Patent Publication Number: US-8522543-B2

Title: Hydraulic control system utilizing feed-forward control

Description:
RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from U.S. Provisional Application No. 61/193,786 by Andrew Krajnik et al., filed Dec. 23, 2008, the contents of which are expressly incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to a hydraulic control system and, more specifically, to a hydraulic control system employing feed-forward control. 
     BACKGROUND 
     Machines such as, for example, excavators, loaders, dozers, and motor graders, often use multiple tool actuators supplied with hydraulic fluid from a hydraulic pump to accomplish a variety of tasks. These tool actuators are typically pilot controlled such that, as an operator moves an input device (e.g., a joystick) an amount of pilot fluid is directed to a tool control valve to move the tool control valve. As the tool control valve is moved, a proportional amount of fluid is directed from the pump to the tool actuators. Various hydraulic control strategies have been implemented to control the amount of fluid flow between the pump and the tool actuators, including a load sensing control strategy. 
     Load sensing control strategies measure a pressure differential between a maximum load pressure of a plurality of tool actuators and a pump delivery pressure. A controller typically receives the pressure differential data and controls a displacement of the pump to deliver the maximum load demand. More specifically, load sensing control systems attempt to control pump displacement to maintain a desired buffer pressure between pump delivery pressure and the maximum load pressure. In order to maintain pump control stability, the pump is typically controlled to deliver fluid at an excess pressure to ensure the maximum load pressure is available to the tool actuators. 
     A control system for regulating pump output is described in U.S. Pat. No. 6,374,722 (the &#39;722 patent) issued to Du et al. on Apr. 23, 2002. The &#39;722 patent describes a system with a variable displacement pump, a controller, a sensor, a servo valve, a servomechanism, and a servo control operable to command adjustment of a swashplate tilt angle and, hence, regulate pump discharge pressure. In the &#39;722 patent, the controller commands adjustment of the swashplate tilt angle based upon the pump discharge pressure. The sensor generates a signal indicative of pump discharge pressure and sends this signal to the controller. Upon receiving the signal and determining an error, the controller commands the servomechanism of the servo valve to vary the swashplate tilt angle, which adjusts pump output. 
     Although the system of the &#39;722 patent may increase regulation precision of pump discharge pressure, certain disadvantages may still persist. For example, a lag between the time at which an error occurs and the time when the error is corrected may cause delayed system response. Further, due to the lag, the system may be difficult to tune and prone to instability. 
     The disclosed system is directed to overcoming one or more of the problems set forth above and/or other problems with prior systems. 
     SUMMARY 
     In one aspect, the present disclosure is directed toward a hydraulic control system. The system may include a pump and a tool actuator configured to move a tool with a flow of pressurized fluid provided by the pump. The system may further include a tool control valve configured to control the flow of pressurized fluid to the tool actuator. The system may also include a controller operably connected with the tool control valve and the pump. The controller may be configured to receive a tool movement request. The controller may further be configured to estimate a change in a flow demand across the tool control valve associated with the tool movement request. The controller may also be configured to command adjustment of a discharge flow rate of the pump based on the estimated change in flow demand to satisfy the tool movement request. 
     In another aspect, the present disclosure is directed toward a method for controlling movement of a tool with a hydraulic control system. The method may include pressurizing fluid with a pump. Additionally, the method may include receiving an operator command to move the tool and estimating a change in a flow demand in the hydraulic control system based on the operator command to move the tool. The method also include adjusting a discharge flow rate of the pump based on the estimated change in the flow demand. The method may additionally include directing at least a portion of the pressurized fluid to move the tool based on the operator command. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of an exemplary machine; 
         FIG. 2  is a schematic of an exemplary hydraulic control system that may be used with machine of  FIG. 1 ; and 
         FIG. 3  is a flow diagram illustrating an exemplary feed-forward and load sensing control process performed by the hydraulic control system of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of a machine  10  is illustrated in  FIG. 1 . Machine  10  may be a mobile or stationary machine able to perform one or more tasks. For example, machine  10  may be a front loader used in the construction industry. It is contemplated that machine  10  may be used in various industries such as transportation, mining, farming, or any other industry. In this embodiment, machine  10  may include a tool  12 , an operator&#39;s station  14 , one or more traction devices  16 , and a power source  18 . 
     Tool  12  may include a variety of different implements such as, for example, a bucket, a fork, a drill, a broom, a hoist, or any other implement apparent to one skilled in the art. Movement of tool  12  may be effected by one or more tool actuators including, for example, a first tool actuator  20  and a second tool actuator  22  (shown in  FIG. 2 ), which may be controlled from operator&#39;s station  14 . First and second tool actuators  20 ,  22  may be a pair of adjacent, double acting, hydraulic actuators configured to move tool  12  (referring to  FIG. 1 ). 
     Operator&#39;s station  14  may include controls for operating and driving machine  10 . One such control may include a tool control device, for example, a joystick  24  operable to regulate the movement of tool  12  by way of first and second tool actuators  20 ,  22 . When manipulated by the machine operator, joystick  24  may initiate a command to hydraulic control system  26  to regulate a flow of pressurized fluid (e.g., hydraulic fluid) to first and second tool actuators  20 ,  22  to move tool  12 . Joystick  24  may regulate both a flow rate and a direction of flow to first and second tool actuators  20 ,  22 , thereby controlling a speed and a movement direction of tool  12 . 
     Referring now to  FIG. 2 , power source  18  may power a hydraulic control system  26  associated with first and second tool actuators  20 ,  22 . Power source  18  may be an engine, such as, for example, a diesel engine, a gasoline engine, a natural gas engine, or any other engine apparent to one skilled in the art. In at least one embodiment, power source  18  may be configured to provide substantially constant rotational power to hydraulic control system  26  by way of a shaft  28 . 
     Hydraulic control system  26  may include a hydraulic circuit  30  and a controller  32 . Controller  32  may control various components of hydraulic control system  26  to control fluid flow through hydraulic circuit  30 . Hydraulic circuit  30  may consist of various hydraulic components used to direct the flow of pressurized fluid within hydraulic control system  26 . For example, hydraulic circuit  30  may include a tank  34 , a pump  36 , first and second tool actuators  20 ,  22 , and other components, as will be discussed below. Pump  36  may utilize rotational power provided by power source  18  to draw fluid from tank  34  and pressurize the fluid for use by first and second tool actuators  20 ,  22 . Controller  32  may be operatively connected to pump  36 , first and second tool actuators  20 ,  22 , and power source  18 , to selectively direct pressurized fluid to move tool  12  that is connected to first and second tool actuators  20 ,  22 . 
     Pump  36  may draw fluid from tank  34  and pressurize it for use within hydraulic circuit  30 . Pump  36  may be, for example, a variable displacement hydraulic pump having a tiltable swashplate  38 . Pump  36  may draw fluid from tank  34  via a pump input port  40  and deliver the fluid under pressure to a hydraulic passageway  42  at a discharge flow rate corresponding to a tilt angle α of swashplate  38  and a rotational speed of shaft  28 . 
     The discharge flow rate of pump  36  may be controlled by varying tilt angle α using a pump actuator, for example, tilt actuator  44 . At a maximum tilt angle α, tilt actuator  44  may cause a maximum discharge flow rate of pump  36 . In contrast, at a minimum tilt angle α, tilt actuator  44  may cause a minimum discharge flow rate of pump  36 . As such, the discharge flow rate, and hence pressure of hydraulic circuit  30 , may be regulated primarily by controlling the movement of swashplate  38  by tilt actuator  44 . 
     Tilt actuator  44  may be any component configured to adjust tilt angle α and, thereby, adjust the discharge flow rate of pump  36 . In one exemplary embodiment, tilt actuator  44  may include a cylinder  46  and a piston  48  arranged to form a first chamber  50  and a second chamber  52 . First chamber  50  may be constantly supplied with pressurized fluid from pump  36  via a first chamber passageway  54 . Second chamber  52  may be selectively supplied with or drained of fluid by way of a second chamber passageway  56 . 
     A pump control valve  58  may be situated in communication with second chamber passageway  56  to control the flow of fluid to and from second chamber  52  to adjust the tilt angle α of swashplate  38 . Pump control valve  58  may be one of various types of control valves including, for example, a spool valve. In one example, pump control valve  58  may be a three-way proportional spool valve. That is, pump control valve  58  may be infinitely variable between three operating states (discussed in more detail below), at which fluid flow is selectively passed through or blocked from three separate passageways. 
     Pump control valve  58  may be actuated using a pump control valve actuator. For example, pump control valve  58  may be actuated using a solenoid, a servomechanism, a pilot operated mechanism, or in any other manner known to one skilled in the art. As shown in the embodiment of  FIG. 2 , a solenoid  60  may be energized by controller  32  to move pump control valve  58  between the first, second, and third states. 
     In the first state (shown in  FIG. 2 ), pump control valve  58  may substantially block fluid flow between hydraulic passageway  42  and second chamber passageway  56 . Additionally, in the first state, fluid flow between second chamber passageway  56  and a pump drain passageway  62  may also be substantially blocked. In the first state, substantially no adjustment of the tilt angle α of swashplate  38  will occur. 
     In the second state, pump control valve  58  may connect second chamber passageway  56  with pump drain passageway  62 , allowing a variable amount of fluid to flow from second chamber  52  to tank  34 , depending on the relative position of the spool within pump control valve  58 , effectively depressurizing second chamber  52 . In this state, pressurized fluid in first chamber  50  may cause piston  48  to retract into cylinder  46 , thereby decreasing the effective length of tilt actuator  44  and increasing the tilt angle α of swashplate  38 . 
     In the third state, pump control valve  58  may connect the output of pump  36  with second chamber passageway  56  by way of hydraulic passageway  42 , allowing a variable amount of fluid to enter second chamber  52 , depending on the relative position of the spool within pump control valve  58 . In this state, pressurized fluid flowing into second chamber  52  may act on piston  48 , causing piston  48  to extend (i.e., enlarging the volume of second chamber  52 ), thereby increasing the effective length of tilt actuator  44  and reducing the tilt angle α of swashplate  38 . Alternatively, it is contemplated that tilt actuator  44  may be reconfigured such that an extension of piston  48  may cause an increase in tilt angle α and a retraction of piston  48  may cause a decrease in tilt angle α of swashplate  38 , if desired. In either the second state or the third state, the position of the spool of pump control valve  58  may be varied within a range to vary the rate of flow to or from tilt actuator  44 . 
     A tool control valve  64  may receive a flow of pressurized fluid via hydraulic passageway  42  from pump  36  to supply fluid into first and second tool actuators  20 ,  22  to move tool  12 . Fluid may be directed to first and second tool actuators  20 ,  22  via a first tool supply passageway  66  (i.e., for extending tool actuators  20 ,  22 ) or a second tool passageway  68  (i.e., for retracting tool actuators  20 ,  22 ), depending on the operating state of tool control valve  64 . Fluid from first and second tool actuators  20 ,  22  may be drained via a tool drain passageway  70 . Tool control valve  64  may be actuated by a tool control valve actuator including, for example, a servomechanism, a solenoid, a pilot operated mechanism, or in any other manner known to one skilled in the art. As shown in the embodiment of  FIG. 2 , a servomechanism  72  may be energized by controller  32  to move tool control valve  64  to move tool  12 . 
     A machine operator may command movement of tool  12  using joystick  24 , and a control sensor  74  may be situated to generate signals indicative of the operator command. That is, control sensor  74  may generate and transmit a signal to controller  32  that is proportional to a displacement of joystick  24  away from a neutral position. This signal may be received by controller  32 , and controller  32  may determine a command (discussed in greater detail below) to responsively energize servomechanism  72  to move tool control valve  64  a corresponding amount that results in the desired adjustment of first and second tool actuators  20 ,  22  to move tool  12 . 
     Controller  32  may embody a single microprocessor, or multiple microprocessors that include a means for controlling and operating components of hydraulic control system  26 . One or more maps relating various system parameters may be stored in the memory of controller  32 . Each of these maps may include a collection of data in the form of tables, graphs, equations and/or another suitable form. The maps may be automatically or manually selected and/or modified by controller  32  or an operator to affect actuation of components attached to machine  10 . It is also contemplated that hydraulic control system  26  may permit controller  32  to access other control functions (e.g., equations, look-up tables), in lieu of using a map. 
     As first and second tool actuators  20 ,  22  extend or retract to move tool  12  according to operator input, the fluid moving into first and second tool actuators  20 ,  22  may affect the pressure across tool control valve  64 . A pressure drop across tool control valve  64  may be sensed by one or more pressure sensors. For example, a first pressure sensor  76  may be positioned along hydraulic passageway  42  to sense a fluid pressure between pump  36  and tool control valve  64 . More specifically, first pressure sensor  76  may be positioned in close proximity to tool control valve  64 . Similarly, a second pressure sensor  78  may be positioned along first tool supply passageway  66  to sense a fluid pressure between tool control valve  64  and first and second tool actuators  20 ,  22 , for example, during an extension of tool  12 . Likewise, a third pressure sensor  80  may be positioned along second tool supply passageway  68  to sense a fluid pressure between tool control valve  64  and first and second tool actuators  20 ,  22 , for example, during a retraction of tool  12 . First, second, and third pressure sensors  76 ,  78 ,  80  may transmit pressure signals to controller  32 . Controller  32  may receive pressure signals from first, second, and third pressure sensors  76 ,  78 ,  80  and compare these signals to determine an actual pressure gradient value across tool control valve  64 . 
     Controller  32  may store in its memory a loading sensing control map  100  relating actual pressure gradient values across tool control valve  64  with one or more predetermined pressure gradient values. It is contemplated that loading sensing control map  100  may include various predetermined pressure gradient values for different operating conditions. Although, it is also contemplated that a single predetermined pressure gradient value may be stored for use under all operating conditions. Using load sensing control map  100 , if controller  32  determines that the actual pressure gradient value across tool control valve  64  deviates from the predetermined pressure gradient value, by more than an acceptable amount, controller  32  may identify an error and generate a load sense control signal to regulate pump control valve  58 . Based upon the load sense control signal, controller  32  may cause pump control valve  58  to vary the flow of fluid to tilt actuator  44 . 
     For example, if the actual pressure gradient value is lower than the predetermined pressure gradient value, controller  32  may command pump control valve  58  to operate in the second state, thereby, increasing the discharge flow rate of pump  36 . In contrast, if the actual pressure gradient value is higher than expected, controller  32  may command pump control valve  58  operate in the third state, thereby decreasing the discharge flow rate of pump  36 . In this manner, a substantially constant pressure gradient across tool control valve  64  may tend to be maintained, at least when flow demand by hydraulic control system  26  is not overly abrupt or transient. 
     In order for controller  32  to command movement of pump control valve  58 , one or more pump control valve maps related to operation of pump control valve  58  may be used. For example, controller  32  may store in its memory a pump control valve position map  102  relating a position of pump control valve  58  to a discharge flow rate of pump  36 . Pump control valve position map  102  may be used by controller  32  to determine an adjustment of the position of tool control valve  58  required to attain desired movements of tilt actuator  44 . In some situations, it may also be necessary to calculate a force required by solenoid  60  to properly position pump control valve  58  to attain a desired fluid flow rate. To facilitate this calculation, controller  32  may store in its memory a pump control valve force map  104  relating a position of pump control valve  58  and to a force (e.g., fluid pressure) required to move pump control valve  58  into position. Specifically, pump control valve force map  104  may contain a constant “k” associated with a biasing device, for example, return spring  82  acting against solenoid  60 , and relate a corresponding force required of solenoid  60  to move pump control valve  58  with an energizing current, which may help controller  32  command an adjustment of the discharge flow rate of pump  36 . Further, controller  32  may store in its memory a pump control valve current map  106  relating energizing current and/or fluid pressure to the required solenoid force. That is, pump control valve current map  106  may also help controller  32  command an adjustment of the discharge flow rate of pump  36 . 
     To improve responsiveness of hydraulic control system  26 , a feed-forward control may be employed. While it is contemplated that feed-forward control may be used as an alternative to load sense control, it may be desirable to use feed-forward in combination with load sensing in order to take advantage of the responsive characteristics of feed-forward control and the ability of load sensing control to verify the accuracy of the feed-forward adjustments and correct for any inaccuracy. Feed-forward control may be capable of estimating a change in flow demand associated with a tool movement request of tool  12  by a machine operator. Further, the feed-forward control may be capable of estimating a change in a pressure gradient across tool control valve  64 . The estimated change in the pressure gradient may be related to the estimated change in flow demand and may be associated with the activation of tool  12 . For example, fluid flowing into hydraulic passageway  42  from pump  36  may tend to cause an increase in pressure within hydraulic passageway  42 . Alternatively, fluid flowing out of hydraulic passageway  42  into first and second tool actuators  20 ,  22  may tend to cause a decrease in pressure within hydraulic passageway  42 . 
     Based on the estimated flow demand changes, feed-forward control may regulate the discharge flow rate of pump  36  and may compensate for pressure changes across tool control valve  64  resulting from actuation of tool  12 . Feed-forward control may be used to vary the supply of fluid before it is required, or as it is required, by hydraulic control system  26 . As used herein, feed-forward control may refer to a control system that responds to an estimated disturbance in a predefined way. For example, when movement of tool  12  is commanded, feed-forward control may respond to an estimated change in flow demand at about the same time as a corresponding change in pressure occurs. It is contemplated that feed-forward control may adjust the discharge flow rate of pump  36  to accommodate for the estimated change in flow demand across tool control valve  64 . The adjustment of the discharge flow rate of pump  36  may occur at about the same time as, or before controller  32  commands actuation of tool  12  via tool control valve  64 . 
     In one exemplary embodiment of feed-forward control, controller  32  may receive signals generated by control sensor  74 . These signals, for example, may be indicative of the position of joystick  24 , as manipulated by the machine operator. Upon receiving signals generated by control sensor  74 , controller  32  may begin to calculate a feed-forward control response. The feed-forward control response may include commands made by controller  32  to adjust the discharge flow rate of pump  36  at about the same time as, or before, tool  12  is moved as commanded by the machine operator. 
     Feed-forward control response may be determined by controller  32  using a feed-forward control map  108  stored in the memory of controller  32 . For example, controller  32  may compare the signal received from control sensor  74  to feed-forward control map  108  relating the tool movement request (i.e., the position of joystick  24 ) to a change in the discharge flow rate of pump  36 . Controller  32  may then use feed-forward control map  108  to estimate a change in flow demand required by first and second tool actuators  20 ,  22  to move tool  12 . For example, if the operator initiates a tool movement request indicative of an increase in flow demand, controller  32  may increase the discharge flow rate of pump  36 . Conversely, if the operator initiates a tool movement request indicative of a decrease in flow demand, controller  32  may decrease the discharge flow rate of pump  36 . 
     Upon determining the new discharge flow rate of pump  36 , controller  32  may implement at least one of pump control valve maps  102 ,  104 ,  106  related to pump control valve  58  to determine a command from controller  32  to solenoid  60  of pump control valve  58 . That is, controller  32  may employ pump control valve position map  102  relating the discharge flow rate of pump  36  to a position of pump control valve  58  required to adjust tilt actuator  44  to implement the changed discharged flow rate of pump  36 . Since pump control valve  58  may be biased by return spring  82 , controller  32  may employ pump control valve force map  104  containing the spring constant “k” to determine the force required by solenoid  60  to move pump control valve  58 . Finally, controller  32  may employ pump control valve current map  106  relating the force required to move pump control valve  58  into the position required to cause the changed discharge flow rate of pump  36  to an energizing current required by solenoid  60 . 
     In a situation when controller  32  indicates an increased flow demand, controller  32  may, for example, command pump control valve  58  into its second state to increase the discharge flow rate of pump  36 . Likewise, in a situation when controller  32  indicates a decreased flow demand, controller  32  may, for example, command pump control valve  58  into its third state to decrease the discharge flow rate of pump  36 . 
     It is contemplated that controller  32  may adjust the discharge flow rate of pump  36  by commanding the feed-forward control response in combination with the load sense control response. Each of these responses may be commanded by controller  32  to occur independently or at about the same time, as will be discussed in more detail below. 
     In order to control tool  12  in accordance with operator input from joystick  24 , controller  32  may also utilize signals from control sensor  74  to command a tool control response command. The tool control response command may be commanded by controller  32  at about the same time as, or just after commanding the feed-forward control response. The tool control response command may be executed by controller  32  to actuate first and second tool actuators  20 ,  22  to move tool  12  as desired by the machine operator. 
     Controller  32  may store in its memory a tool control map  110  relating a tool movement request to a fluid output of tool control valve  64 . Tool control map  110  may be used to determine a change in fluid output of tool control valve  64 . In order to command an adjustment of tool control valve  64  in accordance with the determined change of fluid output of tool control valve  64 , controller  32  may store in its memory a tool command map  112  relating the determined change in fluid output of the tool control valve  64  to a position of the tool control valve  64 . Upon determining the position required by tool control valve  64 , controller  32  may command servomechanism  72  to adjust tool control valve  64  to move tool  12 , for example, by passing fluid through one of first and second supply passages  66 ,  68  and draining fluid into tank  34  via drain passage  70 . 
     INDUSTRIAL APPLICABILITY 
     The disclosed hydraulic control system may find potential application in any machine regulating the discharge flow rate of a pump. The disclosed solution may find particular applicability in hydraulic tool systems, especially hydraulic tool systems for use onboard mobile machines. 
     As shown in  FIG. 3 , a machine operator may initiate a process of regulating operation of hydraulic control system  26  by implementing feed-forward control in combination with load sensing control to adjust a discharge flow rate of pump  36  to meet variable flow demands of machine  10 . During operation of hydraulic control system  26 , a machine operator may provide operator input, for example, via joystick  24  to initiate a tool movement request (Step  200 ). Operator input may be sensed by control sensor  74  (Step  202 ), which may generate an operator input signal (e.g., tool movement request) that is forwarded to controller  32 . The operator input signal (e.g., tool movement request) may be received by controller  32  and may be used in combination with one or more maps to generate at least two response signals (e.g., a feed-forward response signal or a tool response signal). 
     Controller  32  may determine a feed-forward response signal using one or more maps. More specifically, controller  32  may use, for example, feed-forward control map  108  to estimate a change in flow demand across tool control valve  64  caused by operator input (Step  204 ). To compensate for the estimated change in flow demand and, thus, a corresponding related change in pressure gradient, controller  32  may determine an adjustment of the discharge flow rate of pump  36  sufficient to meet the estimated flow demand rate. Further, controller  32  may implement one or more pump control valve maps  102 ,  104 ,  106  to determine a command for pump control valve  58  sufficient to implement the adjustment of discharge flow rate of pump  36  (Step  206 ). Once controller  32  estimates the adjustment of pump discharge flow rate for feed-forward control and determines a command for pump control valve  58 , controller  32  may command pump control valve  58  to adjust tilt actuator  44  in order to modify the discharge flow rate of pump  36  in accordance with the estimated change in flow demand (Step  208 ). 
     Subsequently or concurrently, controller  32  may generate a tool response signal using one more maps. More specifically, controller  32  may utilize tool control map  110  to determine a tool response signal using operator input sensed by control sensor  74  and tool command map  112  to determine how to command tool control valve  64  to implement the tool control (Step  210 ). Controller  32  may then send the tool control response command to servomechanism  72  to adjust tool control valve  64 , which may extend or retract first and second tool actuators  20 ,  22  to move tool  12  (Step  212 ). Fluid flow to first and second tool actuators  20 ,  22  may cause a change in pressure drop across tool control valve  64 , which may be sensed by first, second, and third pressure sensors  76 ,  78 ,  80 . 
     Controller  32  may implement load sensing control by first receiving pressure signals from first, second, and third pressure sensors  76 ,  78 ,  80  to determine an actual pressure gradient value (Step  214 ). Controller  32  may implement one or more maps to determine a pump discharge flow adjustment in response to load sensing control. More specifically, controller  32  may implement load sensing control map  100  to determine if there is an error. That is, an error may be defined when the actual pressure gradient value deviates by more than an acceptable amount from the predetermined pressure gradient value (Step  216 ). Based on an error determined between the actual pressure gradient value and the predetermined pressure gradient value, controller  32  may initiate a command to correct the error by comparing the error to a correction factor in load sensing control map  100 . Further, controller  32  may implement one or more pump control valve maps  102 ,  104 ,  106  to determine a command for pump control valve  58  to correct the error (Step  206 ). Once controller  32  determines the correction factor for load sensing control and determines a command for pump control valve  58  to cause adjustment of the discharge flow rate based on the error, controller  32  may command pump control valve  58  to adjust tilt actuator  44  to adjust of the discharge flow rate of pump  36 . 
     In a first example, a machine operator may command tool  12  to be lifted at a rate corresponding to, for example, twenty percent of the maximum lift rate. Referring now to  FIG. 3 , the machine operator may command this lift rate by manipulating joystick  24 . As a result of the operator&#39;s command, control sensor  74  may generate a signal (e.g., tool movement request) indicative of the twenty percent desired tool lift rate. Upon receiving the signal, controller  32  may implement feed-forward control map  108  in combination with pump control valve maps  102 ,  104 ,  106  to determine a command to adjust pump control valve  58  into the second state to increase the discharge output of pump  36 . At the same time, or about the same time, as implementing feed-forward control, controller  32  may implement tool control via tool control map  110  and tool command map  112  to command tool control valve  64  to move tool  12 . Further, controller  32  may implement load sensing control by utilizing first, second, and third pressure sensors  76 ,  78 ,  80  to monitor the pressure gradient across tool control valve  64 . Upon receiving the pressure signals, controller  32  may implement load sensing control map  100  in combination with pump control valve maps  102 ,  104 ,  106  to determine if an actual pressure drop value deviates more than an acceptable amount compared to a predetermined pressure drop and then to determine a command to adjust pump control valve  58  in accordance with the adjustment of the discharge flow rate of pump  36 . That is, load sensing control may help determine if feed-forward control misestimated the flow demand. For example, in a situation when feed-forward control overestimated the flow demand, controller  32  may command a corrective decrease in the discharge flow rate of pump  36 . 
     In a second example, the machine operator may adjust the lift rate from a lift rate corresponding to twenty percent of the maximum lift rate down to a rate corresponding to five percent of the maximum lift rate. The machine operator may command this lift rate by manipulating joystick  24 . As a result of the operator&#39;s command, control sensor  74  may generate a signal (e.g., tool movement request) indicative of the five percent desired tool lift rate. Controller  32  implementing feed-forward control may recognize a decrease in flow demand and controller  32  may act accordingly to decrease discharge output of pump  36 . Further, controller  32  implementing load sensing control may sense via first, second and third pressure sensors  76 ,  78 ,  80  that feed-forward control misestimated the flow demand. For example, in a situation when feed-forward control underestimated the flow demand, controller  32  may command a corrective increase in the discharge flow rate of pump  36 . 
     While the disclosed embodiment includes a plurality of maps (e.g.,  100 ,  102 ,  104 ,  106 ,  108 ,  110 , and  112 ), any number or organization of maps sufficient to command controller  32  to regulate the flow of fluid within hydraulic control system  26  may be utilized. For example, one or more of the plurality of maps may be combined into a single map or divided into additional maps. Further, hydraulic control system  26  may include various components to regulate flow demand of machine  10 . For example, in situations when machine  10  includes a plurality of tools or various actuators to operate the tools, hydraulic control system  26  may include any number or type of components sufficient to implement feed-forward control and load sensing control. 
     The disclosed method and apparatus may increase system stability and system response by estimating, anticipating, and/or counteracting changes in the flow demand before they occur. By employing hydraulic control system  26  that utilizes feed-forward control, to quickly anticipate and respond to flow demand, in combination with load sensing control, to verify that the flow demand commanded in response to feed-forward control is within an acceptable range, hydraulic control system  26  may enable more responsive and more accurate regulation of the pressure gradient than previous systems. 
     It will be apparent to those skilled in the art that various modification and variations can be made to the disclosed hydraulic control system, without departing from the scope of the disclosure. Other embodiments of the disclosed hydraulic control system will be apparent to those skilled in the art from consideration of the specification and practice disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope being indicated by the following claims and their equivalents.