the received pressure signals which may indicate a low circuit load (step  204 ). For example, a low circuit load may be the result of an associated load aiding in the retraction of hydraulic actuator  12 , e.g., the associated load may be pushing on piston assembly  52 . As such, it may be desired to retract hydraulic actuator  12  slowly so as to increase the stability of hydraulic actuator  12  and correspondingly increase the stability of moving the associated load. 
   Referring to  FIG. 3 , controller  30  may perform hysteretic filter logic  300  to determine the desired pressure drop across head-end drain valve  22 . For example, the functional relationship for increasing circuit loads f 1 (x k ) may be a linear relationship, such as, f 1 (x k )=x, and the functional relationship for decreasing load pressures f 2 (x k ) may be a linear relationship, such as, f 2 (x k )=x+1. Also for example, the input (step  304 ) of the previous determined pressure drop y k−1  may be set to zero for the first sequence of hysteretic filter logic  300 . As such, the maximum threshold value may be:
 
 x   k max   =f   1   −1 ( y   k−1 )= y= 0
 
and the minimum threshold value may be:
 
 x   k min   =f   2   −1 ( y   k−1 )= y− 1=−1.
 
Accordingly, if the determined circuit load x k  functionally relates to be less than or equal to 0 and greater than or equal to −1, the desired pressure drop may remain at the previous determined pressure drop, y k =y k−1 =0. However, because the determined circuit load may be greater than the maximum threshold value, a desired pressure drop may be established based on the determined circuit load x k  and the functional relationship for increasing circuit loads f 1 (x k ). Hysteretic filter logic  300  may be repeated as desired to determine desired pressure drops in response to changing circuit loads. It is noted that for clarification purposes only the functional relationships are represented with simple numerals and that actual relationships account for orders of magnitude, units, and/or other factors necessary and/or desired to relate circuit loads and desired pressure drops.
 
   Referring again to  FIG. 2 , controller  30  may determine the desired flow area of head-end drain valve  22  (step  208 ) based on the desired flow of pressurized fluid and the determined pressure drop. Controller  30  may communicate a control signal via communication line  40  to displace the valve element of head-end drain valve  22  to establish the desired flow area (see  FIG. 1 ). For example, if hysteretic filter logic  300  establishes the desired pressure drop to be substantially equal to the previous pressure drop, y k =y k−1 , e.g., the determined circuit load did not exceed the threshold values, the determined flow area will be substantially equal to the previous determined flow area and controller  30  may not displace the valve element of head-end drain valve  22 . Similarly, if hysteretic filter logic  300  establishes the desired pressure drop based on the functional relationship for increasing circuit loads y k =f 1 (x k ), e.g., the determined circuit load exceeded the maximum threshold value, the determined flow area may be different than the previous determined flow area and controller  30  may displace the valve element of head-end drain valve  22 . A similar relationship is applicable if hysteretic filter logic  300  establishes the desired pressure drop based on the functional relationship for decreasing circuit loads y k =f 2 (x k ), e.g., the determined circuit load exceeded the minimum threshold value. 
   Method  200  and, in particular, hysteretic filter logic  300 , may be substantially continuously repeated for a given operator command to retract hydraulic actuator  12 . Accordingly, subsequent pressure signals may be received by controller  30  from first and second pressure sensors  32 ,  34 , subsequent circuit loads may be determined and compared to subsequent threshold values, subsequent pressure drops may be determined, and subsequent control signals may be communicated to head-end drain valve  22 . As such, the displacement of the valve element of head-end drain valve  22  may only be actuated in response to pressure changes that establish a circuit load that exceeds the threshold values. Hysteretic filter logic  300  may establish a circuit load deadband which must be overcome before valve element displacement may occur. Such a deadband may effectively prohibit small pressure oscillations from affecting valve displacement while allowing large pressure fluctuations to affect valve displacement without undesirable delay. 
   Because hysteretic filter logic  300  establishes threshold values, minor oscillations in pressure acting on hydraulic actuator  12  may not result in corresponding movement of the valve element of head-end drain valve  22 . As such, the stability of head-end drain valve  22  may be increased by reducing overactive displacements. Also, because overrunning operations may be established, unnecessarily restrictive pressure drops across drain valves may be reduced to increase the efficiency of hydraulic system  10 . Additionally, because the threshold values are determined in each sequence of hysteretic logic  300 , the threshold range of circuit loads that may not establish new pressure drops, adjusts as the circuit load increases and decreases. As such, the threshold range may track with the circuit load and may provide increased flexibility in control of head-end drain valve  22 . Furthermore, because head-end drain valve is based in part on circuit loads, lag time between changes in circuit loads and valve element actuation may be reduced. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed valve having a hysteretic filtered actuation command. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed valve. 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.