Abstract:
A method of controlling a hydraulic system having a variable displacement pump operatively coupled to an engine. The method includes detecting a speed of the engine, and determining a desired power value of the pump. The method also includes identifying an allowable power value that may be expended by the pump at the detected speed. The method also includes selecting a pump power value. The selected pump power value is the lower of the allowable power value and the desired power value. The method further includes adjusting the pump to deliver the selected pump power value.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a system and a method of controlling a hydraulic system. 
     BACKGROUND 
     Construction machines having hydraulically controlled implements often include one or more variable displacement hydraulic pumps that are driven by an internal combustion engine. As the operator manipulates the implements through levers or other input devices in the operator cabin, the hydraulic system responds by directing hydraulic fluid flow to appropriate hydraulic circuits. To move an implement carrying a load in a desired direction at a desired velocity, the operator may operate one or more levers that direct flow of a hydraulic fluid, to apply force, and move the implement. As the operator requested hydraulic effort increases, the hydraulic control system increases the displacement of the variable displacement hydraulic pump such that the amount of hydraulic flow increases. Since the amount of power required to drive the hydraulic pump is a function of pressure and flow, as flow increases, a higher amount of engine power is expended to operate the implement. Load on the engine, is therefore, a function of hydraulic flow and pressure. Under some operating conditions, the amount of hydraulic power exceeds the amount of power the engine is capable of producing at that engine speed. When this occurs, the rotational speed of the engine decreases along its lug curve. This condition is typically referred to as engine lug. 
     When the engine lugs, operator perception of engine power may be adversely affected. In extreme cases, the engine may even stall if the requested hydraulic power becomes too high. To reduce lug and avoid stalling the engine, the operator may reduce the amount of hydraulic power being requested when they sense a loss of engine speed. While this action avoids engine stall, the operator may overcompensate and reduce the amount of hydraulic work to a greater extent than needed to prevent stall. As a result, machine productivity may be reduced. Fuel combustion in the engine during lug may become less efficient, resulting in increased emissions and reduced fuel economy. As a result, it may also be desirable to reduce engine lug to decrease emissions and fuel consumption. Some level of engine lug, however, may be desirable to operate the machine at maximum capacity. Therefore, the hydraulic system may be controlled to ensure that the machine is working at maximum capacity while limiting emissions and fuel consumption. 
     U.S. Pat. No. 5,525,043 (&#39;043 patent), issued to Lukich on Jun. 11, 1996 and assigned to the assignee of the current disclosure, describes a hydraulic control system to reduce engine lug. In the control system of the &#39;043 patent, multiple sensors are used to detect various operating parameters of the hydraulic system including the pump and an engine driving the pump. Based on these sensor inputs, a parameter signal that indicates the load on the engine is calculated. When the load on the engine increases above a predefined level, the displacement of the variable displacement pump is reduced to allow the engine speed to increase to the predefined level. In the control system of the &#39;043 patent, the parameter signal is determined based on a number of operating parameters of the hydraulic system to accurately control engine speed with a minimum amount of oscillation (overshoot and undershoot). While quick and accurate control of engine speed by preventing oscillations, as disclosed in the &#39;043 patent, may be important in some applications, it may not be as important in other applications. Although the control system of the &#39;043 patent may effectively control engine lug, the complexity of the methodology employed may make the system expensive for some applications. The present disclosure is directed to solving one or more of the problems set forth above and/or other problems in the relevant art. 
     SUMMARY OF THE INVENTION 
     In one aspect, a method of controlling a hydraulic system having a variable displacement pump operatively coupled to an engine is disclosed. The method includes detecting a speed of the engine, and determining a desired power value of the pump. The method may also include identifying an allowable power value that may be expended by the pump at the detected speed. The method may also include selecting a pump power value. The selected pump power value may be the lower of the allowable power value and the desired power value. The method may further include adjusting the pump to deliver the selected pump power value. 
     In another aspect, a hydraulic system is disclosed. The hydraulic system includes an engine, and a variable displacement pump. The hydraulic power delivered by the pump may be a function of a piston displacement of the pump. The system may also include an implement fluidly coupled to the pump. The implement may be operable by the power delivered by the pump. The system may also include a sensor configured to measure engine speed, and a control system configured to identify an allowable power value that may be expended by the pump from a map that relates allowable power value to engine speed. The control system may also be configured to determine a desired power value. The desired power value may be a power value that is requested to operate the implement. The control system may be further configured to adjust the piston displacement to deliver a lower of the allowable power value and the desired power value to the implement. 
     In yet another aspect, a method of operating a machine having an engine and a hydraulically powered implement fluidly coupled to a pump is disclosed. The method includes detecting a signal indicative of a desired power value to operate the implement, and sensing a speed of the engine. The method may further include choosing an allowable power value that may be directed to the implement from a map that relates allowable power value to the sensed engine speed. The method may further include adjusting the pump to direct a lower of the allowable power value and the desired power value to the implement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of an exemplary construction machine; 
         FIG. 2  is an exemplary hydraulic system of the construction machine of  FIG. 1 ; 
         FIG. 3  is a schematic illustration of an algorithm used by the hydraulic system of  FIG. 2 ; and 
         FIG. 4  is a flow chart that illustrates the functioning of the hydraulic system of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary construction machine  100  having a hydraulic implement  50 . Construction machine  100  may include any type of machine, such as an excavator. The hydraulic implement  50  may include any type of device that is operated by the force of a hydraulic fluid. Construction machine  100  may include a hydraulic system  10  that directs fluid under pressure to operate implement  50 . One or more devices, such as hydraulic cylinders  40 , coupled to construction machine  100 , may assist in using the pressure of the hydraulic fluid to operate implement  50 . An operator may operate implement  50  by controlling one or more levers  16 A located in cab  22 . Hydraulic system  10  may include one or more pumps  20  that direct the fluid to the hydraulic cylinders  40  in response to the operator&#39;s commands, and an internal combustion engine  12  that drives pump  20 . Internal combustion engine  12  may be any type of engine known in the art. In some embodiments, in addition to driving pump  20 , engine  12  may also drive other systems, such as a traction system  44 , of machine  100 . In these embodiments, the power output of engine  12  may be shared by pump  20  and traction system  44 . Although, not discussed hereafter, hydraulic system  10  may also include devices, such as control valves  18 A and safety devices  18 B, that are typical in hydraulic systems known in the art. 
       FIG. 2  is a schematic illustration of hydraulic system  10 . For the sake of brevity, only those features of hydraulic system  10  that will be useful to describe the disclosed control method is illustrated in  FIG. 2 . An operator sitting in cab  22  may control the operation of machine  100 . Part of the operator&#39;s control of machine  100  may include controlling levers  16 A and pedals  16 B. Signals in response to the operator&#39;s control of levers  16 A and pedals  16 B may be directed into an operator interface  28 . For instance, operator may depress and release pedal  16 B to change the speed of engine  12 . A signal indicative of the position of pedal  16 B may be input into operator interface  28 . In response to this signal, operator interface  28  may change the speed of engine  12 . Similarly, operator may control lever  16 A to operate implement  50 . A signal indicative of the lever position may also be input into operator interface  28 . The lever position may indicate the amount of hydraulic power that the operator desires to be directed to implement  50 . Based on this desired power, operator interface may determine the amount of flow that is to be directed to a particular hydraulic circuit to operate implement  50 . 
     Engine  12  of hydraulic system  10  may function in response to operator input from operator interface  28 . The operation of engine  12  is well known in the art, and therefore, will not be described herein. An engine speed sensor  14  may be operatively coupled to engine  12  to measure the speed of engine  12 . Any type of speed sensor known in the art may be used as engine speed sensor  14 . Engine  12  may be operatively coupled to pump  20  to drive an input shaft of the pump. 
     Pump  20  may be a variable displacement type of pump in which the stroke of the pistons (displacement) may be varied, while the pump is running. This piston displacement may correspond to the amount of fluid pumped per revolution of the input shaft. Since the cross-sectional area of the cylinders are a constant, as the stroke of the pistons increase, the amount of fluid pumped per revolution of the input shaft correspondingly increase. In some embodiments, pump  20  may have several pistons reciprocating in cylinders. A swashplate may be connected to the pistons at one end. The angle, or orientation, of the swashplate may determine the displacement of the pistons in the cylinders. A rotary valve, located at an end of the cylinder opposite the swashplate, may alternately connect each cylinder to fluid supply and delivery lines. By changing the angle of the swashplate, the displacement of the pistons may be varied continuously. Pump  20  may include mechanisms (such as, mechanical links or electronic devices) that enable the swashplate angle to be changed in response to commands from a control system  35 . 
     Pump parameter sensors  24  may be coupled to pump  20  to measure operating parameters of pump  20 . In this disclosure, pump parameter sensors  24  are used to collectively refer to all sensors that measure operating parameters of pump  20 . These sensors may include sensors that measure the discharge pressure (P d ) of pump. Discharge pressure (P d ) is the pressure of the fluid that exits pump  20 . In some embodiments, pump parameter sensors  24  may also include sensors that indicate the current displacement (d current ) of pump  20 . Current displacement of pump  20  may be determined by the position of the swashplate of pump  20 . 
     Input from engine speed sensor  14  and pump parameter sensors  24  may be directed to a pump feed forward control  26 . Based on these sensor inputs, pump feed forward control  26  may determine an allowable torque T allowable  that may be expended to operate implement  50 . Pump feed forward control  26  may send a signal indicative of the allowable torque and data measured by pump parameter sensors  24  to a pump displacement control  30 . Based on the determined T allowable , pump displacement control  30  may determine the allowable displacement (d allowable ) of pump  20 . Operator input from operator interface  28  may also indicate a desired displacement (d desired ) of pump  20 . The desired displacement may be a function of the operator requested hydraulic effort to operate implement  50 . Based on the determined allowable displacement d allowable  and operator desired displacement d desired , pump displacement control  30  may set the displacement of pump  20 . 
     Operator interface  28 , pump feed forward control  26 , and pump displacement control  30  may be hardware of software modules of control system  35  of machine  100 . In some embodiments, one or more of these modules may be combined together. Control system  35  may be a standalone part or may be part of a larger electronic control unit of machine  100 . Control system  35  may include memory and computational devices as is common in control systems known in the art. The memory devices may store maps and other specifications of the hydraulic system  10 . 
       FIG. 3  illustrates a schematic of a control algorithm used in hydraulic system  10 . The maximum torque that may be expended by pump  20  to operate implement  50 , T allowable , may be determined by pump feed forward control  26  based on a map  26 A. Map  26 A may be stored in control system  35  or may be calculated based on stored and measured values. Map  26 A may specify T allowable  at a measured value of engine speed N. The shape of map  26 A may depend upon the application. In general, T allowable  may vary from a maximum torque T max  to a minimum torque value T min . At high engine speeds, T allowable  may be set to T max , and at low engine speeds, T allowable  may be set to T min . In the exemplary map  26 A depicted in  FIG. 3 , at engine speeds below 1500 rpm, T allowable  may be set to T min , and at engine speeds above about 1550 rpm, T allowable  may be set to T max . The absolute values of T max  and T min  may also depend upon the application. In some embodiments, T max  may be the maximum rated torque of pump, and T min  may be a fraction of the T max  value (such as, for example 50% of T max ). The maximum rated torque of pump may be a value specified by the manufacturer or determined from the specifications of pump  20 . For instance, in cases where the maximum permissible displacement (or the maximum discharge volume) and maximum discharge pressure P d  of pump  20  are specified, T max  may be obtained as a function of the product of the maximum discharge pressure and the maximum permissible displacement (that is, T max =Max discharge volume×maximum discharge pressure) of pump  20 . In some embodiments, map  26 A may specify T allowable  as a percentage of T max . T allowable  determined from map  26 A may be input to pump displacement control  26 . 
     Pump parameter values, such as P d  and d current , may also be directed to pump displacement control  30 . In some embodiments, d current  may not be measured by pump parameter sensor  24 . In these embodiments, d current  may be value of the most recent pump displacement value d pump  input to pump  20 . Based on T allowable  and P d , pump displacement control  30  may determine the allowable displacement d allowable  of pump  20 . In some embodiments, d allowable  may be determined as a function of T allowable /P d . Pump displacement control  30  may compare the allowable pump displacement d allowable  value to the desired pump displacement d desired , input from operator interface  28 . As mentioned earlier, d desired  may be determined by operator interface  28  based on the position of lever  16 A. Pump displacement control  30  may then set pump displacement d pump  to be the lower of d allowable  and d desired . Pump displacement value d pump  may then be input into pump  20  to change the location of the pump swashplate. If the desired pump displacement value is lower than the allowable value (that is, d desired &lt;d allowable ), the displacement of the pump may be set to the desired value. In this case, the operator power demand may be completely satisfied. However, if d desired  is greater than d allowable  (d desired &gt;d allowable ) then the pump displacement may be set to the maximum allowable value. In this case, the operator&#39;s power demand may not be completely satisfied, and only the maximum allowable power at the current engine speed may be delivered to implement  50 . In some embodiments, pump displacement control  30  may also compare the computed pump displacement value d pump  to the current pump displacement value d current , and not change the pump displacement if d pump  is within a predetermined range of d current . 
     Engine speed sensor  14  and pump parameter sensors  24  may continue to monitor the operating parameters of hydraulic system  10 , and change d pump  in response to changes in engine speed and desired pump displacement d desired . Although the description above describes the pump displacement value as being selected based on a comparison between a desired and allowable pump displacement, it is contemplated that any variable that is indicative of pump power (pump displacement, torque, flow, etc.) may be used for the comparison. That is, in some embodiments, torque expended by pump may be determined based on a comparison of allowable torque to a desired torque, while in some other embodiments, flow delivered by pump may be determined based on an allowable flow to a desired flow. Therefore, in this disclosure, the term power value is used to represent any parameter (such as, for example, pump displacement, torque, flow, etc.) that is indicative of pump power. 
     INDUSTRIAL APPLICABILITY 
     The disclosed embodiments relate to a system and a method of controlling a hydraulic system. The hydraulic control system may be used to limit the power used by a pump to below a desired value when the speed of the engine decreases below a prescribed limit. By limiting the power used by the pump, further reduction in engine speed and engine lug may be avoided. By limiting the pump power only when it is truly needed and only to the extent that is needed to prevent lug, machine performance and operator perception of machine power may be enhanced. To illustrate the application of the disclosed hydraulic control system, an exemplary embodiment will now be described. 
     Pump  20 , rated to produce a maximum pressure of P max  and a maximum displacement of d max  may be fluidly coupled to a hydraulic cylinder  40  that operates implement  50  of construction machine  100 . Pump  20  may be operatively coupled with an engine  12  that also drives a propulsion system  44  of machine  100 . An operator may control machine  100  by operating levers  16 A and pedals  16 B located in cab  22  of machine  100 . During a construction task, such as lifting a load using implement  50 , operator may control lever  16 A to increase the hydraulic power directed to implement  50 . The speed of engine  12  at this time may depend on the loads (such as the torque used by the propulsion system  44 ) on engine  12  and the position of lever  16 B. Based on the measured operating parameters of machine  100 , hydraulic system  10  may determine the amount of power that may be directed to implement  50 . 
       FIG. 4  illustrates the steps used by hydraulic system  10  to determine the amount of power directed to implement  50 . Current operating parameters, such as engine speed N and pump discharge pressure P d , and desired operating conditions, such as d desired , are collected (step  110 ). The allowable torque (T allowable ) that may be directed to pump  20  at the measured engine speed N may be read off map  26 A (step  120 ). The pump displacement corresponding to the determined T allowable  (that is, d allowable ) may be calculated as d allowable =f(T allowable /P d ) (step  130 ). This allowable displacement (d allowable ) may be compared with the operator desired pump displacement (d desired ), and the lower of these displacement values may be used to set the displacement (d pump ) of pump  20  (steps  140  and  150 ). The operating parameters of hydraulic system  10  may be continuously monitored and d pump  updated when conditions change. 
     When the engine speed is high, the hydraulic system may not limit the amount of torque that may be used to operate the implement. When the load on the engine is high, engine speed decreases, and the system may limit the allowable power that may be used to operate the implement. At these conditions, the allowable power may be limited to an such an extent that the engine operates at maximum capacity. Limiting the power used by the implement at low engine speed, may allow the engine to operate at maximum capacity without causing the engine to lug. Determining the allowable torque used by the implement using a minimal number of operating parameters of the hydraulic system reduces system complexity and cost. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed system and method of controlling a hydraulic system. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed system and method of controlling a hydraulic system. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.